Twisted Donor-Ï-Acceptor Carbazole Luminophores With Substituent

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Twisted Donor-#-Acceptor Carbazole Luminophores With Substituent-Dependent Properties of Aggregated Behavior (ACQ to AIEE) and Mechano-Responsive Luminescence Huichao Zhu, Pengfei Chen, Lin Kong, Yupeng Tian, and Jiaxiang Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05638 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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The Journal of Physical Chemistry

Twisted Donor-π-Acceptor Carbazole Luminophores With Substituent-Dependent Properties of Aggregated Behavior (ACQ to AEE) and Mechano-Responsive Luminescence Huichao Zhu, Pengfei Chen, Lin Kong, Yupeng Tian and Jiaxiang Yang* College of Chemistry and Chemical Engineering, Anhui University, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Hefei 230601, P. R. China. E-mail: [email protected].

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Abstract In order to get insight how the donor (D)/acceptor (A) substitutions and their substitution pattern affect optical properties in solution and in the solid-state, four novel carbazole derivatives C1-C4 with different terminal groups (N(CH3)2, H, Br, CN), have been employed. Their single crystal X-ray structures confirm that the twisted structures, intermolecular interactions and loose molecular packing are function of the terminal groups. Interestingly, they displayed substituent-dependent aggregation induced behaviors and mechanochromic (MC) properties. C1 showed aggregation-caused quenching (ACQ), due to the strong intermolecular π-π interaction. While C2, C3 and C4 exhibit strong aggregation-enhanced emission (AEE) characteristics, which contributed to the formation of J-aggregates. Different twisted conformations and molecular arrangements induce different mechano-stimuli response behaviors for C1-C4 between green-yellow to red color along with large red-shifts of 63, 30, 56, and 49 nm, respectively. C1, C3 and C4 displayed obvious mechanochromic characteristics, but C2 performed irreversible mechano-stimulus behavior. Powder X-ray diffraction and single crystal X-ray analysis revealed that the planarization of twisted structure extended the π-conjugation and the destroy of crystalline structure account for the tuned solid-state fluorescence.

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INTRODUCTION Organic luminescent materials that can change their solid-state fluorescence emission color in response to external mechanical stimuli (such as grinding, cutting, etc), named mechanochromic (MC) materials1-7, have gained increasing interest for their wide applications as information security, memory device and mechano-sensors.8-14 A number of MC materials have been synthesized to date.15-20 However, MC materials, compared with other materials, remain limited in number, owing to the lack of a clear guideline on the design strategy.11,19,21-23 Especially, another main reason is the fact that many luminophores with π-conjugated system generally suffer from aggregation-enhanced quenching (ACQ) effect and display very weak emission or even become non-emissive in their aggregated states, which great hinder their practical use.24-26 To overcome this problem, the aggregation-induced emission (AIE) or aggregation-enhanced emission (AEE)-active molecules that can perform strong emission in the aggregated states, is a ideal building blocks for MC materials. In 2010, Park and his partners reported a novel cyano-distyrylbenzene derivative that displayed AIE and MC characteristics.15 Recently, a number of MC materials based on AIE-active (AIE-MC) molecules have been developed.27-31 However, even according to the AIE or AEE concept, the definite relationship between the structure and the MC behaviors is not clear. Therefore, a deep understanding between the molecular structure and the properties are still important, and carrying out more extensive explorations to broaden the MC family are attractive and challenging.32-36 Many investigations confirmed a slight different in chemical structure can strongly affect the molecular conformation, intermolecular interactions and packing modes in the solid state, which inducing different photophysical properties, especially for MC molecules.16, 37-40 Therefore, studies on a group of structural derivative are an effective way to understand the relationship between structure and MC characteristics. Recently, we found that these molecules based on carbazole-malononitrile display obvious MC characteristics, and strong solid-state luminescence.41-42 Herein, we synthesized four new twisted D-π-A molecules C1, C2, C3 and C4 (Figure 1). The four fluorophores 3



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possess the same molecular conjugated backbone functioned by different substituents (N(CH3)2, H, Br, CN) to adjust the molecular conformation, intermolecular interactions and stacking modes and investigated the influence of molecular structures on photo-physical properties (such as AEE and MC properties). It was found that the aggregated fluorescence emission dependent upon substituent on the terminal unit.

