Controllable luminescent behaviors with pyrazine and benzimidazole groups: Syntheses, crystal structures and properties

Optical Materials 98 (2019) 109432

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Controllable luminescent behaviors with pyrazine and benzimidazole groups: Syntheses, crystal structures and properties Yong-Tao Wang *, Yu-Song Wu, Gui-Mei Tang Department of Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, China

A R T I C L E I N F O

A B S T R A C T

Keywords: 2–(pyrazin-2-yl)–1H–benzimidazole salts Crystal structures Supramolecular interactions X-ray diffraction Luminescent lifetime

A set of new salts containing benzimidazole and pyrazine groups, namely, [HPZBI]þ X– nH2O (X ¼ Cl (n ¼ 1) (1), NO3 (n ¼ 1) (2), ClO4 (n ¼ 0) (3) and H2PO4 (n ¼ 0) (4)) [PZBI ¼ 2–(pyrazin-2-yl)–1H–benzimidazole], were obtained by the reaction of PZBI and a set of inorganic acid, respectively. The emission maxima can be found at 444, 472, 471, 432 and 443 nm for PZBI and its compounds 1–4 in the solid state at room temperature, respectively. Compared with the free PZBI, the emission maxima of compounds 1–4 are obviously blue/redshifted for 1–4, indicating that the emission maxima are relative to the intermolecular packing distances. Their photoluminescent lifetime and quantum yield are 1.10 ns and 24.5% for 1, 0.99 ns and 26.4% for 2, 0.94 ns and 21.65% for 3, 2.86 ns and 20.57% for 4, respectively. Compounds 1–4 were structurally characterized by X–ray single crystal diffraction, FT-IR, and UV–Vis spectra. Single crystal X-ray diffraction reveals that π⋯π packing interactions and a set of hydrogen bonds can be observed. The shortest distances of π⋯π stacking in­ teractions are 3.613, 3.534, 3.731 and 3.492 Å in compounds 1–4, respectively. Additionally, the thermal sta­ bilities of compounds 1–4 were investigated in detail.

1. Introduction N-heterocyclic compounds are essential building blocks of a lot of organic/polymeric materials [1], pharmaceuticals [2] and natural products [3]. In particularly, benzimidazoles are an important class of heterocyclic system which display show a broad spectrum of good pharmacological and biological properties such as anti-hypertensive [4], anti-fungal [5], anti-bacterial [6], anti-ulcer [7], anti-cancer [8], anti-inflammatory [9], and anti-HIV activities [10]. This is due to the fact that they have a similar structure with a naturally occurring purine [2g]. Apart from biological applications, benzimidazoles have also found applications in scientific areas including luminescent materials [11], gas absorption/separation [12], pigments [13], solar cells [14], sensors [15], and thermostable membranes for fuel cells [16]. On the other hand, the anions play an important role in the con­ struction of supramolecular structures and the coordination polymers, but the systematic investigations on the influence of the inorganic an­ ions on the supramolecular structures have been sparse [17]. So far, it is very significant to construct the molecular interactions among these benzimidazole-based compounds and to mediate luminescence behav­ iors through the supramolecular interactions and counter-anions.

Therefore, it is still a great challenge to explore and understand the relationships between construction and properties for the pyrazine/benzimidazole-based compounds. Prompted by the aforementioned facts and literature on heterocyclic benzimidazole nucleus with the interesting physical properties, we synthesized one of N-heterocyclic compounds with pyrazine and benz­ imidazole groups, namely, 2–(pyrazin-2-yl)–1H–benzimidazole (PZBI, Scheme 1), which is beneficial to generate benzimidazole salts [17h,18]. Herein, we like to report the syntheses, crystal structures, and lumi­ nescence of a set of new pyrazine/benzimidazolium salts, namely, [HPZBI]þ X– nH2O (X ¼ Cl (n ¼ 1) (1), NO3 (n ¼ 1) (2), ClO4 (n ¼ 0) (3) and H2PO4 (n ¼ 0) (4)) (Scheme 2), which were obtained through the mixture of PZBI and a set of inorganic acid, respectively. Compared with the free PZBI, the blue/red-shifted results, high luminescent quantum yields, and fluorescent lifetime can be found in its salts, which shows the mediated photoluminescent behaviors. The present work aimed to synthesize a new series of benzimidazole salts and investigate their luminescent behaviors.

* Corresponding author. Tel./fax: þ86 0531 89631207. E-mail address: [email protected] (Y.-T. Wang). https://doi.org/10.1016/j.optmat.2019.109432 Received 25 May 2019; Received in revised form 13 September 2019; Accepted 29 September 2019 Available online 9 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.

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Optical Materials 98 (2019) 109432

Scheme 1. Synthetic route of PZBI.

2.2.1. The synthesis of [HPZBI]þ Cl ⋅H2O (1) The compound PZBI (0.10 g, 0.50 mmol) was solved in a mixture of methanol (10 mL) and water (5 mL). The reaction mixture was stirred for 20 min. Then, the condensed HCl (0.50 mmol) was drop wisely added. After the solution was stood about several days, colorless plate crystals of 1 suitable for single crystal X–ray diffraction were obtained. Yield: 0.096 g, 76.6%. M.p. 231–232 � C. Elemental analysis calcd (%) for C11H11ClN4O (250.69): C, 52.70; H, 4.42; N, 22.35; Found: C, 52.84; H, 4.41; N, 22.41. IR (KBr, cm 1): 3363(br, s), 3221(w), 3143(w), 3088(w), 3016(w), 2956(w), 2906(w), 2845(w), 2800(w), 2711(m), 2696(m), 2627(m), 2532(w), 2403(w), 2345(w), 2148(w), 1625(m), 1516(m), 1487(m), 1454(s), 1402(s), 1363(s), 1317(m), 1251(m), 1228(s), 1174 (m), 1151(m), 1047(m), 1016(s), 962(m), 935(m), 860(m), 817(m), 769 (s), 617(m), 567(m), 526(s), 435(m).

Scheme 2. The benzimidazole salts.

2. Experimental

2.2.2. The synthesis of [HPZBI]þNO3 ⋅H2O (2) Compound 2 was synthesized by an analogous procedure except that HNO3 (0.50 mmol) took place of HCl. Colorless needle crystals of 2 were collected in a 72.9% yield (0.101 g). M.p. 214–216 � C. Elemental anal­ ysis calcd (%) for C11H11N5O4 (277.24): C, 47.66; H, 4.00; N, 25.26; Found: C, 47.78; H, 4.01; N, 25.20. IR (KBr, cm 1): 3547(br), 3373(br), 3215(w), 3153(w), 3089(w), 3055(w), 3014(w), 2927(w), 2852(w), 2721(w), 2628(w), 2540(w), 2426(w), 2364(w), 2345(w), 2175(w), 1629(m), 1519(m), 1490(m), 1454(s), 1423(m), 1384(vs), 1334(s), 1255(m), 1230(s), 1170(m), 1159(m), 1120(w), 1045(m), 1016(s), 943 (m), 896(m), 817(m), 769(s), 725(w), 707(w), 617(m), 567(m), 530(m), 432(m).

