Crystal Structure and Aggregation-induced Emission of an Azine Derivative①

LI Qiao NIU Zhi-GangLIU Yan-Ling WANG En-Ju

Crystal Structure and Aggregation-induced Emission of an Azine Derivative①

LI Qiao NIU Zhi-GangLIU Yan-Ling WANG En-Ju②

(571158)

An AIE-active azine derivative(1) was facilely synthesized by aldehyde-amine condensation of2-hydroxy-1-naphthaldehyde and 3-methyl-2-benzothiazolinone hydrazone. It crystallizes in the monoclinic space group21/with= 8.2176(5),= 13.1733(7),=14.5731(8) Å,=90.521(5)º, and= 4. Compound 1exhibits aggregation-induced emission characteristics. In dilute solution, it is non-emissive, while strong emission was observed in the aqueous medium as a result of the molecular aggregation in poor solvent. The powder and crystals of 1 also exhibit strong fluorescence. In its crystal lattice, the molecules stack in a face-to-face style, but there is no-stacking interaction due to the long distance between adjacent molecules. It is the loose stacking mode that blocks the nonradiative decay channel resulting in its AIE effect.

aggregation-induced emission, crystal structure, 2-hydroxy-1-naphthaldehyde, azine;

1 INTRODUCTION

In 2001 Tang's group firstly reported an unusual silole derivative showing no fluorescence in dilute solution, but showing highly emissive behavior in the aggregated state, which was termedas aggregation- induced emission (AIE)[1]. Since then, the photolumi- nescent materialswith AIE characteristics have become an important research territory and attractedmuch research interest. To date, sometypical examples of AIE systems, including siloles[2], arylenevinylene derivatives[3], distyrylanthracene[4]and diphenylacrylonitrile[5]have been developed. The technological applications of AIE materials in a wide variety of high-tech areas have been achieved, such as biological probes[6], chemical sensing[7], optoelectronic devices[8]andsmart materials[9].

The AIE effect has mainly been attributed to the restricted intramolecular rotations (RIR) and the restricted intramolecular vibrations (RIV) in the aggregate state[10, 11]. The luminogens based on RIR mechanism commonly takes a propeller-like shape, while those based on RIV mechanism commonly show a butterfly-like molecular conformation. The peculiar molecular shapes result in difficulties in the synthesis of AIE-active molecules. In the course of our continuing efforts in fluorescent probes for metal ions[12-15],some Schiff bases of 2-hydroxy-1- naphthaldehyde were discoveredexhibiting the aggregation-induced emission. Above all, they are easy to be synthesized. Herein, an AIE-active azine derivative(1) was facilely synthesized by the condensation of2-hydroxy-1-naphthaldehyde and 3-methyl-2-benzothiazolinone hydrazone (Scheme 1) and its structure was determined by single-crystal X-ray diffraction. The origin of AIE characteristics was speculated byanalyzing the molecular confor-mation and molecular stacking of 1.

Scheme 1. Synthesis of compound 1

2 EXPERIMENTAL

2.1 Reagents and apparatus

All chemicals were obtained from commercial suppliers and directly used without further purifi- cation. Analytical grade acetonitrile and deionized water were used as solvents for spectral measure- ments.1H NMR and13C NMR spectra were recorded on a Bruker Av400 NMR spectrometer. ESI-MS spectra were performed on a Bruker Esquire HCT mass spectrometer. Fluorescence spectra were taken on a Hitachi F-7000 fluorescence spectrometer. The fluorescent picture of the crystals was taken using a Nikon Eclipse TS100 inverted microscope.

2.2 Synthesis of 1

2-Hydroxy-1-naphthaldehyde (344 mg, 2.0 mmol), 3-methyl-2-benzothiazolinone hydrazone hydro- chloride (431 mg, 2.0 mmol) and triethylamine (274 μL, 2.0 mmol) were added into 20 mL absolute ethanol and stirred for 4 h atroom temperature. The resulting precipitation was collected by filtration and then washed three times with ethanol. After drying, an azine derivative was obtained in a high yield (533 mg, 80%).1H NMR (400 MHz, CDCl3)(ppm) 12.67 (s, 1H), 9.40 (s, 1H), 8.14 (d, 1H, J = 8.0 Hz), 7.76 (d, 2H, J = 8.0 Hz), 7.50 (t, 1H, J = 8.0 Hz), 7.43 (d, 1H, J = 8.0 Hz), 7.34 (t, 1H, J = 8.0 Hz), 7.28 (t, 1H, J = 8.0 Hz), 7.25 (d, 1H, J = 8.0 Hz), 7.07 (t, 1H, J = 8.0 Hz), 7.01 (d, 1H, J = 8.0 Hz), 3.61 (s, 1H).13C NMR (100 MHz, CDCl3)(ppm) 165.1, 159.0, 152.2, 141.0, 132.3, 132.2, 129.0, 128.2, 127.2, 126.5, 123.5, 123.3, 122.3, 122.0, 120.3, 119.0, 109.5, 109.1, 30.9. ESI-MS m/z calculated for [M+H]+334.10, found 334.0 (Fig. S1-4).

