Clear piezochromic behaviors of AIE-active organic powders under hydrostatic pressure

Abstract

A novel diphenylacrylonitrile derivative (Z)-3-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-2-(4-methoxyphenyl)-acrylonitrile (β-CN-TPA) containing a twisted triphenylamine and diphenylacetonitrile was synthesized via Knoevenagel condensation and Suzuki coupling reactions. These molecules exhibited aggregation enhanced emission (AIE) effects. Interestingly, their mechano-fluorochromic properties were invisible upon grinding with a pestle. However, when hydrostatic pressure in a diamond anvil cell (DAC) was applied on the crystals of β-CN-TPA, the distinct piezochromic behaviors of the compound were observed. The fluorescence color changed from light green (530 nm) to red (665 nm) with a significant red-shift of 135 nm. The powder X-ray diffraction and high-pressure Raman studies indicated that the as-synthesized and ground samples had the same crystalline structures, while the compressed samples had an evident change in inter-molecular interactions. Comparative tests and theoretical analysis further confirmed that the distinct fluorescence behaviors of the desired dye during the different stress conditions were associated with the various inter-molecular interactions that existed with adjacent molecules.

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Introduction

Organic solids possessing mechano- and piezo-chromic fluorescent (MCF and PCF) phenomenon under mechanical grinding and hydrostatic pressure, respectively, have attracted wide-spread attention due to their promising potential applications in sensors, optical data storage devices, and security inks.¹ There are obvious differences between mechanical grinding and isotropic compression. In the former process, rubbing forces, shearing forces and crushing forces are produced; however, these forces do not exist in the latter. Thus, MCF or PCF mechanisms in a certain material can definitely be different. Recently, an anthracene derivative prepared by Tian's group² showed approximately 124 nm red-shift (Δλ_em) under varied hydrostatic pressures (from 0 to 7.9 GPa). In sharp contrast, the change in the anthracene derivative's photoluminescent (PL) wavelength was decreased to 33 nm upon anisotropic grinding. Lately, our group reported a typical intra-molecular charge-transfer (ICT) molecular diphenylacrylonitrile derivative pCN-TPA with a green emission (Δλ_em = 507 nm) in the crystalline state.³ By grinding the crystals, the emission colour was largely red-shifted to red (λ_em = 618 nm) with a 111 nm red-shift upon hydrostatic pressure treatment (Δλ_em = 99 nm, from 0 to 6.1 GPa). Obviously, the same luminescent molecules revealed distinct luminescent responses to both mechanical grinding and hydrostatic pressure. Thus, in-depth investigations on the origin of this difference in optical properties is still a target, and will definitely contribute a lot for further understanding MCF mechanisms.

In 2007, Tang's group discovered a new phenomenon of the aggregation (crystallization)-induced emission (AIE and CIE) effect, in which the fluorophore indicated faint fluorescence in the amorphous state, but became strongly emissive in the crystalline state.⁴ It was found that a variety of secondary bonds (C–H⋯O, C–H⋯N and C–H⋯π) contributed to locking and restricting the intramolecular vibrational and rotational motions and thus clearly enhanced the crystalline-state emissions of the material. Moreover, Ma et al. reported a crystal structure of a CIE active fluorophore, namely, cyano substituted oligo(para-phenylene vinylene);⁵ its secondary bond interactions lessen the possibility of cis–trans isomerization around the double bond, which made the crystals exhibit a rather strong fluorescence in the solid state. Clearly, these secondary bond interactions play a critical role in the optical properties of fluorescent molecules in the aggregated state. To date, the luminescence properties of the reported crystals undergoing the obvious variation upon grinding were mainly attributed to the change of intermolecular stacking modes and intramolecular conformations.⁶ However, considering the complicated factors that exist in the compression process, there are only a few reports that describe the responses to hydrostatic pressure. Moreover, the investigations into the relationship between secondary bond interactions and piezo-chromic luminescent behavior are rarely performed. More and systematic research is still urgently needed to reveal the deeply rooted mechanisms of the phenomenon.

