Molecular packing and photoluminescence efficiency in odd-membered oligothiophene S,S-dioxides

The single-crystal X-ray structures of three odd-membered thiophene oligomers bearing one central thienyl-S,S-dioxide moiety-trimer, pentamer, and heptamer-are reported. Absolute photoluminescence quantum yields in microcrystalline powders are given for all compounds. The solid-state photoluminescence efficiencies of the trimer (45%) and the pentamer (12%) were up to 1 order of magnitude higher than those generally measured in conventional oligothiophenes, while that of the heptamer amounted to only 2%. These results are accounted for in terms of molecular packing characteristics; which, owing to the competing effects of dipolar intermolecular interactions between the sulfonyl groups and intra- and intermolecular C-H ... O hydrogen bondings and S ... S interactions, change dramatically on changing the oligomer size. While the trimer is highly distorted and crystallizes in a chiral orthorhombic space group with the molecular long axes markedly tilted with respect to one another, the heptamer displays a coplanar conformation with the molecules packing in strictly parallel layers. Contrary to the solid state, the photoluminescence intensity in solution increases on increasing the oligomer size and shows marked solvent dependence.