488
J . Phys. Chem. 1991, 95,488-492
and helpful suggestions throughout this research. The authors graciously acknowledge the Research Corporation for funding this research. The access to the high-field NMR spectrometers provided by Dr. Clayton Radke (Department of Chemical Engineering at University of California, Berkeley) and Dr. F. Ann Walker (Department of Chemistry at San Francisco State University) was instrumental to a more complete understanding
of these viscoelastic systems and is gratefully acknowledged. The Department of Chemistry, San Francisco State University, also acknowledges grants from the National Institutes of Health (RR 02684) and the National Science Foundation (DMB-8516065) for purchase of the NMR spectrometers. The financial support of Saint Mary's College is acknowledged for providing an additional stipend for T.A.N.
Energy- and Electron-Transfer Shuttling by a Soluble, Bifunctional Redox Polymer Janet N. Younathan,+ Wayne E. Jones, Jr., and Thomas J. Meyer* Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27599-3290 (Received: May 7, 1990)
A soluble, bifunctional polymer ( [PS-An28,SPTZI,S]) based on derivatized polystyrene was prepared in which there are both energy-transfer acceptors (modified anthracene, An) and electron-transfer donors (derivatized phenothiazene, PTZ). The polymer was successfully incorporated into a photochemical electron-transfer sequence based on [Ru(bpy)J2' (bpy is 2,2'-bipyridine) in which separate oxidative and reductive equivalents were generated in solution. In the sequence, sensitized occurred by diffusion and formation of the triplet excited state of the polymer-bound anthryl sites ( [PS-3An*An27.5PTZl,s]) energy transfer from [Ru(bpy)J2+* following visible excitation of [Ru(bpy)J2+. In the presence of the oxidative quencher and monomeric paraquat ( PQ2+),a series of electron-transfer steps led,ultimately, to the appearance of [PS-An2s,sPTZ+PT~,5] PQ' in solution. The recombination rate constant between PQ+ and [PS-An2,,sPTZ+PTZo,s]was reduced by a factor of 7 relative to back electron transfer between the unbound, IO-methylphenothiazene cation (10-MePTZ') and ?Q+.
-
SCHEME I
Introduction
In reactions 1-3, visible excitation followed by a sequence of bimolecular electron-transfer events leads to the conversion of light into transiently stored oxidative and reductive equivalents (bpy is 2,2'-bipyridine; PQ2+ is paraquat, IO-MePTZ is 10-methylphenothiazene).' Ru(bpy)F Ru(bpy)p' Ru(bpy),3t
+ PO2+
+ 10-MePTZ
hv
Ru(bpy)$'
(1)
Ru(bpy),& + PO'
(2)
RU(bpy)?
(3)
+ 10-MePTZ'
(9-MeAn)'
0022-3654/91/2095-0488$02.50/0
[PS-PTZs]
relay between the light absorbing sensitizer and paraquat, eqs 5 and 6. Prompt recombination within the solvent cage between + 9-AnC0;
319-AnC0.'i
Present address: Eastman Kcdak Company, Rochester, NY 14650.
9;MeAn'
[PS-PTZ~PT~]
RU(bpy)T'
Attempts have been made to maximize yields and inhibit back electron transfer for such reactions by selective modification of the reaction microenvironement. This has involved the utilization of micelles, colloids, microemulsions, or polyelectrolytes.2 Another strategy has involved anchoring the Ru(I1) sensitizer and/or viologen quencher to macromolecules. This can reduce the rate of diffusional encounter but, at the same time, decreases the rate of back electron t r a n ~ f e r . ~ - ' ~ An inherent limitation in the [ R ~ ( b p y ) ~ l ~ + / PsensitizerQ~+ quencher combination is the relatively low cage escape yield associated with eq 2. In homogeneous solutions containing [Ru(bpy)J2+* and enough PQ2+to quench the excited state with near unit efficiency, PQ+ yields are 0.25 or l ~ w e r . ' ~ - 'The ~ separation efficiency can be even lower with polymer-bound reagents,4-6,8,9,1 1-13 The results of studies by Johansen, Mau, and Sasse2OS2'have demonstrated how separation efficiencies can be enhanced by incorporating 9-anthracene-carboxylate (9-AnC02-) as an energy
+
I
9-MeAn,
t
Po2+
___t
Ru(bpY)$ 9-AnC02'
t
'(9-AnCO;)'
(5)
+ PQ'
(6)
COi (9-AnCOf) ~
~
~~~
