Redox and Photoisomerization Switching the Second-Order Nonlinear

ARTICLE pubs.acs.org/JPCC

Redox and Photoisomerization Switching the Second-Order Nonlinear Optical Properties of a Tetrathiafulvalene Derivative Across Six States: A DFT Study Chun-Guang Liu,†,‡ Zhong-Min Su,*,† Xiao-Hui Guan,*,‡ and Shabbir Muhammad§ †

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China ‡ College of Chemical Engineering, Northeast Dianli University, Jilin City, 132012, P. R. China § Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

bS Supporting Information ABSTRACT: The switching of second-order nonlinear optical (NLO) properties for a tetrathiafulvalene (TTF) derivative across the six stable states has been studied by using the density functional theory (DFT) calculations. The redox-active TTF unit and a photoisomerized chromophore 1,2-dithienylperfluorocyclopentene (DTE) have been implemented to switch the secondorder NLO responses. Our DFT calculations with three functionals demonstrate that introduction of the DTE moiety into the π-conjugated bridge can significantly enhance the second-order NLO response relevant to the donor/acceptor end in this work. Our DFT calculations illustrate that photoisomerization bring forth a large change in the geometry of the series of compounds. The closed-ring form possesses a good π-conjugation relative to the open-ring form and thus a large second-order NLO response. The electronic structure analysis shows that the TTF unit will perform as an oxidation center in the one- and two-electronoxidation processes. The one- and two-electron-oxidized species have better planar structures of TTF unit than its neutral compound, which ultimately leads to the low excited energy and enhances the static first hyperpolarizability. Our present DFT calculations using three functionals show that the TTF derivative 4 can switch the second-order NLO properties across six stable states, which is a rare example in previously reported second-order NLO switches.

1. INTRODUCTION The development of molecule-based digital-information-processing components has provided the focus for theoretical and experimental studies of the molecular switches and logic gates over the last two decades.1 The prerequisite for the molecular switch is that a molecule must exist in at least two stable states with different chemical or physical properties for matching the binary zero and one. Bistable molecules thus have been regarded as a good candidate for binary digital architecture because one property, such as linear optical properties (absorbance, fluorescence), can be interconverted by redox, r 2011 American Chemical Society

magnetic, protonation, or photocyclization, and so forth. In principle, ternary or higher-order digit representations may have much superiority relevant to binary for permitting smaller device components.2 This indicates that molecules should exist in more than two stable and independently addressable states for these high-order digit representations relative to binary.1a,2 However, it is poorly explored. Received: May 29, 2011 Revised: October 28, 2011 Published: October 28, 2011 23946

dx.doi.org/10.1021/jp2049958 | J. Phys. Chem. C 2011, 115, 23946–23954

The Journal of Physical Chemistry C Chart 1. Sequential and Reversible Redox Reaction of the TTF

To data, molecular switches exhibiting changes in some properties including, color,3 luminescence,4 optical nonlinearity,5 or magnetic properties6 have been reported. Among them, the second-order nonlinear optical (NLO) property may be a potentially important procedure to explore molecular switches. Secondorder NLO effects, especially electro-optic (EO) modulation is important in interfacing massive amounts of electronic data to wideband optical communication.7 At the molecular level, to achieve high EO efficiency, a second-order NLO molecule must have large the first hyperpolarizability (β). The typical organic second-order NLO chromophores have mostly focused on the donor-π conjugated-acceptor (DπA) structure.8 A series of the molecular switches with DπA structure has been reported.5 The first hyperpolarizability of these DπA molecules can be manipulated by reversibly modifying the donor and acceptor capacity and the nature of the π-conjugated bridge. The reported methods include redox, protonation, and photocyclization, and so forth.5c However, most of them are restricted to switching between three states at most. To obtain the effective switching of the molecular first hyperpolarizability, several questions should be considered. The stability of two forms relative to the switching of ‘on’ and ‘off ’ is important because they can preferably be switched from one to other in an easily controlled way and the fast response time.5c The redox-active tetrathiafulvalene (TTF) unit thus has been focused by our group because it is able to exist in three different stable redox states (TTF, TTF 3 +, and TTF2+). The TTF unit can be oxidized to the radical cation and dication sequentially and reversibly at low potentials.9 The 14 π-electron TTF unit is nonaromatic according to the H€uckel rule, but its oxidation species, the radical cation TTF 3 + and dication TTF2+, are aromatic in the H€uckel sense, which possess aromaticity with one and two 6 π-electrons, respectively (Chart 1). Moreover, the radical cation TTF 3 + and the dication TTF2+ are thermodynamically stable species because of the aromatic character. The electron absorption spectra of the TTF, TTF 3 +, and TTF2+ are decisively different from one another. According to the sum-over-states description, any modification of the absorption spectrum of a molecule would contribute to the modification of the first hyperpolarizability.10 Thus, the excellent redox properties of the TTF moiety and good thermodynamic stability of its oxidized species, especially the different aromaticity relevant to the various oxidation states, provides an ideal model for redox switching of second-order NLO responses. A series of redox-switchable second-order NLO molecules containing the TTF unit has been probed by our group based on the density functional theory (DFT) calculations.11 However, all of them are restricted to switching between three states. Photochromic materials have attracted considerable attentions due to their potential applications in a variety of practical fields, including the optoelectronic devices, memories, and switches.12 Thiophene derivatives of perfluorocyclopentene are a particularly interesting class of photochromic systems undergoing reversible photochemical cleavage of the CC bond in a six-membered ring. These compounds have been extensively

