Cadmium(II) supramolecular complexes constructed from a phenylbenzoxazole-based ligand: self-assembly, structural features and nonlinear optical properties

Abstract

A new phenylbenzoxazole-based ligand bearing a pyridyl group (abbreviated as L) was designed and applied for self-assembly with corresponding cadmium(II) salts to generate four coordination complexes, namely, C₁₉H₁₅CdCl₂N₃O (1), C₁₉H₁₅Cd₂Br₂N₃O (2), C₄₀H₃₈Cd₂I₄N₆O₄ (3) and C₂₁H₁₅CdN₅OS₂ (4). All the complexes 1–4 were fully characterized by single crystal X-ray diffraction. In 1 and 4, new supramolecular architectures are built mainly through synergistic effects of coordination bonds, hydrogen bonding interactions based on the anions and the benzoxazolyl nitrogen (oxygen) atom. There are more C–H⋯π and π–π stacking interactions constructing the higher dimensional structure in 2 and 3, respectively. In addition, in complex 3, we observe that CH₃OH molecules are dispersed in the supramolecular structure and play a vital role in building the supramolecular structure. The nonlinear optical (NLO) properties of the complexes were investigated using the Z-scan technique, which showed that complexes 1 and 3 have obvious nonlinear absorption compared to ligand L.

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Introduction

The design and synthesis of metal–organic complexes based on strong coordinate bonds and multiple, weak non-covalent forces have become one of the most active fields in coordination chemistry and crystal engineering not only for their intriguing variety of architectures but also due to their tremendous potential applications in nonlinear optics, catalysis, luminescence, magnetism and medicine.¹ On the other hand, polymeric Cd(II) complexes with d¹⁰ configuration have also attracted great interest because of their attractive fluorescence properties and potential applications as new advanced luminescent materials.^2–4

To date, several cadmium supramolecular complexes with specific topologies and excellent properties have been synthesized by the assembly of cadmium salts and organic ligands.^5–7 However, it is still a challenge for chemists to predict and control the structures of such metal–organic complexes, owing to the complicated influencing factors of the assembly reactions such as the organic ligands, anions, nature of transition metal ions and experimental conditions such as solvent, metal-to-ligand ratio and reaction temperature.^8,9 Among these factors, the organic ligand, anions and metal centers are the bases of the coordination complexes.

Organic ligands with N- or O-donors, as effective building blocks, also play important roles in the construction of cadmium complexes.^10–12 Among the N-donors, imidazolyl, triazolyl and pyridyl ligands have been intensively engaged in forming complexes associated with interesting structures, topologies and properties in our systematic studies.^13,14 Benzoxazole-containing ligands with pyridyl groups have been used to coordinate with iridium(III), ruthenium(II) and rhenium(II) ions, and rarely coordinate with cadmium(II) ions.^15,16

Based on our previous study and inspired by the following facts: (1) the previously prepared Schiff base phenylbenzoxazole-derivaties exhibited distinct aggregation-induced enhanced emission (AIEE) behavior, which was used in various fields such as optoelectronic devices and chemosensors;^17,18 (2) in designing coordination complexes, pyridyl has been widely used as a ligand due to its ability to coordinate to several metal centers in various modes; (3) the –C–N– single bond has greater flexibility than the –C=N– double bond and can rotate freely to meet the different geometric requirements of the metal ions and for the formation of weak interactions.^19,20 Hence, we designed and synthesized a new ligand, namely, N-(pyridyl-2-methyl)-4-benzo[d]oxazol-2-ylphenylamine (L) based on phenylbenzoxazole via a one-step reaction (Scheme 1).

Scheme 1 The molecular structure of ligand L.

Moreover, self-assembly of L with various CdX₂ salts (X = Cl, Br, I, SCN) was carried out. Four new Cd(II) complexes were obtained. Herein, the synthesis and optical properties of Cd(II) complexes with different coordination anions were systematically investigated.

