Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes

Absorbance and fluorescence spectra of dilute solutions and thin films of p olyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl gro ups at the 9-position define the effect of polymer chain interactions on ex cited state relaxation. Film morphology is controlled by annealing of 250 n m thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, i n turn, a function of the length of the attached alkyl substituents. Alkyl substituents also affect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, a ggregate and excimer formation is evidenced by a broadening of the absorban ce band, an increase in the intensity of the low energy emission, the appea rance of new long-lived fluorescent species, and structure-dependent change s in observed fluorescence quantum yields.