Iridium(III) 2-Phenylbenzimidazole Complexes: Synthesis, Structure, Optical Properties, and Applications in Dye-Sensitized Solar Cells

Rational design of efficient photosensitizers based on cyclometalated iridium(III) complexes with 2-arylbenzimidazole and aromatic 1,3-diketone ligands

A judicious selection of substituents in cyclometalating 2-arylbenzimidazoles and an ancillary aromatic 1,3-diketone enabled the creation of heteroleptic iridium(III) complexes demonstrating strong light absorption up to 500 nm (ε ≈ 10 000-12 000 M-1 cm-1). The complexes, which were studied by various spectroscopic techniques, single-crystal X-ray diffraction and cyclic voltammetry, displayed tunable absorption maxima depending on the nature of substituents and their positions. The experimental study was corroborated by quantum chemical calculations, which showed an increased contribution of intraligand charge transfer transitions to the visible light absorption in the case of complexes containing electron-withdrawing substituents in the ligands. Despite being of high intensity, some of these transitions are responsible for the formation of the excited states located at large distances from the 'anchoring' fragment incorporated in the ancillary ligand. In turn, incorporation of electron-donating substituents at the para-position to the Ir-C bonds increases the number of excited states located on the ancillary ligand. The destabilization of the HOMO, which is caused by the increase in the electron-donating ability of the substituents in the metalated rings, translated into negative shifts of the Ir4+/Ir3+ redox potential, affecting, in some cases, the degree of electrochemical reversibility of the complexes. Several complexes having strong light-harvesting characteristics and undergoing reversible oxidation in the appropriate potential range were used for coating the TiO2 photoanodes, which reached an efficiency of 2.15% upon irradiation with the standard AM 1.5 spectrum.

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.

Synthesis and Crystal Structures of Halogen-Substituted 2-Aryl-N-phenylbenzimidazoles

Four 2-arylbenzimidazoles (aryl = 4-Br-phenyl (1), 3-Br-phenyl (2), 4-I-phenyl (3), 3-I-phenyl (4)) were synthesized and characterized by 1H, 13C{1H} NMR, UV–Vis spectroscopy and single-crystal X-ray diffraction. Both pairs of benzimidazoles bearing the halogen atom at the same position form isostructural crystals, in which para-substituted compounds 1 and 3 are assembled by weak C–H···π and π···π interactions while their meta-isomers 2 and 4 are linked via intermolecular halogen···nitrogen and C–H···π contacts.

Small Molecules Containing Amphoteric Imidazole Motifs as Sensitizers for Dye-Sensitized Solar Cells: An Overview

Organic dyes, porphyrins and inorganic complexes containing imidazole (IM) motifs have been demonstrated as a new class of sensitizers in dye-sensitized solar cells (DSSCs). Particularly, the amphoteric nature of IM-based motifs allows them to be used as donors (D), auxiliary donors (D_A), linker/branch (π), or acceptors (A) in D-π-A-based organic dyes and porphyrins and also employed as cyclometalated heteroleptic and ancillary ligands in the Ru(II) and Ir(III) complexes for DSSCs. It is noteworthy that the introduction of IM chromophores in the dyes of D-π-A configuration can improve the light-harvesting properties and prohibit the charge recombination reactions due to the extension of the π-conjugated structures and hydrophobic nature. Similarly, in the case of inorganic complexes, the presence of IM motifs as ligands can improve the light-harvesting ability, give facilely tuned HOMO and LUMO energy levels, increase the charge recombination resistance and photostability. This results in enhanced photocurrent (J_SC) and photovoltage (V_OC) and consequently solar-to-power conversion efficiency (η) of DSSC devices based on Ru(II) and Ir(III) complexes. Considering the interesting DSSC applications of IM-derived molecules, in this review, we therefore comprehensively discuss their photophysical, electrochemical and photovoltaic properties reported so far and establish their structure-activity relationship to further advance the η of DSSCs. To the best of our knowledge, there is no such a review interpreting the importance of molecules possessing IM-motifs for DSSC applications to date.

