Efficient Generation of the Ligand Field Excited State of Tris-(2,2‘-bipyridine)-ruthenium(II) through Sequential Two-Photon Capture by [Ru(bpy)3]2+ or Electron Capture by [Ru(bpy)3]3+

Photophysical Studies of Helicate and Mesocate Double-Stranded Dinuclear Ru(II) Complexes

The metal-ligand charge transfer (3MLCT) and phosphorescence-quenching metal-centered (3MC) states of the helicate and mesocate diastereoisomers of a double-stranded dinuclear polypyridylruthenium(II) complex have been investigated using ultrafast transient absorption spectroscopy. At 294 K, transient signals of the helicate decayed significantly slower than those of the mesocate, whereas at 77 K, no clear contrast in kinetics was observed. Contributions to excited-state decay from high-lying 3MLCT states were identified at both temperatures. Spectroscopic data (294 K) suggest that the 3MC state of the helicate lies above the 3MLCT and that the reverse is true for the mesocate; this was further validated by density functional theory calculations. The stabilization of the 3MC state relative to the 3MLCT state in the mesocate was explained by a reduction in ligand field strength due to distortion near the ligand bridge, which causes further deviation from octahedral geometry compared to the helicate. This work illustrates how minor structural differences can significantly influence excited state dynamics.

Computational Analysis of Photophysical Properties and Reactivity of a New Phototherapeutic Cyclometalated Au(III)-Hydride Complex

Gold(III) complexes have recently emerged as new versatile and efficacious metal containing anticancer agents. In an attempt to reconcile the specific affinity of such complexes for target sulfur containing biomolecules with their capability to strongly bind thiol-containing compounds widely distributed in non-tumoral cells, a new series of cyclometalated Au(III)-hydride complexes has been proposed as photoactivatable anticancer prodrugs. Here, the computational exploration of the photophysical properties and reactivity in dark and under light irradiation of the first member of the series, named 1 a, is reported. Complex 1 a low hydricity in dark together with facile hydride substitution leading to H₂ elimination under excitation by visible light have been examined by means of DFT and TD-DFT computations. Both singlet and triplet excited states have been characterized, allowing the identification of the active species involved in photoactivation pathways leading to the controlled detachment of the hydride ligand. Also the viable two-photon activation at the ideal phototherapeutic window has been investigated.

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

Understanding light-induced ligand exchange processes is key to the design of efficient light-releasing prodrugs or photochemically driven functional molecules. Previous mechanistic investigations had highlighted the pivotal role of metal-centered (MC) excited states in the initial ligand loss step. The question remains whether they are equally important in the subsequent ligand capture step. This article reports the mechanistic study of direct acetonitrile coordination onto a 3MC state of [Ru(bpy)3]2+, leading to [Ru(bpy)2(κ1-bpy)(NCMe)]2+ in a 3MLCT (metal-to-ligand charge transfer) state. Coordination of MeCN is indeed accompanied by the decoordination of one pyridine ring of a bpy ligand. As estimated from Nudged Elastic Band calculations, the energy barrier along the minimum energy path is 20 kcal/mol. Interestingly, the orbital analysis conducted along the reaction path has shown that creation of the metallic vacancy can be achieved by reverting the energetic ordering of key dσ* and bpy-based π* orbitals, resulting in the change of electronic configuration from 3MC to 3MLCT. The approach of the NCMe lone pair contributes to destabilizing the dσ* orbital by electrostatic repulsion.

Ligand photodissociation in Ru(ii)–1,4,7-triazacyclononane complexes enhances water oxidation and enables electrochemical generation of surface active species

Ligand transformations involved in metal complexes during water oxidation (WO), such as ligand decomposition, partial oxidation, or complete dissociation have been reported, however, ligand photodissociation has not been reported yet.

H/D solvent isotope effects on the photoracemization reaction of enantiomeric the tris(2,2′-bipyridine)ruthenium(ii) complex and its analogues

This work assessed solvent isotope effects on the photoracemization rate and emission lifetime for [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) in water.

Azole-containing cationic bis-cyclometallated iridium(iii) isocyanide complexes: a theoretical insight into the emission energy and emission efficiency

Using a density functional theory approach, we explore the emission properties of a family of bis-cyclometallated cationic iridium(iii) complexes of general formula [Ir(C^N)2(CN-tert-Bu)2]+ that have tert-butyl isocyanides as neutral auxiliary ligands. Taking the [Ir(ppy)2(CN-tert-Bu)2]+ complex (Hppy = 2-phenylpyridine) as a reference, the effect of replacing the pyridine ring in the cyclometallating ppy ligand by a five-membered azole ring has been examined. To this end, two series of complexes differing by the nature of the atom (either nitrogen or carbon) linking the azole to the phenyl ring of the cyclometallating ligand have been designed. Each series is composed of three molecules having an increasing number of nitrogen atoms (2 to 4) in the azole ring. The emission energies computed for the azole-containing [Ir(C^N)2(CN-tert-Bu)2]+ complexes show a generalized blue-shift compared to [Ir(ppy)2(CN-tert-Bu)2]+, in agreement with the experimental data available for two of the six complexes designed here. The electronic nature of the lowest-lying triplet (T1) is clearly established as a ligand-centred (3LC) state associated with the cyclometallating ligands, and cannot be described as a simple HOMO → LUMO promotion. Therefore, no clear trend based on the sole use of molecular orbital energies can be inferred to predict the emission properties. The significant oscillation in the emission quantum yield (ranging from 0.1% to 52%) experimentally reported is rationalized by the energy gap between the emitting T1 state and a non-radiative triplet state having metal-centred (3MC) d-d* nature. On the basis of such a model, two of the here proposed systems are expected to display significant emission quantum yields in the blue region of the visible spectra, which make them good candidates for electroluminescent applications.

