Quinoxaline Group Research Articles

Substituent Controlled Tunable Fluorescence from Green to Red and pH Stimuli‐Induced Reversible Fluorescence Switching in Triphenylamine–Quinoxaline Derivatives

A series of triphenylamine‐quinoxaline donor‐acceptor (bipolar) compounds (TPA‐QH, TPA‐QMe, TPA‐QCOOH and TPA‐QNO2) with different substituents were synthesized and investigated their fluorescence properties, including stimuli‐induced fluorescence responses in solution and the solid‐state. Single crystal structural analysis revealed non‐planar molecular conformation, substituent controlled intermolecular interactions and molecular packing in the crystal lattice. TPA‐QH, TPA‐QMe, TPA‐QCOOH and TPA‐QNO2 showed tunable solid‐state emission from green to red. TPA‐QH showed strong fluorescence at 487 nm (quantum yield (Φf) = 28.3%) whereas NO2 substituted TPA‐QNO2 exhibited relatively weak fluorescence at 610 nm (Φf = 4.3%). Density functional theoretical (DFT) calculations also indicated reduction of optical bandgap with substituting electron withdrawing group. The donor‐acceptor structure with intramolecular charge transfer (ICT) resulted solvent polarity dependent fluorescence tuning. The presence of acid responsive quinoxaline group was utilized to demonstrate pH‐responsive fluorescence switching and dual state fluorescence was utilized for fabricating rewritable fluorescent platform. Thus, the present study provides a structural insight to develop dual state emissive triphenylamine–quinoxaline based bipolar materials for optoelectronic applications.

Effect of chalcogen atoms on the electronic band gaps of the quinoxaline containing donor-acceptor-donor type semiconducting polymers: a systematic DFT investigation.

Due to the widely known positive contributions of the quinoxaline group in organic semiconductors, we conducted a fully computational study using quantum mechanical methods to investigate the effect of quinoxaline in the electron acceptor unit with the combination of different chalcogen atoms on the band gap of a series of donor-acceptor-donor type conjugated polymers. Using density functional theory, we mainly calculated the electronic band gap values of the structures containing four different chalcogen atoms (O, S, Se, and Te) in the electron donor and acceptor units. While chalcogendiazoloquinoxaline groups were used as the electron acceptor units, furan, thiophene, selenophene, and tellurophene were used as the donor units. Our theoretical results showed that the use of heavy chalcogen atoms in both donor and acceptor units resulted in a low band gap. Besides this, the effect of heavy chalcogen atoms used in the electron donor units is much more pronounced compared to the ones used in the acceptor units. More importantly, our findings proved that the inclusion of the chalcogendiazoloquinoxaline group instead of benzochalcogenadiazole as the acceptor unit significantly decreases the electronic band gap of the conjugated polymer. The lowest band gap was found to be 0.10eV for the 4,9-di(tellurophen-2-yl)-[1,2,5]telluradiazolo[3,4-g]quinoxaline polymer. Conformational analysis of the monomers and their corresponding oligomers was performed at the B3LYP/LANL2DZ level of theory. Then, long-range corrected hybrid functional LC-BLYP in a combination with the LANL2DZ basis set was utilized for the calculation of electronic properties and HOMO and LUMO energy gaps of monomers and oligomers through the reoptimization of the lowest energy conformers obtained from the B3LYP/LANL2DZ calculations in the previous step. All energy minimum structures were confirmed through vibrational frequency analysis at both calculation levels. The Gaussian 09 rev. D.01 software was used for all calculations, and GaussView 5.0.9 for visualizations.

