Effect of Temperature and Water Concentration on CO₂ Absorption by Tetrabutylphosphonium Formate Ionic Liquid

The CO₂ absorption capacities for binary solutions of tetrabutylphosphonium formate ionic liquid (IL) and water are measured at 0.1 MPa of CO₂ for various compositions (up to 10 in mole ratio of water to the IL) as a function of temperature (−24 to 60 °C). The capacities measured as the mole of CO₂ with respect to 1 mol of the IL varied in a wide range from 0.01 to 1.0. Capacities decreased monotonously with temperature at a fixed absorbent composition. When the water concentration is varied at a fixed temperature, capacity takes a maximum when the water mole ratio is approximately 1. In contrast to CO₂ absorbents made from acetate ILs, the present system loses affinity to CO₂ in the absence of water. The equilibrium constant for the chemisorption is defined in two ways by assuming that CO₂ is captured as a free bicarbonate ion (HCO₃^–) or a complex ion with formic acid ([(HCOO) (HCO₃)­H] ^–) . The van’t Hoff plot of these two equilibrium constants are both linear. However, the estimated enthalpy change of absorption is ca. 20 kJ/mol larger for the former scenario than the latter

Ab Initio Study on an Excited-State Intramolecular Proton-Transfer Reaction in Ionic Liquid

An excited-state intramolecular proton transfer (ESIPT) reaction of 4′-<i>N</i>,<i>N</i>-dimethylamino-3-hydroxyflavone in room temperature ionic liquid is theoretically investigated using RISM-SCF-SEDD, which is a hybrid method of molecular liquid theory and ab initio molecular orbital theory. The photo-excitation and proton-transfer processes are computed by considering the solvent fluctuation. The calculated absorption and emission energy are in good agreement with the experiments. The changes in the dipole moment indicate that the drastic solvation relaxation is accompanied by the excitation and an ESIPT process, which is consistent with the remarkable dynamic Stokes shift observed in the experiments. We calculated the nonequilibrium free-energy contour as a function of the proton coordinate and the solvation coordinate. We conclude that although immediately after the excitation the barrier height of the ESIPT process is relatively small, the barrier becomes larger as the solvation relaxation to the excited normal state proceeds. The solvation relaxation process is also investigated on the basis of microscopic solvation structure obtained by RISM calculations

Electron Transfer Reaction Dynamics of p‑Nitroaniline in Water from Liquid to Supercritical Conditions

Photoexcitation dynamics of p-nitroaniline (pNA) have been investigated by femto-second transient absorption spectroscopy in water from liquid to supercritical conditions; along the isochoric line from the ambient condition to 664 K at 40.1 MPa and along the isothermal line from 40.1 to 36.1 MPa at 664 K. The rates of the back electron transfer reaction from the photoexcited charge transfer state to the electronic ground state was determined by the bleach recovery of the ground state absorption, and the successive vibrational relaxation in the electronic ground state was determined by the hot-band decay which was apparent at the red edge of the absorption. The variation of the back electron transfer rate was compared with the prediction based on the electron transfer theory including the Franck–Condon active vibrational modes. The results indicated that both the free energy change of the reaction and the change of the intramolecular vibrational reorganization energy cause the characteristic density (or temperature) dependence of the back electron transfer rate. The density dependence of the vibrational relaxation rate was compared with the collision frequency and the coordination number of the solvent molecule around the solute estimated by the molecular dynamics simulations. The density dependence of the coordination of a water oxygen atom to an amino hydrogen atom of pNA was found to be correlated with the density dependence of vibrational relaxation rate

Comparison of 2‑Arylnaphtho[2,3‑<i>b</i>]phospholes and 2‑Arylbenzo[<i>b</i>]phospholes: Effects of 2‑Aryl Groups and Fused Arene Moieties on Their Optical and Photophysical Properties

