Role of intermolecular interactions and conformational changes in polymorphism and vitrification process of 2,2''-bis-substituted para-terphenyls

For the last decades para-terphenyls have been attracting tremendous attention due to polymorphism and conformational diversity. In this work we report synthesis, crystal structure, polymorphism and dielectric properties of two 2,2''-substituted para-terphenyls: 2,2''-bis(hydroxymethyl)-para-terphenyl and 2,2''-bis(acetyloxymethyl)-para-terphenyl. Based on the calorimetric and X-ray studies we showed that the latter compound occurs in at least four polymorphic forms of melting points equal to 364, 345, 341 and 326 K, differentiated also in terms of thermodynamic stability and crystal symmetry. The most stable polymorph I is characterized by P21/n space group. 2,2''-Bis(hydroxymethyl)-para-terphenyl crystallizes in monoclinic P21/c space group. Both 2,2''-bis-substituted para-terphenyls can undergo vitrification process, which is highly exceptional feature for this class of chemical compounds and had not been reported before. Consequently the molecular dynamics and conformational changes in glassy and supercooled liquid states was analyzed by means of IR and broadband dielectric spectroscopy. Two relaxation processes were observed for both compounds: structural α-relaxation, connected with reorientational motions of molecules in supercooled liquid and intermolecular γ-relaxation, ascribed to rotational motions of substituents of para-terphenyl skeleton. Taking into account ongoing discussion about conformational diversity of para-terphenyl skeleton we showed that, although free rotation of benzene rings is suppressed, the molecules in glassy and liquid states can adopt both twisted and helical conformations, what results in diversity of polymorphic forms.