1937
J. Phys. Chem. 1985,89, 1937-1945 rings, respectively. This suggestion is consistent with the observed Si/Al ratio for ferrierite. The present calculations further indicate that diagonal pairing is favored relative to the meta or ortho pairings in accordance with electrostatic principles and the aluminum avoidance rule. 2. As observed in our previous work on mordenite and ZSM-5, the analysis of the formal atomic charges distributions in the clusters indicates the highly covalent nature of aluminosilicate frameworks. The spreading of the negative charge; associated with the presence of Al, indicates that the anionic framework behaves as weak but a soft base.
The results presented in this paper complement our earlier findings16J7and confirm the general principles which have been delineated to describe the behavior and understand the chemistry of zeolite structures.
Acknowledgment. The authors acknowledge stimulating discussions with Dr. Z. Gabelica and Mr. P. Bodart. They are also indebted to Mrs. B. Norberg and Mr. G. Galet for technical assistance. Registry No. Ferrierite, 12173-30-7;aluminum, 7429-90-5.
Charge Separation in Photolnitlated Electron-Transfer Systems with Polyviologen Polyelectrolytes as Quenchers Richard E. Sassoon, Shlomo Gershuni, and Joseph Rabani* Energy Research Center and Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: March 28, 1984; In Final Form: January 25, 1985)
Various polyviologen polyelectrolyteswhich may participate in photoinitiated electron-transfer reactions have been investigated as quenchers. A spectral study was made of the reduced polyviologen radicals on reducing the polyviologen species using the 2-propanol radical. Some of the mono radicals produced, possessing absorption spectra similar to that of the methylviologen radical cation, are found to produce multiradical species in the time scale of seconds and minutes. The nature of the final reduced products and the types of reactions generating them are discussed. The abilities of the polyviologensto quench the emission of the lowest excited states of the two photosensitizers, R ~ ( b p y ) , ( c N )and ~ Ru(bpy)?+, were investigated together with the quantum yields of electron transfer and the rates of their back-reactions by using the laser flash photolysis technique. The highest yield of electron transfer was found for the Ru(bpy),2+-poly(o-xylylviologen) system where the quantum yield of photoinitiated electron transfer was determined to be 0.57. The results were compared to those obtained previously with methylviologen and other polymeric viologen systems. The higher than expected rates of quenching and back-reactions were attributed to hydrophobic interactions between the bipyridine groupingsof the photosensitizer and quencher which may overcome the repulsive Coulombic forces between them.
Introduction Much work has been concerned with the investigation of the reactions of ions bound to polyelectrolytes in order to learn more about the binding of ions and the ability of polyelectrolytes to accelerate or retard the rates of chemical reactions and to increase their yields in photochemical systems,'-' and this may have particular application in the field of solar energy conversion and storage.'-s Several polymeric photo sensitizer^^-'^ and polymeric quench(1) (a) D. Meisel and M. S. Matheson, J . Am. Chem. Soc., 99, 6577 (1977); (b) C. D. Jonah, M. S. Matheson, and D. Meisel, J . Phys. Chem., 83, 257 (1979); (c) D. Meisel, J. Rabani, D. Meyerstein, and M. S. Matheson, ibid., 82, 985 (1978). (2) S. Kelder and J. Rabani, J . Phys. Chem., 85, 1637 (1981). (3) D. Meyerstein, J. Rabani, M. S. Matheson, and D. Meisel, J . Phys. Chem., 82, 1879 (1978). (4) R. E. Sassoon and J. Rabani, J . Phys. Chem., 84, 1319 (1980). (5) (a) R. E. Sassoon and J. Rabani, Isr. J . Chem., 22, 138 (1982); (b) R. E. Sassoon,Z. Aizenshtat, and J. Rabani, J. Phys. Chem., 89, 1182 (1985). (6) I. A. Taha and H. Morawetz, J . Am. Chem. Soc., 93, 829 (1971). (7) N. J. Turro and T. Okubo, J . Phys. Chem., 86, 1535 (1982). (8) (a) J. M. Clear, J. M. Kelly, D. C. Peppcr, and J. G. Vos, Inorg. Chim. Acta, 33, L139 (1979); (b) M. Furue, K. Sumi, and S. Nozakura, Chem. Lett., 1349 (1981); (c) M. Kaneko, S. Nemoto, A. Yamada, and Y. Kurimura, Inorg. Chim. Acta, 44, L289 (1980). (9) J. M. Calvert and T. J. Meyer, Inorg. Chem., 20, 27 (1981). (10) (a) H. D. Abruna, P. Denisevich, M. Umana, T. J. Meyer, and R. W. Murray, J . Am. Chem. SOC.,103, 1 (1981); (b) J. M. Calvert, J. V. Caspar, R. A. Binstead, T. D. Westmoreland, and T. J. Meyer, ibid., 104, 6620 (1982). (11) (a) M. Kaneko, A. Yamada, and Y. Korimura, Inorg. Chim. Acta, 45, L73 (1980); (b) M. Kaneko, M. Ochiai, N. Kinosita, and A. Yamada, J . Polym. Sci., Polym. Chem. Ed., 20, 1011 (1982). (12) (a) Y. Itoh, Y. Morishima, and S. Nozakura, J. Polym. Sci., Polym. Lett. Ed., 21, 167 (1983); (b) Y. Morishima, T. Hashimoto, Y. Itoh, M. Kamachi, and S. Nozakura, Mukromol. Chem., Rapid Commun., 2, 507 (1981).
0022-3654/85/2089- 1937$01.50/0
TABLE I: Polyviologens Used in This Study Polyviologen
Abbrevi at i on
Structural Formula
Poly o-xylylviologen dibromide \
Polypropylviologen di bromide
c'"3
Poly 2,d-ionenedibromide
N+-(cH,),
\ '
P2.4-MeV methyl viologen dibromide
ers14J5have been synthesized and used in photochemical systems with the intention of preparing a fixed chemical matrix containing 0 1985 American Chemical Society
1938 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
many chromophoric sites and in order to study their ability to physically adsorb onto electrode^^*'^-^^ or colloidal parti~les'~J' in solution, thus creating an interface for charge transfer. Indeed polymeric species containing both photosensitizer and quencher chromophores on the same polymer have been prepared where R ~ ( b p y ) , ~and + viologen moieties are covalently linked to a polystyryl backbone.18 Although emission from the excited ruthenium(I1) centers in the polymer is negligibly small as compared to that from Ru(bpy)32+*,due to efficient quenching by the pendant viologen groups, the overall yield of electron-transfer products on addition of the sacrificial compound, EDTA, is much lower than in the homogeneous system of Ru(bpy)32+and methylviologen (MV2+). This is probably due to low net quantum yields of electron-transfer products and their rapid back-reaction within the polymer. In this study several polyviologen polyelectrolytes were prepared and their reactions with the photoexcited states of Ru(bpy),(CN), and Ru(bpy);+ were investigated. The polyviologens used and their structural formulas are listed in Table I. Polyviologen species have been well-known in the literature for several years,14J5*'9-23 and many of them have been found to be highly conducting redox polymers due to transmission of charge between the viologen moieties along the chain.21,22Some work concerning photoinduced electron transfer using polymer viologen species has been performed as in, for example, the work of Lee et al.,I4 in which viologen derivatives of poly(vinylbenzy1 chloride) and poly(viny1 chloroacetate) were used as quenchers of Ru( b ~ y ) , ~ + *These . polymeric viologens were found to be inferior in every stage of the photochemical system as compared to methylviologen as far as energy storage is concerned. This was attributed to fast migration of charge along the polymer chain and to the low charge density of the polyelectr01yte.l~ The polyviologens prepared for this investigation were chosen in order to study the effects of charge density of the polyviologen polyelectrolytes, their ability to conduct electrons along the polymer chain, and the importance of the position of the viologen chromophore in the polyelectrolyte.
