3MC State Research Articles

Exceptional Excited-State Lifetime of an Iron(II)-N-Heterocyclic Carbene Complex Explained.

Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron-N-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (∼9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Fe(II) complexes. In addition, a shallow (3)MLCT potential energy surface with a low-energy transition path from the (3)MLCT to (3)MC and facile crossing from the (3)MC state to the ground state are identified as key features for the excited-state deactivation.

Intramolecular π Stacking in Cationic Iridium(III) Complexes with Phenyl‐Functionalized Cyclometalated Ligands: Synthesis, Structure, Photophysical Properties, and Theoretical Studies

AbstractThe syntheses of two new heteroleptic cationic iridium complexes containing 2,6‐diphenylpyridine (Hdppy) and 2,4,6‐triphenylpyridine (Htppy) as the cyclometalated ligands, namely, [Ir(dppy)2phen]PF6 (1, phen = 1,10‐phenanthroline) and [Ir(tppy)2phen]PF6 (2), are described. The X‐ray crystal structure of 2 reveals a distorted octahedral geometry around the Ir center and close intramolecular face‐to‐face π–π stacking interactions between the pendant phenyl rings at the 2‐position of the cyclometalated ligands and the NN ancillary ligand. This represents a new π–π stacking mode in charged Ir complexes. Complexes 1 and 2 are green photoemitters: their photophysical and electrochemical properties are interpreted with the assistance of density functional theory (DFT) calculations. These calculations also establish that the observed intramolecular interactions cannot effectively prevent the lengthening of the Ir–N bonds of the complexes in their metal‐centered (3MC) states. Complexes 1 and 2 do not emit light in light‐emitting electrochemical cells (LECs) under conditions in which the model compound [Ir(ppy)2phen]PF6 (3) emits strongly. This is explained by degradation reactions of the 3MC state of 1 and 2 under the applied bias during LEC operation facilitated by the enhanced distortions in the geometry of the complexes. These observations have important implications for the future design of complexes for LEC applications.

Accesses to electronic structures and the excited states of iridium complexes containing pyrazolyl or benzimidazoly ligands: A DFT/TDDFT exploitation

The ground state, 3MLCT and 3MC excited states of two Ir complexes Ir(dfbpzb)(ppy)Cl (1) and Ir(Mebib)(ppy)Cl (2) where dfbpzb=1,5-difluore-2,4-bis(3-methylpyrazolyl)benzene, ppy=2-phenylpyridine, and Mebib=bis(N-methylbenzimidazoly)benzene, have been investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The substitution from dfbpzb ligand to Mebib ligand induces a significant change on the luminescent properties of complexes 1 and 2. The complex 1 does not emit at room temperature, while complex 2 does. The UV–Vis absorption spectra of both complexes are well reproduced by TD-DFT calculations. Importantly, the triplet metal-to-ligand charge transfer (3MLCT) and metal-centered (3MC) states were discussed in detail by excited state calculations, and the 3MLCT→3MC deactivation pathway was demonstrated as an important factor for the lack of emission for the investigated complex 1.

Photosolvolysis ofcis-[Ru(α-diimine)2(4-aminopyridine)2]2+Complexes: Photophysical, Spectroscopic, and Density Functional Theory Analysis

The photochemical and photophysical properties of the cis-[Ru(II)(α-diimine)2(4-APy)2](2+) complexes, where α-diimine = 1,10-phenanthroline (phen) and 4-APy = 4-aminopyridine I, 4,7-diphenyl-1,10-phenanthroline (Ph2phen) II, 2,2'-bipyridine (bpy) III, and 4,4'-dimethyl-2,2'-bipyridine (Me2bpy) IV, are reported. The four complexes were characterized using high-performance liquid chromatography, (1)H NMR, UV-visible, emission, and transient absorption spectroscopy. Upon photolysis in acetonitrile solution these complexes undergo 4-APy dissociation to give the monoacetonitrile complex (for II, III, and IV) or the bis(acetonitrile) complex (for I). A fairly wide range of excitation wavelengths (from 420 to 580 nm) were employed to explore the photophysics of these systems. Quantum yields and transient spectra are provided. Density functional theory (DFT) and time-dependent DFT analysis of singlet and triplet excited states facilitated our understanding of the photochemical behavior. A detailed assessment of the geometric and electronic structures of the lowest energy spin triplet charge transfer state ((3)MLCT) and spin triplet metal centered state ((3)MC) (dπ → σ* transitions) for species I-IV is presented. A second, previously unobserved, and nondissociative, (3)MC state is identified and is likely involved in the primary step of photodissociation. This new (3)MC state may indeed play a major role in many other photodissociation processes.

