2468
J. Phys. Chem. 1985,89, 2468-2412
Charge Separation and Photoreduction of Zinc Tetrakis(sulfonat0phenyi)porphyrin by Nitrobenzene and Methylvioiogen in Aqueous Solutions Gad S. Nahor and Joseph Rabani* Department of Physical Chemistry and Energy Research Center, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: September 12, 1984)
The photochemical reactions of zinc tetrakis(sulfonatopheny1)porphyrin (ZnTPPS’) in aqueous solutions containing nitrobenzene (NB) or methylviologen (MV*+)are reported. NB quenches the singlet state by electron transfer without charge separation. When the triplet is quenched by NB, charge-transfer products are observed. The rate constants for quenching and back electron transfer are reported. MV2+quenches the singlet through a static effect of complexation. The stability constant of the complex is reported. In the presence of high concentrations of NaC10, and ZnS0, the complexation is avoided. Under these conditions the triplet is formed and quenched through electron transfer. Charge separation is reported. Similar results were obtained when a negative polyelectrolyte poly(viny1 sulfonate) (PVS) was introduced. PVS completely avoided the complexation and enabled charge separation after photoreduction of the triplet.
Introduction The quenching of porphyrins and metalloporphyrins through electron-transfer processes have been extensively studied.149 In
(1) “The Porphyrins”; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 2, Chapters 5 , 6; Vol. 3, Chapters 1-3; Vol. 5 , Chapters 1-4. See also references cited therein. (2) Harel, Y.; Manassen, J. J . Am. Chem. SOC.1978, 100, 6228. (3) Neta, P.; Scherz, A,; Levanon, H. J . Am. Chem. Soc. 1979, 101, 3624. (4) Potter, W.; Levin, G. Photochem. Photobiol. 1979, 30, 225. (5) Pileni, M.-P. Chem. Phys. Left. 1980, 75, 540. (6) Netzel, T. L.; Bergkamp, M. A,: Chang, C.-K.; Dalton, J. J . Photochem. 1981, 17, 451. (7) Neta, P. J . Phys. Chem. 1981, 85, 3679. (8) Nahor, G. S.; Rabani, J.; Grieser, F. J . Phys. Chem. 1981, 85, 697. (9) “Book of Abstracts, 4th International Conference on Photochemical Conversion and Storage of Solar Energy”; Weizmann: Jerusalem, 1982; Chapter 111. (10) Baral, S.; Neta, P. J . Phys. Chem. 1983, 87, 502. (1 1) Smalley, F.; Feldberg, S. W.; Brunschmig, B. S. J . Phys. Chem. 1983, 87, 1757. (12) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. (13) Schmehl, R. H.; Whitten, D. G. J . Phys. Chem. 1981, 85, 3473. (14) Pileni, M.-P.; GrBtzel, M. J . Phys. Chem. 1980, 84, 1822. (15) Harriman, A,; Porter, G.; Searle, N. J . Chem. SOC.,Faraday Trans. 2 1979, 75, 1515. (16) Harriman, A,; Porter, G.; Wilowska, A. J . Chem. SOC.,Faraday Tram. 2 1983, 79, 807. (17) Harriman, A,; Richoux, M.-C.; Neta, P. J . Phys. Chem. 1983, 87, 4957. (18) Starodubva, N. A,; Maslov, D. G.; Bumistrava, T. I.; Titov, V. I. Russ. J. Phys. Chem. (Engl. Transl.) 1979, 53, 691. (19) yhairutdinov, R. I.; Brikenstein, E. Kh.; Strekava, L. N. Bull. Acad. Sci. USSR,Phys. Ser. (Engl. Transl.) 1982, 7 , 1341. (20) Berezin, B. D.; Khelevina, 0. G.; Brin, Q ,P. Russ. J . Phys. Chem. (Engl. Transl.) 1982, 56, 902. (21) Tabushi, F.: Kugimiya, S . : Mitzutanie, T. J . Am. Chem. SOC.1983, 105, 1658. (22) Tanno, T.; Wohrle, D.; Kanoko, M.; Yamada, A. Ber. Bunsenges. Phys. Chem. 1980,84, 1032. (23) McLendon, G.; Miller, D. S. J . Chem. Soc., Chem. Commun. 1980, 533. (24) Dorwen, J. R. J . Chem. SOC.,Chem. Commun. 1980, 805. (25) Peradevan, G. 0.;Pileni, M.-P. J . Chim. Phys. 1981, 78, 203. (26) Maillard, P.; Gaspord, G.; Krausz, P.; Gianoti, C. J . Organomef. Chem. 1982, 212, 185. (27) Lever, A. B. P.; Ramaswamy, B. S.; Licoccia, S. J . Phofochem. 1980, 19, 173. (28) Rougee, M.; Ebessen, T.; Ghetti, F.; Bensasson, R. V . J . Phys. Chem. 1982, 86, 4404. (29) Shelnutt, J . A. J . Am. Chem. SOC.1983, 105, 7179.
