Triplet Ground-State-Bridged Photochemical Process: Understanding the Photoinduced Chiral Inversion at the Metal Center of [Ru(phen)₂(l-ser)]⁺ and Its Bipy Analogues

Abstract

One of the main concerns in the photochemistry and photophysics of ruthenium complexes is the de-excitation of the triplet metal centered ligand-field state MC. To understand the mechanism by which theMC states in some reversible photochemical reactions could avoid the fate of fast decay and ligand dissociations, the photoinduced chiral inversion at the metal center of the complexes [Ru(diimine)(l-ser)] (diimine = 1,10-phenanthroline or 2,2'-bipyridine, l-ser = l-serine) has been analyzed at the first principle level of theory. The calculated equilibrium constants and ECD curves for the photoinduced equilibrium mixtures are in agreement with the observed ones. The results showed that the reversible photochemical process Δ(δS) ⇌ Λ(δS) on the potential surface of the lowest triplet excited state proceeds in three steps: CT ↔ MC, MC ↔ MC, MC ↔ CT, where the first and the third steps involve mainly the elongation and compression of the octahedral core of the reactant Δ(δS) and product Λ(δS), respectively. The chiral inversion Δ ↔ Λ takes place in the second step through a much distorted square-pyramid-like transition state, and actually proceeds on the triplet ground state MC due to the crossover of the triplet T and singlet S states. Inspecting the transient structures at the crossing points, we found that they become less distorted and their lowest or imaginary-frequency displacement vectors in triplet state still dominate the reaction path, which makes the reaction reversible without ligand release. Thus, the triplet ground-state-bridged photoinduced mechanism offers a new angle of view to understand the related reversible photochemical reactions.

Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry

“…4 Very recently this type of geometry has also been reported to be involved in Δ/Λ isomerism, and its involvement in ligand loss was suggested. 5 In relation with the reported (relative) photoinstability of the archetypical [Ru(bpy) 3 ] 2+ , 6,7,8,9,10 we were interested in testing the hypothesis that such MC states had a more general existence and could also be found in homoleptic tris(bidentate) complexes. The aim of this work was therefore to search for such a 3 MC cis minimum on the triplet excited potential energy surface of [Ru(bpy) 3 ] 2+ and we report its successful characterization here.…”

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

“…The latter is a stretch mode and thus may lead to ligand dissociation by coupling with corresponding vibration. All these features are similar to the photoinduced chiral inversion of the [Ru­(bipy) 2 ­(L-ser)] + series complexes . So it is also a triplet ground-state bridged photochemical process, where the ground-state character of 3 TS A guarantees the chiral inversion proceeding.…”

“…From Figure we see that the second step 3 MC Δ ⇌ 3 MC Λ is the rate-determining step (RDS) for both the bipy and phen complexes, because it has the highest energy barrier of ∼7.55 kcal/mol (bipy) or 8.42 kcal/mol (phen), while those of the first and the third steps are less than 3.79 kcal/mol. This is quite different from that of the [Ru­(diimine) 2 (L-ser)] + complexes . In the latter case, the first forward and the third backward steps are the RDS, while the second is a fast process of nanoseconds, which guarantees their chiral inversion Δ­(δ S )⇌Λ­(δ S ) to be reversible with no ligand release or substitutions.…”

“…The Δ↔Λ chiral inversion occurs in the second step via a much-distorted square-pyramid-like transition-state structure 3 TS A . All these features are same as the photoinduced chiral inversion of the [Ru­(bipy) 2 (L-ser)] + series complexes . So it is also a triplet ground-state bridged photochemical process.…”

On the Possible Coordination on a 3MC State Itself? Mechanistic Investigation Using DFT-Based Methods

“…For more than a decade, we have been able to optimize 3 MC excited states [3] and to explore the topology of the lowest triplet potential energy surface using DFT-based methods [4][5][6][7][8][9]. Spectacular bond elongations and angular distortions have been characterized in numerous Ru(II) 3 MC states [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and for other metals, e.g., Ir(III) [29][30][31]. Our contribution to the field covers photoisomerization mechanisms [32][33][34] and photoluminescence quenching mechanisms [35,36], as well as exploratory ruthenium(II) [37,38] and iron(II) [39][40][41][42] photophysics, in a constant dialogue with experimental chemists.…”