Visualization of Single-Crystal-to-Single-Crystal Phase Transition of Luminescent Molecular Polymorphs

Chao Ge, Jie Liu, Xin Ye, Quanxiang Han, Leilei Zhang, Shuangyue Cui, Qing Guo, Guangfeng Liu, Yang Liu,* and Xutang Tao*

J. Phys. Chem. C 2018, 122, 15744−15752

Abstract

Single-crystal-to-single-crystal (SCSC) phase transition is an ideal model to study the structural correlation between the polymorphs in molecular level. In this regard, a transition process with concomitant emission color change is in favor of direct visualization by fluorescence microscope. Here we report an SCSC transition on the luminescent single crystal of large conjugated molecules, which is accompanied by a drastic luminescence color change from red to orange upon heating. The transition process was clearly recorded under both fluorescence and polarized light. Combining with crystallographic analysis, the results indicate that the existence of molecular layers and the oriented motion of the interface between the daughter and the parent phase preserved the integrity of the single crystal, despite of remarkable changes of both conformational and supramolecular structure. Thus the transition is rationalized to proceed by a nucleation-and-growth mechanism but not a martensitic one. This work on one hand deliver intuitive cognition about the polymorph-dependent optical properties and the mechanism of the phase transition between the polymorphs, and also proved the paramount importance of direct microscopy observation about the actual transition process.