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• High-performance organic fluorescent materials are of great interest to chemical and medical scientists because they can be applied in mechanochromic devices, security systems, fluorescence imaging, organic light-emitting diodes (OLEDs), etc. [1–24]. At present, the reported excited state intramolecular proton transfer (ESIPT) fluorescent materials based on 2-(2′-hydroxyphenyl)benzoxazoles feature planarity due to the intramolecular hydrogen-bonding fixation on the dihedral angle of the diaryl skeleton and, therefore, are apt to aggregate in the solid state, resulting in a quenching enol-form or weak keto-form emission, which is unfavorable for mechanochromic luminescence applications, imaging, and high-performance OLEDs fabrication [4–7,25–28]. Therefore, developing high-performance ESIPT fluorescent materials based on 2-(2′-hydroxyphenyl)benzoxazoles is still highly desirable.

  Herein, we constructed high-performance fluorescent materials based on triphenylamine (TPA)-containing 2-(2′-hydroxyphenyl)benzoxazoles (2a-2c) using the efficient Rh(III)-catalyzed oxidative C–H/C–H cross-coupling reaction (Scheme 1). The phenol-containing natural product estrone underwent the coupling with S1 to afford product 2b with a 67% yield. Furthermore, 2-(benzo[d]oxazol-2-yl)phenol (2aa) has also been synthesized as a reference molecule (Fig. 1, Section III in Supporting information). The emission spectra of compounds 2a-2c and 2aa were measured in toluene and shown in Fig. 1. Compound 2aa without a strongly electron-donating TPA group only displays the excited-state intramolecular proton transfer (ESIPT) keto-form emission at 487 nm with a low fluorescence quantum yield (2.0%) in toluene. TPA-containing 2a exhibits sole the enol-form emission at 417 nm with a higher fluorescence quantum yield (25.6%) than 2aa in toluene.

  Scheme 1

  

  Scheme 1.  Synthesis of TPA-containing 2-(2′-hydroxyphenyl)benzoxazoles (2a-2c) and their photophysical properties in toluene solution (5 × 10⁻⁵ mol/L). Absolute quantum yield was determined with a calibrated integrating sphere system.

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  Figure 1

  

  Figure 1.  (a) ESIPT mechanism of 2aa; photophysical properties in toluene solution (5 × 10⁻⁵ mol/L). (b) ESIPT mechanism of 2a. (c) ESIPT mechanism of 2c. (d) Emission spectra of 2a-2c in toluene solution (5 × 10⁻⁵ mol/L).

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  Compound 2aa is a classical ESIPT molecule, and its mechanism has been confirmed (Fig. 1 and Fig. S1a in Supporting information) [3–7]. For TPA-containing 2-(2′-hydroxyphenyl)benzoxazoles (2a), the energy level of the keto-form is higher than that of the enol form in the lowest excited (Fig. 1 and Fig. S1b in Supporting information). As a result, 2a only exhibits sole the enol-form emissions and lack of keto-form emission (Fig. 1d). Moreover, 2a also displays sole the enol-form emission in other solutions (Fig. S3 in Supporting information). Bis(TPA)-containing 2c shows the weak enol-form emission and the strong keto-form emission in toluene (Fig. 1 and Fig. S1c in Supporting information). The excited-state enol form of the 2c has a close energy level with its keto form. In addition, the fluorescence quantum yields of 2b-2c were measured to be 20% and 23% respectively. These results demonstrate that TPA-containing 2-(2′-hydroxyphenyl)benzoxazoles are a novel fluorescent material.

