Theoretical Studies On The Mechanism Of Photoinduced Chiral Inversion At The Metal Center Of The Ru-Diimine Complexes

In this paper,the mechanism on the photoinduced spontaneous chiral inversion at metal center of the Ru-diimine complexes{prototype compounds[Ru?diimine?₃]²⁺and mixed-ligand Chelates[Ru?diimine?₂?L-aa?]⁺including L-amino acid}has been analysed at the first principle level of theory,of which the diimine represents 1,10-phenanthroline?phen?or 2,2'-bipyridyl?bipy?,the optically active L-aa is L-serine?L-ser?and L-tryptophane?L-trp?,respectively.Particular attention is paid to three aspects,including triplet ground-state-bridged photochemical process,key factor dominating the competition between photolysis and photoracemization,and effect of structural decoration on the inversion mechanism.Some main conclusions and significances are summarized as follows:First,common conclusions:?1?For above three investigated systems,the calculated equilibrium constants and the simulated ECD curve reproduces all of the qualitative features observed experimentally including most of the major peaks with the correct sign and similar band shape,validating the reliability of our DFT calculated results.?2?On the ground state S₀ surface,both Ray-Dutt and Bailar twist mechanisms involve high-energy barriers,and thus the spontaneous chiral inversion is not facile at room temperature.?3?On the triplet state T₁,the?????43?chiral inversion includes two kinds of different excited states in nature,namely the metal-to-ligand charge transfer state ³CT and metal centered state ³MC.During the conversion process from ³CT to ³MC,there exhibits a remarkable spin contraction phenomena.?4?The geometries of the key triplet transition states ³TS_A and ³TS_B are distinctly different from theirs S₀,which possess a flattened square pyramid?FSP?structures and two much weakened coordinated bonds.Strictly speaking,the chiral inversion form of the complex in the triplet state is neither a typical Ray-Dutt twist nor Bailar twist,but partly mixed with the bond rupture form.?5?The reversible photoinduced inversion reaction takes place on the triplet T₁ state potential energy surface?PES?,and proceeds at least in three steps:³CT_??³MC_?,³MC_??³MC_?and ³MC_??³CT_?.During the second inversion step of?????,T₁ becomes the ground state over one-half of the pathway at both sides of the ³TS_A/B state,due to the crossover of the triplet T₁and singlet S₀ states,which prolongs the lifetime of ³TS and ensures the chiral inversion proceeding more easily.Obviously,the key chiral inversion of Ru-diimine complexes actually proceeds on the triplet ground state PES,and the barrier heights undergo consumedly decrease compared to theirs singlet ground state S₁,which become thermodynamically more favorable.?6?For the Ru-diimine compounds,the photoinduced spontaneous inversion???mechanism of the chiral configuration,is nothing but a triplet ground-state-bridged reversible photochemical process.Under the irradiation of the visible light,the initial S₀ state reactant is promoted to one of the singlet excited states S₁ first,and then it rapidly relaxes to the lowest-energy triplet state T₁ through efficient nonradiative intersystem crossing?ISC?.After the excited reactant ³CT_?passes through the key ³MC_????³MC_?chiral inversion step occurring on the triplet ground state PES,the triplet product³MC_?deexcites and returns to its ground state,and finally ends with phosphorescence emitting.Clearly,the role of photons is promoting the reactant from its singlet ground state S₀ to triplet ground state T₁ of higher energy,on which the chiral inversion could take place.As far as this point is concerned,there is no essential difference between the photochemical reaction and the general chemical reaction.Second,individual conclusions:?1?For the[Ru?diimine?₂?L-ser?]⁺series complexes,the first step of³CT_R???³MC_R is the rate determining step?RDS?on the T₁ state PES;while the key second step ³MC_R???³MC_P is a fast process of nanoseconds,which inhibits the accumulation of ³MC state,and thus avoids the fate of fast decay and ligand dissociations,and guarantees the spontaneous inversion of the chiral configuration.Additionally,the lowest or imaginary-frequency displacement vectors of the crossing points in the triplet state dominate the reaction path;furthermore,between the two points,the de-excitation of ³MC is impossible due to the triplet ground-state character.Both are benefit to restrain the ligand dissociations of the chiral complexes.Such mechanism is consistent with the experimental phenomena that no any ligand release product is detected.?2?Different from the L-ser systems,the key second inversion process³MC_??³MC_?of the prototype complexes[Ru?diimine?₃]²⁺is the RDS with a time scale of microseconds,which makes plentiful ³MC states lingering around theirs bottom near the triplet-singlet state crossing points after the first step,and therefore,greatly enhances the possibility of deexcitation via non-radiative transitions or ligand dissociations:certain active ³MC could return to theirs S₀ state equilibrium geometries,but may also interact with solvent molecules,yielding a photolysis or photosubstitution product.This accounts for the fact that the photochemical reactions of the prototype complexes are dominated by the photolysis or photosubstitutions,not by the photoracemization.Evidently,RDS is the key factor dominating the competition between photolysis and photoracemization in these and related complexes.?3?Relative to the first two systems,the pathway at both sides of the inversion transition state ³TS_A/B of the[Ru?diimine?₂?L-trp?]⁺system is significantly asymmetric,due to the influence from the chiral discrimination interaction of the L-trp group.At the reactant side of the rhombic channel,not only involves new convertion beween the triplet charge transfer states ³CT_a?³CT_b,but also includes several convertions among metal centered states ³MC_a?…?³MC_d,where the process over ³MC_b?³TS_A is a multi-step,low-barrier,and full of potholes complicated process.The coexistence of the charge transfer ³CT_a and ³CT_b states,mainly originates from their?-component complementarity among certain low-energy orbitals,e.g.,the?*_?F1-phen?of³CT_a corresponds to the?*_?F2-phen?of ³CT_b.In addition,since the?-orbital types of the product ³CT_P exactly match with that of ³CT_b locationg at the reactant side,therefore,considered the symmetry,³CT_b is the essential state during the whole inversion pathway.Clearly,the structural decoration could directly act on the path or mechanism of the photochemical reaction.Third,significances:?1?The discovery of“triplet-ground-state-bridged”feature corrects the traditionary intuition on“photochemical reaction definitely proceeds through the excited state”,which offers a new angle of view to establish a relationship between the photochemical reactions and the normal chemical ones.?2?The photoracemization and photolysis of the Ru-diimine complexes have a competition relation,where the rate determining step?RDS?dominates the main reaction type.It is not a slave relation as implied by the photoracemization model suggested in literature.This conclusion is helpful to understanding and controlling the photochemical behavior of the complexes in practice.?3?The structural decoration of different ligand could directly act on the pathway and mechanism of the photochemical reaction,which not only deepens our comprehension on the chirality phenomenon and the chiral interaction principle,but also should be valuable for the design and synthesis of improved chiral drugs,chiral materials and chiral catalysis.