Steric Features Research Articles

Precise Steric Features Control Aminoacyl-tRNA Accommodation on the Ribosome.

Protein synthesis involves a complex series of large-scale conformational changes in the ribosome. While long-lived intermediate states of these processes can be characterized by experiments, computational methods can be used to identify the interactions that contribute to the rate-limiting free-energy barriers. To this end, we use a simplified energetic model to perform molecular dynamics (MD) simulations of aminoacyl-tRNA (aa-tRNA) accommodation on the ribosome. While numerous studies have probed the energetics of the early stages of accommodation, we focus on the final stage of accommodation, where the 3'-CCA tail of aa-tRNA enters the peptidyl transferase center (PTC). These simulations show how a distinct intermediate is induced by steric confinement of the tail, immediately before it completes accommodation. Multiple pathways for 3'-CCA tail accommodation can be quantitatively distinguished, where the tail enters the PTC by moving past a pocket enclosed by Helix 89, 90, and 92, or through an alternate route formed by Helix 93 and the P-site tRNA. C2573, located within Helix 90, is shown to provide the largest contribution to this late-accommodation steric barrier, such that sub-Å perturbations to this residue can alter the time scale of tail accommodation by nearly an order of magnitude. In terms of biological function, these calculations suggest how this late-stage sterically induced barrier may contribute to tRNA proofreading by the ribosome.

Exploring the Gallic and Cinnamic Acids Chimeric Derivatives as Anticancer Agents over HeLa Cell Line: An in silico and in vitro Study.

Cervical cancer is one of the most aggressive and important cancer types in the female population, due to its low survival rate. Actually, the search for new bioactive compounds, like gallic and cinnamic acid, is one of the most employed options to finding a treatment. In the present study, 134 phenolic compounds with cytotoxic activity over HeLa cell line were used to generate a descriptive ( =0.76) and predictive ( =0.69 and =0.62) QSAR model. Structural, electronic, steric, and hydrophobic features are represented as different molecular descriptors in our QSAR model. From this model, nine gallate-cinnamate ester derivatives (N1-N9) were designed and synthesized. Furthermore, in vitro cytotoxic activity was evaluated against HeLa and non-tumorigenic cells. Derivatives N6, N5, N1, and N9 were the most active molecules with IC50ExpHeLa values from 7.26 to 11.95 μM. Finally, the binding of the synthesized compounds to the colchicine binding site on tubulin was evaluated by molecular docking as a possible action mechanism. N1, N5 and N6 can be considered as templates for the design of new cervical anticancer compounds.

Pd on cyclotriphosphazen-hexa imine decorated boehmite as an efficient catalyst for hydrogenation of nitro arenes under mild reaction condition

Cyclotriphosphazen-hexa imine ligand was prepared through successive reactions of phosphonitrilchloride trimer with 4-hydroxybenzaldehyde and (3-aminopropyl) triethoxy silane. The as-prepared ligand was then covalently grafted on boehmite to furnish an efficient support for the immobilization of Pd nanoparticles and synthesis of a novel heterogeneous catalyst for hydrogenation of nitro arenes. The catalytic tests revealed that the catalyst had excellent catalytic activity for hydrogenation of various nitro arenes with different steric and electronic features under mild reaction condition in aqueous media. Noteworthy, the catalyst was highly selective and in the substrates with ketone or aldehyde functionalities, reduction of nitro group was only observed. The catalyst was also recyclable and only slight loss of activity was detected after each recycling run. Hot filtration test also approved true heterogeneous nature of catalysis.

