Biphenyl contains a direct bond between two phenyl functional groups, and the dihedral angle between the two phenyl groups has been discussed in terms of electronic and steric stabilization. Facile rotation about the C–C bond between the two phenyl components is expected to occur at high temperatures. Here, we report observations of the rotational dynamics of a biphenylene dirotor in a crystalline state in a novel molecular gyrotop. The structure and rotational dynamics of the rotor exhibit remarkable temperature dependence. In accordance with the amplification of the angular displacement of biphenylene inside a crystal with increasing temperature, the birefringence of the single crystal decreased.

Three-dimensional arrays of dipolar rotors were constructed as single crystals of molecular gyrotops, which are macrocage molecules with a bridged dipolar rotor. In this study, we synthesized novel molecular gyrotops with a five-membered heteroring, i.e., furan-diyl (1), thiophene-diyl (2), and selenophene-diyl (3), and investigated the temperature-dependent orientation and rotation of the dipolar rotors inside the crystal. Unfortunately, furan derivative 1 did not crystallize; however, crystal structures of the other molecular gyrotops, i.e., 2 and 3, showed three-dimensional arrays of dipolar rotors. Thermal order–disorder transitions of the dipolar rotor orientation inside the crystal were observed in 2 and 3 with the transition temperature in selenophene derivative 3 being lower than that of thiophene derivative 2. This may be ascribed to subtle differences in the molecular structure, e.g., the intersection angle between two Si–C(aryl) bonds corresponding to the rotation axis. In accordance with the thermal change of the crystal structure, temperature-dependent optical properties of a single crystal were observed by analysis of birefringence of the crystal.

Pyrene is a common organic luminescent material. To improve the fluorescence properties of pyrene, we have designed a pyrene-2,7-diyl bridged macrocage in which the pyrene moiety is sterically protected by the outside alkyl chains. The macrocage shows intense fluorescence from a monomeric excited state without excimer fluorescence even in saturated solutions, although the parent pyrene shows excimer fluorescence in highly concentrated solutions. These results indicate that the steric shielding by the cage prevents the formation of the excimer. Intensities of florescence in the presence of nitrobenzene were investigated to clarify the cage effects on fluorescence quenching. Lower efficiency of the fluorescence quenching caused by intermolecular collision between the caged pyrene (fluorophore) and nitrobenzene (quencher) was revealed by the analysis of the bimolecular quenching constants kq.

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).

Macrocage molecules with a bridged rotor have been synthesized as molecular gyroscopes. The kinetics of the oxidation reaction of the thiophene-bridged molecular gyroscope, whose thiophene ring was bridged inside a silaalkane cage, was investigated. A remarkable kinetic stabilization against the oxidation of the thiophene moiety induced by the molecular cage framework was observed.

Macrocage molecules with a bridged phenylene rotor have been synthesized as molecular gyrotops, whose cages were constructed by ring-closing metathesis (RCM) of bis(trialkenylsilyl)benzenes. An analysis of the yields of the products in the RCM reaction under various temperature conditions revealed that the desired cage, i.e., a molecular gyrotop, was produced in good yield under reflux, indicating that the cage is a thermodynamically controlled product.

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) Å) was observed in the crystal structure of the germanium derivative as compared to the Si–C(Ph) bond length (1.885(2) Å) 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.

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.

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.

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.

Macrocage molecules with a bridged phenylene rotor have been synthesized as molecular gyrotops, whose cages were constructed by ring-closing metathesis (RCM) of bis(trialkenylsilyl)benzenes. An analysis of the yields of the products in the RCM reaction under various temperature conditions revealed that the desired cage, i.e., a molecular gyrotop, was produced in good yield under reflux, indicating that the cage is a thermodynamically controlled product.

Pyrene is a common organic luminescent material. To improve the fluorescence properties of pyrene, we have designed a pyrene-2,7-diyl bridged macrocage in which the pyrene moiety is sterically protected by the outside alkyl chains. The macrocage shows intense fluorescence from a monomeric excited state without excimer fluorescence even in saturated solutions, although the parent pyrene shows excimer fluorescence in highly concentrated solutions. These results indicate that the steric shielding by the cage prevents the formation of the excimer. Intensities of florescence in the presence of nitrobenzene were investigated to clarify the cage effects on fluorescence quenching. Lower efficiency of the fluorescence quenching caused by intermolecular collision between the caged pyrene (fluorophore) and nitrobenzene (quencher) was revealed by the analysis of the bimolecular quenching constants kq.

Macrocage molecules with a bridged rotor have been synthesized as molecular gyroscopes. The kinetics of the oxidation reaction of the thiophene-bridged molecular gyroscope, whose thiophene ring was bridged inside a silaalkane cage, was investigated. A remarkable kinetic stabilization against the oxidation of the thiophene moiety induced by the molecular cage framework was observed.