Photochemistry and Photobiology Vol 51 NO4 pp 419--426 1990 (X13I-865519() SoIiXl+ll110 Printed in Great Britain All rights reserved Copyright copy 1990 Pergamon Press pIlt

PHOTOPHYSICAL PROPERTIES OF 2-NITRO-5101520shyTETRA-p-TOLYLPORPHYRINS

DEVENS GUSTmiddot THOMAS A MOORE 1 DAVID K LUTTRULL GILBERT R SEELY EDITH BITTERSMANN RENE V BENSASSON~ MICHEL ROUGEE~ EDWARD J LAND- F C DE

SCHRYVER~ and M VAN DER AUWERAER~

Department of Chemistry Arizona State University Tempe AZ 85287-1604 USA 1Laboratoire de Biophysique ERA 951 du CNRS Museum National dHistoire Naturelle 75005 Paris France middotPaterson Institute for Cancer Research Christie Hospital and Holt Radium Institute Manchester

M20 9BX UK and ~Afdeling Organische Scheikunde Katholieke Universiteit Leuven Celestijnenlaan 200F Leuven Belgium

(Received 18 July 1989 accepted 4 October 1989)

Abstract-Tetraarylporphyrins substituted with nitro groups at f3-pyrrolic positions are potential candishydates for electron-accepting pigments in model systems for photosynthesis The photophysics of 2shynitro-5101520-tetramiddotpmiddottolylporphyrin and its zinc analog have been studied in order to evaluate this potential The ground state absorption spectrum the triplet-triplet absorption spectrum the fluorescshyence emission spectrum and associated photophysical parameters have been determined The molecules have short singlet lifetimes and anomalous temperature- and solvent-dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type IT - NOI in which the nitro group is twisted about its bond to the porphyrin relative to the ground state conformation

INTRODUCTION for meso-tetraarylporphyrins nitration of tetrashyphenylporphyrin at the pyrrole ~-position is facile

Synthetic porphyrins covalently linked to qui nones (Evans et al bull 1978 Shine et albull 1979 Giraudeau et or other electron acceptors have been widely used

albull 1979 Baldwin et al 1982 Catalano et al to model various aspects of photosynthetic electron 1984) and the resulting mononitro product is a transfer reactions (see the recent review by Conshy better electron acceptor than meso-tetraphenylporshynolly and Bolton 1988) Tetraarylporphyrins have phyrin itself by = 350 mV (Giraudeau et al bull 1979)also been employed in a variety of more complex However favorable redox behavior is not the onlymolecular triads and tetrads which mimic the photoshy requirement that a tetraarylporphyrin must satisfy initiated multistep electron transfer processes found in order to be a viable component of a photosynshyin natural photosynthetic reaction centers (Gust et thetic electron transfer model system spectroscopic af 1988ab Gust and Moore 1989 and references properties must also be considered These have cited therein) The design of artificial photosynshy

been largely unknown for nitroporphyrins Herein thetic devices of this general type requires methods we report the results of steady state and time to control the redox potentials of the porphyrin resolved spectroscopic studies of the singlet and moieties because the rates and therefore the triplet states of 2-nitro-51O1520-tetra-p-tolylporshyefficiencies of electron transfer processes are sensishy phyrin (TTP-NO~)t and its zinc analog (ZnTTPshytive functions of thermodynamic driving force

NO~) Although some degree of control may be achieved

by altering the substituents on the aryl rings of meso-tetraarylporphyrins (see for example Moore MATERIALS AND METHODS et al 1989) large changes in redox properties may

Syllthesis 2-Nitro-5101520-tetra-p-tolylporphyrin (TTPmiddotbe achieved by direct substitution on the porphyrin N01) was prepared by a method analogous to thatmacrocycle In one such case Cowan and Sanders employed by Giraudeau et al (1979) for 2-nitroshy

(1985) have reported alterations of porphyrin redox 5 101520-tetraphenylporphyrin Mass spectrum mle 715 potentials via substitution of nitro or amino groups (M-) H NMR (CDG 400 MHz) li 266 (3 H s 20shyat the meso positions of octaalkylporphyrins ArCH) 270 (9 H s 5lOl5-ArCH) 751 (2 H d

J=79 Hz 20-Ar C-35) 756 (6 H d J=72 Hz 51015shyAlthough suhstitution at this position is impossible Ar CmiddotJ5) 805-1Ul9 (6H m 51015-Ar C-26) 813 (2 H d J=79 Hz 20-Ar C-26) 1l71 (2 H AB

middotTo whom correspondence should ne addressed J=12 Hz C-1213) 811S (I H d J=52 Hz C-17) 889 +AbbIiatiom ICT intramolecular charee transfer state (I H d J=middotUl Hz CoS) 894 (I H d J=48 Hz C-7)

TlCT twisted intramolecular charlc~ transfer state 199 (I H d J=52 Hz C-Ill) 904 (I H s C-3) TPP 5 10 1520IItraphenylporphyrin TTP-NO~ 2shy Zinc 2-nitro-5101520middottetramiddotpmiddottolylporphyrin (ZnTTP nitro-5 10 I520middottetramiddotpmiddottolylporphyrin ZnTPP zinc NO~) was prepared ny stirring a dichloromethane solution 5101520-tetraphenylporphyrin ZnTTP-N01 zinc 2shy of TTP-NO~ with an excess of zinc acetate at room temshynitro-5IO1520-telra-p-lOlylporphyrin perature for 120 min and purifying the resulting ZnTTPmiddot

~19

~------------~

~ bullbull~ ~ ~ I bull

( o-

S shy

fJbullshy ~~

l)~

I- ~~ ~ 420 DEVENS GUST et 0

-tt-

RR

TPP R=X=H

Scheme I

NOo by chromatography on silica gel (hexanetoluene 11) Mass spectrum mle 778 (M+) H NMR (CDO 400 MHz) 1) 266 (3 H s 211-ArCH) 270 (9 H s 51O15-ArCH) 748 (2 H d J=78 Hz 20-Ar C-35) 755 (6 H d J=78 Hz 51015-Ar C-35) 804-806 (8 H m 5101520-Ar C-26) 887-893 (5 H m Cshy7HIl317) H96 (1 H d J=48 Hz C-18) 920 (I H s C-3)

Spectroscopic measurements All spectroscopic measurements were performed on samples in toluene solshyution at ambient temperatures unless otherwise noted UV-VIS spectra were obtained using an HP-8450 spectroshyphotometer Steady state fluorescence spectra were recshyorded and quantum yields were calculated by procedures described by Brune et 0 (1988) Fluorescence decay measurements were made on = I x 10- M solutions in toluene using the time-correlated single photon counting method Two different setups were used In one the excitation source was a mode-locked argon ion laser coupled to a synchronously pumped cavity dumped dye laser operating at 590 nm Experimental details are proshyvided elsewhere (Boens et 0 1984) In the other the excitation source was a frequency doubled mode-locked Coherent Antares Nd-YAG laser coupled to a synchronshyously pumped cavity dumped Coherent 701-3 dye laser with excitation at 590 or 610 nm This laser system proshyvided pulses of - 9 ps with a 38 MHz repetition rate Thc sample was excited by a vertically polarized laser beam The emission was selected via a polarizer set at the magic angle (547deg) collected by a camcra kns (Pentax Hi mm) and imaged onto the entrance slit of a double monochromator (Jobin-Yvon DH-IO H nm bandwidth) The collimated light from the monochromator was detecled hy a microchannel-platc photomultiplier (Hamshyamatsu R2K1l9U-lll) The resulting signal was amplified with a fasl amplifier (Phillips Scientific 69i4B-IO) disshycriminated hv a constant fraction discriminal0r (Tennekc TC 4ii) and uscd to provide the start signal for a Tennclec TC Kh4 time-tll-amplitude convertcr Approximately 20 of the excitation heam was directed to a fast photodiode (Teldunkcn BPW~Ii) The signal from the phOlolliolle was dclayed (Ortcc DB4hJ delay hox) discriminated (Tenncshy

lec TC 453) and taken as the stop input for the lime-toshyamplitude converter The output from the time-to-amplishytude converter was accumulated in a Nucleus multichannel analyzer Usually_ about 05--1 of the laser pulses resulted in detection of an emitted photon The instrument response function was - 35 ps The instrument response function measured at the excitation wavelength with a scattering sample and the fluorescence decay were transshyferred to a Microvax 3200 workstation for data analysis Multiexponential decay curves were computed with an iterative reconvolution program using a semilinear Marshyquardt algorithm The quality of the fits was judged by the reduced XO and inspection of the residuals and the autocorrelation function (Demas 1983 OConnor and Phillips 1984 Wendler and Holzwarth 1987)

The pulse radiolysis equipment has been described by Butler et 0 (1990) Measurements were made in benzene solutions which were argon-flushed and irradiated in 25 cm quartz capillary cells with 10 ns = 25 Gy pulses of 9-12 MeV electrons from a Vickers electron linear accelerator The resulting changes in light absorption were monitored as a function of time at each wavelength by using photoelectric detection (EMI 9558QA or Hamamshyatsu R928 photomultiplier)

Transient absorption spectra were obtained on = 10- M solutions in benzene which were deoxygenated by bubbling with argon or nitrogen gas Two different spectrometers were used In one the excitation source was a Quantel frequency-doubled YAG laser providing = 5 ns pulses of 532 nm light The analyzing light was a = 20 s xenon flash whose intensity was constant within I for more than 20 s (Bensasson et 0 1972) In the other pulsed excitation was provided by a Lambda Physik FL2000 dye laser using Rhodamine 6G pumped by a Lambda Physik EMG 50E xenon chloride excimer laser at 308 nm The output of the dye laser consisted of = 5 mJ 20 ns pulses at 590 nm with a repetition rate of 10 Hz The analyzing light was provided by a quartz-halogen lamp The detection scheme has been described by Gust et 0 (1986) and Davis et 0 (1987)

