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One-electron reduction of chromium(II1) porphyrins has been studied by radiolytic and electrochemical methods with several porphyrin ligands in variousĀ ...
J. Phys. Chem. 1992, 96,4459-4466

4459

One-Electron Reduction of Chromium( I I I) Porphyrins. Formation of Chromium( I I ) Porphyrins or Chromium( I I I ) Porphyrin ?r-Radical Anions Dirk M. Guldi,' P. Hambright: D. Lexa? P. Neta,*J and J.-M. Sav6ant3 Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Chemistry, Howard University, Washington, DC 20059, and Laboratoire d'Electrochimie Mollculaire, Universitl de Paris 7, 75251 Paris Cedex 05, France (Received: October 9, 1991; In Final Form: January 27, 1992)

One-electronreduction of chromium(II1) porphyrins has been studied by radiolytic and electrochemicalmethods with several porphyrin ligands in various solvents. The absorption spectra of the first reduction products were monitored by pulse radiolysis within microseconds after the pulse. Two types of differential spectra were observed; intense broad absorptions at 600-800 nm asckibed to the s-radical anions or weaker sharp absorption at various wavelengths due to chromium(I1) porphyrins. Cr111TSPP(LIL2)[chromium(III) tetrakis(4-sulfonatopheny1)porphyrin with two axial ligands L, and L2] was reduced to the s-radical anion in water, alcohols, and N,N-dimethylformamide (DMF), where L, and L2 are HzO, OH-, ROH, RO-, or DMF. Cr"'TPP(C1)L (tetraphenylporphyrin)and Cr"'TMP(C1)L (tetramesitylporphyrin), on the other hand, were reduced on the metal in neutral alcohol (L = ROH) but gave the *-radical anions when either the C1- was exchanged with OH(in the presence of KOH) or L was replaced with pyridine, DMF, or dimethyl sulfoxide (DMSO). Cr"'OEP(C1)L (octaethylporphyrin) in alcohol and Cr"'MSP(0H)L (mesoporphyrin-IX) in aqueous alcohol were reduced to Cr"P even in the presence of base. With added pyridine, however, reduction took place on the ligand to form the CrlIIP'- species. Cyclic voltammetry and thin layer spectroelectrochemistry experiments were carried out to determine the redox potential and to confirm the above assignments. In conclusion, all chromium porphyrins studied are reduced on the metal center when this bears axial ligands that are weak electron donors, but stronger axial ligands on the Cr center direct the reduction toward the porphyrin s-system.

Introduction Reduction of chromium(111) porphyrins generally has been suggested to lead to formation of the divalent state, C P P . Electrochemical reduction of chromium(II1) complexes of OEP and TPPSv6under various conditions showed two or three reversible steps which were interpreted as leading sequentially to C P P , C r W - , and Crt1P2-. Chemical reduction of these porphyrins produced the Cr1'P,5q7v8which were characterized spectroscopically7 and were found to react rapidly with 02.The possibility of one-electron reduction of the Cr"'P state to the *-radical anion, Cr"'P'-, has been generally dismissed despite the fact that the potential for its first one-electron reduction is very close to that of the corresponding free base, H2P. It has been shown recently that one-electron reductione1' and oxidationI2of Ni"-porphyrins may take place on the ligand or on the metal center and that the outcome of the reaction was greatly dependent on the structure of the porphyrin and on the medium. Since reduction of Cr"'-porphyrins was studied only for OEP and TPP under aprotic conditions, we examined the (1) NIST. (2) Howard University. (3) Universitl de Paris 7. (4) Fuhrhop, J.-H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. Soc. 1973, 95, 5140. (5) Cheung, S. K.; Grimes, C. J.; Wong, J.; Reed, C.A. J. Am. Chem. Soc. 1976, 98, 5028. (6) (a) Bottomley, L. A.; Kadish, K. M. J . Chem. SOC.,Chem. Commun. 1981, 1212; (b) Inorg. Chem. 1983,22,342. (c) Kelly, S. L.; Kadish, K. M. Inorg. Chem. 1984, 23,679. (d) OBrien, P.; Sweigart, D. A. J . Chem. SOC., Chem. Commun. 1986, 198. (7) Reed, C. A.; Kouba, J. K.; Grimes, C. J.; Cheung, S. K. Inorg. Chem. 1978, I?, 2666. (8) Scheidt, W. R.; Reed, C. A. Inorg. Chem. 1978,17,710. Scheidt, W. R.; Brinegar, A. C.; Kirner, J. F.; Reed, C. A. Inorg. Chem. 1979,18,3610. (9) Lexa, D.; Momenteau, M.; Mispelter, J.; Saveant, J.-M. Inorg. Chem. 1989, 28, 30. (IO) Nahor, G. S.; Neta, P.; Hambright, P.; Robinson, L. R.; Harriman, A. J. Phys. Chem. 1990, 94,6659. (1 1) Kadish, K. M.; Franzen, M. M.; Han, B. C.; Araullo-McAdams, C.; Sazou, D. J. Am. Chem. SOC.1991, 113, 512. (12) Nahor, G. S.;Neta, P.; Hambright, P.; Robinson, L. R. J . Phys. Chem. 1991, 95,4415.

reduction of these and other Cr1ILporphyrins in various solvents, including water and alcohols, and identified the stable products as well as the short-lived intermediates produced immediately upon the first one-electron reduction. We find that CrlILporphyrins are reduced either to the a-radical anion or to CrIIP, depending on the porphyrin, solvent, and other conditions.