0022-3654/85/2089-2468$01.50/0
a number of systems net formation of products is observed when they possess the same charge. On the other hand, it seems that when an electron-transfer process produces products with opposite charges, quenching of the excited state of the porphyrins leads to no net product formation. Apparently contradicting results have been reported for the methylviologen/ZnTPPS4- system. While McLendon and Miller,23Rougee et Okura et a1,,45,46and Brugger et al.48 reported net formation of photochemical electron-transfer products, Kalyanasundaram and Gratze130as well as Richoux and Harriman4’ reported only quenching of the excited states without product formation. The purpose of this work is to elucidate the reactions which take place in the ZnTPPS4-/MV2+ system under various experimental conditions and to provide a full explanation of the different observations as mentioned above. In addition, our aim is to gain further insight into the effect of ions’ charges on net photochemical products formation. This is achieved by comparison between a neutral and a positively charged quencher in a system where the photosensitizer is a negative porphyrin ion, ZnTPPS4-, or its free base (TPPS4-). The neutral quencher was chosen to be nitrobenzene (NB) which compares with the positively charged MV2+. The two quenchers have similar redox potentialsSo( E o = -0.486 (30) Kalyanasundaram, K.; Grltzel, M. Helv. Chim. Acta 1980, 63, 478. (31) (a) Harriman, A,; Richoux, M.-C.; J . Chem. SOC.,Faraday Trans. I , 1980, 76, 1618. (b) J . Photochem. 1980, 14, 253. (32) (a) Harriman, A.; Porter, G.; Richoux, M.-C. J . Chem. Soc.,Faraday Tram. 2 1981, 77, 833. (b) Ibid. 1981, 77, 1175. (c) Ibid. 1982, 78, 1955. (33) Harriman, A.; Porter, G.; Walters, B. J . Chem. Soc., Faraday Trans 2, 1983, 79, 1335. (34) Harriman, A,; Richoux, M.-C. J . Photochem. 1981, 15, 335. (35) Carnieri, N.; Harriman, A. J . Phofochem. 1981, 15, 341. (36) Parmon, V. N.; Lymon, S. V.; Tsvetrov, I. M.;Zamarev, K. I . J . Mol. Cafal. 1983, 21, 353. (37) Okura, I.; Kim-Thuan, N. (a) J . Mol. Cafal. 1979, 6 , 227. (b) J . Chem. SOC.,Chem. Commun.1980,84. (c) J . Chem. SOC.,Faraday Trans. 1 1980, 76, 2209. (38) Okura, I.; Takeuchi, M.; Kim-Thuan, N. Chem. Lett 1980, 765. (39) Tabushi, I.; Yazaki, A.; Koga, N.; Iwasaki, K. Tefrahedron Lett. 1980, 21, 373.
(40) Maillard, P. J . Orgunomer. Chem. 1981, 212, 185. (41) Okura, I.; Aono, S.; Takeuchi, M.;Kusunoki, S. Bull. Chem. SOC. Jpn. 1982, 55, 3637. (42) Kano, K.; Sato, T.; Yamada, S.; Ogawa, T. J . Phys. Chem. 1983,87, 556. (43) Ohno, T.; Kato, S.; Yamada, A,; Tanno, T.J . Phys. Chem. 1983,87, 775.
(44) Okura, I.; Kusunoki, S.; Aono, S. Inorg. Chim. Acta 1983, 7 7 , L99. (45) Okura, I.; Kusunoki, S.; Aono, S. Inorg. Chem. 1983, 22, 3828. (46) Okura, I.; Kusunoki, S.; Aono, S. Bull. Chem. SOC.Jpn. 1984, 57, 1184. (47) Richoux, M.-C.; Harriman, A. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 1873. (48) Brugger, P.-A,; Gratzel, M.;Guarr, T.; McLendon, G. J . Phys. Chem. 1982, 86, 944. (49) Harriman, A.; Porter, G.; Wilowska, A. J . Chem. SOC.,Faraday Trans. 2 1984, 80, 191.
