Synergistic Intramolecular Non-Covalent Interactions Enable Robust Pure-Blue TADF emitters

doi:10.21203/rs.3.rs-4370798/v1

Download PDF

Article

Synergistic Intramolecular Non-Covalent Interactions Enable Robust Pure-Blue TADF emitters

https://doi.org/10.21203/rs.3.rs-4370798/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

Stability-issues of organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) require further advancements, especially in pure-blue range of CIE_y<0.20, existing a dilemma between color purity and device lifetime. Though improving bond-dissociation-energy (BDE) could effectively improve material intrinsic stability, strategies to sufficiently improve BDE without negative effects on photophysical performances are still lacking. Herein, we disclose that synergistic intramolecular non-covalent interactions (Intra-NI) can achieve not only the highest C-N BDE among blue TADF materials, but enhanced molecular-rigidity, near-unity photoluminescent quantum yields and short delayed lifetime. Pure-blue TADF-OLEDs based on proof-of-concept TADF material realize high external-quantum-efficiency and record-high LT₈₀@500 cd m^-2 of 109 h with CIE_y=0.16. Furthermore, deep-blue TADF-sensitized devices exhibit state-of-the-art LT₈₀@500 cd m^-2of 81 h with CIE_y=0.10. Our findings provide new insight into the critical role of Intra-NI in OLED materials and open the way to tackling vexing stability issues for developing robust pure-blue organic emitters and other functional materials.

Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Organic LEDs

Physical sciences/Chemistry/Materials chemistry/Optical materials

Organic light-emitting diodes (OLEDs) materials based on thermally activated delayed fluorescence (TADF) show great potential in the cutting-edge flat-panel displays with higher quality and lower energy consumption because they could harvest non-emissive triplet excitons via reverse intersystem crossing (RISC) process without any rare metals¹. External-quantum-efficiency (EQE) of TADF-OLEDs in full visible light region have been high up over 40%². However, operational lifetime of efficient TADF-OLEDs still lags behind and has been a major conundrum of commercialization^3–6, especially in pure-blue range of CIE_y<0.20. With the decrease of CIE_y value, the device lifetime appears to be dramatically shortened⁷ (Fig. 1a), leaving a dilemma between color purity and device lifetime. It has been recognized that the degradation of TADF materials mainly involves long-lifetime triplet excitons^3–17. Thus, in principle, one could suppress material degradation from the kinetic aspect such as increasing the rate of RISC (k_RISC) ^3–6 or from the thermodynamic aspect such as enhancing “longevity gene”, i.e. the difference between bond dissociation energy of fragile bond and the first triplet state energy (BDE-E_T1)¹⁶. Over the past decade, many efforts have been devoted to the former one^3,4,18–24. However, some of them would impair longevity gene, such as introducing heavy atoms¹⁸ or using 6-membered donors with low BDEs²⁵. Compared with various strategies from kinetic aspect, only a few strategies were proposed to effectively enhance BDE-E_T1 for TADF materials^26–31. Considering the relatively fixed E_T1 value for efficient blue emission, it is urgently and highly desired to develop effective strategies to simultaneously improve BDE and photophysical properties for realizing robust pure-blue TADF materials.

Nowadays, intramolecular non-covalent interactions (Intra-NI) have been employed to boost photophysical performances of OLED materials^19–23. For example, Liao and Jiang et al. demonstrated that non-covalent interactions between donor and acceptor aromatic-rings can produce effective through-space-charge-transfer (TSCT) interactions, which can enhance the molecular rigidity, suppress non-radiative rate (k_nr), and increase k_RISC³². As for the effect of Intra-NI on material stability, focusing on the widely-used ortho-linkages, considerable studies take it granted that ortho-linkages would incur repulsion forces due to steric hindrance and are often thought to elongate local bond length and decrease BDE^17,29,33,34. However, in our recent work, we found that blue TADF materials with larger BDE-E_T1 values (> 1.7 eV) and longer device lifetime all feature large ortho-linkages and strong Intra-NI (Fig. 1b, Supplementary Fig. 1–2, and Supplementary Table 1). More importantly, molecules with strong and multiple Intra-NI all have larger BDE values, which is opposed to previous knowledge. For example, as shown in Fig. 1c, 4CzBN⁸ and DCzTrz³⁵ are both high-efficiency TADF emitters based on carbazole (Cz) with blue emission around 440 nm and E_T1 value around 2.65 eV. 4CzBN features large ortho-groups and strong Intra-NI while DCzTrz does not. The fragile C-N BDE and BDE-E_T1 values of 4CzBN are 4.46 and 1.76 eV, respectively, which are both larger than those of DCzTrz (4.24 and 1.6 eV). Consequently, 4CzBN exhibits longer LT₈₀@500 cd m^− 2 (80% of initial luminance of 500 cd m^− 2) than DCzTrz (20 vs 6 h). These results suggest that a comprehensive understanding for the effect of Intra-NI on BDE is in great need.

