A new family consisting of three luminescent neutral Ir(III) complexes with the unprecedented [Ir(C^N^C)(N^N)Cl] architecture, where C^N^C is a bis(six-membered) chelating tridentate tripod ligand derived from 2-benzhydrylpyridine (bnpy) and N^N is 4,4′-di-tert-butyl-2,2′-bipyridine (dtBubpy), is reported. X-ray crystallography reveals an unexpected and unusual double C–H bond activation of the two distal nonconjugated phenyl rings of the bnpy coupled with a very short Ir–Cl bond trans to the pyridine of the bnpy ligand. Depending on the substitution on the bnpy ligand, phosphorescence, ranging from yellow to red, is observed in dichloromethane solution. A combined study using density functional theory (DFT) and time-dependent DFT (TD-DFT) corroborates the mixed charge-transfer nature of the related excited states.

The reaction of the β-triketonate ligands tris(4-methylbenzoyl)methanide and tribenzoylmethanide with the trivalent lanthanoids Eu3+, Er3+, and Yb3+ in the presence of Cs+ afforded polymeric structures where the repeating units are represented by bimetallic tetranuclear assemblies of formulation {[Ln(Cs)(β-triketonate)4]2}n. The only exception is the structure formed by the reaction of tris(4-methylbenzoyl)methanide, Yb3+, and Cs+, which yielded a polymeric assembly where the repeating units are mononuclear Yb3+ complexes bridged by Cs+ cations. Photophysical measurements on the obtained materials confirmed efficient sensitization from the ligand excited states to the 4f* excited states of the three lanthanoids. According to transient absorption data, Er3+ and Yb3+ are sensitized via energy transfer from the triplet state of the β-triketonate ligands. On the other hand, energy transfer to Eu3+ seems to occur via an alternative pathway, possibly directly via the singlet state or through ligand to metal charge transfer states. The emission measurements confirm efficient sensitization for all three lanthanoids and bright near-infrared emission for Er3+ and Yb3+, a characteristic that seems to be linked to the specific chemical structure of the β-triketonate ligands.

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low-cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light-emitting diode (OLED) and related EL devices. TADF emitters are cross-compared within specific color ranges, with a focus on blue, green–yellow, orange–red, and white OLEDs. Organic small-molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.

The first examples of phosphorescent platinum(II) complexes bearing pentafluorosulfanyl (–SF5) substituted cyclometalating ligands (C^N) are reported. These complexes are of the form [Pt(C^N)(pivacac)], where pivacac is 2,2′,6,6′-tetramethylheptane-3,5-dionate. Modifying the phenyl ring of the C^N ligand to incorporate one strongly electron-withdrawing –SF5 group has important effects on the photophysical and electrochemical properties of the complex that are dependent on the regiochemistry of the substituent. In a meta position with respect to the Pt–CC^N bond, the substituent exerts a predominantly stabilising effect on the lowest triplet excited state that red-shifts the emission of the complex compared to the reference [Pt(ppy)(pivacac)], 1, where ppy is 2-phenylpyridinato. When the –SF5 group is located para to the Pt–CC^N bond, it does not affect the triplet state directly, and the electron-withdrawing group stabilises the metal-based orbitals, resulting in a blue-shift of the emission. In the solid-state all three complexes are mechanochromic, and can display excimeric emission originating from intermolecular π–π* interactions, but the relative emission intensities of the monomeric and dimeric excited states correlate with the steric congestion of the metal centre, and in particular the regiochemistry of the –SF5 group. We relate these findings with observations in the crystal structures.

Two cationic Ir(III) complexes bearing 2-phenylpyridinato cyclometalating ligands and bithiazole-type ancillary ligands have been synthesized and optoelectronically characterised. These emitters exhibit unusually deep red-to-near-infrared emission at room temperature, thereby rendering them as attractive emitters in solution-processed light emitting electrochemical cell (LEEC) electroluminescent devices.

