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-2C6 (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.