âCloser is Better and Two is Superior to Oneâ: Third ... - ACS Publications

Article pubs.acs.org/JPCC

“Closer is Better and Two is Superior to One”: Third-Order Optical Nonlinearities of a Family of Porphyrin−Anderson Type Polyoxometalate Hybrid Compounds Sadaf ul Hassan,‡ Hafiz Muhammad Asif,‡ Yunshan Zhou,* Lijuan Zhang,* Ningning Qu, Jiaqi Li, and Zonghai Shi State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: A family of covalently bonded hybrid compounds composed of Anderson type polyoxometalate (POM) moiety and porphyrin moiety have been synthesized and thoroughly characterized. The compounds all show remarkable nonlinear reverse saturable absorption and self-defocusing effect at 532 nm with a pulse duration τ = 6 ns, rendering them promising candidate materials for device applications in photonics and optoelectronics. More importantly, it is found that the hybrid wherein POM is coupled covalently to porphyrin through shorter bridge has an NLO response superior to the hybrid wherein POM is bonded via longer bridge to porphyrin, and the hybrid having two porphyrins connected to POM shows more enhancement than the hybrid having single porphyrin fused to POM. Disclosure of the inherent structure−property relationship is expected to be instructive for exploration of new porphyrin-POM based NLO materials. Meantime, the hybrid compounds have optical-limiting thresholds lower than 1.0 J/cm2, implying their high potential as lower power OL materials.

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INTRODUCTION Recently, the field of nonlinear optical (NLO) materials has attracted substantial research interest because of their potential applications in optical fibers, image processing, optical computing, optical information processing, optical communication, optical data storage, optical limiting and optical switching, and so on.1−7 Conjugated organic molecules are thought to be the most suitable models to investigate because they can possess characteristically large electronic polarizabilities.8,9 In this context, porphyrins are more promising candidates for third-order NLO properties, not only because of their highly delocalized aromatic π-electron systems but also due to their high stability, conformational flexibility, and possible versatile structural modifications, which is another fascinating feature that is often used to tune the NLO responses.10−30 In order to improve and optimize NLO properties of single/simple porphyrin in the early stage, a number of different strategies were adopted (i.e., insertion of different metals into the porphyrins cores, addition of suitable substituents to form symmetrically/asymmetrically substituted porphyrin).10,24−26 Later on various multiple porphyrins,13−16 expanded porphyrins,17−20,23 porphyrin oligomers,21,22 porphyrin arrays with further π-extensions,27,28 porphyrin-based hybrids,29−31 and so on, have also been widely synthesized with significantly improved optical nonlinearities. However, it is © XXXX American Chemical Society

found that multiple porphyrins exhibiting significant NLO responses are rather confined because some limitations are associated with such kinds of complex systems; for example, it is difficult to synthesize, purify, and characterize31 the complex structures as well as it is also not so easy to arrange porphyrins in appropriate positions,18 especially as these large π-conjugated porphyrin systems show a saturation effect with respect to optical nonlinearity.32 These limitations become the common obstacle toward getting better NLO materials with sound physical data. On the other hand, polyoxometalates (POMs), inorganic metal−oxygen clusters formed by the early transition metals, feature remarkable structural diversity and chemical composition variety with various applications.33,34 Particularly, POMs being good electron acceptor, can exchange number of electrons without any significant structural changing. This remarkable property makes them fascinating building blocks which can play an important role in hybrid systems and therefore they may act as a potential candidates in optical field due to their unique electron-accepting ability.35−41 Since the diverse chemical and physical properties of porphyrins and Received: October 1, 2016 Revised: November 6, 2016

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DOI: 10.1021/acs.jpcc.6b09951 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of Compounds 1−6

and the hybrid having two porphyrins connected to POM show more enhancement than the hybrid having single connected porphyrin to POM. The instructive disclosure of the structure− property relationship will be of importance in guiding rational design and synthesis of more promising compounds with more soundable NLO responses.

POMs are in many ways complementary rather than overlapping,42−66 so far, several porphyrin−POM based articles where porphyrin and POM moieties are connected either by electrostatic interactions62 or by covalent bonding66 are reported regarding either second-order 6 1 or thirdorder62−64,66 NLO studies. All of the reported systems show remarkable NLO responses, clearly indicating that the porphyrins-POMs hybrid system is a new type of excellent candidates for NLO materials. In this context, however, deep understanding on the structure−property relationship of porphyrin−polyoxometalates (POMs) hybrid systems and exploration of new porphyrin-POM based NLO materials with more soundable NLO responses is far to be accomplished. In the course of investigating third-order NLO properties of porphyrin−POM hybrid systems,62−64,66 herein we report a family of hybrid compounds based on Mn-Anderson-type POM, namely, N(C 4 H 9 ) 43 CH 3 C(CH 2 O) 3 MnMo 6 O 18 (OCH2)3CCH3·2H2O (1), N(C4H9)43{H3CC(CH2O)3MnMo6O18(OCH2)3CNH2·2H2O (2), N(C4H9)43{C44H29N4NHCOC6H4CONHC(CH2O)3MnMo6O18(OCH2)3CCH3}· CH3CON(CH3)2 (3), N(C4H9)43C44H29N4CONHC(CH2O)3MnMo 6 O 18 (OCH 2 ) 3 CCH 3 ·CH 3 C(O)N(CH 3 ) 2 (4), N(C4H9)43MnMo6O18(OCH2)3CNHCOC6H4CONHC44H29N4· 2H2O·CH2Cl2 (5), and N(C4H9)43{MnMo6O18(OCH2)3CNHCOC44H29N42} (6), which are synthesized as shown in Scheme 1 and thoroughly characterized by elemental analyses, 1 H NMR, ESI-MS, FT-IR, UV−vis, fluorescence spectra, TG/ DTA, and cyclic voltammetry measurements. Investigations on their third-order NLO properties by using the well-established Z-scan technique prove that all the new porphyrin−POM hybrids exhibit remarkable nonlinear reverse saturable absorption and self-defocusing effect under open- and closedaperture configuration, respectively, and render them promising candidate materials for device applications in photonics and optoelectronics. Importantly, the results reveal that the hybrid wherein POM is covalently embedded to porphyrin via shorter bridge is better in view of NLO response than the hybrid wherein POM is linked through longer bridge to porphyrin,

