Number Of Methyl Substituents Research Articles

Tailoring the dimensionality of metal complexes via ligand modifications.

A series of CuII complexes obtained under the same reaction conditions has been analyzed to gain insight into the effect of the ligand composition on the final reaction product. Dipodal ligands containing N-donor imidazole rings and a benzene ring as a spacer with different numbers of methyl substituents on the aromatic rings were selected for the study such as 1,3-bis(imidazol-1-ylmethyl)benzene (L1), 1,3-bis(imidazol-1-ylmethyl)-5-methylbenzene (L2), 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (L3), 1,3-bis(2-methylimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (L4). L4 has not been reported previously and was synthesized for this study. The formed metal complexes show the presence of polymeric (ligand with no or one methyl substituent; 1-4), or discrete motifs (3- or 5-methyl substituents; 5-7). The new metal complexes 3, 5 and 6 were analyzed using single-crystal X-ray diffraction and powder diffraction. In addition, the structural analyses were supported by computational methods.

Halo-bridged Cu(II) complexes with pyridine–pyrazole ligands: Influence of halogen and ligand lipophilicity on antimicrobial activity

A series of five coordinated, doubly halo bridged Cu(II) complexes [Cu2(μ-X)2(Ln)2×2] (X=Cl and Br; n = 1, 2 and 3), CuL1Cl, CuL1Br, CuL2Cl, CuL2Br, CuL3Cl and CuL3Br (where L1 = 2-((1H-pyrazol-1-yl)methyl)pyridine, L2 = 2-((3-methyl-1H-pyrazol-1-yl)methyl)pyridine, L3 = 2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)pyridine) have been synthesized and structurally characterized using FT-IR and UV–Vis spectroscopy analysis. Moreover, CuL1Cl, CuL1Br and CuL3Cl structures have also been confirmed by single crystal X-ray structural analysis. Copper ion in CuL1Cl, CuL1Br and CuL3Cl displayed distorted squared pyramidal geometry and formed dimers via bridged μ‑chloro/μ‑bromo coordination in the crystal lattice. The intermolecular C–H···Cl hydrogen bonding interaction further connected the dimers of CuL1Cl and CuL3Cl with four neighboring dinuclear complex units. Copper complexes were evaluated for their antimicrobial activity against Gram-negative and Gram-positive bacteria. Interestingly, copper complexes exhibited selective antimicrobial activity against Gram-positive bacteria. Further the increasing ligand lipophilicity showed the increase of antibacterial activity of copper complexes. The lipophilicity of ligands was regulated by number of methyl substituents on the ligands. The chloro bridged copper complexes exhibited relatively higher antibacterial activity compared to the bromo bridged copper complexes. Thus, the present structure-activity of simple copper complexes might provide insight on the structural design of new antimicrobial agents based on coordination complexes.

Biodegradation levels of oils from the Chepaizi Uplift, Junggar Basin (NW China) evaluated by a full-range biodegradation index as constrained by adamantane, diamantane homologs and carboxylic acids

Biodegradation usually masks or even radically alters the original characteristics of molecular biomarkers, restricting the oil sources identification and the exploration of ultra-heavy oils (also known as oil sands, tar sands, asphalt or bitumen). The oils from the Lower Neogene Shawan Formation (N1s), Lower Cretaceous Tugulu Group (K1tg), and Upper Carboniferous Xibeikulasi Formation (C2x) in the Chepaizi Uplift, Junggar Basin (NW China) were biodegraded to different extent, consequently, many commonly used biomarker parameters became less powerful for the evaluation of the extent of biodegradation. To address this problem, the resistance of many adamantane and diamantane homologs and carboxylic acids to biodegradation was analyzed, and those with high bioresistance were selected to yield a more precise and extensive characterization of biodegradation from light to extreme. In contrast, oils from the eastern Chepaizi Uplift were generated from Permian sources, and have been subjected to heavy to extreme levels of biodegradation. In this study, fifteen oil samples obtained from the Chepaizi Uplift were analyzed by a set of mass spectrometric techniques. The results were applied to investigate the variations of commonly used biomarkers in the saturates and aromatics, to qualitatively rank oil biodegradation levels based on the PM scale and to provide a detailed assessment of the variation in carboxylic acids and adamantane and diamantane homologs in heavy oils from varying degrees of biodegradation. The results demonstrate that adamantane and diamantane homologs with various methyl groups, and carboxylic acids with various double bond equivalents (DBEs), exhibit an intact homolog series distribution and certain hypermethylation trend as biodegradation levels progress from light to extreme, which confirmed their reliability in refining the evaluation scale and characterizing the extent of biodegradation. In addition, the full-range biodegradation index (FRBI) presented here consists of the ratios of adamantane/diamantane homologs with different numbers of methyl substituents and the ratios of carboxylic acids with different DBE values, instead of the five Manco scores of 0–4 used previously, which can effectively improve the ranking precision of biodegradation of oils between subtle alteration changes, and universally unraveling to the full range of biodegradation, beginning from light to extreme.

