Self-assembled metallasupramolecular cages towards light harvesting systems for oxidative cyclization

Abstract

Designing artificial light harvesting systems with the ability to utilize the output energy for fruitful application in aqueous medium is an intriguing topic for the development of clean and sustainable energy. We report here facile synthesis of three prismatic molecular cages as imminent supramolecular optoelectronic materials <i>via</i> two-component coordination-driven self-assembly of a new tetra-imidazole donor (<b>L</b>) in combination with 180°/120° di-platinum(ii) acceptors. Self-assembly of 180° <i>trans</i>-Pt(ii) acceptors <b>A1</b> and <b>A2</b> with <b>L</b> leads to the formation of cages Pt₄ <b>L</b> ₂(<b>1a</b>) and Pt₈ <b>L</b> ₂(<b>2a</b>) respectively, while 120°-Pt(ii) acceptor <b>A3</b> with <b>L</b> gives the Pt₈ <b>L</b> ₂(<b>3a</b>) metallacage. PF₆ ^- analogues (<b>1b</b>, <b>2b</b> and <b>3b</b>) of the metallacages possess a high molar extinction coefficient and large Stokes shift. <b>1b-3b</b> are weakly emissive in dilute solution but showed aggregation induced emission (AIE) in a water/MeCN mixture as well as in the solid state. AIE active <b>2b</b> and <b>3b</b> in aqueous (90% water/MeCN mixture) medium act as donors for fabricating artificial light harvesting systems <i>via</i> Förster resonance energy transfer (FRET) with organic dye rhodamine-B (<b>RhB</b>) with high energy efficiency and good antenna effect. The metallacages <b>2b</b> and <b>3b</b> represent an interesting platform to fabricate new generation supramolecular aqueous light harvesting systems with high antenna effect. Finally, the harvested energy of the LHSs (<b>2b</b> + <b>RhB</b>) and (<b>3b</b> + <b>RhB</b>) was utilized successfully for efficient visible light induced photo-oxidative cross coupling cyclization of <i>N</i>,<i>N</i>-dimethylaniline (<b>4</b>) with a series of <i>N</i>-alkyl/aryl maleimides (<b>5</b>) in aqueous acetonitrile with dramatic enhancement in yields compared to the reactions with <b>RhB</b> or cages alone.