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

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posted on 2016-02-19, 02:52 authored by Nathan L. Bill, Masatoshi Ishida, Steffen Bähring, Jong Min Lim, Sangsu Lee, Christina M. Davis, Vincent M. Lynch, Kent A. Nielsen, Jan O. Jeppesen, Kei Ohkubo, Shunichi Fukuzumi, Dongho Kim, Jonathan L. Sessler
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.