A helically-twisted ladder based on 9,9′-bifluorenylidene: synthesis, characterization, and carrier-transport properties

We synthesized a ladder-shaped 9,9'-bifluorenylidene cyclic dimer (CBF), in which the two 9,9'-bifluorenylidene units are connected directly with two covalent bonds. CBF has a pair of enantiomers which are alternately packed in a perpendicular fashion in the crystal, and these enantiomers exhibit rapid interconversion in solution. Owing to the ladder-type connection, the two 9,9'-bifluorenylidene units in CBF electronically interact upon redox, and thus the structure of CBF is changed stepwise during the oxidation and reduction processes, which was confirmed by electrochemistry and quantum chemical calculations. The pi-network of CBF in the solid state leads to its hole transport properties.