Abstract/Summary

Density functional theory (DFT) calculations were performed on the clusters [Os3(CO)10(α-diimine)], for α-diimine = 2,2'-bipyridine (BPY), N-isopropyl 2-iminomethylpyridine (IMP), and N,N'-diisopropyl-l,4-diaza-1,3-butadiene (DAB), together with their spectroscopic study. This important family of clusters is known to convert upon irradiation with visible light into short-lived biradicals and long-lived zwitterions from a σ* (SBLCT) excited state that, however, has not been described accurately thus far by quantum mechanical calculations. Based on the combined DFT, UV-vis absorption and resonance Raman data, the lowest-lying visible absorption band is assigned to a σ(Os1–Os3)-to-*(α-diimine) CT transition for α-diimine = bpy and IMP, and to a strongly delocalized σ(Os1‒Os3)*-to-σ*(Os1‒Os3)* ¬transition for conjugated non-aromatic α-diimine = DAB. The DFT calculations rationalize the experimentally determined characteristics of this electronic transition in the studied series: (i) the corresponding absorption band is the dominant feature in the visible spectral region, (ii) the CT character of the electronic excitation declines from α-diimine = bpy to IMP and vanishes for DAB, (iii) the excitation energies decrease in the order α-diimine = DAB > BPY > IMP, (iv) the oscillator strength shrinks in the order α-diimine = DAB > IMP > BPY. Reference photoreaction quantum yields measured accurately for the formation of a cluster zwitterion from [Os3(CO)10(IMP)] in strongly coordinating pyridine, demonstrate that the optical population of the lowest-energy 1σ* and relaxed 3σ* excited states in the DFT model scheme is still capable of inducing the initial homolytic Os1‒Os3 σ-bond splitting, although less efficiently than the optical excitation into neighbor higher-lying electronic transitions due to a higher potential barrier for the reaction from a dissociative (σσ*) state.