J. Phys. Chem. 1995, 99, 1166-1171
1166
Electronic Structure of Triplet States of Zinc(I1) Tetraphenylporphyrins Valerie A. Waiters,*$' Julio C. de Paula,*3* Brian Jackson,' Charles Nutaitis; Kelly Hall,? Jeffrey Lind,' Kevin Cardozo,' Kartik Chandran,? Duane Raible,t and Charles M. Phillips& Department of Chemistry, Lafayette College, Easton, Pennsylvania 18042; Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041; and Regional Laser and Biotechnology Laboratov, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received: June 14, 1994; In Final Form: September 20, 1994@
The effects of ligation and meso substitution on the energies of two triplet states of zinc(I1) tetraphenylporphyrin (ZnTPP) were investigated. Phosphorescence and triplet-triplet absorption spectroscopy were used to determine the energies of the T I state and a higher energy T, state, of a series of para-phenyl-substituted ZnTPPs in both a ligating and a nonligating solvent. The para-phenyl substituents were found to have a greater effect on the energy of the T, state than on that of the T1 state. The magnitude and direction of energy shifts caused by the various electron-donating/withdrawingsubstituents on the T, state suggest a greater resonance interaction between the phenyl and porphyrin rings in the T, state, possibly caused by a decrease in the average dihedral angle between the ring systems. Ligation by pyridine causes the energy of the porphyrin TI state to drop by about 300 cm-' on average and has a slightly lesser effect on the energy of the T, state. Our results indicate that the electronic structures of the T1 and T, states of ZnTPP differ significantly around the meso-carbons and less so around the pyrrole nitrogens. The results of this phosphorescence/triplettriplet absorption study and previously obtained time-resolved resonance Raman spectra of ZnTPP are combined to yield a description of the dynamics of these two triplet states.
Introduction The structure of porphyrins and porphyrin-based macrocycles in low-energy excited states regulates the dynamics of photoinduced energy- and electron-transfer processes from these states. Such processes include the sensitization of singlet oxygen in photodynamic therapy' and ultrafast electron transfer reactions in solar energy conversion systems.2 Molecular properties of relevance to photochemistry, such as excited-state energies and redox potentials are dependent upon the electronic structure. Modification of porphyrin electronic structure brought about by such factors as metal or ring substitution, ligation, n-n interactions, aggregation, and variation in solvent polarity can lead to changes in the excited-state properties and, therefore, the efficiency of the photoinduced processes. The triplet states of metalloporphyrins are important in photodynamic therapy.' Energy transfer from the porphyrin T1 state forms excited singlet oxygen which attacks tumors. A transition from the T I state to a higher energy triplet state is responsible for a strong band in the triplet-triplet absorption spectrum of many porphyrins. This band appears in the blue region at a slightly longer wavelength than the ground-state Soret absorption band. For zinc(I1) tetraphenylporphyrin (ZnTPP), the absorption maximum of the strong triplet-triplet band has been reported at 460-470 nm in different solvent^^-^ (about 40-50 nm to the red of the Soret absorption band.) The higher energy triplet state is referred to in this paper as the T, state, s referring to the proximity in wavelength of the triplet-triplet absorption to the ground-state Soret absorption. Both the Ti and T, states are 3(n,n*)states.6-8 The electronic structures of the low-lying (n,n*) excited states of porphyrins, including the Ti and T, states, have been Lafayette College.
