Volume 83, Issue 4

Latest in a long series of successful conferences, the XXIIIrd IUPAC Symposium on Photochemistry was held in Ferrara, Italy on 11-16 July 2010. The conference venues were the Opera Theatre and the Estense Castle, in the historic center of the city. The contrasting mix of modern science and ancient environment was a special trait of the Ferrara symposium. The symposium was attended by over 500 delegates (including some 130 Ph.D. students) from 40 different countries. The scientific program consisted of 8 plenary lectures, 23 invited lectures, 97 selected oral presentations, as well as 354 posters. A highlight of the symposium was the presentation of the Porter Medal to Prof. David Phillips of Imperial College London, UK, in recognition of his outstanding contributions to several fields of photochemistry. The title of his lecture was “Targeted sensitizers for photodynamic therapy”. The wide variety of fields encompassed by modern photochemistry is reflected by the list of sessions held within the symposium: Electron and Energy Transfer, Molecular Switches and Machines, Organic Photochemistry, Inorganic Photochemistry, Photochromic Systems, Solar Energy, Supramolecular Photochemistry, Nanoparticles, Photocatalysis, Ultrafast Spectroscopy, Theoretical Photochemistry, Exciton and Charge Dynamics, Microscopy, Nanoscopic Systems, Singlet Oxygen and Phototherapy, Photobiology, Fluorescent Labels, Photoactive Materials, Applied Photochemistry, and Organized Media. Most of the topics discussed were characterized by a fertile combination of fundamental insight, advanced techniques, and practical application. This issue of Pure and Applied Chemistry collects a number of papers based on plenary and invited lectures delivered at the symposium. I hope that this collection will help illustrate modern photochemistry not only as a lively and exciting research field but also as a powerful resource toward the solution of important practical problems. Franco Scandola Conference Editor

The sensitizers in common use for photodynamic therapy (PDT) are summarized, and approaches to the improvement of these outlined. Selectivity in the targeting of sensitizers to tumor cells and tissue is highly desirable, as is water solubility and prevention of aggregation. Some new free sensitizers are described, based upon the pyropheophorbide a (PP a ) structure, and their photophysical properties, distribution in cells via confocal fluorescence microscopy, and cell kill properties described. A novel approach to targeting is to covalently attach such sensitizers to monoclonal antibody fragments, and recent work on the attachment of pyropheophorbide a to such monoclonal antibody fragments is reviewed, with a demonstration of the increased efficiency of cell kill, and the treatment of a solid human tumor in a mouse model described. Finally, an alternative method of achieving selectivity based upon two-photon excitation (TPE) using porphyrin dimer sensitizers is reviewed, and the use of these to kill tumor cells is compared with the use of a commercially available PDT sensitizer (Visudyne). TPE of a porphyrin dimer sensitizer is shown to be capable of sealing blood vessels in a mouse model.

In order for solar energy to serve as a primary energy source, it must be paired with energy storage on a massive scale. At this scale, solar fuels and energy storage in chemical bonds is the only practical approach. Solar fuels are produced in massive amounts by photosynthesis with the reduction of CO 2 by water to give carbohydrates but efficiencies are low. In photosystem II (PSII), the oxygen-producing site for photosynthesis, light absorption and sensitization trigger a cascade of coupled electron-proton transfer events with time scales ranging from picoseconds to microseconds. Oxidative equivalents are built up at the oxygen evolving complex (OEC) for water oxidation by the Kok cycle. A systematic approach to artificial photo-synthesis is available based on a “modular approach” in which the separate functions of a final device are studied separately, maximized for rates and stability, and used as modules in constructing integrated devices based on molecular assemblies, nanoscale arrays, self-assembled monolayers, etc. Considerable simplification is available by adopting a “dye-sensitized photoelectrosynthesis cell” (DSPEC) approach inspired by dye-sensitized solar cells (DSSCs). Water oxidation catalysis is a key feature, and significant progress has been made in developing a single-site solution and surface catalysts based on polypyridyl complexes of Ru. In this series, ligand variations can be used to tune redox potentials and reactivity over a wide range. Water oxidation electrocatalysis has been extended to chromophore-catalyst assemblies for both water oxidation and DSPEC applications.

