Long-lived excited states of platinum(ii)-porphyrins for highly efficient photocatalytic hydrogen evolution

Abstract

It is believed that platinum(II)-porphyrins possessing long-lived triplet photoexcited states should be beneficial for the performance of photocatalytic hydrogen evolution (PHE) systems. Herein, two new Pt(II)-porphyrins conjugated with a naphthalimide (NI) chromophore, PtT(p-NI)PP and PtD(p-NI)PP, are synthesized in excellent yields and characterized by photophysical, electrochemical, and density functional theory studies. Both Pt(II)-porphyrins possess an efficient Förster resonance energy transfer (FRET) between the NI energy donor and the porphyrin energy acceptor. Especially, PtD(p-NI)PP conjugated with two NI units shows a stronger phosphorescence with a higher triplet quantum yield (Φ_T) and singlet oxygen quantum yield (Φ_Δ), and a longer triplet electron lifetime (τ_T) than PtT(p-NI)PP with four NI units. Both porphyrins are capable of showing UV-visible light driven cocatalyst-free PHE due to the involvement of their long-lived triplet excited states in the PHE mechanism. PtD(p-NI)PP exhibits a higher PHE rate (ηH₂) of 750 μmol g⁻¹ h ⁻¹ than PtT(p-NI)PP with a ηH₂ of 300 μmol g⁻¹ h⁻¹. The higher ηH₂ of the PtD(p-NI)PP porphyrin than that of PtT(p-NI)PP is attributable to the longer triplet excited state τ_T and more favourable thermodynamic driving force for accepting electrons from the sacrificial donor, which led to facile and faster electron transfer from the photoexcited triplet states of porphyrin macrocycles to the proton and consequently water reduction. More importantly, the two Pt(II)-porphyrins are highly photostable, which is verified by their continuous hydrogen evolution from the initial time to 40 h of light irradiation. This work provides a fundamental understanding to develop new photosensitizers based on Pt(II)-porphyrins for efficient PHE.