Remarkable Enhancement in the Nonlinear Optical Responses of


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Remarkable Enhancement in the Nonlinear Optical Responses of Porphyrins Realized by Combination with Polyoxometalates via Covalent Bonding: A Case Study Sadaf ul Hassan,† Yunshan Zhou,*,†,‡ Lijuan Zhang,*,† Zonghai Shi,† Di Yang,† Hafiz Muhammad Asif,† and Ningning Qu† †

State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡ Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: Integrating polyoxometalates into porphyrin moieties via covalent bond is expected to be a new approach to tune and enhance the nonlinear optical responses of porphyrins. The effectiveness and efficiency of this proposal has been examined and witnessed by studying two new hybrid compounds as a prototype, namely, (Bu4N)4K2[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2(C4H9NO) (1) and (Bu4N)4K2[{C52H34N5O2}HNC(CH2O)3P2V3W15O59]·3(C4H9NO) (2), in which porphyrins acting as electron donor and Dawson type polyoxometalate acting as electron acceptor are connected via short tether through covalent bond. The new compounds are systematically characterized by means of elemental analyses, FT-IR, 1H NMR, ESI-MS, TG/DTA, UV−vis, fluorescence emission spectra, and cyclic voltammetry measurement. Remarkably, great enhancement in reverse saturation absorption is achieved in 1 and 2 which is ca. 1 order of magnitude greater than their individual reactants. Additionally, optical-limiting thresholds are obtained being 0.484 J/cm2 for 1 and 0.501 J/cm2 for 2, respectively, implying their high potential as low-power optical-limiting materials.

1. INTRODUCTION Nonlinear optical (NLO) materials are the object of a growing interest because of their potential applications as photonic devices, optical limiting, optical data storage, light frequency converter, and so on.1−3 Among the various classes of NLO materials, porphyrins are generally more attractive not only because of their highly delocalized aromatic π-electron system, high stability, and small HOMO−LUMO energy difference,4−6 but also because of their remarkable NLO responses which can easily be modulated through conformational design, molecular symmetry, metal complexation, size and degree of conjugation of the π−systems, and so forth.7−20 After extensive studies in the early stage conducted on simple monomeric porphyrin systems, it is realized that improved optical nonlinearities of porphyrins can be achieved by insertion of different metals in the porphyrin core or by addition of suitable substituents to form asymmetrically substituted push−pull porphyrins.9 Subsequent work shifted more and more to multiple porphyrins with further π-extensions such as porphyrin di-, tri-, nona-, and dendramers;10−13 symmetric, asymmetric, and expanded porphyrins; porphyrin arrays; porphyrin oligomers; and porphyrin based hybrids to improve the NLO properties.14−19 However, it is found that multiple porphyrins exhibiting significant NLO responses are rather confined because some limitations are associated with such complex © XXXX American Chemical Society

systems; for example, it is difficult to synthesize, purify, and characterize15 the complex structures as well as being not so easy to arrange porphyrins in appropriate positions,18 especially these large π-conjugated porphyrin systems show saturation effects with respect to optical nonlinearity.20 These limitations become the common obstacle toward getting better NLO materials with sound physical data. On the other hand, polyoxometalates (POMs), a rich class of molecular metal oxygen clusters, being particularly good electron acceptors, can exchange a number of electrons without any significant structural change.21 Due to this distinctive property, POMs are considered fascinating building blocks which can play an important role in hybrid systems and are thought to be potential candidates in the optical fields.22−27 It is worth noting that our previous and preliminary work28−30 showed that polyoxometalates can tune nonlinear optical responses of porphyrins to a certain degree where polyoxometalate and porphyrin are bonded via electrostatic interactions, which imply that remarkable enhancement with respect to the nonlinear optical responses of porphyrins can be Received: February 2, 2016 Revised: March 19, 2016

