Dihydroazulene‐Azobenzene‐Dihydroazulene Triad Photoswitches

Photoswitch triads comprising two dihydroazulene (DHA) units in conjugation with a central trans‐azobenzene (AZB) unit were prepared in stepwise protocols starting from meta‐ and para‐disubstituted azobenzenes. The para‐connected triad had significantly altered optical properties and lacked the photoactivity of the separate photochromes. In contrast, for the meta‐connected triad, all three photochromes could be photoisomerized to generate an isomer with two vinylheptafulvene (VHF) units and a cis‐azobenzene unit. Ultrafast spectroscopy of the photoisomerizations revealed a fast DHA‐to‐VHF photoisomerization and a slower trans‐to‐cis AZB photoisomerization. This meta triad underwent thermal VHF‐to‐DHA back‐conversion with a similar rate of all VHFs, independent of the identity of the neighboring units, and in parallel thermal cis‐to‐trans AZB conversion. The experimental observations were supported by computation (excitation spectra and orbital analysis of the transitions). The importance of connections: The photoactivity of dihydroazulene‐azobenzene‐dihydroazulene (DHA‐AZB‐DHA) triad photoswitches depends strongly on the para/meta connectivity at the central azobenzene unit. The synthesis and switching properties of two triad systems are presented here. The meta connectivity allowed reversible photo‐/thermoswitching between DHA‐trans‐AZB‐DHA and VHF‐cis‐AZB‐VHF where VHF is the vinylheptafulvene photoisomer of DHA. Ultrafast spectroscopy revealed the DHA‐to‐VHF photoisomerization to be faster than the trans‐to‐cis AZB photoisomerization.