Fast Triplet Population in Iridium(III) Complexes with Less than Unity Singlet to Triplet Quantum Yield

We performed a comprehensive study of the fast relaxation processes in three metal–organic Ir­(III)-complexes in liquid solutions at room temperature in order to estimate their singlet–triplet conversion quantum yields, ΦST. Experimental pico- and femtosecond transient absorption double pump–probe spectroscopy techniques were employed to reveal the peculiarities of the triplet population processes, and the corresponding values of ΦST were obtained based on a six-level electronic model. Our results confirm the fast population of triplets that has been observed before in Ir­(III) complexes with characteristic times of ≤1–2 ps. Such fast transfer to the triplet usually indicates a triplet yield of unity; however, we observe that ΦST is significantly less than the unity value previously reported for this class of compounds and expected. A six-level model explains the apparent contradiction between ultrafast triplet formation and subunity triplet quantum yield. It also explains the observation of simultaneous photoemission from singlet and triplet states.