Theoretical studies on photo-triggered second-order nonlinear optical switches in a series of polyoxometalate-spiropyran compounds

Journal of Organometallic Chemistry 716 (2012) 245e251

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Theoretical studies on photo-triggered second-order nonlinear optical switches in a series of polyoxometalate-spiropyran compounds Lan-He Zhang a, b, *, Ying Wang b, Fang Ma a, Chun-Guang Liu b, c a

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China College of Chemical Engineering, Northeast Dianli University, Jilin City 132012, PR China c Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 March 2012 Received in revised form 12 June 2012 Accepted 5 July 2012

The second-order nonlinear optical (NLO) properties of a series of the polyoxometalate (POM)-spiropyran derivatives have been studied by using quantum chemical calculations. The photochemical reaction of spiropyran compounds will produce two different species, spiropyran and merocyanine. The merocyanine species has a large second-order NLO responses because of its good p-conjugated character. And thus a large contrast of the first hyperpolarizability has been obtained. According to quantum chemical calculation, this photo-triggered switchable effect derived from the spiropyran-merocyanine conversion has been enhanced because of introduction of the POM unit. Meanwhile, the substituted effect of the POM species also has been considered in this work. It can be found that introduction of the electron donor/acceptor group into the POM derivative does not significantly affect the magnitude of the first hyperpolarizability. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Spiropyran Polyoxometalate Molecular switch Nonlinear optics Density functional theory Finite field

1. Introduction Nonlinear optical (NLO) properties of molecular materials have attracted considerable attention because of their potential application in optical fibers [1], data storage [2], optical limiting [3], optical computing [4], optical switching [5], signal processing [6], etc. Compared with conventional inorganic crystalline materials, organic NLO materials have some advantages, including facile processing and reduced production cost, lower dielectric constants, and higher electro-optic coefficients et al. In the past two decades, much effort has been devoted to develop organic second-order NLO materials. A bandwidth of 150 GHz and a detectable terabit signal has been achieved by using organic NLO polymer materials [7]. The drive voltage Vp of modulators also has been reduced to below 1 V based on the organic NLO molecule [8]. Polyoxometalates (POMs) are early transition metal oxo clusters, this class of inorganic anion is unmatched not only in terms of molecular structural diversity, but also due to their rich electronic and optical properties, one of which is their electron-accepting capability which makes POMs excellent candidates as “electron

* Corresponding author. State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China. Tel./ fax: þ86 0432 64806620. E-mail address: [email protected] (C.-G. Liu). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.07.007

reservoirs” [9]. The hybrid involving inorganic POMs and organic species containing a delocalized p-conjugated system is highly desired because this hybrid entity would possess the rich scientific heritage of both species. Especially, the electronic interactions between inorganic POM cluster and the organic p-network are very interesting feature relative to the development of the second-order NLO molecular materials. In the past two decades, the organoimido derivatives of POM clusters have been extensively explored, the majority of the work mainly focused on the hexamolybdate ion [Mo6O19]2
, that is the so-called Lindquist structure [10]. A series of organic-inorganic hybrid materials based on covalently linked inorganic Lindquist-typical POM and various interesting organic ligands have been synthesized [11e13]. Reversible changes of NLO properties of molecular materials are interesting because of their potential application in molecular switches [14e22]. Second-order NLO switches may be an important procedure because they are potentially useful for the development of electro-optic devices [23]. Spiropyran compounds, which are photochromic systems undergoing reversible photochemical cleavage of the CeO bond in the spiropyran, have attracted much interest because of their good fatigue resistance and photostability [24e28]. The photochemical reaction of spiropyran compounds will produce two different species, spiropyran and merocyanine (see Chart 1). The merocyanine species has a good p-conjugated nature relevant to the

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L.-H. Zhang et al. / Journal of Organometallic Chemistry 716 (2012) 245e251

Chart 1. The photochemical reaction of the spiropyran compound.