Modulation of Phase Separation Morphology by Configuration Engineering in Bulk Heterojunction Organic Solar Cells

For bulk‐heterojunction organic solar cells (OSCs), molecular structure design to control molecular stacking is crucial to obtain ideally phase‐separated morphology and high device performance. Herein, the investigation focuses on two polythiophene‐quinoxaline (PTQ) derivatives (PTQ8 and PTQ10) blended with Y6, utilizing coarse‐grained molecular dynamics simulations based on the Lennard–Jones static potential (LJSP) method. The study reveals that the diminished photovoltaic efficiencies of PTQ8:Y6 blends, compared to PTQ10:Y6 blends, are not solely attributed to reduced driving forces. The introduction of fluorine‐substituted sites in the thiophene group of PTQ polymer is identified as a significant factor. This alteration causes PTQ polymers in PTQ8:Y6 blends to coil, compromising the crystalline structure. PTQ8's bifluorine group induces a repulsive effect on the quinoxaline group, leading to a coiled‐chain structure that hinders chain stacking. Conversely, PTQ10 exhibits a straighter chain conformation. Additionally, PTQ8's high solubility in chloroform prevents effective aggregation, further impeding suitable morphology formation. Coarse‐grained simulations employing LJSP prove effective in precisely exploring the morphology of OSCs, offering crucial insights for materials in this field.

Combination of quinoxaline with pentamethinium system: Mitochondrial staining and targeting

Pentamethinium indolium salts are promising fluorescence probes and anticancer agents with high mitochondrial selectivity. We synthesized two indolium pentamethinium salts: a cyclic form with quinoxaline directly incorporated in the pentamethinium chain (cPMS) and an open form with quinoxaline substitution in the γ-position (oPMS). To better understand their properties, we studied their interaction with mitochondrial phospholipids (cardiolipin and phosphatidylcholine) by spectroscopic methods (UV–Vis, fluorescence, and NMR spectroscopy). Both compounds displayed significant affinity for cardiolipin and phosphatidylcholine, which was associated with a strong change in their UV–Vis spectra. Nevertheless, we surprisingly observed that fluorescence properties of cPMS changed in complex with both cardiolipin and phosphatidylcholine, whereas those of oPMS only changed in complex with cardiolipin. Both salts, especially cPMS, display high usability in mitochondrial imaging and are cytotoxic for cancer cells. The above clearly indicates that conjugates of pentamethinium and quinoxaline group, especially cPMS, represent promising structural motifs for designing mitochondrial-specific agents.

Experimental Confirmation of a Predicted Porous Hydrogen‐Bonded Organic Framework

AbstractHydrogen‐bonded organic frameworks (HOFs) with low densities and high porosities are rare and challenging to design because most molecules have a strong energetic preference for close packing. Crystal structure prediction (CSP) can rank the crystal packings available to an organic molecule based on their relative lattice energies. This has become a powerful tool for the a priori design of porous molecular crystals. Previously, we combined CSP with structure‐property predictions to generate energy‐structure‐function (ESF) maps for a series of triptycene‐based molecules with quinoxaline groups. From these ESF maps, triptycene trisquinoxalinedione (TH5) was predicted to form a previously unknown low‐energy HOF (TH5‐A) with a remarkably low density of 0.374 g cm−3 and three‐dimensional (3D) pores. Here, we demonstrate the reliability of those ESF maps by discovering this TH5‐A polymorph experimentally. This material has a high accessible surface area of 3,284 m2 g−1, as measured by nitrogen adsorption, making it one of the most porous HOFs reported to date.

Experimental Confirmation of a Predicted Porous Hydrogen-Bonded Organic Framework.

Hydrogen-bonded organic frameworks (HOFs) with low densities and high porosities are rare and challenging to design because most molecules have a strong energetic preference for close packing. Crystal structure prediction (CSP) can rank the crystal packings available to an organic molecule based on their relative lattice energies. This has become a powerful tool for the a priori design of porous molecular crystals. Previously, we combined CSP with structure-property predictions to generate energy-structure-function (ESF) maps for a series of triptycene-based molecules with quinoxaline groups. From these ESF maps, triptycene trisquinoxalinedione (TH5) was predicted to form a previously unknown low-energy HOF (TH5-A) with a remarkably low density of 0.374 g cm-3 and three-dimensional (3D) pores. Here, we demonstrate the reliability of those ESF maps by discovering this TH5-A polymorph experimentally. This material has a high accessible surface area of 3,284 m2 g-1 , as measured by nitrogen adsorption, making it one of the most porous HOFs reported to date.

