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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Investigation of Regulating Third-Order Nonlinear Optical Property by Coordination Interaction Yujie Zhao,† Honghong Li,† Zhichao Shao,† Wenjuan Xu,† Xiangru Meng,*,† Yinglin Song,*,‡ and Hongwei Hou*,† †

The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan, 450001, P. R. China Department of Applied Physics, Harbin Institute of Technology University, Harbin, Heilongjiang, 150001, P. R. China

‡

Inorg. Chem. Downloaded from pubs.acs.org by KEAN UNIV on 03/28/19. For personal use only.

S Supporting Information *

ABSTRACT: In this paper, a series of organic compounds (L1−L6) with a D-π-A conjugation system were prepared. The investigations of third-order nonlinear optical (NLO) properties indicate that L1−L6 show different degrees of third-order NLO responses. It is surprising that introducing metal ions can effectively regulate their third-order NLO properties and even change the type of nonlinear absorption signal from reverse saturable absorption to saturable absorption, which can be attributed to the formation of coordination bonds between metal ions and L1−L6. It has aroused our tremendous interests in regulating third-order NLO performance. The regulation mechanisms were also discussed through the pump−probe measurements and the density functional theory. The enhancement of electron transfer efficiency is considered to be the key to improving NLO performance. Furthermore, we also obtained two coordination complexes [Cu(L1)2(NO3)2] (1) and [Cd(L1)2I2] (2) based on L1, which further proved the coordination between metal ions and L1−L6. Ligand-to-metal or metal-to-ligand charge transfer makes more electronic delocalization, leading to better third-order NLO properties. This work provides new ideas and explorations for the excogitation of third-order NLO materials.

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for an uncoordinated unit.16−21 So we concentrated our attention on exploring the influence of metal ions on the NLO properties. Chalcone derivatives are selected as outstanding molecules, which are found to be useful in NLO materials with the development of the optical communication technique.22,23 Anthracene as a preeminent NLO group has also received much attention in the optoelectronic materials field due to its inherent rich electronic characteristics. Meanwhile, the nitrogenous heterocyclic is chosen as the electron acceptor. It is easier to design target molecules with donor/acceptor groups to gain better NLO properties. In this sense, we design and synthesize six chalcone compounds (L1−L6) combining nonlinear functional building blocks (anthracene) and an electron-deficient group (nitrogenous heterocyclic)24,25 shown in Scheme 1. Hence, L1−L6 can not only show ideal NLO properties26,27 but also provide a possibility for the NLO regulation of metal ions because of the existence of nitrogen atoms. The main theme of this work is to elucidate the impact of metal ions on NLO responses. L1−L6 show desired NLO properties, and the introduction of metal ions indeed results in better NLO responses. The complexes 1 and 2 also confirm

INTRODUCTION Elaborately designed third-order NLO materials are widely used in various fields such as optical communications, optical switching, optical data processing, optical limiting, photodynamic therapy, and so on.1−3 Up to now, many researches still focus on the design of novel third-order NLO materials to meet the requirements of various applications.4,5 It is generally known that the process of electron transfer is usually considered as the significant factor for favorable NLO response,6 and the third-order NLO property enhancement is usually accompanied by a larger cross section (δ). According to the previous reports, δ increases with increasing donor/ acceptor strength, conjugation length, and planarity of the πcenter.7 So, many attempts had been devoted to the presence of a large conjugate system consisting of an electron donor, an acceptor, or the existence of both types of these groups simultaneously, for instance, donor-π-donor (D-π-D), donor-πacceptor (D-π-A), acceptor-π-acceptor (A-π-A), etc.8−12 On the basis of the above considerations, organic compounds with excellent NLO properties already have been reported, and metal complexes with electron donors and acceptors have emerged as promising candidates for NLO materials on account of the distinctive structure and fast response ability.13−15 Metal ions also play critical roles in the NLO properties of complexes. Electron-pushing/pulling effects of metal centers caused higher NLO responses than that observed © XXXX American Chemical Society