spiropyran species. And thus it will be efficiently switching of second-order NLO responses between two states. Champagne et al have reported that the effective switchable behavior of the series of spiropyran derivatives according to their time-dependent HatreeFock (TDHF), coupled perturbed (CP) HF, and MøllerePlesset (MP2) calculations [29]. Thus, the direct combination of the two interesting molecular units (Lindquist-typical POM and spiropyran) into the same molecule provides an intriguing molecular system for designing second-order NLO molecular switch. In order to explore the switchable behavior of the inorganicorganic hybrid, POM-spiropyran species, we describe here a theoretical study on a series of POM-spiropyran derivatives for switching of second-order NLO responses between two states. 2. Computational details All of geometries were optimized at the B3LYP [30e32]/6-31g(d) level (SDD basis sets on metal atoms). The static hyperpolarizabilities were calculated by using the finite field (FF) with BHandHLYP [33] and CAM-B3LYP [34] functionals, respectively. And the frequency-dependent hyperpolarizabilities have been calculated using CPHF method with CAM-B3LYP functional. A wavelength of 1064 nm was adopted. Solvent effects have been considered and modeled using the polarized continuum model (PCM), and two different solvent, acetonitrile and dimethylsulfoxide (DMSO) were used. The 6-31g(d) basis sets on main group atoms and SDD basis sets on metal atoms were used for all hyperpolarizability calculations in this work. In density functional theory (DFT), all of the molecular properties are determined by the electron density solely. It is generally evaluated by solving the KohneSham equation which includes kinetic, Coulombic, exchange, and correlation terms. The quality of the DFT results depends on the choice of the XC functional. For the first hyperpolarizability calculation, the DFT-derived result sometimes overestimates the first hyperpolarizabilities of donor-p-conjugated bridge-acceptor (D-p-A) systems because of the incorrect longrange charge transfer behaviors between donor and acceptor. To overcome this problem, some long-rang-corrected functionals have been developed recently. In the present paper, long-range-corrected functional, CAM-B3LYP has been employed to calculate the first hyperpolarizability. However, the improvements of these functionals do not hold for all properties. Champagne and Nakano have reported that the BHandHLYP functional successfully reduced the overestimated behavior of the (hyper)polarizability from the traditional functional B3LYP for a medium-size system [35]. As a check on the long-rang-corrected effects in CAM-B3LYP functional for hyperpolarizability, hybrid functional BHandHLYP (containing 50% HF exchange) was also used for hyperpolarizability calculations. The static first hyperpolarizability, btot, for all compounds was calculated by using the following equation (eq. (1)):




btot ¼ b2x þ b2y þ b2z

1 2

In order to obtain a more intuitive description of the trends in the NLO behavior of the studied compounds, TDDFT methods were used to describe the molecular electron spectrum at CAM-B3LYP/631g(d) level [36e39]. It has been proved that TDDFT is a usefully accurate approach for many applications [40], especially, low-lying single excitations. TDDFT is the most popular method for calculating excitations currently. All of the calculations in this work were carried out by using the GAUSSIAN 09 program package [41]. 3. Results and discussion 3.1. Spiropyrans Although the second-order NLO switchable behavior based on spiropyran/merocyanine couple has been reported detailedly according to TDHF and MP2 calculations [29], we review some important results and recalculate the static first hyperpolarizability by using HF, MP2, BHandHLYP, and CAM-B3LYP functionals with 6-31g(d) basis sets here since it will be necessary for our analysis of the switching of the static first hyperpolarizability in the latter POM derivatives. Fig. 1 shows the structural formula of studied spiropyran/merocyanine couple. The calculated static first hyperpolarizability has been listed in Table 1. The MP2 calculation is the highest level among them. For the closed-ring form spiropyran (1-c), it can be found that the BHandHLYP, CAM-B3LYP, and MP2 methods provide roughly same static hyperpolarizability, and the HF method underestimates the static first hyperpolarizability when compared with the MP2 result. The HF result is w2 times as small as that of MP2-derived result. However, the DFT and HF calculations underestimates the b values of the open-ring form merocyanine (1-o) when compare with the MP2 calculation, and thus a large b contract has been achieved according to MP2/6-31g(d) calculations relevant the DFT and HF calculations. The calculated b value of 1-o is w4 times as large as that of 1-c. Although the absolute value of the DFT and HF results are different, a 2-fold increase of the static first hyperpolarizability in the spiropyran-merocyanine conversion has been confirmed (see Table 1). On the basis of the complex sum-over-states expression, Oudar and Chemla [42,43] established a simple link between the molecular hyperpolarizability and a low-lying energy charge transfer transition through the two-level model,




bf mee
mgg

 m2 ge 2 DEge

;

where mgg and mee are the ground and excited state dipole moments, mge is the transition dipole moment, and DEge is the transition energy. Those factors (meeemgg, DEge and mge) are all intimately related, and are controlled by electron properties of the donor/acceptor and the nature of the conjugated bridge. The optimal combination of these factors will provide the maximal b value. In order to get more insights of the second-order NLO responses of the studied compound, the TDDFT calculations have been performed for the excited state. The TDDFT calculated excited energy, oscillator strengths, and associated orbital transitions have

