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.

Optimizing a coarse-grained space for approximate normal-mode vibrations of molecular heterodimers.

We applied the method of coarse-graining the intermolecular vibrations to molecular heterodimers assembled by double hydrogen bonding. This method is based on principal component analysis, by which the original atomic displacement vectors are projected onto a lower-dimensional space spanned by a basis set of translations, librations, and intramolecular vibrations of the constituent molecules. Compared with homodimers, the following points are particularly noted: (1) alignment of the constituent molecules in a non-symmetric atomic arrangement of the whole system and (2) the scheme of reordering the bases to construct an optimal coarse-grained space. We tested three schemes for reordering the intramolecular vibration vectors to determine that the best one is equivalent to size reduction based on the singular value decomposition. The coarse-graining analysis affords three parameters, Φintra, Φinter, and Φapp, which are relevant to the mechanical nature of the molecular assembly. The Φintra values account for the internal stiffness of molecules, while the Φinter values are true stiffness constants of the intermolecular force and show a good correlation with the association energies of the dimers. The Φapp values are the apparent intermolecular stiffness smaller than Φinter, as a result of compensation for neglecting intramolecular vibrations. All these values are consistent with each other under the coupled oscillator model, showing that the present coarse-graining analysis is valid for heterodimers as well as homodimers.

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).

Indices to evaluate the reliability of coarse-grained representations of mixed inter/intramolecular vibrations.

We propose some methods for quantifying the reliability of coarse-grained representations of displacement vectors of normal mode vibrations. In the framework of our basic theory, the original displacement vectors are projected onto a lower-dimensional (i.e., a coarse-grained) space. Four types of functions denoted fidelity indices were introduced as measures of the similarity of the original to the restored displacement vectors. These indices were applied to several hydrogen-bonded homodimers, and the behavior of each index was examined. We found that a coarse-grained representation with high reliability resulted in the accurate restoration of properties such as eigenfrequency, modal mass, and modal stiffness.

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.

Influence of intermolecular interactions on solid state luminescence of imidazopyridines: theoretical interpretations using FMO-TDDFT and ONIOM approaches.

6-Cyano-2-(2'-hydroxyphenyl)imidazo[1,2-a]-pyridine (6CN-HPIP) shows polymorph-dependent luminescence with the three different crystal forms exhibiting the packing-controlled tuning of bright colors, orange, yellow, and red. The distinctive emission in aggregated states was treated with finite cluster models and analyzed by means of quantum chemistry calculations. The influence of structural displacements and intermolecular interactions in the crystalline state on solid state luminescence was examined in detail using the Fragment Molecular Orbital (FMO) scheme, suitable for studies of aggregated molecular systems. The FMO pair interaction analysis of the S1-S0 emission maxima indicated that the intermolecular side-to-side interactions cause hypsochromic shifts; facial interactions induce bathochromic shifts; and crystal packing effects in total induce hypsochromic shifts. The FMO predictions of the emission maxima offered qualitatively satisfactory agreements with the experiments. However, the small cluster models including up to 17 molecules did not reach quantitative convergence, i.e., the emission colour order among them was not well reproduced.

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.

Excited-state intramolecular proton transfer (ESIPT) emission of hydroxyphenylimidazopyridine: computational study on enhanced and polymorph-dependent luminescence in the solid state.

Although 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP) is only weakly fluorescent in solution, two of its crystal polymorphs in which molecules are packed as stacked pairs and in nearly coplanar conformation exhibit bright excited-state intramolecular proton transfer (ESIPT) luminescence of different colors (blue-green and yellow). In order to clarify the enhanced and polymorph-dependent luminescence of HPIP in the solid state, the potential energy surfaces (PESs) of HPIP in the ground (S(0)) and excited (S(1)) states were analyzed computationally by means of ab initio quantum chemical calculations. The calculations reproduced the experimental photophysical properties of HPIP in solution, indicating that the coplanar keto form in the first excited (S(1)) state smoothly approaches the S(0)/S(1) conical intersection (CI) coupled with the twisting motion of the central C-C bond. The S(1)-S(0) energy gap of the keto form became sufficiently small at the torsion angle of 60°, and the corresponding CI point was found at 90°. Since a minor role of the proximity effect was indicated experimentally and theoretically, the observed emission enhancement of the HPIP crystals was ascribed to the following two factors: (1) suppression of efficient radiationless decay via the CI by fixing the torsion angle at the nearly coplanar conformation of the molecules in the crystals and (2) inhibition of excimer formation resulting from the lower excited level of the S(1)-keto state compared to the S(0)-S(1) excitation energy in the enol form. However, the fluorescence color difference between the two crystal polymorphs having slightly different torsion angles was not successfully reproduced, even at the MS-CASPT2 level of theory.

Facile preparation of a fully π-conjugated metallopolymer composed of fused salphen complexes.

Novel π-conjugated compounds composed of fused salphen groups as a repeating unit, embedded with four or an indefinite number of zinc nuclei have been synthesized. The solid-state absorption spectra, IR spectra, and powder XRD patterns of these compounds are investigated and compared with those of analogous oligomers. The presumptive polymer contains at least five units of the salphen complex and its assembly is highly ordered in the solid state.

Evaluation of coupling terms between intra- and intermolecular vibrations in coarse-grained normal-mode analysis: does a stronger acid make a stiffer hydrogen bond?

Using theory of harmonic normal-mode vibration analysis, we developed a procedure for evaluating the anisotropic stiffness of intermolecular forces. Our scheme for coarse-graining of molecular motions is modified so as to account for intramolecular vibrations in addition to relative translational/rotational displacement. We applied this new analytical scheme to four carboxylic acid dimers, for which coupling between intra- and intermolecular vibrations is crucial for determining the apparent stiffness of the intermolecular double hydrogen bond. The apparent stiffness constant was analyzed on the basis of a conjunct spring model, which defines contributions from true intermolecular stiffness and molecular internal stiffness. Consequently, the true intermolecular stiffness was in the range of 43-48 N m(-1) for all carboxylic acids studied, regardless of the molecules' acidity. We concluded that the difference in the apparent stiffness can be attributed to differences in the internal stiffness of the respective molecules.

Comparison of the spectroscopic properties of π-conjugated, fused salphen triads embedded with Zn-homo-, Ni-homo-, and Ni/Zn-heteronuclei.

Stepwise condensation reactions of 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde and a phenylenediamine with concomitant binding of metal ions afforded a trinuclear complex of a fully π-conjugated, fused salphen ligand. By changing the synthetic pathway, we obtained a series of homo- and heteronuclear complexes containing selected combinations of nickel(II) and zinc(II) ions. Comparison of the trinuclear complexes' spectroscopic features with those of analogous dinuclear complexes revealed that the absorption spectrum of each trinuclear complex is composed of a salphen-centered absorption at 400 nm and a naphthalene-centered absorption around 500-600 nm, suggesting that the π-conjugated system is divided into several compartments, each of which independently undergoes electronic excitation. Molecular orbital calculations revealed that the formal fusion of the salphen moieties increases the highest occupied molecular orbital (HOMO) level by ∼0.4 eV, which in turn causes the low-energy absorption observed in the spectra. In contrast, interorbital interactions mediated by the N(2)O(2) metal coordination site are small, even though this site is bridged by an o-phenylene linkage. These results suggest that the coordination site effectively breaks electronic communication between the compartments, which in turn affect various spectroscopic properties of the π-conjugated metallo-polysalphens.