  Subsequently, the mechanochromic luminescence properties of 2a-2c were investigated. Grinding of pristine powder 2a displays a blue shift with an emission color change from yellow-green (λ_em = 512 nm) to blue (λ_em = 437 nm), approximately 75 nm (Fig. 2). The reference compound 2aa does not exhibit piezochromic behavior. For 2b, a significant enhancement of the enol-form emission at 433 nm is observed after grinding, and its keto-form emission is blue-shifted from 535 nm to 500 nm (Fig. 2). The ratio of the two emission peaks remarkably changes before and after grinding. After grinding, the intensity of the enol-form emission of 2b exceeds its keto-form emission. The weak enol-form emission of bis(TPA)-containing 2c almost disappears and the strong keto-form emission induces a slight red-shift after grinding treatment (Fig. 2).

  Figure 2

  

  Figure 2.  The fluorescent images (under UV light) and emission spectra of 2a-2c before and after grinding (λ_ex = 370 nm).

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  To gain insight into the piezochromic mechanism, the phase characteristics of 2a were studied by differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD) analysis. A transition between the metastable state and the stable state of 2a was found because the ground of 2a presented a slight exothermal peak in the DSC experiment (Fig. S6 in Supporting information). In addition, the crystalline morphological transition to amorphous phases was demonstrated due to the pristine 2a showing sharp and intense reflection peaks and corresponding sharp peaks disappearing at the ground 2a state (Fig. S7 in Supporting information). We further obtained two single crystals of yellowish-green-emitting 2bs₁ (CCDC: 2261804) and green-emitting 2bs₂ (CCDC: 2261803) by slow diffusion of EtOH or n-hexane vapor into a solution of 2b in CHCl₃ at room temperature (Fig. 3 and Fig. S8 in Supporting information). The yellowish-green-emitting single crystal 2bs₁ shows emission peaks at 433 nm (minor) and 518 nm, which is close to those of pristine 2b. Furthermore, by grinding the crystal, the enol-form emission is enhanced, and keto-form emission is slightly blue-shifted, which shows the apparent dual-emission of the enol-form (λ_em = 433 nm) and keto-form (λ_em = 503 nm) (Fig. S9 in Supporting information). The single crystal 2bs₁ exhibits a twisted conformation and a strong π–π stacking mode. Although the benzoxazole group and phenol are close to coplanar and have a dihedral angle of 4.08°, the dihedral angle between the benzoxazole ring and the phenyl plane in the TPA is 40.01°.

  Figure 3

  

  Figure 3.  (a) Photograph of single crystal of 2bs₁ under UV light (365 nm). (b) The yellowish-green-emitting single crystal 2bs₁ (CCDC: 2261804) and (c) its packing motif. (d) Photograph of single crystal of 2bs₂ under UV light (365 nm). (e) The green-emitting single crystal 2bs₂ (CCDC: 2261803) and (f) its packing motif.

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  Moreover, the dihedral angles between the phenyl and the other two phenyls in the TPA reveal 62.99° and 68.66° (Fig. 3). The green-emitting single crystal 2bs₂ exhibits a weak enol-form emission (λ_em = 433 nm) and a strong keto-form emission (λ_em = 503 nm) (Fig. S9). By grinding the crystal 2bs₂, the two emission peak positions almost keep unchanged. However, the enol-form emission significantly enhanced, which roughly coincides with those of ground 2b (Fig. S9). The single crystal 2bs₂ reveals 70.10°–73.67° dihedral angles between the phenyl and the other two phenyls in the TPA, and a 42.53° dihedral angle between the benzoxazole ring and the phenyl plane in the TPA. Two molecules are stacked in reverse parallel with distances of 3.567–3.697 Å, and four molecules form a V-shaped structure in the single crystal 2bs₁ (Fig. 3). The shorter distance and tight molecular packing indicate potential π–π stacking among neighboring molecules. The single crystal of 2bs₂ shows the reverse parallelism of two molecules with distances of 5.521–6.357 Å. These results demonstrate that the green-emitting single crystal 2bs₂ displays a more distorted conformation and loose accumulation than the yellowish-green-emitting single crystal of 2bs₁.