(1)

where bi is defined by (eq. (2))

bi ¼ biii þ

X

 

bijj þ bjij þ bjji 3

isj

(3)

(2) Fig. 1. Structural formula of 1-c and 1-o.

L.-H. Zhang et al. / Journal of Organometallic Chemistry 716 (2012) 245e251 Table 1 The static first hyperpolarizability b (  10
30 esu) for 1-c and 1-o obtained by HF/ MP2/BHandHLYP/CAM-B3LYP/6-31g(d) calculations. Compounds Functional 1-c

btot

Compounds Functional

HF 7.566 1-o MP2 14.246 BHandHLYP 13.900 CAM-B3LYP 14.268

HF MP2 BHandHLYP CAM-B3LYP

btot

btot(O)/ btot(C)

15.460 61.218 26.903 29.820

2.0 4.3 1.9 2.1

been listed in Table 2. The crucial excited state is defined as the lowest optically allowed excited state with substantial oscillator strength in this work. The TDDFT calculations show that the crucial excited state of the closed-ring form 1-c is the 8th excited state. It mainly contains the HOMO-3 / LUMO orbital transition (see Table 2). As shown in Table 2, the HOMO-3 is mainly localized on the electron acceptor end, electron donor end also has some contributions. And the LUMO entirely localized over the electron acceptor end. Thus, the orbital transition from HOMO-3 and LUMO would generate a charge transfer on acceptor end mixing charge transfer from donor to acceptor end. By contrast, the crucial excited state of the open-ring form 1-o is the first excited state, which arises from the HOMO / LUMO orbital transition. As shown in Table 2, HOMO and LUMO are entirely delocalized over the whole molecule. Thus, the charge transfer caused by the HOMO / LUMO orbital transition is not substantial. The openring form 1-o has the planar arrangement, and provides a good p-conjugated nature. This may account for this difference on the charge transfer excitation. In the two-level model, the square of the excited energy is inversely proportional to the b value. Thus, the low excited energy is the decisive factor in the large first hyperpolarizability. The crucial excited energies of closed-ring and open-ring forms are compared in Table 2. It can be found that the crucial excited energy of the open-ring form 1-o is w2 times as small as that of 1-c. We also note that the oscillator strength of the crucial excited state of the open-ring form 1-o is w2 times as large as that of the closedring form 1-c. The relevant low excited energy and large oscillator strength will generate a large increase in the static hyperpolarizability, which is well in agreement with the DFT-FF calculations with both functionals.

247

second-order NLO responses in the spiropyran-merocyanine conversion, the inorganic POM unit has been introduced into spiropyran/merocyanine couple to generate two new molecular systems, 2-c and 2-o. Due to the large size and consideration of the electron correction, all of the static first hyperpolarizability of the series of POM derivatives has been calculated using BHandHLYP and CAM-B3LYP functionals with 6-31g(d) basis sets in this work. The calculated static first hyperpolarizability of the series of POM derivatives has been listed in Table 3. It can be found that the introduction of the POM unit effectively enhances the second-order NLO responses. A w1.5-fold enhancement of the static first hyperpolarizability has been acquired for the closed-ring form (see Table 1 and Table 2, 2-c vs 1-c). By contrast, the b value of the openring form 2-o gets a w10 increase when compared with the merocyanine species 1-o. And thus a large contrast of second-order NLO responses has been achieved by introduction of the POM unit. The photo-triggered switchable effect on the second-order optical nonlinearity in the spiropyran-merocyanine conversion has been amplified in the POM derivative according to our DFT-FF calculation. The calculated static hyperpolarizability b value of the open-ring form is w10 times as large as that of the closed-ring form. The next question we are now concerned with is that the substituted effect on the POM derivative. It is well-known that the second-order NLO properties of D-p-A molecules are closely associated with the D/A strength and the nature of the p-conjugated bridge. Two new systems have designed to fulfill this demand in this work, where the strong electron withdrawing group, three fluorine atoms, and the typical electron donor, ferrocene group, has been introduced into POM derivative to generate systems 3-c, 3-o, 4-c, and 4-o, respectively (see Fig. 2). The calculated static hyperpolarizabilities have been listed in Table 3. Although the substituted effect affects second-order NLO responses of the series of POM derivatives according to our DFT calculations with both functionals, the change in the static first hyperpolarizability is not substantial. Such as the static hyperpolarizability of the closed-ring 4-c is w1.4 times as large as that of the closed-ring POM derivative 2-c. And the calculated b value of the open-ring form is w1.2 times as large as that of 2-o. The TDDFT calculated excited energies, oscillator strengths, and associated orbital transitions of the crucial excited state for 2c, 2-o, 3-c, 3-o, 4-c, and 4-o have been listed in Table 4. It can be Table 3 The static first hyperpolarizability b(10
30 esu) for the series of POM derivatives 2c, 2-o, 3-c, 3-o, 4-c, and 4-o (BHandHLYP/CAM-B3LYP/6-31g(d) calculations).

