Charge Transfer Switching in Donor-Acceptor Systems Based on BN

Article Cite This: J. Org. Chem. 2018, 83, 5577−5587

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Charge Transfer Switching in Donor−Acceptor Systems Based on BNFused 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 S Supporting Information *

ABSTRACT: Six-membered azaborine rings have been straightforwardly fused on naphthalimide-based donor−acceptor systems, and a series of BN-containing heteroaromatic compounds BN1−BN3 were constructed. Electron-donating triphenylamines were functionalized in the extended direction of the 3- or/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 the azaborine ring has a great effect on the ICT properties of the D−A systems based on BN-fused naphthalimides. For the precursors without boron, the extension of an electron donor from the 3-position of naphthalimide is superior in enhancing the D−A interactions. On the contrary, upon fusion of the azaborine ring on naphthalimide, the dominant orientation of the ICT interactions conversely converts to the extended direction of the 4-position of naphthalimide in the D−A molecules based on BN-fused naphthalimides. Most interestingly, upon coordinating the boron by a fluoride ion, the ICT interactions are dramatically enlarged and the substitution position of the triphenylamino group has a 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 a 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 © 2018 American Chemical Society

can also be considered as zwitterionic double bonds in the neutral state.8 Hence, most reported BN-heteroacene systems display a similar aromaticity in comparison to those all-carbon analogues, whereas they exhibit different electronic and optical properties, making them promising candidates for various optoelectronic applications. Up until now, the azaborine chemistry has been well established by several groups.9 However, a 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 the BN unit are rarely documented.11 Therefore, investigation of the Received: March 5, 2018 Published: April 26, 2018 5577

DOI: 10.1021/acs.joc.8b00597 J. Org. Chem. 2018, 83, 5577−5587

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The Journal of Organic Chemistry

Chart 1. Chemical Structures of BN-Fused Naphthalimides BN0−BN3 Together with Their Precursors N0−N3 and FluorideCoordinated Analogues FBN0−FBN3

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The straightforward synthetic approach to BN-fused naphthalimides BN0−BN3 is depicted in Scheme 1. It started from commercially available N-octyl-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 3position of naphthalimide. Then the key intermediate N-octyl3-amino-4-bromo-1,8-naphthalimide (c) was obtained by reducing the 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 were generally used as a typical solvent mixture for the Suzuki coupling reaction. Unfortunately, attempting to achieve these precursors in either the toluene/ H2O or THF/H2O solvent mixture was not applicable. An alternative approach was conducted by using the 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 a catalyst in toluene. Finally, the target BN-fused naphthalimides BN0− BN3 were prepared by reacting precursors N0−N3 with an 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

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, 1,8-naphthalimide was selected as an electron-deficient core where a six-membered azaborine ring was straightforwardly fused on. Azaborines BN1−BN3 (Chart 1) with triphenylamine functionalized in the extended direction of the 3- or/and 4-position of naphthalimide were designed and synthesized. 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 a Lewis base, such as a fluoride ion.16 Upon binding with the fluoride ion, the electron-withdrawing boron shifted to an electron-donating unit, which significantly affected the electron distribution and the ICT interactions in the fluoridecoordinated azaborines 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. 5578


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The Journal of Organic Chemistry Scheme 1. Synthetic Route for BN-Fused Naphthalimides BN0−BN3a

a 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.

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

mass spectrometry (HRMS) and were found to be consistent with the proposed structures. The resulting BN compounds exhibited an 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 the 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 a 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)°. (CCDC no. 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 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.

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 of the compounds 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 is located at 420 nm. However, upon introduction of the triphenylamino group from the extended direction of the 3position of naphthalimide, the ICT band slightly hypsochromically shifts to 417 nm by 3 nm for BN1. A 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 and the ICT interactions. On the contrary, when the triphenylamino group is attached from the other extended direction of the 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 5579


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Moreover, to evaluate the effect of the 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 an opposite effect on the ICT interactions. When the electrondonating moiety is functionalized in the extended direction of the 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 the 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 BNfused naphthalimide. The corresponding PL spectra of BN-fused naphthalimides BN0−BN3 and their precursors N0−N3 in THF solutions (ca. 5 × 10−6 M) were also recorded and shown in Figure 3. Unlike

Table 1. Photophysical Properties of BN-Fused Naphthalimides BN0−BN3 in Different Solvents (ca. 5 × 10−6 M) compound

solvent

Δfa

λabs (nm)

λPL (nm)

Stokes shift (cm−1)

BN0

hexane toluene EA THF DCM acetone DMF hexane toluene EA THF DCM acetone DMF hexane toluene EA THF DCM acetone DMF hexane toluene EA THF DCM acetone DMF

−0.006 0.014 0.20 0.21 0.22 0.27 0.28 −0.006 0.014 0.20 0.21 0.22 0.27 0.26 −0.006 0.014 0.20 0.21 0.22 0.27 0.28 −0.006 0.014 0.20 0.21 0.22 0.27 0.28

