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Donor-#-Acceptor Type Unsymmetrical Triarylborane-based Fluorophores: Synthesis, Fluorescence Properties, and Photostability Masato Ito, Emi Ito, Masato Hirai, and Shigehiro Yamaguchi J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01015 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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The Journal of Organic Chemistry

Donor-π-Acceptor Type Unsymmetrical Triarylborane-based Fluorophores: Synthesis, Fluorescence Properties, and Photostability Masato Ito,†,§ Emi Ito,†,§ Masato Hirai,‡ and Shigehiro Yamaguchi*,†,‡ Department of Chemistry, Graduate School of Science and Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan ‡ Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan †

Supporting Information ABSTRACT: A two-step synthesis to prepare tricoordinate organoboron compounds bearing three different aryl groups has been developed. After the first aryl substitution to an aryl boronic ester took place, the intermediate species, that is, bis(diarylborinate) species was isolated as an air- and moisture-stable solid, which allowed the second aryl substitution to carry out in a selective manner. Subsequently, a series of unsymmetrical triarylboranes possessing a sterically bulky aryl group, triarylamine moiety, and para-functionalized phenyl ring was synthesized. Not only did these triarylboranes exhibit remarkable solvent-dependent fluorescence as expected for donor-π-acceptor (D-π-A) systems, they were also accompanied by profound persistence against photoirradiation especially for that bearing a 1,3,5-tri-tert-butylphenyl ring. This survey exemplifies that sufficient electronic and steric modification is key to construct photostable D-π-A type triarylborane-based fluorophores.

INTRODUCTION One of the most sophisticated strategies to alter the peculiarities of π-conjugated organic materials is incorporation of main-group elements, particularly from those of groups 13-16, into the π-skeleton. In this regard, boron is particularly attractive because of efficient p-π* interaction between the vacant p orbital of the boron atom and the π* orbital of the πconjugated framework. This orbital interaction lowers the LUMO energy level and thus increases the electron affinity. The bespoke property of boron becomes especially appealing when a π-electron-donor is attached to a triarylborane electron acceptor. Such compounds possessing a donor-π-acceptor (D-π-A) scaffold tend to have large electronic dipoles in the excited state, which induces effective intramolecular charge transfer (ICT). Consequently, those boron compounds exhibit a bathochromic shift in emission with a large Stokes shift as the solvent polarity increases.1 Adequate D-π-A systems are thus promising tools for a broad range of applications such as anion sensing,2 organic light-emitting diodes (OLEDs),3 nonlinear optics,4 and bioimaging.5 The earliest example of such D-π-A type triarylboranes was reported in 1972 by Williams and coworkers. They described that compound 1, which links electron-accepting dimesitylboryl and electron-donating diphenylamino moieties through a phenyl spacer, certainly enjoyed the list of aforementioned photophysical properties (Figure 1a).6 A more recent development by Marder and coworkers revealed that incorporation

of an appropriate D-π-A sequence can even push the emission band close to the near infrared (NIR) region.2h As one of the few examples, compound 2, composed of both strongly electron-accepting boryl and electron-donating amino groups, emits light at λem = 638 nm and records a high fluorescence quantum yield (ФF = 0.96) even in polar acetonitrile.2h This outcome is of importance because it highlights how sufficient molecular engineering can flexibly adjust the emissive nature of triarylboranes and cover the spectral window from violet to NIR region. In spite of these profound optoelectronic properties, stabilization of triarylboranes is still indisputably challenging. It has been widely acknowledged that the p orbital of the boron center could accommodate external nucleophiles to produce a tetracoordinate boron complex and eventually decompose. Some strategies to prevent such disintegration pathway have been consolidated to date.7 One of the simplest approaches is to kinetically stabilize triarylborane scaffolds by protecting the inherently reactive boron center with sterically bulky aryl groups, such as 1,3,5-trimethylphenyl (mesityl) and other related groups as seen in compounds 1 and 2. Moreover, many reports have proclaimed that organoboron compounds are quite often prone to photolysis under specific circumstances. In fact, tetracoordinate organoboron compounds are extensively known to undergo a photoinduced di-π-borate rearrangement8 or a photochromic B-C/C-C rearrangement.9 Photochemical reactions of tricoordinate organoboranes, however, are much less documented to date.10

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possible variations of triarylboranes prepared by this method are rather limited. In this context, unsymmetrically substituted triarylboranes are highly advantageous because the properties of such compounds can be more precisely and accurately tuned by simply adjusting the electronic and steric effects of the three aryl rings. Taking advantage of this disposition, we designed a series of unsymmetrical triarylboranes 3a-d and 4, which all incorporate: 1) an electron-donating diarylaminosubstituted thienyl group for the construction of a D-π-A moiety, 2) a sterically demanding aryl group for kinetic stabilization of the boron center, and 3) a phenyl ring decorated with an electron-donating or -accepting substituent on the para-position for electronic modification (Figures 1b and 1c). In this study, we synthesized these compounds and investigated their substituent effects on the photophysical properties and photostability.

