A Key Factor Dominating the Competition between Photolysis and Photoracemization of [Ru(bipy)3]2+ and [Ru(phen)3]2+ Complexes

Photolysis and photoracemization are two important photochemical phenomena of the prototype complexes [Ru(bipy)(3)](2+) and [Ru(phen)(3)](2+) (bipy 2,2'-bipyridine, phen 1,10-phenanthroline), but little is known about their relations. To solve this issue, the photoinduced chiral inversion Delta -><-Lambda of the complexes was analyzed theoretically. The results indicated that the photoracemization reaction proceeds on the lowest triplet potential energy surface in three steps (CT Delta)-C-3 & HARR;(MC Delta)-M-3, (MC Delta)-M-3 & HARR;(MC Lambda)-M-3, and (MC Lambda)-M-3 & HARR;(CT Lambda)-C-3 (CT charge transfer state; MC metal-centered state). Where the first and third steps are fast processes of picoseconds, the second is the rate-determining step (RDS) of microseconds. Such a slow step for the racemization leads to the excited molecule lingering around the bottom of (MC)-M-3 state after the first step and, therefore, greatly enhances the possibility of deexcitation and photolysis mostly at the triplet-singlet crossing point. In other words, the photoracemization and photolysis of the complexes have a competition relation, not a slave relation as assumed by the photoracemization model suggested in literature. They are dominated by the RDS. This conclusion is also consistent with the Delta(delta S)reversible arrow Lambda(delta S) chiral inversion of the [Ru(bipy)(2)(L-ser)](+) series complexes, which is reversible with no detectable photolysis, as its second step is a fast one. Note that, although the photoracemization of the prototype complexes is very slow, it passes through the three steps reversibly and ends with a photon emitting, which could be detected with the time-resolved circularly polarized luminescence and related techniques. These findings are helpful to understand and control the photochemical behavior of the complexes in practice.