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.