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Oct 16, 2017 - •S Supporting Information. ABSTRACT: Intramolecular charge-transfer characteristics of a series symmetric methoxy -substituted bi-1,3...
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Article Cite This: J. Phys. Chem. A 2017, 121, 8399-8407

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Substituent Effect on Intramolecular Charge Transfer of Symmetric Methoxy-Substituted Bi-1,3,4-oxadiazole Derivatives Fangyi Chen,† Wanxi Zhang,† Taiji Tian,† Binglian Bai,‡ Haitao Wang,*,†,§ and Min Li*,† †

Key Laboratory of Automobile Materials (MOE) & College of Materials Science and Engineering, Jilin University, Changchun 130012, China ‡ College of Physics, Jilin University, Changchun 130012, China § Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China S Supporting Information *

ABSTRACT: Intramolecular charge-transfer characteristics of a series symmetric methoxy -substituted bi-1,3,4-oxadiazole derivatives with various substituted positions and quantities have been studied with a combination of experimental techniques and theoretical calculations to investigate the substituent effect. Different degrees of fluorescence red shift in polar solvents are observed in these compounds. The metasubstituted molecule (BOXD-m-OCH3) exhibits a larger red shift (82 nm) than the other two monosubstituted molecules, BOXD-o-OCH3 (40 nm) and BOXD-p-OCH3 (37 nm); the polysubstituted molecules BOXD-D1 and BOXD-T1 show 80 and 104 nm red shifts, respectively, which are obviously larger than the monosubstituted molecules. The changes of molecular dipole moment between the ground state and charge transfer (CT) excited state are calculated to be on the same order with the degree of red shift (7.56 D in BOXD-o-OCH3, 12.07 D in BOXD-m-OCH3, 7.38 D in BOXD-p-OCH3, 14.79 D in BOXD-D1, and 16.80 D in BOXD-T1). Theoretical calculations at the density functional theory level reveal that the first singlet excited state of all of these compounds shows both π−π* and CT characteristics and the charge has been proven to transfer from the terminal methoxy phenyl group to the central bioxadiazole group. The analysis of charge transfer based on electron density shows that the greater the amount substituent, the more charge would be involved in the intramolecular charge transfer. In addition, the negative barycenter has a tendency to locate close to the methoxy substituent, which would cause the difference in the charge-transferred distance. The transferred charge and CT distance work jointly and finally lead to differences in dipole moment variation. These findings could provide very good guidance for the design of molecules with intramolecular chargetransfer characteristics.

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and excellent electron-transporting property.31−34 In our previous study, it has been proved that symmetric molecular structure could effectively enhance the ICT characteristic in OXD derivatives.35 In addition, methoxy-substituted bi-1,3,4oxadiazole derivatives are found to be a kind of favorable ICT material with simultaneous excellent fluorescence performance.36 However, only the ortho-substituted compound has been studied; the ICT characteristics of other methoxysubstituted bi-1,3,4-oxadiazole derivatives and the substituent effect on ICT characteristics are still beyond our understanding. So in this paper, we designed and synthesized a series symmetric methoxy-substituted bi-1,3,4-oxadiazole derivatives (Scheme 1) in which both the substituted positions and the quantities are varied to see whether it shows influence on the ICT characteristics on symmetric molecules. Furthermore, a combination of experimental techniques and theoretical

INTRODUCTION Organic molecular materials with intramolecular charge transfer (ICT) characteristic have attracted considerable attention due to their widely applications in dye-sensitized solar cells (DSSCs),1−3 organic photovoltaic cells (OPVCs),4−7 and organic light-emitting diodes (OLEDs).8−10 The practical behaviors such as fluorescence quantum yield and fluorescence lifetime of these organic functional molecules are strongly dependent not only on the surrounding environments but also on their own ICT characteristic.11−14 Furthermore, a large ICT could effectively help to reduce the band gap for red emission.15−18 The ICT characteristics are highly decided by their molecular structures. It has been reported that the ICT characteristic could be widely tuned through appropriate modification of the push−pull electronic effect of donor and acceptor substitutes, conjugated linkers, or the molecular conformations.15,19−30 1,3,4-Oxadiazole (OXD) ring is a kind of important structural unit in organic photoelectric functional materials for its good luminance, stable thermal, chemical performance, © 2017 American Chemical Society

Received: September 5, 2017 Revised: October 16, 2017 Published: October 16, 2017 8399