Experimental The following abbreviations are used for the porphyrins: TPP (tetraphenylporphyrin), TMP (tetramesitylporphyrin), TSPP [tetrakis(4dfonatophenyl)porphyrin], OEP (octaethylporphyrin), MSP (mesoporphyrin-IX), and MSPDME (mesoporphyrin-IX dimethyl ester). Cr"'TPP(C1) was prepared by the method described before.I4 Cr"'OEP(Cl), Cr"'TMP(Cl), and Cr"'MSPDME(C1) were prepared by similar procedures. The watersoluble Cr"'MSP(0H) was made by hydrolysis of the dimethyl ester. It is soluble in water when the two carboxyl groups are dissociated. Cr"'TSPP(0H) was prepared by literature meth0ds.ls This porphyrin bears four negative charges due to the sulfonate groups on the ligand and is soluble at all pH values. The negative charges on the MSP and TSPP ligands are omitted from the abbreviations to simplify the form of presentation and also because their effect on the behavior of the metalloporphyrin is only secondary. In the above abbreviations for the chromium(II1) porphyrins, the third positive charge of the metal ion is neutralized by a chloride or hydroxide anion bound at the axial position. To fulfill the octahedral configuration of the Cr"' complex, a second axial position is assumed to be occupied by a solvent molecule. Pyridine and N,N-dimethylformamide (DMF) were vacuum distilled prior to use. 1,2-Dichloroethane was distilled over P20s. All other solvents and reagents were of analytical grade purity and used as received. Water was purified with a Millipore Su(13) The identification of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified are necessarily the best available for the purpose. (14) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J . Inorg. Nucl. Chem. 1970, 32, 2443. Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am. Chem. SOC.1977, 99, 8195. (15) Fleischer, E. B.; Krishnamurthy, M. J . Coord. Chem. 1972, 2, 89.

0022-3654/92/2096-44S9%03.00/00 1992 American Chemical Society

4460 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992

per-Q system. Solutions containing 0.05-0.1 mM chromium porphyrin in the desired medium were freshly prepared before use and deoxygenated by bubbling with ultra-high-purity Ar or NzO. Steady-state irradiations employed a Gammacell 220 @To source with a dose rate of 95 Gy/min. Absorption spectra before and after irradiation were recorded with a Cary 219 spectrophotometer. Pulse radiolysis experiments were performed with the apparatus described before,1Ā° which utilizes 50-11s pulses of 2-MeV electrons from a Febetron Model 705 pulser. The dose per pulse, determined by KSCN dosimetry, was varied between I and 40 Gy, which in aqueous solutions gives between 4 and 24 pM radicals. Cyclic voltammetry measurements were performed with a Pt (1-mm diameter) or a glassy carbon working electrode, polished with 1-pm diamond paste before each run, a pt counter electrode, and an aqueous standard calomel electrode as reference. The instrumentation for the cyclic voltammetry and the thin-layer spectroelectrochemistrywas the same as previously described.I6 The solutions contained 0.1 M tetrabutylammonium tetrafluoroborate or tetrabutylammonium hexafluorophosphate (recrystallized three times from 1,2-dichloroethane)as supporting electrolyte and were deoxygenated by bubbling with ultra-highpurity Ar.

Guldi et al.

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Results and Discussion Rndidytic Redudion Reactions.The chromium porphyrins were reduced by radiolytically produced radicals in various media. In deoxygenated aqueous solutions containing 2-propanol, they are reduced by the hydrated electrons and by the radicals produced via the reaction of OH' and H' with 2-propanol.

--

+ H' + OH' + H+ + H2 + H2O2 (CH,),CHOH + OH' (H') (CH&COH + H20(Hz) CrP + e,; (CrP)CrP + (CH3),COH (CrP)- + (CH,),CO + H+ H20

e,;

--

(1)

+ OH- a (CH,),CO- + H 2 0

CrP + (CH,),CO-

-

(CrP)-

(16) Lexa, D.; SavQnt,

I

800

I

I

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(4)

(6)

overall yield of reducing equivalents in the aqueous 2-propanol system is -0.6 pmol J-I. Similar reduction reactions take place in neat alcohols, although the yields are somewhat lower (-0.5 pmol J-' in 2-PrOH). In the presence of pyridine, solvated electrons react with this solute to form the pyridinyl radical, which subsequently transfers an electron to the porphyrin. The reducing 2786.

I

700

(3)

pK, = 12 ( 5 )

+ (CH3)2C0

I

600

(2)

(CrP)- represents a reduced chromium porphyrin, without specifying the site of reduction. The reactions of e, - with many metalloporphyrins are known to be diffusion controhd,l7 but the reactions of (CHJ2COH have lower rate constants.18 In the case of TSPP and TPP complexes, reduction by (CH,),COH was sufficiently rapid to be observed in the pulse experiments. We determined a rate constant k4 = (8.0 f 1.0) X lo8 M-' s-I for CrI1'TSPP(OH2), at pH 5.1 from the rate of buildup of the T radical anion absorption at 680 nm as a function of porphyrin concentration. The OEP and MSP complexes, on the other hand, which have more negative reduction potentials, were reduced by (CH3)zCOH more slowly: we estimated k4 I4 X lo7 M-l s-l for Cr111MSP(OH2)2 at pH 6.6. Therefore, reduction of these porphyrins in neutral solutions was inefficient in the pulse radiolysis experiment, where the lifetime of the reducing radicals is relatively short. All porphyrins, however, were readily reduced by the anionic form of the alcohol radical at high pH, where we find k6 = (5.9 f 1.0) X 108 M-Is-l for CrII'MSP(OH), at pH 12.1. The (CH&COH

I

500

J.-M.: Zickler, J. J . Am. Chem. Soc. 1977, 99.

(17) Buxton, G. v.;Greenstock, c. L.;Helman, w.p.; ROSS, A. B. J . Phys. Chem. Ref.Dura 1988, 17, 513. (18) (a) Neta, P. J . Phys. Chem. 1981.85,3678. (b) Morehouse. K.M.: Neta, P. J . Phys. Chem. 1984, 88, 1575.

-50

!

I--

500

I , nm Figure 1. Transient differential absorption spectra observed following pulse radiolytic reduction of several Cr1ILporphyrins. (a) CrTSPP in N,O-saturated aqueous 2-PrOH (5%) solution at pH 12.5, dose 7 Gy/ pulse. (b) CrTPP in deoxygenated 2-PrOH, dose 40 Gy/pulse; (A) neutral, (0)with 0.4 mM 2-PrO-. (c) CrTMP in deoxygenated 2-PrOH, dose 40 Gy/pulse; (A)neutral, (0)with 0.4 mM 2-Pr0-.

radical formed from 2-PrOH does not reduce the neutral form of pyridine and thus reacts directly with the porphyrin. In DMF and DMSO as solvents, the porphyrins may be reduced by the small yield of solvated electrons produced in the radiolysis of these solvents. However, because of the short lifetime of the solvated electrons under these conditions, the yield of their reaction with the low concentrations of porphyrins was low. Higher yields were obtained by using 5% acetone to scavenge the electrons efficiently, to produce a longer-lived reducing radical, (CH3),CO-, which then reacted with the porphyrins. Oae-~R~ProductsObbinedbvR~lseRediolvsis. The nature of the species formed upon one-eleciron reduction"was determined from the differential absomtion swctra. monitored in the 450-850-nmregion, immediately at the end of &e reduction reaction (5200 after the pulse). ~~t~~~~braad absorptions at X 600-800 nm are attributed to the *-radical anions (CrlIIP*), in parallel with absorption spectra of other metalloporphyrin

