Volume 73, Issue 3

In the 20th century, photochemistry blossomed from a poorly defined to a highly sophisticated science. Early breakthroughs in exploratory photochemistry and the underlying physical principles led to new diverse, yet inter-related areas of research. The alluring goal of asymmetric synthesis with circularly polarized light proved elusive. The discovery of the electron brought a gradual awakening to the idea of electron transfer. Time-resolved spectroscopy developed from ms to fs resolution. The field of photosynthesis progressed from an interest in function and structure of photosynthetic pigments to the isolation and structure elucidation of photosynthetic reaction centers (rhodobacter sphaeroides), to the detailed kinetics of sequential electron-transfer steps in natural and synthetic light-harvesting systems.

Solvatochromism and thermochromism of 4-aminophthalimide and 4-amino- N -methylphthalimide were studied by absorption and steady-state and time-resolved emission spectroscopy in solvent mixtures of toluene­ethanol and toluene­acetonitrile at different temperatures. Emission spectra shift to the red upon addition of a polar solvent (PS) to toluene. Solvent mixtures show a much greater thermochromic shift to the blue in emission than the neat solvents. This is explained by the decrease in temperature of the exothermic association of the polar solvent to the excited state. Emission spectra are time dependent in solvent mixtures in the ns timescale. The time evolution of this emission is interpreted on the basis of the different solvation of the ground state and the emitting excited state. Stern­Volmer plots are obtained for the dependence of the spectral-shift characteristic time with [PS].

Photoresponsive molecular receptors based on the photophysical properties of the pyrene chromophore were designed to exhibit tunable fluorescence emission upon binding of transition-metal ions.

The binding of acetyl­pepstatin to the Q7K/L33I/L63I mutant of HIV-1 protease was studied by fluorescence, phosphorescence, and 500-ps molecular dynamics. The protease is a homodimer with two tryptophans per monomer. Maximum entropy method (MEM) analysis and acrylamide quenching results show two tryptophyl, tryptophan (Trp) populations in the apoenzyme that merge into one in the complex. These results are in agreement with molecular dynamics simulations indicative of Trp asymmetry in the apoenzyme as revealed by the occurrence of nonequivalent Trp42 indole rotamer interconversions, not observed for the complex. Analysis of the local Trp42B environments of the apoenzyme with respect to possible quencher groups shows that the c 2 interconversions do not influence the lifetime, while the c 1 interconversions do. Upon binding the inhibitor, Trp42B acquires a single conformation with the same lifetime and orientation as that of Trp42, and also with less quenching accessibility. Thus, protein conformational dynamics become constrained with inhibitor binding. This conclusion is supported by red-edge effect experiments and phosphorescence lifetime measurements. The low temperature t p (~5.8 s) is quenched to ~200 ms as protein motions become activated around the glass transition temperature. In the case of the complex, the phosphorescence lifetime data show a more cooperative activation of the quenching mechanisms.

An overview of the recent activity of the laboratory in the determination of the photophysical properties of porphyrinic arrays, with emphasis on the theme of photoinduced charge separation, is presented.

Conjugated polymers now provide a class of processible, film-forming semiconductors and metals. We have worked on the development of the semiconductor physics of these materials by using them as the active components in a range of semiconductor devices. Polymer light-emitting diodes show particular promise, and recent developments in color range (red, green, and blue), efficiency (above 20 lumen/W for green emitters), and operating lifetime are discussed. Progress on their application to displays, with integration with active-matrix TFT drive, and with patterned deposition using inkjet printing techniques is also reviewed. The role played by interfaces between electrode and semiconducting polymer is also discussed.

The photodynamics of individual molecules and fluorescent proteins has been investigated in real time. In the case of organic molecules, both the triplet state lifetime and intersystem crossing yield appear to vary in time and space. In the case of autofluorescent proteins, light-driven "on-off" fluorescence emission has been observed, with a behavior that is consistent with the existence of a long-lived dark state.

