Acceptor Ligands Research Articles

Aggregation‐Enhanced Photodynamic Activity of Organometallic Ru(II)–Arene Complexes

ABSTRACTOrganometallic Ru(II)–arene complexes have emerged as potential chemotherapeutic agents for improved cancer treatment. However, the application of Ru(II)–arene complexes as photosensitizers has been rarely explored. In consideration of their excellent biological performance, herein, three Ru(II)–arene complexes 1–3 bearing 2,3,8,9,14,15‐hexamethyl‐5,6,11,12,17,18‐hexaazatrinapthalene (HATN‐Me6) as acceptor ligand have been designed and investigated as potential photosensitizers. Distinctively, complexes 1–3 produced negligible singlet oxygen (1O2) generation in DMF. The mechanism investigations indicated that the large change in equilibrium displacement between the excited and ground states rather than narrow energy gap results in the increased vibrational wave function overlap and non‐radiative decay rate. Remarkably, complexes 1–3 induced greatly enhanced 1O2 generation efficiency in an aggregated state due to the suppression of the non‐radiative decay process, implying that the electronic structures of excited states play an essential role in the photochemical properties and functions of the organometallic Ru(II)–arene complexes. The biological evaluation showed that complexes 1–3 exhibited potent photocytotoxicity against cancer cells, highlighting that organometallic Ru(II)–arene scaffolds are potential platforms for the fabrication of efficient photosensitizers.

Visible-light activation of a Ru(II) complex catalyst via a carbazole-modified donor–acceptor ligand for CO2 reduction

Abstract Visible-light activation of photocatalysts is crucial for effectively utilizing solar energy in molecular conversions. This study demonstrates that modifying the bipyridine ligand with carbazole enables visible-light activation of a Ru(II) carbonyl complex catalyst, which was originally developed as an electrocatalyst for CO2 reduction but is inactive as a photocatalyst on its own. The present study opens avenues for individually designing light-absorbing ligands and catalytic functions within one mononuclear complex.

Reabsorption‐Free Scintillating Hetero‐Ligand MOF Crystals Activated by Ultrafast Energy Transfer

AbstractFast photoluminescence and scintillation with a Stokes shift larger than 1 eV is achieved in hetero‐ligand metal–organic framework (MOF) crystals comprising inorganic linking nodes and fluorescent conjugated ligands. By finely engineering the MOF composition with the use of ligands with strictly complementary emission and absorption properties and highly delocalized molecular electronic orbitals, the singlet excitons diffusion is enhanced through the ligand framework, fully exploiting both Förster and Dexter energy transfer mechanisms. This allows for the sensitization of energy acceptor ligand fluorescence by ultrafast non‐radiative energy transfer with a rate up to the THz range. This efficient antenna effect instantly activates the MOF scintillation with a Stokes shift as large as 1.3 eV in the blue spectral range, matching the highest sensitivity spectral window of the best photodetector available. This is obtained using a minimal doping level of the energy acceptor species, with a consequent elimination of emission re‐absorption that allows the achievement of a 500% increment of the MOFs scintillation efficiency and the detection of the radioactive krypton isotope 85Kr from the gas phase with an improved sensitivity compared with the reference material.

Direct observation of electron transfer in solids through X-ray crystallography

Nanoscale electron transfer (ET) in solids is fundamental to the design of multifunctional nanomaterials, yet its process is not fully understood. Herein, through X-ray crystallography, we directly observe solid-state ET via a crystal-to-crystal process. We first demonstrate the creation of a robust and flexible electron acceptor/acceptor (A/A) double-wall nanotube crystal ([(Zn2+)4(LA)4(LA=O)4]n) with a large window (0.90 nm × 0.92 nm) through the one-dimensional porous crystallization of heteroleptic Zn4 metallocycles ((Zn2+)4(LA)4(LA=O)4) with two different acceptor ligands (2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridine (LA) and 2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridin-9(10H)-one (LA=O)) in a slow-oxidation-associated crystallization procedure. We then achieve the bottom-up construction of the electron donor incorporated-A/A nanotube crystal ([(D)2⊂(Zn2+)4(LA)4(LA=O)4]n) through the subsequent absorption of electron donor guests (D = tetrathiafulvalene (TTF) and ferrocene (Fc)). Finally, we remove electrons from the electron donor guests inside the nanotube crystal through facile ET in the solid state to accumulate holes inside the nanotube crystal ([(D•+)2⊂(Zn2+)4(LA)4(LA=O)4]n), where the solid-state ET process (D – e– → D•+) is thus observed directly by X-ray crystallography.