2.1. Materials and measurements Reagents and solvents were commercially available and not further purified. C, H and N microanalyses were carried out with a Perkin–Elmer 240 elemental analyzer. Thermogravimetric analysis (TGA) data were collected with a TA SDT Q600 analyzer in N2 at a heating rate of 10 � C min 1 in the range of 10–600 � C. The FT–IR spectra were recorded from KBr pellets in the range 400–4000 cm 1 on a Bruker Tensor4 spec­ trometer. UV–Vis absorption spectra were recorded on a SHIMADZU UV–2600 spectrophotometer with 0.1 nm resolution. The solution/sol­ id–state fluorescence spectra were recorded on a HITACHI–4500 spec­ trometer at room temperature. The fluorescent lifetime decay was carried on Edinburgh FLS980 fluorescent spectrometer.

2.2.3. The synthesis of [HPZBI]þClO4 (3) The preparation of 3 was similar to that of 1 except that HClO4 (0.50 mmol) substituted HCl. Pale yellow crystals of compound 3 suitable for single crystal X–ray diffraction analyses were obtained. Yield: 0.098 g, 66.1% based on PZBI). M.p. 270–272 � C. Elemental

2.2. The synthesis of 2–(pyrazin-2-yl)–1H–benzimidazole (PZBI) The compound PZBI were synthesized according to the literature reported by our research [17h]. Table 1 Crystal data and structure refinement parameters for compounds 1–4. Compound reference

1

2

3

4

Chemical formula Formula Mass Crystal system a/Å b/Å c/Å α/� β/� γ/� Unit cell volume/Å3 Temperature/K Space group No. of formula units per unit cell, Z Radiation type Absorption coefficient, μ/mm 1 No. of reflections measured No. of independent reflections R i nt Final R1 values (I > 2σ(I))a Final w R(F2) values (all data)b Goodness of fit on F2 CCDC number

C11H9N4,H2O,Cl 250.69 Monoclinic 18.413(3) 7.2456(14) 19.552(5) 90 115.962(2) 90 2345.3(8) 296 C2/c 8 Mo Kα 0.315 9776 2672 0.027 0.0402 0.1174 1.05 795434

C11H9N4,NO3,H2O 277.25 Triclinic 6.9422(19) 9.697(3) 10.059(3) 109.900(3) 104.880(3) 94.680(3) 604.6(3) 296 P-1 2 Mo Kα 0.120 5178 2681 0.027 0.0491 0.1489 1.03 795435

C11H9N4,ClO4 296.67 Monoclinic 6.923(4) 16.546(9) 11.095(6) 90 101.816(6) 90 1244.0(12) 296 P21/c 4 Mo Kα 0.327 10611 2873 0.052 0.0783 0.2498 1.11 795436

C11H9N4,H2O4P 294.21 Monoclinic 16.1124(5) 10.1149(3) 7.6376(3) 90 95.881(1) 90 1238.19(7) 150 P21/c 4 Mo Kα 0.243 10455 2831 0.073 0.0944 0.2571 1.08 1843081

a b

P P R1 ¼ ((Fo|-|Fc((/ |Fo|. P 2 2 2 P wR2 ¼ [ [w(Fo-Fc ) ]/ [w(F2o)2]]1/2. 2

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Optical Materials 98 (2019) 109432

Table 2 Selected bond lengths (Å) and angles (� ) for complexes 1–4. 1 N1–C1 N2–C2 N3–C5 N4–C5 O1W–H1WA C1–N1–C4 N1–C4–C3 N3–C11–C10 N1–C1–C2 N3–C5–N4 C5–N3–C11 N2–C3–C4 N4–C6–C11 H1WA-O1W–H1WB C4–N1–C1–C2 C3–N2–C2–C1 C11–N3–C5–C1 C6–N4–C5–N3 C1–N1–C4–C3 C5–N3–C11–C10 2 N1–C1 N1–C4 N4–C11 N3–C6 O1–N5 O3–N5 O1–N5–O3 C1–N1–C4 C5–N4–C11 N1–C4–C3 N4–C5–C1 N4–C11–C10 C5–N3–C6 N1–C1–C5 N3–C6–C11 C4–N1–C1–C2 C3–N2–C2–C1 C6–N3–C5–C1 C11–N4–C5–N3 C4–N1–C1–C5 C2–N2–C3–C4 3 N(1)–C(1) N(3)–C(5) N(2)–C(2) N(2)–C(3) Cl(1)–O(1) Cl(1)–O(2) C(1)-N(1)-C(4) N(1)-C(4)-C(3) N(4)-C(11)-C(10) N(1)-C(1)-C(2) N(3)-C(5)-N(4) C(5)-N(3)-C(6) N(1)-C(1)-C(5) N(4)-C(11)-C(6) O(1)-Cl(1)-O(2) O(2)-Cl(1)-O(3) O(3)-Cl(1)-O(4) C(4)-N(1)-C(1)-C(2) C(1)-N(1)-C(4)-C(3) C(2)-N(2)-C(3)-C(4) C(5)-N(3)-C(6)-C(7) C(11)-N(4)-C(5)-C(1) C(3)-N(2)-C(2)-C(1) 4 *N1-*C1 *N3-*C8 *N2-*C6 *N2-*C7 P1–O1 P1–O2 P1–O3 *C8-*N3-*C9

Table 2 (continued )

1.340(2) 1.334(3) 1.343(2) 1.334(3) 0.87(4) 115.42(19) 122.4(2) 131.1(2) 121.97(19) 109.42(17) 108.51(16) 122.5(2) 106.48(18) 110(3) 0.5(3) 0.3(3) 178.55(16) 0.43(19) 0.4(3) 177.2(2)

N1–C4 N2–C3 N3–C11 N4–C6 O1W–H1WB C5–N4–C6 N4–C5–C1 C2–N2–C3 N2–C2–C1 N4–C6–C7 N1–C1–C5 N3–C5–C1 N3–C11–C6

1.329(3) 1.323(3) 1.386(3) 1.389(2) 0.75(3) 108.80(16) 125.40(16) 115.8(2) 121.96(19) 131.77(18) 116.01(18) 125.19(18) 106.79(16)

C4–N1–C1–C5 C2–N2–C3–C4 C5–N3–C11–C6 C6–N4–C5–C1 C11–N3–C5–N4 C5–N4–C6–C7

179.76(16) 0.4(3) 0.6(2) 178.75(15) 0.6(2) 178.95(19)

1.343(2) 1.331(3) 1.399(3) 1.390(3) 1.248(2) 1.253(2) 118.97(17) 115.73(17) 108.31(16) 122.02(17) 127.32(18) 131.36(18) 108.51(16) 115.41(18) 106.72(17) 1.0(3) 0.4(3) 179.72(19) 0.5(2) 178.27(18) 1.1(3)

N2–C2 N2–C3 N4–C5 N3–C5 O2–N5 O1–N5–O2 O2–N5–O3 C2–N2–C3 N1–C1–C2 N2–C2–C1 N3–C5–N4 N3–C6–C7 N2–C3–C4 N3–C5–C1 N4–C11–C6 C5–N3–C6–C11 C5–N4–C11–C6 C6–N3–C5–N4 C1–N1–C4–C3 C11–N4–C5–C1 C5–N3–C6–C7

1.335(3) 1.326(3) 1.333(2) 1.337(3) 1.241(2) 121.06(16) 119.98(17) 116.11(18) 122.04(18) 121.69(17) 109.95(17) 131.33(17) 122.39(18) 122.73(17) 106.50(16) 0.7(2) 0.0(2) 0.8(2) 0.5(3) 180.0(2) 179.7(2)

1.336(4) 1.340(4) 1.331(5) 1.329(4) 1.348(6) 1.384(4) 116.4(3) 121.5(3) 132.4(3) 122.0(3) 108.4(3) 109.4(3) 115.7(2) 106.5(3) 109.1(4) 110.5(5) 102.7(5) 0.7(5) 0.4(6) 1.2(5) 179.9(3) 176.7(3) 0.8(5)

