Carbazole-based highly solid-state emissive fluorene derivatives with various mechanochromic fluorescence characteristics

Journal Pre-proof Carbazole-based highly solid-state emissive fluorene derivatives with various mechanochromic fluorescence characteristics Shuai Tan, Ya Yin, Wenzhuo Chen, Zhao Chen, Wei Tian, Shouzhi Pu PII:

S0143-7208(19)33026-8

DOI:

https://doi.org/10.1016/j.dyepig.2020.108302

Reference:

DYPI 108302

To appear in:

Dyes and Pigments

Received Date: 25 December 2019 Revised Date:

21 February 2020

Accepted Date: 21 February 2020

Please cite this article as: Tan S, Yin Y, Chen W, Chen Z, Tian W, Pu S, Carbazole-based highly solidstate emissive fluorene derivatives with various mechanochromic fluorescence characteristics, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108302. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Carbazole-based highly solid-state emissive fluorene derivatives with various mechanochromic fluorescence characteristics Shuai Tana,1, Ya Yina,1, Wenzhuo Chenb,1, Zhao Chena,*, Wei Tianb, Shouzhi Pua,** Synopsis Four carbazole-based fluorene derivatives were prepared. These compounds showed color-tunable highly aggregative-state fluorescence. Furthermore, the emission behaviors of these luminogens could be tuned by mechanical force.

Carbazole-based highly solid-state emissive fluorene derivatives with various mechanochromic fluorescence characteristics Shuai Tana,1, Ya Yina,1, Wenzhuo Chenb,1, Zhao Chena,*, Wei Tianb, Shouzhi Pua,**

a

Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China.

b

MOE Key Laboratory of Material Physics and Chemistry under Extraordinary

Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, School of Science, Northwestern Polytechnical University, Xi’an 710072, P. R. China

Tel: +86-791-83831996 *

**

1

Fax: +86-791-83831996

Corresponding author E-mail: [email protected] Corresponding author E-mail: [email protected] These authors contributed equally to this work.

ABSTRACT Four carbazole-based fluorene derivatives 1-4 have been successfully prepared. All these compounds showed highly solid-state emissive feature with various fluorescence. The aggregation-induced emission effect of compound 1 was investigated by the systematic research of photoluminescence spectroscopy, and

the

results

indicated

that

luminogen

1

displayed

obvious

aggregation-induced yellow light-emitting phenomenon. In addition, the 1

solid-state emission behaviors of these fluorescent molecules could be tuned by mechanical force. More specifically, luminogens 1 and 2 showed reversible mechanochromic fluorescence conversion between blue-green and yellowish brown emission colors, luminogens 3 and 4 exhibited reversible mechanochromic fluorescence conversion involving color changes from green or yellow to yellowish brown. Furthermore, the repeatabilities of their mechanofluorochromism phenomena were excellent. The powder XRD results confirmed that the morphology conversion between crystalline and amorphous phases was responsible for the mechanofluorochromic characteristics of 1-4. This work provides valuable reference for the exploitation of high-contrast mechanochromism materials. Keywords: Carbazole; Fluorene; Various fluorescence; Aggregation-induced emission; Mechanofluorochromism; Morphology conversion

1. Introduction Over the recent decades, organic and organometallic mechanochromic fluorescence dyes that can exhibit tunable solid-state emission characteristics have attracted substantial interest because these “smart” materials can potentially be applied in various fields such as sensors and memory devices [1-16]. Meanwhile, the development of highly solid-state emissive organic fluorescent dyes has also attracted widespread attention on account of their promising applications in organic light-emitting diodes and biological imaging [17-24]. However, it is well known that the corresponding luminescence is often weakened or quenched because of the 2

