Photophysical dynamics of the efficient emission and photosensitization of [Ir(pqi)₂(NN)]⁺ complexes

The photophysical dynamics of three complexes in the highly-emissive [Ir(pqi)₂(NN)]⁺ series were investigated aiming at unique photophysical features and applications in light-emitting and singlet oxygen sensitizing research fields. Rational elucidation and Franck-Condon analyses of the observed emission spectra in nitrile solutions at 298 and 77 K reveal the true emissive nature of the lowest-lying triplet excited state (T₁), consisting of a hybrid ³MLCT/LC_{Ir(pqi)→pqi} state. Emissive deactivations from T₁ occur mainly by very intense, yellow-orange phosphorescence with high quantum yields and radiative rates. The emission nature experimentally verified is corroborated by theoretical calculations (TD-DFT), with T₁ arising from a mixing of several transitions induced by the spin-orbit coupling, majorly ascribed to ³MLCT/LC_{Ir(pqi)→pqi} and increasing contributions of ³MLCT/LLCT_Ir(pqi)→NN. The microsecond-lived emission of T₁ is rapidly quenched by molecular oxygen, with an efficient generation of singlet oxygen. Our findings show that the photophysics of [Ir(pqi)₂(NN)][PF₆] complexes is suitable for many applications, from the active layer of electroluminescent devices to photosensitizers for photodynamic therapy and theranostics.