Conf ormational def ects impact upon the A or C state in polyaniline and exert memory ef f ects. The syn-syn and anti-anti conf ormations of the f urandicarboxylate moiety promote A and C domains respectively, in poly(butylene-2,5-f urandicarboxylate). The thiophene content in poly(3-hexylthiophene-co-thiophenes) shows an inverse correlation with the degree of crystallinity, and the ratio between diketopyrrolopyrrole and benzodithiadiazole units was used to tune the crystallinity in random copolymers based on them. The A state is of ten benef icial f or pharmaceutical compounds; a study of celecoxib showed that while the C f orm had an ordered network of H-bonds involving all the electron donor groups, the A f orm showed relatively stronger H-bonding interactions. Enhanced luminescence in aggregates, supramolecular assemblies and the solid phase of select materials, in contravention of the generally expected concentration quenching ef f ects, is a phenomenon of special interest. Popularly known as 'aggregationinduced emission', it holds great application potential in various technologies including displays, sensing and imaging. A f ine balance of f actors that inhibit non-radiative excited state energy loss channels and intermolecular energy transf er pathways, is key to the realization of the enhanced emission response. Families of molecules that have been developed in this context include tetraphenylethylenes, siloles, triphenylamines, acetylantharacenes, 8,8adihydrocyclopenta[a]indenes and diaminodicyanoquinodimethanes (DADQs). Fluorescence emission switching due to the modif ication of molecular struc-ture and supramolecular assembly motif in specif ic systems has been reported widely. However, the f luorescence signature has not been used to establish the molecular structural control of the A/C states in molecular solids, through a systematic investigation of a whole f amily of compounds. The nature of supramolecular assembly in terms of A or C organization can inf luence the solid state f luorescence response signif icantly. Study of a large number of DADQs have unraveled several interesting scenarios in this regard. Solvent vapor induced A → C transf ormation with a prominent f luorescence switching was demonstrated with molecular microparticles conf ined in a polymer thin f ilm; the transf ormation can also be monitored in real time. Solvent polarity tuning was gainf ully employed to achieve a hierarchical variation of the crystallinity and hence the f luorescence response. Reversible thermally induced A  C transf ormations with concomitant f luorescence changes led to the f irst realization of a f unctional molecular phase change material; 32 such phase changes could also be demonstrated in polymer nanocomposite thin f ilms through laser irradiation, enabling direct laser write-read-erase. While the A-C phase changes can be exploited to modulate the f luorescence responses in molecular materials, the f luorescence characteristics in turn can serve as a signature of the phase transf ormation providing valuable insig ht into phenomena such as the 2-step nucleation. With a view to developing basic molecular level structuref unction correlations that underlie the predilection towards the A or C f orm of supramolecular assembly, we have carried out a detailed investigation of a series of DADQs with a systematic variation of the substituents on the amino moiety. Alkoxyalkyl substituted DADQs (ROR′-DADQs) were selected f or the f ollowing reasons: (i) the f luorophore units and hence the basic photophysical responses of the individual molecules are similar across the series, (ii) f acile structural characterization by X-ray dif f raction is f easible, (iii) the chain substituents are known to lower the melting temperature of DADQs enabling meltquench experiments without thermal decompositio n, and (iv) a f ine tuning of the intermolecular dipolar and dispersion interactions mediated by the aromatic co re units and the substituent chains respectively, is possible. Five each of the alkoxyethyl and alkoxypropyl DADQs were synthesized and their mo lecular and crystal structures determined, so that the variation of the supramolecular assembly is precisely known. Electronic absorption and f luorescence emission studies demonstrated the f luorescence enhancement phenomena typical of the DADQs across the series. Temporal monitoring of the specially prepared A f o rm of the solids by f luorescence spectroscopy and X -ray dif f raction was used to decipher the tendency towards crystallization. Dif f erential scanning calorimetry investigations unraveled the systematic impact of the substituent groups that promote the A and C f orms o f supramolecular assembly, and the f eatures that enable reversible transf ormations. These could be corre-lated to the molecular structural f eatures, the R and R′ moieties in particular, and the supramolecular organization and interactions in the crystal. This study thus provides unique insights into the molecular structural control of the assembly that directly also impacts upon the f luorescence emission enhancement. As a predictive exercise we have explored a borderline system and established its PCM characteristics.