J. Phys. Chem. 1995,99, 3959-3964
3959
Resonance Raman Spectra of the Triplet State of Free-Base Tetraphenylporphyrin and Six of Its Isotopomers Steven E. J. Bell,* Ala H. R. Al-Obaidi, Martin J. N. Hegarty, and John J. McGarvey School of Chemistry, The Queens University of Belfast, Belfast BT9 JAG, U.K.
Ronald E. Hester Department of Chemistry, University of York, York YO1 5DD, U.K. Received: July 28, 1994; In Final Form: January 3, 1995@ The resonance Raman spectra of the ground state and the lowest excited triplet state of free-base tetraphenylporphyrin and six of its isotopomers have been obtained using two-color time-resolved techniques. Ground-state spectra were recorded using low-energy 447 nm probe laser pulses, and triplet-state spectra were probed, with similar pulses, 30 ns after high-energy excitation with 532 nm pump pulses. Polarization data on both the ground and triplet states are also reported. The resonance Raman spectrum of the triplet is very different from that of the ground state but the combination of extensive isotope substitution with polarization data allows bands in the ground state to be assigned and corresponding bands in the triplet state to be located. Isotope shifts of the same bands in the SO and TI states are similar, implying that the compositions of the vibrational modes do not change significantly on excitation. Two of the strongest bands in the TI spectra are associated with phenyl ring substituents; these are shifted less than 5 cm-I between the SO and T I states so that bonding in the phenyl substituents is barely affected by excitation to the T I state. The changes in position of the porphyrin ring bands are larger, but still only tens of cm-I or less, the main changes in the spectra being due to differences in relative band intensities in the two states. The relatively small shifts in the porphyrin ring band positions which are observed show that the excitation energy is not localized on a single small region of the molecule but is delocalized over the entire porphyrin skeleton. This picture of an excited species with high chemical reactivity, but with individual bonds only slightly perturbed from the ground state, is contrasted with molecules, such as benzophenone, where excitation causes a large perturbation in the bonding within a single functional group.
Introduction The potential of time-resolved resonance Raman (TR3) spectroscopy to probe the structure of transient species has been recognized for over a decade.’ However, despite this potential, the literature contains relatively few examples of TR3 studies in which the vibrational bands have been unambiguously assigned using extensive isotopic substitution. Most of the systems where this has been done are representative molecules from important classes of materials. Notable examples are biphenyl,2 ben~ophenone,~ and carotenoid^.^ In many cases, such as in studies of meta-polypyridyl complexes with novel ligands, the essential information (for example the degree of electron localization in the excited states) can be derived from simple fingerprinting experiments without the need for detailed vibrational analy~is.~For such systems, extensive isotopic labeling of each new complex would require considerable effort so that studies have concentrated on the generic 2,2’-bipyridyl ligand.6 Recently, TR3 techniques have been applied to the excited states of free-base porphyrins and metalloporphyrins, which again constitute an important class of photoactive molecules. Already it is clear that the spectra of the excited-state species are sufficiently distinct from their ground-state precursors that they will be useful for simple “fingerprinting” purposes, where they may, for example, be used to determine which species are present following photoexcitation of complex mixtures of porphyrins and redox-active species such as quinones or viologens. Only one excited-state metalloporphyrin has been treated in detail; this is Zn(TPP) (where TPP = tetraphenylporphyrin). @
Abstract published in Advance ACS Abstrucrs, March 1, 1995.
There are two reports in the literature’s8 of the resonance Raman spectra of the excited triplet (TI) state of Zn(TPP), Zn(TPP)d20, in which the phenyl rings are deuterated, and Zn(TPP)-dg, where the protons on the 8, carbons are substituted. Despite some disagreements been the two reported data sets this work establishes Zn(TPP) as a good representative of the closed-shell metalloporphyrins. The other major class of porphyrins which are widely used as photosensitizers are the free-base materials, in which two opposite of the four central nitrogen atoms are protonated. These systems have an idealized D2h symmetry, as opposed to the idealized D4h symmetry of the complexes. This symmetry lowering has a marked effect on the UV/visible absorption spectra and similarly perturbs the ground-state resonance Raman spectra. Two groups have reported preliminary resonance Raman spectra of the lowest excited singlet and triplet states of the free-base form of TPP and made tentative assignments of the observed vibrational mode^.^.'^ These assignments were based both on arguments by analogy with the published data on triplet-state Zn(TPP) and on the expected changes in bond strengths which can be estimated from simple pictures of the molecular orbitals involved in the ground-to-excited-statetransition. A similar approach has been applied to the excited states of free-base octaethylporphyrin.Il Our ultimate goal is to use Raman spectroscopy to probe the structures of the excited states and, at least in part, to test the molecular orbital pictures which have already been derived for these systems. As a first step in this process we have undertaken a more detailed study of the vibrational spectra of the excited triplet states of free-base TPP and several of its isotopomers (see Scheme 1 for the nomen-
0022-3654/95/2099-3959~09.0010 0 1995 American Chemical Society
Bell et al.
