J. Phys. Chem. 1996, 100, 18037-18041
18037
Resonance Raman Study of the T1 Excited State of Zinc(II) Complexes of β-Substituted Chlorins Milton E. Blackwood, Jr., Ranjit Kumble, and Thomas G. Spiro* Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544 ReceiVed: April 24, 1995; In Final Form: July 8, 1996X
Nanosecond time-resolved transient absorption (TA) and time-resolved resonance Raman (TR3) spectroscopies have been used to study the first triplet excited state, T1, of Zn(II) octaethylchlorin (ZnOEC). Additionally, TR3 spectra are reported for Zn(II) etiochlorin I (ZnEtI) and the meso-d4 and 15N isotopomers of ZnOEC. Formation of the T1 state in ZnOEC results in a red shift of the Soret absorption band from 400 to 421 nm. Features in the resonance Raman spectra of the ground and T1 states are assigned on the basis of the isotope shift patterns and of normal coordinate analyses of MOECs. Bands at 1464 and 1597 cm-1 in the T1 spectrum are assigned to CRCm stretching modes, ν3 and ν10 respectively, downshifted by 21 and 19 cm-1 from their ground state frequencies. The only observed CβCβ stretching mode seen in the T1 spectrum is at 1542 cm-1 and is assigned to ν2, downshifted from 1571 cm-1 in the ground state. The CRN and CRCβ stretching modes located in the 1350-1400 cm-1 region of the ground state downshift in the triplet excited state to an unresolved set of peaks in the 1325-1380 cm-1 region. The frequency downshifts indicate that the observed peaks arise from vibrational modes which are localized on bonds that are weakened in the excited state. Peaks corresponding to substituent vibrational modes are not shifted in the excited state spectrum, implying a lack of hyperconjugation in the T1 state.
Introduction Excited states of metallochlorins are intermediates in the photosynthetic energy and electron transfer processes of green plants.1 Because of the sensitivity of vibrational frequencies to structural and electronic properties, resonance Raman (RR) spectroscopic studies of the excited states of metallochlorin can provide insight into the excited state dynamics. While RR spectroscopy has been employed extensively to characterize natural and synthetic metallochlorins in the ground state,2 studies of their excited states have been limited.3-5 RR studies of the excited states of metallochlorin model compounds have been confined to the more stable mesotetraphenylchlorins. In a nanosecond time-resolved Raman (TR3) spectroscopic study of ZnTPC (TPC ) tetraphenylchlorin) the lowest triplet state, T1, of 3(π,π*) character was detected.3 When the closed-shell Zn(II) is replaced by d9 Cu(II), de Paula et al. identified a (π,d) charge transfer state in the difference spectrum of Raman spectra of CuTPC obtained at high and low power pulsed laser excitation.4 Natural metallochlorins are substituted at the pyrrole β positions. We report the first transient absorption (TA) and RR spectra for the T1 state of two β-substituted zinc(II) chlorins, ZnOEC (OEC ) octaethylchlorin) and ZnEtI (EtI ) etiochlorin I) (see structural diagram in Figure 1), along with the meso-d4 and pyrrole 15N isotopomers of ZnOEC. Several RR studies of radicals of metalloporphyrins and metallochlorins have correlated observed structural changes, reflected in vibrational frequency shifts, with predicted bond order changes from calculated electronic structure.3,6,7 Here, we extend this technique to the T1 excited state of β-substituted metallochlorins to better understand changes in their excited state geometries. * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01190-2 CCC: $12.00
Figure 1. Structures of zinc(II) octaethylchlorin (ZnOEC) and zinc(II) etiochlorin I (ZnEtI).
