Hiroshi Hiratsuka

Emitting states of benzyl, p-fluorobenzyl and p-cyanobenzylradicals

Abstract Polarization studies of the fluorescence emission and excitation spectra have been carried out by the photoselection method for the benzyl, p -fluorobenzyl and p -cyanobenzyl radicals in poly (vinyl alcohol) films at 77 K. The emitting states have been assigned to the 1 2 A 2 electronic state for the benzyl and p -fluorobenzyl radicals and the 2 2 B 2 state for the p -cyanobenzyl radical. Flu lifetimes have been determined to be 1.22±0.10, 0.36±0.04 and 0.43±0.04 μs for the three radicals, respectively.

Absorption, emission and reaction kinetics of dimethylsilylene

Abstract The transient absorption of dodecamethylcyclohexasilane in methylcyclohexane at 470 nm obtained by laser photolysis is due to dimethylsilylene, which decays exponentially with a lifetime of ≈200 ns at 293 K. The reaction rate of dimethylsilylene with methanol or triethylsilane is close to the diffusion-controlled rate (9.1 × 10 9 M −1 s −1 or 3.6 × 10 9 M −1 s −1 respectively). The 0-0 1 B 1 - 1 A 1 transition energy for dimethylsilylene is estimated as 53.7 kcal mol −1 from absorption (453 nm, 63.1 kcal mol −1 ) and emission (645 nm, 44.5 kcal mol −1 ) band peaks in MP glass at 77 K. The corresponding fluorescence lifetime is 1.3 ns.

Photophysical Property and Photostability of J-Aggregate Thin Films of Thiacyanine Dyes Prepared by the Spin-Coating Method

By use of electrostatic interactions of dye molecules and poly(diallyldimethylammonium chloride) (PDDA), the spin-coating technique has been successfully applied to the preparation of stable J-aggregate thin films of thiacarbocyanine dyes on a polycarbonate or quartz plate. The J-aggregate thin films were prepared by the spin-coating of PDDA aqueous solution on dye thin films prepared on a substrate by the spin-coating of 2,2,3,3-tetrafluoro-1-propanol solution of dyes. Photophysical properties of the dye thin films and J-aggregate thin films were studied by measuring the fluorescence spectra, quantum yields, and lifetimes. Coherent size of the J-aggregates was estimated to be 3-12 by means of the absorption bandwidth (full width at half maximum) or radiative lifetime. Photostability of the J-aggregate thin films was also studied in terms of photodegradation efficiency under argon and oxygen in comparison with the dye thin films, and J-aggregate thin films were found to be more stable than the corresponding dye thin films.

Dual emission from ion pairs produced by excited-state proton transfer in the naphthol–amine system at 77 K. Steric orientation of contact and separated ion pairs

Fluorescence in the 1-naphthol (ROH)–aliphatic amine (NR3) hydrogen-bonded system in non-polar rigid matrices at 77 K is found to be dual in nature. By means of nanosecond time-resolved emission spectroscopy the total fluorescence spectra can be resolved into two component spectra ascribable to contact and separated ion pairs produced by excited-state proton transfer in the hydrogen-bonded complex. The contact ion pair (CIP) is revealed to have a relatively shorter lifetime (5.3 ns) and a shorter-wavelength fluorescence peak (395 nm) compared to those (13.8 ns and 419 nm, respectively) of the separated ion pair (SIP). The fraction of SIP formation (α) is obtained as 0.27 in a PE film and 0.20 in MP glass, showing that CIP formation is greater than that of SIP in the ROH–NEt3 system. A significant increase in the value of α for SIP formation in the MeROH–NEt3 system is observed in comparison with that in the ROH–NEt3 system. By comparing the α values for SIP formation in different naphthol–amine systems it is possible to assign an out-of-plane (but not perpendicular) relative orientation between the excited naphtholate anion RO–* and the ammonium ion H[graphic omitted]R3 to SIP and an in-plane orientation to CIP. Changing the size of the alkyl group R of the amine gives no significant effect on excited-state ion-pair formation.

Silylation Improves the Photodynamic Activity of Tetraphenylporphyrin Derivatives In Vitro and In Vivo

The effects of silyl and hydrophilic groups on the photodynamic properties of tetraphenylporphyrin (TPP) derivatives have been studied in vitro and in vivo. Silylation led to an improvement in the quantum yield of singlet oxygen sensitization for both sulfo and carboxy derivatives, although the silylation did not affect other photophysical properties. Silylation also improved the cellular uptake efficiency for both sulfo and carboxy derivatives, enhancing the in vitro photodynamic activity of the photosensitizer in U251 human glioma cells. The carboxy derivative (SiTPPC4 ) was found to show higher cellular uptake efficiency and in vitro photodynamic activity than the corresponding sulfo derivative (SiTPPS4 ), which indicates that the carboxy group is a more promising hydrophilic group than the sulfo group in the silylated porphyrin. SiTPPC4 was found to show high selective accumulation efficiency in tumors, although almost no tumor selectivity was observed for the nonsilylated porphyrin. The concentration of SiTPPC4 in tumors was 13 times higher than that in muscle 12 h after drug administration. We also studied tumor response after treatment and found that silylation enhanced in vivo photodynamic activity significantly. SiTPPC4 shows higher photodynamic activity than NPe6 with white light irradiation.

