Tetraphenyethylene-Fused Coumarin Compound Showing Highly

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

A Tetraphenyethylene Fused Coumarin Compound Showing Highly Switchable Solid-State Luminescence Yu-Xin Peng, Hua-Qi Liu, Rong-Guang Shi, Fan-Da Feng, Bin Hu, and Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12567 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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The Journal of Physical Chemistry

A Tetraphenyethylene Fused Coumarin Compound Showing Highly Switchable Solid-State Luminescence Yu-Xin Peng,a Hua-Qi Liu,b Rong-Guang Shi,a Fan-Da Feng,a Bin Hu,b and Wei Huang*a aState

Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu Province, 210093, P. R. China bSchool of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang, Jiangxi, 330063, P.R. China

ABSTRACT: This paper is to explore high-contrast mechanoresponsive luminescence (MRL) turn-on materials by combining the features of the coumarin and typical aggregationinduced emission enhancement (AIEE) tetraphenyethylene groups. So fluorogen C3Ph is obtained by fusing triphenylvinyl and coumarin units, which exhibits typical AIEE characteristics with high solid-state photoluminance (PL) efficiency in amorphous state. The formation of a unique 2D hydrogen-bonded organic framework (HOF) is responsible for the crystalline-state PL quenching but guarantees the luminescence turn-on to the external stimuli. Furthermore, the reversible solid-state fluorescence on-off cycle for C3Ph can be achieved by alternate grinding and melting-solidifying. This work also offers a way to construct high-contrast MRL turn-on materials by using the HOF-involved molecules. INTRODUCTION Mechanoresponsive luminescence (MRL) materials have attracted considerable attention because of their interesting applications in sensors, security papers, memory chips, optical storage, smart inks, etc.1-10 To be an ideal switchable material, the emission of light should be sensitive to the external stimulus such as pressing, grinding, crushing, rubbing, and stretching. To this point, high-contrast MRL switching materials, especially for those achieving MRL turn-on, are imperative. However, high-contrast luminescence recording requires dramatic switching in the luminescence intensity that is not easily achieved from the traditional MRL materials.11-14 To date, only few MRL systems showing high-contrast turn-on luminescence characteristics have been reported.15-19 Among them, the key on-off MRL luminescence emission is mainly modulated by the processes of photodimerizaiton,15 photoinduced electron transfer,16 intramolecular charge transfer,17 and so on. It is already known that intermolecular interactions, such as π–π stacking and hydrogen-bonding, have close relation to the solidstate luminescence properties of fluorescent materials.20-23 Especially, the morphology or conformation switching accompanied by the perturbation of π–π interactions can affect the photophysical process of excited state in the solid phase, which is vital in constructing on-off luminescence switching.24-27 For example, by taking advantage of the defect-sensitive feature of diaminomaleonitrile-functionalized Schiff base with multiple π–π interactions, Tang and co-workers reported a “turn-on” type of mechanofluorochromic material.26 Recently, Li and co-workers have reported two symmetrical benzene-dicyanovinyl based carbazole derivatives with remarkable MRL turn-on characteristics, where the disturbing of π-π stacking is considered to be responsible for the mechanochromisms.27

Coumarin 7 (C7),28 a commercial dye bearing benzimidazole and diethylamino groups at 3 and 7 positions of the coumarin heterocycle ring (Chart 1), has been widely used in the fields of coloring of textile fibers, lasers, photosensitizers, and so on.29-32 The solution of C7 can emit extremely strong fluorescence, but the solid-state emission is much weaker, which is similar to most conventional fluorescent molecules exhibiting aggregation-caused quenching (ACQ) effects.28,33-35 It is deduced that the weak solid emission for C7 is associated with the strong π–π packing interactions between the aromatic rings observed in its singlecrystal structure,36 which commonly turn “off” the light emission. In this sense, the turn “on” state for solid-state C7 might be achieved if the strong π–π stacking is destroyed by the mechanical stimulus. The morphology-dependent photoluminance (PL) emission of C7 in the solid state has been carried out in our initial studies (Figures 5B-C), and the amorphous powder of C7 (Фa = 13.32%) shows much stronger solid-state PL emission than the crystalline sample (Фa = 0.09%). Moreover, grinding the crystalline sample of C7 can result in the PL emission enhancement as expected, but the luminescence contrast ratio is not high enough (ca. 50-fold). H N

