Orthogonal- and Path-Dependent Photo/Acidoswitching in an Eight-State Dihydroazulene-Spiropyran Dyad

Abstract

Molecules comprised of two or more switches have potential to exist in a variety of states, which may only be accessible by a “correct” sequence of stimuli on account of the influence of the neighboring unit. Here we present a straightforward synthesis of a dyad incorporating two photoswitchable units, dihydroazulene (DHA) and spiropyran (SP). The photoswitching properties of both chromophores and the acido-triggered switching of the SP were retained in the dyad, allowing selective switching between eight states with distinct absorption and fluorescence properties (corresponding to light-controlled fluorescence on/off switching). Photoisomerization of DHA and protonation of SP allowed an orthogonal control of the individual sites. Conversely, photoisomerization of SP to its merocyanine (MC) isomer could not be performed without also addressing DHA-to-vinylheptafulvene (VHF) switching. Access to the DHA-MC state thus appeared to be path-dependent as it could exclusively be reached by deprotonation of the thermally stable DHA-E-MCH.