Quantum chemical study on first hyperpolarizabilities of mono- and bimetal Pt(II) diimine complexes

Journal of Organometallic Chemistry 718 (2012) 1e7

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Quantum chemical study on first hyperpolarizabilities of mono- and bimetal Pt(II) diimine complexes Meng-Ying Zhang, Na-Na Ma, Shi-Ling Sun, Xiu-Xin Sun, Yong-Qing Qiu*, Bin Chen** Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 May 2012 Received in revised form 31 July 2012 Accepted 2 August 2012

The static first hyperpolarizabilities (bvec) of ligand L (N,N0 -bis(4-methoxyphenyl)ethylenediimine) and its Pt(II) chelated complexes have been calculated by density functional theory (DFT) method. The results show that the bvec values of Pt(II) diimine complexes range from w4.2 to w198.8 times larger than that of ligand L, and the bvec values of Pt(II) complexes with coordinated atom O are larger than those of the corresponding complexes with S. Notably, the bimetallic complex 3a possesses the largest bvec value in the studied systems, w1498.86  1030 esu, which is w46.6, w30.5, and w3.6 times as large as those of complexes 1a, 2a and 3b, respectively. In addition, the dynamic first hyperpolarizabilities (bvec(u)) of complex 1a are also calculated at the input photon energy from 0.1 to 1.8 eV. Our present work would be beneficial for further theoretical and experimental studies on large second-order nonlinear optical (NLO) responses of metal complexes. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Diimine ligand Pt(II) diimine complex Second-order NLO DFT

1. Introduction Nonlinear optical (NLO) materials have played critical role in the domains of photoelectronic modulators for optical telecommunications [1], and photonic applications such as optical datamanipulations, storage, and electrooptical devices [2e4], so that searching for the excellent NLO molecular materials has become the spotlight for researchers. Among NLO materials, the organic molecules have been in the focus owing to the structure with electron-donor and electron-acceptor groups bridged through a pdelocalized backbone [5e11]. However, compared to conventional organic materials, organometallic complexes represent the additional advantages of optical properties, high environmental stability and a variety of molecular structures in relation to the metal nd configuration, spin state, and oxidation state [12e16]. Moreover, organometallic complexes possess several intense, lowenergy metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT) or metal-to-metal/intervalence charge transfer (MM/IVCT) excitations [17e19]. Therefore, organometallic complexes have been the excellent NLO materials. In recent years, diimine chromophore with strong electronacceptor characteristic has been widely used to tune the NLO * Corresponding author. Fax: þ86 431 85098768. ** Corresponding author. E-mail addresses: [email protected] (Y.-Q. Qiu), [email protected] (B. Chen). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.08.001

responses of molecular materials [20,21]. For instance, the novel donoreacceptor M (M ¼ Pt, Pd, Ni) diimine dithiolate complexes, which are thermally stable noncentrosymmetric metal diimine complexes, have been synthesized and measured with sizable second-order NLO responses by Cummings and co-workers [22]. Moreover, the second-order NLO responses of metal diimine complexes could be enhanced by modifying the central metal and pushepull moieties [23e25]. In general, these metal diimine complexes show the typical donorepeacceptor (DepeA) structure, in which the metal ion acts as a bridge or an electron-donor and plays an important role in controlling the molecular second-order NLO responses. A series of a-diimine catecholatoePt(II) complexes, such as monometallic platinum complexes [Pt(II)(dbcat)L0 ], (L0 ¼ 4-[(pyridine-2-ylmethylene)amino]phenol, dbcat ¼ 3,6-di-ter-butyl-cateolato), [Pt(II)(O2phen)L0 ] (O2phen ¼ 1,10-phenanthroline-5,6diolate) and bimetallic platinum complex [Pt(II)(O2phen)Pt(II)(dbcat)L0 ] has been studied by Heinze and co-workers [26]. The report points out that this class of complexes displays intense green color because of the catecholato / diimine ligand-to-ligand charge transfer (LLCT) bands. Recently, a novel platinum complex [Pt(II)(dbcat)L], (L ¼ N,N0 -bis(4-methoxyphenyl)ethylenediimine) (1a in Fig. 1) has also been synthesized and characterized by Liu and Heinze [27]. They point out that the central Pt(II) in complex [Pt(II)(dbcat)L] adopts a nearly square planar coordination geometry and the diimine moiety shows a strong electron-acceptor characteristic. Although these complexes with the DepeA

