9403
J. Phys. Chem. 1991,95,9403-9405
Electron-Transfer Quenching of a Photoexcited Ruthenium Complex by Stearyl Viologen in Barium Stearate Langmuir-Blodgett Films Tokuji Miyashita,* Department of Biochemistry and Engineering, Tohoku University, Aoba Aramaki Aoba- ku, Sendai. 980, Japan
Yutaka Hasegawa, and Minoru Matsuda Chemical Research Institute of Nonaqueous Solutions, Tohoku University, Katahira 2- 1- 1, Aoba-ku, Sendai 980, Japan (Received: October 23, 1990)
Photoinduced electron transfer of tris(4,7-diphenyl-1, IO-phenanthro1ine)ruthenium (Ru(dpphen)t+) to stearyl viologen (sV2+) in barium stearate LB assembly systems was investigated by a steady-state quenching method. The electron transfer from the excited state of Ru(dpphen):+ to SV2+ across two barium stearate monolayers was impossible by electron tunneling during the lifetime of Ru(dpphen):+*. Even when the monolayer of the ruthenium complex is in direct contact with the layer of SV2+quencher at the hydrophilic interface, the quenching of the ruthenium complex had low efficiency, indicating that R~(dpphen),~+ is located at a more hydrophobic region. In an assembly where both Ru(dppneh)32+and SV2+exist within the same monolayer, the quenching efficiency depended strongly on the two-dimensional density of SV2+. A good linear relationship of In (lo/l)- 1) vs the distance between the quencher molecules, which is calculated from the twdimensional density, was obtained. This shows that electron-transfer rate decreases exponentially with distance and supports the conclusion that the electron-transfer quenching proceeds via an electron-tunneling mechanism. The half-quenching distance, which is defined as the distance giving half the emission intensity, was found to be 1.8 nm.
Introduction Monolayers and Langmuir-Blodgett (LB) multilayers have been studied with an interest in fabricating films with a controllable thickness and a well-defined molecular orientation.’,’ Recently, incorporation of various photoredox species into LB films has been examined from the viewpoint of molecular design for functional devices related to photoelectric conversion.” We have attempted to prepare LB films containing various chromop h o r e ~ . ~ - ’ ~In our previous paper,7 we reported that Ru(dpphen)32+,which has no long alkyl chain substituent, could be molecularly dispersed into fatty acid monolayers by ion-pair formation with the carboxylate anion. The LB films containing the ruthenium complex uniformly were obtained. The ruthenium complex, of which the excited state has both electron-donor and -acceptor properties, is attractive as a sensitizer for light energy conversion systems.13-17 In this paper, photoinduced electron ( I ) Blodgett, K. B.; Langmuir, I. Phys. Reu. 1937, SI, 964. (2) Gaines. G. L., Jr. Insoluble Monolayers at Liquid-Gas Interface; Wiley-Interscience: New York, 1966. (3) Mbbius, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 848. (4) Mbbius, D. AEC.Chcm. Res. 1981, 14, 63. (5) Mobius, D. Kinetics of Nonhomogeneous Processes; Wiley-lnterscience: New York. 1987. (6) Roberts, G. G.;McGinnity, T. M.;Barlow, W. A.; Vincett, P. S.Thin Solid Films 1980, 68, 223. ( 7 ) Murakata, T.; Miyashita, T.; Matsuda, M. J. Phys. Chem. 1988, 92,
6040. (8) Murakata, T.; Miyashita, T.; Matsuda, M. hngmuir 1986, 2, 786. (9) Murakata, T.; Miyashita, T.: Matsuda, M. Macromolecules 1989,22, 2706. (IO) Murakata, 7.;Miyashita, T.; Matsuda, M. Macromolecules 1988.21, 2730.
( I I ) Miyashita, T.; Yatsue, T.; Mizuta, Y.; Matsuda, M. Thin Solid Films 1989. 179, 439. (12) Miyashita, T.; Yatsue, T.; Matsuda, M. J . Phys. Chem. 1991, 95, 2448. (13) Miyashita, T.; Murakata, T.; Yamaguchi, T.; Matsuda, M. J . Phys. Chem. 1985, 89. 497. (14) Miyashita, T.;Matsuda, 7.Bull. Chem. Soc. Jpn. 1985, 58, 3031.
