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 sp(2)-CH center dot center dot center dot N C hydrogen bonds in each crystal. The existence of the sp(2)-CH center dot center dot center dot 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.