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. However, a precise understanding of the molecular features and interactions that govern such assemblies has rarely been realized. The enhanced fluorescence of select classes of molecular materials with various applications in displays, sensing and imaging, is strongly influenced by their crystallinity; the subtle balance between the A and C forms is key to their use as functional phase change materials. We have synthesized and structurally characterized two series of alkoxyalkyl diaminodicyanoquinodimethanes (ROR′-DADQs) exhibiting enhanced and tunable fluorescence from the solution, to the A state, to the C state. An exhaustive study of their photophysical responses, thermal characteristics and A-C phase changes reveal significant correlations with the molecular structural features (R and R′ moieties), key interactions like H-bonds, and molecular orientations in the lattice. These observations provide novel insights into the molecular structural control of the A/C states in molecular solids, and A-C phase change systems; fluorescence switching is exploited as a signature of the phase transformations. The factors promoting the A or C forms of molecular solids can guide the design of novel materials and devices exploiting such supramolecular structures and their interconversions.