Novel photo-controllable third-order nonlinear optical (NLO) switches based on azobenzene derivatives

Dyes and Pigments 170 (2019) 107599

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Novel photo-controllable third-order nonlinear optical (NLO) switches based on azobenzene derivatives

T

Qiong Xiea, Zhichao Shaoa, Yujie Zhaoa, Linpo Yangb, Qiong Wua, Wenjuan Xua, Kai Lia,∗∗, Yinglin Songb, Hongwei Houa,∗ a b

The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, PR China Department of Applied Physics, Harbin Institute of Technology, Harbin, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Reverse saturation absorption Self-focusing effect Two-photon absorption Properties change

Photo-controllable third-order nonlinear optical (NLO) switches are of high interest for extensive and significant applications. In this work, three new azobenzene-based photo-controllable third-order NLO switches were designed and synthesized. Before UV light irradiation, the compounds exhibit strong reverse saturation absorption (RSA). After irradiated by 365 nm light for 1 min, strong self-focusing behaviors (positive refraction) are observed. The transformations are reversible with good fatigue resistance. From UV–vis absorption spectra and 1H NMR spectra we can find that the transformations are caused by the reversible trans ↔ cis interconversion in their azobenzene moieties. The results of time-resolved pump-probe with phase object (POPP) experiment show that the mechanism of RSA behaviors is two-photon induced excited state absorption, and self-focusing behaviors are originated from Kerr induced excited state refraction. The different electron-density of frontier orbitals and the natural charges possibly lead the transformation of the third-order NLO response from RSA to selffocusing behavior. The photoswitch behaviors with the response in high-speeds, good sensitivity and properties change exceed the reported materials. This work provides a new exploration for the design of photo-controllable third-order NLO switches.

1. Introduction Molecular switches have attracted high attention in the past few years [1]. The NLO switches are especially significant in molecular switches and play a vital role in basic research, biological imaging, optical conversion and optical memory devices [2]. Up to now, a lot of photo-controllable second-order NLO materials have been designed and investigated with excellent performance [3]. However, second-order NLO properties are affected by the space symmetry structure of materials and the phenomena in experiments are not rich compared with third-order NLO responses, thus, the application prospect of third-order NLO materials is more widely. Third-order NLO materials show strong advantages in terms of performance accuracy and application fields [4]. For example, the third-order NLO refraction effect (self-focusing or selfdefocusing) of materials can simultaneously change the NLO refractive index and phase by controlling the laser intensity, further realizing alloptical control [5]. It is difficult to achieve ideal third-order NLO photoswitch materials. Only few examples have been reported, where there exist the shortcomings in the effect of linear absorption on NLO,

∗

the response speeds, sensitivity and properties transformation [6]. Therefore, it is indispensable to exploit controllable and multifunctional third-order NLO materials and extend the application value of photocontrollable third-order NLO switches. Organic third-order NLO materials, which are low in cost, relatively simple to synthesize, easy to modify and tailor, are generally considered to be the most promising third-order NLO material candidates [7]. Meanwhile, the materials can be combined with metal ions to form complexes, further optimizing and adjusting third-order NLO performance owing to more electronic delocalization in metal complexes [8]. Organic third-order NLO materials with photosensitivity groups can have structural transformations through the change of external physical light factors, possibly bring the change of third-order NLO properties [9]. Among many available organic third-order NLO materials, azobenzene (AB) and its derivatives represent one of the most studied organic NLO materials due to the advantages of short response time and good invertibility [10]. AB are common photosensitivity groups and can undergo reversible trans ↔ cis interconversion under the UV irradiation [11]. The conversion may cause a difference in the distribution of

Corresponding author. Corresponding author. E-mail addresses: [email protected] (K. Li), [email protected] (H. Hou).

∗∗

https://doi.org/10.1016/j.dyepig.2019.107599 Received 30 April 2019; Received in revised form 28 May 2019; Accepted 29 May 2019 Available online 31 May 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

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Scheme 1. The synthesis and isomerization of 1-3.

dimethyl ester was prepared according to literature procedures [12]. 1H NMR spectra were recorded on a Bruker 600 Avance NMR spectrometer operated at 600 MHz. The data of elemental analyses (C, N and H) were collected by a FLASH EA 1112 elemental analyzer. LED UV flashlight (5V, 365 nm) was used as a light source to induce photoisomerization. UV–vis spectra were measured by JASCO-750 UV–vis spectrophotometer. 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. Powder X-ray Diffraction (PXRD) patterns were obtained by Cu Kα1 radiation on a PANalytical X'Pert PRO diffractometer.