O

O

PK

N

H N

O

N C3Ph

O N

C7

Chart 1. Chemical structures of coumarin based compounds C3Ph and C7. In order to explore high-contrast and even switchable MRL turn-on materials, a tetraphenyethylene (TPE) fused coumarin compound C3Ph is designed and synthesized in this work (Chart 1), in which the molecular assemblies of C3Ph could be subjected to two counter-acting forces. On the one hand, the presence of planar fluorogen benzimidazole/coumarin moiety with multiple heteroatoms in C3Ph may lead to strong π–π and hydrogenbonding interactions, which could both effectively quench the solid-state emission. On the other hand, the existence of twisted conformation of propeller-like TPE may weaken the dipole interactions and open the non-radiate channel. Such a conflicting situation is anticipated to generate a metastable turn-off state of fluorescence, which plays an important role in accessing sensitive MRL turn-on properties.37-38 EXPERIMENTAL SECTION Syntheses of Compounds. Compound 1: A mixture of 1bromo-1,2,2-triphenylethene (1.00 g, 2.98 mmol), 4-formyl-3hydroxyphenylboronic acid pinacol ester (0.90 g, 3.63 mmol),

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Pd(PPh3)4 (0.06 g, 0.05 mmol) and Cs2CO3 (2.91 g, 8.94 mmol) was dissolved in a degassed mixture of dioxane (100 mL) and H2O (20 mL), put into a degassed three-necked flask and refluxed under argon atmosphere for 32 h. After being cooled to room temperature, the solution was removed under a vacuum. The crude product was finally separated by silica gel column chromatography using CHCl3 as the eluent to give pure compound 1 as pale-yellow solid (0.63 g, 56%). 1H NMR (400 MHz, CDCl3) δ: 10.90 (s, 1H), 9.71 (s, 1H), 7.23 (d, J = 8.39 Hz, 2H), 7.13-7.08 (m, 8H), 7.06-7.04 (m, 2H), 7.01-6.99 (m, 4H), 6.69-6.68 (m, 2H); 13C NMR (100 MHz, CDCl ) δ: 195.9, 161.2, 153.5, 143.4, 143.0, 3 142.9, 142.6, 139.6, 132.8, 131.3, 131.2, 131.2, 128.0, 128.0, 127.8, 127.3, 127.0, 127.0, 123.4, 120.2, 118.9; ESI-MS: m/z: 375.33 [M+H]-; elemental analysis calcd (%) for C27H20O2: C, 86.14; H, 5.36; found: C, 86.01; H, 5.62. Compound C3Ph: Compound 1 (0.50 g, 1.33 mmol), 2benzimidazolylacetonitrile (0.25 g, 1.60 mmol) and a catalytic amount of piperidine (two drops) was dissolved in ethyl alcohol (60 mL). The mixture was stirred at room temperature for overnight, the resulting slurry was cooled to 0 oC and filtered. The solid was dispersed into HCl solution (50 mL, 6 M), and refluxed for overnight. After cooled to room temperature, the resulting slurry was filtered. The solid was rinsed with distilled water, recrystallized from CHCl3-hexane (6:1), and dried in a vacuum to give pure compound C3Ph as a yellow solid (0.53 g, 77 %). 1H NMR (400 MHz, CDCl3) δ: 11.20 (s, 1H), 8.85 (s, 1H), 7.67-7.66 (m, 1), 7.41-7.39 (m, 1H), 7.30 (d, J = 8.39 Hz, 1H), 7.18-7.16 (m, 4H), 7.08-6.94 (m, 17H); 13C NMR (100 MHz, CDCl3) δ: 161.0, 153.3, 149.8, 146.3, 143.9, 143.3, 142.9, 142.8, 142.7, 141.7, 139.1, 133.6, 131.3, 131.3, 131.3, 128.7, 128.1, 128.1, 127.8, 127.5, 127.1, 123.7, 123.0, 119.3, 119.2, 117.4, 115.3, 111.6; ESIMS: m/z: 515.58 [M+H]-; elemental analysis calcd (%) for C36H24N2O2: C, 83.70; H, 4.68; N, 5.42; found: C, 83.45; H, 4.87; N, 5.21.

