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.