Search

A Negishi coupling based synthesis of 1,2,4,5-tetra(9-anthryl)benzene derivatives, possessing X-shaped molecular structures, is described. The results of X-ray crystallographic analysis show that two-dimensional highly ordered packing structure of the crystalline state of the unsubstituted derivative is a consequence of intermolecular π-π and CH-π interactions between anthracene units. Photoirradiation of the unsubstituted derivative as a precipitated solution promotes intramolecular [4+4] photocycloaddition reactions between both adjacent pairs of anthracene units to produce a crystalline polycyclic product having a unique 1.700 Å long carbon-carbon single bond. Furthermore, charge-transfer complexes, displaying near-infrared absorption and emission, are generated by co-crystallization of the X-shaped unsubstituted member of the group with electron-acceptor molecules.

Fluorescence probes are widely used to assess the molecular environment based on their photo-physical properties. Specifically, flexible and aromatic photo-functional system (FLAP) is unique viscosity probe owing to the excited-state planarization of anthracene wings. We have previously applied fluorescence spectroscopy to monitor the evaporative crystallization of solvents. The fluorescence color and spectral changes, which depend on the aggregation form, enable direct fluorescence visualization during evaporative crystallization. The fluorescence visualization of the liquid-like cluster intermediate proposed in the two-step nucleation model for the nucleation process has been achieved. However, the physical properties of these clusters, especially the viscosity, molecular motion, and intermolecular interactions, are still unclear. In this study, FLAPs are used as probes for local-viscosity changes and space limitations of the liquid-like cluster state during evaporative crystallization by observing the fluorescence-spectral changes and using hyperspectral-camera (HSC) imaging. Green emission originates from the monomer in the solution owing to the free-flapping motion. The fluorescence color turns blue with increasing viscosity under crowding conditions. If the survival time of the liquid-like cluster state is sufficient, crystalline phase (R-phase) formation proceeds via a 2-fold π-stacked array of the V-shaped molecules. It is difficult to form the V-shaped stacked columnar structures in the liquid-like cluster state region, resulting in the deposition of head-to-tail dimer structures, such as the yellow-emissive phase (Y-phase). In the case of the FLAP, the stacking intermediate does not form during solvent evaporation in the liquid-like cluster; rather, it is deposited in an amorphous form that exhibits blue emission (B-phase). These findings suggest that it is important to the maintenance of the survival time of the liquid-like cluster states to organize and rearrange the stacking forms. We have achieved the fluorescence probing of viscosity changes at local molecular motion with solvent depletion during solvent evaporation for the first time.

New synthetic protocols to the nitrogen-embedded flapping molecules have been developed. Gram-scale synthesis of a key precursor, tetraamine of dibenzo[a,e]cyclooctatetraene has been established for designing flapping quinoxaline and flapping phenazineimide. The impact of the nitrogen substitution on the photophysical properties and the viscosity-probing function has been investigated in comparison with the reported flapping anthraceneimide.

Flexible and aromatic photofunctional system (FLAP) has been recognized as an emerging class of versatile π-conjugated molecules. Here we report a viscosity-probing function of flapping peryleneimi...

The shape of the lowest singlet excited-state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10-20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

Overcrowded ethylenes composed of 10-methyleneanthrone and two bulky aromatic rings contain a twisted carbon-carbon double (C=C) bond as well as a folded anthrone unit. As such, they are unique frustrated aromatic enes (FAEs). Various colored crystals of these FAEs, obtained using different solvents, correspond to multiple metastable conformations of the FAEs with various twist and fold angles of the C=C bond, as well as various dihedral angles of attached aryl units with respect to the C=C bond. The relationships between color and these parameters associated with conformational features around the C=C bond were elucidated using experimental and theoretical studies. Owing to the fact that they are separated by small energy barriers, the variously colored conformations in the FAE crystal change in response to various external stimuli, such as mechanical grinding, hydrostatic pressure and thermal heating.

