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Tuning the Mechanochromic Luminescence of BOPIM Complexes by Rational Introduction of Aromatic Substituents Changxiang Guo, Mengwei Li, Wei Yuan, Kai Wang, Bo Zou, and Yulan Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09667 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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The Journal of Physical Chemistry

Tuning the Mechanochromic Luminescence of BOPIM Complexes by Rational Introduction of Aromatic Substituents Changxiang Guo,1 Mengwei Li,1 Wei Yuan,1 Kai Wang,*2 Bo Zou2 and Yulan Chen*1

1. Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Tianjin University, Tianjin, 300354, P. R. China

2. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R. China *E-mail: [email protected]; [email protected]

Phone/Fax: +86-022-27404118

1

ACS Paragon Plus Environment


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Abstract Considering the great advantages of boron 2-(2’-pyridyl)imidazole (BOPIM) based dyes, such as facile synthesis, high fluorescence quantum yield, large Stocks shift and good thermal stability, three BOPIM complexes (BOMPIM-Ph, BOPIM-Th and BOPIM-TTh) with different aromatic side groups have been developed. Their crystal structures,

photophysical

and

mechanochromic

luminescence

properties

were

systematically studied. It was found that their solid state emission and mechanochromic behaviours

are

related

to

the

side

group.

High

color

contrast

reversible

mechanochromism and piezochromism were observed for the three analogues, while BOPIM-TTh with more bulky thienothiophene groups showed a more sensitive and pronounced mechanochromic response. Steric and electronic effects of the aromatic donating substitutes were proved playing significant roles in regulating the intermolecular interactions and intramolecular charge transfer effect. Fine-tuning the molecular structures of such kind of BOPIM dyes allows convenient modulation of their optical properties and mechano-responsive behaviors.

Introduction In the past few decades, mechanochromic luminescent (MCL) dyes have attracted widespread attention.1-4 This important research effort has been propelled by the need for specific solid state materials that can convert mechanical events into a measurable output, in mechanosensors, optoelectronic devices, security papers, and data storage.5-8 A number 2


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of MCL materials have been documented to date.9-13 However, owing to the complicated nature of mechanoresponsive process, there are still lack of universal design principles of MCL molecules, therefore, the search for novel dyes that are subjected to systematical investigations are by no means complete and remain a challenging target. Studies on a group of structural analogues are usually an effective way to understanding the mechano-responsive mechanisms and elucidate the corresponding structural essence.14-18 Generally, highly efficient emission in the solid state is the prerequisite for excellent MCL materials. Besides, the organic dye should contain a rigid structure to maintain its molecular structure under force as well as a relatively loose packing mode to provide enough space for molecular rearrangements.19-21 Recently, the construction of sophisticated fluorescent dyes around a tetrahedral boron (III) center is a particular approach that has fuelled the creativity of chemists.22-28 Among numerous conjugated borate complexes, boron 2-(2’-pyridyl)imidazole (BOPIM) complexes represent an outstanding type of fluorescent dyes, regarding their intense fluorescence in solution, large Stocks shift and good thermal stability. Structural tailoring of BOPIM dyes for fine-tuning of their optical and physical properties has provided great opportunities towards luminescent organic functional materials.29-32 Although simple synthetic protocols with unusual spectroscopic behavior of BOPIM chromophores well suited to the requirements of MCL dyes, compared to other borate complexes, such as boron diketones,33 up to now, they are less explored as MCL materials. 3



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Herein, we developed a family of new BOPIM based luminescent materials (BOPIM-Ph, BOPIM-Th and BOPIM-TTh, Figure 1), in which different aromatic units are selected to attach to the imidazole ring of BOPIM to afford highly twisted molecular conformation. The control over solid state emission was available, that was, the optical properties of the BOPIM based dyes were delicately dependent upon the substituent on the BOPIM core. All the crystals show reversible mechanochromic characteristics upon grinding-fuming treatments with high color contrast, for instance, between yellow and red with wavelength shift up to 45 nm. More importantly, they can also sense exact hydrostatic pressure higher than 10 GPa with good reversibility and reproducibility, accompanied by visible color changes. The emission wavelengths have a linear relationship with the applied pressures, which enables them as a good candidate for a convenient colorimetric pressure sensor.

