Crystallization-Induced Phosphorescence of Pure Organic Luminogens at Room Temperature

TLDR

Phosphorescence is rarely seen in pure organic chromophores at room temperature. The study reports efficient room‑temperature phosphorescence from crystals of benzophenone derivatives and related compounds. Crystalline packing restricts intramolecular motions and locks conformations via hydrogen‑bonding and other interactions, preventing nonradiative decay of triplet excitons. The rigidified crystal structure yields highly phosphorescent luminogens at room temperature.

Abstract

Phosphorescence has rarely been observed in pure organic chromophore systems at room temperature. We herein report efficient phosphorescence from the crystals of benzophenone and its derivatives with a general formula of (X-C6H4)2C═O (X = F, Cl, Br) as well as methyl 4-bromobenzoate and 4,4′-dibromobiphenyl under ambient conditions. These luminogens are all nonemissive when they are dissolved in good solvents, adsorbed on TLC plates, and doped into polymer films, because active intramolecular motions such as rotations and vibrations under these conditions effectively annihilate their triplet excitons via nonradiative relaxation channels. In the crystalline state, the intramolecular motions are restricted by the crystal lattices and intermolecular interactions, particularly C−H···O, N−H···O, C−H···X (X = F, Cl, Br), C−Br···Br−C, and C−H···π hydrogen bonding. The physical constraints and multiple intermolecular interactions collectively lock the conformations of the luminogen molecules. This structural rigidification effect makes the luminogens highly phosphorescent in the crystalline state at room temperature.