Toxicological effects of different ionic liquids on growth, photosynthetic pigments, oxidative stress, and ultrastructure of Nostoc punctiforme and the combined toxicity with heavy metals

Ionic liquids (ILs1) are used widely because of their excellent properties. However, their ecotoxicity for environment has aroused great concern. Here we studied, the toxicity of three ILs with different numbers of methyl substituents and anions as well as the combined effect of heavy metals to edible algae Nostoc punctiforme. The results show that fresh weight and chlorophyll content decreased, indicating that the growth and photosynthesis were adversely affected. Polysaccharides and soluble protein contents decreased, resulting in a reduced nutritional value of Nostoc punctiforme. ILs can produce many reactive oxygen species (ROS), which lead to increased the malondialdehyde (MDA) content. In order to remove excessive ROS, antioxidant enzymes activity is increased, but decreases under high IL concentration, because the structure and function of the enzymes became damaged. ILs cause stress to algae, as the cell ultrastructure is indicating by increased amounts of starch and osmiophilic globules. The combined action of heavy metals with ILs decreases the antioxidant enzymes activity and chlorophyll content, and increases the MDA content. The results show that the order of toxicity is [C8MIM]Cl >[C8MIM]Br> [C8DMIM]Br. The combination of heavy metals and ILs cause an increase of the toxicity to Nostoc punctiforme.

Interplay of the Ring and Steric Strains in the Highly Substituted Cyclopropanes.

Using the concept of a complete set of homodesmotic reactions for the analysis of molecular energetics of polysubstituted methyl- and fluorocyclopropanes allows assessing the strain energy SE of cyclopropanes, free from interfering effects, in full accordance with the IUPAC definition ("relative to a reference ... hypothetical 'strainless' structure"). The correct SE calculation requires quantifying nonvalence interactions in the products of formal homodesmotic reactions (HDRs) using a routine multiregression analysis. The complete HDR set provides the information necessary for the analysis, namely, the heat effects of HDRs calculated by the G4 composite method and the wide set of reference compounds with various combinations of nonvalence effects. We have found that the SE value for methylcyclopropanes lies in the range from 117.0 (1.1-dimethylcyclopropane) to 146.1 kJ/mol (hexamethylcyclopropane). It is the sum of the ring strain energy RSE = 117.9 ± 0.3 kJ mol, which does not depend on the number of methyl substituents, and the Pitzer strain energy of 4.4±0.1 kJ/mol per one contact (the standard deviation is shown as an error of determination). In the series of fluorocyclopropanes, SE varies from 137.9 (monosubstituted cyclopropane) to 260.0 kJ/mol (hexafluorocyclopropane) and well correlates with the ∑DBCP parameter deduced from the QTAIM analysis of the electron density of the compound, representing the total deviation of bond critical points from geometrical C-C bond lines of CC bonds. The ∑DBCP parameter characterizes the curvature of banana-like bonds in cyclopropanes.

Mineralization of sulfonamides from wastewater using ozone-based systems.