electron cloud, thereby greatly affecting the third-order NLO behavior. The function of the third-order nonlinear optical switches can be realized in case of materials containing AB with photo-induced reversible third-order NLO properties transformation. If AB modified by coordination groups can be achieved, metal coordination complexes will become more excellent third-order NLO switching materials. Based on the above ideas, we designed and synthesized two novel azobenzene compounds, dimethyl 5-((4-(4(ethoxycarbonyl)phenylazo) phenoxy)methyl) isophthalate (1), 5-((4-(4-carboxyphenylazo)phenoxy)methyl)isophthalic acid (H3L, 2), and a metal complex [Zn(HL) (H2O)]n (3). Interestingly, third-order NLO properties of 1–3 transformed obviously from RSA to self-focusing behavior under 365 nm irradiation. The reason is the reversible trans ↔ cis interconversion around N=N double bond (Scheme 1), which is confirmed by UV–vis absorption spectra and 1H NMR spectra. According to DFT and POPP experimental results, the third-order NLO response result from twophoton induced excited state absorption and Kerr induced excited state refraction, and the different electron-density variation may cause the transformation from RSA to self-focusing behavior.

2.2. Preparation of compounds Synthesis of dimethyl 5-((4-(4 (ethoxycarbonyl)phenylazo) phenoxy) methyl) isophthalate (1). 5-(Bromomethyl)isophthalic acid dimethyl ester(517 mg, 1.8 mmol), 4-(4-Hydroxy-phenylazo)benzoic acid ethyl ester (270 mg, 1 mmol) and K2CO3 (207 mg, 1.5 mmol) was dissolved in 50 mL DMF in a 100 mL flask. The reaction system was stirred and heated at 85 °C for 10 h to form precipitate. A mass of water was added and filtered to obtain the cuticolor solid product 1 with a yield of 95%. 1H NMR (600 MHz, CDCl3) δ 8.67 (s, 1H), 8.34 (s, 1H), 8.18 (d, J = 7.9 Hz, 1H), 7.97 (d, J = 8.1 Hz, 1H), 7.91 (d, J = 7.9 Hz, 1H), 7.12 (d, J = 8.1 Hz, 1H), 5.24 (s, 1H), 4.41 (q, J = 6.7 Hz, 1H), 3.97 (s, 2H), 1.43 (t, J = 6.8 Hz, 1H). Elemental analysis results found: C, 65.50; H, 5.10; N, 5.84%; C26H24N2O7 requires C, 65.54; H, 5.05; N, 5.88%.

2. Experimental section 2.1. Materials and physical measurements All materials were commercially available and used without any further purification. Compound 5-(Bromomethyl)isophthalic acid 2

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asymmetric unit contains one Zn2+, one HL2− and one coordinated H2O molecule. Zn1 ion shows a five-coordinate mode, which displays a distorted trigonal bipyramid coordination geometry, and is bonded to four O atoms (O1, O2, O3, O7) from four different HL2− and one O atom (O9) atom from the coordinated H2O molecule. The Zn–O bond lengths fall in the range of 1.9331(12)-2.1646(13) Å, and the O–Zn–O bond angles are about in the range of 85.81(5)-131.83(6) °, which are similar to those found in other zinc complexes [17]. The distance of Zn1 and Zn2 is 5.1473 Å, and the two atoms are linked together by carboxylate groups to obtain a chain with [Zn2(CO2)2] units (Fig. 1(b)). For HL2−, para-carboxyl units in the azobenzene don't coordinate with Zn, and the Zn coordinate to 5-methoxyisophthalic. The para-carboxyl units are located on both sides of the [Zn2(CO2)2] chain. The chains are linked by isophthalic acid, leading the formation of the two-dimensional complex (Fig. 1(c)), where the azobenzene units shows transconformation in 3. The measured PXRD pattern was well comparable to the corresponding simulated one based on the single-crystal X-ray data, indicative of a pure phase of 3 (Figure S1). The UV–vis absorption spectra (Figure S2) of 1–3 all exhibit absorption peaks at 360 nm, which are the characteristic absorption peaks of trans-conformation azobenzene units [18]. Thus, the azobenzene units of 1 and 2 are also transconformation.

