AIEgen configuration transition and aggregation enable dual prompt emission for single‐component nondoped white OLEDs

The dual emission (DE) feature in materials holds great potential to revolutionize the development of one‐component system white organic light‐emitting diodes (WOLEDs). However, the reported DE materials remain scarce owing to the formidable challenge of breaking Kasha's rule and managing the intricate energy/charge transfer processes. Herein, we have introduced a groundbreaking DE AIEgen, 2CzAn‐TPE, which possesses a simple structure and undergoes Z‐to‐E isomerization and exhibits yellow and red fluorescence powders for pre‐ and post‐sublimation, respectively. With relatively lower potential energy, Z‐conformation ((Z)‐1,2‐diphenyl‐1,2‐bis(4‐(10‐(9‐phenyl‐9H‐carbazol‐3‐yl)anthracen‐9‐yl)phenyl)ethene) of 2CzAn‐TPE can be readily transformed into E‐conformation ((E)‐1,2‐diphenyl‐1,2‐bis(4‐(10‐(9‐phenyl‐9H‐carbazol‐3‐yl)anthracen‐9‐yl)phenyl)ethene) via vacuum sublimation. The utilization of X‐ray diffraction and grazing‐incidence‐wide‐angle X‐ray scattering techniques confirms the structural transformation, while the crystallographic analysis reveals the establishment of numerous intermolecular CH···π interactions between the tetraphenylethene (TPE) moiety and both the anthracene and carbazole units. This allows a densely packed molecular arrangement, thereby offering propitious conditions for excimer generation in the E‐conformation aggregated state. By utilizing the sublimated 2CzAn‐TPE as an emitter, a nondoped one‐component WOLED was prepared, exhibiting an exceptionally high external quantum efficiency (EQE) of 5.0%, which represents one of the highest performances among all one‐component WOLEDs. This research introduces a novel, simple, and efficient approach to realize highly efficient one‐molecule WOLEDs.


INTRODUCTION
[3][4][5] Until now, WOLEDs have primarily been achieved by blending two (blue and yellow) or 11] To realize dual emission (DE), Kasha's rule that photon emission generated from the lowest excited state of a singlet, or a triplet should be broken, therefore DE should be generated from emitters with two emitting states existing in two independent or correlated emitters.However, in general, despite considerable efforts, the development of DE materials remains a challenge, with DE emitters being rare and even infrequent for single-component DE applied in WOLEDs. [12]Furthermore, most DE emitters exhibit delayed and prompt emission, resulting in evident roll-off, and some of them are not suitable for electroluminescent devices. [13,14]In contrast, certain single molecules of polycyclic aromatic hydrocarbons (PAHs) can emit from both the monomer and red-shifted structureless excimer states, utilizing singlet states in both photoluminescent (PL) and electroluminescent (EL) processes. [15]These molecules are highly desirable as they can produce stable white emission under both PL and EL conditions with minimal roll-off. [12]herefore, the investigation of new DE materials with both simple structures and fluorescence holds significant potential to revolutionize the fields of DE and WOLED. [16]he anthracene unit, a member of the PAH family, is a widely studied component in organic light-emitting devices (OLEDs) due to its numerous attractive features and deep blue emission capabilities. [17,18]However, its fluorescence is susceptible to quenching upon aggregation, primarily due to its planar structure, resulting in aggregation-caused quenching (ACQ).This issue reduces its efficiency and limits further application.In contrast to ACQ, [19] aggregation-induced emission (AIE) can enhance emission efficiency in the solid state, which is observed in some twisted molecules like tetraphenylethene (TPE), siloles, cyanostilbenes, etc. [20][21][22][23][24][25] The introduction of AIEgen can endow ACQ molecules with AIE characteristics. [26]Besides, carbazole is one of the most employed building blocks for organic emitters due to its excellent electron-donating property, ease of modification, and good hole-transporting character.29][30] In this work, a symmetric AIE-active emitter, 2CzAn-TPE was designed and synthesized.The carbazole (Cz) endgroup, anthracene (An) spacer, and tetraphenyelene (TPE) core were linked together, creating twisted angles between each group that effectively interrupted conjugation, and resulted in the pristine synthesized 2CzAn-TPE exhibiting blue emission (Figure 1A).Interestingly, upon sublimation, the emitter exhibited distinct dual emissions of blue and red.Further experimental investigations revealed that this DE feature was due to Z-/E-isomerization and excimer formation.Moreover, key factors for excimer formation were proposed by comparing the two Z-/E-isomers with the reference molecule Cz-An-TPE-Cz.Finally, a non-doped one-component cool-WOLED with an exceptionally high external quantum efficiency (EQE) of 5.0% were achieved, representing one of the highest performances among all one-component WOLEDs.This study provides a comprehensive investigation into the inherent characteristics of this compound, encompassing its behavior in both solution and condensed solid states, as well as its potential utilization in WOLEDs.

