Electronic Structure and Geometry of the Lowest 2LMCT State of Fe(III) Potential Fluorescent Emitters†.

Metal complexes with a 3d6 electron count are emerging as an alternative to 4d6-based photosensitizers, emitters, or photoredox catalysts. In recent years, several Fe(II) potential emitters have been proposed, based on strongly donating ligand sets. Those tend to facilitate oxidation to their 3d5 species, whose photophysics is based on low-lying ligand-to-metal charge-transfer (LMCT) states. The geometry and electronic structure of 2LMCT states are unveiled in this work.

Merging Grubbs second-generation catalyst with photocatalysis enables Z-selective metathesis of olefins: scope, limitations, and mechanism.

Olefin cross-metathesis is a cornerstone reaction in organic synthesis where stereoselectivity is typically governed by the structure of the catalyst. In this work, we show that merging Grubbs second generation catalyst, a classical E-selective catalyst, with a readily available photocatalyst, enables the exclusive formation of the contra-thermodynamic Z-isomer. The scope and limitations of this unprecedented approach are discussed based on both computational and experimental mechanistic data.

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.

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)2(N∧N)]2+ (bpy = 2,2'-bipyridyl; N∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N∧N ligand. Complexes 2 to 6 all release the ligand N∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)2(NCMe)2]2+, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (3MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of 3MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a 3MCcis state but promotes the population of 3MCtrans states which rapidly deactivate 3MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of 3MCcis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Photoinduced Ligand Exchange Dynamics of a Polypyridyl Ruthenium Complex in Aqueous Solution.

The understanding of photoinduced ligand exchange mechanisms in polypyridyl ruthenium(II) complexes operating in aqueous solution is of crucial importance to rationalize their photoreactivity. Herein, we demonstrate that a synergetic use of ab initio molecular dynamics simulations and static calculations, both conducted at the DFT level, can provide a full understanding of photosubstitution mechanisms of a monodentate ligand by a solvent water molecule in archetypal ruthenium complexes in explicit water. The simulations show that the photoinduced loss of a monodentate ligand generates an unreactive 16-electron species in a hitherto undescribed pentacoordinated triplet excited state that converts, via an easily accessible crossing point, to a reactive 16-electron singlet ground state, which combines with a solvent water molecule to yield the experimentally observed aqua complex in less than 10 ps. This work paves the way for the rational design of novel photoactive metal complexes relevant for biological applications.

On the Spin-State Dependence of Redox Potentials of Spin Crossover Complexes.

Resistance switching properties of nanoscale junctions of spin crossover molecules have received recently much interest. In many cases, this property has been traced back to the variation of molecular orbital energies upon spin transition. However, one can also expect a substantial reorganization of the molecular structure due to charge localization, which calls for a better understanding of the relationship between the redox potential and the spin state of the molecule. To investigate this issue, we carried out a detailed density functional theory and variable temperature cyclic voltammetry investigation of the benchmark compound [Fe(HB(1,2,4-triazol-1-yl)3)2] in solution. We show that, for a correct thermodynamical picture, it is necessary to take into account the charge transfer-induced electronic and structural reorganization as well as spin equilibria in the oxidized and reduced species.

Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N

A complete mechanistic picture for the photochemical release of bipyridine (bpy) from the archetypal complex [Ru(bpy)3]2+ is presented for the first time following the description of the ground and lowest triplet potential energy surfaces, as well as their key crossing points, involved in successive elementary steps along pathways toward cis- and trans-[Ru(bpy)2(NCMe)2]2+. This work accounts for two main pathways that are identified involving (a) two successive photochemical reactions for photodechelation, followed by the photorelease of a monodentate bpy ligand, and (b) a novel one-photon mechanism in which the initial photoexcitation is followed by dechelation, solvent coordination, and bpy release processes, all of which occur sequentially within the triplet excited-state manifold before the final relaxation to the singlet state and formation of the final photoproducts. For the reaction between photoexcited [Ru(bpy)3]2+ and acetonitrile, which is taken as a model reaction, pathways toward cis and trans photoproducts are uphill processes, in line with the comparative inertness of the complex in this solvent. Factors involving the nature of the departing ligand and retained "spectator" ligands are considered, and their role in the selection of mechanistic pathways involving overall two sequential photon absorptions versus one photon absorption for the formation of both cis or trans photoproducts is discussed in relation to notable examples from the literature. This study ultimately provides a generalized roadmap of accessible photoproductive pathways for light-induced reactivity mechanisms of photolabile [Ru(N^N)(N^N')(N^N″)]2+-type complexes.

Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C∧N)2(N∧N)]+ Complexes.

Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C∧N)2(N∧N)]+ (HC∧N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N∧N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room-temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centered (3LC) (1a, 2a, and 2b) or ligand-to-ligand charge transfer (3LL'CT) character (1c, 2c, and 3a-c). Particularly striking is the observation that pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL'CT states. At 77 K, the 3MLCT/3LL'CT component is lost from the photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, while for 2c switching from 3MLCT/3LL'CT- to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a, and 2b which persist throughout the 3 ns time frame of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL'CT states. Transient data for 1b reveals intermediate behavior: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than that observed for 1c and 2c, behavior assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL'CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL'CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a, and 2b distinguished by upon which of the two C∧N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL'CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL'CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL'CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Furthermore, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.

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

We have identified a new 3MC state bearing two elongated Ru-N bonds to the same ligand in [Ru(bpy)3]2+. This DFT-optimized structure is a local minimum on the 3PES. This distal MC state (3MCcis) is destabilized by less than 2 kcal/mol with respect to the classical MC state (3MCtrans), and energy barriers to populate 3MCcis and 3MCtrans from the 3MLCT state are similar according to nudged elastic band minimum energy path calculations. Distortions in the classical 3MCtrans, that is, elongation of two Ru-N bonds toward two different bpy ligands, are not expected to favor the formation of ligand-loss photoproducts. On the contrary, the new 3MCcis could be particularly relevant in the photodegradation of Ru(II) polypyridine complexes.

Linkage Photoisomerization Mechanism in a Photochromic Ruthenium Nitrosyl Complex: New Insights from an MS-CASPT2 Study.

The N → O linkage photoisomerization mechanism in a ruthenium nitrosyl complex, [RuCl(NO)(py)4]2+, for which a quasicomplete photoconversion between the stable nitrosyl (N-bonded) and metastable isonitrosyl (O-bonded) isomers has been observed under continuous irradiation of the crystal at 473 nm ( Cormary et al. Acta Cryst. B 2009 , 65 , 612 - 623 ), is investigated using multiconfigurational second-order perturbation theory (CASPT2). The results support efficient intersystem crossing pathways from the initially excited singlet states to the lowest triplet excited state of metal-to-ligand charge transfer character (3MLCT). The topology of the involved potential energy surfaces corroborates a complex sequential two-photon photoisomerization mechanism involving nonadiabatic processes in agreement with experimental observations and previous density functional theory calculations.

A Theoretical Study of the N to O Linkage Photoisomerization Efficiency in a Series of Ruthenium Mononitrosyl Complexes.

Ruthenium nitrosyl complexes are fascinating versatile photoactive molecules that can either undergo NO linkage photoisomerization or NO photorelease. The photochromic response of three ruthenium mononitrosyl complexes, trans-[RuCl(NO)(py)4]2+, trans-[RuBr(NO)(py)4]2+, and trans-(Cl,Cl)[RuCl2(NO)(tpy)]⁺, has been investigated using density functional theory and time-dependent density functional theory. The N to O photoisomerization pathways and absorption properties of the various stable and metastable species have been computed, providing a simple rationalization of the photoconversion trend in this series of complexes. The dramatic decrease of the N to O photoisomerization efficiency going from the first to the last complex is mainly attributed to an increase of the photoproduct absorption at the irradiation wavelength, rather than a change in the photoisomerization pathways.

Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes.

