The role of ultrafast torsional relaxation in the emission from polythiophene aggregates

An understanding of aggregation effects in organic semiconductors is essential for their effective use in optoelectronic devices. Typically, the electronic dynamics in such systems are heavily dependant upon the aggregation state, and dynamics often occur on subnanosecond timescales. Here, we determined the singlet exciton population within isolated and aggregated P3HT regions using time-resolved photoluminescence measurements, and find a strong decay pathway in the aggregated case only. Comparison of the emission from the lowest two vibronic bands demonstrates a changeover from isolated chain to aggregate-like emission within ~14 ps corresponding to timescales for torsional relaxation in these materials. We conclude that formation of an aggregate excited state in conjugated polymers is mediated by vibrational relaxation from a low-symmetry to a high-symmetry, ordered state for the ensemble.