beta-Iminoenamine-BF2 complexes: Aggregation-induced emission and pronounced effects of aliphatic rings on radiationless deactivation

The synthesis, photophysical, and electrochemical attributes of a novel class of boron difluorides containing an aromatic-fused alicyclic/hetero- alicyclic ring built on a β-iminoenamine chromophoric backbone are reported. The compounds displayed large Stokes shifts (86-121 nm), and were emissive in the solid state. The quantum yields obtained in solution at room temperature were unusually lower by an order of magnitude compared to those in the solid state. Some of the tested compounds displayed aggregation-induced emission (AIE). Single crystal XRD analyses revealed a lack of interplanar π-π interactions, which are presumed to be absent owing to non-planarity of the alicyclic component in the molecule. For most of the studied compounds, time-dependent DFT (TD-DFT) calculations invariably reveal intramolecular charge transfer (π-π*) characteristics with the frontier orbitals concentrated on the boron-nitrogen heterocycle. The participation of boron and fluorine atoms was found to be negligible.