**4.2 Supramolecules of ureas and thioureas**

Urea group is one such powerful building obstruct that formulates obstinate chains that bind through hydrogen bonding in various surroundings, from solutions [54] to gels and fibrous materials [55], also with crystals [47]. An approach that has been comprehensively discovered since the initial research in crystallography of disubstituted ureas in the late 1960s is the use of symmetrical or asymmetrical N,N′-di-substituted ureas that can deliver widespread multiple building congers for designing of organic solids of crystalline nature. N,N′-di-substituted ureas have the ability to act as H-bond donors by using their two N▬H protons, and play the role of acceptors through utilizing the presence of the lone electron pairs of CO group [36]. Robust one-dimensional hydrogen bonded chains having self-association reinforced promising complementarity between both groups (**Figure 8**) [56], which have been reconnoitered for developing the crystalline networks repeatedly.

Nanostructured materials on the basis of cylindrical or columnar constructions have been innovated more recently. Thioureas, despite the fact that they can also practice comparatively robust H-bonded motifs [57], have not been as much searched out for the coherent assemblage of solids of crystalline nature as compared to ureas [58, 59].
