Charge Transfer Switching in Donor-Acceptor Systems Based on BN

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Charge Transfer Switching in Donor-Acceptor Systems Based on BN-Fused Naphthalimides Guangfeng Li, Yijing Chen, Yanjun Qiao, Yifan Lu, and Gang Zhou J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00597 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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The Journal of Organic Chemistry

Charge Transfer Switching in Donor-Acceptor Systems Based on BN-Fused Naphthalimides

Guangfeng Li, Yijing Chen, Yanjun Qiao, Yifan Lu, and Gang Zhou* Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, P. R. China.

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Abstract: Six-membered azaborine rings have been straightforward fused on naphthalimide-based donoracceptor systems and a series of BN-containing heteroaromatic compounds BN1-BN3 were constructed. Electron donating triphenylamine were functionalized in the extended direction of 3or/and 4-position of the naphthalimide unit. For comparison, reference BN0 without triphenylamine was also prepared. The intramolecular charge transfer (ICT) interactions in the resulting BN-fused naphthalimides (BN0-BN3) together with their precursors (N0-N3) and fluoride coordinated analogues (FBN0-FBN3) have been systematically investigated by photophysical, electrochemical, and theoretical approaches. It is found that the fusion of azaborine ring has great effect on the ICT properties of the D-A systems based on BN-fused naphthalimides. For the precursors without boron, extension of electron donor from 3-position of naphthalimide is superior in enhancing the D-A interactions. On the contrary, upon fusion of azaborine ring on naphthalimide, the dominant orientation of the ICT interactions conversely converts to the extended direction of 4-position of naphthalimide in the D-A molecules based on BN-fused naphthalimides. Most interestingly, upon coordinating the boron by fluoride ion, the ICT interactions are dramatically enlarged and the substitution position of triphenylamino group has negligible effect on the ICT properties of the fluoride coordinated analogues.


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Introduction Charge transfer (CT) in conjugated organic semiconductors has been extensively exploited to construct donor–acceptor (D–A) molecules with low band gap for optoelectronic devices, such as light-emitting diodes (LEDs),1 organic photovoltaics (OPVs),2 field-effect transistors (FETs),3 nonlinear optics,4 and fluorescent chemosensors.5 Incorporation of heteroatoms into π-conjugated frameworks is a promising strategy to acquire novel D–A molecules with unique properties and tailored functions.6 Among a variety of possible dopants, boron and nitrogen have received particular attention due to their distinct electronic and optical characteristics.7 Recently, one popular example for combination of both boron and nitrogen into aromatic system is substituting one or more C=C units by isosteric and isoelectronic B−N units, which can be also considered as zwitterionic double bonds in the neutral state.8 Hence, most reported BN-heteroacene systems display similar aromaticity in comparison to those all-carbon analogues, whereas exhibit different electronic and optical properties, making them as promising candidates for various optoelectronic applications. Up to date, the azaborine chemistry has been well established by several groups.9 However, little research has focused on BN-embedded aromatic hydrocarbons containing electron-deficient moieties.10 Moreover, the D–A interactions and the intramolecular charge transfer (ICT) properties upon the incorporation of BN unit are rarely documented.11 Therefore, investigation of the relationship between the chemical structures of the azaborines and the related ICT properties is still challenging. Naphthalimide12 is the fundamental structure of several star materials, such as naphthalenediimide,13 perylenediimide,14 and terrylenediimide.15 In this contribution, 1,8naphthalimide was selected as electron-deficient core where six-membered azaborine ring was straightforward fused on. Azaborines BN1-BN3 (Chart 1) with triphenylamine functionalized in the extended direction of 3- or/and 4-position of naphthalimide were designed and synthesized.


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Reference BN compound BN0 (Chart 1) without triphenylamine was also prepared for comparison. The ICT properties of the four target BN-fused naphthalimides along with their precursors without boron (N0-N3) were comparatively investigated by photophysical, electrochemical, and theoretical approaches. Moreover, the vacant orbital on boron could be occupied by the electron-pair on Lewis base, such as fluoride ion.16 Upon binding with fluoride ion, the electron-withdrawing boron shifted to an electron-donating unit, which significantly affected the electron distribution and the ICT interactions in the fluoride coordinated azaborines

Chart 1. Chemical structures of BN-fused naphthalimides BN0-BN3 together with their precursors N0-N3 and fluoride coordinated analogues FBN0-FBN3.


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FBN0-FBN3 (Chart 1). Such distinct ICT switching among the resulting BN-fused naphthalimides together with their precursors and fluoride coordinated analogues may provide in-depth insight into the intramolecular charge transfer in azaborine based D-A systems.

Results and discussion Synthesis and Structural Characterization. The straightforward synthetic approach to BNfused naphthalimides BN0-BN3 is depicted in Scheme 1. It started from commercial available Noctyl-4-bromo-1,8-naphthalimide (a) which was converted to N-octyl-3-nitro-4-bromo-1,8naphthalimide (b) by nitration with sodium nitrate at the 3-position of naphthalimide. Then the key intermediate N-octyl-3-amino-4-bromo-1,8-naphthalimide (c) was obtained by reducing the

Scheme 1. Synthetic route for BN-fused naphthalimides BN0-BN3. Reagents and conditions: (i) NaNO3, H2SO4, –10 °C to rt, 3 h; (ii) SnCl2, ethyl acetate/tetrahedrofuran, 70 °C, 6 h; (iii) Pd(PPh3)4, NaHCO3, dioxane/H2O, reflux, 8 h; (iv) Pd2(dba)3, t-BuONa, toluene, 95 °C, 15 h; (v) o-dichlorobenzene, 180 °C, 8 h.


