Search

Isotope effect (IE) has been widely applied to probe the kinetics and mechanisms of organic reactions, and hole/electron transport behaviors of semiconductors. The subtle isotope substitution on fluorophores could intrinsically influence their luminescence efficiency and the operational lifetime of organic light-emitting diodes (OLEDs). So far, the IE on the photophysical properties, thermal stabilities and electroluminescence performance of thermally activated delayed fluorescence (TADF) emitters has not been reported. In this work, two deuterium-substituted TADF emitters, TPAAP-D and TPAAQ-D, have been designed and synthesized to investigate the IE in TADF. The comparison study showed that the deuterium substitution led to a decrease in the non-radiative decay rate of S1 (KnrS) and a decrease in the intersystem crossing rate constant (KISC), thus enhancing the luminescence efficiency of deuterated TADF materials. The origin of this positive IE was correlated with the increase in normal-mode displacement (ΔQ) and Huang-Rhys factor (Sj) of deuterated TADF. Furthermore, TPAAP-D and TPAAQ-D exhibited remarkably higher decomposition temperatures than their native counterparts. Importantly, TPAAP-D can sensitize a near infrared (NIR) dye (BF) with the electroluminescence (EL) peak at 760 nm (FWHM = 45 nm) and EQE of 2.8%, which was the best result among the reported fluorophore emitters.

Deep-red and near-infrared organic light-emitting diodes (DR/NIR-OLEDs) showed a wide range of applications such as healthcare and night-vision. However, red emitters generally demonstrated severe non-radiative emission due to energy gap law, and the development of efficient deep-red and near-infrared thermally activated delayed fluorescence (DR/NIR-TADF) materials was still under-researched by now. Herein, we designed and synthesized a series of efficient red/DR TADF emitters. Their emission wavelengths were rationally regulated into DR region via introducing strong electron-withdrawing groups from pyridinyl (PY) to trifluoromethyl (CF3) and to cyano (CN). The internal conversion (IC) process of these emitters was suppressed greatly via the incorporation of fused polycyclic aromatics (dibenzo[a,c]phenazine). Furthermore, the structural relaxation of their excited states was also reduced due to the restricted σ bond rotation between donor (D) and acceptor (A) units. As a result, these red/DR emitters showed high photoluminescence quantum yields (PLQYs) and excellent electrofluorescence performance. Particularly, OLEDs based on CN-TPA showed remarkably high maximum external quantum efficiencies (EQEs) of 22.80% at 668 nm, 18.41% at 688 nm and 15.05% at 698 nm, which were among the best values for reported TADF emitters. This result may provide a straightforward design strategy for efficient DR/NIR TADFs through optimizing the acceptor strength of rigidified polyaromatic frameworks.

Organic materials with multi-stimulus response (MSR) properties have magnificent application prospects in the optoelectronics field. Herein, a new asymmetric donor-spiro-acceptor type hybridized local and charge-transfer excited state (HLCT) compound based on benzothiadiazole derivative (2P-BT) moiety, Spiro-2P-BT-TPA, was designed and synthesized. The fluorene unit has a rigid linker to position the acceptor and donor subunit in close vicinity with control over their spacing and molecular structure and to achieve MSR properties, such as quasi-aggregation-induced emission (quasi-AIE), mechanochromic and solvatochromic. Moreover, Spiro-2P-BT-TPA possesses a high photoluminescence quantum yield (PLQY) of 80.9% when it is incorporated in a solid matrix, owing to spatial confined D/A stacked structure, multi-inter/intramolecular interactions as well as quasi-AIE properties. OLEDs based on Spiro-2P-BT-TPA achieve an EQE of 6.6% and the exciton utilization is 40%. To the best of our knowledge, this is a novel example for HLCT materials based on intramolecular spatial charge transfer as well as MSR properties.