3960 J. Phys. Chem., Vol. 99, No. 12, 1995 SCHEME 1: Nomenclature of the Isotopomers Investigated
a
b
.u x cn
clature of the isotopomers investigated). This approach allows us to assign the vibrational modes in the excited-state spectra without the need for guidance from molecular orbital calculations and should therefore allow the vibrational data to be used as an independent test of the validity of the molecular orbital models.
c
c e,
Y
C
- d
e
Experimental Section Natural abundance TPP, hereafter nu-TPP, and its isotopomers TPP-ds, TPP-d2o, and I3C4-TPP were prepared by condensation of appropriate isotopically substituted pyrrole and benzaldehyde using a standard method.I2 Pyrrole-& used in the preparation of TPP-d8 and benzaldehyde-d6 used in the preparation of TPPd20 were prepared by the method of Fajer et u1.I3 Benzaldehydedg was prepared from toluene-ds (Aldrich) by oxidation with cerium(1V) nitrate. I3C4-TPP was prepared from 13C-benzaldehyde (M.S.D. Isotopes). Isotopic purity was determined by mass spectrometry and NMR and was found to be '95% for all the above compounds. TPP-d2, TPP-dlo, and TPP-d22 were prepared from nu-TPP, TPP-ds, and TPP-d20 by isotopic exchange reactions carried out in situ. In this process, samples were dissolved in tetrahydrofuran which had previously been dried and then treated by addition of ca. 2% w/v of D20. Isotopic exchange of the central protons with deuterium from the solvent was complete in less than one hour. TR3 spectra were obtained using two pulsed Nd:YAG lasers (Spectra Physics DCR2 and GCR3) with a pump wavelength of 532 nm and probe wavelength of 447 nm (second Stokes Raman shifted from 355 nm in CH4). Pump/probe time delay was controlled using a digital pulse generator (Stanford Research Instruments DG 535); timing jitter was measured with a fast response photodiode and was found to be 5 2 ns. The pump and probe pulses were 7-9 and 5-7 ns duration, respectively. The triple spectrometer and gated diode array detection system have been described previously." Samples were studied as approximately 5 x mol ~II-~ solutions and were circulated through a 1 mm internal diameter glass capillary at a rate sufficient to ensure that each pump/probe pulse pair encountered a fresh volume of sample. Raman scattering was collected at 90" to the excitation beam.
Results Figure 1 shows the resonance Raman spectra in the 97016.50 cm-I region of a series of isotopomers of TPP, obtained using low-energy probe laser pulses only. The spectra agree well with literature data for the ground state (SO)of na-TPPI5 and with ground-state spectra of the isotopomers, which we have obtained using CW 457.9 nm excitation from an Ar+ laser (spectra not shown). Interfering solvent bands have been removed by subtracting a pure solvent spectrum scaled to the strong 914 cm-' solvent band. Figure 2 shows representative polarization data for the nu, d ~and , 4"' isotopomers in their
f
9
1000
1200
1400
1600
Wavenumber /cm-'
Figure 1. Resonance Raman spectra of ground-state (a) nu, (b), dz, (d) ds, (d) dlo, (e) d20. (0d22. and (g) I3C4-TPPin THF. Solvent bands have been removed from the spectra as described in the text.
ground states. Again the spectra were obtained with probe laser pulses only and have had pure solvent spectra obtained with parallel and perpendicular polarization digitally subtracted. Figure 3 shows the pump-and-probe spectra of the same series of isotopomers as are shown in Figure 1. These were obtained in pump-and-probe experiments with a time delay of 30 ns and again have had both solvent and residual ground-state bands digitally subtracted, as previously described.I0 Figure 4 shows polarization data obtained in pump-and-probe experiments on nu-TPP and TPP-ds. Again solvent bands and ground-state contributions obtained with parallel and perpendicular polarization have been subtracted as in Figure 2.