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nitro group in compound b at the presence of SnCl2. The synthesis of the precursors N0 and N2 was carried out by Suzuki coupling17 of intermediate c and the related arylboronic ester. It should be noted that toluene/H2O or tetrahydrofuran (THF)/H2O was generally used as a typical solvent mixture for Suzuki coupling reaction. Unfortunately, attempting to achieve these precursors in either toluene/H2O or THF/H2O solvent mixture was not applicable. An alternative approach was conducted by using dioxane/H2O (7:3) solvent system, which produced the product with a high yield of around 80%. Subsequently, precursors N1 and N3 were provided via Buchwald coupling18 of naphthalimide N0 or N2 and triphenylamine bromide with Pd2(dba)3 as catalyst in toluene. Finally, the target BN-fused naphthalimides BN0-BN3 were prepared by reacting precursors N0-N3 with excess amount of dichlorophenylborane in o-dichlorobenzene at the presence of triethylamine. All of the new compounds were characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry (HRMS) and were found to be consistent with the proposed structures. The resulting BN-compounds exhibited excellent chemical stability in both solid and solution states for several months. No obvious change could be observed in the 1H NMR spectra for the BN-compounds after long-term storage. To verify the successful fusion of azaborine on naphthalimide moiety, single crystal X-ray analysis was performed to provide the most direct description of the molecular structural features. The single crystal of BN2 was obtained as red needle-shaped crystal by diffusion of hexane into its dichloromethane solution and slow evaporation at room temperature. Single crystal X-ray analysis (Figures 1 and S1, Table S1) demonstrates that the molecules of BN2 are aligned in a triclinic unit cell with a = 4.8209(2) Å, b = 19.1877(7) Å, c = 23.9230(8) Å, α = 109.218(2)°, β = 95.529(2)°, and γ = 96.646(2)° (Ref: CCDC 1813352 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif). The length


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(a)

(b)

Figure 1. (a) ORTEP diagram with ellipsoid contour probability level of 50% and (b) crystal packing of BN-fused naphthalimide BN2.

of the BN bond in BN2 is 1.413(3) Å (Table S2), which is much shorter than the expected value for a B-N single bond (1.58 Å), but close to the localized B=N double bond (1.40 Å).19 This suggests the effective conjugation extension through the BN bond and aromaticity of the azaborine ring, which is further supported by nucleus-independent chemical shift (NICS) calculations. Moreover, the dihedral angle between the fused azaborine ring and the naphthaline unit is 5.3(3)°, indicating a relatively planar structure for the BN-fused naphthalimide backbone. The crystal packing diagram of BN2 is shown in Figure 1b. A distinct face-to-face π-π stacking pattern is observed and the distance of two neighboring azaborine rings is ∼3.53 Å, which agrees with the fact that BN2 possesses an extended π-conjugated backbone.


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Photophysical Properties. The as-prepared BN-fused naphthalimides BN0-BN3 are well soluble in common organic solvents, such as dichloromethane, THF, and toluene. The optical properties of BN-fused naphthalimides BN0-BN3 in diluted solutions were investigated by UV−vis absorption and photoluminescence (PL) spectroscopies (Figures S2 and S3), and the corresponding photophysical data are listed in Table 1. Figure 2 displays the UV-vis absorption spectra of BN0-BN3 in THF solutions (ca. 5×10-6 M). It can be found that all the compounds

Table 1 Photophysical properties of BN-fused naphthalimides BN0-BN3 in different solvents (ca. 5×10-6 M). Comp.

Solvent

∆ƒa

λAbs (nm)

Hexane -0.006 416 Toluene 0.014 421 EA 0.20 417 THF 0.21 420 DCM 0.22 421 Acetone 0.27 421 DMF 0.28 417 Hexane -0.006 417 BN1 Toluene 0.014 420 EA 0.20 416 THF 0.21 417 DCM 0.22 419 Acetone 0.27 416 DMF 0.26 419 Hexane -0.006 460 BN2 Toluene 0.014 464 EA 0.20 461 THF 0.21 462 DCM 0.22 463 Acetone 0.27 462 DMF 0.28 467 Hexane -0.006 461 BN3 Toluene 0.014 474 EA 0.20 468 THF 0.21 470 DCM 0.22 477 Acetone 0.27 472 DMF 0.28 474 a Orientational polarizability (∆ƒ) of the solvent.20 BN0

λPL (nm)

Stokes shift (cm-1)

431 449 455 458 462 466 478 488 509 512 527 538 551 570 518 553 581 589 609 610 613 545 556 590 596 611 637 654

836 1481 2003 1975 2108 2294 3060 3489 4163 4507 5005 5279 5889 6322 2434 3469 4480 4667 5178 5252 5100 3343 3111 4418 4498 4909 5488 5806


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exhibit two distinct absorption bands. The absorption band in the lowest energy region typically originates from the ICT transition from the electron-donating unit to the electron-withdrawing group, which is ascribed to the highest occupied molecular orbital (HOMO) → the lowest unoccupied molecular orbital (LUMO) transition, as revealed by theoretical calculations. The other one in the higher energy region can be assigned to the π−π* transitions of the aromatic backbone. As shown in Figure 2, the ICT band of BN0 without triphenylamine locates at 420 nm. However, upon introduction of triphenylamino group from the extended direction of 3-position of naphthalimide, the ICT band slightly hypsochromically shifts to 417 nm by 3 nm for BN1. Little contribution of the incorporated donor on the ICT interactions is probably owing to the twisted molecular skeleton caused by the steric hindrance, which greatly affects the conjugation

1 Normalized Abs. Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N0 (431 nm) N1 (462 nm) N2 (432 nm) N3 (463 nm)

0 1

0

1

0 300

BN0 (420 nm) BN1 (417 nm) BN2 (462 nm) BN3 (470 nm)

FBN0 (550 nm) FBN1 (556 nm) FBN2 (578 nm) FBN3 (582 nm)

400 500 600 Wavelength (nm)

700

Figure 2. UV-vis absorption spectra of BN-fused naphthalimides BN0-BN3 together with their precursors N0-N3 and fluoride coordinated FBN0-FBN3 in THF solutions.


