Dimerization and Isomerism Effects on Two-Photon Absorption of

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Dimerization and Isomerism Effects on Two-Photon Absorption of Tetraphenylethene Derivatives and Molecular Design for TwoPhoton Absorption Materials Fu-Qing Wang, Ke Zhao,* Mei-Yu Zhu, and Chuan-Kui Wang Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, 250014 Jinan, Shandong, China ABSTRACT: The two-photon absorption (TPA) properties of a new tetraphenylethene derivative and its covalent dimers have been calculated employing the density functional response theory. It is found that linear arrangement of branches can give rise to a cooperative TPA behavior. Partial planarity and linear arrangement are the possible reasons for the observed aggregation-induced TPA enhancement. On the basis of the model molecule, we have designed a series of tetraphenylethene derivatives which differ by donor moieties, connection modes, or central bridges after taking the structure−property relationship of TPA mechanism into account. The TPA spectra of the designed molecules have been calculated, and their TPA properties are analyzed at length. Our results suggest that the change of the connection mode of the carbazole group and the introduction of a vinylene or ethynylene linkage into a molecule can enhance TPA intensity greatly. It can be expected that all of the designed molecules could possess high TPA features. This research is helpful for the design of efficient TPA materials. donor and acceptor group and dimerization for fluorine derivatives using time-dependent density functional theory calculations and found that dimer formation seems more efficient than substitution for enhancing TPA cross section.17 In addition to cooperative enhancement, additive or decreased behaviors have also been observed. Todescato et al. presented a combined experimental and theoretical study on a push−pull molecule and its dimer. The TPA cross section of dimer is almost twice that for monomer, suggesting additive behavior.12 Terenziani and co-workers reported a strongly decreased behavior in covalent antiparallel dimers of a dipolar chromophore.11 Their modeling suggested that changing the relative orientation of the branches would obtain cooperative TPA enhancement. Isomerism is a quite common phenomenon in chemistry. However, the effects of isomerism on TPA properties of organic molecules have seldom been discussed in the literature.19,20 Arnbjerg et al. reported that the TPA cross section of porphycenes is much larger than that of its isomer, porphyrin.19 In our previous work, we investigated the TPA properties of a series of dipolar molecules and their dimers.21,22 The influences of dimerization and isomerism were analyzed for achieving cooperative TPA enhancement. Recently, a novel aggregation-induced emission active luminophore (ENPOMe) constituted by tetraphenylethene

1. INTRODUCTION Organic two-photon absorption (TPA) materials, which can be designed theoretically by tailoring the structure of the molecules, have attracted a great deal of interest in recent years, because of their significant applications in the fields of two-photon fluorescence microscopy,1 three-dimensional optical data storage,2 optical limiting,3 and photodynamic therapy,4 to name just a few examples. The established structure and property relationships have provided useful guidelines for molecular design and synthesis.5−8 It is well-known that expansion of π-electron conjugation and enhancement of intramolecular charge transfer will increase the TPA cross section values of a compound. Dimerization is a bottom-up strategy to design and synthesize multichromophoric structure from a simple molecular unit based on the knowledge of the properties of the individual chromophores and their mutual arrangement.9−12 On one hand, the study of dimerization effects is valuable to exploit supermolecular structure with amplification of TPA intensity at desirable wavelength. On the other hand, it is also the first fundamental step to mimic real possible aggregate situations.11,13−17 Drobizhev et al. observed that the conjugated porphyrin dimers possess very large TPA cross section values, which are several hundred times larger than that of monomer.18 Brédas et al. performed quantum chemistry calculations on porphyrin dimers and found that the electronic coupling strength between the two units plays an important role in TPA.16 Zein et al. investigated the influence of the © 2016 American Chemical Society

Received: June 7, 2016 Revised: August 8, 2016 Published: August 26, 2016 9708

DOI: 10.1021/acs.jpcb.6b05761 J. Phys. Chem. B 2016, 120, 9708−9715

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The Journal of Physical Chemistry B

molecule are depicted in Figure 1. The ENPOMe molecule is composed of TPE, acrylonitrile, and methylphenate groups and

(TPE) and acrylonitrile moieties with high two-photon fluorescence was synthesized and fully characterized by Xu et al.23 It was found that the ENPOMe system has an exceptionally large TPA cross section of 5548 GM when excited by a laser pulse at 740 nm. In order to achieve cooperative TPA enhancement and design TPA materials, in this paper, we have carried out a theoretical study on the TPA properties of the experimental ENPOMe molecule and its covalent dimers using the density functional response theory. The dimerization and isomerism effects on TPA have been analyzed. More importantly, making good use of the influences of donor moieties, isomerism, and central π-conjugated bridges in TPA properties, on the basis of the model molecule, we have designed a series of TPA active structures and have calculated their TPA properties. The present research would provide guidelines to achieve cooperative enhancements of TPA cross section and would be helpful to design efficient TPA materials.

