Intraligand Charge Transfer Research Articles

Zinc– and Copper–Salicyaldimine complexes: Simultaneous observation of both metal–ligand coordination and weak CH⋯N contact about a single N-donor and the transmetallation reactions

Two salicyaldimine ligands, HsalCl-3-py and HsalOMe-3-py, functionalized with the pyridyl group on the imine side and their Zn and Cu complexes with a general formula [ML2], where M ​= ​Zn, L ​= ​salCl-3-py (1), salOMe-3-py (2); M ​= ​Cu, L ​= ​salCl-3-py (3), salOMe-3-py (4), have been synthesized and characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS), infrared (IR) spectroscopy, elemental analysis (EA), scanning electron microscope (SEM) and further by single-crystal X-ray diffraction analysis and X-ray powder diffraction (XRPD). The crystal structures of Zn– and Cu–salicyaldimine complexes exhibited that the M(II) center is coordinated by two N,O-chelated salicyaldimine ligands in a square-planar geometry, and two further pyridyl donating groups from two neighboring molecules at the axial positions, resulted in self-aggregation to form a two-dimensional (4,4)-sheet. Interestingly, apart from the strong M–N(pyridyl) coordination bond, the weak CH⋯N(pyridyl) hydrogen bonding interactions are simultaneously formed between the pyridyl N-donor and one pyridyl hydrogen of the equatorial salicyaldimine ligand. The electronic absorption spectra of the Zn– and Cu–salicyaldimine complexes in methanol solutions showed the appearance of a metal-to-ligand charge transfer absorption other than the intraligand charge transfer transitions after the complexation of azomethine (–CN–) with M(II) center. On the other hand, salicyaldimine ligands and Cu–salicyaldimine complexes showed no or weak fluorescence emissions. By contrast, Zn–salicyaldimine complexes emitted intense cyan and green light fluorescence in both solid-state and methanol solutions. Noteworthy, transmetallation of Zn–salicyaldimine complexes with Cu2+ ions would be achieved, producing corresponding Cu–salicyaldimine complexes. This implies the stronger binding ability of Cu2+ than that of Zn2+ to salicyaldimine ligands.

Novel blue-phosphorescent platinum(II) ternary complexes with β-diketonate and fluorinated phenylimidazole for organic light-emitting diodes

In this study, the one-pot synthesis of two novel Pt(II) β-diketonate compounds, namely Pt(tpim)(O^O) (1) and Pt(mtpim)(O^O) (2) (tpim = 1-([1,1′:3′,1′′-terphenyl]-2′-yl)-2-(4-fluorophenyl)-1 H-imidazole, mtpim = 2-(4-fluorophenyl)-1-(5′-methyl-[1,1′:3′,1′′-terphenyl]-2′-yl)-1 H-imidazole, and O^O = acetylacetonato), was achieved. Both compounds had a distorted square-planar geometry without direct intermolecular Pt(II)—Pt(II) interactions, which was attributed to the bulky terphenyl group of the C^N chelate as well as strong intermolecular Pt(II)—H(terphenyl/imidazole) interaction. Compounds 1 and 2 exhibited bright sky-blue-phosphorescent emission with a λmax = 490 nm and photoluminescence quantum yields of 0.50 and 0.47, respectively. The time-dependent density functional theory calculations revealed that the electronic transitions of both compounds were attributed to the combination of intraligand charge transfer (π(phenylimidazole)–π*(terphenyl)) and ligand-to-ligand charge transfer (π(acac)–π*(terphenyl)), in addition to a contribution from the metal-to-ligand charge transfer (Ptd–π*(terphenyl)). Compounds 1 and 2, as the dopants, and 3′-di(9 H-carbazol-9-yl)-1,1′-biphenyl:9-(3'-carbazol-9-yl-5-cyano-biphenyl-3-yl)-9 H-carbazole-3-carbonitrile, as the mixed host were used to fabricate multilayer phosphorescent organic light-emitting diodes. Compound 1 doped in the emissive layer at a 10% doping level demonstrated a maximum current efficiency of 21.7 cd A−1 at 100 cd m−2.

