Sterically Overcrowded Alkenes; a Stereospecific Photochemical and Thermal Isomerization of a Benzoannulated Bithioxanthylidene

Enantiomerically pure sterically overcrowded 12H-benzo[a]thioxanthenyl-12-(2'-methyl-9'H-thioxanthene-9'-ylidene) shows a stereospecific photochemical and thermal isomerization of the P-trans into the M-cis isomer (and vice versa), with a large thermal isomerization barrier (ΔG‡ = 28.6 kcal mol–1) (1 cal = 4.184 J), making this type of molecule especially suitable as the key structural unit for applications as chiroptical molecular switches.