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 Fe^IFe^I→Fe^IFe⁰ 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.

Key words: [FeFe] hydrogenase, osmium(Ⅱ) bis-terpyridine complex, electron transfer, energy transfer