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and the ICT interactions. On the contrary, when the triphenylamino group is attached from the other extended direction of 4-position of naphthalimide, a significant bathochromic shift of 42 nm can be observed for BN2 (λmax = 462 nm). This is obviously due to the enhanced ICT interactions as compared to BN0. Similarly, when double triphenylamino groups are introduced from both directions, the ICT band of BN3 further bathochromically shifts to 470 nm. These results suggest that the extension direction of electron donating moiety has significant effect on the ICT interactions in BN-fused naphthalimides. Moreover, to evaluate the effect of BN unit on the ICT interactions in the D-A molecules, the absorption spectra of the precursors N0-N3 before BN-fusion were also measured and shown in Figure 2. It can be found that BN-fusion has opposite effect on the ICT interactions. When the electron donating moiety is functionalized in the extended direction of 3-position of naphthalimide, a significant hypsochromic shift of 45 nm can be observed for BN1 in comparison to its precursor N1. On the contrary, when triphenylamine is linked in the extended direction of 4-position of naphthalimide, the absorption maximum bathochromically shifts from 432 nm for N2 to 462 nm for BN2 by 30 nm upon incorporation of boron atom. However, when triphenylamino groups are introduced from both directions, a slight bathochromic shift of 7 nm can be found for BN3 as compared with N3, suggesting an additive effect on the ICT interactions in BN-fused naphthalimide. The corresponding PL spectra of BN-fused naphthalimides BN0-BN3 and their precursors N0N3 in THF solutions (ca. 5×10-6 M) were also recorded and shown in Figure 3. Unlike the absorption spectra, all the PL curves demonstrate only one resolved ICT emission band. As shown in Figure 3, BN0 displays the PL maximum at 458 nm with a small Stokes shift of 38 nm due to its coplanar characteristic. Upon the functionalization of triphenylamine in the extended


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N0 (515 nm) N1 (542 nm) N2 (561 nm) N3 (562 nm)

1

Normalized PL Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 BN0 (458 nm) BN1 (527 nm) BN2 (589 nm) BN3 (596 nm)

1

0 1

FBN0 (634 nm) FBN1 (638 nm) FBN2 (654 nm) FBN3 (639 nm)

0 500

600 Wavelength (nm)

700

Figure 3. PL spectra of BN-fused naphthalimides BN0-BN3 together with their precursors N0N3 and fluoride coordinated FBN0-FBN3 in THF solutions.

direction of 3-position of naphthalimide, the PL maximum bathochromically shifts to 527 nm with a remarkably enhanced Stokes shift of 110 nm for BN1 due to the more twisted molecular structure. Moreover, both BN0 and BN1 exhibit hypsochromically shifted PL maxima as compared with those for their precursors N0 and N1 similar to their absorption maxima. When the tripheneylamine is functionalized in the extended direction of 4-position of naphthalimide, the PL maximum bathochromically shifts to 589 and 596 nm for BN2 and BN3, respectively, due to the enhanced ICT interactions. Furthermore, the ICT properties of the BN-fused naphthalimides BN0-BN3 are quantitatively analyzed by Lippert-Mataga equation (Eq. 1):20 2(µ E − µG ) 2 ∆υ = υ a − υe = ∆f + constant hca 3


(1)

11


where ∆ν is the stokes shift, νa and νe represent the maximum absorption and the emission wavenumbers (cm−1), respectively, h is Planck’s constant, c is the speed of light, and a corresponds to the Onsager solvent cavity radius. The term ∆ƒ is the orientational polarizability, which is expressed as Eq. 2:20

∆f =

ε −1 n2 −1 − 2ε +1 2n2 +1

(2)

where ε is the dielectric constant and n is the refractive index of the solvent. The relationship of the Stokes shift (∆υ) of BN0-BN3 and the orientational polarizability (∆ƒ) of the solvents is plotted in Figure 4 and the corresponding photophysical data are summarized in Table 1. It can be found that all the luminophores display linear dependence of ∆υ on ∆ƒ together with large slope, indicating that the excited state is polarized and has a larger dipolar moment than the ground state. Similarly, a bathochromic shift can be observed for the PL maximum of BN2 and

BN3 in comparison to those for the precursors N2 and N3, respectively. This is consistent with the trend in their absorption spectra. Therefore, the extension of the electron donating moiety BN0 BN1 BN2 BN3

6000

∆υ (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000

2000

0.0

0.1

0.2

0.3

∆f Figure 4. Stokes shift of BN0-BN3 as a function of the orientational polarizability (∆ƒ) for a series of organic solvents.


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from the direction of 4-position of naphthalimide is super to 3-position in enhancing the ICT interactions in the D-A molecules based on BN-fused naphthalimides. It is well known that three-coordinated boron contains an empty orbital which can be occupied by Lewis base, such as fluoride ion.16 To investigate the influence of coordinating fluoride on the ICT interactions, the BN-fused naphthalimides BN0-BN3 were titrated with tetrabutylammonium fluoride. As shown in Figure 5, the colors of BN0-BN3 turned from yellow to purple for FBN0-

FBN3. The absorption and PL spectra of the resulting fluoride coordinated analogues FBN0FBN3 in diluted THF solutions (ca. 5×10-6 M) were recorded and shown in Figures 2 and 3. A significant bathochromic shift can be found for the absorption and PL maxima of FBN0-FBN3 in comparison to those for BN0-BN3 in both absorption (Figure 2) and PL spectra (Figure 3), similar to some reported push-pull systems in which the ICT shifts occur supramolecularly.21 In general, three-coordinated boron acts as an electron acceptor due to its empty orbital. Upon coordinating a fluoride ion, the empty orbital is occupied by two electrons and thus the anionic four-coordinate boronate is converted from an electron acceptor to an electron donor. Consequently, the coordination of fluoride on the azaborine unit strengthens the ICT interactions and bathochromically shifts their absorption and PL maxima. However, when Lewis acid, such as ethanol or deionized water, was dropwisely added into the solutions of four-coordinate boronate

Figure 5. Photographs of BN-fused naphthalimides BN0-BN3 together with their precursors N0N3 and fluoride coordinated FBN0-FBN3 in THF solutions.


