The role of photoinduced electron transfer processes in photodegradation of the $[Fe_4(\mu_3-S)_3(NO)_7]^{-}$ cluster

Spectroscopic and electrochemical study of the $[Fe_4(\mu_3-S)_3(NO)_7]^{-}$ photochemical reaction and thermodynamic calculations of relevant systems demonstrate the redox character of this process. The photoinduced electron transfer between substrate clusters in excited and ground state (probably via exciplex formation) results in dismutation yielding unstable $[Fe_4(\mu_3-S)_3(NO)_7]^{2-}$ and $[Fe_4(\mu_3-S)_3(NO)_7]^{-0}$. Back electron transfer between the primary products is responsible for fast reversibility of the photochemical reaction in deoxygenated solutions. In the presence of an electron acceptor (such as $O_{2}$, $MV^{2+}$ or NO) an oxidative quenching of the *$[Fe_4(\mu_3-S)_3(NO)_7]^{-}$ is anticipated, although NO seems to participate as well in the reductive quenching. The electron acceptors can also regenerate the substrate from its reduced form ($[Fe_4(\mu_3-S)_3(NO)_7]^{2-}$), whereas the other primary product ($[Fe_4(\mu_3-S)_3(NO)_7]^{0}$) decomposes to the final products. The suggested mechanism fits well to all experimental observations and shows the thermodynamically favored pathways and explains formation of all major ($Fe^{2+}$, $S^{2-}$, NO) and minor products ($N_{2}O$, $Fe^{3+}$). The photodissociation of nitrosyl ligands suggested earlier as the primary photochemical step cannot be, however, definitely excluded and may constitute a parallel pathway of $[Fe_4(\mu_3-S)_3(NO)_7]^{-}$ photolysis.