Structural Optimization of Luminescent Sulfur Dots for Solar Light Induced Efficient and Selective Oxidative Coupling Reactions of Aromatic Amines: A Complete Metal-Free Approach
Srayee Mandal, Bramhaiah Kommula, Santanu Bhattacharyya
Abstract
It is highly desirable to perform aerobic oxidation of aromatic amine to imines through a green photocatalytic approach. Therefore, development of a metal-free complete green photocatalyst is extremely important in this regard. Herein, we have synthesized two different types of luminescent sulfur dots (S dots) from totally earth abundant bulk sulfur with different structural and elemental features through a simple chemical etching method, followed by a solvent extraction based post growth mechanism. The structural and elemental features of both S dots are further correlated with the underpinning photophysics through in-depth steady state and time-resolved spectroscopy. It is further supported by detailed electrochemical and photoelectrochemical studies. Notably, cross-sectional light absorption, charge separation, and free carrier accumulation are maximum for the specific type of S dots with almost defined crystalline nature and maximum extent of molecular sulfur as the constituent elements, while normal S dots show polycrystallinity and predominantly S-based functional groups. Finally, the as-synthesized S dots have been utilized for the photocatalytic coupling reactions of benzylamine at complete ambient conditions. The optimized S dots show almost ∼90% conversion with ∼99.5% selectively for the production of specific aromatic imines. The photocatalytic mechanism was crucially investigated by tracking the photoinduced free radicals and intermediates.