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

作者：Takai Atsuro^*; Freas Dylan J; Suzuki Toshikane; Sugimoto Manabu; Labuta Jan; Haruki Rie; Kumai Reiji; Adachi Shin ichi; Sakai Hayato; Hasobe Taku; Matsushita Yoshitaka; Takeuchi Masayuki^*

来源：Organic Chemistry Frontiers, 2017, 4(5): 650-657.

DOI：10.1039/c7qo00125h

We synthesized methyl-substituted 9,9'-bifluorenylidene (9,9'-BF) derivatives in which two planar fluorene units are connected through a C=C 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 p-planes of a 1,1'-dimethyl-substituted 9,9'-BF is 56 degrees, which is clearly larger than those of pristine 9,9'-BF (42 degrees) and 1-methyl-substituted 9,9'-BF (50 degrees). 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.

• 出版日期2017-5-1