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IntroductionThe fac-(b)ReI(CO)3L (where b is a diimine ligand) chromophore has been widely used for stud-

ies of photoinduced intramolecular energy and electron transfer.1 A series of complexes with varyingphotophysical 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 emis-sion attributed to this excited state is generally broad and structureless. Thus, it is difficult toaccurately determine the energy of the triplet MLCT excited state without tedious and sometimesinconsistent Franck-Condon line-shape analysis.4,5 An alternative technique for measuring theenergy of an MLCT state uses Laser-Induced Optoacoustic Spectroscopy (LIOAS).6 This techniquehas 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 measureexcited state energies and/or yields for molecules of interest. Before studying molecules withhitherto unknown excited state properties, we decided it would be prudent to carry out a calibra-tion study to characterize the accuracy of the LIOAS apparatus and signal analysis software byusing a series of excited states with “known” energy and variable lifetime. Thus, we selected aseries of fac-(b)ReI(CO)3(py-Bz)+ complexes (where b is a series of diimine ligands of varying elec-tron demand and py-Bz is 4-benzylpyridine, Scheme 1) for use in this calibration study. This seriesof complexes was selected for the study because: (1) the energy and lifetime of the Re → diimineMLCT state varies systematically with the electron demand of the diimine ligand; and (2) theMLCT state is luminescent, allowing us to use emission spectroscopy to estimate the energy of therelaxed excited state. In this report we present the results of the LIOAS calibration study of therhenium complex series and compare the energies of the MLCT states in the complexes as deter-mined by LIOAS and Franck-Condon bandshape analysis of the emission spectra.

Continued on page 3

Determination of Triplet Energies in Rhenium PolypyridineComplexes with Laser-Induced Optoacoustic Spectroscopy

Keith A. Walters and Kirk S. Schanze*,Department of Chemistry, University of Florida

O

OEt

NEt2

ON

N

N

Re

CO

CO

CO

R1

(tmb): R1 = CH3, R2 = CH3(dmb): R1 = H, R2 = CH3(bpy): R1 = H, R2 = H

R2

R1

(deb): R1 = H, R2 =

R2

(damb): R1 = H, R2 =

Scheme 1

From the Executive Director

D. C. Neckers, Executive Director, Photochemical Sciences, Bowling Green State University

In This Issue

Determination of Triplet Energies in Rhenium Polypyridine Complexes with Laser-Induced Optoacoustic Spectroscopy ................................................................................... ................................... 1From the Executive Director .................................................................................................... ................................................. 2Photochemistry in Organic Synthesis ............................................................................................ ....................................... 10Scans Yield Mummy Clues ........................................................................................................ .............................................. 16Center for Photochemical Sciences Publications ................................................................................. ............................... 18

The Spectrum Page 2

During the 1980s, I was a member of Mead Imaging’s Scientific Advisory Committee. Among the chosen six wasNick Negroponte, founder of MIT’s Media Lab. You know Nick’s kind. When coffee breaks would come, a secretarywould walk in and hand him a pack of phone messages. Sometime after the next session had begun, Nick wouldreappear 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 thisseemed a little odd to me, but who was a mere chemist from Bowling Green to ask impertinent questions of anarchitect from MIT?

A couple of years after our Mead assignment, Sue and I entertained Nick and Elaine at our Chautauqua Lakevacation home. Nick was speaking at the Chautauqua Institution the next day and, after knowing him for 10 years orso and a nice bottle of wine or two, I finally found out who he was. A year or so later the book Being Digital waspublished. While Nick was living in France, he sent me a copy of Wired. The cat was out of the bag. Friend Nick wasto the cybergeneration what the Beatles were to 1960s America—a clairvoyant folk hero of prodigious intellect whoseenergies helped change our generation. I was reminded of Nick again recently (June 28, 1998) when US Today hadthree 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 theblank) 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 betweentwo kids traveling alone, and the Widdlesticks Club taking their first trip to see Elvis. Nevermind that your flight isfrom Montreal to Atlanta. Some travel agents and airline pricing schemes make it cheaper to fly from Chicago toNashville through Montreal. The interstate highways are perpetually under repair. It is 60 miles from Detroit toToledo. But the interstate has been one lane for the last five years and, given the quality of work of the governmentthese days, will remain that way. As soon as one section gets repaired, the section last completed needs it again. Trafficis so bad in the New York area that even an inefficient flight from the hinterlands can get a traveler to Newark quickerthan 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 someupsides. No insurance agent, stockbroker or university alumni organization can interrupt your evening dinner withan 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 heprobably is, the keystroke will soon be as obsolete as a punch card. Voice activation is next, and after that thoughtactivation.

On the downside soon only the basics of human existence will require two persons together in the same room. Anddepending on the quality of the cyberloveaffair, even those times of togetherness may be mercifully short.

Page 3 The Spectrum

Continued from page 1

The LIOAS TechniqueLIOAS is a photothermal technique that allows quanti-

tative determination of the amount of heat evolved whena photoexcited state decays.9 The release of thermal energyconcomitant with non-radiative excited state decay resultsin the production of an acoustic pressure wave. A piezo-electric transducer pressed against the side of a samplecuvette is used to monitor the temporal evolution of theacoustic wave. In the simplest implementation of LIOAS,calorimetric information is extracted by comparing theacoustic wave amplitude of an unknown sample with thatof a calorimetric reference.10 However, this method onlyprovides the fraction of excitation energy released into thesample as heat in a fixed time domain. This elementaryuse of LIOAS does not allow one to partition the heat depo-sition into various non-radiative excited state decay pro-cesses that occur in different time domains. A more detailedanalysis of the time-resolved acoustic response using a non-linear least squares and iterative reconvolution method(hereafter referred to as “deconvolution analysis”) allowsone to extract the amplitudes and lifetimes for multiplenon-radiative decay processes that occur subsequent tophotoexcitation.11 In the present study we have imple-mented a commercially available software package thatuses the latter method to afford time-resolved LIOAS data.