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EXPERIMENTAL SECTION Materials and Apparatus. All organic solvents used for the reactions in this work were freshly dried according to standard methods before use. All other chemicals and solvents were commercially available as reagent grade and used as received. 5-(4-Aminophenyl-3-carbomoylbenzoyl chloride)10,15,20-triphenylporphyrin (denoted as F@TPP1),66 [N(C 4 H 9 ) 4 ] 4 (α-Mo 8 O 2 6 ), 6 7 5-[4-N-(1,3-dihydroxy-2(hybdroxymethyl)propan-2-yl]benzamide-10,15,20-triphenylporphyrin (denoted as F@TPP2),68 5-(4-aminophenyl)10,15,20-triphenylporphyrin (denoted as NH2TPPH2),69 and N(C4H9)43MnMo6O18{(CH2O)3C(NH2)2} (denoted as TrisPOM)70 were synthesized and characterized according to previously reported references. Pyrrole and benzaldehyde for porphyrin synthesis were freshly distilled prior to use. All other chemicals were purchased commercially and used without further purification. Elemental analyses for C, H, and N were performed on a PerkinElmer Vario El element analyzer. UV/vis spectra were recorded with a UV-2550 spectrophotometer (Shimadzu). Infrared spectra were recorded at room temperature on a Nicolet 470 FT−IR spectrophotometer using a KBr pellet in the 4000−400 cm−1 region. Peak intensities are indicated by the following abbreviations: weak (w), medium (m), and strong (s). The 1H NMR spectra were recorded on a Bruker AV 400 spectrometer in DMSO/CD3CN at room temperature. Chemical shifts are given in ppm (δ). Coupling constants (J) are given in Hz. Multiplicities are indicated by the following abbreviations: s = singlet, d = doublet, t = triplet, dd doublet of doublets, m = multiple, and br = broad for 1H NMR. Thin-layer chromatography (TLC) was performed using precoated silica gel 60 F254. Silica gel (size: 200−300 mesh) B


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The Journal of Physical Chemistry C

Synthesis of Compound 3. F@TPP1 (0.8 g, 0.1 mmol) was dissolved in 25 mL of degassed N,N-dimethylacetamide under N2 atmosphere. Compound 2 (0.188 g, 0.1 mmol), K2CO3 (0.035 g) was dissolved into 10 mL of N,N-dimethylacetamide and then this clear yellow solution was added dropwise into the above mixture. The resulting solution was protected from light and allowed to reflux at 80−90 °C under a N2 atmosphere for 48 h. The mixture was cooled to room temperature and then added dropwise into the diethyl ether (100 mL). The violet colored precipitates were collected by centrifugation and purified by using column chromatography (85% ethyl acetate: hexane) Yield: 34 mg, 23.5% based on Mo. 1H NMR (400 MHz, 25 °C, DMSO): δ = 9.23 (d, J = 4.5 Hz, 24H), 8.36 (d, J = 8.5 Hz, 4H), 7.65 (4H, phenyl-NH), 7.17 (s, 2H, -NH−), 6.49 (2s, 12H, CH2−O), 5.09 (s, 3H, −CH3-terminal), 3.53 (m, 24H, TBA+), 3.23 (9H, N,N-dimethylacetamide), 2.77 (m, 24H, TBA+), 1.65 (m, 24H, TBA+), 0.78 (t, 36H, TBA+), −2.58 (s, 2H) ppm. IR (cm−1): νmax = 3463 (br), 3322 (w), 2975 (w), 2943 (w), 2880 (w), 1676 (s), 1613 (w), 1555 (w), 1489 (w), 1446 (m), 1397 (w), 1349 (w), 1240 (w), 1211 (w), 1182 (w), 1114 (w), 1091 (w), 1000 (w), 953 (s), 907 (s), 798 (s), 665 (s), 561 (w), 480 (w). UV/vis (DMF): λmax (ε, mM−1 cm−1) = 320 (3.60), 413 (31.76), 511 (1.50), 544 (0.71), 588 (0.48), 646 (0.43). Anal. Calcd (%) for C113H167MnMo6N10O27:C, 49.75; H, 6.17; N, 5.13. Found; C, 49.70; H, 6.24; N, 5.11. Synthesis of Compound 4. [N(C4H9)4]4(α-Mo8O26) (0.06 g, 0.028 mmol), Mn(OAc)3·2H2O (0.007 g, 0.028 mmol), F@ TPP2 (C49H39N5O4.MeOH) (0.021 g, 0.028 mmol) and CH3C(CH2OH)3 (0.003 g, 0.028 mmol) were introduced into a round-bottom flask under argon atmosphere. N,NDimethylacetamide (2 mL, dried on molecular sieve) was added, and the resulting suspension was heated at 80−90 °C under argon for 48h. The mixture was then cooled to room temperature, filtered, and added dropwise to diethyl ether (40 mL). The precipitate was collected by centrifugation and washed thoroughly with ether to remove unreacted porphyrin (0.039 g, 65% yield based on Mo). 1H NMR (400 MHz, 25 °C, DMSO): δ = 8.82−8.62 (d, J = 4.5 Hz, 23H), 8.21 (d, J = 8.5 Hz, 4H), 7.97 (s, 1H, NH−CO), 6.74 (2s, 12H, CH2−O), 6.11−6.23 (s, 3H, −CH3-terminal), 3.31 (m, 24H, TBA+), 3.05 (9H, N,N-dimethylacetamide), 1.73 (m, 24H, TBA+), 1.47 (m, 24H, TBA+), 0.88 (t, 36H, TBA+), −2.37 (s, 2H) ppm. IR (cm−1): νmax = 3453 (br), 3328 (w), 2967 (w), 2939 (w), 2879 (w), 1655 (s), 1555 (w), 1495 (w), 1484 (m), 1442 (m), 1401 (w), 1349 (w), 1290 (w), 1253 (w), 1219 (m), 1170 (m), 1114 (w), 1074 (w), 1006 (w), 955 (s), 906 (s), 800 (s), 669 (s), 561 (w), 503 (w). UV/vis (DMF): λmax (ε, mM−1 cm−1) = 320 (3.98), 412 (31.52), 510 (1.82), 545 (1.07), 589 (0.86), 645 (0.79). Anal. Calcd (%) for C106H162MnMo6N9O26: C, 48.79; H, 6.26; N, 4.83. Found; C, 48.83; H, 6.19; N, 4.81. Synthesis of Compound 5. A 0.1 mmol solution of TrisPOM (0.188 g in 2 mL of DMF) was added into the 0.2 mmol solution of NH2TPPH2 (0.013 g in 5 mL of THF) and kept stirring for 10 min under room temperature. The above resultant solution was added into the 0.2 mmol solution of isophthalyol chloride (0.038 g in 2 mL of THF) with constant stirring and then refluxed for 12 h further on. The reaction mixture was diluted with methylene chloride. The organic layer was washed with aqueous sodium bicarbonate, dried over sodium sulfate, and concentrated in a rotary evaporator. A violet color residue was obtained, which was purified by column chromatography on silica gel (80% CH2Cl2−hexane) to give purple solid of titled compound (0.045 g, 24% based on Mo).