Experimental

Materials and measurements

All commercially available chemicals and solvents are of reagent grade and were used as received without further purification. The ¹H NMR and ¹³C NMR spectra were obtained at 25 °C on a Bruker Avance 400 spectrometer and the chemical shifts are reported as parts per million (ppm) from TMS. The mass spectra were obtained with an LTQ Orbitrap XL. IR spectra were acquired with a Nicolet FT-IR NEXUS 870 spectrometer (KBr discs) in the 4000–400 cm⁻¹ region. The solid state luminescence spectra were obtained on a F-4500 FL spectrophotometer. For time-resolved fluorescence measurements, the fluorescence signals were collimated and focused onto the entrance slit of a monochromator with the output plane equipped with a photomultiplier tube (HORIBA HuoroMax-4P). The decays were analyzed by least-squares. The quality of the exponential fits was evaluated by the goodness of fit (χ²). Z-Scan measurements were performed by a femtosecond laser pulse and Ti:95 sapphire system (700–860 nm, 80 MHz, 140 fs). The laser beam was focused into a 1.00 mm thick quartz cell, in which the sample was placed. The solution concentration of L and complexes 1–4 in DMF was 1.0 × 10⁻³ mol L⁻¹.

X-ray crystallography and structure solution

X-ray diffraction data of single crystals were collected on a Bruker SMART CCD area detector. Determination of unit cell parameters and data collections were performed with Mo-K_α radiation (λ = 0.71073 Å). Unit cell dimensions were obtained from least-squares refinements and all structures were solved by direct methods using SHELXS-97.²¹ The remaining non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed using full matrix least-squares refinement on F² with anisotropic thermal parameters for non-hydrogen atoms. Hydrogens were added theoretically, riding on the supporting atoms. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1435491 (1), 1435494 (2), 1435493 (3), 1435495 (4).†

Synthesis of M

M was prepared by the procedures mentioned in the literature.²²

Synthesis of N-(pyridyl-2-methyl)-4-benzo[d]oxazol-2-yl phenylamine (L)

Intermediate M (0.18 g, 0.6 mmol) was fully dissolved in ethanol (20 mL) at room temperature. Then, NaBH₄ (0.06 g, 1.8 mmol) was added to the solution and the reaction mixture was stirred at room temperature for 3 h. After completion of the reaction (monitored by thin layer chromatography, TLC), the resulting reaction mixture was removed under reduced pressure. The residue was washed thoroughly with water, vacuum filtered and then dried to give the crude product, which was recrystallized from ethanol to give a yellow powder. Yield: 0.13 g (72%). ¹H NMR (400 MHz, DMSO-d₆, δ): 8.57–8.56 (d, J = 5.6 Hz, 1H), 7.92–7.90 (d, J = 8.8 Hz, 2H), 7.79–7.75 (m, J = 18.4 Hz, 1H), 7.69–7.65 (m, J = 16.8 Hz, 2H), 7.39–7.24 (m, J = 60.8 Hz, 5H), 6.78–6.75 (d, J = 8.8 Hz, 2H), 4.494–4.479 (d, J = 6.0 Hz, 2H); ¹³C NMR (100 MHz, DMSO-d₆, δ): 164.00, 155.54, 152.45, 150.37, 142.58, 129.28, 128.68, 128.26, 125.32, 124.80, 124.48, 119.29, 119.19, 115.43, 113.20, 112.41, 110.72, 41.38. IR (KBr, cm⁻¹): 3415.82 (s), 3058.70 (w), 1620.66 (vs.), 1572.18 (w), 1517.82 (m), 1452.24 (m), 1435.47 (w), 1341.64 (w), 1244.29 (m), 1176.28 (m), 1049.44 (w), 999.25 (w), 825.13 (w), 744.97 (m), 618.07 (m), 523.90 (w). ESI-MS: found: [M + H]⁺ 302.1293; molecular formula C₁₉H₁₅N₃O requires [M + H]⁺ 302.1215.

Synthesis of complexes

C₁₉H₁₅CdCl₂N₃O (1). A methanol solution (15 mL) of CdCl₂ (0.10 g, 0.55 mmol) was carefully layered onto a dichloromethane solution (15 mL) of L (0.01 g, 0.033 mmol). The clear mixture solution was left to evaporate slowly at room temperature for a few days and white, block-shape crystals of 1 suitable for single crystal X-ray diffraction were obtained. The yield was 0.0035 g (21.91%). IR (KBr, cm⁻¹): 3448 (m), 3302 (m), 1619 (s), 1502 (s), 1453 (m), 1293 (w), 1179 (m), 1051 (m), 1019 (w), 864 (m), 763 (s), 618 (m), 523 (w).

C₁₉H₁₅Cd₂Br₂N₃O (2). Complex 2 was prepared by a similar procedure to that for 1, using CdBr₂ instead of CdCl₂ and white, block-shape crystals of 2 suitable for single crystal X-ray diffraction were obtained. The yield was 0.0041 g (24.85%). IR (KBr, cm⁻¹): 3449 (m), 3307 (m), 1619 (s), 1502 (s), 1451 (m), 1293 (w), 1243 (w), 1181 (m), 1108 (w), 861 (w), 765 (m), 746 (m), 620 (m), 550 (w).