Experimental Section Materials. Poly(o-xylylviologen dibromide) (Po-XV) and poly(m-xylylviologen dibromide) (Pm-XV) were prepared and purified by the method of Factor and Heinsohn,21and the molecular weight of these polymers was around l l OO0.21 Poly(npropylviologen dibromide) (PPrV) and poly(n-butylviologen dibromide) (PBuV) were synthesized by refluxing equimolar amounts of 4,4'-bipyridine and the appropriate alkyl dibromide (13) (a) Y. Itoh, Y. Morishima, and S.Nozakura, J. Polym. Sci., Polym. Chem. Ed., 20,467 (1982); (b) Y. Morishima, Y. Itoh, T. Hashimoto, and S. Nozakura, ibid., 20, 2007 (1982). (14) P. C. Lee, M. S. Matheson, and D. Meisel, Isr. J . Chem., 22, 133 (1982). (15) T. Nishijima, T. Nagamura, and T. Matsuo, J. Polym. Sci., Polym. Left. Ed., 19, 65 (1981). (16) (a) G. J. Samuels and T. J. Meyer, J . Am. Chem. Soc., 103, 307 (1981); (b) H. D. Abruna and A. J. Bard, ibid., 103, 6898 (1981); (c) P. Martigny and F. C. Anson, J. Elecfroanal. Chem. Inferfacial Electrochem., 139, 383 (1982); (d) Y. Morishima, M. Isono, Y. Itoh, and S.Nozakura, Chem. Lett., 1149 (1981); (e) H. Akahoshi, S. Toshima, and K. Itaya, J . Phys. Chem., 85, 818 (1981) (17) R. Laible and K.Hamann, Adu. Colloid Interface Sci., 12,65 (1980). (18) T. Matsuo, T. Sakamoto, K. Takuma, K. Sakura, and T. Ohsako, J. Phys. Chem., 85, 1277 (1981). (19) (a) M. Okawara, T. Hirose, and N. Kamiya, J . Polym. Sci., Polym. Chem. Ed. 17, 927 (1979); (b) M. Okawara, T. Endo, E. Fujiwara, and T. Hirose, J . Macromol. Sei., Chem., A13, 441 (1979). (20) (a) M. S.Simon and P. T. Moore, J. Polym. Sci., Polym. Chem. Ed., 13, 1 (1975); (b) M. Furue, S.Yamanaka, L. Phat, and S. Nozakura, ibid., 19,2635 (1981); (c) R. N. Dominey, N. S.Lewis, J. A. Bruce, D. C. Bookbinder, and M. S.Wrighton, J. Am. Chem. SOC.,104, 467 (1982). (21) A. Factor and G. E. Heinsohn, J . Polym. Sci., Polym. Len. Ed., 9, 289 (1971). (22) (a) Y. Saotome, T. Endo, and M. Okawara, Macromolecules, 16,88 1 (1983); (b) K. Ageishi, T. Endo, and M. Okawara, ibid., 16, 884 (1983). (23) K. Ageishi, T. Endo, and M. Okawara, J . Polym. Sci., Polym. Chem. Ed., 21, 293 (1983). \ -
Sassoon et al.
TABLE I 1 Spectral Data for the Polyviologens' polyviologen radicalb
A,,
polyviologen
nm ,,,e
Po-xv Pm-XV PPrV PBuV P2,4-V P2,4-MeV
264 263
MV2+
257.S
26lC 26lC 260 258
M-' cm-'
,A,,
nm ,,e,
M-I cm-I
20 OOOC 20 OOOC 25 400 12200 =20000d =20000d
610 602 595 595 600 601
12400 14 400 11 600 13 200 11 300 11 700
20100'
609
11 900'
'All spectra were recorded in aqueous solution. Spectra of monoradical or monomer species of the reduced viologen. Reduced polyviologen is prepared by pulse radiolysis or y-irradiation of a heliumsaturated 1 X M solution of the polyviologen containing 1 % v/v acetone and 1% v/v 2-propanol. CTakenfrom ref 21. cannot be determined accurately and is assumed to be similar to that of methylviologen. e r a k e n from J. W. Verhoeven, A.-M. A. Verhoeven, A. Masson, and R. Schwyzer, Helv. Chim. Acta, 57,2503 (1974). /Taken from ref 31 where the viologen radical is produced by the same procedure. in dry acetonitrile for 24 h. The product which precipitated out was filtered off, washed with a little acetonitrile, and dried in vacuo overnight over NaOH. The poly(2,4-ionene viologen) polymer (P2,4-V) was prepared by adding 4,4'-bipyridine to an equimolar mixture of N,N,N',N'-tetramethylethylenediamine and 1,Cdibromobutane in a 1:1 DMF:methanol mixture. The reaction mixture was left at room temperature for 3 days, and the precipitated polymer was washed with benzene and then acetone and finally dried in a vacuum oven at 40 OC for 3 h. N-Methyl-4,4'-bipyridine iodide was prepared according to the method of Hirose et a1.,24and the poly(2,4-ionene methylviologen) polymer (P2,4-MeV) was synthesized as given above for P2,4-V. The molecular weight of the 2,4-ionene polymers was estimated to be around 7000.25 cis-Dicyanobis(2,2'-bipyridine)ruthenium(II) dihydrate, Ru(bpy),(CN),.ZHzO, was prepared and purified according to the literature method,26and all other compounds were of the highest purity available. Water was distilled once and passed through a Millipore Milli-Q water purification system. When necessary, the pH of the solutions was adjusted by addition of HC104 or NaOH, but buffers were not used so that the ionic strengths of the solutions would not be affected. Unless otherwise stated, the pH of the solutions was in the range 5-7. Methods. Steady-state irradiation of solutions was carried out from a Radiation Machinery Corp. cesium-1 37 y-source which gave an absorbed dose of about 1500 rd/min. Dosimetry was carried out using the Fricke d~simeter.~'The pulse radiolysis setup containing a Varian V77 15B linear accelerator has been described before.28 It was operated at 5 MeV with a constant current of 200 mA. Electron pulses of 1-pus duration irradiated the reaction solutions producing about 1 X M total concentration of radicals per pulse. The analytical light source was a 150-W xenon lamp, and the monitoring light passed three times through a 1-cm irradiation cell before reaching the detection system. Absorption spectra were recorded on a Bausch and Lomb Spectronic 2000 spectrophotometer while laser flash photolysis experiments were performed with a Model DL-200 Molectron tunable dye laser (450 pJ, 10 ns) pumped by a Molectron UV-14 pulsed N2 laser (4.0 mJ, 10 ns). The optical setup has been described p r e v i o ~ s l y . ~The ~ signals received by a 1P28 photo~
~~~
~
~~
(24) T. Hirose, N. Shihtani, and M. Okahara, Chem. Absfr. Jpn., 93, P8755 (1980). (25) A. Rembaum and H. Noguchi, Macromolecules, 5, 261 (1972). (26) J. N. Demas, T. F. Turner, and G.A. Crosby, Inorg. Chem., 8 , 674 (1969). (27) K. Sehested, "Manual on Radiation Dosimetry", N. W. Holm and R. J. Berry, Eds., Marcel Dekker, New York, 1970, p 313. (28) R. E. S a w n , Ph.D. Thesis, Hebrew University of Jerusalem, 1983. (29) D. Lougnot, G. Dolan, and C. R. Goldschmidt, J . Phys. E, 12, 1051 (1979).
