Fluorene-based mononuclear gold(I) complexes: the effect of alkyl chain, aggregation-induced emission (AIE) and mechanochromism characteristics

Abstract

In this paper, three fluorene-based gold(I) complexes have been synthesized and characterized. All of these mononuclear gold(I) complexes exhibit obvious aggregation-induced emission (AIE) behavior. Furthermore, the luminogen 3 without containing alkyl chains also shows reversible mechanochromism property involving fluorescence color changes from green to faint yellow. The mechanism for AIE characteristics of these gold(I) complexes and mechanochromic behavior of luminogen 3 possibly involves a change in weak intermolecular π⋯π and C–H⋯F interactions and the generation of intermolecular gold–gold interactions.

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Introduction

The development of highly emissive fluorescence materials has attracted much attention due to their potential applications in photoelectronic devices and display systems.¹ However, the infamous aggregation caused quenching (ACQ) phenomenon leads to poor aggregate state emission efficiency, which has significantly limited the practical application of luminescent materials.² Fortunately, in 2001, Tang et al. reported an aggregation-induced emission (AIE) phenomenon, which is opposite to the ACQ phenomenon.³ Luminogens with AIE property can emit efficiently in aggregated/solid states. Therefore, the discovery of AIE effect has contributed to the effective application of fluorescence materials. Mechanochromic materials, as a kind of “smart material”, have also attracted substantial attention on account of their promising applications in security papers and sensors.⁴ Bright solid state fluorescence and high color contrast are two extremely important factors for mechanochromism.⁵ Thus, it is very significative to synthesize mechanochromic luminogens possessing AIE property. So far, a number of AIE-active mechanochromic materials have been proven to be organic compounds.⁶ In contrast, AIE-active metal complexes exhibiting mechanochromic behavior are still rare. It is well known that gold(I) chemistry has attracted the widespread attention of scientists over the last two decades.⁷ To date, some mechanochromic gold(I) complexes have been reported.⁸ Nonetheless, gold(I) complexes with mechanochromism and AIE characteristics are still inadequate.⁹ On the other hand, fluorene-based derivatives are very valuable candidates in the domain of light emitting diodes.¹⁰ Unfortunately, the ACQ effect is one of the main bottlenecks that obstruct the efficient development of these fluorene-based materials. Hence, it is a meaningful research topic to develop AIE-active mechanochromic gold(I) complexes based on a fluorene skeleton. Herein, three fluorene-based mononuclear gold(I) complexes (Chart 1) have been designed and synthesized. The corresponding synthesis and characterization of these luminogens are depicted in detail. Furthermore, their photophysical and mechanochromic characteristics are explored as well. All of these gold(I) complexes exhibit excellent AIE characteristics. More interestingly, these gold(I) complexes show alkyl chain-dependent mechanofluorochromism. In addition, the crystalline state of solid sample 3 exhibits much stronger fluorescence than its amorphous state, which is very significative.¹¹

Chart 1 Chemical structures of the mononuclear gold(I) complexes 1–3.

Result and discussion

Synthesis

The fluorene-based mononuclear gold(I) complexes 1–3 were prepared according to the reasonable routes described in Scheme 1. The intermediate products 1-c, 2-c and 3-b as starting materials were reacted with C₆F₅Au(tht) (tht = tetrahydrothiophene) to get the expected gold(I) complexes 1–3 in high yields, respectively.

Scheme 1 Synthesis routes of gold(I) complexes 1–3.

Aggregation-induced emission (AIE) properties of luminogens 1–3

In order to study the AIE properties of gold(I) complexes 1–3, their UV-Vis and photoluminescence (PL) spectra were systematically investigated. As shown in Fig. 1, the UV-Vis absorption spectra of luminogens 1–3 in DMF–H₂O mixtures with high water fraction (f_w) showed level-off tails in the visible region. Such tails can be attributed to the Mie scattering effect,¹² which commonly indicates the presence of nano-aggregates.

