Borylated Arylamine–Benzothiadiazole Donor–Acceptor Materials as

DOI: 10.1021/acs.organomet.7b00188. Publication Date (Web): April 25, 2017. Copyright © 2017 American Chemical Society. *E-mail: michael.ingleson@man...
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Borylated Arylamine−Benzothiadiazole Donor−Acceptor Materials as Low-LUMO, Low-Band-Gap Chromophores Daniel L. Crossley, Rosanne Goh, Jessica Cid, Inigo Vitorica-Yrezabal, Michael L. Turner,* and Michael J. Ingleson* School of Chemistry, University of Manchester, Manchester M13 9PL, United Kingdom S Supporting Information *

ABSTRACT: Fused and ladder-type benzothiadiazole−arylamine donor−acceptor C,N-chelated boron complexes were synthesized through direct electrophilic C−H borylation. The frontier molecular orbital energy levels of the borylated materials then could be modulated through variation of the exocyclic boron substituents by transmetalation with different diarylzinc reagents. The borylated materials possessed low band gaps and low LUMO energy levels with a number of examples also showing significant absorbance at >700 nm; however, low photoluminescence quantum yields were found for all these borylated compounds.

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INTRODUCTION

would be particularly desirable to red shift the absorbance and shift the emission maxima to beyond 700 nm. The incorporation of boron into π-conjugated D−A molecules by forming C,N-chelated boracycles was pioneered by Yamaguchi and Wakamiya.11 This approach has since become established as a useful means to modulate photophysical and electronic properties of conjugated materials.12−19 One effect of boracycle incorporation into D−A materials is a substantial decrease in the energy of the LUMO, resulting in a red-shifted absorbance and emission relative to the unborylated precursor.11−13 We have contributed to this area by using electrophilic C−H borylation to functionalize BT-containing A−D−A and D−A−D materials to generate fused structures with low LUMO energies.20−22 However, to date, the electrophilic C−H borylation of BT-containing D−A materials has been limited to the borylation of fluorene and thiophene substituents. Herein we report the extension of this methodology to BT-arylamine-containing D−A materials generating C,N-chelated fused and ladder-type molecules (Figure 2 bottom). These show smaller band gaps than the previously reported borylated BT-fluorene analogues (Figure 2, top). While the electrophilic borylation of 2-arylamine-substituted quinolines has been recently reported,19 the weaker acceptor character of quinoline (relative to BT) results in materials with larger band gaps (absorption maxima 80% yields by silica gel chromatography. The 11B NMR resonances for 1-BPh2 and 1-B(C6F5)2 were centered at 1 and −5 ppm, respectively, consistent with four-coordinate boron centers, and this combined with the 1H NMR spectra each showing only one species indicates that the four-coordinate compounds are the dominant species in solution, suggesting that minimal cleavage of the dative bond occurs in solution, in contrast to BMes2 analogues.23 Compound 1-BPh2 crystallized with two molecules in the asymmetric unit with the bond distances and angles within the boracycles being closely comparable to previously reported borylated BT-fluorene structures.21 The boracycle has only a minor deviation of the boron atom out of the plane generated by the other five atoms in the boracycle (as indicated by the out-of-plane para-C (e.g., C6)−centroid−B angles being 173° and 178°, Figure 3, and the distance between B and the mean plane of the other five atoms (e.g., C8−C7−C6−C1−N1) being 0.21 and 0.04 Å). This deviation combined with the compression of the endocyclic N−B−C (e.g., C8−B1−N1) angle (104.5(2)° and 105.1(2)°) indicates some degree of strain in the boracycle. This presumably arises from the longer bond distances to boron (C8−B1 = 1.629(4) and 1.627(4) Å and N1−B1 = 1.629(4) and 1.614(4) Å) relative to the C−C and C−N bonds in the boracycle (in the range 1.341(3) to 1.462(4) Å) combined with the annulated system containing one five-membered ring orientating the nitrogen lone pair nonoptimally for binding to boron as recently highlighted.23 Optoelectronic Properties. As anticipated a large bathochromic shift in absorbance is observed upon borylation of 1 (Figure 4 and Table 1). Compound 1-BPh2 results in a

Figure 2. Comparison of fluorene borylated D−A structures with the BT-arylamine borylated structures reported herein.

materials reported herein have absorption maxima up to 680 nm and significant absorption beyond 700 nm.