A Panchromatic Cyclometalated Iridium Dye Based on 2-Thienyl-Perimidine

Though 2-arylperimidines have never been used in iridium(III) chemistry, the present study on structural, electronic and optical properties of N-unsubstituted and N-methylated 2-(2-thienyl)perimidines, supported by DFT/TDDFT calculations, has shown that these ligands are promising candidates for construction of light-harvesting iridium(III) complexes. In contrast to N-H perimidine, the N-methylated ligand gave the expected cyclometalated μ-chloro-bridged iridium(III) dimer which was readily converted to a cationic heteroleptic complex with 4,4′-dicarboxy-2,2′-bipyridine. The resulting iridium(III) dye exhibited panchromatic absorption up to 1000 nm and was tested in a dye-sensitized solar cell.

Bis-Heteroleptic Cationic Iridium(III) Complexes Featuring Cyclometalating 2-Phenylbenzimidazole Ligands: A Combined Experimental and Theoretical Study

In this report, we investigate a new family of cationic iridium(III) complexes featuring the cyclometalating ligand 2-phenylbenzimidazole and ancillary ligand 4,4'-dimethyl-2,2'-bipyridine. Our benchmark complex IrL¹₂ (L¹ = 2-phenylbenzimidazole) displays emission properties similar to those of the archetypical complex 2,2'-dipyridylbis(2',4'-phenylpyridine)iridium(III) in deaerated CH₃CN (Φ = 0.20, λ_em = 584 nm and Φ = 0.14, λ_em = 585 nm, respectively) but exhibits a higher photoluminescence quantum yield in deaerated CH₂Cl₂ (Φ = 0.32, λ_em = 566 nm and Φ = 0.20, λ_em = 595 nm, respectively) and especially a lower nonradiative constant (k_nr = 6.6 × 10⁵ s^-1 vs k_nr = 1.4 × 10⁶ s^-1, respectively). As a primary investigation, we explored the influence of the introduction of electron-donating and electron-withdrawing groups on the benzimidazole moiety and the synergetic effect of the substitution of the cyclometalating phenyl moiety at the para position with the same substituents. The emission energy displays very good correlation with the Hammett constants of the introduced substituents as well as with ΔE_redox values, which allow us to ascribe the phosphorescence of these series to emanate mainly from a mixed metal/ligand to ligand charge transfer triplet excited state (³M/LLCT*). Two complexes (IrL⁵₂ and IrL⁸₂) display a switch of the lowest triplet excited state from ³M/LLCT* to ligand centered (³LC*), from the less polar CH₂Cl₂ to the more polar CH₃CN. The observed results are supported by (TD)-DFT computations considering the vibrational contributions to the electronic transitions. Chromaticity diagrams based on the maximum emission wavelength of the recorded and simulated phosphorescence spectra demonstrate the strong promise of our complexes as emitting materials, together with the very good agreement between experimental and theoretical results.

Study of a phosphorescent cationic iridium(III) complex displaying a blue-shift in crystals

We report the synthesis and the characterization of a new cationic iridium(III) complex featuring two 1-(p-methoxyphenyl)-5-methoxybenzimidazole cyclometallating ligands and a dimethylbipyridine ancillary ligand. The complex has been fully characterized by 1D and 2D NMR (¹H, ¹³C, ¹⁹F and ³¹P), elemental analysis and high-resolution mass spectrometry (HRMS). The photoluminescence studies performed in a solution, on amorphous powder and on crystals revealed an unexpected behavior. Indeed, the emission spectra observed in both solution (CH₂Cl₂) and amorphous powder samples are centered at around 580 nm, whereas in crystals the emission displays a large hypsochromic shift of ∼800 cm^-1 (λ_em = 558 nm). X-ray diffraction experiments, photophysical studies and DFT calculations allow for rationalizing the hypsochromic shift.