Chemical Scavenging Yields for Short-Lived Products from the Visible Light Photoionization of the Tris(bipyridine)ruthenium(II) Triplet Metal-to-Ligand Charge-Transfer Excited State

The rate of visible light photoionization of the tris(bipyridine)ruthenium(II) triplet metal-to-ligand charge-transfer excited state (³MLCT) is very strongly dependent on the acid concentration in aqueous solution, and the pattern of this dependence is similar to that reported for the photoionization of iodide. With 405 nm visible irradiation of ³MLCT, less than 15% of the photoionized products appear as free solvated electrons in bulk solution, while more than 75% of the photoproducts appear to be solvent-separated, (oxidized substrate)-electron ion pairs that efficiently recombine with the photo-oxidized complex in the absence of an electron scavenger. The quantum yield of free solvated electrons generated by these 405 nm irradiations is approximately 0.004, but the net quantum yield of scavengeable electrons is estimated to be about 0.04. A visible-region photoionization threshold energy for the ³MLCT is consistent with thermodynamic expectations, and similar behavior is expected for many redox-active complexes.

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(δ S)⇌Λ(δ S) 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.

Microsecond 3MLCT excited state lifetimes in bis-tridentate Ru(ii)-complexes: significant reductions of non-radiative rate constants

In this paper we report the photophysical properties of a series of bis-tridentate Ru^II-complexes, based on the dqp-ligand (dqp = 2,6-di(quinolin-8-yl)pyridine), which display several microsecond long excited state lifetimes for triplet metal-to-ligand charge transfer (³MLCT) at room temperature. Temperature dependence of the excited state lifetimes for [Ru(dqp)₂]²⁺ and [Ru(dqp)(ttpy)]²⁺ (ttpy = 4'-tolyl-2,2':6',2''-terpyridine) is reported and radiative and non-radiative rate constants for the whole series are reported and discussed. We can confirm previous assumptions that the near-octahedricity of the bis-dqp complexes dramatically slows down activated decay at room temperature, as compared to most other and less long-lived bis-tridentate Ru^II-complexes, such as [Ru(tpy)₂]²⁺ with τ = 0.25 ns at room temperature (tpy = 2,2':6',2''-terpyridine). Moreover, the direct non-radiative decay to the ground state is comparatively slow for ∼700 nm room-temperature emission when considering the energy-gap law. Analysis of the 77 K emission spectra suggests that this effect is not primarily due to smaller excited state distortion than that for comparable complexes. Instead, an analysis of the photophysical parameters suggests a weaker singlet-triplet mixing in the MLCT state, which slows down both radiative and non-radiative decay.

Theoretical illumination of highly original photoreactive3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes

Elucidation of the photoreactive mechanism of ruthenium(ii) complexes is reported along with identification of crucial and highly original metal-centred states.

Syntheses and properties of phosphine-substituted ruthenium(ii) polypyridine complexes with nitrogen oxides

The substitution lability of the nitrogen oxide ligands of novel phosphine-substituted ruthenium(ii) polypyridine complexes is discussed in comparison with that of the corresponding acetonitrile complexes.

Experimental evidence of ultrafast quenching of the 3MLCT luminescence in ruthenium(II) tris-bipyridyl complexes via a 3dd state

Ultrafast transient absorption spectroscopy serves to identify the (3)dd state as intermediate quencher state of the (3)MLCT luminescence in the non-luminescent ruthenium complexes [Ru(m-bpy)3](2+) (m-bpy = 6-methyl-2,2'-bipyridine) and [Ru(tm-bpy)3](2+) (tm-bpy = 4,4',6,6'-tetramethyl-2',2'-bipyridine). For [Ru(m-bpy)3](2+), the population of the (3)dd state from the (3)MLCT state occurs within 1.6 ps, while the return to the ground state takes 450 ps. For [Ru(tm-bpy)3](2+), the corresponding values are 0.16 and 7.5 ps, respectively. According to DFT calculations, methyl groups added in the 6 and 6' positions of bipyridine stabilize the (3)dd state by ~4000 cm(-1) each, compared to [Ru(bpy)3](2+).

[Ru(bpy)3]2+* and other remarkable metal-to-ligand charge transfer (MLCT) excited states

In 1974, the metal-to-ligand charge transfer (MLCT) excited state, [Ru(bpy)3]2+*, was shown to undergo electron transfer quenching by methylviologen dication (MV2+), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a “modular approach”. In the intervening years, application of synthesis, excited-state measurements, and theory to [Ru(bpy)3]2+* and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assemblies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated.

