Aggregation of Amphiphilic Pyranines in Water: Facile Micelle Formation in the Presence of Methylviologen

Four amphiphilic pyranines in which the acidic hydrogen of pyranine was replaced by an octyl, dodecyl, hexadecyl, and eicosyl group, POCn (n = 8, 12, 16, and 20), were prepared, and their spectroscopic properties and aggregation behaviors in water were investigated. The critical micelle concentration (cmc) of the amphiphilic pyranines was found to be relatively large even in POC20 having a long hydrophobic alkyl chain (ca. 3 × 10-3 M). 1H NMR studies revealed that POC16 and POC20 exist in the compact structure with the pyranine nucleus wrapped with a long methylene chain in water. As in the case of parent pyranine, the addition of methylviologen (MV2+) to an aqueous solution of POCn resulted in the absorption spectral change and efficient quenching of POCn fluorescence. In the case of POC8 and POC12, these spectral changes induced by the MV2+ addition were thoroughly explained in terms of the formation of the electrostatic complex POCn/MV2+ with a complexation constant of ∼3 × 104 M-1. On the other hand, an unexpectedly large dependence of the absorption spectral change, as well as fluorescence quenching, on the total concentration of MV2+ was observed in POC16 and POC20. The Stern−Volmer plot for quenching of the fluorescence of POC16 and POC20 gave a curve deviating largely upward from a straight line. The plot was successfully analyzed by the equation induced by assuming the aggregate formation of the complex POCn/MV2+, which revealed the considerably small cmc values of the complexes, 3.6 × 10-7 and 3.6 × 10-8 M for POC16/MV2+ and POC20/MV2+, respectively. Experimental evidence in support of the aggregate formation was obtained by 1H NMR and dynamic light scattering studies

Oxidative Formation of Thiolesters in a Model System of the Pyruvate Dehydrogenase Complex^†

In the presence of a catalytic amount of 3-butyl-4-methylthiazolium bromide, the reaction of benzaldehydes with azobenzene in dichloromethane containing octanethiol and Et3N gave the corresponding S-octyl thiobenzoates in good yields. The thiolesters were produced by trapping of the 2-benzoylthiazolium salts with the thiol, which were generated through the azobenzene oxidation of the active aldehydes. This is the first example for the thiolester formation mimicking the function of the pyruvate dehydrogenase complex. An electron-withdrawing substituent at the 4-position of benzaldehyde enhanced the reaction rate. The effect of benzaldehyde substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to a nonlinear correlation of log(kobs) with Hammett's substituent constants (σ). The origin of the nonlinear Hammett plot was interpreted in terms of a shift in the rate-determining step of the multistep reaction with change of the electronic nature of substituent. Further support for this assumption was given by the observation that the reaction constant (ρ) of the Hammett plot for the azobenzene substituent effect on the oxidation rate of 4-bromobenzaldehyde was much smaller than that of 4-cyanobenzaldehyde

Photochemistry of 2-(1-Naphthyl)-2<i>H</i>-azirines in Matrixes and in Solutions: Wavelength-Dependent C−C and C−N Bond Cleavage of the Azirine Ring

The photochemistry of 3-methyl-2-(1-naphthyl)-2H-azirine (1a) was investigated by the direct observation of reactive intermediates in matrixes at 10 K and by the characterization of reaction products in solutions. As already reported, the photolysis of the azirine 1a with the short-wavelength light (>300 nm) caused the C−C bond cleavage of the 2H-azirine ring to produce the nitrile ylide 2. However, the products derived from the C−N bond cleavage were exclusively obtained in the irradiation of 1a with the long-wavelength light (366 nm) both in matrixes and in solutions. When 1a was irradiated in the presence of O2 with the long-wavelength light, acetonitrile oxide (6) was produced through the capture of the biradical 4 generated by the C−N bond cleavage of 1a with O2. An introduction of a nitro group into the naphthyl ring of 1a resulted in an acceleration of the decomposition in the long-wavelength irradiation and an extension of the wavelength region where the products derived from the C−N bond cleavage were selectively obtained. On the basis of molecular orbital calculations with the INDO/S method, the reason for the wavelength-dependent selective C−C and C−N bond cleavage of the azirine ring of 1a is discussed

Photochemical Reactions of Mesityl Azide with Tetracyanoethylene: Competitive Trapping of Singlet Nitrene and Didehydroazepine

Irradiation of the title azide 4 in the presence of TCNE gives a mixture of two stable adducts. One of them is identified as the azomethine ylide 5, the structure of which is strictly determined by X-ray crystallography. The other is spectroscopically assigned to the spiroazepine 6. The effect of wavelength of the light employed in the photolysis reveals that the TCNE−4 charge-transfer complex (λmax 454 and 550 nm in dichloromethane) does not participate in the adduct formation. The ratio of the adducts obtained in the photolysis is dependent linearly upon the initial concentration of TCNE, which strongly suggests that the adducts 5 and 6 are produced by competitive trapping of singlet mesitylnitrene (8S) and trimethyldidehydroazepine (9), respectively. The rate constant for the reaction of 8S with TCNE is estimated to be on the order of 109 M-1 s-1 or greater. The PM3 calculation indicates that the azomethine ylide 5 is thermodynamically more stable than the aziridine 7, which is thought to be initially formed by the reaction of 8S with TCNE. Thus, we propose that these findings make the first example of competitive trapping of singlet arylnitrene and its ring-expanded isomer with an alkene, which definitely reveals the intervention of singlet nitrene in the photolysis of an aryl azide

Direct Observation and Characterization of <i>p</i>-Phenylenebisnitrene. A Labile Quinoidal Diradical

Direct Observation and Characterization of p-Phenylenebisnitrene. A Labile Quinoidal Diradica