Understanding and tuning the electronic structure of pentalenides.

Here we report the first example of systematic tuning of the electronic properties of dianionic pentalenides through a straightforward synthetic protocol which allows the controlled variation of substituents in the 1,3,4,6-positions to produce nine new compounds, representing the largest pentalenide study to date. Both electron-withdrawing as well as electron-donating aromatics have been incorporated to achieve different polarisations of the bicyclic 10π aromatic core as indicated by characteristic 1H and 13C NMR shifts and evaluated by DFT calculations including nucleus-independent chemical shift (NICS) scans, anisotropy of the induced current density (ACID) calculations, and natural bond orbital (NBO) charge distribution analysis. The introduction of methyl substituents to the pentalenide core required positional control in the dihydropentalene precursor to avoid exocyclic deprotonation during the metalation. Frontier orbital analyses showed arylated pentalenides to be slightly weaker donors but much better acceptor ligands than unsubstituted pentalenide. The coordination chemistry potential of our new ligands has been exemplified by the straightforward synthesis of a polarised anti-dirhodium(i) complex.

Terpyridine Based Heteroleptic Ruthenium Complexes: Influence of Donor and Acceptor Ligands in Photosensitization

AbstractWe report heteroleptic ruthenium complexes of terpyridine (tpy) ligands with directly linked carboxylic acid anchors. These complexes feature methyl or methoxy‐substituted 4′−Phtpy as donor ligands. We prepared these heteroleptic complexes from the ruthenium (II) precursor via a milder route to preclude the homoleptic complex formation. The donor−acceptor arrangement of tpy ligands in these ruthenium complexes renders visible light absorption giving metal and ligand‐to‐ligand charge transfer excitations at c.a. 490 nm. We evaluate the effect of the tpy donor substituents on the light‐harvesting ability in Dye‐Sensitized Solar Cells (DSSCs) and compare their photosensitizing ability with heteroleptic complexes bearing phenyl spacer at the acceptor end. Further, scrutinizing their photovoltaic performance, we studied their electron transfer kinetics in DSSCs using electrochemical impedance spectroscopy. This paper presents the structure‐photosensitization relationship of these heteroleptic ruthenium complexes through a combined experimental and computational approach.

Design of Near-Infrared-Triggered Metallo-Photosensitizers via a Self-Assembly-Induced Vibronic Decoupling Strategy.

Development of highly efficient near-infrared (NIR)-excited photosensitizers is hampered by the fast nonradiative vibrational relaxation process regulated by the energy gap law. Here, from the fundamental perspective we propose that the intermolecular coupling of well-designed photosensitizers has the potential to facilitate exciton delocalization and hence reduce the exciton-vibration coupling, thereby boosting their phototherapeutic efficacy via inhibition of the vibrational relaxation pathway. Such conceived NIR-excited metallo-photosensitizers (IrHA1 and IrHA2) were prepared and studied for experimental validation. The resulting iridium complexes exhibited a little singlet oxygen (1O2) production in the monomeric state, but produced substantially enhanced 1O2 generation efficiency via benefiting from the exciton-vibration decoupling in the self-assembly state. Particularly, IrHA2 exhibits an unprecedented high 1O2 quantum yield of 54.9% (FDA-approved NIR dye indocyanine green: ΦΔ = 0.2%) under 808 nm laser irradiation with negligible heat generation, probably attributed to the suppression of vibronic couplings from the stretching mode of the acceptor ligand. In phototherapy, IrHA2-NPs with high biocompatibility and low dark toxicity can induce substantial tumor regression with 92.9% tumor volume reduction in vivo. This self-assembly-induced vibronic decoupling strategy would offer an effective approach to the design of high-performance NIR-excited photosensitizers.

