MLCT Excited State Research Articles

Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States.

Halogen substitution at the 6 and 6″ positions of terpyridine (6,6″-Cl2-2,2:6',2″-terpyridine = dctpy) is used to produce a room-temperature high-spin iron(II) complex [Fe(dctpy)2](BF4)2. Using UV-vis absorption, spectroelectrochemistry, transient absorption, and TD-DFT calculations, we present evidence that the quintet metal-to-ligand charge-transfer excited state ((5)MLCT) can be accessed via visible light absorption and that the thermalized (5,7)MLCT is long-lived at 16 ps, representing a > 100 fold increase compared to the (1,3)MLCT within species such as [Fe(bpy)3](2+). This result opens a new strategy for extending iron(II) MLCT lifetimes for potential use in photoredox processes.

Chromophore-labelled, luminescent platinum complexes: syntheses, structures, and spectroscopic properties.

Ligands based upon 4-carboxamide-2-phenylquinoline derivatives have been synthesised with solubilising octyl hydrocarbon chains and tethered aromatic chromophores to give naphthyl (), anthracenyl () and pyrenyl () ligand variants, together with a non-chromophoric analogue () for comparison. (1)H NMR spectroscopic studies of the ligands showed that two non-interchangeable isomers exist for and while only one isomer exists for and . Supporting DFT calculations on suggest that the two isomers may be closely isoenergetic with a relatively high barrier to exchange of ca. 100 kJ mol(-1). These new ligands were cyclometalated with Pt(ii) to give complexes [Pt()(acac)] (acac = acetylacetonate). The spectroscopically characterised complexes were studied using multinuclear NMR spectroscopy including (195)Pt{(1)H} NMR studies which revealed δPtca. -2785 ppm for [Pt()(acac)]. X-ray crystallographic studies were undertaken on [Pt()(acac)] and [Pt()(acac)], each showing the weakly distorted square planar geometry at Pt(ii); the structure of [Pt()(acac)] showed evidence for intermolecular Pt-Pt interactions. The UV-vis. absorption studies show that the spectral profiles for [Pt()(acac)] are a composite of the organic chromophore centred bands and a broad (1)MLCT (5d → π*) band (ca. 440 nm) associated with the complex. Luminescence studies showed that complexes [Pt()(acac)] are dual emissive with fluorescence characteristic of the tethered fluorophore and long-lived phosphorescence attributed to (3)MLCT emission. In the case of the pyrenyl derivative, [Pt()(acac)], the close energetic matching of the (3)MLCT and (3)LCpyr excited states led to an elongation of the (3)MLCT emission lifetime (τ = 42 μs) under degassed solvent conditions, suggestive of energy transfer processes between the two states.

Demonstration of intramolecular energy transfer in asymmetric bimetallic ruthenium(ii) complexes.

A new family of bimetallic Ru(ii) complexes derived from an asymmetric bridging ligand (tpy-Hbzim-dipy) consisting of both bipyridine and terpyridine chelating sites covalently connected via phenyl-imidazole spacer were designed in this work to demonstrate intramolecular energy transfer from one component to the other in asymmetric dyads. To fine tune the photo-redox properties, both bidentate and tridentate terminal ligands in the complexes were varied systematically. Both steady state and time-resolved luminescence spectral results indicated photo-induced intramolecular energy transfer from the excited MLCT state of the [(bpy/phen)2RuII(dipy-Hbzim-tpy)] component to the MLCT state of the tpy-containing unit [(dipy-Hbzim-tpy)RuII(tpy-PhCH3/H2pbbzim)] in dyads with rate constant values on the order of 106-107 s-1. Temperature-dependent luminescence studies indicated an enhancement in the luminescence intensity and excited state lifetimes upon decreasing the temperature.

ChemInform Abstract: Photochemical Reactions of Fac‐Rhenium(I) Tricarbonyl Complexes and Their Application for Synthesis

AbstractReview: 79 refs.

