BN-Functionalized Benzotrithiophene-Based Azaborines: Synthesis

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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BN-Functionalized Benzotrithiophene-Based Azaborines: Synthesis, Structures, and Anion Binding Properties Chao-Jing Sun,† Nan Wang,† Tai Peng,† Xiaodong Yin,† Suning Wang,†,‡ and Pangkuan Chen*,† †

Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology of China, Beijing 102488, People’s Republic of China ‡ Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada

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S Supporting Information *

strategy for the incorporation of multiple BN substitution in 4a− 4c. We chose an electron-donating benzotrithiophene (BTT) as the core of heteroatom-doped PAHs in view of the possibility of iterative functionalization at the three thiophene rings. Systematic investigations of the effect of BN substitution on the assembled superstructure, electronic behavior, and anion response will be demonstrated as a function of the number of BN units. We started the synthesis of BTT with reactions previously reported in the literature.26 Several BTT derivatives 2a−2c were subsequently prepared by selective electrophilic substitution at the α positions of the thiophene using n-BuLi/I2 at low T (Scheme 1). Next, the Pd-catalyzed Suzuki coupling reactions

ABSTRACT: Facile synthesis of BN-functionalized azaborines (4a−4c) was accomplished via Suzuki coupling reactions followed by the electrophilic C−H borylation of benzotrithiophene (BTT). The core structure of BTT enables multiple modifications at the three thiophene rings. Molecular structures of 4a−4c were confirmed by NMR, high-resolution mass spectrometry, and X-ray crystallographic analysis. Their electronic properties were also examined through photophysical and electrochemical measurements as well as density functional theory computations. The red-shifted absorption and emission of these molecules were demonstrated upon fluoride titration in response to anion binding, leading to a remarkable decrease of the oxidation potential in the electrochemical differential pulse voltammetry scans. This work may provide a new pathway to robust redox-active materials for catalytic applications.

Scheme 1. Synthesis of BTT-Based Building Blocksa

H

eteroatom-doped polyaromatic hydrocarbons (PAHs) have emerged and currently captured much attention in many research fields such as (opto)electronic materials, batteries, capacitors, sensors, and catalysis.1−4 Of particular interest is the incorporation of B, N, S, and P atoms into all-Cbased PAHs, which usually show new structures and easy modulation of the optical and (photo)physical properties. As main-group-doped motifs, the synthesis of organoboron compounds has been pursued for decades because B analogues may significantly enhance or intrinsically modify the electronic properties in comparison to all-C analogues because of the electron deficiency of B moieties.5−15 Replacement of the CC bond with an isoelectronic B−N bond in a synthetic aromatic skeleton was recently established to achieve advanced functional materials.16 The nature of strong polarity in isostructual BN systems can sufficiently change the frontier orbitals and chemical reactivity. Some examples feature BN-substituted π-extended PAHs, such as planar acenes,17 pyrene,18 coronens,19 indene,20 ullazine,21 nonplanar helicenes,22 and corannulene.23 Among the aromatic heterocycles, thiophene derivatives with S-donor groups represent a class of remarkable building blocks for the construction of π-conjugated systems with tunable or enhanced photoluminescence. Considerable efforts have been performed to modify the conjugated thiophene-based moieties in research groups of Pei, Feng, etc.24,25 In this work, we herein describe a versatile design © XXXX American Chemical Society

a Reagents and conditions: (i) n-BuLi (2.0 equiv), C6H13I (2.0 equiv), THF, 0−80 °C, 12 h, under N2. (ii) n-BuLi (1.2 equiv), I2 (2.0 equiv), THF, −78 °C (1.5 h) to rt, 12 h. (iii) n-BuLi (2.5 equiv), I2 (4.0 equiv), THF, −78 °C (1.5 h) to rt, 12 h. (iv) n-BuLi (3.5 equiv), I2 (4.0 equiv), THF, −78 °C (1.5 h) to rt, 12 h.

