The effect of molecular symmetry on the mechanofluorochromic properties of 4H-pyran derivatives

Abstract

Two series of D-π-A 4H-pyran derivatives using 1H-indene-1,3(2H)-dione as an electron-withdrawing, namely asymmetric AP-1-AP-3 and symmetric SP-1-SP-3, were synthesized to investigate the effect of the molecular symmetry on their mechanofluorochromic (MFC) activities. All these compounds with twisted molecular conformations exhibit obvious aggregation-induced emission properties caused by the restriction of intramolecular rotation in the aggregated sate. Although AP-1 and SP-1 are MFC-inactive, methyl-substituted AP-2 and trifluoromethyl-substituted AP-3 with low symmetry display reversible MFC activities, whereas the corresponding compounds SP-2 and SP-3 with high symmetry have no obvious solid-state emission color changes upon grinding. The results indicate the effect of the introduction of methyl and trifluoromethyl groups on the MFC properties of these compounds is controlled by the molecular symmetry. The MFC properties of AP-2 and AP-3 are ascribed to the phase transition from a crystalline state to another and an amorphous state, respectively, as revealed by X-ray diffraction and differential scanning calorimetry experiments. Our results indicate that the adjustment of the molecular symmetry can be regarded as a way of constructing a new and/or high-contrast MFC-active material from a particular MFC-inactive molecule.