Metal-to-Ligand Charge-Transfer Emissions of Ruthenium(II) Pentaammine Complexes with Monodentate Aromatic Acceptor Ligands and Distortion Patterns of their Lowest Energy Triplet Excited States

This is the first report of the 77 K triplet metal-to-ligand charge-transfer ((3)MLCT) emission spectra of pentaammine-MDA-ruthenium(II) ([Ru(NH3)5(MDA)](2+)) complexes, where MDA is a monodentate aromatic ligand. The emission spectra of these complexes and of the related trans-[Ru(NH3)4(MDA) (MDA')](2+) complexes are closely related, and their emission intensities are very weak. Density functional theory (DFT) calculations indicate that the energies of the lowest (3)MLCT excited states of Ru-MDA complexes are either similar to or lower than those of the lowest energy metal-centered excited states ((3)MC(X(Y))), that the barrier to internal conversion at 77 K is large compared to kBT, and that the (3)MC(X(Y)) excited states are weakly bound. The [Ru(NH3)5py](2+) complex is an exception to the general pattern: emission has been observed for the [Ru(ND3)5(d5-py)](2+) complex, but its lifetime is apparently very short. DFT modeling indicates that the excited state distortions of the different (3)MC excited states are very large and are in both Ru-ligand bonds along a single Cartesian axis for each different (3)MC excited state, nominally resulting in (3)MC(X(Y)), (3)MC((X)Y), and (3)MC(Z) lowest energy metal-centered states. The (3)MC(X(Y)) and (3)MC((X)Y) states appear to be the pseudo-Jahn-Teller distorted components of a (3)MC((XY)) state. The (3)MC(X(Y)) states are distorted up to 0.5 Å in each H3N-Ru-NH3 bond along a single Cartesian axis in the pentaammine and trans-tetraammine complexes, whereas the (3)MC(Z) states are found to be dissociative. DFT modeling of the (3)MLCT excited state of [Ru(NH3)5(py)](2+) indicates that the Ru center has a spin density of 1.24 at the (3)MLCT energy minimum and that the (3)MLCT → (3)MC(Z) crossing is smooth with a very small barrier (<0.5 kcal/mol) along the D3N-Ru-py distortion coordinate, implying strong (3)MLCT/(3)MC excited state configurational mixing. Furthermore, the DFT modeling indicates that the long-lived intermediate observed in earlier flash photolysis studies of [Ru(NH3)5py](2+) is a Ru(II)-(η(2)(C═C)-py) species.

Energy Dependence of the Ruthenium(II)-Bipyridine Metal-to-Ligand-Charge-Transfer Excited State Radiative Lifetimes: Effects of ππ*(bipyridine) Mixing.

The variations in band shape with excited state energy found for the triplet metal to ligand charge transfer ((3)MLCT) emission spectra of ruthenium-bipyridine (Ru-bpy) chromophores at 77 K have been postulated to arise from excited state/excited state configurational mixing. This issue is more critically examined through the determination of the excited state energy dependence of the radiative rate constants (kRAD) for these emissions. Experimental values for kRAD were determined relative to known literature references for Ru-bpy complexes. When the lowest energy excited states are metal centered, kRAD can be anomalously small and such complexes have been identified using density functional theory (DFT) modeling. When such complexes are removed from the energy correlation, there is a strong (3)MLCT energy-dependent contribution to kRAD in addition to the expected classical energy cubed factor for complexes with excited state energies greater than 10 000 cm(-1). This correlates with the DFT calculations which show significant excited state electronic delocalization between a π(bpy-orbital) and a half-filled dπ*-(Ru(III)-orbital) for Ru-bpy complexes with (3)MLCT excited state energies greater than about 16 000 cm(-1). Overall, this work implicates the "stealing" of emission bandshapes as well as intensity from the higher energy, strongly allowed bpy-centered singlet ππ* excited state.