* Haverford College 5 @
Regional Laser and Biotechnology Laboratory. Abstract published In Advance ACS Absrracfs, January 1, 1995
0022-365419512099- 1166$09.00/0
described by the Gouterman four orbital model.'-1° In this model, the nearly degenerate highest occupied orbitals are designated a', and aZu, and the lowest unoccupied orbitals are the degenerate eg orbitals (in D4h symmetry). Consideration of the electron density distribution of the a[, and a2, orbitals leads to predictions of the effects of ligation and the addition of substituents at the meso-carbon positions, on the energies of states formed from these orbitals.10-12 Since the a2, orbital has electron density at the meso-carbons and pyrrole nitrogens, Lewis base ligands, and electron-donating meso substituents are expected to increase the energy of the a2, orbital while electronwithdrawing meso substituents should lower it. The energies eg promotion should, therefore, of states formed by an a2, be altered by ligation and meso substitution. Since the al, orbital has nodes at these positions, ligation and meso substitution should not affect significantly the energy of this orbital. By measuring the relative magnitudes of these expected effects on the energies of two states formed by a2, eg promotion, it should be possible to acertain relative features of the electronic structure of the two states. For ZnTPP, the TI state results from an a2, eg promotion and the T, state results from a2, eg, al, eg p r o m o t i ~ n s ~ . ' .Factors ~~. that could modify the effect of ligation and meso substitution on the energies of these states include geometry distortions and the accompanying electron density changes. This possibility is particularly relevant given the theoretical and experimental evidence for a Jahn-Teller effect in the TI state of ZnTPP.s.8,13-17 Also, it is likely that the effect of a substituent on the phenyl rings of ZnTPP will depend on the dihedral angle between the phenyl ring and the porphyrin core. The purpose of this paper is to investigate the effect of ligation and para-phenyl substitution at the meso carbons on the energies of the T I and T, states of ZnTPP, and from that information to gain insight into the electronic structures of these states. The energy of the TI state is obtained from phosphorescence measurements and the energy of the T, state is derived from a
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0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 4, 1995 1167
Triplet States of Zinc(I1) Tetraphenylporphyrins
j
0 R
R = OCH3
ZnTMPP
CH3
ZnlTP
H
ZnTPP
F
ZnTpFPP
CI
ZnTpClPP
Br
ZnTpBrPP
NO2
ZnTpNOZPP
Figure 1. Structures of the substituted zinc(I1) tetraphenylporphyrins (ZnTF'Ps) used in this study.
combination of phosphorescence and triplet-triplet absorption spectroscopy. There have been several previous reports of the phosphorescence12,18-26and triplet-triplet spectra3-6,24,27,28 of ZnTPP and some of the meso-substituted derivatives studied here. We extend previous work to investigate systematically the phosphorescence and triplet-triplet absorption spectra of ZnTPP and six derivatives with electron-donating/withdrawing para-phenyl substituents, in both a ligating and nonligating solvent environment. To our knowledge, ours is the first experimental investigation of the electronic character of the T, state in a series of para-phenyl-substituted ZnTPPs and of the effect of ligation on the energies of both the TI and T, states.
Experimental Section Synthesis and Materials. The structures of the seven studied ZnTPPs are shown in Figure 1. All porphyrins (except ZnTpN02PP) were synthesized according to literature metho d ~ .The ~ ~ZnTpN02PP was purchased from Midcentury Chemicals (Posen, IL). The purity of all of the porphyrins was checked by UV-vis and fluorescence spectroscopies and found to be free of free base and chlorin impurities. Toluene was used as the solvent in obtaining spectra of porphyrins in unligated form. A solution of 0.5% pyridine in toluene was used to obtain spectra of ligated porphyrins. The toluene and pyridine were purchased from Aldrich (spectrophotometric grade) and used without further purification. Spectroscopic Measurements. Phosphorescence spectra were acquired using an excitation wavelength of 420 nm from a nitrogen laser-pumped dye laser (Photon Technology International, South Brunswick, NJ).30 The dye solution was either Carbostyril 124 in methanol or Bis MSB in p-dioxane. Light emitted by the porphyrins was collected at 90" with respect to the excitation beam and passed through a long-pass filter to remove scattered excitation light before entering a l/4 m monochromator. The 1200 grooves/mm grating in the monochromator was blazed at 750 nm. Signal from a Hamamatsu R928 PMT detector was collected using a Stanford Research
