Cyclometalated Ligand Research Articles

Tris-homoleptic cyclometalated Ir(III) complexes based on oligothiophene-derived ligands as electron donors for organic solar cells

Herein, two octahedral homoleptic tris-Ir(III) complexes namely Py3T3Ir and Py5T3Ir are designed from pyridine/oligothiophene hybridized cyclometalated ligands with different conjugation lengths. Py5T3Ir with enlarged conjugation shows bathochromic absorption, strengthened absorptivity in longer-wavelength region and lower-lying energy levels compared to Py3T3Ir. Moreover, Py3T3Ir bearing small-sized backbones presents stronger triplet characteristic with boosted phosphorescence and prolonged exciton lifetimes. Blending ZY-4Cl acceptor to construct all-small-molecule organic solar cells, Py5T3Ir performs evidently increased power conversion efficiency of 8.62% than 3.04% of Py3T3Ir. The pronounced improvement in photovoltaic performance is originated from the advantageous carrier dynamics and superior blend morphology. Our results indicate coordinated Ir complexes are expected as potential photovoltaic materials by judicious molecular design.

A theoretical study on the electronic structures and phosphorescence properties of light-emitting electrochemical cell champions reported in early 2022

The electronic structure, absorption properties, phosphorescence quantum efficiency and emission spectra, of a series of iridium(III) complexes containing phenyltriazole-type cyclometallated ligands (1–4) were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The simulated electronic/photophysical properties of the iridium complexes are in good agreement with the experimental observations. The results of the calculations show that the lowest lying singlet absorptions for 1–4 are located at 3.83, 3.91, 3.01, and 2.98 eV, respectively. All complexes show emissions characterised as a mixture of triplet ligand-to-ligand charge transfer (3LLCT) and metal-to-ligand charge transfer (3MLCT) states. The fluorination of the attached phenyl groups (complex 2) and the introduction of –CF3 moieties (complex 3) in the phenyltriazole cyclometallated ligands results in a strong interaction in the T1 states, due to the contracted Ir–Ns bonds with the bpy ancillary ligand. Therefore, both complexes show a moderate contribution of the metal character (%Mc), a lower energy gap between the T1 and the S1 states (ΔET1-S1) together with the largest transition dipole moment (μS1), resulting in an enhanced phosphorescence quantum efficiency. The introduction of –CH3 groups (complex 4) in the phenyltriazole cyclometallated ligands stabilises the triplet metal-centred (3MC) state, resulting in a fast nonradiative decay along with a lower quantum yield. The vertical emission energies calculated at the B3LYP level for the 1–4 complexes are located at 2.60, 2.38, 2.68, and 2.62 eV, respectively. Based on these results, the theoretical calculations can provide an accurate prediction of the electronic and the photophysical properties of iridium complexes, and then can be used to guide the synthesis of new phosphorescent materials.

Impact of ligand modification on the photovoltaic performance of cyclometalated Ir(III)-complexes based organic solar cells

In this work, systematical comparisons are performed between two Ir-complex based electron donor materials. Compared to the control TBzIr with alkylterthiophene coupled benzo[d]thiazole as cyclometalated ligand, the new-developed TBzRIr is designed by attaching additional strong electron-accepting dicyanorhodanine endcaps to the main ligands. The enlarged ligand conjugation length and amplified intraligand donor-acceptor interactions induces TBzRIr to show less efficient spin-orbital coupling with obviously weakened triplet emission and shorter triplet lifetimes than TBzIr. Notably, TBzRIr demonstrates beneficial physical parameters for organic solar cells with deeper energy levels, remarkably enhanced light-harvest capacity and visibly increased neat-film hole mobility than TBzIr. However, TBzRIr conversely yields inferior device efficiency than TBzIr in the corresponding organic solar cells, with sharply decreased power conversion efficiency from 5.36% for TBzIr:Y6 to 3.05% for TBzRIr:Y6. Besides on the influence of improved film morphologies for TBzIr:Y6, the benefits of stronger triplet feature of TBzIr is also expected to facilitate exciton dissociation, suppress charge recombination, and thus to enhance photovoltaic performance.

Enhanced Deep-Red Phosphorescence in Cyclometalated Iridium Complexes with Quinoline-Based Ancillary Ligands.

