Formation and dissociation of rhodamine 800 dimers in water: steady-state and ultrafast spectroscopic study

We investigated the fundamental photophysics and photochemistry of a cationic dye rhodamine 800 (R800) in water using steady-state and ultrafast time-resolved spectroscopies. In the ground state, the monomer and dimer coexist in equilibrium, which causes significant concentration dependence of UV-visible (vis) absorption spectra. We determined the equilibrium constant as well as the molar absorption spectra of the monomer and dimer from a global fitting analysis of the UV-vis spectra. The obtained pure dimer spectrum indicates that it is a nonparallel H-dimer. In contrast to the absorption spectra, the steady-state fluorescence spectra do not show any noticeable concentration dependence. The fluorescence lifetime was determined as 0.73 ns regardless of the concentration, and the fluorescence of R800 in water was solely attributed to the monomer. In femtosecond time-resolved absorption measurements, we observed the S(n) <-- S1 absorption bands of the monomer and the dimer, as well as the ground-state bleaching signals. It was found that the S1 dimer dissociates to produce the S1 monomer (and the S0 monomer) or relaxes to the S0 dimer with a time constant of as short as 3.0 ps, which brings about the absence of dimer fluorescence.     

Synthesis of endotoxic principle of bacterial lipopolysaccharide and its recognition by the innate immune systems of hosts

A new stage of endotoxin research was brought about by structure elucidation and chemical synthesis of lipid A, the lipophilic partial structure of the lipopolysaccharide (LPS) of Gram-negative bacteria. Synthetic lipid A exhibited full endotoxic activity, which gave unequivocal evidence for the concept that lipid A is the active entity of endotoxin. Various lipid A analogues, as well as their radiolabeled derivatives and more complex partial structures of LPS, were also synthesized. By the use of these synthetic homogeneous preparations, not only simple studies on structure-activity relationships but precise and detailed analyses became possible on how this typical bacterial component is recognized by the innate immune receptor complex of mammalian cells.     

On the 2-part of the ideal class group of the cyclotomic <Emphasis Type="BoldItalic">Z</Emphasis><Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>-extension over the rationals

Let p be an odd prime number, and $\mbox {\boldmath $\mbox {\boldmath the cyclotomic $\mbox {\boldmath $\mbox {\boldmath -extension over the rationals. We show that the 2-part of the ideal class group of $\mbox {\boldmath $\mbox {\boldmath is trivial for p<500.     

Synthesis and reactivity of a (mu-1,1-hydroperoxo)(mu-hydroxo)dicopper(II) complex: ligand hydroxylation by a bridging hydroperoxo ligand

A new tetradentate tripodal ligand (L3) containing sterically bulky imidazolyl groups was synthesized, where L3 is tris(1-methyl-2-phenyl-4-imidazolylmethyl)amine. Reaction of a bis(mu-hydroxo)dicopper(II) complex, [Cu2(L3)2(OH)2]2+ (1), with H2O2 in acetonitrile at -40 degrees C generated a (mu-1,1-hydroperoxo)dicopper(II) complex [Cu2(L3)2(OOH)(OH)]2+ (2), which was characterized by various physicochemical measurements including X-ray crystallography. The crystal structure of 2 revealed that the complex cation has a Cu2(mu-1,1-OOH)(mu-OH) core and each copper has a square pyramidal structure having an N3O2 donor set with a weak ligation of a tertiary amine nitrogen in the apex. Consequently, one pendant arm of L3 in 2 is free from coordination, which produces a hydrophobic cavity around the Cu2(mu-1,1-OOH)(mu-OH) core. The hydrophobic cavity is preserved by hydrogen bondings between the hydroperoxide and the imidazole nitrogen of an uncoordinated pendant arm in one side and the hydroxide and the imidazole nitrogen of an uncoordinated pendant arm in the other side. The hydrophobic cavity significantly suppresses the H/D and 16O/18O exchange reactions in 2 compared to that in 1 and stabilizes the Cu2(mu-1,1-OOH)(mu-OH) core against decomposition. Decomposition of 2 in acetonitrile at 0 degrees C proceeded mainly via disproportionation of the hydroperoxo ligand and reduction of 2 to [Cu(L3)]+ by hydroperoxo ligand. In contrast, decomposition of a solid sample of 2 at 60 degrees C gave a complex having a hydroxylated ligand [Cu2(L3)(L3-OH)(OH)2]2+ (2-(L3-OH)) as a main product, where L3-OH is an oxidized ligand in which one of the methylene groups of the pendant arms is hydroxylated. ESI-TOF/MS measurement showed that complex 2-(L3-OH) is stable in acetonitrile at -40 degrees C, whereas warming 2-(L3-OH) at room temperature resulted in the N-dealkylation from L3-OH to give an N-dealkylated ligand, bis(1-methyl-2-phenyl-4-imidazolylmethyl)amine (L2) in approximately 80% yield based on 2, and 1-methyl-2-phenyl-4-formylimidazole (Phim-CHO). Isotope labeling experiments confirmed that the oxygen atom in both L3-OH and Phim-CHO come from OOH. This aliphatic hydroxylation performed by 2 is in marked contrast to the arene hydroxylation reported for some (mu-1,1-hydroperoxo)dicopper(II) complexes with a xylyl linker.     

