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The effect of a highly twisted C[double bond, length as m-dash]C double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states

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Abstract

We synthesized methyl-substituted 9,9′-bifluorenylidene (9,9′-BF) derivatives in which two planar fluorene units are connected through a C[double bond, length as m-dash]C double bond. The C[double bond, length as m-dash]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 cistrans 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[double bond, length as m-dash]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.

Graphical abstract: The effect of a highly twisted C [[double bond, length as m-dash]] C double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states

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Publication details

The article was received on 14 Feb 2017, accepted on 09 Mar 2017 and first published on 16 Mar 2017


Article type: Research Article
DOI: 10.1039/C7QO00125H
Citation: Org. Chem. Front., 2017, Advance Article
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    The effect of a highly twisted C[double bond, length as m-dash]C double bond on the electronic structures of 9,9′-bifluorenylidene derivatives in the ground and excited states

    A. Takai, D. J. Freas, T. Suzuki, M. Sugimoto, J. Labuta, R. Haruki, R. Kumai, S. Adachi, H. Sakai, T. Hasobe, Y. Matsushita and M. Takeuchi, Org. Chem. Front., 2017, Advance Article , DOI: 10.1039/C7QO00125H

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