First Synthesis of Phenylazomethine Dendrimer Ligands and Structural Studies

Novel dendritic polyphenylazomethines (DPAs), which consist of a π-conjugated backbone, were synthesized up to the fourth generation by the convergent method via dehydration of aromatic ketones with aromatic amines in the presence of titanium(IV) tetrachloride. The obtained dendrimers, DPA G1−4 (designated as GX, where X is the generation number), show high thermostability (Td_10% 521 °C in DPA G4) and high solubility for the common solvents such as chloroform, THF, and DMSO unlike the conventional linear polyphenylazomethines, which have very low solubilities. The DPA G4 molecule was confirmed to have a spherelike structure by GPC measurement and a molecular model based on the crystal structure of DPA G2. Crystal data for DPA G2:  monoclinic space group P2₁/a, a = 25.352(4) Å, b = 8.577(2) Å, c = 16.151(2) Å, β = 106.25(1)°, V = 3371.6(10) ų, Z = 2, D_calc = 1.168 g/cm³, μ(Cu Kα) = 0.536 cm^-1, final R = 0.089, and R_w = 0.287. The molecular modeling reveals that a DPA G4 molecule has a spherelike structure, in which the height, width, and depth are 2.3, 2.9, and 2.5 nm, respectively. The TEM and AFM pictures show the DPA G4 molecules to have a spherelike structure (the diameter:  2.3 nm) and are regularly assembled on a plate by casting. The occupied area of one DPA G4 molecule in a monolayer on water was estimated by π−A measurements to be 3.8−4.2 nm² (the calculated diameter 2.2−2.3 nm, which agreed with the TEM result). NMR studies (¹H NMR at 130 °C and T₁ measurements) supported a conformational rigidity of DPA G4 in solution.