J. Phys. Chem. 1995, 99, 4330-4334
4330
Photophysical Consequences of Porphyrin Tautomerization. Steady-State and Time-Resolved Spectral Investigations of a Zinc Isoporphyrin Steve Gentemann,lBSam H. Leung,lb Kevin M. Smithtb Jack Fajer,*Jc and Dewey Holten*Ja Department of Chemistry, Washington University, St. Louis, Missouri 63130, Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973, and Department of Chemistry, University of Califomia, Davis, Califomia 95616 Received: August 30, 1994; In Final Form: December 6, 1994@
Isoporphyrins are porphyrin tautomers with a saturated meso carbon and thus an interrupted n system. We report here steady-state optical absorption, fluorescence, and fluorescence polarization data as well as timeresolved results that detail the significant effects of porphyrin tautomerization on the photophysical properties of a metallo-isoporphyrin, zinc 2,3,5,5’,7,8,12,18-octamethyl13,17-bis( 3-methoxy-3-oxopropy1)isoporphyrin perchlorate (2). Besides the red-shifted, low-energy absorption bands diagnostic of metallo-isoporphyrins, 2 exhibits a large Stokes shift of its fluorescence emission (-600 cm-l) and an unusually short singlet excitedstate lifetime at room temperature (130 f 15 ps), photophysical properties distinctly different from those of the canonical porphyrin tautomers. The only porphyrins to exhibit marginally similar perturbations of their photophysical properties are those with severely nonplanar macrocycles whose n systems are significantly destabilized by the conformational distortions and thus approach the interrupted ~d systems of isoporphyrins (Gentemann et al. J. Am. Chem. SOC.1994, 116, 7363). In addition to providing the first insights into the photophysical consequences of porphyrin tautomerization, the results for the isoporphyrin further document the sensitivity of the fundamental electronic and excited-state properties of porphyrinic chromophores to modulation of their ~dsystems in vitro and, by extrapolation, in vivo as well.
Introduction Isoporphyrins (l), tautomers of porphyrins (1’) with a saturated meso carbon and thus an interrupted n system, were originally postulated by Woodward2 (Figure 1). Replacement of the lone NH proton by insertion of a divalent metal into 1 should yield a cation, and, indeed, the fist metallo-isoporphyrin was reported by Dolphin et aL3 who prepared zinc 5’-methoxy5,10,15,20-tetraphenylisoporphyrin perchlorate by nucleophilic attack of methanol on the oxidized dication of zinc tetraphenylporphyrin. Since then, isoporphyrins, particularly tetraaryl derivatives, have been synthesized chemically, electrochemically, and phot~chemically!-~ The impetus for the studies arises from the occurrence of isoporphyrins in reactions of oxidized porphyrins with nucleophiles, as byproducts of heme oxidations, and as putative intermediates in the biosynthesis of
chlorophyll^.^-^ Although tetraaryl metallo-isoporphyrins are readily synthesized, their chemistry is complicated by their tendency to revert to porphyrin^.^*^*^ The recently synthesized8zinc isoporphyrin (2) prevents back-tautomerization and should allow a fuller characterization of the chemical and physical properties of isoporphyrins. Indeed, 2 is sufficiently stable to allow its molecular structure to be determined by X-ray diffra~tion.~ The crystallographic results confirm that the complex is cationic and incorporates a saturated mesu carbon, features characteristic of a metallo-isoporphyrin. The overall pattern of bond distances is most consistent with the resonance forms of the interrupted n system illustrated in Figure 1. We report here steady-state optical absorption and fluorescence and fluorescence polarization data as well as time-resolved
* Author to whom correspondence @
should be addressed. Abstract published in Advance ACS Abstracts, March 1, 1995.
results that detail the significant effects of the porphyrin tautomerization on the photophysical properties of isoporphyrins.
Experimental Section The preparation and characterization of 2 has been described HPLC grade solvents were dried just prior to use in all spectroscopic measurements. Ground-state absorption spectra were obtained on a Perkin-Elmer PE330 spectrophotometer. Fluorescence spectra were acquired on a Spex Fluorolog II operated in a right angle detection geometry employing either a cooled Hamamatsu R406 photomultiplier tube or an RCA C30956E Si avalanche photodiode and lock-in detection. Spectra were corrected for variation of detection wavelength sensitivity. Fluorescence excitation polarization was measured in 2-methyltetrahydrofuran ( 2 - M e w ) glasses at 78 K or glycerol solutions at 295 K. Glan-Taylor prisms were used for polarization of excitation and detected light. Correction was made for any polarization bias of the detection system, as described further in the Results. Time-resolved absorption spectra were obtained using a pump-probe apparatus previously described.1° Excitation flashes (1 d, 30 ps, 532 nm) pumped the -100 pM solutions of porphyrin while a weak white-light pulse probed transient species up to 12 ns after excitation.
