5939
J. Phys. Chem. 1992, 96, 5939-5942
Photoinduced Electron Transfer in a Monolayer at the Air-Water Interface Mutsuyoshi Matsumoto,*9+Ramesh C. Ahuja, and Dietmar Mobius Max-Planck-Institut fur Biophysikalische Chemie, Postfach 2841, 0-3400 Gottingen, FRG (Received: November 7, 1991; In Final Form: March 13, 1992)
Photoinduced electron transfer from the amphiphilic oxacyanine (donor) to the amphiphilic viologen (acceptor) embedded on a DMPC matrix is investigated in the monolayer. Steady-state fluorescence intensity-area isotherms were measured simultaneously with surface pressurearea isotherms. In the absence of the acceptor, the fluorescence intensity normalized to surface density increased with an increase in surface pressure, which was suggested to be due to an increase in the lifetime of the excited state of the donor. Dimer formation of the donor is not found in the present case even at the donor density of 0.22 nm-*, contrary to the LB film case where it is found at donor densities as low as 0.01 nm-2 with cadmium arachidatelmethyl arachidate = 111 as a matrix. This shows the important role of the matrix in this type of work. In the presence of the acceptor, the relative fluorescence intensity decreases strongly with increasing surface pressure and molar fraction of the acceptor. This is due to the electron transfer from the excited state of the donor to the ground state of the acceptor. The relative fluorescence intensity depends on the densities of the donor and the acceptor and also on the lifetime of the excited state of the donor. The behavior in the fluorescence intensity is simulated well by a model which is a two-dimensional equivalent of the hard-sphere model in solution and which explicitly shows the dependence on the lifetime of the excited state of the donor. The critical distance for the electron transfer is 0.9 nm at 2 mN m-I and 1.5 nm at 40 mN m-l. The close match between the observed and simulated values shows that the energy delocalization via incoherent exciton hopping is not significant in the present monolayer case as opposed to the LB film systems where the donor and acceptor are localized at the same interface. The discrepancy may be due to the larger values of the donor-to-acceptor ratio in the LB film case.
Introduction Electron transfer is of prime importance in many fields of chemistry, physics, and biology.14 In particular, photoinduced electron transfer between an excited donor molecule and an electron acceptor molecule has been investigated extensively with the aim of capturing and storing solar energy. Supramolecules in which donor and acceptor molecules/components are covalently-linked were synthesized to examine the parameters involved in this process.s+6 In this respect, the Langmuir-Blodgett (LB) technique is one of the most promising tools for the fabrication of the molecular assembly? By use of the LB technique, molecular organizates may be obtained where components like the donor and the acceptor are arranged in such a manner that the donor and the acceptor parts are assembled either at the same interface or at different interfaces!-'* The fluorescence of the excited donor is quenched by electron transfer to an acceptor molecule which is located either in an adjacent monolayer a t the same interface or in a monolayer at a different interface. The relative fluorescence intensity is mainly governed by the donor-acceptor distance and orientation, free energy of the process, and electronic coupling. We have already reported on the photoinduced electron transfer from oxacyanine to viologen located at the same interface in monolayer a~semblies.~ The donor and acceptor molecules were incorporated in different monolayers by using matrix molecules, but the two monolayers faced each other with their hydrophilic parts, realizing the location of the donor and acceptor at the same interface. Relative donor fluorescence intensities calculated from the excited-state lifetime data were found to be significantly larger than the observed values, indicating the contribution of energy delocalization via incoherent exciton hopping. This is consistent with the excitation energy migration among monomer sites before being trapped a t dimers.I3 In the present study, photoinduced electron transfer is examined in the monolayer a t the air-water interface. The donor and the acceptor molecules are incorporated in the same monolayer with L-(adimyristoy1)phosphatidylcholine (DMPC) as a matrix, and the steady-state fluorescence quenching was measured simultaneously with the surface pressumrea isotherm. The investigation a t the air-water interface allows a continuous variation of the lateral distance between the donor and acceptor molecules through the variation of surface pressure and the molar fraction of the components. On leave of absence from the National Chemical Laboratory for Industry,
Tsukuba, Ibaraki 305, Japan.
Experimental Section The N,N'-dioctadecyloxacyanine perchlorate (OC) and N,N'-dioctadecylviologen perchlorate (OV) were synthesized according to the method of Sondermann.I4 The chemical structures of OC and OV are shown in Figure 1. DMPC was obtained from Sigma Chemical Co. and used without further purification. The subphase for all monolayer experiments was Milli-Q-filtered water which has a p H of ca. 5.6 when in equilibrium with the atmosphere. The spreading solvent for all amphiphilic molecules was HPLC-grade chloroform stabilized with 2% ethanol. All experiments were done on a Fromherz-type's round trough equipped with a Wilhelmy balance for surface pressure measurement and a fluorescence detector head. The trough was enclosed in a dark chamber maintained at 288 K. The fluorescence detector head consisted of two quartz optical fiber bundles (diameter 3 mm). The excitation fiber bundle was inclined to the water surface a t an angle of 30°, and the emission fiber bundle was positioned normal to the water surface. Both of the fiber bundles were placed ca. 5 mm above the water surface. The light source was a 30-W deuterium lamp. The light after passing through a monochromator (AA 8 nm) was directed to the excitation fiber bundle. The emission light after passing through the emission fiber bundle was coupled to the emission monochromator (AA 8 nm), the exit slit of which was connected through a lens to a photomultiplier. The photomultiplier was thermoelectrically cooled to 233 K and was operated in the photon-counting mode. The counts were collected in a multichannel analyzer operating in the multichannel scaling mode. The fluorescence signal was integrated over 10 s. A chloroform solution of donor (OC), the acceptor (OV), and DMPC in the desired composition was spread on the Millipore water. The solution was spread a t the water surface using the continuous-flow technique rather than the normally used dropwise addition of the solution to the water surface. The flow technique has been found to avoid the formation of patches and aggregation of dye molecules in the spreading phase. This technique consists of keeping the outlet of the syringe touching the water surface and of slowly spreading the solution. The monolayer was compressed ca. 30 min after the spreading. The surface pressurearea isotherms were measured stepwise: the moving barrier stops when the mean area per molecule decreases by 0.02 nm2 or when the surface pressure increases by 2 mN m-I, and the surface pressure was recorded after 30 s of relaxation. Monolayers were prepared with OC/ OV/DMPC = l/r/lO where 0 I r I2. The steady-state fluorescence intensity-area isotherm was measured with the ex-
-
-
0022-3654/92/2096-5939%03.00/0@ 1992 American Chemical Society
Matsumoto et al.
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
5940
oc:
0.15
q
&
C
H