@ 1985 American Chemical Society
Zinc Tetrakis(sulfonatopheny1)porphyrin
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2469
!20
and -0.446 V for NB/NB- and MV2+/MVt, respectively; potentials are vs. NHE); both are aromatic molecules, and both were found to quench the excited state of ruthenium tris(bipyridy1) with similar rate constant^.^^
40
Experimental Section
1
Materials. All reagents were of highest purity available. The sodium salt of TPPS4- was from Strem Chemicals Inc. and was used without further purification. Its spectrum was recorded, and the following extinction coefficients (M-I cm-') were used: 16 700 P (516 nm), 6840 (553 nm), 6400 (581 nm), 3500 (635 nm), and 2- I0 1960 (460 nm). These values are in good agreement with preE viously given values.52 The Zn complex, ZnTPPS', was prepared W and purified according to the method of Cheung et al.53 The extinction coefficients of the ground state and the first triplet were 550 600 650 700 based on ref 54, while those of the reduced form (ZnTPPS3-) were Xnm taken from ref 5 5 . The values used are 6ZnTpB4- (M-l cm-') = Figure 1. Fluorescence spectrum of 2 X M ZnTPPS4- in the pres2500 (460 nm), 9000 (600 nm); E3ZnTpm4-' (M-* cm-') = 5 5 200 ence of N B and MV2+; excitation wavelength A,, = 530 nm: (a) [NB] (460 nm), t Z n T P P(M-I S ~ cm-') = 8500 (460 nm), 3100.(600 nm). = [MV2+] = 0; (b) [NB] = 8.0 X lo-) M , [MVzt] = 0; (c) [NB] = 0, Nitrobenzene (BDH) was purified by vacuum distillation prior [MVzt] = 2.2 X M. to use. Methylviologen (Sigma) was used as received. The MV+ was detected at 600 nm (E = 1.1 X lo4 M-I cm-I ) and at 460 nm (E = 1000 M-' cm-I). NaC104 (Baker) and ZnS04 (Baker) were used as received. PVS was donated by Dr. R. E. Sassoon after purification according to the method of Meisel et al.56 Unless otherwise stated, all solutions contained distilled water and 2 X M ZnTPPS" or TPPS' and were deaerated by bubbling high purity H e (Matheson). Apparatus. Absorption spectra were recorded on a Bausch and Lomb Model Spectronic 2000 spectrophotometer. Emission spectra were taken on a Perkin-Elmer Model LS-5 luminescence spectrometer or a S L M 4800 spectrofluorimeter. Laser photolysis at 530 nm was carried out on a Molectron s" DL200 dye laser, 450 kJ, 10-ns fwhm, pumped by a Molectron UV 14N2laser (dye: Molectron 70731). The irradiation cell was 1-cm long; the analytical light and laser beam were oppositely coaxial. Full details of this setup have been previously g i ~ e n . ~ ' , ~ ~ The resolution time of this setup was 2 ks. For shorter time measurements a Nd:YAG laser was used. Its excitation beam was 11.O-mm long and 2-mm wide (this was done with a cylindrical lens) and entered the 1-cm cell at an angle of 90° to the analytical light. This setup had a resolution time of 25 ns. In 0.0 1.0 [MV"] 2.0 10-4M 3.0 4.0 both laser setups a 150-W Xe lamp was the analytical source. The signal was transferred through a B & L monochromator to an Figure 2. Stern-Volmer plots of solutions containing 2 X M IP28 photomultiplier. Analysis was carried out using either a ZnTPF'Se and (a) NB, (b) MVZt;detection wavelengths, he,= 605 (O), Biomation Model 8100 with a Nicolet 1170 (dye laser) or on a 650 (0) nm. bo and 4 are the values of emission in the absence and Textronix 7912AD digitizer with a Textronix 4052 data station presence of quencher, respectively. (Nd:YAG laser). The temperature was 22 f 3 "C.
