Photophysical change of poly(9,9-di(2-ethylhexyl)fluorene) and its copolymer with anthracene in solvent-non-solvent: Roles of interchain interactions on the formation of non-emissive and emissive aggregates

This contribution investigates photophysical change of poly(9,9-di(2-ethylhexyl)fluorene) (PF2/6) and its random copolymer with anthracene in solvent-non-solvent system. Isolated chains of the polymers in good solvent are driven to associate by addition of a poor solvent. The increase of interchain interactions upon decreasing solvent quality is found to cause the growth of a broad red-shift shoulder in absorption spectra of PF2/6. The measurements of photoluminescence (PL) spectra detect the increase of red-shift peaks at ∼440 nm and ∼525 nm. Detailed analyses of site-selective PL and PL excitation (PLE) spectra indicate the formation of emissive aggregates in this system. However, direct comparison of PLE and absorption spectra accompanied with the decrease of quantum yield suggests that different type of aggregates, the non-emissive one, also forms. The extent of interchain interactions dictates the existence of two types of aggregates. Our results signify the roles of interchain interactions on photophysical properties of PF2/6. Parallel studies of statistical copolymer of PF2/6 with anthracene groups (5 mol.%) observe drastic drop of the red-shift peak at ∼525 nm in PL spectra. The anthracenes exist as steric groups along the conjugated backbone, prohibiting the formation of emissive aggregates and/or excimers due to the increase of interchain proximity.