Results Ground-State Absorption Spectra. The optical spectrum of 2 in CH3CN is shown in Figure 2. The major absorption bands occur at -320,420 and 790 nm ( E 3 x lo4 M-l cm-’). The 320 nm band consists of at least three overlapping transitions, and the 790 nm band is flanked by weaker features near 725 and 660 nm. There is also a weak band at 520 nm. The distinctive spectral features of the isoporphyrin are insensitive to solvent or counterion; the optical spectrum of the chloride
0022-3654/95/2099-4330$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 12, 1995 4331
Photophysical Consequences of Porphyrin Tautomerization
Isoporphyrin (1)
Porphyrin (1')
Me Me
Me\,
Me
\
\
?
I
7+
7
Zn(ll)isoporphyrin+CIOL (2) Figure 1. Tautomeric structures of isoporphyrin (1) and porphyrin (1'). Resonance structures of the zinc isoporphyrin (2) based on its X-ray ~tructure.~
salt of 2 in CH2C1z8 is nearly identical to those of the perchlorate salt in CH2C12, dimethyl sulfoxide, butyronitrile, butyronitrile plus pyridine, acetonitrile, 2-methyltetrahydrofuran, and glycerol. Substitution of an ester group for one of the methyl groups at the saturated meso carbon does not affect the spectrum either.8 A similar trend is observed for meso-tetraarylisoporphyrins which display similar spectral features that are nearly independent of the metal (Zn, Fe, Sn, Pd, In), counterion, or substituent at the tetrahedral ~ a r b o n . ~The - ~ spectrum shown in Figure 2 thus represents an authentic spectral signature of the metalloisoporphyrin framework. Fluorescence Spectra. The steady-state fluorescence spectra of 2 in CH3CN at 295 K and in 2-MeTHF glass at 78 K are shown in Figure 3. The lowest-energy features in the groundstate absorption spectrum at 295 K are also shown. The fluorescence maximum occurs at -830 nri~at both temperatures, although the shoulder at 900 nm is better resolved in the 78-K spectrum. The fluorescencequantum yield, &, is 0.004 f 0.002 at 295 K, determined by comparison with the fluorescence of bacteriochlorophyll a (& = 0.211). The fluorescence profile symmetrically mirrors the lowenergy absorption manifold (Figure 3). The spacing between the strong band and the weaker, flanking features is roughly 1200 cm-' both in absorption and emission, a value typical for vibronic spacings in metalloporphyrins.'* Similarly, the relative intensities of the main and weaker bands are comparable in the absorption and emission spectra. These results suggest that in both the absorption and emission spectra, the strong feature in the vicinity of 800 nm reflects the electronic origin transition and the weaker features are vibronic overtone bands. Fluorescence and absorption maxima are separated by -600 cm-'. This Stokes shift is larger than the -50 cm-' observed for ZnOEP, for example.12 The persistence of the fluorescence at 78 K confirms that the emission is primarily prompt fluorescence from the lowest excited singlet state rather than being delayed
0.4 El 0.2 .3 0.0 c . , -0.2
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.
a.
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0.2 . . 0.0
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3
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2 300 400 500 600 700 800 900
Wavelength (nm)
Figure 2. Ground-state absorption spectrum of 2 in CHsCN at room temperature (lower panel). Upper panels show the polarization of fluorescence of the two emission bands indicated.
fluorescence arising from thermal repopulation of S1 from a lower-energy state such as the triplet excited state. Fluorescence Excitation Polarization. The value of the polarization ratio, P , for a given pair of excitatioddetection
4332 J. Phys. Chem., Vol. 99, No. 12, 1995
I
I
I
I
I
I
I
800
900
Emission
Qi Absorption A
Gentemann et al.
