TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T-1=3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S-1 -> S-0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m(-2) with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state ((LE)-L-3), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

Automated summary

The high triplet boracycle SO2B was successfully applied in TADF emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B, achieving near-complete separation of highest occupied and lowest unoccupied molecular orbitals and leading to low oscillator strength of the S-1 -> S-0 CT transition. OLEDs using CZ-Dipp-SO2B as the luminescent dopant displayed high external quantum efficiency and maximum luminance with low efficiency roll off. The rISC in CZ-Dipp-SO2B is mediated through vibronic coupling of two CT states, allowing stable OLED performance to be achieved in different host environments.