Toshiki Mutai

Piezochromic luminescence of amide and ester derivatives of tetraphenylpyrene—role of amide hydrogen bonds in sensitive piezochromic response

A series of amide and ester derivatives of 1,3,6,8-tetraphenylpyrene (TPPy) were prepared, and their solid-state luminescence, molecular packing, and piezochromic responses were studied in order to improve and refine the design concept of piezochromic luminescent materials based on the use of multiple hydrogen bonds and close packing of disk-shaped aromatic cores as competing factors in the control of molecular packing. The amide derivative of TPPy having hexyl side chains (1b) showed a sensitive luminescence response at a relatively low applied pressure, which was ascribed to the hydrogen bond-directed H-type columnar packing. Application of higher pressure induced considerable destruction of the hydrogen bond-directed structure, leading to the state where close packing was the overriding factor directing the molecular assembly and showed similar piezochromic response to that of the ester derivative. Based on the results, the role of the amide hydrogen bonds and the side chains was discussed in detail.

Mechanochromic luminescent liquid crystals based on a bianthryl moiety

Smectic liquid crystals consisting of an aromatic moiety based on 10,10′-bis(phenylethynyl)-9,9′-bianthryl at the central part and one and two mesogenic moieties on each side connected through oxyethylene spacers have been prepared to develop stimuli-responsive materials. The compound having two mesogenic moieties on each side (i.e. four total) forms an ordered smectic phase. Mechanochromic luminescence is observed for the smectic liquid crystal. A sample of this molecule formed by thermal annealing shows green photoluminescence at room temperature. The photoluminescent color changes from green to blue-green with mechanical stimulation at room temperature. The change of the photoluminescent color by mechanical shearing is ascribed to the disturbance of the π–π interactions between the adjacent molecules. Moreover, the original green photoluminescent color is recovered by heating followed by cooling to room temperature. The effects of the number, and thus volume of mesogenic moieties were also examined. Smaller mesogenic moieties lead to stronger ground state interactions between adjacent luminescent molecules and red-shifted emission. In contrast, the compound having three mesogenic moieties on each side shows no mechanochromic luminescence. The large mesogenic moieties disturb intermolecular interactions, resulting in no detectable change in the assembled structure of luminescent moieties upon mechanical stimulation. These results suggest that the present molecular design is useful for fabricating new stimuli-responsive materials with liquid-crystalline behavior.

6-Amino-2,2′-bipyridine as a new fluorescent organic compound

6,6′-Diamino-2,2′-bipyridine (1a) has been found to exhibit a strong fluorescence in the near-UV region. Some amino and/or chloro substituted bipyridines (bpys) have been synthesized and studied to show that only 6-amino-substituted derivatives exhibited a strong emission. The emission of 6-amino-6′-chloro-bpy (3a) was the strongest (λmax= 429.0 nm; Φ= 0.78 in ethanol) among them. On the other hand, little or no emission was observed for monochloro-, dichloro- and 4-amino- derivatives.

Phenyl-substituted 2,2′:6′,2″-terpyridine as a new series of fluorescent compounds—their photophysical properties and fluorescence tuning

Several phenyl-substituted 2,2′:6′,2″-terpyridines (tpy) were synthesized and it was found that 4′-phenyl tpy (ptp, 3) exhibited the most effective fluorescence, whose quantum yield was up to 0.64 in cyclohexane. For further study on tuning the fluorescence properties of ptp, different substituents were introduced into the p-position of the phenyl group. While Br- 10, Cl- 11, and CH3-ptp 12 showed their absorption and fluorescence in the same region as 3, those of NH2- 14 and Me2N-ptp 15 were observed at much longer wavelengths. In addition, fluorescence maxima of 14 and 15 showed large (>130 nm) solvent dependence. The difference between ground and excited state dipole moment (Δμ) for 15 was estimated to be 15.2 D by the Lippert–Mataga equation, indicating the intramolecular charge transfer (ICT) process. Semi-empirical MO calculation (MOPAC/AM1) demonstrated that the HOMO-1, HOMO and LUMO of 3, 10–12 were mainly localized on the phenyl (πph), tpy (πtpy) and tpy (π*tpy) part, respectively, indicating that the lowest energy absorption band of 3, 10–12 was the local excitation (πtpy–π*tpy). In the case of 14 and 15, which have an electron-donating substituent, πph instead of πtpy became the HOMO. Thus, the lowest energy absorption of 14 and 15 was an ICT transition (πph–π*tpy), and a large red shift of the fluorescence occurred. In these compounds, the energy level of πph is controlled without affecting that of πtpy and π*tpy, suggesting a novel approach for tuning the color of fluorescence.

