Volume 11, Issue 2
Summer 1998
ISSN 1044-5536
Determination of Triplet Energies in Rhenium Polypyridine
Complexes with Laser-Induced Optoacoustic Spectroscopy
Keith A. Walters and Kirk S. Schanze*,
Department of Chemistry, University of Florida
1998 Center for Photochemical Sciences
R1
R2
CO
(tmb): R1 = CH3, R2 = CH3
(dmb): R1 = H, R2 = CH3
(bpy): R1 = H, R2 = H
O
CO
(damb): R1 = H, R2 =
CO
N
Re
N
O
NEt2
N
R2
R1
A Quarterly Publication
Bowling Green State University
Bowling Green, Ohio 43403
The Spectrum
Introduction
The fac-(b)ReI(CO)3L (where b is a diimine ligand) chromophore has been widely used for studies of photoinduced intramolecular energy and electron transfer.1 A series of complexes with varying
photophysical properties for the dπ (Re) → π* (diimine) metal-to-ligand charge transfer (MLCT)
state can be easily achieved by varying the substituents on the diimine.2,3 Unfortunately, the emission attributed to this excited state is generally broad and structureless. Thus, it is difficult to
accurately determine the energy of the triplet MLCT excited state without tedious and sometimes
inconsistent Franck-Condon line-shape analysis.4,5 An alternative technique for measuring the
energy of an MLCT state uses Laser-Induced Optoacoustic Spectroscopy (LIOAS).6 This technique
has been successfully applied to determine the energy of the MLCT state in RuII(2,2’-bipyridine)3.7,8
We recently implemented LIOAS in our laboratory and plan to use the instrument to measure
excited state energies and/or yields for molecules of interest. Before studying molecules with
hitherto unknown excited state properties, we decided it would be prudent to carry out a calibration study to characterize the accuracy of the LIOAS apparatus and signal analysis software by
using a series of excited states with “known” energy and variable lifetime. Thus, we selected a
series of fac-(b)ReI(CO)3(py-Bz)+ complexes (where b is a series of diimine ligands of varying electron demand and py-Bz is 4-benzylpyridine, Scheme 1) for use in this calibration study. This series
of complexes was selected for the study because: (1) the energy and lifetime of the Re → diimine
MLCT state varies systematically with the electron demand of the diimine ligand; and (2) the
MLCT state is luminescent, allowing us to use emission spectroscopy to estimate the energy of the
relaxed excited state. In this report we present the results of the LIOAS calibration study of the
rhenium complex series and compare the energies of the MLCT states in the complexes as determined by LIOAS and Franck-Condon bandshape analysis of the emission spectra.
(deb): R1 = H, R2 =
Scheme 1
Continued on page 3
OEt
The Spectrum
Page 2
From the Executive Director
D. C. Neckers, Executive Director, Photochemical Sciences, Bowling Green State University
During the 1980s, I was a member of Mead Imaging’s Scientific Advisory Committee. Among the chosen six was
Nick Negroponte, founder of MIT’s Media Lab. You know Nick’s kind. When coffee breaks would come, a secretary
would walk in and hand him a pack of phone messages. Sometime after the next session had begun, Nick would
reappear with apologies. “If it hadn’t been the President of MIT (or Sony or Nintendo or…) I’d have made him wait”.
I didn’t really know what Nick did for a living those days. When I asked him what the Media Lab did he said,
“Studies the interactions of man with machines”. Since Nick’s faculty appointment at MIT was in architecture this
seemed a little odd to me, but who was a mere chemist from Bowling Green to ask impertinent questions of an
architect from MIT?
A couple of years after our Mead assignment, Sue and I entertained Nick and Elaine at our Chautauqua Lake
vacation home. Nick was speaking at the Chautauqua Institution the next day and, after knowing him for 10 years or
so and a nice bottle of wine or two, I finally found out who he was. A year or so later the book Being Digital was
published. While Nick was living in France, he sent me a copy of Wired. The cat was out of the bag. Friend Nick was
to the cybergeneration what the Beatles were to 1960s America—a clairvoyant folk hero of prodigious intellect whose
energies helped change our generation. I was reminded of Nick again recently (June 28, 1998) when US Today had
three pages, including much of the front page, devoted to the Media Lab.
We’re experiencing a quiet revolution. Hard copy is too much hassle, takes too much time, and has to be stored.
Telephones are out of the question. Ever try and connect with a Dean or a University Vice President? “Dean (fill in the
blank) is in conference.” “When can I talk with him?” “Try a week from Thursday.”
Face to face business meetings have to be on the decline. Flying from there to here puts you in a center seat between
two kids traveling alone, and the Widdlesticks Club taking their first trip to see Elvis. Nevermind that your flight is
from Montreal to Atlanta. Some travel agents and airline pricing schemes make it cheaper to fly from Chicago to
Nashville through Montreal. The interstate highways are perpetually under repair. It is 60 miles from Detroit to
Toledo. But the interstate has been one lane for the last five years and, given the quality of work of the government
these days, will remain that way. As soon as one section gets repaired, the section last completed needs it again. Traffic
is so bad in the New York area that even an inefficient flight from the hinterlands can get a traveler to Newark quicker
than the Connecticut Yankee can get there from New Haven.
So Negroponte’s media, cyberspace, is becoming the primary method of human social intercourse. This has some
upsides. No insurance agent, stockbroker or university alumni organization can interrupt your evening dinner with
an unwanted email unless you chose. Cyberspace gets to places no mailman can. Ever try and send a letter to Russia?
To use it one should be courteous. It’s hard to slam a keystroke in another’s ear. Of course if Nick’s right, and he
probably is, the keystroke will soon be as obsolete as a punch card. Voice activation is next, and after that thought
activation.
On the downside soon only the basics of human existence will require two persons together in the same room. And
depending on the quality of the cyberloveaffair, even those times of togetherness may be mercifully short.
In This Issue
Determination of Triplet Energies in Rhenium Polypyridine Complexes with
Laser-Induced Optoacoustic Spectroscopy ...................................................................................................................... 1
From the Executive Director ..................................................................................................................................................... 2
Photochemistry in Organic Synthesis ................................................................................................................................... 10
Scans Yield Mummy Clues ...................................................................................................................................................... 16
Center for Photochemical Sciences Publications ................................................................................................................ 18
Page 3
The Spectrum
Continued from page 1
The LIOAS Technique
LIOAS is a photothermal technique that allows quantitative determination of the amount of heat evolved when
a photoexcited state decays.9 The release of thermal energy
concomitant with non-radiative excited state decay results
in the production of an acoustic pressure wave. A piezoelectric transducer pressed against the side of a sample
cuvette is used to monitor the temporal evolution of the
acoustic wave. In the simplest implementation of LIOAS,
calorimetric information is extracted by comparing the
acoustic wave amplitude of an unknown sample with that
of a calorimetric reference.10 However, this method only
provides the fraction of excitation energy released into the
sample as heat in a fixed time domain. This elementary
use of LIOAS does not allow one to partition the heat deposition into various non-radiative excited state decay processes that occur in different time domains. A more detailed
analysis of the time-resolved acoustic response using a nonlinear least squares and iterative reconvolution method
(hereafter referred to as “deconvolution analysis”) allows
one to extract the amplitudes and lifetimes for multiple
non-radiative decay processes that occur subsequent to
photoexcitation.11 In the present study we have implemented a commercially available software package that
uses the latter method to afford time-resolved LIOAS data.
