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Exploring Second-Order Nonlinear Optical Properties and Switching Ability of a Series of Dithienylethene-Containing, Cyclometalated Platinum Complexes: A Theoretical Investigation Jian Lin,†,‡ Rongjian Sa,† Mingxing Zhang,‡ and Kechen Wu*,† †

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ College of Mechanical and Electrical Engineering, Fujian Agriculture and Forest University, Fuzhou, Fujian 350002, People’s Republic of China S Supporting Information *

ABSTRACT: The second-order nonlinear optical (NLO) properties of a series of dithienylethene- (DTE-) containing Pt(II) complexes have been investigated by density functional theory calculations. The first hyperpolarizabilities β of studied systems can be greatly enhanced by simple ligand substitutions. Because of the nature of DTE units, the β values also can be varied by the use of lights in the studied systems. The highest β difference between photoisomers can over 1000 × 10−30 esu, with the contrast around five times. Thus, the studied systems can act as effective photoswitchable second-order NLO materials. The time-dependent density functional theory calculations revealed that the charge transfer patterns of studied systems have special characters compared to other reported DTE-containing NLO switched chromogens, the DTE units mainly act as electron-donors in studied systems, and the variation of β can be viewed as alternation of donor abilities of DTE units; thus, our work also proposed a new mechanism for designing photoswitched NLO multifunctional materials.

1. INTRODUCTION

diarylethenes etc. may be effective ways to design photoswitchable second-order NLO materials.16−20 The dithienylethene (DTE) and its derivatives received growing interests in the domain of molecular switches in recent years. This is due to their good fatigue resistant, photostability, and rapid and high-yield switching behavior.21−27 When irradiate by light, the DTE will undergo reversible photocyclization process, which cause the molecular structures to commute between less conjugated open-forms and more conjugated closed-forms. Thus, DTE-containing systems may be potential photoswitched NLO materials. Compared to pure organic complexes, the organometallic complexes have some better characteristics. For example, the organometallics often possess better thermal stability compare to their organic counterparts. Besides, due to the additional flexibility brought in by the metal, the NLO active chargetransfer may stronger than the pure organic counterparts, thus resulting in a larger NLO response. Therefore, coordinating with metal centers may lead the DTE-containing systems to possess both a larger NLO response and better physicochemical properties. Although lots of DTE-containing, photochromic

Because of the potential applications in the molecular photonic and electronic devices, molecular switches have attracted a great attention in the past 2 decades. To date, switching various properties such as linear and nonlinear optical properties allowed magnetic properties to be reported.1−4 Among them, the researches for molecules with switchable second-order nonlinear optical (NLO) properties received growing interest in recent years, and some reviews have been published.5−9 At the molecular level, complexes possess switchable second-order NLO properties is that the molecular first hyperpolarizability β can be varied by external stimulus. While the fundamental researches of the NLO materials have been devoted mostly to pursue large NLO response, the researches for switchable NLO materials have another goal to achieve: a notable difference of NLO outputs between different molecular states, these requests post a challenge to rational design such materials. According to Coe,10since most molecules possess large second-order NLO response have a typical donor-bridge-acceptor structures, designing alterable donor or acceptor moieties, or variable πbridge can make the molecular NLO properties respond to various external stimulus (optical, redox, electrical, magnetic, and pH variation etc.).11−15 Among them, coordinating with photochromic moieties such as azobenzenes, spiropyrans, © XXXX American Chemical Society

Received: April 9, 2015 Revised: June 12, 2015

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Since expensive computational cost limit the high-level methods such as coupled-cluster methods to be used in these large-sized systems, the static first hyprpolarizabilities βtot of studied model complexes were calculated at the DFT level too. The Gaussian 09 package provide analytical energy derivatives to calculate the components βijk of the first hyperpolarizability. In this work, the static first hyperpolarizability βtot were evaluated by eq 1:

metallo-complexes have been reported, just a few examples of NLO properties have been measured or calculated in order to explore their potential usage as NLO switches.28−36 Recently, Yam et al. reported the synthesis and characterization of a new class of DTE-containing cyclometalated Pt(II) complexes37(Figure 1). Qiu et al. studied the strategy for

