A quantum chemical investigation of the metal-to-ligand charge-transfer photochemistry

According to a number of recent experiments reported for a class of a-diimine mono- and di-nuclear transition metal carbonyls, these molecules may either photodissociate leading to highly reactive intermediates or manifest the photophysics of metal-to-ligand-charge-transfer (MLCT) complexes. The duality between these two extremes of behavior may be used to promote different applications like catalytic activity or energy/electrons transfers. The potential energy surfaces (PESs) associated with the low-lying singlet and triplet excited states of Mn(H)(CO)(3)(DAB) (DAB = 1,4-diaza-1,3-butadiene) calculated for the Mn-H bond homolysis and for the dissociation of an axial carbonyl ligand illustrate the complexity and the richness of the photochemistry and photophysics of this class of molecules. It is shown that the presence of significant energy barriers on the reaction path corresponding to the homolysis of the Mn-H bond are responsible for the low efficiency of this process in related complexes. The simulation of the dynamics of the (MLCT)-M-1 excited state, calculated at 26 000 cm(-1) and contributing mainly to the visible absorption spectrum, indicates that the most probable primary reaction is an ultrafast dissociation (< 200 fs) of the carbonyl ligand. The relative positions of the quasi-bound MLCT states with respect to the sigma bond-to-ligand-charge-transfer (SBLCT) excited state corresponding to the sigma(Mn-H)-->pi*(DAB) excitation will govern the photoreactivity of the molecules. A comparative study of the sequence, the nature of the low-lying excited states and of the associated state correlation diagrams for the hydride complex and the ethyl analog Mn(Et)(CO)(3)(DAB) points to a better reactivity of the ethyl-substituted complex in agreement with the experimental data. This is a consequence of a lowering of the excited states, an increase in the density of states in the visible energy domain and a mixing of the triplet SBLCT and MLCT states on going from the hydride to the ethyl complex. The influence of the metal center on going from the manganese to the rhenium compound is expressed by an important mixing of the MLCT and SBLCT states and a lowering of the excited states due to a relativistic destabilization of the d shells of the metal center and an indirect stabilization of the pi*(DAB) orbital through its interaction with the 6p of the rhenium. These heavy atom effects should induce a better photoreactivity for the third-row transition metal complexes than for the first-row analogs. (C) 1998 Elsevier Science S.A. All rights reserved.