Conductance Study of 1 : 1 19-Crown-6 Complexes with Various Mono- and Bivalent Metal Ions in Water

Formation constants (ML) of 1 : 1 19-crown-6 (19C6) complexes with mono- (M+) and bivalent metal ions (M2+) were determined in water at 25 °C by conductometry. The KML value of 19C6 for M+ and M2+ decreases in the order Rb+ K+ > Tl+ > Na+ = Ag+ > Li+ Cs+ and Pb2+ > Ba2+ > Sr2+ > Cd2+ > Ca2+, respectively. The selectivity for the neighboring alkali metal ions in the periodic table is lower for 19C6 than for 18-crown-6 (18C6) except for the case of Rb+ and Cs+. The same is true for the alkaline earth metal ions. Generally, the KML values of 19C6 with M2+ are greater than those with M+. For Na+ and the ions which are smaller in size than Na+ (Li+, Ca2+, Cd2+), the KML value is larger for 19C6 than for 18C6, but the contrary holds for all the other ions of larger sizes than Na+. The limiting ionic molar conductivity (°) of the 19C6–K+ complex in water at 25 °C was determined to be 43. Although 19C6 is larger than 18C6, the 19C6–K+ complex is much more mobile in water than the 18C6–K+ complex.     

Extraction of Alkali Metal (Li, Na, K) Picrates with Benzo-15-crown-5 into Various Organic Solvents. Elucidation of Fundamental Equilibria Determining the Extraction-ability and -selectivity

To quantitatively elucidate the effects of the benzo group on the extraction-selectively and -ability of benzo-15-crown-5 (B15C5)for alkali metal ions, the constants of the overall extraction (K_ex), thedistribution for various diluents having low dielectric constants (K_D,MLA), and the aqueousion-pair formation (K_MLA) of B15C5-alkali metal (Li, Na, K) picrate 1:1:1 complexes (MLA) weredetermined at 25 °C. The partition constants of B15C5were also measured at 25 °C. The log K_MLA values for Li⁺, Na⁺, and K⁺ are -0.32 ± 0.22, 2.66 ± 0.19, and 0.71 ± 0.47, respectively. In going from 15-crown-5 (15C5) to B15C5, the benzo group considerably decreasesthe K_MLA value for the same alkali metal ion. The distributionbehavior of B15C5 and its 1:1:1 complexes with the alkali metal picrates closely obeys regularsolution theory, omitting chloroform. Molar volumes and solubility parameters of B15C5and the 1:1:1 complexes were determined. For every diluent, the K_ex valuefor B15C5 increases in the order Li⁺ < K⁺ < Na⁺. K_D,MLA makes anunfavorable contribution to the Na⁺ extraction-selectivity of B15C5 because of the smallest molar volume of the Na(B15C5)A complex. The Na⁺ extraction-selectivity of B15C5 is determined completely by much the highest K_Na(B15C5)A value.The extraction-ability and -selectivity of B15C5 for the alkali metal picrates are compared with those of 15C5on the basis of the underlying equilibrium constants.     