N(1)–C(4) N(3)–C(6) N(4)–C(5) N(4)–C(11) Cl(1)–O(3) Cl(1)–O(4) C(5)-N(4)-C(11) N(4)-C(5)-C(1) C(2)-N(2)-C(3) N(2)-C(2)-C(1) N(3)-C(6)-C(7) N(2)-C(3)-C(4) N(3)-C(5)-C(1) N(3)-C(6)-C(11) O(1)-Cl(1)-O(3) O(2)-Cl(1)-O(4) O(1)-Cl(1)-O(4) C(4)-N(1)-C(1)-C(5) C(6)-N(3)-C(5)-N(4) C(5)-N(3)-C(6)-C(11) C(5)-N(4)-C(11)-C(6) C(6)-N(3)-C(5)-C(1) C(11)-N(4)-C(5)-N(3)

1.331(5) 1.387(4) 1.346(4) 1.377(4) 1.339(9) 1.364(6) 109.5(2) 127.5(2) 116.8(3) 121.2(3) 132.4(3) 122.0(3) 124.0(2) 106.2(3) 113.0(7) 111.2(4) 110.2(6) 177.6(3) 0.7(3) 0.2(3) 0.8(3) 177.0(3) 1.0(3)

1.365(10) 1.391(11) 1.378(10) 1.288(17) 1.504(3) 1.515(3) 1.566(4) 120.0(6)

*N1-*C7 *N3-*C9 *N4-*C10 *N4-*C11 P1–O4 O3–H3O O4–H4O *N1-*C1-*C6

1.375(17) 1.389(11) 1.390(12) 1.390(12) 1.572(4) 0.84 0.84 108.1(6)

*N4-*C10-*C9 *C1-*N1-*C7 *C10-*N4-*C11 *N2-*C6-*C5 O3–P1–O4 O2–P1–O3 P1–O3–H3O O1–P1–O2 C7–N1–C1–C6 C1–N1–C7–C8 C7–N2–C6–C5 C6–N2–C7–N1

120.0(6) 105.8(9) 120.0(6) 133.9(7) 107.3(2) 110.9(2) 109 115.56(18) 1.2(10) 179.1(10) 178.8(10) 0.3(13)

*N4-*C11-*C8 *C6-*N2-*C7 *N1-*C1-*C2 O1–P1–O3 O1–P1–O4 O2–P1–O4 P1–O4–H4O C7–N1–C1–C2 C1–N1–C7–N2 C7–N2–C6–C1 C9–N3–C8–C7 C6–N2–C7–C8

120.0(6) 108.9(9) 131.9(7) 106.22(19) 111.3(2) 105.33(19) 110 179.3(9) 0.9(13) 0.5(10) 179.4(9) 178.3(11)

Table 3 Hydrogen bond geometries in the crystal structure of 1–4. D-H∙∙∙A 1 O1W–H1WA∙∙∙Cl1a O1W–H1WB∙∙∙Cl1 N3–H3⋯Cl1b N4–H4⋯O1Wc C2–H2A∙∙∙Cl1b C7–H7A∙∙∙N2d 2 O1W–H1WA∙∙∙O2e O1W–H1WB∙∙∙O1f N3–H3⋯O1W N4–H4⋯O3g C2–H2A∙∙∙O3g C8–H8A∙∙∙O3h 3 N(3)–H(3)∙∙∙N(2)i N(4)–H(4)∙∙∙O(2)j C(2)-H(2A)∙∙∙N(1)k 4 *N10 -H10 N⋯O1 *N1–H1N⋯O1 *N20 -H20 N⋯O2l *N2–H2N⋯O2l O3–H3O⋯O1m O4–H4O⋯O2n *C2–H2⋯N4’o *C9–H9⋯N4p *C10–H10⋯O4q *C100 -H10’∙∙∙O3r *C11–H11⋯O2l

D-H (Å)

H∙∙∙A (Å)

D∙∙∙A (Å)

D-H∙∙∙A (o)

0.87(4) 0.75(3) 0.99(3) 0.91(3) 0.93 0.93

2.34(3) 2.47(3) 2.08(3) 1.86(3) 2.83 2.49

3.203(2) 3.216(2) 3.0607(19) 2.723(3) 3.736(2) 3.318(3)

177(3) 174(3) 170(2) 159(3) 166 148

0.90(3) 0.89(3) 0.94(3) 1.03(3) 0.93 0.93

1.97(3) 1.92(3) 1.81(3) 1.71(3) 2.36 2.51

2.851(3) 2.793(3) 2.666(2) 2.733(2) 3.256(3) 3.426(3)

167(3) 168(3) 151(3) 174(2) 161 166

0.85(4) 0.83(5) 0.93

2.04(4) 2.08(5) 2.55

2.858(4) 2.857(5) 3.227(4)

163(3) 156(5) 130

0.88 0.88 0.88 0.88 0.84 0.84 0.93 0.93 0.93 0.93 0.93

1.81 1.83 1.81 1.77 1.78 1.78 2.61 2.61 2.32 2.30 2.50

2.657(9) 2.648(9) 2.643(9) 2.638(9) 2.623(5) 2.614(5) 3.291(9) 3.452(10) 3.240(7) 3.217(7) 3.339(8)

161 155 157 167 178 176 131 151 172 167 149

Symmetry codes. a 1/2-x, 1/2 þ y, 1/2-z. b 1/2-x, 1/2-y, 1-z. c -x, y, 1/2-z. d -1/2 þ x, 1/2 þ y, z. e x, 1þy, z. f -x, 1-y, -z. g 1-x, 2-y, 1-z. h 1-x, 1-y, 1-z. i x, 1/2-y, 1/2 þ z. j 1-x, 1-y, 1-z. k x, 1/2-y, 1/2 þ z. l x, 3/2-y, 1/2 þ z. m x, 1/2-y, 1/2 þ z. n x, 1/2-y, 1/2 þ z. o -x, 1/2 þ y, 1/2-z. p 1-x, 1/2 þ y, 3/2-z. q 1-x, 1/2 þ y, 3/2-z. r -x, 1/2 þ y, 1/2-z.

analysis calcd (%) for C11H9N4ClO4 (296.67): C, 44.54; H, 3.06; N, 18.89; Found: C, 44.42; H, 3.05; N, 18.84. IR (KBr, cm 1): 3435(w), 3385(w), 3265(w), 3159(w), 3099(w), 3064(w), 2995(w), 2945(w), 2854(w), 2814(w), 2752(w), 2736(w), 2661(w), 2628(w), 2520(w), 2416(w), 23445(w), 2162(w), 2027(w), 1629(m), 1521(m), 1489(w), 3

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Fig. 1. (a) A perspective view of 1. (b) A one-dimensional supramolecular chain along the b direction. (c) A two-dimensional supramolecular layer along the bc plane. (d) A packing diagram in 1.

1450(m), 1404(m), 1373(m), 1315(m), 1265(w), 1226(w), 1172(m), 1157(m), 1105(s), 1058(s), 1024(m), 929(w), 894(w), 854(m), 769(s), 705(w), 613(s), 563(m), 528(m), 437(m), 406(m).

the calculated sites and included in the final refinement in the riding model approximation with displacement parameters derived from the parent atoms to which they were bonded. Special computations for the crystal structure discussions were carried out with PLATON for Win­ dows [22]. A summary of the crystallographic data and structure re­ finements are listed in Table 1. Corresponding hydrogen bond and packing interactions data for compounds 1–4 are listed in Tables 2 and 3, respectively.

