Crystallization-Induced Emission Enhancement and Amplified Spontaneous Emission from a CF3-Containing Excited-State Intramolecular-Proton-Transfer Molecule

In this work, a simple but effective molecular design strategy is developed for the generation of intense blue emission in the solid state including single crystals through the multiple secondary intermolecular interactions such as CF...HC hydrogen bonding. The synthesized novel imidazole-based excited-state intramolecular proton transfer (ESIPT) molecule, 2-(1-(3,5-bis(trifluoromethyl)phenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenol (HPI-CF3), shows significantly enhanced blue fluorescence in single crystal (phi(F) = 0.67) compared to faint emission in solution (phi(F) < 0.05). It is considered that tight but slipped stacking structure of HPI-CF3 molecules in the single crystal not only effectively suppresses the nonradiative decay pathways such as twist intramolecular charge transfer but also induces highly allowed transition character. Taking advantage of the crystallization-induced emission enhancement characteristics and four-level ESIPT photocycle process of HPI-CF3, an efficient amplified spontaneous emission at 475 nm with a threshold of 13.1 mJ cm(-2) is observed from the single crystal by picosecond laser optical pumping.