Corp. SR250 gated integrator and sent to a Stanford Research Corp. SR245 computer interface. The spectral bandwidth of the monochromator system was estimated to be 2 nm at 823 nm. Just prior to obtaining the phosphorescence spectra, the porphyrin solutions were degassed to 5 mTorr in a 4 mm diameter quartz tube attached to a removable Teflon stopcock. The degassed samples were kept at liquid nitrogen temperatures inside a custom-made dewar with an attached 1.0 cm cuvette (Wilmad, Buena, NJ). The phosphorescence spectra were calibrated with the 823.16 and 828.01 nm lines from a xenon discharge lamp. Since this work focuses on the relative shifts of the band maxima, the phosphorescence spectra shown and used for analysis were not corrected for the wavelength dependent sensitivity of the detector. For the phosphorescence spectra of the porphyrins (except ZnTpN02PP) in the pyridine/toluene solution, the concentration M. One set of phosphorescence spectra of the was 1 x porphyrins (except ZnTpN02PP) in toluene was obtained using a concentration of 1.0 x and another set was obtained using a concentration of 1 x M. Although there were some slight differences in the band contours of the spectra obtained with different concentrations in toluene, the peak maxima were fairly constant and the wavelength values for both sets of data were averaged together. ZnTpN02PP had very low solubility in both solvent systems and the concentration is M. The solvents used did not readily estimated at (1-5) x form smooth glasses at 77 K. A comparison was made of spectra obtained with and without the sample in a smooth glass state, and there was no apparent shift in the band maxima. Triplet-triplet absorption spectra were obtained at the Regional Laser and Biotechnology Laboratory at the University of Pennsylvania. Porphyrins were excited to the lowest triplet state with 532 nm radiation (1-5 mJ) from a Quanta-Ray DCRl Nd: YAG laser which was focused with a cylindrical lens onto the lower region of a degassable 1 cm x 1 cm cuvette containing the porphyrin sample. A xenon flash lamp probed the sample in both the laser-irradiated region and a region slightly above. Signal from each region was collected by a bifurcated fiber optic and directed to a 0.25 m Spex Minimate spectrograph, where it was detected by a gated Princeton Instruments dual-diode-array detector. This technique permitted subtraction of a simultaneously obtained ground-state absorption spectrum from the raw excited-state absorption spectrum. Background noise from the detector was also subtracted, and the two arrays were corrected for variations in pixel sensitivity. The delay between the triggering of the laser pulse and the xenon lamp was 100 ns and provided by a Princeton Research digital delay generator. For the triplet-triplet absorption spectra, the concentration of the porphyrins in toluene and the 0.5% pyridine in toluene M. The solutions were degassed solution was (1-2) x immediately prior to investigation. For both the phosphorescence and triplet-triplet absorption spectra, the integrity of the samples was assessed by obtaining visible absorption spectra of the ground state before and after laser irradiation. No decomposition was observed in the spectra used for analysis.
Results Phosphorescence and triplet-triplet absorption spectra of the studied ZnTPPs in toluene and in a 0.5% pyridine in toluene solution were obtained. Toluene is nonligating, while pyridine has been shown to ligate to zinc(I1) porphyrins.31 The binding constants for the ligation of pyridine to five of the seven studied porphyrins have been reported.32 The log K values were found to generally increase with the value of the Hammett substituent
1168 J. Phys. Chem., Vol. 99, No. 4, 1995
Walters et al.
Figure 3. Triplet-triplet absorption spectra of ZnTpN02PP (solid lines) and ZnTMPP (dashed lines) in toluene and in a 0.5% pyridine in toluene solution, obtained 100 ns after 532 nm excitation. See text for additional experimental details.
700 7 4 0 7 8 0 8 2 0 8 6 0
Wavelength ( n m )
Figure 2. Phosphorescence spectra at 77 K of ZnTMPP, ZnTTP, ZnTpFPP, and ZnTpN02PP in toluene (solid lines) and in 0.5% pyridine in toluene (dashed lines). See text for experimental details. TABLE 1: Summary of Spectral Data for the Triplet States of Para-Phenyl-Substituted Zinc(I1) Tetraphenylporphyrins wavelengths of band maxima (nm) phosphorescence" triplet-triplet absorption 0.5% pyridine 0.5% pyridine porphyrin toluene in toluene toluene in toluene 8094 1 49 1 488 ZnTMPP 781 4 3 ZnTTP 778& 1 798 & 1 481 475 ZnTPP 789k 1 474 469 770f 1 ZnTpFPP 77242 7894 1 47 1 469 ZnTpClPP 788 k 1 482 478 771 f 1 ZnTpBrPP 770 & 1 789 & 1 480 479 ZnTpNOzPP 775 4 3 783 1 465 468 &standard deviation.