2. COMPUTATIONAL METHOD In the case of resonant degenerate TPA, the sum-overstate expression for the two-photon matrix element can be written as24 Sαβ =

⎛ ⟨0|μα |s⟩⟨s|μβ |f ⟩

∑ ⎜⎜ s

⎝

ωsi − ω

+

⟨0|μβ |s⟩⟨s|μα |f ⟩ ⎞ ⎟⎟ ωsi − ω ⎠

(1)

where μα(β) is the dipolar operator in the direction α, β ∈ (x, y, z), ω is the fundamental frequency of the laser, and the resonant condition, 2ω = ωf, is assumed. The summation here includes all intermediate, initial, and final states, where ωsi represents the excitation energies for the intermediate state |s⟩. In response theory, the two-photon matrix element Sαβ can be calculated through the single residues of the quadratic response function.25 The total TPA cross section of molecules excited by a linear polarized single beam can be expressed as24,26 δtp =

Figure 1. Chemical structures and corresponding optimized geometries for ENPOMe monomer M1−M4.

has a donor−acceptor−donor (D−A−D) structure. Because the two vinyl groups in the TPE and acrylonitrile constituents have cis and trans forms with respect to the connected benzene ring and the positions of cyano and methylphenate groups can be exchanged, the geometry optimization gives four equilibrium conformations M1−M4, as shown in Figure 1. The frequency calculations for these geometries did not produce any imaginary frequencies. One can see that M1 and M2 have extensional backbones, while M3 and M4 take on bended structures. All of the TPE parts in these isomeric monomers have propeller-like conformations, which can avoid tight packing in the aggregated state and alleviate intermolecular π−π interaction.29 The geometry of M1 is nearly the same as M2, and M3 is also similar to M4. The dihedral angles between the planes of benzene rings are listed in Table 1. It can be seen that the dihedral angles of the D−E pair in M1 and M2 are much lower than those in M3 and M4. In Prasad’s work, it was found that not full but partial planarization occurs by aggregation and the aggregation-induced enhancement of TPA is ascribable to the

* + 4SαβSαβ *) ∑ (2SααSββ α ,β

(2)

The macroscopic TPA cross section that can be directly compared with the experimental measurement is defined as26 σtp =

4π 2a0 5α ω 2 δtp 15c0 Γf

(3)

Here α0 is the Bohr radius, c0 is the speed of light, and α is the fine structure constant. The level broadening Γf of final state is assumed to have the commonly used value Γf = 0.1 eV. TPA cross section is the unit of GM, 1 GM = 10−50 cm4 s/photon. In this work, the geometries of molecules are optimized at a hybrid B3LYP level with the 6-311G(d) basis set in the Gaussian 09 program.27 The TPA cross sections are calculated by response theory using the B3LYP functional with the 631G(d) basis set in the Dalton 2013 package.28 Our previous work has shown that the B3LYP functional calculations can give reasonable TPA properties which are consistent with the trend of experimental observations.21 The use of larger basis sets could probably provide better numerical results, but we believe that the overall picture would not change.

Table 1. Dihedral Angles of the Selected Planes of the Monomers M1−M4 and Constrained Geometry AM

M1 M2 M3 M4 AM

3. RESULTS AND DISCUSSION 3.1. Effects of Dimerization and Isomerism. 3.1.1. Molecular Structure. The chemical structures of the ENPOMe 9709

A−D

A−B

C−D

B−C

D−E

75.2 75.1 75.8 74.9 60.5

60.1 60.2 60.1 60.1 31.7

59.8 59.7 59.3 60.2 36.2

75.8 76.1 76.2 75.8 65.0

42.8 42.1 56.8 54.6 21.8

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Figure 2. Chemical structures for dimers S1−S6.

Figure 3. Optimized geometries for dimers S1−S6.

