EQCM Analysis of the Insertion Phenomena in a n-Doped Poly-Alkyl-Terthiophene With Regioregular Pattern of Substitution.

Danilo Dini,Franco Decker,Elisabetta Salatelli

doi:10.3389/fchem.2021.711426

Danilo Dini, Franco Decker + Show 1 more

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https://doi.org/10.3389/fchem.2021.711426

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Abstract

In the present work, we have undertaken the study of the n-doping process in poly-3,3″-didodecyl-2,2′:5′,2″-terthiophene (poly-33″-DDTT) employing the electrochemical quartz crystal microbalance (EQCM). The present study aims at understanding how cathodic charge in n-doped poly-33″-DDTT is compensated. For this purpose, the in situ analysis of the variations of the polymeric mass has been considered. Poly-33″-DDTT was obtained as a thin coating onto a metallic substrate via the anodic coupling of the corresponding monomer 3,3″-didodecyl-2,2′:5′,2″-terthiophene (33″-DDTT). When subjected to electrochemical n-doping in the polarization interval -2.5 ≤ E appl ≤ 0 V vs. Ag/Ag+, the films of poly-33″-DDTT varied their mass according to a mechanism of cations insertion during n-doping and cations extraction during polymer neutralization. In fact, the electrochemical doping of polythiophenes requires the accompanying exchange of charged species to maintain the electroneutrality within the structure of the polymer in all states of polarization. At the end of a full electrochemical cycle (consisting of the n-doping and the successive neutralization of poly-33″-DDTT), the polymer retains a fraction of the mass acquired during n-doping, thus manifesting the phenomena of mass trapping. The combined analysis of electrochemical and microgravimetric data suggests that poly-33″-DDTT in the n-doped state undergoes (or electrocatalyzes) uncontrolled electrochemical reactions that are not accompanied by mass variations.

Highlights

Polythiophenes (PTs) are heterocyclic polymers with an extended area of electronic delocalization (Brédas et al, 2016), which possess high chemical and physical stability (Tourillon and Skotheim, 1986)
During the process of poly-3,3′′-DDTT n-doping, the electrochemical quartz crystal microbalance (EQCM) detected variations in the mass of the polymer-modified electrode, which were consistent with the uptake of TBA+ cation for compensating the excess of negative charge in reduced poly3,3′′-DDTT
The value of 2.5 mg C−1 would correspond to the exchange of a species with a molecular mass of amu, when the exchanged species carries one positive charge (TBA+ has a mass of amu)

Summary

Introduction

Polythiophenes (PTs) are heterocyclic polymers with an extended area of electronic delocalization (Brédas et al, 2016), which possess high chemical and physical stability (Tourillon and Skotheim, 1986). The resulting polymer poly-3,3′′-didodecyl-2,2′: 5′,2′′-terthiophene (poly-33′′-DDTT; Figure 1, right sketch) is obtained as a thin film on a supporting metallic substrate when the polymerization of 33′′-DDTT monomer (Figure 1, left sketch) is conducted electrochemically through a potentiodynamic route In this context, we have analyzed the mass variations of poly-33′′-DDTT during electrochemical n-doping employing the electrochemical quartz crystal microbalance (EQCM) (Naoi et al, 1991; Buttry and Ward, 1992; Dini et al, 2021), i.e., a non-invasive tool that detects in situ the mass changes accompanying a solid-state electrochemical process (Schiavon et al, 1994; Ward and Meyers, 2000). EQCM has demonstrated its usefulness in the evaluation of the dynamics of polymerization in the case of the potential pulse sequence-based electrochemical deposition of polypyrrole (Plausinaitis et al, 2015) and the verification of spontaneous adsorption phenomena consisting in the formation of adlayers of pyrrole on metallic substrates prior to any sequence of electrical polarization (Plausinaitis et al, 2017)

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