6648
J. Phys. Chem. 1995, 99, 6648-6651
Effect of Amino Acid Residue Model on the Photoinduced Long-Distance Electron Transfer from the Excited Ru(bpy)32+ to Methylviologen in a Polymer Film Keiji NagaiJ Jiro Tsukamoto, and Nobuo Takamiya Department of Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169, Japan
Masao Kaneko*g$ The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi, Saitama 351-01, Japan Received: October 1I, 1994@ Photoluminescent R ~ ( b p y ) 3 ~complex, + electron-accepting methylviologen, and an amino acid residue model compound were dispersed in a polysiloxane film, and photoinduced electron transfer from the excited Ru( b p ~ ) 3 ~to+methylviologen and the effect of amino acid residue model on the electron-transfer distance have been studied. The electron-transfer reaction was analyzed by a static model, and the electron-transfer distance was calculated considering the distance distribution between the nearest neighboring molecules as well as their excluded volume effect. In the absence of the amino acid residue model, the electron-transfer distance was obtained to be 1.4 nm. p-Cresol, a model compound of tyrosine residue, did not affect the electrontransfer distance, but 3-methylindole, a model of tryptophan residue, increased the electron-transfer distance to twice (2.7 nm) that without it.
Introduction There exist a lot of arguments as to an electron tunneling pathway for long-range electron transport. In recent years, electron transport in biological systems has attracted much attention in this regard. It is suggested that some amino acid residues of proteins may behave as electron pathway molecules in biological systems. In ruthenium-modified cytochrome c-cytochrome c peroxidase systems, electron transfer from ruthenium(I1) to iron(II1) was studied2 and analyzed by considering electronic coupling of a donor center, amino acid residues, and an acceptor m ~ l e c u l e . ~In the photosynthetic center of green plants, it has been reported that electron transfer takes place through aromatic amino acid tyrosine residue^.^ In some artificial electron-transfer systems in which donor (photoexcitation center) and acceptor molecules are covalently bound, electron-transfer rate depends on the chemical structure of the binding spacer such as conjugatednonconjugated structure or singlebadder type chain.5 In these electron-transfer studies, modified enzyme systems or designed donor-acceptor relay systems have been used, but such modification, synthesis, or characterization sometimes has a difficulty. One of the present authors found that solid-phase electron transfer from photoexcited Ru(bpy)32+to methylviologen takes place in a natural polymer matrix.6 Afterward, some other polymer systems containing electron donors and acceptors have also been studied, and an electron-transfer distance was obtained considering the distance distribution between electron donor and a ~ c e p t o r . ~Although -~ the distribution of molecules has to be considered to obtain an electron-transfer distance, such a dispersion system has merits such as simple structure, ease of the preparation, and direct applicability to practical device^.'^-'^ The electron-tunneling pathway effect in such a solid-phase random dispersion system has not been reported yet. We have
* Address correspondence to this author. Present address: Kanagawa Academy of Science and Technology, Tokyo Institute of Polytechnics, 1583 Iiyama, Atsugi-shi, Kanagawa 24302, Japan. Present address: Department of Chemistry, Ibaraki University, 2-1-1 Bunkyo, Mito 310, Japan. @Abstractpublished in Advance ACS Abstracts, April 1, 1995.
investigated the pathway effect in electron transfer in a polymer film system containing tris(2,2'-bipyridine)ruthenium(II) (Ru( b ~ y ) 3 ~ +and ) l,l'-dimethyl-4,4'-bipyridinium (methylviologen: MV2+), for which electron transfer takes place from the photoexcited Ru(bpy)3*+to MV2+. Such a system can also work as a photoinduced charge separation center model for photoenergy conversion. l4,I5 The electron-transfer reaction has been studied by using the quenching of photoluminescence from the excited R~(bpy)3~+ by MV2+. p-Cresol and 3-methylindole are used as the model compound for tyrosine and tryptophan amino acid residues, respectively. We have found that the electrontransfer distance doubles in the presence of 3-methylindole and report the result.
