Linear [3]Spirobifluorenylene: An S-Shaped Molecular Geometry of p-Oligophenyls

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Abstract
An S-shaped p-oligophenyl derivative was synthesized, as exemplified by the linear [3]spirobifluorenylene (R,P,R/S,M,S)-1, which consisted of two p-sexiphenyls tied at the spiro-carbons of three spirobifluorenes. The direct linking of spirobifluorenes results in the formation of strained p-oligophenyl chains, which support each other to maintain the strained structure. This curved structure was successfully constructed by Suzuki-Miyaura cross-coupling of the corresponding spirobifluorene building blocks. Chiral (R,P,R)-1 and (S,M,S)-1 were also synthesized using chiral monomers, and their chiroptical properties were investigated by CD and CPL spectroscopy. The benzene rings in (R,P,R/S,M,S)-1 are distorted with the largest bend angle of 11.6°. The strain energy of (R,P,R/S,M,S)-1 was estimated to be 88.4 kcal/mol, which lies between those for [6] and [7]CPP. Through-space orbital interactions (spiroconjugation) were evaluated by TD-DFT calculations, and the results suggest the contribution of a 15 nm red-shift in (S,M,S)-1 in comparison with the single strand molecule 10. In addition, spiroconjugation is discussed in terms of molecular orbitals, and the splitting of both HOMO and LUMO was revealed by the calculations. The LUMO splitting is particularly interesting, since it is in contrast to the degenerate LUMOs in a simple spirobifluorene. Desymmetrization based on the distortion of the spirobifluorene moieties induces the LUMO splitting.

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