2.2.4. The synthesis of [HPZBI]þ H2PO4 (4) The preparation of 4 was analogous to that of 1 except that H3PO4 (0.50 mmol) replaced HCl. Pale yellow crystals of compound 4 suitable for single crystal X–ray diffraction analyses were obtained. Yield: 0.108 g, 73.4% based on PZBI). M.p. 274–276 � C. Elemental analysis calcd (%) for C11H11N4O4P (294.21): C, 44.91; H, 3.77; N, 19.04; Found: C, 44.80; H, 3.78; N, 19.09. IR (KBr, cm 1): 3049(w), 2927(w), 2715(w), 2611(w), 2409(w), 1878(w), 1624(m), 1523(m), 1456(m), 1408(w), 1377(m), 1317(m), 1261(m), 1230(s), 1085(s), 1012(m), 960(s), 891(m), 873(m), 819(m), 754(s), 711(m), 611(w), 567 (w), 524(s), 507(m), 428(m), 408(m).

3. Results and discussion 3.1. Description of the crystal structures 3.1.1. Structure description of [HPZBI]þ Cl ⋅H2O (1) The X–ray crystallographic analysis reveals that compound 1 belongs to the monoclinic C2/c space group with the monoclinic system. In an asymmetric unit, there exist one protonated PZBI cation, one chloride anion as well as one free water molecule (Fig. 1a). Notably, in four ni­ trogen atoms, N-donor in the benzimidazole ring can be protonated. In other word, only the benzimidazole ring can be protonated, while the pyrazine ring cannot be protonated in this case. The present results reveal that the basicity of N atom within the pyrazine group is weaker than that within benzimidazole ring. The imidazole-yl nitrogen atom is more easily protonated compared with other nitrogen atom of the functional group. The bond lengths of N3–C5 and N4–C5 in the imid­ azole ring are 1.343(2) and 1.334(3) Å (Table 2), respectively, sug­ gesting that two bonds are almost equal each other because the imidazole group has been protonated. Additionally, the dihedra angle of 7.39� can be observed between the benzimidazole and the pyrazine rings, suggesting the existence of the coplanar structure between these two aromatic rings.

2.3. Single-crystal structure determination Single-crystal structures of compounds 1–4 were measured by a Bruker Smart CCD equipped with graphite–monochromator Mo Kα ra­ diation (λ ¼ 0.71073 Å). The lattice parameters were obtained by a leastsquares refinement of the diffraction data. All the measured independent reflections were used in the structural analysis, and semi-empirical ab­ sorption corrections were applied using the SADBASE program [19]. The program SAINT was used for integration of the diffraction profiles [20]. The structure was solved by direct methods using the SHELXS and OLEX2 program of the SHELXTL package and refined with SHELXL [21]. All non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by full-matrix least-s­ quares methods with anisotropic thermal parameters for all the non-hydrogen atoms based on F2. The hydrogen atoms were placed in 4

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Table 4 The selected stacking interactions in compounds, respectively. 1

2

4

π∙∙∙π

Distance (Å)

Cg1∙∙∙Cg1a Cg1∙∙∙Cg3b Cg3∙∙∙Cg1b

3.6132(14) 3.6433(15) 3.6433(15)

Cg1∙∙∙Cg1c Cg1∙∙∙Cg1d Cg1∙∙∙Cg3d

3.6139(16) 3.6457(16) 3.5343(16)

Cg4∙∙∙Cg6e Cg5∙∙∙Cg1e Cg6∙∙∙Cg4f Cg1∙∙∙Cg3f Cg1∙∙∙Cg4e Cg1∙∙∙Cg5f Cg2∙∙∙Cg4f Cg3∙∙∙Cg1e Cg4∙∙∙Cg1f Cg4∙∙∙Cg2e

3.581(5) 3.566(5) 3.582(5) 3.575(5) 3.492(6) 3.566(5) 3.533(5) 3.574(5) 3.493(6) 3.533(5)

Symmetry codes. a -x, 1-y, 1-z. b -x, -y, 1-z. c -x, 1-y, 1-z. d 1-x, 1-y, 1-z. e x, 3/2-y, 1/2 þ z. f x, 3/2-y, 1/2 þ z. Cg(1) ¼ N3–C5–N4–C6–C11; Cg (3) ¼ C6–C7–C8–C9–C10–C11 for 1. Cg(1) ¼ N3–C5–N4–C11–C6; Cg (3) ¼ C6–C7–C8–C9–C10–C11 for 2. Cg(1) ¼ N1–C1–C6–N2–C7; Cg (2) ¼ N3–C8–C11–N4–C10–C9; Cg(3) ¼ C1–C2–C3–C4–C5–C6; Cg(4) ¼ N10 -C10 C60 -N20 -C7’; Cg(5) ¼ N30 -C80 -C110 -N40 -C100 -C9’; Cg(6) ¼ C10 -C20 -C30 -C40 -C50 C60 for 4.

Typically, there exist supramolecular interactions in the supramo­ lecular structures, which contain the strong hydrogen bonds, π⋯π as well as C–H⋯π stacking interactions. Interestingly, five types of hydrogen bonds can be found (Fig. 1 and Table 3): (a) the O–H⋯Cl hydrogen bonds come from the oxygen atoms of the water molecule and the chloride anion (O1W–H1WA⋯Cl1a, 3.203(2) Å, a1/2-x,1/2 þ y,1/2z; O1W–H1WB⋯Cl1, 3.216(2) Å), where these bond angles are 177 and 174� , respectively. (b) The intermolecular hydrogen bond of N–H⋯Cl (N3–H3⋯Cl1b, 3.0607(19) Å) is generated through the nitrogen atom of the imidazole group and the chloride anion, where the bond angle is 170� . (c) The hydrogen bond of N–H⋯O (N4–H4⋯O1Wc, c-x, y, 1/2-z, 2.723(3) Å) is observed through the imidazole-yl nitrogen atom and the oxygen atoms of the crystallized water molecule. The bond angle is 159� . (d) The hydrogen bonding of C–H⋯Cl (C2–H2A⋯Clb, 3.736(2) Å, b 1/2-x, 1/2-y, 1-z) occur from the pyrazine ring and the chloride ion. (e) The C–H⋯N hydrogen bond originates from the phenyl carbon atom and the pyrazine-yl nitrogen atom. The distance and bond angle of C7–H7A⋯N2 ( 1/2 þ x, 1/2 þ y, z) are 3.318(3) Å and 148� , respectively. Additionally, it occurs π⋯π packing interactions among the aromatic rings such as Cg(1)(imidazole ring)⋯Cg(1)a(imidazole ring) (a-x, 1-y, 1z), Cg(1)(imidazole ring)⋯Cg(3)b(phenyl ring) as well as Cg(3)(phenyl ring)⋯Cg (1)b(imidazole ring) (b-x,-y,1-z), where the packing distances are 3.6132(14), 3.6433(15) and 3.6433(15) Å, respectively (Table 4). The C–H⋯π packing interactions cannot be clearly observed. Two adjacent PZBI cations make using of stacking interactions be­ tween the aromatic imidazole rings, which results into the formation of a dimer (Fig. 1b). In a dimer structure, the PZBI cations align in antiparallel and offset formation. The free water molecule and the chlo­ ride anion act as a glue connecting two adjacent PZBI cations, which plays a key role in strengthening the stability of the dimer. We can find that there exist hydrogen bonds such as N4–H4⋯O1W, N3–H3⋯Cl1, C2–H2A⋯Cl as well as O1W–H1WA⋯Cl1, through which a dimer can further be reinforced. Subsequently, the adjacent two dimers can be aligned to a one-dimensional (1D) supramolecular chain along the

