Polymorph-Dependent Multicolor-Switchable Mechanofluorochromism of 1-(9-Anthryl)vinyl-3-(3-pyridyl)vinylbenzene: Molecular Conformation versus Intermolecular Interaction
Yexin Li, You-Gui Chen, Zhenfeng Yu, Xiaofeng Yang, Yong Nie, Yu Cui, Guoxin Sun
Abstract
The mechanism of most mechanofluorochromic (MFC) materials is related to a noncovalent change in aggregate packing. It is still a challenge to clarify the respective effects of molecular conformation and intermolecular interaction on the MFC performance. Herein, a new (9-anthryl)vinyl derivative APYB with a polymorph-dependent multicolor-switchable MFC property is reported. APYB has only one torsion unit in the molecule configuration and could form four different crystal phases (APYB-a, APYB-b, APYB-c, and APYB-d). Furthermore, all polymorphs are MFC-active. Therefore, it provides a rare and direct case to correlate the aggregate packing with MFC performance. The solid-state emission could be finely tuned from 465 to 500 nm by virtue of polymorphism. Crystal structure analyses and optical studies reveal that an increasing twist angle could result in a blue shift in the fluorescence spectrum and a high-contrast MFC performance. In contrast, strong intermolecular interactions could lead to high emission efficiency and amorphization difficulty during the mechanical grinding. A tricolor-switchable MFC phenomenon was observed for APYB-a, while both APYB-b and APYB-d display a reversible bicolor-changing MFC behavior. As for APYB-c, mechanical grinding only causes a minor change in the emission spectrum. However, two competitive reverse transformation routes were found when treating the ground sample with organic solvent vapor. XRD analyses show that the MFC mechanism for APYB-a, APYB-c, and APYB-d is a crystal-to-amorphous transformation, while only partial amorphization was observed in the MFC behavior of APYB-b. The acidochromism effects of APYB-a and APYB-b were also investigated. They also display multicolor changes due to the polymorphic nature of APYB. Interestingly, in the deprotonation process, NH3 vapor could induce a different transformation route and color change from NEt3.