Change in Molecular Orientation with Condensation of 4,4′-Bis(trihydroxysilyl)biphenyl Crystals

Abstract

Abstract Molecular orientation in 4,4′-bis(trihydroxysilyl)biphenyl plate-like square crystals (BPSi) and orientation change with heating-induced condensation were investigated using polarization optical microscopy. The biphenylylene groups in BPSi were arranged along a diagonal of the square plane with a tilt angle of 21° from the planar normal. This arrangement was maintained up to 200 °C for several minutes, but heating at 250 °C promoted a condensation reaction which formed siloxane bonds, causing changes in molecular orientation to a tilt angle of 0°, and altering the shape of BPSi crystals from square to rhombic. The molecular arrangement and its change after condensation were qualitatively confirmed by molecular-mechanics simulation. The absorption and fluorescence bands of the biphenylylene groups were significantly red-shifted by the condensation due to the change in molecular arrangement, which resulted from a notable decrease in biphenylylene–biphenylylene separation.