Figure 1. Chemical structures of BOPIM dyes in this work.

Experimental Section Materials and methods. All the raw materials were used as received without further purification. All the solvents were purchased from Jiangtian chemical technology (Tianjin, 4


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China) and redistilled before use. All reactions were performed under an atmosphere of nitrogen and monitored by TLC. Column chromatography was carried out on silica gel (200–300 mesh). 1H and

13

C NMR spectra were recorded on a Bruker instrument at 400

MHz under 25 °C by using CDCl3 or DMSO-d6 as the solvent, in which chemical shifts of 1H NMR and

13

C NMR were referenced to residual solvent peaks (δ in parts per

million (ppm); 1H NMR: CDCl3, 7.26 ppm;

13

C NMR: CDCl3, 77.23 ppm). Coupling

constants were assigned as observed. The UV-vis absorption spectra were obtained on a PerkinElmer Lambda 750 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. The high resolution electrospray mass spectra (HR-ESI-MS) of products were recorded on a miorOTOF-QII mass spectrometer (Bruker Daltonics). Thermogravimetric analyses (TGA) were carried out using a TA Instruments Q-50 with a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) measurements were conducted using the TA Instruments Q-20 with a scan rate of 10 °C/min. The powder XRD patterns were obtained with a Rigaku SmartLab (9 kW) X-ray diffractometer. Single crystals were obtained in ethyl acetate for BOPIM-Ph and BOPIM-Th or in the mixture of CH2Cl2 and EtOH by a slow solvent diffusion method for BOPIM-TTh. The single crystal X-ray diffraction was recorded on a Rigaku SCX-mini diffractometer with graphite monochromatic Mo-Kα radiation (λ = 0.7173 Å) by ω scan mode. Density functional theory (DFT) calculations were performed in Gaussian 09 software at the B3LYP functional with the 6-31G* basis set level. 5



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High pressure experiments were carried out using a diamond anvil cell (DAC). The culet diameter of the diamond anvils was 0.5 mm. T301 stainless steel gaskets were preindented to a thickness of 60 µm, and center holes of 0.16 mm were drilled for the sample. The ruby chip was used for pressure determination using the standard ruby fluorescent technique. Silicone oil was used as the pressure-transmitting medium. All experiments were performed at room temperature. High-pressure absorption spectra were recorded by an optical fiber spectrometer (Ocean Optics, QE65000). Optical photographs were obtained using an imaging camera (Canon EOS 5D Mark II) equipped on a Nikon Ti-U fluorescence microscope. In situ FL measurements under high pressure were performed in the reflection mode using a 355 nm DPSS laser with a spot size of 20 mm and a power of 10 mW as the excitation source. Thieno[3,2-b]thiophene-2-carbaldehyde was synthesized according to the previous reference.34 L1. 2-Cyanopyridine (1.06 g, 10 mmol), benzaldehyde (2 ml, 20 mmol) and NH4OAc (2.31 g, 30 mmol) were combined and heated in 10 mL of acetic acid at 170 °C for 10 h in a round-bottom flask. After that, the mixture was added to the freshly prepared saturated solution of NaHCO3, brought pH to 7, and extracted with CH2Cl2; then the organic phase was dried by anhydrous MgSO4, filtered and evaporated under vacuum. The residue was purified by flash column chromatography eluting with ethyl acetate/petroleum ether (1:8, v/v) to afford a colorless solid (1.62 g, yield: 55%). 1H 6


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NMR (400 MHz, CDCl3, ppm) δ 10.61 (s, 1H), 8.41 (d, J = 4.7 Hz, 1H), 8.22 (d, J = 7.9 Hz, 1H), 7.71 (t, J = 7.7 Hz, 1H), 7.61 (d, J = 7.5 Hz, 2H), 7.40 (d, J = 7.2 Hz, 2H), 7.32-7.23 (m, 5H), 7.21-7.13 (m, 2H).

