Synthesis, characterization and nonlinear optical effects of M₄(µ₄-O) core complexes with large two-photon absorption cross-section

Abstract

Four tetranuclear 2-mercaptobenzothiazole cluster compounds [M₄O(MBT)₆] (M = Fe, Co, Ni, Cu; MBT = 2-mercaptobenzhothiazole) were synthesized and characterized by elemental analyses, IR and UV–vis spectra, and single crystal X-ray crystallography. The crystal structures of [M₄O(MBT)₆] (M = Fe, Co, Ni) confirm that the four metal(II) atoms locate four capsheaves of a tetrahedral skeletal structure and a tetracoordinate O²⁻ as an interstitial atom occupies the centre position of this tetrahedron. The metal atoms all possess slightly distorted tetrahedral geometry. The thermal gravimetry data indicate that these clusters all have good thermal stability. The nonlinear absorption of four cluster solutions (in DMF) were measured by open-aperture Z-scan technique at a 532 nm wavelength. The results of Z-scan experiments show that these clusters all have remarkable and very strong nonlinear optical absorption effects. The largest two-photon absorption cross-section is 172880 GM for cluster [Fe₄O(MBT)₆].

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1. Introduction

The design and synthesis of materials with large two-photon absorption (TPA) cross-sections are attracting intense research activity because of their potential applications in optical power limiting,^1,2 two-photon up-conversion lasing,^3–5 two-photon fluorescence excitation microscopy,^6–8 three-dimensional optical data storage,^9,10 and photodynamic therapy.¹¹ Extensive studies of TPA materials did not arise until the 1990s, the main difficulty being that the TPA cross-section σ of many substances is too small and little research of value was undertaken. So, finding materials with a large TPA cross-section σ and good physico–chemical stability has always been the object of many researchers. To the best of our knowledge, most of the reported materials with strong TPA or two photon fluorescence (TPF) are organic chromophores.^12–18 Less study was devoted to metal complexes,^19–21 which usually exhibit better physicochemical stability and more structural changeability. It is very necessary and significant to explore TPA materials with a big TPA cross-section σ in the metal complex field. Recently, there are some reported porphyrin metal complexes which have large TPA cross-section σ.^22–24 Bernard et. al. reported some clusters which can be used as new potential candidates for two-photon absorption processes.²⁵ The thiometallic clusters also exhibit good nonlinear optical properties because of these clusters combining the advantages of inorganic heavy atoms, organic ligands, and versatile cluster structures.^26–29 However, direct technical applications are often frustrated by the cluster's low solubility in common organic solvents. One way to circumvent this practical problem is to introduce bulky organic ligands into the clusters to increase their solubility. 2-mercaptobenzothiazole (C₇H₅NS₂, in the following MBT) ligand was selected for this purpose. 2-mercaptobenzothiazole is an important heterocyclic molecule, which can be used as corrosion inhibitors³⁰ and can be vaporized in a vacuum onto a metal surface to form absorbed layers.³¹ It also can be used as organic ligand to coordinate to metal ions.³² So far, there are few examples of their clusters having been reported. In this paper, we report the synthesis, single crystal structures of four clusters [M₄O(MBT)₆] (M = Fe, Co, Ni, Cu), and studied their properties of two-photon absorption, and found that these four clusters all have the larger two-photon absorption (TPA) cross section σ and TPA coefficient β comparing with those reported earlier.

2. Experimental

2.1 Physical measurements

Elemental analyses were measured with a Perkin-Elmer 1400C analyzer. Solid state electronic spectra were measured on a UV–Vis–NIR spectrophotometer, furnished with a reflectance attachment using BaSO₄ as the reference sample. UV–vis solution spectra were recorded on a Shimadzu 3100 spectrophotometer in DMF solution. Infrared spectra were recorded on a Nicolet 170SX spectrometer using pressed KBr plates in the 4000-400 cm⁻¹ ranges. Thermal gravity (TG) and differential thermal analysis (DTA) were recorded on an SDT 2980 simultaneously for the samples of ca. 10 mg under a nitrogen atmosphere (150 mL min⁻¹ ) in the temperature range from 20 °C to 800 °C, with a heating rate of 10 °C min⁻¹.