*
constant, q,; a measure of the electronic influence of substituents in para-phenyl positions.33 Methoxy has the lowest 0, of our substituents and from the log K of ZnTMPP (3.690),32 we calculated that greater than 99% of all the studied porphyrins in the 0.5% pyridine in toluene solution are ligated. Phosphorescence Spectra. In the covered 700-880 nm region, each of our 77 K phosphorescence spectra of the studied ZnTPPs in toluene shows one band with a maximum between 770 and 787 nm. The phosphorescence spectra of ZnTMPP, ZnTTP, ZnTpFPP, and ZnTpN02PP in toluene at 77 K are shown in Figure 2 (solid lines). The wavelengths of the phosphorescence band maxima are given in Table 1. Our data are in good agreement with those reported for ZnTPP18,22,26 and ZnTpClPP23,26in a variety of solvents. In particular, we note one study26 which reported a very slight difference (2 nm) in the band maxima of ZnTPP and ZnTpClPP in 5:5:2 diethyl ether:petroleum ether:isopropyl alcohol; we also observe an essentially negligible shift. Small differences in our wavelength values from previously reported values are most likely due to solvatochromism.
Table 1 shows that fluorine, chlorine, and bromine atoms in the para positions of the four phenyl rings have little effect on the phosphorescence band maxima in toluene. Table 1 also shows that the presence of methyl substituents in ZnTTP and methoxy substituents in ZnTMPP causes a red-shift of the phosphorescence bands by 8 and 17 nm, respectively. relative to the ZnTPP band. The phosphorescence band of ZnTpNO2PP also appears slightly red-shifted (by 5 nm) from that of ZnTPP. Phosphorescence spectra of ZnTMPP, ZnTTP, ZnTpFPP, and ZnTpN02PP in the pyridine/toluene solution at 77 K are also seen in Figure 2 (dashed lines). Again, for all porphyrins, only one band is observed in the covered wavelength region and the wavelengths of the phosphorescence band maxima, shown in Table 1, range from 783 to 809 nm. Excluding ZnTpN02PP, the relationships between the wavelength maxima of the porphyrins in this solution are similar to those found in toluene; the bands for the halogenated porphyrins are essentially unshifted and the bands for ZnTTP and ZnTMPP are red-shifted, from that of ZnTPP. Interestingly, the phosphorescence band of ZnTpN02PP in the 0.5% pyridine in toluene solution is blueshifted by 6 nm relative to the ZnTPP band. Table 1 also shows that the phosphorescence bands of the porphyrins, except ZnTpNO*PP, in the pyridine/toluene solution appear at wavelengths 17-22 nm longer than the bands of the same porphyrins in toluene. In contrast, the ZnTpN02PP band is red-shifted by only 8 nm upon addition of the ligand. The bands observed in all of our phosphorescence spectra are ascribed to the T(0,O) 23.26 emission from the Ti state, in accord with earlier ~tudies.'~-~' Triplet-Triplet Absorption Spectra. Triplet-triplet absorption spectra of ZnTpN02PP and ZnTMPP are shown in Figure 3. These spectra include the Soret and Q(1,O) bleaching bands, at 420-440 and 550-570 nm, respectively. In each case, a clearly definable absorption maximum appears to the red of the Soret bleaching band and the wavelength maxima are given in Table 1. As noted by Rodriguez et a1.,6 shoulders appear on these bands, indicating other weaker transitions in the same region. Our triplet-triplet data on ZnTPP and ZnTTP agrees well with that of previous s t ~ d i e s . ~ , ~ , ~ . ~ ~ For all porphyrins but ZnTpN02PP, the shifts of the absorption bands exhibit similar trends in both the ligating and nonligating solvents; the ZnTMPP, ZnTTP, ZnTpClPP, and ZnTpBrPP bands are all red-shifted from the ZnTPP and ZnTpFPP bands, which appear at similar wavelengths. Relative to the ZnTPP and ZnTpFPP bands, the ZnTpN02PP band appears at a slightly shorter wavelength in toluene but at essentially the same wavelength in the pyridine/toluene solution. It is interesting to note that, in all cases, addition of the pyridine
J. Phys. Chem., Vol. 99, No. 4, 1995 1169
