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Article pubs.acs.org/JPCA

Strategy for Enhancing Second-Order Nonlinear Optical Properties of the Pt(II) Dithienylethene Complexes: Substituent Effect, π‑Conjugated Influence, and Photoisomerization Switch Meng-Ying Zhang, Cun-Huan Wang, Wen-Yong Wang, Na-Na Ma, Shi-Ling Sun, and Yong-Qing Qiu* Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, People’s Republic of China S Supporting Information *

ABSTRACT: The second-order nonlinear optical (NLO) properties of a series of Pt(II) dithienylethene (DTE) complexes possessing the reversible photochromic behavior have been investigated by density functional theory (DFT) combined with the analytic derivatives method. The results show that the calculated static first hyperpolarizabilities (βtot) of the open-ring and closed-ring systems significantly increase in the range of 2.1−4.5 times through strengthening of the electron-withdrawing ability of the substituent R (R = H, CF3, NO2) and an increase of the number of thiophene rings. Moreover, there is a large enhancement of the βtot values from the open-ring systems to the corresponding closed-ring systems. This efficient enhancement is attributed to the better delocalization of the π-electron system, the more obvious degree of charge transfer, and the larger fos/Egm3 (fos is the oscillator strength, and Egm is the transition energy between the ground and the excited states) values in the closed forms according to the bond length alternation (BLA) and time-dependent density functional theory (TDDFT) calculations. In addition, the dispersion has less influence on the frequency-dependent first hyperpolarizabilities (βtot(ω)) of the studied systems at the low-frequency area ω (0.000−0.040 au). Our present work would be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.

1. INTRODUCTION In the past decades, the nonlinear optical (NLO) materials based on molecular compounds are of considerable interest because they hold promise for applications in biological imaging, advanced optoelectronic and all-optical data processing technologies.1−4 Among the NLO materials, although purely organic compounds have received much attention, organometallic complexes are especially interesting in this context. Because the organometallic complexes offer versatility for combining NLO effects with other characteristics, such as ultrafast response times, low dielectric constants, good processability as thin-film devices, and enhanced nonresonant responses in view of producing novel multifunctional materials.5−11 Besides, organometallic complexes also show advantages over traditional organic molecules owing to their several low-energy metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT) or metal-to-metal/ intervalence charge transfer (MM/IVCT) excitations,12−15 redox properties, and the tuning effect of the metal on the electronic properties of the organic fragments.16−18 Therefore, organometallic complexes have become the excellent NLO materials. There has been a growing interest in the design and construction of new organometallic complexes displaying large second-order NLO properties.19−22 It is found that organometallic complexes with donor π-conjugated bridge acceptor © 2013 American Chemical Society

(D−π−A) type structure may possess large molecular NLO responses. For instance, a series of D−π−A structural metal Schiff base complexes have been synthesized and measured with sizable second-order NLO responses by Di Bella and coworkers.23,24 They also find that the introduction of the strong donor/acceptor group can decrease the molecular transition energy, leading to the enhancement of molecular second-order NLO responses. Interestingly, Liu et al.25 have investegated the effect of the π-conjugation length between the donor and acceptor groups on the second-order NLO responses of the Ni(II) Schiff base complexes by using the density functional theory (DFT). They point out that the second-order NLO responses of these Ni(II) Schiff base complexes increase with the increased number of double bond (n = 1−8) in the conjugated bridge between the electron donor/acceptor substituent. Especially, the β value of the complex for n = 8 (3977.782 × 10−30 esu) is ∼96 times as large as that of the complex for n = 1 (41.429 × 10−30 esu). Moreover, the research on photoswitchable NLO properties of photochromic compounds has received much attention in the past two decades, because the ability to switch on and off the NLO activity of a molecule is of relevance to the Received: April 26, 2013 Revised: September 1, 2013 Published: November 1, 2013 12497

dx.doi.org/10.1021/jp4041265 | J. Phys. Chem. A 2013, 117, 12497−12510

The Journal of Physical Chemistry A

Article

Figure 1. Geometric structures of the studied systems.

development of molecular photonic devices.26−28 Thereinto, the dithienylethene (DTE) unit undergoes reversible interconversion between a nonconjugated open form and a πconjugated closed form when irradiated in the UV and visible spectral ranges, respectively, which has received much attention and opened up new perspectives for molecular photoswitchable NLO materials.29 Aubert and co-workers30 have studied the first example of photochromic dipolar zinc(II) DTE complexes allowing an efficient switching of the second-order NLO properties. They point out that a large increase of the secondorder NLO activity is observed from the ring-open to the ringclosed isomers, and the μβ values of ring-closed molecules [(1020−1800) × 10−48 esu] are in the range 11.3−13.6 times as large as those of the ring-open molecules [(75−160) × 10−48 esu]. This efficient enhancement clearly reflects the delocalization of the π-electron system and the formation of strong push−pull chromophores in the closed forms. The metal Pt(II) center possesses d8 electronic configuration. It is easy to form a square-planar four coordination compound, thus resulting in promoting the charge transfer. Therefore, it is obvious that the Pt(II) complexes have received an increasing

amount of attention because these Pt(II) complexes possess good NLO responses.31,32 For example, Espa et al.33 have investigated the second-order NLO property of the Pt(II) dithiolene complex and its redox switching character of the NLO response. They indicate that the β value of the Pt(II) dithiolene complex has been increased about 5-fold through the reversible redox process, which achieves a switchable NLO response. In addition, we15 have investigated the static first hyperpolarizabilities (β vec ) of ligand L (N,N′-bis(4methoxyphenyl)ethylenediimine) and its Pt(II) chelated complexes. The results show that the bimetallic Pt(II) complex possesses the largest βvec value in the studied systems, ∼1498.86 × 10−30 esu, which is ∼198.8 and ∼46.6 times as large as those of ligand L and monometallic Pt(II) complex, respectively. Recently, Chan and co-workers34 have reported the synthesis, characterization, and photoisomerization of a new class of photochromic diarylethene-containing platinum(II) complexes, where the open-ring forms are [Pt(thpy-DTE)(acac)] (thpy = 2-(2′-thienyl)pyridyl, DTE = dithienylethene, acac = acetylacetonato) (1ao), [Pt(CF3-thpy-DTE)(acac)] (2ao), [Pt(tthpy)(acac)] (tthpy = 2-(2′-thieno-thienyl) pyridyl) 12498