Discussion No extensive isotopic substitution data have been reported for free-base TPP in its ground electronic state and for this reason we have reproduced, in Figure 1, representative groundstate spectra of all the isotopomers studied. Although no normal-mode calculations are available for free-base TPP, they are available for both free-base porphineI6 and Ni(TPP),I7 which considerably simplifies the assignment of TPP modes; throughout this discussion the mode numbering from ref 16 will be used for the porphine skeleton with appropriate labeling for phenyl ring substituents taken from ref 17. In general, most of the bands are straightforward to assign, since they lie close to corresponding bands in the well-characterized nickel and freebase systems and also show similar isotope shifts. Since freebase TPP has idealized D2h symmetry, polarization data can be used in some cases to resolve overlapping bands. In this symmetry A, modes should have depolarization ratios '18 < e < 314 while Bzg modes should be depolarized with 314 < Q < -.I6 The data shown in Figure 1 and the results of depolarization measurements are brought together in Table 1, which
Triplet State of Free-Base Tetraphenylporphyrin
I *'
J. Phys. Chem., Vol. 99, No. 12, 1995 3961
n
I
1000 I
1200
,
1600
Wovenumber /cm-'
Figure 2. Polarized resonance Raman spectra of ground-state (a) nu, (b) d8 and (c) I3C4-TPPin THF. For each pair the top spectrum is recorded with parallel polarization and the bottom is recorded with perpendicular polarization. Solvent bands have been removed by digital subtraction of pure solvent spectra recorded under appropriate polarization conditions.
lists the band positions and assignments of ground-state TPP and a series of its isotopomers along with approximate mode descriptions. In this work the ground-stqte data are used only for comparison with excited-state features. Ground-state spectra were recorded at the same probe wavelength as was used in the pump-and-probe experiments. For a complete vibrational analysis of the molecule, a range of excitation wavelengths would be necessary but the present data set is adequate for our purposes. Few of the assignments of the ground-state spectra need much comment except to observe that the relative intensity of the peak at ca. 10oO cm-' compared to other bands in the spectra depends strongly on the nature of the isotopic substitution. Its intensity decreases on going from nu-TPP to the d2 and d8 isotopomers, falling to a minimum in the dlo isotopomer. The most obvious explanation for the loss in intensity would be that the band has two components which move with respect to each other in the various isotopomers, exactly overlapping in some spectra and lying beside each other in others. This spectral region is known to be extremely complex but, since the polarization data (Figure 2a) show that the band is polarized with e = I/d, any possible pair of components would each need to have A, symmetry. This excludes many of the possible assignments. The only A, porphyrin ring modes expected in this region are Y6 and ~ 1 5 . We assign the band at 963 cm-I in nu-TPP to Y15; it lies 40 cm-' lower than Y6, similar to the 36 cm-I difference found for Hz(porphine).i6 This leaves v6 which would be expected to be shifted to lower cm-' in the d2, d8, and dlo isotopomers just as is observed. Second possible components lying close
1200
1400
I
1600
Wovenumber /cm-'
Figure 3. Resonance Raman spectra of triplet state (a) na, (b) d?, (c) dg, (d) dlo, (e) dzo. (f) d22, and (g) 13C4-TPPin THF. Solvent and
residual ground-state bands have been removed from the spectra as described in the text. to v6 are & and 48, which are internal phenyl ring modes.I7 However, r#q and 48 should be sensitive to deuteration of the aryl rings while the 1003 cm-I band is not. Since we can find no evidence for two components, we are forced to conclude that the 1003 cm-I band is Y6 and that the relative intensity changes arise from Fermi resonance with an overtone or combination band which is strongest in the na material and decreases as v6 shifts away from its position in the other isotopomers. The only other bands needing comment are those at ca. 1550 cm-I which are resolved in the d8, dlo, and I3C4 isotopomers but which are not resolved in the natural abundance, d2, or d20 spectra. The polarization data clearly show that the strongly polarized v2 band is shifted to lower wavenumber in the d8 and dlo isotopomers while Y I O is sensitive to I3C4 substitution, as is expected for a mode with a large Ca-C, component. This assignment means that the position of Y I O is 54 cm-' lower in free-base TPP than in free-base porphine. The shift is much larger than is observed for any other porphyrin ring modes between these two systems but entirely consistent with the shift of 54 cm-l to lower wavenumber that has already been reported for v10 in the metallated (Ni) forms of the porphine and TPP.I7 The pump-and-probe experiments were designed specifically to probe the lowest excited triplet (TI) state of TPP. This state forms via intersystem crossing from the lowest excited singlet (SI)state, which in our experiment was generated by optical pumping at 532 nm.I8 The pump-probe time delay of 30 ns was chosen to be considerably longer than the lifetime of the SI state to eliminate the possibility of probing the initially formed SIexcited state. The 30 ns delay is still considerably shorter than the lifetime of the TI state, which we have previously measured both by transient UVhisible absorption spectroscopy and TR3 to be ca. 2 ,us under these conditions. The TI state formed by the pump pulse would therefore still be
Bell et al.
