A phenyl-substituted diazaperylenediium dication (DAP) was synthesized that shows concentration-dependent aggregation in water. The host−guest complexation of DAP was studied with the macrocyclic host cucurbit[n]uril (n = 7, 8) in water. With CB[7], it forms a 1:2 complex by placing two aryl substituents inside the CB[7]; whereas, with CB[8], a supramolecular polymer is formed. The resulting complex and supramolecular polymer have been characterized by ITC (isothermal titration calorimetry), ESI-MS (electrospray ionization mass spectrometry), 1 H NMR (proton nuclear magnetic resonance), 2D NOESY (two-dimensional nuclear Overhauser enhancement spectroscopy), and DOSY (diffusion-ordered spectroscopy) spectra.