Received: November 9, 2018

A

DOI: 10.1021/acs.inorgchem.8b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(E)-3-(Anthracen-9-yl)-1-(pyridin-2-yl)prop-2-en-1-one (L3). A procedure similar to that for L2 was employed. L3 was synthesized from 9-anthraldehyde (206 mg, 1 mmol) and acetylpyrazine (195 mg, 1.6 mmol), and a yellow powder was obtained with a yield of 79.8% based on 9-anthraldehyde. 1H NMR (400 MHz, CDCl3) δ (ppm): 9.48 (s, 1H), 8.99 (d, 1H), 8.77 (s, 1H), 8.65 (s, 1H), 8.48 (s, 1H), 8.34 (d, 2H), 8.18 (d, 1H), 8.03 (d, 2H), 7.52 (t, 4H). 13C NMR (400 MHz, CDCl3) δ (ppm): 188.45, 148.47, 147.52, 146.95, 144.26, 143.53, 142.77, 131.32, 129.27, 128.98, 128.89, 128.53, 127.89, 127.45, 126.63, 125.96, 125.32, 125.09. HRMS: m/z calculated for C21H14N2O [M + H]+ 310.11, found 311.10. Elemental analysis found (%) for L3, C21H14N2O: C, 80.89, H, 4.52, N, 8.98; calcd: C, 81.26, H, 4.51; N, 9.03. (2E,4E)-5-(Anthracen-9-yl)-1-(pyridin-4-yl)penta-2,4-dien-1-one (L4). 3-(9-Anthryl)acrolein (232 mg, 1 mmol) and 4-acetylpyridine (194 mg, 1.6 mmol) were dissolved in 15 mL of ethanol in a 50 mL flask; then the pH was adjusted to 8−9 with sodium hydroxide solution. The reaction system was stirred for 12 h with the precipitate formed. The crude product was purified by washing three times with ethanol and methylene chloride. Finally, an orange-yellow powder was obtained with a yield of 80% based on 3-(9-anthryl)acrolein. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.88 (s, 1H), 8.48 (s, 1H), 8.27 (d, 2H), 8.08−8.02 (m, 3H), 7.95 (dd, 1H), 7.82 (d, 2H), 7.54 (dd, 4H), 7.10 (s, 1H), 7.04 (d, 1H), 7.00−6.94 (m, 1H). 13C NMR (400 MHz, CDCl3) δ (ppm): 189.81, 150.83, 146.23, 144.26, 140.25, 135.36, 131.37, 130.57, 129.46, 128.97, 128.16, 126.30, 126.17, 125.66, 125.41, 125.27, 124.86. HRMS: m/z calculated for C24H17NO [M + H]+ 335.13, found 336.20. Elemental analysis found (%) for L4, C24H17NO: C, 84.63, H, 4.98, N, 4.28; calcd: C, 85.94, H, 5.07; N, 4.18. (2E,4E)-5-(Anthracen-9-yl)-1-(pyridin-2-yl)penta-2,4-dien-1-one (L5). A procedure similar to that for L4 was employed. L5 was synthesized from 3-(9-anthryl)acrolein (232 mg, 1 mmol) and acetylpyrazine (195 mg, 1.6 mmol), and a yellow powder was obtained with a yield of 77% based on 9-anthraldehyde. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.76 (d, 1H), 8.46 (s, 1H), 8.31 (d, 2H), 8.24 (d, 1H), 8.06−8.03 (m, 3H), 7.99 (s, 1H), 7.95−7.90 (m, 2H), 7.53 (dd, 5H), 7.05 (dd, 1H). 13C NMR(400 MHz, CDCl3) δ (ppm): 189.77, 154.23, 148.89, 144.52, 138.76, 137.11, 136.28, 131.40, 129.48, 128.86, 128.75, 127.79, 126.93, 126.12, 125.94, 125.91, 125.51, 125.36, 125.28, 125.05, 122.93. HRMS: m/z calculated for C24H17NO [M + H]+ 335.13, found 336.20. Elemental analysis found (%) for L5, C24H17NO: C, 85.82, H, 5.31, N, 4.08; calcd: C, 85.94, H, 5.07; N, 4.18 (2E,4E)-5-(Anthracen-9-yl)-1-(pyridin-2-yl)penta-2,4-dien-1-one (L6). A procedure similar to that for L5 was employed. L6 was synthesized from 3-(9-anthryl)acrolein (232 mg, 1 mmol) and 2acetylpyridine (194 mg, 1.6 mmol), and a yellow powder was obtained with a yield of 76% based on 9-anthraldehyde. 1H NMR (400 MHz, CDCl3) δ 9.42 (s, 1H), 8.75 (d, 2H), 8.47 (s, 1H), 8.30 (d, 1H), 8.07 (d, 4H), 7.77 (d, 1H), 7.53 (d, 4H), 7.29 (s, 1H), 7.18 (s, 1H), 7.05 (s, 1H). 13C NMR (400 MHz, CDCl3) δ (ppm): 188.74, 148.66, 147.24, 145.34, 144.85, 143.36, 139.88, 135.93, 131.40, 130.88, 129.49, 128.91, 128.79, 128.01, 126.22, 125.38, 124.25. HRMS: m/z calculated for L6, C23H16N2O [M + H]+ 336.13, found 337.20. Elemental analysis found (%) for C23H16N2O: C, 82.53, H, 4.65, N, 8.60; calcd: C, 82.11, H, 4.76, N, 8.33. Synthesis of [Cu(L1)2(NO3)2] (1). L1 (0.015 g, 0.05 mmol) and Cu(NO3)2 (0.0094 g, 0.05 mmol) were dissolved with methanol/ dichloromethane (10 mL, v/v = 3:2). Then, the solution was kept at room temperature for 48 h in a sealed glass bottle. Finally, navy crystals appeared and were separated by filtration and dried in air. Yield: 80%. IR (KBr, cm−1): 3441 (s), 3054 (w), 1672 (s), 1592 (s), 1552 (w), 1518 (w), 1486 (s), 1443 w), 1419 (w), 1384 (s), 1351 (m), 1284 (s), 1267 (m), 1144 (s), 1058 (m), 1015 (s), 993 (m), 896 (m), 847 (m), 828 (m), 737 (s), 747 (m), 673 (s). Elemental analysis found (%) for 1, C22H15Cu0.5N2O4: C, 65.37, H, 3.68, N, 6.80; calcd: C, 65.48, H, 3.72, N, 6.95. Synthesis of [Cd(L1)2I2] (2). L1 (0.015 g, 0.05 mmol) and CdI2 (0.0130 g, 0.05 mmol) were dissolved with methanol/dichloro-

