Excited-State Absorption of Tris(phenanthroline)rhodium(III). A Handle on the Excited-State Behavior of a Powerful Photochemical Oxidant

Laser flash photolysis of Rh(phen)33+ in aqueous solution at room temperature and in 1:1 water:ethylene glycol clear glasses at 77 K reveals an intense transient absorption. On the basis of comparison with emission lifetimes under the same experimental conditions, the transient absorption is assigned as an excited-state absorption (ESA) associated with the ligand-centered π-π* triplet state. The ESA offers a precious handle on the otherwise elusive (practically nonemitting, at room temperature) excited state of this powerful photochemical oxidant. Some aspects of the photophysics of Rh(phen)33+ are also elucidated by ESA, emission, and electron-transfer quenching studies. In particular, it is shown that (i), at room temperature, the π-π* triplet is in fast thermal equilibrium with a metal-centered d-d triplet state and (ii) the quantum yield of intersystem crossing at room temperature is unity. © 1984 American Chemical Society.