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The static first hyperpolarizability (βtot) is calculated according to eq 1

(1bo), and [Pt(CF3-tthpy)(acac)] (2bo) (Figure 1). The four DTE-containing Pt(II) complexes exhibit drastic color changes because the DTE unit undergoes reversible interconversion between a nonconjugated open form and a π-conjugated closed form when irradiated in the UV/visible and near-infrared spectral ranges, respectively. Moreover, they find that these DTE-containing Pt(II) complexes are sensitive to the substitution on the pyridyl ring and the extent of π-conjugation of the C∧N ligand (a cyclometalating thpy or tthpy ligand containing the photochromic DTE unit). Interestingly, the structural changes from a nonconjugated open form to a πconjugated closed form will affect the molecular NLO responses. In this study, we not only explore the influences of the strong electron-withdrawing substituent and the number of thiophene rings of the N∧C ligand on the second-order NLO properties but also consider the photochromism effect. Therefore, we designed the open-ring systems 3ao and 3bo that are produced by the introduction of the strong electronwithdrawing substituent R = NO2 on pyridine deriving from the open-ring systems 1ao and 1bo, respectively. Moreover, the open-ring systems co (1co, 2co, 3co) and do (1do, 2do, 3do) were designed by increasing the two thiophene rings in the N∧C ligands originating from the open-ring systems ao (1ao, 2ao, 3ao) and bo (1bo, 2bo, 3bo), respectively. In addition, their corresponding closed-ring systems ac (1ac, 2ac, 3ac), bc (1bc, 2bc, 3bc), cc (1cc, 2cc, 3cc), and dc (1dc, 2dc, 3dc) were designed by irradiating in the UV/visible light according to the nature of the DTE unit (Figure 1).

βtot =

βx 2 + βy 2 + βz 2

(1)

in which βi is defined by eq 2 βi =

3 (β + βijj + βikk ) 5 iii

i, j, k = x, y, z

(2)

and the frequency-dependent first hyperpolarizability βtot(ω) is calculated according to eq 3 βtot (ω) =

βx 2(ω) + βy 2(ω) + βz 2(ω)

(3)

in which βi(ω) is defined by eq 4 βi (ω) =

⎡ 3⎢ 1 β (ω) + 5 ⎢⎣ iii 3

⎤

∑ (βijj(ω) + βjij(ω) + βjji(ω))⎥⎥ ⎦

j≠i

i, j = x, y, z

(4)

The time-dependent density functional theory (TDDFT) method is one of the most successful and extensively used methods to calculate the excitation energies in quantum chemistry owing to its efficiency and accuracy.43 The absorption spectra for all studied systems were calculated at TD-CAM-B3LYP/6-31+G(d)/LanL2DZ level in benzene solution.

3. RESULTS AND DISCUSSION 3.1. Molecular Geometries. Three hybrid functionals B3LYP, PBE1PBE, and CAM-B3LYP (the 6-31G(d) basis set for nonmetal atoms and LanL2DZ basis set for Pt(II)) were chosen to optimize systems 1ao and 2ao derived from their crystal structures. The results show that the selected bond lengths and bond angles around the metal centers for both systems 1ao and 2ao obtained by PBE1PBE functional are in the best consistent with their experimental crystal data (Table S1, Supporting Information). Therefore, the geometry structures of all studied systems with all real frequencies have been obtained at PBE1PBE/6-31G(d)/LANL2DZ level. The bond length alternation44 (BLA, the difference between single and double bond lengths) of the studied systems are listed in Table 1, and the corresponding single and double bond plotted in the black bold are shown in Figure 1. From Table 1, it is obvious that the trend of the BLA values of the open-ring systems is consistent with that of the closed-ring systems. The BLA values of the studied systems become small with strengthening of the electron-withdrawing ability of the substituent R (R = H, CF3, NO2) on the pyridine and an increase in the number of thiophene rings of the C∧N ligand,

2. COMPUTATIONAL DETAILS All of the calculations were carried out using the Gaussian 09W program package.35 It is well-known that the choice of functionals is crucial to generate reliable and accurate results. Three hybrid functionals B3LYP,36,37 PBE1PBE,38 and CAMB3LYP39,40 were chosen to optimize systems 1ao and 2ao derived from their crystal structures. The standard 6-31G(d) polarized double-ζ basis set is for H, C, N, O, and S atoms, and the effective core potential (ECP) double-ζ(DZ) basis set of LanL2DZ is for Pt(II) to take into account the relativistic effect. The calculated geometric parameters and crystallographic data of systems 1ao and 2ao are shown in Table S1 (Supporting Information), and the atom labels are shown in Figure 1. From Table S1 (Supporting Information), the selected bond lengths and bond angles around the metal centers for both systems 1ao and 2ao obtained by PBE1PBE functional are the most consistent with their experimental crystal data. Therefore, the geometries of the studied systems have been fully optimized without symmetry constraint by using the PBE1PBE functional and were all characterized as minima by frequency analysis. On the basis of the optimized molecular geometries, the static first hyperpolarizabilities (βtot) of the studied systems were calculated by analytical third energy derivatives, which is more efficient and less expensive than numerical derivatives.41 To check the consistency of our calculations, the three hybrid functionals PBE1PBE, mPWPW91*,42 and CAM-B3LYP have been chosen to calculate the βtot values of the studied systems. The 6-31+G(d) basis set for nonmetal atoms and LanL2DZ basis set for Pt(II) were used for all studied systems. Finally, the frequency-dependent first hyperpolarizabilities (βtot(ω)) of each system were calculated by the coupled perturbed density function theory (CPDFT) at CAM-B3LYP/6-31+G(d)+LanL2DZ level.

