Excited-state proton transfer in the crystals of 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile

The crystal and molecular structures of 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile (HBMN) were determined at room temperature. The molecule is almost planar and packed along the b axis with a spacing of approximately 3.7 Å in a monoclinic unit cell. The molecules include a weak hydrogen bonding between the 4-hydroxy proton and one of the two nitrogen atoms of the malononitrile group along the a axis within the a–c plane. Polycrystalline HBMN exhibits very strong green fluorescence in the 490-nm region at low temperature, while the crystal shows a weak fluorescence at 450–460 nm at room temperature. The large temperature dependence of the fluorescence spectra is discussed in terms of an excited-state proton transfer in the weak hydrogen bonded system in the crystals.