C=C bond enables difluorideboron β-diketonate derivatives with high contrast mechanoresponsive luminescence for reversible writing and information encryption

Abstract

Promoting the contrast of mechanoresponsive luminescent materials is of great significance in the application of information storage and encryption. In this work, two D–π–A difluorideboron β-diketonate derivatives were developed by varying the structure of the π-bridge. By incorporating a C=C bond into the phenyl π-bridge of BF₂-PhCz, BF₂-PVCz with a phenyl vinyl π-unit achieved bathochromic fluorescence with a higher quantum yield and a higher contrast of mechanoresponsive luminescence (MRL). Theoretical calculations and experimental characterization revealed the C=C bond contributed to the enhanced oscillator strength of the molecule and head-to-head molecular packing in the crystal. The applications of reversible writing and information deep-encryption were then realized based on the high-contrast and reversible MRL behavior of BF₂-PVCz.

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1. Introduction

Organic mechanoresponsive luminescent (MRL) materials that alter their luminescence in response to mechanical stimuli are highly desirable in the fields of information storage and encryption because they can produce contrasting luminescent signals under mechanical stimuli conditions.^1–4 Among them, red luminescent materials with high solid-state emission efficiency are particularly preferred due to their strong penetrating ability, low excitation energy, and minimal background interference.^5–7 Although the red MRL molecules with delocalized π-conjugation demonstrate high efficiency in solvents, they usually suffer from aggregation-caused quenching (ACQ) due to the π–π interaction in the solid state which limits their practical application.^6,8,9 Therefore, the development of red MRL materials with efficient solid-state emission is still challenging.¹⁰ Electron donor–acceptor (D–A) molecules can exhibit aggregation-induced emission (AIE) properties by restricting intramolecular rotation, but they have to encounter low photoluminescence quantum yields (PLQYs) due to the spatial separation of frontier molecular orbitals.¹⁰

On the other hand, enhancing the reversibility and contrast of MRL materials has been a key prerequisite for applications in information storage and encryption.^11–18 The strategies for developing high-contrast MRL materials typically involve reducing the types of noncovalent interactions through heterocyclic effects,¹⁹ utilizing solvents to adjust the packing arrangement,²⁰ increasing steric hindrance to produce loose arrangement, etc.^21–24 Among the different types of molecules identified for their bright fluorescence properties, the difluoroboron β-diketonate derivatives are well-known for their mechanofluorochromic behavior.^25–28 For example, tetraphenylene based difluorideboron β-diketonate derivatives showed MRL properties from yellow to orange emission.²⁹ Acylamino substituted difluorideboron β-diketone derivatives exhibited green to yellow emission through crystalline-to-amorphous transformation.²⁵ Azepane-substituted β-diketone difluorideboron complexes realized multi-stimuli responsive fluorescence switching.^28,30 Nevertheless, some of them still suffer from low contrast, poor reversibility or low fluorescence quantum yields. Moreover, red emitting candidates with high solid-emission efficiency and contrast remained challenging due to the balance between the rigid packing structure (for high quantum yields) and adjustable intermolecular interactions (for high-contrast MRL).

To achieve highly emissive red mechanoresponsive luminescent (MRL) materials with satisfactory contrast, in this work, two D–π–A molecules, BF₂-PhCz and BF₂-PVCz, were developed with carbazole (Cz) and difluoroboron β-diketonate (BF₂bdk) as the electron donor and acceptor, respectively. Phenyl (Ph) and phenyl vinyl (PV) groups were chosen as the π-bridges, respectively, to regulate the band gap, emission efficiency and solid-state luminescence behaviors. In comparison with the phenyl π-bridge in BF₂-PhCz, the phenyl vinyl group with more extended π-conjugation in BF₂-PVCz significantly reduced the band gap and enhanced the luminescence efficiency by increasing the oscillator strength. Moreover, in the crystals of BF₂-PVCz, the molecules formed “head-to-head” arrangement, which inhibited dipole–dipole interactions and resulted in crystallization-induced blue-shift in emission. Upon grinding, BF₂-PVCz crystals achieved significantly improved MRL contrast (Δλ = 54 nm) by crystalline-to-amorphous transition accompanied by trans-to-cis isomerization of the C=C bond. Moreover, the reversibility of the MRL process enabled successful applications in reversible writing and information deep-encryption.

2. Results and discussion

2.1. Theoretical calculation

Target compounds BF₂-PhCz and BF₂-PVCz were obtained with carbazole, 4-fluoroacetophenone and 4-fluorophenone as the starting materials in three and two steps, respectively (Scheme S1, ESI†).