Compounds with good photoluminescence quantum yields (ΦPL) in the deep-red to near-infrared parts of the spectrum are desired for a variety of applications in optoelectronics, imaging, and sensing. However, in this region of the spectrum, quantum yields are usually modest, which is explained by the energy gap law and the inherently slower radiative decay rates for low-energy emitters according to the second-order perturbation theory. In this work, we outline a new direction in deep-red luminescence, introducing a new suite of bis-cyclometalated iridium complexes with efficient luminescence beyond 650 nm. Seven new complexes are prepared using two different cyclometalating (C^N) ligands with four quinoline-derived ancillary ligands (L^X). The chosen cyclometalating ligands are well-established to produce deep-red phosphorescence and include a metalated phenyl ring appended to a conjugated heterocycle. The ancillary ligands combine a rigid quinoline or benzoquinoline "L" donor with a variable anionic "X" donor comprised of an O-donor aryloxy or carboxylate or an N-donor amidate. These complexes phosphoresce in the deep-red region with wavelengths between 650 and 700 nm and solution quantum yields between 0.018 and 0.42.

Luminescent bis‐alkynylgold(III) complexes with bidentate cyclometalating ligands: A comparison study with related tridentate cyclometalated complexes

AbstractBackgroundThere has been a continuous interest in the development of luminescent gold(III) complexes as emitters for OLED applications in recent years.ObjectiveTo design and synthesize a series of luminescent bis‐alkynylgold(III) complexes with bidentate C^N‐type ligands and to compare their properties with related tridentate cyclometalated complexes.MethodsA series of bis‐alkynylgold(III) complexes with bidentate C^N‐type ligands derived from 2‐phenylpyridine and 7,8‐benzoquinoline has been designed and synthesized. The photophysical properties of the complexes have been studied by electronic absorption and emission spectroscopies.ResultsAll of the complexes were observed to exhibit rich photophysical and electrochemical properties. In particular, the complexes were strongly emissive at ambient conditions, showing vibronic‐structured emission bands peaked at 433–557 nm in dichloromethane solutions at 298 K.ConclusionsBy comparing the structural features, luminescence properties and electrochemistry of the bidentate complexes with related tridentate C^N^C and C^N^N‐type alkynylgold(III) complexes, a better understanding on the structure‐property relationship of the alkynylgold(III) system has been obtained.

Photoluminescent iridium(III) complexes with phenylvinylpyridine-base cyclometalating ligands: Synthesis, photophysical properties, and photocyclization

Photoresponsive materials have recently garnered significant research interest and have been designed using various molecules. A viable approach to develop photoresponsive materials is by integrating photoisomeric structures into the organic ligands of phosphorescent transition metal complexes. In this study, we designed two photoresponsive cyclometalating ligands, 2-phenyl-3-vinylpyridine (HL1) and trans-4-methyl-2-phenyl-3-styrylpyridine (HL2), via the replacement of a phenyl group with a pyridyl group in vinylbiphenyl. Upon photoirradiation under an argon atmosphere, both ligands underwent photocyclization and demonstrated hypochromism and bathochromic shifts in absorption. Notably, HL2 underwent additional oxidative dehydrogenation upon photoirradiation in the presence of oxygen, resulting in an elongated conjugation structure that exhibited significant fluorescence intensity increase at 363 nm. Additionally, three photoresponsive iridium(III) complexes cyclometalated with HL1 or HL2 were synthesized and characterized. The photocyclization process of the complexes was much faster than that of the free ligands because the phenylpyridine moiety became coplanar upon cyclometalation. Upon photoirradiation under an argon atmosphere, these complexes exhibited significant luminescence enhancement by about 10–20 folds because of photocyclization resulting in a rigid closed structure that limited nonradiative decay of the excited complexes. These molecules present a unique strategy to construct photocyclization complexes and provide important insights into the development of photoresponsive materials.

Deep Red Emitting Heteroleptic Ir(III) Complexes that Incorporate Unsymmetrical 4‐quinoline Carboxylic Acid Derived Ligands

AbstractSix disubstituted ligands based upon 2‐(2′‐pyridinyl/pyrazinyl)quinoline‐4‐carboxylic acids have been synthesised, solvent‐free, in one step from a range of commercially available isatin derivatives. These species behave as ancillary chelating ligands for Ir(III) complexes of the form [Ir(C^N)2(N^N)]PF6 (where C^N=cyclometalating ligand; N^N=2‐(2′‐pyridinyl/pyrazinyl)quinoline‐4‐carboxylic acids). An X‐ray crystallographic study on one complex shows a distorted octahedral geometry wherein a cis‐C,C and trans‐N,N coordination mode is observed for the cyclometalating ligands. DFT calculations predicted that variations in N^N ligand from 2,2′‐bipyridine to L1–6 should localise the LUMO on to the Ln ligand and that the complexes are predicted to display MLCT/LLCT character. All complexes displayed luminescence in the deep red part of the visible region (674–679 nm) and emit from triplet states, but with little apparent tuning as a function of L1–6. Further time‐resolved transient absorption spectroscopy supports the participation of these triplet states to the excited state character.