A ratiometric flapping force probe that can evaluate the nanoscale stress concentration in the polymer chain network of common organogels has been developed. Stress-dependent dual-fluorescence properties of the chemically doped flapping force probe has been demonstrated even when the probe is solvated in the wet materials (Figure 1). The fluorescence ratiometric analysis is robust against the local concentration change induced by the macroscopic polymer deformation. While the force-responsive FRET dyads, widely used in mechanobiology, are sensitive to the distance and orientation of the two chromophores, the flapping fluorophore works as a single-component flexible force probe regardless of the FRET efficiency. Realtime and reversible spectral response to the mechanical stress is observed with a low threshold on the order of sub-MPa compression due to its conformational flexibility. The previously reported flapping probe only shows a negligible response in the solvated environments because the undesired spontaneous planarization occurs in the S1 excited state, even without mechanical force. The excited-state engineering by changing the flapping wings from the anthraceneimide units to the pyreneimide units endows this molecule with the force probe function in the wet conditions. The structurally modified force probe also has an advantage in terms of a wide dynamic range of the fluorescence response in solvent-free elastomers, which enabled the ratiometric fluorescence imaging of the molecular-level stress concentration during the crack growth in a stretched polyurethane film. The percentage of the stressed force probes has been experimentally estimated to be approximately 30–40% before the fracture of the elastomers. The flapping force probe is useful for elucidating the toughening mechanism of recently focused unique topological gels and elastomers at molecular level.

Understanding the microviscosity of soft condensed matter is important to clarify the mechanisms of chemical, physical or biological events occurring at the nanoscale. Here, we report that flapping fluorophores (FLAP) can serve as microviscosity probes capable of detecting small changes. By the ratiometric fluorescence analysis, one of the FLAP probes detects a macroscopic viscosity change of a few cP, occurring at the thermal phase transition of a nematic liquid crystal. We discuss the impact of the chemical structure on the detection capability, and the orientation of the FLAP molecules in the ground and excited states. This work contributes to experimentally providing a molecular picture of liquid crystals, which are often viewed as a continuum.

On-surface chemical reaction has become a very powerful technique to synthesize nanostructures by linking small molecules in the bottom-up approach. Given the fact that most reactants are simultaneously activated at certain temperatures, a sequential reaction in a controlled way has remained challenging. Here, we present an on-surface synthesis of multi-block co-oligomers from trifluoromethyl (CF3 ) substituted porphyrin metal complexes. The oligomerization on Au(111) is demonstrated with a combination of bond-resolved scanning probe microscopy and density functional theory (DFT) calculations. Even after the first oligomerization of single monomer unit, the termini of the oligomer keep the CF3 group, which can be used as a reactant for further coupling in a sequential order. Consequently, copper, cobalt, and palladium complexes of bisanthracene-fused porphyrin oligomers were linked with each other in a designed order.

Nanoporous materials with anionic pentacoordinate organosilicon frameworks were prepared using organoalkoxysilanes, tris-catechol linkers (2,3,6,7,10,11-hexahydroxytriphenylene, HHTP), and primary ...

The air-water interface, which is the boundary of two phases with a large difference in polarity, gives a distinct environment compared with bulk water or air. Since the interface provides a field for various biomolecules to work, it is important to understand the molecular behaviors at the interface. Here, polarity-independent flapping viscosity probes (FLAP) equipped with hydrophobic/hydrophilic substituents have been synthesized and studied at the air-water interface. In situ fluorescence (FL), which is related to the internal motion and orientation, of three different FLAPs were investigated at the interface, and the internal motion of the molecule was indicated to be suppressed at the interface. In addition, the molecular response was compared with that of conventional viscosity probes (molecular rotors), which indicates the different behaviors of FLAP probably due to the distinct molecular orientation as well as molecular motion.

Single-molecule spectroscopy (SMS) of a dual fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free volume dynamics in polystyrenes. The SMS study was realized by structural improvement of a previously reported flapping molecule by nitrogen substitution, leading to increased brightness (22 times) of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps between short- and long-wavelength forms, the latter one indicating planarization of the molecule in the excited state. The observed spectral jumps were statistically analyzed to reveal the dynamics of the molecular environment. The analysis together with MD and QM/MM calculations show that the excited-state planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 280 A3 is available close to the molecule, and that such free volume lasts for an average of 1.2 s.

Single-molecule spectroscopy (SMS) of a dual-fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free-volume dynamics in polystyrenes. The SMS study was realized by structural modification of a previously reported flapping molecule by nitrogen substitution, leading to 20 times increased brightness of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps between short- and long-wavelength forms, the latter one indicating planarization of the molecule in the excited state. The spectral jumps were statistically analyzed to reveal the dynamics of molecular environment. The analysis together with MD and QM/MM calculations demonstrate that the planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 200 Å^3 is available close to the molecule, and that such free volume lasts for an average of 1.17 seconds.