Controlling the Excited State and Photosensitizing Property of a 2-(2-Pyridyl)benzo[b]thiophene-Based Cationic Iridium Complex through Simple Chemical Modification

Bis-cyclometalated cationic iridium (Ir) complexes 1-4 comprising two 2-(2-pyridyl)benzo[b]thiophene (btp) ligands and one 2,2'-bipyridyl (bpy) ancillary ligand with different substituents were prepared as new visible light-absorbing sensitizers and examined for their photophysical and electrochemical properties. Complex 1 was prepared as a parent complex without any substituents. Complexes 2-4 contained methyl-, methoxy-, and trifluoromethyl groups at 4,4'-positions on the bpy ancillary ligand. Systematic investigation of these complexes revealed that such a simple chemical modification selectively controls the excited-state lifetime, while the absorption and emission spectral features remain unchanged. Specifically, the phosphorescence lifetimes of complexes 2 and 3 with electron-donating groups (τ = 3.50 μs, 3.90 μs) were found to be much longer than that of complex 1 (τ = 0.273 μs), and complex 4, possessing strong electron-withdrawing trifluoromethyl groups, did not exhibit detectable phosphorescence at room temperature. The large differences in excited-state lifetimes of complexes 1-3, as well as the nonemissive character of complex 4, are attributed to a strong influence of the substituents on the ligand field strength. The increased σ-donating ability of the ancillary ligand in complexes 2 and 3 destabilizes a short-lived, nonemissive triplet metal-centered ((3)MC) state and increases the energy separation between the (3)MC state and emissive triplet ligand-centered ((3)LC) state based on the btp ligand. For complex 4, however, the (3)MC state is close in energy to the (3)LC state because of the decreased σ-donating ability of the ancillary ligand. Additional evidence of the (3)MC state associated with the changeable excited state was also provided via low-temperature phosphorescence measurements and density functional theory calculations. Ir complexes 1-4 were tested as sensitizers in photoinduced electron-transfer reaction of triethanolamine and methylviologen chloride (MVCl2). As a result, complexes 2 and 3 exhibited much better photosensitizing property compared to complex 1 since their long-lived excited states promoted an oxidative quenching pathway. This Study has first demonstrated that simple substitution on the diimine ancillary ligand can control the (3)MC state of the bis-cyclometalated cationic Ir complex to finely tune the excited-state lifetime and photosensitizing property.

Revisited Photophysics and Photochemistry of a Ru-TAP Complex Using Chloride Ions and a Calix[6]crypturea

The effects of the nonprotonated and protonated calix[6]crypturea 1/1(•)H(+) on the PF6(-) and Cl(-) salts of a luminescent Ru-TAP complex (TAP = 1,4,5,8-tetraazaphenanthrene) were investigated. Thus, the phototriggered basic properties of this complex were examined with 1(•)H(+) in acetonitrile (MeCN) and butyronitrile (BuCN). The Ru excited complex was shown to be able to extract a proton from the protonated calixarene, accompanied by a luminescence quenching in both solvents. However, in BuCN, the Cl(-) salt of the complex exhibited a surprising behavior in the presence of 1/1(•)H(+). Although an emission decrease was observed with the protonated calixarene, an emission increase was evidenced in the presence of nonprotonated 1. As the Cl(-) ions were shown to inhibit the luminescence of the complex in BuCN, this luminescence increase by nonprotonated 1 was attributed to the protection effect of 1 by encapsulation of the Cl(-) anions into the tris-urea binding site. The study of the luminescence lifetimes of the Ru-TAP complex in BuCN as a function of temperature for the PF6(-) and Cl(-) salts in the absence and presence of 1 led to the following conclusions. In BuCN, in contrast to MeCN, in addition to ion pairing, because of the poor solvation of the ions, the luminescent metal-to-ligand charge transfer ((3)MLCT) state could reach two metal-centered ((3)MC) states, one of which is in equilibrium with the (3)MLCT state during the emission lifetime. The reaction of Cl(-) with this latter (3)MC state would be responsible for the luminescence quenching, in agreement with the formation of photosubstitution products.

