Theoretical Exploration of Photoisomerization-Switchable Second

Article pubs.acs.org/JPCA

Theoretical Exploration of Photoisomerization-Switchable SecondOrder Nonlinear Optical Responses of Two-Dimendional Λ- and W‑Shaped Polyoxometalate Derivatives of Dithienylperfluorocyclopentene Teng-Ying Ma, Na−Na Ma, Li-Kai Yan,* Ting Zhang, and Zhong-Min Su* Institute of Functional Material Chemistry, Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Ren Min Street No.5268, Changchun, Jilin 130024, PR China ABSTRACT: The switchable second-order nonlinear optical (NLO) properties on two-dimensional (2D) molecules based on Lindqvist-type [Mo6O19]2− and dithienylperfluorocyclopentene (DTE) have been investigated at density functional theory (DFT) level. The CAM-B3LYP and M06-2X functionals were employed to study the switching behavior on NLO properties by photoisomerization reaction. The βtot value of system 2c (closed-ring form) is 15920.5 au, which is 150.1 times larger than that of the corresponding open-ring form (system 2o). The timedependent DFT calculations predict that the charge transfer from DTE to polyoxometalate, and DTE intramolecular charge transfer in closed-ring systems effectively improve the static first hyperpolarizability. Furthermore, the Λ-shaped systems possess a larger u value than those of W-shaped systems owing to different orientation for substituent groups.

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INTRODUCTION

The molecule-based second-order nonlinear optical (NLO) materials involving new scientific phenomena and offering potential application in emerging optoelectronic technologies1−3 have attracted a lot of interest. In the past few years, molecular switching processes have evoked increasing attention,4−8 especially the significant interesting in the reversible of NLO properties. The initial studies were focused on the preparation and design of NLO materials. To date, the switching of NLO properties has been achieved by protonation/deprotonation, oxidation/reduction, and photoisomerization.9−13 In this paper, the photoisomerization was used to switch the NLO properties. Photochromism is a light-induced reversible chemical phenomenon between two isomers, which exhibit distinct absorption spectra under a certain wavelength and intensity of light.14 Apart from the color, the two isomers also differ in various chemical and physical properties, and in molecular structure. Recently, the photochromic molecules have been widely studied as photoswitching units in a variety of photoresponsive molecules, and the most studied photoswitches are derivatives of dithienylethene.15−18 This class of molecules is switchable between open and closed forms accompanied by light with reversible cyclization and cycloreversion reactions for the C−C bond in the six-membered ring. Among this kind of photochromic compound, dithienylperfluorocyclopentene (DTE) has been the focus of attention due to its outstanding thermal stability and low photofatigue (Figure 1).19,20 Furthermore, density functional theory calculations have revealed that the introduction of the © 2013 American Chemical Society

Figure 1. Photoisomerization of the DTE.

DTE moiety as the π-conjugated bridge not only plays the switch role but also significantly enhances the second-order NLO response relevant to the organic donor/acceptor end.21 Although the organic materials possess outstanding advantages, disadvantages are not ignored, such as low thermal stability, facile relaxation to random orientation, and so on. The new materials are required to overcome the existing shortcomings. Previous works have shown that the organic−inorganic hybrid materials are a good choice,22−24 which are combined with the advantages of the organic and inorganic materials to realize the value-added properties. Polyoxometalates (POMs) have constituted a rich class of molecular metal oxygen clusters and exhibit remarkable chemical, physical properties, which have been applied to a variety of fields, such as medicine, catalysis, biology, analytical chemistry, materials science, and so on.25−28 Over the past few years, there has been increasing interest in the investigations of Received: July 23, 2013 Revised: September 15, 2013 Published: September 18, 2013 10783

