Shear- and UV-induced fluorescence switching in stilbenic π-dimer crystals powered by reversible [2 + 2] cycloaddition

We have designed and synthesized asymmetric cyano-stilbene derivatives containing trifluoromethyl (-CF₃) substituents with the aim of producing tightly packed π-dimer systems that as crystals exhibit reversible [2 + 2] cycloaddition with characteristic fluorescence modulation. (Z)-3-(3′,5′-Bis(trifluoromethyl) biphenyl-4-yl)-2-(4′- (trifluoromethyl)biphenyl-4-yl)acrylonitrile (CN(L)-TrFMBE) and its derivatives were found to form antiparallel π-dimer stacks in crystals due to their specific intermolecular interactions, including C-F⋯H and C-F⋯π interactions. The CN(L)-TrFMBE π-dimer crystals (and powder) are not at all fluorescent initially but switch to a highly fluorescent state (Φ_PL = 24%) when an external shear-strain and/or prolonged UV (365 nm) irradiation is applied. Our experimental and theoretical investigations show that the fluorescence modulation in this particular system is due to the external and/or internal (in the case of UV irradiation) shear-induced lateral displacement of the π-dimer molecular pair, which effectively turns the fluorescence emission on at the cost of frustrated [2 + 2] cycloaddition. Further, the fluorescence 'off' state can be restored by thermal annealing, which regenerates the tightly packed π-dimer by reverse displacement together with the thermal dissociation of the [2 + 2] cycloaddition product. This system provides a very rare example of high-contrast reversible fluorescence switching that is driven by a change in the molecular packing mode in the solid state, which enables piezochromic and photochromic responses.