International audience

The shape of the lowest singlet excited-state (S

Flapping fluorophores (FLAP) with a flexible 8π ring are rapidly gaining attention as a versatile photofunctional system. Here we report a highly photostable "flapping peryleneimide" with an unprecedented fluorogenic mechanism based on a bent-to-planar conformational change in the S1 excited state. The S1 planarization induces an electronic configurational switch, almost quenching the inherent fluorescence (FL) of the peryleneimide moieties. However, the FL quantum yield is remarkably improved with a prolonged lifetime upon a slight environmental change. This fluorogenic function is realized by sensitive π-conjugation design, as a more π-expanded analogue does not show the planarization dynamics. With strong visible-light absorption, the FL lifetime response synchronized with the flexible flapping motion is useful for the latest optical techniques such as FL lifetime imaging microscopy (FLIM).

The designed synthesis of inorganic cyclic compounds is a significant topic because of their many potential applications. In this study, we used a building block approach to synthesize siloxane-based macrocycles that resemble zeolite apertures. We synthesized a regioselectively functionalized cubic octasiloxane having two adjacent corners modified with Si–O–C bonds via the reaction of octa(hydridosilsesquioxane) (H8Si8O12) with 2,2′-(o-phenylenedioxy)diethanol. Hydrolysis and condensation of the Si–O–C bonds yield the cyclic compounds consisting of three, four, and five cage siloxane units. These compounds have more rigid ring structures than conventional cyclic organosiloxanes. Such an approach will lead to the design of a new class of host materials and molecular channels for transport and separation.

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

The simple anti-aufbau DFT approach for estimating singlet excited state aromaticity suggested in a recent Communication published in this journal is shown to produce incorrect results because it targets a linear combination of the singlet and triplet configurations involving the HOMO and LUMO rather than the first singlet excited state. If the S1 state of a molecule is dominated by the HOMO→LUMO excitation, a comparably simple but theoretically consistent and qualitatively correct approximation to the S1 wavefunction can be achieved by performing a small "two electrons in two orbitals" CASSCF(2,2) calculation which can be followed by the evaluation of magnetic aromaticity criteria such as NICS.

Graphene nanoribbons (GNRs), low-dimensional platforms for carbon-based electronics, show the promising perspective to also incorporate spin polarization in their conjugated electron system. However, magnetism in GNRs is generally associated with localized states around zigzag edges, difficult to fabricate and with high reactivity. Here we demonstrate that magnetism can also be induced away from physical GNR zigzag edges through atomically precise engineering topological defects in its interior. A pair of substitutional boron atoms inserted in the carbon backbone breaks the conjugation of their topological bands and builds two spin-polarized boundary states around them. The spin state was detected in electrical transport measurements through boron-substituted GNRs suspended between the tip and the sample of a scanning tunneling microscope. First-principle simulations find that boron pairs induce a spin 1, which is modified by tuning the spacing between pairs. Our results demonstrate a route to embed spin chains in GNRs, turning them into basic elements of spintronic devices., We gratefully acknowledge financial support from Spanish Agencia Estatal de Investigación (AEI) (MAT2016-78293, PID2019-107338RB, FIS2017-83780-P, and the Maria de Maeztu Units of Excellence Programme MDM-2016-0618), from the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant. No. 863098), the Basque Departamento de Educación through the PhD fellowship No. PRE_2019_2_0218 (S.S.), the Xunta de Galicia (Centro de Investigación de Galicia accreditation 2019–2022, ED431G 2019/03), the University of the Basque Country (Grant IT1246-19), and the European Regional Development Fund (ERDF). I. P. also thanks Xunta de Galicia and European Union (European Social Fund, ESF) for the award of a predoctoral fellowship

Flapping molecules (FLAP) have been developed as a new series of photofunctional systems. The molecular design is based on the concept of the rigid/flexible hybridization of π conjugated units. FLAP bearing bulky substituents works as a ratiometric viscosity probe that shows no polarity dependence, while FLAP with long alkyl chains provides a “light-melt adhesive,” a highly cohesive columnar liquid crystals that can be melt by ultraviolet light irradiation.