A series of modified γ-cyclodextrins (CDs) were prepared as chiral host for catalyzing the enantiodifferentiating photocyclodimerization of methyl 3-methoxy-2-naphthoate (NA). The complexation behavior of NA with modified γ-CDs was studied by UV–vis, fluorescence, and circular dichroism spectroscopies. All of the modified γ-CDs formed stable 1:2 host–guest complex with NA, and binding affinities for two-step complexation critically depended on the structure of modified γ-CDs. The enantioselectivity of NA photocyclodimerization was also significantly affected by the modification of γ-CD. The secondary rim-modification considerably reduced the enantioselectivity, for which the interrupted hydrogen-bonding network, leading to a flexible CD skeleton, is most probably responsible.

Luminescent Os(II) and Ir(III) complexes containing a tripodal-type structure terminalized with three thiol derivatives are described. The tripod is introduced through derivatization, with a rigid spacer, of a phenanthroline ligand coordinated to the metal ion, and the entire structure possesses axial geometry. The geometry of the complexes combined with the three anchoring sites, the thiol groups, allows the complexes to adopt an almost perpendicular arrangement to the surfaces and the formation of a well-packed monolayer on Au substrates. The photophysical and electrochemical behavior of the complexes is studied in solution and on surfaces. Furthermore, a self-assembled monolayer (SAM) of Os(II) complexes on an ultraflat Au surface is used to fabricate a metal–molecule–metal junction with Au and In Ga eutectic as electrodes. The Os(II) SAM in the tunneling junction exhibits rectification behavior which is opposite in direction to that which we have previously shown for Ru(II) SAMs.

Pterins, a family of heterocyclic compounds derived from 2-aminopteridin-4(1 H )-one, are widespread in living systems and participate in important biological functions, such as metabolic redox processes. Under UV-A excitation (320–400 nm), aromatic pterins (Pt) can generate reactive oxygen species (ROS), as a consequence of both energy- and electron-transfer processes from their triplet excited state. Quantum yields of singlet oxygen ( 1 O 2 ) production depend largely on the nature of the substituents on the pterin moiety and on the pH. Formation of the superoxide anion by electron transfer between the pterin radical anion and molecular oxygen leads to the production of significant amounts of hydrogen peroxide (H 2 O 2 ) by disproportionation. Dihydropterins (H 2 Pt) do not produce 1 O 2 but are oxidized by this species with high rate constants yielding pterins as well as H 2 O 2 . In contrast to aromatic derivatives, H 2 Pt are oxidized by H 2 O 2 , and rates and products strongly depend on the nature of the substituents on the H 2 Pt moiety. Aromatic pterins have been found in vivo under pathological conditions, e.g., biopterin or 6-carboxypterin are present in the skin of patients affected by vitiligo, a depigmentation disorder. The biomedical implications of the production of ROS by pterin derivatives and their reactivity with these species are discussed.

Development of nanotechnology requires that the interaction between components of nanoscale devices can be studied and understood better. Photoinduced processes between small (2–5 nm in core diameter) gold nanoparticles (GNPs) and chromophores, including porphyrin, fullerene, and phthalocyanine, were studied. Two methods were selected for controlled assembly of chromophores and gold particles at close distances: thin films and covalent attachment, which yields chromophore-functionalized particles. Time-resolved spectroscopic and photoelectrical measurements carried out further confirmed the known strong effect of GNPs on photoexcited chromophores that leads to fast photoinduced processes. When GNPs are combined with suitable chromophores, the particles can act as electron donors or acceptors. GNPs are efficient energy acceptors, and some indications of their ability to act as energy donors were obtained. The participation of the GNPs in photoinduced charge transfer in addition to energy-transfer processes makes them attractive components for photoactive devices.

