J. Phys. Chem. 1995,99, 7246-7250
7246
Resonance Raman Characterization of the Triplet State of Zinc Tetraphenylchlorin S. E. Vitols, Shin-ichi Terashita? Milton E. Blackwood Jr., Ranjit Kumble, Yukihiro Ozaki; and Thomas G. Spiro* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received: July 28, 1994; In Final Form: December 23, 1994@
The TI excited state of zinc(I1) tetraphenylchlorin, ZnTPC, has been studied by nanosecond time-resolved transient absorption (TA) and transient resonance Raman (TR3) spectroscopy. The TA difference spectrum * ) The most striking shows an induced absorption at 450 nm that is assigned as arising from a 3 ( ~ , nstate. feature of the 3(ZnTPC) TR3 spectrum is the significant frequency downshifts and strong intensity enhancements of the pyrrole half-ring stretch, v4, and of other pyrrole ring motions which together produce a broad band in the 1150-1300-~m-~region. In addition, the enhancement of C,C, modes ( ~ and 2 V I I )changes with their phasing between the SOand TI states. Weaker intensity enhancements, with no frequency shifts, are seen for the phenyl substituent ring mode, 0 4 and V I ,and C,-phenyl stretch. These findings suggest that there is less electronic participation from the phenyl rings in the resonant triplet state, T,, for ZnTPC than for its parent metalloporphyrin, ZnTPP.
The excited states of metalloporphyrins and metallochlorins play an integral role in biological energy and electron transfer, photodynamic therapy, and photocatalysi~.’-~The lowest-lying excited states of metalloporphyrins have been studied by resonance Raman (RR)6a-Cand resonance coherent anti-Stokes Raman’”-‘ (RCAR) spectroscopy in order to obtain detailed structural information on the excited states. In comparison to metalloporphyrins, far less is known about the excited state dynamics of metallochlorins. Although ground state absorption, resonance Raman, and infrared spectra of metallochlorins have been studied in some detail,8-’s there have been few reports dealing with their excited state spectra. Hydroporphyrins such as ZnTPC (TPC = tetraphenylchlorin) represent a distinct class of biologically relevant compounds.16-’8 The reduction of one pyrrole ring in the chlorin macrocycle lowers the symmetry from D4h to Czv, resulting in further separation of the highest occupied molecular orbitals, HOMOS, and a loss of degeneracy in the lowest unoccupied molecular orbitals, LUMOs. The electronic and structural changes alter the photophysical, redox, and ligand-binding properties of metallochlorins relative to those of the parent metalloporphyrins.18a-b However, it is not clear how the reduction of the pyrrole ring affects the excited state dynamics of metallochlorins relative to metalloporhyrins. In order to address this question, we have investigated the character of the lowest-lying excited state of ZnTPC by transient absorption and TR3 spectroscopy. ZnTPC was purchased from Midcentury Chemical Co. (Posen, IL). Its punty was checked via thin layer chromatography. ZnTpC-d20 and ZnTPC-d8 isotopomers were synthesized according to standard literature preparation^.'^-*' The ZnTPC complexes were purified on 1000-pm silica gel plates (Analtech), using a degassed 3:l mixture of THF and hexane as eluant. The purification was done in a glovebox under an inert atmosphere (Vacuum Atmosphere Model D1-00 1-SD equipped with an He-493 Dri-Train). The nanosecond time-resolved absorption and resonance Raman apparatus has been previously described.22 The ground state RR spectra were recorded on a
* To whom correspondence
should be addressed. Department of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya, Hyogo 662, Japan @Abstractpublished in Advance ACS Absrructs, May 1, 1995.
’Permanent address:
ZnTPC in THF
f
ground slate absorplion
N
0.2
0.1
450 nm 100 N Imnr*nl
8 d
* u
4 -0.4
0.0
-02
450
500
550
600
650
W Q U e h g f h (nm)
Figure 1. Ground state absorption spectrum (top panel) for ZnTPC in THF from 425 to 700 nm. Soret and Qr band maxima occur at 420 and 622 nm, respectively. The sample concentration was 1 pM. Transient difference spectra, measured at 100 ns after excitation, show a strong induced feature at 450 nm (middle panel) as well as bleaching of ground state Soret and Qr bands (lower panel). Excitation was provided by a 2-mJ, 7-ns pulse at 416 nm (10 Hz) for all samples. Note the magnitudes of the bleaches are not comparable because of the difference of the sample concentration in the blue (10 pM) vs red (50 pM) TA difference spectra.
