Crystalline arrays of molecular rotors with complex dynamics such as correlated motions and multiple rotational potentials, thermal dynamics coupled to lattice elasticity with a change in the crystal birefringence response or coupled to the electronic response of the system as in switchable dielectrics, and the emerging phenomenon of quantum dissipation addressing the difference of dynamics of the rotors in solids with different electrical properties are of intense current interest. The control of complex dynamic molecular systems at the nanoscale is an essential issue for the development of molecular machines capable of performing useful work. Materials design that includes deliberate use of halogen- and hydrogen-bonding interactions, as well as variable-temperature X-ray and 1H spin-lattice relaxation experiments, and calculations of rotational barriers, provide an in-depth understanding of the switching mechanism of the rotational barriers and of the frequency of associated rotational motion.
We have observed that the monoclinic unit cell of a single crystal of the rod-like molecular rotor shown in the figure, abruptly changes below 105 K, experiencing an expansion by seven times its volume to encompass three and a half independent rotators at 90 K. As a result, after the transition there is a static modulation wave of arrays of halogen interactions. The remarkable finding is that the total 1H spin-lattice relaxation rate of unprecedented complexity to date in molecular rotors, can be decoded, on the basis of DFT calculations which provide understanding on how the H…H and H…I interactions influence the rotational motion, as the weighted sum of the relaxation rates of the four contributing rotors relaxation rates, each with distinguishable exchange frequencies reflecting Arrhenius parameters with different activation barriers and attempt frequencies. This allowed understanding how the dynamics of molecular rotors are able to decode structural information from their surroundings with remarkable nanoscale precision.
Static Modulation Wave of Arrays of Halogen Interactions Transduced to a Hierarchy of Nanoscale Change Stimuli of Crystalline Rotors Dynamics
Nano Letters 18, 3780-3784 (2018)
(top) Variable-temperature 1H spin−lattice relaxation time T1 −1 at 57 MHz for 1,4-bis((4′-(iodoethynyl)phenyl)ethynyl) bicyclo[2.2.2]octane rotor and curves for each of the four components of the total relaxation (T < Tc). (bottom) The four different arrays of molecular rotators at 90 K, below the phase transition, because of the static modulation wave of halogen interactions.