Hybridized local and charge-transfer excited states of blue OLEDs based on phenanthroimidazole derivates with a narrow FWHM of 24 nm†

Xiao Song^a, Shengbo Zhu*^a, Yongliang Liu^a, Tongyue Shi^a, Lei Yang^b, Yuan He^b, Xiaoling Niu^a, Zhongchen Yang^a, Jinhu Yuan^a and Zhen Feng*^b
aShaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an, 710021, China. E-mail: zhushengbo@xatu.edu.cn
^bShaanxi Lighte Optoelectronics Material Co., Ltd., Xi’an, 710065, China. E-mail: fengzhen@ltom.com

First published on 14th August 2024

---

---

1. Introduction

Organic light-emitting diodes (OLEDs) offer a wide array of benefits including a broad color gamut, minimal power consumption and the ability to create flexible displays, thus progressively fulfilling the growing demand for superior display quality. Nevertheless, achieving high efficiency and stable pure blue OLEDs continues to present a significant challenge in the field.^1–4

To improve the exciton utilization efficiency (EUE) of fluorescence molecules, researchers have developed fluorescent materials with a theoretical EUE of 100%. An example of this kind of material is thermally activated delayed fluorescence (TADF) molecules, which are poor blue-emission molecules due to their strong donor and acceptor properties.^5–8 Therefore, the strategy of designing high purity blue-emission TADF molecules requires strict consideration, resulting in complex synthesis.^9,10 Another kind is hybridized local charger-transfer (HLCT) molecules, adopting a moderate donor and acceptor structure, that are expected to achieve high purity blue-emission.^11–14 Since Ma et al. first developed HLCT materials, there has been significant advancement in their development. These materials provide a synthesis method that distorts the donor–acceptor molecules.¹² Subsequently, Ma et al. enhanced the local excited component by incorporating a π bridge to achieve higher exciton utilization and improved off-device quantum efficiency.¹⁵ Since then, an increasing number of researchers have been developing HLCT materials using various types of PI.^16–18 To sum up, three methods are widely introduced to adjust the HLCT state: (1) selecting a donor (D) or acceptor (A) with different electron pushing or absorbing capacities; (2) changing the distance between the donor and acceptor; (3) changing the distortion angle between the donor and acceptor.^19–22 However, the majority of these studies have focused on introducing different donors or π bridges at the PI 2-position. This method of introducing functional groups results in relatively strong vibrational coupling of the S1 excited state, causing most devices to fail in meeting the requirement of CIE_y < 0.08 or achieving a narrow FWHM.²³ This can only be achieved by adjusting the components of the local excited (LE) state or charge transfer (CT) state along the long axis of the molecule, which is not conducive to fine-tuning its excited state properties. Besides, systematic investigations into the introduction of different groups at the PI 1-position have been limited,^11,24–28 and the issue of color purity remains unresolved. The vibrational coupling of the S1 energy level and the vibrational coupling during the S1 → S0 transition often result in a fluorescence emission spectrum with broad FWHM due to changes in molecular vibrational energy levels.²⁹ Therefore, some researchers introduce groups along the short axis of the molecule that can adjust the CT or LE state properties. This approach aims to suppress vibrational coupling, thereby enhancing fluorescence radiative transition efficiency and color purity. Zhong reported that the properties of excited states can be adjusted by introducing different substituents on the short axis of the molecule.³⁰ Cui reported that by introducing a simple methyl group, they finely tuned the ΔE_ST without significantly affecting the optical bandgap.³¹ Therefore, finely tuning the excited state properties along the short axis of the molecule to suppress vibrational coupling of the excited state or adjust the fluorescence emission FWHM is a feasible approach.

Here, a series of D–A molecules based on the structure of 3a,11b-dihydro-1H-phenanthroimidazole (PI) and triphenylamine (TPA) are reported. The introduction of the phenanthrene structure is to improve the rigidity of imidazole, thereby reducing the occurrence of strong ICT. A moderate donor, TPA, was introduced horizontally into the 2-position of the PI, which maintains conjugation to elevate the LE state.³² However, a variety of functional groups are introduced at the phenzimidazole 1-position to fine-tune the excited state properties. The impacts of various functional groups on the HLCT state are investigated, while the HLCT states are confirmed through theoretical calculations and solvatochromic experiments. Systematic analyses of thermal properties and photophysical properties of various PT molecules are conducted, summarizing the influence of different functional groups on these properties. Among them, Me-PT and TfMe-PT were selected for OLED fabrication and highly pure blue-emission was realized. This investigation resulted in a straightforward synthesis route coupled with a cost-effective purification method, rendering these HLCT materials conducive for possible commercial-scale production.