In this work, we revisited the effect of Intra-NI on OLED materials through theoretical calculations, molecule dynamic (MD) analysis and comprehensive experiments. Taking the ubiquitous and fragile exocyclic C-N single bond in various OLED molecules as subjects, we found that attraction forces arising from London dispersion actually exist in all molecules of interest, though often being underestimated. Through synergetic and multiple Intra-NI, C-N BDEs can be rationally regulated and improved by ~ 0.7 eV. Combing the advancements of Intra-NI in improving photophysical properties, a proof-of-concept pure-blue TADF material 3',5'-di-tert-butyl-2,3,5,6-tetrakis(3,6-di-tert-butyl-9H-carbazol-9-yl)-[1,1'-biphenyl]-4-carbonitrile (4TDTBN) was developed. It exhibits pure-blue emission, near-unity photoluminescent quantum yields (PLQY), and high k_RISC over 10⁶ s^− 1. TADF-devices based on 4TDTBN achieve high EQE of 27.6% and long LT₈₀@500 cd m^− 2 of 109 h with CIE_y=0.16, which is the longest device lifetime of bottom-emitting pure-blue TADF-OLEDs ever reported (Fig. 1a). Furthermore, owing to the enhanced Intra-NI, 4TDTBN has rigid molecule structure and largely suppressed k_nr value, which fulfill the key requirements for an ideal TADF-sensitizer exemplified by Monkman et al.³⁶. As evidence, TADF-sensitized (TSF)/ hyperfluorescence (HF) OLED employing 4TDTBN as sensitizer exhibits deep-blue emission of CIE_y=0.10, high maximum EQE (EQE_max) over 25%, alleviated efficiency roll-off and pleasing device lifetime of LT₈₀@500 cd m^− 2 ~81 h, which is amongst the longest device lifetimes for bottom-emitting deep-blue TSF devices with CIE_y≤0.10. Our findings elucidate the critical role of Intra-NI on OLED materials and pave avenues to developing robust pure-blue and deep-blue OLED materials.

Understand and regulate the effect of Intra-NI on BDE.

Firstly, to explore the comprehensive effect of Intra-NI on BDE and rationally regulate these effects, we designed a series module molecules based on 9-phenyl-9H-carbazole (9-PhCz), a widely-used substructure in typical OLED molecules (Fig. 2a). The module molecules were divided into two groups: non-ortho-substituted derivatives of 9-PhCz in group A without Intra-NI and their ortho-substituted isomers in group B with Intra-NI. The BDE values of the exocyclic C-N bonds were calculated by M06-2X-D3/6–31 + G* on inclusion of dispersion correction, which was proved suitable for describing Intra-NI (details in Supplementary Fig. 3–5, Supplementary Table 2–3). As shown in Fig. 2b and Supplementary Table 4, all C-N BDE values of molecules in group A are comparable to that of 9-PhCz, which is in line with previous results that non-ortho-substituents on C side have little influence on BDE²⁹. In comparison, all BDEs of molecules in group B show a large disparity depending on the specific ortho-substituents. Compared with 9-PhCz, BDE of B1 is decreased by 0.15 eV with ortho-tBu to Cz. This result is consistent with previous cognition that ortho-substituents with large steric hindrance would decrease BDE^29,33. However, in B2 with an ortho-Cz group, BDE is increased by 0.20 eV instead. Then in B3, the BDE of C-N bond from central Cz with two ortho-Cz groups is further increased by 0.37 eV. These results are opposed to previous report on ortho steric effects^29,33. In addition to adding more ortho groups, modifications to ortho groups could also increase BDE. Compared with 9-PhCz, BDE values in B4 and B5 with tBu/phenyl on 3,6 sites of ortho-Cz are increased by 0.39 eV and 0.43 eV, respectively. Notably, in B6, C-N BDE of central tBu-Cz is dramatically increased by 0.68 eV, almost two times increase of B3.

Independent gradient model (IGM)³⁷ analysis was performed to explore why introduction of tBu/phenyl can bring such remarkable increase in BDE. The size of green and blue areas in IGM plots can reflect the strength of attractive part of Intra-NI, which is beneficial to improving BDE^38–40. Figure 2c clearly shows H∙∙∙H contacts⁴¹ between tBu groups in B4 and B6. Such H∙∙∙H contacts further reduce the bending angles between two Cz π planes (eg. 37° vs 17° for B2 and B4), which means a better coplanarity and a closer distance of π-groups in B4 and B6. Accordingly, in B4, such H∙∙∙H contacts between tBu groups and π-π interaction between adjacent Cz groups work together to facilitate a greater improvement in BDE. The synergistic effect in B5 is the extra π-π interaction between four phenyl groups. Importantly, in B6, such H∙∙∙H contacts and bilateral ortho-substitutions work synergistically to endow C-N bond with the highest BDE up to 4.92 eV (Fig. 2b), which clearly demonstrates greater influence of bilateral substitutions than unilateral substitution on the same C-N bond, also in the case for B3 and B2. For common OLED molecules with other donors, such as 9,9-dimethyl-9,10-dihydroacridine, phenoxazine, and phenothiazine derivatives, Intra-NI likewise improves their BDEs (Supplementary Fig. 6).

