Glass versus crystal : a balancing act between competing intermolecular interactions

Abstract

Hydrogen (H) bonds can be used either for crystal engineering or for designing compounds capable of forming very stable glassy phases. Herein, a second type of directional noncovalent interaction, π–π stacking, is introduced to establish the interplay between both types of interactions on crystal and glass formation. For this purpose, two mexylaminotriazine derivatives incorporating 2,3,4,5,6-pentafluorostilbene groups (with or without a H-bond donor at the 2 position of the triazine) and their nonfluorinated analogues were synthesized to compare their glass-forming and crystallization properties. Although all four compounds showed glass-forming ability, only the fluorinated compounds showed crystallization with kinetics strongly affected by the presence or absence of the H-bond donor group. X-ray diffraction of the pentafluorostilbene-containing derivatives revealed an extended π–π stacking interaction different from that of 2,3,4,5,6-pentafluorostilbene. These results, combined with infrared spectroscopy measurements, suggest that π–π stacking promotes crystallization whereas hydrogen bonding impedes it due to the higher conformational constraints imposed by π–π stacking, thereby decreasing the degrees of liberty and the possibilities for secondary interactions with other molecules.