Efficient Solid‐State Emission Control by Positional Isomerism for Platinum(II) Complex with Donor–π–Acceptor Ligand

AbstractSquare‐planar platinum (Pt)(II) complexes have attracted considerable attention owing to their unique luminescent properties, and the development of facile methods to control them is very important. A series of Pt(II) complexes containing a donor–π–acceptor (D–π–A) isomer ligand (ortho‐, meta‐, and para‐position; Pt‐O, Pt‐M, and Pt‐P) is prepared to investigate the positional isomerism on the Pt(II) complex. In particular, the metal‐to‐ligand charge transfer and ligand intramolecular charge transfer characteristics are examined depending on the positional isomer and compared with the donor‐free Pt(II) complex. All Pt(II) complexes observe slightly red‐shifted emission within 10 nm in the order of Pt‐O < Pt‐M < Pt‐P. Experimental data and theoretical calculations show that the Pt(II) complex lowest unoccupied molecular orbital energy level is controlled systematically by the donor position dependency. In particular, the small perturbation of the electronic state by positional isomerism in the single molecule induces significant phosphorescence color disparities of more than 82 nm in the solid‐state emission spectra and aggregation‐induced emission. The intriguing phosphorescent behavior is explained by the ligand‐dependent macroscopic molecular array with the Hirshfeld analysis. Consequentially, this paper reports a facile emission control method by the D–π–A ligand positional isomerism in the Pt(II) complex.

Ab initio study of the energy level alignment at semiconductor/organic interface: Coadsorption of acceptor and donor ligands with organic dyes

Conditions for ab initio evaluation of the surface band alignment in slab models are critically reviewed. Based on the employed approach, we study the relative energy levels arrangement between aromatic dye and wurtzite semiconductor surface and its dependence on the coadsorption of the representative ligands. The obtained results demonstrate the energy level shifts mediated by surface chemistry at the interface. Predictable control of the electronic properties of organically modified semiconductor surfaces can be used as a versatile adjustable instrument in the performance optimization of optoelectronic devices.

Aggregation of Charge Acceptors on Nanocrystal Surfaces Alters Rates of Photoinduced Electron Transfer.

Semiconductor nanocrystals (NCs) interfaced with molecular ligands that function as charge and energy acceptors are an emerging platform for the design of light-harvesting, photon-upconverting, and photocatalytic materials. However, NC systems explored for these applications often feature high concentrations of bound acceptor ligands, which can lead to ligand-ligand interactions that may alter each system's ability to undergo charge and energy transfer. Here, we demonstrate that aggregation of acceptor ligands impacts the rate of photoinduced NC-to-ligand charge transfer between lead(II) sulfide (PbS) NCs and perylenediimide (PDI) electron acceptors. As the concentration of PDI acceptors is increased, we find the average electron transfer rate from PbS to PDI ligands decreases by nearly an order of magnitude. The electron transfer rate slowdown with increasing PDI concentration correlates strongly with the appearance of PDI aggregates in steady-state absorption spectra. Electronic structure calculations and molecular dynamics (MD) simulations suggest PDI aggregation slows the rate of electron transfer by reducing orbital overlap between PbS charge donors and PDI charge acceptors. While we find aggregation slows electron transfer in this system, the computational models we employ predict ligand aggregation could also be used to speed electron transfer by producing delocalized states that exhibit improved NC-molecule electronic coupling and energy alignment with NC conduction band states. Our results demonstrate that ligand aggregation can alter rates of photoinduced electron transfer between NCs and organic acceptor ligands and should be considered when designing hybrid NC:molecule systems for charge separation.

Front Cover: Bond Strength of a Diatomic Acceptor Ligand: A Reliable Measure of Its Antibond Occupation and Its Charge? (Eur. J. Inorg. Chem. 32/2022)

The Front Cover shows, pictorially as the collapse of a house of cards, the repeal of the customary correlation of a nitrosyl ligand's charge and the strength of its N−O bond. Isomeric couples comprising both a linear and a bent MNO moiety show, in fact, a weaker bond of the bent form in terms of a markedly red-shifted NO stretch. However, the charge of the NO group is not a function of the bond strength. Hence, the assignment of an 1NO− to a bent-bonded nitrosyl and NO+ to a linear-bonded one is unjustified as unraveled by local-mode and QTAIM analysis. More information can be found in the Research Article by J. Popp and P. Klüfers.

Aggregation-Dependent Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence from Chiral Carbene-CuI -Amide Enantiomers.

