Stimuli-response fluorescence behaviors of dimesitylboron functionalized with tetraphenylethylene

Tetrahedron Letters xxx (2016) xxx–xxx

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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

Stimuli-response fluorescence behaviors of dimesitylboron functionalized with tetraphenylethylene Junhui Jia a,b, Pengchong Xue a, Ran Lu a,⇑ a

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China Key Laboratory of Magnetic Molecules and Magnetic Information Material, Ministry of Education, College of Chemistry and Material, Shanxi Normal University, Linfen 041004, PR China b

a r t i c l e

i n f o

Article history: Received 9 March 2016 Revised 27 April 2016 Accepted 29 April 2016 Available online xxxx Keywords: Stimuli-response Tetraphenylethylene Dimesitylboron Mechanofluorochromism AIE Fluoride anion

a b s t r a c t New dimesitylborylthiophene derivative with terminal tetraphenylethene TPETB has been synthesized. It was found that TPETB exhibited aggregation-induced emission behavior and reversible mechanofluorochromism. For instance, the emission intensity of TPETB in THF/H2O with water fraction of 90% was ca. 21 times higher than that in THF. In addition, grinding the as-prepared crystal of TPETB the emitting color changed from sky blue to yellowish green. Meanwhile, the ground powder could recover to crystalline structure with sky blue emission when it was exposed to CH2Cl2 vapor. It was suggested that the transformation between crystalline and amorphous states led to the changes of solid emitting colors in response to external mechanical forces. It should be noted that TPETB could detect F selectively, and the detection limit was 1.67  10 8 mol/L in CH2Cl2. Ó 2016 Elsevier Ltd. All rights reserved.

Introduction Recently, due to the potential applications in the fields of organic solid-state lasers, organic light-emitting diodes (OLEDs), organic light-emitting field-effect transistors, and organic fluorescent sensors,1 lots of p-conjugated compounds with strong solid emission have been synthesized. Among them, some fluorescent organic molecules exhibited tunable and switchable solid-state luminescence by external mechanical stimuli, such as smearing, grinding, and pressing,2 termed as mechanofluorochromic (MFC) materials. They are a class of ‘smart’ materials, and have been widely used in sensors,3 memory chips,4 and security inks.5 To date, many different kinds of organic molecules bearing non-planar p-conjugated structures have been found to exhibit MFC properties. Because of the loose molecular stacking in crystal states, the solid emitting colors of tetraphenylethene (TPE), 9,10-divinylanthracene, oligo(p-phenylene vinylene) derivatives, and boron-containing complexes could be changed under mechanical stimuli.6–8 For example, Liu and Zhang have reported that methoxy-substituted tetraphenylethylene derivatives of tetra(4-methoxyphenyl) ethylene and tetra(3,4-dimethoxyphenyl)ethylene exhibited MFC behaviors with multi-color emission and high solid fluorescence ⇑ Corresponding author. E-mail address: [email protected] (R. Lu).

quantum yields.9a A cyano-substituted distyrylbenzene dye with D–p–A structure was synthesized by Wang’s group and its luminescent colors could be tuned by different external stimuli, including mechanical force, organic vapor, heating, acid, and base.9b We have reported that the triphenylacrylonitrile derivatives functionalized with phenothiazine2f and the benzoxazole derivatives bearing triphenylamine or phenothiazine2c,2d gave excellent MFC properties. On the other hand, it is known that the p-conjugated boron-containing compounds have been widely used in nonlinear optical materials,10–14 two-photon fluorescence materials,15–17 organic light-emitting devices (OLEDs)18,19 due to their special electronic structures. Additionally, due to the existence of empty pp orbital in boron center, significant changes in luminance would happen when triarylboranes interacted with Lewis bases via interrupting the pp–p conjugation. Therefore, triarylboranes could be employed as sensors for detecting F .20 To the best of our knowledge, no MFC material based on triarylborane has been reported. Considering that TPE-based derivatives usually showed AIE as well as MFC activities, we introduced TPE into a non-planar dimesitylboron, and synthesized a new dimesitylborylthiophene derivative (TPETB, Scheme 1) so as to gain multifunctional fluorescence dye. It was found that the emission of TPETB was weak in solution, but strong blue emission could be detected in solid state, illustrating AIE in aggregated state. Meanwhile, TPETB exhibited reversible mechanofluorochromism upon the