  Thus, the emission of 2a and keto-form emission of 2b are blue-shifted after grinding their powders can be ascribed to lessened coplanarity of molecules and weakened intermolecular π–π interactions. The enhanced enol-form emission of 2b, and the disappeared enol-form emission of 2c after grinding may be attributed to a new ESIPT equilibrium accompanied by changes in the molecular packing modes, molecular planarity, and intramolecular hydrogen-bond strength [15]. Furthermore, the external force stimuli can strengthen intermolecular π–π stacking interactions, which leads to a slight red-shifted keto-emission of 2c. In addition, a shift of the ESIPT equilibrium towards an enhanced keto-form emission in the solid leads to 2c showing the very weak enol-form emission and the strong keto-form emission. After grinding, the disappearance of very weak enol-form emission, and a slight red-shifted keto-emission result in the 2c displays inconspicuous mechanofluorochromism (Fig. 2).

  Furthermore, using Poloxamer 188 as a matrix, the estrone-containing 2b was fabricated as water-dispersed nanoparticles (NPs) through a thin-film hydration method (Fig. S10 in Supporting information). The 2b NPs exhibit an apparent dual-emission of the enol-form (λ_em = 441 nm) and keto-form (λ_em = 503 nm) (Fig. 4a). Dynamic light scattering (DLS) measurement indicates that 2b NPs have a hydrodynamic diameter of about 76.4 nm (polydispersity index value of 0.136) (Fig. 4b). Scanning electron microscopy (SEM) images reveal that the 2b NPs have a well-defined spherical shape with diameters of approximately about 80 nm (Fig. S13 in Supporting information), which are roughly equal in size to those of DLS measurement. Such appropriate nanoscale sizes have great potential for fluorescence imaging of tumor cells via enhanced permeability and retention effects [20]. Besides, the zeta potential of the as-prepared 2b NPs is approximately 18.38 mV.

  Figure 4

  

  Figure 4.  (a) The fluorescence spectrum of the 2b NPs. (b) Size distribution of the 2b NPs. (c) The single-photon-excited confocal laser scanning microscopy fluorescence imaging 2b nanoparticles channel, (d) bright field. (e) Two-photon-excited confocal laser scanning microscopy fluorescence imaging 2b nanoparticles channel, (f) bright field.

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  To assess the safe usability of 2b NPs for biomedical applications, the cytotoxicity experiments in HeLa cells were conducted using the standard methyl thiazolyl tetrazolium (MTT) assay (Fig. S14 in Supporting information). The cell viability values demonstrated that 2b NPs show almost no toxicity in HeLa cells (Fig. S14). Even at a high concentration, 2b NPs do not exhibit significant cytotoxicity, further confirming its good biocompatibility. To gain study the cellular imaging capability of 2b NPs, staining experiments of 2b NPs in HeLa cells were performed, and then the fluorescent images were captured by confocal laser scanning microscopy (CLSM). The single-photon-excited blue-green fluorescence signal can be observed in HeLa cells after being treated with the 2b NPs for 2 h (Figs. 4c and d, Figs. S15 and S16 in Supporting information). In addition, the cellular two-photon-excited imaging capability of 2b NPs was employed in HeLa cells using CLSM equipped with a femtoseconds laser. The obvious blue-green fluorescence signals were detected in HeLa cells by two-photon irradiation, indicating that 2b NPs have admirable two-photon excitation fluorescence imaging properties (Figs. 4e and f, Figs. S15 and S16). Thus, we conclude that the 2b NPs can be efficiently endocytosed into the HeLa cells and applied for blue-green imaging probes, which is the first reported of dual emission ESIPT systems with promising potential in both single-photon-excited and two-photon-excited fluorescence imaging [3–7].

