Thermally Activated Delayed Fluorescence of a Dinuclear Platinum(II) Compound: Mechanism and Roles of an Upper Triplet State.

A dinuclear Pt(II) compound was reported to exhibit thermally activated delayed fluorescence (TADF); however, the luminescence mechanism remains elusive. To reveal relevant excited-state properties and luminescence mechanism of this Pt(II) compound, both density function theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out in this work. In terms of the results, the S 1 and T 2 states show mixed intraligand charge transfer (ILCT)/metal-to-ligand CT (MLCT) characters while the T 1 state exhibits mixed ILCT/ligand-to-metal CT (LMCT) characters. Mechanistically, a four-state (S 0 , S 1 , T 1 , and T 2 ) model is proposed to rationalize the TADF behavior. The reverse intersystem crossing (rISC) process from the initial T 1 to final S 1 states involves two up-conversion channels (direct T 1 →S 1 and T 2 -mediated T 1 →T 2 →S 1 pathways) and both play crucial roles in TADF. At 300 K, these two channels are much faster than the T 1 phosphorescence emission enabling TADF. However, at 80 K, these rISC rates are reduced by several orders of magnitude and become very small, which blocks the TADF emission; instead, only the phosphorescence is observed. These findings rationalize the experimental observation and could provide useful guidance to rational design of organometallic materials with superior TADF performances.