2. Results and discussion

2.1 Synthesis and characterization

The synthetic routes for Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT are presented in Fig. 1. The synthesis method of the different molecules is relatively simple and the final products are synthesized through the Debus–Radziszewski reaction.³³ The reaction entails a one-step cyclization process, and purification methods such as trituration and recrystallization are employed, avoiding the use of column chromatography methods typically confined to laboratory settings. This approach proves advantageous for large-scale commercial production. The characterization of the molecules is confirmed using ¹H NMR, ¹³C NMR, ¹⁹F NMR, HRMS and elemental analyses, the purity of Me-PT and TfMe-PT is determined using HPLC and detailed information is depicted in the ESI.†

2.2 Theoretical calculations

In order to verify the rationality of the molecular design ideas, density functional theory (DFT) calculations are conducted by using the Gaussian 09W package. The calculations are carried out at the B3LYP/6-31G(d) level for the analysis of ground state properties.³⁴ As depicted in Fig. 2(a), the optimized geometric configurations reveal that all six molecules possess similar torsion angles. Notably, they exhibit significant torsion angles in the minor axis of the molecule, with values of 80.03° for Me-PT, 81.00° for MeO-PT, 80.02° for MTM-PT, 76.91° for TfMe-PT, 78.90° for TfMeO-PT and 76.41° for TfMTM-PT. This substantial torsion angle allows for orbital separation and hinders molecular aggregation in the solid state. Concurrently, the torsion angles of the six molecules in the major axis of the molecule are relatively small, measuring 24.35° for Me-PT, 23.46° for MeO-PT, 24.62° for MTM-PT, 27.51° for TfMe-PT, 26.21° for TfMeO-PT, and 27.64° for TfMTM-PT. This facilitates the hybridization of CT and LE states, optimizing photophysical properties.¹² As depicted in Fig. 2(b), the frontier molecular orbitals (FMOs) can clearly show the electron-cloud distributions of the six PT molecules. In the FMOs of Me-PT, MeO-PT and MTM-PT, the electron density of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is evenly distributed across the PI and TPA skeleton. The introduction of –CH₃, –OCH₃, and –SCH₃ substituents presents a substantial orbital overlap. Furthermore, the HOMO and LUMO energy levels of Me-PT, MeO-PT and MTM-PT are highly comparable, with the introduction of oxygen and sulfur atoms showing minimal impact on these energy levels. The LUMO and HOMO of TfMeO-PT are not completely separated due to the oxygen atom which has a certain strong electric absorption effect. However, in TfMe-PT and TfMTM-PT, a notable orbital separation occurs. To clarify the cause of this phenomenon, the molecular electrostatic potential (ESP) map is depicted in Fig. 2(c), the –CF₃ in TfMe-PT and TfMTM-PT appears distinctly green, while the benzene ring is light blue, indicating that the –CF₃ is electron-rich and the benzene ring is electron-deficient due to the strong electron-withdrawing effect of fluorine. This highlights the clear difference between –CF₃ and its surrounding environment. Therefore, the transfer of LUMO to fluorine atoms is attributed to fluorine's strong electron-withdrawing effect rather than its inductive effect. After incorporating the strong electron-absorbing fluorine group, the HOMO and LUMO energy levels of TfMe-PT, TfMeO-PT and TfMTM-PT experience a decrease. Notably, the LUMO exhibits a more significant decrease compared to the HOMO, resulting in a reduction in the energy gap (E_g). In the process of OLED electron injection, electrons need to overcome the energy barrier, and the reduced LUMO energy level can reduce the energy barrier, making the injection of OLED electrons more favorable. The increased electrical absorption resulting from the introduction of a fluorine atom also enhances CT effects, which means the transition between the excited state and the ground state of the electron is accompanied by more energy level transitions, which will impact the color purity.³⁵ This effect is evident from the FWHM of the fluorescence emission spectrum in THF solutions, as indicated in Table S4 (ESI†). Molecules lacking fluorine atom-absorbing groups typically exhibit narrower FWHM values.

The excited state properties are calculated using the Gaussian 09W package with the method of TD-DFT B3LYP/6-31G(d). The natural transition orbitals (NTOs) and inter-fragment charge transfer (IFCT) analysis were calculated with the Multiwfn package.^36,37 IFCT analysis can quantitatively assess the electronic excited state properties of molecular fragments. Electrons undergo either CT when transferring between fragments or LE when remaining within a fragment post-excitation. This dual characteristic analysis supports a comprehensive assessment of the excited state properties. For convenience, each of the six molecules is categorized into three fragments: the phenanthroimidazole core, a benzene ring with diverse functional groups at the 1-position, and the triphenylamine group. As depicted in Fig. S1 and Table S2 (ESI†), molecules Me-PT, MeO-PT, MTM-PT and TfMeO-PT display a combination of both LE and CT characteristics during the S0 → S1 transition process. In Fig. 3(a), the charge transfer amount (CT%) is 53.02%, the corresponding local excited amount (LE%) is 46.98%, therefore, Me-PT in the S1 excited state shows the equally distributed excited state characteristics of ¹CT and ¹LE, so it presents ¹HLCT characteristics. IFCT analysis in MeO-PT, MTM-PT and TfMeO-PT also confirmed that they all have the characteristic of ¹HLCT. Conversely, in Fig. 3(b), TfMe-PT prominently demonstrated ¹CT characteristics due to the electron-absorbing capability of trifluoromethyl groups and the CT% is 92.40%. Furthermore, the oscillator strengths are considerably smaller, measuring 0.067. The transition process of TfMe-PT at S0 → S2 is further analyzed. In comparison to the S0 → S1 transition, the CT% decreases while the LE% increases. Additionally, the oscillator strength for this transition is 0.736. In this process, it is observed that the TfMe-PT molecules still exhibit hybridization between LE and CT. This situation also applies to TfMTM-PT, which demonstrates ¹CT in the S1 excited and ¹HLCT in the S2 excited states. Hence, it can be concluded that six HLCT molecules have been successfully designed in theory.

