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Template-Directed Synthesis of a Conjugated Zinc Porphyrin Nanoball

Abstract : We report the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This structure consists of two intersecting nanorings of 6 and 10 porphyrin units. Fluo-rescence up-conversion spectroscopy experiments demonstrate that electronic excitation delocalizes over the whole 3D π-system in less than 0.3 ps, if the nanoball is bound to its templates , or over 2 ps if the nanoball is empty. Spurred by the discovery of buckminsterfullerene, 1 chemists have sought rational strategies for the synthesis of 3D π-conjugated geodesic cages. Stepwise chemical synthesis has not yet been used to prepare any fullerenes, except C60 (ref. 2), but several fullerene-like ball-shaped π-conjugated hydrocarbons have been synthesized recently. 3 The high dimensionality of these π-systems is expected to enhance their electronic delocalization, compared with 1D or 2D molecular semiconductors. 4-6 Shape-persistent molecular cages are also in demand for their gas adsorption properties, which mimic those of zeolites. 7 The best routes to molecular cages or capsules use reversible reactions that allow error correction, such as metal coordination 8 or boronic ester condensation, 9 but these reactions do not produce π-conjugated connections. In principle, reversible reactions such as imine formation, 10 alkene metathe-sis 11 and alkyne metathesis 12 could be used to construct π-conjugated cages, but so far the cages made by these reactions lack long-range conjugation. 7,10-12 Template-directed coupling, under kinetic control, is an alternative strategy for preparing large macrocycles and cages. 13 Here we show how simple molecular templates can be used to synthesize the first fully π-conjugated porphyrin ball b-P14 (Figure 1). This 14-porphyrin prolate ellipsoidal cage consists of two perpendicular intersecting conjugation pathways, containing 6 and 10 porphyrin units respectively. Structures of this type are valuable models for photosynthetic light-harvesting systems. 13-15 When all 14 porphyrin units in b-P14 are bound to templates, locking the conformation, excited state energy delocalization occurs over the whole system within 0.3 ps, whereas in the absence of the templates excitation is distributed over the ball with a time constant of about 2 ps. Figure 1. (a)-(c) Three orthogonal views of the structure of the 14-porphyrin ball template complex b-P14·T6·(T4)2 and (d) the templates T6 and T4. The meso-aryl side groups on the porphy-rins are omitted for clarity. The porphyrin nanoball b-P14 was synthesized as shown in Figure 2, using templates T6 and T4 (Figure 1). This route starts from a 6-porphyrin nanoring template complex with four terminal alkyne substituents, c-P6(H)4·T6, which was prepared as reported previously. 16 This ring was coupled to four porphyrin dimers P2-CPDIPS, followed by removal of the CPDIPS protecting groups, to give the extended nanoring c-P6(P2)4·T6. The four-legged template T4 was then used to
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Submitted on : Tuesday, April 17, 2018 - 9:59:45 AM
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Jonathan Cremers, Renée Haver, Michel Rickhaus, Juliane Gong, Ludovic Favereau, et al.. Template-Directed Synthesis of a Conjugated Zinc Porphyrin Nanoball. Journal of the American Chemical Society, American Chemical Society, 2018, ⟨10.1021/jacs.8b02552⟩. ⟨hal-01768194v1⟩

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