βtot =

βx 2 + βy 2 + βz 2

(1)

where the βi are defined by eq 2: βi = βiii + βijj + βikk , Figure 1. Molecular structures of cyclometalated Pt(II) complexes synthesized by Yam et al.

i, j, k = x, y, z

(2)

(Note that, due to the different definition of coefficient in eq 2, the calculated βtot values by Qiu et al. must be divide by 0.6 to compare with our results). As a balance between accuracy and computational cost, the 6-31+G(d) basis set, LANL2DZ basis set and ECP were used for nonmetal atoms and Pt, respectively. While the DFT methods were used to calculate the first hyperpolarizability, the choice of exchange-correlation (XC) functional is crucial to get the reliable βtot values. Traditional DFT functional suffer a shortcoming called “short-sightedness”, this is due to the incorrect asymptotic behavior of XC potential.46,47 This shortcoming often causes the functional to underestimate long-range CT excitation energies, thus leads to overestimate the β values. One correcting method is called “range-separated”, since exact exchange has correct asymptotic behavior, range-separated method mix large amount of exact exchange in the long-range to correct the shortcoming (at least partially), while keep the short-range being regular. In this study, we chose two range-separated functional: CAMB3LYP,48,49 and ωB97X.50 Note that, these two functional have different amount of exact exchange in the long-range, while the former is 65%, the latter is 100%. For gaining more insights into the excited states of studied systems, time-dependent density functional theory (TDDFT)51,52 calculations at 6-31+G(d)/LANL2DZ level were invoked to simulate the lower excite states, the CAM-B3LYP functional was used, because it was proven to give the results match the experimental data of original systems best.

enhancing the second-order NLO properties in these systems by theoretical method.38 By increasing the thiophene rings of the C∧N (cylometalating 2-(2′-thienyl)pyridyl (thpy)) ligand, and introducing stronger electron-withdrawing substituents on pyridyl ring, the β values of both open-forms and closed-forms have several times enlargement. However, from the viewpoint of designing second-order NLO switches, such results are unsatisfactory, because we find that, with the enlargement of the β values, the contrasts (define as the ratio of βtot values of closed-forms over open-forms) are decreased. Besides, the Dvalues (the different values between the closed-forms and the open-forms) remain almost unchanged or even become smaller. It means that, for practical application as NLO switches, the modified structures which have larger β values are not necessarily better than the original one. In this work, in order to give insight on the potential usage of these systems as photoswitchable NLO materials, we reexamined the second-order NLO properties of these systems by theoretical calculations. The molecular structure and presence of Pt may possess a larger NLO response;39−43 thus, in the presence of DTE units, good physicochemical properties may make them to be appealing candidates for photoswitchable metallo-NLO materials. By using simpler ligand substitution, we designed a series of model complexes, which show very large β values and β variations. Both are far better than the results reported by Qiu et al. In these systems, not only can the β values be easily enlarged but also the variations of β can also be very prominent. Besides, insights into the electronic structures, and their relations to the NLO properties have also been analyzed by theoretical calculations; the results proposed a new mechanism to design photoswitched NLO materials.

3. RESULTS AND DISCUSSION 3.1. Geometric Structures. The geometrical structures of five model complexes are depicted in Figure 2.The model 1

2. COMPUTATIONAL DETAILS All calculations in this work were performed by using the Gaussian 09 program package.44 The original structures synthesized by Yam were obtained through CCDC database, then the modified geometries were optimized at the density functional theory (DFT) level in the gas phase by using PBE0 hybrid exchange-correlation (XC) functional,45 since this functional match the original structures best. The 6-31G(d) basis set was used for nonmetal atoms, while the LANL2DZ basis set and effective core potential (ECP) were used for Pt atom. All the optimized geometries were examined by frequency calculations to ensure them as true minima.