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1939
Polyviologen Polyelectrolytes as Quenchers 1
I
400
i
I
I
I
500
600
Wavelength
-+-
1 700
400
(nm)
Wavelength
I
400
I
I
500
600
Wavelength
I
I
700
400
700
(nm)
I
I
500
600
I
700
(nm)
Wavelength ( nm) Figure 1. Visible absorption spectra of monomeric viologen radicals of (a) PO-XV,(b) Pm-XV, (c) PPrV, and (d) PBuV measured from the absorption signals observed approximately 5 ws after pulse irradiation of a He-saturated 1 X lo-) M solution of the polyviologen containing 1% v/v acetone and 1% v/v 2-propanol. Concentration of radicals produced is 1.1 X M.
multiplier were transferred to a Nicolet Series 1170 signal-averaging system via a Biomation Model 8100 wave form recorder. Irradiation was carried out at 421 nm, and actinometry was performed to determine the number of photons per pulse by measuring either the bleaching signal a t 450 nm produced approximately 1 ps after laser irradiation of a solution of Ru(bpy),*+/Fe3+ in 0.25 M HC10:O or the absorption signal a t 350 nm of the lowest excited state of R ~ ( b p y ) * ( C N ) 2 . ~
1.2
In
Results and Discussion Spectral Properties of the Polyuiologens and Their Radicals. The UV absorption spectra of all the polyviologens used in this study are summarized in Table I1 and are very similar to that of methylviologen with a maximum at around 260 nm. The extinction coefficients of the absorption peaks were in the range (2-3) X lo4 M-' cm-' except for the case of poly(butylvio1ogen) which was somewhat .lower. Note that, unless otherwise stated, the concentration of the polyviologens is given in terms of viologen units and not in terms of polymer units. The extinction ooeffcients for the P2,4-V and P2,4-MeV polymers are assumed to be similar to that of methylviologen since their values could not be estimated. This is because the samples of P2,4-V and P2,4-MeV used in this study also contained 2,440nene polymer species carrying no viologen groupings, and hence the exact concentration of viologen chromophores could not be calculated. The polyviologen species were reduced by either pulse radiolysis or y-irradiation of deaerated solutions of the polyviologen containing 1% v/v (0.137 M ) acetone and 1% v/v (0.131 M) 2propan01.~' The visible absorption spectra of the monoradical polyviologen species of Po-XV, Pm-XV, PPrV, and PBuV in the wavelength range 400-700 nm were obtained by using the pulse (30) (a) C. T. Lin and N. Sutin, J . P h y ~Chem., . 80,97 (1976); (b) C. T. Lin, W. Bottcher. M. Chou, C. Creutz, and N. Sutin, J . Am. Chem. Soc., 98, 6536 (1976). (31) D.Mcisel, W. A. Mulac, and M. S. Matheson, J. Phys. Chem., 85, 179 (1981).
Wavelength (nm) Figure 2. Visible absorption spectra of reduced PBuV radical species obtained on y-irradiationof a He-saturated 2 X lo-' M solution of PBuV containing 1% v/v acetone and 1% v/v 2-propanol. Concentration of radical species produced is (a) 1.9 X M, (b) 9.2 X M, and (c) 18.3 X M. Path length = 1 cm.
radiolytic method by recording the absorption signal observed 5 after irradiation. The characteristic absorption spectrum of the reduced viologen radical, with a peak a t around 600 nm, is observed, and they are shown in Figure 1. The wavelengths and extinction coefficients of their absorption peaks are summarized in Table 11. The spectra of the reduced polyviologen species are however found to change with time, and the final absorption spectrum obtained depends on the concentrations of the polyviologen and its radical form in solution. The absorption spectra of the final set of reduced products were therefore more conveniently observed following y-irradiation of the polyviologen solutions which gave identical results to those observed using the pulse radiolytic method. Figures 2 and 3 show the visible absorption spectra of reduced PBuV species at various percentage conversions of the polymer. In Figure 2 a solution of 2 X M poly(butylvio1ogen) containing ps
1940 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
600
400
Wavelength
800
produced on y-irradiation of a He-saturated 1 X lo4 M solution of PBuV containing 1% v/v acetone and 1% v/v 2-propanol. Concentration of radical species produced is (a) 1.2 X M, (b) 2.2 X M,(c) 4.1 X M,(d) 6.1 X M, (e) 7.8 X lo-' M, and (f) 9.8 X M. Path length = 1 cm. I
I
Wavelength
I
400
I
(nm)
Figure 4. Visible absorption spectra of reduced Po-XV radical species obtained on y-irradiation of a He-saturated 2 X lo-) M solution of Po-XV containing 1% v/v acetone and 1% v/v 2-propanol. Concentration of radical species produced is (a) 1.9 X M, (b) 9.2 X M, and (c) 18.3 X M. Path length = 1 cm.
2-propanol and acetone was y-irradiated to give the reduced polyviologen species at up to about 10%conversion of the viologen units, while Figure 3 gives the absorption spectra obtained on y-irradiation of a 1 X lo-" M PBuV solution where between 10% and 100%conversion of PBuV to its monoreduced species takes place. At less than 1% conversion of the viologen units (Figure 2a), the absorption spectrum possesses a maximum at 545 nm, while vestiges of the absorption peak at around 600 nm are also observed. On production of more of the reduced PBuV radical species (Figure 2b,c), the peak at around 600 nm gradually disappears while the maximum observed at 545 nm becomes more dominant and shifts to slightly lower wavelengths. On reduction of a 1 X lo4 M solution of PBuV, where a higher fraction of the polyviologen was reduced, as may be seen in Figure 3, the peak at 600 nm disappears completely while the peak initially observed a t 545 nm appears at even lower wavelengths and converges with a lower wavelength shoulder to yield a maximum a t 526 nm at 97.5% conversion. Poly (m-xylylviologen) and poly(propylvio1ogen) behave in a very similar manner to poly(butylvio1ogen) on reduction by y-irradiation, yielding absorption maxima at 524 and 498 nm, respectively, at 97.5% conversion to their reduced forms. The reduction products of poly(o-xylylviologen) are somewhat different from those of the other polyviologens discussed, as may be observed with reference to Figures 4 and 5. At low percentage reductions of this polyviologen, an absorption spectrum, with a peak at 630 nm and a shoulder a t 584 nm, is observed, which remains virtually the same up to about 5% reduction of the polyviologen (Figure 4a,b). Only at percentage reductions of 10% or more of the viologen units does the peak characteristic of a viologen dimer appear at 540 nm which, at the highest percentage reductions studied, converges with a lower wavelength shoulder
800
600
Wavelength
( nm)
Figure 3. Visible absorption spectra of reduced PBuV radical species
1.51
Sassoon et al.
( nm)
Figure 5. Visible absorption spectra of reduced Po-XV radical species produced on y-irradiation of a He-saturated 1 X M solution of Po-XV containing 1% v/v acetone and 1% v/v 2-propanol. Concentration of radical species produced is (a) 1.2 X M, (b) 2.2 X M, (c) 4.1 X M,(d) 6.1 X M,(e) 7 . 8 X M, and (f) 9.8 X M. Path length = 1 cm.
