Cationic polyfluorene: Conformation and aggregation in a “good” solvent

Abstract

The conjugated polyelectrolyte (CPE) poly{9,9′-bis[6″-( N, N, N-trimethylammonium)-hexylfluorene- alt- co-phenylene] dibromide} (PFPN +Br −) demonstrates a high solubility in methanol in comparison to other more hydrophilic or hydrophobic solvents. We have employed a combination of pulsed-field-gradient-NMR, photoluminescence (PL), and Raman spectroscopy to establish the conformation and aggregation behavior of PFPN +Br − in methanol, with the aim to attain information on how to design CPEs with a high solubility in a preferred solvent. We find that the diffusion coefficient and PL spectrum of PFPN +Br −, as well as the Raman-active methyl rocking mode of methanol, all exhibit a strong dependence on PFPN +Br − concentration. We rationalize our findings with a model in which PFPN +Br − forms aggregates via π–π interactions between main-chain segments, while the ionic side chains are surrounded and electrostatically screened by the methanol solvent. Accordingly, the notably high solubility of PFPN +Br − in methanol is rationalized by favorable interactions between the ionic side chains and the methanol molecules. We propose that an appropriate design of a high-solubility CPE should consider a matching of the mixed hydrophobic/hydrophilic character of the ionic side chain with that of the preferred solvent.