MOLECULAR CONTROL OF LUMINESCENCE FROM POLY(3-HEXYLTHIOPHENES)

Photoluminescence (PL) from poly(3-hexylthiophenes) (P3HT) possessing various degrees of regioregularity has been studied. Wavelengths of ma ximum absorption and emission are blue-shifted for polymers possessing increasing head-to-head (HH) content due to steric interactions betwe en adjacent thienyls. These repulsive forces are partially relieved in the excited state. Stokes shifts increase with increasing HH dyad con tent, reflecting the greater relief from conformational strain. Quantu m yields of fluorescence from polymer solutions increase with decreasi ng head-to-head dyad content of the polymer due to a larger conjugatio n length, higher rigidity, and consequently, a decrease in nonradiativ e, torsional-mode shunt processes (intramolecular decay manifolds). In contrast, quantum yields of thin films of the polymers are largest fo r the less planar configurations containing significant fractions of h ead-to-head dyads. Low quantum yields associated with planar conjugate d structures can be explained by classical concentration quenching eff ects which arise from nonemissive excimer complexes (intermolecular de cay channels). The results are correlated with the quasi-one-dimension al band model.