[Development of Methods to Generate Radical Species and Synthetic Applications]

Radicals are chemical species bearing an isolated single electron. They have developed in a complementary manner to the two-electron species such as anions and cations. Radical species are classified into different groups according to their electronic states, such as cation radicals, neutral radicals, anion radicals, and biradicals, each of which has high reactivity and induces specific reactions. The authors have been developing studies on radical species to establish the generation methods and to control their reactivity. The author has found that heavy atom-containing compounds can undergo photochemical reactions that generate radical species through direct S₀→T_n transitions. The S₀→T_n absorption band exists in a longer wavelength region than the corresponding S₀→S_n band, and thus light in the near-visible light region can be used for the reactions. Although the absorption efficiency of the S₀→T_n transition is not high, it is possible to selectively excite heavy atom-containing molecules by irradiation of near-visible light, thus making it possible to control the generation and reactivity of radical species. The author also succeeded in developing a ligand that is activated by visible light irradiation to generate the monovalent palladium radical species. By using this ligand, it was possible to efficiently generate radical species of transition metals. Furthermore, depending on the valence of the palladium used, radical species with opposite properties could be generated.