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RESULTS AND DISCUSSION Borylated D−A−D Materials. The addition of BCl3 to a dichloromethane (DCM) solution of compound 1 (Figure 3)

Figure 3. (Top) Synthesis of 1-BPh2 and 1-B(C6F5)2. (Bottom) Solid-state structure of 1-BPh2. Only one of two independent molecules in the asymmetric unit is shown, and disordered solvent molecules and hydrogen atoms are omitted for clarity; ellipsoids are at the 50% probability level. Blue = nitrogen, gray = carbon, yellow = sulfur, pink = boron, red = centroid of C8, C7, C6, C1, and N1.

resulted in the coordination of boron to the basic nitrogen site of BT and the borylation of a proximal C−H group of the triphenylamine unit in the position meta to the NPh2 moiety. Borylation was performed in the presence of 2,4,6-tri-tertbutylpyridine (TBP) to prevent any protonation of the amine units, which would otherwise deactivate the π system toward electrophilic substitution. As previously observed with fluorene and thiophene analogues of 1, no C−H borylation of the second donor unit to form a doubly borylated D−A−D material occurs even in the presence of excess BCl3 and longer reaction times. This is in contrast to the report by Fang and coworkers where a thienyl−pyrazine−thienyl D−A−D material underwent double electrophilic C−H borylation.13 This disparity is presumably due to the greater nucleophilicity of pyrazine relative to BT, which enables coordination of a second boron moiety to the pyrazine unit post the initial borylation. Post borylation of 1, the initial product, 1-BCl2, which is sensitive to H2O, was transformed in situ into the air- and

Figure 4. UV−vis−NIR absorbance and emission spectra of 1, 1BPh2, and 1-B(C6F5)2 (1 × 10−5 M toluene solutions). No emission is observed from 1-B(C6F5)2.

bathochromic shift of 170 nm in the absorbance maximum and a reduction in the optical band gap of 0.69 eV relative to unborylated 1. The absorbance maximum could be shifted further into the NIR region with an increase in the absorbance maximum by 49 nm and a decrease in the optical band gap by 0.14 eV, respectively, upon installation of electron-withdrawing C6F5 substituents onto boron in 1-B(C6F5)2. On the basis of previous work this decrease in the optical band gap is attributed B

DOI: 10.1021/acs.organomet.7b00188 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Comparison of the Photophysical and Redox Properties of 1, 1-BPh2, 1-B(C6F5)2, and Compound C compound 1 1-BPh2 1-B(C6F5)2 compound Cg

λmax abs/nm (ε/M−1 cm−1)a 461 631 680 584

(22 200) (12 200) (11 000) (13 900)

λmax em/nm (Φf %b)a

Eoxonset (V)d

Eredonset (V)d

HOMO (eV)d

LUMO (eV)d

590 810 (700 nm, particularly for 1-B(C6F5)2, is notable. To gain further insight into the trends observed in the UV− vis absorbance spectra, DFT calculations (M06-2X/6-311G(d,p)) were performed (Figure 5). Calculations of 1 and 1-

Figure 5. Molecular orbital energy levels and molecular orbital contours (isovalue = 0.04) of the HOMO and LUMO of 1, 1-(BPh2)2, and 1-(B(C6F5)2)2 at the M06-2X/6-311G(d,p) level.

Figure 6. Cyclic voltammetry plots for 1, 1-BPh2, and 1-B(C6F5)2, measured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting electrolyte at a scan rate of 50 mV s−1 (potentials are relative to ferrocenium/ferrocene).

BPh2 show the expected significant lowering of the LUMO (by 0.70 eV) and the minor increase in the HOMO (by 0.07 eV) upon borylation and chelation of BPh2, which is consistent with observations of related borylated BT based D−A−D systems. There is an increased localization of the HOMO onto the borylated triphenylamine unit in 1-BPh2 relative to 1 due to the positive inductive effect of four-coordinate boron−C sigma bonds, which raises in energy the orbitals associated with the borylated Ph-NPh2 unit relative to the nonborylated PhNPh2. Compound 1-B(C6F5)2 showed a further decrease in the LUMO (by 0.20 eV) with only a minor decrease in the HOMO level (by 0.07 eV) relative to 1-BPh2, which is consistent with the lower band gap observed in the UV−vis absorbance spectra. Comparison of the calculated frontier orbitals of 1-BPh2 with compound C is of note and reveals minimal change in the LUMO (calculated LUMO energy = −2.46 eV for C and −2.44 eV for 1-BPh2 with the LUMO localized on BT in both

reduction and oxidation waves are reversible, and the trends observed by the density functional theory (DFT) calculations and UV−vis absorption measurements correlated extremely well with those observed by CV. Significantly reduced reduction potentials of the borylated compounds were observed relative to 1. Furthermore, the oxidation potential was less positive for 1-BPh2 relative to C, consistent with the higher HOMO energy from a structure containing a borylated Ph-NPh2 unit. Diborylated A−D−A Materials. With 1-BPh2 and 1B(C6F5)2 being weakly emissive we targeted a more highly fused borylated-BT-arylamine structure in an attempt to prepare NIR emitters with an enhanced fluorescence quantum C

