In this study, we designed and synthesized three series of blue emitting homoleptic iridium(III) phosphors bearing 4(5-mfcp) cyclometalates, respectively. These iridium complexes exhibit intense phosphorescence in the high energy region of 435−513 nm in the solution state at RT, to which the relatively large T 1 → S 0 transition dipole moment is beneficial for serving as a pure emitter and an energy donor to the multiresonance thermally activated delayed fluorescence (MR-TADF) terminal emitters via Forster resonance energy transfer (FRET). The resulting OLEDs achieved true blue, narrow bandwidth EL with a max EQE of 16−19% and great suppression of efficiency rolloff with ν-DABNA and t-DABNA. We obtained the FRET efficiency up to 85% using titled Ir(III) phosphors f-Ir(mfcp) 3 and f-Ir(5-mfcp) 3 to achieve true blue narrow bandwidth emission. Importantly, we also provide analysis on the kinetic parameters involved in the energy transfer processes and, accordingly, propose feasible ways to improve the efficiency roll-off caused by the shortened radiative lifetime of hyperphosphorescence.
Two new 2,3‐dicyanopyrazinophenanthrene‐based acceptors (A) p‐QCN and m‐QCN were synthesized to blend with a donor (D) CPTBF for the exciplex formation. The energy levels of p‐QCN and m‐QCN are modulated by the peripheral substituents 4‐ and 3‐benzonitrile, respectively. Exciplex‐forming blends were identified by the observation of the red‐shifted emissions from various D : A blends with higher ratios of donor for suppressing the aggregation of acceptor. The two‐component relaxation processes observed by time‐resolved photoluminescence support the thermally activated delayed fluorescence (TADF) character of the exciplex‐forming blends. The device employing CPTBF : p‐QCN and (2 : 1) and CPTBF : m‐QCN (2 : 1) blend as the emitting layer (EML) gave EQEmax of 1.76 % and 5.12 %, and electroluminescence (EL) λmax of 629 nm and 618 nm, respectively. The device efficiency can be further improved to 4.32 % and 5.57 % with CPTBF : p‐QCN and (4 : 1) and CPTBF : m‐QCN (4 : 1) as the EML, which is consistent with their improved photoluminescence quantum yields (PLQYs). A new fluorescent emitter BPBBT with photoluminescence (PL) λmax of 726 nm and a high PLQY of 67 % was synthesized and utilized as the dopant of CPTBF : m‐QCN (4 : 1) cohost system. The device employing CPTBF : m‐QCN (4 : 1): 5 wt.% BPBBT as the EML gave an EQEmax of 5.02 % and EL λmax centered at 735 nm, however, the weak residual exciplex emission remains. By reducing the donor ratio, the exciplex emission can be completely transferred to BPBBT and the corresponding device with CPTBF : m‐QCN (2 : 1): 5 wt.% BPBBT as the EML can achieve EL λmax of 743 nm and EQEmax of 4.79 %. This work manifests the high efficiency near infrared (NIR) OLED can be realized by triplet excitons harvesting of exciplex‐forming cohost system, followed by the effective energy transfer to an NIR fluorescent dopant.
The excited-state solvent-catalysed proton transfer of PyrQs requires a relay of ≧3 methanol molecules, where the N(8) proton-accepting site is the rate-determining step for the intrinsic proton tunnelling kpt.
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