Photoinduced Crystal-to-Liquid Transition of Acylhydrazone-Based Photoswitching Molecules.

A photoinduced crystal-to-liquid transition (PCLT) behavior of new acylhydrazone derivatives (NCs) is reported. The photoswitching of the NCs was identified as a negative photochromism with a high E-to-Z conversion yield (>98%). A kinetic analysis shows a half-life of almost one month. Owing to these high photoswitching performances, we successfully isolated both E- and Z-forms, evaluated their crystal structures, and observed distinct thermal behaviors. The Z-form melts at a lower temperature than the E-form by several tens of degrees. The PCLT occurs at even lower temperatures. UV irradiation induces the E-to-Z conversion in the crystalline state, thereby inducing a eutectic melting. In addition to the PCLT, we observed a photomechanical behavior of the crystals, which suggests that the presented acylhydrazones can be new members of the photoresponsive crystalline materials.

Spectroscopic Tracking of Salicylideneaniline Photocolored Crystals: An Attempt to Quantify Polymorph-Dependent Features toward Precise Structure-Function Correlation Analysis.

Three polymorphs of salicylideneaniline (SA) were prepared, and their photochromic behavior was examined using a recently developed single-crystal microscopic UV-vis spectroscopy approach. This system enabled us to acquire absorption data during the bleaching process as a function of temperature and visible light intensity. First, we demonstrated that, in contrast to the generally accepted assumption, the bleaching curves were notably influenced by the degree of photosaturation at the initial stage. By modifying our kinetic model to include the term representing the initial degree of photosaturation, we successfully obtained the kinetic parameters intrinsic to each crystal structure. Second, we further analyzed the kinetic parameters to show that the bleaching process was accelerated by visible light irradiation to a significantly higher degree than by thermal relaxation. The two bleaching-prompting effects were quantitatively compared between two photochromic polymorphs, α1 and α2; the long life of the photoproduct in α2 was attributed to efficient self-shielding from visible light irradiation enabled by its structural features. These results prompted us to reexamine the simple dualistic photochemical and thermal classification of photochromic systems and will provide a foundation for the precise structure-function analysis of crystalline materials, including SAs.

Cold Crystallization and the Molecular Structure of Imidazolium-Based Ionic Liquid Crystals with a p-Nitroazobenzene Moiety.

The cold crystallization mechanism of 1-{[4'-(4″-nitrophenylazo)phenyloxy]}hexyl-3-methyl-1H-imidazol-3-ium tetrafluoroborate ionic liquid crystal was investigated based on thermal analysis, structural analysis, infrared spectroscopy, and quantum chemical calculations. By conducting thorough structural characterization, we found that the prerequisite for cold crystallization is the irreversible molecular conformational alteration induced by the initial heating of the as-grown crystal into a smectic liquid crystal. The originally linear cation molecule bends and forms a step-stair conformation that persists throughout the subsequent heating and cooling processes as phase transition occurs from the crystal phase to the liquid crystal phase and then to the isotropic liquid phase. The formation of cold crystal occurs because of the choice of molecular stability over crystalline stability. Given the exothermic anomaly exhibited upon heating generic crystals to cold crystals, these findings demonstrate the promising potential of this ionic liquid crystal for thermal energy storage applications.

Ratchet-like mechanism in a long-life photoproduct of salicylideneaniline enclathrated in a pillared-layer guanidinium disulfonate structure.

Salicylideneaniline (SA) was found to exhibit extraordinary long-life photochromism upon being included in a cavity of guanidinium organosulfonate and exhibited a lifetime of ∼30 times that of a pure SA crystal. Crystal structure analysis suggested that the sulfonate molecule in the apo-host provided a flexible cavity space that kinetically trapped SA in its photo-isomerized form, as if it was locked by a ratchet-like mechanism.

A superelastochromic crystal.

