EXCITATION-ENERGY RELAXATION OF OXACYANINE IN LANGMUIR-BLODGETT MONOLAYER FILMS - PICOSECOND TIME-RESOLVED FLUORESCENCE STUDIES

Picosecond time-resolved fluorescence spectra and fluorescence decay curves of NN'-dioctadecyloxacyanine in Langmuir-Blodgett (LB) monolayers have been measured in the dye concentration ranging from 0.02 to 17.3 mol %. At concentration lower than 0.2 mol %, oxacyanine monomers excited at 358 nn show approximately a single exponential decay with a 1.6-ns lifetime. As the concentration is increased from 0.2 to 3.0 mol %, the excitation energy relaxation is interpreted in terms of the energy transfer from monomers to dimers. At concentrations higher than 3 mol %, oxacyanine dimers are excited and they undergo quenching at higher aggregates. It is found that oxacyanine in the ground state forms effectively a face-to-face dimer in which the long axis of oxacyanine is parallel and the short axis is perpendicular to the monolayer surface. The oxacyanine LB monolayer is found to be essentially different in structures and in relaxatioin dynamics from the rhodamine B LB monolayers presented in our previous work (Can. J. Phys. 1990, 68, 1013).