RESULTS AND DISCUSSION Figure 1. (a) Examples of D-π-A type tricoordinate organoboron fluorophores, (b) molecular design of unsymmetrically substituted triarylboranes, and (c) compounds studied in this work. Accordingly, clarification of what dictates the photostability of triarylboranes is a key step forward to develop useful fluorophores. To this end, we decided to engineer the aryl substituents of the triarylboranes to illuminate the impact of electronic and steric effects towards photostability. Such knowledge should facilitate rational design of robust D-π-A type fluorophores with tunable properties. In order to consider the electronic and steric contributions separately, this study is devoted to tricoordinate organoboron compounds with three different aryl substituents on the boron center. Scheme 1. Synthesis of triarylboranes containing two to three different aryl groups.

Only a few strategies to synthesize unsymmetrical triarylboranes have been reported thus far.2b,11 One of the protocols was reported by Jäkle and coworkers who developed an elegant synthesis of unsymmetrically substituted triarylboranes based on the consecutive arylation of boron tribromide with different arylmetal reagents in one pot (Scheme 1b).2b,12 This method uses hazardous organotin and/or organocuprate reagents as well as corrosive boryl halide precursors. In contrast, our synthesis is based on a two-step synthesis through an environment-friendly approach starting from readily accessible arylboronic esters (Scheme 1c). The key factor of this strategy is that the intermediate species after the first aryl substitution are stable enough to isolate in air, and therefore the second aryl substitution could be carried out in a selective manner. Scheme 2. Synthesis of unsymmetrical boranes 3a-d.

(a) Traditional synthesis of C2 symmetric triarylboranes

(b) One-pot synthesis of unsymmetrical triaryboranes reported by Jäkle group

(c) This work: Two-step synthesis of unsymmetrical triarylboranes via isolable bis(diarylborinate) intermediates

Most of triarylboranes reported in literature contain two of the same aryl groups, mainly because of synthetic easiness. Those compounds are generally obtained by reaction of metallated agents such as organolithium and Grignard reagents with monoaryl- or diaryl-boryl halide precursors (Scheme 1a). Hence, structural modification is difficult and

Compounds 3a-d, which all consist of 1,3,5triisopropylphenyl (Tip) group, were prepared under the same conditions (Scheme 2). First, arylboronic esters 5a-d were treated with 2,4,6-triisopropylphenylmagnesium bromide to afford bis(diarylborinate) species 6a-d as air-stable colorless solids upon recrystallization in ethanol. The structure of one of the bis(diarylborinate) species, compound 6a, was confirmed by single crystal X-ray analysis (see Figure S1 in the SI). We speculate that the driving force of the reaction originates from the formation of a stable dimagnesium salt which precipitates out of solution. Successive reaction of 6a-d with 2-(4-diphenylaminophenyl)-5-thienyllithium furnished


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Table 1. Photophysical properties of triarylboranes 3a-d, 4, and 7 in various solvents. Compd

Solvents

λaba / nm

ε / × 104 M-1 cm-1

λemb / nm

Stokes shift [cm-1]

ФFc

τ [ns]

kr [108 s-1]

knr [108 s-1]

3a

cyclohexane

404

3.84

445

2280

0.93

2.1

4.4

0.34

CHCl3

410

3.45

490

3982

0.92

2.7

3.5

0.30

CH2Cl2

411

3.44

518

5025

0.90

3.2

2.8

0.32

acetone

409

3.72

532

5652

0.90

3.4

2.6

0.29

CH3CN

408

3.39

554

6459

0.89

3.9

2.3

0.31

cyclohexane

399

4.24

441

2387

0.94

2.1

4.5

0.31

CH3CN

404

3.63

536

6096

0.92

3.6

2.6

0.24

cyclohexane

405

3.72

446

2269

0.91

2.1

4.3

0.42

CH3CN

409

3.49

556

6464

0.88

4.0

2.2

0.31

cyclohexane

418

3.45

462

2278

0.89

2.3

3.8

0.45

CH3CN

420

3.05

604

7253

0.34

2.6

1.3

2.5

cyclohexane

405

3.65

444

2169

0.94

2.0

4.8

0.21

CH3CN

409

3.72

542

6000

0.88

3.5

2.7

0.20

cyclohexane

399

3.30

439

2284

0.96

2.3

4.0

0.26

CH3CN

401

3.30

531

6105

0.93

3.7

2.4

0.32

3b 3c 3d 4 7

aOnly the longest absorption maximum wavelengths are shown. bEmission maxima upon excitation at 380 nm for 3a–c, 4 and 7 and 400 nm for 3d. cAbsolute fluorescence quantum yields determined by a calibrated integrating sphere system within ±3% error.