-

~

One-Electron Reduction of Chromium(II1) Porphyrins

TABLE I:

The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4461

pulse Radiolytic Reduction of Chromium(II1) Porphyrins

porphyrina TSPP

TPP

TMP OEP

MSP

conditions H 2 0 , 5% 2-PrOH, pH 7, Ar HSO, 5% 2-PrOH, pH 7, N 2 0 H20, 5% 2-PrOH, pH 12.5, N 2 0 MeOH, 5 mM KOH, N 2 0 MeOH:2-PrOH (1:9), N 2 0 DMF, 5% acetone, Ar DMF, KOH sat., Ar 2-PrOH (or EtOH, MeOH), N 2 0 MeOH (or EtOH), 1 mM KOH, N 2 0 2-PrOH, 0.4 mM 2-PrO-, Ar 2-PrOH, 1% pyridine, Ar DMSO, 5% acetone," Ar DMSO, 5% pyridine, Ar DMF, 5% acetone, Ar 2-PrOH, Ar 2-PrOH, 0.4 mM 2-Pr0-, Ar 2-PrOH, N 2 0 2-PrOH, 0.4 mM 2-PrO-, Ar 2-PrOH, 0.01% pyridine, N 2 0 2-PrOH, 1% (or 5%) pyridine, N 2 0 2-PrOH, KOH, 5% acetone, 10% DMSO, Ar H20:2-PrOH ( l : l ) , 1 mM KOH, N 2 0 H 2 0 , 5% 2-PrOH, 1 mM KOH, N 2 0 H20:2-PrOH (l:l), 1% pyridine, N 2 0

-

peaks, nm

speciesb

680, -780 675, -780 700, -800 680, -800 680, c 680, -800 680, c 630, 710 680, -800 680, -800 670, 800 sh 690, 800 690, -800 680, c 640, 730 680, 800 650, 710-720 640, 690 sh, 725, 750 sh, 800 (600), 640, 730 600, 680, 730 610, 680 sh, 760 730 620-640, 720 605, 680, 730 sh

'See Experimental Section for the abbreviations of the porphyrins. The starting materials were in the Cr"' state. *The species produced was either the *-radical anion, abbreviated as Cr"'P'- without the axial ligands, or that reduced at the metal center, CrlIP. T h e position of the -800-nm peak was not determined. With and without 1 mM ascorbate as a scavenger for the oxidizing radical in DMSO.

*-radical anions.lg Changes in the redox state of the metal in a metalloporphyrin are hown to result in minor shifts of the peaks without strong absorption in the red.10J8*20Therefore, in the cases in which no intense broad absorptions at 600-800 nm appeared, and the differential spectrum reflected only a small shift in the Q-bands, we attribute the spectrum to the species reduced on the metal, CrIIP. This was confirmed also by the spectra of Cr"P formed as stable products in the y-radiolysis experiments. In all cases we observed bleaching of the Q-bands of the CrlI1state upon reduction. Typical differential spectra obtained by pulse radiolysis are shown in Figures 1 and 2. The results (Table I) show that the nature of the initial oneelectron-reduction product of Cr111P(LlL2) depends on the porphyrin ligand (P) and on the axial ligands (L, and L2). CrlIITSPP(OH)(OH2) dissolved in water, alcohols, or DMF, under various conditions, was reduced to an intermediate exhibiting the typical absorptions of the porphyrin *-radical anion (Figure la), with an intense band at 675-700 nm and a weaker one at 780-800 nm. On the other hand, CrII'TPP(C1)L in neutral alcohols (L = ROH) was r e d u d on the metal to give the divalent state, which exhibits weak absorptions at 630 and 710 nm (Figure lb). In alcohols containing KOH (where C1 is replaced with OH) or pyridine (where the axial ligands are pyridine molecules) and in DMF and DMSO as solvents (where at least one axial ligand is presumably a solvent molecule), reduction took place on the ligand. The a-radical anion exhibits an intense peak at 680 nm and a weaker peak at 800 nm (Figure lb), similar to those found with Cr1I1TSPP(OH)(0Hz).Cr"'TMP(C1)L also was reduced on the metal in neutral 2-PrOH (L = ROH) but on the ligand in alkaline 2-PrOH (L = RO-), in both cases giving spectra similar to those observed with the TPP complex (Figure IC). The three porphyrins discussed above are similar in that they are all related in structure to the meso-substituted tetraphenylporphyrin, and they exhibit somewhat similar behavior. In neutral alcohols, the TPP and TMP complexes exist in the form of Cr1I1P(C1)(ROH)l4and are reduced to the Cr"P state. In the (19) See, for example: Neta, P.; Scherz, A.; Levanon, H. J . Am. Chem.

Soc. 1979,101,3624;Baral, S.;Neta, P.; Hambright, P. Radiat. Phys. Chem. 1984, 21,245; Baral, S.;Hambright, P.; Neta, P. J . Phys. Chem. 1984.88,

1595; Richoux, M.-C.; Neta, P.; Harriman, A.; Baral, S.; Hambright, P. J . Phys. Chem. 1986, 90, 2462; and ref 17. (20) Dolphin, D.; Niem, T.; Felton, R. H.; Fuita, I. J . Am. Chem. SOC. 1975,97,5288. Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970,92,2982.

