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Abstract A single‐crystal X‐ray study of a fullerene‐imidazole adduct at nine temperatures (80 K≤T≤480 K), accompanied by energy calculations, strongly suggested thermal motion of the C₆₀moiety with respect to the imidazolium heterocycle. Analysis of the anisotropic displacement parameters, calculations of frequencies, and the refinement of disorder models for the crystal at four temperatures (230 K≤T≤380 K) lead to the conclusion that the rotator is moving at all temperatures. The rotation barrier is low, with one preferred crystallographic site and several other energy minima. 

Not AvailableAmphidynamic crystals are a type of condensed matter that blends two extremes of the dynamic spectrum: rigid components forming a static lattice and rapidly moving parts. Among them, ordered rotor arrays within metal-organic frameworks (MOFs) constitute a promising platform to explore unchartered territories, such as gas phase-like dynamics in the crystalline state. Through quantum mechanical (QM) calculations and molecular dynamics (MD) simulations we verified that nearly barrierless cubane rotators in CUB-5 display rotational dynamics that transitions from continuous or inertial at high tempera-ture, to chaotic behavior, and ultimately to discrete jumps, as the temperature decreases from room temperature down to cry-ogenic conditions. 1H NMR spin-lattice (T1) relaxation measurements corroborate our theoretical predictions, with experi-mental rotational activation energy of 0.17 kcal/mol and an attempt frequency of 1.03×1012 s-1 that compare well with calcu-lated values of 0.15 kcal/mol and 0.38×1012 s-1, respectively. 

Ground state destabilization is a promising strategy to modulate rotational barriers in amphidynamic crystals. DFT studies of polar phenylenes installed as rotators in pillared-paddle wheel metal-organic frameworks were performed to investigate the effects of ground state destabilization on their rotational dynamics. We found that as the steric size of phenylene substituents increases the ground state destabilization effect is also increased. Specifically, a significant destabilization of the ground state energy occurred as the size of the substituents increased, with values ranging from 2 kcal/mol to 11.7 kcal/mol. An evalua-tion of the effects of substituents on dipole-dipole interaction energies and rotational barriers suggest that it should be possi-ble to engineer amphidynamic crystals where the dipole-dipole interaction energy becomes comparable to the rotational barri-ers. Notably, dipole-dipole interaction energies reached values ranging from 0.6 kcal/mol to 2.4 kcal/mol. We propose that careful selection of polar substituents with different size may help create temperature-responsive materials with switchable collective polarization.