Experimental Section R~(bpy)3C12*6H20~~ and polysiloxane containing carboxyl groups at side chains17were prepared by the methods reported earlier. Methylviologen dichloride was purchased from Sigma Chemical Co. and recrystallized from a methanol solution. 3-Methylindole and p-cresol were of commercially available purest grade. 3-Methylindole was recrystallized from its ligroin solution. Fixed amounts of polysiloxane, Ru(bpy)3Cly6H20, methylviologen dichloride, and 3-methylindole (or p-cresol) were dissolved in ethanol to obtain a concentration of the total solutes of 0.02 g / d . The solution was cast on a glass plate and allowed to dry under vacuum at 35 "C to prepare a polymer film. The film thickness was estimated to be ca. 1 pm. The R ~ ( b p y ) 3 ~concentration + in the film was fixed to be 0.05 M (M = mol per film volume dm3, estimated by using a film density of 1.3 g ~ m - to ~ )minimize concentration quenching of the photoluminescence from R~(bpy)3~+*.'* At this concentration, only 10% of the photoexcited R~(bpy)3~+ is subjected to concentration quenching.19 All the measurements were carried out at 25 "C under argon. The irradiation was with 450-nm monochromatic light incident on the front surface of the film, and the emission was monitored from the back side of the glass plate at a right angle from the excitation light to minimize the excitation light scattering effect. The emission decay was
0022-3654/95/2099-6648$09.oo/o 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 17, 1995 6649
Effect of Amino Acid Residue Model
4'0 3.5
t
bl O /
L
0.00
0.05
0 15
0.10
0.20
[MV"]
g
Figure 2. Stem-Volmer plots for the electron-transfer quenching of photoexcited Ru(bpy)3*+;the upper curve is based on the relative Q1 ksV[MV2+]; the lower line emission intensity (0),( Q w ~ + ~ o ) /= is based on the longer emission lifetime (O), t& = 1 -tkSV[MV2+].
0.5
+
E 0.0 0.00
0.05
0.10
[MPl
Figure 1. Lifetime
0.15
0.20
0.25
[mol dm-7
its fraction of excited,Ru(bpy)?+ in a polysiloxane film as a function of M V + concentration. (TI)and
measured by a time-correlated single photon counting apparatus (Hitachi-Horiba NAES-1100) equipped with a 10 atm hydrogen lamp; the emission was monitored through a cutoff filter (Toshiba 0-58) to minimize the scattering.
Results and Discussion The visible absorption and emission spectra of the films showed maxima at 460 and 600 nm, respectively, similar to those of a conventional Ru(bpy)P solution. All the films used in the present experiment were homogeneously transparent and orange colored. At f i s t the electron-transfer quenching of the emission from R~(bpy)3~+* by MVZ+ was studied without a mediator. The emission quantum yield decreased with higher MV2+ concentration. The emission decay curve of the film did not show a single-exponential decay but could be analyzed satisfactorily by a double-exponential decay (eq l), Z(t)
= A l exp(-f/z,)
+ A, exp(-t/z,)
(1)
where Z(t), t l and z2, and A1 and A2 represent the intensity of the emission at t seconds, decay times of the emission, and preexponential factors, respectively. The major and longer decay time (TI) was 1.1 ps and independent of the MVZ+ concentration (Figure 1). In the time region before 80 ns of the emission decay curve, the shorter lifetime (z2) is influenced to some extent by scattering of the excitation light, which is often a problem for such solid systems, but z1 is reliable. The relative emission yield of the shorter lifetime (z2) component obtained here [A&(Altl Azz~)]was 5-20%, indicating most of the emission was caused by the t1 component. Generally, a dynamic quenching which takes place by diffusion and collision of molecules is suppressed entirely or to a great extent in a solid matrix, and a static quenching occurs. However, dynamic quenching can take place partly even in a solid matrix.* If the quenching obeys a dynamic mechanism, the emission lifetime decreases with the increase of the MV2+ concentration. In a static quenching, when a quencher molecule is present in a quenching sphere (radius Ro) around a luminescent probe, the emission is quenched entirely. A probe which does not contain any quencher in the quenching sphere is not quenched at all. Therefore, when only static quenching takes place, the emission lifetime does not change with the degree of the quenching. In the present quenching system, in spite of the change of the relative emission yield with MV2+ concentration, the emission lifetime did not change with the MV2+ concentration
as shown in the Stern-Volmer plots (Figure 2 ) . Therefore, the quenching is regarded to take place by a static mechanism. Generally, the probability that the nearest neighbor to a molecule occurs between r (nm) and r dr (nm) is represented by P(r,c) dr at random dispersion, and the probability density P(r,c) is expressed by eq 2,20