Triplet States of Zinc(I1) Tetraphenylporphyrins ligand shifts the triplet-triplet absorption bands by only 1-6 nm. Since our triplet-triplet absorption spectra were obtained 100 ns after the pump pulse, the observed spectral bands reported in Table 1 are ascribed to transitions from the T1 state to the higher energy T, state. Of particular interest in the triplet-triplet spectrum of ZnTMPP in toluene is the appearance of a weaker, broad absorption in the 440-470 nm region, near the Soret bleaching band. An early calculation of the ZnTPP triplet spectrum by Gouterman7 predicted two absorption bands in the wavelength region near the ground-state Soret transition. These two bands have been observed in other studies of the triplet-triplet spectra of ZnTPP that looked more closely at wavelengths at or lower than the Soret a b s o r p t i ~ n . ~Since . ~ the (stronger) triplet-triplet band of ZnTMPP is the most red-shifted, it is possible that the 440-470 nm absorption is part of a second absorption band, which is obscured in the other porphyrins due to its proximity to the Soret bleaching band. Interestingly, significant absorption in this region is not observed in the triplet-triplet spectrum of pyridine-ligated ZnTMPP.
-a
34400 T
34000 n
7
-
33600
E 0
33200
x
cn 32800 E W
a,
13200
+ U
3;
12800
+
a, -
.-a i
12400
k
12000
Discussion Triplet-State Configurations of the ZnTPPs. The lower energy singlet and triplet states of porphyrins are described by the Gouterman four orbital m ~ d e l . ~ -In ' ~this model, transitions from each of the two nearly degenerate highest occupied orbitals, and a2, (n), to the degenerate lowest unoccupied orbitals, al, (n) eg (JC*),yield nearly degenerate alueg and azUegconfigurations for both singlet and triplet states, in D4h symmetry. The electron interaction integral which couples the al, eg and azU eg transitions to form the lowest triplet states vanishes on symmetry grounds in D4h symmetry, and to a good approximation, the triplet alueg and aZuegstates do not mix.8913319934 It has been established that the T1 state of ZnTPP is a2ueg10935. The T1 states of ZnTMPP and ZnTTP must also be a2,eg, since the electron-donating substituents in these porphyrins should increase further the energy of the aZu orbital over that of the al, orbital. The halogen substituents can be both electron-donating (via resonance) and electron-withdrawing (via induction). The electron-donating and -withdrawing abilities of these halogens may nearly cancel each other out. (This hypothesis is supported by the lack of phosphorescence band shift upon halogen substitution.) Hence, it is likely that the T1 states of our halogenated porphyrins are also a2,eg. The nitro substituent is electron-withdrawing by resonance and induction. Sufficient lowering of the a2, orbital energy due to electron withdrawal could cause the identity of the T1 state of ZnTpNOZPP to be alueg. Note that since ligation selectively increases the a2,,, orbital energy, the T1 state configuration of ZnTMPP, ZnTTP, ZnTPP, ZnTpFPP, ZnTpClPP, and ZnTpBrPP in the 0.5% pyridine in toluene solution, should all be a2,eg. Ligation also increases the likelihood of an a2uegT1 state for ZnTpNOzPP. We expect that the configuration of the T, state for all the porphyrins (including ZnTpN02PP) is a2,alUegeg;the configuration suggested for the ZnTPP T, ~ t a t e . ~ . ~ TI State Energies. By adding the energies of the triplettriplet transitions to the T1 state energies calculated from the phosphorescence wavelength maxima, we calculated the T, state energies of the studied Z ~ T P P S The . ~ ~T1 and T, state energies of the porphyrins in toluene and pyridine/toluene are plotted in Figure 4. The plot shows an insensitivity to para-halogen substitution on the location of T(0,O) phosphorescence bands. This has previously been observed for ZnTpClPP, ZnTpBrPP, and ZnTpIPP21$24 in 1:l petroleum ether:propanol, and our work suggests that this is not an accident of solvatochromism but
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-
Figure 4. (a) T,state energies of porphyrins in toluene. (b) T, state energies of porphyrins in a 0.5% pyridine in toluene solution. (c) TI state energies of porphyrins in toluene. (d) TI state energies of porphyrins in a 0.5% pyridine in toluene solution. (Since the wavelengths in Table 1 are reported to the nearest nm, the energies are rounded to the nearest 100 cm-I.) See text for experimental details.