To shed light on the mechanism of improved BDE by stabilizing Intra-NI, we comparatively analyzed BDE values, molecular electronic energy E(M), and total electronic energy of fragments E_tot(F) for isomers in group A/B (Fig. 3a). We found that E(M) values of ortho-substituted molecules are substantially lowered by the stabilizing Intra-NI compared with those of non-ortho-substituted counterparts. Meanwhile, every pair of non-ortho- and ortho-substituted isomers (such as A1 and B1) has almost identical E_tot(F) values after bond cleavage. As a result, the difference of BDE (ΔBDE) between every pair of isomers almost equals the difference in their molecular energy (ΔE(M)). As shown in Fig. 3b, there is an excellent linear correlation between ΔBDE and ΔE(M) within ~ 1 eV, which just reflects the distinct stabilizing effects of Intra-NI in molecules.

Moreover, we employed functional-group partitioned symmetry-adapted perturbation theory (F-SAPT)⁴² to decompose Intra-NI of molecules B1, B2, B4, and B6 into electrostatic interaction, repulsive exchange interaction, induced interaction, and attractive dispersion interaction (Fig. 3c and Supplementary Table 5). In principle, the sum of four interactions is approximately equal to ΔE(M). As shown in Fig. 3c, B1 has greater repulsive exchange interaction than attractive dispersion interaction between tBu and Cz groups, which will destabilize the molecule and decrease BDE. In comparison, B2 exhibits greater attractive interaction including dispersion and induction between two ortho-Cz groups, which will stabilize molecule and increase BDE instead. Things are slightly different in B4 and B6, which show almost the same magnitude increase of dispersion interaction and exchange interaction, so that their effects cancel each other out. Nonetheless, the tBu substitution and bilateral ortho-groups promote the attractive electrostatic interaction and induction interaction considerably, thus providing more stabilizing Intra-NI to improve the BDE. The results of F-SAPT analysis confirm that even though repulsion interaction between ortho-substitutions do harm to BDE, as long as the substitution groups can bring enough attraction interactions, the overall effect of Intra-NI would improve BDE rather than impair it. Moreover, the stronger the stabilizing Intra-NI, the larger the BDE values. This result provides a new perspective for rationally regulating BDE by Intra-NI and makes up the puzzle of why 4CzBN with significant Intra-NI have large BDE and BDE-E_T1 values (4.46 and 1.76 eV) than DCzTrz (4.24 and 1.60 eV) in introduction.

Design robust pure-blue TADF material employing Intra-NI

On the basis of phenomenal effects of Intra-NI on BDEs, we attempted to design a robust pure-blue TADF emitter by combining the advancements of Intra-NI in improving photophysical properties. We chose 4TCzBN and 5TCzBN with top two BDE-E_T1 values (Fig. 1b) as parent molecules and targeted to address their weaknesses for robust pure-blue emission. For 4TCzBN, its fragile C1-N BDE and BDE-E_T1 value are increased to 4.67 eV and 2.07 eV from 4CzBN of 4.46 eV and 1.76 eV credited to the extra tBu groups at 3,6-sites of Cz (Fig. 1 and Fig. 4). Correspondingly, the device lifetime of 4TCzBN is longer than 4CzBN (54 h vs 20 h). However, its frontier molecule orbitals (FMOs) have considerable distribution at unprotected para-site of cyan group (Supplementary Fig. 7), being vulnerable to be attacked. Moreover, the absence of substitution at para-site of cyan group brings larger probability for C-N bonds rotation and reduces molecule rigidity, which is not desired for an ideal TADF sensitizer³⁶. As for 5TCzBN, it is not a pure-blue emitter due to five strong donors of tBu-Cz, leading to emission peak at 477 nm and device CIE_y>0.40⁸. Meanwhile, the BDE value of fragile C1-N bond in 5TCzBN is relatively lower than that in 4TCzBN (4.56 eV vs 4.67 eV) due to strong Intra-NI between ortho C2-Cz and C3-Cz in fragment after C1-Cz bond cleavage (Fig. 4a). To address these issues, we added one 1,3-di-tert-butylbenzene (DTPh) group onto para-site of cyan in 4TCzBN to regulate Intra-NI, in other words, replacing the para tBu-Cz in 5TCzBN by a relatively electron-inert group DTPh and designed 4TDTBN (Fig. 4a). As expected, this targeted modification indeed results in desired improvements. For 4TDTBN, C-N BDE values are sufficiently improved to 4.68 and 4.86 eV owing to enhanced stabilizing Intra-NI (Fig. 4b), which are the highest C-N BDEs for blue TADF materials ever reported (Supplementary Table 1). At the same time, emission peak of 4TDTBN is successfully blue-shifted to 465 nm compared with 5TCzBN of 477 nm, which leads to remarkable reduction in CIE_y from 0.31 to 0.18 for pure-blue emission (Fig. 4a and Supplementary Fig. 8).