+
P(r,c) = 4n?NA10-24c exp[-4n(r3 - ~ ~ ) N , l O - ~ ~ c / 3 (2) ] where NA, s, and c are Avogadro's number, the radius of the excluded sphere in which another molecule center cannot exist (nm), and the concentration of the dispersed molecule (mol dm-3),respectively. The factor of transforms the volume unit dm3 to nm3, In this equation, the preexponential factor, ~ X ~ N A ~ Ois -the ~ ~probability C, density that a molecule exists on a surface of the sphere with r (nm) radius. The exponential term, exp[-4n($ - s3)NA10-24c/3],is the probability that no molecule exists within the same sphere. When the excluded volume is approximated as a sphere, its radius (s) is the sum of the radii of the probe and the quencher. The radius of Ru( b p ~ ) 3 ~was + obtained from the crystallographic data as 0.41 nm,21,22 and that of MV2+was taken as 0.41 nmZ3in a maximum case, which led to the s value of 0.82 nm. Generally, in a static quenching in a rigid matrix, the probe containing a quencher in its quenching sphere (the distance is shorter than the quenching sphere radius: Ro) is quenched, and therefore the relative emission ratio (@/(@'[MVZ+I=O)) is equal to the probability that MV2+ is out of the quenching sphere (K$(r,c) dr) (eq 3). (3)
+
In the present system, Ro is regarded as the electron-transfer distance (center to center) between R~(bpy)3~+* and MVZ+.In the present system without an amino acid residue model, the conventional Stem-Volmer plots showed an upward deviating curve (Figure 2 ) . However, they showed a linear relationship according to the plots of l n [ ( " [ ~ ~ z + ~ = ~ )versus / @ ] [MV2+] as shown in Figure 3, and from the slope (=4z(Ro3 - S ~ ) N A ~ O - ~ ~ / 3) the electron-transfer distance Ro is calculated to be 1.4 (nm). This electron-transfer distance is similar to the previous reports on electron-transfer reactions in biological and artificial
system^.^*^-^^^^ In order to study the effect of the amino acid residue model compound on the present electron-transfer reaction, a model compound was coexisted in the film. p-Cresol and 3-meth-
6650 J. Phys. Chem., Vol. 99, No. 17, 1995 1.01
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/
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Nagai et al.
0.05
0.10
0.15
case A
case B
0.20
(MV"]
W
Figure 3. Plots of ln[(@~~vz+]=o)/@] versus MVZ+concentration for the electron-transfer quenching of excited Ru(bpy)sZ+by MVZf in
polysiloxane film in the absence of 3-methylindole.
case C
14-
1210-
J
0.00
t
1
0.05
0.10
0.15
0.20
[MPl
Figure 4. Stem-Volmer plots for the electron-transfer quenching of photoexcited R~(bpy)3~'in the presence of IND. Relative emission intensity in the presence of 0.2 mol dm-3 IND (0)and 0.05 mol dm-3 IND (0)and without IND (0). Relative emission lifetime (0)in the presence and absence of IND. 2.5-
0
0.00
005
0.10
0.15
0.20
[MV"]
Figure 5. Plots of ln[(@w~z+l=o)/@] versus MV2+ concentration for the electron-transfer quenching of excited Ru(bpy)sz+by MVZf in polysiloxane film in the presence of 0.2 mol dm-3 IND (0)and 0.05 mol dm-3 IND (0)and without IND (0);the line and curve show theoretical values based on eqs 4 and 6, respectively, calculated by using a nonlinear least-square method.
ylindole were used as the model compounds of tyrosine and tryptophan residues, respectively. It was confirmed that both the model compounds did not work as quenchers for the photoexcited Ru(bpy)32+. p-Cresol did not affect the electrontransfer quenching from Ru(bpy)3*+*to MV2+. However, the electron-transfer quenching was enhanced by 3-methylindole as shown in the Stem-Volmer plots (Figure 4). The longer and major lifetime component tl (1.1 ps) was independent of the MV2+ concentration, similar to that in the absence of 3-methylindole. The relative emission yield of the shorter lifetime ( t 2 ) component (6-20%) was low. From the Stem-Volmer plots in the presence of 3-methylindole, the quenching is regarded to take place by a static mechanism also in the presence of 3-methylindole. The ln((@[Mvz+l=o)/@)versus [MVZ+]plots in the presence and absence of 3-methylindole (Figure 5 ) show enhancement of the quenching by 3-methylindole. As for the effect of this compound, there would exist four possibilities. One might be an interaction between IND and MV2+. The second would be
Figure 6. Schematic representation of the quenching of R~(bpy)3~+* by MVZf in the absence and the presence of mediator (IND).Case A, MVZ+is out of the quenching sphere whose radius is Ro; case B, MVZ+ is within the quenching sphere; case C, Ru(bpy)3z+*is quenced by MVZ+mediated by IND, for which the distance from the Ru(bpy)3*+ center to the nearest neighboring MVZfis more than ROand less than R'o, and the mediator exists within the volume V,.
