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Electro-reduction of Viologen-phenyl Diazonium Salts as A Strategy to Control Viologen Coverage on Electrodes liangcheng cao, Gan Fang, and Yuechuan Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04317 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017
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Electro-reduction of Viologen Phenyl Diazonium Salts as A Strategy to Control Viologen Coverage on Electrodes Liangcheng Cao,a,* Gan Fang,a Yuechuan Wangb a.
Chongqing Institute of Green and Intelligent Technologies, Chinese Academy of Sciences.
Fangzheng Avenue 266, Chongqing 400714, China. b.
College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials,
Sichuan University. South section of 1st ring road 24, Chengdu 610065, China.
ABSTRACT: Majority of the reported electrografting of aryldiazonium salts results in the formation of covalently attached films with a limited surface coverage below 5 nmol.cm-2. Herein, we reported the preparation of higher thickness redox active viologen grafted electrodes from electroreduction of viologen phenyl diazonium salts, by either cyclic voltammetric (CV) sweeps or electrolysis using a fixed potential. Both of the methodologies, were successfully applied for various conductive surfaces, including glassy carbon, gold disc, ITO glass, mesoporous TiO2 electrodes and 3D compacted carbon fibers. A robust maximal viologen coverage, Γviologen= 9.5 nmol was achieved on a glassy carbon electrode by CV electro-reduction. Electro-reduction holding at a fixed potential at Eappl. = −0.3V, can fabricate viologen grafted
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electrode with Γviologen in the range of 0~37 nmol.cm-2 in a controllable way, by simply adjusting the electrodeposition time tappl.. Time dependent Γviologen were found as: 10 nmol.cm-2@2 min, 20
[email protected] min, 30 nmol.cm-2@7 min. Furthermore, TiO2 electrode coupled with Γviologen of 140 nmol.cm-2 exhibited electrochromic performance with color changes from pale yellow to blue and red-brown.
INTRODUCTION
Viologens (N,N′-disubstituted-4,4′-bipyridinium) functionalized electrodes were promising candidates for electrochromic devices,1-5 electrochemical biosensors,6-8 organic radical batteries911
and electron mediators,12-14 taking the advantage of their reversible redox chemistry
accompanied by reversible color changes. Unfortunately, the exploitation of viologens for these applications was hampered by difficulties in fabricating environmentally stable viologen layer in a nano- or micro-meter scale onto an electrode. Strategies developed in preparing viologen functionalized electrodes can be generally divided into at least four classes: physical adsorption using anchoring groups (e.g., -PO3H2, -Si(OC2H3)3, -SH, cucurbits)15-19 or by electrostatic interactions;20 electrodeposition of viologen containing conjugated oligomer blocks, e.g., thiophene21-22 and pyrrole;23-24 polymerization of vinyl containing viologens;25 and selfassembling via either Langmuir–Blodgett or bottom-up methods.26-29 However, still there was a lack of one-step procedure for electrode modification with viologens using a covalently attaching method in a controllable way.
Electrochemical reduction of arydiazonium derivatives has been widely used for the purpose of covalent modification of electrodes.30-34 Modified electrodes of specified functions in the fields of molecular electronics, energy conversions, and chemical/biological sensors, can be tailored
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with specific reactive group at the para position.35-38 Monolayer or multilayers could be formed with a limited surface coverage below 5 nmol.cm-2. Pedersen & Daasbjerg have revealed that cyclic voltammetric sweeping on aryldiazonium salt carrying an electroactive moiety (nitrobenzene, anthraquinone, or benzophenone) allows the formation of thick conducting films, even in the micrometer size range.39-41 In this work, redox active viologen phenyl diazonium salts (4a) were synthesized, for the first time, to fabricate viologen functionalized electrode surfaces. Viologen modified electrodes by means of diazonium coupling have been previously reported by Downard42 and Holm,43 both of the procedures required multi-step reactions, long periods (typically 4−5 days), but resulted in low coverages ( 3x10−10 mol.cm−2). Stable and high surface coverage up to 37 nmol.cm-2, that was approximately 95 layers, can be achieved within 10 minutes. Importantly, desired coverage can be designed by simply adjusting the coupling time. And this one-step procedure was found working for flat conducting glassy carbon (GC), indium tin oxide (ITO) glass slides, gold (Au) surfaces, three dimensional mesoporous TiO2 electrode and compacted carbon fiber.
EXPERIMENTAL
Materials and Synthesis All the chemicals and solvents were commercial available either from Sigma-Aldrich or Alfa Aesar and used as received without any treatment. Solvents for the electro- spectroscopy analysis were of 99.9% purity. Mesoporous TiO2 modified electrode with a TiO2 thickness of ~5 ݑm was prepared according to the literature.44 SIGRATHERM® carbon fiber KFA 5 (A >1.5 m2/g) and GFA 5 (A