With the aim to realize pure-blue emission, we comparatively explored the effects of Intra-NI on material and device performances of 4TDTBN and 4TCzBN. The lowest singlet excited state (S₁) energies are estimated to be 2.91 and 2.92 eV for 4TDTBN and 4TCzBN, respectively, from the onset of fluorescence spectra (Fig. 4c), and T₁ energies are estimated to be 2.70 and 2.60 eV from the first peak of phosphorescence spectra (Supplementary Fig. 9). Accordingly, their ∆E_ST values are calculated to be 0.21 eV and 0.32 eV, respectively. PLQY values of 4TDTBN and 4TCzBN in degassed solution are 99% and 98%, and decrease to 89% and 79% in doped film with a doping concentration of 30 wt% in stable exciplex-forming host comprising 9-(3-(triphenylsilyl) phenyl)-9H-3,9′-bicarbazole (SiCzCz) and 9,9′-(6-(3-(triphenylsilyl) phenyl)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (SiTrzCz2) with a ratio of 6:4. The smaller decrease in PLQY of 4TDTBN from solution to film-state could be attributed to the eliminated vulnerable para-site and well-protected FMOs, which increase intermolecular distance and suppresses annihilation of excited states⁸ (Supplementary Fig. 10a). Importantly, the emission peak of 4TDTBN in film-state shows only 1 nm red-shift to 466 nm compared with 4TCzBN of 8 nm (Fig. 4c). MD analysis shows that this result could be credited to the enhanced molecule rigidity of 4TDTBN, which suppresses the dihedral-angle inhomogeneity and lower-energy conformers in film-state (Supplementary Fig. 10–11). Prompt/delayed fluorescence lifetimes (τ_p/τ_d) of 4TDTBN and 4TCzBN are 5.85 ns/7.44 µs and 3.22 ns/10.65 µs, respectively (Fig. 4d and Supplementary Table 6). Radiative rate (k_r), k_nr, and k_RISC values are correspondingly calculated to be 1.88×10⁷, 0.41×10⁶, 10.9×10⁵ s^− 1 for 4TDTBN and 4.66×10⁷, 7.64×10⁶, 4.95×10⁵ s^− 1for 4TCzBN with the assumption of k_nrT=0 (More calculation details is in Supplementary Table 7–8). For 4TDTBN, the larger τ_p and smaller k_r could be ascribed to its smaller oscillator strength of 0.12 than 4TCzBN of 0.17, while the shorter τ_d and larger k_RISC could be attributed to smaller ∆E_ST and extra TSCT channels originating from the weak electron-withdrawing ability of DTPh (Supplementary Fig. 12) ⁴³. Importantly, k_nr value of 4TDTBN is more than 10 times smaller than that of 4TCzBN. This surprising result could be credited to enhanced molecule rigidity and suppressed C-N bond rotation by improved Intra-NI. It is worth remembering that apart from the above satisfying photophysical properties, 4TDTBN has increased BDE values, eliminated vulnerable site, and well-protected molecule structure benefiting from enhanced Intra-NI, suggesting 4TDTBN would exhibit better performance as a pure-blue TADF emitter than 4TCzBN. Moreover, according to the key requirements for an ideal sensitizer of TSF-OLEDs³⁶, the longer τ_p and enhanced molecule rigidity likewise guarantee 4TDTBN a promising TADF sensitizer for robust deep-blue TSF-OLED.

Device performances of pure-blue TADF and deep-blue TSF-OLEDs.

To validate the superior performance of 4TDTBN, TADF-OLEDs were fabricated with a multilayered structure of ITO/HATCN (5 nm)/NPB (30 nm)/SiCzCz (10 nm)/SiCzCz: SiTrzCz2: TADF emitter (0.60:0.40:0.30) (40 nm)/SiTrzCz2 (5 nm)/DPPyA:Liq (1:1, 30 nm)/LiF (0.5 nm)/Al (150 nm) (Fig. 5a and Supplementary Fig. 13). Electroluminescence (EL) peaks of 4TDTBN and 4TCzBN are 461 nm and 458 nm, respectively, which are both blue-shifted compared with PL peaks in film-state (Supplementary Fig. 14). The blue-shifted EL peaks result in CIE_y of 0.16 for 4TDTBN and 0.15 for 4TCzBN. Figure 5b shows EQE-luminance characteristics, revealing similar and high EQE_max values of 27.6% for 4TDTBN and 26.8% for 4TCzBN, owing to their high PLQY values and effective utilization of triplet excitons. Figure 5b also illustrates the device operational lifetimes measured at a constant current density of 10 mA cm^− 2. As expected, device based on 4TDTBN exhibits a longer device lifetime of 6.2 h with an initial luminance L₀ of 2577 cd m^− 2 compared with 4TCzBN (5.1 h with L₀ of 2319 cd m^− 2). By adopting a degradation acceleration factor that relates luminance to lifetime, viz. LT₈₀ (500 cd m^− 2) = LT (L₀) × (\(\frac{{L}_{0}}{500}\))ⁿ with n = 1.75³, LT₈₀@500 cd m^− 2 for 4TDTBN and 4TCzBN are 109.3 and 74.8 h, respectively. The longer device lifetime of 4TDTBN is originated from harmonious improvement in material intrinsic stability and photophysical properties including well-protected molecule structures, enhanced BDEs, and higher k_RISC values. Encouragingly, the device of 4TDTBN is the most stable pure-blue TADF-OLEDs with CIE_y<0.20 ever reported (Fig. 1a and Supplementary Table 9). Notably, the positive correlation between device lifetime and BDE-E_T1 could also be applied to 4TDTBN (Fig. 1b), which again confirms the strong power of “longevity gene” in describing material intrinsic stability and even corresponding device lifetime¹⁶.