Fig. 2. (a) A ball-and-stick drawing of 2 in the asymmetric unit. (b) A onedimensional chain in compound 2. (c) A two-dimensional supramolecular network along the bc plane in 2. (d) The packing diagram of 2.

crystallographic b direction through the stacking interactions such as Cg (1)(imidazole ring)⋯Cg(3)(phenyl ring) as well as Cg(3)(phenyl ring)⋯Cg (1)(imidazole ring), where these dimers can be found in anti-parallel and offset conformation (Fig. 1b). These supramolecular chains can be further stacked to a supramolecular two-dimensional (2D) layer along the crystallographic bc plane through the O1W–H1WB⋯Cl1 hydrogen bond interaction (Fig. 1c), which make these supramolecular 2D layers extend to a three-dimensional (3D) supramolecular architecture along the crystallographic b direction (Fig. 1d). 3.1.2. Structure description of [HPZBI]þNO3 ⋅H2O (2) When HNO3 replaced HCl, a new compound 2 can be obtained under the similar reaction conditions. Single–crystal X–ray analysis reveals that compound 2 crystallizes in the triclinic system with the P-1 space group. The asymmetric unit consists of one protonated PZBI cations, one 5

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Optical Materials 98 (2019) 109432

structure between the two N-heterocyclic rings. The bond lengths around the nitrate N atom are 1.248(2), 1.241(2) and 1.253(2) Å; the bond angles are 121.06(16)� , 118.97(17)� and 119.98(17)� . Generally, it occurs supramolecular interactions in the supramolec­ ular frameworks. For examples, the strong hydrogen bonds and packing interactions can usually be found (Fig. 2b). As shown in Table 2, there exist hydrogen bonds between the oxygen atoms of the crystallized water molecule and the nitrate ions (O1W–H1WA⋯O2e, O1W–H1WB⋯O1f (ex, 1þy, z, f-x, 1-y, -z), where the distances and the bond angles of hydrogen bonds are 2.851 Å and 167� , 2.793 Å and 168� , respectively. The nitrogen atoms of imidazole group and the oxygen atoms from the water molecule and the nitrate anion generate another type of hydrogen bonds such as N3–H3⋯O1W and N4 –H4⋯O3g (g1-x,2y,1-z), where the bond lengths are 2.666 and 2.733 Å, respectively; the bond angles are 151 and 174� , respectively. Additionally, the weak C–H⋯O hydrogen bonds can be observed from the pyrazine-yl/phenyl carbon atoms and the nitrate oxygen atoms. The hydrogen bond dis­ tances of C2–H2A⋯O3g and C8–H8A⋯O3h (h1-x, 1-y, 1-z) are 3.256 and 3.426 Å, respectively. Apart from these hydrogen bonds, the stacking interactions can also be found among the imidazole and phenyl rings. The distances of Cg1⋯Cg1c, Cg1⋯Cg1d and Cg1⋯Cg3d (c-x, 1-y, 1-z; d1x, 1-y, 1-z) are 3.6139(16), 3.6457(16) and 3.5343(16) Å, respectively, where the rings Cg1 and Cg3 stand for the imidazole and phenyl groups, respectively. Notably, one PZBI cation and one nitrate anion generate a neutral aggregate. In the aggregate structure, the N/C–H⋯O (N4–H4⋯O3 and C2–H2A⋯O3) hydrogen bonds of can be observed. Additionally, the hydrogen bond of C8–H8A⋯O3 can also be found, through which these aggregates can be further expanded to a supramolecular 1D zig-zag chain along the crystallographic b axis (Fig. 2b). There exist another aggregate containing two free water molecules and two nitrate anions, where the hydrogen bonds of O1W–H1WA⋯O2 and O1W–H1WB⋯O1 can be found. Consequently, the specific hydrogen bond pattern A is represented as R44 ð14Þ (Fig. 2c), through which two adjacent supramo­ lecular 1D chains can be stretched to a supramolecular 2D layer along the crystallographic bc plane (Fig. 2c). In the 2D layer structure, we can find two other hydrogen bond pattern B and C, where can be featured as R22 ð7Þ and R88 ð28Þ, respectively. Meanwhile, the π⋯π stacking in­ teractions (Cg1⋯Cg1d and Cg1⋯Cg3d (d1-x, 1-y, 1-z) occur between the imidazole ring groups and phenyl ones (Table 4). The distances among the aromatic rings are 3.646 and 3.534 Å, respectively. As a result, another 2D supramolecular layer with a depth of 0.5 nm can be observed between two adjacent 2D thin layer. Apart from these packing in­ teractions, there exist another stacking interaction (3.6139 Å) between two imidazole rings, through which a 3D supramolecular framework is formed along the crystallographic b direction (Fig. 2d). 3.1.3. Structure description of [HPZBI]þClO4 (3) A new compound 3 can be obtained when HClO4 was used instead of HCl. The results of the X–ray crystallographic analysis reveal that compound 3 belongs to the space group P21/c with the monoclinic system. As shown in Fig. 3a, the asymmetric unit structure is comprised of one protonated PZBI cation and one perchlorate anion. The bond lengths of N(3)–C(5) and N(4)–C(5) are 1.340(4) and 1.346(4) Å, respectively, which are comparable to that found in compounds 1 and 2. The present results also reveal that the imidazole ring has easily been protonated. The dihedra angle between the pyrazine and imidazole rings is 7.627� , indicating the existence of coplanar structure between the aromatic rings. In the perchlorate anion, the bond lengths around chlorine center are 1.348, 1.384, 1.339 and 1.364 Å. The values of the bond angles fall in the range of 102.7–113.0� . Apart from that the hydrogen bonds can be found in the supramo­ lecular structures, there exist π⋯π packing interactions. In compound 3, there exist three types of hydrogen bonds: (i) The intermolecular hydrogen bond of N(3)–H(3)⋯N(2) (x, 1/2-y, 1/2 þ z) comes from the

Fig. 3. (a) A perspective view of 3. Some disordered cations in structures. (b) A one-dimensional chain along the a axis. (c) A two-dimensional supramolecular layer along the bc plane. (d) A packing diagram in 3.

nitrate anion as well as one free water molecule (Fig. 2a). As illustrated in Fig. 2a and Table 2, the values of 1.337(3) and 1.333(2) Å can be observed for the distances of N3–C5 and N4–C5, respectively, revealing that there exists the protonated imidazole group in the conjugated for­ mation, which is similarly found in other benzimidazole-based de­ rivatives reported previously. The dihedra angle between the imidazole and pyrazine rings is 4.418� , which is smaller than that found in com­ pound 1. The present observation reveals that it occurs the coplanar 6

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Optical Materials 98 (2019) 109432

Table 5 The stacking interactions and the maximal peaks for compounds of 1–4. Compound

π∙∙∙π

Shortest distance (Å)

λem (nm) in the solid

λem (nm) in the solution

PZBI 1 2 3 4

Cg1∙∙∙Cg1 Cg1∙∙∙Cg3 Cg1∙∙∙Cg3 Cg1∙∙∙Cg4

3.613(1) 3.534(2) 3.731(3) 3.492(6)

