Colored Charge-transfer Complexes Research Articles

Colored supramolecular charge-transfer host system using 10,10′-dihydroxy-9,9′-biphenanthryl and 2,5-disubstituted-1,4-benzoquinone

A colored charge-transfer (CT) host complex is formed using racemic ( rac )-10,10′-dihydroxy-9,9′-biphenanthryl, which has a large and widely π-conjugated phenanthrene ring, as the electron donor and 2,5-disubstituted-1,4-benzoquinone as the electron acceptor. This CT host complex can include aromatic molecules as guests and its color and diffuse reflectance spectra (DRS) change according to the type of guest molecules included. Characteristically, it is possible to tune the color and DRS of the inclusion CT complex by changing the type of the component 2,5-disubstituted-1,4-benzoquinone.

A study on the mechanism of reaction between BNAH and chloranil: Evidence for electron transfer

The reaction between chloranil and N−benzyldihydronicotinamide (BNAH) in borate buffer/DMF was investigated. The reaction mixture gave a strong esr signal, which is consistent with that of chloranil anion radical, and tetrachlorohydrophenol (QH2) and N−benzylnicotinamide (BNA+) were obtained as the sole products. When the reaction was run in benzene solution, a green coloured charge-transfer complex between the reactants could be isolated, which decomposed in polar solvents to give BNA+ and QH2. Based on kinetic studies by esr spectroscopy by the stopped-flow technique, a two-step electron-transfer mechanism for the reaction is proposed in contrast to the hydride-transfer mechanism reported in the literature.

Utility of certain σ- and π-acceptors for the spectrophotometric determination of ganciclovir in pharmaceutical formulations

Studies were carried out, for the first time, to investigate the charge–transfer reactions of ganciclovir as n-electron donor with the σ-acceptor iodine and various π-acceptors: 7,7,8,8-tetracyanoquinodimethane; tetracyanoethylene; 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; p-chloranilic acid; 2,3,5,6-tetrabromo-1,4-benzoquinone; 2,3,5,6-tetrachloro-1,4-benzoquinone and 2,4,7-trinitro-9-fluorenone. The formation of the colored charge–transfer complexes was utilized in the development of simple, rapid and accurate spectrophotometric methods for the analysis of ganciclovir in pure form as well as in its pharmaceutical formulation (capsules). Different variables affecting the reactions were studied and optimized. Under the optimum reaction conditions, linear relationships with good correlation coefficients (0.9993–0.9998) were found between the absorbance and the concentration of ganciclovir in the range of 2.0–240 μg mL −1. For more accurate analysis, Ringbom optimum concentration range was found to be between 5.0 and 225 μg mL −1. The limits of detection ranged from 0.36 to 2.45 μg mL −1 and the limits of quantification ranged from 1.20 to 8.17 μg mL −1. A Job's plot of the absorbance versus the molar ratio of ganciclovir to each of acceptors under consideration indicated (1:1) ratio. The proposed methods were applied successfully for simultaneous determination of ganciclovir in capsules with good accuracy and precision and without interferences from common additives. The recovery percentages ranged from 99.45 ± 0.73% to 100.35 ± 1.40%. The results were compared favourably with the reported method.

Exploiting π-acceptors for the determination of thyroid hormones (T3 and T4) using a single interface flow system

A fully automated methodology was developed for the determination of the thyroid hormones levothyroxine (T4) and liothyronine (T3). The proposed method exploits the formation of highly coloured charge–transfer (CT) complexes between these compounds, acting as electron donors, and π-acceptors such as chloranilic acid (CLA) and 2,3-dichloro-5,6-dicyano- p-benzoquinone (DDQ). For automation of the analytical procedure a simple, fast and versatile single interface flow system (SIFA) was implemented guaranteeing a simplified performance optimisation, low maintenance and a cost-effective operation. Moreover, the single reaction interface assured a convenient and straightforward approach for implementing Job's method of continuous variations used to establish the stoichiometry of the formed CT complexes. Linear calibration plots for levothyroxine and liothyronine concentrations ranging from 5.0 × 10 −5 to 2.5 × 10 −4 mol L −1 and 1.0 × 10 −5 to 1.0 × 10 −4 mol L −1, respectively, were obtained, with good precision (R.S.D. <4.6% and <3.9%) and with a determination frequency of 26 h −1 for both drugs. The results obtained for pharmaceutical formulations were statistically comparable to the declared hormone amount with relative deviations lower than 2.1%. The accuracy was confirmed by carrying out recovery studies, which furnished recovery values ranging from 96.3% to 103.7% for levothyroxine and 100.1% for liothyronine.