Development of 3D-QSAR and pharmacophoric models to design new anti-Trypanosoma cruzi agents based on 2-aryloxynaphthoquinone scaffold

ABSTRACT In this work we have collected a set of 30 trypanosomicidal naphthoquinones and developed pharmacophoric and 3D-QSAR models as tools for the design of new potential anti-Chagasic compounds. Firstly, qualitative information was obtained from SAR and pharmacophoric models identifying some fragments around the 2-aryloxynaphthoquinone scaffold important for the antiparasitic activity. Then, 3D-QSAR CoMFA and CoMSIA models were developed. The models showed adequate statistical parameters where the steric, electrostatic, and hydrophobic features explain the trypanosomicidal effect. Therefore, to validate our models, we carried out the design, synthesis, and biological evaluation on T. cruzi epimastigotes of five new compounds (33a-e). According to CoMFA model, three out of five compounds showed pIC50 values within one logarithmic unit of deviation. The two compounds that did not fit the predictions were those with high lipophilicity, which agreed with the SAR and pharmacophore models. Docking and molecular dynamic studies were performed on T. cruzi trypanothione reductase, in a proposed binding site for this type of naphthoquinone. Interestingly, 33a-e showed the same interaction pattern as a naphthoquinone inhibitor (2). Finally, predicted drug-likeness properties indicated that 33a-e have optimal oral bioavailability. Thus, this study provides new in silico models for obtaining novel trypanosomicidal compounds.

Bovine Serum Amine Oxidase and Polyamine Analogues: Chemical Synthesis and Biological Evaluation Integrated with Molecular Docking and 3-D QSAR Studies

Natural polyamines (PAs) are key players in cellular homeostasis by regulating cell growth and proliferation. Several observations highlight that PAs are also implicated in pathways regulating cell death. Indeed, the PA accumulation cytotoxic effect, maximized with the use of bovine serum amine oxidase (BSAO) enzyme, represents a valuable strategy against tumor progression. In the present study, along with the design, synthesis, and biological evaluation of a series of new spermine (Spm) analogues (1-23), a mixed structure-based (SB) and ligand-based (LB) protocol was applied. Binding modes of BSAO-PA modeled complexes led to clarify electrostatic and steric features likely affecting the BSAO-PA biochemical kinetics. LB and SB three-dimensional quantitative structure-activity relationship (Py-CoMFA and Py-ComBinE) models were developed by means of the 3d-qsar.com portal, and their analysis represents a strong basis for future design and synthesis of PA BSAO substrates for potential application in oxidative stress-induced chemotherapy.

Two new 5-mercapto-1-phenyl-1H-tetrazole-based Cu(I) coordination polymers with double layer structures: Crystal structures, substituent effects and sensing responses to NACs

During the construction of coordination compounds, a minor variation in the steric and electronic characteristics of the substituent on a ligand might have a great impact on the structures and properties of the target compounds. To investigate this issue, two new coordination polymers with double layer structures, [Cu2(MPTZ)I]n (1) and {[Cu(MHPTZ)]·0.5H2O}n (2), have been successfully synthesized through the reactions of CuI and the sulfhydryl tetrazole derivatives (HMPTZ ​= ​5-mercapto-1-phenyl-1H-tetrazole and HMHPTZ ​= ​5-mercapto-1-(4-hydroxyphenyl)- 1H-tetrazole) and characterized by single crystal X-ray diffraction, powder X-ray diffraction, elemental analysis, infrared spectra and thermogravimetric analysis. In compounds 1 and 2, both (MPTZ)- and (MHPTZ)- coordinate with four Cu(I) ions with distorted tetrahedral geometry in the same coordination modes (μ4-1қN3, 2қN4, 3қS1, 4қS1) to construct the similar two-dimensional (2D) double-layer structures, however, different substituents on ligands have important effects on the introduction of iodide ions and supramolecular forces. For compound 1, a 2D inorganic wavy (CuI)n cationic double-layer structure containing Cu2I2 secondary structural unit with the Cu⋯Cu distance of 2.65 ​Å is observed, but I¯ anions are not involved in the coordination with Cu(I) in compound 2. The difference in crystal structures and compositions of compounds 1 and 2 leads to changes of their fluorescence properties. Compound 1 shows the apparent yellow emission with the maximum at 558 ​nm when exciting at 352 ​nm, which shows sensitive luminescence quenching response to nitrobenzene compounds. However, fluorescent signals of compound 2 were not monitored under UV illumination or on fluorescence spectrophotometer. The results indicate that a minor variation in the substituent on a ligand has a great impact on the structures and properties of the target coordination compounds.