2.3 Photophysical properties

The ultraviolet-visible (UV-vis) absorbance spectrum and photoluminescence (PL) spectrum of different PT molecules are recorded in THF (10⁻⁵ M) and in the neat films. As depicted in Fig. 4(a) and (d), the shape and position of the absorption peaks of molecules are relatively similar (detailed data are listed in Table 1). The maximum absorption peaks (λ_abs) are ranging about 360 nm in THF solution and 400 nm in the neat films (Fig. S4, ESI†), which can be ascribed to a newly generated CT transition from donor to acceptor. The PL curves show slight variation among the analogues, with blue emission wavelengths centered around 420 nm. As depicted in Fig. 4(b) and (e), notably, the redshift compared in solution states intensifies with increasing electron-withdrawing properties of the molecules, which is caused by π–π stacking between solid molecules. In the solid state, molecules are relatively fixed, with better orientation and less energy disturbance compared in solutions, leading to a narrower FWHM. Meanwhile, the experimental ΔE_ST is measured by using the fluorescence spectrum at 300 K and the phosphorescence spectrum at 77 K, listed in Table 1. The S1 energy level and the T1 energy level are estimated by using the intercept method, demonstrating a significant ΔE_ST gap which can help avoid RISC from T1 to S1. The solvent polarity has a significant impact on the photophysical behavior of these molecules, effectively affecting their excited emission states. As depicted in Fig. 4(c) and (f), with the increasing polarity from n-hexane to acetonitrile, the maximum emission (λ_PL) of Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT is red-shifted by 31 nm, 32 nm, 33 nm, 33 nm, 42 nm and 29 nm, respectively (detailed data are listed in Table S3, ESI†). In low-polarity solvents such as n-hexane or toluene, molecules including Me-PT, MeO-PT, MTM-PT, TfMe-PT and TfMeO-PT exhibit dual-peak fluorescence emission. This corresponds to well-defined vibrational structures, indicating that the excited states of these molecules in such solvents are primarily LE states. Specifically, the shorter wavelength emission peak represents the LE state, while the longer wavelength peak corresponds to the CT state.³⁸ As the solvent polarity increases, the fine structure of the dual-peak emission gradually diminished and eventually disappeared. During this process, the excited state of the derivates transitions progressively to a HLCT state. In high polarity solvents, all the studied molecules exhibit only single-peak emission. This can be attributed to the impact of intramolecular charge distribution and intermolecular interactions by the solvent polarity. Notably, for TfMTM-PT, due to its significant charge transfer characteristics in the S1 excited state (with a remarkable CT% of 95.31%), it predominantly shows a single-peak emission in low-polarity solvents. Although TfMe-PT also exhibits typical ¹CT characteristics, it still shows a fine dual-peak emission in hexane. This is related to the introduction of sulfur, leading to molecular conformational instability, which broadens the emission FWHM. In low-polarity solvents, the LE state is the dominator, which appears as a bimodal emission. However, as the polarity increases, the CT state increases, forming a hybrid of LE and CT, as the HLCT states, which appears as a single-peak emission with a narrow FWHM. Ultimately, in high-polarity solvents, a singular CT state emission is observed, which appears as a unimodal emission with obvious redshift and wider FWHM.

---

The excited state dipole (μ_e) of the six molecules is determined using the Lippert–Mataga model,³⁹ and the plots of the Stokes shift (υ_a–υ_ƒ) versus the solvent polarity function (ƒ) are provided in Fig. S6 (detailed data are listed in Table S3, ESI†). The polarization fitting lines of Me-PT, MeO-PT, TfMe-PT, TfMeO-PT and TfMTM-PT display two sections. In the low polarity solvents (0 < f < 0.1), the μ_e values are 12.2 D, 13.6 D, 13.6 D, 12.1 D and 9.7 D, respectively, indicating typical LE characteristics. Meanwhile, the μ_e value in the high polarity solvents (f > 0.2) are 19.4 D, 20.4 D, 20.0 D, 21.4 D and 20.8 D, respectively, indicating the existence of CT characteristics in the excited state. In the middle polarity solvents (0.1 < f < 0.2), the coexistence of both LE and CT states was observed in all five molecules. The polarization fitting line of the MTM-PT compound showed a consistent relationship in both low polar and high polar solvents, yielding an estimated excited dipole moment magnitude of 18.8 D. Consequently, in medium polar solvents, all six molecules exhibited the simultaneous presence of both LE and CT states. Furthermore, the CIE coordinates of the six molecules in ten different solvents resulted in deep blue emission (detailed data are listed in Table S4, ESI†).