The interesting emission properties of cyclometalated iridium(III) polypyridine complexes, originated from a range of excited states, have been well documented. The intense and long-lived emission of these complexes has been exploited in various areas of research including photovoltaic cells, chemosensors, and light-emitting devices. Additionally, there is an emerging interest in the applications of these luminescent complexes in various biological studies. In this paper, we summarize our recent work on the utilization of luminescent cyclometalated iridium(III) polypyridine complexes as biomolecular and cellular probes.

The steady-state as well as transient absorption spectroscopy of a series of 2-bromo-aryl ketones have been comprehensively examined to gain insights concerning (i) the transient phenomena (absorption spectral attributes as well as lifetimes), (ii) rates of C–Br homolysis, and (iii) the behavior of 2-aroylaryl radicals thus generated. The X-ray crystal structure analyses of selected ketones in which the mesomeric effects operate differently reveal that the two aryl rings are drastically twisted about the C=O bond. The twisting manifests itself in the spectral features of the transients, attributed to triplet–triplet (T–T) absorptions, such that they are not readily comparable in some cases to the transients of parent diaryl ketones that lack the 2-bromo group. By associating triplet decays with C–Br cleavage rates, the absolute rate data have been determined for diverse 2-bromoaryl ketones. With the exception of 2-bromo ketones containing meta -methoxy substituents, all other ketones are found to undergo efficient C–Br bond cleavage with rates of ca. 0.1–1.0 × 10 8 s –1 . For m -methoxy-substituted ketones, intriguingly slower deactivation of the triplets was observed. Based on solvent-dependent variation of the lifetimes (longer lifetimes in polar solvents), intramolecular charge transfer has been proposed. The preparative photochemistry and transient phenomena permit invaluable inferences as to the reactivity of 2-aroylaryl radicals in general. Quantum yield determinations and product analyses reveal that highly electrophilic aryl radicals undergo radical recombination, in a poor hydrogen-donating solvent, almost exclusively (>90 %) in the absence of incentive for stabilization via conversion to π-conjugated hydrofluorenyl radicals. Of course, when the latter is feasible, Pschorr cyclization leads to productive photochemical outcome. Moderately electrophilic radicals that lack stabilization via conversion to hydrofluorenyl radicals lend themselves to intramolecular 1,5-hydrogen shifts in conjunction with the formation of dehalogenated diaryl ketones and cyclized fluorenones (Fls) or its analogs.

A series of homoleptic Ru(II) star complexes were synthesized and bound to semiconductor metal oxide nanoparticle films. In the nanosized, highly symmetrical “star”-shaped complexes, the Ru(II) center is coordinated to three identical bipyridine ligands carrying conjugated oligophenylenethynylene (OPE) rigid linker units terminating with carboxylic anchor groups. In this report, we describe how this concept is being applied toward the development of chromophoric units that are isolated from the surface of the semiconductor, and some possible advantages of this surface engineering strategy.

Laser trapping of molecules and proteins in solution at room temperature is made possible by irradiating 1064-nm continuous-wave (CW) laser with power around 1 W. Although conventional small molecules are not trapped at the focal point, molecules that can form clusters upon assembling and proteins whose size is close to 10 nm are gathered, giving unique assembly structure. Glycine in H 2 O shows crystallization, urea in D 2 O gives a millimeter-sized giant droplet, and cobalt oxide-filled ferritin protein confirms assembly followed by precipitation. Solute concentration, solvent, and laser power are important factors for determining trapping and assembling phenomena, and the laser focal position is very critical. These unique behaviors are realized by setting the irradiation at the air/solution surface, inside the solution, and at the glass/solution interface. Laser trapping-induced crystallization, liquid/liquid phase separation, and precipitation are compared with the previous results and considered. After summarizing the results, we describe our future perspective and plans.

