Kiyull Yang

Blue Phosphorescent Ir(III) Complex with High Color Purity: fac-Tris(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(III)

A blue phosphorescent iridium(III) complex (1) bearing fluorine-substituted bipyridine (dfpypy) has been synthesized and characterized to investigate the effect of the substitution and replacement of the phenyl ring in ppy (phenylpyridine) with pyridine on the solid state structure and its photoluminescence. The optical properties and electrochemical behaviors of 1 have also been systematically evaluated. The structure of 1 has also been determined by a single-crystal X-ray diffraction analysis. There are varied intermolecular interactions caused by the pyridine and fluorine substituents, such as C-H...N, C-H...F, and pi...pi interactions of either face-to-face type or edge-to-face C-H...pi and halogen...pi in crystal packing. In electrochemistry, the remarkably higher oxidation potential than that of FIrpic was observed. The emission lambda(max) of 1 at room temperature is at 438 nm with a higher PL quantum efficiency. Complex 1 exhibits intense blue emission with high color purity (CIE x = 0.14, y = 0.12), which has been attributed to metal-to-ligand charge-transfer triplet emission based on DFT calculations.

Alkali metal ion catalysis and inhibition in nucleophilic displacement reactions at phosphorus centers: ethyl and methyl paraoxon and ethyl and methyl parathion.

We report on the ethanolysis of the P=O and P=S compounds ethyl and methyl paraoxon (1a and 1b) and ethyl and methyl parathion (2a and 2b). Plots of spectrophotometrically measured rate constants, kobsd versus [MOEt], the alkali ethoxide concentration, show distinct upward and downward curvatures, pointing to the importance of ion-pairing phenomena and a differential reactivity of free ions and ion pairs. Three types of reactivity and selectivity patterns have been discerned: (1) For the P=O compounds 1a and 1b, LiOEt > NaOEt > KOEt > EtO-; (2) for the P=S compound 2a, KOEt > EtO- > NaOEt > LiOEt; (3) for P=S, 2b, 18C6-crown-complexed KOEt > KOEt = EtO(-) > NaOEt > LiOEt. These selectivity patterns are characteristic of both catalysis and inhibition by alkali-metal cations depending on the nature of the electrophilic center, P=O vs P=S, and the metal cation. Ground-state (GS) vs transition-state (TS) stabilization energies shed light on the catalytic and inhibitory tendencies. The unprecedented catalytic behavior of crowned-K(+) for the reaction of 2b is noteworthy. Modeling reveals an extreme steric interaction for the reaction of 2a with crowned-K(+), which is responsible for the absence of catalysis in this system. Overall, P=O exhibits greater reactivity than P=S, increasing from 50- to 60-fold with free EtO(-) and up to 2000-fold with LiOEt, reflecting an intrinsic P=O vs P=S reactivity difference (thio effect). The origin of reactivity and selectivity differences in these systems is discussed on the basis of competing electrostatic effects and solvational requirements as function of anionic electric field strength and cation size (Eisenmans theory).

Spectroscopic Properties of Fluoroquinolone Antibiotics in Water–methanol and Water–acetonitrile Mixed Solvents¶

Abstract The fluorescence properties of ofloxacin (OFL), norfloxacin (NOR) and flumequine (FLU) were studied in H2O–CH3OH and H2O–CH3CN mixed solvents because these solvents were thought to behave as a biological mimetic system. The emission spectra of OFL and NOR were very sensitive to the composition of the solvents. In the Lippert–Mataga analysis of the steady-state fluorescence data of OFL and NOR, clear reverse solvatochromism was exhibited in both mixed solvents. This observation can be explained by the twisted excited-state intramolecular charge transfer, which is accelerated by water. Theoretical treatments further support these results. The radiative and nonradiative rate constants were analyzed as a function of solvent dipolarity–polarizability (π*) and hydrogen-bond donor acidity (α). These results were well consistent with the suggested mechanism of the excited-state chemical process of OFL and NOR, which depended upon the solvent–solute interactions such as bulk dielectric effects and specific hydrogen-bonding interactions. However, the influence of dielectric effects was more significant. The solvent structures of H2O–CH3CN and the preferential solvation by water were also examined. The emission spectra of FLU do not exhibit any characteristic responses to the properties of the environment.