In order to understand the mechanism behind the properties of the excited state, the fluorescence decay of six molecules in THF (10⁻⁵ M) and in neat film excitation state at 370 nm is measured using time-correlated single photon counting technology,⁴⁰ which is shown in Fig. S7 (ESI†). The lifetime of the six molecules in THF solvent is as follows: Me-PT (1.81 ns), MeO-PT (1.54 ns), MTM-PT (1.61 ns), TfMe-PT (2.27 ns), TfMeO-PT (1.65 ns), and TfMTM-PT (2.31 ns),. and the lifetime in neat films is as follows: Me-PT (1.29 ns), MeO-PT (1.31 ns), MTM-PT (0.90 ns), TfMe-PT (0.89 ns), TfMeO-PT (1.25 ns), and TfMTM-PT (0.83 ns). In the solid film, the radiative transition rate k_r is calculated, as depicted in Fig. 3 and Table 2. Me-PT and TfMe-PT have remarkable radiative transition rates of 6.21 × 10⁸ S⁻¹ and 9.85 × 10⁸ S⁻¹, respectively. The lifetime of typical TADF molecules comprises two components: a nanosecond prompt fluorescence lifetime induced by a rapid S1 → S0 process, and a notable delayed fluorescence resulting from a slower T1 → S1 → S0 process.^41,42 Therefore, to further eliminate the TADF mechanism, the transient PL decay curves at 300 K are measured, as depicted in Fig. S8 (ESI†) and Table 1, and the single-component and nanosecond lifetime in all six molecules confirm the high-RISC in the Tm → Sn rather the T1 → S1. Therefore, six molecules are verified to be HLCT materials.

---

The PLQY of the molecules in different polarity solvents is determined (Table S4, ESI†). The Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT, and TfMTM-PT showed φ_PL values in THF, which are 29.9%, 30.21%, 27.89%, 44.25%, 30.86%, and 14.39%, respectively. While they are 80.10%, 77.21%, 20.66%, 87.64%, 62.52% and 56.04% in neat films, respectively. As depicted in Fig. S9 (ESI†), the PLQY of most molecules exhibits a gradual decrease with increasing solvent polarity. In solid states, a higher PLQY indicates that molecules have lower degrees of freedom and better orientation. This suppression of vibrational coupling leads to a higher PLQY and a narrower FWHM.

The introduction of an oxygen atom has little effect on the PLQY of MeO-PT and TfMeO-PT in the solution state. However, in solid states, the introduction of oxygen reduces the PLQY to varying degrees, indicating an adverse effect on the PLQY. For the sulfur atom, the PLQY of MTM-PT is minimally affected in solution, however, the stronger ¹CT in TfMTM-PT exacerbates the intramolecular charge effect, significantly attenuating the PLQY. In the solid state, the PLQY decreases by varying degrees, suggesting that the introduction of sulfur also adversely affects the PLQY. From the perspective of introducing fluorine, in the solution state: (1) the introduction of TfMe-PT enhances the CT effect, leading to increased EUE and higher PLQY; (2) TfMeO-PT shows minimal effect on the PLQY; (3) the decreased PLQY in TfMTM-PT is attributed to its stronger intramolecular charge effect. In the solid state, both TfMe-PT and TfMTM-PT exhibit a higher PLQY due to the introduction of appropriate CT excited states promoting the PLQY. However, TfMeO-PT shows a decrease, possibly due to molecular arrangement issues. Overall, introducing appropriate CT excited states generally enhances the PLQY. The introduction of oxygen and sulfur adversely affects the PLQY in both solution and solid states, with a significant impact noted for sulfur. Even with the introduction of adjustable CT by the fluorine unit, TFMTM-PT still shows a lower PLQY compared to TfMeO-PT, and this effect of sulfur introduction is therefore greater than that of fluorine introduction.

2.4 Thermal properties

The fabrication of an OLED requires the materials to have good thermal properties.⁴³ In order to confirm the thermal stability of the six molecules, the six molecules were evaluated via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Fig. 5, Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT have high thermal decomposition (T_d, 5% weight loss), with the decomposition temperatures of 424.3 °C, 439.3 °C, 438.2 °C, 366.2 °C, 409.0 °C and 429.0 °C, respectively. The melting temperature (T_m) of Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT is 255.1 °C, 291.3 °C, 253.5 °C, 260.5 °C, 235.5 °C and 269.2 °C, respectively. The high thermal decomposition, attributed to the rigid skeleton of PI, ideally contributes to the stability of the device evaporation process. Meanwhile, the distorted spatial configuration of TPA is advantageous for minimizing molecular aggregation.

According to TGA analysis, the introduction of fluorine decreased the T_d of TfMe-PT, TfMeO-PT and TfMTM-PT. Conversely, introducing oxygen and sulfur increased the T_d. According to the DSC analysis, after the introduction of oxygen, the T_m of MeO-PT significantly increases, likely due to possible intermolecular hydrogen bonding. This phenomenon is observable in TfMeO-PT, where replacing hydrogen with fluorine eliminates intermolecular hydrogen bonds, resulting in a lower T_m. Introducing sulfur further increases the melting temperature. However, after introducing fluorine, there is a varying degree of decrease, indicating that fluorine reduces molecular interactions. Conversely, introducing oxygen and sulfur enhances molecular interactions.

