Access to Multifunctional AEEgens via Ru(II)-Catalyzed Quinoxaline-Directed Oxidative Annulation

Abstract

Multifaceted application potential of AEEgens in bioimaging, theranostics, chemo/biosensors, mechanochromics, solar cells, and organic photoelectronics opens up a new research paradigm to develop and design more such compounds. Herein, quinoxaline N-directed Ru(II)-catalyzed oxidative annulation of 2-arylquinoxalines with internal alkynes leads to the formation of highly luminescent annulated quaternary ammonium salts in the presence of a Cu(OAc)2·H2O oxidant. While the synthesized compounds exhibit emissions in the green-to-yellow region with large Stokes shifts and reasonable quantum yields, their DFT calculations display a 3D twisted conformation bearing a donor−π–acceptor (D-π-A) configuration where the two phenyl moieties could serve as the donors and the extended quinoxaline core as the acceptor. Single-crystal analysis of the quaternary salt 3aa depicts the presence of multiple intermolecular noncovalent and weak π–π interactions that are possibly responsible for the luminescence behavior in crystalline and solid states. The advent of aggregation-enhanced emission in quinoxalinium salt 3aa in DMF/water is due to the restriction of intramolecular motion and suppression of intermolecular charge transfer in the aggregated state. AEEgen 3aa unveils reversible mechanochromism on changing from crystalline to the amorphous state upon grinding and returning back to the crystalline state upon DCM fuming, where a few of such compounds are utilized for development of latent fingerprints on adhesive tape. Furthermore, a representative group of synthesized luminescent quinoxalinium salts portrays dose-dependent cell growth inhibition of HeLa cells with concomitant cell arrest in G1 phases. Hence, these AEE luminogens are not only attractive as luminescent “light-up” probes for cell imaging but also important as anticancer agents.