<p>Over the last several decades, molecular materials have emerged as an important class within the broader field of materials science and chemistry. The critical impact of the structure of molecular building blocks, as well as their mode of assembly, have been exploited to develop a wide range of functional molecular materials. The most rudimentary level of molecular assembly that can be visualized in this context is the amorphous or crystalline form; their mutual transformations also serve as the basis for various potential applications. This review outlines the basic concepts in this domain and uses several case studies with the diaminodicyanoquinodimethane (DADQ) family of molecules to illustrate or model them. The enhancement and tuning of the fluorescence emission of DADQs, from the solution to the amorphous, to the crystalline states, provide a facile handle for monitoring the assembly modes and their transformations. This has also led to the formulation of fundamental molecular structure–phase correlations. This review is expected to trigger new ideas that leverage even the most basic forms of molecular assembly to achieve useful responses and functions in molecular materials.</p> Graphical abstract <p>Following an overview of the development of molecular materials, the relevance of amorphous and crystalline assemblies as well as their reversible interconversions is highlighted. Exploitation of the sensitive changes in the fluorescence emission of diaminodicyanoquinodimethane (DADQ) family of molecules to monitor and model the assemblies and their transformations is discussed.</p> <p></p>