Percutaneous absorption and exposure risk assessment of organophosphate esters in children’s toys

The percutaneous penetration and exposure risk of organophosphate esters (OPEs) from children’s toys remains largely unknown. Percutaneous penetration of OPEs was evaluated by EPISkin™ model. Chlorinated OPEs (Cl-OPEs) and alkyl OPEs, except tris(2-ethylhexyl) phosphate, exhibited a fast absorption rate and good dermal penetration ability with cumulative absorptions of 57.6–127 % of dosed OPEs. Cumulative absorptions of OPEs through skin cells were inversely associated with their molecular weight and log octanol-water partition coefficient. Additionally, a quantitative structure-activity relationship model indicated that topological charge and steric features of OPEs were closely related to the transdermal permeability of these chemicals. With the clarification of the factors affecting the transdermal penetration of OPEs, the level and exposure risk of OPEs in actual toys were studied. The summation of 18 OPE concentrations in 199 toy samples collected from China ranged from 6.82 to 228,254 ng/g, of which Cl-OPEs presented the highest concentration. Concentrations of OPEs in toys exhibited clear type differences. Daily exposure to OPEs via dermal, hand-to-mouth contact, and mouthing was evaluated, and dermal contact was a significant route for children’s exposure to OPEs. Hazard quotients for noncarcinogenic risk assessment were below 1, indicating that the health risk of OPEs via toys was relatively low.

Photocatalytic Hydrogen Production from Aqueous Solutions of Glucose and Xylose over Layered Perovskite-like Oxides HCa2Nb3O10, H2La2Ti3O10 and Their Inorganic-Organic Derivatives.

Nowadays, the efficient conversion of plant biomass components (alcohols, carbohydrates, etc.) into more energy-intensive fuels, such as hydrogen, is one of the urgent scientific and technological problems. The present study is the first one focused on the photoinduced hydrogen evolution from aqueous D-glucose and D-xylose using layered perovskite-like oxides HCa2Nb3O10, H2La2Ti3O10, and their organically modified derivatives that have previously proven themselves as highly active photocatalysts. The photocatalytic performance was investigated for the bare compounds and products of their surface modification with a 1 mass. % Pt cocatalyst. The photocatalytic experiments followed an innovative scheme including dark stages as well as the control of the reaction suspension’s pH and composition. The study has revealed that the inorganic−organic derivatives of the layered perovskite-like oxides can provide efficient conversion of carbohydrates into hydrogen fuel, being up to 8.3 times more active than the unmodified materials and reaching apparent quantum efficiency of 8.8%. Based on new and previously obtained data, it was shown that the oxides’ interlayer space functions as an additional reaction zone in the photocatalytic hydrogen production and the contribution of this zone to the overall activity is dependent on the steric characteristics of the sacrificial agent used.

Role of a "Magic" Methyl: 2'-Deoxy-2'-α-F-2'-β-C-methyl Pyrimidine Nucleotides Modulate RNA Interference Activity through Synergy with 5'-Phosphate Mimics and Mitigation of Off-Target Effects.