Synthesis and photophysical properties of quinoxaline-based blue aggregation-induced emission molecules

A series of quinoxaline-based compounds 1–4 have been synthesized by a palladium-catalyzed cross-coupling reaction and their photophysical properties have been extensively studied. Compounds 1–4 show deep blue light emission both in solution (λem ≤ 425 nm, Commission Internationale de L’Eclairage y (CIEy) ≤ 0.03) and in the solid state. Moreover, compounds 1–3 show a non-typical aggregation-induced enhanced emission (AIEE), which would be effective deep blue light-emitting materials. The DFT calculation indicated that the HOMO energy levels of compounds 1–3 are distributed throughout the molecule, and the LUMO energy levels are mainly concentrated on the quinoxaline group. However, the HOMO of compound 4 is mainly on the benzene ring at 2,3 position, and the LUMO is distributed both of the quinoxaline and the benzaldehyde group at the 6,7 position.

Trifunctional quinoxaline‐based maleimide and its polymer alloys with benzoxazine: Synthesis, characterization, and properties

AbstractNovel quinoxaline‐based trismaleimide (2,3‐di[3‐maleimido]phenyl‐6‐ maleimidoquinoxaline, namely TQMI) and its polymer alloys with 3‐phenyl‐3,4‐dihydro‐2H‐1,3‐benzoxazine (P‐a) were successfully prepared and characterized. Differential scanning calorimetry investigation of TQMI exhibited distinct double exothermic peaks, which implied that the curing behavior of different type of maleimide group was discrepant. The curing temperature of P‐a/TQMI prepolymers was lower than that of both neat TQMI and P‐a monomer. The temperatures at 5% (T5) and 10% (T10) weight loss and the char yield (Yc) of cured TQMI at 800°C reached 513°C, 524°C, and 63.5%, respectively, which were much higher than the record of traditional 4,4′‐bismaleimideodiphenylmethane (BMDPM) resin as well as most of other reported bismaleimide resins. Moreover, the addition of TQMI enhanced thermal stability, glass transition temperature (Tg), and limiting oxygen index of benzoxazine resin dramatically. Attractively, the Tg value of P‐a/TQMI copolymer at 30 wt% TQMI loadings was approximately 20°C higher than that of P‐a/BMDPM copolymer owing to the bulky quinoxaline group.

On the effect of pattern substitution and oligo(ethylene oxide) side-chain modification on thiophene-quinoxaline copolymers and their applications in photovoltaic cells

Abstract The molecular design of conjugated polymers for efficient organic photovoltaic devices (OPVs) providing advantageous processing properties, namely solubility in low toxic solvents as ethanol, is an important issue for the progress of OPV technology. In this work, the pattern substitution and chemical nature of the side chain, either n-alkoxy or oligo(ethylene oxide) groups, in alternating thiophene-quinoxaline copolymers is varied and their effect in polymer chain grow, solubility and optical spectra is thoroughly analyzed in light of quantum mechanical calculations. Although the polymer with oligo(ethylene oxide) groups exhibited good solubility in ethanol, it was found that substituting in both meta- and para-positions of the peripheral phenyl rings attached to the quinoxaline groups causes smaller molecular weight, lower solubility in chlorinated solvents and poorer performance in photovoltaic devices. The theoretical studies revealed differences in the most energetically favored conformers for each case of the polymers'substitution pattern that can justify the observed results.