Synthesis, thermal stability, electrochemical, and molecular basic information
The molecular architecture of AIEgen 2CzAn-TPE is illustrated in Figure 1A and was achieved with high yield through Suzuki coupling between the TPE precursor TPE-2BE and the Br-containing building block Cz-An-Br, as demonstrated in Scheme S1-S3 for a fully synthetic pathway.The preceding AIE precursor TPE-2BE, was crafted by adhering to the previously reported McMurry approach followed by nucleophilic substitution.However, the McMurry coupling often results in the production of mixtures of Z-and E-isomers that prove difficult to distinguish due to their corresponding molecular compositions and polarities. [31]Despite potential differences in properties resulting from distinct conformations, these isomers are typically not separated for subsequent reactions and further applications in optoelectronic and biological fields. [31]ollowing careful purification and characterization, the AIEgen 2CzAn-TPE (the pristine) was confirmed by nuclear magnetic resonance (NMR) and matrix-assisted laser desorption Electrospray Ionization Time-of-Flight Mass Spectrometer (ESI-TOF-MS) analysis.A portion of this pristine AIEgen was further subjected to sublimation to prepare another batch of AIEgen (2CzAn-TPE after sublimation).The molar mass of both batches was nearly identical (approximately 1166.4600g mol −1 in Figure S1-2), and the NMR analysis also proved that these two batches had nearly identical chemical spectra, only with minor differences in distribution (Figure 1B).Surprisingly, the material's characteristics had changed-the pristine 2CzAn-TPE was a greenish amorphous powder, exhibiting blue emission under UV light (Figure 1C-D).However, the powder after sublimation transformed into a red glassy solid, displaying clearly separated dual emissions of blue and orange under UV light (Figure 1E-F).This DE phenomenon is relatively rare in organic compounds, particularly for the two emissions with large energy gaps (blue and red), as it should overcome energy/charge transfer and electron delocalization, which usually occur to organic π-conjugated molecules, as well as Kasha's rule that photon emission generated from the lowest excited state of a singlet or a triplet should be broken in this process. [12]It is speculated that the red emission was derived from the excimer, like most anthracene derivatives. [13,14]However, considering that the pristine powder exclusively manifested blue emission, it is plausible to surmise that the molecular conformation underwent a transformation after sublimation, concomitant with a modification in the molecular stacking pattern.
Despite changes in emissive behaviors after sublimation, this AIEgen still exhibited excellent thermal and electrochemical stability.Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out on both batches of AIEgen.As illustrated in Figure S3, the thermal properties of the isomers were compa-