to yield a single maximum at 522 nm as may be seen from Figure 5. To account for these changes in optical absorbances, the type of process which the reduced polyviologen species undergoes must first be considered. These may be either an intrapolyelectrolyte process which may involve electron transfer between the active groups in the polyviologen or an interpolyelectrolyte process which could involve reactions such as disproportionation or dimerization of the reduced polyviologen radicals. Evidence that one process occurs rather than the other may be obtained from the effect of concentration of the radical species. An intrapolyelectrolyte process will yield spectra independent of the radical concentration at least up to the limit of one radical per polymer while the spectra produced from species formed in an interpolyelectrolyte process will be dependent on the radical concentration. Our results for the reduction of Pm-XV, PPrV, and PBuV clearly suggest that an interpolyelectrolyte process occurs since the absorption spectrum of the final products strongly depends on the concentration of the reduced species even when the nature of the initial reduced polyviologen remains unchanged, Le., when all the polymer radicals produced contain only one radical per polymer molecule. We therefore conclude that the absorbance changes observed in the time range of seconds or even minutes after the electron pulse represent interpolymer processes. The proposed interpolymer mechanism is also supported by the fact that the reduced PBuV, PPrV, and Pm-XV species generated by the pulse radiolytic method are found to reach equilibrium in a time range of several tens of seconds, which is of the same order of magnitude as the time required for reaction between positively charged polybrene radicals under similar condition^.^^ The resemblance of some of the spectra, obtained minutes after pulsed or y-irradiation (Figures 2 and 3), to that of the methylviologen cation radical dimer, in which a peak at 545 nm is typical, might suggest that the interpolymer reactions may involve dimerization of the polyviologen radicals. However, dimer formation in these systems is not likely to occur for the following reasons. Firstly, the stability constant for the formation of the dimer of MV+- has been reported to be around 500 M-] and a viologen polymer radical carrying a multiple positive charge would be expected to have a much lower stability of its dimer due to strong electrostatic repulsions between the two polymer radicals in the dimer. This therefore suggests that no dimer formation occurs under our experimental conditions. We therefore suggest, from the dependence of the absorption spectra on the radical concentration, that an interpolyelectrolyte process occurs to yield as the final set of reduced PPrV, PBuV, or Pm-XV products diradicals or even multiradicals. These may possibly further react to produce an intramolecular dimer species, the spectrum of which is similar to the spectrum of the methylviologen dimer. (32) R. E. Sassoon and J. Rabani, J . Phys. Chem., 88, 6389 (1984).
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1941
Polyviologen Polyelectrolytes as Quenchers TABLE III: Rate Constants of Quenching and Back-Reaction and Net Yields of Electron Transfer for the Ru(bpy)2(CN)2-Polyviologen Systems" polyviologen 1O4kl, M-' s-' db 104k2, M-I s-I
Po-xv Pm-XV PPrV PBuV P2,4-V P2,4-MeV MV2+'
1.1 1.1 1.6 2.2 2.5 2.8 4.1
0.13
2.3
0.10 0.1 1 0.07
2.4
2.8
0.08
2.6 3.1 3 .O
0.09
9.4
0.09
OResults obtained by laser flash photolysis and averaged over the signals from 64 pulses. Wavelength of excitation is 421 nm. Number of photons deposited per unit volume per laser pulse is 1.3 X 10l6 photons/cm3 as measured by absorption signal of R~(bpy),(cN)~* in Ru(bpy),(CN), solution. [R~(bpy)~(CN),1 = 5.5 X M; [viologen quencher] = 3 X M. bQuantum yields of electron-transferproducts from reaction 1, corrected for imcomplete quenching and calculated from absorption signal of reduced polyviologen product at 600 nm measured 5 ps after laser pulse. Results of reference experiment using methylviologen as quencher. In the case of poly(o-xylylviologen) no evidence for an interpolyelectrolyte process was observed, and we therefore believe that the reduced Po-XV species produced at low-percentage convsrsion of the parent compound is predominantly a monoradical species in which the additional negative charge may be conjugated over both the 0-xylyl and the viologen groupings. This process, which is completed already 2 s after the electron pulse, will give additional stability to the monoradical species such that it is the major final product at low-percentage conversions of the polyviologen to its reduced state (Figure 4a,b). Only a t percentage conversions greater than about 10% does the presence of multiradical species or intramolecular dimers with the characteristic spectrum of the viologen dimer become significant, as may be seen in Figure 5. Moreover, when the initial concentration of the parent Po-XV to 1 X M, compound in solution is changed from 2 X the spectrum of the final reduced viologen species at these lowpercentage conversions remains identical, suggesting no participation of the polyviologen itself in its formation as may be expected for an intrapolyelectrolyte process. It should be noted here that the phenomena described in order to explain the absorption spectra of all the reduced polyviologens mentioned above occur together with migration of electrons within the polymer chains. Such movement of electronic charge within the reduced polymers takes place via hopping of the electrons between the viologen ~ n i t s ' and ~ ~ is' the ~ ~reason ~ ~ for the very high conductive properties of these polyviologens?' However, this transmission of charge alone will not lead to any overall change in the absorption spectra of the reduced polyviologen species. The two other viologen polyelectrolytes which were investigated in this study, poly(2,4-ionene viologen) (P2,4-V) and poly(2,4ionene methylviologen) (P2,4-MeV), gave on reduction the typical absorption spectrum of the methylviologen cation radical with an absorption maximum at around 600 nm. These spectra were found to remain unchanged over several hours and were obtained even when most of the polymer species were reduced. This reflects the fact that each polyelectrolyte carries only one viologen unit per polymer, and hence no interactions between neighboring viologen group or conduction of charge may occur, leading to any distortion of the regular viologen radical cation spectrum. Of course, negligible dimer formation between two viologen radical species occurs because of the high electrostatic repulsive forces between them. Systems Containing R ~ ( b p y ) ~ ( c I Vas) ,Photosensitizer. The rate constants for the quenching of the lowest excited emitting state of Ru(bpy),(CN);! by the various polyviologens, the yields of net electron transfer, and the rate constants for the subsequent back electron transfer reactions in the photochemical systems are summarized in Table 111. The technique of laser flash photolysis was used to investigate these systems, and laser excitation was carried out at 421 nm. Absorption by Ru(bpy)z(CN)z*at this
wavelength is negligible,4 and hence no biphotonic effects occur even at the highest intensities of the laser light. The quenching reaction is described by eq 1 where PV2+ rep-
+
R~(bpy),(CN)2* PV2+
-
Ru(bpy),(CN)z+
+ PV'.
(1)
resents a polyviologen, and the quenching rate constants were calculated by following the rate of decay of the emission signal from R ~ ( b p y ) , ( c N ) ~at * 600 nm after laser excitation. The net quantum yields of electron transfer and the rate of the back-reaction, given by eq 2, were calculated by following the absorption Ru(bpy)Z(CN)2+
-
+ PV+*
R~(bpy)z(CN)2+ PV2+ (2)
changes at 600 nm where the PV+- species absorbs strongly. Absorption by the R ~ ( b p y ) , ( c N ) ~species + was neglected at this wavelength since the difference spectrum measured in the range 550-650 nm, produced several microseconds after laser irradiation, confirmed that the reduced monoradical polyviologen is the sole absorbing species at 600 nm. The results obtained for these systems were compared to the photoelectron-transfer system containing Ru(bpy)z(CN), as photosensitizer and methylviologen as quencher in order to estimate the polyelectrolyte effects of the polyviologens. The rate of quenching of the emission of R ~ ( b p y ) ~ ( c N )by~ *methylviologen in aqueous solution was measured to be (4.1 f 0.4) X lo9 M-I s-l, which is about 20% lower than the value determined by G a i n e ~ .Quenching ~~ rate constants for all the polyviologens studied were found to be between 1.5 and 4 times lower than the value for MV2+and were found to be pH independent in the pH range 4-10 where measurements were carried out. Several parameters may contribute to the overall rate of quenching of the excited state of R U ( ~ ~ ~ ) , ( C byNthe ) ~polyviologens, and these are discussed below. (1) The rate of quenching of Ru(bpy),(CN),* by MV2+ is quite close to the calculated diffusion-controlled limit of 7 X lo9 M-' s-IJ3 and most of the rate constants for quenching by the polyviologens given in Table I11 are also probably quite close to their diffusion-controlled values. It should, however, be noted that diffusion of the polymers in solution is expected to be considerably lower than that of MVZ+because of their much higher molecular weights, and hence the rate constants of quenching by the polyviologens may be reduced to about half the value obtained for quenching by monomeric viologen species. This is indeed the usual reason given for the reduced rate of reaction observed between a neutral species capable of diffusing freely in solution and a charged species trapped by a polyelectrolyte with a much lower ability to d i f f ~ s e . ~It, has ~ also been suggested as the reason for the lowering of the quenching rate constants of Ru(bpy)32+*with other polymeric viologen species.I4 In high molecular weight polyviologens containing many viologen units per polymer, the viologen moieties are concentrated in specific regions in solution, and hence the average distance each R ~ ( b p y ) , ( c N ) ~molecule * must initially diffuse in order to collide with a quenching viologen unit is greater than in systems containing only low molecular weight species. This will lead to a larger fraction of R ~ ( b p y ) , ( c N ) ~escaping * quenching by the polyviologen and hence to a lower overall rate constant for the quenching reaction. (2) Since the quenching reaction given by eq 1 involves electron transfer, the rate of quenching may be expected to show some dependence on the free energy change of reaction and hence on the reduction potentials of the polyviologens, as has been observed with other viologen q ~ e n c h e r s . ~Very ~ little reliable data are available concerning the redox potentials of the polyviologens although they are probably less negative than that of methylviologen by comparison with values given for similar polyviologens which have appeared in the literature.,O This may therefore cause small increases in the quenching rates although no such increases ~~
(33) G. L. Gaines, J . Phys. Chem., 83, 3089 (1979). (34) (a) E. Amouyal, B. Zidler, P. Keller, and A. Moradpour, Chem. Phys. Lett., 74, 314 (1980); (b) E. Amouyal and B. Zidler, Isr. J . Chem., 22, 117 (1982).
