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-C 6 H 4 ) 2 CdO (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.
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Intrinsic luminescence of naturally occurring proteins provides considerable information about their structure and dynamics and thus is often used to study protein folding, associated reaction, and other biochemical processes. [1,2] Generally, protein emission is believed to be originated from three aromatic amino acids, namely phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp). [2,3] Little attention, however, has been paid to nonaromatic amino acids, presumably due to their lack of classic discrete fluorophores. Recently, several groups reported the intrinsic emission form the aggregates of some biomolecules. [4][5][6][7] Discovery of the bright intrinsic emission of peptides and proteins allows for direct imaging of their folding and aggregation properties. [6a,7a] In particular, the study of the intrinsic fluorescence from proteins permits sensitive information about the native protein to be obtained with little or no structural modification, thus highly valuable in biochemical researches. [7a] Despite exciting advances have been achieved, there still deserves more insightful understanding on the emission mechanism, for which different and even controversial hypotheses were proposed.
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