Solid-state molecular rotors with perdeuterated stators: mechanistic insights from biphenylene rotational dynamics in...

Samples of 4,4'-bis(3,3,3-tri-d(5)-phenylpropynyl)biphenyl 2, 9,10-bis(3,3,3-tri-d(5)-phenylpropynyl)anthracene 3, 1,4-bis(3,3,3-tri-d(5)-phenylpropynyl)naphthalene 4, and 4,4'-bis(3,3,3-tri-d(5)-phenylpropynyl)-1,1'-binaphthyl 5 were prepared via a Sonogashira coupling of 3,3,3-tri-d(5)-phenylpropyne 7 and the appropriate aryl dibromide. Single crystal X-ray diffraction structures were obtained for an o-xylene clathrate of 2 and for solvent-free crystals of 3. All four molecular rotors were characterized by CPMAS (13)C NMR experiments with varying contact times in order to determine whether the carbon signals of the central rotator group could be selectively enhanced and studied without interference or overlap of signals from the deuterated stator, which is insensitive to the {(1)H}-(13)C cross-polarization method. It was shown that the (13)C signals of the natural abundance rotator group can be selectively observed with short contact times (ca. 50 micros) without interference from other (13)C signals in the molecule. Variable-temperature CPMAS (13)C NMR studies with a crystalline o-xylene solvate of biphenylene rotor 2 suggested a 2-fold flipping process in the fast exchange regime, even at temperatures as low as 199 K (-74 degrees C). Indirect support for this was obtained by studies carried out with a disordered, solvent-free solid, obtained by fast precipitation from hexanes and dichloromethane, which displayed slower dynamics within the same temperature range with an activation energy of 8.7 kcal/mol and a pre-exponential factor of 4.9 x 10(9) s(-1). Confirmation of an exchange process in the megahertz regime for the crystalline solvate was obtained by variable-temperature quadrupolar echo (2)H NMR data acquired with samples prepared with a deuterated biphenylene rotator and a natural abundance stator. Although rotational exchange occurs in the solvated samples with a slightly lower barrier of 7.4 kcal/mol, the main difference with the precipitated solid comes from the pre-exponential factor, which is nearly 3 orders of magnitude greater with a value of 2.5 x 10(12) s(-1). On the basis of these differences, we speculate that efficient rotational motion in the solvated crystals may take advantage of long-range lattice vibrations that couple with molecular modes and that the lack of long-range order may be responsible for the low pre-exponential factor observed in the disordered crystals.