Synthesis, photophysical properties and NIR photochromism of photoresponsive difluoroboron β-diketonate complex based on dithienylethene unit

Accepted Manuscript Synthesis, photophysical properties and NIR photochromism of photoresponsive difluoroboron β-diketonate complex based on dithienylethene unit Ziyong Li, Mengna Li, Guoxing Liu, Yangyang Wang, Guohui Kang, Chaoyang Li, Hui Guo PII:

S0143-7208(18)31725-X

DOI:

10.1016/j.dyepig.2018.08.034

Reference:

DYPI 6948

To appear in:

Dyes and Pigments

Received Date: 4 August 2018 Revised Date:

18 August 2018

Accepted Date: 19 August 2018

Please cite this article as: Li Z, Li M, Liu G, Wang Y, Kang G, Li C, Guo H, Synthesis, photophysical properties and NIR photochromism of photoresponsive difluoroboron β-diketonate complex based on dithienylethene unit, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.08.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Synthesis, Photophysical Properties and NIR Photochromism of Photoresponsive Difluoroboron β-Diketonate Complex Based on Dithienylethene Unit

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Ziyong Li,a,* Mengna Li,a Guoxing Liu,b Yangyang Wang,a Guohui Kang,c Chaoyang Li,a Hui Guoa,*

Key Laboratory of Organic Functional Molecules, Luoyang City, College of Food and Drug, Luoyang Normal University, Luoyang 471934, P. R. China

c

College of Chemical Engineering and Environment, Henan University of Technology, Zhengzhou 450001, P. R. China

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b

College of Chemistry and Chemical Engineering, and Henan Key Laboratory of Function-Oriented Porous Materials,

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Luoyang Normal University, Luoyang 471022, China

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Corresponding author E-mail: [email protected]; [email protected]

ABSTRACT: The development of stimuli-responsive difluoroboron β-diketonate materials has attracted more and more attention for their potential application in optoelectronics and functional bio-materials. In this study, we have designed and synthesized a novel photoresponsive dithienylethene-containing difluoroboron β-diketonate complex 1 by Knoevenagel condensation reaction. Its structure have been confirmed by 1H NMR, 13C NMR and HRMS (ESI). And it exhibited solvent-dependent photophysical properties in solution before visible light irradiation. Subsequently, photochromism of 1 was investigated in solution, and it was found that 1 revealed solvent-dependent and visible light-triggered NIR photochromic behaviors. Moreover, it also displayed excellent fluorescence switching behavior and photochromism in the film state upon visible light irradiation. Accordingly, the dithienylethene 1 may be 1

ACCEPTED MANUSCRIPT acted as a novel visible light-responsive fluorescence dyes in optoelectronic materials. Keywords: Dithienylethene; NIR photochromism; Photoresponsive; Difluoroboron β-diketonates (BF2bdks); Fluorescent switch.

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1. Introduction The development of stimuli-responsive materials has recently attracted increasing attention in the material science community owing to their potential applications for sensors, memory storage, displays, security inks, OLED development, etc [1-6]. Over the past decades, there have been continuous efforts to

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develop novel organic luminescent materials, among which organoboron materials are a research area of much interest given their tunable optical properties [7-9]. Difluoroboron β-diketonates (BF2bdks) are a

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class of highly fluorescent organoboron complexes with intriguing photophysical properties such as strong fluorescence in both solution and solid state, tunable fluorescent emission, large extinction coefficients, two-photon excited fluorescence, intramolecular charge transfer (ICT) character and emission-sensitive to environments [10]. According to the literature, BF2bdks have displayed numerous stimuli-responsive features, for example, oxygen sensing and oxygen-sensitive room temperature

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phosphorescence in rigid polymer media [11-16], organic vapor sensitivity [17-20], mechanochromic luminescence [21-27], aggregation induced emission [17, 28-30], and thermally responsive emission [31]. However, the development of novel regulation methods will further strengthen and enrich the control

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pathway of stimuli-responsive BF2bdks materials.

