Photophysical and Photochemical Properties of Viologen-Linked N-Alkylporphyrin Diads and Their Metal Complexes

A new type of viologen-linked or -bridged N- alkylporphyrin and metalloporphyrin diads was synthesized and characterized by 1 H NMR spectroscopy. Although the viologen moiety of free base diads linked with a methylene chain is flexible in solution, the insertion of metal ions results in the extended conformation, where the viologen moiety is located nearly perpendicular to the N- alkylporphyrin plane. The diad, having two porphyrin units bridged by a viologen moiety, is more rigid in conformation than the diad containing one porphyrin unit. The fluorescence quantum yields in the viologen-linked and -bridged N- alkylporphyrins become larger by insertion of H +, Mg 2+, Al 3+, and Si 4+ into the N- alkylporphyrins. This may arise because the extended and orthogonal conformer due to the electrostatic repulsion between the viologen and the central metal ion decelerates the ET quenching of N- alkylporphyrin by the bound viologen unit.