the corresponding triarylboranes 3a-d as bright yellow solids in yields ranging from 17% to 44%. In turn, we also endeavored to synthesize triarylborane 4, which embodies a more sterically hindering aryl group than that of Tip group, namely, 1,3,5-tri-tert-butylphenyl (Mes*) group, by adopting the same protocol for the preparation of its Tip congener. However, the bis(diarylborinate) intermediate could not be obtained, probably due to excessive steric demandingness of the Mes* group, and thus the disturbance of nucleophilic aryl substitution to the boron center. We therefore employed the method reported by Jäkle, which led to successful synthesis of target compound 4 in three steps (Refer to Scheme S1 in the SI for the synthetic scheme).2b For comparison, relevant triarylborane analogue 7 possessing a dimesitylboryl acceptor moiety was also prepared according to the procedure reported by Shirota (Refer to Scheme S2 in the SI for the synthetic scheme).3a The structures of all triarylboranes were unequivocally characterized by multinuclear NMR spectroscopy and HRMS measurements. Each compound displayed a broad singlet at 60-63 ppm in the 11B NMR spectrum in CDCl3, which confirms the presence of a triarylborane moiety. It is noteworthy that the purification processes involved aqueous two-phase extraction as well as silica gel column chromatography in air, verifying that these triarylboranes have substantial stability towards air and moisture. With the synthesized triarylboranes in hand, we next examined the photophysical properties by recording the UV-vis absorption and fluorescence spectra in various solvents. Their data are summarized in Table 1 and the spectra of 3a are depicted in Figure 2 as a representative example, while the others can be found in the SI. In the absorption spectra in cyclohexane, for instance, all triarylboranes showed strong absorption bands centered at 399-418 nm. The absorption maximum wavelengths were almost unchanged irrespective to the solvent used, while their molar absorption coefficients tend to show subtle decrease as the solvent polarity increases. The small solvent effects suggest that the ground state of all compounds is steadily nonpolar and devoid of significant substituent effects. In stark contrast, the fluorescence spectra showed exceptional solvent dependence, a feature commonly seen in D-π-A fluorophores. In cyclohexane, 3a exhibited a

blue fluorescence with the maximum at 445 nm and a quantum yield ФF of 0.93. As the solvent polarity increased, the emission maximum shifted accordingly in the bathochromic direction and eventually reached λem = 554 nm in CH3CN, while maintaining a high ФF value of 0.89. The degree of such solvatochromism prominently increased along with enhanced electron-withdrawing nature of the para-substituted phenyl groups. The most electron-withdrawing 4-(trifluoromethyl)phenyl-substituted derivative 3d displayed the largest solvatochromic shift from λem = 462 nm in cyclohexane to λem = 604 nm in CH3CN. Its fluorescence quantum yield, however, suffered from a vast suppression with respect to increased solvent polarity (ФF = 0.89 in cyclohexane; ФF = 0.34 in CH3CN). 5

c-hexane acetone

4

CHCl3 CH3CN

CH2Cl2

Normalize fluorescence intensity

ε / 104 M -1cm-1

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


3 2 1 0 300

400

500 Wavelength / nm

600

700

Figure 2. UV-vis absorption and emission spectra of 3a in various solvents; dotted lines represent absorption, while solid lines show emission. The fact that the fluorescence quantum yields are remarkably high even in polar solvents despite the significant solvatochromic shift of λem is a notable virtue of the boron-based D-π-A fluorophores. This feature differs from the general trend of other D-π-A fluorophores. To gain deep insight into this characteristic, we assessed the relationship between the fluorescence quantum yields and the excited-state character. The dipole moments in the excited state (µE) were estimated by Lippert-Mataga method, where the Stokes shift values (Δʋ) and the orientation polarizability (Δf) of solvents were found



Finally, the photostability of the D-π-A type triarylboranes was assessed in dry and degassed acetonitrile by irradiation with a Xe discharge lamp (300 W) equipped with a 325 nm cut-off filter (λ = 460 nm, 100 W/m2). The change of the absorption maxima was monitored over time (See Figures S1217 in the SI for the absorption spectra). To uncover the electronic effects toward photostability, we compared the photodegradation rates of compounds 3a-d (Figure 3a). Notably, methoxyphenyl-substituted 3b was the most durable out of all four compounds and 4 h of irradiation induced the absorbance to decrease by 8%. This is in contrast to compound 3d, which persisted the least and decomposed by 30% under the same conditions. Considering how the absorbance of 3a and 3c lessened by 16% and 12%, respectively, it is obvious that electron-withdrawing components accelerate the photochemical reactions. Indeed, the rate of reaction increased concomitantly with respect to more positive Hammett substituent constants, which are ascending electron-withdrawing charac-

ter of the para-substituted groups (See Figure S18 in the SI for details). (a)

1

(b) 1

0.9

3a 3b 3c 3d

0.8

0.7

0

60 120 180 240 Irradiation time / min

Relative absorbance A/A 0

to be directly proportional to each other (R2 > 0.91), advocating the absence of perceptible interaction between the triarylboranes and solvent molecules. Compound 3d has the largest value of µE = 28.2 D compared to those of the rest (3a, 22.7 D; 3b, 22.8 D; 3c, 24.2 D), which verifies the presence of a most polarized excited state in 3d. While the µE values should be relevant to decrease in fluorescence quantum yields, the plot of ФF values for 3a-d in CH3CN as a function of µE values shows no linear relationship (See Figure S10 in the SI for the plot). The dynamics of the excited state are also linked to the rate constants of radiative (kr) and nonradiative (knr) decay from the lowest excited singlet state (S1), which are estimated based on the ФF values and the fluorescence lifetime τ. For compounds 3a-c, the kr values gradually declined along with increased solvent polarity. The plots of kr values as a function of the orientation polarizability Δf values of the solvents showed high linearity with a negative gradient slope (See Figure S11 in the SI for the plots). On the other hand, the knr values were rather unaffected and only decreased by a margin. This trend tends to be a unique yet significant feature of triarylborane-based D–p–A type fluorophores and may be responsible for the high solvent-independent ФF values of these compounds. This tendency, however, was not the same for compound 3d, which bears the strongest push-pull character. For instance, when the solvent changed from cyclohexane to acetonitrile, the kr value of 3d plummeted from 3.8 × 108 s-1 to 1.3 × 108 s-1, while the knr value increased from 0.45 × 108 s-1 to 2.5 × 108 s-1. The combination of decreased kr and increased knr values with respect to elevated solvent polarity is responsible for the relatively low ФF value of 3d particularly in polar solvents. Meanwhile, the fluorescence properties were compared among the sterically differing 3a, 4, and 7. It is worth noting that simply replacing the most frequently utilized dimesitylboryl group with (triisopropylphenyl)(phenyl)boryl (TipPhB) group results in a 20 nm red shift of the lem in CH3CN (554 nm and 531 nm for 3a and 7, respectively), while both maintaining the high fluorescence quantum yields. This result suggests the utility of TipPhB group for tuning the fluorescence wavelength without introducing any electron-withdrawing group. The corresponding red shift most likely results from the differences in the extent of structural relaxation in the excited state as well as electron-donating effect of the aryl group on the boron atom.

