Spin mixed charge transfer states of iridium complex Ir(ppy)3: transient absorption and time-resolved photoluminescence

Abstract

Nanosecond transient absorption and time-resolved spectroscopic techniques were applied to study an organometallic phosphorescent emitter Ir(ppy)₃ complex, tris[2-phenylpyridinato-C²,N] iridium(III) dissolved in tetrahydrofuran (THF) under degassed conditions at ambient temperature. Transient absorption curves obtained at a pump pulse of 355 nm and at a probe wavelength of 430–600 nm show positive and negative signals, indicating triplet–triplet (T–T) absorption and triplet–singlet (T–S) emission. Tri-exponential global fitted transient absorption curves and time-resolved photoluminescence (PL) decays demonstrated the presence of four close low lying triplet states, of which, two emit green PL with a lifetime of 210 ns and 1.71 μs. The emitting states are spin mixed metal-to-ligand charge transfer (³MLCT) states produced from a non-equilibrium ¹MLCT state following fast intersystem crossing (ISC), whereas the ¹MLCT state is produced directly and indirectly depending on the excitation wavelength. Moreover, the electronic structures for the ground state and low-lying excited states of Ir(ppy)₃ were studied using quantum chemistry calculations.