The nonradiative decay mechanism of dinuclear iridium complexes: a density functional theory study

Dinuclear metal complexes are a promising class of compounds applicable to photoluminescence and catalysis. However, an understanding of the mechanism of the nonradiative decay process of dinuclear metal complexes remains very limited. Herein, the mechanism of the nonradiative decay process of dinuclear iridium(III) complexes (D1 and D2) and their mononuclear iridium(III) complex (M1) is elucidated by using density functional theory (DFT). Our results reveal that the nonradiative decay process occurs on a weak Ir-N bond and therefore results in metal-centered triplet excited (³MC) states. The deactivation pathways connecting the Franck-Condon region and the minimum energy seam of crossing (MESX) were further identified to be the determining step, which is the thermal deactivation pathways of ³MLCT → TS → ³MC→ MESX. The smaller energy barrier from the T₁ minimum to the MESX state for D1 (9.48 kcal mol^-1) and D2 (8.64 kcal mol^-1) relative to that for M1 (10.95 kcal mol^-1) plays a key role in observed weak emissions of D1 and D2 in the red region compared to that of M1. Moreover, by introducing the electron-withdrawing Cl atom at the para- or meta-position of the 2-phenylpyrimidine (ppd) moiety, a large energy barrier between the ³MC state and the T₁ minimum is obtained. Our work not only provides the possibility of the nonradiative decay process of dinuclear iridium(III) materials, but also paves a promising way for reducing the nonradiative process and developing saturated efficient red dinuclear iridium(III) materials for broader potential application.