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compounds FBN0-FBN3, the absorption and PL maxima hypsochromically shifted to the original ones before coordination with fluoride ions. This reversible ICT switching is due to the fact that the binding ability of fluoride ion to the boron in azaborine is weaker than that for Lewis acid. Therefore, the dissociation of fluoride ion from the four-coordinate boronate leads to the hypsochromic shifts and the reversible ICT interactions.

Electrochemical Properties. To further investigate the electronic properties of the BN-fused naphthalimides BN0-BN3 together with their precursors N0-N3 and fluoride coordinated FBN0-

FBN3, the electrochemical behavior was investigated by differential pulse voltammetry (DPV) measurements which were carried out using a typical three-electrode electrochemical cell with a glassy carbon electrode as the working electrode, a Pt electrode as the counter electrode, and an Ag/Ag+ electrode as the reference electrode in a solution of Bu4NPF6 (0.1 M) with a scan rate of 100 mV/s at room temperature under nitrogen. As shown in Figure 6, precursor N0 displays two redox peaks at 0.52 and 0.79 V (relative to Fc/Fc+, the same below), which can be assigned to the oxidation of the amino and thienyl groups substituted at 3- and 4-position of naphthalimide, respectively. Under the same condition, N1 displays a first oxidation peak at 0.18 V, which is attributed to the removal of electron from electron-donating triphenylamine moiety by comparing with reference N0. The negatively shifted oxidation potentials for amino and thienyl groups can be explained by contributing inductive effect22 caused by the introduction of electron-donating moiety from the extended direction of 3-position of naphthalimide. When triphenylamine is substituted in the extended direction of 4-position of naphthalimide, the DPV curve of N2 exhibits a first oxidation peak at 0.56 V with a positive shift of 0.38 V in comparison to that form

N1. This indicates that extension the conjugation from 3-position of naphthalimide is superior to 4-position in enhancing the D-A interactions for the precursors without boron. Compared with


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N0 N1 N2 N3 BN0

Current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BN1 BN2 BN3 FBN0 FBN1 FBN2 FBN3

0 0.4 0.8 + Potential (V vs. Fc/Fc ) Figure 6. Differential pulse voltammograms of BN-fused naphthalimides BN0-BN3 together with their precursors N0-N3 and fluoride coordinated FBN0-FBN3 in dichloromethane solutions.

the DPV curves of N1 and N2 with single triphenylamine substituent, precursor N3 shows two oxidation peaks at 0.19 and 0.48 V, assigning to the oxidation of the two triphenylamino groups substituted in the extended direction of 3- and 4-position of naphthalimide, respectively. Upon fusion of azaborine ring, the oxidation peaks of the substituted amino groups completely disappear for BN0-BN3, indicating the loss of electron-donating property for the amino group after the formation of azaborine ring. The first oxidation potentials of BN0-BN3 locate at 0.85, 0.18, 0.50, and 0.42 V, respectively. It should be noted that although BN1 displays negatively shifted first oxidation potential in comparison to BN2, the ICT interactions are more strengthened in BN2 when electron donor is substituted in the extended direction of 4-position of


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naphthalimide instead of 3-position. This is probably owing to the vertically twisted molecular skeleton in BN1, which dramatically weakens the ICT interactions. Another plausible explanation may be due to the small integral area of the first oxidation peak for BN1, which suggests that only partial charge can be transferred from the electron-donating triphenylamine moiety.23 Moreover, BN-fused naphthalimide BN3 displays two anodic redox steps at 0.42 V and 0.51 V, respectively, assigning to the sequential removal of electrons from the two triphenylamine moieties to form radical cation BN3+• and dication BN32+. However, the first oxidation peak corresponds to the removal of electron from the triphenylamino group substituted in the extended direction of 4-position of naphthalimide, which is on the contrary of the case for precursor N3. The assignment is further verified by theoretical calculations discussed below and indicates that under competition conditions, the charges on the electron donor substituted in the extended direction of 4-position of naphthalimide are more facile to transfer in the BN-fused naphthalimides, exactly opposite to the precursors. When the boron in the azaborine is coordinated by fluoride ion, the first oxidation potentials of

FBN0-FBN3 locate at 0.08, 0.11, 0.04, and 0.02 V. A significant negative shift of 0.77, 0.07, 0.46 and 0.40 V, respectively, can be found in comparison to those for BN0-BN3. Such negatively shifted first oxidation potential is attributed to the anionic four-coordinate boronate converted from an electron acceptor into a strong electron donor. Consequently, the coordination of fluoride on the azaborine unit strengthens the ICT interactions in the fluoride coordinated analogues. Moreover, negligible differences can be observed for the first oxidation potentials among FBN1-FBN3, suggesting that the electron-donating part is centered on the fourcoordinate boronate moiety and the substitution position of electron donor has little effect on the ICT properties of the fluoride coordinated analogues.


