Two-photon spectroscopy of inductively perturbed naphthalenes
by Richard Dwight Jones
A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in
Chemistry
Montana State University
© Copyright by Richard Dwight Jones (1987)
Abstract:
Perturbative effects on the one - and two -photon spectra of naphthalene caused by fluorine, chlorine
and aza (nitrogen in the place of carbon) substitution are investigated. The results are in general accord
with the pseudoparity selection rules of Callis, Scott and Albrecht J. Chem. Phys. 7 8, 16 (1983).
Enhancement of two-photon absorptivity in the second excited singlet state (La) by fluorine
substitution is not as dramatic in the fluoronaphthalenes, as it is in fluorobenzene. As seen in the
one-photon spectra, fluorine substitution has a greater inductive effect in the 2- position of naphthalene,
but has less than aza substitution.
Polarized two-photon fluorescence excitation spectra in the laser wavelength range 450-650 nm are
compared to one-photon absorption spectra for solution-phase naphthalene, 1- and 2 fluoronaphthalenes , 1- and 2-chloronaphthalenes, isoquinoline and its cation. Results of INDO/S
calculations using singly as well as singly and doubly excited configuration interaction are presented
for naphthalene, vibrationally distorted naphthalenes, the fluoronaphthalenes, the azanaphthalenes and
their cations. The calculations underestimate the inductive effect of fluorine, especially in the 2position.
A method is developed for properly normalizing two-photon spectral data which greatly reduces the
uncertainties involved in matching spectral segments from different laser dyes. The usual linear
reference detector is replaced with a quadratic detector based on powders of nonlinear optical materials
such as potassium dihydrogen phosphate (KDP). The source of the uncertainties resulting from
normalizing two-photon excited fluorescence to the square of a linear reference is due primarily to
changes in laser temporal pulse widths as the dye laser is scanned. TWO-PHOTON SPECTROSCOPY OF INDUCTIVELY PERTURBED
NAPHTHALENES
by
Richard Dwight Jones
A thesis submitted in partial fulfillment
of the requirements for the degree
of
Doctor of Philosophy
in
Chemistry
MONTANA STATE UNIVERSITY
Bozeman, Montana
December 1987
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of a thesis submitted by
Richard Dwight Jones
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This work is dedicated to the memory of Professor Ray
Woodriff whose encouragement and inspiring example led the
author to graduate s t u d i e s .
V
ACKNOWLEDGMENTS
.
This project grew from the beautiful piece of work
done by Bruce Anderson during his last year of graduate
research,
in which he designed and built the t w o -photon
spectrometer used by the a u t h o r .
The willingness of David
Theiste to share his computer expertise is greatly
appreciated.
It was a pleasure working and recreating
with these talented f r i e n d s .
The patience and support of
Professor Patrik Callis is also appreciated.
Financial support from the M.S.U.
Department,
Chemistry
and the National Institutes of Health made
this work possible.
vi
TABLE OF CONTENTS
Pa Se
LIST OF TABLES
..........................................
vii
........ '.....•...........................
viii
LIST OF FIGURES
ABSTRACT
........... :....................................
OVERVIEW
..............................
PART I .
SPECTRA OF PERTURBED NAPHTHALENES
INTRODUCTION
Statement of Problem
PROCEDURES
I
........ \
........................................
Historical Background
xi'
■
2
3
........................
4
.........................
16
................
19
Two -Photon Experiments
.......................
19
One -Photon Experiments
.......................
27
Theoretical Computations
RESULTS
........
28
..... . . .,..............,....................
DISCUSSION
..........................................
.31
,54
■
CONCLUSIONS
PART II.
........... '.............. ................
NORMALIZATION OF TWO-PHOTON SPECTRA
........
71
........................
72
..........................................
76
INTRODUCTION
PROCEDURES
. . . ...........
RESULTS AND DISCUSSION
CONCLUSIONS
REFERENCES
70
........
79
.........................................
89
................................................
90
vii
LIST OF TABLES
Table
1.
Page
Sources of chemicals used in spectroscopic
experiments ..........................................
19
2.
Scan ranges and pulse energies of the dyes used
24
3.
T w o -photon excited fluorescence intensities
relative to naphthalene ............................
47
One-photon excited fluorescence quantum yields
relative to naphthalene ............................
47
Spectroscopic properties of naphthalene from
INDO/S calculations using SCI (singly excited
configuration interaction with Hataga-Nishimoto
electron repulsion) and SDCI (singly and doubly
excited configuration interaction with OhnoKlopman electron repulsion) ■.......................
49
Two-photon a b s o r p t i v i t y , Sg, induced in the
naphthalene L^ transition by MNDO vibrational
modes for singly and doubly excited configuration
interaction in INDO/S ..............
50
Spectroscopic properties of substituted
naphthalenes from INDO/S ...........................
52
Two-photon cross -sections of the L^ bands of
vibronically perturbed naphthalene and
equilibrium geometry I- and 2- fluoronaphthalenes
with different carbon-fluorine bond lengths .....
58
Temporal pulse widths ( n s , F W H M ) at the middle
and short and long wavelength ends of dye scans
79
4.
5.
6.
7.
8.
9.
..
..
viii
LIST OF FIGURES
Figure
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Page
Pi molecular orbitals of e y e lodecapentaene in
the atomic orbital basis ...........................
7
Pi molecular orbitals of naphthalene in the
atomic orbital basis ................................
9
O n e -photon (dotted line) and two-photon (solid
line) spectra of benzene (upper) and
f luorobenzene (lower) from reference 28 with
author's (P.R. C a l l i s ) permission .................
17
Polarized two -photon fluorescence excitation
apparatus .................................... ........
21
Linearly polarized two -photon excitation
spectrum of naphthalene (solid line) and
polarization ratio (dotted l i n e ) ..................
32
Linearly polarized two -photon excitation
spectrum of I -fluoronaphthalene (solid line)
and polarization ratio (dotted line) .............
34
Linearly polarized two-photon excitation
spectrum of 2-f luoronaphthalene (solid line)
and polarization ratio (dotted line) ..............
35
Linearly polarized two-photon spectrum of
I ,2,3,4-tetrafluoronaphthalene (solid line)
and polarization ratio (dotted l i n e ) .............
36
Linearly polarized two -photon excitation
spectrum of I -chloronaphthalene (solid l i n e )
and polarization ratio (dotted line) .............
38
Linearly polarized two-photon excitation
spectrum of 2-chloronaphthalene (solid l i n e )
and polarization ratio (dotted line) .......
39
ix
Figure
11 .
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
Page
Linearly polarized two-photon excitationspectrum of isoquinoline (solid l i n e )
and polarization ratio (dotted line) ............
41
Linearly polarized two -photon spectrum of
isoquinolinium cation (solid l i n e )
and polarization ratio (dotted line) ............
42
One-photon absorption spectrum of.
naphthalene in cyclohexane ........................
43
One-photon absorption spectrum of
1 - fluoronaphthalene in cyclohexane
..............
44
O n e -photon absorption spectrum of
2 - f luoronaphthalene in cyclohexane
..............
45
T w o -photon (solid l i n e ) and one-photon
(dotted line) spectra of naphthalene ............
60
Two -photon (solid line) and one-photon
(dotted l i n e ) spectra of I -f luoronaphthalene
....
62
Two -photon (solid l ine) and o n e -photon
(dotted line) spectra of 2-f luoronaphthalene
....
63
Two-photon (solid line) and one -photon
(dotted line) spectra of I -chloronaphthalene
....
64
Two -photon (solid line) and one -photon
(dotted l i n e ) spectra of 2-chloronaphthalene
....
65
Two-photon (solid line) and one-photon
(dotted line) spectra of isoquinoline ...........
66
Two-photon (solid line) and one-photon
(dotted l i n e ) spectra of isoquinolinium cation
68
..
Quantum counter detection of laser intensity,
(Q ) , for dyes used (upper panel) and second
harmonic intensity, (S }, normalized to (Q)^ ....
80
T w o -photon excited f l u o r e s c e n c e , ( F ) , of
naphthalene normalized to the square of
quantum counter intensity, ( Q ) (upper p a n e l )
and normalized to second harmonic intensity, (S ) ,
(lower panel) .......................................
82
X
Figure
Page
25.
Two-photon excited f l u o r e s e n c e , (F ) , of
1- fluoronaphthalene normalized to the. square of
quantum counter i n t e n s i t y , (Q )^ , (upper' p a n e l )
and normalized to second harmonic i n t e n s i t y , (S ),.
(lower p a n e l ) .......................................
83
26.
Two -photon excited f l u o r e s c e n c e , (F) , of
2- f luoronaphthalene normalized to the square of
quantum counter intensity, ( Q ) (upper p a n e l )
and normalized to second harmonic intensity, ( S ) ,
(lower p a n e l ) .......................................
84
The ratio (l^)/(l)^ from temporal and spatial
beam profiles (upper p a n e l ) and their convolution
compared to data from KDP powder (lower panel) ..
88
27.
xi
ABSTRACT
Perturbative
effects
on
the
one - and
two -photon
spectra of naphthalene caused by fluorine, chlorine and
aza (nitrogen in the place of carbon) substitution are
investigated.
The results are in general accord with the
pseudoparity selection rules of C a l l i s , Scott and Albrecht
J. C h e m . Phys . 7_8 , 16 (1983).
Enhancement of two-photon
absorptivity in the second excited singlet state (La ) by
fluorine substitution is not as dramatic in the fluoronaphthalenes , as it is in f l u o r o b e n z e n e . As seen in the
one-photon spectra, fluorine substitution has a greater
inductive effect in the 2- position of naphthalene, but
has less than aza substitution.
Polarized two-photon fluorescence excitation spectra
in the laser wavelength range 450-650 nm are compared to
one-photon
absorption
spectra
for
solution-phase
naphthalene, I- and 2 -f luoronaphth'alenes , I- and 2-chloron a p h t h a l e n e s , isoquinoline and its cation.
Results of
INDO/S calculations using singly as well as singly and
doubly excited configuration interaction are presented for
naphthalene,
vibrationally
distorted naphthalenes,
the
fluoronaph t h a l e n e s , the azanaphthalenes and their cations.
The calculations underestimate the inductive effect of
fluorine, especially in the 2- position.
A method is developed for properly normalizing twophoton
spectral
data
which
greatly
reduces
the
uncertainties involved in matching spectral s e g m e n t s ■from
different laser dyes.
The usual linear reference detector
is replaced with a quadratic detector based on powders of
nonlinear optical materials such as potassium dihydrogen
phosphate
(KDP).
The
source
of
the
uncertainties
resulting from normalizing two-photon excited fluorescence
to the square of a linear reference is due primarily to
changes in laser temporal pulse widths as the dye laser is
scanned.
I
OVERVIEW
The
two
Initial
part
of
the
was
to
use
thesis
spectroscopies
two
goal
lowest
thalenes.
research
described
one-photon
and
in
this
two-photon
to compare the absorption strengths of the
excited
The
singlet
major
states
scientific
of
substituted
contribution
from
naph­
this
wor k resides in the two-photon spectra of these m o l e c u l e s ,
which
are
presented
spectra in Part I .
semiempirical
wit h
their
respective
one-photon
Also included in Part I are results of
molecular
orbital
calculations
on
species
pertinent to this study.
To
the
confidently
lowest
two-photon
the
excited
detector
frequency
wh i c h
heretofore
the
absorptivities
method
was
by
harmonic
generated
from
of
in
a
the
powders
Most
of
constructing
using
of
of
normalizing
developed.
involved
eliminated
second
was
revealed
concerning
was
u pon
optical materials.
insights
new
fluorescence
spectra
based
two-photon
states, a
excited
uncertainty
long-range
compare
reference
excitation
of
nonlinear
Part II describes this m e thod and the
by
temporal
causes
of
and
the
spatial
beam
uncertainty
previously plagued two-photon spectroscopy.
profiles
which
has
2
PART I
SPECTRA OF PERTURBED NAPHTHALENES
3
INTRODUCTION
Two-photon
(TP)
absorption
is
a
nonlinear
optical
phenomenon in which two quanta of light are simultaneously
absorbed
to
excite
transition
energy
energies.
atom
is
the
or molecule
sum
of
to a
the
state whose
single
photon
The probability for such a process is quadrati-
cally
dependent
light,
whereas
linearly
upo n
for
the
are
those
powerful
OP
lasers
thirty
the
excitation
absorption
cross-sections
orders
absorption.
are
of
(OP)
Moreover,
some
for
intensity
one-photon
dependent.
absorption
than
an
of
magnitude
Consequently
required
to
induce
it
is
for
TP
smaller
moderately
observable
TP
absorption.
