Synthesis and Photophysical Study of a [NiFe] Hydrogenase Biomimetic Compound Covalently Linked to a Re-diimine Photosensitizer

The synthesis, photophysics, and photochemistry of a linked dyad ([Re]-[NiFe₂]) containing an analogue ([NiFe₂]) of the active site of [NiFe] hydrogenase, covalently bound to a Re-diimine photosensitizer ([Re]), are described. Following excitation, the mechanisms of electron transfer involving the [Re] and [NiFe₂] centers and the resulting decomposition were investigated. Excitation of the [Re] center results in the population of a diimine-based metal-to-ligand charge transfer excited state. Reductive quenching by NEt₃ produces the radically reduced form of [Re], [Re]^- (k_q = 1.4 ± 0.1 × 10⁷ M^-1 s^-1). Once formed, [Re]^- reduces the [NiFe₂] center to [NiFe₂]^-, and this reduction was followed using time-resolved infrared spectroscopy. The concentration dependence of the electron transfer rate constants suggests that both inter- and intramolecular electron transfer pathways are involved, and the rate constants for these processes have been estimated (k_inter = 5.9 ± 0.7 × 10⁸ M^-1 s^-1, k_intra = 1.5 ± 0.1 × 10⁵ s^-1). For the analogous bimolecular system, only intermolecular electron transfer could be observed (k_inter = 3.8 ± 0.5 × 10⁹ M^-1 s^-1). Fourier transform infrared spectroscopic studies confirms that decomposition of the dyad occurs upon prolonged photolysis, and this appears to be a major factor for the low activity of the system toward H₂ production in acidic conditions.