Structural and electronic properties in asymmetric binuclear Zn(II) amphiphilic compounds

A new type of asymmetric binuclear Zn(II) complex was synthesized from a suitable mononuclear precursor. One Zn(II) was coordinated to a salphen ligand (salphen = N,N '-disalicylidene-1,2-phenylenediamine) and the other to a modified salphen bearing OC12H25 chain in the 4,4' positions, the two being joined by phenylene rings. The molecular structure, determined by DFT calculations (ADF/BP86(COSMO:THF)/TZ2P), showed the non-planarity of each of the two complexes, the dihedral angle at the junction being similar to 36 degrees. The absorption spectra of 2 and its precursor 1 in DMSO showed a strong band in the visible at 427 and 408 nm, respectively, assigned based on TDDFT calculations mainly to intra- and interligand pi ->pi* transitions. Complex 2 displayed emission at 531 nm in DMSO and 534 nm in other solvents. The long alkyl chain of its substituents promoted self-assembly of these amphiphilic molecules. No gels were formed in DMSO, but several bands (420, 418 and 480 nm) and shoulders appeared in other solvents, and gels were detected when increasing concentration. The formation of aggregates was studied by scanning electron microscopy and atomic force microscopy, and the images found in the three solvents reflected different supramolecular arrangements. These studies revealed that the binuclear compound formed stable gels above 8.88 mM for tetrahydrofuran and 13.3 mM for toluene and chloroform.