Unusual photophysical properties of conjugated, alternating indigo–fluorene copolymers

Abstract

Alternating indigo–fluorene copolymers have been synthesized by the coupling of didromoindigo and fluorendiboronic ester monomers. The low solubility of the copolymers only allowed for the synthesis of moderate molecular weight copolymers, with a degree of polymerization (DP) up to 11. The syntheses were accomplished through a 10% excess of the fluorene-based monomer component in an AA/BB-type polycondensation mixture. Next, a comprehensive spectroscopic (singlet–singlet and transient – from fs to μs – absorption, fluorescence and phosphorescence spectra) and photophysical investigation (fluorescence, phosphorescence and triplet lifetimes together with fluorescence and triplet occupation and singlet oxygen sensitization quantum yields) of the copolymers was carried out. The experiments were complemented with the spectroscopic results from a fluorene–indigo–fluorene model compound, as well as by TDDFT calculations. Based on our kinetics analysis, singlet energy transfer from the fluorene to indigo moieties is found to be inefficient. Besides the low energy indigo-related absorption band, an additional intermediate energy absorption band is also observed between 400 nm and 500 nm, both for the copolymer and for the model compound. Excitation into this band causes an emission of the indigo moiety. The triplet state is found to be mainly localized at the fluorene moiety; however, the decrease of the phosphorescence quantum yield (ϕ_Ph) when going from the monomeric 9,9-bis(dodecyl)fluorene (0.075) to the model trimer (0.003) and copolymer (ϕ_Ph = 0.008) suggests that excitation energy transfer occurs in the triplet state. This is further confirmed by the higher level of delocalization of the transient triplet–triplet absorption spectra of the copolymer relative to the monomeric 9,9-bis(dodecyl)fluorene.