Trinuclear Cyclometalated Iridium(III) Complex Exhibiting Intense Phosphorescence of an Unprecedented Rate.

Herein, we present two novel cyclometalated Ir(III) complexes of dinuclear and trinuclear design, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, respectively, where dppm is 4,6-di(4-tert-butylphenyl)pyrimidine ligand and acac is acetylacetonate ligand. In both cases, rac-diastereomers were isolated during the synthesis. The materials show intense phosphorescence of outstanding rates (kr = ΦPL/τ) with corresponding radiative decay times of only τr = 1/kr = 0.36 µs for dinuclear Ir2(dppm)3(acac)2 and still shorter τr = 0.30 µs for trinuclear Ir3(dppm)4(acac)3, as measured for doped polystyrene film samples under ambient temperature. Measured under cryogenic conditions, radiative decay times of the three T1 substates (I, III, and III) and substate energy separations are τI = 11.8 µs, τII = 7.1 µs, τIII = 0.06 µs, ΔE(II-I) = 7 cm-1, and ΔE(III-I) = 175 cm-1 for dinuclear Ir2(dppm)3(acac)2 and τI = 3.1 µs, τII = 3.5 µs, τIII = 0.03 µs, ΔE(II-I) ≈ 1 cm-1, and ΔE(III-I) = 180 cm-1 for trinuclear Ir3(dppm)4(acac)3. The determined T1 state ZFS values (ΔE(III-I)) are smaller compared to that of mononuclear analogue Ir(dppm)2(acac) (ZFS = 210-1 cm). Theoretical analysis suggests that the high phosphorescence rates in multinuclear materials can be associated with the increased number of singlet states lending oscillator strength to the T1 → S0 transition.

Thermally activated delayed fluorescence in a deep red dinuclear iridium(iii) complex: a hidden mechanism for short luminescence lifetimes.

The high luminescence efficiency of cyclometallated iridium(iii) complexes, including those widely used in OLEDs, is typically attributed solely to the formally spin-forbidden phosphorescence process being facilitated by spin-orbit coupling with the Ir(iii) centre. In this work, we provide unequivocal evidence that an additional mechanism can also participate, namely a thermally activated delayed fluorescence (TADF) pathway. TADF is well-established in other materials, including in purely organic compounds, but has never been observed in iridium complexes. Our findings may transform the design of iridium(iii) complexes by including an additional, faster fluorescent radiative decay pathway. We discover it here in a new dinuclear complex, 1, of the form [Ir(N^C)2]2(µ-L), where N^C represents a conventional N^C-cyclometallating ligand, and L is a bis-N^O-chelating bridging ligand derived from 4,6-bis(2-hydroxyphenyl)-pyrimidine. Complex 1 forms selectively as the rac diastereoisomer upon reaction of [Ir(N^C)2(µ-Cl)]2 with H2L under mild conditions, with none of the alternative meso isomer being separated. Its structure is confirmed by X-ray diffraction. Complex 1 displays deep-red luminescence in solution or in polystyrene film at room temperature (λem = 643 nm). Variable-temperature emission spectroscopy uncovers the TADF pathway, involving the thermally activated re-population of S1 from T1. At room temperature, TADF reduces the photoluminescence lifetime in film by a factor of around 2, to 1 µs. The TADF pathway is associated with a small S1-T1 energy gap ΔEST of approximately 50 meV. Calculations that take into account the splitting of the T1 sublevels through spin-orbit coupling perfectly reproduce the experimentally observed temperature-dependence of the lifetime over the range 20-300K. A solution-processed OLED comprising 1 doped into the emitting layer at 5 wt% displays red electroluminescence, λEL = 625 nm, with an EQE of 5.5% and maximum luminance of 6300 cd m-2.

Unusual Excimer/Dimer Behavior of a Highly Soluble C,N Platinum(II) Complex with a Spiro-Fluorene Motif.

In this work, we introduce a spiro-fluorene unit into a phenylpyridine (CN)-type ligand as a simple way to deplanarize the structure and increase the solubility of the final platinum(II)···complex. Using a spiro-fluorene unit, orthogonal to the main coordination plane of the complex, reduces intermolecular interactions, leading to increased solubility but without significantly affecting the ability of the complex to form Pt···Pt dimers and excimers. This approach is highly important in the design of platinum(II) complexes, which often suffer from low solubility due to their mainly planar structure, and offers an alternative to the use of bulky alkyl groups. The nonplanar structure is also beneficial for vacuum-deposition techniques as it lowers the sublimation temperature. Importantly, there are no sp3 hybridized carbon atoms in the cyclometalating ligand that contain hydrogens, the undesired feature that is associated with the low stability of the materials in OLEDs. The complex displays high solubility in toluene, ∼10 mg mL-1, at room temperature, which allows producing solution-processed OLEDs in a wide range of doping concentrations, 5-100%, and EQE up to 5.9%, with a maximum luminance of 7400 cd m-2. Concurrently, we have also produced vacuum-deposited OLEDs, which display luminance up to 32 500 cd m-2 and a maximum EQE of 11.8%.

