Synthesis and photophysical properties of inclusion complexes between conjugated polyazomethines with γ-cyclodextrin and its tris-O-methylated derivative

Abstract

In this study, we investigate the influence of γ-cyclodextrin (γCD) or octakis-(2,3,6-tri-O-methyl)-γ-cyclodextrin (TMe-γCD) encapsulation on the solubility, thermal, optical, electrochemical, morphological, and electrical properties of an aromatic polyazomethine (PA). PA was prepared by oxidative CC coupling of pyrenyl groups from the monomer MA using FeCl3 as catalyst in DMF. After threading of γCD or TMe-γCD macrocycles onto the PA chains, the 1H NMR spectra were in line with the formation of the respective inclusion complexes, PA·γCD and PA·TMe-γCD, which showed distinct improvements of their solubility in common organic solvents, higher thermal stability, as well as better film-forming ability. Fluorescence spectroscopy revealed that the emission maximum shifted from 388 nm for PA to 425 nm and 428 nm for PA·γCD and PA·TMe-γCD, which is in good agreement with encapsulation of PA backbones into the hydrophobic cavities. Fluorescence lifetime measurements indicated multiexponential decays for all compounds, which is in accordance with different conformers in the excited state. Atomic force microscopy indicated that the surfaces of PA·γCD and PA·TMe-γCD in the solid state showed needle-shaped morphologies with widths of 105 ± 25 nm and 46 ± 13 nm, respectively, whereas the surface of the naked PA polymer displayed an irregular morphology with embedded particles and width of 158 ± 29 nm. The electrochemical band gaps (ΔEg) of PA·γCD and PA·TMe-γCD were lower than that of non-encapsulated PA. The electrical conductivities of the investigated compounds were about 10−11 S cm−1, which is in the range of insulating materials.