Synthesis and crystal structures of N-H, N-phenyl and N-benzyl-2-(4-hexyl-oxyphen-yl)benzimidazoles

The title compounds, 2-(4-hexyl-oxyphen-yl)-1H-benzimidazole (C19H22N2O; 1), 2-(4-hexyl-oxyphen-yl)-1-phenyl-1H-benzimidazole (C25H26N2O; 2) and 1-benzyl-2-(4-hexyl-oxyphen-yl)-1H-benzimidazole (C26H28N2O; 3) were synthesized and their structures were determined by single-crystal X-ray analysis. The N-substituent at the imidazole moiety slightly affects the inter-planar angle between the 4-hexyl-oxyphenyl ring and the benzimidazole system. The unsubstituted benzimidazole (1) forms inter-molecular N-H⋯N bonds while in the crystal structures of 2 and 3, the mol-ecules are assembled only through π-π and C-H⋯π inter-actions.

Sterically hindered phenanthroimidazole ligands drive the structural flexibility and facile ligand exchange in cyclometalated iridium(III) complexes

A series of bis-cyclometalated iridium(iii) complexes with 2-arylphenanthroimidazole "antenna" ligands containing electron-donor or withdrawing substituents and a more flexible ancillary aromatic β-diketone bearing the "anchoring" carboxymethyl function has been prepared. Thorough X-ray study of the complexes revealed significant structural strains caused by bulky cyclometalated 2-arylphenanthroimidazoles resulting in dramatic distortions of the iridium octahedron and even in twist of the phenanthrene fragment. The crystal data were corroborated by gas-phase DFT calculations whereby the geometry of the complexes was distorted in the same way. While redox potentials, absorption and emission maxima of the complexes displayed expected change upon the variation of the electron-donating ability of the cyclometalated ligands, the complexes readily exchanged the bidentate ancillary ligand in the presence of a negligible amount of protons that was inspected in solution by UV-Vis spectroscopy. Moreover, after hydrolysis of the carboxymethyl group the resulting complexes readily react with the surface of titanium dioxide giving unique binuclear structures in which the deprotonated carboxy group of the coordinated β-diketonate binds the second bis-cyclometalated unit by forming a four-membered metallacycle. Though the enhanced reactivity of the complexes is contrary to the common idea of the high inertness of iridium(iii) compounds it can be seen as a consequence of the interplay between the steric hindrance induced by the ligands and the strong preference of the iridium(iii) ion for octahedral geometry. This study demonstrates that the use of bulky ligands provides access to light-harvesting iridium(iii) complexes with required extent of lability which may be promising as photocatalysts and biologically active molecules.

A photoactivated Ir(iii) complex targets cancer stem cells and induces secretion of damage-associated molecular patterns in melanoma cells characteristic of immunogenic cell death

The new iridium complex selectively targets cancer stem cells and has potential to induce immunogenic cell death in cancer cells.

Synthesis and hybridization properties of iridium(III) polypyridyl complex-conjugated oligonucleotide

An Ir(III) polypyridyl complex-conjugated 14-mer oligonucleotide (Ir^III-DNA) was synthesized and its hybridization properties with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) were evaluated by UV-melting experiments. The stabilities of the duplexes of Ir^III-DNA with 14-, 20-, and 26-mer ssDNAs were higher than those of the unconjugated oligonucleotides. The triplex of Ir^III-DNA with 14-mer dsDNA was also stabilized. However, the triplexes of Ir^III-DNA with 20- and 26-mer dsDNAs, flanked by 3 and 6 base pairs at the both ends of 14-mer dsDNA target, were destabilized. This is presumably because of steric repulsion between the Ir(III) complex and the protruding 3- and 6-mer dsDNA moieties which are inflexible compared to ssDNA.

Photochromism and photocatalysis of organic–inorganic hybrid iodoargentates modulated by argentophilic interactions

A series of organic–inorganic hybrid iodoargentates exhibit photochromic and photocatalytic properties, which could be effectively modulated by argentophilic interactions.

