Molecular Structural Control of the Amorphous‐Crystalline Balance in Solids: Structure‐Phase Correlation in DADQs with Enhanced/Switchable Fluorescence

The amorphous/crystalline (A/C) assembly in molecular solids has a direct bearing on their attributes and applications, including mechanical, pharmaceutical, electronic and photophysical. A systematic analysis of the molecular features and interactions that determine the predilection towards the A, C or bi-stable A-C states is critical. This fundamental problem is addressed through an exhaustive investigation of a large family of alkoxyalkyl diaminodicyanoquinodimethanes (ROR'-DADQs); enhancement of their fluorescence from the solution, to the A, to the C state serves as an excellent signature of the phase preference and temporal stability. Their thermal characteristics and single crystal structure investigations coupled with Hirshfeld surface analysis of the intermolecular interactions, not only confirm the spectroscopic observations, but also reveal the significant impact of the molecular structural features, key interactions like H-bonds, and molecular orientations in the lattice, on the phase stability. These explorations lead to a basic structure-phase correlation; its predictive power is demonstrated by identifying the borderline systems as functional phase change materials, and the subsequent verification through the reversible cycles of fluorescence switching between the A/C states. The factors promoting the A or C forms of molecular solids can guide the design of novel materials exploiting such supramolecular structures and their interconversions.