Aggregation-induced emission triggered by the radiative-transition-switch of a cyclometallated Pt(ii) complex

Aggregation-induced emission (AIE) has been a vitally important research topic on luminescent materials. AIE-active emitters are usually developed based on the mechanisms of restriction of intramolecular rotation (RIR) or restriction of intramolecular vibration (RIV). Herein, we report a simple phosphorescent Pt(ii) complex DPA-Pt that shows impressive AIE behavior mainly because of a new mechanism. In THF/H₂O mixture solutions with the water fraction (f_w) increasing from 0 to 60%, DPA-Pt shows rather weak phosphorescence with the intensities slowly increasing and the emission profiles remaining almost unchanged. Once the f_w reached 70-90%, the emissions are greatly enhanced with new peaks appearing and shifting to longer wavelengths. The emission of the vacuum-deposited DPA-Pt neat film is further red-shifted to the deep-red/near-infrared (NIR) region with the photoluminescence quantum yield (PLQY) markedly increased to 0.33. The experimental and theoretical results indicate that the significant enhancement and red-shift of emissions from in dilute solution to aggregation are mainly triggered by the radiative-transition-switch (RTS) from the intra-ligand charge transfer (ILCT) to the metal-metal-to-ligand charge transfer (MMLCT), indicating that RTS is a new mechanism behind this AIE phenomenon. The non-doped OLED based on DPA-Pt exhibits deep-red/NIR emission with a peak external quantum efficiency of 9.8%, which is not only higher than that of the 6 wt% doped device based on DPA-Pt, but also much higher than those reported for the state-of-the-art non-doped deep-red/NIR OLEDs based on other AIE-active emitters.