13

C NMR (100 MHz, CDCl3, ppm) δ 148.70,

148.33, 145.55, 139.20, 137.14, 134.82, 130.73, 128.74, 128.40, 127.96, 127.92, 127.85, 127.07, 123.12, 120.14. HRMS (ESI- TOF) m/z: [M + H]+ Calcd for C20H16N3 298.1344; found 298.1344. L2. 2-Cyanopyridine (2.12 g, 20 mmol), thiophene-2-carbaldehyde (3.7 ml, 40 mmol) and NH4OAc (4.62 g, 60 mmol) were combined and heated in 20 mL of acetic acid at 170 °C for 10 h in a round-bottom flask. After that, the mixture was added to the freshly prepared saturated solution of NaHCO3, brought pH to 7, and extracted with CH2Cl2; then the organic phase was dried by anhydrous MgSO4, filtered and evaporated under vacuum. The residue was purified by flash column chromatography eluting with ethyl acetate/petroleum ether (1:8, v/v) to afford a gray solid (2.1 g, yield: 34%). 1H NMR (400 MHz, CDCl3, ppm) δ 10.70 (s, 1H), 8.50 (d, J = 4.7 Hz, 1H), 8.27 (d, J = 7.9 Hz, 1H), 7.79 (t, J = 7.7 Hz, 1H), δ 7.38 (d, J = 5.0 Hz, 1H), 7.34 (d, J = 3.0 Hz, 1H). 7.28-7.23 (m, 3H), 7.09 (t, J = 4.0 Hz, 1H), 7.02 (t, J = 4.0 Hz, 1H). 13C NMR (100 MHz, CDCl3, ppm)

δ 148.66, 147.85, 145.59, 137.22, 137.02, 135.14, 130.85, 127.69, 127.48, 127.31, 126.65, 124.88, 124.68, 123.44, 121.82, 120.52. HRMS (ESI- TOF) m/z: [M + H]+ Calcd for C16H12N3S2 310.0473; found 310.0464. L3. 2-Cyanopyridine (2.12 g, 20 mmol), thieno[3,2-b]thiophene-2-carbaldehyde (6.72 g, 7



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40 mmol) and NH4OAc (4.62 g, 60 mmol) were combined and heated in 20 mL of acetic acid at 170 °C for 10 h in a round-bottom flask. After that, the mixture was added to the freshly prepared saturated solution of NaHCO3, brought pH to 7, and extracted with CH2Cl2; then the organic phase was dried by anhydrous MgSO4, filtered and evaporated under vacuum. The residue was purified by flash column chromatography eluting with ethyl acetate/petroleum ether (1:4, v/v) to afford a yellow solid (1.9 g, yield: 23%). 1H NMR (400 MHz, CDCl3, ppm) δ 10.50 (s, 1H), 8.54 (s, 1H), 8.28 (d, J = 7.8 Hz, 1H), 7.81 (t, J = 7.7 Hz, 1H), 7.57 (s, 1H), 7.53 (s, 1H), 7.46 (d, J = 4.8 Hz, 1H), 7.31-7.24 (m, 4H).

13

C NMR (100 MHz, DMSO-d6, ppm) δ 149.62, 148.48, 146.47, 140.11, 139.40,

139.33, 139.09, 138.83, 137.90, 134.85, 132.37, 129.37, 128.36, 124.21, 123.42, 121.62, 120.76, 120.63, 120.46, 117.41. HRMS (ESI- TOF) m/z: [M + H]+ Calcd for C20H12N3S4 421.9914; found 421.9912. BOPIM-Ph. Triethylamine (8 mL, 57.8 mmol) was added to a stirred mixture of L1 (1.188 g, 4 mmol) in dry CH2Cl2 (60 mL) under N2 at room temperature. After 5 min, boron trifluoride etherate (8 mL, 63.4 mmol) was added dropwise. The solution was stirred overnight. The reaction was quenched by adding water (20 mL) and the mixture was extracted with CH2Cl2 (250 mL). The organic phase was dried by anhydrous MgSO4, and concentrated to dryness. The crude product was purified by silica gel column chromatography with ethyl acetate/petroleum ether (1:4, v/v), then recrystallized from DCM and petroleum ether to afford BOPIM-Ph as a green powder (0.5 g, yield: 36%). 8


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1

H NMR (400 MHz, CDCl3, ppm) δ 8.36 (d, J = 5.6 Hz, 1H), 8.10 (t, J = 7.8 Hz, 1H),