Ozone, UV/ozone, ozone/persulfate (PS) and UV/ozone/PS systems were used to mineralize sulfonamides. Sulfadiazine (SDZ), sulfamerazine (SMR) and sulfamethazine (SMZ) were the target compounds. The novel contribution of this study is its determination of the effects of PS addition, sulfonamide structure, pH and salinity on sulfonamide mineralization in ozone-based systems. The mineralization rate of sulfonamides satisfied pseudo-first-order kinetics. The SMZ mineralization rate constant in ozone, UV/ozone, ozone/PS and UV/ozone/PS systems at pH 5 were 0.0058; 0.0101; 0.0069 and 0.0802 min-1, respectively, and those at pH 7 were 0.0075; 0.0116; 0.0083 and 0.0873 min-1, respectively. The increase in the number of methyl substituents in the heterocyclic group of SMZ and the corresponding increase in the steric hindrance of radical addition, reduced mineralization rates below those of SMR and SDZ. The addition of PS promoted sulfonamide mineralization in the ozone-based systems; conversely, salinity inhibited sulfonamide mineralization.

Oxidative stress response mechanism of Scenedesmus obliquus to ionic liquids with different number of methyl-substituents

Ionic liquids (ILs) have become persistent contaminants in water because of their good solubility and low biodegradability. The oxidative stress responses of Scenedesmus obliquus to three imidazole ILs with different number of methyl-substituents, i.e., 1-decyl-imidazolium chloride ([C10IM]Cl), 1-decyl-3-methylimidazolium chloride ([C10MIM]Cl), and 1-decyl-2,3-dimethylimidazolium chloride ([C10DMIM]Cl), were studied. There was a positive correlation between ROS level and IL concentration. The activities of antioxidant enzymes, i.e., superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and glutathione peroxidase, and the content of antioxidants, i.e., ascorbic acid and glutathione, changed in IL treatment with a concentration-dependent effect. Proline accumulation increased with increasing IL concentration. Integrated biomarker response (IBR) index analysis, based on the eight oxidative stress response indicators, revealed that the toxicity order was: [C10IM]Cl < [C10DMIM]Cl < [C10MIM]Cl. Proteomic analysis showed that IL affect the type and distribution of proteins in S. obliquus. Chloroplast and photosystem II were affected as cellular component, and the proteins related to oxidative stress are annotated in GO categories. IBR index and proteomic analysis indicate that oxidative stress response is one of the main biomarkers of IL stress.

Tuning of Sensitivity in Thermochromic Luminescence by Regulating Molecular Rotation Based on Triphenylamine‐Substituted o‐Carboranes

AbstractThis study shows tuning of sensitivity in thermochromic luminescence from triphenylamine‐substituted o‐carboranes by regulating rotation barriers with a substituent effect. A series of o‐carborane dyads with various numbers of methyl substituents were prepared, and their optical properties were compared. Initially, significant dual‐emission properties consistent of the locally excited (LE) state and the induced charge transfer (ICT) state were observed from the solid samples of all dyads. In particular, diverse responses were obtained by cooling from the dyads. The pristine and mono‐substituted dyads presented thermally independent emission during temperature changes, whereas the detected thermochromic luminescence with drastic color change from red to blue originated from the alteration of intensity ratios between LE and CT emission bands. From mechanistic studies (experimental and theoretical), it was clearly shown that the difference in the rotation barrier of the o‐carborane unit caused by the substituent effect should play a key role in variable sensitivity in luminescent chromic behaviors in temperature change.

Rational Design of BODIPY-Diketopyrrolopyrrole Conjugated Polymers for Photothermal Tumor Ablation.

Conjugated polymers (CPs) have drawn growing attention in cancer phototherapy and imaging due to their large extinction coefficients, robust photostability, and good biocompatibility. Herein, we propose a new type of photothermal therapy materials on the basis of BODIPY-diketopyrrolopyrrole CPs, where the number of methyl substituents at the β and β' positions on BODIPYs is variable, allowing us to investigate the interplay between the structure of the monomers and the related properties of CPs. Combining the experimental data with theoretical calculations, we concluded that with the decrease of the number of methyl moieties on the β and β' positions of BODIPY, the polymerization degree and the solubility of the obtained CPs improved and the polymeric spatial planarization and degrees of conjugation increased, inducing the bathochromic shift of absorption, which resulted in the absorption spectra getting closer to the near-infrared region and more conducive to the application of the conjugated polymers in vivo. Afterward, the CP nanoparticles were constructed and their photothermal activity in cancer therapy was validated by a series of in vitro and in vivo experiments. In this paper, we provide a new way to manipulate properties of CPs with great potential in photothermal therapy through structural engineering.

