articol cu stelian in revista de chimie-2009

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REV. CHIM. (Bucureºti) ♦60♦ Nr.3 ♦ 2009248

Synthesis and Electrochemical Characterization of New Water

Soluble Thiophene Derivatives

STELIAN LUPU1*, PAUL CATALIN BALAURE2,*, ION COSTEA2, CECILIA LETE3, MARIANA MARIN3,

CRISTIAN ENACHE4

1 Department of Analytical Chemistry and Instrumental Analysis, Faculty of Applied Chemistry and Materials Science,

 University “Politehnica” of Bucharest, 1-3 Polizu Str., 011061, Bucharest, Romania2 Department of Organic Chemistry, Faculty of Applied Chemistry and Materials Science, University “Politehnica”

 of Bucharest, 1-3 Polizu Str., 011061, Bucharest, Romania3 Institute of Physical Chemistry “Ilie Murgulescu”, 202A Splaiul Independentei, 060021, Bucharest, Romania4 Central Laboratory for Phytosanitary Quarantine, 11 Afumati Str., Bucharest, Romania

The synthesis and the electrochemical characterisation of new water soluble thiophene derivativesare reported. (3-Thenyl)triethylammonium bromide and (3-thenyl)triethylammonium perchloratewere obtained by bromination of 3-methylthiophene and using the corresponding bromoderivativeas alkylation agent in reaction with triethylamine in dry benzene. The new thiophene derivativeswere characterised by 1 H-NMR, 13C-NMR, heteronuclear correlations experiments, and ESI-MS techniques. The electrochemical characterization of the new compounds has been performed inaqueous solution by means of cyclic voltammetry.

Keywords: thiophene derivatives, quaternary ammonium salts, cyclic voltammetry

* email: stelianl@yahoo.com

During the last two decades much interest has beendevoted to the synthesis and characterization of neworganic conducting polymers due to their electronic andelectrochemical properties. Organic conducting polymerscan be deposited as thin films onto conventional electrodesubstrates allowing the preparation of new chemically modified electrodes. A large number of conductingpolymers has been prepared by using various monomers,such as aniline, pyrrole and thiophene as well as their

derivatives [1-5]. Usually the chemical or electrochemicalsynthesis is carried out in organic media and the resultedpolymer is characterised in view of the applications inorganic media. The use of solvents is related to safety healthand environmental pollution problems and nowadaysseveral procedures are used to reduce their impact on thequality of life and environment. For these reasons newsynthetic procedures and new organic conductingpolymers have been developed in order to allow the use of friendly environmental and safety compounds. Design of new substances and materials that can be used to preparemodified electrodes suitable for analytical applications inaqueous media is therefore a research area of greatinterest. Quaternary ammonium salts are among thosesubstances used to achieve this goal. They have been usedin order to increase the electrochemical flux of larger redoxspecies such as erythromycin that, otherwise, cannot bedetected at a Nafion® modified electrode [6, 7]. In abovementioned references a Nafion® membrane incorporated with quaternary ammonium bromides, prepared by simpleco-casting the quaternary ammonium bromide with asuspension of the perfluorinated sulfonate polymer, waspipetted onto the electrode surface followed by solventevaporation and a final thermal treatment (annealing) of the modified electrode surface. Quaternary ammoniumsalts can also be immobilized on silica gel by chemically modifying silica on treatment, for instance, with 3-

aminopropyltriethoxysilane and alkylation of the terminalamino group [8]. By supporting this material on a mixtureof graphite powder with epoxy resin, a potentiometricsensor for perchlorate ions was prepared. Synthesis of new

thiophene derivatives bearing appropriate moieties thatmake them water soluble and able to polymerize inaqueous media, by electrochemical or chemical means,giving rise to highly conjugated electroactive polymerssuitable for electrochemical sensing of a large scale of analytes is also of major interest [9]. For instance,copolymerization between 3-[ N -succinimido (tetraethoxy)oxy]-4-methyl thiophene and 1-methyl-3-[3-(2-ethoxy)-4-methylthiophene]imidazolium bromide leads to a water

soluble copolymer that was used with good results asbiosensor for alkali metals ions and nucleic acids [10, 11].Other quaternary ammonium salts, such as tetrabutyl-ammonium (3-thienyl)ethoxybutanesulfonate (TEBSTA)and tetrabutylammonium (3,4-ethylenedioxy) methoxy-butanesulfonate (EDOTSTBA), contain as their anion asulfonated thiophene derivative, and can be easily polymerized in water with iron trichloride [12, 13], theresulting conducting polythiophenes being also watersoluble. Due to this special property thin films of thesepolymers have been spray coated on a SnO2 substrate [12]. With respect to this growing interest for new water solublemonomers, we report the synthesis and electrochemicalcharacterization of some new water soluble quaternary ammonium salts containing a 3-thenyl moiety.

Experimental partThe NMR spectra were recorded on a 200 MHz VARIAN

spectrometer, using D2O as a solvent. ESI mass spectra were recorded on a 1200 L/MS/MS VARIAN triple-quadrupolmass spectrometer by flow injection analysis (FIA)technique, using water as a solvent, nitrogen as nebulisergas, and air as drying gas.

( 3-Thenyl) triethylammonium bromide (3).To 1.2 g (6.78 mmol) of 3-thenylbromide were added

dropwise with cooling and stirring 2 mL of triethylamine.