Directional charge transfer and highly reducing and oxidizing excited states of new dirhodium(ii,ii) complexes: potential applications in solar energy conversion

Two new series of dirhodium(II,II) complexes cis-[Rh2(μ-DTolF)2(L)2][BF4]2 and cis-[Rh2(μ-F-form)2(L)2][BF4]2 were synthesized and fully characterized (DTolF = p-ditolylformamidinate, F-form = p-difluorobenzylformamidinate; L = the chelating diimine ligands dpq (dipyrido[3,2-f:2′,3′-h]quinoxaline), dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppn (benzo[i]dipyrido[3,2-a:2′,3′-h]quinoxaline). The complexes undergo facile oxidation and exhibit directed ligand-to-ligand charge transfer (LLCT) excited states upon excitation from the corresponding formamidinate to the diimine ligand. Time-resolved studies reveal that the LLCT states decay with lifetimes that range from 16 to 100 ps and generate a longer-lived excited state. For complexes with dpq and dppz ligands, the longer-lived excited state, with lifetimes that range from 40 to 100 ns, has been assigned as 3MC (MC = metal-centered) arising from the Rh2(π*) → Rh2(σ*) transition. In the case of cis-[Rh2(μ-DTolF)2(dppn)2]2+ and cis-[Rh2(μ-F-form)2(dppn)2]2+, the dppn-centered 3ππ* excited state is observed from ∼10 ps to ∼2 ns, but following its decay, the 3MC state of each complex is observed with lifetimes of 2.4 and 3.0 μs, respectively. The long lifetimes observed for the dppn complexes is explained by a pre-equilibrium of the low-lying 3ππ* and 3MC states. The excited state oxidation potentials, E*ox(1LLCT), for the complexes are calculated to lie between −2.5 and −2.8 V vs. SCE and E*ox(3MC) ∼ −1.8 V vs. SCE. The 1LLCT excited states of the complexes are also good oxidizing agents, with E*red(1LLCT) ∼ +1.3 V vs. SCE, making them significantly better oxidizing and reducing agents than commonly used Ru(II) complexes.

Near infra-red emission from a mer-Ru(II) complex: consequences of strong σ-donation from a neutral, flexible ligand with dual binding modes.

A rare example of dual coordination modes by a novel tridentate ligand gives rise to unique fac-and mer-Ru((II/III)) complexes. The mer-Ru(II)-complex displays the farthest red-shift of a triplet metal-to-ligand charge transfer ((3)MLCT) emission with a tridentate ligand for a mononuclear complex. This observation is a consequence of large bite angle and strong σ-donation by the ligand, the combined effect of which helps to separate the energy of the (3)MLCT and (3)MC states.

Computational Studies on the Excited States of Luminescent Platinum(II) Alkynyl Systems of Tridentate Pincer Ligands in Radiative and Nonradiative Processes

Platinum(II) alkynyl complexes of various tridentate pincer ligands, [Pt(trpy)(C≡CR)](+) (trpy = 2,2':6',2″-terpyridine), [Pt(R'-bzimpy)(C≡CR)](+) (R'-bzimpy = 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine and R' = alkyl), [Pt(R'-bzimb)(C≡CR)] (R'-bzimb = 1,3-bis(N-alkylbenzimidazol-2'-yl)benzene and R' = C4H9), have been found to possess rich photophysical properties. The emission in dilute solutions of [Pt(trpy)(C≡CR)](+) originated from a triplet alkynyl-to-tridentate pincer ligand-to-ligand charge transfer (LLCT) excited state, with mixing of a platinum-to-tridentate pincer ligand metal-to-ligand charge transfer (MLCT) excited state, while that of [Pt(R'-bzimb)(C≡CR)] originated from a triplet excited state of intraligand (IL) character of the tridentate ligand mixed with a platinum-to-tridentate ligand MLCT character. Interestingly, both emissions were observed in [Pt(R'-bzimpy)(C≡CR)](+) in some cases. In addition, [Pt(R'-bzimb)(C≡CR)] displayed a photoluminescence quantum yield higher than that of [Pt(R'-bzimpy)(C≡CR)](+). Computational studies have been performed on the representative complexes [Pt(trpy)(C≡CPh)](+) (1), [Pt(R'-bzimpy)(C≡CPh)](+) (2), and [Pt(R'-bzimb)(C≡CPh)] (3), where R' = CH3 and Ph = C6H5, to provide an in-depth understanding of the nature of their emissive origin as well as the radiative and nonradiative processes. In particular, the factors governing the ordering of the triplet excited states and radiative decay rate constants of the emissive state ((3)ES) have been examined. The potential energy profiles for the deactivation process from the (3)ES via triplet metal-centered ((3)MC) states have also been explored. This work reveals for the first time the potential energy profiles for the thermal deactivation pathway of square planar platinum(II) complexes.