Application of pyrrolo[2,3-b]quinoxaline with an N,N-bis(pyridin-2-ylmethyl)ethylenediamine chain for Zn(II) detection in Candida albicans and fibroblast cells

In the present study, a new 'turn on' fluorescent sensor, (E/Z)-enaminone containing a pyrrolo[2,3-b]quinoxaline group as the fluorophore and a N,N-bis(pyridin-2-ylmethyl)ethylenediamine group as a specific chelator capable of selective detection of Zn2+ is reported. The structure of the ligand was determined by IR, MS, and 1D and 2D NMR, including 1H-15N HMBC correlations. X-ray analysis for (E/Z)-2 confirmed the presence of 90.3% E-diastereoisomer and 9.7% Z-one in the structure of the crystals obtained from acetonitrile. The mechanism of zinc ion recognition is related to the restriction of E/Z-isomerisation of enaminone occurring upon Zn2+ binding that alters the electronic structure of the ligand. Sensing of Zn2+ was confirmed in detail using UV–vis, fluorescent, and 1H NMR titrations, which allowed us to propose the binding mode for complexes formed in the solution during the zinc ion recognition. The ligand functions under physiological conditions and retains its activity in buffer solutions over a wide pH range 2.4–10.6. The developed sensor allowed for intracellular sensing of Zn2+ in human (fibroblast) and fungal (Candida albicans) cells through fluorescence imaging studies.

Nonlinear optical response of a series of small molecules: quantum modification of π-spacer and acceptor

In this research article, a series of compounds have been designed to investigate the nonlinear optical (NLO) response. The electronic structures, absorption spectra and nonlinear optical response have been calculated by employing quantum chemical methods. The new design of dyes has been proposed by the structural modification of π-spacers/conjugated systems (CS) and terminal acceptors (TA). The quantum modeling and design is based on three phases; in every phase different π-spacer (CS1–CS2) with the acceptors (TA1–TA5) are used around central quinoxaline group. DFT and TDDFT calculations are performed to shed light on how structural modification influences the NLO properties. Polarizability (α), hyperpolarizability (β) and absorption wave length are calculated. These strategies indicate that the thiazole-based conjugated systems (CS2) offer relatively an effective way to improve NLO response. This theoretical framework might be useful to design other organic dyes in the field of electro-optics. The DFT and TDDFT calculations have been performed to shed light on how structural modification influences the NLO properties. Impact of various terminal acceptors (TA) and conjugated systems (CS) has been systematically studied.

Effect of substituent groups on quinoxaline-based random copolymers on the optoelectronic and photovoltaic properties

Abstract In this study, three low band gap donor-acceptor (D-A) type quinoxaline-based random copolymers were synthesized. The effect of varying substituent groups on quinoxaline groups on optoelectronic properties and the performance of bulk-heterojunction polymer solar cells was investigated. Electrochemical and optical studies indicate that these copolymers are promising materials for both electrochromic and polymer solar cells applications. Two random polymers were found as excellent candidates for NIR electrochromic devices owing to their 60% and 94% optical contrast values respectively in NIR region with less than 1 s switching times. Polymer solar cells were constructed using the copolymers as the donor components together with PC71BM as the acceptor in the active layer. Among all, the highest PCE was reported as 2.13%.

Pharmacokinetics and Metabolism of Cyadox and Its Main Metabolites in Beagle Dogs Following Oral, Intramuscular, and Intravenous Administration.

