Impact of the 2,2′-Bithienyl Framework on the Charge-Transfer

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Article Cite This: ACS Omega 2018, 3, 12730−12736

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Impact of the 2,2′-Bithienyl Framework on the Charge-Transfer Emission of Triarylborane-Based o,o′-Substituted Biaryls Sheng-Yong Li, Zuo-Bang Sun, and Cui-Hua Zhao* School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, Jinan 250100, People’s Republic of China

ACS Omega 2018.3:12730-12736. Downloaded from pubs.acs.org by 46.148.112.126 on 10/14/18. For personal use only.

S Supporting Information *

ABSTRACT: Two new triarylborane-based o,o′-substituted 2,2′-bithienyls, BT-BNMe2 and BT-BNBn2, which contain BMes2 and NMe2/NBn2 groups at the 3,3′-positions, have been synthesized. Similar to the o,o′-substituted biphenyl analogues, BP-BNMe2 and BP-BNBn2, which contain BMes2 and NMe2/NBn2 groups at the 2,2′-positions, the steric effect of the amino group has significant influence on the conformation of the 2,2′-bithienyl skeleton. The boryl and amino groups are located at the same side of 2,2′bithienyls axis with a short B···N distance (3.63 Å) for the NMe2substituted BT-BNMe2. On the contrary, the two substituents are arranged on the two different sides of the 2,2′-bithienyls axis for BT-BNBn2, which is modified with bulky NBn2. Despite the remarkable differences in the steric structure, the two 2,2′-bithienyls display fluorescence at close wavelengths, which is in sharp contrast to the much red-shifted fluorescence of BPBNMe2 than BP-BNBn2. The theoretical calculations demonstrated that the two 2,2′-bithienyls have close highest occupied molecular orbital−lowest unoccupied molecular orbital gaps in the excited state, which firmly support the experimental results. Thus, the parent main chain framework can exhibit great impact on the charge-transfer emission of o,o′-substituted biaryls.

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provides another novel way for the tuning of fluorescence wavelength. For the biphenyl BP-BNMe2, which is modified with dimethylamino (NMe2), NMe2 and BMes2 are located at the same side of the biphenyl axis with a short B···N distance (3.59 Å), as indicated by its X-ray crystal structure. In contrast, the boryl and amino groups are arranged on the two opposite sides of the biphenyl axis in BP-BNBn2, which is modified with bulky dibenzylamino (NBn2). The change of the amino group from NMe2 to NBn2 causes a 71 nm hypochromic shift of emission in cyclohexane. Moreover, BP-BNMe2 is highly emissive in both the solution and solid state with a rather long fluorescence wavelength (λ = 521 nm in cyclohexane). The interesting properties of BP-BNMe2, and the great influence of the amino steric effect on the photophysical properties prompted us to turn our attention to 3,3′-substituted 2,2′bithienyl derivatives, BT-BNMe2 and BT-BNBn2, which contain boryl and amino groups at 3,3′-positions of the 2,2′bithienyl framework (Figure 1). Compared with biphenyl derivatives, the emissions of bithienyl compounds were initially expected to shift to longer wavelength. Furthermore, the 5,5′positions of 2,2′-bithienyl are easily functionalized, which would make it possible to further tune the emission properties by attachment of extra substituents. A large shift of 72 nm in emission was in fact observed from BP-BNBn2 to BT-BNBn2

INTRODUCTION Organic fluorophores have found extensive applications in organic light emitting devices, chemical probes, medical diagnosis, and biomolecular labels.1 In the design of organic fluorophores, one important issue is the fine-tuning of fluorescence emission wavelength. To tune the fluorescence emission wavelength, the following general strategies have been adopted, such as change of the parent π-conjugated framework,2 choosing substituents with a different electronic effect,3 and change of substituent positions.2b,4 In our recent research on a triarylborane-based biphenyl π-system, which contains an electron-accepting dimesitylboryl (BMes2) and an electrondonating N,N-disubstituted amino (NR2) at the o,o′-positions of the biphenyl framework (Figure 1),5 it was revealed that the change of stereo geometry, which is influenced greatly by the steric effect of the amino group, can induce a remarkable difference in the fluorescence emission wavelength,5c which

Received: August 13, 2018 Accepted: September 21, 2018 Published: October 5, 2018

Figure 1. Molecular structures of triarylborane-based (a) 1,1′biphenyls and (b) 2,2′-bithienyls. © 2018 American Chemical Society