The synthesis and structural and photophysical characterisation of four novel, cationic iridium(III) complexes are reported. These complexes were designed to emit in the blue region of the visible spectrum without employing sp2 carbon–fluorine bonds, which have been shown to be electrochemically unstable. Two different C∧N (where C∧N is a bidentate cyclometalating ligand possessing a nitrogen–carbon chelate) ligands [5-(4-methylpyridin-2-yl)-2,4-dimethoxypyrimidine (Mepypyrm) and 5-(5-(trifluoromethyl)pyridine-2-yl)-2,4-dimethoxypyrimidine (CF3pypyrm)] combine electron-withdrawing pyrimidyl nitrogen atoms (in a para relationship with respect to the metal) with methoxy groups in a meta relationship with respect to the metal, which both inductively withdraw electron density from the metal centre, stabilizing the highest occupied molecular orbital. The result is highly efficient (ΦPL = 73–81%) green to blue (λPL = 446–515 nm) emission for complexes 1–4 in MeCN solution. Complex 1 exhibits a broad, unstructured charge transfer (CT) emission profile, while complexes 2–4 exhibit structured, vibronic emission profiles. Density Functional Theory (DFT) calculations corroborate these findings with spin density calculations predicting a T1 state that is metal-to-ligand and ligand-to-ligand (C∧N to N∧N) charge transfer (3MLCT/3LLCT) in nature for complex 1, while complexes 2–4 are predicted to exhibit ligand-centred (3LC) states with spin density localised exclusively on the C∧N ligands. These complexes were used as emitters in sky-blue and blue-green light-emitting electrochemical cells (LEECs). The bluest of these devices (CIE: 0.23, 0.39) is among the bluest reported for any iridium-based LEEC. It is noteworthy that although the electroluminescence intensity decreases rapidly with time (t1/2 = 0.1–20 min), as is typical of blue-green LEECs, for devices L1, L3 and L4 we have observed for the first time that this decay occurs without an accompanying red-shift in the CIE coordinates over time, implying that the emitter does not undergo any chemical degradation process in the non-doped zones of the device.

This perspective illustrates our approach in the design of heteroleptic cationic iridium(III) complexes for optoelectronic applications, especially as emitters in electroluminescent devices. We discuss changes in the photophysical properties of the complexes as a consequence of modification of the electronics of either the cyclometalating (C^N) or the ancillary (N^N) ligands. We then broach the impact on these properties as a function of modification of the structure of both types of ligands. We explain trends in the optoelectronic behaviour of the complexes using a combination of rationally designed structure–property relationship studies and theoretical modelling that serves to inform subsequent ligand design. However, we have found cases where the design paradigms do not always hold true. Nevertheless, all these studies contribute to the lessons we have learned in the design of heteroleptic cationic phosphorescent iridium(III) complexes.

We present dynamic supramolecular systems composed of a Ru(II) complex of the form of [Ru(dtBubpy)2(qpy)][PF6]2 (where dtBubpy is 4,4′-di-tert-butyl-2,2′-dipyridyl and qpy is 4,4′:2′,2′′:4′′,4′′′-quaterpyridine) and zinc tetraphenylporphyrins (ZnTPP), through non-covalent interactions between the distal pyridine moieties of the qpy ligand and the zinc of ZnTPP. The optoelectronic properties of the assemblies and the electronic interactions between the chromophoric units have been comprehensively characterized by computational investigations, and steady-state and time-resolved emission spectroscopy. Upon photoexcitation of ZnTPP, electron transfer to the ruthenium center is thermodynamically favorable and, as a result, strong emission quenching of both units occurs.

Two deep blue thermally activated delayed fluorescence (TADF) emitters (imCzDPS and imDPADPS) that contain charged imidazolium groups tethered to the central luminophore were designed and synthesized as small molecule organic emitters for light-emitting electrochemical cell (LEEC) electroluminescent devices. The emission profile of the doped thin films (5 wt% in PMMA) is very blue and narrow (λPL: 414 nm and 409 nm; full width at half maximum (FHWM): 62 nm and 46 nm for imCzDPS and imDPADPS, respectively) with good photoluminescence quantum efficiencies (ΦPL: 44% and 49% for imCzDPS and imDPADPS, respectively). In neat films, emission maxima occur at 440 nm and 428 nm for imCzDPS and imDPADPS, respectively, with comparable ΦPL values of 44 and 61%, respectively. Both emitters exhibit biexponential emission decay kinetics (nanosecond prompt and microsecond delayed fluorescence) in both MeCN solution and thin films, characteristic of TADF behaviour. While imDPADPS did not show any emission in the LEEC device, that of imCzDPS gave an electroluminescence (EL) maximum at 470 nm and CIE coordinates of (0.208, 0.250), which makes this device amongst the bluest reported to date. However, the maximum device luminance achieved was 2.5 cd m−2 and this poor brightness was attributed to the electrochemical instability of the emitter in the LEEC architecture, as evidenced by the additional peak at around 550 nm observed in the EL spectrum.