An important limitation of LIOAS arises because ultra-sonic piezoelectric transducers respond to acoustic signalsover a relatively narrow frequency range. For example, thefrequency 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 thisdetector only responds to pressure waves with frequencies in the 0.2 - 2 MHz range. Acoustic waves at frequenciesoutside of this range will yield either a partially or fully damped (i.e., zero) response.12 Qualitatively, there is aninverse relationship between the lifetime of a non-radiative decay process and the frequency of the resulting acousticwave (i.e., a short lifetime gives a high frequency acoustic wave). Consequently, in LIOAS measurements: (1) non-radiative decay processes with lifetimes that are “short” compared to the upper bound of the transducer frequencyresponse will appear as a “fast” or “prompt” acoustic wave component that is in-phase with a reference sample thathas a very short excited state lifetime (e.g., < 1 ns); (2) non-radiative decay processes that are “intermediate” withrespect to the transducer response will appear as an acoustic wave that is damped in amplitude and phase-shiftedwith respect to the reference sample; and (3) non-radiative decay processes that are “slow” with respect to the trans-ducer response will be fully damped and therefore undetected. Although it is possible to simulate the relationshipbetween an excited state decay lifetime (i.e., the lifetime of the non-radiative decay process) and the resulting acousticpressure wave for a transducer with a given characteristic frequency and damping time constant,6 in practice it isnecessary to empirically characterize an LIOAS system. The most important questions that must be addressed in anLIOAS 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-radiativedecay 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 forthe lifetime. When this limit is exceeded, the lifetime and amplitude of the non-radiative decay component will bedistorted due to the limited frequency response of the transducer.

Figure 1: Transducer response spectra: (a) PanametricsV103 1 MHz Transducer, (b) Panametrics V105 5 MHzTransducer.

The Spectrum Page 4

A principal objective of the present study is to delineate the time domain over which our LIOAS system can beused 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 wasexamined. In particular, with this series the lifetime of the MLCT state varies from 100 to 1500 ns.5 This timescalerange is ideal for characterizing the ability of our 1 and 5 MHz transducers to resolve non-radiative decay processes.

Materials and MethodsSteady-State Emission Spectroscopy and Emission Quantum Yields. Corrected steady-state emission spectra were re-

corded on solutions of each complex with a SPEX F-112 fluorimeter. Samples were contained in 1 cm x 1 cm quartzcuvettes and excited at 350 nm. Emission quantum yields (φem) were calculated relative to two actinometers, and thevalues 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 ofapproximately 0.14 at the excitation wavelength.

Table 1. Emission Data for (b)Re I(CO)3(4-benzylpyridine) a

ν 00/cm-1 c

Ligand φem τem/ns ν max/cm-1 b (kcal mol-1)

tmb 0.25 1473 18700 19400(55.5)

dmb 0.06 274 17450 18375(52.5)

bpy 0.045 208 17210 17975(51.4)

damb 0.026 116 16700 17280(49.4)

deb 0.0145 93 15360 16050(45.9)

aArgon-degassed CH3CN solutions, 298 K. Estimated errors: φem, ± 15%; τem, ± 5%; ν max, ± 100 cm-1; ν 00, ± 500 cm-1.bEmission maximum. c0-0 emission energy estimated by Franck-Condon analysis as explained in text.

Emission Lifetimes. Time-correlated single-photon counting (FLI, Photochemical Research Associates) was used tomeasure 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. Lifetimeswere 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 wereexcited with the third harmonic of a pulsed Nd:YAG laser (355 nm, 10 ns fwhm, 10 Hz). The beam was attenuatedwith 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 responsesat the selected experimental energy. The Gaussian beam was passed through a 1.25 mm slit and a 10 cm focal lengthlens which focused the beam into the cell (the beam diameter was ca. 0.5 mm within the cell). A 1 cm x 1 cm quartzcuvette was firmly fixed in contact with a Panametrics V103 (1 MHz) or V109 (5 MHz) transducer that was positionedperpendicular to the excitation beam. A film of vacuum grease was used to assure good acoustical contact betweenthe cell wall and transducer element. The cell and transducer were fixed in a holder mounted on a linear stage toallow adjustment of the distance between the excitation beam and transducer, which was necessary to move thetemporal position of the LIOAS signal away from the initial RF noise resulting from the laser shot. Signals wereamplified 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 theauthors. Great care was taken to insure that the cell geometry did not change between reference and sample dataacquisition. Therefore, solvent rinse and sample solutions were carefully transferred into and out of the cell with a

Page 5 The Spectrum

syringe. Acetonitrile samples were nitrogen degassed in thefixed LIOAS cell for 20 minutes.

LIOAS data was analyzed using Sound Analysis Version1.14 (Quantum Northwest, Inc.). A reference signal (fer-rocene in acetonitrile) was used to determine the “transducerresponse function”. The reference waveform was acquiredimmediately prior to each sample to insure that the laserenergy and/or beam position was unchanged between ref-erence and sample signal acquisitions. Four measurementswere made on fresh solutions of each complex with eachtransducer. LIOAS signals were normalized for absorptiondifferences between the reference and sample.11 However,absorbance values were always the same within 10%. Theacoustic waveform for each sample was subjected todeconvolution analysis with two heat-deposition compo-nents (φ1 and φ2 are the amplitudes of the “fast” and “slow”heat components, respectively). The analysis software wasrun with the lifetime of the “fast” component (τ1) fixed at 10ns and 1 ns for the 1 MHz and 5 MHz transducers, respec-tively. The amplitudes of the “fast” and “slow” lifetime com-ponents and the lifetime of the “slow” component (i.e., φ1,φ2, 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 statisticalanalysis of the fits for all of the studied rhenium complexesare listed in Table 2.

The Energy of the 3MLCT StateThe energy of the lowest MLCT excited state in the

(b)Re(CO)3(py-Bz)+ complexes can be calculated from bothcomponents recovered from deconvolution analysis of theLIOAS data. The lowest excited state in this family of com-plexes has predominantly triplet spin character and willhereafter be referred to as 3MLCT. In principle, the “fast”heat deposition process (τ1 and φ1) corresponds to the heatreleased 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) corre-sponds to heat released concomitant with non-radiative decay of 3MLCT. Based on these definitions, the amplitudescan be related to the energy of 3MLCT by the following equations,

ν

ν −=φ

h

Th1 E

EE(1)

( )ν

Φ−=φ

h

emT2 E

1E(2)

where Ehν is the excitation energy (355 nm = 80.5 kcal mol-1), Φem is the quantum yield for emission from 3MLCT andET is the energy of 3MLCT. Table 2 contains (1) the average normalized amplitudes (φi) recovered from deconvolutionanalysis of four independent LIOAS measurements on each (b)Re(CO)3(py-Bz)+ complex with the two ultrasonictransducers; (2) the average lifetime of the “slow” heat-deposition component (τ2); and (3) ET values calculated fromthe experimental φi values by using equations 1 and 2.