Scheme 1. Chemical Structures of Compounds L1−L6

the coordination effect between L1−L6 and metal ions. By means of DFT, we found that the efficiency of the charge transfer was greatly enhanced after introducing metal ions, which caused a stronger delocalization of the whole system, leading to the enhancement of NLO response.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All materials were commercially available and used without any further purification. 9Anthraldehyde, 3-(9-anthryl)acrolein, 4-acetylpyridine, 2-acetylpyridine, and acetylpyrazine were purchased from Energy Chemical Co., Beijing, China. The other materials were purchased from Sinopharm Chemical Reagent Beijing Co., Beijing, China. 1H NMR spectra were recorded on a Bruker 400 Avance NMR spectrometer operated at 400 MHz. Mass spectra (MS) were recorded on a Bruker Esquire 3000 plus ion trap mass spectrometer (Bruker-Franzen Analytik GmbH, Bremen, Germany). Elemental analyses were carried out on a FLASH EA 1112 analyzer. UV−vis diffuse reflectance spectra were measured by Agilent Cary 5000 UV-vis-NIR Spectrometer. The Fourier Transform Infra-Red (FT-IR) spectra were carried out on a Bruker Tensor 27 spectrophotometer in the range of 400−4000 cm−1. Synthesis. (E)-3-(Anthracen-9-yl)-1-(pyridin-4-yl)prop-2-en-1one (L1).28 9-Anthraldehyde (206 mg, 1 mmol) and 2-acetylpyridine (194 mg, 1.6 mmol) were dissolved in 15 mL of ethanol in a 50 mL flask; then the pH was adjusted to 8−9 with sodium hydroxide solution. The reaction system was stirred for 6 h with the precipitate formed. The crude product was purified by washing three times with ethanol, and a yellow powder was obtained with a yield of 80% based on 9-anthraldehyde. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.86 (q, 3H), 8.50 (s, 1H), 8.25 (dd, 2H), 8.03 (dd, 2H), 7.85 (dd, 2H), 7.65−7.47 (4H). 13C NMR (400 MHz, CDCl3) δ (ppm): 189.2, 151.2, 144.1, 131.5, 130.1, 129.9, 129.5, 129.3, 129.25, 127.0, 125.7, 125.2, 121.8. HRMS: m/z calculated for C22H15NO [M + H]+ 309.12, found 310.09. Elemental analysis found (%) for L1, C22H15NO: C, 85.32, H, 4.22; N, 4.48; calcd: C, 85.40, H, 4.85; N, 4.52. (E)-3-(Anthracen-9-yl)-1-(pyridin-2-yl)prop-2-en-1-one (L2).29 A procedure similar to that for L1 was employed. L2 was synthesized from 9-anthraldehyde (206 mg, 1 mmol) and 2-acetylpyridine (194 mg, 1.6 mmol), and a yellow powder was obtained with a yield of 79% based on 9-anthraldehyde. 1H NMR (400 MHz, CDCl3) δ (ppm): 9.48 (d, 1H), 9.00 (d, 1H), 8.79 (dd, 1H), 8.66 (dd, 1H), 8.50 (s, 1H), 8.36 (dd, 2H), 8.20−8.14 (m, 1H), 8.05 (dt, 3H), 7.56−7.50 (m, 4H), 7.44 (dt, 1H). 13C NMR (400 MHz, CDCl3) δ (ppm): 189.45, 149.49, 148.55, 147.90, 145.24, 145.55, 143.73, 130.34, 129.30, 128.90, 127.80, 127.45, 126.65, 125.90, 125.35, 125.03. HRMS: m/z calculated for C22H15NO [M + H]+ 309.12, found 310.10. Elemental analysis found (%) for L2, C22H15NO: C, 84.89, H, 4.82, N, 4.60; calcd: C, 85.40, H, 4.85; N, 4.52. B

