Singlet Oxygen–mediated Hydroxyl Radical Production in the Presence of Phenols: Whether DMPO–·OH Formation Really Indicates Production of ·OH?¶

The reaction of singlet oxygen (1O2) generated by ultraviolet-A (UVA)-visible light (lambda > 330 nm) irradiation of air-saturated solutions of hematoporphyrin with phenolic compounds in the presence of a spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), gave an electron spin resonance (ESR) spectrum characteristic of the DMPO-hydroxyl radical spin adduct (DMPO-*OH). In contrast, the ESR signal of 5,5-dimethyl-2-pyrrolidone-N-oxyl, an oxidative product of DMPO, was observed in the absence of phenolic compounds. The ESR signal of DMPO-*OH decreased in the presence of either a *OH scavenger or a quencher of *O2 and under anaerobic conditions, whereas it increased depending on the concentration of DMPO. These results indicate both 1O2- and DMPO-mediated formation of free *OH during the reaction. When DMPO was replaced with 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO), no DEPMPO adduct of oxygen radical species was obtained. This suggests that 1O2, as an oxidizing agent, reacts little with DEPMPO, in which a strong electron-withdrawing phosphoryl group increases the oxidation potential of DEPMPO compared with DMPO. A linear correlation between the amounts of DMPO-*OH generated and the oxidation potentials of phenolic compounds was observed, suggesting that the electron-donating properties of phenolic compounds contribute to the appearance of *OH. These observations indicate that 1O2 reacts first with DMPO, and the resulting DMPO-1O2 intermediate is immediately decomposed/reduced to give *OH. Phenolic compounds would participate in this reaction as electron donors but would not contribute to the direct conversion of 1O2 to *OH. Furthermore, DEPMPO did not cause the spin-trapping agent-mediated generation of *OH like DMPO did.     

Elongation and contraction of molecular springs. Synthesis, structures, and properties of bridged [7]thiaheterohelicenes

A series of bridged [7]thiaheterohelicenes 3a-c and 4 with a variety of helical pitches have been prepared from racemic and optical pure 2,13-bis(hydroxymethyl)dithieno[3,2-e:3',2'-e']benzo[1,2-b:4,3-b']bis[1]benzothiophene (1) in order to investigate the helical structures in solution. Recrystallizations of (PM)-3a, (PM)-3b, (PM)-3c, and (P)-4 from hexane-dichloromethane gave crystals suitable for X-ray crystallography, while recrystallization of (PM)-4 with benzene gave an inclusion complex with a stoichiometry of (PM-4)(4).(C(6)H(6)). X-ray analyses of (PM)-3a-c, (PM-4)(4).(C(6)H(6)), and (P)-4 indicate that the dihedral angles between terminal thiophene rings of the helical framework significantly vary from 22 degrees for 4 to 59 degrees for 3c. This represents as increase of 37 degrees or 168%. Although the (13)C NMR and UV absorption spectra of bridged helicenes 3a-c and unbridged helicene 5 are essentially the same, the molar rotation of 5 is very large compared with those of 3a-c and 4. A red shift (15 nm) in the circular dichroism (CD) spectrum is observed for 4, suggesting that this compound is more planar than 3a-c in solution. In the series of [7]thiaheterohelicenes studied, the minimum helical pitch is 2.70 A for 4.     

Polarized near-edge x-ray-absorption fine structure spectroscopy of C60-functionalized 11-amino-1-undecane thiol self-assembled monolayer: molecular orientation and evidence for C60 aggregation

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of pi* resonance associated with C60 pi network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57 degrees with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid C60 and /or 1-ML C60/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2 orbitals. The latter implied hybridization between N Pz of -NH2 group of thiolate SAM and pi levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C60 from Ih to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to pi* graphitic LUMO, signified the formation of aggregated clusters, (C60)n of C60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n approximately 5.     

Formation of Supramolecular Polymers Constructed by Cyclodextrins with Cinnamamide

-Cyclodextrin having cinnamamide at 6- or 3-positions (6-CiNH--CD, 3-CiNH--CD) and -cyclodextrin with cinnamamide on 6-position (6-CiNH--CD) have been prepared. Supramolecular structures were formed in the solid state or aqueous solutions and characterized by measurements of NMR and vapor pressure osmometry (VPO). The results indicate that 6-CiNH--CD formed insoluble supramolecular polymers in the solid state, while 6-CiNH--CD and 3-CiNH--CD formed supramolecular complexes in aqueous solutions. 6-CiNH--CD was found to form a dimer in an aqueous solution. 3-CiNH--CD formed intermolecular complexes to give supramolecular polymers. The differences of the position of guest part on cyclodextrins caused to give a variety of supramolecular structures in aqueous solutions.     

