Effect of the dilution in polypropylene on photophysical properties of poly(3-alkylthiophenes)

Optical properties on the basis of absorption and photoluminescence (PL) have been examined for regioregular (RR-) poly(3-octylthiophene) (P3OT) and regiorandom (RRa-) P3OT diluted in polypropylene (PP). RR-P3OT gives blue-shifted spectra in both absorption and PL against the ones in the spin coated (SC) film, contrary to the case when diluted in poly-(methyl methacrylate) (PMMA). Spectroscopic features indicate that isolated species are primarily generated for RR-P3OT by the dilution using PP. The comparison in PL spectra between in the SC film and in diluted sample demonstrates that the species responsible for PL in the SC film are interchain excitons in RR-P3OT and intrachain excitons in RRa-P3OT. We further identified the enhancement in the PL quantum efficiency of RRa-P3OT as well as RR-P3OT by the dilution using PP, indicating that PL of conjugated polymers can be enhanced in the solid state by dilution using moderate inert polymers. We infer that RR-P3OT takes a highly rod-like structure in PP and that RRa-P3OT primarily takes a much less linear conformation in PP.