2.2 Preparation of four clusters

All chemicals were of analytical reagent grade and used directly without further purification. NaMBT was prepared by reacting 2-mercaptobenzothiazole (4.00 g, 0.024 mol) with EtONa (1.70 g, 0.025 mol) in dry ethanol at room temperature. The MSO₄·nH₂O (M = Fe, Co, Ni, Cu; n = 5, 6, 7) aqueous solution (50 ml, 30 wt%) was added dropwise in above solution with stirring under aerobic conditions. The precipitates were formed (brown-yellow for Fe, green for Co, dark brown for Ni and brown for Cu), and were collected by filtration, washed with water, and air-dried. Single crystals suitable for X-ray measurements were obtained by recrystallization from tetrahydrofuran (THF) (brown, yield 57% for Fe; deep green, yield 59% for Co, deep purple, yield 61% for Ni and deep brown, yield 63.4% for Cu, all based on MBT). They were submitted for elemental analysis. An X-ray crystallographic study has confirmed the existence of [M₄O(MBT)₆] (M = Fe, Co, Ni) cluster. We did not get the single crystal of Cu complex. From the element analysis below and the spectra characteristics, we guess that the complex of 2-mercaptobenthiazole copper is also the cluster and form to [Cu₄O(MBT)₆]. Anal. Calc. for C₄₂H₂₄N₆Fe₄OS₁₂1: C, 40.79%; H, 1.96%; N, 6.80%; S, 31.11%. Found: C, 40.49%; H, 1.97%; N, 6.57%; S, 30.58%. Calc. for C₄₂H₂₄N₆Co₄OS₁₂2: C, 40.39%; H, 1.94%; N, 6.73%; S, 30.81%. Found: C, 40.23%; H, 2.16%; N, 6.32%; S, 31.13%. Calc. for C₄₂H₂₄N₆Ni₄OS₁₂3: C, 40.41%; H, 1.94%; N, 6.73%; S, 30.83%. Found: C, 40.78%; H, 1.75%; N, 6.19%; S, 30.37%. Calc. for C₄₂H₂₄N₆Cu₄OS₁₂4: C, 39.80%; H, 1.91%; N, 6.63%; S, 30.35%. Found: C, 39.78%; H, 2.35%; N, 6.07%; S, 31.02%.

2.3 Crystallographic data collection and solution of structure

The diffraction data were collected on a four-cycle CAD4 diffractometer for cluster [Co₄O(MBT)₆] and on SMART APEX CCD diffractometer for clusters [Fe₄O(MBT)₆] and [Ni₄O(MBT)₆] with graphite monchromatic Mo-K_α(λ = 0.71073 Å, T = 293 K) radiation. Empirical absorption correction was carried out by using the SADABS program.³³ The structures were solved by direct methods and refined by least squares on F_obs² by using the SHELXTL³⁴ software package. All non-H atoms were anisotropically refined. The hydrogen atoms were located by difference synthesis and refined isotropically. The molecular graphics were plotted using SHELXTL. Atomic scattering factors and anomalous dispersion corrections were taken from International Tables for X-ray Crystallography.³⁵ CCDC reference number are 257276 for [Fe₄O(MBT)₆], 257277 for [Co₄O(MBT)₆], and 257278 for [Ni₄O(MBT)₆]. For the crystallographic data in CIF format see ‡.

2.4 NLO measurements

The TPA cross-section σ and coefficient β, were determined by the Z-scan technique. NLO measurements were performed on dimethylform amide (DMF) solutions of clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆] which were contained in a 1 mm thick quartz cell with concentrations of 5.0 × 10⁻⁴ M. The excited light source was a Q-switched Nd:YAG laser providing a pulse at the second-harmonic 532 nm. The pulse duration was 20 ns and the repetition frequency was 1 Hz. The pulse energy was recorded on an energy detector (EPM2000, Molectron). The pump laser beam came from a mode-locked Ti:sapphire laser system operating at 800 nm, pulse duration <200 fs, and repetition rate 76 MHz (Coherent Mira900-D).

