Template Synthesis of Disilacycloalkanes Utilizing the Reactivity of a Siloxane Bond.

The investigation of large and flexible macrocyclic compounds has garnered significant attention due to their functions as host molecules and linkers. Although the synthetic yields of such compounds, achieved by linking two molecular fragments, are often hindered by the flexibility of the molecular skeleton, one of the effective solutions is template synthesis for the macrocycles. In this study, a novel template synthesis for disilacycloalkanes by leveraging the reactivity of a siloxane bond was investigated. The yields obtained through the template methods surpassed those of the nontemplate approach, and the introduction of substituents to the silicon atoms was also accomplished with success. All of the resulting disilacycloalkanes crystallized exceptionally well, enabling their structural determination through X-ray crystallography. Notably, the stability of these structures was elucidated by analyzing dispersion forces between alkyl chains, using density functional theory (DFT) calculations. This template synthesis method demonstrates its efficacy in synthesizing molecular systems that encompass two functional moieties linked with macroalkanes.

Synthesis of Dimethyldisilabicycloalkanes: Cage-Size Effects on the Relative Stabilities between In,Out and Twist-Out,Out Forms.

In large bicycloalkanes, several in/out forms exist, wherein substituents on the bridgehead atoms are oriented either outside or inside the cage. The relative stability between the in,out and twist-out,out forms, which can interconvert through homeomorphic conversion, was found to depend upon the cage size. The in,out form demonstrated thermodynamic stability in the smaller C10 derivative, whereas the twist-out,out form prevailed in the larger derivatives of C14 and C18 plausibly as a result of dispersion forces among the chains.

Synthesis and structure of stannane-based molecular bevel gears having substituents on a tin linker.

Ditriptycylmethanes are known as molecular bevel gears because the two triptycyl groups act as wheels and display correlated rotation in solution. However, introduction of substituents on the methylene junction for chemical modification of the bevel gears has been difficult due to steric hindrance of the triptycyls. In this study, stannane-based molecular bevel gears with organic substituents on the tin linker were achieved by utilizing long Sn-C bonds. Ditriptycylstannanes with methyls or phenyls on the tin atoms were synthesized. Their structures were characterized by X-ray crystallography, and the angle of triptycyl-Sn-triptycyl in the diphenyl derivative was observed to be narrower than that of the dimethyl derivative due to steric hindrance of the phenyls. The gear rotation of these derivatives was observed by NMR spectroscopy through observations of the phase isomers of the corresponding methyltriptycyl derivatives. Gear slippage was observed in the high-temperature region, and the activation energy of the diphenyl derivative was higher than that of the dimethyl derivative. The observed relationship between gear rotation and substituents on the tin can aid in the molecular design of functional molecular bevel gears.

Synthesis and rotational dynamics of diazamacrocycles having bridged 1,4-naphthylene as framed molecular rotors.

The rotation of a part of a molecule has attracted attention because of its possible role in the development of an artificial molecular rotor. In this study, 1,4-naphthylene bridged diazamacrocycles, in which the 1,4-diaminonaphthalene moiety is bridged by two long alkyl chains, were designed as novel framed molecular rotors, and the dependence of the rotation on the frame size were investigated. Framed rotors CnNp (n = 12, 14, 16, 18), where the number in the compound name indicates the length of a side chain, were synthesized by direct cyclization of 1,4-naphthalenediamine with α,ω-dihaloalkane in the presence of Na2CO3 as the base. The rotation of the naphthylene rotor in C12Np, C14Np, and C16Np was nearly suppressed in solution, whereas the rotor in C18Np showed rotation, as confirmed by the temperature-dependent coalescence of the NMR signals of the α-CH2.

Synthesis and Characterization of Silyl[n]acediynes (Linearly Fused Benzodehydro[12]annulenes): Utilizing Bulkiness of Silyl Groups to Improve Selectivity.

[n]Acediynes have unique properties owing to their highly strained and extended π-electron system. We have previously reported the selective synthesis of a silyl[1]acediyne, i.e., silyldibenzotetradehydro[12]annulene, utilizing the bulkiness of the silyl groups to improve the selectivity of the reaction. Herein, we report the facile synthesis of high-order silyl[n]acediynes via Eglington coupling between silyldiethynylbenzene and silyltetraethynylbenzene. In fact, silyl[2]acediyne as well as silyl[1]acediyne can be isolated from the reaction mixture. Their structures were determined using X-ray crystallography and discussed with reference to our previous report, and their spectroscopic properties, including absorption, fluorescence, Raman, and IR spectra, were fully characterized with DFT calculations. A drastic redshift in their absorption and fluorescence spectra occurred with an increase in the order number n of the [n]acediynes. The antiaromaticities of the dehydro[12]annulene moieties at the center of the annulene ring in these silylacediynes were also evaluated using NICS values. The calculated values were small and positive, which confirmed the weak antiaromatic character of the rings.

Design of rotational potential in a phenyltriptycene molecular rotor by exploiting CH/π-interaction between tripticil hydrogen and phenyl.

The chemistry of artificial molecular rotors has recently attracted considerable attention in the field of molecular machines. Phenyltriptycene could be used as a stepwise molecular rotor because it is composed of a phenyl rotor and a triptycene stator, in which the rotational potential can be designed by introducing substituents. In this study, a novel design of the relative energies among three rotamers of a substituted phenyltriptycene by exploiting the CH/π-interaction between a peri-hydrogen and phenyl was investigated. First, the structures of two different phenyltriptycenes were compared to confirm CH/π-interactions. Second, the effects of the substituents of 1,4,5-trichloro-3',5'-dimethoxyphenyltriptycene on the relative energies were investigated by structural analysis, temperature-dependent nuclear magnetic resonance studies, and density functional theory calculations. The obtained results should facilitate the design of novel molecular switches and/or molecular rotors.

