Wataru Setaka

Order–Disorder Transition of Dipolar Rotor in a Crystalline Molecular Gyrotop and Its Optical Change

Successful control of the orientation of the π-electron systems in media has been achieved in certain liquid crystals, making them applicable to devices for optical systems because of the variation in the optical properties with the orientation of the π-electron system. However, because of close packing, changing the orientation of molecules in the crystalline state is usually difficult. A macrocage molecule with a bridged thiophene rotor was synthesized as a molecular gyrotop having a dipolar rotor, given that the dipole derived from the thiophene can rotate even in the crystal. The thermally induced change in the orientation of the dipolar rotors (thiophene ring) inside the crystal, i.e., order-disorder transition, and the variation in the optical properties in the crystalline state were observed.

Thermal modulation of birefringence observed in a crystalline molecular gyrotop

Recently, functional organic materials have been put into practical use. The application of molecular motions has the potential to create new molecule-based materials. For this reason, considerable attention has been focused on the chemistry and properties of molecular machines in which mechanical motions of parts of the molecules are observed. In particular, phenylene rotation in the crystalline state has been investigated using framed molecular gyrotops having a phenylene rotor encased in three long alkyl spokes. In this study, we show thermal modulation of birefringence in a crystal due to the states of dynamic equilibrium of a novel molecular gyrotop. A macrocage molecule having a bridged phenylene rotor was synthesized as a novel molecular gyrotop. Rapid rotation of the phenylene rotor of the molecular gyrotop was confirmed by solid-state 2H NMR spectroscopy that showed changes in the optical properties of a single crystal, i.e., the thermal modulation of birefringence. These results are the first application of the dynamic states in a crystal causing an optical change. These phenomena were also confirmed by control experiments using a molecular gyrotop with a nonrotating xylene rotor. We anticipate our finding to be a starting point for the creation of a new field of material chemistry that will make use of the dynamic states of molecules.

Introduction of Clutch Function into a Molecular Gear System by Silane−Silicate Interconversion

Introduction of the clutch-declutch mechanism into a new gear system, bis(4-methyl-9-triptycyl)difluorosilane 1, is achieved by the reversible attachment of fluoride ion giving the corresponding fluorosilicate 2. Although the phase isomers of 1 (1(dl) and 1(meso)) cannot be separated because of the equilibrium via a slow gear slippage process (DeltaH(double dagger) = 17.2 +/- 0.2 kcal x mol(-1) and DeltaS(double dagger) = 0.9 +/- 0.9 cal x mol(-1) x K(-1)), 1 works as meshed molecular gears in solution at room temperature. On the other hand, silicate 2 in the solid state has quite an unusual TBP structure having two organic triptycyl groups at the apical positions and three electronegative fluorine atoms at the equatorial positions against the Muetterties rule. Rotation of the two triptycyl groups around Si-C bonds in 2 is facile and independent to each other in solution. Silicate 2 is reverted to the corresponding silane mixture by treating with excess water.

A molecular balloon: expansion of a molecular gyrotop cage due to rotation of the phenylene rotor.

A macrocage molecule with a bridged phenylene rotor has been reported as a molecular gyrotop, because the rotor can rotate even in a crystalline state. Although the most stable cage structure of the molecular gyrotop in a crystal was folded and shrunken at low temperature, expansion of the cage was observed at high temperature due to rapid rotation of the phenylene in a crystal. This phenomenon is analogous to the deflation and inflation of a balloon. Moreover, the unusually large thermal expansion coefficient of the crystal was estimated in the temperature range in which the expansion of the cage was observed, indicating a new function of dynamic states of the molecules.

Synthesis of crystalline molecular gyrotops and phenylene rotation inside the cage.

Phenylene-bridged macrocage molecules were synthesized as molecular gyrotops because the rotor can rotate even in a crystal. The chain-length-dependent properties of the molecular gyrotops were investigated in order to explore the potential to create new molecular materials. The formation of the cage in the synthesis of each molecular gyrotop depended on the length of the alkyl chains of the precursor. The rotation modes and energy barriers for phenylene rotation inside the crystals of the molecular gyrotops were changed by varying the chain length of the cage.

1,4-Naphthalenediyl-Bridged Molecular Gyrotops: Rotation of the Rotor and Fluorescence in Solution.

