Amplification of dissymmetry factors by dihedral angle engineering in donor–acceptor type circularly polarized luminescence materials

Abstract

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.

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Circularly polarized luminescence (CPL) materials have received broad attention during the past few decades^1–15 due to their potential applications in full-colour 3D displays,¹⁶ biological detection,¹⁷ encrypted data storage,¹⁸etc. In particular, various CPL materials have been developed for electroluminescent applications in circularly polarized organic light-emitting diodes (CP-OLEDs).^19–26 How to realize a high luminescence dissymmetry factor (g_lum), which characterizes the level of polarization and is defined by g_lum = 2(I_L − I_R)/(I_L + I_R) (where I_L and I_R are the emission intensity of left-handed and right-handed circularly polarized light, respectively), has been the key challenge in the field to promote the practical applications of CPL 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 (ΔE_ST) 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 g_lum 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 g_lum of D–A type CPL materials limits the rational improvement of g_lum.

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 g_lum of CPL materials (Fig. 1). Theoretical studies reveal a substantial dependence of g_lum 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 g_lum is achieved experimentally, providing a novel strategy to boost the g_lum of D–A type CPL materials by regulating the dihedral angle of the D–A moieties.

Fig. 1 Molecular design of new D–A type CPL materials in this work.

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|cos θ||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, |cos θ| 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, g_lum 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 g_lum can be expected in theory by engineering the dihedral angle of the D–A type CPL materials.

Fig. 2 (a) The structure model of Trz-A7H investigated in this work and the illustration of the dihedral angle between the donor and acceptor units. (b) |μ| and |m| as a function of the dihedral angle. (c) g_lum as a function of the dihedral angle.

Based on the theoretical results, we synthesized two molecules (Trz-A7H and DMTrz-A7H) for experimental verification of the g_lum 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.

Scheme 1 Synthetic routes to (a) Trz-A7H and (b) DMTrz-A7H.

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) g_lum 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 (|g_abs|) of DMTrz-A7H and Trz-A7H are plotted as a function of wavelength according to the equation |g_abs| = |Δε|/ε. The |g_abs| of DMTrz-A7H is significantly larger than that of Trz-A7H beyond 400 nm (S₀ to S₁ transition), indicating that the |g_abs| 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 |g_lum| 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 g_lum 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 cos θ is kept at the maximum value of 1, which indicates the negligible effect on cos θ 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|cos θ||m|/|μ|, these results suggest that the main reason why |g_lum| 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.

Fig. 4 Transition electric and magnetic dipole moments of (a) Trz-A7H and (d) DMTrz-A7H, where the length of the transition magnetic dipole moment vector is multiplied by 120 for clarity. The transition electric dipole moment density (only for the x axis component) of (b) Trz-A7H and (e) DMTrz-A7H. The transition magnetic dipole moment density (only for x axis component) of (c) Trz-A7H and (f) DMTrz-A7H. Both transition electric and magnetic dipole moment densities are calculated for the S₁ → S₀ transition displayed with an iso-surface value of 0.002 a.u.

In summary, we have disclosed the effect of dihedral angles on g_lum 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 g_lum 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 g_lum has been demonstrated. This work thus provides a novel strategy to boost the g_lum 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.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (No. 21901128, 22071120 and 92256304), the National Key R&D Program of China (2020YFA0711500), the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology), and the Fundamental Research Funds for the Central Universities.

Notes and references

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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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