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

Metal endohedral [60]fullerenes (M@C 60 ) have a unique structure encapsulating a metal atom (M) inside the π −conjugated spherical C 60 cage. Lithium cation endohedral fullerene (Li + @C 60 ) is a commercially available M@C 60 and has a potential for application in nano material fields. The cationic Li + @C 60 crystallizes with weakly coordinated anions such as SbCl 6 − [1], PF 6 − [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 + @C 60 is shifted to positive direction by 0.7 eV than that of pristine C 60 . As a result, Li + @C 60 has larger electron acceptability than C 60 [1]. Li + @C 60 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 + @C 60 and 5,10,15,20-tetrakis(4–sulfonatophenyl) zinc porphyrin anion (ZnTPPS 4 – ), [Li + @C 60 – ZnTPPS 4 – ] with the binding constant of ca. 10 5 M – 1 shows a long−lived electron charge separation state by photoinduced electron transfer and can be used in photovoltaic cells [5]. The [Li + @C 60 – ZnTPPS 4 – ] 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 + @C 60 . The first one is neutral 5,10,15,20-Tetrakis(1,2,4,5-tetrafluorophenyl) porphyrin (H 2 TTFPP). H 2 TTFPP was synthesized by Lindsey method and characterized by 1 H NMR, 19 F 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 (ZnTFPP4TC 4 − 4TBA + ). ZnTFPP4TC 4 − 4TBA + was synthesized according to the modified Lourenço method [8] and characterized by 1 H NMR, 19 F NMR, UV-vis, CV, and DPV. H 2 TTFPP was confirmed to form supramolecular structure with Li + @C 60 in the co-crystal by X-ray structure analysis. ZnTFPP4TC 4 − was confirmed to form a supramolecule with Li + @C 60 in solution by UV-vis titration experiment. The binding constant of [ZnTFPP4TC 4 − − Li + @C 60 ] is significantly larger than that of [H 2 TTFPP − Li + @C 60 ] because of strong coulomb interactions between anionic ZnTFPP4TC 4 − and cationic Li + @C 60 . References [1] S. Aoyagi et al., Nature Chem ., 2 , 678–683 (2010). [2] S. Aoyagi et al., Angew. Chem. Int. Ed. , 51 , 3377–3381 (2012). [3] S. Aoyagi et al. , R Soc Open Sci. , 5 , 180337 (2018). [4] H. Ueno et al ., Angew. Chem. Int. Ed. , 54 , 3707–3711 (2015). [5] K. Ohkubo et al., Chem. Commun. , 48 , 4314–4316 (2012). [6] Y. Kawashima et al. , J. Phys. Chem. B , 119 , 7690−7697 (2015). [7] K. Ohkubo et al., Chem. Commun. , 49 , 4474–4476, (2013). [8] L. M.O. Lourenço et al., J. Porphyrins Phthalocyanines , 18 , 967–974, (2014).