We have identified highly novel photoreactive 3MC states of ruthenium(ii) 4,4'-bi-1,2,3-triazolyl (btz) complexes of the form [Ru(N^N)(btz)2]2+ and have elucidated the mechanism of the highly unusual experimental observations of photochemical ligand dechelation and concomitant ligand rearrangement reactivity to form unusual photoproducts trans-[Ru(N^N)(κ2-btz)(κ1-btz)(solvent)]2+. The triplet metal-to-ligand charge-transfer (3MLCT) states and classical Jahn-Teller type triplet metal-centred (3MC) states of the series of complexes [Ru(N^N)3-n(btz)n]2+ (btz = 4,4'-bi-1,2,3-triazolyl; N^N = 2,2'-bipyridyl (bpy), n = 0 (1), 1 (2), 2 (3), 3 (5); N^N = 4-(pyrid-2-yl)-1,2,3-triazole (pytz), n = 1 (4)) have been optimised by density functional theory (DFT) and characterised. There is a gradual and significant destabilisation of the 3MLCT states as the triazole content of the complexes increases, which occurs with a slight stabilisation of the 3MC states. Whilst consistent with the promotion of photochemical reactivity in the heteroleptic complexes of the series relative to 1, these classical 3MC states fail to account for the extraordinary ligand rearrangement processes that accompany ligand ejection. Thorough theoretical exploration of the lowest excited triplet potential energy surface (3PES) here reveals the existence of a new type of 3MC state and the role it plays in the photochemical reactivity of the complexes. This newly discovered state, called MC(F), displays a flattened geometry (indicated by the 'F' in the parentheses) which makes it clearly on the path to achieving the coplanarity of the bidentate ligands in the experimentally observed trans-photoproduct. Further novel 'pentacoordinate' 3MC states with coplanar bidentate ligands, called MC(P) (where the 'P' in the parentheses denotes the pentacoordinate character), were then identified and optimised. The energy barriers between the different triplet states were confirmed to be small which makes all triplet states accessible. Solvent trapping, which occurs on the singlet PES according to Wigner's rules, is finally achieved by a singlet pentacoordinate species to yield the monosolvento photoproduct. Thus, our calculations not only reveal highly novel 3MC states but more significantly demonstrate their crucial role in the formation of the experimentally observed photoproducts.

Is photoisomerization required for NO photorelease in ruthenium nitrosyl complexes?

The factors that explain the competition between intramolecular NO linkage photoisomerization and NO photorelease in five ruthenium nitrosyl complexes were investigated. By applying DFT-based methods, it was possible to characterize the ground states and lowest triplet potential energy surfaces of these species, and to establish that both photoisomerization and photorelease processes can occur in the lowest triplet state of each species. This work highlights the crucial role of the sideways-bonded isomer, a metastable state also known as the MS2 isomer, in the photochemical loss of NO, while the results obtained also indicate that the population of the triplet state of this isomer is compulsory for both processes and show how photoisomerization and photorelease interfere. Graphical Abstract Illustration of the crucial role of the 3MS2 state in the photoreactivities of ruthenium nitrosyl complexes.

Computational Estimate of the Photophysical Capabilities of Four Series of Organometallic Iron(II) Complexes.

In this study, we examine a large range of organometallic iron(II) complexes with the aim of computationally identifying the most promising ones in terms of photophysical properties. These complexes combine polypyridine, bis(phosphine), and carbon-bound ligands. Density functional theory has allowed us to establish a comparative Jablonski diagram displaying the lowest singlet, triplet, and quintet states. All of the proposed FeN5C or FeN3P2C complexes unfavorably possess a lowest triplet state of metal-centered (MC) nature. Among the FeN4C2 and FeN2P2C2 series, the carbene complexes display the least favorable excited-state distribution, also having a low-lying (3)MC state. Validating our design strategy, we are now able to propose seven iron(II) complexes displaying a lowest excited state of triplet metal-to-ligand charge-transfer nature.

Pivotal Role of a Pentacoordinate (3)MC State on the Photocleavage Efficiency of a Thioether Ligand in Ruthenium(II) Complexes: A Theoretical Mechanistic Study.

A mechanistic study of the photocleavage of the methylthioethanol ligand (Hmte) in the series of ruthenium complexes [Ru(tpy)(N-N)(Hmte)](2+) (tpy = 2,2':6',2″-terpyridine, N-N = bpy (2,2'-bipyridine), biq (2,2'-biquinoline), dcbpy (6,6'-dichloro-2,2'-bipyridine), dmbpy (6,6'-dimethyl-2,2'-bipyridine)) was performed using density functional theory. These studies reveal the decisive role of two quasi-degenerate triplet metal-centered states, denoted (3)MChexa and (3)MCpenta, on the lowest triplet potential energy surface. It also shows how the population of the specific pentacoordinate (3)MCpenta state, characterized by a geometry more accessible for the attack of a solvent molecule, is a key step for the efficiency of the photosubstitution reaction. The difference in the photosubstitution quantum yields experimentally observed for this series of complexes (from φ = 0.022 for N-N = bpy up to φ = 0.30 for N-N = dmbpy) is rationalized by the existence of this (3)MCpenta photoreactive state and by the different topologies of the triplet excited-state potential energy surfaces, rather than by the sole steric properties of these polypyridinyl ligands.