Both compounds possessed moderate distorted conformation, and the dihedral angles of carbazole and phenyl groups were 48.9° and 50.0° for BF₂-PhCz (Fig. 1a) and BF₂-PVCz (Fig. 1b), respectively. Comparing with organic luminophores with planar structures, these molecules with moderately twisted geometries may be better candidates to form various packing patterns in the aggregation state, which may promote MRL with phase transition.³¹ The molecules showed negative and relatively positive electrostatic potentials distributed on the difluoroboron groups and the carbazolyl unit, respectively, implying the dipole–dipole interactions would contribute to the intermolecular interactions. The twisted conformation, combined with the separate electrostatic potentials of the molecules, suggested that the π–π intermolecular interactions would be inhibited and efficient emission in the solid state could be realized. Then Multiwfn 3.8 software package and VMD 1.9.3 were used to analyze the charge distribution.^32–35 In the excited state (S₁), the holes of the molecules were distributed on the carbazole group, while the electrons were mostly located on the BF₂bdk group and the π bridge, indicating the charge transfer (CT) characteristic of the excited state. In comparison with BF₂-PhCz, BF₂-PVCz with a more extended π-bridge showed a narrower band gap in both ground and excited states (Fig. 1 and Fig. S1, ESI†). The singlet excited state energies of BF₂-PhCz and BF₂-PVCz were 3.07 and 2.94 eV with oscillator strengths (f) of 0.0309 and 0.0539 calculated from the excited state transition dipole moment (Fig. S2, ESI†), respectively, indicating redder emission with a higher PLQY of BF₂-PVCz.

Fig. 1 Molecular structures, electrostatic potentials and singlet excited state properties of molecules (a) BF₂-PhCz and (b) BF₂-PVCz.

2.2. Photophysical properties

To further investigate the photophysical properties of the compounds, the UV-vis absorption and PL spectra in toluene solution were characterized (Fig. 2a and b). Both compounds showed two broad absorption bands around 350–390 and 430–460 nm, which were attributed to the π–π* transition of the skeleton and the intramolecular charge transfer (ICT) from the carbazole to boron difluoride β-diketone segment, respectively. In comparison with BF₂-PhCz (2.39 eV), BF₂-PVCz exhibited a lower band gap (E_g) of 2.32 eV. BF₂-PhCz and BF₂-PVCz exhibited structureless PL emission peaking at 526 (Φ_F = 25.08%) and 566 nm (Φ_F= 31.83%), respectively. Compound BF2-PVCz (Δλ = 102 nm) has a larger Stokes shift than BF₂-PhCz (Δλ = 93 nm), indicating that the extended π-conjugation between the donor and acceptor units could help to inhibit self-absorption effectively. As illustrated in Fig. S3 (ESI†), the emission bands were bathochromically shifted from 490 nm to 601 nm and from 500 nm to 616 nm upon increasing the solvent polarity from nonpolar n-hexane to polar THF, respectively, which was a typical intermolecular charge transfer (ICT) effect.

Fig. 2 UV-vis absorption and PL spectra of (a) BF₂-PhCz and (b) BF₂-PVCz in toluene solution, and PL spectra of (c) BF₂-PhCz and (d) BF2-PVCz in DMSO/water mixtures with different water fractions (the inset shows the images of the solutions with 0% and 90% water fractions under a 365 nm UV lamp).

In DMSO/H₂O mixtures, both compounds exhibited typical aggregation induced emission (AIE) properties. As shown in Fig. 2c and d, the emission was quenched when the water fraction was lower than 50% due to the large polarity of the solvent, and dramatically turned on when the water content was above 50% because of the aggregation of molecules (Fig. S4, ESI†).³⁶ Impressively, the aggregation of molecules achieved highly efficient orange (λ_PL = 611 nm, 21.25%) and red (λ_PL = 649 nm, 22.68%) emissions when the water content was 90%, respectively, implying the efficient solid-state luminescence capability. Both the PL performance in solution and aggregation confirmed that BF₂-PVCz possessing more extended π-conjugation realized bathochromic emission with higher fluorescence quantum yields, which verified the calculation prediction and overcame the challenge of “the redder, the brighter”.

The crystals that were cultivated from a mixture of n-hexane and acetone solution exhibited PL emission peaking at 649 (Φ_F = 15.34%) and 587 nm (Φ_F = 26.76%) for BF₂-PhCz and BF₂-PVCz, respectively. Comparing with the aggregation of molecules obtained from the DMSO/H₂O mixture, the BF₂-PhCz crystal showed bathochromic shift emission, while BF₂-PVCz exhibited hypsochromic shift emission (Fig. S5, ESI†). Single crystal analysis of BF₂-PVCz (CCDC No. 2369031†) revealed that the molecule adopted a linear and twisted skeleton with a trans-conformational C=C bond and a dihedral angle between carbazolyl and phenyl ring units of 52.9° (Fig. 3a), which was consistent with the theoretical calculations. In a long-range view, the molecules formed “head-to-head” arrangement and packed in overlapped layers without any typical π⋯π interactions (Fig. 3b), which would suppress the dipole–dipole interactions and inhibit red-shifted emission. In the crystal, one BF₂-PVCz molecule interacted with four adjacent molecules, resulting in four kinds of dimers with illustrated intermolecular interactions (Fig. 3b and Fig. S6, ESI†). In particular, the strong F⋯H hydrogen-bond interaction between the phenyl ring and boron difluoride (d₁ = 2.482 Å) in Dimer 1 could fix the bond angle and rotation of the phenyl ring. This rigidified structure would lead to a smaller reorganization energy than that in aggregates, which was indispensable to realize crystal-induced blue-shifted emission.^37–39

