10521
J. Phys. Chem. 1993,97, 10521-10523
A Time-Resolved Resonance Raman Study of the TI Excited State of Zinc(I1) Octaalkylporphyrins Ranjit Kumble, Songzhou Hu, Glen R. Loppnow, S. E. Vitols, and Thomas G. Spiro' Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received: August 23, 1993'
The T1 excited states of zinc(I1) octaethylporphyrin (ZnOEP) and zinc(I1) etioporphyrin I (ZnEtio) have been studied by nanosecond time-resolved resonance Raman spectroscopy. The spectra reveal identical features for both compounds, which we attribute to the lowest-lying triplet state. Slight downshifts of CBCB stretching modes v2 and u1I are seen from the ground-state (SO) values, determined by an analysis of /3-l3C-1abeled zinc(I1) etioporphyrin I. The magnitude of these shifts are smaller than those observed for the same modes in the T1 state of zinc(I1) tetraphenylporphyrin (ZnTPP), consistent with expected differences in electronic configuration between the TI states of meso-tetraphenyl and 0-octaalkyl substituents [3(a2ueg)vs 3(alueg), respectively]. The direction of frequency shifts seen for these modes is opposite to that observed for the T1 state of free-base octaethylporphyrin (HzOEP); we speculate that these differences result from a Jahn-Teller effect occurring in the T1 states of ZnOEP and ZnEtio. None of the modes observed showed sensitivity to meso-d4 substitution, implying that they do not contain significant C,-H stretching or bending character.
The excited states of porphyrins and hydroporphyrins (chlorophylls and bacteriochlorophylls) act as primary intermediates in processes ranging from biological energy transfer' and electron transfer2 to photodynamic therapy3 and photo~atalysis.~The low-lying (T,T*)excited states of these compounds have been described by a four-orbital model.5 The electronic properties of thesestates have been studied by a number of techniques including static and time-resolved emission and absorption spectroscopies,a magnetic resonance spectroscopy? and resonance Raman spectroscopy.'&+ Time-resolved vibrational studies of singlet and triplet (T,T*)and (d,d) excited states of metalloporphyrins have provided insight into the nature of structural changes accompanying formation of these states.1Oa-e The structural distortions reflect changes in the distribution of electron density characteristic of the new electronic configuration as well as symmetry-lowering processes arising due to the Jahn-Teller effect.5 The structure of the TI excited state of meso-substituted Zn(I1) tetraphenylporphyrin (ZnTPP) has been characterized by nanosecond time-resolved RR spectroscopy;lObSC &alkylsubstituted Zn( 11) porphyrins are observed to have different triplet emission properties'l and sublevel decay rates,'2 and accordingly it has been proposed that theT, electronicconfigurationis different from that of meso-tetraphenylporphyrins. Studies of &substituted metalloporphyrins are particularly important in recognition of the exclusive location of substituentsat these positions in naturally occurring chromophores. We report RR spectra of the T I excited states of two 0-octaalkylporphyrins: zinc(I1) octaethylporphyrin (ZnOEP) and zinc(I1) etioporphyrin I (ZnEtio). In ZnEtio, alternating ethyl substituents are replaced by methyl groups (Scheme I). ZnOEP and ZnEtio were purchased from Midcentury Chemical Co. (Posen, IL) and used without further purification. The meso-deuterated dd-H,OEP was prepared from natural abundance H 2 0 E P by exchange in *H2SO4 (99% H2) according to the proceduredescribedin ref 13. The (2,7,12,17-'3C)-etioporphyrin I ( p ' C H 2 E t i o ) was synthesized from ethyl acetoacetonate-(3'3C) following the procedure of Momenteau et aL'4.15 Zinc insertion was performed by standard procedures.l3 The nanosecond time-resolved RR apparatus has been described previously.'Ob Ground-state RR spectra (recorded with 436-nm laser pulses) of ZnOEP, d4-ZnOEP, ZnEtio, and (@-13C)-ZnEtioare
* To whom correspondence should be addressed.
Abstract published in Advance ACS Absrracrs, October 1, 1993.
