UV-Vis SPECTROSCOPY,
CHEMOMETRICS AND NONBONDING INTERACTIONS IN
SOLUTION:
qualitative conclusions
and
quantitative estimations
Lecture Outline:
• UV-Vis Spectroscopy background
(structural information, quantitative
analysis)
• Crown ethers complex formation
• Self-association of dyes
• Difficult cases for quantitative analysis
of equilibria in solution (chemometrics)
UV-Vis Spectroscopy:
What is Optical Spectroscopy?
Maxwell: The light is an
The study of molecular
structure andfield
electromagnetic
dynamics throughcharacterized
the absorption,
by a
emission and scattering
of light.
frequency
n, and
wavelength l.
Light obeys the
relationship
c=n.l
With energy of photons
E=h.n
UV-Vis Spectroscopy:
The Electromagnetic Spectrum:
E=h.n
n=c/l
UV-Vis Spectroscopy:
The Electromagnetic Spectrum:
Electronic
excitation
1020
1018
-rays
X-rays
1016
UV
Visible
Cosmic
rays
n [Hz]
Vibration
1014
Rotation
1012
Infrared
108
Radio
Bond breaking
and ionization
Microwave
E=h.n
Visible Spectrum
400
500
600
n=c/l
700
UV-Vis Spectroscopy:
Molecular Spectroscopy:
UV-Vis Spectroscopy:
Optical Spectroscopy and Non-Bonding
Interactions (IR vs UV-Vis):
UV-Vis
Spectroscopy:
IR
Spectroscopy:
Electronic transitions
– 1000
Vibrational
motion (l(200
> 1000
nm);nm)
quantitative
analysis;
More suitable for structural
identification;
Low concentration
High
concentration range
range (<
(> 10
10-4-3 mol/l)
mol/l);
Large number
Limited
numberofofsolvents,
solvents;including
water;
Simple instrumentation, low running costs
UV-Vis Spectroscopy:
1940 - Single beam Beckman B
UV-Vis spectrometer
1941 - Dual beam scheme Cary & Beckman
Beckman DU-2 UV-Vis spectrometer (19411975, 35 000 units produced)
UV-Vis Spectroscopy:
UV-Vis Spectroscopy:
Frank-Condon principle:
Describes the intensities of vibronic
transitions, or the absorption or
emission of a photon. When a
molecule is undergoing an electronic
transition the nuclear configuration
experiences no significant change.
This is due in fact that nuclei are much
more massive than electrons and the
electronic transition takes place faster
than the nuclei can respond. When
the nucleus realigns itself with the new
electronic configuration, the theory
states that it must undergo a
vibration.
UV-Vis Spectroscopy:
ABSORBANCE
position of the band –
energy of the transition
Amax
integral intensity (area) –
oscillator strength (not molar
absorptivity)
Dn1/2
Spectral shape:
Amax /2
nmax
ENERGY
- position of the band
- intensity
- half-band width
UV-Vis Spectroscopy:
Structural information:
- chromophore is
needed
(conjugation and/or
auxochrome(s))
- broad bands
(usefulness for
identification is
limited)
UV-Vis Spectroscopy:
Quantitative analysis:
Single compound:
Beer-Lambert’s law
Mixture:
Additivity principle
l
l
A   .c.l
n
A  l.  i .ci
l
i 1
l
Crown Ethers:
Fathers of Modern Supramolecules:
Charles J.
Donald J.
Jean-Marie
Pedersen
Cram
Lehn
Nobel Prize in Chemistry 1987
Crown Ethers:
Modern Supramolecules:
crown
ethers
Charles J.
Pedersen
container
molecules
Donald J.
Cram
cryptands
Jean-Marie
Lehn
Crown Ethers:
Crown Ethers:
Charles J.
Pedersen
JACS 89, 7017 (1967)
12-Crown-4
Crown Ethers:
Crown Ethers:
- flexible ring structure, containing several
ether groups;
- size fit effect of complex formation with
metals (very strong complexes: 18-crown6 has high affinity for potassium cation,
15-crown-5 for sodium cation, and 12crown-4 for lithium cation);
Crown Ethers:
Crown Ethers:
- the oxygen atoms are well situated to
coordinate with a cation located at the
interior of the ring, whereas the exterior of
the ring is hydrophobic. The resulting
cations often form salts that are soluble in
nonpolar solvents, and for this reason
crown ethers are useful in phase transfer
catalysis.
Crown Ethers:
Crown Ethers:
- contain electron-rich atoms (O,S,N)
crown
aza-crown
cyclen
Crown Ethers:
Crown Ethers:
BUT: no spectra, no possibilities for optical
sensors
SOLUTIONS:
- crown ether becomes part of a
chromophore
- aza-crown ether linked to a
chromophore through N-atom
Crown Ethers:
- crown ether becomes part of a
chromophore, but
reduced flexibility;
reduced electron density at O-atoms
Aza Crown Ethers:
- a distinct and specific interaction
between the nitrogen(s) and cations
may improve the selectivity of
ionophores based on size-fit effect;
- the ionophore nitrogen may be a part of
the conjugated system improving the
potential of the whole molecule to act as
a sensor;
- the nitrogen may facilitate the
synthesizing of three-dimensional cavities,
which improve the receptor selectivity.
Crown Ethers:
UV-Vis spectroscopy:
- quantitative (spectral shift upon
complexation or/and change in the
quantum yield);
- qualitative (stoichiometry of the complex
in solution; binding constant, which might
be used as measure for sensitivity (one
ion) and selectivity (set of ions));
CK