Figure 1: Transducer response spectra: (a) Panametrics
An important limitation of LIOAS arises because ultraV103 1 MHz Transducer, (b) Panametrics V105 5 MHz
sonic piezoelectric transducers respond to acoustic signals
Transducer.
over a relatively narrow frequency range. For example, the
frequency responses of the 1 and 5 MHz transducers used
in the present study are illustrated in Figure 1. Taking the 1 MHz transducer as an example, it can be seen that this
detector only responds to pressure waves with frequencies in the 0.2 - 2 MHz range. Acoustic waves at frequencies
outside of this range will yield either a partially or fully damped (i.e., zero) response.12 Qualitatively, there is an
inverse relationship between the lifetime of a non-radiative decay process and the frequency of the resulting acoustic
wave (i.e., a short lifetime gives a high frequency acoustic wave). Consequently, in LIOAS measurements: (1) nonradiative decay processes with lifetimes that are “short” compared to the upper bound of the transducer frequency
response will appear as a “fast” or “prompt” acoustic wave component that is in-phase with a reference sample that
has a very short excited state lifetime (e.g., < 1 ns); (2) non-radiative decay processes that are “intermediate” with
respect to the transducer response will appear as an acoustic wave that is damped in amplitude and phase-shifted
with respect to the reference sample; and (3) non-radiative decay processes that are “slow” with respect to the transducer response will be fully damped and therefore undetected. Although it is possible to simulate the relationship
between an excited state decay lifetime (i.e., the lifetime of the non-radiative decay process) and the resulting acoustic
pressure wave for a transducer with a given characteristic frequency and damping time constant,6 in practice it is
necessary to empirically characterize an LIOAS system. The most important questions that must be addressed in an
LIOAS calibration study are: “How fast is fast? and How slow is slow?”!
As noted above, deconvolution analysis allows one to determine the amplitudes and lifetimes for non-radiative
decay processes that occur on “fast” and “intermediate” timescales with respect to the transducer response. In theory,
this method is also able to resolve heat deposition signals that approach the detection limit of a fully damped “slow”
component. However, because of the finite S/N of experimental LIOAS data there is a finite upper detection limit for
the lifetime. When this limit is exceeded, the lifetime and amplitude of the non-radiative decay component will be
distorted due to the limited frequency response of the transducer.
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Page 4
A principal objective of the present study is to delineate the time domain over which our LIOAS system can be
used with deconvolution analysis to accurately recover lifetimes and amplitudes of “prompt” and “intermediate”
timescale non-radiative decay processes. It was for this reason that the series of (b)ReI(CO)3(py-Bz)+ complexes was
examined. In particular, with this series the lifetime of the MLCT state varies from 100 to 1500 ns.5 This timescale
range is ideal for characterizing the ability of our 1 and 5 MHz transducers to resolve non-radiative decay processes.
Materials and Methods
Steady-State Emission Spectroscopy and Emission Quantum Yields. Corrected steady-state emission spectra were recorded on solutions of each complex with a SPEX F-112 fluorimeter. Samples were contained in 1 cm x 1 cm quartz
cuvettes and excited at 350 nm. Emission quantum yields (φem) were calculated relative to two actinometers, and the
values are listed in Table 1. RuII(bpy)3 in degassed water (φem = 0.055)13 and 9,10-dicyanoanthracene in ethanol
(φem = 0.89)14 were used as actinometers. All solution concentrations were adjusted to result in optical densities of
approximately 0.14 at the excitation wavelength.
Table 1. Emission Data for (b)ReI(CO)3(4-benzylpyridine)a
Ligand
φem
τem/ns
ν max/cm
tmb
0.25
1473
18700
dmb
0.06
274
17450
bpy
0.045
208
17210
damb
0.026
116
16700
deb
0.0145
93
15360
-1 b
ν 00/cm-1 c
(kcal mol-1)
19400
(55.5)
18375
(52.5)
17975
(51.4)
17280
(49.4)
16050
(45.9)
Argon-degassed CH3CN solutions, 298 K. Estimated errors: φem, ± 15%; τem, ± 5%; ν max, ± 100 cm-1; ν 00, ± 500 cm-1.
b
Emission maximum. c0-0 emission energy estimated by Franck-Condon analysis as explained in text.
a
Emission Lifetimes. Time-correlated single-photon counting (FLI, Photochemical Research Associates) was used to
measure emission lifetimes. The excitation and emission wavelengths were selected with bandpass filters (excitation,
Schott UG-11; emission, 550 nm interference filter). Samples were contained in 1 cm x 1 cm quartz cuvettes. Lifetimes
were calculated with DECAN fluorescence lifetime deconvolution software (v. 1.0) and are listed in Table 1.
LIOAS Measurements. LIOAS measurements were conducted on an apparatus built in our lab. The samples were
excited with the third harmonic of a pulsed Nd:YAG laser (355 nm, 10 ns fwhm, 10 Hz). The beam was attenuated
with a beam splitter and neutral density filters to produce energies incident on the sample cell of ca. 8-30 µJ pulse-1
depending on the transducer used. Energy calibrations were routinely performed to avoid nonlinear region responses
at the selected experimental energy. The Gaussian beam was passed through a 1.25 mm slit and a 10 cm focal length
lens which focused the beam into the cell (the beam diameter was ca. 0.5 mm within the cell). A 1 cm x 1 cm quartz
cuvette was firmly fixed in contact with a Panametrics V103 (1 MHz) or V109 (5 MHz) transducer that was positioned
perpendicular to the excitation beam. A film of vacuum grease was used to assure good acoustical contact between
the cell wall and transducer element. The cell and transducer were fixed in a holder mounted on a linear stage to
allow adjustment of the distance between the excitation beam and transducer, which was necessary to move the
temporal position of the LIOAS signal away from the initial RF noise resulting from the laser shot. Signals were
amplified with a Panametrics 5670 Pre-Amplifier (40 dB) and fed into a Tektronix TDS 540 digitizing oscilloscope.
Data was averaged over 1000 pulses and captured on a 160 MHz pentium computer with software written by the
authors. Great care was taken to insure that the cell geometry did not change between reference and sample data
acquisition. Therefore, solvent rinse and sample solutions were carefully transferred into and out of the cell with a
Page 5
The Spectrum
LIOAS Amplitude
1.0
Reference
Sample
Simulation
0.5
Residuals
0.0
a.
-0.5
350
400
450
500
550
600
Channel
1.0
LIOAS Amplitude
Reference
Sample
Simulation
0.5
Residuals
0.0
-0.5
b.
350
400
450
500
550
syringe. Acetonitrile samples were nitrogen degassed in the
fixed LIOAS cell for 20 minutes.
LIOAS data was analyzed using Sound Analysis Version
1.14 (Quantum Northwest, Inc.). A reference signal (ferrocene in acetonitrile) was used to determine the “transducer
response function”. The reference waveform was acquired
immediately prior to each sample to insure that the laser
energy and/or beam position was unchanged between reference and sample signal acquisitions. Four measurements
were made on fresh solutions of each complex with each
transducer. LIOAS signals were normalized for absorption
differences between the reference and sample.11 However,
absorbance values were always the same within 10%. The
acoustic waveform for each sample was subjected to
deconvolution analysis with two heat-deposition components (φ1 and φ2 are the amplitudes of the “fast” and “slow”
heat components, respectively). The analysis software was
run with the lifetime of the “fast” component (τ1) fixed at 10
ns and 1 ns for the 1 MHz and 5 MHz transducers, respectively. The amplitudes of the “fast” and “slow” lifetime components and the lifetime of the “slow” component (i.e., φ1,
φ 2, and τ2) were optimized by non-linear least squares.
Figure 2 illustrates the results of a typical analysis for
(dmb)ReI(CO)3(py-Bz)+, and the averages and statistical
analysis of the fits for all of the studied rhenium complexes
are listed in Table 2.