8.00 (d, J = 8.1 Hz, 1H), 7.58-7.46 (m, 4H), 7.40 (t, J = 6.6 Hz, 1H), 7.41-7.27 (m, 6H). 13

C NMR (100 MHz, CDCl3, ppm) δ 145.81, 145.55, 145.22, 144.73, 141.33, 134.75,

134.42, 130.79, 128.75, 128.67, 128.42, 128.19, 127.36, 122.81, 117.68. HRMS (ESITOF) m/z: [M + H]+ Calcd for C20H15BF2N3 346.1327; found 346.1362. BOPIM-Th. Triethylamine (8 mL, 57.8 mmol) was added to a stirred mixture of L2 (1.236 g, 4 mmol) in dry CH2Cl2 (60 mL) under N2 at room temperature. After 5 min, boron trifluoride etherate (8 mL, 63.4 mmol) was added dropwise. The solution was stirred overnight. The reaction was quenched by adding water (20 mL) and the mixture was extracted with CH2Cl2 (250 mL). The organic phase was dried by anhydrous MgSO4, and concentrated to dryness. The crude product was purified by silica gel column chromatography with ethyl acetate/petroleum ether (1:4, v/v), then recrystallized from DCM and petroleum ether to afford BOPIM-Th as a yellow solid (0.459 g, yield: 32%). 1

H NMR (400 MHz, CDCl3, ppm) δ 8.47 (d, J = 5.5 Hz, 1H), 8.21 (t, J = 7.8 Hz, 1H),

8.08 (d, J = 8.1 Hz, 1H), 7.57-7.49 (m, 2H), 7.40-7.28 (m, 3H), 7.11 (t, J = 3.8 Hz, 1H), 7.05 (t, J = 3.8 Hz, 1H).

13

C NMR (100 MHz, CDCl3, ppm) δ 144.27, 143.92, 143.73,

140.40, 138.96, 135.23, 129.91, 127.11, 127.03, 126.40, 126.30, 125.81, 125.10, 124.66, 122.11, 116.96. HRMS (ESI- TOF) m/z: [M + H]+ Calcd for C16H11BF2N3S2; 358.0455 found 358.0453. BOPIM-TTh. Triethylamine (20 mL, 144.5 mmol) was added to a stirred mixture of L3 9



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(1.68 g, 4.0 mmol) in dry toluene (100 mL) under N2 at room temperature. After 5 min, boron trifluoride etherate (20 mL, 158.5 mmol) was added dropwise. The solution was stirred overnight at 100 °C for 12 h. The reaction was quenched by adding water (50 mL) and the mixture was extracted with CH2Cl2. The organic phase was dried by anhydrous MgSO4, and concentrated to dryness. The crude product was purified by silica gel column chromatography with ethyl acetate/petroleum ether (1:4, v/v), then recrystallized from DCM and EtOH to afford BOPIM-TTh as a yellow solid (0.595 g, yield: 32 %). 1H NMR (400 MHz, CDCl3, ppm) δ 8.49 (d, J = 5.4 Hz, 1H), 8.24 (t, J = 7.9 Hz, 1H), 8.12 (d, J = 8.1 Hz, 1H), 7.71 (s, 1H), 7.61-7.52 (m, 2H), 7.43 (d, J = 5.2 Hz, 1H), 7.37 (d, J = 5.2 Hz, 1H), 7.24 (m, 2H). 13C NMR (100 MHz, CDCl3, ppm) δ 144.85, 141.54, 140.50, 139.43, 139.37, 138.14, 132.43, 128.17, 127.17, 123.35, 120.58, 119.60, 119.54, 118.35, 118.22. HRMS (ESI- TOF) m/z: [M + H]+ Calcd for C20H11BF2N3S4; 469.9897 found 469.9901.