Experimental Section Free base H2OEC and H2EtI were prepared from Fe(III) octaethylporphyrin‚Cl (FeOEP‚Cl) and Fe(III) etioporphyrin‚Cl I (FeEtioI‚Cl) (Midcentury Chemicals), respectively, using the method of Whitlock et al.8 The 15N isotopomer was prepared from 15N-labeled H2OEP, synthesized from Na15NO2 (Cambridge Isotopes) using the technique reported by Paine and coworkers.9 The free base meso-d4 isotopomer was prepared from natural abundance (NA) OEC by allowing the meso hydrogens to exchange in D2SO4/D2O (90%/10% v/v) overnight. Zinc(II) was inserted into the macrocycles by refluxing with zinc(II) acetate in N,N dimethylformamide, except that the meso-d4 free base was refluxed with Zn(II) chloride in a chloroform/ deuterated methanol solution (95%/5% v/v) in order to prevent back-exchange of hydrogens at the meso positions. All compounds were purified on 1000 µm alumina chromatograph plates (Analtech) using a hexane/THF eluant (60%/40% v/v). Transient absorption and RR spectra were obtained in THF that had been distilled over sodium and then thoroughly © 1996 American Chemical Society
18038 J. Phys. Chem., Vol. 100, No. 46, 1996
Blackwood et al.
Figure 2. Absorption spectrum of ZnOEC in tetrahydrofuran. The inset illustrates the frontier orbital excitations that are responsible for the observed electronic transitions.
degassed with four freeze-pump-thaw cycles. Solutions were prepared in a glovebox under an inert atmosphere (Vacuum Atmosphere, Model D1-001-SD equipped with a He-493 DriTrain). Ground state RR spectra were obtained in spinning NMR tubes using a 0.75 m single-stage spectrograph equipped with a liquid N2 cooled CCD detector (1024 × 256 EEV, Princeton Instruments), utilizing 413.1 nm Kr+ laser excitation and 135° backscattering geometry. The laser powers at the sample were approximately 15 mW and spectral acquisition times were 10 min. The integrity of the sample was checked by measuring the absorption spectrum of the solution before and after the Raman experiments. The nanosecond time-resolved absorption and resonance Raman systems have been previously described.10 The transient absorption spectrum of the T1 state was obtained 100 ns after photoexcitation with a 397 nm pulse generated by Raman shifting (first Stokes-shifted line of D2) the third harmonic of a Nd:YAG laser (Quanta Ray, 7 ns fwhm, 10 Hz). For the TR3 experiment 416 nm probe and the 503 nm pump pulses were generated by hydrogen Raman shifting the third harmonic of the Nd:YAG laser. A 15 ns delay was introduced with an optical delay path. Spectra with longer delays (50 ns) were obtained with two synchronized Nd:YAG lasers, to ensure that the spectra were not contaminated by residual singlet excited state features. The pump and probe pulses beams were spatially superimposed using a dichroic beamsplitter and were focused with a cylindrical lens onto the sample that was contained in an airtight 1 mm quartz cuvette. Scattered radiation was collected at 135o and dispersed in a triple-stage spectrometer equipped with an OMA detector (Princeton Instruments). A series of probe-only and pump/probe spectra were taken to avoid systematic errors from photodecomposition. The RR spectra were obtained by the subtraction of the summed probeonly spectra from the summed pump/probe spectra (a total of 1 h acquisition time for each). Standard solvents were used for frequency calibration, and data processing was performed using the Lab Calc software package (Galactic Industries Corp.). Results a. Ground State Absorption and RR Spectra. Metallochlorins differ from metalloporphyrins in having one reduced
Figure 3. Ground state resonance Raman spectra for natural abundance (NA) ZnOEC, its 15N and meso-d4 isotopomers, and ZnEtI in tetrahydrofuran.