Synthesis of 5-((4-(4-carboxyphenylazo)phenoxy)methyl) isophthalic acid (H3L, 2). 1(476 mg, 1 mmol) dissolved in 50 mL THF was added to NaOH (4 g, 100 mmol) dissolved in 50 mL H2O and the resulting solution was stirred under reflux for 24 h. The mixture was acidified with aq. HCl, and then filtered. The pure product 2 was obtained in a 93% yield as orange-yellow solids. 1H NMR (600 MHz, DMSO) δ 8.45 (s, 1H), 8.18 (s, 2H), 8.10 (d, J = 7.8 Hz, 1H), 7.94 (d, J = 8.2 Hz, 1H), 7.88 (d, J = 7.9 Hz, 1H), 7.27 (d, J = 8.3 Hz, 1H), 5.37 (s, 1H). Elemental analysis results found: C, 62.91; H, 3.79; N, 6.70%; C22H16N2O7 requires C, 62.86; H, 3.84; N, 6.66%. Synthesis of [Zn(HL)(H2O)]n (3). A mixture of Zn(NO3)2‧6H2O (9 mg, 0.03 mmol), 2 (4.2 mg, 0.01 mmol), HNO3(0.05 mL, 2.8 M), water (6 mL) and CH3CN (2 mL) was sealed in a 20 mL Teflon-lined stainless steel vessel, and heated at 120 °C for 3 days. After the autoclave was cooled over a period of 14 h at a rate of 5 °C h−1, orange block-shaped crystals were collected, washed with distilled water, and dried in air. Yield: 2.6 mg, 52% based on Zn. Elemental analysis results found: C, 52.69; H, 3.17; N, 5.60%; C22H16N2O8Zn requires C, 52.66; H, 3.21; N, 5.58%. IR data (KBr, cm−1): 3599(s), 3083(s), 2543(s), 1676(s), 1596(s), 1501(m), 1386(m), 1255(s), 838(m), 776(s). 2.3. Single crystal X-ray crystallography

3.2. The third-order NLO studies

The crystallographic data of 3 was collected on a Bruker D8 VENTURE diffractometer with Mo Kα radiation (λ = 0.71073 Å). The integration of the diffraction data, as well as the intensity corrections for the Lorentz and polarization effects, were performed using the SAINT program [13]. Semiempirical absorption correction was performed using SADABS program [14]. The structures were solved by direct methods and refined with a full matrix least-squares technique based on F [2] with the SHELXL-2014 crystallographic software package [15]. The hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically using the riding model. Crystallographic data and structure processing parameters are summarized in Table S1. Selected bond lengths and bond angles of it are listed in Table S2.

The relative uncertainty of data extraction can be remarkably reduced by using a quartz glass plate with the solution of CuPcTs/DMSO (2.8 × 10−4) as a standard sample in a comparative Z-scan measurement [19]. Solvents were selected on the basis of a combination of solubility and minimization of overlap between the linear absorption spectra and the Z-scan measurement wavelength. The Z-scan measurements were first performed on DMSO and CHCl3 in the picosecond Z-scan experiment. The results revealed that no NLO absorption and refraction could be observed with the input energy (600 nJ) used in the experiment (Figures S3 and S4). Therefore, the observed NLO responses of the solution should come from the solute. Normalized Z-scan curves of 1, 2 and 3 were shown in Fig. 2. The blue scatter dots were experimental data, while the blue solid lines were the theoretical curves fitted by Sheik Bahae's theory [20]. It is obvious that the theoretical curves reproduced the observed experimental data well. The hollow of normalized transmittance at equal distances from the focus indicates a reverse saturable absorption (RSA). The valley-topeak occurred at zero position represents a self-focusing behavior. 1, 2 and 3 exhibit strong RSA and very weak self-focusing behavior. The values of NLO properties were summarized in Table 1. Third-order NLO absorptive coefficient (β) were calculated to 4.2 × 10−12 mW−1 for 1, 4.5 × 10−12 mW−1 for 2, and 3.9 × 10−12 mW−1 for 3, respectively. Though 1 and 2 have the difference substituent groups, 1 and 2 possess the close NLO response. It reveals that the difference doesn't cause the distinct electron cloud density and further influence on the NLO response. Since 3 is so insoluble that the maximum concentration is only one fifth of 2, we can indirectly confirm that 3 is more advantageous than 2 in terms of third-order NLO properties. From the second-order hyperpolarizability (γ) values excluding the effect of concentration of 1–3 (Table 1), 3 possesses the largest value and best third-order nonlinear performance. The stability of 3 in solution has also been proved by 1H NMR. According to the experimental results, proton a and proton b of isophthalic (Figure S5) shift to the low field compared with 2. The three compounds all present very week NLO refractive behavior with the refractive index n2 of 0.3 × 10−18 m2W−1 for 1, 1.1 × 10−18 m2W−1 for 2 and 0.8 × 10−18 m2W−1 for 3.