was used for column chromatography with suitable solvents as the eluent. Cyclic voltammograms (CV) were obtained on a CHI 660B electrochemical analyzer in dry DMF at room temperature at the rate of 10 mV/s in the presence of 0.1 M (nBu4N)PF6 as a supporting electrolyte. A glassy carbon electrode was used as a working electrode, Ag/AgCl as a reference electrode and a Pt wire as an auxiliary electrode. The scan range of the CV were selected carefully from the wide range to small range to ensure the peak used for calculating the HOMO or LUMO levels is the onset peaks. Electron spray ionization mass spectra (ESI-MS) were measured by using the Waters ACQUITY UPLC and Xevo G2 Qt of MS systems. For ESIMS analysis, the 10−5 M solutions in acetonitrile were used. Measurements of the third-order nonlinear optical properties of all the compounds were done using an EKSPLA NL303 Qswitched Nd:YAG laser. Photoluminescence measurements were recorded by using a Hitachi F-7000 FL fluorescence spectrophotometer with both excitation and emission slits of 5 nm, using a xenon arc lamp as the light source (150w), the photomultiplier tube voltage was 400 V, the scan speed was 1200 nm·min−1. Synthesis of Compound 1. A mixture of [N(C4H9)4]4(αMo8O26) (4.00 g, 1.85 mmol), Mn(OAc)3·2H2O (0.8 g, 2.8 mmol) and (HOCH2)3CCH3 (0.78 g, 6.4 mmol) in 75 mL of acetonitrile was refluxed for 16 h. The orange solution was cooled to room temperature and filtered to remove a very fine black solid. The filtrate was exposed to ether vapor. After 2 h, a white precipitate was filtered off. The orange filtrate was again exposed to ether vapor for several days. Large orange crystals were obtained. They were isolated by filtration, washed with a small amount of acetonitrile and ether, and dried under vacuum. Yield: 3.5 g, 78% based on Mo. 1H NMR (400 MHz, 25 °C, DMSO): δ = 4.48 (2s, 12H, CH2−O), 4.01 (2s, 6H, CH3-terminal), 2.38 (4H), 3.59 (m, 24H, TBA+), 1.73 (m, 24H, TBA+), 0.91 (m, 24H, TBA+), 0.28 (t, 36H, TBA+) ppm. IR (cm−1): νmax = 3432 (br), 2969 (w), 2948 (w), 2881 (w), 1481 (s), 1379 (m), 1162 (w), 1287 (w), 1107 (w), 1061 (w), 1031 (w), 951 (s), 925 (s), 910 (s), 852 (m), 799 (s), 656 (s), 557 (w), 500 (w). UV/vis (DMF): λmax (ε, mM−1 cm−1) = 311 (9.21). Anal. Calcd (%) for C58H130MnMo6N3O26: C, 36.35; H, 6.84; N, 2.19. Found; C, 36.31; H, 6.89; N, 2.18. Synthesis of Compound 2. A mixture of [N(C4H9)4]4(αMo8O26) (4.00 g, 1.85 mmol), Mn(OAc)3·2H2O (0.8 g, 2.8 mmol), and (HOCH 2 ) 3 CCH 3 (0.78 g, 6.4 mmol) (HOCH2)3CNH2 (0.78 g, 6.4 mmol) in 200 mL of extra dried acetonitrile was refluxed for 24 h. The dark orange solution was cooled to room temperature and filtered to remove a very fine black solid. The filtrate was exposed to ether vapor and again remove the black solid. After 5 days large orange needle like crystals had been obtained. They were isolated by filtration, washed with a small amount of acetonitrile and ether, and dried under vacuum. Yield: 3.5 g, 78% based on Mo. 1H NMR (400 MHz, 25 °C, DMSO): δ = 9.10 (s, 2H, (terminal) NH2), 6.20 (s, 6H, (OCH2)3-NH2), 5.59 (6H, (OCH 2 ) 3 -CH 3 ), 3.59 (m, 24H, TBA + ), 2.00 (s, 3H, (terminal)−CH3C) 1.73 (m, 24H, TBA+), 0.91 (m, 24H, TBA+), 0.28 (t, 36H, TBA) ppm. IR (cm−1): νmax = 3371 (br), 2968 (w), 2946 (w), 2874 (w), 1640,(s) 1475 (s), 1375 (m), 1151 (w), 1121(w), 1061 (w), 1021 (w), 940 (s), 920 (s), 896 (s), 836 (m), 801 (s), 660 (s), 572 (w), 461 (w). UV/vis (DMF): λmax (ε, mM−1 cm−1) = 313 (9.23). Anal. Calcd (%) for C57H129MnMo6N4O26: C, 35.71; H, 6.78; N, 2.92. Found; C, 36.11; H, 6.80; N, 3.05. C


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Figure 1. 1H NMR spectra of compounds 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f) in DMSO.

