The Many Facets of Photochemistry

George S. Hammond was one of the initiators of the IUPAC Symposium on Photochemistry, which first took place in 1964 in Strasbourg, France. Since then, this conference has been held biannually in Europe. This year marked the first time the conference was held outside of Europe—in Kyoto, Japan, with Masahiro Irie (Kyushu University) and Atsuhiro Otsuka (Kyoto University) acting as chairmen. The opening presentation of the conference was devoted to George S. Hammond. In an obituary, David Whitten (University of New Mexico, Albuquerque) paid tribute to the personality and scientific work of this pioneer of photochemistry and physical organic chemistry, who passed away in 2005 at the age of 84 (see Angew. Chem. 2006, 118, 1034; Angew. Chem. Int. Ed. 2006, 45, 1018). As in previous years, the numerous presentations this year reflected the interdisciplinary character of photochemistry. The spectrum of topics ranged from the classical fields of chemistry—inorganic, organic, physical, and theoretical chemistry—to physics, biological chemistry, and materials science. Spectroscopy was one of the central themes. Accordingly, W. E. Moerner (Stanford University, USA) reported on “Single Molecules as Nanoscale Reporters in Biophysics, Chemistry, and Materials Science”. Frans De Schryver (University of Leuven, Belgium) addressed the same topic with the objective of ”Probing Molecular Properties and the Role of the Environment at Simple Molecule Level“. Niek F. van Hulst (University of Twente, The Netherlands) reported on how ”Ultrafast Femtosecond Single Molecule Spectroscopy Reveals the Effect of Disorder on Exciton Dynamics“. Spectroscopic time resolution plays, of course, a crucial role in this context. Paul Corkum (National Research Council of Canada, Ottawa) discussed in impressive fashion the concept of ”Attosecond Imaging“, a technique that differs fundamentally from femtosecond spectroscopy and will offer completely new insights into reaction dynamics. Another topic that was covered was photoinduced electron transfer (PET). This process possesses such fundamental significance that it was discussed in many of the presentations from chemistry, materials science, and biologically oriented disciplines. For instance, electron transfer played an important role in Klaus Müllen’s (Max Planck Institute, Mainz, Germany) talk on “Nanoemitters by Design”. Yet this lecture also addressed further important aspects of modern dye chemistry, for example the control of absorption in fluorescence and energy transfer through, among other means, supramolecular assembly. Frank Würthner (University of Würzburg, Germany) also addressed this aspect in his presentation on “Photoluminescent Arrays and Aggregates Based on Perylene Bisimide Dyes”. Aggregates composed of linear arenes and the role of energy transfer within self-assemblies were discussed by Jean-Pierre Desvergne (Université Bordeaux I, France). PET as a tool of modern synthetic chemistry remains a topic of current interest. Norbert Hoffmann (Université de Reims, France) reported on the “Efficient Radical Addition of Tertiary Amines to Alkenes Using Photochemical Electron Transfer”. By providing excellent examples, he demonstrated the wide applicability of the technique he developed. The basics of PET, as well as its role in biological systems, simple model systems, and materials, were discussed in a workshop. Dirk M. Guldi (University of Erlangen, Germany) outlined “Novel Opportunities for Electron Transfer—Versatile Carbon Nanostructures”. David I. Schuster (New York University, NY, USA) reported on “Topological Effects on Electron Transfer in Porphyrin–Fullerene Hybrids”. “PET to Triplet Flavins” was presented by Silvia E. Braslavsky (Max Planck Institute, Mülheim, Germany). Michael R. Wasielewski (Northwestern University, USA) discussed “Photoinduced Charge Separation and Transport in Self-Assembled Biomimetic Systems”. Two engaging and quite controversial presentations on “Design of Long-Lived Charge-Separated States and Applications” and “Long-Lived Charge Transfer States in Compact Dyads; Pitfalls and Opportunities” were given by Shunichi Fukuzumi (Osaka University, Japan) and Jan W. Verhoeven (University of Amsterdam, The Netherlands), respectively. The latter speaker seriously questioned the unusually long-lived charge-transfer states by presenting his own extensive experiments. Biological and other aspects of electron transfer and energy transfer were treated in the following presentations. Frederick D. Lewis (Northwestern University, USA) spoke about “DNA Photonics”, Gary B. Schuster (Georgia Institute of Technology, Atlanta, USA) about “Long Distance Radical Cation Migration in Duplex DNA”, Monique M. Martin (Ecole Normale Supérieure, Paris, France) about “Charge Transfer as the Driving Force for the Photoactivity of the Chromophore of Photoactive Yellow Protein in Solution”, and Jaques-E. Moser (EPFL, Lausanne, Switzerland) about “Dynamics of Light-Induced Interfacial Electron Transfer in the Dye-Sensitization of Nanocrystalline Oxide Semiconductors”. At a conference organized by Masahiro Irie, it came as no surprise that the subject of photoswitches took on a wide scope. Though the groundwork is already established for this field, photoswitches are currently garnering much interest in chemistry, physics, and the life sciences. This trend has come thanks to Masahiro Irie, as the diaryl ethylene