2.4. Z-scan and time-resolved pump-probe with phase object measurements (POPP) Open aperture and closed aperture Z-scan experiment were conducted to measure the third-order NLO responses. The light sources included a Qswitched and mode locked Nd: YAG laser (PL2143B, EKSPLA) working at 532 nm pulses with 21ps pulse width (FWHM). The repetition rate was set to 10 Hz to avoid heat accumulation in the sample. Compound 1 was dissolved in CHCl3. Compound 2 and 3 were dissolved in DMSO. And the concentrations of solutions were 5 × 10−3 mol/L, 5 × 10−3 mol/L and 1 × 10−3 mol/L, respectively. Cuvettes (2 mm) containing sample solution were placed on a translate stage and moved along Z axis. Detectors (Rj-765a, Laser Probe) were used to record the energy data. The NLO absorption results were evaluated under OA, and the NLO refraction results were obtained from the ratio of CA transmittance divided by the OA transmittance. All systems were controlled through computer. A Q-switched and mode locked Nd: YAG laser (PL2143B, EKSPLA, 532 nm, 21 ps) is employed as laser source in POPP. The details of POPP method could be found elsewhere [16]. The samples and detecting systems in POPP are identical with those in Z-scan experiment. 3. Results and discussion 3.1. Structures of compounds 1-3

3.3. Study on third-order NLO switches The X-ray single-crystal diffraction analysis declares that 3 is a twodimensional (2D) framework complex and crystallizes in the monoclinic system with space group P21/c. As shown in Fig. 1(a), the

Due to the excellent photosensitivity of azobenzene units, structural transformations of 1–3 via UV irradiation may occur. Then the third3

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Fig. 1. (a) Coordination environment around Zn2+ in 3. (b) The [Zn2(CO2)2] unit chain. (c) 3D supramolecular structure.

transition. When the compounds were irradiated by 365 nm light, the absorption peaks of π-π* transition were significantly reduced, and the weak forbidden n-π* transition absorption peaks appeared between 430 and 480 nm, which are the characteristic absorption peaks of cis-isomer 1a, 2a and 3a. These results suggested the trans-isomer converted to cisisomer when excited by 365 nm light. After 2 and 3 were irradiated by 365 nm light, a large number of trans-isomers converted into cis-isomers, and were able to reach equilibrium quickly for 5s. For 1, the transformation needed 10s to reach equilibrium. This may be due to the influence of molecular structures and solvents. When the light was removed, the absorption peaks at 360 nm gradually enhanced, and the absorption peaks at 430–480 nm gradually decreased, indicating that cis-isomers transform to trans-isomers. Cis→trans isomerization was much slower than trans→cis isomerization. Therefore, the photoinduced reversible trans ↔ cis interconversions produce the third-order NLO switches. In order to fully confirm that 1, 2 and 3 show cis-trans isomerization under 365 nm light, 1H NMR experiments were performed to monitor the cis-trans isomerization process [23]. As shown in Fig. 4, under different illumination time the intensities of original signal peaks decrease, some new peaks appear and the integral areas increase. Taking 2 as an example, before illumination the peaks at 8.10 (d), 7.94 (d), 7.27 (d) and 7.88 (d) are characteristic chemical shifts of trans-azobenzene; after illumination these peaks decrease and some new signals at 7.85 (d), 6.89 (dd), and 6.98 (d) appear due to cis-azobenzene. The chemical shifts of the azobenzene have relatively large variation (Table S3). It can be calculated to 54.7% of the cis-isomer after 15 min of illumination, 64% after 30 min, 73.1% after 45 min, and 84% after 1 h. The 1H NMR experimental results of 1 and 3 (Figures S6, S7, and Table S4) also demonstrate the photoinduced azobenzene cis-trans isomerization. Combined with UV and NMR, compounds 1–3 show transisomer before irradiation and about 84% trans-isomers converted into cis-isomer rapidly after irradiation. When the lamp was removed, cisisomers gradually transformed to trans-isomers. It can be found that weak self-focusing behavior before illumination is due to the presence of little cis-isomers in system, and after irradiation existing little trans-