3.2. POM-spiropyran derivatives Although the 2e4 fold increases of the b value has been achieved in the spiropyran-merocyanine conversion according to HF, DFT, and MP2 calculations, the switchable effect is not substantial when compared with the other reported second-order NLO molecular switches. Such as, Castet and Pozzo demonstrated that a series of indolino[2,1-b]-xazolidine derivatives possess distinct NLO contrast between open and closed ring form though pH variations [44e46]. In order to reinforce the contrast of the

Compounds Functional

btot

2-c

21.101 2-o 19.266 17.611 3-o 15.974 22.719 4-o 27.668

3-c 4-c

BHandHLYP CAM-B3LYP BHandHLYP CAM-B3LYP BHandHLYP CAM-B3LYP

Compounds Functional BHandHLYP CAM-B3LYP BHandHLYP CAM-B3LYP BHandHLYP CAM-B3LYP

btot

btot(O)/ btot(C)

242.155 261.388 259.061 264.257 307.090 320.683

11.5 13.6 14.7 16.5 13.5 11.6

Table 2 The TDDFT result for spiropyran and merocyanine species obtained by the CAM-B3LYP/6-31g(d) calculations.

DEge (eV)

Contributions

0.4931

5.10

HOMO-3 / LUMO (63%)

0.8785

2.77

HOMO / LUMO(98%)

Compound

Excited state

f

1-c

S8

1-o

S1

os

Occupied orbital

Unoccupied orbital

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L.-H. Zhang et al. / Journal of Organometallic Chemistry 716 (2012) 245e251

Fig. 2. Structural formula of the series of POM derivatives 2-c, 2-o, 3-c, 3-o, 4-c, and 4-o.

found that the first excited state of the closed-ring POM derivative, 2-c, is generated by the promotion of one electron from HOMO to LUMO. The HOMO localizes on the MoeN moiety and the benzene ring attached the POM unit, and LUMO delocalizes over the electron acceptor end. This excitation consists of strong

charge transfer from MoeN moiety and benzene ring to electron acceptor. However, the little overlap between the two orbitals forbidden such electron transition, reflecting the small oscillator strength. The crucial excited state of 2-c is also shown in Table 4. It mainly comes from the HOMO / LUMOþ4 transition according

Table 4 The TDDFT result for all compounds obtained by the CAM-B3LYP/6-31g(d) calculations.

DEge (eV)

Contributions

0.04

2.90

HOMO / LUMO(95%)

S5a

0.46

3.52

HOMO / LUMOþ4(57%)

2-o

S1a

1.61

2.42

HOMO / LUMO(85%)

3-c

S5a

0.48

3.52

HOMO / LUMOþ4(52%)

3-o

S1a

1.51

2.45

HOMO / LUMO(86%)

4-c

S12a

0.57

3.54

HOMO / LUMOþ4(50%)

4-o

S3a

1.71

2.40

HOMO / LUMO(84%)

Compound

Excited state

f

2-c

S1

a

The crucial excited state.