RESULTS

UV- VIS absorption spectra

The absorption spectra of TIP-N01 and ZnTIPshyN02 in toluene are shown in Fig 1 The waveshylengths of the major absorption bands and the correshysponding extinction coefficients are listed in Table

E u 20

5

15 C0 ~

0 ltJ

c 2 U

~ W

500 100

-_

400

Wayelength (nm)

Figure I Absorption spectra of TTP-NOo (--) and ZnTTP-NO ( ) in toluene solution normalized to the cJltinclion codticienl~ a~ ~hown on the ordinate The insert is a vertical cxpansion (X 3) of the 500-750 nm region

~oC ~Imiddotmiddot~middotmiddot~lgtrmiddotmiddot - 1 ~ ~

t ~)

plimiddot nd h~s

~nt

Ibe

h a

an larmiddot t hy the and

by ~cnc

J in ulses ncar wrre h hy lamshy

on ated rent urce iding

as a ilhin n the hsik b a laser

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heen 1~7)

lITPshyaveshyllrre-Table

J ) and d 10 the insert fl-middotgilln

Photophysics of nitroporphyrins 421

Table 1 Electronic absorption bands of nitroporphyrins and analogs

Compound

TIP-NO

TPP

Soret

430 (229000)

419 (470000)

A m om E (M-I em-I)

IV

528 (15800)

515 (18700)

III

56 (5000)

548 (8100)

II

606 (3720)

592 (5300)

668 (8640)

647 (3400)

ZnTIP-NO

ZnTIph

Soret

434 (195000)

423 (544 000

J3

562 (13000)

548 (22800)

Q

604 (9020)

586 (3680)

In benzene (Badger et albull 1964) hIn benzene (Quimby and Longo 1975)

1 along with those for 5lO1520-tetraphenylporshy 12

phyrin (TPP) and its zinc analog (ZnTPP) The general features of the absorption spectra of TIPshy

poundl bullc

NO~ and ZnTIP-NO~ are similar to those reported 01 for the nitrotetraphenylporphyrin analogs in differshy ent solvents (Baldwin et al 1982 Evans et al ~bull1978) It is clear from Table 1 that incorporation of i middotthe nitro group into the free base porphyrin alters i 04 middotIt middot middotmiddotboth the wavelengths of the absorption bands and their relative extinction coefficients The Soret bands of TIP-NO~ and ZnTIP-N02 are both red 00

shifted by 11 nm relative to TPP and ZnTPP The Q-bands are also red shifted in the nitroporphyrins with the largest shifts being seen for the longest wavelength absorptions (21 and 18 nm for TTP-N02

and ZnTTP-N021 respectively) Nitro substitution has broadened the Soret bands in both compounds and decreased their maximum intensity and the longest wavelength absorptions have gained oscilshylator strength at the expense of the other Q-bands in both TTP-N02 and ZnTIP-NO~

Fluorescence spectra

The corrected fluorescence emission spectra for TTP-NO~ and ZnTTP-NO~ in toluene appear in Fig 2 The emission maxima are at 720 and 672 nm respectively The shapes and emission maxima are independent of concentration Tetraphenylporphyshyrin itself shows two emission bands of similar intenshysity at 652 and 718 nm in benzene solution whereas the zinc analog ZnTPP shows two bands at 598 and M7 nm with a small shoulder at 560 nm (Quimby and Longo 1975) Thus the nitro substitution gives rise to very large red shifts in the emission maxima and broad featureless emissions which contrast sharply with the structured bands usually observed in tetraarylporphyrins and their zinc analogs The apparent lack of mirror-image symmetry between

510 130 110 730 710 130

Wvlngth (nm)

Figure 2 Corrected fluorescence emission spectra for TIP-NO (--) and ZnTIP-NO ( ) in toluene at ambient temperature with intensities normalized to I at

the emission wavelength maxima

the absorption and emission spectra and the large Stokes shift suggest that the potential surface for the emitting state differs considerably from that of the ground state for both nitro compounds

Solvent effects upon the emission spectrum of TIP-N02 were investigated Figure 3 shows the fluorescence emission spectra of TIP-NO~ at room temperature in four solvents of different polarities As the solvent is changed from isooctane to acetoshynitrile the emission wavelength maximum changes from 692 to 758 nm (Note that the emission in isooctane still shows some intensity in the longer wavelength region) In general the emission intenshysity falls off with peak broadening as the polarity of the solvent is increased

The emission spectrum of TIPmiddotNO~ exhibits a dramatic temperature dependence Figure 4 shows the fluorescence spectra in 2-methyltetrahydrofuran at ambient temperature and at liquid nitrogen temshyperature (77 K) where the solvent is a rigid glass

00

0_lt T_ _ bull ~ 0 ~ 0 bull l bull ~ bull shy

~~~~f~J(~ ~ o~

422 DEVENS GUST et al

12 T---------------_

~ 08

~ ~ - 1ii i 04 rr

~ 00 ~-O- ------_---__l

840 880 720 780 00

Wavlnglh (nm)

Fi~ure 3 Corrected fluorescence emission spectra of TIPshyNO in four different solvents isooctane(--) toluene () 2-methyltetrahydrofuran (---) and acetonitrile

(_- )

At ambient temperature a broad emission similar to that in toluene and other solvents is observed with a maximum at 720 nm At 77 K the spectrum features a structured emission with the same general shape as that of TPP with two relatively narrow bands at 695 and 752 nm The emission intensity of TIP-N02 at the maximum increases by roughly an order of magnitude as the sample is cooled from ambient to 77 K The absorption spectrum of TIPshyN02 in 2-methyltetrahydrofuran on the other hand shows only some limited band narrowing when the temperature is changed from ambient to 77 K

The fluorescence quantum yields for TIP-N02

and ZnTIP-N02 in air saturated toluene solution at ambient temperatures were determined to be 0059 and 0047 respectively The corresponding yield for TPP is 011 in air saturated benzene (Seybold and Gouterman 1969) and 0075 in propashynol (Gradyushko and Tsvirko 1971) whereas that for ZnTPP is 00315 in air saturated toluene (Grashydyushko and Tsvirko 1971) and 003 in benzene (Quimby and Longo 1975 Seybold and Goutershyman 1969)

Fluorescence lifetimes

Fluorescence decay curves for TIPmiddotN02 and ZnTIP-N02 were measured in toluene solution at ambient temperatures The decay for ZnTIP-N02

at 655 nm could be satisfactorily (X 2 = 103) fit by a single exponential with a lifetime of 12 = 01 ns The free base TIP-N02 deeay measured at 750 nm was also fit by a single exponential with a lifetime of 25 = 01 ns (X 2 = 095) In contrast ZnTPP in toluene has a fluorescence lifetime of 22 ns (Grashydyushko and Tsvirko 1971) and free base TPP a lifetime of 106 ns in methanol (Bonnett et al 1988) and 90 ns in propanol (Gradyushko and Tsvirko 1971) Thus the singlet lifetimes of both the free base and mdallated nitroporphyrins are reduced substantially relative to those of the unsubstituted

tetraphenylporphyrin analogs

12--- -

c 08bullbull

C ~ 1ii 04i rr

25 875 725 775 828 00 +-=--------------1

Wavlngth (nm)

Figure 4 Corrected fluorescence emission spectra of TIPshyNO in 2-methyltetrahydrofuran at room temperature (--) and at 77 K ( ) with intensities normalized to

IE 80 u- 70I E 600

50-~ 40 u JO 8 u 20 c a 10 u ~ 00

1 at the emission maxima

-free base TTP-N02 -zinc TIP-N02

ttmiddot

middotmiddot~

-shyw -10 +---r-------------r---r-----+

440 490 540 590 640 690 740 790 840 Wavelength (nm)

Figure 5 Transient absorption spectra (t-EG) following laser flash excitation of TTP-NO (e) and ZnTIP-N02 () in benzene solution at ambient temperatures The same spectral shapes were obtained with excitation at either 532 or 590 nm and when taken at a variety of times from = 100 ns to several microseconds following

excitation

Triplet-triplet absorption specTra

Figure 5 shows the triplet-triplet absorption specmiddot tra (trtG) of TIPmiddotN02 and ZnTIP-N02 in deaershyated benzene at ambient temperatures as obtained using flash photolysis In both cases the maximum absorption occurs at about 480 nm The major absorption bands for TIP-N02 and ZnTIP-N02

are somewhat red shifted from those of TPP and ZnTPP (Pekkarinen and Linschitz 1960) but othershywise the main spectral features are similar to those of the parent compounds

Extinction coefficients for the triplet states (tT-tC) were measured in benzene solution at ambishyent temperatures using pulse radiolysis The energy transfer method which involves comparing the tripshylet extinction of the unknown porphyrin with that of the triplet of a biphenyl standard [(trtG) = 27 100 M- I em-I at 360 nm in benzene) was employed The details of the method are given by Bensasson and Land (1971) and discussed by Carshymichael and Hug (1986) and by Carmichael eT 01

~

825

Tpmiddot ure Jto

I 840

ing 00

The n al of ing

i1ecshyaershyned lUm

ljor 0 and hershylose

ales nhishyrgy ripshythat )=

as 1 hy arshyI al