Chromism-color changes by external stimuli-has been intensively studied to develop smart materials because of easily detectability of the stimuli by eye or common spectroscopy as color changes. Luminescent chromism has particularly attracted research interest because of its high sensitivity. The color changes typically proceed in a one-way, two-state cycle, i.e. a stimulus-induced state will restore the initial state by another stimuli. Chromic systems showing instant, biphasic color switching and spontaneous reversibility will have wider practical applicability. Here we report luminescent chromism having such characteristics shown by mechanically controllable phase transitions in a luminescent organosuperelastic crystal. In mechanochromic luminescence, superelasticity-diffusion-less plastic deformation with spontaneous shape recoverability-enables real-time, reversible, and stepless control of the abundance ratio of biphasic color emissions via a single-crystal-to-single-crystal transformation by controlling a single stimulus, force stress. The unique chromic system, referred to as superelastochromism, holds potential for realizing informative molecule-based mechanical sensing.

Development of Imidazo[1,2- a]pyridine Derivatives with an Intramolecular Hydrogen-Bonded Seven-Membered Ring Exhibiting Bright ESIPT Luminescence in the Solid State.

Imidazo[1,2- a]pyridine derivatives with different hydroxyaryl units (1-3), which could potentially form an intramolecular hydrogen-bonded seven-membered ring in either a planar or a twisted conformation, were newly developed, and the effect of conformation and steric repulsion on the excited-state intramolecular proton transfer (ESIPT) luminescence was evaluated. Among them, 1 and 2 formed an intramolecular hydrogen-bonded seven-membered ring in the crystalline state and exhibited efficient ESIPT luminescence in the solid state (quantum yield up to 0.45).

Single-crystal UV-vis spectroscopic examination of a striking odd-even effect on structure and chromic behaviour of salicylidene alkylamines.

We have found that a series of N-(5-bromosalicylidene) alkylamines exhibited distinct chromic behaviour depending on the parity of their alkyl chain length. A group with an even number of carbon atoms in the alkyl chain showed photochromism, while another group with odd number showed thermochromism.

Systematic investigations on fused π-system compounds of seven benzene rings prepared by photocyclization of diphenanthrylethenes.

We studied the photoproducts of 1-(n-phenanthryl)-2-(m-phenanthryl)ethenes (nEm; n, m = 1, 3 and 9) for understanding photocyclization patterns based on NMR spectroscopy. The crystal structures of the photoproducts were analyzed by X-ray crystallography, and the photophysical features of the photocyclized molecules were investigated based on emission and transient absorption measurements. Phenanthrene derivatives substituted at the 1- and 3-positions were prepared for synthesizing nEm by photocyclization of stilbene derivatives. We obtained four types of primary photoproducts (n@m) from the corresponding nEm. Two of them were found to have racemic molecular structures in the single crystal determined by X-ray crystallography. Besides the primary photoproducts, two types of secondary photoproducts (n@mPP) were isolated. Fluorescence quantum yields and lifetimes of the obtained photoproducts were determined in solution whereas the definite fluorescence quantum yields were obtained in the powder. Observation of the triplet-triplet absorption spectra in solution by laser photolysis techniques showed that intersystem crossing to the triplet state competes with the fluorescence process.

Synthesis and Properties of Salicylaldehydes Fine-Tuned by Modular Assembly using "Plug-and-Socket"-Type Extendibility.

A salicylaldehyde derivative bearing four pyridine arms, 3,5-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-2,4-dihydroxybenzaldehyde (Hbpsal) as a "socket", was prepared and used to derive a series of zinc complexes with various extra anions "plugged" into their vacant site. The crystal structure and 1 H NMR spectra were noticeably influenced by the extra anions, allowing fine-tuning of the properties by "plug-and-socket"-type modification. Similar to unsubstituted salicylaldehyde, the zinc complexes reacted with primary amines to afford Schiff-base compounds. Because of the potential chirality around the coordination sphere, reaction with a chiral amine resulted in an equilibrium system between diastereomers, the potential of which as chiral sources tunable by the extra anions is discussed. Some of the complexes were further converted into zinc- or nickel-salphen (=N,N-bis(salicylidene)-1,2-phenylenediamine) complexes. The electrochemical properties of the nickel complex were slightly modified by the extra anions, whereas the photophysical properties of the zinc complex appeared unchanged.

Spontaneous ligand nitrosation and self-assembly into a pentacopper metallacrown complex.

Upon mixing acetylacetonedioxime and copper(ii) nitrate in water, the acetylacetonedioxime is spontaneously nitrosated at the central α-carbon and four of the nitrosated ligand molecules and five Cu ions self-assemble into a pentanuclear metallacrown complex, whose structure has been revealed by single crystal X-ray analysis and magnetic interactions between the Cu ions in the complex have been probed. The lability of the core Cu ion in the complex is suggested.