os

Occupied orbital

Unoccupied orbital

L.-H. Zhang et al. / Journal of Organometallic Chemistry 716 (2012) 245e251

to our TDDFT calculation, these excitations mostly consist of charge transfer from the benzene ring to the POM unit. This indicates that the nitrobenzene moiety does not display the electron acceptor character because the non-planar structure reduces the molecular conjugation. By contrast, the crucial excitation of the open-ring POM derivative, 2-o, is the first excited state, and it arises from the HOMO / LUMO transition. As shown in Table 4, the electron density in the two orbitals is mainly localized on the organic segment. The POM unit has hardly any contributions. However, a detailed comparison shows that introduction of the POM unit increases that electron density distribution on the benzene ring adjacent the POM unit when compared with the open-ring non-derivative system 1-o (HOMO of 1-o vs HOMO of 2-o), and thus gives a stronger charge transfer than that of the 1-o. which would be favorable to enhance second-order NLO response of the POM derivative. As shown in Table 4, the excited energies of the crucial excited state of openring form 2-o are lower than that of closed-ring form 2-c because of its good conjugated character. We also note that the oscillator strength of the crucial excited state of the open-ring form 2-o is w3 times as large as that of the closed-ring form 2-c. As mentioned above, the relevant low excited energy and large oscillator strength will enhance the second-order NLO responses. This result is supported by the DFT-FF calculations in this work. Compared with the POM derivative 2-c and 2-o, introduction of the three fluorine atom and ferrocene group does not significantly affect the magnitude of the crucial excited state, and thus they have analogous second-order NLO responses. For the ferrocene derivative 4-c and 4-o, excited energy and associated orbital transitions of the crucial excited state are basically similar to the POM derivatives 2-c and 2-o. But the oscillator strengths of 4-c and 4-o are slightly larger than that of the POM derivatives 2-c and 2-o. Compared with the 4-c, 4-o gives a relevant large change on the excited energy and oscillator strength, and thus it gives a large enhancement on the static first hyperpolarizability relative to 2-o. 3.3. Conformers in POM derivatives For the merocyanine forms, rotation around the two CeC single bonds may generate four different stable conformers. In the present paper, we took the POM derivative 4-o as an example to discuss the conformer consideration. As shown in Fig. 3, the four conformers have been optimized at B3LYP/6-31g(d) level. The calculated energy and the static first hyperpolarizability of the four conformers have been listed in Table 5. It can be found that the magnitude of the energy decreases in the order, Conformer-

249

Table 5 The static first hyperpolarizability b (10
30 esu) (BHandHLYP/CAM-B3LYP/6-31g(d) calculations) and energy (B3LYP/6-31g(d) calculations) for the four POM conformers (SDD basis sets on the metal atom). Compounds

Energy (kcal/mol)

Conformer-1 Conformer-2 Conformer-3 Conformer-4

2.44 1.70 2.16 0.00

btot CAM-B3LYP

BHandHLYP

330.164 321.153 375.310 320.683

316.967 308.626 358.792 307.090

1 > Conformer-3 > Conformer-2 > Conformer-4. The stable species Conformer-4 is only w2.44 kcal mol
1 more stable than that of the highest energy species Conformer-1. The small differences on the energy suggest that it may be a statistical distribution of the four possible conformers. Thus, it is necessary to check second-order NLO properties of these conformers because the experimental measurement on the second-order NLO response will obtain the average values of these conformers. As shown in Table 5, the static first hyperpolarizability of the four conformers changes in the range from the 300  10
30 esu to 380  10
30 esu. Thus the conformer effect on the second-order NLO property is not substantial. 3.4. Frequency-dependent hyperpolarizability The frequency-dependent hyperpolarizabilities in acetonitrile and DMSO have been calculated using CPHF method with CAMB3LYP functional for all studied compounds. The secondharmonic generation (SHG) process, frequency-dependent hyperpolarizability b (-2u; u, u) has been calculated in this work. It is important to produce theoretical results that compare more directly to the experiment for the SHG. In all the studied systems, the charge transfer axes are chosen as Z-axis. As a result, the b tensor will be dominated by the b1064 zzz component. In the experiments, combinations of different components are often used. Thus, also has been listed in Table 6. the b1064 z As shown in Table 6, the solvent and frequency dispersion effects lead to a large enhancement of the frequency-dependent hyperpolarizability for each compound relevant to its static value. For the POM derivative, the calculated frequency-dependent hyperpolarizabilities are sensitive to the substituted effect. But of all POM derivatives are roughly on the the magnitudes of b1064 z same order. The relative values of closed- and open-ring forms are very important in this work because it is directly associated with the

Fig. 3. Structural formula of four POM conformers.

