Trifluoroborate-modification of both pyridine and N-alkyldiarylamine-based π-conjugated polymer films: tuning the electronic communication and the mean conjugated length based on two types of nitrogen in the conjugated main segments

Abstract

We have demonstrated trifluoroborate-modification of both poly(2,6-pyridine-alt-9,9-dioctylfluorene-2,7-diyl) and poly(4,4′-diphenyl-N-hexylamine-alt-9,9-dioctylfluorene-2,7-diyl) (PPF and PAF, respectively) films to tune the electronic interactions and the mean conjugated lengths in the conjugated main segments. After BF₃–OEt₂ vapor treatment, both the absorption and fluorescence spectra of the PPF film were red-shifted due to the charge transfer interactions between the electron-accepting trifluoroborate-pyridine moiety and the weak electron-donating 9,9-dialkylfluorene unit. On the other hand, the transformation from an sp²- to an sp³-nitrogen in the main segment of the PAF film via Lewis acid (BF₃–OEt₂) vapor treatment resulted in a short mean conjugated length thus leading to a blue-shifted absorption band. Coordination between the BF₃ and nitrogen moieties in the PPF and PAF films was successfully confirmed by high performance solid-state ¹⁹F MAS NMR spectroscopic measurements. The 5% weight loss decomposition temperatures of both unmodified and BF₃-modified polymers were almost the same under nitrogen. After Lewis base (Et₃N) vapor treatment to restore these modified polymer films to the unmodified films, the absorption and fluorescence bands of the PPF film blue-shifted and corresponded to that of the unmodified PPF. In contrast, the bands of the PAF film were red-shifted due to extension of the π-conjugated system by transforming from an sp³- to an sp²-nitrogen, and corresponded to that of the unmodified PAF film. It is noteworthy that the reversible process could be cycled many times.