Crystal structures and third-order optical properties of three manganese(II) complexes constructed from N-heterocyclic and polycarboxylate ligands

1 Introduction

Investigations of materials with large optical nonlinearities and fast third-order nonlinear optical (NLO) responses are of current interest, driven by their variety of applications in optical switching, signal processing, and optical limiting [1], [2], [3], [4]. In the past decades, investigations in this active research area focused on inorganic semiconductors, organic molecules, conjugated polymers, and organometallic compounds [5], [6], [7]. Recently, several transition metal complexes showing strong NLO properties have been reported in the literature [8]. The results on transition metal complexes have indicated that the delocalization of π electrons and the transfer of electron density between the metal and the ligands can enhance the NLO properties of materials. Terpyridine and its derivatives are a class of important N-heterocyclic ligands involving π-conjugated systems, and have been widely used to synthesize metal complexes. Recently, several terpyridine transition metal complexes with different biological and chemical activities have been reported in the literature [9], [10], [11], [12]. However, investigations on the third-order NLO properties of the terpyridine transition metal complexes are scarce. Aromatic polycarboxylates ligands, including 1,3,5-benzenetricarboxylate and 4,4′-oxydibenzoic acid, constitute an important family of multidentate O-donor ligands and have been extensively used in the preparation of complexes [13], [14]. The combination of carboxylate linkers along with N-heterocyclic ligands is a good choice for the construction of novel complexes. Based on the above considerations, we have synthesized three transition metal complexes derived from 4′-phenyl-2,2′:6′,2″-terpyridine and 4′-(furan-2-yl)-2,2′:6′,2″-terpyridine and used the Z-scan technique to determine the sign and magnitude of the nonlinear refractive index, the nonlinear absorption coefficient, and the third-order NLO susceptibility of three terpyridine manganese(II) complexes.

2 Experimental section

All chemicals were of analytical reagent grade and used without further purification (all chemicals supplied by XiYa Reagent Co., Ltd., Shandong, China). The ligands 4′-phenyl-2,2′:6′,2″-terpyridine (PTP) and 4′-(furan-2-yl)-2,2′:6′,2″-terpyridine (FTP) were prepared by the methods reported previously [15]. H₃BTA and H₂ODA stand for benzene-1,3,5-tricarboxylic acid and 4,4′-oxydibenzoic acid, respectively. Elemental analyses for carbon, hydrogen, and nitrogen were carried out with a Perkin-Elmer 2400 CHN elemental analyzer. Electronic spectra (Beijing Puxi analysis Co., Ltd., Beijing, China) were measured on a TU1901 spectrophotometer.

3 X-ray structure determination

Data collections of crystals of complexes 1–3 were performed on a Bruker Smart Apex CCD diffractometer at 293 K. The diffractometer was equipped with graphite monochromated MoK_α radiation (λ=0.71073 Å). Multiscan absorption corrections were applied. All the structures were solved by direct methods and refined by full-matrix least-squares on F² using the shelx crystallographic software package [16]. Nonhydrogen atoms of the complexes were refined anisotropically. The positions of the hydrogen atoms on carbon atoms were calculated. The O-bonded H atoms in complexes 1 and 3 were located from difference Fourier maps and refined as riding atoms with U_iso=1.5×U_eq(O). A summary of the crystal data and refinement parameters for 1–3 are listed in Table 1. Selected bond lengths and angles are presented in Table 2.

CCDC 1497372–1497374 for 1–3 contain the supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

4 Results and discussion

4.1 Crystal structure of {[Mn(PTP)]₂[Mn(PTP)(H₂O)]}(BTA)₂·2H₂O (1)

X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic crystal system with space group P2₁/c. The asymmetric unit contains three Mn(II) atoms, three PTP ligands, two anionic BTA ligands, two solvate water molecules, and one aqua ligand, as shown in Fig. 1a. The Mn1 center is placed in a distorted trigonal bipyramidal coordination sphere, defined by three nitrogen atoms from one PTP ligand, plus two carboxylate oxygen atoms from two different BTA ligands. Both the Mn2 and Mn3 centers adopt a distorted octahedral coordination environment, but the sources of donor atoms are somewhat different. Mn2 is coordinated by three nitrogen atoms from one PTP ligand, two carboxylate oxygen atoms from two different BTA ligands, plus one aqua ligand. Mn2 is surrounded by three nitrogen atoms from one PTP ligand, and two carboxylate oxygen atoms from different BTA ligands. The Mn–O bond lengths range between 2.064(3) and 2.429(3) Å, whereas the Mn–N bond lengths fall in the range of 2.200(3) and 2.274(4) Å (Table 2), which are comparable to those observed in other manganese(II) complexes [18]. There are two independent BTA ligands in the asymmetric unit. The ligand containing O3 and O4 is tridentate, and uses three carboxylate groups binding three Mn(II) centers in monodentate mode. The other one containing O9 and O10 is tetradentate. The two carboxylate groups containing O8 and O9 are coordinated to the metal center by using one of the carboxylate oxygen atoms in monodentate mode, whereas the third carboxylate group chelates the metal center in μ₁: η¹, η¹ mode. In this complex, each BTA ligand is coordinated to three Mn(II) atoms acting as a μ₃-bridge, whereas each Mn(II) atom is surrounded by two BTA ligands and one PTP ligand. This connection mode produces a layered structure containing two types of ring structures (A and B ring, Fig. 1b). The A ring is composed of four Mn(II) centers and four BTA ligands. The B ring contains eight Mn(II) centers and eight BTA ligands. To further understand the structure of 1, a topological analysis by reducing the multidimensional structure to a simple node-and-linker net was performed. The BTA ligands and the Mn(II) centers can be viewed as 3-connectors and 2-connectors, respectively. The analysis of the topology was carried out by using the TOPOS (V4.0) program. The result shows that the structure of 1 is a 3-nodal net with a point symbol {16}{8.16²}₂{8}₂ (Fig. 1c).