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J. Jia et al. / Tetrahedron Letters xxx (2016) xxx–xxx

Scheme 1. The synthetic routes for compound TPETB.

treatment of grinding/fuming with CH2Cl2. Besides, it could recognize fluoride anion selectively in CH2Cl2. Such multi stimuliresponse fluorescence dye would be used as sensory material to detect F and external mechanic force. Results and discussion The synthetic route for TPETB is shown in Scheme 1. Compound 1 ((2-(4-bromophenyl)ethene-1,1,2-triyl)tribenzene) was synthesized according to the reported procedure from diphenylmethane and (4-bromophenyl)phenylmethanone.21 Compound 2 (tributyl (thiophen-2-yl)stannane) was synthesized according to the literature.22 The Stille coupling reaction between compounds 1 and 2 catalyzed by Pd(PPh3)2Cl2 under N2 atmosphere afforded compound 3 (2-(4-(1,2,2-triphenylvinyl)phenyl)thiophene) in a yield of 80%. TPETB was prepared by the lithiation of compound 3 with n-BuLi at 78 °C in dry THF, followed by the nucleophilic substitution of dimesitylboron fluoride with the corresponding thiophene lithium. The new compounds were characterized with FT-IR, 1H NMR, 13C NMR, MALDI/TOF mass spectroscopy and high resolution mass spectroscopy or elemental analysis. The target compound TPETB showed good solubility in most organic solvents, including toluene, CH2Cl2, THF, CHCl3, DMF, etc. As shown in Figure 1, two obvious absorption bands for TPETB appeared at 331 nm and 384 nm, respectively, in CH2Cl2 (1.0  10 6 M). The former one was attributed to p–p⁄ transition and the latter one was ascribed to the intermolecular charge transfer (ICT) transition. In addition, the maximal emission peak of TPETB emerged at 498 nm was ascribed to ICT emission,12c which could be confirmed by the solvent-dependent fluorescence emission spectra (Fig. S1 and Table S1). Although no obvious change was detected for the absorption of TPETB in different solvents, the emission bands red-shifted obviously with

Figure 1. Normalized UV–vis absorption and fluorescence emission spectra (kex = 385 nm) of TPETB in CH2Cl2 (1.0  10 6 M).