(1) (a) Young, R. C.; Meyer, T. J.; Whitten, D. G . J . Am. Chem. Soc. 1975, 97,4781. (b) Meyer, T. J. Isr. J . Chem. 1977, I S , 200. (c) Bock, J. A.; Connor, A. R.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G . ; Sullivan, B. P.; Nagel, J. K. J. Am. Chem. Soc. 1979, I O / , 4815. (d) Sutin, N.; Creutz, C. Pure Appl. Chem. 1980,52,2717. (e) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159 and references therein. (2) For example, see: (a) Rabani, J.; Sasson, R. E. J. Photochem. 1985, -29, 7. (b) Gratzel, M., Ed. Energy Resources through Photochemistry and Cafalysis;Academic: New York, 1983. (c) Kalyanasundaram, K. Phorochemistry in Microheterogeneous Systems; Academic: New York, 1987. (d)
McLendon, G., et al. In Photochemical Energy Conversion, Proc. Int. Conf. Photochem. Convers. Solar Energy Storage, Elsevier: New York, 1989, 47-59. (e) Rabani, J. In Photoinduced Electron Transfer, Part B; Fox, M. A., Chanon, M., Eds.; Elsevier: New York, 1988; p 642. (3) Kaneko, M.; Nakamura, H. Macromolecules 1987, 20, 2265. (4) Kaneko, M.; Hou, X.-H.; Yamada, A. Bull. Chem. Soc. Jpn. 1987,60, 2523. (5) Kaneko, M.; Nakamura, H. Makromol. Chem. 1987, 188, 201 1. (6) Hou, X.-H.; Kaneko, M.; Yamada, A. J . Polym. Sci., Polym. Chem. Ed. 1986, 24, 2749.
0 1991 American Chemical Society
Energy and Electron Transfer by a Redox Polymer
CHzR
--
CH2R'
x -30
y 1
28.5 1.5
cii20-
R=
I
R'=
[ps-~28.s~i.51
Figure 1. Structure and abbreviation of the redox polymer.
the anthrylcarboxy radical and the paraquat cation radical is spin forbidden,22and the separation efficiency of the monomeric redox products can approach unity. We are interested in the utilization of soluble polymers as a vehicle for preparing complex molecular assemblies that contain controlled combinations of chromophores, quenchers, and relays. The goal is to apply these materials to problems in photochemical electron and energy transfer. We previously reported on a three-component, polystyrene-based photoredox system.23 When a polymer-bound derivative of [ R ~ ( b p y ) ~ (][PS-Ru113]) ~+ was excited in the presence of 9-methylanthracene (9-MeAn) and separate polymers containing electron-transfer acceptors based on PQ2+ ( [PS-PQ2+,]) or donors based on phenothiazene ([PSPTZ,]), separated oxidative and reductive equivalents were created on isolated polymeric strands (Scheme I). Because of slow diffusion by the polymers, the rate constant for recombination by back electron transfer between the transiently generated [PS-PQ+]/[PS-PTZ+] pair was reduced by a factor of 27 relative to IO-MePTZ+ and PQ+. We have prepared a soluble, bifunctional polystyrene polymer that contains derivatives of both anthracene and phenothiazene. In this polymer there is a high loading of the anthracene derivative and a low loading of the phenothiazine derivative with the two sites distributed randomly among individual polymeric sites. The structure and abbreviation used for the polymer are illustrated in Figure 1. In the abbreviation, the average number of anthracene or phenothiazine derivatives that are present per polymeric strand are indicated by the subscripts. The polymer was prepared by the nucleophilic displacement of CI- from a 1:l styrene/m,p-chloromethylstyrene copolymer of 30 repeating units. The synthetic procedure allows for the stepwise preparation
-
(7) Ennis, P. M.; Kelly, J. M.; OConnell, C. M. J . Chem. SOC.,Dalton Trans. 1986, 2485. (8) Sassoon, R. E.; Gershuni, S.; Rabani, J. J. Phys. Chem. 1985,89, 1937. (9) Sumi, K.; Furue, M.; Nozakura, S.4. J . Polym. Sci., Polym. Chem. Ed. 1985, 23, 3059. (IO) Ohsako, T.; Sakamoto, T.; Matsuo, T. J . Phys. Chem. 1985,89,222. ( I I ) Kaneko, M.; Yamada, A.; Tsuchida, E.; Kurimura, Y. J . Phys. Chem. 1984.88, 106 I . (12) Kelly, J. M.; Long, C.; OConnell, C. M.; Vos, J. G.;Tinnemans, A. H . A. Inorg. Chem. 1983, 22, 2818. (13) Ohsako, T.; Sakamoto, T.; Matsuo, T. Chem. Letr. 1983, 1675. (14) Matsuo, T.; Sakamoto, T.; Takuma, K.; Sakura, K.; Ohsako, T. J . Phys. Chem. 1981, 85, 1277. ( I 5 ) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helu. Chim. Acta 1978, 61, 2720. (16) Maestri. M.; Sandrini, D. Nouu. J . Chim. 1981, 5 , 637. (17) Chan, S.-F.;Chou, M.; Creutz, C.; Matsuhara, T.; Sutin, N. J . Am. Chem. SOC.1981, 103, 369. ( I 8 ) Kalyanasundaram, K.; Neuman-Spallart, M. Chem. Phys. Lett. 1982, 88, 7. (19) Mandal, K.; Hoffman, M. Z. J . Phys. Chem. 1984, 88, 185. (20) Johansen, 0.;Mau, A. W.-H.; Sasse, W. H. F. Chem. Phys. Lert. 1983, 94, 107. (21) Johansen, 0.;Mau, A. W.-H.; Sasse, W. H. F. Chem. Phys. Left. 1983, 94, I 1 3. (22) Olmstead, J., 111; Meyer, T. J . J . Phys. Chem. 1987, 91, 1649. (23) Olmsted, J., 111; McClanahan, S.F.; Danielson, E.; Younathan, J. N.; Meyer. T. J . J . Am. Chem. SOC.1987, 109, 3297.