3. Results and discussion

3.1 Crystal structures

Information concerning crystallographic data and structure refinement of iron, cobalt and nickel clusters are given in Table 1. Selected bond distances and angles for three clusters are listed in Table 2. Their typical molecular structure is illustrated in Fig. 1. Fig. 2 shows a perspective view of the crystal packing in the unit cells for the compound [Fe₄O(MBT)₆]. The crystal packing of compounds [Co₄O(MBT)₆] and [Ni₄O(MBT)₆] are similar to that of [Fe₄O(MBT)₆]. The three clusters are isomorphous and isostructural. In these clusters, the unit cells all consist of monomeric [M₄O(MBT)₆] (M = Fe, Co, Ni) units and each metal(II) ion has a distorted tetrahedral geometry. The basic moiety of these molecule structures all contains six 2-mercaptobenzothiazole ligands and the core structure [M₄O]⁶⁺ with a four-coordinate oxygen atom occupying the center of a tetrahedron of divalent metal atoms. Four metal atoms all locate four vertex of a tetrahedral skeletal structure with the M–M–M angles ranging from 57.60 to 61.20° for iron, 57.66 to 61.17° for cobalt, and 57.78 to 61.11° for nickel, respectively.

Fig. 1 Molecular structure of [M₄O(MBT)₆] [M = Fe, Co, Ni] with the atomic numbering scheme. Thermal ellipsoids enclose 50% probability.

Fig. 2 Packing diagram of the unit cell of [Fe₄O(MBT)₆] down the a axis.

Table 1 Summary of crystallographic data of [M₄O(MBT)₆] (M = Fe,Co,Ni)

Compound	[Fe4O(MBT)6]	[Co4O(MBT)6]	[Ni4O(MBT)6]
^Fe w = 1/[σ^2(F^2) + (0.0084P)^2], where P = (F^2 + 2F^2c)/3; ^Cow = 1/[σ^2(F^2) + (0.0271P)^2], where P = (F^2+ 2F^2c)/3; ^Niw = 1/[σ^2(F^2) + (0.0480P)^2], where P = (F^2 + 2F^2c)/3.
Empirical formula	C42H24N6Fe4OS12	C42H24N6Co4OS12	C42H24N6Ni4OS12
Formula weight	1236.79	1249.11	1248.23
Temperature/K	293(2)	293(2)	293(2)
Wavelength/Å	0.71073	0.71073	0.71073
Crystal system, space group	Trigonal, R-3	Trigonal, R-3	Trigonal, R-3
Unit cell dimensions/Å,(°)	a = 18.217(6) α = 90	a = 18.264(3) α = 90	a = 18.261(3) α = 90
	b = 18.217(6) β = 90	b = 18.264(3) β = 90	b = 18.261(3) β = 90
	c = 24.865(7) γ = 120	c = 24.878(5) γ = 120	c = 24.862(5) γ = 120
Volume/Å^3	7146(4)	7187(2)	7180(2)
Z, Calculated density/Mg m^−3	6, 1.724	6, 1.732	6, 1.732
Abs coeff./mm^−1	1.763	1.926	2.115
F(000)	3732	3756	3780
Theta range for data collection	1.53 to 24.49 (°)	1.53 to 25.97 (°)	1.53 to 23.99 (°)
Limiting indices	−13≤h≤21, −21≤k≤20, −28≤l≤28	−22≤h≤15, −15≤k≤22, −24≤l≤30	−20≤h≤20, −20≤k≤20, −28≤l≤18
Reflections collected/unique	8955/2507 [Rint = 0.2066]	9648/3086 [Rint = 0.1031]	8814/2452[Rint = 0.2156]
Completeness to theta	94.7%	98.3%	97.3%
Refinement method	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2	Full-matrix least-squares on F^2
Data/restraints/parameters	2507/0/197	3086/0/196	2452/0/196
Goodness-of-fit on F^2	1.023	0.982	1.028
Final R indices [I > 2σ(I)]	R 1 = 0.0755, wR2 = 0.1298	R 1 = 0.0423, wR2 = 0.0681	R 1 = 0.0955, wR2 = 0.1798
R indices (all data)	R 1 = 0.1167, wR2 = 0.1483	R 1 = 0.1131, wR2 = 0.0820	R 1 = 0.1551, wR2 = 0.2095
Largest diff. peak and hole	0.503 and −0.582 e. Å^−3	0.408 and −0.324 e. Å^−3	0.827 and −0.731 e. Å^−3