Luminescent rhenium(I)–chromone bioconjugate: Synthesis, photophysical properties, and confocal luminescence microscopy investigation

Luminescent Re(I) complexes represent important materials for bioimaging applications. Herewith a new Re-complex Re(CO)3(phen)L1, where phen = 1,10-phenanthroline and L1 = 3-CCCH2NHCOCH2CH2-6-bromo-chromone, is reported. The new conjugate displays moderately intense orange-red luminescence stemming from a 3MLCT excited state associated with the organometallic Re(CO)3(phen) fragment. At ambient temperature, the emission quantum yield amounts to 0.2–0.9 % in aerated organic solvents and 7% in poly(methyl methacrylate). The emission decay times are in the order of several tens of nanoseconds to a few microseconds depending on the polarity and viscosity of the medium. Re(CO)3(phen)L1 penetrates into the mouse C2C12 cancer cells where it accumulates in cytoplasm, mitochondria, and endoplasmic reticulum, as it was determined by means of confocal fluorescence microscopy. Furthermore, the complex shows high photostability in live cells. No photobleaching was observed under prolonged intense irradiation (λ = 405 nm, I = 0.8 W/cm2, t = 30 min), whereas at similar conditions significant photobleaching of commonly used fluorescent stains, e.g. DAPI, occurs.

The substituent effect on the MLCT excited state dynamics of Cu(I) complexes studied by femtosecond time-resolved absorption and observation of coherent nuclear wavepacket motion.

The substituent effect on the excited-state dynamics of bis-diimine Cu(I) complexes was investigated by femtosecond time-resolved absorption spectroscopy with the S1← S0 metal-to-ligand charge transfer (MLCT) photoexcitation. The time-resolved absorption of [Cu(phen)2](+) (phen = 1,10-phenanthroline) showed a slight intensity increase of the S1 absorption with a time-constant of 0.1-0.2 ps, reflecting the flattening distortion occurring in the S1 state. The transient absorption of the 'flattened' S1 state was clearly observed, although its fluorescence was not observed in the previous fluorescence up-conversion study in the visible region. The flattened S1 state decayed with a time constant of ∼2 ps, and the S0 bleaching recovered accordingly. This clarifies that the S1 state of [Cu(phen)2](+) is predominantly relaxed to the S0 state by internal conversion. The time-resolved absorption of [Cu(dpphen)2](+) (dpphen = 2,9-diphenyl-1,10-phenanthroline) showed a 0.9 ps intensity increase of the S1 absorption due to the flattening distortion, and then exhibited a 11 ps spectral change due to the intersystem crossing. This excited-state dynamics of [Cu(dpphen)2](+) is very similar to that of [Cu(dmphen)2](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline). In the ultrafast pump-probe measurements with 35 fs time resolution, [Cu(phen)2](+) and [Cu(dpphen)2](+) exhibited oscillation due to the nuclear wavepacket motions of the initial S1 state, and the oscillation was damped as the structural change took place. This indicates that the initial S1 states have well-defined vibrational structures and that the vibrational coherence is retained in their short lifetimes. The present time-resolved absorption study, together with the previous time-resolved fluorescence study, provides a unified view for the ultrafast dynamics of the MLCT excited state of the Cu(I) complexes.

Synthesis and characterization of rhenium(I) complexes based on O, N, N coordinating ligands: DFT/TDDFT studies on the electronic structures and spectral properties

Mononuclear rhenium(I) complexes having fac-[Re(CO)3]+ moiety of general formula fac-[Re(CO)3(L)] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with HL1 and HL2 in a ratio of 1:1 in methanol. Here L− is deprotonated form of 1-[(quinolin-8-ylimino)methyl]naphthalene-2-ol (HL1) and 1-[(quinolin-8-yl-diazenyl)]naphthalene-2-ol (HL2). The elemental analysis and ESI mass spectroscopic measurements ensure the formations of desired complexes. Molecular structures of fac-[Re(CO)3(L1)] and fac-[Re(CO)3(L2)] were confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. A remarkable change in optical properties is observed in case of fac-[Re(CO)3(L2)] over fac-[Re(CO)3(L1)] where the ligands bearing a NN function (HL2) instead of coordinated CN function (HL1). The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of Re(I) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited is associated with the 3MLCT excited state. The emission like transition is consistent with the strong 3MLCT character.