EXPERIMENTAL SECTION Materials and methods The absorption spectra in the solid state were recorded by Hitachi U-41000 UV-Vis-NIR spectrometer. The fluorescence spectra were obtained by using a Hitachi F-4500 fluorescence spectrophotometer with the samples placed between quartz plates. The absolute solid-state fluorescence quantum yields (ΦF) were measured on HORIBA FluoroMax-4 spectrofluorometer. The 1H and

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C NMR spectra were

carried out on a Bruker Avance 400 MHz (CDCl3 as solvent) with tetramethylsilane (TMS) as the internal reference. The powder X-ray diffraction (PXRD) measurements were conducted on a Rigaku Smart Lab X-ray diffractometer with the 2θ range from 5° to 80°. Single crystal XRD patterns were collected with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) on a Bruker SMART II CCD area detector. CCDC Nos. 1505339 (C1), 1541386 (C2), 1509254 (C3), 1554722 (C4) contain the supplementary crystallographic data for this paper.

Figure 1 Molecular Structures of compounds C1-C4

The target compounds C1-C4 were prepared according to our reported works.41, 42 The products were characterized by NMR spectra and mass spectra. C1. Red-color powder (Yield: 43%) 1H NMR (400 MHz, CDCl3) δ: 0.99 (t, J = 4


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7.32 Hz, 3H), 1.42-1.51 (m, 2H), 1.87-1.95 (m, 2H), 3.06 (s, 6H), 4.35 (t, J = 7.20 Hz, 2H), 6.63 (d, J = 8.92 Hz, 2H), 6.90 (d, J = 15.16 Hz, 1H), 7.27 (d, J = 7.20 Hz, 1H), 7.41-7.54 (m, 7H), 8.10 (m, 2H).

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C NMR (100 MHz, CDCl3) δ: 13.9, 20.6, 31.2,

40.1, 43.2, 108.9, 109.2, 111.8, 114.8, 115.5, 119.7, 120.0, 120.7, 122.0, 122.5, 122.6, 122.9, 124.2, 126.6, 127.0, 131.2, 141.0, 141.8, 150.3, 152.6. MS (APCI) m/z calcd for C30H28N4: 444.23, found 445.23 [M + H]+. C2: Yellow-color powder (Yield: 40%) 1H NMR (400 MHz, CDCl3) δ: 1.00 (t, J = 7.32 Hz, 3H), 1.42-1.51 (m, 2H), 1.87-1.95 (m, 2H), 4.36 (t, J = 7.24 Hz, 2H), 6.99 (d, J = 15.52 Hz, 1H), 7.29 (d, J = 7.40 Hz, 1H), 7.39-7.56 (m, 9H), 7.68 (d, J = 15.52 Hz, 1H), 8.10 (d, J = 7.76 Hz, 1H), 8.17 (s, 1H).

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C NMR (100 MHz, CDCl3) δ: 13.9,

20.6, 31.2, 43.2, 109.1, 109.3, 113.7, 114.5, 120.0, 120.8, 122.2, 122.4, 123.1, 123.3, 125.5, 126.8, 127.0, 128.7, 129.2, 131.4, 134.6, 141.1, 142.2, 149.3, 172.2. MS (APCI) m/z calcd for C28H23N3: 401.19, found 402.20 [M + H]+. C3: Yellow-color powder (Yield: 70%) 1H NMR (400 MHz, CDCl3) δ: 0.99 (t, J = 7.32 Hz, 3H), 1.41-1.51 (m, 2H), 1.86-1.94 (m, 2H), 4.36 (t, J = 7.24 Hz, 2H), 6.90 (d, J = 15.56 Hz, 1H), 7.29 (d, J = 6.92 Hz, 1H), 7.37-7.39 (m, 2H), 7.46 (d, J = 8.24 Hz, 1H), 7.51-7.56 (m, 5H), 7.64 (d, J = 15.56 Hz, 1H), 8.10 (d, J =7.72 Hz, 1H), 8.15 (s, 1H). 13C NMR (100 MHz, CDCl3) δ: 13.9, 20.6, 31.2, 43.2, 109.2, 109.3 109.5, 113.6, 114.3, 120.0, 120.8, 122.2, 122.4, 123.0, 123.1, 125.9, 126.0, 126.9, 129.9, 132.4, 133.5, 141.1, 142.2, 147.6, 171.8. MS (APCI) m/z calcd for C28H22N3Br: 479.10, found 480.58 [M + H]+. C4: Yellow-color powder (Yield: 46%) 1H NMR (400 MHz, CDCl3) δ: 0.99 (t, J = 7.32 Hz, 3H), 1.41-1.50 (m, 2H), 1.87-1.94 (m, 2H), 4.36 (t, J = 6.76 Hz, 2H), 6.95 (d, J = 15.60 Hz, 1H), 7.30 (d, J = 7.48 Hz, 1H), 7.47-7.75 (m, 9H), 8.10 (d, J =7.76 Hz, 1H), 8.16 (s, 1H). 13C NMR (100 MHz, CDCl3) δ: 13.9, 20.6, 31.2, 43.3, 109.3, 109.4, 113.2, 113.3, 114.0, 118.2, 120.2, 120.8, 122.2, 122.3, 122.7, 123.2, 126.9, 127.0, 128.7, 128.8, 132.8, 138.6, 141.1, 142.3, 146.1, 171.1. MS (APCI) m/z calcd for C29H22N4: 426.18, found 427.19 [M + H]+.

