Towards multifold cycloswitching of biphotochromes: investigation on a bond-fused dihydroazulene/vinylheptafulvene and dithienylethene/dihydrothienobenzothiophene

The concept of molecular switching within a cyclic four-stage process is in troduced on the basis of two structurally fused photochromic systems (A-D). The synthesis of 1,8a-dihydro-2,3-bis(2,5-dimethyl-3-thienyl)azulene-1,1-d icarbonitrile (open/closed A) is described; irradiation with UV light leads to an equilibrium of open/open B, open/closed A and closed/closed C, where as B is thermally rearranged to A by ring closure of the vinylheptafulvene substructure.