~~~--~I-_bull bullbullbullbullbullbullbullbull

Photophysics of nitroporphyrins 423

(1987) For TIP-NO ErEG was 47000 7 000 M-I cm -I at 475 nm whereas for ZnTIPshy

N02 the value was 69 000 10 000 M-I cm -I at 500 nm Pekkarinen and Linschitz (1960) measured ET for ZnTPP in benzene by flash photolysis using the complete conversion method and obtained a value of 74 ()()() at 470 nm where the ground state does not absorb appreciably The triplet of TPP has a maximum which overlaps the Soret band of the absorption spectrum this can lead to relatively large errors in extinction coefficient and this factor commiddot bined with the spectral shift resulting from nitration makes comparison with the extinction coefficient obtained for TIP-N02 difficult

Triplet quantum yields lttgtT were obtained by the comparative method (Bensasson et al 1978 1983) using the following equation which is valid when the optical densities at the laser excitation waveshylength of solutions of the unknown and a reference molecule are both low and the same

lttgtPo = lttgtR x 10~~b x (ErEc)R T T 10M_G (ErEci)P0

In this equation lttgtt and lttgtyen are respectively the triplet quantum yields of the porphyrin being stud- ied and a reference material of known quantum yield lODt~h and lOD~_G are the changes in optical density at selected wavelengths following laser flash excitation (taken in domains where the optical density change is linear with laser power as determined by 5-7 measurements at different powers) and the (ET-Ed values are the respective triplet minus singlet difference extinction coefshyficients at the same selected wavelengths Because the main thrust of this work is comparison of the results for the nitro substituted porphyrins with those for the unsubstituted TPP and ZnTPP the reference material chosen was ZnTPP itself The triplet quantum yield of ZnTPP was taken as 088 (Harriman et al 1981 Dzhagarov 1970 Kalyanmiddot asundaram and Neumann-Spallart 1982) and (ErE() as 32 250 at 500 nm and 74 ()()() at 470 nm for this compound (Pekkarinen and Linschitz 1960) The TIP-NO and ZnTIP-N02 triplet absorptions were monitored at 475 and 500 nm and the extinction coefficients reported above were employed On this basis lttgtT values of 062 and 065 were calculated for TIP-NO and ZnTIP-NO respectively Thus the triplet quantum yield for ZJTP-N02 is reduced somewhat relative to ZnTPP whereas that for TIP-NO is about the same as that for the TPP parent compound in benshyzene (lttgt1 = 067 Bonnett el al 1988)

The lifetimes of the triplet states of TIP-NO and ZnTIPmiddotNO in 2-methyltetrahydrofuran glass at 77 K were I 7 and 2 I ms respectively as measured by the transient absorption method These are fairly typical lifetimes for porphyrins in organic glasses (Gradyushko and Tsvirko 1971 Harriman el al 1981 )

DISCUSSION

The absorption spectra of TIPmiddotN02 and ZnTIPshyN02 are characterized by red shifts and in the case of the free base a change in the relative intensities of the Q-bands compared to TPP itself Similar behavior has been observed in 5-nitrooctaethylpormiddot phyrin (Dvornikov et al 1986) and the tendency of electron withdrawing substituents on the periphshyery of the porphyrin to cause shifts to longer waveshylengths of the visible and Soret bands is well known (Falk 1964) The triplet-triplet absorption bands of TIP-N02 and its zinc analog are also somewhat red shifted relative to the parent compounds The ET-EG value determined for ZnTIP-N02 is comparable to that found for ZnTIP given the relatively large errors inherent in such measurements

In emission however the nitro substituted porshyphyrins differ strikingly from their parent molecules In the first place their singlet lifetimes are substanshytially decreased The shorter lifetimes cannot be due only to enhanced fluorescence or intersystem crossing based upon the lttgtF and lttgtT values obtained and more rapid internal conversion by some mechanism must be involved These results suggest that the singlet emitting states of the nitroshysubstituted porphyrins are structurally significantly different from those of their normal porphyrin analshyogs Reduced fluorescence yields and lifetimes have also been observed in tetraarylporphyrins bearing nitro groups on the aryl rings (Harriman and Hosie 1981ab) and in 5-nitrooctaethylporphyrin and related materials (Dvornikov et al 1986) Formashytion of an intramolecular charge transfer state P~shyNO- was suggested by both groups as a possible explanation This is a reasonable suggestion and is presumably applicable to TIP-N02 and ZnTIPmiddot N02 as well

There is an additional aspect of the problem which is evident from the TIP-N02 data The emisshysion spectra are broad and unstructured and the red shift is strongly solvent dependent with shifts to longer wavelength in more polar solvents Tetramiddot arylporphyrins with nitro substitution on the aryl rings and also evidently 5-nitrooctaethylporphyrin have normal porphyrin emission bands with the expected fine structure (Harriman and Hosie 1981ab Dvornikov et al 1986) The unusual behavior of TPPmiddotN02 and its zinc analog is remishyniscent of that of aromatic amino compounds which form twisted intramolecular charge transfer (TICf) states (Rettig 1986 1988 and references cited therein Rotkiewicz el al 1973 Grabowski et al bull 1978 1979 Lippert et al 1987) In these molecules electron transfer from the amino nitrogen atom in the excited singlet state of the molecule to the aroshymatic system is accompanied by twisting of the amino group about its bond to the aromatic system so as to achieve an essentially perpendicular orienshytation of the 7T-electron system of the aromatic

~ bullbull~ ~ ~ I bull

( o-

S shy

fJbullshy ~~

l)~

I- ~~ ~ 420 DEVENS GUST et 0

-tt-

RR

TPP R=X=H

Scheme I

NOo by chromatography on silica gel (hexanetoluene 11) Mass spectrum mle 778 (M+) H NMR (CDO 400 MHz) 1) 266 (3 H s 211-ArCH) 270 (9 H s 51O15-ArCH) 748 (2 H d J=78 Hz 20-Ar C-35) 755 (6 H d J=78 Hz 51015-Ar C-35) 804-806 (8 H m 5101520-Ar C-26) 887-893 (5 H m Cshy7HIl317) H96 (1 H d J=48 Hz C-18) 920 (I H s C-3)

Spectroscopic measurements All spectroscopic measurements were performed on samples in toluene solshyution at ambient temperatures unless otherwise noted UV-VIS spectra were obtained using an HP-8450 spectroshyphotometer Steady state fluorescence spectra were recshyorded and quantum yields were calculated by procedures described by Brune et 0 (1988) Fluorescence decay measurements were made on = I x 10- M solutions in toluene using the time-correlated single photon counting method Two different setups were used In one the excitation source was a mode-locked argon ion laser coupled to a synchronously pumped cavity dumped dye laser operating at 590 nm Experimental details are proshyvided elsewhere (Boens et 0 1984) In the other the excitation source was a frequency doubled mode-locked Coherent Antares Nd-YAG laser coupled to a synchronshyously pumped cavity dumped Coherent 701-3 dye laser with excitation at 590 or 610 nm This laser system proshyvided pulses of - 9 ps with a 38 MHz repetition rate Thc sample was excited by a vertically polarized laser beam The emission was selected via a polarizer set at the magic angle (547deg) collected by a camcra kns (Pentax Hi mm) and imaged onto the entrance slit of a double monochromator (Jobin-Yvon DH-IO H nm bandwidth) The collimated light from the monochromator was detecled hy a microchannel-platc photomultiplier (Hamshyamatsu R2K1l9U-lll) The resulting signal was amplified with a fasl amplifier (Phillips Scientific 69i4B-IO) disshycriminated hv a constant fraction discriminal0r (Tennekc TC 4ii) and uscd to provide the start signal for a Tennclec TC Kh4 time-tll-amplitude convertcr Approximately 20 of the excitation heam was directed to a fast photodiode (Teldunkcn BPW~Ii) The signal from the phOlolliolle was dclayed (Ortcc DB4hJ delay hox) discriminated (Tenncshy

lec TC 453) and taken as the stop input for the lime-toshyamplitude converter The output from the time-to-amplishytude converter was accumulated in a Nucleus multichannel analyzer Usually_ about 05--1 of the laser pulses resulted in detection of an emitted photon The instrument response function was - 35 ps The instrument response function measured at the excitation wavelength with a scattering sample and the fluorescence decay were transshyferred to a Microvax 3200 workstation for data analysis Multiexponential decay curves were computed with an iterative reconvolution program using a semilinear Marshyquardt algorithm The quality of the fits was judged by the reduced XO and inspection of the residuals and the autocorrelation function (Demas 1983 OConnor and Phillips 1984 Wendler and Holzwarth 1987)

The pulse radiolysis equipment has been described by Butler et 0 (1990) Measurements were made in benzene solutions which were argon-flushed and irradiated in 25 cm quartz capillary cells with 10 ns = 25 Gy pulses of 9-12 MeV electrons from a Vickers electron linear accelerator The resulting changes in light absorption were monitored as a function of time at each wavelength by using photoelectric detection (EMI 9558QA or Hamamshyatsu R928 photomultiplier)

Transient absorption spectra were obtained on = 10- M solutions in benzene which were deoxygenated by bubbling with argon or nitrogen gas Two different spectrometers were used In one the excitation source was a Quantel frequency-doubled YAG laser providing = 5 ns pulses of 532 nm light The analyzing light was a = 20 s xenon flash whose intensity was constant within I for more than 20 s (Bensasson et 0 1972) In the other pulsed excitation was provided by a Lambda Physik FL2000 dye laser using Rhodamine 6G pumped by a Lambda Physik EMG 50E xenon chloride excimer laser at 308 nm The output of the dye laser consisted of = 5 mJ 20 ns pulses at 590 nm with a repetition rate of 10 Hz The analyzing light was provided by a quartz-halogen lamp The detection scheme has been described by Gust et 0 (1986) and Davis et 0 (1987)

RESULTS

UV- VIS absorption spectra

The absorption spectra of TIP-N01 and ZnTIPshyN02 in toluene are shown in Fig 1 The waveshylengths of the major absorption bands and the correshysponding extinction coefficients are listed in Table