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RESULTS & DISCUSSION SECTION Theoretical calculations To well investigated the electronic features of C1-C4 constituting four terminal groups, the density functional theory (DFT) calculation on C1-C4. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated by using the B3LYP/6-31G* level. As shown in Figure 2, the LUMOs of these compounds are located mainly at the substituted phenyl ring and malonitrile unit, which is due to the strong electron-withdrawing of malonitrile unit. When the substitution is a strong electron donor (R = N(CH3)2), the HOMOs is mainly centered on the substituted phenyl ring unit. If no electron terminal donor appears (R = H, Br, CN), the HOMOs are distributed over the carbazole unit, which is obviously different to that of C1. The calculated band gaps of C1-C4 are 2.608, 2.507, 2.232, 2.040 eV, respectively. The band gaps showed a decrease with the ability of electron-donating of terminal groups decreasing. The results indicate that the electronic structure of these compounds can be influenced by the terminal groups with different electron-donating ability, which may tune their photo-physical properties.

Figure 2 The molecular frontier orbitals of C1-C4

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Single crystal X-Ray diffraction study

Figure 3 The crystal structures of compound C1-C4

The single crystals of C1-C4 suitable for X-ray diffraction (XRD) analysis, were obtained by slow evaporation (DCM and ethanol) at room temperature. The corresponding crystallographic data are listed in Table S1. The results showed that crystals of C1, C2 and C3 are monoclinic with the P21/c space group, while the unit cell of C4 are belong to orthorhombic with the space group P212121. As shown in Figure 3, the crystal structures of these compounds adopt twisted structures. In C1, the dihedral angles between the carbazole unit (plane A) and the malononitrile unit (plane B), the plane A and the styrene unit (plane B) are 63.86° and 8.98°, respectively, and the angle between the plane B and plane C is 55.40°. In C2, the corresponding dihedral angles are 60.85°, 8.31° and 55.64, respectively, while these are 57.21°, 28.87°, 76.18°, respectively for C3 and 62.57°, 29.64°, 83.83°, respectively for C4. The dihedral angle between the plane B and C increased with the electron-donating decreasing of terminal groups (55.40° < 55.64° < 76.18° < 83.83°), which can indicate that the molecular structures are more twisted. We further investigated the molecular interactions and the packing modes of C1-C4 (Figure 4). The molecules of C1 are stabilized by C-H···π (2.577 Å) and hydrogen bond interaction C-H···N (2.724 Å). Additionally, there existed strong π-π stacking interaction with a distance 3.366 Å between the benzene rings connected the terminal (dimethylamino) group. These multiple intermolecular interactions induced C1 with a 7



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stagger layer arrangement, and the stagger layer arrangement connected by the π-π stacking interaction of 3.511 Å between the carbazole rings. Table 1 The molecular interactions of crystals C1-C4 C1

C2

C3

C4

C9-H9A···π 2.577 Weak interaction

C9-H9B···N3

(Å)

π···π

3.366

π···π

3.511

2.724

C7-H7···N3 C12-H12···N3

2.722

C15-H15···π 2.878

2.640

C24-H24···π 2.856

C10-H10···N3

2.617

C2-H2···N3

C13-H13···N4

2.658

C14-H14···N4

2.735

2.727

For C2, the molecules connected by C-H···N (2.722 Å) and C-H···π (2.878 Å) with a linear monolayer J-type packing. And between benzene rings connecting the terminal unit exits weak intermolecular interaction with a distance of 7.814 Å. While C3 also adopts a characteristic of J-type packing with the molecule connected by molecular interactions C-H···N (2.727 Å) and C-H···π (2.878 Å). Additionally, there exits weak intermolecular interaction with a distance of 7.909 Å. When the terminal unit is CN group, C4 showed a inserted J-type packing that can considered as herringbone packing mode, with the intermolecular distance of 8.193 and 8.227 Å. These results indicated that the terminal groups have an effective influence on the molecular conformation, intermolecular interactions and packing modes.

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Figure 4 The molecular arrangement and molecular interactions of C1 (a) C2 (b), C3 (c) and C4 (d).