In this paper, we report a novel ICT-type compound β-CN-TPA (Scheme 1), belonging to the donor–acceptor (D–A) system consisting of diphenylacrylonitrile as the electron-acceptor and triphenylamine (TPA) as the electron-donor, according to previous studies by park's⁷ and our group.^3,8 The desired dye presented aggregation induced emission (AIE) behavior and high emission efficiency (over 45%). However, it did not exhibit slight fluorescence color changes upon mechanical grinding. Interestingly, under hydrostatic pressure from 1 atm to 9.97 GPa, the emission color of the green-emission crystal gradually became deep-red with the red-shift as high as 135 nm.

Scheme 1 Molecular structure of β-CN-TPA.

Results and discussion

Synthesis and structural characterization

The molecular structure of the TPA-based cyanostilbene derivative β-CN-TPA is illustrated in Scheme 1 and the reaction process is shown in Scheme S1.† The key intermediate 4′-(diphenylamino)-[1,1′-biphenyl]-4-carbaldehyde (TPA1) was prepared by the palladium-catalyzed Suzuki coupling reaction of (4-(diphenylamino)phenyl)boronic acid (1.73 g, 6 mmol, 1.2 equiv.) with 4-bromobenzaldehyde (0.92 g, 5 mmol, 1 equiv.) and tetrakispalladium (0.11 g, 0.1 mmol, 0.02 equiv.) in a 30 mL mixture of toluene/THF (5/3, v/v) and 5 mL K₂CO₃ aqueous solution (2 M) stirred at 90 °C for 24 h (yield = 75%). Then, the Knoevenagel condensation reaction of TPA1 with 4-methoxybenzonitrile was carried out at room temperature. 4-Methoxybenzonitrile (0.80 g, 6 mmol, 3 equiv.), TPA1 (0.70 g, 2.00 mmol, 1 equiv.) and sodium methoxide (0.13 g, 6 mmol, 3 equiv.) were dissolved and stirred in anhydrous ethanol (30 mL) for 6 hours to obtain (Z)-3-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-2-(4-methoxyphenyl)acrylonitrile (β-CN-TPA) in a yield of 89%. Finally, the raw dye was recrystallized from the chloroform/ethanol (1/20, v/v) mixture to obtain the β-CN-TPA crystal.

The desired luminophore β-CN-TPA was fully characterized by ¹H NMR (Fig. S1†), ¹³C NMR (Fig. S2†), HRMS (Fig. S3†), and elemental analysis. ¹H NMR (500 MHz, CDCl₃): δ 7.94 (d, J = 8.5 Hz, 2H), 7.67 (d, J = 13 Hz, 2H), 7.63 (d, J = 6.5 Hz, 2H), 7.53 (d, J = 8 Hz, 2H), 7.46 (s, 1H), 7.30 (t, J = 7.5 Hz, 4H), 7.16 (d, J = 6.5 Hz, 6H), 7.04 (d, J = 15.6 Hz, 2H), 6.98 (t, J = 9 Hz, 2H), 3.87 (s, 3H). ¹³C NMR (126 MHz, DMSO): δ (ppm) 39.02, 38.86, 38.69, 38.52, 38.35, 38.19, 38.02. MS m/z: 478.2104. Anal. calcd for C₃₄H₂₆N₂O: C, 85.33; H, 5.48; N, 5.85; O, 3.34; found: C, 85.21; H, 5.39; N, 5.97; O, 3.43. MP: 175.6 °C (from DSC).