Table 1. BLA Values (Å) of the Studied Systems

12499

1ao

2ao

3ao

1ac

2ac

3ac

0.103 1bo

0.049 2bo

0.035 3bo

0.099 1bc

0.045 2bc

0.032 3bc

0.086 1co

0.042 2co

0.030 3co

0.085 1cc

0.041 2cc

0.029 3cc

0.075 1do

0.038 2do

0.027 3do

0.072 1dc

0.035 2dc

0.025 3dc

0.067

0.035

0.026

0.065

0.033

0.024



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Table 2. Static First Hyperpolarizability β (×10−30 esu) of the Open-Ring Systems (Frequency ω = 0)

Table 3. Static First Hyperpolarizability β (×10−30 esu) of the Closed-Ring Systems (Frequency ω = 0)

complex

method

βx

βy

βz

βtot

complex

method

βx

βy

βz

βtot

1ao

PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP

−1.1 −1.0 −1.2 −1.0 −0.5 −0.8 1.4 0.9 −0.1 0.4 0.1 −0.3 0.5 0.3 −0.1 1.6 1.1 0.3 1.5 0.7 −0.1 2.3 1.4 0.4 9.8 6.8 4.4 3.4 2.2 1.3 7.3 5.5 4.1 12.0 8.5 6.0

0.2 0.1 0.1 1.0 0.9 0.8 1.0 0.8 1.0 1.1 0.9 0.9 1.6 1.4 1.2 4.3 3.5 3.1 0.3 0.2 0.2 −1.8 −1.6 −1.5 −0.1 −0.1 −0.1 0.3 0.2 0.1 3.3 2.7 2.2 6.6 5.1 4.1

16.4 14.2 13.5 23.0 20.0 18.5 69.8 56.7 50.0 28.6 24.8 23.2 41.3 36.1 33.2 111.1 91.4 80.4 43.1 36.5 33.3 64.6 55.4 49.7 161.5 130.8 112.2 59.9 49.3 43.4 94.2 78.4 67.8 226.9 178.7 148.0

16.5 14.2 13.5 23.0 20.0 18.5 69.8 56.7 50.0 28.6 24.8 23.2 41.3 36.2 33.2 111.2 91.5 80.4 43.1 36.5 33.3 64.7 55.4 49.7 161.8 131.0 112.3 60.0 49.3 43.4 94.5 78.6 67.9 227.3 178.9 148.1

1ac

PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP PBE1PBE mPWPW91* CAM-B3LYP

−0.7 −1.5 −2.7 −1.8 −2.8 −4.3 2.6 −2.7 −7.3 9.1 6.1 4.1 18.7 14.0 10.9 47.1 34.6 26.2 0.4 0.1 −0.2 −0.6 −1.4 −1.6 −2.9 −6.0 −6.4 14.1 8.1 3.8 28.5 18.3 11.1 78.1 47.2 28.5

5.1 4.3 4.1 8.1 7.2 7.0 17.7 15.8 14.7 −0.3 −0.2 −0.2 −0.9 −0.7 −0.6 −3.6 −2.3 −1.6 1.7 1.3 1.2 −0.5 −0.3 −0.2 −13.2 −9.1 −6.6 1.0 0.7 0.4 4.7 3.3 2.4 11.7 7.7 5.2

39.7 34.3 31.9 60.4 53.7 50.0 198.6 167.3 145.7 57.9 45.5 38.0 94.0 74.8 61.8 278.4 200.9 153.5 79.6 60.2 48.6 137.4 104.6 82.6 391.2 268.2 196.0 97.5 69.8 52.9 182.0 130.2 96.2 509.9 324.5 221.9

40.0 34.6 32.3 61.0 54.3 50.6 199.4 168.1 146.6 58.6 45.9 38.3 95.8 76.1 62.8 282.4 203.9 155.7 79.6 60.2 48.6 137.4 104.6 82.6 391.4 268.5 196.2 98.5 70.3 53.1 184.2 131.5 96.9 516.0 328.0 223.8

2ao

3ao

1bo

2bo

3bo

1co

2co

3co

1do

2do

3do

2ac

3ac

1bc

2bc

3bc

1cc

2cc

3cc

1dc

2dc

3dc

B3LYP, HF = 100%). However, the three methods show the same trend of the second-order NLO responses. The βtot values progressively enhance with strengthening of the electronwithdrawing ability of the substituent R (R = H, CF3, NO2) on the pyridine and an increase of the number of thiophene rings of the C∧N ligand, and the βtot values of the open-ring systems are smaller than those of the corresponding closed-ring systems. For clarity, we only take the calculated static first hyperpolarizabilities with CAM-B3LYP functional as an example to evaluate the qualitative second-order NLO responses of the studied systems. 3.2.1. Static First Hyperpolarizabilities of Open- and Closed-Ring Systems. The calculated βtotvalues of the openand closed-ring systems at CAM-B3LYP/6-31+G(d)/LanL2DZ level are listed in Tables 2 and 3, respectively. From Table 2, for the open-ring systems ao (1ao, 2ao, 3ao), system 3ao possesses the largest βtot value of 49.960 × 10−30 esu, which is about ∼3.7 and ∼2.7 times as large as those of systems 1ao and 2ao, respectively. It is no doubt that the open-ring systems ao (1ao−3ao) have a considerable enhancement of the βtot values with gradually strengthening the electron-withdrawing ability of the substituent R (R = H, CF3, NO2). The same trend

which decreases in the ranges 0.009−0.054 and 0.001−0.017 Å, respectively. Moreover, the BLA values of the open-ring systems are larger than those of the corresponding closedring systems because of the structural changes from the nonconjugated open form to the π-conjugated closed form; that is, the unconnected atoms C2 and C3 are connected and plane configurations are formed under the irradiation of UV/ visible light (Figure 1). This implies that the π-conjugation of the closed-ring systems is stronger than that of the corresponding open-ring systems. These changes on the structures will inevitably affect the molecular second-order NLO properties. 3.2. Static First Hyperpolarizabilities. The static first hyperpolarizabilities (βtot) of the studied systems were calculated by three different functionals PBE1PBE, mPWPW91*, and CAM-B3LYP on the basis of the optimized geometries. The βtot values together with the components of βx, βy, and βz values for the open-ring and the closed-ring systems are listed in Tables 2 and 3, respectively. From Tables 2 and 3, the calculated βtot values for the studied systems become small as the increase of the HF exchange for the three functionals (PBE1PBE, HF = 25%; mPWPW91*, HF = 40%; CAM12500



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Table 4. Detailed TD-CAM-B3LYP Calculations of the Open-Ring Systems complex 1ao 2ao 3ao 1bo 2bo 3bo 1co 2co 3co 1do 2do 3do a

λ (nm) 380.8 397.5 441.3 399.8 417.4 464.2 409.1 426.2 472.6 418.1 435.2 482.0

(438)a (462)a (462)a (485)a

Egm (eV)

fos

3.26 3.12 2.81 3.1 2.97 2.67 3.03 2.91 2.62 2.97 2.85 2.57

0.3114 0.3498 0.5720 0.5232 0.5680 0.8017 0.7600 0.8074 1.0423 1.0185 1.0547 1.2561

contributions H H H H H H H H H H H H

→ → → → → → → → → → → →

L(78%) L(67%) L(58%) L (85%) L (82%) L(76%) L (85%) L(81%) L (74%) L(85%) L(81%) L(73%)

H−1 → L(13%) H−1 → L(23%) H−1 → L (25%) H−1 → L (6%) H−1 → L (7%)

H → L+1(5%)

H−1 → L(8%) H−1 → L(10%)

H → L+1(5%)

H−1 → L(8%) H−1 → L(12%)

H → L+1(5%)

Experimental values in parentheses are taken from ref 34.

