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

Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the effect of polymer chain interactions on excited state relaxation. Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in 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, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.