A
0.0
4
d
-0.1 600
700
800
900
1000
Wavelength (nm) Figure 3. Room temperature and 78 K fluorescence spectra of 2 and room temperature absorption bands in the Q’yregion. Room temperature spectra in CH3CN, 78 K emission spectrum in 2-MeTHF.
wavelengths is calculated from emission intensity, I, according to
where superscripts and subscripts refer to the polarization direction of the excitation and detection light, respectively, and the factors in parentheses correct for any polarization bias of the detection system (e.g. I: is the horizontally polarized emission intensity obtained with vertically polarized excitation light).13 The curves in the top two panels of Figure 2 give the fluorescence excitation polarization spectra of 2 in glycerol for detection in each of the two emission bands. The top curve represents detection in the fluorescence origin (830 nm) while the lower polarization trace corresponds to detection in the emission vibronic band (910 nm). Both fluorescence excitation polarization traces in the region of the long-wavelength absorption band exhibit maximum values approaching the theoretical limit of 0.5 expected for excitation and detection of the same electronic tran~iti0n.l~ The polarization in the near-UV region of the spectrum falls to a value below -0.2 in the band at 410 nm and rises to a substantially positive value on the shortwavelength side of the multiple-banded absorption feature centered near 320 nm. The bands at -800 and 320 nm are thus polarized in the same direction, whereas the band at -410 nm possesses an orthogonal polarization. Transient Absorption. Figure 4 displays transient absorption difference spectra of 2 in CHzC12 obtained in the region of the lowest-energy ground-state absorption bands. At 60 ps after excitation, the long-wavelength ground-state origin and overtone absorption bands are bleached and only a weak positive absorption appears above these bleachings near 600 nm. The transient spectrum decays to zero with a time constant of 130 f 15 ps. After 600 ps, the ground-state bleaching and excitedstate absorption are no longer observed. This 600 ps null spectrum thus indicates that the excited state responsible for the spectrum at earlier times has decayed to the ground electronic state with ’90% yield.
Discussion The interrupted z systems of metallo-isoporphyrins profoundly affect their optical properties, particularly the lowenergy transitions. This effect has been noted repeatedlf-’ since the original preparation of a metallo-isoporphyrin3 and is
600
700
Wavelength (nm) Figure 4. Transient absorption difference spectra of 2 in CHzClz at room temperature obtained at the delay times indicated after excitation with a 30 ps, 532 nm flash. Essentially the same spectra are observed in CH3CN and 2-MeTHF.
obvious in the spectrum of 2 shown in Figure 2. For comparison, taking zinc 2,3,7,8,12,13,17,18-octaethylporphyrin (ZnOEP) as the approximately equivalent porphyrin tautomer of 2, the first absorption bands of ZnOEP in a variety of solvents (and even in the presence of complexing anions) range between 530 and 580 nm or 2.34 and 2.14 eV.15 In contrast, the lowenergy transition of 2 occurs at 798 nm (1.55 eV) and the corresponding fluorescence emission at 830 nm (1.49 eV). Also, the redox properties of the two chromophores differ markedly, particularly the reduction potentials. In butyronitrile, ZnOEP undergoes a one-electron oxidation to a n cation radical at a potential of = 0.63 V vs SCE and a one-electron reduction to a z anion radical at E Y = -1.61 V.16 In porphyrins and hydroporphyrins, the difference between the two redox potentials, AE = Eox - EWD,corresponds approximately to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), neglecting solvation effects.17J8 Indeed, the observed AE of 2.24 V in ZnOEP corresponds nicely with the optical energy gap of 2.17 eV for the compound in the same s01vent.l~ In contrast to the redox behavior of ZnOEP, 2 in butyronitrile is reversibly oxidized and reduced at 1.09 and -0.29 V, respectively, for a AE of 1.38 V,9,20in reasonable agreement with the optical transition of 1.55 eV. (Note that solvation effects may be significant in the redox chemistry of 2 since it is a cationic complex. Oxidation leads to a dication radical, whereas reduction results in a neutral