Mesoporous black arsenic (MBA): synthesis and red-light-enhanced photocatalytic activity in C─H arylation

The global demand for sustainable energy and chemical transformations are increasing day by day. To improve that, photocatalytic systems that can be activated by solar light provides greener approaches and thus have driven interest specifically for harnessing visible light for organic and environmental transformations. Recently, the group of 15 elements of the periodic table, named as pnictogens, have become increasingly popular due to their tunable band structures, optoelectronic properties, and strong visible light absorption abilities. Despite the popularity of pnictogens, arsenic, a semimetal member of the group, remains largely unexplored due to synthetic challenges and stability issues. In this thesis, bottom-up wet-chemical synthesis of mesoporous black arsenic (MBA), a structurally ordered As–As networked material featuring an exceptionally high surface area and robust chemical stability, is successfully reported. The detailed characterization of the resulting material with advanced analytical techniques (X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, N2 Adsorption and Desorption (BET), Transmission Electron Microscopy (TEM), Scanning Electron Microscope (SEM), Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV–Vis DRS) and Mott–Schottky analysis) revealed a semiconductor material with a band gap of 1.75 eV (conduction band potential= -1.40 V vs NHE and valence band potential= +0.44 V vs NHE), with orthorhombic crystalline domains and defect-rich mesoporous structure with a surface area of 18.57 m2/g. Moreover, the yielded MBA showed a photo-enhanced activity in the photoredox C─H arylation of heteroarenes under red-light irradiation. After completing structural and photophysical characterizations, the MBA went through cytotoxicity tests to clear the conflict about a new arsenic-based material's toxicity. The cytotoxicity tests were evaluated in two different cell lines at various concentrations. It was proven that MBA has no cytotoxicity up to concentration of 1 µg/mL and 70% cell viability rate was observed even at high concentrations up to 50 µg/mL. Moreover, MBA showed a much less toxicity than that of arsenic (III) chloride at concentrations greater than 2 µg/mL. The ability of MBA as a photocatalyst evaluated for C─H arylation reactions with heteroarenes and variety of diazonium salts. Considering the band gap and electronic absorption spectrum of MBA, low intensity red LED (625 nm) was chosen as the light source. The C─H arylation of furan and 4-chlorobenzenediazonium tetrafluoroborate was chosen as the model reaction and all optimization parameters including light source, reaction time and temperature were assessed. The highest yield for the model reaction was obtained under 10 °C, 2 hours by using red-light irradiation. After the optimization experiments, mechanistic insights were obtained by performing experiments using different sacrificial agents: 1,4-Benzoquinone (BQ), (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), and ascorbic acid, confirming that MBA predominantly follows a SET-based photoredox pathway. The substrate scope of MBA for photoredox C─H arylation reactions were tested on 13 different substrates, and moderate to excellent yields were obtained. Overall, the first-time synthesis of black arsenic via the bottom-up wet chemical method was successfully achieved. The synthesized material is surpassing most conventional semiconductors in redox potential and enabling efficient visible-light-driven single-electron transfer (SET) under low-energy red light (625 nm). The MBA's potential for photoredox C─H arylation reaction is shown by this study, which opens a conceptual path for the usage of black arsenic-based materials as photocatalysts with high reduction potentials in the literature.