DOI: 10.1021/acs.jpca.7b08845 J. Phys. Chem. A 2017, 121, 8399−8407

The Journal of Physical Chemistry A

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RESULTS AND DISCUSSION Photophysical Properties. Absorption and Fluorescence Emission Spectra. The UV−vis absorption and fluorescence emission spectra of all of these compounds in solvents with different polarity have been conducted to study their solvatochromism. The solvents used in the measurements include cyclohexane (CHEX), tetrahydrofuran (THF), dichloromethane (DCM), acetonitrile (ACN), and ethanol (ETO). The spectra of the parent compound molecule BOXD (Scheme 1), in which there is no substituent on the terminal benzene ring, are presented for comparison. As shown in Figure 1, BOXD shows an intense absorption at ∼300 nm. It

Scheme 1. Molecular Structures of Methoxy-Substituted Bi1,3,4-oxadiazole Derivatives Investigated

calculations was carried out to illustrate the ICT characteristic more clearly and get a deep understanding of the mechanism of the substituent effect.

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EXPERIMENTAL SECTION The synthesis of BOXD, BOXD-o-OCH3, BOXD-p-OCH3, and BOXD-T1 was previously reported,36,37 whereas BOXD-mOCH3 and BOXD-D1 were synthesized for the first time through the reported synthetic route shown in Figure S1. The purity of these compounds was verified by FT-IR, 1H NMR, and elemental analysis (Supporting Information). All of these solvents for the spectroscopic measurements were of spectroscopic grade and used as received. 1HNMR spectra were recorded with a Mercury-300BB 300 MHz spectrometer, using CDCl3 or DMSO-d6 as solvents and tetramethylsilane (TMS) as an internal standard (δ 0.00). FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B), and the sample was in the form of a pressed tablet with KBr. UV−vis absorption spectra were recorded on a Shimazu UV2550 spectrometer, and photoluminescence was measured on a Perkin-Elmer LS 55 spectrometer. The room-temperature luminescence quantum yields in solutions were determined relative to quinine sulfate acid aqueous solution (0.546) and calculated according to the following equation: Φunk = Φstd(Iunk/Aunk)(Astd/Istd)(ηunk/ηstd)2, where Φunk is the radiative quantum yield of the sample, Φstd is the radiative quantum yield of the standard, Iunk and Istd are the integrated emission intensities of the sample and standard, respectively, Aunk and Astd are the absorbance of the sample and standard at the excitation wavelength, respectively, and ηunk and ηstd are the indexes of refraction of the sample and standard solutions (pure solvents were assumed), respectively. In theoretical calculations, all of the calculations were carried out with Gaussian 09 software package (version A.02) at the CAM-B3LYP3/6-311+G** level.38 The bulk solvent effect was considered in the calculation of excited-state single point using a well known polarizable continuum model (PCM).39 Five kinds of solvents (CHEX, DCM, THF, ETO, and ACN) are used in the calculation, and their dielectric constant is 2.06, 8.93, 7.43, 24.85, and 35.69, respectively. All geometry optimization calculations were performed with equilibrium linear-response PCM (LR-PCM) approach, whereas the TDDFT energy calculations were performed with state-specific PCM (SS-PCM) approaches. In addition, there is no symmetry constraints imposed on the ground-state geometry. Multiwfn and VMD were employed for visualizing the molecular orbital, electron density variation, and charge transfer.40,41

Figure 1. Normalized UV−vis absorption and fluorescence emission spectra of BOXD.

should be caused by a π−π* type of transition. From the nonpolar solvent (CHEX) to the strong polar solvent (ETO), the maximum absorption peak red-shifts only ∼6 nm in wavelength (Figure 1 and Table 1), which indicates that the electronic and structural nature of the ground-state and Franck−Condon (FC) excited states does not change, even in different solvents. As for the fluorescence emission spectra (Figure 1), the maximum emission peaks of BOXD in those solvents are at ∼366 nm and show only small changes in wavelength during the polarity of solvent increased (∼6 nm). No solvatochromism effect observed in both the absorption and fluorescence emission spectra means that no ICT process occurred and the molecular dipole moment in the excited state is almost the same as the ground state in the parent compound BOXD. Introduction of the methoxy group on the terminal benzene ring could greatly affect the ICT characteristic. The orthosubstituted compound BOXD-o-OCH3 (Scheme 1) shows obvious solvatochromic behaviors in different polar solvents.36 It exhibits 8 and 40 nm red shifts in the maximum absorption and in the maximum emission with increasing polarity of solvent, respectively (Figure S2). It has been demonstrated that the ICT occurred during the excited-state molecular geometry relaxation in solutions and reorganization of the solvent molecules. BOXD-p-OCH3 (Scheme 1) is an allotrope of BOXD-o-OCH3 in which a methoxy group was introduced in para position on the benzene ring. Its absorption and fluorescence emission spectra in various solvents are presented in Figure 2. BOXD-p-OCH3 shows an intense absorption at ∼320 nm with nearly no noticeable shifts from the nonpolar solvent (CHEX) to the polar solvents (ETO). In fluorescence 8400

