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

Abstract

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 CF…HC 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 (ΦF = 0.67) compared to faint emission in solution (ΦF < 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.

Null Exciton Splitting in Chromophoric Greek Cross (+) Aggregate

“…Geometrical rulers, that is, relative spatial orientation, π‐overlap, and intermolecular distances ( d ) between the chromophores govern the electronic communication and synchronously the optical response of the crystalline scaffold . Inevitably, assemblies of π‐chromophores typically encounter bottlenecks, such as concentration fluorescence quenching mediated by trap states and/or exciton coupling . Molecular exciton theory accounts coherent coupling among the co‐facial, parallel transition dipoles (H‐aggregate) for the quenched fluorescence (Scheme ) .…”

Natural Born Laser Dyes: Excited-State Intramolecular Proton Transfer (ESIPT) Emitters and Their Use in Random Lasing Studies

“…Efficient solid-state emitters find significant applications including security printing, optoelectronics and amplified stimulated emission (ASE) devices [27]. Among ASE dyes, very few are based on an ESIPT process and are all studied in the crystalline state [28,29,30,31,32,33] which can be tricky to obtain, since the presence of alkyl groups on the molecular core, usually required to ensure solubility during the synthesis, can impede the crystallization processes. The unique and peculiar spectral properties of ESIPT dyes make them ideal candidates for studies on light enhancement phenomena such as random lasing (RL) [6,7,27].…”

Highly Fluorescent Liquid Crystals from Excited‐State Intramolecular Proton Transfer Molecules

“…The requisite of chemical structures for ESIPT is the presence of an intramolecular hydrogen bond (H‐bond) between the proton donor (OH and NH 2 ) and the proton acceptor (N and CO) groups close to one another in a single molecule. Recent notable reports on the ESIPT fluorophores include the polymorph‐dependent emissive crystals, double proton transfer process with an extra‐large Stokes shift, amplified spontaneous emission, lasing system, environment‐sensitive multicolored materials, and the use as emitters with electrically generated intramolecular proton transfers . Most of ESIPT molecules show significant fluorescence in the solid states, not suffering from “concentration quenching” and thus serving as aggregation‐induced emission enhancement (AIEE) materials .…”