presence of base (OH- or 2-Pr0-), the C1- is replaced by the stronger donor ligand to form CrII'P(OH)(ROH) or CrlIIP(OR)(ROH),21which are found to be reduced on the a-system of the porphyrin.22 In the presence of pyridine, DMF, or DMSO (abbreviated as ligands L), but in the absence of other anions, the porphyrin forms CrlllP(Cl)(L),Z' which again is reduced on the porphyrin ring. (Possibly, the chloride ligand is also exchanged with a solvent molecule to give Cr111P(L)2.) Cr111TSPP(OH2)z in aqueous solution exhibits pK, values of 7.6 [to CrII'TSPP(OH)(OH2)] and 11.4 [to Cr111TSPP(OH)z]?3 and is reduced at both neutral and alkaline conditions to the a-radical anion. In alcohols and DMF (where axial OH2 is replaced with ROH or DMF) it undergoes similar reactions. Therefore, it appears that the site of reduction, whether at the metal center or at the asystem, depends on the donor strength of the axial ligand and thus indirectly on the solvent and other solutes. With one C1- and one alcohol ligand, reduction to Cr"P is favored, but when the alcohol is replaced with a stronger electron donor as axial ligand (DMF, DMSO, pyridine) or when the C1- is replaced with another stronger donor ligand (alkoxide, hydroxide), reduction occurs at the *-system. The reason probably lies in the fact that stronger donor ligands increase the electron density on the central Cr ion. In the case of Ni"-porphyrins, the site of reduction was found to be affected by the axial ligands possibly through their effect on the position of the large Nit with respect to the plane of the porphyrin ring." In the case of Cr, however, both CrlI1and Cr" have been shown to be centered in the plane of the porphyrin ring? In contrast with the meso-phenyl-substituted porphyrins, Cr"IOEP(C1)L and Cr1I1MSP(OH)L have electron-donating substituents on the pyrrole rings and show more negative reduction potentials. It is expected that the relative reactivity of the porphyrin ring and the metal center will be affected by the nature of the substituents, and in fact, these @-pyrrolederivatives dem(21) The change in axial ligands is reflected in the spectra, particularly as a shift in the Soret peak, as demonstrated in Table 11, and in agreement with previous reports: Fleischer, E.B.; Srivastava,T. S. Inorg. Chim. Acra 1971,5, 151; Gouterman, M.; Hanson, L.K.; Khalil, G.-E.; Lecnstra, W. R.; Buchler, J. W. J . Chem. Phys. 1975, 62, 2343. (22) In fact, the base is not necessary. When the CI- ligand was replaced with an oxygen ligand by the action of Ag20 on CrlIITPP(C1) in neutral 2-PrOH (as evident from a 12-nm blue shift of the Soret peak), pulse radiolytic reduction gave the radical anion. (23) Ashley, K. R.; Leipoldt, J. G.; Joshi, V . K. Inorg. Chem. 1980, 19, 1608.

4462 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 TABLE II: y-Radiolytic Reduction of Chromium(II1) Porphyrins porphyrin" conditions* TSPP H 2 0 , 5% 2-PrOH, pH 12.3 HzO, 5% 2-PrOH, pH 7 H 2 0 , 5% 2-PrOH, 1 M HC104 TPP

2-PrOH DMSO, 5% pyridine 2-PrOH, 0.4 mM 2-Pro-

TMP

2-PrOH, 0.4 mM 2-Pr0-

OEP

2-PrOH, Ar 2-PrOH, 0.4 mM 2-Pr02-PrOH, 0.01% pyridine 2-PrOH, 1% pyridine

MSP

H20:2-PrOH (l:l), 1 mM KOH H 2 0 , 5% 2-PrOH, 1 mM KOH H 2 0 , 5% 2-PrOH, pH 7 H 2 0 , 10% 2-PrOH, 1% pyridine

MSPDME

2-PrOH, 0.4 mM 2-Pro-

Guldi et al.

peaks: nm 435, 520, 564,600, 635 457, 508, 850 444, 520, 560, 598, 635 428, 520 sh, 590 sh, 622 445, 520, 562, 598,635 408, 520 sh, 578, 622 440, 520 sh, 560, 597 402 sh, 419, 530, 600 sh 446, 504, 578, 620 448, 524, 565, 605 63gr 432, 520, 558, 596 420, 600, 650 sh, 685, 780 438, 522, 560, 596 414,650, 705, 790 435, 492, 540, 514, 755 475, 524, 628 422,498, 541, 514, 130 404, 534, 558, 645, 725 436, 496, 542, 514, 758 404, 478, 524, 558,628, 728 458, 496, 542, 515, 784 482, 524 422, 538, 572, 736 402, 525, 552, 644, 685, 725 422, 539, 572, 730 385,464, 530, 552, 644, 728 422, 539, 512, 760 388, 464, 528, 552, 644, 130 458, 544, 572, 782 480, 525,630 422, 542, 574,645, 730 402, 528, 555, 646,125

species Cr111TSPP(OH)2 Cr"'-phlorin Cr111TSPP(OH)(OH2) Cr"'-chlorind CrlIITSPP(OH 2) Cr"'-chlorin Cr1I1TPP(C1)(ROH) CrllTPP(ROH)z Crlkhlorin CrlIITPP(py), Cr"kh1orin Cr1I1TPP(R0-)(ROH) Cr"'-phlorin CrlIITMP(RO-)(ROH) CrIILphlorin Cr"'OEP(CI)(ROH)

f

CrI1IOEP(RO-)(ROH) Cr"OEP( ROH)2 CrlllOEP(Cl)(ROH) Cr110EP(ROH)2g CrlI'OEP(py), h Cr1I1MSP(OH), Cr"MSP(OH)(OH2) Cr111MSP(OH)2 Cr"MSP(0H) (OHz) Cr1ILMSP(OH) (OHz) Cr11MSP(OH2)2 CrlIrMSP(py), h Cr"'MSPDME( RO)(ROH) Cr"MSPDME( ROH)2

Experimental Section for the abbreviations of the porphyrins. bAll solutions were deoxygenated by bubbling with Ar. CThepeaks are given for the starting material in the first line and for the product in the second line. In the early stages of irradiation a peak at 700 nm appeared which subsequently decreased and was replaced by the chlorin peaks, it is not clear whether the 700-nm absorption is due to a remnant of the radical anion or to another unstable product. rThe radiolytic reduction yield in this system was very small; the chlorin peak at 638 nm was observed but the peaks of the original porphyrin were also present. 'Although the initial reduction product in this case is Cr"OEP, as seen in the pulse radiolysis, subsequent reactions of the alcohol radical take place preferentially on Cr"P rather than CrlllP and yield a product with a C r C bond, which will be discussed in a future paper. #The peaks at 404 and 728 nm identify Cr"0EP in this case, but the other peaks indicate a mixture with a Cr-C product as discussed in footnote f, *The stable reduction products under these conditions are probably the four-electron-reduced species, which upon addition of O2 are oxidized back to the original porphyrin with partial formation of the chlorin.