C L .C salt
Ligand + Metal Salt  Complex
Crown Ethers:
Spectrophotometric titration:
- quantitative
- qualitative
CK

C L .C salt
Ligand + Metal Salt  Complex
Crown Ethers:
Aza Crown Ethers:
direct part of
conjugated donor30000
acceptor system
MOLAR ABSORPTIVITY
40000
20000
10000
0
210
310
410
WAVELENGTH [nm]
510
Crown Ethers:
20,000
15,000

5,000
10,000
1
0


Aza Crown Ethers:
linked to conjugated
donor-acceptor
system via spacer 7
300
400
l / nm

500
600
Crown Ethers:
Aza Crown Ethers:
non-conjugated donor-acceptor system
Crown Ethers:
Crown Ethers & UV-Vis spectroscopy:
Relatively simple case for study
IF:
- The binding constant is large enough;
- The complex stoichiometry is simple;
- The process of complexation is simple as
a mechanism;
- The binding site is linked to a donoracceptor system and the complexation
influence either donor or acceptor;
Aggregation:
Beer-Lambert’s law:
l
l
Deviations: in the
textbooks is
written – at high
concentrations
WRONG!
Absorbance
A   .c.l
A4
Ax
A3
A2
A1
C1
C2
C3
C4
Concentration
Aggregation:
Beer-Lambert’s law:
20000
l
l
A   .c.l
c.l  const.
16000
12000
8000
path length
from 0.01 mm
to 100 mm
4000
0
350
400
450
500
550
600
Aggregation:
Aggregation of dyes:
- affecting their colouristic and spectral properties
- increases with an increase of dye concentration or
ionic strength;
- decreases with temperature rising or organic solvents
adding;
- addition to the dye structure of ionic solulilizing groups
(as sulphonate group) decreases aggregation;
- inclusion of long alkyl chains increases aggregation
because of higher hydrophobic interaction in solution
Aggregation:
Exciton theory:
spectral changes observed upon
aggregation are caused by
electronic interactions between the
dye molecules in the aggregate.
Simple Dimer Model:
0  1 .  2
1
'
'
 
.(1.2  1 .2 )
2
1
'
'
 
.(1.2  1 .2 )
2
Aggregation:
Exciton theory:
transition dipols
parallel (sandwich)
in-line (head-to-tail)
Aggregation:
Exciton theory:
transition dipols
parallel (sandwich)
in-line (head-to-tail)
Aggregation:
J-Aggregates:
- red (bathochromic) shift in the absorption
with increased intensity;
- red shift in the emission;
- aggregation caused by concentration,
solvent or salt addition;
- typical example: cyanine dyes (non-planar
structures) forming helix patern
Aggregation:
H-Aggregates:
20000
- blue (hypsochromic)
shift in the absorption;
16000
- weak or no emission;
- aggregation
caused by concentration,
12000
solvent or salt addition;
- typical 8000
example: ionic planar dyes forming
rods (rarely, mainly dimers)
4000
0
350
400
450
500
550
600
Aggregation:
UV-Vis Spectroscopy and Dimerization:
- determination of the
type of aggregate;
- estimation of the
distance between
monomer molecules in
the dimer;
- estimation of the angle
of transition dipols in the
dimer
Aggregation:
Structural Parameters of The Dimers:
- require finding the
constant of dimerization
and the spectra of the
monomer and dimer
D
i
M 2
i
c
KD 
(c )
A   .c .l   .c .l
*
i
M
M
i
i
D
D
i i
Chemometrics:
Difficult cases for quantitative UV-Vis
spectral analysis:
- complexation with low stability constant of the
complex, which does not allow to obtain
experimentally the pure spectrum of the complex
with addition of metal salt;
- dimerization with large dimeric constant, which
does not allow to obtain experimentally the pure
monomer spectrum upon dilution
- dimerization with low dimeric constant, which
does not allow to obtain the pure dimer spectrum
Chemometrics:
raw spectral data
set of spectra
Soft & Hard Modeling:
Variant II
Variant I
Beer's law
mass
balance
model
definition
assumptions
assumptions
comp
Ak (li )  l.  ck ,s . (l )
optimization
s 1
Beer's law
mass
balance
o
s
i
optimization
RESULTS:
components'
concentrations and
individual spectra
non linear
modeling
RESULTS:
fundamental
process constants
fitting the model
non linear
modeling
concentrations of
the components and
individual spectra
fundamental
process constants
Chemometrics:
Soft Modeling (dimerization):
strongly bound to the model
ciD
KD  M 2
(ci )
Ai*   M .ciM .li   D .ciD .li


 A*  1  8.c .K D  1 .( AM  AD )  AD 

i, j
j
j
j


4
.
c
.
K
i 1 j 1
D


S 22 
p.m
p
m
*
i
*
i
2
Chemometrics:
Hard Modeling (complexation):
gives flexibility and might be used for
complicated equilibria
O
N
H
COOH
COOH
N
H
N
H
COOH
COOH
N
H
O
BR
Bilirubin (BR)
1
AuBR
O
N
H
Biliverdin (BVD)
N
H
N
N
H
O
k1
BVD
2
AuBVD
Conclusions:
UV-Vis spectroscopy is very suitable for
study of equilibria in solution, but has
some limitations in both quantitative and
qualitative analysis.
Always use in combination with other
instrumental methods for analysis. Do not
forget theoretical approaches.