Monitoring Molecular Microparticles through the Amorphous-to-Crystalline Transformation and Fluorescence Enhancement/Tuning

Impact of the mode of assembly of molecules on the properties and functions of molecular materials is a problem of fundamental and practical relevance. Understanding the transition of amorphous assemblies to the crystalline state has important implications for the nucleation/growth process as well as applications like phase change materials. We present a novel diaminodicyanoquinodimethane derivative with diphenylpropyl groups, BDPPDQ, which exhibits strongly enhanced fluorescence in the crystalline state. Drop-cast films of BDPPDQ show the formation of size-tunable amorphous microparticles with markedly weaker and red-shifted fluorescence. Facile transformation of these particles to the crystalline state upon exposure to solvent vapors is monitored through sequential imaging by electron microscopy and atomic force microscopy and confirmed by X-ray and electron diffraction. The dipolar zwitterionic core of the molecule coupled with the bulky, labile substituent groups and the resulting interplay of intermolecular interactions lead to the molecular assembly in the drop-cast films and the gradual structural transitions. The amorphous-to-crystalline transformation is accompanied by a prominent tuning of the fluorescence wavelength and intensity, which is highlighted also through fluorescence lifetime imaging. Computational investigations shed light on the impact of molecular assembly on the photophysical responses. The present study provides a graphic demonstration of the gradual amorphous-to-crystalline transition in molecular solids with concomitant fluorescence tuning and insight into the subtle role of molecular assembly.