2.3 Structure determination

Colorless rod-like crystals of 1 suitable for X-ray analysis were obtained by slowly evaporatingthe DCM/EtOH (1/3) solution of 1 in a refrigerator. X-ray diffraction data were collected on a Gemini A Ultra diffractometer (Mo,= 0.71073 Å) at 123(2) K.A total of 27874 reflections were collected in the range of 4.168°≤2≤52.744°, of which 3230 were unique (int= 0.0541,sigma= 0.0294) and used in all calculations. The final=0.0436 (> 2()) and= 0.1193 (all data). Crystal data were provided in supporting informa- tion (Table S1).The structure was solved by direct methods and refined by full-matrix least-squares on2. All non-hydrogen atoms were refined with anisotropic thermal parameters.The hydrogen atoms were determined with theoretical calculations and refined isotropically.

3 RESULTS AND DISCUSSION

3.1 Crystal structure description

Compound 1 crystallizes in monoclinic21/space group. The selected bond lengths and bond angles are listed in Table 1. As shown in Fig. 1, all atoms in the molecule, except for hydrogen atoms in the methyl group, are coplanar and form a conjugated system. The dihedral angle between the naphthalene ring and the benzothiazoline moiety is only 1.433(4)°. An intramolecular six-membered ring hydrogen bond O(1)–H(1)···N(1) links the phenolic hydroxy with the imine nitrogen (d(H×××N) 1.857,ÐOHN 147°). The molecules stack in a face-to-face style along theaxis to form a one-dimensional molecular chain (Fig. 2). The distance between adjacent molecular planes is about 3.5 Å, which is in the range of-stacking interaction. Nevertheless, the horizon- tal lateral displacement of adjacent aromatic rings is about 2.1～2.2 Å (Fig. S5), which are too long for-stacking interaction (The horizontal lateral dis-placement should be less than 1.5 Å for effective-stacking interaction). As a result, there is no-stacking interaction between the adjacent molecules.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Fig. 1. Crystal structure of 1 shown at 50% probability

Fig. 2. Crystal packing showing one-dimensionalmolecular chain along the-axis

3.2 Aggregation-induced emission characteristics

ESIPT (excited-state intramolecular proton trans- fer) means a red-shifted emission and a large Stokes shift. Therefore, integrating ESIPT with AIE can improve the photophysical properties of AIE systems. Compound 1 is a typical ESIPT molecule with AIE characteristics. Its fluorescence spectra were measuredin MeCN/H2Omixtures with dif- ferent volume fractions of water. As illustrated in Fig. 3, green luminescence at about 520 nm was observed when the water fraction reaches 80% and goes up with the increase of waterunder 390 nm excitation. The Stokes shift is up to 130 nm. The fluorescence turn-on should beattributed to the formation of aggregates and consequentlythe res- triction of intramolecular rotation. The fluorescence photos of 1 in its powder state and crystal state are shown in Fig. 4. It can be seen that 1 glows green in the two solid states under UV light excitation, while its natural color is pale yellow.

Aromatic Schiff bases usually show a planar con- jugated configuration which is favorable for photo-luminescence. Nevertheless, the planar configura- tions frequently result in the face-to-face intermole- cular-stacking in their crystal states, which leads to radiationless relaxation and gives rise to the phenomenon of aggregation-caused quenching (ACQ)[16]. Fortunately, many 2-hydroxy-1-naphthal- dehyde Schiff base derivatives are AIE-active[17, 18]. The stacking manner of compound 1 may illustrate their AIE effects. X-ray diffraction analysis has indicated that the planar molecules of 1 stack in a face-to-face style with a distance of about 3.5 Å that restricts the intramolecular rotations, while a large horizontal lateral displacement about 2.1～2.2 Å blocks the-stacking interaction. It is the unique stacking pattern that results in its AIE effect. As suggested above, the 2-hydroxy-1-naphthaldehyde Schiff base derivatives are an important class of AIE-active molecules with many good properties, such as simplicity in structure, accessibility in synthesis and large Stokes shift.

Fig. 3. (a) Fluorescence spectra of 1 (10 μM) in CH3CN/H2O mixtures with different water fractionswhen excited at 390 nm. (b) Dependence of the fluorescence intensity at 520 nm on the water fraction. (c) Fluorescence photos in CH3CN/H2O mixtures with 0～99% water content under 365 nm irradiation

Fig. 4. Luminescent pictures of 1 in crystal state (a) powder state (b) under UV light excitation, and picture under natural light (c)

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20 May 2019;

8 September 2019 (CCDC 1875772)

① This project was supported by the Natural Science Foundation of Hainan Province (No. 20162028) and the Program for Innovative Research Team in University (IRT-16R19)

. E-mail: enjuwang@163.com

10.14102/j.cnki.0254-5861.2011-2464