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Chart 2. Photocyclization of the Thiophene Derivatives of Perfluorocyclopentene

studied because of their fatigue resistance and good photostability (Chart 2).13 The photocyclization of this chromophore (open- and closedring forms) significantly changes the π-electron conjugation. The complete photocyclization of its derivative may alter secondorder NLO responses between two states because of the significant change on the π-conjugation.14 To explore the higher-order digit representations more than three states for second-order optical nonlinearity, we describe here a theoretical study of a series of new molecules for switching second-order NLO responses among six stable states.

2. COMPUTATIONAL DETAILS The geometries of all compounds were optimized and characterized as energy minima at B3LYP/6-31 g (d) level.15 The static first hyperpolarizability β was calculated by using the finite field (FF) method16 with field of 0.0010 au at B3LYP/CAMB3LYP/LC-BLYP/6-31 g(d) level (The numerical stability of the calculated β value has been checked, Supporting Information). It should be stressed that the B3LYP hybrid functional sometimes overestimates the first hyperpolarizabilities of DπA systems because of the incorrect long-range charge transfer behaviors between donor and acceptor. As a checking, the long-range corrected functionals, CAM-B3LYP17 and LCBLYP18 were also used for all hyperpolarizability calculations. The static first hyperpolarizability, βtot, for all compounds was calculated by using the following equation (eq 1): βtot ¼ ðβ2x þ β2y þ β2z Þ1=2

ð1Þ

where βi is defined by (eq 2) " βi ¼ βiii þ

∑

i6¼ j

# ðβijj þ βjij þ βjji Þ 3

ð2Þ

Moreover, to obtain a more intuitive description of trends in the second-order NLO behavior of the series of molecules, timedependent (TD)DFT methods were used to descript the molecular electronic spectrum. It has been proved that TDDFT is a usefully accurate approach for many applications,19 especially, low-lying single excitations. TDDFT is the most popular method for calculating excitations currently. The calculation of the natural bond orbital (NBO) analysis is performed at the B3LYP/6-31 g(d) level. Spin unrestricted calculations were performed for all of the open-shell systems in this work. And the calculated square of total spin is quite close to its eigenvalues, which indicates that the spin contamination is minor. All of the calculations in this work were carried out by using the GAUSSIAN 09 program package.20 23947

dx.doi.org/10.1021/jp2049958 |J. Phys. Chem. C 2011, 115, 23946–23954

The Journal of Physical Chemistry C

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Figure 1. Structural formula of the series of the DTE derivative, where the DTE unit has been supposed as donor (1 and 10 ), acceptor (2 and 20 ), and πconjugated bridge (3 and 30 ).

