Near Infra-Red S0↔T1 Transitions of Iridium(III) Phthalocyanine: Degeneracy of the T1 State Evidenced by Magnetic Circular Dichroism

Abstract

The lowest excited triplet (T1) state of an iridium(III) phthalocyanine (IrPc) complex was investigated by electronic absorption, magnetic circular dichroism (MCD), and emission spectroscopy. The introduction of the iridium(III) center, which possesses strong spin–orbit coupling, induced the appearance of transitions between the singlet ground state (S0) and the T1 state (S0↔T1). These transitions were characterized by room-temperature observations of: (i) the relatively intense, sharp T1→S0 phosphorescence in the near infrared (NIR) region and (ii) the distinct S0→T1 absorption band. Further, in addition to two Faraday A terms corresponding to the Q- and B-bands, a third Faraday A term was successfully observed for the S0→T1 transition, indicating that the T1x and T1y states were degenerate in accordance with the expected D4h symmetry of the π-conjugated system of IrPc. To the best of our knowledge, this is the first observation of the Faraday A term of the S0→T1 transition in porphyrin compounds.