Development of Strong Visible-Light-Absorbing Cyclometalated Iridium(III) Complexes for Robust and Efficient Light-Driven Hydrogen Production

Weak light absorption of common Ir(III) complexes (e. g., using phenylpyridine as the ligand) has hindered their applications in photocatalytic hydrogen generation from water as an efficient photosensitizer. To address this issue, a series of cyclometalated Ir(III) complexes (Ir1–Ir5), featuring different electron-donating substituents to enhance the absorptivity, have been synthesized and studied as photosensitizers (PSs) for light-driven hydrogen production from water. Ir6–Ir7 were prepared as fundamental systems for comparisons. Electron donors, including 9-phenylcarbazole, triphenylamine, 4,4′-dimethoxytriphenylamine, 4,4′-di(N-hexylcarbazole)triphenylamine moieties were introduced on 6-(thiophen-2-yl)phenanthridine-based cyclometalating (C^N) ligands to explore the donor effect on the hydrogen evolution performance of these cationic Ir(III) complexes. Remarkably, Ir4 with 4,4′-dimethoxytriphenylamine achieved the highest turn-over number (TON) of 12 300 and initial turnover frequency (TOF_i) of 394 h⁻¹, with initial activity (activity_i) of 547 000 μmol g⁻¹ h⁻¹ and initial apparent quantum yield (AQY_i) of 9.59 %, under the illumination of blue light-emitting diodes (LEDs) for 105 hours, which demonstrated a stable three-component photocatalytic system with high efficiency. The TON (based on n(H₂)/n(PSr)) in this study is the highest value reported to date among the similar photocatalytic systems using Ir(III) complexes with Pt nanoparticles as catalyst. The great potential of using triphenylamine-based Ir(III) PSs in boosting photocatalytic performance has also been shown.     

Visible light induced catalytic water reduction without an electron relay

Protons from water are reduced by a catalytic system composed of a heteroleptic iridium(III) photosensitizer [Ir(ppy)2(bpy)]+, platinum catalyst, and sacrificial reductant. The hydrogen quantum yield reaches 0.26 in this study, which proceeds via reductive quenching of the excited photosensitizer by triethanolamine. This simplified approach allows the characterization of degradation products that are otherwise obscured in more complex systems. A novel 16-well setup for parallel kinetic analysis of H2 evolution enables high-throughput screening of reaction conditions and quantization of the decaying reaction rate. DFT calculations rationalize the differences between this and previous studies on tris-diimine ruthenium(II) photosensitizers.     

Surface-immobilized single-site iridium complexes for electrocatalytic water splitting

Water into oxygen: Mono-iridium complexes (see picture; L=PO(3) H(2) or COOH) were immobilized on an indium tin oxide (ITO) surface to form a molecular electrocatalytic water oxidation assembly that mimics photosystem?II in producing molecular oxygen with high turnover numbers (TONs). The catalyst shows TONs for O(2) higher than 210?000 and turnover frequencies higher than 6.7?s(-1) during electrochemical catalytic water splitting.     

Synthetically tailored excited states: phosphorescent, cyclometalated iridium(III) complexes and their applications

Phosphorescent iridium(III) complexes are being widely explored for their utility in diverse photophysical applications. The performance of these materials in such roles depends heavily on their excited-state properties, which can be tuned through ligand and substituent effects. This concept article focuses on methods for synthetically tailoring the properties of bis-cyclometalated iridium(III) materials, and explores the factors governing the nature of their lowest excited state.     

Tricyclometalated Iridium Complexes as Highly Stable Photosensitizers for Light‐Induced Hydrogen Evolution

The development of an efficient and stable artificial photosensitizer for visible‐light‐driven hydrogen production is highly desirable. Herein, a new series of charge‐neutral, heteroleptic tricyclometalated iridium(III) complexes, [Ir(thpy)₂(bt)] ( 1 – 4 ; thpy=2,2′‐thienylpyridine, bt=2‐phenylbenzothiazole and its derivatives), were systematically synthesized and their structural, photophysical, and electrochemical properties were established. Three solid‐state structures were studied by X‐ray crystallographic analysis. This design offers the unique opportunity to drive the metal‐to‐ligand charge‐transfer (MLCT) band to longer wavelengths for these iridium complexes. We describe new molecular platforms that are based on these neutral iridium complexes for the production of hydrogen through visible‐light‐induced photocatalysis over an extended period of time in the presence of [Co(bpy)₃]²⁺ and triethanolamine (TEOA). The maximum amount of hydrogen was obtained under constant irradiation over 72 h and the system could regenerate its activity upon the addition of cobalt‐based catalysts when hydrogen evolution ceased. Our results demonstrated that the dissociation of the [Co(bpy)₃]²⁺ catalyst contributed to the loss of catalytic activity and limited the long‐term catalytic performance of the systems. The properties of the neutral complexes are compared in detail to those of two known non‐neutral bpy‐type complexes, [Ir(thpy)₂(dtb‐bpy)]⁺ ( 5 ) and [Ir(ppy)₂(dtb‐bpy)]⁺ ( 6 ; ppy=2‐phenylpyridine, dtb‐bpy=4,4′‐di‐tert‐butyl‐2,2′‐dipyridyl). This work is expected to contribute toward the development of long‐lasting solar hydrogen‐production systems.     

