Abstractmagnified imageThe 3,6‐bis{(1E)‐2‐[4‐(dodecyloxy)phenyl]ethenyl}pyridazine (1a) was regio‐ and stereoselectively brominated or chlorinated to the (Z,Z)‐compounds 3 and 6, respectively. The conjugated core of these systems represents a mesogen for smectic (and nematic) liquid crystals and, moreover, a chromophore with an absorption at the UV/VIS border. Irradiation and efficient intersystem crossing to the triplet state leads to isomerization reactions (Z,Z)→(E,Z) in solution as well as in the thermotropic LC phases. Since (E,Z)‐configurations are only tolerated to a very small extent by the mesophases, the photoinduced transition S→N→I of 6 can be used as an imaging technique.
3,6-Bis(styryl)pyridazines 1a–f with 2–6 alkoxy groups show on irradiation in solution a stereoisomerization which leads to a photostationary state of (E,E)- and (E,Z)-isomer. Sensitizing and quenching experiments reveal that the (E,E)→(E,Z) route is a pure triplet process, whereas the (E,Z)→(E,E) route can have a minor singlet by-reaction.