radical.) Clearly, 2 is substantially easier to reduce than ZnOEP (-0.29 V vs - 1.61 V) and somewhat harder to oxidize (1.09 V vs 0.63 V). At least part of these differences must arise from the cationic nature of 2. These electrostatic effects can be estimated from the redox chemistry of charged porphyrin radicals; oxidation of porphyrin n cations to dications typically occurs at potentials which are 0.3-0.4 V more ~0sitive.l~ For example, ZnOEP+ ZnOEP2+ occurs at 1.02 V, a potential -0.4 V more positive than required for the first oxidation.16 The redox potentials of porphyrins, chlorins, isobacteriochlorins, and bacteriochlorins have been shown to track the calculated relative energies of their HOMO’S and LUMO’S.’~ As a HOMO is stabilized, its energy level is lowered and the molecule becomes harder to oxidize. Conversely, if the LUMO is lowered, the molecule becomes easier to reduce.18 Extrapolation of these trends to 2 and ZnOEP clearly suggests that the LUMO of the isoporphyrin has shifted down significantly more
e
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Photophysical Consequences of Porphyrin Tautomerization than the HOMO relative to ZnOEP and it is this resulting smaller gap between the HOMO and LUMO that causes the observed red shift in the optical spectrum. The classical four-orbital model developed by Gouterman12 accounts for the major optical features of porphyrins and hydroporphyrins in terms of transitions consisting of HOMO LUMO, HOMO LUMO 1, HOMO - 1 LUMO, and HOMO - 1 LUMO 1. In bacteriochlorins, which exhibit spectra and oscillator strengths similar to those of 2, these transitions and their polarization correspond to Qy, B,, and By, in increasing order of energy. If the four-orbital model is extended to isoporphyrins, then the optical bands at 798,520,420, and 320 nm would be assigned to the equivalent states Q’y, Q’,, B’,, and Bjy with the 798 and 320 nm bands having the same polarization and the 520 and 410 nm bands having the orthogonal polarization, as shown in Figure 2.21The two strong features at a slightly longer wavelength than the B’y band at 320 nm may derive from contributions of high-energy configurations, at least one x-polarized, not accounted for by the simple four-orbital model. Calculations will be needed to address this issue. Since the additional optical bands at 660 and 725 nm appear to be vibronic overtones of the 798 nm electronic transition, we label these as Q;(2,0), l,O), and Q’y(O,O), respectively, and the fluorescence band at 900 nm as Q’y(O,l). Interestingly, all these transitions have essentially the same strong positive polarization. Several factors may contribute to the observation that the Q’y vibronic overtone bands of 2 have the same high polarization ratio as the electronic origin. First, Herzberg -Teller (vibronic) coupling of the Q’ and B’ states may be reduced in 2 compared to the other systems due to their larger energy separation associated with the substantial red shift of the transition, as in bacteriochlorins. Second, the vibronic bands may derive considerable intensity via a Franck-Condon effect, in which case the bands should maintain the Polarization of the electronic origin. A redistribution of Franck-Condon intensity from the origin to overtone bands would occur if the potential energy surface for the SI excited state is displaced from that for the ground state due to changes in structural parameters. Supporting evidence for such a structural displacement derives from the rather large -600 cm-’ shift between the absorption and fluorescence maxima. This shift is substantially larger than the values of go%) deactivation to the ground state via internal conversion, with only a small (< 10%) probability of triplet formation. From these data, we can obtain a lower limit of ZISC > 130 ps1O.l = 1.3 ns for the inherent time constant for intersystem crossing. This value is not substantially different than the estimates of 2-4 ns for z~scobtained from photophysical data for typical metalloporphyrins, chlorins, and bacterio~hlorins.