To further shift the EL spectra to deep-blue range with CIE_y≤0.10, we selected the popular deep-blue multi-resonance material v-DABNA as terminal emitter and fabricated TSF-OLED devices. The large overlaps between fluorescence spectra of 4TDTBN, 4TCzBN and absorption spectrum of v-DABNA in doped film guarantee that 4TDTBN and 4TCzBN would both well sensitize v-DABNA (Supplementary Fig. 15). At present, TSF-OLEDs have been widely employed to improve device operational stability and color purity^{2,3,5,18,17,36,44}. However, to date, few blue TADF sensitizer was ever reported to be appropriate for robust deep-blue TSF emission^3,5. The formidable challenge is still how to simultaneously improve material intrinsic stability and photophysical properties of TADF sensitizers themselves. Monkman et al. recently proposed key requirements that an ideal TADF sensitizer should possess rigid molecule structure to suppress the low excited energy conformations and long τ_p value to achieve efficient energy transfer, which is exemplified by a spiro-linked TADF molecule ACRSA³⁶. However, C-N BDE in ACRSA (3.70 eV) is much lower than that in Cz (4.24 eV), which results in a much poorer longevity gene of 0.90 eV than 4TDTBN (1.98 eV). Consequently, LT₈₀@500 cd m^− 2 of TADF and TSF devices based on ACRSA are reported to be 2.5 h and 6.2 h, respectively³⁶, leaving a large room for improvement. Herein, as exemplified in 4TDTBN, enhancing Intra-NI is a superior strategy to simultaneously improve molecule rigidity and longevity gene.

As shown in Fig. 5c, TSF devices based on 4TDTBN and 4TCzBN have similar EL peak of 467 nm while the full width at half maximum (FWHM) of 4TDTBN is 17 nm, which is smaller than 4TCZBN of 18 nm. More notably, 4TCzBN shows larger tail at the long wavelength region, which suggests incomplete energy transfer³⁶, resulting in a larger CIE_y of 0.12 than 4TDTBN of 0.10. The deeper blue emission of devices based on 4TDTBN could be attributed to higher molecular rigidity induced by enhancing Intra-NI and longer τ_p value as aforementioned. Figure 5d shows the EQE-luminance characteristics, revealing high EQE_max> 20% for both 4TDTBN and 4TCzBN. The EQE_max of 4TDTBN is 25.4% and EQE value at 1000 cd m^− 2 (EQE₁₀₀₀) is 22.2%, representing a suppressed efficiency roll-off of 13% compared with 4TCzBN of 21%. This could be attributed to elimination of vulnerable para-site in 4TDTBN and well-protected FMOs by peripheral tBu groups, which largely suppresses triplet-triplet annihilation (TTA) process⁸. Furthermore, similar to TADF devices, TSF-OLED based on 4TDTBN exhibits remarkable LT₈₀@500 cd m^− 2 of 81.5 h (Table 1). It is worth mentioning that, in deep-blue emission range with CIE_y≤0.10, only one TSF-device has been reported with device lifetime of LT₈₀@500 cd m^− 2=7.6 h⁴⁵ (Supplementary Table 10), which again demonstrates the formidable challenge to develop robust TADF sensitizer for deep-blue TSF-devices. Most importantly, TSF-device based on 4TDTBN exhibited smaller efficiency roll-off and longer device lifetime, strongly confirming that targeted regulation of Intra-NI has the capability to tackle the above formidable challenge.

Table 1

Summary of device performances based on 4TCzBN and 4TDTBN.
Device Type	Emitter/ Sensitizer	V_on^a (V)	λ_EL (nm)	FWHM (nm)	EQE^b (%)	Roll-off^c (%)	CIE^d	LT₈₀^e (h)
TADF	4TDTBN	2.7	461	55	27.6/24.2/22.5	18	(0.15,0.16)	109.3
TADF	4TCzBN	3.0	458	52	26.8//23.9/21.9	18	(0.15,0.15)	74.8
TSF	4TDTBN	3.0	467	17	25.4/24.9/22.2	13	(0.13, 0.10)	81.5
TSF	4TCzBN	3.1	467	18	21.5/18.2/16.9	21	(0.13, 0.12)	20.4

^a V_on values were recorded at 1 cd m^− 2. ^b EQE values recorded at maxima, 500 and 1,000 cd m^− 2. ^c Efficiency roll-off value is calculated by (1-\(\frac{{\text{E}\text{Q}\text{E}}_{1000}}{{\text{E}\text{Q}\text{E}}_{\text{m}\text{a}\text{x}}}\))×100%. ^d CIE values at the initial luminance of 1,000 cd m^− 2. ^e LT₈₀ values at the initial luminance of 500 cd m^− 2.

