Terpyridine Complexes Research Articles

Solar‐Driven Water Splitting in Photovoltaic Electrolysis Systems Using Copper Terpyridine Complexes as Oxygen Evolution Catalysts

Copper complexes, as kinds of molecular catalysts bioinspired from the copper O2‐activating enzymes, are attractive targets for water oxidation, which can undergo facile and reversible OO bond formation and cleavage under mild conditions. Herein, three copper(II) complexes of Cu(tpy), Cu(ctpy), and Cu(pctpy) are investigated as oxygen evolution catalysts, which are designed and synthesized using α,α′,α″‐terpyridyl(tpy) derivatives as ligands. In the electrochemical measurements, the carboxylic‐functionalized catalysts of Cu(ctpy) and Cu(pctpy) exhibit high water oxidation turnover frequencies of 1027 and 3363 s−1, respectively. The astonishing rates of O2 production exceed the oxygen evolution complex in natural photosystem II. Moreover, the photovoltaic electrolysis (PVE) system is constructed by connecting Si solar panels with the copper complex‐decorated electrodes. Under the irradiation of AM1.5G‐simulated sunlight, a high photocurrent density of 5.47 mA cm−2 and solar‐to‐hydrogen efficiency of 6.81% are achieved for solar water splitting without external bias. This is the first report PVE system using copper complex as a oxygen evolution catalyst molecular catalyst, which is of great significance to simulate the catalytic water oxidation process of natural enzymes and build an artificial photosynthesis system to achieve green, efficient, and stable decomposition of aquatic oxygen.

Synthesis and Theoretical and Photophysical Study on a Series of Neutral Ruthenium(II) Complexes with Donor–Metal–Accepter Configuration

In order to construct a new type of ruthenium(II) terpyridine complexes with activated triplet metal-centered (3MC) states, as well as stabilized triplet metal-to-ligand charge transfer (3MLCT) states, conducive to fine emissive performances, Ru-1, Ru-2, Ru-3, and Ru-4 were synthesized. Compared with the [Ru(terpyridine)2]2+ prototype (0.25 ns), this series of ruthenium(II) terpyridine complexes exhibit lengthened excited state lifetime (43.3 ns for Ru-1, 52.7 ns for Ru-2, 43.6 ns for Ru-3, and 53.4 ns for Ru-4). Interfragment charge transfer analysis illustrates the electron transfer direction of the four complexes, manifesting their intramolecular charge transfer characteristic. When excited, their lowest-lying triplet states are assigned as 3MLCT based on spin-density surface distribution. The singlet excited states and 3MLCT states were thoroughly studied by UV-visual absorption and nanosecond time-resolved transient absorption spectra, respectively. Photoluminescence spectra revealed their weak broadband near-infrared emission at room temperature and red phosphorescence at 77 K. The low molecular weight and the good thermal stability make Ru-1 and Ru-2 suitable for vaporization coating, while the fine solubility in common organic solvents makes Ru-3 and Ru-4 suitable for solution processing. Furthermore, the intrinsic electroneutrality and favorable energy levels endow them with new potential to be applied in the optoelectronic field.

Ruthenium based terpyridine complexes as both luminescent and NLO materials

Currently, a lot of attention is paid to seeking new organic low-molecular chromophores, which can provide efficient optical or nonlinear optical (NLO) response. In the last few decades, the idea of preparing multifunctional ligands and their corresponding metal complexes has become more common. In this contribution we describe the synthesis of a new multifunctional terpyridine ligand L and its corresponding heteroleptic (C1) and homoleptic (C2) complexes. We have investigated the influence of the ligands on the electron cloud distribution, which is straightforwardly connected with provided spectroscopic features. By using Ru(II) metal cation it was possible to enhance NLO response in the manner of third harmonic of light generation. In addition, we describe the synthesis strategy, full spectroscopic characterization, and other 3rd order nonlinear optical properties, in both liquid and solid states. The experimental data were supported by the DFT calculations. A theoretical model explaining a light-emitting mechanism is thus presented. The bis(terpyridine) ruthenium(II) coordination complexes are valuable candidates for opto-electronic devices, like light modulators or generators.