Structures and Oxidation Properties of Phenylene-Bridged Diazacycloalkanes: Ring Size Effects on Structures and Properties.

Phenylenediamine derivatives have been investigated as functional molecules because of their characteristic oxidation properties. In this study, phenylene-bridged diazacycloalkanes, that is, C10, C12, and C14, in which numbers indicate the lengths of a side chain, were synthesized, and the effects of the macroring size on their structures and oxidation properties were investigated. X-ray crystallography revealed that the structures around the nitrogen atoms were remarkably dependent on the chain lengths. The benzene plane of C10 is arranged almost perpendicular to the macroring to avoid steric contact. However, the benzene plane of C14 and alkyl frame were co-planar. Stabilization resulting from conjugation was comparable to destabilization caused by macroring strain according to DFT calculations. Structural differences between C10 and C14 caused changes in the NMR chemical shifts of the inner methylene protons and first oxidation potentials in solution. Notably, the properties of C12 can be analyzed as those of distributed structures, which is of interest with respect to exploiting the metastable structure of the molecules. The observed relationships between the steric structures and properties can facilitate the design of functional molecules containing phenylenediamine moieties.

Simultaneous synthesis and characterization of in/out-isomers of disilabicyclo[14.14.14]alkanes.

Facile and simultaneous synthesis of diphenyl-disilabicyclo[14.14.14]alkane in/out-isomers was achieved by using organosilicon chemistry. Although the formation of several in/out-isomers would be conceivable, only two diastereomers, i.e. the (traditional-)out,out-isomer and the twist-out,out-isomer, could be isolated because of homeomorphic isomerization. Crystal structures of the diastereomers were confirmed.

Steric effects on the intramolecular charge transfer fluorescence of benzo[b]thiophene-1,1-dioxide bridged macrocages.

Intramolecular charge transfer (ICT) fluorescence has been widely investigated and exploited in sensor molecules. However, steric effects on the ICT fluorescence properties have rarely been reported so far, although research in this area would promote an understanding of the basics of solvation. Herein, we report the detailed fluorescence properties of bis(trimethylsilyl)benzo[b]thiophene-1,1-dioxide (TMSBTO2) and its caged cyclophanes and non-cage isomers, which demonstrate ICT fluorescence in various solutions. The fluorescence band maxima for these benzo[b]thiophene-1,1-dioxides (BTO2s) showed a red-shift with increasing solvent polarity, confirming ICT fluorescence characteristics. The linearity of the Lippert-Mataga plots was confirmed for all ICT fluorescence measured in hexane, toluene, AcOEt, CH2Cl2, and EtOH. The slopes of the plots decreased in the following order: TMSBTO2, non-cage isomers, and caged BTO2s. It is concluded that the Onsager radii for these BTO2s were increased in the abovementioned order, assuming that the difference in the dipole moments between the excited and ground states for these BTO2s was identical.

Structure and Dynamics of Crystalline Molecular Gyrotops with a Difluorophenylene Rotor.

Macrocage molecules with a bridged π-electron system could be assumed as crystalline molecular gyrotops because of the structural similarity and the rotatable π-electron system. In this study, 1,2-difluoro-3,6-phenylene-bridged macrocages were designed and synthesized as crystalline molecular gyrotops with a dipolar rotor. The thermal ring dynamics of the dipolar rotor in the crystal were investigated by solid-state NMR and dielectric spectroscopy. The gyrotop that was surrounded by three C14-alkyl chains exhibited an exchange between two stationary positions in the crystalline state. In contrast, the gyrotop cage consisting of C18 chains exhibited no dynamics in the crystalline state. Although the corresponding phenylene derivatives exhibit a facile rotational motion of the phenylene group in the crystalline state, the dynamics of each derivative was observed to be different. The reason for this difference is ascribed to the difference in the bulkiness between the fluorine and hydrogen atoms in the rotor.

Gear Slippage in Molecular Bevel Gears Bridged with a Group 14 Element.

Ditriptycilmethanes are known as molecular bevel gears because the two triptycil groups show correlated rotation. In this report, molecular bevel gears bridged with a group 14 element, bis(methyltriptycil)X (X = SiH2, GeH2, GeF2), were synthesized, and their gearing properties were investigated. Gear slippage, that is an error in gear rotation, is observed in high-temperature solutions of molecular bevel gears. Heavy atom derivatives undergo gear slippage more easily due to the long bond lengths and wide angles between the two triptycil units and the bridging group 14 element. Activation energies of gear slippages were estimated by temperature-dependent NMR spectroscopy and DFT calculations, and theoretical thermodynamic parameters for gear slippage were found to be in excellent agreement with experimental values. The results indicate that theoretical calculations for gear rotation in molecular bevel gears can accurately reproduce experimental phenomena.

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.

Dielectric Relaxation of Powdered Molecular Gyrotops Having a Thiophene Dioxide-diyl as a Dipolar Rotor.

The dielectric properties of powdered molecular gyrotops with a thiophenedioxide-diyl are reported. Crystals without a solvent molecule show usual dielectric relaxation spectra due to orientation polarization of the dipolar rotor, while a crystal having ethanol as the crystalline solvent molecule showed novel temperature-dependent dielectric relaxation switching by crystal-to-crystal phase transition, which is induced by hydrogen-bonding interactions between thiophene dioxide and ethanol.

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).

A pyrene-bridged macrocage showing no excimer fluorescence.

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.

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.

Ring-closing metathesis for the synthesis of a molecular gyrotop.

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.

Kinetic stabilization against the oxidation reaction induced by a silaalkane cage in a thiophene-bridged molecular gyroscope.

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.

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.

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.