Tunable Emission of Organic Fluorescent Crystals through Polymorphic Manipulation

Development of tunable-emission solid fluorescent materials remains a challenge due to tedious synthesis strategies. Polymorphism plays a critical role in tuning solid state photophysical properties of crystalline materials. In this work, tunable emission of organic fluorescent crystals was achieved through polymorphic manipulation. The X-shaped tetrasubstituted molecule 2,5-bis(5-methyl-2-thienyl) terephthalaldehyde was synthesized as a model compound, which is pH insensitive and has large Stokes shifts. It adopts an extended conjugated system and enhanced conformational flexibility and occupies an intercrossed excited state of charge-transfer (CT) and local excitation (LE). Two polymorphs were prepared by controlling the cooling rate. The polymorphism-dependent emission and the interconversion between Form I (530 nm) and Form II (570 nm) provide an opportunity to unveil the inherent mechanism of different emission and the possible relationship between structure, packing, and function. With the incorporation of X-ray photoelectron spectroscopy, solid-state 13 C NMR spectra, crystal structure analysis, and computational simulation, the dissimilar molecular structure and packing modes were intuitively and systematically investigated. Noncovalent interaction (NCI), atoms in molecules (AIM) approaches, and Hirshfeld surface analysis were used to further explain the fluorescence tuning from both different molecular packing modes and intermolecular interactions. The results demonstrated that electron delocalization degree and overlap of adjacent aromatic systems were responsible for the dissimilar photophysical properties.