While photosubstitution reactions in metal complexes
are usually
thought of as dissociative processes poorly dependent on the environment,
they are, in fact, very sensitive to solvent effects. Therefore, it
is crucial to explicitly consider solvent molecules in theoretical
models of these reactions. Here, we experimentally and computationally
investigated the selectivity of the photosubstitution of diimine chelates
in a series of sterically strained ruthenium(II) polypyridyl complexes
in water and acetonitrile. The complexes differ essentially by the
rigidity of the chelates, which strongly influenced the observed selectivity
of the photosubstitution. As the ratio between the different photoproducts
was also influenced by the solvent, we developed a full density functional
theory modeling of the reaction mechanism that included explicit solvent
molecules. Three reaction pathways leading to photodissociation were
identified on the triplet hypersurface, each characterized by either
one or two energy barriers. Photodissociation in water was promoted
by a proton transfer in the triplet state, which was facilitated by
the dissociated pyridine ring acting as a pendent base. We show that
the temperature variation of the photosubstitution quantum yield is
an excellent tool to compare theory with experiments. An unusual phenomenon
was observed for one of the compounds in acetonitrile, for which an
increase in temperature led to a surprising decrease in the photosubstitution
reaction rate. We interpret this experimental observation based on
complete mapping of the triplet hypersurface of this complex, revealing
thermal deactivation to the singlet ground state through intersystem
crossing.