Branching mechanism of photoswitching in an Fe(II) polypyridyl complex explained by full singlet-triplet-quintet dynamics

It has long been known that irradiation with visible light converts Fe(II) polypyridines from their low-spin (singlet) to high-spin (quintet) state, yet mechanistic interpretation of the photorelaxation remains controversial. Herein, we simulate the full singlet-triplet-quintet dynamics of the [Fe(terpy)2]2+ (terpy = 2,2’:6’,2”-terpyridine) complex in full dimension, in order to clarify the complex photodynamics. Importantly, we report a branching mechanism involving two sequential processes: a dominant 3MLCT→3MC(3T2g)→3MC(3T1g)→5MC, and a minor 3MLCT→3MC(3T2g)→5MC component. (MLCT = metal-to-ligand charge transfer, MC = metal-centered). While the direct 3MLCT→5MC mechanism is considered as a relevant alternative, we show that it could only be operative, and thus lead to competing pathways, in the absence of 3MC states. The quintet state is populated on the sub-picosecond timescale involving non-exponential dynamics and coherent Fe-N breathing oscillations. The results are in agreement with the available time-resolved experimental data on Fe(II) polypyridines, and fully describe the photorelaxation dynamics.

Cancer stem cell activity of copper(II)-terpyridine complexes with aryl sulfonamide groups.

Copper(II)-terpyridine complexes are endowed with the ability to generate reactive oxygen species (ROS) and induce cancer cell death. Here we report the synthesis, characterisation, and anti-breast cancer stem cell (CSC) properties of a series of copper(II)-terpyridine complexes containing aryl sulfonamide groups (1-5). All of the copper(II)-terpyridine complexes adopt distorted square pyramidal geometries and are suitably stable in biologically relevant solutions (PBS and cell culture media). The p-toluene sulfonamide-bearing copper(II)-terpyridine complex 1 is 6-8-fold more potent towards breast CSCs than salinomycin (an established anti-CSC agent) and cisplatin (a metal-based anticancer drug). The copper(II)-terpyridine complex 1 also reduces the formation, size, and viability of three-dimensionally cultured mammospheres, to a similar or better extent than salinomycin and cisplatin. Mechanistic studies show that 1 successfully enters breast CSCs, generates intracellular ROS at short exposure times, partially induces endoplasmic reticulum stress, and triggers apoptosis. To the best of our knowledge, this is the first study to investigate the anti-breast CSC properties of copper(II)-terpyridine complexes.

Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission.

A series of complexes L22-M (L2: 6,6″-bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2″-terpyridine, M: Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were synthesized by coordinating p-methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p-methoxyphenyl substituents endowed complexes L22-M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L12-M (L1: 4'-phenyl-2,2':6',2″-terpyridine). The chiral helix geometry was observed for L22-M, in which 2,2':6',2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L12-M, resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L22-Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L22-Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands.

Neural network and decision tree-based machine learning tools to analyse the anion-responsive behaviours of emissive Ru(II)-terpyridine complexes.