E u 20

5

15 C0 ~

0 ltJ

c 2 U

~ W

500 100

-_

400

Wayelength (nm)

Figure I Absorption spectra of TTP-NOo (--) and ZnTTP-NO ( ) in toluene solution normalized to the cJltinclion codticienl~ a~ ~hown on the ordinate The insert is a vertical cxpansion (X 3) of the 500-750 nm region

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t ~)

plimiddot nd h~s

~nt

Ibe

h a

an larmiddot t hy the and

by ~cnc

J in ulses ncar wrre h hy lamshy

on ated rent urce iding

as a ilhin n the hsik b a laser

~d of lIe of h a

heen 1~7)

lITPshyaveshyllrre-Table

J ) and d 10 the insert fl-middotgilln

Photophysics of nitroporphyrins 421

Table 1 Electronic absorption bands of nitroporphyrins and analogs

Compound

TIP-NO

TPP

Soret

430 (229000)

419 (470000)

A m om E (M-I em-I)

IV

528 (15800)

515 (18700)

III

56 (5000)

548 (8100)

II

606 (3720)

592 (5300)

668 (8640)

647 (3400)

ZnTIP-NO

ZnTIph

Soret

434 (195000)

423 (544 000

J3

562 (13000)

548 (22800)

Q

604 (9020)

586 (3680)

In benzene (Badger et albull 1964) hIn benzene (Quimby and Longo 1975)

1 along with those for 5lO1520-tetraphenylporshy 12

phyrin (TPP) and its zinc analog (ZnTPP) The general features of the absorption spectra of TIPshy

poundl bullc

NO~ and ZnTIP-NO~ are similar to those reported 01 for the nitrotetraphenylporphyrin analogs in differshy ent solvents (Baldwin et al 1982 Evans et al ~bull1978) It is clear from Table 1 that incorporation of i middotthe nitro group into the free base porphyrin alters i 04 middotIt middot middotmiddotboth the wavelengths of the absorption bands and their relative extinction coefficients The Soret bands of TIP-NO~ and ZnTIP-N02 are both red 00

shifted by 11 nm relative to TPP and ZnTPP The Q-bands are also red shifted in the nitroporphyrins with the largest shifts being seen for the longest wavelength absorptions (21 and 18 nm for TTP-N02

and ZnTTP-N021 respectively) Nitro substitution has broadened the Soret bands in both compounds and decreased their maximum intensity and the longest wavelength absorptions have gained oscilshylator strength at the expense of the other Q-bands in both TTP-N02 and ZnTIP-NO~

Fluorescence spectra

The corrected fluorescence emission spectra for TTP-NO~ and ZnTTP-NO~ in toluene appear in Fig 2 The emission maxima are at 720 and 672 nm respectively The shapes and emission maxima are independent of concentration Tetraphenylporphyshyrin itself shows two emission bands of similar intenshysity at 652 and 718 nm in benzene solution whereas the zinc analog ZnTPP shows two bands at 598 and M7 nm with a small shoulder at 560 nm (Quimby and Longo 1975) Thus the nitro substitution gives rise to very large red shifts in the emission maxima and broad featureless emissions which contrast sharply with the structured bands usually observed in tetraarylporphyrins and their zinc analogs The apparent lack of mirror-image symmetry between

510 130 110 730 710 130

Wvlngth (nm)

Figure 2 Corrected fluorescence emission spectra for TIP-NO (--) and ZnTIP-NO ( ) in toluene at ambient temperature with intensities normalized to I at

the emission wavelength maxima

the absorption and emission spectra and the large Stokes shift suggest that the potential surface for the emitting state differs considerably from that of the ground state for both nitro compounds

Solvent effects upon the emission spectrum of TIP-N02 were investigated Figure 3 shows the fluorescence emission spectra of TIP-NO~ at room temperature in four solvents of different polarities As the solvent is changed from isooctane to acetoshynitrile the emission wavelength maximum changes from 692 to 758 nm (Note that the emission in isooctane still shows some intensity in the longer wavelength region) In general the emission intenshysity falls off with peak broadening as the polarity of the solvent is increased

The emission spectrum of TIPmiddotNO~ exhibits a dramatic temperature dependence Figure 4 shows the fluorescence spectra in 2-methyltetrahydrofuran at ambient temperature and at liquid nitrogen temshyperature (77 K) where the solvent is a rigid glass

00

0_lt T_ _ bull ~ 0 ~ 0 bull l bull ~ bull shy

~~~~f~J(~ ~ o~

422 DEVENS GUST et al

12 T---------------_

~ 08

~ ~ - 1ii i 04 rr

~ 00 ~-O- ------_---__l

840 880 720 780 00

Wavlnglh (nm)

Fi~ure 3 Corrected fluorescence emission spectra of TIPshyNO in four different solvents isooctane(--) toluene () 2-methyltetrahydrofuran (---) and acetonitrile

(_- )

At ambient temperature a broad emission similar to that in toluene and other solvents is observed with a maximum at 720 nm At 77 K the spectrum features a structured emission with the same general shape as that of TPP with two relatively narrow bands at 695 and 752 nm The emission intensity of TIP-N02 at the maximum increases by roughly an order of magnitude as the sample is cooled from ambient to 77 K The absorption spectrum of TIPshyN02 in 2-methyltetrahydrofuran on the other hand shows only some limited band narrowing when the temperature is changed from ambient to 77 K

The fluorescence quantum yields for TIP-N02

and ZnTIP-N02 in air saturated toluene solution at ambient temperatures were determined to be 0059 and 0047 respectively The corresponding yield for TPP is 011 in air saturated benzene (Seybold and Gouterman 1969) and 0075 in propashynol (Gradyushko and Tsvirko 1971) whereas that for ZnTPP is 00315 in air saturated toluene (Grashydyushko and Tsvirko 1971) and 003 in benzene (Quimby and Longo 1975 Seybold and Goutershyman 1969)

Fluorescence lifetimes

Fluorescence decay curves for TIPmiddotN02 and ZnTIP-N02 were measured in toluene solution at ambient temperatures The decay for ZnTIP-N02

at 655 nm could be satisfactorily (X 2 = 103) fit by a single exponential with a lifetime of 12 = 01 ns The free base TIP-N02 deeay measured at 750 nm was also fit by a single exponential with a lifetime of 25 = 01 ns (X 2 = 095) In contrast ZnTPP in toluene has a fluorescence lifetime of 22 ns (Grashydyushko and Tsvirko 1971) and free base TPP a lifetime of 106 ns in methanol (Bonnett et al 1988) and 90 ns in propanol (Gradyushko and Tsvirko 1971) Thus the singlet lifetimes of both the free base and mdallated nitroporphyrins are reduced substantially relative to those of the unsubstituted

tetraphenylporphyrin analogs

12--- -

c 08bullbull

C ~ 1ii 04i rr

25 875 725 775 828 00 +-=--------------1

Wavlngth (nm)

Figure 4 Corrected fluorescence emission spectra of TIPshyNO in 2-methyltetrahydrofuran at room temperature (--) and at 77 K ( ) with intensities normalized to

IE 80 u- 70I E 600

50-~ 40 u JO 8 u 20 c a 10 u ~ 00

1 at the emission maxima

-free base TTP-N02 -zinc TIP-N02

ttmiddot

middotmiddot~

-shyw -10 +---r-------------r---r-----+

440 490 540 590 640 690 740 790 840 Wavelength (nm)

Figure 5 Transient absorption spectra (t-EG) following laser flash excitation of TTP-NO (e) and ZnTIP-N02 () in benzene solution at ambient temperatures The same spectral shapes were obtained with excitation at either 532 or 590 nm and when taken at a variety of times from = 100 ns to several microseconds following

excitation

Triplet-triplet absorption specTra

Figure 5 shows the triplet-triplet absorption specmiddot tra (trtG) of TIPmiddotN02 and ZnTIP-N02 in deaershyated benzene at ambient temperatures as obtained using flash photolysis In both cases the maximum absorption occurs at about 480 nm The major absorption bands for TIP-N02 and ZnTIP-N02

are somewhat red shifted from those of TPP and ZnTPP (Pekkarinen and Linschitz 1960) but othershywise the main spectral features are similar to those of the parent compounds

Extinction coefficients for the triplet states (tT-tC) were measured in benzene solution at ambishyent temperatures using pulse radiolysis The energy transfer method which involves comparing the tripshylet extinction of the unknown porphyrin with that of the triplet of a biphenyl standard [(trtG) = 27 100 M- I em-I at 360 nm in benzene) was employed The details of the method are given by Bensasson and Land (1971) and discussed by Carshymichael and Hug (1986) and by Carmichael eT 01

~

825

Tpmiddot ure Jto

I 840

ing 00

The n al of ing

i1ecshyaershyned lUm

ljor 0 and hershylose

ales nhishyrgy ripshythat )=

as 1 hy arshyI al

~~~--~I-_bull bullbullbullbullbullbullbullbull

Photophysics of nitroporphyrins 423

(1987) For TIP-NO ErEG was 47000 7 000 M-I cm -I at 475 nm whereas for ZnTIPshy

N02 the value was 69 000 10 000 M-I cm -I at 500 nm Pekkarinen and Linschitz (1960) measured ET for ZnTPP in benzene by flash photolysis using the complete conversion method and obtained a value of 74 ()()() at 470 nm where the ground state does not absorb appreciably The triplet of TPP has a maximum which overlaps the Soret band of the absorption spectrum this can lead to relatively large errors in extinction coefficient and this factor commiddot bined with the spectral shift resulting from nitration makes comparison with the extinction coefficient obtained for TIP-N02 difficult

