Ultrafast Triplet–Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens

Abstract

Narrowband emissive organoboron emitters featuring the multi-resonance (MR) effect have now become a critical material component for constructing high-performance organic light-emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high-efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet-to-singlet reverse intersystem crossing (RISC). However, RISC involving spin-flip exciton upconversion is generally the rate-limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter (CzBSe) doped with a selenium atom, demonstrating a record-high RISC rate exceeding 10 8 s À 1 , which is even higher than its fluorescence radiation rate. Furthermore, the spin-flip upconversion process in CzBSe can be accelerated by factors of � 20000 and � 800, compared to those of its oxygen-and sulfur-doped homologs (CzBO and CzBS), respectively. Unlike CzBO and CzBS, the photophysical rate-limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time-delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR-induced blue narrowband emission and significantly alleviated efficiency roll-off features.

Space‐Confined Donor‐Acceptor Strategy Enables Fast Spin‐Flip of Multiple Resonance Emitters for Suppressing Efficiency Roll‐Off

“…In addition, MR‐TADF molecules with sulfur or selenium atom were developed to enhance SOC ST based on heavy‐atom effect [5a, 11] . Despite significant progress, up to now, only a limited number of examples could reach a k RISC of 10 6 s −1 [10a, 11b–d] . Considering that the efficiency roll‐off of MR‐OLEDs is still unsatisfactory, especially for green and red MR‐OLEDs, the further improvement of k RISC is urgently needed yet challenging.…”

“…With the contribution of partial molecular structures, the excited states exhibit a larger difference in the excitation character between the S 1 state and the T 2 or T 3 state than between the S 1 state and the T 1 state, which gives higher SOC S1‐T2 and SOC S1‐T3 than SOC S1‐T1 . Consequently, the efficient multichannel RISC process enables to increase k RISC [11b–d, 12] …”

Boron‐Based Narrowband Multiresonance Delayed Fluorescent Emitters for Organic Light‐Emitting Diodes

“…To promote the k RISC in MR‐TADF emitters, Yasuda et al proposed a heavy atom doping approach and consequently they developed three blue MR‐TADF emitters by doping the different heteroatoms (O, S, and Se), viz., CzBO , CzBS , and CzBSe . [ 109 ] With the aid of prominent MR‐effect, the compounds showed low Δ E ST below 0.15 eV. Further, the SOC of the compounds improved from O to Se attributed to the heavy atom effect of Se.…”

Enhancement of Thermally Activated Delayed Fluorescence (TADF) in Multi-Resonant Emitters via Control of Chalcogen Atom Embedding

“…However, a major drawback of the MR-TADF molecules is that they usually have low RISC rates, typically on the order of 10 4 s –1 . , An appealing strategy to solve this issue is to increase the spin–orbit couplings between the singlet and triplet states that contribute to RISC. − In this context, very recently, we reported that the inclusion of sulfur or selenium atoms into the core of organoboron molecules leads to a substantial increase in SOC while keeping intact the multi-resonant nature of the frontier orbitals. For instance, when the oxygen atoms are substituted by sulfur atoms [ selenium atoms ], the SOC between the S 1 and T 1 states of DOBNA (Figure ) increases by a factor of 6 [ 131 ] and the overall RISC rate (when combining the contributions from the T 1 and T 2 states) is evaluated to increase by a factor of 2.45 [ 2.42 × 10 3 ] …”