Engineered [FeFe]-Hydrogenase Mimics Featuring Heteroaryl Linkers: Molecular Design and Photocatalytic Hydrogen Evolution Under Visible Light

Inspired by the active site of [FeFe]-hydrogenase, we have developed synthetic mimics engineered from the reaction of heteroaryl thioketone derivatives ferrocenyl(5-(4-(diphenylamino)phenyl)thiophen-2-yl)methanethione (PS-Fc-1), ferrocenyl(5'-(4-(di-phenylamino)phenyl)-[2,2'-bithiophen]-5-yl)methanethione (PS-Fc-2) and phenyl(5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophen]-5-yl)methanethione (PS-Ph) as pro-ligands with Fe3(CO)12. The resulting complexes contain thiolato ligands which enable a close linkage between heteroaryl chromophores and the catalytic center, thereby promoting efficient photocatalytic hydrogen evolution under visible light irradiation. These mimics incorporate a push-pull organic chromophore, consisting of triphenylamine and (bi)thiophene groups, designed to facilitate direct photoexcitation into a charge-separated state. Electrochemical properties were examined using cyclic voltammetry, and photophysical characteristics were determined by steady-state spectroscopy and nanosecond-transient absorption supported by (TD-)DFT simulations. Whilst both catalytically active species revealed the formation of charge-separated states, directly upon excitation, fast deactivation due to relaxation into low-lying ferrocene-located states prevents the formation of long-living excited states in the ferrocene-linked dyad which explains the reduced activity for hydrogen generation of the dyad containing the ferrocene moiety compared to phenyl one.