A

DOI: 10.1021/acs.jpcc.6b01118 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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dissolved in dichloromethane (150 mL). Nitric acid 65% (2.1 mL, 30.87 mmol) was added with a dropping funnel at 0−5 °C over a 2 h period. The reaction was monitored at different intervals by TLC to ensure total conversion of starting material (Rf = 0.87) to product (Rf = 0.77). When it was converted to the desired product the solution was washed 3 × 150 mL with saturated solution of NaHCO3, 3 × 150 mL with H2O, and dried over sodium sulfate. The solvent was removed under reduced pressure and the crude product was dissolved in HCl (65 mL). Tin chloride (2 g, 10.5 mmol) was added and the solution was refluxed overnight. The mixture was neutralized by addition of aqueous ammonia, washed with ethyl acetate (3 × 150 mL), and finally solvent was removed. The product was isolated by silica column chromatography (CH2Cl2 used as eluent) to obtain the desired product as a purple solid (64 mg, 63%). Elemental analysis for C 44 H 31 N 5 ·C 4 H 8 O 2 (viz., C48H39N5O2; Fw = 717.31) (denoted as NH2TPPH2) (%). Calcd: C, 80.31; H, 5.48; N, 9.76. Found: C, 81.73; H, 5.41; N, 10.21. UV−vis (THF): λmax (ε, mM−1 cm−1) = 422 (14.13), 516 (1.21), 552 (0.88), 594 (0.82), 650 nm (0.8). 2.3.2. 5-(4-Aminophenyl)-10,15,20-triphenyl-Zn Metalated-Porphyrin. The title compound was prepared by the similar fashion of compound NH2TPPH233 except that ZnTPP was used in place of TPP. The desired product was obtained as a purple solid (70 mg, 64.2%). Elemental analysis for C44H29N5Zn·H2O (viz., C44H31N5OZn; Fw = 709.18) (denoted as NH2TPPZn) (%). Calcd: C, 74.31; H, 4.39; N, 9.85. Found: C, 74.73; H, 4.41; N, 9.61. UV−vis (THF): λmax (ε, mM−1 cm−1) = 423 (100.34), 558 (4.3), 596 nm (2.08). 2.3.3. 5-(4-Aminophenyl-3-carbomoyl benzoyl chloride)10,15,20-triphenyl Porphyrin. To a solution of NH2TPPH2 (130.0 mg, 0.210 mmol), a catalytic amount of 4(dimethylamino)pyridine (2 mg), and pyridine (0.5 mL, 6.18 mmol) in THF (5 mL) was added a solution of isophthaloyl chloride (4.0 mg, 0.210 mmol) in THF (1 mL). After being stirred overnight, the reaction mixture was diluted with methylene chloride. The organic layer was washed with aqueous sodium bicarbonate, dried over sodium sulfate, and concentrated in vacuo. The residue was purified by column chromatography on silica gel (90% ethyl acetate/hexane) to give a purple solid of title compound (25.0 mg, 18%). Elemental analysis for C52H34N5O2Cl·C4H8O2·H2O (viz., C56H44ClN5O5; Fw = 901.3) (denoted as [email protected]) (%). Calcd: 75.53; H. 4.91; N, 7.76. Found: C, 75.71; H, 5.16; N, 7.72. UV−vis (THF): λmax (ε, mM−1 cm−1) = 404 (101.41), 512 (5.21), 549 (2.79), 588 (2.21), 636 nm (2.12). 2.3.4. 5-(4-Aminophenyl-3-carbomoyl benzoyl chloride)10,15,20-triphenyl-Zn metalated-porphyrin. The synthetic procedure for the title compound was the same as that used for compound [email protected] except that NH2TPPZn was used instead of NH2TPPH2. (Yield 24.3 mg, 17.89%). Elemental analysis for C 5 2 H 3 2 N 5 O 2 ClZn·C 4 H 8 O 2 ·2H 2 O (viz., C56H44ClN5O6Zn; Fw = 981.2) (denoted as [email protected]) (%). Calcd: C, 68.37; H, 4.51; N, 7.12. Found: C, 68.41; H, 4.61; N, 7.21. UV−vis (THF): λmax (ε, mM−1 cm−1) = 409 (100.42), 564 (5), 607 nm (3.74). 2.3.5. Hybrid Compound 1. [email protected] (0.2 g, 0.0250 mmol) was dissolved in 20 mL of degassed N,N-dimethylacetamide (DMA) under N2 atmosphere. Tris-P2V3W15 (0.13 g, 0.0250 mmol) and K2CO3 (0.023 g) was dissolved into 10 mL DMA and then this clear yellow solution was added dropwise into the above mixture over half an hour. The resulting solution was protected from light and allowed to reflux at 80−90 °C

realized assuming that POMs and prophyrins are integrated in a proper fashion. We herein report a new approach to tune and enhance the nonlinear optical responses of porphyrins, i.e., integrating polyoxometalates into porphyrin moieties via covalent bonding. The effectiveness and efficiency of this new approach has been examined by studying two new organic−inorganic hybrid compounds as a prototype, namely, (Bu 4 N) 4 K 2 [{C52H32N5O2Zn}HNC(CH2O) 3P2V3W15O59]·2(C 4H9NO) (1) and (Bu 4 N) 4 K 2 [{C 52 H 34 N 5 O 2 }HNC(CH 2 O) 3 P 2 V 3 W15O59]·3(C4H9NO) (2), which are systematically characterized by elemental analyses, 1H NMR, ESI-MS, FT-IR, TG/ DTA, UV−vis, fluorescence emission spectra, and cyclic voltammetry measurement. Investigations of third-order NLO properties of the hybrid compounds 1 and 2 using Z-scan method prove that the new designed approach is simple yet efficient to enhance the optical nonlinearities of porphyrin.