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Table 2 Selected bond lengths (Å) and angles (°)

	[Fe4O(MBT)6]	[Co4O(MBT)6]	[Ni4O(MBT)6]
M(1)-O(1)	1.941(4)	1.940(2)	1.942(6)
M(1)-N(1)	2.027(6)	2.040(3)	2.026(8)
M(2)-O(1)	1.904(9)	1.916(5)	1.91(1)
M-S(2)	2.314(3)	2.312(1)	2.318(2)
M-M	3.091(2)	3.097(2)	3.207(1)
O(1)-M(1)-N(1)	111.7(2)	112.00(9)	112.0(3)
O(1)-M(1)-S(2)	106.8(2)	106.6(1)	106.5(3)
O(1)-M(2)-N(2)	103.9(2)	104.1(1)	103.8(3)
S(2)-M(1)-S(4)	108.2(1)	107.80(5)	107.8(1)
O(1)-M(1)-S(4)	105.1(2)	105.4(1)	105.2(3)

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In the clusters of [M₄O(MBT)₆] (M = Fe, Co, Ni), six MBT ligands have the similar coordination modes. They all offer one nitrogen atom and one sulfur atom of mercapto-group to coordinate to metal atoms. Four metal(II) ions in the clusters of [M₄O(MBT)₆] (M = Fe, Co, Ni) have the different coordination environment although each metal atom possesses a slightly distorted tetrahedral geometry. One metal atom is coordinated by three nitrogen atoms and the interstitial O atom, and the others are coordinated by the interstitial O atom, one nitrogen atom and two sulfur atoms.

In the cluster of [Fe₄O(MBT)₆], the Fe₄ tetrahedron is slightly distorted, the iron–iron distances can be classified in two groups: a short distance of 3.092(2) Å for Fe(1)–Fe(2)^#1 and Fe(1)–Fe(2)^#2, and a long distance of 3.209(2) Å for Fe(1)–Fe(2), Fe(2)–Fe(2)^#1 and Fe(2)–Fe(2)^#2, which compare with the intermetallic distances in the tetrairon(II) cluster.³⁶ The tetrahedral arrangement of iron ions bonded to a single central oxo ion. The Fe–O–Fe angles vary from 105.6(3) to 113.1(3)°. The coordination geometry around each iron center is also tetrahedral, with a center oxygen atom coordinating to four iron atoms. Fe(2) atom is bound to the central oxygen atom and three nitrogen. The Fe(2)–O(1) and Fe(2)–N distances are 1.904(9) Å and 2.030(6)Å, respectively. The other iron atom, Fe(1), Fe(1)^#1 and Fe(1)^#2, are bound to the central oxo ion, one nitrogen atom and two sulfur atoms. The Fe(1)–O(1) and Fe(1)–N(2)^#2 distances are 1.941(4) Å and 2.027(6) Å, respectively. The two Fe–S bond lengths are 2.312(3) Å and 2.313(3) Å, respectively. These data are all in agreement with the similar tetrahedron iron complexes reported before.³⁷

The benzothiazole ring [N(2), S(3), C(8)∼C(14)] in the cluster with the conjunction sulfur atom S(4) and metal atom Fe(1) are all fairly planar, the largest deviation from the least squares plane through the ring atoms is 0.026 Å. The dihedral angles among the benzothiazole ring moieties are 65.53°, 4.86° and 65.05°, respectively.

