Xing-Yu
Chen
a,
Ji-Kun
Li
a,
Wen-Long
Zhao
b,
Cheng-Zhuo
Du
a,
Meng
Li
b,
Chuan-Feng
Chen
b and
Xiao-Ye
Wang
*ac
aState Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: xiaoye.wang@nankai.edu.cn; Web: https://wang.nankai.edu.cn
bBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
cState Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China
First published on 13th December 2022
By employing aza[7]helicene as the chiral donor and triazine as the acceptor, we have developed a new type of circularly polarized luminescence material. The influence of the dihedral angle between the donor and acceptor moieties on the dissymmetry factors has been revealed for the first time, providing a novel strategy to amplify dissymmetry factors by engineering the dihedral angles in donor–acceptor type chiral materials.
On the other hand, to maximize the electroluminescence efficiency, thermally activated delayed fluorescence (TADF) materials, which can effectively convert triplet excitons into singlet excitons through reverse intersystem crossing (RISC) to achieve 100% internal quantum efficiency, have played a pivotal role in OLEDs.27–29 Traditional TADF emitters usually consist of donor (D) and acceptor (A) moieties to furnish effective separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), which can significantly reduce the energy gap between the singlet and triplet states (ΔEST) to boost RISC. Introducing chiral units into classical D–A type TADF materials is an effective strategy to construct emitters for CP-OLED applications.30–42 Nevertheless, the glum values of D–A type CPL materials are generally low (in the range of 10−3 to 10−5).43–54 Besides, the lack of understanding of the impact of structural variations on the glum of D–A type CPL materials limits the rational improvement of glum.
Herein, we design a new type of CPL material by introducing aza[7]helicene10 as the helical donor and triazine28 as the acceptor, and for the first time, investigate the influence of the dihedral angle between the D–A parts on the glum of CPL materials (Fig. 1). Theoretical studies reveal a substantial dependence of glum on the dihedral angles. To further verify the structure–property correlations, we design and synthesize two CPL molecules with different dihedral angles by controlling the steric hindrance between the D–A units. A nearly two-fold amplification of glum is achieved experimentally, providing a novel strategy to boost the glum of D–A type CPL materials by regulating the dihedral angle of the D–A moieties.
First, we introduced aza[7]helicene as the unprecedented chiral donor and 2,4,6-triphenyl-1,3,5-triazine as the acceptor to construct D–A type molecule Trz-A7H for theoretical investigations. Based on the optimized geometry of the singlet excited state (S1), we varied the dihedral angle (C1–C2–N3–C4) to carry out a rigid scanning (Fig. 2). The impact of dihedral angles (from 45° to 135°) on the transition electric dipole moment (μ), the transition magnetic dipole moment (m), and their angle θ was first studied because they jointly determine the g factor according to the calculation formula g = 4|cosθ||m|/|μ| (|μ| ≫ |m|) for common organic molecules (Fig. 2b).11 It can be seen that with the increase of the dihedral angle, the |μ| of Trz-A7H decreases first and then increases, resulting in a minimum value of 2.56 × 10−18 esu cm at 105°. Meanwhile, with the increase of the dihedral angle, the |m| of the molecule shows an opposite trend and reaches the maximum value of 0.70 × 10−20 erg G−1 at 100°. Besides, |cosθ| does not change significantly and keeps close to 1 because μ and m are aligned in parallel during the scanning of the dihedral angle. As a result, glum increases first and then decreases as the dihedral angle is raised, reaching 10.9 × 10−3 as the maximum value at 105° (Fig. 2c). Therefore, a more than two-fold amplification of glum can be expected in theory by engineering the dihedral angle of the D–A type CPL materials.
Based on the theoretical results, we synthesized two molecules (Trz-A7H and DMTrz-A7H) for experimental verification of the glum amplification strategy (Scheme 1). The dihedral angle between the donor and acceptor moieties is regulated by introducing two methyl groups ortho to the aza[7]helicene substituent to increase the steric hindrance. Density functional theory (DFT) calculations show that the introduction of methyl groups increases the dihedral angle between the donor and acceptor parts from 54° (Trz-A7H) to 92° (DMTrz-A7H) in the ground state (S0), and from 47° to 74° in the S1 state. The synthetic routes to Trz-A7H and DMTrz-A7H are depicted in Scheme 1. The aza[7]helicene derivative (3) was first synthesized according to the reported procedure.10 Then Trz-A7H and DMTrz-A7H were obtained through aromatic nucleophilic substitution reactions of 2-(4-fluorophenyl)-4,6-diphenyl-1,3,5-triazine (1) and 2-(4-fluoro-3,5-dimethylphenyl)-4,6-diphenyl-1,3,5-triazine (2) with 3 in 30% and 26% yields, respectively. Their structures were confirmed by 1H and 13C NMR spectroscopies and high-resolution mass spectrometry.