The fabrication and use of metal nanoparticles to influence electronic transitions in a given molecule is of growing interest; there is much to be gained by developing and exploiting methods to enhance weak optical signals. Singlet molecular oxygen, O 2 (a 1 ∆ g ), which is an important intermediate in many oxidation reactions, particularly in biological systems, is ideally monitored by the 1275-nm O 2 (a 1 ∆ g ) → O 2 (X 3 Σ g – ) phosphorescent transition. Unfortunately, the latter is highly forbidden and, as such, often presents a severe limitation in the application of this optical probe. In this paper, we describe how this weak phosphorescent transition can be enhanced by using localized surface plasmons (LSPs) from specially engineered gold nanostructures. In an attempt to elucidate the mechanism of this process, data were recorded from samples in which we decoupled the component of the plasmon resonance that absorbs incident light from the component that scatters incident light. We find that the latter appears to be the feature of significance in the process through which singlet oxygen phosphorescence is enhanced. In this work, we also illustrate how the singlet oxygen system provides an ideal model for a general study of metal-enhanced radiative rate constants.

The photophysical properties of molecules containing anthracene, pyrene, or phenyleneethynylene chromophores bearing complementary triple H-bonding terminal units, namely, 2,6-di(acetylamino)pyridine (donor–acceptor–donor, DAD) and uracyl (acceptor–donor–acceptor, ADA) have been investigated as a function of solvent polarity. For asymmetric systems, presenting only one H-bonding unit, a solvatochromic effect is found, suggesting a charge-transfer character of the lowest electronic excited state. Systematic absorption and emission studies carried out as a function of temperature show that phenylene-ethynylenes having linear geometry and H-bonding functionalities at both ends undergo reversible self-aggregation in cyclohexane (CHX), leading to the formation of spherical nanoparticles, as evidenced by wide-field fluorescence microscopy (WFM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A combination of an anthracene derivative bearing only one ADA terminal functionality and a linear phenylene-ethynylene derivative possessing two DAD terminal groups in CHX (2:1 molecular ratio) leads to the formation of vesicular nanostructures. The interaction of linear phenylene-ethynylenes possessing two terminal 2,6-di(acetylamino)pyridine functionalities with that bearing bis uracylic units gives origin to nanofibers, while the assembly of the former with bisuracylic units exhibiting bent geometry leads to the formation of helical nanofibers. The length of these fibers can be controlled by addition of the anthracene derivative having only one uracyl group which effectively blocks the extent of H-bonding, prompting the formation of shorter nanorods.

The ketone-photoinduced formation of Au, Ag, and Cu nanoparticles from their corresponding ions in solution has been carried out using benzoin photoinitiators. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver, but rather because of the reducing free radicals they can generate. Efficient photochemical nanoparticle generation thus requires a careful selection of substrates and experimental conditions such that free radical generation occurs with high quantum efficiency, where metal ion precursors do not inhibit radical formation. A key consideration to achieve nanoparticle synthesis with short exposure times is to minimize excited-state quenching by metal ions. Applications of nanostructures in catalysis require control of the nanoparticle characteristics, such as dimension, morphology, and surface properties. Part of this article describes the strategies to modify photochemically prepared particles. Finally, we illustrate some of the nanoparticle applications that interest us, with some emphasis on plasmon-mediated processes.

This glossary of terms covers phenomena considered under the very wide terms photocatalysis and radiation catalysis . A clear distinction is made between phenomena related to either photochemistry and photocatalysis or radiation chemistry and radiation catalysis. The term “radiation” is used here as embracing electromagnetic radiation of all wavelengths, but in general excluding fast-moving particles. Consistent definitions are given of terms in the areas mentioned above, as well as definitions of the most important parameters used for the quantitative description of the phenomena. Terms related to the up-scaling of photocatalytic processes for industrial applications have been included. This Glossary should be used together with the Glossary of terms used in photochemistry , 3 rd edition, IUPAC Recommendations 2006: (doi:10.1351/pac200779030293) as well as with the IUPAC Compendium of Chemical Terminology , 2 nd ed. (the “Gold Book”, 2006– doi:10.1351/goldbook) because many terms used in photocatalysis are defined in these documents.