2. EXPERIMENTAL DETAILS 2.1. Reagents and Chemicals. [(Bu4N)4H2]{H2NC(CH2O)3P2V3W15O59} (denoted as Tris-P2V3W15)31 and tetraphenyl porphyrin (denoted as TPP)32 were prepared according to the literature and characterized by elemental analysis and IR. Acetonitrile, pyrrole, and benzaldehyde were freshly distilled prior to use. All other chemicals and solvents were commercially available as reagent grade and used as received. 2.2. Instruments and Measurements. Elemental analyses for C, H, and N were performed on a PerkinElmer Vario El element analyzer. ICP analyses for K, P, V, W, and Zn were determined by ULTIMA type analytical instrument. 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 as KBr pellet in the 4000−400 cm−1 region. The 1H NMR spectra were recorded on a Bruker AV 400 spectrometer in CDCl3 at room temperature. Thin-layer chromatography (TLC) was performed using precoated silica gel 60 F254. Silica gel (zcx II: 200−300 mesh) 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 a scan rate of 10 mV/s in the presence of 0.1 M (n-Bu4N)PF6 as a supporting electrolyte. A glassy carbon electrode was used as a working electrode, Ag/ AgCl as a reference electrode, and Pt wire as an auxiliary electrode. Electron spray ionization mass spectra (ESI-MS) were measured by using the Waters ACQUITY UPLC and Xevo G2 QTof MS systems. For ESI-MS analyses, the 10−5 M solutions in acetonitrile were used. The observed experimental values were consistent, within the experimental values with accuracy of ±2 in the m/z range of 400−3500. Measurements of the third-order nonlinear optical properties of all the compounds were done using an EKSPLA NL303 Q-switched 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 (150 W), the photomultiplier tube voltage was 400 V, the scan speed was 1200 nm· min−1. 2.3. Synthetic Procedures. 2.3.1. 5-(4-Aminophenyl)10,15,20-triphenylporphyrin. The title compound was prepared according to the reported literature.33 In a typical procedure, tetraphenyl porphyrin (1 g, 1.62 mmol) was B

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at 2976, 2950, 2884 cm−1 and 2973, 2940, 2889 cm−1 are assigned to ν(C−H) of Bu4N+ for compounds 1 and 2, respectively.34,35 The phenyl ring modes of [email protected] are observed at 1575 and 1440 cm−1. The peaks at 1635, 1660, 1630, and 1632 cm−1 are assigned to ν(CO) stretching vibration for [email protected], [email protected], compounds 1 and 2, respectively, which clearly indicate the formation of covalent bond between Tris-P2V3W15 and [email protected] (M = Zn, 2H). In the low wavenumber region of the spectra, the bands for [email protected] TPPZn and [email protected] can be seen at 1350, 1314, 1090, 966, 798, 721 and 1381, 1337, 1065, 997, 981, 801, 795, and 749, respectively. Corresponding bands 1350, 1314, 1099, 948, 803, 755 cm−1 in compounds 1 and 1351, 1310, 1085, 950, 805, and 740 cm−1 in compound 2, respectively, are observed. Compared to the corresponding bands for Tris-P2V3W15,31 the vibrational bands of M-Oa, M-Ob-M in compounds 1 and 2 are showing the red shift while M-Od, M-Oc-M band are showing blue shift which are indicating that M-Oa, M-Ob-M bands are weakened while M-O d, M-Oc -M bands are strengthened in the porphyrin-POM hybrids.31,36,37 The FTIR studies not only clearly indicate that both structures of TrisP2V3W15 moiety and [email protected] (M = Zn, 2H) moiety keep intact but also confirm the two moieties are covalently bonded with each other and result in formation of the hybrid compounds 1 and 2. The 1 H NMR spectra of NH 2 TPPH 2 , [email protected] 2 , NH2TPPZn, [email protected], and compounds 1 and 2 are shown in Figure 2. All the spectra clearly show resolved signals that can be unambiguously assigned. For NH2TPPZn (Figure 2a), the −NH2 proton signal is observed at 4.03 ppm while the signals corresponding to the aromatic protons linked with the −NH2 show doublets at 7.06−7.67 ppm. The protons of pyrrole show different peaks at 8.89 and 8.83 ppm due to different environments, while the remaining aromatic protons of porphyrin present signals at 7.99−8.19 ppm (Table S1).38 In the same fashion, −NH2 proton signal is observed at 4.01 ppm and the aromatic protons which are linked with the −NH2 show doublets at 7.04−7.78 ppm, while the remaining aromatic protons of porphyrin present at 8.01−8.24 ppm, the protons of DCM show peaks at 5.30 ppm, and the pyrrolic hydrogen signal presents at −2.71 ppm for NH2TPPH2 (Figure 2c).39 For [email protected], the signal observed at 8.14 ppm corresponds to −NHCO that moved downfield because of linker (i.e., isophthaloyl chloride linked to porphyrin represents as linker) attachment and the linker aromatic protons signals are presented at 7.75−7.87 ppm. The signals corresponding to the aromatic protons of porphyrin group are present at 8.19− 8.75 ppm and the remaining aromatic protons are observed at 8.89 and 8.74 ppm for the pyrrole ring (Figure 2b). Similarly, the signal observed at 8.48 ppm corresponds to −NHCO and the aromatic proton signals for phenyl aromatic rings are present at 7.18−8.31 ppm for [email protected] (Figure 2d). The signals corresponding to the aromatic protons of the pyrrole group appear at 9.06 ppm. The pyrrolic hydrogen signal is present at −1.98 ppm for [email protected] Upon grafting of [email protected] TPPZn to Tris-P2V3W15, all these signals exhibit an upfield chemical shift (Figure 2e). In compound 1, grafting of the [email protected] TPPZn onto the paramagnetic Tris-P2V3W15 is evidenced by a broad singlet at 6.37 ppm for the −NHCO which is directly attached with −CH2O groups and peak at 3.41 ppm corresponds to −CH2O groups. The singlet at 7.21 ppm represents the −NHCO which is directly linked to the porphyrin ring and the region, i.e., 7.75 to 8.37 ppm,