3962 J. Phys. Chem., Vol. 99, No. 12, 1995 7-
1200 Wovenumber /cm-
1600 1
Figure 4. Polarized resonance Raman spectra of triplet-state (a) na and (b) TPP-d~in THF. For each pair the top spectrum is recorded with parallel polarization and the bottom is recorded with perpendicular polarization. Solvent bands have been removed by digital subtraction of pure solvent spectra recorded under appropriate polarization condi-
tions. present as the major component when the probe pulse encountered the sample. The 447 nm probe wavelength was chosen to fall within the strongest absorption band of the triplet state,I9 to maximize resonance enhancement of the transient Raman scattering. Even at this probe wavelength, which is near a minimum in the ground-state absorption spectrum, the Raman scattering cross sections of the strongest bands in the groundstate spectra are comparable to those of the T I state and, although the degree of conversion achieved in our experiments was high (ca. 80-90%), it was not possible to achieve 100% conversion to the excited state. As discussed previously,10the pump-and-probe spectra still contain some residual ground-state bands. These can readily be removed by careful subtraction of a scaled multiple of the probe-only spectrum where the scaling factor is chosen so that no negative bands appear at the positions of any of the ground-state features. The data shown for the isotopomers in Figure 3 have all been treated in this fashion.
Triple-State Assignments The much lower signal-to-noise levels in the pump-and-probe experiments make vibrational mode assignment more difficult than for the ground state. In essence the excited-state assignments depend on the assumption that the band shifts accompanying a given isotopic substitution will be similar in both the ground and TI states even if the absolute positions of the bands are different. This will only be true if the compositions of the vibrational modes are similar in both states. On the basis of previously published data for Zn(TPP),7s8the assumption that mode compositions are not seriously perturbed on excitation does seem to hold and the assignments (see below) for the
triplet-state bands of free-base TPP are entirely consistent with this view. The polarization data can also be used to assist in the assignment of vibrational modes. Such an approach is strictly valid only if the symmetry of the TI state is the same as that of the ground state but again this approximation does appear to hold. The resonance Raman spectra of the TI states of the isotopomers of free-base TPP are much richer in bands than those of the TI state of Zn(TPP). The most straightforward bands to assign are those associated with the aryl ring substituents, which are shifted by tens of wavenumbers in the d20 and d22 isotopomers but are almost unchanged in the other isotopomers. Strong bands in the na material at 1600 and 1233 cm-I are associated with an internal phenyl ring mode $4, and vi, which is a C,-phenyl ~ibrati0n.l~ These bands are close to their ground-state positions and are shifted by 38 and 51 cm-I, respectively, in both the d20 and d22 isotopomers. These isotope shifts are similar to those seen in the ground state of TPP (see Table 1) and Ni(TPP).I7 $4 is much stronger in the T I spectra than in the SO spectra; a similar effect has been observed for Zn(TPP).8 Semiempirical calculations of H2TPP suggest that this increase may be due to increased phenyl ring character in the orbitals involved in the TI-T, transition of the T I state.