1942 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
are observed in Table 111 probably due to opposing factors. (3) Strong interaction between the 2,2'-bipyridine ligands of the R ~ ( b p y ) ~ ( c N photosensitizer )~ and the 4,4'-bipyridine groups of the polyviologens may be present and may cause considerable acceleration of the quenching reaction. These interactions will also of course occur in the reaction with methylviologen and hence may not seriously affect the ratio of the rate constant of quenching for methylviologen to those for the polyviologens. These interactions will be discussed in more detail in a later section. The overall effects of the various factors discussed above thus result in only small and specific changes in the quenching ability of the polyviologens studied. In addition, since the CN- ligands in R u ( b ~ y ) , ( c N )are ~ in a cis conformation, any dipole which the molecule may possess will be small and should not greatly affect the distribution of the photosensitizer molecules away from the polyviologens. It should finally be noted that the results given in Table 111 show little if any effect of the position of the viologen moiety in the polymer on the rate of quenching. For it is observed that the rate of quenching of the emission of Ru(bpy),(CN),* by P2,4MeV, where the viologen unit is sited at the end of the polymer, is only about 10% higher than that by P2,4-V, and this is about the same as the experimental error in the measurements. The quantum yields of net electron transfer (corrected for incomplete quenching) and the rates of the energy-wasting back-reaction in the R~(bpy)~(CN),-polyviologen systems are also given in Table 111. It should be noted that the back-reactions were found to follow good second-order kinetics and no evidence was found for any complex formation between the electron-transfer products as was observed in the R~(bpy)~~+-dibenzylsulfonateviologen (BSV) ~ y s t e m . ~ The net quantum yields of electron-transfer products were found to vary between 0.06 and 0.13 for the various polyviologens, and these values are all quite close to the value of 0.09 obtained for methylviologen. Thus, no great increase is observed in the net quantum yields of electron transfer as was found for the reaction of excited R ~ ( b p y ) , ( c N ) with ~ Fe(CN)63- on addition of the positive polyelectrolyte polybrene! The rates of the back-reactions were only found to be between 3 and 4 times slower than the rate of the back-reaction between Ru(bpy),(CN)2+ and M V . . The highest quantum yield for electron transfer and the slowest rate of back-reaction measured for the polyviologens investigated in this study were found using Po-XV as the quencher. The lower diffusion of the reduced polyviologen product is expected to reduce the rate of back-reaction to about half the value for the corresponding monomer viologen. The results in Table 111 show that any additional retardation of the back-reaction by further electrostatic repulsion of the photoinitiated electron-transfer products is very small indeed. The charge densities of the polyviologens chosen for this study were all quite high compared to previous viologen polyelectrolytes inve~tigated,'~J~ and hence the potential field of these polyions could be expected to cause a high degree of inhibition. The nonconducting polymers, P2,4-V and P2,4-MeV, give yields of electron transfer and rates of back-reaction similar to those of the other polyviologen quenchers studied in this work, and hence minimal changes are observed by positioning the viologen unit a t the end of the polymer rather than in the center. An explanation previously suggested for higher than expected rates of back-reaction between reduced polyviologens and oxidized photosensitizer molecules was that electron conduction along the polymer chain occurs,leading to a greater cross section for reaction with the reduced polyviologen m o l e c ~ l e . ' ~However, J~ although such electron transmission occurs in the reduced Pm-XV, PPrV, and PBuV species and probably in the reduced Po-XV polymer, it will certainly not take place in the P2,4-V and P2,4-MeV reduced polymer systems which only possess one viologen unit per polymer. Now since the rates of back-reaction between Ru(bpy),(CN),+ and the reduced polymers are very similar for all the polyviologens studied in this work, whether they are conducting or not, electron transmission within the polymer does not seem to provide a feasible explanation for the results obtained. Further
Sassoon et al. TABLE IV: Rate Coastants of Quencl~i~tg and Back-Reaction and Net Yields of Electron Traosfer for the Ru(bpy),*+-Polyviologen Systems4 polyviologen 10-*kp,M-l s-' .j~~ 10-9kd,M-I s-I Po-xv 0.5 0.57 0.9 Pm-XV 1.6 0.12 1.9 PPrV 1 .o 0.16 1.7 PBuV 1.1 0.15 3.9 P2,4-V 5.4 0.14 2.2 P2,4-MeV 13.4 0.14 3.4
MVZCC
0.20
4.5
5.0
Results obtained by laser flash photolysis and averaged over the signals from 64 pulses. Wavelength of excitation is 421 nm. Number of photons deposited per unit volume per laser pulse is 7.2 X lois photons/cm3 as measured by a R~(bpy),~+/Fe~+ actinometer solution. = 3.75 X M; [viologen quencher] = 3 X [Ru(b~y)~*+] M. Quantum yields of electron-transfer products from reaction 3, corrected for incomplete quenching and calculated from average of bleaching and absorption signals measured 5 ps after laser pulse at 450 and 600 nm, respectively. Reference experiment using methylviologen as quencher. a
evidence suggesting that charge migration within the polyviologen radical is not the cause of the high rates of back-reaction observed in photochemical systems containing polyviologen quenchers is given in the next section concerning R ~ ( b p y ) , ~as + the photosensitizer where other possible explanations are also considered. Systems Containing R ~ ( b p y ) ~ ,as ' Photosensitizer. An investigation of photoredox systems containing Ru(bpy),,+ as photosensitizer was also undertaken in this work. Quenching occurs by electron transfer via reaction 3 followed by a reverse electron-transfer reaction regenerating the original ground-state species, given by equation 4. In these systems we wished to
+ PV2+ R ~ ( b p y ) 3 ~++ PV+-
Ru(bpy),,+*
+
+
+ + PV2+
R~(bpy),~+ P V .
