Dual emission from donor-modified MR-TADF emitter: Evidence for coexistence of TICT and MR excited states

Multiresonance thermally activated delayed fluorescence (MR-TADF) materials have attracted wide attention due to their large radiation rate constant ( k r ) and high color purity. Attaching additional electron donor or acceptor was established as an effective strategy to optimize the optoelectronic parameters of MR-TADF emitters, such as the reverse intersystem crossing rate constant ( k RISC ). However, these is no report that both the MR and twisted intramolecular charge transfer (TICT) excited states can be detected simultaneously from a single molecule yet. Herein, a group of MR-TADF emitters BNCz-SAF, BNCz-DMAC and BNCz-PXZ are designed and synthesized through introducing different donor, spiroacridine (SAF), acridine (DMAC) and phenoxazine (PXZ), at the para-site of the boron atom of a famous MR core BNCz. All these three emitters maintained intrinsically excellent properties of MR-TADF molecules with high k r over 10 8 s −1 and high k RISC over 10 5 s −1 . Interestingly, with gradually increasing the donor strength to PXZ , dual emissions from the MR core and the CT excited state between PXZ and BNCz moieties are detected simultaneously for BNCz-PXZ in either strong polar solvents or doped film in highly polar host, both of which are confirmed as the TADF with the lifetimes of about 4 μs. Independent of the host, the organic light-emitting diodes (OLEDs) of BNCz-SAF and BNCz-DMAC exhibited sky-blue electroluminescence with high external quantum efficiencies (EQE max ) of 25.3% and 23.8% and narrow full widths at half maximum (FWHM) of 23–25 nm. In contrast, BNCz-PXZ exhibited a single-band (482 nm) sky-blue EL in mCBP host with EQE max of 20.9%, while exhibited dual-band EL at 484 and 548 nm from MR and CT states in higher polar host DPEPO, realizing a warm white emission with CIE (0.31, 0.48) and EQE max of 16.4%. For the first time both the short-range CT (i.e. MR) and long-range CT (i.e. TICT) excited states are experimently detected and confirmed simultaneously from a donor-modified B,N-based polycyclic hydrocarbon skeleton, which may be a potential strategy to bulid up broad-spectra or white emission. Dual TADF was experimentally confirmed from the donor modified multiresonance TADF framework through tuning the donor strength, from the coexistent short-range charge transfer (CT) state of the MR core and the long-range twisted intramolecular CT state between MR core and the PXZ donor. Warm white OLED was realized with CIE (0.31, 0.48) and EQE of 16.4% when BNCz-PXZ was doped in polar host. • Three MR-TADF emitters were developed by introducing different donors into the BNCz framework. • Both high k r over 10 8 s −1 and high k RISC over 10 5 s −1 were achieved for all these emitters. • Weak donor modified emitters exhibited single-band blue emission, resulting in EQE of 25.3% and 23.8% in OLEDs. • Dual TADF was detected for strong donor based BNCz-PXZ, from SRCT state of the MR core and the long-range TICT state. • When DPEPO was used as host for BNCz-PXZ, single-molecule warm white emission was achieved in its OLED with EQE of 16.4%.