This chapter introduces flapping molecules and their versatile functions, whose structure is composed of rigid aromatic wings and a conformationally flexible joint. Photophysical properties and molecular assembly modes can be tailored in a series of the flapping systems. The chapter describes the applications particularly as a viscosity probe and a light-melt adhesive. Conformational rigidity plays an important role in the molecular chemistry and materials science of π-conjugated systems. The inherent rigidity of p-skeletons means the difficulty in providing convertible photophysical properties, which would originate from the flexibility of molecular structures. Although conformational flexibility would be the key to create dynamic functions, not every flexible p-molecule displays convertible photophysical properties. Viscosity mapping by synthetic fluorescent probes is a useful technique because it can visualize local viscosity distribution in heterogeneous media. Photoinduced phase transition has been well studied on materials containing isomerization dyes such as azobenzene, diarylethene and spiropyran.

This paper presents a frequency-modulation controlled resonant converter with the push-pull class-E inverter and the class-D rectifier. The Class-E inverter has no high-side switch in its topology, which is suitable to high-frequency operation. Additionally, it is possible to satisfy the ZVS condition in the entire ranges of the specified input-voltage and load variations by applying the class-E inverter. The ZVS region and output-voltage contours are illustrated on the parameter space, which are helpful for determine the component values. The validity of the design was confirmed by a quantitative agreements between the experimental results and the numerical predictions.

A variety of fluorescent molecular viscosity probes have been widely used for mapping the local viscosity in cells and for monitoring the microenvironments in materials. However, their viscosity-sensing structural design still relies strongly on molecular rotors featuring intramolecular rotational dynamics. Here we report flapping molecules (FLAP) as a ratiometric viscosity-sensing fluorophore that shows polarity-independent dual fluorescence. Viscosity-sensing mechanism is based on a unique V-shaped-to-planar conformational change in the singlet excited state (S1), in which the flexible motion of an eight-membered ring plays an important role. Fast conformational dynamics have been studied by time-resolved spectroscopies, and the viscochromic properties have been quantitatively analyzed. Application of FLAP to monitoring the curing process of epoxy resins has also been demonstrated, in which other typical environment-sensitive dyes did not work as a local viscosity probe.

The solution-processed fabrication of thin films of organic semiconductors enables the production of cost-effective, large-area organic electronic devices under mild conditions. The formation/dissociation of a dynamic B-N coordination bond can be used for the solution-processed fabrication of semiconducting films of polycyclic aromatic hydrocarbon (PAH) materials. The poor solubility of a boron-containing PAH in chloroform, toluene, and chlorobenzene was significantly improved by addition of minor amounts (1 wt % of solvent) of pyridine derivatives, as their coordination to the boron atom suppresses the inherent propensity of the PAHs to form π-stacks. Spin-coating solutions of the thus formed Lewis acid-base complexes resulted in the formation of amorphous thin films, which could be converted into polycrystalline films of the boron-containing PAH upon thermal annealing. Organic thin-film transistors prepared by this solution process displayed typical p-type characteristics.

Liquid crystal (LC) provides a suitable platform to exploit structural motions of molecules in a condensed phase. Amplification of the structural changes enables a variety of technologies not only in LC displays but also in other applications. Until very recently, however, a practical use of LCs for removable adhesives has not been explored, although a spontaneous disorganization of LC materials can be easily triggered by light-induced isomerization of photoactive components. The difficulty of such application derives from the requirements for simultaneous implementation of sufficient bonding strength and its rapid disappearance by photoirradiation. Here we report a dynamic molecular LC material that meets these requirements. Columnar-stacked V-shaped carbon frameworks display sufficient bonding strength even during heating conditions, while its bonding ability is immediately lost by a light-induced self-melting function. The light-melt adhesive is reusable and its fluorescence colour reversibly changes during the cycle, visualizing the bonding/nonbonding phases of the adhesive., Liquid crystals are used in many applications, such as removable adhesives, but this requires both good bonding strength, and its rapid disappearance under an external stimulus. Here, Saito and others report a dynamic liquid crystal material that loses is bonding strength under photo irradiation.

