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

The first chlorin-fullerene electron donor-acceptor (DA) compounds have been synthesized. The synthesis produced C-2' R and S epimers, both of which exhibited atropisomerism regarding the mutual orientation of the fullerene ball and the propionic acid residue of the phytochlorin unit. The atropisomerism arises from the short linkage between the C-60 ball and the chlorin ring, which hinders free rotation of the bulky ball. Dynamic H-1 NMR and molecular modelling were used in concert to investigate the atropisomerism in the metal-free DA molecules 5. The dynamic NMR-measurements showed a lower energy barrier [E-a = 21.4(5) kcal mol(-1)] for one stereoisomer and a higher one [E-a = 23.0(8) kcal mol(-1)] for the three other stereoisomers. MM+ molecular mechanic calculations were performed for each C-2' epimer to assess 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. Solvation phenomena were proposed to play an essential role in the stabilization of the isomers. Fast intramolecular photoinduced electron transfer from the chlorin donor to the fullerene acceptor was observed in polar solvents.