Development of Aldehyde Functionalized Iridium(III) Complexes Photosensitizers with Strong Visible-Light Absorption for Photocatalytic Hydrogen Generation from Water

Four iridium(III) dyes functionalized with aldehyde functional group in the cyclometalating (C^N) ligands, bearing either diethyl [2,2′-bipyridine]-4,4′-dicarboxylate or tetraethyl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) anchoring groups, coded as Ir1–Ir4, are synthesized and explored as photosensitizers. The synthetic route is described and all of the complexes are characterized with respect to their electrochemical and photophysical properties. Density functional theory (DFT) calculation was used to gain insight into the factors responsible for the photocatalytic properties of Ir1–Ir4 as effective photosensitizers for photocatalytic hydrogen generation. Relative to common iridium(III) dyes, such as [Ir(ppy)2(dcbpy)]+ (ppy = 2-phenylpyridine), the absorption spectra of our dyes are broader, which is attributed to the extended π-conjugation in their C^N ligands. All of the new iridium(III) dyes were used as photosensitizers for visible-light driven hydrogen production by attaching to platinized TiO2 nanoparticles (Pt–TiO2) in the presence of sacrificial electron donor (SED) of ascorbic acid (AA) in a purely aqueous solution. A H2 turnover number (TON) up to 5809 was demonstrated for 280 h irradiation. Complexes with tetraethyl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) anchoring groups were found to outperform those with classical diethyl [2,2′-bipyridine]-4,4′-dicarboxylate, which may be one of the important steps in developing high-efficiency iridium(III) photosensitizers in water splitting hydrogen generation.

Iridium(III) complex of fluorinated cyclometalating ligands and imidazolium-bipyridine as an effective lifetime based phosphates sensor

A new bis-heteroleptic cyclometallated Ir(III) complex, Ir-1 (C46H36F28IrN8P3), containing [3,5- difluoro-2-(5-trifluoromethyl-2-pyridinyl)phenyl] [dF(CF3)ppy] and 3,3′-([2,2′-bipyridine]-5,5′-diylbis(methylene))bis(1-ethyl-1H-imidazol-3-ium) has been synthesized and the solution structure was fully characterized by ESI-MS, NMR, UV–vis spectroscopy, and single crystal X-ray diffraction analysis. The fluorinated complex Ir-1 acted as a highly selective luminescent lifetime-based sensor for phosphates in CH3CN over other competitive anions. It has shown superiority in sensing phosphates as compared to previously reported non-fluorinated analog Ir-2 in CH3CN in terms of lower detection limit, enhanced binding constant, and longer excited state lifetimes. The implementation of [dF(CF3)ppy] ligand instead of 2-phenylpyridine (ppy) has improved the photoluminescence properties of Ir-1 as compared to Ir-2. Further, 1H NMR titration experiment established that the binding with these anions occurred exclusively through solitary neutral CH⋯anion hydrogen-bonding interactions.

Design, Synthesis and Photophysical Studies of Luminescent Rhodium(III) Complexes in Near‐Infrared Region

AbstractA series of near‐infrared (NIR)‐emitting cyclometalated rhodium(III) complexes have been designed, synthesized and characterized. The NIR luminescence has been realized by rational design of strong σ‐donor cyclometalating (C^N) ligand with extended π‐conjugation structure for decreasing the energy level of intraligand (IL) state. In addition, the investigation of substituents on the benzo[g]quinoxaline moiety as the carbon‐donor demonstrated that the luminescence can be further shifted to the red by the introduction of electron‐donating thienyl groups. The luminescence maxima of these complexes are ranging from 763 nm to 792 nm with the luminescence quantum yield (Φlum) of 0.41 %–0.66 % in dichloromethane solution. This work demonstrates the first example of NIR‐phosphorescent rhodium(III) complexes and provides an alternative for diversifying the development of NIR materials.

