Triplet Energy Transfer from Carbazole to Bromonaphthalene in a Two-Dimensional Chromophore Plane Prepared by Poly(octadecyl methacrylate) Langmuir-Blodgett Films

Abstract

The triplet energy migration and transfer in Langmuir-Blodgett (LB) films prepared by the copolymers of octadecyl methacrylate with 2-(9-carbazolyl)ethyl methacrylate (donor unit) and (4-bromo-1-naphthyl)-methyl methacrylate (acceptor unit), were investigated. Compared with the singlet energy transfer, a more strict spatial arrangement of chromophores is required in the triplet energy transfer because of the short range interaction of the electron exchange mechanism. For the control of triplet energy transfer, the position of chromophores has to be regulated within the range narrower than the thickness of a layer of LB film (∼3 nm). The dependence of the quenching efficiency of carbazole phosphorescence upon the acceptor density at a low donor density in the film plane was reproduced by the active sphere model with a planar distribution of chromophores in a monolayer. This indicates that the chromophores distribute in a two-dimensional plane within the range of a few angstroms. In the present LB films, the spatial distribution of chromophores in a vertical direction can be regulated enough to control the triplet energy transfer although the distribution in the lateral direction is random. When the donor density was high, the apparent radius of the active sphere was longer than the value at a low donor density. This finding suggests that the energy transfer occurs by a dynamic process, i.e., a few steps of energy migration among donors, and then is followed by energy transfer to the acceptor.