Excited State Tuning of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores by Push–Pull Effects and Bite Angle Optimization: A Comprehensive Experimental and Theoretical Study

The synergy of push-pull substitution and enlarged ligand bite angles has been used in functionalized heteroleptic bis(tridentate) polypyridine complexes of ruthenium(II) to shift the (1) MLCT absorption and the (3) MLCT emission to lower energy, enhance the emission quantum yield, and to prolong the (3) MLCT excited-state lifetime. In these complexes, that is, [Ru(ddpd)(EtOOC-tpy)][PF6 ]2 , [Ru(ddpd-NH2 )(EtOOC-tpy)][PF6 ]2 , [Ru(ddpd){(MeOOC)3 -tpy}][PF6 ]2 , and [Ru(ddpd-NH2 ){(EtOOC)3 -tpy}][PF6 ]2 the combination of the electron-accepting 2,2';6',2''-terpyridine (tpy) ligand equipped with one or three COOR substituents with the electron-donating N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine (ddpd) ligand decorated with none or one NH2 group enforces spatially separated and orthogonal frontier orbitals with a small HOMO-LUMO gap resulting in low-energy (1) MLCT and (3) MLCT states. The extended bite angle of the ddpd ligand increases the ligand field splitting and pushes the deactivating (3) MC state to higher energy. The properties of the new isomerically pure mixed ligand complexes have been studied by using electrochemistry, UV/Vis absorption spectroscopy, static and time-resolved luminescence spectroscopy, and transient absorption spectroscopy. The experimental data were rationalized by using density functional calculations on differently charged species (charge n=0-4) and on triplet excited states ((3) MLCT and (3) MC) as well as by time-dependent density functional calculations (excited singlet states).

Inverse Kinetic Isotope Effect in the Excited-State Relaxation of a Ru(II)–Aquo Complex: Revealing the Impact of Hydrogen-Bond Dynamics on Nonradiative Decay

Photophysics of the MLCT excited-state of [Ru(bpy)(tpy)(OH2)](2+) (1) and [Ru(bpy)(tpy)(OD2)](2+) (2) (bpy = 2,2'-bipyridine and tpy = 2,2':6',2″-terpyridine) have been investigated in room-temperature H2O and D2O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the (3)MLCT state that initiates H-bond dynamics with the solvent and the (3)MC state that serves as the principal pathway for nonradiative decay.

Enhancing the luminescence properties and stability of cationic iridium(iii) complexes based on phenylbenzoimidazole ligand: a combined experimental and theoretical study

Herein we designed and synthesized a series of cationic iridium(III) complexes with a phenylbenzoimidazole-based cyclometalated ligand, containing different numbers of carbazole moieties from zero to three (complexes 1-4). The photophysical and electrochemical properties of this series have been systematically investigated. The complexes exhibit strong luminescence in both solution and in neat films, as well as excellent redox reversibility. Introducing carbazole groups into the complexes is found to lead to substantially enhanced photoluminescence quantum efficiency in the neat film, but has little effect on the emitting color and excited-state characteristics as supported by density functional theory (DFT) results. DFT calculations also suggest that functionalized complexes 2-4 reveal better hole-transporting properties than 1. More importantly, all complexes effectively reduce the degradation reaction to some extent in metal-centered (³MC) excited-states, demonstrating their stability. Further studies indicate that restriction of opening of the structures in the ³MC state is caused by the unique molecular conformation of the phenylbenzoimidazole ligand, which is first demonstrated here in cationic iridium(III) complexes without intramolecular π-π stacking. These results presented here would provide valuable information for designing and synthesizing highly efficient and stable cationic iridium(III) complexes suitable for the optical devices.