dx.doi.org/10.1021/jp407314a | J. Phys. Chem. A 2013, 117, 10783−10789

The Journal of Physical Chemistry A

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POMs.29−31 POMs covalent bonds with organic groups or organometallic groups via linkages have constituted an exceptionally interesting class of organic−inorganic hybrid materials, which exhibit remarkably large NLO response.32−34 POMs as electron acceptors enable the formation of hybrid materials in which the delocalized electrons coexist in both the organic network and the inorganic clusters. Also, this kind of hybrid materials has been rapidly developed owing to their widely potential application in various fields. Moreover, the first and second hyperpolarizability of organic−inorganic hybrid POM derivatives with one-dimensional π-conjugated have been studied with density functional theory (DFT) by our group.35,36 Typical second-order NLO chromophores are one-dimensional π-conjugated systems modified with donor (D) and acceptor (A) moieties. Extending the conjugation and optimizing the donor/acceptor strengths can effectively improve the molecular hyperpolarizability. Beyond the classical one-dimensional dipolar systems, a concept based on molecules with two-dimensional (2D) geometries has been proposed.37 The 2D compounds have been observed to possess better phase-matching than one-dimensional chromophores because of their larger off-diagonal components.38,39 A series of 2D NLO molecules have been synthesized and their second-order NLO responses have been measured.40,41 Moreover, the twodimensional Λ-shaped molecules possessing significant secondorder NLO responses have also been observed owing to the large off-diagonal β tensor component.42−47 It suggests that these kinds of 2D systems have potential applications in the field of nonlinear optics. In this paper, we design a series of 2D systems based on Lindqvist-type [Mo6O19]2− and dithienylperfluorocyclopentene. The POMs were modified with dimethylphenyl and nitro to extend the π-conjugated and enhance the donor strength. Due to the different positions of the substituent groups on POM, two kinds of Λ-shaped and W-shaped molecules48 were designed. Herein, we named opposite angle position substituted systems as Λ-shaped molecules and vertical angle position substituted systems as W-shaped molecules. The molecular structure and switchable second-order NLO responses are investigated in detail using density functional theory method. The calculation models are shown in Figure 2. In this paper, the open-ring forms are named as 1o−5o, and the corresponding closed-ring forms are 1c−5c.

Article

COMPUTATIONAL DETAILS The density functional theory (DFT) with Becke’s threeparameter exchange functional combined with the Lee−Yang− Parr correlation functional (B3LYP)49−51 has been widely used to optimize the molecular geometries. Herein, the geometries of all systems were optimized at the B3LYP/6-31G(d) level (LANL2DZ basis set for metal atoms). The ground states of studied systems were closed-shell singlet states. The finite field (FF) method was broadly applied because this methodology can be used in concert with the electron structure method to compute the second-order polarizability (β).52−54 When a molecule is subjected to a static electric field (F), the energy (E) of the molecule is expressed by eq 1 E = E(0) − μi Fi −

1 1 1 αijFF β FF γ FF i j − i jFk − i jFkFl 2 6 ijk 24 ijkl (1)

− ... (0)

In this expression, E is the energy of the molecule in the absence of an electronic field, μi represents the components of the dipole moment vector, α is the linear polarizability tensor, and β and γ are second- and third-order polarizability tensors, respectively. The subscripts i, j, and k label x, y, and z components. It is clear that the values of μ, α, β, and γ can be obtained by different E with respect to F. In this article, we have calculated the static second-order polarizability using the FF method55,56 with a field frequency of 0.0010 au at two functionals, hybrid functional M06-2X, and CAM-B3LYP functional containing long-range corrected effects used Coulomb-attenuating method. The static second-order polarizability, βtot, was noted as (eq 2) βtot = (βx 2 + βy 2 + βz 2)1/2

(2)

where βi = (βiii + βijj + βikk), i, j, k = x, y, z. To better describe the second-order NLO behavior of systems, the excitation energies of systems were calculated using the time-dependent (TD) DFT method due to its efficiency and accuracy. And the natural bond orbital (NBO) analysis was performed by the NBO program57 at the B3LYP/ 6-31G(d)-LANL2DZ level. All the calculations in this work were carried out by using the GAUSSIAN 09W program package.58

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RESULT AND DISCUSSION 1. Geometric and Electronic Structure. From the optimized structures, the open-ring and the closed-ring forms have significant difference. The obvious difference is reflected in the dihedral angle between the two thiophenes of DTE. For open-ring forms, systems 1o−5o, two thiophene rings are nonplanar and the dihedral angle is ∼79°. In contrast, the dihedral angle decreases to ∼32° for closed-ring form. These changes in geometric structure effectively improve πconjugation for closed-ring form. The Mo−N−C angles for open-ring forms are ∼174−175°, which are larger than those of closed-ring form (∼172° for systems 1c, 2c, 4c, and 5c). In addition, the Mo−N−C angle for system 3c is ∼162°, which is ∼10° smaller than other closed-ring systems. The large discrepancy in the geometry may be result in different electronic structure. The frontier molecular orbitals of all systems have something in common. Taking systems 1o and 1c for examples and the

Figure 2. Calculation models of systems 1o−5o, and systems 1c−5c defined corresponding to closed-ring forms of 1o−5o. 10784



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Figure 3. Frontier molecular orbitals, orbital energies, and HOMO−LUMO energy gaps of systems 1o and 1c.

