Interface specific variations in the steady-state optical properties of polyfluorene thin films

The top and bottom interfaces of spin cast poly(di-n-octylfluorene) (PF8) and poly[bis(2-ethylhex-yl)fluorene] (PF2/6) films are investigated by optical absorption and emission spectroscopy. In this study both the pure polyfluorene homopolymers and random copolymers comprised of linear di-n-octyl (F8) and the branched bis(2-ethylhexyl) (F2/6) units are used. PF2/6 is noted for forming a single conformationally disordered helix whereas PF8 adopts a number of near-planar conformational isomers. PF2/6, PF8 and their copolymers form asymmetric films in which the local structure at or near the top and bottom interfaces are distinctly different. This asymmetry is least apparent in pure PF2/6 films but increases in F8 containing polymers. In polymers containing a large fraction of F2/6 monomers the substrate-polymer side exhibits chains with slightly improved intrachain structural order and also increased green band emission. The effects of thermal and solvent vapor annealing are also reported.