Highly coplanar very long oligo(alkylfuran)s: A conjugated system with specific head-to-head defect

Well-defined monodisperse conjugated oligomers, which have planar backbones and are free from the disturbance of substituents, attract broad interest. Herein, we report a series of symmetrical, isomerically pure oligofurans, namely, the 16-mer 16F-6C₆ together with the related nF-2C ₆ (n = 4, 6, 8). Through computational studies and detailed spectroscopic and X-ray characterization, for the first time, we show that the planarity of the furan backbone is almost unaffected by the head-to-head defect which is known to cause considerable twists in its oligo- or polythiophene analogues. We present that the properties of these rigid oligo(alkylfuran)s are strongly influenced by the conjugation length. As the longest monodisperse α-oligofuran synthesized to date, 16F-6C₆ was observed to be stable and highly fluorescent. Experimental and computational studies of the redox states of these oligo(alkylfuran)s reveal that 16F-6C₆ has singlet biradical (polaron-pair) character in the doubly oxidized ground state: the open-shell singlet (〈S₂〉 = 0.989) is 3.8 kcal/mol more stable than the closed-shell dication.