Anisotropic Photophysical Properties of Highly Aligned Crystalline Structures of a Bulky Substituted Poly(thiophene)

The photophysical properties of a phenyl-substituted poly(thiophene), poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT), were studied as a function of polarization and degree of orientation of the crystalline structure. Under well-chosen controlled conditions, large-sized spherulitic crystals of PDOPT were successfully prepared from the melt. From polarized optical microscopy and X-ray diffraction, the molecular orientation of PDOPT within the spherulite was determined, indicating that the fastest growth direction of the spherulite was the a-axis. This implied that crystallization of PDOPT was directed by the packing of the side chains rather than the backbones, which are significantly separated. As the crystalline lamellae were all radially oriented, the local absorbance strongly depended on the polarization of the incoming light. Compared to randomly oriented crystals in a quenched and thus rapidly crystallized sample, PDOPT spherulites displayed red-shifted absorption and emission spectra, combined with a reduced photoluminescence quantum yield. Even for these markedly separated polymer backbones (1.47 nm), the reduced photoluminescence suggests an enhancement of interchain interactions of highly ordered bulky substituted polythiophene induced by crystallization.