Chiral Triptycene-Based Red Thermally Activated Delayed Fluorescence Polymers and Their Organic Light-Emitting Diodes★

doi:10.6023/A23040153

Abstract

Chiral triptycene acridine (TpAc) has unique 3D structure, homoconjugation effect and two separate reaction sites. By using the design strategy of chiral electron donor-acceptor (D*-A) copolymerization, the triptycene-based non-conjugated chiral red thermally activated delayed fluorescence (TADF) polymers (R,R)-pTpAcBTZ and (S,S)- pTpAcBTZ were directly polymerized by using TpAC as chiral electron donor and 4,7-dibromobenzo[c][1,2,5]thiadiazole (BTZ) as the strong electron acceptor. The molecular weights of these chiral polymers were characterized by gel permeation chromatography (GPC), and the GPC analysis indicated that the number-average molecular weight (M_n) ranged from 37.9 kDa to 39.3 kDa with the polydispersity index (PDI) values from 1.99 to 2.02. The chiral polymers exhibited good solubility in common organic solvents, such as chlorobenzene, tetrahydrofuran, dichloromethane and chloroform, ensuring good film quality during solution processing. In addition, the chiral TADF polymers showed excellent thermal stability with thermal decomposition temperature (T_d) (corresponding to a 5% weight loss) around 440 ℃, and glass transition temperatures (T_g) were also detected in these chiral polymers when they were heated to 300 ℃, which could be ascribed to their highly rigid and twisted structures. According to the onset of the oxidation curve, the highest occupied molecular orbital (HOMO) energy levels of the chiral polymers were estimated to be -5.27 eV. Their corresponding optical energy band gaps were estimated to be 2.21 eV from the onset of their ultraviolet-visible (UV-Vis) absorption spectra in toluene. Consequently, the corresponding lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be -3.06 eV. The S₁ and T₁ values of pTpAcBTZ were 2.13 eV and 2.05 eV, respectively, which were determined at 77 K, and the corresponding ΔE_ST was 0.08 eV which were favorable for reverse intersystem crossing of excitons from T₁ to S₁ states. The chiral TADF polymers showed efficient red TADF (λ_toluene=663 nm) activity, and they also displayed mirror-image red circularly polarized luminescence (CPL) signals, with the |g_lum| of approximately 1.4×10^-3. By using the chiral red polymers as emitters, the obtained solution-processed red OLEDs displayed maximum electroluminescence peak of about 658 nm, and its turn-on voltage is 3.6 V at a brightness of 1.0 cd/m². The device exhibited well device efficiency, with a maximum external quantum efficiency of 2.0%, maximum current efficiency of 1.1 cd/A, maximum power efficiency of 0.8 lm/W. The design of the non-conjugated chiral polymers and their red TADF activity are conducive to promoting the development of related research fields such as chiral luminescent materials and red luminescence devices.

Key words: thermally activated delayed fluorescence, circularly polarized luminescence, polymer, triptycene, red luminescence

Cite this article

Yinfeng Wang, Meng Li, Chuanfeng Chen. Chiral Triptycene-Based Red Thermally Activated Delayed Fluorescence Polymers and Their Organic Light-Emitting Diodes^★[J]. Acta Chimica Sinica, 2023, 81(6): 588-594.

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Fig. & Tab. 8

Figure 1. Design strategy and chemical structure of red TADF polymers based on chiral triptycene

Scheme 1. Synthetic route of polymers (R,R)-pTpAcBTZ and (S,S)-pTpAcBTZ

Figure 2. (a) LUMO and (b) HOMO electron cloud distribution of pTpAcBTZ

Figure 3. (a) UV-Vis absorption and (b) PL spectra of (R,R)-pTpAcBTZ in different solvents (λex=500 nm), (c) PL spectra at 77 K in (R,R)-pTpAcBTZ (w=15%) doped mCP (λex=500 nm) and (d) transient PL spectra of (R,R)-/(S,S)-pTpAcBTZ in doped films (λem=660 nm, τIRF≈1 μs)

Figure 4. (a) CD and (b) CPLspectra of (R,R)-/(S,S)-pTpAcBTZ

Figure 5. Energy level diagram and molecular structures of the materials employed in OLEDs

Figure 6. (a) EL spectra and (b) EQE-density characteristics of (R,R)-/(S,S)-pTpAcBTZ-based OLEDs, (c) current density-voltage-luminance and (d) current efficiency-current density-power efficiency characteristics of (R,R)-pTpAcBTZ-based OLEDs, and (e) current density-voltage-luminance and (f) current efficiency-current density-power efficiency characteristics of (S,S)-pTpAcBTZ-based OLEDs

Device^a	V_T^b/V	λ_EL^c/nm	EQE_max^d/%	CE_max^e/(cd•A^-1)	PE_max^f/(lm•W^-1)	L_max^g/(cd•m^-2)
A	3.6	658	2.0	1.1	0.8	1058
B	3.5	658	1.8	1.0	0.8	949

Table 1. Device performances of OLEDs based on (R,R)-/(S,S)-pTpAcBTZ

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