775
J. mys. Chem. 1903, 87,775-781
nated surface if there are sufficient electronic states in it. Such peculiar electronic states are occasionally contained even in well-treated semiconductor surfaces, and become abundant by damaging photocatalyst surfaces and/or by loading metals on the surface.
Acknowledgment. This research was supported by a Grant-in-Aid for Scientific Research (No. 57550501) from
the Ministry of Education. Registry No. Lead dioxide, 1309-60-0;palladium, 7440-05-3; ruthenium dioxide, 12036-10-1;platinum, 7440-06-4; titanium dioxide, 13463-67-7;polypyrrole, 30604-81-0; silver, 7440-22-4; gallium phosphide, 12063-98-8;cadmium sulfide, 1306-23-6;silicon, 7440-21-3;palladium chloride, 7647-10-1;ruthenium trichloride, 10049-08-8;chloroplatinicacid, 16941-12-1;pyrrole, 109-97-7;silver perchlorate, 7783-93-9.
Electron Transfer Reactions of the Photoexcited Triplet State of Chloroaluminum Phthalocyanine with Aromatic Amines, Benzoquinones, and Coordination Compounds of Iron( II)and Iron( III) Takeshl Ohno,’ Shunjl Kato, Aklra Yamada,t and Takashl Tanno§ Chemistry Department, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan (Received: June 7, 1982; I n Final Form: October 28, 1982)
Electron transfer reactions of the photoexcited triplet state of chloroaluminum phthalocyanine (AlPcCl) in aqueous dimethylacetamide (DMA) and aqueous Me2S0solutions were studied by means of nanosecond laser photolysis-kinetic spectroscopy. Four 1,4-benzoquinonesin the quenching of 3A1PcClefficiently produced the one-electron oxidized radical, A1Pc2+.,whose absorption spectrum consists of four bands at 420, 520, 730, and 845 nm. 3A1PcCl was reduced to AlPc. by the aromatic amines N,N,N’,N’-tetramethylbenzidine(TMB), phenothiazine (PT),N,N,N’,N’-tetramethyl-p-phenylenediamine(TMPD), and 3,3’-dimethylbenzidine (DMB) with an efficiency higher than 0.5. Low-spin compounds of Fe(I1) and Fe(II1) did not efficiently produce the phthalocyanine radical in the quenching process. The fraction of radical production in the quenching process (F,)was measured. The AlPc2+.and AlPo radicals disappeared with second-order rate constant, the ratios of which to the rate constant of the encounter complex formation (Fz)are nearly equal to 1- Fl. The relation of F, + F2= 1indicates that (i) every quenching event produces a geminate radical pair and (ii) failure of the radical production in the quenching is ascribed to fast reverse electron transfer in the geminate radical pair. Rates of the reverse electron transfer were accelerated by large negative Gibbs energy change and spin-orbit interaction.
Electron transfer reactions involving electronically excited molecules are of considerable interest because of their potential application in chemical conversion of solar energy. Recent studies on free-radical formation in the quenching of excited triplet molecules revealed that (i) every quenching event yields a geminate radical pair in polar medium, followed by reverse electron transfer or dissociation into the and (ii) the fraction of freeradical formation in the quenching reaction of excited triplet molecules can be estimated from the reaction rate of reverse electron transfer between free radicals from the The proposed mechanism is as follows: 3D
+A
k, +
-
3J(D+.,A--) 3,1(D+.,A-.)
3J(D+-,A--)
2D+. + 2A-.
kdh
ke
k,
D
+A
zD+.+ 2A-.
(1) (2)
(3)
3”(D+.,A-.)
(4) where spin inversion takes place in the course of reactions 1 and 2. Rate constants of radical formation in the forward quenching process, ket, and radical disappearance in the *Instituteof Chemistry and Physics, Wako, Saitama 351, Japan. 5Riken Co., Ltd., Chiyoda, Tokyo 102, Japan. 0022-3654/83/2087-0775$01.50/0
reverse process, kret, are written by using these rate constants for the elementary processes lZet kret
=
kqkdi8/(kr
=
kekr/(kr
+ kdis)
(5)
+ kdis)
(6)
Provided the efficiency of geminate radical pair formation is unity in the triplet quenching, it is generally expected that the fraction of free-radical formation in the triplet quenching ( k e t / k q )can be estimated from the rate of the thermal reverse electron transfer from the bulk (k,. which either is experimentally measured or can be estimated in a semiempirical manner from some parameters (Gibbs energy change, reorganization energy, nonadiabaticity, and so on). The most difficult problem in the estimation of kretor kret/keis the estimation of the adiabaticity parameter of the spin-inverted electron transfer processes of reactions 1 and 2. The spin inversion in the processes can occur via two mechanisms, that is, hyperfine interaction in the weakly interacted pair and spin-orbit (1)Ohno, T.; Lichtin, N. N. J . Am. Chem. SOC.1980, 102, 4636-43. (2) Ohno, T.; Lichtin, N. N. J . Phys. Chem. 1982, 86, 354-60. (3) Ohno, T.; Kato, S.; Lichtin, N. N. Bull. Chem. SOC.Jpn. 1982,55, 2753-9. (4) Hoselton, M. A,; Lin, C.-T.; Schwarz, H. A.; Sutin, N. J.Am. Chem. SOC. 1978, 100, 2383-8. (5) Periasamy, N.; Linschits, H. Chem. Phys. Lett. 1979, 64, 281-5.