Thermally Activated Delayed Fluorescence of a Dinuclear Platinum(II) Compound: Mechanism and Roles of an Upper Triplet State.

A dinuclear Pt(II) compound was reported to exhibit thermally activated delayed fluorescence (TADF); however, the luminescence mechanism remains elusive. To reveal relevant excited-state properties and luminescence mechanism of this Pt(II) compound, both density function theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out in this work. In terms of the results, the S1 and T2 states show mixed intraligand charge transfer (ILCT)/metal-to-ligand CT (MLCT) characters while the T1 state exhibits mixed ILCT/ligand-to-metal CT (LMCT) characters. Mechanistically, a four-state (S0 , S1 , T1 , and T2 ) model is proposed to rationalize the TADF behavior. The reverse intersystem crossing (rISC) process from the initial T1 to final S1 states involves two up-conversion channels (direct T1 →S1 and T2 -mediated T1 →T2 →S1 pathways) and both play crucial roles in TADF. At 300 K, these two channels are much faster than the T1 phosphorescence emission enabling TADF. However, at 80 K, these rISC rates are reduced by several orders of magnitude and become very small, which blocks the TADF emission; instead, only the phosphorescence is observed. These findings rationalize the experimental observation and could provide useful guidance to rational design of organometallic materials with superior TADF performances.

Protonation and Non-Innocent Ligand Behavior in Pyranopterin Dithiolene Molybdenum Complexes.

The complex [TEA][Tp*MoIV(O)(S2BMOPP)] (1) [TEA = tetraethylammonium, Tp* = tris(3,5-dimethylpyrazolyl)hydroborate, and BMOPP = 6-(3-butynyl-2-methyl-2-ol)-2-pivaloyl pterin] is a structural analogue of the molybdenum cofactor common to all pyranopterin molybdenum enzymes because it possesses a pyranopterin-ene-1,2-dithiolate ligand (S2BMOPP) that exists primarily in the ring-closed pyrano structure as a resonance hybrid of ene-dithiolate and thione-thiolate forms. Compound 1, the protonated [Tp*MoIV(O)(S2BMOPP-H)] (1-H) and one-electron-oxidized [Tp*MoV(O)(S2BMOPP)] [1-Mo(5+)] species have been studied using a combination of electrochemistry, electronic absorption, and electron paramagnetic resonance (EPR) spectroscopy. Additional insight into the nature of these molecules has been derived from electronic structure computations. Differences in dithiolene C-S bond lengths correlate with relative contributions from both ene-dithiolate and thione-thiolate resonance structures. Upon protonation of 1 to form 1-H, large spectroscopic changes are observed with transitions assigned as Mo(xy) → pyranopterin metal-to-ligand charge transfer and dithiolene → pyranopterin intraligand charge transfer, respectively, and this underscores a dramatic change in electronic structure between 1 and 1-H. The changes in electronic structure that occur upon protonation of 1 are also reflected in a large >300 mV increase in the Mo(V/IV) redox potential for 1-H, resulting from the greater thione-thiolate resonance contribution and decreased charge donation that stabilize the Mo(IV) state in 1-H with respect to one-electron oxidation. EPR spin Hamiltonian parameters for one-electron-oxidized 1-Mo(5+) and uncyclized [Tp*MoV(O)(S2BDMPP)] [3-Mo(5+)] [BDMPP = 6-(3-butynyl-2,2-dimethyl)-2-pivaloyl pterin] are very similar to each other and to those of [Tp*MoVO(bdt)] (bdt = 1,2-ene-dithiolate). This indicates that the dithiolate form of the ligand dominates at the Mo(V) level, consistent with the demand for greater S → Mo charge donation and a corresponding increase in Mo-S covalency as the oxidation state of the metal is increased. Protonation of 1 represents a simple reaction that models how the transfer of a proton from neighboring acidic amino acid residues to the Mo cofactor at a nitrogen atom within the pyranopterin dithiolene (PDT) ligand in pyranopterin molybdenum enzymes can impact the electronic structure of the Mo-PDT unit. This work also illustrates how pyran ring-chain tautomerization drives changes in resonance contributions to the dithiolene chelate and may adjust the reduction potential of the Mo ion.