under a N2 atmosphere for 7 days. The mixture was cooled to room temperature and then added dropwise into the diethyl ether (200 mL). The greenish purple precipitates were collected by centrifugation and washed thoroughly with diethyl ether, acetonitrile, and water to remove all unreacted reactants, and finally dried over anhydrous Na2SO4. Yield: 54 mg, 41.5% based on Tris-P2V3W15. Elemental analysis for (Bu4N)4K2[{C52H32N5O2Zn}HNC(CH2O) 3P2V3W15O59]·2(C 4H9NO) (viz., C128H201K2N12O66P2V3W15Zn; Fw = 6079.97 g/mol) (denoted as compound 1) (%). Calcd: C, 25.29; H, 3.33; N, 2.76; K, 1.29; P, 1.01; V, 2.51; W, 45.35; Zn, 1.08. Found: C, 25.01; H, 3.55; N, 2.69; K, 1.19; P, 0.99; V, 2.41; W, 46.38; Zn, 1.00. UV−vis (THF): λmax (ε, mM−1 cm−1) = 386 (67.08), 417 (100.37), 567 (4.62), 628 nm (15.54). 2.3.6. Hybrid Compound 2. Compound 2 was prepared by the similar procedure for compound 1 except that [email protected] was used in the place of [email protected] Yield: 47 mg, 39.97% based on Tris-P2V3W15. Elemental analysis for (Bu4N)4K2[{C52H34N5O2}HNC(CH2O)3P2V3W15O59]·3(C4H9NO) (viz., C132H212K2N13O67P2V3W15; Fw = 6103.71 g/mol) (denoted as compound 2) (%). Calc.: C, 25.97; H, 3.50; N, 2.98; K, 1.28; P, 1.01; V, 2.50; W, 45.18. Found: C, 25.62; H, 3.66; N, 2.94; K, 1.18; P, 1.07; V, 2.41; W, 45.67. UV−vis (THF): λmax (ε, mM−1 cm−1) = 373 (65.83), 413 (100.12), 479 (3.21), 517 (1.75), 569 (2.46), 626 nm (15.53).

3. RESULTS AND DISCUSSION 3.1. IR Spectra, 1H NMR Spectra, Electron Spray Ionization Mass Spectra, and TG Analysis. From the FTIR spectra of Tris-P2V3W15, [email protected], [email protected], compounds 1 and 2 (Figure 1), and the data listed in Table 1 for

Figure 1. Comparative FT-IR studies of compounds; Tris-P2V3W15 (black), [email protected] (red), [email protected], (blue), hybrid compounds 1 (purple) and 2 (green).

