Spectroscopic study of phosphine-substituted oligothiophenes

The spectroscopic behaviour (solution and solid-state absorption and emission) of a series of mono(diphenylphosphino) and bis-(diphenylphosphino) alpha-substituted oligothiophenes varying in length from one to three thienyl rings is described. The absorption spectra red-shift with increasing oligomer length, and the spectra of the bisphosphines are red-shifted, relative to the monophosphines that contain the same number of thienyl groups. In solution, emission from the mono- and bithienyl compounds is broad and shows a large Stokes shift, attributed to a planar excited state due to the n –> pi{*} nature of the excitation. Emission from the terthienyl compounds shows a much smaller Stokes shift and is attributed to a thienyl-based pi –> pi{*} transition. In the solid state, similar effects are observed in both absorption and emission spectra. Density functional theory (DFT) calculations show that the contribution of the P lone pair to the highest occupied molecular orbital (HOMO) diminishes as the oligomer length increases, supporting the experimental results.