-
?
li!cYLA
Results and Discussion
The photolysis of ZnTPPS" as well as the nature of its singlet and triplet have been previously Our ob~~
(50) (a) Meisel, D.; Neta, P. J. Am. Chem. Soc. 1975,97,5198. (b) Clark, W. M. "Oxidation Reduction Potentials of Organic Systems"; Robert E. Krieger Publishing Co.: Huntington, New York, 1972. (51) (a) Bock, C. R.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. SOC. 1974, 96,4712. Gaines Jr., G. L. J. Phys. Chem. 1979,83, 3088. Keller, P.; Moradpoar, A,; Amouyal, E.; Kagan, H. B. Nouv. J. Chim. 1980,4,377 and references cited therein. Harriman, A.; Mills, A. J. Chem. SOC.,Faraday Trans. 2 1981, 77, 2111. (b) Meyerstein, D.; Rabani, J.; Matheson, M. S.; Meisel, D. J . Phys. Chem. 1978, 82, 1879. (52) Fleischer, E. B.; Cheung, S . K. J. Am. Chem. SOC.1976, 98, 8 3 8 1 . (53) Cheung, S. K.; Dixon, F. L.; Fleischer, E. B.; Jeter, D. Y.; Krishnamurthy, M. Bioinorg. Chem. 1973, 2, 281. (54) Kalyanasundaram, K.; Neumann-Spallart, M. J . Phys. Chem. 1982, 86, 5166. (55) Neumann-Spallart, M.; Kalyanasundaram, K. Z . Naturforsch. 1981, 36, 596. (56) Meisel, D.; Rabani, J.; Meyerstein, D.; Matheson, M. S. J. Phys. Chem. 1978, 82, 985. (57) LOURnOt, - D. J.; Dolan, G.; Goldschmidt, C. R. J . Phys. E 1979, 22, 1057. (58) Sassoon, R. E.; Rabani, J. J. Phys. Chem. 1980, 84, 1319.
servations were in full agreement with these findings. We have also observed an identity between the absorption and excitation spectra of ZnTPPS4- in the wavelength range 400-650 nm. Experiments with NB. When N B is added to a solution conM ZnTPPS" at neutral pH it has only a small, taining 2 X if any, effect on the absorption spectrum of the ZnTPPS", up to the highest concentrations used (8 X M). However, it has strong effects on the lifetimes of the excited states. The fluorescence of ZnTPPS4- is quenched in the presence of NB. There is no change in the fluorescence spectrum as can be seen in Figure 1. The quenching reaction 1 is presumed to involve 'ZnTPPS"'
+ NB
kl
fast
'(ZnTPPS3---NB-) ZnTPPS4-
+ NB
(1)
electron transfer from the porphyrin to the NB to yield a charge-transfer complex. N o net formation of redox products is observed. N o energy transfer could have taken place since N B does not have the appropriate energy levels. From the Stern-Volmer plot ~~~~
~
(59) Kalyanasundaram, K. J. Chem. SOC.,Faraday Trans 2 1983, 79, 1365.
2470
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
Nahor and Rabani
A
0.03
a
I
I
i
I
~
0.02
1.
1
I
b
c 0 .-
A
0
c
Q L
450
0
m
500
550
Xnm
6CO
650
700
Figure 4. Comparison of the triplet absorbance (A)and (0,scale: X IO), with the absorbance after process 3 ( 0 ) . bX= 530, [ZnTPPS4-]= 2 X
-D
5
10-5 M.
8
in a second-order process attributed to the back-electron-transfer reaction 4. The observed second-order rate constant for this
C
NB-
Time Figure 3. Typical absorbance changes of solutions containing 2 X M ZnTPPS4-(a), and 1 X IO4 M NB (b and c). A,, = 530 nm, Adst = 460 nm.
