METAL-TO-LIGAND CHARGE-TRANSFER PHOTOCHEMISTRY - QUANTUM-CHEMISTRY AND DYNAMICS OF THE SYSTEMS RM(CO)(3)(DAB) (M = MN, R = H, METHYL, ETHYL, M = RE, R = H, DAB = 1,4-DIAZA-1,3-BUTADIENE)

The photodissociation dynamics of the model system HMn(CO)(3)(alpha-di imine), representative of a class of transition metal complexes charac terized by low-lying metal-to-ligand charge-transfer excited states, i s reported for the following elementary processes: (i) absorption from the singlet electronic ground state to the low-lying singlet excited states; (ii) dissociation on nine kinetically coupled potentials corre sponding to the low-lying singlet excited states and to the upper trip let dissociative state; (iii) photodissociation under visible irradiat ion; (iv) photodissociation under UV irradiation. By extension of the strategy applied with success to small di- and tri-atomic molecules to multidimensional transition metal complexes, the dynamics are simulat ed using a time-dependent wave packet propagation technique on ab init io CASSCF/CCI potentials calculated along the Mn-H elongation for the singlet/triplet excited states. The nature of the photoactive excited stales is determined without ambiguity, as well as the time scales and some important features of the absorption spectrum (either in the UV region or in the visible region). On the basis of this one-dimensional complete study and on the calculation of the lowest excited states in RM(CO)(3)(DAB) (M=Mn and R= methyl, ethyl; M = Re, R = EI) a tentativ e rationalization of the photochemical and photophysical properties of this class of molecules is proposed. (C) 1998 Elsevier Science S.A.