Volatility, thermodynamic properties and dispersion interactions of sulfur-containing tricyclic molecular materials

Organic heterocyclic molecules play the role of precursors for various sophisticated materials from semiconductors to pharmaceuticals. Knowledge of their volatility is always an important factor to minimize hazards associated with their use and potential toxicity. Nevertheless, thermodynamic properties of polycyclic heterocycles have been very scarcely studied experimentally. In silico approaches can help provide the required data in many cases, but established benchmarks assessing the performance of first-principles models of the sublimation equilibrium for molecular crystals do not cover sulfur-based heterocyclic materials. This work aims at filling these obvious knowledge gaps at both experimental and computational sides. In this work, reference experimental sublimation data are established for four nitrogen- or sulfur-based heterocyclic compounds containing a structural motif of three fused rigid rings. Vapor pressure measurements and calorimetric experiments across broad temperature ranges are carried out to provide reliable reference data for stringent benchmarking of first-principles models of the crystal cohesion. Since the selected materials possess a very limited to no potential for hydrogen bonding, other non-covalent interactions such as dispersion or π-π stacking govern their cohesion. An accurate description of the dispersion interactions in heterocyclic polyaromatic molecules is challenging within density functional theory (DFT). Accuracy of popular post hoc dispersion corrections to DFT is benchmarked for the target heterocyclic materials, revealing that adopting the latest D4 dispersion model along with a lower-tier DFT functional does not necessarily lead to improvements of the computational accuracy over older dispersion models for this class of materials.