TP
spectroscopy
complementary
just
In
as
Raman
inaccessible
spectroscopy
centrosymmetric
absorption,
g<— »g
transitions,
to
or
be
provides
or
g<— »u,
probed.
molecules
u<— >u,
which
Even
for
symmetry,
states
strong
a b s o r b e r s , so
one
TP
spectrum
dependence
which
information
to
does
to
the
are
allow
common
opposite
different
wea k
OP
information
rules
sets
a
for
TP
for
OP
of
states
center
absorbers
are
absent
hidden
is. revealed by the other.
of TP absorption upo n
absorption.
those
without
is
experiments,
infrared
selection
molecules
are
OP
which
or
of
often
in
In addition,
the
the polarization of
the
4
excitation light provides knowledge about
the symmetry of
the excited state.
Benzene and naphthalene were among the first organic
molecules
reason
to
being
be
investigated
to
examine
by
TP
theories
spectroscopy
of
their
-
the
electronic
structure wh i c h had been built upo n OP spectroscopic data.
The
following
of theories
section recounts
explaining the
the historical development
spectral features of these and
related m o l e c u l e s .
Historical Background
In the early
1940's , patterns began to appear in the
OP electronic absorption spectra of aromatic hydrocarbons
and their substituted derivatives
bands were
observed
in most
of
(I - 3).
Three types of
these ultraviolet
absorp­
tion spectra:
1)
w e a k b a n d s , usually of lowest e n e r g y , with
complex vibrational s t r u c t u r e , having
intensities and positions sensitive to
substituents,
2)
moderately intense bands unchanged by
s u b s t i t u e n t s , w ith regular vibrational
s t r u c t u r e , and .
3)
strong bands of highest energy wit h little
vibrational structure.
5
Theoretical
descriptions
of benzene's
excited states
(4,5) and the effects of substituents on its least intense
band (6,7) were complex and could not be applied to larger
molecules.
Not
until
a
simple
unified
theory
was
developed could these spectral features be understood.
Platt
(8),
describing
and
b e n z e n e -Iike
present
in 1949,
introduced an
classifying
the
electronic
hydrocarbons, which
understanding.
The
elegant method
forms
idea
of
the
tt
of
states
basis
of
electrons
of
the
in
an
unsaturated system being "mobile” (9), delocalized or free
had
been
use d
mechanical
the
to
describe
particles
spectra
of
in a box
linear
polyacenes
perimeter)
are
like
one-dimensional
perimeter.
component
angle.
of
orbitals
as
quantum
(11).
Platt
postulated
in cata-condensed hydrocarbons
which
those
loop
Such
of the form
in
electrons
(10) and applied to explain
polyenes
that the n and tt* orbitals
(those
those
all
carbon
atoms
lie
on
a
of a free electron moving on a
constant
are
potential
around
the
described by wavefunctions
, where m is the quantum number for the z
of
angular
momentum
and
<fi
is
the
azimuthal
For the orbitals of cyclic polyenes,
- (2n + l ) < m < + ( 2n + I )
where 2(2n + 2 )
Orbitals
degenerate
is the number of carbon atoms in the ring.
wit h
the
in e n e r g y , but
same
absolute
along w ith
value
this
of
m
are
energy pairing
6
there
is
also
a
mirror
image
pseudo-pairing
across
the
barycenter of e n e r g y .
These properties are illustrated in Figure I by the tt
molecular
orbital
tional
orbitals
of
eyelodecapentaene
representation.
to
orbital
the
and
The
coefficient
the
of
size
the
of a circle
the
respective
filled and unfilled
and positive coefficients,
in
circles
respectively.
atomic
is p r o p o r ­
2pn
atomic
are negative
In this f o r m , or
representation,
the molecular orbitals are not necessarily
eigenfunctions
of the angular momentum o p e r a t o r , but they
do retain the nodal patterns of the e^m ^ eigenfunctions of
this o p e r a t o r .
The
mirror
present
in
pair.
For
coefficients
molecular
image
each
pseudo-pairing alluded
primed
and
unprimed
e x a m p l e , in molecular
have
orbital
the
I'
same
the
molecular
orbital
magnitude
coefficients
to above
are
orbital
I all
and
of
the
sign.
the
is
In
same as in
I, but the sign of alternate coefficients has c h a n g e d .
It
would soon be discovered that this pseudo-pairing property
resulted
in
into
distinct
two
states
configuration
behaved
interaction
s u b s e t s , and
differently
different p e r t u r b a t i o n s .
that
under
states
these
the
which
two
fell
types
influence
of
of
7
O
>
O
cr
LlI
Z
Ll I
O
Figure I .
Pi molecular orbitals of e y e lodecapentaene in
the atomic orbital basis
8
If the carbon atoms of e y e lodecapentaene are numbered
sequentially,
formed
one
by
and
starting
removing
six
and
cross-linking
the
even
the
process
which contain no
the
top,
hydrogen
bonding
numbered
one and six.
at
these
has
no
then
atoms
bonded
carbons
effect
to
the
is
carbons
together.
on
eyelodecapentaene
naphthalene
This
energies
molecular
of
orbitals
contributions from 2pn orbitals on atoms
H o w e v e r , the odd numbered molecular orbitals
do have nonvanishing coefficients at atoms one and six and
the energies
the
two
coefficients
they have
which
of these molecular orbitals are decreased if
have
opposite sign.
shows
the
the
same
This
molecular
sign
or
increased
if
is illustrated in Figure 2
orbitals
of n a p h t h a l e n e , again
in the atomic orbital representation.
As Huckel had done in the case of benzene (12), Platt
summed
the
electrons
orbital
to
particular
angular
obtain
the
electronic
momenta
total
ring
state.
His
of
all
the
n
(free)
quantum number
model
for
identified
a
the
three absorption bands in the c a t a -condensed h y d r o c a r b o n s .
In
his
nomenclature,
quantum number
based
on
u pon excitation,
changes
in
the weak band
L]-,, the moderate band L a , and the strong band
in
the
cyclic
description
this wo r k
polyenes)
included
is only
Ba
triplet
or
as
.
well
as
total
is
ring
labeled
(degenerate
A l t hough
Platt's
singlet
states,
concerned w ith the latter and will not
label states by their multiplicity.
9
>
O
cr
LU
Z
LU
Figure 2.
Pi molecular orbitals of naphthalene in the
atomic orbital basis
10
In a later wor k extending the treatments of Sklar (6)
and Forster
for the
of
(7), Platt presented a s e m i -quantitative model
intensity induced in the I/y (lowest energy)
mono-,
di-
determined
and
t r i -substituted
relative
different
polyacenes
"spectroscopic
substituents
including
aza (nitrogen in the place of c a r b o n ) .
here
for
future
reference
that
in
(13).
moments”
fluorine,
bands
He
for
25
chlorine
and
It should be noted
this
vector
model
the
signs of the moments induced by fluorine and chlorine were
taken as positive and that of aza was negative.
A
more
quantitative
spectra
of
Moffitt
(14).
linear
the
complete
cata-condensed hydrocarbons
To
Platt's
combination
of
free-electron
spectrum
and
of
atomic
he
cyclic
perturbed
cross-linking,
molecules,
such
as
orbitals
the
OP
formulated
by
(LCAO)
Starting
estimated
hypothetical
by
was
of
perimeter model he applied the
orbitals.
benzene,
model
polyene
the
wit h
naphthalene,
form
the
properties
molecules,
to
instead
which
the
of
known
of
he
the
then
cata-condensed
a z u l e n e , anthracene,
and
phenanthrene.
One
of
treatment
into
the
was
most
his
revealing
bisection
of
"odd"
and
"even”
parts.
illustrates
the
division
of a
even m a t r i c e s :
discoveries
the
The
3 x
of
Moffitt's
perturbation
following
3 matrix
into
matrix
equation
odd and
6
.7
8
9,
==
4
0
6
0
8
0.
+
0
3 ’
0
5
0
VO
5
'l
O
4
O
3'
CM
2
O
’I
■vi
11
Although the.elements of the first matrix on the right are
even numbers the matrix is " o d d " , by M o f f i t t 's definition,
because
the
sum of
the
is an odd n u m b e r .
indices
of every non-zero element
Moffitt also realized that each of the
cyclic polyene
states
and of the
lower e n e r g y , dipole -forbidden states,
two
possessed an even or odd c h a r a c t e r ,
one
(Lj3) would be coupled to the dipole-allowed states only by
even perturbations
bations.
and the other
Cross -linking
respective
cyclic
(La ) only by odd p e r t u r ­
- forming polyacenes
polyenes
(such
as
from
their
naphthalene
from
c y clodecap e n t a e n e ) - is an odd perturbation since the new
matrix
bation
element
is
couples
naphthalene
to
in the
the
the
1,6 position.
otherwise
allowed
B
This
forbidden
state
odd p e r t u r ­
La
thereby
state
inducing
of
OP
absorptivity.
He
continued
substituents
by
considering
as being
”even"
the
effect
perturbations
associated w i t h changes in the diagonal
the
Coulomb matrix.
This
analysis
of
inductive
since they are
(even) elements of
resulted
in excellent
agreement wit h experimental intensities of the Lj3 bands of
ortho -, metahe
and para-
demonstrated
that
d i s u b s tituted benzenes.
stretching
vibrations,
Likewise
as
odd
12
perturbations,
could couple L a to a dipole-allowed state,
provided
have
they
the
proper
syimmnetry
as
dictated
by
group t h e o r y .
McLachlan
odd
(15) and Donath
character
involving
Moffitt
promotion
molecular
orbital
antibonding
configurations.
of
j,
orbital
had
a
(16) explained the even or
utilized.
n
electron
for example,
j ' , are
Configurations
into
termed
A n y nonsymmetric
from
its
a
bonding
corresponding
symmetrically
configuration
excited
j->kr , for
example, has a related (degenerate in the cyclic polyenes)
configuration k-»j' .
Linear
combinations
of
nonsymmetric
configuration pairs are divided into two distinct classes:
the
additive
tive
combinations,
combinations,
configurations
to
produce
(including
termed
may
plus
the
termed plus,
mix
through
states.
ground
minus.
configuration
minus
configuration)
among themselves resulting in minus states.
state
and
La
pseudoparity,
dictates
(even
is a plus
state.
subtrac­
Pariser
and
plus
interaction
configurations
interact
only
L]-, is a minus
(17)
showed
that
as this even or odd character is n o w called,
electric
«--»
the
Symmetric
Likewise,
state
and
odd),
dipole
in
selection
addition
to
r u l e s , plus
those
of
+—> minus
group
theory.
Further refinements were made
in the analysis of vibronic
effects
of
by
Albrecht
(18)
and
inductive
(substituent) effects by Petruska (19).
and
mesomeric
13
Modern
computers
calculations
the
of
molecular
simplicity
Moffitt
model
have
and
have
obviated
pencil-and-paper
spectroscopic
qualitative
proven
it
i n f o r m a t i o n , but
accuracy
of
to
useful
be
a
interpreting recent experimental r e s u l t s .
the TP
spectra of aromatic molecules
the
In
have
Platt
tool
in
particular,
been
explained
on the basis of this m o d e l .
In
1931
Goeppert-Mayer
simultaneous
absorption
1961 , after
the
observed
was
flurry
of
this
laser
(21).
sample
of
The
predicted
two
was
photons
invented
first
organic
I -chloronaphthalene
spectroscopic
the
possibility
(20).
was
Not
this
of
until
phenomenon
molecule use d as a TP
(22)
activity.
which
By
the
initiated
beginning
a
of
■d e c a d e , TP spectra of more than one hundred aromatic
compounds had been gathered and once again patterns became
apparent.
Goodman
and
coworkers
produced
spectra of substituted benzenes
work of Sklar
(6), Forster
many
(23-25),
(7), Moffitt
vapor-phase
TP
and extended the
(14) and Petruska
(19) to explain the intensity regularities in the ky bands
of
polysubstituted
benzenes.
Their
work
entitled
"Two-
Photon Spectroscopy of Perturbed B e n z e n e s " is an excellent
review
(26).
finding that
the
TP
formally
Pertinent
inductive
absorption
to
present
substituents have
strength
T P -forbidden
the
but
of
the
gains
work
is
their
little effect on
L-]-, b a n d ,
absorptivity
which
is
through
14
vibrational
This was
perturbation
of
the
hexagonal
symmetry.
also seen by C a l l i s , Scott and Albrecht in their
solution-phase TP spectrum of pyrimidine (27).