C₄₀H₃₈Cd₂I₄N₆O₄ (3). Complex 3 was prepared by a similar procedure to that for 1 using CdI₂ instead of CdCl₂. White block crystals suitable for single crystal X-ray diffraction were obtained. The yield was 0.0032 g (22.16%). IR (KBr, cm⁻¹): 3422 (m), 3161 (m), 1607 (s), 1493 (s), 1452 (s), 1245 (s), 1214 (m), 1179 (m), 1022 (s), 854 (m), 764 (s), 527 (w).

C₂₁H₁₅CdN₅OS₂ (4). Complex 4 was synthesized by a similar method to 1, except using Cd(SCN)₂ instead of CdCl₂. Pale-yellow block crystals suitable for single crystal X-ray diffraction were obtained. The yield was 0.0044 g (25.43%). IR (KBr, cm⁻¹): 3463 (s), 3314 (m), 2093 (s), 1616 (m), 1500 (m), 1450 (m), 1241 (w), 1178 (w), 867 (w), 764 (w), 750 (w), 616 (m), 520 (w).

Results and discussion

Description of X-ray crystal structures

Compounds 1–4 were obtained as supramolecular complexes by coordinating L with different inorganic CdX₂ salts (X = Cl, Br, I, SCN). Well-shaped X-ray quality crystals of 1–4 were obtained by direct metal–ligand assembly in CH₂Cl₂–CH₃OH solvent. The crystallographic data and details of refinements for compounds 1–4 are summarized in Table 1, and selected bond lengths and angles are listed in Table S1.†

Table 1 Crystallographic data

	1	2	3	4
Empirical formula	C19H15CdCl2N3O	C19H15Cd2Br2N30	C40H38Cd2I4N6O4	C21H15CdN5OS2
Formula weight	484.64	573.56	1399.16	529.90
Crystal system	Monoclinic	Triclinic	Triclinic	Monoclinic
Space group	P21/c	P1̄	P1̄	P21/c
a [Å]	7.0414(17)	7.3599(10)	9.051(5)	8.313(5)
b [Å]	16.004(4)	8.8756(12)	9.426(5)	27.927(5)
c [Å]	16.841(4)	15.374(2)	15.226(5)	9.630(5)
α [°]	90.000(5)	78.3570(10)	85.016(5)	90.000(5)
β [°]	98.670(3)	80.819(2)	77.951(5)	110.883(5)
γ [°]	90.000(5)	81.586(2)	62.098(5)	90.000(5)
V [Å^3]	1876.1(8)	964.2(2)	1122.6(9)	2088.8(17)
Z	4	2	1	4
T [K]	296(2)	296(2)	296(2)	296(2)
Dcalcd [g cm^−3]	1.716	1.976	2.070	1.685
μ [mm^−1]	1.462	5.289	3.742	1.269
θ range [°]	1.77–25.00	1.36–25.00	1.37–25.00	1.46–25.00
Total no. data	13 126	6712	7858	14 665
No. unique data	3299	3345	3888	3680
No. params refined	235	235	255	275
R1	0.0220	0.0215	0.0314	0.0219
wR2	0.0926	0.0662	0.1069	0.0565
GOF	1.001	1.090	0.932	1.060

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Structure of C₁₉H₁₅CdCl₂N₃O (1). Crystal structure analysis reveals that complex 1 crystallizes in the monoclinic crystal system with P2₁/c space group. The asymmetric unit of complex 1 is found to be composed of one L ligand, one Cd²⁺ (Cd1), two types of chloride ion (Cl1, Cl2), as illustrated in Fig. 1a. In the repeating unit, as depicted in Fig. 1b, the Cd(II) ion locates in a six-coordinated environment, which coordinates four chlorine atoms and two nitrogen atoms (N2, N3) derived from one ligand. Each Cd(II) center is in an octahedral geometry and links two neighboring Cd(II) centers via four bridging chlorine atoms, which further link into a 1D H-like chain (Fig. 1c) with the smallest repeating unit. Intermolecular hydrogen bonding interactions C5–H5⋯Cl1 (H5⋯Cl1 2.663 Å) and C18–H18⋯N1 (H18⋯N1 2.696 Å) result in a 2D structure (Fig. 1d). Multiple intermolecular interactions based on the 2D structure contribute to the formation of the 3D supramolecular structure (Fig. 1e).