increasing solvent polarities. For example, the emission at 478 nm for TPETB in n-hexane shifted to 506 nm in DMF. It should be noted that the emission of TPETB was weak in solutions, and the fluorescence quantum yield (UF) was only 0.01 in THF. It is known that the intramolecular bond rotations would result in the weak emission of tetraphenylethene derivatives in solutions.22 To further understand the electronic structure of TPETB, the frontier molecular orbitals were obtained at B3LYP/6-31 level with the GAUSSIAN 2012W program package. As shown in Figure 2, the HOMO was mainly located in tetraphenylethene and thiophene units, and the LUMO was predominantly situated on the boron atom and the surrounding aromatic rings. It revealed the D–p–A feature of TPETB, so that ICT might occur. Although the emission of TPETB was weak in solutions, intense greenish blue light was observed from the solid of TPETB under UV illumination (Fig. S2), and the solid fluorescence quantum yield was as high as 0.40. We deduced that AIE might happen. Compared with the UF of 0.01 in THF, an AIE factor (aAIE = UF,a/UF,s, where UF,a and UF,s are the quantum yield in the aggregated state and in solution, respectively)23 for the solid of TPETB reached 400. In order to confirm the occurrence of AIE, the emission spectra of TPETB in the mixtures of THF/H2O containing different amounts of water are shown in Figure 3. It was clear that the emission spectra of TPETB in THF/H2O with water fraction below 30% were quite similar to that in THF. In THF/H2O with water fraction of 30–70%, the emission of TPETB red-shifted to 510 nm compared with that in THF (495 nm) due to the increase of the solvent polarity, accompanying with a slight increase of the fluorescence intensity. However, a dramatic enhancement of the luminescence was observed when the water fraction was over 80%. For example, the emission intensity of TPETB in THF/H2O with water fraction of 90% was ca. 21 times higher than that in THF, and the UF increased to 0.33 (Table S2). It meant that the intramolecular rotations would be suppressed in aggregates formed in THF/H2O with high water fraction.24 Due to the introduction of a bulky group of dimesitylborylthiophene into tetraphenyletnene derivative, the MFC property of TPETB was expected since the molecules would pack loosely in crystal, in which the crystalline structure can be destroyed easily by external force. As shown in Figure 4, the as-prepared crystal of TPETB gave strong sky blue emission with a maximum at 460 nm, and it could be transformed into the ground powder emitting yellowish-green fluorescence centered at 494 nm upon gently grinding with a spatula. Upon fuming the ground powder with CH2Cl2 vapor for about 30 s, the original sky blue emission was restored. Such MFC process can be repeated for many times (Fig. 4c).12c–e To gain insights into the MFC mechanism, X-ray diffraction (XRD) patterns of TPETB in different solid states are shown in Figure 4d. We found that the as-prepared sample gave several intense and sharp diffraction peaks, indicative of crystalline nature. However, no obvious diffraction peak could be detected for the ground powder. It meant that the crystalline structures could be destroyed when the as-prepared crystal was ground. Fuming the ground powder with CH2Cl2, sharp diffraction peaks reappeared, implying the recovery of an ordered crystalline structure. Therefore, the mechanofluorochromism of TPETB can be realized via the transformation between the crystalline and amorphous states. It has been known that dimesitylboron derivatives can be used as ratiometric fluorescence sensory materials to detect F .10–14 In order to study the sensory ability of TPETB to anions, the tetrabutylammonium salts of F , Cl , Br , I , OAc , H2PO4 , and HSO4 were selected as analysts. As shown in Figure S3, only F could lead to the color change of TPETB from yellowish-green to colorless. Meanwhile, the emission of TPETB could be weakened obviously upon the addition of F . However, other anions could not induce

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Figure 2. The frontier orbital plots of the HOMO and LUMO of TPETB.

Figure 3. Fluorescence emission spectra (a) and the emission intensity (b) of TPETB in THF/water with different water fractions (kex = 385 nm). Inset: the photos of TPETB in THF/water with water faction of 90% (left) and in THF (right). The concentration is maintained at 1.0  10 6 M.

Figure 4. Photographs TPETB in different solid states under UV light (365 nm) (a); fluorescence emission spectra of TPETB in different solid states (kex = 410 nm, b); emission wavelengths of TPETB upon repeated grinding and fuming treatments (c); XRD patterns of TPETB in different solid states (d).

the color change of TPETB in CH2Cl2. Thus, TPETB can detect F selectively. Additionally, the UV–vis absorption and fluorescence emission spectra of TPETB in CH2Cl2 changed obviously upon adding F instead of other anions (Figs. 5, S4 and S5). To further

reveal the interaction between F and TPETB, the spectral titration experiments were carried out. As shown in Figure 5a, the absorption bands of TPETB at 329 nm and 382 nm blue-shifted gradually to 316 nm and 363 nm, respectively, with increasing the amounts

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Figure 5. UV–vis absorption spectra (a), the maximal absorption intensity at 382 nm (b), fluorescence emission (c, kex = 385 nm) spectra and the maximal fluorescence emission intensity at 498 nm (d) of TPETB (1.0  10 6 M) upon adding different equiv of F in CH2Cl2.

