Iwao Amada

Laser photolysis studies of electron transfer between triplet naphthoquinones and amines

Abstract Electron transfer between triplet naphthoquinones ( 3 Nap ∗ ; 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone and 2,3-dimethyl-1,4-naphthoquinone in the triplet state) and N , N -dimethylaniline (DMA) or triethylamine (TEA) in acetonitrile (MeCN) and a mixture of MeCN and H 2 O (4:1, v/v) was studied by laser photolysis techniques at 295 K. By analysis of the transient absorption spectrum observed after the completion of electron transfer, the absorption spectra and coefficients of the naphthoquinone anion radicals (Nap .− ) were determined on the basis of those of the DMA cation radical (DMA .+ ) in MeCN and MeCNH 2 O (4:1, v/v). The values of the rate constants ( k et ) for electron transfer between 3 Nap ∗ and DMA were found to be close to those for diffusion-controlled processes as predicted by the Rehm-Weller theory on electron transfer. Using the determined molar absorption coefficients of Nap .− , the efficiencies ( Γ rad ) for the information of Nap .− and DMA .+ were shown to be unity since the radical ion pair produced ( 3 [Nap .− + DMA .+ ]) readily dissociates into Nap .− and DMA .+ due to the triplet spin multiplicity.

Laser flash photolysis studies on hydrogen atom abstraction from phenol by triplet naphthoquinones in acetonitrile

By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (Iµ) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 7100 dm3 mol–1 cm–1 at 368 nm in acetonitrile (MeCN) and MeCN–H2O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and Iµ values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (3NQ* and 3DMNQ*) from PhOH occurred in a collision process with quenching rate constants, kq= 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. On the basis of the obtained values of kq and quantum yields (ΦHA), the efficiencies (ψHA) for HA by both 3NQ* and 3DMNQ* from PhOH were obtained to be unity. The rate constants (kHA) for HA of 3NQ* and 3DMNA* were determined to be 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. The difference in kHA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of 3(π,π*) character mixed in with the 3(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (IµT–Tλ) were determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 5200 dm3 mol–1 cm–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φisc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of 3NQ* and 3DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, kSQ= 9.7 × 108 and 1.2 × 108 dm3 mol–1 s–1, respectively.

Hydrogen atom abstraction and electron transfer of triplet 2,3-dihalo-1,4-naphthoquinones studied by the cidep technique and laser flash photolysis

Investigations using CIDEP techniques showed that an H-atom abstraction from phenol and the electron transfer from 1,2,4,5-tetramethoxybenzene to photoexcited 2,3-dibromo-1,4-naphthoquinone (DBNQ) and 2,3-dichloro-1,4-naphthoquinone (DCNQ) in the polar media originated from their triplet states. Nanosecond laser photolysis at 355 nm was carried out to determine the absorption spectra and coefficients of the corresponding triplet states, semiquinone, and anion radicals for a quantitative investigation of the mechanisms involved in the H-atom abstraction and the electron transfer. The steric hindrance of the substituted groups was indifferent to H-atom abstraction. The electronic structures of triplet DBNQ and DCNQ at 295 K were both revealed to be the mixed states of 3(n,π*) with a 3(π,π*) character.

Laser flash photolysis studies on radical formation by H-atom abstraction of triplet vitamin K3 and by H-atom transfer via the triplet exciplex between triplet methyldihydroxynaphthalene and benzophenone

Nanosecond laser photolysis techniques were incorporated to obtain (1) the absorption spectra and coefficients of triplet vitamin K3 (2-methyl-1,4-naphthoquinone, MNQ) and its ketyl radical (2-methylnaphthosemiquinone, 2MNQH*) in acetonitrile (ACN) as well as to reveal (2) the mechanisms for hydrogen atom abstraction of triplet MNQ (3MNQ*) from phenol which proceeded in a diffusion process with an efficiency of unity. On the other hand, the hydroxymethylnaphthoxy radical was produced with the benzophenone ketyl radical (BPK) by the hydrogen atom transfer from triplet 2-methyl-1,4-dihydroxynaphthalene (MDHNp) sensitized by triplet benzophenone to benzophenone (BP) via the triplet exciplex. The question to be addressed was, which was produced in the MDHNp-BP system, the 2-methyl or 3-methylnaphthosemiquinone radical? Comparing the absorption spectrum and coefficient of the radical produced via the triplet exciplex with those of the 2MNQH* obtained by H-atom abstraction of 3MNQ*, the radical formed with BPK was revealed to be 2MNQH*. The reasons for the preferable formation of 2MNQH* are discussed for H-atom abstraction as well as the transfer reactions.

Inclusion Complex of Cinnarizine with β-Cyclodextrin in Aqueous Solution and in Solid State

Inclusion complex formation of cinnarizine(CN) with s-cyclodextrin (s-CD) in aqueous solution and in solid state was confirmed by the solubility method, powder X-ray diffractometry, differential scanning calorimetry(DSC) and proton nuclear magnetic resonance(1H-NMR) spectroscopy. The apparent stability constant, K’, of the complex in water at 20°C was estimated as 6.2x103M-1. The stoichiometry of the complex was given as the ratio 1:2 of CN to s-CD. The dissolution rate of CN/s-CD complex which could be prepared three different methods, coprecipitation method, neutralization method and spray-drying method, was much more rapid than intact CN, i.e., about 30 times or more. The degradation of CN in acidic solution was found to be of pseudo first-order reaction. The pseudo first-order rate constant with s-CD decrease with an increase in concentration of s-CD at pH 1.20. The inclusion complex prepared by spray-drying method was very stable under heating conditions and under high humid conditions. There was no difference in the bioavailability of CN between oral administration of s-CD complex and that of CN alone. The absorption of CN decreased significantly when CN administered with NaHCO3. However, there was observed no decrease in the case of CN/s-CD inclusion complex.