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As the first step in developing synthetic analogues of the siroheme prosthetic group of nitrite and sulfite reductases, octaethylisobacteriochlorin (H2(OEiBC)) and its Zn(II) complex (Zn(OEiBC)) have been prepared and certain physicochemical properties investigated. Details of the synthesis and purification of H2(OEiBC) are presented. From 360-MHz nuclear magnetic resonance evidence, the free base as obtained is most probably a mixture of two nearly equally abundant isomers with trans-reduced pyrroline rings. Quantitative spectral data for F)2(OEiBC) and Zn(OEiBC) are reported. By absorption and fluorescence emission spectral criteria, H2(OEiBC) proves to be a viable model of sirohydrochlorin, the metal-free form of siroheme whose complete structure has recently been elucidated. H2(OEiBC) and Zn(OEiBC) undergo two one-electron oxidations to the radical cations and dications. Comparison of potentials for the two primary oxidations with those for corresponding octaethylporphyrin (OEP) and octaethylchlorin (OEC) species reveals that the ease of oxidation follows the order OEiBC > OEC > OEP. Successive potential intervals are 0.2-0.3 V. The electron paramagnetic resonance spectra of H2(OEiBC)+• and Zn(OEiBC)4"• exhibit no resolved hyperfine splittings. Absorption spectra of the radical cations were measured by a spectroelectrochemical technique using an OTTLE cell. When electrolyzed at potentials sufficient to generate the cation radical, solutions of H2(OEiBC) in acetonitrile and dichloromethane eventually develop an intense royal blue chromophore, whose formation is more rapid at potentials sufficient to generate H2(OEiBC)2+. The blue species was also obtained by reaction of H2(OEiBC) in dichloromethane with 2 equiv of the strong one-electron oxidant (p-BrCeFEhN"1"•. This species was identified as the protonated chlorin EQfOEC)"1" and is proposed to be formed by dehydrogenation of H2(OEiBC)2+.

We present the first measurements of the excited-state relaxation dynamics of a bimetallic class III mixedvalence molecule. The 800 nm absorption of [Ru 2 TIEDCl 4 ] + (TIED ) tetraiminoethylenedimacrocycle) relaxes in 250 and 1000 fs to at least two different intermediate states that can be followed with transient absorption spectroscopy. These states decay in 1.3 and 11.5 ps, and the absorption of the 1.3 ps intermediate displays a large amplitude, very low frequency, highly damped vibrational coherence that completely modulates the absorption. The coherence frequency is 20 ( 5 cm -1 , and the dephasing times range from 360 to 730 fs over the wavelength range of the absorption band. The occurrence of a low-frequency coherence at room temperature, the nearly 100% modulation amplitude, and the phase properties as a function of wavelength are consistent with a nonradiative rate modulation rather than the typical impulsive mechanism that creates a coherent Franck-Condon modulation of the absorption. A nonradiative rate modulation can occur from a vibronic coupling mechanism that is created by breakdown of the Born-Oppenheimer approximation. This type of electronic state coupling likely occurs via nontotally symmetric vibrations, and this is the first time domain measure of a vibronic coupling frequency for inorganic complexes. The resonance Raman activity of the ground-state absorption is consistent with very small mode displacements for the optically connected ground and excited states, as expected for a class III molecule. Since similar nonradiative rates are measured for both the optically excited-state and intermediate-state decays, they both require similar energy gaps in the range of 5000-7000 cm -1 . With these energy gaps, we infer that vibronic coupling matrix elements from 4500 to 11 200 cm -1 can explain the observed nonradiative decay time of 250 fs. These experiments show that class III molecules, and probably many other inorganic complexes, can have fast nonradiative decay channels from vibronic coupling when electronic states are available at lower energies. Therefore, applications with such molecules require careful molecular design to compete with or reduce rates of return to the ground state.