Ultrafast Fluorescence Detection in Tris(2,2‘-bipyridine)ruthenium(II) Complex in Solution:  Relaxation Dynamics Involving Higher Excited States

The excited-state dynamics of a transition metal complex, tris(2,2‘-bipyridine)ruthenium(II), [Ru(bpy)3]2+, has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck−Condon singlet excited state in this molecule was missing. In this study, emission from the Franck−Condon excited singlet state of [Ru(bpy)3]2+ has been observed for the first time, and its lifetime has been estimated to be 40 ± 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited 1(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 ± 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin−orbit coupling is proposed. This enhances the radiative rate constant, k r, of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56−1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)3]2+ are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.