Fig. 1 (a) UV spectra of complex 1 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with various water contents (0–90%). (b) UV spectra of complex 2 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with various water contents (0–90%). (c) UV spectra of complex 3 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with various water contents (0–90%).

As presented in Fig. 2, dilute DMF solution (2.0 × 10⁻⁵ mol L⁻¹) of complex 1 exhibited a single emission band with a maximum (λ_max) at 350 nm, and the solution was practically nonluminescent under irradiation with UV light at 365 nm. However, when the f_w value in the DMF solution was increased to 40%, a new yellow emission band with λ_max at around 565 nm was observed. Moreover, when the water content was increased to 90%, the peak intensity of luminogen 1 at 565 nm was approximately 130-fold higher than the intensity in pure DMF, and the bright yellow fluorescence could be obtained under 365 nm UV light. This is because complex 1 is insoluble in water, and thus increasing the f_w value in DMF–H₂O mixtures resulted in the formation of aggregated particles. Actually, the aggregated particles obtained in DMF–H₂O mixtures with high f_w value have been confirmed by dynamic light-scattering (DLS) analysis (Fig. S1, ESI†). When a high percentage of water was added to the DMF solution containing small amounts of luminogen 1, nano-aggregates were formed, and the generation of intermolecular aurophilic interactions was possibly responsible for the strong yellow emission.¹³ Obviously, the yellow emission of luminogen 1 was associated with aggregate formation, and therefore 1 exhibited typical AIE property. As can be seen in Fig. 3, the aggregation-induced characteristics of complexes 2 and 3 in DMF–H₂O mixtures with various f_w value were similar to those of complex 1. As a consequence, 2 and 3 are also AIE-active luminogens.

Fig. 2 (a) PL spectra of the dilute solutions of complex 1 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–H₂O mixtures with different water fractions (f_w). Excitation wavelength = 330 nm. The inset shows the corresponding emission images of luminogen 1 (2.0 × 10⁻⁵ mol L⁻¹) in pure DMF as well as 90% water fraction under 365 nm UV light. (b) The changes in emission intensity of luminogen 1 at 565 nm in DMF–H₂O mixtures with various volume fractions of water (0–90%).

Fig. 3 (a) PL spectra of the dilute solutions of complex 2 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–H₂O mixtures with different water fractions (f_w). Excitation wavelength = 330 nm. The inset shows the corresponding emission images of luminogen 2 (2.0 × 10⁻⁵ mol L⁻¹) in pure DMF as well as 90% water fraction under 365 nm UV light. (b) PL spectra of the dilute solutions of complex 3 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–H₂O mixtures with different water fractions (f_w). Excitation wavelength = 330 nm. The inset shows the corresponding emission images of luminogen 3 (2.0 × 10⁻⁵ mol L⁻¹) in pure DMF as well as 90% water fraction under 302 nm UV light.

Mechanochromism behavior of complex 3

The reversible mechanochromic property of complex 3 was investigated by PL spectroscopy. As evident from Fig. 4, the solid-state fluorescence spectrum of complex 3 exhibited two emission peaks with λ_max at 467 nm and 498 nm, and the solid powder of 3 showed green luminescence upon UV irradiation at a wavelength of 365 nm. Intriguingly, two new emission bands were observed with λ_max at 457 nm, 461 nm and 596 nm upon gentle grinding of solid powder 3, and the corresponding emission color was converted from green to faint yellow. Indeed, mechanofluorochromic phenomenon involving color changes from green to faint yellow is very rare, and it is different from our previous work.^9d,9e Furthermore, the faint yellow luminescence reverted to its original unground green color after a 1 min treatment of the ground powder with dichloromethane vapor. In addition, as Fig. 5 shows, the repeatability of this reversible mechanochromic transformation is very superior. It was speculated that the observed mechanochromism behavior may be related to the change of molecular morphology. In order to better understand the possible mechanochromic mechanism, diverse solid states of luminogen 3 were tested by powder X-ray diffraction (XRD) measurements. As shown in Fig. 6, the XRD diffractogram of unground solid powder 3 exhibited many intense and sharp reflection peaks, which is indicative of its crystalline nature. However, the grinding gave rise to weakening and broadening of all the diffraction peaks, which suggested the use of mechanical force led to the transformation from crystalline state to amorphous state.