formation of detrimental species such as excimers and exciplexes when luminous materials are aggregated in the condensed phase [25]. Generally, this phenomenon is called aggregation-caused quenching (ACQ). Obviously, the ACQ phenomenon is disadvantage to the practical application of solid-state light-emitting materials. Fortunately, the discovery of aggregation-induced emission (AIE) effect provides a straightforward and effective solution to overcome the notorious ACQ problem [26-28], and luminophors with AIE behavior usually show strong solid-state emission. Indeed, the exploitation of highly solid-state emissive luminogens is currently a hot research topics. Bright solid-state emission and a prominent color contrast before and after grinding are two very important factors for the practical applications of mechanical stimulus-responsive materials [29-31]. In general, luminogens containing some rotatable units are very beneficial to the realization of bright solid-state fluorescence. Therefore, the design and preparation of luminogens with rotatable groups offers the possibility for the acquisition of high-performance mechanical force-responsive materials, and this kind of mechanochromic fluorophores are promising candidates for high-contrast mechanofluorochromic materials. Oligomers and polymers with fluorene or carbazole as a skeleton are very valuable candidates in the field of optoelectronic devices because of their easily tunable electronic properties and facile structural fragility. Unfortunately, the ACQ effect largely hinders the effective application of these fluorene or carbazole-based materials. Thus, the design and synthesis of fluorene or carbazole-based luminogens 3

with highly solid-state emission feature is urgent and challenging. In this work, we reported four novel fluorescent compounds containing carbazole and fluorene units. Introducing the trifluoromethyl groups into the molecules 2-4 possibly induced the formation of weak intermolecular C-H···F interaction, which was advantageous to the generation

of

mechanochromic

phenomena.

Solid-state

emission

and

mechanochromic fluorescence characteristics of these luminogens 1-4 were investigated by solid-state photoluminescence (PL) spectroscopy. Interestingly, all these compounds exhibited different solid-state fluorescence. Furthermore, these fluorescent molecules showed reversible mechanofluorochromic behaviors.

Chart 1. The molecular structures of compounds 1-4. 2. Materials and methods 2.1. Experimental General: All operations were carried out under an argon circumstance using standard Schlenk

techniques,

unless

otherwise

(4-(9H-carbazol-9-yl)phenyl)boronic 2,7-dibromo-9H-fluoren-9-one,

stated. acid,

The

starting

9H-fluoren-9-one,

(2-(trifluoromethyl)phenyl)boronic 4

materials

acid,

(3-(trifluoromethyl)phenyl)boronic acid, (4-(trifluoromethyl)phenyl)boronic acid purchased from Alfa Aesar were used as received. The other starting materials were purchased from J&K Chemical Ltd. (Shanghai, China). All reagents were obtained as analytical-grade from commercial suppliers and used without further purification. Compounds 1-1, 2-1, 3-1 and 4-1 were prepared by procedures described in the literature [32]. 1H NMR and

13

C NMR spectra were collected on American Varian

Mercury Plus 400 spectrometer (400 MHz) and Bruker AVANCE NEO 500 MHz FT-NMR Spectrometer (500 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).

13

C NMR chemical shifts reported in ppm (δ) relative to the

central line of triplet for CDCl3 at 77 ppm. Mass spectra were obtained using Thermo scientific DSQⅡand Bruker AmaZon SL Ion Trap Mass spectrometer. Elemental analyses (C, H, N) were carried out with a PE CHN 2400 analyzer. Fluorescence spectra were recorded on a Hitachi-F-4600 fluorescence spectrophotometer. 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). The X-ray crystal-structure determination of compound 4 was obtained on a Bruker APEX DUO CCD system. Fluorescence lifetimes were tested by FLS 1000, 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. 5

2.2. Crystallographic Details Single crystals of compound 4 suitable for X-ray analysis were obtained by slow diffusion of n-hexane into a dichloromethane solution containing small amounts of 4. A crystal of 4 with approximate dimensions of 0.26 × 0.24 × 0.22 mm3 for 4 was mounted on a glass fiber for diffraction experiment. Intensity data were collected on a Nonius Kappa CCD diffractometer with Mo Kα radiation (0.71073 Å) at room temperature. The structures were solved by a combination of direct methods (SHELXS-97) [33] and Fourier difference techniques and refined by full-matrix least-squares (SHELXL-97) [34]. All non-H atoms were refined anisotropically. The hydrogen atoms were placed in the ideal positions and refined as riding atoms. Detailed crystal information are showed in Table S1. Crystallographic data for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplemental publication CCDC 1971079. 2.3. Synthesis 2.3.1. General procedure for the synthesis of compound 1.

