External Electric Field Effects on Absorption and Fluorescence Spectra of Monomer and Dimer of Oxacarbocyanine in Langmuir−Blodgett Films

Abstract

The dimer of oxacarbocyanine (OCC) formed in mixed Langmuir−Blodgett (LB) films shows very different electric field effects from the monomer both in the absorption spectrum and in the fluorescence spectrum. The magnitude of the change in electric dipole moment between the S1 excited state and the ground state of the dimer along the normal to the surface is much larger than that of the monomer. The fluorescence quantum yield of the OCC dimer also shows a remarkable electric field effect. The sign of the field-induced change in the quantum yield of the dimer fluorescence becomes opposite when the direction of the applied electric field is inverted, suggesting that the excitation dynamics of the dimer that is affected by an external electric field is a vectorial process along the normal to the surface. The process affected by an electric field is attributed to the interlayer electron transfer between the excited state of the OCC dimer and the cadmium salt of the fatty acid deposited in contact with each other.