Symmetrical Alkyl Research Articles

Molecular Ordering Manipulation in Fused Oligomeric Mixed Conductors for High-Performance n-Type Organic Electrochemical Transistors.

Advanced n-type organic electrochemical transistors (OECTs) play an important part in bioelectronics, facilitating the booming of complementary circuits-based biosensors. This necessitates the utilization of both n-type and p-type organic mixed ionic-electronic conductors (OMIECs) exhibiting a balanced performance. However, the observed subpar electron charge transport ability in most n-type OMIECs presents a significant challenge to the overall functionality of the circuits. In response to this issue, we achieve high-performance OMIECs by leveraging a series of fused electron-deficient monodisperse oligomers with mixed alkyl and glycol chains. Through molecular ordering manipulation by optimizing of their alkyl side chains, we attained a record-breaking OECT electron mobility of 0.62 cm2/(V s) and μC* of 63.2 F/(cm V s) for bgTNR-3DT with symmetrical alkyl chains. Notably, the bgTNR-3DT film also exhibits the highest structural ordering, smallest energetic disorder, and the lowest trap density among the series, potentially explaining its ideal charge transport property. Additionally, we demonstrate an organic inverter incorporating bgTNR-3DT OECTs with a gain above 30, showcasing the material's potential for constructing organic circuits. Our findings underscore the indispensable role of alkyl chain optimization in the evolution of prospective high performance OMIECs for constructing advanced organic complementary circuits.

Liquid crystalline behaviour of symmetrical thermotropic Schiff base compounds

A series of new symmetrical liquid crystal compounds with Schiff base as a linking unit attached to different terminal alkyl chains were synthesized and characterized. The intermediates of 4-(decyloxy)benzaldehyde, 4-(dodecyloxy)benzaldehyde, and 4-(tetradecyloxy)benzaldehyde were synthesized through alkylation reaction between 4-hydroxybenzaldehyde with three series of bromoalkanes. Next, it further reacted with 1,4-phenylenediamine through condensation reaction to produce symmetrical Schiff base linkage with different terminal group chain which indicated as N-(substitutedbenxylidene)benzene-1,4-diamine. The compounds were characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and CHN elemental analysis. The Schiff base linkages are detectable in the FTIR spectra within the range of 1601–1604 cm⁻¹ and in the NMR spectra at δ 8.47–8.48 ppm. The mesophase transition phase of this compounds were determined using polarized optical microscope (POM) and further confirmed using differential scanning calorimetry (DSC). The Schiff base compounds were found to be mesogenic with smectic C and nematic transitions phases. These compounds attached to decyl, dodecyl, and tetradecyl terminal chains were found to exhibit smectic C phase at 142.47, 135.60, and 131.90 °C and nematic phase at 199.62, 197.11, and 188.76 °C, respectively. The incorporation of a Schiff base linking unit with a symmetrical alkyl chain induces the mesogenic effect in the molecules. A long alkyl chain allows the compounds to adopt a lamellar packing, facilitating the formation of a smectic phase. Incorporating the Schiff base into the molecules maintains an appropriate permanent dipole moment, which enables the homologs to arrange in a parallel orientational order, thereby adopting a nematogenic character.

Symmetric effect on electrical double-layer characteristics and molecular assembly interplay in imidazolium-based Ionic liquid electrolytes in supercapacitor models.

Studies on the ion-layer formation of imidazolium-based ionic liquids have extensively explored how to improve in-depth knowledge of electrical double-layer (EDL) properties. In this computational study, 1-alkyl-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]), namely, [C1mim][NTf2] and [C2mim][NTf2], inside a simulated supercapacitor were investigated to expose an symmetric alkyl chain effect. Molecular dynamic simulations of a supercapacitor model with graphite electrodes were conducted. Changes in charging dynamics and EDL structures at different voltages were studied. Although [C1mim][NTf2] equilibrated much quicker than [C2mim][NTf2], surface charge development on the symmetrical imidazolium ionic liquid was slower than that on the asymmetrical counterpart. Physical EDL structural analysis showed that [C1mim][NTf2] could not rearrange in a rigid co-ion layer, whereby the [C1mim]+ cation stayed adsorbed on the positive electrode throughout all the tested voltages. The strongly attached [C1mim]+ on the electrode surface contributed to low responsiveness in symmetrical [C1mim][NTf2], which was supported by lower overall differential capacitance (CD) magnitude and less sharp CD wings at high voltage when compared to [C2mim][NTf2].