single stage spectrograph equipped with a CCD detector (Princeton Instruments) utilizing 135’ backscattering geometry. Figure 1 shows the ground state absorption (top panel) and the transient absorption spectra of ZnTPC in THF in the visible region from 400 to 700 nm (lower panels). The transient absorption difference spectra, obtained 100 ns after excitation
0022-3654/95/2099-7246$09.00/00 1995 American Chemical Society
J. Phys. Chem., Vol. 99,No. 19, 1995 7247
The Triplet State of Zinc Tetraphenylchlorin TABLE 1: Ground State and TI State Vibrational Freauencies (cm-9 for ZnTPC vibrational mode
ZnTPC
AZnTPC-Qo
3(ZnTPC)
AS0 - T I
-8
+17
-2
+1
-1
1004
-16
15
1012
1075
>-loo
-3
1076
-50
1232
-6
-50
0
1294
-5
-8
-5 I
1524
-50
+2
-7
1541
-11
-15
-2
-20 '-100
1190
VI
1232
v(CmPh) v36 v(CmPh)
1249
-7
-32 -4
v4 I
1316
v(pyr half-ring),,, v4 v(pyr half-ring),,,
1345
-17
-2
VI2
1360
-11
-9
v(pyr half-ring),y, v3 v(CaCm)sym
1455
-27
+3
VI I
1531
-28
-11
V(CBCd v2 v(CaCm)asym
1543
- 10
1596
0
-3
-32
0
1596
r1345 v4
v)
A3(ZnTPC-&)
+5
v(pyr breathing) v9 d(C$fLym v3 d(C$Oa,ym
v8
A3(ZnTPC-&)
995
v6 v(pyr breathing) VI5
AZnTPC-ds
v1
1' 5
0
-35
v,
NA v
Q, Q,
2
1
950
1000
1100
1200
1300
1400
1500
1600
1035
1120
1205
1290
1375
1460
1545
1630
A v (cm-1) Figure 3. TR3 difference spectra of 3(ZnTPC) [NA], 3(ZnTPC-d20)
ZnTPC (NA), ZnTPC-dzo (D20), and ZnTPC-ds (D8) in DMF (ca. 1 mM). Spectra were obtained with 413.1-nm excitation at 25 mW.
[D20] and 3(ZnTPC-ds) [D8] isotopomers (3.5-mJ, 7-ns, 450-nm probe and 6 - d , 7-ns 416-nm pump, both at 10 Hz) taken in THF (ca. 330 pM) solutions. The delay between the pump and probe pulses was 100 ns. The difference spectra were obtained by subtracting the probe only spectra from the pump plus probe spectra.
of the sample, show bleaching of the ground state Soret band at 420 nm and the Q, band at 622 nm. An induced transient, with a maximum at 450 nm, occurs on the low energy side of the ground state Soret band. This induced absorption persists, without change in its intensity, until 1 ,us (the final time point taken in the measurements). The 450-nm absorption is characteristic of '(n,n*) and 3 ( ~ , nstates * ) observed in metallopor-
p h y r i n ~ .The ~ ~ duration of the excited state is too long for it to be a singlet state since the lifetimes of '(n,z*) states for metalloporphyrins range from 700 ps to 12 ns.23 Moreover, there are no negative transient features to the red of the Q band bleach, whereas a stimulated emissions band in this region is characteristic of '(n,n*) states.23 The transient absorption is therefore asigned to the 3(n,n*) TI state.
A u(cm-l) Figure 2. Resonance Raman spectra of ground state, natural abundance
1248 J. Phys. Chem., Vol. 99, No. 19, 1995 a2
Vitols et al.
(4
0.3322
-0.0997
w
0.W4
0.Y38
-02031
.O.Ols
Figure 4. Molecular orbitals for HOMO (a2) and LUMO (bl) orbitals in ZnTPC and bond order changes calculated for excitation.
