Showing papers in "Journal of Photochemistry and Photobiology C-photochemistry Reviews in 2000"

Titanium dioxide photocatalysis

Abstract: Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO 2 ), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO 2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO 2 and its applications are presented.

Clay minerals as photochemical reaction fields

Abstract: This article surveys the intercalation characteristics of a number of clay minerals which can be utilized as host materials for photofunctional organic–inorganic hybrid systems as well as photochemical anisotropic reaction fields. The introduction of this review describes the chemical compositions and the structural features of various kinds of artificial as well as naturally produced clays. The chemical structures of these clays are often considered to be complicated due to their unfamiliar names which are sometimes taken from the areas where they originate although, structurally, only slight differences actually exist. The second chapter summarizes the adsorption and aggregation properties of typical organic dyes in clays. The aggregation behavior is sensitive to the kind and amount of metallic ions present in the lattice of the clays, the amount of adsorbed water as well as the structure of the organic guests to be intercalated. All of these factors are discussed in relation to the adsorption and aggregation behavior. In the third chapter, the photochemical reactions of excited metal complexes adsorbed in clays are discussed. Electron transfers for [Ru(bpy)3]2+ in clays are described, showing that the clay layers provide reaction fields which stabilize the resulting charge separated species. This article aims to provide important insights into the fundamental mechanisms and properties of anisotropic reaction fields for the applications in design of promising, new photochemical and photobiological systems and processes.

Controlling photochemical reactions via confinement: zeolites

Abstract: Zeolites, aluminosilicates, with well-defined internal architecture containing cavities, channels or cages, can accommodate a large number of organic guest molecules. Internal structure of zeolites is studded with cations, the counter-ions of the anionic framework. The internal constrained structure and the cations serve as handles for chemists to control the behaviour of guest molecules included within zeolites. Photochemical reactions carried out in zeolite are highlighted in this review.

Large molecules in high-intensity laser fields

Abstract: New research fields have opened up that are related to the interactions between molecules and high-intensity optical fields where the laser intensity ranges from 1012–1017 W cm−2. A broad outline of this area will be described from the perspective of products and new techniques for beam generation. Studies of large molecules have begun and some examples are introduced herein. Parent ions with little fragmentation are found to form in the intensity region below 1016 W cm−2. The formation of intact ions can be used in femtosecond laser mass spectrometry. In the intensity region above 1016 W cm−2, electrons are stripped from the molecules by optical field ionization and the highly charged ions can undergo a Coulomb explosion. Coulomb explosions of benzene and C60 have been demonstrated, and the mechanism can be analyzed by means of molecular dynamics simulations. A high intensity femtosecond laser beam can be converted to radiation sources of coherent VUV light, X-rays etc. and some possibilities for new chemical applications will be discussed.

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.

Photochemistry of cyclopropanes, methylenecyclopropanes, and vinylidenecyclopropanes

Abstract: This review deals with the recent advances in the photochemistry of cyclopropanes, methylenecyclopropanes, and vinylidenecyclopropanes cis–trans Photoisomerization of 1,2-diarycyclopropanes via excited singlet and triplet states and radical cations, photochemical polar addition to arylcyclopropanes, and photooxygenation of arylcyclopropanes and methylenecyclopropanes giving cyclic peroxides are described The new photochemistry of vinylidenecyclopropanes including cis–trans photoisomerization, (3+2) photocycloadditions, and photorearrangements is also discussed

Near-field fluorescence spectroscopy and photochemistry of organic mesoscopic materials