Fluorescence behaviors in the aggregated state The photophysical properties of C1-C4 in the aggregated state were conducted in DMF/H2O mixtures, with the concentration is 1 × 10-5 mol/L. DMF is a good solvent for these compounds and water as a poor solvent, the degree of aggregation can be adjusted by changing the water fraction from 0 to 100%.43-45 As shown in Figure 5, the fluorescence emission of C1 showed an obvious decrease and a red-shift band with the water added into the mixtures. While C2-C4 displayed excellent aggregation-enhanced emission (AEE) properties. For C2-C4, C4 as an example, when the water fraction (ƒw) was lower than 50%, the fluorescence emission showed weak or no fluorescence emission and no obvious change, when more water was added (50-70%), the fluorescence intensities extremely increased and the fluorescence increases are 55-fold for C2, 24-fold for C3 and 60-fold for C4, when the fluorescence intensities achieve the maximum. However, with the addition of more water (ƒw > 70%), the fluorescence intensity decreased. The fluorescence images of the compounds in the DMF/H2O mixture solvents under 365 nm UV light were shown in Figure S10. It is clear that C1 showed strong fluorescence in DMF solution, but very weak fluorescence in DMF/H2O mixture with a high ƒw. While C2, C3 and C4 displayed weak fluorescence emission in DMF and strong fluorescence emission in DMF/H2O mixture with a high water fraction. This further indicating that the compound C1 had an ACQ effect, while C2, C3, and C4 exhibited strong AIE characteristics.

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Figure 5 The fluorescence emission spectra of C1 (a), C2 (b), C3 (c) and C4 (d) in DMF/H2O solvents with different water fraction

For C2-C4, in lower water fraction, the molecules were in isolated state, in which the molecules can freely rotate and induce a weak fluorescence emission. With the water was added to the mixture system, the absorption band for C2-C4 showed obvious decrease and a red-shift with a long tail, indicating the transition from the isolated molecules to the nano-aggregates. The formation of aggregates suppressed the free rotation of molecules and resulted in an enhancement of fluorescence, and the aggregates is J-type, which confirmed by the single crystal X-ray analysis (Figure 4a).46-47 However, in a higher water faction, the molecules aggregate quickly to nano-particles, which may be the reason to the decrease of fluorescence intensities. And the J-aggregation formation should also be account for the red-shift of the fluorescence emission. For C1, multiple molecular interactions may induced C1 with a stagger layer arrangement, and exhibit strong π-π interaction (3.366 Å), which would be responsible for the fluorescence decrease for C1.

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Figure 6 The fluorescence spectra (a) and fluorescence emission decay curves (b) of C1-C4.

The fluorescence behaviors in the solid state of C1-C4 were also investigated. As shown in Figure 6 and Table 2, the emission colors of these compounds can be tuned from yellow-green to red by the terminal moieties, which are derived from different electron-donating abilities. The emission maximums in the solid samples of C1-C4 were observed at 616, 554, 534, and 568 nm, respectively. In crystal analysis, the molecular conformation of C1 adopts a good planar compare with that of C2-C4. Additionally, multi C-Hπ interactions and strong π-π stacking induced C1 with a longer π-conjugated system, which would be the reason to the long wavelength fluorescence emission (Figure 4). With the terminal group was changed, twisted conformation and weak molecular interactions of C2 and C3 made the molecule arrangement more loose and the fluorescence emission showed blue-shift compare with that of C1. For C4, four kind of strong hydrogen bond interactions induced that the molecule are tightly bound together and showed a compact J-aggregation arrangement, which is benefit to the longer π-conjugated system. Thus, comparing with C2 and C3, the red-shifted emission of C4 would be considered to the result of the longer π-conjugated system. Additionally, the changing of the terminal groups also can influence the fluorescence quantum yield and the fluorescence lifetime of C1-C4 (Figure 6b and Table 2).

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Table 2 The fluorescence parameters of C1-C4 in the solid state Fluorescence decay samples

λem (nm)

Φ (%)

τ (ns)

Kf (s-1)

Knr (s-1)

C1

616

14

0.58

2.41 × 108

1.48 × 109

C2

554

20

0.43

4.65 × 108

1.86 × 109

C3

534

8.9

2.90

3.07 × 107

3.14 × 108

C4

568

11

0.91

1.21 × 108

9.78 × 108

Mechanochromic behavior

Figure 7 The photographs of all compounds in different solid state under UV lamp (365 nm).

