Host–Guest Interactions in the C59N•⊂[10]CPP Supramolecular Radical

Abstract

A remarkable stability of a supramolecular radical comprising an azafullerene (C₅₉N^•) radical sterically protected by a [10]cycloparaphenylene ([10]CPP) nanoring, C₅₉N^•⊂[10]CPP, has recently been observed by various experimental probes. In order to investigate the host–guest interaction in these supramolecular complexes, we carried out electron paramagnetic resonance (EPR) measurements, theoretically supported by density functional theory (DFT) calculations. The continuous wave (CW) EPR spectrum shows the presence of two active spin components: C₅₉N^•⊂[10]CPP monomers that can in certain cases polymerize into oligomeric radicals. Two- and three-pulse electron spin echo envelope modulation (ESEEM) measurements allow for determination of experimental hyperfine coupling constants for ¹³C and ¹H atoms and also show the strong coupling to the ¹⁴N atom adjacent to the radical C of the azafullerene. These experimental hyperfine coupling constants reasonably agree with those calculated for the DFT optimized supramolecular structures. The results are consistent with a small spin delocalization from the guest (C₅₉N^•) to the host ([10]CPP), thereby explaining weak but non-negligible interaction between them. Our study demonstrates that ESEEM experiments in alliance with DFT computations can offer valuable insights into the radical host–guest structures.