a solvation of MV2+ by IND. The third would be simple plasticizer effect by which the diffusivity of the dispersed molecule increases to result in dynamic quenching and thereby to enhance the process. The fourth would be enhancement by making a pathway for the electron transfer. We found no change in the W-vis and IR absorption spectra of MV2+by the addition of IND.Therefore, we would conclude that there is no significant interaction between MV2+ and IND. Generally, the emission lifetime reflects the microenvironmental change of the probe. In the present Ru(bpy)3*+ system, the emission lifetime in the presence of IND was the same as that in the absence of IND.This show that IND does not change the microenvironment around the Ru complex, which indicates that the second possibility of solvation by IND can be neglected. As described later, the quenching takes place by a static mechanism, even in the presence of IND. As stated before, IND does not work as a quencher by itself. The DSC (differential scanning calorimetry) result of PCPMS containing 3-methylindole showed a Tg (glass transition temperature) of -25 "C, similar to that in its absence. This result and the static quenching in the presence of 3-methylindole indicate that 3-methylindole does not behave as a plasticizer. These results exclude the first to third possibilities about the effect of IND, so that the effect as a pathway for the electron transfer would be most probable. In Figure 3 the plots of ln((@[~v2+1=0)/@) versus MVZ+ concentration showed a high quenching efficiency with a downward deviating curve in the presence of 3-methylindole. To analyze this, the following model (Figure 6) is proposed for which a pathway molecule (IND) works between donor (Ru( b ~ y ) 3 ~ and + ) acceptor (MV2+). In the absence of a pathway
Effect of Amino Acid Residue Model molecule, the excited Ru(bpy)3*+ is quenched when the quencher exists within the quenching radius (Ro) (Figure 6, case B), but the rest of the excited Ru(bp~)3~+ gives emission (Figure 6, case A). In the presence of a pathway molecule, some of the quencher present out of the quenching sphere can accept an electron because of the pathway effect (case C). In this case the emission is quenched in addition to case B under the following conditions: The distance between Ru(bp~)3~+ and MV2+ is more than Ro and less than R'o, and the pathway molecule exists within the volume V, in which the it can lengthen the electron-transfer distance. The center of V, is present on the line which connects the R~(bpy)3~+ and MV2+ centers. The total probability for electron transfer is the sum of the probabilities case B (without pathway molecule) and case C (through a pathway molecule). The relative emission yield (W(@[W~+~=O)) is expressed by the probability of case A minus the probability of case C as follows:
. I . Phys. Chem., Vol. 99, No. 17, 1995 6651
The present electron transfer is the first example of a longdistance electron transfer taking place in the presence of an amino acid residue model molecule in a synthetic polymer matrix.