*O An additional phenyl ring mode is also present in the tripletstate spectra of the d20 isotopomer and, more clearly resolved in the spectra of the d22 isotopomer, at 1391 cm-'. This mode is assigned as $ 5 , which lies at 1399 cm-I in Ni(TPP)-dlo. $5 lies at 1510 cm-' in na-Ni(TPP)I7 and should therefore be detectable around this position in the T I spectra of nu-TPP and the other isotopomers where the phenyl rings are undeuterated. However, this spectral region also contains a porphyrin ring mode at ca. 1500 cm-I, clearly visible in the d2o spectrum, so that unambiguous detection of the phenyl ring mode in the spectra where the phenyl rings are undeuterated is not possible. The only other band in the T I state spectra which involves the phenyl ring substituents is v27, at 1308 cm-I, which is the B,, counterpart of v1 (Le., a C,-phenyl stretch). This mode is not observed in the ground-state (probe only) spectra recorded with 447 nm excitation, where the totally symmetric bands dominate the spectra, but it is readily assigned on the basis of its position and characteristic sensitivity to phenyl ring deuteration. The appearance of an enhanced ~ 2 7band has previously been observed for the T I state of Zn(TPP).* In assigning the porphyrin ring modes of the TI state, we began by looking initially for the totally symmetric A, ring vibrations in the expectation that, since we are in resonance with a strong electronic absorption band, "A' term enhancement is likely to be dominant. The isotope shifts for all the groundstate A, modes in this spectral region were available from the probe-only spectra, where the totally symmetric ring vibrations dominate the spectra. Only after assigning all of the A, modes visible in the T I spectra did we turn to lower symmetry Blg modes, which account for a much smaller number of bands in both the ground and T I spectra. Even for these non-totally symmetric vibrations, no modes appeared in the excited-state spectra which were not visible in ground-state spectra. Indeed, with the exception of the two modes involving the phenyl ring substituents which were discussed above, all the bands in the TI spectra also appear in the ground-state spectra, although their relative intensities are completely different. Characteristic shifts on dg substitution allowed the C D - C ~ stretching vibrations v2 and v1 1 to be readily identified although in both the dg and dlo spectra this region is complicated by the appearance of a broad polarized feature which we have not been able to assign. vf, and vl5. which are ring breathing modes
J. Phys. Chem., Vol. 99, No. 12, 1995 3963
Triplet State of Free-Base Tetraphenylporphyrin
TABLE 1: Band Positions and Mode A~signmentslh~~ of Ground- and Triplet-State na-TPP and Its Isotopomers ground state triplet state na d2 dg dlo d20 d22 I3C4 nu d2 ds dio d20 d22 I3C4 1600 1604 1603 1602 a a 1602 1600 1604 1599 1600 1562 1562 1599 44,phenyl 1555 1555 1537 1535 1556 1553 1547 1540 1544 1511b 1511b 1542 1540 1539 vz,Cp-Cp(pyr) 1556 1555 a 1531 CaCm a a a ~~
~
vi03
1502 1465
1501 1447
1461 1461
1461 1461
-
-
-
-
1362
1358
1351
1352
1501 1459
-
1360
1499 1454
-
1493 1442 -
1353
1358
-
-
-
-
-
-
-
-
-
-
1296 1238 1003 963
1294 1236 998 959
1295 1235 998 957
1293 1235 994 953
1296 1184 1001 965
1291 1182 994 962
1295 1230 999 959
-
-
1500
1492
148ob
-
-
1369' 1373' 1308 1261 1233 1018 959
1364 1364" 1294 1261 1232 1008 948
-
1366
14606
-
1361
-
-
1302 1257 1232 1010 954
1297 1254 1231 1005 951
1499
1487
1492
-
-
-
1391 1376 1376"
1390 1357 1357"
-
-
-
1252 1182 1017 956
1256 1182 1007 954
136W 137W 1289 1253 1226 1015 954
~li,CpC/j v3, CaCm 45. phenyl v4, Pyrhalf-ring ~26,CpH' v27, C,-phenyl v12,Pyrhalf-ring v~,C,-phenyl v6,Pyrbreathing v15,Pyrbreathing
a Assumed to be lying under a stronger band. Obtained by curve fitting to a broad unresolved feature; checked by polarization measurements. Resolved from overlapping band by polarization measurement. Tentative assignment.
d2
A cm-1
d8
d10
a20
d22
C13
Excited-State Structure
-20 -30 -40
-50
-60 d2
d8
d10
d20
d22
C13
0 -2
(b)
A cm-l
made for bands in the TIspectra are compared with those of the ground state in Table 1.