comparison with C7 (452 nm), indicating that the triphenylvinyl group is a weaker electron donor than the diethylamino group. When irradiating the dilute solution of C3Ph, dark-green fluorescence could be observed, whereas its powder emits strong yellow-green light under the 365 nm UV light (Figures 4A and 4C). In contrast, the solid sample of C7 shows much weaker PL intensity than in solution (Figures 5A and 5C). The distinct fluorescence behaviors for C7 and C3Ph in solution and in the solid state hint their distinguishing aggregation nature.

a Figure 1. UV-vis absorption emission (b) for compounds C7 acetone solutions at room concentration of 3×10−5 M. λex = C3Ph.

HO

Br

H N

O B O

Pd(PPh3)4, K2CO3, dioxane / H2O

N

CHO OH

H N CN

O

b spectra (a) and fluorescence (black) and C3Ph (red) in their temperature with the same 450 nm and 400 nm for C7 and

Aggregation-Induced Emission Behavior. The aggregation behaviors of C7 and C3Ph are further studied by recording the UV-vis absorption and PL spectra in the acetone/water mixtures with different fractions of water (fw) (Figure 2). The dilute acetone solution of C7 exhibits strong emission at 488 nm (Ф = 78%) when excited at 450 nm. When fw is increased from 0% to 90% gradually, the leveled off tails of the absorption spectra appear in the long-wavelength region (Figure 2a), indicative of the formation of aggregates. Moreover, the solution fluorescence is quenched step by step with the increase of fw values displaying typical ACQ effects (Figures 2b and 5A). Different from C7, C3Ph shows typical aggregation-induced emission enhancement (AIEE) nature, in which the pure acetone solution gives weak PL emission and the Ф value is only 1.31%. When the water fractions are lower than 50%, no obvious alterations could be observed in both the absorption and emission spectra (Figures 2c-2d). When the water fraction is further increased, the PL intensity enhances dramatically, and the highest fluorescence intensity (Фs,aggr = 31.21%) is recorded at fw = 90% (Figure 4A).

RESULTS AND DISCUSSION Syntheses and Photophysical Property. As can be seen in scheme 1, our synthsis started from the key intermidiate 1, which was prepared from the Pd-catalyzed crosscoupling reaction between 1-bromo-1,2,2-triphenylethene and 4-formyl-3hydroxyphenylboronic acid pinacol ester. Then, the target coumarin based compound C3Ph was produced via two steps reactions: 1, the iminolactone was firstly obtained via the basecatalysed condensation between 1 and 2benzimidazolylacetonitrile. 2, the precursor iminolactone was allowed to refluxing in dilute aqueous hydrochloric solution to offer the final target compound C3Ph.

OHC

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O

N

1, piperidine, EtOH 2, HCl / H2O

1, 56 %

C3Ph, 77 %

Scheme 1. Synthetic route of C3Ph. a For the sake of comparison, the photophysical behaviors of C3Ph and C7 in solution were recorded at the same concentration of 3×10−5 M (Figure 1). Generally, C7 could be considered as a typical donor-acceptor chromogenic compound, where the diethylamino group is a strong electron donor and the carbonyl group serves as the electron acceptor.28,29,39 Herein, the maximum absorption peak of C3Ph (401 nm) is obviously blue-shifted in

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c

d

Figure 2. UV–Vis absorption (a and c for C7 and C3Ph ) and fluorescence emission spectra (b and d for C7 and C3Ph ) in acetone/water mixtures with the same starting concentration of 3.0 × 10−5 mol L−1 and different water volume fractions (fw). λex = 450 nm and 400 nm for C7 and C3Ph. Moreover, powder X-ray diffraction (PXRD) measurements have been performed to reveal the different aggregated states of C7 and C3Ph. The aggregated sample of C7 gives a diffraction pattern very close to its crystalline solid (Figure 3a). In contrast, the PXRD spectrum of aggregated C3Ph shows almost no peaks corresponding to the amorphous state (Figure 3b). In fact, multiple π–π stacking interactions could be observed in the crystal packing of both C7 and C3Ph (see the following structural analyses), which are believed to significantly quench the solid state fluorescent emission.26,40 Thus, it is suggested that the restricted intramolecular motions of TPE structure for C3Ph can overwhelm its dipole-dipole interactions and lead to the emission enhancement when aggregated into nanosuspensions.