The reaction mixture was stirred for two hours at roomtemperature and allowed to stay overnight in order tocomplete the chemical reaction. The resulted precipitate was filtered off with suction and washed well on filter with10 mL of dry benzene. Yield 1.82g (96%).

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The ESI mass spectrometry data are consistent withthe structures of compounds 3  and 4. Operation in thepositive mode reveals the molecular mass of (3-thenyl)triethylammonium cation, m/z =198. In afragmentation experiment along with the molecular masspeak, m/z =198, two other peaks,  i. e., m/z = 97 and m/z= 59 appear. A fragmentation pattern consistent with thesepeaks is depicted in scheme 2.

The peak at m/z = 97 is obviously due to loss of neutraltriethylamine from cation 5. Further elimination of cyclopropenyl carbene gives rise to protonatedpropargylthiol, which is responsible for the other peakobserved in the fragmentation experiment.

 As expected, operating in the negative mode two peaksat m/z = 79 and m/z = 81 in a ratio 1:1 appear in the massspectrum of bromide 3 corresponding to the two naturally occurring isotopes of bromine. In the case of perchlorate4, operation in the negative mode reveals two peaks at m/ z = 99 and m/z = 101 in a ratio of 3 : 1.

 Electrochemical characterisation.The electrochemical behaviour of the new water soluble

thiophene derivatives 3 and 4 has been studied in aqueoussolution by using cyclic voltammetry. Figure 1 shows thecyclic voltammograms of (3-thenyl)triethylammoniumbromide recorded at platinum electrode in 0.1 M KClO4aqueous solution at a scan rate of 50 mV/s for different

concentrations. The electrode potential was sweptbetween 0.0 and 1.3 V, when a redox wave has beenobserved at about 0.95 V. A peak to peak potentialseparation of 100 mV has been measured. Both the anodicand cathodic peak currents increase with the concentrationof the thiophene derivative 3. The oxidation of the thiophenederivative 3 occurs at the potential value 1.01 V, which ishigher than that observed for the electrochemicalpolymerization of 3,4-ethylenedioxythiophene in aqueoussolution [17]. Despite of the presence of a well definedredox wave, no electrochemical polymerization takesplace.

These results demonstrate that the water solublethiophene derivative 3 can not be used for theelectrochemical generation of thin polymeric films untoelectrode surface.

Based on these results and taking into account theelectrochemical behaviour of 3,4- ethylenedioxythiophene,it is worth to compare the electrochemical behaviour of 

thiophene derivative 3 with that of bromide ions at variousconcentrations. Figure 2 reports the cyclic voltammogramsof 1 mM and 5 mM (3-thenyl)triethylammonium bromideoverlayed with those of 1 mM and 5 mM potassiumbromide. The voltammograms show that the values of theanodic and cathodic peak potentials for these species arequite similar, which suggests that the thiophene derivative3 is not electroactive.

Table 2

  13C-NMR SPECTRUM OF (3-THENYL)TRIETHYLAMMONIUM CATION (5)

Scheme 2

Fragmentation pattern for (3-thenyl)triethylammonium cation

Fig,1. Cyclic voltammograms of 1mM (b) and 5mM (3-

thenyl)triethylammonium bromide (c) in 0.1 M KClO4. Cyclic

voltammogram of 0.1 M KClO4 at platinum electrode (a).

Potential scan rate: 50 mV/s

Fig, 2. Cyclic voltammograms of 1mM (dotted line), 5mM

(3-thenyl)triethylammonium bromide (thin solid line), 1mM

(thick solid line) and 5mM (trash line) KBr, respectively, in 0.1

M KClO4 on platinum electrode. Potential scan rate: 50 mV/s

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Fig, 3. Cyclic voltammogram of 500 µL of saturated solution of (3-thenyl)triethylammonium perchlorate in 0.1 M KClO

4 on

platinum electrode. Potential scan rate: 50 mV/s

For this reason, the counter ion bromide was replacedby perchlorate ion as described in the experimental section.The electrochemical behaviour of (3-thenyl) triethyl-ammonium perchlorate was investigated in aqueoussolution containing 0.1 M KClO4. Figure 3 shows the cyclic voltammogram of 500 µL of saturated solution of (3-thenyl)triethylammonium perchlorate in 0.1 M KClO4 onplatinum electrode at 50 mV/s. In this case an anodic peakpotential around 0.82 V could be observed.

This electrochemical behaviour is quite different thanthat reported for thiophene derivatives in aqueous solution.Further studies are necessary in order to clarify theelectrochemical behaviour of both organic cation andcounter anions.

These studies will provide new data and furtherdevelloping our previous results concerning the use of modified electrodes in aqueous solutions forelectrochemical sensing [18].

ConclusionsIn this work the synthesis and the characterization of 

new water soluble thiophene derivatives have beenreported. The NMR and mass spectra results clearly sustain

the chemical structures of the newly synthesisedcompounds. The obtained results suggest that furtherstudies are required in order to design new chemicalstructures characterized by improved electrochemicalproperties.

 Acknowledgments. We thank Professor Dr. Renato Seeber and Dr.

Chiara Zanardi from the Department of Chemistry, University of 

 Modena and Reggio Emilia, I taly, for the NMR measurements.

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Manuscript received: 25.02.2008

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