Charge‐Neutral Amidinate‐Containing Iridium Complexes Capable of Efficient Photocatalytic Water Reduction

Two new charge‐neutral iridium complexes, [Ir(tfm‐ppy)₂(N,N′‐diisopropyl‐benzamidinate)] ( 1 ) and [Ir(tfm‐ppy)₂(N,N′‐diisopropyl‐4‐diethylamino‐3,5‐dimethyl‐benzamidinate)] ( 2 ) (tfm‐ppy=4‐trifluoromethyl‐2‐phenylpyridine) containing an amidinate ligand and two phenylpyridine ligands were designed and characterised. The photophysical properties, electrochemical behaviours and emission quenching properties of these species were investigated. In concert with the cobalt catalyst [Co(bpy)₃]²⁺, members of this new class of iridium complexes enable the photocatalytic generation of hydrogen from mixed aqueous solutions via an oxidative quenching pathway and display long‐term photostability under constant illumination over 72 h; one of these species achieved a relatively high turnover number of 1880 during this time period. In the case of complex 1 , the three‐component homogeneous photocatalytic system proved to be more efficient than a related system containing a charged complex, [Ir(tfm‐ppy)₂(dtb‐bpy)]⁺ ( 3 , dtb‐bpy=4,4′‐di‐tert‐butyl‐2,2′‐dipyridyl). In combination with a rhodium complex as a water reduction catalyst, the performances of the systems using both complexes were also evaluated, and these systems exhibited a more efficient catalytic propensity for water splitting than did the cobalt‐based systems that have been studied previously.     

Synthesis,structure and antiproliferative activity of organometallic iridium(III) complexes containing thiosemicarbazone ligands

A series of half‐sandwich iridium complexes ( 1 – 4 ) with thiosemicarbazone ligands in two types of coordination modes were synthesized and characterized. The molecular structures of compounds 1 , 2 and 3 were determined using single‐crystal X‐ray diffraction analysis. The nature of the complexes was studied using density functional theory calculations. The stability of the complexes was investigated using UV–visible absorption spectroscopy. The compounds were further evaluated for their in vitro antiproliferative activities against HeLa, HepG2, CNE‐2, SGC‐7901, KB and HEK‐293 T cell lines. Compound 2 displays the highest antiproliferative activity among the other analogues and cisplatin.     

Photocatalytic hydrogen generation from water with iron carbonyl phosphine complexes: improved water reduction catalysts and mechanistic insights

An extended study of a novel visible-light-driven water reduction system containing an iridium photosensitizer, an in situ iron(0) phosphine water reduction catalyst (WRC), and triethylamine as sacrificial reductant is described. The influences of solvent composition, ligand, ligand-to-metal ratio, and pH were studied. The use of monodentate phosphine ligands led to improved activity of the WRC. By applying a WRC generated in situ from Fe(3) (CO)(12) and tris[3,5-bis(trifluoromethyl)phenyl]phosphine (P[C(6)H(3)(CF(3))(2)](3), Fe(3)(CO)(12)/PR(3)=1:1.5), a catalyst turnover number of more than 1500 was obtained, which constitutes the highest activity reported for any Fe WRC. The maximum incident photon to hydrogen efficiency obtained was 13.4% (440 nm). It is demonstrated that the evolved H(2) flow (0.23 mmol H(2) h(-1) mg(-1) Fe(3)(CO)(12)) is sufficient to be used in polymer electrolyte membrane fuel cells, which generate electricity directly from water with visible light. Mechanistic studies by NMR spectroscopy, in situ IR spectroscopy, and DFT calculations allow for an improved understanding of the mechanism. With respect to the Fe WRC, the complex [HNEt(3)](+)[HFe(3)(CO)(11)](-) was identified as the key intermediate during the catalytic cycle, which led to light-driven hydrogen generation from water.     