Coupling
selection
this
rules
Albrecht
observation
in
(28,29)
OP
wit h
the
spectroscopy,
pseudoparity
Callis,
formulated selection rules
Scott
and
for O P ,TP and
three-photon spectroscopies of alternant hydrocarbons.
By
extending the TP spectra of benzene and fluorobenzene into
the
La
region
the
latter
unaffected by inductive
group
saw
that,
substitution,
while
L]-, is
L a is substantially
enhanced (28).
The
group
selection
theory
properties
They
these
with
of
showed
rules
those
alternant
that
combine
imposed
by
the
hydrocarbon
transition
states fall
restrictions
bond
imposed
pseudoparity
electronic
order
by
matrices
states.
between
into the even and odd m a trix categories
Hoffitt had seen in the perturbation Hamiltonian matrices.
Transition
same
bond
order
pseudoparity
matrices
are
odd
between
matrices
states
while
having
those
the
between
states of different pseudoparity are even.
To
states,
determine
if
a
perturbation,
$ ^ and W g , the matrix
evaluated.
element
K' , can
couple
(^rI [X'
two
must be
Another way to write this is:
tr (X' |»2 ><*ll ) " Cr 0 O 12 ) ,
where,
cable
a first-order reduced density matrix
in
this
case
since X ' is
a
one-electron
(appli­
operator)
15
and
tr
means
symmetrized
the
trace
density
of
the
matrix
or
matrix
product.
transition
m a t r i x , B , is given by B 12 = l/2(p12 + p 2 1 ).
demonstrated as Donath
(16) had,
matrix
it
is
odd
(even)
can
The
bond
order
Finally they
that if the perturbation
only
couple
states
whose
transition bond order matrix is odd ( e v e n ) .
In
the
OP
spectra
of
perturbed by vibrations
rules
predict a w eak
bond
order
matrix.
wit h
transition
matrix.
bond
and n a p h t h a l e n e , only
(odd p e r t u r b a t i o n s ) , the selection
(minus) band since its transition
the
However,
benzene
ground
La
order
has
with
state
plus
the
(minus)
is
an
pseudoparity
ground
state
even
and
is
its
an
odd
Its contraction w ith the vibrational or c r o s s ­
link perturbation matrix
(also odd in the p o l y a c e n e s , but
even in the case of a z u l e n e , for example) does not vanish,
resulting
in
other hand,
moderate
absorptivity
in
the
La .
On
the
fluoro- and aza- benzenes and naphthalenes are
inductively perturbed
(odd perturbation matrix)
resulting
in enhancement of L]-, absorptivity wit h little effect upon
La .
These predictions are confirmed in the OP spectra of
benzene,
f Iuorobenzene
and
pyridine
(azabenzene)
and
n a p h t h a l e n e , quinoline and isoquinoline (a z a n a p h t h a lenes).
C a l l i s , SCott
spectra
of
vibrational
process
these
and
Albrecht
compounds
perturbations
p l u s -to-plus
or
realized
the
are
roles
reversed,
minus -to-minus
that
of
in
the
inductive
since
in
transitions
TP
and
a
TP
are
16
induced.
gains
According
intensity
should
be
inductive
to
the
through
unaffected
selection
vibrational
by
are verified
enhance
the
L]-, state
perturbations
inductive
perturbations will
predictions
rules
effects.
and
However
the L a b a n d .
These
in the TP spectra of benzene and
f Iuorobenzene (28), which are shown in Figure 3 with their
OP spectra.
Statement of Problem
As
thalene
to
the
two
(32,22)
test
project
began,
TP
spectra
of
naph­
- the aromatic alternant hydrocarbon most similar
benzene
only
present
- had
been
substituted
published
naphthalenes,
and acenaphthene
the pseudoparity
(30,31) .
TF
of
I -chloronaphthalene
(31) were available.
selection rules
spectra
To further
regarding
inductive
perturbations which appeared to hold in these c a s e s , this
work under t o o k the task of obtaining TP spectra of f l u o r o and aza- n a p h t h a l e n e s .
During
the
early
part
of
this
research,
Rava
and
Goodman (33) published vapor-phase TP spectra of I- and 2f luoron a p h t h a l e n e s .
Their
spectra
of
the
L]-, bands
of
these compounds display totals of 7 and 5 n m respectively.
Their finding that the inductive perturbation of fluorine
especially
origin
is
in
in
the
sharp
2-
position
contrast
pseudoparity selection r u l e s .
to
greatly
the
enhanced
predictions
the
L^
of
the
17
BENZENE
F L U OR O B E NZ E NE
20 0
220
24 0
260
WAVELENGTH (NM )
Figure 3.
One -Photon (dotted line) and T w o -Photon (solid
line) spectra of benzene (upper) and fluorobenzene (lower) from reference 28 with author's
(P .R . C a l l i s ) permission
18
At
the time these results appeared,
had produced
preliminary TP
spectra of
the present work
I- and
2- fluoro-
naphthalenes which agreed not with the results of Rava and
Goodman
but
Albrecht.
of Rava
with
the
In order
predictions
to challenge
and Goodman,
it was
of
Callis,
Scott
the v a p o r -phase
essential
and
results
to be confident
in
the relative peak heights of these solution-phase spectra.
The
joining
of
wi t h different
art"
II
(34).
has
excitation
at that time,
data
obtained
kn o w n as a
"black
Development of the technique described in Part
in
naphthalenes
sections.
fluorescence
dyes was,
reduced
resulted
TP
the
uncertainty
reliable
which
spectra
are
in
the
of
inductively
presented
TP
in
experiment
the
and
perturbed
following
19
PROCEDURES
Two-Photon Experiments
The chemicals used as samples in this work are listed
in Table I.
All were fractionally distilled under 0.1-1.0
m m Hg vacuum with the help of Richard R . C o p p , Jr.
the
author
is most
fluorescence
excitation
spectral
grateful.
emission
spectra
wavelengths
analysis
Purities were
and
performed
taken
by
by
at
to whom
confirmed by
three
different
chromatographic/mass
L . Joseph
Sears
at
the
M.S.U. mass spectrometry facility.
Table I .
Sources of chemicals used in spectroscopic
experiments
Chemical
Source
n a p h t h a Iene
Chem Service
I -fluoronaphthalene
Aldrich
2 -f luoronaphthalene
Pfaltz and Bauer
isoquinoline
Aldrich
I -chloronaphthalene
Chem Service
2 -chloronaphthalene
Eastman
I ,2,3,4-tetrafluoronaphthalene
research sample
donated by
Dr. Robert Filler
Dept, of Chemistry
111. Inst. , of Tech,
20
Solutions
naphthalenes
of naphthalene and the fluoro-
were
made
spectrophotometric
dissolved
in
at
grade
0.100
0.200 M.
cation was
prepared
in Aldrich
cyclohexane.
spectroscopic
tration of
M
and c h l o r o -
grade
Gold
Label
Isoquinoline
ethanol
at
a
was
concen­
A 0.20 M solution of isoquinolinium
by using
0.2
H
hydrochloric
acid
in
ethanol as the solvent.
The
was
polarized
designed
detail
in
apparatus
by
his
TP
fluorescence
Bruce
E . Anderson
doctoral
made
during
and
with
thesis
the
replacing the homebuilt
amplifiers
detector
quadratic
of
units,
cooled
quantum
reference
in
Figure
4,
in
use
Changes
this
response,
IS
is
in
work
in
his
include
of a monochromator
in
the
and use of a photodiode as
fluorescence.
based
on
KDP
The
powder
is
A diagram of the apparatus is shown
which
H
is
a
monochromator,
visible-absorbing/ultraviolet-transmitting
a Hamamatsu R955
described
photomultiplier
counter
detector
described in Part II.
of
is
apparatus
stepping motor drivers and signal
commercial
thermoelectrically
and
(35).
course
fluorescence detection channel,
the
excitation
photomultiplier
a diffusing
F
filter,
is
PHT
a
is
tube configured for fast
integrating
sphere,
QC
is
a
quantum counter solution of Rhodamine B or Nile Blue A, PD
is a fast
response
the
cell
2
mm
(back biased)
containing
75-150
PIN photodiode
pm
particle
and X
size
(potassium dihydrogen phosphate) powder in decalin.
is
KDP
21
......... V.... 1
ATTENUATOR
POLARIZER
FRESNEL
RHOMB
SAM PLE
Figure 4.
Polarized two -photon fluorescence excitation
apparatus
22
The
one
replacement
major
of
improvement
the
in
troublesome
the
NRG
apparatus
was
the
nitrogen-pumped
dye
laser with a Lumonics N d :Y A G -pumped dye l a s e r .
Adaptation
of existing data acquisition software to the n e w dye laser
was done by David Theiste and the a u t h o r .
The
data
first
step
in collecting
meaningful
TP
spectral
is to confirm the quadratic dependence of TP excited
fluorescence
each
sample
on
the
at
the
laser
intensity".
maximum
This
intensity
was
done
wavelength
of
for
each
dye and/or at a maximum in the TP fluorescence excitation
spectrum.
polarizer
laser
Two methods were used to test for this:
and
neutral
light.
density
Reducing
the
filter
laser
attenuation
intensity
to
crossed
of
the
50% should
reduce the fluorescence to 25%.
A criterion for acceptable data in this work was that
Q , as defined b e l o w , should be within the range I. 9- 2 .0 .
r<F)
Q
=
i
f(l>
l<F'>J
(F>
is
is the average
the
average
I
l o g
l o g
!(I')]
integrated fluorescence
integrated
laser
intensity
i n t e n s i t y , <l)
and
the
primed
quantities are those measured when the beam is attenuated.
Host Q values were in the 1.95-2.00 range except those for
isoquinoline
and
quantum yields
resulting
to-noise
its
cation which
both have
fluorescence
approximately a tenth that of n a p h t h a l e n e ,
in weak
ratios.
fluorescence
signals
and
lower
sign a l -
If the Q value was u n a c c e p t a b l e , photon
23
flux
in
the
sample
was
reduced
by
either
defocus ing
the
beam or decreasing the laser p o w e r .
Once
this
criterion was
satisfied,
a software option
could be used to insert three neutral density filters
(79,
63 and 50 %T) into the beam and step the laser wavelength.
At several wavelengths in the lasing region of a dye,
values
of
(F) / ( l) ^
wavelength
(using
These
n
the
-
absorption
calculated.
relative
I =
tests
were
that
commonly
stationary
fluoresce
a
deviation
3 w e i g h t i n g ) was
insured
did
standard
At
instead
states
of
four
particular
among
less
these
than
5%.
excited by TP
undergoing
further
excitation into non-fluorescing states.
After
it
was
confirmed
depended
quadratic a l Iy
collected
at
on
that
fluorescence
laser
0.5 nm increments
intensity,
from 550 to
intensity
data
were
650 nm and at
1.0 n m increments from 450 to 550 nm.
Dyes were d i l u t e d ,
mixed
scan
or
solvent
shifted
to
produce
ranges
overlapped by at least 5 nm or 10 data points.
their
scan
ranges
and
maximum
pulse
which
Dyes u s e d ,
energies
(at
the
s a m p l e ) are listed in Table 2.
The
the
signal
changing
The
s i g n a l -to-noise
amplifiers
and
limited
by
linearity
of
i n t e g r a t o r s , was
optimized
by
the photomultiplier voltages
consequence
segments
ratio,
must
be
of
these
fit
or amplifier g a i n s .
adjustments
together
by
is
that
multiplying
TP
an
data
entire
24
segment
by a constant
determined
to
intersect
its n e i g h ­
boring segment at the center of their overlap region.
Table 2.
Scan ranges and pulse energies of the dyes used
Dye
Max i m u m Energy
(mJ/oulse')
Scan Range
(nm)
Coumarin 460
450 -480
3
Coumarin 480
465-500
4
Coumarin 500
490-530
6
Coumarin 485
510-560
4
Coumarin 540A
530-580
5
Rhodamine 590
552-574
14
562-585
10
578-596
12
589-612
8
Rhodamine 640
605-623
12
D icyanome t h y Iene
617-665
8
Rhodamine 590
+ Rhodamine 610
Rhodamine 610
Rhodamine 610
+ Rhodamine 640
In
method
the
spectral
presented
absorption
segments
of
of
in
second
(F)/(l)^
m e t h o d , i.e. , matched
the
normalization
region 450-550
nm
Part
not
be
the
filters.
II
could
harmonic
in
at
this
the
by
region were
midpoint
of
used
fit
by
their
due
to
Data
the
old
overlap
range and averaged after deletion of curled-up points.