Table 1. Analysis of NBO for Systems 1o and 1c system

natural bond orbitals

occupancy

1o (right)

Mo−N Mo−N Mo−N Mo−N Mo−N Mo−N

1.90 1.83 1.76 1.90 1.84 1.70

1c (right)

orbital coefficients and hybrids 0.45 0.54 0.57 0.45 0.55 0.55

(sp1.73d1.62)Mo + 0.89(sp0.77)N (pd4.08)Mo + 0.84 (p)N (pd4.61)Mo + 0.82 (p)N (sp1.75d1.58) Mo + 0.89(sp0.78)N (pd4.47)Mo + 0.83 (p)N (pd3.91)Mo + 0.84 (p)N

orbital type σ π π σ π π

Table 2. Static First Hyperpolarizability βtot (au), and In-Plane Nonlinear Anisotropy u of Systems 1o−5o and 1c−5c systems

functionals

βxxy

βyyy

1o (Λ)

M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP M06-2X CAM-B3LYP

−461.9 −414.5 −238.8 −249.9 −478.6 −92.3 5409.3 −5533.8 2095.3 1574.6 −13610.9 −13262.3 −16023.9 −15605.5 −8135.5 −8263.6 −24657.3 −24237.5 −8880.6 −8640.1

196.2 180.7 153.1 149.9 −1022.3 −971.5 170.6 175.3 7253.2 7451.2 −5.4 −71.8 −142.0 −191.6 −1734.6 −1768.3 −34.5 −69.9 8552.7 8812.0

2o (Λ) 3o (W) 4o (Λ) 5o (W) 1c (Λ) 2c (Λ) 3c (W) 4c (Λ) 5c (W)

u= βxxy /βyyy −2.29 −1.67 0.10 −31.57 0.21 184.74 81.44 4.67 346.97 −0.98

βtot 304.6 278.4 106.3 106.1 1611.1 1544.5 5259.7 5378.5 10919.4 11178.2 13709.4 13440.5 16300.8 15920.5 10065.7 10286.9 24834.1 24447.0 1993.8 1941.5

interaction for two parts. The strong interaction was attributed to the MoN triple bond, which has been studied in arylimido−hexamolybdate.34 To explain this interaction, we performed NBO calculations on all systems to understand the bonding characteristics between molybdenum and nitrogen. It is found that the bonding characteristics for the left and right in one molecule are nearly same. Studying systems 1o and 1c (Table 1) by NBO analysis reveals that the Mo−N triple bond is composed of one MoN σ bond and two MoN π bonds. In system 1o, the MoN σ bond is formed by a Mo (sp1.73d1.62) orbital and a N (sp0.77) orbital, whereas the two

frontier molecular orbitals are displayed in Figure 3. In system 1o, the highest occupied molecular orbital (HOMO) and HOMO−1 locates on the d−p π-bond that comes from d orbital of Mo atom and p orbital of N atom, and the π-bond is from the p-carbon orbital of the adjacent thiophene. The lowest unoccupied molecular orbital (LUMO) and LUMO+1 mainly concentrate on the Mo atoms and the bridge O atoms. Compared with system 1o, the closed-ring form (system 1c) shows good delocalization. For systems 1o and 1c, the orbital populations in the bonding orbitals between inorganic and organic parts exhibit a large overlap that illustrates strong 10785



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results demonstrate that the Λ-shaped systems possess better 2D character of the optical nonlinearity. 2.2. Switchable Properties. To emphasize the switching behavior of the first hyperpolarizability duo to the structure photoisomerization, the βtot values calculated by different functionals of all systems have been compared in Figure 4. It