Suzuki–Miyaura cross-coupling reactions were used in the divergent synthesis of a series of 2-arylnaphtho­[2,3-<i>b</i>]­phosphole <i>P</i>-oxides and their benzo­[<i>b</i>]­phosphole counterparts. We elucidated the electronic and steric effects of the 2-aryl groups and fused arene moieties on the optical and photophysical properties of these two types of phosphole-based π-systems

Excitation Wavelength Dependence of Excited State Intramolecular Proton Transfer Reaction of 4′‑<i>N</i>,<i>N</i>‑Diethylamino-3-hydroxyflavone in Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence Measurement

Excited state intramolecular proton transfer reactions (ESIPT) of 4′-<i>N</i>,<i>N</i>-diethylamino-3-hydroxyflavone (DEAHF) in ionic liquids have been studied by steady-state and time-resolved fluorescence measurements at different excitation wavelengths. Steady-state measurements show the relative yield of the tautomeric form to the normal form of DEAHF decreases as excitation wavelength is increased from 380 to 450 nm. The decrease in yield is significant in ionic liquids that have cations with long alkyl chains. The extent of the decrease is correlated with the number of carbon atoms in the alkyl chains. Time-resolved fluorescence measurements using optical Kerr gate spectroscopy show that ESIPT rate has a strong excitation wavelength dependence. There is a large difference between the spectra at a 200 ps delay from different excitation wavelengths in each ionic liquid. The difference is pronounced in ionic liquids having a long alkyl chain. The equilibrium constant in the electronic excited state obtained at a 200 ps delay and the average reaction rate are also correlated with the alkyl chain length. Considering the results of the steady-state fluorescence and time-resolved measurements, the excitation wavelength dependence of ESIPT is explained by state selective excitation due to the difference of the solvation, and the number of alkyl chain carbon atoms is found to be a good indicator of the effect of inhomogeneity for this reaction

Synthesis of 3,5-Disubstituted BODIPYs Bearing <i>N</i>‑Containing Five-Membered Heteroaryl Groups via Nucleophilic C–N Bond Formation

Aromatic substitution reactions were applied to the divergent synthesis of a series of symmetrically and unsymmetrically 3,5-disubstituted BODIPYs bearing <i>N</i>-heteroaryl groups. Furthermore, the effect of <i>N</i>-heteroaryl substituents at the 3- and 5-positions on the optical and photophysical properties of the BODIPY π-electron system was elucidated

Acrostichum flagelliferum

The synthesis, structures, optical and electrochemical properties, and aromaticity of a series of 5,15-diaza-10,20-dimesitylporphyrins (M–DAP; M = Pb, H₂, Ni, Pd, Pt, Zn; mesityl = 2,4,6-trimethylphenyl) are reported. Treatment of mesityl-substituted bis­(5,5′-dibromodipyrrin) with sodium azide in the presence of lead­(II) acetylacetonate afforded Pb–DAP, which was quantitatively converted to H₂–DAP by acidolysis. The free base H₂–DAP reacted with palladium­(II), platinum­(II), and zinc­(II) salts to give Pd–DAP, Pt–DAP, and Zn–DAP, respectively. The crystal structures, optical and electrochemical properties, and aromaticities of these β-unsubstituted M–DAPs were comprehensively investigated by X-ray crystallography, UV–vis absorption/fluorescence spectroscopy, nanosecond flash photolysis, cyclic and differential pulse voltammetry, NMR spectroscopy, and density functional theory calculations. The obtained data show that replacement of the 5- and 15-methine carbons with nitrogen atoms alters the intrinsic properties of the porphyrin 18π system as follows: (i) the coordination spheres at the N₄ core become contracted while keeping high planarity; (ii) the Q bands are red shifted and largely intensified; (iii) the electron-accepting ability is enhanced, whereas the electron-donating ability is reduced; (iv) the radiative decay rates from the S₁ state are enhanced; and (v) the aromaticity of the 18π circuit is slightly reduced in terms of both geometric and magnetic criteria. These optical and electrochemical properties of M–DAPs stem from their characteristic frontier orbitals; two HOMOs and two LUMOs are nondegenerate as a result of the incorporation of the electronegative nitrogen atoms at the two meso positions. In addition, the group 10 metals incorporated at the core finely tune the fundamental properties of DAP π systems through inductive effects as well as dπ–pπ antibonding orbital interactions; the HOMO–LUMO gaps of the group 10 metal complexes increase in the order Ni–DAP < Pd–DAP < Pt–DAP