Singlet molecular oxygen generation by water-soluble phthalocyanine dendrimers with different aggregation behavior

Phthalocyanines having hydrophilic or lipophilic dendrons were synthesized to investigate the efficiencies of singlet molecular oxygen (1Δg) formation. The introduction of higher generation of dendrons to the central metal (Si) of phthalocyanine in vertical direction to their ring plane has resulted in the successful improvement in avoiding aggregate formation that resulted in efficient generation of 1Δg even in water.

Laser-induced fluorescence of cyclohexadienyl (c-C6H7) radical in the gas phase.

A laser-induced fluorescence spectrum was observed in the 500-560 nm region when a mixture of 1,4-cyclohexadiene and oxalyl chloride was photolyzed at 193 nm. The observed excitation spectrum was assigned to the A (2)A(2)<--X (2)B(1) transition of the cyclohexadienyl radical c-C6H7, produced by abstraction of a hydrogen atom from 1,4-cyclohexadiene by Cl atoms. The origin of the A<--X transition of c-C(6)H(7) was at 18 207 cm(-1). From measurements of the dispersed fluorescence spectra and ab initio calculations, the frequencies of several vibrational modes in both the ground and excited states of c-C(6)H(7) were determined: nu(5)(C-H in-plane bend)=1571, nu(8)(C-H in-plane bend)=1174, nu(10)(C-C-C in-plane bend)=981, nu(12)(C-C-C in-plane bend)=559, nu(16)(C-C-C out-of-plane bend)=375, and nu(33)(C-C-C in-plane bend)=600 cm(-1) for the ground state and nu(8)=1118, nu(10)=967, nu(12)=502, nu(16)=172, and nu(33)=536 cm(-1) for the excited states.

Assignments of near-ultraviolet absorption and intramolecular charge-transfer emission in phenylpentamethyldisilane

Assignments of the u.v. absorption and intramolecular charge-transfer (c.t.) emission in phenylpentamethyldisilane have been studied by measurements of m.c.d. spectra and fluorescence polarization spectra with the aid of CNDO/S calculations. From the m.c.d. spectra the first absorption band at 38.5 × 103 cm–1(Iµ= 300 mol–1 dm3 cm–1) is assigned to the 1B2→1A1(or 1Lb→1A) transition, which corresponds to that of 1B2u→1A1g in benzene. Using m.c.d. measurements and molecular-orbital calculations, the mechanism of charge migration from the phenyl ring to the disilanyl group in the first excited state is established. On the basis of fluorescence polarization, in addition to the m.c.d. results, the c.t. emission of phenyldisilane is ascribed to that from the 1(2pπ, 3dπ) c.t. state produced by the 2pπ* orbital (the phenyl ring) to the vacant 3dπ orbital (the Si—Si bond) intramolecular charge-transfer transition.

Photophysical and photochemical processes of 9,10-dihydro-9-silaanthracenes: photochemical generation and electronic structure of 9-phenyl-9-silaanthracene

Abstract Photophysical processes of 9,10-dihydro-9-silaanthracene ( 1 ) and its 9-phenyl derivative ( 2 ) have been studied in 3-methylpentane at 77 K. Photolyses of these compounds have been carried out at 77 K and reaction intermediates have been studied. Carbon-centered radicals and silylenes have been observed for 1 , while for 2 , formation of carbon-centered radicals and 9-phenyl-9-silaanthracene has been confirmed by use of UV–vis absorption spectrum, fluorescence spectrum, fluorescence polarization and results of semi-empirical MO calculations. The electronic structure of 9-silaanthracene has been discussed in comparison with that of anthracene.

Photochemical processes of benzyltrimethylsilane at 77 K remarkable solvent effects and reaction mechanism

Photophysical and photochemical processes of benzyltrimethylsilane have been studied in 3-methylpentane and ethanol glass at 77 K. Remarkable solvent effects were observed in the UV photolysis: in ethanol glass, benzyl radicals were produced, while in 3-methylpentane glass α-trimethylsilylbenzyl radicals and benzyltrimethylsilane radical cations were produced. It was confirmed that the α-trimethylsilylbenzyl radicals and benzyltrimethylsilane radical cations were formed via the lowest triplet state and the benzyl radicals via the excited singlet state or higher triplet state. The molecular structure of the excited-state benzyl-trimethylsilane has been studied by MNDO-PM3 calculations. Based on the results, a possible mechanism involving the excited state of radical pair character has been proposed for the photochemical process of benzyltrimethylsilane.