Direct Observation of the S0 → T2 Transition in Phosphorescent Platinum(II) Octaethylporphyrin, Evidenced by Magnetic Circular Dichroism.

In recent years, the importance of analyzing excited triplet states has increased dramatically because of their relevance in the design and development of photofunctional molecules. Since the second lowest excited triplet (T2) state plays an important role in enhancing the nonradiative intersystem crossing (ISC) process from the lowest excited singlet (S1) state to the lowest excited triplet (T1) state, it is strongly desired to develop direct observation methods for the nonluminescent, short-lived T2 state. In this study, the excited triplet states of platinum(II) octaethylporphyrin (PtOEP), which was used as the first phosphorescent organic light emitting diode and oxygen sensor, are investigated using UV-vis absorption, magnetic circular dichroism, and phosphorescence spectroscopies. At low temperature, in highly concentrated solutions, we observe a distinct Faraday A term for the S0 → T1 transition, as well as for the S0 → T2 transition. The novel spectroscopic methodology applied allows resolution of the excited-state properties of a wide variety of molecular systems.