the observed FT-IR bands and their assignments along with mode numbers, it is known that all the C−H, −NH, −NH CO, CC stretching vibration of benzene ring, CN stretching vibration of pyrrole ring, as well as the CC stretching vibration are found in the FT-IR spectra of compounds 1 and 2. In detail, the stretching primary amines {ν(−NHCO)} bands are observed at 3450, 3448, 3438, and 3440 cm−1 for [email protected], [email protected], compounds 1 and 2, respectively. The band observed at 3342 cm−1 for [email protected] and 3331 cm−1 for compound 2 assigned to stretching N−H which is not found in case of [email protected] and compound 1 because they contain metal inside porphyrins cores. The bands C

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[email protected] H2

compound 1

compound 2

assignment

3450 1635 1575 1438 1350,1314 1090 966 798 721 643 -

3448 3341 1660 1575 1440 1381, 1337 1065, 997 981 801 795/749 709 -

3438 2938, 2950, 2884, 1630 1575 1440 1350, 1314 1099 948 803 755 658 1062, 1025 910 851 798

3440 3331 2973, 2940, 2889 1632 1575 1437 1351, 1310 1085 950 805 740 655 1069, 1052 909 854 797

primary amines ν(−NHCO) ν(N−H) ν(C−H) ∼ Bu4N+a ν(−HNCO) Phenyl Phenyl ν(Pyr half-ring) ν(Pyr half-ring) + Phenyl/ν(Pyr half-ring) δ(Pyr breath)d/ δ(Pyr breath)p + Phenyl δ(Pyr def)/ δ(Pyr def) δ(Pyr def)/ δ(Pyr def) γ(Cβ-H)sym + Phenyl M-Oa M-Od M-Ob-M M-Oc-M

ν = stretching, a = tetra-butyl ammonim, δ = in-plane bending, γ = out-of-plane bending modes, subscripts sym = symmetric, subscripts asym = asymmetric modes, def = deformation, and pyr = pyrrole, superscript p = protonated pyrrole, and superscript d = deprotonated pyrrole. a

compound 1, peaks at 1093, 1316, 1367, 1418, 1755, 1822, 1890, 1958, and 3039 correspond to the formula, {(Bu4N)[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2C4H9NO}5−, {K2[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]· 2C4H9NO}4−, {(Bu4N)K[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2C4H9NO}4−, {(Bu4N)2[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2C4H9NO}4−, {K3[{C52H32N5O2Zn}HNC(CH 2 O) 3 P 2 V 3 W 1 5 O 5 9 ]·2C 4 H 9 NO} 3 − , {(Bu 4 N)K2[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]· 2C4H9NO}3−, {(Bu4N)2K[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2C4H9NO}3−, {(Bu4N)3[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2C4H9NO}3−, and {(Bu4N)4[{C52H32N5O2Zn}HNC(CH2O)3P2V3W15O59]·2C4H9NO}2−, respectively, and in the same pattern for compound 2, peaks at 1098, 1322, 1358, 1424, 1763, 1832, 1898, 1967, and 3051 correspond to the formula{(Bu4N)[{C52H34N5O2}HNC(CH 2 O) 3 P 2 V 3 W 15 O 59 ]·3C 4 H 9 NO} 5− , {K 2 [{C 52 H 34 N 5 O 2 }HNC(CH 2 O) 3 P 2 V 3 W 15 O 59 ]·3C 4 H 9 NO} 4− , {(Bu 4 N)K[{C 52 H 34 N 5 O 2 }HNC(CH 2 O) 3 P 2 V 3 W 15 O 59 ]·3C 4 H 9 NO} 4− , {(Bu 4 N) 2 [{C 5 2 H 3 4 N 5 O 2 }HNC(CH 2 O) 3 P 2 V 3 W 1 5 O 5 9 ]· 3C 4 H 9 NO}} 4− , {K 3 [{C 52 H 34 N 5 O 2 }HNC(CH 2 O) 3 P 2 V 3 W 15O 59 ]·3C 4 H 9NO}}3− , {(Bu 4 N)K2 [{C 52H 34 N 5O 2 }HNC(CH2O)3P2V3W15O59]·3C4H9NO}}3−, {(Bu4N)2K[{C52H34N5O2}HNC(CH2O)3P2V3W15O59]·3C4H9NO}3−, {(Bu4N)3[{C 52 H 34 N 5 O 2 }HNC(CH 2 O) 3 P 2 V 3 W 15 O 59 ]·3C 4 H 9 NO} 3− , and {(Bu 4 N) 4 [{C 52 H 34 N 5O 2 }HNC(CH 2 O) 3 P 2V 3 W15O 59]· 3C4H9NO}2−, respectively. All the observed peak values match well with calculated values via [M+K]+ in the present case. Among all the peaks, the maximum relative intensities appear at m/z = 3039 (99.98%) for compounds 1 and at m/z = 3051 (99.93%) for compound 2, unequivocally confirming the assigned formulas for compound 1 and compound 2. Obviously, the results from the observed mass and charge corresponding to each of these peaks match very well with the assigned formula as well as the simulated spectra, further confirming the successful preparation of the compounds. The TG analysis of compounds 1 and 2 (Figure S1) shows that the first weight loss of 18.88% (calculated value: 18.80%) in the temperature range 40−260 °C corresponds to the loss of two DMA molecules and decomposition of four Bu4N+ moieties for compound 1, while the weight loss of 20.15%

