Observation of a non-conventional Horner–Wadsworth–Emmons olefination product and the effect of the lateral ethyl substitution on the solid state fluorescence

Abstract

Donor-acceptor-substituted diphenylbutadienes, namely 1-(p-methoxyphenyl)-4-(p-cyanophenyl)buta-1E,3E-diene (MCB) and 1-(p-N,N-dimethylaminophenyl)-4-(p-cyanophenyl)buta-1E,3E-diene (ACB) exhibit fluorescence properties in their solid that are distinctly different from their fluorescence in solution. The red-shifted absorption and emission spectra observed in the solid state are attributed to the formation of J aggregates. Preparation of these derivatives via a Horner–Wadsworth–Emmons reaction, wherein the phosphonate obtained by treating p-cyanobenzyl bromide with triethyl phosphite was condensed with corresponding cinnamaldehydes, also yielded the non-conventional olefination products 1-(p-methoxyphenyl)-4-(p-cyanophenyl)-4-(ethyl)buta-1E,3E-diene (MCBE) and 1-(p-N,N-dimethylaminophenyl)-4-(p-cyanophenyl)-4-(ethyl)buta-1E,3E-diene (ACBE), which bear an ethyl group substituent on their butadiene chain. The formation of these products suggests a base-catalyzed 1,3-migration of an ethyl group from an oxygen center to the benzylic position in the initially formed phosphonate. The presence of the ethyl group in an otherwise planar molecule was observed to significantly hinder aggregation in the solid state, resulting in molecule-like fluorescence even in their bulk state.