Synthesis and Characterization of Supramolecules of Li + @C60 and Fluorinated Porphyrins

Abstract

Metal endohedral [60]fullerenes (M@C60) have a unique structure encapsulating a metal atom (M) inside the π−conjugated spherical C60 cage. Lithium cation endohedral fullerene (Li + @C60) is a commercially available M@C60 and has a potential for application in nano material fields. The cationic Li+@C60 crystallizes with weakly coordinated anions such as SbCl6 − [1], PF6 − [2], Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion (TFPB−) [3], and bis(trifluoromethanesulfonyl)imide anion (TFSI−) [3]. Due to the inside Li+ cation, the first reduction potential of Li+@C60 is shifted to positive direction by 0.7 eV than that of pristine C60. As a result, Li+@C60 has larger electron acceptability than C60 [1]. Li+@C60 also forms a guest–host type supramolecule with [10]cycloparaphenylene ([10]CPP) [4] and π−π stacking supramolecules with porphyrins or phthalocyanines [5][6]. A supramolecule of Li+@C60 and 5,10,15,20-tetrakis(4–sulfonatophenyl) zinc porphyrin anion (ZnTPPS4 –), [Li+@C60 – ZnTPPS4 –] with the binding constant of ca. 105 M– 1 shows a long−lived electron charge separation state by photoinduced electron transfer and can be used in photovoltaic cells [5]. The [Li+@C60 – ZnTPPS4 –] based photovoltaic cell shows maximum incident photon-to-photocurrent efficiency (IPCE) value of 78% under the certain condition [7]. The performance of the photovoltaic cells would be changed by the chemical modification of porphyrins.Here, we synthesized two kinds of porphyrins to obtain new supramolecules of Li+@C60. The first one is neutral 5,10,15,20-Tetrakis(1,2,4,5-tetrafluorophenyl) porphyrin (H2TTFPP). H2TTFPP was synthesized by Lindsey method and characterized by 1H NMR, 19F NMR, UV-vis, CV, DPV, and XRD. The second one is anionic 5,10,15,20-Tetrakis(4-carboxymethylthio-2,3,5,6-tetrafluorophenyl) zinc porphyrin tetrabutylammonium (ZnTFPP4TC4 −4TBA+). ZnTFPP4TC4 −4TBA+ was synthesized according to the modified Lourenço method [8] and characterized by 1H NMR, 19F NMR, UV-vis, CV, and DPV.H2TTFPP was confirmed to form supramolecular structure with Li+@C60 in the co-crystal by X-ray structure analysis. ZnTFPP4TC4 − was confirmed to form a supramolecule with Li+@C60 in solution by UV-vis titration experiment. The binding constant of [ZnTFPP4TC4 − − Li+@C60] is significantly larger than that of [H2TTFPP − Li+@C60] because of strong coulomb interactions between anionic ZnTFPP4TC4 − and cationic Li+@C60.