Light, especially visible light, seems to an attractive external trigger for manipulation of organic

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luminescent materials because it offers highly spatiotemporal resolution [32,33], which is generally noninvasive, and does not cause the contamination of the sample. The light-sensitivity is generally acquired by the incorporation of molecular photoswitches into luminescent materials, among which diarylethene derivatives bearing two thiophene or benzothiophene groups have been extensively investigated by virtue of their excellent thermal stability, rapid response and fatigue resistance [34-41]. Recently, Yam and coworkers designed and synthesized a novel class of photoresponsive difluoroboron β-diketonate complexes based on photochromic dithienylethene moiety [42-44]. However, we still know very little about these compounds, especially in the field of visible light-responsive materials [45,46]. We deem that it will be of great interest to prepare novel photochromic materials by appending BF2bdk 2

ACCEPTED MANUSCRIPT fragment on one side position of dithienylethene. Herein we present an unprecedented example of trifluoromethyl-substituted dithienylethene-containing difluoroboron β-diketonate complex 1 (Scheme 1), in which the dithienylethene moiety act as photoresponsive unit, moreover, BF2bdks group not only functions as fluorophore, but can make the dithienylethene derivatives have the intramolecular charge

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transfer (ICT) characteristics, thus showing solvent-dependent behaviors. As expected, it exhibited solvent-dependent photophysical and visible light-triggered NIR photochromic properties in the solution. Moreover, photoinduced color and fluorescence changes in the film state have been observed upon irradiation.

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2. Experimental section

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2.1. Materials and general methods

All manipulations were carried out under a nitrogen atmosphere by using standard Schlenk techniques unless otherwise stated. THF / Toluene were distilled under nitrogen from sodium-benzophenone. DMF was dried with magnesium sulfate and then distilled under vacuum. 1, 2-Bis(5-chloro-2-methylthiophen -3-yl)cyclopent-1-ene 2 [47], 4 [48] were prepared by modified literature methods, respectively. All other

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starting materials were obtained commercially as analytical-grade and used without further purification. The relative quantum yields were determined by comparing the reaction yield with the known yield of the compound 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene [49]. 1H and 13C NMR spectra were collected on German BRUKER AVANCE III 400 MHz. 1H and 13C NMR chemical shifts are relative to

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TMS. Mass spectra were obtained on ABSCIEX X500R QTOF (ESI mode). UV-vis spectra were obtained on a Persee TU-1810 UV-Vis spectrophotometer, and fluorescence spectra were obtained on a Hitachi

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Model F-4500 fluorescent spectrophotometer. In the photoisomerization reaction, the visible light irradiation experiment was carried out using a photochemical reaction apparatus with a 500 W Hg lamp and CEL-HXF300 14 V 50 W xenon lamp with cut-off filter. 2.2. Synthesis

2.2.1. Synthesis of intermediate 5 To a solution of 4 (438 mg, 1.0 mmol) in anhydrous THF (10 mL) was added n-BuLi (0.40 mL of 2.5 M solution in hexane, 1.0 mmol) under nitrogen at 0 °C in 10 portions using a syringe. The resultant solution was stirred for 1 h at 0 °C, then anhydrous DMF (32 mg, 1.5 mmol) was added in one portion, and stirred for another 1 h at 0 °C. After the reaction was completed, the reaction solution was poured into 50 3

ACCEPTED MANUSCRIPT mL saturated NH4Cl solution. The product was extracted with EA (3 × 30 mL), and the combined organic layer was washed with the saturated NaCl solution (2 × 30 mL). The organic layer was dried over Na2SO4, filtered and concentrated in vacuo. And then the residue was purified by column chromatography (silica gel: 200-300, PE : EA = 9 : 1) to obtain 5 as a yellow viscous solid in a yield of 72 %. 1H NMR (400 MHz,

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CDCl3): δ 9.74 (s, 1H), 7.54-7.59 (m, 4H), 7.47 (s, 1H), 7.06 (s, 1H), 2.81-2.87 (m, 4H), 2.11-2.17 (m, 2H), 2.09 (s, 3H), 1.98 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 182.61, 146.67, 140.07, 138.65, 138.02, 137.78, 137.71, 136.59, 136.29, 136.26, 133.81, 126.04, 126.00, 125.96, 125.93, 125.43, 125.19, 38.62, 38.47, 23.10, 15.56, 14.56. HRMS (ESI-TOF) m/z: [M + NH4]+ Calcd for C23H23F3NOS2 450.1168; Found

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450.1168. 2.2.2. Synthesis of 1

