Cation and Anion Radicals of (5,15-Dimethyl-2,3,7,8,12,13,17,18

J. Phys. Chem. 1995,99, 8045-8049

8045

Cation and Anion Radicals of (5,15-Dimethyl-2,3,7,8,12,13,17,18-oc~ethyl-5~,15~porphinato)nickel(II): Comparison of the Nickel Complexes of Porphodimethene and Chlorin Chromophores Mark W. Renner" Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973

Johann W. Buchler Institut jiir Anorganische Chemie, Technische Hochschule Darmstadt, 0-64287 Darmstadt, Germany Received: October 31, 1994; In Final Form: February 27, 1995@

The metalloporphodimethene (5,15-dimethyl-2,3,7,8,12,13,17,18-octaethyl-5H, 15H-porphinato)nickel(II) [ l , Ni(OEPMe2)l is electrochemically oxidized or reduced to its n cation radical, 1+, or n anion radical, 1-, respectively. The half-wave potentials of these processes are +0.64 and -1.52 V, respectively (CH2C12, NBu4C104, SCE). The optical and X-band EPR spectra of 1+ (g = 2.003, CHZC12) and 1- (g = 2.000, THF) are characteristic of macrocycle radicals. The spin densities for 1+ and 1- obtained using iterative extended Huckel (IEH) calculations agree well with the observed EPR spectra. The EPR spectrum of 1- has a large relative spin density of 0.19 at CIOand (220 expressed by the large two-proton coupling of 5.06 G, which was verified by simulations. The n-n* transitions at 433 and 546 nm in the optical spectrum of 1 are reproduced by the intermediate neglect of differential overlap spectroscopic (INDOh) method (428 nm; 556 nm). The HOMO of 1 looks like a porphyrin al, orbital in which the two pyrromethene chromophores are conjugated through the nickel atom, while in the LUMO the two pyrromethenes are not coupled. Optically, 1+ is similar to 1, but 1- is more characteristic of a porphomethene. A comparison of 1, 1+, and 1- with nickel octaethylchlorin [2, Ni(OEC)], 2+,and 2- reveals that the redox properties (Eox - &ed = 2.16 V for 1 and 1.98 V for 2) and unpaired spin profiles are very similar despite structural and electronic differences within the otherwise isomeric dihydroporphyrin chromophores.

Introduction Conformational changes in porphyrin-containing enzymes and proteins are believed to play a crucial role in determining their electron-transfer and catalytic properties. In vivo the protein environment can be responsible for altering the porphyrin structure.'-4 Other important factors are the sites and degree of saturation in the porphyrin macrocycle. Progressively saturating the porphyrin macrocycle causes the conformational flexibility to increase. In the case of nickel porphyrins there are several structures5-' which range from planar to ruffled depending upon the porphyrin substituents and the crystallization conditions. In low-spin nickel(I1) hydroporphyrins, Le., 2,3dihydroporphyrin (chlorins), the increased flexibility as the macrocycle is reduced leads to shorter Ni-N distance^.^,^-" Model studies of porphyrins have shown that these changes are important in modulating their structural, optical, and redox properties as well as their r e a ~ t i v i t y . ~ ~ ~ ~As* .a' ~continuation -'~ of these studies, we present here the physical properties of a nickel(I1) 5,15-dihydroporphyrin. Dihydroporphyrinsare derivatives of porphyrins in which the macrocycle is reduced at one of the double bonds by addition of two hydrogen atoms. The most common example of dihydroporphyrins found in nature are chlorins, such as chlorophylls, which are the prosthetic group in ~ 1 a n t s . I ~Iron chlorins are also found in several proteins.I6 Porphodimethenes are isomers of chlorins in which two hydrogen atoms are added at opposite meso positions of the porphyrin macrocycle, thus @

Abstract published in Advance ACS Abstmcts, April 15, 1995.