Fig. 4 (a) PL spectra of complex 3 before grinding, after grinding, and after treatment with dichloromethane. Excitation wavelength: 365 nm. Photographic images of luminogen 3 under the irradiation of UV light at 302 nm: (b) the unground sample. (c) The partially ground sample. (d) The entirely ground sample. (e) The sample after treatment with dichloromethane solvent vapor.

Fig. 5 Invertible grinding-fuming processes of the photoluminescence of complex 3 at 596 nm. Excitation wavelength: 365 nm.

Fig. 6 XRD patterns of complex 3: unground, ground and after treatment with dichloromethane solvent vapor.

Moreover, the initial clear and intense reflection peaks reappeared upon treatment of the ground sample with dichloromethane solvent vapor, which implies a restoration of its original crystalline phase state. Thus, the XRD experimental results proved that the mechanochromic property of 3 can be ascribed to the morphology conversion between stable crystalline state and metastable amorphous state. In contrast, as Fig. 7 shows, no mechanochromism phenomenon was observed for luminogens 1 and 2 containing alkyl chains of different lengths.

Fig. 7 (a) Photographic images of luminogen 1 under the irradiation of UV light at 365 nm: the unground sample and the entirely ground sample. (b) Photographic images of luminogen 2 under the irradiation of UV light at 365 nm: the unground sample and the entirely ground sample.

Experimentals

General

All manipulations were accomplished 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 1-a,¹⁴ 2-a¹⁴ and C₆F₅Au(tht) (tht = tetrahydrothiophene)¹⁵ were synthesized 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 measured using EI (TRACE MS2000). Elemental analyses (C, H, N) were performed by the Microanalytical Services, College of Chemistry, CCNU. UV-Vis spectra were recorded on U-3310 UV Spectrophotometer. Fluorescence spectra were obtained 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 various water fractions were prepared by slowly adding ultra-pure water into the DMF solution containing small amounts 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 ZEN3600 (Malvern, UK) with a 173° back scattering angle and He–Ne laser (λ = 633 nm).

Synthetic procedures for 1-b and 2-b

A mixture of compound 1-a or 2-a (5 mmol) and formic acid (30 ml) was stirred for overnight at 110 °C. After completion of present reaction, formic acid was removed from reaction system by 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 product in a yield of 64%, 68%, respectively. 1-b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.78–8.42 (m, 2H), 7.67–7.62 (m, 2H), 7.49–7.45 (m, 1H), 7.33–7.27 (m, 3H), 7.08–7.05 (m, 1H), 1.99–1.90 (m, 4H), 1.13–1.02 (m, 12H), 0.78–0.73 (m, 6H), 0.60 (t, J = 8 Hz, 4H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.1, 159.0, 152.6, 151.9, 150.5, 150.3, 140.5, 140.1, 138.5, 137.7, 136.2, 135.7, 126.9, 126.8, 126.7, 122.7, 120.6, 119.9, 119.4, 119.3, 118.4, 117.7, 114.5, 113.3, 55.2, 40.3, 31.4, 29.6, 23.6, 22.5, 13.9. EI-MS: m/z = 377.25[M]⁺. Anal. calcd for C₂₆H₃₅NO: C, 82.71; H, 9.34; N, 3.71. Found: C, 82.77; H, 9.38; N, 3.66. 2-b: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.74–8.42 (m, 1H), 7.67–7.44 (m, 3H), 7.31 (s, 3H), 7.19–7.02 (m, 2H), 1.96 (t, J = 6 Hz, 4H), 1.10–1.04 (m, 4H), 0.69–0.57 (m, 10H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.5, 158.8, 145.1, 144.4, 143.2, 142.9, 141.1, 140.8, 139.3, 138.6, 135.6, 135.3, 127.0, 126.8, 126.5, 125.0, 120.8, 120.2, 119.7, 119.6, 118.7, 118.1, 117.1, 116.0, 37.0. EI-MS: m/z = 321.26[M]⁺. Anal. calcd for C₂₂H₂₇NO: C, 82.20; H, 8.47; N, 4.36. Found: C, 82.26; H, 8.51; N, 4.31.