444 472 471 432 443

461 462 463 418 460

Fig. 5. TGA of compounds PZBI and its salts 1, 2, 4.

nitrogen atoms of imidazole-yl and pyrazine-yl groups, where the bond distance and the bond angle are 2.858(4) Å and 163� , respectively. (ii) The N–H⋯O intermolecular hydrogen bond (N(4)–H(4)⋯O(2)j j1-x, 1-y, 1-z) occur between the nitrogen atom from the HPZBI cation and the perchlorate oxygen atom, where the bond length is 2.857(5) Å. (iii) Another kind of intermolecular hydrogen bond of C–H⋯O (C(2)-H (2A)⋯N(1)k, 3.227(4) Å, kx, 1/2-y, 1/2 þ z) originates from the carbon and nitrogen atom of pyrazine-yl groups. The hydrogen bonds of C/N–H⋯O connect the imidazole and pyr­ azine rings in two adjacent HPZBI cations, which aligns in the antiparallel mode along the crystallographic c axis. Therefore, a 1D supra­ molecular chain can be observed along the crystallographic c direction (Fig. 3b). Interestingly, the perchlorate anion acts as a glue linking two adjacent HPZBI cations through the hydrogen bonds (C3–H3A⋯O3, C4–H4A⋯O3, C7–H7A⋯O1 and C8–H8A⋯O1), which results into consolidating the stability of supramolecular chain. Apart from these hydrogen bonds, there exists π⋯π packing interaction between the imidazole and phenyl functional groups. The distance of Cg(1)⋯Cg(3) is 3.731(3) Å, where the rings Cg(1) and Cg(3) represent the imidazole and phenyl groups, respectively. Consequently, these 1D chains can be further stretched to a 2D supramolecular layer along the crystallo­ graphic bc plane (Fig. 3c). Finally, a 3D supramolecular architecture can be observed along the crystallographic c direction (Fig. 3d). 3.1.4. Description of [HPZBI]þ H2PO4 (4) A new compound 4 can be obtained when H3PO4 was used instead of HCl. The results of the X–ray crystallographic analysis reveal that compound 4 crystallizes to the monoclinic system with P21/c space group. As shown in Fig. 4a, there exist one protonated PZBI cation and one dihydric phosphate anion in the asymmetric unit structure. Notably, the presence of disordered HPZBI cation can be observed. In the imid­ azole structure, the bond lengths of N1–C7 and N2–C7 are 1.288 and 1.375 Å, respectively, which are comparable to that found in compounds 1–3. The present results also show that the imidazole ring can easily accept the proton. There exists the dihedra angle of 10.82� observed between the pyrazine and imidazole rings, indicating that it occurs the disappearance of coplanar structure between the aromatic rings.

Fig. 4. (a) A perspective view of 4. The disordered cations in structure. (b) A one-dimensional chain along the b axis. Some disordered cations in structure. (c) A two-dimensional supramolecular layer along the ab plane. Some disor­ dered cations in structure. (d) A packing diagram in 4. Some disordered cations in structure. 7

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Fig. 6. IR spectra of 1–4.

Fig. 7. UV–Vis absorption spectra of PZBI and its compounds 1–4 in the mixed solution (10-5 M) containing methanol and water.

Fig. 8. Fluorescent emission spectra of PZBI and 1–4 in the solid state at room temperature.

Obviously, the value of the dihedra angle in compound 4 is higher than those in compounds 1–3, which may be assigned to the disordered structure of the protonated PZBI cation. In the dihydric phosphate anion, the bond distances around the phosphor atom are 1.504, 1.515, 1.566 and 1.572 Å. The bond angles of O–P–O range from 105.33� to 111.3� . Usually, hydrogen bonds and π⋯π packing interactions can be observed in the supramolecular structures. In 4, four types of hydrogen bonds can be found: (i) The intermolecular hydrogen bonds of N–H⋯O construct between the imidazole-yl nitrogen atoms and the dihydrogen phosphate oxygen atoms. The bond distances of N1–H1N⋯O1 and

N2–H2N⋯O2 are 2.648(9) and 2.638(9) Å, respectively. (ii) The O–H⋯O intermolecular hydrogen bonds (O3–H3O⋯O1 and O4–H4O⋯O2) generate from the oxygen atoms from the dihydrogen phosphate anions, where the bond lengths are 2.623(5) and 2.614(5) Å. (iii) Another kind of intermolecular hydrogen bonds of C–H⋯N (C2–H2⋯N40 and C9–H9⋯N4) can be found, which occur from the carbon atoms of the pyrazine-yl/phenyl groups and the pyrazine-yl ni­ trogen atom. The distances and the bond lengths are 3.291(9) Å and 131� , 3.452(10) Å and 151� , respectively. (iv) The weak intermolecular hydrogen bonds (C10–H10⋯O4 and C11–H11⋯O2) come from the benzimidazole and the dihydrogen phosphate anion, where the bond 8

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the host. The further weight loss occurs in the range of 193–381 � C, suggesting the departure of the organic component (obsd. 90.12%, cald. 92.82%). For 2, there exists a weight loss ranging from room tempera­ ture to 112 � C, indicating the release of water molecules (obsd. 6.90%, cald. 6.50%). The present case indicates the existence of interactions of hydrogen bonds between the host and the crystallized water molecules. When the temperature is further increased, the second weight loss starts at 144 � C, and ends at 360 � C, showing the disappearance of the organics (obsd. 90.50%, cald. 93.50%). For 4, the plateau can be found from room temperature to 240 � C, suggesting that there do not exist water molecules and other solvents. Further enhancing the temperature, it occurs to decompose at 251 � C. 3.3. FT–IR spectra studies Fig. 9. Fluorescent emission spectra of PZBI and its compounds 1–4 in the solution containing methanol and water at room temperature (c ¼ 10 5 M).

The FT-IR spectra of its PZBI-containing compounds 1–4 are shown in Fig. 6, respectively. As shown in Fig. 6a, there exists strong and sharp peak located at 3363 cm 1 in compound 1, which can be attributed the stretching vibration of –OH group. The present results indicate the ex­ istence of free water molecule in salt 1. The stretching vibration of νNH is observed at 2345–3088 cm 1, being relative to the presence of the imidazole ring. The bands between 1625 and 1454 cm 1 correspond to – C and C– – N stretching vibration. The band found at 962 cm 1 the C– can be tentatively ascribed to the γ (CH) modes. The bands observed at 769 and 617 cm 1 are relative to the γ(CH) and δ(CH) modes [23]. As shown in Fig. 6b, the broad peak of the stretching vibration for the –OH group can be observed at 3547 cm 1 in compound 2, which sug­ gests the existence of water molecule. The strong band can be observed at 3373 cm 1, which indicates the characteristic stretching vibration of N–H. The peaks range from 2628 to 2175 cm 1 can be ascribed to the stretching vibration of N–H for the protonated benzimidazole aromatic – C and C– – N stretching modes can be observed in the range ring. The C– of 1423–1629 cm 1. The very strong and sharp peak found at 1384 cm 1 is the characteristic feature of the stretching vibration of the nitrate anion. The bands fall in the range of 617–817 cm 1, suggesting that there exist the in–plane and out––plane bending modes of –CH located at aromatic rings. For compound 3, its IR spectrum is shown in Fig. 6c. The band located at 3385 cm 1 can be easily found, indicating the presence of the stretching vibration of νN-H. The bands located at the range of 2661–2814 cm 1 can be observed, which can correspond to the stretching vibration of N–H from imidazole group. The bands located in the region of 1450–1629 cm 1 can assigned to the stretching vibration – C and C– – N of HPZBI. The bands appeared at 528–854 cm 1 bands of C– can be ascribed to the in-plane and out-plane of the bending vibration of C–H from phenyl and pyrazine rings. The specific band of the stretching vibration for the perchlorate anion is found at 1105 cm 1 [24]. In IR spectrum of compound 4, several distinguishable peaks can be observed from 4000 to 400 cm 1 (Fig. 6d). The N–H stretching vibration can be observed at 3049 cm 1. The broad bands located at 2715 and 2611 cm 1 may be ascribed to the stretching vibration of O–H from dihydrogen phosphate anion. The bands can be observed at – N and 1456–1624 cm 1, which reveals that there exist the aromatic C– – C stretching vibrations. The specific stretching vibration of P– – O can C– be observed at 1230 cm 1. In the present case, the band observed at 1085 cm 1 in the IR spectrum is characteristic feature of the stretching vibrational mode of dihydrogen phosphate [25]. The bands ranged from 567 to 873 cm 1 are assigned to the in-plane and out-plane bending vibrational mode of aromatic C–H.

