J. Phys. Chem. 1985,89, 470-474
470
Equations 27 and 28 establish the most general relationship between the phenomenological first-order rate constants of the dynamical system of interconverting sites which will result in a reduction in the dimensionality of the master equation to 2-fold. The question naturally arises as to the nature of the microscopic physical processes associated with hole burning and thermal annealing which will result in eq 27 and 28. Any phenomenological kinetic model admits a number of possible mechanisms. We discuss only one possible mechanism here. The rate constants k, which appear in the master equation are associated with a variety of processes for interconversion between sites, including classical thermally activated processes, quantummechanical tunneling, and photoassisted processes. If we neglect tunneling events and assume that at sufficient low ambient temperatures all thermally assisted processes are quenched in the resorufin/PMMA sample, then none of the sites populated (set
B ) during hole burning reconvert to a site belonging to the depleted set (set A ) . Thus, all kji = 0 and eq 28 is trivially satisfied. To satisfy eq 27, we must make a further assumption about the nature of the hole-burning process. If it is assumed that the high instantaneous energy density associated with the use of a pulse laser for hole burning produces highly energetic sites in which all of the initial intermolecular structural information which characterized the initial site is lost, then it follows that the sum of all rate constants for the decay of these sites into the distribution of final sites (set B ) must be a constant for all members of the set. If the relaxation event is the rate-determining step, then eq 27 is satisfied. Further experiments are currently in progress to test this hypothesis by using low-power CW sources for hole burning in resorufin/PMMA. Registry No. PMMA (homopolymer), 901 1-14-7; resorufin, 635-78-9.
Laser Photolysis Studies on the Electron-Transfer Reaction from the Photoexcited Triplet State of Chloroindium(I I I ) Tetraphenylporphyrin to Methylviologen in Methanol Solutions Mikio Hoshino,* Hiroshi Seki, Solar Energy Research Group, The Institute of Physical and Chemical Research, Wako, Saitama 351, Japan
and Haruo Shizuka Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: May 15, 1984; In Final Form: October 3, 1984)
Laser photolysis studies were carried out for chloroindium(111) tetraphenylporphyrin, ClIn"'TPP, in methanol solutions. The triplet states of (1n"')'TPP and methylviologen, MV2+,were found to form a triplet exciplex with an association constant of 6.5 X IO2 M-I. The triplet exciplex partly dissociatesto the cation radical of (In"')'TPP, [(In"')+TPP+.], and methylviologen cation radical, MV'., followed by the back electron transfer from MV+. to [(In"')+TPP+.] to regenerate MV2+and (1n"')'TPP. The triplet exciplex reacts with triethanolamine,TEA, presumably to produce a new triplet exciplex, 3[(In111)+TPP(TEA)(MV2+)], in which a TEA molecule is considered to occupy the axial position. No ionic dissociation from this triplet exciplex was M MV2+gives rise to the formation observed. Photolysis of the methanol solution of CIIn"lTPP containing 0.5 M TEA and of MV+. as a final product. The absorption spectroscopic study revealed that C1In"'TPP in a methanol solution at 0.5 M TEA is transformed to [In111TPP(TEA)2]+[C1-],in which two TEA molecules are located in the axial positions. On the basis of the laser photolysis study the triplet state of [In"'TPP(TEA),]+ is confirmed to undergo efficient electron transfer toward MV2+, resulting in the formation of MV'..
Introduction The excited states of metalloporphyrins in solutions have long been recognized to undergo the electron-transfer reaction in the presence of suitable electron Particular attention has been paid to electron transfer between the photoexcited porphyrins and quinones in order to elucidate the primary photochemical processes in photosynthesis.5* Recently solar energy (1) Quinlan, J. J. Phys. Chem. 1968, 72, 1797-1799. (2) Holten, D.; Windsor, M. W.; Parson, W. W.; Gouterman, M. Photochem. Photobiol. 1978, 28, 951-961. (3) Seely, G. R. Photochem. Photobiol. 1978, 27, 639-654. (4) Shiozawa, M.; Yamamoto, H.; Fujita, Y. Bull. Chem. SOC.Jpn. 1977, 50. 2177-2178. -- ( 5 ) Netzel, T. L.; Bergkamp, M.A.; Chang, C. K. J. Photochem. 1981, 17, 451-460. (6) Lever, A. B. P.; Ramswamy, B. S.; Licoccia, S. J. Photochem. 1982, -19 - , -171-1112 . - - - -. (7) Netzel, T. L.; Bergkamp, M. A,; Chang, C. K. J. A m . Chem. SOC. 1982, 104, 1952-1957. ( 8 ) Migita, M.; Okada, T.; Mataga, N.; Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S . Chem. Phys. Lett. 1981, 84, 263-266.
