Sadaaki Yamamoto

Mechanisms of deactivation of triplet 1-anthrol and 2-naphthol by aromatic n-heterocycles

Abstract A flash photolytic investigation to determine the efficiency of the hydrogen atom transfer reaction in the triplet state was carried out for the systems of 1-anthrol—quinoline and 2-naphthol—pyridine in cyclohexane. The triplet states of 2-naphthol and 1-anthrol react respectively with pyridine and quinoline at a reaction efficiency of unity. In contrast, the reaction efficiency of the triplet 1-anthrol hydrogen bonded with quinoline is 0.59 and that of the triplet 2-naphthol hydrogen bonded with pyridine is 0.80. The difference in reaction efficiencies between the free and the hydrogen-bonded species implies that the hydrogen atom transfer reaction of the free species does not occur in the hydrogen-bonded triplet state following the encounter state but directly in the encounter state.

Hydrogen atom transfer reaction from excited carbazole to pyridine

Abstract The hydrogen bonding interaction between excited carbazole and pyridine was investigated in cyclohexane by an emission—absorption flash technique. Triplet carbazole is deactivated by pyridine with a rate constant of 4.9 × 10 7 M −1 s −1 , yielding the carbazyl radical with a reaction yield of unity. The triplettriplet absorption of carbazole hydrogen bonded with pyridine was not observed. By means of the triplet energy transfer from N -ethylcarbazole to the hydrogen-bonded carbazole it was found that the triplet state of the hydrogen-bonded carbazole yields the carbazyl radical wtih a reaction yield of 0.7. Excited singlet carbazole is deactivated by pyridine with a diffusion-controlled rate, yielding the carbazyl radical with a reaction yield of 0.1. Flashing of the hydrogen-bonded carbazole does not yield carbazyl radical. The difference in the reaction yields between the free and the hydrogen-bonded species indicates that the dynamic hydrogen atom transfer reaction occurs from the encounter state in competition with hydrogen bond formation.

Deactivation of the excited states of 1-anthrol by aromatic N-heterocycles

Abstract The interactions of the excited 1-anthrol with pyridine, quinoline, and acridine have been investigated in cyclohexane. Fluorescence of 1-anthrol is quenched dynamically by pyridine and quinoline at about a diffusion controlled rate. 1-Anthrol hydrogen bonded with pyridine or quinoline in the ground state does not produce any transient species detectable by a conventional flash apparatus and is non-fluorescent. The bimolecular interaction between the excited singlet 1-anthrol and pyridine leads to neither the enhancement of intersystem crossing nor reaction. The triplet 1-anthrol is not deactivated by pyridine, but by quinoline and acridine with the rate constants of 1.1 – 1.4 × 109 and 1.1 × 109 M−1 s−1, respectively, yielding 1-anthroxyl radical. By means of the triplet energy transfer to the hydrogen bonded species, it was found that the radical is formed from the triplet state of the hydrogen bonded 1-anthrol. Acid dissociation constants of 1-anthrol were determined as pK = 10.0, ∼10, and −0.07 for the ground, the triplet, and the singlet excited states, respectively. It was suggested that the acidities of both the hydrogen bonding donor and acceptor are closely correlated with the facility of the hydrogen atom transfer reaction in the excited states.

Hydrogen-atom transfer reaction from phenol to excited 7,8-benzoquinoline

Abstract The hydrogen-bonding interaction between excited 7,8-benzoquinoline and phenol was investigated in cyclohexane by an emission-absorption flash technique. The dynamic quenching of the excited singlet and triplet 7,8-benzoquinoline with phenol gives rise to a hydrogen-atom transfer reaction with efficiencies of 0.25 and 1.0 respectively. Independent of whether the excited state is a hydrogen-bond donor or an acceptor, the efficiency for the excited singlet state changes considerably among hydrogen-bonding systems of the type , whereas the efficiency for the triplet state is unity for all the reacting systems. For these reactions the dynamic quenching is regarded as resulting from the competitive reactions of hydrogen-atom transfer and hydrogen-formation.

The initial photochemical process of triosmiumdodecacarbonyl adsorbed on the surface of silica as studied by picosecond diffuse reflectance laser photolysis and matrix isolation

Abstract The transient absorption spectrum of the coordinatively unsaturated trinuclear osmium carbonyl cluster Os 3 (CO) 11 adsorbed on the surface of silica was observed for the first time by the ps time-resolved diffuse reflectance laser photolysis of Os 3 (CO) 12 . The present results demonstrated that Os 3 (CO) 11 is the photoprimary product responsible for the net oxidative addition of surface hydroxyls to Os 3 (CO) 12 adsorbed on the surface of silica to form the surface attached trinuclear species HOs 3 (CO) 10 -OSi. It was revealed that the addition of surface hydroxyls to Os 3 (CO) 11 is completed within the time of our instrumental response (⩽35 ps), which is comparable to the coordination of solvent to coordinatively unsaturated metal carbonyls.