0 1983 American Chemical Society
776
Ohno et al.
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983
interaction in a strongly interacted pair formed on the reaction coordinate. The former contribution is limited: while the latter can become considerable. Recently, it was reported that spin-orbit interaction is the most important factor determining the efficiency of radical production in the quenching of excited triplet molec~les.~It was also recognized2that k, for the reactants containing no heavy atom is weakly dependent on the Gibbs energy change of the process, though a large decrease in Gibbs energy increases k, for the reactants containing heavy atom(s). In other words, a low adiabaticity parameter due to spin conservationcan be reflected in the dependence of k, upon the Gibbs energy change. From this point of view, experimental study of spin-inverted electron transfer reaction is of great importance to estimate a fraction of radical formation in the triplet quenching. The study will help in understanding rates of electron transfer reactions between transition-metal compounds having various spin states. Dependence of the spin-inverted electron transfer rate upon the Gibbs energy change in the process is also pursued. In this report, rate constants of three kinds, electron transfer quenching, free-radical formation, and reverse electron transfer, are discussed in terms of Gibbs energy change and nonadiabaticity due to spin-multiplicity restriction. Chloroaluminum phthalocyanine (AlPcC1) was used as an excited donor or acceptor. Electron transfer reactions of photoexcited metal phthalocyanine compounds have been recently studied by many workers: the present authors, T.O. and S.K.,3 T.T. and A.Y.,8 T.T., A.Y., T.O., and S.K.? Darwent, McCubbin, and Phillips,lo Harriman and Richoux," and Lever, Licoccia, Ramaswamy, Kandil, and Stynes.12 As acceptors, we used low-spin iron(II1) comp~unds-Fe"'(CN)~(bpy)~+,Fe111(CN)2(phen)2+, Fe"'(CN)((bpy)-, Fe"'(CN):(bpy = 2,2'-bipyridyl; phen = l,l0-phenanthroline)-1,4-benzoquinone (BQ), 2,5-dichloro-1,4-benzoquinone (2,5-DCBQ), 2,6-dichloro-1,4benzoquinone (2,6-DCBQ),2,3,5,6-tetrachloro-1,4-benzoquinone (TCBQ), and methylviologen (MV2+ClZ2-).As donors, we used low-spin iron(1I) compounds-ferrocene and Fe11(CN)4(bpy)2--and aromatic amines-N,N,N',N'-tetramethylbenzidine (TMB), 3,3'-dimethylbenzidine (DMB), N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), and phenothiazine (PT). Experimental Section Apparatus. A NEC SLG-2018 Q-switched ruby laser capable of providing up to 0.85 J per flash at 694.3 nm with nominal pulse width of 20 ns was used in most of the experiments. Details of the laser and monitoring apparatus have been described else~here.~ Flash intensity of the laser was 0.2-0.3 J per flash in this work and transient changes of absorption were fed into a transient recorder, Kawasaki Electronica M-500T, followed by computer analysis using a laboratory-constructed minicomputer in most of the experiments. Data for dilute solution of AlPcCl in a 5- or 10-cm long cell were obtained with a xenon flash apparatus. The light duration of the two flash lamps, Xenon Corp. (6) Schulten, K.; Staerk, H.; Weller, A.; Werner, H.-J.; Nickel, B. Z. Phys. Chem. (Wiesbaden) 1976, 101, 371-90. (7) Steiner, U.; Winter, G. Chem. Phys. Lett. 1978, 55, 364-8. (8) Tanno, T.; Wohrle, D.; Kaneko, M.; Yamada, A. Ber. Bunsenges. Phys. Chem. 1980,84, 1032-4. (9) Tanno, T.; Yamada, A.; Ohno, T; Kato, S. Photochem. Photobiol., t o be submitted for publication. (IO) Darwent, J. R.; McCubbin, I.; Phillips, D. J. Chem. SOC.,Faraday Trans. 2 1982. 78. 347-57. (11) Har&an,'A.; Richoux, M.-C. J. Chem. SOC.,Faradaq Trans. 2 1980, 76, 1618-26. (12) Lever, A. B. P.; Licoccia, S.; Ramaswamy, B. S.; Kandil, S.A,; Stynes, D. V. Inorg. Chim. Acta 1981, 51, 169-76.
I
lb 0 2'
I
I
400
X/nm
500
'
600
'02
Flgure 1. Difference spectra of 8 pM AlPcCl solution at 10 fis after flashing: (1) 70% DMA aqueous solution; (2) 80% Me,SO aqueous solution.
FP-4, was 1.2 ps with 45 J using a coaxial capacitor (0.3 pF) of Hivoltage Components 5195-25.