DOI: 10.1021/acs.jpca.7b08845 J. Phys. Chem. A 2017, 121, 8399−8407

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BOXD-p-OCH3 also possesses an ICT characteristic during the excitation. In addition, we have also characterized another allotrope BOXD-m-OCH3 (Scheme 1) that has a meta-substituted methoxy group on the terminal benzene ring to see whether the position of methoxy substituent could have influence on ICT characteristic. Figure 3 presents the absorption and

Table 1. Photophysical Characteristics of the Bi-1,3-4oxadiazole Derivatives Investigated in Different Solvents (∼5 × 10−6 M) at Room Temperature compound BOXD

BOXD-o-OCH3

BOXD-m-OCH3

BOXD-p-OCH3

BOXD-D1a

BOXD-T1a

solvent

λabs (nm)

λem (nm)

Δυst (cm−1)

Φ

CHEX DCM THF ACN ETO CHEX DCM THF ACN ETO CHEX DCM THF ACN ETO CHEX DCM THF ACN ETO CHEX DCM THF ACN CHEX DCM THF ACN ETO

297 301 298 295 297 315 323 318 318 323 298 298 297 294 296 317 321 316 316 320

362 368 367 365 366 364 380 377 390 404 357 379 378 413 439 368 389 382 397 405 379 418 409 459 386 407 436 476 490

6046 6049 6309 6501 6348 4274 4644 4921 5806 6207 5546 7172 7215 9801 11005 4372 5446 5468 6457 6559

0.82 0.79 0.74 0.79 0.74 0.62 0.46 0.70 0.49 0.13 0.86 0.52 0.77 0.78 0.26

308 305 303 328 325 322

8544 8337 11216

0.43 0.17

8364 7833 10047

0.12 0.37 0.40

Figure 3. Normalized UV−vis absorption and fluorescence emission spectra of BOXD-m-OCH3 (λex: 316 nm in CHEX, 298 nm in DCM, 306 nm in THF, 298 nm in ACN, 298 nm in ETO).

fluorescence emission spectra of BOXD-m-OCH3 in different solvents. The same as the former compounds (BOXD-o-OCH3 and BOXD-p-OCH3), the maximum absorption peaks of BOXD-m-OCH3 show only 4 nm red shifts, but the fluorescence emission spectra show appreciable red shifts (82 nm) as the polarity of solvent increases (Table 1). The extent of the bathochromic shift is obviously larger than the other two methoxy-substituted derivatives, indicating larger dipole moment change in the excited state of BOXD-m-OCH3. The reason for the larger dipole moments variation in metasubstituted compound will be discussed in the following text. Because the meta-substituted compound exhibits such difference in the solvatochromism behaviors, the molecule BOXD-D1 (Scheme 1) in which two methoxy groups are introduced in meta-(3,5) position on the benzene ring is designed and synthesized with the expectation of obtaining enhanced ICT characteristics of phenyl-substituted bi-1,3,4oxadiazole derivatives. As shown in Figure 4 (the spectra in ETO cannot be obtained for the poor solubility), the solvatochromic of the fluorescence emission spectra of BOXD-D1 in different solvents can be observed. When the polarity of solvent increased from the low polar solvent (CHEX) to the strong polar solvent (ACN), BOXD-D1 exhibited a large red shift of 80 nm: 379 nm in CHEX, 418 nm in DCM, 409 nm in THF, and 459 nm in ACN (Table 1). Furthermore, BOXD-T1 (Scheme 1), in which three methoxy groups are introduced in meta-(3,4,5) position on the benzene ring, also exhibits nearly identical absorptions with a peak around 325 nm in different solvents (Figure 5); whereas their emission spectra demonstrate an obvious solvatochromism with emission maxima red shifts of ∼104 nm (386 nm in CHEX, 407 nm in DCM, 436 nm in THF, 476 nm in ACN, and 490 nm in ETO) (Table 1). The poly-substituted molecules BOXD-D1 and BOXD-T1 show obviously larger red shifts than the monosubstituted molecules do.

a

Nonexistent data are due to their poor solubility in corresponding solvents.