~~ On the other hand, the inherent time constant of internal conversion TIC < 130 PSI 0.9 130-140 ps is at least a factor of 50 shorter than the values of 7-20 ns typical of porphyrins and hydroporphyrins. Again, notable exceptions to the latter values are found for the metal-free porphyrins mentioned above that show substantial deviation from planarity.” The shortened excited-state lifetimes of these distorted porphyrins (400-850 ps) compared to planar free-base porphyrins (10-20 ns) are due in part to an 1.5 ns inherent time constant for S1 SO nonradiative relaxation that is about a factor of 50 shorter than normal. Interestingly, there is also a substantial enhancement of intersystem crossing in the nonplanar porphyrins,22 a situation we do not find for 2. Whether this difference reflects the presence of the central metal ion or different modes of structural distortions is not clear at present and is under investigation. Nonetheless, both the distorted free-base porphyrins and 2 exhibit substantially enhanced internal conversion of the ‘(x,n*) excited state to the ground state, with a greater enhancement in the isoporphyrin. Within the context of the Fermi Golden Rule, the inherent rate of internal conversion is the product of a density-of-statesweighted Franck-Condon (vibrational overlap) factor and an electronic matrix element. As a starting point for a discussion
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4334 J. Phys. Chem., Val. 99, No. 12, 1995
of the internal conversion rates, we will not consider the latter factor nor deviations from this framework involving BomOppenheimer breakdown. The Franck-Condon factor is governed by the relationship between two parameters: (i) the magnitude of the coordinate displacement (the horizontal shift) between the potential surfaces for the excited and ground states and (ii) the vertical energy gap between the energy minima of the surfaces (ignoring, for simplicity, vibrational frequency differences between S1 and SO). The optimum Franck-Condon factor arises when the “reorganization energy” is the same as the energy gap, in which case the potential energy surface for SO crosses at the minimum in that for SI. We have proposed for the distorted metal-free porphyrins that the -50-fold increase in the rate of internal conversion compared to relatively planar porphyrins arises primarily from a coordinate shift in the excited state along a collective coordinate reflecting the degree of nonplanarity, evidenced by the large Stokes shift of -900 cm-’. In the case of 2, although the Stokes shift of -600 cm-’ is somewhat smaller than for the distorted free-base porphyrins, the energy gap between SI and SO is considerably reduced, as indicated by the bathochromic shift of the ground-state absorption band to 800 nm. Hence, one would also expect an improved Franck-Condon factor for internal conversion in 2 compared to typical porphyrins. However, whether the magnitude of the substantially increased S1 SO radiationless deactivation of 2 is imbedded entirely in the Franck-Condon factor or involves additional factors will require further studies on a larger series of isoporphyrins.28 The results presented above for the zinc isoporphyrin illustrate the significant changes that can be induced in porphyrinic systems by a tautomerization that disrupts the n system of the chromophore. We have also called attention to the many similarities between the isoporphyrin and nonplanar porphyrins in which the n systems are destabilized. The cumulative results for the isoporphyrins and the nonplanar porphyrins thus illustrate the sensitivity of the fundamental electronic and excited-state properties of porphinoid chromophores to modulation of their n systems in vitro and, by extrapolation, in vivo as well. In addition, these results suggest that such chromophores can be tailored to exhibit specijk photophysical and chemical properties by synthetic manipulation of their conformations and n systems.
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Acknowledgment. This work was supported by NIH Grant GM34685 (D.H.), NSF Grant CHE-93-05577 (K.M.S.), and the Division of Chemical Sciences, U.S.Department of Energy, under Contract DE-AC02-76CH00016 (J.F.). References and Notes (1) (a) Washington University. (b) University of California. (c) Brookhaven National Laboratory. (2) Woodward, R. B. Ind. Chim. Belge 1962, 27, 1293. (3) (a) Dolphin, D.; Felton, R. H.; Borg, D. C.; Fajer, J. J. Am. Chem. SOC.1970, 92, 743. (b) Dolphin, D.; Muljiani, Z.; Rousseau, K.; Borg, D. C.; Fajer, J.; Felton, R. H. Ann. N. Y.Acad. Sci. 1973, 206, 177. (4) (a) Guzinski, J. A.; Felton, R. H. J . Chem. Soc., Chem. Commun. 1973, 715. (b) Shine, H. J.; Padilla, A. G.; Wu, S. M. J. Org. Chem. 1979,