Materials. AlPcCl was prepared and purified by the usual m e t h ~ d .All ~ the iron compounds except for K3[Fe"'(CN),] were prepared by the method of Schilt13and their purities were verified by elementary analysis. Analytical-grade K3[Fen1(CN),Jwas supplied by Merck. Aldrich TMPD was purified by vacuum sublimation. Aldrich TMB was recrystallized from ethanol. Wako BQ was recrystallized from ethanol, followed by vacuum sublimation. Tokyo Kasei DMB, PT, 2,5-DCBQ, TCBQ, MV2+C12, Wako 2,6-DCBQ, Dojin analytical-grade EDTA, and Kishida NaC10,.H20 were used as supplied. For oxidative quenching work, the solvent consisted of Wako dimethylacetamide (DMA) and water (7:3 by volume). For reductive quenching work, the mixed solvent consisted of Wako dimethyl sulfoxide (Me2SO) and water (8:2 by volume). Water was purified by passage through a Millipore deionizer and filter. Procedure. All the sample solutions were deaerated by purging with nitrogen of 99.99% purity for 15 min. Decay of transient absorbance for oxidative quenching and for reductive quenching was measured at 15 f 2 and 28 f 2 "C, respectively. Production and decay of the excited triplet AlF'cCl were monitored at 450 or 490 nm = 2.66 X lo4 M-' cm-'). Production of the reduced radical and oxidized radical of AlPcCl was monitored at 570 nm (e570 = 4.6 X lo4 M-' cm-l) and at 520 nm (e520 = 2.9 X lo4 M-' cm-I), respectively. Rates of recovery and bleaching of AlPcCl were monitored at 676 nm (€676 = 2.08 X lo5 M-' cm-' in the DMA aqueous solution and 1.9 X lo5M-' cm-' in the Me2S0 aqueous solution). Results 1. Triplet State and Its Quenching. AlPcCl is stable in both mixed solvents of DMA-water (7:3) and Me2SOwater (8:2). On exposure to 694.3-nm light, the S1-So absorption of AlPcCl was reduced to about one-third and a transient species was observed with a lifetime of 0.3 ms, whose absorption spectrum is very similar to the absorption spectra of the excited triplet zinc phthalocyanine3and is independent of solvents as shown in Figure 1. The molar extinction coefficient of the T-T absorption band at 490 nm is calculated to be 2.66 X lo4 M-' cm-' in both solvents on the assumption that the molar extinction coefficient of the triplet state at 676 nm is so small com113) Schilt, A. A. J . Am. Chem. SOC.1960, 82, 3000-5.
The Journal of Physical Chemlstry, Vol. 87, No. 5, 1983 777
Photoexcited Triplet State of AlPcCl
0
6 50
I-
E .lo-
ea 0
40 -
g
1
.'
I
'. ,
i
.
a .05.
Abtte
..
'
I
'
1
,'
I
600
500
400
700
900
800
n / n m
Figure 4. Transient difference spectrum of photolyzed solution containing AlPcCl (5 pM), 2,5-DCBQ (0.3 mM), and NaCIO, (1 M) in DMA-H,O (7:3)at 100 ~s after flashing.
io
20
-
40
30
5b
Ab t
Flgure 2. Analysls of 3AIPcCIdecay in 70% DNA aqueous solutlon. k,: the rate constant of firstorder decay. k,: the rate constant of secondorder decay. = '6 time Interval (2 X lo4 s). e: the molar extinction coefflclent of T-T absorptlon at 450 nm (2 X lo4 M-' cm-'). I:the optical length.
O.* 0.6 C A R C161c C3AIPcCII,
t
10.8
.
'
a,
,'
0
.
-06
A *
0
tAIPcC61, +AlPcClb
/' /'
o O O?
1
2
3
I O 1.4/lo4
4 MS
Flgure 5. Dependence of the ratio of free-radical production to 3AiPcCI production upon [Qllk,: (0)2,6-DCBQ in DMA-H,O; (0)TMB in Me,SO-H,O.
O:
2
i
6
8
io'
[Q1/1T5M
-re 3. Dependence of the decay rate constant of 3AiPcCImonitored at 460 nm in the presence of a quencher (0)2,6DCBQ in DMA-H,O (0)TMB in Me,SO-H,O.
pared to that of AlPcCl that the production of 3AlPcCl can be estimated from the reduction in the absorbance at 676 nm at 1 ps after flashing. The decay of the triplet state of AlPcCl without quencher can be analyzed by a kind of Guggenheim method; l4the rate constants of the fiist-order decay and the second-order decay are 2.7 X lo3 s-l and 2.8 X lo8 M-' s-', respectively (see Figure 2). The rate of the decay of the triplet state increased on addition of oxidant or reductant. Their redox potentials are shown in Table I. Analysis of a pseudefirst-order decay gives a bimolecular rate constant of oxidative quenching with Fe111(CN)2(bpy)z+, Fel"(CN)2(phen)2+,Fen1(CN)4(bpy)-,Fenl(CN)a-, TCBQ, 2,5DCBQ, 2,6-DCBQ, or BQ. Bimolecular rate constants of reductive quenching with ferrocene, Fe=(cN),(b~y)~-, PT, DMB, TMB, and TMPD were also obtained (see Figure 3). Since a quenching rate constant (k,) with an anionic quencher was decreased and that with a cationic one was increased on addition of 1M NaClO,, the most probable form of the triplet state is 3A1Pc+in the solution. All the kq's are shown in Tables I1 and 111. 2. Absorption Spectra of the Radicals Formed i n the Quenching and the Fraction of Radical Formation. A second transient species was formed in the oxidative ~~
(14) Shank,N. E.;Dorfman, L.M. J . Chem. Phys. 1970,52,4441-7.