DOI: 10.1021/acs.organomet.7b00188 Organometallics XXXX, XXX, XXX−XXX

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Organometallics yield. Therefore, the BT-carbazole-BT compound, 2, containing the “swallow tail” substituent (CH(C8H17)2) to provide solubility, was synthesized via standard methodologies. Carbazole was selected, as it is a fused arylamine well documented to undergo high-yielding electrophilic C−H borylation.26 Furthermore, double borylation of 2 will generate analogues of previously double-borylated BT-fluorene-BT compounds (e.g., compound D, Figure 2 top), allowing for structure−property relationships to be assessed. The diborylation of compound 2 was achieved using the same methodology as described for compound D (Scheme 1),

Scheme 2. Synthesis of 2-B(Aryl2)2

(BPh2)2 was also attempted via a borocation-mediated borodestannylation reaction;22 however, the addition of an excess (6 equiv) of PhnBu3Sn to [2-(BCl)2][AlCl4]2 resulted only in the formation of trace amounts of the desired product. Transmetalation of 2-(BCl2)2 with Zn(C6F5)2 was also low yielding (24% isolated yield). Attempts to improve the yield of 2(B(C6F5)2)2 through extended heating times at 60 °C or heating at 100 °C resulted in decomposition and only traces of the desired product. To gain some insight into the transmetalation step, the 19 1 F{ H} NMR of the reaction mixture containing 2-(BCl2)2 in toluene was monitored (Figure 7). Upon addition of 4 equiv of

Scheme 1. Synthesis of 2-(BCl2)2

although the reaction time was significantly reduced (from 2 days to 4 h) presumably due to the more nucleophilic nature of carbazole at the 3 and 6 positions (relative to fluorene). Again four equivalents of AlCl3 are required due to the propensity to form the diborocation [2-(BCl)2][AlCl4]2 (Scheme 1), as previously observed for the fluorene analogue.20 The C−H borylation of compound 2 with BCl3 (with or without TBP) does not proceed; instead the addition of BCl3 to 2 led to a color change to red and the formation of a sharp 11B resonance at 5.0 ppm, but no reduction in the number of aromatic resonances (combined 12H integral relative to aliphatic resonances). This is consistent with coordination of BCl3 to the less hindered nitrogen positions as previously reported.20 Furthermore, attempts using Murakami’s conditions, BBr3/ Hünigs base,27 were also unsuccessful. While 2-(BCl2)2 could be produced in situ in high conversion (it is the only carbazole-containing species observed by in situ NMR spectroscopy), it proved sensitive to protic species as expected; therefore, arylation at boron was attempted. Unlike the fluorene analogue of 2-(BCl2)2, which undergoes transmetalation in DCM at 20 °C within 3 h with organozinc reagents (ZnPh2 or Zn(C6F5)2), 2-(BCl2)2 undergoes transmetalation much more slowly. The conditions that were successfully employed for formation of compound D in moderate isolated yield (53%) resulted in only trace amounts of 2-(BPh2)2. In our hands, the best conditions for the transmetalation of 2-(BCl2)2 with ZnPh2 proved to be heating the reaction mixture at 60 °C in toluene for 2 h. Further heating resulted in decomposition to unidentified species. Even under these optimized conditions 2-(BPh2)2 was isolated in only 17% yield (Scheme 2) and was characterized based on multinuclear NMR spectroscopy (including a broad 11B resonance centered at 2 ppm) and mass spectroscopy. 2(BPh2)2 is stable to silica and H2O; thus the low isolated yield is not from decomposition during isolation. The synthesis of 2-

Figure 7. Partial in situ 19F{1H} NMR spectra monitoring the transmetalation of 2-(BCl2)2 with Zn(C6F5)2 (−153 to −164 ppm region in toluene with a d6-DMSO capillary insert at 298 K).