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1-phenylbutane-1,3-dione 6 (163 mg, 1.0 mmol) and BF3·OEt2 (0.19 mL, 47%, 1.5 mmol) were dissolved in toluene (3 mL) and under nitrogen at 65 °C for 2 h. And then the aldehyde 5 (432 mg, 1.0 mmol) dissolved in the minimum amount of toluene (2 mL) was added into the solution, followed by tributyl borate (0.54 mL, 2.0 mmol). The solution was stirred for 30 min and n-butylamine (50 µL, 0.5 mmol) was added dropwise, and the resulting solution was kept stirring at 65 °C for 24 h. After cooling to

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room temperature, upon removel of solvent under reduced pressure and purified on a silica gel column using petroleum ether/dichloromethane (2:1 v/v) as the eluent to obtain the target dithienylethene 1 as a orange solid in a yield of 69 %. 1H NMR (400 MHz, CDCl3): δ 8.09 (d, J = 16.0 Hz, 1H), 8.04-8.06 (m, 2H), 7.64 (t, J = 8.0 Hz, 1H), 7.58 (s, br, 4H), 7.51 (t, J = 8.0 Hz, 2H), 7.16 (s, 1H), 7.06 (s, 1H), 6.54 (s,

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1H), 6.38 (d, J = 16.0 Hz, 1H), 2.80-2.87 (m, 4H), 2.10-2.15 (m, 2H), 2.08 (s, 3H), 2.00 (s, 3H). 13C NMR

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(101 MHz, CDCl3): δ 181.31, 180.57, 143.93, 140.59, 138.64, 138.62, 137.72, 136.66, 136.63, 136.37, 136.26, 136.01, 134.86, 133.78, 132.31, 129.20, 128.85, 128.78, 126.02, 125.98, 125.44, 125.21, 118.00, 97.65, 38.62, 38.44, 23.11, 15.49, 14.61. HRMS (ESI-TOF) m/z: [M + NH4]+ Calcd for C33H30BF5NO2S2 642.1726; Found 642.1720.

3. Results and discussion

3.1. Synthesis and characterization The stepwise synthesis of the dithienylethene-functionalized difluoroboron β-diketonate complex 1 was outlined in Scheme 1. 1,2-Bis-(5-chloro-2-methylthiophen-3-yl) cyclopent-1-ene 2 was used as the starting material and was treated with n-BuLi and tributyl borate in succession, followed by a Suzuki 4

ACCEPTED MANUSCRIPT cross-coupling reaction with 1-bromo-4-(trifluoromethyl)benzene 3 to afford the corresponding intermediate 4 in a field of 64%. And then the continuous treatment of 4 with n-BuLi followed by addition of DMF gave the monoaldehyde intermediate 5 in a field of 72%. Subsequently, the target complex 1 was synthesized by Knoevenagel condensation reaction between the corresponding aldehyde 5 and

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1-phenylbutane-1,3-dione 6 in a yield of 69%. To avoid a Knoevenagel condensation at the C-2 atom of 1-phenylbutane-1,3-dione 6 (As illustrated in Scheme 1), the β-diketone moiety need to be fixed into the enol form by the formation of BF2 complex between 1-phenylbutane-1,3-dione 6 and BF3·Et2O. Therefore, 6 should firstly react with BF3 at 65 °C for 2 h, followed by successive addition of the aldehyde, tributyl

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borate and n-butylamine, in which B(OBu)3 was used to scavenge water being produced during the reaction, while n-BuNH2 was typical base used as catalysts for this type of reaction. The chemical

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structures of all new intermediates and the target complex were characterized by standard spectroscopic characterization methods (1H NMR, 13C NMR, and HRMS) (Figure S17-22). In the 1H NMR spectrum of the BF2 complex 1, the coupling constants of peaks at ca. 8.09 ppm and 6.38 ppm were 16.0 Hz, meaning

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the trans-form of the vinyl group.

Scheme 1. Synthetic route of dithienylethene 1.