Catching up with tetrazines: coordination of Re(I) to 1,2,4-triazine facilitates an inverse electron demand Diels-Alder reaction with strained alkynes to a greater extent than in corresponding 1,2,4,5-tetrazines.

Inverse electron demand Diels Alder (IEDDA) reactions of 1,2,4-triazines are of interest to biorthogonal chemistry but suffer from slow kinetics. It is shown here that coordination of Re(I) to a 1,2,4-triazine ring speeds up the IEDDA reaction with bicyclooctyne (BCN) by a factor of 55. Comparative analysis with corresponding 1,2,4,5-tetrazine analogues reveals that the origin of the increased reactivity is markedly different and more profound than in tetrazine analogues. DFT calculations and subsequent analysis indicated the greater increase for the triazine than the tetrazines on coordination could be attributed to the triazine's lower distortion energy and more favourable interaction energy for the triazine, the latter attributable to lower Pauli repulsion than the tetrazines rather than to favourable frontier orbital energies.

Electroluminescence of Tetradentate Pt(II) Complexes: O

Alkylation of one of the phenolic hydroxyl groups in a salen-type tetradentate ligand changes the coordination mode from O^N^N^O to the cyclometallating C^N^N^O type. The ligand was used to synthesize a new cyclometalated luminescent Pt(II) complex 2. While in solution the complex is poorly luminescent, in the solid state the emission is reinstated, which allowed one to evaluate complex 2 as a phosphorescent emitter in organic light-emitting diodes. 2 displays external quantum efficiency (EQE) = 9.1% and a maximum luminance of 9000 cd m-2 in a vacuum-deposited device. We carried out comparative analysis of photo- and electroluminescence of complex 2 with O^N^N^O complex 1 and demonstrated that the similar luminescent properties of the O^N^N^O and C^N^N^O complexes are rather coincidental because they display different excited-state landscapes. Surprisingly, the two complexes display a dramatically different electrochemical behavior, with O^N^N^O coordination leading to the formation of a stable electropolymer but C^N^N^O coordination fully preventing electropolymerization.

Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear Ir(III) Complexes.

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes fac-Ir(dppm)3 (Hdppm-4,6-bis(4-(tert-butyl)phenyl)pyrimidine), Ir(dppm)2(acac) (acac-acetylacetonate), and Ir(ppy)2(acac) (Hppy-phenylpyridine). The heteroleptic Ir(dppm)2(acac) is found to emit with efficiency above 80% and feature a remarkably high rate of emission. As measured under ambient temperature, Ir(dppm)2(acac) emits with the unusually short (sub-µs) radiative decay time of τr = τem/ΦPL = 1/kr = 0.91 µs in degassed toluene and τr = 0.73 µs in a doped polystyrene film under nitrogen. Investigations at cryogenic temperatures in glassy toluene showed that the emission stems from the T1 state and thus represents T1 → S0 phosphorescence with individual decay times of the T1 substates of T1,I = 66 µs, T1,II = 7.3 µs, T1,III = 0.19 µs, and energy gaps between the substates of ΔE(T1,II-T1,I) = 14 cm-1 and ΔE(T1,III-T1,I) = 210 cm-1. Analysis of the electronic structure of Ir(dppm)2(acac) showed that such a high rate of phosphorescence may stem from the two dppm ligands, with extended π-conjugation system and π-deficient character due to the pyrimidine ring, being serially aligned along one axis. Such alignment, along with the quasi-symmetric character of Jahn-Teller distortions in the T1 state, affords a large chromophore, comprising four (het)aryl rings of the two dppm ligands. This affords an exceptionally large oscillator strength of the MLCT-character singlet state spin-orbit coupled with the T1 state and thus brings about enhancement of the phosphorescence rate. These findings reveal molecular design principles paving the way to new phosphors of enhanced emission rates.

Exceptionally fast radiative decay of a dinuclear platinum complex through thermally activated delayed fluorescence.

