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

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.