Fluorinated distyrylbenzene chromophores: Effect of fluorine regiochemistry on molecular properties and solid-state organization

A series of fluorinated distyrylbenzene (DSB) derivatives were synthesized and studied in order to probe the effect of fluorine substitution on molecu lar properties and on the arrangement of molecules in the solid state. Reac tion of 1,4 -diiodobenzene with 4-fluorostyrene or pentafluorostyrene in th e presence of Pd(OAc)(2) gives trans-trans-bis(4-fluorostyryl)benzene (2F(t )) and trans-trans-1,4-bis(pentafluorostyryl)benzene (10F(t)), respectively . Bis(2,5-difluorostyryl)benzene (4F(t) was prepared by the reaction of 2,5 -difluorobenzal dehyde with p-xylylenebis-(triphenylphosphonium bromide) in the presence of LiOEt in EtOH. Coupling of 1,4-dibromo-2,5-difluorobenzene with styrene, 4-fluorostyrene, or pentafluorostyrene using Pd(OAc)(2) give s 1,4-bis(styryl)-2,5-difluorobenzene (2F(c)), 1,4-bis(4-fluorostyryl)-2,5- difluorobenzene (2F(c)2F(t)), and 1,4-bis-pentafluorstyryl)-2,5-difluoroben zene (2F(c)10F(t)), respectively. Absorption spectroscopy shows that DSB, 2 F(t), 2F(c), 2F(t),2F(t), 4F(t), 10F(t), and 2F(c),10F(t) have a lambda(max ) at approximately 350 nm. Addition of dimethylaniline to hexane solutions results in exciplex emission with lambda(max) ranging from 458 to 514 nm, d epending on fluorine regiochemistry. Cyclic voltammetry shows that, as the fluorine load increases, the reduction of the DSB framework becomes more fa cile. It is also shown that the regiochemistry of substitution makes an imp act on the inductive ability of fluorine to facilitate reduction. The latti ce properties of 2F(c), 2F(c),2F(t), 4F(t),10F(t), and 2F(c),10F(t) were de termined by X-ray diffraction experiments. Two structural motifs emerge fro m these studies. One is the tendency of the DSB framework to stack cofacial ly and form vertical "coIumns" within the crystal. The second motif is the alignment of these "columns" to maximize C-H ... F electrostatic registry.