disrupting the ring conjugation. They are also possible intermediates in the Rothemund synthesis of porphyrin~'~and products in the photoreduction of porphyrins.1s Dolphin has isolated the zinc porphodimethene in the synthesis of zinc tetraphenyloctamethylporphyrin, which has a characteristic absorption at 519 nm.I7 Since porphodimethenes are unstable and tend to undergo oxidative dehydrogenation to the corresponding porphyrin, there have been few studies of their physical properties. Stable metalloporphodimethenes have been synthesized by placing alkyl groups at the saturated meso positions, thereby preventing oxidation to the p ~ r p h y r i n . ' ~ - ~ ~ There are three possible stereoisomers for Ni(I1) porphodimethenes which are related by the stereochemistry of the alkyl groups at the C5 and Cl5 positions. These isomers are denoted syn-axial, syn-equatorial, and anti, of which the synaxial isomer is the most stable. Unlike Ni(I1) porphyrins and chlorins, which range from planar to S4 ruffled structures, a common feature of porphodimethenes is folding of the macrocycle about the C5-Cl5 axis and ruffling of the For the Ni(I1) complex this affords a "rooflike" structure with the apex along the C5-Ni-Cl5 axis, which allows the lowspin nickel to achieve short Ni-N distances, av = 1.908 A.24 We present here the electrochemical, optical, and EPR properties of the one-electron oxidation and reduction products of (5,15dimethy1-2,3,7,8,12,13,17,18-octaethy1-5H, 15H-porphinato)nickel(II)24 [Ni(OEPMe2), 1 refers to the syn-axial configuration] to investigate how the electronic and structural differences between Ni(I1) chlorins and porphodimethenese alter the physical properties of these dihydroporphyrins.

0022-365419512099-8045$09.00/0 0 1995 American Chemical Society

8046

J. Phys. Chem., Vol. 99, No. 20, 1995

Renner and Buchler

0.25r , ,

Ni (OEPMe2), 1

,

,

, ,,

, ,

,

,

, , ,

,

, , , ,

,,.

,

,

,,,,,,,

,

, ,

,,

,

, ,

,

Ni (OEC),2

Experimental Section Methylene chloride (CH2C12) and tetrahydrofuran (THF) were Wavelength (nm) purified using standard technique^.^^ Tetrabutylammonium perchlorate (TBAP) was prepared and purified using published technique^.^^ Imidazole was purchased from Aldrich and purified by sublimation. (5,15-Dimethyl-2,3,7,8,12,13,17,18octaethy1-5H,15H-porphinato)nicke1(11)[Ni(OEPMez), 11 was prepared as described in the l i t e r a t ~ r e . ~ ~ Optical spectra were recorded on a Cary 2300 spectrophotometer. EPR spectra were obtained on a Bruker-IBM ER200 spectrometer equipped with an Aspect 2000 data acquisition system. Spectroelectrochemicaland cyclic voltammetry experiments were performed using a BAS lOOA electrochemical system and were carried out under a nitrogen atmosphere using platinum working and counter electrodes, a saturated calomel reference electrode (SCE), TBAP as the supporting electrolyte, and previously described ~ e l l s . ~The ~ , EPR ~ ~ spectra were simulated using least-squares fitting by Monte Carlo methods.34 The iterative extended Hiickel (IEH)35-37 and intermediate 200 300 400 500 600 700 800 900 1000 neglect of differential overlap/spectroscopic (INDO/S)~*-~O Wavelength (nm) programs have been described previously and used the crystalFigure 1. Optical spectra of Ni(OEPMe2) 1 (-), (a) 1+ (* * -) in CH2lographic coordinates of l.24The INDO/s method consists of Clz, and (b) 1- * -) in THF at room temperature with TBAP as the a self-consistent-field calculation followed by a monoexcited supporting electrolyte. Intermediate spectra (- - -) in the (a) oxidation configuration interaction (CI). The CI included all the singleand (b) reduction reactions show clean isosbestic points. excited configurations from the HOMO through HOMO- 15 d and macrocycle n orbitals, which could lead to conjugation to the LUMO through LUMOS.15 of 1. of the pyrromethene chromophores via the nickel. The calcu(e