Combining a sterically bulky, electron-deficient 2-(2,4-difluorophenyl)-4-(2,4,6-trimethylphenyl)pyridine (dFMesppy) cyclometalating C∧N ligand with an electron rich, highly rigidified 1,1′-(α,α′-o-xylylene)-2,2′-biimidazole (o-xylbiim) N∧N ligand gives an iridium complex, [Ir(dFMesppy)2(o-xylbiim)](PF6), that achieves extraordinarily bright blue emission (ΦPL = 90%; λmax = 459 nm in MeCN) for a cationic iridium complex. This complex is compared with two reference complexes bearing 4,4′-di-tert-butyl-2,2′-bipyridine, and solution-processed organic light emitting diodes (OLEDs) have been fabricated from these materials.

Phthalocyanines and their main group and metal complexes are important classes of organic semiconductor materials but are usually highly insoluble and so frequently need to be processed by vacuum deposition in devices. We report two highly soluble silicon phthalocyanine (SiPc) diester compounds and demonstrate their potential as organic semiconductor materials. Near-infrared (λEL = 698–709 nm) solution-processed organic light-emitting diodes (OLEDs) were fabricated and exhibited external quantum efficiencies (EQEs) of up to 1.4%. Binary bulk heterojunction solar cells employing P3HT or PTB7 as the donor and the SiPc as the acceptor provided power conversion efficiencies (PCE) of up to 2.7% under simulated solar illumination. Our results show that soluble SiPcs are promising materials for organic electronics.Keywords: near-IR emission; organic solar cells; silicon phthalocyanines; single crystals; solution-processable organic light-emitting diodes

The structure–property relationship study of a series of cationic Ir(III) complexes in the form of [Ir(C^N)2(dtBubpy)]PF6 [where dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C4 of the phenyl substituent, i.e., −CF3 (1), −OCF3 (2), −SCF3 (3), −SO2CF3 (4)] has been investigated. The physical and optoelectronic properties of the four complexes were comprehensively characterized, including by X-ray diffraction analysis. All the complexes exhibit quasireversible dtBubpy-based reductions from −1.29 to −1.34 V (vs SCE). The oxidation processes are likewise quasireversible (metal + C^N ligand) and are between 1.54 and 1.72 V (vs SCE). The relative oxidation potentials follow a general trend associated with the Hammett parameter (σ) of the EWGs. Surprisingly, complex 4 bearing the strongest EWG does not adhere to the expected Hammett behavior and was found to exhibit red-shifted absorption and emission maxima. Nevertheless, the concept of introducing EWGs was found to be generally useful in blue-shifting the emission maxima of the complexes (λem = 484–545 nm) compared to that of the prototype complex [Ir(ppy)2(dtBubpy)]PF6 (where ppy = 2-phenylpyridinato) (λem = 591 nm). The complexes were found to be bright emitters in solution at room temperature (ΦPL = 45–66%) with microsecond excited-state lifetimes (τe = 1.14–4.28 μs). The photophysical properties along with density functional theory (DFT) calculations suggest that the emission of these complexes originates from mixed contributions from ligand-centered (LC) transitions and mixed metal-to-ligand and ligand-to-ligand charge transfer (LLCT/MLCT) transitions, depending on the EWG. In complexes 1, 3, and 4 the 3LC character is prominent over the mixed 3CT character, while in complex 2, the mixed 3CT character is much more pronounced, as demonstrated by DFT calculations and the observed positive solvatochromism effect. Due to the quasireversible nature of the oxidation and reduction waves, fabrication of light-emitting electrochemical cells (LEECs) using these complexes as emitters was possible with the LEECs showing moderate efficiencies.