-4 -6 -8 -10 -12 -14
Figure 5. Bar charts illustrating the isotope shifts of (a) V I and (b) V6 from their position in nu-TPP. Solid bars show shifts in the ground state and hatched bars the triplet state. involving the pyrrole and pyrrolenine rings, were sensitive to deuteration at both the Cp and pyrrole ring N atoms.16 The only ambiguous region of the spectrum is around 1350-1390 cm-l where a strong and broad feature is observed. This band changes significantly both in intensity and in shape between the various isotopomers. As discussed above, part of the reason for these changes is that in the d20 and d22 isotopomers a phenyl ring mode (&) moves down to cu. 1390 cm-' and, since it is only clearly resolved in the spectrum of the d22 isotopomer, in the d20 isotopomer it merely broadens even further the feature around 1370 cm-l. However, even in the isotopomers where the phenyl rings are undeuterated, and q55 is at much higher wavenumber, significant differences between the isotopomers are still observed. Polarization data show that one of the components in the band is the strongly polarized v4 pyrrole halfring stretching mode at 1369 cm-I,l6 while we tentatively assign a second component at 1373 cm-I as v26. v26 is Strongly d8 sensitive (it involves bending of the Cp-H bond) and this can account for the loss in intensity of the band at cu. 1370 cm-l in the d8 and dlo isotopomers. The change in band shape in the I3C4isotopomer, where neither v4 or v26 would be expected to change significantly from their positions in the nu material, suggests that a further component contributes to the band at cu. 1370 cm-l, but our data do not allow us to speculate on the nature of a third feature which we cannot resolve even in our data set which covers the seven isotopomers, together with polarization measurements. The assignments which we have
At first sight, the resonance Raman spectra of the T I states of na-TPP and its isotopomers are very different from those of their ground-state counterparts. A combination of changes in the relative intensities of the various vibrational modes and their wavenumber positions acts to make few of the vibrational modes of the TI state species recognizable as ground-state counterparts; V I is really the only band which is easily recognized in both states. Nonetheless, it is possible to trace essentially equivalent bands from the ground to excited states. This is because not only do the absolute positions of the bands in the spectra move by only a few tens of wavenumbers at most upon excitation but also because the band shifts on isotopic substitution are similar for both states. Figure 5 compares the isotope shifts of two modes, one a porphyrin ring mode and the other associated with the phenyl ring substituents, through the various isotopes studied. It is clear from these comparisons that the mode composition of the phenyl band, as judged by the isotope shifts, is very similar in both the ground and T I states. In the case of the porphyrin ring mode the agreement is less conclusive because isotopic substitution gives only small band shifts in the isotopomers. This increases the relative importance of the experimental errors in the band positions, particularly in the T I spectra, where we estimate the accuracy of even well-resolved bands to be f3 cm-'. Nonetheless, even for this mode there is a clear correlation between the isotope shifts in both states suggesting that the mode compositions of the porphyrin ring modes also remain similar in the T I state. Excitation of TPP to the TI state gives rise to only small shifts in cm-l of the vibrational modes which we have assigned, primarily because the ~t molecular orbitals involved are delocalized over a large ring system. This contrasts strongly with systems such as benzophenone3 and biphenyl2 where excitation to excited states involves much more localized orbitals centered mainly on single functional groups. In benzophenone the orbitals involved are nonbonding and n* orbitals on the carbonyl group and excitation results in a shift of '400 cm-' in the C - 0 vibration, corresponding to a large change in bond order.3 Because of the delocalized nature of the excitation in the porphyrins, changes in the strengths of specific bonds are expected to be much smaller. Indeed, semiempirical calculations predict changes in bond lengths of 0.03A at most on excitation of Zn(TFP) to its TIstate2' and we have calculated similarly small changes for free-base TPP.22 Fortunately, even these small changes would be expected to give differences in Raman band
Bell et al.