Reference

Sample

Simulation

Residuals

350 400 450 500 550 600-0.5

0.0

0.5

1.0

a.

LIO

AS

Am

plitu

de

Reference

Sample

Simulation

Residuals

350 400 450 500 550 600-0.5

0.0

0.5

1.0

b.

LIO

AS

Am

plitu

de

Channel

Channel

Figure 2: LIOAS data for (dmb)Re(CO)3(4-benzylpyridine):(a) 1 MHz Transducer (Fit Parameters: φ1 = 0.3364; τ1 =10 ns (fixed); φ2 = 0.6892; τ2 = 279 ns), (b) 5 MHzTransducer (Fit Parameters: φ1 = 0.3326; τ1 = 1 ns (fixed);φ2 = 0.6191; τ2 = 273 ns).

The Spectrum Page 6

The energy of 3MLCT can also be estimated by using a single-mode Franck-Condon line-shape analysis of theemission band observed from the (b)Re(CO)3(py-Bz)+ complexes in CH3CN solution at ambient temperature.4,5 Ac-cordingly, the experimental emission spectra for the complexes were fitted by using the following equation,

( ) ( )∑=ν

ν

ν∆

ων+ν−ν−ν

ν

ων−ν=ν5

0

2

2/1,0

mm00

m

m

3

00

mm00

m

m

2ln4exp!

SI

hh(3)

where ( )νI is the relative emission intensity at energy ν , 00ν is the energy of the zero-zero transition (i.e., the energyof 3MLCT), νm is the quantum number of the average medium frequency vibrational mode, mωh is the average ofmedium frequency acceptor modes coupled to the MLCT transition (1450 cm-1)4, Sm is the Huang-Rhys factor (i.e., theelectron-vibration coupling constant), and 2/1,0ν∆ is the half-width of the individual vibronic bands. The 00ν valuesobtained by application of equation 3 for the (b)ReI(CO)3(py-Bz)+ series are listed in Table 1, and fitted emissionspectra are shown in Figure 3 along with the fit parameters. These fit parameters are consistent with a previous studyon similar complexes.4

Table 2. LIOAS Data for (b)Re I(CO)3(4-benzylpyridine) a

1 MHz Transducer ET/cm-1 b ET/cm-1 c

Ligand φ1 φ2 τ2/ns (kcal mol-1) (kcal mol-1)

tmb 0.2698 ± 0.02 0.4402 ± 0.05 1109 ± 294 20570 ± 630 16540 ± 1850(58.8 ± 1.8) (47.3 ± 5.3)

dmb 0.3281 ± 0.02 0.6267 ± 0.09 264 ± 11 18610 ± 450 18780 ± 2550(53.2 ± 1.3) (53.7 ± 7.3)

bpy 0.3059 ± 0.04 0.6410 ± 0.01 196 ± 9 19550 ± 1110 18890 ± 420(55.9 ± 2.9) (54.0 ± 1.2)

damb 0.3534 ± 0.08 0.5908 ± 0.06 103 ± 6 18220 ± 2310 17070 ± 1750(52.1 ± 6.6) (48.8 ± 5.0)

deb 0.3070 ± 0.02 0.6493 ± 0.07 56 ± 9.7 19520 ± 700 18540 ± 2100(55.8 ± 2.0) (53.0 ± 6.0)

5 MHz Transducer ET/cm-1 b ET/cm-1 c

Ligand φ1 φ2 τ2/ns (kcal mol-1) (kcal mol-1)

tmb 0.3122 ± 0.03 0.5123 ± 0.04 773 ± 180 19380 ± 730 19240 ± 1400(55.4 ± 2.1) (55.0 ± 4.0)

dmb 0.3486 ± 0.04 0.6302 ± 0.03 249 ± 17 18360 ± 1010 18890 ± 870(52.5 ± 2.9) (54.0 ± 2.5)

bpy 0.3587 ± 0.04 0.6780 ± 0.05 197 ± 14 18050 ± 1150 20010 ± 1430(51.6 ± 3.3) (57.2 ± 4.1)

damb 0.4238 ± 0.03 0.5475 ± 0.04 111 ± 6 16230 ± 910 15840 ± 1120(46.4 ± 2.6) (45.3 ± 3.2)

deb 0.5156 ± 0.12 0.5499 ± 0.10 79 ± 9.5 13640 ± 3360 15700 ± 2800(39.0 ± 9.6) (44.9 ± 8.0)

aArgon-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 firstdeconvolution amplitude (equation 1 in text). cTriplet energy calculated from the second deconvolution amplitude(equation 2 in text).

Page 7 The Spectrum

Figure 3 illustrates the emission spectra andcompares the energies of 3MLCT for the five(b)Re(CO)3(py-Bz)+ complexes obtained from theLIOAS data using the 1 MHz and 5 MHz trans-ducers (four ET values for each complex) and fromthe Franck-Condon analysis.

Discussion and ConclusionsA primary objective of the study presented

herein is to identify the conditions under whichLIOAS coupled with deconvolution analysis maybe safely applied to determine the energy of anexcited state. In order to assess the validity of theLIOAS data, we make several assumptions. First,we rely upon the emission spectra and the Franck-Condon bandshape analysis ( 00ν ) to provide thebest available estimate for the energy of the 3MLCTstate. Second, our analysis of the LIOAS data ne-glects contributions to the acoustic wave arisingfrom the volume change which occurs concomi-tant with 3MLCT decay. Previous work by othergroups suggests that the latter assumption is validfor LIOAS studies where no net photoreactionoccurs.15

Three major conclusions can be drawn concern-ing the recovery of excited state energies and life-times using the LIOAS apparatus coupled withdeconvolution analysis.