Relative absorbance A/A 0

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

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0.9

3a

0.8

4 7

0.7 0

60 120 180 240 Irradiation time / min

Figure 3. Plots of relative absorbance values of (a) 3a-d and (b) 3a, 4, and 7 in acetonitrile as a function of photoirradiation time to estimate the photostability. Plot (a) is to compare the electronic effects, while plot (b) is to compare the steric effects. In parallel, compounds 3a, 4, and 7 were examined to elucidate the correlation between photostability and steric effects (Figure 3b). After 4 h of photoirradiation, 86% of dimesitylboryl-susbstituted 7 persisted, which is comparable to that of 3a. Conversely, bulkier 1,3,5-tri-tert-butylphenylsubstituted 4 scarcely reacted and 98% of the fluorophore remained intact under the same irradiation time span. Since the excited-state character barely differs from one another, we conjecture that the phenomenal photostability of 4 in all likelihood arises from the sufficient steric protection about the boron center. In other words, the boron center may be involved in the photodegradation process. Alternatively, the absence of the benzylic protons may have inhibited the possibility of photoinduced cyclization via [1,6]-sigmatropic rearrangement, similar to what our group observed recently.10h Owing to sluggish reactivity, however, we were unable to access enough product for satisfactory structural characterization. Identification of the origin of the photodegradation as well as their corresponding products would therefore further improve the novel design of photostable D-π-A type triarylboranes.

CONCLUSION We described the synthesis, photophysical properties, and photostabilities of a series of D-π-A type triarylboranes built upon electron-donating triarylamine and unsymmetrically substituted electron-accepting triarylborane moieties. We showed a two-step synthesis of these compounds starting from a variety of arylboronic esters. An important feature of this compound class is the high fluorescence quantum yields retained even in polar solvents while showing significant solvatochromism in the fluorescence spectra. This characteristic is highly dependent on the degree of the excited-state polarization, which is relevant to the electron-accepting ability of the triarylborane moiety. The increased polarization of the excited states also decreases the photostability. As a way to improve the persistence against photoirradiation, we quantitatively measured and proved that sufficient steric protection around the boron center is effective. In this regard, the introduction of 1,3,5-tri-tert-butylphenyl ring, which is not only sterically bulky but also lacks benzylic protons, was


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The Journal of Organic Chemistry found to be greatly effective to prevent any possible photochemical reaction. As part of our ongoing interest, we will continue to investigate the utility of D-π-A type triarylborane-based fluorophores in OLEDs and bioimaging.

EXPERIMENTAL SECTION General Comments. Melting points (mp) or decomposition temperatures were determined by a Yanaco MP-S3 instrument (MP-S3). 1H, 13C{1H}, 11B{1H}, and 19F NMR spectra were recorded on a JEOL AL-400 or ECS-400 spectrometer in CDCl3 or acetone-d6 (400 MHz for 1H, 100 MHz for 13C, 128 MHz for 11B, and 376 MHz for 19F). The chemical shifts in 1H NMR spectra are reported in δ ppm using the residual proton of CHCl3 (7.26 ppm) in CDCl3, or acetone (2.05 ppm) in acetone-d6 as internal standards. The chemical shifts in 13C NMR spectra are reported using the solvent signals of CDCl3 (77.16 ppm) as internal standards. The chemical shifts in 11B NMR spectra are reported using BF3·OEt2 (0.00 ppm) as an external standard. The chemical shifts in 19F NMR spectra are reported using CF3COOH (–78.50 ppm) as an external standard. Mass spectra were taken on a Bruker microTOF Focus spectrometer with the APCI ionization method or a Thermo Fisher Scientific Exactive spectrometer with the ESI ionization method. Thin layer chromatography (TLC) was performed on plates coated with 0.25 mm thickness of silica gel 60F254 (Merck). Column chromatography was performed using PSQ100B (Fuji Silysia Chemicals). Recycling preparative HPLC was performed using LC-918 (Japan Analytical Industry) equipped with silica gel column (Wakosil-II 5-Prep, Wako). Recycling preparative gel permeation chromatography (GPC) was performed using LCForte/R (YMC TECHNOS CORPORATION) equipped with polystyrene gel columns (YMC-GPC T2000 and YMC-GPC T4000, YMC TECHNOS CORPORATION). Anhydrous solvents were purchased from Kanto Chemicals and further purified by Glass Contour Solvent Systems. 2-Bromo-5-[4-(N,Ndiphenylamino)phenyl]thiophene13 and 2,4,6-tri-tertbutylphenyllithium14 were prepared according to the literatures. Representative synthesis of bis(diarylborinate)s 6a–d (General Procedure 1). To a solution of arylboronic ester 5a– d in anhydrous THF (⁓0.4 M) was added a THF solution of 2,4,6-triisopropylphenylmagnesium bromide (0.42 M, 1.1 equiv) at room temperature, and the mixture was stirred at 70 °C for 3 h. The volatiles were removed under reduced pressure and the resulting mixture was dissolved in hexane and then filtered through Celite® to remove the magnesium salts. The filtrate was concentrated in vacuo and the mixture was subjected to silica gel column chromatography and further purified by recrystallization from EtOH to afford bis(diarylborinate)s 6a–d. Representative synthesis of triarylboranes 3a-d (General Procedure 2). To a solution of 2-bromo-5-[4-(N,Ndiphenylamino)phenyl]thiophene in anhydrous THF (0.5 M) was added dropwise a pentane solution of t-BuLi (1.64 M, 2 equiv) at –78 °C over 10 min. After stirring for 1 h to generate the corresponding lithium salt, a solution of 6a-d (0.55 equiv) in anhydrous THF (1.0 mL) was added at –78 °C and the mixture was further stirred at 0 °C for 18 h. Upon removal of the solvent under reduced pressure, the mixture was dissolved in hexane and filtered through Celite® to remove the lithium salts. The filtrate was concentrated in vacuo and was subsequently subjected to silica gel column chromatography followed by GPC or HPLC to afford the desired triarylboranes.