25
The
light
ratio
to
that
of
TP
absorption
of
linearly
of
circularly
polarized
assigning excited state symmetry.
light
polarized
is
useful
This ratio was measured
at one nanometer increments for all the s a m p l e s .
polarizer was
circularly
rotated 45°
polarized
by a
light
in
stepping motor
with
the
Fresnel
The Glan
to produce
rhomb.
The
ratio of (F)/(l)2 with circularly polarized light to
(F)/( i )^
with
linearly
polarized
light
was
computed
at
each wavelength.
TP
were
excited
compared
at
spectral
region
adjusted
to
were
fluorescence
wavelengths
covered.
insure
quadratic
several
and
that
the
intensities
largest
smallest
all
samples
throughout
Experimental
the
of
parameters
fluorescence
were
above
the
were
signals
noise
level.
Each sample was
placed in the cell holder three different
times while
ratio
for 400
the
laser
shots.
(F)/(l)^ was
The
measured and
three values
averaged
obtained for each
sample were averaged and normalized to that of naphthalene
at
that
wavelength.
detectors were not
numbers
can be used
Since
the
changed during
beam
parameters
and
these m e a s u r e m e n t s , the
in conjunction with relative fluores­
cence quantum yields to determine- relative TP crosssections .
Relative
linearly
polarized
spectra were
constructed
using the following relationship analogous to the LambertBeer law:
26
<F> - GLC$f 5 l l n <l ) 2
where G is a collection of instrumental parameters , 5 j_j_n
is the TP absorptivity for linearly polarized l i g h t , $£ is
the
fluorescence
dependent, C
pathlength
settings
is
in
at
a
quantum
the
the
yield
sample
which
may
be
concentration
sample.
For
and
constant
particular wavelength,
the
wavelength
L
is
the
instrumental
TP
absorptivity
of sample 2 relative to that of sample I is given by:
The
^lin(Z)
(F(2))$f(I)C(I)
^lin(I)
<F(l)>$f(2)C(2)
measurement
intensities,
relative
TP
excited
(F ( 2 ))/(F ( I )}, was described
paragraph.
fluorescence
in the previous
Relative fluorescence quantum yields,
(2 )/$f(I)
section.
of
were
measured
as
described
in
the
following
Absolute concentrations were k n o w n .
The procedure used to construct a TP spectrum of each
sample consisted of matching and averaging the eleven data
segments.
compared
Digital
numerically
fluorescence
number was
Each
cence
files
to the
through
each
their
of
the
relative
spectrum of naphthalene.
spectra
TP
were
excited
One average
calculated by which to multiply each spectrum.
spectrum
was
then
quantum yield
relative
of
divided
(naphthalene
concentration.
The
by
its
=
prominent
relative
I) and
fluores­
then by
vibronic
peak
its
at
601 n m in the naphthalene spectrum was given a 6 H n value
27
of u n i t y .
Fluorescence
quantum yields
for
TP
excitation
were assumed to be the same as those for OP e x c i t a t i o n .
At
the
transition
emission
wavelength
of
each
spectra
These
were
fluorescence
the
most
sample,
were
cence monochromator
of
T P -excited
obtained
using
0.5
by
mm
obtained
for
emission
spectra
intense
fluorescence
scanning
slits
the
fluores­
(bandwidth 4 nm) .
comparison
to
low-energy
with
O P -excited
the
often-made
test
assumption that emitted fluorescence is independent of the
means of excitation.
One-Photon Experiments
Relative
measured
detector
light
for
OP excited fluorescence
all
as was
from
a
monochromator
samples
used
and
extinguished
the
Since
source
was
UV
constant
fluorescence
and
lamp
the
passed
in
the
isoquinoline
beam
in
a
intensities
of
geometry
0.10
at
tenths
M
a
(or
quartz
0.20
solutions
of
a
complete,
the
samples
M
which
millimeter.
a particular
was
and
Ultraviolet
through
cation)
few
intensity
absorption
same
TP experiments .
was
focused
isoquinoline
the
in the
xenon
and
with
quantum yields were
wavelength
the
relative
reflects
their
relative fluorescence quantum yields.
OP excited fluorescence emission spectra of the 0.10M
or 0.20 M samples were obtained with a Spex fluorometer in
the
front-face
geometry.
Right-angle
geometry
and
28
concentrations
of approximately
instrument
obtain
to
IO""*"1' M were
OP-excited
used
fluorescence
on
this
excitation
spectra.
OP
absorption
spectra
were
obtained
visible-ultraviolet spectrometer.
difference
to
four
significant
with
a
Cary
14
Solutes were weighed by
figures
and
dissolved
in
spectral grade solvent to make two known concentrations of
each c o m p o u n d .
Quartz cells with pathlengths of 100,
2 0 , 10 and 5 mm were used to collect spectra vs.
filled cell
were
a solvent
of the same pathlength and material.
digitized
at
0.5
nm
increments
and
50,
Spectra
matched
by
concentration and optical pathlength using the LambertBeer
law.
The
resulting
digital
spectra
were
transferred
to
computer files as molar decadic extinction coefficient v s .
wavelength.
Theoretical Computations
Calculations
absorptivities
using
program
IND0/S
of
of
OP
excited
(36),
a
parameterized
Calculation
oscillator
singlet
of vibrational
singly
out:
normal
configuration
excited
were
molecular
spectroscopic
naphthalene used HNDO (37).
carried
states
s e m i -empirical
for
strengths
TP
performed
orbital
information.
displacements
for
Two sets of calculations were
interaction
configurations
electron repulsion parameters
mode
and
using
(Cl)
involving
Mataga-Nishimoto
(SCI), and Cl involving both
29
singly and doubly excited configurations wit h Ohno-Klopman
electron repulsion ( S DCI).
to
determine
the
These two approaches were used
inductive
and
vibronic
effects
on
the
spectroscopic properties of n a p h t h a l e n e .
The
spectrum
study
of
of
vibrational
naphthalene
effects
began
by
on
the
electronic
optimizing
the
ground
state equilibrium geometry of the molecule using an option
in
MNDO.
force
With
another
field,
option
frequencies
in
and
displacements were c a l c u l a t e d .
the
same
program,
Cartesian
normal
the
mode
Twelve in-plane modes with
significant carbon motion and symmetry capable of inducing
TP absorptivity were chosen from the total of 48.
a
large majority
of
the TP vibronic activity
Though
is known to
be due to bg^ modes coupling Iq3 to the ground state, modes
with
b]_u
in-plane
symmetry
(the
irreducible
only
other
non-totally
representation
in
the
symmetric
D 2h
point
g r o u p ) were also included in this analysis to determine if
any
TP-allowed
excited
states
are
coupled
to
Iq-,.
The
naphthalene molecule was distorted one zero-point rootmean- square amplitude from its equilibrium geometry along
each
of
these
normal
coordinates
(38)
and
used
as
input
geometry for the INDO/S c a l c u l a t i o n s .
The
inductive
effect
of
substituting
fluorine
hydrogen or nitrogen for carbon at the I- or 2 of
naphthalene
calculations
on
was
I-
determined
and
by
for
positions
performing
INDO/S
2 -, f l u o r o n a p h t h a l e n e , q u i n o l i n e ,
30
isoquinoline,
cation.
Boggs
(39)
quinoIinium
and
isoquinolinium
The naphthalene crystal geometry of Sellers and
was used
for all
fluorine bond
lengths of
to
any
determine
parameter.
cation
these calculations.
.130,
dependence
.133 and
of
the
Carbon-
.136 n m were used
results
upon
this
31
RESULTS
The
excitation
spectrum
cyclohexane
is
in
spectrum
that
dotted
TP
is
line
shown
of
spectrum
Figure
linearly
is
of
the
0.1
M
5.
naphthalene
The
polarized
solid
light
polarization
line
while
ratio
in
fi .
the
This
polarization spectrum is in excellent agreement with that
of Dick and Hohlneicher (31).
Only two differences in the
linearly polarized spectrum are noticeable:
of
the
vibronic
spectrum are
peaks
all
the
at
5 7 7,
same and
584
and
2 ) the
I) the heights
589
nm
in
their
relative heights
of
the feature at 476 nm to that of the sharp peak at 601 nm
is
much
larger
in their work as well as
in the
spectrum
of Hikami and Ito (30).
More recent work by W i r t h , et a l . (40) as well as the
original spectrum of Hikami and Ito are in accord with the
peak heights of the 577-601 n m series shown in Figure 5.
The
height
of
the
476
nm
shoulder
having
high
polarization relative to that of the low polarization peak
at
601
nm
was
found
to
be
1.3
in
this
work.
Dick
and
Hohlneicher found this ratio to be 6 while Mikami and Ito
found
it
to
be
8.
The
476
stationary state of naphthalene,
nm
state • is
a
TP-allowed
having B^g symmetry (31),
and might be expected to have a larger TP absorbance
the vibronically induced false origin at 601 nm.
than
S r e l a t iv e
= S c i RCULAR/S l INEAR
500
550
601
LASER WAVELENGTH (NM)
Figure 5.
Linearly polarized two-photon excitation spectrum of
naphthalene (solid line) and polarization ratio (dotted Line)
33
Figure 6 is the TP excitation spectrum of I -f l u o r o naphthalene
(IFN)
naphthalene
aside
This TP
to
which
looks
very
from a 2 n m red-shift
spectrum and all others
naphthalene
unity at
similar
which
was
to
that
of
of the L ^3 band.
in this work are relative
assigned
a
TP
absorptivity
of
the strong vibronically induced peak at 601 nm.
The L a band
of
IFN is enhanced by approximately
50% over
that of naphthalene.
The high polarization ratio at 475nm
indicates
naphthalene -like
that
the
hidden in this spectrum,
B^g
state,
though
is still present.
In Figure 7 is the TP spectrum of 2-fluoronaphthalene
(2 F N )
in
which
the
La
band
is
approximately 400% enhanced over
should be noted
nm)
is
but
somewhat
still
a
predominant
feature
that of naphthalene.
that the true origin of the L-g band
larger
only
10%
relative
the
to
height
that
of
of
the
It
(635
naphthalene,
sharp
vibronic
feature at 605 nm.
The
(TFN)
is
perturbed
TP
spectrum
shown
of
in Figure
naphthalene
I ,2 ,3 ,4 -tetrafluoronaphthalene
8.
The L-g band
molecule
is
only
of
this
slightly
highly
shifted
whereas the low polarization strongly TP allowed band with
maximum beyond 450 nm is red-shifted such that it overlaps
with
the L a band which appears to be enhanced 3 -fold over
that of n a p h t h a l e n e .
1 .5
5
S r e l a t iv e
= S c i RCULAR/S l INEAR
500
550
60<
0.0
LASER WAVELENGTH (NM)
Figure 6.
r
Linearly polarized two-photon excitation spectrum of 1-fluoronaphthalene (solid line) and polarization ratio (dotted line)
1 .5
5
S r e l a t iv e
= S c i RCULAR/S l INEAR
0.0
LASER WAVELENGTH (NM)
Figure 7.
Linearly polarized two-photon excitation spectrum of 2-fluoronaphthalene (solid line) and polarization ratio (dotted line)
1 .5
5
S r e l a t iv e
= S c IRCULAR/S l INEAR
0.0
LASER WAVELENGTH (NM)
Figure 8.
Linearly polarized two -photon spectrum of I ,2,3,4-tetrafluoronaphthalene (solid line) and polarization ratio (dotted line)
The TP spectrum of I -chloronaphthalene
(ICN) shown in
Figure 9 is similar to that of IFN except for a larger L-y
origin.
et
This spectrum is different from that of Friedrich
a l . (41)
samples.
which
was
obtained
from highly
concentrated
Their work found that TP excited fluorescence of
this compound was quadratic a l Iy dependent upo n conc e n t r a ­
tion
in the L a r e g i o n .
neat
(7.3
channel
M)
to
did
1.5
not
Their concentrations
M
and
have
a
their
ranged from
fluorescence
monochromator
to
detection
discriminate
between monomer and eximer f l u o r e s c e n c e .
Such
comparing
0.05 M
a
the
dependence
was
TP
fluorescence
excited
solutions
of
ICN and
sought
in
this
work
by
0.1,
and
2CN at two wavelengths .
For
from 0.2,
ICN the ratios of TP excited fluorescence of
were
I .83/1/0.457
at
609 n m and
indicating no appreciable
.2M/.1M/.05M
I .81/1/0.469
at
550 nm ,
concentration dependence
of the
spectral f e a t u r e s .
