Luminescence quenching by inter-chain aggregates in substituted polythiophenes

Time-resolved photo luminescence spectra measured in solid films of two pol ythiophene derivatives with different chain packing allow to distinguish em ission of intra-chain excitations from the luminescence of inter-chain aggr egates. Aggregate luminescence is red shifted by about 0.1 eV relative to i ntra-chain emission and shows vibronic coupling to the C=C bond stretch wit h the Huang-Phys factor of S approximate to 1.5, which is twice bigger than that of the intra-chain emission. Combining time resolved luminescence dat a with femtosecond transient absorption, we show that the dynamic quenching of the luminescence in films with dense chain packing is mainly due to exc itation energy transfer to aggregates. The radiative lifetime of the lowest excited state of the aggregate with the optical gap of 1.84-1.9 eV is esti mated to be about 20 ns. The aggregate contribution to the total luminescen ce in the polythiophene films with dense chain packing is about 50% and doe s not change significantly with temperature. (C) 2001 Elsevier Science B.V. All rights reserved.