The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991 489 of multiderivatized polymers containing controlled loadings of redox-active sites.24 The anthracene-phenothiazine-containing polymer represents an intermediate stage in the preparation of more complex molecular assemblies that contain the components that provide the basis for Scheme I. Ultimately, we hope to utilize these polymeric arrays to achieve long-range electron or energy transfer or to promote multiple energy- and electron-transfer events within a single molecular f r a m e ~ o r k . ~ , In , ~ this ~ study, our goal was to exploit the properties of the bifunctional polymer in a photochemical scheme or schemes. In particular, the goal was to access the combined functions of energy-transfer acceptor at the anthracene groups and the electron-transfer donor capabilities of the phenothiazene groups. Experimental Section Materials. The synthesis and characterization of [PSAn28,SPTZ,,5]have been described previously.24 The starting polymer was a 1: 1 copolymer of styrene and m,p-chloromethylstyrene (60/40),27which consisted of approximately 30 repeating units. The polydispersity of this atactic polymer was found to be -2 by gel-permeation chromatography. After derivatization the individual polymeric strands contain an average of 28.5 anthryl and 1.5 PTZ sites. The anthryl-containing polymer [PS-An2,] used in the Stern-Volmer quenching studies was prepared in a similar manner. Spectral grade methylene chloride and acetonitrile were supplied by Burdick and Jackson and used as received. The compound 10-methylphenothiazine was recrystallized twice from toluene and stored in a desiccator protected from light. The salts [Ru( b p ~ ) J ( p F and ~ ) ~[PQ](PF6)229 ~~ were obtained and purified by standard methods. Methods and Instrumentation. Emission spectra for SternVolmer quenching studies were recorded on a Spex Fluorolog-2 emission spectrometer for steady state measurements. Time-resolved emission measurements were made by using a PRA LN 1000/LN 102 nitrogen laser/dye combination for excitation (A = 460 nm). Emission was monitored at right angles by using a PRA B204-3 monochromator and a cooled, 10-stage, Hamatsu R928 photomultiplier. In these experiments the concentration of [PS-AII~~] was varied from (0-3.0) X lo-, M in a degassed [Ru(bpy),](PF,), solution with an optical density of 0.2 at the excitation wavelength. Solutions for transient absorbance measurements were prepared by combining [PS-An28.sPTZ1,5](0.33 g) and 2.5 mL of a 1 mM acetonitrile solution of [PQ](PF6)2. The solution was diluted to 25 mL with methylene chloride. Under these conditions, [PQ] (PF6)2, which is insoluble in methylene chloride, remains dissolved. Sufficient [Ru(bpy),] (PF6)2was added to achieve an optical density of 0.6-0.7 at 450 nm. Conventional flash photolysis measurements were conducted by using a standard flash lamp apparatus.jO A Corning filter (no. 3-73) was employed to exclude light of X C 400 nm. Samples were thoroughly bubble deaerated with solvent-saturated argon and were maintained under an argon blanket during the experiment. These measurements yielded changes in transmittance with time and were converted to changes in absorbance (AA) for kinetic analysis. Second-order rate constants were extracted from the (24) Younathan, J. N.; McClanahan, S. F.; Meyer, T. J. Macromolecules 1989, 22, 1048. (25) Strouse, G. F.; Worl, L. A.; Younathan, J. N.; Meyer, T. J. J . Am. Chem. Soc. 1989, I l l , 9101. (26) (a) Worl, L. A.; Danielson, E.; Strouse, G. F.; Younathan, J. N.; Baxter, S.; Meyer, T. J. J . Am. Chem. SOC.1990, 112, 7571. (b) Worl, L.