416 421 417 420 421 421 417 417 420 416 417 419 416 419 460 464 461 462 463 462 467 461 474 468 470 477 472 474

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

BN1

BN2

BN3

a

Orientational polarizability (Δf) of the solvent.20

Figure 3. PL spectra of BN-fused naphthalimides BN0−BN3 together with their precursors N0−N3 and fluoride-coordinated FBN0−FBN3 in THF solutions.

the absorption spectra, all of 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 direction of the 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 the 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 the Lippert− Mataga equation (eq 1):20

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

directions, the ICT band of BN3 further bathochromically shifts to 470 nm. These results suggest that the extension direction of the electron-donating moiety has a significant effect on the ICT interactions in BN-fused naphthalimides. 5580


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The Journal of Organic Chemistry Δν = νa − νe

2(μE − μG )2 ℏca3

Δf + constant

analogues FBN0−FBN3 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, the 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 fourcoordinate 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 a Lewis acid, such as ethanol or deionized water, was dropwisely added into the solutions of fourcoordinate boronate 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 the fluoride ion to the boron in azaborine is weaker than that for the Lewis acid. Therefore, the dissociation of the 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 fluoridecoordinated 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 the 3- and 4positions of naphthalimide, respectively. Under the same

(1)

where Δν is the Stokes shift, νa and νe represent the maximum absorption and the emission wavenumbers (cm−1), respectively, ℏ is Planck’s constant, c is the speed of light, and a corresponds to the Onsager solvent cavity radius. The term Δf is the orientational polarizability, which is expressed as eq 2:20 Δf =

ε−1 n2 − 1 − 2 2ε + 1 2n + 1

(2)

where ε is the dielectric constant and n is the refractive index of the solvent. The relationship of the Stokes shift (Δν) of BN0− BN3 and the orientational polarizability (Δf) of the solvents is plotted in Figure 4, and the corresponding photophysical data

Figure 4. Stokes shift of BN0−BN3 as a function of the orientational polarizability (Δf) for a series of organic solvents.

are summarized in Table 1. It can be found that all of the luminophores display linear dependence of Δν on Δf together with a 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 from the direction of the 4position of naphthalimide is superior to the 3-position in enhancing the ICT interactions in the D−A molecules based on BN-fused naphthalimides. It is well-known that the three-coordinated boron contains an empty orbital, which can be occupied by a Lewis base, such as a fluoride ion.16 To investigate the influence of the 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

Figure 5. Photographs of BN-fused naphthalimides BN0−BN3 together with their precursors N0−N3 and fluoride-coordinated FBN0−FBN3 in THF solutions.

Figure 6. Differential pulse voltammograms of BN-fused naphthalimides BN0−BN3 together with their precursors N0−N3 and fluoridecoordinated FBN0−FBN3 in dichloromethane solutions. 5581


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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.

condition, N1 displays a first oxidation peak at 0.18 V, which is attributed to the removal of an electron from the electrondonating triphenylamine moiety by comparing with reference N0. The negatively shifted oxidation potentials for amino and thienyl groups can be explained by contributing an inductive effect22 caused by the introduction of an electron-donating moiety from the extended direction of the 3-position of naphthalimide. When triphenylamine is substituted in the extended direction of the 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 for N1. This indicates that the extension of the conjugation from the 3position of naphthalimide is superior to the 4-position in enhancing the D−A interactions for the precursors without boron. Compared with the DPV curves of N1 and N2 with a single triphenylamine substituent, precursor N3 shows two oxidation peaks at 0.19 and 0.48 V, which were assigned to the oxidation of the two triphenylamino groups substituted in the extended direction of the 3- and 4-positions of naphthalimide, respectively. Upon fusion of the azaborine ring, the oxidation peaks of the substituted amino groups completely disappear for BN0−BN3, indicating the loss of the electron-donating property for the amino group after the formation of an azaborine ring. The first oxidation potentials of BN0−BN3 were located at 0.85, 0.18, 0.50, and 0.42 V, respectively. It should be noted that, although BN1 displays a negatively shifted first oxidation potential in comparison to BN2, the ICT interactions are more strengthened in BN2 when an electron donor is substituted in the extended direction of the 4-position of naphthalimide instead of the 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 a 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 and 0.51 V, respectively,

which are assigned to the sequential removal of electrons from the two triphenylamine moieties to form the radical cation BN3+• and dication BN32+. However, the first oxidation peak corresponds to the removal of an electron from the triphenylamino group substituted in the extended direction of the 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 the 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 a fluoride ion, the first oxidation potentials of FBN0−FBN3 were located 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 a negatively shifted first oxidation potential is attributed to the anionic fourcoordinate 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 four-coordinate boronate moiety and that the substitution position of the electron donor has a little effect on the ICT properties of the fluoride-coordinated analogues. Theoretical Approach. To have a 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 the basis of the single-crystal structure of BN2. As shown in Figure 7, the NICS(1) values for BN0−BN3 were 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) 5582