( I S ) Sprintschink. G.; Sprintschnik. H. W.; Kirsch, P. P.; Whitten, D. G. J . Am. Chem. Soc. 1977, 99, 4947. (16) Gains, G. L., Jr.; Behnken, P. E.; Velenty, S. J. J . Am. Chem. SOC. 197a,ioo. 6549. (17) Seefeld, K. P.; Mbbius, D.; Kuhn, H. Helu. Chim. Acta 1977, 60, 2608.
0022-3654/9 1/2095-9403$02.50/0
CHART I
Ru(dpphen),,
dpphen :
sv2+:
transfer from the ruthenium complex to stearyl viologen in two types of LB multilayer assembly systems (Chart 11) is investigated by a quenching method: (1) the ruthenium complex and SV2+ are located at different layers (A and B in Chart 11). and (2) both are in the same monolayer (C). Experimental Section Ru(dpphen)?+ was synthesized by modification of a procedure for the preparation of Ru(bpy)$+ (bpy = 2,2’-bipyridine) and was purified by recrystallization.Is Stearyl viologen (N,N’-distearyl-4,4’-bipyridinium dibromide) was prepared from the reaction of 4,4’-bipyridine with steary bromide. Stearic acid (Wako Chemical Co.) was purified by recrystallization from methanol. Distilled water was used (Millipore Milli-QII). An automatic working Langmuir trough (Kyowa Kaimen Kagaku HBM-AP with a Wilhelmy-type film balance) was used for the measurements of surface pressure-area isotherms (x-A isotherms) and the preparation of LB films. The quartz slides used for the deposition of monolayers were previously cleaned in boiling H2SO4-HNO, (2/1) solution and were made hydrophobic with dichlorodimethylsilane. They were coated in advance by four layers of barium stearate monolayer to prepare a uniform surface and to remove the influence of bare quartz slide. Fluorescence (18) Fujita, 1.; Kohyashi, H. Eer. Bunsen-Ges. Phys. Chem. 1972. 76, 1 IS.
0 1991 American Chemical Society
9404 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 CHART I1
1
: sv2+
Miyashita et al.
: St
A
B
C
spectra were measured with a Hitachi 850 spectrofluorometer.
Results and Discussion In the previous study,' it was found that Ru(dpphen):+ could be dispersed molecularly into a barium stearate monolayer matrix with a molecular area of 1.2 nm2/molecule. The condensed mixed monolayer could be transferred onto a quartz slide giving a uniform LB film with a yellow color due to the absorption by Ru( d ~ p h e n ) , ~ +The . absorption and emission spectra for the ruthenium in the LB film were the same as those in homogeneous solutions. The photoexcited state of (bipyridy1)ruthenium complexes is quenched by dialkyl viologen quenchers via an electron-transfer reaction, and the quenching has been investigated in various molecular assemblies.Ie2' The quenching of R ~ ( d p p h e n ) ~ ~ + * by SV2+ quencher in LB assemblies is studied by means of steady-state fluoroescence spectroscopy: R~(dpphen),~+* SV2+ R~(dpphen),~+ + SV+ ( I ) Quenching of the Photoexcited Ruthenium by Stearyl Viologen Located at Different Layers (Assemblies A and B). The electron transfer from the excited ruthenium complex incorporated in the barium stearate monolayer ( [ R ~ ( d p p h e n ) ~ ~[St] + ] / = 1 / 10) to the stearyl viologen located in the different layers was investigated with LB assemblies A and B by fluorescence quenching. Although stearyl viologen itself does not form a stable monolayer, the mixed monolayer with barium stearate gave a condensed stable monolayer.I2 Especially, in mixed monolayers more, diluted than St/SVZ+= 5/1, the condensed stable monolayer is formed. The SV*+ molecule occupies a constant surface area (0.4 nm2/molecule) in the mixed monolayer.I2 In assembly B, where the viologen layer is separated by two barium stearate layers (about 5.0-nm distance8vZZ)from the ruthenium complex layer, no emission quenching for R ~ ( d p p h e n ) ~ ~was + * observed. This result is consistent with the fact that the quenching proceeds via an electron-transfer mechanism, not via an energy-transfer mechanism. Mobius et al. also reported no emission quenching of an excited cyanine dye of SV2+ separated with two arachidate monolayers (5.4 nm).,s4 On the other hand, when the layer of stearyl viologen was face to face with the ruthenium layer at the hydrophilic interface (assembly A), the emission (600 nm) of R~(dpphen),~+* was quenched slightly and decreased with the two-dimensional density of SV2+in the adjacent layer (Figure 1). It is expected that R~(dpphen),~+ is placed at a hydrophilic part in the barium stearate monolayer on a water surface, because
+
-
(19) Balzani, V.; Maggi, L.; Manfrin, M. F.; Bolletta, F.: Laurence, G.S. Coord. Chem. Rev. 1975, I S , 321. (20) Miyashita, T.;Matsuda, M.Macromolecules 1990, 23, 2598. (21) Turro,N. J.; Grittzel, M.: Braun, A. M. Angew. Chem., Inr. Ed. Engl. 1980, 19, 675. (22) Srivasta, V. K.; Verma, A. R.Solid State Commun. 1966, 4, 367.
