Volume 75, Issue 8

The making and breaking of σ bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of σ delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.

Various continuous-wave and pulse electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) experiments performed on the radical species occurring in photosynthetic reaction centers of plants and bacteria during light-induced charge separation are reviewed here. Emphasis is placed on time-resolved experiments performed on short-lived intermediate states such as radical pairs and triplet states for which also hyperfine information can be obtained from pulse ENDOR spectroscopy. Detailed insight into the electronic structure of these intermediates and their interaction with the protein environment is now becoming available.

The ability of time-resolving enthalpy (Δ H ) and structural volume (Δ V ) changes in the nano- to µs time range offered by laser-induced optoacoustic spectroscopy (LIOAS) opens the possibility of a stepwise thermodynamic analysis of chromophore-medium interactions upon photoinduced reactions in biological systems. We applied LIOAS to biological photoreceptors, as well as to model systems, with the purpose of understanding the origin of Δ V in electron-transfer (ET) reactions in those systems. The linear correlation between the counterion-dependent volume changes and ΔH for the free-radical formation upon ET quenching of erythrosin dianion triplet, 3 Er 2- , by Mo(CN) 8 4- and of Ru(bpy) 3 2+ by MV 2+ is interpreted in terms of an enthalpy–entropy compensation owing to the strong influence of the counterions on the water hydrogen-bond network in which the reactants are embedded. The relatively large entropic term determined for radical formation thus originates in water rearrangements during the process. The increasing contraction in acetonitrile, propionitrile, butyronitrile, and valeronitrile for the ET quenching of 3 Zn-tetraphenylporphin by 1,4-benzoquinone is understood by considering the increasing interaction strength between the electron-pair donor nitriles and ZnTPP + . Thus, in polar environments, specific chromophore-medium (solvent or proteins) interactions, in addition to electrostriction, should be considered to explain the time-resolved Δ V and Δ H values.

Attempts to stabilize the triplet state of carbenes to the extent that they can survive under normal conditions were made, in which triplet carbenes having a half-life of 19 min are realized. A way to use the resulting persistent carbene as a unit to construct photosensitive magnetic materials is also discussed.

Regio- and stereoselective photoallylation of electron-deficient alkenes by use of allylic silanes via photoinduced electron transfer has been described. Similar photoinduced  functionalization reactions such as arylmethylation, alkylation, and silylation can be achieved by using a variety of organosilicon compounds. These photoreactions proceed via radical cations of organosilicon compounds and radical anions of electron-deficient alkenes as reactive intermediates. The key step of the photoreactions is the attack of carbon radicals, which are generated from the radical cations of organosilicon compounds, on the radical anions of alkenes. The mechanism of the regio- and stereoselective photofunctionalization is discussed.

Two classes of rotaxanes are described in which photoinduced processes modulate a large-amplitude motion. In the first type, E–Z-isomerization of a fumaric diamide unit to a maleic diamide leads to a substantial weakening of the hydrogen bonds between the diamide and the macrocyclic ring that surrounds it. As a result, the rate of the pirouetting motion is increased approximately by six orders of magnitude. In the second type, intermolecular photoinduced electron transfer is used to induce a reversible shuttling motion on a time scale of microseconds. Medium effects on the rate of shuttling are presented.

Encapsulation of chromophores within Cram-type hemicarcerands allowed the investigation of fundamental photophysical phenomena, such as long-range triplet energy transfer, electron transfer, and the remote heavy atom effect. Furthermore, novel water-soluble hemicarcerands are being used to develop unique hybrid materials composed of semiconductor nanoparticles and host–guest assemblies. Photoinduced charge injection from the “incarcerated” guest into the conduction band of the semiconductor has been demonstrated.

This article describes the utilization of porphyrin and fullerene architectures—yielding artificial light-harvesting antenna and reaction center mimics—to tune the electronic coupling element between electron donor and electron acceptor and ultimately to affect the total reorganization energy. Most importantly, the effects that these parameters have on the rate, yield, and lifetime of the energetic charge-separated states are discussed.

Novel crown-containing 2-styrylbenzothiazoles were prepared to obtain optical ditopic receptors that can bind alkaline-earth metal cations by a crown ether fragment and could serve as host for the heavy metal cations and proton owing to the presence of the heterocyclic part. Photochemical transformations of the ditopic receptors depend on the type of metal cations present in solution. They include trans–cis -isomerization, [2+2]-cycloaddition, and electrocyclic reaction.