Synthesis of Compound 6. A mixture of [N(C4H9)4]4(αMo8O26) (0.06 g, 0.028 mmol), Mn(OAc)3·2H2O (0.011 g, 0.042 mmol) and F@TPP2 (0.042 g, 0.042 mmol) were introduced into a flask under argon atmosphere. N,NDimethylacetamide (2 mL) was added, and the resulting suspension was heated at 80−90 °C under argon for 60 h. The mixture was then cooled to room temperature, filtered, and added dropwise to diethyl ether (40 mL). The precipitate was collected by centrifugation and washed thoroughly with ether to remove unreacted porphyrin (0.042 g, 70% yield based on Mo). 1H NMR (400 MHz, 25 °C, DMSO): δ = 8.88−8.82 (2d, J = 4.5 Hz, 46H), 8.24 (2d, J = 8.5 Hz, 8H), 7.95 (2s, 2H, NH− CO), 7.63 (2s, 12H, CH2−O), 3.11 (m, 24H, TBA+), 1.61 (m, 24H, TBA+), 1.41 (m, 24H, TBA+), 0.97(t, 36H, TBA+), −2.88 (2s, 4H). IR (cm−1): νmax = 3435 (br), 3330 (w), 2973 (w),

H NMR (400 MHz, DMSO): δ = 8.79 (2d, J = 4.5 Hz, 48H), 7.89 (2d, J = 8.5 Hz, 8H), 7.43 (2s, 8H, phenyl-NH), 6.77 (2s, 4H, −NH−), 5.92 (2s, 12H, CH2−O), 3.25 (m, 24H, TBA+), 2.98 (6H), 2.89 (m, 24H, TBA+), 1.51 (m, 24H, TBA+), 1.03 (t, 36H, TBA), −2.59 (2s, 4H) ppm. IR (cm−1): νmax = 3445 (br), 3325 (w), 2971 (w), 2940 (w), 2879 (w), 1680 (s), 1555 (w), 1521 (w), 1481 (w), 1438 (m), 1400 (w), 1355 (w), 1294 (w), 1243 (w), 1213 (m), 1179 (w), 1117 (w), 1103 (w), 1079 (w), 1041 (w), 1010 (w), 949 (s), 920 (s), 906 (s), 853 (m), 805 (s), 666 (s), 558 (w), 504 (w). UV/vis (DMF): λmax (ε, mM−1 cm−1) = 320 (3.87), 414 (31.49), 511 (1.59), 544 (0.82), 588 (0.67), 647 (0.65). Anal. Calcd (%) for C161H196Cl2MnMo6N15O30: C, 54.89; H, 5.61; N, 5.96. Fund; C, 54.89; H, 5.61; N, 5.94. 1

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Figure 2. Negative-ion ESI mass spectra of the compounds 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f).

of terminal CH3 (Figure 1b). The signals corresponding to the aromatic protons are present at 9.23 for compound 3, 8.82− 8.62 for compound 4, 8.79 ppm for compound 5, and 8.88− 8.82 ppm for compound 6, respectively.72 The proton signals at 6.49 in compound 3, 6.74 in compound 4, 5.92 in compound 5 and 7.63 ppm in compound 6 represent the O3−(CH2)3−, which is directly linked to the POM, respectively. Proton signals of the −NH−CO, which is directly linked with the Anderson POM via V3 cap μ-oxo ligands (i.e., O3−(CH2)3C−), appear at 7.17 for compound 3, 7.97 for compound 4, 6.77 for compound 5, and 7.95 ppm for compound 6, respectively, which is the clear indication of covalent bonding between porphyrin and POM. The signals at 7.65 ppm, and 7.43 ppm correspond to the OC−phenyl−CO group which for compounds 3 and 5. The protons of three tetrabutyl ammonium cations (TBA+) for all compounds 1−6 are presented in the range of 3.59−0.28 ppm, respectively, and pyrrolic hydrogen atoms show signal in the range of −2.88 to −2.37 ppm for compounds 3−6 (Figure 1).68 Electron spray ionization mass spectra studies (ESI-MS). Compounds 1−6 exhibit the clear ESI-MS spectra as shown in Figure 2. All of the hybrid compounds show distinct groups of peaks corresponding to different charge states of the parent compounds or their parent anions, more prominently −1 in the

2944 (w), 2881 (w), 1666 (s), 1644 (w), 1555 (m), 1524 (w), 1487 (m), 1443 (m), 1417 (w), 1386 (w), 1340 (m), 1298 (w), 1253 (w), 1207 (m), 1176 (m), 1159 (w), 1108 (w), 1071 (w), 1028 (w), 1002 (w), 952 (s), 924 (s), 910 (s), 852 (w), 798 (s), 661 (s), 555 (w), 500 (w). UV/vis (DMF): λmax (ε, mM−1 cm−1) = 320 (3.42), 413 (31.54), 510 (1.58), 544 (0.92), 588 (0.67), 646 (0.62). Anal. Calcd (%) for C146H180MnMo6N13O26: C, 55.43; H, 5.73; N, 5.76.Found; C, 55.51; H, 5.77; N, 5.73.

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RESULTS AND DISCUSSION H NMR Studies. All the 1H NMR spectra shown in Figure 1 and selected data listed in Table S1 (the Supporting Information) of compounds 1−6 clearly show resolved signals that can be unambiguously assigned.71 In compound 1, the signals at 4.01 and 4.48 ppm correspond to the protons of −CH3 and the O3−(CH2)3 group (Figure 1a), respectively. Compound 2 which is asymmetric having two different terminals exhibits important signal at 9.10 ppm for two protons of terminal NH2. The broader peak at 6.22 ppm is assigned for 6 protons of O3−(CH2)3−C attached with terminal NH2 and another broader peak at 5.63 ppm is ascribed to the 6 protons of another O3−(CH2)3−C attached with CH3 terminal, respectively. The signal at 2 ppm corresponds to three protons 1