of F , accompanying with the decrease of the absorption intensities. In addition, two isoabsorptive points emerged at 301 nm and 361 nm, respectively. It illustrated that strong interaction would happen between F and TPETB. Moreover, the emission intensity at 498 nm for TPETB decreased gradually with increasing the amount of F (Fig. 5c). When 5 equiv of F was added, the fluorescence quenching reached to 16%. Moreover, the binding constant (Kass) and the binding number (n) of TPETB for fluoride anion were determined from the fluorescence titration experiment.3c,25 The lg Kass value was found to be 6.09 (Fig. S6), illustrating the strong interaction between TPETB and F .26 The binding number (n) was evaluated to be 1.05, indicating the formation of 1:1 complex between TPETB and F .27 Additionally, the competition experiments were performed by addition of 5 equiv of F to the solutions of TPETB containing 50 equiv of other anions (Cl , Br , I , OAc , HSO4 , H2PO4 ). As shown in Figure S7, the recognition of F by TPETB could not be affected by other anions. The detection limit for F was measured to be 1.67  10 8 mol/L (Fig. S8). Conclusions In summary, a new dimesitylborylthiophene functionalized with tetraphenylethene TPETB was synthesized. Although the emission of TPETB in solutions was very weak, its solid fluorescence quantum yield reached 0.40. Additionally, we found that the emission intensity of TPETB in THF/water with water fraction of 90% was ca. 21 times higher than that in THF. Such AIE behavior of TPETB might be originated from the suppression of the intramolecular rotations. Moreover, the as-prepared crystal of TPETB emitted strong sky blue light, and the emitting color changed into yellowish green when it was ground. The ground powder could be turned into crystalline state with sky blue emission upon fuming with CH2Cl2, illustrating reversible MFC process. It was found that TPETB could be used as fluorescent sensory material

to detect F selectively in CH2Cl2 with the detection limit of 1.67  10 8 mol/L. Therefore, the luminescent materials with multi stimuli-response to external microenvironments can be used as sensory materials to sense analysts, mechanical forces, and so on. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21374041), the Open Project of the State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201615). Supplementary data Supplementary data (synthesis of compounds; photophysical data; 1H NMR, 13C NMR and MALDI/TOF mass spectra; UV–vis absorption and fluorescence emission spectra; Fluorescence sensory properties) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.04.110. These data include MOL files and InChiKeys of the most important compounds described in this article. References and notes 1. (a) Zhang, X. Q.; Chi, Z. G.; Zhang, Y.; Liu, S. W.; Xu, J. R. J. Mater. Chem. C 2013, 1, 3376–3390; (b) Kwon, M. S.; Gierschner, J.; Seo, J.; Park, S. Y. J. Mater. Chem. C 2014, 2, 2552–2557; (c) Cheng, X.; Li, D.; Zhang, Z. Y.; Zhang, H. Y.; Wang, Y. Org. Lett. 2014, 16, 880–883; (d) Zhang, Y. J.; Sun, J. W.; Zhuang, G. L.; Ouyang, M.; Yu, Z. W.; Cao, F.; Pan, G. X.; Tang, P. S.; Zhang, C.; Ma, Y. G. J. Mater. Chem. C 2014, 2, 195–200; (e) Yuan, W. Z.; Gong, Y. G.; Chen, S. M.; Shen, X. Y.; Lam, J. W. Y.; Lu, P.; Lu, Y. W.; Wang, Z. M.; Hu, R. R.; Xie, N.; Kwok, H. S.; Zhang, Y. M.; Sun, J. Z.; Tang, B. Z. Chem. Mater. 2012, 24, 1518–1528; (f) Gong, Y. Y.; Zhang, Y. R.; Yuan, W. Z.; Sun, J. Z.; Zhang, Y. M. J. Phys. Chem. C 2014, 118, 10998–11005; (g) Gong, Y. Y.; Tan, Y. Q.; Liu, J.; Lu, P.; Feng, C. F.; Yuan, W. Z.; Lu, Y. W.; Sun, J. Z.; He, G. F.; Zhang, Y. M. Chem. Commun. 2013, 4009–4011. 2. (a) Chi, Z. G.; Zhang, X. Q.; Xu, B. J.; Zhou, X.; Ma, C. P.; Zhang, Y.; Liu, S. W.; Xu, J. R. Chem. Soc. Rev. 2012, 41, 3878–3896; (b) Sun, J. W.; Lv, X. J.; Wang, P. J.; Zhang, Y. J.; Dai, Y. Y.; Wu, Q. C.; Ouyang, M.; Zhang, C. J. Mater. Chem. C 2014, 2,

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Please cite this article in press as: Jia, J.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.04.110