The effect of fluorine substituents on the physical and structural properties of conjugated molecular materials

Abstract

A series of selectively fluorinated tolans of the general formulae C(_6)F(_5)-C=C-C(_6)H(_4)X and C(_6)H(_5)-C=C-C(_6)F(_4)X (where X = I, Br, CI) have been synthesized via homogeneous palladium-catalysed Sonogashira cross-coupling and organolithium chemistry. Several of their crystal structures have been solved from X-ray diffraction data, and their molecular packing is described in terms of arene-perfluoroarene and halogen-halogen interactions. Diffraction-quality crystals of a number of binary arene-perfluoroarene complexes of hexafluorobenzene and octafluoronaphthalene with several mismatched polyaromatic hydrocarbons have been obtained and their crystal structures solved from X-ray diffraction data. All of the structures have been shown to consist of infinite stacks of alternating components. The individual structures are compared and contrasted in detail, and those of the HFB complexes are found to closely resemble those predicted from ab initio DFT calculations, which implies that the interactions are over 90 % electrostatic in nature, in contrast with previous calculations on related complexes. A number of selectively fluorinated 4,4'-bis(phenylethynyl)tolan (BPET) derivatives containing fluorinated and non-fluorinated phenyl rings, have been synthesized from palladium-catalysed Sonogashira cross-coupling of various tolan-based precursors. They are observed to strongly absorb in the UV range 336 - 342nm, which are directly comparable to the absorptions for similarly fluorinated 1,4-bis(phenylethynyl)benzene derivatives which suggests that an effective conjugation length (ECL) of 3-4 repeat units is applicable for these phenylene ethynylene systems. They are observed to fluoresce very strongly in the range 372-410 nm. The diethynylbenzene derivatives 1,4-diethynyltetrafluorobenzene and 1,4- diethynyl-2,5-difluorobenzene have been synthesized from the hydrodesilation of their trimethylsilylated precursors. Their crystal structures have been solved from X-ray diffraction data, and are described in terms of C=C-H F and C=C-H π(C=C) interactions.