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listed in Table 1. The amino and boryl-bonded thienyl rings are labeled as P1 and P2, respectively. The phenyl ring of the mesityl group located above the amino-bonded thienyl is labeled as P3, and the phenyl ring of the another mesityl group is labeled as P4. Regarding the X-ray crystal structures, BT-BNMe2 and BTBNBn2 display some characteristics similar to the corresponding biphenyl derivatives. (1) The conformations of the bithienyl skeleton are also totally different. For BT-BNMe2, which is substituted with less bulky NMe2, the amino and boryl groups are located at the same side of bithienyl axis. The distance between boron and nitrogen centers is 3.63 Å, which implies the existence of some attractive B−N interactions. In contrast, the amino and boryl groups are arranged at two different sides of the bithienyl axis in BT-BNBn2, which is substituted by more bulky NBn2. Owing to the greater steric congestion in BT-BNMe2, the bithienyl framework is more twisted than BT-BNBn2 (dihedral angle between P1 and P2: 58.8° for BT-BNMe2 and 32.0° for BT-BNBn2). (2) The boron centers are both perfectly planar with the sum of the C− B−C bond angles equal to 360°. It was also noted that the boryl-bonded thienyl ring is more coplanar with the BC3 plane, as illustrated by much smaller dihedral angles (22.0° for BTBNMe2 and 35.3° for BT-BNBn2) than the two phenyl rings of mesityl groups (48.4°−64.1°). Consequently, the B−C19 bond length is shorter than B−C1 and B−C10 to some extent, suggesting that the B center is conjugated with the bithienyl framework more efficiently. On the contrary, the nitrogen center in BT-BNMe2 exhibits highly pyramidalized geometry. The sum of the C−N−C bond angle is around 340°. (3) There possibly exists π−π interactions between P1 and P3 in BT-BNMe2, as evidenced by the small torsion angle (26.8°) and short centroid−centroid distance (3.89 Å). Compared with the biphenyl analogues, some structural differences were also observed. (1) The nitrogen center of BT-BNBn2 is still highly pyramidalized despite the much released steric congestion. (2) The main chain of bithienyl derivatives turns more coplanar compared with the corresponding biphenyl analogues, probably because of less steric congestion of the bithienyl skeleton (dihedral angle between two phenyl rings: 70.7° for BP-BNMe2 and 50.0° for BP-BNBn2). Photophysical Properties. The two 2,2′-bithienyl derivatives contain both electron-accepting BMes2 and electrondonating NR2 groups.7 They were expected to display intramolecular CT transitions, which would lead to great fluorescence solvatochromism. To eliminate the influence of the solvent polarity, the photophysical properties were first examined in cyclohexane. Figure 3 shows the UV−vis absorption and fluorescence spectra, and the corresponding data are listed in Table 2. Regarding the photophysical properties, the most notable feature for the biphenyl derivatives is that the fluorescence of NMe2-substituted BP-BNMe2 is significantly red-shifted than NBn2-substituted BP-BNBn2 (Δλ = 71 nm in cyclohexane), although their absorption spectra are not very different (Figure 3). From BP-BNBn2 to BT-BNBn2, both the absorption and emission of BT-BNBn2 are significantly red-shifted (Δλ = 52 nm for absorption and 72 nm for emission), suggesting the remarkable effect of the parent framework on the photophysical properties. Thus, the longest absorption band of the BT-BNBn2 is located at 414 nm with a moderate intensity (log ε = 3.73), and the fluorescence band was observed at 522 nm. Considering the very similar structural features between

for the NBn2-substituted derivatives, just as expected. However, the NMe2-substituted compounds, BP-BNMe2 and BT-BNMe2, emit fluorescence at almost the same maximum wavelengths. As a result, the emissions of two bithienyl derivatives are very close, which naturally evoked a question why the two π-systems display emissions in such a completely different manner. The elucidation of this point will lay a great foundation for the fine-tuning of the emission wavelength. Herein, the X-ray single-crystal structures, photophysical properties, and theoretically calculated structures in the ground state and excited states of the two bithienyl compounds were fully characterized to investigate the impact of the 2,2′bithienyl parent framework on the photophysical properties.

