Photophysical and Spectroscopic Investigations on (Oligo)Thiophene-Arylene Step-ladder Copolymers. The Interplay of Conformational Relaxation and On-Chain Energy Transfer

An optical spectroscopy and photophysics study on four (oligo)thiophene-phenylene and (oligo)thiophene-naphthylene step-ladder type copolymers in solution (room and low temperature) and in the solid state (thin film) is presented. The study involves absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation), excited-state lifetimes, and singlet and triplet energies. The overall data allow for a determination of the rate constants for all decay processes and from these several conclusions could be drawn: (1) in solution the main deactivation channels are radiationless processes (S-1 similar to-->S-0 internal conversion and S-1 similar to-->T-1 intersystem crossing); (2) from time-resolved fluorescence decays in the picosecond time domain three decay components are seen: a fast decay (10-20 ps) at short wavelengths, which becomes a rising component at longer wavelengths, an intermediate decay component (120-190 ps) most probably associated to isolated conjugated segments, and a third exponential related to the emission of the fully relaxed polymer. The assignment of the fast decay component to an on-chain energy transfer/migration is based of the dependence of the decay time on the solvent viscosity in combination with the investigation of an oligomeric model compound. Here, the absence of any significant changes of the decay parameters (decay times and pre-exponential factors) upon going from a less (toluene) to a more viscous (decalin) solvent together with the monoexponential fluorescence decay of the oligomeric model compound allow us to differentiate between deactivation of the singlet excited state by conformational relaxation and on-chain energy/transfer migration.