Synthesis, Characterization, and the N Atom Transfer Reactivity of a Nitridochromium(V) Complex Stabilized by a Corrolato Ligand.

Metal complexes bearing nitrido ligands (M≡N) are at the forefront of current scientific research due to their resemblances with the metal complexes involved in the nitrogen fixation reactions. An oxo(corrolato)chromium(V) complex was used as a precursor complex for the facile synthesis of a new nitrido(corrolato)chromium(V) complex. The nitrido(corrolato)chromium(V) complex was characterized by various spectroscopic techniques. Density functional theory (DFT) calculations were performed on the nitrido(corrolato)chromium(V) complex to assign the vibrational and electronic transitions of this complex. The chromium–nitrogen (nitrido) bond distance obtained in the DFT-optimized structure is 1.530 Å and matches well with the earlier reported authentic Cr≡N bond distances obtained from the single-crystal X-ray diffraction data. This nitrido(corrolato)chromium(V) compound exhibited a sharp Soret band at 438 nm and a Q band at 608 nm. DFT calculations deliver that the origin of the bands at 438 and 608 nm is due to the intraligand charge transfer transitions. The nitrido(corrolato)chromium(V) complex showed one reversible oxidation and one reversible reduction couple at +0.53 and −0.06 V, respectively, vs the Ag/AgCl reference electrode. The simulation of the electron paramagnetic resonance data of the nitrido(corrolato)chromium(V) compound provided the following parameters: giso = 1.987, A53Cr = 26 G, and A14N = 2.71 G. From all these analyses, we can conclude that the electronic configuration in the native state of nitrido(corrolato)chromium(V) can be best described as [(cor3–)CrV(N3–)]−. Reactions of nitrido(corrolato)chromium(V) with the chloro(porphyrinato)chromium(III) complex resulted in a complete intermetal N atom transfer reaction between chromium corrole and chromium porphyrin complexes. A second-order rate constant of 4.29 ± 0.10 M–1 s–1 was obtained for this reaction. It was also proposed that this reaction proceeds via a bimetallic μ-nitrido intermediate.

Unveiling the sensing mechanism and luminescence property of a new ESIPT-based fluorescent sensor for detecting Zn2+

Recently, based on the mechanism of excited-state intramolecular proton transfer (ESIPT), a new fluorescent probe named 3-(benzo[d]thiazol-2-yl)-5-bromosalicylaldehyde-4N-phenyl thiosemicarbazone (BTT) was successfully synthesized [Analyst 146 (2021) 4348–4356.]. However, the importance of ESIPT processes of BTT probe and the mechanism of detecting Zn2+ ions have not been studied in detail. In this study, the photochemical behavior of ESIPT-chromophore and the photophysical changes of detecting Zn2+ ions were explained at the molecular level for the first time. The calculated spectral values were in agreement with the experiment. We not only confirmed the excited state hydrogen-bond strengthening by interaction region indicator (IRI), but also scanned the potential energy curves of BTT molecule in different electronic states, which confirmed that the hydrogen proton is easier to transfer in the first excited state. In addition, we had given the reasonable structure of the BTT-Zn2+ complex (L1) by comparing the binding free energies. The hole-electron distribution and interfragment charge transfer (IFCT) methods proved the excitation type of intraligand charge transfer (ILCT). Finally, the photophysical phenomenon of BTT for detecting Zn2+ ions is explained by calculating the electronic spectra and the energy gap (Egap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

An Orange‐Emitting Phosphorescent Iridium(III) Complex Featuring Three Strong Electron‐Donating N‐Embedded π‐Conjugation Units

A new facial phosphorescent iridium complex fac-Ir(NO)3 with three strong electron-donating N-embedded π-conjugation units has been prepared. It can display a distinct orange phosphorescence emission with a narrow half-peak width. In addition, it also shows both distinct intraligand charge transfer (ILCT) and partial unusual ligand-to-metal charge transfer (LMCT) excited state properties due to strong electron-donating capacity of three N-embedded π-conjugation units.