Despite hundreds of cationic bis-cyclometalated iridium(III) complexes having been explored as emitters for light-emitting electrochemical cells (LEECs), uniformly their composition has been in the form of a racemic mixture of Λ and Δ enantiomers. The investigation of LEECs using enantiopure iridium(III) emitters, however, remains unprecedented. Herein, we report the preparation, the crystal structures, and the optoelectronic properties of two families of cyclometalated iridium(III) complexes of the form of [(C^N)2Ir(dtBubpy)]PF6 (where dtBubpy is 4,4′-di-tert-butyl-2,2′-bipyridine) in both their racemic and enantiopure configurations. LEEC devices using Λ and Δ enantiomers as well as the racemic mixture of both families have been prepared, and the device performances were tested. Importantly, different solid-state photophysical properties exist between enantiopure and racemic emitters, which are also reflected in the device performances.Keywords: cationic iridium(III) complexes; enantiopure complexes; light-emitting electrochemical cells; photophysical properties; solid-state packing;

We report the first examples of highly luminescent di-coordinated Pd(0) complexes. Five complexes of the form [Pd(L)(L′)] were synthesized, where L = IPr, SIPr or IPr* NHC ligands and L′ = PCy3, or IPr and SIPr NHC ligands. The photophysical properties of these complexes were determined in degassed toluene solution and in the solid state and contrasted to the poorly luminescent reference complex [Pd(IPr)(PPh3)]. Organic light-emitting diodes were successfully fabricated but attained external quantum efficiencies of between 0.3 and 0.7%.

Two novel charged organic thermally activated delayed fluorescence (TADF) emitters, 1 and 2, have been synthesized. Their TADF behavior is well-supported by the multiexponential decay of their emission (nanosecond and microsecond components) and the oxygen dependence of the photoluminescence quantum yields. Spin-coated electroluminescent devices have been fabricated to make light-emitting electrochemical cells (LEECs) and organic light-emitting diodes (OLEDs). The first example of a non-doped charged small organic molecule LEEC is reported and exhibited an external quantum efficiency (EQE) of 0.39% using 2. With a multilayer architecture, a solution-processed OLED device using neat 2 as the emitting layer gave an EQE of 5.1%, the highest reported to date for a nondoped solution-processed small molecule organic TADF OLED. These promising results open up a new area in light-emitting materials for the development of low-cost TADF LEECs.

A new class of cationic iridium(III) complexes of the form [(C∧N)2Ir(N∧N)][PF6] is reported, where C∧N = cyclometallating 2-phenylpyridinato, ppy, or 2-(2,4-difluorophenyl)-5′-methylpyridinato, dFMeppy, and N∧N = guanidyl-pyridine, gpy, or -pyrazine, gpz, as the ancillary ligand. A large blue-shift in the emission coupled with a 7-to-9 fold enhancement in photoluminescence quantum yield and microsecond emission lifetimes were observed for the complexes containing the partially saturated gpy ligand as compared to the benchmark complex [(ppy)2Ir(bpy)][PF6], C1, where bpy is 2,2′-bipyridine.

We report on four cationic iridium(III) complexes [Ir(C^N)2(dtBubpy)](PF6) that have sulfur pentafluoride-modified 1-phenylpyrazole and 2-phenylpyridine cyclometalating (C^N) ligands (dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridyl). Three of the complexes were characterized by single-crystal X-ray structure analysis. In cyclic voltammetry, the complexes undergo reversible oxidation of iridium(III) and irreversible reduction of the SF5 group. They emit bright green phosphorescence in acetonitrile solution and in thin films at room temperature, with emission maxima in the range of 482–519 nm and photoluminescence quantum yields of up to 79%. The electron-withdrawing sulfur pentafluoride group on the cyclometalating ligands increases the oxidation potential and the redox gap and blue-shifts the phosphorescence of the iridium complexes more so than the commonly employed fluoro and trifluoromethyl groups. The irreversible reduction of the SF5 group may be a problem in organic electronics; for example, the complexes do not exhibit electroluminescence in light-emitting electrochemical cells (LEECs). Nevertheless, the complexes exhibit green to yellow-green electroluminescence in doped multilayer organic light-emitting diodes (OLEDs) with emission maxima ranging from 501 nm to 520 nm and with an external quantum efficiency (EQE) of up to 1.7% in solution-processed devices.