Crystallographic data for β-CN-TPA: C₃₄H₂₆N₂O, M = 478.57 g mol⁻¹, monoclinic, a = 21.031(3) Å, b = 6.0833(8) Å, c = 21.443(3) Å, β = 109.880(2)°, V = 2579.9(6) ų, T = 296 K, space group P2(1)/c, D_calc = 1.232 mg m⁻³, Z = 4, the final R indices were R₁ = 0.0523(3504), wR₂ = 0.1686(5052), CCDC 893984.†

Methods

A single crystal of β-CN-TPA was obtained from the mixture of n-hexane/dichloromethane at room temperature. Organic nanoparticles were prepared by a general re-precipitation method. The purified materials were dissolved in solvent (THF) to a concentration of 1 × 10⁻⁴ M. 1 mL of this solution was injected into 9 mL THF and water, and was stirred for 3 minutes. The mixture was kept stationary to stabilize the nanostructures, and maintained at room temperature for 1 day. One drop of suspension was placed onto a quartz plate for SEM examination. A diamond anvil cell was used (DAC) to execute the high-pressure experiments. A ruby chip was used for pressure determination using the standard ruby fluorescent technique. The mixture of methanol/ethanol (4/1, v/v) was used as the pressure-transmitting medium. All the experiments were performed at room temperature.

Aggregation-induced emission

As depicted in Fig. 1 and 3, the absorption and emission spectrum of β-CN-TPA was obtained in THF–water mixtures to understand the AIE effect. In the absorption spectrum of β-CN-TPA, the excitation band was influenced by the fractions of water in THF/water mixtures, which indicated the competition between polarity and nanoparticles in the THF/water mixture.⁹ The absorption band was located at 390 nm, which mainly originated from the π–π* transition of the β-CN-TPA molecules in the mixture. β-CN-TPA dye is a D–A dipole molecule, which indicated an evident intra-molecular charge-transfer (ICT) effect.^9b,10 As shown in Fig. 2, the frontier molecular orbitals revealed that the corresponding electronic density distribution of the frontier molecular orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) were located on the donor triphenylamine (TPA) and the acceptor cyanostilbene group, respectively. This resulted in a charge-transfer (CT) transition. Such an electron distribution further proved its intrinsic ICT properties.¹¹ With the increase in f_w (water fraction in the mixture) from 0% to 70%, the absorption intensity gradually weakened, and the absorption peaks position were maintained between 390 and 394 nm. This behaviour was related to the alterations in the polarity of mixture. However, when the f_w was above 80%, the absorption band presented a sudden boost and a high energy tail. As shown in Fig. S4,† a large amount of nanoparticles were observed at a high f_w of 90% that did not exist in the pure solution. Thus, these tails could be attributed to Mie scattering caused by nanoscale particles.¹²

Fig. 1 The UV-vis absorption spectra of β-CN-TPA in the THF/water mixtures with different fractions of water (f_w). Solution concentration: 10 μM.

Fig. 2 Pictorial representations of frontier molecular orbitals of dye β-CN-TPA calculated at B3LYP/3-21G**.

In contrast, the images under UV light (356 nm) and the emission spectra of the β-CN-TPA obtained exhibited a dramatic change in the luminescence behavior in THF/water mixtures. As the f_w increased from 0% to 70%, the emission spectra continually red-shifted from 520 nm to 555 nm, along with weakened emission intensities (Fig. 3b). However, luminescence was remarkably enhanced at f_w = 80%, and reached the intensity maximum at 90% of water content. These results revealed that the emission peaks depended on the solvent polarity (f_w < 70%). With the f_w over 80%, the molecule became gradually less soluble, after which nanoparticles appeared. Therefore, the enhanced luminescence can be attributed to the aggregation of β-CN-TPA molecules.

Fig. 3 (a) Digital images under UV light (excitation wavelength: 356 nm) and (b) emission spectra of β-CN-TPA in the THF/water mixtures with different fractions of water. Solution concentration: 10 μM.

Mechanochromic fluorescence behaviors

β-CN-TPA is a type of triphenylamine derivative that holds a distorted molecular conformation. Generally, such molecules contribute to the formation of mechanochromic behaviors.¹³ However, in this case, there was no visible color change after the β-CN-TPA crystals were completely grinded with a pestle. Their fluorescence color was still light green, which was similar to the original state, as shown in Fig. S5.† In addition, the fluorescence emission spectra of the β-CN-TPA crystal and the ground sample superimposed on each other, which was coincident with the fluorescence color. This phenomenon demonstrated that the compound did not reveal any mechano-chromic properties.