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Theoretical Approach. To have deep insight into the aromaticity of the resulting BN-fused naphthalimides BN0-BN3, nucleus-independent chemical shift (NICS)24 calculations were carried out. The geometric data were optimized on basis of the single crystal structure of BN2. As shown in Figure 7, the NICS(1) values for BN0-BN3 are calculated to be -5.34, -5.16, -5.44, and -5.15 ppm, respectively, which are lower than those for the C=C analogues and indicate moderate aromaticity of the azaborine ring.25 On the other hand, only negligible changes can be observed for the NICS(1) values of the benzene and thiophene rings neighboured to the azaborine ring as compared with those for the C=C analogues (Figure 7), suggesting that BN-fusion has little influence on the aromaticity of the surrounded aromatic rings. To investigate the electronic properties of the resulting BN compounds together with their precursors and fluoride coordinated analogues, time-dependent density functional theory (TDDFT) calculations were carried out with the Gaussian 03 program using B3LYP method and 631G* basis set. 26 The calculated energies, oscillator strength and compositions of major

N

N

H H B N

B N -5.34 -10.69

S

O

B N -5.16

-9.47 -11.18

N C8H17 -10.48 S

N

-9.47 -11.16

O

-5.44

O N C8H17

-9.28

N

-9.33

S

-11.18

O

BN0

B N -5.15

O N C8H17

-8.22

O -9.26

S

N C8H17

-11.12

O

BN1

O

BN2

BN3

N N H

H -10.54 -10.22

S

-10.19

O -10.04

-8.64 -11.46

N C8H17 O

CC0

-9.98

S

N

O

-8.57 -11.44

-11.42

-9.69

O N

-8.43

S

N C8H17 O

CC1

-8.95

N C8H17

-8.72

S

O -9.43

-11.40

O

CC2

N C8H17 O

CC3

Figure 7. Calculated NICS(1) values of the six-membered azaborine rings in BN-fused naphthalimides BN0-BN3 and the benzene rings in the related C=C analogues CC0-CC3.


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electronic transitions for the target compounds are listed in Tables S15-S26. It can be found that the S0-S1 transition for each target compound is mainly attributed to HOMO → LUMO transition with contribution over 98%, while S0-S2 transitions are composed of HOMO-1 → LUMO and HOMO → LUMO+1 transitions. To further understand the distinct ICT switching among the resulting BN-fused naphthalimides together with their precursors and fluoride coordinated analogues, the calculated frontier orbital distributions of the resulting compounds were plotted and shown in Figure 8. Remarkable HOMO distribution differences can be observed among the precursors, the azaborines, and the fluorides. As shown in Figure 8, the HOMO of N1 mainly delocalizes on the triphenylamine moiety with extension to the naphthaline unit. However, upon the incorporation of azaborine, BN1 demonstrates a partially delocalized HOMO on triphenylamine without any distribution on naphthaline. Moreover, after coordinating with fluoride ion, the HOMO of FBN1 is centered on the four-coordinate boronate instead of triphenylamine. On the contrary, N2 exhibits a delocalized HOMO on triphenylamine with negligible distribution on naphthaline, while the HOMO of BN2 mostly delocalizes on the triphenylamine group extending to the naphthalimide moiety. Moreover, the HOMO distribution differences among BN3 series are more complicated. N3 shows a delocalized HOMO distribution on the triphenylamine substituted in the extended direction of 3-position of naphthalimide with slight extension on naphthaline, which is similar to N1. However, the HOMO distribution of BN3 shifts to the other triphenylamine substituted in the extended direction of 4-position of naphthalimide. These results indicate that under competition conditions, the HOMO of the precursor tends to delocalize on the electron donor functionalized in the extended direction of 3-position of naphthalimide, while the HOMO of the BN-fused naphthalimide is likely to distribute on the donor part functionalized in the extended direction of 4-position of naphthalimide. Furthermore, similar to FBN1, both FBN2 and FBN3 display


18

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Figure 8. Calculated HOMO and LUMO distributions of the resulting BN compounds BN0-BN3 along with their precursors N0-N3 and fluoride coordinated analogues FBN0-FBN3.


19


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delocalized HOMOs centered on the four-coordinate boronate owing to its electron donating characteristic. On the other hand, the LUMOs of the related compounds were calculated and shown in Figure 8. The LUMOs delocalize mainly on the naphthalimide moiety with extension to the neighbored aromatic rings for all the compounds and no significant differences can be found among each series. To further compare the electron-withdrawing properties among these compounds, the Mulliken population analysis27 is utilized to compare the partial charges on the naphthalimide moieties. As shown in Table 2, the Mulliken charge on naphthalimide moiety increases from -0.019 for N0 to -0.009 for BN0, and then decreases to -0.255 for FBN0. A more

Table 2. Calculated partial charges of the naphthalimide moiety from Mulliken population analyses and photophysical properties for BN-fused naphthalimides BN0-BN3 together with their precursors N0-N3 and fluoride coordinated analogues FBN0-FBN3. Mulliken Charge

a λAbs max (nm)

a λPL max (nm)

N0

-0.019

431

515

BN0

-0.009

420

458

FBN0

-0.255

550

634

N1

-0.034

462

542

BN1

-0.017

417

527

FBN1

-0.254

556

638

N2

-0.033

432

561

BN2

-0.037

462

589

FBN2

-0.239

578

654

N3

-0.035

463

562

Compound

BN3 -0.111 470 596 FBN3 -0.492 582 639 Abs PL a Absorption ( λmax ) and PL ( λmax ) maxima were measured in THF solutions (~5 × 10-6 M).


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negative Mulliken population on the electron acceptor suggests stronger ICT interactions and hence bathochromically shifted absorption maximum, and vice versa. Although the chemical structures of the electron-deficient naphthalimide moieties in N0, BN0, and BN1 keep identical, the changed Mulliken charges on naphthalimide moieties indicate the varied ICT interactions, which explains the phenomenon that the absorption and PL maxima hypsochromically shift from 431 and 515 nm for N0 to 420 and 458 nm for BN0, and then bathochromically shift to 550 and 634 nm for FBN0, respectively. Similarly, the Mulliken charge on naphthalimide moiety increases from -0.034 for N1 to -0.017 for BN1, and then reduces to -0.254 for FBN1, while it decreases from -0.033 and -0.035 for N2 and N3, respectively, to -0.037 and -0.111 for BN2 and

BN3, and then further to -0.239 and -0.492 for FBN2. This perfectly predicts the trend of the absorption and PL maximum shifts among the BN-fused naphthalimides along with their precursors and fluoride coordinated analogues.