1. LIOAS measurements are accurate only whenthe excited state lifetime is greater than 200 ns.

It can clearly be seen in Figure 3 that for the tmb,dmb, and bpy complexes the energies of 3MLCTderived from LIOAS and from emission spectralfitting are in reasonable agreement. However, forthe damb and deb complexes the ET values derivedfrom LIOAS vary widely and are in poor agree-ment with the energies derived from emissionspectral fitting. Importantly, the 3MLCT lifetimesof the former three complexes range from 210 to1500 ns, while for the latter two complexes the life-times range from 90 to 120 ns (see emission life-times 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 theexcited state under study and (with our system) the lower lifetime limit for reliability is τ ≥ 200 ns. The probable originfor this lower limit is that the measurements are ultimately limited by the acoustic transit time (τa) of the LIOASapparatus.16 This parameter is defined as

aa V

R=τ (4)

where R is the radius of the excitation beam and Va is the velocity of sound in the sample medium. If we assume abeam radius of 0.25 mm and a velocity of sound in acetonitrile of 1300 m-s-1, an acoustic transit time of 192 ns isobtained. Therefore, given this acoustic transit time it is not possible to accurately resolve the “prompt” relaxation of

Em

issi

on In

tens

ity (

Nor

mal

ized

)

0.00.20.40.60.81.0 a. (tmb)

υ00 = 19400 cm-1

S = 1.1∆υ1/2 = 2900 cm-1

0.00.20.40.60.81.0 b. (dmb)

υ00 = 18375 cm-1

S = 1.1∆υ1/2 = 2900 cm-1

0.00.20.40.60.81.0

0.00.20.40.60.81.0 d. (damb)

υ00 = 17280 cm-1

S = 1.1∆υ

1/2 = 2700 cm-1

Emission Energy / 103 cm-1

12 14 16 18 20 22 24

0.00.20.40.60.81.0 e. (deb)

υ00 = 16050 cm-1

S = 0.9∆υ1/2 = 2550 cm-1

Emission υ00

LIOAS φ1 , 1 MHzLIOAS φ2 , 1 MHz

LIOAS φ1 , 5 MHz

LIOAS φ2 , 5 MHz

c. (bpy)υ00 = 17975 cm-1

S = 1.1∆υ1/2 = 2800 cm-1

Figure 3: Corrected emission spectra of (b)ReI(CO)3(4-benzyl-pyridine) in degassed CH3CN solution at 298 K: (a) b = (tmb); (b) b= (dmb); (c) b = (bpy); (d) b = (damb); (e) b = (deb). Points areexperimental data, and solid lines are spectra calculated usingFranck-Condon analysis (see text) with the fitting parameters listedon each spectrum. MLCT energies are presented on each plot withappropriate errors.

the Franck-Condon state to the 3MLCT state from the “slow” non-radiative decay of the 3MLCT state for complexeswith 3MLCT lifetimes less than 200 ns (i.e., the damb and deb rhenium complexes). While it is theoretically possible toreduce the acoustic transit time by decreasing the size of the excitation beam, studies have shown that photon satura-tion 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 tothose derived from the first (prompt) heat deposition.

The experiments carried out in this study indicate that the precision and accuracy (i.e., reproducibility and close-ness to the “true value”, respectively) of the energies determined by using φ2 are generally lower compared with thosederived from φ1. Simulations of LIOAS responses indicate that φ1 is determined primarily by the quality of the fit inthe 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 thesecondary oscillation region of the LIOAS signal is complex and relatively noisy, which inherently decreases theprecision (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 forthese three complexes range from 100 - 300 ns. Since LIOAS accurately recovers these lifetimes, we conclude that thisis 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 thelifetime of the tmb complex (τem ≈ 1500 ns) is less accurate and with the 1 MHz transducer the lifetime of the debcomplex (τem ≈ 90 ns) is less accurate. This indicates that, as expected, the “intermediate” time domain where lifetimescan be accurately recovered shifts to shorter lifetimes as the characteristic response frequency of the transducer in-creases. While this study demonstrates that given sufficient information regarding the characteristics of a LIOASsystem it is possible to accurately recover excited state lifetimes from deconvolution analysis, the narrow range overwhich lifetimes can be accurately recovered severely limits the general usefulness of the method.

SummaryThe 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” heatdeposition 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 aredetermined by time-resolved emission. With the above conclusions in mind, LIOAS can be a useful technique for thedetermination of excited state energies and/or yields.

References1. 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.

The Spectrum Page 8

Page 9 The Spectrum

About the AuthorsKeith 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 receivedthe ACS Outstanding Senior award. His research interests include photothermal chemistry, design and implementa-tion 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 FloridaState University and his Ph.D. from the University of North Carolina. His research interests include metal-complexphotochemistry and photophysics, electron transfer, π-conjugated polymers, and luminescence imaging. His addressis Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, e-mail: kschanze@chem.ufl.edu.

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 accessvia 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. Wewill remove you from our paper mailing list. Please browse our Web site for up-to-date information about the Centerand its programs.

Center for Photochemical Sciences Awards Fellowships

Robin Lasey, a Ph.D. student in Photochemical Sciencesat Bowling Green State University, is the first recipient ofthe Hammond Fellowship in the Photochemical Sciencesat the University. The fellowship was established in rec-ognition of Professor George S. Hammond’s enormouscontributions to the photochemical sciences and in honorof his being awarded the 1994 National Medal of Science,the nation’s highest scientific award. American studentswho display outstanding academic and research abilityare eligible for the award. Robin is a student in Dr. MichaelOgawa’s laboratory and is conducting research on elec-tron transfer in model proteins. Professor Hammond is avisiting distinguished professor at Bowling Green StateUniversity.

The Center also awards McMaster Fellowships, avail-able through the generous donation of Helen and HaroldMcMaster, to doctoral students. Students who display out-standing academic ability after the first year in the Ph.D.program are named Outstanding Academic Doctoral Fel-lows. This year’s recipients are Mikhail Chamachkine,Anna Fedorova, Haiyan Gu, Maxim Makarov, and IouliaZotova. Students who exhibit exceptional research abilityafter the second year receive Outstanding DoctoralFellowships.This year’s recipients are Wenyi Cao, XinhuaGu, Anna Kornilova, Evgueni Piatnitski, ZinaidaRomanova, Xiaosong Wu, and Ningning Zhao.Robin Lasey, Hammond Doctoral Fellow, (front) and

George S. Hammond

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 theoccasion of their sixtieth birthdays.

IntroductionStudents 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 “OrganicSyntheses“. Up to the last volume in the former series only three out of almost 200 chapters deal with light-inducedreactions, while in the latter from more than 3,000 procedures only 26 are found under the heading ”photochemicalreaction“ (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 forelectrochemical reactions or procedures--no chapter at all in the former and only seven procedures in the latterseries--are even lower. This short article is therefore intended to help synthetic organic chemists to overcome theirreluctance in using light sources as instruments for preparing compounds. It lists some important applications oflight induced reactions in preparative organic chemistry rather than representing an exhaustive survey (for this latterpurpose the reader is referred to more general references ).1-6 Two general types of reactions will be discussed (Figure1), first the light induced formation of products (or intermediates) which are also accessible from the same startingmaterial by alternative (dark) methods (1), and then ”unique“ or ”selective“ reactions wherein the conversion toproducts or to intermediates requires the population of an excited state of either the starting compound or of anadditional reagent (2).