To further understand the optical properties discussed above, a single crystal of β-CN-TPA dye was obtained. The optical behaviors were closely associated with the molecular conformation and stacking modes. Fig. 4 shows the stacking mode of β-CN-TPA in the single crystal state. Two specific inter-molecular interactions were observed between adjacent molecules. One type of molecule adopted J-type aggregation with head-to-tail stacking, in which the inter-molecular interactions were rather weak. The distance of multiple secondary bond interactions, for instance, C–H⋯π and π⋯π existing in two neighbouring molecules, were 3.482 Å and 3.473 Å, respectively. The secondary bond interactions played a significance role in crystal stability. Moreover, T-type molecular packing was observed, which was stabilized by other secondary bonds, as depicted in Fig. 4D. Thus, these multiple secondary bonding interactions induced tight packing and limited intramolecular rotation, without face-to-face packing, activating the strong fluorescence response of the crystals. Powder X-ray diffraction spectra of the original crystal and ground samples were obtained, as depicted in Fig. S6.† The diffraction peaks of the compound in two different states (before and after grinding) were mutually consistent, which suggested that both of them had the same molecular packing. The results were consistent with the same fluorescence of the two phases as mentioned above. Subsequently, the time-resolved emission decay behaviors of β-CN-TPA powders before and after grinding were also studied, as illustrated in Table S1.† The pristine crystal decayed with a short lifetime of 2.7 ns, which got very close to that of the ground powders (2.4 ns). The β-CN-TPA molecule and (Z)-2-(4′-(diphenyl amino)-[1,1′-biphenyl]-4-yl)-3-(4-methoxy-phenyl)acrylonitrile (α-CN-TPA, Chart S1†) are isomers.¹⁴ In the crystalline state, the α-CN-TPA molecule possess H-type stacking, which induces a long lifetime (20.8 ns) due to the formation of an excimer. For β-CN-TPA, the J-type and T-type packing aggregation effectively prevented excimer formation.¹⁵ Thus, the lifetime of the crystalline β-CN-TPA should be shorter than that of α-CN-TPA. In addition, the emission efficiency of β-CN-TPA in the crystal state was higher than that of α-CN-TPA, and was almost constant after grinding. These results indicated that intermolecular interactions after grinding did not alter obviously in β-CN-TPA.

Fig. 4 Illustration of secondary bond interactions (C–H⋯π and π⋯π) in the β-CN-TPA single crystal.

Piezochromic fluorescence behaviors

The piezochromic luminescence of the β-CN-TPA was further investigated by hydrostatic pressure. As depicted in Fig. S7,† the clear red-shift and the enhanced intensity appeared with pressure increases from 1 atm to 1.07 GPa. During 1.07 GPa to 9.97 GPa, the fluorescence spectra exhibited a drastic red-shift, but the intensity declined. As depicted in Fig. 5, the fluorescence images of the β-CN-TPA crystal under hydrostatic pressure also showed color changes from bright green to red. The fluorescence spectra and colors returned to the original state after releasing the pressure. The β-CN-TPA molecules contained a twisted triphenylamine unit, and its intra-molecular benzene rings were connected with each other via rotatable single bonds, which resulted in the non-radiative transition. At low hydrostatic pressures from 1 atm to 1.07 GPa, the emission intensity exhibited an evident enhancement. According to the restriction of intra-molecular rotation (RIR) hypothesis for the mechanism of AIE-active materials, the inter-molecular interactions limited the rotation and vibration of the aromatic parts, which reduced energy loss through non-radiative rotational relaxation, and thus enhanced the PL efficiency.¹⁶ As compression went beyond 1.07 GPa, close packing of the panel parts of the molecules were generated in the crystal state, quenching the excited states to some extent; this should be responsible for the emission attenuation. Moreover, the inter-molecular π⋯π interaction was gradually strengthened, and thus induced the emission color change from a green emission to a red emission, which accounted for the huge red-shift of the PL spectrum at higher pressures.