Although 2′-deoxy-2′-α-F-2′-β-C-methyl (2′-F/Me) uridine nucleoside derivatives are a successful class of antiviral drugs, this modification had not been studied in oligonucleotides. Herein, we demonstrate the facile synthesis of 2′-F/Me-modified pyrimidine phosphoramidites and their subsequent incorporation into oligonucleotides. Despite the C3′-endo preorganization of the parent nucleoside, a single incorporation into RNA or DNA resulted in significant thermal destabilization of a duplex due to unfavorable enthalpy, likely resulting from steric effects. When located at the terminus of an oligonucleotide, the 2′-F/Me modification imparted more resistance to degradation than the corresponding 2′-fluoro nucleotides. Small interfering RNAs (siRNAs) modified at certain positions with 2′-F/Me had similar or better silencing activity than the parent siRNAs when delivered via a lipid nanoparticle formulation or as a triantennary N-acetylgalactosamine conjugate in cells and in mice. Modification in the seed region of the antisense strand at position 6 or 7 resulted in an activity equivalent to the parent in mice. Additionally, placement of the antisense strand at position 7 mitigated seed-based off-target effects in cell-based assays. When the 2′-F/Me modification was combined with 5′-vinyl phosphonate, both E and Z isomers had silencing activity comparable to the parent. In combination with other 2′-modifications such as 2′-O-methyl, the Z isomer is detrimental to silencing activity. Presumably, the equivalence of 5′-vinyl phosphonate isomers in the context of 2′-F/Me is driven by the steric and conformational features of the C-methyl-containing sugar ring. These data indicate that 2′-F/Me nucleotides are promising tools for nucleic acid-based therapeutic applications to increase potency, duration, and safety.

Acid/Base-Free Acyclic Anionic Oxoborane and Iminoborane Bearing Diboryl Groups.

Anionic oxoboranes and neutral iminoboranes, which are isoelectronic to ketones and alkynes, respectively, have attracted much attention because of their unique structures and various reactivity. However, acid/base-free oxoboranes and iminoboranes are still limited, and readily accessible examples with diverse electronic and steric characteristics are highly desirable. Herein, we report the first syntheses of the acyclic anionic oxoborane 2 and iminoborane 4 bearing two boryl ligands, both of which are acid/base-free. Spectroscopic analysis, X-ray crystallography, and theoretical calculations reveal that 2 and 4 possess a polarized terminal B═O double bond and central B≡N triple bond, respectively.

Steric and Electronic Origins of Fluorescence in GFP and GFP-like Proteins.

Fluorescent proteins have become routine tools for biological imaging. However, their nanosecond lifetimes on the excited state present computational hurdles to a full understanding of these photoactive proteins. In this work, we simulate approximately 0.5 nanoseconds of ab initio molecular dynamics to elucidate steric and electronic features responsible for fluorescent protein behavior. Using green fluorescent protein (GFP) and Dronpa2─widely used fluorescent proteins with contrasting functionality─as case studies, we leverage previous findings in the gas phase and solution to explore the deactivation mechanisms available to these proteins. Starting with ground-state analyses, we identify steric (the distribution of empty pockets near the chromophore) and electronic (electric fields exerted on chromophore moieties) factors that offer potential avenues for rational design. Picosecond timescale simulations on the excited state reveal that the chromophore can access twisted structures in Dronpa2, while the chromophore is largely confined to planarity in GFP. We couple ab initio multiple spawning (AIMS) and enhanced sampling simulations to discover and characterize conical intersection seams that facilitate internal conversion, which is a rare event in both systems. Our AIMS simulations correctly capture the relative fluorescence profiles of GFP and Dronpa2 within the first few picoseconds, and we attribute the diminished fluorescence intensity of Dronpa2, relative to GFP, to flexible chromophore intermediates on the excited state. Furthermore, we predict that twisted chromophore intermediates produce red-shifted intensities in the Dronpa2 fluorescence spectrum. If confirmed experimentally, this spectroscopic signature would provide valuable insights when screening and developing novel fluorescent proteins.

Single-Site Iridium Picolinamide Catalyst Immobilized onto Silica for the Hydrogenation of CO2 and the Dehydrogenation of Formic Acid.

The development of an efficient heterogeneous catalyst for storing H2 into CO2 and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol–gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L3-edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO2. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

Resonant Ionic, Covalent Bond, and Steric Characteristics Present in 1Σu+ States of Li2

The molecular bonding in the excited states of the alkali dimers involves the resonant ionic, covalent bond and steric interactions. We show here the case of the 1Σu+ states of Li2 by ab initio calculation. These interactions as functions of the internuclear distance lead to complex potential energy curves, providing an important application for high resolution laser spectroscopy. The spectroscopic constants for the 4 and 5 1Σu+ states are obtained for the first time.

