Self-assembled [2 + 3] organic-imine cage as an artificial light harvester for the photocatalytic organic transformation in an aqueous medium

Abstract

Designing an efficient visible light-mediated photocatalyst based on artificial light harvesting systems (LHSs) contemporary to photosynthetic complexes provides a powerful approach for energy capture and conversion. Herein, we report the synthesis of a supramolecular [2 + 3] organic-imine cage CA1 via dynamic covalent amine condensation of a triazine-based tri-aldehyde and cis-(1R,2S)-cyclohexyl-diamine. The designed cage CA1 features an electron-deficient interior cavity that efficiently encapsulates the aggregation-induced emissive (AIE) tetraphenylethylene (TPE) fluorophore. Encapsulation of TPE in a cage complex suppresses non-radiative decay and yields intense blue emission. The host–guest system CA1⊃TPE exhibits a 10-fold emission enhancement due to the AIE effect in the aggregate state (90% water/THF solution) with a large stokes shift. The morphological characterization of the aggregate is well established through SEM and DLS analysis, revealing the formation of spherical nano-aggregate particles. Blue emissive nano-aggregate of CA1⊃TPE exhibits strong spectral overlap with Rhodamine B (RhB) and is utilised for the fabrication of artificial LHSs by Förster resonance energy transfer (FRET) between energy donor CA1⊃TPE and energy acceptor RhB in the 90% water/THF mixture. The FRET system CA1⊃TPE@RhB exhibits a maximum energy efficiency of 88.9% and an antenna effect of 11 at a 10 : 1 donor:acceptor ratio. The light-harvester complex CA1⊃TPE@RhB is further utilised as a visible light-driven photocatalyst for the condensation reaction involving substituted benzaldehyde and malononitrile in a green aqueous environment, demonstrating an improved yield compared to CA1⊃TPE and RhB alone.