Cathodic co-reactant electrogenerated chemiluminescence of water-soluble heteroleptic iridium(III) complexes bearing N-methyl(pyridyl)pyridinium cyclometalating ligands

Heteroleptic iridium(III) complexes containing N-methyl-(2′-pyridyl)pyridinium cyclometalating ligands were examined as luminophores for cathodic electrogenerated chemiluminescence (ECL) with peroxydisulfate co-reactant. These water-soluble iridium(III) complexes exhibit reduction potentials that are far less negative than those of the archetypal tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) luminophore. As observed with tris(2,2′-bipyrazine)ruthenium(II) ([Ru(bpz)3]2+), these shifts in potential are favourable for cathodic ECL because they lower the oxidative quenching of the excited state, and improve the stability of the reduced metal complex in aqueous solvent. The ECL intensities increased with repeated scanning or pulsing of the applied potential, due to the deposition of the less-soluble reduced species at the electrode surface. Iridium(III) complex ‘Ir4’, which contains cyclometalating ligands derived from the 1-methyl-4-(2ʹ-pyridyl)pyridinium cation, produces ECL with peroxydisulfate co-reactant in aqueous solution that is far more intense than that of [Ru(bpy)3]2+, and comparable to [Ru(bpz)3]2+, but in contrast to the red emissions of the conventional ruthenium(II) complexes, the iridium(III) species exhibits green luminescence, creating opportunities for novel spectrally and potentially resolved ECL systems.

A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes

A novel 1,2-azaborine (i.e., 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, 1a) has been synthesized and used for the first time as a B-N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (i.e., 2a, 3, and 4) of general formula [Ir(C∧N)2(N∧NB)], where C∧N indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex 2a was compared to the isoelectronic C═C iridium(III) complex 2b, obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand 1b. Due to the dual cyclometalation mode of such C═C ligand, the isomeric complex 2c was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of 2a was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (e.g., λmax = 523 vs 577 nm for 2a vs 2b, in acetonitrile solution at 298 K). On the other hand, the 3LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its C═C pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical C═C counterparts (e.g., photoluminescence quantum yield (PLQY) = 16 vs 22% for 2a vs 2b, in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.

Spin-vibronic mechanism at work in a luminescent square-planar cyclometalated tridentate Pt(II) complex: absorption and ultrafast photophysics.

The absorption spectrum of [Pt(dpybMe)Cl] (dpyb = 2,6-di-(2-pyridyl)benzene), representative of luminescent halide-substituted tridentate cyclometalated square planar Pt(II) neutral complexes, has been revisited by means of non-adiabatic wavepacket quantum dynamics. The early photophysics has been investigated on the basis of four singlet and five triplet excited states, namely nineteen "spin-orbit states", coupled with both vibronic and spin-orbit couplings, and includes eighteen normal modes. It is shown that in-plane scissoring and rocking normal modes of the cyclometalated tridentate ligand are responsible for the vibronic structure observed at around 400 nm in the experimental spectrum of the complex. The ultrafast decay of [Pt(dpybMe)Cl], within 1 ps, follows a spin-vibronic mechanism governed by excited state electronic characters, spin-orbit, and active tuning mode interplay. Both spin-orbit coupling and Pt(II) coordination sphere stretching modes and in-plane scissoring/rocking of the cyclometalated ligand activate the ultrafast decay within 20 fs of absorption. At longer time-scales (>100 fs) an asynchronous stretching of the Pt-C and Pt-N bonds activates the depopulation of the upper "reservoir" electronic states to populate the two lowest luminescent T1 and T2 electronic states. The in-plane rocking motion of the ligand controls the T1/T2 population exchange which is equilibrated at about 1 ps. Stabilization of the upper non-radiative metal-centered (MC) states by out-of-plane ligand distortion of low frequency is not competitive with the ultrafast spin-vibronic mechanism discovered here for [Pt(dpybMe)Cl]. Modifying the Pt-C covalent bond position and rigidifying the cyclometalated ligand will have a dramatic influence on the spin-vibronic mechanism and consequently on the luminescence properties of this class of molecules.

Controlling emission energy in metal–organic frameworks featuring cyclometalated iridium(III) linkers

Efficient solid-state luminescent materials are critical components in varied optoelectronic devices. Here, we report three metal–organic frameworks combining calcium(II) with heteroleptic iridium(III) metalloligands containing the same 1,2-diimine ancillary ligand but different cyclometalating ligands. The synthesised frameworks exhibit similar supramolecular structure but different emission properties depending on the cyclometalating ligand. Binding calcium(II) to the metalloligands further affects framework emission depending on the relative contribution of triplet charge transfer (3MLLCT) or ligand-centred (3LC) transitions to the emissive excited state.

