The effect of a highly twisted C
C double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states†
Abstract
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 C
C 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 C
C 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.
- This article is part of the themed collection: Novel π-electron molecular scaffolds