A

mixture

of

compound

1-1

(1.0

mmol,

0.33

g),

(4-(9H-carbazol-9-yl)phenyl)boronic acid (2.2 mmol, 0.63 g), Na2CO3 (10 mmol),

6

Pd(PPh3)4 (0.2 mmol) were stirred in THF (50 ml) and H2O (5 ml) for two days under an argon atmosphere at 80℃. After completion of present reaction, the mixture was extracted with dichloromethane (3 × 60 mL), and the combined organic layers were washed with brine, dried (Na2SO4), and concentrated in vacuo. The residues were purified by column chromatography using petroleum ether: dichloromethane (volume ratio = 1:1) as eluent, affording the expected white solid product in a yield of 69.2%. 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.18 (d, J = 8 Hz, 4H, Ar-H), 7.77 (d, J = 8 Hz, 2H, Ar-H), 7.72 (s, 8H, Ar-H), 7.59 (d, J = 8 Hz, 4H, Ar-H), 7.48 (t, J = 8 Hz, 4H, Ar-H), 7.34 (t, J = 6 Hz, 6H, Ar-H), 7.08 (t, J = 8 Hz, 2H, Ar-H), 6.86 (d, J = 8 Hz, 2H, Ar-H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 143.2, 141.5, 140.8, 140.7, 138.5, 137.9, 135.7, 131.5, 128.2, 127.4, 126.7, 126.1, 124.9, 123.7, 120.5, 120.3, 119.6, 109.8. EI-MS: m/z= 660.5[M]+. Anal. Calcd. For C50H32N2: C, 90.88; H, 4.88; N, 4.24. Found: C, 90.81; H, 4.85; N, 4.29. 2.3.2. General procedure for the synthesis of compound 2.

A

mixture

of

compound

2-1

(1.0

mmol,

0.62

g),

(4-(9H-carbazol-9-yl)phenyl)boronic acid (2.2 mmol, 0.63 g), Na2CO3 (10 mmol), Pd(PPh3)4 (0.2 mmol) were stirred in THF (50 ml) and H2O (5 ml) for two days under an argon atmosphere at 80℃. After completion of present reaction, the mixture was extracted with dichloromethane (3 × 60 mL), and the combined organic 7

layers were washed with brine, dried (Na2SO4), and concentrated in vacuo. The residues were purified by column chromatography using petroleum ether: dichloromethane (volume ratio = 1:2) as eluent, affording the expected white solid product in a yield of 66.6%. 1H NMR (500 MHz, CD2Cl2): δ (ppm) = 8.06 (d, J = 10 Hz, 4H, Ar-H), 7.79 (d, J = 10 Hz, 2H, Ar-H), 7.68 (t, J = 7.5 Hz, 6H, Ar-H), 7.53 (d, J = 10 Hz, 4H, Ar-H), 7.45 (t, J = 7.5 Hz, 2H, Ar-H), 7.40 (t, J = 7.5 Hz, 2H, Ar-H), 7.23 (m, 12H, Ar-H), 7.08 (s, 4H, Ar-H), 6.85 (s, 2H, Ar-H).

13

C NMR data of the

compound were not obtained due to the poor solubility. HRMS-ESI (m/z): 948.796 [M]+. Anal. Calcd. For C64H38F6N2: C, 81.00; H, 4.04; N, 2.95. Found: C, 81.05; H, 4.01; N, 2.89. 2.3.3. General procedure for the synthesis of compound 3.

