J . Phys. Chem. 1990, 94, 4123-4121 with a temperature-independent concentration, in contrast to previous model^.^^-^^ The relaxation of ("CH), is unusual. In the case of ("CD), the I3C are strongly coupled to the defect but are not strongly coupled to each other. Whereas in the case of (CH), the protons are not strongly coupled to the defect (and, presumably, are strongly coupled to each other), in (I3CH), the nuclei are not only strongly coupled to the defect but are strongly coupled to each other. In this case, and only this case, is the strong nuclear-nuclear coupling an efficient mechanism for defect spin relaxation. The nature of diffusion of magnetization through nuclei strongly coupled to an electron remains a largely unexplored subject. We have outlined how both one-dimensional and three-dimensional nuclear diffusion could contribute to R I for the electron defect. It appears that nuclear spin diffusion within the region of the defect
4723
has markedly different effects on the defect relaxation than distant nuclei. We suggest that 13Cand H nuclei are quasi-iso-spins in the presence of the electron; the energy mismatch between them is accommodated by the defect, and diffusion for matrix nuclei can be a very efficient process. Acknowledgment. We are most grateful to Dr. L. R. Dalton and Dr. Street for the polyacetylene samples used in these studies and for many helpful discussions. We are also grateful to Drs. H. Thomann, G. Drobny, and J. M. Schurr for many helpful suggestions and stimulating discussions. We acknowledge the support of the National Science Foundation DMB-87-06175, the Research Foundation, IBM, Exxon Educational Foundation, and the National Science and Engineering Research Council of Canada.
Photochemistry In Polymers. Photoinduced Electron Transfer between Phenosafranine and Triethylamine in Perfluorosulfonate Membrane K. R. Gopidas and Prashant V. Kamat* Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: July 25, 1989; In Final Form: January 15, 1990)
Absorption and emission properties of protonated and unprotonated forms of phenosafranine dye are investigated in the perfluorosulfonate ion-exchange membrane (Nafion) and polymer solution. Only the unprotonated form of phenosafranine is photoactive and undergoes electron transfer with triethylamine upon excitation with visible light. The triplet excited state of phenosafranine and the electron-transferproducts in the Nafion film are characterized by laser flash photolysis with 532-nm excitation. In a dry evacuated film the triplet lifetime of phenosafranine is greater than 1.5 ms. During the steady-state photolysis, the semireduced dye undergoes disproportionation to accumulate a colorless two-electron-reduction product.
Introduction Photoactive polymers have important applications in photoresists, xerography, photocuring of paints and resins, and solar energy conversion systems.' These polymeric systems can be broadly classified into two categories: (1) in which chromophores are directly attached to the backbone of the polymer and ( 2 ) in which the polymer film acts as a host to the photosensitizing molecules. Photoactive guest molecules can be incorporated in a polymer film via electrostatic or hydrophobic interactions. The type and degree of interaction between the dye molecule and the polymer determines the course of a photochemical reaction. Various aspects of photochemical and photophysical processes in polymers have been discussed in detail elsewhere.I4 Nafion (manufactured by Du Pont Corp.) is a perfluorosulfonate ionic polymer with a wide variety of applications in electrochemistry and photochemistry. For example, an electrode surface modified with Nation film containing electroactive species exhibits interesting electrocatalytic proper tie^.^,^ Nafion membrane has been used in integrated chemical systems for the purpose of solar enegy c o n ~ e r s i o n . The ~ ~ ~solid polymer matrix has been found useful in the preparation of quantized semiconductor particles.*I2 The strong acidic microenvironment of the protonated form of Nafion facilitates organic rearrangement^'^*'^ and isomerization^.'^ The fluorocarbon network of the swollen Nafion film consists of solvated -SO3- head groups and counterionsolvent clusters (-40 A in diameter) that are interconnected by short channels (- 10 A).ls This biphasic structure has been compared with the structure of reverse micelles.16 Luminescent molecules such as pyreneI7 and Ru(bpy),Z+'6,18q19have been used to probe the excited-state interactions with the polymer and to characterize To whom correspondence should be addressed.
