Spectroelectrochemical investigation of the anodic oxidation of dibenzo-18-crown-6
Introduction
Crown ethers are of great interest since their discovery due to their complexing properties towards alkaline and alkaline-earth metal cations [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. These properties have been exploited in the construction of ion-selective electrodes and in the development of selective electrochemical recognition systems [11], [12], [13], [14], [15], [16], [17], [18]. It is well known that polymacrocyclic molecules have higher binding abilities towards some cations; therefore, such molecules and their polymers are good candidates for the extraction of metallic cations [19]. For example, DB18C6 is a macrocyclic monomer and its conductive polymer films have complexing properties towards cations especially in their reduced (neutral) form which makes them a good candidate for this application [20].
Le Berre et al. [21] reported that DB18C6 having an analogous structure to veratrol (1,2-dimethoxybenzene) undergoes trimerization reaction during its electrochemical oxidation. However, due to the sensitivity of crown ethers towards strong acidic media, they proposed the use of dichloromethane or acetonitrile in the presence of freshly activated neutral alumina to reduce the trace amount of water in the solvents used. Thus, they were able to obtain a black and amorphous polymeric conducting film on the platinum working electrode during constant potential electrolysis (CPE), for the first time [4], [20], [21], [22]. Later on it was reported by the same group that the best condition for the formation of conducting polymer films from DB18C6 is using a mixture of dichloromethane + acetonitrile (1:1, v/v) as the solvent [2].
Some properties of the poly-DB18C6 films: cyclic voltammetric (CV), [2], [21], [22] chrono-amperometric, microcoulometric [23] measurements, uses as modified electrodes [23] and polymeric membranes [24], paramagnetic [21], electronic conductive [21] and complexing properties with metallic cations [14], [19] were also reported.
Although synthesis of poly-DB18C6 is well documented [2], [21], [22], information about its spectroelectrochemical (SPEL), ESR and thermal properties is still lacking. In this work, the anodic polymerization of DB18C6 under extra-dry conditions was first monitored by using in situ ESR and in situ UV–Vis spectroscopic techniques. Then, the SPEL and thermal properties of the resulting polymer films were also studied by conventional methods.
Section snippets
Experimental
DB18C6 (Fluka), supporting electrolytes TBABF4 (Aldrich) and TBAPF6 (Fluka) were used without any further purification. Acetonitrile was first dried over CaH2 and distilled once from P2O5 under argon and stored over molecular sieves. Dichloromethane was also dried over CaH2 and distilled over CaH2 under nitrogen. Both solvents were treated with freshly activated alumina prior to use.
Polished platinum electrodes were used as working and counter electrodes. For SPEL studies indium tin oxide
Electrochemical polymerization
Since it is reported that satisfactory synthesis of conducting polymer films requires anhydrous conditions to avoid the rapid inhibition of the anodic oxidation [2], all solvent-supporting electrolyte solutions were treated with freshly activated neutral alumina prior to use and CPE was conducted in the presence of freshly activated neutral alumina. In fact, water reacts with crown ether radical cation and leads to the formation of quinone units which prevent polymerization.
The electrochemical
Conclusion
In this work amorphous semiconducting polymer films were prepared by electrochemical oxidation of DB18C6 in a mixture of acetonitrile and dichloromethane as solvent at room temperature. Both in situ ESR and UV–Vis spectroscopic measurements revealed the formation of triphenylene radical cations during the electrochemical polymerization. UV–Vis and CV measurements confirmed that the polymer film on ITO can be cycled reversibly between 0.0 and 0.9 V. The band gap Eg of the polymer was found to be
Acknowledgements
Acknowledgement is made to the Middle East Technical University Research Fund for support of this work. A.M. Önal thanks Prof. Dr. J. Simonet for his valuable suggestions during the synthesis of poly-DB18C6.
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