Analytical study for the charge-Transfer complexes of gabapentin

Studies were carried out, for the first time, to investigate the charge-transfer reactions ofgabapentin as n-electron donor with the σ-acceptor: iodine and various π-acceptors: 7,7,8,8-tetracyanoquino-dimethane (TCNQ), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid, pCA), tetracyanoethylene (TCNE) and 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil). Different colored charge-transfer complexes and radical anions were obtained. Different variables affecting the reactions were studied and optimized. The formations of the colored complexes were utilized in the development of simple, rapid and accurate spectrophotometric methods for the analysis ofgabapentin in pure form as well as in its pharmaceutical preparation. Under the optimum reaction conditions, linear relationships with good correlation coefficients (0.9995 - 0.9999) were found between the absorbance and the concentrations of gabapentin in the range of 6.0 - 200 µg ml-1. The limits of assays detection ranged from 0.39 to 7.59 µg ml-1. No interference could be observed from the additives commonly present in the pharmaceutical preparation. The methods were success-fully applied to the analysis of pharmaceutical preparation that contain gabapentin, with good accuracy and precision; the percentages recovery ± S.D. ranged from 99.29 ± 0.651 to 100.36 ± 1.56. The results were compared favorably with the reported method. Key words: Charge-transfer complexes, gabapentin, pharmaceutical analysis, spectrophotometry.

Origins of Selectivity in a Colorimetric Charge-Transfer Sensor for Diols

Bispyridyl hydrogen bonding receptor 1 forms colored charge transfer (CT) complexes with complementary phenols and naphthols. Despite its low association constants of approximately 10(1) M(-1), receptor 1 was highly selective forming CT complexes of varying color and intensity with different diol guests. The selectivity of 1 was correlated with the ability of its CT band to simultaneously yield information about the association constant and the electronic structure of the phenols and naphthols.

Spectrophotometric Analysis of Selective Serotonin Reuptake Inhibitors Based on Formation of Charge-Transfer Complexes with Tetracyanoquinodimethane and Chloranilic Acid

A simple, accurate, and sensitive spectrophotometric method for analysis of selective serotonin reuptake inhibitors (SSRIs) has been developed and validated. The analysis was based on the formation of colored charge-transfer complexes between the intact molecule of SSRI drug as an n-electron donor and each of tetracyanoquinodimethane (TCNQ) or p-chloranilic acid (pCA) as electron acceptors. The formed complexes were measured spectrophotometrically at 842 and 520 nm for TCNQ and pCA, respectively. Different variables and parameters affecting the reactions were studied and optimized. Under the optimum reaction conditions, linear relationships with good correlation coefficients (0.9975-0.9996) were found between the absorbances and the concentrations of the investigated drugs in the concentration ranges of 4-50 and 20-400 microg/mL with TCNQ and pCA, respectively. With all the investigated drugs, TCNQ gave more sensitive assays than pCA; the limits of assay detection were 2.5-4.8 and 20-40 microg/mL with TCNQ and pCA, respectively. The intra- and interassay precisions were satisfactory; the relative standard deviations did not exceed 2%. The proposed procedures were successfully applied to the analysis of the studied drugs in pure form and pharmaceutical formulations with good accuracy; the recovery values were 98.4-102.8 +/- 1.24-1.81%. The results obtained from the proposed method were statistically comparable with those obtained from the previously reported methods.

Analytical study for the charge-transfer complexes of losartan potassium

Studies were carried out, for the first time, to investigate the charge-transfer reactions of losartan potassium (LOS-K) as n-electron donor with the σ-acceptor iodine and various π-acceptors: 7,7,8,8-tetracyanoquinodimethane, 1,3,5-trinitrobenzene, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, p-chloranilic acid, tetracyanoethylene, 2,3,5,6-tetrabromo-1,4-benzoquinone, 2,3,5,6-tetrachloro-1,4-benzoquinone, and 2,4,7-trinitro-9-fluorenone. Different colored charge-transfer complexes and radical anions were obtained. Different variables affecting the reactions were studied and optimized. The formed complexes and the site of interaction were examined by UV–vis, IR, and 1H NMR techniques, and computational molecular modeling. The formation of the colored complexes were utilized in the development of simple, rapid and accurate spectrophotometric methods for the analysis of LOS-K in pure form as well as in its pharmaceutical tablets. Under the optimum reaction conditions, linear relationships with good correlation coefficients (0.9985–0.9998) were found between the absorbances and the concentrations of LOS-K in the range of 2–200 μg ml −1. The limits of assays detection ranged from 0.61 to 19.65 μg ml −1. No interference could be observed from the co-formulated hydrochlorothiazide (HCTZ), as well as from the additives commonly present in the tablets. The methods were successfully applied to the analysis of tablets from different manufacturers that contain LOS-K, alone or combined with HCTZ, with good accuracy and precision; the recovery percentages ranged from 98.96 ± 1.62% to 101.58 ± 1.29%. The results were compared favourably with the reported method.

