Thermooxidative stability of spectra of fluorene-based copolymers

The origin of the low-energy emission of fluorene-based homo- and copolymers still remains controversial. In this work, the effect of thermal treatment on the emission properties of poly[(9,9-dihexylfluorene)-alt-co-(1,4-phenylene)] (PF6P) and its four derivatives modified by attaching different lengths of alkoxy side chains on the phenylene rings has been systematically investigated. By comparing the photoluminescence (PL) spectra of PF6P and the modified polymers, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and PL lifetime measurements have revealed that the long wavelength emission could be attributed to the formation of fluorenone-based excimers rather than to the localized fluorenone π-π* transition, the energy transfer from fluorene segments to the fluorenone moieties, or the fluorenone defects generated by thermal oxidation during thermal treatment. Compared with PF6P, the attachment of alkoxy side chains on the phenylene rings effectively inhibits the aggregation of backbone chains, thus restrains the formation of fluorenone-based excimers and remarkably improves thermal stability of the spectra.