2469
J. Phys. Chem. 1986, 90, 2469-2415
Coulombic Effect on Photoinduced Electron-Transfer Reactions between Phenothiazines and Viologens Yuji Kawanishi, Noboru Kitamura, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori- ku, Yokohama 227, Japan (Received: September 5, 1985)
Photoinduced electron-transfer reactions between excited phenothiazine derivatives (PTH) and viologens (V) were investigated in aqueous acetonitrile at room temperature. Ionic substituents on PTH or V strongly affected the reaction kinetics, and the Coulombic effects on the reactions were interpreted by the work terms w,and wp, where r and p represent reactants and products, respectively. Fluorescence quenching reactions proceeded with the diffusion-controlled rate constants which were dependent on w, and were explained by the Debye-Smoluchowski equation. Charge separation efficiency (F)corresponding to the cage escape yield of geminate ion radical pair strongly depended on (wp - w,),which indicated that (i) the backelectron-transfer rate determined F and (ii) the Coulombic work terms decided the back-electron-transfer rate. Quantitative treatments of the present results demonstrated that Coulombic effects can be used effectively to generate highly efficient photoredox systems.
SCHEME I: Synthetic Routes of PTH Derivatives
Introduction
The importance of photoinduced electron-transfer reactions is well recognized especially in the chemical conversion of solar energy. Theoretical as well as semiempirical treatments of electron-transfer reactions in photochemical processes describe the results of fluorescence quenching as a function of the overall free energy changes.' The generally accepted view is that fluorescence quenching via electron transfer is a diffusion-controlled p r o w s if the process is exoergic by more than 10 kcal mol-'. Complete fluorescence quenching can be achieved easily by a proper choice of electron acceptors or donors. However, this does not lead to a complete charge separation, because of the occurrence of competitive back-electron-transfer processes resulting in photoredox reactions. A target of research on photoredox reactions is, consequently, to suppress back electron transfer. Use of Coulombic effects to prevent back electron transfer is an effective approach, as shown by photoinduced electron-transfer reactions on micellar and/or polyelectrolyte s ~ r f a c e s . ~Although .~ the use of heterophases may be of practical significance, homogeneous systems are undoubtedly plausible for the understanding of Coulombic effects. Recently we have demonstrated that the quantum yield of photooxidation of leuco dyes in homogeneous systems can be improved by the use of cationically charged sensitizers or by the addition of neutral salts such as tetraalkylammonium halides4 Similarly, Richoux and Harriman have reported the Coulombic effects on the photoreduction of methylviologen by ionically charged porphyrin as a sensitizer in homogeneous ~ o l u t i o n . ~ Coulombic attraction between a sensitizer and a substrate after a one-electron transfer from one to another is reduced in the presence of neutral salts (primary salt effect) or when ionic substituents are attached to the reactants to neutralize Coulombic attraction. This general trend of the Coulombic effect is understandable. The details however are not at all clear. Coulombic effects will influence the diffusion constants of charged species (kinetic factor) as well as the free energy change of both forward and back electron transfer (thermodynamic factor). More fun(1) D. Rehm and A. Weller, Zsr. J . Chem., 8, 259 (1970); D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 73, 834 (1969). (2) N. J. Turro, M. Grltzel, and A. M. Braun, Angew. Chem., Znt. Ed. Engl., 19, 675 (1980); B. Katusin-RaZem, M. Wong, and J. K. Thomas, J. Am. Chem. SOC.,100, 1679 (1978). (3) A. Slama-Schwork, Y. Feitelson, and J. Rabani, J . Phys. Chem., 85, 2222 (1981). (4) S.Tazuke, Y. Kawasaki, N. Kitamura, and T.Inoue, Chem. Lett., 251 (1980); N. Kitamura, Dr. Thesis, Tokyo Institute of Technology, 1983. (5) M. C. Richoux and A. Harriman, J. Chem. SOC.,Faraday Trans. I , 78, 1873 (1982).
