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

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

Design and Evaluation of a Crystalline Hybrid of Molecular Conductors and Molecular Rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis­(carboxyethynyl)­bicyclo[2.2.2]­octane (BABCO) functional rotor. One, [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂, is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH₂)₂⁺ (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO^–] rotors. Using variable-temperature (5–300 K) proton spin–lattice relaxation, ¹H T₁^–1, we were able to assign two types of Brownian rotators in [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂. We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol^–1. Rotors on the BABCO^– sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol^–1. In contrast, the BABCO^– rotors within the highly conducting crystals of (EDT-TTF-CONH₂)₂⁺[BABCO^–] are essentially “braked” at room temperature. Notably, these crystals possess a conductivity of 5 S cm^–1 at 1 bar, which increases rapidly with pressure up to 50 S cm^–1 at 11.5 kbar. Two regimes with different activation energies <i>E</i>_a for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C_sp3–H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol^–1. Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers

Design and Evaluation of a Crystalline Hybrid of Molecular Conductors and Molecular Rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis­(carboxyethynyl)­bicyclo[2.2.2]­octane (BABCO) functional rotor. One, [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂, is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH₂)₂⁺ (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO^–] rotors. Using variable-temperature (5–300 K) proton spin–lattice relaxation, ¹H T₁^–1, we were able to assign two types of Brownian rotators in [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂. We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol^–1. Rotors on the BABCO^– sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol^–1. In contrast, the BABCO^– rotors within the highly conducting crystals of (EDT-TTF-CONH₂)₂⁺[BABCO^–] are essentially “braked” at room temperature. Notably, these crystals possess a conductivity of 5 S cm^–1 at 1 bar, which increases rapidly with pressure up to 50 S cm^–1 at 11.5 kbar. Two regimes with different activation energies <i>E</i>_a for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C_sp3–H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol^–1. Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers

Design and Evaluation of a Crystalline Hybrid of Molecular Conductors and Molecular Rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis­(carboxyethynyl)­bicyclo[2.2.2]­octane (BABCO) functional rotor. One, [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂, is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH₂)₂⁺ (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO^–] rotors. Using variable-temperature (5–300 K) proton spin–lattice relaxation, ¹H T₁^–1, we were able to assign two types of Brownian rotators in [<i>n</i>Bu₄N⁺]₂[BABCO]­[BABCO^–]₂. We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol^–1. Rotors on the BABCO^– sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol^–1. In contrast, the BABCO^– rotors within the highly conducting crystals of (EDT-TTF-CONH₂)₂⁺[BABCO^–] are essentially “braked” at room temperature. Notably, these crystals possess a conductivity of 5 S cm^–1 at 1 bar, which increases rapidly with pressure up to 50 S cm^–1 at 11.5 kbar. Two regimes with different activation energies <i>E</i>_a for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C_sp3–H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol^–1. Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers

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