Insights into the radical-radical and radical-substrate dimerization processes for substituted phenylmethylenepyrans
Introduction
The reversible dimerization of organic radicals has for a long time fascinated electrochemists, essentially because the analysis of the experimental data, in particular cyclic voltammetry (CV) and chronoamperometry (CA), can give straightforward information on the mechanism of dimerization as well as the kinetics/thermodynamics of the process [[1], [2], [3], [4], [5], [6], [7], [8]]. Different types of organic dimers can be found according to the mode of interaction between monomeric units, upon redox activation. Most of studies have been focused on organic dimers generated by the formation of σ-σ carbon-carbon bond [[9], [10], [11]]. Also, π-π dimers have been more recently developed with supramolecular organic systems [[12], [13], [14], [15]]. While most of reported studies have been carried out for fundamental purposes, recent works have shown some remarkable applications in the domain of redox switches and electrochromic devices for systems displaying optical properties in the UV–Visible spectroscopic domain [9,11,[16], [17], [18], [19], [20]]. Among them, methylenepyrans (MPs) are organic molecules which have been fully exploited as dyes for photovoltaic cells [21], but also as component parts of push-pull compounds for non-linear optic (NLO) applications and luminescent devices [[22], [23], [24]]. As their dithiafulvalene (DTF) analogues [25], MPs are able to form σ−σ CC bound dimers upon oxidation at a relatively low redox potential. The generated bis-pyrylium compounds can be reduced chemically and electrochemically to yield back the initial MPs. Our experimental and computational studies on a series of substituted-MPs have emphasized that the redox and spectroscopic properties of these compounds could be modulated by adequate variation of the substituting group [[26], [27], [28], [29], [30]]. Noteworthy, we recently demonstrated by time-resolved spectroelectrochemistry that R-substituted phenyl-MPs 1a-f (Scheme 1) could be switched between their mono-pyran and bis-pyrylium states by redox triggering over several cycles, leading to a concomitant change of their spectroscopic signatures [26]. Moreover, the electrophilic character of the substituting group R on the phenylmethylene moiety (Scheme 1) was shown to affect in a significant manner the oxidation potential of the MP. Contrarily, the reduction potential of the bis-pyrylium species (2a-f)2+ remained almost insensitive to the nature of the R group. These experimental results were fully supported by electronic structure and thermochemical calculations [26]. In complement to these first results, we present here our further investigations on the dimerization mechanism and the kinetics associated to the oxidation of compounds 1a-f, by the combination of electrochemical and computational approaches. In particular, the influence of the R-substituting group on the dimerization process has been deeply explored, with the help of low-temperature and high scan rate cyclic voltammetry.
Section snippets
Synthesis of compounds 1a-f
All syntheses and characterization of the phenylmethylenepyran derivatives 1a-f were previously reported [26].
Electrochemical methods
The electrochemical studies were performed in a glovebox (Jacomex) (O2 < 1 ppm, H2O < 1 ppm) with a homemade three-electrode cell (WE, Pt; RE, Ag wire; CE, Pt), such that the volume of the solution (hence concentration) was constant all over the duration of measurement (no need for gas bubbling). Ferrocene was added at the end of each experiment to determine the redox potential values.
Electrochemical studies
Cyclic voltammetric (CV) studies of compounds 1a-f have been first carried out at a Pt working electrode in dichloromethane at room temperature. As previously stated [26], all PhMPs display the same redox behavior, i.e. a single irreversible oxidation peak on the forward scan at Epa(1) and an irreversible reduction peak on the backward scan at Epc(2) (see Table 1 for electrochemical data), the value of Epa(1) and Epc(2) varying with R. For example, Fig. 1 displays the CV obtained for 1a at v
Conclusions
The voltammetric studies of the R-substituted phenylmethylenepyrans 1a-f have shown that the irreversible oxidation potential leads to the formation of bis-pyrylium σ-dimers (2a-f)2+. Although the oxidation potential is fully dependent on the electrophilic property of the substituent, the potential of reduction of (2a-f)2+ cleaving the CC bond and yielding back 1a-f, does not vary significantly with R. Our voltammetric analyses also demonstrate that the intermolecular dimerization follows
Acknowledgments
Financial support by Agence Nationale pour la Recherche (ANR-13-BSO7-0018), Conseil Général du Finistère, and Université de Bretagne Occidentale. I.L. acknowledges support from the European Commission under the Campus Prestige progamme (PRESTIGE-2015-2-0019).
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