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 molecular properties and on the arrangement of molecules in the solid state. Reaction of 1,4-diiodobenzene with 4-fluorostyrene or pentafluorostyrene in the presence of Pd(OAc)2 gives trans-trans-bis(4-fluorostyryl)benzene (2Ft) and trans-trans-1,4-bis(pentafluorostyryl)benzene (10Ft), respectively. Bis(2,5-difluorostyryl)benzene (4Ft) was prepared by the reaction of 2,5-difluorobenzaldehyde 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 gives 1,4-bis(styryl)-2,5-difluorobenzene (2Fc), 1,4-bis(4-fluorostyryl)-2,5-difluorobenzene (2Fc2Ft), and 1,4-bis(pentafluorostyryl)-2,5-difluorobenzene (2Fc10Ft), respectively. Absorption spectroscopy shows that DSB, 2Ft, 2Fc, 2Fc2Ft, 4Ft, 10Ft, and 2Fc10Ft have a λmax at approximately 350 nm. Addition of dimethylaniline to hexane solutions results in exciplex emission with λmax ranging from 458 to 514 nm, depending on fluorine regiochemistry. Cyclic voltammetry shows that, as the fluorine load increases, the reduction of the DSB framework becomes more facile. It is also shown that the regiochemistry of substitution makes an impact on the inductive ability of fluorine to facilitate reduction. The lattice properties of 2Fc, 2Fc2Ft, 4Ft, 10Ft, and 2Fc10Ft were determined by X-ray diffraction experiments. Two structural motifs emerge from these studies. One is the tendency of the DSB framework to stack cofacially and form vertical “columns” within the crystal. The second motif is the alignment of these “columns” to maximize C−H···F electrostatic registry.