Nickel (II), Copper (II), and Zinc (II) Complexes of N-bis (4-methoxybenzyl) Dithiocarbamate: Synthesis, Characterization Studies, and Evaluation of Antitumor Activity

Three new complexes with the general formula [M(L)2] (L=N-bis(4-methoxybenzyl) dithiocarbamate; M= Ni(II) 1, Cu(II) 2, and Zn(II) 3 ), have been synthesized, and characterized by elemental analysis, IR, UV–visible, 1H, 13C NMR and mass spectrometry. The magnetic behavior of nickel and copper complexes was measured and found to be diamagnetic and paramagnetic, respectively. Complex 3 crystallized in the monoclinic space group P21/c and possessed a centrosymmetric dimeric structure stabilized by weak C-H···S, C-H···π, and C-H…O intermolecular interactions. Electronic absorption spectral studies of the complexes show the absorption band in the range of 270-331 nm, which is due to intraligand charge transfer transitions. Complexes 1 and 2 also show transitions in the range of 325-475 nm due to ligand to metal charge transfer transition. The antitumor activities of the complexes have been screened, and the results obtained show that the metal complexes have significant antitumor activity against Dalton's lymphoma cells in a concentration-dependent manner, and the metal complexes were relatively tolerant to normal human cells with low or marginal toxicity against both lymphocytes and monocytes.

Blue phosphorescent platinum(IV) complex bearing bipyridine ligand for potential application in organic light-emitting diodes (OLEDs)

A cyclometalated Pt(IV) compound, Pt(dfpypy)2(Me)Cl (1), with blue phosphorescence was prepared using 2′,6′-difluoro-2,3′-bipyridine (dfpypy) as a C^N chelate ligand. Compound 1 exhibits distorted octahedral geometries around the platinum ion and has strong intermolecular interactions via interatomic contacts (H···N/F/Cl) involving hydrogen bond interactions based on Hirshfeld surface analysis. This compound shows bright blue phosphorescence emissions with λmax = 427 and 455 nm and a high photoluminescence quantum efficiency (ΦPL = 0.60). Furthermore, a good blue colour purity with Commission Internationale de L'Eclairage (1931) coordinates ( x, y) of (0.14, 0.17) was observed in the thin film, which was close to pure blue. TD-DFT calculations revealed that the electronic transition can be attributed to a combination of intraligand charge transfer (πbpy–π*bpy) and ligand-to-ligand charge transfer [pz(Cl)–π*(dfpypy)] with a small contribution from metal-to-ligand charge transfer (Ptd−π*bpy). Owing to the introduction of the electronegative C^N chelate dfpypy, the emission of compound 1 was blueshifted by 20 nm compared with its analog [Pt(ppy)2(Me)Cl] ( λmax = 447), where ppy is 2-phenylpyridine.

Synthesis and characterization of alkaloid derived hydrazones and their metal (II) complexes

Abstract Alkaloids have been known overtime to have medicinal uses. Exploring alkaloid derived hydrazones and their complexes as potential therapeutic agents with a view to improving the medicinal uses of alkaloids are imperative. 1,8-dichloroacridone hydrazone hydrochloride, 1-chloro pilocarpine nitrate-3-chlorophenyl hydrazone and 1-phenethyl-4-piperidone formyl hydrazone ligands were synthesized via Wolff–Kishner condensation reaction. Five metal (II) complexes of cobalt, nickel and manganese were prepared by stirring the ligands with the respective metal salts. The ligands and complexes were characterized using elemental analyses, molar conductivity, FTIR, 1H and 13C nmr, UV–Vis spectra, melting point and solubility. Antimicrobial activities of the ligands and their complexes were tested against four bacteria (Staphylococcus aureus, Streptococcus faecalis Escherichia coli, Salmonella paratyphimurium) and a fungus (Candida albican). The molar conductance values indicate that they are 1:1 and 1:2 type electrolytes while the elemental analyses of the complexes reveals a 1:1 metal to ligand stoichiometry. The relevant IR bands suggest coordination is through the C=N, C=O, N=C–O and C–N groups. Both 1H and 13C nmr corroborated the elemental analysis while the UV–Vis reveals intra-ligand charge transfer while the complexes exhibited the expected metal transitions bands. A proposed octahedral geometry is supported by spectral data. Only 1, 8-dichloroacridonehydrazone hydrochloride ligand was found to be active against all the organisms. Cobalt and nickel complexes of 1,8-dichloroacridonehydrazone hydrochloride were active against S. paratyphimurium and S. aureus, respectively, while cobalt complex of 1-chloropilocarpinenitrate-3-chlorophenylhydrazone was active against S. faecalis and S. paratyphimurium. The minimum inhibitory concentrations (MIC) were also recorded.