Synthetic procedure for 3-a

A mixture of compound 2-aminofluorene (11 mmol) and formic acid (30 ml) was stirred for overnight at 110 °C. After completion of present reaction, collecting the white solid product by suction filtration. Yield = 58%. ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 8.75–8.41 (m, 1H), 8.11–7.91 (m, 1H), 7.75–7.70 (m, 2H), 7.53 (t, J = 8 Hz, 1H), 7.40–7.28 (m, 3H), 7.10 (d, J = 4 Hz, 1H), 3.89 (d, J = 8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 162.5, 158.8, 145.1, 144.4, 143.2, 142.9, 141.1, 140.8, 139.3, 138.6, 135.6, 135.3, 127.0, 126.8, 126.5, 125.0, 120.8, 120.2, 119.7, 119.6, 118.7, 118.1, 117.1, 116.0, 37.0. EI-MS: m/z = 209.02[M]⁺. Anal. calcd for C₁₄H₁₁NO: C, 80.36; H, 5.30; N, 6.69. Found: C, 80.41; H, 5.34; N, 6.63.

Synthetic procedures for 1-c and 2-c

A CH₂Cl₂ suspension (15 ml) of 1-b or 2-b (2.5 mmol) and triethylamine (5 ml) was cooled to 0 °C. To the mixture was added dropwise a CH₂Cl₂ solution (10 ml) of triphosgene (2.8 mmol). The mixture was refluxed under an argon atmosphere 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 light yellow liquid or green solid in a yield of 67%, 66%, respectively. 1-c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.70–7.66 (m, 2H), 7.35–7.33 (m, 5H), 1.98–1.92 (m, 4H), 1.13–1.03 (m, 12H), 0.77 (t, J = 6 Hz, 6H), 0.60–0.52 (m, 4H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.5, 152.0, 151.0, 142.2, 139.3, 128.1, 127.0, 125.3, 124.9, 122.9, 120.9, 120.2, 120.2, 55.3, 40.1, 31.4, 29.5, 23.6, 22.5, 13.9. EI-MS: m/z = 359.41[M]⁺. Anal. calcd for C₂₆H₃₃N: C, 86.85; H, 9.25; N, 3.90. Found: C, 86.89; H, 9.32; N, 3.84. 2-c: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.71–7.67 (m, 2H), 7.37–7.33 (m, 5H), 2.02–1.90 (m, 4H), 1.13–1.03 (m, 4H), 0.68 (t, J = 8 Hz, 6H), 0.59–0.51 (m, 4H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.8, 152.0, 150.9, 142.2, 139.2, 128.1, 127.0, 125.3, 124.9, 122.9, 120.9, 120.2, 120.1, 55.3, 39.9, 25.8, 22.8, 13.6. EI-MS: m/z = 303.21[M]⁺. Anal. calcd for C₂₂H₂₅N: C, 87.08; H, 8.30; N, 4.62. Found: C, 87.14; H, 8.35; N, 4.57.

