Crystallization-Induced Phosphorescence of Pure Organic Luminogens at Room Temperature

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)(2)C=O (X = F, Cl, B) 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 center dot center dot center dot O, N-H center dot center dot center dot O, C-H center dot center dot center dot X (X = F, Cl, Br), C-Br center dot center dot center dot Br-C, and C-H center dot center dot center dot pi 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.