Figure 2. Normalized UV−vis absorption and fluorescence emission spectra of BOXD-p-OCH3 (λex: 318 nm in CHEX, 328 nm in DCM, 323 nm in THF, 322 nm in ACN, 328 nm in ETO).

emission spectra, it shows an intense emission at ∼368 nm with fine vibrational structures in nonpolar CHEX and red-shifted ∼37 nm (λem = 405 nm) in polar ETO with fine structures disappeared. The maximum emission peaks in other solvents are laid between the emission peak of CHEX and ETO (Figure 2 and Table 1). BOXD-p-OCH3 exhibits a comparable red shift in emission maximum with BOXD-o-OCH3, which means 8401

DOI: 10.1021/acs.jpca.7b08845 J. Phys. Chem. A 2017, 121, 8399−8407

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Figure 6 shows the Δυst versus Δf plots for all of these compounds in a series of aprotic solvents (Table S1). The

Figure 4. Normalized UV−vis absorption and fluorescence emission spectra of BOXD-D1 (λex: 304 nm in CHEX, 310 nm in DCM, 309 nm in THF, 304 nm in ACN). Figure 6. Plot and fit line of Stokes shift against the solvent polarity parameters for the bi-1,3-4-oxadiazole derivatives investigated.

experimental data obey the linear relationship predicted by the Lippert−Mataga equation well in the range of solvents. The ICT characteristics are typically dependent on the molecular structures with different substituted positions and quantities. It is found that the changes in the dipole moment between the CT excited state and the ground state (Δμeg) are significantly large in BOXD-D1 (14.79 D) and BOXD-T1 (16.80 D), and the value of Δμeg is found to be about 7.38, 7.56, and 12.07 D for BOXD-p-OCH3, BOXD-o-OCH3, and BOXD-m-OCH3, respectively. Because the structures of all of these bi-1,3,4oxadiazole derivatives are highly symmetric, which would not exhibit molecule dipole moments, it is worth noting that these calculated dipole moment variations should be assigned to the changes in local dipole moments.35 The dipole moment variation can be considered with strength in the order BOXD-p-OCH3, BOXD-o-OCH3, BOXD-m-OCH3, BOXDD1, and BOXD-T1, which is in agreement with the observed solvatochromic shifts as mentioned above. The relatively large change in the dipole moment is a direct evidence of the CT characteristic of the excited state. Quantum Yields. The fluorescence quantum yields for all of these methoxy-substituted compounds in different solvents have also been measured, and the data are collected in Table 1. All monosubstituted compounds show high quantum yields in different solvents. As for the poly substituted compounds, the quantum yields are smaller and can be obtained in only THF, DCM, and ACN due to their poor solubility in CHEX and ETO. In addition, the fluorescence quantum yields show a decreased trend with the increase in the solvent polarity. It is understandable that the compound with ICT characteristic interacts strongly with polar solvent, which would enhance the nonradiative decay rate and result in lower quantum yield.15,44,45 Theoretical Study. Electronic-State Transitions. Theoretical calculations at density functional theory (DFT) level are carried out to reveal the physical picture of the electronic excitation, excited-state molecular geometry, and the light emission processes. CAM-B3LYP approach was applied in all of our study considering its much more excellent performance than other approaches in exploring the mechanism of charge

Figure 5. Normalized UV−vis absorption and fluorescence emission spectra of BOXD-T1 (λex: 313 nm in CHEX, 336 nm in DCM, 333 nm in THF, 329 nm in ACN, 331 nm in ETO).

