Inter- and intra-stacking-influenced charge hopping interactions in core-substituted squaraine dye derivatives: a computational investigation

Abstract

The stacking arrangement and associated interactions within the crystal packing of ten core-substituted squaraine (CSQ) dyes, which were symmetric dicyanovinyl (DCV) acceptor group-functionalized squaraine derivatives with distinct electron-rich side donor moieties containing various substituents, were assessed using computational methods. Out of the three sets, two contained indolenine and quinolinium donor moieties as side substitutions. In the first set, the halogen substituents on indolenine donor moieties (such as –Cl, –Br, and –I) exhibited monoclinic polymorphism, while the –F substituted one exhibited triclinic polymorphism. Triclinic and orthorhombic polymorphs are known for the absence of any substituents. Second set of two CSQ dyes has quinolinium donor units that differ in length of alkyl chain substitution found in monoclinic polymorphism. The third set of CSQ contains monoclinic polymorphism in which one indolenine CSQ contains a chiral center while the other one is with benzothiazole ring. Herein, we investigated the impact of the DCV acceptor moiety present at the central C₄ ring of squaraine dyes and the functionalization of side donor groups on the inter- and intra-stacking patterns during crystal packing to establish a charge transfer protocol. Theoretical calculations, including geometrical analysis, internal reorganization energy, charge-transfer integral, drift mobility and interaction energy, along with a complete investigation of the Hirshfeld surface analysis, were performed. The distinctive geometrical changes upon electron gain and loss were observed at different moieties, i.e. side aromatic rings and the central C₄ ring, respectively, of the respective CSQs. Inter- or intra-stacking cofacial interactions between side-substituted donor rings offered electron transfer character, while the participation of slip-stacked central C₄ rings provided a hole-transfer character to all CSQs. Although the very meager participation of the DCV group in the inter- and intra-stacking interactions of all motifs was studied, indirect influence was noted through the geometrical impacts. Such packing effects provide insights into inter- and intra-stacking interactions, possibly guiding experimentalists' ideas for shaping and using promising squaraine derivatives in advanced functional materials.