Fig. 1 (a) ORTEP drawing of 1, (b) the repeating unit molecular structure of 1, (c) the one-dimensional structure of the coordination polymer, (d) the two-dimensional polymeric structure of 1 formed by multiple C5–H5⋯Cl1 and C18–H18⋯N1 hydrogen bonds, (e) the three-dimensional network of 1. The colored dotted lines represent the weak interactions and hydrogen atoms not participating in hydrogen bonds are omitted for clarity.

Structure of C₁₉H₁₅Cd₂Br₂N₃O (2). Complex 2 crystallizes in the triclinic system, space group P1̄. The asymmetric unit of complex 2 involves one Cd²⁺ (Cd1), one L ligand and two types of bromine atoms (Br1, Br2), as depicted in Fig. 2a. The coordination geometry around Cd1 is octahedral with two nitrogen atoms (N2, N3) from one L ligand and four bromine atoms (Br1, Br2, Br1^#1, Br2^#2), in which Br2, Br1^#1, Br2^#2, N3 atoms occupy the corners of a square, and Br1 and N2 atoms are at the apical positions (Fig. 2b). Through the bridging bromine atoms, Cd atoms were linked with the distances between two adjacent Cd atoms from 3.996 Å to 4.046 Å, which further assemble into a 1D infinite chain structure (Fig. 2c). As shown in Fig. 2d, numerous hydrogen bonding interactions C16–H16⋯N1 (H16⋯N1 2.609 Å) can also be observed, which further extend these 1D chains into a 2D architecture. A 3D supramolecular structure is well organized via π–π stacking interactions from adjacent molecules with a centroid–centroid distance of 4.036 Å (Fig. 2e).

Fig. 2 (a) ORTEP drawing of 2, (b) the repeating unit molecular structure of 2, (c) the one-dimensional structure of the coordination polymer, (d) the two-dimensional polymeric structure of 2 formed by multiple C16–H16⋯N1 hydrogen bonds, (e) the three dimensional network of 2 formed by multiple π–π stacking interactions. The colored dotted lines represent the weak interactions and hydrogen atoms not participating in hydrogen bonds are omitted for clarity.

Structure of C₄₀H₃₈Cd₂I₄N₆ (3). Complex 3 is isostructural with 2, which also crystallizes in the triclinic crystal system with P1̄ space group. The molecular structure with numbering scheme for 3 is given in Fig. 3a, which shows that the Cd(II) ion exhibits a penta-coordinated geometry completed by three iodine atoms (I1, I2, I1^#1) and two nitrogen atoms (N2, N3) from one ligand. At the same time, the two Cd(II) centers are bridged by two iodine atoms, which act as bridged auxiliary ligands to form a rhombus-like motif; the distance across this dinuclear center is 4.1833 Å. It is noted that the CH₃OH solvent molecule in the smallest repeating unit cannot be neglected because it plays a vital role in generating the 2D supramolecular structure. As illustrated in Fig. 3b, the neighboring repeating units are linked to each other via multiple C(14)–H(14B)⋯π and π–π stacking interaction with H(14B)⋯centroid and centroid-to-centroid distances of 2.802 Å and 3.684 Å, respectively, giving rise to a 1D structure. Also contributed to by another C(20)–H(20B)⋯π interaction and a H(2)⋯(O2) hydrogen bonding interaction based on the CH₃OH solvent molecule with distances of 1.961 Å and 3.456 Å, respectively, a 2D structure is produced, as shown in Fig. 3c.

Fig. 3 (a) Coordination environment of Cd atom with the atom numbering scheme. (b) 1D framework of 3 showing the C–H⋯π and π–π stacking interactions. (c) The 2D framework of complex 3 showing the H2⋯O2 hydrogen interactions, C–H⋯π and π–π stacking interactions. Hydrogen atoms not participating in hydrogen bonding are omitted for clarity.