Table 1. Static First Hyperpolarizability (1030 esu), β, for 1, 2, and 3 (βtot(O)), and Relevant Photocyclized Species 10 , 20 , and 30 (βtot(C)) (B3LYP/CAM-B3LYP/LC-BLYP/6-31g(d) Calculations) compounds 1

2

3

functionals

βtot(O)

compounds 10

functionals

βtot(C)

βtot(C)/βtot(O)

B3LYP

9.5

B3LYP

84.8

8.9

CAM-B3LYP

27.2

CAM-B3LYP

134.7

4.9

LC-BLYP

37.5

LC-BLYP

107.7

2.9

B3LYP

118.4

B3LYP

743.7

6.3

CAM-B3LYP

62.1

CAM-B3LYP

443.1

7.1

LC-BLYP

39.1

LC-BLYP

365.7

9.4

B3LYP

874.4

B3LYP

2915.7

3.3

CAM-B3LYP LC-BLYP

230.9 170.3

CAM-B3LYP LC-BLYP

2196.8 1538.0

9.5 9.0

3. RESULTS AND DISCUSSION The NLO properties of molecules containing 1,2-dithienylperfluorocyclopentene (DTE) fragment already have been reported,2 but a detail discussion about the role of DTE in the DπA structure is still lacking. We thus design three systems, where the DTE fragment is supposed as donor (1), acceptor (2), and π-conjugated bridge (3), respectively (Figure 1). These systems contain the strong electron donor N,N-bis-(4-methoxyphenyl)phenyl-amino and electron acceptor 2-dicyanomethylen-3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-2,5-dihydrofuran (Figure 1).

20

30

The β values calculated by different functionals in GAUSSIAN 09 program have been listed in Table 1. It can be found that the β value of 1, 2, and 3 are functional-dependent, and the long-range corrected functionals, CAM-B3LYP and LC-BLYP, seem to underestimate the hyperpolarizabilities of 2 and 3 and overestimates that of 1 when compared with the B3LYP result (Table 1). All of the functionals yield the same order of β values, 3 > 2 > 1. The β values of the relevant closed-ring forms caused by the photocyclization, 10 , 20 , and 30 also have been calculated at the same levels (Figure 1). Three functionals also provide the same 23948



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order of the β values, 30 > 20 > 10 . This indicates that introduction of the DTE into to the π-conjugated bridge segment may be a good option for the DπA molecule. The calculated β value of the closed-ring form 30 is ∼3 and ∼9 times as large as that of the open-ring form 3 according to B3LYP and long-range corrected functionals (CAM-B3LYP and LC-BLYP), respectively. Our optimized calculations provide a clearly different geometry between the open-ring form 3 and the closed-ring form 30 , where the two thiophene rings of the DTE are nonplane for 3, the dihedral angel between two thiophenes is ∼82°, and the dihedral angel between two thiophene rings of the DTE decreases to ∼32° for 30 (Figure 2). This change on the geometry will enhance π-conjugation. The total energies including electronic and zero-point corrected energies of the closed-ring and openring forms have been calculated. The results show that the openring form is 9.16 cal mol1 more stable than that of the closedring form. On the basis of the complex sum-over-states expression, Oudar and Chemla21 established a simple link between the molecular hyperpolarizability and a low-lying energy charge transfer (CT) transition through the two-level model, β µ ðμee μgg Þ

fos ΔE3ge

ð3Þ

where μgg and μee are the ground and excited state dipole moments, fos is the oscillator strength, and ΔEge is the transition energy. Those factors (μee μgg, ΔEge, and fos) are all intimately related and are controlled by electron properties of the donor/acceptor and the nature of the conjugated bridge. The optimal combination of these factors will provide the maximal β value. To get more insights of the difference on the second-order NLO response between open- and closed-ring forms, we have performed the TDDFT calculation on the excited state. The

Figure 2. Optimized structure of the open-ring form 3 and closed-ring form 30 obtained by B3LYP/6-31 g(d) calculations.

