The effect of molecular symmetry on the mechanofluorochromic properties of 4H-pyran derivatives. (March 2019) - Wang, Zhiqiang
Abstract: Two series of D-π-A 4 H -pyran derivatives using 1 H -indene-1, 3(2 H )-dione as an electron-withdrawing, namely asymmetricAP-1 -AP-3 and symmetricSP-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. AlthoughAP-1 andSP-1 are MFC-inactive, methyl-substitutedAP-2 and trifluoromethyl-substitutedAP-3 with low symmetry display reversible MFC activities, whereas the corresponding compoundsSP-2 andSP-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 ofAP-2 andAP-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. Graphical abstract: Highlights: Synthesis and characterization of two series of new D-π-A 4 H -pyran derivatives. These 4 H -pyran derivatives exhibit aggregation-induced emission properties. The molecular symmetry plays an important role in determining the generation of MFC phenomenon.