Resonance Raman spectra of the ground and TI states of ZnTPC and its ZnTPC-d2o and -dg isotopomers are shown in Figures 2 and 3, respectively. Assignments (Table 1) are made by analogy to ZnTPP, for which ground and excited state modes have been assigned.22 Experience with NiOEC8 shows that ring reduction does not greatly affect the mode compositions (except for the reduced CpCp bonds). However, because of the symmetry lowering, more modes are RR active for ZnTPC than ZnTPP. Pending a normal coordinate analysis on ZnTPC, these asignments are considered preliminary. The broadness of the features in the T I ZnTPC spectrum are intrinsic to this excited state and do not result from product decomposition, as "probeonly" spectra obtained after the pump-pulse spectra are identical to the original ground state spectra. Broad features have also been reported for the T I state of ZnTPP22 and ZnOEP.11,31We tentatively assign the breadth of the bands to overlapping contri'htions from multiple modes, which become activated as a result of bond order changes in the T I state, as discussed below for v4. Another possibility is that photoexcitation alters the axial ineractions of the Zn metal with solvent molecules, producing heterogenous broadening, but changing the solvent to DMF does not produce noticeable spectral changes. Modes involving the phenyl rings can be identified by their sensitivity to phenyl ring deuteration. Thus, the large ZnTPC-
d20 downshift of 35 cm-I identifies the phenyl ring stretch, @4,
at 1596 cm-I in both the ground and excited states. Likewise, the C,-phenyl (Cm-Ph) stretching mode, V I , is identified, in both the ground and excited states, via its 50-cm-I ZnTPC-d20 downshift. The ground state band at 1249 cm-' also undergoes a large (32-cm-I) downshift upon d20 substitution and is assigned to another C,,-Ph mode, v36. This mode is only active in the IR spectra of metallotetraphenyl porphyrins, but the loss of the inversion center renders v36 Raman active in ZnTPC. It is not seen, however, in the ZnTPC excited state spectrum. Sensitivity of Cp deuteration can be used to assign Cp-H bending and CpCp stretching modes. The band at 1075 cm-I, in both the ground and the excited states RR spectra, shifts out of the spectral region upon ZnTPC-dg substitution and is assigned to the Cp-H bending mode, vg. In the ground state, the bands at 1531 and 1455 cm-' are identified with CpCp stretches via their large (27-cm-I) ZnTPC-d8 shifts. They are associated with the TPP modes vll and v3, respectively. The v11 mode is identified with the 1524-cm-' band in the TI spectrum via its large ZnTPC-4 shift. In the ground state spectrum, the three bands between 1300 and 1400 cm-I are assigned to the three expected pyrrole half-ring stretches, v4, v12, and v41, while the pair of bands at 1004 and 995 cm-I are assigned to pyrrole breathing modes, v.5 and v15. The isotope shift pattems are similar to those seen in NiTPP.24 In the TI
The Triplet State of Zinc Tetraphenylchlorin spectrum, one of the latter pyrrole breathing modes appears at 1012 cm-I and is assigned as V I 5 from its isotope shifts. The remaining bands are assigned to skeletal modes by considering the bonding changes expected upon excitation from SOto T I . Figure 4 shows the pattern of the HOMO and LUMO orbitals of ZnTPCZ5 and the calculated bond order changes between SO and TI.*^ Significant changes are predicted for all the skeletal bonds, but the pattern is far from uniform around the ring. The most striking effect is the alteration of the two pairs of CaCm bonds opposite the reduced ring. The farther CaC, bonds contract while the nearer bonds expand, by a substantial amount in both cases. (A similar alteration is seen for the CaCm bonds near the reduced ring, but the effect is smaller.) This phasing of the bond order changes is expected to selectively affect the vibrational modes which involve asymmetric stretching of the CaCm bonds, namely, YIO,vl9, and ~ 3 7 . Of these, VIO and a component of ~ 3 are 7 totally symmetric in ZnTPC, and we therefore assign the TI RR band at 1541 cm-' to one of the them. (VIOis chosen arbitrarily.) Its appreciable (15 cm-') ZnTPC-d2o shift is compatible with asymmetric CaCm stretching. The frequency of this mode should be elevated in SOrelative to T I , given the bond order changes, but a higher frequency skeletal mode is not seen in the SO spectrum. Rather a band is seen at essentially the same frequency, 1543 cm-l, with no ZnTPc-d~oshift. This band is assigned to the v2 mode, which involves symmetric stretching of the C,C, bonds. Thus the enhancement of CaCm modes in the two states changes with their phasing. Another feature of the SO-TI bond order changes is the predicted weakening of both CaCp and C,N bonds in the pyrrole ring opposite the reduced ring, with little change in the adjacent rings. We attribute the very broad 1291-cm-' band in the TI spectrum to these irregular changes in the pyrrole bonding. The peak