Theoretical study of the effect of D-A type polymer donors on the performance of Y6-based organic solar cells

Non-fullerene organic solar cells (OSCs) based on Y6 molecule have developed rapidly in recent years, and the power conversion efficiency (PCE) varies greatly when the OSC device constructed by Y6 and donor molecules with small structural difference. In this work, the influence of six donor-acceptor (D-A) polymer donors on Y6-based OSCs were focused. The ground-state properties (planarity, frontier molecular orbital energy levels, driving force), excited-state properties of donors (primary indicators, Coulomb attraction) and donor/acceptor interfaces (Frenkel excitation (FE) and charge transfer (CT) states, electron coupling and charge separation rates) were discussed. The results show that symmetrical alkyl chains in the acceptor unit is favorable for maintaining planarity of molecule, while fluorination induces twisting in the thiophene bridge and acceptor unit. And the investigated high-performance molecules have desirable driving force, smaller Coulomb attraction and significant degree of holes and electrons delocalization. Also, the highest-performance system has multiple charge transfer pathways, which can improve charge separation efficiency. At the same time, high-performance systems often have better planarity and more favorable substituent positions. The high-performance donor/Y6 systems have more advantage in charge separation than the low-performance systems. This work provides a fundamental understanding of the impact of D-A type donor materials on the cell performance and theoretical guidance for further optimization of polymer donor materials in Y6-based OSCs.

A novel formulation of triethyl orthoformate mediated durable, smart and antibacterial chitosan cross-linked cellulose fabrics

Antibacterial, durable and smart cotton fabrics was developed using chitosan-based formulation. The cellulose was covalently cross-linked with chitosan using TEOF. The antibacterial activity of prepared smart fabrics and CS was studied against S. aureus and E. coli strains. The FTIR, SEM and XRD were employed to confirm the linkage of CS molecules with cellulose in cotton fabrics. The CS of 160 KDa extracted from shrimp shell showed the optimum antibacterial activity. The prominent asymmetric, symmetric alkyl CH peaks of CS were shifted to 2930 and 2845 (cm−1), respectively. Moreover, the shifted peaks at 1590 and 1400 (cm−1) indicate the CO stretching and NH2 bending bands of CS, respectively. This confirm the existence of new imine functional group that was generated after cross-linking of NH2 groups of CS. The SEM results showed more uniform morphology of TEOF cross-linked fabrics versus CS coated fabrics, which revealed a promising microbial growth inhibition activity. The TEOF as a cross-linker has been unveiled, showcasing the effectiveness of this innovative crosslinking approach. The fabric treated with cross-linked CS exhibited remarkable antibacterial properties that endured even after undergoing 30 washing cycles. These antibacterial textiles possess substantial commercial potential across a diverse range of industries.

An Asymmetric Non‐fullerene Acceptor with Low Energy Loss and High Photovoltaic Efficiency

Comprehensive SummaryMinimizing energy loss (Eloss) plays a key role in improving the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, to reveal the feasibility of the asymmetric molecular strategy in designing high‐efficiency and low Eloss OSC materials, we adopt the alkyl‐alkoxy modification to design an asymmetric non‐fullerene acceptor (NFA) named OC8‐4F, where its symmetric alkyl‐ and alkoxy‐substituted counterparties (2OC8‐4F, eC9‐4F) are also prepared. The results suggest that the introduction of a symmetric alkoxy at the edge of eC9‐4F can effectively decrease the lowest unoccupied molecular orbit level without greatly changing the highest occupied molecular orbit level, leading to a mediated bandgap. In the devices, the OC8‐4F possesses well‐balanced charge generation and Eloss, giving the highest PCE of 18%. Our results imply that finely tuning the asymmetric structure can be used as an effective molecular design strategy to improve the photovoltaic performance of OSCs.

