Ultrafast Photodriven Intramolecular Electron Transfer from a Zinc Porphyrin to a Readily Reduced Diiron Hydrogenase Model Complex

Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe-2(mu-R)(CO)(6-n)(L)(n)], where commonly R = alkyl or aryl dithiolate and L = CO, CN-, or PR3, are a promising class of catalysts for use in photodriven H-2 production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate duron complex [NMI-Fe2S2(CO)(6), 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe2S2(CO)(6) donor-acceptor dyad (2). The electron-withdrawing nature of the NMI group makes the diiron complex among the most easily reduced hydrogenase mimics reported to date (-0.74 V vs SCE). In the presence of triflic acid, the cyclic voltammogram of 1 showed an increase in current at the first reduction wave at -0.78 V and a new reduction wave at -1.4 V. As the acid concentration was increased, the current at -0.78 V remained constant while the current at -1.4 V increased significantly, which is consistent with a catalytic proton reduction process. Selective photoexcitation of the Zn porphyrin in 2 with 553 nm, 110 fs laser pulses in both toluene and CH2Cl2 yielded transient absorption spectra showing a distinct peak at 616 nm, which has been assigned to [NMI-Fe2S2(CO)(6)](-) on the basis of spectroelectrochemical measurements on 1. The 616 nm peak was used to monitor the charge separation (CS) and charge recombination (CR) dynamics of 2, which yielded tau(cs) = 12 +/- 1 ps and tau(CR) = 3.0 +/- 0.2 ns in toluene and tau(CS) = 24 +/- 1 ps and tau(cR) = 57 +/- 1 Ps in CH2Cl2. Photoexcitation of the disulfide precursor to 2 in both toluene and CH2Cl2 produced only the singlet and triplet excited states of the Zn porphyrin, showing that electron transfer is favorable only when the diiron complex is present. Photoexcitation of 2 in the presence of trifluoroacetic acid was shown to generate H-2.