Exciplex formation in the phenoxathiin-thioxanthone system

Abstract

Amines and other nitrogen-containing compounds, due to their electron donor character, can form excited state emissive complexes in systems that comprise an electron acceptor. Sulphur-containing heteroaromatic compounds such as phenoxathiin are also well-known for their electron donor character. Starting from these two considerations, we aimed at finding exciplexes in which the electron donor contains sulphur. We studied several D-A pairs in solvents of different polarity and found out that the system phenoxathiin-thioxanthone presents a new emission band at longer wavelength than the D and A bands in non-polar solvents, which is quenched in polar ones. In order to explain the exciplex formation, which is determined by an electron transfer process, the standard free enthalpy for the latter was calculated according to the Rehm-Weller equation. The equilibrium constants and the activation barrier for the exciplex formation were determined as well from steady-state emission data at different donor concentrations and at different temperatures, respectively.