Substituent group-tunable hydrogen evolution activity observed in isostructural Cu(ii)-based coordination polymer photocatalysts

Abstract

Elucidations on the structure–activity correlations of non-Pt coordination polymer (CP)-based photocatalysts are highly significant for both the enhancement in catalytic activity and large-scale industrial applications of sustainable hydrogen from water splitting. Herein, three isostructural [Cu(HL)₂(R-BDC)]_n (denoted as Cu-CP-R, HL = 4′-(4-hydroxyphenyl)-4,2′:6′,4′′-terpyridine, R-BDC = 2-R-1,4-benzenedicarboxylate, R = NO₂, OH and Br) CPs were solvothermally synthesized by varying the substituents attached to benzenedicarboxylate, which together with two previously reported analogues (R = NH₂ and H) were used as photocatalysts to systematically explore the substitution effect on the hydrogen evolution activity. These five CPs feature isomorphic layered motifs with axially elongated Cu^II octahedra extended alternately by ditopic HL and R-BDC²⁻ connectors, in which R behaves structurally as a non-coordinate group. The hydrogen production rate over the Cu-CP-R photocatalysts increased from 0.21 to 2.34 mmol g⁻¹ h⁻¹, which followed the order of –NH₂ > –NO₂ > –H > –OH > –Br. Furthermore, the combined experimental and theoretical investigations reveal that the free R moiety significantly dominates the photocatalytic activity by shifting the d states of the Cu^II ion towards the Fermi level, controlling the potential of the conduction band and quickening the charge transfer ability. These important findings can provide informative hints for the design of high-performance, earth-abundant non-noble metal CP-based semi-conductive photocatalysts.