2.5 Electrochemical properties

The electrochemical properties of the six molecules are investigated in DMF solution by cyclic voltammetry (CV). The obtained CV spectra are shown in Fig. S10 (ESI†), and the corresponding data are listed in Table 3. All molecules exhibited an oxidation/reduction process in the positive potential range and a reduction/oxidation process in the negative potential range, indicating that they possess both electron and hole transport capabilities.⁴⁴ The E_HOMO is calculated from the first oxidation potential, while the E_LUMO is calculated in combination with the UV-vis absorbance spectra.⁴⁵ The first oxidation potential of Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT is 0.44 eV, 0.45 eV, 0.42 eV, 0.37 eV, 0.42 eV and 0.38 eV, respectively. The E_HOMO of Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT is −5.16 eV, −5.09 eV, −5.17 eV, −5.14 eV, −5.14 eV and −5.10 eV, respectively. The band energy of Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT is 3.127 eV, 3.100 eV, 3.106 eV, 3.106 eV, 3.097 eV and 3.097 eV, respectively. In addition, the first oxidation potential, the E_HOMO and the band energy are almost similar ∼0.41 eV, ∼−5.1 eV and ∼3.1 eV.

---

2.6 Pure blue OLED properties

According to the test results, the corresponding doped devices are designed by Shaanxi Lighte Optoelectronics Material Co., Ltd. (LTOM). The OLED devices with doped configurations of ITO/LHT124: p-D (2%) (10 nm)/LHT124 (99.5 nm)/LHT108 (5 nm)/MBH513: Me-PT; TfMe-PT (2%) (20 nm)/LET101 (5 nm)/LET109:LiQ = 1:1 (30 nm)/Yb (1.5 nm)/Mg:Ag = 1:9 (12.5 nm). Among them, the ITO (indium tin oxide) is the anode, LHT124: p-D 2% is the hole injection layer, LHT124 is the hole transporting layer, LHT108 is the electron blocking layer, the emission materials layers (EMLs) are Me-PT and TfMe-PT, respectively, with the doped concentration of 2 wt% in MBH513, LET101 is the hole blocking layer, LET109:LiQ is the electron transporting layer, Yb is the electron injection layer, and Mg:Ag is the cathode. The OLED structure is shown in Fig. 6(a). The related data are listed in Table 4. The two devices achieve EQE_max of 5.52% and 5.47% with Me-PT and TfMe-PT, respectively, indicating the rationality of the molecular design and the suitability of the device structures. As shown in Fig. 6(c) and (d), both devices achieve blue emission, with λ_EL at 465 nm and 466 nm with CIE coordinates of (0.140, 047) and (0.139, 0.049), respectively. Furthermore, Me-PT and TfMe-PT are incorporated into the host material, significantly restricting molecular vibration and freedom. During the evaporation process, molecules are more orderly arranged compared to pure solid films, effectively suppressing molecular vibration coupling. As a result, the FWHM of the EL spectra narrows to 24 nm and 25 nm, respectively.⁴⁶ Table 4 lists the maximum power efficiency (CE_max) and the maximum luminance efficiency (PE_max) of Me-PT and TfMe-PT as 2.614, 2.678 cd A⁻¹, and 1.992, 2.033 lm W⁻¹, respectively. The CE curves and PE curves are depicted in Fig. S11 (ESI†).

---

At the same time, lifespan tests are conducted on the two groups of devices, with the time taken for the brightness to drop to 95% of its initial value considered as the device's lifetime. Relevant data are listed in Fig. S12 (ESI†) and Table 4. A stable constant current of 4.5 mA to the devices at room temperature was applied. The lifetime of Me-PT is much higher than that of TfMe-PT, with a value of 18.5 h compared to 0.1 h, respectively. The introduction of –CF₃ allows the electron injection process to cause the material to decompose, resulting in a much shorter device lifetime than Me-PT. According to Fig. S12(b) (ESI†), the voltage is stable within the range of 0.5 V, which ensures the luminance stability of the light source when the device is working. As depicted in Fig. S13 (ESI†), the capacitance of the device changes with the voltage, and the curve can reflect the accumulation of interface charge. From V_inj onwards, the device begins to accumulate charge, at which point the electrons break through the barrier and facilitate the transport of charge carriers. OLED devices can be considered as a capacitor, but with charge accumulation C_p-Area to a certain extent (up to V_max), the ability to store charge begins to decline. From Fig. S13 (ESI†) it can be observed that the V–C curves of the two devices are very close, but their maximum capacitance values are different, which are 4.40 nF and 4.46 nF respectively. The larger capacitance in TfMe-PT may be due to the fact that –CF₃ has a stronger ability to absorb electrons, allowing it to absorb and hold more charges. The more stable structure of Me-PT makes the storage of final charge higher than that of TfMe-PT, 0.749 nC and 0.728 nC, respectively.

Moreover, to verify the EUE in the device and guide molecular design work, additionally, the EUE of the Me-PT and TfMe-PT are calculated by using the relevant formulas,⁴⁷ resulting in values of 34.46% and 31.21%, respectively. We observe that the EUE of the devices exceeds the traditional 25%. The transient PL decay curve test shows a single-stage decline trend, and phosphorescence spectra indicate a lower T1 level, resulting in a significant difference in ΔE_ST, confirming the formation of a “hot exciton channel”. The reason for the low EUE is that the two molecules have small gaps in the T1 and T2 energy levels, making triplet state excitons prone to IC transition to the T1 energy level, leading to exciton annihilation. Consequently, the device experiences a severe roll-off issue. To achieve higher exciton utilization in molecules capable of forming “hot exciton channels”, adjust the excited state properties of the Tm (m ≥ 2) levels to ³CT characteristics to accelerate the high-RISC rate from high triplet excitons to singlet excitons Sn (n ≥ 1),⁴⁸ or further separating the T1 and T2 levels is necessary.