quenching process of 3AlPc+. A difference spectrum of the sample solution containing 2,5-DCBQ after the complete decay of 3AlPc+consists of four bands at 420,540,730, and 845 nm (see Figure 4). This spectrum is ascribed to a one-electron oxidized radical (AlPP.),since the absorption ..~ spectrum of A1Pc2+.is so similar to that of Z ~ P C + The molar extinction coefficient of Ape2+at 520 nm, c5%, can be calculated from the depletion of AlPcCl on the assumption that €676 of AlPc2+.is negligible compared to e676 = 2.08 X lo5 M-l cm-' of AlPcCl. The rate constants of free-radical production (k,,) were obtained by using eq 8, where the subscript 0 means initial (7) [A1Pc2+-]o= he,( [Q]/kd)[3A1PC+]o production, [Q] is the quencher concentration, and kd is the pseudo-first-order decay rate constant of 3AlPc+in the presence of quencher (see Figure 5). Both k,, and k, were unaffected by addition of 1 M NaC104except for TCBQ, with which k,, was reduced to three-quarters. A different kind of radical was formed in the reductive quenching of 3A1Pc+with EDTA, ferrocene, Fe"(CN),(bpy)2-,PT, DMB, TMB, and TMPD, whose absorption bands at 410, 580, 625 nm and in the IR region can be ascribed to a one-electron reduced form of AlPc+ (AlPe), because the same four bands, which are in agreement with those of chemically reduced A I P C C ~appeared ,~~ irrespective of the reductants as shown in Figure 6. Transient absorption bands other than those described above are ascribed to the one-electron oxidized radical of the reductant, for example, an intense peak at 465 nm to DMB-+,at 475 nm to TMB+..16 Absorption bands at 570 and 610 nm of TMPD+. overlapping with the bands of AlPe are not apparent because of the small extinction coefficients.16 Rate (15) Clack, D. W.; Yandle, J . R. Inorg. Chem. 1972, 11, 1738-42. (16) Rao, P. S.; Hayon, E.J . Phys. Chem. 1975, 79, 1063-6.
770
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983
Ohno et al.
TABLE I: Energy Levels of Excited Triplet States and Redox Potentials levels of triplet compd state, eV E" vs. SCE AlPc' F~"'(CN),(~PY)~+ Fe"'(CN),(phen),' Fel"( CN),( bpy )FelI1(CN), 3TCBQ 2,5-DCBQ BQ MV2+ PT DMB Fel'( CN),( bpy),' ferrocene TMB TMPD AlPc'
1.la 2.2 5-1.3 2' 2.2 5-1.3 2' 2.2 5-1.3 2' 2. 25f
1.15b 0.54d 0.32e 0.228 0.02h -0. 18h -0.54 -0.5lj 0.54h 0.46h 0.32e 0.36n 0.32h -0.02h -0.53'
2.24' 2.65' 2.93-1.56l
1.88'" 2.64O l.1°
couples (AIPcZ+./AIPc+) ( Fe'" (CN )A ~ P l2+ Y /Fe" (CN
~ P l2Y)
(Fe"'(CN ) , ( ~ P Y)Y/FeI'(CN),(bpy ( FelI1( CN ), '-/Fe"( CN)," )
)'-I
(TCBQ/TCBQ-.) (2,5-DCBQ/2,5-DCBQ-) (BQ/BQ-. ( MV2+/MV'* ) (PT+./PT) (DMB'. /DMB)
(Fe"l(CN),(b~~)-/Fef'(CN),(b~~)Z~) ( ferrocenium iodferrocene) (TMB'. /TMB) (TMPD+./TMPD) ( AlPc'/AlPc. )
'
Giraudeou, A. H. ; Fan, F.-R. a Assumed to be close to those of phthalocyanine compounds of nontransition meta1s.P Assumed to be between the excited triplet states of Fe111(CN),3-f F.; Bard A. J. J. A m . Chem. SOC. 1980, 102, 5157-63. Schilt, L. A. Anal. Chem. 1963, 35, 1599-602. e George, P.; Hanoania, G. I. H.; Irvine, D. H. J. and Fe1h(bpy),3t.q Chem. Soc. 1959, 2548-54. f Noiman, C. S. J. Chem. Phys. 1961, 35, 323-8. g Jordan, J.;Ewing, G. J. Inorg. Chem. 1962, 3, 5 8 7 ~ 9 1 . Mann, C. K. ; Barnes, K. K. "Electrochemical Reactions in Nonaqueous Systems"; Marcel Dekker, New Hunig, S.; Grass, J.; Shenk, W. Yor, 1970. Briegleb, G.; Herre, W.; Wolf, D. Spectrochim. Acta, Part A 1969, 25, 39-46. Alkaitis, S. A.; Beckad, G.; Gratzel, M. J. A m . Chem. SOC.1975, 97, 5723. RefJustus Liebigs A n n . Chem. 1973, 324. Mason, J. G.; Rosenblum, Kikuchi, M.; Kikuchi, K.; Koizumi, M. Bull. Chem. SOC. Jpn. 1974, 47, 1331-3. erence 1. Kalanter, A. H.; Albrecht, A. C. Ber. Bunsenges. Phys. Chem. 1964, 63, 361. M. J. A m . Chem. SOC.1960, 82, 4206-8. P Vincett, P. S.; Voigt, E. M.; Rieckhoff. K. E. J. Chem. Phys. 1971, 55, 4131-40. 4 Feltham, R. D.; Silverthorn, W. Inorg. Chem. 1970, 9, 1207-10.
