Molecular and supramolecular structural inf ormation are essential to understand the various spectroscopic and thermal characteristics of the ROR′-DADQs and develop rational design strategies. We have grown single crystals of all the ROR′-DADQs (Table , Figure ) f rom acetonitrile, ethyl acetate or methanol solutions and carried out X-ray crystallographic investigations. Two of these have been reported in an earlier study; the molecular structures of the remaining compounds f rom the X-ray dif f raction study are s ho wn in Figure , and their crystallographic data are collected in Tables ; those molecules with disorder in the alkoxyalkyl chains were modeled with multiple occupancies f or the relevant atoms (Figure ). The 6-member ring of the f luorophore unit is essentially benzenoid, thanks to the push-pull ef f ect of the diaminomethylene and dicyanomethylene units and the resulting zwitterionic nature of the DADQ f ramework. All the molecules show the torsional twist between the diaminomethylene unit and the benzenoid core ring (), characteristic of DADQ molecules (Figure ); the angles  are f ound to be in the range 38 -51 o (Table ). Unit cell packing diagrams of selected RO2-DADQs and RO3-DADQs are shown in Figure , and the complete set of the two series are shown Figures S11 and S12 respectively. A notable observation is that nearly all the RO3-DADQs (except 1O3 with a water molecule of crystallization) show segregated nonpolar and polar domains dominated by the alkoxyalkyl chains and the dipolar f luorophore units respectively; the longer R′ units appear to trigger this, as similar eff ects are not seen in the RO2-DADQs. We show later that these trends correlate well with the pref erence towards the A or C f orms. Some of the signif icant structural parameters such as the intra and intermolecular H-bond distances, and the distance and angle between the near neighbor benzenoid ring planes show interesting trends (Table , Figure ). The RO2-DADQs, except 1O2 have well-def ined intramolecular H-bonding between the amino group and the oxygen atom of the alkoxyethyl chain; notably, the RO3-DADQs do not possess such intramolecular interactions. While the RO2-DADQs show a single intermolecular H-bonding between the amino group of a molecule and the cyano N Please do not adjust margins Please do not adjust margins Please do not adjust margins Please do not adjust margins Table . Significant molecular and intermolecular structural features of ROR′-DADQs from single crystal X-ray diffraction analysis; θ = dihedral twist angle (average of θa-b-c-d and θa′-b-c-d′ in Figure ), r = H-bond distance, R = distance between the centroids of the nearest neighbor benzenoid units,  = angle between the mean planes of the nearest neighbor benzenoid units (see also Figure ). Shading of the cells indicate the dominant tendencies towards A (green), C (cyan) and A/C (white) states, and potential structural correlations (see the main text for details). ). While the RO3-DADQs show relatively higher values compared to RO2-DADQs as expected based on the molecular size, the increase of the molecular volume with the R chain length is steeper in the latter compared to the f ormer. This is once again an indication of the improved packing of the RO3-DADQs. The trends observed in the intra and intermolecular H-bonds, the dipolar f luorophore packing, and the tendency to f orm more compact supramolecular assembly, bear a direct correlation to the dominant tendency of the RO3-DADQs towards the f ormation of the C state with higher FEE revealed in the various thermal and spectroscopic studies discussed earlier. Most important ly , the RO2-DADQ with the longest R chain (n-4O2), and the RO3-DADQs with the shorter R chains (1O3, 2O3) f all in the borderline domain in terms of the intermolecular structural parameters, once again consistent with their C-A bistable character revealed through the spectroscopic and thermal studies. We have carried out powder X-ray dif f raction experiments on melt-quench samples of the ROR′-DADQs as a f unction of time, to directly probe their tendency to remain in the A state or transition to the C state. The dif f raction patterns collected in Figures (consistent with the simulations shown in Figure ) show clearly that 1O2, 2O2, n-3O2 and i-3O2 do not exhibit any sign of crystallization even af ter a week, conf irming their predilection towards the A f orm. On the other hand, n-3O3, i-3O3 and n-4O3 exhibit a clear dif f raction pattern within an hour af ter the meltquench, conf irming their pref erence f or the C f orm. n-4O2, 1O3 and 2O3 develop a crystalline dif f raction pattern within 2 -4 days revealing their A-C balance. Discovery of 2O3 as the f irst well def ined example of a f unctional molecular PCM and the observation of similar ef f ect in 1O3, are consistent with the current observations.