Assemblies of functional small-sized molecules having 4-amino-2,2,6,6- tetramethylpiperidine-1-oxyl responsive to heat and pH in water and their water proton relaxivities

1,3,5-Triureabenzene derivatives carrying alkyl (C _n) and poly(ethylene glycol) (Eg _m) chains C _nEg ₃ (1, 2, and 3, n = 6, 7, and 8, respectively) and C _nN(X)Eg _m (4 and 5, X = M (methyl), n = 6 and 8, respectively, m = 3; 6 and 7, X = T (2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPO), n = 6, m = 3 and 6, respectively) were prepared. All compounds in aqueous solutions exhibited the lower critical solution temperature (LCST) phenomena unique for small-sized molecules and formed self-assemblies above the transition temperature, T _t, of the LCST. Only compound 3 formed a hydrogel with a minimum gelation concentration of 0.5 mM (0.05 wt %). In 1.0 mM aqueous solution, the T _t values were determined to be in the range of 12-40 °C. In addition, the T _t values for 4-7 containing tertiary amine also responded to the solution pH with high sensitivity. The LCST behaviors for all compounds were reversible in the cycles of warming and cooling. The water proton relaxivities, r ₁, for 6 and 7 carrying TEMPO were altered below and above T _t and were largely reduced by the formation of self-assemblies above T _t. Compound 6 showed r ₁ values at 25 °C of 0.92 and 0.23 mM ^-1 s ^-1 at pH 7.0 and 6.0, respectively. In transmission electron microscopy (TEM) images, globular particles with polydispersity were observed, and their average hydrodynamic diameters (D _H) were determined to be in the range of 2400-730 nm by dynamic light scattering. In the TEM and scanning electron microscopy images of a xerogel sample of 3, bundles of fibers with a diameter of ca. 10 nm and a network structure, respectively, were observed.