Multi-mode switching based on dihydroazulene/vinylheptafulvene photochromism: Synergism of photochromism and redox switching in heteroaryl-functionalized systems

The syntheses of the dihydroazulenes (DHAs) DHA-a-DHA-f containing covalently linked heteroaromatic subunits derived from dibenzodioxin, thianthrene, phenoxathiine, N-methylphenothiazine, N-methylphenoxazine, and N,N'-dimethylphenazine groups are described, and their spectroscopic and analytical data are reported. The dihydroazulene/vinylheptafulvene (DHA/VHF) photochromism (photochemical forward and thermal back reaction) depends with high sensitivity on the electronic properties of the functional groups. Whereas the dimethylphenazine derivative DHA-f is photochemically inactive towards rearrangement, all other DHAs (DHA-a-DHA-e) were found to isomerize to the corresponding vinylheptafulvene forms under irradiation. Cyclic voltammetry revealed that the DHA and the VHF forms have significantly different oxidation and reduction waves. The products of the oxidative one-electron transfer are characterized by UV/Vis/NIR spectroelectrochem istry. Those DHAs having weaker donor substituents (DHA-a-DHA-c) undergo oxidative dimerization whereas DHAs with stronger donating heterocyclic subunits (DHA-d-DHA-f) form stable radical cations.