Carbazole-based gold(I) complexes with alkyl chains of different lengths: tunable solid-state fluorescence, aggregation-induced emission (AIE), and reversible mechanochromism characteristics

Abstract

In this paper, seven carbazole-based mononuclear gold(I) complexes with alkyl chains of different lengths have been synthesized and reported. All of these gold(I) complexes exhibit outstanding AIE characteristics. Furthermore, these various solid-state light-emitting AIE-active gold(I) complexes all show reversible mechanochromic fluorescent behaviors. The possible mechanism explaining these interesting AIE and mechanochromism phenomena involves a variation in weak multiple intermolecular C–H⋯F and π⋯π interactions and the formation or alteration of aurophilic interactions.

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Introduction

Luminescent organic materials, especially tunable solid-state organic smart fluorescent materials showing a response to environmental stimuli, have attracted considerable interest due to their tremendous application potential in various fields, such as fluorescent switches, data storage, and photoelectronic devices.^1–5 Mechanochromic fluorescent materials are an important class of smart luminescent materials, the fluorescent properties of which can be adjusted through a change in an external mechanical stimulus. High aggregate-state emission and remarkable color contrast are key factors for the application of mechanochromic luminescent materials. However, conventional organic fluorescent materials suffer from the notorious aggregation-caused quenching (ACQ) effect,⁶ which results in a poor aggregate state emission efficiency and has greatly limited the high-technology applications of numerous conventional fluorescent molecules.^7,8 Fortunately, in 2001, Tang et al. reported an interesting and unusual luminescence phenomenon, which is widely known as aggregation-induced emission (AIE).⁹ Next, in 2002, Park et al. found an important aggregation-induced emission enhancement (AIEE) phenomenon.¹⁰ Both AIE and AIEE are useful methods for overcoming ACQ, allowing for bright aggregate-state emission. To date, most luminescent materials exhibiting AIE behavior have been organic compounds.^11,12 Occasionally, AIE complexes based on transition metals have also been discovered. For more than a decade, gold(I) chemistry has attracted substantial attention due to the existence of intriguing aurophilic Au–Au interactions.^13–21 To date, however, limited AIE luminogens with mechanochromic behavior based on gold(I) complexes have been reported.²² On the other hand, it is well known that carbazole-based derivatives are extremely promising candidates in the field of electronic devices.²³ Unfortunately, the ACQ effect seriously limits the efficient application of these materials with carbazole skeletons. Furthermore, a carbazole-based gold(I) complex showing AIE properties has not yet been reported. Thus, it is a very meaningful challenge to design and synthesize AIE-active gold(I) complexes with a carbazole scaffold structure. Meanwhile, the solid-state emission of fluorescent molecules is closely related to their molecular structures. Therefore, adjusting the solid-state fluorescence of luminogens by introducing alkyl chains of different lengths is a reasonable and effective method. Herein, we report a series of carbazole-based mononuclear gold(I) complexes 1–7 (Chart 1) with alkyl chains of different lengths. Their synthesis and characterization are described in detail, as well as investigations of their AIE and mechanochromic properties. Moreover, most of these complexes were obtained in crystalline form, and their crystal structures permit a reasonable interpretation of their different solid-state fluorescence properties.

Chart 1 Chemical structures of the mononuclear gold(I) complexes 1–7.

Result and discussion

Synthesis

The carbazole-based gold(I) complexes 1–7 were prepared in accordance with the corresponding synthetic routes presented in Scheme 1. The starting materials 1c–7c were reacted with C₆F₅Au(tht) (tht = tetrahydrothiophene) to afford the respective target gold(I) complexes 1–7 in high yields.

Scheme 1 Synthesis routes of luminogens 1–7.