A

mixture

of

compound

3-1

(1.0

mmol,

0.62

g),

(4-(9H-carbazol-9-yl)phenyl)boronic acid (2.2 mmol, 0.63 g), Na2CO3 (10 mmol), Pd(PPh3)4 (0.2 mmol) were stirred in THF (50 ml) and H2O (5 ml) for two days under an argon atmosphere at 80℃. After completion of present reaction, the mixture was extracted with dichloromethane (3 × 60 mL), and the combined organic layers were washed with brine, dried (Na2SO4), and concentrated in vacuo. The residues were purified by column chromatography using petroleum ether: dichloromethane (volume ratio = 1:2) as eluent, affording the expected faint yellow 8

solid product in a yield of 65.1%. 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.18 (t, J = 4 Hz, 4H, Ar-H), 7.90 (d, J = 8 Hz, 2H, Ar-H), 7.82 (d, J = 8 Hz, 4H, Ar-H), 7.77 (t, J = 8 Hz, 6H, Ar-H), 7.64 (d, J = 4 Hz, 1H, Ar-H), 7.62 (d, J = 4 Hz, 1H, Ar-H), 7.45 (t, J = 8 Hz, 4H, Ar-H), 7.40 (t, J = 4 Hz, 4H, Ar-H), 7.34 (m, 8H, Ar-H), 7.22 (m, 4H, Ar-H).

13

C NMR (100 MHz, CDCl3): δ (ppm) = 142.0, 141.1, 140.5, 134.0,

139.6, 138.4, 138.3, 131.5, 129.6, 129.5, 127.5, 127.3, 126.1, 124.0, 123.8, 123.7, 120.4, 120.3, 109.7. HRMS-ESI (m/z): 948.779 [M]+. Anal. Calcd. For C64H38F6N2: C, 81.00; H, 4.04; N, 2.95. Found: C, 81.03; H, 4.09; N, 2.92. 2.3.4. General procedure for the synthesis of compound 4.

A

mixture

of

compound

4-1

(1.0

mmol,

0.62

g),

(4-(9H-carbazol-9-yl)phenyl)boronic acid (2.2 mmol, 0.63 g), Na2CO3 (10 mmol), Pd(PPh3)4 (0.2 mmol) were stirred in THF (50 ml) and H2O (5 ml) for two days under an argon atmosphere at 80℃. After completion of present reaction, the mixture was extracted with dichloromethane (3 × 60 mL), and the combined organic layers were washed with brine, dried (Na2SO4), and concentrated in vacuo. The residues were purified by column chromatography using petroleum ether: dichloromethane (volume ratio = 1:2) as eluent, affording the expected yellow solid product in a yield of 63.9%. 1H NMR (500 MHz, CD2Cl2): δ (ppm) = 8.13 (d, J = 5 Hz, 4H, Ar-H), 7.86 (d, J = 5 Hz, 2H, Ar-H), 7.78 (d, J = 10 Hz, 4H, Ar-H), 7.73 (d, 9

J = 5 Hz, 4H, Ar-H), 7.59 (d, J = 10 Hz, 2H, Ar-H), 7.46 (d, J = 5 Hz, 4H, Ar-H), 7.40 (t, J = 7.5 Hz, 8H, Ar-H), 7.31 (t, J = 7.5 Hz, 4H, Ar-H), 7.26 (t, J = 10 Hz, 4H, Ar-H), 7.22 (s, 2H, Ar-H). 13C NMR data of the compound were not obtained due to the poor solubility. HRMS-ESI (m/z): 948.444 [M]+. Anal. Calcd. For C64H38F6N2: C, 81.00; H, 4.04; N, 2.95. Found: C, 81.08; H, 3.99; N, 2.99.

Scheme 1. Synthesis of the compounds 1-4. 3. Results and discussion 3.1. Synthesis As can be seen in Scheme 1, the fluorene-based derivatives 1-4 were prepared in