The Energy of the 3MLCT State
600
The energy of the lowest MLCT excited state in the
(b)Re(CO)3(py-Bz)+ complexes can be calculated from both
Figure 2: LIOAS data for (dmb)Re(CO)3(4-benzylpyridine):
components recovered from deconvolution analysis of the
(a) 1 MHz Transducer (Fit Parameters: φ1 = 0.3364; τ1 =
LIOAS data. The lowest excited state in this family of com10 ns (fixed); φ2 = 0.6892; τ2 = 279 ns), (b) 5 MHz
plexes has predominantly triplet spin character and will
Transducer (Fit Parameters: φ1 = 0.3326; τ1 = 1 ns (fixed);
hereafter be referred to as 3MLCT. In principle, the “fast”
φ2 = 0.6191; τ2 = 273 ns).
heat deposition process (τ1 and φ1) corresponds to the heat
released when the Franck-Condon “singlet” MLCT state (the
vertical state produced by photoexcitation) relaxes to 3MLCT (here we assume that relaxation from the Franck-Condon
“singlet” MLCT state to 3MLCT occurs with unit efficiency). The second heat deposition process (τ2 and φ2) corresponds to heat released concomitant with non-radiative decay of 3MLCT. Based on these definitions, the amplitudes
can be related to the energy of 3MLCT by the following equations,
Channel
φ1 =
E hν − E T
E hν
φ2 =
E T (1 − Φ em )
E hν
(1)
(2)
where Ehν is the excitation energy (355 nm = 80.5 kcal mol-1), Φem is the quantum yield for emission from 3MLCT and
ET is the energy of 3MLCT. Table 2 contains (1) the average normalized amplitudes (φi) recovered from deconvolution
analysis of four independent LIOAS measurements on each (b)Re(CO)3(py-Bz)+ complex with the two ultrasonic
transducers; (2) the average lifetime of the “slow” heat-deposition component (τ2); and (3) ET values calculated from
the experimental φi values by using equations 1 and 2.
The Spectrum
Page 6
The energy of 3MLCT can also be estimated by using a single-mode Franck-Condon line-shape analysis of the
emission band observed from the (b)Re(CO)3(py-Bz)+ complexes in CH3CN solution at ambient temperature.4,5 Accordingly, the experimental emission spectra for the complexes were fitted by using the following equation,

 ν 00 − ν m hω m
I(ν ) =

ν 00

ν m =0 

∑
5
3

 ν − ν 00 + ν m hω m
 (Sm )νm

exp  − 4 ln 2

∆ ν 0,1 / 2

 ν m!






2

 


(3)
()
where I ν is the relative emission intensity at energy ν , ν 00 is the energy of the zero-zero transition (i.e., the energy
of 3MLCT), νm is the quantum number of the average medium frequency vibrational mode, hω m is the average of
medium frequency acceptor modes coupled to the MLCT transition (1450 cm-1)4, Sm is the Huang-Rhys factor (i.e., the
electron-vibration coupling constant), and ∆ν 0 ,1 / 2 is the half-width of the individual vibronic bands. The ν 00 values
obtained by application of equation 3 for the (b)ReI(CO)3(py-Bz)+ series are listed in Table 1, and fitted emission
spectra are shown in Figure 3 along with the fit parameters. These fit parameters are consistent with a previous study
on similar complexes.4
Table 2. LIOAS Data for (b)ReI(CO)3(4-benzylpyridine)a
1 MHz Transducer
φ1
φ2
τ2/ns
(kcal mol-1)
ET/cm-1 c
(kcal mol-1)
tmb
0.2698 ± 0.02
0.4402 ± 0.05
1109 ± 294
dmb
0.3281 ± 0.02
0.6267 ± 0.09
264 ± 11
bpy
0.3059 ± 0.04
0.6410 ± 0.01
196 ± 9
damb
0.3534 ± 0.08
0.5908 ± 0.06
103 ± 6
deb
0.3070 ± 0.02
0.6493 ± 0.07
56 ± 9.7
20570 ± 630
(58.8 ± 1.8)
18610 ± 450
(53.2 ± 1.3)
19550 ± 1110
(55.9 ± 2.9)
18220 ± 2310
(52.1 ± 6.6)
19520 ± 700
(55.8 ± 2.0)
16540 ± 1850
(47.3 ± 5.3)
18780 ± 2550
(53.7 ± 7.3)
18890 ± 420
(54.0 ± 1.2)
17070 ± 1750
(48.8 ± 5.0)
18540 ± 2100
(53.0 ± 6.0)
ET/cm-1 b
(kcal mol-1)
ET/cm-1 c
(kcal mol-1)
19380 ± 730
(55.4 ± 2.1)
18360 ± 1010
(52.5 ± 2.9)
18050 ± 1150
(51.6 ± 3.3)
16230 ± 910
(46.4 ± 2.6)
13640 ± 3360
(39.0 ± 9.6)
19240 ± 1400
(55.0 ± 4.0)
18890 ± 870
(54.0 ± 2.5)
20010 ± 1430
(57.2 ± 4.1)
15840 ± 1120
(45.3 ± 3.2)
15700 ± 2800
(44.9 ± 8.0)
Ligand
ET/cm-1 b
5 MHz Transducer
φ1
φ2
τ2/ns
tmb
0.3122 ± 0.03
0.5123 ± 0.04
773 ± 180
dmb
0.3486 ± 0.04
0.6302 ± 0.03
249 ± 17
bpy
0.3587 ± 0.04
0.6780 ± 0.05
197 ± 14
damb
0.4238 ± 0.03
0.5475 ± 0.04
111 ± 6
deb
0.5156 ± 0.12
0.5499 ± 0.10
79 ± 9.5
Ligand
Argon-degassed CH3CN solution, 298 K. Reported values are averages of 4 runs for each sample, and errors are ± 1σ.
τ1 fixed at 10 ns for the l MHz transducer and 1 ns for the 5 MHz transducer. bTriplet energy calculated from the first
deconvolution amplitude (equation 1 in text). cTriplet energy calculated from the second deconvolution amplitude
(equation 2 in text).
a
Page 7
The Spectrum
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Figure 3 illustrates the emission spectra and
compares the energies of 3MLCT for the five
(b)Re(CO)3(py-Bz)+ complexes obtained from the
LIOAS data using the 1 MHz and 5 MHz transducers (four ET values for each complex) and from
the Franck-Condon analysis.
a. (tmb)
υ00 = 19400 cm -1
S = 1.1
∆υ1/2 = 2900 cm -1
b. (dmb)
υ00 = 18375 cm -1
Discussion and Conclusions
S = 1.1
∆υ1/2 = 2900 cm -1
Emission Intensity (Normalized)
A primary objective of the study presented
herein is to identify the conditions under which
LIOAS coupled with deconvolution analysis may
c. (bpy)
be safely applied to determine the energy of an
1.0
υ00 = 17975 cm -1
excited state. In order to assess the validity of the
0.8
S = 1.1
0.6
∆υ1/2 = 2800 cm -1
LIOAS data, we make several assumptions. First,
0.4
we rely upon the emission spectra and the Franck0.2
Condon bandshape analysis (ν 00 ) to provide the
0.0
best available estimate for the energy of the 3MLCT
d. (damb)
1.0
-1
υ00 = 17280 cm
state. Second, our analysis of the LIOAS data ne0.8
S = 1.1
0.6
glects contributions to the acoustic wave arising
∆υ = 2700 cm -1
1/2
0.4
from the volume change which occurs concomi0.2
tant with 3MLCT decay. Previous work by other
0.0
groups suggests that the latter assumption is valid
e. (deb)
1.0
υ00 = 16050 cm -1
for LIOAS studies where no net photoreaction
0.8
S = 0.9
occurs.15
-1
0.6
∆υ1/2 = 2550 cm
0.4
Three major conclusions can be drawn concern0.2
ing the recovery of excited state energies and life0.0
times using the LIOAS apparatus coupled with
12
14
16
18
20
22
24
deconvolution analysis.
3
-1
Emission Energy / 10 cm
1. LIOAS measurements are accurate only when
Emission υ
the
excited state lifetime is greater than 200 ns.