(Figure 2), Ksv = (80f 5 ) M-' is derived, yield kl = (4.7 f 0.3) X 1Olo M-' s-I on the basis of the actual fluorescence lifetime Tf0 = 1.7 ns.49954*59 This value is similar to k('TPPS''+NB) = (1.9 f 0.3) X 1OloM-l s-l.' As in the case of TPPS',' no net products have been found following the quenching of the singlet state. This is attributed to the fast back-electron-transfer process in the charge-transfer complex, which is spin a l l ~ w e d . ' * ~When ~~~~ solutions containing only ZnTPPS' are employed with laser pulses, absorbance changes are observed and attributed to the triplet formation. These absorbance changes decay away by process 2 with k2 = (450 f 100) s-I. This value is in agreement with previous data.54 When 1 X 104-8 X 10-3 M N B is added, the
k2
3ZnTPPS"' ZnTPPS4(2) initial absorbance change due to the triplet formation decreases in parallel with the decrease in the fluorescence of the singlet. The absorbance change decays away in two processes, as presented in Figure 3. The first process, attributed to reaction 3, is first order in both 3ZnTPPS4-' and NB, with k3 = (1.6 f 0.2) X lo9 M-1 s-l . This value is by 3 orders of magnitude higher than that 3ZnTPPS4-'
+ NB
k3
ZnTPPS3- + NB-
(3)
observed with TPPS", (3.2 f 0.5) X lo6 M-' s-l.' The difference may be related to the redox potentials" (Eop+P. = 4 . 3 4 and -0.75 for TPPS' and ZnTPPS", respectively). "he spectrum at the end of the fast decay process (Figure 4) is considerably different from the triplet s p e c t r ~ m . This ~ ~ , is ~ ~attributed to the formation of the products written on the right-hand side of eq 3. Indeed, there is a resemblance to the known spectrum of ZnTPPS3.54 The absorbance changes remaining at the end of reaction 3 decay away (60) (a) Holten, P.; Gouterman, M.;Parson, W. W.; Windsor, W.; Rockley, M. G. Photochem. Phorobiol. 1976, 23,415. (b) Gouterman, M.; Holten, D. Ibid. 1977, 25, 85. (c) Holten, D.; Windsor, W.; Parson, W. W.; Gouterman, M. Ibid. 1978, 28, 951.
+ ZnTPPS3- -% NB + ZnTPPS4-
(4) reaction is k4 = (2 f 1) X 10' M-I s-' taking = (7000 f 700) M-I cm-l. The value is by one order of magnitude lower than the corresponding rate constant in the TPPS" system.' The difference again may be connected with the appropriate redox potentials. The quantum yield of reaction 3 is determined to be unity by comparison of the absorption at the end of reaction 3 to the initial absorption of the triplet. Experiments with M p + . Upon the addition of MV2+ ( 1 . 1 X 10-5-4.4 X M) to 2 X M ZnTPPS4-, the absorption spectrum shows a slight broadening of the peaks, a change in their relative heights, and a small red shift. This is in agreement with previously reported r e s ~ l t s . No ~ ~change ~ ~ ,in~the~ fluorescence spectrum was observed, but a very efficient quenching (Figure 1) occurred. The Stern-Volmer plot (Figure 2) yields Ksv = 5 X lo4 M-I. This value is somewhat higher that the one reported by Brugger et al. (1.5 X lo4 M-1).48 Taking Tf0 = 1.7 ns49,54959 we obtain K s v / ~ f o= 3 X 10I3 M-I s-I for the quenching of the singlet. This value is far too high if attributed to any dynamic effect such as reaction 5, and therefore a static process must be
MV2+
+ IZnTPPS4-'
-
MV2+
+ ZnTPPS4- + heat
(5)
considered. A formation of a ground-state complex according to , ~ is ~ ,in~ agreement ~~~ with our eq 6 has been s ~ g g e s t e d ~ 'and K
MV2+
+ ZnTPPS4- & MV2+-.ZnTPPS2-
(6)
results. It also accounts for the changes in the ground-state absorption s p e ~ t r u m We . ~suggest ~ ~ ~that ~ ~the~excited complex does not emit light, and hence there is no effect on the shape of the emission spectrum. The emission is solely by the free IZnTPPS4-'. K6 can be calculated under a variety of MV2+ concentrations. The results are summarized in Table I. The complex concentration (Camp)is determined from the expression (Cpo- C,,,)/CPo = 4/+o. Cpois the total porphyrin concentration while C#I and C#Io are the emission intensities in the presence and absence of MV2+,respectively. Since by changing [MV2+] the ionic strength also changes, a value at zero ionic strength is calculated. The ionic strength is calculated taking into account the formation of the complex. The value of K6 at zero ionic strength, K60, is (9.3 f . 0 . 7 ) X lo4 M-I. This value is in good agreement with the value reported by Rougee et aL2' (K6 = 1500 M-'). Their value is based on absorption measurements at constant ionic strength, p = 0.1 M, and reported yields &O = 1.4 X lo5 M-'. However, Harriman et a lower value for K6, namely 5200 M-l at w = M, which corresponds to a value one order of magnitude smaller at w = 0 as compared to our value. As in the case of N B the quenching in the complex is also expected to be via an electron-transfer route. However, no direct
