The effect of a highly twisted CC double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states

We synthesized methyl-substituted 9,9′-bifluorenylidene (9,9′-BF) derivatives in which two planar fluorene units are connected through a CC double bond. The CC double bond is twisted owing to the steric crowding between the fluorene units, and by introducing substituents at 1,1′-positions (inner space of 9,9′-BF) it becomes more twisted. Indeed, single crystal X-ray structural analysis and theoretical calculation reveal that the dihedral angle between two fluorene π-planes of a 1,1′-dimethyl-substituted 9,9′-BF is 56°, which is clearly larger than those of pristine 9,9′-BF (42°) and 1-methyl-substituted 9,9′-BF (50°). The twisted conformation of 1,1′-dimethyl-substituted 9,9′-BF facilitates the cis–trans isomerization process which we assessed quantitatively by variable-temperature NMR measurements. The 9,9′-BF derivatives with different numbers of methyl groups also exhibit remarkable changes in optoelectronic properties, primarily because of the change in the twisting angle of the central CC double bond. Theoretical calculation further indicates that the electronic structures of methyl-substituted 9,9′-BF derivatives in the excited states are considerably different from those of pristine 9,9′-BF.