Ru(bpy),,+
(3)
(4)
investigate how the positive potential fields of the polyviologen could reduce the rates of quenching and back electron transfer with the multipositively charged R ~ ( b p y ) , ~ +and * R~(bpy),~+ species, respectively. The results obtained are given in Table IV. Laser excitation was carried out a t 421 nm, and the intensity of the laser light was reduced until the biphotonic effect caused by the absorption of an additional photon of laser light by Ru( b p ~ ) ~ ~was + *negligible.35 The yields of the photoinitiated electron-transfer products and their subsequent reactions were monitored by the bleaching and absorption signals at 450 and 600 nm, respectively. The values for the yields of the electron-transfer products and for the rates of their subsequent back-reactions given in Table IV are averaged over the measurements taken at both the monitoring wavelengths which always agreed within 15% of each other, and the back-reactions followed reasonable secondorder kinetics over 2 half-lives at both wavelengths. It may be seen that the trends found in these results with Ru(bpy)t+ as the photosensitizer are quite similar to those found with Ru(bpy),(CN), as the photosensitizer. The most significant feature of the results summarized in Table IV is, with the exception of Po-XV, the lack of any great retardation effect on the rates of quenching and back-reaction and the lack of any large increase in the quantum yields of electron transfer due to the high positive potential fields of the polyelectrolytes, as compared to the methylviologen quencher. However, it should be noted that the rate constants for quenching of R ~ ( b p y ) , ~ +are * about an order of magnitude less than those for quenching of Ru(bpy),(CN),*, and this is also true for quenching by methylviologen itself. It therefore appears that, in the quenching of the excited ruthenium photosensitizers by viologen compounds, electrostatic repulsion between the two reactants may be important in determining the quenching ~
~
~
(35) (a) D. Meisel, M. S. Matheson, W. A. Mulac, and J. Rabani, J . Phys. Chem., 81, 1449 (1977); (b) R. V. Bensasson, C. Salet, and V. Balzani, C. R.Seances Acad. Sci., Ser. B, 289, 41 (1979).
Polyviologen Polyelectrolytes as Quenchers
The Journal of Physical Chemistry, Vola89, No. 10, 1985 1943
rate constant on increasing the positive charge of the ruthenium center. However, the additional electrostatic repulsion on attaching the viologen unit to a positive polymer chain appears to have only a relatively small effect on the rate of quenching. If one compares the rates of quenching of the emission of R ~ ( b p y ) , ~ +by* the various polyviologens, the same factors that were discussed concerning the quenching of Ru(bpy)Z(CN)2* should again contribute to the final quenching rate constant of R ~ ( b p y ) , ~ + *The . P2,4-V and P2,4-MeV polymers quench the emission of Ru(bpy):+* at somewhat higher rates than the other polyviologens. The solutions containing P2,4-V and P2,4-MeV also contain 2,440nene polymers possessing no viologen chromophores. Thus, the total concentration of charged species was much higher than in the other polymer solutions. This may result in a weakening of the potential field of the polymer due to the higher density of oppositely charged ions close to the polymer. The rate constant for quenching by P2,4-MeV is about 2.5 times that for quenching by P2,4-V, and hence the position of the viologen moiety on the polymer is of some importance. When the viologen unit is sited at the end of a polymer, the ease by which R ~ ( b p y ) , ~ +may * react with it appears to be considerably larger than when it is found in the polymer center. The lowest rate constant for quenching of R ~ ( b p y ) ~ is ~ +found * for Po-XV although it produces the highest yield of net separated electrontransfer products of all the polyviologens studied and the rate of their back-reaction is considerably lower than that obtained with the other polyviologens. The quantum yield of 0.57 obtained for * Po-XV is, to our electron-transfer quenching of R ~ ( b p y ) , ~ +by knowledge, the highest yield of charge-separated products ever obtained for quenching of R ~ ( b p y ) , ~ +by * a viologen. The anomalous results for Po-XV as a quencher are probably related to its less negative reduction potential as compared to the other polyviologensZ1and hence to the structure of the various Po-XV species which may lead to an increase in the local charge density on the polymer. The quantum yields of charge-separated products of electron transfer for the other polyviologens are all quite similar, being 20-40% lower than that obtained with methylviologen. The rates of back-reaction between the photoinduced electron-transfer products are all somewhat lower than found with methylviologen, usually by a factor of about 2, with the exception of poly(propylviologen) which is somewhat higher. The reduction in the values of these rate constants can probably be attributed to the lower diffusion of the polyviologen radicals, and any electrostatic repulsion between the photoproduced electron-transfer products is either very small or is compensated by other factors. h e should also note that whereas the rate constants for quenching of Ru( b ~ y ) ~ by ~ +methylviologen * and the polyviologens are about an order of magnitude less than for quenching of R ~ ( b p y ) ~ ( c N ) ~ * , the rates of back electron transfer are approximately the same for both Ru(bpy),,+ and R U ( ~ ~ ~ ) ~ ( CThe N ) rate ~ + .constants for back-reaction of the P2,4-V. and P2,4-MeV. species with Ru(bpy),,+ are similar to the average of those for the other polyviologens, and thus there appears to be only a small ionic strength effect, if any, although back-reaction with viologen units at the end of a polymer chain may be a little faster than when they are sited in the center of the polyion. The results given above in Tables I11 and IV seem to clearly show that electrostatic repulsion of the positively charged Ru( b ~ y ) 2 ( C N ) ~R + ,~ ( b p y ) , ~ + and * , R ~ ( b p y ) ~species ~ + by polyviologen polycations is largely ineffective in inhibiting the rates of the appropriate electron-transfer reactions. Indeed in some cases, such as the back-reaction of Ru(bpy),,+ with PPrV., the rate is faster than that of the corresponding reaction in the methylviologen system. Previous work with polyviologens of lower charge density than those investigated in this study also gave back-reaction rates with Ru(bpy),,+ much larger than that for methylviologen, and they even exceeded the diffusion-controlled limit.I4 The question must therefore be asked of why significant inhibition of reactions of cationic polyviologens with positively charged bipyridine complexes of ruthenium does not occur. It should be noted here that inhibition factors of more than 2 orders
TABLE V Addition of Ferricyanide to the Ru(bpy)?-Polyviologen Systems‘
wlwioloaen Po-xv PPrV P2,4-V
10-*k..6 M-’ 1.1 1.4 6.1
s-l
&c
&d
0.23
0.42
0.09
0.17 0.15
0.10
‘Experimental conditions as in Table IV but with addition of 2 X lo4 M Fe(CN)& to solutions. bRate constant for quenching of Ru(bpy)?+*. cQuantum yield of reduced polyviologen product as measured by absorption signal at 600 nm. dQuantum yield of Ru(bpy),)+ as measured by bleaching signal at 450 nm. of magnitude have been obtained on addition of a polyelectrolyte to chemical systems containing reactants of opposite charge.3638 It has previously been suggested that fast migration of charge within the polyviologen leads to a greater effective reaction cross section which counteracts the effect of electrostatic repulsion between the reacting species thereby leading to a faster rate of r e a ~ t i 0 n . lMoreover, ~ the polyviologens studied before this work all possessed relatively low charge densities and hence significant electrostatic repulsion by these polyions may not be attained. The polyviologens investigated in this work, however, were both of much higher charge density, and furthermore only some were capable of conducting charge along the polymer on reduction. The results obtained in this study seem to suggest, therefore, that electron transmission is not the reason for the fast rates of reaction, particularly on consideration of the following points. Firstly, the behavior of P2,4-V and P2,4-MeV, which possess only one viologen unit per polymer and hence cannot transmit charge, is very similar to that of the other polyviologens regarding their reactions in the photochemical systems containing Ru(bp~)~(cN and ) ~Ru(bpy),2+. The apparently high rates of quenching of Ru(bpy)?+* by these two polyviologen species are attributed to the higher concentration of anions. Secondly, the polyviologens in their oxidized forms are not expected to conduct electrons, and hence no explanation is provided by the electron migration theory for why little if any inhibition by Coulombic repulsion between the two reactants is observed in the quenching reaction of Ru(bpy):+* by all the polyviologens as compared to quenching by methylviologen. Finally, additional experiments were carried out by adding 2 X 10-4 M ferricyanide to solutions of Ru(bpy)?+ and polyviologens with the intention of “doping” the polymer chains with low-energy acceptors which may trap any migrating electrons. The results are summarized in Table V. Evidence that the ferricyanide ion is situated in the potential field of the polyviologen is observed by a red shift and a lowering of the 420-nm absorption maximum in the ferricyanide spectrum. The rates of quenching of the emission of Ru(bpy)t+* in these solutions are seen to be somewhat larger, and in the case of Po-XV, the quenching rate constant is more than double that in the absence of Fe(CN)63-. This may be attributed to partial neutralization of the polyelectrolytic charge by the ferricyanide ion, leading to a faster rate of reaction 3 as well as perhaps to direct reaction of R ~ ( b p y ) , ~ +with * Fe(CN)63via reaction 5 . It is not easy to distinguish whether one or both of the quenching processes are occurring although an experiment
was performed with a similar solution to those used in Table V except where the polyviologens were replaced with the inert positive polyelectrolyte polybrene. N o quenching of the R ~ ( b p y ) , ~ + * excited state by ferricyanide was detected under these conditions. However, an analogous situation appears not to pertain to the polyviologen solutions since the lower repulsive forces against the (36) (a) H.Morawttz and J. A. Shafer, J . Phys. Chem., 67,1293 (1963); (b) T.Okubo, T. Maruno, and N. Ise, Proc. R.SOC.London, A, 370, 501 (1980). (37)S. Kunugi and N . Ise, 2.Phys. Chem. (Wiesboden),92,69 (1974). (38)T.Ishiwatari, T.Maruno, M. Okubo, T. Okubo, and N. Ise, J . Phys. Chem., 85,47 (1981).
