7-Azaindolyl- and 2,2′-dipyridylamino-functionalized molecular stars with sixfold symmetry: Self-assembly, luminescence, and coordination compounds

Two novel star molecules functionalized with 7-azaindolyl and 2,2′-dipyridylamino groups have been synthesized. Both molecules possess a sixfold rotation symmetry. Molecule L1 is based on the hexaphenylbenzene core with the formula of hexa[p-(7-azaindolyl)phenyl]benzene, while molecule L2 is based on the hexakis(biphenyl)benzene core with the formula of hexa[p-(2,2′-dipyridylamino)biphenyl]benzene. Both compounds have been characterized by single-crystal X-ray diffraction analyses. Molecule L1 forms extended two-dimensional layered structure, while L2 forms interpenetrating columnar stacks in the solid state, as revealed by X-ray diffraction analyses. Nanowire structures based on columnar stacks through self-assembly of L2 on a graphite surface were revealed by an STM study. Molecules L1 and L2 are capable of binding to metal ions, resulting in unusual structural motifs. Two Ag ¹ complexes with the formulae of [(AgNO₃)₂(L1) (1) and [(AgNO₃)₃(L1) (2) were isolated from the reactions of AgNO₃ with L1. Compound 1 displays extended intermolecular π-π stacking interactions that are responsible for its extended two-dimensional structure in the crystal lattice, Complex 2 has a "bowl" shape and forms polar stacks in the crystal lattice. A Cu ^II complex with the formula of [{Cu(NO₃)₂} ₆(L2) (3) was isolated from the reaction of Cu(NO₃) ₂ with compound L2. The six Cu^II ions in 3 are chelated by the 2,2′-dipyridylamino groups of the star ligand L2. Intermolecular Cu-O (nitrate) bonds lead to the formation of an extended two-dimensional coordination network of 3. Both L1 and L2 are blue luminescent. Their interactions with Ag^I or Cu^II cause drastic quenching of emission. In addition, the luminescence of L1 and L2 is sensitive to the presence of protons, which cause a reduction of emission intensity and a red shift of the emission energy.