Synthesis, Stability and Sensitised Lanthanide Luminescence of Heterobimetallic d/f Terpyridine Complexes

AbstractThe synthesis, solution behavior and photophysical properties of several heteronuclear bimetallic d/f complexes that utilise a RuII bis‐terpyridine moiety as the sensitiser for EuIII, NdIII and YbIII luminescence are reported and compared to a GdIII analogue. The former compounds display sensitised emission in the visible and near‐infrared (NIR) regions depending on the choice of the LnIII cation. We illustrate that sensitised lanthanide emission can operate by two distinctly different pathways that involve either the triplet ligand‐centred (3LC) excited state of the organic ligand or the triplet metal‐to‐ligand charge transfer (3MLCT) excited state of the RuII complex. Owing to the electronic structure of this antenna and the higher density of metal‐centred accepting states, a higher sensitization efficiency with NdIII was observed than with YbIII and EuIII. Lastly, we have observed unusual sensitised emission from the upper (4F5/2, 2H9/2) excited state of NdIII at approximately 800 nm, which we report here for the first time from a molecular species in solution.

A Homoleptic Trisbidentate Ru(II) Complex of a Novel Bidentate Biheteroaromatic Ligand Based on Quinoline and Pyrazole Groups: Structural, Electrochemical, Photophysical, and Computational Characterization

We synthesized a new homoleptic, tris-bidentate complex [Ru(QPzH)3](2+) based on the novel biheteroaromatic, 8-(3-pyrazolyl)-quinoline ligand QPzH. The QPzH ligand was designed to reduce the distortions typically observed in complexes incorporating the 8-quinolinyl group into the ligand framework. This was indeed observed, and was also, as anticipated, found to facilitate the formation of tris-homoleptic Ru(II) complexes; [Ru(QPzH)3](2+) is the first reported tris-homoleptic complex with ligands based on the 8-quinolinyl group. The synthesis can either result in a statistical 3:1 mer/fac ratio of the complex, or, through controlled exposure to light, be tweaked to allow isolation of the pure mer isomer only. X-ray crystallography reveals three nonequivalent ligands, with significantly less strain than other quinoline-based bidentate ligands. The complex exhibits a nearly octahedral coordination geometry but shows large differences in bond lengths between the Ru core and the quinoline and pyrazoles, respectively. The Ru-N(pyrazole) bond distances are ∼2.04 Å, while the corresponding distances for Ru-N(quinoline) are ∼2.12 Å. Structural, photophysical, electrochemical, and theoretical characterization revealed a mer-Ru(II) complex with a low oxidation potential (0.57 V vs ferrocene(0/+)) attributed to the incorporation of the pyrazolyl group, a ground state absorption that is sensitive to the local environment of the complex, and a short-lived (3)MLCT excited state.

A density functional theory and spectroscopic study of intramolecular quenching of metal-to-ligand charge-transfer excited states in some mono-bipyridine ruthenium(II) complexes

The complexes [Ru(NCCH3)4bpy]2+ and [Ru([14]aneS4)bpy]2+ ([14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane, bpy = 2,2′-bipyridine) have similar absorption and emission spectra but the 77 K metal-to-ligand charge-transfer (MLCT) excited state emission lifetime of the latter is less than 0.3% that of the former. Density functional theory modeling of the lowest energy triplet excited states indicates that triplet metal centered (3MC) excited states are about 3500 cm−1 lower in energy than their 3MLCT excited states in both complexes. The differences in excited state lifetimes arise from a much larger coordination sphere distortion for [Ru(NCCH3)4bpy]2+ and the associated larger reorganizational barrier for intramolecular electron transfer. The smaller ruthenium ligand distortions of the [Ru([14]aneS4)bpy]2+ complex are apparently a consequence of stereochemical constraints imposed by the macrocyclic [14]aneS4 ligand, and the 3MC excited state calculated for the unconstrained [Ru(S(CH3)2)4bpy]2+ complex (S(CH3)2 = dimethyl sulfide) is distorted in a manner similar to that of [Ru(NCCH3)4bpy]2+. Despite the lower energy calculated for its 3MC than 3MLCT excited state, [Ru(NCCH3)4bpy]2+ emits strongly in 77 K glasses with an emission quantum yield of 0.47. The emission is biphasic with about a 1 μs lifetime for its dominant (86%) emission component. The 405 nm excitation used in these studies results in a significant amount of photodecomposition in the 77 K glasses. This is a temperature-dependent biphotonic process that most likely involves the bipyridine-radical anionic moiety of the 3MLCT excited state. A smaller than expected value found for the radiative rate constant is consistent with a lower energy 3MC than 3MLCT state.

