2H NMR reveals liquid state-like dynamics of arene guests inside hexameric pyrogallol[4]arene capsules in the solid state

Abstract

The dynamics of guests in molecular encapsulation complexes have been studied extensively in solution, but the corresponding behavior of those guests when the capsules are present in the solid state is not as well understood. Here we report on comparative solution ¹H and solid-state ²H NMR measurements of encapsulation complexes of fluorene(-d₂), fluoranthene(-d₁₀), and pyrene(-d₁₀) in pyrogallol[4]arene hexamers assembled in the solid state by ball milling. In solution, the ¹H spectra show that these rigid guests tumble and exchange positions quickly within the capsules’ interiors, with the exception of pyrene, which has slower tumbling and positional exchange. Static solid-state ²H NMR using the deuterated guests shows that, when the capsules are in the solid state, their guests retain the liquid state-like dynamics observed for the capsules in solution. When the pyrogallol[4]arene hexamers’ pendant decyl groups were substituted with propyl groups, guest dynamics in the solid state were slowed. We propose that these pendant alkyl groups form an interdigitated and dynamic waxy domain surrounding the capsules in the solid state, and that the greater mobility of the decyl groups is translated across the walls of the host, resulting in more rapid guest dynamics in the capsules’ interiors.