SCHEME I
ZnOEP : R, = R2 = -C2H5
ZnEtio : R1= -CH, R2 = -C2Hrj
shown in Figure 1; the bands are labeled with reference to the previous assignments for nickel(I1) octaethylporphyrin (NiOEP).I6 As expected, strong enhancement of totally symmetric porphyrin skeletal modes is seen along with weak enhancement of certain nontotally symmetric and substituent modes. Similar frequencies are observed for ZnOEP and ZnEtio, indicating that the ethyl/ methyl replacements do not constitute a significant perturbation in this case. Deuteration at the meso positions of ZnOEP results in a 13-cm-' downshift of the C,C, stretching mode YIO. Substitution of I3C at a single 0-position of each pyrrole ring in ZnEtio results in significant downshifts of the symmetric and nontotally symmetric C& stretching modes u2 and (28 and 23 cm-I, respectively). The vibrational mode u5 (C,-Co + C r C,) is seen at 1135 cm-l in ZnOEP and 1133 cm-' in ZnEtio: it is observed to downshift by 13 cm-1 in (@-W)-ZnEtio. RR spectra of the TI excited state obtained by subtraction of ground-state features from pump/probe spectra are shown in Figure 2. The triplet-state/ground-state ratio is estimated to be 60% at a delay of 100 ns following photoexcitation; this time delay was chosen due to the 20-11stime jitter between pump and probe pulses. Excited-state features are not observed at time delays exceeding 1 gs. Most of the bands appear at the same frequencies for ZnOEP and ZnEtio, indicating that their triplet
0022-3654/93/2097-10521~04.00/00 1993 American Chemical Society
Letters
10522 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 X
T IExcited States
Ground Slates
7'
R
c 4
I I
C.
0
N,
I -
800
1000
1200
1400
1600
800
1000
I200
1400
1600
Raman Shift ( c m - I )
Raman Shift (cm-l )
Figure 1. Ground-state RR spectra of &substituted zinc(I1) octaalkylporphyrins (NA = natural abundance) using 436-nm probe laser pulses ( 7 ns fwhm, 0.08 mJ/pulse at 10 Hz) focused on a 0.5 mM solution (in tetrahydrofuran)contained in an airtight spinning cell. Scatteredradiation was dispersed by a SPEX triple spectrometer ( 1 800 grooves/mm grating) and detected with an optical multichannel analyzer (Princeton Instruments). Asterisks mark artifacts arising from subtraction of the THF
solvent features.
-
states have the same structures. In both spectra the highestfrequency feature is a broad band centered at 1560 cm-l(l559 cm-I for ZnOEP and 1564 cm-1 for ZnEtio) which shifts down by 22 cm-I for (@-13C)-ZnEtio.We therefore assign this to the excited-state C&,+ stretching modes; the broad width suggests that 1 2 and uII both contribute to this band. The remaining features are insensitive to the available isotopic substitution, and their assignment is therefore uncertain. The 1320-cm-1 band is suggested to arise from CH2 wagging modes, which appear at the same position in thegroundstate. Theabsenceofanyddsensitivity in the ZnOEP TI spectrum implies that none of the modes that involve or interact with C,-H bending or stretching are resonance enhanced. In particular, the v10C,-C,stretching mode expected in the 1600-cm-1 region is not seen, although it is observed in the TI RR spectrum of ZnTPP. The TI states of ZnOEP and ZnEtio on the one hand, and ZnTPP on the other, have different electronic configurations, 3(alueg)and 3(a2,eg),respectively.12 To zeroth order, the structural changes upon photoexcitation are expected to be the result of the changes observed upon radical cation ( 2 A ~and , 2A2u,respectively) and radical anion (2Eg) formation. To these changes must be added the Jahn-Teller distortion arising from single occupation of the degenerate es(r*) orbitals;17thisdistortion may be different for the TI state and the radical anion. In ZnTPP the u2 (AIg) C r C , +stretching mode downshiftlobgcfrom SOto TI, 40 cm-1, was close to the sum of the downshifts for the radical cation (32 cm-l)20and anion (16 cm-I) formation,lobreflecting the expected reduction in CFCB bond order upon extracting an electron from the azu orbital (C,&,+ bonding) and placing it in the eg orbital
Figure 2. TIexcited-state RR spectra for (a)-(d): ZnOEP, meso-d4-
ZnOEP, ZnEtio and (@-13C)-ZnEtio@-I)C.Pump/probe spectra were recorded using 532-nm photoexcitation (7 ns fwhm, 0.4 mJ/pulse at 10 Hz) and 436-nm probe (same as Figure 1 ) at delay times of -100 ns (yielding ground-statefeaturesonly) and +100ns (yielding featuresfrom ground state + TIexcited state). Ground-state features were then subtracted to yield triplet-state RR spectra, using a subtraction factor sufficient to avoid negative features. Asterisks mark artifacts arising from subtraction of the THF solvent features.
