PHOTOINDUCED ELECTRON-TRANSFER IN A MONOLAYER AT THE AIR-WATER-INTERFACE

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 pressure-area 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-2, contrary to the LB film case where it is found at donor densities as low as 0.01 nm-2 with cadmium arachidate/methyl arachidate = 1/1 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-1 and 1.5 nm at 40 mN m-1. 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 thc same interface. The discrepancy may be due to the larger values of the donor-to-acceptor ratio in the LB film case.