Scheme 2. Photochromism of dithienylethene 1. 5

ACCEPTED MANUSCRIPT 3.2. Photophysical properties in solution The photophysical properties of dithienylethene-containing complex 1 in solvents with different polarity before light irradiation were firstly investigated, as shown in Figure 1 and Table 1. It was clear that an absorption band appeared at 463 - 479 nm, which was ascribed to intramolecular π - π* transition

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[50]. For example, the maximal absorption band of dithienylethene 1 located at 469 nm (ε = 5.45 × 104 cm-1 M-1) in Toluene bathochromic-shifted gradually with increasing the polarity of the solvents, and it reached 479 nm in DMSO (Figure 1A). Accordingly, it exhibited obvious solvatochromic behavior from its absorbance spectra in various solvents (Figure 1C). As illustrated in Figure 1B and 1D, it displayed

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strong fluorescence emission in a nonpolar or less polar solvent (such as Toluene, DCM, THF and EA), and the fluorescence emission bands of 1 red-shifted significantly with increasing the solvent polarity. For

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instance, 1 exhibited a strong green fluorescence in toluene and the emission maximum appeared at 537 nm, which red-shifted to 588 nm in DCM (reddish orange fluorescence). Whereas very weak fluorescence emission was present in Acetone and DMSO. Meanwhile, its fluorescence quantum yields (ФF) in different solvents were determined using Rhodamine 6G as standard, and it was found that the ФF values decreased with the increasing the solvent polarity from 0.44 in Toluene to 0.06 in DMSO (Table 1), which may be

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attributed to nonradiative decay process due to the dipole-dipole interaction between excited molecules and highly polar solvent. Consequently, complex 1 exhibited distinct solvent-dependent absorption and emission spectra in various solvents, which was attributed to intramolecular charge transfer (ICT)

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transition from dithienylethene moiety to the BF2bdk group. Table 1. The photophysical data of 1 in different solvents at 298 K (2.0 × 10-5 mol/L).

λabsa (nm)

ε × 104 b (cm-1 M-1)

λemc (nm)

Φfd

Toluene DCM THF EA Acetone DMSO

469 477 468 463 467 479

5.45 7.40 8.35 3.90 8.31 6.12

537 588 560 558 544 547

0.44 0.39 0.38 0.43 0.08 0.06

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a

Absorption maximum.

b

Extinction coefficients calculated at the absorption maxima.

c

Fluorescence emission maxima.

d

Fluorescence quantum yield determined by a standard method with rhodamine 6G in water (Φf = 0.75, λex = 488 nm) as reference.

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Wavelength / nm

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Normalized absorbance

0.8

Toluene DCM THF EA Acetone DMSO

1.0

Normalized emission

Toluene DCM THF EA Acetone DMSO

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Figure 1. Normalized UV-vis absorption spectra of 1 in different solvents (2.0 × 10-5 mol/L) (A); normalized fluorescence emission spectra of 1 (λex = 463 - 479 nm) in different solvents (2.0 × 10-5 mol/L) (B); corresponding solution color of 1 in different solvents before photoirradiation (C); fluorescent photograph of 1c in different solvents under UV irradiation (365 nm) (D).

3.3. Photochromic properties and fluorescent switching behavior in solution

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Owing to the introduction of the dithienylethene unit, the photoisomerization behavior of dithienylethene 1 in various solvents was investigated at room temperature. It underwent photoisomerization reaction between ring-open isomer and ring-closed isomer upon alternating irradiation

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with > 402 nm and > 600 nm visible light, as illustrated in Scheme 2. And it was found that it exhibited near-infrared (NIR) photochromic behavior with a gradually increasing low-energy absorbance band at

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700 - 738 nm in the various solvents. As shown in Figure 2A, the absorption maximum of 1 in DCM at room temperature was observed at 477 nm (ε = 7.40 × 104 cm-1 M-1). Upon irradiation with > 402 nm visible light, the absorption peak at 477 nm gradually shifted to 427 nm (ε = 4.73 × 104 cm-1 M-1), and a new absorption band centered at 734 nm (ε = 5.51 × 104 cm-1 M-1) appeared concomitant with an obvious color change from yellow to green, as a result of formation of the corresponding ring-closed isomer 1c. Moreover, a well-defined isosbestic point was observed at 536 nm, which indicated that ring-open isomer 1o was cleanly converted into the photocyclized product. Upon irradiation with λ > 600 nm visible light, the green ring-closed isomer 1c underwent a cycloreversion reaction and returned to the initial 1o. In particular, 1 showed very good reversibility and no apparent deterioration (about 5%) was observed after 7

