Fig. 2: Experimental evidence and DFT calculations for the formation of NU rosettes.

a Stacked ¹H-NMR spectra of (R, R)-NU in DMF-d₇ with increasing deuterated water fractions (f_w). The peaks between 7.9 and 8.2 ppm are ascribed to the solvent residual of DMF. [NU] = 0.1 mM. The labels of a to m at the bottom refer to the aromatic protons of NU, whose chemical structure is inserted in (2b). b Diffusion-ordered spectroscopy of (R, R)-NU in DMF-d₇/D₂O mixed solvent, f_w = 30%, [NU] = 0.1 mM. D: diffusion coefficient, d: hydrodynamic diameter. c High-resolution electrospray ionization mass spectroscopy of (R, R)-NU in acetonitrile. The inserted image represents experimental results (upper) and calculated results (bottom) of isotopically resolved m/z signals of a NU hexamer [6NU+Na]⁺. d The cryo-TEM image of NU assemblies in DMF/H₂O mixed solvents, f_w = 30%, [NU] = 0.1 mM. The inserted image is an enlarged micrograph of the observed nanostructure. e The statistical particle size distribution of nanostructures in (2 d). f, g DFT-optimized structure of a rosette by Gaussian 09 program at B3LYP 6-31 G* level. The red, blue, white, and gray balls represent oxygen, nitrogen, hydrogen, and carbon atoms, respectively. Except for hydrogen atoms in urea groups, other hydrogen atoms are omitted for clarity. h The space-filling mode of the cyclic hydrogen bond array extracted from a DFT-optimized rosette.