Interligand Charge Transfer Research Articles

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study.

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates.

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Why Ligand Doping Increases the Fluorescence of an Anthracene-Based Metal-Organic Framework.

Metal-organic frameworks (MOFs) built from fluorescent ligands frequently exhibit enhanced fluorescence when doped with inert ligands. This study focuses on a MOF of the UiO-68 structure, which is built from a fluorescent dibenzoate-anthracene ligand doped with a dibenzoate-benzene ligand. Our investigation aims to understand the mechanism behind the doping-enhanced emission of this MOF. We rule out several possible mechanisms, including exciton coupling, electron transfer between ligand and metal center, and ligand intersystem crossing induced by the metal center. Inhibition of the interligand charge transfer is considered a possible way to enhance emission. Furthermore, we propose that the conformational change of the anthracene-based ligand in the MOF cavity is also a way for enhancement. Our molecular dynamics simulations of the MOF structure filled with solvents reveal that the steric crowding in the cavity induces a conformational change at different doping levels, affecting the rate of intersystem crossing of the ligand.

Comparative (magneto)optical, redox, and photophysical properties of phosphorous octacarbazole- and tetra-tert-butyltetrabenzotriazacorroles

The spectroscopy and electronic structure of the novel phosphorous(V) octacarbazole tetrabenzotriazacorrole (TBC(Cz)8P=O) compound were evaluated using a large array of experimental and theoretical methods, and compared to the reference terta-tert-butyl tetrabenzotriaazacorrole (TBC(tBu)4P=O). The first coordination sphere in TBC(Cz)8P(XY) and TBC(tBu)4P(XY) in a solid state and solution was probed by MALDI-TOF mass spectrometry, 31P NMR spectroscopy, XPS, UV–Vis, MCD, and elemental analysis. Based on the 31P NMR, mass spectrometry, UV–Vis, and MCD data, it was demonstrated that the initial pentacoordinated TBC(tBu)4P=O complex can be easily transformed to the hexa-coordinated species in the presence of water and methanol following TBC(tBu)4P=O ⇌ TBC(tBu)4P(OH)2 ⇌ TBC(tBu)4P(OH)(OMe) ⇌ TBC(tBu)4P(OMe)2 equilibrium. In wet solvents, the UV–Vis and MCD spectroscopies revealed that the two structures possess similar absorbance profiles; however, the TBC(Cz)8P(OH)2 compound had a more intense and red-shifted Q-band than the TBC(tBu)4P(OH)2 compound. The first oxidation under spectroelectrochemical conditions was found to be completely reversible for the TBC(Cz)8P(OH)2 compound; conversely, these experiments were completely irreversible for the TBC(tBu)4P(OH)2 compound. DFT and TDDFT calculations were performed using three different exchange-correlation functionals. The DFT results revealed several (16 in total) carbazole-centered MOs at energies in between the classical Gouterman's a1u and a2u frontier orbitals while the four Gouterman's frontier orbitals were found to be energetically sequential for the TBC(tBu)4P(OH)2 compound. The TDDFT calculations provided a very reasonable agreement between theory and experiment and demonstrated the presence of the inter-ligand (carbazole-to-TBC) charge-transfer transitions in TBC(Cz)8P(OH)2 in ∼500 nm region. Steady-state fluorescence spectroscopy data are indicative of the mid-range fluorescence quantum yields in both compounds. Transient absorption spectroscopy was utilized to probe triplet state formation and lifetimes (τT) which were found to be 112 μs for TBC(Cz)8P(OH)2 and 91 μs for TBC(tBu)4P(OH)2.

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes.

Two-coordinate carbene-MI-amide (cMa, MI = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (ΦPL > 0.95) and fast radiative rates (kr = 4 × 106 s-1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole-electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔEST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.

Interligand Charge-Transfer Processes in Zinc Complexes

Electron donor–acceptor (EDA) complexes are characterized by charge-transfer (CT) processes between electron-rich and electron-poor counterparts, typically resulting in a new absorption band at a higher wavelength. In this paper, we report a series of novel 2,6-di(imino)pyridine ligands with different electron-rich aromatic substituents and their 1:2 (metal/ligand) complexes with zinc(II) in which the formation of a CT species is promoted by the metal ion coordination. The absorption properties of these complexes were studied, showing the presence of a CT absorption band only in the case of aromatic substituents with donor groups. The nature of EDA interaction was confirmed by crystallographic studies, which disclose the electron-poor and electron-rich moieties involved in the CT process. These moieties mutually belong to both the ligands and are forced into a favorable spatial arrangement by the coordinative preferences of the metal ion.