ACCEPTED MANUSCRIPT repeating the above process six times, indicating excellent fatigue resistance (Figure S6). The cyclization and cycloreversion quantum yields of 1 were 0.64 (φo-c) and 0.0087 (φc-o), respectively. Similar photochromic properties were obtained when solutions of dithienylethene 1 in other solvents (Toluene, THF, EA, Acetone and DMSO) were irradiated with the same visible light, as shown in Figure 2B, 2D,

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Figure S1-S5, S7-S11 and Table 2. Furthermore, the optical response rate for 1 in different solvents was compared, and it was found that the response rate in THF was the fastest and the response rate in DMSO was the slowest (Figure 2C). It was worth noting that dithienylethene 1 in a nonpolar or less polar solvent (Toluene, DCM, THF and EA) reached photostationary states more efficiently than that in highly polar

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solvent (Acetone and DMSO). Therefore, the photochromic reaction for dithienylethene 1 also exhibited

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characteristics of solvent polarity dependence.

1.8 > 402 nm

Absorbance

1.4 > 402 nm

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> 600 nm

1.0 0.8 0.6 0.4

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Toluene DCM THF EA Acetone DMSO

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Toluene DCM THF EA Acetone DMSO

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> 600 nm

Normalized absorbance

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Irrad. time / s

Figure 2. Absorption spectral changes of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in DCM (2.0 × 10-5 mol/L) at 298 K (A); Absorption spectra of the closed-isomer 1c in various solvents (2.0 × 10-5 mol/L) (B); Optical response rate of dithienylethene 1 upon > 402 nm light irradiation in various solvents monitored at the maximum absorption wavelength for 1 (C); Corresponding color changes of dithienylethene 1c in various solvents upon photoirradiation (> 402 nm) (D).

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ACCEPTED MANUSCRIPT Table 2. Absorption characteristics and photochromic quantum yields of 1 in different solvents at 298 K.

λabsa (nm)

ε × 104 b (cm-1 M-1)

φo-cc

φc-od

Toluene DCM THF EA Acetone DMSO

706 734 708 700 712 738

4.05 5.51 5.94 2.84 3.81 1.83

0.70 0.64 0.59 0.54 0.27 0.23

0.0091 0.0087 0.0083 0.0089 0.0068 0.0054

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Absorption maximum of closed-ring isomers. b Extinction coefficients calculated at the absorption maxima.

c

Quantum yields of open-ring isomers. d Quantum yields of closed-ring isomers.

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Figure 3. Partial 1H NMR spectral changes of 1 upon visible light irradiation in CDCl3 at room temperature.

The photochromic behavior of complex 1 was examined by means of 1H NMR spectrometry and the photocyclization yield at the photostationary state was measured. As shown in Figure 3, the resonance of numerous protons in the ring-closed isomer 1c showed an obvious upfield shift (such as Hb’, Hc’, Hh’, He’ and Hg’) with respect to that of the corresponding ring-open isomer 1o, which was mainly due to the electronic shielding effect of the closed ring isomers with large conjugated systems. For example, proton signals of trans-vinyl group linking dithienylethene and BF2bdks moieties for 1o appeared at δ = 6.38 ppm (Hb) and 8.09 ppm (Hc), while the ones for 1c appeared at δ = 6.13 ppm (Hb’) and 7.97 ppm (Hc’), respectively. Moreover, the photocyclization yield of dithienylethene was 24% according to 1H NMR 9

ACCEPTED MANUSCRIPT analysis. Subsequently, the fluorescence changes of 1 induced by photoirradiation in the above solvents were explorated at room temperature. As illustrated in Figure 4, its emission intensity in Toluene gradually decreased concomitant with an obvious green fluorescence fading upon irradiation with λ > 402 nm light,

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which was attributed to the formation of the ring-closed isomer. On reaching the photostationary state, the emission intensity of 1 was quenched by ca. 86%. Moreover, the photoirradiation with visible light (λ > 600 nm) could return the original emission as a result of the formation of the ring-opened isomer. Similar results were obtained when other solutions of 1 were irradiated with the same light (Figure S12-S16).

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These results indicated that dithienylethene 1 can be utilized as a potential fluorescent switch in

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optoelectronic materials.

