Femtosecond Excited-State Dynamics and Nitric Oxide Photorelease in a Prototypical Ruthenium Nitrosyl Complex

Excited-state relaxation of a prototypical ruthenium nitrosyl complex (pentachloronitrosylruthenate) in water is studied by means of ultrafast dispersed, broadband transient absorption spectroscopy. Excitation pulses (duration, 40-70 fs) utilized at seven different wavelengths in the range from 675 to 335 nm populated excited electronic states of different orbital nature. The second excited singlet state of π_NO^* nature relaxes into the lowest triplet ³π_NO^* state in 100 fs via the ¹d-d intermediate (lowest excited singlet) state with ca. 80 fs lifetime. The ³π_NO^* lifetime is 3.2 ps, and all three states are inert toward NO release, which happens in less than 200 fs from higher excited states. The vibrational coherences observed are attributed to the Jahn-Teller effect in the ¹π_NO^* state and nitric oxide loss and provide important insights into the nature of the reaction coordinate in the course of the ultrafast excited-state relaxation dynamics.