Crystalline Arrays of Pairs of Molecular Rotors: Correlated
Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking
Due to Conformational Mutations
posted on 2013-06-26, 00:00authored byCyprien Lemouchi, Konstantinos Iliopoulos, Leokadiya Zorina, Sergey Simonov, Pawel Wzietek, Thomas Cauchy, Antonio Rodríguez-Fortea, Enric Canadell, Jiřı́ Kaleta, Josef Michl, Denis Gindre, Michael Chrysos, Patrick Batail
The
rod-like molecule bis((4-(4-pyridyl)ethynyl)bicyclo[2.2.2]oct-1-yl)buta-1,3-diyne, 1, 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 C2/c 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 T1–1, 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 1 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.