corresponds to the phenyl aromatic group. The protons of four tetrabutyl ammonium cations (Bu4N+) are present at 1.02−3.10 (Figure 2e). The 1H NMR spectrum for compound 2 also shows a quite similar pattern in which the observed sharp singlet peak at 3.34 ppm corresponds to the −CH2O groups which are direct covalently attached to the Tris-P2V3W15 (Figure 2f). The proton of the −NHCO group which is directly linked to −CH2O shows a singlet at 5.98 ppm while the proton of −NHCO which is linked to the porphyrin ring shows a singlet at 6.86 ppm. The remaining aromatic protons of the porphyrin ring are observed at the 7.3−7.87 ppm region. The protons of four Bu4N+ are present at 1.09−3.00 while the pyrrolic hydrogen shows a signal at −2.59 ppm (Figure 2f).40 From the 1H NMR analysis results, the same conclusion can be made as that from the FT-IR studies above. In addition, mass spectra of all compounds clearly show distinct groups of peaks (Figure 3). The mass spectra show clear strong peaks at 717.3, 709.1, 901.3, and 980.9 which are clearly assigned to the corresponding compounds NH2TPPH2 (Figure 3a), NH2TPPZn (Figure 3b), [email protected] (Figure 3c), and [email protected] (Figure 3d), respectively. The ESI-MS mass spectra of compounds 1 (Figure 3e) and 2 (Figure 3f), which are almost similar, clearly show distinct groups of peaks which correspond to different charge states of the parent anions. The detection of such multiple-charged ions of different charge states due to the attachment of different numbers/types of counterions is quite normal in the ESI-MS spectra of POMs.41−43 As expected, the majority of the peaks in a particular group follow an isotopic distribution with a Gaussian shape, because of the presence of many tungsten atoms in hybrid compounds 1 and 2. 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. The ESI-MS spectra of hybrid compounds 1 and 2 clearly show that both compounds have the four group peaks with different charges, i.e., 5-, 4-, 3-, and 2- within the mass range of m/z = 1000−3300. All these major peaks observed at different m/z values for compounds 1 and 2 could be satisfactorily assigned to the formulas with different combinations of Bu4N+ and K+ countercations (Table S2), i.e., for D

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Figure 2. 1H NMR spectra of NH2TPPZn (a), [email protected] (b), NH2TPPH2 (c), [email protected] (d), compound 1 (e), and compound 2 (f) in CDCl3.

(calculated value: 20.16%) in the temperature range 60−295 °C corresponds to the loss of three DMA molecules and decomposition of four Bu4N+ moieties for compound 2, and the second weight loss of 16.92% (calculated value: 16.66%) and 15.89% (calculated value:15.60%) in the temperature range 300−650 °C corresponds to the loss of the [email protected]

(∼C 5 2 H 32 N 5 O 2 Zn) for compound 1 and [email protected] 2 (∼C52H34N5O2) for compound 2, respectively. The total weight loss of 35.8% and 36.04% approximately matches the calculated values, i.e., 35.46% and 35.76%, for compounds 1 and 2, respectively, in the temperature range of 30−700 °C. Correspondingly, the DTA curve shows two endothermic peaks E

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Figure 3. Molecular diagrams of NH2TPPH2 (a), NH2TPPZn (b), [email protected] (c), and [email protected] (d), and the negative-ion ESI mass spectra of compound 1 (e) and compound 2 (f).

Figure 4. Z-scan curves for NH2TPPH2 (violet), NH2TPPZn (black), Tris-P2V3W15 (blue), [email protected] (sky blue), [email protected] (purple), compound 1 (red), and compound 2 (green) in DMF (all the compounds have the concentrations of 3.25 × 10−4 M). (a) open-aperture curves; (b) closedaperture curves. The triangle indicates the measured data and the solid lines represent the theoretical fit. F

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Table 2. Z-Scan Data for the Compounds at Concentration of 3.25 × 10−4 M in DMF at 532 nm with I0 Being 2.107 × 1012 W/ m2 (E = 20 μJ), and the Obtained HOMO−LUMO Gaps Eg, LUMO Energy Levels ELUMO, and ΔE(porphyrin−POM)a compound

β ×10−6 (esu)

n2 ×10−11 (esu)

