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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. 

Photochemical valence bond isomerization of a crystalline Dewar benzene diacid monoanion salt with an acetophenone-linked piperazinium cation that serves as an intramolecular triplet energy sensitizer (DB-AcPh-Pz) exhibits a quantum chain reaction with as many as 450 product molecules per photon absorbed (F ≈ 450). By contrast, isomorphous crystals of the Dewar benzene diacid monosalt of an ethylbenzene-linked piperazinium (DB-EtPh-Pz) lacking a triplet sensitizer showed a less impressive quantum yield of ca. F ≈ 22. To establish the critical importance of a triplet excited state carrier in the adiabatic photochemical reaction we prepared mixed crystals with DB-AcPh-Pz as a dilute triplet sensitizer guest in crystals of DB-EtPh-Pz. As expected from the their high structural similarities, solid solutions were easily formed with the triplet sensitizer salt in the range of 0.1% to 10%. Experiments carried out under conditions where light is absorbed by the triplet sensitizer-linked DB-AcPh-Pz can be used to initiate a triplet state adiabatic reaction from 3DB-AcPh-Pz to 3HB*-AcPh-Pz, which can serve as a chain carrier and transfer energy to an unreacted DB-EtPh-Pz where exciton delocalization in the crystalline solid solution can help carry out an efficient energy transfer and enable a quantum chain employing the photoproduct as a triplet chain carrier. Excitation of mixed crystals with a little as 0.1% triplet sensitizer resulted in an extraordinarily high quantum yield F ≈ 517.