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naphthaline, while the HOMO of BN2 mostly delocalizes on the triphenylamine group extending to the naphthalimide moiety. Moreover, the HOMO distribution differences among the BN3 series are more complicated. N3 shows a delocalized HOMO distribution on the triphenylamine substituted in the extended direction of the 3-position of naphthalimide with a 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 the 4position 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 the 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 the 4position of naphthalimide. Furthermore, similar to FBN1, both FBN2 and FBN3 display 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 an extension to the neighbored aromatic rings for all of 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

values of the benzene and thiophene rings neighbored to the azaborine ring as compared with those for the CC analogues (Figure 7), suggesting that BN fusion has a 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 fluoridecoordinated analogues, time-dependent density functional theory (TD-DFT) calculations were carried out with the Gaussian 03 program using the B3LYP method and 6-31G* basis set.26 The calculated energies, oscillator strength, and compositions of major 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 a 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 BNfused 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

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 compound

Mulliken charge

a λabs max (nm)

a λPL max (nm)

N0 BN0 FBN0 N1 BN1 FBN1 N2 BN2 FBN2 N3 BN3 FBN3

−0.019 −0.009 −0.255 −0.034 −0.017 −0.254 −0.033 −0.037 −0.239 −0.035 −0.111 −0.492

431 420 550 462 417 556 432 462 578 463 470 582

515 458 634 542 527 638 561 589 654 562 596 639

a

PL Absorption (λabs max) and PL (λmax) maxima were measured in THF solutions (∼5 × 10−6 M).

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.

charge on the naphthalimide moiety increases from −0.019 for N0 to −0.009 for BN0 and then decreases to −0.255 for FBN0. A more negative Mulliken population on the electron acceptor suggests stronger ICT interactions and hence a bathochromically shifted absorption maximum and vice versa. Although the chemical structures of the electron-deficient naphthalimide moieties in N0, BN0, and BN1 stay identical, the changed Mulliken charges on the 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 the naphthali-

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 the 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 a negligible distribution on 5583


Article

The Journal of Organic Chemistry mide 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 fluoridecoordinated analogues.

Synthesis of N-Octyl-4-bromo-3-nitro-1,8-naphthalimide (b). To a solution of N-octyl-4-bromo-1,8-naphthalimide (a) (1.5 g, 4.52 mmol) in sulfuric acid (40 mL) was added sodium nitrate (0.45 g, 5.3 mmol), and the solution was stirred for 30 min at −10 °C and then for 3 h 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 a brown product (1.41 g, 71%). The product was used in the 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), and the mixture was heated to 70 °C. Then SnCl2 (0.60 g, 3.20 mmol) was added, and the reaction mixture was stirred for 6 h at 70 °C. After cooling to room temperature, the solution was filtered, and 150 mL of a 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 a 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 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. 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). 13C 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. 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). 13C 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), 4bromotriphenylamine (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 the solvents were evaporated. The crude product was separated by column chromatography to give a brown solid. Compound N1. 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). 13C 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.

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CONCLUSIONS In summary, a family of BN-fused naphthalimides with an electron-donating triphenylamine functionalized at different positions has been designed and successfully synthesized. The photophysical, electrochemical, and theoretical approaches reveal that the fusion of an azaborine ring dramatically affects the ICT properties of the D−A system based on BN-fused naphthalimide. The extension of the conjugation from the 3position of naphthalimide is superior to the 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 an electron donor is substituted in the extended direction of the 4-position of naphthalimide instead of the 3-position. Most interestingly, after coordinating with a fluoride ion, the ICT interactions of the D−A system based on BN-fused naphthalimide are dramatically enlarged and the substitution position of the triphenylamino group has a negligible effect on the ICT properties of the fluoride-coordinated analogues. Our findings may provide a pathway for the construction of novel BN-fused aromatics with electron-deficient moieties and a further understanding of the intramolecular charge transfer in azaborine-based D−A systems.

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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 a standard inert atmosphere and Schlenk techniques. N-Octyl-4-bromo-1,8-naphthalimide,28 N,N-diphenyl-4-[5-(4,4,5,5-tetramethyl-1,3,2-ioxaborolanyl)thienyl]aniline,29 and 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 a Bruker MicroTOF II spectrometer by using the positive mode. UV−Vis absorption spectra were recorded on a Shimadzu UV-2550PC spectrophotometer. PL spectra were measured using a 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 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 the basis of the single-crystal structure of BN2 using the default convergence criteria without any constraints. 5584



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Compound N3. 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). 13C 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) were 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. 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), 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. 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). 13C 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. 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). 13C 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. Compound BN3. 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.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b00597. 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 (PDF) Crystallographic data for BN2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-21-5163-0350. ORCID

Gang Zhou: 0000-0002-1533-7795 Notes

The authors declare no competing financial interest.

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

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REFERENCES

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Charge Transfer Switching in Donor-Acceptor Systems Based on BN

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