various sites in such clusters. The effect of acidic environment on the excited triplet of xanthone and benzophenone20 and the kinetic properties of singlet oxygen generation in Nafion film2' (1) Guillet, J. Polymer Photophysics and Photochemistry; Cambridge University Press: New York, 1985. (2) Farid, S.;Martic, P. A.; Daly, R. C.; Thompson, D. R.; Specht, D. P.; Hartman. S. E.: Williams. J. L. R. Pure Aool. Chem. 1979. 51. 241. (3) Kalyansundaram, K.Photochemistry'h MicroheterogeneousSystems; Academic Press: New York, 1987; p 255. (4) Kamat, P. V.; Fox, M. A. In Applicationr of Lasers in Polymer Science and Technology; CRC Press, in press. (5) (a) Rubinstein, I.; Bard, A. J. J . Am. Chem. Soc. 1981, 103, 5007. (b) Henning, T.P.; White, H. S.;Bard, A. J. J. Am. Chem. Soc. 1981, 103, 3937. (c) Martin, C. R.; Rubinstein, I.; Bard, A. J. J . Am. Chem. SOC.1982, 104, 4817. ( 6 ) (a) Buttry, D. A.; Anson,
F. C. J . Am. Chem. Soc. 1982, 104,4824. (b) Tsou, Y.-M.; Anson, F. C. J . Phys. Chem. 1985.89, 3818. (7) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. SOC. 1983, 105, 7002. (8) Meisner, D.; Memming, R.; Kastening, B. Chem. Phys. Leu. 1983,96, 34. (9) Hilinski, E. F.; Lucas, P. A.; Wang, Y. J . Chem. Phys. 1988,87,3435. (IO) Gopidas, K. R.; Kamat, P. V . Mater. Lett., in press. (11) Bard, A. J. Eer. Bunsen-Ges. Phys. Chem. 1988, 92, 1187. (12) Olah, G. A,; Meidar, D. Nouu. J. Chim. 1979, 3, 269. (13) Demuth, M.; Mikhail, G.; George, M. V. Helu. Chim. Acta 1981,64, 2759. (14) Childs, R. F.;Mika-Gibala, A. J. Org. Chem. 1982, 47, 4204. (15) Yeo, S.C.; Eisenberg, A. J . Appl. Polym. Sei. 1977, 21, 875. (16) Lee, P. C.; Meisel, D. J . Am. Chem. Sot. 1980, 102, 5477. (17) Lee, P. C.; Meisel, D. Photochem. Photobiol. 1985, 41, 21. (18) Prieto, N. E.; Martin, C. R. J. Electrochem. SOC.1984, 131, 751. (19) Szentirmay, M. N.; Prieto, N. E.;Martin, C. R. J . Phys. Chem. 1985, 89, 3017. (20) Weir, D.; Scaiano, J. C. Tetrahedron 1987, 43, 1617. (21) Lee, P. C.; Rodgers, M. A. J. J . Phys. Chem. 1984,88, 4385.
OQ22-3654/90/2094-4123$02.50/0 0 1990 American Chemical Society
4724
Gopidas and Kamat
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990
have also been studied. Though the feasibility of utilizing photochemical processes in Nafion films has been suggested in several applications,'2-'6~22little effort has been made to probe the electron-transfer processes in polymer hosts. Such a heterogeneous microenvironment can be useful for effecting and controlling photochemical processes more efficiently than can be accomplished in homogeneous solutions. In the present study we have chosen a cationic phenazine dye, phenosafranine, since its photochemical and photoelectrochemical applications have been widely studied.23-27 A laser flash photolysis study which elucidates the mechanistic details of the electron transfer between excited phenosafranine (PHNS+) and triethylamine (TEA) in Nation film is described here.