Bright fluorescent molecules with long fluorescence lifetimes are important for the development of lifetime-based fluorescence imaging techniques. Herein, a molecular design is described for simultaneously attaining long fluorescence lifetime (τ) and high brightness (ΦF ×ɛ) in a system that features macrocyclic dimerization of fluorescent π-conjugated skeletons with flexible linkers. An alkylene-linked macrocyclic dimer of bis(thienylethynyl)anthracene was found to show excimer emission with a long fluorescence lifetime (τ≈19 ns) in solution, while maintaining high brightness. A comparison with various relevant derivatives revealed that the macrocyclic structure and the length of the alkylene chains play crucial roles in attaining these properties. In vitro time-gated imaging experiments were conducted as a proof-of-principle for the superiority of this macrocyclic fluorophore relative to the commercial fluorescent dye Alexa Fluor 488.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Mechanical control of the molecular energy landscape is an important issue in modern materials science. Mechanophores play a unique role in that the mechanical responses are induced against the activation barrier for intramolecular transformation with the aid of external forces. Here we report an unprecedented activation process of a flexible flapping mechanophore. Namely, thermal void collapse in a crystalline phase triggers mechanophore compression in a definite proportion. Unfavored conformational planarization of the flapping mechanophore is compulsorily induced by packing force, leading to a total energy gain in crystal packing. Fluorescence chromism indicates extended π conjugation resulting from the mechanophore compression, giving rise to an energy transfer from the unpressed to compressed conformers.

Mobius aromatic molecules have attracted great attention as new functional materials because of their π-orbital cyclic conjugations lying along the twisted Mobius topology. To elucidate the electronic character of the lowest excited triplet (T1) state of a Mobius aromatic [28]hexaphyrin, we employed a time-resolved electron paramagnetic resonance (TREPR) method with applied magnetophotoselection measurements at 77 K. Analyses of the EPR parameters have revealed that the T1 state possesses intramolecular charge-transfer (CT) character together with local excitation character residing at one side in the Mobius strip ring. We have also demonstrated that the CT character between orthogonal unpaired orbitals triggers quick triplet deactivation by spin-orbit coupling. This deactivation can be an important barometer to represent the “antiaromaticity” because of a connection between the orthogonal CT character and instability by a weakened spin-spin exchange coupling in the T1 state.

Fluorescent molecular force probes have been developed for rheology and mechanobiology. Single covalent bond is generally cleaved by nano-Newton force, which has been confirmed by the analyses of AFM and optical tweezers. To quantitatively evaluate stress concentration in stretched polymeric materials or biological systems before they break, pico- Newton force must be detected at molecular scale, and therefore molecular force probes based on a bond-breaking mechanism cannot be used for this purpose. Here we have explored flexible force probes that show fluorescence response by a conformational change of flapping π-conjugated molecules (FLAP), which have the potential to realize fluorescence response to pico-Newton forces. In this presentation, a series of FLAPs synthesized in our laboratory will be introduced in connection with the force mapping technology.

A series of novel benzene centered mono-, bis- and tris-1,3,5-triazapentadiene ligands 6a–e was synthesized and investigated with respect to their reactivity towards triphenylborane. The resulting blue-fluorescent boron complexes 14a–e with a six-membered ring chelate structure show excellent thermal and chemical stability. All title compounds were completely characterized including X-ray diffraction studies for 14a–c and 14e. Whereas the absorption spectra of all three classes of compounds are similar, the fluorescence spectra show distinct differences. Thus, the emission spectra of 14a,b show Stokes shifts of 4100–6700 cm−1 with low quantum yields both in solution and in the solid state. However, the more bulky compounds 14c–e show markedly larger molar extinction coefficients and smaller bathochromic shifts compared to 14a,b. For all compounds, we observe significantly more intense red-shifted fluorescence in the solid state compared to that in dichloromethane solutions. For the interpretation of the absorption properties TD-DFT studies were performed based on DFT geometry optimizations.

Boron is a unique element in terms of electron deficiency and Lewis acidity. Incorporation of boron atoms into an aromatic carbon framework offers a wide variety of functionality. However, the intrinsic instability of organoboron compounds against moisture and oxygen has delayed the development. Here, we present boron-doped graphene nanoribbons (B-GNRs) of widths of N=7, 14 and 21 by on-surface chemical reactions with an employed organoboron precursor. The location of the boron dopant is well defined in the centre of the B-GNR, corresponding to 4.8 atom%, as programmed. The chemical reactivity of B-GNRs is probed by the adsorption of nitric oxide (NO), which is most effectively trapped by the boron sites, demonstrating the Lewis acid character. Structural properties and the chemical nature of the NO-reacted B-GNR are determined by a combination of scanning tunnelling microscopy, high-resolution atomic force microscopy with a CO tip, and density functional and classical computations., Incorporation of boron atoms into an aromatic carbon framework offers a wide variety of functionality. Here, the authors present boron-doped graphene nanoribbons by on-surface chemical reaction and characterize the structures and properties using scanning probe microscopy at the atomic-scale.