(C& antib~nding).'~The YIO (BI,) frequency, on the other hand, is unaffected in the radical cation but decreases by 19 cm-I in the radical anionlob and 48 cm-I in the TI state,Iobcsignaling a Jahn-Teller distortion about the C,-C, bonds. The pattern for ZnOEP is less clearcut, since v10 is not enhanced in the TI state and the u2 shift is obscured by overlap with uIl. It is known, however, that the u2and YI1 frequencies both increase upon radical cation formation (by 19 and 28 cm-I, respectively)2I due to the C r C , +antibonding character of the al, orbital and appear to be unaffected in the radical anion.22 Thus, an increase in 19and u1I frequencies might have been expected in the TI state and this is not observed. Instead, there is a downshift, by as much as -25 cm-I in u2 and/or as much as 5 cm-l in VI\; these shifts, are however,smaller than the v2 So-TI downshift in ZnTPP (40 cm-l).lob+cWe speculate that the Jahn-Teller effect is responsible for the lack of the expected v2 and/or V I I upshift. In support of this suggestion is the observation by Sato et al. that v2 does shift up, by 14 cm-I, in the TI state of H20EP.'OB The protons in HzOEP lift the degeneracy of the es(ir*) orbitals, thereby eliminating the Jahn-Teller effect. Further isotopic and depolarization experiments are planned to assign the remaining bands in the ZnOEP and ZnEtio TI spectra. Their lack of @ - I T or meso-do sensitivity suggests substantial involvement of the ethyl (and methyl) substitutent modes. The ZnTPP TI RR spectrum is dominated by bands due to the phenyl modes,l0bSc occurring at essentially the same frequencies as in the SOstate. For ZnOEP, however, theT, bands are shifted substantially from candidate substituent frequencies (except for the CH2 wag),*6 suggesting appreciable electronic involvement of the substituents in the TI state.
Letters
Acknowledgment. This work was supported by DOE Grant DE-FG02-93ER 14403. References and Notes (1) Sauer, K. Annu. Rev. Phys. Chem. 1978,30, 155. (2) Parson, W. W. Biochim. Biophys. Acta 1968,153,248. (3) Kessel, D. Photochem. Photobiol. 1984,39, 851. (4) Harriman, A. J . Chem. SOC.,Faraday Trans. I 1980,76, 1978. (5) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978;Vol. 111, Chapter 1 and references therein. (6) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M. Coord. Chem. Rev. 1982,44,83. (7) Holten, D.; Gouterman, M. In Optical Properties and Structure of Tetrapyrroles; Blauer, G.,Sund, H., Eds.; Walter D. Gruyter: New York, 1985;pp 63-88. (8) Dzhagarov, B. M.; Chirvonyi, V. S.; Gurinovich, G. P. In Laser Picosecond Spectroscopy and Photochemistry of Biomolecules; Letokhov, V. S., Ed.; Adam Hilger: Philadelphia, 1987;Chapter 3. (9) van der Waals, J. H.; van Dorp, W. G.;Schaafsma, T. J. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979;Vol. IV, Chapter 5. (10) (a) Sato, %-I.; Asano-Someda, M.; Kitagawa, T. Chem. Phys. Lett. 1992,189,443.(b) Reed, R. A.; Purrello, R.; Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991,95,9720.(c) Walters, V. A.; de Paula, J. C.; Babcock,
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