Solution cis-Proline Conformation of IPCs Inhibitor Aureobasidin A Elucidated via NMR-Based Conformational Analysis.

Aureobasidin A (abA) is a natural depsipeptide that inhibits inositol phosphorylceramide (IPC) synthases with significant broad-spectrum antifungal activity. abA is known to have two distinct conformations in solution corresponding to trans- and cis-proline (Pro) amide bond rotamers. While the trans-Pro conformation has been studied extensively, cis-Pro conformers have remained elusive. Conformational properties of cyclic peptides are known to strongly affect both potency and cell permeability, making a comprehensive characterization of abA conformation highly desirable. Here, we report a high-resolution 3D structure of the cis-Pro conformer of aureobasidin A elucidated for the first time using a recently developed NMR-driven computational approach. This approach utilizes ForceGen's advanced conformational sampling of cyclic peptides augmented by sparse distance and torsion angle constraints derived from NMR data. The obtained 3D conformational structure of cis-Pro abA has been validated using anisotropic residual dipolar coupling measurements. Support for the biological relevance of both the cis-Pro and trans-Pro abA configurations was obtained through molecular similarity experiments, which showed a significant 3D similarity between NMR-restrained abA conformational ensembles and another IPC synthase inhibitor, pleofungin A. Such ligand-based comparisons can further our understanding of the important steric and electrostatic characteristics of abA and can be utilized in the design of future therapeutics.

Strategies to Control the Cis-Trans Isomerization of Peptoid Amide Bonds.

Peptoids are oligomers of N-substituted glycine units. They structurally resemble peptides but, unlike natural peptides, the side chains of peptoids are present on the amide nitrogen atoms instead of the α-carbons. The N-substitution improves cell-permeability of peptoids and enhance their proteolytic stability over natural peptides. Therefore, peptoids are ideal peptidomimetic candidates for drug discovery, especially for intracellular targets. Unfortunately, most peptoid ligands discovered so far possess moderate affinity towards their biological targets. The moderate affinity of peptoids for biomacromolecules is linked to their conformational flexibility, which causes substantial entropic loss during the peptoid-biomacromolecule binding process. The conformational flexibility of peptoids is caused by the lack of backbone chirality, absence of hydrogen bond donors (NH) in their backbone to form CO⋅⋅⋅HN hydrogen bonds and the facile cis-trans isomerization of their tertiary amide bonds. In recent years, many investigators have shown that the incorporation of specific side chains with unique steric and stereoelectronic features can favourably shift the cis-trans equilibria of peptoids towards one of the two isomeric forms. Such strategies are helpful to design homogenous peptoid oligomers having well defined secondary structures. Herein, we discuss the strategies developed over the years to control the cis-trans isomerization of peptoid amide bonds.

Electronic and steric factors affecting the ligands redistribution reaction of [Cu(2,2′-biquinoline)(PR3)2]+ systems in fluid solutions