'
TABLE 11: Rate Constants and Fractions of Electron Transfer in Oxidative Quenching in DMA-Water quencher Fe"'(CN)z(bpy)z+ Fe"'(CN),(phen),+ FelI1( CN),( bpy )Fe111(CN),3' TCBQ
4.4 5.2 2.8 0.11 4.5
2,5-DCBQ 2,6-DCBQ BQ MVZ'
5.5 4.8 0.092
0 (0.14) 0 0 0.60 0.46' 0.53d 0.59d 0.40
3.4e 3.4e
0.49 0.49
1.02 1.08
10-2
'
With 1 M NaClO,. F , in the solution containing 1M NaCIO, is the same as that with a F , = ket/kq. F , = kret/ke. no NaClO,. e Measured in the solution containing 1 M NaC10,. TABLE 111: Rate Constants and Fractions of Electron Transfer in Reductive Quenching in Me,SO-Water
lo-* x quencher FeI'(CN),(bpy)'ferrocene' PT DMB TMB TMPD TMPD~
x
12
M-I'L-I
~
,
-~
5.8 6.6 0.01 0.79 1.7 19.0 7.2
0 0.07 0.52 0.52 0.72 0.85 0.84
F , = k,,/k,. F 2 = kret/ke. In DMA-water (7:3).
kret, aM - I s - ~~
8.9 4.9 8.6 3.1 3.5
, b
0.47 0.26 0.45 0.16 0.27
F , -1 F,
0.99 0.78[ 1.17 1.01 1.11
In Me,SO-water ( 9 : l ) .
constants of free-radical formation (ket) in the quenching were calculated by using eq 7 and the molar extinction coefficient of 4.6 X lo4M-' cm-' obtained in Me2SO-water (9:l by v ~ l u m e ) . ~ 3. Reverse Electron Transfer between the Radicals in the Bulk. Both radical species of A h 2 + .and AlPc. formed in the quenching process were reconverted to APc+ within several milliseconds via reverse electron transfer with the radical coming from the quencher. The transient absorption bands due to A1Pc2+.monotonically decayed with the same rate and did not produce any secondary transient
species unlike the complex behavior of Z ~ P C +No . . ~degradation of AlPcCl was found a t all. Addition of 1 M NaC104 to the sample solution increased the bimolecular rate constant between A1Pc2+-and the reduced radical of the oxidant by about 3 times but had no effect on the rate constant of the reverse electron transfer reaction between AlPe and the oxidized radical of the quencher. The salt effect on the rate indicates that (i) the one-electron reduced form of AlPcCl has no charge and (ii) the precursor, the triplet state, has a charge of 1+ as mentioned above. Consequently, the most probable form of the one-electron oxidized species must be A1Pc2+.. All the kretare tabulated in Tables I1 and 111. Discussion I . Triplet State of AlPcCl. The triplet state of AlPcCl in both solvents exhibits a difference absorption spectrum with a peak at 490 nm as 3ZnPcdoes in 70% DMA aqueous ~olution.~ A real absorption spectrum of 3A1Pc+,obtained by making a correction due to the depletions of So-S1 and So-S2 absorption, has another strong hand in the nearultraviolet region. Its molar extinction coefficient (2.66 x lo4 M-' cm-') at 490 nm is a little smaller than those of 3ZnPc (3.25 X lo4M-' cm-') and 3CuPc (3.6 X lo4 M-' cm-' (ref 17)). Bimolecular decay of 3AlPc+may be ascribed
The Journal of Physlcal Chemktry, Vol. 87, No. 5, 1983 779
Photoexcited Triplet State of AlPcCi
TMPD
0.9mM
E D T A 2.lmM
..
.
DMB *
ImM
. *
. ..
e
.
. *
TMB 0.3mM
. . ..
. .
400
.
I
500
.,
.. . 700
600
h
800
1
!
nm
F W e 6. Transient dmerence spectra of photolyzed solution containing AlPcCl(4-18 FM)and amine in Me2SO-H20(8:2) after disappearance of 3AIPcCI.
to T-T annihilation, because no electron transfer product was produced. 2-1. Quenching of 3A1Pc+. Bimolecular quenching of triplet excited states is known to occur via the following mechanisms: (a) energy transfer to a quencher via exchange interaction, (b) electron transfer to or from a quencher, (c) complex formation resulting in chemical reaction. The quenching mechanism is most clearly proved by the identification of the products-the excited state of the quencher, the electron transfer products, or the complex-but it is not always possible. Therefore, a correlation between the quenching rate and the Gibbs energy change involved in the process has been sought instead of direct proof for the quenching mechanism. Gibbs energy change for electron transfer to take place is required to be negative, which is written in eq 8, where AG,, = -F(*Eo(Q) F E"(T))
+ AV
(8)
E o ( Q ) is the redox potential of a quencher, E o ( T )is the redox potential of 3A1Pc+,the upper signs correspond to oxidative quenching and the lower ones to reductive quenching, and AV means the change of reactant-reactant interaction energy involving solvation energy accompanied with electron transfer. In Table I, the redox potentials and the excitation energies of the reactants are shown. The energy level of 3A1Pc+is probably close to 1.1 eV as the lowest triplet levels of most non-transition-metal phthalocyanines are.18 In order for energy transfer to occur, the energy level of the triplet state of the quencher must be lower than that of AlPc+ (1.1 eV). Reorganization energy of molecular configuration with the transfer process decreases the Franck-Condon factor of energy transfer or electron transfer, which can be es(17)McVie, J.; Sinclair, R.-S.;Truscott, T. G. J. Chem. SOC.,Faraday Trans. 1 1976,72,187C-9. (18)Vincett, P.S.; Voigt, E. M.; Rieckfoff, K. E. J.Chem. Phys. 1971, 55,4131-40.