The clusters [Co₄O(MBT)₆] and [Ni₄O(MBT)₆] have the same structure as [Fe₄O(MBT)₆]. The Co–Co distances of 3.097(2) and 3.211(2) Å, the Ni–Ni distances of 3.100(2) and 3.208(2) Å, are all in agreement with the intermetallic distances in the similar cluster.^38,39 The distances; Co–O of 1.940(2) and 1.916(5)Å, Co–N of 2.029(3) and 2.040(3)Å, Co–S of 2.312(1) and 2.320(1)Å, and Ni–O of 1.906(1) and 1.943(5)Å, Ni–N of 2.034(7) and 2.037(7)Å, Ni–S of 2.315(3) and 2.321(3)Å, all fall in the normal range.^40,41

There are some π–π stacking interactions^42,43 and C–H⋯π supramolecule interactions⁴⁴ between the molecules in the crystal lattice. For [Fe₄O(MBT)₆], there are two types of π–π stacking interactions; thiazole ring–thiazole ring and thiazole ring–phenyl ring. The centroid–centroid distances are 3.940 and 3.880 Å, respectively. The shortest interplanar distances above are 3.560 and 3.537 Å, respectively. There are three types of C–H⋯π supramolecule interactions between C–H and aromatic rings. The distances between C(1)–H(1A) and C(2)–H(2A) to thiazole ring, C(2)–H(2A) to phenyl ring are 3.223, 3.387 and 2.859 Å, respectively. For [Co₄O(MBT)₆], there are three types of π–π stacking interactions; thiazole ring (X, Y, Z)-thiazole ring (1/3 − X, 2/3 − Y, 2/3 − Z), thiazole ring (X, Y, Z)-phenyl ring (1/3 − X, 2/3 − Y, 2/3 − Z) and phenyl ring (X, Y, Z)-thiazole ring (1/3 − X, 2/3 − Y, 2/3 − Z). The centroid-centroid distances are 3.942, 3.881 and 3.880 Å, respectively. The shortest interplanar distances above are 3.542, 3.559 and 3.518 Å, respectively. There are one type of C–H⋯π supramolecule interactions between C–H and aromatic rings. The distance between C(9)–H(9A) to phenyl ring is 2.878 Å. For [Ni₄O(MBT)₆], there are two types of π–π stacking interactions; thiazole ring–thiazole ring and thiazole ring–phenyl ring. The centroid–centroid distances are 3.948 and 3.891 Å, and the shortest interplanar distances are 3.563 and 3.538 Å, respectively. There is C(2)–H(2A)⋯π supramolecule interactions between C–H and aromatic rings. The distance between C(2)–H(2A) to phenyl ring is 2.883 Å. In the solid state, all above intermolecular interactions in these three structures stabilized the crystal structure.

3.2 Spectra characteristics

The IR spectra (See Fig. S3 in the Electronic supplementary information (ESI)†) of four clusters show a little difference, compared with that of 2-mercaptobenzothiazole (MBT) ligand reported.⁴⁵ The bands at about 2840–3113 cm⁻¹ for [Fe₄O(MBT)₆], at 2848–3059 cm⁻¹ for [Co₄O(MBT)₆], at 2850–3060 cm⁻¹ for [Ni₄O(MBT)₆], and at 3061 cm⁻¹ for [Cu₄O(MBT)₆] 3061 cm⁻¹ are assigned to the C–H stretching vibration of the benzothiazole ring. They all exhibit characteristic strong bands at about 1597 and 1498 (C=C), 1458 and 1428 (C=N), 1078 and 1035 (C=S), 752 (ν_C–H benzene ring) and 670 cm⁻¹ (ν_C–H thiazole ring) for the uncoordinated 2-mercaptobenzothiazole ligands. There is a little different v(C=S) band at 1085 and 1030 cm⁻¹ for [Co₄O(MBT)₆], and 1085 and 1032 cm⁻¹ for [Ni₄O(MBT)₆], and 1080 and 1023 cm⁻¹ for [Cu₄O(MBT)₆], indicating sulfur coordination.⁴⁶ The bands at 1457 and 1427 cm⁻¹ for [Fe₄O(MBT)₆], 1453 and 1370 cm⁻¹ for [Co₄O(MBT)₆], 1452 and 1396 cm⁻¹ for [Ni₄O(MBT)₆], and 1467 and 1397 cm⁻¹ for [Cu₄O(MBT)₆], are shifted from their positions for the free 2-mercaptobenzothiazole ligand, indicating nitrogen coordination. The band at 426, 428 and 427 cm⁻¹ for these cluster is tentatively attributed to ν(Fe–N), ν(Co–N), ν(Ni–N) and ν(Cu–N), respectively.

