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

4-Bromo-trans-cinnamic acid (1) displays some interesting behaviour in the solid-state undergoing a photo-induced head-to-head [2 + 2] cydoaddition 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 P2(1)/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 (1) over bar. In all cases, the structures were found to be stabilised by the presence of O-(HO)-O-... hydrogen bonding and C-(HO)-O-... interactions. The results of these investigations are presented herein.