2

M.-Y. Zhang et al. / Journal of Organometallic Chemistry 718 (2012) 1e7

O

CH3

O

N

N

N

N

CH3 O

CH3

N

X Pt

N

X

N

Pt X

N

O

O

O

CH3

CH3

X

CH3

X=O 2a X=S 2b

X=O 1a X=S 1b

L

O

CH3

z 1

N1

X1

N3

Pt 1

Pt 2

2

N2

x

X3

X2

N4

X4

O CH3

X=O 3a X=S 3b Fig. 1. Structural formulas of the studied systems.

structure may possess the exceptional NLO responses, their first hyperpolarizabilities have not been investigated. Moreover, Cummings et al. have investigated that a series of metal complexes bearing diimine accepter and dithiolate donor ligands possesses the large second-order NLO responses [22]. In this paper, we not only explore the influences of mono- and bimetallic complexes on the second-order NLO responses, but also consider the effect of metal coordinated atom (X ¼ O, S). Thus, the studied systems of the phenolate-substituted diimine ligand (L) and its Pt(II) chelated complexes (series a and b complexes) have been designed (Fig. 1). The series a complexes are monometallic and bimetallic Pt(II) complexes bearing different ligands (dbcat, O2phen), and the series b complexes are obtained by replacing the metal coordinated atom O with S. 2. Computational details The Becke’s three-parameter exchange functional [28] combined with the LeeeYangeParr correlation functional [29] (B3LYP) shows the advantage to optimize the geometries of

medium-sized molecules [30]. Thus, the geometries of all the studied systems have been fully optimized without symmetry constraint by using the B3LYP functional. The standard 6e31G(d) polarized double-z basis set is for H, C, N, O and S atoms, and taking into account the relativistic effect, the effective core potential (ECP) double-z (DZ) basis set of LanL2DZ is for Pt(II). All geometries were characterized as minima by frequency analysis (Nimag ¼ 0). Time-dependent density functional theory (TD-DFT) has been used to predict the optoelectronic properties of molecules in quantum chemistry [31,32], and PBE1PBE functional has great accuracy of analyzing charge transfer bands and transition energies in metal complexes for both solution and gas phase calculations [33,34], thus the electron absorption spectra of the studied systems have been calculated at the PBE1PBE/6e31 þ G(d) (LanL2DZ basis set on Pt(II)) level. However, according to the earlier reports [14,35], the solvent effect exerts critical influence on the absorption spectra features. Thus, considered the solvent effect, the electronic excitation was calculated by using polarizable continuum model (PCM) [36,37] in acetonitrile (CH3CN) solution for the studied systems.

M.-Y. Zhang et al. / Journal of Organometallic Chemistry 718 (2012) 1e7

The hybrid DFT functional mPWPW91* is an applicable approach for the first hyperpolarizabilities of transition metal complexes [14,38], and M06 as a novel hybrid metal functional shows good accuracy “across-the-board” for transition metals [39]. Therefore, the static first hyperpolarizabilities of the studied systems were calculated by the finite field (FF) approach at mPWPW91*/6e31 þ G(d) and M06/6e31 þ G(d) (LanL2DZ basis set on Pt(II)) levels. Finally, the frequency-dependent first hyperpolarizabilities of complex 1a were calculated by the coupled perturbed density function theory (CPDFT) by the M06 functional and the same basis set. The static first hyperpolarizability (bvec) is calculated according to Equation (1)

