Using potassium bromide pellets and optical spectroscopy to assess the photodimerization of two -(trifluoromethyl)-cinnamic acid compounds

The potential solid-state [2 + 2] photodimerization of two trans -cinnamic acid derivatives, trans -4-(trifluoromethyl)cinnamic acid (4-tfmca) and trans -3-(trifluoromethyl)cinnamic acid (3-tfmca), has been studied using potassium bromide (KBr) pellets and optical spectroscopy. As opposed to taking a powder or single crystal material as a sample, herein, we used polycrystalline samples incorporated in the KBr matrix (pellet) and were able to follow the course of the photochemical reaction via optical spectroscopy. The results show that the photodimerization of 4-tfmca within the KBr matrix yields a photodimer in roughly 100% conversion, while the changes observed for 3-tfmca within the KBr matrix suggest a subtle photochemical change which could not be observed when irradiating the neat powder. This suggests that the KBr matrix helps to couple light into the crystallites of the embedded sample and shows that the KBr matrix reveals photochemistry which might not have been found in the single crystal or powder state of the sample. Solid state photoreactions are an attractive route to produce special isomers due to the crystal packing of the starting molecules. We use powder samples pressed into potassium bromide pellets to investigate photoreactions with optical spectroscopy.