Triplet quantum yields lttgtT were obtained by the comparative method (Bensasson et al 1978 1983) using the following equation which is valid when the optical densities at the laser excitation waveshylength of solutions of the unknown and a reference molecule are both low and the same

lttgtPo = lttgtR x 10~~b x (ErEc)R T T 10M_G (ErEci)P0

In this equation lttgtt and lttgtyen are respectively the triplet quantum yields of the porphyrin being stud- ied and a reference material of known quantum yield lODt~h and lOD~_G are the changes in optical density at selected wavelengths following laser flash excitation (taken in domains where the optical density change is linear with laser power as determined by 5-7 measurements at different powers) and the (ET-Ed values are the respective triplet minus singlet difference extinction coefshyficients at the same selected wavelengths Because the main thrust of this work is comparison of the results for the nitro substituted porphyrins with those for the unsubstituted TPP and ZnTPP the reference material chosen was ZnTPP itself The triplet quantum yield of ZnTPP was taken as 088 (Harriman et al 1981 Dzhagarov 1970 Kalyanmiddot asundaram and Neumann-Spallart 1982) and (ErE() as 32 250 at 500 nm and 74 ()()() at 470 nm for this compound (Pekkarinen and Linschitz 1960) The TIP-NO and ZnTIP-N02 triplet absorptions were monitored at 475 and 500 nm and the extinction coefficients reported above were employed On this basis lttgtT values of 062 and 065 were calculated for TIP-NO and ZnTIP-NO respectively Thus the triplet quantum yield for ZJTP-N02 is reduced somewhat relative to ZnTPP whereas that for TIP-NO is about the same as that for the TPP parent compound in benshyzene (lttgt1 = 067 Bonnett el al 1988)

The lifetimes of the triplet states of TIP-NO and ZnTIPmiddotNO in 2-methyltetrahydrofuran glass at 77 K were I 7 and 2 I ms respectively as measured by the transient absorption method These are fairly typical lifetimes for porphyrins in organic glasses (Gradyushko and Tsvirko 1971 Harriman el al 1981 )

DISCUSSION

The absorption spectra of TIPmiddotN02 and ZnTIPshyN02 are characterized by red shifts and in the case of the free base a change in the relative intensities of the Q-bands compared to TPP itself Similar behavior has been observed in 5-nitrooctaethylpormiddot phyrin (Dvornikov et al 1986) and the tendency of electron withdrawing substituents on the periphshyery of the porphyrin to cause shifts to longer waveshylengths of the visible and Soret bands is well known (Falk 1964) The triplet-triplet absorption bands of TIP-N02 and its zinc analog are also somewhat red shifted relative to the parent compounds The ET-EG value determined for ZnTIP-N02 is comparable to that found for ZnTIP given the relatively large errors inherent in such measurements

In emission however the nitro substituted porshyphyrins differ strikingly from their parent molecules In the first place their singlet lifetimes are substanshytially decreased The shorter lifetimes cannot be due only to enhanced fluorescence or intersystem crossing based upon the lttgtF and lttgtT values obtained and more rapid internal conversion by some mechanism must be involved These results suggest that the singlet emitting states of the nitroshysubstituted porphyrins are structurally significantly different from those of their normal porphyrin analshyogs Reduced fluorescence yields and lifetimes have also been observed in tetraarylporphyrins bearing nitro groups on the aryl rings (Harriman and Hosie 1981ab) and in 5-nitrooctaethylporphyrin and related materials (Dvornikov et al 1986) Formashytion of an intramolecular charge transfer state P~shyNO- was suggested by both groups as a possible explanation This is a reasonable suggestion and is presumably applicable to TIP-N02 and ZnTIPmiddot N02 as well

There is an additional aspect of the problem which is evident from the TIP-N02 data The emisshysion spectra are broad and unstructured and the red shift is strongly solvent dependent with shifts to longer wavelength in more polar solvents Tetramiddot arylporphyrins with nitro substitution on the aryl rings and also evidently 5-nitrooctaethylporphyrin have normal porphyrin emission bands with the expected fine structure (Harriman and Hosie 1981ab Dvornikov et al 1986) The unusual behavior of TPPmiddotN02 and its zinc analog is remishyniscent of that of aromatic amino compounds which form twisted intramolecular charge transfer (TICf) states (Rettig 1986 1988 and references cited therein Rotkiewicz el al 1973 Grabowski et al bull 1978 1979 Lippert et al 1987) In these molecules electron transfer from the amino nitrogen atom in the excited singlet state of the molecule to the aroshymatic system is accompanied by twisting of the amino group about its bond to the aromatic system so as to achieve an essentially perpendicular orienshytation of the 7T-electron system of the aromatic

~oC ~Imiddotmiddot~middotmiddot~lgtrmiddotmiddot - 1 ~ ~

t ~)

plimiddot nd h~s

~nt

Ibe

h a

an larmiddot t hy the and

by ~cnc

J in ulses ncar wrre h hy lamshy

on ated rent urce iding

as a ilhin n the hsik b a laser

~d of lIe of h a

heen 1~7)

lITPshyaveshyllrre-Table

J ) and d 10 the insert fl-middotgilln

Photophysics of nitroporphyrins 421

Table 1 Electronic absorption bands of nitroporphyrins and analogs

Compound

TIP-NO

TPP

Soret

430 (229000)

419 (470000)

A m om E (M-I em-I)

IV

528 (15800)

515 (18700)

III

56 (5000)

548 (8100)

II

606 (3720)

592 (5300)

668 (8640)

647 (3400)

ZnTIP-NO

ZnTIph

Soret

434 (195000)

423 (544 000

J3

562 (13000)

548 (22800)

Q

604 (9020)

586 (3680)

In benzene (Badger et albull 1964) hIn benzene (Quimby and Longo 1975)

1 along with those for 5lO1520-tetraphenylporshy 12

phyrin (TPP) and its zinc analog (ZnTPP) The general features of the absorption spectra of TIPshy

poundl bullc

NO~ and ZnTIP-NO~ are similar to those reported 01 for the nitrotetraphenylporphyrin analogs in differshy ent solvents (Baldwin et al 1982 Evans et al ~bull1978) It is clear from Table 1 that incorporation of i middotthe nitro group into the free base porphyrin alters i 04 middotIt middot middotmiddotboth the wavelengths of the absorption bands and their relative extinction coefficients The Soret bands of TIP-NO~ and ZnTIP-N02 are both red 00

shifted by 11 nm relative to TPP and ZnTPP The Q-bands are also red shifted in the nitroporphyrins with the largest shifts being seen for the longest wavelength absorptions (21 and 18 nm for TTP-N02

and ZnTTP-N021 respectively) Nitro substitution has broadened the Soret bands in both compounds and decreased their maximum intensity and the longest wavelength absorptions have gained oscilshylator strength at the expense of the other Q-bands in both TTP-N02 and ZnTIP-NO~

Fluorescence spectra

The corrected fluorescence emission spectra for TTP-NO~ and ZnTTP-NO~ in toluene appear in Fig 2 The emission maxima are at 720 and 672 nm respectively The shapes and emission maxima are independent of concentration Tetraphenylporphyshyrin itself shows two emission bands of similar intenshysity at 652 and 718 nm in benzene solution whereas the zinc analog ZnTPP shows two bands at 598 and M7 nm with a small shoulder at 560 nm (Quimby and Longo 1975) Thus the nitro substitution gives rise to very large red shifts in the emission maxima and broad featureless emissions which contrast sharply with the structured bands usually observed in tetraarylporphyrins and their zinc analogs The apparent lack of mirror-image symmetry between

510 130 110 730 710 130

Wvlngth (nm)

Figure 2 Corrected fluorescence emission spectra for TIP-NO (--) and ZnTIP-NO ( ) in toluene at ambient temperature with intensities normalized to I at

the emission wavelength maxima

the absorption and emission spectra and the large Stokes shift suggest that the potential surface for the emitting state differs considerably from that of the ground state for both nitro compounds

Solvent effects upon the emission spectrum of TIP-N02 were investigated Figure 3 shows the fluorescence emission spectra of TIP-NO~ at room temperature in four solvents of different polarities As the solvent is changed from isooctane to acetoshynitrile the emission wavelength maximum changes from 692 to 758 nm (Note that the emission in isooctane still shows some intensity in the longer wavelength region) In general the emission intenshysity falls off with peak broadening as the polarity of the solvent is increased

The emission spectrum of TIPmiddotNO~ exhibits a dramatic temperature dependence Figure 4 shows the fluorescence spectra in 2-methyltetrahydrofuran at ambient temperature and at liquid nitrogen temshyperature (77 K) where the solvent is a rigid glass

