β <i>-</i> Thiophene-Fused Push–Pull Porphyrins: Synthesis, Characterization, and Efficient Electrocatalytic Hydrogen Evolution by Their Copper(II) Complexes

Porphyrin-based catalysts are gaining widespread applications in water-splitting reactions for the production of hydrogen and oxygen gases. Based on the catalytic performance of previously reported porphyrins, we have synthesized a family of thiophene-fused porphyrins, H2TPPTh(X)2 (where X = Br, PE, CN, and CF3), by introducing various electron-withdrawing groups at the antipodal positions of the β-thiophene-fused pyrrolic position. All the synthesized push–pull β-thiophene-fused porphyrins were fully characterized by 1H NMR, UV–visible, fluorescence, and mass spectrometry. A distinctive red or bathochromic shift in UV–vis absorption was observed upon antipodal β-substitution via electron-withdrawing groups on the β-thiophene porphyrin backbone and followed the order H2TPPTh(CF3)2 > H2TPPTh(CN)2 > H2TPPTh(PE)2 > H2TPPTh(Br)2 > H2TPPTh. Single-crystal X-ray analysis of H2TPPTh(Br)2 and H2TPPTh(CF3)2 revealed wave and quasi-planar conformations, respectively. Further, we unveiled the electrocatalytic performances of their Cu(II) complexes for the hydrogen evolution reaction in water splitting. These electrocatalysts, CuTPPTh(X)2, exhibited efficient electrocatalytic performance toward water splitting with overpotential values of 314 (X = PE), 331 (X = CF3), 421 (X = CN), 426 (X = Br), and 477 (X = H) mV for HER in 0.5 M H2SO4 using Pt wire as a counter electrode in comparison to the unsubstituted parent porphyrin, CuTPP, with an overpotential of 662 mV under similar conditions (for a standard current density of 10 mA cm–2). Upon replacing the Pt with a graphite counter electrode, the values slightly increase to 425 (X = PE), 460 (X = CF3), 522 (X = CN), 539 (X = Br), 584 (X = H), and 764 mV for unsubstituted CuTPP due to the combined effects, viz., unavoidable carbon electrode oxidation and lack of Pt cluster deposition on the working electrode. These results suggested that β-thiophene-fused push–pull Cu(II) porphyrins with tuned electronic properties are much more efficient electrocatalysts for HER than unsubstituted porphyrins.