Ligand field states dominate excited state decay in trans-[Ru(py)4Cl2] MLCT chromophores

Abstract

In this contribution, the ultrafast excited state dynamics of a family of trans-[Ru(L)4Cl2] metal to ligand charge transfer chromophores, where L = pyridine, 4-tert-butylpyridine, 4-methoxypyridine and 4,4′-bipyridine, are studied using a combination of femtosecond transient absorption spectroscopy and (TD)DFT theoretical methods. Ligand field states were directly observed and found to dominate the excited state decay cascade, leading to ligand photosubstitution. The internal conversion process that feeds ligand field states, indirectly after light absorption, occurs prior to thermal relaxation in <2 ps for L = pyridine, 4-tert-butylpyridine and 4-methoxypyridine, suggesting a downhill process. This is consistent with DFT calculations, that indicate the lowest triplet is a ligand field state. The exception is for L = 4,4′-bpy, where a slower internal conversion of 34 ps points to a thermally relaxed population undergoing an uphill process with a small energy barrier. Matching this notion, DFT yields a metal to ligand charge transfer character for the lowest triplet of this compound.