Table 5. Detailed TD-CAM-B3LYP Calculations of the Closed-Ring Systems

a

complex

λ (nm)

Egm (eV)

fos

1ac 2ac 3ac

336.7 (364)a 347.6 (378)a 479.0

3.68 3.57 2.59

0.3864 0.3582 0.4274

1bc 2bc

405.5 421.0

3.06 2.94

0.8206 0.8320

3bc

462.0

2.68

0.8670

1cc

418.0

2.97

0.9002

2cc

435.2

2.85

0.8542

3cc

476.0

2.6

0.7659

1dc

423.7

2.93

1.2203

2dc

439.9

2.82

1.1773

3dc

481.1

2.58

1.0877

contributions H−2 → L(82%) H−2 → L(79%) H → L+1(56%) H−1 → L(10%) H−1 → L(44%) H → L+1(40%) H → L+2(6%) H → L+1(41%) H → L+2(10%) H → L+4(5%) H−1 → L(40%) H−1 → L+1(6%) H → L+1(41%) H → L(6%) H → L+1(36%) H → L(10%) H−1 → L(47%) H−1 → L+1(8%) H−1 → L(47%) H → L(7%) H−1 → L(40%) H → L(11%)

H−1 → L(8%) H → L+2(17%) H → L+4(5%) H → L+1(35%) H−1 → L(38%) H−1 → L(26%) H → L(9%) H → L+1(39%) H−1 → L(39%) H−1 → L(31%) H → L+2(9%) H → L+1(30%) H → L+1(30%) H → L+1(26%) H → L+2(7%)

Experimental values in parentheses are taken from ref 34.

of the βtot values is found in the open-ring systems bo (1bo, 2bo, 3bo), co (1co, 2co, 3co), and do (1do, 2do, 3do). That is to say, the order of the βtot values for these systems is βtot(1bo) < βtot(2bo) < βtot(3bo), βtot(1co) < βtot(2co) < βtot(3co) and βtot(1do) < βtot(2do) < βtot(3do). In addition, for the openring systems 1o (1ao, 1bo, 1co, 1do), system 1do has the largest βtot value (43.387 × 10−30 esu), which is about ∼3.2, ∼1.9, and ∼1.3 times as large as those of systems 1ao, 1bo, and 1co, respectively. Analogously, with the increasing number of thiophene rings of the C∧N ligand, the order of the βtot values for the open-ring systems 2o (2ao, 2bo, 2co, 3do) and 3o (3ao, 3bo, 3co, 3do) is βtot(2ao) < βtot(2bo) < βtot(2co) < βtot(2do) and βtot(3ao) < βtot(3bo) < (3co) < βtot(3do), respectively. These clearly indicate that the open-ring systems show an increase in βtot values with the increasing number of thiophene rings of the C∧N ligand. The same trend of the βtot values is also found in the closed-ring systems (Table 3). It is obvious that strengthening the electron-withdrawing ability of the substituent R is significantly helpful to increase the βtot

values of the closed-ring systems ac (1ac−3ac), bc (1bc−3bc), cc (1cc−3cc), and dc (1dc−3dc), respectively. That is, the order of the βtot values is βtot(1ac) < βtot(2ac) < βtot(3ac), βtot(1bc) < βtot(2bc) < βtot(3bc), βtot(1cc) < βtot(2cc) < βtot(3cc), and βtot(1dc) < βtot(2dc) < βtot(3dc). In addition, for the closed-ring systems 1c (1ac, 1bc, 1cc, 1dc), 2c (2ac, 2bc, 2cc, 3dc), and 3c (3ac, 3bc, 3cc, 3dc), there is a significant increase in the βtot values with the increasing number of thiophene rings of the C∧N ligand, respectively (βtot(1ac) < βtot(1bc) < βtot(1cc) < βtot(1dc), βtot(2ac) < βtot(2bc) < βtot(2cc) < βtot(2dc), and βtot(3ac) < βtot(3bc) < βtot(3cc) < βtot(3dc)). As described in the Molecular Geometries section, the BLA values of the open- and closed-ring systems become small with the strengthening of the electron-withdrawing ability of substituent R on the pyridine and an increase of the number of thiophene rings of the C∧N ligand. These indicate that the smaller BLA values, the stronger molecular π-conjugation, and thus the calculated βtot values of the open- and closed-ring systems significantly increase, respectively. 12501



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Figure 2. Molecular orbitals related to the dominant electron transitions of the open-ring systems (assignment: H = HOMO, L = LUMO, H−1 = HOMO−1, L+1 = LUMO+1, etc.).

configurational mixture of the various CT transitions and π → π* transition. In addition, the molecular orbitals corresponding to the dominant electron transitions with the maximum oscillator strength are depicted in Figures 2 and 3. The following conclusions can be drawn from Figure 2: (i) for the open-ring systems ao (1ao−3ao), the occupied molecular orbital HOMO−1 and HOMO of system 1ao mainly localize on the thiophene ring, DTE moiety, and metal Pt(II), whereas the lowest unoccupied molecular orbital LUMO is the π* orbital localized on the thienylpyridine ligand. It is obvious that the electron transitions of HOMO → LUMO(78%) and HOMO−1 → LUMO(13%) are both categorized as the ILCT transition from the thiophene and DTE moieties to the pyridine ring and from the thienylpyridine to the thienylpyridine (π → π*), and the MLCT transition from the metal Pt(II) to the pyridine ring. Interestingly, with strengthening the electron-withdrawing ability of the substituent R on the pyridine, the HOMO−1 and HOMO of systems 2ao (R = CF3) and 3ao (R = NO2) are both similar to those of system 1ao, whereas the LUMOs of systems 2ao and 3ao are not only localized on the thienylpyridine ligand but also localized on the substituents −CF3 and −NO2, respectively. That means the degree of the ILCT and MLCT becomes progressively obvious from systems 1ao to 3ao because of the introduction of the strong electron-withdrawing substituent R on the pyridine, which thus plays an important role in the enhancement of the βtot values. Likely, the electron transitions of systems bo (1bo− 3bo), co (1co−3co), and do (1do−3do) are all mainly attributed to the ILCT and MLCT transitions. The degree of