DOI: 10.1021/acs.inorgchem.8b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry methane (10 mL, v/v = 1:1). Then, the solution was kept at 100 °C for 12 h in a sealed glass bottle. Finally, orange crystals appeared and were separated by filtration and dried in air. Yield: 82%. IR (KBr, cm−1): 3443 (s), 3046 (w), 1673 (s), 1596 (s), 1552 (w), 1517 (w), 1442 (w), 1416 (m), 1351 (m), 1325 (w), 1291 (w), 1268 (m), 1058 (m), 1039 (s), 989 (m), 895 (m), 847 (m), 826 (m), 735 (s), 719 (w), 669 (s). Elemental analysis found (%) for 2, C44H30CdI2N2O2: C, 53.93, H, 3.01, N, 6.81; calcd: C, 53.61, H, 3.05, N, 2.84. Single-Crystal X-ray Crystallography. The structures of 1 and 2 were tested on a Bruker D8 VENTURE diffractometer with Mo Kα radiation (λ = 0.71073 Å). The SAINT program was used to control the integration of the diffraction data, polarization effects, and the intensity corrections for the Lorentz. We used the SADABS program to perform semiempirical absorption correction and solved structures by immediate ways, and refined by a full-matrix least-squares technique relied on F2 with the SHELXL-2014 software package. The hydrogen atoms were generated geometrically and refined isotropically using the riding model. The summary of crystallographic data for 1 and 2 is listed in Table S1. Corresponding bond lengths (Å) and bond angles (deg) are provided in Table S2. Z-Scan Technique and Pump−Probe Experiments Measurements. The third-order NLO properties of all the samples were tested by the standard picosecond Z-scan technique in the openaperture (OA) mode. A Q-switched Nd:YAG 532 nm laser (GKPPL1064-1-10, Beijing GK Laser technology Co., Ltd.) was used as the light source, which provided linearly polarized 21 ps pulses with a repetition rate of 10 Hz at 532 nm; another is conducted under excitation with laser pulses of wavelength tunable output from a femtosecond laser system: a 190 fs laser pulse, and the sample solution were both in 2 mm quartz cell. The pump−probe experiments were performed on a TNLO-TR transient nonlinear refractometer (Suzhou micronano laser photon technology Co., Ltd.). The laser source used for ps-pulsed time-resolved pump−probe experiment was the same as those used for the open-aperture Z-scan technique.

at the same concentrations (0.01 M) and DMF was selected as the sample solvent on the basis of a combination of solubility and minimization of overlap between the solvent linear absorption spectra of the organic compounds and the Z-scan measurement wavelength. Furthermore, DMF itself does not show third-order NLO response, which ensures the accuracy of our researches. The third-order NLO absorptive process of samples can be well described by the following equations. ∞

T (z , s = 1) =

∑ m=0

q 0 (z ) =

[−q0(z)]m (m + 1)3/2

(1)

βeff I0(t )Leff 1 + z 2 + z02

(2)

Light transmittance (T) is a function of the sample’s Z position (with respect to the focal point at Z = 0), Leff is the effective length of the sample given by Leff = (1 − exp(−αL))/α (α is the linear absorption coefficient and L is the optical path), t is the time, I0 is the intensity of irradiation at the Z position, Z0 is the Rayleigh range, λ is the laser wavelength, and βeff is the effective NLO absorption coefficient. For the NLO absorption curve, the intensity of light reaches the maximum at the focal point (Z = 0), and βeff > 0, the third-order NLO of the sample exhibits RSA, and βeff < 0, the sample exhibits saturable absorption (SA). As to the investigated results in Figure 2, the third-order NLO responses of L1−L3 all show RSA signals. They just have the differences for the number and the location of heteroatoms. And the differences will cause the distinct electron cloud density, further resulting in the distinctions of NLO response. According to the fitting data, the NLO absorption coefficient βeff values of L1 (0.85 × 10−11 m/W) and L2 (0.9 × 10−11 m/W) are similar, whereas the βeff value of L3 (1.4 × 10−11 m/W) is obviously advanced in comparison with those of L1 and L2. The preeminent third-order NLO property of L3 is kept in line with the actual measure results. All of them contain a large π-conjugated chain with D-π-A structural motifs, but L3 shows the significantly better NLO response. It can be caused by the difference in the number of heteroatoms, and the existence of heteroatoms can promote the electron-withdrawing property, which improves the delocalization of the system. To further study the influence of the length of the conjugated chain on the third-order NLO properties,30,31 L4−L6 were synthesized. And the structural differences with L1−L3 originate in increasing a CC double bond. The results of NLO measurement showed that the third-order NLO properties of L4−L6 have significant improvement. Their βeff values are 2.7 × 10−11 m/W, 1.3 × 10−11 m/W, and 6.6 × 10−11 m/W, respectively. Both the experimental curves and fitting data proved that introducing CC double bonds has obvious impacts on NLO properties exactly.32 Increasing the CC double bonds has efficiently extended the length of the π-conjugation chain and the channel of electron-flowing, further enhancing the performance of NLO properties.33−36 Regulation of Metal Ions to Third-Order NLO Properties. According to the previous reports on NLO properties of metal complexes, metals played in control roles of NLO properties.37−43 We would like to carry on investigations of adjusting third-order NLO properties by introducing metal ions (M(NO3)X, M = Cd2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Pb2+) into L1−L6 (0.01 M). The metal ions of different

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RESULTS AND DISCUSSION The Third-Order NLO Properties Studies of L1−L6. In order to gain insight into the influence of structural difference on third-order NLO properties, we have designed and synthesized six organic compounds with a little structural difference (Scheme 1). The UV−vis absorption spectra of L1− L6 were performed in DMF (5 × 10−5 M) at room temperature. Shown in Figure 1, the main absorption peaks of L1−L6 are observed within the range of 400−470 nm, which are assigned to the π−π* transition of the anthracene groups and originate from the intramolecular charge transfer (ICT). Besides, the third-order NLO properties were assessed

Figure 1. UV/vis spectra of L1−L6 in DMF. C

DOI: 10.1021/acs.inorgchem.8b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) The open-aperture Z-scan results at 532 nm for L1−L3; (b) the open-aperture Z-scan results at 532 nm for L4−L6 (the blue graphs represent the experimental data, and the red curve is the theoretical data).