Light‐Driven Multi‐Charge Separation in a Push‐Pull Ruthenium‐Based Photosensitizer – Assessed by RASSCF and TDDFT Simulations

AbstractThe performance of photosensitizers in the field of, for example, solar energy conversion, relies on their light‐harvesting efficiency in the visible region, population of long‐lived charge‐separated intermediates, as well as their charge‐accumulation capacity amongst other properties. In this computational study, we investigate the photophysical properties of a bis(bipyridyl)ruthenium(II)‐based black dye (Ru) incorporating a chromophoric unit based on a thiazole donor‐acceptor push‐pull motif. The combination of two light‐harvesting units, that is, the Ru(II)polypyridyl and the thiazole‐based organic dye, yields close‐lying metal‐to‐ligand charge transfer (MLCT) states, involving both ligand spheres as well as intra‐ligand charge transfer (ILCT) states of the organic dye. Due to the combination of inorganic and organic chromophores the computational modelling of the photophysics of Ru is challenging. To this aim, time‐dependent density functional theory and multiconfigurational methods are applied. The excited‐state properties obtained for the states of interest are rationalized by electronic absorption and resonance Raman spectroscopies. The CAM‐B3LYP functional was found to accurately describe the ground‐ and excited‐state properties of Ru. Finally, excited‐state relaxation pathways and the multi‐charge‐accumulation capacity were addressed. Despite the unidirectional nature of the MLCTthia and ILCTthia transitions, the thiazole unit is merely capable to store one redox equivalent.

Effect of ancillary ligand on the photoluminescent and electroluminescent properties of blue Ir(III) complexes bearing main bipyridine ligand

Homoleptic and heteroleptic cyclometalated blue iridium(III) complexes, namely, 1 and 2, incorporating 2′,6′-dimethoxy-5-trimethylsilyl-2,3′-bipyridine (OMe2py-TMSpy) as the main ligand and acetylacetonate (acac) as the ancillary ligand, are developed for the comparison of their structure, photophysical properties, and electroluminescent characteristics. Both 1 and 2 exhibit distorted octahedral geometries around the iridium center, and the aspect ratios are 1.0 for 1 and 0.76 for 2, respectively. The interatomic contacts involving hydrogen bond interactions (H···N/O) in 1 are greater than those in 2, based on Hirshfeld surface analysis. TD-DFT calculations reveal that the electronic transition for 1 may be attributed to intra-ligand charge transfer (ILCT, πbpy−π*bpy) mixed with metal-to-ligand charge transfer (MLCT, Ird−π*bpy), while for 2, it is attributed to the combination of ILCT/MLCT and additional ligand-to-ligand charge transfer (LLCT, πacac−π*bpy). Owing to the introduction of the ancillary ligand, the emission of complex 2 is red-shifted by only 6 nm compared to complex 1. However, the photoluminescent quantum efficiency of 2 is higher than that of 1 owing to the high radiative decay rate. Furthermore, phosphorescent organic light-emitting diodes (PhOLEDs) based on 2 achieve a high external quantum efficiency of 26.7%, which is one of the highest performances observed among reported blue PhOLEDs.

