Donor-Acceptor Boron-Ketoiminate Complexes With Pendent N-Heterocyclic Arms: Switched-On Luminescence Through N-Heterocycle Methylation

Abstract

A series of intramolecular, donor-stabilized BF2 complexes supported by phenanthridinyl-decorated, β-ketoiminate chelating ligand scaffolds is described, along with their characterization by spectroscopy and X-ray diffraction. In solution, the relative orientation of the pendent phenanthridinyl arm is fixed despite not coordinating to the boron center, and a well-resolved through-space interaction between a phenanthridinyl C-H and a single fluorine atom can be observed by 19F-1H NOE NMR spectroscopy. The neutral compounds are nonetheless only weakly luminescent in fluid solution, ascribed to non-radiative decay pathways enabled by rotation of the N-heterocyclic unit. Methylation of the phenanthridinyl nitrogen restricts this rotation, “switching on” comparably strong emission in solution. Modelling by density functional theory (DFT) and time-dependent DFT (TDDFT) indicates that the character of the lowest energy excitations changes upon methylation, with shallow calculated potential energy surfaces of the neutral complexes consistent with their lack of significant radiative decay.