0022-3654/86/2090-2469$01.50/0
H
CH3
damentally, a difficulty lies in the fact that the theory of electrolyte solution is based on stationary thermodynamic conditions assuming a dielectric continuum and high dilution of ions. These conditions are violated when a transient ion radical pair is considered: First, this is not an equilibrium state. Second, ions are locally concentrated. Dielectric saturation may also take place.6 Although the primary salt effect on ionic reactions, either positive or negative, is explainable by the common Coulombic effect outlined by Debye-Huckel theory, there are many features unpredicted by the theory such as specific salt effects. Despite these unavoidable difficulties, we set about to try to interpret the origin of the Coulombic effects on photoinduced electron-transfer reactions. We chose the combination of phenothiazine (PTH) as a reducing sensitizer with viologens (V) as an oxidant. Ionic substituents were introduced to both sensitizer and oxidant moieties. The PTH derivatives are capable of reducing V under photoirradiation in the absence of additional reducing agent, such as triethanolamine or EDTA. This is the difference from and the advantage over more common sensitizers such as tris(2,2'-bipyridine)ruthenium(II) or porphyrins. On the other hand, they suffer from short singlet lifetimes and low fluorescence quantum yields caused by efficient intersystem crossing. Nevertheless, this is an appropriate reaction system to investigate the role of Coulombic effects on both fluorescence quenching and subsequent charge separation under identical conditions since the reaction with any sacrificial reagents are excluded. (6) E. Sacher, and K. J. Laidler, Trans. Faraday SOC.,59, 396 (1963).
0 1986 American Chemical Society
2470
The Journal of Physical Chemistry, Vol. 90, No. 1 1 , 1986
Experimental Section Reagents and Materials. Methylviologen (Nakarai Chemicals, guaranteed reagent grade) was used as supplied. Solvents for spectroscopic and electrochemical measurements (water, acetonitrile, methanol, and ethanol) were spectroscopic grade (Kanto Chemicals) and used without further purification. Tetraethylammonium perchlorate ( T E A P Tokyo Kasei, extra pure grade) as a supporting electrolyte was recrystallized from an ethanol(Tokyo Kasei, methanol mixture. 2,5-Dimethyl-2,4-hexadiene extra pure grade) as a triplet quencher was distilled under vacuum prior to use. Synthesis of Phenothiazine Derivatives and Zwitterionic Viologen (Scheme I). 10-Methylphenothiazine (2) was prepared from phenothiazine (1) by the method of Normant et a].' The crude product was purified by column chromatography (Si gel/n-hexane + benzene) followed by repeated recrystallizations from ethanol (yield: 76%). Anal. Calcd for C13H,,NS: C (73.20), H (5.20), N (6.57), S (15.03). Found: C (73.50), H (5.00), N (6.56), S (14.62). ' H N M R (CCI4): 6 7.0 (8 H, m, aromatic), 3.3 (3 H, s, -CH3). l0-(3-Bromopropyl)phenothiazine( 3 ) and 1 O-(6-Bromohexy1)phenothiazine ( 4 ) were prepared from the corresponding alkyl dibromides with the sodium salt of phenothiazine as described for the preparation of 2 and purified by column chromatography (A1203/n-hexane) (yield: 18% (3) and 46% (4)). ' H N M R (CDCI3) for 3: 6 7.0 (8 H, m, aromatic), 4.1 (2 H, t, -CH2-C2H4Br), 3.5 (2 H, t, -C,H,-CH,-Br), 2.3 (2 H, dt, -CH2-CHZ-CH2Br). 'H N M R (CDCI,) for 4: 6 7.0 (8 H, m, aromatic), 3.8 (2 H, t, -CH2-C5HloBr), 3.4 (2 H, t, -C5H,,CH2Br), 1.0-2.0 (8 H, broad m, -CH2-C4H8-CH,Br). 