--.--
utilization has become one of the important subjects of photochemistry. From this viewpoint metalloporphyrins serve as useful photosensitizers because they efficiently absorb visible light from the s ~ n . ~ - I * Metalloporphyrins in the ground or the excited state are known to form molecular complexes with electron donors or acceptor^.'^-'^ A few studies have been carried out on the role of these molecular complexes in photochemistry.'6*17 For example, the triplet states (9) Harriman, A.; Porter, G.; Richoux, M. C. J. Chem. SOC.,Faraday Trans. 2 1981, 77, 833-844. (10) Okura, I.; Thuan, N. K. J. Chem. SOC.,Faraday Trans. 1 1980.76, 2209-221 1. .. __.. (11) Matsuo, T.; Ito, K.; Takama, K. Chem. Lett. 1980, 1009-1012. (12) Kiwi, J.; Gratzel, M. J. Am. Chem. SOC.1979, 101, 7214-7217. (13) Gouterman, M.; Stevenson, P. E. J. Chem. Phys. 1962, 37, 2266-2269. (14) Roy, J. K.; Carroll, F. A.; Whitten, D. G. J. Am. Chem. SOC.1974, 96, 6349-6355. (15) Barry, C. D.; Hill, H. A. 0.;Mann, B. E.; Sadler, P. J.; Williams, R. P. J. J. A m . Chem. SOC.1973, 95, 4545-4551.
0022-3654/85/2089-0470$0lSO/O0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 471
C1In"'TPP in Methanol Solutions of palladium(I1) tetraphenylporphyrin, PdI'TPP, and N,N-dimethylaniline establish a triplet exciplex which leads to the formation of the reductive adducts." The triplet exciplex is known to exhibit the phosphorescence and absorption spectra which are respectively very similar to those of the triplet Pd"TPP. In the present study, we have carried out laser photolysis of chloroindium(II1) tetraphenylporphyrin (l), ClIn"'TPP, in methanol solutions containing methylviologen (Z), MV2+, and triethanolamine (3), TEA, with the aim of throwing light on the role of the triplet exciplex in the electron-transfer reaction.
o.9i
A
0.8
Bl
c H3- ~ KC H -C (2)
(1)
(3)
Experimental Section Chloroindium(II1) tetraphenylporphyrin, ClIn"'TPP, was prepared and purified according to the literature.18 Methylviologen, MV2+,was recrystallized once from a methanol solution. Reagent grade methanol and TEA were used without further purification. Absorption spectra were recorded on a Hitachi 200-20 spectrophotometer. Laser photolysis studies were carried out by using a Nd:YAG laser equipped with second (532 nm),third (355 nm), and fourth (266 nm) harmonic generators: the duration of a laser pulse was approximately 20 ns. The second harmonic (ca. 100 mJ/cm2 per pulse) was used for the excitation of C1In"'TPP in methanol solutions. The detection system for measurements of transient spectral9 consisted of a 150-W xenon lamp (Ushio UXL 150-D) as an analyzing light beam source, a monochromator (Ritsu Model MC-20 N), a photomultiplier (Hamamatsu R 758), and an oscilloscope (Tektronix Model 7904). The analyzing light beam was intensified by a factor of ca. 20 during the detection of the transient spectra. The transmitted light beam through a sample cell was led into the entrance slit of the monochromator. The output from the photomultiplier attached to the exit slit of the monochromator was displayed on the oscilloscope. Results Absorption Spectrum of C1In"'TPP in Methanol Solutions. The absorption spectrum of C1In"'TPP in a methanol solution has absorption peaks at 516, 554, and 593 nm with molar absorption coefficients of 3.67 X lo3, 2.07 X lo4, and 1.06 X lo4 M-l cm-l, respectively. These peak wavelengths are red-shifted by ca. 5 nm in comparison with those observed for C1In"'TPP in a benzene solution. This result suggests that C1In"'TPP in a methanol solution dissociates as follows:
CIInl"TPP
-
(In"')+TPP
+ C1-
The similar ionic dissociation has been observed for chloromanganese(II1) and chlorocobalt(II1) tetraphenylporphyrins in alcohol solutions.2b22 Figure 1 shows the absorption spectra of CIInlllTPP in methanol solutions containing TEA at various concentrations. The ab(16) Whitten, D. G.; Lopp, I. G.; Wildes, P. D. J. A m . Chem. SOC.1968, 90, 7196-7200. (17) Mercer-Smith, J. A.; Sutcliff, C. R.; Schmehl, R. H.; Whitten, D. G . J. Am. Chem. SOC.1979, 101, 3995-3997. (18) Bhatti, M.; Bhatti, W.; Mast, E. Nucl. Chem. Leu.1972, 8, 133-137. (19) Hoshino, M.; Imamura, M.; Watanabe, S.; Hama, Y . J . Phys. Chem. 1984,88, 45-49. (20) Boucher, L. J. J . Am. Chem. SOC.1968, 90, 6640-6645. (21) Hoshino, M.; Konishi, S . ; Imamura, M. Bull. Chem. SOC.Jpn., in press. (22) Yamamoto, K. Sei. Pap. Inst. Phys. Chem. Res. (Jpn.) 1977, 71, 11 1-115.
Figure 1. Absorption spectra of C1In"'TPP (3.4 X M) in methanol solution: (A) without TEA; (B) 5 X lo-) M TEA; (C) 1 X lo-* TEA; M TEA; (E) 5 X lo-' M TEA. (D) 5 X 700
'
""""""
t
'
11
12
i
WAVELENGTH i nm I 600 550 500
800
450 "
" " " " " " " ' "
I \ ' "$ 13
14
15 16 17 18 19 20 WAVENUMBER ( k K )
21
22
'I I
23
Figure 2. Transient absorption spectrum of CI1n"'TPP in methanol solution observed at 0.5 ps after a 532-nmlaser pulse.
sorption peaks are gradually red-shifted with an increase in the concentration of TEA. Two-step equilibria are recognized from the spectral changes: one is observed in the concentration range 0-5 X lo-* M TEA and the other, 5 X 10-2-5 X IO-' M TEA. The two equilibria are represented by
+ TEA s (In"')+TPP(TEA) (In"')+TPP(TEA) + TEA s (In"')+TPP(TEA), (In"')+TPP
K1
K2
Since the first-step equilibrium is predominant at low concentrations of TEA, the equilibrium constant, K , (=[(In"')+TPP(TEA)]/ [(In"')+TPP] [TEA]), is computed from ( D , - Dho)/ G E A = Ki(DXM- Dh)
(1)
Here Cm is the concentration of TEA, DAMis the optical density at a wavelength of X when (In"')+TPP is completely transformed to In"'TPP(TEA), and DAand D: are the optical densities at X at CTEA and in the absence of TEA, respectively. A plot of (Dh - Dxo)/C~EA against Dx in the TEA concentration range 0-3 X lo-* M gives a straight line. From the slope and the intercept of the line, K , and DAMcan be calculated: the value of K , is determined to be (1.0 f 0.1) X 10, M-'. When the concentrations of TEA exceed lo-' M, the secondstep equilibrium becomes predominant in the solutions. The equilibrium constant, K2 (= [(In"')+TPP(TEA),]/ [(In"')+TPP(TEA)] [TEA]), is also computed from ( D , - DX')/G~A = K2(Dim- DJ
(11)
where D," stands for the optical density at an infinite concentration of TEA and the other symbols have the same meaning as those against D, in the TEA used in eq I. A plot of (D, - DAM)/.CTEA concentrations CT, 1 2 X lo-' M gives a straight line. The slope of the line gives K2 = I f 2 M-I. The TEA molecule(s) in (In"')+TPP(TEA) and (In"')+TPP(TEA), is considered to occupy the axial position(s).