Figure
thalene
10
(2CN)
shows
the
TP
spectrum
of
in which the L a band is not
2 -chloronaph­
so enhanced as
in 2FN but the Ly origin is so much more intense that the
second
peak
in
its
F r a n c k -Condon progression
is
resolved
at 627 nm.
For
2 CN
.2M/.1M/.5M
I .77/1/0.443
the
ratios
solutions
at
550
of
were
nm,
concentration d e p e n d e n c e .
TP
excited
I .75/1/0.420
again
showing
fluorescence
nm
of
at
612
and
no
appreciable
S r e l a t iv e
= Scircular/Slinear
500
55U
ooi
LASER WAVELENGTH (NM)
Figure 9.
Linearly polarized two -photon excitation spectrum of 1-chloronaphthalene (solid line) and polarization ratio (dotted line)
S r e l a t iv e
= S c IRCULAR/S l INEAR
500
laser
Figure 10.
550
bUi
WAVELENGTH (NM)
Linearly polarized two-photon excitation spectrum of 2-chloronaphthalene (solid line) and polarization ratio (dotted line)
40
Figures
(ISQ)
and
have
been
quantum
11 and
its
12 are the TP spectra of isoquinoline
cation
(CAT)
corrected
yields
not
relative
respectively.
only
to
for
These
their
naphthalene
spectra
fluorescence
but
also
for
the
decrease in their quantum yields toward shorter excitation
wavelengths.
spectrum
rather
The
is
the
high
different
major
point
greatly
that
of
interest
in
La
which
enhanced
polarization.
from
of
The
band
CAT
naphthalene
the
spectrum
and
shows
ISQ
has
is
a
quite
a
peak
from
OP
of
high polarization at the onset of the L a b a n d .
Fluorescence
TP
excitation
structure.
20%
over
included
emission
of all
compounds
Deviations
the
100
in this
spectra
resulting
studied were
and
identical
in
in relative heights were less than
nm(UV)
range.
thesis,
but
are
These
spectra
available
are
not
in the l a b o r ­
atory of Dr. Patrik C a l l i s .
OP absorption spectra of IFN and 2FN vapors have been
published
(42),
however
they
coefficients
and
other
naphthalene.
or
to
therefore
do
not
cannot
include
be
Figures
extinction
compared
13,
14
to
and
solution-phase absorption spectra of naphthalene,
2FN,
respectively.
excellent
those
of
spectra.
The
agreement
IFN
and
with
2FN
are
naphthalene
published
similar
in
15
shape
(43),
to
are
IFN and
spectrum
results
each
the
is
in
while
vapor
1 .5
S r e l a t iv e
= &CIRCULAR/&LINEAR
0.0
LASER WAVELENGTH (NM)
Figure 11.
Linearly polarized two-photon excitation spectrum of
isoquinoline (solid line) and polarization ratio (dotted line)
1.2
S r e l a t iv e
= SciRCULARyStlKIEAR
0.0
500
550
600
LASER WAVELENGTH (NM)
Figure I2.
LiLriearLy polarized two -pLioLori spectrum of i socjuinol inium
cation (solid line) and polarization ratio (dotted line)
(LIT E R /M O L-C M )
4000
2000
280
300
WAVELENGTH (NM )
Figure 13.
One-photon absorption spectrum of naphthalene in cyclohexane
4000
Zi 2000
280
300
WAVELENGTH (N M )
Figure 14.
O n e -photon absorption spectrum of I -fluoronaphthaIene
in cyclohexane
.4 0 0 0
-.2000
280
300
WAVELENGTH (N M )
Figure 15.
One -photon absorption spectrum of 2 -fluoronaphthalene
in cyclohexane
46
The L]-) origin of
coefficient
of
1950
liter
mol "-^cm-
cyclohexane
solution
peak
but
is
8 25
which
available
its
in
height
origin
of
OP
is
again
will
the
of
Lfo
in
a
50
Lfo band
under
the
appear
of
the
can
IFN
inert
spectrum
be
and
first
during
This
the
as
this
but
Callis.
is
From
p e a k , the
a maximum
is
in
such
thesis
vibronic
is
2FN
vibronic
solvent
this
cm-1.
2FN
of
naphthalene
D r . Patrik
given
extinction
that
obtained
in
first
liter m o l -1
in
of
an
laboratory
to
and
origin
hidden
not
naphthalene
a molar decadic
m o l -^ c m -
The
relative
coefficient
the
liter
resolvable
perfluorohexane,
research
IFN has
Lfo
extinction
enhancement
signature
of
of
an
inductive perturbation.
TP
excited
fluorescence
naphthalene
at
Table
These
3.
seven
laser
data
wavelengths
were
used
are
shown
relative
TP
spectra
record.
OP
excited fluorescence
to naphthalene
tion
and
intensities
as measured by
apparatus
are
here
to
presented
in
constructing
as
a
Table
4.
0.17;
(44)
ICN,
relative
are:
0.01;
values
quinoline
are
solvents
(45).
naphthalene,
2 C N , 0.02
for
known
In
0. IH
to
in
0.05;
good
solutions
show
dilute
no
cyclohexane
I F N , 0.11;
agreement
in Table
with
4.
fluorescence
ethanol
detec­
Literature
values of fluorescence quantum yields of 10
solutions
of
relative
TP fluorescence
in
the
matter
quantum yields
the
displayed
in
are
relative
solution
2FN,
the
ISQ and
in
aprotic
the
room
47
temperature
(46)
and
fluorescence
that
of
limit of 0.0001.
quantum
quinoline
has
yield
of
ISQ
is
0.012
been given
(47)
an upper
This unfortunate circumstance precluded
the study of quinoline in this work.
Table 3.
Two -photon excited fluorescence intensities
relative to naphthalene
Laser Wavelength (nm)
Comoound
635
620
590
570
550
500
485
IFN
2FN
TFN
ISQ
CAT
ICN
5.29
10.5
I .37
.94
9.46
3.34
5.76
1.46
.740
2.55
4.32
.904
I .70
6.39
3.85
I .85
.841
2.41
1.35
.98
I .18
2CN
2.79
3.00
6.53
I .12
.549
2.10
.437
1.04
2.77
I .55
2.35
2.11
.62
.18
.18
.23
.29
(see text)
Table 4.
3.38
.268
1.07
.285
.570
.468
.903
.465
2.29
I .33
1.43
One-photon excited fluorescence quantum yields
relative to naphthalene
Excitation Wavelength
Comoound
(see text)
IFN
2FN
TFN
ISQ
CAT
ICN
. 2 CN
(nm)
310
300
290
280
270
260
250
2.21
2.27
2.49
' .623
.119
.120
.226
.346
2.24
2.44
.636
.107
.115
.219
.347
2.33
2.58
.667
.105
.118
.227
.361
2.18
2.43
.648
.089
.108
.212
.344
2.09
2.37
.616
.078
.100
.206
.330
2.14
2.42
.617
.073
.084
.209
.323
2.47
.653
.126
.119
.239
.332
All quantum yields were assumed constant except those
of
ISQ
and
CAT
which
decrease
toward
shorter
excitation
48
wavelengths.
The
relative
quantum
yield
of
ISQ
was
assumed to drop linearly throughout its range of
excitation while that of CAT was assumed to be constant at
0.12 down to 280 n m and then drop linearly.
The results of theoretical computations are presented
here
as
a key
perturbed
to understanding
naphthalenes.
naphthalene
First
molecule
in
reviewed.
Presented
next
vibrations
on
its
computations
on
its
spectral
the
lower
the
and
of
geometry
effects
finally
inductively
behavior
states
equilibrium
are
spectrum
the
of
the
perturbed
of
the
are
molecular
results
of
(substituted)
naphthalenes.
Dick and Hohlneicher (31) performed the same kinds of
computations on naphthalene in its equilibrium geometry as
those done
in this
with
presented
those
configuration
energies
allowed
to
work.
in
interaction
states
states
are
Their results
Table
(SCI)
observed
misplaced
5.
are
.While
in agreement
singly
gives accurate
in
OP
in
excited
transition
s p e c t r o s c o p y , the
energy.
Aside
from
TPthe
erroneously high transition energies predicted with singly
and
doubly
excited
configuration
interaction
(SDC I ) , the
spectral properties resulting from this calculation are in
good
agreement
properly
are
well
inclusion
ordered
with
and
experiment.
the
represented.
of
triply
OP
It
excited
and
has
The
TP
SDCI
states
transition
been
shown
configurations
are
strengths
(48)
can bring
that
the
49
transition energies down to more
reasonable v a l u e s , while
maintaining correct energy order.
Table 5.
Spectroscopic properties of naphthalene from
INDO/S calculations using SCI (singly excited
configuration interaction with Mataga-Nishimoto
electron repulsion) and SDCI (singly and doubly
excited configuration interaction wit h OhnoKlopman electron repulsion)
(140 excited configurations)
SCI
3
4
5
foregoing
naphthalene
origins,
and
but
results
yield
are
vibrations
can
cause
states
forbidden
results
with
character
of
SCI
and
allowed
is
known
weak
SDCI
character
as
to
undistorted
strengths
electronic transitions to become allowed.
mixing
I .50
0.53
13.2
192
0
applicable
absorption
.
0
0
0.0004
0.0780
0 .0000
0.0000
I .5572
42756
43450
53706
56129
58057
The
0.29
I .50
1370
0
678
C152 excited configurations)
SDCI
I
2
3
4
5
0
0
0.0027
0.1584
0.0000
1.6928
0 .0000
32350
37065
44356
44576
45889
I
2
TP Absorptivity
0
(IO-^ScmG)
Oscillator
Strength
Transition
Energy
(cm-I )
Excited
State
or
band
forbidden
This process of
into
vibronic
calculations
of
on
those
coupling.
the
with
The
vibronic
50
coupling of TP absorption into the
state of naphthalene
are presented in Table 6 .
Frequency
O
BI
H
Symmetry
SCI
391
0.091
0.280
0 .001
0
Mode #
10
b Iu
b 2u
b Iu
b 2u
b Iu
b 2u
b 2u
b Iu
b 2u
b Iu
b 2u
b Iu
16
18
29
31
32
35
37
38
39
43
45
the
MNDO
SDCI
0.008
0.025
0.007
0.3.50
0.048
7.969
6.830
0.539
0.006
1.019
0.001
5.291
I .309
18.996
13.229
normal
modes
those of Scherer
from
(49)
this
or the
Pulay and Boggs
The Cartesian displacements also do not agree w e l l .
However
the
set which
ments.
eight MNDO
spans
the
Moreover
Sellers,
et
electronic state.
which
of
4 3 cm6)
0.727
22.855
I .892
0.065
0.925
0.014
recent ab initio results of Sellers,
(50)
and
1154
1197
1209
1267
1356
1422
1431
1637
1735
not as accurate as
work are
more
632
798
frequencies
The
H
O
Two-photon a b s o r p t i v i t y , 5 q , induced in the
n a p h t h a Iene Li3 transition by MNDO vibrational
modes for singly and doubly excited
configurati on interaction in INDO/S
Table 6 .
bgu displacements
subspace of bg u vibrational displace-
these modes
al
form a complete
are
Formally,
as well as
normal
modes
those
of
Scherer
of
the
ground
it is the excited state modes
should be used for vibronic coupling c a l c u l a t i o n s .
51
It has been shown,
mixing
(Duschinsky
state b 2u modes
in the case of benzene,
rotation)
occurs
v ^4 and 1/3.5 leading
between
that mode
the
ground
to one excited state
b2u mode which actively induces TP absorption into the L 3-,
state
(51).
Such
almost all
the
is
the
case
in naphthalene
(52)
where
of the vibronic activity is from one b 2u mode,
frequency
of which is higher
in the L]-, state than in
the ground state (53).
The point of this argument i s : the MNDO modes may not
be
accurate
modes
is
a
but
the
good
sum
of
estimate
the
of
S q 's induced
the
vibronically
absorption in the Iq3 band of n a p h t h a l e n e .
Sg's of the six b 2u modes
and
represents
an
by
the
induced
b 2u
TP
The sum of SDCI
in Table 6 is 14.054 x 10 ^ c m ^
estimated
upper
limit
to
the
TP
absorptivity of the vibronic progression beginning at
601 n m .
The
OP
and
TP properties
have been calculated by Callis
vein,
to
the
this
spectroscopic
study
Naphthalene
were
of
substituted
benzenes
(54) using C N D 0 / S .
properties
calculated
in
its
equilibrium
the
OP
extinction
and
of
are
molecules
given
geometry
is
In this
relevant
in Table
included
7.
for
reference.