X-ray transient absorption structural characterization of the 3MLCT triplet excited state of cis-[Ru(bpy)2(py)2]2+

The excited state dynamics and structure of the photochemically active complex cis-[Ru(bpy)2(py)2](2+) have been investigated using optical transient absorption (OTA) and X-ray transient absorption (XTA) spectroscopy, and density functional theory (DFT). Upon light-excitation in aqueous solution cis-[Ru(bpy)2(py)2](2+) undergoes ultrafast dissociation of one pyridine ligand to form cis-[Ru(bpy)2(py)(H2O)](2+). OTA measurements highlighted the presence of two major time components of 1700 ps and 130 ps through which the system decays to the ground-state and evolves towards the photoproduct. XTA data were acquired after 150 ps, 500 ps, and 3000 ps from laser excitation (λexc = 351 nm) and provided the transient structure of the (3)MLCT state corresponding to the longer time component in the OTA experiment. In excellent agreement with DFT, XTA shows that the (3)MLCT geometry is characterized by an elongation of the dissociating Ru-N(py) bond and a shortening of the trans Ru-N(bpy) bond with respect to the ground state. Conversely, calculations show that the (3)MC state has a highly distorted structure with Ru-N(py) bonds between 2.77-3.05 Å.

Photophysical and electrochemical properties of polypyridine imine ruthenium(ii) complexes: a comparative experimental and theoretical study

The photophysical and electrochemical properties of two ruthenium complexes formulated as [Ru(bpy)2LL′]2+ (bpy = 2,2′-bipyridine, LL′ = pyrim=phenylpyridin-2-ylmethylene-amine; and LL′ = Mes-dab = 1,4-dimesityl-1,4-diazabutadiene) have been investigated by a combined theoretical and experimental study. The UV/Vis absorption spectra of both complexes are dominated by absorption bands in the 400–600 region, which are assigned, by means of TD-DFT calculations, to metal-to-ligand charge transfer (MLCT) transitions, and the bands observed at 300 nm have a dominant intraligand character. The pyrim complex shows luminescence at both low and ambient temperature, whereas for the diaza Mes-dab complex no emission was observed in the visible region at all temperatures. Calculations showed that the presence of the Mes-dab ligand results in a strong decrease in the emission energy from the lowest triplet 3MLCT. As a result, we predict that the Mes-dab complex could be luminescent in the infrared region, while the pyrim complex emits in the visible region. Furthermore, in both cases, triplet metal-centered (3MC) states are inaccessible from the lowest triplet states, which is a requirement for efficient emission. Electrochemical data are typical of Ru(II) bipyridine complexes. DFT calculations, in the gas and the solvated phase, on the different oxidized and reduced species are consistent with metal-based oxidations and ligand-based reductions. The first reduction is always localized on the LL′ ligand. For the first time, the second reduction process was investigated for these kinds of complexes. It occurs at the same potential for the two complexes, as it corresponds to the reduction of the bipyridine ligands. The calculations match the experimental trend in electrochemical shifts.

Intramolecular π-stacking in cationic iridium(iii) complexes with a triazole–pyridine type ancillary ligand: synthesis, photophysics, electrochemistry properties and piezochromic behavior

To make Ir(III)-based complexes potentially multifunctional materials, two new cationic Ir(III) complexes with a 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (Phtz) ancillary ligand were designed and synthesized. By introducing the pendant phenyl ring into the ancillary ligand, the two complexes possess desired intramolecular π–π stacking between the pendant phenyl ring of the Phtz ligand and one of the phenyl rings of the cyclometalated ligand, which renders the complexes more stable. Density functional theory calculation indicates that the intramolecular π–π interactions in both complexes can reduce the degradation reaction in metal-centered (3MC) states to some extent, which further implies their stability. With these results in combination with their reversible oxidation and reduction processes as well as excellent photophysical properties, the stable light-emitting cells (LECs) would be expected. Furthermore, the two synthesized complexes exhibit reversible piezochromism. Their emission color can be smartly switched by grinding and heating, which is visible to the naked eye. In light of our experimental results, the present piezochromic behavior is due to interconversion between crystalline and amorphous states.