3. Conclusion

In summary, six PT molecules with HLCT properties, confirmed by theoretical calculations and experimental tests, are synthesized and characterized. The narrower FWHM and higher PLQY observed in the thin film state indicate that the purified crystalline molecules are better arranged in the solid state, leading to suppressed vibrational coupling. The introduction of oxygen and sulfur negatively affects the PLQY; however it improves the thermal stability. Conversely, introducing fluorine significantly enhances the PLQY but adversely affects thermal stability. The OLED based on Me-PT demonstrated a better comprehensive performance compared with that based on TfMe-PT. It realizes an EQE_max of 5.52%, with CIE coordinates of (0.140, 0.047), close to the NTSC standard blue CIE coordinates of (0.14, 0.08). The λ_EL reaches 465 nm, accompanied by a narrow FWHM value of 24 nm, indicating high color purity. Additionally, the radiative transition rates of both have reached remarkable levels of 6.21 × 10⁸ S⁻¹ and 9.85 × 10⁸ S⁻¹, respectively. These results prove that the moderate electric-withdrawing group at the 1-position in the molecule is crucial in adjusting the color purity and PLQY. As a result, this study provides a strategy based on HLCT materials which can realize high purity blue emission, and provides significant guidance for high purity blue OLEDs.

4. Experimental section

4.1 Materials and characterization

The reagents and solvents required for the synthesis were purchased from commercial sources. The synthesis method of the different molecules is relatively simple and the final products were synthesized using the Debus–Radziszewski reaction.^{33 1}H NMR, ¹³C NMR and ¹⁹F NMR spectra were recorded on a Bruker AVANCE instrument with tetrame-thylsilane (TMS) as the internal standard. High resolution mass spectra (HRMS) were measured with a Bruker maXis mass spectrometer. Elemental analysis (C H N) was carried out on a VARIO-EL-III elemental analyzer. High performance liquid chromatography (HPLC) was carried out with a CLC-3200.

4.2 Synthesis measurements

The synthesis methods of Me-PT, MeO-PT, MTM-PT, TfMe-PT, TfMeO-PT and TfMTM-PT are described in Fig. 1. The synthetic details are described in the ESI.† The ¹H NMR, ¹³C NMR, ¹⁹F NMR and HRMS spectra are depicted in Fig. S14–S25 (ESI†).

4.3 Theoretical calculation measurements

The quantum chemical calculations were performed with the Gaussian 09W package using density functional theory (DFT). The optimized ground structure and the HOMO and LUMO distributions of the complexes were calculated using the DFT B3LYP/6-31G(d) function. The excited states were calculated with the TD-DFT B3LYP/6-31G(d). The natural transition orbitals (NTOs) and inter-fragment charge transfer (IFCT) analysis were calculated with the Multiwfn package.

4.4 Photophysical measurements

UV-vis absorbance spectra of the molecules in n-hexane, toluene, chloroform, ethyl ether, ethyl acetate, tetrahydrofuran, methylene chloride, dimethyl formamide, acetone, and acetonitrile solution (1 × 10⁻⁵ M) were measured with a UV2500 spectrophotometer. The emission spectra, quantum yield and the CIE coordinates of the molecules in different solvents (1 × 10⁻⁵ M) were measured with a Quanta Master 8000 steady-state transient fluorescence spectrometer. The luminescence decay curves in THF solvent (1 × 10⁻⁵ M) were measured with a Quanta Master 8000 steady-state transient fluorescence spectrometer.

4.5 Thermal measurements

Differential scanning calorimetry (DSC) was performed on a DSC 823e instrument in the temperature range of 25–300 °C at a heating rate of 20 °C min⁻¹ and an argon flow rate of 60 mL min⁻¹.

Thermal gravimetric analysis (TGA) was performed using a Q50V20.7 instrument in the temperature range of 20–810 °C at a heating rate of 20 °C min⁻¹ and a nitrogen flow rate of 60 mL min⁻¹.

4.6 Electrochemical measurements

Cyclic voltammograms (CV) were conducted on an electrochemical workstation (CIMPS-2, Zahner, Germany) at a scan rate of 100 mV s⁻¹ under a N₂ atmosphere. Working electrode: glassy carbon; counter electrode: Pt wire; reference electrode: non-aqueous Ag/Ag⁺ electrode; supporting electrolyte: N,N-dimethylformamide (DMF) solution containing 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF₆). The redox potentials were calibrated with ferrocene/ferrocenium (Fc/Fc⁺) as the internal reference.