From
compounds , the
inductive
greatly underestimated
no
enhancement
at all
by
and
effects
IND0/S .
coefficients
upon
the
In the
of
these
Iq-, origin
are
SCI , 2FN shows
IFN only a 4- fold enhancement,
52
as
noted
tions
for
for
f Iuorobenzene
quinoline
and
by
Callis
isoquinoline
agreement with experimental r e s u l t s .
more
realistic
except
for
the
(54) .
The
are
qualitative
in
predic­
The SDCI results are
prediction
that
IFN has
a
stronger Iq3 origin than 2 F N , where in fact it is less than
half as s t r o n g .
Table 7.
Spectroscopic properties of substituted
naphthalenes from INDO/S
Compound
OP Absorptivity
TP Absorptivity
&c(io- 4jcm&)
(oscillator strength)
Lb
Lb
La
0.00
4.27
7.06
0.00
'SCI
.158
.181
.147
.144
,121
.162
.0082
.0027
naphthalene
.0118
I -fluoronaphthaIene
.0026
2 -fluoronaphthalene
.0308
quinoline
.0541
isoquinoline
.113
quinoline cation
isoquinoline cation . .157
9.62
9.76
399
637
2.07
8.44
109
160
660
840
0.00
0.00
I .74
I .99
4.10
I .13
17.9
116
3.65
7.64
SDCI
naphthalene
I -f luoronaphthalene
2 -f luoronaphthalene
quinoline
isoquinoline
quinoline cation
isoquinoline cation
The
results
.0780
.0966
.0675
.0710
.0518
.144
.0121
.0004
.0140
.0063
.0359
.0620
.0543
.114
complementarity
in enhancement
of
of . TP
Sg
and
OP
4.76
17.2
158
108
spectroscopies
in the L a band by
inductive
53
perturbations.
selection rules
It
is
Reasonable agreement with the pseudoparity
is seen in both the SCI and SDCI r e s u l t s .
interesting
that
SDCI
predicts
TP
absorptivity
in
the L a of quinoline to be almost four times less than that
of isoquinoline,
more
effective
indicating that inductive perturbation is
in
the
2-
position.
the OP and TP spectra of IFN and 2 F N .
This
is
observed
in
54
DISCUSSION
Two
spectra
questions
in
regarding
this work must
be
the
reliability
addressed, before
of
the
discussing
their importance:
1)
Is
there
evidenced by
in
the
a
the
long
ratio
naphthalene
range
slope
to
these
of peak heights
spectrum
here
spectra
as
(476 n m )/ (601 nm)
and
those
published
of
and
previously (30,31)?
2)
Is
Goodman
it
(33)
possible
are
that
t r u e , i.e.
the
findings
the
origin of
Rava
2FN has
the
same intensity as the next vibronically induced peak?
Regarding
used
a
laser
the
first
p h o t o -transitor
intensity,
while
of
q u e s t i o n , Mikami
undisclosed
Dick
and
and
origin
Hohlneicher
Molectron J2-05 pyroelectric detector.
Ito
to
(30)
measure
(31)
used
a
The latter authors
scaled their measurements of energy by the laser frequency
to
normalize
their' fluorescence
data
photon
number.
However,
in
neither
length
dependent
response
of
their
The
response
photodiodes
400
nm.
the
square
(electrons
per
to
work
the
square
was
the
detectors
photon)
of
of
wave­
considered.
blue -sensitive
can decrease by a factor of three from 600 to
Since
of
the
the
length dependent
fluorescence
reference
response
signal
detector
is more
is
normalized
signal,
than enough
the
to
wave­
to account
55
for
the difference
experiment
and
Nile
cence
avoided
Blue
A
quantum
in relative peak h e i g h t s .
this
difficulty
quantum
yields
counter
of
which
by
The present
using
Rhodamine
s o l u t i o n s , the
are
independent
B
fluores­
(within
a
few precent) of excitation wavelength (55).
Another argument which lends credence to the relative
peak
heights
except
that
ratio.
The
found
of
CAT,
two
unperturbed
in
by
this
show
work
nearly
transitions
the
is
in
the
as
pseudoparity
selection
rules,
behave
like
those
naphthalene.
results
of
two
this
transitions
work
to
found
be
equilibrium naphthalene
the
very
and
TP
all
same
question
substituents,
of
that
peak
are
essentially
be
The
6-15
results,
x
the
same:
Then,
according
to
work
put
naphthalene
of Dick
the
of
these
in
SDCI
is in the
computational
of TP absorptivity of the 47 6 nm peak
to that at 601 nm is in the range 0.9-2.2.
this
to
state #3 has S q = 13.2 x 10'^^cm^
lO'^^cm^.
ratio
the
computational
absorptivity
the
by
expected
and for the bg^ v i brationalIy induced L-g the S q
range
height
predicted
may.
nearly
spectra,
ratio
I. 2- 1.8 whereas
spectrum of Mikami and Ito
and Hohlneicher
(31).
areas rather than peak heights,
The spectra in
it
is
8 in
the
(30) and 6 in that
An analysis
based on peak
though more meaningful,
is
not easily applied to solution-phase spectra.
The
present
second
work.
question
Rava
and
is
of pivotal
Goodman
(33)
importance
to
found
vapor-
from
the
56
phase TP fluorescence excitation that the
has
intensity
with
one
also
found
equal
quantum of
the
Lg
to
that
the
of
1535
origin
origin of 2FN
the Lg transition induced
cm" ■*- vibration
of
IFN to
be
intensity
of
the
v 21 P ea-k which
is
blue
of
the
origin.
dyes were
side
experiment
and
the
data
three
Another
swift
they may have
segments,
possible
drop
in
Three
cause
as
for
is
the
(laser)
the
used
illustrated
results
quantum
They
one -fourth
had uncertainty
their
fluorescence
30 nm
^ 21 ■
to
in their
in matching
in
Part
II.
may have been a
yield
toward
shorter
excitation w a v e l e n g t h s .
The
the
latter possibility was
2 FN
vapor
OP
investigated by comparing
fluorescence
obtained by the present author,
tion
spectrum
peak heights
ing
the
of
Iredale
and
excitation
spectrum,
with the OP vapor a b s o r p ­
White
(42) .
The
relative
in the two spectra agree within 10% indicat­
quantum
yield
of
2FN
vapor
does
not
drop
appreciably.
Suppose
Figure
7
is
one argues
very
and
Goodman
claim,
Condon progression
peak
(c f . OP
the TP
distorted
matching data segments.
Rava
that
due
spectrum
to
of
2FN
in
uncertainties
in
If the Lg origin is as intense as
the
third
peak
in
its
Franck -
should be 60% the height of the origin
solution-phase absorption spectrum of 2FN in
Figure
15)
and will
lie directly beneath the v 21 vibronic
peak.
From the 2FN TP spectrum,
the polarization r a t i o , fi
57
of
the
L]-,
vibronic
origin
peak
would
is
transition
position
of
come
origin.
the
origin
is
0.93, and
in
from
=
0.45
naphthalene.
indeed
i/gl,then
Q
as
the
the
Assuming
intense
60% of the
for
as
the
that
the
at
the
peak
intensity
pure
of the latter
third Franck -Condon
peak
of
the
Therefore 60% of fi in this peak would be that of
origin
b a n d .If
this
were
the
case
the
0
value
measured at 605 n m would be:
0.6 0 (origin) + 0.4 0 (pure i/gl)
= 0.6 ( .93 ) + 0.4 ( .45 )
=
which
is
far
outside
the
.74
experimental
uncertainty
in
the
measured value of 0.48.
Rava
and
Goodman
conclude
their
work
by
explaining
the apparent
enhancement of the Li3 origin in 2FN as being
due
resonance
to
the
effect
of
charge
transfer
states
(fluorine 2p7r electron promoted to naphthalene tt* orbital)
contaminating
the
L-y w a v e f u n c t i o n .
They
go
on
to
show
that the resonance effect will be predominant in 2- rather
than
that
I -substituted n a p h t h a l e n e .
d i -substituted
greater
effect
predominates,
enhancement
than
naphthalene
They close by mentioning
would
they observed.
2 , 3 -difluoronaphthalene
than
exhibit
Since
even
2 - substitution
should
I ,4-difluoronaphthalene.
an
show greater
The
present
work investigated 1,2,3., 4- tetraf Iuof onaphthalene since the
difluoro
compounds
spectrum
of
TFN
were
shows
not
no
easily
such
obtainable.
enhancement.
It
The
is
TP
also
noted
that in SCI,
the IFN and 2FN L]-, states contain less
than 2% amplitude in charge transfer configurations.
Further
corroboration
of
the
spectra
from
this
work
is available in the SDCI computational r e s u l t s , from which
are
taken
TP
in units
of
photon'l).
cross -sections
for
Goeppert-Mayers
The
pertinent
linearly polarized
(IGM =
data
10" ^
for
light
crrA sec mlcl ~^
this
discussion
are
given in Table 8.
Table 8.
Two -Photon cross -sections of the L-g bands of
vibronically perturbed naphthalene and
equilibrium geometry I- and 2- fluoronaphthalenes with different carbon-fluorine
bond lengths
Molecular Species
5 Iinear (CM)
I .684
I .050
.440
.356
naphthalene + MNDO mode #32
naphthalene + MNDO mode #43
IFN
C - F = .130 nm
C - F = .133 nm
IFN
- F = .136 nm
C
IFN
F = .130 nm
C
2FN
= .133 nm
F
C
2 FN
=
.136 nm
F
C
2FN
The
revealed
one
spectrum
that
excited
tions
by
agreement
of
.289
.340
.284
.236
naphthalene
crystal
at
4 . 2K
has
the L-g false origin is induced primarily by
state
Marconi
with
r e s u l t s , Rava
m o d e , ^ 21
and
this
and
(52).
Orlandi
Theoretical
(53)
experiment.
Goodman estimated
are
From
in
the
the TP
c alcula­
excellent
theoretical
cross -section
59
of
the
v 21 -induced
invoking
L]-,
transition
to
be" .4 G M .
the Dnschinsky rotation observed
Again
in benzene's TP
L-)-, false origin (51) to construct one active b 2u mode from
the two
2.73
in- Table 8 this cross-section is calculated to be
CM.
This
cross-section
thalene
is
six
computed
true
times
in
origin.
greater
this
work
The
than
for
spectra
the
a
largest
fluoronaph-
obtained
in
the
present work show L^3 false origins ten times the height of
the true origin.
The
Goodman
conclusion
apparently
spectroscopy
selection
and
rules
of
fell
hope
can
these
arguments
victims
remains
apply
to
to
is
the
that
black
that
the
Rava
art
and
of
TP
pseudoparity
inductively
perturbed
naphthalenes.
The TP
presented
spectra with linearly polarized light will be
again
with
their
respective
OP
spectra
to
emphasize the inductive effect on the OP L^3 band and TP L a
band.
which
Figure
.the
evident.
16
displays
similarities
the
with
spectra
of
benzene
in
naphthalene
Figure
3
in
are
In the OP spectrum (both L^3 and L a are formally
forbidden in benzene) L a gains oscillator strength through
cross-linking
dipole-allowed
behavior,
vibronic
states.
The
coupling
TP
to
spectra
higher
reverse
energy
this
L^3 and L a are formally forbidden in both benzene
and naphthalene
into L^3 .
and
but vibronic perturbations
bring activity
1.0
6000
RELATI VE
€ (LITER/MOL-CM )
; x/
280
300
WAV ELE NGTH ( N M )
Figure 16.
T w o -photon (solid line) and one -photon (dotted line) spectra
of naphthalene
61
The
at
appearance
of the
true
origin of naphthalene
632 n m has been attributed to asymmetry of the solvent
cage
(30).
could
Non-vanishing
very
energies
well
of
be
the L a
TP absorption
attributed
states
of
to
the
in the L a region
same
alternant
effect.
and non-alternant
hydrocarbons
are known to shift in different
TP
of
spectrum
naphthalene
p e r f luorohexane
might
in
resolve
an
inert
this
The
solvents.
solvent
question
as
A
such
as
well
as
pinpoint the weakly T P -allowed Bgg transition at 476 nm
OP and
shown
in
features
TP
Figures
of
I F N , 2 F N , I C N , 2CN and
spectra of
the
17,
OP
18,
19,
and TP
20,
and
21.
The
ISQ are
vibronic
spectra of naphthalene
remain,
but n o w inductive effects are predicted to increase the L^
oscillator
strength.