Effect of differently methyl-substituted ionic liquids on Scenedesmus obliquus growth, photosynthesis, respiration, and ultrastructure

Concerns have been raised regarding the ecotoxicity of ionic liquids (ILs) owing to their wide usage in numerous fields. Three imidazolium chloride ILs with different numbers of methyl substituents, 1-decyl-imidazolium chloride ([C10IM]Cl), 1-decyl-3-methylimidazolium chloride ([C10MIM]Cl), and 1-decyl-2,3-dimethylimidazolium chloride ([C10DMIM]Cl), were examined to assess their effects on growth, photosynthesis pigments content, chlorophyll fluorescence, photosynthetic and respiration rate, and cellular ultrastructure of Scenedesmus obliquus. The results showed that algal growth was significantly inhibited by ILs treatments. The observed IC50,48h doses were 0.10 mg/L [C10IM]Cl, 0.01 mg/L [C10MIM]Cl, and 0.02 mg/L [C10DMIM]Cl. The chlorophyll a, chlorophyll b, and total chlorophyll content declined, and the chlorophyll fluorescence parameters, minimal fluorescence yield (F0), maximal fluorescence yield (Fm), maximum quantum yield of PSII photochemistry (Fv/Fm), effective quantum yield of PSII [Y(II)], non-photochemical quenching (NPQ) and non-photosynthetic losses yield [Y(NO)] were notably affected by ILs in a dose-dependent manner. ILs affected the primary photosynthetic reaction, impaired heat dissipation capability, and diminished photosynthetic efficiency, indicating negative effects on photosystem II. The photosynthetic and respiration rates of algal cells were also reduced due to the ILs treatments. The adverse effects of ILs on plasmolysis and chloroplast deformation were examined using ultrastructural analyses; chloroplast swelling and lamellar structure almost disappeared after the [C10MIM]Cl treatment, and an increased number of starch grains and vacuoles was observed after all ILs treatments. The results indicated that one-methyl-substituted ILs were more toxic than non-methyl-substituted ILs, which were also more toxic than di-methyl-substituted ILs. The toxicity of the examined ILs showed the following order: [C10IM]Cl < [C10DMIM]Cl ≤ [C10MIM]Cl.

Effect of methyl substituents in the reactivity of methylxanthines.

The methylxanthines have attracted interest due to the changes on their biological activities and physicochemical properties in terms of the number and position of the methyl groups present in the xanthine moiety. We report a theoretical study of the influence of the methyl substituent in the basicity and reactivity of xanthine and its methylated derivatives. Our results provide that when the xanthine increases the number of methyl substituents, the gas phase basicity increases (reactivity to proton increases), and the global hardness decreases. The result is in agreement with the maximum hardness principle (MHP) that states, "at equilibrium, chemical systems are as hard as possible" (Pearson, R.G., J. Chem. Educ., 1987, 64, 561-567, and Parr R.G., Chattaraj P.K., J. Am. Chem. Soc. 1991, 113, 1854-1855). Graphical abstract Xanthine and its methyl derivatives.

Surface Structuring and Water Interactions of Nanocellulose Filaments Modified with Organosilanes toward Wearable Materials.

Colloidal dispersions of cellulose nanofibrils (CNFs) are viable alternatives to cellulose II dissolutions used for filament spinning. The porosity and water vapor affinity of CNF filaments make them suitable for controlled breathability. However, many textile applications also require water repellence. Here, we investigated the effects of postmodification of wet-spun CNF filaments via chemical vapor deposition (CVD). Two organosilanes with different numbers of methyl substituents were considered. Various surface structures were achieved, either as continuous, homogeneous coating layers or as three-dimensional, hairy-like assemblies. Such surface features reduced the surface energy, which significantly affected the interactions with water. Filaments with water contact angles of up to 116° were obtained, and surface energy measurements indicated the possibility of developing amphiphobicity. Dynamic vapor sorption and full immersion experiments were carried out to inquire about the interactions with water, whether in the liquid or gas forms. Mechanical tests revealed that the wet strength of the modified filaments were almost 3 times higher than that of the unmodified precursors. The hydrolytic and mechanical stabilities of the adsorbed layers were also revealed. Overall, our results shed light on the transformation of aqueous dispersions of CNFs into filaments that are suited for controlled interactions with water via concurrent hydrolysis and condensation reactions in CVD, while maintaining the moisture buffering capacity and breathability of related structures.