Stabilities in Polar Nonaqueous Solvents and Transfer Activity Coefficients from Water to Nonaqueous Solvents of 19-Crown-6-Alkali Metal Ion Complexes

Formation constants of 1 : 1 19-crown-6(19C6) complexes with alkali metal ions weredetermined conductometrically at 25 °Cin acetonitrile(AN), propylene carbonate (PC), methanol, DMF, andDMSO. 19C6 always forms the most stable complex withK⁺. The selectivity order of 19C6 forheavy alkali metal ions isK⁺ > Rb⁺ > Cs⁺.The selectivity for Na⁺ varies withthe solvent; that for Li⁺ is the second lowest(AN, DMSO) or the lowest (PC). Transfer activity coefficients(^SH _{2 O}) of19C6 from water to the nonaqueous solvents (S) weremeasured at 25 °C. The contributions of a methylenegroup and an ether oxygen atom to thelog ^SH _{2 O}value of a crown ether wereobtained. The ^SH _{2 O}values of the 19C6–alkali metal ion complexes(^SH _{2 O} (ML⁺)) werecalculated, M⁺ and L denoting an alkali metal ionand a crown ether, respectively. For AN, PC, andCH₃OH, although the M⁺ ion is more stronglysolvated by water than by AN, PC, or CH₃OH, thelog ^SH _{2 O} (ML⁺) islarger than the correspondinglog ^SH _{2 O} (L)expect for the case of M⁺ = Li⁺.The higher lipophilicity of the19C6 complex ion is attributed to an enforcement ofthe hydrogen-bonded structure of water for the complexion caused by the greatly decreased hydrogen bondingbetween ether oxygen atoms and water uponcomplexation. For DMF and DMSO, thelog ^SH _{2 O} (ML⁺) is also greater thanthe correspondinglog ^SH _{2 O} (L).It was concluded from thisfinding that the unexpectedly lowest stability of the19C6 complex ion in water is due to the hydrogenbonding between 19C6 and water. The stabilities and thelog ^SH _{2 O}of 19C6–alkali metal ion complexes were compared with those of 18C6complexes.     

Phase behaviour of the thermotropic cubic mesogens 1,2-bis-(4-n-undecyl- and 4-n-dodecyl-oxybenzoyl)hydrazine under pressure

The phase transition behaviour of two optically isotropic, thermotropic cubic mesogens 1,2-bis-(4-n-undecyloxy- and 4-n-dodecyloxy-benzoyl)hydrazine, BABH(11) and BABH(12), was investigated under hydrostatic pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope equipped with a high pressure optical cell. It is found that for BABH(11) and BABH(12), a smectic C (SmC) phase is induced between the isotropic liquid (I) and the cubic (Cub) phases by applying pressures above 10-12 and 16-17 MPa, respectively. A sea-island texture consisting of bright sand-like sea regions (SmC phase) and areas of dark islands (Cub phase) appears in the mesophase under pressures up to 140 MPa, while the sand-like texture of the SmC phase is formed predominantly on cooling under pressure. These observations indicate the destabilization of the cubic phase with increasing pressure. The phase transition sequence of BABH(11) and BABH(12), Cr-Cub-I at atmospheric pressure, changes to Cr-Cub-SmC-I under intermediate pressures and would change to Cr-SmC-I under elevated pressure.     

Intrinsic reaction coordinate for the gas-phase pyrolysis of ethyl formate

Fukui's IRC is obtained for the pyrolysis of ethyl formate within the level of MNDO molecular orbital calculations. The activation enthalpy and entropy can be evaluated from the calculated partition functions and compared with the observed values. It is suggested that the pyrolysis would be significantly promoted when it occurs in an aprotic polar solvent.