Determination of some psychotropic phenothiazine drugs by charge-transfer complexation reaction with chloranilic acid

A spectrophotometric method is described for the determination of four commonly used psychotropic phenothiazine drugs. The method is based on the measurement of the intensely coloured charge-transfer complex formed by the interaction of these drugs as n-electron donors with chloranilic acid as the π-acceptor. The coloured species measured exhibits maximum absorption at 535 nm. The molar-combining ratio and optimum assay conditions are reported. Beer’s law is obeyed over the range, 25–250 μg ml –1 with apparent molar absorptivity in the range, 0.93 × 10 3–1.45 × 10 3 l mol –1 cm –1. The limits of detection and quantification are reported. The proposed method was applied to the determination of these psychotropics in pharmaceutical formulations and the results demonstrated that the method is equally accurate, precise and reproducible as the official methods. The validity of method was established by recovery studies via standard-addition technique with satisfactory results.

Determination of imipramine in presence of iminodibenzyl and in pharmaceutical dosage form

Two spectrophotometric methods for the determination of imipramine in presence of iminodibenzyl as an impurity are described. The first method is a ratio-spectra first derivative spectrophotometry, the signals were measured at 240.2 nm for imipramine. Calibration graph was found linear in the range 5–30 μg ml −1. The second method is based on the reaction of imipramine base, being an electron donor, with p-chloranilic acid, being π acceptor, to form a purple colored charge transfer complex. The absorbance was measured at 520.5 nm without interference with iminodibenzyl. Both methods are rapid, simple and do not require any preliminary separation or treatment of the samples. Furthermore, the two methods were applied to pharmaceutical dosage form.

Use of charge-transfer complexation in the spectrophotometric analysis of certain cephalosporins

Three simple, rapid and sensitive spectrophotometric procedures were developed for the analysis of cephapirin sodium ( 1), cefazoline sodium ( 2), cephalexin monohydrate ( 3), cefadroxil monohydrate ( 4), cefotaxime sodium ( 5), cefoperazone sodium ( 6) and ceftazidime pentahydrate ( 7) in pure form as well as in their pharmaceutical formulations. The methods are based on the reaction of these drugs as n-electron donors with the σ-acceptor iodine, and the π-acceptors: 2,3-dichloro-5,6-dicyano- p-benzo-quinone (DDQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Depending on the solvent polarity, different coloured charge-transfer complexes and radicals were developed. Different variables and parameters affecting the reactions were studied and optimized. The obtained charge-transfer complexes were measured at 364 nm for iodine (in 1,2-dichloroethane), 460 nm for DDQ (in methanol) and 843 nm for TCNQ (in acetonitrile). Ultraviolet–visible, infrared and 1H-nuclear magnetic resonance techniques were used to study the formed complexes. Due to the rapid development of colours at ambient temperature, the obtained results were used on thin-layer chromatograms for the detection of the investigated drugs. Beer's plots were obeyed in a general concentration range of 6–50, 40–300 and 4–24 μg ml −1 with iodine, DDQ and TCNQ, respectively, with correlation coefficients not less than 0.9989. The proposed procedures could be applied successfully to the determination of the investigated drugs in vials, capsules, tablets and suspensions with good recovery; percent ranged from 96.47 (±1.14) to 98.72 (±1.02) in the iodine method, 96.35 (±1.62) to 98.51 (±1.30) in the DDQ method, and 95.98 (±0.78) to 98.40 (±0.87) in the TCNQ method. The association constants and standard free energy changes using Benesi–Hildebrand plots were studied. The binding of cephalosporins to proteins in relation to their molar absorptivities was studied.

Spectrophotometric Estimation of Nimesulide and its Formulations

Four simple and sensitive visible spectrophotometric methods (A–D) have been described for the assay of nimesulide (NMD) either in pure form or in pharmaceutical formulations. Methods A and B are based on the oxidative coupling between the reduced product of NMD (RNMD) and p-N,N-dimethyl phenylenediamine dihydrochloride (DMPD) in presence of chloramine – T (CAT) or 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) in presence of ferric chloride (Fe III) to form coloured products with λmax at 540 nm and 600 nm respectively. Method C is based on the diazotization of RNMD with excess nitrous acid (HNO2) and estimating the consumed HNO2 with cresyl fast violet acetate (CFVA). Method D is based on the formation of the coloured charge-transfer complex, when RNMD is treated with metol (p-methyl aminophenol sulphate, PMAP) in presence of potassium dichromate. All variables have been optimized and the reaction sequence is presented. The concentration measurements are reproducible within a relative standard deviation of 1.0%. Recoveries are 98.6–100.2%.

