Crystal Structures and Fluorescence Spectroscopic Properties of Cyano-Substituted Diphenylhexatrienes

The crystal structures and spectroscopic
properties of (E,E,E)-1,6-di­(x-cyanophenyl)-1,3,5-hexatrienes (2: x = 2, 3: x = 3, 4: x = 4) have been investigated.
The single crystal
X-ray structure analysis reveals that the molecules are linked via
sp2-CH···NC hydrogen bonds in each
crystal. The existence of the sp2-CH···NC
hydrogen bonds in crystals 2–4 is
spectroscopically evidenced by the fact that the IR peaks due to CN
stretching vibration shift to lower wavenumbers in the solid state
relative to those in solution. The molecules of 3 are
hydrogen bonded to form orthogonally oriented ribbons, whereas those
of 2 and 4 are organized into molecular
sheets. The distance and displacements between the nearest stacking
molecules are larger in 3 than those in 2 and 4. For each triene, the absorption and fluorescence
spectra in the solid state shift to longer wavelengths than those
in solution. The magnitude of the red shift is similar for 2–4 in the absorption spectra, but larger for 2 and 4 than for 3 in the fluorescence
spectra. We conclude that the origin of the solid-state fluorescence
is monomeric for 3 and excimeric for 2 and 4. The relationship between crystal structure and fluorescence
properties is discussed.