Temperature dependent spectroscopy of poly[bis-(2-ethyl) hexylfluorene]/ (9,9-di- $n$ -octylfluorene) copolymers*

A series of random polyfluorene (PF8) copolymers comprised of linear di-$n$-octyl (F8) and branched bis(2-ethylhexyl) (F2/6) units has been synthesized and characterized by temperature dependent steady-state absorption and emission spectroscopy. The F2/6 polymer is already well known for forming conformationally disordered five fold helices while the F8 polyfluorene adopts a number of distinct near-planar type conformational isomers. One of these conformational sequences is an unusual low energy absorption and emission band known as the $\beta$ phase. In these copolymers the PF chains must temporize between differing interchain packing motifs, pentagonal and planar type structures, and different main chain morphologies. Increasing the content of F2/6 monomers strongly affects the formation of the $\beta$ phase conformer, enhances the extent of conformational disorder (and the effective electron-phonon coupling strengths) and also alters the bulk structural phase behavior. There are only weak correlations between the overall phase behavior and the observed spectroscopy at temperatures below 100~$^\circ$C.