good

yields

by

reacting

1-1,

2-1,

3-1,

(4-(9H-carbazol-9-yl)phenyl)boronic acid at a 1:2.2 ratio in THF/H2O. 10

4-1

and

3.2. Aggregation-induced emission (AIE) behavior of compound 1 To survey the AIE phenomenon of compound 1, the absorption spectra of compound 1 (with a concentration of 10-5 M) in DMF-H2O mixtures with various water contents were recorded at room temperature (Supporting information: Fig. S1). Level-off tails could be clearly observed in the long-wavelength region as the water fraction (fw) values were increased. In general, such observed tails are the signals of the formation of nano-aggregates [35]. Subsequently, the photoluminescence (PL) spectra of 1 (10 µM) in DMF-H2O mixtures with different fw values were systematically studied. As shown in Fig. 1, compound 1 in pure DMF (10 µM) emitted hardly any fluorescence upon 365 nm UV photoexcitation, and the absolute fluorescence quantum yield (Φ) was as low as 0.03%. However, a new broad emission band with a maximum (λmax) at 555 nm was visible when the fw value in the DMF solution was increased to 40%. Meanwhile, a yellow emission could be observed. Furthermore, as the water content in the DMF solution reached 90%, a bright yellow fluorescence (Φ = 10.01%) was noticed. Obviously, the quantum yield of 1 (fw = 90%) was approximately 334-fold higher than that in pure DMF. It is well-known that water is a nonsolvent of luminogen 1, when water was added to the DMF solution, nano-aggregates were formed, and the intramolecular motion of compound 1 was restricted. Meanwhile, it was possible that the molecular conformation of compound 1 became more planar with an increase in fw value, and thus the corresponding fluorescence spectra of the DMF-water mixtures with high water fractions showed obvious red shifts. As a result, the strong yellow fluorescence 11

was triggered by aggregate formation, and luminogen 1 showed typical AIE effect. Indeed, as presented in Fig. 2, the nano-aggregates (fw = 90%) were characterized by dynamic light scattering (DLS). The aggregation-induced behaviors of luminogens 2, 3 and 4 were not further explored due to their poor solubilities. Nevertheless, 2, 3 and 4 exhibited bright solid-state blue-green, green and yellow fluorescence, respectively. The strong solid-state emission characteristics of 2, 3 and 4 are very beneficial to their practical applications.

Fig. 1. (a) PL spectra of the dilute solutions of compound 1 (10-5 M) in DMF-water mixtures with various water contents (0-90%). Excitation wavelength = 365 nm. (b) Fluorescence images of 1 (10-5 M) in DMF-water mixtures with various fw values under 365 nm UV irradiation.

12

Fig. 2. Size distribution curve of compound 1 (10-5 M) in DMF-water mixtures with fw = 90%. 3.3. Mechanofluorochromism behavior of luminogens 1-4 To further investigate the solid-state fluorescence characteristics of compounds 1-4, the PL spectra of luminogens 1-4 in the solid state were tested, and the obtained PL spectra are summarized in Fig. 3 and Fig. 4, luminogens 1-4 exhibited one emission band with the λmax at 492 nm, 484 nm, 497 nm and 538 nm, respectively. Compounds 1 and 2 emitted blue-green fluorescence with the absolute luminescence quantum yield of 1.46% (1) or 16.50% (2) upon UV illumination at 365 nm. However, compounds 3 and 4 showed green and yellow fluorescence with the absolute luminescence quantum yield of 18.47% (3) or 1.35% (4) under irradiation with UV light at 365 nm. The density functional theory (DFT) calculations for the luminogens 1-4 were performed at the B3LYP/6-31G* level with the Gaussian 09 program. As shown in Fig. S2 (Supporting information), the calculated energy gaps (∆E) of 1-4 were 3.2997567 eV (molecule 1), 3.2194872 eV (molecule 2), 3.1988076 eV (molecule 3) and

13

3.1852026

eV

(molecule 4),

respectively.

Therefore,

the

presence

of

trifluoromethyl groups and the position of trifluoromethyl groups had slight influences on the HOMO and LUMO energy levels of 1-4. Indeed, the various solid-state emission behaviors of compounds 1-4 were possibly attributed to their different molecular packings [36-43]. Next, the mechanofluorochromic phenomena of compounds 1-4 were explored via solid-state PL spectroscopy. As presented in Fig. 5, the fluorescence spectrum of as-prepared solid sample 1 displayed an emission band with a λmax at 492 nm, and 1 showed bright blue-green emission under 365 nm UV illumination, and the corresponding emission lifetime was 6.94 ns (Fig. 6). Interestingly, it was noticed that the fluorescence peak was redshifted from 492 nm to 570 nm upon gentle grinding of powder sample 1 using a spatula or a pestle, and the blue-green fluorescence that arose from UV irradiation at 365 nm was transformed into yellowish brown fluorescence, and its emission lifetime was 1.36 ns (Fig. 6). Furthermore, the original blue-green emission was completely restored upon treatment of the ground sample with fuming dichloromethane vapor for 30 s. Obviously, 1 showed reversible mechanofluorochromic conversion between blue-green and yellowish brown emission colors. Moreover, its mechanochromic transition could be repeated numerous times without fatigue (Fig. 7).

