Electronic states of diarylethene switch-gold nanocluster hybrids

Diarylethenes (DAEs) are promising molecular photoswitches for optoelectronic devices and light-responsive nanomaterials. For potential applications, they can be mounted on metal surfaces or nanoparticles, which, in turn, may affect their switching behaviour. In this work, the electronic structure, reaction energetics, and optical properties of open- and closed-ring stiff DAE adsorbed on Au _n clusters (n = 10, 19, and 20) are investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The latter is combined with fraction of transition density matrix (FTDM) analysis to characterize excited states by distinguishing between molecule-localized, gold-cluster-localized, and charge-transfer excitations. We find that interaction with gold clusters lowers the calculated ground-state isomerization barrier (through the formation of a molecular radical), reduces the energetic separation between the open and closed forms, and introduces new low-energy electronic transitions. The low-lying excited states of the hybrid systems show mixed molecular, cluster-localized, and bidirectional charge-transfer character. The obtained results demonstrate that nanoscale gold can tune both the energetics and the optical response of DAE photoswitches, providing microscopic insights essential for the design of light-controlled molecule-metal hybrid systems.