Thermodynamic stability of hydrogen-bonded nanostructures: A calorimetric study

The self-assembly of hydrogen-bonded aggregates (rosettes) in solvent mixtures of different polarity has been studied by calorimetry. The C-50 parameter, the concentration when 50% of the components are incorporated in the assembly, is used to compare assemblies with different stoichiometry. C50 for the single rosette 1(3)(.)(BuCYA)(3) (1 = N,N-di(4-tert-butylphenyl)melamine; BuCYA = n-butylcyanuric acid) in 1,2-dichloroethane is 25 muM, whereas for double rosettes 2a(3)(.)(BuCYA)(6) and 2b(3)(.)(BuCYA) (2 = calix[4]arene-dimelamine) it is 0.7 and 7.1 muM, respectively. DeltaGdegrees, DeltaHdegrees, and TDeltaSdegrees values indicate that the thermodynamics of double rosettes reflect the independent assembly of two individual single rosette structures or two rosettes reinforced by additional stabilizing interactions. In more polar solvents the stability of double rosettes decreases. From the correlation of DeltaGdegrees with solvent polarity it is predicted that it should be possible to assemble double rosettes in methanol or water. The assembly of 2b(3)(.)(BuCYA)(6) in 100% MeOH was proven by H-1 NMR and CD spectroscopy.