Yasuhiro Shigemitsu

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.

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.

Synthesis, solid-state fluorescence properties, and computational analysis of novel 2-aminobenzo[4,5]thieno[3,2-d]pyrimidine 5,5-dioxides

Summary New fluorescent compounds, benzo[4,5]thieno[3,2-d]pyrimidine 5,5-dioxides (3a–g), 2-amino-4-methylsulfanylbenzo[4,5]thieno[3,2-d]pyrimidine (6), and 2-amino-4-methylsulfanyl-7-methoxybenzo[4,5]furo[3,2-d]pyrimidine (7), were synthesized in good yields from heterocyclic ketene dithioacetals (1a–c) and guanidine carbonate (2a) or (S)-methylisothiourea sulfate (2b) in pyridine under reflux. Among the fused pyrimidine derivatives, compound 3c, which has an amino group at the 2-position and a benzylamino group at the 4-position of the pyrimidine ring, showed the strongest solid-state fluorescence. The absorption and emission properties of the compounds were quantitatively reproduced by a series of ab initio quantum-chemical calculations.

DEVELOPMENT OF FLUORESCENT 2-PYRONE DERIVATIVES USING KETENE DITHIOACETALS FOR ORGANIC EL DEVICES

In recent years, organic electro luminescent (EL) materials have received considerable attention due to their potential application in next-generation display devices with high brightness levels, wide color ranges, and wide viewing angles. Fluorescent 2-pyrone derivatives have been synthesized using a convenient method involving ketene dithioacetals, which emit fluorescence (red, green, and blue) in both solution (dichloromethane and ethanol) and solid state. The structure-activity relationships of various 2-pyrone derivatives have been clarified.

Synthesis and electronic spectra of novel merocyanine dyes bearing a maleimide ring incorporated into the methine chains

Abstract The present article describes the synthesis of new polymethine dyes with a heterocyclic ring incorporated into the methine chain. 1-Alkyl-2 or 4-methylpyridinium salts and related compounds 1a–e reacted at the exocyclic double bond with cyano- or methoxycarbonyl heterocycles bearing a methylthio group 2a–d to give new polymethine dyes 3a–n in good yield. This reaction occurred by an addition-elimination mechanism, involving nucleophilic attack of cyclic enamines at carbon deficient atoms on the heterocycles followed by elimination of methylmercaptan. The polymethine dyes obtained are red, violet, and blue in color with the absorption peaks at 524–614 nm. Semi-empirical as well as ab-initio quantum chemical calculations were used to theoretically characterize their π−π* absorption maxima in the visible region which plays a decisive role in their color appearances.

Electronic spectra of cycl[3.3.2]azine and related compounds: solvent effect on vibronic couplings.

Quantitative ab initio calculations are presented for the ultraviolet-visible peaks of cycl[3.2.2]azine and its mono- and dibenzannulated polycyclic compounds at the multistate CASPT2 (MS-CASPT2) level of theory, with 11 nm deviation from the experimental S0 → S1 absorption. The electrophilic substitution reactions of cycl[3.2.2]azine, benzo[a]/[g]annulated cycl[3.2.2]azines, and 6-dimethylamino[2.2.3]cyclazine-1-carboxylates with 3-cyano-4-methylthiomaleimide gave the corresponding functionalized cycl[3.2.2]azine derivatives, which exhibited the absorption maxima around 510-630 nm. The first intense peaks were investigated by means of time-dependent density functional theory (TD-DFT). These peaks were systematically underevaluated by ∼50 nm, within the acceptable accuracies of TD-DFT. Furthermore, we calculated vibronic coupling constants of the electronic excited states of cycl[3.2.2]azine and simulated absorption spectra both in vacuo and in ethanol. The solvent effect is found to enhance oscillator strengths and vibronic couplings. This is because the solvent effect gives rise to changes in the electron density difference on the phenyl ring, and in turn, the intensified overlap between the electron density difference and the potential derivative in the phenyl ring leads to enhanced vibronic couplings in ethanol.

Synthesis of Quinolizino[3,2-a]quinolizine Derivatives and Their Fluorescence

Polycyclic quinolizine derivatives, quinolizino[3,2-a]quinolizines (4a-c), were synthesized in moderate yields by a one-pot method using ketene dithioacetal, methyl bis(methylsulfanyl)methylene-cyanoacetate (2), alkyl 2-pyridylacetates (1a, b), and 2-pyridylacetonitrile (1c). Compounds 4a, b exhibited red fluorescence (Em max: 576 nm) in solid state.

One-Pot Synthesis of 6-Substituted Amino-2,4-diaminopyrimidine Derivatives Using Ketene Dithioacetals with Amines and Guanidine Carbonate

6-Substituted amino-2,4-diaminopyrimidine derivatives were prepared by one-pot synthesis using ketene dithioacetals, amine compounds, and guanidine carbonate in pyridine. These pyrimidine products displayed blue fluorescence in the solid state.

Computational Analysis of Solute–Solvent Coupling Magnitude in the Z/E Isomerization Reaction of Nitroazobenzene and Benzylideneanilines

The dynamic solvent effect often arises in solution reactions, where coupling between chemical reaction and solvent fluctuation plays a decisive role in the reaction kinetics. In this study, the Z/E isomerization reaction of nitoroazobenzene and benzylideneanilines in the ground state was computationally studied by molecular dynamics simulations. The non-equilibrium solvation effect was analyzed using two approaches: (1) metadynamics Gibbs energy surface exploration and (2) solvation Gibbs energy evaluation using a frozen solvation droplet model. The solute–solvent coupling parameter (Ccoupled) was estimated by the ratio of the solvent fluctuation Gibbs energy over the corresponding isomerization activation Gibbs energy. The results were discussed in comparison with the ones estimated by means of the analytical models based on a reaction–diffusion equation with a sink term. The second approach using a frozen solvation droplet reached qualitative agreement with the analytical models, while the first metadynamics approach failed. This is because the second approach explicitly considers the non-equilibrium solvation in the droplet, which consists of a solute at the reactant geometry immersed in the pre-organized solvents fitted with the solute at the transition state geometry.

Accelerated Molecular Dynamics Study of Z/E Isomerization of Azobenzene: Kramers’ Theory Validation

The reaction rate in a condensed phase is retarded by solvent friction and deviates from the transition state theory (TST) in a highly viscous solvent. The Z/E isomerization kinetics of 4-dimethylamino-4′-nitroazobenzene in solution have been experimentally analyzed in a wide range of pressure in our previous studies. Herein the non-TST behaviors of the rate constant were theoretically modelled by means of Kramers’ theory and its extensions and by molecular dynamics (MD) simulations. The Z/E isomerization takes place in milliseconds or longer and so is not reachable by normal MD simulations. The accelerated MD method was employed to simulate the rare events using a biased isomerization potential barrier.