BMIm][BARF] imidazolium salt solutions in alkyl carbonate solvents: Structure and interactions

Solutions of weakly coordinating ionic liquids (ILs) in alkyl carbonates are gaining growing attention, as the latter are “green” solvents with high solvation power, but the phase behavior and structure of ILs in organic polar solvents are still poorly understood. Here, we study the interactions and nanoscale structure of 1‑butyl‑3-methylimidazolium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [BMIm][BARF], in three symmetrical alkyl carbonate solvents with increasing alkyl chain-length. Electrical conductivity and nuclear magnetic resonance measurements showed that [BMIm][BARF] was mostly undissociated in these solvents, especially at lower IL concentration. Small angle X-ray scattering patterns evidenced the presence of rod-like nanostructures in the IL/solvent mixtures. At higher IL concentration, [BMIm][BARF] is increasingly more dissociated in solvents with lower dielectric constant, as confirmed by analysis of the solvents’ carbonyl stretching band via Fourier transform infrared spectroscopy. This trend is opposite to that exhibited by BMIm ILs with less bulky counterions. The bulky BARF− is weakly coordinating and has no ability to give strong H-bonding, thus short-range anisotropic van der Waals forces are likely key in the interaction of the ion pairs. The slower self-diffusion of the ions in alkyl carbonates with lower dielectric constants might partially hinder close contact needed for self-assembly into local nano-sized structures. Overall, our results shed light on interactions and self-organization in imidazolium salt-alkyl carbonate mixtures, with potential impact in applicative fields spanning from batteries, catalysis and extraction, up to bio-applications (antimicrobial and bioengineering).

Influence of the Alkyl Side Chain Length on the Assembly and Thermochromic Solid-State Properties in Symmetric and Asymmetric Monoalkoxynaphthalene–Naphthalimide Donor–Acceptor Dyads

We have been investigating monoalkoxynaphthalene–naphthalimide (MAN–NI) donor–acceptor dyads that undergo a unique and dramatic orange-to-yellow thermochromic transition in the solid state. This study reports a new series of these stimuli-responsive solids, based on dyads with symmetric and relatively long alkyl side chains, as well as asymmetric derivatives. The length of the alkyl side chains influenced the structures, properties, and dynamic behavior of the resulting solids. All of the symmetric alkyl side chain derivatives can crystallize with identical packing (allowing for alkyl chain length differences) in a lasagna-like fashion with alternating alkyl and stacked aromatic layers. The fully extended alkyl side chains are aligned in an interdigitated, “cork and bottle” fashion. The aromatic layers feature head-to-head stacking geometries of the aromatic dyad cores. The crystalline solids derived from each of these derivatives undergo dramatic thermochromic orange-to-yellow transition in the solid state. Crystal structures were obtained for three of the four asymmetric dyad derivatives. In each case, layered structures were observed, although the mismatch in alkyl side chain lengths results in a unique hybrid alkyl/aromatic layer. Two of the asymmetric derivatives exhibit almost identical head-to-head dyad core stacking geometries, while one exhibits a head-to-tail dyad core stacking geometry. Of the four asymmetric dyads studied, only two asymmetric derivatives with the longer alkyl side chains exhibit dynamic solid-state behavior. Overall, our results demonstrate the relatively general nature of the thermochromic orange-to-yellow color-changing mechanism of the MAN–NI dyads and verify the link between the alkyl side chain interactions and this interesting stimuli-responsive behavior in the solid-state.

Efficient preparation of unsymmetrical disulfides by nickel-catalyzed reductive coupling strategy

Disulfides are widely found in natural products and find a wide range of applications in life sciences, materials chemistry and other fields. The preparation of disulfides mainly rely on oxidative couplings of two sulfur containing compounds. This strategy has many side reactions and other shortcomings. Herein, we describe the reductive nickel-catalyzed cross-electrophile coupling of unactivated alkyl bromides with symmetrical alkyl- and aryltetrasulfides to form alkyl-alkyl and aryl-alkyl unsymmetrical disulfides. This approach for disulfide synthesis is practical, relies on easily available, unfunctionalized substrates, and is scalable. We investigated the mechanism of this transformation and found that the tetrasulfide compound does not selectively break the central S–S bond, but regio-selectively generates trisulfide intermediates.

Thermally Stable Organic Field-Effect Transistors Based on Asymmetric BTBT Derivatives for High Performance Solar-Blind Photodetectors.