TDDFT calculated excited energies, oscillator strengths, and associated orbital transitions have been listed in Table 2. It can be found that the first excited state of the open-ring form 3 is generated by the promotion of one electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The frontier molecular orbitals of the open-ring form 3 have been shown in Table 2. It can be found that the HOMO is mainly localized on the electron donor end, ethylene fragment, and one thiophene ring of the DTE, and the LUMO is mainly localized on the electron acceptor and another thiophene ring of the DTE, which indicates that the occupied donor orbital (HOMO) and the empty acceptor orbital (LUMO) are separated completely because of the nonplanar geometry of the open-ring form 3. As is clearly seen from Table 2, an excitation corresponding to the HOMO f LUMO orbital transition would give the significant CT from electron donor to the acceptor end. However, because of the hardly any overlap between the two orbitals, such electron transition is forbidden, reflecting the small oscillator strength (0.08). It cannot be viewed as the crucial excited state. The crucial excited state is defined as the lowest optically allowed excited state with substantial oscillator strength in this work. Our TDDFT calculations show that the crucial excited state of the open-ring form 3 is the second excited state. It contains the HOMO-2 f LUMO orbital transition (Table 2). As shown in Table 2, the HOMO-2 is delocalized over the electron acceptor end, combination of the electron distribution in the LUMO, which indicates that the crucial excitation of the open-ring form 3 can be viewed as a very weak CT transition on the acceptor end. This indicates that the N,N-bis-(4-methoxyphenyl)phenyl-amino unit does not display the electron donor character because the nonplanar structure reduces the molecular π-conjugation. By contrast, the closedring form 30 incorporating the relevant good planar arrangement of the DTE provides a good π-conjugation. This leads to a significantly different nature for the crucial excited state relevant to the nonplanar open-ring form 3. According to our TDDFT calculations, the crucial excitation of the closed-ring form 30 is the first excited state, which arises from the HOMO f LUMO orbital transition. As shown in Table 2, it mostly consists of a strong CT from electron donor to acceptor across the π-conjugated bridge. The crucial excited energies of closed-ring and open-ring forms are compared in Table 2. It can be found that the crucial excited energies of closed-ring form 30 are lower than that of open-ring form 3; the crucial excited energies of the closed-ring form 30 is ∼2 times as small as that of 3.

Table 2. TDDFT Result for 3 and 30 Obtained by the CAM-B3LYP/6-31g(d) Calculations

*

The crucial excited state. 23949



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Figure 3. Structural formula of the TTF derivative, 4 and 40 , and its one- and two-electron-oxidized species.

The relevant low excited energy and significant CT transition of the closed-ring form 30 will generate a large increase in the static hyperpolarizability, which is well in agreement with the DFT-FF calculations with three functionals On the basis of the large second-order NLO response of 3, the DTE unit has been chosen as the π-conjugated bridge in this system. The effective DπA model has been adopted to design the new system in this work. The electron donor of 3, N,N-bis-(4methoxyphenyl)phenyl-amino group, has been replaced with the redox-active TTF unit to generate to 4 (Figure 3). The calculated β value of 4 has been listed in Table 3. It can be found that the replacement of the electron donor N,N-bis-(4-methoxyphenyl)phenyl-amino group with the TTF unit cannot effectively enhance the second-order NLO response according to B3LYP/ CAM-B3LYP/LC-BLYP/6-31 g(d) calculations, but the differences between them is not substantial. The calculated β value of 3 is ∼1.7 times as large as that of 4 according to long-range

Table 3. Static First Hyperpolarizability ( 1030 esu), β, for 4 and 40 , and Its One- and Two-Electron Oxidized Species (B3LYP/CAM-B3LYP/LC-BLYP/MP2/6-31g(d) Calculations) compounds 4

2 1e

4

oxidized

functionals

βtot

compounds 40

4

oxidized

B3LYP

3439.0

B3LYP

270.6 131.8

LC-BLYP

106.2

LC-BLYP

667.1

MP2

199.1

MP2

1321.4

B3LYP

37681.6

B3LYP

2623.2

LC-BLYP B3LYP

593.8 13898.1

CAM-B3LYP 1713.5 LC-BLYP 23950

βtot

CAM-B3LYP

CAM-B3LYP 1102.1

2 0 1e 4 oxidized

CAM-B3LYP 2085.8 3 2e

functionals

155825.2

CAM-B3LYP 123699.9 3 0 2e 4 oxidized

LC-BLYP B3LYP

2912.2 690.0

CAM-B3LYP 1257.2 LC-BLYP

2072.0



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Table 4. TDDFT Calculations for 4 and 40 and Its One- and Two Electron-Oxidized Species Obtained by the CAM-B3LYP/631g(d) Calculations