TA B L E 1
The physic properties of AIEgen 2CzAn-TPE.a) The HOMO of Z-/E-isomer was obtained from DFT, while the pristine batch and the batch after sublimation were calculated from cyclic voltammetry.
b) The LUMO of Z-/E-isomer was obtained from DFT, while the pristine batch and the batch after sublimation were calculated from the equation of LUMO = (HOMO + E g ). c ) The E g of Z-/E-isomer was obtained from DFT, while the pristine batch and the batch after sublimation were calculated from on-tail absorption;.d) The energy of S 1 state of Z-/E-isomers was obtained from DFT, while the pristine batch and the batch after sublimation were calculated from emission.
rable before and after sublimation.The temperatures of 5% weight loss were 504 and 528 • C. The glass transitional temperatures were measured to be 236 and 237 • C for the AIEgen before and after sublimation.In general, compared to the pristine AIEgen, the batch after sublimation exhibited a slightly increased thermal stability, which could be attributed to the higher purity and a more compact packing arrangement after sublimation.Cyclic voltammetry (CV) was utilized to evaluate the electrochemical stability and explore the practical HOMO and LUMO levels of both batches of AIEgen.As illustrated in Figure S4 and Table 1, the presublimation and postsublimation profiles exhibited an extraordinary degree of congruity, primarily due to the closely aligned frontier molecular orbitals and energy gaps between the Z-and Eisomers.According to the onset oxidation potentials (E OX ) and the equation of where E (Fc/Fc+) was the oxidation potential of ferrocene, both batches of AIEgen shared the same HOMO energy level of −4.83 eV, while the LUMO energy levels were slightly different, calculated to be −1.91 eV and −1.99 eV, based on the equation of LUMO = (HOMO + E g ) eV, where the value of E g was determined by their on-tail absorption wavelength.
To gain insight into the relationships between the molecular structure and property, spatial configuration, frontier molecular orbitals (FMO) and energy levels, density functional theory (DFT) calculations were conducted on both E-and Z-isomers based on B3LYP/6-31G (d,p) level.The results of these calculations were illustrated in Figure S5 and Table 1.The anthracene moieties exhibited nearly vertical orientation to adjacent carbazole and TPE groups, with the dihedral angle of 80.32 and 84.38 • for Cz-An and An-TPE in E-isomer and 101.36 and 97.67 • in Z-isomer.Due to the nearly perpendicular configuration between adjacent groups, the conjugation in both isomers was effectively interrupted, resulting in blue emission.Furthermore, compared to its Ecounterpart, the Z-configuration exhibited relatively larger twisted angles due to increased steric hindrance caused by two bulky adjacent anthracene rings on the same side.Consequently, the Z-isomer exhibited a higher potential energy than its E-configuration counterpart (Table S1).That's why the Z-isomer could transform to E-isomer during the sublimation process, as molecules always choose to exist at the lowest potential energy state.Regarding the FMOs, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were primarily concentrated on the anthracene ring, indicating that anthracene served as the primary electronically active cores.The energy levels of both E-and Z-isomers are also summarized in Table 1, with LUMO energy levels at −1.54 and −1.55 eV for Z-and E-isomers, and HOMO at −4.99 and −4.96 eV, respectively.Thus, the E-isomer has a slightly smaller energy gap compared to Z-isomer (3.41 vs. 3.45 eV).

Photophysical property
The ultraviolet (UV) absorption spectra of both batches of AIEgen in THF solution and neat films were depicted in Figure 2A.The AIEgen exhibited distinct absorption peaks at 360, 376, and 396 nm in solution, attributed to typical ππ* transitions of anthracene unit.The onset absorption was observed at 425 and 438 nm in the film for the pristine and after sublimated AIEgen, respectively, corresponding to energy gaps of 2.80 and 2.76 eV.The sublimated AIEgen showed a slight red-shifted absorption compared to the pristine one, possible due to increased E-isomer after sublimation, known as E/Z isomerization (EZI), [3,32] which was corroborated by the DFT results that indicated the E-isomer had a smaller energy gap.The emission peaks for the pristine and after sublimated AIEgen in solution were located at 444 and 451 nm, respectively.The red-shift of the main peak can also be attributed to the E-isomer, whose S 1 energy was slightly lower than its counterpart.These minor differences were indicative of a transition in molecular conformation.Additionally, it was noteworthy that compared to film emission of the pristine batch, an additional faint emission band with a peak at 538 nm appeared in the AIEgen after sublimation, originating from the excimer emission.Based on the divergences arising from experimental and computational analyses, it can be inferred that the molecule undergoes a conformational change after sublimation.This transition in molecular conformation does exert some degree of influence on material properties.However, it is not substantial enough to bring about significant changes in the emissive properties.Therefore, we conducted an investigation into the photoluminescent (PL) properties of their aggregated states in order to gain a deeper understanding of the phenomenon.By utilizing THF/water mixed solvent, the AIE properties of both batches were investigated (Figure S6a and 6b).Upon the addition of water to the THF solution, the molecules began to aggregate due to the poor solubility of water for the organic emitters.In the case of the pristine batch, the shape of the emissive spectrum remained unchanged, with only a slight redshift observed.However, there was a significant enhancement in the photoluminescent (PL) intensity, verifying the AIE property of the pristine batch.In contrast, the sublimated batch exhibited complex emissive behaviors in the mixed solvents.Specifically, it maintained the same emissive spectra as the pristine counterpart until the water fraction (f w ) in the mixed solvents reached 40 vol%.At this point, it exhibited only one peak at c.a. 463 nm in the THF/water mixed solvent.However, an additional emission of broad and longer-wavelength profile, accompanied by a long tail region, emerged at higher water fractions.The PL intensity of the shorter-wavelength peak exhibited a gradual increase as the f w increased from 0% to 80% but experienced a slight decrease at 90%.Meanwhile, the intensity of longerwave emission was gradually intensified as the water fraction increased.Moreover, for in-depth investigations, both batches were immersed in dichloromethane solution with diverse concentrations, encompassing a spectrum from unsaturated to saturated (Figure 2B-C).In the case of the pristine compound, all solutions emitted blue fluorescence under UV excitation and demonstrated nearly identical emission characteristics.The individual blue emissions were slightly redshifted from 445, 450 to 458 nm in solutions with concentrations ranging from 0.1 to 10 g mL −1 , respectively.In contrast, the sublimation batch exhibited distinct emission behavior at different concentrations.At lower concentrations, the solution emitted blue light, whereas at higher concentrations, an additional emission band at 550 nm wavelength emerged, resulting in white light emission.Moreover, the luminescent properties of the deposited film were also investigated (Figure S7a), and the phenomenon observed was aligned with that appeared in highly concentrated solutions.Subsequently, time-resolved photoluminescence (TRPL) measurements were performed at 460 nm and 540 nm to analyze the two exciton sources (Figure S7b).Their respective exciton lifetimes for both sources were found to be within 10 ns, indicating that the DE follows a dual prompt fluorescent model.Furthermore, a schematic diagram illustrating the photodynamic process was presented in Figure S7c.

