Crucial role of N...Si interactions in the solid-state coloration of disilylazobenzenes.

2,2'-Bis(fluorodiphenylsilyl)azobenzenes bearing two methyl and butyl groups at the 4- and 4'-positions were synthesized, and their X-ray crystallographic analyses revealed that they have tetracoordinated and pentacoordinated silicon atoms, respectively. Although the two compounds showed almost the same spectra in solution, the reflectance spectra of the solid revealed that the pentacoordinated state had a weaker absorbance in the long-wavelength region compared with that of the tetracoordinated state, which is ascribed to the shift of the n-pi* transition. This difference in the absorptions due to the n-pi* transition drastically affects the color of the azobenzenes: the color of the pentacoordinated state becomes apparently paler than that of the tetracoordinated state.