order NLO properties achieve change [21]. We studied the third-order NLO properties of 1, 2 and 3 under 365 nm irradiation (the compounds after irradiation were defined as 1a, 2a and 3a), and the results were exhibited in Fig. 2. From Fig. 2(a), (c) and (e), it can be found that RSA decreased with the increase of illumination time and reached the weakest after 1 min. According to the fitting data in Table 1, the β values decreased from 4.2 × 10−12 mW−1 to 1 × 10−12 mW−1, from 4.5 × 10−12 mW−1 to 0.8 × 10−12 mW−1 and from 3.9 × 10−12 mW−1 to 0.55 × 10−12mW−1, respectively, which suggest the strong RSA effects to weaken. The NLO refract properties from Fig. 2(b), (d) and (f) obviously improve and possess the strongest self-focusing behaviours after 1 min irradiation. And the n2 values of 1a, 2a and 3a reach to 2 × 10−18 m2W−1 (1: 0.3 × 10−18 m2W−1), 4 × 10−18 m2W−1 (2: 1.1 × 10−18 m2W−1) and 3.8 × 10−18 m2W−1 (3: 0.8 × 10−18 m2W−1), respectively. Comparing 1a with 2a, the n2 values of 2a was 2 times of 1a, which indicates that 2a shows the significantly better self-focusing behavior. It may result from different substituent groups, and carboxyl groups can better promote the electron-withdrawing and the delocalization of the system than ester. Although n2 values of 2a and 3a are very close, 3a exhibits better selffocusing behavior than 2a due to the lower concentration of 3a. The reason for NLO performance improvement may contribute to the coordination action between metal ions and carboxyl. The intramolecular charge-transfer transition efficiency is enhanced significantly for the coordination action between Zn2+ and 2a. These results suggest that the third-order NLO properties of 1, 2 and 3 transformed quickly from RSA to self-focusing behavior under 365 nm irradiation. When the UV light was removed, the third-order NLO properties gradually transformed from self-focusing effects to RSA after 210min for 1, 90min for 2 and 3. The reversible transformation suggest that the compounds could be used as photo-controllable third-order NLO switches with fast conversion rate from RSA to self-focusing behavior. To investigate whether photoinduced cis-trans isomerization caused their third-order NLO properties changes, we monitored the linear absorption spectra of 1, 2 and 3 at room temperature [22]. As shown in Fig. 3, the strong UV band (λmax∼360 nm) arose from the allowed π-π* 4

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Fig. 2. Normalized Z-scan curves at 532 nm with the increase of illumination time. (a), (c) and (e) are the open aperture Z-scan results for 1, 2, and 3. (b), (d) and (f) are the closed aperture Z-scan results for 1, 2, and 3. Conditions: [1] = [2] = 5 × 10−3 mol/L, [3] = 1 × 10−3 mol/L, 2 mm quartz cells. 1 is dissolved in CHCl3 solution. 2 and 3 are dissolved in DMSO.

isomers leads to weak RSA. Meanwhile, as optical switches, 1–3 exhibit good reversibility and fatigue resistance. As shown in Figure S8, they are toggled repeatedly for six times while the absorbance at 360 nm remained constant without degradation. This result ensures that the optical switching properties can be controlled in two different wavelength channels. Thus, they are ideal photo-controllable third-order NLO switches. To well understand the originate of third-order NLO switches, the electronic structure and the photo-physical properties of these compounds were studied by using density functional theory (DFT) at the

level of B3LYP/6–31 + g (d, p) and Lanl2dz. The frontier molecular orbital, the energy of HOMO-LUMO gap (E gap, Figure S9) and natural charge (Table S5) were estimated at the same level of theory [24]. All the calculations were performed using the Gaussian 09 quantum software. DFT are applied to demonstrate the electron cloud distribution of the compounds and implied an easier intramolecular charge transfer (ICT) process after photo excitation, which is an important factor for NLO properties. As for 1 and 1a, the electron-density of HOMOs are evenly populated on azobenzene. The electron-density distribution of LUMOs mainly appears on N=N bonds and phenyls with esters,

Table 1 The third-order NLO parameters of 1, 2 and 3. Compounds

Concentration (10−3molL−1)

Irradiation time(s)

