β-Functionalized Push–Pull Monobenzoporphyrins: Synthesis, Photophysical, Structural Properties, and Electrocatalytic Hydrogen Evolution by Their Co(II) Complexes

Hydrogen production through water splitting is a promising strategy toward sustainable energy, driving the need for efficient catalysts. This study presents a series of electronically tunable β-functionalized push-pull monobenzoporphyrins, MTPP(Benzo)(R)2 (R = CH3, Ph, tBu-Ph, Py; M = 2H and Co(II)), and evaluates their cobalt(II) derivatives for the electrocatalytic hydrogen evolution reaction (HER). The successful synthesis of these compounds was confirmed through NMR, mass spectrometry, and UV-vis spectroscopy. Single-crystal X-ray analysis revealed a distinct saddle-shaped conformation for CoTPP(Benzo)(CH3)2, with ΔCβ = 0.406 and Δ24 = 0.245 Å, further validated through DFT calculations. The electrocatalysts (CoTPP(Benzo)(R)2; R = CH3, Ph, tBu-Ph, and Py) demonstrated excellent HER activity in 0.5 M H2SO4, showing significantly reduced overpotentials (254-487 mV) compared to unsubstituted CoTPP (689 mV) at a current density of 10 mA cm-2. Lower Tafel slopes (238-298 mV dec-1) relative to CoTPP (302 mV dec-1) reflected the enhanced kinetic behavior in the HER. Further, the catalytic efficiencies, evaluated via EIS, chronoamperometry, ECSA, and contact angle measurements, demonstrated that push-pull CoTPP(Benzo)(R)2 catalysts exhibit significantly lower overpotentials, improved kinetics, and enhanced stability, offering critical structural insights and a sustainable alternative to rare and costly metal-based electrocatalysts for hydrogen evolution and future energy conversion technologies.