To provide an origin understanding on the first hyperpolarizabilities of the open- and closed-ring systems, we focused on the transition states of these models. To choose suitable calculated methods, the electron absorption spectra of the synthesized systems (1ao, 2ao, 1bo, 2bo, 1ac, 2ac, 1bc, and 2bc) were simulated using TD-B3LYP and TD-PBE1PBE methods and their corresponding long-range-corrected functional TD-CAM-B3LYP and TD-LC-wPBE45,46 methods associated with conductor-like polarizable continuum model (CPCM)47,48 in benzene solution. The calculated absorption wavelengths of the synthesized systems at TD-B3LYP, TDPBE1PBE, TD-CAM-B3LYP, and TD-LC-wPBE methods are listed in Table S2 (Supporting Information) and relevant experimental results are listed in Tables 4 and 5. The results show that the absorption wavelengths obtained by CAMB3LYP functional are in the best reasonable agreement with the experimental data. Therefore, the absorption spectra for all studied systems were calculated at the TD-CAM-B3LYP/631+G(d)/LanL2DZ level to gain an intuitive understanding of the origin of the static first hyperpolarizabilities. The maximum oscillator strengths ( fos) and the corresponding absorption wavelengths (λ), transition energies (Egm), and contributions for all studied systems are shown in Tables 4 and 5, respectively. From Table 4, the absorption wavelengths are red shift with strengthening the electron-withdrawing ability of the substituent R (R = H, CF3, NO2) and the increasing number of thiophene rings. The same trend of the absorption wavelengths red shift is found in the closed-ring systems (Table 5), except for systems 3c (3ac−3dc) because of the different 12502



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Figure 3. Molecular orbitals related to the dominant electron transitions of the closed-ring systems (assignment: H = HOMO, L = LUMO, H−1 = HOMO−1, L+1 = LUMO+1, etc.).

βtot values from systems 1ao to 1do. Similarly, systems 2o (2ao−2do) and 3o (3ao−3do) exhibit the ILCT and MLCT transitions, where the degree of the CT for each system (2o and 3o) is also obvious with the increasing number of thiophene rings, thus resulting in a remarkable increase in the second-order NLO responses. However, the different trend of the electron transition is found in the closed-ring systems (Figure 3). From Figure 3, for the closed-ring systems ac (1ac− 3ac), the HOMO−2 of system 1ac is centered not only on the thiophene-DTE moieties and metal Pt(II) but also a little on the acac moiety, whereas the LUMO is mainly localized on the pyridine and thiophene rings. It is obvious that the electron transition of HOMO−2 → LUMO(82%) for system 1ac is categorized as the LLCT transition from the acac moiety to the pyridine ring, the ILCT transition from the thiophene-DTE moieties to the pyridine ring and from the thiophene ring to the thiophene ring, and the MLCT transition from the metal Pt(II) to the pyridine ring. Interestingly, the HOMO−2 →

the charge transfer (CT) for each system (bo, co, and do) has an efficient increase with strengthening the electron-withdrawing ability of the substituent R. As expected, the increasing CT degree has the significant contribution to enhancing the molecular βtot values. (ii) for systems 1o (1ao−1do), the HOMO → LUMO(78%) and HOMO−1 → LUMO(13%) of system 1ao are mainly assigned as the ILCT [π(thiopheneDTE) → π*(pyridine)] and π(thienylpyridine) → π*(thienylpyridine) transitions, and the MLCT [d(Pt(II)) → π*(pyridine)] transition. Notably, the HOMOs of systems 1bo, 1co, and 1do all spread on the metal Pt(II), thiophene, and DTE moieties, whereas their LUMOs all mainly center on the pyridine moiety and a few localize on the thiophene rings. There is no doubt that the HOMOs → LUMOs of systems 1bo, 1co, and 1do are all ascribed as the ILCT and MLCT transitions, and the increasing number of thiophene rings is helpful to the long-rang CT and the increase of the CT degree. Therefore, there is a remarkable enhancement of the calculated 12503



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Figure 4. Relationship between the βtot values (black star) and the corresponding fos/Egm3 values (blue circle) for the open-ring systems.

the pyridine, which leads to a substantial increase in the βtot values. Additionally, for the closed-ring systems 1c (1ac−1dc), system 1ac exists as the LLCT [π(acac) → π*(pyridine)] transition, the ILCT [π(thiophene-DTE) → π*(pyridine)] and [π(thienylpyridine) → π*(thienylpyridine)] transitions, and the MLCT [d(Pt(II)) → π*(pyridine)] transition. Moreover, for system 1bc, the HOMO−1 → LUMO(44%) and HOMO → LUMO+1(35%) are assigned as the ILCT [π(thienothiophene-DTE) → π*(pyridine)] and [π(thienylpyridine) → π*(thienylpyridine)] transitions and the MLCT [d(Pt(II)) → π*(pyridine)] transition. Notably, there are similar electron transitions between systems 1cc and 1dc. They both possess HOMO−1 → LUMO, HOMO → LUMO+1, and HOMO−1 → LUMO+1, which are attributed to the ILCT and MLCT transitions. It is obvious that the increasing number of thiophene rings is helpful to the long-range CT and the increase of the CT degree. Therefore, there is an enhancement of the βtot values from systems 1ac to 1dc. Likely, the electron transitions of systems 2c (2ac−2dc) and 3c (3ac−3dc) are all mainly attributed to the LLCT/ILCT and MLCT transitions, where the degree of the CT for each system (2c and 3c) is