Theoretical Modeling of Thermal Decomposition of Methylnaphthalene Derivatives: Influence of Substituents

The kinetics and thermodynamics of thermal decomposition of naphthalene and several mono- and dimethylnaphathalene derivatives have been established through reactive molecular dynamics simulations using ReaxFF. These results were compared to previous theoretical and experimental studies and also compared to density functional theory calculation results. This work demonstrates that the kinetics and thermodynamics of the initial activation reaction is directly impacted by the position and number of methyl substituents. Subsequently, the activated molecules react to form either small organic or large char molecules. The char formation mechanism is shown to occur in three steps: activation, dimerization/trimerization, and condensation. Finally, temperature effects on the char formation reaction were also studied.

Binding Mode Selection Determines the Action of Ecstasy Homologs at Monoamine Transporters.

Determining the structural elements that define substrates and inhibitors at the monoamine transporters is critical to elucidating the mechanisms underlying these disparate functions. In this study, we addressed this question directly by generating a series of N-substituted 3,4-methylenedioxyamphetamine analogs that differ only in the number of methyl substituents on the terminal amine group. Starting with 3,4-methylenedioxy-N-methylamphetamine, 3,4-methylenedioxy-N,N-dimethylamphetamine (MDDMA) and 3,4-methylenedioxy-N,N,N-trimethylamphetamine (MDTMA) were prepared. We evaluated the functional activities of the compounds at all three monoamine transporters in native brain tissue and cells expressing the transporters. In addition, we used ligand docking to generate models of the respective protein-ligand complexes, which allowed us to relate the experimental findings to available structural information. Our results suggest that the 3,4-methylenedioxyamphetamine analogs bind at the monoamine transporter orthosteric binding site by adopting one of two mutually exclusive binding modes. 3,4-methylenedioxyamphetamine and 3,4-methylenedioxy-N-methylamphetamine adopt a high-affinity binding mode consistent with a transportable substrate, whereas MDDMA and MDTMA adopt a low-affinity binding mode consistent with an inhibitor, in which the ligand orientation is inverted. Importantly, MDDMA can alternate between both binding modes, whereas MDTMA exclusively binds to the low-affinity mode. Our experimental results are consistent with the idea that the initial orientation of bound ligands is critical for subsequent interactions that lead to transporter conformational changes and substrate translocation.

Control of morphology for energy dissipation in carbon nanotube forests

This study focuses on the effect of carbon precursor on the carbon nanotube (CNT) morphology and energy dissipation. Benzene, toluene, and m-xylene were used as carbon precursors for the synthesis of CNT forests following a chemical vapor deposition process. The results indicate that substituents on the benzene ring increase entanglement in the CNT forests. The absorbed energy was slightly greater for CNT forests synthesized using m-xylene than for toluene, but was much smaller for benzene. When compressed to a strain of 0.67, the toluene CNTs absorbed more energy than the m-xylene CNTs. The restitution was much higher for the forests synthesized with m-xylene than toluene while it further decreased for the forests made with benzene. A strong correlation is also observed between the average diameter of the CNTs and the number of methyl substituents on the benzene ring. The control of the entanglement of the CNT forests can potentially be used to design high energy absorbing composites for blast energy dissipation.

FTIR gas-phase kinetic study on the reactions of some acrylate esters with OH radicals and Cl atoms.