pyrrole ring; the idealized symmetry is lowered from D4h to C2V. The optical properties of metallochlorins are understood within the framework of the Gouterman four-orbital model (Figure 2).11 In metalloporphyrins the HOMOs (a1u and a2u) are accidentally nearly degenerate and the LUMO is a doubly degenerate eg* orbital. The symmetry lowering in metallochlorins destroys the degeneracy of the LUMO and lifts the accidental degeneracy of the HOMOs. The energies of the a1uand egx*-like orbitals are increased relative to those of metalloporphyrins, while the energies of the a2u- and egy*-like orbitals are only slightly altered. The x-polarized excitations are nearly degenerate, and strong configuration interaction leads to porphyrin-like intense Bx and weak Qx transitions. The y-polarized excitations are far apart in energy, however, and produce By and Qy transitions of similar intensities. The Bx and By energies are nearly the same, and only a single strong B absorption band is seen (Figure 2), but the Qx and Qy have very different energies and intensities. Vibronic sidebands, Qx1 and Qy1, are also seen. Figure 3 shows the ground state RR spectra for ZnOEC and its meso-d4 and 15N isotopomers, as well as ZnEtI, obtained with 413.1 nm excitation, which falls in the B band. The NA and meso-d4 spectra of ZnOEC have been reported previously.12 Vibrational assignments of the Raman peaks (Table 1) are based upon previous resonance Raman studies and normal mode analyses (NMA) for β-substituted metallochlorins.12-19 The peak labeling is based on the parent porphyrin modes, as worked out for NiOEC.18 In the higher frequency region of the spectrum (1300-1600 cm-1), Raman peaks arising from skeletal stretching modes can
Zinc(II) Complexes of β-Substituted Chlorins
J. Phys. Chem., Vol. 100, No. 46, 1996 18039
TABLE 1: Ground and T1 State Vibrational Frequencies (cm-1) for ZnOEC and ZnEtI vibrational mode
ZnOEC
ν10 (CRCm) ν37 (CRCm) ν2 (CβCβ) ν19 (CRCm) ν11 (CβCβ) ν3 (CRCm) ν40 (CRN, CRCβ) ν20 (CRN, CRCβ) ν29 (CRN, CRCβ) ν4 (CRN, CRCβ) ν41 (CRN, CRCβ) ν21 (δCCH, δCmH) -CH2 twist CβC1, CRN C1C2 C1C2
1616 1580 1571 1565 1533 1486 1397 1389 1381 1365 1351 1318 1258 1137 1018 1012
∆meso-d4 11 0 16 1 9
∆15N
ZnEtI
3ZnOEC
ZnEtI
∆T1-S0
1 +1
1597
9
0
+3
-19
1542
0
4
+4
-29
1464
10
5
0
-22
1 1 1 2 0 1
+2 +3 0
∆meso-d4
∆15N
-7 -21
1374 1348 +3 0
0 +2 5
+14
be identified. Meso-d4 downshifts identify four of the eight expected CRCm stretching modes: ν10 (1616 cm-1), ν37 (1580 cm-1), ν19 (1565 cm-1), and ν3 (1486 cm-1). These peaks do not shift in ZnEtI, which has different substituents on the Cβ atoms, or upon 15N substitution, indicating little mixing with other internal coordinates. Two of the three expected CβCβ double bond stretches are assigned to peaks at 1571 cm-1 (ν2) and 1533 cm-1 (ν11) on the basis of small upshifts in the ZnEtI spectrum. These upshifts are due to the smaller effective mass of one of the β carbon substituents in EtI. RR spectra in the 1330-1400 cm-1 range are much more complex for metallochlorins than metalloporphyrins due to symmetry lowering and resultant mixing among CRCβ and CRN stretching coordinates. The assignment of five bands in this region to ν40, ν20, ν29, ν4, and ν41 is somewhat arbitrary, but is consistent with previous studies.18 Modes involving bending coordinates and substituent vibrations dominate the lower frequency region of the spectrum of