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Figure 4. Optimized geometries for dimers IS1−IS6.

planarization of π-conjugation by intermolecular steric interaction.14 We have then expected that the ENPOMe molecule could have a better planarity in the aggregated state. In order to prove the possible reason for TPA enhancement, we have constructed a structure AM with partial planarization through changing some dihedral angles of M1 and have performed a constrained geometry optimization. The optimized geometric parameters are also listed in Table 1. One can see that all of the dihedral angles of the selected planes of AM are reduced with respect to the free optimized geometry M1, which means structure AM possesses better planarity than M1. After taking the different connection positions and the cis and trans isomerization into account, first, we link two M1 monomers and form six dimeric conformers S1−S6 through a single covalent bond. Their chemical structures and the corresponding optimized geometries are shown in Figures 2 and 3. It is interesting to find that the two methylphenate branches in these dimers have different mutual orientations. Both in dimers S1 and S2, the two branches locate at a nearly linear backbone. In S3 and S5, they take on antiparallel and parallel orientations, respectively, while they are crossed in S4 and S6. The structures with different branched orientations could model the various arrangements of the aggregates. It has been demonstrated that some relative orientations of the branches in multi-branching chromophores could achieve cooperative TPA enhancement.22,30 From Figure 3, one can also notice that the central two benzene rings do not lie on one plane. The torsional angle between them is about 38° for all of the isomers. Then, we use two M3 monomers to form six isomeric dimers IS1−IS6 by single bonds and the optimized geometries are given in Figure 4. The branched orientations are similar to S1− S6, but the conjugation channels are bent to some extent in various directions. This would give lower charge transfer ability.

The central two benzene rings are also not coplanar, and the dihedral angles are in the range 37−40° for these isomers. 3.1.2. Two-Photon Absorption. The TPA properties of the monomers M1−M4 and AM have been calculated, and the simulated TPA spectra with Lorentz broadening are illustrated in Figure 5. Because the TPA properties of M2 and M4 are

Figure 5. TPA spectra of monomers M1, M3, and AM.

nearly the same as those of M1 and M3, respectively, their spectra are not plotted in the figure. We pay attention to the TPA behavior in the two-photon wavelengths above 600 nm, where the lowest excited electronic states are located. All of the structures have one absorption peak. The peak position of M1 is at 707 nm and has a 10 nm red-shift with respect to M3. The largest cross section of M1 is 979GM, which is much higher than the value of M3, 325GM. This demonstrates that the isomerism can have significant effects on the TPA of the molecule. In M1, the electron donors and acceptors are located nearly in a line. Thus, the TPA intensity of M1 is supposed to be stronger than that of M3. For the structure AM, the calculated results show that the absorption position is redshifted to 775 nm and the absorption intensity is increased significantly. The largest cross section comes to a value of 9711

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The Journal of Physical Chemistry B 1268GM. This result is consistent with our expectation mentioned above. It is very interesting to see how different TPA spectra of various dimers are. Figure 6 shows the calculated TPA spectra

Figure 7. TPA spectra of dimers IS1−IS6.

intensity is multiplied by 2. Similar to the case of the S series dimers, the spectral profiles of IS1, IS3, and IS4 are still very close to IS2, IS6, and IS5, respectively. IS2 has the strongest TPA intensity among these IS series dimers. In comparison with M3, cooperative enhancements appear and the maximum absorption bands are red-shifted or blue-shifted greatly for all of the dimers. The largest cross section of IS1 is located at 761 nm with 1293GM, which is much higher than the value of 2 times of M3, 650GM. Also, the peak position is red-shifted by 64 nm with respect to M3. For IS3−IS6, the maximum absorption bands are all blue-shifted significantly. IS3 has the maximum cross section of 1161GM at 625 nm. IS4 has several absorption peaks with comparative intensities, and the biggest value is 771GM centered at 637 nm. To sum up the dimerization effects, Table 2 gives the maximum TPA wavelength λmax and the cross section σmax for

Figure 6. TPA spectra of dimers S1−S6.