TABLE III: Pulse Radiolvtic Reduction of CODDW and Z h c POrDbVhS porphyrin conditions CuIITSPP HzO, 1% 2-PrOH, pH 7, N 2 0 CuIITPP 2-Pr0H:acetone:pyridine (17:2:1) CuIIOEP 2-PrOH, 7 mM 2-Pr0-, Ar 2-PrOH, 7 mM 2-PrO-, 2% pyridine Zn"0EP 2-PrOH, 7 mM 2-Pr0-, Ar 2-PrOH, I mM 2-Pr0-, 1% pyridine

onstrate a greater tendency to be reduced at the metal. Unlike the phenylporphyrins, Cr"'OEP(LIL2) and Cr111MSP(L,L2)are found to form Cr"P even in alkaline 2-PrOH or water and in the presence of low concentrations of pyridine (0,018, serving as a base, not as an axial ligand) (Figure 2a, Table I). Axial ligation of Cr1110EP(LIL2)and Cr1I1MSP(LlL2)with pyridine (present at 1 1 % concentrations) leads to onaelectron-reducedspecies which exhibit intense peaks at 600 and 680 nm (Figure 2a), different than those of the Cr"P species. Although these transient spectra are somewhat different from those of the CrlIIP'- species derived from TSPP and TPP, we ascribe these absorptions to the radical anions, based on comparison with the spectra obtained in the pulse radiolysis of ZnOEP, CuOEP, CuTPP, and CuTSPP (Figure 2b,c, Table 111). The peah for ZnOEP- in the region of 600-800 nm are shifted from those of ZnTPP'-24and ZnTSPP'-,18Pand the peak for CuIIOEP- are similarly shifted as compared with those of CuIITPP- and CuIITSPP-; the shifts are in the same direction as found for CrII'OEP- as compared with CrIIITPP-. Therefore, we conclude that strong axial ligands, such as pyridine, direct the ~