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RESULTS AND DISCUSSION Synthesis. The two target 2,2′-bithienyl derivatives were easily prepared in four steps starting from the commercially available 3,3′-dibromo-2,2′-bithienyl (Scheme 1). The first Scheme 1. Synthesis of BT-BNMe2 and BT-BNBn2

Pd(0)-catalyzed monocoupling with benzophenone imine and subsequent hydrolysis with concentrated hydrochloric acid afforded the main synthetic intermediate 3-amino-3′-bromo2,2′-bithienyl (BT-BrNH2).6 The following N-methylation and N-benzylation of the amino group with methyl iodide and benzyl bromide gave the bromide precursors, BT-BrNMe2 and BT-BrNBn2, respectively. Finally, BT-BNMe2 and BT-BNBn2 were obtained by the lithiation of the corresponding bromide precursors followed by quenching with dimesitylfluoroborane. All the reactions went successfully with yields ranging from 65 to 99%. These two triarylborane-based 2,2′-bithienyl derivatives are stable to air either in the solution or in solid state, and their structures were confirmed by 1H NMR and 13C NMR spectroscopy, high-resolution mass spectrometry, and X-ray diffraction analysis. Crystal Structures. The structures of BT-BNMe2 and BTBNBn2 determined by X-ray diffraction are shown in Figure 2. The selected important bond lengths and dihedral angles are

Figure 2. Crystal structures of (a) BT-BNMe2 and (b) BT-BNBn2. Hydrogen atoms are omitted for clarity. 12731

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Table 1. Selected Bond Lengths (Å), Distances (Å), Bond Angles (deg) and Dihedral Angles (deg) for Structures of Triarylborane-Based 2,2′-Bithienyls in the X-ray Crystal Structure, DFT-Optimized S0-Geometry, and TD-DFT-Optimized S1Geometry [Calculated at PBE0/6-31G(d) Level of Theory] BT-BNMe2 B1−C1 B1−C10 B1−C19 N1−C24 ∑∠C−B−C ∑∠C−N−C ∠P2−BC3 ∠P3−BC3 ∠P4−BC3 ∠P1−P2 ∠P1−P3 centroid−centroid distance (P1−P3) B···N distance

BT-BNBn2

crystal structure

S0-geometry

S1-geometry

crystal structure

S0-geometry

S1-geometry

1.588 1.570 1.565 1.399 359.9 341.2 22.0 56.4 61.4 58.8 26.8 3.89 3.63

1.584 1.577 1.568 1.410 359.8 340.5 25.2 52.8 56.9 59.6 25.7 3.91 3.65

1.593 1.593 1.546 1.356 360.0 357.7 16.9 56.4 53.0 63.4 17.7 3.67 3.81

1.574 1.574 1.566 1.414 359.8 344.8 35.3 48.4 64.1 32.0

1.580 1.583 1.568 1.408 359.8 345.5 35.2 51.5 54.3 36.2

1.566 1.577 1.588 1.384 359.7 348.0 42.2 41.1 47.8 28.5

BP-BNBn2). On the basis of the radiative and nonradiative decay rate constants, which are calculated according to the equations Φ F = kr × τ and kr + kr = τ−1, the decrease in the fluorescence efficiency of the bithienyl system is mainly ascribed to the acceleration of the nonradiative decay process. The main reason for this is probably the heavy atom effect of sulfur that facilitates the intersystem crossing.8 Consequently, the change of amino groups from NMe2 to NBn2 exhibits a trivial effect on the photophysical properties of triarylboranebased 2,2′-bithienyl derivatives. Considering the donor−π−acceptor structure and thus possible intramolecular CT characteristics of two bithienyl derivatives, the absorption and emission spectra were also examined in solvents with different polarities. The related data are summarized in Table 3. With the increased solvent polarity,

Figure 3. Absorption and fluorescence spectra of triarlyborane-based o,o′-substituted biaryls (blue solid line: BT-NBn2; red solid line: BTNMe2; blue dashed line: BP-NBn2; red dashed line: BP-NMe2).

