Shear- and UV-induced Fluorescence Switching in Stilbenic Pi-dimer Crystals Powered by Reversible [2 + 2] Cycloaddition

We have designed and synthesized asymmetric cyano-stilbene derivatives containing trifluoromethyl (-CF(3)) substituents with the aim of producing tightly packed pi-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 pi-dimer stacks in crystals due to their specific intermolecular interactions, including C-F...H and C-F...pi interactions. The CN(L)-TrFMBE pi-dimer crystals (and powder) are not at all fluorescent initially but switch to a highly fluorescent state (Phi(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 pi-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 pi-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.