Fluorescence Modulation through the Inverted Energy Gap Law in Triply N-B←N-Containing Windmill-Shaped Triazines

A series of windmill-shape heterocyclic molecules containing three N-B←N units, TBN and its derivatives, with quasi-planar C₃ symmetric backbone, are synthesized. The parent TBN exhibits a strongly allowed, doubly degenerate lowest excited state but suffers from very low fluorescence, due to very fast nonradiative decay rate through a conical intersection (CI) as revealed by femtosecond transient absorption spectroscopy and quantum-chemical calculations. Introducing peripheral phenyl- or thienyl-groups (Ph-TBN or Th-TBN) induces pronounced bathochromic shifts and enhances fluorescence, which is beneficial from inhibited nonradiative pathway by the increased energy barriers to access the CI at excited state. The understanding of this rather uncommon behaviour may open routes for the design of novel fluorescence materials.