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The Journal of Physical Chemistry C m/z range of 400−3500. The detection of such a multiple charged ions of different charge states due to the attachment of different number/type of counterions is quite normal in the ESI-MS spectra of POMs.70 All important peaks observed at different mass to charge ratio (m/z) values could be assigned satisfactorily to the formula with different combinations of counterions, particularly TBA+ in these compounds (Table S2). For compound 1 and 2 there is only one major peak at 1926 (Figure 2a) and 1928 (Figure 2b), respectively, indicating that these are neutral compounds having formula N(C4H9)43CH3C(CH 2 O) 3 MnMo 6 O 18 (OCH 2 ) 3 CCH 3 ·2H 2 O (1) and N(C 4H 9 ) 43{H 3CC(CH 2 O) 3MnMo 6O 18(OCH 2 ) 3CNH 2 ·2H 2O (2), respectively. The ESI-MS spectrum of compound 3 shows signals at m/z = 1281, 2011, and 2738 (Figure 2c), which can be assigned as {H2CONHC(CH2O)3MnMo6O18(OCH2)3CCH 3(CH3 CON(CH 3 ) 2 )} −, {H2 C 44 H 29N 4 NHCOC 6 H 4CONHC(CH2O)3MnMo6O18(OCH2)3CCH3·CH3CON(CH3)2}−, and N(C 4 H 9 ) 43 C 44 H 29 N 4 NHCOC 6 H 4 CONHC(CH 2 O) 3 MnMo6O18(OCH2)3CCH3·CH3CON(CH3), respectively. Similarly, for compound 4 the major peaks appear at 1074 and 2619 (Figure 2d) corresponding to formulas {H2(CH2O)3MnMo6O21}− and N(C4H9)43C44H29N4CONHC(CH2O)3MnMo6O18(OCH2)3CCH3·CH3C(O)N(CH3)2, respectively. In same fashion of fragmentation for compound 5 the major peaks appear at 1450, 1570, and 3532 (Figure 2e) which are evidence for the corresponding formulas {H2NH{COC6H4 CONHC(CH2O)3}2MnMo6O18}−, (N(C4H9)4)2{C(CH2O)3MnMo6O21}−, and N(C4H9)43MnMo6O18{(OCH2)3CNHCOC6H4CONHC44H29N4}2·2H2O·CH2Cl2, respectively. Likewise, compound 6 also has prime peaks at 1223, 1346, and 3173 (Figure 2f) with their corresponding formulas {HMnMo6O18(OCH2) 3CNHCOC44H29N42}2−, {(N(C4H9)4)MnMo6O18{(OCH2)3CNHCOC44H29N4}2}2−, and N(C4H9)43{MnMo6O18(OCH2)3CNHCOC44H29N42}, respectively. One other important observation of the ESI-MS spectra of all compounds is their simplicity, probably due to the nonfragmentation of the samples under the experimental conditions in spite of their large size. Also, no major impurity peaks were observed, and this gives some indication of the purity of the samples (with respect to the presence of other intrinsically charged species). Obviously, the results that the observed mass and charge corresponding to each of these peaks match very well with the assigned formula as well as the above 1 H NMR spectra unequivocally confirm the successful preparation of the hybrid compounds. FT-IR Spectroscopy Studies. As shown in Figure 3, the IR spectra of the porphyrin precursors F@TPP1, F@TPP2, [N(C4H9)4]4(α-Mo8O26) and compounds 1−6 are compared/studied and the data for the observed FT-IR bands and their assignments along with mode numbers are listed in Table 1. For the IR spectra of porphyrin precursors F@TPP1 and F@TPP2, the bands observed at 3341 and 3321 cm−1 are assigned to stretching vibration of N−H. The bands at 1660, 1686 are assigned to ν(−NH−CO) stretching vibration, respectively. The phenyl ring modes of F@TPP1 and F@TPP2 are observed in the range of 1583−1400 cm−1. In low wavenumber region of the spectra, the bands for F@TPP1 and F@TPP2 can be found at 1800−1381 cm−1.68 In the case of compounds 1−6, the bands in the range of 2975−2881 cm−1 are assigned to ν(C−H) of TBA+ for compounds 1−6.67 The strong bands observed in the range of 1000−900 cm−1are assigned to ν(MoO) while the range of broad band around 670−650 cm−1 represent to ν(br. Mo−O−Mo) which

Figure 3. Comparative FT-IR studies of F@TPP1, F@TPP2, and compounds 1−6.

is characteristic bands of Anderson type POM.67,68 For the hybrids 3−6, some variations in bands along with new bands are observed. The bands observed at 3322, 3328, 3325, and 3330 cm−1 for compounds 3, 4, 5, and 6, respectively, are assigned to stretching ν(N−H). The bands at 1676, 1655, 1680, and 1666 cm−1 are assigned to ν(−NH−CO) stretching vibration for compounds 3, 4, 5, and 6, respectively, unequivocally indicating the formation of covalent bond between Anderson type POM MnMo6O18 moiety and porphin precusors of F@TPP1 and F@TPP2, respectively. The skeleton vibration of phenyl ring are observed in the range of 1575− 1400 cm−1 for compounds 2, 3, 4, 5, and 6. In summary, from the FT-IR spectra of the porphyrin precursors of F@TPP1, F@ TPP2 and compounds 3−6 (Figure 3) and the detail assignment in Table 1, it is known that Anderson type POM and porphyrin are covalently bonded with each other and result in hybrid structures where structures of the porphyrin moiety F@TPP1 and F@TPP2 have been kept intact in the resulting porphyrin−POM hybrids. Thermogravimetric Analysis of Compounds 1−6. The TG curve (Figure S1a) shows that compound 1 is decomposed in two steps. First weight loss of 1.86% (calculated value of 1.86%) in the temperature range 100−140 °C corresponding to the loss of two water molecules. In the second step, loss of 46.97% (calcd 47%) in the temperature range 330−570 °C corresponded to the loss of two (C5H9O3) molecules and three counter cations (TBA +). The compound 2 is mainly decomposed into two steps (Figure S1b). In the first step, the weight loss of 1.77% (calculated value of 1.87%) in the temperature range 110−130 °C corresponding to the loss of two water molecules. In the second step, loss of 52.52% (calcd 52.75%) in the temperature range 250−570 °C corresponded to the loss of two ligands {i.e., (C5H9O3) and (C4H8NO3)} and three counter cations (TBA+). The compound 3 is decomposed in two steps (Figure S1c). First weight loss of 3.17% (calculated value of 3.18%) in the temperature range 180−200 °C corresponded to the loss of one N,N-dimethylacetamide (C4H9NO) molecule. In the second step, loss of 64.89% (calcd. 64.91%) in the temperature range 330−560 °C corresponded to the loss of (C49H36O4N5), (C5H9O3) molecules and three counter cations (TBA+). The TG curve (Figure S 1d) shows that compound 4 decomposed in two steps. First weight loss of 3.32% (calculated value of 3.32%) in the temperature range 180−200 °C corresponded to the loss of one N,N-dimethylacetamide molecule. In the second step, loss of 63.24% (calcd 63.27%) in the temperature range 330−555 F