In this study, a series of four formyl-substituted chloro-bridged iridium(III) dimers were prepared. Their absorption, photophysical and electrochemical properties were studied in dichloromethane solution. It was found that as the formyl content increased on the cyclometalating ligands, emission unexpectedly became brighter. Organic light-emitting diodes (OLEDs) were fabricated using each of these iridium dimers as the emitter. The OLED fabricated using the brightest of the series, 2b, as the dopant afforded a decent external quantum efficiency (EQE) of 2.6%. This suggests that chloro-bridged iridium dimers are potential candidates as solid-state emitters.

Five cationic platinum(II) complexes bearing a 2-(3′-substituted aryl)pyridine cyclometalating ligand (C∧N) and a neutral Ar-BIAN ligand have been synthesized: [Pt(ppy)(PhBIAN)]PF6 (1), [Pt(3Fppy)(PhBIAN)]PF6 (2), [Pt(3MeOppy)(PhBIAN)]PF6 (3), [Pt(3MeOppy)(4-FPhBIAN)]PF6 (4), [Pt(ppy)(4-MeOPhBIAN)]PF6 (5). All complexes have been characterized by NMR spectroscopy and mass spectrometry. Complexes 2 and 3 have been characterized by X-ray crystallography. Structure–property relationships were established from UV–visible spectroscopy and cyclic voltammetry studies. Interestingly, we found that when both the C∧N and the Aryl-BIAN ligands contained electron-donating MeO groups the absorption spectrum for the platinum complex extended out to 650 nm. The electrochemical studies of these complexes established that they are electronically compatible dye molecules for dye-sensitized solar cells.

There is presently a lack of efficient and stable blue emitters for light-emitting electrochemical cells (LEECs), which limits the development of white light emitting systems for lighting. Cyclometalated iridium complexes as blue emitters tend to show low photoluminescence efficiency due to significant ligand-centred character of the radiative transition. The most common strategy to blue-shift the emission is to use fluorine substituents on the cyclometalating ligand, such as 2,4-difluorophenylpyridine, dFppy, which has been shown to decrease the stability of the emitter in operating devices. Herein we report a series of four new charged cyclometalated iridium complexes using methoxy- and methyl-substituted 2,3′-bipyridine as the main ligands. The combination of donor groups and the use of a cyclometalated pyridine has been recently reported for neutral complexes and found electronically equivalent to dFppy. We describe the photophysical and electrochemical properties of the complexes in solution and use DFT and TDDFT calculations to gain insights into their properties. The complexes exhibit bluish-green emission with onsets around 450 nm, which correspond to the maximum emission at 77 K. Furthermore, photoluminescence quantum yields in solution are all above 40%, with the brightest in the series at 66%. Finally, LEECs were prepared using these complexes as the emissive material to evaluate the performance of this particular design. Compared to previously reported devices with fluorine-containing emitters, the emitted colours are slightly red-shifted due to methyl substituents on the coordinating pyridine of the main ligand and overall device performances, unfortunately including the stability of devices, are similar to those previously reported. Interestingly within the series of complexes there appears to be a positive effect of the methoxy-substituents on the stability of the devices. The poor stability is therefore attributed to the combination of cyclometalated pyridine and methoxy groups.

Despite the differing size of the Cl− and PF6− counter-ions, the structures of the heteroleptic iridium(III) complexes, [Ir(dFphtl)2(btl)]Cl, [1]Cl, and [Ir(dFphtl)2(btl)]PF6, [1]PF6, (where dFphtl = 1-benzyl-4-(2,4-difluorophenyl)-1H-1,2,3-triazole and btl = 1,1′-dibenzyl-4,4′-bi-1H-1,2,3-triazolyl) are found to exhibit similar morphologies in which both structures adopt hydrogen-bonded networks driven by the hydrogen-bond donor and acceptor demands of the triazole functional group. The triazole thus can be used as a supramolecular synthon to control the internuclear distance in the solid-state.