We implemented both neural network and decision tree-based machine learning tools to analyse the anion-responsive behaviours of two heteroleptic Ru(II) complexes based on two tridentate ligands, 2,6-bis(benzimidazole-2-yl)pyridine (H2pbbzim) and substituted terpyridine ligands, tpy-Ar with Ar = 2-naphthyl and 9-anthryl groups. The secondary coordination sphere of the complexes is decorated with two imidazole NH moieties, benefitting from the anion sensing characteristics of the complexes previously reported by us. Considerable change in their absorption, emission as well as electrochemical and spectroelectrochemical responses occur in the presence of selected anions. Restoration of their initial states is made possible by acid and the process is reversible. We utilized their spectral, electrochemical and spectroelectrochemical responses upon the influence of anions and acid to mimic the operations of YES-NOT and set-reset flip-flop logic gates. We also implemented machine learning tools such as artificial neural networks (ANNs), adaptive neuro-fuzzy inference system (ANFIS) and decision tree (DT) regression to analyse and forecast the experimental data and can thus reduce the time and expenditure associated with the execution of comprehensive sensing experiments. The outcomes of the ANN, ANFIS and DT methods were also tallied with the experimental results. Among the three models, the outcomes derived from DT regression analysis turned out to be excellent with almost zero error. Thus, the applied machine learning based tools could be regarded as a prospective anion-responsive data model for the studied complexes.

Cobalt(ii) terpyridine complexes: synthesis, characterization, antiproliferative activity and molecular docking with proteins and DNA

Nine Co(ii)-terpyridine complexes were synthesized and their anticancer activity was investigated. Their interaction with DNA has been experimentally and computationally studied.

Platinum(terpyridine) complexes with N-heterocyclic carbene co-ligands: high antiproliferative activity and low toxicity in vivo.

Pt(terpyridine) complexes are well-known DNA intercalators. The introduction of an NHC co-ligand rendered such a complex highly antiproliferative in cancer cells compared to its chlorido derivative. Despite the high potency, zebrafish embryos tolerated the compound well, especially compared to cisplatin. DNA interaction studies support a mode of action related to intercalation.

Luminescent alkynylplatinum(II) terpyridine-containing conjugated polymers: synthesis, characterization and photophysical studies

A series of alkynylplatinum(II) terpyridine complexes and alkynylplatinum(II) terpyridine-containing conjugated polymers with different polymer backbones has been synthesized, and their spectroscopic properties and Förster resonance energy transfer (FRET) processes has been investigated. The platinum(II)-containing polymers exhibit dual emissive features with emission maxima at ca. 416-465 nm and ca. 671-673 nm, which are assigned to be originated from singlet intraligand (1IL) excited states from the polymer backbone and triplet metal-metal-to-ligand charge transfer (3MMLCT) excited states from the platinum(II) pendants, respectively. The Förster radii (R0) of the platinum(II)-containing conjugated polymers have been determined, and their distinctive thermo-responsive luminescence changes have also been observed. The present work has demonstrated the utilization of “click” reaction for the preparation of platinum(II)-containing conjugated polymers, which show unique photophysical and spectroscopic properties. Through the judicious design, this type of platinum(II)-containing polymer is found to be sensitive to temperature, resulting in ratiometric emission changes. This study has provided valuable insights into the preparation of metal-containing polymeric systems for different applications.

Dinuclear coumarin-containing alkynylplatinum(ii) terpyridine complexes with supramolecular assembly-assisted photodimerization

A series of coumarin-containing alkynylplatinum(ii) terpyridine complexes features supramolecular-assembly assisted photodimerization, attributed to the preorganized CC bond of coumarin moieties in close proximity by directional Pt⋯Pt interaction.

New gold(III) chlorophenyl terpyridine complex: Biomolecular interactions and anticancer activity against human oral squamous cell carcinoma

