Recent advances with π-conjugated salen systems

Schiff base ligands have long been successfully employed as ligands in combination with various metals to give catalysts capable of realizing a variety of synthetic transformations. One of the most widely used Schiff base ligands, the "salen" ligand, has been extensively researched. Recently, there has been increased interest in π-conjugated salen systems, known as "salphen" ligands, as a result of the differences in reactivity of the complexes in catalytic applications compared with the salen analogues. Complexes of salphen ligands display interesting photophysical and supramolecular properties which are not always observed with salen systems as a result of their π-conjugation. This tutorial review therefore describes the most significant advances recently made with salphen and related π-conjugated ligand systems.     

A strategy for improving the room-temperature luminescence properties of Ru(II) complexes with tridentate ligands

Several ruthenium(II) complexes with new tridentate polypyridine ligands have been prepared, and their photophysical properties have been studied. The new tridentate ligands are tpy-modified systems (tpy = 2,2':6',2' '-terpyridine) in which aromatic substituents designed to be coplanar with the tpy moiety are introduced, with the aim of enhancing delocalization in the acceptor ligand of the potentially luminescent metal-to-ligand charge-transfer (MLCT) state and increasing the MLCT-MC energy gap (MC = metal-centered excited state). Indeed, the Ru(II) complexes obtained with this new family of tridentate ligands exhibit long-lived luminescence at room temperature (up to 200 ns). The enhanced luminescence properties of these complexes support this design strategy and are superior to those of the model Ru(tpy)22+ compound and compare favorably with those of the best Ru(II) complexes with tridentate ligands reported so far.     

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.     

Sensitization of NO‐Releasing Ruthenium Complexes to Visible Light

We report a combined spectroscopical–theoretical investigation on the photosensitization of transition metal nitrosyl complexes. For this purpose, ruthenium nitrosyl complexes based on tetradentate biscarboxamide ligands were synthesized. A crystal structure analysis of a lithium‐based ligand intermediate is described. The Ru complexes have been characterized regarding their photophysical and nitric oxide (NO) releasing properties. Quantum chemical calculations have been performed to unravel the influence of the biscarboxamide ligand frame with respect to the molecular electronic properties of the NO‐releasing pathway. A quantitative measure for the ligand design within photosensitized Ru complexes is introduced and evaluated spectroscopically and theoretically by using time‐dependent density functional theory.     

联三吡啶配体及其衍生物合成进展

金属-配体间的配位作用是超分子化学中最重要的相互作用之一, 寡聚吡啶配体可以与许多过渡金属离子配位, 形成具有独特磁、光物理和电化学性质的过渡金属络合物, 因此联三吡啶配体的合成及其过渡金属络合物性能研究引起化学家的广泛关注. 综述了联三吡啶配体及其衍生物的合成方法, 主要包括成环缩合反应、过渡金属催化的偶联反应以及其它方法, 并选取具有代表性的实例对联三吡啶配体的结构和合成方法进行详细地阐述.     

环金属钌配合物的合成、结构多样性及氧化还原调控

环金属钌配合物具有良好的氧化还原和光物理性质,在诸多光电领域如染料敏化太阳能电池、电致变色、电子转移等方面具有重要应用。环金属钌配合物的合成方法主要包括“后期金属化”、“前期金属化”、“转金属化”三种方法。环金属配合物具有丰富的结构多样性。环金属配合物由环金属配体和辅基配体与金属螯合形成。环金属配体包括N^∧C、N^∧N^∧C、N^∧C^∧N和C^∧C^∧C-类型多齿配体。辅基配体主要包括吡啶、咪唑、三唑、嘧啶等杂环。碳-金属键的引入大大降低了钌配合物的氧化还原电位。通过改变环金属配体和辅基配体的结构,可以对金属的氧化还原电位进行有效调控。金属钌配合物的氧化还原电位对敏化电池的性能以及电子转移的过程具有重要的影响。     

Highlights on the recent advances in gold chemistry--a photophysical perspective

The presence of inter- and/or intra-molecular aurophilic interactions among the closed-shell gold(i) centres in various systems has been studied from various aspects, including synthetic, spectroscopic and theoretical approaches. The employment of different ligands can impose a significant influence on these factors and give rise to new complexes with interesting structural and photophysical properties. In this tutorial review, a number of recent examples are selected to illustrate the fascinating properties and chemistry, as well as versatility of gold(i) in these aspects and their potential applications to newcomers in this field. An emerging class of luminescent gold(iii) complexes is also described.     

