Multi-Photochromic Molecules Based on Dihydroazulene Units.

Multi-photochromic systems incorporating individually addressable switching units are attractive for development of advanced data storage devices. Here, we present the synthesis and properties of a selection of such molecular systems incorporating the dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermoswitch. The influence of the linker (meta-phenylene vs. azulene-1,3-diyl vs. thiophene-2,5-diyl) separating two DHA units on the switching properties was investigated. An azulene-1,3-diyl spacer largely inhibited both the DHA-to-VHF photoisomerizations and the thermal VHF-to-DHA back-reactions; the latter occurred ten times slower than for the related compound with ameta-phenylene spacer. A DHA trimer containing three DHA units around a central benzene ring was found to undergo stepwise DHA-to-VHF photoisomerizations, whereas the thermal back-reactions occurred at similar rates for the three VHF entities. Ameta-phenylene-bridged DHA dimer was subjected to further structural modifications at position C-1 of each DHA, having strong implications for the switching events, and synthetic steps for further functionalizations at position C-7 of each DHA were investigated. Finally, the molecular structure (from X-ray crystallographic analysis) between themeta-phenylene-bridged DHA dimer and Cu(I)is presented.