Table 3. UV−Vis Absorption and Fluorescence Data of BTBNMe2 and BT-BNBn2 in Various Solvents

biphenyl and 2,2′-bithienyl derivatives, the fluorescence of NMe2-substituted bithienyl BT-BNMe2 was initially expected to be shifted to about 600 nm, which is much longer than BTBNBn2. It was completely unexpected that the emission spectra of BT-BNMe2 and BT-BNBn2 are almost identical (Δλ = 3 nm), which is a sharp contrast to about 71 nm red shift from BP-BNBn2 to BP-BNMe2. As for the absorption, BT-BNMe2 shows absorption maximum at 402 nm (log ε = 3.31), which is slightly blue-shifted with a lower intensity than BT-BNBn2. It was also noted that the fluorescence of two 2,2′bithienyl compounds is not very intense (ΦF = 0.075 for BTBNMe2 and 0.025 for BT-BNBn2). The time-resolved fluorescence study revealed that the fluorescence lifetimes of two 2,2′-bithienyl derivatives are much shorter than the biphenyl compounds (17.8 ns for BP-BNMe2 and 9.8 ns for

BT-BNMe2

BT-BNBn2

solvent

λabs (nm)a

λabs (nm)

Δν (cm−1)b

cyclohexane CHCl3 THF MeCN cyclohexane CHCl3 THF MeCN

402 408 411 413 414 416 417 415

525 544 554 564 522 531 536 539

5828 6127 6280 6483 4998 5206 5324 5544

ac

Δμd

6.34

6.3

6.73

6.3

a

The longest absorption maxima. bStokes shift. cCalculated radius of cavity. dCalculated dipole moment change (μe − μg) from the ground state to the excited state.

Table 2. UV−Vis Absorption and Fluorescence Data of BT-BNMe2 and BT-BNBn2 in Cyclohexane absorption BT-BNMe2 BT-BNBn2

emission

Stokes shift

excited dynamics

λabs (nm)a

log ε

λem (nm)

Φ Fb

Δν (cm−1)

Δλ

τ (ns)c

kr (s−1)d

knr (s−1)d

402 414

3.31 3.73

525 522

0.075 0.025

5828 4998

123 108

0.26/3.26 (0.28/0.72) 0.29/3.15 (0.95/0.05)

2.4 × 107 1.9 × 107

2.9 × 108 7.3 × 108

The longest absorption maxima. bCalculated by using fluorescein as a standard. cAmplitudes of two lifetimes are given in the parentheses. Calculated using the average lifetime, that is, derived following the equation τ = ∑Aiτi2/∑Aiτi, in which Ai is the amplitude of each lifetime.

a

d

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the fluorescence spectra are red-shifted obviously in spite of unobvious solvent dependence on absorption. This fact clearly suggests the more polarized structures in the excited state than in the ground state, denoting the intramolecular CT characteristics of the first excited state. It was noted that the fluorescence solvatochromism of bithienyls is not significant as the biphenyl analogues. From nonpolar cyclohexane to highly polar MeCN, the fluorescence shifts are only 39 and 17 nm for BT-BNMe2 and BT-BNBn2, respectively. In contrast, the corresponding shifts of BP-BNMe2 and BP-BNBn2 are 59 and 58 nm, respectively. The polarization degree of the excited state can be compared in terms of the Lippert−Mataga equation [eq 1],9 in which Δν is the Stokes shift, μe and μg correspond to the excited and ground state dipole moments,respectively, Δf is the solvent polarity, and C is a constant. Δf is given by eq 2, where n refers to the optical Δ;ν = νA − νF = Δf =