Aggregation-induced emission (AIE) properties of complexes 1–7

To survey the AIE behavior of complexes 1–7, their UV and photoluminescence (PL) spectra were studied in DMF/H₂O mixtures with different water fractions (f_w). Luminogen 1 exhibited similar absorption spectra with absorption maxima at around 280, 328, and 342 nm in DMF/H₂O mixtures with various f_w (Fig. S1, ESI†). Meanwhile, level-off tails could clearly be observed in the visible region as the f_w values were increased. Such tails can be attributed to the Mie scattering effect, which usually indicates the formation of nanoscopic aggregates.²⁴ As shown in Fig. 1, luminogen 1 in pure DMF (1.0 × 10⁻⁵ mol L⁻¹) are practically nonluminescent. However, when the water content in the DMF solution was increased to 70%, a new green emission band was observed with λ_max at 515 nm. When the f_w value in the DMF solution reached 90%, the emission intensity of luminogen 1 at 515 nm was approximately 715-fold higher than that in pure DMF, and a strong green fluorescence was observed under UV light at a wavelength of 365 nm. This was because water is a nonsolvent for luminogen 1. Thus, the PL intensity of luminogen 1 is greatly strengthened by adding water into the DMF solution containing small amounts of 1 to induce aggregate formation. Dynamic light-scattering (DLS) analysis confirmed the formation of nano-aggregates in DMF–H₂O mixtures with high water contents (Fig. S2, ESI†). When water was added to the DMF solution containing small amounts of complex 1, the corresponding nano-aggregates were formed. The formation of the significative nano-aggregates resulted in a variation in the intermolecular gold–gold interactions. Possessing intermolecular aurophilic interactions that were possibly responsible for the green fluorescence.^22a Thus, the green emission of 1 was clearly induced by aggregate formation, and therefore 1 must be considered as an AIE-active species. The aggregation-induced emission characteristics of complexes 2, 3, 4, 5, 6, and 7 in DMF/H₂O mixtures with various water contents were similar to those of 1 (Fig. S3–S8, ESI†), indicating that these complexes also show AIE properties.

Fig. 1 (a) PL spectra of the dilute solutions of luminogen 1 (1.0 × 10⁻⁵ mol L⁻¹) in DMF–H₂O mixtures with different water fractions (f_w). Excitation wavelength = 330 nm. (b) Changes in the emission intensity of 1 at 515 nm in DMF–H₂O mixtures with various volume fractions of water (0–90%). (c) The fluorescence images of 1 (concentration: 1.0 × 10⁻⁵ mol L⁻¹) in diverse DMF–H₂O mixtures with various f_w values (0–90%) under 365 nm UV irradiation.

X-ray structures

Crystallographic details. Single crystals of complexes 1, 3, 4, 5, and 7 suitable for X-ray structure analysis were obtained by slow diffusion of n-hexane into dilute solutions of the respective complexes in dichloromethane (Chart 2). Selected crystals of 1, 3, 4, 5, and 7 were mounted on glass fibers for diffraction experiments. Intensity data were collected on a Nonius Kappa CCD diffractometer employing Mo-K_α radiation (0.71073 Å) at room temperature. The structures were solved by a combination of direct methods (SHELXS-97)²⁵ and Fourier difference techniques, and refined by full-matrix least-squares (SHELXL-97).²⁶ All non-H atoms were refined anisotropically. The hydrogen atoms were placed in ideal positions and refined as riding atoms. Detailed crystal data and further information on the data collection are provided in Tables S1–S5 (ESI†). The bond angles and distances can be found in Tables S6–S10 (ESI†).

Chart 2 (a) Single crystal structure of complex 1. (b) Single crystal structure of complex 3. (c) Single crystal structure of complex 4. (d) Single crystal structure of complex 5. (e) Single crystal structure of complex 7.

X-ray crystal structures of complexes 1, 3, 4, 5, and 7. As shown in Fig. 2, the shortest intermolecular Au⋯Au distances in 1, 3, 4, 5, and 7 are different, indicating the diversity of intermolecular gold–gold interactions in these carbazole-based gold(I) complexes with alkyl chains of different lengths. According to the preceding literature, significantly strong intermolecular gold–gold interactions occur when the distance between the intermolecular gold atoms is within the range 2.7–3.3 Å.¹⁴ Therefore, the structures of 1, 3, 5, and 7 have no strong intermolecular gold–gold interactions, with shortest Au⋯Au distances of 3.407, 8.305, 4.938, and 3.357 Å, respectively. However, the structure of 4 features a strong intermolecular gold–gold interaction, with a shortest Au⋯Au distance of 3.126 Å. Thus, a carbazole-based gold(I) complex with a strong intermolecular gold–gold interaction was successfully obtained by adjusting the length of the alkyl chain. On the other hand, the structures of 1, 3, 4, 5 and 7 all involve weak intermolecular C–H⋯F and π–π interactions (Fig. S21–S25, ESI†), which allow slipping of the molecules under the influence of external mechanical stimuli.

