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.

Aggregation-induced emission effect on turn-off fluorescent switching of a photochromic diarylethene.

Background: Diarylethenes are well-known photochromic compounds, which undergo cyclization and cycloreversion reactions between open- and closed-ring isomers. Recently, diarylethene derivatives with photoswitchable fluorescent properties were prepared. They are applicable for fluorescence imaging including bio-imaging. On the other hand, a new system called "excited state intramolecular proton transfer (ESIPT)" is reported. In the system, absorption and emission bands are largely separated due to the proton transfer, hence it showed strong fluorescence even in the crystalline state. We aimed to construct the photochromic system incorporating the ESIPT mechanism. Results: A diarylethene incorporating a fluorescent moiety that exhibit ESIPT behavior was prepared. The ESIPT is one of the examples which express the mechanisms of aggregation-induced emission (AIE). This compound emits orange fluorescence with a large Stokes shift derived from ESIPT in aprotic solvents such as THF or hexane, while it exhibits only a photochromic reaction in protic solvents such as methanol. In addition, it shows turn-off type fluorescence switching in an aprotic solvent and in crystals. The fluorescence is quenched as the content of closed-ring isomers increases upon UV light irradiation. Conclusions: A diarylethene containing an ESIPT functional group was prepared. It showed fluorescent turn-off behavior during photochromism in aprotic solvents as well as in crystalline state upon UV light irradiation. Furthermore, it showed AIE in THF/water mixtures with blue-shift of the emission.

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).

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.

Excited-State Intramolecular Proton Transfer and Global Aromaticity.

A general survey of excited-state intramolecular proton transfer (ESIPT) processes was made from the viewpoint of global aromaticity. For most ESIPT processes studied, a tautomeric product in the first excited electronic state was found to have a larger topological resonance energy (TRE) than the reactant in the same excited state. Conversely, if a transient tautomer is much less aromatic in the excited state than the reactant, an appreciable aromaticity-imposed energy barrier to the reaction will result. Thus, excited-state aromaticity is a very important factor, although not a definitive one, in determining the allowedness of ESIPT.

Colorless, transparent, dye-doped polymer films exhibiting tunable luminescence color: controlling the dual-color luminescence of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine derivatives with the surrounding matrix.

Colorless, transparent, polymer films including 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP) derivatives, 1 and 2, are prepared by a spin-coating method. The observed emission spectra upon photoexcitation of these polymer films were composed of dual emission bands: the normal luminescence at 370-410 nm (purple) and the greatly Stokes-shifted emission at 520-580 nm (yellow) assigned as the excited-state intramolecular proton transfer (ESIPT) luminescence. Relative intensity of the two emissions varied according to the polymer matrices, resulting in change in the luminescent color of the dye-doped polymer films. Particularly, the luminescence properties of 6-cyano HPIP, 2, are highly susceptible to the surrounding environment, and therefore successfully tuned to produce a wide range of colors, from purple to orange, by changing its concentration within and the type of the polymer matrix. This observation can be ascribed to the formation of a relatively weak intramolecular hydrogen bond resulting from the electron-withdrawing 6-cyano group. Thus, we demonstrate large variations in emission color can be achieved using interactions of the single component with the surrounding matrix. These results offer promise as a convenient and effective method for a wide-range tuning the luminescence colors of dye-doped polymer films.

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.

Tuning of excited-state intramolecular proton transfer (ESIPT) fluorescence of imidazo[1,2-a]pyridine in rigid matrices by substitution effect.

2-(2'-Hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP, 1) and its derivatives are synthesized, and their fluorescence properties are studied. Although all the compounds show faint dual emission (Φ ≈ 0.01), which is assigned to the normal and excited-state intramolecular proton transfer (ESIPT) fluorescence in a fluid solution, they generally display efficient ESIPT fluorescence (Φ up to 0.6) in a polymer matrix. The introduction of electron-donating and electron-withdrawing groups into the phenyl ring causes blue and red shifts of the ESIPT fluorescence emission band, respectively. On the other hand, the introduction of such groups into the imidazopyridine part results in fluorescence shifts in the opposite directions. The results of ab initio quantum chemical calculations of the intramolecular proton-transferred (IPT) state are well in line with the ESIPT fluorescence energies. The plots of the calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels against the Hammett substituent constants (σ) show good linearity with different slopes, which can rationalize the effect of the substituent and its position on the IPT state. Therefore, we have developed a series of HPIPs as new ESIPT fluorescent compounds and demonstrate that ESIPT fluorescence properties would be rationally tuned using quantum chemical methods.