Fig. 3 (a) BF₂-PVCz crystal structure, (b) crystal packing and intermolecular interactions of Dimer 1, (c) rigid potential energy surface scan map, (d) Hirshfeld surface analysis fingerprint, (e) F⋯H and (f) CH⋯π weak interaction ratio in the fingerprint of the compound, (g) reduced density gradient (RDG) isosurface maps (isovalue = 0.5) and (h) scatter plot of RDG versus sign(λ₂)ρ for Dimer 1.

In order to further analyze the intra/intermolecular interactions in Dimer 1, Hirshfeld surface analysis using Multiwfn was employed.³² As depicted in Fig. 3c, there are efficient intermolecular interactions on the molecular surface. The red isosurface portion, indicative of strong intermolecular contacts, was predominantly F⋯H interactions (contributing 8.41% to the total) between the phenyl group and F atoms, which could effectively inhibit the dipole–dipole interaction, promote the blue-shifted emission and improve the luminescence efficiency (Fig. 3d and e). To more clearly visualize the F⋯H interactions in the molecular aggregates, reduced density gradient (RDG) analysis was performed on Dimer 1.^32,40 The green region in the isosurface map of the RDG (Fig. 3g) and the prominent spikes in the region where sign(λ₂)ρ (isovalue = 0.5) in the scatter plot (Fig. 3h) indicated the presence of intermolecular interactions between the F and the H atoms in the adjacent phenyl and BF₂ groups. On the right-hand side of the finger pattern, the yellowish plates were mainly distributed by the CH⋯π (contributing 20.4% to the total; Fig. 3d and f) interaction between the phenyl and carbazolyl groups, inhibiting the π⋯π interaction and promoting the radiative transition rate. As a result, the abundant intra/intermolecular interactions such as F⋯H, B⋯H and CH⋯π (Fig. S6 and S7, ESI†) were expected to enhance molecular rigidity, which decreased nonradiative rotations and vibrations, ultimately supporting the high PLQY.

2.3. Mechanoresponsive luminescent properties

The twisted structure of BF₂-PhCz and BF₂-PVCz led to diverse molecular aggregations, which is sensitive to the dynamic process. Herein the crystal samples were taken to investigate the MRL properties. As shown in Fig. 4a and b, after fully grinding, the PL properties of BF₂-PhCz remained stable (λ_PL = 647 nm, Φ_F = 13.00%). In contrast, BF₂-PVCz exhibited distinct mechanoresponsive luminescence (λ_PL = 641 nm, Φ_F = 6.00%) with a bathochromic-shifted emission of 54 nm and a high contrast from yellow to red. Meanwhile, the fluorescence lifetime of BF₂-PVCz became shorter after being ground (Fig. S8, ESI†), which could be attributed to the increase of nonradiative pathways in the molecular arrangement. The radiative transition rates (k_rO/k_rG = 0.033/0.0088 ns⁻¹, where “O” represents the original state and “G” represents the ground state) were decreased and the non-radiative transition rates (k_nrO/k_nrG = 0.089/0.138 ns⁻¹) were increased after grinding (Table S2, ESI†). Moreover, the MRL process exhibited good reversibility during ground-fumed (with CH₂Cl₂ vapor) cycles, maintaining its mechanoresponsive luminescence properties even after 10 cycles.

Fig. 4 PL spectra of (a) BF₂-PhCz and (b) BF₂-PVCz (the inset shows the fluorescent images of the samples before and after grinding under a 365 nm lamp, and the repeated cycles of the ground–fuming processes); PXRD patterns of (c) BF₂-PhCz and (d) BF₂-PVCz in the original and ground states, respectively; the FT-IR spectra (e) and DSC curves (f) of BF₂-PVCz before and after grinding.