bvec ¼

X m bi i m j j i ¼ x;y;z

chosen the long-range-corrected functional CAM-B3LYP method [43,44] to optimize complex 1a. The optimized geometric parameters of complex 1a obtained by the two methods are both in good agreement with its crystallographic data, however, the B3LYP method costs less than CAM-B3LYP method. And the optimization of our designed bimetallic Pt(II) complexes may cost more than those of the monometallic Pt(II) complexes. Therefore, the geometry structures of ligand L and its Pt(II) chelated complexes series a (X ¼ O) and b (X ¼ S) with all real frequency have been obtained by B3LYP/6e31G(d) (LanL2DZ basis set on Pt(II)) method. The selected experimental and calculated geometric parameters are listed in Table 1 and the atom labels are shown in Fig. 1. From Table 1, compared with ligand L, the C1 ¼ N1 and the C2 ¼ N2 bond distances of metal complexes increase by 0.035e0.051  A, 0.029e 0.050  A, respectively, whereas the C1eC2 bond distances decrease by 0.051e0.067  A. These indicate that the conjugation of ligand L in the metal complexes changes greatly, and the strong interactions of PteN coordinated bond lead to the longer C]N bond distances. Furthermore, for the metal complexes, the coordinated bond distances PteN and PteX, bond angles N1ePt1eN2 and X1ePt1eX2 change slightly from 1a to 3a, and 1b to 3b. However, there is a large diversity between series a and b complexes. For instance, the bond distances Pt1eN and Pt1eO of complex 1a are about 0.070  A and 0.292  A shorter than the bond distances Pt1eN and Pt1eS of complex 1b, respectively, and the bond angle O1ePt1eO2 of complex 1a is about 5 smaller than the bond angle S1ePt1eS2 of complex 1b. Likewise, a similar change is found between 2a and 2b, 3a and 3b. These imply that the interactions between metal and coordinated atom of series a complexes are stronger than those of series b complexes. These changes on the structures will inevitably affect the molecular NLO properties.

(1)

in which bi is defined by Equation (2)

bi ¼

 3 b þ bijj þ bikk 5 iii

i; j; k ¼ x; y; z

(2)

and the frequency-dependent first hyperpolarizability bvec(u) is calculated according to Equation (3)

bvec ðuÞ ¼

X m b i ð uÞ i jmj i ¼ x;y;z

(3)

in which bi(u) is defined by Equation (4)

bi ðuÞ ¼ biii þ

 1 X b þ bjij þ bjji 3 jsi ijj

i; j ¼ x; y; z

3

(4)

where bvec and bvec(u) refer to the static first hyperpolarizability and frequency-dependent first hyperpolarizability along the molecular dipole moment, respectively, and mi denotes the dipole moment along i direction of ground state. All of the calculations were carried out using the Gaussian 09W program package [40]. Density-of-States (DOS) was calculated using the AOMix program [41,42].

3.2. HOMO and LUMO analysis In order to get a further understanding of the structures of ligand L and its Pt(II) complexes, their electronic structures have been probed. The calculated orbital energies and orbital distributions of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are shown in Fig. 2. From Fig. 2, the HOMO and LUMO energy gap (Egap) values of Pt(II) complexes are smaller than that of ligand L. And the Egap values show a decreasing trend from 1a to 3a and 1b to 3b, which may gradually increase their bvec values. Moreover, the Egap values of series a complexes are in the range of 0.32e0.40 eV smaller than those of series b complexes, which implies that the bvec values of series a complexes may be larger than those of series b complexes.

3. Results and discussion 3.1. Chemical models and geometries B3LYP method shows the good accuracy to optimize the geometries of medium-sized molecules [30], and we have also

Table 1 Selected experimental and bond distances ( A) and bond angles ( ) of the studied systems obtained at the B3LYP/6e31G(d)/LanL2DZ level. System

L

1a

1aexp

2a

3a

1b

2b

3b

Pt1eN1 Pt1eN2 Pt1eX1 Pt1eX2 N1 ¼ C1 N2 ¼ C2 C1eC2 N1ePt1eN2 X1ePt1eX2 Pt2eN3 Pt2eN4 Pt2eX3 Pt2eX4 N3ePt2eN4 X3ePt2eX4

e e e e 1.283 1.283 1.468 e e e e e e e e

2.023 2.022 1.996 1.996 1.333 1.333 1.402 79.3 80.8 e e e e e e

1.992 1.990 1.975 1.974 1.315 1.312 1.417 78.8 82.1 e e e e e e

2.017 2.016 2.012 2.011 1.334 1.334 1.402 79.7 81.2 e e e e e e

2.022 2.020 2.018 2.018 1.331 1.330 1.406 79.7 81.5 2.037 2.039 1.992 1.992 80.3 82.7

2.089 2.087 2.288 2.288 1.323 1.323 1.412 78.0 86.3 e e e e e e

2.082 2.082 2.304 2.303 1.323 1.322 1.413 78.3 87.3 e e e e e e

2.085 2.083 2.307 2.307 1.319 1.319 1.417 78.3 87.5 2.105 2.105 2.289 2.289 78.2 86.6

exp

Experimental values are from Ref. [27].