A novel dinuclear platinum(ii) complex featuring a ditopic, bis-tetradentate ligand has been prepared. The ligand offers each metal ion a planar O^N^C^N coordination environment, with the two metal ions bound to the nitrogen atoms of a bridging pyrimidine unit. The complex is brightly luminescent in the red region of the spectrum with a photoluminescence quantum yield of 83% in deoxygenated methylcyclohexane solution at ambient temperature, and shows a remarkably short excited state lifetime of 2.1 µs. These properties are the result of an unusually high radiative rate constant of around 4 × 105 s-1, a value which is comparable to that of the very best performing Ir(iii) complexes. This unusual behaviour is the result of efficient thermally activated reverse intersystem crossing, promoted by a small singlet-triplet energy difference of only 69 ± 3 meV. The complex was incorporated into solution-processed OLEDs achieving EQEmax = 7.4%. We believe this to be the first fully evidenced report of a Pt(ii) complex showing thermally activated delayed fluorescence (TADF) at room temperature, and indeed of a Pt(ii)-based delayed fluorescence emitter to be incorporated into an OLED.

Non-Stereogenic Dinuclear Ir(III) Complex with a Molecular Rack Design to Afford Efficient Thermally Enhanced Red Emission.

Cyclometalated complexes containing two or more metal centers were recently shown to offer photophysical properties that are advantageous compared to their mononuclear analogues. Here we report the design, synthesis, and luminescent properties of a dinuclear Ir(III) complex formed by a ditopic N^C^N-N^C^N bridging ligand (L1) with pyrimidine as a linking heterocycle. Two dianionic C^N^C terminal ligands were employed to achieve a charge-neutral and nonstereogenic dinuclear complex 5. This complex shows a highly efficient red emission with a maximum at λem = 642 nm as measured for a toluene solution. The decay time and emission quantum yield of the complex measured for the degassed sample are τ = 1.31 µs and ΦPL = 80%, respectively, corresponding to the radiative rate of kr = 6.11·105 s-1. This rate value is approximately fourfold faster than for the green-emitting mononuclear analogue 3. Cryogenic temperature measurements show that the three substrates of the lowest triplet state T1 of 5 emit with decay times of τ(I) = 120 µs, τ(II) = 7 µs, and τ(III) = 1 µs that are much shorter compared to those of the mononuclear complex 3, which has values of τ(I) = 192 µs, τ(II) = 65.6 µs, and τ(III) = 3.6 µs. These data indicate that the spin-orbit coupling of state T1 with the singlet states is much stronger in the case of complex 5, which results in a much higher T1 → S0 emission rate. Indeed, a computational analysis suggests that in the dinuclear complex 5 the T1 state is spin-orbit coupled with twice the number of singlet states compared to that of mononuclear 3, which is a result of the electronic coupling of two coordination sites. The investigation of the temperature dependence of the emission rates of 3 and 5 shows that the room-temperature emission of both complexes is mainly contributed by a thermally populated excited state lying above the T1 state. To the best of our knowledge, complexes 3 and 5 are the first examples of Ir(III) complexes that show photophysical behavior reminiscent of thermally activated delayed fluorescence (TADF).

Halide-Enhanced Spin-Orbit Coupling and the Phosphorescence Rate in Ir(III) Complexes.

The spin-forbidden nature of phosphorescence in Ir(III) complexes is relaxed by the metal-induced effect of spin-orbit coupling (SOC). A further increase of the phosphorescence rate could potentially be achieved by introducing additional centers capable of further enhancing the SOC effect, such as metal-coordinated halides. Herein, we present a dinuclear Ir(III) complex Ir2I2 that contains two Ir(III)-iodide moieties. The complex shows intense phosphorescence with a quantum yield of ΦPL(300 K) = 90% and a submicrosecond decay time of only τ(300 K) = 0.34 µs, as measured under ambient temperature for the degassed toluene solution. These values correspond to a top value T1 → S0 phosphorescence rate of kr = 2.65 × 106 s-1. Investigations at cryogenic temperatures allowed us to determine the zero-field splitting (ZFS) of the emitting state T1 ZFS(III-I) = 170 cm-1 and unusually short individual decay times of T1 substates: τ(I) = 6.4 µs, τ(II) = 7.6 µs, and τ(III) = 0.05 µs. This indicates a strong SOC of state T1 with singlet states. Theoretical investigations suggest that the SOC of state T1 with singlets is also contributed by halides. Strongly contributing to the higher occupied molecular orbitals of the complex (e.g., HOMO, HOMO - 1, and so forth), iodides work as important SOC centers that operate in tandem with metals. The examples of Ir2I2 and of earlier reported analogous complex Ir2Cl2 reveal that the metal-coordinated halides can enhance the SOC of state T1 with singlets and, consequently, the phosphorescence rate. A comparative study of Ir2I2 and Ir2Cl2 shows that the share of halides in total contribution (halides plus metals) to the SOC of state T1 with singlets increases strongly upon exchange of chlorides for iodides. The exchange also led to the decrease in values of ZFS of the T1 state from ZFS(III-I) = 205 cm-1 for Ir2Cl2 to T1 ZFS(III-I) = 170 cm-1 for Ir2I2. This results in a more efficient thermal population of the fastest emitting T1 substate III, thus further enhancing the room-temperature phosphorescence rate.

