White organic light-emitting diode (WOLED) has received significant development in the past few years owing to its great potential in the next-generation solid-state lighting such as eye protection, power conservation, intrinsic lightweight, and flexibility [1–6]. In generally, white lighting emission is composed of two-color emitters (blue and yellow) or three-color emitters (blue, green, and red), and therefore WOLED is usually fabricated based on multiple emission layers with different color emitters or single emission layer including mixed fluorophores or phosphors emitters [7–11]. Despite the success of WOLEDs based on multi-emitters, the relatively complicated production process of devices leads to unsatisfactory color stability and reproducibility, moreover, increased cost [12–15]. Recently, many reported single small organic molecules can exhibit white emission with dual or more emission bands and then several WOLEDs based on single-emitter were successfully fabricated, which provided potential to solve the above issue [16–22]. However, the external quantum efficiency (EQE) of them are relatively unsatisfactory resulted from the restricted exciton utilization in these multi emission systems. Among them, small organic molecule based on phenothiazine (PTZ) unit is most commonly employed for single-emitter WOLED due to its dual emission bands caused by conformational isomerism, in which the blue traditional fluorescence comes from quasi-axial (ax) conformer and orange thermally activated delayed fluorescence (TADF) comes from quasi-equatorial (eq) conformer [19–21]. Since the traditional fluorescence of "ax" conformer can only utilize singlet excitons and its internal quantum efficiency (IQE) is limited to be only 25%, an alternative method to improve efficiency of single-emitter WOLED is to develop highly efficient TADF of "eq" conformer which can also utilize triplet excitons via reverse intersystem crossing (RISC) to realize nearly 100% IQE due to small energy gap (ΔE_ST) between the lowest singlet excited state (S₁) and the lowest triplet excited state (T₁) [23]. In addition, in such dual emission system, the energy will transfer from host to "ax" conformer and "eq" conformer simultaneously, while part of energy on "ax" conformer will transfer to "eq" conformer and then radiate by TADF channel. Obviously, "eq" conformer plays a key intermediary role in this process and therefore highly efficient TADF of "eq" conformer is necessary to ensure high exciton utilization in such dual emission system.

Trifluoromethyl (CF₃) is a strong electron withdrawing group resulted from electron-inductive-effect [24]. The large steric hindrance of CF₃ will be able to reduce intermolecular interaction and then result in high photoluminescence quantum yield (PLQY) [25,26]. The F atoms in CF₃ with large electronegativity can form hydrogen bonds between adjacent molecules, and therefore enhance the rigidity and thus suppress the nonradiative deactivation [27]. In addition, owing to inductive effect, the CF₃ can be used to adjust electrochemical property, excited-state property and luminescence property of TADF emitter [28–30]. Recently, many TADF emitters with CF₃ substituents were successfully synthesized and exhibited excellent performance [24–33]. For example, in 2021, Duan et al. introduced CF₃ group into TADF emitter as secondary acceptor aiming to optimize molecular electronic and photophysical properties and then improved performance of OLEDs with EQE over 30% [31]. In 2023, they first developed multiresonance (MR) acceptor with CF₃ substituents to modify the distribution of frontier molecular orbitals (FMO) and optimize excited-state property. Finally, the OLED exhibited high EQE over 40% [32]. These excellent performances indicate the effectiveness of the CF₃ substituent strategy in TADF emitters.

