Dihydroazulene-Azobenzene-Dihydroazulene Triad Photoswitches

Photoswitch triads comprised of two dihydroazulene (DHA) units in conjugation to a central trans-azobenzene (AZB) unit were prepared in stepwise protocols starting from meta- and paradisubstituted azobenzenes. The para-connected triad had significantly altered optical properties and lacked the photoactivity of the separate photochromes. Instead, 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. The photoisomerizations were studied by ultrafast spectroscopy, revealing a fast DHA-to-VHF photoisomerization and a slower trans-to-cis AZB photoisomerization. This meta triad underwent thermal VHF-to-DHA back-conversions 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 computations (excitation spectra and orbital analysis of the transitions).