A fifth-generation poly(propylene imine) dendrimer decorated with palmitoyl- and azobenzene-containing alkyl groups forms giant vesicles in aqueous solutions with diameters from 50 nm up to 20 mm and a multilaminar onion-like structure. Dense and ordered arrangement of the azobenzene chromophores in the bilayer structure leads to fluorescence with lmax= 600 nm. The fluorescence intensity can be increased by irradiation with blue light, and at low pH a distinctive blue shift of the spectrum is observed. With the aid of a single-beam optical tweezers it is possible to trap vesicles and direct them in a billiard-like fashion against each other using forces in the range of several pN. In collision experiments, the vesicles behave like hard spheres, and merging is not observed.

Fast time-resolved infrared (TRIR) spectroscopy has been used to study a wide range of organometallic alkane and noble-gas complexes at ambient temperature. We have shown that the reactivity of the n-heptane complexes decreases both across and down Groups V, VI, and VII, and that the corresponding xenon complexes have similar reactivities.

Long-distance transfer of a positive charge through DNA can be described by a hopping model. In double strands where the (A:T)n bridges between the guanines are short (n 3), the charge hops only between guanines, and each hopping step depends strongly upon the guanine to guanine distances. In strands where the (A:T)n sequences between the guanines are rather long (n 4), also the adenines act as charge carriers. To predict the yields of the H2O-trapping products one has to take into account not only the charge-transfer rates but also the rates of H2O-trapping reactions.

A new mechanism for spectral sensitization of silver halide is described, which can potentially double the sensitivity of photographic emulsions. The photooxidized sensitizing dye is trapped using an organic donor molecule, which fragments to form a cation and a reducing radical, which injects an electron into the conduction band of the silver halide. In this way, two conduction-band electrons can be produced for each absorbed photon.

Learning from the concepts used by green plants, we have developed a photovoltaic cell based on molecular light absorbers and mesoporous electrodes. The sensitized nanocrystalline injection solar cell employs organic dyes or transition-metal complexes for spectral sensitization of oxide semiconductors, such as TiO2, ZnO, SnO2, and Nb2O5. Mesoporous films of these materials are contacted with redox electrolytes, amorphous organic hole conductors, or conducting polymers, as well as inorganic semiconductors. Light harvesting occurs efficiently over the whole visible and near-IR range due to the very large internal surface area of the films. Judicious molecular engineering allows the photoinduced charge separation to occur quantitatively within femtoseconds. The certified overall power conversion efficiency of the new solar cell for standard air mass 1.5 solar radiation stands presently between 10 and 11. The lecture will highlight recent progress in the development of solar cells for practical use. Advancement in the understanding of the factors that govern photovoltaic performance, as well as improvement of cell components to increase further its conversion efficiency will be discussed.

The synthesis and photophysical and photochemical characterization of four novel bacterio-chlorin-fullerene dyads with fullerene C60 coupled at various positions and by various bridges to the chlorin ring are described. All dyads undergo rapid charge separation in the sub-picosecond to picosecond time range as characterized by time-resolved fluorescence and transient absorption spectroscopy.

The effects of temperature, pressure, solvent, and substrate concentration upon several enantio- and diastereodifferentiating photochemical reactions have been investigated and discussed in detail. These entropy-related external factors, which manipulate the weak electronic and steric interactions in the excited state, dynamically control the stereochemical outcome of asymmetric photochemical reactions, and even switch the product chirality. A global view on the "entropy chemistry" and its potential applications are also discussed.

Certain drugs are known to elicit photosensitivity side effects. A satisfactory understanding of the involved mechanistic aspects is necessary to anticipate the photosensitizing potential. We have used tiaprofenic acid (TPA), a photosensitizing nonsteroidal antiinflammatory drug, to illustrate the methodology followed to address this problem. After studying the photophysical and photochemical properties of TPA, the attention has been directed towards the reactivity of its lowest lying p-p* triplet with biomolecules. Photosensitized lipid peroxidation occurs by a mixed type I (radicals) and type II (singlet oxygen) mechanism. In the case of proteins, the photosensitized reactions include Tyr, Trp, and His photodegradation, protein­protein photocrosslinking and drug­protein photobinding. This involves direct quenching of the drug triplet by the amino acid residues (Tyr and Trp) or by oxygen, followed by singlet oxygen oxidation (His and Trp). With DNA, the studies have included comet assay, induction of single-strand breaks in supercoiled DNA, and reaction with 2'-deoxyguanosine and thymidine. Product studies, together with time-resolved measurements, have shown that the fastest reaction occurs with purine bases, by a mechanism involving both radical and singlet oxygen processes. The employed methodology can be of general use to investigate the mechanistic aspects of photosensitization by drugs.