Solvatochromic Measurements and Dipole Moments. The Stokes shifts of all of these compounds are summarized in Table 1. From the low polar solvent to the strong polar solvent, the Stokes shifts of these methoxy-substituted compounds increased along with the increasing of solvent polarity, indicating that the CT state can be stable in the polar solvents. The changes in the dipole moment between the CT excited state and the ground state can be estimated from the slope of the plot of the Stokes shift against the solvent parameters using the Lippert−Mataga equation42,43 2 Δνst = × (μCT − μg )2 × Δf (ε , η) + const (4πε0)(hcα 3) Δf (ε , η) =

η2 − 1 ε−1 − 2 2ε + 1 2η + 1

where Δvst is the Stokes shift and h, c, and ε0 are the Planck’s constant, velocity of light, and permittivity of vacuum, respectively. μg is the dipole moment of the ground state, μCT is the dipole moment of the CT excited state, a is the solvent cavity (Onsager) radius obtained by optimizing the molecular geometry at the CAM-B3LYP/6-311+G** level of theory, and ε and n are the dielectric constant and refractive index of the solvent, respectively. 8402

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Table 2. Computed Excitation Energy (in eV) and Oscillator Strengths (in Parentheses) in Gas Phase for the Five Lowest Excited States of All of These Compounds excited state

BOXD

S1 S2 S3 S4 S5

4.41 (1.27) 5.08 (0) 5.08(0.02) 5.18 (0) 5.41 (0.0008)

BOXD-o-OCH3 4.18 4.57 4.84 5.25 5.27

(1.16) (0) (0.11) (0) (0.0003)

BOXD-m-OCH3 4.41 4.58 4.63 5.19 5.41

(1.17) (0) (0.23) (0) (0.0007)

transfer in OXD derivatives.35 The theoretical calculated results of BOXD-p-OCH3 have been reported in our previous work;35 for the sake of comparison, they are also discussed here. Table 2 lists the excitation energy and oscillator strengths of the five lowest electronic transitions in the gas phase of all of these compounds. In the gas phase, S0 to S1 transition is highly allowed in all compounds except for BOXD-D1, in which the highest allowed transition is S0−S3 ( f = 1.18), which could account for the low quantum yields in solutions. The excitation energies are 4.41 eV in BOXD, 4.18 eV in BOXD-o-OCH3, 4.41 eV in BOXD-m-OCH3, 4.20 eV in BOXD-p-OCH3, 4.33 eV in BOXD-D1, and 4.11 eV in BOXD-T1. They all show comparable values to the experiment results, which means the calculated results agree with the experiments results well and could be used to explain the experiments results. In addition, the oscillator strength (f) in S0−S1 is 1.27, 1.16, 1.17, 1.52, and 1.37 in BOXD, BOXD-o-OCH3, BOXD-m-OCH3, BOXD-pOCH3, and BOXD-T1, respectively. Other allowed transitions involved in the lowest five excited states are S0−S3 (f = 0.02 in BOXD, 0.11 in BOXD-o-OCH3, 0.23 in BOXD-m-OCH3, 0.02 in BOXD-p-OCH3, and 0.06 in BOXD-T1) and S0−S5 ( f = 0.0008 in BOXD, 0.0003 in BOXD-o-OCH3, 0.0007 in BOXDm-OCH3, 0.001 in BOXD-p-OCH3, and 0.020 in BOXD-T1). Figure 7 presents the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital

BOXD-p-OCH3 4.20 4.87 4.97 4.98 5.46

(1.52) (0) (0.02) (0) (0.001)

BOXD-D1 4.33 4.34 4.46 5.15 5.40

(0.28) (0) (1.18) (0) (0.0007)

BOXD-T1 4.11 4.50 4.51 4.63 5.30

(1.37) (0) (0.06) (0) (0.020)