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
412
Hoshino et al. 0-1 X lo-, M. These results indicate that the equilibria (a) and (b) are not responsible for the leveling off shown in Figure 3. We,
6
therefore, consider that the leveling off is ascribed to the triplet exciplex formation (case (c))
+ MV2+
T
'I
T
1
o
~
kl
T
"
"
'
'5 '
"
' 10' ' " [MV2+] X 1 0 3 , '
'
" " 1 5' M
+ MV2+ E
20
Figure 3. The first-order rate constants, koMr of the triplet state measured a t 460 nm as a function of MV2* concentration. The solid curve is calculated by using eq 9, with k , = 1.8 X lo4 s-I, k2 = 5.2 X lo4 s-I, and K = 6.5 X IO2 M-l.
-
k,
k
E
(i)
(InlI1)+TPP (In"')+TPP
(ii)
+ MV2+
(Inlll)+TPP+ MV2+
k2
(In"')+TPP+.
(iii) (iv)
+ MV+.
where T and E denote the triplet (Inlll)+TPP and the triplet exciplex, '[ (In"')+TPP(MV2+)], resptively. The triplet exciplex, as will be mentioned later, partly dissociates to produce MV+. and (In"')+TPP+.. The assumption that kf[MV2+],kb >> k l + k, [MV2+], k2 leads toz4
Laser Photolysis Studies of CIZnlllTPPin Methanol Solutions. Figure 2 shows the transient spectrum of C1In"'TPP in a degassed methanol solution observed at 0.5 pus after a 532-nm laser pulse. The spectrum having a peak maximum at 460 nm is very similar to the triplet-triplet (T-T) absorption of usual metallotetraphenylporphyrins with diamagnetic central metal^.'^-^^ The decay rate of the transient spectrum follows the first-order kinetics with a rate constant of (1.8 f 0.1) X lo4 s-l. For the aerated solution the decay rate becomes as large as 5.2 X lo5s-l due to quenching of the transient by oxygen. From these results we conclude that the transient can be ascribed to the triplet state of (In"')+TPP. Addition of TEA into methanol solutions of C1In"'TPP gives rise to the formation of (In'")+TPP(TEA) and (In"')+TPP(TEA), depending on the concentration of TEA as mentioned previously. The triplet lifetimes, as well as the triplet spectra, are scarcely altered by the addition of TEA. We, therefore, consider that the intramolecular charge-transfer.interaction between (1n"')'TPP and the axial TEA molecule(s) is very weak in the triplet state of (In"')+TPP(TEA) or (In1'1)+TPP(TEA)2. Figure 3 represents the first-order rate constants of the decay for the triplet (InlI1)+TPPas a function of the MV2+concentration in methanol. The rate constant increases with an increase in the MVZ+concentration. However, there is a leveling off at higher concentrations. This result cannot be interpreted in terms of a simple quenching mechanism of the triplet (In"')+TPP by MV2+. Since the absorption spectrum of (In"')+TPP is not altered by the addition of MV2+in the concentration range 0-2 X M, no formation of the ground-state complex of (In"')+TPP with MV2+ is concluded. The leveling off, therefore, is suggested to be due to the equilibrium in the excited states. The decay of the T-T absorption spectrum of (1n"')'TPP observed for the methanol solution containing 1.5 X lOw3-2.OX lo-, M MV2+follows the first-order kinetics beyond 0.5 ps after a laser pulse. It, therefore, is likely that the equilibrium in the excited states is completed within 0.5 ps after the pulse. Three excited-state equilibria are supposed as a cause of the leveling off (a) triplet energy migration, '[(In"')+TPP] + MV2+ G (In"')+TPP + '[MV2+], (b) electron transfer (ion-pair formation), 3[(In111)+TPP] MVZ+F? ( [(Inrrl)+TPP]+...MV+.), and (c) triplet exciplex formation, 3[(In11')+TPP] + MV2+ F? 