Among the many important roles played by ozone in the atmosphere is the role it plays in the generation of OH radicals, which are responsible for initiating the oxidation of a wide variety of atmospheric trace constituents. The OH production occurs dominantly from the formation of the excited O(1D) species in the UV photolysis of ozone, followed by the reaction of O(1D) with H2O vapor. The photochemistry of ozone is very complex, as the relatively weak bonds in ozone allow different states of the O and O2 photoproducts to be accessed. Recent detailed studies have now revealed that different photolysis channels are occurring in the 290­375 nm spectral range, the region of importance for the generation of OH radicals in the lower atmosphere. The measured temperature-dependent quantum yields for the production of O(1D) atoms reflect the importance of the longer "wavelength tail" formation with regard to the enhanced OH production. Other significant atmospheric photolysis processes involving carbonyl compounds are reported. Direct photodissociation rates were measured in the outdoor photoreactor EUPHORE in Valencia and compared with model calculations. For most of the carbonyl compounds the effective quantum yields are significantly below unity.

Novel polymeric photosensitizers for singlet oxygen production in water are described. The polymers contain rose bengal (4,5,6,7-tetrachloro-2',4',5',7'­tetraiodo-fluorescein) chromophores covalently attached to the polymer chain. The quantum yields of singlet oxygen formation and photooxidation of phenol in water have been determined. The methods of separation of photosensitizer from the reactants after completion of reaction have been established.

The optical absorption and photoluminescence properties of PPE-type p-conjugated oligomers that contain a 2,2-bipyridine-5,5¢-diyl metal coordinating unit have been examined. The spectra of the free oligomers are compared with those that contain ­Re(CO)3Cl and ­Ru(bpy)22+ chromophores chelated to the bpy-diyl unit.

The principles of photochemistry continue to fuel progress in luminescent sensors and photonic switches. Examples of sensors based on photoinduced electron transfer (PET) are discussed, including those which form the basis of successful systems used in physiology and medicine. More complex formats usually involve multiple receptors. One progression takes us to lanthanide complexes enabled with sensory capabilities. Another path takes us to molecular-scale implementation of logic gates such as AND and INHIBIT. Such luminescent switches can be enriched by combination with nonluminescent cousins. The latter are based on internal charge-transfer excited states (ICT). An example of rudimentary arithmetic at the molecular scale is presented by running a luminescent AND gate in parallel with a nonluminescent XOR gate. Thus, small molecules can process small numbers for the first time outside of our brains.

Dynamics of two proteins [myoglobin and photoactive yellow protein (PYP)] initiated by photoexcitation were studied by the time-resolved transient grating (TG) and photo-acoustic methods. For both proteins, the temporal profiles of the TG signals showed new features that have not been detected before by the transient absorption method. These dynamics were attributed to the volume change, which is caused by the protein dynamics apart from the chromophores.

Chemically induced dynamic electron polarization (CIDEP) spectra generated by photoinduced electron-transfer reactions have been studied using continuous-wave time-resolved EPR and FTEPR spectroscopy. It has been suggested that the energy difference (DEST) between the singlet and triplet states of radical ion pairs (RIPs) is dependent on the charge recombination free energy (­DGCR) and reorganization energy (l). The result is interpreted in terms of the spin-selective stabilization or destabilization of RIP states by electronic coupling with the S0 and locally excited triplet states of the donor­acceptor pair at equilibrium distance in the reaction coordinate. It has been verified that a charge transfer-type exchange interaction governs the DEST. Magnitudes as well as the sign of DEST can be predicted at the distance of the solvent-separated RIP based on DGCR and l values.