(LUMO) of all of these compounds. The other frontier molecular orbital involved in the five lowest transitions can be obtained in Figures S3−S7. As can be seen, almost all of the occupied orbitals mainly consist of π-bonding orbital, except for orbital H-5, H-4 in BOXD, and H-4 in BOXD-m-OCH3, which are contributed by the n orbital of N atoms. On the contrary, the unoccupied orbitals are mainly composed of the πantibonding orbital. An inspection of the composed orbital (Tables S2−S6) makes it clear that the main absorption should be assigned as a π−π* type and the energy absorption band should show π−π* characteristic. In BOXD, both HOMO and LUMO are perfectly delocalized over the whole π-system. In BOXD-o-OCH 3 , HOMO and LUMO are still mainly delocalized on the benzene ring and oxadiazole ring; in addition, HOMO is also delocalized on the methoxy group. BOXD-p-OCH3 shows similar behavior as in BOXD-o-OCH3. However, in BOXD-m-OCH3, although HOMO and LUMO remain spread over the whole molecule, there is a tendency that the HOMO and LUMO are separated. It could be seen that the HOMO tends to delocalize on the benzene ring and methoxy group and the LUMO tends to concentrate on the oxadiazole ring. As in BOXD-D1, the HOMO is obviously localized on the methoxy phenyl ring and LUMO is mainly localized on the bi1,3,4-oxadiazole unit. In BOXD-T1, the HOMO is also obviously localized on terminal methoxybenzene ring and LUMO is mainly localized on the central bi-1,3,4-oxadiazole unit as in BOXD-D1. Because the transition S0−S1 is mainly composed of the electron transitions from orbital HOMO to LUMO in all of these compounds, it can be inferred that the electron transitions in all of these compounds are different from one another considering the distribution of the electron density: There is no ICT characteristic in BOXD, and for BOXD-o-OCH3 and BOXD-p-OCH3, ICT would occur, but its degree is smaller than that in BOXD-m-OCH3, and many more electron transitions would take place in BOXD-D1 and BOXDT1, which is highly consistent with the experimental result. The theory calculation has also been conducted including solvents by a PCM model,39 (Tables S2−S5) and molecular geometries optimized with solvents have been applied. As shown by the calculated results, the inclusion of solvents and the increase in the solvent polarity exhibit similar behaviors within the gas phase no matter in the absorption transitions or the excitation energy and the oscillator strengths. Thus it is clear that the electronic and structural natures of these compounds are not sensitive to the solvent polarity in the ground and FC excited states. The lack of structural and electronic sensitivity toward solvents is consistent with the low correlation between absorbance behavior and solvent observed experimentally. Molecular Geometry and Intramolecular Charge Transfer. The DFT-optimized molecular geometry of the ground state and first excited state has been present in Figures S8−S11; the bond, angle, and dihedral are listed in Tables S7−S18. The

Figure 7. Frontier molecular orbitals (HOMO and LUMO) of all of these compounds computed with CAM-B3LYP/6-31G** method (1: BOXD; 2: BOXD-o-OCH3; 3: BOXD-m-OCH3; 4: BOXD-p-OCH3; 5: BOXD-D1; 6: BOXD-T1). 8403

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The Journal of Physical Chemistry A calculated results reveal that all of these compounds show good planarity and a large extent of conjugation no matter in the ground state or in the first excited state. In addition, the introduction of substituent does not cause any charges on the molecular geometrical structures, while in solutions, small changes would be found in the bond of oxadiazole ring and the linkers between benzene ring and oxadiazole rings as compared with that in the gas phase. Because all of these compounds have a CT characteristic in the first excited state, charge transfer based on electron density between the ground state and the first excited state is analyzed to quantify the extent of CT during the excitation in these compounds.21,46 Figure 8 (left) shows the electron density

We further try to calculate the electron density differences of half-molecules of all of these compounds to eliminate the influence of molecular symmetry on the distribution of the electron density. Although the ICT in the half-molecule is not stronger than that in the symmetric molecule, the half-molecule exhibits similar spectroscopic behavior and same ICT process with the whole symmetric molecule.35 Therefore, the electron density variation and charge transfer of the half-molecule of all of these compounds are meaningful to qualitative analysis of the substituent effect. The corresponding half molecules are named OXD, OXD-oOCH3, OXD-m-OCH3, OXD-p-OCH3, OXD-D1, and OXDT1, respectively. As can be seen in Figure 9, the electron

Figure 8. Electron density difference between the ground and the first excited states of all of the bi-1,3,4-oxadiazole derivatives investigated (1: BOXD; 2: BOXD-o-OCH3; 3: BOXD-m-OCH3; 4: BOXD-pOCH3; 5: BOXD-D1; 6: BOXD-T1).

