Extending Excited-state Lifetimes by Interchromophoric Triplet-state Equilibration in a Pyrene-Ru(II)diimine Dyad System

The synthesis and the spectroscopic properties of a bichromophoric ruthenium trisbipyridyl-1,4-diethynylenebenzene-pyrene system (Ru-b-Py) and the corresponding pyrene ligand (b-Py) are reported. The ruthenium model systems Ru-b-OH, Ru-b-Ph are also presented. UV-Vis absorption and emission at room and low temperature and time-resolved spectroscopy are discussed. For the Ru-b-Py dyad, a mixing of the MLCT state of the ruthenium-based component and the triplet state of pyrene, 3Py, is observed. Time-resolved transient absorption studies performed on the Ru-b-Py and on the b-Py ligand show that the lowest energy absorption is due to the population of the triplet state localized on the pyrene-component. Time-resolved studies also evidenced a relatively slow forward triplet equilibration rate, in the order of 2 × 105 s-1 (5 μs), and an even slower back energy transfer rate, 3.3 × 104 s-1, still faster than the intrinsic decay time of the pyrene (200 μs).