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.

Propylamino-connected fluorescent terpyridine dimer and trimer: syntheses, photophysical properties and formation of duplex-type complexes with Cd(II).

By using propylamine as a connector, a dimer (L2) and trimer (L3) of 2,2':6',2''-terpyridine (tpy) are synthesized. They showed the lowest energy π­π* absorption at 354 nm and blue fluorescence at 420 nm with a moderate quantum yield in solution. Spectral titrations, electrospray ionization mass spectrometry (ESI-MS), (1)H­(1)H rotating-frame nuclear Overhauser effect correlation spectroscopy (ROESY), and structure simulation (MOPAC/PM5) indicated that both L2 and L3 form duplex-type multi-component complexes with Cd(II), [Cd(2)(L2)(2)](4+) and [Cd(3)(L3)(2)](6+), respectively. While the luminescence of these complexes was quite weak in solution (quantum yield Φ < 0.005), the complexes showed enhanced emission in the solid state with remarkable increase of the quantum yield (Φ = 0.13).

Fabrication of colorless organic materials exhibiting white luminescence using normal and excited-state intramolecular proton transfer processes.

Organic, white luminescent materials were fabricated using a mixture of proton-transfer and nonproton-transfer fluorophores. 2'-Methoxy and 2'-hydroxy derivatives of 2-phenylimidazo[1,2-a]pyridine (PIP) have similar UV-absorption properties; however, they exhibit mechanistically different luminescence respectively ascribable to the normal (∼420 nm) and excited-state intramolecular proton transfer processes (∼530 nm) in the solid state. UV-irradiation of mixed solids excites both components concurrently and results in efficient white luminescence composed of two independent emissions without involving energy transfer process. White luminescent solids are easily transformed into vapor-deposited films under mild conditions, and a colorless and transparent thin film by dissolving in PMMA.

Tuning of fluorescence properties of aminoterpyridine fluorophores by N-substitution.

Several N-alkyl and N-phenyl derivatives of 6-amino- () and 6,6'-diamino-2,2':6',2''-terpyridine () were synthesized, and their fluorescence properties were studied. A successive red-shift was observed as the number of the N-substituted groups increased. It was also shown that the susceptivity of the fluorophores to a solvent varied considerably according to the mode of the N-substitution. While the monoamino-tpys (tpy: 2,2':6',2''-terpyridine) suffered almost complete quenching of their fluorescence in ethanol, the fully N-alkylated diamino-tpys and retained their fluorescence. The results show that N-substitution is a useful way to tune both the radiation energy and solvent susceptivity of the fluorescence of the amino-tpys.

Reproducible on-off switching of solid-state luminescence by controlling molecular packing through heat-mode interconversion.

Organic luminescent solids are attracting increasing interest in various fields of application. Modification or alteration of the chemical structures of their component molecules is the most common approach for tuning their luminescence properties. However, for dynamic tuning or switching of solid-state luminescence with high efficiency and reproducibility successful examples are limited as chemical reactions in the solid state frequently encounter insufficient conversion, one-way reactions or loss of their luminescence properties. One promising approach is to control the luminescence properties by altering the mode of solid-state molecular packing without chemical reactions. Here, we show that 2,2':6',2''-terpyridine, practically non-luminescent in the form of amorphous solid or needle crystal, shows strong blue luminescence upon formation of a plate crystal. Efficient and reproducible on-off switching of solid-state luminescence is demonstrated by heat-mode interconversion between the plate and needle crystals. Because alteration of the mode of molecular packing does not require chemical reactions, the present findings would open the way for the development of novel organic luminescent solids that can be switched on and off by external thermal stimuli.

A distance-controlled oligopeptide linker as a novel photo-induced energy transfer switch by secondary structural transition.

By using an oligopeptide chain as a functional linker and introducing coumarin 2 and coumarin 343 at the chain ends, an effective photo-induced energy transfer system was constructed and energy transfer from coumarin 2 to coumarin 343 was switched on and off by a solvent-induced helix-coil secondary transition of the oligopeptide chain.