ZnOEC. The peak at 1318 cm-1 is assigned to ν21 (CβC1H, C1C2H, and CmH bending)20 on the basis of its anomalous polarization (not shown) and large meso-d4 shift. The 1258 cm-1 mode is assigned to a -CH2 twisting mode because of the similarity in its isotope sensitivity to a similar mode in Ni(II) octaethylporphyrin (NiOEP).20 The intense peak at 1137 cm-1 shows a 9 cm-1 downshift in the 15N spectrum and is assigned to ν5, a CRN and CβC1 stretching mode. A pair of peaks at 1012 and 1018 cm-1 are assigned to modes comprised predominately of the -C1C2 stretching motion of the ethyl substituents. The lower frequency component upshifts to 1026 cm-1 in the meso-d4 spectrum, consistent with the NiOEC calculations.20 This band loses intensity in the ZnEtI spectrum as it does in the corresponding etioporphyrin spectrum.21 b. T1 Excited State Absorption and RR Spectra. The transient absorption spectrum (380-620 nm) for ZnOEC in THF obtained 100 ns after excitation with a 397 nm pump pulse shows a bleaching of the 400 nm ground state absorption and an induced absorption at 421 nm (Figure 4). The induced absorption is assigned to a 3(ππ*) excited state since the lifetimes of 1(ππ*) states are 700 ps to 12 ns for metallochlorins.22 A similar long-lived red-shifted absorption peak has been assigned to the T1 state of ZnTPC.3 TR3 spectra of the T1 state (Figure 5) were generated by pumping into the Qx(0,1) vibronic band with a 503 nm pulse and probing at 416 nm near the maximum (421 nm) of the induced absorption. The RR enhancement patterns differ significantly between the So and T1 states, but peak correlations can nevertheless be made on the basis of the isotope and substituent sensitivities (Table 1). All of the observable skeletal modes are at appreciably lower frequencies in the excited state: ν10 and ν3 (CRCm stretching),
0 2 0
1258
1
2
0
1014
+7
+3
+2
Figure 4. ZnOEC transient absorption spectrum in tetrahydrofuran (100 ns delay).
ν2 (CβCβ stretching), and ν29 and ν4 (CRN and CRCβ stretching). The assignment of ν29 and ν4 is somewhat arbitrary because of the mode crowding in the 1300-1400 cm-1 region, as noted above, but there is no doubt that the entire band envelope in this region is shifted down upon photoexcitation. There is a possibility that the band assigned to ν2 is actually ν11, which would then have experienced a 9 cm-1 upshift (Table 1). Neither mode is sensitive to the available isotopes. However the ν2 assignment is preferred since ν11 is the weaker mode in the ground state spectrum and could easily escape detection in the T1 spectrum. The two identifiable T1 substituent modes, the -CH2 twist and -C1C2 stretches, are at essentially the same frequencies as in the ground state. Additional bands are seen in the T1 spectra, 1191, 1172, and 1045 cm-1 (NA) and 994 cm-1 (15N). The latter three bands are assigned to ν14 (CRN stretch and CRNCR bend), ν44a (CβCβ stretch of the reduced pyrrole), and ν31 (-C1C2 stretch and CRCmCR and NCRCm bend), respectively, on the basis of the isotope pattern observed or calculated for the analogous modes of NiOEC.20 The 1191 cm-1 band, which is 15N and d4 sensitive, does not correlate with any of the NiOEC modes and is currently unassigned. Discussion Utilizing the molecular orbital calculations performed by Sekino and Kobayashi,23 the expected skeletal bond order
18040 J. Phys. Chem., Vol. 100, No. 46, 1996
Blackwood et al.