A reproducible formation of strongly bent crystals was accomplished by structurally restraining macrocyclic π-conjugated molecules. The model π-units consist of two 9,10-bis(2-thienylethynyl)anthracenes with a strong propensity for stacking, which are connected in a macrocyclic fashion via two alkylene linkers. The correlation between the crystalline morphology and the macrocyclic structures restrained by a variety of flexible alkylene linker combinations was systematically studied. Bent crystals were obtained only with specific alkylene linkers of appropriate chain length. The alkylene linkers can adopt different conformations in the crystal packing, so as to fill voids within the macrocycle. The ability to form several similar molecular structures with different alkylene conformations gives rise to contaminations of different crystalline phases within a single crystal, and it is these phase contaminations which are responsible for the bending of the crystals.

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.

Liquid crystal (LC) provides a suitable platform to exploit structural motions of molecules in a condensed phase. Amplification of the structural changes enables a variety of technologies not only in LC displays but also in other applications. Until very recently, however, a practical use of LCs for removable adhesives has not been explored, although a spontaneous disorganization of LC materials can be easily triggered by light-induced isomerization of photoactive components. The difficulty of such application derives from the requirements for simultaneous implementation of the following essential requisites: (i) adequate strength for a temporary bond (more than 1 MPa) even under heating conditions, (ii) significant reduction of the bonding strengths by light irradiation, and (iii) quick photoresponse for the separation of bonded materials. Here we present a liquid crystal (LC) material that satisfies all of the above-mentioned requisites for the light-melt adhesives, namely, (i) a shear strength over 1 MPa up to 110 °C for bonding glass plates, (ii) an 85% reduction of the strength by ultraviolet (UV) irradiation, and (iii) an instant photomelting of the LC film in a few seconds. Moreover, this material is reusable as an adhesive, and the transformation between the LC and melted phases is associated with an informative color change in fluorescence. We envision that composite materials with the light-melt function will further improve the performance in manufacturing processes, which will accelerate the on-demand photoseparation technology complementary to the other switchable adhesion approaches.

The synthesis of a planarized trinaphthylborane with partially fused structure is presented. This compound shows not only high chemical and thermal stability but also sufficient Lewis acidity to form Lewis adducts with pyridine derivatives in solution. The B-N Lewis adducts exhibit unprecedented photodissociation behavior in the excited state, reminiscent of the photogeneration of carbenium ions from triarylmethane leuco dyes. Consequently, these B-N Lewis adducts exhibit dual fluorescence emission arising from the initial tetracoordinate B-N adducts and the photodissociated tricoordinate boranes.

A series of nitrogen-containing heterotriangulenes expanded at the bridging positions has been synthesized. Among them, a dibenzo[c,g]fluorenylidene-substituted derivative has a highly twisted conformation for the overcrowded alkene moieties, which impart a highly electron-accepting character to the electron-donating heterotriangulene skeleton and thereby an NIR absorption as well as multiredox properties with a low reduction potential.

We have synthesized a series of new fluorescent boron systems 1a-c and 2a-d based on nitrogen (NNN) or nitrogen and oxygen (ONO)-containing tridentate ligands. These novel dyes are characterized by high thermal and chemical stability. They show large Stokes shifts (mostly above 3200 cm(-1)) and quantum yields in solution and in the solid state up to 40%. The easy, modular synthesis facilitates the convenient variation of the axial substituent on the central boron atom, allowing the functionalization of this dye for biochemical use. Introducing a long alkyl chain with a phenyl spacer at this axial position enables the self-assembly of the boron compound 2d to form a fluorescent vesicle, which is able to encapsulate small molecules such as sulforhodamine. Additionally, boron compound 2d was found to serve as a dye for cell imaging since it has the capability of binding to the nuclear membranes of HeLa cells. With phospholipids such as DOPC, giant unilamelar vesicles (GUV) are formed. These results demonstrate the wide applicability of this new boron system in supramolecular and medicinal chemistry.