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

Dalton Trans. 2018 Jan 23;47(4):1179-1188. doi: 10.1039/c7dt03930a.

Abstract

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/LCIr(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/LCIr(pqi)→pqi and increasing contributions of 3MLCT/LLCTIr(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.