Structure of C₂₁H₁₅CdN₅OS₂ (4). Single crystal X-ray diffraction analysis reveals that complex 4 crystallizes in the monoclinic system, space group P2₁/c. SCN⁻ is a highly ambidentate ligand with two donor atoms, which give rise to linkage isomers or polymers and form a variety of different coordination modes in coordination polymers.²³ Fig. 4a shows the coordination environment of the Cd(II) ion with the atom numbering scheme. Each Cd(II) coordinates with two nitrogen atoms of one ligand, two nitrogen atoms of SCN⁻ and two sulphur atoms of another two SCN⁻ ligand. The neighboring cadmium(II) ions are bridged by double SCN⁻ ligands to form an eight-membered ring Cd(SCN)₂Cd unit with a Cd⋯Cd separation of 5.369 Å. The Cd(SCN)₂Cd units construct an infinite 1D polymeric chain (Fig. 4b). In the ab plane, one-dimensional chains are also interconnected by C19–H19⋯S2 hydrogen bonds (H19⋯S2 2.922 Å) to aggregate into a two-dimensional construction (Fig. 4c). The two-dimensional constructions further extend into three-dimensional supramolecular structures through C16–H16⋯O1 (H16⋯O1 2.922 Å) and C17–H17⋯O1 (H17⋯O1 2.710 Å) hydrogen bonding interactions, as illustrated in Fig. 4d.

Fig. 4 (a) ORTEP drawing of 4, (b) the one-dimensional structure of coordination polymer, (c) the two-dimensional polymeric structure of 4 formed by multiple C19–H19⋯S2 hydrogen bonds, (d) the three-dimensional network of 4. The colored dotted lines represent the weak interactions and hydrogen atoms not participating in hydrogen bonds are omitted for clarity.

Structural comparison and effect of anion on the structures

Both 1 and 2 present a similar coordination mode and molecular packing structure in which Cd(II) ion locates in a six coordination environment coordinated by four chlorine (bromine) atoms and two nitrogen atoms derived from one ligand. In complex 1, numerous hydrogen bonding interactions based on the anion can be found; however, in complex 2, the multiple π–π interactions play a significant role in the molecular packing of the crystal. Making a comparison, in complex 3, the Cd(II) ion exhibits a penta-coordinated geometry completed by three iodine atoms and two nitrogen atoms from one ligand. In the crystal packing of 3, the two-dimensional structure is formed through multiple π–π stacking and weaker C–H⋯π interactions; only a few hydrogen interactions based on the CH₃OH solvent molecule are found, which may be attributed to the greater size and smaller electronegativity of the iodide anion.²⁴ Moreover, in the case of complex 4, each Cd(II) coordinates with two nitrogen atoms of one ligand, two nitrogen atoms of SCN⁻ and two sulphur atoms of another two SCN⁻ ligands. There are C–H⋯O and C–H⋯S hydrogen interactions constructing the higher dimensional structure in 4, which may due to the stronger coordinating ability, smaller steric hindrance and electron-rich property of the linear type SCN⁻ ligand. As a result, different halide ions lead to various intermolecular interactions in the crystals, and eventually to different stacking patterns.

Optical properties

Luminescent properties. Coordination complexes formed by d¹⁰ metal centers show fluorescence properties and are promising candidates for photoactive materials having potential applications. Therefore, the luminescent properties of complexes 1–4 as well as the free ligand were determined in the solid state at room temperature. The solid state emission spectra of L and its cadmium(II) coordination complexes 1–4 are presented in Fig. 5. If excited at 358 nm, the free ligand exhibits a fluorescence emission maximum at 408 nm with a shoulder peak at 522 nm, which are presumably associated with the π–π* and/or n–π* transitions.²⁵ Excitation of the coordination complexes of 1, 2 and 4 at 373, 365, and 345 nm, respectively, produces luminescence with one main peak at 425, 403, 396 nm, respectively. In general, they may be attributed to intra-ligand (π–π* or n–π*) emission because of the resemblance of the emission spectra to that of the free ligand.²⁶ In our case, it is noted that the emission spectrum of complex 3 shows a main peak at 472 nm with a shoulder peak at 504 nm when excited at 365 nm. The twin peaks appearing in the emission spectrum may originate from supramolecular interactions, such as C–H⋯π and hydrogen bond interactions, in complex 3, which not only affect the molecular packing mode but also have a significant influence on the ability to transport charges and consequently lead to multiple electronic transitions with close transition energies.²⁷ In addition, for complex 3, a distinct red shift (64 nm) of the luminescence emission is observed in comparison with the emission peak of the free ligand, which may be ascribed to the cooperative effects of intra-ligand emission.²⁸ It can be noted that complex 4 exhibits stronger emission intensities than that of L and the other three complexes. It is because the molecular unit contains a highly delocalized conjugated system formed by coordination of the anion of SCN. To further understand the fluorescent properties, the fluorescence decay profiles of L and complexes 1–4 were measured at their optimal excitation wavelengths in the solid state at room temperature. The fluorescence lifetimes of L (0.16 ns) is quite short, as is that of complex 4 (1.56 ns) shows the longer fluorescence lifetime than other complexes, which may result from the less efficient nonradiative relaxation process due to the higher rigidity of the ligand.²⁹ The results indicated that different anions and coordination environments of metal centers may also have an effect on the luminescent properties of the complexes.³⁰

Fig. 5 Solid-state emission spectra of L and complexes 1–4 at room temperature, inset: solid-state normalized emission spectra for L and complexes 1–4.

