4-Bromo-trans-cinnamic acid: structural characterisation and crystallographic investigation into the solid state [2 + 2] cycloaddition reaction and temperature induced phase transition

Abstract

4-Bromo-trans-cinnamic acid (1) displays some interesting behaviour in the solid-state undergoing a photo-induced head-to-head [2 + 2] cycloaddition reaction in addition to a reversible phase transition. Differential scanning calorimetry established that the thermal phase transition shows a slight hysteresis occurring at 252 K upon cooling and 256 K upon warming. Interestingly it was noted that upon photoirradiation above the phase transition temperature the compound appears to undergo a conversion to the low temperature form, prior to undergoing a cycloaddition reaction. Crystal structures of 1 were determined at 120, 240 and 300 K and found to be in the monoclinic space group P2₁/n at all three temperatures. The crystal structure of 1i was obtained, after recrystallisation of a powdered sample of 1 that had been irradiated at room temperature, and was found to be in the triclinic space group P. In all cases, the structures were found to be stabilised by the presence of O–H⋯O hydrogen bonding and C–H⋯O interactions. The results of these investigations are presented herein.