Zn(C6F5)2, the 19F{1H} NMR spectrum showed an upfield shift and significant broadening of the resonances associated with the para- (−153.4 ppm) and meta- (−161.1 ppm) substituted fluorines of Zn(C6F5)2 with no resonances associated with the transmetalated product observed at short reaction times (longer reaction times at 20 °C did not lead to a significant increase in transmetalation). Upon heating (for up to 16 h), 19F{1H} resonances associated with the transmetalated product 2(B(C6F5)2)2 are present, but the majority of the Zn(C6F5)2 remains unreacted. The broad meta and para resonances suggest a dynamic interaction between Zn(C6F5)2 and a Lewis base that is retarding transmetalation. However, no interaction is observed when Zn(C6F5)2 is added to N-octylcarbazole, indicating that Lewis adduct formation between carbazole and Zn(C6F5)2 is unlikely. There is also no significant production of HC6F5 (precluding reaction of Zn(C6F5)2 with TBP-H) and no evidence for deboronation (no BCl3 or B(C6F5)3 by 11B NMR spectroscopy). Thus, currently the reason for the low yields of 2-B(Aryl2)2 is unclear. D

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Organometallics Table 2. Comparison of the Photophysical and Redox Properties of 2, 2-(BPh2)2, 2-(B(C6F5)2)2, and Compound D compound 2 2-(BPh2)2 2-(B(C6F5)2)2 compound Df

λmax abs/nm (ε/M−1 cm−1)a 401 540 548 538

(24 600) (17 000) (15 200) (19 500)

λmax em/nm (Φf %b)a

Eoxonset (V)c

Eredonset (V)c

HOMO (eV)c

LUMO (eV)c

491 727 (0.3) 719 (0.3) 636 (18)

0.75 0.50 0.67 0.72

−1.89 −1.31 −1.13 −1.28

−6.14 −5.89 −6.06 −6.11

−3.50 −4.08 −4.26 −4.11

band gap (eV) 2.74,d 1.98,d 1.95,d 2.02,d

2.64e 1.81e 1.80e 2.00e

1 × 10−5 M solution in toluene. bRelative fluorescence quantum yield, estimated by using cresyl violet as standard (Φf = 54% in methanol);24 estimated error ±20%. Fluorescence spectra were measured by exciting the solutions at their absorption maxima. cMeasured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting electrolyte at a scan rate of 50 mV s−1; potentials are given relative to the Fc/Fc+ redox couple, which is taken to be 5.39 eV below vacuum.25 dOptical band gap estimated from onset of absorption. eElectrochemical band gap. fFrom ref 20. a

Optoelectronic Properties. The effect on the optical properties of the incorporation of two B(Ar)2 units into the structure of compound 2 was investigated by UV−vis spectroscopy in toluene (Table 2). It should be noted that, due to the larger band gap of the parent compound 2 (relative to 1), the borylated congeners have blue-shifted absorption and emission relative to 1-BAryl2. Again a large bathochromic shift in absorbance is observed upon borylation; for 2-(BPh2)2 and 2-(B(C6F5)2)2 the bathochromic shifts in the absorbance maxima are 139 and 147 nm, respectively (Figure 8).

increase in the HOMO (by 0.37 eV) for 2′-(BPh2)2 relative to the unborylated compound 2′ were found. However, this is not due to any increased orbital delocalization on planarization, with borylation actually decreasing the degree of delocalization of the HOMO and LUMO (Figure 9). The energy changes

Figure 9. Molecular orbital energy levels and molecular orbital contours (isovalue = 0.04) of the HOMO and LUMO of 2′, 2′(BPh2)2, and 2′-(B(C6F5)2)2 at the M06-2X/6-311G(d,p) level.

Figure 8. UV−vis absorbance spectra of 2, 2-(BPh2)2, and 2(B(C6F5)2)2 (1 × 10−5 M toluene solutions).

observed on borylation are therefore attributed to the electronwithdrawing effect that boron dative bond formation to BT has on the LUMO energy and the positive inductive effect of the four-coordinate B−C bond raising the HOMO energy. The calculations also indicate that, upon replacing the Ph boron substituents for C6F5, there is a decrease in the energy of the HOMO and LUMO by approximately the same degree (0.21 and 0.22 eV, respectively). This is consistent with the similar optical band gaps observed in the UV−vis absorbance spectra. Additionally, the calculations indicate a complete localization of the LUMO onto the BT unit and a complete localization of the HOMO onto the carbazole unit in 2′-(BPh2)2 and 2′(B(C6F5)2)2. This is in contrast to the calculated structure of a model of compound D, where the HOMO is delocalized over both BT units and the fluorene unit.20 Again the change in the character of the HOMO is consistent with the stronger donor nature of the arylamine unit relative to fluorene. The greater spatial separation of HOMO and LUMO in 2-(BPh2)2 relative to compound D is also consistent with the larger Stokes shift (187 nm) observed for 2-(BPh2)2, suggesting a greater degree of intramolecular charge transfer character. The absence of any spatial HOMO−LUMO overlap may also contribute to the observed low quantum yield values, as this will result in a lower oscillator strength and thus a low radiative transition rate.10

