Porphyrins Fused with Strongly Electron-Donating 1,3-Dithiol-2-ylidene Moieties: Redox Control by Metal Cation Complexation and Anion Binding

A new class of redox-active free base and metalloporphyrins fused with the 1,3-dithiol-2-ylidene subunits present in tetrathiafulvalene, termed MTTFP (M = H₂, Cu, Ni, Zn), have been prepared and characterized. The strong electron-donating properties of MTTFP were probed by electrochemical measurement and demonstrated that oxidation potentials can be tuned by metalation of the free base form, H₂TTFP. X-ray crystal structures of H₂TTFP, ZnTTFP, and CuTTFP revealed that a severe saddle-shape distortion was observed with the dithiole rings bent out of the plane toward one another in the neutral form. In contrast, the structure of the two-electron oxidized species (CuTFFP²⁺) is planar, corresponding to a change from a nonaromatic to aromatic structure upon oxidation. A relatively large two-photon absorption (TPA) cross-section value of H₂TTFP²⁺ (1200 GM) was obtained for the free base compound, a value that is much higher than those typically seen for porphyrins (<100 GM). Augmented TPA values for the metal complexes were also seen. The strong electron-donating ability of ZnTTFP was further enhanced by binding of Cl^– and Br^– as revealed by thermal electron-transfer between ZnTTFP and Li⁺-encapsulated C₆₀ (Li⁺@C₆₀) in benzonitrile, which was “switched on” by the addition of either Cl^– or Br^– (as the tetrabutylammonium salts). The X-ray crystal structure of Cl^–-bound ZnTTFP was determined and provided support for the strong binding between the Cl^– anion and the Zn²⁺ cation present in ZnTTFP.