3964 J. Phys. Chem., Vol. 99, No. 12, 1995 positions of tens of cm-l, which are within the accuracy of typical TR3experiments. The calculations also demonstrate that, despite that fact that the excitation of closed-shell porphyrins is essentially z-n* in character, some bond lengths actually decrease upon excitation. To a first approximation this agrees with the Raman data which show some bands moving to higher cm-I on excitation. A major problem with interpretation of the excited-state Raman data is that the vibrational modes of free-base porphyrins are concerted motions of many atoms within the molecules. It is not a simple process to directly correlate the changes in cm-' of particular modes with changes in strength of specific bonds with the molecules. One rigorous approach to this problem is to develop a force field for the ground state and then modify it, using the changes in band position on excitation, to give a force field for the excited state. Direct comparison of these force fields would give information on changes in bond strengths within the excited-state species. We are currently following this approach but are conscious of the fact that, even with considerably more vibrational data than we now have available for these systems, the force fields derived will not be unique. A simpler, qualitative, approach is to interpret the excitedstate data on the basis of mode compositions of the ground state. This is a reasonable approach in this case since the data show that the modes retain the same general character in the TI state as in the ground state. Although no reliable force field is available for H;?TPP a full analysis of free-base porphine has been published.I6 We have taken our mode compositions from this work and made suitable allowance for the differences in modes involving the C, and aryl ring substituents where appropriate. We would not expect the mode compositions obtained in this way to be accurate in detail but the major contributing motions should certainly be correct. The protonated and unprotonated (pyrrole and pyrrolenine) rings in free-base TPP are not equivalent, so that it is necessary to treat the changes in bonding within these rings separately when discussing changes in the structure of the molecule on excitation. Several of the vibrational modes assigned involve vibrations of both types of ring simultaneously. For example, v2 is primarily a Cp-Cp stretching mode involving both the pyrrole and pyrrolenine rings.I6 The shift to lower wavenumber of v2 suggests that there is an overall decrease in bond strength in the Cp-Cp bonds on excitation to the T I state. However, whether this decrease is localized on just the pyrrole or the pyrrolenine rings, or if it occurs on both to the same extent, is not clear. v6 is a ring breathing mode mainly involving just the protonated ringsI6 and its shift to higher wavenumber implies an increase in bonding in the N-C, and C,-Cp regions of the pyrrole rings on excitation. The corresponding mode for the unprotonated pyrrolenine rings, ~ 1 5 does , not shift so much on excitation, implying that little change in bonding occurs for N-C, and C,-Cp of these rings. The shift to higher wavenumber of v4,which involves both N-C, and C,-Cp stretching vibrations of the pyrrole and pyrrolenine rings,I6 is consistent with the shift to higher wavenumber of Y6, increasing bond strengths of the N-C, and C,-CB in the protonated rings giving rise to most of the change. The overall picture which emerges is one in which excitation to the triplet state decreases the strength of the Cp-Cp bonds and has little effect on either the N-C, and C,-Cp bonds of the unprotonated rings or the C,-phenyl bonds and phenyl substituents, but increases bonding in the N-C, and C,-Cp region of the protonated rings. It is worth stressing that these changes in wavenumber, which accompany excitation to the T I state, correspond to changes in force constants which are
extremely small, of the order of 1%, but that the overall effect of excitation on the chemical reactivity of the system is significant. For example, these systems are reducing agents in the TI state and have been extensively investigated as components in solar-energy conversion systems.23 The TI state lies ca. 140 kJ above the ground state; it is apparent that in this state the energy is not localized in small regions of the conjugated system but is spread throughout the system. This gives minor changes in bond lengths and energies of large numbers of bonds, in contrast to the large changes in bonding within localized regions which are observed in the case of ben~ophenone,~ for example. This delocalization would, of course, be expected for the triplet state of any large aromatic molecule, but the relative ease of isotopic substitution and the extensive data base now becoming available make the free-base porphyrins ideal systems for the investigation of such effects.