2Δf hca3

(μe − μg )2 + C

BNBn2 and BT-BNMe2 modified with different amino groups display photophysical properties in a complete different trend with the corresponding biphenyl analogues BP-BNBn2 and BP-BNMe2. Theoretical Calculations. To further shed light on the photophysical properties of 2,2′-bithienyl, comprehensive theoretical calculations were performed by the Gaussian 09 program.11 The geometries of the ground state (S0) were first optimized by DFT calculations using the X-ray crystal structure as the initial structure. Then, the vertical excitation energies were calculated via time-dependent DFT (TD-DFT) calculations. A frequency calculation was made following all geometry optimizations to make sure to get the minimum optimized geometry. The optimized structure of S0 reproduces the features that are essentially in agreement with those of the X-ray crystal structures (Table 1). For example, NR2 and BMes2 groups are located at the same side of the bithienyl axis in BT-NMe2, while they lay on two opposite sides in BT-NBn2. Moreover, the short centroid−centroid distance (3.91 Å) and small dihedral angle (25.7°) between P1 and P3 in BT-NMe2 again suggest possible π−π interactions between them. Figure 5 shows the pictorial drawing, energy levels, and transitions of two bithienyl compounds. In the S0 state, both the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of BTBNMe2 and BT-BNBn2 can spread over the whole bithienyl skeleton with great contributions from the amino group and the dimesitylboryl group, respectively. In BT-BNMe2, the benzene ring P3 also contributes to the HOMO, suggesting the intramolecular π−π interaction between P1 and P3. Probably because of the more efficient conjugation of the bithienyl skeleton in BT-BNBn2 as the result of higher coplanarity between two thienyl rings, BT-BNBn2 shows higher HOMO and lower LUMO energy levels compared with BT-BNMe2. The TD-DFT calculations suggest that the first excited states of BT-BNBn2 and BT-BNMe2 are assignable to HOMO → LUMO transitions, which is a characteristic of intramolecular CT to some extent. Consequently, BT-BNMe2 has a higher vertical excitation energy with smaller oscillator strength. This is in good agreement that BT-BNMe2 shows the longest absorption band at a shorter wavelength with a lower intensity relative to BT-BNBn2. Compared with the biphenyl derivatives, the most noteworthy feature for the 2,2′-bithienyls is that no obvious red shift of fluorescence was observed for the NMe2-substituted BT-BNMe2 relative to the NBn2-substituted BT-BNBn2. To clarify the effect of the amino group on the fluorescence property, the optimizations of the first excited state (S1) geometries were also carried out.10 With regard to the structure of the S1 state, the following four points are noted. (1) The boryl and amino groups are still located on the same side and opposite sides of the 2,2′-bithienyl axis for BTBNMe2 and BT-BNBn2, just as in the ground state. (2) The NC3 plane of BT-BNMe2 becomes almost completely planar, whereas the N center in BT-BNBn2 still remains highly pyramidalized. Meanwhile, the N−C24 bond turns shorter compared with the ground state, especially for BT-BNMe2. These facts might indicate the more efficient conjugation between the amino group and 2,2′-bithienyl skeleton in the excited state. (3) The BC3 plane of BT-BNMe2 turns more coplanar with P2 in the S1 state along with the shortening of the corresponding B−C19 bond. On the contrary, the

(1)

ε−1 n2 − 1 − 2 2ε + 1 2n + 1

(2)

refractive constant and ε refer to the dielectric constant. In fact, the linear relationships were obtained for the plots of Δν as a function of Δf for BT-BNMe2 and BT-BNBn2 (Figure 4). 10

Figure 4. Lippert−Mataga plots of BT-BNMe2 and BT-BNBn2.

From the slope of two plots, the dipole moment change (μe − μg) from the ground state to the excited state was calculated with the assumption of the molecular radius as the cavity radius. The examined molecules are not spherical in nature. The molecular radii were estimated by density functional theory (DFT) calculations and are summarized in Table 3.11 The dipole moment changes from the ground state to excited state were also calculated and are also summarized in Table 3. The two bithienyl compounds show distinct dipole moment changes from the ground state to the excited state, suggesting the intramolecular CT characteristics of emission. However, the dipole moment changes are much less significant than the corresponding biphenyls. In addition, the dipole moment changes are the same for the two bithienyls, which is different from that the dipole moment change of the NBn2-substituted biphenyl BP-BNBn2 is higher than the NMe2-substituted biphenyl BP-BNMe2 (μe − μg: 14.8 debye for BP-BNMe2 and 17.7 for BP-BNBn2). The abovementioned results clearly suggest that the simple change of the parent framework from biphenyl to bithienyl exhibits substantial influence on the photophysical properties. The bithienyl compounds BT12733

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Figure 5. Kohn−Sham energy levels, pictorial drawing of frontier orbitals, and transitions of (a) BT-BNMe2 and (b) BT-BNBn2 at different geometries, calculated at PBE0/6-31g(d).

almost the same, which is very different from those of the NBn2-substituted biphenyl BP-BNBn2, which exhibits greater dipole moment changes than the NMe2-substituted biphenyl BP-BNMe2. The similar dipole moment changes of 2,2′bithienyls are well consistent with the experimental results obtained from the fluorescence solvatochromism. Therefore, the above mentioned calculated results reasonably support the experimental results that both 2,2′-bithienyls emit fluorescence at very close wavelength, and their fluorescence solvatochromism is not very significant. Thus, for the triarylborane-based 3,3′-subsituted 2,2′-bithienyls, the change of the amino group with different steric effects is not effective to tune the fluorescence wavelength.