Furthermore, there is a reduction in the optical band gap of 0.76 and 0.79 eV, respectively, relative to unborylated 2. The similar optical band gaps for the two 2-(B(Aryl)2)2 compounds is attributed to the replacement of the Ph boron substituents with C6F5 having a similar effect on both the HOMO and LUMO energy levels, as observed in fluorene based A−D−A borylated ladder structures.12,20 In contrast, in the monoborylated D−A−D systems the LUMO is reduced in energy more than the HOMO on exchange of Ph for C6F5. 2-(BPh2)2 and 2-(B(C6F5)2)2 showed significantly red-shifted emission and larger Stokes shifts (λmax > 700 nm) relative to their fluorene analogues, e.g., compound D, but significantly lower Φf values. Thus, the increased degree of fusion in 2-(BAryl2)2 relative to 1-BAryl2 has not significantly enhanced the Φf values. Calculations on model compounds of 2, 2-(BPh2)2, and 2(B(C6F5)2)2 (replacing N-CH(C8H17)2 with N-Me, referred to as 2′, 2′-(BPh2)2, and 2′-(B(C6F5)2)2, respectively) show that upon diborylation there is a considerable reduction in the torsion angle from 46.3° to 3.0° between the carbazole and BT units. A significant lowering of the LUMO (by 0.70 eV) and an E

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Organometallics

TOF) measurements were performed by the Mass Spectrometry Service, School of Chemistry, University of Manchester. MALDI-TOF analyses were performed using a Shimadzu Axima Confidence spectrometer using a 4k PPG as a calibration reference. All UV−vis absorption spectra were recorded on a Varian Cary 5000 UV−vis− NIR spectrometer at room temperature in spectroscopic grade solvents. Emission spectra were recorded on a Varian Cary Eclipse fluorimeter at room temperature in spectroscopic grade solvents, and the solutions were excited at their relative absorbance maxima. Cyclic voltammetry was performed using a CH-Instrument 1110C electrochemical/analyzer potentiostat under a nitrogen flow. The DFT calculations were performed using the Gaussian09 suite of programs.30 Geometries were optimized with the DFT method using the M06-2X functional31 and 6-311G(d,p) as a basis set with inclusion of a PCM model for solvent correction (DCM).32 All stationary geometry optimizations were full, with no restrictions. Structures were confirmed as minima by frequency analysis and the absence of imaginary frequencies. Accurate combustion data were not obtainable. Consistently low % C content was observed and persisted even when V2O5 was used as an oxidant. Purity was indicated by multinuclear NMR spectroscopy in organic solvents (in which the sample fully dissolved) and is supported by MS analysis. Compound 1-BPh2. BCl3 (1 M) in DCM (0.35 mL, 0.35 mmol) was added to a solution of 1 (93 mg, 0.15 mmol) in DCM (∼8 mL). The reaction mixture was stirred at ambient temperature for 1 h under the dynamic flow of nitrogen, over which time period the reaction mixture had changed color from orange to dark blue. 2,4,6-Tri-tertbutylpyridine (38 mg, 0.15 mmol) was then added to the reaction mixture. The solvent and other volatiles were removed under reduced pressure, and the resulting blue residue was dissolved in DCM (∼10 mL). ZnPh2 (109 mg, 0.50 mmol) was added to the reaction mixture, which was then stirred at ambient temperature for 3 h. The reaction mixture was filtered through a plug of silica gel, and the resulting solution was evaporated to dryness under reduced pressure. The resulting blue residue was purified via column chromatography [eluent: DCM/hexane (1:9 followed by 2:8)]. The desired product was obtained as a blue solid (yield 102 mg, 86%). 1 H NMR (400 MHz, CDCl3) δ = 8.12 (d, 3JHH = 7.8 Hz, 1 H), 7.91 (d, 3JHH = 8.7 Hz, 1 H), 7.81−7.88 (m, 3 H), 7.28−7.39 (m, 5 H), 7.14−7.26 (m, 20 H), 7.04−7.14 (m, 6 H), 6.97−7.04 (m, 2 H), 6.93 ppm (dd, 3JHH = 8.6, 4JHH = 2.3 Hz, 1 H); 13C{1H} NMR (101 MHz, CDCl3) δ = 154.6 (br), 154.0 (br), 153.5, 148.4, 148.0, 147.8, 147.2, 147.1, 133.3, 130.4, 130.0, 129.5, 129.4, 129.1, 128.7, 128.0, 127.7, 127.4, 125.8, 125.0, 124.6, 124.4, 123.6, 123.2, 123.0, 122.9, 122.5, 120.8; 11B NMR (128 MHz, CDCl3) δ ≈ 1 (br); MALDI-TOF calc.d for C54H39BN4S+ [M]+ = 786.3, found 786.3. Compound 1-B(C6F5)2. BCl3 (1 M) in DCM (0.25 mL, 0.25 mmol) was added to a solution of 1 (69 mg, 0.11 mmol) in DCM (∼8 mL). The reaction mixture was stirred at ambient temperature for 1 h under the dynamic flow of nitrogen, over which time period the reaction mixture had changed color from orange to dark blue. 2,4,6Tri-tert-butylpyridine (28 mg, 0.11 mmol) was then added to the reaction mixture. The solvent and other volatiles were removed under reduced pressure, and the resulting blue residue was dissolved in DCM (∼10 mL). Zn(C6F5)2 (73 mg, 0.33 mmol) was added to the reaction mixture, which was then stirred at ambient temperature for 3 h. The reaction mixture was filtered through a plug of silica gel, and the resulting solution was evaporated to dryness under reduced pressure. The resulting blue residue was purified via column chromatography [eluent: DCM/petroleum ether (1:9 followed by 2:8)]. The desired product was obtained as a blue solid, which was then washed with pentane (yield 87 mg, 82%). 1 H NMR (400 MHz, CDCl3) δ = 8.22 (d, 3JHH = 7.8 Hz, 1 H), 7.90 (d, 3JHH = 7.8 Hz, 2 H), 7.79−7.87 (m, 2 H), 7.29−7.37 (m, 4 H), 7.16−7.26 (m, 10 H), 7.03−7.14 ppm (m, 10 H); 13C{1H} NMR (101 MHz, CDCl3) δ = 153.6, 149.0, 148.8, 147.6, 147.5 (br d,1J (13C, 19F) = 240 Hz), 147.1, 146.9, 140.0 (br d,1J (13C, 19F) = 250 Hz), 137.2 (br d,1J (13C, 19F) = 250 Hz), 131.0, 130.2, 129.7, 129.5, 129.1, 128.1, 126.8, 126.3, 125.2, 124.9, 124.7, 123.8, 123.5, 123.2, 122.5, 122.3, 121.2; 19F{1H} NMR (376 MHz, CDCl3) δ = −131.91 (dd, 3JFF =

