The conformational equilibrium and thermodynamic parameters of cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (1a) and its trans derivative 1c are presented. Irradiation of both compounds was carried out by using 365 nm light, which affects the cis/trans isomerisation process, whereas irradiation with 405 nm light preferentially promotes the formation of the cyclised product. Prolonged irradiation of 1a at 365 nm leads to the formation of a new derivative showing photochromic properties. Evidence for the formation of the novel derivative is presented, and details of its photochromic properties were investigated; a tentative mechanism has also been proposed. The equilibrium between parallel and antiparallel forms of cis- and trans-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene was studied by NMR spectroscopy. Photocyclisation or cis/trans isomerisation was induced by irradiation with 405 or 365 nm light. Extended irradiation of the cis compound generated two peroxy derivatives that thermally convert into a new coloured compound with its own photochromic properties.