Fig. 5 Fluorescence images of the β-CN-TPA crystal under hydrostatic pressure (from compression to decompression).

In situ Raman spectra of β-CN-TPA was obtained to further understand the piezochromic performance. As depicted in Fig. 6, all Raman modes showed blue-shifts. The major blue-shifts in the Raman spectra appeared at 1185 cm⁻¹, 1196 cm⁻¹, 1547 cm⁻¹ and 2212 cm⁻¹, which were attributed to the C–H in-plane swing mode, benzene ring stretching, biphenyl C–C stretching and cyano group stretching, respectively. These blue-shifts were due to the enhancement of the secondary bonds and π⋯π interactions with increasing pressure.¹⁷ Thus, the secondary bonds were enhanced under lower pressures and the intra-molecular rotation part was restricted, which unblocked the radiation channel. Therefore, the fluorescence intensity was obviously improved in the low-pressure stage. With increased pressure, the π⋯π interaction existing in the diphenylacrylonitrile segment of the molecules was strengthened, which resulted in the decrease of fluorescence intensity and the red-shift of emission color. No discontinuities of the Raman mode was detected with increasing pressure. Thus, it could be concluded that the structure of the crystal state was kept stable up to 9.97 GPa.¹⁸ The evolution of high-pressure Raman spectra matched well with the hypothesis of the PL spectra and the fluorescence behaviors under high pressures. Notably, when the pressure gradually returned to atmospheric pressure, the fluorescence spectra turned back to the original state. This reversible pressure dependence of the fluorescence phenomenon was noteworthy as a promising reversible response piezochromic effect. Inter-molecular interactions and π⋯π interactions were reduced, which caused the blue-shift of the emission band. The fluorescent images were in line with the fluorescence spectra.

Fig. 6 Raman spectra of the β-CN-TPA crystal at hydrostatic pressures in different spectral regions: (a) 890–1250 cm⁻¹, (b) 1450–1720 cm⁻¹, and (c) 2050–2350 cm⁻¹. The arrows denote the original peaks and their blue-shifts.

The corresponding Raman spectra of the pressure releasing process were investigated, as shown in Fig. S8.† Majority of the Raman peaks were red-shifted due to the gradual restoration of inter-molecular interactions. Compared to the Raman spectra of β-CN-TPA in the uncompressed and released states, it was found that both of them significantly overlapped. The results indicated that the piezochromic behavior of β-CN-TPA dye was reversible at high pressures.

Conclusions

In summary, we demonstrate the clear piezochromic behavior of β-CN-TPA stimulated by hydrostatic pressure. Under mechanical grinding, β-CN-TPA exhibits an invisible mechano-chromic performance due to the intrinsic stability of the compound endowed by various strong secondary bonds. In contrast, the high pressure experiment of the dye using a DAC displays dramatic piezochromic behaviors. Importantly, the emission color transforms from light green to red with a remarkable red-shift (over 135 nm) in the PL spectra. Under low pressures before 1.07 GPa, the emission intensity exhibited an evident enhancement due to the restriction of rotation about single bonds. During 1.07 GPa to 9.97 GPa, compression leads to the distinct enhancement of intermolecular interactions, which result in a further red-shift and a reduction in PL intensity.

Acknowledgements

We highly appreciate Prof. Bo Zou and Dr Kai Wang, Jilin university, for the help of high-pressure experiment. This study was supported by the National Natural Science Foundation of China (51203138, 51273179, 51403060, 51573165), Natural Science Foundation of Zhejiang Province (LY15E030002, LY15E030006, LQ14B040003), the Natural Science Foundation of Huzhou (2014YZ02) and national scholarship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 893984. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21218a

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