Construction of a Helical Structure with a Parallel Alignment of Molecular Dipoles in Crystals by Utilizing a Halogen-3 Synthon and a Bulky Silyl Spacer

The arrangement of dipoles in a molecular crystal is one of the most important factors determining the electronic and photophysical properties in the solid state. However, it remains a significant challenge to design the dipole arrangement in crystals because of the difficulty in predicting the crystal structure. Herein, we report the formation of linear structures with antiparallel dipoles and helical structures with parallel dipoles in crystals by introducing halogen substituents. Polar 2,3-difluorophenylene derivatives 1–3 consist of a bulky triisopropylsilyl moiety on one side as a spacer and a hydrogen (4-H, 1) or halogen substituent (4-Br, 2; 4-I, 3) at the para-position of the silyl group. Single-crystal X-ray diffraction studies revealed that crystals 2 and 3 form triangle "halogen-3 synthon" by halogen bonding among Br or I substituents, resulting in helical structures with parallel alignment of the dipoles, whereas the dipoles are in antiparallel alignment in linear structures to cancel their polarity in crystal 1.