Photophysics and solvent-induced aggregation of 2,7-carbazole-based conjugated polymers

We report on the optical, photophysical, and solvatochromic properties of five carbazole-based polymers, namely poly(N-octyl-2,7-carbazolediyl) (POC), poly(N-(2'-ethylhexyl)-2,7-carbazolediyl) (PEHC), poly(N-octyl-2,7-carbazolediyl-alt-2,5-thiophene) (POCT), poly(N-(2'-ethylhexyl)-2,7-carbazolediylalt-2,5-thiophene) (PEHCT), and poly(N-octyl-2,7-carbazoleneethynylene) (POCE). Compared to their corresponding triads, the absorption and fluorescence spectra of polycarbazoles in chloroform are red-shifted due to an increase of the effective conjugation length. Among the polycarbazoles investigated, the optical spectra of POCT are the more red-shifted due to the electron donor properties of the thiophene rings. The variation of the photophysical properties of the polycarbazoles mainly involves radiative processes (k(F)). From steady-state and time-resolved fluorescence studies in chloroform/methanol mixtures, all polymers have revealed solvatochromic effects, which have been related to the formation of H- and/or J-aggregates. On one hand, the aggregation process found on homopolymers (POC and PEHC) and copolymers involving thiophene units (POCT and PEHCT) is accompanied by a strong quenching of the photoluminescence efficiency. On the other hand, for POCE, the molecular interactions between the polymeric chains are not strong enough to induce a deactivation of the fluorescence. Finally, it is observed that the nature of alkyl chains on the nitrogen atoms of the carbazole moieties has a significant effect on the emission properties of the polymers.