timated from the rate of self-exchange reaction of excitation energylgor electron.20 Rate constanta of self-exchange of electron are known to be 8 X l@M-' s-l for ferrocenium ion-ferrocene,21 3.5 X lo4 M-' s-l for Ferr'(CN),*-Fen(CN)6dr,22 and lower than 6 X 10, M-l C'for W + - M V + S , ~ respectively. Spin restriction and/or the lack of electron exchange interaction seriously suppress the transfer rate. In most electron transfer reactions of aromatic compounds, reorganization energy is seldom taken into consideration because of its small value24except for MV2+-MV+.. 2-2. Oxidative Quenching with Fe(III) Compounds, Quinones, and MV+.All the quenchers studied in this work are unable to accept the excitation energy of 3AlPc+ judging from the energy levels of the triplet state which are shown in Table I. On the other hand, the redox potential of 3A1Pc+( A ~ P c ~ + . / ~ A ~isPlower c + ) than those of the four low-spin iron(II1) compounds and is close to those of TCBQ and 2,5-DCBQ. Production of A1Pc2+. and one-electron reduced radical of the quenchers, TCBQ, 2,5-DCBQ, 2,6-DCBQ, W +Fem(CN)2(phen)2+, , strongly indicates electron transfer quenching, which will be discussed in section 3-1. A mechanism of oxidative quenching is suggested by the very small quenching rate constant with BQ, which has the lowest oxidation potential, and MV2+, which has an unusually small exchange rate. 2-3. Reductive Quenching with Fe(II) Compounds and Aromatic Amines. All the reductants, ferrocene, Fe"(CN)4(bpy)2-,PT, DMB, TMB, TMPD, andSEDTA, are able to reduce 3ALpc+judging from the free energy change calculated from the redox potentials of 3A1Pc+and the reductants. As Tables I and I1 show, k , increases with a decrease in the redox potential or in AGev This trend fits the expectation based on the mechanism of electron transfer quenching which was originally proposed by Rehm and Weller for explanation of fluorescence quenching.26 The large rate constant of self-exchange reaction between ferrocene and ferrocenium ion compared to other metal compounds (6.6 X 10, M-ls-') is responsible for the large quenching rate constant of 6.6 X lo8 M-l s-'. The mechanism of electron transfer quenching will be firmly proved by evidence of production of radicals in the following section. However, any alternative quenching mechanism via energy transfer is almost impossible because the triplet state of the quencher is higher than 3AlPc+. 3-1. Fraction of Radical Formation in Both Processes of Quenching and Reverse Electron Transfer. Electron transfer quenching does not always bring about free-radical formation in the bulk. The fact that a fraction of freeradical formation in the quenching event, defined as Fl = ket/kq,is lower than unity has been ascribed to a reverse or nonelectron transfer in the geminate radical radiative degradation of exciplex formed in the quenching p r o ~ e s s . ~ In ' ~ the ~ ~ electron transfer quenching of methylene and Z ~ P Cit ,has ~ been recognized that (i) ~
~~
~
(19)Balzani, V.; Bolletta, F. J. Am. Chem. SOC.1978,100,7404-6. (20)Marcus, R. A. J. Phys. Chem. 1963,67,853-7. 1973,95, (21)Pladziewicz, J. R.;Espenson, J. H. J . Am. Chem. SOC. 56-63. (22)Shporer, M.;Ron, G.; Loewenstein, A.; Navon, G. Inorg. Chem. 1965,4,361-4. (23)Bock, C.R.; Conner, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J.Am. Chem. SOC.1979,101,4815-24. (24)Suga, K.; Aoyagi, S. Bull. Chem. SOC.Jpn. 1973,46, 755-61. (25)Marcus, R. A.;Sutin, N. Znorg. Chem. 1975,14,213-6. (26)Rehm, D.;Weller, A. Ber. Bunsenges. Phys. Chem. 1969, 73, 834-9. (27)Steiner, U.;Winter, G.; Kramer, H. E. A. J . Phys. Chem. 1977, 81,1104-10. (28)Tamura, %I.; Kikuchi, K.; Kokubun, H.; Weller, A. 2. Phys. Chem. (Wiesbaden) 1980,121,165-72.
780
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983
Ohno et al.