distance and the bond length are 3.240(7) Å and 172� , 3.339(8) Å and 149� , respectively. Apart from these hydrogen bonds, it occurs the stacking interactions among these aromatic rings such as imidazole, pyrazine and phenyl groups. The packing interactions (Cg1∙∙∙Cg3f, 3.575(5) Å; Cg1∙∙∙Cg4e, 3.492(6) Å; Cg1∙∙∙Cg5f, 3.566(5) Å; Cg3∙∙∙Cg1e, 3.574(5) Å; Cg4∙∙∙Cg1f, 3.493(6) Å; Cg5∙∙∙Cg1e, 3.566 (5) Å) can be constructed between the imidazole and pyrazine rings, imidazole and phenyl rings, imidazole and pyrazine rings, where the rings Cg1, Cg3, Cg4 and Cg5 represent the imidazole, phenyl, disordered imidazole and disordered pyrazine rings, respectively (Table 5). Other packing interactions can be found such as Cg2∙∙∙Cg4f, Cg4∙∙∙Cg2e, Cg4∙∙∙Cg6e as well as Cg6∙∙∙Cg4f, which the interaction distances are 3.533(5), 3.533(5), 3.581(5) and 3.582(5) Å, respectively. The rings Cg2, Cg4 and Cg6 indicate the pyrazine, disordered imidazole, disor­ dered phenyl rings, respectively. Two pyrazine groups make use of the C–H⋯N (*C9–H9 ⋯N4) hydrogen bonds linking two adjacent PZBI cations, which aligns in the anti-parallel mode along the b direction. Therefore, a 1D supramolecular zig-zag chain can be generated along the crystallographic b axis (Fig. 4b). The dihydrogen phosphate decorates the supramolecular chain through the N1–H1N⋯O1 hydrogen bond, which play a key role in connecting to the HPZBI cation. Other hydrogen bond (*C2–H2 ⋯N40 ) further stretches these 1D supramolecular chains to a 2D supra­ molecular sheet along the crystallographic ab plane (Fig. 4c). Finally, the residue hydrogen bonds (*N20 -H20 N⋯O2, *N2 –H2N⋯O2, O3–H3O⋯O1, O4–H4O⋯O2, *C10–H10⋯O4, *C100 -H10’∙∙∙O3 and *C11–H11⋯O2) and the stacking interactions make these 2D supra­ molecular layers extend to a 3D supramolecular architecture along the crystallographic c direction (Fig. 4d). 3.2. Powder X-ray diffraction and thermogravimetric analysis To confirm the bulk samples with the same composition as the selected single crystals, powder X-ray diffraction (PXRD) data of bulk samples was measured at room temperature. As shown in Fig. S1, it is evidently observed that the simulated and experimental XRD patterns of compounds 1–4 are in good accordance with each other. The present results reveal the phase purity of the products. To evaluate the thermal stabilities of these compounds, the pre­ liminary thermogravimetric experiment on samples containing powdered crystals was taken under N2 atmosphere with heat rate of 10 � C min 1. The thermogravimetric analysis (TGA) results are shown in Fig. 5. The TGA curve of PZBI shows that it occurs a platform before 184 � C. It starts to decompose when the temperature was further increased. For 1, the first weight loss can be observed from 25 to 140 � C, which is in agreement with the loss of free water molecules (obsd. 6.37%, cald. 7.18%). The present results reveal that there obviously exist the hydrogen bond interactions between the water molecules and

3.4. UV–Vis spectra UV–Vis absorption spectra of PZBI and its compounds 1–4 were carried out in mixed solution containing methanol and water solution in the ration of 1:1 (1 � 10 5 M). As shown in Fig. 7a, the bands at 200–400 nm can be clearly found in compounds PZBI and its salts 1–4. 9

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Optical Materials 98 (2019) 109432

Fig. 10. The fluorescence decay curves of PZBI and its salts 1–4 at room temperature.

The bands at ca. 237 and 327 nm can be found, which can be tentatively – C and C– – N bonds in PZBI. The ascribed to the π–π* transition of C– absorption peaks can be observed at 234 and 324 for 1, 235 and 324 nm for 2, 237 and 324 nm for 3, and 236 and 324 for 4, respectively. The peaks located at about 234 and 324 nm can be clearly found, which can be ascribed to the π–π* transition of the phenyl and pyrazine ring, respectively. There exists a large conjugated system formed by benz­ imidazole and pyrazine ring [26].

3.5. Luminescent properties Given that benzimidazole-based compounds show interestingly luminescent properties, we primarily examined the photoluminescent behaviors of compounds 1–4. Hence, the solid state photoluminescent emission spectra of 1–4 were investigated at room temperature. As shown in Fig. 8, the spectrum of compound 1 reveals that one emission band located at 472 nm can be observed when the excitation wavelength 10

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Optical Materials 98 (2019) 109432

