The Frontiers of Quinoidal Stability in Long Oligothiophenes: Raman Spectra of Dicationic Polaron Pairs

The transformation of bipolarons into polaron pairs in long oligothiophene dications has been reported by Raman spectroscopy. These polaron-pair dicationic species possess singlet open-shell biradicaloid ground electronic states. The formation of biradical polaron pairs marks the end of the quinoidal stability promoted by the intrinsic proaromatic character. The quinoidal stability in TCNQ oligothiophenes in comparison with dicationic oligothiophenes has been addressed.