4

M.-Y. Zhang et al. / Journal of Organometallic Chemistry 718 (2012) 1e7 Table 2 Experimental and theoretical data calculated at the TD-PBE1PBE/6e31 þ G(d)/ LanL2DZ level for the studied systems (wavelength l (nm), transition energy DE (eV), oscillator strength (f), and the corresponding dominant MO transitions). System

l (nm)

DE (eV)

f (a.u.)

Contributions

L

438

2.83

0.9237

H / Lb(95%)

1a

502 421(430)a

2.47 2.95

0.4384 0.1169

H-2 / L(94%) H-5 / L(88%)

2a 3a 1b 2b 3b

561 560 495 515 656

2.21 2.21 2.50 2.41 1.89

0.2646 0.3255 0.4722 0.2526 0.3031

H-1 H-3 H-2 H-2 H-2

a b

/ / / / /

L(81%) L(83%), H-8 / L(7%) L(97%) L(71%), H-4 / L(19%) L(97%)

Experimental value in parentheses comes from Ref. [27]. H ¼ HOMO and L ¼ LUMO.

Fig. 2. The energies and distributions of HOMO and LUMO for the studied systems.

Analyzing the HOMO and LUMO distributions as plotted in Fig. 2, two interesting results are observed: (1) For ligand L, the HOMO and LUMO are both localized on diimine and phenyl moieties, thus there is no obvious charge transfer from HOMO to LUMO. (2) For Pt(II) diimine complexes, their LUMOs are mainly located on diimine, phenyl moieties and Pt1(II), whereas their HOMOs are mainly located on the right terminal moieties. For series a complexes, the HOMO of complex 1a is mainly centered on dbcat moiety, the HOMO of complex 2a is mainly centered on O2phen moiety, and the HOMO for complex 3a is chiefly centered on Pt2(II) and terminal dbcat moiety. Obviously, the degrees of the charge transfers from HOMO to LUMO are increasing from complexes 1a to 3a. For series b complexes, the similar charge transfers from HOMO to LUMO are observed. The increases of the degrees of charge transfers may lead to the enhancement of the static first hyperpolarizabilities from 1a to 3a and 1b to 3b, respectively.

diimine moiety and the LMCT transition from phenyl to Pt1(II). Compared with complex 1a, the intense absorption peaks of complexes 2a and 3a display red shifts and locate at 561 nm and 560 nm, respectively. Those are attributed to the O2phen / diimine LLCT transition and Pt2(II) / diimine MLCT, Pt2(II) / Pt1(II) MMCT, dbcat / diimine LLCT, O2phen / diimine LLCT transitions, respectively. Notably, compared with complexes 1a and 2a, complex 3a possesses an extra Pt2(II) / Pt1(II) MMCT transition along the negative x-axis. That is, the bimetallic Pt(II) complex is more helpful for larger CT. Likewise, for series b complexes, their intense absorption bands are found at 495 nm, 515 nm, and 656 nm, respectively, which emerges red shifts from complexes 1b to 3b. The absorption band of 495 nm for complex 1b can be characterized as the phenyl / diimine LLCT transition. While the absorption bands complexes 2b and 3b are attributed to the phenyl / diimine and S2phen / diimine LLCT transitions, and S2phen / diimine LLCT transition, respectively. It’s obvious that the degree of LLCT transition is increasing from complexes 1b to 3b.