Fig. 2 (a) The structural organization of complex 1. (b) The structural organization of complex 3. (c) The structural organization of complex 4. (d) The structural organization of complex 5. (e) The structural organization of complex 7.

Mechanochromism behaviors of complexes 1–7

Subsequently, the mechanochromic behaviors of complexes 1–7 were investigated by PL spectroscopy. As shown in Fig. 3, the emission spectrum of solid powder 1 showed a weak, broad emission peak, corresponding to a weak white emission, upon UV irradiation at a wavelength of 365 nm. Intriguingly, upon gentle grinding, a new emission band centered at 511 nm appeared, and the emission color of the ground sample turns to a strong green. Furthermore, after the ground solid powder was treated with dichloromethane vapor for 1 min, it was observed that the ground sample could revert to its original unground emission color. As a consequence, the solid-state emission of 1 could be switched by mechanical stimulus, such that it exhibited reversible mechanochromic luminescence behavior. Moreover, this interesting mechanochromic luminescence transformation between the weak white and strong green emissions could be repeated many times through repeated grinding-exposure treatments (Fig. S26, ESI†). In order to further probe the possible mechanism responsible for the observed mechanochromism property, the structural transition of the solid sample of 1 was tested by powder X-ray diffraction (XRD) analysis. As shown in Fig. 4, the XRD pattern of unground solid sample 1 exhibited a lot of sharp and intense diffraction peaks, indicative of its crystalline nature. In contrast, upon grinding, the sharp reflection peaks vanished, which suggested that grinding resulted in the conversion from a crystalline to an amorphous phase. After treatment of the ground sample by exposure to dichloromethane vapor, the clear and intense diffraction peaks reappeared, implying that the original crystalline phase state was restored through molecular repacking. Hence, the XRD experimental results indicated that a morphology conversion between a stable crystalline phase and a metastable amorphous phase was responsible for the reversible mechanochromism phenomenon of 1. As can be seen in Fig. 5, consistent with the mechanochromic properties of 1, luminogens 3 and 5 (ref. 27) also showed similar mechanochromic behavior involving fluorescence color changes from white to green. Furthermore, the results of XRD analysis and repetitive experiments on 3 and 5 were similar to those for 1 (Fig. S28, S30, S34, and S35, ESI†). When solid samples of luminogens 1, 3, and 5 were ground, the metastable states were formed, possessing the intermolecular aurophilic interactions that were possibly responsible for the generation of green fluorescence.^22b,c As shown in Fig. 2c, the structure of complex 4 features strong intermolecular gold–gold interactions, with a shortest Au⋯Au distance of 3.126 Å, which is within the range of strong aurophilic bonding (2.7–3.3 Å).¹⁴ Therefore, as indicated in Fig. 6, the solid-state fluorescence spectrum of luminogen 4 showed two emission bands with λ_max at 554 and 596 nm, and the solid powder of 4 emitted a bright-yellow fluorescence under 365 nm UV irradiation. After grinding of the sample, a new broad emission band with λ_max at 513 nm was observed, and the yellow solid-state light emission of 4 was converted into a green luminescence. Treatment of ground luminogen 4 with dichloromethane vapor induced reversion to its original state. The solid-state emission of 4 could clearly be switched between yellow and green states by mechanical stimulus, such that it also exhibited reversible mechanochromic luminescence behavior. As shown in Fig. 7, the corresponding XRD experimental results revealed that the mechanochromism phenomenon of 4 could be ascribed to a crystalline-to-metastable morphology phase transition. In addition, repetitive experiments on the switchable solid-state luminescence indicated superior repeatability of the mechanochromism behavior of 4 (Fig. S29, ESI†). Similarly, as indicated in Fig. 8, luminogens 2, 6, and 7 showed analogous mechanochromic luminescence processes involving a fluorescence color change from yellow to green. Meanwhile, the results of XRD analysis and repetitive experiments on 2, 6, and 7 were similar to those for 4 (Fig. S27, S31–S33, S36, and S37, ESI†). The mechanochromism characteristics of complexes 1–7 can be influenced by the alkyl chain length. This is because the stacking states of these luminogens can be adjusted by the alkyl chain length. Thus, solid-state fluorescence of these luminogens can be affected by the alkyl chain length.

Fig. 3 (a) PL spectra of luminogen 1 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographic images of 1 under irradiation with UV light at 365 nm: (b) the unground solid sample. (c) The entirely ground sample. (d) The sample after treatment with dichloromethane vapor.

