Effect of Substituents on the Structure, Stability, and π‑Dimerization of Dithienylpyrrole Radical Cations

A series of 2,5-di­(2-thienyl)-<i>N</i>-methylpyrrole derivatives <b>1a</b>–<b>1d</b> with methylthio end-capping groups and electron-donating substituents at the 3-position of the thiophene rings was synthesized, and the effects of the substituents on the structure, stability, and π-dimerization ability of the radical cation were investigated using UV–vis–NIR and electron spin resonance spectra and density functional theory (DFT) calculations. Among the electron-donating methyl, methoxy, and methylthio substituents, the methoxy derivative <b>1c</b> gave the most stable radical cation, which persisted in dichloromethane at room temperature under nitrogen for several hours without any apparent decomposition. In addition, <b>1c</b>^•+ had the largest π-dimerization enthalpy among <b>1a</b>^•+–<b>1d</b>^•+. DFT calculations with the M06-2X method revealed that methyl and methylthio derivatives <b>1b</b>^•+ and <b>1d</b>^•+ as well as <b>1c</b>^•+ adopt a cis–cis conformation, in contrast to the trans–trans conformer of unsubstituted <b>1a</b>^•+, while the π-dimers of all of these compounds were shown to have a cis–cis conformation. On the basis of further detailed analyses, the preformed cis–cis conformation and the weaker intramolecular and intermolecular steric repulsions were considered to explain why <b>1c</b>^•+ has the largest π-dimerization enthalpy