Results and Discussion The structures and synthetic routes of BOPIM based dyes in this work are depicted in Figure 1 and Scheme 1. The pivotal roles of boron(III) chelation is to render the π system planar, thereby enhancing conjugation and charge transfer along the main molecular axis. Moreover, to favor mechanical response, relatively loose packing mode is achieved by substitution of the imidazole ring with two aromatic groups. Phenyl, thienyl and 10


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thieno[3,2-b]thienyl units with varied electron density were selected, with expectation to change the electronic environment of the chromophore core. In this way, the emission and mechanical response behaviors of BOPIMs may be tuned. Starting from the corresponding aromatic aldehydes 1-3, modification of BOPIMs relied on the formation of their 2-(1H-imidazol-2-yl)pyridine ligands L1-3 through condensation in acidic media with the yields of 23%-55% as a key step, followed by BF2 complexation. All the target compounds (BOPIM-Ph, BOPIM-Th, BOPIM-TTh) were obtained in moderate yields (32%-36%) and were unambiguously characterized with 1H and

13

C NMR spectra, and

HR-ESI mass spectroscopy.

Scheme 1. Synthetic routes to BOPIM-Ph, BOPIM-Th and BOPIM-TTh.

Single crystals of BOPIM-Ph and BOPIM-Th suitable for X-ray measurements were obtained by slow evaporation of ethyl acetate solutions in hexane atmosphere. BOPIM-TTh single crystal was obtained by slowly evaporating the mixed solvent of 11



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dichloromethane and ethanol. Crystal information showed that both the BOPIM-Ph and BOPIM-Th crystals belonged to a monoclinic space group P2(1)/n, whereas BOPIM-TTh crystals belonged to a monoclinic space group P-1 (Table S1-3, Supporting Information). As shown in Figure 2, the boron–fluorine chromophore core is almost in the same plane with 4-substituted aromatic ring, however, 5-aromatic rings adopt a nonplanar orientation, with large dihedral angles (58.80o, 63.55o and 78.44o for BOPIM-Ph, BOPIM-Th and BOPIM-TTh, respectively) between the two planes (Figure S1, Supporting Information). The packing mode showed that all the unit cells of BOPIM-Ph, BOPIM-Th and BOPIM-Th contain two molecules with multiple intermolecular interactions, such as B-F⋅⋅⋅H-C, B-F⋅⋅⋅C and F-B⋅⋅⋅C interactions, with distances in the range of 2.4–3.9 Å (Figure 2 and Figure S2, Supporting Information). As is shown in Figure 2d-f, all the BOPIM cores are arranged in an antiparallel way, with partial overlap between the neighboring BOPIM cores, suggesting they form dimers by π−π stacking interactions. These observations are resemble to the reported cases,31,32 in which the twisted geometry of the two aryl rings may offer steric hindrance, thereby the π-π stacking area decreased. Moreover, the multiple intermolecular interactions rigidified the molecular structure in the crystal lattice, which was expected to prevent excitons from a non-radiative dissipation.

12


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Figure 2. Single crystal structures of a) BOPIM-Ph, b) BOPIM-Th, and c) BOPIM-TTh. And packing diagrams of d) BOPIM-Ph, e) BOPIM-Th, and f) BOPIM-TTh.

Detailed

spectroscopy

measurements

were

performed

to

elucidate

the

structure-dependent optophysical properties of the three BOPIM complexes and all the photophysical data are collected in Table 1. Firstly, as an inherent D-A type chromophore,35 the UV-vis absorption and FL emission spectra of these BOPIM dyes were investigated in various solvents. It was found that in nonpolar solvent, such as hexane, the absorption bands for BOPIM-Ph were located at 339 nm and 428 nm. The former one might be derived from intramolecular π−π* transition,36 and the lowest energy absorption maxima could be ascribed to charge-transfer (CT) transition (Figure 3a). The ICT transition peak for BOPIM-Th and BOPIM-TTh red-shifted to 445 nm and 460 nm, respectively, which might be due to the increased conjugation and electron-donating ability through their appended aromatic groups to lower the transition energy. In addition, solvatochromic effect was observed for all the three dyes, that was, the lowest-energy absorption bands were marked blue-shift with solvent polarity 13



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Page 14 of 32

increasing, corresponding to classic ICT characteristics37 (Figure 3b and Figure S3a, c, Supporting Information). It’s evident that enhancing electron-donating ability of the attached aromatic rings significantly affects the ICT efficiency,29,

38-43

indeed,

BOPIM-TTh bearing the strongest electron-donating units displayed the most remarkable variation of absorption maximum (∆λabs = 49 nm) upon increasing the solvent polarity from hexane to acetonitrile.