Fig. 1:

(a) Coordination environment of Mn in 1; hydrogen atoms are omitted for clarity; (b) infinite two-dimensional sheets of 1; (c) schematic representation of the packing of the molecules in 1.

4.2 Crystal structures of [Mn(PTP)(ODA)]₂ (2) and [Mn(FTP)(ODA)]₂·H₂O (3)

Complex 2 has the same molecular structure with complex 3, only the terminal substituent of the terpyridine is different (Fig. 2a and b). Here, the structure of compound 2 is described in detail as a representative. There is one unique Mn(II) ion, one ODA ligand and one FTP ligand in the asymmetric unit. The Mn1 center is six-coordinated by three oxygen atoms from two ODA ligands, plus three nitrogen atoms from a tridentate chelating FTP ligand. The Mn–O and Mn–N distances are comparable to those in 1 and other reported manganese(II) complexes [18]. The ODA ligand is hexadentate. The carboxylate group containing O3 and O4 is coordinated to a Mn(II) center in chelating mode. The other one is monodentately coordinated to a Mn(II) center. The ODA ligand uses two carboxylate groups to connect two Mn(II) atoms into a ring-like structure in μ₂-coordination mode. The Mn–Mn separation within the ring is 11.62(3) Å. In 1, these binuclear units are linked by π···π stacking with a distance of 3.39(5) Å between pyridine rings of TPP ligands into a chain along the c axis (Fig. 2c).

Fig. 2:

(a) Coordination environment of Mn in 2; (b) coordination environment of Mn in 3; (c) the packing of the molecules in the structure of 2.

4.3 UV–visible absorption spectra of 1–3

The electronic spectra of the three complexes in the solid state were measured on a UV–vis double beam spectrophotometer in the 230–850 nm range (Fig. 3a–c). The three complexes exhibit a wide range of absorption bands. Those in the region of 280–292 nm correspond to π···π* transitions of the PTP ligands, whereas those in the 330–360 nm are assigned to intraligand charge transfer transitions. These absorption peaks are far from 532 nm, indicating that the three complexes are not prone to resonance absorption in a strong light-electric field.

Fig. 3:

(a–c) UV–vis spectra of 1–3.

4.4 NLO properties of 1–3

The NLO properties of complexes 1–3 were investigated as films with 532 nm laser pulses of 7 ns duration. The NLO absorption components were evaluated by Z-scan experiments under an open aperture configuration. Examples of open aperture Z-scan traces are depicted in Fig. 4a–c. When the sample was moved to the focal point, the transmittance decreased obviously. The transmittance is symmetric with respect to the focus (z=0), where it has a minimum transmittance, thus an intensity-dependent absorption effect is observed. The obvious minimum transmittance at the focus indicates that the NLO absorption effect can be attributed to a two-photon absorption effect. The transmittance at the focus drops to 0.55 for 1, 0.61 for 2, and 0.65 for 3, respectively. The third-order NLO absorptive coefficients β are 1.32×10⁻⁶, 3.01×10⁻⁶, and 9.42×10⁻⁶ m W⁻¹, respectively. These β values are comparable to the values of clusters [19], [20], [21]. The NLO refractive property of 1 was assessed by dividing the normalized Z-scan data obtained under closed aperture configuration by the normalized Z-scan data obtained under open aperture configuration. The normalized transmittance through a closed aperture is shown in Fig. 5. The Z-scan trace presents a pattern of a valley in prefocus and a peak in postfocus. This valley–peak configuration corresponds to a positive nonlinear refractive index and a characteristic self-focusing behavior of the propagating wave in the compound. The corresponding refractive index γ of complex 1 is 1.25×10⁻¹² m² W⁻¹. The real part of the third-order nonlinear susceptibility χ⁽³⁾ is related to γ through Reχ⁽³⁾=2n²ε₀cγ, and the imaginary part is related to the nonlinear absorption β by Imχ⁽³⁾=n²ε₀cλβ/2π, where n is the linear refractive index, ε₀ is the permittivity of the vacuum, and c is the velocity of light. The corresponding third-order susceptibility χ⁽³⁾ of the three complexes is 47.55×10⁻⁸, 4.85×10⁻⁸, and 15.16×10⁻⁸ esu, respectively. These results are much larger than those of the coordination polymers reported for solutions and for pure semiconductors [22], [23], [24], [25].

Fig. 4:

(a–c) Open-aperture Z-scan traces for 1–3.

Fig. 5:

Closed-aperture Z-scan trace for 1.

5 Conclusion

In summary, the hydrothermal syntheses, crystal structures, and third-order NLO properties of three new complexes have been reported. Complex 1 exhibits a layered structure with a topology of {16}{8.16²}₂{8}₂. Complexes 2 and 3 show discrete ring-like structures, which are extended by π···π stacking between the pyridine rings of the TPP ligands into chain structures. The three new complexes all present excellent third-order NLO properties as thin films. The third NLO susceptibilities are much larger than those of other CPs reported previously. Therefore, the three complexes seem to be promising candidates for third-order NLO materials.