Correlation between the microscopic morphology and the solid-state photoluminescence properties in fluorene-based polymers and copolymers

The microscopic morphology of a series of substituted fluorene-based conjugated polymers and copolymers are analyzed with tapping-mode atomic force microscopy. Different structures are observed depending on the nature of the substituents. Thin deposits of polyfluorenes substituted with linear alkyl groups are made of long fibrils, with lateral dimensions on the order of a few nanometers; polymers with branched alkyl or aromatic substituents form homogeneous, featureless films. To understand how polymer chains pack into these structures, comparisons are made with molecular modeling calculations; the simulation results highlight the dependence between intermolecular pi-pi interactions and steric hindrance among substituents: linear alkyl substituents allow for a close packing of the conjugated chains into very long, regular pi-pi stacks, in contrast to the bulkier substituents. A strong correlation is established between the degree of order in the thin deposits and the solid-state photoluminescence spectra; a red shift and the formation of a broad emission band in the green region are observed for deposits showing long-range organization, which is attributed to the formation of aggregates of well-organized, densely packed molecules.