Synthesis of 2‑Alkenyl- and 2‑Alkynyl-benzo[<i>b</i>]phospholes by Using Palladium-Catalyzed Cross-Coupling Reactions

Heck, Stille, and Sonogashira reactions of 2-bromobenzo[<i>b</i>]phosphole <i>P</i>-oxide afforded a series of 2-alkenyl- and 2-alkynyl-benzo[<i>b</i>]phosphole <i>P</i>-oxides. The charge-transfer character of the new benzo[<i>b</i>]phosphole π-systems in the excited state is enhanced by the terminal electron-donating substituents. Furthermore, the C–Sn cross-coupling of the bromide was applied to the facile synthesis of a new Stille-coupling precursor, 2-stannylbenzo[<i>b</i>]phosphole

Anomalous Dependence of Translational Diffusion on the Water Mole Fraction for Solute Molecules Dissolved in a 1‑Butyl-3-methylimidazolium Tetrafluoroborate/Water Mixture

Translational diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) were determined in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) and water using transient grating spectroscopy at different mole fractions of water (xw). While DPA exhibited a larger diffusion coefficient than DPCP at low water mole fractions (xw < 0.7), as observed for conventional liquids and ionic liquids (ILs), it was smaller at high mole fractions (xw > 0.9). The apparent molecular radius of DPA determined using the Stokes–Einstein equation at xw > 0.9 is close to the radius of an IL cluster in a water pool as determined from small-angle neutron scattering experiments (J. Bowers et al., Langmuir, 2004, 20, 2192–2198), suggesting that the DPA molecules are trapped in IL clusters in the water pool and move together. The solvation state of DPCP in the mixture was studied using Raman spectroscopy. Dramatically strong water/DPCP hydrogen bonding was observed at higher water mole fractions, suggesting that DPCP is located near the cluster interfaces. The large diffusion coefficient of DPCP suggests that hopping of DPCP between IL clusters occurs through hydrogen bonding with water

Excited-State Proton Transfer of Cyanonaphthols in Protic Ionic Liquids: Appearance of a New Fluorescent Species

Excited-state proton transfer (ESPT) of 5-cyano-2-naphthol (5CN2) and 5,8-dicyano-2-naphthol (DCN2) in three different protic ionic liquids (PILs), triethylammonium trifluoromethanesulfonate ([N₂₂₂H]­[CF₃SO₃]), triethylammonium methanesulfonate ([N₂₂₂H]­[CH₃SO₃]), and triethylammonium trifluoroacetate ([N₂₂₂H]­[CF₃COO]), was studied by time-resolved fluorescence. In [N₂₂₂H]­[CF₃SO₃], both 5CN2 and DCN2 showed fluorescence only from ROH* (normal form of substituted naphthol in the excited states), indicating that no ESPT occurred in [N₂₂₂H]­[CF₃SO₃]. For 5CN2 in [N₂₂₂H]­[CH₃SO₃], fluorescence bands from ROH* and RO^–* (anionic form of substituted naphthol in the excited states) were observed, indicating that 5CN2 could dissociate proton to surrounding solvents and form RO^–*. More interestingly, 5CN2 in [N₂₂₂H]­[CF₃COO] and DCN2 in [N₂₂₂H]­[CH₃SO₃] and [N₂₂₂H]­[CF₃COO] showed an anomalous fluorescence band around 470 nm (5CN2) or around 520 nm (DCN2) which has not been reported previously. The kinetics of each fluorescent component of 5CN2 and DCN2 was analyzed on the basis of the time profile of fluorescence intensity. Plausible ESPT schemes of 5CN2 and DCN2 were discussed on the basis of the kinetics and the basicity of anion in PILs