MoN π bonds are made up of a hybrid Mo orbital and a pure p-N orbital, respectively. Furthermore, the closed-ring system 1c has similar bonding characteristics with system 1o. The NBO result coincides with orbital analysis. Figure 3 also lists the orbital energies of HOMO and LUMO for systems 1o and 1c. It can be seen that the HOMO energy of system 1c is higher than that of system 1o, whereas the LUMO energy is lower than that of system 1o. As a result, the calculated energy gap between the HOMO and LUMO (H−L) for system 1c is smaller than that of system 1o by ∼1.48 eV. It suggests that the change of geometry accompanying with the interlinkage of the C−C bond in six members ring heightens the energy level of HOMO and reduces the energy level of LUMO for the closed-ring form. Therefore, the H−L energy gaps of closed-ring systems are smaller than corresponding open-ring systems. Furthermore, other pair systems exhibit the same feature in energy gap. Generally, a lower H−L energy gap is in favor of larger NLO response.59,60 In the following work, the values of the static first hyperpolarizability (βtot) have been calculated. 2. NLO Properties. 2.1. 2D NLO Character of Open-Ring Systems 1o−5o. The static first hyperpolarizability (βtot) has been calculated using the FF method at CAM-B3LYP and M06-2X functionals with the 6-31G(d) basis set (LANL2DZ basis set for Mo atoms) in this work. As the data shown in Table 2 indicate, the hybrid functional M06-2X and the longrange correction functional CAM-B3LYP yield the same order of βtot values. Furthermore, we compared the βtot values of five systems with the open-ring DTEc monomer, which is 278.1 au of the CAM-B3LYP functional.61 The comparative results show that introducing the POMs to the DTE scarcely impacts the first hyperpolarizability of system 1o (278.4 au). But the first hyperpolarizability is significantly affected when the DTE is modified by the POM derivative. It can be seen that introducing the 2,6-dimethyphenyl to the system, gives βtot values of 106.1 and 1544.5 au for systems 2o and 3o. We further introduce a strong electron-withdrawing group (−NO2) (systems 4o and 5o); then the first hyperpolarizabilities enormously increase. According to the CAM-B3LYP functional calculation, the βtot values of systems 4o and 5o are calculated to be 5378.5 and 11178.2 au, which are 19.3 and 40.2 times larger than that of the DTE monomer (278.1 au). In addition, the W-shaped systems 3o and 5o exhibit larger βtot values than the corresponding Λ-shaped systems 2o and 4o, respectively. This phenomenon may result from the different position for substituent groups and the ground state geometric structure. For the sake of describing the 2D NLO character of the studied systems, we introduce the ratio u = βxxy /βyyy (the molecular with 2D structure in the xy plane, two β tensor components βyyy and βxxy should be substantial), which defines the “in-plane nonlinear anisotropy”.62−64 The u value is a very sensitive function of the off-diagonal tensor components and a relevant parameter for describing the 2D character of the molecule. The calculated u (u = βxxy/βyyy) values with CAMBLYP functional have been summarized in Table 2. From Table 2, there is an obvious difference in u values between the two types systems (Λ-shaped and W-shaped). For the Λ-shaped systems (systems 1o, 2o, and 4o), the off-diagonal tensor βxxy is larger than the diagonal tensor βyyy, that is, u > 1. For the Wshaped systems 3o and 5o, a significant decline of the in-plane nonlinear anisotropy has been observed, the u values of systems 3o and 5o decrease to 0.10 and 0.21, respectively. All of these

Figure 4. Comparison of the static first hyperpolarizability of systems 1o−5o and 1c−5c.

can be found that the βtot values of closed-ring systems are usually larger than those of open-ring systems, which are consistent with the trend of calculated energy gaps. Ordinarily, the lower H−L energy gap is helpful for enhancing the NLO response. For instance, the calculated βtot value of the closedring system 1c by the CAM-B3LYP functional is 13440.5 au, which is 48.3 times larger than that of open-ring system 1o. Moreover, the βtot value of system 1c is larger than that of DTEo monomer (208.0 au). The multiplying factor for system 2 (β2c/β2o) is 150.1 times by the CAM-B3LYP method, and other pairs of systems also show good switching properties on second-order NLO coefficients. From the structure−property point, the larger βtot values of closed-ring systems are mainly due to the better conjugated geometry structure. It proposes that the photoisomerization reaction significantly affects the static first hyperpolarizability. So these kinds of complexes might be promising candidates for switchable NLO materials. It is worthwhile to note that the open-ring system 5o possess a larger βtot value than the closed-ring system 5c. The multiplying factor for system 5 (β5o/β5c) is 5.8, which is different from other systems. Therefore, we analyze the possible reason that leads to the above result. We found that the structure of system 5c is very unusual compared with other systems. The dihedral angle between the benzene ring and the Mo4O4 ring of system 5c is ∼62°, and other systems are smaller than ∼10°. The change in the geometry in system 5c effectively reduces πconjugation, which causes the smaller βtot value comparing with system 5o. In addition, the in-plane nonlinear anisotropy of the five pairs systems exhibits the amplificatory relations. For example, the u value of Λ-shaped system 2c is 81.44, which is 48.8 times larger than that of system 2o. The closed-ring systems possess larger u values compared to corresponding open-ring systems. In other words, the photoisomerization reaction from the open-ring form to the closed-ring form heightens the in-plane anisotropy. 10786



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Figure 5. Molecular orbitals of systems 1o and 1c involved in the dominant electron transition.