Figure 2. Molecular structures of model complexes 1−5.

complexes were obtained by introducing an auxiliary ligand (NN−ph) instead of the original substituents in order to increase the π-conjugation. Note that, in the work of Qiu et al., the increasing of π-conjugation is through increasing the number of thiophene rings of the C∧N ligand. Our method was proven to be much more effective on the NLO properties (see next section) and simpler. Because TD-DFT calculations reveal B

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Table 2. As seen from tables, the bond length and bond angles around the metal atoms do not have significant difference between open-forms and closed-forms. The major geometric differences come from the DTE units. For all 5 complexes, the two rotatable thiophene rings of DTE units exhibit antiparallel conformations (that is, the R3 subsituents of the DTE units point to opposite direction), with the dihedral angles drastically decrease from open-forms to closed-forms. Therefore, the variation of βtot values should be mainly assigned to the rotational effects of DTE units. It is worth noted that, unlike most other DTE-containing NLO switching systems, in which the DTE units act as variable π-bridges, the DTE units act as electron donor ligands in these systems (see section 3.3), thus the charge transfer patterns not only were affected by the ring open-closure action of the six-member rings, but mainly affected by the rotations of two thiophene rings with respect to the thienylpyridine ligands. Besides the rotations, the reorganization of the single/double bonds also play an important role on the structures change of DTE, e.g. the C2−C3 bonds have relatively obvious changes (around 0.08 Å) when commute between open-forms and closed-forms. 3.2. The NLO Properties. We investigated the NLO properties of 5 model complexes. The calculated results were listed in Table 3. As can see, the βtot values are functional dependent, but the trends calculated by 2 functional are similar (see below discusses), therefore the results obtained by two functional reach consistency. The dominated components are βxxx in all 5 complexes, which indicate that the charge transfer mainly occur along the x direction, this is further been confirmed by TD-DFT calculations. According to the paper by Qiu et al., the βtot of original system synthesized by Yam is 22.6 × 10−30 esu for open-form, 54.1 × 10−30 esu for closed-form (calculated by CAM-B3LYP, the same below), with the contrast be 2.4 and D-values be 31.5 × 10−30 esu. When added four thiophene rings on the C∧N ligand, the calculated βtot values of the open-form was 72.3 × 10−30 esu, the closed-form was 88.5 × 10−30 esu. Though the βtot increased, the contrast decrease to 1.2, the D-values also decrease to 16.2 × 10−30 esu, thus such modification is not suitable from the viewpoint of designing NLO switches. The βtot of our model complex 1 is 142.2 × 10−30 esu for open-form, 432.4 × 10−30 esu for closed-form, not only the contrast increase to 3.0, but the D-value also increase to 290.2 × 10−30 esu. These are distinct enlargement. Note that, we achieve these results by a simple modification, thus proving there are great NLO potentials in these systems. Motivated by above results, we further investigate the substitute effects in these systems. The model complex 2 is obtained through replacing the methyl at the R1 position with trifluoromethyl, this substitution further increase the contrast

the charge transfer patterns of these model complexes (see section 3.3), we further design model complexes 2−5 to study the substitute effects on these systems. The selected bond lengths, bond angles and dihedral angles of open- and closed-form species are listed in Table 1 and Table 1. Selected Bond Lengths (Å) and Angles (deg) of the Open-Form (o) of Model Complexes 1−5 1o Pt−O1 Pt−O2 Pt−N1 Pt−C1 C2−C3 N1−Pt−O2 C1−Pt−O1 N1−Pt−C1 O1−Pt−O2 N1−Pt−O1 C1−Pt−O2 C2−C3−C4−C5 C3−C2−C6−C7

2o

3o

Bond Lengths 2.098 2.116 2.111 2.011 2.030 2.028 2.016 2.010 2.010 1.982 1.983 1.982 1.392 1.394 1.395 Bond Angles 175.67 176.69 176.65 172.91 172.8 172.64 81.42 81.60 81.60 91.19 90.27 90.32 91.50 91.20 91.04 95.89 96.91 97.01 Dihedral Angle 54.38 54.40 54.04 57.86 57.34 57.37