Conclusions. In summary, a family of BN-fused naphthalimides with electron-donating triphenylamine functionalized at different positions has been designed and successfully synthesized. The photophysical, electrochemical, and theoretical approaches reveal that the fusion of azaborine ring dramatically affects the ICT properties of the D-A system based on BN-fused naphthalimide. Extension of the conjugation from 3-position of naphthalimide is superior to 4-position in enhancing the D-A interactions for the precursors without boron. On the contrary, upon fusion of azaborine on naphthalimide, the ICT interactions are more strengthened when electron donor is substituted in the extended direction of 4-position of naphthalimide instead of 3-position. Most interestingly, after coordinating with fluoride ion, the ICT interactions of the D-A system based on BN-fused naphthalimide are dramatically enlarged and the substitution position of


21


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triphenylamino group has negligible effect on the ICT properties of the fluoride coordinated analogues. Our findings may provide a pathway for construction of novel BN-fused aromatics with electron-deficient moieties and further understanding of the intramolecular charge transfer in azaborine based D-A systems.

Experimental Section Materials and Reagents. All chemicals and reagents were purchased from commercial sources and used as received. Anhydrous tetrahydrofuran (THF) and toluene were distilled from sodium benzophenone ketyl. Dichloromethane (DCM) and chloroform were distilled from CaH2. All reactions and manipulations were carried out with the use of standard inert atmosphere and Schlenk

techniques.

N-octyl-4-bromo-1,8-naphthalimide,28

tetramethyl-1,3,2-ioxaborolanyl)thienyl]aniline,29

and

N,N-diphenyl-4-[5-(4,4,5,5-

2-pinacolborylthiophene30

were

synthesized according to the previously published methods.

Measurement and Characterizations. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured on a Varian Mercury Plus-400 spectrometer. The splitting patterns are designated as follows: s (singlet); d (doublet); t (triplet); m (multiplet). HRMS spectra were recorded with Bruker MicroTOF II spectrometer by using positive mode. UV-vis absorption spectra were recorded on Shimadzu UV-2550PC spectrophotometer. PL spectra were measuring using Shimadzu RF-5301PC spectrophotometer. Electrochemical redox potentials were obtained by DPV measurements, which were performed by on an electrochemical workstation (CHI660E) using a typical three-electrode electrochemical cell in a solution of tetrabutylammonium hexafluorophosphate (0.1 M) in anhydrous DCM at a scan rate of 100 mV/s at room temperature under nitrogen. A glassy carbon electrode was used as the working electrode, a Pt wire as the counter electrode, and an Ag/Ag+ electrode as the reference electrode. The potential of the


22

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reference electrode was calibrated with ferrocene. TD-DFT calculations were conducted with the Gaussian 03 program using the B3LYP method and 6-31G* basis set. The geometries were optimized on basis of the single crystal structure of BN2 using the default convergence criteria without any constraints.

Synthesis of N-octyl-4-bromo-3-nitro-1,8-naphthalimide (b). To a solution of N-octyl-4bromo-1,8-naphthalimide (a) (1.5 g, 4.52 mmol) in sulfuric acid (40 mL), sodium nitrate (0.45g, 5.3 mmol) was added and the solution stirred for 30 minutes at -10 oC and then for 3 hours at room temperature. The mixture was added slowly to ice-water (300 mL) and then the suspension was filtered, washed with water and dried to give brown product (1.41 g, 71%), the product was used in next step without further purification.

Synthesis of N-octyl-4-bromo-3-amino-1,8-naphthalimide (c). Under a nitrogen atmosphere, compound b (0.15 g, 0.40 mmol) was dissolved in a mixture of ethyl acetate (20 mL) and THF (10 mL), heated to 70 °C. Then SnCl2 (0.60 g, 3.20 mmol) was added and the reaction mixture was stirred for 6 hours at 70 °C. After cooling to room temperature, the solution was filtered and 150 mL saturated Na2CO3 solution was added. The resulting suspension was then extracted with ethyl acetate, washed with brine, and dried over anhydrous sodium sulfate. After removal of the organic solvent by rotary evaporator, the residue was purified by column chromatography (PE/DCM = 7/1, Rf = 0.7) to give a yellow solid (0.10 g, 72%, mp 188 − 189 °C). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.35 (d, J = 7.2 Hz, 2H), 8.09 (s, 1H), 7.72 (t, J = 8.0 Hz, 1H), 4.69 (s, 2H), 4.13 (t, J = 8.2 Hz, 2H), 1.71 – 1.69 (m, 2H), 1.37 – 1.33 (m, 10H), 0.86 (t, J = 6.8 Hz, 3H).

General synthetic procedure for compounds N0 and N2. Under a nitrogen atmosphere, in a Schlenk flask, compound (c) (1.0 equiv.), arylboronic acid (1.5 equiv.), NaHCO3 (2.0 equiv.), and Pd(PPh3)4 (5% equiv.) were mixed and dissolved in a dioxane/water solution (7:3). The


23


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mixture was stirred and heated to reflux for 8 h. After cooling, the solvent was evaporated and the residue was purified by column chromatography to give the product.