Laser energy E (μJ)

T0

β (10−12 mW−1)

n2 (10−18 m2W−1)

χ(3) (10−13 esu)

γ (10−31 esu)

1

5

2

5

3

1

0 10 60 0 10 60 0 10 60

0.60 0.61 0.61 0.63 0.62 0.62 0.63 0.62 0.60

0.73 0.69 0.63 0.70 0.68 0.65 0.83 0.79 0.79

4.2 3 1 4.5 1.9 0.80 3.9 2.1 0.55

0.3 1 2 1.1 3.6 4 0.80 2 3.8

2.6 8.0 16 9.2 30 33 6.7 16 31

0.25 0.76 1.5 0.8 2.6 2.9 2.9 7.2 14

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Fig. 3. UV–vis absorption spectra upon photoisomerization (a) and (b) are for 1. (c) and (d) are for 2. (e) and (f) are for 3. Conditions: [1] = [2] = [3] = 5 × 10−5 mol/L, 2 mm quartz cells. 1 is dissolved in CHCl3 solution. 2 and 3 are dissolved in DMSO solution.

indicating ICT from azobenzene to phenyl. The natural charges of 1 and 1a are obviously different, which may cause the transformation in third-order NLO properties. The electron cloud distributions of 2 and 2a are distinct in the electron-density of LUMO, appearing on N=N bonds and phenyl for 2, and benzyl and azobenzene for 2a. And the difference also exists in the natural charges of 2 and 2a. The isomerization lead to the rearrangement of electron cloud and promotes the transformation in third-order NLO performances. These are possible origins of the third-order NLO properties transforming from RSA to self-focusing behavior under 365 nm irradiation. As for 3, the electron clouds are mainly localized on metal Zn at the HOMO. At the LUMO, the electron clouds dominate azobenzene and carboxyl. Zn has a great influence on ICT, and most of the electrons transfer directly to the empty orbit of azobenzene and carboxyl. The electron transformation process from HOMO to LUMO can be regarded as the π-electron delocalization. The Egap of 3 (0.3eV) is the lowest comparing with 1, 1a, 2, and 2a, which is beneficial for the third-order NLO properties in picosecond test. The results are consistent with the study of third-order NLO properties in which 3 has the best third-order NLO properties. These help us further

comprehend the electronic structures and the optical physical behaviors of these compounds. The different electron-density of HOMO and LUMO and the natural charges lead the third-order NLO response to transform from RSA to self-focusing behavior. Compared with a few reported photoswitches, the compounds in this work perfectly and quickly realize the photoswitches behavior [6(a), 25]. In the photo-controllable third-order NLO switch based on the rhodamine B salicylaldehyde hydrazone Zn(II) metal complex, a strong linear absorption peak at 532 nm can not rule out the effect of linear absorption on NLO [4]. The azobenzene-phthalocyanine materials can simultaneously weakly increase in absorption and refraction by photoresponsive aggregation behavior, host–guest interactions or different film thickness [25]. These compounds posses poor sensitivity and properties transformation. Thus, 1, 2, and 3 all overcome the influence of linear absorption on NLO properties and posses the response in high-speeds, and the behaviors are higher than those of the reported materials.

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Fig. 4. The portion of the 1H NMR spectra of a concentrated solution of 2 in [D6] DMSO taken at various times under irradiation.

βTPA (0.7 × 10−12 mW−1) < β (4.5 × 10−12 mW−1) in 2 and σS0(6.7 × 10−23 m2) < σS1(8.1 × 10−23 m2), βTPA (0.3 × 10−12 mW−1) < β (3.9 × 10−12 mW−1) in 3 coincide with those of 1. And the third-order NLO absorption are also from TPA and ESA. After UV irradiation, the transient nonlinear refractive traces of 1a, 2a and 3a in the CA condition show sharp peaks and the ultrafast signals near the zero-delay time (Fig. 5 (b)(d) and (f)), which is derived from Kerr refraction. The mechanism may be that the light field cause the change of molecular orientation, which is consistent with the photoisomerization. The transmittances of 1a, 2a and 3a in the positive-delay are higher than those in the negative-delay, suggesting the presence of excited state refraction. The △η (1.2 × 10−29 m3 for 1a, 2.4 × 10−29 m3 for 2a and 2.6 × 10−29 m3 for 3a) values are greater than zero, which is self-defocusing behavior. n2 kerr (1 × 10−19 m2W−1 for 1a, 2.1 × 10−19 m2W−1 for 2a and 2.2 × 10−19 m2W−1 for 3a) are less than the thirdorder nonlinear refractive index (n2) in Z-scan study. The third-order nonlinear refraction is from Kerr refraction and excited state refraction. Photoinduced cis-trans isomerism lead to the structural torsion of compounds, which further cause a transformation from RSA to selffocusing behavior corresponding to changes from absorption cross sections to the refractive volumes. τs1 of 1, 2 and 3 are about 1.5 ns, corresponding to ESA. Therefore, the mechanisms of RSA and self-focusing behavior are two-photon induced excited state absorption and Kerr induced excited state refraction, respectively.

