草場 光博

Photo-oxidation of water in a system consisting of perfluorinated p-terphenyl (F-OPP-3) as a photosensitizer, O-2 as an oxidative quencher and benzene as a hydroxyl radical scavenger is mechanistically studied by laser flash photolysis and gamma-ray and pulse radiolysis. The radical cation of F-OPP-3 (F-OPP3(.+)) is formed via the oxidative quenching of its excited singlet state by O,. The decay of F-OPP-3(.+) observed in transient spectroscopy is enhanced by the addition of water to the system, indicating that F-OPP-3(.+) plays a crucial role in the one-electron oxidation of water.

2,2′:5′,2″-Terpyridine (OPy-3) and 2,2′:5′, 2″:5″,2‴-quaterpyrldine (OPy-4) are prepared by Ni O-complex-catalysed polycondensation of 2,5-dichloropyridine in the presence of an excess of 2-chloropyridine. These two compounds show more efficient photocatalysis of H2 evolution by the reduction of water than p-terphenyl and p-quaterphenyl in the presence of triethylamine as an electron donor, and RuCl3 or K2PtCl6 as a source of RuO or PtO colloids which function as an electron mediator. The primary photochemical processes of OPy-n (n = 3 or 4) and the successive reactions have been investigated by γ-radiolysis, pulse radiolysis and laser flash photolysis. The OPy-n molecules are converted to their anion radicals through reductive quenching of the excited states. A mechanism for the photocatalysis of hydrogen evolution is proposed in which the anion radicals supply electrons to protons on the metal colloids. The anion radicals undergo protonation in competition with electron transfer, resulting in the loss of photocatalytic activity.

Photoreduction of carbon dioxide (CO2) to formate (HCO2-) can be achieved by UV-irradiation of the system involving phenazine (Phen) as a photosensitizer, a cobalt complex of cyclam (Co(III)L, L = cyclam = 1,4,8, 11-tetraazacyclotetradecane) as an electron mediator, and triethylamine (TEA) as an electron donor. Reduction products from the system are HCO2- and a small quantity of CO and H-2. The quantum yield is 0.07 for the formation of HCO2- at lambda = 313 nn. Preferential electron transfer from the photoformed radical anion of phenazine (Phen(.-)) to Co(III)L is confirmed by EPR analysis and the reaction between Phen(.-) and Co(III)L with a second-order rate constant, k = 4.3 x 10(9) M(-1) s(-1). The resulting Co(II)L reacts with phenazinyl radical,1 giving [CoL(H)](2+) by hydrogen transfer from phenazinyl radical. The effective insertion of a CO2 molecule into [CoL(H)](2+) yields HCO2- selectively.

Photoreduction of CO2 to formate (HCO2-) can be achieved by the phenazine-catalyzed system consisting of cobalt cyclam complex (Co-cyclam, cyclam = 1,4,8,11-tetraazacyclotetradecane) as an electron mediator and triethylamine as an electron donor. Flash photolysis revealed that the catalytic system should involve electron transfer from the photoformed radical anion of phenazine (P-.-) to Co(III)cyclam and hydrogen transfer from phenazinyl radical (PH.) to intermediary Co(II)cyclam. The resulting cobalt hydride complex, Co-cyclam(H), provides formate through Co(III)formate complex formed by CO2 insertion.

Quantum yields of hydrated electrons of Cl-, Br-, I-, SO42-, SCN and OH- in aqueous solution have been determined by means of 193 nm laser flash photolysis. Shorter wavelength excitation showed higher quantum yields for I- and SCN- ions in three wavelengths of 193, 222 and 248 nm. The highest yield was 0.73 for SO42- at 193 nm and the lowest one was 0.015 for SCN-at 248 nm.

Photoreduction yields of Eu3+ to Eu2+ in aqueous and alcoholic solutions were determined. The highest quantum yields were found to be 0.97 +/- 0.16 for EuCl3 in methanol under 308 nm excitation and 0.94 +/- 0.17 in 2-propanol under 248 nm excitation. The lowest quantum yield was 0. 12 +/- 0.02 for Eu(ClO4)3 in aqueous solution under 193 nm excitation. The high photoreduction yields for EUCl3 indicate that Cl. reacts efficiently with neighbouring alcohols. The yield was found to depend on the excitation wavelength for EuCl3 in methanol solution.

