Photochemical properties of phthalocyanines with transition metal ions

This review focuses on phthalocyanine (Pc) complexes with transition metal ions, especially the second- and third-row transition metal ions, and summarizes their photochemistry in terms of spin-allowed singlet–singlet (S0-S1) absorption, spin-forbidden singlet–triplet (S0-Tn) absorption, excited-state dynamics, and photofunctions. Characteristic photophysical and photochemical properties are identified from numerous data based on relevant theories. Firstly, it is demonstrated that the Q absorption bands show a blue or red shift with/without broadening owing to the interactions with transition metal ions, which is explained by the energy relationship among the d, π, and π* orbitals. Secondly, it is shown that the Tn energies can be determined based on the strong spin–orbit coupling of transition metal ions: the T1 energy can be evaluated by observing spin-forbidden singlet–triplet (S0-Tn) absorption, whereas the T2 energy of low-symmetry Pc complexes can be determined by analyzing zero-field splitting. Thirdly, numerous data on the excited-state dynamics are summarized and discussed in terms of spin-forbidden processes, such as phosphorescence decay rate and intersystem crossing. Finally, some attractive photofunctions are introduced. To provide a systematic understanding of the roles of the transition metal ions in relation to the characteristic photophysical and photochemical properties of Pcs, brief analyses are provided. This review provides direction for designing various photofunctions based on metal Pc complexes.