chromophore that was developed by him certainly represents one of the most stable and widely applicable photoswitches known. In the numerous poster and oral presentations on the subject, the focus was not just about switching between molecular properties, rather also applications of a quite different sort: switching of spin states, of chirality, of optoelectronic features, of reactivity, of energy and electron transfer, and so on. Jochen Mattay (University of Bielefeld, Germany), in his lecture titled “On the Way to Supramolecular Photochemistry at the Single Molecule Level”, reported on calixarene–anthracene photoswitches, which were investigated at the single-molecule level by atomic force microscopy. The first application of dynamic force spectroscopy enabled not only the determination of the off rates, but also a ”glimpse of the binding pocket“ of the host–guest complex. Neil Branda (Simon Fraser University, Canada) reported on ”Relating Photochromism and Reactivity in Molecular Switching“. This lecture as well dealt with the topic of ”Irie diaryl switches“, which had however been modified in various ways and ”trimmed” to meet specific reactions. Of course, attention was also given in this context to other photochromic systems: as always the classical azobenzene, but also fulgides und especially the spiropyranes, which drastically change polarity upon switching from the “closed” to “open” state. Light as a “traceless reagent” and control element in biological systems also received much attention. The topics spanned a wide range—from the development of new technologies for imaging, for example the “Molecular Engineering of Two-Photon Absorbing Molecules for Bioimaging” as described by Mireille Blanchard-Desce (Université de Rennes, France), to “Drug-Biomolecule Interactions in the Excited State”, to which the lecture by Miguel A. Miranda (Universidad Politécnica de Valencia, Spain) was devoted. Heiko Ihmels (University of Siegen, Germany) presented his latest results in his lecture on the “Detection of Biomolecules with Fluorescent Light-Up Probes”, and Isao Saito (Nihon University, Japan) reported on “Highly Selective Fluorescent Nucleobases for Designing Base-Discriminating Fluorescence Probes”. The extent to which photochemistry can play a deciding role in addressing future issues such as solving energy shortages was elucidated in the lecture by Kazunari Domen (The University of Tokyo, Japan), who spoke on an “Overall Water Splitting Reaction on Non-Oxide Based Photocatalysts”. The use of fundamental photophysical processes and light-induced charge separation as potential means of energy storage, as they are used in photosynthesis, were also comprehensively investigated in model systems. Franco Scandola (University of Ferrara, Italy) reported on “Photophysics of Metal-mediated Assemblies of Porphyrins and Perylene Bisimides”, Dongho Kim (Yonsei University, Seoul, Korea) on “Ultrafast Excitation Energy Migration Processes in Various Multiporphyrin Arrays”, Villy Sundström (Lund University, Sweden) on “Light-induced Processes in Organic Semiconductors”, David Whitten (University of New Mexico, Albuquerque, USA) on “Cooperative Self-Assembly of Cyanines and Carboxy-methylamylose”, and Andrew Beeby (University of Durham, UK) on “An Interesting Twist of Molecular Wires”. An interesting and technologically meaningful application of photochemical methods was discussed by Lei Jiang (Center of Molecular Sciences, CAS Beijing, P. R. China), who reported on a “UV-Manipulated Switch between Superhydrophobicity and Superhydrophilicity”. There were also two workshops devoted to the application of OLEDs and photocatalysis. Peter I. Djurovich (University of Southern California, Los Angeles, USA) reported on “Blue Phosphorescent Materials for OLEDs”, and Chihaya Adachi (Kyushu University, Japan) on “Prospects for Organic Semiconductor Lasers from an Organic Solid-State Film”. In the workshop on photocatalysis, Jean-Marie Hermann (Université Lyon, France) gave a talk on “Photocatalysis in Environmental Green Chemistry”, Akihiko Kudo (Tokyo University of Science, Japan) on “Development of Powdered Photocatalysts for Water Splitting”, and Jae Sung Lee (Pohang University, Korea) on “Engineered Nanocomposite Photocatalysts for Visible Light Harvest”. Out of a total of 462 posters, 131 were selected to be short presentations, and three authors were additionally recognized with prizes: Hisao Shimizu (Toyama University, Japan), A. O. Chapman (University of Nottingham, UK), and Jong Won Chung (Seoul National University, Korea). The 21st IUPAC Symposium ended with one additional highlight, the awarding of the Porter Medal to Howard E. Zimmerman (University of Wisconsin, Madison, USA) and Hiroshi Masuhara (Osaka University, Japan). Howard Zimmerman, also one of the cofounders of this conference and a pioneer of organic photochemistry, is also known to organic chemists from the Zimmerman–Traxler model, which plays a significant role in stereochemistry. Hiroshi Masuhara has made important contributions in laser physics and laser chemistry which have had an impact in many areas of materials and biological sciences. This IUPAC Symposium has once again shown just how diverse the many facets of photochemistry are, and it has aroused interest in the next conference in this series, which is anticipated to take place in July 2008 in Göteborg (Sweden).

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