Figure 4. (A) PL spectra of C3Ph in acetone (black) and aggregates at fw = 90% (red), together with the photographs taken under UV illumination. (B) Emission spectra of crystalline C3Ph before and after grinding. (C) Morphology-dependent PL emission photographs and the switching processes: I, grinding with a glass sheet and II, melting-solidifying. (D) Plot of solid luminescence quantum yields versus repeated I and II cycles. Mechanoresponsive Luminescence Properties. Similar to C7 (Figures 5B-5C), the crystalline sample of C3Ph is nearly nonemissive with a ultralow Фs value of 0.13% (Figures 4B-4C), whereas its amorphous powder could emit much stronger fluorescence (Фs = 57.12%) exhibiting strong morphologydependent PL properties. The MRL behavior of C3Ph is further investigated (Figure 4C), in which its crystals is placed between two pieces of quartz glasses and ground into powder (process I). As anticipated, non-emissive C3Ph crystals became emissive and the Фs value increases from 0.13% to 25.2% (193 fold). Besides the drastic increment of emission intensity, the emission maxima of C3Ph is blue-shifted from 545 nm to 529 nm after grinding. This process is different from most of the TPE based AIE-MRL materials, where the solid-state maximum emission peaks are redshifted due to the grinding induced conformational planarity.3,37,38 Moreover, the emission for ground C3Ph crystals could be quenched by melting and subsequent in situ solidifying (process II) and easily recovered by grinding again (Process I). It is noted that good reversibility could be found for both processes, in which the quantum yields display neglectable fatigue after being repeated for several cycles (Figure 4D). With regard to the crystalline sample of C7, it gives a 47-fold PL intensity enhancement after grinding accompanied by a slight blue-shift of 3 nm (Figure 5B). A reversible but low-contrast solid-state fluorescence on-off cycle for C7 can also be recorded by alternate grinding and melting-solidifying (Figure 5D). In addition, some other reported quenching methods, such as thermal annealing and/or organic solvent fuming,38 have been tried to turn off the solid-state emission of C3Ph and C7, but neither of them works.

a b Figure 3. PXRD spectra of (a) C3Ph in aggregated (bottom) and amorphous (top) states, and (b) C7 in aggregated (bottom) and crystalline (top) states.



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Figure 6. (A) ORTEP drawing of C3Ph (top view), (B) π–π Interactions with the centroid-to-centroid separations (a = 3.524, b = 3.562, c = 3.777, d = 3.605 Å) and (C) hydrogen bonding (e = 2.507, f = 2.817, g = 2.844 Å) in the single-crystal lattice of C3Ph. (D) Layer packing structure of HOF C3Ph.