~~

~

(24) Closs, G. L.; Closs, L.E.J . A m . Chem. Soc. 1963, 85, 818.

~~~

peaks, nm 620, 660, 840 670,1840 600,680 sh, 740, 820 600, 660 sh, 740, 820 630, 825 630, 825

species Cu"P'CUW Cu"P'Cu"P'ZnVZnV-

reduction of the Cr111P(LIL2) toward forming the *-radical anion even in the case of OEP and MSP, where the porphyrin ligands are more difficult to reduce than TPP or TSPP. We note here that no evidence was found for the production of Cu'P from the aforementioned Cu"P reactants. Thus, CuilP is similar in this respect to Ad1[-, InlIL, SnIV-, SbV-, V"Q-, EuII'-, and Lu111-porphyrins,25which have been shown to be reduced on the porphyrin ring, as are Zn'Lporphyrins. Mn"'-, FelIL, and ColILporphyrinsare known to be reduced at the metal center. In the case of Co11iTMPyP(LIL2),however, addition of CN- as strong axial ligands favors reduction of the porphyrin ring,26as observed here with chromium porphyrins. Stable products Obtahed by y-R.didysLa The *-radical anions monitored by pulse radiolysis were short-lived and decayed over milliseconds to seconds. To examine the stable products, we (25) Richoux, M.-C.; Neta, P.; Harriman, A,; Baral, S.; Hambright, P. J. Phys. Chem. 1986, 90, 2462. Oliver, F. W.; Thomas, C.; Hoffman, E.; Sutter, T. P.G.; Hambright, P.; Haye, S.;Thorpc, A. N.; Quoc, N.; Haniman, A.; Neta, P.; Mosseri, S. Inorg. Chim. Acra 1991, 186, 119. (26) Mosseri, S.;Neta, P.; Harriman, A.; Hambright, P. J. Inorg. Biochem. 1990, 39, 93.

One-Electron Reduction of Chromium(II1) Porphyrins

The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4463

50t8

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1.12

0 0.8

3.08

Q,

0 C

e

0 v,

a

Q 0.4

0

0.04

I

-100

I

I

I

500

600

700

800

I

I

I

I

I

50

I

I

0.0

I

600

400

I

aoo

c

1.61

0.00

10.16

C.

0 ...... ., ....., ... ., .........., ., ........., ...., .., .., .. ......................., ..., ...... .., . .

t

t

i

I..

I..

I.

i 500

I 700

600

I

I

I

I

' I

800

A , nm Figure 2. Transient differential absorption spectra observed following pulse radiolytic reduction of several metallooctaethylporphyrins. (a) CrIIIOEP in deoxygenated 2-PrOH; (A) with 0.01% pyridine, dose 40 Gy/pulse, showing reduction to CrI'OEP, and (0)with 1% pyridine, dose 13 Gy/pulse, showing reduction to the ?r-radical anion. (b) ZnIIOEP in deoxygenated 2-PrOH containing 7 mM 2-Pr0-, dose 13 Gy/pulse. (c) CuIIOEP in deoxygenated 2-PrOH containing 7 mM 2-Pr0- and 2% pyridine, dose 40 Gy/pulse.

carried out y-radiolysis experiments with similar solutions and recorded the optical absorption spectra before and after several irradiation intervals. In all cases the peaks of the starting material, Cr111P(L,L2),disappeared gradually upon irradiation and new absorptions were formed (Figure 3, Table 11). The *-radical anions decay to give the products of two-electron reduction and protonation, Le., chlorins (in which one pyrrole double bond is saturated) and phlorin anions (reduced and protonated at a meso position). Generally, phlorin anions were produced in alkaline solutions, were characterized by a broad peak at 800-900 nm (Figure 3a), and yielded the original porphyrin quantitatively upon reoxidation with 02.

+

2Cr111P'- H+

+

CrlIIP

+ CrlIIPH-

(phlorin anion)

(7)

Chlorins were formed in neutral and acid solutions, had a sharp

A, nm Figure 3. Spectral changes upon y-radiolytic reduction of CrlI1porphyrins. The solid lines are the spectra before irradiation, and the dashed and dotted lines are after various irradiation times. (a) CrTSPP in N 2 0 saturated aqueous solution containing 5% 2-PrOH at pH 12.5, irradiation times 66 and 132 s. (b) CrTSPP in N20-saturatedaqueous solution containing 5% 2-PrOH and 1 M HClO,, irradiation times 66 and 102 s. (c) CrTPP in deoxygenated neutral 2-PrOH solution, irradiation times 126 and 192 s. (d) CrMSP in deoxygenated aqueous 2-PrOH (5%) solution at pH 11, irradiation times 36 and 72 s.

peak at 620-630 nm (Figure 3b) and were not readily reoxidized with 02. 2Cr111P'- 2H+ CrlIIP + Cr111PH2 (chlorin) (8)

+

-

Guldi et al.

4464 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992

\

- 2.0

-1.6

E [ V vs SCEl

Figure 4. Cyclic voltammograms of CrlllTPP in dichloroethane (in deoxygenated solutions containing 0.1 M Bu4NPF6,glassy carbon electrode, sweep rate 0.1 V/s, temperature 17 "C). (a) 9.4 X lo4 M porphyrin, (b) with 0.01 M Et,NCI added, and (c) 4.6 X lo4 M porphyrin, 20% DMSO in dichloroethane.

Radiolytic reduction of Cr'I'TPP(C1)L in neutral alcohol solutions led to the formation of Cr"TPP which was sufficiently stable to observe in some experiments (Figure 3c), but it disappeared within several hours to form the chlorin, probably by disproportionation, as suggested before for Ni1P.Io 2Cr"P

+ 2H+

-

Cr"'P

+ Cr111PH2

(9)

-1.6

-1

.a \ 7 -0.8

V

-0.4

0.0

E [ V vs SCEl

Figure 5. Cyclic voltammograms of CrlIIOEP in dichloroethane (in deoxygenated solutions containing 0.1 M Bu4NPF6,glassy carbon electrode, sweep rate 0.1 V/s, temperature 17 " C ) . (a) 8.0 X lo-' M porphyrin, solid line - without added CI-, dashed line - with 0.024 M Et,NCI added, (b) 5.5 X lo4 M porphyrin, with 20% DMSO in dichloroethane, and (c) 4.0 X lo4 M porphyrin, with 10% pyridine in dichloroethane.

The Cr"P products from Cr"'OEP(C1)L and Cr"'MSP(C1)L were longer lived and were observed following y-radiolysis (Figure 3d) in neutral or alkaline alcohol solutions. Upon introduction of O2 into the solution, all Cr"P products were immediately oxidized to Cr"'P(LlL2) quantitatively. In the presence of pyridine, however, reduction of these porphyrins did not yield either the Cr"P species or the chlorins: the products exhibit absorptions at 480 and 525 nm, probably due to the four-electron-reduction product^,^' which after reoxidation by O2 formed some chlorin and some original porphyrin. Cyclic Voltammetric Measurements. A cyclic voltammogram of Cr1''-octaethylporphyrin in DMSO between -0.7 and -1.8 V vs SCE showed two reversible reduction steps and the onset of a third reduction. The half-wave potentials were found at -1.1 1 and -1.32 V, in good agreement with literature values determined under similar condition^.^ The MSP and MSPDME complexes exhibited very similar behavior, with the first reduction occurring at E I j 2= -1.15 and -1.12 V, respectively. The first reduction of the phenyl-substitutedporphyrins took place at more positive potentials: E , values for the first and second reduction steps of Cr111TPP(DhS0)2were -0.86 and -1.3 1 V, also in agreement with earlier measurements in the same solvent.6a The behavior of Cr"'TMP(DMSO), was quasi-reversible, with the first E l = -0.94 V, and the voltammograms for CrlllTSPP(DMSO{, showed a much lower degree of reversibility (peak separation of 200 mV), with E l l 2 = -0.92 V. Thus, reduction of the phenyl-