3.4. Study on the mechanism of third-order NLO switches There are many mechanisms about the generation of NLO phenomena, such as excited state absorption (ESA), two-photon absorption (TPA), Kerr refraction and excited state refraction et al. [26]. In order to gain an insight into third-order NLO properties about 1, 2 and 3, POPP experiment was carried out at 532 nm under open aperture (OA) and closed aperture (CA) conditions. 1, 2 and 3 were selected for POPP experiment using 38.5 μJ laser pulses under OA and 32 μJ under CA. Generally, the electronic energy levels and transitions of a molecule are typically described by a five-level model containing singlet and triplet states. Because the time of intersystem crossing is longer than the pulse width of picosecond pulse, the change of triplet particles is neglected and the three-level model is used to describe the change in particle number (Figure S10). In this model, there are three singlet states (S0, S1 and Sn). S0 is the ground state, and S1 and Sn are the singlet-first excited state and higher excited state. The relationship between the absorption cross section σ and the refractive volume η of S0 and S1 will affect the layout of the number of particles on each energy level, thereby affecting the change of the transmittance of the probe light. The ground state absorption cross section σS0 and the excited state absorption cross section σS1 are the mainly determinants of the nonlinear absorption characteristics. △η is the difference in refractive volume between S0 and S1, which is the main parameter of the nonlinear refraction characteristics. By fitting the experimental data, we can obtain σS0, σS1, △η and τs1 (Table S6). σS1 > σS0 means that the sample shows RSA, and σS1 < σS0 means that the sample has SA property. When △η > 0 the sample possesses self-focusing behavior. △η < 0 means that the sample exhibits self-defocusing effect. In the OA condition (Fig. 5 (a)(c) and (e)), there are sharp valleys near the zero-delay time, which indicates that the nonlinear absorption of 1, 2 and 3 are RSA and mainly from two-photon absorption (TPA) [27]. TPA is the simultaneous absorption of two photons to excite an electron from S0 to Sn. The transmittances of 1, 2 and 3 in the positivedelay are lower than those in negative-delay corresponding to linear absorption, indicating the presence of excited state absorption. It can be found from the numerical simulation for 1 that σS1 (3.4 × 10−23 m2) is obviously greater than σS0 (2.3 × 10−23 m2), which is typical RSA. TPA coefficient βTPA (1.2 × 10−12 mW−1) is less than the third-order NLO absorptive coefficient β (4.2 × 10−12 mW−1) in Z-scan study. The mechanism of RSA property can be recognized as TPA and excited state absorption (ESA). Both σS0 (2.6 × 10−23 m2) < σS1(3.9 × 10−23 m2),

4. Conclusions In summary, two novel azobenzene compounds and a metal complex have been successfully designed and synthesized as photo-controlled third-order NLO switches. The light regulated reversible trans ↔ cis interconversions can lead the third-order NLO change from RSA to self-focusing behavior. The change is possible from the different electron-density of HOMO and LUMO and the natural charges. The mechanism of third-order NLO variation is that two-photon induced excited state absorption and Kerr induced excited state refraction cause the third-order NLO response. This work not only expands the application scope of the materials, but provides a new idea for designing photo-controllable third-order NLO switches as well, which quickly achieve the conversion from RSA to positive refraction in one material by UV irradiation.

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Fig. 5. (a)(c) and (e) are the open aperture POPP results for 1, 2, and 3. (b)(d) and (f) are the closed aperture pump-probe results for 1a, 2a, and 3a. Conditions: [1] = [2] = 5 × 10−3 mol/L, [3] = 1 × 10−3 mol/L, 2 mm quartz cells. 1 is dissolved in CHCl3 solution. 2 and 3 are dissolved in DMSO.

Acknowledgment

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