4,4-Dimethyl-1,7-diphenyl-1,7-bis(2,4,6-triisopropylphenyl)2,6-dioxa-1,7-diboraheptane (6a). This compound was synthesized by following General Procedure 1, starting from 5,5dimethyl-2-phenyl-1,3,2-dioxaborinane (compound 5a; 5.71 g, 30.1 mmol). Upon recrystallization, 6a (8.16 g, 11.9 mmol) was obtained in 79% yield as block-shaped colorless crystals: mp 149.1–149.9 °C; 1H NMR (400 MHz, acetone–d6) δ 7.66 (dt, J = 6.6 Hz, 1.6 Hz, 4H), 7.45 (tt, J = 7.4 Hz, 1.4 Hz, 2H), 7.35 (tt, J = 7.4 Hz, 1.4 Hz, 4H), 7.10 (s, 4H), 3.92 (s, 4H), 2.94 (sep, J = 6.8 Hz, 2H), 2.48 (sep, J = 6.8 Hz, 4H), 1.29 (d, J = 6.8 Hz, 12H), 1.21 (s, 6H), 1.16 (d, J = 6.8 Hz, 12H), 0.98 (d, J = 6.8 Hz, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 150.0, 149.1, 138.6, 135.5, 133.4, 131.3, 127.5, 120.3, 74.0, 37.5, 35.2, 35.0, 34.4, 24.9, 24.7, 24.2, 21.9; 11B{1H} NMR (128 MHz, CDCl3) δ 47.0; HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C47H67B2O2 685.5322; Found 685.5332. 1,7-Bis(4-methylphenyl)-4,4-dimethyl-1,7-bis(2,4,6triisopropylphenyl)-2,6-dioxa-1,7-diboraheptane (6b). This compound was synthesized by following General Procedure 1, starting from 2-(4-methoxyphenyl)-5,5-dimethyl-1,3,2dioxaborinane (5b) (6.61 g, 30.0 mmol). Upon recrystallization, 6b (8.45 g, 11.3 mmol) was obtained in 76% yield as block-shaped colorless crystals: mp 142.9–143.5 °C; 1H NMR (400 MHz, CDCl3) δ 7.60 (dt, J = 8.6 Hz, 2.4 Hz, 4H), 6.97 (s, 4H), 6.81 (dt, J = 8.6 Hz, 2.4 Hz, 4H), 3.81 (s, 6H), 3.80 (s, 4H), 2.92 (sep, J = 6.8 Hz, 2H), 2.48 (sep, J = 6.8 Hz, 4H), 1.29 (d, J = 6.8 Hz, 12H), 1.13 (d, J = 6.8 Hz, 12H), 1.13 (s, 6H), 0.98 (d, J = 6.8 Hz, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 162.2, 150.0, 149.0, 137.4, 133.8, 131.0, 120.2, 112.9, 73.9, 55.1, 37.5, 35.1, 34.4, 24.9, 24.7, 24.2, 21.9; 11B{1H} NMR (128 MHz, CDCl3) δ 47.0; HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C49H71B2O4 745.5533; Found 745.5518. 1,7-Bis(4-fluorophenyl)-4,4-dimethyl-1,7-bis(2,4,6triisopropylphenyl)-2,6-dioxa-1,7-diboraheptane (6c). This compound was synthesized by following General Procedure 1, starting from 2-(4-fluorophenyl)-5,5-dimethyl-1,3,2-dioxaborinane (5c) (6.25 g, 30.1 mmol). Upon recrystallization, 6c (8.61 g, 11.9 mmol) was obtained in 79% yield as blockshaped colorless crystals: mp 160.7–161.2 °C; 1H NMR (400 MHz, CDCl3) δ 7.66-7.63 (m, 4H), 6.98 (s, 4H), 6.97 (t, J = 8.8 Hz, 4H), 3.82 (s, 4H), 2.92 (sep, J = 6.8 Hz, 2H), 2.42 (sep, J = 6.8 Hz, 4H), 1.29 (d, J = 6.8 Hz, 12H), 1.13 (s, 6H), 1.12 (d, J = 6.8 Hz, 12H), 0.97 (d, J = 6.8 Hz, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 165.2 (d, J = 251.1 Hz), 150.0, 149.4, 137.7 (d, J = 7.7 Hz), 134.6, 133.1, 120.4, 114.6 (d, J = 20.0 Hz), 74.0, 37.4, 35.2, 34.4, 24.9, 24.7, 24.2, 21.9; 11B{1H} NMR (128 MHz, CDCl3) δ 46.5; 19F NMR (376 MHz, CDCl3) δ –108.6 (q, J = 9.0 Hz); HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C47H65B2F2O2 721.5133; Found 721.5152. 4,4-Dimethyl-1,7-bis[4-(trifluoromethyl)phenyl]-1,7-bis(2,4,6triisopropylphenyl)-2,6-dioxa-1,7-diboraheptane (6d). This compound was synthesized by following General Procedure 1, starting from 2-(4-trifluoromethylphenyl)-5,5-dimethyl-1,3,2dioxaborinane (5d) (7.62 g, 29.5 mmol). Upon recrystallization, 6d (8.85 g, 10.8 mmol) was obtained in 73% yield as block-shaped colorless crystals: mp 160.0–160.6 °C; 1H NMR (400 MHz, CDCl3) δ 7.76 (d, J = 7.8 Hz, 4H), 7.53 (d, J = 7.8 Hz, 4H), 7.00 (s, 4H), 2.93 (sep, J = 6.8 Hz, 2H), 2.40 (sep, J = 6.8 Hz, 4H), 1.30 (d, J = 6.8 Hz, 12H), 1.15 (s, 6H), 1.14 (d, J = 6.8 Hz, 12H), 0.98 (d, J = 6.8 Hz, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 150.1, 149.8, 142.2, 135.6, 132.9 (J = 32.2 Hz), 132.5, 124.4 (J = 272.0 Hz), 124.3 (J = 3.2 Hz), 120.6, 74.3, 37.5, 35.4, 34.5, 24.9, 24.8, 24.2, 21.9; 11B{1H} NMR (128 MHz, CDCl3) δ 47.1; 19F NMR (376 MHz, CDCl3) δ –62.6; HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C49H65B2F6O2 821.5069; Found 821.5070.