Fig. 4 XRD patterns of complex 1: unground, ground and after treatment with dichloromethane.

Fig. 5 (a) PL spectra of luminogens 3 and 5 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographic images of 3 and 5 under irradiation with UV light at 365 nm: (b) the unground solid sample. (c) The entirely ground sample. (d) The sample after treatment with dichloromethane vapor. The upper is data of luminogen 3. The subjacent is data of luminogen 5.

Fig. 6 (a) PL spectra of luminogen 4 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographic images of 4 under irradiation with UV light at 365 nm: (b) the unground solid sample. (c) The entirely ground sample. (d) The sample after treatment with dichloromethane vapor.

Fig. 7 XRD patterns of complex 4: unground, ground and after treatment with dichloromethane.

Fig. 8 (a) PL spectra of luminogens 2, 6 and 7 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographic images of 2, 6 and 7 under irradiation with UV light at 365 nm: (b) the unground solid sample. (c) The entirely ground sample. (d) The sample after treatment with dichloromethane vapor. The upper is data of luminogen 2. The middle is data of luminogen 6. The subjacent is data of luminogen 7.

Experimentals

General

All manipulations were carried out under an argon atmosphere by using standard Schlenk techniques, unless otherwise stated. All starting materials were obtained commercially as analytical-grade and used without further purification. Compounds 1a,²⁸ 2a,²⁹ 3a,³⁰ 4a,³¹ 5a,³² 6a,³¹ 7a,³³ N-(4-hydroxyphenyl)formamide³⁴ and C₆F₅Au(tht) (tht = thiophane)³⁵ were prepared by procedures described in the corresponding literatures. ¹H NMR (400 MHz) and ¹³C NMR (100.6 MHz) spectra were collected on American Varian Mercury Plus 400 spectrometer (400 MHz). ¹H NMR spectra are reported as followed: chemical shift in ppm (δ) relative to the chemical shift of TMS at 0.00 ppm, integration, multiplicities (s = singlet, d = doublet, t = triplet, m = multiplet), and coupling constant (Hz). ¹³C NMR chemical shifts reported in ppm (δ) relative to the central line of triplet for CDCl₃ at 77 ppm. ¹⁹F NMR chemical shifts are relative to C₆F₆ (δ = −163.00). EI-MS was obtained using Thermo scientific DSQII. Elemental analyses (C, H, N) were performed by the Microanalytical Services, College of Chemistry, CCNU. UV/Vis or UV-Vis spectra were obtained on U-3310 UV Spectrophotometer. Fluorescence spectra were recorded on a Fluoromax-P luminescence spectrometer (HORIBA JOBIN YVON INC.). Column chromatographic separations were carried out on silica gel (200–300 mesh). TLC was performed by using commercially prepared 100–400 mesh silica gel plates (GF₂₅₄) and visualization was effected at 254 nm. The DMF/water mixtures with different water fractions were prepared by slowly adding ultra-pure water into the DMF solution of samples. XRD studies were recorded on a Shimadzu XRD-6000 diffractometer using Ni-filtered and graphite-monochromated Cu Kα radiation (λ = 1.54 Å, 40 kV, 30 mA). Dynamic light scattering (DLS) measurements were performed on the Zetasizer instrument ZEN 3600 (Malvern, UK) with a 173° back scattering angle and He–Ne laser (λ = 633 nm). The X-ray crystal-structure determinations of complexes 1, 3, 4, 5 and 7 were obtained on a Bruker APEX DUO CCD system.