In this work we synthesized new monofunctional gold(III) complex [Au(Cl‐Ph‐tpy)Cl]Cl2 (Cl‐Ph‐tpy = 4′‐[4‐chlorophenyl]‐2,2′:6′, 2″‐terpyridine). This complex was characterized by UV–Vis, NMR, IR, and ESI‐MS spectrometry. The kinetic study of the substitution reactions of the Au‐Cl‐Ph‐tpy complex with biomolecules showed that the rate constants depend on the nature of the entering nucleophile. Based on the calculated values of entropy (∆H≠ > 0) and enthalpy (∆S≠ < 0) the proposed substitution mechanism is associative. Additionally, the relative stability and thermodynamic properties of Au‐Cl‐Ph‐tpy complex were compared with the analogue Au‐tpy complex by the B3LYP/def2‐svp method. DNA/BSA binding studies showed that Au‐Cl‐Ph‐tpy complex interacts with CT DNA through the intercalation and moderately quenches the fluorescence of tryptophan residues in serum albumin (BSA). Molecular docking confirmed results obtained by spectroscopic experiments and suggested site I (subdomain IIA) for binding of Au complex to BSA. We demonstrated that the Au chlorophenyl terpyridine complex possessed significant in vitro cytotoxic activity against human oral squamous carcinoma cells (CAL‐27), induced apoptosis, inhibited proliferation of CAL‐27 cells, and induced cell cycle disturbance. Treatment of CAL‐27 cells with the Au complex enhanced expression of cyclin‐dependent kinase inhibitors p21 and p27, resulting in cell cycle arrest in the G1 phase, reduced the percentage of CAL‐27 cells in S phase and decreased expression of Ki‐67. Additionally, Au complex reduced expression of phosphorylated STAT3 and downstream regulated molecules associated with cancer stemness, NANOG, and Sox2 protein.

Photoinduced Processes in Rhenium(I) Terpyridine Complexes Bearing Remote Amine Groups: New Insights from Transient Absorption Spectroscopy.

Photophysical properties of two Re(I) complexes [ReCl(CO)3(R-C6H4-terpy-κ2N)] with remote amine groups, N-methyl-piperazinyl (1) and (2-cyanoethyl)methylamine (2), were investigated. The complexes show strong absorption in the visible region corresponding to metal-to-ligand charge transfer (1MLCT) and intraligand-charge-transfer (1ILCT) transitions. The energy levels of 3MLCT and 3ILCT excited-states, and thus photoluminescence properties of 1 and 2, were found to be strongly affected by the solvent polarity. Compared to the parent chromophore [ReCl(CO)3(C6H5-terpy-κ2N)] (3), both designed complexes show significantly prolonged (by 1-2 orders of magnitude) phosphorescence lifetimes in acetonitrile and dimethylformamide, contrary to their lifetimes in less polar chloroform and tetrahydrofuran, which are comparable to those for 3. The femtosecond transient absorption (fsTA) measurements confirmed the interconversion between the 3MLCT and 3ILCT excited-states in polar solvents. In contrast, the emissive state of 1 and 2 in less polar environments is of predominant 3MLCT nature.

Mechanistic insights into CO2 conversion chemistry of copper bis-(terpyridine) molecular electrocatalyst using accessible operando spectrochemistry

The implementation of low-cost transition-metal complexes in CO2 reduction reaction (CO2RR) is hampered by poor mechanistic understanding. Herein, a carbon-supported copper bis-(terpyridine) complex enabling facile kilogram-scale production of the catalyst is developed. We directly observe an intriguing baton-relay-like mechanism of active sites transfer by employing a widely accessible operando Raman/Fourier-transform infrared spectroscopy analysis coupled with density functional theory computations. Our analyses reveal that the first protonation step involves Cu-N bond breakage before the *COOH intermediate forms exclusively at the central N site, followed by an N-to-Cu active site transfer. This unique active site transfer features energetically favorable *CO formation on Cu sites, low-barrier CO desorption and reversible catalyst regeneration, endowing the catalyst with a CO selectively of 99.5 %, 80 h stability, and a turn-over efficiency of 9.4 s−1 at −0.6 V vs. the reversible hydrogen electrode in an H-type cell configuration. We expect that the approach and findings presented here may accelerate future mechanistic studies of next-generation CO2RR electrocatalysts.