Third-order nonlinear optical properties. The third-order NLO properties were measured with a concentration of 1.0 × 10⁻³ mol L⁻¹ in DMF solution using the Z-scan technique. For the experiments, the pulse length was 140 fs and the repetition rate was kept at 10 Hz. Because 2 and 4 have no nonlinear absorption in DMF, only the third-order optical nonlinearities of complexes 1 and 3 are presented. The excitation wavelengths of 1 and 3 are 820 and 780 nm, respectively. The open aperture Z-scan curves are shown in Fig. 6. The filled squares represent the experimental data and the solid line is the theoretical data fitted using the following equations:^31,32

where x = z/z₀, z is the distance of the sample and beam focus, z₀ = πω₀²/λ is the diffraction length of the beam with ω₀ the spot size at focus, λ is the wavelength of the beam, β is the TPA coefficient, I₀ is the input intensity at the focus (z = 0) calculated by the input energy divided by πω₀², L_eff = (1 − e^−αL)/α is the effective length with α the linear absorption coefficient and L the sample length. Furthermore, the molecular TPA cross-section (σ) could be determined using the following relationship:

σ = hγβ/N_Ad × 10⁻³

where h is Planck's constant, γ is the frequency of input intensity, N_A is the Avogadro constant, and d is the concentration of the sample. Based on the abovementioned equations, the values of β and σ of the complexes are given in Table S2.†

Fig. 6 Z-Scan data for 1 (λ = 820 nm) and 3 (λ = 780 nm) in DMF at 1.0 × 10⁻³ mol L⁻¹ obtained under an open aperture and closed aperture configuration. The filled squares represent the experimental data and the solid line the theoretical data.

The nonlinear refraction indexes of 1 and 3 were determined by the close aperture Z-scan technique (Fig. 6). According to the figures, the reported compounds exhibit a self-defocusing effect (Δn < 0) with a peak-to-valley configuration. The effective third-order NLO susceptibility χ⁽³⁾ of the sample can be calculated by the following equations:³³

Reχ⁽³⁾ = 10⁻⁴n₀²ε₀c²γ/π

Imχ⁽³⁾ = 10⁻²n₀²ε₀c²βλ/4π²

where ε₀ is the vacuum permittivity, c is the velocity of light in vacuum and n₀ is the linear refractive index of DMF. In this case, the third-order nonlinear refractive index γ can be derived from the equations:³⁴

ΔT_pv = 0.406(1 − S)^0.25|Δφ|, Δφ = KL_effγI₀

In the equations, ΔT_pv is the difference between the peak and the valley of the normalized transmission, S is the fraction of the transmitted beam through the aperture, and Δφ is the on-axis phase shift, where ΔT_pv = 0.095(1), 0.055(3), S = 0.1, K = 2π/λ. The obtained γ and χ⁽³⁾ are listed in Table S2.† The results indicated complexes 1 and 3 possess third-order optical nonlinearities at the excitation wavelength, which may result from cooperative intermolecular interactions in the complexes. To further elucidate the proposed reasons, we measured the dihedral angles between benzoxazolyl and the adjacent benzene ring in a complex unit; these are 6.91° (1), 15.66° (2), 13.39° (3), 12.27° (4), respectively (Fig. 7). Apparently, the dihedral angle between the two groups is smaller in complex 1, 3, 4, which will be conductive to transport of charges and consequently influence the nonlinear optical properties. In addition, from the bond lengths of C7–C8 in each complex, such as 1.458 Å in 1, 1.445 Å in 2, 1.452 Å in 3, and 1.461 Å in 4, it was revealed that there are highly π-electron delocalized systems in the molecules 1, 2 and 3. Taken together, in complex 1 and 3, there is a greater degree of electron delocalization and more effective intramolecular charge transfer, which may be responsible for the third-order optical nonlinearities.

Fig. 7 The related dihedral angles and bond lengths of complexes 1–4.