LIOAS φ , 1 MHz
LIOAS φ , 1 MHz
It can clearly be seen in Figure 3 that for the tmb,
LIOAS φ , 5 MHz
dmb,
and bpy complexes the energies of 3MLCT
LIOAS φ , 5 MHz
derived from LIOAS and from emission spectral
fitting are in reasonable agreement. However, for
Figure 3: Corrected emission spectra of (b)ReI(CO)3(4-benzylthe damb and deb complexes the ET values derived
pyridine) in degassed CH3CN solution at 298 K: (a) b = (tmb); (b) b
from LIOAS vary widely and are in poor agree= (dmb); (c) b = (bpy); (d) b = (damb); (e) b = (deb). Points are
ment with the energies derived from emission
experimental data, and solid lines are spectra calculated using
spectral fitting. Importantly, the 3MLCT lifetimes
Franck-Condon analysis (see text) with the fitting parameters listed
of the former three complexes range from 210 to
on each spectrum. MLCT energies are presented on each plot with
1500 ns, while for the latter two complexes the lifeappropriate errors.
times range from 90 to 120 ns (see emission lifetimes in Table 1 and τ2 in Table 2).
This result clearly indicates that the reliability of the LIOAS data is strongly influenced by the lifetime of the
excited state under study and (with our system) the lower lifetime limit for reliability is τ ≥ 200 ns. The probable origin
for this lower limit is that the measurements are ultimately limited by the acoustic transit time (τa) of the LIOAS
apparatus.16 This parameter is defined as
00
1
2
1
2
τa =
R
Va
(4)
where R is the radius of the excitation beam and Va is the velocity of sound in the sample medium. If we assume a
beam radius of 0.25 mm and a velocity of sound in acetonitrile of 1300 m-s-1, an acoustic transit time of 192 ns is
obtained. Therefore, given this acoustic transit time it is not possible to accurately resolve the “prompt” relaxation of
The Spectrum
Page 8
the Franck-Condon state to the 3MLCT state from the “slow” non-radiative decay of the 3MLCT state for complexes
with 3MLCT lifetimes less than 200 ns (i.e., the damb and deb rhenium complexes). While it is theoretically possible to
reduce the acoustic transit time by decreasing the size of the excitation beam, studies have shown that photon saturation effects at exceedingly small beam diameters decrease the effectiveness of the LIOAS measurement.11
2. Excited state energies derived from the second (slow) heat deposition process are less precise and less accurate compared to
those derived from the first (prompt) heat deposition.
The experiments carried out in this study indicate that the precision and accuracy (i.e., reproducibility and closeness to the “true value”, respectively) of the energies determined by using φ2 are generally lower compared with those
derived from φ1. Simulations of LIOAS responses indicate that φ1 is determined primarily by the quality of the fit in
the initial “peak” region of the LIOAS signal, while φ2 is determined mainly by the quality of the fit in the secondary
“oscillations” region of the LIOAS signal that occurs after the initial peak. It can clearly be seen in Figure 2 that the
secondary oscillation region of the LIOAS signal is complex and relatively noisy, which inherently decreases the
precision (and presumably also the accuracy) with which we are able to recover φ2.
3. Lifetimes can be accurately measured for “intermediate” decay processes.
Comparison of the data in Tables 1 and 2 indicates that the 3MLCT lifetimes recovered from LIOAS are in good
agreement with the emission lifetimes (i.e., the “true” values) for the dmb, bpy and damb complexes. The lifetimes for
these three complexes range from 100 - 300 ns. Since LIOAS accurately recovers these lifetimes, we conclude that this
is the “intermediate” time domain within which LIOAS can be used to accurately determine lifetimes. By contrast,
LIOAS underestimates the lifetimes of both the deb and tmb complexes. Note that with the 5 MHz transducer the
lifetime of the tmb complex (τem ≈ 1500 ns) is less accurate and with the 1 MHz transducer the lifetime of the deb
complex (τem ≈ 90 ns) is less accurate. This indicates that, as expected, the “intermediate” time domain where lifetimes
can be accurately recovered shifts to shorter lifetimes as the characteristic response frequency of the transducer increases. While this study demonstrates that given sufficient information regarding the characteristics of a LIOAS
system it is possible to accurately recover excited state lifetimes from deconvolution analysis, the narrow range over
which lifetimes can be accurately recovered severely limits the general usefulness of the method.
Summary
The use of LIOAS to measure triplet MLCT energies in a series of inorganic complexes having excited states with
lifetimes ranging from 90 to 1500 ns has been demonstrated. Excited state energies derived from the “prompt” heat
deposition component are relatively precise and accurate for complexes with lifetimes longer than 200 ns. However,
the method fails to accurately recover excited state energies for complexes with lifetimes less than the 200 ns limit.
Lifetimes for the 3MLCT state are also reasonably measured by LIOAS in comparison to the “true” values which are
determined by time-resolved emission. With the above conclusions in mind, LIOAS can be a useful technique for the
determination of excited state energies and/or yields.
References
1. Schanze, K.S.; Walters, K.A. In Organic and Inorganic Photochemistry; Ramamurthy, V., Schanze, K.S., Eds.;
Marcel-Dekker: New York, 1998; p 75.
2. Schanze, K.S.; MacQueen, D.B.; Perkins, T.A.; Cabana, L.A. Coord. Chem. Rev. 1993, 122, 63.
3. Worl, L.A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T.J. J. Chem. Soc., Dalton Trans. 1991, 849.
4. Caspar, J.V.; Kober, E.M.; Sullivan, B.P.; Meyer, T.J. J. Am. Chem. Soc. 1982, 104, 630.
5. Wang, Y.; Schanze, K.S. Inorg. Chem. 1994, 33, 1354.
6. Rudzki, J.E.; Goodman, J.L.; Peters, K.S. J. Am. Chem. Soc. 1985, 107, 7849.
7. Goodman, J.L.; Herman, M.S. Chem. Phys. Lett. 1989, 163, 417.
8. Song, X.; Endicott, J.F. Chem. Phys. Lett. 1993, 204, 400.
9. Braslavsky, S.E.; Heibel, G.E. Chem. Rev. 1992, 92, 1381.
10. Lynch, D.; Endicott, J.F. Appl. Spect. 1989, 43, 826.
11. Rudzki-Small, J.; Libertini, L.J.; Small, E.W. Biophys. Chem. 1992, 42, 29.
12. Song, X.; Endicott, J.F. Inorg. Chem. 1991, 30, 2214.
13. Harriman, A. J. Chem. Soc., Chem. Commun. 1977, 777.
14. Murov, S.L.; Carmichael, I.; Hug, G.L. Handbook of Photochemistry; Marcel-Dekker: New York, 1993.
15. Hung, R.R.; Grabowski, J.J. J. Am. Chem. Soc. 1992, 114, 351.
16. Isak, S.J.; Komorowski, S.J.; Merrow, C.N.; Poston, P.E.; Eyring, E.M. Appl. Spectros. 1989, 43, 419.
Page 9
The Spectrum
About the Authors
Keith A. Walters is a second year graduate student and Grinter fellow at the University of Florida. He received his
B.S. in chemistry from Furman University in 1996, where he worked under Dr. Noel A.P. Kane-Maguire and received
the ACS Outstanding Senior award. His research interests include photothermal chemistry, design and implementation of spectroscopic instruments, and inorganic photochemistry.
Kirk S. Schanze is a professor of chemistry at the University of Florida. He received his B.S. in chemistry at Florida
State University and his Ph.D. from the University of North Carolina. His research interests include metal-complex
photochemistry and photophysics, electron transfer, π-conjugated polymers, and luminescence imaging. His address
is Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, e-mail: kschanze@chem.ufl.edu.