Figure 3. (a) Regulate the third-order NLO performance of L1 through introducing metal ions (+Mx+ = L1 + Mx+). (b) The exceedingly excellent result through introducing Cu2+ (the blue graphs represent the experimental data, and the red curve is the theoretical data).

response is caused by Cu2+. We have tested the NLO properties of metal salts before studying the “metal salt effect”. The results show that the metal salts do not have any thirdorder NLO properties, so the enhancements of NLO responses excluded the NLO response of metal salts. The βeff value of L1 + Cu2+ is 5 × 10−11 m/W, nearly 6 times that of L1. The reason for NLO performance improvement may be the coordination between metal ions and the nitrogen atoms. After introducing Cu2+, the intramolecular charge-transfer efficiency enhances significantly because of the strong coordination between Cu2+ and L1, which is later proved by the DFT of 1. Similar to the L1, introducing metal ions into L2 (Figure S5) also advances remarkably on the third-order NLO absorption. Relative to differences in the structure of L1 and L2, the nitrogen atom and carbonyl of L2 are easier to chelate with metals. It results in different degrees of intramolecular charge delocalization, which expresses different results of NLO absorption adjustment compared with L1. From the experimental results, L2 is supposed to be easily combined with Zn2+ to form stable complexes and exhibits the greatest third-order NLO properties, while the introduction of Cu2+ has the weakest response to the third-order NLO absorption of L2. The third-order NLO property of L3 promoted markedly after adding Ni2+ (Figure S6), and the βeff value increases from 1.4 × 10−11 m/W to 3.2 × 10−11 m/W. The influences of introducing other metal ions on L3 are basically similar, and the βeff values are about 2.5 × 10−11 m/W. Notably, Cu2+ is also the promising candidate for L4 and L6 (Figures S7 and S8) to control the NLO performance. Both of the NLO properties

equivalents were added to the solutions of L1−L6. The other conditions were consistent with the experimental conditions of previous Z-scan measurements. Shown in Table S3 and Figure S1, the best response to the mole ratio of L1−L6 to metal ions is 2:1. Shown in Figure S2, the position of the linear absorption peak of L1 did not appear red-shifted or blue-shifted after adding metal ions. The effect of charge transfer between L1− L6 and the metal ions on linear absorption (Figure S3) is very small and cannot be observed on linear absorption spectra directly. It can be deduced that the linear absorption peaks of the complexes are also caused by the π−π* transition of ligands (Figure S4). The introduction of metal ions cannot affect the π−π* transition of L1−L6, so the linear absorption peaks of the complexes are consistent with those of L1−L6. The coordination of metal ions and L1−L6 in solution leads to the enhancement of third-order NLO response. As to the L5 + Cu2+, the position of the absorption peak did not move compared with that of L5 (Figure S2), but the absorbance decreased significantly. It shows that the introduction of Cu2+ did not change the energy levels of electron transitions, only increased the number of electron transitions. L5 possesses a large conjugate structure including an anthracene ring, pyridine ring, and CC double bonds, and the effect of π−π* transition is more obvious than the charge-transfer transition. After introducing Cu2+, the intramolecular charge-transfer efficiency enhances significantly because of the strong coordination between Cu2+ and L5. The metal ions had positive effects on NLO response of L1 (Figure 3), and the most obvious enhancement of NLO D

DOI: 10.1021/acs.inorgchem.8b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. (a) The NLO properties of L5 in DMF; (b) the NLO properties of L5 in DMF upon the addition of 0.5 equiv of Cu2+; (c) the NLO properties of L5 with 0, 0.2, 0.4, and 0.5 equiv of Cu2+ (the blue graphs represent the experimental data, and the red curve is the theoretical data).

