Photodissociation dynamics of Mn2(CO)1 in solution on ultrafast time scales

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Abstract
A study of the photodissociation dynamics of Mn2(CO)10 and the vibrational relaxation of its subsequent photoproducts in solutions has been carried out using picosecond time‐resolved laser techniques. The molecule predissociates in less than 2–3 ps after excitation with a 295 nm photon. Two dissociation channels are open for this excitation wavelength, namely, the Mn–Mn bond breaking and the Mn–CO bond breaking, generating internally hot ⋅Mn(CO)5 and Mn2(CO)9, respectively. These two species have a different absorption spectrum in the visible region and are probed independently by varying the probe laser wavelength. The vibrational relaxation of these nascent photoproducts is observed for the first time. In cyclohexane the vibrationally hot Mn2(CO)9 reaches thermal equilibrium with the solvent through two distinct decay channels with time constants of 15 and 170 ps, respectively. The vibrationally cold Mn2(CO)9 then persists for many nanoseconds. The vibrational relaxation is found to be faster in the 2‐propanol solution with time constants of 10 and 145 ps. On the other hand, the ⋅Mn(CO)5 species cools down in less than 10 ps and then exists in the solution for many nanoseconds as well. This result indicates that energy transfer from the internally hot ⋅Mn(CO)5 species to the solvent is much faster than from Mn2(CO)9. Comparison is made with Cr(CO)5 in similar solvents.

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