Many photochemical processes involving the breaking and formation of chemical bonds take place on a femtosecond time scale. By means of ultrafast laser pulses it became possible to monitor these events in real time with a temporal resolution in the sub-20 fs regime. Moving beyond the observation to the control of chemical reactions requires additional theoretical and experimental effort. We illustrate with the aid of selected model systems the concepts and possibilities of "passive" and "active" control and their differences.

Quinone methides are widely encountered reactive intermediates in the chemistry of phenols and related compounds. This paper summarizes our recent progress in uncovering new and general photochemical methods for forming quinone methides of various structural types in aqueous solution. Their mechanism of formation and subsequent chemistry are also discussed. New examples of excited-state intramolecular proton transfer (ESIPT) have been uncovered in these studies. We have also discovered that appropriately designed biphenyls and terphenyls display photochemistry that is best rationalized by highly polarized and planar excited states of these ring systems, which can efficiently lead to the corresponding extended quinone methides.

Reversible electronic transduction of photonic processes occurring on electrodes is the conceptual method to develop molecular and biomolecular optoelectronic systems. Cyclic photochemical activation of molecular or biomolecular monolayer redox-functions provides a general methodology for the amperometric transduction of photonic information that is recorded by the chemical assembly. Alternatively, photoisomerizable monolayers associated with electrodes act as "command interfaces" for controlling the interfacial electron transfer between molecular redox-species or redox-proteins. The systems use a photonic input for the generation of an electronic output and act as information processing assemblies. Programmed arrays of photosensitizer/electron acceptor cross-linked Au-nanoparticle arrays are assembled on indium tin oxide (ITO) for photoelectrochemical applications.

A series of soluble di- and polynuclear transition-metal acetylides with rich luminescence behavior have been designed and successfully isolated. The photophysical and photochemical properties have been studied. Luminescent polynuclear metal complexes have also been obtained based on the metal chalcogenide building block. These high-nuclearity transition-metal chalcogenide complexes have been structurally characterized and shown to display rich luminescence behavior. Various approaches and strategies to design and synthesize luminescent polynuclear metal complexes that may find potential applications as chemosensors and luminescence signalling devices will also be described.

As opposed to animals, plants have to cope with the resources, environmental restrictions, herbivores, and pathogens they find in the particular spot where they are bound to grow. Hence, resource sequestration, predation and competition relationships, and adaptation to various sources of other environmental stresses and their seasonal variation must be flexible enough to ensure survival and successful reproduction. Plants express this fitness by a combination of biological traits and chemical arsenals which operate under the reign of a genome of considerable plasticity. For the great majority of plants it is either the biological characters or the chemical composition that are explored independently to understand their fitness. But only in a few instances is the combination of these two avenues examined jointly. The extensive studies on the ecology, chemistry, and toxicology of bracken (Pteridium aquilinum) make this fern one of the few examples where a reasonable explanation for its extraordinary success is beginning to emerge by the combined perception of these two most important aspects of plant life. It is the purpose of this article to briefly review how the sum of biological and chemical traits cooperates to make of bracken one of the five most pernicious weeds in the world today.

The plant kingdom offers a rich source of structural biodiversity in the form of a variety of natural products. Our work on new bioactive compounds from medicinal plants has led to the isolation and structure elucidation of a number of exciting new pharmacophores. Bioassay-guided fractionation has recently led to the discovery of a series of new acetylcholinesterase, urease and a-glucosidase inhibitors, antioxidants and other classes of bioactive compounds, which will be presented. A theory based on hydrogen bonding on the chemical basis of memory storage in the brain is presented.

In this article we highlight the contribution of molecular biology and lignin genetic engineering toward a better understanding of lignin biosynthesis and spatio-temporal deposition of lignin. Specific examples from the literature and from our laboratory will serve to underline the chemical flexibility of lignins, the complexity of the regulatory circuits involved in their synthesis, and the specific behavior of different cell types within the xylem. We will also focus on strategies aiming to reduce the lignin content or to modify the lignin composition of plants and present their impact on plant development. We will show that the ectopic expression of a specific transgene may have a different impact, depending on the genetic background, and that plants with a severe reduction in lignin content may undergo normal development. Lignification is currently benefiting enormously from recent developments in molecular biology and transgenesis, and the progress made opens the way for future developments to study how the walls of lignified plant cells are built and organized.

