Onset and Morphological Evolution of Cooperativity in Glass-Forming Liquids Composed of Anisotropically Shaped Molecules.

Many theories attribute the dramatic slowdown in glass-forming liquid dynamics to a transition from independent to cooperative molecular motion. Although the concept of cooperative rearranging regions (CRRs) is well established, the microscopic nature of these regions and their evolution upon cooling remain subjects of active investigation. Molecules with anisotropic shapes offer a new perspective, as molecular reorientations along their long and short axes act as distinct and complementary probes of emerging cooperativity. Here, we introduce model molecules with tailored dipole orientations and demonstrate that the temperature-dependent evolution of a bimodal dielectric response captures the transition from independent to cooperative dynamics. This clear spectroscopic signature marks a significant advance in our understanding of how cooperativity emerges and evolves in glass-forming systems. Upon cooling, CRRs adopt increasingly compact morphologies, leading to a reduction in effective shape anisotropy. This transition from anisotropic to isotropic CRR geometries reveals previously unrecognized mechanisms by which anisotropy becomes suppressed during vitrification.