Regulating the Spatially Folded Arrangement of Donor and Acceptor Units To Achieve Efficient Orange-Red Thermally Activated Delayed Fluorescence.

Achieving efficient long-wavelength organic light-emitting diodes (OLEDs) remains a challenge due to the energy gap law, which leads to increased non-radiative decay rates as the emission wavelength shifts to longer regions. Herein, a strategy of constructing folded three-dimensional architectures is proposed to explore new orange-red thermally activated delayed fluorescence (TADF) emitters with through-space charge transfer characteristics. Innovatively, naphthalene is selected as a bridge to connect O-bridged triphenylamine donor and planar dibenzo[a,c]phenazine acceptor respectively via simple Suzuki-Miyaura Coupling. In this way, a series of rigid orange-red emitters with "U"-shaped, folded "Z"-shaped and "W"-shaped configurations are elaborately constructed by modifying end groups, adjusting the numbers of naphthalene and donor, and regulating the linkage sites. The excited state natures and photophysical properties of the emitters can be effectively regulated and optimized by changing three-dimensional architectures. Finally, the prepared emitter QX36 achieves a lower non-radiative transition rate, a higher radiative rate and a higher photoluminescence quantum efficiency. Solution-processed OLEDs based QX36 present the excellent electroluminescent performance with a maximum external quantum efficiency (EQE) of up to 32.3 % and EQE of 20.6 % at 1000 cd m, which are the leading values of solution-processed orange-red OLEDs. This work demonstrates the promising potential of folded TADF based naphthalene backbone as emitters for future efficient solution-processed long-wavelength OLEDs.