LUMO(79%) and HOMO−1 → LUMO(8%) of system 2ac (R = CF3) are classified as the LLCT [π(acac) → π*(pyridineCF3)] transition, the ILCT [π(thiophene-DTE) → π*(pyridine-CF 3 ) ] and [ π( t h ie n ylp yr id i ne) → π *(thienylpyridine)] transitions, and the MLCT [d(Pt(II)) → π*(pyridine-CF3)] transition. For system 3ac (R = NO2), the HOMO → LUMO+1(56%), HOMO → LUMO+2(17%), HOMO−1 → LUMO(10%), and HOMO → LUMO+4(5%) are assigned as the LLCT [π(thiophene-DTE) → π*(acac)] transition, the ILCT [π(thiophene-DTE) → π*(pyridineNO2)] and [π(thienylpyridine) → π*(thienylpyridine)] transitions, and the MLCT [d(Pt(II)) → π*(pyridine-NO2)] transition. Obviously, the introduction of the strong electronwithdrawing substituent R on the pyridine is helpful to increase the degree of the CT, thus resulting in an effective enhancement of the βtot values from systems 1ac to 3ac. Similarly, the electron transitions of systems bc (1bc−3bc), cc (1cc−3cc), and dc (1dc−3dc) are all mainly attributed to the LLCT, ILCT, and MLCT transitions. The degree of the CT for each system (bc, cc, and dc) is increased through the strengthening electron-withdrawing ability of substituent R on 12504



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Figure 5. Relationship between the βtot values (black star) and the corresponding fos/Egm3 values (blue circle) for the closed-ring systems.

strengthening of the electron-withdrawing ability of substituent R (R = H, CF3, NO2), that is, fos/Egm3(1ao) < fos/Egm3(2ao) < fos/Egm3(3ao), fos/Egm3(1bo) < fos/Egm3(2bo) < fos/Egm3(3bo), fos/Egm3(1co) < fos/Egm3(2co) < fos/Egm3(3co), and fos/ Egm3(1do) < fos/Egm3(2do) < fos/Egm3(3do). These increasing trends are consistent with those of the enhanced βtot values for the systems ao (1ao−3ao), bo (1bo−3bo), co (1co−3co), and do (1do−3do), respectively. Interestingly, similar trends are found in systems 1o, 2o, and 3o in Figure 4b. The order of the fos/Egm3 values is fos/Egm3(1ao) < fos/Egm3(1bo) < fos/Egm3(1co) < fos/Egm3(1do), fos/Egm3(2ao) < fos/Egm3(2bo) < fos/Egm3(2co) < fos/Egm3(2do), and fos/Egm3(3ao) < fos/Egm3(3bo) < fos/ Egm3(3co) < fos/Egm3(3do), which is also in accordance with the trend of the enhanced βtot values for systems 1o (1ao−1do), 2o (2ao−2do), and 3o (3ao−3do), respectively. Similarly, for the closed-ring systems, although the fos/Egm3 value of system

enhanced with the increasing number of thiophene rings, causing a remarkable enhancement of the βtot values for the closed-ring systems. Above all, strengthening the electronwithdrawing ability of substituent R and increasing the number of thiophene rings of the C∧N ligand are helpful to enhance the second-order NLO responses of the open- and closed-ring systems. Moreover, we consider the two-level model that links the βtot value and the low-lying CT transition.49 According to the twolevel model, the third power of the Egm value is inversely proportional to the βtot value and the fos value is proportional to the βtot value. The relationship between the βtot values and the corresponding fos/Egm3 values for the open- and closed-ring systems are shown in Figures 4 and 5, respectively. From Figure 4a, it is clearly seen that the fos/Egm3 values of the open-ring systems show an increasing trend with the gradually 12505



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Figure 6. Relationship of the fos/Egm3 values between the open- (black star) and closed-ring systems (red circle).

ability of substituent R on the pyridine and increasing the number of thiophene rings of the C∧N ligand cause a great increase in the fos/Egm3 values of the open- and closed-ring systems, leading to the enhancement of the second-order NLO responses. 3.2.2. Photoisomerization Effects on NLO Switching. Under the irradiation of UV/visible light, the configurations of open-ring systems distort, then C2 and C3 are connected, and the plane configurations are formed. When near-infrared is absorbed, bond C2−C3 is broken and configurations are restored. That is, the structural changes from the nonconjugated open form to the π-conjugated closed form may affect the second-order NLO responses. It can be found in Tables 2 and 3, the βtot values for all open-ring systems increase when the DTE unit is closed. The βtot values of all closed-ring systems are in the range of 1.2−2.9 times as large as those of the corresponding open-ring systems, because the BLA values

2ac (0.00787) is a little larger than that of the system 1ac (0.00775), the fos/Egm3 values of the closed-ring systems (ac, bc, cc, dc) all show an increasing trend with the strengthening of the electron-withdrawing ability of substituent R (R = H, CF3, NO2) on the pyridine (Figure 5a). These trends of increasing fos/Egm3 values for the closed-ring systems (ac, bc, cc, dc) are in quantitative agreement with the trends of the enhanced βtot values. Interestingly, similar trends are found in Figure 5b; that is, the fos/Egm3 values of the closed-ring systems (1c, 2c) also show an increasing trend with the increasing number of thiophene rings of the C∧N ligand, except for the closed-ring systems 3c (3ac−3dc), in which the order of the fos/Egm3 values is fos/Egm3(3ac) < fos/Egm3(3cc) < fos/Egm3(3bc) < fos/Egm3(3dc). However, the degree of CT is enhanced with the increasing number of thiophene rings, which has a significant influence on improving the βtot values from systems 3ac to 3dc. Therefore, strengthening the electron-withdrawing 12506



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Figure 7. Relationship between the βtot(c)/βtot(o) values (black star) and the corresponding ΔL values (blue circle) and the electron density maps (EDDM) with maximal oscillator strengths for the systems a and d.

