Coordination properties of diethylenetriamine in relation to zinc phthalocyanine

Abstract

The coordination properties of diethylenetriamine (DETA) as a ligand containing three donor nitrogen atoms, two terminal primary amino groups and one secondary central amino group, with respect to divalent zinc ion in ZnPc were examined. The ZnPcDETA complex in the powder form was obtained by the solvothermal reaction of ZnPc with DETA. Recrystallization of the crude product from 3,4-lutidine yields non-centrosymmetric monoclinic solvated crystals (ZnPcDETA)3·3,4-lut (1) with the space group of Cc. As show the X-ray single crystal analysis in ZnPcDETA complex, the DETA coordinates via terminal amine group to the divalent zinc ion of a planar ZnPc molecule in axial position. Interaction of the N atom of DETA containing a lone electron pair with divalent zinc ion of ZnPc and the formation of the Zn-N coordination bond leads to deviation of the Zn from the N4-isoindole plane of Pc by ~ 0.5 Å toward the N-atom of the axial DETA ligand as well as to saucer-shaped distortion of the Pc macrocycle. Arrangement of ZnPcDETA molecules in the 3,4-lutidine solvate crystal with the composition of (ZnPcDETA)3·(3,4-lutidine) - (1) is determined by the van der Waals forces and by the weak NH⋯N hydrogen bonds. The lack of the π⋯π interaction between the phthalocyaninate(2-) macrocycles in the crystals (1) is clearly evidenced by the Hirshfeld surface analysis, and increases significantly their solubility in most common solvents, even in water, when comparing to the parent ZnPc pigment with limited solubility in solvents and insoluble in water. This feature together with the strong absorption in the therapeutic window within the 600–900 nm makes it a potentially good photosensitizer. DFT calculations performed for the ZnPc-derivatives with DETA, coordinated via terminal or central amine group of DETA as well as for the bridged complex (through the terminal amino groups of the DETA ligand of two ZnPc molecules) show the possibility to obtain all three of these complexes. UV–Vis absorption spectra ZnPcDETA complex (1) in solutions as well as the diffuse reflectance spectroscopy (DSR) supported by the TD-DFT calculations were used for the characterization of the spectroscopic properties. SHG efficiency of 1 is ~10% in relation to that of KDP.