Photooxidation reactions of 1,5-dihydroxynaphthalene (DHN) have been carried out in the presence of cyclometalated neutral and cationic iridium (Ir) complexes 1-6 as singlet oxygen ((1)O(2)) sensitizers in order to investigate the (1)O(2) generation quantum yield and photosensitizing durability of the complexes. The reactions allowed a successful kinetic study to provide the pseudo-first-order rate constants and the initial rates of DHN consumption, which in turn led to the (1)O(2) generation quantum yields. The results revealed that cationic Ir complexes [Ir(ppy)(2)(phen)(+) (4) and Ir(ppy)(2)(bpy)(+) (5), where ppy = 2-phenylpyridine, phen = 1,10-phenanthroline, bpy = 2,2'-bipyridyl] have high (1)O(2) generation quantum yields (Φ(Δ) = 0.93, 0.97). On the other hand, neutral complexes with lower oxidation potentials were considered to have a more efficient charge-transfer (CT) interaction with molecular oxygen, which decreased the efficiency of singlet oxygen formation. Additionally, a steric factor of the ligands was reflected in (1)O(2) generation quantum yield. High yields of the oxidized product for the photoreactions using the cationic complexes indicated their excellent photosensitizing durability, originating from the high photochemical stability upon irradiation.

Bis-cyclometalated cationic iridium (Ir) complexes 1-4 comprising two 2-(2-pyridyl)benzo[b]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 1 was prepared as a parent complex without any substituents. Complexes 2-4 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 2 and 3 with electron-donating groups (τ = 3.50 μs, 3.90 μs) were found to be much longer than that of complex 1 (τ = 0.273 μs), and complex 4, possessing strong electron-withdrawing trifluoromethyl groups, did not exhibit detectable phosphorescence at room temperature. The large differences in excited-state lifetimes of complexes 1-3, as well as the nonemissive character of complex 4, 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 2 and 3 destabilizes a short-lived, nonemissive triplet metal-centered ((3)MC) state and increases the energy separation between the (3)MC state and emissive triplet ligand-centered ((3)LC) state based on the btp ligand. For complex 4, however, the (3)MC state is close in energy to the (3)LC state because of the decreased σ-donating ability of the ancillary ligand. Additional evidence of the (3)MC state associated with the changeable excited state was also provided via low-temperature phosphorescence measurements and density functional theory calculations. Ir complexes 1-4 were tested as sensitizers in photoinduced electron-transfer reaction of triethanolamine and methylviologen chloride (MVCl2). As a result, complexes 2 and 3 exhibited much better photosensitizing property compared to complex 1 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 (3)MC state of the bis-cyclometalated cationic Ir complex to finely tune the excited-state lifetime and photosensitizing property.

We report a novel system of visible-light-driven CO2 reduction to CO in an aqueous solution, in which DPPC vesicles dispersed in the solution act as a photocatalyst using ascorbate (HAsc(-)) as an electron source. In the vesicles metal complexes [Ru(dtb)(bpy)2](2+) and Re(dtb)(CO)3Cl (dtb = 4,4'-ditridecyl-2,2'-bipyridyl) are incorporated, which act as a photosensitizer and a catalyst for CO2 reduction, respectively. The reaction is initiated with the reductive quenching of the (3)MLCT excited state of the Ru complex with HAsc(-), followed by an electron transfer from the reduced Ru complex to the Re complex to give a one-electron reduced Re species having catalytic ability for CO2 reduction. In order to search for optimum conditions for the CO production, the dependence of the initial rate of CO formation, vi, on the concentration of the metal complexes and HAsc(-) in the vesicle solution was examined. Consequently, we obtained ∼3.5 μmol h(-1) and 190 for vi and the turnover number for CO formation with respect to the Re catalyst, respectively. On the basis of the dependence of vi on the incident light intensity, we have concluded that the photocatalytic reduction of CO2 to CO with HAsc(-) in this system requires only one photon, and propose that HAsc(-) donates an electron not only to the excited state of the Ru complex, but also to the Re-CO2 adduct involved in the catalytic cycle for CO2 reduction.

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 O(2) 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 O(2). 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.