Photoactivation of peroxymonosulfate (PMS) over a CuO-ZnO p-n heterojunction for the selective C2 trimerization of indoles

In this study, a CuO-ZnO p-n heterojunction was found to be an unprecedented photocatalyst for the selective C2 trimerization of indoles. The hetero-conjunction of monoclinic CuO with the wurtzite lattice structure of ZnO was evident from TEM and Mott-Schottky analysis. Upon irradiation with UV light, the CuO-ZnO photocatalyst with a band gap of 3.1 eV activated peroxymonosulfate (PMS) forming highly active hydroxyl (OH) and sulphate (SO4-) radicals. Both these radicals actively participated in the selective C2 trimerization of indoles. Mechanistic investigations revealed an S-scheme catalytic pathway for the reaction. Photo-luminescence (PL) spectroscopic analysis indicated the involvement of radical species in the reaction. The presence of suitable band-edge potential along with the capacity to reduce electron (e-)-hole (h+) recombination make CuO-ZnO a highly promising low-cost photocatalyst. The present protocol is advantageous in terms of reaction time, product selectivity, substrate scope and the recyclability of the catalyst. The reaction occurred within a short time span of 15-45 min without employing any harsh conditions or additives. Four new molecules, including di-substituted indoles and indoles with a -COOH functional group, were isolated in 76-87% yields. The partial and total density of states (PDOS and TDOS) were evaluated to understand the contribution of individual metals to the band structure and mechanistic aspects. Activation of peroxymonosulfate (PMS) over a CuO-ZnO p-n junction for the selective synthesis of C2-trimerized products of indoles.