Optical Absorption and Emission of Metal Complexes

Transition metal complexes often exhibit a variety of bright colours, which are due to the magnitude of the d–d splitting associated with complex formation corresponding to the visible region of the electronic absorption spectrum. This chapter discusses the electronic structures and photochemical properties of transition metal complexes based on the interaction between light and metal ions. First, the Franck–Condon principle is described, followed by the processes of light absorption and emission by metal complexes, and the photoreactions that occur upon photoexcitation. As the complexes contain several d electrons, a number of excited states arise depending on the electronic configuration. For electronic configurations satisfying the Pauli exclusion principle, spectral terms arising from Russell–Saunders coupling (LS coupling) and ligand field symmetry are discussed. The transition probability from the ground state to the excited state determines the optical absorption intensity, and, in this context, selection rules are introduced for the presence or absence of spin multiplicity changes and inversion symmetry. Finally, the formation of MOs resulting from the interaction between d orbitals and ligand orbitals is discussed, d–d absorption and charge transfer absorptions are explained, and, on the basis of these, absorption spectra will be interpreted for the actual complexes.