Acknowledgment. M. Hegarty acknowledges receipt of a Musgrave Studentship and support from The Arbuthnot Fund of The Queens University of Belfast. The authors also acknowledge financial support from the S.E.R.C.E.P.S.R.C. (grant no. GR/F83389 and GIUJ01905). References and Notes ( I ) Atkinson, G. H. In Advances in Infrared and Raman Spectrscopy; Hester, R. E., Clark, R. J. H., Eds.; Heyden: London, 1981; Vol. 9, Chapter 1. (2) Buntinx, G.; Benbouazza, A.; Poizat, 0.;Guichard, V. Chem. Phys. Lett. 1988, 153, 297. (3) Tahara, T.; Hamaguchi, H-0.; Tasumi, M. J . Phys. Chem. 1987, 91, 5875. (4) Koyama, Y.: Mukai, Y.; Kuki, M. In Laser Spectroscopy of Biomolecules; Korppi-Tommola, J. E. I., Ed.; Proc. SPIE: 1992: SPIE: Bellingham, MA, Vol. 1921, p 191. ( 5 ) Bradley, P. G.; Kress, N.; Homberger, B. A,; Dallinger, R. F.: Woodruff, W. H. J . Am. Chem. Soc. 1981, 103, 7441. (6) Mallick, P. K.; Danzer, G. D.; Strommen, D. P.; Kincaid, J. R. J . Am. Chem. Soc. 1990, 112, 1686. (7) (a) Walters, V. A.: dePaula, J. C.; Babcock, G. T.; Leroi, G. E. J . Am. Chem. Soc. 1989, 111, 8300. (b) Nam, H.; Walters, V. A,; dePaula, J. C.; Babcock, G. T.; Leroi, G. E., In Proceedings of the 12th lntemational Conference on Raman Spectroscopy; Durig, J. R., Sullivan, J. F., Eds.; Wiley: New York, 1990; p 618. (8) Reed, R. A.; Puerello, R.; Prendergast. K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9270. (9) de Paula, J. C.; Walters, V. A,; Nutatis, C.; Lind, J.; Hall, K. J . Phys. Chem. 1992, 96, 10591. (10) (a) Bell, S. E. J.; Al-Obaidi, A. H. R.; Hegarty, M.: Lefly, C.: Hester, R. E.: McGarvey, J. J. In Laser Spectroscopy of Biomolecules; KorppiTommola, J. E. I., Ed. Proc. SPIE; SPIE: Bellingham, MA, 1992; Vol. 1921, p 274. (b) Bell, S. E. J.; AI-Obaidi, A. H. R.; Hegarty, J. N. M.; Hester, R. E.; McGarvey, J. J. J . Phys. Chem. 1993, 97, 11599. (1 1) Sato, S.; Asano-Someda, M.; Kitagawa, T. Chem. Phys. Lett. 1992, 189, 443. (12) Lindsay, J. S.: Wagner, R. W. J . Org. Chem. 1989, 54, 828. (13) Fajer, J.; Borg, D. C.; Forman. A,: Felton, R. H.: Vegh, L.; Dolphin, D. Ann. New York Acad. Sci. 1973, 206, 70. (14) Gordon, K. C.; McGarvey, J. J. Inorg. Chem. 1992, 30, 2986. (15) Stein, P.; Ulman, A.; Spiro, T. G. J . Phys. Chem. 1984, 88, 369. Zigierski, M. Z. J . Phys. Chem. 1991, 95, 4268. Czemuszewicz, R. S . ; Kincaid, J . R.: Su,Y.0.: Spiro, T. G. J . Phys. Chem. 1990, 94, 31. (18) Kalyanasundaram, K.; Neumann-Spallart, M. J . Phys. Chem. 1982, 86, 2725. (19) Rodriguez, J.; Kirmaier, C.; Holten, D. J . Am. Chem. Soc. 1989, 111, 6500. (20) Bell, S. E. J.; Hegarty, J. N. M.; Al-Obaidi, A. H. R.: McGarvey, J. J. In Proceedings of the 14th International Conference on Raman Spectroscopy; Yu, N.-T., Li, X.-Y., Eds., Wiley: Chichester, U.K., 1994: p 5 16. (21) Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9728. (22) Bell, S. E. J.; AI-Obaidi, A. H. R.; Hegarty, J. N. M.; McGarvey, J . J.; Lefley, C. R.; Moore, J. N.; Hester, R. E. J . Chem. Soc., Faraday Trans. 1995, 91, 418. (23) Gaines, G. L.; O'Neil, M. P.; Svec, W. A,; Niemcyzk, M. P., Wasielewski, W. R. J . Am. Chem. Soc. 1991, 113, 719. (b) Harriman, A.: West, M. A. Phofogeneration of Hydrogen; Academic Press: London. 1982. JP94 1964D