Abstract: In this Review, an emerging research field of near-field fluorescence and photochemistry studies on molecular materials is introduced and relevant background, instrumentation, attractive topics, and future perspectives are discussed. Principles of near-field scanning optical microscope and technically important points are described, and our picosecond near-field fluorescence microspectroscopic system is explained. Its high performance of 100 nm spatial, a few ps temporal, and 1 nm spectral resolutions makes it possible to correlate topography, fluorescence image, fluorescence spectrum, and fluorescence rise and decay curve with each other. Near-field fluorescence spectroscopy reveals thickness-dependent fluorescence spectra of tetracene microcrystals, relations between photophysics and morphology of charge transfer microcrystals, and inhomogeneous inner structure of single microcrystals in anthracene-tetracene films. Similar fluorescence and morphology studies are described for polymer films, Langmuir-Blodgett films, and J aggregates. Some anthracene solids show interesting photothermal and photochemical nanometer morphological changes, while photoisomerization in organic crystals, and photolithography and ablation of polymer films upon near-field excitation are introduced. Future perspectives near-field excitation with shorter wavelength and/or higher intensity and various kinds of pump-probe measurement are discussed in view of photochemistry studies.

Primary processes in plant photosynthesis : photosystem I reaction center

Abstract: The photosystem I (PSI) pigment-protein complex of plants converts light energy into a transmembrane charge separation, which ultimately leads to the reduction of carbon dioxide. Recent studies on the dynamics of primary energy transfer, charge separation, and following electron transfer of the reaction center (RC) of the PSI prepared from spinach are reviewed. The main results of femtosecond transient absorption and fluorescence spectroscopies as applied to the P700-enchied PSI RC are summarized. This specially prepared material contains only 12–14 chlorophylls per P700, which is a special pair of chlorophyll a and has a significant role in primary charge separation. The P700-enriched particles are useful to study dynamics of cofactors, since about 100 light-harvesting chlorophylls are associated with wild PSI RC and prevent one from observing the elementary steps of the charge separation. In PSI RC energy and electron transfer were found to be strongly coupled and an ultrafast up-hill energy equilibration and charge separation were observed upon preferential excitation of P700. The secondary electron-transfer dynamics from the reduced primary electron acceptor chlorophyll a to quinone are described. With creating free energy differences (ΔG0) for the reaction by reconstituting various artificial quinones and quinoids, the rate of electron transfer was measured. Analysis of rates versus ΔG0 according to the quantum theory of electron transfer gave the reorganization energy, electronic coupling energy and other factors. It was shown that the natural quinones are optimized in the photosynthetic protein complexes. The above results were compared with those of photosynthetic purple bacteria, of which the structure and functions have been studied most.

Photochemistry of metalladichalcogenolene complexes

Abstract: In Section 1 we have provided a classification of the photoreactions. Later, we have applied it to the photoreactions of the metalladichalcogenolene complexes. We recognize that metalladichalcogenolene complexes show various types of photoreactions. The study of metalladichalcogenolene complexes is a versatile field of photochemistry.

Intramolecular photoexcitation dynamics and magnetic field effects in an intermediate-case molecule

Abstract: Molecular vibration and rotation play a significant role in the intramolecular photoexcitation dynamics of the so-called intermediate-case molecule, and the fluorescence intensity, decay and polarization of s-triazine vapor are shown to depend on the excited rovibronic level of the S1 state. Fluorescence characteristics are interpreted by assuming three zero-order states: (1) a zero-order singlet state that carries the absorption intensity and emits fluorescence with sharp structure; (2) zero-order singlet states that do not carry the absorption intensity but emit broad fluorescence; and (3) zero-order triplet states. The interaction among these states depends not only on the vibrational level but also on the rotational level excited. It is suggested that the number of triplet states coupled to the singlet state increases with increasing excess vibrational energy. It is also suggested that K-scrambling occurs both in the triplet manifold following intersystem crossing (ISC) and in the singlet manifold following intramolecular vibrational energy redistribution (IVR). The fluorescence intensity and decay of s-triazine vapor are significantly influenced by a magnetic field, and the field effects are interpreted in terms of the spin decoupling in the triplet manifold following ISC; the role of external magnetic fields is to mix the spin sublevels of different rovibronic levels coupled to the excited singlet state. Magnetic depolarization of fluorescence also occurs because of the efficient interaction between the excited singlet state and the triplet state.