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 (<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)

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 (<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)

Reversible Photolysis of Nitrosobenzene <i>cis</i>-Dimer Monitored In Situ by Single Crystal Photocrystallography

A single crystal of the <i>cis</i>-dimer of nitrosobenzene was directly observed by photocrystallography to transition to a pair of monomers and reversibly redimerize. The remarkable displacement of the nitrogen atoms within the crystalmoving a total distance of 2.97(5) Å for the two atomssuggests that the breadth of solid-state photochemical reaction systems susceptible to X-ray diffraction studies need not be limited to those with very small atomic displacements

Reversible Photolysis of Nitrosobenzene <i>cis</i>-Dimer Monitored In Situ by Single Crystal Photocrystallography

A single crystal of the <i>cis</i>-dimer of nitrosobenzene was directly observed by photocrystallography to transition to a pair of monomers and reversibly redimerize. The remarkable displacement of the nitrogen atoms within the crystalmoving a total distance of 2.97(5) Å for the two atomssuggests that the breadth of solid-state photochemical reaction systems susceptible to X-ray diffraction studies need not be limited to those with very small atomic displacements

Structure–Reactivity Correlations and Mechanistic Understanding of the Photorearrangement and Photosalient Effect of α‑Santonin and Its Derivatives in Solutions, Crystals, and Nanocrystalline Suspensions

The synthesis, crystal packing and photochemical reactions of α-santonin (<b>1a</b>) and its methyl, ethyl, <i>n</i>-propyl, and <i>n</i>-butyl derivatives (<b>1b</b>–<b>e</b>) are described to explore the effect that a photochemically benign yet structurally significant synthetic modification can have on the solid-state photochemical reactivity. The structures of the derivatives were determined using single crystal X-ray diffraction and compared against the packing of α-santonin. A cage dimer (<b>12a</b>) found exclusively upon irradiation of <b>1a</b> in the solid state was not found when the other derivatives were exposed to light, because the alkylation proved to perturb the crystal packing away from an optimal dimerization alignment. Using a high-speed camera, we monitored the photosalient effect of α-santonin and found it to occur at an angle orthogonal to the <i>b</i>-axis of the unit cell, which we suspect is caused by the formation of the cage dimer <b>12</b>. The photochemistry of <b>1a</b>–<b>e</b> in solution and crystalline suspensions was also analyzed. The solution photochemistry was in accord with literature precedence, and the crystalline suspensions yielded a variety of photoproducts including a tertiary alcohol (<b>7b</b>–<b>e</b>), which is not commonly observed in neutral water. An exocyclic alkene photoproduct (<b>8b</b>) was also discovered, and its presence is hypothesized to be caused by an intermolecular deprotonation caused by a water molecule present in the crystal