In situ illumination with inelastic neutron scattering: a study of the photochromic material cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE)

For the first time we successfully demonstrate in situ illumination of a photochromic material during an inelastic neutron scattering (INS) experiment at cryogenic temperatures. cis-1,2-Dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE) is a well-studied and commercially available photochromic compound that undergoes a cyclisation reaction when exposed to light with a wavelength of around 400 nm. CMTE was therefore used as a proof of concept material to demonstrate the new sample environment used on the TOSCA indirect geometry spectrometer. When combined with simulated INS spectra from DFT calculations, the resulting difference spectra were interrogated to obtain insight into how the photoisomerisation affects the vibrational modes of the material. It was found that rigidification of the molecule after illumination, caused by the cyclisation of the central six-membered ring in CMTE, led to a blue-shifting in the methyl group torsion modes as well as methyl group scissor modes.