every quenching event produces a geminate radical pair, of which some fraction (Fl)dissociates into the radicals and the residual fraction, 1- F,, performs reverse electron transfer yielding the original reactanta, and (ii) the radicals in the bulk form an encounter complex which either degradates with fraction of F2or dissociates to the radicals with fraction of F1. F1and F, can be experimentally determined as the following ratios:
F1 = ket/kq
= kr/(kr
+ kdiJ
b
d
0
0
i
e '
C 0
I
f
-'Or
9
- 2.0
1
-60
(lob)
As eq 10 show, the sum of Fland F2is unity when every quenching yields geminate radical pair. However, the reverse electron transfer rate of the geminate radical pair is different from that of the encounter pair formed in the bulk reverse process, because one-quarter of the encounter pairs are singlet, while most of the geminate pair are triplet. Unless spin conservation is broken, k, of the geminate pair is smaller than that of the encounter pair. When spin conservation is firm, Fl is unity and F, is 0.25 at maximum and the sum is 1.25 at maximum. Tables I1 and I11 list the values of F1and F2 obtained, the latter of which are evaluated by assuming the largest quenching rate constant to be k, (7.3 X lo8 M-' s-l in DMA-water and 19 X lo8 M-l s-l in Me2SO-water). The sums of Fl and F2are close to unity for both reactions of 3AlPc+with the quinones and the aromatic amines except for DMB. Consequently, it is most likely that every quenching event produces a geminate radical pair followed by reverse electron transfer competing with dissociation to the free radicals. This conclusion offers a ground for estimation of radical yield in electron transfer quenching. 3-2. Dependence of Reverse Electron Transfer upon S p i n Multiplicity. Neither iron(II1) nor iron(I1) compounds converted 3AIPc+to its radical in the quenching reaction except that Fe11r(CN)2(phen)z+ and ferrocene yielded the radical with Fl less than 0.2. Small or nil production of AlPc2+.can be explained in terms of fast reverse electron transfer in the geminate pair, which not only is spin allowed as eq 11 shows but also is favorable 2AlPc2+-+ Fe11(CN),(bpy)2AlPc+ + 2Ferr*(CN),(bpy)-(11)
-
(-0.8 eV) with respect to the Gibbs energy change. The fact that the doublet compound as Fe"'(CN),3- and ferrocenium ion failed in producing ZnPc+. in the triplet quenching of ZnPc was also explained by spin-allowed reverse electron transfer in the life of the doublet geminate pair.3 It is recalled that fast reverse electron transfer has been pointed out to be responsible for small production of free radicals in electron transfer quenching of fluoresc e n ~ e .In~ this ~ case, formation of free radicals can only be expected on the occasion that (i) kret is reduced when the Gibbs energy change is large and positive and (ii) strong interaction between an excited molecule and a quencher stabilizes the exciplex slowing down its decay via radiationless degradation. The small yield of AlPe in the 3 A l P ~quenching + with the iron(I1) compounds may be ascribed to fast reverse (29) Seely, G. R.Photochen. Photobid. 1978, 27, 639-54.
I
l a
(94
FZ= kret/ke (9b) where k, is the encounter complex formation rate. These fractions can be also written by using the rate constants of the elementary processes (eq 2 and 3). FI = kdis/(kr + kdis) (104 F2
1
1.0,
-50
-40
-30
-20
AGret /RT
Flgure 7. Dependence of In ( ? I F , - 1 ) on AG,,/RT: (a) BQ, (b) 2,5-DCBQ, (c) TCBQ, (d) PT, (e) DMB, (f) TMB, and (g) TMPD.
electron transfer caused by both large spin-orbit interaction due to a heavy atom, iron in this case, and large Gibbs energy change. The present author (T.O.) reported that the large Gibbs energy decrease in the reverse electron transfer (AG,,,) enhanced spin-inverted electron transfer between MBH+. (or MB.) and iron(II1) compounds.2 For example, 1- F1increased to 0.9 with AGret of -0.8 eV for MBS-ferrocenium ion. Therefore, it is reasonable that AGIet of -0.8 eV for the reactions 2AlPc2+* + 2ferrocenium ion and 2AlPc2+, ?Fem(CN),(bpy)-is sufficiently negative to enhance the reverse electron transfer. As for organic oxidants and reductants consisting of nonheavy atoms, however, F2or 1 - F1weakly correlates with AGret. The dependence of the rate of reverse electron transfer (k,) upon AGret is visualized in Figure 7 on plotting In (l/Fl - 1) = In (k,/kdi,) vs. -AG,,,/RT. The plot of these data has a good linearity with a slope of 0.07 for the amines and a slope of 0.05 for the quinones. Meanwhile, a rate constant of electron transfer between molecules i and j can be expressed by the following equation kij = p Z exp(-AG*ij/RT) (12)
+
where p is the probability of electron transfer, 2 is the collision number in the solution, and AG*ij is the free energy of activation for the electron transfer process which can be written according to Rehm and WellerZ6 AG*ij = AGoij/2
+ ([AGoij/2J2+ [AG*ij(0)]2)1/2
AG*jj(O) = [AG*ii(O)
+ AG*jj(O)]/2
(13) (14)
where AGO,. is the Gibbs energy change with electron transfer ana AG*ii(0)is the free energy of activation for the self-exchangereaction of molecule i. When (1)AG*,,(O) is assumed to be small for many aromatic compounds, and (2) the process is adiabatic, kij or k, is approximated by Z exp(-AGoij/RT). The free energy of activation, AG*di8, for the dissociation process of the geminate pair between AlPe and a cation radical in Me2SO-water is expected to be constant with the cations and to be different from that for the pair between A1Pc2+. and an anion radical in DMA-water, since AG*dis reasonably depends on the charge of the geminate pair and the solvent. The change, A(ln k,/k,), with the donor or the acceptor can be roughly expressed by A(-AGO, + AG*,,)/RT = -A(AGo,)/RT. Under these conditions, the slope in Figure 7 , A(ln k,/ kdis)/A(-AGo,/RT), would not be far from 1 and is inconsistent with the observed slopes, which are comparable
