Ryoichi Nakagaki

Excited-state proton transfer of 7-hydroxyquinoline in a non-polar medium: mechanism of triple proton transfer in the hydrogen-bonded system

Abstract Excited-state proton transfer (ESPT) of 7-hydroxyquinoline (7HQ) has been investigated in hexane solutions containing a small amount of alcohol (methanol, ethanol, 2-propanol, tert -butanol and 2,2,2-trifluoroethanol) by means of picosecond time-resolved fluorescence spectroscopy. Formation of hydrogen-bonded complexes with 1:2 stoichiometry, 7HQ·(alcohol) 2 , is concluded from measurements and analysis of steady-state absorption spectra. The formation of cyclic complexes in the ground state is supported by semi-empirical MO calculations. The rate constants of ESPT within the complexes are found to increase with increasing acidity of the alcohols. The mechanism of triple proton transfer is discussed on the basis of the Kamlet–Taft acidity and basicity scales of alcohols.

Magnetic field effects on the fluorescence of intramolecular electron-donor-acceptor systems

Abstract The external magnetic field effects on the exciplex fluorescence of α-(4-dimethylaminophenyl)-ω-(9-phenanthryl)alkanes have been studied by photostationary, time-resolved, and magnetic field modulation fluorescence spectroscopy. The singlet-triplet degeneracy of the radical ion pair is suggested to occur at a methylene chain length of about ten.

Infrared and Raman spectra of 4-(dimethylamino)benzonitrile and isotopomers in the ground state and vibrational analysis

Abstract Vibrational analysis of 4-(dimethylamino)benzonitrile (DMABN) has been performed. Infrared and Raman spectra of DMABN and its four different isotope-labeled species have been recorded. Assignments of the vibrational bands are given on the basis of normal coordinate calculation on an empirical force field and of the Hartree–Fock and the density functional calculations. Most of the vibrational bands observed have been successfully assigned, and the isotope shifts are reasonably explained. It is clearly demonstrated from the measurements on the isotopomers that assignments of some bands in the literature are not correct. The characteristics of the empirical and the quantum-chemical force fields are compared. It has been found that the strong interaction between the dimethylamino group and the benzene ring is directly reflected in the force field.

Hydrogen bonding of aromatic amines in hydroxylic solvents 2. Absorption and emission spectroscopy of substituted 7-aminocoumarins and 7-aminocarbostyrils

Abstract The absorption and fluorescence spectra of 7-aminocoumarins and 7-aminocarbostyrils with different degrees of alkylation were studied in 2-propanol (IP), polyfluorinated alcohols and water. The spectral properties of substituted 7-aminocoumarins and 7-aminocarbostyrils in hexafluoro-2-propanol (HFP) are very different from those in 2-propanol due to the strong hydrogen-bonding (HB) interaction between the solute and the solvent (HFP). The spectral behaviour can be explained in terms of the strength of the HB interaction which depends on the degree of alkylation of the amino group and the electron affinity of the electron-accepting moiety. The absorption spectra indicate that a structural change at the amino nitrogen is induced on formation of strong hydrogen bonds.

Photochemistry of bichromophoric chain molecules containing electron donor and acceptor moieties: External magnetic field effects upon the photochemistry of N-[ω-(p-nitrophenoxy]alkyl]anilines

Abstract Magnetic field effects upon the photochemistry of chain molecules, A-(CH2) n-D, where A is a p-nitrophenoxy group and D an anilino or p-fluoroanilino group, have been studied by steady-state photolysis. Magnetic field effects due to the hyperfine coupling mechanism are observed when the chain length n is larger than 8.

Photochemistry of bichromophoric chain molecules containing electron donor and acceptor moieties. Dependence of reaction pathways on the chain length and mechanism of photoredox reaction of N-|ω-(p-nitrophenoxy)alkyl|anilines

Abstract Switching of photochemical reaction pathways was found for a series of bichromophoric species. D(CH 2 ) n A, consisting of electron donor and acceptor moieties by changing the number of methylene groups linking two chromophores. Long-chain molecules ( n ⩾ 8) show a photoredox reaction, while short-chain compounds ( n ⩽ 6) exhibit the photo-Smiles rearrangement when D  anilino and A  p -nitrophenoxy group.

Fluorescence on–off switching mechanism of benzofurazans

Many fluorescent reagents with a benzofurazan (2,1,3-benzoxadiazole) skeleton have been developed and widely used in bio-analyses. In this study, we try to elucidate the fluorescence on-off switching mechanism of three fluorogenic reagents and their derivatives. Ten 4,7-disubstituted benzofurazans were used for this purpose and the measurements of their fluorescence, phosphorescence, photolysis, and time-resolved thermal lensing signal in acetonitrile were obtained in order to understand the relaxation processes of these compounds. These results indicate that the competition of fluorescence with a fast intersystem crossing or fast photoreaction plays a key role in the fluorescence on-off switching. Semi-empirical molecular orbital calculations show that the existence of the triplet n pi* state is responsible for the fast intersystem crossing while the proximity of the reactive second single pi pi* state to the first singlet pi pi* state contributes to the fast photoreaction in the excited states.

Substituent effects in benzene derivatives—a local mode analysis of ring CH stretching overtone spectra of methylated and chlorinated benzenes

Abstract Methylation and chlorination of benzene induce red and blue shifts, respectively, in overtone spectra of the ring CH stretching vibrations. Except for the distinctly different sign of spectral shifts, both substitutions exhibit similar effect on the magnitude of A terms in the Birge—Sponer relationship, Δ E (ν - 0) = ( A + B ν)ν. The observed changes in the A terms are due to the polar effect and quantitatively analyzed by the use of a simple model based upon the local mode concept assuming additivity of the substituent effects.

Laser flash photolysis studies of the magnetic field effects on the hydrogen abstraction reaction of 2-naphthylphenylcarbene in micellar solution

Abstract 2-Naphthylphenylcarbene in the triplet state, photochemically generated from 2-naphthylphenyldiazomethane, abstracts a hydrogen atom from a surfactant in aqueous micellar solution. The lifetime of the radical pair nad the yield of the free radicals generated in the reaction increase significantly on application of magnetic fields (less than 1 T). The effects of magnetic field on the reaction of diphenylcarbene and fluorenylidene in micellar solution are also discussed for comparison.

Twisted S1 excited state geometries in 4-dimethylaminobenzonitrile and dimethylaniline: New -d6 origin bands

The S1←S0 electronic spectra of 4-dimethylaminobenzonitrile-h6 and -d6 (DMABN) and dimethylaniline-h6 and -d6 have been reexamined, and new electronic origins have been observed for the -d6 species, approximately 65 cm−1 lower in energy than previously reported. The spectra of DMABN-h3d3 and several other isotopomers of DMABN are reported for the first time. A prominent low-frequency progression is assigned to dimethylamino torsion, and the S1 states are found to be twisted by about 26° with a small 190 cm−1 barrier to planarity. Other bands are tentatively assigned to inversion and methyl torsional motions.