Teijiro Ichimura

Recent progress on photoreactions in microreactors

A microreaction system for organic photoreactions was developed, and the processes of diastereo-differentiating photosensitized reaction, photocatalytic oxidation and reduction of organic compounds, and amine N-alkylation were examined in microspace. These model reactions proceeded very rapidly with considerably large photonic efficiencies because of some distinct properties of microreactors for photoreactions, such as higher spatial illumination homogeneity and better light penetration through the entire reactor depth, and large surface-to-volume ratio in comparison with large-scale batch reactors. These results suggest feasibility of microreaction systems on organic photoreactions.

Photolysis of monochlorobenzene in gas phase

Monochlorobenzene vapor has been photolyzed in the presence of ethane at 184.9, 206.2, 228.8, and 253.7 nm. The products benzene, ethylene, and n‐butane were detected. Quantum yields were found to be dependent on the excitation wavelength, while they were independent of the pressure of ethane above 30 torr. The possible reaction mechanism is discussed and the decomposition rate of the C–Cl bond in the molecule is estimated.

Photofragmentation of chlorobenzene: translational energy distribution of the recoiling Cl fragment

Abstract Laser photodecomposition of a C6H5Cl molecular beam was studied by the measurement of time-of-flight (TOF) distributions of the Cl photofragment at 193 and 248 nm. In a 193 nm photolysis of C6H5Cl, a distribution of the total translational energy obtained from the TOF spectrum in a 193 nm photolysis is well explained by the superposition of three distribution functions; one Boltzmann, and two Gaussian distribution functions. While a photolysis of C6H5Cl at 248 nm gave rise to a translational energy distribution which consists of two distributions, one Boltzmann and one Gaussian. In contrast to the above, a 193 nm photolysis of C6F5Cl (pentafluorochlorobenzene) resulted in a remarkably reduced kinetic energy distribution of the Cl fragments whose entire profile can be well expressed by single Boltzmann distribution function. These results can lead to a conclusion that the photodecomposition of the CCl bond in chlorobenzene by the 193 nm excitation takes place through three different dissociation channels with probabilities of similar magnitudes; (1) a direct dissociation or very fast predissociation, (2) a channel via vibrationally excited triplet levels, and (3) a channel via highly excited vibrational levels of the ground electronic state (hot molecules). The photodecomposition of C6H5Cl at 248 nm occurs dominantly via the second and third channels. Laser excited C6F5Cl, however, dissociated through hot molecules.

Photofragmentation of chlorotoluenes and dichlorobenzenes: Substituent effects on the dissociation mechanism, and angular distribution of the Cl fragment

Time-of-flight spectra of the Cl photofragments were measured for molecular beams of o-, m-, and p-chlorotoluene (ClC6H4CH3) and o-, m-, and p-dichlorobenzene (ClC6H4Cl) irradiated by a 193 nm excimer laser pulse. The observed translational energy distributions of photofragments revealed that these chlorinated benzene derivatives dissociate via three different channels: (1) very fast predissociation and/or a direct dissociation, (2) predissociation through the triplet state, and (3) predissociation via highly excited vibrational levels of the ground electronic state (hot molecules). The three dissociation channels for dichlorobenzene have similar probabilities (∼0.3) in accord with those for chlorobenzene, indicating no significant change caused by the additional chlorine atom. The methyl substituent on chlorobenzene (chlorotoluene), however, remarkably induces dissociation through triplet states, probably due to the enhanced intersystem crossing by the methyl group. The angular distribution of the photof...

Excited-State Dynamics of 6-Aza-2-thiothymine and 2-Thiothymine: Highly Efficient Intersystem Crossing and Singlet Oxygen Photosensitization

Significant influences of sulfur and aza substitution on excited-state dynamics in thymine analogues, 6-aza-2-thiothymine (ATT) and 2-thiothymine (2TT), were intensively studied by means of nanosecond transient absorption and time-resolved luminescence spectroscopy. Transient absorption spectral measurements gave distinct spectral features and sufficiently longer lifetimes for both molecules attributable to the T(1) (pi pi*) state under Ar-saturated condition. Additionally, another long-lived subsequent transient was also observed for ATT, which suggests hydrogen abstraction from a ground-state molecule itself takes place only for triplet ATT. Quantum yields for intersystem crossing (Phi(ISC)) were determined to be unity as well as other pyrimidine analogues we reported previously, and efficient photosensitized singlet oxygen O(2)* ((1)Delta(g)) formation was also observed in the presence of dissolved molecular oxygen with a quantum yield (Phi(Delta)) of 0.69 +/- 0.02 for ATT whereas it almost halved for 2TT. These findings have shown the combination of subtle substitutions can possibly control photophysical or photochemical properties such as O(2)* ((1)Delta(g)) formation or reactivity in excited states in addition to the substantial intersystem crossing caused by replacing oxygen O2 in thymine by sulfur (referred to S2 substitution henceforward). The experimental results were corroborated by quantum chemical calculation at the B3LYP/6-31+G(d,p)/PCM level.

