Correlation of Structure with Crystalline to Amorphous Phase Transitions of 1,3,6-Substituted Fulvene-Derived Molecular Glasses.

An investigation into the crystalline to amorphous phase transitions of prepared 1,3,6-substituted pentafulvenes showed the expected reversible heated melt and cooling recrystallization in only a few examples. Systematic incorporation of bulky substituents at the 6-position of the fulvene ring led to the nonreversible thermal behavior, rendering phases that were locked into glassy, vitrified states. These molecular glasses produced physically translucent and amorphous features with glass transition temperatures in the range of 61-77 °C, comparable with high-strength plastics such as polyethylene terephthalate. Additionally, the melting point transitions and the resulting heat of fusion values were found to be directly influenced by the nature of the 6-position substituent. Single crystal X-ray crystallography showed that in some cases, fulvenes possessing fused aromatics exhibited a high degree of intermolecular π-π stacking. These results point to a class of molecular glass formers as host materials possessing tunable bulk properties for potentially new optical applications.