Photoinduced ligand isomerisation in a pyrazine-containing ruthenium polypyridyl complex

Photochemically-induced ligand rearrangements for the N2 and N4 coordination isomers of the complex [Ru(bpy)(2)(Hpztr)](2+) and its deprotonated analogue [Ru(bpy)(2)(pztr)](+), where bpy is 2,2'-bipyridyl and Hpztr is pyrazine-1,2,4-triazole ligand, are reported. (1)H NMR spectroscopic and HPLC studies indicate that in acetone and acetonitrile the complexes are photostable when the triazole ring is deprotonated. Irradiation of the protonated N2 isomer in acetone results in formation of the N4 isomer, with the N4 isomer being photostable. In acetonitrile both isomers show photolability of the triazole-based ligand and full dissociation to form [Ru(bpy)(2)(CH(3)CN)(2)](2+) is observed. The activation parameters for the population of the (3)MC state from the lowest (3)MLCT manifold, as obtained from temperature-dependent emission lifetime studies, are reported and their relevance to the observed photochemical behaviour is considered. The results obtained are discussed in relation the analogous pyridine-triazole complexes.

An improved synthesis, crystal structures, and metallochromism of salts of [Ru(tolyl-terpy)(CN) 3] −

The previously reported complex [Ru(ttpy)(CN) 3] − [ttpy = 4′( p-tolyl)-2,2′:6′,2″-terpyridine] is conveniently synthesised by reaction of ttpy with Ru(dmso) 4Cl 2 to give [Ru(ttpy)(dmso)Cl 2], which reacts in turn with KCN in aqueous ethanol to afford [Ru(ttpy)(CN) 3] − which was isolated and crystallographically characterised as both its (PPN) + and K + salts. The K + salt contains clusters containing three complex anions and three K + cations connected by end-on and side-on cyanide ligation to the K + ions. The solution speciation behaviour of [Ru(ttpy)(CN) 3] − was investigated with both Zn 2+ and K + salts in MeCN, a solvent sufficiently non-competitive to allow the added metal cations to associate with the complex anion via the externally-directed cyanide lone pairs. UV–Vis spectroscopic titration of (PPN)[Ru(ttpy)(CN) 3] with Zn(ClO 4) 2 showed a blue shift of 2900 cm −1 in the 1MLCT absorption manifold due to the ‘metallochromism’ effect; a series of distinct binding events could be discerned corresponding to formation of 4:1, 1:1 and then 1:3 anion:cation adducts, all with high formation constants, as the titration proceeded. In contrast titration of (PPN)[Ru(ttpy)(CN) 3] with the more weakly Lewis-acidic KPF 6 resulted in a much smaller blue-shift of the 1MLCT absorptions, and the titration data corresponded to formation of 1:1 and then 2:1 cation:anion adducts with weaker stepwise association constants of the order of 10 4 and then 10 3 M −1. Although association of [Ru(ttpy)(CN) 3] − resulted in a blue-shift of the 1MLCT absorptions, the luminescence was steadily quenched, as raising the 3MLCT level makes radiationless decay via a low-lying 3MC state possible.

Electronic peculiarities of the excited states of [RuN5C]+vs. [RuN6]2+ polypyridine complexes: insight from theory

The ground state, oxidized ground state, (3)MLCT and (3)MC excited states have been studied by DFT and TDDFT for two Ru(II) complexes bearing an N(6) or N(5)C coordination sphere. The effect of replacing one Ru-N dative bond by one Ru-C covalent bond have been studied and quantified on their ground state by the means of geometry optimization, NBO analysis and calculation of their IR vibrations. IR fingerprints of the Ru-C bond have been found at 945 and 1113 cm(-1). In addition, this study confirmed and quantified the effects of N→C(-) substitution on the spectroscopic properties of the [RuN(5)C](+) complex: a broader and bathochromically-shifted absorption spectrum, a smaller ground-(3)MLCT energy gap and a highly energetic (3)MC state are the major characteristics of the carbon-containing monocationic complex.

Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes

The photophysical properties for a series of facial (fac) cyclometalated Ir(III) complexes (fac-Ir(C--N)(3) (C--N = 2-phenylpyridyl (ppy), 2-(4,6-difluorophenyl)pyridyl (F2ppy), 1-phenylpyrazolyl (ppz), 1-(2,4-difluorophenyl)pyrazolyl) (F2ppz), and 1-(2-(9,9'-dimethylfluorenyl))pyrazolyl (flz)), fac-Ir(C--N)(2)(C--N') (C--N = ppz or F2ppz and C--N' = ppy or F2ppy), and fac-Ir(C--C')(3) (C--C' = 1-phenyl-3-methylbenzimidazolyl (pmb)) have been studied in dilute 2-methyltetrahydrofuran (2-MeTHF) solution in a temperature range of 77-378 K. Photoluminescent quantum yields (Phi) for the 10 compounds at room temperature vary between near zero and unity, whereas all emit with high efficiency at low temperature (77 K). The quantum yield for fac-Ir(ppy)(3) (Phi = 0.97) is temperature-independent. For the other complexes, the temperature-dependent data indicates that the luminescent efficiency is primarily determined by thermal deactivation to a nonradiative state. Activation energies and rate constants for both radiative and nonradiative processes were obtained using a Boltzmann analysis of the temperature-dependent luminescent decay data. Activation energies to the nonradiative state are found to range between 1600 and 4800 cm(-1). The pre-exponential factors for deactivation are large for complexes with C--N ligands (10(11)-10(13) s(-1)) and significantly smaller for fac-Ir(pmb)(3) (10(9) s(-1)). The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir-N (Ir-C for fac-Ir(pmb)(3)) bond rupture leading to a five-coordinate species that has triplet metal-centered ((3)MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and (3)MC states. The energy level for the (3)MC state is estimated to lie between 21,700 and 24,000 cm(-1) for the fac-Ir(C--N)(3) complexes and at 28,000 cm(-1) for fac-Ir(pmb)(3).

Photoinduced Electron-Transfer Processes Based on Novel Bipyridine−Ru(II) Complex: Properties of cis-[Ru(2,2‘-bipyridine)2(5,6-bis(3-amidopyridine)-7-oxanorbornene)](PF6)2 and cis-[Ru(2,2‘-bipyridine)2(3-aminopyridine)2](PF6)2 Complexes

This paper presents the synthesis, MO calculations, and photochemical and photophysical properties of cis-[Ru(bpy)2(3Amdpy2oxaNBE)](PF6)2 (2), where bpy is 2,2'-bipyridine and 3Amdpy2oxaNBE is the novel 5,6-bis(3-amidopyridine)-7-oxanorbornene chelate-ligand (1). Complex 2 is considered in relation to the cis-[Ru(bpy)2(3Amnpy)2](PF6)2 (3) analogous complex, where 3Amnpy is 3-aminopyridine. Complexes 2 and 3 exhibit absorptions near 350 nm and in the 420-500 nm region attributable to a contribution from MLCT transitions (dpi-->bpy and dpi-->L; L=3Amdpy2oxaNBE or 3Amnpy). Whereas complex 3 is photochemically reactive, complex 2 shows luminescence either at 77 K or at room temperature in fluid solution. The emission of 2 assignable as an MLCT (Ru-->bpy) emission is characterized by a long lifetime at room temperature (650 ns in CH3CN and 509 ns in H2O). It is independent of lambdairr, but it is temperature dependent; i.e., it increases as the temperature is lowered. Considering the chelate ring of 1 contributes to the stability of the complex 2 under continuous light irradiation, the difference in the primary photoprocesses of 3 (loss of 3Amnpy) and 2 (luminescence) may be caused by a lowering of the lowest excited state from 3 to 2. The surface crossing to the lowest MC state value of 987 cm-1 (similar to that of [Ru(bpy)3]2+) will be prevented in the case of complex 2, and as a result, efficient 3Amdpy moiety loss cannot occur. The electronic depopulation of the {Ru(bpy)2} unit and population of a bpy* orbital upon excitation are evident by comparing the photophysical properties with those of a [Ru(bpy)3]2+ related complex. Moreover, a reduction of a bpy ligand in the MLCT excited state is indicated by time-resolved spectra that show features typical of bpy*-. The photocatalytic property of 2 is spectroscopically demonstrated by oxidative quenching using either methylviologen2+ or [RuCl(NH3)5]+2 electron-acceptor ions.