The field of luminescent carbene-metal-amide (CMA) complexes and chiroptical-active materials has been blossoming in recent years, although chiroptical-active CMA complexes have not been reported so far. For the first time, a pair of chiral CuI -based CMA enantiomers, (R,R)-PSIPr*-Cu-DMAC and (S,S)-PSIPr*-Cu-DMAC, have been developed by using chiral phenyl-substituted N-heterocyclic carbenes as acceptor ligands in the CMA motif. The CuI -based CMA enantiomers exhibited aggregation-induced circularly polarized luminescence with a large luminescence dissymmetry factor of up to +0.027, the first reported for CMA complexes. This success originates from the limited ligand-ligand rotation freedom and asymmetrical packing pattern (helical structure) of the CMA enantiomers in the crystals. Moreover, these CuI enantiomers displayed inspiring aggregation-dependent thermally activated delayed fluorescence properties. These findings bring new insights into the optical properties of chiral CMA complexes from the perspective of aggregation states.

Aggregation‐Dependent Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence from Chiral Carbene‐CuI‐Amide Enantiomers

AbstractThe field of luminescent carbene‐metal‐amide (CMA) complexes and chiroptical‐active materials has been blossoming in recent years, although chiroptical‐active CMA complexes have not been reported so far. For the first time, a pair of chiral CuI‐based CMA enantiomers, (R,R)‐PSIPr*‐Cu‐DMAC and (S,S)‐PSIPr*‐Cu‐DMAC, have been developed by using chiral phenyl‐substituted N‐heterocyclic carbenes as acceptor ligands in the CMA motif. The CuI‐based CMA enantiomers exhibited aggregation‐induced circularly polarized luminescence with a large luminescence dissymmetry factor of up to +0.027, the first reported for CMA complexes. This success originates from the limited ligand‐ligand rotation freedom and asymmetrical packing pattern (helical structure) of the CMA enantiomers in the crystals. Moreover, these CuI enantiomers displayed inspiring aggregation‐dependent thermally activated delayed fluorescence properties. These findings bring new insights into the optical properties of chiral CMA complexes from the perspective of aggregation states.

Bond Strength of a Diatomic Acceptor Ligand: A Reliable Measure of Its Antibond Occupation and Its Charge?

AbstractA nitrosyl ligand is bonded to a central metal mainly via a mostly covalent normal bond and a coordinative metal‐to‐NO π‐backbond. A recent analysis had unravelled similar bonding characteristics of both linear and bent CoNO moieties in terms of ligand charge and antibond occupation. Thus, there should be no justification for the usual assignment of an NO+ ligand to a linear MNO unit and a singlet‐NO− ligand to a bent one. This claim seems to contradict that bending an MNO unit weakens the N−O bond with a marked red‐shift of the N−O stretch as one indicator. In this work, the failure of Dewar‐Chatt‐Duncanson‐derived conclusions is demonstrated for linear/bent isomer couples by the analysis of M−N and N−O bond strengths. Instead of DCD behavior, lateral electrostatic influence on NO and other diatomic ligands modulates the intraligand bond strength in a similar way as has been shown in former work for polar interaction of a charge with CO in the ‘non‐classical’ carbonyls. Methodologically, local‐mode analysis is used to determine bond strengths. Oxidation states are determined by the effective‐oxidation‐state (EOS) method.

Machine learning the frontier orbital energies of SubPc based triads.

Organic photovoltaic devices are promising candidates for efficient energy harvesting from sunlight. Designing new dye molecules suitable for such devices is a challenging task restricted by the rapid increase of computational cost with system size. Solar cell material properties are closely related to the electronic structure of the dye, and an effective molecular orbital energy screening method for a family of dyes is therefore desired. In this work, a machine learning approach is used to sort through the chemical space of peripheral double-substituted boron-Subphthalocyanine dyes. A database of 12,102 PM6 optimized structures was built and for each of the structures time-dependent density functional theory (LC-[Formula: see text]HPBE/6-31+G(d)) calculations were performed. We investigated the changes of the molecular orbital energies of the molecular orbitals related to reduction and oxidation of the compounds. With the Electrotopological-state index moleculear representation all the tested algorithms, Support Vector Machine, Random Forest Regression, Neural Network, and Simple Linear Regression, captured the calculated frontier orbital energies with a prediction root-mean-square-error in the order of 0.05 eV. Finally, frontier orbital energies were predicted for more than 40,000 new structures by the trained Support Vector Machine algorithm. Compared to the parent boron-Subphthalocyanine structure, 237 and 132 functionalized dyes were predicted to have upshifted molecular orbital energies using the Electrotopological-state index and OneHot encoding feature vector, respectively. Out of 27 investigated donor and acceptor ligands, the acetamide and hydroxyl ligands gave rise to the desired increase in frontier molecular orbital energy.

Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters.

Carbene–metal–amides (CMAs) are an emerging class of photoemitters based on a linear donor–linker–acceptor arrangement. They exhibit high flexibility about the carbene–metal and metal–amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 106 s–1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin–orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.

Energy‐Transfer Photocatalysis Using Lead Halide Perovskite Nanocrystals: Sensitizing Molecular Isomerization and Cycloaddition

AbstractA relatively new addition to the application portfolio of lead halide perovskites is to photosensitize molecular triplets for a variety of photochemical applications. Here we report visible‐light‐driven isomerization and cycloaddition of organic molecules sensitized by spectrally‐tunable perovskite nanocrystals. We first demonstrate with stilbene as the substrate molecule that photoisomerization can proceed efficiently and rapidly by either directly grafting carboxylated stilbene onto nanocrystal surfaces or using triplet–acceptor ligands as the energy relay. The relay approach is more generally applicable as it does not require anchoring‐group functionalization of substrate molecules, allowing us to facilely extend it to isomerization of a series of substituted stilbene molecules and ring‐closing isomerization of diarylethene, as well as intermolecular [2+2] cycloaddition of acenaphthylene. This study opens an avenue of energy‐transfer photocatalysis using perovskite nanocrystals.

Energy‐Transfer Photocatalysis Using Lead Halide Perovskite Nanocrystals: Sensitizing Molecular Isomerization and Cycloaddition

A relatively new addition to the application portfolio of lead halide perovskites is to photosensitize molecular triplets for a variety of photochemical applications. Here we report visible-light-driven isomerization and cycloaddition of organic molecules sensitized by spectrally-tunable perovskite nanocrystals. We first demonstrate with stilbene as the substrate molecule that photoisomerization can proceed efficiently and rapidly by either directly grafting carboxylated stilbene onto nanocrystal surfaces or using triplet-acceptor ligands as the energy relay. The relay approach is more generally applicable as it does not require anchoring-group functionalization of substrate molecules, allowing us to facilely extend it to isomerization of a series of substituted stilbene molecules and ring-closing isomerization of diarylethene, as well as intermolecular [2+2] cycloaddition of acenaphthylene. This study opens an avenue of energy-transfer photocatalysis using perovskite nanocrystals.

A direct high-throughput protein quantification strategy facilitates discovery and characterization of a celastrol-derived BRD4 degrader

We describe a generalizable time-resolved Förster resonance energy transfer (TR-FRET)-based platform to profile the cellular action of heterobifunctional degraders (or proteolysis-targeting chimeras [PROTACs]) that is capable of both accurately quantifying protein levels in whole-cell lysates in less than 1h and measuring small-molecule target engagement to endogenous proteins, here specifically for human bromodomain-containing protein 4 (BRD4). The detection mix consists of a single primary antibody targeting the protein of interest, a luminescent donor-labeled anti-species nanobody, and a fluorescent acceptor ligand. Importantly, our strategy can readily be applied to other targets of interest and will greatly facilitate the cell-based profiling of small-molecule inhibitors and PROTACs in a high-throughput format with unmodified cell lines. We furthermore validate our platform in the characterization of celastrol, a p-quinone methide-containing pentacyclic triterpenoid, as a broad cysteine-targeting E3 ubiquitin ligase warhead for potent and efficient targeted protein degradation.

Aligned chromophores in a host–guest MOF crystal for switchable polarized nonlinear optical response

By preparing a host–guest MOF to elaborately arrange donor–acceptor ligand and dye guests, second-harmonic generation and two-photon-pumped lasing can be switched in a MOF crystal by orthogonally manipulating the excitation polarization direction.