Imχ(3) ×10−11 (esu)

Reχ(3) ×10−11 (esu)

χ(3) ×10−11 (esu)

γ ×10−29 (esu)

Eg (eV)

ELUMO (eV)

ΔE (eV)

NH2TPPH2 NH2TPPZn Tris-P2V3W15 [email protected] [email protected] compound 1 compound 2

2.291 2.548 1.248 3.189 2.337 17.921 15.973

−13.811 −12.735 −14.444 −13.985 −13.391 −19.939 −16.998

0.312 0.315 0.160 0.326 0.299 2.298 2.048

−2.031 −2.042 −2.187 −2.118 −2.028 −3.022 −2.574

2.081 2.138 2.193 2.142 2.050 3.795 3.289

3.177 3.335 3.414 3.755 3.192 5.913 5.122

3.230 1.853 1.904 1.859 1.873

−4.319 −2.685 −3.094 -

1.634 1.225

a

ΔE(porphyrin-POM) = [ELUMO([email protected]) − ELUMO(Tris-P2V3W15), (M = Zn, 2H)].

Figure 5. (a) Optical-limiting properties of compounds 1 (black) and 2 (red); (b) comparison of optical-limiting results of compounds 1 and 2 in 3.25 × 10−4 M DMF solutions with 70% linear transmittance at 532 nm.

at 156.7 and 366.8 °C for compound 1 and 196.8 and 379.8 °C for compound 2. The TG analysis results match well with the results of elemental analysis and ESI-MS analysis. 3.2. Nonlinear Optical Properties. From the Z-scan curves along with the corresponding fits44,45 for the compounds (Figure 4, Supporting Information) it can be seen that 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 4a, 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 to 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 (Figure 4b). After careful comparison of the calculated nonlinear refractive index n2, nonlinear absorption coefficient β, resulting third-order optical nonlinear susceptibility Imχ(3), Reχ(3), and χ(3), and the molecular second hyperpolarizability γ listed in Table 2, it is found that while n2 values of compounds 1 and 2 do not show considerable change as compared to their individual reactants, the β value of compound 1 is 7.0 times greater than that of NH2TPPZn, 14.4 times greater than that of Tris-P2V3W15, and 5.6 times greater than that of [email protected], and the β value of compound 2 is 6.9 times greater than that of NH2TPPH2, 12.8 times greater than that of Tris-P2V3W15, and 6.8 times greater than that of [email protected] TPPH2. This result clearly shows that the resultant hybrid

compounds 1 and 2 show excellent improvement especially in nonlinear reverse saturated absorption compared with their individual reactants reflected by the corresponding Imχ(3) value as compared to their individual reactants. It should be noted that the β value of compound 1 (17.921 × 10−6 esu) is comparatively higher than that of compound 2 (15.973 × 10−6 esu), which must be ascribed to the presence of the divalent Zn(II) within the tetrapyrolic porphyrin ring of compound 1, which is normal after insertion of metals in the porphyrin core.46 Due to the remarkable nonlinear reverse saturation absorption of compounds 1 and 2, their optical limiting properties are investigated with the same laser source in DMF (Figure 5). Compounds 1 and 2 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 optical limiting threshold, defined as the incident energy at which the transmittance is half of the initial linear transmittance,47 was revealed to be ca. 0.484, 0.501 J/cm2 for compounds 1 and 2, respectively. As shown in Figure 5a, the transmittance is reduced from 70% to 15.1% for compound 1 and 16.3% for compound 2 at 45 μJ energy, 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 low-power optical-limiting materials based on nonlinear RSA and refraction. 3.3. HOMO−LUMO Energy Gaps (Eg) Calculations. As it is commonly known, the NLO property is related to the values of the transition energy.48,49 The HOMO−LUMO gap Eg G

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Figure 6. CV spectra of ferrocene (a), Tris-P2V3W15 (b), [email protected] (c), and [email protected] (d) in dry DMF (all the compounds have the concentration of 1 mM) obtained at room temperature at a scan rate of 10 mV/s in the presence of 0.1 M [n-Bu4N]PF6 as a supporting electrolyte.