References and Notes
(1) Farver, 0.;Pecht, I. J. Am. Chem. SOC.1992, 114, 5764. (2) Beraton, D. N.; Onuchic, J. N.; Betts, J. N.; Bowler, B. E.; Gray, H. B. J. Am. Chem. SOC. 1990, 112, 7915. (3) Siddarth, P.; Marcus, R. A. J . Phys. Chem. 1993, 97, 2400. (4) Barry, B. A. Photochem. Photobiol. 1993, 57, 179. (5) Electron Transfer in Inorganic, Organic and Biological Systems: ACS Advances in Chemistry Series No. 228; Bolton, J. R., Mataga, N., McLendon, G., Eds.; American Chemical Society: Washington, DC, 1991. (6) Kaneko, M.; Motoyoshi, J.; Yamada, A. Nature 1980, 285, 468. (7) Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1985,89, 1830. (8) Colbn, J. L.; Yang, C.-Y.; Clearfeld, A.; Martin, C. R. J. Phys. Chem. 1990, 94, 874. (9) Turbeville, W.; Robins, D. S.; Dutta, P. K. J. Phys. Chem. 1992, 96, 5024. (10) Oyama, N.; Anson, F. C. J. Am. Chem. SOC. 1979, 101, 739. (11) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368. = exp{-(V, - V,)NA10-z4[MV2']} (12) Leddy, J.; Bard, A. J.; Maloy, J. T.; Saveant. J. Elecrroanal. Chem. @rM"z+]=o 1985, 187, 205. (13) Kaneko, M.; Wohrle, D. Adv. Polym. Sci. 1988, 84, 141-228. [ l - ~X~(-V,N~~O-~~[IND])]~~(~.[MV~+]) dr x (14) Kaneko, M.; Teratani, S.; Harashima, K. J. Electroanal. Chem. 1992, 325, 325. [Vo = 4nR,3/3 (nm3), V, = 4ns3/3 (nm3)] ( 5 ) (15) COMOlly, J. S., Ed. Photochemical Conversion and Storage of Solar Energy; Academic Press: New York, 1981. (16) Kaneko, M.; Yamada, A. Photochem. Photobiol. 1981, 33, 793. In this equation, JpP(r,[MV2+])dr is the probability that the (17) Nemoto, N.; Asano, M.; Asakura, T.; Ueno, Y.; Ikeda, K.; nearest neighboring h V 2 + center exists in the distance between Takamiya, N. Makromol. Chem. 1989, 191, 497. Ro and R'o from the Ru(bpy)3*+center, and [ l - exp(-VdA(18) The mechanism of the concentration quenching of the excited Ru( b p ~ ) 3 ~has + been attributed to the external spin-orbit coupling effect,* 10-z4[IND])]is the probability that 3-methylindole (IND)exists and we have shown that the relation between the quenching radius and in the volume V, in which the IND can behave as a pathway emission lifetime obeys the Dexter mechanism: Nagai, K., Takamiya, N., molecule. Equation 5 is transformed as follows: Kaneko, M. J. Inorg. Organomet. Polym. 1994, 4, 391. (19) Nagai, K.; Takamiya, N., Kaneko, M. J. Photochem. Photobiol. A: Chem. 1994, 84, 271. = exp{ -(Vo - V,)NA10-24[MVZ+]}(20) Torquato, S.; Lu, B.; Rubinstein, J. Phys. Rev. A 1990, 41, 2059. rM"Z+] =o (21) The radius of Ru(bpy)3*+ was estimated by examining crystallographic data of the Ru(bpy)3Xn complex as follows. First, twice the distance from the molecular center to the 4- or 4'-position hydrogen atom of the bpy ligand (0.588 nm) plus the van der Waals radius of hydrogen (0.12 nm) was considered as the excluded radius (0.71 nm) of the complex. On this basis, the contact distance between the Ru complex is 1.42 nm. However, the crystallographic data of [Ru(bpy)3](PF& show that the nearest distance between the Ru centers is 0.82 nm, indicating that the distance Equation 6 was applied to the analysis of the emission in the between the central Ru atom can be shorter than that estimated by the presence of 3-methylindole (Figure 5 ) by using a nonlinear leastremotest hydrogen atom position (1.4 nm). Since the crystallographic square method. As for the Ro and s values, 1.4 and 0.82 nm analysis of Ru(bpy)sCls used in the present work has not been done because of the difficulty in preparing its single crystal, we adopt here s = 0.82 nm were used, respectively, as described before. From the fitting as an approximation from the data of [Ru(bpy)s](PF&." of the data, the electron-transfer distance through a pathway (22) Rillema, D. P.; Jones, D. S.; Levy, H. A. J. Chem. SOC., Chem. molecule (R'o)and the volume V, in which IND can behave as Commun. 1979, 849. a pathway were obtained to be 2.7 nmZ1and 14 nm3, respec(23) The volume of MV2+ was estimated on a cuboid model whose length, width, and thickness are 1.34 nm, 0.63 nm, and 0.34 nm, respectively. tively. The electron-transfer distance R'o is twice that in the In order to estimate the contact distance with R~(bpy)3~+, the shape of MVz+ absence of 3-methylindole (1.4 nm), indicating a long-distance was assumed as a sphere whose volume is the same as the cuboid form of electron transfer through the pathway molecule which is a model MV2+ (0.29 m3); this sphere radius is calculated as 0.41 nm. (24) Guam, T.; McGuire, M.; Strauch, S.; McLendon, G . J. Am. Chem. for tryptophan residue. When V, is assumed as a sphere, its SOC. 1983, 105, 616. radius (R,) is calculated to be 1.5 nm. This R , value is close @
@
to half of the R'o value.
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