Reactions Taking Place From BothExcited- and Ground States

The homolytic cleavage of relativelyweak covalent bonds as S-H in thiols, C-Clin α-chloroketones, or C-N in α-diazoke-tones is a process which can be typicallyinduced either by light or by alternativemethods, e.g. starting a radical chain by de-composition of either AIBN or Bu3SnH,7 orcatalyzing the diazo compound cleavagewith a transition metal,8 respectively. Ex-amples for such reactions as the synthesesof 3-substituted thiolanes via a radical ad-dition/cyclization sequence,9 of the nonste-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 ofbonds between carbon and other atoms occur under mild conditions only if a stabilized carbenium ion is formed. Thelight induced cleavage of the exocyclic C-O bond in 10-alkoxy-10-phenyl-xanthenes in aq. CH3CN makes the pixylgroup an interesting photocleavable protecting group for primary alcohols,13 which can be used in the synthesis ofoligoribonucleosides as the deprotecting step occurs at neutral pH-values, i.e. it avoids the (thermal) use ofstrong acids.

Cathodic reduction is a conventional way of generating radical anions from ground state molecules. Suchintermediates are also often accessible by interaction of an excited molecule with a (sacrificial) electron donor, as

Photochemistry in Organic Synthesis

Paul Margaretha Institute of Organic Chemistry, University of Hamburg, Germany

M* M + R*

M M + R

[I] P [I] P

M* M + R*

[I] P [I] Phv

M

(1)

(2)

M + R

The Spectrum Page 10

Figure 1. Generalized schemes for M -> P conversion for “alternative”reactions (1) and “excited state selective” reactions (2). (M = startingcompound, R = additional reagent, I = intermediate, P = product.

Page 11 The Spectrum

exemplified by the reductive ring opening of excitedcyclopropyl ketones by an amine as shown in Figure3. These reactions are commonly run in CH3CN con-taining LiClO4 in order to minimize the reduction tothe corresponding alcohol.14, 15 Single electron trans-fer (SET) between two ground state molecules is usu-ally highly endothermic due to their large HOMO-LUMO gaps. This situation is reversed with electroni-cally excited states and therefore photosensitizedoxydation16 of e.g. alkenes, ethers or aromatic hydro-carbons can easily be achieved by using excited aro-matic nitriles as naphthalene-1,4-dicarbonitrile or an-thracene-9,10-dicarbonitrile. Many applications of thismethod make use of the facile fragmentation of radi-cal cations, as shown for the preparation (and trap-ping) of α-hydroxymethyl radicals.17 Alternatively analkene radical cation can undergo cycloaddition to asecond alkene moiety to afford a cyclobutane radicalcation which is then reduced by back electrontransfer from the nitrile radical anion.18 This samesequential principle can be achieved by irradiatingCu(I) complexes of hepta-1,6-dien-3-ols to affordbicyclo[3.2.0]heptan-2-ols.19

Excited-State Selective ReactionsLight-induced homolysis of a single bond in an

acyclic molecule affords first a radical pair and thentwo (separated) radicals. Only if one of the two radi-cals is long lived, rearrangement of the other one, e.g.by H-transfer, and sequential recombination representa useful synthetic sequence, as illustrated (Figure 4)for the so-called Barton reaction20 of organic nitrites.In contrast to radical pairs, biradicals have emergedas highly important intermediates in a multitude ofphotochemical reactions. Only very recently somethermal reactions starting from enediynes or enyne-allenes 21 proceeding via such intermediates have beendeveloped. One classic approach to such intermedi-ates consists in the homolytic cleavage of a single bondin a cyclic molecule. This process can be terminatedby intramolecular H-transfer,22 cyclization to a consti-tutional isomer of the starting material,23, 24 or followedby elimination of a molecular fragment, e.g. CO25-28 orketene.29 Biradicals can also be formed by intramolecu-lar H-transfer to the oxygen atom of an excited carbo-nyl group (Figure 5), the most common example be-ing the so-called Yang reaction,30 wherein a 1-hydroxytetramethylene-1,4-biradical cyclizes to acyclobutanol. An extension of this reaction has beenused for converting N-phenacyl-δ-valerolactone to abicyclic azetidinol of > 99% optical purity by irradiat-ing a 1:1 clathrate of the lactam and an enan-tiomerically pure spirodioxolane.31 A related reaction

CO2R

HHSCH2

H CO2R

HSCH2

H

.S

CH2CO2R

85%

[9]

O

Cl

R

O

R.

R

CO2H

hv

hv H2O[10]

70%R = i-C4H9

O

N2hv

O

..

CO2H

HO

H2O[11]

80% (3:1 mixture)

O

Ph OR

hv

O

Ph+

H2O

+ RO

O

Ph OH

+

ROH

(78-97% for various R)

[13]

-

Figure 2. Examples of reactions where the (primary) bondbreaking step can be induced either by light or by other (dark)methods.

Figure 3. Examples of reactions induced by photo-electron-transfer (PET).

O

CH3CN / LiClO4

hv / N(C2H5)3

O

50%

[15]

R3SiO SiR3

CH3CN / MeOH

N

HOCH2O

O

42%

[17]

C6H6

O

O

O

O

O

O

O

O

80% (3:2 mixture)

[18]

HOEt2O

HO88-92%

[19]

9,10-DCA / hv

N-Methylmaleimide

1,4-DCN / hv

CuOTf / hv

The Spectrum Page 12

is the conversion of 2-alkylbenzophenones to stableο-xylylenols,32 which can be trapped by dienophiles,e.g. by fullerenes.33

Electrocyclic ring closure and ring opening canbe easily achieved with light, the product most of-ten being diastereomeric to that of the correspond-ing thermal reaction (conrotatory vs disrotatory re-actions depending on the number of pi electrons).Examples of such reactions include the valenceisomerization of cyclopentadiene to bicyclo-pentene,34 or the conversion of Z-stilbenes todihydrophenanthrenes, which are usually irrevers-ibly dehydrogenated to phenanthrenes,35 while thereversible reaction of substituted dithienylethenesemphasizes the use of such molecules as a photo-chromic system.36 The ring opening of thietes37 rep-resents the first synthetic approach to alicyclicenthiones (Figure 6) and the ring opening of provi-tamin D to previtamin D can be conveniently runby using a two-step laser as light source.38

The conversion of an E-alkene to its (thermody-namic) less stable Z-diastereoisomer can be easilyachieved by excitation.39 Similarly, Z-cycloalkenescan be converted to highly strained E-diastereoiso-mers, which can then be trapped, e.g. by protona-tion.40 Some light-induced rearrangements (Figure6) 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 twopi-electron systems in excited molecules. On irradiation, 1,4-dienes undergo bridging between C(2) and C(4). Themethano-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 over-all reaction is known as di-pi-methane rearrangement(or Zimmerman reaction).45 A related reaction is theconversion of β,γ-unsaturated ketones to cyclopro-pyl ketones,46 the so-called oxa-di-pi-methane rear-rangement (Figure 7).