Figure 6. Calculated bond order changes for the T1 excited state of a metallochlorin, based on themolecular orbital coefficients reported by Sekino and Kobayashi.23
Figure 5. Time-resolved resonance Raman spectra for (NA) ZnOEC, its 15N and meso-d4 isotopomers, and ZnEtI in tetrahydrofuran (15 ns delay).
changes for metallochlorins in the T1 state can be calculated (Figure 6). There is a pattern of alternating increased and decreased bond orders for bonds of a given type. One might therefore expect the vibrational frequencies to show both upand downshifts. Yet only downshifts are observed in the frequencies of the RR bands (with the possible exception of ν11; see previous section) in the T1 state, relative to the corresponding bands in the S0 state. Of course not all vibrational modes are observed, and the missing modes might include those with increased T1 frequencies. In the case of the CRCm bonds, the calculated decreases in bond order exceed the increases, so that the average bond order is diminished. This may partly explain the observed downshifts of the CRCm modes. For the CβCβ double bonds, however, the average bond order is calculated to remain the same. One of the CβCβ bonds (the one trans to the reduced ring) experiences an increase in bond order that is twice as large as the decrease experienced by the other two CβCβ double bonds. These two bonds must be the main contributions to ν2, which shifts down in the T1 spectrum. The trans CβCβ bond must contribute mainly to one of the CβCβ modes that is not detected in the T1 spectrum. We conclude that the pattern of exclusively downshifted T1 RR bands must be due to selective enhancement of certain modes, those with large contributions from bonds that are weakened in the excited state. It is possible that this selectivity is associated with the polarization of the excited state electronic transition [T1 f Tn] which is resonant at the excitation wavelength. We note that the CβCβ bond which is trans to the reduced ring, and which experiences an increase in bond order, is parallel to the y axis
of the chlorin (Figure 1), while the other two CβCβ bonds, which are cis to the reduced ring, are parallel to the x axis. Likewise the CRCm bonds whose bond orders increase in the T1 state (the bonds adjacent to the reduced ring and those adjacent to the trans ring, Figure 6) are aligned with the y axis, while those whose bond orders decrease are aligned with the x axis. If the resonant electronic transition is polarized along the x axis (the character of the Tn state is unknown), then selective enhancement of modes with dominant Rxx′ Raman polarizability tensor elements might explain the observation of exclusively downshifted RR bands. Although the skeletal modes are all downshifted, the two observed substituent modes, the -CH2 twist and the ethyl -C1C2, stretch are at the same frequency in the T1 state as in the S0 state (Table 1). This is a matter of some interest, since the significant enhancement of these modes in the S0 state implies an appreciable displacement of the resonant excited state, S2 (the B state), along the substituent coordinates.20 These displacements have been suggested to result from hyperconjugative interactions of the ethyl groups with the porphyrin π systems.20 The expectation is that these modes should experience noticeable frequency shifts in the S2 state. However, the absence of any frequency shifts in the T1 state implies that the hyperconjugative mechanism is unimportant in this state. This disparity is attributable to the S2-T1 energy difference. The S2 state is much closer in energy to the ethyl σ* orbitals that are involved in the proposed hyperconjugation interactions.20 Conclusions The present work illustrates the utility of TR3 spectroscopy in studying the excited state properties of metallochlorins. Only downshifts in skeletal modes are observed in the RR spectra of the T1 excited state of β-substituted metallochlorins, suggesting that these modes are localized to the bonds that are weakened in the excited state. The vibrational frequencies of substituents were found to be minimally perturbed in the T1 excited state of ZnOEC, implying that hyperconjugation is less important for the T1 state than the S2 state. Acknowledgment. The authors thank Dr. Songzhou Hu for helpful discussion and for his assistance in preparing the 15N ZnOEC isotopomer. This work was supported by DOE grant DE-FG02-93ER14403. References and Notes (1) Stryer, L. In Biochemistry, 4th ed.; W. H. Freeman and Co.: New York, 1995; pp 653-682.
