Dual-State Emission in Molecular Rotors with Reorientable Benzotriazole Acceptors

Dual-State Emission (DSE) of organic molecules in both solution and solid-state is an elusive property that has recently drawn significant interest. In this work, we report two molecular rotors 1 and 2 with exceptional photoluminescence (PL) in solution (ФPL = 0.53 and ФPL = 0.43) and in the solid state (ФPL = 0.92 and ФPL = 0.84). To the best of our knowledge, this is the first report containing Donor-pi-Acceptor-pi-Donor (D-pi-A-pi-D) compounds with high quantum yields in both states that features a rotary acceptor. Furthermore, we describe the marked variations in PL in solution upon polarity and viscosity changes. TD-DFT computations indicate that these changes are facilitated by the fast molecular rotation in these molecules. The symmetry (or lack of it) in 1 and 2 plays a crucial role in producing noticeable differences in the X-ray crystal arrays with variations in the solid-state PL. We attribute this DSE behavior to the favorable combination of the D--A--D architecture and the twisted conformations adopted in the solid state.