3[(In11')+TPP(MV2+)]. In the former two cases, (a) and (b), it is expected that (1) the intensity of the T-T absorption spectrum of (In"')+TPP measured at 0.5 ps after a laser pulse decreases with an increase in the MV2+concentration and (2) new absorption bands ascribed to '[MV2+] or ([(In"')+TPP]+....MV+.) appear in the transient spectra. However, the transient spectrum observed for (1n"')'TPP in the methanol solution containing 6 X lo-' M MVZ+was found to be very similar to the T-T absorption spectrum of (In"')+TPP, and the intensities of the transient spectra monitored at 460 nm were invariant in the MV2+concentration range
The plot of Yagainst kobd gave a straight line, revealing that eq 10 fairly holds in the present system. From the slope and the intercept of the line we obtained K = (6.5 f 0.5) X 10, M-' and (Kk, + k,) = (3.4 f 0.2) X lo7 M-' s-I. The solid line in Figure 3 represents, as a function of [MV2+],the koW values calculated numerically by using eq 9, with k l = 1.8 X lo4 s-I, (Kk, + ks) = (3.4 f 0.2) X lo7 M-' s-I, and K = 6.5 X 10, M-I. The values of kobd obtained experimentally are in good agreement with the calculated ones. On the assumption that kf in reaction i is a diffusion-controlled process, kb is estimated as kb N 1.5 X lo7 s-l by using K = kf/kb = (6.5 f 0.5) X lo2 M-' and kf 1 O l o M-I s- l . From (Kk, + k ) = (3.4 f 0.2) X .lo7M-l s-I , we obtain k, 5: 3.4 X lo7 M-' s-l and k, 5 5.2 X lo4 s-I. These values (kf, kb, k l , k,, and k2)
(23) Pekkarinen, L.; Linschitz, H. J . Am. Chem. SOC. 1969, 82, 2407-241 1
(24) Birks, J. B. 'Photophysics of Aromatic Molecules";Wiley-Interscience: New York, 1970; pp 309-31 1.
+
K = kf/kb = [E]/[T][MV2+]
(1)
The optical density, D, of the transients at a given wavelength is given as the sum of those of T and E (DT and DE, respectively): D = DT DE = cT[T] €E[E] (2)
+
+
Here, and tE are the molar absorption coefficients of the triplet and the exciplex, respectively. Therefore, we obtain dD/dt = dDT/dt + dD,/dt = € ~ d [ T ] / d t+ t ~ d [ E ] / d t (3) From reactions i-iv, two rate equations are derived as -d[T1/dt =
(kl
+ (kf + kq)[MVZ+I)[Tl - kb[El
-d[E]/dt = (k2
+ kb)[E] - kf[MV2+][T]
(4) (5)
Substitution of eq 4 and 5 into eq 3 gives -dD/dt = [tT(kl
+ ( k f + k,)[MV2+])
- t ~ k f [ M v ~ + ] ] [ T+] [EE(k2+ kb) - €Tkbl (6)
By use of eq 1 and 2, eq 6 is transformed to -dD/dt = k,bdD
(7)
Here kobsd
= [€Tkl + ikf(€T- CE)
+ kqtT + K(cE(kZ + kb) t ~ k b )[MV2+]](€~ ] + CEK[MV~+])-'(8)
Since the transient absorption spectra obtained for (Inl")+TPP in the absence and presence of MV2+ are identical, we assume tT = tE. This assumption and eq 8 lead to kobd = (kl
(Kk,
k,)[MVZf])(l
K[MV2+])-' (9)
Equation 9 is transformed to Y = (kobsd- kl)[MV2+]-l = (Kkz
+ k,) - Kkobsd
(lo)
CIInlllTPP in Methanol Solutions
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 473 I.
45
2.0'Y)
c
X
+
1
Y
v 1
2
3
4
5
6
7
8 1
[TEA)X102, M
The first-order rate constants, k,,of the triplet state measured for C1In"'TPP in methanol solutions containing 1.O X M MV2+and 0-5 X M TEA.
2
kob,j = kl
+ kq[MVZ+]
4
[MV']
Figure 4.
justify the use of the assumption (kf[MV2+],kb >> k l + k,[MV2+],k2) leading to eq 1 . It should be noted that, when K N 0, eq 9 is transformed to the usual expression of kow for the quenching of the excited state:
3
5
6
7
8
X103. M
The first-order rate constants, k,, of the triplet state measured for C1In"'TPP in methanol solutions containing 0.5 M TEA and 0-6 X 10-3 M MV+. Figure 5.