were performed under standard conditions, followed by electrophilic borylation with PhBCl2 in refluxed 1,2-dichlorobenzene solutions for 12 h, leading to the isolated products 4a−4c in high yields (56−83%; Scheme 2). These compounds were fully characterized by 1H, 13C, and 11B NMR spectroscopy. As evidenced by high-resolution mass spectrometry (MS) measurements, the molecular weights are consistent with the theoretical values [see the Supporting Information (SI)]. The solid-state structures of 4a−4c were determined by X-ray crystallographic analysis. Single crystals were grown via the slow Received: December 27, 2018

A

DOI: 10.1021/acs.inorgchem.8b03579 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Synthetic Approach to Compounds 4a−4ca

nucleus-independent chemical shift (NICS) was calculated to be −4.69 ppm for 4a and approximately −3.40 ppm for 4b and 4c at the BN ring centers (Figure S10). These negative values are much lower than NICS(0) = −9.8 ppm for Ph, indicating a decreased aromaticity of the BNC4 cycles in 4a−4c. A close inspection revealed that the assembly of compound 4a leads to a layered superstructure via CH−π interactions (3.61 Å). The neighboring molecules within a layer are arranged in an antiparallel mode (Figure 1b). In contrast, expansion of the superstructures of 4b and 4c is dominated by face-to-face stacking interactions with a π−π distance of ∼3.4−3.8 Å. Adjacent molecules in the π stacks are arranged in the opposite direction (Figure 1e,g). To investigate the electronic structures, we examined their photophysical and electrochemical properties, and the results are summarized in Table 1. The UV−vis absorption spectra in Table 1. Summary of the Results from Photophysical, Electrochemical, and Computational Data

a

Reagents and conditions: (i−iii) BR, Pd(PPh3)4, K3PO4, DMF, 80 °C, 12 h, under N 2. (iv−vi) PhBCl 2, Et 3N, anhydrous odichlorobenzene, rt (15 min) to 180 °C, 12 h.

evaporation of solution [1:1 (v/v) CHCl3/MeOH] at ambient temperature. The molecular structures of these compounds are shown in Figure 1. As detailed in Figure 1a,d,f, the B−N bond

4a 4b 4c

λabsa (nm)

λem (nm)

EHOMOb (eV)

ELUMOb (eV)

Egapc (eV)

ETD‑DFTd (eV/nm)

Eopte (eV)

353 353 353

407 412 413

−5.19 −5.10 −5.09

−1.29 −1.28 −1.18

3.90 3.82 3.91

3.43/361 3.31/375 3.46/358

3.22 3.18 3.10

Recorded in THF (c = 1.0 × 10−5 M). Only the longest λmax was given. bObtained by DFT calculations (B3LYP and 6-31G*). c HOMO−LUMO energy gap: Egap = ELUMO − EHOMO. dVertical excitation of the lowest transition (S0 → S1) calculated by TD-DFT (B3LYP and 6-31G*). eCalculated from the experimental absorption onset. a

tetrahydrofuran (THF) exhibit vibronic fine structures with an absorption maximum at 353 nm for 4a−4c (Figure 2a). The

Figure 2. (a) UV−vis spectra recorded in THF (c = 1.0 × 10−5 M). (b) CV (vs Fc/Fc+) recorded in CH2Cl2 for 4a−4c with [NBu4]PF6 as the electrolyte (c = 0.1 M). ν = 100 mV/s.

longest wavelength absorption bands (S0 → S1) around 370 nm are ascribed to the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) transition, which is strongly supported by time-dependent density functional theory (TD-DFT) calculations (B3LYP and 6-31G*) at the optimized S0 state (see the SI). As shown in Figure 3, the excitation energies required for HOMO−LUMO transitions (361 nm, 3.43 eV, and f = 0.1622 for 4a; 375 nm, 3.31 eV, and f = 0.0301 for 4b; 358 nm, 3.46 eV, and f = 0.0124 for 4c) are well consistent with the optical energy gaps (3.10−3.22 eV; Table 1). The emission bands (λex = 353 nm; Figure S4) are observed for 4a−4c at 407−413 nm with poor quantum efficiency and short lifetimes in common organic solvents. Cyclic voltammetry (CV) scans show irreversible oxidation waves in CH2Cl2 (Figure 2b), and no reduction waves were acquired. The HOMO energy levels (−5.19, −5.10, and −5.09