Synthetic procedures for 1b–7b

A mixture of compounds 1a, 2a, 3a, 4a, 5a, 6a or 7a (1 mmol), N-(4-hydroxyphenyl)formamide (1.5 mmol), K₂CO₃ (6 mmol) were stirred in DMF (50 ml) for overnight under an argon atmosphere at 60 °C. After completion of present reaction, DMF was removed from reaction system by vacuum distillation. The residual mixture was extracted with dichloromethane (3 × 20 ml). The combined organic layers were washed with brine, dried (Na₂SO₄), and concentrated in vacuo. The residues were purified by column chromatography, affording the expected white solid products 1b–7b in a yield of 71%, 69%, 68%, 75%, 73%, 70%, 65%, respectively. The NMR data of 1b and 2b were not obtained due to the poor solubility. 3b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.49–8.30 (m, 1H), 8.11 (d, J = 8 Hz, 2H), 7.49–7.39 (m, 5H), 7.23 (d, J = 8 Hz, 2H), 7.08–6.97 (m, 2H), 6.83–6.80 (m, 2H), 4.41 (t, J = 4 Hz, 2H), 3.92 (t, J = 6 Hz, 2H), 2.08 (s, 2H), 1.84 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.99, 158.83, 156.77, 155.89, 140.26, 129.84, 129.43, 125.62, 122.77, 121.67, 121.51, 120.35, 118.79, 115.34, 114.70, 108.58, 67.75, 67.65, 42.62, 26.90, 25.78. EI-MS: m/z = 358.42[M]⁺. Anal. calcd for C₂₃H₂₂N₂O₂: C, 77.07; H, 6.19; N, 7.82. Found: C, 77.13; H, 6.12; N, 7.87. 4b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.49–8.31 (m, 1H), 8.11 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 5H), 7.23 (t, J = 8 Hz, 2H), 7.03–6.97 (m, 2H), 6.83–6.80 (m, 2H), 4.36–4.32 (m, 2H), 3.89 (t, J = 6 Hz, 2H), 1.99–1.92 (m, 2H), 1.82–1.76 (m, 2H), 1.58–1.51 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.67, 158.59, 157.10, 156.18, 140.41, 129.79, 129.30, 125.62, 122.87, 121.81, 121.72, 120.37, 118.81, 115.54, 114.91, 108.59, 68.02, 42.94, 29.06, 28.79, 23.89. EI-MS: m/z = 372.50[M]⁺. Anal. calcd for C₂₄H₂₄N₂O₂: C, 77.39; H, 6.49; N, 7.52. Found: C, 77.45; H, 6.56; N, 7.44. 5b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.49–8.30 (m, 1H), 8.10 (d, J = 8 Hz, 2H), 7.58–7.39 (m, 5H), 7.22 (d, J = 8 Hz, 2H), 7.07–6.97 (m, 2H), 6.83–6.80 (m, 2H), 4.32 (t, J = 6 Hz, 2H), 3.88 (t, J = 6 Hz, 2H), 1.91 (s, 2H), 1.72 (s, 2H), 1.47 (s, 4H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.03, 158.82, 156.92, 156.02, 140.30, 129.71, 129.29, 125.54, 122.70, 121.62, 121.46, 120.28, 118.69, 115.32, 114.67, 108.57, 67.87, 42.82, 28.95, 28.85, 26.97, 25.81. EI-MS: m/z = 386.37[M]⁺. Anal. calcd for C₂₅H₂₆N₂O₂: C, 77.69; H, 6.78; N, 7.25. Found: C, 77.65; H, 6.76; N, 7.28. 6b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.49–8.27 (m, 1H), 8.10 (d, J = 8 Hz, 2H), 7.78–7.37 (m, 6H), 7.24–7.13 (m, 2H), 6.98 (d, J = 8 Hz, 1H), 6.85–6.80 (m, 2H), 4.32–4.28 (m, 2H), 3.87 (t, J = 6 Hz, 2H), 1.88 (s, 2H), 1.72 (s, 2H), 1.39 (s, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.11, 158.90, 156.94, 156.03, 140.29, 129.70, 129.28, 125.52, 122.67, 121.60, 121.40, 120.26, 118.65, 115.29, 114.65, 108.57, 68.03, 67.95, 42.89, 29.06, 28.82, 27.14, 25.80. EI-MS: m/z = 400.34[M]⁺. Anal. calcd for C₂₆H₂₈N₂O₂: C, 77.97; H, 7.05; N, 6.99. Found: C, 77.89; H, 7.09; N, 6.90. 7b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.49–8.25 (m, 1H), 8.11–7.92 (m, 3H), 7.48–7.37 (m, 5H), 7.21 (d, J = 8 Hz, 2H), 6.98 (d, J = 8 Hz, 1H), 6.84–6.80 (m, 2H), 4.30–4.26 (m, 2H), 3.88 (t, J = 6 Hz, 2H), 1.87–1.82 (m, 2H), 1.37–1.32 (m, 10H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.16, 158.95, 156.93, 156.03, 140.27, 129.69, 129.28, 125.49, 122.64, 121.59, 121.33, 120.24, 118.62, 115.27, 114.62, 108.57, 68.09, 68.00, 42.89, 29.22, 29.08, 28.84, 27.11, 25.81. EI-MS: m/z = 414.41[M]⁺. Anal. calcd for C₂₇H₃₀N₂O₂: C, 78.23; H, 7.29; N, 6.76. Found: C, 78.29; H, 7.34; N, 6.70.