High‐performance solar‐blind photodetectors are widely studied due to their unique significance in military and industrial applications. Yet the rational molecular design for materials to possess strong absorption in solar‐blind region is rarely addressed. Here, an organic solar‐blind photodetector is reported by designing a novel asymmetric molecule integrated strong solar‐blind absorption with high charge transport property. Such alkyl substituted [1]benzothieno[3,2‐b][1]‐benzothiophene (BTBT) derivatives Cn‐BTBTN (n = 6, 8, and 10) can be easily assembled into 2D molecular crystals and perform high mobility up to 3.28 cm2 V−1s−1, which is two orders of magnitude higher than the non‐substituted core BTBTN. Cn‐BTBTNs also exhibit dramatically higher thermal stability than the symmetric alkyl substituted C8‐BTBT. Moreover, C10‐BTBTN films with the highest mobility and strongest solar‐blind absorption among the Cn‐BTBTNs are applied for solar‐blind photodetectors, which reveal record‐high photosensitivity and detectivity up to 1.60 × 107 and 7.70 × 1014 Jones. Photodetector arrays and flexible devices are also successfully fabricated. The design strategy can provide guidelines for developing materials featuring high thermal stability and stimulating such materials in solar‐blind photodetector application.

Asymmetric Hybrid Siloxane Side Chains for Enhanced Mobility and Mechanical Properties of Diketopyrrolopyrrole-Based Polymers.

High performance organic field effect transistor devices based on intrinsically scalable materials are of great significance in wearable electronics. In this work, an exclusive approach is reported to rationale the carrier mobility and stretchability of the conjugate polymers (CPs) by modifying the symmetry of the side chains species. Semiconductor CPs with symmetrical alkyl side chains (P(C-C)), symmetrical siloxane side chains (P(Si-Si)), and asymmetrical silicon-carbon side chains (P(C-Si)) are synthesized to investigate the influence of these side chains on the carrier mobility and mechanical behavior. The result shows that silicon-carbon asymmetric side chains can modulate the aggregation degree of polymer chains with a coherence length of 134 Å and maintain the mobility at 0.90 cm2 V-1 s-1 . P(C-Si) exhibits superior tensile properties that even elongation up to 100% the value of mobility retains a majority properties. The main reason is that the lowest coherence length of P(C-Si) polymer leads to an increased proportion of amorphous zones in its polymer film, which efficiently dissipates mechanical stresses. This study provides an efficient strategy for the design and synthesis of the CPs with high carrier transport properties-mechanical stability.

Introducing Siloxane-Terminated Side Chains in Small Molecular Donors for All-Small-Molecule Organic Solar Cells: Modulated Molecular Orientation and Enhanced Efficiency.

In this work, three small molecular donors (SMDs) S35, S35-1Si, and S35-2Si, with 3,5-difluorophenyl-substituted benzodithiophene as the central 2-dimensional unit to combine different numbers of siloxane-terminated side chain, were synthesized for all-small-molecule organic solar cells (ASM-OSCs). The three SMDs showed comparable film absorption peaks at 570 nm and optical band gaps of 1.8 eV. Relative to S35 and S35-1Si with symmetric alkyl side chains and asymmetric side chains on the central unit, respectively, the S35-2Si carrying two symmetric siloxane-terminated side chains displayed largely elevated melting and crystalline temperatures, lowered surface energy, and modulated molecular orientation. The three SMDs possessed edge-on dominated molecular orientations of their neat films; however, a big difference was found for their blend films with nonfullerene acceptor Y6. The S35:Y6 and S35-1Si:Y6 blends exhibited edge-on and face-on bimodal orientations but the S35-2Si:Y6 blend showed pure face-on orientation, indicating quite different donor:acceptor intermolecular interactions. Some large domains existed in the S35:Y6 and S35-1Si:Y6 blends, but could be suppressed by the S35-2Si:Y6 blend, leading to a more balanced charge transport. In ASM-OSCs, the two S35:Y6 and S35-1Si:Y6 active layers showed comparable power conversion efficiencies (PCE) of ∼12% but a much higher efficiency of 13.50% could be achieved with the S35-2Si:Y6 active layer. Our results suggest that the siloxane-terminated side chain is promising to regulate crystalline ability of a SMD, paving a way for high performance ASM-OSCs.