4.7 OLED fabrication

LTOM is solely responsible for the fabrication of OLED devices. First, ITO glass is etched to prepare the required anode pattern. Then the substrate surface was washed with a mixture of isopropyl alcohol and acetone. The substrate was placed in deionized water containing about 5% detergent, heated to 40 °C, ultrasonically cleaned at a frequency of 70 kHz for 15 min, then ultrasonically cleaned in acetone at 40 °C for 15 min, and finally ultrasonically cleaned in isopropyl alcohol solution at room temperature for 15 min. The ultrasonically treated ITO glass was removed from isopropyl alcohol and dried with N₂. Then it was put into an ultraviolet oven for ultraviolet irradiation. The treated ITO glass substrate was put into a vacuum evaporation chamber, when the vacuum degree was below 3 × 10⁻⁴ Pa, LHT124: p-D (2%), LHT124, LHT108, MBH513: Me-PT/TfMe-PT (2%), LET101, LET109:LiQ (1:1) and Yb was steamed successively and finally the Mg:Ag (1:9) metal cathode was steamed.

Data availability

The data supporting this study are available in the published article and in its ESI.† The data related to this paper may be requested from the corresponding author.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Qin Chuangyuan “Scientist + Engineer” Team of Key Research and Development Program of Shaanxi Province (No. 2023KXJ-177), and the excellent doctoral dissertation Cultivation Fund project of Xi’an Technological University (No. YB202207).

