Multiple Resonance Organoboron OLED Emitters with High Efficiency and High Color Purity Via Managing Long‐ and Short‐Range Charge‐Transfer Excitations

Multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters are very promising in the high-resolution and high-efficiency displays, due to their narrow and highly efficient optical emissions. Early MR-TADF cores that show only short-range charge-transfer (CT) electronic excitations hardly afford ideal performances (e.g., show low efficiencies) in organic light-emitting diodes (OLEDs). This work thus designs and synthesizes two MR-TADF emitters (TCzBN-BP and TCzBN-FP), where the same MR core TCzBN is chemically modified by the acceptor fragments benzophenone/9-fluorenone (BP/FP) to incorporate long-range CT excitations in the two molecules. OLEDs exploiting TCzBN-BP as emitter, in which short-range CT excitation is dominant in the first singlet (S-1) excited state, achieve a maximum external quantum efficiency (EQE) of 35.6% and a narrow emission bandwidth of 35 nm. In contrast, OLEDs exploiting TCzBN-FP with an overloaded long-range CT excitation in the S-1 state exhibit a maximum EQE of 27.2% and a broadened emission bandwidth of 56 nm. This work not only shows the importance of careful management of long- and short-range CT excitations, but also provides a new insight into the structure-property relationship in the MR-TADF emitters, which thus promotes the design of more novel MR-TADF emitters with high efficiencies and high color purity.