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 metalloporph yrin diads was synthesized and characterized by H-1 NMR spectroscopy. Altho ugh 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+, Mg2+, Al3, and Si4+ into the N-alkylporphyrins. This may arise because the extended and orthogonal conformer due to the electrostatic repulsion between the vio logen and the central metal ion decelerates the ET quenching of N-alkylporp hyrin by the bound viologen unit.