1,5-Hexadienes photoisomerize to both bi-cyclo[2.2.0]- and bicyclo[2.1.1]hexanes. An exampleof such a diastereomer differentiating reaction isillustrated (Figure 7) for two diastereomericcyclopentenylpyrrole derivatives.47 As a matter offact the (intramolecular) cyclobutane formation ofcyclopent-2-enones or cyclohex-2-enones containingan additional (exocyclic) alkene moiety48 representsone of the most often used photochemical reactionsin organic synthesis. An example for such a processis shown in the multistep isomerization of analkenyl-alkynyl substituted cyclohex-2-enone to atetracyclic cyclohexanone.49 Less often used, butsimilarly powerful as a synthetic tool are photo-isomerizations of alkenylarenes to di- or tri-quinanes.50 For lack of space it will just be noted that

CH3

ONO

CH2

OH

CH2NO

OH

[20]hv NO

..

RN

O

C2H5

hv

RN

O

C2H5

..RN C3H7

CO

N

R

CH2CO2CH3C3H7

CH3

O.

MeOH [22]

R = COOCH3 66%

S

O

O

hv

S

O

O

.. SO

O

[23]

90%

S O

R’

R

hv

S O

R’

R ..- CO

SR

R’

[28]

70-90%

OO

R O

hv

OO

R O

. . - CH2CO

H

R

O

O

R = t.Bu 75%[29]

N O

Ph

O

H

Hhv **

N O

Ph

HO

H..

N

O

PhHO

99% ee

[31]

**1: 1 clathrate with (-)

O

OC(Ph)2OH

C(Ph)2OH

O

Ph

CH3

hv

Ph

OH C60

OHPh

SiO2

C

OPh

CHC58

57%

[33]

Figure 4. Light-induced cleavage reactions.

Figure 5. Light-induced H-transfer reactions.

Page 13 The Spectrum

the previously mentioned photoisomerizations (Figure 8) comprising the interaction of two chromophoric groups inone molecule obviosly extend to intermolecular reactions between an excited molecule and a (second) reactant (enone+ alkene cycloadditions,51 arene + alkene cycloadditions,52 etc.).

Figure 6. Light-induced electrocyclic reactions.

Figure 8. Formation of (intramolecular) enone + alkene and arene + alkene photocycloadducts.

hv

C6H5OHH+

MeOH

OMe

52%

[40]

N

O

hv

C6H6 NO

O

N

50%

[41]

S

Ohv

S

O.. S

O

95%

[42]

O O

C6H6

hv

C6H6O O

. .

O

O

70%

[43]

XR R’

hv

XR R’. .

RR’

X

[45]: X = CR2

[46]: X = O

Figure 7. Light-induced reactions involving C-C doublebonds.

hv

THF40-60%

[34]

hvHH

Ox

75-85%

[35]

S

CF2

CF2CF2

S

hv

hv’ S S

CF2

CF2CF2

[36]

λmax = 234 nm λmax = 534 nm

hv

hv’

t.Bu

R

[37]

λmax = 237 nm λmax = 554 nm

R

t.Bu

S

NRO

NRO

NR OHH

NR OHH

R = CO2t.Bu [47]

O

t.Bu

hv

hv

hv

O

CH2

t.Bu

..O

[49]

47%

OMe

hv

C5H12

C5H12

OMe MeO

+

1) Br2

2) Bu3SnH

3) KOH / H2NNH2

35% overall

[50]

The Spectrum Page 14

Finally, photooxygenation53 represents an important exampleof the light-induced formation of an excited ”reagent“ (singlet oxy-gen), which reacts with alkenes to afford either allylic hydroperox-ides54 or 1,2-dioxetanes, the former being easily reduced to allylicalcohols (Figure 9).

ConclusionAs summarized in the last chapter, electronic isomers of ground

state molecules, with well defined multiplicity (singlet or tripletstates) undergo a big variety of synthetically useful transforma-tions. In addition to ”conventional“ conversions, such reactions canbe run down to a very low temperature (in solution or in a matrix)and also on pure solids, both crystals or thin films. This lattermethod 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 mixturesbecause such intermediates react unselectively“ are literally renouncing to a now established and well-understoodpart of their science. Hopefully they will be convinced by browsing through the references cited in this article, or toput it in one slogan: ”There is no life without light and no organic chemistry without photochemistry.“

References1. Schoenberg, A.; Schenck, G.O.; Neumueller, O.A. Preparative Organic Photochemistry; Springer-Verlag, 1968.2. Margaretha, P. Topics in Curr. Chem. 1982, 103, 1.3. Horspool, W.A. Synthetic Organic Photochemistry; Plenum Press, 1984.4. Coyle, J.D. Photochemistry in Organic Synthesis; RSC, 1986.5. Kopecky, J. Organic Photochemistry: A Visual Approach; VCH, 1992.6. Mattay, J.; Griesbeck, A. Photochemical Key Steps in Organic Synthesis; VCH, 1994.7. Giese, B.; Kopping, B.; Goebel, T.; Dickhaut, J.; Thoma, G.; Kulicke, K.J.; Trach, F. Org. React. 1996, 48, 301.8. Ye, T.; McKervey, A. Chem. Rev. 1994, 94, 1091.9. Kiesewetter, R.; Margaretha, P. Helv. Chim. Acta 1989, 72, 83.