1944 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
approach of Ru(bpy)?+* may allow quenching to take place via both of the reactions 3 and 5. The quantum yields of charge-separated electron-transfer products were determined at the monitoring wavelengths, 450 and 600 nm, where the relative yields of R ~ ( b p y ) , ~and + reduced polyviologen could be observed, respectively. At 450 nm the yields of Ru(bpy),,+ remain virtually unchanged by the addition of Fe(CN)63- except when Po-XV is the quencher when the yield of R ~ ( b p y ) , ~is+ reduced by about a quarter. The yields of the reduced polyviologen radicals are however all lowered to just about half of the corresponding Ru(bpy),,+ yield on addition of 2 X lo4 M Fe(CN),3- in these experiments. The higher yield of Ru(bpy),,+ over reduced polyviologen may be explained if quenching via reaction 5 leads to the production of chargeseparated products or if rapid scavenging of some of the polyviologen radicals by ferricyanide takes place before they escape into the bulk of solution. The subsequent reactions of the electron-transfer products were also followed at the two monitoring wavelengths, and at both wavelengths the signals decayed to zero by complex kinetics which were neither first nor second order. The reactions responsible for these decays are probably those given by eq 4,6, and 7. Diffusion
Ru(bpy),)+
+ Fe(CN);'
-
R ~ ( b p y ) , ~++ Fe(CN),,-
(7)
of Fe(CN)63- in the potential field of the polyviologen followed by reaction with the polyviologen radical according to eq 6 may compete with the regular back-reaction given by eq 4. The Ru(bpy)?+ remaining may then react with the ferrocyanide produced via reactions 5 and 6 in reaction 7 . It should be noted that the decay of electron-transfer products in these reactions was about an order of magnitude shorter than in the absence of ferricyanide which is the opposite of what is expected if fast electron migration along the polymer chain in the reduced polyviologen is the reason for the lack of any inhibition of the back-reactions. For in such a case, the localizing of the reduced site on one ion in the polymer field would be expected to lead to a lower reaction cross section and hence to a slow back electron transfer via either reaction 4 or 7. N o slow decay of any residual signal corresponding to an impairment of either of these reactions was observed. It therefore appears that electron migration within the polyviologens is not responsible for the higher than expected rates of reaction with the cationic polypyridine ruthenium complexes. This implies that the electron-hopping process is too slow to be important, and no significant charge transmission along the polymer takes place by this mechanism in the duration of an encounter. A possible explanation for the phenomena observed in this investigation may lie in the fact that interactions which occur between a polyion and other species in solution may not necessarily be purely electrostatic in nature. In many cases association between the hydrophobic residues of the interacting entities in solution may also occur.7~13*3841 Quenching of the fluorescence of amphiphilic polymers containing both polyelectrolytic and hydrophobic segments has indeed sometimes been found to take place at above the diffusion-controlled rate because of hydrophobic association between the polymer-linked photosensitizer and quencher.', Other forms of specific interactions also occur, leading to site binding of ions on a polyelectrolyte via the formation of chelate complexes with coordination p o l y i o n ~or~ ~of chargetransfer complexes with electron-donating or -accepting polyions.43,44 (39) (a) T. Okubo and N. Ise, J . Am. Chem. SOC.,95, 4031 (1973); (b) N. J. Turro and I. F. Pierola, J . Phys. Chem., 87, 2420 (1983). (40) Y. Kurimura, H. Yokota,K. Shigehara, and E. Tsuchida, Bull. Chem. SOC.Jpn., 55, 55 (1982). (41) T. Takagishi, H. Kozuka, and N. Kuroki, J . Polym. Sci., Polym. Chem. Ed., 21,447 (1983). (42) (a) H. Morawetz, A. M. Kotliar, and H. Mark, J . Phys. Chem., 58, 619 (1954); (b) M. Mandel and J. C. Leyte, J . Polym. Sci., A l , 2883, 3771 (1964); (c) Y. Ueba, K. J. Zhu, E. Banks, and Y. Okamoto, J . Polym. Sci., Polym. Chem. Ed., 20, 1271 (1982); (d) A. Szilagyi and I. Vancso-Szmercsanyi, ibid., 21, 2225 (1983).
Sassoon et al. Thus, in the photochemical systems studied in this work, we invoke the existence of, in addition to the strong Coulombic repulsive forces between the reacting species, other highly specific interactions between the 2,2'-bipyridine ligands of the ruthenium complexes and the 4,4'-bipyridine groupings of the polyviologens, similar to those responsible for ion pairing between R ~ ( b p y ) , ~ + and m e t h y l v i ~ l o g e nand ~ ~between ~ ~ ~ Ru(bpy)? and BSV-S.~They may also be similar to the strong hydrophobic interactions observed between the 2,2'-bipyridine or 1,lO-phenanthroline ligands and the phenyl group of poly(styrenesu1fonate). For the rate of the complexation reaction of these ligands with NiZ+is accelerated by a factor of up to 20 on addition of this anionic polyele~trolyte~~ whereas no such acceleration was observed in the presence of poly(ethylenesu1fonate). Similarly, Ru(bpy),2+ is found to be much more tightly bound to poly(styrenesu1fonate) than to poly(viny1 sulfate) from measurements of the adsorption of Ru(bpy),2+ on cation-exchange resins in the presence of the polyanions.@ This is again attributed to hydrophobic association of the bipyridine ligands of R ~ ( b p y ) , ~to+ the phenyl groups of poly(styrenesu1fonate) .40 No evidence for complex formation between the polyviologens and the ruthenium complexes could be observed, and the rates of both the forward and backward reactions taking place in the photochemical systems were not found to exceed the diffusioncontrolled limit as has been observed previously for reactions of other p o l y v i ~ l o g e n s ' ~and ~ ' ~other polyelectrolytes where hydrophobic interactions are important.', This is attributed to the fact that any net attractive force which exists after repulsive electrostatic interactions are overcome will usually be quite small and of very short range. Thus, only a small fraction of the ruthenium species in solution will be associated with the polyviologens, and the rate-determining steps of the processes being observed will involve diffusion of the excited or oxidized photosensitizer molecules to the polyviologens. It should be noted that the nonCoulombic interaction may affect both the oxidized and reduced forms of all the polyviologens including P2,4-V and P2,4-MeV and hence may explain the relatively fast rates of quenching of Ru(bpy)32+*by the polyviologen. The same forces will also of course act on the reacting species even in the presence of ferricyanide. Conciusions
In this work an investigation of polyviologens as polyelectrolytic quenchers of the emission of R u ( b ~ y ) ~ ( c Nand ) ~ *Ru(bpy)?+* has been investigated. The most striking aspect of this work is probably the importance of hydrophobic interactions in determining the kinetics of the systems. Such interactions were sometimes found to completely dominate the electrostratic forces present in the system. However, the results from these photochemical systems have suggested other, as yet unresolved, questions. For example, how does the molecular weight of the polyelectrolyte affect the kinetics of these systems? The charge density of the polyions may influence their conformation in solution-what roles do these factors play in such systems? Interesting results may be obtained by varying the ionic strength of the reaction solutions. In the case of polyviologens charge migration along the polymer occurs, but the question still remains of whether pendant viologen groups can arrange for charge migration more easily than viologen moieties lying in the polymer backbone as studied in this work. A real measure of the rate of charge migration could be determined perhaps by following the disappearance of the reduced polyviologen radical, produced radiolytically or in a sacrificial photochemical system, in the presence of one or two Fe(CN),,- ions per molecule in polyviologens of various molecular weights and with various degrees of doping of viologen units. Further work on these and other questions concerning the role of polyelectrolytes in photochemical systems and (43) W. Slough, Trans. Faraday SOC.,55, 1030 (1959). (44) W. Slough, Trans. Faraday SOC.,58, 2360 (1962). (45) (a) M. A. J. Rodgers and J. C. Becker, J . Phys. Chem., 84, 2762 (1980); (b) K. Kalyanasundaram and M. Neumann-Spallart, Chem. Phys. L e x , 88, 7 (1982).