Synthesis, photophysical and electrochemical properties, and protein-binding studies of luminescent cyclometalated iridium(III) bipyridine estradiol conjugates

A new series of luminescent cyclometalated iridium(III) bipyridine estradiol conjugates [Ir(N-C)2(N-N)](PF6) (N-N = 5-(4-(17alpha-ethynylestradiolyl)phenyl)-2,2'-bipyridine, bpy-est, HN-C = 2-phenylpyridine, Hppy (1 a), 1-phenylpyrazole, Hppz (2 a), 7,8-benzoquinoline, Hbzq (3 a), 2-phenylquinoline, Hpq (4 a), 2-((1,1'-biphenyl)-4-yl)benzothiazole, Hbsb (5 a); N-N = 4-(N-(6-(4-(17alpha-ethynylestradiolyl)benzoylamino)hexyl)aminocarbonyl)-4'-methyl-2,2'-bipyridine, bpy-C6-est, HN-C = Hppy (1 b), Hppz (2 b), Hbzq (3 b), Hpq (4 b), Hbsb (5 b)) was synthesized, characterized, and their photophysical and electrochemical properties studied. Upon photoexcitation, all the complexes displayed intense and long-lived emission in fluid solutions at 298 K and in low-temperature glass. The emission of complexes 1 a-3 a and 1 b-3 b was assigned to a triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir)-->pi*(bpy-est and N-C-)) state mixed with some triplet intraligand ((3)IL) (pi-->pi*) (N-C- and N-N) character. However, the emissive states of the pq- and bsb- complexes 4 a, 4 b, 5 a, and 5 b showed substantial (3)IL (pi-->pi*) (pq-/bsb-) character. The lipophilicity of all the complexes was determined by reversed-phase HPLC. Upon binding to estrogen receptor alpha, all of these iridium(III) estradiol conjugates exhibited emission enhancement and lifetime extension, rendering them a novel series of luminescent probes for this receptor.     

Selective reduction of aqueous protons to hydrogen with a synthetic cobaloxime catalyst in the presence of atmospheric oxygen

Protons preferred: A cobalt catalyst is reported that evolves H(2) electro- and photocatalytically at room temperature, in pH-neutral water, and in the presence of atmospheric O(2) . The catalyst shows respectable Faradaic efficiencies under N(2) and 21?% O(2) in N(2) , and can be used under both homogeneous and heterogeneous conditions.     

Square-planar iridium(II) and iridium(III) amido complexes stabilized by a PNP pincer ligand

Squaring the circle: the novel dienamido pincer ligand N(CHCHPtBu(2))(2)(-) affords the isolation of the unusual square-planar iridium(II) and iridium(III) amido complexes [IrCl{N(CHCHPtBu(2))(2)}](n) (n=0 (1), +1 (2)). In contrast, the corresponding iridium(I) complex of the redox series (n=-1) is surprisingly unstable. The diamagnetism of 2 is attributed to strong N→Ir π donation.     

A triad based on an iridium(III) bisterpyridine complex leading to a charge-separated state with a 120-micros lifetime at room temperature

A triad D-Ir-A, where Ir is an Ir(III) bisterpyridine complex connected through an amidophenyl spacer to D, a triphenylamine electron donor, and to A, a naphthalene bisimide electron acceptor, has been synthesized and electrochemically investigated. The photoinduced processes in the triad, which is more than 4-nm long, have been characterized by steady-state and time-resolved optical spectroscopy by comparison with the model dyads D-Ir, Ir-A, and the reference monomers D, Ir, and A. A sequential electron transfer occurs upon excitation of the D and Ir units, leading to the charge-separated state D+-Ir- -A in 100 % yield and subsequently to D+-Ir-A- in about 10 % yield. The final charge-separated state has a lifetime at room temperature of 120 micros in air-free acetonitrile and of 100 micros in air-equilibrated solvent. Excitation of the A units does not yield intramolecular reactivity, but the resulting triplet-excited state localized on the acceptor, D-Ir-3A, displays intermolecular reactivity.     

Amido PNP complexes of iridium: Synthesis and catalytic olefin and alkyne hydrogenation

In situ lithiation of HN(o-C₆H₄PPh₂)₂ (H[ 1a ]) or HN(o-C₆H₄PiPr₂)₂ (H[ 1b ]) with nBuLi in THF at −35°C followed by addition of [Ir(μ-Cl)(COD)]₂ (COD = 1,5-cyclooctadiene) in toluene at −35°C generates 5-coordinate [ 1a ]Ir(η⁴-COD) ( 2a ) or 4-coordinate [ 1b ]Ir(η²-COD) ( 2b ), respectively. Oxidative addition of N-H in H[ 1b ] to [Ir(μ-Cl)(COD)]₂ affords square pyramidal [ 1b ]Ir(H)(Cl) ( 3b ). Metathetical reaction of 3b with LiBHEt₃ in the presence of 1 atm of H₂ in toluene produces [ 1b ]Ir(H)₂ ( 4b ). Both 2a and 4b are active for catalytic hydrogenation of olefins and alkynes under extremely mild conditions.     