Figure 1. X-ray crystallographic analysis: (a, d, and f) 4a−4c with selected B−N and B−C(th) bond lengths (50% thermal ellipsoids). (b) View of the coupled molecules of 4a in an antiparallel array. (c, e, and g) Molecular packing motifs of 4a−4c with major intermolecular interactions. H atoms are omitted for clarity.

lengths [1.436(5) Å for 4a, 1.432(4) and 1.438(4) Å for 4b, and 1.438(4) and 1.440(4) Å for 4c] are comparable to those in typical B−N aromatics (1.45−1.47 Å)27 but much greater than those of BN bonds (1.37−1.40 Å).23 Meanwhile, the B− C(th) bond lengths are observed in the range of 1.538(5)− 1.556(4) Å for 4a−4c, likely indicating single-bond character. The large difference in bond length (>0.1 Å) between B−N and B−C is indicative of a less aromatic character of the BN-doped heterocycles (BNC4) compared with the typical Ph ring. The B


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Inorganic Chemistry

in conjugated triarylboranes, which typically show blue-shifted absorption as a result of anion-binding-induced conjugation interruption. The red shift of the absorption band was previously reported on several examples of azaborines. 25c,28 This absorption change could be attributed to the reduced HOMO−LUMO band gaps of DFT-optimized fluoride adducts, leading to a decrease (∼0.4 eV) of those gaps upon F− complexation (Figure S6). We also encountered a similar observation in their emission spectra. As shown in Figure 4b,d,f, new emission bands appear to increase at 475 nm for 4a and 4b and 465 nm for 4c as the original bands gradually attenuate. Interestingly, an unusual emission enhancement was determined for 4b and 4c upon full binding with excess F− ions rather than commonly observed emission quenching by fluoride, which may apply to luminescent sensing materials based on anion responses. Electronic interactions of 4a−4c with fluoride ions were confirmed by NMR spectroscopy in CDCl3. As shown in Figures S12−S14, the addition of fluoride resulted in the broadening of 1 H NMR signals and the pronounced upfield shift of aromatic protons. Moreover, the 11B NMR signals completely shifted from ∼40 to ∼2 ppm, typically indicating that all B atoms are 4coordinated as a consequence of full binding with fluoride (Figure S15). To further explore the electronic properties of these fluoride adducts, electrochemical measurements were performed under N2 conditions. As illustrated in Figures 5 and

Figure 3. Kohn−Sham molecular orbitals (HOMO and LUMO; isovalue = 0.02) and oscillator strength (f) involved in the S0−S1 transition for simplified 4a−4c (TD-DFT, B3LYP, and 6-31G*). Hexyl groups on the C(th) and N atoms are replaced by methyl groups.

eV for 4a−4c, respectively) were obtained by DFT computations (B3LYP and 6-31G*), in agreement with the values estimated from differential pulse voltammetry (DPV). To identify how distinctly these BN-doped molecules respond to anion complexation, we monitored the fluoride-ion titrations in THF using UV absorption and fluorescence spectroscopy in a N2 atmosphere. Compounds 4a−4c show a new strong redshifted band in their absorption spectra around 425−430 nm, while the original vibronic structure band (∼353 nm) gradually decreases with the addition of a F− anion (Figure 4a,c,e). This observation is in sharp contrast with what we commonly found

Figure 5. Top: DPV scans recorded in CH2Cl2 for (a) 4a−4c and (b) fluoride adducts with [NBu4]PF6 as the supporting electrolyte (c = 0.1 M) at ν = 100 mV/s, vs Fc/Fc+ (for clarity, the oxidation band of Fc is not shown). Bottom: Summary of the first oxidation potential (E1ox) in electrochemical DPV data and the HOMO−LUMO energy gap (Egap) derived from DFT calculations.