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80 > 402 nm

> 600 nm

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537 nm

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Figure 4. Emission spectral changes of of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in Toluene (2.0×10-5 mol/L) at 298 K, (Inset) Corresponding fluorescence photographs upon photoirradiation (A); The variation curve

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of fluorescent intensity of dithienylethene 1 upon irradiation with > 402 nm light in Toluene (B).

3.4. Photophysical and photochromic properties in film state For practical applications in future optical devices, it was of great importance that photochromic materials can keep excellent photochromism in the solid state or polymer medium [51-53]. Then the photoresponsive behavior of 1 was investigated in film state. The PMMA film was prepared by solubilizing 5.0 mg dithienylethene 1 and 100 mg PMMA in chloroform (2 mL), then evenly coated the homogeneous solution on the filter paper and silica gel plate. The film was dried in air and kept in darkness at room temperature. As shown in Figure. 5A and B, it exhibited strong yellow fluorescence when the dilute chloroform solution of 1 dried completely on filter paper and silica gel plate. After irradiation with > 10

ACCEPTED MANUSCRIPT 402 nm Vis light, the fluorescence emission of 1 was almost completely quenched. Simultaneously, dithienylethene 1 also displayed good photochromism in film state. Upon irradiation with > 402 nm visible light, an obvious color change was observed from yellow to green (as illustrated in Figure 5C and D). The fluorescence and colour of 1-embedded PMMA film could returned to the initial state when irradiating by

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another visible light (λ > 600 nm). These results were basically consistent with those in solution. Furthermore, the film loaded dithienylethene 1 on silica gel plate was irradiated upon the same visible light, it could be seen that the nonirradiated region (letters ‘‘LY’’) exhibited very bright words compared to the irradiated region (Figure 5E). This result indicated that dithienylethene 1 can be used as a photoresponsive

(B)

Vis

(D)

Vis

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(E)

(C)

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(A)

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material in optical printing application.

Figure 5. Fluorescence and photochromism images of 1-embedded PMMA film before and after irradiation with > 402 nm

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Vis light. (A) Fluorescence images on paper; (B) Fluorescence images on silica gel plate; (C) Photochromism images on paper; (D) Photochromism images on silica gel plate; (E) Irradiating area other than the letter ‘LY’ on silica gel plate.

3.5. Theoretical calculation

In order to gain an insight into the electronic features and photoreactivity of 1, its optimized molecular geometries and electron densities have been calculated by dependent density functional theory (DFT) in Gaussian 09 B3LYP/6-31G* level. Details of the optimized structures and molecular orbital correlation diagrams of ring-opened and ring-closed isomers were displayed in Figure 6. The energy-minimized structure of the open-isomer 1o presented a classical antiparallel conformation. And the intramolecular distance between the two reactive carbons was 3.218 Å (Scheme S17), which was short enough for the 11

ACCEPTED MANUSCRIPT cyclization reaction (less than 4.2 Å) [54]. And the computing results indicated that 1o could undergo photocyclization in solution or solid state, which was consistent with the experimental results. Moreover, the thiophene segment with difluoroboron β-diketonate (BF2bdk) linked by a vinyl unit was almost on the same plane, implying that intramolecular charge transfer (ICT) process from dithienylethene moiety to the

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BF2bdk moiety was easy to occur. On the other hand, the HOMO orbital energy of 1o was mainly localized on the central dithienylethene moieties due to great effect of electron deficient boron atom and CF3 group, while its orbital energy of the LUMO was largely distributed over the BF2bdk core. Looking at the electron density distribution, consequently, one can clearly see a definite shift in the electron density towards the

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BF2bdk which indicated ICT. And higher HOMO-LUMO band gap (2.77 eV) for the open-isomer 1o was observed. For the closed-ring isomer 1c, it presented an almost planar conjugated skeleton, and its HOMO

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was mainly delocalized on dithienylethene unit while the LUMO coefficient was nearly on the whole molecular backbone. As expected, 1c presented a narrower energy band gap (1.33 eV) in comparison to 1o due to the extended π conjugation system. Consequently, the theoretical calculations further validated the

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above experimental results of the optical behavior for 1 in solution and solid state.