5-(2,4,6-Triisopropylphenyl)(phenyl)boryl-2-[4-(N,Ndiphenylamino)phenyl]thiophene (3a). This compound was synthesized according to General Procedure 2 using 6a (188 mg, 0.274 mmol) as the bis(borinate) precursor. After purification by silica gel chromatography (hexane/toluene = 3/1; Rf = 0.60) followed by preparative GPC (CHCl3), 3a (139 mg, 0.224 mmol) was obtained in 44% yield as a yellow solid: mp 170.1–171.1 °C; 1H NMR (400 MHz, acetone-d6) δ 7.94 (dt, J = 6.8 Hz, 1.2 Hz, 2H), 7.75 (d, J = 4.0 Hz, 1H), 7.71 (dt, J = 9.0 Hz, 2.4 Hz, 2H), 7.67 (d, J = 4.0 Hz, 1H), 7.57–7.54 (m, 1H), 7.52– 7.48 (m, 2H), 7.36–7.32 (m, 4H), 7.14–7.11 (m, 8H), 7.05 (dt, J = 9.0 Hz, 2.4 Hz, 2H), 2.96 (sep, J = 6.8 Hz, 1H) , 2.52 (sep, J = 6.8 Hz, 2H), 1.31 (d, J = 7.2 Hz, 6H) , 1.09 (d, J = 6.4 Hz, 6H), 1.01 (d, J = 6.8 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 156.9, 149.4, 148.5, 148.3, 147.4, 144.6, 144.3, 142.3, 140.2, 137.3, 131.7, 129.5, 127.9, 127.3, 124.9, 124.4, 123.5, 123.3, 120.1, 35.5, 34.3, 24.4, 24.4, 24.3 (1 signal for the carbon atoms were not observed due to the overlap with another signals); 11B NMR (128 MHz, CDCl3) δ 63.0; HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C43H45BNS 618.3366; Found 618.3356. 5-(2,4,6-Triisopropylphenyl)(4-methoxyphenyl)boryl-2-[4(N,N-diphenylamino)phenyl]thiophene (3b). This compound was synthesized according to General Procedure 2 using 6b (208 mg, 0.279 mmol) as the bis(borinate) precursor. After purification by silica gel column chromatography (hexane/toluene = 3/1; Rf = 0.43) followed by prepartive HPLC (hexane/toluene = 3/1), 3b (62.3 mg, 0.0962 mmol) was obtained in 19% yield as a yellow solid: mp 204.2–205.0 °C; 1H NMR (400 MHz, acetone–d6) δ 7.95 (dt, J = 8.8 Hz, 2.4 Hz, 2H), 7.71–7.62 (m, 3H), 7.62 (d, J = 3.6 Hz, 1H), 7.36–7.32 (m, 4H), 7.13–7.06 (m, 12H), 2.95 (sep, J = 6.8 Hz, 1H), 2.52 (sep, J = 6.8 Hz, 2H), 1.31 (d, J = 6.8 Hz, 6H), 1.07 (d, J = 6.8 Hz, 6H), 1.00 (d, J = 6.8 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 162.9, 155.9, 149.4, 148.3, 148.2, 147.5, 144.6, 143.7, 140.5, 140.0, 134.3, 129.5, 128.1, 127.3, 124.9, 124.2, 123.5, 123.4, 120.1, 113.4, 35.3, 34.3, 24.4, 24.4, 24.3 (3 signals for the carbon atoms bound to the boron atom were not observed due to the quadrupolar relaxation); 11B{1H} NMR (128 MHz, CDCl3) δ 60.3; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C44H46BNOS 648.3471; Found 648.3461. 5-(4-Fluorophenyl)(2,4,6-triisopropylphenyl)boryl-2-[4-(N,Ndiphenylamino)phenyl]thiophene (3c). This compound was synthesized according to General Procedure 2 using 6c (198 mg, 0.275 mmol) as the bis(borinate) precursor. After purification by silica gel column chromatography (hexane/toluene = 3/1; Rf = 0.55) followed by preparative GPC (CHCl3), 3c (54.8 mg, 0.0862 mmol) was obtained in 17% yield as a yellow solid: mp 196.2–196.9 °C; 1H NMR (400 MHz, acetone–d6) δ 8.03– 7.99 (m, 2H), 7.76 (d, J = 3.9 Hz, 1H), 7.71 (dt, J = 8.8 Hz, 2.4 Hz, 2H), 7.67 (d, J = 3.9 Hz, 1H), 7.36–7.32 (m, 4H), 7.28 (tt, J = 8.8 Hz, 2.4 Hz, 2H), 7.13–7.09 (m, 8H), 7.05 (dt, J = 8.8 Hz, 2.4 Hz, 2H) , 2.96 (sep, J = 6.8 Hz, 1H) , 2.49 (sep, J = 6.8 Hz, 2H), 1.31 (d, J = 6.8 Hz, 6H) , 1.08 (d, J = 6.8 Hz, 6H), 1.00 (d, J = 6.8 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 165.5 (J = 253.0 Hz), 157.0, 149.4, 148.7, 148.4, 147.4, 144.3, 139.8 (J = 8.3 Hz), 129.5, 127.8, 127.3, 124.9, 124.4, 123.6, 123.2, 120.2, 115.1 (J = 19.8 Hz), 35.5, 34.3, 24.4, 24.4, 24.3 (3 signals for the carbon atoms bound to the boron atom were not observed due to the quadrupolar relaxation); 11B{1H NMR (128 MHz, CDCl3) δ 61.7; 19F NMR (376 MHz, CDCl3) δ –107.4 (q, J = 9.0 Hz); HRMS (ESITOF) m/z: [M + H]+ Calcd for C43H44BFNS 636.3266; Found 636.3239. 5-[4-(Trifluoromethyl)phenyl](2,4,6triisopropylphenyl)boryl-2-[4-(N,Ndiphenylamino)phenyl]thiophene (3d). This compound was

