Substitution- and packing-driven charge and exciton transport in core-substituted squarylium dye crystals: a computational study

Abstract

A comprehensive computational study was performed to elucidate structure–property relationships in a series of core- and side-substituted CSQ dyes. Six crystal systems (CSQ1–CSQ6) were investigated by systematically varying substituents at the electron-deficient central C₄ ring (oxo-dicyanovinyl group (DCV), thio-DCV, and bis-DCV) and the electron-donating indoline and pyrylium side rings. Structural analyses reveal that oxosquaraines exhibit smaller dihedral angles between the central core and terminal rings, leading to enhanced molecular planarity and π-conjugation, whereas thio- and bis-DCV-substituted derivatives display increased torsional distortion. Intramolecular hydrogen bonding induces elongation of carbonyl bonds, indicating localized electronic effects, while ionization primarily perturbs bond lengths within the central C₄ ring. Intermolecular interactions, including intra stacked, inter stacked, and slip stacked motifs, were analyzed using Hirshfeld surface analysis, interaction energy calculations, and electronic coupling analysis. The reorganization energy reveals that the pyrylium-based CSQs preferentially facilitate electron transport, whereas indoline-based CSQs favor hole transport. Hole mobility is enhanced by intra stacked interactions between central C₄ rings, while electron transport benefits from inter stacked interactions involving donor side rings. Slip stacked interactions between DCV acceptor groups and pyrylium donors significantly enhance both hole and electron transport through increased charge-transfer (CT) character. Exciton coupling analysis shows that oxo-substituted CSQs exhibit stronger excitonic interactions than thio- and bis-DCV-substituted analogues, arising from contributions of both long-range Coulombic and short-range CT-mediated coupling. Notably, CSQ4 displays the strongest CT-mediated exciton coupling due to its favorable slip stacked packing geometry, highlighting the critical role of molecular substitution and crystal packing in tuning the optoelectronic properties of CSQ-based materials.