Four cationic iridium(III) complexes of the form [Ir(C^N)2(N^N)]+ bearing either a 2,5-dipyridylpyrazine (2,5-dpp) or a 2,2′:5′,2′′-terpyridine (2,5-tpy) ancillary ligand and either 2-phenylpyridine (ppy) or a 2-(2,4-difluorophenyl)-5-methylpyridine (dFMeppy) cyclometalating ligands were synthesized. The optoelectronic properties of all complexes have been fully characterized by UV-visible absorption, cyclic voltammetry and emission spectroscopy. The conclusions drawn from these studies have been corroborated by DFT and TDDFT calculations. The four complexes were assessed as emitters in light-emitting electrochemical cells. Complex 1a, [Ir(ppy)2(2,5-dpp)]PF6, was found to be a deep red emitter (666 nm) both in acetonitrile solution and in the electroluminescent device. Complex 2a, [Ir(ppy)2(2,5-tpy)]PF6 was found to be an orange emitter (604 nm) both in solution and in the LEEC. LEECs incorporating both of these complexes were stable over the course of around 4–6 hours. Complex 1b, [Ir(dFMeppy)2(2,5-dpp)]PF6, was also determined to emit in the orange (605 nm) but with a photoluminescent quantum yield (ΦPL) double that of 2a. Complex 2b, [Ir(dFMeppy)2(2,5-tpy)]PF6 is an extremely bright green emitter (544 nm, 93%). All four complexes exhibited quasireversible electrochemistry and all four complexes phosphoresce from a mixed charge-transfer excited state.

Combining a sterically bulky, electron-deficient 2-(2,4-difluorophenyl)-4-(2,4,6-trimethylphenyl)pyridine (dFMesppy) cyclometalating C∧N ligand with an electron rich, highly rigidified 1,1′-(α,α′-o-xylylene)-2,2′-biimidazole (o-xylbiim) N∧N ligand gives an iridium complex, [Ir(dFMesppy)2(o-xylbiim)](PF6), that achieves extraordinarily bright blue emission (ΦPL = 90%; λmax = 459 nm in MeCN) for a cationic iridium complex. This complex is compared with two reference complexes bearing 4,4′-di-tert-butyl-2,2′-bipyridine, and solution-processed organic light emitting diodes (OLEDs) have been fabricated from these materials.

Four cationic iridium(III) complexes of the form [Ir(C^N)2(N^N)]+ bearing either a 2,5-dipyridylpyrazine (2,5-dpp) or a 2,2′:5′,2′′-terpyridine (2,5-tpy) ancillary ligand and either 2-phenylpyridine (ppy) or a 2-(2,4-difluorophenyl)-5-methylpyridine (dFMeppy) cyclometalating ligands were synthesized. The optoelectronic properties of all complexes have been fully characterized by UV-visible absorption, cyclic voltammetry and emission spectroscopy. The conclusions drawn from these studies have been corroborated by DFT and TDDFT calculations. The four complexes were assessed as emitters in light-emitting electrochemical cells. Complex 1a, [Ir(ppy)2(2,5-dpp)]PF6, was found to be a deep red emitter (666 nm) both in acetonitrile solution and in the electroluminescent device. Complex 2a, [Ir(ppy)2(2,5-tpy)]PF6 was found to be an orange emitter (604 nm) both in solution and in the LEEC. LEECs incorporating both of these complexes were stable over the course of around 4–6 hours. Complex 1b, [Ir(dFMeppy)2(2,5-dpp)]PF6, was also determined to emit in the orange (605 nm) but with a photoluminescent quantum yield (ΦPL) double that of 2a. Complex 2b, [Ir(dFMeppy)2(2,5-tpy)]PF6 is an extremely bright green emitter (544 nm, 93%). All four complexes exhibited quasireversible electrochemistry and all four complexes phosphoresce from a mixed charge-transfer excited state.