corrected functionals CAM-B3LYP and LC-BLYP, and the β values of both systems are on the same order. TDDFT calculations show that the crucial excited state of 4 consists of the HOMO-2 f LUMO(44%) and HOMO f LUMO(41%) orbital transitions (Table 4). As shown in Table 4, this excitation contains a very weak CT transition on the acceptor end mixing a strong CT from donor to acceptor (Table 4). The calculated crucial excited energy of 4 is ∼1.1 times as large as that of 3. It is well known that the first hyperpolarizability is inversely proportional to the cube of excited energy and proportional to the oscillator strength for the crucial excited state, and the small difference on the crucial excited state implies a small decrease on the second-order NLO responses for 3 and 4, which is well in agreement with the DFT-FF calculations. Compared with the open-ring form 4, the relevant closed-ring form 40 possesses a large second-order NLO responses. The calculated β value of closed ring form 40 is ∼12, 8, and 6 times as large as that of the open-ring form 4 according to B3LYP/CAMB3LYP/LC-BLYP/6-31 g(d) calculations, respectively. TDDFT calculations show that the crucial excited state of the closed-ring form 40 is the first excited state, and it contains the HOMO-1 f LUMO(56%) and HOMO f LUMO(37%) transitions (Table 4). As show in Table 4, it can be viewed as a strong CT excitation from the electron donor to acceptor across the

π-conjugated bridge DTE. The crucial excited energy of the closed-ring form 40 is 0.55 times as large as that of the open-ring form 4. The relevant low excited energy and significant CT imply a large enhancement of the first hyperpolarizability, which is well in agreement with the DFT-FF calculations. The question we are now concerned with is the redox properties of 4 (open-ring form) and its photoisomerized species, the closed-ring form 40 . There is no question that the redox property of the molecule is closely associated with the nature of the frontier molecular orbital. The HOMOs of both compounds have been listed in Table 4. It can be found that the HOMO of open-ring form 4 mainly localized on the TTF unit, and the adjacent benzene ring and the ethylene segment also have some contributions. Also, the HOMO of the closed-ring form 40 also mainly localized on the TTF unit. Both of the HOMOs support that the TTF unit will act as the oxidized center in the series of oxidized processes. Unrestricted calculations have been employed to check the molecular predictions in this work. UB3LYP/6-31 g(d) calculations of the one-electron-oxidized species, 241eoxidized and 240 1eoxidized (the one-electron-oxidized species has one unpaired electron, and thus a doublet state) show that the spin densities are mainly localized on the TTF unit (∼0.86 and 0.57), and this indicates that the TTF unit is the oxidized center in the one-electron-oxidized process, which is 23951



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well in agreement with the molecular orbital predictions. For the two-electron-oxidized species, two possible spin, singlet and triplet states of each compound have been considered. Our DFT calculations show that the triplet state of both compound are more stable than that of singlet state. The calculated total energy including the electronic and zero-point corrected energies of triplet state is ∼6.9 kcal mol1 more stable than the singlet state for both systems, respectively. The small differences in energy suggest that it may be a statistical distribution of the two possible spin states, and thus the first hyperpolarizability of the two-electron-oxidized species should be a statistically average value of the two spin states. For the low-energy triplet state, 342eoxidized and 340 2eoxidized, the Mulliken spin population analysis shows that the spin density is still mainly localized on the TTF unit (∼ 0.92) in both compounds, and this indicates that the TTF unit is still the oxidized center in the two-electron-oxidized process, and is well in agreement with the molecular orbital prediction. Table 5 lists the NBO charges of TTF unit for all derivatives of TTF studied here. For neutral species 4 and 40 , the calculated NBO charge for the TTF unit is quite small, 0.032 for 4 and 0.001 for 40 , whereas the NBO charge of TTF unit gets a significantly increase in the one- and two-electron-oxidized processes for both species (∼0.7 for the one-electron-oxidized species, and ∼0.8 for the two-electron-oxidized species). The result also supports that TTF unit is the oxidized center in the series of oxidized processes. Our optimized calculation gives an arched geometry for TTF unit in 4 and 40 (Figure 4). The arched arrangement of the TTF has been confirmed by electron diffraction data,22 and also has been observed in its derivative, the bisTTF-porphyrin, according to X-ray measurement.23 Theoretically, the arched arrangement also has been found in a series of TTF-porphyrins and TTF Schiff-based metal complexes based on DFT calculations using the different levels of theory.11 As mentioned above, the oxidization center of 4 and 40 is the TTF unit. It indicates that the oneand two-electron-oxidized processes will affect the geometrical structure of the TTF unit. For the one- and two-electron-oxidized species, 241eoxidized, 240 1eoxidized, 342eoxidized, and 340 2eoxidized, our DFT calculations show that the arched TTF unit changes Table 5. NBO Charges of the TTF Unit for All Derivatives of TTF at the B3LYP/6-31g(d) Level fragment

