Photoreversible Birefringence Change of Diarylethene Single Crystals as Revealed by Change in Molecular Polarizability Anisotropy

Stimuli-responsive organic crystals represent a new frontier of material chemistry. Recently, we have reported photoreversible interference color change to multicolor in single crystals composed of a photochromic diarylethene derivative, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a), accompanied by the photochromic reaction. The origin of the interference color change is due to the photoinduced birefringence change in the photoisomerization of diarylethenes. In this study, we newly found that single crystals composed of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2a) also exhibit a photoreversible interference color change. The birefringence value for crystal 2a increased with the photocyclization conversion, while that for crystal 1a decreased. The relationship between the photoinduced birefringence changes for crystals 1a and 2a and their molecular structures was discussed based on the change in the molecular polarizability anisotropy accompanied by the photochromic reaction. These results would provide not only new opportunities for the application of photochromic crystals but also useful strategies for the design of crystalline materials that exhibit the desired birefringence change.

Kinetic Stabilization of Carbazole Nitroxides by Inclusion in a Macrocage and Their Electron Spin Resonance Characterization

Some nitroxides, for example, tetramethylpyridiniumoxide, are known as stable radicals; however, carbazole nitroxide is less stable. The kinetic stabilization of labile radicals by introduction of bulky substituents is usually effective to investigate intrinsic properties of the molecule because of small electronic perturbation induced by the substituents. In this study, macrocage molecules with a carbazole nitroxide connected by covalent bonds were newly designed as kinetically stabilized carbazole nitroxides. The nitroxides were prepared and characterized by electron spin resonance spectroscopy. The caged nitroxides presented long half-lives (∼50 h) by kinetic analysis.

Synthesis of dipolar molecular rotors as linkers for metal-organic frameworks

We report the synthesis of five dicarboxylic acid-substituted dipolar molecular rotors for the use as linker molecules in metal-organic frameworks (MOFs). The rotor molecules exhibit very low rotational barriers and decent to very high permanent, charge free dipole moments, as shown by density functional theory calculations on the isolated molecules. Four rotors are fluorescent in the visible region. The linker designs are based on push-pull-substituted phenylene cores with ethynyl spacers as rotational axes, functionalized with carboxylic acid groups for implementation in MOFs. The substituents at the phenylene core are chosen to be small to leave rotational freedom in solids with confined free volumes. The dipole moments are generated by electron-donating substituents (benzo-1,3-dioxole, benzo-1,4-dioxane, or benzo-2,1,3-thiadiazole annelation) and withdrawing substituents (difluoro, or dicyano substitution) at the opposite positions of the central phenylene core. A combination of 1,4-dioxane annelation and dicyano substitution generates a theoretically predicted, very high dipole moment of 10.1 Debye. Moreover, the molecules are sufficiently small to fit into cavities of 10 Å3. Hence, the dipolar rotors should be ideally suited as linkers in MOFs with potential applications as ferroelectric materials and for optical signal processing.

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. Although this line shape is typically associated with fast-moving molecular components, our VT 2H NMR experiments, along with X-ray diffraction analyses and periodic DFT computations show that this spectroscopic feature can also be originated from low-frequency intramolecular rotations of the central phenylene with a cone angle of 54.7° that is attained by the cooperative motion of the entire structure that distorts the molecular axis to rotation. In contrast, two isomeric structures (2 and 3) do not show a noticeable intramolecular rotation, because their crystallographic arrays showed very restricting close contacts. Our findings clearly indicate that the multiple components and phase transitions in crystalline molecular machines can work in concert to achieve the desired motion.

Polarized fluorescence of a crystal having uniaxially oriented molecules by a carbazole-diyl-bridged macrocage

An axially oriented π-electron system is achieved in a single crystal of a macrocage molecule, and polarized fluorescence of the single crystal was observed.

Precessional Motion in Crystalline Solid Solutions of Ionic Rotors

The order-disorder phase transition associated with the uprise of reorientational motion in (DABCOH2)²⁺ , in the supramolecular salts of general formula [1⋅(DABCOH₂ )]X₂ (where 1=12-crown-4, DABCO=1,4-diazabicyclo[2.2.2]octane, and X=Cl^- or Br^- ), has been investigated by variable temperature X-ray diffraction on single crystals and powder samples, as well as by DSC and solid-state NMR spectroscopy (SSNMR). The two compounds undergo a reversible phase change at 292 and 290 K, respectively. The two crystalline materials form solid solutions [1⋅(DABCOH₂ )]Cl_2x Br_2(1-x) in the whole composition range (0 < x<1), with a decrease in the temperature of transition to a minimum of ca 280 K, corresponding to x=0.5. Activation energy values for the dynamic processes, evaluated by variable-temperature ¹³ C magic-angle spinning (MAS) SSNMR and line-shape analysis are ca. 50 kJ mol^-1 in all cases. Combined diffraction and spectroscopic evidence has allowed the detection of a novel dynamic process for the (DABCOH₂ )²⁺ dications, based on a room temperature precessional motion that is frozen out below the disorder-order transition; to the best of the authors' knowledge this phenomenon has never been observed before.

Synthesis of Structurally Diverse Emissive Molecular Rotors with Four-Component Ugi Stators

The use of the multicomponent Ugi reaction to rapidly prepare a library of dumbbell-like molecular rotors is highlighted here. The synthetic strategy consisted of the atom-economic access to 15 bulky and structurally diverse iodinated stators, which were cross-coupled to the 1,4-diethynylphenylene rotator. From those experiments, up to six rotors 1a-c and 1l-n were obtained, with yields ranging from 35 to 69% per coupled C-C bond. In addition to the framework diversity, five of these compounds showed aggregate-enhanced emission properties thanks to their conjugated 1,4-bis(phenylethynyl)benzene cores, a property that rises by increasing the water fraction (f_w) in their THF solutions. The results highlight the significance of the diversity-oriented synthesis of rapid access to new molecular fluorescent rotors.

Exploiting rotational motion in molecular crystals

Rotational motion within molecular crystals is a prototypical concept to build future functional materials and solid-state molecular machines.