Setsuo Takamuku

Stilbene Dimer Radical Cations in the Radiolyses of Stilbenes and 1,2,3,4-Tetraphenylcyclobutanes

The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.

Singlet photosensitization of simple alkenes. Part 1. cis–trans-Photoisomerization of cyclo-octene sensitized by aromatic esters

The use of aromatic carboxylic esters, e.g. methyl benzoate, as sensitizers gave an unusual cis–trans-photoisomerization of cyclo-octene, in which the trans : cis-ratio in the photostationary state was anomalously high, provided the usual triplet sensitization is operating (Table 1). From Stern–Volmer quenching studies and the consequent comparison of the decay ratios for three different sensitizers in two different solvents with those for direct or triplet-sensitized photoisomerizations (Table 4), it is concluded that the present photoisomerization takes place via a singlet mechanism which involves non-vertical singlet energy transfer from an excited aromatic ester to the cyclo-octene moiety in an exciplex resulting in 90° twisted cyclo-octene (1p), which in turn decays to ground state cis- or trans-cyclo-octene in the ratio of 0.56 : 0.44. The asymmetric cis–trans-photoisomerization induced by chiral aromatic esters also supports the involvement of an exciplex.

Asymmetric cis–trans photoisomerization of cyclo-octene sensitized by chiral aromatic esters

The asymmetri cis→trans photoisomerization of cyclo-octene (1) in the presence of the chiral aromatic esters, (–)-menthyl benzoate (2), cholesteryl benzoate (3), or di-(–)-menthyl isophthalate (4) gave (R)-(–)-trans-cyclo-octene with a maximum enantiomeric excess of 4·0%.

Synthesis of 2,′:5′,2″-terpyridine and 2,2′:5′,2″:5″,2‴-quaterpyridine and their photocatalysis of the reduction of water

2,2′:5′,2″-Terpyridine (OPy-3) and 2,2′:5′,2″:5″,2‴-quaterpyridine (OPy-4) are prepared by Ni°complex-catalysed polycondensation of 2,5-dichloropyridine in the presence of an excess of 2chloropyridine. 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.

Photochemical carbon–phosphorus bond cleavage of nitro-substituted benzylphosphonic acids

Upon u.v.-irradiation, the C–P bond of p-nitrobenzylphosphonate is more readily cleaved to give a monomeric metaphosphate anion as intermediate than those of o- and m-derivatives.

Mechanistic studies of the one-electron oxidation of water to hydroxyl radicals photosensitized by perfluorinated p-terphenyl

Photo-oxidation of water in a system consisting of perfluorinated p-terphenyl (F-OPP-3) as a photosensitizer, O2 as an oxidative quencher and benzene as a hydroxyl radical scavenger is mechanistically studied by laser flash photolysis and γ-ray and pulse radiolysis. The radical cation of F-OPP-3 (F-OPP-3˙+) is formed via the oxidative quenching of its excited singlet state by O2. 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.

PHOTOLYSIS OF ARYL ESTERS OF TRI- AND TETRACOORDINATE PHOSPHORUS COMPOUNDS

Abstract Upon. UV excitation in methanol, some diaryl esters of alkyl- or alkenylphosphonates underwent an elimination of two aryl groups to give biaryls and the corresponding alkyl- or alkenylphosphonic acids. Tris(4-methoxyphenyl) phosphite also underwent a similar elimination to give 4,4′-dimethoxybiphenyl and 4-methoxyphenyl phosphonate. This interesting biaryl elimination was confirmed to proceed via a singlet intramolecular excimer by means of fluorescence spectra and Stern-Volmer analysis.

Photoluminescence lifetime of GaP/AlP superlattices grown by gas‐source molecular‐beam epitaxy

The photoluminescence lifetimes of GaP/AlP short‐period superlattices (GaP)11/(AlP)3 and (GaP)9/(AlP)5, grown on GaP(001) by gas‐source molecular‐beam epitaxy are studied by a time‐correlated single photon counting method at 4.2 and 298 K. Two components of the photoluminescence lifetime are found for each sample. The fast component is about 2 ns accompanied by a slow component of about 20 ns. Discussion of the possible explanations of the two components includes the co‐existence of direct‐ and indirect‐band‐gap transitions in the superlattice, which was fabricated from the indirect‐band‐gap constituents.

CONVENIENT SYNTHETIC ROUTE TO MONO-OR DIALKYL PHOSPHATE FROM INORGANIC PHOSPHORUS ACIDS

Abstract Mono- or dialkyl phosphate was synthesized in a favorable yield by oxidation of phosphonic or phosphinic acid in alcohol with oxygen at the presence of a catalytic amount of copper (II) chloride. The reaction may proceed via the formation of corresponding phosphorochloridate or phosphorochloridite.

PHOTOCHEMICAL C[sbnd]P BOND CLEAVAGE OF BENZOYLBENZYLPHOSPHONIC ACIDS

Abstract Benzoylbenzylphosphonic acids (p- and m-) easily underwent photochemical C[sbnd]P bond cleavage in a basic aqueous ethanol solution to give methylbenzophenone, orthophosphate, and ethyl phosphate.