J. Phys. Chem. 1985,89, 1945-1947 their consequences for solar energy conversion is being carried out in this laboratory.
Acknowledgment. This research was supported by the Israel National Research Council, The Balfour Foundation, and the
1945
Schrieber Hebrew University Center for Hydrogen. Registry No. Po-XV, 31531-96-1; Pm-XV, 32218-99-8; PPrV, 69860-51-1; PBuV, 37584-31-9; P2,4-V, 95864-72-5; P2,4-MeV, 95864-73-6; Ru(bpy),(CN),, 20506-36-9; Ru(bpy),*+, 15158-62-0; Fe(CN);, 13408-62-3.
Correct Assignment of the Low-Temperature Lumlnescence from 9-Nltroanthracene Satoshi Hirayama,* Faculty of Textile Science, Kyoto Technical University, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Ybichi Kajiwara, Toshihiro Nakayama, Kumao Hammoue,* and Hiroshi Teranishi Department of Chemistry, Faculty of Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku. Kyoto 606, Japan (Received: May 14, 1984; In Final Form: December 26, 1984)
9-Nitroanthracene (9-NA) is found to exhibit low-temperature luminescencewith band maxima at 685 and 760 nm in EPA (ether/isopentane/ethanol = 5 5 2 in volume ratio) at 77 K. This luminescence is assigned as the intrinsic phosphorescence of 9-NA on the basis of the spectral shape of the luminescence, the excitation spectrum, the emission lifetime, and the photochemical reactivity of 9-NA. Thus the previous assertion made by Snyder and Testa that 9-NA does not phosphoresce is refuted (Snyder, R.; Testa, A. C. J . Phys. Chem. 1981, 85, 1871).
Introduction The nitro group is well-known as a fluorescence quenching substituent.’ For instance, fluorescence in 9-nitroanthracene (9-NA) is totally absent even at 77 K. In this molecule, the nitro group appears to enhance singlet to triplet intersystem crossing significanty by providing a triplet n?r* intermediate state as has previously been proposed by us.* Recently, Snyder and Testa3 disputed our proposal, since they were unable to observe any phosphorescence or triplet state for 9-NA which should be generated as a result of such an efficient intersystem crossing from the lowest excited singlet TA* to higher triplet n?r* states. From their study on the photolysis of 9-NA at 77 K, they were led to the conclusion that we did not measure the T’ TI absorption and lifetime inherent to 9-NA but rather those of anthraquinone formed via an efficient photochemical reaction. If their conclusion is correct, a photochemical reaction must be responsible for the lack of fluorescence of 9-NA. Unfortunately, however, their conclusion is based on insufficient experimental results and erroneous interpretation. Therefore, it is the purpose of the present work to show that 9-NA itself actually phosphoresces in the same wavelength region in which the phosphorescence of anthracene is observed.
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Experimental Section Materials. 9-NA was synthesized according to the literature4 and purified by recrystallization from ethanol. When necessary, it was further purified by sublimation or thin-layer chromatography. Anthracene of a scintillation grade was used as received. Isopentane of a guaranteed grade was purified by passing through a column of silica gel (200 mesh). Spectroscopic grade ethanol, diethyl ether, and methylcyclohexane were used as received. (1) Wehry, E. L.; Rogers, L. B. ‘Fluorescence and Phosphoresccnce Analysis”; Hercules, D. M., Ed.; University of Tokyo Press: Tokyo, 1966; p 89. (2) Hamanoue, K.; Hirayama, S.;Nakayama, T.; Teranishi, H. J . Phys. Chem. 1980.84, 2074. ( 3 ) Synder, R.;Testa, A. C. J . Phys. Chem. 1981, 85, 1871. (4) Braun, C. E.; Cook, C. D.; Merritt, C. Jr.; Roasseau, J. E. Org. Synth. 1951, 31, 17.
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Apparatus and Procedures. Irradiations were carried out at 77 K using an USH-SOOD super-high-pressure mercury lamp. Light of 366-nm monochromatic wavelength was selected by the combination of Toshiba UV-35 and UV-D35 glass filters and a filter solution (CuSO4.5Hz0, 100 g dm-’, pathlength 3 cm). The absorption spectrum was taken at 77 K with a Hitachi 200-20 spectrometer. Phosphorescence spectra were taken on either a Hitachi M P F 4 spectrophosphorometer or a home-built phosphorometer which consists of a Ritsu N20 monochromator with a Toshiba Y-46 glass filter to cut off the light below 460 nm, a rotating sector, a transparent Dewar vessel, a 500-W high-pressure mercury arc with a Toshiba UV-DIB glass filter to isolate the 366-nm light, and an N F Model LI-574A lock-in amplifier. In either case, an HTV R928 photomultiplier whose spectral response well extends to 900 nm was employed. The excitation spectra of the phosphorescence were measured m the M P F 4 spectrophosphorometer. The phosphorescence and excitation spectra were not corrected for the spectral response of the equipment. Both commercial and home-built spectrophosphorometers gave almost the same phosphorescence spectra, but because of the higher exciting light intensity, weak phosphorescences could be taken on the latter with less noise. Thus, the phosphorescence lifetime was measured on the latter. The procedure to measure the phosphorescencelifetime was the same as that previously r e p ~ r t e d . ~ The sample solutions were degassed by several freeze-pumpthaw cycles.
Results When weak luminescence is measured, great care must be taken if highly luminescent photoproducts are produced. This is true of 9-NA which readily yields anthraquinone upon irradiation. In fact, Synder and Testa’ observed that the phospohrescence intensity due to anthraquinone increased during irradiation of 9-NA in ethanol at 77 K, but they failed to observe any luminescence inherent to 9-NA. We obtained the same result during the repeated spectral measurements in aerated ethanol and EPA (ether/isopentane/ethanol = 5:5:2 in volume ratio) at 77 K. On ( 5 ) Hirayama, S . J. Chem. SOC.,Faraday Trans. 1 , 1982, 78, 2411.
0 1985 American Chemical Society