J. Phys. Chem. 1983, 87, 781-788
with that (0.05) for MB+-aromatic amines., The small slopes can be explained in the following way. When the probability of electron transfer (p) is small and dependent on the interaction energy (Ifif)between the initial state and the final state in a nonadiabatic process such as spin-inverted electron transfer, the rate of nonadiabatic process is proportional to both exp(-AGo,/Rn as just mentioned and the square of the interaction energy. Since Hi, is proportional to the reciprocal of the energy difference (aif) between the initial state and the final state according to perturbation theorem, the variation of l/(aEif)2 with AGO, partially cancels the variation of exp(-AGO,/RT). Therefore, the slope, A(ln k , / k , J / A(-AGor/RT) is expected to deviate from 1 in the spininverted electron transfer. When the interaction energy, Hif,between the initial triplet state and the final singlet state increases with increase in the spin-orbit mixing, the rate of electron transfer reaction depends mainly on (exp(-AGO,/RZJ. This expectation is realized by a large slope (0.16) observed in the plot of In (l/Fl - 1) vs. -AGo,,,/RT for the quenching of methylene blue by ferrocenes.,
781
A weak salt effect on Itet or F1observed for AlPc+-BQ, DCBQ, and TCBQ indicates that an effect of C10, coordination to AlPc+ before electron transfer is too weak to force the geminate radical pair, AlPc2+-BQ-., to dissociate, and/or coordination reaction of Na+ and Clod- to AlPc2+-BQ-. after electron transfer is too slow. The weak effect due to the simple salt on ion-pair dissociation is in contrast to a dramatic increase of R u " ' ( C N ) , ( ~ ~ ~for)~+ mation in the quenching of 3Ru11(CN)2(bpy)2 by Fe"'(CN):- located near a cation polymer, on which the positive electric field may reject R u " ' ( C N ) ~ ( ~ ~ ~ into ) , +the bulk to reduce the chance of geminate reverse electron transfer.30 Registry No. AlPcCl, 14154-42-8;Fenl(CN)z(bpy)z+, 4759723-9; Fem(CN)z(phen)z+, 28850-37-5;Fem(CN)&bpy)-, 22337-23-1; Fe111(CN)63-, 13408-62-3;TCBQ, 118-75-2;2,5-DCBQ,615-93-0; 2,6-DCBQ, 697-91-6;BQ, 106-51-4;MV2+,4685-14-7;Fe11(CN)4(bpy)2-, 17455-56-0; ferrocene, 102-54-5;PT, 92-84-2; DMB, 119-93-7;TMB, 366-29-0;TMPD, 100-22-1;AlPP, 84370-48-9; AlPc, 84370-49-0. (30) Sassoon, R. E.; Rabani, J. J. Phys. Chem. 1980, 84, 1319-25.
ENDOR Study of Bis(acetylacetonato)copper(I I ) in Solid Solution Burkhard Klrste and Hans van Wllllgen" Department of Chemistry, University of Massachusetts at Boston, Boston, Massachusetts 02125 (Received: June 11, 1982; In Final Form: October 25, 1982)
The bis(acetylacetonato)copper(II) (Cu(acac).J complex and its adducts with methanol and pyridine in frozen solution have been studied by using ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE). The ENDOR spectra yield the values of the principal components of the hyperfine tensors of the CH and CH3protons. Signs of the hyperfine components were determined with the aid of TRIPLE and computer simulations. Measurements of quadrupole splittings in 2HENDOR spectra of Cu(acac-3-dIzoriented in a frozen liquid crystal allowed the assignment of the in-plane CH couplings to specific molecular axes and also provided information on the alignment of the molecule in the nematic phase. It is shown that TRIPLE can be used to reduce a powder ENDOR spectrum to a single-crystal-typespectrum that can give semiquantitative information on the relative orientation of hyperfine tensor axes. The proton hyperfine interactions in Cu(acac), are similar to those in VO(a~ac)~, notwithstandingthe difference in ground-state orbital of the unpaired electron. Adduct formation does not have much effect on the hyperfine components of the CH and CH3protons. The ENDOR data indicate that methanol and pyridine bind in a vacant axial position.
Introduction In recent papers we described the results of an ENDOR study of bis(acetylacetonato)oxovanadium(IV), VO(a~ac)~, and some of its adducts randomly oriented in solid solution' and partially aligned in frozen liquid crystals., It was shown that, through the measurement of hyperfine and quadrupole interactions of ligand nuclei, detailed information is obtained on the structure of these complexes. Here we report on an extension of this study to the Cu(acac), complex. This study appeared of interest for a number of reasons. While Cu(acac)2has been the subject of numerous magnetic resonance these have focused on structural (1) Kirste, B.; van Willigen, H. J. Phys. Chem. 1982, 86, 2743. (2) Kirste, B.; van Willigen, H. Chem. Phys. Lett. 1982, 87, 589. (3) Maki, A. H.; McGarvey, B. R. J. Chem. Phys. 1958,29, 31. (4) Gersmann, H. R.; Swalen, J. D. J. Chem. Phys. 1962, 36, 3221.
information that can be derived from measurements of Cu hyperfine-, quadrupole-, and g-tensor components. These spectral parameters primarily serve to characterize the (electronic) structure of the metal ion and its immediate environment and may not give a very good picture of the system as a whole. By supplementing the data with information on hyperfine (and possibly quadrupole) interaction components of ligand nuclei, a more complete description of the structure of the system can be given. For instance, as in the case of V0(acac),,lp2 the ENDOR data may give a better insight into the effects of adduct formations-10on the structure of the complex. Furthermore, (5) Rollmann, L. D.; Chan, S.I. J. Chem. Phys. 1969,50, 3416. (6) So, H.; Belford, R. L. J. Am. Chem. SOC.1969,91, 2392; Belford, R. L.; Duan, D. C. J. Magn. Reson. 1978, 29, 293. (7) Kita, S.; Hashimoto, M.; Iwaizumi, M. J. Magn. Reson. 1982, 46, 361.
0022-3654/83/2087-0781$01.50/0 0 1983 American Chemical Society