New cyclometalated transition-metal based photosensitizers for singlet oxygen generation and photodynamic therapy

The aim of this review article is to introduce recent studies on an emergent class of singlet oxygen photosensitizers of potential applications to the photodynamic therapy,with a primary focus on the cyclometalated transition-metal complexes.Singlet oxygen photosensitization performances of various cyclometalated Ir and Pt scaffolds are reviewed,and the general photophysical properties of relevant systems and the mechanisms of singlet oxygen production via photo-sensitization are also briefly discussed.Thus far,investigations of singlet oxygen sensitization by such Ir and Pt complexes are mainly carried out in organic solvents and under non-physiological conditions,while some research efforts have been made at examining the feasibility of applying pertinent cyclometalated complexes to photodynamic therapy.     

New fluorescence PET systems based on N2S2 pyridine-anthracene-containing macrocyclic ligands. spectrophotometric, spectrofluorimetric, and metal ion binding studies

Three new fluorescent devices for protons and metal ions have been synthesized and characterized, and their photophysical properties have been explored; these are the macrocycles 7-(9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L1) and 7-(10-methyl-9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L2) and the bis macrocycle 7,7'-[9,10-anthracenediylbis(methylene)]bis-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L3). All these systems have a pyridil-thioether-containing macrocycles as a binding site and an anthracene moiety as a signaling agent. The coordination properties of these ligands toward Cu(II), Co(II), Ni(II), Zn(II), and Pd(II) have been studied in solution and in the solid state. The addition of these metal ions to dichloromethane solutions of L1, L2, and L3 produce strong changes in the absorption and emission spectra of these ligands. The stoichiometry of the species, formed at 298 K, have been determined from absorption and fluorescence titrations. The Co(II) and Cu(II) complexes of L1 have been studied by EPR spectroscopy. This last complex and its free ligand have also been characterized by X-ray crystallography.     

Increased antenna effect of the lanthanide complexes by control of a number of terdentate N-donor pyridine ligands

Three europium complexes with the terdentate N-donor ligand 2,6-bis(1H-pyrazol-3-yl)pyridine (L) have been synthesized, and their crystal structures have been determined. The ligand/metal ratios in these complexes are 3, 2, and 1. The photophysical properties of the complexes indicate more efficient ligand sensitization of europium emission for the homoleptic complex.     

Insight into luminescent iridium complexes: Their potential in light-emitting electrochemical cells

Cyclometallated iridium complexes possess fascinating electrochemical and photophysical properties that make them excellent candidates for a variety of photonic and optoelectronic applications. In particular, light-emitting electrochemical cells (LEECs) based on iridium-containing ionic transition-metal complexes (Ir-iTMCs) are a promising alternative to conventional organic light-emitting diodes with several advantages, including a simpler device structure, solution processability, and reduced manufacturing costs. This review aims to provide a comprehensive and systematic overview of the current status of Ir-iTMC-based LEECs using the archetypal complex [Ir(ppy)₂(bpy)]PF₆ as a reference emitter. After a discussion of the device fundamentals and important photophysical and device parameters, key strategies for tuning the emission characteristics and device stability through LUMO and HOMO stabilization/destabilization are presented using numerous examples from the literature, with a particular focus on ligand modification with hydrophobic, electron-withdrawing, and electron-donating substituents, π-stacking interactions, and alternative ancillary and cyclometalated ligand skeletons. Comprehensive data tables summarizing the photophysical and LEEC properties of the various classes of iridium complexes reported to date are also provided. Finally, in an effort to highlight promising directions for future research, the current champion iridium complexes for fabricating state-of-the-art LEECs are identified, and the merits and limitations of existing approaches are discussed.     

Facile Tuning of Luminescent Platinum(II) Schiff Base Complexes from Yellow to Near‐Infrared: Photophysics,Electrochemistry, Electrochemiluminescence and Theoretical Calculations

The photophysical and related properties of platinum(II) Schiff base complexes can be finely and predictably tuned over a wide range of wavelengths by small and easily implemented changes to ligand structure. A series of such complexes, differing only in the number and positioning of methoxy substituents on the phenoxy ring, were synthesised and their photophysical, electrochemical and electrochemiluminescent (ECL) properties investigated. Theoretical calculations were performed in order to gain further insight into the relationship between structure and properties in these materials. By positioning methoxy groups para and/or ortho to either the imine or the oxygen group on the ligand, electron density could be directed selectively toward the LUMO or HOMO as required. This allowed the emission colour (both photoluminescent and electrochemiluminescent) to be tuned over a wide range between 587 and 739 nm. The variation in orbital energies was also manifested in the positions of the absorption bands and the redox properties of the complexes, as well as in the NMR shifts for the uncoordinated ligands. All reported complexes displayed intense electrochemiluminescence (ECL), which could be initiated either by annihilation or co‐reactant pathways. The relationship between the electrochemical and photophysical properties and the efficiency of the ECL is discussed. For two of the complexes solid‐state ECL could be generated from electrodeposited layers of the complex.     