obvious increase of the βtot(c)/βtot(o) values with the increasing number of thiophene rings of the C∧N ligand. We have taken systems a and systems d as examples to explain the decrease of βtot(c)/βtot(o) values with the increasing number of thiophene rings of the C∧N ligand. The relationship between the βtot(c)/βtot(o) values and the ΔL values (the BLA difference between the open forms and closed forms, Å) and the electron density difference maps (EDDM) of the ground state and excited state corresponding to the dominant electron transitions are plotted in Figure 7. The purple color represents the electron donor, and the blue color delegates the electron acceptor. From Figure 7a, for systems 1a (1ao, 1ac) and 1d (1do, 1dc), the order of the ΔL values is as follows: ΔL(1a) > ΔL(1d). That means the π-conjugation difference (between the open forms and closed forms) of system 1a is larger than that of system 1d. Therefore, the order of the βtot(c)/βtot(o) values is βtot(1ac)/βtot(1ao) > βtot(1dc)/βtot(1do). Similarly, for systems 2a (2ao, 2ac) and 2d (2do, 2dc), the ΔL value of systems 2a is larger than that of systems 2d. The decreasing ΔL values cause the declining βtot(c)/βtot(o) values. Interestingly,

of the closed-ring systems are all smaller than those of the corresponding open-ring systems according to the Molecular Geometries calculations. This indicates that the smaller BLA values, the stronger π-conjugation, and thus the βtot values of the studied systems increase from the open forms to the closed forms. In addition, the relationship of the fos/Egm3 values between the open-ring systems and the corresponding closedring systems are depicted in Figure 6. It is obviously found that the fos/Egm3 values of the closed-ring systems are all larger than those of the corresponding open-ring systems, and it shows an increasing trend with gradual strengthening of the electronwithdrawing ability of substituent R (Figure 6a) and an increase in the number of thiophene rings of the C∧N ligand (Figure 6b). Therefore, the photoisomerization causes a large enhancement of the second-order NLO responses. Thereinto, the difference on the βtot values for system 1d from open form (1do) to closed form (1dc) is the smallest, which is βtot(1dc) = 1.2βtot(1do). In addition, the difference on the βtot values for system 3a from open form (3ao) to closed form (3ac) is the biggest, which is βtot(3ac) = 2.9βtot(3ao). Clearly, there is no 12507



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Figure 8. Frequency-dependent first hyperpolarizability βtot(ω) values for the open-ring systems 1ao, 2ao, 1bo, and 2bo.

investigate the frequency-dependent first hyperpolarizabilities. The βtot(−ω;ω,0) and βtot(−2ω;ω,ω) values are plotted in Figure 8. As shown in Figure 8, the βtot(−ω;ω,0) values of systems 1ao, 2ao, 1bo, and 2bo vary slowly at a frequency range from 0.000 to 0.0650 au. However, the βtot(−2ω;ω,ω) value of system 1ao dramatically increases up to 2281.43 × 10−30 esu at ω = 0.060 au (λ = 759.6 nm). The λ/2 (379.8 nm) value is almost equal to the maximum absorption peak (380.8 nm) of system 1ao, resulting in a remarkable enhancement of the βtot(−2ω;ω,ω) value of system 1ao because of the large resonance or dispersion. Analogously, the βtot(−2ω;ω,ω) value of system 2bo drastically rises up to 2826.797 × 10−30 esu at ω = 0.055 au (λ′ = 828.7 nm) because of its larger resonance or dispersion at 417.4 nm (Table 4). In addition, the βtot(−2ω;ω,ω) values of systems 2ao and 1bo both increase rapidly up to 176.448 × 10−30 and 264.123 × 10−30 esu at 0.055 au, respectively, which is attributed to their larger resonances or dispersions at 397.5 and 399.8 nm (Table 4). Therefore, to avoid the effect of resonance absorption on the frequencydependent first hyperpolarizabilities, a frequency range from 0.000 to 0.040 au might be considered a better choice to get the experimental second-order NLO responses.

this trend between the systems 3a (3ao, 3ac) and 3d (3do, 3dc) is also similar to that between systems 1a and 1d, where the smaller the ΔL values, the lesser the βtot(c)/βtot(o) values. This indicates that the π-conjugation difference between the open forms and closed forms becomes smaller with the increasing number of thiophene rings of the C∧N ligand, which leads to the decreased βtot(c)/βtot(o) value. Additionally, from the EDDM in Figure 7b, there is a similar trend between systems a (1a−3a) and d (1d−3d); that is, the difference on the CT degree between open- and closed-ring forms for systems a is more obvious than that for corresponding systems d. Obviously, this also suggests that the βtot(c)/βtot(o) values of systems a is larger than those of the corresponding systems d. As mentioned above, photoisomerization could significantly tune the second-order NLO responses of our chosen molecules, and thus the DTE-containing Pt(II) complexes can act as the efficient second-order NLO switching materials. 3.3. Frequency-Dependent First Hyperpolarizabilities. To investigate the effect of dispersions, the frequencydependent first hyperpolarizabilities (βtot(ω)) were calculated by using coupled perturbed density functional theory (CPDFT) at CAM-B3LYP/LanL2DZ/6-31+G(d) level. The βtot(−ω;ω,0) and βtot(−2ω;ω,ω) values of the studied systems at a frequency range from 0.000 to 0.065 au are listed in Tables S3 and S4, Supporting Information. The βtot(−ω;ω,0) values of each system vary slowly at a frequency range from 0.000 to 0.065 au. However, the trend of the βtot(−2ω;ω,ω) values of each system is different from that of the βtot(−ω;ω,0) values. The βtot(−2ω;ω,ω) values dramatically increase at a frequency range from 0.050 to 0.060 au, which can be attributed to its larger resonance or dispersion at 336.7−482.0 nm according to the TD-DFT results (Table 4). For clarity, we only chose four open-ring systems 1ao, 2ao, 1bo, and 2bo as an example to

4. CONCLUSIONS A systematic DFT calculation has been carried out to investigate the geometry structures and the second-order NLO responses on a series of Pt(II) dithienylethene (DTE) complexes. The main conclusions observed are as follows: (1) For the open-ring and closed-ring systems, the molecular BLA values become small with introduction of the strong electron-withdrawing substituent R on pyridine and an increase in the number of thiophene rings of the C∧N 12508



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ligand, which results in a better π-conjugation and a remarkable enhancement of the static first hyperpolarizabilities (βtot). Moreover, the TD-CAM-B3LYP calculations show that the introduction of the strong electron-withdrawing substituent R and the increase of the number of thiophene rings can substantially raise the degree of molecular charge transfer and increase the fos/ Egm3 values, except closed-ring systems 3c (3ac−3dc), thus improving the second-order NLO responses. (2) The βtot values of closed-ring systems are in the range of 1.2−2.9 times as large as those of the corresponding open-ring systems because the closed-ring systems possess smaller BLA values and larger fos/Egm3 values than the corresponding open-ring systems. In addition, for systems a (1a−3a) and d (1d−3d), the βtot(c)/βtot(o) values of systems d are all smaller than those of the corresponding systems a. Because the ΔL values become small and the difference on degree of charge transfer between open- and closed-ring systems becomes not obvious with the increasing number of thiophene rings. (3) The dispersion has less influence on the studied systems at the low-frequency area ω (0.000−0.040 au). The low frequency of incident light should been chosen to measure the molecular second-order NLO coefficients in experiment.