There is presently a lack of efficient and stable blue emitters for light-emitting electrochemical cells (LEECs), which limits the development of white light emitting systems for lighting. Cyclometalated iridium complexes as blue emitters tend to show low photoluminescence efficiency due to significant ligand-centred character of the radiative transition. The most common strategy to blue-shift the emission is to use fluorine substituents on the cyclometalating ligand, such as 2,4-difluorophenylpyridine, dFppy, which has been shown to decrease the stability of the emitter in operating devices. Herein we report a series of four new charged cyclometalated iridium complexes using methoxy- and methyl-substituted 2,3′-bipyridine as the main ligands. The combination of donor groups and the use of a cyclometalated pyridine has been recently reported for neutral complexes and found electronically equivalent to dFppy. We describe the photophysical and electrochemical properties of the complexes in solution and use DFT and TDDFT calculations to gain insights into their properties. The complexes exhibit bluish-green emission with onsets around 450 nm, which correspond to the maximum emission at 77 K. Furthermore, photoluminescence quantum yields in solution are all above 40%, with the brightest in the series at 66%. Finally, LEECs were prepared using these complexes as the emissive material to evaluate the performance of this particular design. Compared to previously reported devices with fluorine-containing emitters, the emitted colours are slightly red-shifted due to methyl substituents on the coordinating pyridine of the main ligand and overall device performances, unfortunately including the stability of devices, are similar to those previously reported. Interestingly within the series of complexes there appears to be a positive effect of the methoxy-substituents on the stability of the devices. The poor stability is therefore attributed to the combination of cyclometalated pyridine and methoxy groups.

In this study, a series of four formyl-substituted chloro-bridged iridium(III) dimers were prepared. Their absorption, photophysical and electrochemical properties were studied in dichloromethane solution. It was found that as the formyl content increased on the cyclometalating ligands, emission unexpectedly became brighter. Organic light-emitting diodes (OLEDs) were fabricated using each of these iridium dimers as the emitter. The OLED fabricated using the brightest of the series, 2b, as the dopant afforded a decent external quantum efficiency (EQE) of 2.6%. This suggests that chloro-bridged iridium dimers are potential candidates as solid-state emitters.

We report the first examples of highly luminescent di-coordinated Pd(0) complexes. Five complexes of the form [Pd(L)(L′)] were synthesized, where L = IPr, SIPr or IPr* NHC ligands and L′ = PCy3, or IPr and SIPr NHC ligands. The photophysical properties of these complexes were determined in degassed toluene solution and in the solid state and contrasted to the poorly luminescent reference complex [Pd(IPr)(PPh3)]. Organic light-emitting diodes were successfully fabricated but attained external quantum efficiencies of between 0.3 and 0.7%.

This perspective illustrates our approach in the design of heteroleptic cationic iridium(III) complexes for optoelectronic applications, especially as emitters in electroluminescent devices. We discuss changes in the photophysical properties of the complexes as a consequence of modification of the electronics of either the cyclometalating (C^N) or the ancillary (N^N) ligands. We then broach the impact on these properties as a function of modification of the structure of both types of ligands. We explain trends in the optoelectronic behaviour of the complexes using a combination of rationally designed structure–property relationship studies and theoretical modelling that serves to inform subsequent ligand design. However, we have found cases where the design paradigms do not always hold true. Nevertheless, all these studies contribute to the lessons we have learned in the design of heteroleptic cationic phosphorescent iridium(III) complexes.

We present the first examples of dynamic supramolecular systems composed of cyclometalated Ir(III) complexes of the form of [Ir(C^N)2(N^N)]PF6 (where C^N is mesppy = 2-phenyl-4-mesitylpyridinato and dFmesppy = 2-(4,6-difluorophenyl)-4-mesitylpyridinato and N^N is 4,4′:2′,2′′:4′′,4′′′-quaterpyridine, qpy) and zinc tetraphenylporphyrin (ZnTPP), assembled through non-covalent interactions between the distal pyridine moieties of the qpy ligand located on the iridium complex and the zinc of the ZnTPP. The assemblies have been comprehensively characterized by a series of analytical techniques (1H NMR titration experiments, 2D COSY and HETCOR NMR spectra and low temperature 1H NMR spectroscopy) and the crystal structures have been elucidated by X-ray diffraction. The optoelectronic properties of the assemblies and the electronic interaction between the iridium and porphyrin chromophoric units have been explored with detailed photophysical measurements, supported by time-dependent density functional theory (TD-DFT) calculations.

A new class of cationic iridium(III) complexes of the form [(C∧N)2Ir(N∧N)][PF6] is reported, where C∧N = cyclometallating 2-phenylpyridinato, ppy, or 2-(2,4-difluorophenyl)-5′-methylpyridinato, dFMeppy, and N∧N = guanidyl-pyridine, gpy, or -pyrazine, gpz, as the ancillary ligand. A large blue-shift in the emission coupled with a 7-to-9 fold enhancement in photoluminescence quantum yield and microsecond emission lifetimes were observed for the complexes containing the partially saturated gpy ligand as compared to the benchmark complex [(ppy)2Ir(bpy)][PF6], C1, where bpy is 2,2′-bipyridine.