Novel structured functional hybrid materials can be obtained by incorporating photoresponsive organic components into the silicate walls of periodic mesoporous silicates. Two such materials are described, containing either viologens or stilbene analogs. Thermal or photochemical activation of viologen units generates the corresponding radical cations, whose lifetime can vary from months to milliseconds, depending on the surfactant content of the solid. In a second example, the porosity and tortuosity of the silicate can be modulated by irradiation of an organosilica containing a stilbene-like moiety.

Propagation rate coefficients, k p , as a function of temperature, are reported for bulk free-radical homopolymerizations of oxiranylmethyl, cyclohexyl, and benzyl methacrylate at ambient pressure and low conversion. The data were obtained from experiments combining pulsed-laser initiated polymerization and size-exclusion chromatography. The data determined from experiments carried out in independent laboratories obey the consistency criteria established for this technique. The rate coefficients for the three monomers are well represented by a single Arrhenius relation.

In view of the significance of environmental analytical data, it is essential that the quality of both sampling strategy and analysis be assured and that procedures used, as well as all relevant additional information, are reported. There is a minimum level of information required in order to guarantee the fitness-for-use of the data. Emanating from discussions on the fundamental problems of the analysis of environmental samples for chemical or biological contaminants, a general guidance is given regarding the minimum information that should be provided to adequately describe the sampling strategy, method of sampling, sample properties, handling between sampling and analysis (including storage conditions, pretreatments, homogenization, subsampling), and the analytical methodology (including calculation and validation procedures). Special attention and specific guidance are given for the environmental compartments soil, pore water, groundwater, inland surface water, sediment, seawater, precipitation water, and air.

The biennial review of atomic-weight, A r (E), determinations and other cognate data have resulted in changes for the standard atomic weights of the following elements:  FromToZinc65.39 ± 0.0265.409 ± 0.004Krypton83.80 ± 0.0183.798 ± 0.002Molybdenum95.94 ± 0.01 95.94 ± 0.02Dysprosium162.50 ± 0.03162.500 ± 0.001 Presented are updated tables of the standard atomic weights and their uncertainties estimated by combining experimental uncertainties and terrestrial variabilities. In addition, this report again contains an updated table of relative atomicmass values and half-lives of selected radioisotopes. Changes in the evaluated isotope abundance values from those published in 1997 are relatively minor and will be published in a major review of each element in 2003. Many elements have a different isotopic composition in some nonterrestrial materials. Some recent data on parent nuclides that might affect isotope abundances or atomic-weight values are included in this report for the information of the interested scientific community.

National governments introduced residue limits and guideline levels for pesticide residues in water when policies were implemented to minimize the contamination of ground and surface waters. Initially, the main attention was given to drinking water. Regulatory limits for pesticide residues in waters should have the following characteristics: definition of the type of water, definition of the residue, a suitable analytical method for the residues, and explanation for the basis for each limit. Limits may be derived by applying a safety factor to a no-effect-level, or from levels occurring when good practices are followed and also passing a safety assessment, or from the detection limit of an analytical method, or directly by legislative decision. The basis for limits and guideline values issued by WHO, Australia, the United States, New Zealand, Japan, Canada, European Union, and Taiwan is described, and examples of the limits are provided. Limits have been most commonly developed for drinking water, but values have also been proposed for environmental waters, effluent waters, irrigation waters, and livestock drinking waters. The contamination of ground water is of concern because it may be used as drinking water and act as a source of contamination for surface waters. Most commonly, drinking water standards have been applied to ground water. The same terminology may have different meanings in different systems. For example, guideline value (GV) in WHO means a value calculated from a toxicology parameter, whereas in Australia, a GV is at or about the analytical limit of determination or a maximum level that might occur if good practices are followed. In New Zealand, the GV is the concentration where aesthetic significance is influenced.The Australian health value (HV) is conceptually the same as the WHO GV. The New Zealand maximum acceptable value (MAV) and the Canadian maximum acceptable concentration (MAC) are also conceptually the same as the WHO GV. Each of the possible ways of defining the residues has its merits. A residue limit in water expressed as the sum of parent and toxicologically relevant transformation products makes sense where it is derived from the acceptable daily intake (ADI). For monitoring purposes, where it is best to keep the residue definition as simple as possible for the sake of practical enforcement and economy, theparent or a marker residue is preferable. It is also possible for parent and degradation products (hydrolysis and photolysis products and metabolites) to become physically separated as the water moves through soil strata, which suggests that separate limits should be set for parent and important degradation products. The Commission has made 12 recommendations for regulatory limits for pesticide residues in water. The recommendations will act as a checklist for authorities introducing or revising limits or guidelines for pesticide residues in water.