44,4069. (c) Gold, A.; Ivey, W.; Toney, G. E.; Sangaiah, R. Inorg. Chem. 1984,23,2932. (d) Lee, W. A.; Bruice, T. C. Inorg. Chem. 1986,25, 131. (e) Takeda, Y.; Takahara, S.; Kobayashi, Y.; Misawa, H.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1990, 2103. (5) (a) Dolphin, D.; Halko, K. J.; Johnson, E. C.; Rousseau, K. In Porphyrin Chemisiy Advances; Longo, F. R., Ed.; Ann Arbor Science: Ann Arbor, MI, 1979; p 119. (b) Cavaleiro, J. A. S.; Evans, B.; Smith, K. M. Ibid. p 335. (6) (a) Kadish, K. M.; Rhodes, R. K. Inorg. Chem. 1981, 20, 2961. (b) Guilard, R.; Jagerovic, N.; Tabard, A.; Naillon, C.; Kadish, K. M. J. Chem. SOC.,Dalton Trans. 1992, 1957. (c) Hinman, A. S.; Parelich, B. J.; Kondo, A. E. J. Electroanul. Chem. Interfacial Electrochem. 1987, 145 (7) (a) Mosseri, S.; Mialocq, J. C.; Perly, B.; Hambright, P. J. Phys. Chem. 1991, 95, 2196. (b) Harriman, A.; Porter, G.; Walters, P. J. Chem. SOC., Faraday Trans. I 1983, 76, 1335. (c) Richoux, M. C.; Neta, P.; Christensen, P. A.; Hamiman, A. J. Chem. Soc., Faraday Trans. 2 1986, 82,235. (d) Szulbinski, W.; Strojek, J. W. J. Electroanul. Chem. Interfacial Electrochem. 1988, 252, 323. (8) Xie, H.; Smith, K. M. Tetrahedron Lett. 1992, 33, 1197. (9) Barkigia, K. M.; Renner, M. W.; Xie, H.; Smith, K. M.; Fajer, J. J. Am. Chem. SOC.1993, 115, 7894. (10) Kim, D.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305. (11) Tait, C. D.; Holten, D. Photobiochem. Photobiophys. 1983,6,201. (12) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, p 1. (13) Cehelnik, E. D.; Mielenz, K. D.; Velapoldi, R. A. J. Re. Natl. Bur. Stand., Sect. A. 1975, 79A, 1. (14) Gouterman, M.; Stryer, L. J. Chem. Phys. 1%2, 37, 2260. (15) Fajer, J.; Borg, D. C.; Forman, A.; Felton, R. H.; Vegh, L.; Dolphin, D. Ann. N. Y.Acad. Sci. 1973, 206, 349. (16) Fuhrhop, J. H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. SOC. 1973, 95, 5140. (17) (a) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 5, p 53. (b) Davis, D. G. Ibid. p 127. (18) Chang, C. K.; Hanson, L. K.; Richardson, P. F.; Young, R.; Fajer, J. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 2652. (19) The compensating effects of solvation, configuration interaction, etc. in this agreement have been noted in ref 17. (20) Similar trends are observed for the chloride salt of 2 in dimethyl sulfoxide and dichloromethane: Fawcett, W. R.; Fedurco, M.; Smith, K. M.; Xie, H. J. Electroanul. Chem. Interfacial Electrochem. 1993,354,281. (21) The primes on the Q and B‘ state designations are used since the molecular symmetry axes (d and y’) are chosen to pass through the meso carbons rather than the nitrogens of the ring. This convention is analogous to that used for adjacent tetrahydro porphyrins (Gouterman, M.; Wagniere, G.; Snyder, L. J. Mol. Spectrosc. 1963, 11, 108). Also in analogy to the situation for typical porphyrins, we have retained the (O,O), (O,l), etc. designations of the bands. However, because of the coordinate shift between SO and S1 implied by the absorptiodfluorescence shift, it is possible that the (0,O)transition(s) of the mode(s) coupled to the optical transitions may not be completely resolved. (22) Gentemann, S.;Medforth, C. J.; Forsyth, T. P.; Nurco, D. J.; Smith, K. M.; Fajer, J.; Holten, D. J. Am. Chem. SOC. 1994, 116, 7363. (23) (a) Dwyer, P. N.; Buchler, J. W.; Scheidt, W. R. J. Am. Chem. SOC. 1974,%, 2789. (b) Buchler, J. W.; Dreher, C.; Lay, K. L.; Lee, J. A.; Scheidt, W. R. Inorg. Chem. 1983, 22, 888 and references therein. (24) Barkigia, K. M.; Fajer, J.; Adler, A. D.; Williams, G. J. B. Inorg. Chem. 1980, 19, 2057. (25) Scheidt, W. R.; Lee, Y. J. Stmct. Bonding (Berlin) 1987, 64, 1. (26) Gradyushko, A. T.; Tsvirko, M. P. Opt. Spectrosc. (Engl. Transl.) 1971, 31, 291. (27) Connolly, J. S.; Samuel, E. B.; Janzen, A. F.; Photochem. Photobiol. 1982, 36, 565. (28) For example, it is possible that the substantially decreased porphyrin LUMO energy together with an increase in the energy of a metal orbital drops the energy of a metal-to-ring charge-transfer state sufficiently to state of the isoporphyrin. facilitate radiationless deactivation of the ’(n,n*) JP9423256
