Tuning the solid-state color properties of pigments tailored for specific applications is extremely important. With the aim of designing new organic pigment materials, binary and ternary solid-state forms of the natural plant pigment quercetin (Qur) were synthesized with two dipyridine derivatives 4,4′-ethylenedipyridine (BPA) and 4,4′-azobispyridine (AP) based on the supramolecular synthon approach. The obtained two cocrystals and four cocrystal solvates were identified and characterized thoroughly with powder and single-crystal X-ray diffractions as well as thermal analysis containing thermogravimetric analysis, differential scanning calorimetry, and hot-stage microscopy. Similar lattice parameters, molecular conformations, packing patterns, and intermolecular interactions revealed that the binary cocrystals Qur−BPA and Qur−AP are isostructural. Meanwhile, the robust porous structure framework composed of tetramers and the volume of solvent molecules play a critical role in the formation of isostructural ternary cocrystal solvates (Qur−AP−acetone and Qur−AP−dioxane). The quantitative similarity parameters via CrystalCMP and XPac programs further proved the corresponding isostructurality. These obtained solid-state forms of Qur exhibit colors ranging from pale-yellow, earthy yellow, orange, to dark red. An explanation for the observed different colors of the multicomponent crystals was rationalized using a combination of UV−vis absorption spectra and density functional theory calculations as well as π−π stacking interactions. Finally, the accelerated stability experiments were conducted and the results show that Qur−AP presents better stability than Qur−BPA, which can offer promising development value for new organic pigments.

Crystal nucleation determining the formation and assembly pathway of first organic materials is the central science of various scientific disciplines such as chemical, geochemical, biological, and synthetic materials. However, our current understanding of the molecular mechanisms of nucleation remains limited. Over the past decades, the advancements of new experimental and computational techniques have renewed numerous interests in detailed molecular mechanisms of crystal nucleation, especially structure evolution and solution chemistry. These efforts bifurcate into two categories: (modified) classical nucleation theory (CNT) and non-classical nucleation mechanisms. In this review, we briefly introduce the two nucleation mechanisms and summarize current molecular understandings of crystal nucleation that are specifically applied in polymorphic crystallization systems of small organic molecules. Many important aspects of crystal nucleation including molecular association, solvation, aromatic interactions, and hierarchy in intermolecular interactions were examined and discussed for a series of organic molecular systems. The new understandings relating to molecular self-assembly in nucleating systems have suggested more complex multiple nucleation pathways that are associated with the formation and evolution of molecular aggregates in solution.