In summary, our work for the first time revealed the critical but often overlooked effect of Intra-NI on BDE and material intrinsic stability, unveiling that multiple Intra-NI including π-π interactions, H∙∙∙H contacts, C-H∙∙∙π interactions can synergistically achieve targeted improvement of BDE with a significant increase by ~ 0.7 eV. Such phenomenal effect is attributed to the considerably lowered E(M) values by stabilizing Intra-NI, in which the critical role of large and attractive London dispersion should be taken seriously. Proof-of-concept pure-blue TADF emitter 4TDTBN with targeted regulation of Intra-NI demonstrates essentially improved intrinsic stability and photophysical behaviors. Moreover, the enhanced molecular rigidity by Intra-NI efficiently suppresses dihedral-angle inhomogeneity and lower-energy conformers in film-state, making 4TDTBN a desirable pure-blue emitter and a great deep-blue TSF sensitizer. As evidence, TADF devices based on 4TDTBN achieve high EQE_max of 27.6% and long LT₈₀@500 cd m^− 2 of 109 h with CIE_y=0.16, which is the longest device lifetime of bottom-emitting pure-blue TADF-OLEDs ever reported. Furthermore, its TSF-OLEDs exhibit deep-blue emission with CIE_y=0.10, high EQE_max over 25%, alleviated efficiency roll-off and remarkable device lifetime of LT₈₀@500 cd m^− 2 ~81 h. Our work paves avenues to rationally regulate BDE of organic molecules and provides a powerful strategy to develop robust pure-blue TADF emitters and address the dilemma between color purity and device lifetime for ideal pure-blue/deep-blue emission.

These findings reveal an interesting blind-spot in stability issues of TADF/TSF materials. Due to the long-standing underestimation of attractive London dispersion interaction, one would subconsciously consider large ortho-groups inducing Intra-NI will decrease BDE and impair molecule stability. After revisiting the effects of Intra-NI, a re-evaluation of the critical role of BDE in material intrinsic stability for many previously reported studies is desirable (Supplementary Fig. 16 and Supplementary Table 11). More broadly, since Intra-NI widely exists in all molecular fragments, one could simultaneously improve BDE values and freely tune the emission spectra in a wide range by employing different substitutions with different electrical properties. Importantly, addition to TADF materials, Intra-NI also widely exists in a variety of structures including fluorescence, phosphorescence, and host materials (Supplementary Fig. 17), which suggests great potential of targeted regulation of Intra-NI for expected performances in various organic functional materials.

Materials. 4TDTBN were synthesized and characterized according to the methods described in Synthesis process of the Supplementary Information.

Steady-state spectra, lifetime and photoluminescence quantum yield measurements. Steady-state emission spectra were recorded by HITACHI F-7000 fluorescence spectrometer. The photoluminescence quantum yields were obtained by absolute method on Hamamatsu C9920. The transient PL measurements were conducted on Edinburgh FL920P.

Computational details. Calculations and analysis in this work were performed with Gaussian 16, Multiwfn (3.7) ⁴⁶, and Psi4⁴⁷ BDE was calculated as the energy changes of the bond cleavage reactions at 298.15K and 1 atm (gas phase). The corresponding geometry optimizations and frequency analysis were performed at the density functional theory (DFT) level using M06-2X functional with dispersion correction (D3) and 6–31 + G* basis set. SAPT0 was employed here to conduct the F-SAPT analysis, in which the second-order perturbation theory was used to consider Hartree-Fock wavefunctions and describe dispersion terms. We used the jun-cc-pVDZ basis set, which was demonstrated to provide reliable results in the calculation of Intra-NI.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information file.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (No. 2022YFB3603002) and the National Natural Science Foundation of China (No. 52273184). We are grateful to Prof. Lian Duan, Prof. Lei Liu, Prof. San-zhong Luo, Dr. Jin-dong Yang, Dr. Dong-dong Zhang and Dr. Tian-yu Huang in Tsinghua University, and Prof. Ge Zhang in Jiangxi Science and Technology Normal University for their valuable suggestions on our work. We thank the Center of High-Performance Computing, Tsinghua University and Tsinghua Xuetang Talents Program for supporting the computational resources.

Author Contributions

Q. M. and H. S. synthesized 4TDTBN used in this study and prepared the samples and measured their properties. Q. M. carried out quantum-chemical calculations. Q. M. fabricated OLED devices. Q. M. and H. S. measured device performances. C. Y. performed molecule dynamic calculations. R. W., Y.W., and X. W. provided inspiring suggestions for improvement. J. X. and Y. D. provided suggestions on the synthesis of 4TDTBN. J.Q. conceived and supervised the project. All authors contributed to data analysis and writing the paper.