Conclusion

In summary, four novel Cd(II) supramolecular complexes (1–4) based on a benzoxazole-phenyl-pyridine derivative have been prepared and structurally characterized. The luminescence properties of the ligand and the complexes were investigated. The results reveal that different anions and coordination environments of the metal center have an effect on the emission wavelength, intensity, and fluorescence lifetimes of the complexes. In addition, the third-order NLO properties of the complexes were investigated in detail, and the results showed that complexes 1 and 3 may be good candidates for nonlinear optical materials.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21271003, 21271004, 51432001 and 51472002), the Science and Technology Plan of Anhui Province (1604b0602016), the Ministry of Education of the People's Republic of China, Higher Education Revitalization Plan Talent Project of Anhui Province (2013), the Undergraduate Training Program for Innovation and Entrepreneurship of Anhui University (201610357007), and the Undergraduate Training Program for Scientific Research of Anhui University (2016).

Notes and references

1. X. L. Wang, N. Xu, X. Z. Zhao, J. W. Zhang, C. H. Gong and T. J. Li, CrystEngComm, 2015, 17, 7038–7047.
2. C. P. Li and M. Du, Inorg. Chem. Commun., 2011, 14, 502–513.
3. (a) M. Zhang, J. A. Gallagher, M. B. Coppock, L. M. Pantzar and M. E. Williams, Inorg. Chem., 2012, 51, 11315–11323; (b) B. A. Blight, R. Guillet-Nicolas, F. Kleitz, R. Y. Wang and S. N. Wang, Inorg. Chem., 2013, 52, 1673–1675.
4. (a) P. Yang, X. X. Wu, J. Z. Huo, B. Ding, Y. Wang and X. G. Wang, CrystEngComm, 2013, 15, 8097–8109; (b) G. Sivaraman, T. Anand and D. Chellappa, Analyst, 2012, 137, 5881–5884.
5. D. X. Xue, Y. Y. Lin, X. N. Cheng and X. M. Chen, Cryst. Growth Des., 2007, 7, 1332–1336.
6. Y. M. Badiei, A. Krishnaswamy, M. M. Melzer and T. H. Warren, J. Am. Chem. Soc., 2006, 128, 15056–15067.
7. H. Scheel, J. Wiederkehr, K. Eichele, H. A. Mayer, F. Winter, R. Pöttgen and L. Wesemann, Dalton Trans., 2014, 43, 11867–11876.
8. (a) M. T. Li, J. Q. Sha, X. M. Zong, J. W. Sun, P. F. Yan, L. Li and X. N. Yang, Cryst. Growth Des., 2014, 14, 2796; (b) M. G. Liu, P. P. Zhang, J. Peng, H. X. Meng, X. Wang, M. Zhu, D. D. Wang, C. L. Meng and K. Alimaje, Inorg. Chem., 2012, 12, 1273.
9. (a) T. Hirano, K. Kikuchi, Y. Urano and T. Nagano, J. Am. Chem. Soc., 2002, 124, 6555–6562; (b) L. Fabbrizzi, M. Licchelli, G. Rabaioli and A. Taglietti, Coord. Chem. Rev., 2000, 205, 85.
10. B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629–1658.
11. A. J. Blake, N. R. Champness, P. Hubberstey, W. S. Li, M. A. Withersby and M. Schröder, Coord. Chem. Rev., 1999, 183, 139–155.
12. P. Haack, A. Kärgel, C. Greco, J. Dokic, B. Braun, F. F. Pfaff, S. Mebs, K. Ray and C. Limberg, J. Am. Chem. Soc., 2013, 135, 16148–16160.
13. S. S. Chen, Q. Liu, Y. Zhao, R. Qiao, L. Q. Sheng, Z. D. Liu, S. Yang and C. F. Song, Cryst. Growth Des., 2014, 14, 3727–3741.
14. (a) L. K. Wang, Y. Y. Zhu, Z. Q. Wu, Z. R. Li, H. P. Zhou, J. Y. Wu and Y. P. Tian, Polyhedron, 2015, 100, 326–332; (b) S. S. Chen, M. Chen, S. Takamizawa, M. S. Chen, Z. Su and W. Y. Sun, Chem. Commun., 2011, 47, 752–754.