Substitution‐Controlled Excited State Processes in Heteroleptic Copper(I) Photosensitizers Used in Hydrogen Evolving Systems

Four different heteroleptic [Cu(N^N)(P^P)]PF₆ complexes, which combine classical bidentate diimine ligands and sterically demanding diphosphine ligands, are studied by a combination of ultrafast time‐resolved spectroscopy and quantum chemical calculations. The light‐induced excited state processes, accompanied by a structural change, are discussed with respect to the application of these complexes as a new class of noble‐metal‐free photosensitizers in proton reducing systems. In particular, the influence of different substituents in the ligand backbone on the photophysical properties is highlighted.     

Ruthenium(II) polypyridyl complexes and DNA--from structural probes to cellular imaging and therapeutics

In the last few decades, coordination complexes based on d(6) metal centres and polypyridyl ligand architectures been developed as structure- and site-specific reversible DNA binding agents. Due to their attractive photophysical properties, much of this research has focused on complexes based on ruthenium(II) centres and, more recently, attention has turned to the use of these complexes in biological contexts. As the rules that govern the cellular uptake and cellular localisation of such systems are determined they are finding numerous applications ranging from imaging to therapeutics. This review illustrates how the interdisciplinary nature of this research-which takes in synthetic chemistry, biophysical and in cellulo studies-makes this an exciting area in which an array of further applications are likely to emerge.     

Synthesis of cationic metallocyclophanes with arene and cyclopentadienyl ligands

Previously unknown bridged cationic complexes (metallocyclophanes) with arene and cyclopentadienyl ligands have been obtained by ligand exchange in trifluoroacetic acid:

     

Heteroleptic platinum(II) complexes of 8-quinolinolates bearing electron withdrawing groups in 5-position

A series of novel luminescent platinum(II) complexes bearing orthometalated 2-phenylpyridine ligands (C N), namely 2-phenylpyridine (4) and 3-hexyloxy-2-phenylpyridine (5), and several 5-substituted quinolinolate ligands (5-X-Q), where X = NO2 (a), X = CHO (b), X = Cl (bearing another Cl in 7-position of the Q-ligand) (c) and X = H (d) have been synthesized, characterized and their photophysical properties were studied. All complexes were obtained as a single isomer with N atoms of the C N and Q ligands trans-coordinated to the platinum center as evidenced using single-crystal X-ray crystallography and NMR spectroscopy. Absorbance, luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The compounds under investigation absorb intensively via an intraligand charge transfer (ILCT) in the visible range (460-480 nm) and emit from fluid solution and in the solid state at room temperature at 600-630 nm. The complexes show quantum yields up to 25% and lifetimes in the range of 20-30 micros in deoxygenated organic solvents at room temperature. The emitting state can be best described as a triplet intraligand charge-transfer state localized mainly on the quinolinolate ligand. In these complexes the phenylpyridine ligand can be essentially regarded as an ancillary ligand. Density functional theory (DFT) calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes and revealed the influence of the substitution of the quinolinolate ligand on the HOMO/LUMO energies and the oscillator strengths. Substitution on 3-position of the phenylpyridine ligand does not impact on the transition energies, and is thus suited to introduce other functional moieties, such as a solubilizing hexyloxy group.     

二-2,2′:6′,2″-三联吡啶-4′-(4-苯甲酰)-乙二胺的合成

二、三联吡啶作为螯合配体可以和过渡金属离子生成高度稳定的配合物,并具有有趣的物理性质。因此在过去的几十年里,二、三联吡啶金属配合物受到广泛而深入的研究。文章在前人的研究基础上获得了含官能化双(三联吡啶)单元结构的二-2,2′:6′,2″-三联吡啶-4′-(4-苯甲酰)-乙二胺配体,可以作为非共价聚合物的很有效的构建模块,合成有趣的金属超分子聚合物。     

Bioinspired catalyst design and artificial metalloenzymes

Many bioinspired transition-metal catalysts have been developed over the recent years. In this review the progress in the design and application of ligand systems based on peptides and DNA and the development of artificial metalloenzymes are reviewed with a particular emphasis on the combination of phosphane ligands with powerful molecular recognition and shape selectivity of biomolecules. The various approaches for the assembly of these catalytic systems will be highlighted, and the possibilities that the use of the building blocks of Nature provide for catalyst optimisation strategies are discussed.     

Cyclometalated Gold(III) Complexes: Synthesis,Reactivity, and Physicochemical Properties

This Review showcases the ability of bi‐ and tridentate ligands to stabilize gold in high oxidation states through the formation of mono‐ and biscyclometalated gold(III) complexes. In‐depth studies on the synthesis, intrinsic reactivity, catalytic relevance, and photophysical properties of stabilized gold(III) species have been carried out, setting the stage for exciting developments in various research areas, such as catalysis, inorganic and bioinorganic chemistry, ligand design, and materials science.