Near Infrared Phosphorescent Dinuclear Ir(III) Complex Exhibiting Unusually Slow Intersystem Crossing and Dual Emissive Behavior.

A dinuclear iridium(III) complex IrIr shows dual emission consisting of near infrared (NIR) phosphorescence (λmax = 714 nm, CH2Cl2, T = 300 K) and green fluorescence (λmax = 537 nm). The NIR emission stems from a triplet state (T1) localized on the ditopic bridging ligand (3LC). Because of the dinuclear molecular structure, the phosphorescence efficiency (ΦPL = 3.5%) is high compared to those of other known red/NIR-emitting iridium complexes. The weak fluorescence stems from the lowest excited singlet state (S1) of 1LC character. The occurrence of fluorescence is ascribed to relatively slow intersystem crossing (ISC) from state S1 (1LC) to the triplet manifold. The measured ISC rate corresponds to a time constant τISC of 2.1 ps, which is an order of magnitude longer than those usually found for iridium complexes. This slow ISC rate can be explained in terms of the LC character and large energy separation (0.57 eV) of the respective singlet and triplet excited states. IrIr is internalized by live HeLa cells as evidenced by confocal luminescence microscopy.

Unusual dual-emissive heteroleptic iridium complexes incorporating TADF cyclometalating ligands.

Five new neutral heteroleptic iridium(iii) complexes IrL2(pic) (2-6) based on the archetypical blue emitter FIrpic have been synthesised. The cyclometallating ligands L are derived from 2-(2,6-F2-3-pyridyl)-4-mesitylpyridine (7), 2-(3-cyano-2,6-F2-phenyl)-4-mesitylpyridine (8), 2-(2,6-F2-phenyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (9), 2-(2,6-F2-3-pyridyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (10) and 2-(3-cyano-2,6-F2-phenyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (11) for complexes 2, 3, 4, 5 and 6, respectively. The carbazole-functionalised ligands 9-11 show weak thermally activated delayed fluorescence (TADF) in solution. Complexes 5 and 6 reveal dual emission in polar solvents. A broad charge transfer (CT) band appears and increases in intensity relative to the higher energy emission band as solvent polarity is increased. The dual emission occurs when the energy of the ligand 3CT state is comparable to that of the 3MLCT state of the complex, resulting in fast interconversion between the two. Assignment of the ligand TADF and dual emission properties is supported by hybrid density functional theory (DFT) and time dependent DFT (TD-DFT) calculations. Phosphorescent organic light emitting devices (PhOLEDs) have been fabricated using these complexes as sky-blue emitters, and their performance is compared to devices using FIrpic and the previously reported complex IrL2(pic) 1 (L from the 2-(2,6-F2-phenyl)-4-mesitylpyridine ligand). For identical device structures, the device containing the carbazole complex 4 performs best out of the seven complexes. The dual emission observed in solution for complexes 5 and 6 is not observed in their devices.

Exploring the Subtle Effect of Aliphatic Ring Size on Minor Actinide-Extraction Properties and Metal Ion Speciation in Bis-1,2,4-Triazine Ligands.