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synthesized according to General Procedure 2 using 6d (188 mg, 0.274 mmol) as the bis(borinate) precursor. After purification by silica gel column chromatography (hexane/toluene = 3/1; Rf = 0.60) followed by preparative GPC (CHCl3), 3d (139 mg, 0.203 mmol) was obtained in 40% yield as a yellow solid: mp 189.2–190.0 °C; 1H NMR (400 MHz, acetone-d6) δ 8.09 (d, J = 8.0 Hz, 2H), 7.85 (d, J = 8.0 Hz, 2H), 7.82 (d, J = 3.9 Hz, 2H), 7.73–7.71 (m, 3H), 7.38–7.33 (m, 4H), 7.14–7.10 (m, 8H), 7.05 (dt, J = 8.0 Hz, J = 2.4 Hz, 2H), 2.96 (sep, J = 8.6 Hz, 1H), 2.51 (sep, J = 8.6 Hz, 2H), 1.31 (d, J = 8.6 Hz, 6H), 1.11 (d, J = 8.6 Hz, 6H), 1.02 (d, J = 8.6 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 158.2, 149.4, 149.0, 148.7, 147.3, 146.2, 144.9, 144.1, 139.4, 136.8, 132.6 (J = 32.2 Hz), 129.5, 127.5, 127.4, 125.0, 124.6, 124.5 (J = 4.1 Hz), 123.7, 123.0, 120.3, 35.7, 34.4, 24.5, 24.4, 24.3 (the signal corresponding to the CF3 carbon atom was not observed due to overlap with other signals); 11B{1H} NMR (128 MHz, CDCl3) δ 62.1; 19F NMR (376 MHz, CDCl3) δ –62.6; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C44H44BF3NS 686.3240; Found 686.3217. Phenyl(thiophen-2-yl)(2,4,6-tri-tert-butylphenyl)borane (S8). This compound was synthesized following a procedure reported previously.2b To a solution of BBr3 (4.23 g, 16.9 mmol) in anhydrous CH2Cl2 (2.5 mL) was added dropwise a CH2Cl2 solution (25 mL) of 2-trimethylsilylthiophene (2.36 g, 15.1 mmol) at 0 °C across 30 min and the mixture was stirred at room temperature for 6 h. After all volatiles were removed under reduced pressure, the mixture was dissolved in anhydrous toluene (8.0 mL). The solution was then cooled down to -78 °C and a toluene solution (8.0 mL) of trimethyl(phenyl)tin (2.7 mL, 15.0 mmol) was added dropwise over 1 h. The reaction mixture was gradually warmed up to ambient temperature and was allowed to stir for another 13 h. The volatile components were again removed in vacuo first at ambient temperature then at 60 °C for 6 h to afford a dark red oil. The oil was taken up to anhydrous toluene (6.0 mL) and a toluene solution (10 mL) of 2,4,6-tri-t-butylphenyllithium was subsequently added at ambient temperature and stirred for 84 h. After quenching with water (50 mL), the organic layer was separated, and the aqueous layer was extracted with ethyl acetate (50 mL × 2). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The mixture was subjected to silica gel column chromatography (hexane/CH2Cl2 = 5/1; Rf = 0.65), and further purified by recrystallization from EtOH to afford 4 (1.07 g, 2.57 mmol) in 18% yield as block-shaped colorless crystals: mp 163.7–164.5 °C; 1H NMR (400 MHz, CDCl3) δ 8.09 (d, J = 5.9 Hz, 2H), 7.84 (dd, J = 4.8 Hz, 0.80 Hz, 1H), 7.62 (d, J = 3.4 Hz, 1H), 7.46–7.38 (m, 5H), 7.21–7.19 (m, 1H), 1.39 (s, 9H), 1.11 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3) δ 151.7, 148.7, 148.1, 143.8, 142.1, 138.2, 136.5, 135.8, 131.0, 128.4, 127.9, 122.5, 38.6, 35.1, 34.9, 31.7; 11B{1H} NMR (128 MHz, CDCl3) δ 58.3; HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C28H38BS 417.2782; Found 417.2801. (5-Bromothiophen-2-yl)(phenyl)(2,4,6-tri-tertbutylphenyl)borane (S9). To a solution of S8 (210 mg, 0.504 mmol) in anhydrous THF (1.0 mL) was added dropwise a THF solution of LDA (0.50 M, 1.10 mL, 0.550 mmol) at -78 °C across 10 min. After the mixture was stirred for 1 h, a solution of 1,2-dibromotetrachloroethane (176 mg, 0.539 mmol) in anhydrous THF (1.0 mL) was successively added at -78 °C over 5 min and further stirred at room temperature for 2 h. Subsequently, the reaction mixture was quenched with water (20 mL), the organic layer was separated, and the aqueous layer was extracted with ethyl acetate (20 mL × 2). The combined organic layer was washed with brine, dried over anhy-


