An unexpected phase transition during the [2 + 2] photocycloaddition reaction of cinnamic acid to truxillic acid: Changes in polymorphism monitored by solid-state NMR

We have detected a phase transition during the progress of the solid-state [2 + 2] photocycloaddition reaction of α-trans-cinnamic acid. The reaction was monitored using ¹³C CPMAS experiments as a function of irradiation time of the parent α-trans-cinnamic acid, which forms the product dimer, α-truxillic acid. UV light centered at 350 nm was used for photoirradiation, which is in the "tail" of the absorption band of cinnamic acid. Two different crystal polymorphs of a-truxillic acid are observed (P2₁/n and C2/c) at different stages of conversion of the parent crystal, assigned through ¹³C NMR and powder X-ray diffraction. The two polymorphs showed clear, distinguishable patterns in the ¹³C NMR spectra: a 2-peak versus 3-peak pattern corresponding to sites on the 4-membered sp³ hybridized ring in the photoproduct. A phase transition is observed midway through the reaction, which we have assigned to the conversion of the P2₁/n polymorph to the C2/c polymorph of α-truxillic acid. The packing energy of the resultant mixed crystal of cinnamic acid and truxillic acid changes during the course of the photoreaction, which allows for the C2/c polymorph of truxillic acid to appear. Both phases have been confirmed via X-ray powder diffraction. Two techniques - differential scanning calorimetry and solid-state CPMAS NMR using increasingly fast rotational frequencies - demonstrate that the P2₁/n phase is metastable.