Luminescent magnesium complexes with intra- and inter-ligand charge transfer.

Herein, we report two 2,2'-pyridylpyrrolide (PyPyrH) ligand supported magnesium complexes (1 and 2), which demonstrate bright luminescence with a quantum yield of 22% and 14% in the solid state, respectively. Theoretical calculations reveal that their emissive properties originate from the intra- and inter-ligand charge transfer.

QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

Dicopper complexes with thermally activated delayed fluorescence (TADF) phenomena are important in enriching the arsenal of organic light-emitting diodes materials. However, the TADF mechanism is still elusive, especially in the solid state. Herein, we chose a TADF dicopper complex and investigated its geometric and electronic structures and absorption and emission spectra using DFT, TD-DFT, and QM/MM methods. On the basis of these results, we further estimate the fluorescence emission rate from the S1 state, phosphorescence emission rate from the T1 state, and forward and reverse intersystem crossing (ISC and rISC) rates between S1 and T1. The present work shows that both the S1 and the T1 states have mixed metal-to-ligand and interligand charge-transfer character. Good spatial separation between the HOMO and the LUMO makes the S1–T1 energy gap small, ca. 2.8 kcal/mol. This small energy gap leads to efficient ISC and rISC processes, whose rates are much larger than the fluorescence and phosphorescence emission rates at 300 K, therefore enabling TADF. In contrast, at 77 K, the rISC process is blocked because its rate is much smaller than the phosphorescence emission. Thus, TADF disappears at 77 K. Further analysis shows that high-frequency deformation and low-frequency torsional vibrational modes make a large contribution to the Huang–Rhys factors, but Duschinsky rotation effects are essentially negligible for both the ISC and the rISC processes.

Metal-Organic Frameworks with Novel Catenane-like Interlocking: Metal-Determined Photoresponse and Uranyl Sensing.

The assembly of two tripyridinium-tricarboxylate ligands and different metal ions leads to seven isostructural MOFs, which show novel 2D→2D supramolecular entanglement featuring catenane-like interlocking of tricyclic cages. The MOFs show tripyridinium-afforded and metal-modulated photoresponsive properties. The MOFs with d10 metal centers (1-Cd, 1-Zn, 2-Cd, 2-Zn) show fast and reversible photochromism and concomitant fluorescence quenching, 1-Ni displays slower photochromism but does not fluoresce, and 1-Co and 2-Co are neither photochromic nor fluorescent. It is shown here that the network entanglement dictates donor-acceptor close contacts, which enable fluorescence originated from interligand charge transfer. The contacts also allow photoinduced electron transfer, which underlies photochromism and concomitant fluorescence response. The metal dependence in fluorescence and photochromism can be related to energy transfer through metal-centered d-d transitions. In addition, 1-Cd is demonstrated to be a potential fluorescence sensor for sensitive and selective detection of UO2 2+ in water.

Optical and Infrared Spectroelectrochemical Studies of CN-Substituted Bipyridyl Complexes of Ruthenium(II).

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)x(bpy)3-x]2+ (CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, bpy = 2,2'-bipyridine, and x = 1-3, abbreviated as 12+, 22+, and 32+) undergo four (12+) or five (22+ and 32+) successive one-electron reduction steps between -1.3 and -2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+ and 32+ being separated by 150-210 mV; reduction of the unsubstituted bpy ligand in 12+ and 22+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. -45 cm-1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30 show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•-, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1-, and 2- exhibit a prominent band attributable to the CN-Me-bpy•- moiety between 6000 and 7500 cm-1, whereas bpy•--based absorption occurs between 4500 and 6000 cm-1; complexes 2+, 3+, and 30 also exhibit a band at ca. 3300 cm-1 due to a CN-Me-bpy•- → CN-Me-bpy interligand charge-transfer transition. In the UV-vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by -38 cm-1 and 5-7-fold-enhanced relative to the ground state.

Inter-ligand charge-transfer interactions in a photochromic and redox active zinc–organic framework

A novel Zn(ii)–organic framework with 1D stair-like structure displays reversible photochromic and redox active properties. And inter-ligands charge-transfer interactions exist in this material.

Molybdenum(VI) bis-imido Complexes of Dipyrromethene Ligands.