is assigned to v4, but the band probably has contributions from the other pyrrole half-ring stretches, shifted down from their ground state frequencies, as well as from other CaCp and C,N bond related modes, all contributing to the broad envelope. Turning to the phenyl modes, 0 4 (1596 cm-') and V I (1235 cm-I), we note that they are significantly stronger in the TI spectra than in the SO spectra. However, the excited state enhancement of these phenyl ring modes is much less than in ZnTPP,22where the TI RR spectra are dominated by the phenyl modes. To explain this enhancement pattem, it was suggested that mixing of the porphyrin eg*and phenyl n* orbitals produces a charge transfer character for the TI-T, transition. (The Tn energy, 34.5 kK, was estimated by adding the TI-SO phosphorescence (12.8 kIQ2' and TI-T, absorption (21.7 kK)28 energies.) The proposed charge transfer could produce a substantial change in the phenyl ring geometry and, thus, account for the phenyl mode enhancement. In the ground state, there is little electronic interaction between the phenyl rings and the porphyrin macrocycle as indicated by NMR studies on the effects of asymmetrical phenyl substituents on pyrrole proton shifts.29 Steric repulsion exists between the phenyl and pyrrole H atoms which keep the phenyl ring orthogonal to the porphyrin macrocycle. The absence of shifts for the phenyl ring modes in the 3(ZnTPP) RR spectrum implies that this electronic isolation is maintained in the TI state. In the resonant T, state however, electronic interaction may be sufficient to overcome the steric barriers and induce roation of the phenyl rings into the porphyrin plane in order to maximize overlap between the eg* and phenyl n* orbitals. Since the frequencies of the phenyl ring modes and Y I are essentially the same for ground state ZnTPC and 3(ZnTPC), it is reasonable to suppose that a similar TI-T, transition accounts
J. Phys. Chem., Vol. 99, No. 19, 1995 7249 for the phenyl mode enhancement seen in the 3(ZnTPC) spectrum. However, ZnTPC shows substantially less intensity enhancement for a4 and Y I than ZnTPP, implying that there is less charge transfer character for the TI-T, transition for 3(ZnTPC). The attenuation of intensity for a4 and V I could result from poor orbital overlap between the chlorin macrocycle and the phenyl rings. The diminished intensity enhancement for the phenyl modes and the sharper bandwidth of the TI-T, absorption for ZnTPC suggest that rotation of the phenyl rings into the macrocycle plane in the Tn state may be less favorable for the metallochlorin, possibly due to the increased steric repulsion introduced by the two additional H atoms of the reduced pyrrole ring. Moreover, the crystal structure of ZnTPC indicates substantial ruffling of the porphyrin macrocyle which could further hinder phenyl ring rotation.30 As a result, orbital overlap with the T, state would be diminished, resulting in less CT character for the TI-T, transition. Currently resonance Raman studies are underway on the radical cations and anions of ZnTPC. In addition, a more refined normal coordinate analysis is being undertaken in hopes of making better modes assignments for ZnTPC. The anticipated data should help clarify the preliminary findings on the excited state behavior of ZnTPC.
Acknowledgment. We thank Dr. Songzhou Hu and Ching Yao Lin for their skilled assistance and good humor in synthesizing the ZnTPC isotopomers. This work was supported by Department of Energy Grant No. DE-FG02-88ER13876, References and Notes (1) Gouterman, M. Physical Chemistry, Part A. In The Porphyrins, Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111. (2) Danvent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M. Coord. Chem. Rev. 1982, 44, 83. (3) Holten, D.; Gouterman, M. In Optical Properties and Structure of Tetrapyrroles; Blauer, G., Sund, H., Eds.; Gruyter: New York, 1985; p 63. (4) Dzhagarov, B. M.; Chirvonyi, V. S.; Gurinovich, G. P. In Laser Picosecond Spectroscopy and Photochemistry of Biomolecules; Letokhov, V. S., Ed.; Hilger: Philadelphia, 1987; Chapter 3. (5) Petrich, J. W.; Martin, J. L. Chem. Phys. 1989, 131, 31. (6) (a) de Paula, J. C.; Walters, V. A.; Nutaitis, C.; Lind, J.; Hall, K. J . Phys. Chem. 1992, 96, 10591. (b) Walters, V. A.; de Paula, J. C.; Babcock, G. T.; Leroi, G. E. J . Am. Chem. Soc. 1989, 111, 8300. (c) Findsen, E. W.; Shelnutt, J. A.; Ondrias, M. R. J . Phys. Chem. 1988, 92, 307. (7) (a) Apanasevich, P. A. J. Mol. Struc. 1984,115,233. (b) Chikishev, A. Y.; Kamalov, V. F.; Koroteev, N. I.; Kvach, V. V.; Shkurinov, A. P.; Toleutaev, B. N. Chem. Phys. Lett. 1988, 144, 90. (c) Kamalov, V. F.; Koroteev, N. I.; Toleutaev, B. N. In Time Resolved Spectroscopy; Clarke, R. H., Hester, R. E., Eds.; Wiley: New York, 1989; p 288. (8) Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9728. (9) Sato, S.; Asano-Someda, M.; Kitagawa, T. Chem. Phys. Lett. 1992, 189, 443.
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