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

Tong Liu; Isaac Abrahams; T John S Dennis

doi:10.1021/acs.jpclett.0c01421

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

J Phys Chem Lett. 2020 Jul 16;11(14):5397-5401. doi: 10.1021/acs.jpclett.0c01421. Epub 2020 Jun 24.

Authors

Tong Liu¹, Isaac Abrahams², T John S Dennis¹

Affiliations

¹ School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, U.K.
² School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K.

PMID: 32551707
DOI: 10.1021/acs.jpclett.0c01421

Abstract

The ¹H NMR spectrum of phenyl C₆₁ butyric acid methyl ester ([60]PCBM) was recorded at high resolution (600 MHz). All of the ¹H resonances expected of the C_s-symmetric molecule were observed. The spin-spin couplings in the ¹H 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 σ bonds of the alkyl chain in the ester group, although rotation becomes free at a larger distance from the fullerene bridge carbon (C61). The ¹H 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.