The synthesis and evaluation of three novel bis-1,2,4-triazine ligands containing five-membered aliphatic rings are reported. Compared to the more hydrophobic ligands 1-3 containing six-membered aliphatic rings, the distribution ratios for relevant f-block metal ions were approximately one order of magnitude lower in each case. Ligand 10 showed an efficient, selective and rapid separation of AmIII and CmIII from nitric acid. The speciation of the ligands with trivalent f-block metal ions was probed using NMR titrations and competition experiments, time-resolved laser fluorescence spectroscopy and X-ray crystallography. While the tetradentate ligands 8 and 10 formed LnIII complexes of the same stoichiometry as their more hydrophobic analogues 2 and 3, significant differences in speciation were observed between the two classes of ligand, with a lower percentage of the extracted 1:2 complexes being formed for ligands 8 and 10. The structures of the solid state 1:1 and 1:2 complexes formed by 8 and 10 with YIII , LuIII and PrIII are very similar to those formed by 2 and 3 with LnIII . Ligand 10 forms CmIII and EuIII 1:2 complexes that are thermodynamically less stable than those formed by ligand 3, suggesting that less hydrophobic ligands form less stable AnIII complexes. Thus, it has been shown for the first time how tuning the cyclic aliphatic part of these ligands leads to subtle changes in their metal ion speciation, complex stability and metal extraction affinity.

Iridium(iii) complexes of 1,2,4-triazines as potential bioorthogonal reagents: metal coordination facilitates luminogenic reaction with strained cyclooctynes.

In this paper we describe unprecedented Ir(iii) complexes of 5-(2-pyridyl)-1,2,4-triazine and their reactivity towards the strained cyclooctyne BCN. The coordination of a 1,2,4-triazine ring to an iridium(iii) ion drastically increases the speed of the reaction, showing the second order rate constant of 8 M-1 s-1, the record value to date for a triazine-BCN reaction.

Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design.

The design and detailed photophysical study of two novel Ir(III) complexes featuring mono- and dinuclear design are presented. Emission quantum yield and decay times in solution are ΦPL = 90% and τ(300 K) = 1.16 µs for the mononuclear complex 5, and ΦPL = 95% and τ(300 K) = 0.44 µs for the dinuclear complex 6. These data indicate an almost 3-fold increase in the phosphorescence rate for dinuclear complex 6 compared to 5. Zero-field splitting (ZFS) of the T1 state also increases from ZFS = 65 cm-1 for the mononuclear complex to ZFS = 205 cm-1 for the dinuclear complex and is accompanied by a drastic shortening of the individual decay times of T1 substates. With the help of TD-DFT calculations, we rationalize that the drastic changes in the T1 state properties in the dinuclear complex originate from an increased number of excited states available for direct spin-orbit coupling (SOC) routes as a result of electronic coupling of Ir-Cl antibonding molecular orbitals of the two coordination sites.

Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs.

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near-infrared regions of the spectrum due to the transfer of electronic excited-state energy into vibrations. We describe how this undesirable "energy gap law" can be sidestepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centers. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λmax = 610 nm, Φ = 0.85 in deoxygenated CH2Cl2 at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53 800 M-1 cm-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T1 → S0 phosphorescence process, allowing it to compete effectively with nonradiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.

1,2,4-Triazines in the Synthesis of Bipyridine Bisphenolate ONNO Ligands and Their Highly Luminescent Tetradentate Pt(II) Complexes for Solution-Processable OLEDs.

This article describes a convenient method for the synthesis of ONNO-type tetradentate 6,6'-bis(2-phenoxy)-2,2'-bipyridine (bipyridine bisphenolate, BpyBph) ligands and their platinum(II) complexes. The methodology includes the synthesis of 1,2,4-triazine precursors followed by their transformation to functionalized pyridines by the Boger reaction. Two complementary routes employing 3,3'- and 5,5'-bis-triazines allow a modification of the central pyridine rings in different positions, which was exemplified by the introduction of cyclopentene rings. The new ligands were used to prepare highly luminescent ONNO-type Pt(II) complexes. The position of the cyclopentene rings significantly influences the solubility and photophysical properties of these complexes. Derivatives with closely positioned cyclopentene rings are soluble in organic solvents and proved to be the best candidate for solution-processable organic light-emitting devices (OLEDs), showing efficient single-dopant candlelight electroluminescence.

Pyridazine-bridged cationic diiridium complexes as potential dual-mode bioimaging probes.