substituted CrIILporphyrins is more favorable, by 0.2-0.3 V, as compared with reduction of the pyrrole-substituted CrlI1porphyrins. While a similar difference was noted also in the reduction of the corresponding free base porphyrins,28this does not indicate the site of reduction in the Cr"'P(L,L,) systems. To avoid axial ligation by a solvent molecule, cyclic voltammetric measurements were carried out in a noncoordinating solvent, 1,2-dichloroethane. The effect of axial ligation was then examined by adding to these solutions various concentrations of ligands with increasing donor strength (chloride, DMSO, pyridine). To study the difference between the reduction of phenyl-substituted and pyrrole-substitutedCrlILporphyrins, we examined the electrochemical behavior of the Cr"' complexes of OEP and TPP in more detail. The cyclic voltammogram of CrII'TPP(C1) in dichloroethane (Figure 4) shows a minor reduction wave at -0.88 V vs SCE, whose related oxidation occurs at -0.50 V. In addition, two major redox processes were observed, with E I l 2 = -1.16 and -1.79 V. By adding 0.01 M C1- (tetraethylammonium chloride) the small wave at -0.88 V disappeared, apparently under the next major wave, and the related oxidation wave at -0.50 V shifted to -0.59 V. The redox process at -1.16 V is not influenced by the addition of C1-. Addition of DMSO (20%) instead of C1- resulted in voltammograms with different features. Three redox processes were found at E I j z= -0.92, -1.23, and -1.75 V vs SCE. The first

(27) Mosseri, s.;Nahor, G . s.;Neta, P.; Hambright, P. J . Chem. Soc., Faraday Trans. 1991, 87, 2567.

(28) Worthington, P.; Hambright, P.; Williams, R. F.X.;Reid, J.; Burnham, C.; Shamim, A.; Turay, J.; Bell, D. M.; Kirkland, R.; Little, R. G.; Datta-Gupta, N.; Eisner, U. J. Inorg. Biochem. 1980, 12, 281.

One-Electron Reduction of Chromium(II1) Porphyrins wave was confirmed to be reversible and diffusion-controlled;the other two were quasi-reversible(difference in peak location of 90 mV) . A solution of CrII'OEP(C1) in dichloroethane shows cyclic voltammograms similar to those of Cr"'TPP(CI), except that in the case of OEP two minor waves are found at -1.08 and -1.27 V and their related oxidation waves appear at -0.43 V (Figure 5). In addition to these small waves, one reversible redox process at E,,, = -1.44 V is observed. The two small waves become one reduction wave at -1.3 V by adding increasing concentrations of C1-. The related oxidation wave shifts from -0.43 to -0.56 V. An opposite shift was observed for the major wave, to El = -1.34 V. Addition of 20% DMSO to the Cr%EP(Cl) in dicdoroethane did not change the features of the cyclic voltammogram, but the waves shifted slightly. The two minor reduction waves were at -0.88 and -1.23 V, and their related oxidation wave was at -0.44 V. In contrast, addition of 10% pyridine resulted in the observation of only one dominant redox process at E 1 p= -1.12 V. Assignment of these potentials to the specific reduction processes is discussed below based on the spectroelectrochemical results. Cyclic voltammetry of the Cr'ILporphyrins was also attempted in 2-PrOH solutions under conditions similar to those used in the radiolysis experiments. Two reduction steps were observed, but the processes were much less reversible. The waves were less pronounced and the separation between the first reduction and its reoxidation wave for these chromium porphyrins was 120 mV for OEP, 160 mV for MSP, 250 mV for MSPDME, 300 mV for TPP, and 400 mV for TMP (Cr"'TSPP is not soluble in 2-PrOH). Thia-Layer Spectroelectrochemistry. To assign the waves of the cyclic voltammograms to specific redox processes, spectra of solutions electrolyzed at various potentials were recorded by thin-layer spectroelectrochemistry. Electrolytic reduction of Cr"'TPP(C1) in dichloroethane containing 0.01 M Et,N+Cl- at -1.45 V vs SCE resulted in spectral changes with clean isosbestic points which shifted after longer times, indicating that the reduction involves two consecutive steps. The fmt step is attributed to the formation of the divalent Cr complex. The general features and the location of the absorption maxima are in agreement with those obtained by ?radiolysis in 2-PrOH. The second step leads apparently to the formation of CrIITPP-, with broad absorption maxima at 600,650, and 780 nm. Applying a potential between the two oxidation waves (-0.59 and -1.13 V), for example, -1.0 V, leads back to a clean spactrum of CrIITPP, which shows further reoxidation at -0.4 V to Crll'TPP(L,Lz). In the case of dichloroethane/DMSO (4:l) the spectrum of the species produced upon electrolysis at -1.15 V shows the featum that were attributed in the pulse radiolysis section to the *-radical anion Cr"'TPP'-, i.e., two broad intense peaks at 680 and 780 nm (Figure 6b). The Soret band shows only broadening during this process, in contrast with the clean hypsochromic shift observed when the reduction takes place at the metal center. At more negative potentials (-1.4 V) the spectrum indicates the formation of the dianion Cr"'TPPZwith broad maxima at 600,660, and 790 nm. Reoxidation to the a-radical anion takes place at -1.1 V and is followed by the recovery of Cr"'TPP(L1LZ) at -0.8 V. Crlt1OEP(Cl)in dichloroethane solutions exhibited a behavior similar to that of Crl''TPP(C1) in that reduction took place at the metal center. Applying a potential of either -1.3 or -1.4 V (after the fmt or the second minor waves in the voltammogram) resulted in the same product. Addition of 0.024 M C1- and reduction at -1.6 V resulted in the formation of the same divalent CrIIOEP, which can be reversibly reoxidized to the starting material. A second reduction at a potential of -1.7 V, due to the major wave of the cyclic voltammogram, could not be observed, probably due to reaction with the solvent. Addition of 20% DMSO to the dichloroethane solution of CrIIIOEP(C1) and reduction at either -1.15 or -1.45 V also led to formation of CrWEP. As in the previous solutions, the second wave did not result in observable products and reoxidation to the Cr"'P state took place quantitatively at -0.2 V. The formation of Cr"P in the presence of DMSO is in apparent contrast with the observation by pulse radiolysis, where addition of 10%DMSO

The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4465 1.8

G

J

1

1 0.4

1

I

boo

O.O 0 -

800

A,

700

.-

0.0

800

nm

Figure 6. Spectroelectrochemical results with Cr"'-porphyrins. (a) 4.0 X IO4 M CrIl'OEP, 10%pyridine in dichloroethane, 0.1 M Bu4NPF6, at -1.4 V vs SCE. (b) 4.6 X lo4 CrI'ITPP, 20%DMSO in dichloroethane, 0.1 M Bu4NPF6at -1.15 V vs SCE. The arrows indicate the decrease or increase in absorbance with time. Experiments at room temperature.

to the porphyrin solution in 2-PrOH/KOH changed the site of reduction from the metal center to the ligand. The reason for this difference is probably due to differences in axial ligation; i.e., in the electrochemical experiment the porphyrin is coordinated with a DMSO molecule and a weak C1- ligand whereas in the pulse radiolysis experiment the weak C1- ligand has been replaced with the stronger OH- ligand. The donor strength of the axial ligands is thus crucial in determining the path of reduction. The addition of 10%pyridine changed the path of the reduction from the metal center to the ligand, as observed in the pulse radiolysis experiments. Electrolytic reduction at -1.4 V led to the spectral changes shown in Figure 6a, indicating the formation of the a-radical anion, with absorption peaks at 600,680, and 730 nm, in agreement with the peaks observed by pulse radiolysis. This product could be quantitatively reoxidized to the starting material at -0.8 V. Electrolytic reduction of Cr"'OEP(C1) was also carried out in neat DMSO solutions. At -1.3 V CrIIOEP was formed, which was further reduced at -1.55 V to the a-radical anion, CrI'OEP, with broad absorptions at 570, 670, and 740 nm. The second reduction step could be reversed at -1.2 V, and the product was transformed back to the original material at -0.9 V.