Center for Photochemical Sciences Awards Fellowships
Robin Lasey, Hammond Doctoral Fellow, (front) and
George S. Hammond
Robin Lasey, a Ph.D. student in Photochemical Sciences
at Bowling Green State University, is the first recipient of
the Hammond Fellowship in the Photochemical Sciences
at the University. The fellowship was established in recognition of Professor George S. Hammond’s enormous
contributions to the photochemical sciences and in honor
of his being awarded the 1994 National Medal of Science,
the nation’s highest scientific award. American students
who display outstanding academic and research ability
are eligible for the award. Robin is a student in Dr. Michael
Ogawa’s laboratory and is conducting research on electron transfer in model proteins. Professor Hammond is a
visiting distinguished professor at Bowling Green State
University.
The Center also awards McMaster Fellowships, available through the generous donation of Helen and Harold
McMaster, to doctoral students. Students who display outstanding academic ability after the first year in the Ph.D.
program are named Outstanding Academic Doctoral Fellows. This year’s recipients are Mikhail Chamachkine,
Anna Fedorova, Haiyan Gu, Maxim Makarov, and Ioulia
Zotova. Students who exhibit exceptional research ability
after the second year receive Outstanding Doctoral
Fellowships.This year’s recipients are Wenyi Cao, Xinhua
Gu, Anna Kornilova, Evgueni Piatnitski, Zinaida
Romanova, Xiaosong Wu, and Ningning Zhao.
The Spectrum on the World-Wide Web
The Spectrum is available on the Center’s Web site: http://www.bgsu.edu/departments/photochem/. You can access
via Acrobat Reader. There are instructions for downloading a free copy of Acrobat Reader from the Adobe Web site.
If you plan to access The Spectrum electronically, please send an e-mail to: photochemical@listproc.bgsu.edu. We
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The Spectrum
Page 10
Photochemistry in Organic Synthesis
Paul Margaretha
Institute of Organic Chemistry, University of Hamburg, Germany
This article is dedicated to R.S.H. Liu, D.C. Neckers, J. Saltiel, N.J. Turro, P.J. Wagner and D.G. Whitten on the
occasion of their sixtieth birthdays.
Introduction
Students quite often ask me about ”useful contributions of photochemistry to organic synthesis”. Such questions
seem legitimate if one looks through some typical preparative organic series like ”Organic Reactions” or “Organic
Syntheses“. Up to the last volume in the former series only three out of almost 200 chapters deal with light-induced
reactions, while in the latter from more than 3,000 procedures only 26 are found under the heading ”photochemical
reaction“ (and to make it worse, in one-third of these examples light is just used to initiate a radical chain reaction, e.g.
homolysis of Cl2 or of a diacylperoxide). In this context it cannot be considered as reassuring that the numbers for
electrochemical reactions or procedures--no chapter at all in the former and only seven procedures in the latter
series--are even lower. This short article is therefore intended to help synthetic organic chemists to overcome their
reluctance in using light sources as instruments for preparing compounds. It lists some important applications of
light induced reactions in preparative organic chemistry rather than representing an exhaustive survey (for this latter
purpose the reader is referred to more general references ).1-6 Two general types of reactions will be discussed (Figure
1), first the light induced formation of products (or intermediates) which are also accessible from the same starting
material by alternative (dark) methods (1), and then ”unique“ or ”selective“ reactions wherein the conversion to
products or to intermediates requires the population of an excited state of either the starting compound or of an
additional reagent (2).
M*
Reactions Taking Place From Both
Excited- and Ground States
M + R*
The homolytic cleavage of relatively
weak covalent bonds as S-H in thiols, C-Cl
M
M+R
in α-chloroketones, or C-N in α-diazoketones is a process which can be typically
induced either by light or by alternative
M*
M + R*
methods, e.g. starting a radical chain by de(2)
[I]
P
[I]
P
composition of either AIBN or Bu3SnH,7 or
hv
catalyzing the diazo compound cleavage
M
M+R
with a transition metal,8 respectively. Examples for such reactions as the syntheses
Figure 1. Generalized schemes for M -> P conversion for “alternative”
of 3-substituted thiolanes via a radical adreactions (1) and “excited state selective” reactions (2). (M = starting
dition/cyclization sequence,9 of the nonstecompound, R = additional reagent, I = intermediate, P = product.
roidal antiinflammatory agent Ibuprofen via
an aryl ring migration10 and of norandrostenol-16-carboxylic acids11 or of [4.4.4.5] fenestranes12 by ring contraction
(Wolff rearrangement) of a 2-diazocyclopentanone are summarized in Figure 2. In contrast, heterolytic cleavage of
bonds between carbon and other atoms occur under mild conditions only if a stabilized carbenium ion is formed. The
light induced cleavage of the exocyclic C-O bond in 10-alkoxy-10-phenyl-xanthenes in aq. CH3CN makes the pixyl
group an interesting photocleavable protecting group for primary alcohols,13 which can be used in the synthesis of
oligoribonucleosides as the deprotecting step occurs at neutral pH-values, i.e. it avoids the (thermal) use of
strong acids.
Cathodic reduction is a conventional way of generating radical anions from ground state molecules. Such
intermediates are also often accessible by interaction of an excited molecule with a (sacrificial) electron donor, as
[I]
P
[I]
P
(1)
Page 11
The Spectrum
H
CO2R
HSCH2
H
hv
.SCH
H
CH2CO2R
CO2R
[9]
H
2
S
85%
O
O
Cl
hv
R
H2O
.
R
CO2H
70%
R = i-C4H9
O
CO2H
O
..
hv
N2
[10]
R
H2O
[11]
HO
80% (3:1 mixture)
Ph
Ph
OR
Ph
+
hv
OH
H2O
O
O
O
+ RO
+
[13]
ROH
(78-97% for various R)
Figure 2. Examples of reactions where the (primary) bond
breaking step can be induced either by light or by other (dark)
methods.
O
O
hv / N(C2H5)3
[15]
CH3CN / LiClO4
50%
O
9,10-DCA / hv
R3Si
O
N
SiR3
CH3CN / MeOH
N-Methylmaleimide
[17]
HOCH2
O
42%
O
O
O
O
1,4-DCN / hv
O
O
O
O
[18]
C6H6
80% (3:2 mixture)
CuOTf / hv
[19]
HO
Et2O
HO
88-92%
Figure 3. Examples of reactions induced by photo-electrontransfer (PET).
exemplified by the reductive ring opening of excited
cyclopropyl ketones by an amine as shown in Figure
3. These reactions are commonly run in CH3CN containing LiClO4 in order to minimize the reduction to
the corresponding alcohol.14, 15 Single electron transfer (SET) between two ground state molecules is usually highly endothermic due to their large HOMOLUMO gaps. This situation is reversed with electronically excited states and therefore photosensitized
oxydation16 of e.g. alkenes, ethers or aromatic hydrocarbons can easily be achieved by using excited aromatic nitriles as naphthalene-1,4-dicarbonitrile or anthracene-9,10-dicarbonitrile. Many applications of this
method make use of the facile fragmentation of radical cations, as shown for the preparation (and trapping) of α-hydroxymethyl radicals.17 Alternatively an
alkene radical cation can undergo cycloaddition to a
second alkene moiety to afford a cyclobutane radical
cation which is then reduced by back electron
transfer from the nitrile radical anion.18 This same
sequential principle can be achieved by irradiating
Cu(I) complexes of hepta-1,6-dien-3-ols to afford
bicyclo[3.2.0]heptan-2-ols.19
Excited-State Selective Reactions
Light-induced homolysis of a single bond in an
acyclic molecule affords first a radical pair and then
two (separated) radicals. Only if one of the two radicals is long lived, rearrangement of the other one, e.g.
by H-transfer, and sequential recombination represent
a useful synthetic sequence, as illustrated (Figure 4)
for the so-called Barton reaction20 of organic nitrites.
