Fascinating Supramolecular Assembly through Noncovalent Interactions Involving Anions in Organic Ionic Crystals

Abstract

Organic crystals are not only suitable for fundamental research in supramolecular chemistry but also ideal candidates for high-performance optoelectronic materials. Among them, organic ionic crystals are a field of great potential. Herein, we prepared three ionic crystals 1–3 by the protonation of the pyridine wing in PBA (N 4,N 7-di­(pyridin-2-yl)­benzo­[c]­[1,2,5]­thiadiazole-4,7-diamine). In this work, we found that either unique non-coplanar conformation or interesting supramolecular assembly is triggered by noncovalent interactions involving anions in the crystal. Since protonated PBA molecules with a twisted conformation can act as a synthetic receptor to weaken the strong electrostatic repulsion of anions, a rare “dimer” of nitrate anions has been observed in 1. Unlike semiprotonated 3, the full protonation of the pyridine group in 1 or 2 can support enough noncovalent interactions involving anions to hinder the effective π–π stacking of the BTD cores. Herein, either 1 or 2 shows monomeric BTD luminescence affected by anions, while in 3, typical J-aggregate luminescence is observed. Moreover, the luminescence intensity of 1 or 2 under hydrostatic pressure increases first and then decreases, and the luminescence shift is obviously smaller than that of the neutral PBA crystal. In contrast, due to the dominant π–π interaction similar to that of the neutral PBA crystals, 3 shows the quenched luminescence with a large red shift. Interestingly, due to the different molecular packing modes, 1–2 show isotropic luminescence, while 3 shows anisotropic luminescence.