3.3. Absorption spectra 3.4. Static first hyperpolarizability For a comprehensive qualitative and quantitative analysis of electronic spectra, TD-DFT has been used to probe the electronic spectra and transition energies of crucial excited states for the studied systems. The PBE1PBE functional has great accuracy of analyzing charge transfer bands and transition energies in metal complexes for both solution and gas phase calculations [33,34], and the corresponding long-range-corrected functional LC-wPBE method [45,46] have also been used to calculate the absorption spectrum of the complex 1a, whose experimental absorption peak is at 430 nm. The calculated absorption peak of complex 1a is at 359.8 nm, which is about 70 nm smaller than that of the experimental absorption peak. However the calculated absorption peak by PBE1PBE method is about 9 nm smaller than that of the experimental absorption peak. Hence, the absorption spectra of the studied systems calculated by TD-PBE1PBE functional are reliable. The major absorption wavelengths with the corresponding transition energies, oscillator strengths and contributions on basis of TDPBE1PBE calculations are shown in Table 2. The molecular orbitals corresponding to the dominant electron transitions of all systems are plotted in Fig. 3. As shown in Table 2 and Fig. 3, the experimental absorption peak of 430 nm for complex 1a is attributed to the charge transfers from Pt1(II) and dbcat moiety (HOMO-5) to diimine moiety (LUMO) according to the PCM model calculations. That is categorized as MLCT and LLCT transitions, respectively. Moreover, the calculated spectrum of complex 1a also exhibits a more intense absorption peak at 502 nm with the maximal oscillator strength and the lower transition energy, which is attributed to the LLCT transition from phenyl to

The static first hyperpolarizability (bvec) was calculated by two different functionals mPWPW91* and M06 on the basis of the optimized geometries. The calculated ground state dipole moment m together with the component mx, my, mz, and the bvec together with the component bx, by, bz are listed in Table 3. The two methods show the same trend of the bvec values, however, all the bvec values obtained by the M06 method are smaller than those by the mPWPW91* method except for ligand L and complex 3b. Because the HF exchange percents of the two methods are different, in which the mPWPW91* and M06 methods possess the 40% and 27% of HF exchange, respectively. Consequently, we will discuss the bvec values of the studied systems according to the results obtained by the M06 functional. Moreover, the bvec values increase gradually with the increasing the number of Pt(II) and the p-conjugation groups. These NLO mechanisms of the studied systems have been ascribed to the increased degrees of MLCT/LMCT/MMCT, the extended p-conjugation ligand and the descending transition energy (DE). From Table 3, we can see: (1) Compared with the bvec value of ligand L, the bvec values of Pt(II) diimine complexes are in the range of w4.2 to w198.8 times larger than that of ligand L. This indicates that MLCT/LMCT/MMCT of the Pt(II) complexes lead to the significant enhancement in bvec values according to the TD-DFT calculations. (2) For the Pt(II) diimine complexes, their bvec values increase with the increasing the p-conjugation groups and the number of Pt(II). For series a complexes, the order of bvec values is bvec (1a) < bvec (2a) < bvec (3a). There into, the bvec value of complex 2a

M.-Y. Zhang et al. / Journal of Organometallic Chemistry 718 (2012) 1e7

5

Fig. 3. The molecular orbitals related to the dominant electron transitions for the studied systems.

is w1.5 times as large as that of complex 1a due to the strong pconjugated O2phen groups along the positive x-axis. And the bvec value of complex 3a is w46.6 and w30.5 times as large as those of complexes 1a and 2a, respectively, which attributes to the contribution from the bimetallic Pt(II) ions. As for series b complexes, a similar trend of bvec values is found: bvec (1b) < bvec (2b) < bvec (3b), in which the bvec value of complex 3b is w13.2 and w8.5 times as large as those of complexes 1b and 2b, respectively. (3) Compared series a with b complexes, it is obvious to find the following order: bvec (1a) > bvec (1b), bvec (2a) > bvec (2b), bvec (3a) > bvec (3b). Especially, the bvec value of complex 3a is w3.6 times as large as that of complex 3b. As described in the part

“Chemical Models and Geometries”, the coordinated bond distances PteN and PteS, bond angle S1ePt1eS2 of series a complexes are smaller than those of series b complexes, respectively. These indicate that coordinated atom S weakens the conjugation features of series b complexes, and thus the second-order NLO responses of series a complexes are larger than those of series b complexes. To gain an intuitive understanding of the origin of the static first hyperpolarizability values, we consider the two-level model that linked bvec and a low-lying CT transition [47]. According to the twolevel model, the third power of the transition energy (DE) is inversely proportional to the bvec value, which is a decisive factor in

Table 3 Dipole moment of ground state m (Debye) and static first hyperpolarizability bvec (1030 esu) for the studied systems. System