Spectroscopic Properties of Various Quinolone Antibiotics in Aqueous–organic Solvent Mixtures¶

Abstract The spectroscopic properties of enoxacin (ENO), oxolinic acid (OXO) and nalidixic acid (NAL) were studied in various H2O–CH3OH and H2O–CH3CN mixed solvents because these solvents were thought to behave as a biological mimetic system. ENO has piperazinyl group, but OXO and NAL do not have this substituent. The fluorescence emission spectra of ENO were very sensitive to the composition of the solvents. In the Lippert–Mataga analysis of the steady-state fluorescence data, clear reverse solvatochromism was exhibited for ENO in both mixed solvents. This observation can be explained using the excited state twisted intramolecular charge transfer (TICT) from the nitrogen of the piperazinyl group to the keto oxygen. Theoretical calculations further support this observation. The nonradiative and radiative rate constants of these molecules were analyzed as a function of dipolarity–polarizability (π*) and hydrogen bond donor acidity (α) of the mixed solvents. These results for ENO were consistent with the suggested mechanism of the TICT very well. The influence of bulk dielectric effect was more significant relative to the specific hydrogen bonding interactions. The emission spectra of OXO and NAL do not exhibit any characteristic responses to the properties of the solvent.

Theoretical studies on the formation mechanism and explosive performance of nitro‐substituted 1,3,5‐triazines

To develop new highly energetic materials, we have considered the design of molecules with high nitrogen content. Possible candidates include 1,3,5‐triazine derivatives. In this work, we studied potential synthetic routes for melamine using the MP2/6‐31+G(d,p)//B3LYP/6‐31G(d) level of theory. The mechanisms studied here are stepwise mechanism beginning with the dimerization of cyanamide and one‐step termolecular mechanism. The same type of mechanism is also applied to nitro‐substituted 1,3,5‐triazines. Values for the heat of formation in the solid phase were predicted from density functional theory calculations. Densities were estimated from a regression equation obtained by molecular surface electrostatic potentials. The Cheetah program was used to study the explosive performance of these compounds. In this study, we found that the explosive properties of 2‐amino‐4, 6‐dinitro‐1, 3,5‐triazine (ADNTA), and 2,4,6‐trinitro‐1,3,5‐triazine (TNTA) are similar to those of RDX and HMX, respectively.

Ab initio and DFT studies on hydrolyses of phosphorus halides

Hydrolyses of phosphorus halides, (RO)2POX where R = H or Me and X = F or Cl, in the gas phase and in the reaction field have been investigated theoretically with ab initio and the density functional theory (DFT). The free energy of activation in the reaction field was also estimated using the Onsager method with a correction of entropy change and basis set superposition error (BSSE). The reaction of (MeO)2POF proceeds through a path with bifunctional catalysis regardless of the medium, but the reaction of (MeO)2POCl proceeds through bifunctional and general base catalysis in the gas phase and in water, respectively. The estimated free energy barrier of 23 kcal/mol for the hydrolysis of (MeO)2POF is in good agreement with the experimental values of 24 kcal/mol, and relative barrier of 3 kcal/mol to the (MeO)2POCl is also in good agreement with the experimental values of 5 kcal/mol of diisopropyl phosphorus halides ((PriO)2POX, X = F and Cl).

Limitations of the transition-state variation model. Part 3. Solvolyses of electron-rich benzenesulphonyl chlorides

Rate constants for solvolyses of 4-methoxy-2,6-dimethylbenzenesulphonyl chloride 3(Z = OMe) and of 4-methyl- and 4-methoxybenzenesulphonyl chlorides 4(Z = Me and OMe) are reported for aqueous binary mixtures with acetone, ethanol and methanol. Some additional rate constants are reported for aqueous acetonitrile and dioxane mixtures, as well as product selectivities (S) in alcohol–water mixtures. For each binary mixture, rates of solvolyses of 3(Z = OMe)vs. YCl or Y are approximately bilinear. As water is added to alcohol, S values for solvolyses of 3(Z = OMe) pass through a maximum and for solvolyses of 4-methoxybenzenesulphonyl chloride 4(Z = OMe) the position of the maximum shifts to more aqueous media. For solvolyses of 4-methylbenzenesulphonyl chloride 4(Z = Me), S values are shifted such that they reach a plateau rather than a maximum, and the rate–rate profiles with YCl are approximately linear rather than bilinear. All of the rate–rate profiles show ‘dispersion’ into separate correlations for the various binary mixtures. These substituent effects follow the same trends as corresponding solvolyses of benzoyl chloride and strengthen recent proposals that solvolyses of 3(Z = Me) proceed via competing (dual) reaction channels.

Hydration of acetone in the gas phase and in water solvent

In water solvent, the hydration of acetone proceeds by a cyclic (cooperative) process in which concurrent C–O bond formation and proton transfer to oxygen take place through a solvent and (or) catalyst bridge. Reactivity is determined primarily by the concentration of a reactant complex and not the barrier from this complex. This situation is reversed in the gas phase; although the concentrations of reactive complexes are much higher than in solution, the barriers are also higher and dominant in determining reactivity. Calculations of isotope effects suggest that multiple hydron transfers are synchronous in the gas phase to avoid zwitterionic transition states. In solution, such transition states are stabilized by solvation and hydron transfers can be asynchronous.