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

The single-crystal X-ray structures of three odd-membered thiophene oligome rs bearing one central thienyl-S,S-dioxide moiety-trimer, pentamer, and hep tamer-are reported. Absolute photoluminescence quantum yields in microcryst alline powders are given for all compounds. The solid-state photoluminescen ce efficiencies of the trimer (45%) and the pentamer (12%) were up to 1 ord er of magnitude higher than those generally measured in conventional oligot hiophenes, while that of the heptamer amounted to only 2%. These results ar e accounted for in terms of molecular packing characteristics; which, owing to the competing effects of dipolar intermolecular interactions between th e sulfonyl groups and intra- and intermolecular C-H ... O hydrogen bondings and S ... S interactions, change dramatically on changing the oligomer siz e. While the trimer is highly distorted and crystallizes in a chiral orthor hombic space group with the molecular long axes markedly tilted with respec t to one another, the heptamer displays a coplanar conformation with the mo lecules packing in strictly parallel layers. Contrary to the solid state, t he photoluminescence intensity in solution increases on increasing the olig omer size and shows marked solvent dependence.