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(3) Prasad, N. P.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (4) Zyss, J. Molecular Nonlinear Optics: Materials, Physics and Devices; Academic Press: Boston, 1994. (5) Cariati, E.; Pizzotti, M.; Roberto, D.; Tessore, F.; Ugo, R. Coordination and Organometallic Compounds and Inorganic-Organic Hybrid Crystalline Materials for Second-Order Non-Linear Optics. Coord. Chem. Rev. 2006, 250, 1210−1233. (6) Di Bella, S. Second-order Nonlinear Optical Properties of Transition Metal Complexes. Chem. Soc. Rev. 2001, 30, 355−366. (7) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Design and Construction of Molecular Assemblies with Large Second-Order Optical Nonlinearities. Quantum Chemical Aspects. Chem. Rev. 1994, 94, 195− 242. (8) Manzur, C.; Fuentealba, M.; Hamon, J. R.; Carrillo, D. Cationic Organoiron Mixed-Sandwich Hydrazine Complexes: Reactivity Toward Aldehydes, Ketones, β-Diketones and Dioxomolybdenum Complexes. Coord. Chem. Rev. 2010, 254, 765−780. (9) Nalwa, H. S. Organometallic Materials for Nonlinear Optics. Appl. Organomet. Chem. 1991, 5, 349−377. (10) Verbiest, T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons, A. Second-Order Nonlinear Optical Materials: Recent Advances in Chromophore Design. J. Mater. Chem. 1997, 7, 2175−2189. (11) Yam, V. W. W.; Lo, K. K. W.; Wong, K. M. C. Luminescent Polynuclear Metal Acetylides. J. Organomet. Chem. 1999, 578, 3−30. (12) Trujillo, A.; Fuentealba, M.; Carrillo, D.; Manzur, C.; LedouxRak, I.; Hamon, J. R.; Saillard, J. Y. Synthesis, Spectral, Structural, Second-Order Nonlinear Optical Properties and Theoretical Studies On New Organometallic Donor-Acceptor Substituted Nickel(II) and Copper(II) Unsymmetrical Schiff-Base Complexes. Inorg. Chem. 2010, 49, 2750−2764. (13) Wang, C. H.; Ma, N. N.; Sun, X. X.; Sun, S. L.; Qiu, Y. Q.; Liu, P. J. Modulation of the Second-Order Nonlinear Optical Properties of the Two-Dimensional Pincer Ru(II) Complexes: Substituent Effect and Proton Abstraction Switch. J. Phys. Chem. A 2012, 116, 10496− 10506. (14) Wang, W. Y.; Du, X. F.; Ma, N. N.; Sun, S. L.; Qiu, Y. Q. Theoretical Investigation on Switchable Second-Order Nonlinear Optical (NLO) Properties of Novel Cyclopentadienylcobalt Linear [4]Phenylene Complexes. J. Mol. Model. 2013, 19, 1779−1787. (15) Zhang, M. Y.; Ma, N. N.; Sun, S. L.; Sun, X. X.; Qiu, Y. Q.; Chen, B. Quantum Chemical Study on First Hyperpolarizabilities of Mono- and Bimetal Pt(II) Diimine Complexes. J. Organomet. Chem. 2012, 718, 1−7. (16) Li, X. J.; Sun, S. L.; Ma, N. N.; Sun, X. X.; Yang, G. C.; Qiu, Y. Q. Theoretical Investigations on Electronic Spectra and the RedoxSwitchable Second-Order Nonlinear Optical Responses of Rhodium(I)-9,10-Phenanthrenediimine Complexes. J. Mol. Graphics Modell. 2012, 33, 19−25. (17) Ma, N. N.; Sun, S. L.; Liu, C. G.; Sun, X. X.; Qiu, Y. Q. Quantum Chemical Study of Redox-Switchable Second-Order Nonlinear Optical Responses of D-π-A System BNbpy and Metal Pt(II) Chelate Complex. J. Phys. Chem. A 2011, 115, 13564−13572. (18) Zou, H. Y.; Ma, N. N.; Sun, S. L.; Li, X.; Qiu, Y. Q. Structures and Redox-Switchable Second-Order Nonlinear Optics Properties of N-Legged Piano Stool Shaped 12-vertex Rhenacarborane HalfSandwich Complexes. J. Organomet. Chem. 2013, 728, 6−15. (19) Liu, C. G.; Guan, X. H.; Su, Z. M. Computational Study on Redox-Switchable 2D Second-Order Nonlinear Optical Properties of Push-Pull Mono-tetrathiafulvalene-Bis(Salicylaldiminato) Zn(II) Schiff Base Complexes. J. Phys. Chem. C 2011, 115, 6024−6032. (20) Liu, Y.; Yang, G. C.; Liu, C. G.; Sun, S. L.; Qiu, Y. Q. SecondOder Nonlinear Optical Responses Switching of N∧N∧N Ruthenium Carboxylate Complexes with Proton-Electron Transfer. Int. J. Quantum Chem. 2012, 112, 779−788. (21) Sun, X. X.; Yang, G. C.; Sun, S. L.; Ma, N. N.; Qiu, Y. Q. Effects of the Substituting Groups and Proton Abstraction on the Nonlinear Optical Properties of Heteroleptic Bis-tridentate Ru(II) Complexes. J. Organomet. Chem. 2011, 696, 3384−3391.

ASSOCIATED CONTENT

S Supporting Information *

Calculated bond distances (Å) and bond angles (deg) of systems 1ao and 2ao obtained at different methods compared with their corresponding experimental data (the atom labeling scheme is shown in Figure 1) (Table S1), calculated absorption wavelengths (nm) of open-ring systems 1ao, 2ao, 1bo, 2bo and the corresponding closed-ring systems 1ac, 2ac, 1bc, 2bc (Table S2), the βtot(−ω;ω,0) values of all studied systems at a frequency range from 0.000 to 0.065 au (Table S3), the βtot(−2ω;ω,ω) values of all studied systems at a frequency range from 0.000 to 0.065 au (Table S4), and the validation of calculation method. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Y.-Q. Qiu: e-mail, [email protected]; fax, (+86) 431 85098768. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21173035).

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REFERENCES

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