Cyadox (Cyx) is an antibacterial drug of the quinoxaline group that exerts markedly lower toxicity in animals, compared to its congeners. Here, the pharmacokinetics and metabolism of Cyx after oral (PO), intramuscular (IM), and intravenous (IV) routes of administration were studied to establish safety criteria for the clinical use of Cyx in animals. Six beagle dogs (3 males, 3 females) were administered Cyx through PO (40 mg kg−1 b.w.), IM (10 mg kg−1 b.w.), and IV (10 mg kg−1 b.w.) routes with a washout period of 2 weeks in a crossover design. Highly sensitive high-performance liquid chromatography with ultraviolet detection (HPLC-UV) was employed for determination of Cyx and its main metabolites, 1, 4-bisdesoxycyadox (Cy1), cyadox-1-monoxide (Cy2), N-(quinoxaline-2-methyl)-cyanide acetyl hydrazine (Cy4), and quinoxaline-2-carboxylic acid (Cy6) in plasma, urine and feces of dogs. The oral bioavailability of Cyx was 4.75%, suggesting first-pass effect in dogs. The concentration vs. time profile in plasma after PO administration indicates that Cyx is rapidly dissociated into its metabolites and eliminated from plasma earlier, compared to its metabolites. The areas under the curve (AUC) of Cyx after PO, IM and IV administration were 1.22 h × μg mL−1, 6.3 h × μg mL−1, and 6.66 h × μg mL−1, while mean resident times (MRT) were 7.32, 3.58 and 0.556 h, respectively. Total recovery of Cyx and its metabolites was >60% with each administration route. In feces, 48.83% drug was recovered after PO administration, while 18.15% and 17.11% after IM and IV injections, respectively, suggesting renal clearance as the major route of excretion with IM and IV administration and feces as the major route with PO delivery. Our comprehensive evaluation of Cyx has uncovered detailed information that should facilitate its judicious use in animals by improving understanding of its pharmacology.

The effect of meta-substituted or para-substituted phenyl as side chains on the performance of polymer solar cells

Two donor-acceptor alternative copolymers (P1 and P2) with carbazole group as the donor unit and 2,3-diphenylquinoxaline group as the acceptor unit were designed and synthesized by Suzuki polycondensation reaction. Alkoxy groups were linked to meta-position or para-position of phenyl rings, which were attached on the 2,3-position of quinoxaline group to investigate the effect of the alkoxy position at phenyl rings on the resulting polymer optical, electrochemical and photovoltaic properties. GIWAXS experiments revealed that para-substituted polymer P2 possesses closer packing properties in the solid state. The absorption spectra of para-substituted polymer P2 also displayed larger red-shift than that of meta-substituted polymer P1 when it went from solution to the film, which was beneficial to improve the hole mobilities and short circuit current density (Jsc). When used as the donor materials, devices based on P2:PC71BM exhibited a PCE of 3.33%, which was obviously higher than that based on P1:PC71BM. Our results revealed that the slightly structure change of polymer has a significant influence on the photovoltaic properties of polymer solar cells and adopting para-substituted phenyl ring as the side groups gave superior photovoltaic performance in donor materials based on carbazole and quinoxaline.

Decomposition and mineralization of sulfaquinoxaline sodium during UV/H2O2 oxidation processes

Sulfaquinoxaline sodium (N′1-quinoxalin-2-ylsulphanilamide sodium, SQ-Na) has been widely used to prevent coccidiosis in poultry, swine, and sheep. The quinoxaline group within SQ-Na exhibits mutagenic and carcinogenic properties. The removal of residual SQ-Na from wastewater has not been reported before. In this study, the decomposition of SQ-Na in aqueous solutions in UV/H2O2 oxidation process was investigated. The results show that SQ-Na was completely decomposed during UV/H2O2 oxidation process, and the process fitted a fluence-based pseudo-first-order kinetics model when the initial H2O2 concentration varied from 0 to 160mg/L with rate constant of kobs=0.802×[H2O2]+0.206×10−3, (R2=0.990). The decomposition of SQ-Na via UV/H2O2 process was favorable under acidic and neutral conditions but was inhibited under strong alkaline conditions (pH⩾11). The presence of bicarbonate, chloride and nitrate ions were shown to have slight inhibition on the decomposition, but humic acids significantly decreased the decomposition rate. As compared with in deionized water, the decomposition rate of SQ-Na in surface water and wastewater treatment plant (WWTP) effluent was significantly decreased at 80mg/L of H2O2, but the decrease was mitigated with more H2O2 addition. The decomposition products of SQ-Na were identified using ion chromatography and liquid chromatography with an ion trap and time-of-flight mass spectrometry (LCMS-IT-TOF). Based on these decomposition products, the possible decomposition pathways of SQ-Na in the UV/H2O2 oxidation process were proposed.

Substituted Tetraaza- and Hexaazahexacenes and their N,N'-Dihydro Derivatives: Syntheses, Properties, and Structures.