of dimers S1−S6. For comparison, the TPA spectrum of M1 is also given and its TPA intensity is multiplied by 2. One can see that all of the dimers have several strong TPA peaks in the region 600−900 nm. We attribute this observation to the interactions between branches according to the exciton model.11,31 It is found that these TPA spectra can be classified into three forms. The spectra of S1, S3, and S4 are very close to S2, S6, and S5, respectively. By inspecting the structures of these dimers, one can realize that the similar branched orientation features bring about the similar TPA properties. Take S3 and S6 for example. Although the geometries of them are quite different, the angles between the branch on the left and the central benzene backbone are almost the same, which leads to similar conjugation channels in two dimers. Because the nearly linear backbone is in favor of the charge transfer process, the S1 or S2 conformation has the strongest TPA intensity among these dimers. For dimer S1, the maximum cross section comes to a value of 2535GM at 709 nm, which is increased by 577GM with respect to the value of 2 times of M1. The increase is about 30%. This demonstrates that the cooperative TPA enhancement occurs in dimers S1 and S2. It is also noticed that, in the region of longer wavelength, two strong absorption bands appear at about 795 and 844 nm with cross sections of 1871GM and 1258GM. Hence, dimers S1 and S2 are good candidates for the TPA applications in this wavelength domain. For other dimers, decreased behaviors are observed. S3 has three strong absorption bands with comparative intensities, and the largest cross section occurs at 633 nm with 1300GM, which is smaller than the value of 2 times of M1. As for S4, the peak position is red-shifted to 725 nm and the maximum cross section is 929GM. According to the exciton model, the different mutual orientations between branches could give rise to different coupling and thereby produce cooperative or decreased TPA properties. It should be mentioned that all of the dimers have considerable TPA in the vicinity of 707 nm, where the peak of M1 locates. The TPA spectra of dimers IS1−IS6 have also been calculated, and the results are presented in Figure 7. For comparison, the TPA spectrum of M3 is also given and its TPA

Table 2. Ratio of the Maximum TPA Wavelength λmax and of the Maximum TPA Cross Section σmax Obtained for the Dimers S1−S6 and IS1−IS6 to the Coresponding Values λ0 and 2σ0 Obtained for the Monomers M1 and M3, Repectively M1 S1 S2 S3 S4 S5 S6

λmax/λ0

σmax/2σ0

1 1.00 1.00 0.90 1.03 1.03 0.90

0.5 1.30 1.32 0.66 0.47 0.48 0.63

M3 IS1 IS2 IS3 IS4 IS5 IS6

λmax/λ0

σmax/2σ0

1 1.09 1.10 0.90 0.91 0.91 0.90

0.5 1.99 2.45 1.79 1.19 1.16 1.73

the dimers, in comparsion with the coresponding values λ0 and 2σ0 for the monomers. In the case of the S series, the maximum absorption wavelengths of S1 and S2 are exactly at the value of M1, since both of the λmax/λ0 factors are equal to 1.00. The σmax/2σ0 factors of S1 and S2 are 1.30 and 1.32, which means the cooperative enhancement occurs. The factors λmax/λ0 are equal to 0.9 and 1.03, while the σmax/2σ0 factors are equal to 0.66 and 0.47 in S3 and S4, respectively. These values correspond to a large blue-shift and a slight red-shift with the TPA decreased behaviors. When it comes to the IS series, it is clear to see that all of the dimers present the cooperative enhancements and large red-shifts or blue-shifts. 9712

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Figure 8. Chemical structures for the designed molecules 1−6.

Figure 9. Optimized geometries of 1−6.

two benzene rings with a single bond. It is noticed that the carbazole group would have planarity, which is in favor of TPA. The optimized geometry of 2 shows that there is a large torsional angle between the planar carbazole group and the benzene ring connected with it. Hence, this structure still can reduce the likelihood of π−π stacking efficiently. By examining the structure of 2, we think of its isomeric structure 3 which has a different connecting mode of carbazole. For structure 3, the carbazole group and its connecting benzene ring would have better planarity and longer conjugated length with respect to molecule 2. Our optimization has proved it. At last, the different covalent linkages are considered for constructing various dimers. It has been demonstrated that the solution quantum yield value of biphenyl is increased by 4.4-fold when a methylene linkage is introduced to fasten the phenyl rings in fluorine.32 Therefore, molecule 4 is designed. The methylene bridge makes the central two phenyl rings coplanar in conformation. At the same time, it should be noticed that the whole molecular backbone deviates from linearity slightly. Molecules 5 and 6 are the dimers linked by the vinylene and ethynylene bridges. Similar to 4, both molecules have a nearly planar center. The torsional angles between the central two benzene rings are 6.7 and 5.9° for 5 and 6, respectively. Tang and co-workers have investigated how conjugation affects the TPA and emission of the single bond, vinylene and ethynylene linked TPE systems.29 It was found that the vinylene and ethynylene linked systems are more conjugated than the corresponding single bond linked system. 3.2.2. Two-Photon Absorption. It is curious to know whether our design strategies are efficient. The calculated TPA