Results and Discussion The optical spectra of metalloporphodimethenes are sensitive to the central metal atom, but generally have an intense absorption maximum between 430 and 500 nm and a broad weak band -60 nm to the red.41 Dipyrromethenes and porphomethenes, on the other hand, have absorption maximum at -500 nm.41 The parent optical spectrum has a strong absorbance at 433 nm and a weaker one at 546 nm in CH2Cl2; see Figure la. The optical spectrum of 1 calculated using INDO/s methods predicts two weak transitions at 561 (0.0013) and 556 (0.001 1) nm (oscillator strength) and a strong band at 428 (1.60) nm. These absorption bands originate from JC to JC* transitions. Lowering the symmetry of 1 removes the degeneracy found in porphyrins for the lowest unoccupied (LUMO) molecular orbitals. The macrocycle n and n*molecular orbitals (MO) of 1 using INDO/s and IEH calculations are in agreement; however, the IEH calculation overestimates the nickel character for the n orbital; see Figure 2.42 The separations between the macrocycle n to n-1 and n* to n*+l orbitals are 0.122 and 0.130 eV, respectively, and have opposite symmetries. The calculated signs and patterns of the two n MO are characteristic of porphyrin al, nodes at the meso positions, and bonding n orbitals along the C,-Cp bonds of altemating bond order around the macrocycle. The n molecular orbital has some metal character, due to the close proximity of the filled metal

lated n* MO of 1 has a node along the saturated meso carbon C5-Cl5 axis and is similar to a linear combination of the degenerate porphyrin e, and egyorbitals. However, the signs of the atomic orbitals for the n* MO are unlike what is found for porphyrin^.^^ There is a mirror plane along the C5-Cl5 axis of 1, and the two dipyrromethenes appear as two independent chromophores. The asymmetry in the HOMO and LUMO orbitals from both INDO/s and IEH calculations, see Figure 1, originates from using the crystallographic coordinates of 1 and close n-1 and n*+l orbitals. The optical spectra obtained during the electrochemical oneelectron oxidation and reduction of 1 are shown in parts a and b of Figure 1, respectively. Both redox reactions are reversible one-electron processes with '95% of the starting material regenerated upon reversing the potential. Oxidation of 1, Figure la, results in a blue-shifted A,,, at 383 nm and a broad band at 552 nm. Reduction of 1, Figure lb, causes both absorption bands to blue shift to 373 and 511 nm, in which the Amax corresponds to the later band. The spectral features of 1+ are similar to those of 1, whereas spectra of 1- more closely resemble those of a porphomethene. Optically, if the two pyrromethenes are coupled via the nickel in 1, the similarity between the optical spectra of 1 and 1+ might indicate they are also in conjugation. However, the spectrum of 1- is reminiscent of two noninteracting dipyrromethenes; therefore, the conjugation is removed upon one-electron reduction.

J. Phys. Chem., Vol. 99, No. 20, 1995 8047

Cation and Anion Radicals of Ni(OEPMe2)

V

LUMO

l , , , , l , , , , l , , , , l , , , , l , , , , J

10 G

Figure 3. X-band EPR spectra of (a) 1+ in CHzClz (g = 2.003, line width = 6 G), (b) 1- (g = 2.000), and (c) a simulated spectrum for 1-. The simulation includes a two-proton coupling of 5.06 G and a line width of 4.8 G.

HOMO Figure 2. Relative unpaired spin densities from iterative extended Hiickel calculations for the macrocycle n (HOMO) and n* (LUMO) orbitals. The size of the circles are proportional to the atomic orbital coefficients in which the filled circles represent negative coefficients.

TABLE 1: Oxidation and Reduction Potentials of Ni(I1) Complexes in CH2C12 versus SCE, 0.1 M TBAP, Scan Rate 100 mV/s, at Room Temperature compound Ni(OEPMe2) (1) Ni(0EP) Ni(0EC) (2) Ni(0EiBC)

Ei/zoX

1.01 1.24 1.03 0.86

Ei/zoX 0.64 0.17 0.48 0.21

E1/zTed - 1.52 - 1.46 -1.50

irr

E112 values for NiOEP, OEC, and OEiBC were taken from refs 5 1 and 61.