Figure 5. (A) PL spectra of C7 in acetone (black) and aggregates at fw = 90% (red), together with the photographs taken under UV illumination. (B) Emission spectra of crystalline C7 before and after grinding. (C) Morphology-dependent PL emission photographs and the switching processes: I, grinding with a glass sheet and II, melting-solidifying. (D) Plot of solid luminescence quantum yields versus repeated I and II cycles. Single-Crystal Structures and MRL Mechanisms. As one of the most powerful tools to characterize molecular geometry and related packing fashion, X-ray single-crystal diffraction technique could offer detailed information on the intermolecular interactions, such as hydrogen bonds41 and π–π interactions.42 To further uncover the distinguishable MRL turn-on mechanisms, structural analyses and comparisons for C3Ph and C736 have been carried out (Tables 1-2 and Figures 6-7). Single-crystal structure of C3Ph reveals the formation of one-dimensional (1D) supramolecular chains sustained by multiple π–π stacking interactions between adjacent planar benzimidazole/coumarin moieties with the centroid-to-centroid separations of 3.524~3.777 Å, in which the two heterocycles are essentially coplanar by means of strong intramolecular N–H···O hydrogen-bonding interactions (2.145 Å) with a small dihedral angle of 4.83° (Figures 6A-6B). These structural features are analogous to those in C7 (2.255 Å/3.602~3.787Å/9.26°, Figures S1 and 7A). Because of the presence of spatial crowding TPE group in C3Ph, however, a different two-dimensional (2D) hydrogen-bonded organic framework (HOF) is formed in which the 1D supramolecular chains are further connected by weak intermolecular C–H···O (2.507 Å) and C–H···C (2.817 Å and 2.844 Å) hydrogen bonds (Figures 6C- 6D). In contrast, strong interchain N–H···O (2.280 Å) and complementary C–H···O (2.694 Å) hydrogen bonds are found in the 2D HOF of C7 (Figures 7B-7C). In comparison with C7, one can see herein the integration of planar benzimidazole/coumarin and steric hindrance TPE unit in C3Ph leads to different supramolecular interactions and 2D HOFs. This is believed to be the structural proofs accounting for the distinguishable solid-state fluorescence mechanoresponsiveness before and after grinding, where the destroy of π–π stacking and hydrogen bonding to different extents impacts the final MRL sensitivity.

Figure 7. (A) π–π Interactions with the centroid-to-centroid separations (a = 3.787, b = 3.602, c = 3.657, d = 3.738 Å) and (B) hydrogen bonding (e = 2.280, f = 2.694 Å) in the single-crystal lattice of C7. (C) Layer packing structure of HOF C7. PXRD measurements have also been carried out to explore the MRL mechanism. For the PXRD spectra of C3Ph crystals before and after grinding, the sharp peaks become broad and the intensity turns to be weak (Figure 8a). Moreover, a peak at 2θ = 26.63° (3.34 Å) associated with the interlayer distance between two adjacent benzimidazole/coumarin parts is obviously weakened or even diminished after grinding at the same time (Figure S2). These changes indicate that the grinding treatment (process I) has damaged the crystal packing of C3Ph accompanied by the interlayer slipping, which is responsible for the observed blueshifted MRL turn-on,27,40 instead of red-shifted ones induced by conformational planarity. As for the crystalline sample of C7, only slight decrease of the diffraction intensity could be observed even after the thorough grinding (Figure 8b), which demonstrates


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that the more stability of C7 HOF agrees well with its lower MRL sensitivity.