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Table 1. Some Important FT-IR Data (Unit: cm−1) of F@TPP1, F@TPP2, Compounds 1−6, and Their Corresponding Assignment F@TPP1

F@TPP2

1

2

3

4

5

6

3341 −

3321 −

1660 1575 1488 1400 1381 1337 − 1065 981 795/ 749 −

1686 1569 1490 1442 1383 1341 − 1067 1002 796

−− 2969 2948 2881 − −

3350 2968 2946 2874 1640 1475

−

1381

1162 − − −

1121 1028 − 799 741 940 918 898 658

3322 2975 2943 2880 1676 1555 1489 1446 1397 1349 1182 1091 1000 798

3328 2967 2939 2879 1655 1555 1484 1442 1401 1349 1170 1074 1006 800

3325 2971 2940 2879 1680 1555 1481 1438 1400 1355 1179 1079 1010 805

3330 2973 2944 2881 1666 1555 1487 1443 1386 1340 1176 1076 1002 798

953 907

955 907

665

669

949 920 906 666

952 924 910 661

−

−

−

951 925 910 656

assignmenta ν(N−H) ν(C−H) ∼ TBAa

ν(−HN−CO) phenyl

ν(Pyr half-ring)pasym ν(C−O) ν(Pyr half-ring)dasy + phenyl/ ν(Pyr half-ring)pasym δ(Pyr breath)d/δ(Pyr breath)p + Phenyl δ(Pyr def)aasym/δ(Pyr def)pasym ν(MoO)

ν(br. Mo−O−Mo)

Key: ν = stretching, a = tetrabutyl ammonium; δ = in-plane bending. Subscripts: sym = symmetric; asym = asymmetric modes; def = deformation; Pyr = pyrrole. Superscripts: p = protonated pyrrole; d = deprotonated pyrrole. a

Figure 4. Z-scan data curves for Tris-POM (gray), F@TPP1 (green), F@TPP2 (blue), compound 1 (red), compound 2 (violet), compound 3 (sky blue), compound 4 (pink), compound 5 (orange), and compound 6 (dark yellow) in DMF (all the compounds have the concentrations of 3.25 × 10−4 M) under (a) open- and (b) closed-aperture at 532 nm. Stars indicate the measured data, and the solid lines represent the theoretical fit.

°C corresponded to the loss of (C49H36O4N5), (C5H9O3) molecules and three counter cations (TBA+). Compound 5 also decomposed in two steps (Figure S1e). First weight loss of 3.99% (calcd. 3.51%) in the temperature range 80−140 °C corresponded to the loss of one dichloromethane and two water molecules. In the second step, loss of 72.71% (calcd 72.79%) in the temperature range 330−585 °C corresponded to the loss of two Ligs. (C56H41N6O5) and three counter cations (TBA+) while the compound 6 decomposed into one step (Figure S1f) where the weight loss of 70.63% (calcd. 70.68%) in the temperature range 330−560 °C corresponded to the loss of loss of all organic moieties. In summary, the total weight loss (Table S3) of 48.83%, 54.29%, 68.06%, 66.56%, 76.7%, and 70.63% matches well with calculated values, i.e., 48.86%, 54.62%, 68.09%, 66.59%, 76.3%, and 70.68% for compounds 1, 2, 3, 4, 5, and 6, respectively, in the temperature range of 30−750 °C, indicating successful preparation of the hybrids. Third-Order Nonlinear Optical Measurements. The third-order nonlinear optical properties of all the compounds,

Tris-POM, F@TPP1, F@TPP2, and compounds 1−6, were investigated by using the single beam Z-scan technology.73−75 It is a well-known and efficient technique for the determination of the nonlinear optical parameters of materials, providing simultaneously the third-order nonlinear susceptibilities, the magnitude of nonlinear absorption as well as sign of the nonlinear refraction. An EKSPLA NL303 Q-switched Nd:YAG laser with a wavelength of λ = 532 nm, a pulse duration of τ = 6 ns, and a repetition rate of 10 Hz with intensity of light at the focus E0 of 2.3 μJ was employed as the light source. The linear transmittance of the far-field aperture S (defined as the ratio of the pulse energy passing through the aperture to the total pulse energy) was measured to be 0.25. The laser beam (i.e., GAUSSIAN light beam), after being focused by a 20 cm focal length lens into the sample placed in a L = 1 mm path-length quartz cell, was passed through a large-area beam splitter. Before the measurements, the system was calibrated using CS2 in a quartz cell as reference. The Z-scan curves under open aperture and closed aperture configuration along with the G


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H

− − − 3.817 3.670 0.535 0.542 0.575 0.710 −3.710 −3.135 −3.128 −3.817 −3.670 − − − −

ELUMO (eV) Eg (eV)

3.447 1.878 1.871 3.542 3.444 1.885 1.881 1.876 1.870 − 3.20 7.64 − − 17.13 24.18 31.14 49.39 − 2.05 4.91 − − 10.99 15.53 20.02 31.72 − 0.30 4.32 − − 6.24 11.67 15.46 19.03 − −1.34 −1.51 − − −5.98 −6.25 −8.46 −16.80 − 0.23 3.37 − − 4.87 9.10 12.06 14.84 tris-POM F@TPP1 F@TPP2 1 2 3 4 5 6

− −2.03 −2.34 − − −9.05 −10.22 −12.73 −25.41

γ × 10−29 χ(3) × 10−11 (esu) Im χ(3) × 10−11 (esu) Re χ(3) × 10−11 (esu) n2 × 10−10 (esu) β × 10−5 (esu) compounds

