Synthesis and photophysical and mechanochromic properties of novel 2,3,4,6-tetraaryl-4H-pyran derivatives

Three novel 2,3,4,6-tetraaryl-4H-pyran derivatives with benzene (PR-Ph), triphenylamine (PR-TPA), and 9-phenyl-9H-carbazole (PR-Cz) at the 6-position were designed and synthesized to investigate possible aggregation-induced emission (AIE) and mechanochromic (MC) properties. These three compounds all adopted highly twisted molecular conformations, as confirmed by the theoretical calculation and X-ray crystallographic analyses. PR-Ph emitted no fluorescence in common organic solvents, while PR-TPA and PR-Cz containing strong electron-donating groups exhibited red-shifted solvatochromic activities in the solvents accompanied with the increase of polarity owing to intramolecular charge transfer (ICT). Furthermore, the emission of PR-TPA and PR-Cz in tetrahydrofuran-water mixtures was found to be dominated by the ICT effect with lower water contents and the aggregation-induced emission effect with higher water contents, respectively. In particular, crystalline PR-TPA exhibited a rare bathochromic MC activity with a "turn-on" fluorescence enhancement type through a crystalline-to-amorphous transition, whereas PR-Ph and PR-Cz showed no MC activities due to weak crystallization abilities. Moreover, the red-shifted spectrum of PR-TPA upon grinding should be attributed to the enhanced molecular conjugation, while the enhanced fluorescence quantum efficiency was attributed to the fact that the compact dislocation packing reduced the non-radiative energy loss by inhibiting the intramolecular motions. The results indicate that the introduction of different aromatic groups in a parent structure can be used to regulate the crystallization abilities of fluorescent molecules to determine the MC activities.