Three-color polymorph-dependent luminescence: crystallographic analysis and theoretical study on excited-state intramolecular proton transfer (ESIPT) luminescence of cyano-substituted imidazo[1,2-a]pyridine

Three solid-state luminescence colors, yellow, orange, and red, can be achieved by controlling the crystalline polymorphs of 6-cyano-2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine (2). This study investigates the relationship between the emission properties and the crystal structure of 2. All luminescence is assigned as a singlet excited-state intramolecular proton transfer (ESIPT) emission. X-ray crystallographic analyses of the three crystals show that there are remarkable differences in the molecular packing: herringbone-like, antiparallel dimer stacking and two slip-stacked, parallel stacking modes, with similar coplanar molecular conformation. Density functional theory (DFT) calculations show that the dipole moment of the ground state enol form is much smaller (1.66 D) than that of the parent compound 1 (5.40 D), which may be the reason why parallel stacking is energetically allowed. The luminescence colors are well reproduced from quantum chemical calculations of the intramolecular proton transfer (IPT) species, which are optimized by the two-layer ONIOM cluster models extracted from the corresponding crystal structure. The results indicate that the intermolecular interactions of the π-stacked IPT and enol molecules are a decisive factor in the emission energy of the crystalline polymorphs. Furthermore, the dipole moments of the excited (4.99 D) and ground states (3.70 D) of the IPT species are found to orient in a high-angled manner (ca. 150°). Therefore, the energy levels of the two states shift differently upon environmental variation, resulting in a change in the luminescence energy. Thus, the three-color, polymorph-dependent luminescence of 2 is rationally explained with crystallographic analyses and quantum chemical simulations. The results presented here will contribute to understanding the structure–property relationships of solid-state luminescence at the molecular level and further design of new polymorph-dependent luminescent materials.

Solid-state luminescence of tetraphenylpyrene derivatives: mechano/vapochromic luminescence of 1,3,6,8-tetra(4′-carboxyphenyl)pyrene

The effect of mechanical stress on the solid-state luminescence of 3, a carboxylic acid derivative of 1,3,6,8-tetraphenylpyrene (TPPy), is studied. Grinding the powdery solid of 3 in a mortar results in a blue shift of luminescence (from yellow to green) instead of the red shift observed for amide (1) and ester (2) derivatives of TPPy. The initial yellow luminescence is recovered by exposure to solvent vapour. In contrast, application of compressive stress using an oil press causes practically no change of the initial yellow luminescence of 3. The yellow luminescence is ascribed to the dimeric form of 3, while the green luminescence is ascribed to the monomeric state. The macroscale shear stress applied by grinding induces the dimer-to-monomer transition at the molecular level, which causes the blue shift of the luminescence. Based on the results, the mechanisms of macroscale compressive and shear stresses to induce alteration of nanoscale molecular packing and luminescent properties are discussed.

The development of aryl-substituted 2-phenylimidazo[1,2-a]pyridines (PIP) with various colors of excited-state intramolecular proton transfer (ESIPT) luminescence in the solid state

A series of solid-state luminescent dyes based on 2-phenylimidazo[1,2-a]pyridine (PIP) displaying a wide range of emitting colors from blue to red have been developed. Whereas 2′-methoxy PIP (2′MeOPIP, 10) shows blue luminescence, 2′-hydroxy PIP (HPIP, 1) exhibits emission with large Stokes shift at around 500 nm that is known as the excited-state intramolecular proton transfer (ESIPT) luminescence, which can be tuned from blue-green to red by simply introducing aryl group(s) into HPIP through Pd-catalyzed cross coupling reactions. It is shown that the energy of ESIPT luminescence decreases as the electron-withdrawing nature of the para-substituent on the aryl group increases. Varying the substitution position is also an effective tuning method, because the ESIPT luminescence wavelength is in the order 6-aryl < 8-aryl < 6,8-diaryl. Although the quantum yields of these compounds are quite low in organic solutions (Φ ∼ 0.01), they generally display a much stronger ESIPT luminescence in the solid state. For all compounds except for 9 having long C6-alkyl chains, the similar emission properties in the dilute frozen matrix and the solid state indicated that ESIPT emission in the solid state is from the monomeric species, even though π-stacked motifs of the HPIP cores and the aryl groups introduced are confirmed by crystallographic analysis. Time-dependent DFT calculations reasonably explained the effect of substitution on ESIPT luminescence in the solid state. The results show that aryl-substitution is a convenient approach to tuning the radiation energy of the ESIPT luminescence of HPIPs without suffering the quenching effect due to intermolecular interactions, and thus a series of PIP compounds that exhibit a wide range of luminescence colors can be realized.

Sterically induced polymorphism: ON–OFF control of excited-state intramolecular proton transfer (ESIPT) luminescence of 1-methyl-2-(2′-hydroxyphenyl)benzimidazole

By introduction of ‘appropriate’ steric hindrance into 2-(2′-hydroxyphenyl)benzimidazole, the obtained compound forms polymorphic crystals based on either intramolecular or intermolecular hydrogen bonding, which respectively show strong ESIPT luminescence (ON state; Φ = 0.74) and practically no luminescence (OFF state; Φ = 0.007). Further, the luminescence can be switched off and on by heating and grinding.

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.

Packing-directed tuning and switching of organic solid-state luminescence

Although many fluorescent organic dyes suffer serious concentration quenching in the solid state, novel types of luminophore showing intense solid state emission have developed in recent decades. Many of them are non-emissive or only weakly emissive in solution but become highly emissive upon formation of aggregates or in the solid state by suppression of the radiationless decay pathways. Furthermore, specific molecular conformation and arrangement in the solid state can effectively control the intensity and colour of luminescence by a variety of mechanisms, leading to development of tuneable and switchable solid-state luminophores by the mode of molecular packing. Their applications are also discussed.