14

Fig. 3. (a) Fluorescence spectra of solid-state sample 1 and solid-state sample 2. (b) Photograph of solid-state sample 1 under 365 nm UV illumination. (c) Photograph of solid-state sample 2 under 365 nm UV illumination.

Fig. 4. (a) Fluorescence spectra of solid-state sample 3 and solid-state sample 4. (b) Photograph of solid-state sample 3 under 365 nm UV illumination. (c) Photograph of solid-state sample 4 under 365 nm UV illumination.

Fig. 5. (a) Fluorescence spectra of compound 1 before grinding, after 15

grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographs of 1 under 365 nm UV light: (b) the unground sample. (c) the entirely ground sample. (d) the sample after treatment with dichloromethane.

Fig. 6. (a) Decay curve of unground sample 1 at the peak emission wavelength of 492 nm, Excitation wavelength = 375 nm. (b) Decay curve of ground sample 1 at the peak emission wavelength of 570 nm, Excitation wavelength = 375 nm.

Fig. 7. Repetitive experiment of mechanofluorochromic behavior for compound 1. Similarly, as can be seen in Fig. 8, Fig. 10 and Fig. 12, luminogens 2-4 also exhibited reversible mechanochromic fluorescence behaviors involving color 16

changes from blue-green, green or yellow to yellowish brown. Indeed, 2-4 with different positions of trifluoromethyl groups showed various mechanofluorochromic phenomena. Furthermore, their fluorescence lifetimes before and after stimulating are shown in Fig. 9, Fig. 11 and Fig. 13, respectively. Obviously, the fluorescence lifetimes of solid samples 2-4 were reduced due to the application of mechanical force. It is possible that the excitons in excited states of unground molecules 2-4 are more stable than that of ground molecules 2-4. In addition, the repeatabilities of the mechanofluorochromic behaviors of 2-4 were also good (Supporting information: Figs. S3-S5). The maximum emission wavelengths and average lifetimes of 1-4 in different solid states have been summarized in a table (Supporting information: Fig. S6).

Fig. 8. (a) Fluorescence spectra of compound 2 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographs of 2 under 365 nm UV light: (b) the unground sample. (c) the entirely ground sample. (d) the sample after treatment with dichloromethane.

17

Fig. 9. (a) Decay curve of unground sample 2 at the peak emission wavelength of 484 nm, Excitation wavelength = 375 nm. (b) Decay curve of ground sample 2 at the peak emission wavelength of 541 nm, Excitation wavelength = 375 nm.

Fig. 10. (a) Fluorescence spectra of compound 3 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographs of 3 under 365 nm UV light: (b) the unground sample. (c) the entirely ground sample. (d) the sample after treatment with dichloromethane.

Fig. 11. (a) Decay curve of unground sample 3 at the peak emission 18

wavelength of 497 nm, Excitation wavelength = 375 nm. (b) Decay curve of ground sample 3 at the peak emission wavelength of 536 nm, Excitation wavelength = 375 nm.

Fig. 12. (a) Fluorescence spectra of compound 4 before grinding, after grinding, and after treatment with dichloromethane vapor. Excitation wavelength: 365 nm. Photographs of 4 under 365 nm UV light: (b) the unground sample. (c) the entirely ground sample. (d) the sample after treatment with dichloromethane.

Fig. 13. (a) Decay curve of unground sample 4 at the peak emission wavelength of 538 nm, Excitation wavelength = 375 nm. (b) Decay curve of ground sample 4 at the peak emission wavelength of 551 nm, Excitation wavelength = 375 nm.

