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

Abstract

Photolysis and photoracemization are two important photochemical phenomena of the prototype complexes [Ru(bipy)] and [Ru(phen)] (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 CT↔MC, MC↔MC, and MC↔CT (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 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(δ S)⇌Λ(δ S) chiral inversion of the [Ru(bipy)(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.

Green‐Light Activation of Push–Pull Ruthenium(II) Complexes

“…Two differently substituted bpy ligands can form diastereomers even for mononuclear complexes [Ru(bpy)(bpy R )(bpy R’ )] 2+ . Finally, bidentate ligands are comparably photolabile enabling photoisomerization of the complex or photosubstitution of the ligands . Replacing bpy by the tridentate ligand 2,2’:6’2’’‐terpyridine (tpy) yields achiral, comparably photostable [Ru(tpy) 2 ] 2+ complexes (Scheme ) .…”

Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes

“…Computational studies have provided highly illuminating results in this regard, which have shed light on the nature of these important excited states. Numerous reports have detailed the pivotal role of 3 MC states in mediating photochromic − and photoracemization , reactions of metal complexes and photochemical ligand substitution reactivity. − These studies reveal that metal complexes can access a number of 3 MC states of differing geometric and electronic character . Further, investigations have suggested that structural modification of complexes can modulate the relative accessibility of different 3 MC states, states which may have preferential roles in either promoting ground-state recovery or ligand substitution, thus modulating the photochemical ligand substitution efficiency .…”

“…Computational studies have provided highly illuminating results in this regard, which have shed light on the nature of these important excited states. Numerous reports have detailed the pivotal role of 3 MC states in mediating photochromic 40 − 42 and photoracemization 43 , 44 reactions of metal complexes and photochemical ligand substitution reactivity. 45 − 50 These studies reveal that metal complexes can access a number of 3 MC states of differing geometric and electronic character.…”