Reactions of 2,3-dihydrospiro[1H-4- and 5-azabenzimidazole-2,1′-cyclohexane] with nucleophiles: A potential route to some substituted aromatic heterocycles

The readily available title compounds 2, 16 and 27 react with pseudohalogenes (cyanide, azide), carbon and heterocyclic N-nucleophiles in the presence of manganese dioxide to give the corresponding substituted azaisobenzimidazoles (= 2H-azabenzimidazoles) 4, 8, 23–26 and 29–33, 36 or dihydroazabenzimidazoles (= 2,3-dihydro-1H-azabenzimidazoles 7, 22, 34 and 35. In 8 one of the two imidazolyl-substituents can be replaced by nucleophiles yielding the compounds 9–15. Treatment of 6′-bromo-2,3-dihydro-4-azabenzimidazole 16 with morpholine or piperidine results in loss of the Br-atom presumably by an AEa-mechanism. Reduction of the substituted azaisobenzimidazoles with sodium hydrosulfite followed by fission of the cyclohexane ring leads to substituted o-diaminopyridines. They were cyclised in situ with various condens ing agents to give new heterocyclic systems. Equimolar mixtures of some azaisobenzimidazoles and dihydroazabenzimidazoles lead to the formation of coloured charge transfer complexes stable only in the solid state. Owing to poor electron-acceptor properties the complex dissociates in solution.

Spectrophotometric determination of clozapine in pharmaceuticals

New Spectrophotometric methods for the assay of clozapine (CZP) in pure and dosage forms are described. Method A is based on the oxidative coupling reaction of CZP with 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) in the presence of ceric ammonium sulphate to form a coloured species (λmax 570 nm). Method B is also based on the oxidative coupling reaction of CZP withP-,N,N-dimethylpheny-lenediamine dihydrochloride (DMPD) in the presence of sodium hypochlorite to form a coloured species (λmax 690 nm). Method C is based on the formation of coloured charge-transfer complex between CZP and chloranilic acid (λmax 540 nm). Regression analysis of Beer-Lambert plots showed good correlation in the concentration ranges 2–25, 10–120 and 15–300 μg/ml for methods A, B and C, respectively. The validity of the proposed methods was tested by analysing pharmaceutical dosage forms containing CZP and the relative standard deviation values were within 1.0%.

Charge-transfer complexes of barbiturates and phenytoin

Simple and sensitive spectrophotometric methods are described, for the first time, for the determination of sodium salts of phenobarbital (1), thiopental (2), methohexital (3) and phenytoin (4). The methods are based on the reaction of these drugs as n-electron donors with the σ-acceptor iodine and various π-acceptors: 7,7,8,8-tetracyanoquinodimethane; 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; 2,3,5,6-tetrachloro-1,4-benzoquinone; 2,3,5,6-tetrafluoro-1,4-benzoquinone; 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone; tetracyanoethylene and 2,4,7-trinitro-9-fluorenon. Depending on the solvent polarity, different coloured charge-transfer complexes and radicals were developed. Different variables and parameters affecting the reactions were studied and optimized. The formed complexes were examined by UV/VIS, infrared and 1H-NMR. Due to the rapid development of colours at ambient temperature, the obtained results were used on thin layer chromatograms for the detection of the investigated compounds. Beer's plots were obeyed in a general concentration range of 1–400 μg ml −1 for the investigated compounds with different acceptors. Interference from some co-formulated drugs was also studied. No interference was observed due to additives commonly present in the pharmaceutical preparations. The proposed methods could be applied successfully to the determination of the investigated compounds in pure and pharmaceutical dosage forms with good accuracy and precision, the recoveries ranged from 98.7±0.5 to 101.1±0.5%. The results were compared favourably with the official methods.

