Room temperature phosphorescence from organic materials: Unravelling the emissive behaviour of chloro-substituted derivatives of cyclic triimidazole

Purely organic luminophores that exhibit room temperature phosphorescence (RTP) are receiving an ever-growing attention. The design of efficient RTP materials and the understanding of their emissive behaviour, however, remain a challenging task. Herein, to gain further insights into metal-free RTP, we report the synthesis, characterization, and photophysical study of chloro-derivatives of [1,2-a:1',2'-c:1'',2''-e][1,3,5]triazine or cyclic triimidazole (TT). Mono-(1Cl), di-(2Cl) and tri-chloro-TT (3Cl) derivatives are prepared by mild chlorination with N-chlorosuc-cinimide (NCS). The three compounds are characterized by aggregation-induced emission (AIE) comprising dual fluorescence and multiple phosphorescences, whose origin is disclosed by crystal structure analysis together with accurate DFT/TDDFT calculations. Comparison with previously reported bromine and iodine TT derivatives reveals the new chlorinated compounds as the best performing halogenated AIE members of the family.