The XXIst IUPAC Symposium on Photochemistry was chaired jointly by Profs. Masahiro Irie (Kyushu University, International Committee) and Atsuhiro Osuka (Kyoto University, Local Committee) on 2-7 April 2006 in Kyoto, the beautiful and ancient capital of Japan in the season of cherry blossoms. This series of Symposia was initiated by Prof. George S. Hammond, and the first was held in Strasbourg in 1964. Since then it has been held almost every two years, and this is the first time that it has been held outside Europe.The opening presentation of the Symposium was dedicated to the founding pioneer, Prof. Hammond, who passed away in 2005 at the age of 84. In a eulogy, Prof. David G. Whitten (University of New Mexico) paid a tribute to his memory with an account of his personality and his scientific works in photochemistry and physical organic chemistry.The scientific program of the Symposium featured 25 plenary and invited lectures, and three workshops on the topics "Organic light-emitting diodes: Present and future", "Photocatalysis: Water splitting and environmental purification", and "Photoinduced electron transfer: Present and future". In addition to 462 posters on display, 131 were selected for short oral presentations. The poster sessions were spirited, with the younger participants engaged in stimulating discussions with their more senior colleagues, and the contributions of three young authors were additionally recognized by the award of prizes. The numerous presentations reflected the broad scope and interdisciplinarity of modern photochemistry. The spectrum of topics ranged from the classical fields of inorganic, organic, physical, and theoretical chemistry, to physics, biological chemistry, and materials science. In summary, the exciting scientific program maintained the proud tradition of this series, and demonstrated that photochemistry continues to be a vivid, active, and evolving discipline, some details of which have been introduced by Prof. Jochen Mattay [Angew. Chem., Int. Ed.45, 3570 (2006)]. In addition to the scientific program, the participants and their accompanying guests enjoyed a variety of social and cultural programs provided by the Local Organizing Committee, which took full advantage of the spring season in Kyoto.A highlight of the meeting was the presentation of the Porter Medals, named for 1967 NobelLaureate, George Porter. On this occasion, two medals were awarded, to recognize the outstandingachievements of Prof. Howard E. Zimmerman (University of Wisconsin, Madison, USA), one of thefounders of modern organic photochemistry whose lifetime's work has contributed to our understandingof fundamental processes in excited states, and Prof. Hiroshi Masuhara (Osaka University, Japan)for his pioneering work on the integration of time and spatially resolved measurements on the emergingfields of nanosciences.This issue of Pure and Applied Chemistry offers a representative collection of papers, based upon plenary and invited lectures by the two medallists and a number of internationally recognized scientists, who covered traditional as well as modern photochemistry, and discussed the science as well as related technologies such as materials for photonics, nanotechnology, and photobiology. This demonstrates the broadening scope of photochemistry from an initial central core toward a periphery that is increasingly interactive and interdisciplinary. As editors, we appreciate all the important contributions made by our authors and hope that this collection enables us to convey to readers some of the excitement of photochemistry and the status of recent achievements in the field.Hiroshi MiyasakaMasahiro IrieConference Editors
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