A Sensitive Temperature Sensor with a Large Dynamic Spectral Range Based on a Dual-emissive Thermally Activated Delayed Fluorescence Dendrimer System

Dual emission from organic thermally activated delayed fluorescence (TADF) emitters is often difficult to observe, especially in the solution-state, because most compounds adhere to Kasha’s rule where emission originates from the lowest energy excited state. Two TADF dendrimers with different rigid and planar N-doped polycyclic aromatic hydrocarbons (PAH) as acceptors were designed and synthesized. By modulating the molecular geometry, compound 2GCzBPN that possesses a strongly twisted geometry exhibits TADF, while 2GCzBPPZ, possessing a less twisted geometry, shows dual emission with an emission peak at 475 nm associated with the monomer and one at 575 nm linked to aggregates that is TADF. This dual emission is both concentration-dependent and temperature-dependent in solution. This is the first observation of aggregate emission from TADF dendrimers in solution. The control of the contributions from intramolecular and intermolecular charge-transfer states permits a wide color tuning from sky blue through white to yellow light emission. We demonstrate how 2GCzBPPZ can serve as a temperature sensor and exhibits excellent temperature sensitivity across a very wide temperature range (70 °C to 70 °C) in n-hexane, accompanied by a significant spectral response, ranging from yellow to white, and then blue emission, which is the widest detected temperature range and color response reported for an organic luminescent material in solution and also to the best of our knowledge the first small molecule TADF compound used for colorimetric temperature sensing. By embedding 2GCzBPPZ into paraffin, we demonstrated a spatio-temperature sensor that showed a noticeable emission shift from yellow to green and ultimately to blue as the temperature increased from 20 °C to 200 °C. Finally, solution-processed organic light-emitting diodes (OLEDs) using these two dendrimer emitters showed divergent performance, with a three-times higher maximum external quantum efficiency (EQEmax) of 15.0% for the device with 2GCzBPPZ compared to the device with 2GCzBPN (5.3%).