Competing interests

The authors declare no competing financial interest

Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012).
Lee, H. et al. Efficient pure blue hyperfluorescence devices utilizing quadrupolar donor-acceptor-donor type of thermally activated delayed fluorescence sensitizers. Nat. Commun. 14, 419 (2023).
Zhang, D. et al. Efficient and Stable Deep-Blue Fluorescent Organic Light-Emitting Diodes Employing a Sensitizer with Fast Triplet Upconversion. Adv. Mater. 32, 1908355 (2020).
Cui, L.-S. et al. Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states. Nat. Photonics 14, 636–642 (2020).
Jeon, S. O. et al. High-efficiency, long-lifetime deep-blue organic light-emitting diodes. Nat. Photonics 15, 208–215 (2021).
Kim, E. et al. Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence. Sci. Adv. 8, eabq1641 (2022).
Jeon, S. K., Lee, H. L., Yook, K. S. & Lee, J. Y. Recent Progress of the Lifetime of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescent Material. Adv. Mater. 31, 1803524 (2019).
Zhang, D., Cai, M., Zhang, Y., Zhang, D. & Duan, L. Sterically shielded blue thermally activated delayed fluorescence emitters with improved efficiency and stability. Mater. Horiz. 3, 145–151 (2016).
Freidzon, A. Y. et al. Predicting the Operational Stability of Phosphorescent OLED Host Molecules from First Principles: A Case Study. J. Phys. Chem. C 121, 22422–22433 (2017).
Ahn, D. H. et al. Rigid Oxygen-Bridged Boron-Based Blue Thermally Activated Delayed Fluorescence Emitter for Organic Light-Emitting Diode: Approach towards Satisfying High Efficiency and Long Lifetime Together. Adv. Opt. Mater. 8, 2000102 (2020).
Lim, J., Lee, K. H., Kim, J.-M. & Lee, J. Y. Synergistic Control of Hot Exciton Relaxation and Exciton–Polaron Dispersion Using a Polaron Retarder for Long Lifetime in Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes. Adv. Opt. Mater. 10, 2201575 (2022).
Lee, Y.-T. et al. Tailor-Made Multi-Resonance Terminal Emitters toward Narrowband, High-Efficiency, and Stable Hyperfluorescence Organic Light-Emitting Diodes. Adv. Opt. Mater. 10, 2200682 (2022).
Park, J., Kim, K. J., Lim, J., Kim, T. & Lee, J. Y. High Efficiency of over 25% and Long Device Lifetime of over 500 h at 1000 nit in Blue Fluorescent Organic Light-Emitting Diodes. Adv. Mater. 34, 2108581 (2022).
Kang, H. et al. Designing Stable Deep-Blue Thermally Activated Delayed Fluorescence Emitters through Controlling the Intrinsic Stability of Triplet Excitons. Adv. Opt. Mater. 10, 2102309 (2022).
Liu, D. et al. Management of Host–Guest Triplet Exciton Distribution for Stable, High-Efficiency, Low Roll-Off Solution-Processed Blue Organic Light-Emitting Diodes by Employing Triplet-Energy-Mediated Hosts. Adv. Funct. Mater. 33, 2301327 (2023).
Meng, Q.-Y. et al. Longevity gene responsible for robust blue organic materials employing thermally activated delayed fluorescence. Nat. Commun. 14, 3927 (2023).
Jiang, J. & Lee, J. Y. Degradation mechanisms and lifetime extending strategy of phosphorescent and thermally activated delayed-fluorescence organic light-emitting diodes. Mater. Today 68, 204–233 (2023).
Wang, J.-X. et al. Heavy-atom engineering of thermally activated delayed fluorophores for high-performance X-ray imaging scintillators. Nat. Photonics 16, 869–875 (2022).
Rajamalli, P. et al. A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes. J. Am. Chem. Soc. 138, 628–634 (2016).
Chen, X.-K. et al. Intramolecular Noncovalent Interactions Facilitate Thermally Activated Delayed Fluorescence (TADF). J. Phys. Chem. Lett. 10, 3260–3268 (2019).
Wang, T.-T. et al. π-Stacked Thermally Activated Delayed Fluorescence Emitters with Alkyl Chain Modulation. CCS Chem. 3, 1757–1763.
Tang, X. et al. Highly efficient luminescence from space-confined charge-transfer emitters. Nat. Mater. 19, 1332–1338 (2020).
Wada, Y., Nakagawa, H., Matsumoto, S., Wakisaka, Y. & Kaji, H. Organic light emitters exhibiting very fast reverse intersystem crossing. Nat. Photonics 14, 643–649 (2020).
Fu, Y., Liu, H., Tang, B. Z. & Zhao, Z. Realizing efficient blue and deep-blue delayed fluorescence materials with record-beating electroluminescence efficiencies of 43.4%. Nat. Commun. 14, 2019 (2023).
Meng, Q.-Y. et al. Precise Regulation on the Bond Dissociation Energy of Exocyclic C–N Bonds in Various N-Heterocycle Electron Donors via Machine Learning. J. Phys. Chem. Lett. 4422–4429 (2024).