4o

5o

2.118 2.031 2.009 1.984 1.394

2.114 2.029 2.009 1.983 1.395

176.48 172.78 81.57 90.09 91.21 97.12

176.47 172.62 81.57 90.14 91.05 97.22

53.00 57.69

52.66 57.80

Table 2. Selected Bond Lengths [Å] and Angles [deg] of the Closed-Form (c) of Model Complexes 1−5 1c Pt−O1 Pt−O2 Pt−N1 Pt−C1 C2−C3 N1−Pt−O2 C1−Pt−O1 N1−Pt−C1 O1−Pt−O2 N1−Pt-O1 C1−Pt-O2 C2−C3−C4−C5 C3−C2−C6−C7

2c

3c

Bond Lengths 2.095 2.115 2.111 2.022 2.031 2.031 2.008 2.002 2.002 1.998 1.989 1.987 1.473 1.471 1.471 Bond Angles 173.32 173.34 173.55 171.81 172.77 172.68 81.48 81.56 81.56 90.25 89.51 89.52 90.37 91.49 91.40 97.75 97.17 97.25 Dihedral Angle 7.66 8.31 8.32 16.79 18.6 18.92

4c

5c

2.122 2.044 1.998 2.009 1.473

2.117 2.044 1.999 2.008 1.472

176.02 171.70 82.00 88.28 89.79 99.83

176.31 171.65 82.08 88.37 89.65 99.82

6.14 17.22

6.11 17.54

Table 3. Static βtot (10−30 esu) for Open- and Closed-Forms of Model Complexes 1−5 CAM-B3LYP

ωB97X

complex

1

2

3

4

5

closed open D-value contrast closed open D-value contrast

432.4 142.2 290.2 3.0 287.3 105.2 182.1 2.7

453.1 128.7 324.4 3.5 303.5 93.8 209.7 3.2

947.3 272.6 674.7 3.5 589.0 186.9 402.1 3.2

678.9 145.0 533.9 4.7 511.5 103.2 408.3 5.0

1352.9 304.6 1048.3 4.4 978.9 202.7 776.2 4.8

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λega

Eegb

fegc

major contributionsd

1o

446 313 700 479 370 437 316 725 480 372 463 332 789 507 397 446 328 776 479 389 478 347 849 518 409

2.78 3.97 1.78 2.59 3.35 2.84 3.93 1.71 2.58 3.33 2.68 3.74 1.57 2.44 3.13 2.78 3.78 1.60 2.59 3.19 2.59 3.58 1.46 2.39 3.03

0.8594 0.5224 0.3666 0.4519 0.3334 0.9113 0.2446 0.3534 0.4446 0.4474 1.0803 0.2764 0.4526 0.4550 0.3957 0.8717 0.2837 0.4733 0.4385 0.4670 0.9268 0.2664 0.6185 0.4288 0.4726

H → L (76%) H → L+1 (36%) H → L (84%) H → L+1 (62%) H → L+2 (52%) H → L (61%) H−2 → L (19%) H → L (85%) H → L+2 (61%) H → L+3 (60%) H → L (54%) H−2 → L (39%) H → L (80%) H → L+2 (43%), H → L+3 (32%) H−1 → L (36%) H → L (37%) H−3 → L (53%) H → L (86%) H → L+2 (60%) H−1 → L (47%), H → L+3 (30%) H → L (46%) H−3 → L (43%) H → L (83%) H → L+2 (56%), H → L+3 (27%) H−1 → L (57%)

1c

2o 2c

3o 3c

4o 4c

5o 5c

a

Absorption wavelength (nm). bTransitions energy (ev). cOscillator strength. dH = HOMO; L = LUMO.

