Smaragdyrin–Azobenzene Conjugates: Syntheses, Structure, and Spectral and Electrochemical Properties

Abstract

The syntheses, characterization, and spectral properties of smaragdyrin–azobenzene conjugates are reported. Our synthetic strategy involves linking the azobenzene group in one of the precursors to a dipyrromethane subunit, which was achieved by reaction of azobenzenecarbaldehyde with pyrrole under TFA catalysis. A [3+2] acid-catalyzed oxidative coupling of this precursor with the tripyrrane moiety gave the expected smaragdyrin–azobenzene conjugates. The azobenzene is in the (E) conformation both in the precursor and in the smaragdyrin conjugates, as revealed by its single-crystal X-ray structure. Electronic absorption and emission spectral studies reveal the presence of a moderate electronic interaction between the azobenzene and smaragdyrin π-systems. Irradiation experiments at 360 nm reveal the presence of a reversible (E)/(Z) transformation of the azobenzene moiety in the precursors 3b and 4b. However, in the smaragdyrin conjugates the formation of the (Z) conformer leads to the decomposition of the macrocycle upon prolonged irradiation. Excitation at the azobenzene absorption results in the appearance of the emission band of smaragdyrin, thereby suggesting an energy transfer. The electrochemical data reveal that ring oxidations of the smaragdyrin macrocycle become harder upon azobenzene introduction, which suggests the electron-withdrawing nature of the azobenzene in these conjugates. A significant shortening of the N–N bond (0.067 Å) and elongation of the C–N bonds (0.055 and 0.069 Å) in 7a relative to the azobenzene–dipyrromethane precursor 4a clearly reveal a rearrangement of the electronic π-delocalization pathway in the smaragdyrin–azobenzene conjugates. RhI binds to one amino and one imino nitrogen atom in the smaragdyrin cavity to form a 1:1 complex.