In contrast to radical pairs, biradicals have emerged
as highly important intermediates in a multitude of
photochemical reactions. Only very recently some
thermal reactions starting from enediynes or enyneallenes 21 proceeding via such intermediates have been
developed. One classic approach to such intermediates consists in the homolytic cleavage of a single bond
in a cyclic molecule. This process can be terminated
by intramolecular H-transfer,22 cyclization to a constitutional isomer of the starting material,23, 24 or followed
by elimination of a molecular fragment, e.g. CO25-28 or
ketene.29 Biradicals can also be formed by intramolecular H-transfer to the oxygen atom of an excited carbonyl group (Figure 5), the most common example being the so-called Yang reaction, 30 wherein a 1hydroxytetramethylene-1,4-biradical cyclizes to a
cyclobutanol. An extension of this reaction has been
used for converting N-phenacyl-δ-valerolactone to a
bicyclic azetidinol of > 99% optical purity by irradiating a 1:1 clathrate of the lactam and an enantiomerically pure spirodioxolane.31 A related reaction
The Spectrum
Page 12
is the conversion of 2-alkylbenzophenones to stable
ο-xylylenols,32 which can be trapped by dienophiles,
hv
NO
e.g. by fullerenes.33
[20]
Electrocyclic ring closure and ring opening can
O
ONO
OH
OH
be easily achieved with light, the product most often being diastereomeric to that of the correspondO
O
O
C
ing thermal reaction (conrotatory vs disrotatory re.
hv
[22]
MeOH
RN
.
RN
RN
actions depending on the number of pi electrons).
CH
CH
CH
N
CH CO CH
CH
R
Examples of such reactions include the valence
R = COOCH3
66%
isomerization of cyclopentadiene to bicyclopentene, 34 or the conversion of Z-stilbenes to
O
O
dihydrophenanthrenes, which are usually irrevershv
O
ibly dehydrogenated to phenanthrenes,35 while the
[23]
S
S
S
reversible reaction of substituted dithienylethenes
O
O
O
90%
emphasizes the use of such molecules as a photochromic system.36 The ring opening of thietes37 repR’
R’
R’
resents the first synthetic approach to alicyclic
hv
- CO
R
[28]
enthiones (Figure 6) and the ring opening of proviR
O
R
O
S
S
S
70-90%
tamin D to previtamin D can be conveniently run
by using a two-step laser as light source.38
O
O
R
R
O
The conversion of an E-alkene to its (thermodyR
hv
- CH2CO
namic) less stable Z-diastereoisomer can be easily
H
O
O
O
O
achieved by excitation.39 Similarly, Z-cycloalkenes
O
[29]
can be converted to highly strained E-diastereoisoR = t.Bu 75%
mers, which can then be trapped, e.g. by protonaFigure 4. Light-induced cleavage reactions.
tion.40 Some light-induced rearrangements (Figure
6) occur due to interaction of pi-electrons with
n-electrons of an additional heteroatom, as shown for the ring enlargement of quinoline-N-oxides to benzoxazepines,41
or the ring contraction of thiinones to thietanones.42 A more often encountered process is the interaction of two
pi-electron systems in excited molecules. On irradiation, 1,4-dienes undergo bridging between C(2) and C(4). The
methano-bridged 1,4-biradical can either close to a bicyclo[2.1.0]pentane unit,43, 44 or afford a vinylcyclopropane via a
bond cleavage/bond formation sequence. This overall reaction is known as di-pi-methane rearrangement
Ph
(or Zimmerman reaction).45 A related reaction is the
HO
hv **
H
conversion of β,γ-unsaturated ketones to cycloproN
H
N
O
[31]
H
N
O
pyl ketones,46 the so-called oxa-di-pi-methane rearO
HO
O
rangement (Figure 7).
Ph
99% ee
Ph
1,5-Hexadienes photoisomerize to both biC(Ph)2OH
**
cyclo[2.2.0]and bicyclo[2.1.1]hexanes. An example
1: 1 clathrate with (-)
O
C(Ph)2OH
of such a diastereomer differentiating reaction is
O
illustrated (Figure 7) for two diastereomeric
cyclopentenylpyrrole derivatives.47 As a matter of
O
Ph
fact the (intramolecular) cyclobutane formation of
Ph
OH
Ph
hv
cyclopent-2-enones or cyclohex-2-enones containing
C60
OH
an additional (exocyclic) alkene moiety48 represents
CH3
one of the most often used photochemical reactions
SiO2
in organic synthesis. An example for such a process
CH
[33]
C58
is shown in the multistep isomerization of an
C
57%
alkenyl-alkynyl substituted cyclohex-2-enone to a
tetracyclic cyclohexanone.49 Less often used, but
Ph
O
similarly powerful as a synthetic tool are photoisomerizations of alkenylarenes to di- or triFigure 5. Light-induced H-transfer reactions.
quinanes.50 For lack of space it will just be noted that
.
CH3
CH3
CH2
.
CH2NO
.
2 5
2 5
..
..
. .
.
.
3 7
3 7
2
2
3
Page 13
The Spectrum
the previously mentioned photoisomerizations (Figure 8) comprising the interaction of two chromophoric groups in
one molecule obviosly extend to intermolecular reactions between an excited molecule and a (second) reactant (enone
+ alkene cycloadditions,51 arene + alkene cycloadditions,52 etc.).
OMe
hv
hv
[34]
THF
H+
C6H5OH
40-60%
[40]
MeOH
52%
hv
H
H
Ox
hv
[35]
O
C6H6
N
N
O
.
O
CF2
hv
CF2
CF2
CF2
CF2
O
50%
75-85%
CF2
O
O
.
S
C6H6
S
hv
S
hv’
S
λmax = 234 nm
[42]
S
[36]
S
[41]
N
95%
λmax = 534 nm
O C6H6
O
.
.
hv
S
O
[43]
O
O
70% O
R
R
R
hv
t.Bu
hv
[37]
t.Bu
R
R’
X
X
R
S
hv’
R’
. .
R’
X
[45]: X = CR2
λmax = 554 nm
λmax = 237 nm
[46]: X = O
Figure 6. Light-induced electrocyclic reactions.
Figure 7. Light-induced reactions involving C-C double
bonds.
hv
H H
NR
O
NR
O
H H
NR
[47]
R = CO2t.Bu
hv
O
O
NR
O
O
hv
O
.
C5H12
t.Bu
.
[49]
CH2
t.Bu
47%
OMe
+
hv
C5H12
OMe
MeO
1) Br2
2) Bu3SnH
3) KOH / H2NNH2
35% overall
[50]
Figure 8. Formation of (intramolecular) enone + alkene and arene + alkene photocycloadducts.
The Spectrum
Page 14
1
3Sens + 3O2
O2
+ Sens
[53]
(Sens = Tetraphenylporphyrine, eosine or other dyes)
Finally, photooxygenation53 represents an important example
of the light-induced formation of an excited ”reagent“ (singlet oxygen), which reacts with alkenes to afford either allylic hydroperoxides54 or 1,2-dioxetanes, the former being easily reduced to allylic
alcohols (Figure 9).
OOH
Conclusion
1O2
As summarized in the last chapter, electronic isomers of ground
state molecules, with well defined multiplicity (singlet or triplet
states) undergo a big variety of synthetically useful transforma67%
tions. In addition to ”conventional“ conversions, such reactions can
Figure 9. Formation of singlet oxygen and
be run down to a very low temperature (in solution or in a matrix)
formation of an allylic hydroperoxide by
and also on pure solids, both crystals or thin films. This latter
photooxygenation.
method adds additional options, as sometime photoreactions, e.g.
cyclodimerizations, do not occur at all in solution but efficiently in
the solid state.55 In my modest opinion, synthetic organic chemists refusing to use photochemical methodology, i.a.
due to indoctrination with sentences like ”reactions proceeding via excited states or radicals always afford mixtures
because such intermediates react unselectively“ are literally renouncing to a now established and well-understood
part of their science. Hopefully they will be convinced by browsing through the references cited in this article, or to
put it in one slogan: ”There is no life without light and no organic chemistry without photochemistry.“
[54]
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The Spectrum
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About the Author
Dr. Margaretha received his Ph.D. in chemistry at the University of Vienna (Austria) in 1969. He is currently professor of organic chemistry at the University of Hamburg. His interests focus on mechanistic and preparative organic
photochemistry and also include competitive bridge.
