Photochromic Molecular Gyroscope with Solid State Rotational States Determined by an Azobenzene Bridge

Abstract

We describe the synthesis, characterization, photochemical isomerization, and rotational dynamics of a crystalline molecular gyroscope containing an azobenzene bridge (<i>trans</i>-<b>2</b>) that spans from one end of the stator to other, with the intention of exploring its function as a molecular brake. While single crystal X-ray diffraction analysis of a photochemically inactive dichloromethane solvate was used to confirm the molecular and packing structures of <i>trans</i>-<b>2</b>, a nanocrystalline pseudopolymorph was shown to be photoactive, and it was analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy, and variable temperature solid state ²H NMR before and after photoisomerization. It was shown that the nanocrystalline suspension irradiated with λ = 340 nm reaches a photostationary state with 34% of <i>cis</i>-isomer, as compared to that observed in solution where the corresponding value is 74%. Line shape analysis of solid state ²H NMR spectra of a phenylene-<i>d</i>_<i>4</i> isotopologue, obtained as a function of temperature, indicated that rotation in crystals of the <i>trans-</i><b>2</b> isomer, with a mean activation energy of 4.6 ± 0.6 kcal/mol and a pre-exponential factor exp(29.4 ± 1.7), is ten times faster than that of samples containing the <i>cis-</i><b>2</b> isomer, which has a higher mean activation energy of 5.1 ± 0.6 kcal/mol and a lower pre-exponential factor of exp(27.9 ± 1.3)