was 324 nm. Compound 2 also displays one luminescence emission peak observed at 471 nm upon excitation at 324 nm. For compound 3, its photoluminescent spectrum shows that there exist one maxima located at 432 nm (λex ¼ 324 nm). From the spectrum of compound 4, we can also find one maximal emission peak at 443 nm. To further contribute the luminescent profiles of compounds 1–4, the photoluminescent property of PZBI was also investigated. As shown in Fig. 8, the emission maxima of PZBI was observed at 444 nm upon the excitation at 324 nm, which can be attentively attributed to the intraligand π* → π or π* → n transitions [27]. The luminescent profiles of compounds 1–4 are clearly similar to that of the free PZBI, which can be tentatively attributed to the intraligand π* → π or π* → n transitions. Compared with the free PZBI, the emission maxima of compounds 1–4 are clearly blue/red-shifted about 28, 27, 12 and 1 nm for 1–4, respectively, which may be tenta­ tively attributed to the fact that the combination of receptor molecules and anions, and the contribution from the intramolecular charge transfer can be responsible for it [28]. It is clearly found that the blue/red-shifted amplitudes are relative to the intermolecular packing interactions. To further investigate the luminescent behaviors, we carefully compared the π⋅⋅⋅π packing interactions among these compounds. As discussed in the crystal structures, the shortest intermolecular distances of 3.613, 3.534, 3.731, and 3.492 Å are clearly observed in compounds 1, 2, 3 and 4, respectively (Table 5). Therefore, their stacking distances among aromatic rings are obviously relative to the blue/red-shifted amplitudes. To further confirm the results mentioned above, the photo­ luminescent behaviors of PZBI and its compounds 1–4 were introduced in the solution state at room temperature. As shown in Fig. 9, the spectra of compounds 1–4 and the free PZBI show that the maximal peaks can be found at 461, 462, 418, 460 and 461 nm, respectively. The emission profiles of compounds 1–4 are analogous to those of PZBI, therefore, the luminescence of compounds 1–4 are tentatively assigned to intraligand π* → π or π* → n transitions [29]. From the present results, it was clearly observed that except from compound 1, the red-shifted phenomena cannot be found among these compounds, which can be tentatively ascribed to the disappearence of packing interactions in the solution [30]. To further study the luminescence of these salts, the luminescent quantum yields of compounds were estimated in the mixed solution (1:1 methanol and water) at room temperature by a relative method using coumarins as a standard. The fluorescence quantum yields (φ) are 20.0%, 24.5%, 26.4%, 21.65%, % and 20.57% for compounds PZBI and its salts 1–4, respectively. The present observation reveals that compared with the free PZBI itself, its salts show slightly higher lumi­ nescent quantum yields. To further evaluate the photoluminescent decay of PZBI and its salts 1–4, the primary experiments of the solid luminescent lifetime were performed in the solid state at room temperature. For all of compounds, single exponential functions were applied into fitting the decay curves. As shown in Fig. 10, the solid luminescent decay times of 4.03, 1.10, 0.99, 0.94 and 2.86 ns were fitted for PZBI and its salts 1–4, respectively. Compared with PZBI itself, the lifetime decay of the salts containing PZBI are shorter.

supramolecular frameworks are easily generated. The present results demonstrate that the intermolecular interactions play a key role in the luminescent behaviors of the products, which provide us a new platform to exploring and designing novel luminescent materials. Currently, other benzimidaole/pyrazine-containing salts with promising physical properties is on the waty in our laboratory. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was financially supported by Project of Shandong Prov­ ince Higher Educational Science and Technology Program (J09LB03), Shandong Distinguished Middle-aged Young Scientist Encouragement and Reward Foundation (BS2011CL034) and Shandong Provincial Natural Science Foundation (ZR2017MB041). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109432. References [1] (a) L.F. Zou, Y.J. Sun, S. Che, X.Y. Yang, X. Wang, M. Bosch, Q. Wang, H. Li, M. Smith, S. Yuan, Z. Perry, H.C. Zhou, Porous organic polymers for postcombustion carbon capture, Adv. Mater. 29 (2017), 1700229; (b) H.V. Babu, M.G.M. Bai, M.R. Rao, Functional pi-conjugated two-dimensional covalent organic frameworks, Acs Appl. Mater. Interf. 11 (2019) 11029–11060; (c) R.R. Liang, X. Zhao, Heteropore covalent organic frameworks: a new class of porous organic polymers with well-ordered hierarchical porosities, Org. Chem. Front. 5 (2018) 3341–3356; (d) Q. Pang, B. Tu, Q. Li, Metal–organic frameworks with multicomponents in order, Coord. Chem. Rev. 388 (2019) 107–125; (e) M. Yan, X.-B. Liu, Z.-Z. Gao, Y.-P. Wu, J.-L. Hou, H. Wang, D.-W. Zhang, Y. Liu, Z.-T. Li, A pore-expanded supramolecular organic framework and its enrichment of photosensitizers and catalysts for visible-light-induced hydrogen production, Org. Chem. Front. 6 (2019) 1698–1704; (f) Y.R. Yusran, Q.R. Fang, S.L. Qiu, Postsynthetic covalent modification in covalent organic frameworks, Isr. J. Chem. 58 (2018) 971–984; (g) S.Y. Ding, W. Wang, Covalent organic frameworks (COFs): from design to applications, Chem. Soc. Rev. 42 (2013) 548–568; (h) N. Huang, P. Wang, D.L. Jiang, Covalent organic frameworks: a materials platform for structural and functional designs, Nature Rev. Mater. 1 (2016); (i) S. Yuan, J.S. Qin, J.L. Li, L. Huang, L. Feng, Y. Fang, C. Lollar, J.D. Pang, L. L. Zhang, D. Sun, A. Alsalme, T. Cagin, H.C. Zhou, Retrosynthesis of multicomponent metal-organic frameworks, Nat. Commun. 9 (2018); (j) Y.Y. Zhang, J.X. Li, L.L. Ding, L. Liu, S.M. Wang, Z.B. Han, Palladium nanoparticles encapsulated in the MIL-101-catalyzed one-pot reaction of alcohol oxidation and aldimine condensation, Inorg. Chem. 57 (2018) 13586–13593; (k) Z. Yin, S. Wan, J. Yang, M. Kurmoo, M.-H. Zeng, Recent advances in postsynthetic modification of metal-organic frameworks: new types and tandem reactions, Coord. Chem. Rev. 378 (2019) 500–512; (l) B. Liu, F. Yu, M. Tu, Z.-H. Zhu, Y. Zhang, Z.-W. Ouyang, Z. Wang, M.-H. Zeng, Tracking the process of a solvothermal domino reaction leading to a stable triheteroarylmethyl radical: a combined crystallographic and massspectrometric study, Angew. Chem. Int. Ed. 58 (2019) 3748–3753; (m) S. Yan, J. Chen, L. Cai, P. Xu, Y. Zhang, S. Li, P. Hu, X. Chen, M. Huang, Z. Chen, Phthalocyanine-based photosensitizer with tumor-pH-responsive properties for cancer theranostics, J. Mater. Chem. B 6 (2018) 6080–6088; (n) H. He, E. Ma, J. Yu, Y. Cui, Y. Lin, Y. Yang, X. Chen, B. Chen, G. Qian, Periodically aligned dye molecules integrated in a single MOF microcrystal exhibit single-mode linearly polarized lasing, Adv. Opt. Mater. 5 (2017) 1601040. [2] (a) S. Tahlan, S. Kumar, K. Ramasamy, S.M. Lim, S.A.A. Shah, V. Mani, R. Pathania, B. Narasimhan, Design, synthesis and biological profile of heterocyclic benzimidazole analogues as prospective antimicrobial and antiproliferative agents, Bmc Chem 13 (2019); (b) A. Srivastava, A.P. Mishra, S. Chandra, A. Bajpai, Benzothiazole derivative: a review ON its pharmacological importance towards synthesis OF lead, Int. J. Pharmaceut. Sci. Res. 10 (2019) 1553–1566; (c) S. Rajasekhar, B. Maiti, M.M. Balamurali, K. Chanda, Synthesis and medicinal applications of benzimidazoles: an overview, Curr. Org. Synth. 14 (2017) 40–60; (d) M. Flores-Ramos, F. Ibarra-Velarde, H. Jung-Cook, A. Hernandez-Campos,

4. Conclusions In summary, a set of salts based benzimidazole and pyrazine groups have been successfully synthesized, which show specific luminescent behaviors. Compared with the free benzimidazole, the luminescent emission peaks of its crystal-state salts are relative to the intermolecular packing interactions. Compared with benzimidazole itself, the salts show shorter decay lifetime and more powerful fluorescent quantum yields. Single crystal X-ray analyses show that in these salts, there exist a variety of hydrogen bonding such as C/O/N–H⋯Cl, C/N/O–H⋯O as well as C/N/O–H⋯N, and π⋯π packing interactions, through which 3D 11

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