Difluorenoheteroles: topological control of π conjugation in diradicaloids and mixed-valence radical ions

Two families of difluorenoheterole diradicaloids were synthesized, featuring isomeric ring systems with distinct conjugation topologies. The two types of difluorenoheteroles contain, respectively, a Chichibabin-like motif ( CH ) and a newly introduced heteroatom-linked triphenylmethyl dyad ( TD -X). Combined experimental and theoretical investigations show that the TD -X systems have reduced quinoidal character but the interaction between formal spin centers is sufficiently strong to ensure a singlet ground state. The singlet-triplet energy gaps in the TD -X difluorenoheteroles are strongly affected by the heterocyclic ring, with values of −4.3 and −0.7 kcal mol −1 determined for the pyrrole- and thiophene-containing analogues, respectively. In cyclic voltammetry experiments, the TD -X systems show diminished energy gaps and superior reversibility in comparison with their CH counterparts. The radical anions and cations obtained from these diradicaloids show extremely red-shifted bands, occasionally with λ max > 3500 nm. Computational studies show that some of these ions adopt distonic structures and may be characterized as class-II mixed-valence species. Linking and fusion of triphenylmethyl radicals produces isomeric difluorenoheterole diradicaloids, with heteroatom-dependent paths for spin-spin interactions.