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

We describe the synthesis, characterization,
photochemical isomerization,
and rotational dynamics of a crystalline molecular gyroscope containing
an azobenzene bridge (trans-2) 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 trans-2, 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 cis-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-d4 isotopologue,
obtained as a function of temperature, indicated that rotation in
crystals of the trans-2 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 cis-2 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).