15. (a) N. M. Shavaleev, R. Scopelliti, F. Gumy and J. C. G. Bünzli, Inorg. Chem., 2009, 48, 2908–2918; (b) O. Iasco, G. Novitchi, E. Jeanneau, W. Wernsdorfer and D. Luneau, Inorg. Chem., 2011, 50, 7373–7375.
16. (a) H. Huo, C. Fu, K. Harms and E. Meggers, J. Am. Chem. Soc., 2014, 136, 2990–2993; (b) H. P. Lee, Y. F. Hsu, T. R. Chen, J. D. Chen, K. H. C. Chen and J. C. Wang, Inorg. Chem., 2009, 48, 1263–1265.
17. L. Cui, Y. J. Baek, S. Y. Lee, N. Kwon and J. Y. Yoon, J. Mater. Chem. C, 2016, 4, 2909–2914.
18. Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361–5388.
19. Z. Zheng, Z. P. Yu, M. D. Yang, F. Jin, L. N. Ye, M. Fang, H. P. Zhou, J. Y. Wu and Y. P. Tian, Dalton Trans., 2014, 43, 1139–1150.
20. T. T. Yu, X. H. Tian, H. Li, W. Li, W. J. Zhu, H. P. Zhou, Y. P. Tian and J. Y. Wu, Biosens. Bioelectron., 2016, 82, 93–98.
21. G. M. Sheldrick, SHELXTL V5.1 Software Reference Manual, Bruker AXS Inc., Madison, Wisconsin, USA, 1997.
22. L. K. Wang, Z. Zheng, Z. P. Yu, J. Zheng, M. Fang, J. Y. Wu, Y. P. Tian and H. P. Zhou, J. Mater. Chem. C, 2013, 1, 6952–6959.
23. B. Ding, P. Yang, Y. Y. Liu, Y. Wang and G. X. Du, CrystEngComm, 2013, 15, 2490–2503.
24. Y. F. Hsu, W. Hsu, C. J. Wu, P. C. Cheng, C. W. Yeh, W. J. Chang, J. D. Chen and J. C. Wang, CrystEngComm, 2010, 12, 702–710.
25. I. Nowort, B. Machura and R. Kruszynski, CrystEngComm, 2015, 17, 830–845.
26. F. Jin, H. Z. Wang, Y. Zhang, Y. Wang, J. Zhang, L. Kong, F. Y. Hao, J. X. Yang, J. Y. Wu, Y. P. Tian and H. P. Zhou, CrystEngComm, 2013, 15, 3687–3695.
27. (a) J. Gierschner, M. Ehni, H. J. Egelhaaf, B. M. Medina, D. Beljonne, H. Benmansour and G. C. Bazan, J. Chem. Phys., 2005, 123, 144914; (b) V. Shinto and D. Suresh, J. Phys. Chem. Lett., 2011, 2, 863–873.
28. X. W. Wang, J. Z. Chen and J. H. Liu, Cryst. Growth Des., 2007, 7, 1227–1229.
29. F. Jin, Y. Zhang, H. Z. Wang, H. Z. Zhu, Y. Yan, J. Zhang, J. Y. Wu, Y. P. Tian and H. P. Zhou, Cryst. Growth Des., 2013, 13, 1978–1987.
30. G. A. Senchyk, V. O. Bukh’ko, A. B. Lysenko, H. Krautscheid, E. B. Rusanov, A. N. Chernega, M. Karbowiak and K. V. Domasevitch, Inorg. Chem., 2012, 51, 8025–8033.
31. D. M. Li, Q. Z. Wang, P. J. Y. Wu, Y. H. Kan, Y. P. Tian, H. P. Zhou, J. X. Yang, X. T. Tao and M. H. Jiang, Dalton Trans., 2011, 40, 8170–8178.
32. T. Geethakrishnan and P. K. Palanisamy, Opt. Commun., 2007, 270, 424–428.
33. (a) J. F. Ge, Y. T. Lu, R. Sun, J. Zhang, Q. F. Xu, N. J. Li, Y. L. Song and J. M. Lu, Dyes Pigm., 2011, 91, 489–494; (b) J. Mattu, T. G. Johansson and W. Leach, J. Phys. Chem. C, 2007, 111, 6868–6874.
34. (a) T. Xia, D. J. Hagan, M. Sheik-Bahae and E. W. Van Stryland, Opt. Lett., 1994, 19, 317–319; (b) I. L. Bolotin, D. J. Asunskis, A. M. Jawaid, Y. M. Liu, P. T. Snee and L. Hanley, J. Phys. Chem. C, 2010, 114, 16257–16262.

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Footnote

† Electronic supplementary information (ESI) available: Tables S1–S2. CCDC 1435491 (1), 1435494 (2), 1435493 (3), 1435495 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra16821c

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