Figure 5. (a) The single crystal structure of [Cu(L1)2(NO3)2] (1); (b) crystal structure diagram of π−π accumulation in 1; (c) the single crystal structure of [Cd(L1)2I2] (2); (d) crystal structure diagram of π−π accumulation in 2.

observed. And the NLO absorption type showed the typical SA when 0.4 equiv of Cu2+ was introduced. When the metal ions in the solution reach 0.5 equiv, the saturated absorption signal was further enhanced. We attribute this phenomenon to the strong coordination ability of Cu2+;44−49 it is easier to form stable coordination bonds with nitrogen atoms on the pyridine ring. For L5, the nitrogen and oxygen atoms in the pyridine ring only provide electrons when they form coordination bonds with transition metals, and they cannot accept electrons from metal ions and are difficult to form π-backbonding. The HOMO orbit energy of L5 (Figure S10) is the highest compared with others, and it is easier to give the electrons to

have increased by twice as much. In summary, metal ions can effectively regulate the NLO properties of the compounds, and the different metal ions have various regulating ability by the diversity of coordination between L1−L6 and metal ions. It should be noted that introducing metal ions into the solution of L5 was relatively slight to third-order NLO absorption (Figure S9). However, the third-order NLO absorption signal transformed from RSA to SA when Cu2+ were added (Figure 4). The βeff value is changed from 1.5 × 10−11 m/W to −1.7 × 10−11 m/W. And we also captured the whole process of NLO absorption of L5 + Cu2+ from RSA to SA. Shown in the Figure 4, the NLO absorption of L5 + 0.2 equiv of Cu2+ cannot be E

DOI: 10.1021/acs.inorgchem.8b03154 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. (a) The open-aperture Z-scan results at 532 nm for L1 and 1; (b) the closed-aperture Z-scan results at 532 nm for L1 and 2 (the blue graphs represent the experimental data, and the red curve is the theoretical data).

metal ions. The lone pair electrons on the nitrogen atom of L5 transferred easily to the empty orbit of Cu2+ after being excited by laser light, which led to the electron clouds becoming rearranged in the whole molecule; even a part of the electrons transferred to the metal state of the Cu2+. The introduction of Cu2+ enhanced the interaction between the different energy levels in the coordination compounds, finally promoting the intramolecular charge transfer.50 The Third-Order NLO Properties Studies of 1 and 2. In order to gain insight into the coordination between L1−L6 and metal ions, we got two complexes based on L1. Singlecrystal X-ray diffraction analysis confirms the ratio of L1 to metal is 2:1, which is just the same as our previous experiments. According to single-crystal X-ray measure analysis, both of 1 and 2 belong to the triclinic space group C2/c and exhibit a zero dimensional structure. As shown in Figure 5, each asymmetric unit contains one Cu(II) ion, two nitrates, and two independent ligands. The Cu(II) is surrounded by two nitrates from Cu(NO3)2 and two N atoms from ligands. The Cu−N bond lengths are 1.9905(15) Å, and the bond angles of N−Cu−O fluctuate in the range of 57.04(6)−153.49(7)°. For 2 (Figure 5), each asymmetric unit contains one Cd(II) ion, I atoms, and two independent ligands. The Cd(II) is surrounded by two I atoms from CdI2 and two N atoms from ligands. The Cd−N bond lengths are 2.3047(18) Å, and the bond angle around Cd varies from 94.74(9)° to 125.822(11)°. The measured PXRD patterns were well comparable to the corresponding simulated ones based on the single-crystal X-ray data, indicative of a pure phase of 1 and 2 (Figure S11). Their third-order NLO properties have been greatly enhanced contrasted to L1 (Figure 6). The βeff values are 6.5 × 10−11 m/W for 1 and 1.2 × 10−11 m/W for 2, and the results coincide with L1 + Cu2+ and L1 + Cd2+. The formation of coordination complexes can indeed modulate the thirdorder NLO properties. The introduction of metal ions is more conducive to intramolecular charge flow. The coordinative metal ions play very important roles on the third-order NLO properties of complexes because their existence permits more allowed electronic transitions to take place and exhibits large third-order NLO absorption. The NLO performance of 1 is better than that of 2. It can be caused by different metal ions of the complexes. Moreover, we researched third-order NLO properties of 1 and 2 at different wavelengths with a 190 fs pulse for the sake of ensuring the accuracy of our experiments (Figure 7 and Table S4). The results show that the NLO properties of 1 and 2 are improved to some extent compared

Figure 7. Open-aperture Z-scan of 1 under 515, 532, 680, 750, and 900 nm with a 190 fs pulse.

with L1, and the enhancement under 532 nm is better than the others. The βeff value of 1 is 4.5 × 10−13 m/W, nearly 1.8 times that of L1 under 532 nm. And the βeff values under 680, 750, and 900 nm of 1 are about 1.5 times those of L1. The smallest change appears at 515 nm, only 1.03 times that of L1. In addition, the stability of 2 in solution has also been proved by 1 H NMR (1 cannot be characterized by NMR, because Cu2+ is paramagnetic). According to the experimental results, proton a and proton b of the pyridine ring (Figure S12) shift to the low field compared with L1. This is because the introduction of metal ions makes the electrons on the pyridine ring move toward the central metal, and the density of the electron cloud on the pyridine ring is reduced. In order to verify that complexes are indeed formed in solution after introducing metal ions, we used 1H NMR titration to confirm the complexation of metal ions and L1−L6. We found that proton a and proton b have also shifted to the low field, and reached the end point of titration after adding 0.5 equiv of metal ions. So, the introduced metal ions indeed exist coordination with the L1−L6 in the solution and the optimum ratio of L1−L6 to metal ions is 2:1, which is consistent with those in complexes 1 and 2. In order to confirm the influences of anions on the regulation of third-order NLO properties, we have introduced metal salts with different anions (CuCl2, CdCl2, CdBr2, and CdI2). The experimental results (Figure S13) show that the βeff value of L1 + CuCl2 (4.8 × 10−11 m/W) and that of L1 + Cu(NO3)2 (5 × 10−11 m/W) are close. Besides, the βeff values of L1 + Cd(NO3)2, L1 + CdCl2, L1 + CdBr2, and L1 + CdI2 are 1.1 × 10−11 m/W, 1.0 × 10−11 m/W, 1.1 × 10−11 m/W, and F

