Influence of Chalcogen Bonding on Lattice Dynamics in Isostructural Molecular Solids

Abstract

Solid-state lattice dynamics can significantly impact the expression of material properties in crystalline molecular materials, complicating crystal engineering efforts that approach materials design from a primarily static structural picture. While noncovalent interactions have been extensively studied for their role in directing supramolecular assembly, their effect on lattice dynamics in the resulting crystalline materials remains comparatively underexplored. Here, we investigate how chalcogen bonding (Ch-bonding) interactions influence low-frequency vibrational dynamics in a pair of isostructural molecular solids. Substitution of chalcogen donor atoms from S to Se in naphthalene bis(1,2,6-chalcogenadiazine) (Nap2Thn-I and Nap2Sen-I, respectively) enables systematic tuning of Ch-bond strength without modification to the crystal packing structure, producing a pair of materials with an identical set of vibrational normal modes. Using low-frequency Raman spectroscopy, solid-state density functional theory simulations, and local mode analysis, we demonstrate that strengthening Ch-bonding interactions by exchanging S for Se selectively increases the force constants and frequencies of the normal and local modes in Nap2Sen-I that directly modulate the intermolecular Ch···N coordinate. However, vibrational modes that are not strongly influenced by Ch-bonding interactions are dominated by mass effects, causing the lighter S-based Nap2Thn-I to exhibit higher frequencies. These trends hold at the Brillouin zone center and boundary, and collectively yield a net reduction in the thermal displacement amplitude of the atoms in Nap2Sen-I. Together, these findings demonstrate that directional noncovalent interactions can be harnessed to tune the expression of solid-state lattice dynamics, providing a foundation for the rational design of crystalline molecular materials with targeted dynamic properties.