Structural Study and Fluorescent Property of a Novel Organic Microporous Crystalline Material

Um novo material orgânico microporoso [(2-{2-[2-(bis-metoxicarbonilmetilamino)fenoxi] etoxi}-4-benzimidazol-fenil)metoxicarbonilmetilamino] éster metílico do ácido acético 6 foi sintetizado e caracterizado por difração de raio X de cristal único, espectroscopia no infravermelho por transformada de Fourier (FT-IR), espectrometria de massas de alta resolução com ionização por electrospray (ESI-HRMS), difração de pó (PXRD) e RMN do H e C. 6 cristaliza em grupos de espaço monoclínico centrossimétrico C2/c, com parâmetros de célula unitária a = 35,648(3) Å, b = 14,3240(12) Å, c = 15,3693(13) Å, α = 90,00, β = 94,8190(10), γ = 90,00, V = 7820,16 Å e Z = 8 a 296(2) K. Conforme indicado pelo empacotamento cristalino, os planos de conjugação molecular se organizam ao longo do eixo c para formar microporos devido às ligações de hidrogênio. Além disso, espectro de fluorescência e tempo de vida de luminescência foram estudados para 6.


Introduction
During recent years, research on porous organic crystalline materials is of particular interest for their broad application in chemical separating, 1 capturing, 2 and gas storing. 3Many recent studies have focused on the most successful application: the gas storage.In this field, some measures that aim to obtain an improved performance of microporous materials are adopted, such as optimizing pore size and surface area. 4,5However, other functions of microporous materials excluding gas storage have not been well studied.This motivated us to explore microporous materials with novel properties.Herein, a novel luminescent organic microporous crystalline material [(2-{2-[2-(bismethoxycarbonylmethylamino)phenoxy]ethoxy}-4-benzimidazole-phenyl)methoxycarbonylmethylamino] acetic acid methyl ester 6 is reported.This structure contains benzene and imidazole functionalities to enhance luminescence and shows the property to form microporous skeleton via intramolecular [6][7][8] and intermolecular hydrogen bonds (Figure 1). 8,91][12][13] Most studies are associated with the spectroscopic and structural properties of benzimidazole 14,15 and some of its derivatives coordination compounds containing benzimidazole group 16 .8][19] To improve the photoabsorption properties of imidazole, it is possible to introduce some groups with conjugated structures, such as benzene.
The single crystal X-ray structural study, as an efficient method to reveal molecular conformation, intra-and Vol. 25, No. 1, 2014   intermolecular interactions in solid state of substances, can help us better understand and refine molecular structures in order to optimize certain properties.

Reagents and equipments
Melting points were determined on an XT-4B micro melting point apparatus without correction.FT-IR (Fourier transform infrared spectroscopy) spectra were recorded with KBr pellets on a Bruker EQUINOX-55 FT-IR spectrometer. 1 H and 13 C NMR spectra were recorded on a Varian INOVA-400 spectrometer at 400 and 100 MHz, respectively.Chemical shifts were reported relative to internal standard Me 4 Si.Electron spray ionization-mass spectrometry (ESI-MS) analyses were carried out in positive ion modes using a Thermo Finnigan LCQ Advantage MAX LC/MS/MS.The X-ray diffraction data were collected on a Bruker Smart APEX II CCD diffractometer.The X-ray powder diffraction (PXRD) pattern was recorded with a Pigaku D/Max 3III diffractometer.The fluorescent spectrum was measured with a Hitachi F-4500 FL spectrophotometer.The luminescent lifetime was performed on Edinburge FLS920.
And all the reagents and solvents used for synthesis were commercially available and without further purification unless otherwise noted.The reaction process was monitored by thin-layer chromatography (TLC).The products were purified by recrystallization or column chromatography, and the latter was carried out on silica gel (200-300 mesh).

2-[2-(2-Aminophenoxy)ethoxy]benzenamine 3
Iron powder (3.36 g, 0.06 mol), concentrated hydrochloric acid (0.2 mL), and anhydrous ethanol (10 mL) were added into a dried three-necked flask equipped with a magnetic stirrer.When the mixture was heated to boiling, 2 (3.04 g, 0.01mol) was added in three portions.The mixture was refluxed for 4 h, and then made alkaline to litmus by addition of 15% alcoholic potassium hydroxide solution, the iron powder was removed by filtration afterwards.Into the filtrate, 6 mol L -1 sulfuric acid was added and white precipitate was obtained.After filtration, the precipitate was dissolved in 40 mL of warm water and made alkaline to pH = 8 with saturated sodium hydroxide solution.The generated light yellow solid was collected and recrystallized in MeOH to give white solid 3. Yield 88%; m.p. 116-117 °C; 1  Compound 3 (2.44 g, 0.01 mol) was dissolved in MeCN (10 mL), then (i-Pr) 2 NEt (6 mL) and methyl bromoacetate (3 mL) were added to the mixture with stirring.The reaction mixture was refluxed for 24 h.After the reaction, the mixture was cooled down, poured into EtOAc (20 mL), and filtered to remove the generated white solid.The combined EtOAc filtrates were concentrated in vacuo to give an oily solid, then adding a little methanol, white solid was generated, filtered, air dried and recrystallized in MeOH to give white solid 4. Yield 87%; m.p. 94-95 °C;  POCl 3 (2.4 mL) was added dropwise over 40-45 min into a dry three-necked flask which contained anhydrous DMF (20 mL).The POCl 3 /DMF mixture was stirred at room temperature for 1-2 h and added dropwise into a DMF (20 mL) solution of compound 4 (5.32 g, 0.01mol) afterwards.The reaction mixture was heated at 75 °C for 4 h, concentrated in vacuo, and then poured into ice water.The suspension was filtered and purified by column chromatography (silica gel, V(EtOAc):V(hexane) = 1:1 as eluent) to afford white solid 5. 20  A mixture of 1,2-phenylenediamine (0.11 g, 1 mmol), compound 5 (0.56 g, 1 mmol), H 2 O 2 (30%, 4 mmol, 0.4 mL) and Fe(NO 3 ) 3 •9 H 2 O (0.04 g, 0.1 mmol) was heated at 50 °C for 30 min.After completion of the reaction, the reaction mixture was dissolved in EtOH (10 mL) and then poured into ice-water (30 mL).The pure solid product was filtered, washed with ice-water, dried and subsequently purified by column chromatography (silica gel, V(EtOAc):V(hexane) = 1:1 as eluent) 21,22  A colorless crystal with a size of 0.30 × 0.24 × 0.19 mm 3 was selected for X-ray data collection.The X-ray diffraction measurement was made on a Bruker Smart APEX II CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å).The crystal data was collected at room temperature using ω-2θ scan technique.The structure was solved by direct methods with SHELXS-97 23 and refined using SHELXL-97. 24The crystal data collection and refinement parameters are given in Table 1.