Assay of cisapride in pharmaceutical formulations by visible spectrophotometry

Three spectrophotometric methods (A-C) for the assay of cisapride (CPD) in pure and dosage forms are described. Method A is based on the oxidative coupling reaction of CPD with 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) in the presence of ferric chloride to form a coloured species (λmax; 565 nm.) Method B is based on the oxidation of CPD with Fe (III) and subsequent chelation of Fe(II) to form a coloured complex with 1,10-phenanthroline (λmax: 520 nm). Method C is based on the formation of a coloured charge-transfer complex between CPD and chloranilic acid (λmax; 555 nm). Regression analysis of Beer-Lambert plots showed good correlation in the concentration ranges 2.0–32.0, 0.4–6.4 and 25.0–450.0 μg/ml for methods A,B and C, respectively. The validity of the proposed methods was tested by analysing pharmaceutical dosage forms containing CPD and the relative standard deviations were within 1.0%

Formation of C60F48 and fluorides of higher fullerenes

The effect of contact times and temperatures on fluorination of [60]fullerene by fluorine gas has been analysed by Knudsen cell mass spectrometry. The main product ranges from C60F38/40(4 h, 70 °C) to C60F48(16 h, 315 °C), showing that some control of product according to conditions is possible. The IR spectrum for C60F48(which is white, and crystallises from chloroform or dichloromethane as large colourless prisms) shows fine structure with sharp bands at 1238, 1215, 1199, 1170, 1142, 1126, 771, 754, 724, 650 and 603 cm–1. The 19F NMR spectrum (CDCl3) duplicates precisely that reported in the literature, but couplings not previously observed are evident by 2D (COSY) analysis, thereby confirming further the structure. Fluorination of a sample containing [70]- and higher fullerenes gives rise to C74F44–56, C78F54–60 and C84F56–62. C60F48 is stable towards chlorinated hydrocarbons, but is rapidly degraded by THF, ether and acetone. With benzene and alkylbenzenes, coloured charge transfer-complexes are formed which exhibit a bathochromic shift with increased alkyl substitution of the benzene ring; from toluene solution thick, red needles may be obtained and these slowly become colourless on standing as toluene is lost from the lattice.

Systematic search for spontaneous neutral to ionic and ionic to neutral transitions in organometallic materials based on minimally alkylated ferrocenes and classical polycyanide electron acceptors

Methyl-, ethyl- and n-butyl ferrocene (I–III) form dark colored charge transfer complexes with TCNE both in solution and by trituration of the solid donor and acceptor. Their Mossbauer spectra suggest only minor reduction of the TCNE acceptor. On the other hand, 1,1′-dimethylferrocene (IV) forms a ferricenium complex with TCNE of 1∶2 stoichiometry whose structure is likely similar to that determined for [IV][TCNQ]2. The new 1∶2 ferricenium-TCNE phase undergoes a novel, spontaneous, irreversible solid-state transformation back to a mixture of a low spin FeII ferrocene species and a high spin FeII complex over a period of weeks at ambient temperature. In preliminary studies, irradiation of samples of these materials with white light (including near infrared wavelengths) suggests significant photochemical activity of the type\(D^0 A^0 \xrightarrow{{hv}}D^ + A^ - \) and vice versa in the solid state. This is observable in Mossbauer spectra at 77 K. In the absence of irradiation with tungsten-halogen white light sources, however, the photo product is not permanently trapped at this temperature.

Charge transfer complexes of Schiff bases derived from 2-aminobenzothiazole with some acidic and nonacidic acceptors

When Schiff bases derived from 2-aminobenzothiazole were reacted with di- and trinitrobenzene derivatives as acidic and nonacidic acceptors in the ratio of 1:1, stable coloured charge transfer complexes (CTC's) were obtained. The structure and stoichiometry of the CTC's were studied by elemental analysis.as well as by electronic absorption and infrared and nuclear magnetic resonance spectra. The CTC's derived from nonacidic acceptors were stabilized via intermolecular π–π* and n–π* transitions while those derived from acidic acceptors were formed by intermolecular π–π* transitions and a proton transfer stabilized by resonance. The ionization potentials of the Schiff bases and the electron affinities of the nitrobenzene derivatives were calculated.

Photochemistry of charge transfer complexes of styrene derivatives with tetranitromethane. X-Ray crystal structure of a novel dimeric adduct from p-methylstyrene

Styrene and its derivatives form coloured charge transfer complexes with tetranitromethane as the acceptor. These complexes are stable in acetonitrile under neutral conditions in the dark for several days. Photochemistry of these complexes was found to be highly dependent on the structure and substituent effects on the styrene. The structure of the dimeric adduct 2b from p-methylstyrene is determined by X-ray crystallography and it clearly establishes the ambident reactivity of the trinitromethide anion to act as an oxygen nucleophile in the present case, which is in contrast to its well known reactivity as a carbon nucleophile in the previously reported adducts from other aromatic substrates.