It suggests that this kind of systems possess good switchable 2D NLO properties. 3. TDDFT Calculations. To get more insights into the second-order NLO responses of 2D systems, we have performed the TDDFT calculations on the electron transition. Take systems 1o and 1c, for example; the orbital features corresponding to the maximal absorption bands were listed in Figure 5. To understand the electron density distributions of the corresponding molecular orbitals more intuitively, the Mulliken population analyses for systems 1o and 1c were performed and the results were collected in Table 3. From Table 3. Mulliken Population Analyses of the Molecular Orbitals for Systems 1o and 1c system

orbital

DTE

2N

2POM

1o

HOMO−1 HOMO LUMO+2 LUMO+3 HOMO LUMO

48 54 16 15 68 36

15 13 4 4 4 0

24 22 62 68 15 44

1c

Figure 6. Electronic difference density maps of all systems.

transfer similar to that of system 1o, the small difference is that the nitro substituent in systems 4o and 5o also serves as the electron acceptor, making the larger degree of charge transfer. This may be the reason that systems 4o and 5o exhibit larger βtot values than other open-ring systems. However, the molecules exhibit good conjugation in the closed-ring form systems 1c−5c, such as system 1c, where the mixing transitions of the two types played a significant role in charge transfer. As mentioned above, the mixing transitions may effectively improve the static first hyperpolarizability, which is agreed with the closed-ring systems possessing larger βtot values than open-ring systems, as discussed in the NLO Properties section.

Table 3 and Figure 5, the occupied orbitals (HOMO−1 and HOMO) of system 1o mainly localize on the DTE, 48% and 54%, respectively, and they also contain some contributions from the N atom and POM. The unoccupied orbitals LUMO +2 and LUMO+3 mainly localize on POM (62% and 68%), and the DTE has little contribution, 16% and 15%. The charge transfers were considered from DTE (the p orbitals of carbon atoms) and N atom (the p orbitals) to POM (the d orbitals of molybdenum atoms and p orbitals of oxygen atoms). This illuminated that the DTE and N atom act as the electron donors, the POM acts as the electron acceptor. In system 1c, the HOMO mainly delocalizes on the DTE (68%), and the unoccupied orbital LUMO mainly delocalizes on POM (44%) and DTE (36%), which is assigned the charge transfer from DTE, N atoms to POM, and the DTE intramolecular charge transfer (π → π*). In addition, the π → π* transition is more different from that of the corresponding open-ring system 1o. Moreover, system 1c exhibits a larger βtot value than system 1o. So the mixing transitions for the two transition types may play an important role for NLO properties of system 1c. For simply and intuitively describing the charge transfer, we have plotted the electronic difference density maps (EDDMs) in Figure 6, and the direction of charge transfer is from the purple part to the pink. It can be seen that the open-ring systems have charge

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CONCLUSION In this paper, we systemically investigated the electronic structure, second-order NLO properties of a series of 2D Λshaped and W-shaped systems based on the chromophore DTE and [Mo6O19]2− derivative with DFT and TDDFT methods. The geometric structures of studied systems show that the photoisomerization processes significantly affect the geometrical structure and the closed-ring systems with planar structure provide a good π-conjugation. The photoisomerization accompanying conversions from open-ring form to closedring form lowers the H-L energy gap and leads to a larger static first hyperpolarizability. The TDDFT calculations exhibit that the mixing transitions in closed-ring systems significantly 10787



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improve the NLO properties. The βtot value of system 2c (closed-ring form) is 150.1 times larger than that of open-ring form (system 2o). So these kinds of complexes might be promising candidates for switchable NLO materials. Furthermore, the Λ-shaped systems possess larger u values than Wshaped systems due to the different orientation of substituent groups.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by NSFC (20971020, 21073030, and 21131001), Program for New Century Excellent Talents in University (NCET-10-318), Doctoral Fund of Ministry of Education of China (20100043120007), and the Science and Technology Development Planning of Jilin Province (20100104).

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