(678j) Identifying the Origins of Stability in a Complex, Organic Vapor Deposited Glass Former

The emissive layer of an organic LED screen is a thin layer designed to facilitate light emission by maximizing charge transport. Physical vapor deposition (PVD) has become a key technology to produce this layer, as the process has been shown to form smooth, thin organic glasses with layered structures, where the π-π stacking interactions between aromatic groups enhances charge mobility through the material. To design and optimize the properties of these glasses, we must understand the molecular-level origin of this order as well as its dependence on molecular chemistry and flexibility. Here, we investigate the anisotropic ordering in vapor deposited α-α-A, a small aromatic hydrocarbon, using all-atom molecular dynamics. We bypass expensive deposition simulations via umbrella sampling to directly probe the energetics and stability of the layered structure. We then leverage AniSOAP, an anisotropic machine learning descriptor, to quantitatively characterize the molecular configurations between the layered and isotropic configurations of the α-α-A glass. Our results suggest that combining molecular simulation and data-driven analysis can bridge the gap between molecular geometry and bulk-level organization in organic glasses.