Synthesis, characterization of pyrazino-phenanthroline derivatives and study of their aggregation properties

Abstract

New pyrazinopyraine-phananthroline derivatives with various lengths of oligoether and alkyl substituents and the corresponding pyrazino-phenanthroline derivatives were synthesized for comparison to their photophysical, electrochemical and thermal properties by UV-vis absorption spectroscopy, emission spectroscopy, cyclic voltammetry, DFT and TDDFT calculations. These derivatives were employed to study molecular self-assembly and kinetic factors in the processes. Furthermore, the newly developed compounds with pyrazinopyraine-phananthroline and pyrazino- phenanthroline moieties were applied in coordination with Re(I) metal ion. The photophysical and electrochemical properties of the Re(I) complexes were investigated.

The pyrazinopyraine-phananthroline derivatives showed similar photophysical and electrochemical properties and exhibited bathochromic shifts in the absorption and emission spectra with smaller optical bandgaps than compounds with pyraino bridges. The compounds with pyrazinopyraine bridges showed a positive solvatochromism in charging the polarity of solvents implying the excited states being more polar than the ground states. In the solution of chloroform, pyrazinopyrazine-phenanthroline derivatives were not stable and could be photo-excited under UV light changing color from yellow to amber. From the DFT and TDDFT calculations, the computational results agreed with the experimental results. For the molecular self-assembly, fabrication of nano- or micro-structures of the derivatives was performed by the mixing with acetone and pure water. Various micro-structures were obtained based on differences in molecular geometries. These micro-structures were controlled and modified by changing kinetic factors such as changing solvent ratios and solvent compositions. These self-assembled structures can be potentially applied to develop organic semiconductors and photonic devices.

The new phenanthroline-containing Re(I) complexes displayed unique photophysical and electrochemical properties. Bathochromic shifts in the absorption and emission spectra were identified upon coordination with Re(I) metal ion. The experimental absorption results agreed with the TTDFT calculations with two electron excitations for the complexes. The Re(I) complexes showed weak quantum yields compared with their parent ligands due to self-quenching processes through non-radiative pathways.