Table 2. Z-Scan and HOMO−LUMO Data of Compounds Tris-POM, F@TPP1, F@TPP2, and Compounds 1−6 at a Concentration of 3.25 × 10−4 M (in DMF) at 532 nm with I0 Being 2.11 × 1012 W/m2 (E0 = 2.3 μJ)

corresponding fits76,77 for all the compounds are shown in Figure 4. Reasonably good matches between the experimental data and the theoretical curves are observed suggesting that the experimentally detected NLO effects have an efficient thirdorder characteristic. As shown in Figure 4, all of the samples have notable nonlinear reverse saturated absorption under the open-aperture configuration corresponding to a positive nonlinear absorption coefficient β, a property that is widely applicable in the protection of optical sensors. Each of the closed-aperture Z-scan curves have a peak−valley configuration corresponding to a negative nonlinear refractive index and a characteristic self-defocusing behavior of the propagating wave in all samples. According to the theoretical equations given by Sheik-Bahaeetal and Wang et al. (See Supporting Information), the nonlinear refractive index n2, nonlinear absorption coefficient β, resulting third-order optical nonlinear susceptibility Im χ(3), Re χ(3), and χ(3) and the molecular second hyperpolarizability γ are calculated (Table 2). While n2 and β values of compounds 3−6 show remarkable enhancement as compared to F@TPP1 and F@TPP2, the refractive index (n2) and absorption (β) value for compounds 1 and 2 are neglectable. The n2 × 10−10 (esu) values of F@TPP1, F@ TPP2 and compounds 3− 6 are −1.34, −1.51, −5.98, −6.25, −8.46, and −16.80 and the β × 10−5 (esu) values of F@TPP1, F@TPP2 and compounds 3−6 are 0.23, 3.37, 4.87, 9.10, 12.06, and 14.84, respectively, which clearly show the remarkable enhancement for compounds 3−6 in nonlinear reverse saturated absorption and refractive index compared with their individual reactants. It is also be noted that n2 and β values of compound 3 (one side attached F@TPP1) is less than that of compound 4, the n2 and β values of compound 5 are less than that of compound 6, and compound 6 shows maximum n2 and β values and maximum enhancement among the compounds. The increasing order of n2 and β values of all hybrid compounds is as follows: compounds 1 ≈ 2 ≪ compound 3 < compound 4 < compound 5 < compound 6. Thus, a conclusion is made that the hybrid wherein POM is coupled covalently to porphyrin through shorter bridge is superior to the hybrid wherein porphyrin is bonded via longer bridge to porphyrin, and the hybrid having two porphyrins connected to POM shows more enhancement than the hybrid having single porphyrin fused to POM. UV−Vis Spectroscopy Studies. The linear electronic properties of the compounds are studied by UV−vis., spectroscopy in DMF (Figure 5) and data are summarized in Table S4. The expected strong Soret band associated with the S0 → S2 electronic transitions (420 nm for F@TPP1 and 421 nm for F@TPP2) and four weak Q-bands associated with the S0 → S1 electronic transitions (518, 552, 593, and 649 nm for F@TPP1 and 517, 553, 594, and 649 nm in their electronic absorption spectra in DMF,68 compounds 1, 2 and Tris-POM show a strong adsorption at 311, 313, and 310 nm, respectively, which are ascribed to the Ob,c → Mo charge transfer of the POM skeleton. Hybrid compounds 3−6 exhibit different electronic spectra from their precursor compounds. It is clearly seen that intensity of the absorption bands coming from the POM moiety appearing at around 320 nm for the hybrids 3−6 is much weaker than that (at around 310 nm) for the real/ formal POM precursors, while with some bathochromic-shifts. The strong Soret bands of compounds 3−6 (412−414 nm) show a blue-shifting of ca. 7 nm compared to that of F@TPP1 and F@TPP2 (420, 421 nm respectively). Similarly, blue-

ΔE (porphyrin−POM)



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The Relation between HOMO−LUMO Energy Gaps (Eg) and Second Hyperpolarizability. As it is commonly known that the NLO property is related to the values of the transition energy. The HOMO−LUMO gap Eg values of Anderson POM, F@TPP1, F@TPP2, Tris-POM, and compounds 1−6 are calculated from the absorption edges of the UV−vis spectra (Figure S2) by equation Eg = 1240/λ (λ is absorption edge) (Table 2). With help from cyclic voltammetry (CV) measurement (Figure S3), the lowest unoccupied molecular orbital (LUMO) level of F@TPP1 and F@TPP2 was calculated by ELUMO (eV) = −e(4.8 − EFOC + Eoxi) + Eg (Eoxi is the onset of oxidation potential of porphyrin, EFOC = (Eoxi + Ered)/2 is the energy level of ferrocene used as standard, and Eg is the HOMO−LUMO gap of F@TPP1 and F@TPP2, respectively), while the LUMO levels of the Anderson POM and Tris-POM were obtained by ELUMO (eV) = −e (4.8 − EFOC + Ered) (Ered is the onset of reduction potential of Anderson POM and Tris-POM). To find out the onset peak for the calculation of HOMO or LUMO levels, the CV scan range i.e., +2.5 V to −2.5 V was precisely selected from wide to small range in this article. The lower LUMO level ELUMO = −3.817 eV of Anderson POM and ELUMO = −3.710 eV) of Tris-POM compared with the ELUMO = −3.135 of F@TPP1 and ELUMO = −3.128 V of F@-TPP2 indicated the easy transformation of excited electron from F@TPP1 and F@TPP2 to the Anderson POM and Tris-POM, respectively. Therefore, the resulting porphyrin-POM hybrid systems are more activated when exposed in laser because of excited electron transformation which corresponds to smaller HOMO−LUMO gaps Eg and, subsequently, gives the enhancement in nonlinear response as compare to the individual reactants. Obviously, such kind of covalent-bonded hybrid systems provide us a new insight in exploring new NLO materials with higher second hyperpolarizabilities. In addition, it is noted that the decreasing Eg value of compound 1−6 (i.e., 3.542, 3.444, 1.885, 1.881, 1,876, and 1.87 eV), the increasing discrepancy of LUMO levels ΔE, ΔE (porphyrin−POM) (porphyrin = F@TPP1, F@TPP2; POM = compound 2, Tris-POM) is 3.817, 3.670, 0.535, 0.542, 0.575, and 0.710 eV for the compounds 1−6, respectively, while the increasing second hyperpolarizability γ values of compound 3−6 (17.13, 24.18, 31.14, and 49.39 × 10−29 esu). This observation may implies that the second hyperpolarizability γ values are inversely proportional to the HOMO− LUMO gap Eg of a compound, while are proportional to the discrepancy of LUMO levels ΔE between porphyrin acting as electron donor and the parent polyoxometalate acting as electron acceptor. In other word, in these porphyrin−POM systems low-lying LUMO levels of POMs is thought to play a significant role for the improved second hyperpolarizability γ value, and improvement of third order NLO properties of TPP can be achieved by forming a hybrid with any POM which has a lower LUMO level. Optical Limiting (OL) Studies. On the basis of the remarkable nonlinear reverse saturation absorption of compounds 3−6 the optical limiting (OL) properties are investigated with the same laser source. Figure 7 shows the OL curves of compounds 3−6 in DMF. These compounds show a typical transmittance profile of optical limiting materials with the linear transmittance at 0.7 before the laser input fluence reaches ca. 0.1 J/cm2, which however decreases along with the increase of the input fluence after this point. The OL threshold, defined as the incident energy at which the transmittance is half of the initial linear transmittance,78 was