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L.-H. Zhang et al. / Journal of Organometallic Chemistry 716 (2012) 245e251

Table 6 The frequency-dependent first hyperpolarizability (10
30 esu) of all compounds (CPHF method with CAM-B3LYP/6-31g(d) calculations (SDD basis sets)). Compounds

Solvents

b1064 zzz

b1064 z

Compounds

Solvents

b1064 zzz

b1064 z

b1064 (o)/b1064 (c) z z

1-c

DMSO Acetonitrile DMSO Acetonitrile DMSO Acetonitrile DMSO Acetonitrile


41.32
39.57
457.12
423.71
433.07
401.87
136.22
360.22


123.61
118.36
1328.26
1229.81
1270.11
1176.44
250.28
1007.14

1-o

DMSO Acetonitrile DMSO Acetonitrile DMSO Acetonitrile DMSO Acetonitrile


22.05
23.34
2184.11
1888.62
897.417
795.418
212.80
1819.12


222.08
213.44
6652.47
5746.71
2871.83
2545.03
726.52
4948.13

1.80 1.80 5.01 4.67 2.26 2.16 2.90 4.91

2-c 3-c 4-c

2-o 3-o 4-o

switching effects of the second-order NLO properties. As mentioned above, a w2 fold enhancement of the static first hyperpolarizabilities of the spiropyran molecule for the conversion from the closed-ring form to the open-ring form has been observed according to CAM-B3LYP calculations (see Table 1). The frequencydependent hyperpolarizability calculations in different solvents show that this conversion leads to a 1.8 fold increase in DMSO and acetonitrile for this conversion. This indicates that a static prediction may provide a rational result for switchable effect of the spiropyran molecule to their second-order NLO properties. However, for the POM derivative, the static hyperpolarizability calculations predict that the conversion from the closed-ring form to open-ring form leads a very large enhancement. The calculated static hyperpolarizability of the open-ring form is w10 times as large as that of the closed-ring form in the series of POM derivative (see Table 3). The calculated frequency-dependent hyperpolarizabilities of the series of POM derivative in acetonitrile and DMSO show that this conversion only leads to a 2e5 fold increase (see Table 6). It should be stressed that the highly charged anions do not exist in the gas phase, moreover, the nature of molecular orbital of the charged species is sensitive to the solvent effect in quantum chemical calculations. Thus, this large change on the relative values for the two forms may come from the high charged nature of POM derivative, which indicates that the solvent and frequency dispersion effects of the charged species are important for their second-order NLO properties. Compared with the spiropyran molecule (1-c and 1-o), introduction of the POM cluster still amplifies the switchable behavior on their second-order NLO responses. Such as, the frequency-dependent hyperpolarizability of the open-ring form 2-o is w5 times as large as that of the closed-ring form 2-c in acetonitrile, which is still larger than a 2-fold increase of spiropyran molecule (1-c and 1-o). 4. Conclusion In summary, the static and frequency-dependent first hyperpolarizability of a series of POM-spiropyran derivative has been calculated using quantum chemical calculation. It is found that the photo-triggered switchable effect derived from the spiropyranmerocyanine conversion has been enhanced because of introduction of the POM unit. The substituted effect on the POM derivative also has bee considered in the present paper. Although the substituted effect affects second-order NLO responses of the series of POM derivatives, the change in the static first hyperpolarizability is not substantial. The present TDDFT results rationalize the change of the static hyperpolarizability caused by the series of photoisomerization and substitution effects. Acknowledgments The project was supported by Open Project of State Key Laboratory of Urban Water Resource and Environment and Harbin Institute of Technology (No.QA201013), Scientific Research

Foundation for Doctor of Northeast Dianli University (BSJXM201110). Society Development Foundation for Science and Technology of Jilin Province (20110405), Society Development Foundation for Science and Technology of Jilin City (201132402), Education Research Foundation for Science and Technology of Jilin Province (2012-95).

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