Understanding Electrochromic Processes Initiated by Dithienylcyclopentene Cation-Radicals

Abstract

Simple photochromic dithienylethylenes with either a perfluoro or a perhydro cyclopentene ring, and a variety of substituents (chlorine, iodine, trimethylsilyl, phenylthio, aldehyde, carboxylic acid, and ethynylanisyl), have been prepared and their electrochemical behavior was explored by cyclic voltammetry. All dithienylethylenes present two-electron irreversible oxidation waves in their open form, but the cation-radical of the open isomers can follow two different reaction pathways: dimerization or ring closure, whereas the halogen derivatives follow a dimerization mechanism, the presence of donor groups, such as the phenylthio-substituted compound, promote an efficient oxidative ring closure following an ECE/DISP mechanism. Electrochromic properties are also found in the corresponding ring-closed isomers. Depending on the substituents on the thiophene ring, and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring-closed isomers via an EC mechanism. This reaction pathway is favored by the presence of electron-withdrawing groups in the molecule. For all these compounds, closed or open, the oxidation lies between 0.8 and 1.5 V vs SCE, and provokes a permanent modification of the color, even after an oxidation-reduction cycle. This could be qualified as "electrochromism with memory". On the other hand, the ring-closed electron-rich isomers (E degrees < 0.8 V), which show reversible waves at the cation-radical or even dication level, give rise to "true electrochromism", for which no structural changes are observed. The experimental study was completed by theoretical calculations at the DFT level, using B3LYP density functional, which gave information on the total energy, the geometry, and the electronic structures of several representative compounds, either in the neutral form or in the cation-radical state. These results are important for the potential design of photochromic systems, such as three-state conjugated systems and photoelectrical molecular switching devices.