The redox potentials of 1 along with the corresponding values for Ni(OEP), Ni(0EC) (2), and Ni(0EiBC) are given in Table 1. Under the anaerobic and anhydrous conditions used here 1 interestingly undergoes two reversible oxidations and one reduction with no evidence of dehydrogenation back to the corresponding porphyrin. The trend for the first oxidation in the Ni(I1) porphyrin series in porphyrin > 1 > chlorin > isobacteriochlorin. For Ni(I1) hydroporphyrins the first oxidation potential decreases, a negative shift in the potentials of -200 mV, as the sites of saturation increase, assuming only macrocycle redox hemi is try.^^,^-^' The differences in oxidation potentials observed between 1 and 2 must originate from the conformational and electronic differences. Crystallographic studies of nickel(I1) chlorinsx-'0~12~52-54 indicate that the macrocycle geometry ranges from planar to Sq ruffled, whereas 124 has a folded structure. The macrocycles of 1 and 2 both have 10 n bonds; however, the chlorin macrocycle is aromatic and the macrocycle of 1, which does contain aromatic pyrroles, consists of two extended systems. The reduction potentials

for these tetrapyrroles are insensitive to the sites and degree of saturation. Thus, experimentally the energy of the LUMO is less sensitive to the electronic and structural changes in these dihydroporphyrins. In a study of planar versus ruffled Ni(I1) porphyrins the energy of the LUMO was also shown to be insensitive to conformational changes in the macro cycle^.^ The difference between the E112 values for the first oxidation and reduction in porphyrins is 2.2 f 0.15 V for porphyrin-centered redox p r o c e ~ s e s . ~The ~ , ~Ai5112 ~ , ~ values ~ for 1 and 2 are 2.16 and 1.98 V, respectively. These values follow the trend observed for the relative energy of the first absorption bands (1 > 2) and further indicate that the first oxidation and reduction reactions occur at the macrocycle of 1. It is quite surprising that such a dramatic change in the electronic and structural properties as well as the macrocycle conjugation has only a small effect on their redox properties. The EPR spectra for the one-electron oxidation and reduction products are shown in Figure 3. Both the spectra are indicative of macrocycle n cation and anion radicals. The spectra of 1' in methylene chloride (g = 2.003) and 1- in THF (g = 2.000) are shown in parts a and b of Figure 3, respectively. The unpaired spin densities for 1+ (HOMO) and 1- (LUMO) obtained from the IEH calculation are shown in Figure 2. The spin profile for 1+ has most of the unpaired spin density at the pyrrole a-carbons and the nickel, similar to porphyrin a], radicals. The IEH calculation overestimates the nickel character in the n MO, but predicts the trend observed in the EPR spectrum of 1+ (line width of 6 G and no resolved proton or nitrogen couplings). In n cation radicals of chlorins the unpaired electron also resides in an al, type orbital. IEH calculations reveal that 1- has large unpaired spin density at the two meso carbons, Clo and C20, and to a lesser extent the four P-pyrrole carbons, CZ,CS,C12, and CIS. Simulation of the EPR spectrum using only two protons with a coupling of 5.06 G yields a reasonable fit, Figure 3c. Using the McConnell equation the aH of 5.06 G corresponds to an unpaired spin density Q of 0.19 at the Clo and C20 positions:' similar to what has been observed at the meso positions in hydroporphyrin anion radical^.^^%^* The magnitude of the unpaired spin density of 1- i s similar to that