The structures of a range of natural products from Africa, Nepal, and Chile will be discussed.

Recent advances in the diastereo- and enantioselective synthesis of piperidine, pyrrolidine, and indolizidine alkaloids, based on the highly stereoselective 1,2-addition to the CN double bond of chiral aldehyde-SAMP/RAMP hydrazones, are described. The enantioselective syntheses of the pyrrolidine alkaloids bgugaine and (2S,12¢R)-2-(12¢-aminotridecyl)-pyrrolidine, a defense alkaloid of the Mexican bean beetle are reported. Furthermore, the SAMP/RAMP-hydrazone method was applied to the syntheses of two 5,8-disubstituted indolizidine alkaloids that have been extracted from neotropical poison-dart frogs. The a-alkylation of aldehyde-SAMP/RAMP hydrazones has been used in the enantioselective synthesis of two epimers of stenusine, a 3-substituted piperidine alkaloid and spreading reagent of the beetle Stenus comma.

One of our research lines has been the chemical studies among Calceolaria species (Scrophulariaceae), from which have been isolated 77 new diterpenes of various skeleton types (labdanes, pimaranes, stemaranes, and stemodanes), all coming from a common bicyclic intermediate, having H-9 and C-20 as a cis relationship and C-11 becomes trans with respect to C-20. Another line of research is bringing out chemical changes on diterpenes obtained from this family, using biological systems as the fungus Giberella fujikuroi and Mucor plumbeus.

One of the mysteries of man's contact with nature concerns the question why shamanism and science so often lead to consistent results in the search for useful natural products. To find clues toward an answer via a coherent chemobiological language, structural and functional information is taken into account. The functional information, given by ethnobotanical codes based on traditional knowledge (via number of useful dicotyledon species), is confronted with the structural information, given by metabolic codes based on micromolecular diversification (via number of occurrences in dicotyledons). The challenge to integrate these databases implies in the development of qualitative (via dahlgrenograms) and quantitative (via Sporne indices) models into an evolutionary framework. The follow-up of ethnobotanical qualifications reveals systematic and evolutionary patterns, susceptible to juxtaposition into a phytochemical background based on gallates (GAL) and caffeates (CAF), regulators of intermediate metabolism. Comparison of the two models of information suggests that the "spectral" features of the GAL and CAF-models are coincident with the ethnobotany-guided food- and medicine-plants respectively. Hence, the complementary nature of food and medicine species can be rationalized by oscillations between GAL/CAF. Analyses of these indicators of toxicity would allow advances in the understanding of mechanisms regulating the production of bioactive products.

Manzamine A (1) known as a cytotoxin exhibited potent antimalarial activity with 40% recovery 60 days after single intraperitoneal (i.p.) injection against mice infected with the malaria parasite Plasmodium berghei. Pachastrisamine (6), a simple derivative of a sphingosine, isolated from a sponge, Pachastrissa sp. showed a high level of cytotoxicity. New sesquiterpene carbonimidic dichlorides and related aldehydes (8­12) have been isolated as cytotoxic constituents of the sponge Stylotella aurantium and shown to have moderate cytotoxicity. Cyclic undecapeptides, barangamides A­D (13­16), have been isolated from the sponge Theonella swinhoei. Their structures were established by spectroscopic analysis and chemical reaction.

The preservation of tropical rain forests is an important goal both for the intrinsic value of their cultural and biological diversity and for the well-being of the forest peoples who make these forests their home. In addition, tropical forests are potential sources of new pharmaceutical products which can only be found by chemical prospecting in nature's genetically encoded combinatorial library. A collaborative program to discover potential pharmaceuticals in the rain forest of Suriname is described as part of an effort to integrate biodiversity conservation and drug discovery with economic development. Progress on this project will be reported, including results obtained on the isolation of bioactive diterpenoids, quinones, alkaloids, and polyketides, and the benefits of this general approach to biodiversity and drug discovery will be discussed.