10. Sonawane, H.R.; Nanjundiah, B.S.; Kulkarni, D.G. ref. [6], p 59.11. Wheeler, T.N.; Meinwald, J. Organic Syntheses, Coll. Vol. VI; Noland, W.E., Ed.; 1988, 840.12. Rao, V.B.; George, C.F.; Wolff, S.; Agosta, W.C. J. Am. Chem. Soc. 1985, 107, 5732.13. Misetic, A.; Boyd, M.K. Tetrahedron Lett. 1998, 39, 1653.14. Pandey, B. Tetrahedron 1994, 50, 3843.15. Cossy, J.; Furet, N.; Bouzbouz, S. Tetrahedron 1995, 51, 11751.16. Mella, M.; Fagnoni, M.; Freccero, M.; Fasani, E.; Albini, A. Chem. Soc. Rev. 1998, 27, 81.17. Gutenberger, G.; Steckhan, E.; Blechert, S. Angew. Chem. 1998, 110, 679.18. Mizuno, K.; Otsuji, Y. Chem. Letters 1986, 683.19. Salomon, R.G.; Ghosh, S. Organic Syntheses, Coll. Vol. VII; Freeman, J.P. Ed.; 1990, 177.20. Brun, P.; Waegell, B. In Reactive Intermediates, Vol. 3; Plenum: New York, 1983, 367.21. Wang, K.K. Chem. Rev. 1996, 96, 207.22. Muraoka, O.; Okumura, K.; Maeda, T.; Tanabe, G.; Momose, T. Tetrahedron Asymmetry 1994, 5, 317.23. Kowalewski, R.; Margaretha, P. Angew. Chem. 1988, 100, 1431.24. Crockett, G.C.; Koch, T.H. Organic Syntheses, Coll. Vol. VI; Noland, W.E., Ed.; 1988, 226.25. Maier, G.; Born, D.; Bauer, I.; Wolf, R.; Boese, R.; Cremer, D. Chem. Ber. 1994, 127, 173.26. Turro, N.J.; Leermakers, P.A.; Vesley, G.F. Organic Syntheses, Coll. Vol. V; Baumgarten, H.E., Ed.; 1973, 297.27. Andresen, S.; Margaretha, P. J. Chem. Res. (S) 1997, 345.28. Hinrichs, H.; Margaretha, P. Chem. Ber. 1992, 125, 2311.29. Hobel, K.; Margaretha, P. Helv. Chim. Acta 1987, 70, 995.30. Wagner, P.J.; Park, B.S. Org. Photochem. 1991, 11, 227.31. Toda, F. Jpn. Kokai Tokkyo Koho, Chem. Abstr. 1994, 121, 255656c.

3Sens + 3O21O2

+ Sens

(Sens = Tetraphenylporphyrine, eosine or other dyes)

1O2

OOH

67%

[53]

[54]

Figure 9. Formation of singlet oxygen andformation of an allylic hydroperoxide byphotooxygenation.

Page 15 The Spectrum

32. Wagner, P.J.; Sobczak, M.; Park, B.S. J. Am. Chem. Soc. 1998, 120, 2488.34. Andrist, A.H.; Baldwin, J.E.; Pinschmidt, R.K., Jr. Organic Syntheses, Coll. Vol. VI; Noland, W.E., Ed.; 1988, 145.35. Mallory, F.B.; Mallory, C.W. Org. React. 1984, 30, 1.36. Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem. 1995, 60, 8305.37. Mergenhagen, T.; Margaretha, P. Helv. Chim. Acta 1997, 80, 510.38. Dauben, W.G.; Share, P.E.; Ollmann, R.R., Jr. J. Am. Chem. Soc. 1988, 110, 2548.39. Kropp, P.J. Org. Photochem. 1979, 4, 1.40. Tise, F.P.; Kropp, P.J. Organic Syntheses, Coll. Vol. VII; Freeman, J.P., Ed.; 1990, 304.41. Albini, A.; Bettinetti, G.F.; Minoli, G. Organic Syntheses, Coll. Vol. VII; Freeman, J.P., Ed.; 1990, 23.42. Er, E.; Margaretha, P. Helv. Chim. Acta 1992, 75, 2265.43. Grohmann, K.; Margaretha, P. Helv. Chim. Acta 1982, 65, 556.44. Smith, C.D. Organic Syntheses, Coll. Vol. VI; Noland, W.E., Ed.; 1988, 962.45. Zimmerman, H.E. Org. Photochem. 1991, 11, 1.46. Demuth, M. Org. Photochem. 1991, 11, 37.47. Wrobel, M.N.; Margaretha, P. JCS Chem. Commun. 1998, 541.48. Crimmins, M.T.; Reinhold, T.L. Org. React. 1993, 44, 297.49. Kisilowski, B.; Agosta, W.C.; Margaretha, P. JCS Chem. Commun. 1996, 2065.50. Wender, P.; Siggel, L.; Nuss, J.M. Org. Photochem. 1989, 10, 357.51. Cargill, R.L.; Dalton, J.R.; Morton, G.H.; Caldwell, W.E. Organic Syntheses, Coll. Vol. VII; Freeman, J.P., Ed.;

1990, 315.52. Laarhoven, W.H. Org. Photochem. 1989, 10, 163.53. Denny, R.W.; Nickon, A. Org. React. 1973, 20, 133.54. Margaretha P. In Houben-Weyl, Methoden der Organischen Chemie, Vol. E13; Kropf, H., Ed.; G.Thieme, Stuttgart,

1988, 71.55. Bethke, J.; Kopf, J.; Margaretha, P.; Pignon, B.; Dupont, L.; Christiaens, L.E. Helv. Chim. Acta 1997, 80, 1865.

About the AuthorDr. Margaretha received his Ph.D. in chemistry at the University of Vienna (Austria) in 1969. He is currently profes-

sor of organic chemistry at the University of Hamburg. His interests focus on mechanistic and preparative organicphotochemistry and also include competitive bridge.

Copyright 1998 by the Center for Photochemical SciencesThe Spectrum is a quarterly publication of the Center forPhotochemical Sciences, Bowling Green State University,Bowling Green, OH 43403.Phone 419-372-2033 Fax 419-372-6069Email photochemical@listproc.bgsu.eduWWW http://www.bgsu.edu/departments/photochem/

Executive Director: D.C. NeckersAdministrative Director: Pat GreenPrincipal 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 GreenProduction Editor: Alita Frater

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Articles submitted to The Spectrum will appear at the discretion ofthe editorial staff as space is available.

1 museum specimen thought to be royaltyOne 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 affordfancy 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 fin-

gers through the holes in hisjaw,” she said.

