How do Nanohoops Exercise Their Strain in [5]Helicene Racemization?

The effect of strain in curved carbon nanohoops has a major effect on their optoelectronic properties. How this strain affects their dynamics and reactivity is, however, far less documented in the literature. Here, we investigate the effect of strain on racemization of [5]helicene, which we embedded in three different [<i>m</i>]cycloparaphenylenes (<i>m</i> = 5-7). [5]Helicene is a prototypical organic molecule with helical chirality that racemizes at room temperature. We synthesized and separated the enantiomers of the strained helicene-paraphenylene macrocycles, and we investigated their racemization as a function of temperature. Our results revealed that the configurational stability of the [5]helicene increases with increasing nanohoop size, with activation free energies ranging from 25 to 29 kcal mol^-1. The combination of the experimental data and DFT calculations established a clear relationship between the strain and racemization in helicene-embedded nanohoops.