persists in other solvents, both ligating and nonligating. Figure 4 also shows that of all the substituents, the methoxy groups in ZnTMPP have the greatest effect on the T1 state energy. The difference in T1 state energies for ZnTMPP and ZnTPP is 300 cm-', in both solvent systems. Substituent effects on various properties of tetraphenylporphyrins, including spectral band positions, have often been evaluated using linear free energy relationship^.^^ Plots of the T1 state energy shifts due to para substitution vs the sum of the Hammett substituent constant, 4a+, are shown in Figure 5 for both solvent systems. (The coefficient u+ is another measure of electronic influence which includes more resonance contribution than does q,.33) In both plots of Figure 5, the point for the NO2 substituent (in ZnTpN02PP) was excluded from the least-squares fits. The bottom plot shows that for the porphyrins in the 0.5% pyridine in toluene solution, there exists a reasonable correlation between the T1 state energy shifts and the electrondonating or electron-withdrawing character of the substituent. Note that the point for the NO2 substituent lies on the line representing the 95% confidence limit of the least squares fit. In the Hammett plot for the porphyrins in toluene, the point for the NO2 substituent shows a large deviation from the leastsquares fit line (top plot). This deviation is a result of the redshift of the phosphorescence band of ZnTpN02PP relative to that of ZnTPP in toluene, in contrast to the blue-shift observed in the pyridine/toluene solution. The reason for the anomalous behavior of ZnTpN02PP in toluene may be that the configuration of the T1 state in this solvent is not a2,eg but alueg. Figure 4 also shows that ligation by pyridine causes a decrease in the energy of the T1 state of ZnTpNOzPP by 100 cm-' and of the other porphyrins by about 300 cm-'. Note that for ZnTMPP, the magnitude of the red shift due to meso substitution is the same as that caused by ligation.
1170 J. Phys. Chem., Vol. 99, No. 4,1995
Walters et al.
400 200
-
A
0
I
E
2.-200 x -
a, C
-400
-4
-2
0
2
4
0
2
4
W
+5
in
-4
toluene
-2
4a+ Figure 5. Hammett plot of the difference between the TI state energies of each of the seven studied porphyrins and that of ZnTPP vs 4at, in toluene (top plot) and in a 0.5% pyridine in toluene solution (bottom
plot). Data points are labeled with the corresponding substituent. For each plot, a least-squares fit was performed on all points except that for ZnTpN02PP. (Since the phosphorescence wavelengths are reported to the nearest nm, the TI state energy shifts are rounded to the nearest 100 cm-I.)