Acrylate esters are α,β-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon. These compounds are widely used in the production of plastics and resins. Atmospheric degradation processes of these compounds are currently not well understood. The kinetics of the gas phase reactions of OH radicals with methyl 3-methylacrylate and methyl 3,3-dimethylacrylate were determined using the relative rate technique in a 50 L Pyrex photoreactor using in situ FTIR spectroscopy at room temperature (298 ± 2 K) and atmospheric pressure (708 ± 8 Torr) with air as the bath gas. Rate coefficients obtained were (in units cm(3) molecule(-1) s(-1)): (3.27 ± 0.33) × 10(-11) and (4.43 ± 0.42) × 10(-11), for CH3CH═CHC(O)OCH3 and (CH3)2CH═CHC(O)OCH3, respectively. The same technique was used to study the gas phase reactions of hexyl acrylate and ethyl hexyl acrylate with OH radicals and Cl atoms. In the experiments with Cl, N2 and air were used as the bath gases. The following rate coefficients were obtained (in cm(3) molecule(-1) s(-1)): k3 (CH2═CHC(O)O(CH2)5CH3 + Cl) = (3.31 ± 0.31) × 10(-10), k4(CH2═CHC(O)OCH2CH(CH2CH3)(CH2)3CH3 + Cl) = (3.46 ± 0.31) × 10(-10), k5(CH2═CHC(O)O(CH2)5CH3 + OH) = (2.28 ± 0.23) × 10(-11), and k6(CH2═CHC(O)OCH2CH(CH2CH3)(CH2)3CH3 + OH) = (2.74 ± 0.26) × 10(-11). The reactivity increased with the number of methyl substituents on the double bond and with the chain length of the alkyl group in -C(O)OR. Estimations of the atmospheric lifetimes clearly indicate that the dominant atmospheric loss process for these compounds is their daytime reaction with the hydroxyl radical. In coastal areas and in some polluted environments, Cl atom-initiated degradation of these compounds can be significant, if not dominant. Maximum Incremental Reactivity (MIR) index and global warming potential (GWP) were also calculated, and it was concluded that these compounds have significant MIR values, but they do not influence global warming.

Multichannel-emissive V-shaped boryl-BODIPY dyads: synthesis, structure, and remarkably diverse response toward fluoride.

Three new V-shaped boryl-BODIPY dyads (1-3) were synthesized and structurally characterized. Compounds 1-3 are structurally close molecular siblings differing only in the number of methyl substituents on the BODIPY moiety that were found to play a major role in determining their photophysical behavior. The dyads show rare forms of multiple-channel emission characteristics arising from different extents of electronic energy transfer (EET) processes between the two covalently linked fluorescent chromophores (borane and BODIPY units). Insights into the origin and nature of their emission behavior were gained from comparison with closely related model molecular systems and related photophysical investigations. Because of the presence of the Lewis acidic triarylborane moiety, the dyads function as highly selective and sensitive fluoride sensors with vastly different response behaviors. When fluoride binds to the tricoordinate borane center, dyad 1 shows gradual quenching of its BODIPY-dominated emission due to the ceasing of the (borane to BODIPY) EET process. Dyad 2 shows a ratiometric fluorescence response for fluoride ions. Dyad 3 forms fluoride-induced nanoaggregates that result in fast and effective quenching of its fluorescence intensity just for ∼0.3 ppm of analyte (i.e., 0.1 equiv ≡ 0.26 ppm of fluoride). The small structural alterations in these three structurally close dyads (1-3) result in exceptionally versatile and unique photophysical behaviors and remarkably diverse responses toward a single analyte, i.e., fluoride ion.

A Novel Series of CoIII(salen-type) Complexes Containing a Seven-Membered Metallacycle: Synthesis, Structural Characterization and Factors Affecting the Metallacyclization Rate

A series of electronically tuned trans-[CoIII(chel)(CH2Cl)]2 complexes, where chel is a salen derivative (salen = 2,2′-ethylenebis(nitrilomethylidene)diphenol) containing either two or four methyl substituents in different positions, has been synthesized and characterized, both in solution and in the solid state. These complexes undergo an intramolecular cyclization reaction in methanolic solution to form the corresponding cis β organometallic derivative containing a seven-membered metallacycle, by replacement of the Cl atom of the axial CH2Cl by the salen phenolate oxygen. The cyclization rate increases on going from two to four methyl substituents in the chelate, in agreement with the electrochemical data that evidence a general shift toward more negative values with an increase in the number of methyl substituents. The cyclization rate is also affected by the substituent position, and both electrochemical and kinetic data evidence a remarkable influence of the methyls on the −C═N– groups of the chelate...