Synthesis, structure and application of a simple cadmium(II)-terpyridine complex as sensor material for selective detection of pyrophosphate anion

A simple 1:1 cadmium (II) complex CdCl2L of 4´-[4-(4-aminophenyl)phenyl]- 2,2′:6′,2"-terpyridine ligand L was synthesized, and structure was established by 1H NMR and single-crystal X-ray diffraction study. The crystal structure revealed that Cd(II) ion in the complex adopted a distorted trigonal bipyramidal geometry, and coordinated to three nitrogen atoms of the terpyridine domain and two chlorine atoms. The complex shows selective detection of pyrophosphate anion (P2O74−, PPi) compared to a variety of different anions (HCO3−, I −, SO42−, S2O32−, Cl−, CH3COO−, CO32−, Br−, SO32−, H2PO4−, HPO42−, F−, NO3−) in CH3CN-H2O solution (15:1 v/v) buffered with HEPES, with the lowest limit of detection (LOD) for PPi ion ∼ 2.4 µM. UV-Visible absorption spectroscopy and colorimetric method were used in the detection study. The DFT calculations provide a plausible explanation for the observed selectivity since the PPi is the sole anion capable of substituting the ligand in the CdCl2L complex due to its ability to form a six-membered chelate ring.

Deciphering Distinct Overpotential-Dependent Pathways for Electrochemical CO2 Reduction Catalyzed by an Iron–Terpyridine Complex

[Fe(tpyPY2Me)]2+ ([Fe]2+) is a homogeneous electrocatalyst for converting CO2 into CO featuring low overpotentials of <100 mV, near-unity selectivity, and high activity with turnover frequencies faster than 100 000 s-1. To identify the origins of its exceptional performance and inform future catalyst design, we report a combined computational and experimental study that establishes two distinct mechanistic pathways for electrochemical CO2 reduction catalyzed by [Fe]2+ as a function of applied overpotential. Electrochemical data shows the formation of two catalytic regimes at low (ηTOF/2 of 160 mV) and high (ηTOF/2 of 590 mV) overpotential plateaus. We propose that at low overpotentials [Fe]2+ undergoes a two-electron reduction, two-proton-transfer mechanism (electrochemical-electrochemical-chemical-chemical, EECC), where turnover occurs through the dicationic iron complex, [Fe]2+. Computational analysis supports the importance of the singlet ground-state electronic structure for CO2 binding and that the rate-limiting step is the second protonation in this low-overpotential regime. When more negative potentials are applied, an additional electron-transfer event occurs through either a stepwise or proton-coupled electron-transfer (PCET) pathway, enabling catalytic turnover from the monocationic iron complex ([Fe]+) via an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism. Comparison of experimental kinetic data obtained from variable controlled potential electrolysis (CPE) experiments with direct product detection with calculated rates obtained from the energetic span model supports the PCET pathway as the most likely mechanism. Moreover, we build upon this mechanistic understanding to propose the design of an improved ligand framework that is predicted to stabilize the key transition states identified in our study and explore their electronic structures using an energy decomposition analysis. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.

ATP‐Induced Emergent Circularly Polarized Luminescence and Encryption

AbstractBiomimetic ATP‐driven supramolecular assembly is important to understand various biological processes and dissipative systems. Here, we report an ATP‐driven chiral assembly exhibiting circularly polarized luminescence (CPL) via the interaction of an achiral terpyridine‐based ZnII complex with nucleotides. It was found that while the metal complexes could co‐assemble with the nucleotides to form fluorescent assemblies, only a combination of furan‐substituted terpyridine complex and ATP showed an intense CPL with a dissymmetry factor (glum) as high as 0.20. This means that the complex could recognize ATP using CPL as a readout signal, thus providing an example of ATP encryption. Interestingly, when ATP was transferred into ADP or AMP under enzymatic hydrolysis, the CPL decreases or disappears. Addition of ATP generates CPL again, thus producing an ATP‐induced CPL system. This work presents the first example of ATP‐induced CPL and encryption.

ATP-Induced Emergent Circularly Polarized Luminescence and Encryption.