Impact of the synergistic collaboration of oligothiophene bridges and ruthenium complexes on the optical properties of dumbbell-shaped compounds

The linear and non-linear optical properties of a family of dumbbell-shaped dinuclear complexes, in which an oligothiophene chain with various numbers of rings (1, 3, and 6) acts as a bridge between two homoleptic tris(2,2'-bipyridine)ruthenium(II) complexes, have been fully investigated by using a range of spectroscopic techniques (absorption and luminescence, transient absorption, Raman, and non-linear absorption), together with density functional theory calculations. Our results shed light on the impact of the synergistic collaboration between the electronic structures of the two chemical moieties on the optical properties of these materials. Experiments on the linear optical properties of these compounds indicated that the length of the oligothiophene bridge was critical for luminescent behavior. Indeed, no emission was detected for compounds with long oligothiophene bridges (compounds 3 and 4, with 3 and 6 thiophene rings, respectively), owing to the presence of the (3)π-π* state of the conjugated bridge below the (3)MLCT-emitting states of the end-capping Ru(II) complexes. In contrast, the compound with the shortest bridge (2, one thiophene ring) shows excellent photophysical features. Non-linear optical experiments showed that the investigated compounds were strong non-linear absorbers in wide energy ranges. Indeed, their non-linear absorption was augmented upon increasing the length of the oligothiophene bridge. In particular, the compound with the longest oligothiophene bridge not only showed strong two-photon absorption (TPA) but also noteworthy three-photon-absorption behavior, with a cross-section value of 4×10(-78) cm(6) s(2) at 1450 nm. This characteristic was complemented by the strong excited-state absorption (ESA) that was observed for compounds 3 and 4. As a matter of fact, the overlap between the non-linear absorption and ESA establishes compounds 3 and 4 as good candidates for optical-power-limiting applications.

Excited State Intervalence Transfer in a Rigid Polymeric Film

The ligand-bridged complex cis,cis-[(bpy)2ClRu(pz)RuCl(bpy)2]2+ as the PF6- salt, (1)(PF6)2, is stabilized toward photochemical ligand loss in poly(methyl methacrylate) (PMMA). Stabilization allows measurement of metal-to-ligand charge transfer (MLCT) photophysical properties--emission and transient absorption. This includes appearance of an intervalence transfer absorption band in the near IR spectrum of the photochemically prepared, mixed valence form, [(bpy)2ClRuIII(pz(-*))RuIICl(bpy)2](PF6)2* (1*(PF6)2). Comparison of its IT band properties with those of ground state cis,cis-(bpy)2ClRuIII(pz)RuIICl(bpy)2]3+ in CD3CN allows a comparison to be made between pz and pz(-*) as bridging ligands. A model based on differences between rigid and fluid media provides an explanation for decreased IT band energies and widths in PMMA and provides important insight into electron transfer in rigid media.

Rigid Medium Stabilization of Metal-to-Ligand Charge Transfer Excited States

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.

Direct observation of the fourth MLCT triplet state in ruthenium(ii) tris(2,2′-bipyridine)

The luminescence properties of ruthenium(ii) tris(2,2'-bipyridine) have been recorded in butyronitrile solution and in a transparent KBr disk over a reasonable temperature range. In solution, spectral curve fitting routines indicate that emission arises solely from an ensemble of triplet states, each of which is of Metal-to-Ligand, Charge-Transfer (MLCT) character and of closely comparable energy. At ambient temperature, dual emission is observed for the KBr disk and interpreted in terms of luminescence from both the ensemble and the fourth MLCT triplet state that lies at slightly higher energy. Relative reorganisation energies, energies, Huang-Rhys factors and radiative rate constants have been calculated for the two emissive states. It is confirmed that the fourth MLCT triplet state possesses more singlet character than the ensemble.

Photexcitation of Aqueous Ruthenium(II)-tris-(2,2‘-bipyridine) with High-Intensity Femtosecond Laser Pulses

We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.

An Apparent Angle Dependence for the Nonradiative Deactivation of Excited Triplet States of Sterically Constrained, Binuclear Ruthenium(II) Bis(2,2‘:6‘,2‘ ‘-terpyridine) Complexes

The photophysical properties are reported for a series of binuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes built around a geometrically constrained, biphenyl-based bridge. The luminescence quantum yield and lifetime increase progressively with decreasing temperature, but the derived rate constant for nonradiative decay of the lowest-energy triplet state depends on the length of a tethering strap attached at the 2,2'-positions of the biphenyl unit. Since the length of the strap determines the dihedral angle for the central C-C bond, the rate of nonradiative decay shows a pronounced dependence on angle. The minimum rate of nonradiative decay occurs when the dihedral angle is 90 degrees, but there is a maximum in the rate when the dihedral angle is about 45 degrees. This effect does not appear to be related to the extent of electron delocalization at the triplet level but can be explained in terms of variable coupling with a low-frequency vibrational mode associated with the strapped biphenyl unit.

Chemical approaches to artificial photosynthesis. 2

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.