Cobalt(III) complexes of cyclam (cyclam (L1) = 1,4,8,11-tetraazacyclotetradecane) (Co(III)L1) or related 14-membered tetraazamacrocycles (L2-L8) modiate electron transfer in the photoreduction of CO2 with p-terphenyl (OPP-3) as a photocatalyst and tertiary amines as sacrificial electron donors in methanolic acetonitrile. Tertiary amines (e.g., triethylamine (TEA)) used as electron donors play an important role in the electron mediation of Co(III)L1 through coordination, and the mediation of the amine-coordinatod Co(III)L1 suppresses the degradative and competitive photo-Birch reduction of OPP-3 and enhances the activity of OPP-3, leading to efficient and selective formation of both carbon monoxide (CO) and formate (HCO2-) without producing much H-2. The degradation of OPP-3 is mostly suppressed in the presence of beta-hydroxylated tertiary amines such as triethanolamine (TEOA) and tri-2-propanolamine (TIPOA), leading to much more efficient and selective production of CO and HCO2-. The total quantum yield of CO and HCO2- is 0.25 at 313 nm in the presence of TEOA. Preferential electron transfer from the photoformed radical anion of OPP-3 (OPP-3.-) to the TEA-coordinated Co(III)L1, [Co(III)L1(TEA)2]3+, is confirmed by the quenching of OPP-3.- by [Co(III)L1(TEA)2]3+ with a diffusion-controlled rate (k(s) = 1.1 x 10(10) M-1 s-1). Successive reduction of [Co(II)L1(TEA)]2+ by OPP-3.- results in the formation of [Co(I)L1]+. [Co(I)L1]+ can react with CO2 to give [Co(I)L1(CO2)]+ or react with a proton to give a d6 hydride [Co(III)L1(H-)(TEA)]2+. The extensive charge transfer from metal to bound CO2 and the coordination of tertiary amines may lead to the formation of d6 complexes like [Co(III)L1(CO22-)(TEA)]+, which may react with an electron from OPP-3.- or Co(I) species to form CO, OH-, and Co(II) species such as [Co(II)L1(TEA)]2+. As for the mechanism for the formation of HCO2-, the insertion of CO2 into intermediary hydride complexes such as [Co(III)L1(H-)(TEA)]2+ derived from [Co(I)L1]+ and H+ is proposed. The structural and electrochemical properties of cobalt complexes of the 14-membered tetraazamacrocycles investigated (L2-L8) are also discussed in view of the distribution of the reduction products of CO, HCO2-, and H-2.

We have found that the photoreduction yield from Eu3+ to Eu2+ depends on excitation wavelength within a single charge-transfer band. The quantum-yields phi were 0.12+/-0.02 (at 193 nm) and 0.06+/-0.01 (at 222 nm) in aqueous solution. and 0.42+/-0.07 (at 222 nm ) and 0.35+/-0.06 (at 248 nm) in methanol solution, The escape probability of Eu2+ from the photochemically prepared geminate pair is suggested to be higher with shorter-wavelength excitation.

Photoreduction of CO2 to formic acid (HCO2-) and a small quantity of carbon monoxide (CO) can be achieved in nonaqueous polar solvent by using oligo(p-phenylenes) (OPP-n) as a photocatalyst and triethylamine (TEA) as a sacrificial electron donor under > 290-nm irradiation, where the photocatalysis in N,N-dimethylformamide leads to the most effective formation of HCO2- and CO. Among OPP-n, OPP-3 and OPP-4 show high photocatalytic activity for the formation of HCO2-, in which the apparent quantum yields of HCO2- formation for OPP-3 and OPP-4 are 0.072 and 0.084, respectively. Although photocatalyst OPP-3 itself concurrently undergoes photo-Birch reduction during the photocatalysis, the turnover number for the formation of HCO2- based on the reacted OPP-3 is calculated to be 4, implying a cyclic activity of the reduction system. The laser flash photolysis and pulse radiolysis studies reveal that the photocatalysis initially starts from the reductive quenching of the singlet state of OPP-n by TEA followed by the formation of the radical anion of OPP-n, OPP-n.-, resulting in direct electron transfer from OPP-n.- to CO2 molecules.

Oligo(p-phenylenes) (OPP-n), p-terphenyl (OPP-3) to p-sexiphenyl (OPP-6), catalyze water-reductive H2 formation and reduction of concomitantly formed acetaldehyde to ethanol upon irradiation of heterogeneous suspensions in aqueous organic solution in the presence of triethylamine (TEA) and RuCl3. Colloidal Ru0 is photoformed in situ to work as an electron relay. The activity of OPP-n increases with the number of phenylene units except for the cases of OPP-3 and of the alkylated derivatives, where the net photocatalytic activities are higher, mainly due to the effective homogeneous catalysis, since their solubilities in the solvents employed are significantly larger. The homogeneous photocatalysis of OPP-3 leads not only to H2 evolution but also to effective formation of ethanol in the absence of colloidal Ru0, being accompanied by photo-Birch reduction of OPP-3. Dynamics studies of OPP-3 reveal that photocatalysis should be initiated by formation of the excited singlet state of OPP-3 (1OPP-3*), which is reductively quenched by TEA at a rate controlled by diffusion to produce the OPP-3 radical anion (OPP-3.-) and the TEA radical cation (TEA.+). From laser flash photolysis and pulse radiolysis experiments, it is concluded that electron transfer from OPP-3.- leads to effective reduction of water to H2 catalyzed by Ru0 colloid. Furthermore, it is confirmed that OPP-3.- gives electrons directly to acetaldehyde without any electron relays like colloidal metals, resulting in the formation of ethanol. During photocatalysis, OPP-3 itself undergoes photo-Birch reduction to some extent.