Figure 3. Molecular orbitals of complex 2 involved in the dominant orbital transitions (H-HOMO, L-LUMO) calculated by TD-DFT. (a) openform; (b) closed-form.

and D-values to 3.5 and 324.4 × 10−30 esu, respectively. As seen, two functional show that this increasing is through both reducing the βtot of open-form and enlarge the βtot of closedform. The model complex 3 was obtained by adding strong electron-withdrawing substituents nitro on the R2 position. This substitution has great effects on the hyperpolarizability of both open-form and closed-form. For example, the βtot calculated by CAM-B3LYP is about 2.0−2.2 times larger than complex 1, with the βtot of closed-form near 1000 × 10−30 esu,

and the D-values enlarge to 674.7 × 10−30 esu. But as may be seen, though the hyperpolarizability and D-values have been greatly enlarged by these substituents, the contrasts remain almost unchanged compared to those of complex 2. The model complex 4 were modified from complex 2 by replacing the methyl with amino on the R3 position of DTE units. Jacquemin et al. gave detailed researches on a series of DTE derivatives which have different substituents on these positions,53 their results show that, the βtot and contrasts vary with different substituents, but larger βtot do not accompany D

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The Journal of Physical Chemistry A with larger contrasts or D-values. However, our results show that, using stronger electron-donor substituent on R3 positions obviously enlarge the contrasts and D-values at the same time. Complex 4 possesses the biggest contrast 4.7 in studied systems, with the D-values over 530 × 10−30 esu. As seen, the βtot of open-forms have only slightly increase compared to complex 2, the enlargement mainly due to the distinct increase of the closed-forms. This suggests that, using different electron donor substitutents on the R3 position may be effective way to modify the switching properties of such systems. We design model complex 5 to study the combined effect of above-mentioned substitution. As expected, the βtot values and D-values are all the largest in studied systems. The contrast is a little smaller than complex 4, but still larger than other complexes. The D-value is over 1000 × 10−30 esu (CAMB3LYP), which means the difference of NLO outputs between open-/closed-form will be very significant in this complex, thus proving these systems to be not only very good NLO materials, but also promising NLO photoswitchable materials. 3.3. TD-DFT Analysis. To get a more profound understanding on the NLO responses of studied systems, we have performed the TD-DFT calculations. The calculated major lowlying excited states with its oscillator strength, transition energy and absorption wavelength are listed in Table 4. The TD-DFT calculations revealed that, the charge transfer (CT) patterns for all 5 complexes are similar, thus we take the complex 2 as an example. The major orbital involved in the CT processes of complex 2 were pictured in Figure 3. As can see, for open-form, the HOMO−2 → LUMO and HOMO → LUMO mainly arise from DTE to the pyridyl and substitute ligand; for closed-form, the situation is similar, HOMO → LUMO, HOMO → LUMO +2 and HOMO → LUMO+3 also arise from DTE to the pyridyl and substitute ligand. It can say that, the DTE act mainly as electron-donors in these studied systems. Comparing to other reported DTE-containing NLO switched systems, the NLO-active CT patterns of studied systems are unique. To the best of our knowledge, in other reported systems, although DTE unit may also act as part of electron-donor or -acceptor moieties, they mainly were designed as π-bridges, with the CT along the direction of DTE’s two rotatable thiophene rings. In those systems, the photocyclized processes vary the π-bridges, results in the variation of β-values. In these studied systems, the situation is different. Since the photocyclized processes let DTE forming polycyclic structures, and the CT arise from the whole DTE units, we consider the better way to describe the main function of DTE are variable electron-donor moieties, not just π-bridges. Accompany with the rotations of 2 thiophene rings with respect to the C∧N ligand, the donor ability of DTE units changed, thus lead to the variation of NLO properties. We illustrated our idea schematically in Figure 4. Comparing the studied systems, it can see that, using stronger electron-pushing or electron-withdrawing substitutents on the donor or acceptor moieties have different effects on the βtot contrast. For example, comparing complex 2 to complex 4, the βtot values of open-form of the latter only has a slightly increase than the former, but the closed-form of the latter shows a huge increase than the former. Base on our abovementioned opinion (Viewing the DTE as variable donors), it suggests that, due to the rotations of thiophene rings, the stronger donor ability of substituent has different scales in the open- and closed-forms. Comparing complex 2 to complex 3, we saw a uniform enlargement of open-form and closed-form,

