Nature and dynamics of photoexcited states in an electroluminescent poly(phenylene vinylene-co-fluorenylene vinylene)-based π-conjugated polymer

This paper describes the optical properties of an electroluminescent poly(phenylene vinylene-co-fluorenylene vinylene) (BPPPV-PF)-based π-conjugated polymer using absorption, photoluminescence (PL), time-resolved photoluminescence (TRPL), continuous wave (CW) and transient-photoinduced absorption (PA) spectroscopic techniques. The TRPL decay spectra of BPPPV-PF in chloroform and film exhibit single exponential decay with PL lifetimes of 0.8 ns and 0.2 ns, respectively. The CW-PA spectrum exhibits a single well-defined band centred at 1.5 eV, which is assigned to the T–T* transition of the triplet excitons. The monomolecular lifetime (τ) of the triplet excitons was estimated from the intensity dependence of CW-PA and found to be ∼1.2 ms at 80 K. The temperature dependence of the CW-PA signal at 1.5 eV was studied for a temperature range from 80 K to 298 K. It was observed that the PA signal at 1.5 eV was persistent to relatively high temperatures, which may be due to the bulky side chain of the polymer and morphology of the film. The relaxation process of this triplet excitation was studied by measuring the transient decay of the PA signal for various temperatures. The transient-PA signal shows monomolecular recombination process at all recorded temperatures and the temperature dependence of monomolecular lifetime (τ) was studied. It was found that the monomolecular lifetime (τ) is dependent on temperature but independent of the laser excitation intensity. The obtained results are analyzed based on the molecular structure of the PPV-PF copolymer.