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2471
Zinc Tetrakis(sulfonatopheny1)porphyrin
h
b
th
L
300 sec
C
3
3 e
fh
f
Time
M ZnTPPS4- (Adet = 600 nm). (b) 2 X 10" M ZnTPPS4-, Figure 5. Effect of polyelectrolyte or salt on charge separation. bX= 530 nm. (a) 2 X M ZnTPPS4-, 2 X lo4 M MV2+,0.05 M ZnS04, Adet 460 2 X lo-" M MV2+,0.2 M NaC104, Adc, = 460 nm. (c) As (b), at 600 nm. (d) 2 X M ZnTPPS4-, 2 X lo4 M MV2+,0.01 M PVS, Adc, = 460 nm. ( 9 ) As (f), at 600 nm. nm. (e) As (d), at 600 nm. (f) 2 X
evidence for charge transfer has been found.49 This can be explained considering a fast, spin allowed back electron transfer to the ground-state reactants.a When 0.1-0.2 M NaC104 is added to ZnTPPS', broadening as well as red shifts in both the absorption and emission spectra are observed. This is in agreement with similar effects previously described8s5for other porphyrins. When PVS is added (0.01 M in monomer units) no such effects are observed on either absorption or emission spectra. When [MV2+] is added to solutions containing either 0.01 M PVS or 0.1-0.2 M NaC104, a dramatic decrease in the quenching by [MV2+] is observed. The results are given in Table 11. The NaC104 affects K6 by providing high ionic strength; thus equilibrium (eq 6 ) is shifted to the left, and the amount of ZnTPPS" which is complexed decreases. These findings are in agreement with the results of Brugger et al.a who reported a tenfold decrease in Ksv a t an ionic strength of 0.1 M. The effect of PVS is explained on the basis of the strong negative electrical field in its
vicinity. It removes MVZ+out of the bulk of the solution while rejecting the negative ZnTPPSe and preventing any significant complexation. Table I1 also shows that similar effects are observed for TPPS" (the free base). Laser flash experiments were also carried out. In the presence of 0.2 M NaC104 or 0.05 M Z n S 0 4 (in both cases the ionic strength is 0.2 M) the lifetime of 3ZnTPPS4- is shortened by a factor of about 10, and the decay kinetics obeys neither first- nor second-order rate law. These results are similar to previously reported data, which were attributed to the formation of dimer# or to triplet-triplet annihilation,6' both enhanced by partial neutralization of the four negative charges of the porphyrin. PVS (61) Houlding, V. H.; Kalyanasundaram, K.; Griitzel, M.; Milgrom, L. R. J . Phys. Chem. 1983, 87, 3175. (62) White, W . I. "The Porphyrins";Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, p 303.
2472 The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
Nahor and Rabani
TABLE I: Comdexation of ZnTPPSC and MVZt 1.1 2.2 4.4 11.0 22.0 44.0
0.61 0.45 0.26 0.13 0.005 0.046
0.71 1.65 3.60 10.13 21.08 43.05
3.87 5.47 7.40 8.67 9.17 9.54
6.13 4.53 2.60 1.33 0.83 0.46
0.94 1.11 1.56 3.43 6.68 13.32
10.6 8.9 9.8 9.1 8.4 9.3 av 9.4 f 0.7
10.5 8.8 9.7 8.9 8.2 8.9 9.2 h 0.7
"MV2+initial concentration in units of M. b @ / @ o is the ratio between the emission at 605 nm in the presence and absence of MV2+, respectively. CComplexconcentration in units of 10" M (for calculation details, see text). units of M, C, = CMVz+O- Camp. eConcentrationof noncomplexed ZnTPPSk. C, = Cp" - Cmmpin units of M. 'Ionic strength in units of lo4 M. #K6O (in units of lo4 M-I) is the equilibrium constant at zero ionic strength calculated by the Debye-HUckel theory as log K60/K6' = 1 . 0 2 2 + 2 4 ~ ' / ~1/ (+ 0 1 f i I / ~ ) taking 01 = 1. h K 6 0calculated as in g, taking 01 = 3. TABLE II: Emission of ZnTPPSt in the Presence of Additives" ZnTPPSe TPPSe additive A m b b/bLIC :A,, bIboC none 605 1.00 644 1.00 0.1 M PVS 605 1.00 644 0.2 M NaC104 607 0.92d 657 0.56d 2 X lo4 M MV2+ 605 0.09 644 0.30 f 0.03 0.1 M PVS + 2 X lo4 M MV2+ 605 1.00 644 0.94 f 0.05 0.01 M NaC10, + 2 X lo4 M 645 0.41 MV2+ 0.1 M NaCIO,
MV2+ 0.2 M NaC10, MV2+
+ 2 X lo4 M + 2 X lo4 M
607
0.63
657
1.00
607
0.76
657
1.00
"2 X M ZnTPPSe. Wavelength (nm) of maximum emission. CRatioof emission in the presence and absence of additives. dThese values are used as reference for solutions containing MVZt and 0.1-0.2 M NaCIO,.