Stabilization of ruthenium(II) polypyridyl chromophores on nanoparticle metal-oxide electrodes in water by hydrophobic PMMA overlayers.

We describe a poly(methyl methacrylate) (PMMA) dip-coating procedure, which results in surface stabilization of phosphonate and carboxylate derivatives of Ru(II)-polypyridyl complexes surface-bound to mesoporous nanoparticle TiO2 and nanoITO films in aqueous solutions. As shown by contact angle and transmission electron microscopy (TEM) measurements, PMMA oligomers conformally coat the metal-oxide nanoparticles changing the mesoporous films from hydrophilic to hydrophobic. The thickness of the PMMA overlayer on TiO2-Ru(II) can be controlled by changing the wt % of PMMA in the dipcoating solution. There are insignificant perturbations in electrochemical or spectral properties at thicknesses of up to 2.1 nm with the Ru(III/II) couple remaining electrochemically reversible and E1/2 values and current densities nearly unaffected. Surface binding by PMMA overlayers results in stable surface binding even at pH 12 with up to a ∼100-fold enhancement in photostability. As shown by transient absorption measurements, the MLCT excited state(s) of phosphonate derivatized [Ru(bpy)2((4,4'-(OH)2PO)2bpy)](2+) undergo efficient injection and back electron transfer with pH independent kinetics characteristic of the local pH in the initial loading solution.

Mononuclear rhenium(i) complexes incorporating 2-(arylazo)phenyl benzyl thioethers: synthesis, structure, spectral, DFT and TDDFT studies

The stoichiometric reaction of [Re(CO)5Cl] with 2-(arylazo)phenyl benzyl thioethers [ArNNC6H4SCH2Ph: Ar = Ph, 4-chlorophenyl, 4-nitrophenyl] (L1–L3) in a ratio of 1 : 1 in dry toluene under refluxing condition afforded the Re(I) complexes of general formula fac-[Re(L)(CO)3Cl] in excellent yields. The ligands used in this present work are 2-(phenylazo)phenyl benzyl thioether (L1), 2-(4-chlorophenylazo)phenyl benzyl thioether (L2) and 2-(4-nitrophenylazo)phenyl benzyl thioether (L3). The elemental analysis and ESI mass spectroscopic measurements ensure the formation of desired complexes. The crystal structure of the complexes [Re(L1)(CO)3Cl], 1 and [Re(L2)(CO)3Cl], 2 were determined by X-ray diffractometry study. The molecular structures observed in the solid state were retained in the solution (1H and 13C NMR spectra). The ground and excited-state geometries, NMR, absorption, and luminescent properties of three Re(I) complexes were examined by DFT and TDDFT methods. The introduction of an electron withdrawing group at the para position of the phenyl ring attached to the NN bond varies the frontier molecular orbital energies, compositions and optical properties of these molecules significantly. The lowest lying triplet excited is associated with an admixture of the 3MLCT and 3ILCT excited state having a cis conformation of the azoaryl moiety. The emission like transition having mixed 3MLCT and 3ILCT nature was characterized by natural transition orbital (NTO) and spin density difference map analysis. The presence of the electron withdrawing NO2 group leads to the low quantum yield through enhanced non-radiative deactivation. The cis orientation at the lowest lying triplet excited state (T1) may imply some kind of photoinduced isomerization in the ligand frame and as a result there may be more than one emitting species in the solution and hence bi-exponential decay nature was observed for all the complexes.