Crystallographic and stacking properties
As previously mentioned, molecular configuration exerts a profound influence on the packing arrangement.Therefore, to corroborate this supposition, single crystals of both the pristine and sublimated 2CzAn-TPE were grown via a dichloromethane and ethanol mixture.Surprisingly, both specimens exhibited identical single crystal types, with only the E-type isomer being cultured (Figure S8a).This finding indicates that the E-isomer gathered to form a stable crystal from the mixed solution of Z/E-isomers.However, conformation transition of 2CzAn-TPE, shifting from cistype to trans-type, during the thermal sublimation process (high temperature), resulting in discernible differences in the aggregated state of the pristine and after sublimated forms.Subsequently, to furnish further affirmation regarding the formation of the E-type configuration of 2CzAn-TPE after sublimation, the powder X-ray diffraction measurement was conducted, with the results being depicted in Figure S8b.It was observed that the sublimated 2CzAn-TPE offered a more discernible and well-structured diffraction peak compared to the pristine powder, with the profile aligning impeccably with the calculated pattern derived from the single crystal analysis, within the acceptable tolerance range.This provided a further and clearer evidence for our hypothesis regarding the structural transformation.
Subsequent investigation was carried out on the single crystal of 2CzAn-TPE to elucidate its properties regarding the aggregation state (Figure 3A).The findings revealed a face-to-edge stacking pattern between two monomers, with particular emphasis on the emergence of numerous intermolecular CH⋅⋅⋅π interactions between the TPE component and both the anthracene and carbazole units.This discovery underscores the presence of a robust binding force that promotes the formation of excimers.It is noteworthy that the crystal exhibited no obvious π-π stacking, thereby effectively avoiding the ACQ effect.Therefore, 2CzAn-TPE emerges as a superior candidate for nondoped OLED applications.Subsequently, the 2D grazing-incidence-wide-angle X-ray scattering (GIWAXS) technique was employed to support the description of molecular packing patterns.As shown in Figure 3B and S9, the pristine film exhibited no diffraction halo, indicating that the intermolecular interactions were too weak to be detected.However, a broad diffraction ring was observed for the after sublimation film at the out-of-plane (OOP) direction with the q = 1.25 Å −1 .The d-spacing was calculated to be 5.0 Å according to the equation of d = 2π/q, indicating that the molecular stacking pattern was face-toedge, consistent with the analytical results for single crystals.Furthermore, the stronger signal at the OOP axis also evidenced a face-on stacking direction, which could enhance the light outcoupling efficiency.In short brief, it was indeed the different molecular configurations that led to the varied properties of aggregation state and consequently, the alteration of luminous phenomenon.

