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 Δ⇌Λ of the complexes was analyzed theoretically. The results indicated that the photoracemization reaction proceeds on the lowest triplet potential energy surface in three steps ³CT_Δ↔³MC_Δ, ³MC_Δ↔³MC_Λ, and ³MC_Λ↔³CT_Λ (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 ³MC 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 Δ­(δS)⇌Λ­(δS) 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.