Absolute configuration was established for the following semiochemicals: (S)-polyzonimine (1), (S)-9-methylgermacrene-B (2) and (1S,3S,7R)-3-methyl-a-himachalene (3). The stereoisomers of 2,6-dimethylheptane-1,7-diol monotetrahydropyranyl ether served as useful building blocks for the synthesis of syn- or anti-1,5-dimethylated aliphatic pheromones such as 4 and 5. Synthesis of analogs of the Israeli pine bast scale pheromone 6, which exhibits both pheromonal and kairomonal activities, enabled us to find a strong pheromone mimic 7 without any kairomonal activity.

Some recent results from a research program dealing with the isolation, identification, and synthesis of biologically active compounds for applications in agriculture and forestry will be reported. The solid-phase micro extraction (SPME) technique has been successfully used for identification of the main sex pheromone components of Cydia and Bonagota species (Lepidoptera, Tortricidae). Synthesis of a series of dodecadienols, tetradecadienols, and their corresponding aldehydes and acetates have been performed to be used for testing inhibitory and synergistic effects on pest insects of these species. Behaviorally active volatile compounds emitted from aphids and aphid-infested plants have been identified by SPME-technique, synthesized, and used in olfactometer tests. Methyl salicylate is shown to act in interplant communication released by plants infested by insects. In the presence of the compound, the colonization density of aphids in crop is reduced. Methyl salicylate is a common multifunctional compound, which is active in several biological systems. The action of this compound as an anti-aphrodisiac will be presented. The compound is transferred at mating from the male pierid butterfly Pieris napi to the female and makes the female unattractive for mating.

Chemical communication involves the production and release of specific chemicals (pheromones and other semiochemicals) by the emitter, and the detection and olfactory processing of these signals leading to appropriate behavioral responses in the receiver. In contrast to most of the scarab species investigated to date, the Japanese and Osaka beetles have the ability to detect the allospecific pheromone, which plays a pivotal role in the isolation mechanism between these two species. Each species produces a single enantiomer of japonilure [(Z)-5-(dec1-enyl)oxacyclopentan-2-one], but they have evolved the ability to detect both enantiomers, one as an attractant and the other as a behavioral antagonist (stop signal). There is growing evidence in the literature that the inordinate sensitivity and selectivity of the insect olfactory system is achieved by a combination of various olfactory-specific proteins, namely, odorant-binding proteins (OBPs), odorant receptors (ORs), and odorant-degrading enzymes. The relationship between the pheromone structures and the primary sequences of the proteins suggest that OBPs play a part in the selectivity of the olfactory system in scarab beetles by "filtering" chemical signals during the early olfactory processing (perireceptor events). Nevertheless, it is unlikely that pheromone-binding proteins are "chiral filters" as the Japanese and Osaka beetles each possess only one single binding protein. Upon interaction with negatively charged membranes, OBPs undergo conformational changes that may lead to the release of the ligands.

Crude extracts of Rutales species were tested in vitro against the trypomastigote form of Trypanosoma cruzi at 4 mg/mL, and 20% of them showed significant activity (80%). Their inhibitory activity against the glycolytic enzyme GAPDH from T. cruzi has also been evaluated at the concentrations of 100 and 200 mg/mL. Additionally, the inhibitory activity of 13 purified coumarins were also assayed against T. cruzi-GAPDH. Chalepin was the most active substance with IC50 = 64 mM. The 3D structure of the complex chalepin-enzyme was elucidated by X-ray crystallography, revealing the architecture of the interactions between the inhibitor and the enzyme active site.

An enantioselective synthesis and resolution of the key White intermediate (3) for the synthesis of trisporic acids are described. Attempts to develop a synthetic route toward the antiulcerogenic compound cassiol (2) by an olefination reaction of 3 and an alternative sequence involving a Michael addition followed by an aldol condensation of an open substrate, are also reported.