Conformational Analysis of [60]PCBM Via Second-Order Proton NMR Spin-Spin Coupling Effects.

The H-1 NMR spectrum of phenyl C-61 butyric acid methyl ester ([60]PCBM) was recorded at high resolution (600 MHz). All of the H-1 resonances expected of the C-s-symmetric molecule were observed. The spin-spin couplings in the H-1 NMR spectrum were not as expected at first order. Instead, the effects of AA'BB'-type second-order couplings were clearly observed for the protons attached to both ester carbons C3 and C4, which were analyzed in terms of seven coupling constants. This indicates that there is no free rotation of the sigma bonds of the alkyl chain in the ester group, although rotation becomes free at a larger distance from the fullerene bridge carbon (C61). The H-1 NMR results further indicated that there is a 1:6:1 population ratio of the three staggered conformers (gauche:anti:gauche') about the ester group C3-C4 bond. These results may aid in the understanding of the morphological interactions between [60]PCBM and its surroundings in condensed-phase organic electronic devices such as organic and perovskite photovoltaics.