A summary of our recent studies on multiphase self-assembly of 5-alkoxyisophthalic acid (C_nISA) as a versatile molecular building block is described. An interplay of weak interactions such as hydrogen bonding and van der Waals forces (alkyl chain crystallization) is the key factor in achieving and controlling the multiphase self-assembly of C_nISAs. Self-assembly of pure acids and its stoichiometric mixtures with bifunctional diazines in the solid state is described with typical examples. Supramolecular structures observed at the solid–solution interface (2D crystals) show characteristic similarities and differences with the structures observed in the 3D crystal lattices. C_nISAs with n>16 exhibit thermotropic behavior. An overview of the important features of the mesophase is also discussed. The self-assembly in solution leads to a cyclic structure similar to the one observed in the single crystal lattice of C_nISAs with shorter alkyl chains (n=6–10). Towards designing a functional supramolecular system, properly functionalized C_nISAs are used and the analysis of the results from their photophysical studies at the solid–solution interface is discussed.