structure of BT-BNBn2 changes in an opposite manner. As a result, the conjugation between the boron center and 2,2′bithienyl skeleton is more efficient in the S1 state for BTBNMe2, whereas the conjugation between the boron center and pendent two phenyl rings of mesityl groups are more effective for BT-BNBn2. (4) The π−π interaction between P1 and P3 might become stronger, as indicated by the smaller dihedral angle and shorter centroid−centroid distance. Along with the stereo structure changes from the ground state to the excited state, no much great difference was observed for the electron distribution of frontier orbitals. The energy levels of HOMOs and LUMOs are elevated and lowered, respectively. Notably, the HOMO energy levels of BT-BNMe2 and BTBNBn2 are almost the same and the LUMO energy level of BT-BNBn2 is even a little lower than BT-BNMe2. Thus, BTBNMe2 has a slightly higher HOMO−LUMO gap than BTBNBn2 (0.08 eV difference). Considering the similar HOMO−LUMO gap for the 2,2′-bithienyl molecules, it is reasonable that they emit fluorescence at very close wavelengths. To shed light on the solvent effect on the fluorescence, the dipole moments at the optimized S0 and S1 geometries and at the Frank-Cotton geometries for absorption and emission were also calculated and are summarized in Table 4. The dipole moment is increased from the ground state to the excited state for both the 2,2′-bithienyl compounds. However, the increase extent is less significant than the biphenyl derivatives. In addition, dipole moment changes of the two 2,2′-bithienyls are

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CONCLUSIONS In summary, we have prepared two new triarylborane-based o,o′-substituted 2,2′-bithienyls, BT-BNMe2 and BT-BNBn2, in which BMes2 and NMe2/NBn2 groups are introduced at 3,3′positions of the 2,2′-bithienyl framework. Their X-ray singlecrystal structures, UV-absorption, and fluorescence properties were fully characterized to elucidate the effect of the parent framework change from biphenyl to bithienyl on the photophysical properties. Similar to the previously reported o,o′-substituted biphenyl analogues, BP-BNMe2 and BPBNBn2, the steric effect of the amino group has significant influence on the conformation of the 2,2′-bithienyl skeleton. The boryl and amino groups are located at the same side of the 2,2′-bithienyls axis with a short B···N distance (3.63 Å) for the NMe2-substituted BT-BNMe2. On the contrary, the two substituents are arranged on the two opposite sides of the 2,2′-bithienyls axis for the NBn2-substituted BT-NBn2. Despite the remarkable differences in the steric structure, the two 2,2′bithienyls display fluorescence at the close wavelengths, which is in sharp contrast to the much red-shifted fluorescence of BPBNMe2 than BP-BNBn2. The theoretical calculations demonstrated that the two 2,2′-bithienyls have close energy levels for the frontier orbitals and thus similar HOMO−LUMO gaps, which firmly support the experimental results. Thus, for the triarylborane-based 3,3′-substituted 2,2′-bithienyls, the change of the amino group with different steric effects is not effective to tune the fluorescence wavelength. The parent main chain

Table 4. Theoretical Dipole Moments (debye) of BTBNMe2 and BT-BNBn2, Calculated at PBE0/6-31g(d)a BT-BNMe2 BT-BNBn2

S0

SFC 1

S1

SFC 0

S1 − S0

1.53 1.32

8.46 8.85

8.63 7.88

1.92 1.79

7.10 6.56

a

Calculated at the following points on the ground and lowest singlet excited-state surfaces: S0, the optimized ground-state geometry; S1, the optimized first singlet excited-state geometry; S1Fc, the S1 state at the FC geometry upon excitation; and SFc 0 , the S0 state at the FC geometry upon emission. 12734

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framework can exhibit a great impact on the charge-transfer emission of o,o′-substituted biaryls. Our current systematic results about triarylborane-based o,o′-substituted biaryls are believed to be a very useful basis for the fine-tuning of emission wavelength.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02004. Synthesis, X-ray crystal structure analysis, computation, and fluorescence spectra of BT-BNMe2 and BT-BNBn2 in various solvents; 1H NMR and 13C NMR spectra of all new compounds, and Cartesian coordinates of DFToptimized structures in the ground state (S0) and TDDFT-optimized structures in the first excited state (S1) (PDF) Crystal data for BT-BNMe2 (CCDC 1858684) (CIF) Crystal data for BT-BNBn2 (CCDC 1858685) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-H.Z.). ORCID

Cui-Hua Zhao: 0000-0002-4077-5324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (21572120, 21272141). REFERENCES

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DOI: 10.1021/acsomega.8b02004 ACS Omega 2018, 3, 12730−12736