Twisted chromophore assist to tetrathiafulvalene-spiropyran hybrid driving four-state molecular switch

Abstract

A series of novel molecular hybrids, in which tetrathiafulvalene (TTF) unit and spiropyran (SP) core are covalently linked to various typical π-conjugated bridges, has been designed based on auxiliary donors and acceptors model and twisted intramolecular charge transfer (TICT) strategy. And the second-order nonlinear optical (NLO) responses of these molecular hybrids have been investigated by using density functional theory (DFT) calculations. The results show that grafting of the TTF unit and SP core to the twisted chromophore (TC) can effectively enhances second-order NLO responses. The promoting effect of enhancing NLO response can be attributed to a strongly favorable charge transfer in TTF-TC-SP hybrid. More importantly, we have found that this TTF-TC-SP hybrid with donor-π-conjugated bridge-acceptor structure can efficiently switch the first hyperpolarizability across four states via redox and photochemical stimuli. Meanwhile, time-dependent (TD) DFT calculations were carried out to probe the origin of the second-order NLO response of TTF-TC-SP molecular hybrid. All these findings provide a clear and well understanding of the relationship between the tunable first hyperpolarizability and electronic structure. We hope these findings would be very useful to guide the search for potential polymorphic molecular switch.