Figure 3. a) UV−vis absorption spectra of BOPIM-Ph, BOPIM-Th and BOPIM-TTh in hexane; b) UV−vis absorption spectra of BOPIM-TTh in different solvents; c) fluorescence spectra of BOPIM-Ph, BOPIM-Th and BOPIM-TTh (5 × 10−5 M, excited at 425 nm for BOPIM-Ph and BOPIM-Th, 460 nm for BOPIM-TTh) in hexane; d) fluorescence spectra of BOPIM-TTh (5 × 10−5 M, excited at 460 nm) in different solvents.

Owing to their ICT feature, the FL emission spectra of BOPIM-Ph, BOPIM-Th and 14


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BOPIM-TTh are dependent on solvent polarity as well. As shown in Figure 3c, the solution emission maxima of BOPIM-Ph, BOPIM-Th and BOPIM-TTh in hexane were located at 523 nm, 562 nm and 578 nm, respectively. Typical bathochromic shifts were estimated in more polar solvents.44 Taking BOPIM-TTh for example, it exhibited green to red fluorescence (∆λfl = 74 nm) with high to lower efficiencies (13.38%–0.41%) from hexane to acetonitrile (Figure 3d, Table 1). The Stokes shifts (ν) for the three dyes in all the studied solvents are found to be over 95 nm and there is negligible spectral overlap between its absorption and fluorescence (Figure S3, Supporting Information), which would suppress Forster-type energy transfer and benefit for strong and robust emission. As for BOPIM-TTh, its Stokes shift was up to 118 nm, which was 23 nm larger than that of BOPIM-Ph. With thienothiophene as strong electron donor, such large Stokes shift could be ascribed to the efficient ICT transition from electron-rich groups to electron-accepting BOPIM skeleton. Table 1. Photophysical properties of BOPIMs. BOPIM-Ph λabsa

λemb

ν

(nm)

(nm)

Hexane

428

Toluene

BOPIM-Th d

△λ

ΦFe

λabsa

λemb

ν

(nm)

(nm)

(%)

(nm)

(nm)

523

95

~

41.8

445

418

536

118

~

35.6

Chloroform

414

536

122

~

THF

404

544

140

Acetonitrile

393

562

pristine

417

ground

418 a

c

BOPIM-TTh d

△λ

ΦFe

λabsa

λemb

νc

△λd

ΦFe

(nm)

(nm)

(%)

(nm)

(nm)

(nm)

(nm)

(%)

562

117

~

17

460

578

118

~

13.38

431

587

156

~

10.3

445

601

156

~

8.72

33

427

590

163

~

7.3

440

612

172

~

3.93

~

18.8

416

594

178

~

3.3

425

630

205

~

1.58

169

~

8.2

403

611

208

~

1

411

652

241

~

0.41

514

~

19

23.9

413

549

~

22

6.4

413

566

~

45

5

533

~

20.5

418

571

~

7.4

418

611

~

Maximum absorption;

b

Maximum emission;

c

c

Stokes shift;

d

Red shift value of fluorescence

15


4.1


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maximum after ground; e The fluorescence quantum yields were determined by a relative method for solution with 3-(2-benzothiazolyl)-N,N-diethylumbelliferylamine (in CH2Cl2, ΦF = 76%) as a standard, and with an absolute fluorescence quantum yield spectrometer for solid, respectively.

The D-A nature of these BOPIMs is also confirmed by density functional theory (DFT) studies performed at the B3LYP/6-31G(d) level. As shown in Figure S4 (Supporting Information), the energy minimized structures for the three molecules display twisted conformation of the aromatic rings. The π electrons of the HOMO molecular orbital are mainly located on the 5-substituted aromatic moiety, whereas those of the LUMO are mostly positioned on the boron-fluorine chromophore part. With enhancing the electron-donating ability from phenyl, thiophene to thienothiophene units, the calculated energy gap decreased accordingly. Concerning the structure features mentioned above, it is not surprised that BOPIM-Ph, BOPIM-Th and BOPIM-TTh exhibited solid state fluorescence with good quantum efficiency of 23.9%, 6.4% and 5%, respectively (Table 1). More interestingly, upon grinding, all of the emission peaks of these luminescent materials were clearly red-shifted. The difference between the emissions of the pristine and ground powders is sufficiently large as to be easily distinguished by the naked eyes. In detail, as shown in Figure 4, the emission peak of BOPIM-Ph shifted bathochromically from 514 nm to 533 nm by grinding. As for BOPIM-Th, the corresponding fluorescence color altered more 16