We report the synthesis of high-valent molybdenum(VI) bis-imido complexes 1-4 with dipyrromethene (DPM) supporting ligands of the general formula (DPMR)Mo(NR')2Cl (R, R' = mesityl (Mes) or tert-butyl (tBu)). The electrochemical and chemical properties of 1-4 reveal unexpected ligand noninnocence and reactivity. 15N NMR spectroscopy is used to assess the electronic properties of the imido ligands in the tert-butyl complexes 1 and 3. Complex 1 is inert toward ligand (halide) exchange with bulky phenolates such as KOMes or amides (e.g., KN(SiMe3)2), whereas the use of the lithium alkyl LiCH2SiMe3 results in a rare nucleophilic β-alkylation of the DPM ligand. While the reductions of the complexes occur at molybdenum, the oxidation is centered at the DPM ligand. Quantum-chemical calculations (complete active space self-consistent field, density functional theory) suggest facile (near-infrared) interligand charge transfer to the imido ligand, which might preclude the isolation of the oxidized complex [1]+ in the experiment.

Enhancement of the Luminescent Efficiency in Carbene-Au(I)-Aryl Complexes by the Restriction of Renner-Teller Distortion and Bond Rotation.

A series of (carbene)Au(I)(aryl) complexes are reported. The nature of the lowest excited state in these complexes changes character from metal-to-ligand charge transfer (MLCT) to interligand charge transfer (ICT) with increasing electron-donating strength of the aryl ligand. Complexes that have the MLCT lowest excited state undergo a Renner-Teller bending distortion upon excitation. Such a distortion leads to a large rate of nonradiative decay, on the order of 108 s-1. Renner-Teller-based nonradiative decay does not occur in chromophores with an ICT emissive state. Introducing a julolidine moiety and ortho-methyl substituents to the aryl group makes the molecule rigid and hinders the rotation along the Au-Caryl-coordinate bond. Consequently, the nonradiative decay rates of these ICT emitters are decreased and become lower than the radiative decay rate constants (kr = 105 s-1). Thus, high-luminescent efficiencies (ΦPL = 0.61 and 0.77) along with short lifetimes (τ < 2 μs) are obtained for yellow and green emitters, respectively. Thermally assisted delayed fluorescence behavior is observed, owing to the small exchange energy (ΔEST < 1600 cm-1) in these emitters.

DFT Study of the Structural and Electronic Properties of Selected Organogold(III) Compounds with Characteristic Anticancer Activity

The limitless exploration for the discovery of potential drug candidates for cancer treatment has become a worldwide research priority. More importantly is the chemistry of gold(III) compounds, which has recently undergone intense scrutiny due to their outstanding cytotoxic properties as an emerging class of metal complexes. Herein, a quantum chemical study is presented to analyze the properties of some active organogold(III) complexes at the molecular level using different density functional theoretical methods and quality basis sets. The studied conceptual quantum chemical parameters proved to be significant in evaluating the selectivity, reactivity and stability of gold(III) complexes as potential anticancer moieties. Inter-ligand charge transfer was the predominated actions on investigating the density of state and ultraviolet visible spectra of these compounds. The theoretical 1H and 13C NMR chemical shift values were in agreement with experimental data. An important observation from these electronic calculations is that compounds having two C–Au bonds are very reactive and promising anticancer candidates. The outcome of this in silico study could serve as a useful material towards future design of new anticancer drug.

Spectroscopic and TDDFT studies on the charge-transfer properties of metallated Octa(carbazolyl)phthalocyanines

Four novel phthalocyanine complexes containing carbazole moieties were synthesized and characterized by 1H &13C NMR, IR, and UV–visible spectroscopies, magnetic circular dichroism (MCD), elemental analysis, matrix-assisted laser desorption ionization mass spectrometry (MALDI), and X-ray crystallography. In all new compounds, characteristic interligand charge transfer transitions were observed in the 400–600 nm range and are indicative of interligand charge transfer transitions. The electrochemical and spectroelectrochemical methods used to probe the redox properties of 2–5 suggested phthalocyanine-centered first oxidation and reduction processes. Photophysical measurements of the prepared complexes in different solvents exhibited interesting properties due to the charge transfer effect and consequently prolonged excited state lifetime (5.00 ns) with high singlet oxygen quantum yield (ФΔ = 0.825) were acquired. Due to their remarkable solvatochromism phenomena and enhanced photo-related properties, the presented compounds have high potential in chemo-sensors and optical limiting material applications.

Ratiometric Tuning of Luminescence: Interplay between the Locally Excited and Interligand Charge-Transfer States in Pyrazolate-Based Boron Compounds

In an aim to study the potential application toward organic light-emitting diodes, a series of boron compounds NBN-1 to NBN-5 bearing 1-isoquinolinyl pyrazolate, phenyl, substituted aryl, or fused ...