Synthesis,photophysical and electrochemical properties,and biological labeling studies of cyclometalated iridium(III) bis(pyridylbenzaldehyde) complexes: novel luminescent cross-linkers for biomolecules

We report the synthesis, characterization, photophysical, and electrochemical properties of a series of luminescent cyclometalated iridium(III) complexes containing two aldehyde functional groups [Ir(pba)(2)(N-N)](PF(6)) (Hpba=4-(2-pyridyl)benzaldehyde; N-N=2,2'-bipyridine, bpy (1), 1,10-phenanthroline, phen (2), 3,4,7,8-tetramethyl-1,10-phenanthroline, 3,4,7,8-Me(4)-phen (3), 4,7-diphenyl-1,10-phenanthroline, 4,7-Ph(2)-phen (4)). The X-ray crystal structure of complex 1 has been investigated. Upon photoexcitation, complexes 1-4 exhibit intense and long-lived emission in fluid solutions at 298 K and in low-temperature glass. The luminescence is assigned to a triplet intra-ligand ((3)IL) excited state associated with the pba(-) ligand, probably with mixing of some triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir)-->pi*(pba(-))) character. Since each of these complexes possesses two aldehyde groups, which can react with the primary amine groups of biomolecules to form stable secondary amines after reductive amination, we have investigated the possibility of these complexes as novel luminescent cross-linkers for biological substrates. L-Alanine has been labeled with complexes 1-4 to give the luminescent bioconjugates 1-(Ala)(2)-4-(Ala)(2). These conjugates show strong photoluminescence with long emission lifetimes under ambient conditions. On the basis of the emission energy trend, the excited state of these luminescent bioconjugates is likely to bear a high parentage of (3)MLCT (dpi(Ir)-->pi*(N-N)) character. In addition, the glycoprotein avidin (Av) has also been conjugated with complexes 1-4 to give the bioconjugates 1-Av-4-Av. Upon photoexcitation, these bioconjugates also display intense and long-lived (3)MLCT (dpi(Ir)-->pi*(N-N)) emission in aqueous buffer at 298 K. Furthermore, a heterogeneous competitive assay for biotin has been developed using 2-Av and biotinylated microspheres. We have shown that complexes 1-4 represent a new class of multicolor luminescent cross-linkers for biomolecular species.     

Phosphorescent iridium(III) complexes with nonconjugated cyclometalated ligands

A series of blue phosphorescent iridium(III) complexes 1-4 with nonconjugated N-benzylpyrazole ligands were synthesized and their structural, electrochemical, and photophysical properties were investigated. Complexes 1-4 exhibit phosphorescence with yields of 5-45 % in degassed CH2Cl2. Of the compounds, 1 showed emission that was nearly true blue at 460 nm with a lack of vibronic progression. These photophysical data clearly demonstrate that the methylene spacer of the cyclometalated N-benzylpyrazole chelate effectively interrupts the pi conjugation upon reacting with a third L X chelating chromophore. This gives a feasible synthesis for the blue phosphorescent complexes with a sufficiently large energy gap. In another approach, these complexes were investigated for their suitability for the host material in phosphorescent OLEDs. The device was synthesized by using 1 as the host for the green-emitting [Ir(ppy)3] dopant, which exhibits an external quantum conversion efficiency (EQE) of up to 11.4 % photons per electron (and 36.6 cdA(-1)), with 1931 Commission Internationale de L'Eclairage (CIE) coordinates of (0.30, 0.59), a peak power efficiency of 21.7 lmW(-1), and a maximum brightness of 32000 cdm(-2) at 14.5 V. At the practical brightness of 100 cdm(-2), the efficiency remains above 11 % and 18 lmW(-1), demonstrating its great potential as the host material for phosphorescent organic light-emitting diodes.     

Cyclometalated iridium(III) complexes containing 2-phenyl-2H-indazole ligand: Synthesis,photophysical studies,and DFT calculations

Heteroleptic cyclometalated iridium(III) complexes ( Ir1 – Ir5 ) featuring piz-based ligands and acetylacetone ancillary ligand are synthesized and characterized. Their photophysical and electrochemical properties were studied, and DFT calculations were used to further support the experiment results. All the complexes emit yellow color with quantum yields of 12.2–56.5% in dichloromethane solution at room temperature, and the emission originates from a hybrid ³MLCT/³ILCT/³LLCT excited state.