Molecular Dynamics of Regioregular Poly(3-hexylthiophene) Investigated by NMR Relaxation and an Interpretation of Temperature Dependent Optical Absorption

The molecular Structure and dynamics of regioregulated poly(3-hexylthiophene) (P3HT) were investigated using high-resolution solid-state C-13 nuclear Magnetic resonance (NMR) and optical absorption spectroscopies. A crystal (C)-plastic crystal (PC) transition induced by the molecular motion of the aliphatic side group was observed for P3HT in the temperature dependence analysis of C-13 NMR spectra and spin-lattice relaxation time (T-1C). The aliphatic side group motion in the crystalline state weakened intermolecular pi-pi interaction, resulting in the blue shift of the characteristic absorption of the interchain exciton. Above the transition temperature, the thiophene twisting motion induces not only further collapsing of the intermolecular interaction but also localizing of the intrachain exciton, leading to the blue shift of the absorption of both the inter- and intrachain exciton.