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Pure Appl. Chem., 2010, Vol. 82, No. 4, pp. 843-852

Published online 2010-03-09

New routes to functionalized dihydroazulene photoswitches

Mogens Brøndsted Nielsen*, Søren Lindbæk Broman, Michael Åxman Petersen, Asbjørn Sune Andersson, Tanja Skouw Jensen, Kristine Kilså and Anders Kadziola

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK‑2100 Copenhagen Ø, Denmark

Abstract: Molecular photoswiches are important for the development of advanced materials and molecular electronics devices. The dihydroazulene (DHA) is a particularly attractive molecule as it undergoes a light-induced ring opening to a vinylheptafulvene (VHF) isomer with altered optical properties. While functionalization of the five-membered ring of DHA has been possible for the last 25 years, this was not the case for the seven-membered ring. This article summarizes our synthetic efforts in achieving this goal. Incorporation of an alkyne at C-7 was accomplished by (i) regioselective bromination of DHA, followed by (ii) elimination of HBr, and (iii) Pd-catalyzed cross-coupling with triisopropylsilylacetylene. Light-induced ring opening of this DHA followed by thermal ring closure provided a mixture of 6- and 7-substituted DHAs with different absorption characteristics. The isomer ratio was controlled by the wavelength of irradiation and the solvent polarity. The dibromide formed in the initial step served as a precursor for a 3-bromo-functionalized azulene that was employed as a building block for acetylenic scaffolding. Incorporation of a dithiafulvene (DTF) unit at the five-membered ring of DHA resulted in a significant red-shift in the longest-wavelength absorption and consequently a lowering of the energy required for ring opening. Incorporation of a redox-active tetrathiafulvalene (TTF) unit allowed for redox-controlled photoswitching.