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significantly from green to orange. The most prominent mechanofluorochromic effect was achieved by BOPIM-TTh, in which the red-shift of emission wavelength reached up to 45 nm, with distinctive red fluorescence emitting upon mechanical treatment. Such considerable difference in mechanical response of BOPIM-TTh relative to the other BOPIM dyes could be ascribable to the key role of the side aromatic groups, since the thienothiophene groups not only made the molecule more twisted and loosely packed through their bulky effects,45 but also promoted ICT transition which was well known to contribute largely to MLC nature with higher sensitivity towards mechanical stimuli. Fuming the ground powders with CH2Cl2 vapor led to luminescence recovery of the three samples, and the switching of fluorescence between mechanical force and vapor stimuli could be repeated many times (Figure 4d and Figure S5, Supporting Information).

Figure 4. Normalized fluorescence spectra of a) BOPIM-Ph, b) BOPIM-Th and c) BOPIM-TTh in different solid states (Excited at 425 nm for BOPIM-Ph and BOPIM-Th, 460 nm for BOPIM-TTh). 17



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The insets depict the photographs of BOPIM-Ph, BOPIM-Th and BOPIM-TTh in different solid states under 365 nm UV illumination. d) Mechanochromic fluorescence of “TJU” written on the solid film of BOPIM-TTh cast on a glass plate by using a metal spatula (upper). Reversible switching of emission of BOPIM-TTh by repeated grinding/fuming cycles (lower).

Generally, the optical properties of organic molecules in the solid state were strongly affected by the intramolecular conformation and packing modes.46,47 Powder X-ray diffraction (PXRD) measurements were thereby carried out for each sample to determine the mechanism of the mechanofluorochromic effect (Figure 5). It was found that all the three as-prepared samples displayed intense and sharp diffraction peaks, indicating the crystalline state of the solids. After grinding, some of the sharp peaks became evidently weak and some blended with the background,48 suggesting that the crystalline structure was destroyed through mechanical force. In contrast to BOPIM-Ph and BOPIM-Th, PXRD patterns indicated higher amorphization of BOPIM-TTh induced by grinding. Additionally, the DSC curves (Figure S8, Supporting Information) of the ground powder of BOPIM-TTh displayed a cold-crystallization transition peak at 96 °C prior to melting at 254 °C, manifesting that the ground powder presented in a metastable amorphous phase and converted to a stable crystalline phase via an exothermal recrystallization process. No cold-crystallization process was detected for ground BOPIM-Ph and BOPIM-Th, which demonstrated that BOPIM-TTh possessed an additional metastable 18


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phase in its aggregation state, and grinding led to a change of the solid-state molecular aggregation mode from a thermodynamically stable ordered crystalline state to a metastable amorphous state, which significantly tuned the emission color.48 After fuming, the WAXD curve diffraction peaks were recovered to reform the crystalline structure, implying the reversible nature of these MCL materials. Combined with the solid state molecular absorption bands before and after grinding, which are well preserved in the visible region between 200 and 600 nm (Figure S6, Supporting Information), we thus deemed that the mechanochromic effect was not caused by changes in molecular structure, but due to intermolecular interactions caused by molecular rearrangement.50 The highly twisted conformation most likely results in the weak intermolecular interactions and relatively loose packing, which is more susceptible to external perturbation to endow a sensitive chromic response, as a result, force induced changes in packing allowing for a more planar conformation, which were in accordance with the larger red-shift of fluorescence concomitant with decreased intensity.

Figure 5. PXRD patterns of a) BOPIM-Ph, b) BOPIM-Th, and c) BOPIM-TTh in different solid states.