Complexes with the formula [Cu(bq)(PR3)2]+ {where bq = 2,2′-biquinoline and R = Ph (Cu-Ph-2), C6H4–4-OMe (Cu-OMe-2), C6H4–4-F (Cu-F-2), cyclohexyl (Cu-Cy-2)} were synthesized and characterized. The absorption and emission spectra of the complexes in dichloromethane indicated the presence of [Cu(bq)2]+, resulting from the ligands redistribution reactions. The solvent-dependence of [Cu(bq)2]+ formation from [Cu(bq)(PPh3)2]+ was investigated, highlighting a decrease in the formation constant in the order: DCM > ACE > DMSO > EtOH > MeCN. The presence of coordinating solvents lowers the formation constant; possibly through the formation of stable adducts with the formula [Cu(bq)2(PPh3)(solvent)]+. Moreover, the electronic and steric aspects of the phosphines were explored for the redistribution reaction of [Cu(bq)(PR3)2]NO3 in DCM. The steric environments of the phosphines in Cu-Ph-2, Cu-F-2 and Cu-OMe-2 are identical; the variable electronic properties caused increases in the homoleptic formation constant in the order: Cu-OMe-2 < Cu-Ph-2 < Cu-F-2. Moreover, comparison between the formation constants of [Cu(bq)2]+ from Cu-Ph-2 and Cu-Cy-2 indicated higher values for Cu-Cy-2. Despite the strong electron-donating nature of PCy3, the high steric bulkiness induces the high formation constant. Calculations suggest that Cu-Cy-2 possesses longer Cu-P bonds, rationalizing the fast dissociation rate of PCy3. Overall, the study highlights that the ligand redistribution equilibrium of [Cu(bq)(PR3)2]+ is dependant on the electronic and steric characteristics of the phosphine ligands.

Reactivity of four coordinate iridium complex towards hydrogen: An experimental and computational study

Reaction of a four coordinate, 16-electron Ir complex, [Ir(iPr)4(POCOP)(PPh3)] (4) ((iPr)4(POCOP = 2,6-bis(di-isopropyl phosphinito)benzene, κ3-C6H3-1,3-[OP(iPr)2]2), with H2 resulted in an oxidative addition product, cis-dihydride complex, cis-[Ir(H)2(iPr)4(POCOP)(PPh3)] (cis-5) presumably via the intermediacy of a sigma complex, [Ir(η2-H2)(iPr)4(POCOP)(PPh3)]. The cis-dihydride complex completely isomerizes to the trans-dihydride complex trans-[Ir(H)2(iPr)4(POCOP)(PPh3)] (trans-5) under ambient conditions in about 3 h. It was found that the steric and electronic features on the iridium center have significant influence on the approach of H2 onto the metal center followed by oxidative addition and isomerization. The isomerization process was studied in detail and all the mechanistic aspects have been elucidated using a combination of both experimental work and computation. The cis-dihydride complex isomerizes to the trans-dihydride by compensating the trans influence of the strongly trans-directing hydride ligand. A mechanism involving the exchange of the position of PPh3 with a hydride ligand cis to itself via PPh3 dissociation and re-coordination thereby resulting in the formation of the trans-dihydride complex, has been proposed for the isomerization. The cis-dihydride was found to be a highly active catalyst for hydrogenation of ethylene. A competing reactivity study of cis-dihydride between isomerization versus insertion of C2H4 into the Ir—H bond, was studied experimentally and computationally.

Structural Bioinformatics Used to Predict the Protein Targets of Remdesivir and Flavones in SARS-CoV-2 Infection.

During the current SARS-CoV-2 pandemic, the identification of effective antiviral drugs is crucial. Unfortunately, no specific treatment or vaccine is available to date. Here, we aimed to predict the interactions with SARS-CoV-2 proteins and protein targets from the human body for some flavone molecules (kaempferol, morin, pectolinarin, myricitrin, and herbacetin) in comparison to synthetic compounds (hydroxychloroquine, remdesivir, ribavirin, ritonavir, AMD-070, favipiravir). Using MOE software and advanced bioinformatics and cheminformatics portals, we conducted an extensive analysis based on various structural and functional features of compounds, such as their amphiphilic field, flexibility, and steric features. The structural similarity analysis of natural and synthetic compounds was performed using Tanimoto coefficients. The interactions of some compounds with SARS-CoV-2 3CLprotease or RNA-dependent RNA polymerase were described using 2D protein-ligand interaction diagrams based on known crystal structures. The potential targets of considered compounds were identified using the SwissTargetPrediction web tool. Our results showed that remdesivir, pectolinarin, and ritonavir present a strong structural similarity which may be correlated to their similar biological activity. As common molecular targets of compounds in the human body, ritonavir, kaempferol, morin, and herbacetin can activate multidrug resistance-associated proteins, while remdesivir, ribavirin, and pectolinarin appear as ligands for adenosine receptors. Our evaluation recommends remdesivir, pectolinarin, and ritonavir as promising anti- SARS-CoV-2 agents.

