Glass-forming Schiff bases: Peculiar self-organizing systems with bifurcated hydrogen bonds

Abstract

A combination of quantum computations with calorimetric, dielectric, infrared, and diffraction studies was used to dissect the nature of atypical bifurcated (three-centered) hydrogen bonds (BHBs). Based on the analysis of four glass-forming Schiff bases, this article experimentally proves that self-organization via BHBs may be prevalent in H-bonding systems. Moreover, it explicitly shows that even high screening of the proton-donor and proton-acceptor groups is not a sufficient obstacle to form the BHBs. Even highly sterically hindered Schiff bases form lasting centrosymmetric dimers via a pair of BHBs in the liquid and glass phases. The driving force for their medium-range-scale ordering tendency is a disorder in the molecular scaffold, strictly bound to the N…H…O bond character. As a result, the protons of the BHBs center gain translational freedom. Finally, it is shown that, contrary to the prevailing opinion, Schiff bases can be good glass-forming systems with high glass transition temperatures.