(11)
In order to examine whether or not the triplet exciplex reacts with TEA to produce MV+., the laser photolysis studies were carried out for C1In"'TPP in methanol solutions containing 1 X M TEA. The ratio of [E]/[T] M MV2+ and 0-5 X calculated by eq 1 at [MV2+] = 1.0 X M is obtained to be 6.5, indicating that ca. 90% of the triplet state forms the triplet exciplex. Figure 4 shows the triplet decay rate constant, k,, as a function of TEA concentration. The transient T-T absorption spectrum measured for the solution was identical irrespective of the absence or presence of 5.0 X M TEA. No formation of MV+. from the triplet state was observed in the TEA concentration range 0-5 X M as will be described later. These results imply that the triplet exciplex reacts with TEA without forming MV'.. As shown in Figure 4, k, as a function of TEA concentration asymptotically approaches the decay rate constant of the triplet exciplex, [(Int1')+TPP(TEA)(MV2+)],produced by the reactions
$
0
z Q
a I-
-liz35E
&tk+=ke
100
10
Oscilloscope traces monitored at 590 nm for C1In"'TPP in methanol solutions: (A) without MVZ+and TEA; (B) low2M MV2* without TEA; (C) 1.0 X M MV2+and 3 X M TEA; (D)2 X M MVZ+and 0.5 M TEA. Figure 6.
a methanol solution. A slight increase in transmittance after the pulse corresponds to the bleaching of the ground-state C1In"'TPP resulted from the formation of the triplet state. Part B of Figure 6 shows the oscilloscope trace monitored at 3[(Inttt)+TPP(MV2+)]+ TEA 590 nm after the pulse, obtained for C1In"'TPP in a methanol 3[(In1t1)+TPP(TEA)(MVZ+)]solution containing 1.0 X M MV2+. Since MV+. in a methanol solution has a broad absorption band around 610 nm, [ (In"')+TPP(TEA)] MV2+ ~t a decrease in the transmittance at 590 nm is regarded as being [(In1")+TPP(TEA)(MV2+)] due to the formation of MV+. produced by two reactions: a fast reaction within a pulse width of a laser shot and a slow one that where a TEA molecule in 3[(In111)+TPP(TEA)(MVZ+)] is assumed completes within 20 ws after the pulse. Presumably both the to occupy the axial position. excited singlet state of (InI1')+TPP and the triplet exciplex, Photochemical reactions of (I~I"')+TPP(TEA)~in methanol 3[(Int11)+TPP(MV2+)],are responsible for the formation of MV'.: solutions containing MV2+were investigated by the laser photolysis technique. Figure 5 shows the triplet decay rates of CIInttlTPP '[(In"')+TPP] + MV2+ (In"')+TPP+. MV+. in methanol solutions containing 0.5 M TEA and 0-6 X M 3[(In111)+TPP(MVZ+)] (In"')+TPP+. + MV+. MV2+. As revealed from the study on the absorption spectra, C1In"'TPP in methanol solutions converts to (Int11)+TPP(TEA)2 The cation radical, MV'., decays according to the second-order at 0.5 M TEA. The decay rate of the triplet (In1t1)+TPP(TEA)2 kinetics to regenerate MVZ+. The back electron transfer from (k: = (2.1 f 0.2) X lo4 s-') increases linearly with increasing MV+. to the porphyrin cation radical, (Inlll)+TPP+., is considered of MV2+ concentrations: k, = (2.1 f 0.2) X lo4 (3.4 f 0.5) to result in the decay of MV'.. Absorption spectroscopic meaX 10S[MV2+]in units of s-l. The quenching rate constant, surements of the solution after several laser pulses showed the therefore, is determined to be (3.4 f 0.5) X los M-' s-l. This absence of long-lived products. fact indicates that (In11t)+TPP(TEA)2is effectively quenched by For one-electron oxidation of metalloporphyrins it is important MV2+without forming the triplet exciplex. Presumably, two axial to determine whether the electron is removed from the central TEA molecules prevent the formation of the triplet exciplex bemetal or the porphyrin ring. Our recent studies have shown that cause of their steric effect. y-radiolysis of the tetrachloroethane (TCE) solutions at 77 K Formation Mechanisms of M P . . When the methanol solution provides cationic species of metalloporphyrins which are trapped of C1Int1'TPPcontaining MVZ+and TEA is exposed to visible light, as stable p r o d ~ c t s . ~ The ~ - ~absorption ~ spectrum obtained for the color of the solution turns to blue owing to the formation of MV'.. We have studied the formation mechanisms of MV+. by (25) Hoshino, M.; Konishi, S.; Ito, K.; Imamura, M. Chem. Phys. Lett. monitoring the oscilloscope traces at 590 nm after laser pulses. 1982, 88, 138-141. Part A of Figure 6 shows the oscilloscope trace monitored at ( 2 6 ) Konishi, S.;Hoshino, M.; Imamura, M. J . Am. Chem. Soc. 1982, 104, 590 nm after a 532-nm laser pulse, obtained for C1Id''TPP in 2057-2059
+
-
+
+
414
Hoshino et al.