Zinc(II) Complexes of β-Substituted Chlorins (2) (a) Schick, G. A.; Bocian, D. F. Biochim. Biophys. Acta 1987, 895, 127-154. (b) Kitagawa, T.; Ozaki, Y. Struct. Bond. 1987, 64, 71-114. (c) Procyk, A. D.; Bocian, D. F. Annu. ReV. Phys. Chem. 1992, 43, 465-496. (d) Lutz, M.; Ma¨ntele, W. In Chlorophylls; Scheer, H., Ed.; CRC Press: Boca Raton, 1991, pp 855-902. (3) Vitols, S. E.; Terashita, S.; Blackwood, M. E., Jr.; Kumble, R.; Ozaki, Y.; Spiro, T. G. J. Phys. Chem. 1995, 99, 7246-7250. (4) de Paula, J. C.; Walters, V. A.; Jackson, B. A.; Cardozo, K. J. Phys. Chem. 1995, 99, 4373-4379. (5) Nishizawa, E.; Hashimoto, H.; Koyama, Y. Chem. Phys. Lett. 1989, 164 (2,3), 155-160. (6) Kumble, R.; Loppnow, G. R.; Hu, S.; Mukherjee, A.; Thompson, M. A.; Spiro, T. G. J. Phys. Chem. 1995, 99, 5809-5816. (7) Hu, S.; Lin, C.-Y.; Blackwood, M. E., Jr.; Mukherjee, A.; Spiro, T. G. J. Phys. Chem. 1995, 99, 9694-9701. (8) Whitlock, H. W., Jr.; Hanauser, R.; Oester, M. Y.; Bower, B. K. J. Am. Chem. Soc. 1969, 91, 7485-7489. (9) Paine, J. B., III; Kirshner, W. B.; Moskowitz, D. W.; Dolphin, D. J. Org. Chem. 1976, 41, 3857-3860. (10) Reed, R. A.; Purello, R.; Prendergast, K.; Spiro, T. G. J. Phys. Chem. 1991, 95, 9720-9727. (11) Gouterman, M., Physical Chemistry, Part A. In The Porphyrins, Dolphin, D., Ed.; Academic Press: New York, 1978; Vol III.
J. Phys. Chem., Vol. 100, No. 46, 1996 18041 (12) Procyk, A. D.; Kim, Y.; Schmidt, E.; Fonda, H. N.; Chang, C. K.; Babcock, G. T.; Bocian D. F. J. Am. Chem. Soc. 1992, 114, 6539-6549. (13) Ozaki, Y.; Kitagawa T.; Ogoshi, H. Inorg. Chem. 1979, 18 (7), 1772-1776. (14) Boldt, N. J.; Donohoe, R. J.; Birge, R. R.; Bocian, D. F. J. Am. Chem. Soc. 1987, 109, 2284-2298. (15) Salehi, A.; Oertling, W. A.; Fonda, F. N.; Babcock, G. T.; Chang, C. K. Photochem. Photobiol. 1988, 48, (4), 525-530. (16) Fonda, H. F. Oertling, W. A.; Salehi, A.; Chang, C. K.; Babcock, G. T. J. Am. Chem. Soc. 1990, 112, 9497-9507. (17) Ozaki, Y.; Iriyama, H.; Ogoshi, H.; Ochiai, T.; Kitagawa, T. J. Phys. Chem. 1986, 90, 6105-6112, 6113-6118. (18) Prendergast, K.; Spiro T. G. J. Phys. Chem. 1991, 95, 1555-1563. (19) Perng , J.; Bocian, D. F. J. Phys. Chem. 1992, 96, 10234-10240. (20) Li, X.-Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Stein, P.; Spiro, T. G. J. Phys. Chem. 1990 94, 47-61. (21) Kumble, R.; Hu, S.; Loppnow, G. R.; Loppnow, G. R.; Vitols, S. E.; Spiro, T. G. J. Phys. Chem. 1993, 97, 10521-10523. (22) Rodriguez, J.; Kirmaier, C.; Holten, D., J. Am. Chem. Soc. 1989, 111, 6500-6510. (23) Sekino, H.; Kobayashi H. J. Chem. Phys. 1987, 86 (9), 5045-5052.
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