This work was financially supported by the National Natural Science Foundation of China (No. 21871133) and the National Natural Science Foundation of Jiangsu Province (No. BK20171334). REFERENCES (1) Yoon, S. J.; Park, S. Y. Polymorphic and Mechanochromic Luminescence Modulation in the Highly Emissive Dicyanodistyrylbenzene Crystal: Secondary Bonding Interaction in Molecular Stacking Assembly J. Mater. Chem. 2011, 21, 8338–8346. (2) Chi, Z.; Zhang, X.; Xu, B.; Zhou, X.; Ma, C.; Zhang, Y.; Liu, S.; Xu, J. Recent Advances in Organic Mechanofluorochromic Materials. Chem. Soc. Rev. 2012, 41, 3878–3896. (3) Yang, Z.; Chi, Z.; Mao, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Aldred, M. P.; Chi, Z. Recent Advances in MechanoResponsive Luminescence of Tetraphenylethylene Derivatives with Aggregation-Induced Emission Properties. Mater. Chem. Front. 2018, 2, 861–890. (4) Kunzelman, J.; Kinami, M.; Crenshaw, B. R.; Protasiewicz, J. D.; Weder, C. Oligo(p-Phenylene Vinylene)s as a "New" Class of Piezochromic Fluorophores. Adv. Mater. 2008, 20, 119–122. (5) Zhao, Z.; Wang, Z.; Lu, P.; Chan, C. Y. K.; Liu, D.; Lam, J. W. Y.; Sung, H. H. Y.; Williams, I. D.; Ma, Y.; Tang, B. Z. Structural Modulation of Solid-State Emission of 2,5Bis(trialkylsilylethynyl)-3,4-Diphenylsiloles. Angew. Chem. Int. Ed. 2009, 48, 7608–7611. (6) Sun, H.; Liu, S.; Lin, W.; Zhang, K. Y.; Lv, W.; Huang, X.; Huo, F.; Yang, H.; Jenkins, G.; Zhao, Q.; et al. Smart Responsive Phosphorescent Materials for Data Recording and Security Protection. Nat. Commun. 2014, 5, 3601. (7) Yoon, S. J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M. G.; Kim, D; Park, S. Y. Multistimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular Sheets. J. Am. Chem. Soc. 2010, 132, 13675–13683. (8) Jiang, Y.; Gindre, D.; Allain, M,; Liu, P.; Cabanetos, C.; Roncali, J. A Mechanofluorochromic Push-Pull Small Molecule with Aggregation-Controlled Linear and Nonlinear Optical Properties. Adv. Mater. 2015, 27, 4285–4289. (9) Wang, J.; Mei, J.; Hu, R.; Sun, J. Z.; Qin, A.; Tang, B. Z. Click Synthesis, Aggregation-Induced Emission, E/Z Isomerization, Self-Organization, and Multiple Chromisms of Pure Stereoisomers of a Tetraphenylethene-Cored Luminogen. J. Am. Chem. Soc. 2012, 134, 9956–9966. (10) Fang, X.; Zhang, Y. M.; Chang, K.; Liu, Z.; Su, X.; Chen, H.; Zhang, S. X. A.; Liu, Y.; Wu, C. Facile Synthesis, Macroscopic Separation, E/Z Isomerization, and Distinct AIE properties of Pure Stereoisomers of an Oxetane-Substituted Tetraphenylethene Luminogen. Chem. Mater. 2016, 28, 6628– 6636. (11) Zhao, N.; Zhang, C.; Lam, J. W. Y.; Zhao, Y. S.; Tang, B. Z. An Aggregation-Induced Emission Luminogen with Efficient Luminescent Mechanochromism and Optical Waveguiding Properties. Asian J. Org. Chem. 2014, 3, 118– 121. (12) Ma, Z.; Wang, Z.; Teng, M.; Xu, Z.; Jia, X. Mechanically Induced Multicolor Change of Luminescent Materials. ChemPhysChem 2015, 16, 1811–1828.

a b Figure 8. PXRD spectra of (a) C3Ph and (b) C7 in different states: crystal simulated (top), HOF state (middle) and ground HOF state (bottom). CONCLUSION In summary, we have designed and prepared a TPE fused coumarin compound, C3Ph, to achieve a new type of highcontrast MRL turn-on material. The crystalline C3Ph shows a higher PL contrast ratio than its control compound C7 before and after grinding (193-fold vs 47-fold). Moreover, reversible solidstate fluorescence on-off cycles for both C3Ph and C7 can be achieved by alternate grinding and melting-solidifying. The high luminescence contrast ratio as well as the good reversibility makes C-3Ph an ideal MRL switching candidate. Further X-ray diffraction analyses indicate that the presence of TPE moiety guarantees the AIEE activity of C3Ph and triggers highly solidstate emissive, in the premise of retaining the planar benzimidazole/coumarin unit. Meanwhile, distinguishing dipoledipole, π–π stacking and hydrogen bonding interactions in C3Ph and C7 result in their different MRL behavior. The introduction of spatial hindrance TPE group in C3Ph is suggested to weaken the supramolecular interactions that are sensitive to the external mechanical forces and lead to higher luminescence contrast ratio, which can be further evidenced by corresponding PXRD spectra. So our studies on structure-property relationships demonstrate that our synthetic strategy of integrating two counter-acting forces in C3Ph to construct a metastable HOF makes possible the highly switchable solid-state luminescence, which is suggested to throw certain new light into a class of high-contrast MRL turn-on materials. Supporting Information The Supporting Information is available: Materials and measurements, details of synthetic characterizations, singlecrystal X-ray diffraction data and related perspective view of the packing structures. CCDC reference number 1885442. For Supporting Information and crystallographic data in CIF or other electronic format see DOI: 10.1021/ . This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT



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