Direct Synthesis of Unsymmetrical Glycosyl Disulfides from Glycosyl Bromides

AbstractA significantly fast one step reaction condition has been developed for the synthesis of unsymmetrical anomeric glycosyl disulfide derivatives in excellent yield by the treatment of anomeric glycosyl bromides with a combination of carbon disulfide (CS2) and sodium sulfide nonahydrate (Na2S ⋅ 9H2O) in the presence of symmetrical alkyl, aryl and glycosyl disulfides at room temperature. Exclusive formation of anomerically beta‐disulfides was observed under the reaction conditions. Most of the reactions are high yielding and suitable for scale‐up synthesis.

A new tricomponent reaction for the synthesis of symmetric and asymmetric alkyl bisphosphoramidates

A novel three-component one-pot reaction procedure was designed for the synthesis of symmetric and asymmetric bisphosphoramidate derivatives. Our key reactants were a set of four aliphatic diamines, which individually react with two different phosphoryl chloride derivatives (N,N,N′,N′-tetramethylphosphorodiamide chloride and diphenyl phosphoryl chloride). A total of 10 products, six of them none reported before, were synthetized. The proposed experimental procedure does not involve any catalyst and proceeds under routine experimental conditions (room temperature and 1 atm). Each synthesized compound was characterized by NMR (1H, 13C, and 31P) spectroscopy and mass spectrometry.

Achieving reversible thermochromism of bisdiynamide polydiacetylene via self-assembling in selected solvents

This contribution demonstrates a new approach for achieving reversible thermochromism of bisdiynamide polydiacetylene(bis-PDA). The use of specific solvent as a medium can promote molecular organization and crystal growth of bis-PDA assemblies, which in turn results in reversible thermochromism. In this study, bis-PDAs constituting symmetric alkyl tails were prepared by self-assembling in water or alkanes. The preparation by using water as a medium produces bis-PDA assemblies with relatively small particle size and irregular shape. In this system, the bis-PDA exhibits irreversible thermochromism. Interestingly, the use of nonpolar alkanes promotes crystal growth of the bis-PDA assemblies into micron-size sheet-like structure. We have found that the increase of particle size and molecular ordering results in reversible thermochromism. X-ray diffraction reveals lamellar structure with uncharacteristically high tilting angle of the alkyl side chain with respect to the lamellar plane. Infrared spectroscopy also detects the variation of segmental arrangement within the bis-PDA assemblies. Our study points to new and rather simple approach for achieving reversible thermochromism of bis-PDA-based materials that can extend their applications as colorimetric sensors, smart ink or responsive paint/pigment.

Alkylthio- and alkyl-substituted asymmetric diphenyldiacetylene-based liquid crystals: phase transitions, mesophase and single-crystal structures, and birefringence

ABSTRACTAsymmetric diphenyldiacetylenes (DPDAs) bearing a fixed hexylthio group and different alkyl groups with carbon numbers (n) of 0–8 and 12 at each termini (or 4- and 4ʹ- positions), abbreviated as 6S–DPDA–n, were synthesised and their phase transition behaviour and birefringence were evaluated. It was found that alkyl-unsubstituted and methyl-substituted analogues (6S–DPDA–0 and 6S–DPDA–1, respectively) do not show any mesophases, whereas 6S–DPDA–n (n ≥ 2) form enantiotropic liquid crystal phases. Wherein, the nematic (N) phase for 6S–DPDA–4 is supercooled to room temperature, and 6S–DPDA–12 forms not only N phase but also highly ordered mesophase under room temperature. In light of the fact that symmetrically alkylthio-possessing 6S–DPDA–S6, 6S–DPDA–0 and 6S–DPDA–1 are not mesogenic, the present results strongly suggest that an alkyl group opposite to an alkylthio group plays a vital role to induce mesophases. Single-crystal X-ray analysis revealed anti-parallel packed molecules in the lattice. 6S–DPDA–7 have higher birefringence than 7–DPDA–7 being the symmetric alkyl counterpart, over the entire range of each N phase, which is supported by the enhanced polarisabilities based on density functional theory calculation.