8048 J. Phys. Chem., Vol. 99, No. 20, 1995 observed in chlorin anion radicals (-4.5 G).33,59 In chlorin anions all four meso positions have large unpaired spin densities, whereas in 1- the two saturated meso positions have none. Since 1+ and 1- could not be saturated in the EPR measurements, these radicals most likely have some metal character. For porphyrins and metalloporphyrins the EPR spectra of z anion radicals show no resolved hyperfiie coupling due to Jahn-Teller line broadening from the degenerate n* orbitals.6o Lowering the symmetry from D4h to Cz, in 1- removes this degeneracy and affords a resolved z anion radical EPR spectrum. Despite the electronic differences between porphodimethenes and chlorins both the cation and anion radicals are remarkably similar in their unpaired spin density profiles. Nickel(I1) porphyrins are generally four-coordinate, but are known to bind axial ligands, resulting in a spin state change, low- to high-spin. Addition of an excess of imidazole to 1 in methylene chloride shows no optical changes at room temperature to -70 "C; therefore, the nickel does not coordinate axial ligands to become high-spin. Nickel(I1) chlorins on the other hand are able to form high-spin complexes." When the spin or oxidation state of a metal complex changes there is a concomitant change in the metal-to-ligand bond length. For Ni(I1) porphyrins the site of oxidation or reduction is very sensitive to factors such as macrocycle substituents, solvent, electrolyte, and temperature.46s61-66In Ni(I1) porphyrins the relative energy of the macrocycle and the metal orbitals and the flexibility of the macrocycle are important in determining the redox site. As we have shown, one-electron oxidation or reduction of 1 affords only macrocycle rather than metalcentered redox reactions. Structural studies for a series of metalloporphodimethenes have revealed that the core size (CtN: 1.91-2.06 A) varies to accommodate the coordination requirements of the central metal ion.21-24,27,28This is accomplished by changes in the angle between the normal of the two pyrromethene halves. Therefore, structurally the porphodimethene macrocycle should be able to accept a high-spin Ni(I1); however, energetically this is not favorable. In both 1 and 2 the ordering of the molecular orbitals do not favor either Ni(II1) or Ni(1) formation even though structurally these macrocycles should be able to accept nickel in these oxidation states.

Conclusions The electronic and conformational differences between the two forms of Ni(I1) dihydroporphyrins, chlorins and porphodimethenes, govem their chemical and physical properties. These differences affect their oxidation potentials but not their reduction potentials. The sites of saturation for dihydroporphyrins result in a folded structure for 1, whereas most Ni(I1) chlorins are S4 ruffled. For Ni(I1) hydroporphyrins the relative energies of the macrocycle versus metal orbitals and the conformational flexibility govem the site of oxidation and reduction. The macrocycle core is flexible enough in both 1 and 2 to accept either Ni(II1) or Ni(1) oxidation states; however, the molecular orbitals are not properly ordered. The optical spectra of 1 and 1+ are very similar, whereas the spectrum of 1- resembles that of a porphomethene. Molecular orbital calculations of 1 show that the HOMO resembles the porphyrin al, orbital and the LUMO is similar to the porphyrin z* orbital in which the two halves are not coupled. Optical spectra and molecular orbital calculations of 1 indicate that conjugation of the two dipyrromethene halves via the nickel is possible for the neutral species and the cation radical, whereas the anion radical behaves as two independent dipyrromethenes. The spin profiles for the cation and anion radicals of 1 are very similar

Renner and Buchler to what is observed for chlorins. By lowering the symmetry of Ni(II) porphyrins as in chlorins and 1, the LUMO z* degeneracy is removed, thus affording well-resolved EPR spectra for the anion radicals. The lowered symmetry in both dihydroporphyrins affords large electron densities at the meso positions. It is quite surprising that even though the z system in 1 is interrupted, the redox properties and EPR spectra for the anion and cation radicals are so similar to the spectra of the corresponding derivatives of 2.

