Reversible Control of Crystalline Rotors by Squeezing Their Hydrogen Bond Cloud Across a Halogen Bond-Mediated Phase Transition

We report on a crystalline rotor that undergoes a reversible phase transition at 145 K. Variable-temperature X-ray and ¹H spin–lattice relaxation experiments, and calculations of rotational barriers, provide a description (i) of the way in which the rotators’ dynamics changes back and forth at the onset of the phase transition and (ii) of the mechanism responsible for the abrupt switching of the crystalline rotors from a very low-energy 4-fold degenerate equilibrium state, in which the rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy state associated with a slower motion (2.3 GHz at 145 K). Our results provide evidence that the reversible change observed in the rotational barriers at the transition is due to a cooperative modulation of the C–H_rotator···I_stator hydrogen bond cloud across a C–I_stator···I_stator–C halogen bond-mediated phase transition. In addition, we report evidence for second-harmonic generation from this material, thereby confirming with a second example the benefit of using polarized light to probe the torsional degree of freedom of chiral helix blades, as well as symmetry and dimensionality of large collections of chiral rotors in the solid state

Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations

The rod-like molecule bis­((4-(4-pyridyl)­ethynyl)­bicyclo[2.2.2]­oct-1-yl)­buta-1,3-diyne, <b>1</b>, contains two 1,4-bis­(ethynyl)­bicyclo[2.2.2]­octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic <i>C</i>2/<i>c</i> centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70–300 K) proton spin–lattice relaxation, ¹H <i>T</i>₁^–1, at two different ¹H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol^–1, to the two ¹H spin–lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of <b>1</b> should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis­(3-((trimethylsilyl)­ethynyl)­bicyclo­[1.1.1]­pent-1-yl)­buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response

Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations

The rod-like molecule bis­((4-(4-pyridyl)­ethynyl)­bicyclo[2.2.2]­oct-1-yl)­buta-1,3-diyne, <b>1</b>, contains two 1,4-bis­(ethynyl)­bicyclo[2.2.2]­octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic <i>C</i>2/<i>c</i> centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70–300 K) proton spin–lattice relaxation, ¹H <i>T</i>₁^–1, at two different ¹H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol^–1, to the two ¹H spin–lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of <b>1</b> should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis­(3-((trimethylsilyl)­ethynyl)­bicyclo­[1.1.1]­pent-1-yl)­buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response

Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations

The rod-like molecule bis­((4-(4-pyridyl)­ethynyl)­bicyclo[2.2.2]­oct-1-yl)­buta-1,3-diyne, <b>1</b>, contains two 1,4-bis­(ethynyl)­bicyclo[2.2.2]­octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic <i>C</i>2/<i>c</i> centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70–300 K) proton spin–lattice relaxation, ¹H <i>T</i>₁^–1, at two different ¹H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol^–1, to the two ¹H spin–lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of <b>1</b> should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis­(3-((trimethylsilyl)­ethynyl)­bicyclo­[1.1.1]­pent-1-yl)­buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response