00

0_lt T_ _ bull ~ 0 ~ 0 bull l bull ~ bull shy

~~~~f~J(~ ~ o~

422 DEVENS GUST et al

12 T---------------_

~ 08

~ ~ - 1ii i 04 rr

~ 00 ~-O- ------_---__l

840 880 720 780 00

Wavlnglh (nm)

Fi~ure 3 Corrected fluorescence emission spectra of TIPshyNO in four different solvents isooctane(--) toluene () 2-methyltetrahydrofuran (---) and acetonitrile

(_- )

At ambient temperature a broad emission similar to that in toluene and other solvents is observed with a maximum at 720 nm At 77 K the spectrum features a structured emission with the same general shape as that of TPP with two relatively narrow bands at 695 and 752 nm The emission intensity of TIP-N02 at the maximum increases by roughly an order of magnitude as the sample is cooled from ambient to 77 K The absorption spectrum of TIPshyN02 in 2-methyltetrahydrofuran on the other hand shows only some limited band narrowing when the temperature is changed from ambient to 77 K

The fluorescence quantum yields for TIP-N02

and ZnTIP-N02 in air saturated toluene solution at ambient temperatures were determined to be 0059 and 0047 respectively The corresponding yield for TPP is 011 in air saturated benzene (Seybold and Gouterman 1969) and 0075 in propashynol (Gradyushko and Tsvirko 1971) whereas that for ZnTPP is 00315 in air saturated toluene (Grashydyushko and Tsvirko 1971) and 003 in benzene (Quimby and Longo 1975 Seybold and Goutershyman 1969)

Fluorescence lifetimes

Fluorescence decay curves for TIPmiddotN02 and ZnTIP-N02 were measured in toluene solution at ambient temperatures The decay for ZnTIP-N02

at 655 nm could be satisfactorily (X 2 = 103) fit by a single exponential with a lifetime of 12 = 01 ns The free base TIP-N02 deeay measured at 750 nm was also fit by a single exponential with a lifetime of 25 = 01 ns (X 2 = 095) In contrast ZnTPP in toluene has a fluorescence lifetime of 22 ns (Grashydyushko and Tsvirko 1971) and free base TPP a lifetime of 106 ns in methanol (Bonnett et al 1988) and 90 ns in propanol (Gradyushko and Tsvirko 1971) Thus the singlet lifetimes of both the free base and mdallated nitroporphyrins are reduced substantially relative to those of the unsubstituted

tetraphenylporphyrin analogs

12--- -

c 08bullbull

C ~ 1ii 04i rr

25 875 725 775 828 00 +-=--------------1

Wavlngth (nm)

Figure 4 Corrected fluorescence emission spectra of TIPshyNO in 2-methyltetrahydrofuran at room temperature (--) and at 77 K ( ) with intensities normalized to

IE 80 u- 70I E 600

50-~ 40 u JO 8 u 20 c a 10 u ~ 00

1 at the emission maxima

-free base TTP-N02 -zinc TIP-N02

ttmiddot

middotmiddot~

-shyw -10 +---r-------------r---r-----+

440 490 540 590 640 690 740 790 840 Wavelength (nm)

Figure 5 Transient absorption spectra (t-EG) following laser flash excitation of TTP-NO (e) and ZnTIP-N02 () in benzene solution at ambient temperatures The same spectral shapes were obtained with excitation at either 532 or 590 nm and when taken at a variety of times from = 100 ns to several microseconds following

excitation

Triplet-triplet absorption specTra

Figure 5 shows the triplet-triplet absorption specmiddot tra (trtG) of TIPmiddotN02 and ZnTIP-N02 in deaershyated benzene at ambient temperatures as obtained using flash photolysis In both cases the maximum absorption occurs at about 480 nm The major absorption bands for TIP-N02 and ZnTIP-N02

are somewhat red shifted from those of TPP and ZnTPP (Pekkarinen and Linschitz 1960) but othershywise the main spectral features are similar to those of the parent compounds

Extinction coefficients for the triplet states (tT-tC) were measured in benzene solution at ambishyent temperatures using pulse radiolysis The energy transfer method which involves comparing the tripshylet extinction of the unknown porphyrin with that of the triplet of a biphenyl standard [(trtG) = 27 100 M- I em-I at 360 nm in benzene) was employed The details of the method are given by Bensasson and Land (1971) and discussed by Carshymichael and Hug (1986) and by Carmichael eT 01

~

825

Tpmiddot ure Jto

I 840

ing 00

The n al of ing

i1ecshyaershyned lUm

ljor 0 and hershylose

ales nhishyrgy ripshythat )=

as 1 hy arshyI al

~~~--~I-_bull bullbullbullbullbullbullbullbull

Photophysics of nitroporphyrins 423

(1987) For TIP-NO ErEG was 47000 7 000 M-I cm -I at 475 nm whereas for ZnTIPshy

N02 the value was 69 000 10 000 M-I cm -I at 500 nm Pekkarinen and Linschitz (1960) measured ET for ZnTPP in benzene by flash photolysis using the complete conversion method and obtained a value of 74 ()()() at 470 nm where the ground state does not absorb appreciably The triplet of TPP has a maximum which overlaps the Soret band of the absorption spectrum this can lead to relatively large errors in extinction coefficient and this factor commiddot bined with the spectral shift resulting from nitration makes comparison with the extinction coefficient obtained for TIP-N02 difficult

Triplet quantum yields lttgtT were obtained by the comparative method (Bensasson et al 1978 1983) using the following equation which is valid when the optical densities at the laser excitation waveshylength of solutions of the unknown and a reference molecule are both low and the same

lttgtPo = lttgtR x 10~~b x (ErEc)R T T 10M_G (ErEci)P0

In this equation lttgtt and lttgtyen are respectively the triplet quantum yields of the porphyrin being stud- ied and a reference material of known quantum yield lODt~h and lOD~_G are the changes in optical density at selected wavelengths following laser flash excitation (taken in domains where the optical density change is linear with laser power as determined by 5-7 measurements at different powers) and the (ET-Ed values are the respective triplet minus singlet difference extinction coefshyficients at the same selected wavelengths Because the main thrust of this work is comparison of the results for the nitro substituted porphyrins with those for the unsubstituted TPP and ZnTPP the reference material chosen was ZnTPP itself The triplet quantum yield of ZnTPP was taken as 088 (Harriman et al 1981 Dzhagarov 1970 Kalyanmiddot asundaram and Neumann-Spallart 1982) and (ErE() as 32 250 at 500 nm and 74 ()()() at 470 nm for this compound (Pekkarinen and Linschitz 1960) The TIP-NO and ZnTIP-N02 triplet absorptions were monitored at 475 and 500 nm and the extinction coefficients reported above were employed On this basis lttgtT values of 062 and 065 were calculated for TIP-NO and ZnTIP-NO respectively Thus the triplet quantum yield for ZJTP-N02 is reduced somewhat relative to ZnTPP whereas that for TIP-NO is about the same as that for the TPP parent compound in benshyzene (lttgt1 = 067 Bonnett el al 1988)

The lifetimes of the triplet states of TIP-NO and ZnTIPmiddotNO in 2-methyltetrahydrofuran glass at 77 K were I 7 and 2 I ms respectively as measured by the transient absorption method These are fairly typical lifetimes for porphyrins in organic glasses (Gradyushko and Tsvirko 1971 Harriman el al 1981 )

DISCUSSION

The absorption spectra of TIPmiddotN02 and ZnTIPshyN02 are characterized by red shifts and in the case of the free base a change in the relative intensities of the Q-bands compared to TPP itself Similar behavior has been observed in 5-nitrooctaethylpormiddot phyrin (Dvornikov et al 1986) and the tendency of electron withdrawing substituents on the periphshyery of the porphyrin to cause shifts to longer waveshylengths of the visible and Soret bands is well known (Falk 1964) The triplet-triplet absorption bands of TIP-N02 and its zinc analog are also somewhat red shifted relative to the parent compounds The ET-EG value determined for ZnTIP-N02 is comparable to that found for ZnTIP given the relatively large errors inherent in such measurements

In emission however the nitro substituted porshyphyrins differ strikingly from their parent molecules In the first place their singlet lifetimes are substanshytially decreased The shorter lifetimes cannot be due only to enhanced fluorescence or intersystem crossing based upon the lttgtF and lttgtT values obtained and more rapid internal conversion by some mechanism must be involved These results suggest that the singlet emitting states of the nitroshysubstituted porphyrins are structurally significantly different from those of their normal porphyrin analshyogs Reduced fluorescence yields and lifetimes have also been observed in tetraarylporphyrins bearing nitro groups on the aryl rings (Harriman and Hosie 1981ab) and in 5-nitrooctaethylporphyrin and related materials (Dvornikov et al 1986) Formashytion of an intramolecular charge transfer state P~shyNO- was suggested by both groups as a possible explanation This is a reasonable suggestion and is presumably applicable to TIP-N02 and ZnTIPmiddot N02 as well

There is an additional aspect of the problem which is evident from the TIP-N02 data The emisshysion spectra are broad and unstructured and the red shift is strongly solvent dependent with shifts to longer wavelength in more polar solvents Tetramiddot arylporphyrins with nitro substitution on the aryl rings and also evidently 5-nitrooctaethylporphyrin have normal porphyrin emission bands with the expected fine structure (Harriman and Hosie 1981ab Dvornikov et al 1986) The unusual behavior of TPPmiddotN02 and its zinc analog is remishyniscent of that of aromatic amino compounds which form twisted intramolecular charge transfer (TICf) states (Rettig 1986 1988 and references cited therein Rotkiewicz el al 1973 Grabowski et al bull 1978 1979 Lippert et al 1987) In these molecules electron transfer from the amino nitrogen atom in the excited singlet state of the molecule to the aroshymatic system is accompanied by twisting of the amino group about its bond to the aromatic system so as to achieve an essentially perpendicular orienshytation of the 7T-electron system of the aromatic