Figure 4. Different mechanism of β variations: (a) Alteration of πbridges. (b) Alteration of electron-donor ability.

which is reasonable, since the open-form and closed-form possess the same π-bridges (since the DTEs were regarded as donors). These phenomena also can be semiquantitatively analyzed by the well-known two-level model,54 which states that, if βtot is dominated by one component of tensor (in these systems that is βxxx) and assumes that only one excited state is strongly coupled to the ground state by the applied electric field, then, β ∝((Δμeg feg)/(Eeg3)), where Δμeg is the difference between the dipole moments of the ground and the excited state, feg is the oscillator strength, and Eeg is the transition energy. To get an intuitive understanding, we plot the absorption spectra of complexes 2−4 on Figure 5. As seen,

Figure 5. Calculated absorption spectra of open-forms (a) and closedforms (b) for complex 2 (black), complex 3 (red), and complex 4 (blue). E

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for the open-form, the absorption curve of complex 2 and complex 4 are closely adjoined, whether on the position of absorption maximum or on the intensity, while the curve of complex 3 stands alone, with a prominent red-shift and stronger absorption peak. For the closed-form, the situation is inverted, the absorption peak correspond to the first excited states of complex 3 and 4 closely adjoined and red-shift with higher intensity, while left the complex 2 stands alone. At the end of this section, we give some comments on the NLO properties and switching properties of studied systems. Ideally, NLO switches should possess not only large hyperpolarizabilities, but also large NLO contrasts. In these studied systems, the NLO responses are very large. The reasons are mainly due to the presence of central Pt atoms, which directly coordinate with DTE units, pyridyl rings and O∧O ligands, forming large and planar π-conjugations, and improve the intensity of NLO active CT. The calculated contrasts are moderate (In a recent benchmark studied, Jacquemin et al. found the average contrast of mono-DTE compounds is around 4.53), the prominent characteristics of studied systems is that the β D-values are very large. We can see that the limitation of contrasts mainly due to the relatively large β values of open form (since they still possess relatively large π-conjugations), thus introducing multi-DTE units to coordinate with metal center may be a promising way to further increasing the contrasts and achieve the two goal of NLO switches in DTEcontaining systems (e.g., two recent studies reported by Le Bozec et al.34,35).

AUTHOR INFORMATION

Corresponding Author

*(K.W.) E-mail: [email protected]. Fax: 86-591-83792932. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports from FuJian Natural Science Foundation (Nos. 2011J05017, 2013J05040) and NSFC project (Nos. 91122015, 21201165, and 21171165). The authors thank the Supercomputing Environment of CAS for hardware and software supporting.

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REFERENCES

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4. CONCLUSIONS The DFT calculations on the first static hyperpolarizability of a series of DTE-containing Pt(II) organometallic complexes have been carried out in this article. The results prove the large second-order NLO responds of these systems. Very large static βtot values were achieved by simple ligand substitution. Because of the nature of DTE units, these systems also have stable photoisomers, the βtot differences between photoisomers are very prominent. By simple substitution on the DTE units and the pyridyl ring, the largest difference was observed in model complex 5, with different-value over 1000 × 10−30esu (CAMB3LYP), the contrast also can be enlarged to around 5 times in model complex 4 and 5. It proved that, these DTE-containing, cyclometalated Platinum complexes not only are good NLO materials, but also promising phototriggered NLO switched materials. Through TD-DFT calculations, we analyzed the CT patterns in these model complexes. We found that, the main CT routes are from the DTE units to the other parts of molecule, the DTE units act mainly as electron-donors in these systems, which make them unique from the other reported DTE-containing photoswitched NLO systems. The photoresponse DTE units can also be used as alterable electron-donor or acceptor moieties proposed a new mechanism to design DTE-containing photoswitched NLO materials.

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ASSOCIATED CONTENT

* Supporting Information S

Figures comparing the major orbital transition of the original complex synthesized by Yam et al. and model complex 1 in this paper. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.5b03456. F

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