The structure of each state was investigated by powder X-ray diffraction (XRD) measurement to identify the mechanism of MRL (Fig. 4c and d). The original crystals of BF₂-PhCz and BF₂-PVCz exhibited sharp diffraction peaks. The main peaks in the small angle region were located at 6.27 and 6.10° for BF₂-PhCz and BF₂-PVCz with d-spacings of 14.23 and 14.47 Å, respectively, implying that BF₂-PVCz may possess a larger intermolecular distance and a looser arrangement, which was conducive to the phase transition.⁴¹ Upon grinding, the diffraction peaks of BF₂-PhCz remained almost the same, indicating that grinding could not disturb the crystal arrangement. However, the sharp peaks of BF₂-PVCz vanished after grinding, meaning the sample underwent a crystalline-to-amorphous transformation. The ordered molecular packing was disrupted, and then the dipole–dipole interactions governed by the molecular electrostatic potential distribution became dominant, resulting in a red-shift in luminescence and a decrease in PLQY. In the FT-IR spectra, the stretching vibration peak of the F–B–F bond in the ground sample shifted to 1303 from 1297 cm⁻¹ in the initial state with increased intensity, suggesting grinding resulted in the rupture of the F⋯H hydrogen bond.⁴² Meanwhile, the out-of-plane bending vibration peak of the C–H bond in the vinyl group shifted from 1083 to 770 cm⁻¹, and the stretching vibration peak of the C=C bond shifted from 1623 to 1628 cm⁻¹ (Fig. 4e). These changes indicated that trans–cis isomerization of the C=C bond occurred upon grinding. Upon fuming with CH₂Cl₂ vapor, the characteristic vibration peaks of the C–H bond in the vinyl group and the F–B–F bond were restored from 770 to 1082 cm⁻¹ and 1303 to 1297 cm⁻¹, respectively, which means the good reversibility of the F⋯H hydrogen bond in the grinding and fuming cycles. In contrast, the stretching vibration peak of the F–B–F bond at 1303 cm⁻¹ in BF₂-PhCz did not change before and after grinding, indicating there were probably no variable F···H hydrogen bonds in the solid (Fig. S9, ESI†). In the DSC curve, an exothermic peak (T = 120 °C, ΔH = 4.2 J g⁻¹) emerged in the heating process before melting in the ground sample of BF₂-PVCz, revealing the thermodynamic metastable nature of the ground state and the possibility of reversibility (Fig. 4f).

2.4. Information reversible writing and deep encryption applications

The reversible MRL properties with high contrast enabled BF2-PVCz as an ideal candidate for application in reversible writing and information encryption. Firstly, an ink-free information rewritable system was developed by depositing BF2-PVCz on filter paper. The information can be written and erased reversibly by scratching and fuming (with CH₂Cl₂) for at least 10 cycles (Fig. 5a and b). Then, an information deep-encryption application was designed by a screen printing technique. BF₂-PhCz and BF₂-PVCz inks made from a suspension of ground powders were printed on red paper to form complementary QR codes 1 and 2. Initially, the square was hidden on the red paper under sunlight (Fig. 5c, Stage I), which could be revealed under 365 nm UV light (Fig. 5c, Stage II). The true information contained in the QR code cannot be read out until the paper was exposed to CH₂Cl₂ vapor (Fig. 5c, Stage III). As a result, the information was deeply encrypted and three level decryption was needed in practical application (Fig. 5d). More importantly, the information can be easily erased by grinding, achieving “Burn After Reading” to further increase the security level.

Fig. 5 Schematic diagram of application in (a) reversible writing, (b) the PL wavelength changes of BF₂-PVCz in filter paper during grinding and fumigation cycles, (c) schematic diagram of screen printing, (d) information encryption and decryption.

3. Conclusion

In summary, two D–π–A red-emitting difluoroboron β-diketonate derivatives, BF₂-PhCz and BF₂-PVCz, were developed by varying the π-bridge. The introduction of the phenyl vinyl π-bridge in BF₂-PVCz promoted the redshifted emission and improved the luminescence efficiency. In the crystal, the molecules of BF₂-PVCz formed “head-to-head” arrangement dominated by F⋯H interactions, which inhibited dipole–dipole interactions and resulted in blue-shifted emission. Upon grinding, BF₂-PVCz crystals achieved high contrast mechanoresponsive luminescence from yellow to red in a crystal-to-amorphous process, accompanied by trans-to-cis isomerization of the C=C bond. Information reversible writing and deep encryption applications were demonstrated taking advantage of the efficient MRL switching of BF₂-PVCz. Our work provided a promising strategy for designing considerably emissive red MRL materials with high contrast in the application of information storage and security.

Data availability

The data supporting this article have been included in the paper and/or the ESI.† Crystallographic data for BF₂-PVCz have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition number CCDC 2369031.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (62074109 and 61775155), the Natural Science Foundation of Shanxi Province (No. 20210302123144), and Shanxi Scholarship Council of China (No. 2021-057).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, theoretical characterization, photophysical properties, crystal structure, MRL properties, and NMR spectra. CCDC 2369031. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc04206a

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