Cyclometalated Ru(ii) complexes with tunable redox and optical properties for dye-sensitized solar cells

Cyclometalated Ru(ii) complexes with 2-arylbenzimidazoles demonstrate enhanced light-harvesting properties and good characteristics in dye-sensitized solar cells.

Two photochromic iodoargentate hybrids with adjustable photoresponsive mechanism

Two photochromic hybrid iodoargentates with adjustable photoresponsive mechanism have been synthesized and exhibit fast response and wide color range.

Photoactive layer based on T-shaped benzimidazole dyes used for solar cell: from photoelectric properties to molecular design

Three benzimidazole-based organic dyes, possessing the same triphenylamine donors and cyanoacrylic acid acceptors with the bithiophene π-bridges combined in different nuclear positions of benzimidazole, were investigated in the utility of dye-sensitizer solar cells. The structure, molecular orbital and energy, absorption spectra and some important parameters (such as light harvesting efficiency (LHE), electron injection driving force, the electron injection time, chemical reactivity parameters, vertical dipole moment as well as interaction models of dye-I₂) were obtained according to Newns–Anderson model and DFT calculation. The process and strength of charge transfer and separation were visualized with charge different density and index of spatial extent (S, D and Δq). Current work paid attention to the new T-shaped dyes to reveal the relation between the structure and photoelectric performance. Furthermore, nine dyes (substitution of alkyl chains and π-bridges) have been designed and characterized to screen promising sensitizer candidates with excellent photo-electronic properties.

Preparation of metalated azine complexes of iridium(iii)

The preparation of half-sandwich κ¹-aldazine and κ¹-ketazine complexes of iridium and their metalation reaction affording chelate κ²-azine derivatives are described. The photoluminescence properties of the chelate complexes are also described.

(2-Benzoyl-1-phenylethenolato-κ2O,O′)bis[2-(1-phenyl-1H-benzimidazol-2-yl)phenyl-κC1]iridium(III) dichloromethane disolvate

We present here synthesis and crystal structure of a neutral IrIIIcomplex, [Ir(C19H13N2)2(C15H11O2)]·2CH2Cl2or [Ir(C^N)2O^O]·2CH2Cl2, where C^N is 1,2-diphenyl-1H-benzimidazole and O^O is 2-benzoyl-1-phenylethenolate. The coordination sphere of the IrIIIatom, located on a twofold rotation axis, is that of a slighlty distorted C2N2O2octahedron, with the N atoms in atransconfiguration. In the crystal, complex molecules assemble through weak C—H...π interactions in the range 2.699 (3)–2.892 (3) Å. The solvent CH2Cl2molecules reside in channels aligned along theaaxis and are connected to the complex molecules by C—H...O interactions.

Highly Fluorinated Ir(III)-2,2':6',2″-Terpyridine-Phenylpyridine-X Complexes via Selective C-F Activation: Robust Photocatalysts for Solar Fuel Generation and Photoredox Catalysis

A series of fluorinated Ir(III)-terpyridine-phenylpyridine-X (X = anionic monodentate ligand) complexes were synthesized by selective C-F activation, whereby perfluorinated phenylpyridines were readily complexed. The combination of fluorinated phenylpyridine ligands with an electron-rich tri-tert-butyl terpyridine ligand generates a "push-pull" force on the electrons upon excitation, imparting significant enhancements to the stability, electrochemical, and photophysical properties of the complexes. Application of the complexes as photosensitizers for photocatalytic generation of hydrogen from water and as redox photocatalysts for decarboxylative fluorination of several carboxylic acids showcases the performance of the complexes in highly coordinating solvents, in some cases exceeding that of the leading photosensitizers. Changes in the photophysical properties and the nature of the excited states are observed as the compounds increase in fluorination as well as upon exchange of the ancillary chloride ligand to a cyanide. These changes in the excited states have been corroborated using density functional theory modeling.