  Organic fluorescent molecules with the hybridized local charge transfer (HLCT)-state character were considered high-performance fluorescent materials that can favor improving the utilization of OLED excitons [29–34]. Our recent research has shown that the electron-donating TPA group can adjust the energetics of ESIPT and endow molecules to have the hybridized local charge transfer (HLCT)-state feature [35]. In addition, the thermal and electrochemical properties of 2c were studied (Table S1, Figs. S17 and S18 in Supporting information), indicating that it was suitable for vacuum thermal sublimation for OLED fabrication. Hence, the electroluminescence properties of bis(TPA)-containing 2c in OLED devices were further surveyed.

  The devices A-F were fabricated with the structure of ITO/MoO₃ (3 nm)/TAPC (50 nm)/2c (x wt%): CBZ₂-F₁ or TBCPF (20 nm)/Tm₃PyP₂₆PyB (50 nm)/LiF (1 nm)/Al (100 nm) (Part X in Supporting information). The molecular structures and energy levels of the devices were shown in Fig. S19 (Supporting information). Device D based on 2c (6 wt%) in CBZ₂-F₁ shows a low turn-on voltage and good performance with the maximum EQE (EQE_max), brightness (L_max), power efficiency (PE_max), current efficiency (CE_max), and CIE coordinates of 3.4 V, 3.5%, 8338 cd/m², 11.5 cd/A, 10.7 lm/W and (0.30, 0.52), respectively (Table S2 and Fig. 5). The exciton utilization (η_s) of device D was calculated by equation below:

  Figure 5

  

  Figure 5.  (a) Normalized EL spectra of device at 3.4 V. (b) The EL efficiency–current density curve and EQE–current density curve of device. (c) Luminance–current density–voltage (L–J–V) characteristics of the device. (d) Linear fitting of Lippert–Mataga model (enol-form emission of 2c).

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  where η_out, η_PL, γ is the light-out-coupling efficiency, the photoluminescence (PL) efficiency of 2c (6 wt% in CBZ₂-F₁), and the recombination probability of hole–electron in the emission layer, respectively. The exciton utilization of device D breaks through the theoretical limit of 25% in conventional fluorescent OLEDs. The singlet exciton yield of up to 92% is the one of highest exciton utilization value recorded for the ESIPT molecules with a dual emission system [35–39].

  The HLCT state character of 2c was further studied by the solvation effects on absorption and emission in different solvents (Fig. S21 and Section XI in Supporting ingormation). The vibrational fine structure is found from the PL spectrum of 2c in n-hexane, which demonstrates the existence of the LE state [35,37,40]. The enol-form emission of 2c exhibits a gradually red-shifted solvatochromic behavior, whereas its keto-form emissions have only a slight change in the solvent polarity [35,37,40]. Its enol form exhibits only one slope value of 4781 (r = 0.99) in the linear fitting of the Lippert–Mataga model (Fig. 5d and Fig. S21), demonstrating the CT feature of 2c in the excited state. The excited state dipole moment (μ_e) was estimated to be 15.8 D, which is larger than that of usual LE emitters (ca. 8 D) and smaller than that of the typical CT molecule 4-(N, N-dimethylamino)benzonitrile (ca. 23 D) [35,37,40–43]. Furthermore, the HOMO and LUMO of 2c have intercrossed molecular orbitals, leading to an intercrossed transition of CT and π→π* and bringing the intercrossed character of the LE and CT states (Fig. S23 in Supporting information) [35,43]. Moreover, the PL lifetimes of 2c in solvent and film were investigated (Fig. S22 and Table S4 in Supporting information). Only several lifetime components with nanosecond order were detected, and delayed fluorescence components have not found [35,43].