Structural characterization
The structure of 6 was characterized by IR, ESI-HRMS, PXRD, 1 H and 13 C NMR. IR spectrum of 6 shows typical secondary amine absorption (ν N-H 3506 cm -1 ), aromatic absorption (ν =C-H 3033 cm -1 ; ν C=C 1510, 1478 cm -1 ; d =C-H 746 cm -1 ) and ester absorption (ν C=O 1743 cm -1 ; ν C-O 1171 cm -1 ).The HRMS m/z value, 1 H and 13 C NMR chemical shifts are in accordance with the structure of 6.And powder X-ray diffraction at room temperature was carried out to testify the phase purity of assynthesized samples, the diffraction peaks of both simulated 25,26 and experimental patterns match in the key positions, indicating the pure phase of 6. PXRD pattern of 6 at room temperature can be found in Figure 2.

Crystal structure
The molecular structure and atom numbering of 6 are shown in Figure 3, and the crystal packing of 6 along the c axis is presented in Figure 4.
The supramolecular packing is determined mainly by the hydrogen bonds C-H•••O (Table 2).Due to the intramolecular hydrogen bonds from carboxylic oxygen atoms O4, O7, O9 and ether oxygen atoms O1, O2, microporous skeleton along the c axis is formed.And the intermolecular hydrogen bonds between carbonyl oxygen atoms O4 and O9 from a neighboring molecule are observed to extend supramolecular organic frameworks. 27,28he investigated compound crystallizes in the C2/c space group with eight molecules per unit cell.Table 3 lists some selected bond lengths and torsion angles for 6 to demonstrate the whole molecular skeleton.It can be    atoms in benzene rings, keep certain angles and make the intramolecular hydrogen bonds possible, thus micropores are formed, as indicated clearly by the intramolecular hydrogen bonds from carboxylic oxygen atoms O4, O7, O9 and ether oxygen atoms O1, O2.And the intermolecular hydrogen bonds between carbonyl oxygen atoms O4 and O9 from a neighboring molecule are observed to extend supramolecular organic architecture.

Fluorescent spectrum and luminescent lifetime
The solid state fluorescent spectrum of 6 at room temperature is given in Figure 5. 6 in the solid state displays a blue emission with emission peaks at 389.6 nm and 461.6 nm upon excitation at 310 nm.The emission peak at 389.6 nm can be assigned to π-π * transitions of benzene and imidazole delocalized π electrons, 461.6 nm emission peak is probably due to n-π* transitions of n electrons in nitrogen atoms of imidazole.
The luminescent lifetime τ indicates the average time of a molecule at excitation state.A small value of τ, which means the molecule can recover quickly from the excitation state and allow multi-excitations, always comes along with high sensitivity in luminescence.As shown in Figure 6, the decay lifetime curve of 6 can be well fitted with doubleexponential decay, giving two lifetimes of τ 1 = 0.68 ns and τ 2 = 2.68 ns (c 2 = 1.252), which suggest fluorescence. 29

Conclusions
The investigated molecule 6 is fluorescent and has two luminescent lifetimes.Furthermore, single crystal X-ray diffraction study indicates that the electrons are delocalized and inter-, intramolecular hydrogen bonds are displayed in the molecular system.The planarity of molecular skeleton and luminescence make delocalized electrons transfer and migration possible.These characteristics are unique and clearly originate from the highly ordered structure of 6.  Exploration of functional microporous crystalline materials is a probable way to the development of novel materials.And formation of optoelectronic devices is expected by filling the micropores with photoactive molecules such as electron acceptors in similar crystalline materials of 6, which worth our attention and will be a target for further investigation.

Figure 1 .Scheme 1 .
Figure 1.Schematic representation of void cage in 6 with V void of 20% (total potential solvent occupied volume of 1563.1 Å 3 per unit cell volume).The molecular skeleton with same color located at centrosymmetric position.

Figure 4 .
Figure 4. Crystal packing of 6 along the c axis.

Figure 5 .
Figure 5. Solid state emission of 6 on excitation at 310 nm.

Table 1 .
Crystallographic data and structure refinement parameters for 6