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 (ΦPL = 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.