Figure 6. Frontier molecular orbital profiles of dithienylethene 1 based on DFT calculations at the B3LYP/6-31G* level by using the Gaussian 09 program.

4. Conclusions

In summary, a novel photoresponsive dithienylethene-containing difluoroboron β-diketonate complex 1 was prepared by Knoevenagel condensation reaction. It was found that 1 exhibited solvent-dependent photophysical properties in various solvents (Toluene, DCM, THF, EA, Acetone and DMSO) before visible light irradiation. And it revealed reversible NIR photochromic behaviors with excellent fatigue resistance upon irradiation with visible light in solution, which also showed the solvent-dependent features. 12

ACCEPTED MANUSCRIPT Then the photoresponsive behavior of 1 was investigated in solid state, and it was found that it displayed excellent fluorescence switching behavior and photochromism upon irradiation with the same visible light. Accordingly, the dithienylethene 1 may be acted as a novel visible light-responsive fluorescence dyes in optoelectronic materials.

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Acknowledgment The authors acknowledge financial support from The Key Scientific Research Project of Higher Education of Henan Province (No. 18A150012, 16A350008). The Science and Technology Project of

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Henan Province (No. 162300410008). The Key Project Technology Research Henan Province Department Education (No. 16B150010).

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Supporting Information

The absorption spectral changes, fatigue resistance and emission spectral changes in various solvents of dithienylethene 1; 1H NMR,

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C NMR and HRMS spectra of the interminate 5 and dithienylethene 1

and are available in the Supporting Information.

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Supporting Information for

Synthesis, Photophysical Properties and NIR Photochromism of

Dithienylethene Unit

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Photoresponsive Difluoroboron β-Diketonate Complex Based on

Key Laboratory of Organic Functional Molecules, Luoyang City, College of Food and Drug, Luoyang Normal University,

b

College of Chemical Engineering and Environment, Henan University of Technology, Zhengzhou 450001, P. R. China

College of Chemistry and Chemical Engineering, and Henan Key Laboratory of Function-Oriented Porous Materials,

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Luoyang Normal University, Luoyang 471022, China

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Luoyang 471934, P. R. China

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Ziyong Li,a,* Mengna Li,a Guoxing Liu,b Yangyang Wang,a Guohui Kang,c Chaoyang Li,a Hui Guoa,*

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Table of Contents

Absorption spectral changes of 1 in Toluene, THF, EA, Acetone and DMSO ……………2-3

2.

Fatigue resistance of dithienylethene 1 in various solvents………………………………3-5

3.

Emission spectral changes of of 1 in DCM, THF, EA, Acetone and DMSO………………6-8

4.

The intramolecular distance between the two reactive carbons of of 1 based on DFT

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Appendix: NMR and Mass spectra………………………………………………………9-11

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calculations…………………………………………………………………………………8

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1.2

> 402 nm

> 600 nm

1.0 > 600 nm

0.6 0.4 0.2 0.0 400

500

600

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800

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Wavelength / nm

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Absorbance

> 402 nm 0.8

Figure S1. Absorption spectral changes of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in Toluene (2.0 ×

1.8

> 402 nm

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10-5 mol/L) at 298 K.

> 600 nm

1.6

> 402 nm

1.2 1.0 0.8

> 600 nm

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0.4 0.2 0.0

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Wavelength / nm

Figure S2. Absorption spectral changes of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in THF (2.0 ×

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10-5 mol/L) at 298 K.

0.9 > 402 nm

> 600 nm

0.8

Absorbance

0.7 > 402 nm

0.6

> 600 nm

0.5 0.4 0.3 0.2 0.1 0.0 400

500

600

700

800

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1000

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Wavelength / nm

20

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1.8 > 402 nm

> 600 nm

1.6

1.2

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1.4

1.0 > 402 nm

> 600 nm

0.8 0.6 0.4 0.2

400

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Figure S4. Absorption spectral changes of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in Acetone (2.0 × 10-5 mol/L) at 298 K.

1.4 > 402 nm

> 600 nm

1.2

0.8

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Figure S5. Absorption spectral changes of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in DMSO (2.0 × 10-5 mol/L) at 298 K.

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1.2

0.8 0.6 0.4 0.2 0.0 0

1

2

3

4

5

6

Times

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Figure S6. Fatigue resistance of dithienylethene 1 at 734 nm on alternate excitation at > 402 nm and > 600 nm Vis light

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irradiation over six cycles in DCM (2.0 × 10-5 mol/L) at 298 K.

