Oligoporphyrin Arrays Conjugated to [60]Fullerene: Preparation, NMR Analysis, Photophysical, and Electrochemical Properties

We report the synthesis and physical properties of novel fullerene - oligoporphyrin dyads. In these systems, the C-spheres are singly linked to the terminal tetrapyrrolic macrocycles of rod-like meso,meso-lmked or triply-linked oligoporphyrin arrays. Monofullerene-mono(ZnII porphyrin) conjugate 3 was synthesized to establish a general protocol for the preparation of the target molecules (Scheme 1). The synthesis of the meso,meso-linked oligopophyrin-bisfullerene conjugates 4-6, extending in size up to 4.1 nm (6), was accomplished by functionalization (iodination followed by Suzuki cross-coupling) of the two free meso-positions in oligomers 21-23 (Schemes 2 and 3). The attractive interactions between a fullerene and a ZnII porphyrin chromophore in these dyads was quantified as ?G = -3.3 kcal mol-1 by variable-temperature (VT) 1H-NMR spectroscopy (Table 1). As a result of this interaction, the C-spheres adopt a close tangential orientation relative to the plane of the adjacent porphyrin nucleus, as was unambiguously established by 1H- and 13C-NMR (Figs. 9 and 10), and UV/VIS spectroscopy (Figs. 13-15). The synthesis of triply-linked diporphyrin-bis[60]fullerene conjugate 8 was accomplished by Bingel cyclopropanation of bis-malonate 45 with two C60 molecules (Scheme 5). Contrary to the meso,meso-linked systems 4-6, only a weak chromophoric interaction was observed for 8 by UV/VIS spectroscopy (Fig. 16 and Table 2), and the 1H-NMR spectra did not provide any evidence for distinct orientational preferences of the C-spheres. Comprehensive steady-state and time-resolved UV/VIS absorption and emission studies demonstrated that the photophysical properties of 8 differ completely from those of 4-6 and the many other known porphyrin - fullerene dyads: photoexcitation of the methano[60]fullerene moieties results in quantitative sensitization of the lowest singlet level of the porphyrin tape, which is low-lying and very short lived. The meso,meso-linked oligoporphyrins exhibit 1O2 sensitization capability, whereas the triply-fused systems are unable to sensitize the formation of 1O2 because of the low energy content of their lowest excited states (Fig. 18). Electrochemical investigations (Table3, and Figs. 19 and 20) revealed that all oligoporphyrin arrays, with or without appended methano[60]fullerene moieties, have an exceptional multicharge storage capacity due to the large number of electrons that can be reversibly exchanged. Some of the ZnII porphyrins prepared in this study form infinite, one-dimensional supramolecular networks in the solid state, in which the macrocycles interact with each other either through H-bonding or metal ion coordination (Figs. 6 and 7).