Synthetic procedures for 1c–7c

A CH₂Cl₂ suspension (15 ml) of 1b, 2b, 3b, 4b, 5b, 6b or 7b (0.6 mmol) and triethylamine (5 ml) was cooled to 0 °C. To the mixture was added dropwise a CH₂Cl₂ solution (10 ml) of triphosgene (0.66 mmol). The mixture was refluxed for 3 h, then 10% aq. Na₂CO₃ (50 ml) was added dropwise at room temperature. The mixture was extracted with dichloromethane (3 × 20 ml). The combined organic layers were washed with brine, dried (Na₂SO₄), and concentrated in vacuo. The residues were purified by column chromatography, affording the expected yellow solid products 1c–7c in a yield of 81%, 79%, 82%, 77%, 83%, 78%, 74%, respectively. 1c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.09 (d, J = 8 Hz, 2H), 7.48 (d, J = 4 Hz, 4H), 7.28–7.19 (m, 4H), 6.73 (d, J = 8 Hz, 2H), 4.72 (t, J = 6 Hz, 2H), 4.34 (t, J = 6 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.78, 158.38, 140.36, 127.58, 125.74, 122.98, 120.36, 119.29, 114.86, 108.59, 66.30, 42.23. EI-MS: m/z = 312.26[M]⁺. Anal. calcd for C₂₁H₁₆N₂O: C, 80.75; H, 5.16; N, 8.97. Found: C, 80.71; H, 5.22; N, 8.93. 2c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.09 (d, J = 8 Hz, 2H), 7.39 (d, J = 4 Hz, 4H), 7.29–7.20 (m, 4H), 6.80 (d, J = 8 Hz, 2H), 4.56 (t, J = 6 Hz, 2H), 3.87 (t, J = 6 Hz, 2H), 2.39–2.33 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.58, 158.63, 140.16, 127.57, 125.63, 122.69, 120.23, 118.92, 114.81, 108.34, 64.57, 38.90, 28.31. EI-MS: m/z = 326.33[M]⁺. Anal. calcd for C₂₂H₁₈N₂O: C, 80.96; H, 5.56; N, 8.58. Found: C, 80.91; H, 5.50; N, 8.64. 3c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 4H), 7.27–7.22 (m, 4H), 6.79–6.75 (m, 2H), 4.40 (t, J = 6 Hz, 2H), 3.91 (t, J = 6 Hz, 2H), 2.12–2.04 (m, 2H), 1.87–1.80 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.74, 159.10, 140.33, 127.69, 125.66, 122.90, 120.40, 119.49, 118.91, 114.97, 108.53, 67.83, 42.61, 26.83, 25.70. EI-MS: m/z = 340.32[M]⁺. Anal. calcd for C₂₃H₂₀N₂O: C, 81.15; H, 5.92; N, 8.23. Found: C, 81.07; H, 5.99; N, 8.27. 4c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 4H), 7.28 (s, 2H), 7.23 (d, J = 4 Hz, 2H), 6.78 (d, J = 8 Hz, 2H), 4.35 (t, J = 6 Hz, 2H), 3.89 (t, J = 6 Hz, 2H), 2.00–1.93 (m, 2H), 1.84–1.77 (m, 2H), 1.52 (d, J = 4 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.42, 159.20, 140.32, 127.65, 125.59, 122.80, 120.35, 118.79, 114.94, 108.53, 67.91, 42.81, 28.84, 28.71, 23.75. EI-MS: m/z = 354.28[M]⁺. Anal. calcd for C₂₄H₂₂N₂O: C, 81.33; H, 6.26; N, 7.90. Found: C, 81.39; H, 6.33; N, 7.85. 5c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 4 Hz, 2H), 7.45–7.39 (m, 4H), 7.28–7.23 (m, 4H), 6.78 (d, J = 8 Hz, 2H), 4.33 (t, J = 6 Hz, 2H), 3.89–3.84 (m, 2H), 1.91 (d, J = 8 Hz, 2H), 1.72 (d, J = 8 Hz, 2H), 1.45 (s, 4H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.36, 159.11, 140.21, 127.48, 125.46, 122.64, 120.19, 118.63, 114.77, 108.48, 67.88, 42.67, 28.74, 28.67, 26.82, 25.68. EI-MS: m/z = 368.35[M]⁺. Anal. calcd for C₂₅H₂₄N₂O: C, 81.49; H, 6.57; N, 7.60. Found: C, 81.46; H, 6.55; N, 7.64. 6c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 4H), 7.28–7.21 (m, 4H), 6.81 (d, J = 8 Hz, 2H), 4.31 (t, J = 8 Hz, 2H), 3.89 (t, J = 6 Hz, 2H), 1.90 (t, J = 6 Hz, 2H), 1.73 (t, J = 6 Hz, 2H), 1.40 (d, J = 4 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.31, 159.23, 140.27, 127.56, 125.48, 122.68, 120.25, 119.09, 118.64, 114.84, 108.52, 68.04, 42.86, 29.01, 28.81, 27.11, 25.72. EI-MS: m/z = 382.31[M]⁺. Anal. calcd for C₂₆H₂₆N₂O: C, 81.64; H, 6.85; N, 7.32. Found: C, 81.69; H, 6.93; N, 7.26. 7c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.10 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 4H), 7.28 (t, J = 4 Hz, 2H), 7.25–7.21 (m, 2H), 6.82 (d, J = 8 Hz, 2H), 4.31 (t, J = 6 Hz, 2H), 3.91 (t, J = 6 Hz, 2H), 1.92–1.69 (m, 4H), 1.41–1.33 (m, 8H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.37, 159.36, 140.35, 127.65, 125.52, 122.75, 120.30, 119.19, 118.67, 114.94, 108.57, 68.21, 42.97, 29.26, 29.09, 28.91, 27.17, 25.80, 1.01. EI-MS: m/z = 396.41[M]⁺. Anal. calcd for C₂₇H₂₈N₂O: C, 81.78; H, 7.12; N, 7.06. Found: C, 81.70; H, 7.18; N, 7.01.

