Photosensitized Radical-Anion-Driven Metal-Free Selective Reduction of Aldehydes Using Graphene Oxide as an Electron Relay Mediator under Visible Light

Abstract

Despite the modern boost, developing a new photocatalytic system for the reduction of aldehydes is still challenging due to their high negative reduction potential. Herein, we have used a metal-free photoinduced electron-transfer system based on a cheap and readily available organic dye eosin Y (EY), graphene oxide (GO), and ammonium oxalate (AO) for photocatalytic reduction of structurally diverse aldehydes under sustainable conditions. The protocol shows remarkable selectivity for the photocatalytic reduction of aldehydes over ketones. The decisive interaction of GO and AO with the various states of EY (ground, singlet, triplet, and radical anions), which are responsible for the commencement of the reaction, was examined by various theoretical, optical, electrochemical, and photo-electrochemical studies. The synergetic system of GO, EY, and AO is appropriate for enhancing the separation efficiency of visible-light-induced charge carriers. GO nanosheets act as an electron reservoir to accept and transport photogenerated electrons from the photocatalytic system to the reactant. The reduction of the GO during the process ruled out the back transfer of photoexcited charges. Control experiments explained that the reaction involves two stages: electron transfer and protonation. This process eliminates the necessity of precious-metal-based photocatalysts or detrimental sacrificial agents and overcomes the redox potential limitations for the photoreduction of aldehydes.

Carbon Dots as Ligand Operons to Expand Cluster Size Distribution for High Load‐bearing Liquid Superlubricity

“…A variety of oxygen‐containing functional groups exits on the surface of CDs, such as typical ‐OH (≈3424 cm −1 , ≈1073 cm −1 , it can mostly be removed by annealing, i.e., annealed CDs), ─C≐O (≈1708 cm −1 ), and C─O─C (≈1175 cm −1 ) revealed by FTIR (Figure 1e). [ 32,33 ] Corresponding XPS full‐spectrum data also reveals that the percentage of O in the annealed CDs decreases from 20.4% to 12.3% while C increases from 79.6% to 87.7% (Figure 1f); these O reductions are mainly attributed to the decomposition of C─O (from 42.3% to 19.9% at 286.4 eV), [ 34,35 ] C≐O (from 9.5% to 4.8% at 288.2 eV), [ 36,37 ] and O─C≐O (from 8.1% to 0% at 288.8 eV) [ 35,37 ] decomposition of the three oxygen‐containing functional groups (Figure 1g); the same trend is demonstrated in the corresponding O 1s fine spectra, especially C≐O at 530.4 eV [ 38,39 ] and O─C≐O at 531.4 eV (Figure 1h). [ 40,41 ] Some of the oxygen‐containing functional groups containing H are also verified by their corresponding 1 H‐NMR (Figure S2, Supporting Information), including ─COOH at 10.28 ppm, ─CHO at 9.49 ppm, and ─OH at 2.06 ppm.…”