n. 5
n
0.5
1.0
SV2+ I n o I e c u I e / n n 2 1 Figure 1. Change in emission intensity of Ru(dpphen),*+* by two-dimensional density of SV2+in LB assembly A.
of ion-pair formation with a carboxylate anion,' and complete or efficient quenching takes place as reported by M i i b i ~ s . ~For , ~ an electron-transfer quenching in a solid matrix, an active-sphere model, such as the Perrin m~de1,2~3 for energy-transfer quenching in the absence of diffusion has been considered. If a quencher molecule exists within the active sphere of the excited species, the probability of quenching is unity. As the radius of the active sphere in the quenching of Ru(bpy),2+* by MV2+ (methyl viologen), 1.09 and 1-08nm, which are slightly longer than the distance in direct contact of R ~ ( b p y ) , ~and + MV2+, have been reported for the matrix of a rigid glycerolZSand a layered inorganic solid,26respectively. The low efficiency of the quenching in the assembly A suggests that Ru(d~phen),~+ is located at a more hydrophobic region, apart from the hydrophilic Ba2+cation region. Transferring the R~(dpphen),~+ mixed monolayer on the water surface to a solid support would squeeze out a binding water, and Ru( d ~ p h e n ) , ~seems + to move into a hydrogen region, due to the strong hydrophobic character of the ligands of this ruthenium complex. The plots in Figure 1 cannot be analyzed in detail because of a low change in the emission intensity. Quenching in the Same Monolayer (Assembly C). The quenching of R~(dpphen),~+* by SV2+ existing in the same monolayer (assembly C) was measured as a function of the two-dimensional density of SV2+. R ~ ( d p p h e n ) ~SVZ+, ~ + , and St were mixed with a mixing molar ratio as 1/1/2X/(10 - X ) to vary the two-dimensional density of SV2+. Since the surface area of SV2+is twice that of St, one SV2+ molecule is replaced with two (23) Perrin, F. Ann. Chem. Phys. 1932, 17, 283. (24) Turro, N.J. Modern Molecular Photochemistry: Benjamin/Cummings: Menlo Park, CA 1978; p 317. (25) Colon, J. L.; Yang, C.-Y.: Clearfield, A.: Martin, C. R. J . Phys. Chem. 1990. 94. 874. (26) Guarr, T.; McGuire, M.; Strauch, S.; McLendon, G.J . Am. Chem. SOC.1983, 105, 616.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9405
Electron-Transfer Quenching of Ru Complexes
0
-
c
I I
-Y- -2 0
e
S V ~ + [noiecuIe/nn*) Figure 2. Change in emission intensity of Ru(dpphen)32+*by two-dimensional density of SV2+ in LB assembly C.
molecules of St to keep the density of Ru(dpphen)32+constant. It is known from the previous work7 that the mixed monolayer of Ru(dpphen),2+ with St gives a stable condensed monolayer at a molar ratio of 1 /IO. The surface pressure-area isotherms of the mixed monolayers composed of three components, in practice, indicated the formation of a stable condensed monolayer with a high collapse pressure of ca.40 mN/m. Thus, the monolayer was transferred to a quartz slide at 25 mN/m with a transfer ratio of 1 .O. Two mixed monolayers, which are separated from each other by four barium stearate monolayers (ca. 10.0 nm) were deposited to get a sufficient emission intensity for analysis (assembly C in Chart 11). Figure 2 shows that the relative emission intensity of Ru(dpphen)32+at around 600 nm decreases with the SV2+density. The decrease is due to electron-transfer quenching by SV2+quencher. The following photoelectrochemical processes for the excited Ru(dpphen):+ in barium stearate monolayer matrix in the absence and presence of SV2+quencher can be considered
-!!.