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Figure 8. Open-aperture pump−probe measurements (the blue graphs represent the experimental data, and the red curve is the theoretical data).

1.3 × 10−11 m/W, respectively. So, we can exclude the effects of anions on third-order NLO properties regulation. The Picosecond Time-Resolved Pump−Probe Experiments. There are many mechanisms about the generation of NLO phenomena, such as excited state absorption (ESA), twophoton absorption (TPA), nonlinear scattering, etc.51−54 In order to gain insight into the mechanisms of metal ions on the NLO properties, pump−probe measurement was carried out at 532 nm. Generally speaking, the five-energy-level model (Figure S14) can be used to describe the nonlinear absorption caused by transitions between excited states. In this model, there are three singlet states (S0, S1, and S2) and two triplet states (T1 and T2), and S0 is the ground state. S1 and S2 are the singlet first excited state and higher excited state, while T1 and T2 are the triplet first excited states and certain higher excited states. The electrons transfer from the ground state S0 to the high singlet excited state S1 by the excitation of a pump light. There are three possibilities for electrons in the S1: the first is that the electron relaxes to S0; the second is that the electrons transfer to T1; and the third is that the electron absorbs another photon and transfers to a higher single excited state S2. There are two possible channels for electrons in the T1 state: the first is relaxes to S0 by radiation-free transition, and the

other is to absorb a photon to the higher excited state T2 of the triplet state, which then relaxes back to T1 quickly. The absorption becomes stronger as the incident intensity increases, which results in the NLO absorption effect. The ground state absorption cross section (σ0) and the excited state absorption cross section (σ1) are the main determinants of the nonlinear absorption characteristics. By fitting the experimental data, we can obtain σ0 and σ1. σ1 > σ0 means that the Z-scan curve shows RSA, and σ1 < σ0 means that the Z-scan curve shows SA. According to the pump experiments of L1−L6 (Table S5 and Figure S15), their NLO absorptions all belong to excited state absorption, and their NLO absorption types are also typical reverse saturation absorptions (RSA) (σ0 < σ1). The σ1/σ0 of L1−L3 are 1.4, 1.6, and 2.0, respectively; those of L4−L6 are 7.37, 7.8, and 8.3. The larger value of σ1/σ0 shows the better NLO property. Thus, the presence of a pyrazine ring can make L3 and L6 have the better NLO properties separately compared with L1, L2 and L4, L5. Moreover, the σ1/σ0 of L4−L6 have greatly improved compared with those of L1−L3, which further indicates that CC double bonds as the channel of electron flow can also improve the NLO responses of the samples. The fitting parameters of L1−L6 G

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Theoretical Calculation. For L1−L6, their third-order NLO properties originate from intramolecular charge transfer, and the whole molecules belong to the classical D-π-A structure. As to the complexes, the introduction of metal ions greatly improves the degree of charge transfer. We can deduce from pump−probe experiments that most electrons exist in excited states and do not return to the ground state as quickly as free L1−L6. However, the third-order NLO absorptions of free L1−L6 and the complexes both belong to the excited-state absorption. Essentially, the NLO responses of complexes 1 and 2 originated in the NLO properties of L1. Then, density functional theory (DFT) calculations were applied to demonstrate the electron cloud distribution of ligands and complexes. Both low HOMO energies and the small energy gaps are beneficial for the third-order NLO properties in the picosecond test.59−61 With regard to L1 and 2 (Figure S17), their electron clouds dominate the block anthracyl groups at the HOMO orbits. In addition, LUMO orbits are mainly localized on pyridine units, which verifies the electron transfer behaviors. Therefore, we conclude that the anthracyl unit plays as the quasi-electron donor, whereas the pyridine unit acts as the quasi-electron acceptor, and the origination of the NLO property is just the delocalization of the π-electron clouds. For 1, Cu2+ contributes greatly to the enhancement of the NLO performance. The electron density dominates the block anthracyl group at the HOMO orbit. At the LUMO orbit, the clouds are distributed in the coordination metal. It follows that Cu2+ has a great influence on the transfer of intramolecular charge, and most of the electrons transfer directly to the empty orbit of Cu2+. The electron transformation process from HOMO to LUMO can be regarded as the π-electron delocalization between the ligand and the coordination metal. As to 2, Cd2+ does not participate in the entire electronic delocalization of the π-electron, but compared with the DFT of L1, the introduction of Cd2+ had a relatively obvious influence on the charge transfer of 2. Moreover, the HOMO−LUMO energy gaps of 1 and 2 are far less than that of L1. The small energy gaps and delocalization of the πelectron clouds have led to the enhancement of molecular NLO properties after introducing metal ions.