Tailoring the π-system of benzimidazole ligands towards stable light-harvesting cyclometalated iridium(III) complexes.

The synthesis, structure, optical and redox properties as well as photovoltaic studies of iridium(III) complexes with cyclometalated 2-arylbenzimidazoles decorated with various polyaromatic fragments and an ancillary aromatic β-diketone are reported. Despite the strong preference of the iridium(III) ion to form bis- or tris-cyclometalated complexes in which the metal participates in five-membered metallacycles, the cyclometalation of the benzimidazole ligands containing rigid π-extended systems yields dimeric complexes containing strained five- or six-membered metallacycles and allows for generating an extremely rare monocyclometalated complex. X-ray crystallography shows that the steric strain observed in the dimers is retained in heteroleptic diketonate complexes which is also corroborated by gas-phase DFT calculations. While emission maxima and redox potentials of the heteroleptic complexes exhibit just a moderate variation upon the change of the cyclometalated ligands, the extension of the π-system of the benzimidazole ligands give the complexes remarkable light absorption in the visible spectral range, which meets the requirements for application in dye-sensitized solar cells. At the titania photoanodes, these iridium dyes retain their optical properties and exhibit power conversion efficiencies under standard AM 1.5 G conditions comparable to those of other iridium-based sensitizers. These results demonstrate that the size and position of the π-extended fragment in cyclometalated ligands can modulate not only the electronic structure of the corresponding iridium(III) complexes, but also affect their composition, structure and reactivity that may find implications in future design of emerging iridium dyes, emitters and catalysts.

AIE-active cyclometalated iridium(III) complexes for the detection of lipopolysaccharides and wash-free imaging of bacteria.

Infectious diseases caused by bacteria pose a major threat to human health and are currently one of the leading causes of mortality worldwide. Therefore, the development of probes for rapid detection of bacteria and their pathogenic components is highly essential. Aggregation-induced emission (AIE)-active compounds have shown great promise for the diagnosis of bacterial infections. In this study, we have synthesized three cationic AIE-active cyclometalated iridium(III) polypyridyl complexes viz., [Ir(C^N)2(N^N)]Cl2 (Ir1-Ir3), where C^N is a cyclometalating ligand such as pq = 2-phenylquinoline (in Ir1), pbt = 2-phenylbenzothiazole (in Ir2), and dfppy = 2-(2,4-difluorophenyl)pyridine (in Ir3), and N^N is a 2,2'-bipyridine derivative, for detection of lipopolysaccharide (LPS) in aqueous solution and wash-free imaging of bacteria. These complexes exhibit rapid sensing of LPS also known as endotoxin released by the bacteria, with a detection limit in the nanomolar range being determined by fluorescence spectroscopy within 5 min. Detection of both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria by the complexes is visible to the naked eye and was also observed by fluorescence microscopy imaging. The above features of the complexes make them a promising scaffold for the detection of bacterial contamination in aqueous samples.

2-(Thienyl)quinoxaline derivatives and their application in Ir(III) complexes yielding tuneable deep red emitters.

The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality, including at the thiophene or quinoxaline rings, are described. These species display variable fluorescence properties in the visible region (λem = 401-491 nm) depending upon the molecular structures and extent of conjugation. The series of 2-(thienyl)quinoxaline species were then investigated as cyclometalating agents for Ir(III) to yield [Ir(C^N)2(bipy)]PF6 (where C^N = the cyclometalated ligand; bipy = 2,2'-bipyridine). Eight complexes were successfully isolated and fully characterised by an array of spectroscopic and analytical techniques. Two Ir(III) examples were structurally characterised in the solid state using single crystal X-ray diffraction; both structures confirmed the proposed formulations and coordination spheres in each case showing that the thiophene coordinates via a Ir-C bond. The photophysical properties of the complexes revealed that each complex is luminescent under ambient conditions with a range of emission wavelengths observed (665-751 nm) indicating that electronic tuning can be achieved via both the thienyl and quinoxaline moieties.

Controlling the sensitivity and selectivity for the detection of nitro-based explosives by modulating the electronic substituents on the ligand of AIPE-active cyclometalated iridium(III) complexes.