Nickel(II) complex with an asymmetric tetradentate Schiff base ligand: synthesis, characterization, crystal structure, and DFT studies

A new asymmetric tetradentate Schiff base, bis(5-methoxysalicylidene)-4-methylbenzene-1,2-diamine), H2L, and its Ni(II) complex were prepared and characterized using elemental analyses (CHN), FTIR, UV–Vis, 1H NMR, and 13C{1H} NMR spectroscopic techniques, and crystal structures of both were determined by X-ray crystallography. For both ligand and Ni(II) complex, density functional theory calculations to find geometry parameters, IR frequencies, electronic properties, and natural bond orbital analysis (NBO) were done with M062X method and Def2-TZVP basis set. All calculated data are consistent with the experiments. NBO data for the Ni(II) complex show that the main type of transition in UV-Vis is interligand charge-transfer, which is assigned as π-π*.

Metal–Organic Frameworks with Extended Viologen Units: Metal-Dependent Photochromism, Photomodulable Fluorescence, and Sensing Properties

The photoresponsive viologen unit has been widely used to endow metal–organic materials with photochromic and other photomodulable properties. Herein we report the first examples of the metal–organic materials functionalized by extended viologens (ExVs), of general formula [ML]·2H2O (M = Zn for 1, M = Mn for 2, M = Co for 3, and L is a tetracarboxylate ligand with the p-phenylene-extended viologen spacer). Of the three isomorphic metal–organic frameworks, only 1 is photochromic owing to formation of extended viologen radicals through photoinduced electron transfer (PET). The incapability of 2 and 3 to undergo photochromism can be ascribed to longer intermolecular donor–acceptor contacts, emphasizing the sensitivity of solid-state PET to structural changes. 1 also shows strong fluorescence owing to interligand charge transfer, and the fluorescence can be reversibly modulated and switched on/off in the photochromic process. Furthermore, 1 shows excellent hydrolytic stability and can be used as a sensitive, ...

Interligand Charge-Transfer Interactions in Electroactive Coordination Frameworks Based on N, N'-Dicyanoquinonediimine (DCNQI).

Coordination frameworks containing DCNQI2- (DCNQI = N, N'-dicyanoquinonediimine ligand) are produced by deprotonation of DCNQIH2 in the presence of a metal center and a co-ligand. This approach has yielded two-dimensional (2D) sheet compounds [Cd(DCNQI)(L)2] (where L = pyridine (py) or isoquinoline (isoquin)) that can be partially oxidized via solid-state electrochemical and in situ spectroelectrochemical methods to materials that contain DCNQI as its radical monoanion. The new frameworks display charge-transfer bands that are indicative of interligand charge-transfer interactions as supported by TD-DFT computational calculations. The redox-state dependent spectral properties of the frameworks have been probed using a newly developed solid-state spectroelectrochemical cell. Coupled with computational calculations, the experimental data provide an understanding of the fundamental charge-transfer processes that may underpin long-range functional properties such as conductivity in framework materials.

Interligand Charge Transfer in a Complex of Deprotonated cis-Indigo Dianions and Tin(II) Phthalocyanine Radical Anions with Cp*IrIII.

A diamagnetic complex, {(cis-indigo-N,N)2-(Cp*IrIII)} (1), in which deprotonated cis-indigo dianions coordinate an iridium center through two nitrogen atoms was obtained. By employment of the ability of the iridium center in 1 to coordinate an additional ligand, the complex [(Bu4N+)2{[SnII(Pc•3-)](cis-indigo-N,N)2-Cp*IrIII}•-2·0.5(H2Indigo)·2.5C6H4Cl2 (2), which has two functional ligands coordinating an IrIII center, was obtained. This complex has a magnetic moment of 1.71 μB at 300 K, in accordance with an S = 1/2 spin state. The spin density is mainly delocalized over the Pc•3- macrocycle and partially on (cis-indigo-N,N)2-. Due to an effective π-π interaction, a thermally activated charge transfer from [SnII(Pc•3-)]•- to (cis-indigo-N,N)2- is observed, with an estimated Gibbs energy (-ΔG°) of 9.27 ± 0.18 kJ/mol. The deprotonation of indigo associated with the coordination of IrIII by the indigo releases H+ ions, which protonate noncoordinating indigo molecules to produce leuco cis-indigo (H2Indigo). One H2indigo links two (cis-indigo-N,N)2- dianions in 2 to produce strong N-H···O═C and O-H···O═C hydrogen-bonding interactions.