Chlorophylls. IX. The first phytochlorin-fullerene dyads: synthesis and conformational studies

The first chlorin-fullerene electron donor-acceptor (DA) compounds have bee n synthesized. The synthesis produced C-2' R and S epimers, both of which e xhibited atropisomerism regarding the mutual orientation of the fullerene b all and the propionic acid residue of the phytochlorin unit. The atropisome rism arises from the short linkage between the C-60 ball and the chlorin ri ng, which hinders free rotation of the bulky ball. Dynamic H-1 NMR and mole cular modelling were used in concert to investigate the atropisomerism in t he metal-free DA molecules 5. The dynamic NMR-measurements showed a lower e nergy barrier [E-a = 21.4(5) kcal mol(-1)] for one stereoisomer and a highe r one [E-a = 23.0(8) kcal mol(-1)] for the three other stereoisomers. MM+ m olecular mechanic calculations were performed for each C-2' epimer to asses s the potential energy as a function of the angle of rotation about the C3- C2' single bond. For the C-2' R and S epimers, these calculations produced two potential energy curves that showed a near mirror-image relationship. S olvation phenomena were proposed to play an essential role in the stabiliza tion of the isomers. Fast intramolecular photoinduced electron transfer fro m the chlorin donor to the fullerene acceptor was observed in polar solvent s.