0_lt T_ _ bull ~ 0 ~ 0 bull l bull ~ bull shy

~~~~f~J(~ ~ o~

422 DEVENS GUST et al

12 T---------------_

~ 08

~ ~ - 1ii i 04 rr

~ 00 ~-O- ------_---__l

840 880 720 780 00

Wavlnglh (nm)

Fi~ure 3 Corrected fluorescence emission spectra of TIPshyNO in four different solvents isooctane(--) toluene () 2-methyltetrahydrofuran (---) and acetonitrile

(_- )

At ambient temperature a broad emission similar to that in toluene and other solvents is observed with a maximum at 720 nm At 77 K the spectrum features a structured emission with the same general shape as that of TPP with two relatively narrow bands at 695 and 752 nm The emission intensity of TIP-N02 at the maximum increases by roughly an order of magnitude as the sample is cooled from ambient to 77 K The absorption spectrum of TIPshyN02 in 2-methyltetrahydrofuran on the other hand shows only some limited band narrowing when the temperature is changed from ambient to 77 K

The fluorescence quantum yields for TIP-N02

and ZnTIP-N02 in air saturated toluene solution at ambient temperatures were determined to be 0059 and 0047 respectively The corresponding yield for TPP is 011 in air saturated benzene (Seybold and Gouterman 1969) and 0075 in propashynol (Gradyushko and Tsvirko 1971) whereas that for ZnTPP is 00315 in air saturated toluene (Grashydyushko and Tsvirko 1971) and 003 in benzene (Quimby and Longo 1975 Seybold and Goutershyman 1969)

Fluorescence lifetimes

Fluorescence decay curves for TIPmiddotN02 and ZnTIP-N02 were measured in toluene solution at ambient temperatures The decay for ZnTIP-N02

at 655 nm could be satisfactorily (X 2 = 103) fit by a single exponential with a lifetime of 12 = 01 ns The free base TIP-N02 deeay measured at 750 nm was also fit by a single exponential with a lifetime of 25 = 01 ns (X 2 = 095) In contrast ZnTPP in toluene has a fluorescence lifetime of 22 ns (Grashydyushko and Tsvirko 1971) and free base TPP a lifetime of 106 ns in methanol (Bonnett et al 1988) and 90 ns in propanol (Gradyushko and Tsvirko 1971) Thus the singlet lifetimes of both the free base and mdallated nitroporphyrins are reduced substantially relative to those of the unsubstituted

tetraphenylporphyrin analogs

12--- -

c 08bullbull

C ~ 1ii 04i rr

25 875 725 775 828 00 +-=--------------1

Wavlngth (nm)

Figure 4 Corrected fluorescence emission spectra of TIPshyNO in 2-methyltetrahydrofuran at room temperature (--) and at 77 K ( ) with intensities normalized to

IE 80 u- 70I E 600

50-~ 40 u JO 8 u 20 c a 10 u ~ 00

1 at the emission maxima

-free base TTP-N02 -zinc TIP-N02

ttmiddot

middotmiddot~

-shyw -10 +---r-------------r---r-----+

440 490 540 590 640 690 740 790 840 Wavelength (nm)

Figure 5 Transient absorption spectra (t-EG) following laser flash excitation of TTP-NO (e) and ZnTIP-N02 () in benzene solution at ambient temperatures The same spectral shapes were obtained with excitation at either 532 or 590 nm and when taken at a variety of times from = 100 ns to several microseconds following

excitation

Triplet-triplet absorption specTra

Figure 5 shows the triplet-triplet absorption specmiddot tra (trtG) of TIPmiddotN02 and ZnTIP-N02 in deaershyated benzene at ambient temperatures as obtained using flash photolysis In both cases the maximum absorption occurs at about 480 nm The major absorption bands for TIP-N02 and ZnTIP-N02

are somewhat red shifted from those of TPP and ZnTPP (Pekkarinen and Linschitz 1960) but othershywise the main spectral features are similar to those of the parent compounds

Extinction coefficients for the triplet states (tT-tC) were measured in benzene solution at ambishyent temperatures using pulse radiolysis The energy transfer method which involves comparing the tripshylet extinction of the unknown porphyrin with that of the triplet of a biphenyl standard [(trtG) = 27 100 M- I em-I at 360 nm in benzene) was employed The details of the method are given by Bensasson and Land (1971) and discussed by Carshymichael and Hug (1986) and by Carmichael eT 01

~

825

Tpmiddot ure Jto

I 840

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Photophysics of nitroporphyrins 423

(1987) For TIP-NO ErEG was 47000 7 000 M-I cm -I at 475 nm whereas for ZnTIPshy

N02 the value was 69 000 10 000 M-I cm -I at 500 nm Pekkarinen and Linschitz (1960) measured ET for ZnTPP in benzene by flash photolysis using the complete conversion method and obtained a value of 74 ()()() at 470 nm where the ground state does not absorb appreciably The triplet of TPP has a maximum which overlaps the Soret band of the absorption spectrum this can lead to relatively large errors in extinction coefficient and this factor commiddot bined with the spectral shift resulting from nitration makes comparison with the extinction coefficient obtained for TIP-N02 difficult

Triplet quantum yields lttgtT were obtained by the comparative method (Bensasson et al 1978 1983) using the following equation which is valid when the optical densities at the laser excitation waveshylength of solutions of the unknown and a reference molecule are both low and the same

lttgtPo = lttgtR x 10~~b x (ErEc)R T T 10M_G (ErEci)P0

In this equation lttgtt and lttgtyen are respectively the triplet quantum yields of the porphyrin being stud- ied and a reference material of known quantum yield lODt~h and lOD~_G are the changes in optical density at selected wavelengths following laser flash excitation (taken in domains where the optical density change is linear with laser power as determined by 5-7 measurements at different powers) and the (ET-Ed values are the respective triplet minus singlet difference extinction coefshyficients at the same selected wavelengths Because the main thrust of this work is comparison of the results for the nitro substituted porphyrins with those for the unsubstituted TPP and ZnTPP the reference material chosen was ZnTPP itself The triplet quantum yield of ZnTPP was taken as 088 (Harriman et al 1981 Dzhagarov 1970 Kalyanmiddot asundaram and Neumann-Spallart 1982) and (ErE() as 32 250 at 500 nm and 74 ()()() at 470 nm for this compound (Pekkarinen and Linschitz 1960) The TIP-NO and ZnTIP-N02 triplet absorptions were monitored at 475 and 500 nm and the extinction coefficients reported above were employed On this basis lttgtT values of 062 and 065 were calculated for TIP-NO and ZnTIP-NO respectively Thus the triplet quantum yield for ZJTP-N02 is reduced somewhat relative to ZnTPP whereas that for TIP-NO is about the same as that for the TPP parent compound in benshyzene (lttgt1 = 067 Bonnett el al 1988)

The lifetimes of the triplet states of TIP-NO and ZnTIPmiddotNO in 2-methyltetrahydrofuran glass at 77 K were I 7 and 2 I ms respectively as measured by the transient absorption method These are fairly typical lifetimes for porphyrins in organic glasses (Gradyushko and Tsvirko 1971 Harriman el al 1981 )

DISCUSSION

The absorption spectra of TIPmiddotN02 and ZnTIPshyN02 are characterized by red shifts and in the case of the free base a change in the relative intensities of the Q-bands compared to TPP itself Similar behavior has been observed in 5-nitrooctaethylpormiddot phyrin (Dvornikov et al 1986) and the tendency of electron withdrawing substituents on the periphshyery of the porphyrin to cause shifts to longer waveshylengths of the visible and Soret bands is well known (Falk 1964) The triplet-triplet absorption bands of TIP-N02 and its zinc analog are also somewhat red shifted relative to the parent compounds The ET-EG value determined for ZnTIP-N02 is comparable to that found for ZnTIP given the relatively large errors inherent in such measurements

In emission however the nitro substituted porshyphyrins differ strikingly from their parent molecules In the first place their singlet lifetimes are substanshytially decreased The shorter lifetimes cannot be due only to enhanced fluorescence or intersystem crossing based upon the lttgtF and lttgtT values obtained and more rapid internal conversion by some mechanism must be involved These results suggest that the singlet emitting states of the nitroshysubstituted porphyrins are structurally significantly different from those of their normal porphyrin analshyogs Reduced fluorescence yields and lifetimes have also been observed in tetraarylporphyrins bearing nitro groups on the aryl rings (Harriman and Hosie 1981ab) and in 5-nitrooctaethylporphyrin and related materials (Dvornikov et al 1986) Formashytion of an intramolecular charge transfer state P~shyNO- was suggested by both groups as a possible explanation This is a reasonable suggestion and is presumably applicable to TIP-N02 and ZnTIPmiddot N02 as well

There is an additional aspect of the problem which is evident from the TIP-N02 data The emisshysion spectra are broad and unstructured and the red shift is strongly solvent dependent with shifts to longer wavelength in more polar solvents Tetramiddot arylporphyrins with nitro substitution on the aryl rings and also evidently 5-nitrooctaethylporphyrin have normal porphyrin emission bands with the expected fine structure (Harriman and Hosie 1981ab Dvornikov et al 1986) The unusual behavior of TPPmiddotN02 and its zinc analog is remishyniscent of that of aromatic amino compounds which form twisted intramolecular charge transfer (TICf) states (Rettig 1986 1988 and references cited therein Rotkiewicz el al 1973 Grabowski et al bull 1978 1979 Lippert et al 1987) In these molecules electron transfer from the amino nitrogen atom in the excited singlet state of the molecule to the aroshymatic system is accompanied by twisting of the amino group about its bond to the aromatic system so as to achieve an essentially perpendicular orienshytation of the 7T-electron system of the aromatic

825

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Photophysics of nitroporphyrins 423

(1987) For TIP-NO ErEG was 47000 7 000 M-I cm -I at 475 nm whereas for ZnTIPshy

N02 the value was 69 000 10 000 M-I cm -I at 500 nm Pekkarinen and Linschitz (1960) measured ET for ZnTPP in benzene by flash photolysis using the complete conversion method and obtained a value of 74 ()()() at 470 nm where the ground state does not absorb appreciably The triplet of TPP has a maximum which overlaps the Soret band of the absorption spectrum this can lead to relatively large errors in extinction coefficient and this factor commiddot bined with the spectral shift resulting from nitration makes comparison with the extinction coefficient obtained for TIP-N02 difficult

Triplet quantum yields lttgtT were obtained by the comparative method (Bensasson et al 1978 1983) using the following equation which is valid when the optical densities at the laser excitation waveshylength of solutions of the unknown and a reference molecule are both low and the same