Controlling the Excited State and Photosensitizing Property of a 2‑(2-Pyridyl)benzo[<i>b</i>]thiophene-Based Cationic Iridium Complex through Simple Chemical Modification

Bis-cyclometalated cationic iridium (Ir) complexes <b>1</b>–<b>4</b> comprising two 2-(2-pyridyl)­benzo­[<i>b</i>]­thiophene (btp) ligands and one 2,2′-bipyridyl (bpy) ancillary ligand with different substituents were prepared as new visible light-absorbing sensitizers and examined for their photophysical and electrochemical properties. Complex <b>1</b> was prepared as a parent complex without any substituents. Complexes <b>2</b>–<b>4</b> contained methyl-, methoxy-, and trifluoromethyl groups at 4,4′-positions on the bpy ancillary ligand. Systematic investigation of these complexes revealed that such a simple chemical modification selectively controls the excited-state lifetime, while the absorption and emission spectral features remain unchanged. Specifically, the phosphorescence lifetimes of complexes <b>2</b> and <b>3</b> with electron-donating groups (τ = 3.50 μs, 3.90 μs) were found to be much longer than that of complex <b>1</b> (τ = 0.273 μs), and complex <b>4</b>, possessing strong electron-withdrawing trifluoromethyl groups, did not exhibit detectable phosphorescence at room temperature. The large differences in excited-state lifetimes of complexes <b>1</b>–<b>3</b>, as well as the nonemissive character of complex <b>4</b>, are attributed to a strong influence of the substituents on the ligand field strength. The increased σ-donating ability of the ancillary ligand in complexes <b>2</b> and <b>3</b> destabilizes a short-lived, nonemissive triplet metal-centered (³MC) state and increases the energy separation between the ³MC state and emissive triplet ligand-centered (³LC) state based on the btp ligand. For complex <b>4</b>, however, the ³MC state is close in energy to the ³LC state because of the decreased σ-donating ability of the ancillary ligand. Additional evidence of the ³MC state associated with the changeable excited state was also provided via low-temperature phosphorescence measurements and density functional theory calculations. Ir complexes <b>1</b>–<b>4</b> were tested as sensitizers in photoinduced electron-transfer reaction of triethanolamine and methylviologen chloride (MVCl₂). As a result, complexes <b>2</b> and <b>3</b> exhibited much better photosensitizing property compared to complex <b>1</b> since their long-lived excited states promoted an oxidative quenching pathway. This Study has first demonstrated that simple substitution on the diimine ancillary ligand can control the ³MC state of the bis-cyclometalated cationic Ir complex to finely tune the excited-state lifetime and photosensitizing property

Photochemistry of 2,4-Bis(diazo)-1,2,3,4-tetrahydro- naphthalene-1,3-dione: Selective Photodecomposition of One of the Two Inequivalent Diazo Groups

Photochemistry of 2,4-Bis(diazo)-1,2,3,4-tetrahydro- naphthalene-1,3-dione:  Selective Photodecomposition of One of the Two Inequivalent Diazo Group

Photochemistry of 2,4-Bis(diazo)-1,2,3,4-tetrahydro- naphthalene-1,3-dione: Selective Photodecomposition of One of the Two Inequivalent Diazo Groups

Photochemistry of 2,4-Bis(diazo)-1,2,3,4-tetrahydro- naphthalene-1,3-dione:  Selective Photodecomposition of One of the Two Inequivalent Diazo Group

Remarkable Wavelength-Dependent Photoreactions of the Bis(diazo) Ketone Having Inequivalent Diazo Groups: Studies in Fluid Solutions and in Low-Temperature Matrixes

The photoreactions of the bis(diazo) ketone 11, which has two inequivalent diazo groups, have been investigated in solutions at room temperature and in matrixes at 12 K. Irradiation of 11 in benzene containing methanol gave a mixture of the spironorcaradiene 13 and the diazo ketone 17 as primary isolable photoproducts. The former 13 originated from the diazo ketene 20, which was formed from the initial extrusion of N2 from the 2-position of 11, while the latter product 17 was derived from the diazo ketene 24 which was generated by the initial decomposition of the diazo group at the 4-position of 11. The product distribution was remarkably dependent upon the excitation wavelength:  13 was predominantly obtained in the photolysis with light of >350 nm, while the irradiation with long-wavelength light (>420 nm) exclusively gave 17. The consistent wavelength effects were observed in photoreactions in an Ar matrix at 12 K. The irradiation of 11 matrix-isolated in Ar with light of >350 nm afforded 20 in preference to the isomer 24 as the first-formed intermediates, while 24 was mainly obtained in the long-wavelength irradiation (>420 nm). On the basis of these experimental data, we conclude that the short-wavelength irradiation of 11 causes a preferential cleavage of the diazo group at the 2-position and that the selective extrusion of N2 from the 4-position is practically achieved by the irradiation with long-wavelength light. The reason for the selective cleavage of the two inequivalent diazo groups of 11 is discussed on the basis of theoretical calculations with the PM3 CI method

A Photocatalytic System Composed of Benzimidazolium Aryloxide and Tetramethylpiperidine 1‑Oxyl to Promote Desulfonylative α‑Oxyamination Reactions of α‑Sulfonylketones

A new photocatalytic system was developed for carrying out desulfonylative α-oxyamination reactions of α-sulfonylketones in which α-ketoalkyl radicals are generated. The catalytic system is composed of benzimidazolium aryloxide betaines (BI+–ArO–), serving as visible light-absorbing electron donor photocatalysts, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), playing dual roles as an electron donor for catalyst recycling and a reagent to capture the generated radical intermediates. Information about the detailed nature of BI+–ArO– and the photocatalytic processes with TEMPO was gained using absorption spectroscopy, electrochemical measurements, and density functional theory calculations