Macrocage molecules with a bridged π-electron system have been reported as molecular gyrotops in which the π-electron system can rotate within the cage. We recently reported the dynamics of the rotor in solution using 1,4-naphthalenediyl-bridged molecular gyrotops, which consist of cages formed of three C14, C16, or C18 chains. In this work, we synthesized novel gyrotops with C15 and C17 chains and systematically investigated the activation energies for the rotation of the rotor in solution. The activation energies for rotation in solution were found to decrease with increasing size of the cage. Therefore, a rotational barrier can be designed by adjusting the length of the side chains in these molecular gyrotops. Additionally, these gyrotops were fluorescent in solution; the quantum yields and lifetimes of the fluorescence were investigated. However, these properties were not influenced by the chain length owing to a large difference in time scale between fluorescence (10(-8)-10(-9) s) and the rotational dynamics inside the cage (10°-10(-5) s).

Cage size effects on the rotation of molecular gyrotops with 1,4-naphthalenediyl rotor in solution.

1,4-Naphthalenediyl-bridged macrocages (2, 3, and 4) were synthesized as novel molecular gyrotops. Compound 2 (C14 chains) does not show rotation of the naphthalene ring about an axis in solution. The 1,4-naphthalenediyl moieties of compounds 3 (C16 chains) and 4 (C18 chains) show restricted and rapid rotation inside the cage in solution, respectively. Therefore, steric protective effects on the rotation of the rotor in molecular gyrotops can be controlled by changing the size of the cage.

A crystalline molecular gyrotop with germanium junctions between a phenylene rotor and alkyl spokes

Macrocage molecules with a bridged phenylene have been reported as molecular gyrotops, in which the phenylene moiety can rotate even in the crystalline state. The roles of the atoms in the junctions between the rotor and spokes in molecular gyrotops have not been clarified well. In this study, a molecular gyrotop with germanium junctions was designed and synthesized, and the differences between the properties of the germanium and silicon derivatives were discussed. Notably, a structural isomer of the cage, which is not formed in the synthesis of the silicon derivative, was formed during the synthesis of the germanium derivative. Because the long Ge–C(Ph) bond length (1.958(4) A) was observed in the crystal structure of the germanium derivative as compared to the Si–C(Ph) bond length (1.885(2) A) of the silicon derivative, the activation energy for the rotation of the phenylene moiety inside the crystalline state of the germanium derivative (8.0 kcal mol−1) was lower than that of the silicon derivative (9.0 kcal mol−1). Similar tendencies of temperature-dependent optical properties of the single crystal, i.e., birefringence (Δn), were observed between the germanium and the silicon derivatives, but the temperatures and magnitudes of the discontinuous change in the birefringence were different.

Direct observation of the solvent reorientation dynamics in the “twisted” intramolecular charge-transfer process of cyanophenyldisilane–water cluster by transient infrared spectroscopy

The solvent reorientation dynamics of the intramolecular charge-transfer (ICT) process of the (p-cyanophenyl)pentamethyldisilane-H(2)O (CPDS-H(2)O) cluster was investigated by transient infrared (IR) absorption spectroscopy. Transient IR bands of two distinct charge-transfer (CT) states appeared in both the OH and the CN-stretching vibration regions. Analyses of the IR spectra and the time profiles of the transient bands revealed that the ICT process of the CPDS-H(2)O cluster proceeds in two steps. The first step is a transition from a photo-prepared locally excited (LE) state to the CT state, which is accompanied by a minor reorientation of the H(2)O moiety. In contrast, the second step is an extensive reorientation process of the H(2)O molecule in the CT state. These two reorientation processes exhibit very distinct pico- and nano-second time scales. In the latter case, a relatively slow time constant of 2 ns was related to a large geometric change in the orientation.

Solvent reorientation process in the "twisted" intramolecular charge-transfer process of cyanophenyldisilane-(H2O)2 cluster investigated by transient infrared spectroscopy.

The solvent reorientation process of the intramolecular charge-transfer (ICT) process of the (p-cyanophenyl)pentamethyldisilane-(H2O)2 (CPDS-(H2O)2) cluster in the excited-state was investigated by transient infrared (IR) absorption spectroscopy. It was found that there are at least two isomers in the charge-transfer (CT) state: one of the isomers exhibits a band of a pi-hydrogen-bonded OH stretch of the water moiety. Analyses of the IR spectra in the dominant isomers revealed that water molecules are hydrogen-bonded with each other in the CT state. This indicates that the reorientation process of the water molecules takes place to form such a dimer structure during the ICT process.