Spectroscopic and quantum-chemical investigations of chloro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

The luminescence spectra of cis-[Ru(bpy)2(L)Cl]+ (bpy is 2,2′-bipyridyl; L is pyrazine, pyridine, 4-amino-pyridine, 4-picolin, isonicotinamide, 4-cyanopyridine, or 4,4′bipyridyl) complexes are studied in alcoholic (4: 1 EtOH-MeOH) solutions at 77 K. A linear correlation is found between the energy of the lowest electronically excited metal-to-ligand charge transfer (3MLCT) state dπ(Ru) → π* (bpy) and the parameter pKa of the free 4-substituted pyridines and pyrazine used as ligands L. The [B3LYP/6-31G + LanL2DZ(Ru)] hybrid method of the density functional theory is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of the ruthenium ion. It is shown that there exists a linear unambiguous correlation between the negative charge on the nitrogen atom (qNL) of ligands L coordinated in the complex and the parameters pKa of free ligands. The calculated energies of 3MLCT excited states almost linearly (correlation coefficient 0.958) depend on the charge qNL, which completely agrees with experimental data.

Highly accurate excited-state structure of [Os(bpy)2dcbpy](2+) determined by X-ray transient absorption spectroscopy.

Determining the electronic and geometric structures of photoexcited transient species with high accuracy is crucial for understanding their fundamental photochemistry and controlling their photoreactivity. We have applied X-ray transient absorption spectroscopy to measure the XANES and EXAFS spectra of a dilute (submillimolar) solution of the osmium(II) polypyridyl complex [Os(bpy)2dcbpy](PF6)2 (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) (OsL2L') in methanol at the Os LIII edge. We have obtained spectra of superb quality for both the ground state and the photoinduced (3)MLCT excited state that have allowed us not only to extract detailed information about the Os 5d orbitals but also to resolve very small differences of 0.010 ± 0.008 Å in the average Os-N bond lengths of the ground and excited states. Theoretical calculations using a recently developed DFT-based approach support the measured electronic structures and further identify the nature of the molecular orbitals that contribute to the main absorption bands in the XANES spectra.

Photochemical dihydrogen production using an analogue of the active site of [NiFe] hydrogenase.

Photoproduction of dihydrogen (H2) by a low molecular weight analogue of the active site of [NiFe] hydrogenase has been investigated by reduction of the [NiFe2] cluster, 1, by a photosensitier PS (PS = [ReCl(CO)3(bpy)] or [Ru(bpy)3][PF6]2). Reductive quenching of the (3)MLCT excited state of the photosensitizer by NEt3 or N(CH2CH2OH)3 (TEOA) generates PS(•-), and subsequent intermolecular electron transfer to 1 produces the reduced anionic form of 1. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of 1 and subsequent photocatalytic H2 production from [HTEOA][BF4], which was monitored by gas chromatography. Two structural isomers of the reduced form of 1 (1a(•-) and 1b(•-)) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH3CN and DMF (dimethylformamide), while only 1a(•-) was detected in CH2Cl2. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. 1a(•-) is assigned to a similar structure to 1 with six terminal carbonyl ligands, while calculations suggest that in 1b(•-) two of the carbonyl groups bridge the Fe centers, consistent with the peak observed at 1714 cm(-1) in the FTIR spectrum for 1b(•-) in CH3CN, assigned to a ν(CO) stretching vibration. Formation of 1a(•-) and 1b(•-) and production of H2 was studied in CH3CN, DMF, and CH2Cl2. Although the more catalytically active species (1a(•-) or 1b(•-)) could not be determined, photocatalysis was observed only in CH3CN and DMF.

Organic light‐emitting materials based on iridium(III) complexes bearing phenanthroimidazole ligands

We present the elegant synthesis and the photophysical and electroluminescent properties of a series of cyclometalated iridium(III) complexes [Ir(PPI)2(pic), PPI: 1,2‐diphenyl‐1H‐phenanthro[9,10‐d]imidazole; pic: picolinic acid]. The Ir(PPI)2(pic) complexes showed characteristic phosphorescence with an emission range of 556–579 nm and a high quantum efficiency with microsecond lifetimes. The strongly allowed phosphorescence in these complexes is the result of significant spin–orbit coupling of the Ir center. All bis(PPI) derivatives exhibit intense triplet metal‐to‐ligand charge transfer (MLCT) photoluminescence in the fluid solutions at room temperature. The impact of different solvents, substituents on the phenanthroimidazole ligands and complex concentrations upon their emissive behavior have been examined and demonstrate that their emission energies can be systematically modified. Weak bands located at longer wavelength have been assigned to the 1MLCT ← S0 and 3MLCT ← S0 transitions of iridium complexes. Application of the 3MLCT excited state of the [Ir(PPI)2(pic)] materials in organic light‐emitting devices are described. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Ruthenium Bis-diimine Complexes with a Chelating Thioether Ligand: Delineating 1,10-Phenanthrolinyl and 2,2′-Bipyridyl Ligand Substituent Effects

