Effect of substituents on the structure, stability, and π-dimerization of dithienylpyrrole radical cations

A series of 2,5-di(2-thienyl)-N-methylpyrrole derivatives 1a-1d 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 1c gave the most stable radical cation, which persisted in dichloromethane at room temperature under nitrogen for several hours without any apparent decomposition. In addition, 1c(•+) had the largest π-dimerization enthalpy among 1a(•+)-1d(•+). DFT calculations with the M06-2X method revealed that methyl and methylthio derivatives 1b(•+) and 1d(•+) as well as 1c(•+) adopt a cis-cis conformation, in contrast to the trans-trans conformer of unsubstituted 1a(•+), 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 1c(•+) has the largest π-dimerization enthalpy.