Impact of Unsymmetrical Alkyl–Fluoroalkyl Side Chains over Coil-to-Rod Transition of Soluble Polyacetylenes: Modulation of Electronic Conjugation of Isolated Chains and Their Self-Assembly

Soluble all-trans polyacetylenes (PAs) are most ideal models to study the spectroscopic properties of conjugated polymers for precise control of their backbone configurations, resulting in semiconducting properties for optoelectronic applications. Herein, we report precise control of the backbone configuration and subsequent self-assembly of soluble PAs flanked with alkyl–fluoroalkyl side chains, using light and temperature stimuli respectively in solution. PAs with symmetric alkyl and unsymmetrical alkyl–fluoroalkyl side chains were successfully polymerized by living cyclopolymerization with the olefin metathesis reaction mediated by third-generation Grubbs catalysts. Coil-to-rod-like conformational transition was observed for all of the PAs under ambient light due to photoisomerization, monitored by UV–vis spectroscopy and discussed in terms of the Huang–Rhys parameter. Enrichment of the trans configuration in the conjugated backbones opened up the possibility of aggregation between the isolated conjuga...

Synthesis, structure and thermal investigation of a new volatile iridium (I) complex with cyclooctadiene and methoxy-substituted β-diketonate

The presence of donor groups in volatile metal compounds is interesting both in the thermochemical aspect and as the possibility to form hetero-metallic precursors by a reaction with an acceptor-capable component. Following this trend, the present work deals with synthesis and detailed structural and thermochemical investigation of a first iridium volatile complex with donor-atom-functionalized β-diketonate ligand, namely [Ir(cod)(zis)] (cod = cyclooctadiene-1,5, zis = 2-methoxy-2,6,6-trimethylheptanedionato-3,5). The compound has been characterized by elemental analysis, IR- and NMR-spectroscopy. According to single-crystal X-ray diffraction, the crystal structure of the complex is formed by layered-packed isolated molecules. Within the molecules, the coordination site IrO2C′2 (C′ is the center of C=C bond of the cod-ligand) is implemented to form distorted planar square metal coordination environment with Ir–O and Ir–C′ distances being (2.034–2.046) and (1.965–1.978) A, respectively. TG–DTA study shows that the compound is characterized by extremely low melting point (378 K) and a high thermal stability during evaporation. Then, the temperature dependencies of saturated vapor pressures over both the solid and liquid compounds have been measured by the flow (transpiration) method at (353–376) K and (381–403) K, respectively, giving the molar enthalpy and entropy of sublimation and evaporation processes. In addition, the comparison of the structural and thermal data with the ones for the related [Ir(cod)(L)] complexes containing symmetric alkyl terminal substituents in β-diketonate ligand L has been performed and, thereby, the donor group influence on the characteristics of this type of volatile compounds has been revealed.

Influences of solvent media on chain organization and thermochromic behaviors of polydiacetylene assemblies prepared from monomer with symmetric alkyl tails

This contribution investigates thermochromic behaviors and morphologies of polydiacetylene(PDA) assemblies prepared from a diacetylene monomer constituting symmetric alkyl tails. Properties of solvent media strongly affects molecular organization during the preparation process. The use of water provides PDA assemblies with irregular shape while a sheet-like structure forms in butanol. Thermal analysis and X-ray diffraction detect the variation of crystalline fraction and interlamellar spacing in these structures. The change of solvent media significantly affects their thermochromic behaviors. Thin film of PDA assemblies prepared in butanol exhibits reversible thermochromism with much higher color-transition temperature compared to the system of water.

Europhtal (8020) efficiently catalyzes the aerobic oxidation of in situ generated thiols to symmetric disulfides (disulfanes)

Efficient, odorless and scalable synthetic protocols have been introduced for the preparation of symmetric alkyl disulfides by treatment of the corresponding organic halides with thiourea and NaHCO3 in the presence of commercially available Europhtal catalyst (8020) in both H2O and poly ethylene glycol (PEG-200) media at 80–90 °C. Structurally diverse primary, secondary, allylic and benzylic halides were examined successfully whereas, the result with tert-butyl bromide was not satisfactory. Also, another procedure has been introduced for achieving symmetric aryl disulfides in high yields by reacting aryl halides, thiourea and NaHCO3 in PEG-200 at 115–120 °C using CuI along with Europhtal catalysts. The results showed that the aerobic oxidation of in situ generated thiols proceeded efficiently in the presence of Europhtal catalyst giving the symmetric disulfide without contamination by symmetric sulfide side-product.