Two deep blue thermally activated delayed fluorescence (TADF) emitters (imCzDPS and imDPADPS) that contain charged imidazolium groups tethered to the central luminophore were designed and synthesized as small molecule organic emitters for light-emitting electrochemical cell (LEEC) electroluminescent devices. The emission profile of the doped thin films (5 wt% in PMMA) is very blue and narrow (λPL: 414 nm and 409 nm; full width at half maximum (FHWM): 62 nm and 46 nm for imCzDPS and imDPADPS, respectively) with good photoluminescence quantum efficiencies (ΦPL: 44% and 49% for imCzDPS and imDPADPS, respectively). In neat films, emission maxima occur at 440 nm and 428 nm for imCzDPS and imDPADPS, respectively, with comparable ΦPL values of 44 and 61%, respectively. Both emitters exhibit biexponential emission decay kinetics (nanosecond prompt and microsecond delayed fluorescence) in both MeCN solution and thin films, characteristic of TADF behaviour. While imDPADPS did not show any emission in the LEEC device, that of imCzDPS gave an electroluminescence (EL) maximum at 470 nm and CIE coordinates of (0.208, 0.250), which makes this device amongst the bluest reported to date. However, the maximum device luminance achieved was 2.5 cd m−2 and this poor brightness was attributed to the electrochemical instability of the emitter in the LEEC architecture, as evidenced by the additional peak at around 550 nm observed in the EL spectrum.

We present dynamic supramolecular systems composed of a Ru(II) complex of the form of [Ru(dtBubpy)2(qpy)][PF6]2 (where dtBubpy is 4,4′-di-tert-butyl-2,2′-dipyridyl and qpy is 4,4′:2′,2′′:4′′,4′′′-quaterpyridine) and zinc tetraphenylporphyrins (ZnTPP), through non-covalent interactions between the distal pyridine moieties of the qpy ligand and the zinc of ZnTPP. The optoelectronic properties of the assemblies and the electronic interactions between the chromophoric units have been comprehensively characterized by computational investigations, and steady-state and time-resolved emission spectroscopy. Upon photoexcitation of ZnTPP, electron transfer to the ruthenium center is thermodynamically favorable and, as a result, strong emission quenching of both units occurs.

Herein we present a structure–property relationship study of thirteen cationic iridium(III) complexes of the form of [Ir(C^N)2(P^P)]PF6 in both solution and the solid state through systematic evaluation of six bisphosphine (P^P) ligands (xantphos, dpephos, dppe, Dppe, nixantphos and isopropxantphos). All of the complexes are sky-blue emissive, but their photoluminescence quantum yields (ΦPL) are generally low. However, strong and long-lived blue luminescence (λem = 471 nm; ΦPL = 52%; τe = 13.5 μs) can be obtained by combining the reduced bite angle of the 1,2-bis-diphenylphosphinoethene (dppe) chelate with the bulky 2-(4,6-difluorophenyl)-4-mesitylpyridinato (dFmesppy) cyclometalating ligand. To the best of our knowledge this is the highest ΦPL and the longest τe reported for cyclometalated iridium(III) complexes bearing bisphosphine ligands. Light-emitting electrochemical cells (LEECs) were fabricated using lead complexes from this study, however due in part to the irreversible electrochemistry, no functional LEEC was achieved. Organic light-emitting diodes were successfully fabricated but only attained maximum external quantum efficiencies of 0.25%.

We show that the emission efficiency of blue-green phosphorescent emitters can be controlled through coupling of the excited state to vibrational modes. We controlled this vibrational coupling through choice of different ligands and as a result, complexes with CF3-groups on the ancillary ligand were essentially non-emissive (ΦPL < 1%), whereas with isosteric CH3-groups the complexes were strongly emissive (ΦPL > 50%). Emission of the complexes can be drastically improved (30 times higher ΦPL compared to degassed solution for the CF3-containing complexes) by blending them with an inert solid host such as PMMA, which mitigates metal-ligand vibrations. Solution-processed organic light-emitting diodes made from these materials showed efficiency as high as 6.3%.