B3LYP

mx

Functional

my

mz

bx

by

bz

bvec

m

L

1.82

0.65

0.10

1.94

M06 mPWPW91*

7.82 6.16

0.66 0.42

0.71 0.97

7.54 5.89

1a

6.24

0.26

0.07

6.24

M06 mPWPW91*

32.24 66.58

0.40 1.13

0.86 1.27

32.18 66.46

2a

9.47

1.15

0.54

9.56

M06 mPWPW91*

49.93 98.76

2.47 4.23

0.59 1.81

49.22 97.48

3a

19.81

1.73

0.54

9.56

M06 mPWPW91*

1502.58 8675.36

9.06 66.84

61.46 347.41

1498.86 8654.94

1b

6.07

0.10

0.01

6.08

M06 mPWPW91*

31.34 47.89

0.47 0.43

0.52 0.73

31.32 47.89

2b

10.31

1.44

0.43

10.42

M06 mPWPW91*

49.09 67.43

1.20 1.19

0.75 1.32

48.44 66.61

3b

22.20

2.04

0.84

22.31

M06 mPWPW91*

415.65 225.16

4.09 1.89

1.79 1.00

414.05 224.26

6

M.-Y. Zhang et al. / Journal of Organometallic Chemistry 718 (2012) 1e7

the static first hyperpolarizability, and the oscillator strength (f) is proportional to the bvec value. The transition energies and oscillator strengths of the crucial excited states obtained by the TD-PBE1PBE method are listed in Table 2. The DE value of ligand L is the largest, and the DE values of the complexes show a descending trend from 1a to 2a and 1b to 3b, which is consistent with the calculated order of bvec values (bvec (L) < bvec (1a) < bvec (2a), bvec (L) < bvec (1b) < bvec (2b) < bvec (3b)). Although the DE value of complex 3a is equal to that of complex 2a, the f value of complex 3a is larger than that of complex 2a, thus the bvec value of complex 3a is larger than that of complex 2a. In addition, compared with complexes 3a and 3b, though the crucial DE value of complex 3a is larger than that of complex 3b, the complex 3a possesses an extra Pt2(II) / Pt1(II) MMCT transition according to the TD-DFT calculations (Fig. 3). Hence, the order of bvec values is bvec (3a) > bvec (3b). Naturally, the bvec values of the studied systems are also in accordance with the analyses of the geometry structures, electronic structure and the energy gap (Egap) above. To compare with other realistic Pt(II) complexes, whose experimental first hyperpolarizabilities have been reported, we have searched the corresponding articles and found that Cummings has reported a series of monometallic Pt(diimine)(dithiolate) complexes with experimental first hyperpolarizabilities [22]. The Pt(dmbpy)(tdt) (dmbpy ¼ 4,40 -dimethyl-2,20 -bipyridine, tdt ¼ 3,4toluenedithiol) and Pt(phen)(tdt) (phen ¼ 1,10-phenanthroline) complexes have been chosen from the synthetic monometallic Pt(diimine)(dithiolate) complexes, because their geometry structures are similar to the studied monometallic complexes. The experimental second-order NLO responses of the Pt(dmbpy)(tdt) and Pt(phen)(tdt) complexes are 25  1030 esu and 28  1030 esu, respectively, which are obtained at 1907 nm fundamental radiation in the experiment. The bvec(u) values of these complexes were calculated by M06 and mPWPW91* methods at 1907 nm. The calculated bvec(u) values of the Pt(dmbpy)(tdt) and Pt(phen)(tdt) complexes are 51  1030 esu and 54  1030 esu calculated by M06 method, and 21  1030 esu, 23  1030 esu calculated by mPWPW91* method, respectively. The trend of the bvec(u) values calculated by the two methods is same, however, the bvec(u) values calculated by mPWPW91* method are more approximate to the experimental second-order NLO responses. Hence, the mPWPW91* method may give the better values for our studied systems. Compared with these complexes, the first hyperpolarizabilities of our designed mono- and bimetallic Pt(II) diimine complexes are more larger. There into, the bvec absolute value of complex 3a calculated by mPWPW91* method is about w309 and