The palladium-catalyzed coupling of a substituted o-diaminoanthracene and a substituted o-diaminophenazine to substituted 2,3-dichloroquinoxalines furnishes 10 differently substituted N,N'-dihydrotetraaza- or -hexaazahexacenes with the quinoxaline group of the azaacenes carrying fluorine, chlorine, or nitro groups. The N,N'-dihydrotetraazahexacenes with hydrogen, chlorine, and fluorine subtituents are oxidized to azaacenes, whereas only the parent N,N'-dihydrohexaazahexacenes, with hydrogen substituents, are oxidized by MnO2. The resultant azaacenes are characterized by their optical and spectroscopic data. In addition, single-crystal X-ray structures have been obtained for the parent tetraazahexacenes and their difluoro-substituted derivatives. The di- and tetrachloro derivatives of the N,N'-dihydrohexaazahexacene have also been structurally characterized.

Conjugated polymer dots-on-electrospun fibers as a fluorescent nanofibrous sensor for nerve gas stimulant.

A novel chemical warfare agent sensor based on conjugated polymer dots (CPdots) immobilized on the surface of poly(vinyl alcohol) (PVA)-silica nanofibers was prepared with a dots-on-fibers (DoF) hybrid nanostructure via simple electrospinning and subsequent immobilization processes. We synthesized a polyquinoxaline (PQ)-based CP as a highly emissive sensing probe and employed PVA-silica as a host polymer for the elctrospun fibers. It was demonstrated that the CPdots and amine-functionalized electrospun PVA-silica nanofibers interacted via an electrostatic interaction, which was stable under prolonged mechanical force. Because the CPdots were located on the surface of the nanofibers, the highly emissive properties of the CPdots could be maintained and even enhanced, leading to a sensitive turn-off detection protocol for chemical warfare agents. The prepared fluorescent DoF hybrid was quenched in the presence of a chemical warfare agent simulant, due to the electron transfer between the quinoxaline group in the polymer and the organophosphorous simulant. The detection time was almost instantaneous, and a very low limit of detection was observed (∼1.25 × 10(-6) M) with selectivity over other organophosphorous compounds. The DoF hybrid nanomaterial can be developed as a rapid, practical, portable, and stable chemical warfare agent-detecting system and, moreover, can find further applications in other sensing systems simply by changing the probe dots immobilized on the surface of nanofibers.

Gold(III) Porphyrins Containing Two, Three, or Four β,β′-Fused Quinoxalines. Synthesis, Electrochemistry, and Effect of Structure and Acidity on Electroreduction Mechanism

Gold(III) porphyrins containing two, three, or four β,β'-fused quinoxalines were synthesized and examined as to their electrochemical properties in tetrahydrofuran (THF), pyridine, CH2Cl2, and CH2Cl2 containing added acid in the form of trifluoroacetic acid (TFA). The investigated porphyrins are represented as Au(PQ2)PF6, Au(PQ3)PF6, and Au(PQ4)PF6, where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β'-pyrrolic position of the porphyrin macrocycle. In the absence of added acid, all three gold(III) porphyrins undergo a reversible one-electron oxidation and several reductions. The first reduction is characterized as a Au(III)/Au(II) process which is followed by additional porphyrin- and quinoxaline-centered redox reactions at more negative potentials. However, when 3-5 equivalents of acid are added to the CH2Cl2 solution, the initial Au(III)/Au(II) process is followed by a series of internal electron transfers and protonations, leading ultimately to triply reduced and doubly protonated Au(II)(PQ2H2) in the case of Au(III)(PQ2)(+), quadruply reduced and triply protonated Au(II)(PQ3H3) in the case of Au(III)(PQ3)(+), and Au(II)(PQ4H4) after addition of five electrons and four protons in the case of Au(III)(PQ4)(+). Under these solution conditions, the initial Au(PQ2)PF6 compound is shown to undergo a total of three Au(III)/Au(II) processes while Au(PQ3)PF6 and Au(PQ4)PF6 exhibit four and five metal-centered one-electron reductions, respectively, prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for reduction and UV-visible spectra of the electroreduced species are analyzed using linear free energy relationships.