has no effect on the decay kinetics of the 3ZnTPPS4-*. Solutions containing (2-5) X lo4 M MVZ+, 2 X 10" M ZnTPPS4-, and either 0.01 M PVS, 0.2 M NaC104, or 0.05 M ZnS04 were irradiated by laser pulses. Absorbance changes were followed at 600 and 460 nm. Typical results are given in Figure 5. At 600 nm, absorption appears only when all three components, namely ZnTPPS4; MVZ+,and either PVS or salt are present. The rate of the formation of the absorbance is within our resolution time (2 rs). The absorption decays away in the submillisecond time range. At 460 nm two decay processes are observed. The faster decay is nearly completed within our resolution time. The slower decay is in the same time range as the decay at 600 nm when salts are used, and twice as slow as the decay at 600 nm when PVS is used. W e attribute the fast process to reaction 7 and the slower process to reaction 8. The absorption at 600 nm is assigned to MV+ while at 460 nm the absorption is mainly due 3ZnTPPSe* ZnTPPS3-
+ MV2+
+ MV+
-
-
+ MV+
(7)
+ MV2+
(8)
ZnTPPS3-
ZnTPPS4-
to 3ZnTPPS4-* and ZnTPPS3-. The faster decay of MV', as compared to ZnTPPS3-, in the presence of PVS might be due to dimer formation or reaction with impurities which are concentrated together with MV+ by the PVS. When the salts are used, the
reaction rate constant is k8 = 2 X lo9 M-I s-' in agreement with previously reported values obtained under roughly similar condition~.~~,~~
Conclusions The above results, together with the previously reported res u l t ~ ~make ~ *it possible ~ ~ ~to ~advance ~ ~ a~full~ interpretation - ~ ~ of the ZnTPPS4-/MV2+ system: (1) Strong complexation between the oppositely charged species occurs in the ground state. This complexation causes very efficient quenching of the singlet, presumably via an electron-transfer route. (2) The quenching of the singlet does not involve net product formation. (3) The singlet state of the porphyrin is the triplet's precursor and therefore when the singlet is quenched, no triplet formation occurs. (4) The complexation could be avoided by appropriate media, either high ionic strength provided by inert salts or the presence of a polyelectrolyte. (5) At sufficiently low [MV2+]and [ZnTPPS'], it is possible to obtain the triplet. Since only little complexation takes place, under such conditions, it is possible to observe quenching of 3ZnTPPS4-* by MV2+. However, again, no net formation of electron-transfer products is observed. This is due to the fast back-electron-transfer process which competes successfully with product separation. (6) In the presence of inert salts or polyelectrolyte, charge separation of the photochemical products is enhanced, and the production of MV' and ZnTPPS3- is observed. It should be noted here that in all previous cases where charge separation was reported the ionic strength was high (either by the addition of an inert salt or due to the buffer used). In contrast to the MV2+when the negatively charged porphyrin is quenched by a neutral acceptor such as NB, the repulsion between the negative charges of the products seems to be sufficient to cause the charge separation and retard the back electron transfer. Acknowledgment. This work was supported by the National Council for Research and Development, by the Balfour Foundation, and by the Schreiber Foundation. Registry No. NB, 98-95-3; MV2+,4685-14-7; MV+, 25239-55-8; pvs, 26101-52-0; ZnTPPS', 80004-36-0; ZnTPPS3-,94603-08-4; NaCI04, 7601-89-0; ZnS04, 7733-02-0.