Spectroscopic tracking of Schiff base compounds' hydrogen bonding reorganization associated with solid-to-solid phase transition.

A series of 2,6-dihydroxynaphthalene-1-methylidene alkylamines whose alkyl chain lengths ranged from 9 to 12 was spectroscopically examined. Transmission ultraviolet-visible absorption microspectroscopy revealed that the spectra of solid thin-films of the crystalline samples showed two distinct profiles depending on polymorphs as well as on alkyl chain length. We concluded that these spectral changes occurred not because of conventional intramolecular proton transfer but because of the molecules' interactions with an external proton source, that is, the intermolecular proton transfer. The spectral changes were accompanied by changes in the intermolecular hydrogen bonding network. When a crystal of a sample compound was heated, its spectrum changed dramatically before the crystal underwent a solid-to-solid phase transition to another polymorph. We concluded that these spectral changes indicated strengthening of intermolecular hydrogen bonding or intermolecular proton transfer, which would have triggered a drastic change in the hydrogen bonding network structure.

Spontaneous helical folding of bis(Ni-salphen) complexes in solution and in the solid state: spectroscopic tracking of the unfolding process induced by Na(+) ions.

Four kinds of bis(Ni-salphen) complexes containing a 2-methylenepropane-1,3-diyl linker were synthesised and characterised. Crystal structural analysis revealed that one of the complexes folded into a helical structure, in which two Ni atoms were at a distance of 3.2 Å from each other. NMR measurements suggested that a similar folded state was maintained in solution, and the energy barrier for refolding through an unfolded state was 47 kJ mol(-1). The folded complex also exhibited a significant bathochromic shift in its UV-Vis absorption structure. Addition of sodium ions caused unfolding of the complexes, and the corresponding spectra were attributed to a virtually isolated state. DFT calculations reproduced well the energy barrier and folding-induced bathochromic shift.

Mutual interference between intramolecular proton transfer sites through the adjoining π-conjugated system in Schiff bases of double-headed, fused salicylaldehydes.

We synthesized two constitutionally isomeric bis(iminomethyl)-2,6-dihydroxynaphthalenes, namely, α,α-diimines 1 and ß,ß-diimines 2, which can be formally represented as fused salicylaldimines with resonance-assisted hydrogen-bonding sites. Spectroscopic data show that the OH/OH, NH/OH, and NH/NH forms of 1 were in equilibrium in solution and that the proportion of the NH-bearing tautomers increased as the solvent polarity increased. The UV spectra of thin solid films of 1 with various types of hydrogen-bonding networks differed from one another, and the spectral profiles were markedly temperature dependent, whereas the spectra of 1 in the molten state showed quite similar profiles. In contrast, 2 existed predominantly as the OH/OH form irrespective of the solvent polarity or crystal packing. Quantum chemical calculations suggest that the difference between the probabilities of intramolecular proton transfer in 1 and 2 can be explained in terms of the interplay between the resonance-assisted hydrogen-bonding sites and the adjoining π-conjugated system.

Amphiphilic sulfamide as a low-molecular-mass hydrogelator: A novel mode of 3-D networks formed by hydrogen-bond-directed 2-D sheet assemblies.

Asymmetrically substituted amphiphilic sulfamide, N-tetradecyl-N'-(6-dimethylaminohexyl)-sulfamide (3c) having a dimethylamino group at one end of the side chain, showed a strong ability to form two-dimensional (2-D) sheet-like assemblies by the 2-D hydrogen-bond networks between sulfamide moieties. Upon protonation of the amino group with acid, the cationic ammonium form of 3c induced effective hydrogelation (minimum gelation concentration: 0.5wt%) to yield a translucent, self-standing hydrogel. Infrared (IR) spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies confirmed formation of the fibrous assemblies of the hydrogen-bond-directed 2-D nanosheets in the hydrogel. A novel mode of three-dimensional (3-D) networks was formed by branching and recombination of hydrogen-bond networks and knit-like linkages between the assemblies. The storage and loss moduli of the hydrogel (2wt%) were measured to be in the range of 10(2) and 10(3)Pa, showing relatively high mechanical stability.

