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Temperature Dependent Interplay between Emitting Species in Highly Ordered Poly (thiophenes) as Revealed by Optical Spectroscopy

Agumba O John and Fanuel K Mugwanga

The optical response of polymer aggregates has been described by considering coulombic interactions of transition dipoles that can couple in either a side-by-side orientation, (H-aggregates), or aligned along one dimension (J-aggregates). For thin-films of polythiophenes, it has been reported that both optical absorption and the photoluminescence spectra can be well explained by weakly coupled H-aggregates. This is due to dense π-stacking of the polymer chains with short distances between them. According to Kasha’s rule, vibrational relaxation to the lowest excited state takes place on a much shorter time scale than the lifetime of an exciton. The photon emission thus takes place from the lowest energy excited state. In ideal H-aggregates consisting of rigid molecules, the optical transition from the band bottom of the lowest excited state is optically forbidden, hence these aggregates do not show any fluorescence. The absorption and PL spectra of polythiophenes, are usually described in terms of intramolecular interactions coupled to only a single phononic mode. However, for highly organized/ordered single crystals of polythiopheses such as P3HT, this model is so simplistic and cannot fully describe the PL spectra due to the presence of both the intramolecular and intermolecular couplings showing up in the absorption and emission spectra. In this study, the temperature dependent PL spectra measurement has provided us a feasible means to elucidate the nature of the emissive species and the melt transitions in other polythiophenes.