Compound N0. Eluent: PE/DCM = 8/1, Rf = 0.5, 0.41 g, yield 84%, mp 140 – 142 °C . 1H NMR (400 MHz, CDCl3) δ (ppm): 8.34 (d, J = 7.2 Hz, 1H), 8.14 (s, 1H), 7.81 (d, J = 8.0 Hz, 1H), 7.61 (d, J = 5.6 Hz, 1H), 7.55 (t, J = 8.0 Hz, 1H), 7.27 (m, 1H), 7.14 (d , J = 6.8 Hz, 1H), 4.15 (t, J = 6.8 Hz, 2H), 3.26 (s, 2H), 1.75 – 1.67 (m, 2H), 1.41 – 1.26 (m, 10H), 0.86 (t, J = 5.6 Hz, 3H). 13

C NMR (100 MHz, CDCl3) δ (ppm): 164.6, 164.2, 144.1, 133.1, 133.8, 130.5, 129.6, 128.3,

128.1, 127.6, 127.5, 123.8, 122.7, 122.6, 122.0, 117.8, 40.7, 32.0, 29.6, 29.5, 28.4, 27.4, 22.9, 14.3. HRMS (ESI-TOF) m/z: [M + H] + Calcd for C24H27N2O2S 407.1793; Found 407.1787.

Compound N2. Eluent: PE/DCM = 3/1, Rf = 0.6, 0.33 g, yield 82%, mp 188 – 190 °C. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.36 (d, J = 8.0 Hz, 1H), 8.19 (s, 1H), 7.98 (d, J = 8.2 Hz, 1H), 7.57 (t, J = 8.4 Hz, 1H), 7.53 – 7.48 (m, 2H), 7.39 (t, J = 7.2 Hz, 1H), 7.30 – 7.26 (m, 7H), 7.15 7.04 (m, 9H), 4.17 (t, J = 6.8 Hz, 2H), 2.74 (s, 2H), 1.75 – 1.68 (m, 2H), 1.41 – 1.27 (m, 10H), 0.87 (t, J = 6.8 Hz, 3H).

13

C NMR (100 MHz, CD2Cl2) δ (ppm): 164.4, 164.0, 148.0, 147.6,

146.8, 144.7, 133.8, 133.6, 130.7, 130.3, 129.6, 127.8, 127.5, 126.9, 126.8, 124.9, 123.9, 123.6, 123.5, 123.1, 122.8, 122.5, 121.8, 117.1, 40.6, 32.1, 29.9, 29.6, 28.3, 27.4, 22.9, 14.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C42H40N3O2S 650.2841; Found 650.2835.

General synthetic procedure for compounds N1 and N3. Under a nitrogen atmosphere, a mixture of N0 or N2 (1.0 equiv.), 4-bromotriphenylamine (1.25 equiv.), Pd2(dba)3 (0.5 equiv.), DPPF (1.25 equiv.), t-BuONa (1.5 equiv.) and toluene was stirred at 95 °C for 15 h. To this reaction mixture was added aqueous NH4Cl and the aqueous layer was extracted with DCM. The organic layer was dried with Na2SO4 and these solvent were evaporated. The crude product was separated by column chromatography to give a brown solid.


24

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Compound N1. Eluent: PE/DCM = 4/1, Rf = 0.4, 0.37 g, yield 76%, mp 79 – 81 °C. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 8.54 (s, 1H), 8.32 (d, J = 7.2 Hz, 1H), 7.82 (d, J = 8.4 Hz, 1H), 7.68 (d, J = 4.8 Hz, 1H), 7.58 (t, J = 7.8 Hz, 1H), 7.35 – 7.23 (m, 6H), 7.03 – 7.11 (m, 10H), 6.05 (s, 1H), 4.13 (t, J = 8.0 Hz, 2H), 1.75 – 1.68 (m, 2H), 1.44 – 1.29 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H).

13

C NMR (100 MHz, CD2Cl2) δ (ppm): 164.3, 163.9, 148.2, 144.1, 136.5, 134.8, 133.9,

130.6, 130.2, 129.4, 128.5, 128.4, 127.6, 127.5, 125.8, 124.7, 124.3, 124.1, 124.0, 123.9, 123.1, 122.8, 120.6, 120.1, 40.6, 32.1, 29.9, 29.5, 28.3, 27.4, 22.9, 14.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C42H40N3O2S 650.2841; Found 650.2835.

Compound N3. Eluent: PE/DCM = 3/1, Rf = 0.5, 0.44 g, yield 71%, mp 86 – 88 °C. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 8.55 (s, 1H), 8.33 (d, J = 6.8 Hz, 1H), 7.98 (d, J = 8.4 Hz, 1H), 7.68 – 7.50 (m, 3H), 7.43 (d, J = 3.6 Hz, 1H), 7.32 – 7.26 (m, 8H), 7.17 – 7.03 (m, 19H), 6.23 (s, 1H), 4.13 (t, J = 7.4 Hz, 2H), 1.75 – 1.68 (m, 2H), 1.42 – 1.30 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H). 13

C NMR (100 MHz, CD2Cl2) δ (ppm): 164.3, 164.0, 148.0, 147.5, 147.2, 144.1, 142.8, 136.6,

133.9, 133.1, 131.4, 130.7, 129.6, 129.4, 127.7, 127.6, 126.8, 126.7, 125.9, 124.9, 124.0, 123.9, 123.8, 123.6, 123.5, 123.3, 123.2, 123.1, 122.8, 122.7, 120.6, 120.2, 40.6, 32.1, 30.0, 29.5, 28.3, 27.4, 22.9, 14.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C60H53N4O2S 893.3889; Found 893.3883.

General synthetic procedure for compounds BN0-BN3. To a solution of precursor N0-N3 (1.0 equiv.) in o-dichlorobenzene (20 mL) was added triethylamine (3.0 equiv.) and phenyldichloroborane (1.5 equiv.). The mixture was stirred at 180 °C under argon for 8 h. After cooling to room temperature, the solvent was removed through reduced pressure distillation. Then the residue was purified by flash chromatography on silica gel.

