Temperature dependence of intra-chain photoluminescence of a long oligothiophene



The temperature dependence (4 K-300 K) of the photoluminescence (PL) and absorption spectra of 16-mer oligothiophene (16T) diluted in polypropylene (PP) with a weight ratio of 1/30000 has been investigated in order to clarify the intrinsic temperature effect on conjugated polymers with negligible interchain interaction. The PL and absorption spectra are found to blueshift with increasing temperature. Simple model calculations suggest that the effective conjugation length of 16T is reduced to the two-third of the original chain length at room temperature. The magnitude of Stokes shift of 16T increases with increasing temperature. The steady-state PL spectra can be well reproduced by spectral fitting based on Franck-Condon analysis using a single Huang-Rhys (HR) factor over the wide temperature range of the measurement. This analysis demonstrates the temperature dependence of HR factor in PL. The HR factor in PL is found to decrease with increasing temperature, whereas that of the absorption spectra increases with temperature. This observation can be explained by assuming that the lowest energy conformer of 16T is responsible for PL and such a conformer has smaller coupling with phonons. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)