Establishing the Two-Photon Linkage Isomerization Mechanism in the Nitrosyl Complex trans-[RuCl(NO)(py)4](2+) by DFT and TDDFT.

The density functional theory calculations presented in this work allow the first rationalization of the full linkage photoisomerization mechanism of trans-[RuCl(NO)(py)4](2+), in both the forward and reverse directions. These mechanisms are consistent with the experimental data establishing that blue-light irradiation triggers the forward process, while red or IR photons trigger the reverse process. Characterization of the singlet and lowest triplet potential energy surfaces shows that, despite the unfavorable thermodynamic character of the forward process, the topologies of the surfaces and particularly some crucial surface crossings enable the isomerization. In the forward Ru-NO → Ru-ON direction, a sequential two-photon absorption mechanism is unraveled that involves a sideways-bonded metastable state. In contrast, in the reverse reaction, two mechanisms are proposed involving either one or two photons.

Reversing the relative 3MLCT-3MC order in Fe(ii) complexes using cyclometallating ligands: a computational study aiming at luminescent Fe(ii) complexes.

Following a computational approach, the use of strongly electron-donating cyclometallating ligands has allowed us to increase the (3)MC-(3)MLCT gap dramatically in Fe(ii) bis(tridentate) polypyridine complexes, and eventually to reverse the ordering between these states, yielding a (3)MLCT state that is clearly more stable than the (3)MC state. Simultaneously, the quintet excited states ((5)MC and (5)MLCT) are displaced away from the region (in terms of geometry and energy) where classical photophysics occur, allowing us to avoid magnetism. The situation is thus similar to that of classical ruthenium polypyridine complexes. This opens the way towards luminescent iron(ii) complexes, in particular Fe(ii)bis(6-phenyl-2,2'-bipyridine) Fe(NNC)(2).

Unravelling the S → O linkage photoisomerization mechanisms in cis- and trans-[Ru(bpy)2(DMSO)2](2+) using density functional theory.

A mechanistic study of the intramolecular S → O linkage photoisomerization in the cis and trans isomers of [Ru(bpy)2(DMSO)2](2+) was performed using density functional theory. This study reveals that for the cis isomer the linkage photoisomerization of the two DMSO ligands occurs sequentially in the lowest triplet excited state and can either be achieved by a one-photon or by a two-photon mechanism. A mechanistic picture of the S → O photoisomerization of the trans isomer is also proposed. This work especially highlights that both adiabatic and nonadiabatic processes are involved in these mechanisms and that their coexistence is responsible for the rich photophysics and photochemical properties observed experimentally for the studied complexes. The different luminescent behavior experimentally observed at low temperature between the cis and trans isomers is rationalized based on the peculiarity of the topology of the triplet excited-state potential energy surfaces.

The (N4C2)2- donor set as promising motif for bis(tridentate) iron(II) photoactive compounds.

The ground and excited states of iron(II) bis(dipyridylbenzene) were probed by means of DFT and TDDFT. In comparison to the well-known Fe(tpy)2(2+), this neutral complex should not suffer from ligand loss, and it displays a better absorption profile in the visible region. The relative energies of the spectroscopically relevant excited states ((3)MLCT, (3)MC, (5)MLCT, and (5)MC) are quite different from those of its archetypical counterpart and thus make it a promising candidate for photophysics in general.

Can a functionalized phosphine ligand promote room temperature luminescence of the [Ru(bpy)(tpy)]2+ core?

Unexpected room temperature luminescence is observed and rationalized by highly challenging excited state calculations for a functionalized phosphine ligand coordinated on the [Ru(bpy)(tpy)](2+) core.

Adiabatic versus nonadiabatic photoisomerization in photochromic ruthenium sulfoxide complexes: a mechanistic picture from density functional theory calculations.

Polypyridine ruthenium sulfoxide complexes are intriguing compounds which can display both photochromic and electrochromic properties. These properties are based on the Ru-S → Ru-O linkage isomerization capability of the sulfoxide group. The photoisomerization mechanism is of particular importance in order to understand the photophysical properties of such molecules. Density functional theory calculations demonstrate that the main photoisomerization mechanism is nonadiabatic for the system under study in agreement with the experimental observations. Indeed, funnels for efficient radiationless decay back to the ground state are shown to be easily accessible compared to transition states on the adiabatic triplet potential energy surface. However, we highlight for the first time that triplet metal-centered states play a central role in the photoisomerization mechanism of these compounds.