Dicopper(II) Metallacyclophanes with Electroswitchable Polymethyl‐Substituted para‐Phenylene Spacers

Double-stranded anionic dinuclear copper(II) metallacyclic complexes of the paracyclophane type [Cu2L2](4-) have been prepared by the Cu(II)-mediated self-assembly of different para-phenylenebis(oxamato) bridging ligands with either zero-, one-, or four-electron-donating methyl substituents (L=N,N'-para-phenylenebis(oxamate) (ppba; 1), 2-methyl- N,N'-para-phenylenebis(oxamate) (Meppba; 2), and 2,3,5,6-tetramethyl- N,N'-para-phenylenebis(oxamate) (Me4ppba; 3)). These complexes have been isolated as their tetra-n-butylammonium (1 a-3 a), lithium(I) (1 b-3 b), and tetraphenylphosphonium salts (1 c-3 c). The X-ray crystal structures of 1 a and 3 c show a parallel-displaced π-stacked conformation with a smaller deviation from perpendicularity between the two benzene rings and the basal planes of the square planar Cu(II) ions when increasing the number of methyl substituents (average dihedral angles (ϕ) of 58.72(7) and 73.67(5)° for 1 a and 3 c, respectively). Variable-temperature (2.0-300 K) magnetic-susceptibility measurements show an overall increase of the intramolecular antiferromagnetic coupling with the number of methyl substituents onto the para-phenylene spacers (-J=75-95, 100-124, and 128-144 cm(-1) for 1 a-c, 2 a-c, and 3 a-c, respectively; H=-JS1×S2). Cyclic voltammetry (CV) measurements show a reversible one-electron oxidation of the double polymethyl-substituted para-phenylenediamidate bridging skeleton at a relatively low formal potential that decreases with the number of methyl substituents (E1=+0.33, +0.24, and +0.15 V vs. SCE for 1-3, respectively). The monooxidized dicopper(II) π-radical cation species 3' prepared by the chemical oxidation of 3 with bromine exhibits intense metal-to-ligand charge-transfer (MLCT) transitions in the visible and near-IR (λmax=595 and 875 nm, respectively) regions together with a rhombic EPR signal with a seven-line splitting pattern due to hyperfine coupling with the nuclear spin of the two Cu(II) ions. Density functional (DF) calculations for 3' evidence a characteristic iminoquinonoid-type short-long-short alternating sequence of C-N and C-C bonds for both tetramethyl-para-phenylenediamidate bridges and a large amount of spin density of negative sign mainly delocalized along each of the four benzene C atoms directly attached to the amidate N atoms, which is in agreement with a fully delocalized π-stacked monoradical ligand description. Hence, the spins of the two Cu(II) ions (SCu=1/2) that are antiparallel aligned in 3 (OFF state) become parallel in 3' (ON state). Further developments may be then envisaged for this new permethylated dicopper(II) paracyclophane with a redox noninnocent ligand as a prototype for molecular magnetic electroswitch.

Tictoid Expanded Pyridiniums: Assessing Structural, Electrochemical, Electronic, and Photophysical Features

In regard to semirigid donor-spacer-acceptor (D-S-A) dyads devised for photoinduced charge separation and built from an unsaturated spacer, there exists a strategy of design referred to as "geometrical decoupling" that consists in introducing an inner-S twist angle approaching 90° to minimize adverse D/A mutual electronic influence. The present work aims at gaining further insights into the actual impact of the use of bulky substituents (R) of the alkyl type on the electronic structure of spacers (S) of the oligo-p-phenylene type, which can be critical in the functioning of derived dyads. To this end, a series of 12 novel expanded pyridiniums (EPs), regarded as model S-A assemblies, was synthesized and its structural, electronic, and photophysical properties were investigated at both experimental and theoretical levels. These EPs result from the combination of 4 types of pyridinium-based acceptor moieties with the three following types of S subunits connected at position 4 of the pyridinum core: xylyl (X), xylyl-phenyl (XP), and xylyl-tolyl (XT). From comparison of collected data with those already reported for eight other EPs based on the same A components but linked to S fragments of two other types (i.e., phenyl, P, and biphenyl, PP), the following quantitative order in regard to the pivotal S-centered HOMO energy perturbation was derived (sorted by increasing destabilization): P < X ≪ PP ≈< XP ≈< XT. This indicates that spacers (S) are primarily distinguished on the basis of their mono- or biaryl composition and secondarily by their number of methyl substituents (R). The electron-donating inductive contribution of methyl substituents (HOMO destabilization) more than counterbalances the effect of conjugation disruption (HOMO stabilization). This "compensation effect" suggests that mildly electron-withdrawing hindering groups are better suited for "geometrical decoupling", given that high-energy S-centered occupied MOs can assist charge recombination within D-S-A dyads.