Again the trends in energies observed by the DFT calculations and optical spectroscopy are confirmed by CV studies (Figure 10). Of note is that the comparable change in

Figure 10. Cyclic voltammetry plots for 2, 2-(BPh2)2, and 2(B(C6F5)2)2, measured in DCM (1 mM) with [nBu4N][PF6] (0.1 M) as the supporting electrolyte at a scan rate of 50 mV s−1 (potentials are relative to ferrocenium/ferrocene).

the calculated HOMO and LUMO energies on exchanging Ph for C6F5 is consistent with a change in the observed oxidation and reduction onset potentials. Comparison of the onset potentials of 2-(BPh2)2 and compound D revealed that the replacement of fluorene with carbazole results in minimal change in the LUMO energy but a dramatic change in the HOMO (by 0.22 eV), as expected based on the nature of the respective frontier orbitals.

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CONCLUSIONS Electrophilic C−H borylation has been extended to BTarylamine-based D−A materials and represents a facile route to reduce the band gap of these popular low-band-gap materials. The borylated arylamine compounds reported herein possess lower band gaps than the fluorene analogues due to an increase in the HOMO energy achieved by incorporating the borylated arylamine donor units. The borylated D−A−D material 1B(C6F5)2 shows a band gap of ca. 1.5 eV and has significant absorbance at greater than 700 nm. The low-energy LUMO and low band gap of these materials suggest that they (and derivatives thereof) have potential as ambipolar semiconductors, and this is currently being explored.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Unless otherwise indicated, all reagents were purchased from commercial sources and were used without further purification. 128 and 9-(9-heptadecanyl)-9H-carbazole2,7-diboronic acid bis(pinacol) ester29 were prepared according to modified literature procedures. All appropriate manipulations were performed using standard Schlenk techniques or in an argon-filled MBraun glovebox (O2 levels below 0.5 ppm). Solvents were distilled from NaK, CaH2, or K and degassed prior to use. Dichloromethane and tetrahydrofuran (THF) were stored over activated 3 Å molecular sieves, while toluene was stored over a potassium mirror. NMR spectra were recorded using a Bruker AV-400 spectrometer. Unless otherwise stated, all NMR spectra are recorded at 293 K. Carbon atoms directly bonded to boron are not always observed in the 13C{1H} NMR spectra due to quadrupolar relaxation leading to signal broadening. Matrix-assisted laser desorption/ionization time-of-flight (MALDIF