S5, DPV scans of the freshly prepared adducts (4a-F, 4b-2F, and 4c-3F) show oxidation waves shifting to less positive potentials compared with molecules before complexation. More importantly, complex 4b-2F exhibits the first oxidation wave at low potential (E1ox = 0.05 V, vs Fc/Fc+), indicating its capability as a luminescent redox-active material. We envision that these fluoride adducts could be useful in catalytic applications such as the oxygen reduction process. In conclusion, we have achieved the introduction of BN functionality into BTT with multiple sites for further chemical modification. Simply changing the number of BN units gave rise to a series of BTT-based azaborines with distinct supramolecular motifs. We disclosed that 4a−4c show red-shifted absorption and enhanced luminescence in response to anion binding. Furthermore, this work demonstrated the ease of oxidation of

Figure 4. F− anion titration monitored by UV−vis absorption and fluorescence spectroscopy recorded in THF (c = 1.0 × 10−5 M; λex= 353 nm) under N2: (a and b) 4a; (c and d) 4b; (e and f) 4c. C


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Inorganic Chemistry their fluoride complexes under electrochemical conditions. We believe that these key findings may offer new opportunities in luminescent anion sensors, catalysis, and other materials science.

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(8) (a) Hudson, Z. M.; Wang, S. Impact of Donor-Acceptor Geometry and Metal Chelation on Photophysical Properties and Applications of Triarylboranes. Acc. Chem. Res. 2009, 42, 1584. (b) Rao, Y. L.; Amarne, H.; Wang, S. Photochromic Four-Coordinate N,C-Chelate Boron Compounds. Coord. Chem. Rev. 2012, 256, 759. (9) Narita, A.; Wang, X. Y.; Feng, X.; Müllen, K. New Advances in Nanographene Chemistry. Chem. Soc. Rev. 2015, 44, 6616. (10) (a) Lorbach, A.; Hübner, A.; Wagner, M. Aryl(hydro)boranes: Versatile Building Blocks for Boron-Doped π-Electron Materials. Dalton Trans 2012, 41, 6048. (b) Schickedanz, K.; Radtke, J.; Bolte, M.; Lerner, H.; Wagner, M. Facile Route to Quadruply Annulated Borepins. J. Am. Chem. Soc. 2017, 139, 2842. (11) (a) Hu, R.; Feng, J.; Hu, D.; Wang, S.; Li, S.; Li, Y.; Yang, G. A Rapid Aqueous Fluoride Ion Sensor with Dual Output Modes. Angew. Chem., Int. Ed. 2010, 49, 4915. (b) Feng, J.; Tian, K.; Hu, D.; Wang, S.; Li, S.; Zeng, Y.; Li, Y.; Yang, G. A Triarylboron-Based Fluorescent Thermometer: Sensitive Over a Wide Temperature Range. Angew. Chem., Int. Ed. 2011, 50, 8072. (12) Matsuo, K.; Saito, S.; Yamaguchi, S. Photodissociation of B-N Lewis Adducts: A Partially Fused Trinaphthylborane with Dual Fluorescence. J. Am. Chem. Soc. 2014, 136, 12580. (13) Gon, M.; Tanaka, K.; Chujo, Y. A Highly Efficient Near-InfraredEmissive Copolymer with a N = N Double-Bond π-Conjugated System Based on a Fused Azobenzene-Boron Complex. Angew. Chem., Int. Ed. 2018, 57, 6546. (14) (a) Meng, B.; Ren, Y.; Liu, J.; Jäkle, F.; Wang, L. X. p−π Conjugated Polymers Based on Stable Triarylborane with n-Type Behavior in Optoelectronic Devices. Angew. Chem., Int. Ed. 2018, 57, 2183. (b) Min, Y.; Dou, C. D.; Tian, H. K.; Geng, Y. H.; Liu, J.; Wang, L. X. n-Type Azaacenes Containing B←N Units. Angew. Chem., Int. Ed. 2018, 57, 2000. (15) (a) Li, S. Y.; Hui, Y. J.; Sun, Z. B.; Zhao, C. H. A TriarylboraneBased Biphenyl Exhibiting Abrupt Fluorescence Enhancement at a Specific High Temperature. Chem. Commun. 