Circularly Polarized Thermally Activated Delayed Fluorescence Emitters in Through-Space Charge Transfer on Asymmetric Spiro Skeletons

Abstract

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters SFST and SFOT are constructed, whose spiro architectures containing asymmetric donors result in chirality. Upon grafting within the spiro frameworks, the donor and acceptor are fixed proximally in a face-to-face manner. This orientation allows intramolecular through-space charge transfer (TSCT) to occur in both emitters, leading to TADF properties. The donor units in SFST and SFOT have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. SFOT exhibits greatly enhanced EL performance in doped organic light-emitting diodes, with external quantum efficiency (EQE) up to 23.1%, owing to the concurrent manipulation of highly photoluminescent quantum efficiency (PLQY, ∼90%) and high exciton utilization. As a comparison, the relatively larger sulfur atom in SFST introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor–acceptor distance. SFST thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of SFST, i.e., 12.5%, is much lower than that of SFOT. The chirality of these two spiro emitters results in circularly polarized luminescence. Because SFST has a more distorted molecular architecture than SFOT, the luminescence dissymmetry factor (|g lum|) of circularly polarized luminescence of one enantiomer of the former, namely, either (S)-SFST or (R)-SFST, is almost twice that of (S)-SFOT/(R)-SFOT. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with g EL of 1.30 × 10–3 and 1.0 × 10–3 for (S)-SFST and (S)-SFOT, respectively.

Synthesis of Single‐Handed Helical Spiro‐Conjugated Ladder Polymers through Quantitative and Chemoselective Cyclizations**

“…The left-and righthanded helical poly-6R Me and poly-6S Me also exhibited clear CPL signals with opposite signs in the corresponding fluorescence regions of the spiro p-conjugated backbones (Figure 4 C). [15] In contrast, the precursor (poly-5R Me and poly-5S Me ) and defective (poly-6R H ) polymers produced almost no or very weak CPL signals, respectively (Figure 4 C; Supporting Information, Figure S15). These results reveal a crucial role of the defect-free one-handed helical spiro-conjugated ladder structure in their outstanding chiroptical properties.…”

Chiral Spiro‐Axis Induced Blue Thermally Activated Delayed Fluorescence Material for Efficient Circularly Polarized OLEDs with Low Efficiency Roll‐Off

“…Ideally, TADF molecules with fast RICS process and short delayed fluorescence lifetime should be able to help alleviate efficiency roll‐off via the suppression of triplet‐triplet annihilation (TTA) and singlet‐triplet annihilation (STA) [11] . Up to now, only two green CP‐TADF material based devices with maximum external quantum efficiency (EQE max ) above 20.0 % have been reported to exhibit both CPEL property and low efficiency roll‐off (Figure 1 a)) [9c, 10b] . Therefore, it still remains a big challenge to develop efficient CP‐TADF materials with both high g factors and low device efficiency roll‐offs.…”

“…However, spiro‐type TADF molecules containing through space charge transfer (TSCT) process [13] normally suffer from obvious device efficiency roll‐off. Quite recently, great progresses have been made by Jiang and Liao's group [10b, 13a,d–f] . Via a parallel surface strategy, TSCT process is successfully enhanced and some devices exhibit EQE max over 20 % with low efficiency roll‐off [10b, 13a,e,f,i] .…”

“…Quite recently, great progresses have been made by Jiang and Liao's group [10b, 13a,d–f] . Via a parallel surface strategy, TSCT process is successfully enhanced and some devices exhibit EQE max over 20 % with low efficiency roll‐off [10b, 13a,e,f,i] . However, this type of TADF molecules usually display greenish emission, efficient TSCT based TADF molecule with blue emission still remains scarce.…”

Harvesting triplet excitons for near-infrared electroluminescence via thermally activated delayed fluorescence channel

“…As for the molecular design, besides the classical ICT, the emerging through space charge transfer (TSCT) (Tang et al, 2020;Yang et al, 2020c;Peng et al, 2020;Wada et al, 2020;Tsujimoto et al, 2017;Li et al, 2020a), MR-TADF (Hatakeyama et al, 2016;Xu et al, 2020), and these hybridizations would be potential choices for the realization of efficient TADF channels in the NIR region in the future. As for the TSCT-based TADF emitters, electron donors and acceptors usually have a spatial face-to-face alignment through a rigid linker.…”