indicate that the singlet excited state lifetimes of L1−L3 are longer than those of L4−L6. We infer that the addition of C C double bonds in L4−L6 enhances the efficiency of electrons returning from the excited state to the ground state, then giving L4−L6 the shorter excited state lifetimes compared with L1−L3. As to the experimental curve of L1 (Figure 8), an ultrafast decline appeared near the zero point, followed by a recovery and a long flat tail after the pump beam passes through the sample. After introducing metal ions, the NLO mechanism still belongs to the excited state absorption (Figure S16). However, L1 + Cu2+ has almost no recovery and has a longer time at low transmittance, which indicates the longer lifetime of S1 than that of L1. Data of L1 + Cu2+ at the zerodelay time gives β = 0, σ1 = 3.99 × 10−21 cm2, and τ1 = 0.7 ns. The β = 0 as well as the σ1 (3.99 × 10−21 cm2) > σ0 (1.9 × 10−21 cm2) reveals that the NLO absorption of the samples originates from ESA.55−57 For 1 and 2, the NLO absorption mechanisms both are ESA. Their values of σ1/σ0 are 2.6 and 1.7 separately, which are larger than that of L1. And the singlet first excited life of 1 is longer than that of 2, which is analogous with L1 + Cu2+. The first excited state absorption cross section of 1 (σ1 = 1.51 × 10−20 cm2) is obviously greater than that of 2 (σ1 = 0.68 × 10−20 cm2). Therefore, the NLO response of 1 is more significantly preeminent compared with that of 2. It coincides with our previous experiments that the third-order NLO property of L1 + Cu2+ is better compared with L1 + Cd2+. As to L5 and L5 + Cu2+, the most obvious is the transform of the NLO absorption signal from RSA to SA. For L5, the parameters are as follows: σ1 = 1.1 × 10−20 cm2, σ0 = 1.4 × 10−21 cm2, τ1 = 6 × 10−11, in which L5 has a larger cross section of the first excited state than that of the ground state (σ0 < σ1) Most of the electrons exist in S1 and σ1 occupies a dominant position, so the tested Z-scan curve is the focus symmetry single peak, which shows the typical reverse saturated absorption behavior. After introducing Cu2+ (Figure 8), a rapid increase in the normalized nonlinear transmittance appeared at zero-delay time. Once the pump pulse had passed through the sample, the normalized nonlinear transmittance reduced and remained a long flat transmittance tail. Data near the zero-delay time region gives σ1 (8.9 × 10−20 cm2) < σ0 (1.27 × 10−21 cm2) and τ1 < τ2. The tested Z-scan curve is the focus symmetry single peak, which shows the typical saturated absorption behavior. The NLO absorption of L5 originates from the ESA mechanism, and the electrons transfer from S0 to S1 by the excitation of pump light. Then most electrons relax to S0. The longer CC chain makes the pyridine ring of L5 easily reverse,58 and the electrons in S1 may relax quickly to S0. However, the coordination of Cu2+ with nitrogen and oxygen atoms limits the rotation of the pyridine. The lone pair electrons on the nitrogen atom transfer easily to the empty orbit of Cu2+ after being excited by pump light, leading to the electron clouds rearranging in the whole molecule. And the introduction of Cu2+ enhances the interaction between the different energy levels in L5 + Cu2+, and promotes the intramolecular charge transfer. When the electrons transfer from S0 to S1 by the excitation of pump light, the electrons stay in S1 and even transfer to a higher triplet state T1, which is distinct from the charge-transfer process of L5. The different charge-transfer processes cause the NLO absorption cross sections to change from σ1 > σ0 (L5) to σ0 > σ1 (L5 + Cu2+), and finally result in the SA of L5 + Cu2+. This may be the essential reason for the change of the NLO response signal.

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, the regulation of third-order NLO is not only limited to the structural modifications, but the introduction of extra metal ions can also effectively enhance the third-order NLO properties, even change the absorption signal from RSA to SA. Introducing metal ions can availably reduce the energy gap and promote the electronic delocalization in the whole molecule. Central metal ions play the roles of the bridge in the complexes, which lead to the enhancement of the third-order NLO properties. This work admirably expands the application space of the third-order NLO materials, and simultaneously provides a novel exploration for the third-order NLO materials.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03154. Additional experimental details and supporting figures (PDF) H

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CCDC 1841136 and 1841150 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.H.). *E-mail: [email protected] (X.M.). *E-mail: [email protected] (Y.S.). ORCID

Hongwei Hou: 0000-0003-4762-0920 Funding

This work was financially supported by the National Natural Science Foundation (No. 21671174), Zhongyuan Thousand Talents Project, and the Natural Science Foundation of Henan Province (No. 182300410008). Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.inorgchem.8b03154 Inorg. Chem. XXXX, XXX, XXX−XXX