Acknowledgment. This work was supported by the Division of Chemical Sciences, U.S. Department of Energy, under Contract No. DE-AC02-76CH0016 at BNL and the late Professor H. H. Inhoffen while one of the authors (J.W.B.) was working at the Institut fur Organische Chemie der Technischen Universitat, Braunschweig, Germany. The authors thank Dr. J. Fajer for his interest in this research and helpful discussions. References and Notes (1) Deisenhofer, J.; Michel, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 829. (2) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am. Chem. SOC. 1988, 110, 7566. (3) Eschenmoser, A. Ann. N.Y. Acad. Sci. 1986, 471, 108. (4) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.; Smith, K. M.; Fajer, J. J . Am. Chem. Soc. 1993, 115, 3627. ( 5 ) Meyer, E. F. J. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972,828, 2162. (6) Cullen, D. L.; Meyer, E. F., Jr. J . Am. Chem. SOC.1974, 96, 2095. (7) Hoard, J. L. Ann. N.Y. Acad. Sci. 1973, 206, 18. (8) Kratky, C.; Waditschatka, R.; Angst, C.; Johansen, J. E.; Schreiber, J.; Eschenmoser, A. Helv. Chim. Acta 1985, 68, 1312. (9) Gallucci, J. C.; Sweptson, P. N.; Ibers, J. A. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, 838, 2134. (10) Ulman, A.; Fisher, D.; Ibers, J. A. J. Heterocycl. Chem. 1982, 19, 409. (1 1) Renner, M. W.; Furenlid, L. R.; Barkigia, K. M.; Forman, A.; Shim, H.; Simpson, D. J.; Smith, K. M.; Fajer, J. J . Am. Chem. SOC. 1991, 113, 689 1. (12) Waditschatka, R.; Kratky, C.; Jaun, B.; Heinzer, J.; Eschenmoser, A. J . Chem. Soc., Chem. Commun. 1985, 1604. (13) Geno, M. K.; Halpem, J. J . Am. Chem. SOC.1987, 109, 1238. (14) Scheidt, W. R; Lee, Y. J. Struct. Bonding 1987, 64, 1. (15) Chlorophylls; H. Scheer, Ed.; CRC Press: Boca Raton, FL, 1991. (16) Straws, S. H.; Pawlik, M. J.; Skowyra, J.; Kennedy, J. R.; Anderson, 0. P.; Spartalian, K.; Dye, J. L. Znorg. Chem. 1987, 26, 724. (17) Dolphin, D. J . Heterocycl. Chem. 1970, 7, 275. (18) Mauzerall, D. J. Am. Chem. SOC.1962, 84, 2437. (19) Botulinski, A,; Buchler, J. W.; Wicholas, M. J. Am. Chem. SOC. 1987, 26, 1540. (20) Buchler, J. W.; Puppe, L. Justus Liebigs Ann. Chem. 1970, 740, 142. (21) Buchler, J. W.; Lay, K. L.; Smith, P. D.; Scheidt, W. R.; Ruppecht, G.A.; Kenny, J. E. J . Organomet. Chem. 1976, 110, 109. (22) Buchler, J. W.; Dreher, C.; Lay, K. L.; Lee, Y. J. A,; Scheidt, W. R. Inorg. Chem. 1983, 22, 888. (23) Buchler, J. W.; Lay, K. L.; Lee, Y. J.; Scheidt, W. R. Angew. Chem., Int. Ed. Engl. 1982, 21, 432. (24) Dwyer, P. N.; Buchler, J. W.; Scheidt, W. R. J . Am. Chem. SOC. 1974, 96, 2789. (25) Buchler, J. W.; Lay, K. L. Z. Natuiforsch. 1975, 30b, 385. (26) Botulinski, A,; Buchler, J. W.; Tonn, B.; Wicholas, M. Inorg. Chem. 1985, 24, 3239. (27) Botulinski, A,; Buchler, J. W.; Abbes, N. E.; Scheidt, W. R. Liebigs Ann. Chem. 1987, 305. (28) Dwyer, P. N.; Puppe, L.; Buchler, J. W.; Scheidt, W. R. Inorg. Chem. 1985, 14, 1782. (29) Botulinski, A.; Buchler, J. W.; Lay, K. L.; Stoppa, H. Liebigs Ann. Chem. 1984, 1259. (30) Riddick, J. A,; Bunger, W. B. Organic Solvents. Physical Properties and Methods of Pur$cation, 3rd ed.; Wiley-Interscience: New York, 1970; Vol. 11. (31) Fajer, J.; Borg, D. C.; Forman, A,; Dolphin, D.; Felton R. H. J . Am. Chem. SOC.1970, 92, 3451. (32) Fujita, I.; Chang, C. K. J . Chem. Educ. 1975, 61, 913. (33) Fajer, J.; Fujita, I.; Davis, M. S.; Forman, A,; Hanson, L. K.: Smith, K. M. Adv. Chem. Ser. 1982,201, 489. (34) Kirste, B. Anal. Chim. Acta 1992, 265, 191.