Lin, N., Qiao, J., Duan, L., Wang, L. & Qiu, Y. Molecular Understanding of the Chemical Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, Phosphine-Oxide, and Carbonyl-Containing Host Materials. J. Phys. Chem. C 118, 7569–7578 (2014).
Hong, M., Ravva, M. K., Winget, P. & Brédas, J.-L. Effect of Substituents on the Electronic Structure and Degradation Process in Carbazole Derivatives for Blue OLED Host Materials. Chem. Mater. 28, 5791–5798 (2016).
Ihn, S.-G. et al. An Alternative Host Material for Long-Lifespan Blue Organic Light-Emitting Diodes Using Thermally Activated Delayed Fluorescence. Adv. Sci. 4, 1600502 (2017).
Wang, R. et al. Effects of ortho-Linkages on the Molecular Stability of Organic Light-Emitting Diode Materials. Chem. Mater. 30, 8771–8781 (2018).
Wang, R., Meng, Q.-Y., Wang, Y.-L. & Qiao, J. Negative Charge Management to Make Fragile Bonds Less Fragile toward Electrons for Robust Organic Optoelectronic Materials. CCS Chem. 4, 331–343 (2022).
Lee, H. J., Lee, H. L., Han, S. H. & Lee, J. Y. Novel secondary acceptor based molecular design for superb lifetime in thermally activated delayed fluorescent organic light-emitting diodes through high bond energy and fast up-conversion. Chem. Eng. J. 427, 130988 (2022).
Yang, S.-Y., Qu, Y.-K., Liao, L.-S., Jiang, Z.-Q. & Lee, S.-T. Research Progress of Intramolecular π-Stacked Small Molecules for Device Applications. Adv. Mater. 34, 2104125 (2022).
Noda, H., Kabe, R. & Adachi, C. Blue Thermally Activated Delayed Fluorescence Molecule Having Acridane and Cyanobenzene Units. Chem. Lett. 45, 1463–1466 (2016).
Kim, H. S., Lee, S. H., Yoo, S. & Adachi, C. Understanding of complex spin up-conversion processes in charge-transfer-type organic molecules. Nat. Commun. 15, 2267 (2024).
Kim, M., Jeon, S. K., Hwang, S.-H. & Lee, J. Y. Stable Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes with Three Times Longer Lifetime than Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 27, 2515–2520 (2015).
Stavrou, K., Franca, L. G., Danos, A. & Monkman, A. P. Key requirements for ultraefficient sensitization in hyperfluorescence organic light-emitting diodes. Nat. Photonics 1–8 (2024).
Lefebvre, C. et al. Accurately extracting the signature of intermolecular interactions present in the NCI plot of the reduced density gradient versus electron density. Phys. Chem. Chem. Phys. 19, 17928–17936 (2017).
Grimme, S. & Schreiner, P. R. Steric Crowding Can Stabilize a Labile Molecule: Solving the Hexaphenylethane Riddle. Angew. Chem.-Int. Ed. 50, 12639–12642 (2011).
Schreiner, P. R. et al. Overcoming lability of extremely long alkane carbon–carbon bonds through dispersion forces. Nature 477, 308–311 (2011).
Ishigaki, Y., Shimajiri, T., Takeda, T., Katoono, R. & Suzuki, T. Longest C-C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 angstrom. Chem 4, 795–806 (2018).
Echeverría, J., Aullón, G., Danovich, D., Shaik, S. & Alvarez, S. Dihydrogen contacts in alkanes are subtle but not faint. Nat. Chem. 3, 323–330 (2011).
Parrish, R. M., Parker, T. M. & Sherrill, C. D. Chemical Assignment of Symmetry-Adapted Perturbation Theory Interaction Energy Components: The Functional-Group SAPT Partition. J. Chem. Theory Comput. 10, 4417–4431 (2014).
Yin, C. et al. High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers. CCS Chem. 2, 1268–1277 (2020).
Chan, C.-Y. et al. Stable pure-blue hyperfluorescence organic light-emitting diodes with high-efficiency and narrow emission. Nat. Photonics 15, 203–207 (2021).
Fan, T. et al. One-Shot Synthesis of B/N-Doped Calix[4]arene Exhibiting Narrowband Multiple Resonance Fluorescence. Angew. Chem. Int. Ed. 61, e202213585 (2022).
Lu, T. & Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. Journal of Computational Chemistry 33, 580–592 (2012).
Parrish, R. M. et al. Psi4 1.1: An Open-Source Electronic Structure Program Emphasizing Automation, Advanced Libraries, and Interoperability. J. Chem. Theory Comput. 13, 3185–3197 (2017).

There is NO Competing Interest.

SupplementaryInformationfinal.pdf

Download PDF

Version 1

posted

You are reading this latest preprint version

Synergistic Intramolecular Non-Covalent Interactions Enable Robust Pure-Blue TADF emitters

Status:

Version 1

Abstract

Figures

INTRODUCTION

RESULTS AND DISSCUSION

Design robust pure-blue TADF material employing Intra-NI

Discussion

Methods

Data availability

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1