Synthetic procedures for 1–7

A mixture of C₆F₅Au(tht) (0.315 mmol) and 1c, 2c, 3c, 4c, 5c, 6c or 7c (0.3 mmol) was stirred in CH₂Cl₂ (20 ml) over night under an argon atmosphere at room temperature. After completion of present reaction, the solvent was evaporated. A small amount of CH₂Cl₂ was added, and then a lot of n-hexane was added. Collecting the expected white solid products 1–7 by suction filtration in a yield of 85%, 81%, 89%, 88%, 92%, 78%, 82%, respectively. 1: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.10 (d, J = 8 Hz, 2H), 7.50 (d, J = 4 Hz, 4H), 7.36 (d, J = 8 Hz, 2H), 7.28 (t, J = 4 Hz, 2H), 6.82 (d, J = 8 Hz, 2H), 4.77 (t, J = 6 Hz, 2H), 4.42 (t, J = 6 Hz, 2H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.80, −158.28, −163.15. Anal. calcd for C₂₇H₁₆AuF₅N₂O: C, 47.94; H, 2.38; N, 4.14. Found: C, 47.99; H, 2.47; N, 4.03. 2: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.10 (d, J = 8 Hz, 2H), 7.44–7.36 (m, 6H), 7.24–7.20 (m, 2H), 6.87 (d, J = 8 Hz, 2H), 4.57 (t, J = 6 Hz, 2H), 3.90 (t, J = 6 Hz, 2H), 2.41 (t, J = 6 Hz, 2H). ¹⁹F NMR (CDCl₃): δ (ppm) = −117.17, −158.70, −163.53. Anal. calcd for C₂₈H₁₈AuF₅N₂O: C, 48.71; H, 2.63; N, 4.06. Found: C, 48.79; H, 2.75; N, 4.01. 3: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.49–7.40 (m, 4H), 7.26–7.23 (m, 4H), 6.85 (d, J = 8 Hz, 2H), 4.42 (t, J = 8 Hz, 2H), 3.94 (t, J = 6 Hz, 2H), 2.14–2.06 (m, 2H), 1.90–1.84 (m, 2H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.82, −158.35, −163.18. Anal. calcd for C₂₉H₂₀AuF₅N₂O: C, 49.44; H, 2.86; N, 3.98. Found: C, 49.49; H, 2.77; N, 3.90. 4: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.40 (m, 6H), 7.23 (d, J = 8 Hz, 2H), 6.87 (d, J = 8 Hz, 2H), 4.36 (t, J = 8 Hz, 2H), 3.93 (t, J = 6 Hz, 2H), 2.02–1.95 (m, 2H), 1.86–1.79 (m, 2H), 1.59–1.52 (m, 2H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.67, −158.19, −163.03. Anal. calcd for C₃₀H₂₂AuF₅N₂O: C, 50.15; H, 3.09; N, 3.90. Found: C, 50.04; H, 3.00; N, 3.98. 5: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.40 (m, 6H), 7.23 (d, J = 8 Hz, 2H), 6.86 (d, J = 8 Hz, 2H), 4.34 (t, J = 8 Hz, 2H), 3.92 (t, J = 6 Hz, 2H), 1.94 (d, J = 8 Hz, 2H), 1.76 (s, 2H), 1.47 (s, 4H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.98, −158.52, −163.35. Anal. calcd for C₃₁H₂₄AuF₅N₂O: C, 50.83; H, 3.30; N, 3.82. Found: C, 50.76; H, 3.35; N, 3.85. 6: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 6H), 7.22 (t, J = 4 Hz, 2H), 6.92–6.89 (m, 2H), 4.32 (t, J = 8 Hz, 2H), 3.94 (t, J = 6 Hz, 2H), 1.90 (t, J = 8 Hz, 2H), 1.75 (t, J = 8 Hz, 2H), 1.45–1.40 (m, 6H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.71, −158.21, −163.05. Anal. calcd for C₃₂H₂₆AuF₅N₂O: C, 51.48; H, 3.51; N, 3.75. Found: C, 51.36; H, 3.43; N, 3.83. 