Unexpectedly Large Couplings Between Orthogonal Units in Anthraquinone Polymers

The unusual electronic properties of directly linked 1,4-polyanthraquinones (14PAQs) are investigated. The dihedral angle between the molecular planes of anthraquinones (AQs) is found to be close to 90°. Contrary to the prevailing notion that the interaction between orthogonal units is negligible due to broken π-electron conjugation, the coupling between neighboring AQ units does not have a minimum at 90° and is much larger than that expected. The unexpectedly large electronic coupling between orthogonal AQ units is explained by the interaction between the lone pairs of the carbonyl oxygen and the π system of the neighboring unit, which allows favorable overlap between frontier molecular orbitals at the orthogonal geometry. It is shown that this effect, which is described computationally for the first time, can be strengthened by adding more quinone units. The effect of thermal fluctuations on the couplings is assessed through ab initio molecular dynamics simulations. The distributions of the couplings reveal that electron transport is resilient to dynamic disorder in all systems considered, whereas the hole couplings are much more sensitive to disorder. Lone pair-π interactions are described, as a previously largely overlooked conjugation mechanism, for incorporation into a new class of disorder-resilient semiconducting redox polymers.