Physical organic photochemistry

Abstract

Physical organic photochemistry has been concerned with the systematic investigation of structure-reactivity relationships involved in the photophysics and photochemistry of organic molecules. The field relies heavily on the use of physical and computational methods to elucidate mechanistic pathways that allow an understanding of the molecular basis of observed or predicted structure-reactivity relationships. Determination of rate laws through kinetic analysis, syntheses of “tailor made” structures to test theories or mechanisms, and the use of product structure has been the dominant tool in physical organic chemistry and has shown similar validity in physical organic photochemistry. Reactive intermediates, which were postulated to explain observed products, have played an important role in the development of physical organic chemistry. In physical organic photochemistry these reactive intermediates have often been directly detected by time resolved spectroscopic methods. During the 1960s the methods of physical organic photochemistry typically involved steady state kinetics, product analysis and the assumption of triplets as ubiquitous intermediates. During the 1970s time resolved methods became available and traditional reactive intermediates (carbenes, ylides, radicals, biradicals etc.) became accessible to direct spectroscopic observation, characterization and investigation. In addition, singlet states, exciplexes and radical ions were added to the list of commonly occurring reactive intermediates. There was a shift in the theoretical framework for thinking about photochemical reactions from the state diagram level to the energy surface level. During the 1980s the use of sophisticated instrumentation and time resolution of the order of nanoseconds became more common. During this period the pervasive occurrence of electron transfer processes in organic photochemical reactions became apparent, and reactive intermediates such as contact ion pairs and solvent separated ion pairs became familiar in a variety of reactions. A major change occurred as photochemists began to investigate photoprocesses occurring in microheterogeneous systems such as micelles, cyclodextrins, polymers, and porous solids. It was discovered that weak magnetic fields can strongly modify photoprocesses which occur in microheterogeneous media if radical pairs or biradicals are involved . Since my last review in 1984, the basic paradigms for investigating organic photochemistry have not changed very much, but advances in instrumentation have continued to have a major impact on the field. For example