2017, 53, 3446. (b) Zhao, Z. H.; Zhang, M. Y.; Liu, D. H.; Zhao, C. H. Modification of [5]Helicene with Dimesitylboryl: One Way to Enhance the Fluorescence Efficiency. Org. Lett. 2018, 20, 7590. (16) (a) Campbell, P. G.; Marwitz, A. J. V.; Liu, S. Y. Recent Advances in Azaborine Chemistry. Angew. Chem., Int. Ed. 2012, 51, 6074. (b) Bosdet, M. J. D.; Piers, W. E. B-N as a C-C Substitute in Aromatic Systems. Can. J. Chem. 2009, 87, 8. (c) Liu, K.; Lalancette, R. A.; Jäkle, F. B−N Lewis Pair Functionalization of Anthracene: Structural Dynamics, Optoelectronic Properties, and O2 Sensitization. J. Am. Chem. Soc. 2017, 139, 18170. (d) Yang, D. T.; Mellerup, S. K.; Peng, J. B.; Wang, X.; Li, Q. S.; Wang, S. Substituent Directed Phototransformations of BN-Heterocycles: Elimination vs Isomerization via Selective B−C Bond Cleavage. J. Am. Chem. Soc. 2016, 138, 11513. (e) Lu, J. S.; Ko, S. B.; Walters, N. R.; Kang, Y. J.; Sauriol, F.; Wang, S. Formation of Azaborines by Photoelimination of B,N-Heterocyclic Compounds. Angew. Chem., Int. Ed. 2013, 52, 4544. (f) Wang, X. Y.; Wang, J. Y.; Pei, J. BN Heterosuperbenzenes: Synthesis and Properties. Chem. - Eur. J. 2015, 21, 3528. (g) Hatakeyama, T.; Hashimoto, S.; Seki, S.; Nakamura, M. Synthesis of BN-Fused Polycyclic Aromatics via Tandem Intramolecular Electrophilic Arene Borylation. J. Am. Chem. Soc. 2011, 133, 18614. (h) Hashimoto, S.; Ikuta, T.; Shiren, K.; Nakatsuka, S.; Ni, J. P.; Nakamura, M.; Hatakeyama, T. Triplet-Energy Control of Polycyclic Aromatic Hydrocarbons by BN Replacement: Development of Ambipolar Host Materials for Phosphorescent Organic Light-Emitting Diodes. Chem. Mater. 2014, 26, 6265. (i) Wang, X. Y.; Zhang, F.; Schellhammer, K. S.; Machata, P.; Ortmann, F.; Cuniberti, G.; Fu, Y. B.; Hunger, J.; Tang, R. Z.; Popov, A. A.; Berger, R.; Müllen, K.; Feng, X. L. Synthesis of NBN-Type ZigzagEdged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif. J. Am. Chem. Soc. 2016, 138, 11606. (j) Shi, Y. G.; Mellerup, S. K.; Yuan, K.; Hu, G.; Sauriol, F.; Peng, T.; Wang, N.; Chen, P.; Wang, S. Stabilising Fleeting Intermediates of Stilbene Photocyclization with Amino-Borane Functionalisation: the Rare Isolation of Persistent Dihydrophenanthrenes and their [1,5] H-Shift Isomers. Chem. Sci. 2018, 9, 3844. (k) Hou, Q. G.; Liu, L. J.; Mellerup,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03579. Experimental procedures, analytical data (1H, 13C, and B NMR and MS) for all products and intermediates, crystallographic data for products 4a−4c, photophysical and electrochemical data, and DFT computations (PDF)

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Accession Codes

CCDC 1858359−1858361 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nan Wang: 0000-0001-5973-4496 Suning Wang: 0000-0003-0889-251X Pangkuan Chen: 0000-0002-8940-7418 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.-J.S., N.W., T.P., S.W., and P.C. are grateful to the National Natural Science Foundation of China for financial support (Grants 21501011, 21701011, 21571017, and 21772012).

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