ESTUDO ONE-POT DA APLICAÇÃO DO CATALISADOR DE GRUBBS DE PRIMEIRA GERAÇÃO EM REAÇÕES DE METÁTESE DA PIPERINA E EUGENOL: A INFLUÊNCIA DA LIGAÇÃO DUPLA TERMINAL

ONE-POT STUDY OF THE APPLICATION OF CATALYST FIRST-GENERATION GRUBBS IN METATHESIS REACTIONS OF PIPERINE AND EUGENOL: THE INFLUENCE OF THE DOUBLE- BOND TERMINAL. In this paper, the piperine (PIP) and eugenol (EUG) organic compounds were used as substrates in olefin metathesis reactions catalyzed by first-generation Grubbs catalyst (G1) – [RuCl2(PCy3)2(=CHPh)], Cy is cyclohexyl and Ph is phenyl, in a one-pot system. The reactions occurred at 50 ºC, for 24, 48, or 96 h using the two substrates simultaneously in the ratio Ru:PIP:EUG of 1:1:1 and 1:10:10 mol. The results were evaluated by GC-MS, discussed in terms of the electronic and steric characteristics of the G1, and the structural proprieties of the substrates. In the Ru:PIP:EUG ratio of 1:1:1 mol, a dimeric species was observed for the olefin metathesis of EUG, in all time studied and in an additional experiment at 70 ºC for 48 h. When the Ru:PIP:EUG ratio was 1:10:10 mol, this dimer was not observed. Isomerization processes also were observed at all temperatures and catalyst:substrates ratio. The reaction yield increased with the raise of substrates amount. There was no cross-metathesis reaction. This fact was attributed to the chelating effect of the double coordination of the PIP to the catalyst.

Finely tuning electronic and steric structures of spiro[fluorene-9,9′-xanthene] (SFX)-based emitters by isomerization strategy towards efficient electroluminescence

Considering the advantages of spiro[fluorene-9,9′-xanthene] (SFX) on facilitating radiative transition and suppressing concentration quenching, a pair of isomers, namely F-BT-F and X-BT-X, were designed and synthesized with benzo[c][1,2,5]thiadiazole (BT) as an acceptor core to bridge fluorene moieties (F-BT-F) or xanthene moieties (X-BT-X) of SFXs. The pair of isomers with D-A-D architecture presented almost the same energy gap in chloroform solution, providing a delicate model to probe the electronic and steric differences between binary structure of SFX. Single-crystal diffractions reveal that SFX-donor units are in antiparallel arrangement on the both sides of the BT core, and X-BT-X molecules have more discrete and ordered packing compared to F-BT-F in crystal phase. Photophysical, electrochemical measurements and DFT calculations manifest that the pair of emitters possess excellent photoluminescence quantum yields up to 95%, and the X-BT-X shows weaker charge-transfer character compared with the F-BT-F, attributed to the strong electron-donating nature and partly interrupted conjugation pathway of xanthene-end relative to fluorene-end. In doped organic light-emitting devices (OLEDs), the F-BT-F and X-BT-X displayed nearly uniform electroluminescence (EL) with the emission peaks at 515 and 518 nm, as well as the maximum external quantum efficiencies (EQEs) approaching 5.0%. In non-doped OLEDs, however, the X-BT-X had more remarkable EL performance with the emission peak at 525 nm, maximum current efficiencies of 14.02 cd A−1, maximum power efficiencies of 8.81 lm W−1 and maximum EQE of 4.25%, exceeding the efficiency values of 13.75 cd A−1, 7.71 lm W−1, and 4.08% of F-BT-F based device. By illuminating the electronic and steric features of xanthene moiety, this work opens a new door to construct high-efficiency and low-cost SFX-based fluorescence emitters.