Conclusion Chromium(II1) porphyrins undergo one-electron reduction either at the metal center or at the porphyrin ligand. 4l

cr'11-p

+

e-

< :;.-

(loa)

l (1Ob) LZ The pathway of the initial reduction depends strongly on the electron affinity of the porphyrin ligand and on the electron-donor properties of the axial ligands, L1and Lz, which affect the electron density on the metal center. We can classify the axial ligands as weak (L, = Cl-), intermediate (Li = ROH,RO-, HzO, OH-), and strong (L, = pyridine, DMSO) electron donors. The results of both the radiolytic and the electrochemical experiments indicate that the first one-electron reduction of Cr"'-porphyrins, where the macrocycle is of the meso-tetraphenylporphyrintype, leads to formation of Cr"P when the Cr center bears L,Li, but yields CrlIIP'- when the Cr bears LiLi or L,L, (or stronger). Pyrrolesubstituted porphyrins, whose macrocyclic rings have lower electron affinities than TPP and TSPP, are reduced on the T -

4466

J. Phys. Chem. 1992, 96, 4466-4469

system only when the Cr center bears LiL, or stronger ligands but form Cr"P with weaker ligands. The Cr"P products are relatively stable in solution but are sensitive to the presence of 02,which oxidizes them immediately to the Cr'I'P state. The C r l W species, however, are short-lived in protic solvents and undergo disproportionation and protonation to form chlorins in neutral and acid solutions or phloM anions under alkaline conditions. The phlorins are also readily oxidized by O2 to Cr"'P(LIL2). These studies

establish the conditions for production of Cr"-porphyrins and permit us to study the reactions of these products with various alkyl radicals that lead to formation of Cr-C bonds. This will be the topic of a future publication. Acknowledgment. This research was supported by the Office of Basic Energy Sciences of the US Department of Energy. We thank Dr. P. Hapiot for helpful discussions.

Photoinduced Intramolecular Electron Transfer in Vioiogen-Linked Zinc Porphyrins in Dimethyl Sulfoxide Ichiro Okura* and Hideyuki Hosono Department of Bioengineering, Tokyo Institute of Technology, Nagatsuta, Midori- ku, Yokohama 227, Japan (Received: November 7, 1991; In Final Form: January 27, 1992)

Compounds containing a viologen linked to a zinc porphyrin via methylene groups have been synthesized, and photoinduced intramolecular electron transfer between porphyrin and viologen was observed. The photoexcited singlet state of the porphyrin was quenched by the bonded viologen, the photoexcited triplet state was quenched, and the lifetime of the charge separated species was about 1 p s .

Introduction Various viologen-linked porphyrins have been synthesized to develop suitable redox systems for the photochemical utilization of solar As the compound serves as a photosensitizer and an electron carrier in the same molecule, simpler redox systems for solar energy conversion can be established. To improve the quantum yield, the viologen-linked porphyrins with longer lifetimes of the charge-separated species are desired. In this paper we hope to describe the preparation and characterization of the viologen-linked zinc porphyrins with longer charge-separated lifetimes. We prepared a series of two compounds, viologen-linked porphyrins connected with viologen at the 3 (meta) and 4 (para) positions of the pyridine ring. It is hoped that the intramolecular electron-transfer process from the porphyrin ring to the viologen would occur more rapidly in meta compounds than in para compounds because of the conformational effect. Experimental Section The structures of viologen-linked zinc porphyrins are shown in Figure 1. For the preparation of p-ZnPC,V the starting material, 5-(4-pyridyl)-10,15,2O-tritolylporphyrin(PyTP), was synthesized and the byproducts were removed as described in the l i t e r a t ~ r e s . ~ JPyPT ~ was then quaternized with an excess of (1) Harriman, A.; Porter, G.; Wilowska, A. J . Chem. Soc., Furaduy Trans. 2 1984,80, 19 1. (2) Aono, S.; Kaji, N.; Okura, I. J. Chem. Soc., Chem. Commun. 1986,

".

I 7n I ,

(3) Okura, I.; Kaji, N.; Aono, S.;Nishisaka, T. Bull. Chem. SOC.Jpn. 1987,60, 1243. (4) McMahon, R. J.; Force, R. K.; Patterson, H. H.; Wrighton, M. S. J. Am. Chem. SOC.1988, 110, 2670. ( 5 ) Aono, S.;Kaji, N.; Okura, I. J . Mol. Carol. 1988, 45, 175.

(6)Force, P.K.; McMahon, R. J.; Yu,J.; Wrighton, M. S.Spectrochim. Acia, Pari A 1989, 45, 23. (7)Noda, S . ; Hosono, H.; Okura, I.; Yamamoto, Y.; Inoue, Y. J . Chem. SOC.Faraday Trans. 1990,86, 8 11. ( 8 ) Noda, S.;Hosono, H.; Okura, I.; Yamamoto, Y.; Inoue, Y. J. Phoiochem. Phoiobiol., A 1990, 53, 423. (9) Franco, C . ; McLendon, C. Inorg. Chem. 1984, 23, 2370.

a,w-dibromoalkane at 130 "C. The quaternized porphyrin and a 100-fold molar e x m s of 1-methyl-1'-(bromoalky1)bipyridinium were stirred at 80 "C in DMF for 48 h to obtain viologen-linked metal free (protonic) porphyrins (p-PC5V). The protons of the porphyrin ring were replaced by metal cations as follows. To the solution of p-PC5V (3.98 X mol) dissolved in 100 mL of MeCN, zinc chloride (1.84 X lo4 mol) was added and stirred at 30 O C for 48 h without light. After the removal of MeCN by evaporation, the solid was washed with water to remove excess zinc chloride. The solid was dissolved in acetone, and (C2H5),NC1 was added to replace the counteranion to Cl-. The desired product should be insoluble in acetone with C1-. The precipitate was collected and dissolved in MeOH and was developed in Sephadex LH-20 column (1.8 X 10 cm; developer (C2H5),NCl-MeOH solution) without light. After the development the solution corresponding to the second band was collected, and MeOH removed with evaporation. The solid was washed with water to remove (C,H,),NCl, the solid was dissolved in MeOH again, and NH4PF6(0.2 g) was added to replace the counteranion with PF,. The precipitate was collected, washed with MeOH, and then dried under vacuum at room temperature. The purity of the products was established from their 'H N M R spectra. In the case of the preparation of m-ZnPC,V, 5-(3-pyridyl)10,15,2O-tritolylporphyrinwas used instead of 5-(4-pyridyl)10,15,20-tritolylporphyrinas the starting material. The molecular structure of the synthesized viologen-linked porphyrins was characterized by 'H NMR. The compounds were dissolved in dimethyl-d6 sulfoxide (DMSO-d,) for the NMR samples and the concentrations of the samples were 10 mM for p-ZnPC5V and m-ZnPC5V. Chemical shifts were referenced to the residual solvent peak, which in turn was calibrated against tetramethylsilane. Other viologen-linked porphyrins were synthesized by the method reported p r e v i o u ~ l y . ~ ~ ~ The luminescence intensity was measured using a Hitachi-850 spectrometer. The excitation wavelength was 500 nm. In these experiments the concentration of the sample solution was adjusted in order to keep the absorbance at the excitation wavelength (10)

Rosseau, K.; Dolphin, D.Tetrahedron Leii. 1984, 4251

0022-3654/92/2096-4466$03.00/00 1992 American Chemical Society