OP spectra where
The predictions are born out in the
the 0-0
(true origin) with its attendant
F r a n c k -Condon progression increases in the order ICN < 2CN
< IFN < 2FN < I S Q .
was seen by Platt
The
The order of chlorine < fluorine < aza
(13) and Petruska (19).
pseudoparity
inductive .effects will
While
none
dramatic
ICN
<
of
as
IFN
the
selection
rules
2CN
<
enhancements
2FN
predict
that
enhance the L a band in TP spectra.
in
in f luo r o b e n z e n e , they
<
also
<
ISQ.
naphthalenes
increase
The major
are
in the
as
order
departure
from
predicted behavior is the small enhancement caused by I F N ,
for
which
the
explanation.
computational
results
provide
no
direct
5200
€ (LITER / M O L - CM)
280
300
WAV ELE NGTH ( N M )
Figure 17.
Two -photon (solid line) and one -photon (dotted line) spectra
of I -fluoronaphthalene
RELATI VE
€ (LITER/ M O L ' C M )
280
300
WA V E L E N G T H ( N M )
Figure 18.
Two -photon (solid line) and o n e -photon (dotted line) spectra
of 2 -fluoronaphthaIene
7000
RELATIVE
€ ( LI TE R / M O L 1CM )
280
300
WAVELENGTH ( N M )
Figure 19.
Two-photon (solid line) and one -photon (dotted line) spectra
of I -chloronaphthalene
6000
RELATI VE
€ (L IT E R / M O L -CM)
280
300
WAV ELE NGTH ( N M )
Figure 20.
Two-photon (solid line) and one-photon (dotted line) spectra
of 2-chloronaphthalene
4000
€ (LITER / M O L -C M )
280
300
WAV ELE NGTH ( N M )
Figure 21.
Two-photon (solid line) and one-photon (dotted line) spectra
of isoquinoline
67
In the
any
OP and TP spectra of CAT shown in Figure
but
the
signature of inductive perturbation is still evident.
In
the
structure
OP
in
the
spectrum
strength
of
bands
is
L a , while
so
in
has
been
enhanced
the
TP
diffused
22 ,
as
to
be
spectrum
twice
the
the
effect
is
reversed.
It is interesting to note an apparent conservation of
oscillator
strength
in
the OP
into Li3 is lacking in La .
I F N , 2FN and
the
weaker
quinoline
La
becomes
s p e c t r a : intensity
This
is true for the C A T , I S Q ,
spectra where
until
induced
in CAT
the
stronger L-y is,
the L a band has
less
than half the extinction coefficient of naphthalene's L a .
A
complementary
relationship
spectra of these compounds where
is
seen
in
the
TP
the Ly 0-0 is relatively
unchanged while its vibronically induced peak decreases in
intensity as
the L a gains intensity through the inductive
perturbation.
that
the
SpectrOscopists
intensity
of
the
have
ygl'^^duced
previously
peak
assumed
in Ly
should
remain constant under inductive p e r t u r b a t i o n s .
This
relationship
is not
2CN and especially I C N .
approximately
equivalent
mimicked
in
spectra
of
The OP L a maxima are respectively
and
greater
than
thalene and the dominant peak in ICN Ly
vibronic peak.
the
that
is not
of
naph­
0-0 but a
5000
.
......
€ (LITE R / MOL-CM)
RELATI VE
1.2
\
MD
0.0 -
260
280
300
320
WA V E L E N G T H ( N M )
Figure 22.
Two-photon (solid line) and one -photon (dotted line) spectra
of isoquinolinium cation
69
Apparently chlorine (particularly in the I- position)
is less
inductive
than it is m e s o m e r i c , i.e.
its orbitals
mix
substantially with those of n a p h t h a l e n e .
the
TP
spectra
of
greater than that
for
the
purely
The
increase
the ■ series
ICN
and
the
in
origin
is
L]-, false
of n a p h t h a l e n e , contrary to the results
inductive
in
2 CN
Likewise
ratio
fluoro-,
substituents
of
0-0
chloro-
fluorine
and
aza.
to i/^4 "vibronic peak
and
bromobenzene
has
in
been
noted by Goodman and Rava (26).
The
inability
properties
of
of
CNDO/S
f luorobenzene
to
while
predict
OP
providing
and
TP
satisfactory
results for many other substituted benzenes has been noted
(54).
While
effect
in
dominance
solved
(56)
I N D O / S -SDCI
the
of
the
2by
properly
oscillator
the
greater
of
effect
position.
using
new
predict
strengths
c o u l d , through
thalene .
predict
fluoronaphthalenes , it
recently
to
does
Such
fails
a
electron
relative
fluorine
in
the
inductive
to
problem
find
has
the
been
repulsion
schemes
vibronically
induced
in the L^, of benzene.
electron density
some
This approach
changes,
2-
explain
position
of
the
naph­
70
CONCLUSIONS
The pseudoparity selection rules of C a l l i s , Scott and
Albrecht
(29)
inductively
been
perturbed
considerable
activity
have
verified,
in
general,
naphthalenes.
Though
effects
OP
inductive
in
the
IFN
for
shows
b a n d , such
is conspicuously feeble in the TP L a b a n d .
spectrum
of
quinoline
(I-a z a n a p hthalene)
would
A TP
be
an
interesting test of this r e s u l t .
Computational
results
from
INDO/S
are
in
reasonable
agreement wit h experimental r e s u l t s , except in the case of
2 F N , where
in
that
the method predicts less inductive effect than
of
correct
IFN.
this
New
problem
electron
as
well
repulsion
as
schemes
explain
the
may
greater
inductive effect upon the 2 -position of n a p h t h a l e n e .
The
thalene
calculations
indicate
that
the Li3 by v 21
B 3g
TP-allowed
would
be
the
s t a t e , in
by using
then
vibrat i o n a l Iy
TP
distorted
absorptivity
approximately equivalent
not with previous
completed
on
agreement
experiments.
the rest
possible
to
of
with
naph­
induced
to
the
into
that of the
present
but
These calculations may be
the 48 normal
compare
the
modes .
amount
of
It
OP
oscillator strength vibrationally induced and that induced
by the cross-link p e r t u r b a t i o n .
71
PART II
NORMALIZATION OF TWO-PHOTON SPECTRA
72
INTRODUCTION
Two-photon
widely used
tion
in
(TP)
fluorescence excitation has become a
spectroscopic method
1961
by
Kaiser
emphasizing
the
Mahr
M cClain
(57.) ,
theory
and
of
and
TP
since its first obs e r v a ­
Garrett
(21).
processes
Harris
(58)
Reviews
include
and
those
an
of
excellent
introductory article by Friedrich (59).
As
the
technique
spectroscopy
the
light
has
most
F,
time
photon
often
emitted
should be proportional
laser
TP
as
a
characteristics
laser,
change
used.
I
the
with
is
a
excitation
lasers
The
have
become
intensity
to
TP
to the instantaneous
f l u x , I , but
dye
dye
subsequent
in a real experiment.
such
fluorescence
e v o l v e d , pulsed
source
fluorescence,
of
absorption
square of the
function
of
space
In a pulsed multi-mode
spatial
and
wavelength
of
source
temporal
and
to
a
and
beam
lesser
extent from pulse to pulse.
The effects of these changes
on
investigated
TP
absorption
have
been
(60-62)
in
the
context of peak height (of F and I) measurements.
Another
method
measures,
integrated fluorescence and laser
Though
(F)
is
strictly
approximation of normalizing
not
peak
intensity,
proportional
(F)
heights
to
to
normalizing
(F)
to
(l) ^
has
(F) and (l) .
(I^)
,the
(l) ^ is use d because
no method has been developed to measure (I ^ ) .
of
but
been
The result
mentioned
by
some
73
(63-66)
and
in
general
leads
to
anomolous
increases
in
their ratio toward one or both wings of a dye gain c u r v e .
In an
extended
curling-up
the
spectrum several
of
relative
the
spectral
heights
of
dyes
data
spectral
are needed and this
leads
to uncertainty
features whose
in
energies
lie in more than o n e , or even one dye tuning r a n g e .
Three
problem:
absolute
cally
methods
I)
the
attenuation
been
proposed
to
eliminate
this
a TP fluorescence standard calibrated by an
method,
to
have
2)
a
reference
instantaneous
of
coherent
which
laser
reacts
quadrati -
intensity
anti-Stokes
and
Raman
3)
scattering
(C A R S ).
Lytle
and
co-workers
m e t h y l s t y r y I )benzene
standard.
(64,65)
(bis-MSB)
as
have
a
use d
TP
p - b i s (o-
fluorescence
The TP spectrum was obtained from 537 to 694 n m
wi t h a single-mode ring laser for which the ratio
( / ( I)2 is c o n s t a n t .
They then took TP spectral data of
naphthalene and the dye 2-(I -naphthyl)- 5 -phenyloxazole
(ce-NPO) relative to that of bis-MSB using a multi-mode dye
laser.
Their
measurement
of
thte
TP
cross
section
of
bis-MSB at 585 n m is estimated at 6.9 x 10"^"^ erne's/(photon
molecule) .
fluorescence
easily
This
from
visible
is
a
wit h
very
l a r g e , in
millimolar
the
10
fact
solution
mJ/pulse
the
TP
excited
of
bis-MSB
N d :Y A G -pumped
is
dye
laser used in the present work, which attempted to utilize
L y t l e 's technique.
H o w e v e r , the photon fluxes required to
74
produce
detectable
thalenes
are
dependence
TP
too
large
from
establishing
to
fluorescence
obtain
bis-MSB.
a
intensity upo n
excited
As
quadratic
laser
quadratic
mentioned
dependence
intensity
in
the
naph­
intensitypreviously,
of
fluorescence
is a primary criterion
in
collecting meaningful TP d a t a .
has
The
second
been
use d
absolute
laser
TP
approach
for
single
using
has
crystal
of
been
a
quadratic
wavelength
cross-sections.
frequency
single
of
determinations
Second harmonic
generated
potassium
reference
wit h
dihydrogen
a
(SH)
of
of
the
phase-matched
phosphate
(KDP)
(67) and w i t h a quartz plate (68) to measure crosssections
at
the
ruby
laser
wavelength,
694
nm.
These
methods are not amenable to extended wavelength scanning.
The
more
elaborate
the
attenuation
measuring
Raman
scattering
scanned
of
the
third
of
solvent
method
the
as
(69)
coherent
one of
anti-Stokes
the
through a TP resonance of the s o l u t e .
two-beam experiment the results
obtain and again,
involves
lasers
This
is
is a
of which are difficult to
are not amenable to extended wavelength
spectra.
This
w ork
the powder
combined the method
of
SH generation with
technique of Kurtz and Perry (70)
to develop a
quadratic reference detector useful in extended wavelength
TP
spectra.
Though
cross -s e c t i o n s ,
it
it does not yield absolute values
of
does
TP
produce
reliable
relative
75
spectra.
behavior
Further
of dye
studies
laser
pulses
of
the
temporal
revealed
curling-up of TP spectral data.
and
the origins
spatial
of the
76
PROCEDURES
A L u m o n i cs N d :Y A G -pumped dye laser was used, having a
m a x imum
linewidth of
recommended
dye
0.003 n m
(0.09 cm" ^ at
concentrations
were
laser was adjusted to minimize ASE
emission),
(71) was
wavelength
M a x imum
The
and. the
dye
(amplified spontaneous
As a result of these measures, A S E , determined
as background
dye
diluted
580nm).
pulse
and
less than 10% at the least intense
3%
energy
at
at
the
the
most
intense
sample
was
wavelength.
12 mJ/pul.se in a
one m m diameter beam.
Solution
and
samples
of naphthalene,
2 -fluoronaphthalene
cyclohexane were
cells.
TP
detected
at
reduced
these
and
than
samples
d o wn-beam
in spectroscopic
fluorescence
a right
less
0.10 M
from
the
samples
angle wit h a quartz collecting
photomultiplier.
one part
in
Beam
IO6
by
the
sample without
was
lens,
intensity
is
TP absorption
in
so laser intensity measurements
from
grade
contained in Ixl cm^ quartz fluorescence
excited
monochromator
at
I-fluoronaphthalene
can be made
significant
changes
in
be a m characteristics.
After
removes
the beam traverses
10%
detectors.
diffusing
which
The
the sample,
then impinges
linear reference
integrating
sphere
and
on
one
detector
a
Nile
a bea m splitter
of two
reference
consists
Blue
A
of
a
quantum
77
counter
solution
whose
fluorescence
is
detected
by
a
photodiode.