Copyright 1998 by the Center for Photochemical Sciences
The Spectrum is a quarterly publication of the Center for
Photochemical Sciences, Bowling Green State University,
Bowling Green, OH 43403.
Phone 419-372-2033 Fax 419-372-6069
Email photochemical@listproc.bgsu.edu
WWW http://www.bgsu.edu/departments/photochem/
Executive Director:
D.C. Neckers
Administrative Director: Pat Green
Principal Faculty:
G.S. Bullerjahn, J.R. Cable,
K.D. Deshayes, Y.J. Ding,
W.R. Midden, M.V. Mundschau,
D.C. Neckers, M.Y. Ogawa,
M.A.J. Rodgers, D.L. Snavely
The Spectrum Editor:
Pat Green
Production Editor:
Alita Frater
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The Spectrum is not responsible for contents of articles.
Articles submitted to The Spectrum will appear at the discretion of
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The Spectrum
Page 16
Scans Yield Mummy Clues*
by Vanessa Winans, Toledo Blade Staff Writer
1 museum specimen thought to be royalty
One of the Toledo Museum of Art’s two mummies appears to have been an aristocrat—maybe even royalty.
The museum called in three egyptologists to study the results of the mummies’ CAT scans, which were performed
in November at Toledo Hospital. Those results contain many clues to the pair’s lives and identities.
They have looked at the exposed hands and head of the female too.
They noticed an absence of hair on any of the female’s skin, indicating that she was probably a priestess, said
Sandra Knudsen, the museum’s curatorial consultant for ancient art.
Priestesses shaved all the hair off their bodies, she said.
The fingertips and nails of the female mummy had been painted with expensive red henna, which is another sign
of status. Her impeccable teeth—a rarity in ancient Egypt—show that her family had enough money to afford
fancy fare.
All this has excited the egyptologists, although they have not made a final decision on the degree of her pedigree.
“They’ve muttered the word ‘royal’ to us several times,” Ms. Knudsen said.
The man, however, appears to have belonged to the working class. Dentists examining the results have noted his
eroded jawbone and ground-down and missing teeth, casualties of the sand-filled bread that poorer people ate.
“He may have died of acute tooth decay and infection,” Ms. Knudsen said. “His gingivitis was so bad, he lost some
of his teeth, gums, and bone. That’s why you should floss.”
“You could stick your fingers through the holes in his
jaw,” she said.
Ms. Knudsen spoke as she
looked at work in progress at
Spectra Group Limited, Inc.,
a Maumee firm that makes
medical models.
Chemist and computer
technician Dustin Martin is
making a half-sized replica of
the male mummy, using a
fairly new technology called
stereolithography.
The firm takes the computerized axial tomography scan
results and builds a threedimensional copy, with a laser beam turning liquid resin
into the solid plastic model.
The firm mostly makes medical and automotive copies.
Doctors use the exact repDustin Martin of Spectra Group Limited, Inc., trims the support columns from a mummy
licas to plan delicate surgerfigure being recreated in the plastics lab. The firm is making a replica of the male
ies, Mr. Martin said.
mummy using stereolithography. Photo by Chris Walker, Toledo Blade.
Page 17
The Spectrum
In stereolithography, a laser beam leaves its mark during a time
exposure marking one ‘pass’ over the photo-sensitive liquid
plastic. The beam solidifies the plastic, and the mummy,
represented by a series of computerized axial tomography
scans, is recreated. Photo by Chris Walker, Toledo Blade.
Having the scan results turned into threedimensional models allows researchers to see what’s
inside the bodies, previously impossible because the
museum refused to unwrap the mummies, Ms.
Knudsen said.
The firm has donated time, equipment, and
materials to the effort, just as Toledo Hospital did
for the CAT scan.
“It’s kind of been a labor of love,” said Doug
Neckers, a professor of photochemical science at
Bowling Green State University and consultant to
the firm. “It’s a problem to be solved. We solved it.
Who pays for it, we’ll worry about later.”
Firm employees have enjoyed the challenge of
reconstructing a mummy.
“We’ve laughed a lot about this guy who thought
he was going to be reincarnated, and now he’s
being reincarnated—in plastic,” Dr. Neckers said.
Ask Mr. Martin about it, and his face lights up.
“It’s a lot of fun,” he said. “It’s a lot of work. But
to see their faces when they come in and see what’s
been done, it’s worth it.”
Museum officials don’t know yet how they’ll use
the replicas, which take about 12 hours to create,
Ms. Knudsen said. They never expected to have
such models.
“We’re stunned,” she said, looking at the halffinished model. “He is incredible.”
*Reprinted by permission of Toledo Blade, 5/7/98.
Advertising in The Spectrum
In the past we have not had any advertisements in The Spectrum because we are limited in staff. Corporate sponsors
donate money to defray the cost of publishing and mailing The Spectrum to over 7,000 people in 50 countries. Those
sponsors can place ads, if they wish, in four issues and are acknowledged as a sponsor. Some companies have
chosen to remain anonymous. We intend to continue the corporate sponsorship program. This is our preferred way
to help with our costs. However, we receive constant inquiries about placing advertisements. Therefore, we are
going to offer advertising space on a trial basis.
Size:
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Deadlines:
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$1,000 per issue
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$ 500 per issue
Computer file - EPS or PICT file (preferred method)
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We also will reserve the right to limit the number of advertisements per issue based on space. For additional
information about the corporate sponsorship or advertisements, please contact Pat Green (pgreen@bgnet.bgsu.edu)
or 419-372-2033 (phone) or 419-372-6069 (fax).
The Spectrum
Page 18
Center for Photochemical Sciences Publications
306. Manea, V.P.; Wilson, K.J.; Cable, J.R. Conformations and relative stabilities of the cis and trans isomers in a
series of isolated N-phenylamides. J. Am. Chem. Soc. 1997, 119, 2033.
307. Sarker, A.M.; Mejiriski, A.; Neckers, D.C. Novel imaging materials: synthesis and characterization of
poly(methyl methacrylate) with pendant benzophenone borate salt as single component photoimaging
system. Macromolecules 1997, 30, 2268-2273.
308. Hu, S.; Neckers, D.C. Photochemical reactions of sulfide-containing alkyl phenylglyoxylates. Tetrahedron
1997, 53, 12771-12788.
309. Hu, S.; Neckers, D.C. Photochemical reactions of alkoxyl-containing-alkyl phenylglyoxylates: remote
hydrogen abstraction. J. Chem. Soc., Perkin Trans. 2 1997, 1751.
310. Popielarz, R.; Hu, S.; Neckers, D.C. Applicability of decahydroacridine-1,8-dione derivatives as fluorescent
probes for monitoring of polymerization processes. J. Photochem. Photobiol., A: Chem. 1997, 110, 79-83.
311. Hu, S.; Neckers, D.C. Alkyl phenylglyoxylates as radical photoinitiators creating negative photoimages. J.
Mater. Sci. 1997, 7, 1737-1740.
312. Kang, J.U.; Ding, Y.J.; Burns, W.K.; Melinger, J.S. Backward second-harmonic generation in periodicallypoled bulk LiNbO3. Opt. Lett. 1997, 22, 862-864.
313. Ding, Y.J.; Khurgin, J.B. Transversely-pumped counter-propagating optical parametric amplication and
difference-frequence generation. J. Opt. Soc. Am. B 1997, 14, 2161-2166.
314. Khurgin, J.B.; Obeidat, A.; Lee, S.J.; Ding, Y.J. Cascaded optical nonlinearities: microscopic understanding
as a collective effect. J. Opt. Soc. Am. B 1997, 14, 1977-1983.