DOI: 10.1021/acs.organomet.7b00188 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 24.1, 4JFF = 8.6 Hz, 4 F), −156.90 (t, 3JFF = 21.1, 2 F), −162.66 (m, 4 F); 11B NMR (128 MHz, CDCl3) δ ≈ −5 (br); MALDI-TOF calcd for C54H29BF10N4S+ [M]+ = 966.2, found 966.0. Compound 2. 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester (0.62 g, 0.94 mmol), 4-bromo-2,1,3benzothiadiazole (0.46 g, 2.1 mmol), K3PO4.H2O (1.30 g, 5.62 mmol), degassed deionized water (1.35 mL), Pd2(dba)3 (42.9 mg, 0.05 mmol), and S-PHOS (38.5 mg, 0.01 mmol) were dissolved in THF (10 mL). The reaction mixture was stirred at 70 °C for 24 h. The cooled mixture was extracted with DCM (100 mL), washed with brine (1 × 100 mL) and then water (1 × 100 mL), and dried with MgSO4. After evaporation of solvents, the resulting orange residue was purified on base-treated silica gel (5% NEt3 in hexane) column chromatography [eluent: chloroform/hexane (0:1 followed by 2:8)]. The desired product was obtained as a yellow solid (yield 0.48 g, 77%). 1 H NMR (400 MHz, CDCl3) δ = 8.43 (br s, 1 H), 8.31 (br t, 3JHH = 8.4 Hz, 2 H), 8.20 (br s, 1 H), 8.04 (dd, 3JHH = 8.8, 4JHH = 1.0 Hz, 2 H), 7.86 (dd, 3JHH = 6.8, 4JHH = 1.0 Hz, 2 H), 7.83 (br d 3JHH = 7.6 Hz, 2 H), 7.73 (m, 2 H), 4.82 (sept, 3JHH = 5.2 Hz, 1 H), 2.61−2.44 (m, 2 H), 2.14−1.98 (m, 2 H), 1.41−1.11 (m, 24 H), 0.87−0.76 (m, 6 H); 13 C{1H} NMR (101 MHz, CDCl3) δ = 155.7, 153.7, 142.8, 139.3, 135.5, 135.4, 134.9, 134.3, 129.6, 127.7, 127.7, 123.6, 122.3, 120.5, 120.2, 120.1, 113.0, 110.2, 56.4, 33.8, 31.7, 29.4, 29.3, 29.1, 26.8, 22.5, 13.9; hindered N-CH(C8H17)2 rotation leads to lower solution symmetry as previously observed; 33 MALDI-TOF calcd for C41H47N5S2+ [M]+ = 673.3, found 673.8. Compound 2-(BPh2)2. BCl3 (1 M) in DCM (0.38 mL, 0.38 mmol) was added to a stirred solution of 2 (64 mg, 0.095 mmol) and 2,4,6-tri-tert-butylpyridine (48 mg, 0.19 mmol) in DCM (3 mL). AlCl3 (26 mg, 0.19 mmol) was added to the reaction mixture, which was then stirred for 2 h at room temperature. An additional quantity of AlCl3 (26 mg, 0.19 mmol) was added, and the reaction mixture was stirred for 2 h, over which time period the reaction mixture had turned blue. The reaction mixture was evaporated to dryness, the residue was then dissolved in DCM (5 mL), and NMe4Cl (41 mg, 0.38 mmol) was added to the solution, which instantly turned pink. After the removal of the solvent under reduced pressure, the reaction mixture was dissolved in toluene (20 mL), and ZnPh2 (105 mg, 0.49 mmol) was added to the reaction mixture. The reaction was stirred at 60 °C for 2 h, which resulted in a purple solution. After the solvent was removed under reduced pressure, the desired product was purified by basetreated (5% NEt3 in hexane) preparative TLC [eluent: DCM/hexane (3:7)] to afford a purple solid (yield 16 mg, 17%). 1 H NMR (400 MHz, CDCl3) δ = 8.37 (d, 3JHH = 6.3 Hz, 1 H), 8.32 (d, 3JHH = 6.8 Hz, 1 H), 8.21 (s, 1 H), 8.12 (d, 3JHH = 6.8 Hz, 2 H), 8.03 (s, 1 H), 7.96−7.78 (m, 4 H), 7.13−7.31 (m, 20 H), 4.70 (sept, 3 JHH = 4.8 Hz, 1 H), 2.44 (m, 2 H), 2.09 (m, 2 H), 1.43−1.08 (m, 24 H), 0.79 (t, 3JHH = 6.80, 6 H); 13C{1H} NMR (101 MHz, CDCl3) δ = 155.2, 155.1, 147.7, 142.8, 142.6, 139.4, 133.5, 133.0, 131.2, 128.6, 128.2, 127.5, 127.0, 126.7, 125.8, 124.6, 122.9, 118.7, 104.3, 101.6, 56.1, 33.9, 31.9, 31.7, 31.6, 29.7, 29.6, 29.3, 29.2, 27.1, 22.5, 14.0; hindered N-CH(C8H17)2 rotation leads to lower solution symmetry as previously observed;33 11B NMR (128 MHz, CDCl3) δ ≈ 2.0 (br); MALDI-TOF calcd for C59H60B2N5S2+ [M − C6H5]+ = 924.4, found 924.6. Compound 2-(B(C6F5)2)2. BCl3 (1 M) in DCM (0.6 mL, 0.6 mmol) was added to a stirred solution of 2 (100 mg, 0.15 mmol) and 2,4,6-tri-tert-butylpyridine (73 mg, 0.30 mmol) in DCM (3 mL). AlCl3 (40 mg, 0.30 mmol) was then added to the reaction mixture, which was then stirred for 2 h at room temperature. An additional quantity of AlCl3 (40 mg, 0.30 mmol) was added, and the reaction mixture was stirred for 2 h, whereupon the reaction mixture had turned blue. The reaction mixture was evaporated to dryness, the reaction mixture was then dissolved in DCM (5 mL), and NMe4Cl (32 mg, 0.30 mmol) was added to the solution, which instantly turned pink. After the removal of the solvent under reduced pressure, the reaction mixture was dissolved in toluene (20 mL) and Zn(C6F5)2 (237 mg, 0.59 mmol) was added to the reaction mixture. The reaction was left to stir overnight at 60 °C, which resulted in a purple solution. After the solvent was removed under reduced pressure, the desired product was