0.8

Absorbance

0.6

0.4

0.2

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4

5

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Figure S7. Fatigue resistance of dithienylethene 1 at 706 nm on alternate excitation at > 402 nm and > 600 nm Vis light

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irradiation over six cycles in Toluene (2.0 × 10-5 mol/L) at 298 K.

Absorbance

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1.2 1.0 0.8 0.6 0.4 0.2 0.0

0

1

2

3

4

5

6

Times

Figure S8. Fatigue resistance of dithienylethene 1 at 708 nm on alternate excitation at > 402 nm and > 600 nm Vis light irradiation over six cycles in THF (2.0 × 10-5 mol/L) at 298 K. 22

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0.6

0.4 0.3 0.2 0.1 0.0 0

1

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3

4

5

6

Times

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Figure S9. Fatigue resistance of dithienylethene 1 at 700 nm on alternate excitation at > 402 nm and > 600 nm Vis light

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irradiation over six cycles in EA (2.0 × 10-5 mol/L) at 298 K.

0.8 0.7 0.6

Absorbance

0.5 0.4 0.3 0.2 0.1

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-0.1

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Figure S10. Fatigue resistance of dithienylethene 1 at 712 nm on alternate excitation at > 402 nm and > 600 nm Vis light

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irradiation over six cycles in Acetone (2.0 × 10-5 mol/L) at 298 K.

0.40 0.35 0.30

Absorbance

0.25 0.20 0.15 0.10 0.05 0.00

-0.05 0

1

2

3

4

5

6

Times

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ACCEPTED MANUSCRIPT Figure S11. Fatigue resistance of dithienylethene 1 at 738 nm on alternate excitation at > 402 nm and > 600 nm Vis light irradiation over six cycles in DMSO (2.0 × 10-5 mol/L) at 298 K.

90 100 80

> 600 nm

588 nm 70

FL Intensity

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Figure S12. Emission spectral changes of of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in DCM (2.0×10-5 mol/L) at 298 K, (Inset) Corresponding fluorescence photographs upon photoirradiation (A). The variation curve

> 402 nm

80

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560 nm

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FL Intensity

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50 40 30 20 10 0

700

750

0

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40

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80

100

Time / nm

Figure S13. Emission spectral changes of of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in THF (2.0×10-5 mol/L) at 298 K, (Inset) Corresponding fluorescence photographs upon photoirradiation (A). The variation curve of fluorescent intensity of dithienylethene 1 upon irradiation with > 402 nm light in THF (B).

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> 600 nm

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FL Intensity

FL Intensity

558 nm

80

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Figure S14. Emission spectral changes of of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in EA (2.0×10-5

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mol/L) at 298 K, (Inset) Corresponding fluorescence photographs upon photoirradiation (A). The variation curve of

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25

FL Intensity

20

15

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Figure S15. Emission spectral changes of of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in Acetone

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Figure S16. Emission spectral changes of of dithienylethene 1 with > 402 nm and > 600 nm light irradiation in DMSO

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(2.0×10-5 mol/L) at 298 K.

Figure S17. The intramolecular distance between the two reactive carbons of of dithienylethene 1 based on DFT calculations

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at the B3LYP/6-31G* level by using the Gaussian 09 program.

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Appendix: NMR and Mass spectra

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Figure S18. 400 MHz 1H NMR spectrum of intermediate 5 in CDCl3 at room temperature.

Figure S19. 100 MHz 13C NMR spectrum of intermediate 5 in CDCl3 at room temperature.

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Figure S20. HRMS of intermediate 5.

Figure S21. 400 MHz 1H NMR spectrum of dithienylethene 1 in CDCl3 at room temperature.

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Figure S22. 100 MHz 13C NMR spectrum of dithienylethene 1 in CDCl3 at room temperature.

Figure S23. HRMS of dithienylethene 1.

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ACCEPTED MANUSCRIPT Highlights

Photoresponsive difluoroboron β-diketonate complex was synthesized.

•

It exhibited solvent-dependent and visible light-triggered NIR photochromism.

•

Fluorescence switching behavior and photochromism in film state were observed.

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•