J. Phys. Chem., Vol. 99,No. 20, 1995 8049

Cation and Anion Radicals of Ni(OEPMe2) (35) Chang, C. K.; Hanson, L. K.; Richardson, P. F.; Young, R.; Fajer, J. Proc. Natl. Acad. Sci. USA. 1981, 78, 2653. (36) Davis, M. S.; Forman, A,; Hanson, L. K.; Thornber, J. P.; Fajer, J. J . Phys. Chem. 1978, 83, 3325. (37) Zemer, M.; Gouterman, M. Theor. Chim. Acta 1979, 4, 44. (38) %mer, M.; Loew, G.; Kirchner, R.; Mueller-Westerhoff, U.J . Am. Chem. SOC.1980, 102, 589. (39) Ridley, J.; Zemer, M. Theor. Chim. Acta (Berlin) 1973, 32, 111. (40) Ridley, J.; Zemer, M. Theor. Chim. Acta (Berlin) 1976, 42, 223. (41) Scheer, H.; Inhoffen, H. H. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1987; Vol. 2, p 45. (42) Zemer, M.; Gouterman, M. Theor. Chim. Acta 1966, 4 , 44. (43) Gouterman, M. J . Mol. Spectrosc. 1961, 6, 138. (44) Stolzenberg, A. M.; Stershic, M. T. Inorg. Chem. 1987, 26, 1970. (45) Stolzenberg, A. M.; Spreer, L. 0.;Holm, R. H. J . Am. Chem. SOC. 1980, 102, 364. (46) Cham. D.: Malinski. T.: Ulman. A,: Kadish. K. M. Inora. Chem. 1984, 23, 8 1 f (47) Stolzenberg, A. M.; Straws, S. H.; Holm, R. H. J . Am. Chem. SOC. 1981, 103, 4763. 148) Richardson, P. F.; Chang, C. K.; Hanson, L. K.; Spaulding, L. D.; Fajer, J. J . Phys. Chem. 1979, 6, 3420. (49) Richardson, P. F.; Chang, C. K.; Spaulding, L. D.; Fajer, J. J . Am. Chem. SOC.1979, 101, 7736. (50) Chang, C. K.; Fajer, J. J . Am. Chem. SOC.1980, 102, 848. (51) Stolzenberg, A. M.; Stershic, M. T. Inorg. Chem. 1988, 27, 1614. (52) Kratky, C.; Angst, C.; Johansen, J. E. Angew. Chem., Int. Ed. Engl. 1981, 20, 211.

-

(53) Suh, M. P.; Swepston, P. N.; Ibers, J. A. J . Am. Chem. SOC. 1984, 106, 5164.

(54) Barkigia, K. M.; Thompson, M. A.; Fajer, J.; Pandey, R. K.; Smith, K. M.; Vicente, G. H.New J . Chem. 1992, 16, 599. (55) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 5, p 53. (56) Fuhrhop, J.; Kadish, K. M.; Davis, D. G. J. Am. Chem. SOC.1973, 95, 5140. (57) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. 4, p 179. (58) Renner, M. W.; Fujita, E.; Fujita, I.; Procyk, A. D.; Bocian, D. F.; Fajer, J. J . Phys. Chem. 1992, 96, 9597. (59) Assuming that QFH = 27 for an anion radical. (60) Townsend, M. G.; Weissman, S. I. J . Chem. Phys. 1960, 32, 309. (61) Wolberg, A,; Manassen, J. Inorg. Chem. 1970, 9, 2365. (62) Dolphin, D.; Niem, T.; Felton, R. H.;Fujita J . Am. Chem. SOC. 1975, 97, 5288. (63) Johnson, E. E.; Niem, T.; Dolphin, D. Can. J . Chem. 1978, 23, 1381. (64) Kim, D. K.; Miller, L. A.; Spiro, T. G. Inorg. Chem. 1986, 25, 2468. (65) Fuhrhop, J.; Mauzerall, D. J . Am. Chem. SOC.1969, 91, 4174. (66) Erler, B. S.; Scholz, W. F.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A. J . Am. Chem. SOC. 1987, 109, 2644. (67) Stolzenberg, A. M.; Stershic, M. T. J . Am. Chem. SOC. 1988, 110, 6391. JP942928H