Herein, as shown in Fig. 1, CF₃ group was elaborately used to be secondary acceptor connected to quinazoline (PQ), in which PTZ was selected as electron donor to construct asymmetric donor-acceptor-acceptor' (D-A-A') structure with dual emission bands, namely PTZ-PQ-CF3. The molecular PTZ-PQ-F with fluorine (F) atom as secondary acceptor was also synthesized to illustrate the effectiveness of the CF₃ group. The OLEDs exhibit white emission with dual emission bands from "ax" conformer and "eq" conformer at low doping concentration, while single orange emission band from "eq" conformer was obtained at high doping concentration, confirming the energy transfer process from "ax" conformer to "eq" conformer. The C—H···F hydrogen bonds from CF₃ groups in PTZ-PQ-CF3 greatly enhance the rigidity to suppress the nonradiative deactivation and then provide higher PLQY. Owing to strong electron-inductive-effect, the CF₃ groups in PTZ-PQ-CF3 successfully modulate the excited state of "eq" conformer and then minimize its ΔE_ST as only 0.01 eV which is much smaller than that of PTZ-PQ-F (0.13 eV), leading to larger k_RISC (1.9 × 10⁵ s⁻¹) of PTZ-PQ-CF3 than 0.7 × 10⁵ s⁻¹ of PTZ-PQ-F. As the station of energy transfer, the improved performance of "eq" conformer will therefore greatly enhance exciton utilization of the whole dual emission system. As expected, the exciton utilization of single-emitter WOLED increased from 90% (PTZ-PQ-F) to 100% (PTZ-PQ-CF3). Compared with PTZ-PQ-F, the single-emitter WOLED based on PTZ-PQ-CF3 displays higher EQE of 25.5% with smaller roll-off.

As shown in Scheme S1 (Supporting information), PTZ-PQ-F and PTZ-PQ-CF3 were simply synthesized starting from PTZ-pH-Bpin by two-step Suzuki-Miyaura reaction with high yield over 70%. The more detailed information was provided in Supporting information. Their highest occupied molecular orbital (HOMO) energy levels were calculated from the onset of the oxidation potential measured by cyclic voltammetry (CV) as −5.16 eV and −5.15 eV Meanwhile, the lowest unoccupied molecular orbital (LUMO) were obtained as −2.12 eV and −2.17 eV from the optical bandgaps. The LUMO of PTZ-PQ-CF3 is slightly deeper than PTZ-PQ-F because of strong electron-inductive-effect of CF₃.

The single crystal of PTZ-PQ-CF3 was obtained to explore the influence of CF₃ group on stacking mode (Fig. 2). It is noted that only "eq" conformation is observed in single crystal which is different with our previous observation, presumably resulted from the influence of secondary acceptor [21]. The large dihedral angle of 89.31° between phenyl bridge and PTZ will promote separation of the frontier orbital, leading to a small ΔE_ST. The large steric hindrance of CF₃ groups greatly expand the distance between two acceptors of adjacent molecules and then weaken the accumulation of triplet excitons [25,26]. Moreover, the multiple intramolecular and intermolecular C—H···F hydrogen bonds from CF₃ groups will greatly enhance the rigidity and thus suppress the nonradiative deactivation by restricting the molecular vibrations. Unfortunately, we failed to obtain the single crystal of PTZ-PQ-F after many attempts with different method. We estimate the PTZ-PQ-CF3 has more C—H···F hydrogen bonds than that of PTZ-PQ-F due to more fluorine atoms from CF₃, therefore a higher PLQY in PTZ-PQ-CF3 expectedly.

As shown in Fig. 3a, the ultraviolet-visible (UV–vis) absorption, photoluminescence (PL), and phosphorescence (pH) spectra were measured in toluene solution to study the photophysical properties. For UV–vis spectra, the broad absorption peaks (λ_abs) at around 350-450 nm belong to intramolecular charge transfer from PTZ to PQ. Meanwhile, the PL spectra exhibits obvious two different emission bands. According to previous works, the "ax" conformer exhibits a higher energy emission without TADF character and the "eq" conformer exhibits a lower energy emission with TADF character [19–21]. Therefore, the blue emission bands 463 nm of PTZ-PQ-F and 481 nm of PTZ-PQ-CF3 belong to traditional fluorescence "ax" conformers and orange emission bands 588 nm of PTZ-PQ-F and 617 nm of PTZ-PQ-CF3 belong to TADF "eq" conformers [21]. The PTZ-PQ-CF3 shows obvious red-shift emission compared to PTZ-PQ-F due to stronger secondary acceptor of CF₃ group. For pH spectra, PTZ-PQ-F, PTZ-PQ-CF3, and 2Ph-PQ show similar characteristic vibrational structure, indicating the lowest triplet states of them are local excited states from PQ acceptor (³LE_A), which will be able to enhance the spin orbital coupling (SOC) between the singlet and triplet states [34–37]. From the corresponding onsets of PL and pH spectra, the S₁ and T₁ of "ax" conformers are estimated to be 2.98 eV/2.44 eV for PTZ-PQ-F and 2.90 eV/2.41 eV for PTZ-PQ-CF3 and then the ΔE_ST of them are calculated to be 0.54 eV and 0.49 eV, respectively, which are too large for RISC process so that the "ax" conformers may belong to traditional fluorescence. In terms of "eq" conformers (Fig. 3b), due to the strong electron-inductive-effect of CF₃ group, the S₁ of PTZ-PQ-CF3 (2.42 eV) is much lower than 2.57 eV of PTZ-PQ-F, leading to much smaller ΔE_ST of PTZ-PQ-CF3 (0.01 eV) than 0.13 eV of PTZ-PQ-F and then the faster RISC process of PTZ-PQ-CF3 will be expected.