Nitroaromatic compounds are extremely explosive materials that pose a national security risk and raise environmental concerns. The design and development of sensitive and selective compounds for explosive materials are highly desirable. 'Aggregation-Induced Emission' (AIE) active materials are best suited for sensing purposes because of their sensitivity, fast detection time, and easy operation. By rationally incorporating substituents on the cyclometalated (C^N) ligand, four different AIE active iridium(III) based monocyclometalated complexes with the general formula [Ir(PPh3)2(H)(Cl)(C^N)] were synthesized. The phenyl ring of the phenyl pyridine cyclometalated portion of an iridium(III) complex was substituted with the right substituents to adjust the FMO levels thus, leading to appropriate alignment of the energy levels. Each of the resulting complexes displayed a significant property known as 'Aggregation-Induced Phosphorescent Emission' (AIPE). The complexes were subjected to structural characterization, electrochemical analysis, and photophysical property studies. The synthesized complexes were employed for the detection of aromatic nitro explosive compounds such as trinitrophenol (TNP) and trinitrotoluene (TNT) in the aqueous phase with a high degree of sensitivity. The sensing capabilities of each complex were assessed for these nitro explosive compounds and compared to those of the unsubstituted iridium(III) complex (M). Notably, the best limits of detection for TNP and TNT have been achieved with iridium(III) complexes [M1 (489 pM) and M3 (3.6 nM)] within the literature reported until now. For detecting picric acid with M1, FRET was found to be the potential mechanism, and for TNT, PET was found to be the cause of emission quenching by M3. Furthermore, for low-cost detection, filter paper-based sensing was also found effective for each complex. Real-field sensing of PA in soil samples was also performed.

Organometallic Platinum(II) Photosensitisers that Demonstrate Ligand-Modulated Triplet-Triplet Annihilation Energy Upconversion Efficiencies.

A series of 2-phenylquinoxaline ligands have been synthesised that introduce either CF3 or OCF3 electron-withdrawing groups at different positions of the phenyl ring. These ligands were investigated as cyclometalating reagents for platinum(II) to give neutral complexes of the form [Pt(C^N)(acac)] (in which C^N=cyclometalating ligand; acac=acetyl acetonate). X-ray crystallographic studies on three examples showed that the complexes adopt an approximate square planar geometry. All examples revealed strong Pt-Pt linear contacts of 3.2041(6), 3.2199(3) and 3.2586(2) Å. The highly coloured complexes display efficient visible absorption at 400-500 nm (ϵ ≈5000 M-1 cm-1 ) and orange red photoluminescent characteristics (λem =603-620 nm; Φem ≤37 %), which were subtly tuned by the ligand. Triplet emitting character was confirmed by microsecond luminescence lifetimes and the photogeneration of singlet oxygen with quantum efficiencies up to 57 %. Each complex was investigated as a photosensitiser for triplet-triplet annihilation energy upconversion using 9,10-diphenylanthracene as the annihilator species: a range of good upconversion efficiencies (ΦUC 5.9-14.1 %) were observed and shown to be strongly influenced by the ligand structure in each case.

Cationic Iridium(III) Complexes with Benzothiophene-Quinoline Ligands for Deep-Red Light-Emitting Electrochemical Cells.

Three new cationic cyclometalated iridium(III) complexes equipped with differently substituted benzo[b]thiophen-2-ylquinoline cyclometalating ligands and with a sterically demanding tert-butyl-substituted 2,2'-bipyridine ancillary ligand were synthesized and structurally characterized by NMR and X-ray diffraction techniques. To tune the electronic properties of such complexes, the quinoline moiety of the cyclometalating ligands was kept pristine or equipped with electron-withdrawing phenyl and -CF3 substituents, leading to complexes 1, 2, and 3, respectively. A complete electrochemical and photophysical investigation, supported by density functional theory calculations, permits a deep understanding of their electronic properties. The emission of all complexes arises from ligand-centered triplet states in the spectral range between 625 and 950 nm, with excited-state lifetimes between 2.10 and 6.32 μs at 298 K. The unsubstituted complex (1) exhibits the most blue-shifted emission in polymeric matrix at 298 K (λmax = 667 nm, photoluminescence quantum yield (PLQY) = 0.25 and τ = 5.32 μs). The phenyl-substituted complex (2) displays the highest photoluminescent quantum yields (up to 0.30 in polymeric matrix), while the CF3-substituted counterpart (3) shows the most red-shifted emission, peaking at approx. 720 nm, but with lower quantum yields (e.g., 0.10 in polymeric matrix at 298 K). Complexes 1 and 2 were tested in single-layer nondoped light-emitting electrochemical cells (LEECs), using a nozzle-printing technique; both devices display deep-red electroluminescence with an external quantum efficiency close to 20%.