Synthetic procedure for 3-b

A CH₂Cl₂ suspension (15 ml) of 3-a (3.8 mmol) and triethylamine (5 ml) was cooled to 0 °C. To the mixture was added dropwise a CH₂Cl₂ solution (10 ml) of triphosgene (4.2 mmol). The mixture was refluxed under an argon atmosphere 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 light yellow solid in a yield of 80%. ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.60 (d, J = 8 Hz, 1H), 7.55 (d, J = 4 Hz, 1H), 7.41 (d, J = 4 Hz, 1H), 7.33 (s, 1H), 7.27–7.21 (m, 3H), 3.69 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) = 163.6, 144.0, 143.4, 142.6, 139.8, 127.7, 127.0, 125.1, 125.0, 122.9, 120.3, 120.1, 36.6. EI-MS: m/z = 191.16[M]⁺. Anal. calcd for C₁₄H₉N: C, 87.93; H, 4.74; N, 7.32. Found: C, 87.87; H, 4.70; N, 7.38.

Synthetic procedures for 1 and 2

A mixture of C₆F₅Au(tht) (0.51 mmol) and 1-c or 2-c (0.5 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. The residues were purified by column chromatography, affording the expected yellow sticky product or white solid product in a yield of 69%, 63%, respectively. 1: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.79–7.74 (m, 2H), 7.53 (d, J = 4 Hz, 1H), 7.50 (s, 1H), 7.42–7.37 (m, 3H), 2.05–1.92 (m, 4H), 1.14–1.01 (m, 12H), 0.77 (t, J = 8 Hz, 6H), 0.60–0.54 (m, 4H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.86, −158.34, −163.21. Anal. calcd for C₃₂H₃₃AuF₅N: C, 53.12; H, 4.60; N, 1.94. Found: C, 53.21; H, 4.68; N, 1.87. 2: ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.79–7.74 (m, 2H), 7.55–7.47 (m, 2H), 7.44–7.37 (m, 3H), 2.06–1.93 (m, 4H), 1.14–1.05 (m, 4H), 0.69 (t, J = 8 Hz, 6H), 0.61–0.50 (m, 4H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.54, −158.15, −163.02. Anal. calcd for C₂₈H₂₅AuF₅N: C, 50.38; H, 3.78; N, 2.10. Found: C, 50.45; H, 3.70; N, 2.19.

Synthetic procedure for 3

A mixture of C₆F₅Au(tht) (1.03 mmol) and 3-b (1 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 white solid product by suction filtration. Yield = 63%. ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.89–7.83 (m, 2H), 7.71 (s, 1H), 7.63–7.56 (m, 2H), 7.48–7.41 (m, 2H), 3.99 (s, 2H). ¹⁹F NMR (CDCl₃): δ (ppm) = −116.95, −158.43, −163.33. Anal. calcd for C₂₀H₉AuF₅N: C, 43.26; H, 1.63; N, 2.52. Found: C, 43.34; H, 1.69; N, 2.45.

Conclusions

In summary, three fluorene-based complexes containing mononuclear gold(I) units have been synthesized. Through study of their aggregation-induced properties it was revealed that all of these gold(I) complexes exhibited excellent AIE characteristics. Subsequent mechanochromic studies showed that complex 3 possessed reversible mechanochromic behavior, while complexes 1 and 2 containing alkyl chains of different lengths did not exhibit any mechanochromic behavior upon grinding, suggesting the effect of alkyl chain played a very important role in mechanochromism characteristics of complexes 1–3. The mechanism for AIE characteristics of 1–3 and mechanochromic behavior of 3 possibly involves a change in weak intermolecular π⋯π and C–H⋯F interactions and the generation of intermolecular gold–gold interactions. Undoubtedly, the results of this research should be helpful for the design of mechanical-stimuli-responsive gold(I) complexes with AIE property. Further studies on new gold(I) complexes possessing other fascinating characteristics are in progress.

Acknowledgements

The authors acknowledge financial support from National Natural Science Foundation of China (No. 21272088, 21472059 and 21402057).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra, mass spectra, size distribution curves of complexes 1–3 (2.0 × 10⁻⁵ mol L⁻¹) in DMF–water mixtures with 90% volume fraction of water. See DOI: 10.1039/c6ra17806e

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