315. Cui, A.G.; Gorbounova, O.; Ding, Y.J.; Veliadis, J.V.D.; Lee, S.J.; Khurgin, J.B.; Wang, K.L. Evidence of
strong sequential band filling at interface islands in asymmetric coupled quantum wells. Superlattices
Microstruct. 1997, 22, 497-505.
316. Lee, S.J.; Khurgin, J.B.; Ding, Y.J. Directional couplers via the cascaded resonant surface-emitting secondharmonic generation. Opt. Commun. 1997, 139, 63-68.
317. Fry, B.E.; Neckers, D.C. Highly active visible-light photocatalysts for curing a ceramic precursor. Chem.
Mater. 1998, 10, 531-536.
318. Hu, S.; Neckers, D.C. Photoreduction of ethyl phenylglyoxylates. J. Photochem. Photobiol. 1998, 114, 103-108.
319. Babu, C.R.; Arucdchandran, A.; Hille, R.; Gross, E.L.; Bullerjahn, G.S. Reconstitution and characterization
of a divergent plastocyanin from the photosynthetic prokaryote, Prochlorothrix hollandica, expressed in
Escherichia coli. Biochem. Biophys. Res. Commun. 1997, 235, 631-635.
320. Hu, S.; Mejiritski, A.; Neckers, D.C. Photoreactions of polymeric (meth)acryloylethyl phenylglyoxylate reactivity in solution of film. Chem. Mater. 1997, 9, 3171-3175.
321. He, J.; Larkin, H.E.; Li, Y.-S.; Rihter, B.D.; Zaidi, S.I.A.; Rodgers, M.A.J.; Mukhtar, H.; Kenney, M.E.;
Oleinick, N.L. The synthesis, photophysical and photobiological properties, and in vitro structure-activity
relationships of a set of silicon phthalocyanine PDT photosensitizers. Photochem. Photobiol. 1997, 65, 581-586.
Page 19
The Spectrum
322. Aoudia, M.; Cheng, G.; Kennedy, V.O.; Kenney, M.E.; Rodgers, M.A.J. The synthesis of a series of
octabutoxy- and octabutoxybenzophthalocyanines and photophysical properties of two members of the
series. J. Am. Chem. Soc. 1997, 119 (26), 6029-6039.
323. Kozlov, G.V.; Ogawa, M.Y. Electron-transfer across a peptide-peptide interface within a designed
metalloprotein. J. Am. Chem. Soc. 1997, 119, 8377-8388.
324. Hu, S.; Neckers, D.C. Photochemical reactions of alkenyl phenylglyoxylates. J. Org. Chem. 1997, 62,
6820-6826.
325. Hu, S.; Neckers, D.C. Photochemical reactions of halo/arylsulfide-substituted alkyl phenylglyoxylate, an
assessment of the lifetime of the intermediate 1,4-biradical. J. Org. Chem. 1997, 62, 7827-7831.
326. Dwivedi, K.; Sen, A.; Bullerjahn, G.S. Expression and mutagenesis of the dpsA gene of Synechococcus sp.
PCC7942, encoding a DNA-binding protein required for growth during oxidative stress. FEMS Microbiol.
Lett. 1997, 155, 85-91.
327. Tretiakov, I.V.; Cable, J.R. Electronic spectroscopy and molecular structure of jet-cooled diphenylamine and
diphenylamine derivatives. J. Chem. Phys. 1997, 107, 9715.
328. Fernando, S.R.L.; Kozlov, G.V.; Ogawa, M.Y. Distance dependence of electron-transfer along artificial βstrands at 298 K and 77 K. Inorg. Chem. 1998, 37, 1900-1905.
329. Pandey, R.K.; Constantine, S.; Tsuchida, T.; Zheng, G.; Medforth, C.J.; Aoudia, M.; Kozyrev, A.N.; Rodgers,
M.A.J.; Kato, H.; Smith, K.M.; Dougherty, T.J. Synthesis, photophysical properties, in vivo photosensitizing
efficacy, and human serum albumin (HSA) bind properties of some novel bacteriochlorins. J. Med. Chem.
1997, 40, 2770-2779.
330. Ford, W.E.; Wessels, J.M.; Rodgers, M.A.J. Electron injection by photoexcited Ru(bpy)32+ into colloidal SnO2:
analyses of the recombination kinetics based on electrochemical and Auger-capture models. J. Phys. Chem.
B 1997, 101, 7435-7442.
331. Popielarz, R.; Sarker, A.M.; Neckers, D.C. Applicability of tetraphenylborate salts as free radical
coinitiators. Macromolecules 1998, 41, 951-954.
* 332. Lungu, A.; Mejiritski, A.; Neckers, D.C. Solid state studies on the effect of fillers on the mechanical
behavior of photocured composites. Polymer, in press.
* 333. Sarker, A.M.; Kaneko, Y.; Nikolatchik, A.V.; Neckers, D.C. Photoinduced electron transfer reactions: highly
efficient cleavage of C-N bonds and photogeneration of tertiary amines. J. Phys. Chem. 1998, in press.
334. Aoudia, M.; Rodgers, M.A.J. Photoprocesses in self-assembled complexes of oligopeptides with metalloporphyrins. J. Am. Chem. Soc. 1997, 119, 12859-12868.
335. Hu, S.; Neckers, D.C. Photochemically active polymers containing pendant ethyl phenylglyoxylate.
Macromolecules 1998, 31, 322-327.
For reprints of any of these publications, please write the Center for Photochemical Sciences and refer to the reprint
by number. Reprints of articles in press will be provided upon publication of the article.
* As soon as an article is accepted for publication, the Center assigns a number and lists them accordingly for
internal recordkeeping.
The Spectrum
Page 20
Celebration of the “Photochemical Tie to 1938”
In appreciation of the contributions of six photochemists born in the year of 1938, a symposium will be held. The
symposium will take place during the Fall 1998 ACS National Meeting (August 23-27, 1998) in Boston. You are
cordially invited to participate in this historical event.
Honorees (will also speak at the symposium):
R. S. H. Liu
D. C. Neckers
J. Saltiel
N. J. Turro
P. J. Wagner
D. G. Whitten
Speakers at the symposium include the following:
D. R. Arnold
P. F. Barbara
J. K. Barton
I. Bronstein
R. A. Caldwell
W. J. DeGrip
F. C. De Schryver
M. A. El-Sayed
S. Farid
M. D. Forbes
M. A. Fox
E. R. Gaillard
T. Gillbro
R. S. Givens
J. L. Goodman
I. R. Gould
H. B. Gray
G. S. Hammond
E. F. Hilinski
H. Inoue
Y. Inoue
M. Irie
Y. Ito
W. F. Jager
W. S. Jenks
L. B. Johnston
M. Kasha
C. V. Kumar
F. D. Lewis
T. J. Meyer
J. Michl
H. A. Morrison
K. Nakanishi
D. G. Nocera
K. S. Schanze
J. R. Scheffer
D. I. Schuster
G. B. Schuster
R. E. Schwerzel
Y. Shichida
S. C. Shim
M. B. Sponsler
J. K. Thomas
L. M. Tolbert
M. Tsuda
C. Turro
D. H. Waldeck
R. G. Weiss
N. C. Yang
O. C. Zafiriou
R. G. Zepp
H. E. Zimmerman
For details, see the world wide web site at http://www.chem.fsu.edu/photo38.htm.
Alumni and Friends Reception for Dr. Douglas Neckers
A Photochemist Born in 1938
Bowling Green State University
and
The Center for Photochemical Sciences
cordially invite
Bowling Green alumni, friends and colleagues of D. C. Neckers
to a reception in his honor during the ACS meeting in Boston.
The reception is scheduled for Tuesday, August 25, 1998,
5:00-6:30 p.m., at the Sheraton Boston Hotel and Towers, Back Bay Ballroom.