A novel diiridium complex [(N^C^N)2Ir(bis-N^C)Ir(N^C^N)2Cl]PF6 (N^C^N = 2-[3-tert-butyl-5-(pyridin-2-yl)phenyl]pyridine; bis-N^C = 3,6-bis(4-tert-butylphenyl)pyridazine) was designed, synthesised and characterised. The key feature of the complex is the bridging pyridazine ligand which brings two cyclometallated Ir(iii) metal centres close together so that Cl also acts as a bridging ligand leading to a cationic complex. The ionic nature of the complex offers a possibility of improving solubility in water. The complex displays broad emission in the red region (λ em = 520-720 nm, τ = 1.89 µs, Φ em = 62% in degassed acetonitrile). Cellular assays by multiphoton (λ ex = 800 nm) and confocal (λ ex = 405 nm) microscopy demonstrate that the complex enters cells and localises to the mitochondria, demonstrating cell permeability. Further, an appreciable yield of singlet oxygen generation (Φ Δ = 0.45, direct method, by 1O2 NIR emission in air equilibrated acetonitrile) suggests a possible future use in photodynamic therapy. However, the complex has relatively high dark toxicity (LD50 = 4.46 µM), which will likely hinder its clinical application. Despite this toxicity, the broad emission spectrum of the complex and high emission yield observed suggest a possible future use of this class of compound in emission bioimaging. The presence of two heavy atoms also increases the scattering of electrons, supporting potential future applications as a dual fluorescence and electron microscopy probe.

Rigidly linking cyclometallated Ir(iii) and Pt(ii) centres: an efficient approach to strongly absorbing and highly phosphorescent red emitters.

The synthesis and photophysical properties of an unprecedented tetranuclear complex are described, in which a fac-tris-cyclometallated Ir(iii) centre is rigidly connected to three cyclometallated Pt(ii) centres. The complex absorbs strongly up to ∼600 nm and emits red light with unusually high efficiency.

Mesomorphism and Photophysics of Some Metallomesogens Based on Hexasubstituted 2,2':6', 2''-Terpyridines.

The luminescent and mesomorphic properties of a series of metal complexes based on hexacatenar 2,2':6',2''-terpyridines are investigated using experimental methods and density functional theory (DFT). Two types of ligand are examined, namely 5,5''-di(3,4,5-trialkoxyphenyl)terpyridine with or without a fused cyclopentene ring on each pyridine and their complexes were prepared with the following transition metals: Zn(II) , Co(III) , Rh(III) , Ir(III) , Eu(III) and Dy(III) . The exact geometry of some of these complexes was determined by single X-ray diffraction. All complexes with long alkyl chains were found to be liquid crystalline, which property was induced on complexation. The liquid-crystalline behaviour of the complexes was studied by polarising optical microscopy and small-angle X-ray diffraction. Some of the transition metal complexes (for example, those with Zn(II) and Ir(III) ) are luminescent in solution, the solid state and the mesophase; their photophysical properties were studied both experimentally and using DFT methods (M06-2X and B3LYP).

When two are better than one: bright phosphorescence from non-stereogenic dinuclear iridium(III) complexes.

A new family of eight dinuclear iridium(iii) complexes has been prepared, featuring 4,6-diarylpyrimidines L(y) as bis-N^C-coordinating bridging ligands. The metal ions are also coordinated by a terminal N^C^N-cyclometallating ligand L(X) based on 1,3-di(2-pyridyl)benzene, and by a monodentate chloride or cyanide. The general formula of the compounds is {IrL(X)Z}2L(y) (Z = Cl or CN). The family comprises examples with three different L(X) ligands and five different diarylpyrimidines L(y), of which four are diphenylpyrimidines and one is a dithienylpyrimidine. The requisite proligands have been synthesised via standard cross-coupling methodology. The synthesis of the complexes involves a two-step procedure, in which L(X)H is reacted with IrCl3·3H2O to form dinuclear complexes of the form [IrL(X)Cl(µ-Cl)]2, followed by treatment with the diarylpyrimidine L(y)H2. Crucially, each complex is formed as a single compound only: the strong trans influence of the metallated rings dictates the relative disposition of the ligands, whilst the use of symmetrically substituted tridentate ligands eliminates the possibility of Λ and Δ enantiomers that are obtained when bis-bidentate units are linked through bridging ligands. The crystal structure of one member of the family has been obtained using a synchrotron X-ray source. All of the complexes are very brightly luminescent, with emission maxima in solution varying over the range 517-572 nm, according to the identity of the ligands. The highest-energy emitter is the cyanide derivative whilst the lowest is the complex with the dithienylpyrimidine. The trends in both the absorption and emission energies as a function of ligand substituent have been rationalised accurately with the aid of TD-DFT calculations. The lowest-excited singlet and triplet levels correlate with the trend in the HOMO-LUMO gap. All the complexes have quantum yields that are close to unity and phosphorescence lifetimes - of the order of 500 ns - that are unusually short for complexes of such brightness. These impressive properties stem from an unusually high rate of radiative decay, possibly due to spin-orbit coupling pathways being facilitated by the second metal ion, and to low non-radiative decay rates that may be related to the rigidity of the dinuclear scaffold.