According to our calculations, the reason for the experimental aggregation-induced TPA enhancement can be speculated. On one hand, the molecule could have a partial planarization in the aggregated state due to the intermolecular interactions. Hence, the TPA intensity can be improved. On the other hand, the molecules maybe prefer a linear arrangement like the S1 structure by aggregation, which can give rise to a significant TPA enhancement. It can also draw a conclusion that special dimerization is an efficient design strategy for enhancing TPA cross sections. 3.2. Molecular Design. 3.2.1. Molecular Structure. For designing TPA active chromophores, one needs to consider the established TPA structure−property relationships. It has been revealed that several key structural factors, such as the strength of donor and acceptor substituents, the electron richness and planarity of the π-conjugation center, the conjugated length, as well as the nature of the linkers (alkene or alkyne units), play important roles in enhancing the TPA activities of many organic molecules.6−8 On the basis of the model molecule, we have designed six new structures which are supposed to have aggregationenhanced TPA properties. The chemical formulas and the optimized geometries are given in Figures 8 and 9. At first, in molecule 1, the methylphenate group is replaced by the triphenylamine because the triphenylamine group has good electron donating and transporting capability, together with special propeller-like molecular structures, as shown in Figure 9. Then, the carbazole group is conjured up in our mind and molecule 2 is generated. The carbazole structure can be regarded as the restricted form of diphenylamine by connecting 9713

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4. CONCLUSION Effects of dimerization and isomerism on the two-photon absorption properties of tetraphenylethene derivatives have been theoretically studied by quantum chemical calculations. The TPA cross sections of the various monomers M1−M4 and dimers S1−S6, IS1−IS6 are calculated by the density functional response theory. It is found that the linear arrangement of branches can give rise to cooperative TPA and thereby dimerization is an efficient design strategy for enhancing TPA cross sections. A constrained geometry AM with partial planarity is optimized, and the corresponding TPA spectra are simulated and compared with monomers. Our calculations indicate that the partial planarity and the linear arrangement of branches could possibly explain the experimentally observed aggregation-induced TPA enhancement. On the basis of the model molecule, we have designed a series of structures employing different donor groups, isomeric structures, and central π-conjugated linkages. The established TPA structure−property relationships have been taken into account. The TPA spectra of these designed molecules are calculated, and we have found that the isomerism of the carbazole group has an important effect on the TPA property and can be used to enhance the TPA intensity greatly. The vinylene and ethynylene linkages also can improve the TPA intensity significantly due to their strong conjugation. It can be predicted that all of the designed molecules could possess considerably high TPA cross sections. These results provide useful guidelines for the design of efficient TPA materials.

spectra of molecules 1−3 are illustrated in Figure 10a. In comparison with M1, the TPA peak positions of 1−3 are all

Figure 10. TPA spectra of (a) 1−3 and (b) 4−6 in comparison with M1 and S1, respectively.

■

red-shifted significantly and the peak values are increased to different extents. The red-shifts are 96, 130.2, and 61.4 nm, respectively. The maximum cross section of 1 is 1550GM which is enhanced by 58% with respect to M1. Although molecule 2 has a better planarity, the TPA intensity of 2 is lower than that of 1 because the donor strength of carbazole is weaker than that of diphenylamine. As expected, the TPA intensity of 3 is enhanced dramatically in comparison with 2 due to improved planarity and longer conjugated length. The largest cross section of 3 comes to 2310GM which is much larger than the value of 2, 1072GM. Also, the TPA wavelength difference of 2 and 3 is nearly 70 nm. This suggests that the isomerism of the carbazole group has an important effect on TPA property and can be used to raise the TPA intensity greatly. However, we need to realize that the good planarity of the phenyl ring and carbazole group may also reduce the fluorescence quantum efficiency slightly induced by facile π−π stacking. The TPA spectra of the dimers with different linkages are compared in Figure 10b. All of the spectra have several absorption peaks, and large differences are located in the region 750−900 nm. In this region, these designed molecules 4−6 have stronger absorption intensities than that of S1. The calculation shows that the cross section of molecule 4 is equal to 2230GM at 865 nm. When the linkages are vinylene and ethynylene groups, the TPA intensities are improved significantly. The strongest peaks of 5 and 6 are red-shifted to 822 and 808 nm with the largest cross sections of 4236 and 4255GM, respectively. The spectrum of 5 also has a strong absorption band at 880 nm with the cross section of 3565GM. This indicates that the introduction of the vinylene or ethynylene linkages into the molecule could enhance TPA intensity greatly because of their strong conjugation. Our results are in good agreement with the observations of the experiments for the related systems.29

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-531-86180349. Fax: +86-531-86182521. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (Grant Nos. 11374195 and 11404193), the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014AM026), and the Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J14LJ01).

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DOI: 10.1021/acs.jpcb.6b05761 J. Phys. Chem. B 2016, 120, 9708−9715