Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon–Carbon Bond Making/Breaking

Abstract

Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron stimulus are of high interest for the development of functional materials. In particular, devices based on radical dimerization are appropriate because of the effectiveness and speed of carbon-carbon bond making/breaking. Phenylmethylenepyrans are organic chromophores which are well suited for such purposes since their oxidation leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox switching is reversible over several cycles and is accompanied by a significant modification of the UV-vis spectrum of the chromophore, as shown by time-resolved spectroelectrochemistry in thin-layer conditions.

Insights into the radical-radical and radical-substrate dimerization processes for substituted phenylmethylenepyrans

“…Contrarily, the reduction potential of the bispyrylium 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.…”

“…The preparation of the input files and analysis of the output files was carried out using the Avogadro 1.03 software [34]. The input structures for the geometry optimization in the gas phase of 1a, 1a +• , 1f , 1f +• , as well as 2 + and 2 2+ dimers (for R = NMe 2 , H or NO 2 ) were manually built from the X-ray structures of 1b and [BisFcMP] 2+ , respectively [26,30]. A vibrational frequency calculation was carried out on each of the structures to confirm that the stationary point is a minimum in energy and to obtain the free energy correction term.…”

“…All syntheses and characterization of the phenylmethylenepyran derivatives 1a-f were previously reported [26].…”

“…Noteworthy, we recently demonstrated by time-resolved spectroelectrochemistry that Rsubstituted 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.…”

“…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].…”