lttgtPo = lttgtR x 10~~b x (ErEc)R T T 10M_G (ErEci)P0

In this equation lttgtt and lttgtyen are respectively the triplet quantum yields of the porphyrin being stud- ied and a reference material of known quantum yield lODt~h and lOD~_G are the changes in optical density at selected wavelengths following laser flash excitation (taken in domains where the optical density change is linear with laser power as determined by 5-7 measurements at different powers) and the (ET-Ed values are the respective triplet minus singlet difference extinction coefshyficients at the same selected wavelengths Because the main thrust of this work is comparison of the results for the nitro substituted porphyrins with those for the unsubstituted TPP and ZnTPP the reference material chosen was ZnTPP itself The triplet quantum yield of ZnTPP was taken as 088 (Harriman et al 1981 Dzhagarov 1970 Kalyanmiddot asundaram and Neumann-Spallart 1982) and (ErE() as 32 250 at 500 nm and 74 ()()() at 470 nm for this compound (Pekkarinen and Linschitz 1960) The TIP-NO and ZnTIP-N02 triplet absorptions were monitored at 475 and 500 nm and the extinction coefficients reported above were employed On this basis lttgtT values of 062 and 065 were calculated for TIP-NO and ZnTIP-NO respectively Thus the triplet quantum yield for ZJTP-N02 is reduced somewhat relative to ZnTPP whereas that for TIP-NO is about the same as that for the TPP parent compound in benshyzene (lttgt1 = 067 Bonnett el al 1988)

The lifetimes of the triplet states of TIP-NO and ZnTIPmiddotNO in 2-methyltetrahydrofuran glass at 77 K were I 7 and 2 I ms respectively as measured by the transient absorption method These are fairly typical lifetimes for porphyrins in organic glasses (Gradyushko and Tsvirko 1971 Harriman el al 1981 )

DISCUSSION

The absorption spectra of TIPmiddotN02 and ZnTIPshyN02 are characterized by red shifts and in the case of the free base a change in the relative intensities of the Q-bands compared to TPP itself Similar behavior has been observed in 5-nitrooctaethylpormiddot phyrin (Dvornikov et al 1986) and the tendency of electron withdrawing substituents on the periphshyery of the porphyrin to cause shifts to longer waveshylengths of the visible and Soret bands is well known (Falk 1964) The triplet-triplet absorption bands of TIP-N02 and its zinc analog are also somewhat red shifted relative to the parent compounds The ET-EG value determined for ZnTIP-N02 is comparable to that found for ZnTIP given the relatively large errors inherent in such measurements

In emission however the nitro substituted porshyphyrins differ strikingly from their parent molecules In the first place their singlet lifetimes are substanshytially decreased The shorter lifetimes cannot be due only to enhanced fluorescence or intersystem crossing based upon the lttgtF and lttgtT values obtained and more rapid internal conversion by some mechanism must be involved These results suggest that the singlet emitting states of the nitroshysubstituted porphyrins are structurally significantly different from those of their normal porphyrin analshyogs Reduced fluorescence yields and lifetimes have also been observed in tetraarylporphyrins bearing nitro groups on the aryl rings (Harriman and Hosie 1981ab) and in 5-nitrooctaethylporphyrin and related materials (Dvornikov et al 1986) Formashytion of an intramolecular charge transfer state P~shyNO- was suggested by both groups as a possible explanation This is a reasonable suggestion and is presumably applicable to TIP-N02 and ZnTIPmiddot N02 as well

There is an additional aspect of the problem which is evident from the TIP-N02 data The emisshysion spectra are broad and unstructured and the red shift is strongly solvent dependent with shifts to longer wavelength in more polar solvents Tetramiddot arylporphyrins with nitro substitution on the aryl rings and also evidently 5-nitrooctaethylporphyrin have normal porphyrin emission bands with the expected fine structure (Harriman and Hosie 1981ab Dvornikov et al 1986) The unusual behavior of TPPmiddotN02 and its zinc analog is remishyniscent of that of aromatic amino compounds which form twisted intramolecular charge transfer (TICf) states (Rettig 1986 1988 and references cited therein Rotkiewicz el al 1973 Grabowski et al bull 1978 1979 Lippert et al 1987) In these molecules electron transfer from the amino nitrogen atom in the excited singlet state of the molecule to the aroshymatic system is accompanied by twisting of the amino group about its bond to the aromatic system so as to achieve an essentially perpendicular orienshytation of the 7T-electron system of the aromatic

I

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moiety and the p-orbital of the nitrogen Thus the electron donor and acceptor orbitalsare essentially decoupled

The anomalous emission spectra of TIP-N02 and its zinc analog may be due to rotation in one direcshytion or another about the bond joining the nitro group to the porphyrin macrocyde in the leT state Support for this proposal is provided by the fact that the corresponding 2-cyano and 2-bromo derivashytives of TPP do not show such anomalous behavior These molecules have normal emission spectra at ambient temperatures with the expected fine strucshyture These two substituents are like the nitro group strongly electron withdrawing However they have conical symmetry and are thus incapable of demonstrating any effect from the proposed bond rotation

It is well known that the phenyl rings of TPP and related molecules are not coplanar with the porphyrin but form angles of 45-900 with the plane of the macrocycle (see for example Hoard 1971 1975 La Mar and Walker 1973 Dirks et al 1979 Chachaty et al 1984) The equilibrium dihedral angle is determined by the balance between the steric repulsions resulting from interactions with the ~-pyrrole hydrogen atoms and the stabilizing resonshyance interaction of the aromatic group with the porphyrin ring This being the case it is clear from molecular modeling that the nitro group of TIPshyN02 cannot be coplanar with the porphyrin macroshycycle in the ground state because of severe steric interactions with the adjacent aryl group The minishymum steric hindrance is achieved when the planes of the nitro group and the adjacent aryl ring make essentially the same angle with the plane of the macrocycle Thus the nitro group is already parshytially twisted out of plane in the ground state and this may favor bond rotation in the ICT state

The temperature dependence of the fluorescence spectrum of TIP-N02 is also consistent with the proposed bond rotation in the leT state At ambient temperatures in 2-methyltetrahydrofuran the porshyphyrin fluorescence is broad unstructured and red shifted and resembles emission from a TleT state At 77 K however the emission band has the fine structure characteristic of other porphyrins These observations are consistent with the formation of an ICT state at ambient temperatures with a rotated nitro group which is not accessible at low temperashytures in a rigid glass

Given that in the ground states the nitro groups of TIP-~02 and ZnTIP-N02 are twisted at angles greater than 00 but less than 900 with respect to the porphyrin plane there are two possibilities for the proposed bond rotation in the ICT state The nitro group could rotate perpendicular to the porphyrin plane to give a traditional TICT state with full charge separation Alternatively the nitro group might rotate in the other direction toward coplanarshyity with the porphyrin macrocyde This rotation

would increase the conjugation between the nitro group and the macrocycle but would likely require distortions from planarity of the macrocycle itself Either type of rotation in the leT state could in principle give rise to a more stable leT state and to broadening and red shifts of the emission spectra in polar solvents and could be inhibited in a lowshytemperature glass

With regard to the use of nitro substituted tetrashyarylporphyrins as components of covalently linked model systems for photosynthetic electron transfer it is clear that in general the singlet and triplet properties of these molecules compare favorably with those of other porphyrins which have been employed in such models The singlet lifetimes of ZnTIP-N02 and TIP-N02 are somewhat shorter than those of typical tetraarylporphyrins but are still long enough so that electron transfer from attached donors could easily compete with other decay pathways The proposed leT state could conshyceivably enhance the rate of electron transfer to the porphyrin macrocycle under some conditions because of the large amount of positive charge preshysent on this moiety in the excited state

Acknowledgements-We thank Dr Alfred R Holzwarth for providing us with the single photon counting data analysis software for the instrument at Arizona State Unishyversity This work was supported by the National Science Foundation (CHE-8515475 INT-85 14232 INT-8701662 D G and T A M) the Office of Basic Energy Sciences US Department of Energy (DE-FG02-86ERI3620 G R S) and the Cancer Research Campaign UK (E J L) This is publication II from the Arizona State Uni ersity Center for the Study of Early Events in Photosynthcsis The Center is funded by US Department of Energy grant no DE-FG02-88ERI39 as part of the USDADOE NSF Plant Science Center program

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Bensasson R V and E J Land (1971) Triplet-triplet extinction coefficients via energy transfer Tram Farashydal Soc 67 19lt1-1915

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Bonnell R D J McGarvey A Harriman E J Land T G Truscoll and I-J Winfield (19l8) Photophysical properties of mlJ()tclraphenylporphyrin and some meso-tetra( hydroxyphenyl )porphyrins PholOcltem PhOiohiol 48 271-270

UEVENS LJUST et al

Rotkiewicz K K H Grellman and ZR Grabowski radicals ~5 Reactions of zinc tetraphenylporphyrin catshy(1973) Reinterpretation of the anomolous fluorescence ion radical perchlorate with nucleophiles J Org Chern of p-NN-dimethylaminobenzonitrile Chen Phys Lett 44 4069--4075 19 315-31R 21 212 Wendler J and A R Holzwarth (1987) State transitions

Seybold P G and M Gouterman (1969) Porphyrins in the green alga Scenedesrnlls Obliquus probed by timeshyXIII fluorescence spectra and quantum yields J resolved chlorophyll fluorescence spectroscopy and gloshyMolec Spectrosc 31 1-13 bal data analysis Biophys J 52 717-72R

Shine H L A G Padilla and S-M Wu (1979) Ion

- lt --~1 middot1middotmiddotmiddotmiddotmiddot~yen30QtQP amp00 jJQ amp 2 14l( d22 rb