T, State Energies. As with the TI state, the methoxy substituents in ZnTMPP cause the greatest shift in the T, state energy, relative to that of ZnTPP. This T, state energy shift is 1000 and 1100 cm-', in toluene and pyridine/toluene, respectively. This is substantially greater than the 300 cm-' energy shift observed for the TI state. Substituent effects are magnified in the T, state. The pattern of T, state energy shifts is similar in the ligating and nonligating solvent systems. It is noted that the electrondonating methyl and methoxy substituents in ZnTTP and ZnTMPP lower the energy of the T, state, the same effect these substituents have on the TI state energy. The fluoro substituent has little effect on the T, state energy, again similar to its effect on the T1 state. Interestingly, the chloro and bromo substituents cause a significant lowering of the T, state energy. Another notable feature is that the T, state energy of ZnTpN02PP is highest of all porphyrins in both nonligating and ligating solvents. The nitro substituents appear to cause the energy of the T, state to increase. Although the configuration of the T1 state of this porphyrin is not clear and may even be different in toluene (alueg) and pyridine/toluene (a2,eg), the configuration of the T, state in both solvent systems is expected to be the same, a2"alUegeg. In accord with this, the T, state energy shifts for ZnTpN02PP in the two solvent systems exhibit the expected behavior for electron-withdrawing substituents. It is apparent from Figure 4 that ligation causes a 0-300 cm-' decrease in the energy of the T, states of all the ZnTPPs. Generally, then, the effect of ligation by pyridine on the T, state energies is somewhat less than the effect on the T1 state energies. Electronic Structures of the TI and T, States. In this study, we have found that para-phenyl substituents affect the energy of the T, state much more than that of the T1 state. This suggests greater electronic communication between the phenyl and porphyrin rings in the T, state. The magnitude and, particularly, the pattern of energy shifts for the various substituents suggests that the greater electronic communication is accomplished through enhanced resonance interaction. Specifically, the lower
energy of the ZnTpClPP and ZnTpBrPP T, states can be attributed to electron donation via resonance of the chloro and bromo substituents. Although resonance donation from the fluoro substituent should be greater, it is possible that the greater inductive effect of fluorine balances the resonance effect in ZnTpFPP. Significant electron donation via resonance of the chloro and bromo substituents appears to be absent in the T1 state. Greater resonance interaction between the phenyl and porphyrin rings in the T, state suggests a specific geometry difference between the two triplet states. Resonance interaction can increase by rotation of the phenyl rings toward the plane of the porphyrin ring. We conclude, therefore, that the average dihedral angle between the phenyl and porphyrin rings is smaller in the T, state than in the Ti state. To support this, we carried out molecular mechanics calculations to determine the relative energy barriers to phenyl rotation in the T1 state and the ground state of ZnTPP. It is known that the energy barrier is high in the ground-state and X-ray crystallographic studies indicate a dihedral angle of 69-90' for metallated tetraphenylporphyrins in this state.38 We modeled the ground state with atomic coordinates from published crystallographic data.39 The T1 state was modeled with atomic coordinates predicted by the semiempirical calculations of Prendergast and Spiro16 for zinc(I1) meso-tetrafluoroporphine; for this molecule the a2, orbital lies higher in energy than the al, orbital, as is the case for ZnTPP. Our MM2 calculations (Personal CAChe, CAChe Scientific) indicated that the energy barrier in the T1 state is 30% less than in the ground state, hence, is still significant. It is possible that the barrier to phenyl rotation in the T, states of these porphyrins is even lower allowing the average dihedral angle to assume a lower value. The observation that ligation has a generally lesser effect on the T, state energy than on the T1 state energy implies that the electronic structure in the region of the nitrogen atoms is different in the two states, but the difference seems to be less profound than in the meso-carbon region. Dynamics of the Triplet States. The results presented here are in accord with conclusions drawn from earlier investigations of the resonance Raman spectra of the T1 state of ZnTPP and two deuterated derivatives, ZnTPP-d8 and Z ~ T P P - ~ ~The O.~.~~ T, state was the resonant higher energy triplet state in these studies. The most intense peaks in these transient Raman spectra were assigned to phenyl stretching modes. Since the vibrational modes that experience the greater intensity enhancement in resonance Raman spectra are those that mimic the difference in electronic structure between the two resonant states, the high peak intensities were explained by delocalization of electrons onto the phenyl rings in the T, state14 (referred to as the T, state in that reference). The present work supports a conclusion of greater electronic communication between the phenyl and porphyrin rings and specifically points to an enhanced resonance interaction in the T, state, probably due to a reduced dihedral angle. Another interesting feature of the TI state Raman spectrum of ZnTPP is that the frequencies of the phenyl modes are shifted by no more than 5 cm-I from the ground-state values. This suggests that the phenyl-porphyrin interaction is similar in the ground and T1 states. As previously mentioned, the energy barrier to rotation is high in the ground state. Our present computational results suggesting that the rotational barrier is only 30% less in the Ti state than in the ground state and experimental results indicating that the phenyl-porphyrin interaction in the TI state is small (at least compared to the T,
J. Phys. Chem., Vol. 99, No. 4, 1995 1171
Triplet States of Zinc(I1) Tetraphenylporphyrins state) are consistent with this slight shift in phenyl ring vibrational frequencies. The complementarity of the conclusions drawn from this phosphorescence/triplet-triplet absorption study of substituted ZnTPPs and transient resonance Raman spectra of ZnTPP and isotopic derivatives underscores the utility of these combined studies in deriving structural information on porphyrin excited states.