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More controllable stimulation was by isotropic compression with the hydrostatic pressure higher than 10 GPa created by a diamond anvil cell (DAC).47 As presented in Figure 6 and Figure S9-13 (Supporting Information), owing to the rigid BOPIM skeleton, all the three crystals can withstand high hydrostatic pressure, accompanied with observable color and fluorescence changes. For BOPIM-Ph, the absorption spectra displaced towards longer wavelength upon hydrostatic compression (Figure S9a, Supporting Information). The FL maximum red-shifted gradually from 503 nm at ambient pressure to 553 nm at 14.0 GPa with luminescence intensity decreased. And the color of the crystal changed from slight yellow to deep yellow under natural light (Figure S9e and S12, Supporting Information). All the changes in the compression process were reversible upon decompression, by which the UV-vis absorption and luminescence spectra of BOPIM-Ph recovered back to the original one after depressurizing (Figure S9b and S12b, Supporting Information). As for BOPIM-Th, its crystal emitted green fluorescence under ambient pressure. When the external pressure was continuously increased from 1 atm to 12.2 GPa, the fluorescence of the crystal underwent a remarkable variation from green to orange, and further to red, with emission wavelength exhibiting a red-shift of 99 nm (Figure S13, Supporting Information). As demonstrated by Figure 6 and Figure S11 (Supporting Information), resemble piezochromic behavior was detected for BOPIM-TTh, particularly with lower pressure threshold (as verified by the increased 20


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slop derived from the plots of emission wavelength vs. pressure from BOPIM-Ph, to BOPIM-Th and BOPIM-TTh, Figure S14, Supporting Information) and higher color contrast, bathochromic shifted from 561 nm (without compression) to deep-red fluorescence (λfl = 695 nm) at 10.3 GPa (∆λfl = 134 nm), manifesting its greater sensitivity as a luminescent force probe. According to these piezochromic investigations, it is worth noting that, as for the three BOPIM crystals, the emission wavelengths have a good linear relationship with external pressure, and good reversibility and reproducibility were estimated, and modulation of the optical properties as well as the responsive sensitivity were readily achievable at molecular level, all of which are key factors for the potential application as advanced colorimetric sensor.

Figure 6. FL spectra of a BOPIM-TTh crystal during the a) compression and b) decompression process. c) Plots of the relative emission wavelength and intensity vs. pressures under compression. d) 21



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Hydrostatic compression-decompression profiles of BOPIM-TTh based on the luminescence wavenumber at different hydrostatic pressures according to (a, b). e) Fluorescent images of a BOPIM-TTh crystal in DAC during a pressurizing and depressurizing cycle (λex = 355 nm).

Conclusions In summary, a series of solid-emissive BOPIM complexes bearing different aromatic side groups (including phenyl, thiophene and thienothiopene) have been synthesized, which demonstrated mechanochromic and piezochromic luminescence upon mechanical stimuli (by grinding and hydrostatic compression). All the crystals adopted highly twisted conformation and could sense high external pressure with distinct red-shifted luminescence. The reversible structural switch between crystal and amorphous states upon mechanical loading/deloading was confirmed, illustrating good reversibility and reproducibility of mechanochromic luminescence. The solid state emission and mechanochromic behavior of these BOPIM dyes were found structure-sensitive and were well interpreted as arising from both the inter- and intra-molecular effects modulated by the bulky aromatic donating units: enhanced weak intermolecular interactions and intramolecular CT effect were illuminated jointly benefiting for mechanochromic response. The current study thus not only provides a modular design strategy to develop MCL materials with desired merits, such as tunable luminescence, excellent color contrast and high sensitivity, but also inspires our further efforts to promote their practical 22


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application as colorimetric pressure sensor.

Supporting Information Available: Single crystals structure and data, photophysical properties in solution, energy levels calculation, TGA curves, DSC curves, DAC experiment, 1H NMR,

13

C NMR and mass spectra. This material is available free of

charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT Financial support by the National Natural Science Foundation of China (Grant 21522405 and 51503142), Chang Jiang Scholars Program (No. T2016051), the Thousand Youth Talents Plan, and Natural Science Foundation of Tianjin (Grant 15JCYBJC52900) is gratefully acknowledged.

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