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 108 s−1 and high k RISC over 105 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. [Display omitted] • Three MR-TADF emitters were developed by introducing different donors into the BNCz framework. • Both high k r over 108 s−1 and high k RISC over 105 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%. [ABSTRACT FROM AUTHOR]