The solid reflectance electronic spectrum of [Fe₄O(MBT)₆] shows a peak band at 302 nm and a shoulder at 357 nm. The band at 302 nm is ascribed to intraligand transition of the thione ligand,⁴⁷ while the shoulder band at 357 nm is assigned to charge transfer transitions from the iron(II) d orbital to low-energy π* orbital of the ligand. The solid reflectance electronic spectrum of [Co₄O(MBT)₆] shows two broad bands around 320 nm and 600 nm. The band around 320 nm is ascribed to intra-ligand, probably π→π*, transition of the benzothiazole group. The peak at 600 nm are d–d transfer transitions of Co(II), which may be taken as evidence for tetrahedral Co(II) complexes.⁴⁸ The solid reflectance electronic spectrum of [Ni₄O(MBT)₆] shows one broad bands around 330 nm, which is ascribed to intraligand transition of the benzothiazole group.

3.3 Thermal analysis

The four clusters, [M₄O(MBT)₆], all have good thermal stability according to the TG and DTG curves (See Fig. S5 in the ESI†). They decompose in the 273.9–332.2 °C temperature range, and all show an intense endothermic phenomenon in DTG curves at 273.9 °C for [Fe₄O(MBT)₆], at 311.6 °C for [Co₄O(MBT)₆], at 332.2 °C for [Ni₄O(MBT)₆], and 305.5 °C for [Cu₄O(MBT)₆], respectively. The another obvious feature of these four clusters is that they are all sublimation fellow the increasing of temperature, especially for [Fe₄O(MBT)₆], it's final residue is nearly to zero at about 600 °C.

3.4 Nonlinear optical properties

The electronic spectra of tetrahedral clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆] in DMF solution were examined and are depicted in Fig. S4 in the ESI†. It is noticed that there is not absorption in the range of 400 nm–800 nm for these four clusters, which suggests these four clusters are the two-photon absorption at 532 nm. The nonlinear absorption of clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆] in an open-aperture Z-scan can be explained mainly by the two-photon absorption (TPA) mechanism, and the total absorption coefficient can be written as: α(I) = α₀ + βI, where α₀ is the linear-absorption coefficient and β is the TPA coefficient. The TPA coefficient β for the clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆] were determined by the Z-scan technique^49–51 with a concentration of 0.5 mmol l⁻¹ in DMF solution. In our experiments, use of a 1 mm sample cell assured thin-sample approximation. Relative dilute solutions and a slow pulse frequency (1 Hz) reduced the thermal effect and excited state absorption. Generally, if |q(0)| < 1, the open-aperture T–Z curve can be written

Where q₀ (z) = βI₀ (t) L_eff/(1 + z²/z₀²), β is the TPA coefficient, I₀ (t) is the intensity of laser beam at focus (z = 0), L_eff = [1-exp(-α₀L)]/α₀ is the effective thickness with α₀ the linear absorption coefficient and L the sample thickness. z₀ = πω₀²/λ is the Rayleigh diffraction, ω₀ is the radius of the beam at focus, and z is the sample position. In the above equation, only the TPA coefficient β is unknown. The open-aperture data for clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆], are shown Fig. 3. The numerical calculations were performed by choosing the most fitting values for the parameter β. From the solid curves in Fig. 3, the best-fitting values for β are 0.27867 cm GW⁻¹, 0.0769 cm GW⁻¹, 0.0776 cm GW⁻¹, and 0.07177 cm GW⁻¹ for clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆], respectively. In addition, the molecular TPA cross section σ can be determined by the following expression:^52,53