H2N
a::p
8
NH2
c'Q
+0.5
0.0
-0.5
--'
-0.9
V vs. SCE
"' ,,/ 7 O'O
-0.50
V vs. SCE
PHNS'
Experimental Section Materials. Nafion 1 17 in the H+ form and acetonitrile (Gold Label) were obtained from Aldrich. Phenosafranine, 3,7-diamino-5-phenylphenazinium chloride (Sigma), was purified chromatographically over silica gel.24 Triethylamine (Baker) was distilled prior to its use. Sample Preparation. The Nafion film was extracted with methanol for 4-5 h and was dried in the oven at 60 OC for 24 h. The sodium-exchanged form of the Nafion was prepared by soaking the film in an aqueous solution of 1 M NaOH for 24 h and then washing thoroughly with deionized wateraZ0The film was then dried in the oven a t 60 "C. The optically transparent Nafion film was then cut into 0.5- X 4-cm pieces so that the film can conveniently be introduced into a 1-mm-thick optical cell. Phenosafranine was incorporated into the Nafion film by suspending the film in an acetonitrile solution (20 mL) of M P H N S for 20 min with occasional stirring. The color of the film turned pink (or blue) as Na+ (or H+) ions were exchanged with PHNS+ in the Nafion film. The film was then thoroughly washed with deionized water and dried overnight at 65 OC. Optical measurements with dry films were made after evacuating the cell for 2-3 h. The films soaked in CH3CN were degassed by bubbling argon for 30 min. All the experiments were performed at room temperature (23 "C). Electrochemical Measurements. Cyclic voltammetric measurements were performed with a Bioanalytical Systems electrochemical analyzer (Model 100). A three-compartment cell with a working electrode (SnOz or Pt), a counter electrode (Pt wire), and a reference electrode (saturated calomel electrode, SCE) was employed for the electrochemical measurements. The cyclic voltammograms were recorded in deaerated acetonitrile containing -0.1 M tetrabutylammonium perchlorate, TBAP. Polymer film coated Sn02 electrodes (0.5 X 5 cm) were prepared by dipping the Sn02 electrodes in 5% Nafion solution (Aldrich) and drying in air. The film was then soaked in dilute NaOH solution to obtain it in Na+ form. The film was then washed with water and soaked in the dye solution for 10 min before transferring it to the electrochemical cell. Typically, a surface coverage of (0.1-1 ) X 10-9 mol/cm2 of dye in the polymer film was obtained. It may be noted that cast films do not have the same microheterogeneity as that of commercial membranes. (22) Krishnan, M.; Zhang, X.;Bard, A. J. J . Am. Chem. SOC.1984,106, 7371. (23) Gopidas, K. R.; Kamat, P. V. J . Photochem. Photobiol., A 1989, 48, 291. (24) Kamat, P. V.; Gopidas, K. R.; Weir, D. Chem. Phys. Lett. 1988, 149, 491. (25) Gopidas, K. R.; Kamat, P. V. fungmuir 1989, 5, 22. (26) (a) Rohatgi-Mukherjee, K. K.; Roy, M.; Bhowmik, B. B. Sol. Energy 1983,31. 417. (b) Bhowmik, B. B.; Roy, S. Indian J . Chem. 1988,27A, 237. (27) Natarajan, P. J . Macromol. Sci., Chem. 1988, A25, 1285.
Figure 1. (a, top) Cyclic voltammogram of 0.5 m M phenosafranine in acetonitrile with Pt working electrode (scan rate, 50 mV/s; reference electrode, SCE; supporting electrolyte, 0.1 M TBAP). (b, bottom) Cyclic voltammogram of phenosafranine incorporated Nafion film on SnO, electrode (scan rate, 20 m V/s; reference electrode, SCE; electrolyte, aqueous solution of 0.1 M trifluoroacetate buffer (pH 3 ) .
-
Hence, the comparison of the two conditions should be considered with sufficient caution. Optical Measurements. The absorption spectra were recorded with a Perkin-Elmer Model 3840 diode-array spectrophotometer. The corrected fluorescence spectra were recorded with an SLM photon-counting spectrofluorometer. Steady-state photolysis was performed with filtered light (A > 400 nm) from a 150-W halogen lamp. Fluorescence lifetime measurements were performed by the time-correlated single-photon counting technique using the apparatus that has been described elsewhere.28a Laser flash photolysis experiments were performed with a 532-nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta Ray DCR-1 Nd:YAG laser system as the excitation source and a 1000-W xenon lamp as the monitoring source. The excitation was carried out with front-face excitation geometry. The details of the laser flash photolysis setup are described elsewhere.28b A typical experiment consisted of a series of 1-5 replicate shots per single measurement, and the average signal was processed with an LSI-11 microprocessor interfaced to a PDP-l1/55 computer. The point at which the laser beam strikes the sample was changed between the successive measurements. This minimized the contribution of photoinduced irreversible changes to the transient absorption measurements. All experiments were performed at room temperature (23 "C).
