Bis-terpyridine Os (II) complex sensitized [FeFe] hydrogenase mimic systems: synthesis and photophysical study

Abstract

To mimic [FeFe] hydrogenase in nature, a PS-Fe2S2 type molecular dyad 1a and corresponding intermolecular model compounds 1b and 2 were synthesized and characterized. In the case of 1a, bis-terpyridine osmium (II) complex as photosensitizer (PS) is covalently linked to a Fe2S2 cluster of [FeFe] hydrogenase mimic active site by a cyanide group. UV-Vis absorption spectrum revealed that both compounds 1a and 1b show two LC absorptions at the range of 280~330 nm, and 1MLCT absorption at ca. 498 nm. Particularly, 3MLCT absorption at 669 nm was clearly observed in compound 1b. Redox property of 1a and 1b was studied by cyclic voltammetry. Compound 1b shows two reduction potentials E (Os+2/+1) and E (Os+1/0) at -1.60 and -1.87 eV, and one oxidation potential E (Os+2/+3) at 0.51 eV. As compared with 1b, 1a shifts to more negative reduction potentials E (Os+2/+1) and E (Os+1/0) at -1.63 and -1.91 eV, respectively, but to more positive oxidation potential E (Os+2/+3) at 0.58 eV. Compound 1a also show an irreversible reduction potential of FeIFeI → FeIFe0 at -1.77 eV, which shifts 180 mV more negative than the corresponding reduction potential of compound 2. Excitation of the MLCT absorption of compound 1b leads to a strong luminescence at 740 nm with lifetime of 105 ns in CH3CN. However, the luminescence of dyad 1a was significantly quenched with efficiency of 92%. Under the same condition, the luminescence of 1b was slightly quenched by one equivalent of compound 2. The quenching efficiency of 1b was changed from 4% to 65% when the concentration of 2 was increased from 1 to 10 equivalents. According to Rehm-Weller equation, the photoinduced electron transfer from the excited osmium complex to Fe2S2 moiety neither in intramolecular dyad 1a nor in intermolecular (1b+2) system is feasible for the uphill free energy change. Consequently, the luminescence quenching in intramolecular dyad 1a and intermolecular (1b+2) system was tentatively attributed to energy transfer from 3MLCT of osmium complex to Fe2S2 cluster. © 2012 Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.