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
the TCE solution of C1In"'TPP after y-radiolysis at 77 K was very similar to that of the zinc(I1) tetraphenylporphyrin cation radical, Zn"TPP+-. We, therefore, conclude that the electron is removed not from the central metal but from the porphyrin ring of C1In"'TPP upon oxidation: CI1n"'TPP
-
ClIn"'TPP+.
+ e-
Part C of Figure 6 shows the oscilloscope trace monitored at 590 nm obtained for a methanol solution of C1In"'TPP containing 1.0 X M MVZ+and 3.0 X IO-, M TEA. The trace indicates that MV+. is produced instantaneously after a laser pulse. No slow formation of MV+. is observed, indicating that no triplet species is involved in the reduction of MVZ+. Since C1In"'TPP in a methanol solution containing 3.0 X lo-' M TEA is transformed to (In"I)+TPP(TEA), the formation of MV+. is represented as (In"')+TPP(TEA) '[(In"')+TPP(TEA)]
-k
+ MV2+
[(In"')+TPP(TEA)]
-
[(In"')+TPP+.(TEA)]
+ MV+.
Here, [(In"')+TPP(TEA)] stands for the excited singlet state of (In"')+TPP(TEA). Methylviologen cation radicals, MV'., produced in the solution remain as a long-lived product. Part D of Figure 6 shows the oscilloscope trace monitored at 590 nm obtained for the methanol solution of CI1n"'TPP containing 0.5 M TEA and 2 X M MV2+. In contrast to part B, MV+. is produced merely by the slow reaction, indicating that the triplet state is essential to the electron-transfer reaction. Since (In"')+TPP converts to (In"')+TPP(TEA), at 0.5 M TEA, the photochemical reactions are represented as (In1")+TPP(TEA)2 -k [(In11')+TPP(TEA)2] [ (In"')+TPP(TEA),]
--
3[(In111)+TPP(TEA)2]
3[(In111)+TPP(TEA)2]+ MV2+ [(In"')+TPP+.(TEA),]
+ MV+.
The formation of MV+. from 3[(In111)+TPP(TEA)2]is confirmed from the fact that the first-order rate constant obtained from the formation curve of MV+. is in good accord with that from the decay curve of the triplet (In"')+TPP(TEA), monitored at 460 nm. The absorption spectrum of the solution observed after several laser pulses revealed that MV+-was formed as a long-lived product. Because of the low concentrations of MVZ+(6 X M at most), reduction of MV2+ due to the excited singlet state of (In"')+TPP(TEA)2 was not detected. Discussion The triplet (InI")+TPP and MVZ+in a methanol solution are confirmed to form the triplet exciplex. Analogously to the ground-state complexes of metalloporphyrin with electron acceptor molecules,I5 MVZ+is assumed to interact with the porphyrin ring in the triplet state. Since the absorption spectrum of the triplet exciplex, 3[(In111)+TPP(MV2+)], is almost identical with that of the triplet (In"')'TPP, the charge-transfer interaction in the triplet exciplex is considered to be too weak to exert strong effects on the optical absorption spectrum. The lifetime of the triplet exciplex, however, becomes as short as one-third of that of the triplet (In"')+TPP. The triplet exciplex was found to dissociate partly to produce a small amount of MV+. which finally disappears according to the back electron transfer from MV+. to the porphyrin cation radical, In"'TPP+-. In methanol solutions, C1In"'TPP reacts with TEA molecules to form (Intl')+TPP(TEA) and (In111)+TPP(TEA)2. Laser photolysis studies of (In"')+(TEA) in methanol solutions containing MV2+revealed that the triplet (In"')+TPP(TEA) and MV2+ are (27) Konishi, S.; Hoshino, M.; Imamura, M. J. Phys. Chem. 1982, 86, 1412-1 4 14. (28) Hoshino, M.; Konishi, S.;Imamura, M.; Watanabe, S.; Hama, Y. Chem. Phys. Lett. 1983, 102, 259-262.
