Femtosecond pump-probe transient absorption study of the photolysis of [Cp′Mo(CO)3]2 (Cp′ = η5-C5H4CH3):: Role of translational and rotational diffusion in the radical cage effect

Femtosecond pump-probe studies of the photodissociation and subsequent radical cage pair recombination dynamics of the organometallic dimer [Cp'Mo(CO)(3)](2) (Cp' = eta(5)-C5H4CH3) are reported. The dynamics following photodissociation were studied in numerous noncoordinating hydrocarbon solvents. The results indicate that primary geminate recombination occurs on an ultrafast time scale (tau approximate to 5 ps) and the efficiency of cage escape is inversely proportional to solvent viscosity. Investigation of the time-dependent anisotropy in this system allowed for an estimate of the rotational correlation time of the radical fragments (tau approximate to 5-25 ps). Comparison of the rates of rotational motion with the population kinetics shows that the primary solvent cage dynamics and recombination efficiency are controlled by radical diffusion and not by radical rotation.