A.; Jones, W. E.; Danielson, E.; Strouse, G. F.; Meyer, T. J. Manuscript in preparation. (27) Arshady, R.; Reddy, B. S. R.; George, M. H. Polymer 1984,35,716. (28) Caspar, J . V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583. (29) Margerum, L. D.; Meyer, T. J.; Murray, R. W. J. Elecfroanal. Chem. 1983, 149, 279. (30) Young, R. C.; Keene, F. R.; Meyer, T. J. J . Am. Chem. SOC.1977, 99, 2468.
Younathan et al.
The Journal of Physical Chemistry, Vol. 95, No. I, 1991
490
h
m
TABLE I: Rate Constants for Electron or Energy Transfer in 1:9 (v/v) CH3CN/CH2C12 at 295 f 2 K reaction k 4.0 X l o t o M-I s - ~ " (PS-An2,] + Ru(bpy)32t* [PS-3AnAn23] R@PY )3,+ 1.1 X 10" M-' s - I b [PS-An28PTZ1.5]+ Ru(bpy),,'* -,+ [PS-3AnAn2,PTZ,,5] Ru(bpy), 3.8 x I 09 M-1 s-1 b [PS-3AnAn27,5PTZl,5] + PQ2' PS-AntAn27,5PTZ,,5]+ PQ' >1.0 x 106 s-I [PS-AntAn27.5PTZ,,5] [PS-An28,5PTZtPTZo,5] [PS-An28,5PTZtPTZo,5]+ PQt 1 .O X IO8 M-' s-I [PS-An28,5PTZl,5]+ PQ2+ IO-MePTZt + PQt 10-MePTZ PQ2' 6.9 X IO8 M-' s - I C
-
2 X Y
+
- + +
f 200. 0
3 U 0 4
0 0
t
0.0 2.0
8. 0 T i m e (mS)
4'
8. 0
10.0
c
"By Stern-Volmer quenching in CH2C12(f4%). bBy laser flash photolysis (f5%). By conventional flash photolysis (f8%).
a minimum of four cycles and were contained in a cell of local design, which allowed for fresh sample to be used for each measurement. The quenching experiments were conducted under conditions that were pseudo first order in quencher.
400.
h
m
5 X
" 0
5 200. 9
0
8 U 0 d
0 o
0.0
'
15.0
'
2h.O
'
3b.O ' T i m e CmS)
45.0
'
34.0
c
Figure 2. Absorbance vs time traces obtained following conventional, visible flash photolysis of a 1:9 (v/v) CH3CN/CH2C12solution containmM), PQ(PF6)2 (0.1 mM), and 10ing: (a) [ R ~ ( b p y ) ~ ] ( p F , )(0.05 , MePTZ (1 mM); (b) [ R u ( b p ~ ) ~ l ( P (0.05 F ~ ) ~mM), PQ(PF&, ( 1 mM), and [PS-An28,5PTZI,5](2.6 mM in anthryl groups, 0.14 mM in -PTZ groups, 0.09 mM in polymer). Absorbance decays were monitored at A,, for PQt (610 nm). Inserts contain plots of l / [ A ] vs time for the temporal data.
slopes of plots of l / A A vs time based on equal concentration, second-order kinetics, by using the relationship in eq 7,31awith ~ ( 5 1 7 )= 8020 M-lcm-' for PTZ+31band~ ( 6 1 0 )= 13800 M-' cm-' for PQ+.31c In eq 7, s is the slope, Ae(X) is the change in k = -sAe(X)b (7) extinction coefficient between the ground state and the transients produced in the flash at the wavelength X, and b is the cell path length in cm. Laser flash photolysis studies were conducted with a Quanta Ray DCR-2A Q-switched Nd:YAG laser pumping coumarin 460 in a PDL-2 dye laser to produce an excitation pulse of ca. 6 ns at