Results and Discussion Redox Behavior of PHNS+ in Nafion. Phenosafranine readily undergoes reduction at -0.665 V vs SCE ((& + Ep,J/2) in acetonitrile. The difference in peak potential is around 40 mV and is close to the value of 59 mV/2 which is expected for a two-electron reduction process. The cyclic voltammograms of PHNS+ in acetonitrile and in Nafion-coated SnOz electrode are shown in Figure 1. Incorporation of cationic dyes in Nafion films coated on a conducting electrode surface has been demonstrated earlier.s*29 The P H N S in the Nafion film was electroactive and exhibited a reduction peak with E = -0,098 V and Ew = +0.022 V vs SCE (EO = -0.060 V vs E E ) . The uncompensated re(28) (a) Federici, J.; Helman, W.p.; Hug, G.L.; Kane, C.; Patterson, L. K. Comput. Chem. 1985, 9, 171. (b) Nagarajan, V.; Fessenden, R. W. J . Phys. Chem. 1985,89, 2330. (29) Lu, 2.; Dong, S. J . Chem. SOC.,Faraday Trans. 1 1988, 84, 2979.
The Journal of Physical Chemistry, Vol. 94, No. 1 I , 1990 4725
Photochemistry in Polymers
0'5r-----7
TABLE I: Absorption and Emission Characteristics of Phenosafranine Dye
0
A
0'4: 0.3
in CH$N
absorption max, nm emission max, nm fluorescence lifetime, ns triplet-triplet abs max, nm triplet lifetime, ps
in Nafion film"
517 560
520 561
4.1 440, 730, 830
440, 735, 830
3.8
25
-1106
> 1SOW
form. bImmersed in deaerated CH,CN. 400 nm) for 5 min. Similar photobleaching of the dye was also seen in Nafion solutions and films. The semireduced phenosafranine formed as a result of the reaction between excited phenosafranine and TEA undergoes disproportionation to yield a leuco dye (reaction 6). TEA'+ 2PHNS'
-
PHNS-
+PHNS
(6) formed as a result of reaction 5 rearranges to form the a-amino radical.38 This intermediate further undergoes disproportionation, coupling, or oxidation. As a result of this, the back-reaction between TEA'+ and PHNS' is suppressed and the disproportionation of PHNS' (reaction 6) is facilitated. Since TEA acts as a sacrificial donor in this photochemical reaction, one observes accumulation of the colorless product PHNS-. A slow recovery of the dye in acetonitrile (-80% recovery in 75 min) was seen in the dark possibly as a result of the reaction between PHNSand some oxidative i m p ~ r i t i e s . The ~ ~ rate of dye recovery in the polymer film was at least 2-4 times slower than in acetonitrile. The electrostatic and hydrophobic effects caused by the polymer environment can also play an important role in enhancing the yields of photoinduced electron-transfer process and retarding the rate of back-electron-transfer process. The stabilization of charge-transfer products in the polymer films has important applications in the development of photochemical systems for solar energy conversion (e.g., in the design of integrated chemical systems) and artificial photosynthesis. Acknowledgment. We thank Dr. Maria Bohorquez for her assistance with fluorescence lifetime measurements. The work described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-3202 from the Notre Dame Radiation Laboratory. Registry No. PHNS*, 81-93-6; Na+, 17341-25-2; H+, 12408-02-5; Nafion, 39464-59-0; triethylamine, 121-44-8. (37) Delaire, J. A,; Sanquer-Barrie, M.; Webber, S . E. J . Phys. Chem. 1988, 92, 1252. (38) Pienta, N. J. In Photoinduced Electron Trunsfer; Fox, M. A., Chanon, M., Eds.; Elsevier: New York, 1988; p 4217. (39) We would like to thank the referee for bringing this aspect to our
attention.