Figure 9. Charge-transfer diagram between the ground and the first excited states of the half-molecular bi-1,3,4-oxadiazole derivatives investigated (1: OXD; 2: OXD-o-OCH3; 3: OXD-m-OCH3; 4: OXDp-OCH3; 5: OXD-D1; 6: OXD-T1).

variation from S0 to S1 of all these compounds, in which the yellow and cyan denote the increase (positive) and decrease (negative) in electron density relative to the ground state, respectively. As can be seen, the electron density differences are spread over the whole molecule in all of these compounds, including both π−π* and CT characteristics. It is worth noting that for the methoxy-substituted compounds the area of positive domain is obviously larger than that of negative domain in bioxadiazole group; whereas in the phenyl group, the negative domain is larger than the positive domain in methoxy groups, electron density variation is mainly negative. The electron density difference maps clearly show that the electrons in these compounds have been transferred from the terminal methoxy phenyl groups to the central bioxadiazole group. To visualize the CT more clearly, two centroids of charges associated with the positive and negative density regions are calculated (Figure 8, right). Because all of these compounds possess a central symmetric configuration, the barycenters of the positive and negative parts highly overlap in the molecular center with the overlap integral between C+ and C− up to 1, which indicates that the two parts are almost superposed. Not only that, other related parameters are also almost to zero (Table S19). It is insufficient to analyze the ICT characteristic.

density changes in half-molecules are mainly positive over the oxadiazole group and are mainly negative over the phenyl group and methoxy groups, the same as in the whole molecules. Obvious differences are found in the charge transfer that the positive and negative parts of all of these compounds are well separated (Figure 9, right). The positive parts are mainly located at the oxadiazole side, whereas the negative parts are located at the methylbenzene side. It is evident that the direction of electron transfer is from methylbenzene (electron donor) to oxadiazole (electron acceptor). The related parameters, such as the transferred charge (qCT), distance of CT (DCT.), and the dipole moment variation (Δμ), are collected in Table S20. The transferred charge (qCT) is the magnitude of the integral of the electron density variation between the first excited state and the ground state. Distance of CT (DCT.) is defined as the distance between the positive barycenter and negative barycenter. The dipole moment variation (Δμ) between the first excited state and the ground state is the product of transferred charge and CT distance (Δμ = qCT × DCT.). There are 0.265 |e−| involved in ICT of OXD molecule upon S0−S1 excitation, and the distance of CT is 0.773 Å. The transferred charge and distance of CT in OXD-o-OCH3 8404

DOI: 10.1021/acs.jpca.7b08845 J. Phys. Chem. A 2017, 121, 8399−8407

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The Journal of Physical Chemistry A molecule and OXD-p-OCH3 molecule are 0.448 |e−|, 0.403 |e−| and 1.010 Å, 1.179 Å, respectively. OXD-m-OCH3 molecule owns a middle transferred charge (0.429 |e−|) and a larger CT distance (1.671 Å) as compared with OXD-o-OCH3 and OXDp-OCH3 molecule. OXD-D1 molecule and OXD-T1 molecule have obviously larger transferred charges (0.484 |e−| for OXDD1 molecule, 0.436 |e−| forOXD-T1 molecule) than monosubstituted compounds and comparable CT distance (1.777 Å for OXD-D1 molecule, 1.494 Å for OXD-T1 molecule) with OXD-m-OCH3 molecule. Furthermore, it is worth noting that the positive barycenters are almost located at the same place in each compound, whereas the positions of negative barycenter changed in different compounds as the substituent situation changed (see Figure 9, right). (The arrows’ start and end points represent the negative and positive centers, respectively.) In OXD, the negative barycenter is located in the benzene ring, which is close to the oxadiazole ring. In OXD-o-OCH3 and OXD-mOCH3, the positions of the negative barycenters tend to locate on the benzene ring edge closing to the substituent. Furthermore, the negative barycenter in OXD-p-OCH3 is located at the benzene center, with longer distance between the positive and negative barycenter than OXD for the effect of the para-substituted methoxy group. OXD-D1 also shows a negative barycenter on the benzene ring center, which could be caused by the two methoxy in different directions. And in OXD-T1, the negative barycenter is also located at central of benzene ring but slightly toward a meta-substituent. The different CT distance could be beneficial to the different location of the negative barycenter. Because the dipole moment variation is the product of transferred charge and CT distance, it can be obtained in the order OXD < OXD-o-OCH3 < OXDp-OCH3 < OXD-T1 < OXD-m-OCH3 < OXD-D1, which is the same as the calculated results based on the experiment data (except for OXD-T1). It can be concluded that the introduce of methoxy group could effectively bring about ICT characteristic in these symmetric bi-1,3,4-oxadiazole derivatives. The more amount of substituent, the more charge would be involved in the ICT. Furthermore, the CT distance is decided by the position of negative barycenters which have a tendency to locate close to the substituent. The dipole moment variation contains the resulting information on the transferred charge and the distance of CT. These conclusions could provide a guide for the prediction and design of molecules with ICT characteristics. Computed Emission Energy. The emission energy of all of these compounds was calculated with TD-CAM-B3LYP/6311+G** based on the optimized excited-state molecular geometries. A nonequilibrium SS-PCM approach was applied to describe the solvent effect. The results of BOXD-T1 are not included because of the long time of calculation. As shown in Table 3, the calculated first excited-state vertical emission energy of BOXD in CHEX is 3.48 eV. With the polarity of