Fast photodecomposition of chlorobenzene and m-chlorotoluene in molecular beams at 193 nm

Abstract Molecular beams of C 6 H 5 Cl and m -ClC 6 H 4 CH 3 were photodissociated using an excimer laser at 193 nm. The photodissociation of the CCl bond by the excitation to a dissociative continuum was first observed followed by the dissociation presumably through triplet levels by the measurement of time-of-flight distributions of the Cl photofragment.

Fluorescence of excited ethynyl radicals produced by pulsed vacuum ultraviolet photolyses of C2H2, C2D2, and C2HBr

Electronically excited ethynyl radicals were produced by pulsed vacuum ultraviolet photolyses of C2H2, C2D2, and C2HBr. The broad fluorescence spectra of the ethynyl radical extending from 410 to wavelengths longer than 750 nm were found to be insensitive to parent molecules photolyzed, their pressures, and also pressures of an added gas, He. Time profiles of the fluorescence intensity were found to be a superposition of two components. The dominant component is due to excited ethynyl radicals formed within the excitation pulse duration of ∼20 ns. At higher pressures, however, some minor contribution from a long‐lived precursor to the fluorescence was detected. Zero‐pressure decay rates and quenching rate constants were obtained. Formation mechanism of fluorescent ethynyl radicals is discussed briefly.

ArF laser flash photolysis of hexafluorobenzene vapor: Formation of hot molecules and their collisional relaxation

Time‐resolved absorption spectra of hexafluorobenzene vapor have been observed in the time range of 0–2 μs after pulsed ArF laser (193 nm) excitation. The absorption spectrum observed at the time t=0 is attributed to HFB* (S0) (hot hexafluorobenzene with an internal energy of 639 kJ/mol). This transient absorption spectrum can be simulated as a part of the spectrum of the S3(1E1u)←S0 transition at 3050 K on the basis of a modified Sulzer–Wieland model. The collisional deactivation of HFB* molecules is explained in terms of an energy transfer model which assumes that the energy removed per collision depends upon the internal energy.

Photophysics and photochemical dynamics of methylanisole molecules in a supersonic jet

Abstract The fluorescence excitation, dispersed fluorescence and hole burning spectra, and fluorescence lifetimes of jet-cooled o -, m -, and p -methylanisoles (MA) were measured. The low-frequency ring methyl internal rotational bands observed for their S 0 and S 1 states were assigned. In the case of m -MA, the rotational isomers of cis and trans conformers, which arise from the orientation of the OCH 3 group with respect to the CH 3 group, were assigned by hole-burning spectroscopy. The observed level energies and relative intensities of the methyl internal rotation were reproduced by a calculation using a free rotor basis set. Furthermore, their potentials in the S 0 and the S 1 states were determined. The potential barrier heights for the S 0 states of m - and p -MA were quite low, suggesting that the methyl groups are freely rotating, while changing from S 0 to S 1 states, the potential barrier height increases. The potential barrier heights of o -MA drastically decreased in going from S 0 to S 1 states. The decrease would be due to the hydrogen bonding between O atom and one H atom of the methyl group. The torsional bands of the methoxy group (–OCH 3 ) were also observed for p - and o -MA. The –OCH 3 modes are found to couple with the level of the e species for the methyl internal rotation. Fluorescence lifetimes ( τ f ) of the methyl internal rotational bands in the S 1 states of o -, m -, and p -MA were measured in order to investigate the photochemical dynamics. The values of the nonradiative rate constant ( k nr ) were estimated from the τ f values and Franck–Condon factors. The k nr values drastically increased with the excitation of methyl internal rotation. Accordingly, the methyl internal rotation should enhance the nonradiative process, presumably intersystem crossing (ISC). The enhancement should be caused by the increase of the state density ( ρ ) effectively coupled with triplet manifolds. The drastic increase in the ρ value should be caused by level mixing. In addition, the methyl internal rotational motion may enhance the increase of the coupling matrix elements through the vibronic coupling between the excited singlet states. The remarkable rotational quantum species dependence on the ISC rate constant ( k ISC ) value clearly appeared in m -MA. The dependence should result from the difference of the ρ value between a 1 and e species, since the e species are doubly degenerate. The species dependence was apparently related to the potential barrier height, suggesting that the large barrier height should have an influence on the ρ value of the triplet states.

Three dissociation channels for p-dichlorobenzene excited at 193 nm in molecular beams

Abstract A molecular beam of p -ClC 6 H 4 Cl was photolyzed using an excimer laser at 193 nm. Measurements of time-of-flight distributions of the Cl photofragment suggested the presence of three different dissociation processes: direct photodissociation, predissociation presumably through non-thermalized vibrationally excited triplet levels and predissociation probably after thermalization to the ground state.