w346 times larger than that of Pt(dmbpy)(tdt) and Pt(phen)(tdt) complexes, respectively. 3.5. Density of states calculation In order to further investigate on the static first hyperpolarizabilities for the bimetallic complexes 3a and 3b according to their electronic structures, the total density of states (TDOS) and projected partial density of states (PDOS) calculations have been obtained by using the AOMix program. As plotted in Fig. 4, for the complex 3a, HOMO-3 is mainly comprised of 43.57% d orbital from Pt2(II) and 39.05% p orbital from O2phen moiety, and HOMO-8 is mainly comprised of 72.69% p orbital from dbcat moiety and 16.45% d orbital from Pt1(II). Whereas, the LUMO of the complex 3a is mainly made up of 10.57% d orbital from Pt1(II), 70.99% p* orbital from ligand L, and 13.79% orbital from dbcat moiety. These indicate that there are very substantial mixed p (O2phen and dbcat) / p* (diimine) LLCT, d(Pt2(II)) / p* (diimine) MLCT and d(Pt2(II)) / d(Pt1(II)) MMCT transitions, which are helpful for larger bvec value of complex 3a. However, for the complex 3b, HOMO-2 is mainly comprised of 73.3% p orbital from S2phen moiety, and LUMO is mainly made up of 76.50% p* orbital from ligand L, so that the electron transition of complex 3b is attributed to p (S2phen) / p* (diimine) LLCT transition. Obviously, the extra MLCT and MMCT transitions along the negative x-axis for the complex 3a lead to its larger bvec value. 3.6. Frequency-dependent first hyperpolarizability of complex 1a To investigate the effect of dispersions on complex 1a, which has been synthesized, the frequency-dependent first hyperpolarizabilities (bvec(u)) were computed by using CPDFT method at the M06/6e31 þ G(d) (LanL2DZ basis set on Pt(II)) level. Because the M06 method costs less than mPWPW91* method when being used to calculate the static first hyperpolarizabilities of the studied systems. The magnitudes of the b (2u; u, u) and b (u; u, 0) values of complex 1a at the input photon energy from 0.1 to 1.8 eV are plotted in Fig. 5. Clearly, the b (2u; u, u) values vary slowly at the input photon energy from 0.1 to 0.7 eV, and then dramatically increase to 454.38  1030 esu with the input photon energy at 1.4 eV, which can be attributed to its larger resonances or dispersions at 0.7 eV according to the TD-DFT results. While the b (2u; u, u) values decrease at the input photon energy from 1.4 to 1.8 eV. However, the trend of the b (u; u, 0) values is different from those of the b (2u; u, u) values, and the b (u; u, 0) values increase

Fig. 4. Total and partial density of states (TDOS and PDOS) for complexes 3a and 3b.

M.-Y. Zhang et al. / Journal of Organometallic Chemistry 718 (2012) 1e7

Fig. 5. Calculated magnitudes of frequency-dependent first hyperpolarizability values for complex 1a.

slowly at the input photon energy from 0.1 to 1.8 eV. Therefore, in order to avoid the effect of resonance absorption on the dynamic second-order NLO responses, the bvec(u) value of complex 1a can be measured at the low-frequency region ranging from 0.0 to 0.7 eV in the experiment. 4. Conclusions A systematic DFT calculations have been carried out to investigate the geometry structures and second-order NLO responses on the ligand L and its Pt(II) chelated complexes, and the electron absorption spectra for the studied systems were investigated by the TD-DFT method. The results show that the increases of the pconjugated groups and the number of Pt(II) ions enhance the bvec values, because the degrees of the MLCT/LMCT/LLCT are more obvious. The series a complexes have the better conjugation than those of series b complexes. Thus, the bvec values of series a complexes are larger than those of the corresponding series b complexes. Significantly, the bimetallic complex 3a shows the largest bvec value of 1498.86  1030 esu among the studied complexes due to the MMCT transition along the negative x-axis, and the bvec value is w46.6, w30.5, and w3.6 times as large as those of complexes 1a, 2a and 3b, respectively. Moreover, the analysis of the dynamic second-order NLO responses of complex 1a indicates that in order to avoid the effect of resonance absorption, measuring the bvec(u) value of the complex 1a can use the laser wavelength at the low-frequency region ranging from 0.0 to 0.7 eV in the experiment. Acknowledgments The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (No. 21173035), the Natural Science Foundation of Jilin Province (20101154). References [1] N.P. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [2] L.R. Dalton, A.W. Harper, R. Ghosn, W.H. Steier, M. Ziari, H. Fetterman, Y. Shi, R.V. Mustacich, A.K.Y. Jen, K.J. Shea, Chem. Mater. 7 (1995) 1060e1081.

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