Dry micromanipulation of supramolecular giant vesicles on a silicon substrate: highly stable hydrogen-bond-directed nanosheet membrane.

Guanosine derivative 1 forms hydrogen-bond-directed giant vesicles. On a silicon substrate, the vesicles retain their shape and internal water phase even after removal of external water under vacuum. Dry manipulation of the micrometer-sized vesicles was carried out via AFM-tip-induced partition and fusion of the vesicles. For larger vesicles (5-10 µm), external solutions were successfully injected through a microcapillary inserted into the vesicle in air.

Hydrogen-bond-directed giant unilamellar vesicles of guanosine derivative: preparation, properties, and fusion.

By mixing a small volume of THF containing guanosine derivative 1 and tetraethylenegrycol dodecyl ether (TEGDE) with water and subsequently removing TEGDE by gel permeation chromatography, micrometer-sized giant unilamellar vesicles (GUV) of 1 were successfully prepared. The vesicle membrane was a 2-D sheet assembly of thickness 2.5 nm, composed of a 2-D inter-guanine hydrogen-bond network. The GUV dispersion showed high stability because of a large negative zeta potential, which allowed repeated sedimentation and redispersion by centrifugation and subsequent gentle agitation. TEGDE-triggered fusion of GUVs took place within 350 ms, which proceeded by fusion of the vesicle membranes in contact. These unique static and dynamic properties of the GUV membrane assembled by the 2-D hydrogen-bond network are discussed.

Hydrogen-bond-directed 2-D sheet assemblies of sulfamide derivatives: formation of giant vesicles with patchwork-like surface pattern.

Sulfamide derivatives showed high ability to form hydrogen-bond-directed two-dimensional (2-D) sheet assemblies of nanometer thickness. Further, fine-tuning of the side chain structures and preparation conditions allowed for the formation of micrometer-sized giant vesicles of 4b in water by the simple injection method. IR and XRD studies indicated that 4b having tetradecyl and oxyethylene-terminated alkyl side chains formed hydrogen-bond-directed 2-D nanosheet pairs. SEM, AFM, and TEM observation of the dried vesicles revealed that the vesicle membrane was composed of several lamellar-stacked layers of 2-D nanosheets and showed a characteristic patchwork-like pattern on the surface.

Blue/red linear dichroic emission from a highly anisotropic crystal of triarylmethane dye conjugated with phenoxo-zinc complexes.

We have developed a novel triphenylmethane-based hexanuclear zinc complex that exhibits peculiar photochemical and photophysical properties. Upon UV irradiation, the compound turned from colorless to reddish purple, while the color of emission turned from blue to red. The color change was attributed to an oxidation of the ligand part. It was suggested that an intramolecular energy-transfer mechanism operates to give rise to the red emission. The UV treatment of a single crystal results in simultaneous emission of orthogonally polarized blue and red light. This color switching, namely linear dichroic emission was so distinct that one can recognize with by sight through optical microscope. The columnar arrangement of molecules in the crystal clearly accounts for the observed polarization of the emission.

Induction of micronuclei in germinating onion seed root tip cells irradiated with high energy heavy ions.

Effects of high LET charged particles on a perfect in-vivo system are an essential theme for the study of the biological effects of radiation. Germinating onion seeds are independent complete organisms and the radiation induced micronuclei in the root chip cells can be examined quantitatively and theoretically. We irradiated with three types of high energy accelerated heavy ions germinating onion seeds using a synchrotron and observed micronuclei in the root tip cells. Micronuclei induction showed characteristic dose responses of an upward convex bell shape and a steep rise near zero doses for all types of the ions. The bell curve dose responses, however, could be explained by a simple mathematical model. A parameter in the model which indicates micronuclei induction frequency and another parameter which indicates induction frequency of lethal damages (or damages delaying cell divisions) per heavy ion track were both proportional to square of the LET. Because we suspected by-stander effect concerning the dose responses rising steeply near zero doses and tapering off for higher doses, we tested acute irradiation to remove time of information transmittance between cells using a single spill (about 0.3 s) of the synchrotron beam. No difference was detected between normal multiple spill irradiations and single spill.