σ = hνβ/N_Ad

where hν is the energy of the incident photon. N_A is the Avogadro constant. d is the concentration of the TPA compound in mol cm⁻³ units. The TPA cross sections σ for clusters [Fe₄O(MBT)₆], [Co₄O(MBT)₆], [Ni₄O(MBT)₆], and [Cu₄O(MBT)₆], are listed in Table 3. Comparing the β and σ of the clusters with those reported earlier for organic compounds (σ_Max = 5030 GM,¹²σ_Max = 6700 GM,¹⁵ and σ_Max = 8100 GM¹³), and for metal complexes (σ_Max = 5700–9900 GM,¹⁹σ_Max = 9200–12100 GM,²⁰σ_Max = 3000–10000 GM²¹ and σ_Max = 12000–15000 GM,²³) we find that the four clusters all have remarkable and very strong nonlinear optical absorption effects, and exhibit the very large two-photon absorption cross-section. In order to understand the effect of ligand 2-mercaptobenzothiazole (MBT), we have done the optimal excitation and emission of ligand MBT, and found that MBT does not give single-photon fluorescence spectra and single-photon absorption. This indicates that the two-photon absorption and two-photon fluorescence of the clusters could not result from the contribution of π*–π transition of the ligand MBT. This structure containing interstitial tetracoordinate O²⁻ atom may be the key to enhance the TPA cross sections σ. We hypothesise that the cluster form π–D–A–D–π structure (MBT is π bridge, Donor is metal and Acceptor is O²⁻ atom) and form two-photon absorbing chromophores, which lead to an great increase in the σ values. Also, it is interesting to note that the TPA cross section of cluster [Fe₄O(MBT)₆] is much lager than that of clusters [Ni₄O(MBT)₆], [Co₄O(MBT)₆], and [Cu₄O(MBT)₆], even when accounting for the large uncertainty (±25%) in the value for [Fe₄O(MBT)₆], although they have the similar structure. Maybe the d⁶ electron structure is preponderance. This structure–function relationship is the subject of our current investigation.

Fig. 3 Open-aperture Z-scan data: normalized transmittance of clusters [M₄O(MBT)₆] (M = Fe, Co, Ni, Cu). Scatter points are experimental data, and solid curves are theoretical fitting results.

Table 3 TPA coefficient β, cross section σ of clusters

	[Fe4O(MBT)6]	[Co4O(MBT)6]	[Ni4O(MBT)6]	[Cu4O(MBT)6]
β/(cm GW^−1)	0.28	0.077	0.078	0.072
σ/(GM)	172880	48100	47710	44520

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4. Conclusions

We have synthesized four new 2-mercaptobenthiazole metal tetranuclear clusters containing interstitial oxygen, [M₄O(MBT)₆] (M = Fe, Co, Ni and Cu). Their structures have been characterized by elemental analyses, IR and UV–vis spectra, and single crystal X-ray. The cluster structures of Fe, Co, Ni and Cu are all composed of M₄O core and six 2-mercaptobenthiazole ligands. Six MBT ligands have the similar coordination modes, and one four-coordinate oxygen atom occupies the center of a tetrahedron of divalent metal atoms, and each metal atom possesses slightly distorted tetrahedral geometry. The four clusters all exhibit remarkable thermal stability and have highly soluble in common organic solvents. Using the Z-scan technique, we have investigated nonlinear optical absorption of these clusters. These clusters all posses a strong two-photon absorption cross-section. The preliminary studies show that these clusters have very large TPA cross sections up to 172880 GM for [Fe₄O(MBT)₆] cluster. Although the origin of the [Fe₄O(MBT)₆] cluster showing the TPA cross section three times larger than that of the other clusters is not clear, such molecules will be useful for a variety of applications through existing procedures. Given the feasible synthetic routes and superior two-photon absorption properties shown in this paper, it is conceivable that molecules with even larger TPA cross sections could be developed based on the metal clusters design. This could further pave the way for the development of new nonlinear optical devices based on two-photon absorption.

Acknowledgements

We thank Natural Science Foundation of Shandong Province (No.Y2005B04) for support of these studies.

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Footnotes

† Electronic supplementary information (ESI) available: ORTEP diagram, IR spectra, solution UV–vis spectra, TG and DTG curves of the clusters; perspective view of the crystal packing in the unit cell. See DOI: 10.1039/b607731e

‡ CCDC reference numbers 257276–257278. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b607731e

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