Origin of the isotropic motion in crystalline molecular rotors with carbazole stators

Herein we report two crystalline molecular rotors 1 and 4 that show extremely narrow signals in deuterium solid-state NMR spectroscopy.

Spectroscopic data matched with the previously reported.

4-Bromo-2-Nitro-1,1'-biphenyl (8).
2,5-dibromo-1-nitrobenzene (0.100 g, 0.36 mmol) and copper powder (0.079 g, 1.25 mmol) were added to iodobenzene (3 mL) and heated at 140 ºC overnight. The reaction mixture was cooled to room temperature and filtered under silice using 10 mL of ethyl acetate. The crude was purified by silice column chromatography using hexanes as eluent and the product was isolated as a yellow oil (0.081 g, yield 82% General procedure for the synthesis of 2 and 7 bromo-substituted carbazole (7 and 9) from nitro-biphenyl. 0.5 g of nitro-biphenyle derivative (8) or (10) and 2 eq of triphenylphosphine were added to N,N-dimethylformamide (3 mL) and heated at reflux for 5 hours under atmosphere of N2. The reaction mixture was poured into saturated ammonium chloride solution and extracted with dichloromethane (3x10 mL), products were purified by silice column chromatography using hexanes/dichloromethane as eluent and the carbazole bromo-substituted were isolated as white powders.

Data collection were performed at 298 K on a Bruker-APEX-II CCD diffractometer with Mo
Kα-radiation, λ = 0.71073 Å. The structures were solved by direct methods and refined using SHELXL-2014. All non-hydrogen atoms were refined anisotropically.

Crystallization of title compounds
Prisms of rotor 1 were obtained from slow evaporation of a dichloromethane solution at room temperature. The single crystal X-ray data was solved and refined in an orthorhombic space group Pbca with eight molecules in the unit cell (Z=8). Complementarily, thin colorless prisms of molecular rotor 2 were obtained from slow evaporation of an N,Ndimethylformamide solution at room temperature. The X-ray diffraction data was solved and refined in a monoclinic space group C2/c with Z=4. Single crystals of 3 suitable for X-ray diffraction were obtained from a THF solution by slow evaporation at room. They were solved and refined in the monoclinic system, space group P21/c with Z=2. Finally, single crystals of compound 4 were grown by evaporation of a dichloromethane solution at room temperature, solved in the monoclinic system with a C21/c space group.

Solid state 2 H NMR quadrupolar echo spin
Solid state 2 H NMR experiments were carried out on a Bruker AV300 spectrometer operating at a frequency of 46.07 MHz using a 4 mm wide-line probe with a π/2 pulse of 2.5 μs and recycle delay of 20 s (compounds 1 and 3) and a spectrometer in which the 2 H nucleous resonates at 92.1 MHz with a 5 mm wide-line probe and a π/2 pulse of 2.9 μs were used (compound 2). The spectra were acquired by averaging at least 256 scans and processed with a line broadening of 2 kHz. The sample was placed inside a borosilicate glass NMR tube between two glass rods. The temperature inside the probe was calibrated by using the shift of 207 Pb as the reference. The 2 H NMR line shapes were simulated using the NMRWebLab 6.0.4; in all cases a Quadrupolar Coupling Constant (QCC) of 170 kHz was employed.

Powder X-Ray diffraction
Analyses were carried out using Cu-Kα1 = 1.5406Ǻ radiation, data were collected at room temperature in the range of 2Θ= 5-50° (step of 0.017°, step time 40.005 s), comparison can be seen from Figures S38 to S42.

Computational Details
Density functional theory (DFT) computations were carried out within the framework of the Projector-Augmented Wave (PAW) method, 5 as implemented in the Vienna Ab-initio Simulation Package (VASP). 6 The electron-ion interactions were described using the PAW potentials as are in the VASP library. The valence states were expanded in plane waves with an energy cut-off of 550 eV. All computations were done using the GGA-PBE exchangecorrelation functional, 7 augmented by Grimme's D3-dispersion correction. 8,9 The Brillouin zone integration was performed using 2×1×1, 4×1×1, 2×2×2, and 2×1×2 Monkhorst-Pack kpoint meshes. The structural parameters for each crystal were computed by a variable-cell optimization until all forces were smaller than 0.001 eV/Å.  Figure S46 contains the computed energy barriers for compounds 2 and 3, whose values were discussed in the main text. Figure S47 shows the computed rotational barriers for compound 4 with the DCM molecules included. The trend is that the presence of the solvent slightly increases the values of the energy barriers with respect to the solvent-free compound. For the case of the disrotatory motion, the most probable dynamic process, the highest energy barrier is increased from 29.2 kcal/mol to 30.5 kcal/mol due to the inclusion of the DCM molecules.