3 4 10-Phenothiazinyl)propyltriethylammoniurnBromide (5s ) and 6-(1O-Phenothiazinyl)hexyltriethylarnmoniurn Bromide (6a ) and Chlorides (56 and 6 6 ) . A solution of 10-bromoalkylphenothiazine (3 or 4) and excess triethylamine in dry acetonitrile was stirred at 60 OC for 10 h. After evaporation of the solvent and triethylamine, the crude product was chromatographed on Alz03 (CH3CI-methanol). After AI2O3 was filtered off, the solvents were evaporated and the resulting bromide salt was dried under vacuum. Anion exchange of 5a and 6a by DIAION SAIOA (Mitsubishi Chemical Industries Ltd.) gave the chloride salts 5b and 6b (yield: 74% (5a) and 55% (6a)). Anal. Calcd for C24H3SNzSC1.(3/4)H20 (5b): C (64.59), H (7.68), N (7.17), S (8.21), C1 (9.08). Found: C (64.78), H (7.87), N (7.18), S (7.27), C1 (9.83). Anal. Calcd for Cz4H35NzSCI.H20(6b): C (65.95), H (8.53), N (6.41), S (7.34), C1 (8.1 1). Found: C (65.69), H (8.45), N (6.28), S (7.25), C1 (7.84). 'H N M R (CDCI,) for 5b: 6 7.1 (8 H , m, aromatic), 4.2 (2 H, t, -CH,-C2H4-Nf), 3.4 (8 H, q and t, -CH2-N(CH2-CH3)3), 2.3 (2 H, broad s, -CH2CH,-CH,-Nt), 1.2 (9 H, t, -N(CH,-CH3)3). 'H N M R (CDC13) for 6a: S 7.1 (8 H, m, aromatic), 3.8 (2 H, t, -CH,-C,H,,-Nf), 3.3 (8 H , q and t, -CH,-N(CH,-CH,),), 1.0-2.0 (8 H, broad, -CH2-C4Hx-CH2-), 1.3 (9 H, t, -N(CH2-CH3)3). Sodium 3-( 10-Phenothiazinyl)propanesulfonate(7) and Sodium 6-(I0-Phenothiazinyl)hexanesu[fonate( 8 ) . A solution of 10-bromoalkylphenothiazine (3 or 4) and a small excess of Na2S03 in acetonitrile-water (4/1 v/v) was refluxed for 12 h. After the solvents were removed, the crude product was dissolved in a minimal volume of dry methanol, and then dry benzene was added slowly with vigorous stirring. The resulting precipitate was collected, dried, and purified by gel chromatography (TOYOPEARL HW-40/methanol; Toyosoda Industries Ltd.) (yield: 5 1% (7) and 59% (8)). Anal. Calcd for CI5Hl4NS2O3Na (7): C (52.47), H (4.11), N (4.08), S (18.67). Found: C (52.50), H (4.17), N (3.88), S (18.15). Anal. Calcd for CI8H2,NS2O3Na (8): C (56.09), H (5.23), N (3.63), S (16.63). Found: C (55.77), H (5.23), N (3.58), S (16.57). 'H N M R (D20-MezSO-d6 mixture (1/1 v/v) for 7: 6 7.1 (8 H, m, aromatic), 4.0 (2 H, t, -CH,-C2H4S03Na), 2.9 (2 H, dt, -CH2-CH2-CH2-), 2.1 (2 H, t, -CH,-SO,Na). 'H N M R (Me2SO-d,) for 8: 6 7.1 (8 H , m, ( 7 ) H. Normant and T. Curigny, Bull. Sot. Chim. Fr.. 1866 (1965).
Kawanishi et al. TABLE I: Structures and Abbreviations structure
a!m R ' - & m N + - , '
abbrev
R = -CH, MPTH R = -C,H~-N+(C,HS)~ QPTH3 R = - C ~ H I ~ - N + ( C ~ H SQPTH6 )~ R = -C6H,,-SO,-C,H,-SOc
SPTH3 SPTH6
R' = -CH, R' = -C,H,-SO,-
svo
MV2+
aromatic), 3.9 (2 H, t, -CH2-C5HI,SO3Na), 2.4 (2 H, t, -CH,-S03Na), 1.2-1.8 (8 H, broad, -CH2-C4Hx-CH2-). 4,4'-Bipyridinium-l , I '-bis(ethanesu[fonate)(0. A solution . CHz-CH2-SOi
c a
I
of 4,4'-bipyridine and a small excess of sodium bromoethanesulfonate in dry dimethyl sulfoxide was stirred at 90 OC for 20 h. After the solvent was removed by distillation under reduced pressure, the bipyridinium salt was recrystallized twice from ethanol-water (4/1 v/v) and dried under vacuum (yield: 17%). Anal. Calcd for C14H16N2S20,~H20: C (43.07), H (4.65), N (7.18), S (16.42). Found: C (43.39), H (4.73), N (7.18), S (17.33). 