Despite the high π-acidity of thioether donors, ruthenium(II) complexes with a bidentate 1,2-bis(phenylthio)ethane (dpte) ligand and two chelating diimine ligands (i.e., Ru(diimine)2(dpte)(2+)) exhibit room-temperature fluid solution emission originating from a lowest MLCT excited state (diimine = 2,2'-bipyridine, 5,5'-dimethyl-2,2'-bipyridine 4,4'-di-tert-butyl-2,2'-bipyridine, 1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 5-chloro-1,10-phenanthroline, 5-bromo-1,10-phenanthroline, 5-nitro-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, and 3,4,7,8-tetramethyl-1,10-phenanthroline). Crystal structures show that the complexes form 2 of the 12 possible conformational/configurational isomers, as well as nonstatistical distributions of geometric isomers; there also are short intramolecular π-π interactions between the diimine ligands and dpte phenyl groups. The photoinduced solvolysis product, [Ru(diimine)2(CH3CN)2](PF6)2, for one complex in acetonitrile also was characterized by single-crystal X-ray diffraction. Variations in the MLCT energies and Ru(III/II) redox couple, E°'(Ru(3+/2+)), can be understood in terms of the influence of the donor properties of the ligands on the mainly metal-based HOMO and mainly diimine ligand-based LUMO. E°'(Ru(3+/2+)) also is quantitatively described using a summative Hammett parameter (σT), as well as using Lever's electrochemical parameters (EL). Recommended parametrizations for substituted 2,2'-bipyridyl and 1,10-phenanthrolinyl ligands were derived from analysis of correlations of E°'(Ru(3+/2+)) for 99 homo- and heteroleptic ruthenium(II) tris-diimine complexes. This analysis reveals that variations in E°'(Ru(3+/2+)) due to substituents at the 4- and 4'-positions of bipyridyl ligands and 4- and 7-positions of phenanthrolinyl ligands are significantly more strongly correlated with σp(+) than either σm or σp. Substituents at the 5- and 6-positions of phenanthrolinyl ligands are best described by σm and have effects comparable to those of substituents at the 3- and 8-positions. Correlations of EL with σT for 1,10-phenanthrolinyl and 2,2'-bipyridyl ligands show similar results, except that σp and σp(+) are almost equally effective in describing the influence of substituents at the 4- and 4'-positions of bipyridyl ligands. MLCT energies and d(5)/d(6)-electron redox couples of the complexes with 5-substituted 1,10-phenanthroline exhibit correlations with values for other d(6)-electron metal complexes that can be rationalized in terms of the relative number of diimine ligands and substituents.

Phenylcyanamide Ligand Control of Photo-Induced Linkage Isomerism

The photo- and electrochemically induced linkage isomerism of six new complexes [Ru(bpy)2(L)(dmso-S)](+), where dmso is dimethylsulfoxide, bpy is 2,2'-bipyridine, and L(-) is pentachloro-, 2,3,5,6-tetrachloro-, 2,4,5-trichloro-, 2,4-dichloro-, 4-chloro-, and unsubstituted phenylcyanamide anion, were investigated. The quantum yields of linkage isomerism forming the metastable [Ru(bpy)2(L)(dmso-O)](+) complexes are shown to decrease with increasing donor properties of the phenylcyanamide ligand, and it is suggested that the donor properties of the cyanamide ligand stabilize the [Ru(bpy)2(L)(dmso-S)](+) complexes in the (3)MLCT excited state. The cyclic voltammetry of these complexes showed two oxidation processes: a phenylcyanamide L(0/-) couple (an assignment supported by density functional theory (DFT) calculations) and a Ru(III/II) couple at more positive potential. Upon oxidation to Ru(III), the complexes rearranged to form Ru-O linkage isomers, and the scan rate dependent voltammograms permitted estimates of the rates of linkage isomerism.