The influence of polytypic structures on the M011-->M101 solid-solid phase transition of n-C36H74: an application of the oblique infrared transmission method

The molecular displacements on the M011-->M101 phase transition of n-hexatriacontane (n-C36H74) have been investigated with an IR microscope designed for the oblique infrared transmission method. It has been clarified that two polytypic structures of the M011 modification, single-layered structure (Mon) and double-layered one (Orth II), both transform to the M101 modification of single-layered structure with their respective mechanisms. On the M011(Mon)-->M101 transition, the inclination direction of molecular axis is changed by 90 degrees through an intermediate state in which the molecular chain is set perpendicular to the basal plane of the single crystal. On the other hand, a polymorphic-polytypic composite structural change on the M011(Orth II)-->M101 transition is accomplished through two kinds of molecular displacements occurring alternately along the stacking direction of molecular layers.     

Study of thermodynamic stabilities of polytypes of n-C36H74 by solubility measurements and incoherent inelastic neutron scattering

The thermodynamic properties of the two polytypes of n-hexatriacontane (n-C36H74), single-layered structure Mon and double-layered structure Orth II have been investigated by means of solubility measurements and incoherent inelastic neutron scattering. The solubility measurements reveal that Orth II is more stable than Mon by 1.2 kJ/mol because of the advantage of larger entropy. The neutron scattering measurements show that the vibrational modes of Orth II shift to the lower frequencies compared with those of Mon in the frequency region below 120 cm(-1). The advantage of Orth II in vibrational entropy due to the low-frequency shifts is estimated to be 9.6 J K(-1)/mol at 288 K under the harmonic approximation, which nearly agrees with the entropy difference of 6.8 J K(-1)/mol between Mon and Orth II determined by solubility measurements. These results suggest that the difference in vibrational entropy due to low-frequency modes mainly contributes to the relative thermodynamic stabilities of polytypic structures of long-chain compounds. From the frequency of methyl torsional mode, it is suggested that the cohesive force at the lamellar interface is stronger in Mon than in Orth II.     

High resolution emission Fourier transform infrared spectra of the 4p-5s and 5p-6s bands of ArH

In the 2500-8500 cm(-1) region several strong emission bands of (40)ArH were observed by Fourier transform spectroscopy through a dc glow discharge in a mixture of argon and hydrogen. Rotational-electronic transitions of the two previously unstudied 4p-5s and 5p-6s,v = 0-0, bands of (40)ArH were measured and assigned in the 6060 and 3770 cm(-1) regions, respectively. A simultaneous fit of the emission transitions of the 4p-5s and 5p-6s bands and an extended set of transitions of the 6s-4p band observed by Dabrowski, Tokaryk, and Watson [J. Mol. Spectrosc. 189, 95 (1998)] and remeasured in the present work yielded consistent values of the spectroscopic parameters of the electronic states under investigation. In the branch of the 4p-5s band with transitions of type (Q)Q(f(3)e) we observed a narrowing in the linewidths with increasing rotational quantum number N. The rotational dependence of the linewidth is caused by predissociation of the 5s state by the repulsive ground 4s state through homogeneous coupling and changes in overlap integrals of the vibrational wave functions with the rotational level. Analysis was based on the Fermi's golden rule approximation model. In the 4p-5s band region a vibrational sequence ofv(')-v(")=1-1, 2-2, and 3-3 were recorded and a number of transitions belonging to the strongest (Q)Q(f(3)e) form branch of the 1-1 band were analyzed.     

Laser-ionization mass-spectrometric studies on laser ablation of a nitrogen-rich polymer at 532 nm and 1064 nm

Laser-ionization Time-Of-Flight (TOF) mass-spectrometric studies have been carried out on the 532 nm and 1064 nm laser ablation products from a nitrogen-rich polymer. The polymer used had an elemental composition of C_6.0N_8.9H_3.4 and consisted of C=N, C-N, and N-H chemical bonds. The TOF mass spectra observed were composed of various peaks (150 amu) depending on the ablation laser wavelength. The primary peaks were assigned to C⁺, CN⁺, CHnN⁺ ₂ (n=1–3) and C₂H₂N⁺ ₃ for 532 nm ablation, and C⁺, C⁺ ₃, HCN⁺, HCCN⁺, CH₂NH⁺, HNCN⁺, H₃NCN⁺, and C₄H₄N⁺ ₇ for 1064 nm ablation. The flight velocity distributions with peak velocities ranging from 8.6×10³ cm/s to 3.8×10⁴ cm/s were measured for these products. The distinct velocity distributions observed between small and large products indicate the presence of two origins in the fragment ejection process from the polymer for both 532 nm and 1064 nm ablation. Furthermore, we suggest an importance of the translational energy of the fragments for the product generation in the laser plume.