Crystal Structure and Aggregation-induced Emission of an Azine Derivative

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 termed as aggregationinduced emission (AIE)^[1]. Since then, the photoluminescent materials with AIE characteristics have become an important research territory and attracted much research interest. To date, some typical 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] and smart 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 discovered exhibiting 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 of 2-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 by analyzing the molecular conformation and molecular stacking of 1.

Scheme 1

Scheme 1.  Synthesis of compound 1

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2. EXPERIMENTAL

2.1 Reagents and apparatus

All chemicals were obtained from commercial suppliers and directly used without further purification. Analytical grade acetonitrile and deionized water were used as solvents for spectral measurements. ¹H NMR and ¹³C 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 hydrochloride (431 mg, 2.0 mmol) and triethylamine (274 μL, 2.0 mmol) were added into 20 mL absolute ethanol and stirred for 4 h at room 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) δ(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) δ(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 evaporating the DCM/EtOH (1/3) solution of 1 in a refrigerator. X-ray diffraction data were collected on a Gemini A Ultra diffractometer (MoKα, λ = 0.71073 Å) at 123(2) K. A total of 27874 reflections were collected in the range of 4.168°≤2θ≤52.744°, of which 3230 were unique (R_int = 0.0541, R_sigma = 0.0294) and used in all calculations. The final R = 0.0436 (I > 2σ(I)) and wR = 0.1193 (all data). Crystal data were provided in supporting information (Table S1). The structure was solved by direct methods and refined by full-matrix least-squares on F². 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 monoclinic P2₁/n 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, ∠OHN 147°). The molecules stack in a face-to-face style along the a-axis 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 π-π stacking interaction. Nevertheless, the horizontal lateral displacement of adjacent aromatic rings is about 2.1~2.2 Å (Fig. S5), which are too long for π-π stacking interaction (The horizontal lateral displacement should be less than 1.5 Å for effective π-π stacking interaction). As a result, there is no π-π stacking interaction between the adjacent molecules.

Table 1

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

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Bond	Dist.		Bond	Dist.		Bond	Dist.
S(1)–C(12)	1.762(2)		N(3)–C(12)	1.369(2)		N(1)–N(2)	1.399(2)
S(1)–C(18)	1.758(2)		N(3)–C(13)	1.396(2)		N(1)–C(11)	1.289(3)
O(1)–C(1)	1.349(2)		N(3)–C(19)	1.455(3)		N(2)–C(12)	1.294(2)
Angle	(°)		Angle	(°)		Angle	(°)
C(18)–S(1)–C(12)	90.46(10)		C(13)–N(3)–C(19)	124.87(17)		N(3)–C(12)–S(1)	111.00(14)
C(12)–N(3)–C(13)	114.53(16)		C(11)–N(1)–N(2)	114.69(17)		N(2)–C(12)–S(1)	127.28(15)
C(12)–N(3)–C(19)	120.58(17)		C(12)–N(2)–N(1)	110.65(16)		N(2)–C(12)–N(3)	121.73(18)

Figure 1

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

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Figure 2

Figure 2.  Crystal packing showing one-dimensional molecular chain along the a-axis

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3.2 Aggregation-induced emission characteristics

ESIPT (excited-state intramolecular proton transfer) 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 measured in MeCN/H₂O mixtures with different 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 water under 390 nm excitation. The Stokes shift is up to 130 nm. The fluorescence turn-on should be attributed to the formation of aggregates and consequently the restriction 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.

Figure 3

Figure 3.  (a) Fluorescence spectra of 1 (10 μM) in CH₃CN/H₂O mixtures with different water fractions when 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

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Figure 4

Figure 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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Aromatic Schiff bases usually show a planar conjugated configuration which is favorable for photoluminescence. Nevertheless, the planar configurations frequently result in the face-to-face intermolecular π-π 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-naphthaldehyde 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 π-π 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.

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