Intrachain photoluminescence properties of conjugated polymers as revealed by long oligothiophenes and polythiophenes diluted in an inactive solid matrix

The intrinsic intrachain photoluminescence (PL) dynamics of conjugated polymers in the solid state is investigated. We focus on the PL properties of long $\beta$-substituted oligothiophenes (8-mer, 12-mer, and 16-mer) and regio-regular (RR) and regio-random poly(3-octylthiophenes) (P3OTs) diluted in the inactive solid matrix polypropylene (PP). The oligothiophenes have well-resolved 0-0 PL and absorption peaks at $4\mathrm{K}$ and show a good linear relationship with the reciprocal of the ring number. Franck-Condon analyses on the PL spectra reveal that the strength of the electron-phonon coupling represented by the Huang-Rhys factor becomes weak with increasing chain length in the oligomers. In contrast, the presence of stronger electron-phonon coupling is confirmed in RR P3OT despite it being of a much longer chain length than oligomers. This is probably due to its stereoregular chain conformation. The materials used in this work diluted in PP exhibit single-exponential PL decays with lifetimes that decrease with increases in the chain length. The presence of intrachain exciton migration is shown by the time-resolved PL measurements, except for the case of RR P3OT. The energy shifts attributed to intrachain migration are much smaller than those ascribed to interchain migration. We conclude that a change in the torsion angle around the thiophene ring bonds occurs within $50\mathrm{ps}$ following the absorption transition in thiophene derivatives and that the torsional change gives rise to a constant Stokes shift $(60–70\mathrm{meV})$, regardless of the chain length. We propose that the Stokes shift can be utilized for estimating the magnitude of exciton migration in thiophene derivatives, even in the case of undiluted films.