purified by base-treated (5% NEt3 in hexane) preparative TLC [eluent: DCM/hexane (3:7)] to afford a purple solid (yield 48 mg, 24%). 1 H NMR (400 MHz, CDCl3) δ = 8.50 (d, 3JHH = 6.7 Hz, 1 H), 8.54 (d, 3JHH = 6.7 Hz, 1H) 8.20 (s, 1 H), 8.09−7.87 (m, 7 H), 4.69 (sept, 3 JHH = 5.0 Hz, 1 H), 2.46−2.35 (m, 2 H), 2.15−2.04 (m, 2 H), 1.40− 1.07 (m, 24 H), 0.79−0.73 (t, 3JHH = 6.8 Hz, 6 H); 13C{1H} NMR (101 MHz, CDCl3) δ 155.0, 147.6 (br d,1J (13C, 19F) = 238 Hz), 147.1, 143.1, 140.0 (br d,1J (13C, 19F) = 252 Hz), 139.8, 137.3 (br d,1J (13C, 19F) = 250 Hz), 133.4, 130.2, 127.5, 127.1, 125.7, 125.4, 125.2, 124.9, 124.4, 121.6 (br m), 119.4, 104.7, 102.0, 56.5, 33.9, 31.7, 29.5, 29.3, 29.2, 27.0, 22.5, 14.0; 19F{1H} NMR (376 MHz, CDCl3) δ = −131.89 (m, 8 F), −156.79 (m, 4 F), −162.80 (m, 8 F); 11B NMR (128 MHz, CDCl3) δ ≈ −6.0 (br); MALDI-TOF calcd for C59H45B2N5F15S2+ [M − C6F5]+ = 1194.3, found 1194.0.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00188. Analytical data and crystallographic information (PDF) Crystallographic data (CIF) Calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael J. Ingleson: 0000-0001-9975-8302 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from Cambridge Display Technology Limited (Company Number 02672530, CDT/EPSRC Case Award to D.L.C.), the EPSRC (EP/K03099X/1), and the European Research Council (FP/ 2007−2013/ERC Grant Agreement 305868). M.J.I. acknowledges the Royal Society (for the award of a University Research Fellowship), and M.L.T. thanks InnovateUK for financial support of the Knowledge Centre for Material Chemistry. The authors acknowledge the use of the EPSRC UK National Service for Computational Chemistry Software (NSCCS) at Imperial College London in carrying out this work. Dr. Martin J. Humphries at CDT is also thanked for useful discussions. Additional research data supporting this publication are available as supplementary information accompanying this publication.

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DOI: 10.1021/acs.organomet.7b00188 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00188 Organometallics XXXX, XXX, XXX−XXX