Structural and theoretical studies on copper(II) and zinc(II) complexes with a 9-anthraldehyde-derived thiosemicarbazone

Complexes [Cu(ATPh)2]·H2O (1) and [Zn(ATPh)2] (2) were obtained with 9-anthraldehyde-N(3)-phenyl thiosemicarbazone (HATPh). The g|| ≈ 2.15 and A|| ≈ 180 G EPR parameters of complex (1) are compatible with two nitrogen and two sulfur donor atoms in a nearly square planar geometry. The first excited state of complex (1) is not emissive due to fluorescence quenching by the paramagnetic copper(II) ion. The crystal structure of [Zn(ATPh)2]·2DMF (2a) shows that in 2a the anionic thiosemicarbazone ligands are attached to the metal through the NS chelating system in a distorted tetrahedral arrangement. Computational studies revealed that the S1 → S0 emission of complex (2) is attributed to anthracene, the emission band involving two electronic transitions centered at the anthracene rings. The HOMO-1 → LUMO transition is characterized as a ligand-to-ligand charge transfer (LLCT) between anthracene groups, which contributes with 12% and an intra-ligand charge transfer transition (ILCT) centered in the anthracene group of one of the ligands, which contributes with 77% to the emission band. The emission intensity of complex (2) is lower than that of the free thiosemicarbazone, but the presence of two ligand molecules in the complex increases the fluorescence efficiency and the computed fluorescence rate constant for 2 is ∼ 600-fold higher than that of the free ligand.

In situ preparation, structure, photophysical properties of a novel zinc complex

A novel zinc complex [ZnI3(N-benzyl-4,4'-bipy)]·2MeOH·H3O (1) (bipy = bipyridine; MeOH = methanol) is synthesized and characterized. This complex is characterized by an isolated structure. The photoluminescence spectra using solid state samples unveiled that this complex can show a green photoluminescence emission. As revealed by the TDDFT (time-dependent density functional theory) calculation results, the photoluminescence emission should rise from an intraligand charge transfer (ILCT). The complex possesses CIE (Commission Internationale de I'Éclairage) chromaticity coordinates of 0.1305 and 0.3855. A solid-state UV-Visible diffuse reflectance spectrum shows that this complex has a narrow optical band gap of 1.87 eV.

Tuning the photophysical properties of lanthanide(iii)/zinc(ii) 'encapsulated sandwich' metallacrowns emitting in the near-infrared range.

A family of Zn16Ln(HA)16 metallacrowns (MCs; Ln = YbIII, ErIII, and NdIII; HA = picoline- (picHA2−), pyrazine- (pyzHA2−), and quinaldine- (quinHA2−) hydroximates) with an ‘encapsulated sandwich’ structure possesses outstanding luminescence properties in the near-infrared (NIR) and suitability for cell imaging. Here, to decipher which parameters affect their functional and photophysical properties and how the nature of the hydroximate ligands can allow their fine tuning, we have completed this Zn16Ln(HA)16 family by synthesizing MCs with two new ligands, naphthyridine- (napHA2−) and quinoxaline- (quinoHA2−) hydroximates. Zn16Ln(napHA)16 and Zn16Ln(quinoHA)16 exhibit absorption bands extended into the visible range and efficiently sensitize the NIR emissions of YbIII, ErIII, and NdIII upon excitation up to 630 nm. The energies of the lowest singlet (S1), triplet (T1) and intra-ligand charge transfer (ILCT) states have been determined. LnIII-centered total (QLLn) and intrinsic (QLnLn) quantum yields, sensitization efficiencies (ηsens), observed (τobs) and radiative (τrad) luminescence lifetimes have been recorded and analyzed in the solid state and in CH3OH and CD3OD solutions for all Zn16Ln(HA)16. We found that, within the Zn16Ln(HA)16 family, τrad values are not constant for a particular LnIII. The close in energy positions of T1 and ILCT states in Zn16Ln(picHA)16 and Zn16Ln(quinHA)16 are preferred for the sensitization of LnIII NIR emission and ηsens values reach 100% for NdIII. Finally, the highest values of QLLn are observed for Zn16Ln(quinHA)16 in the solid state or in CD3OD solutions. With these data at hand, we are now capable of creating MCs with desired properties suitable for NIR optical imaging.