7: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.11 (d, J = 8 Hz, 2H), 7.48–7.39 (m, 6H), 7.23 (t, J = 8 Hz, 2H), 6.91 (d, J = 8 Hz, 2H), 4.31 (t, J = 8 Hz, 2H), 3.95 (t, J = 6 Hz, 2H), 1.89 (t, J = 6 Hz, 2H), 1.75 (t, J = 8 Hz, 2H), 1.39 (s, 8H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.79, −158.36, −163.17. Anal. calcd for C₃₃H₂₈AuF₅N₂O: C, 52.11; H, 3.71; N, 3.68. Found: C, 52.25; H, 3.65; N, 3.61.

Conclusions

In summary, a series of carbazole-based complexes containing mononuclear gold(I) units have been synthesized. All of these gold(I) complexes possess excellent AIE characteristics. The solid-state fluorescence of these gold(I) complexes can be adjusted by introducing alkyl chains of different lengths. Upon grinding, the respective light-emitting powders are transformed into powders showing similar green fluorescence. Interestingly, all of these green light-emitting powders can be reverted to their respective initial luminescent conditions by infusing them with dichloromethane vapor for 1 min. Thus, all of these gold(I) complexes clearly show tunable solid-state luminescence properties elicited by means of mechanical force. Among them, luminogens 1, 3, and 5 exhibit reversible mechanochromic behavior involving a fluorescence color change from white to green. Luminogens 2, 4, 6, and 7 also exhibit switchable mechanochromism properties involving fluorescence color changes from yellow to green. The mechanism underpinning these outstanding AIE and mechanochromism properties possibly involves a change in weak intermolecular C–H⋯F and π⋯π interactions and the formation or alteration of aurophilic interactions. The results of this study should be beneficial for the design of mechanical-stimuli-responsive metal-bearing functional fluorescent materials with AIE features. Further explorations of new gold(I) complexes with interesting luminescence characteristics are in progress.

Acknowledgements

The authors acknowledge financial support from National Natural Science Foundation of China (No. 21272088, 21472059 and 21402057) and self-determined research funds of CCNU from the colleges' basic research and operation of MOE (CCNU14A05009 and CCNU14F01003).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis absorption spectra of luminogens 1–7 in DMF–H₂O mixtures with different water contents, NMR spectra, mass spectra, details of the crystal data collection, bond distance and angles. CCDC 1425339 for complex 1, CCDC 1425340 for complex 3, CCDC 1423615 for complex 4, CCDC 1044227 for complex 5, CCDC 1425341 for complex 7. Characterization datas mentioned in the paper. See DOI: 10.1039/c5ra19378h

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