Electrochemistry of mono- and bis-porphyrins containing a β,β′-fused tetraazaanthracene group

Electrochemical and thin-layer spectroelectrochemical properties of mono- and bis-porphyrins containing a β,β′-fused tetraazaanthracene (TA) group were examined in CH2Cl2, PhCN and pyridine containing 0.1 M tetra-n-butylammonium perchlorate as supporting electrolyte. The investigated mono-porphyrins are represented as (PTA)M, where M = 2H, Cu(II) or Zn(II) and PTA = the dianion of 5,10,15,20-tetrakis-(3,5-di-tert-butylphenyl)-6′,7′-dimethyl-1′,4′,5′,8′-tetraazaanthraceno-[2′,3′b]-porphyrin, while the bis-porphyrins are represented by M(P)-TA-(P)M, where M is H2, Cu(II) , Zn(II) , Ni(II) or Pd(II) and P is the dianion of 5,10,15,20-tetrakis-(3,5-di-tert-butylphenyl)porphyrin. The effect of the fused TA group on the redox potentials and UV-visible spectra of the neutral, electroreduced and electrooxidized porphyrins is discussed and the data compared to what is observed for related mono- and bis-quinoxalinoporphyrins of the type (PQ)M and (QPQ)M, where Q is a β,β′-fused quinoxaline group. Each TA-linked bis-porphyrin exhibits a strong interaction between the two equivalent porphyrin macrocycles, the magnitude of which is dependent upon the specific metal ion.

Unusual Multi-Step Sequential AuIII/AuII Processes of Gold(III) Quinoxalinoporphyrins in Acidic Non-Aqueous Media

The electrochemistry of gold(III) mono- and bis-quinoxalinoporphyrins was examined in CH(2)Cl(2) or PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) before and after the addition of trifluoroacetic acid to solution. The investigated porphyrins are represented as Au(PQ)PF(6) and Au(QPQ)PF(6), where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β'-pyrrolic position of the porphyrin macrocycle; in Au(QPQ)PF(6) there is a linear arrangement where the quinoxalines are fused to pyrrolic positions that are opposite each other. The porphyrin without the fused quinoxaline groups, Au(P)PF(6), was also investigated under the same solution conditions. In the absence of acid, all three gold(III) porphyrins undergo a single reversible Au(III)/Au(II) process leading to the formation of a Au(II) porphyrin which can be further reduced at more negative potentials to give stepwise the Au(II) porphyrin π-anion radical and dianion, respectively. However, in the presence of acid, the initial Au(III)/Au(II) processes of Au(PQ)PF(6) and Au(QPQ)PF(6) are followed by an internal electron transfer and protonation to regenerate new Au(III) porphyrins assigned as Au(III)(PQH)(+) and Au(III)(QPQH)(+). Both protonated gold(III) quinoxalinoporphyrins then undergo a second Au(III)/Au(II) process at more negative potentials. The electrogenerated monoprotonated monoquinoxalinoporphyrin, Au(II)(PQH), is then further reduced to its π-anion radical and dianion forms, but this is not the case for the monoprotonated bis-quinoxalinoporphyrin, Au(II)(QPQH), which accepts a second proton and is rapidly converted to Au(III)(HQPQH)(+) before undergoing a third Au(III)/Au(II) process to produce Au(II)(HQPQH) as a final product. Thus, Au(P)PF(6) undergoes one metal-centered reduction while Au(PQ)PF(6) and Au(QPQ)PF(6) exhibit two and three Au(III)/Au(II) processes, respectively. These unusual multistep sequential Au(III)/Au(II) processes were monitored by thin-layer spectroelectrochemistry and a reduction/oxidation mechanism for Au(PQ)PF(6) and Au(QPQ)PF(6) in acidic media is proposed.