Hierarchical assembly of a molecular material through the amorphous phase and the evolution of its fluorescence emission

Abstract

Fabrication of an amorphous phase followed by a hierarchy of forms leading to the crystalline state offers a new dimension in the assembly of small molecule based materials. The morphology and extent of crystallinity of the nanostructures in drop-cast thin films of a diaminodicyanoquinodimethane molecule are shown to be tunable through the variation in the composition of the solvent mixture used for drop-casting. The different states of assembly starting from the solvated molecule through amorphous spherical particles, crystalline nanofibers and nano/microcrystals to bulk crystals are shown to be accompanied by a smooth variation of the fluorescence emission, the color evolving from green to blue, and the efficiency increasing steadily with the overall enhancement from the solution to the bulk crystalline state being ∼ 400 times. Quantum chemical computations on the molecule and its H-bonded dimer and π-dimers provide insight into the impact of intermolecular interactions in the crystalline assemblies on the electronic structure. The current study illustrates a significant departure from the conventional molecules-to-materials transition, opening up new hierarchical pathways of assembly of molecular materials.