The photophysical properties in doped films were also measured. As shown in Fig. S3 (Supporting information), at 1 wt% doping concentration, both PTZ-PQ-F and PTZ-PQ-CF3 exhibit white emission with dual emission bands from "ax" conformer and "eq" conformer. The PLQYs of them are measured to be 75% and 83%, respectively. When the doping concentration is 15 wt%, they exhibit only one emission band from "eq" conformer and then the PLQYs are 74% and 79%. The higher PLQY of PTZ-PQ-CF3 confirms the effectiveness of C—H···F hydrogen bonds from CF₃ groups. As shown in Fig. 3c, the transient PL spectra at high energy emission peaks corresponding to "ax" conformers exhibit only prompt fluorescence component of 21 ns for PTZ-PQ-F and 18 ns for PTZ-PQ-CF3, confirming the traditional fluorescence character. Meanwhile, the transient PL spectra at low energy emission peaks corresponding to "eq" conformers exhibit obviously second-order exponential decay, in which the prompt fluorescence lifetimes are 2.1 ns for PTZ-PQ-F and 2.8 ns for PTZ-PQ-CF3 and the delayed fluorescence lifetimes are 54.5 µs for PTZ-PQ-F and 40.3 µs for PTZ-PQ-CF3, confirming the TADF character. Therefore, the k_RISC of "eq" conformers are calculated to be 0.7 × 10⁵ s⁻¹ and 1.9 × 10⁵ s⁻¹ for PTZ-PQ-F and PTZ-PQ-CF3, respectively, in which the k_RISC of PTZ-PQ-CF3 is much larger than PTZ-PQ-F contributed to its smaller ΔE_ST and enhanced SOC. In addition, the k_RISC of non-fluorinated substituted 2PQ-PTZ is obvious slower, indicating the effectiveness of the secondary acceptor in enhancing RISC (Fig. S2 in Supporting information). As expected, the enhanced k_RISC of "eq" conformer in PTZ-PQ-CF3 will be able to increase exciton utilization of the whole dual emission system and therefore a higher EQE in single-emitter WOLED.

To investigate the performances of PTZ-PQ-F and PTZ-PQ-CF3 in OLEDs, multilayer devices were fabricated with a simple architecture of ITO/TAPC (50 nm)/TCTA (5 nm)/x wt% emitter in host (30 nm)/TmPyPB (30 nm)/ Liq (1 nm)/Al (100 nm) (Figs. 4a and b). Among them, 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) was used as hole-transporting layer (HTL) while 3,3′-[5′-[3-(3-pyridinyl)phenyl] [1,1′:3′,1′,-terphenyl]-3,3′,-diyl]bispyridine (TmPyPB) was used as electron-transporting layer (HTL). Tris(4-carbazolyl-9-ylphenyl)amine (TCTA) worked as electron-blocking layer. 1,3-Di(9H-carbazol-9-yl)benzene (mCP) or 3,5-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (35DCzPPy) are separately used as the host, in which the bipolar host 35DCzPPy has more balanced charge transporting character and displays lower triplet energy with T₁ of 2.71 eV than T₁ of 2.91 eV in mCP. The performances of OLEDs are shown in Table 1.

