Investigation of the Ultrafast Dynamics Occurring during Unsensitized Photocatalytic H₂ Evolution by an [FeFe]-Hydrogenase Subsite Analogue

Biomimetic compounds based upon the active subsite of the [FeFe]-hydrogenase enzyme system have been the focus of much attention as catalysts for hydrogen production: a clean energy vector. Until recently, use of hydrogenase subsite systems for light-driven hydrogen production has typically required the involvement of a photosensitizer, but the molecule [(μ-pdt)­(μ-H)­Fe2(CO)4(dppv)]+, (1; dppv = cis-1,2-C2H2(PPh2)2; pdt = 1,3-propanedithiolate) has been reported to catalyze the evolution of hydrogen gas under sensitizer-free conditions. Establishing the molecular mechanism that leads to photohydrogen production by 1 is thus an important step that may enable further development of this family of molecules as solar fuel platforms. Here, we report ultrafast UVpump–IRprobe spectroscopy of 1 at three different excitation wavelengths and in a range of solvents, including under the conditions required for H2 production. Combining spectroscopic measurements of the photochemistry and vibrational relaxation dynamics of 1 with ground-state density functional theory (DFT) calculations shows that, irrespective of experimental conditions, near-instantaneous carbonyl ligand loss is the main photochemical channel. No evidence for a long-lived excited electronic state was found. These results provide the first time-resolved data for the photochemistry of 1 and offer an alternative interpretation of the underlying mechanism of light-driven hydrogen generation.以[铁铁]-氢化酶（[FeFe]-hydrogenase）活性亚位点为基础构建的仿生化合物，作为清洁能源载体——氢气的制备催化剂，一直受到广泛关注。此前，利用氢化酶亚位点体系进行光驱动制氢通常需要光敏剂参与，但已报道的分子[(μ-pdt)­(μ-H)­Fe₂(CO)₄(dppv)]⁺（记为化合物1；dppv = 顺式-1,2-C₂H₂(PPh₂)₂；pdt = 1,3-丙二硫醇盐1,3-propanedithiolate）可在无光敏剂条件下催化氢气生成。因此，阐明化合物1光致制氢的分子机制，是推动该类分子作为太阳能燃料平台进一步发展的关键一步。本文报道了化合物1在三种不同激发波长、多种溶剂体系（包括制氢所需反应条件）下的超快紫外泵浦-红外探测光谱研究。将化合物1光化学过程的光谱测量与振动弛豫动力学数据，结合基态密度泛函理论（DFT）计算结果分析后发现：无论实验条件如何，近乎瞬时的羰基配体解离均为主要光化学通道，未观测到长寿命激发电子态的存在。本研究首次提供了化合物1光化学过程的时间分辨数据，并为光驱动制氢的潜在机制提供了新的解释。