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 (– 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.

本研究合成并表征了一类新型氧化还原活性游离碱型与金属化卟啉，其分子结构融合了四硫富瓦烯（tetrathiafulvalene）所含的1,3-二硫杂环戊烯-2-亚基单元，该类化合物被命名为MTTFP（M = H₂, Cu, Ni, Zn）。通过电化学测试探究了MTTFP优异的给电子性能，结果表明可通过对游离碱形式H₂TTFP进行金属化修饰，调控其氧化电位。对H₂TTFP、ZnTTFP及CuTTFP的X射线晶体结构解析结果显示，中性状态下的化合物存在严重的马鞍形畸变，其二硫杂环戊烯环偏离分子平面并相互靠拢。与之相反，双电子氧化物种（CuTTFP²+）的分子结构呈平面构型，对应氧化过程中从非芳香性到芳香性的结构转变。游离碱形式的H₂TTFP²+展现出较大的双光子吸收（two-photon absorption, TPA）截面，数值达1200 GM，该值远高于常规卟啉类化合物的双光子吸收截面；此外，通过苄腈溶剂中ZnTTFP与锂离子包合富勒烯（Li⁺@C₆₀）之间的热电子转移实验发现，加入四丁基铵盐形式的Cl⁻或Br⁻可“激活”该电子转移过程。解析了结合Cl⁻的ZnTTFP的X射线晶体结构，该结果为ZnTTFP中的Zn²+阳离子与Cl⁻阴离子之间的强结合作用提供了实验支撑。