+ hv Ru(dpphen)32+*A R ~ ( d p p h e n ) ~+~kT + Ru(dpphen)?+*
Ru(dpphen)?+
(2)
(3)
R ~ ( d p p h e n ) ~ -% ~ + * Ru(dpphen),'+ (4) where k f , k,+ and kqt are the rate constants for fluorescence, thermal deactivation, and quenching processes of Ru(dpphen):+*, respectively. The quenching process is due to electron transfer from Ru(dpphen)32+*to SV2+via an electron-tunneling mechanism, because there is no diffusion during the lifetime of the excited Ru(l1) complex. The relative fluorescence intensity in the absence of (lo)and presence of (I) of SV2+ quencher is given as lo/l = 1 k q , / ( k , + kd) = 1 + kq,ro (5)
+
where T~ is the lifetime of R ~ ( d p p h e n ) ~ ~in+ the * absence of quencher. Recent studies of electron-transfer reactions in rigid matrices suggest that the rate constant for electron transfer via an electron-tunneling mechanism depends exponentially on the distance (r) between the donor and a ~ c e p t o r ~ ~ - ~ ~
k,, = k(O) exp(-(r - R o ) / 4
(6)
where a is the range parameter related to the binding energy at the electron donor, or barrier height, and Ro corrects for the finite (27) McLcndon, G. Arc. Chem. Res. 1988, 21, 160. (28) Miller, J. R.; Beitz, J. V. J . Chem. Phys. 1981, 74, 6746. (29) Beitz, J. V.; Miller, J. R. J . Chem. Phys. 1979, 71, 4579.
-4
2.0
0
4.0
R
lnrl Figure 3. Linear relationship between In ( ( f o / I ) - 1 ) and R for the quenching of Ru(dpphen)32+* in assembly C.
size of the reactants. From eqs 5 and 6 , the following relation is obtained:
- 1) = -(r - R o ) / a + In (k(0)so) In ((lo/l)
(7)
The distance between Ru(dpphen)$+* and SV2+depends linearly on the SV2+- SV2+distance (2R),which is calculated from the two-dimensional density (n, (molecules/nm)) of SV2+ in the monolayer matrix ( R = (n,/r)-'/2 (nm)). The plots of In ((Zo/l) - 1) vs R are shown in Figure 3. A good linear relationship was obtained, indicating the electron-transfer quenching via the electron-tunneling mechanism. A similar linear relationship of In ((lo/l)- 1) with the distance has been reported in the quenching of a cyanine dye by SV2+,which is separated by the thickness ( r ) of one m ~ n o l a y e r . ~The - ~ electron-tunneling across a barrier of 2-2.5 nm constituted by the fatty acid monolayer has been propo~ed.~?~ For the quenching in LB assembly systems, the quenching half-distance ( R l / 2 )which , is defined as the distance giving half the fluorescence intensity (Zo/Z = OS), has been often obtained to compare the efficiency of quenching with the results of other systems. Mobius et al. l 7 have been obtained Rl12values for the electron-transfer quenching of various excited donor dyes by SV2+ quencher, which is located a t adjacent monolayer, and showed that R l 1 2depends on the ionization potential in the excited state of the donor dye. The Rl,z for a long alkyl substituted (bipyridy1)ruthenium complex-SV2+ pair was reported to be 0.9-1.2 nm.I7 The R,,, for the present system is calculated to be 1.8 nm from the plots in Figure 3. The larger R , for the present work compared to that for Mobius' case may 6e due to a difference in the LB assembly; that is, the ruthenium complex and SV2+ quencher are located in the same monolayer matrix at the present assembly, resulting in the efficient electron-transfer quenching. The value 1.8 nm seems to be reasonable, considering that the radius of active sphere where complete quenching occurs for the ruthenium complex and SV2+couple has been obtained to be about 1.1 nm. In conclusion, the electron transfer from the excited state of R ~ ( d p p h e n ) ~which ~ + , is incorporated uniformly into the barium stearate monolayer matrix to SV2+located in the same monolayer, proceeds via an electron-tunneling mechanism. The electrontransfer quenching depended strongly on the two-dimensional density of SVz+ in the LB assembly. A good linear relationship of In ((loll)- 1) vs the distance between SV2+ molecules was obtained. The quenching half-distance was estimated to be 1.8 nm.