References

1. D. F. O. B. M. A. Baldo, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151–154 CrossRef .
2. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913–915 CrossRef CAS .
3. K. Tuong Ly, R.-W. Chen-Cheng, H.-W. Lin, Y.-J. Shiau, S.-H. Liu, P.-T. Chou, C.-S. Tsao, Y.-C. Huang and Y. Chi, Nat. Photonics, 2016, 11, 63–68 CrossRef .
4. R. Young, Infor. Disp., 2020, 36, 41–45 Search PubMed .
5. H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234–238 CrossRef CAS PubMed .
6. Z. Yang, N. Qiu, X. Dong, J. Li, P. Zou, D. Yang, D. Ma, B. Z. Tang and Z. Zhao, Adv. Opt. Mater., 2024, 12, 2302677 CrossRef CAS .
7. Y. Fu, H. Liu, B. Z. Tang and Z. Zhao, Nat. Commun., 2023, 14, 2019 CrossRef CAS PubMed .
8. S. Wang, Y. Xu, J. Miao, T. Hua, X. Cao, N. Li, Z. Huang and C. Yang, Chem. Eng. J., 2023, 471, 144644 CrossRef .
9. P. Keerthika and R. K. Konidena, Adv. Opt. Mater., 2023, 11, 2301732 CrossRef CAS .
10. Y. Kondo, K. Yoshiura, S. Kitera, H. Nishi, S. Oda, H. Gotoh, Y. Sasada, M. Yanai and T. Hatakeyama, Nat. Photonics, 2019, 13, 678–682 CrossRef CAS .
11. C. Du, H. Liu, Z. Cheng, S. Zhang, Z. Qu, D. Yang, X. Qiao, Z. Zhao and P. Lu, Adv. Funct. Mater., 2023, 33, 2304854 CrossRef CAS .
12. W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu, B. Yang and Y. Ma, Adv. Funct. Mater., 2012, 22, 2797–2803 CrossRef CAS .
13. R. Wu, K. Sun, G. Shi, Y. Han, T. Gong, Y. Xu, S. T. Zhang and B. Yang, Adv. Funct. Mater., 2024, 2403501 CrossRef .
14. J. Jin, S. Wang, H. Jiang, L. Wang and W. Y. Wong, Adv. Opt. Mater., 2024, 12, 2302354 CrossRef CAS .
15. S. Zhang, L. Yao, Q. Peng, W. Li, Y. Pan, R. Xiao, Y. Gao, C. Gu, Z. Wang, P. Lu, F. Li, S. Su, B. Yang and Y. Ma, Adv. Funct. Mater., 2015, 25, 1755–1762 CrossRef CAS .
16. T. Jairam and W. P. Hong, J. Mater. Chem. C, 2022, 10, 16173–16217 RSC .
17. A. Ivanauskaite, R. Lygaitis, S. Raisys, K. Kazlauskas, G. Kreiza, D. Volyniuk, D. Gudeika, S. Jursenas and J. V. Grazulevicius, Phys. Chem. Chem. Phys., 2017, 19, 16737–16748 RSC .
18. J. Tagare, D. K. Dubey, R. A. K. Yadav, J.-H. Jou and S. Vaidyanathan, Mater. Adv., 2020, 1, 666–679 RSC .
19. Y. Tao, K. Yuan, T. Chen, P. Xu, H. Li, R. Chen, C. Zheng, L. Zhang and W. Huang, Adv. Mater., 2014, 26, 7931–7958 CrossRef CAS PubMed .
20. C. L. Yi, C. Y. Lin, Y. Tang, C. Y. Wang, C. W. Huang, X. Gong, S. Gong, C. C. Wu and K. T. Wong, Adv. Opt. Mater., 2021, 10, 2101791 CrossRef .
21. S. H. Jeon, S. Kang and T. Kim, Adv. Theory Simul., 2023, 7, 2300363 CrossRef .
22. X. Tian, M. Yao, X. Liang, C. Zhou, S. Xiao, Y. Gao, H. Liu, S.-T. Zhang and B. Yang, Dyes Pigm., 2022, 205, 110463 CrossRef CAS .
23. X. Tang, Q. Bai, Q. Peng, Y. Gao, J. Li, Y. Liu, L. Yao, P. Lu, B. Yang and Y. Ma, Chem. Mater., 2015, 27, 7050–7057 CrossRef CAS .
24. M. Sun, C. Ma, M. Xie, L. Chu, Q. Sun, W. Yang and S. Xue, Chem. Eng. J., 2024, 480, 148107 CrossRef CAS .
25. H. Zhang, B. Zhang, Y. Zhang, Z. Xu, H. Wu, P. A. Yin, Z. Wang, Z. Zhao, D. Ma and B. Z. Tang, Adv. Funct. Mater., 2020, 30, 2002323 CrossRef CAS .
26. S. Geng, Z. Liu, H. Li, Z. Zhong, X. J. Feng, Z. Zhao and H. Lu, Adv. Opt. Mater., 2023, 2301344 Search PubMed .
27. C. Kaiyasuan, P. Chasing, P. Nalaoh, P. Wongkaew, T. Sudyoadsuk, K. Kongpatpanich and V. Promarak, Chem. – Asian J., 2021, 16, 2328–2337 CrossRef CAS PubMed .
28. H. Peng, Z. Wei, L. Wu and X. Li, Opt. Mater., 2020, 101, 109726 CrossRef CAS .
29. A. Khan, X. Tang, C. Zhong, Q. Wang, S. Y. Yang, F. C. Kong, S. Yuan, A. S. D. Sandanayaka, C. Adachi, Z. Q. Jiang and L. S. Liao, Adv. Funct. Mater., 2021, 31, 2009488 CrossRef CAS .
30. Z. Zhong, X. Zhu, X. Wang, Y. Zheng, S. Geng, Z. Zhou, X. J. Feng, Z. Zhao and H. Lu, Adv. Funct. Mater., 2022, 32, 2112969 CrossRef CAS .
31. L. S. Cui, H. Nomura, Y. Geng, J. U. Kim, H. Nakanotani and C. Adachi, Angew. Chem., Int. Ed., 2016, 56, 1571–1575 CrossRef PubMed .
32. W. C. Chen, Y. Yuan, Y. Xiong, A. L. Rogach, Q. X. Tong and C. S. Lee, ACS Appl. Mater. Interfaces, 2017, 9, 26268–26278 CrossRef CAS PubMed .
33. H. Debus, Ann. Chem. Pharm., 1858, 107, 199–208 CrossRef .
34. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09 (Revision D.01), Gaussian, Inc., Wallingford CT, 2013 Search PubMed .
35. S. Tretiak, A. Saxena, R. L. Martin and A. R. Bishop, Phys. Rev. Lett., 2002, 89, 097402 CrossRef CAS PubMed .
36. T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580–592 CrossRef CAS PubMed .
37. Z. Liu, T. Lu and Q. Chen, Carbon, 2020, 165, 461–467 CrossRef CAS .
38. V. R. Gopal, A. M. Reddy and V. J. J. J. o O. C. Rao, J. Org. Chem., 1995, 60, 7966–7973 CrossRef CAS .
39. Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev., 2003, 103, 3899–4031 CrossRef PubMed .
40. J. Zhou, B. He, J. Xiang, B. Chen, G. Lin, W. Luo, X. Lou, S. Chen, Z. Zhao and B. Z. Tang, ChemistrySelect, 2016, 1, 812–818 CrossRef CAS .
41. H. L. Lee, C. S. Oh, K. H. Lee, J. Y. Lee and W. P. Hong, ACS Appl. Mater. Interfaces, 2021, 13, 2908–2918 CrossRef CAS PubMed .
42. Y. Im, M. Kim, Y. J. Cho, J.-A. Seo, K. S. Yook and J. Y. Lee, Chem. Mater., 2017, 29, 1946–1963 CrossRef CAS .
43. W. Wang, J. Bian, K. Chen, C. Li, Y. Long, H. Huang, L. Jiang, J. Zhao, S. Liu, Z. Chi, J. Xu and Y. Zhang, Angew. Chem., Int. Ed., 2024, e202318782 CAS .
44. H. M. Hilal, M. A. El Bitar Nehme and T. H. Ghaddar, ACS Appl. Energy Mater., 2018, 1, 2776–2783 CrossRef CAS .
45. J. C. S. Costa, R. J. S. Taveira, C. F. R. A. C. Lima, A. Mendes and L. M. N. B. F. Santos, Opt. Mater., 2016, 58, 51–60 CrossRef CAS .
46. Y.-J. Yu, S.-N. Zou, C.-C. Peng, Z.-Q. Feng, Y.-K. Qu, S.-Y. Yang, Z.-Q. Jiang and L.-S. Liao, J. Mater. Chem. C, 2022, 10, 4941–4946 RSC .
47. O. S. Lee, N. Sharma, T. Matulaitis, A. M. Z. Slawin, Y. Olivier, I. D. W. Samuel, M. C. Gather and E. Zysman-Colman, J. Mater. Chem. C, 2024, 12, 4273–4286 RSC .
48. J. M. Jacob, P. K. Samanta and M. K. Ravva, New J. Chem., 2023, 47, 10552–10563 RSC .

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc02261k

This journal is © The Royal Society of Chemistry 2024