In our previous study, we found that for these π-conjugated compounds with twisted conformation, AIE feature, usually displayed tuned solid-state fluorescence emission by grinding.41-42 To check whether the four luminogens exhibit MC characteristics, the solid fluorescence emission in different state were studied. As shown in Figure 7, the original C1, C2, C3 and C4 solids emit red, yellow, yellow-green and bright yellow light under UV irradiation (365nm), respectively. And upon grinding, their fluorescence color changed into dark red, orange, yellow and deep yellow, respectively. These observations indicate that C1-C4 exhibit obvious 12


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MC behaviors. The fluorescence spectra were conducted to further monitor the fluorescence changes, and the fluorescence emission bands of samples in different solid state (original, ground and fumed) are listed in Figure 8. The fluorescence emission peaks of C1-C4 in original powders are at 616, 554, 534 and 568 nm, respectively, and ground powders, it red-shifted to 679, 589, 590 and 617 nm, respectively. These indicated that the original powders showed an obvious red-shift upon grinding with the spectral shifts of 63, 30, 56, 49 nm for C1-C4, respectively. Additionally, after treated with ethanol, fluorescence color and the maximum emission peaks of C1, C3 and C4, were recovered to the original ones. While after treated by ethanol, the ground powder of C2 turned to be orange-red powder with the wavelength at 589 nm. The fluorescence changes of C1, C3 and C4 in the solid state could be cycled many times, which indicating excellent reversibility of this behavior (Figure S11).

Figure 8 The fluorescence spectra of C1 (a), C2 (b), C3 (c) and C4 (d) in different solid state.

To gain more insight the mechanism of MC behaviors of C1-C4, the XRD measurements were conducted on solid powders in different state and the XRD patterns are illustrated in Figure 9. It is clear that the diffraction patterns of the 13



original samples display intense and sharp diffraction peaks, which indicating that a well crystalline structure. When it was ground, the sharp diffraction peaks disappear or its intensity sharply decrease, suggesting that the crystalline structure was destroyed. After the ground samples of C1, C3 and C4 were treated with ethanol, the sharp and intense diffraction peaks recovered again, which are similar to those of original samples. These results strongly suggest that the C1, C3 and C4 have excellent reversible MC characteristics, which are based on their well crystalline structures destroyed by grinding. Interestingly, the XRD curve of C2 shows that the sharp diffraction peaks of fumed C2 is absolutely different to those of original C2, which indicating a new crystalline structure after treated by ethanol (Figure 9b).

Figure 9 The XRD curves of compound C1 (a), C2 (b), C3 (c) and C4 (d).

It is well known that the destruction of the crystalline structure can lead to the planarization of the molecular conformation.48-50 This would be one of the possible reasons for the increase in the π-conjugated system, which induced a red-shift in the fluorescence spectra. In the crystal analysis, we can obtain that these luminogens exhibit twisted molecular conformation and relatively loose packing modes. However, in this case, the red-shifts upon grinding would be original form the conformation change and molecular packing destroyed, which can effectively extend the 14


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π-conjugation. For C2 and C3, their molecular arrangements with J-type modes are more loose, comparing with C1 and C4, which are due to the weak molecular interactions. Therefore, C2 and C3 displayed excellent MC activities and the planarization of the molecular conformation and the loose packing modes account for these MC behavior.

CONCLUSIONS In summary, four twisted D-π-A type carbazole derivatives substituted different terminal groups, C1, C2, C3 and C4, were synthesized and characterized by NMR, MS and single-crystal analysis. The HOMOs and LUMOs for these luminogens showed obvious change by changing the terminal groups. Importantly, the molecular conformation, aggregation behaviors and MC characteristics were modulated via D/A substituents. Molecules of C1 tend to form strong intermolecular π-π interactions upon aggregation, which cause the fluorescence quenching. While C2-C4 showed excellent aggregation-enhanced emission, which is due to the formation of J-aggregates. These four dyes performed obvious tuned solid-state fluorescence in response to the mechanical stimuli (grinding), which can ascribed to the planarization of the molecular conformation and the loose packing modes destroyed. The terminal groups can influence the molecular spatial conformation by the different steric hindrance, and it also can tune the molecular interactions, induced absolutely different molecular packing modes. The structure-property relationship have been studied, would provide some useful information for designing new AIE-MC luminogens, which can have potential application in optical recording materials.

SUPPORTING INFORMATION AVAILABLE The synthesis routes of C1-C4; NMR spectrum of C1-C4 in CDCl3; Crystal data of C1-C4; The absorption spectra of C1, C2, C3 and C4 in DMF/H2O with different water fraction (ƒw); The fluorescence graphs in DMF/water mixtures under UV illumination (365 nm); Reversible switching of solid-state emission wavelength for 15



C1, C3 and C4 by grinding and fuming.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51673001 and 51432001) and the Educational Commission of Anhui Province of China (KJ2014ZD02).

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