Monolayer Assemblies of Dyes to Study the Role of Thermal Collisions in Energy Transfer

Abstract

Abstract The energy transfer from an oxacyanine to a thiacyanine dye was investigated in monolayer assemblies. No temperature‐dependence of the efficiency was observed between 300 K and 20 K in arrangements where the donor molecules were isolated. However, in arrangements where the donor was organized into a large aggregate and the acceptor was highly diluted in the adjacent monolayer, the efficiency of the energy transfer was proportional to the absolute temperature; the spectral distributions of the fluorescence of the donor and the absorption of the acceptor were equally well matched at low and high temperatures. A simple model is discussed based on the idea that the energetic match needed for energy and electron transfer occurs occasionally due to thermal fluctuations and that this match will not be achieved below a certain temperature. A mechanism based on exciton hopping in the aggregate followed by energy transfer from a molecule in the vicinity of the acceptor to the acceptor does not explain the observed temperature‐dependence. However, the present experimental data can be rationalized by assuming that the excitation extends over a certain domain (consisting of about 10 donor dye molecules at room temperature and about 150 molecules at 20 K) which moves over the aggregate, occasionally reaching the vicinity of an acceptor molecule.