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.

中文翻译：

---

新型2,3,4,6-四芳基-4H-吡喃衍生物的合成及其光物理和变色性质

三个新颖的2,3,4,6-四芳基-4 H-吡喃衍生物，在6-位具有苯（PR-Ph），三苯胺（PR-TPA）和9-苯基-9 H-咔唑（PR-Cz）设计和合成位置以研究可能的聚集诱导发射（AIE）和机械变色（MC）特性。正如理论计算和X射线晶体学分析所证实的，这三种化合物都采用了高度扭曲的分子构象。PR-Ph在普通有机溶剂中不发出荧光，而PR-TPA和PR-Cz含有强供电子基团的分子在溶剂中表现出红移的溶剂变色活性，并且由于分子内电荷转移（ICT）而导致极性增加。此外，发现四氢呋喃-水混合物中PR-TPA和PR-Cz的排放分别受较低含水量的ICT效应和较高含水量的聚集诱导的排放效应的支配。尤其是，结晶的PR-TPA表现出罕见的红移MC活性，并具有从结晶到非晶转变的“开启”荧光增强类型，而PR-Ph和PR-Cz由于结晶能力弱，因此没有MC活性。此外，研磨时PR-TPA的红移光谱应归因于增强的分子共轭作用，而增强的荧光量子效率归因于以下事实：致密的位错堆积通过抑制分子内运动而减少了非辐射能损失。结果表明在母体结构中引入不同的芳族基团可用于调节荧光分子的结晶能力以确定MC活性。