Biomimetic ATP-driven supramolecular assembly is important to understand various biological processes and dissipative systems. Here, we report an ATP-driven chiral assembly exhibiting circularly polarized luminescence (CPL) via the interaction of an achiral terpyridine-based ZnII complex with nucleotides. It was found that while the metal complexes could co-assemble with the nucleotides to form fluorescent assemblies, only a combination of furan-substituted terpyridine complex and ATP showed an intense CPL with a dissymmetry factor (glum ) as high as 0.20. This means that the complex could recognize ATP using CPL as a readout signal, thus providing an example of ATP encryption. Interestingly, when ATP was transferred into ADP or AMP under enzymatic hydrolysis, the CPL decreases or disappears. Addition of ATP generates CPL again, thus producing an ATP-induced CPL system. This work presents the first example of ATP-induced CPL and encryption.

Osmium Complex-Chromophore Conjugates with Both Singlet-to-Triplet Absorption and Long Triplet Lifetime through Tuning of the Heavy-Atom Effect.

Os(II) complexes showing singlet-to-triplet absorption are of growing interest as a new class of triplet sensitizers that circumvent energy loss during intersystem crossing, and they enable effective utilization of input photon energy in various applications, such as photoredox catalysis, photodynamic therapy, and photon upconversion. However, triplet excited-state lifetimes of Os(II) complexes are often too short (τ < 1 μs) to transfer their energy to neighboring molecules. While the covalent conjugation of chromophores has been known to extend the net excited-state lifetimes through an intramolecular triplet energy transfer (IMET), heavy-atom effects of the central metals on the attached chromophore units have rarely been discussed. Here, we investigate the relationship between the spin-density contribution of the heavy metals and the net triplet excited-state lifetimes for a series of Os(II) and Ru(II) bis(terpyridine) complexes modified with perylene units. Phosphorescence lifetimes of these compounds strongly depend on the lifetimes of the perylenyl group-localized excited states that are shortened by the heavy-atom effect. The degree of heavy-atom effect can be largely circumvented by introducing meta-phenylene bridges, where the perylene unit retains its intrinsic long excited-state lifetime. The thermal activation to the short-lived excited states is suppressed, thanks to sufficient but still small energy losses during the IMET process. Involvement of the metal center was also confirmed by the prolonged lifetime by replacing Os(II) with Ru(II) that possesses a smaller spin-orbit coupling constant. These results indicate the importance of ligand structures that give a minimum heavy-atom effect as well as the sufficient energy gap among the excited states and fast IMET for elongating the triplet excited-state lifetime without sacrificing the excitation energy.

Analysis and prediction of anion- and temperature responsive behaviours of luminescent Ru(II)-terpyridine complexes by using Boolean, fuzzy logic, artificial neural network and adapted neuro fuzzy inference models.

Anion- and temperature responsive behaviors of three luminescent Ru(II)-terpyridine complexes are utilized here to demonstrate multiple Boolean (BL) and fuzzy logic (FL) operations. Taking advantage of the imidazole NH protons, anion-promoted alteration of the photophysical characteristics of the complexes was thoroughly investigated via absorption, and emission spectral and lifetime measurements. In their free state, the complexes display luminescence representing the "on-state", whereas quenching of luminescence is observed with anions demonstrating the "off-state". Likewise, lowering of temperature induces a substantial increase of luminescence and lifetime demonstrating the "on-state", while the increase of temperature induces a significant decrease of emission intensity and lifetime indicating the "off-state" and the process is reversible in both cases. The complexes thus can act as anion- and temperature-responsive molecular switches. The spectral signatures of the complexes under the influence of anions and temperature were employed to mimic multiple BL and FL functions. Performing very detailed sensing studies by varying the analyte concentration within a wide domain is very tedious, time-consuming and expensive. In order to overcome the lacuna, we implemented machine learning and soft computing tools such as artificial neural networks (ANNs), fuzzy-logic and adaptive neuro-fuzzy inference system (ANFIS) to predict the experimental anion sensing data of the complexes.