Ms. Knudsen spoke as shelooked at work in progress atSpectra Group Limited, Inc.,a Maumee firm that makesmedical models.

Chemist and computertechnician Dustin Martin ismaking a half-sized replica ofthe male mummy, using afairly new technology calledstereolithography.

The firm takes the comput-erized axial tomography scanresults and builds a three-dimensional copy, with a la-ser beam turning liquid resininto the solid plastic model.The firm mostly makes medi-cal and automotive copies.

Doctors use the exact rep-licas to plan delicate surger-ies, Mr. Martin said.

Scans Yield Mummy Clues*

by Vanessa Winans, Toledo Blade Staff Writer

The Spectrum Page 16

Dustin Martin of Spectra Group Limited, Inc., trims the support columns from a mummyfigure being recreated in the plastics lab. The firm is making a replica of the malemummy using stereolithography. Photo by Chris Walker, Toledo Blade.

Page 17 The Spectrum

Having the scan results turned into three-dimensional models allows researchers to see what’sinside the bodies, previously impossible because themuseum refused to unwrap the mummies, Ms.Knudsen said.

The firm has donated time, equipment, andmaterials to the effort, just as Toledo Hospital didfor the CAT scan.

“It’s kind of been a labor of love,” said DougNeckers, a professor of photochemical science atBowling Green State University and consultant tothe 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 ofreconstructing a mummy.

“We’ve laughed a lot about this guy who thoughthe was going to be reincarnated, and now he’sbeing 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’sbeen done, it’s worth it.”

Museum officials don’t know yet how they’ll usethe replicas, which take about 12 hours to create,Ms. Knudsen said. They never expected to havesuch models.

“We’re stunned,” she said, looking at the half-finished model. “He is incredible.”

*Reprinted by permission of Toledo Blade, 5/7/98.

In stereolithography, a laser beam leaves its mark during a timeexposure marking one ‘pass’ over the photo-sensitive liquidplastic. The beam solidifies the plastic, and the mummy,represented by a series of computerized axial tomographyscans, is recreated. Photo by Chris Walker, Toledo Blade.

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In the past we have not had any advertisements in The Spectrum because we are limited in staff. Corporate sponsorsdonate money to defray the cost of publishing and mailing The Spectrum to over 7,000 people in 50 countries. Thosesponsors can place ads, if they wish, in four issues and are acknowledged as a sponsor. Some companies havechosen to remain anonymous. We intend to continue the corporate sponsorship program. This is our preferred wayto help with our costs. However, we receive constant inquiries about placing advertisements. Therefore, we aregoing to offer advertising space on a trial basis.

Size: Full page - $1,000 per issueHalf page - $ 500 per issue

Specifications: Computer file - EPS or PICT file (preferred method)Camera-ready hard copy

Deadlines: Fall Issue - September 1Winter Issue - December 1Spring Issue - March 1Summer Issue - June 1

We also will reserve the right to limit the number of advertisements per issue based on space. For additionalinformation 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 aseries 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 ofpoly(methyl methacrylate) with pendant benzophenone borate salt as single component photoimagingsystem. Macromolecules 1997, 30, 2268-2273.

308. Hu, S.; Neckers, D.C. Photochemical reactions of sulfide-containing alkyl phenylglyoxylates. Tetrahedron1997, 53, 12771-12788.

309. Hu, S.; Neckers, D.C. Photochemical reactions of alkoxyl-containing-alkyl phenylglyoxylates: remotehydrogen 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 fluorescentprobes 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 periodically-poled bulk LiNbO3. Opt. Lett. 1997, 22, 862-864.

313. Ding, Y.J.; Khurgin, J.B. Transversely-pumped counter-propagating optical parametric amplication anddifference-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 understandingas 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 ofstrong sequential band filling at interface islands in asymmetric coupled quantum wells. SuperlatticesMicrostruct. 1997, 22, 497-505.

316. Lee, S.J.; Khurgin, J.B.; Ding, Y.J. Directional couplers via the cascaded resonant surface-emitting second-harmonic 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 characterizationof a divergent plastocyanin from the photosynthetic prokaryote, Prochlorothrix hollandica, expressed inEscherichia 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-activityrelationships 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 ofoctabutoxy- and octabutoxybenzophthalocyanines and photophysical properties of two members of theseries. 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 designedmetalloprotein. 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, anassessment 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 anddiphenylamine 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 β-strands 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 photosensitizingefficacy, 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 radicalcoinitiators. Macromolecules 1998, 41, 951-954.

* 332. Lungu, A.; Mejiritski, A.; Neckers, D.C. Solid state studies on the effect of fillers on the mechanicalbehavior of photocured composites. Polymer, in press.

* 333. Sarker, A.M.; Kaneko, Y.; Nikolatchik, A.V.; Neckers, D.C. Photoinduced electron transfer reactions: highlyefficient 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 metallo-porphyrins. 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 reprintby 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 forinternal recordkeeping.

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. Thesymposium will take place during the Fall 1998 ACS National Meeting (August 23-27, 1998) in Boston. You arecordially invited to participate in this historical event.

Honorees (will also speak at the symposium):

R. S. H. Liu N. J. TurroD. C. Neckers P. J. WagnerJ. Saltiel D. G. Whitten

Speakers at the symposium include the following:

D. R. Arnold R. S. Givens M. Kasha Y. ShichidaP. F. Barbara J. L. Goodman C. V. Kumar S. C. ShimJ. K. Barton I. R. Gould F. D. Lewis M. B. SponslerI. Bronstein H. B. Gray T. J. Meyer J. K. ThomasR. A. Caldwell G. S. Hammond J. Michl L. M. TolbertW. J. DeGrip E. F. Hilinski H. A. Morrison M. TsudaF. C. De Schryver H. Inoue K. Nakanishi C. TurroM. A. El-Sayed Y. Inoue D. G. Nocera D. H. WaldeckS. Farid M. Irie K. S. Schanze R. G. WeissM. D. Forbes Y. Ito J. R. Scheffer N. C. YangM. A. Fox W. F. Jager D. I. Schuster O. C. ZafiriouE. R. Gaillard W. S. Jenks G. B. Schuster R. G. ZeppT. Gillbro L. B. Johnston R. E. Schwerzel H. E. Zimmerman

For details, see the world wide web site at http://www.chem.fsu.edu/photo38.htm.

The Spectrum Page 20

Alumni and Friends Reception for Dr. Douglas NeckersA 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.