third-order nonlinear optical properties of the covalently bonded hybrid systems. It should also be mentioned that the fluorescence emission of [email protected] and [email protected] is strongly quenched after forming the hybrid compounds 1 and 2 (Figure 7) with emission intensities at 601 and 662 nm are about 20% and 18% those of

values of [email protected], [email protected], and compounds 1 and 2 are calculated from the absorption edges of the UV−vis spectra by Eg = 1240/λ (λ is absorption edge, Figure S3 and Table 2).48 With help from cyclic voltammetry (CV) measurements (Figure 6),50 the lowest unoccupied molecular orbital (LUMO) level of [email protected] was calculated by ELUMO (eV) = −e(4.8 − EFOC + Eox) + Eg (Eox is the onset of oxidation potential of [email protected], EFOC = (Eox + Ered)/2 is energy level of ferrcene used as standard, Eg is the HOMO−LUMO gap of [email protected] TPPM), while the LUMO levels of the Tris-P2V3W15 were obtained by ELUMO (eV) = −e(4.8 − EFOC + Ered) (Ered is the onset of reduction potential of Tris-P2V3W15).51 To find out the onset peak for the calculation of HOMO or LUMO levels, the scan range of the CV was selected carefully from a wide range (2.5 V to −2.5 V) to a small range to ensure the peak used for calculating the HOMO or LUMO levels is the onset peaks. It is found that the LUMO level (ELUMO = −4.319 eV) of Tris-P2V3W15 is lower than that of [email protected] (M = Zn, 2H; ELUMO = −2.685 eV and ELUMO = −3.094 eV, respectively), which corresponds to smaller HOMO−LUMO gaps Eg (Table 2), also indicating the easy transformation of excited electrons from the porphyrin moiety to the Tris-P2V3W15 when exposed in the laser. The emergence of a facile electron transformation herein is thought to be responsible for the enhancement in the

Figure 7. Room temperature emission fluorescence spectra of solid [email protected] (blue), [email protected] (pink), compound 1 (green), and compound 2 (navy blue) at the excitation wavelength λexc = 415 nm where compounds 1 and 2 have the strongest absorption in UV−vis spectra. H

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[email protected] and [email protected], respectively. The strong fluorescence quenching for compounds 1 and 2 compared to that of [email protected] and [email protected] not only indicates the formation of covalent bonding between Tris-P2V3W15 and porphyrin moiety, but also shows that strong electron transformation and/or energy transfer takes place from the porphyrin moiety acting as electron donor to Tris-P2V3W15 moiety acting as electron acceptor. In addition, it is noted that the Eg value of compound 1 (1.859 eV) is less than that of compound 2 (1.873 eV), and the discrepancy of LUMO levels ΔE, [ΔE (porphyrin-POM) = [ELUMO([email protected]) − ELUMO(Tris-P2V3W15), (M = 2H, Zn)] is 1.634 eV for compound 1 and 1.225 eV for compound 2, while the second hyperpolarizability γ value of compound 1 (5.913 × 10−29 esu) is greater than that of compound 2 (5.122 × 10−29 esu). This observation may imply that the second hyperpolarizability γ values are inversely proportional to the HOMO−LUMO gap Eg of a compound, while they are proportional to the discrepancy of LUMO levels ΔE between the porphyrin moiety acting as electron donor and the polyoxometalate acting as electron acceptor. In other words, in these porphyrin-POM systems low-lying LUMO levels of POMs are thought to play a significant role for the improved second hyperpolarizability γ values,28−30 and enhancement in third-order NLO properties of porphyrin may be achieved by forming a hybrid with a POM which has a lower LUMO level.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] Tel.: +86-10-64414640. *E-mail: [email protected] Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Financial support of the NSFC and PCSIRT (No. IRT1205) and Beijing Engineering Center for Hierarchical Catalysts is greatly acknowledged. Prof. Xue Duan of Beijing University of Chemical Technology is greatly acknowledged for his kind support.



REFERENCES

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4. CONCLUSIONS Two new covalently bonded hybrid compounds composed of porphyrin moiety and Dawson-type polyoxometalate moiety were synthesized and systematically characterized. Investigations on their NLO properties showed that integrating polyoxometalates into porphyrin moieties via covalent bonds is found capable of significantly enhancing nonlinear optical responses, especially reverse saturation absorption of porphyrins. In these porphyrin-POM systems, strong electron transformation and/or energy transfer between the porphyrin moiety acting as electron donor and the Tris-P2V3W15 moiety acting as electron acceptor are thought to play a significant role in the improved second hyperpolarizability γ values. In perspective, given the large number of porphyrins and polyoxometalates of various structures, constitutions, and properties reported in the literature, the presented results may provide us with a new guide to design and explore porphyrin-based NLO materials possessing excellent optical nonlinear responses by proper combination with polyoxometalates.



Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01118. Selected 1H NMR data listed in Table S1. Different m/z values and their corresponding formulas with relative intensities by using ESI-MS spectrum for compounds 1 and 2 listed in Table S2. TG curves of compounds 1 and 2. Third-order nonlinear optical measurements and calculation methods. UV−vis spectra highlighting the absorption edges corresponding to the HOMO−LUMO gap Eg values of the compound. (PDF) I

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