suggested to form the triplet exciplex without undergoing ionic dissociation. However, the excited singlet state of (In"')+TPP(TEA) reacts with MV2+to produce (Inl")'TPP+. and MV+.. The fact that MV+. produced in the solution remains as a long-lived product suggests that the fast one-way intramolecular electron transfer occurs from an axial TEA to the TPP'. moiety in [ (In"')+TPP+.(TEA)], resulting in the reduction of the TPP+moiety to produce (In"')+TPP [(In"')+TPP+.(TEA)] [(III"~)+TPP(TEA+.)] followed by [(In"')+TPP(TEA+-)] and TEA+-
-
-
(In"')+TPP
+ TEA'.
decomposition products
The cation radical, TEA'., is regarded to decompose quickly. Consequently, MV+. is given as a long-lived product owing to the suppression of the back electron transfer from MV+. to [(In"')+TPP+-(TEA)] or TEA'.. In contrast to the triplet states of (Inl")+TPP and (In"')+TPP(TEA), the triplet state of (In"')+TPP(TEA)2 readily transfers an electron toward MVZ+to produce (In111)+TPP+.(TEA)2and MV+-. The formation of MV+. as a long-lived product is interpreted on the assumption that the back electron transfer from MV+. to [(In111)+TPP+.(TEA)2]becomes unable due to the fast intramolecular electron transfer in [ (In"')+TPP+.(TEA),] as suggested in the case of [(In"')+TPP+-(TEA)]: [(In111)+TPP+.(TEA)]2 (In"')+TPP(TEA) + TEA'.
-
The present study shows that the triplet (In"')+TPP(TEA) and MVZ+ form the triplet exciplex without undergoing electron transfer in methanol solutions. On the other hand, the triplet (In"')+TPP(TEA), transfers an electron to MV2+ to produce MV+. without forming the triplet exciplex. The triplet exciplex of (In"')+TPP(TEA), and MVZ+,if present, is considered to have a very short lifetime because of the fast ionic dissociation to produce (In11')+TPP+.(TEA)2 and MV'.. Since TEA is an electron-donating molecule, two TEA molecules in the axial positions of (In111)+TPP(TEA)2may increase the apparent electronegativity of the central In a t ~ m , ' resulting ~ , ~ ~ in the decrease in the oxidation potential of the porphyrin ring. As a result, the triplet (In"')+TPP(TEA), transfers an electron toward MV2+more readily than the triplet (In"')+TPP or (In"')'TPP(TEA). It seems that the effect of one TEA molecule in the triplet (In"')+TPP(TEA) is not enough to undergo a direct reduction of MV2+. Conclusion Laser photolysis studies on C1In"'TPP in methanol solutions in the absence and presence of TEA revealed that (1) the triplet (In"')+TPP and MVZ+establish the triplet exciplex, 3[ (In"')+TPP(MVZ+)],which partly dissociates to produce In"'TPP+. and MV'., (2) the triplet exciplex reacts with a TEA molecule to form [(In111)+TPP(TEA)(MV2+)]without undergoing electron transfer, (3) the triplet (In"')+TPP(TEA), effectively transfers an electron toward MVZ+to produce [(In"')+TPP+.(TEA),] and MV'., (4) (In"')+TPP+. and MV+. undergo the back electron transfer to regenerate (In"')+TPP and MV2+,and (5) the TPP'. moiety in [ (In"')+TPP+.(TEA)] or [ (Inlll)+TPP+.(TEA),] is reduced by one-way intramolecular electron transfer from the axial TEA molecule.
Acknowledgment. This work was supported by Research Grants for "Solar Energy Conversion" given by the Japan Science and Technology Agency and by a Scientific Research grant-in-aid (59045094) of the Ministry of Science, Education, and Culture of Japan. Registry No. 1, 63128-70-1; 2, 4685-14-7; 3, 102-71-6; MV'., 25239-55-8; In"'TPP+, 56551-52-1; In"'TPP(TEA)+, 94024-71-2; In"1TPP(TEA)2', 94024-72-3. (29) Kooyman, R. P. H.; Schaafsma, T. J.; Cleisbeuker, J. F. Photochem. Photobiol. 1971, 26, 235-240.