QM/MM study on thermally activated delayed fluorescence mechanism of a three-coordinated Au(I) complex: Key roles of crystal environments

Three-coordinate Au(I) complexes with thermally activated delayed fluorescence (TADF) have recently gained experimental attention. However, its luminescence mechanism is elusive. Herein, we have employed density functional theory (DFT), time-dependent DFT (TD-DFT), and QM/MM methods to investigate the excited-state and emission properties of the Au(I) complex in both gas and crystal phases. In both environments, the S1 and T1 emitting states mainly involve HOMO and LUMO and show clear metal-ligand charge transfer and intraligand charge transfer characters. The good spatial separation of HOMO and LUMO minimizes the S1–T1 energy gap, which benefits the reverse intersystem crossing (rISC) from T1 to S1. At 300 K, the rISC rate is faster than the T1 phosphorescence emission, which enables the TADF emission. However, at 77 K, such a rISC process is blocked and TADF disappears; instead, only phosphorescence is recorded experimentally. Importantly, this work highlights the importance of environments in regulating luminescence properties and contributes to understanding the TADF emission of organometallic complexes.

In Situ Assembled Polynuclear Zinc Oxo Clusters Using Modified Schiff Bases as Ligands.

A series of different cores and nuclearity zinc metal clusters 1–5 have been synthesized using Zn(ClO4)2·6H2O, Schiff-base primary ligands, and dibenzoyl methane (DBM) or monoethanolamine (MEA) as co-ligand in a room-temperature reaction. The structure of the complexes is characterized using single-crystal X-ray diffraction. Among them, (1) [Zn(L1)(DBM)] is mononuclear; (2) [Zn4(L2)2(DBM)4], (3) [Zn4(L2)4(H2O)2(ClO4)2]·2CH2Cl2, and (4) [Zn4(L3)2(DBM)4] have a cubane core; and (5) [Zn4(L4)4(MEA)2(ClO4)2] has a ladderlike core structure. Compounds 1–5 have also been characterized using UV–vis absorption and emission spectroscopies. For an in-depth understanding of the absorption spectra of 1 and 3, density functional theory (DFT) calculations have been performed, which suggest that the transitions correspond to the π → π* intraligand charge transfer (ILCT) transitions.

In-Depth Studies of Ground- and Excited-State Properties of Re(I) Carbonyl Complexes Bearing 2,2':6',2″-Terpyridine and 2,6-Bis(pyrazin-2-yl)pyridine Coupled with π-Conjugated Aryl Chromophores.

In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3ILaryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-functionalized ligands were found to have predominant 3ILpyrene/3ILCTpyrene→terpy character. The 3IL/3ILCT nature of the lowest energy excited state of [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3MLCT and 3IL/3ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO)3(4′-(1-naphthyl)-terpy-κ2N)] and [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] were determined by femtosecond transient absorption studies.

Three coordination polymers as a multi-responsive luminescent probe for the detection of Fe3+, Cr2O72− and antibiotic in aqueous media

Three coordination polymers (CPs), namely {[Co(L)(dpa)]·H2O}n (1), {[Zn(L)2(dpb)]}n (2) and {[Zn(L)(dpe)]}n (3), have been assembled through a dual-ligand strategy of a semi-rigid dicarboxylic acid (H2L = 1,4-bis(4-phenoxy)benzenedicarboxylic acid) with the help of different N-donor co-ligands (dpa = di(pyridin-4-yl)amine, dpb = 1,4-di(pyridin-4-yl)benzene), dpe = trans-1,2-di(4-pyridyl)ethylene). Single crystal X-ray diffraction analyses reveals that compound 1 is a 4-fold interpenetrating 2D sql network and compound 2 is a 2-fold interpenetrating 2D sql network, although the final structures are different because of various inter-layer stacking modes. Compound 3 is a 8-fold interpenetrating 3D dia framework containing an octahedral cage. A photoluminescence investigation shows that compounds 2 and 3 exhibit strong fluorescence originating from the intra-ligand charge transfer transitions. Benefiting from its chemical stability and luminescent properties, compound 2 possesses multi-functional fluorescent responses toward the Fe3+ ion, the Cr2O72− ion and the NZF antibiotic in aqueous medium. The KSV values of compound 2 are 4.68 × 104 M−1 for the Fe3+ ion, 7.23 × 104 M−1 for the Cr2O72− anion, and 1.03 × 104 M−1 for the NZF antibiotic. Additionally, the possible mechanism for the luminescence quenching has been investigated.