Engineering of Cyano Functionalized Benzo[d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes

Abstract

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.

Bis-tridentate Ir(III) Phosphors and Blue Hyperphosphorescence with Suppressed Efficiency Roll-Off at High Brightness

“…Their pincer carbene chelate E possesses a planar configuration from which the peripheral carbene chelates exhibit slightly longer Ir–C­(carbene) bond distances (i.e., 2.054(5)–2.060(5) Å) in comparison to those of the central Ir–C­(aryl) distance (1.964(4)–1.965(4) Å). The variation of these Ir–C distances are different from those observed in the tris-bidentate Ir­(III)-based carbene complexes to which the Ir–C­(carbene) distances are always shorter than the respective Ir–C­(aryl) distance. − On the other hand, these bonding patterns are consistent with the bis-tridentate Ir­(III) complexes bearing the analogous carbene pincer chelate . Hence, we concluded that the shortened Ir–C­(aryl) distances of carbene pincer chelate were caused by the geometrical constraint imposed by the peripheral carbene entities .…”

Selective syntheses of homoleptic Ir(iii) complexes bearing di-CF3-functionalized benzoimidazol-2-ylidenes for generation of blue phosphorescence

“…Hence, these hyper-OLEDs also exhibited lower turn-on voltages and higher maximum EQE and EQE at a brightness of 1000 cd m −2 in comparison to the pure MR-TADF devices recorded using ν -DABNA under identical device architecture. 69 Furthermore, the EL spectrum also exhibited a large reduction in FWHM from 58 nm for the parent device to 19 nm and 29 nm for terminal emitters ν -DABNA and t -DABNA, respectively.…”

“…5a. 69 The hole-transport layer (HTL) and electron-transport layer (ETL) consisted of mCP and 3TPYMB, respectively. Thin layers of 4 wt% ReO 3 doped mCP and Liq were also applied to enhance the carrier and charge injection from the respective electrodes.…”

Peripheral engineering of Ir(iii) emitters with imidazo[4,5-b]pyrazin-2-ylidene cyclometalates for blue organic light emitting diodes

“…Particularly, MR-TADF terminal emitters are the preferred choice for giving narrowband blue emission. 41 With this in mind, we examined hyper-OLEDs by incorporating f -ct4a , f -ct4b , and f -ct4c as sensitizers along with 1–2 wt% of DtBuCzB 42 as a blue terminal emitter (Fig. 6 and Table 4).…”