As shown in Fig. 4c, both PTZ-PQ-F and PTZ-PQ-CF3 exhibit white emission with two obvious emission bands, confirming that the "ax" and "eq" conformers can also coexist and emit individual emission from each other under electroexcitation in OLED. The long wavelength emission peaks show red-shift with the doping concentration increased which is attributed to polarization phenomena, confirming the strong ICT character of "eq" conformer. Since the limited exciton utilization of "ax" conformer, part of energy on "ax" conformer will transfer to "eq" conformer and then radiate by TADF channel, leading to increased proportion of orange emission band from "eq" conformer in EL spectra. This process will be enhanced with the increased doping concentration and therefore pure orange OLEDs are obtained at high doping concentration of 15 wt%. As the station of energy transfer, "eq" conformer plays a key intermediary role to ensure high exciton utilization in this process. Consequently, owing to higher k_RISC of "eq" conformer in PTZ-PQ-CF3, both single-emitter WOLEDs and pure orange OLEDs based on PTZ-PQ-CF3 exhibit higher EQE and smaller roll-off than that of PTZ-PQ-F. As shown in Fig. 4d and Fig. S6 (Supporting information), for mCP host, at 0.3 wt% doping concentration, the single-emitter WOLED based on PTZ-PQ-CF3 is turned on at 3.4 eV and exbibit higher current efficiency (CE), power efficiency (PE), and EQE of 30.0 cd/A, 25.4 lm/W, and 13.0% with nearly standard CIE of (0.35, 0.36). For 35DCzPPy host, due to the balanced bipolar carrier transport capability and more efficient Dexter energy transfer (DET) process from host to "eq" conformers, the single-emitter WOLED exhibit higher EQE and more warm white emission than that in mCP host. At 1 wt% doping concentration, the single-emitter WOLED based on PTZ-PQ-CF3 shows much higher CE, PE, and EQE of 69.2 cd/A, 62.1 lm/W, and 25.5% than that of PTZ-PQ-F (CE, PE, and EQE of 57.7 cd/A, 51.8 lm/W, and 20.3%, respectively). According to Eq. S13 (Suporting information) [38,39], the exciton utilization of single-emitter WOLED based on PTZ-PQ-F is 90% while the PTZ-PQ-CF3 is up to nearly 100%. As a result, the more excellent performance and higher exciton utilization of single-emitter WLEDs based on PTZ-PQ-CF3 confirm the effectiveness of the secondary acceptor strategy based on CF₃ group.

In conclusion, two novel asymmetric emitters PTZ-PQ-CF and PTZ-PQ-CF3 were successfully designed and synthesized by employing secondary acceptor. They can emit white lighting with dual emission bands from "ax" conformer and "eq" conformer, and then the experimental results prove that "ax" conformer belongs to traditional fluorescence and "eq" conformer belongs to TADF. The CF₃ groups in PTZ-PQ-CF3 provide steric hindrance and C—H···F hydrogen bonds to suppress the nonradiative deactivation and then improve PLQY. Owing to inductive effect, the CF₃ groups decrease S₁ energy of "eq" conformer in PTZ-PQ-CF3 and then successfully minimize its ΔE_ST, which therefore greatly enhance k_RISC. As a result, the exciton utilization of the whole dual emission system increases to nearly 100% in PTZ-PQ-CF3. Furthermore, highly efficient single-emitter WOLED with CIE of (0.35, 0.36) and EQE of 13.0% and warm single-emitter WOLED with CIE of (0.40, 0.47) with EQE of 25.5% were obtained. These excellent performances confirm the effectiveness of our molecular design strategy for the single-emitter WOLED, which will provide a promising method for the development of high-performance WOLED.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

These authors would like to thank the supports by the National Natural Science Foundation of China (No. 22175049) and the Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University (No. JDGD-202213). Y. Zhang thanks the support of the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109118.