4

TTF

0.032

2 1e 4 oxidized 342eoxidized

0.735

0.804

40 0.001

2 0 1e 4 oxidized 340 2eoxidized

0.683

0.881

to a fully planar geometry (Figure 4). All of the results show that the oxidization processes will enhance the molecular π-conjugation and affect the electron donor strength of the TTF unit and thus switching of the second-order NLO properties of both compounds. Because the S atom and the πCdC bond of TTF unit are closed to the common plane, hyperconjuation effects exist between the filled lone electron pairs of S atom (donor) and the empty π*CdC bond (acceptor). NBO second-order perturbation analysis has proved to be an effective tool for chemical interpretation of hyperconjugative interaction and electron density transfer from the filled lone electron pairs into the unfilled antibond π* orbital. According to perturbation theory, the NBO second-order perturbation stabilization energy can be estimated as24 Eð2Þ ¼ ΔEij ¼ qi

F 2 ði, jÞ εj εi

ð4Þ

where qi is the ith donor orbital occupancy, εi, εj are diagonal elements (orbital energy) and Fi,j is off-diagonal elements respectively associated with the NBO Fock matrix. The NBO second-order stabilization energy (nS f π*CdC) has been listed in Table S1 of the Supporting Information. The nS f π*CdC hyperconjuation strength of each S atom in TTF unit is indicated in terms of the magnitude of the NBO second-order perturbation stabilization energies. For open-ring species, the second-order perturbation stabilization energies follow the trend 4 > 241eoxidized ≈ 342eoxidized, and 40 > 240 1eoxidized ≈ 340 2eoxidized for closed-ring species. The second-order perturbation stabilization energy of neural species is about 2 times as large as that of the one- and two-electron-oxidized species for both species, respectively. Hence the series of redox processes lead to a decrease in nSfπ*CdC hyperconjuation interaction. As shown in Figures 3 and 4, six stable states can be given by the oxidization/reduction and photoisomerization. The secondorder NLO properties of the six states have been calculated at the B3LYP/CAM-B3LYP/LC-BLYP/6-31 g(d) levels in this work. The calculated β values of the six states have been listed in Table 3. It can be found that the results are functional dependent, but all of the functionals show the large differences in secondorder NLO properties for the six states. This indicates that this compound can afford the potential application in multistate second-order NLO switches. We employed the CAM-B3LYP result as an example to analyze the switchable effect on second-order NLO properties for 4. As mentioned above, the TTF unit can be oxidized

Figure 4. Optimized geometries of 4 and 40 , and its one- and two-electron-oxidized species obtained by B3LYP/6-31 g(d) calculations. 23952