Compound BN0. Eluent: PE/DCM = 4/1, Rf = 0.6, 0.22 g, yield 89%, mp 190 – 192 °C . 1H NMR (400 MHz, CDCl3) δ (ppm): 9.37 (d, J = 8.4 Hz, 1H), 8.58 (s, 1H), 8.53 (d, J = 6.8 Hz, 1H),


25


Page 26 of 34

8.39 (s, 1H), 7.95 – 7.86 (m, 4H), 7.74 (d, J = 5.2 Hz, 1H), 7.54 (d, J = 3.6 Hz, 3H), 4.11 (t, J = 7.6 Hz, 2H), 1.75 – 1.67 (m, 2H), 1.42 – 1.25 (m, 10H), 0.86 (t, J = 6.4 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ (ppm): 164.6, 163.8, 147.8, 142.6, 137.2, 133.5, 131.6, 130.9, 129.9, 128.9, 128.7, 128.6, 128.0, 127.6, 126.0, 124.8, 123.3, 121.5, 120.5, 109.9, 40.9, 32.1, 29.6, 29.5, 28.3, 27.4, 22.9, 14.3. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C30H30BN2O2S 493.2121; Found 493.2150.

Compound BN1. Eluent: PE/DCM = 2/1, Rf = 0.4, 0.18 g, yield 72%, mp 233 – 235 °C. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 9.70 (d, J = 8.4 Hz, 1H), 8.66 (d, J = 7.4 Hz, 1H), 8.64 (s, 1H), 8.05 (t, J = 8.0 Hz, 1H), 7.81 (d, J = 5.2 Hz, 1H), 7.66 (d, J = 5.2 Hz, 1H), 7.41 (d, J = 7.4 Hz, 2H), 7.31 – 7.36 (m, 7H), 7.18 – 7.03 (m, 10H), 4.15 (t, J = 7.2 Hz, 2H), 1.76 – 1.69 (m, 2H), 1.43 – 1.28 (m, 10H), 0.89 (t, J = 6.6 Hz, 3H).

13

C NMR (100 MHz, CD2Cl2) δ (ppm): 164.3,

163.7, 147.8, 147.6, 147.2, 140.2, 138.0, 133.6, 133.4, 132.9, 132.8, 131.6, 130.5, 129.6, 129.4, 129.2, 128.7, 128.0, 127.7, 127.6, 127.4, 124.8, 124.6, 124.5, 123.5, 123.3, 121.7, 121.2, 40.7, 32.0, 29.6, 29.4, 28.3, 27.4, 22.9, 14.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C48H43BN3O2S 736.3169; Found 736.3204.

Compound BN2. Eluent: PE/DCM = 1/1, Rf = 0.4, 0.21 g, yield 77%, mp 112 – 114 °C. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 9.29 (d, J = 8.6 Hz, 1H), 8.47 (d, J = 7.4 Hz, 2H), 8.35 (s, 1H), 7.93 – 7.91 (m, 3H), 7.87 – 7.83 (m, 1H), 7.64 (d, J = 8.6 Hz, 2H), 7.57 – 7.55 (m, 3H), 7.33 (t, J = 7.6 Hz, 4H), 7.18 – 7.16 (m, 4H), 7.13 – 7.09 (m, 4H), 4.12 (t, J = 6.8 Hz, 2H), 1.75 – 1.67 (m, 2H), 1.44 – 1.27 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H).

13

C NMR (100 MHz, CD2Cl2) δ

(ppm): 164.1, 163.5, 148.4, 147.5, 146.2, 146.1, 133.9, 133.5, 130.8, 129.8, 129.6, 128.6, 128.5, 128.2, 127.7, 127.5, 127.1, 125.8, 125.5, 125.3, 125.1, 124.9, 123.7, 123.3, 123.1, 121.1, 120.2, 109.9, 40.7, 32.1, 30.0, 29.5, 28.3, 27.4, 22.9, 14.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C48H43BN3O2S 736.3169; Found 736.3193.


26

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Compound BN3. Eluent: PE/DCM = 2/1, Rf = 0.3, 0.19 g, yield 74%, mp 232 – 234 °C. 1H NMR (400 MHz, CD2Cl2) δ (ppm): 9.57 (d, J = 8.4 Hz, 1H), 8.56 (t, J = 8.0 Hz 2H), 7.95 (t, J = 8.4 Hz 2H), 7.63 (d, J = 8.2 Hz, 2H), 7.41 – 7.30 (m, 14H), 7.13 – 7.04 (m, 18H), 4.10 (t, J = 8.2 Hz 2H), 1.74 – 1.67 (m, 2H), 1.44 – 1.30 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H). 13C NMR (100 MHz, CD2Cl2) δ (ppm): 164.2, 163.6, 148.5, 148.4, 147.8, 147.7, 147.5, 146.4, 146.3, 146.1, 145.6, 145.4, 139.9, 139.5, 139.4, 137.9, 133.6, 131.3, 130.5, 129.6, 128.9, 128.9, 127.8, 127.7, 127.6, 127.43, 127.40, 127.14, 127.12, 127.10, 127.06, 125.1, 124.9, 124.6, 123.7, 123.4, 123.3, 123.1, 121.7, 120.7, 40.7, 32.1, 29.6, 29.5, 28.3, 27.4, 22.8, 14.1. HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C66H56BN4O2S 979.4217; Found 979.4221.

SUPPORTING INFORMATION UV-vis absorption and PL spectra of BN0-BN3 in different solvents, 1H and 13C NMR spectra of the target compounds, crystal data for BN2, and computational data for the target compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *Corresponding author: E-mail: [email protected], Tel./Fax: +86-21-5163-0350.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51722301, 21674023, and 21733003).

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Relationships

in

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Donor-Acceptor

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Charge Transfer Switching in Donor-Acceptor Systems Based on BN

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