'H N M R (DzO): 6 9.3 (4 H, d, Ha), 8.6 (4 H, d, Hb), 5.2 (4 H, t, H,.), 3.7 (4 H, t, Hd). Apparatus. Absorption and emission spectra were recorded with a Shimadzu UV-ZOOS spectrophotometer and a Hitachi MPF-4 spectrofluorometer,respectively. Emission lifetimes were measured by a single-photon-counting system (Photochemical Research Associates). Cyclic voltammetry was carried out with a Hokuto Denko HA-301 potentiostat, a HB-104 function generator, and a conventional H-type cell equipped with a Pt working electrode, a Pt counterelectrode, and a saturated calomel reference electrode. Continuous photoirradiation experiments were carried out with a 500-W Xe lamp (Ushio Electric) and a combination of glass filters (a Toshiba IR-25s IR cutoff filter, a UV-D33S filter, and a plate glass for UV cutoff) to obtain light of narrow wavelength around 365 nm. The quantum yield of the methylviologen photoreduction in the MPTH-MV,' system was determined by using a United Detector Tech. Inc. 21A optical power meter and a spectroirradiator consisting of a 2-kW Xe lamp (Ushio Electric) and a monochromator (JASCO CT-25C). The excitation wavelength was 365 nm. Sample Preparations. For fluorescence quenching experiments, the concentration of PTH was 5.0 X M. All solutions were deoxygenated by argon gas purging for 30 min. For continuous photoirradiation experiments, the concentrations and 5.0 X lo-) M, respectively. of PTH and V were 5.0 X Deaerated samples were prepared by several freeze-pumpthaw cycles. Results and Discussion Spectroscopic Properties and Oxidation Potentials of PTH. Structures and abbreviations of the samples used in this study are shown in Table I. Absorption and fluorescence spectra of MPTH in acetonitrile-water (4/ l v/v) at 298 K are shown in Figure l , and the spectral data including fluorescence lifetimes (TO), fluorescence quantum yields (af), and oxidation potentials (Elj2(PTH+/PTH) of PTH are summarized in Table 11. The spectroscopic properties of QPTH3, QPTH6, SPTH3, and SPTH6 are almost identical with those of MPTH showing absorption and fluorescence maxima around 305 and 450 nm, respectively. The absorption around 305 nm was assigned to the n-** transition by Gratzel et aL8 (8) R. Humphry-Baker, A. M. Braun, and M. Gratzel, Helo. Chim.Atla, 64. 2036 (1981).
Reactions between Phenothiazines and Viologens
The Journal of Physical Chemistry, Vol. 90,No. 1 I, 1986 2471
TABLE 11: Spectral and Electrochemical Data of PTH Derivatives absorption emission AE"': abbrev max," nm (log c) max,' nm eV
io,'ns
w
E1/Z(PTHt/PTH),C V (vs. SCE)
EII2(PTHt/PTH*)," V (vs. SCE)
1.9 1.9 1.9 2.0 2.0
0.0081 0.0078 0.0091 0.0090 0.0094
0.64 0.7 1 0.62 0.59 0.59
-2.26 -2.24 -2.30 -2.33 -2.31
MPTH QPTH3 QPTH6 SPTH3 SPTH6
252 251 253 253 253
(4.60), (4.51), (4.57), (4.57), (4.52),
306 (3.71) 301 (3.61) 306 (3.67) 305 (3.65) 306 (3.61)
452 446 45 1 452 454
2.90 2.95 2.92 2.92 2.90
'In CH3CN-Hz0 (4/1 v/v) mixture at room temperature. bCalculated with the emission maxima at 77 K in ethanol-methanol glass. 'In 0.1 M TEAP/CH3CN-H20 (4/1 v/v) mixture at 25 "C. "Calculated by subtracting A* from E l I z (PTHt/PTH).
Io/I or
po l
TOIT
WAVELENGTH (nrn)
Figure 1. Absorption (-) and fluorescence (---) spectra of MPTH in acetonitrile-water (4/1 v/v) mixture at 298 K.
However, its relatively large molar absorptivitiy (e = 5000 M-l cm-*) suggests that the transition is not a pure n-?r* transition. Indeed, Mantsch and Dehler9 have concluded from the results of quantum chemical calculations as well as from spectroscopic measurements at 80 K that this absorption consisted of three closely located T-A* transitions. The values of T~ and afwere determined to be -2 ns and