19

In order to gain further insight into the possible mechanofluorochromic mechanism of compounds 1-4, the morphology structures of 1-4 in various solid states were determined by powder X-ray diⅡraction (XRD) measurements. As shown in Fig. 14, the as-synthesized solid sample of 1 showed a number of intense and sharp reflection peaks, which indicated its crystalline nature. In sharp contrast, the powder XRD patterns after grinding did not exhibit any noticeable diⅡraction peaks, implying the formation of amorphous morphology. When the ground sample was fumigated with dichloromethane vapor, the initial sharp diⅡraction peaks was attained again, suggesting the recovery of original crystalline state. Therefore, the experiment data suggested that the reversible mechanofluorochromic phenomenon of 1 was directly caused by crystalline to amorphous phase transition. As shown in Figs. S7-S9 (Supporting information), the powder XRD results also confirmed that the reversible mechanofluorochromic phenomena for 2-4 were associated with the mutual transformation between crystalline and amorphous phases.

Fig. 14. XRD patterns of compound 1: unground, ground, and after treatment with dichloromethane vapor.

20

Fortunately, single crystals of compound 4 were also successfully obtained by means

of

recrystallization

involving

slow

evaporation

of

its

n-hexane/dichloromethane solution. To further investigate the mechanism of bright solid-state emission and mechanofluorochromic characteristics of 1-4, the molecular packing of the obtained single crystal of 4 was studied systematically. As can be seen in Fig. 15, the molecular structure of compound 4 showed a twisted conformation due to the presence of carbazole groups, which was responsible for the bright solid-state fluorescence of 4. Although the C-H…π（2.738 Å, 2.689 Å and 2.669 Å）interactions effectively facilitated molecular packing, the absence of a strong intermolecular acting force resulted in a loose packing motif, and thus the molecular packing might change upon mechanical grinding, and a metastable state could be formed, and the corresponding solid-state fluorescence could be changed.

Fig. 15. The structural organization of compound 4. 4. Conclusions In summary, we developed four novel highly solid-state emissive fluorescent compounds containing carbazole and fluorene units. Interestingly, their different solid-state emission behaviors could be adjusted by mechanical force. More 21

specifically, compounds 1 and 2 showed reversible mechanofluorochromic conversion between blue-green and yellowish brown emission colors, compounds 3 and 4 exhibited mechanochromic characteristics involving fluorescent color changes from green or yellow to yellowish brown. The crystalline-to-amorphous morphology transition was responsible for the mechanofluorochromic characteristics of 1-4. This research work provides valuable reference for the design and development of high-contrast mechanochromic materials.

Acknowledgments

The authors are grateful for the financial support from the National Natural Science Foundation of China (21702079 and 41867053). Supporting Information Available UV/visible-absorbance spectra of compound 1 (1.0 × 10-5 mol L-1) in DMF-water mixtures with various water contents (0-90%). DFT calculates of compounds 1-4. Repetitive experiments of mechanochromic behaviors for compounds 2, 3 and 4. The maximum emission wavelengths and average lifetimes of compounds 1-4 in different solid states. XRD patterns of compounds 2, 3 and 4 in various solid states. Copies of NMR spectra and Mass spectra. Conflicts of interest There are no conflicts to declare. References [1] Chi Z, Zhang X, Xu B, Zhou X, Ma C, Zhang Y, et al. Recent advances in organic 22

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28

with

reversible

Highlights


Four carbazole-based fluorene derivatives were synthesized.
All these compounds showed highly solid-state emissive feature with various fluorescence.
Luminogen 1 exhibited obvious aggregation-induced emission behavior.
These luminogens exhibited highly reversible mechanofluorochromic characteristics.
The repeatabilities of mechanofluorochromic phenomena of these luminogens were good.

Zhao Chen designed the research; Zhao Chen and Shouzhi Pu supervised the work; Zhao Chen collected the data and wrote the manuscript; Shuai Tan and Ya Yin performed the synthesis of compounds 1-4; Ya Yin and Wenzhuo Chen investigated the photophysical characteristics of 1-4; Ya Yin performed the DFT calculates of compounds 1-4; Wei Tian helped optimize the research; The authors declare no competing financial interests.

There are no conflicts to declare.