Figure 5. UV−vis. spectra for Tris-POM (gray), F@TPP1 (green), F@TPP2 (blue), compound 1 (red), compound 2 (violet), compound 3 (sky blue), compound 4 (pink), compound 5 (orange), and compound 6 (dark yellow) in DMF (all the compounds have the concentrations of 5.32 × 10−5 M).

shifting of the weak Q bands are also observed with respect to F@TPP1 and F@TPP2. This result not only shows the formation of covalent bond between functionalized Anderson POMs and porphyrin but also indicates strong electronic interaction between the POM moiety and the porphyrin moiety in the ground state. Steady State Fluorescence Spectroscopic Studies. Fluorescence spectra analysis is another powerful tool for the investigation of linear electronic absorption spectra. The two bands in the emission spectra of compounds 3−6 at 600 and 653 nm are identical to that of F@TPP1, F@TPP2 indicating that the emission is from the porphyrin ligands in compounds 3−6, whereas the fluorescence is partly quenched for all the hybrid compounds 3−6 (Figure 6). The fluorescence

Figure 6. Emission fluorescence spectra of F@TPP1 (green), F@ TPP2 (blue), compound 3 (sky blue), compound 4 (pink), compound 5 (orange), and compound 6 (dark yellow) in 1 M DMF at the excitation wavelength λexc = 415 nm where the compounds 3−6 have the strongest absorption in UV−vis spectra.

intensities are ca. 40%, 36.8%, 38%, and 37.8% times less for compounds 3−6 respectively vs F@TPP1and F@TPP2. This fluorescence quenching for compounds 3−6 compared to F@ TPP1, F@TPP2 is thought to be ascribed to the intramolecular interactions between porphyrin and Anderson type POM in compounds 3−6 where either energy or electron transfer process occurs from porphyrin to POM directly via linker.65 I


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Figure 7. Nonlinear transmission measurements (a) and optical limiting properties (b) of compounds 3 (sky blue), 4 (pink), 5 (orange), and 6 (dark yellow) in 3.25 × 10−4 M DMF solutions with 70% linear transmittance at 532 nm.

revealed to be ca. 0.987, 0.831, 0.697, and 0.51 J/cm2 for compounds 3−6, respectively. As shown in Figure 7a demonstrating the energy dependent transmission measurement results of compounds, the transmittance is reduced from 70% to 8.58%, 7.38%, 7.13%, and 6.25% for compound 3−6, respectively. With the continuous increasing of laser energy, there is no considerable change in transmittance. This result indicates that they possess great potential acting as OL materials based on nonlinear RSA and refraction.

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CONCLUSIONS In conclusion, we have reported a family of new compounds in order to investigate the structure−property relationship of porphyrin−POMs hybrid systems and thoroughly characterized by using different techniques. The Z-scan investigations clearly show the remarkable enhancement in nonlinear reverse saturable absorption and self-defocusing effect under openand closed-aperture configuration for compounds 3−6, respectively, while compounds 1 and 2 show neglectable response. The results further show that closer is better and two is superior to one: i.e., POM linked via a shorter bridge to porphyrin is better than that embedded covalently through longer bridge to porphyrin, and two porphyrins connected to POM show more enhancement than a single connected porphyrin. Factors that cause to enhance the NLO response of resultant hybrid systems are briefly discussed in this article. The charge transfer from porphyrin to POM in the hybrids plays the key role for the enhancement of NLO response while the low-lying LUMO level of POM is found to be a principal factor for the improvement of γ value. The present paper provides important insight into the structure−property relationship of new type of porphyrin−POM hybrid, and it is expected that it will guide us to explore the different kinds of new porphyrin−POM-based new NLO materials with higher hyperpolarizabilities.

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measurements of the third-order nonlinear optical properties, electronic absorption data of Tris-POM, F@ TPP1, F@TPP2, and compounds 1−6 in DMF (Table S4), UV−vis. spectra of Tris-POM, F@TPP1, F@TPP2, and compounds 1−6 in DMF highlighting the absorption edges corresponding to the HOMO− LUMO gap Eg values of the compounds, respectively (Figure S2), and CV spectra of ferrocene, Tris-POM), F@TPP2, compound 2, and F@TPP1 in dry DMF (Figure S3) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(Y.Z.) E-mail: [email protected]. Telephone: +86 18511945529. *(L.Z.) E-mail: [email protected]. Telephone: +86 13717916378. ORCID

Yunshan Zhou: 0000-0003-0143-7370 Author Contributions

‡ The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors denoted by the footnote contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express thanks for the financial support of the Natural Science Foundation of China and the Open Projects Fund of the Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences. Prof. Xue Duan of Beijing University of Chemical Technology is greatly acknowledged for his kind support.

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

(1) Zyss, J. Molecular Nonlinear Optics. Materials, Physics and Devices; Academic Press: Boston, MA, 1994. (2) Prasad, P.-N.; Williams, D.-J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (3) Chemla, D.- S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: Orlando, FL, 1987. (4) Bosshard, C. Organic Nonlinear Optical Materials; Gordon and Breach Science Publishers: 1995. (5) Hann, R.-A.; Bloor, D. Organic Materials for Nonlinear Optics II; Royal Society of Chemistry: London, U.K., 1991.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09951. Some important 1H NMR data selected of compounds 1−6 in ppm (Table S1), peaks observed at different m/z values and their corresponding formulas with intensities by using ESI-MS spectrum for compounds 1, 2, 3, 4, 5, and 6 (Table S2), TG-DTA plot (Figure S1) and the TG data calculations (Table S3) of compounds 1−6, Z-scan J


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