Acknowledgment. J.C.dP. thanks the Olin Charitable Trusts of the Research Corporation (Grant C3007R), the Petroleum Research Fund (Grant 26491-B), and Howard Hughes Medical Institute for financial support. J.C.dP. and V.A.W. also thank Pew Charitable Trusts for partial funding of this project. References and Notes (1) For example, see: (a) Mironov, A. F.; Seylanov, A. S.;Seylanov, J. A,; Pizhik, V. M.; Deruzhenko, I. V.; Ju Nockel, A. J. Photochem. Photobiol. B: Biol. 1992, 16, 341. (b) Borland, C. F.; McGarvey, D.J.; Morgan, A. R.; Tmscott, T. G. J . Photochem. Photobiol. B: Biol. 1988,2, 427. (c) Schermann, G.; Volcker, A.; Seikel, K.; Schmidt, R.; Brauer, H.D.; Montforts, F.-P. Photochem. Photobiol. 1990,51,45.(d) Jon, G. Radiat. Phys. Chem. 1987, 30, 375. (2) For example, see: (a) Kay, A.; Gratzel, M. J . Phys. Chem. 1993, 97, 6272. (b) Harriman, A.; Porter, G.; &chow, M.-C. J . Chem. Soc., Faraday Trans 2 1981, 77, 833. (c) Kalyanasundaram, K.; NeumannSpallart, M. J . Phys. Chem. 1982, 86, 5163. (3) Pekkarinen, L.; Linschitz, H. J . Am. Chem. SOC.1960, 82, 2407. (4) Harriman, A. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1281. (5) Reed, R. A,; Purrello, R.; Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9720. (6) Rodriguez, J.; Kirmaier, C.; Holten, D. J . Am. Chem. SOC.1989, 111, 6500. (7) Gouterman, M. J. Chem. Phys. 1960, 33, 1523. (8) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, Chapter 1. (9) (a) Gouterman, M. J . Chem. Phys. 1959,30, 1139. (b) Gouterman, M. J . Mol. Spectrosc. 1961, 6, 138. (10) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. lnorg. Chem. 1980, 19, 386. (1 1) Shelnutt, J. A.; Ortiz, V. J . Phys. Chem. 1985, 89, 4733. (12) Ohno, 0.; Kaizu, Y.; Kobayashi, H. J . Chem. Phys. 1985,82, 1779. (13) Gouterman, M. Ann. N.Y. Acad. Sci. 1973, 206, 70. (14) (a) Walters, V. A.; de Paula, J. C.; Babcock, G. T.; Leroi, G. E. J . Am. Chem. SOC.1989, 111, 8300. (b) Nam, H.; Walters, V. A.; de Paula, J. C.; Babcock, G. T.; Leroi, G. E. In Proceedings of the XIIth Intemational Conference on Raman Spectroscopy;Durig, J. R., Sullivan, J. F., Eds.; Wiley and Sons: New York, 1990; p 618. (15) de Paula, J. C.; Walters, V. A,; Nutaitis, C.; Lind, J.; Hall, K. J . Phys. Chem. 1992, 96, 10591. (16) Prendergast, K.; Spiro, T. G. J . Phys. Chem. 1991, 95, 9728. (17) (a) 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,
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