The Journal of Physical Chemistry C sequentially to generate to the fully planar one- and two-electronoxidized species. The CAM-B3LYP calculations show that the one-electron-oxidized process significantly enhances the secondorder NLO responses when compared with the neutral species, whereas the two-electron-oxidized process largely reduces the second-order NLO responses in the open-ring and closed-ring forms when compared with their one-electron-oxidized species (342eoxidized vs 241eoxidized and 340 2eoxidized vs 240 1eoxidized). However, the β values of the two-electron-oxidized species in both forms are still larger than that of the neutral species 4 and 40 . For the open-ring form, the β value of the one-electronoxidized species 241eoxidized is ∼15.8 times as large as that of the neutral species 4, and the β value of the two-electron-oxidized species 342eoxidized is ∼0.05 times as large as that of the oneelectron-oxidized species 241eoxidized. For the closed-ring form, the β value of the one-electron-oxidized species 240 1eoxidized is ∼112.2 times as large as that of its reduced parents 40 , and the β value of the two-electron-oxidized species 340 2eoxidized is ∼0.01 times as large as that of the one-electron-oxidized species 2 0 1e 4 oxidized. Our DFT-FF calculations show that the photochromic reaction significantly alerts the static first hyperpolarizability for these compounds. For the neutral compound 4 and its one-electronoxidized species 241eoxidized, the photoisomerization causes a large enhancement of the second-order NLO response. The calculated β value gets a ∼8-fold improvement for the neutral compound 4 and a ∼60-fold improvement for the one-electronoxidized species 241eoxidized, respectively (4 vs 40 and 241eoxidized vs 240 1eoxidized). By contrast, the photoisomerization cannot largely affect second-order NLO responses for the two-electron-oxidized species. In DFT, all of the molecular properties are determined by the electron density solely. It is generally evaluated by solving the KohnSham equation, which includes kinetic, Coulombic, exchange, and correlation terms. The quality of the DFT results depends on the choice of the XC functional. For the first hyperpolarizability calculation, the DFT-derived result sometimes overestimates the first hyperpolarizabilities of DπA systems because of the incorrect long-range CT behaviors between donor and acceptor. To overcome this problem, some long-rang-corrected functionals have been developed recently. However, the improvements of these functionals do not hold for all properties. Thus, the static first hyperpolarizabilities of 4 and 40 have been calculated at MP2/6-31 g(d) levels in this work. The result indicates that the MP2-derived results are similar to the CAM-B3LYP results for both systems. But the B3LYP method provides the larger β value than that of MP2 method. Although the LC-BLYP results are smaller than that of MP2 results for both systems, the relative values between them do not change largely. The β value of 40 is ∼6.6 times as large as that of 4 according to MP2/6-31 g(d) calculations, and ∼6.3 times at LC-BLYP/6-31 g(d) level (Table 3). The TDDFT results calculated at CAM-B3LYP/6-31 g(d) level for 4 and 40 and their one- and two-electron-oxidized species have been listed in Table 4. It can be found that the first and crucial excited energies of one- and two-electron-oxidized species are lower than that of its neutral compound 4 and 40 . For the closed-ring species, the crucial excited energy of the oneelectron-oxidized species 240 1eoxidized is ∼3 times as small as that of the neutral compound 40 . The relevant low excited energy will generate a large increase in the static first hyperpolarizability, which is well in agreement with the DFT-FF calculations. For the

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two-electron-oxidized species 340 2eoxidized, the crucial excited energy is larger than that of the one-electron-oxidized species 2 0 1e 4 oxidized, but it is smaller than that of the neutral compound 40 . For the open-ring form, the TDDFT calculations also show that the large difference on the crucial excited energy. We also note that the photocyclization of these compounds also affect the first and crucial excited energies of the relevant compounds. This indicates that redox and photoisomerized processes significantly affect the excited state nature, and thus the second-order NLO responses.

4. CONCLUSIONS In conclusion, our DFT calculations have spotlighted a novel relationship among excitation spectrum, redox, and switchable second-order NLO properties of a series of the TTF derivatives. Our DFT-FF calculations with three functionals support the fact that the introduction of photochromic chromophore DTE into π-conjugated bridge segment is a good option for second-order NLO molecule with D-π-A model. Furthermore, the replacement the electron donor, N,N-bis-(4-methoxyphenyl)phenyl-amino group, with the TTF unit cannot largely affect the second-order NLO responses. The calculated β values of the TTF derivative 4 and 3 are on the same order according to our DFT-FF calculations. On the basis of the oxidization/reduction and photoisomerization, 4 has been adopted to probe the switchable effects on the second-order NLO properties. Our DFT calculations confirm that the TTF unit will perform the oxidized center in the one- and two-electron-oxidized processes, and thus the redox processes significantly affect the geometry of the TTF unit. The present results explain that the one- and two-electron-oxidized species have better planar structure for the TTF unit relative to its neutral compound. This effect lowers the allowed transitions and thus enhances the static first hyperpolarizabilities. Besides this, a photoisomerization process also changes the geometry of the series of compounds, where a closed-ring form possesses a relevant good πconjugation, and thus a large second-order NLO response. DFT-FF calculations with three functionals show that 4 is a best choice to switching of the first hyperpolarizability across its six states. ’ ASSOCIATED CONTENT

bS

Supporting Information. Second-order perturbation theory analysis of Fock matrix in NBO basis and the static first hyperpolarizability for 3 and 30 with field 0.0010, 0.0020, 0.0030 au studied here. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.-M.S.), [email protected]. cn (X.-H.G.).

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project No. 20971020), Program for Changjiang Scholars and Innovative Research Team in University (IRT0714), Department of Science and Technology of Jilin Province (20082103), the Training Fund 23953


The Journal of Physical Chemistry C of NENU’s Scientific Innovation Project (STC07017), and Science Foundation for Young Teachers of Northeast Normal University (20090401).

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