The quadratic reference detector consists
of KDP
or urea
having
sieves to 75-150 ftm.
path
length
quartz
particle
sizes
graded
of powders
by
standard
These powders were contained in 2 nun
cells
filled wit h
spectroscopic
grade
decahydronaphthaIene (decalin) as an index-matching f l u i d .
Beam
diameter
on
the
interaction volume
randomly oriented.
from
its
powder
contains
was
8
mm.
The
IO^ p a r t i c l e s , assumed
to be
Second harmonic radiation was separated
fundamental
transmitting
samples
filters
by
and
v i s i b l e -absorbing/ultravioletdetected
by
a
Hamamatsu
R955
photomultiplier.
The
were
for
signals
integrated
average
and
of
each
for
100
signal,
laser
shot
shots.
(f ) ,
The
average
(20 Hz)
values
second
(S ),and average photon number s i g n a l ,
recorded
dependence
from
averaged
fluorescence
harmonic signal,
(Q) , were
resulting
at
(f ) and
0.5 n m
increments.
{S) u p o n
(Q)
The
as well as
quadratic
the linear
dependence of (Q) u pon laser intensity were confirmed with
crossed
polarizer
attenuation
at
the
end
and
central
wavelengths of each dye.
For a
easily
the
light pulse with gauss ian time p r o f i l e , it is
shown that the ratio,
reciprocal
profiles
of
dye
of
the
laser
(I2 )/<l)2 is proportional to
pulse width.
pulses
are
Because
known
to
the temporal
change
during
78
scans
(72),
wavelengths
these
for
pulse
each
shapes
dye
were
using
recorded
a
at
Hamamatsu
several
R1328U-02
biplanar phototube and a PAR 162 boxcar averager and Model
163
Sampled
Integrator
with Tektronix
S -2
sampling h e a d .
Time resolution of the total system is less than 150 p s .
Representative results are given in Table 10.
Rhodamine 610,
For the dye
temporal profiles were digitally stored and
numerically integrated to calculate the ratio
Beam
and
scans were
photodiode
Rhodamine
610.
at
By
also
the
performed with a 50 fj,m pinhole
central
and
end
incrementally moving
wave lengths
the
pinhole
of
and
d e t e c t o r , a 1500 point digital intensity grid was obtained
at
each
wavelength
from
which
the
ratio
was
calculated by the same method used for the time p r o f i l e s .
79
RESULTS AND DISCUSSION
The gain profiles of the dyes are shown in Figure 23
as
(Q)
laser
vs
wavelength.
normalized to the
Second
This is the ratio (l^)/(l)^ as a
function of laser wavelength.
to (Q) ^ at any wavelength,
function
and/or
of
spatial
intensity
square of the quantum counter signal is
also shown in Figure 23.
a
harmonic
Though (S)
the proportionality constant is
w a v e l e n g t h , or more
properties
is proportional
of
the
e x a c t l y , the
beam which
temporal
change
with
i
wavelength.
a
dye
scan
The
increase of (l^)/(l)^ toward the ends of
agrees
qualitatively with
width measurements in Table 9.
fine adjustments
the
temporal
pulse
It is noted here that even
of the dye laser can change the shape of
these c u r v e s .
Table 9.
Temporal pulse widths (ns, F W H H ) at the middle
and short and long wavelength ends of dye scans
Dye
short
Wavelength
middle
long
Rhodamine 590
6.52
7.08
5.85
Rhodamine 590
+ Rhodamine 610
6.69
7.20
5.85
Rhodamine 610
6.24
7.12
5.71
Rhodamine 610
+ Rhodamine 640
6.46
7.41
5.93
Rhodamine 640
6.61
7.12
5.41
80
R610
M630
R640
DCM
<S>/ < ( ] > <
<Q>
R590 M 6 0 0
LASER
WAVELENGTH
(NM )
Figure 23.
Quantum counter detection of laser i n t e n s i t y ,
(Q ) , for dyes used (upper panel) and second harmonic
i n t e n s i t y , (S ) , normalized to (Q)^
81
Two-photon
thalene
to
are
fluorescence
shown
excitation
in Figure
24.
data
for
naph­
In the data normalized
(Q ) ^ , the curvature due to the changes in (l^)/(l)2 ± s
obvious
and
producing
a
exaggerate
work.
being
introduces
full
the
In one
sought
a
troublesome
spectrum.
extent
case
of
These
this
uncertainty
data
effect
by
into
no
means
observed .in
other
it completely obscured the transition
(66).
Similar
data
for
IFN
and
2FN
are
presented in Figures 25 and 26.
The
harmonic
same
intensity
lower panel
ment
in
of
the
deviations
the
fluorescence
regions
plotting
segments
from KDP
each figure
from
have
data normalized
of
perfect
powder
and
dye
In
overlap.
all
been
joined
presented
are
In
the
some
within
spectra,
at
the
second
in
the
show much improved a g r e e ­
overlap
pen.
are
to
cases
the width
overlapping
middle
the
point
of
of
dye
their
overlapping region and no averaging has been d o n e .
Although
SH
from powders
provides
a good
reference,
it still has a slowly varying dependence on wavelength.
order
for
harmonic
this
method
efficiency
calculated.
to
be
spectrum
most
of
useful,
KDP
powder
the
has
In
Second
been
<F>/<S> (KDP)
<F>/ <Q>
82
575
LASER
Figure 24.
600
625
WAVELENGTH ( N M )
Two-photon excited fluorescence, ( F ) , of
naphthalene normalized to the square of
quantum counter intensity, (Q)2 , (upper panel)
and normalized to second harmonic intensity,
(S ), (lower panel)
< F >/ ( S >
(F )/ (Q)
83
575
600
625
LASER W A V E L E N G T H ( N M )
Figure 25.
Two-photon excited f l u o r e s c e n c e , ( F ) , of
I -fluoronaphthalene normalized to the square
of quantum counter i n t e n s i t y , (Q)2 , (upper
panel) and normalized to second harmonic
i n t e n s i t y , (S) (lower panel)
F/ SHG ( K D P )
F/ ( QC)
84
LASER WAVELENGTH ( N M )
Figure 26.
Two-photon excited f l u o r e s c e n c e , ( F ) , of
2-fluoronaphthalene normalized to the square
of quantum counter intensity, (Q)2 , (upper
panel) and normalized to second harmonic
intensity, (S ) , (lower panel)
85
Kurtz and Perry (70) derived an expression for second
harmonic
i n t e n s i t y , I(2w),
matchable powders under
gence
and
particle
coherence
shows
were
length,
that
particle
the conditions
dispersion.
sizes
as
A
further
much greater
rpj^/sin0m .
second
size
by angular averaging of p h a s e -
harmonic
long
as
assumption
was
that
than the angle-averaged
The
following
intensity
the
of small b i r e f r i n ­
is
expression
independent
of
latter assumption is v a l i d .
Their result w a s :
62 I(Ct))
327T
I (2w )
[n(to) + 1]
[n( 2 w ) + I].
[dPMt2^ l2 V k " P H
where:
I (w ) = intensity of the fundamental frequency
A = wavelength of the fundamental
(^m)
n(w) = average refractive index at angular frequency w
dpM.( 2«) = effective nonlinear coefficient at the phase
matching a n g l e , ©m
L = powder layer thickness
For KDP and A = 600 nm,
(/m).
rpM/sin0m = 6.4 /im which is much
smaller than the particle sizes used in this w o r k .
It has been shown experimentally (73) as well as
theoretically (74) that d p ^ ( 2 w ) is proportional to
[n(2w)-I ] [n(w)-I ]2 .
assumptions
Using
this
proportionality
of Kurtz and P e r r y , R(w),
and
the
the relative second
harmonic efficiency as a function of fundamental frequency
(proportional to I (2w)/I2 (w)) is given by:
86
[n2 (2w )-I ]2 [n(w)-l ]4 n ( 2w) sin 8 ^
R(w) = ---------- o---------- 4---------:
-------------[n( 2 w) + l]
[n(w) + l]
n.^(w)[n^(2 w ) - n ^ ( 2w)]
The
phase-matching
angle
refractive indices
is
expressed
in
terms
of
the
(74) by:
n e (2to)
n 0 2 (2to) - n 0 2 (w)
n ri(to )
n 0 2 (2 w) - n e 2 (2u>)
sin 0 ,
and the dispersion equations for the indices of refraction
for
ordinary,
known (76),
n 0 (w),
so R(w)
and
e x t r a o r d i n a r y , n e (w),
rays
are
is calculable.
At 650 n m R(w) has a value of 0.400 and at 550 n m it
is 0.576
- a 44% increase.
Using Shen's formula
(77) for
the width of acceptance about the phase-matching angle,
it
is found that this width increases 22% from 650 to 550 nm.
It
is
(78)
also
noting
that
for
a
1.23
cm KDP
crystal
this width is only about 0 .1 °, whereas for a 100 /tm
crystal
angle
wo r t h
it
is
several
acceptance
degrees.
width
is
interaction length (77),
efficiency
length.
increases
Thus,
it
inversely
the
phase-matching
proportional
to
the
it has been observed (78) that SH
as
is
While
the
very
square
many,
of
the
interaction
extremely
inefficient
powder particles producing SH in these s a m p l e s .
After averaging of overlapping data,
the TP spectrum
is multiplied by R(w) to obtain the TP fluorescence
87
excitation
spectrum.
The
visible-absorbing/ultraviolet-
transmitting filters used in this work begin to absorb at
wavelengths shorter than 2 9 0 n m , but the quantum efficiency
of
the
R955
(24% at
photomultiplier
650 run and 26% at
tube
changes
550 nm) .
only
slightly
Correction for these
latter effects are not included in this w o r k .
Urea
powder
quadratic
was
also
reference
wavelengths near
used
capable
500 n m
in
of
(77).
the
hope
extending
of
dow n
having
to
a
laser
Unfortunately the visible-
absorbing/ultraviolet -transmitting filters begin to absorb
strongly
at
wavelengths
investigation
Nevertheless
was
it
shorter
limited
is
to
interesting
sharp
5 -fold increase
laser
wavelength
than
the
270
range
to note
nm,
so
this
presented.
that
there was
a
in second harmonic intensity as the
approached
600
nm
from
the
blue
side.
This is the onset of Type TI phasematching (79).
The digitized temporal and spatial profiles have been
used
to
deconvolute
properties
for
ratios
the
the
dye
Rhodamine
from these
27 where
the
curvature
changes
is
in
these
610.
The
calculated
shown
in Figure
profiles
seen
to
are
arise
two
completely
beam
from
the temporal c h a n g e s , while the spatial changes are almost
linear
wi t h
wavelength.
The
lower
part
of
Figure
27
compares the product of the temporal and spatial data with
the
experimental
data
obtained
using
KDP
powder.
These
preliminary results show good agreement w ith the SH data.
88
1.5
X
CX
- - - - - - - - - - - - 1—
X
<I2 >/<I>
X
X
x
1.0 "
0
x
o
"
<I2 >/<I>2
B
\
TEMPORAL
SPATIAL
I
I
°\
1.0 KDP DATA
O P R O F I L E DATA
•
57 5
585
595
LASER W A V E L E N G T H ( N M )
Figure 27.
The ratio (I 2 )/(l )2 from temporal and spatial
beam profiles (upper p a n e l ) and their co n v o l u ­
tion compared to data from KDP powder
(lower p a n e l )
89
CONCLUSIONS
This wor k has demonstrated that powders of nonlinear
optical
materials
references
harmonic
for
can
to
successfully
two -photon
efficiency
calculated
be
TP
P -BaB 2 0 4 , better
tubes,
or
quartz
second harmonic
of
KDP
fluorescence
normalized by this method.
as
quadratic
The
powder
second
has
excitation
been
spectra
N e w nonlinear m a t e r i a l s , such
f i l t e r s , solar-blind
prisms
as
spectroscopy.
spectrum
correct
used
to
could extend
separate
this
photomultiplier
fundamental
technique
to
from
cover
the
visible s p e c t r u m .
Analysis
of
a
that
dye
a
due to
laser
large
spectral
of the temporal and spatial characteristics
data
b eam
at
portion
toward
the
the wings
for collecting
indicates
in
of
gain curve
the dye
two-photon
laser p u l s e s .
is
Use of a
accurate and faster method
spatial profile data than the pinhole beam
Acquisition
accomplished
a more
wavelengths
curling-up
shortening in time of the
reticon array would be
scans.
of
different
wit h
a
of
space-time
profiles
pinhole/phototube-boxcar
apparatus but would be extremely time consuming.
may
be
averager
Further
experimental and theoretical w o r k in this area is needed.
90
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