  In summary, the high-performance and multifunctional fluorescent materials based on TPA-containing 2-(2′-hydroxyphenyl)benzoxazoles (2a-2c) were obtained efficiently by rhodium-catalyzed C–H activation reaction. Compound 2a exhibit sole the enol-form emission with a high fluorescence quantum yield and blue-shifted piezochromic luminescence properties. The ESIPT molecule 2b displays significantly enhanced enol-form emission, as well as blue-shifted keto-form emission after grinding. In contrast, bis(TPA)-containing 2c shows the weak enol-form emission disappeared and the strong keto-form emission slightly red-shift after grinding treatments. This discovery represents the first example of opposite mechanochromic trends in ESIPT molecules with a dual emission system. The piezochromic blue shift after grinding can be ascribed to lessened coplanarity of molecules and weakened intermolecular π–π interactions, and the enol-form emission alteration after grinding may be attributed to a new ESIPT equilibrium accompanied by changes in the molecular packing modes, molecular planarity, and intramolecular hydrogen-bond strength. The estrone-containing 2b has been fabricated as water-dispersed NPs and exhibited an apparent dual-emission. The NPs further were used as a blue-green imaging probe in both single-photon-excited and two-photon-excited fluorescence imaging. In addition, bis(TPA)-containing 2c-based devices exhibit dual-emission with good performance with an EQE_max of 3.5% and a maximum brightness of 8338 cd/m². The singlet exciton yield of the device is 92% breaking through the theoretical limit of 25% in conventional fluorescent OLEDs. This is one of the highest exciton utilization value recorded for the ESIPT molecules with a dual emission system. This work unlocks an opportunity to rapidly synthesize the high-performance and multifunctional fluorescent materials, which can be used in mechanochromic luminescence, imaging, and high-performance OLED fabrication.

  Declaration of competing interests

  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.

  Acknowledgments

  We thank Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (No. BM2012110), the fundamental research funds for the central universities (No. 2023CDJYGRH-YB17), the Venture & Innovation Support Program for Chongqing Overseas Returnees (No. cx2022061), the Natural Science Foundation of Chongqing (No. CSTB2022NSCQ-MSX1123), the Chongqing Talents: Exceptional Young Talents Project (No. cstc2021ycjh-bgzxm0067), Changzhou University, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center (No. ACGM2022–10–10), and National Natural Science Foundation of China (Nos. 21702019, 62174160) for financial support. We also thank Prof. Jingsong You for the helpful discussions, and Mrs. Qin Deng (Chongqing University analysis and testing center) for fluorescence imaging.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109554.

  
  
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• 

  Scheme 1  Synthesis of TPA-containing 2-(2′-hydroxyphenyl)benzoxazoles (2a-2c) and their photophysical properties in toluene solution (5 × 10⁻⁵ mol/L). Absolute quantum yield was determined with a calibrated integrating sphere system.

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  Figure 1  (a) ESIPT mechanism of 2aa; photophysical properties in toluene solution (5 × 10⁻⁵ mol/L). (b) ESIPT mechanism of 2a. (c) ESIPT mechanism of 2c. (d) Emission spectra of 2a-2c in toluene solution (5 × 10⁻⁵ mol/L).

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  Figure 2  The fluorescent images (under UV light) and emission spectra of 2a-2c before and after grinding (λ_ex = 370 nm).

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  Figure 3  (a) Photograph of single crystal of 2bs₁ under UV light (365 nm). (b) The yellowish-green-emitting single crystal 2bs₁ (CCDC: 2261804) and (c) its packing motif. (d) Photograph of single crystal of 2bs₂ under UV light (365 nm). (e) The green-emitting single crystal 2bs₂ (CCDC: 2261803) and (f) its packing motif.

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  Figure 4  (a) The fluorescence spectrum of the 2b NPs. (b) Size distribution of the 2b NPs. (c) The single-photon-excited confocal laser scanning microscopy fluorescence imaging 2b nanoparticles channel, (d) bright field. (e) Two-photon-excited confocal laser scanning microscopy fluorescence imaging 2b nanoparticles channel, (f) bright field.

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  Figure 5  (a) Normalized EL spectra of device at 3.4 V. (b) The EL efficiency–current density curve and EQE–current density curve of device. (c) Luminance–current density–voltage (L–J–V) characteristics of the device. (d) Linear fitting of Lippert–Mataga model (enol-form emission of 2c).

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•