A computational probe granting insight into intra and inter-stacking interactions in squaraine dye derivatives‡

In order to decipher structure function relationships regarding the optoelectronic properties of organic functional materials, two sets of anilinosquaraine dye derivatives were analysed computationally. 2,4-Bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine exhibits orthorhombic and monoclinic polymorphs (motif-1 and motif-2) in bulk and spin coated films, respectively, and concomitant J- and H-aggregate spectral features. In the second set, in spite of variation in the n-alkyl chain of the anilinosquaraine derivatives (n-propyl vs. n-butyl; motif-3 and motif-4), both acquired triclinic morphology and intermolecular charge transfer had been identified as the origin of their panchromaticity. The differences between these motifs were closely inspected at the theoretical level including geometrical parameters, internal reorganization energies, charge transfer integrals, drift mobilities, interaction energies and lattice energies along with a comprehensive Hirshfeld surface analysis. A correlation was established between C⋯C type intra-stack interactions with the observed red-shifted absorption patterns in the order motif-1 > motif-3 > motif-4. Hydrophobic interactions, π–π interactions and hydrogen bonds were found to influence the crystal packing patterns and concomitantly the overall molecule planarity and thereby the extent of π-bond delocalization/resonance. Insights into intra-stack and inter-stack interactions based on crystal packing effects are provided, which support and potentially guide the experimentalists’ key ideas for shaping and encompassing the applications of promising squaraine derivative materials.