Theoretical calculation and analysis
To deepen our comprehension of the packing model of 2CzAn-TPE in both E-and Z-isomers, the calculation of the molecular electrostatic potential (ESP) was performed. [33,34]s depicted in Figure 4A, the TPE and An fragments displayed negative potentials, whereas the peripheral carbazole units exhibited more concentrated positive potentials for both the E-and Z-isomers.Consequently, it is highly probable that intermolecular interactions would occur on these two specific parts, aligning with the analysis conducted on the sin-gle crystals.Considering the unavailability of a single crystal structure for the Z-isomer, molecular dynamics simulations were performed using the GROMACS software package to simulate the packing mode of Z-isomer.The details of these simulations are outlined in Figure S10.The configuration at the end of 50 ns simulation is visually depicted in Figure 4B.The dimer of the E-isomer, extracted from the single crystal, and a representative frame selected from the molecular dynamics trajectory were chosen for optimization using the WB97XD/6-31G (d,p) set. [35]Notably, the intermolecular C-H⋅⋅⋅π exhibited significantly greater strength and abundance in the E-isomer system compared to the Zisomer.The binding energies were calculated to be 6.63 for the Z-isomer and 9.43 kcal/mol for the E-isomer, indicating that the E-isomer formed a more tightly packed molecular structure, whereas Z-isomers exhibited much looser packing units.Furthermore, compounds Cz-An-TPE-Cz with a single anthracene and 2Cz-TPE (previous work), [36] which lacks the anthracene unit (Scheme S4), were synthesized for comparison.Notably, neither of these materials exhibited the DE phenomenon (Figure S11a).The complexation energy of dimer-like Cz-An-TPE-Cz was calculated to be 7.44 kcal/mol (Figure S11b), providing further evidence that the formation of excimers may require a certain level of binding force.

CONCLUSION
This research involved the synthesis of a symmetrical AIEgen with a TPE core, anthracene spacer, and carbazole end-group,

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data supporting the findings of this study are available from the corresponding authors upon request.

F I G U R E 1
The chemical structure of AIEgen 2CzAn-TPE and its emission before and after sublimation.(A) The chemical structure of 2CzAn-TPE; (B) The nuclear magnetic resonance (NMR) spectra of the AIEgen before sublimation (pristine) and after sublimation (after sbl.); (C and D) the photos of the pristine AIEgen under sunlight and UV light; (E and F) the photos of the AIEgen after sublimation under sunlight and UV light.

F I G U R E 2
The photophysical behaviors of AIEgen 2CzAn-TPE before and after sublimation.(A) The absorption and emission spectra of AIE properties of 2CzAn-TPE before and after sublimation in solution and as film; (B and C) emissive behaviors of 2CzAn-TPE before and after sublimation in dichlorom (DCM) at different concentrations.

F
I G U R E 4 (A) The electrostatic potential (ESP) distribution of the Z-/E-isomers and (B) dimer-like structures of the Z-/E-isomers, respectively.F I G U R E 5 The 2CzAn-TPE-based non-doped OLED architecture.(A) The configuration of the OLED structure; (B) the Commission Internationale de L'Eclairage (CIE) coordinate of the OLED; (C) the photoluminescence of deposited film and spin-coated film and the electroluminescence; (D) the current density and the luminance of the OLED.
which exhibited impressive AIE characteristics.The pristine compound emitted blue light under UV illumination, while after sublimation, showed dual emission of blue and red simultaneously.Aggregation experiments in mixed solutions and saturated solutions indicate that the heating process can prompt a transformation of the molecular configuration from Z-to E-isomer, as further confirmed by X-ray diffraction and GIWAXS techniques.Thanks to the tighter intermolecular arrangement and the potent intermolecular CH⋅⋅⋅π interactions in the E-isomer, the excimer can be formed in the aggregated state.Notably, the absence of π-π stacking in the 2CzAn-TPE single-crystal effectively avoided the ACQ effect, making it a candidate for non-doped OLED applications.By utilizing the sublimated 2CzAn-TPE as an emitter, nondoped one-component OLEDs emitting cool-white light were successfully prepared.These devices displayed an impressive maximum brightness of 25000 cd m −2 and stable EQE values of 2.4% within the range of 1 cd m −2 to 1000 cd m −2 .By optimizing the device structure, the maximum EQE value was enhanced to 5.0%, which is one of the highest performances among all one-component WOLEDs.This study presents a pioneering, uncomplicated, and efficacious methodology for attaining highly efficient single-molecule WOLEDs.A C K N O W L E D G M E N T SThis work is financially supported by the National Science Fund for Distinguished Young Scholars (grant number: 21925506), National Natural Science Foundation of China (grant numbers: U21A20331, 51773212, 81903743, and 52003088), Ningbo Key Scientific and Technological Project (grant numbers: 2022Z124 and 2022Z119).