solvent increased, the emission energy exhibit red shifts and the emission energy is ∼3.38 eV in ACN. In BOXD-o-OCH3, from CHEX to ACN, the emission energy red-shifted by 0.18 eV (∼20 nm). As for BOXD-m-OCH3 and BOXD-D1, the calculated energy is 3.49 eV in CHEX and 3.37 eV in ACN. The calculated vertical emission energy gives a solvatochromic trend, but only minor solvatochromism shifts were predicted, which may be caused by the coarse approach we used in computing emission energy.

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CONCLUSIONS The ICT characteristics of a series symmetric methoxy substituted bi-1,3,4-oxadiazole derivatives, in which methoxy groups are introduced on the benzene ring at different positions or with different quantities, have been studied in this paper by a combination of spectroscopic techniques and theoretical calculations. It has been found that the introduction of methoxy group on the symmetric phenyl-substituted bi-1,3,4oxadiazole derivatives could effectively bring about ICT characteristic. Small red shifts are found in UV−vis absorption spectra, as the electronic and structural natures of all of these compounds are not sensitive to the solvent polarity in the ground and FC excited states. Strong fluorescence red shifts in polar solvents are observed in all of these compounds with different degrees. The dipole moment variations between the ground state and CT excited state are calculated to be on the same order with the extent of red shift in all of these compounds (7.56 D in BOXD-o-OCH3, 12.07 D in BOXD-mOCH3, 7.38 D in BOXD-p-OCH3, 14.79 D in BOXD-D1, and 16.80 D in BOXD-T1). What’s more, all of the monosubstituted compounds show higher quantum yields in different solvents than the polysubstituted compounds do. Theoretical calculations explain the experimental phenomenon well. It has been proved that the first singlet excited state of all of these compounds show both π−π* and CT characteristics. In addition, all of these compounds show good planarity and a large extent of conjugation, no matter in ground state or in the first excited state. Charges have been demonstrated to transfer from the terminal methoxy phenyl group side to the central bioxadiazole group. Analysis on charge transfer revealed that the existence of methoxy group could effectively introduce ICT characteristics in bi-1,3,4-oxadiazole derivatives. Furthermore, the greater the amount of substituent, the more charge would be involved in the ICT. The location of substituent could also affect the distribution of the charge barycenter and finally cause the difference in CT distance. As a consequence, the changes of molecular dipole moment show different value in different molecules. These findings would be of great value for understanding structure−property relationships and the rational design of functional materials for photoelectric applications.

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S Supporting Information *

Table 3. Vertical Emission Energies for the First Excited States of All of These Compounds in Solutions

BOXD BOXD-o-OCH3 BOXD-m-OCH3 BOXD-D1

CHEX

DCM

THF

ACN

ETO

3.48 3.35 3.49 3.49

3.40 3.23 3.40 3.40

3.41 3.24 3.40 3.41

3.38 3.17 3.37 3.37

3.38 3.19 3.37 3.38

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b08845. Synthesis of BOXD-m-OCH3 and BOXD-D1, absorption and emission spectra of BOXD-o-OCH3, frontier molecular orbitals, optimized molecular geometry, and related parameters of charge transfer of all of these compounds. (PDF) 8405

DOI: 10.1021/acs.jpca.7b08845 J. Phys. Chem. A 2017, 121, 8399−8407

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The Journal of Physical Chemistry A

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AUTHOR INFORMATION

Corresponding Authors

*H.W.: E-mail: [email protected]. *M.L.: E-mail: [email protected]. ORCID

Haitao Wang: 0000-0003-4143-0317 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Science and Technology Develop Programme of Jilin Province (20170520127JH), Postdoctoral Science Foundation of China (2012T50294), the Fundamental Research Funds for the Central Universities.

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