UV-vis absorption spectra show that Trz-A7H and DMTrz-A7H have very similar optical properties (Fig. 3a). The lowest-energy absorption maximum of DMTrz-A7H is at 417 nm (ε = 1.16 × 104 M−1 cm−1), which is slightly blue-shifted compared with Trz-A7H (420 nm, ε = 1.33 × 104 M−1 cm−1). Both DMTrz-A7H and Trz-A7H exhibit blue emission peaking at 426 nm and 431 nm, respectively (Fig. 3b). These results imply that the change of dihedral angle has only a negligible effect on the photophysical properties of the two molecules. Based on the electrochemical characterizations, the HOMO/LUMO energy levels of DMTrz-A7H and Trz-A7H are −5.61/−2.79 eV and −5.59/−2.77 eV, respectively (Fig. S7, ESI†). The electrochemical gaps are both 2.82 eV for DMTrz-A7H and Trz-A7H, which is consistent with their optical gaps (2.89 eV and 2.87 eV, respectively). The photoluminescence quantum yield (PLQY) of DMTrz-A7H in degassed toluene is 43%, which is slightly lower than that of Trz-A7H (55%).
Fig. 3 (a) UV-vis absorption and (b) emission spectra of Trz-A7H and DMTrz-A7H in toluene (c = 1 × 10−5 M). (c) CPL spectra and (d) glum of Trz-A7H and DMTrz-A7H in toluene (c = 1 × 10−5 M). |
Chiral separation of DMTrz-A7H and Trz-A7H was successfully achieved through high-performance liquid chromatography (HPLC) by using the Daicel Chiralpak IE column, thanks to their high configurational stability as revealed by transition-state calculations (Fig. S8 and S9, ESI†). The chiroptical properties of DMTrz-A7H and Trz-A7H were thus characterized. Each pair of enantiomers (DMTrz-A7H and Trz-A7H) has circular dichroism (CD) responses ranging from 280 to 450 nm with opposite Condon effects (Fig. S5, ESI†). The configuration of P and M was confirmed by comparing the experimental CD spectra with the simulated result (Fig. S11, ESI†). The absolute absorption dissymmetry factors (|gabs|) of DMTrz-A7H and Trz-A7H are plotted as a function of wavelength according to the equation |gabs| = |Δε|/ε. The |gabs| of DMTrz-A7H is significantly larger than that of Trz-A7H beyond 400 nm (S0 to S1 transition), indicating that the |gabs| can be increased by changing the dihedral angle of D–A type molecules in the ground state. Meanwhile, DMTrz-A7H and Trz-A7H show similar CPL responses between 390 nm and 500 nm, which are assigned to the transition from S1 to S0. The strongest CPL response of DMTrz-A7H (427 nm, 24 mdeg) is about twice that of Trz-A7H (427 nm, 12 mdeg) at the same concentration (Fig. 3c). Moreover, the maximum |glum| of DMTrz-A7H (3.5 × 10−3) is also about twice that of Trz-A7H (2.1 × 10−3), which is basically consistent with the theoretical results (Fig. 3d).
In order to further understand why glum of DMTrz-A7H is higher than that of Trz-A7H after changing the dihedral angle between the donor and acceptor units, the vector and density distributions of m and μ of DMTrz-A7H and Trz-A7H were calculated.55,56 The calculation results show that the m and μ directions of both compounds are the same, so that cosθ is kept at the maximum value of 1, which indicates the negligible effect on cosθ by changing the dihedral angle (Fig. 4a and d). The transition magnetic dipole moment densities of positive (purple) and negative (blue) transitions are completely distributed on the donor part, indicating their absolute contributions to m, with 0.59 × 10−20 erg G−1 for DMTrz-A7H and 0.58 × 10−20 erg G−1 for Trz-A7H, respectively (Fig. 4c and f). Meanwhile, the transition electric dipole moment density of the two molecules (Fig. 4b and e) has a similar distribution on the donor moiety, but some positive (purple) contributions are distributed on the triazine core in Trz-A7H, but not in DMTrz-A7H, leading to the smaller |μ| of DMTrz-A7H (3.64 × 10−18 esu cm) than that of Trz-A7H (5.16 × 10−18 esu cm). According to the formula g = 4|cosθ||m|/|μ|, these results suggest that the main reason why |glum| of DMTrz-A7H is higher than that of Trz-A7H is the decrease of |μ| due to the increase of the dihedral angle between the donor and acceptor, which is consistent with the trend of |μ| obtained by theoretical calculations.
In summary, we have disclosed the effect of dihedral angles on glum in D–A type CPL materials for the first time through theoretical simulation and experimental verification. We have introduced aza[7]helicene as the unprecedented helical donor and 2,4,6-triphenyl-1,3,5-triazine as the acceptor to develop new D–A type CPL molecules. Theoretical calculations have revealed that glum is highly dependent on the dihedral angles, reaching a maximum value at an ideal angle. Experimentally modulating the dihedral angle has been achieved by introducing additional methyl groups to Trz-A7H, and a nearly two-fold amplification of glum has been demonstrated. This work thus provides a novel strategy to boost the glum of D–A type CPL materials by modulating the dihedral angles. In light of the vital role of D–A molecules in TADF OLEDs, this work will further promote the development of high-performance CPL materials for CP-OLED applications.
Footnote |
† Electronic supplementary information (ESI) available: Synthetic procedures, characterization data, and theoretical calculations. See DOI: https://doi.org/10.1039/d2tc04848e |
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