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The Journal of Organic Chemistry drous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was subjected to silica gel column chromatography (hexane/CH2Cl2 = 5/1; Rf = 0.70), and further purified by recrystallization from EtOH to afford S9 (208 mg, 0.419 mmol) in 83% yield as block-shaped colorless crystals: mp. 159.5-160.5 °C; 1H NMR (400 MHz, CDCl3) δ 8.02 (d, J = 6.6 Hz, 2H), 7.47–7.38 (m, 5H), 7.34 (d, J = 3.4 Hz, 1H), 7.14 (d, J = 3.4 Hz, 1H), 1.38 (s, 9H), 1.11 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3) δ 151.8, 150.8, 148.9, 143.2, 142.0, 138.0, 135.3, 131.6, 131.2, 128.0, 123.8, 122.6, 38.6, 35.1, 34.9, 31.6; 11B{1H} NMR (128 MHz, CDCl3) δ 59.1; HRMS (APCI-TOF) m/z: [M + H]+ Calcd for C28H37BBrS 495.1887; Found 495.1903. 5-(2,4,6-Tri-tert-butylphenyl)(phenyl)boryl-2-[4-(N,Ndiphenylamino)phenyl]- thiophene (4). To a solution of S9 (166 mg, 0.336 mmol) and 4-(diphenylamino)phenylboronic acid (108 mg, 0.374 mmol) in toluene/water (3.0 mL/3.0 mL) was added Pd2(dba)3·CHCl3 (19.1 mg, 18.5 µmol), Xantphos (21.2 mg, 36.6 µmol) and Na2CO3 (40.5 mg, 0.382 mmol), and the mixture was stirred at 100 °C for 16 h. After addition of water (50 mL), the organic layer was separated, and the aqueous layer was extracted with ethyl acetate (50 mL × 2). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was subjected to silica gel column chromatography (hexane/CH2Cl2 = 5/1; Rf = 0.70), and further purified by GPC (CHCl3) to afford 6 (139 mg, 0.210 mmol) in 63% yield as a yellow solid: mp 187.8–188.8 °C; 1H NMR (400 MHz, acetone-d6) δ 8.09 (m, 2H), 7.70 (d, J = 8.8 Hz, 2H), 7.60– 7.47 (m, 7H), 7.36–7.31 (m, 4H), 7.13–7.04 (m, 8H), 1.40 (s, 9H), 1.17 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3) δ 154.6, 151.7, 148.5, 148.1, 147.5, 146.7, 144.0, 143.5, 138.1, 136.3, 130.8, 129.5, 128.2, 127.8, 127.1, 124.8, 123.7, 123.5, 123.4, 122.5, 38.6, 35.1, 34.9, 31.6; 11B{1H} NMR (128 MHz, CDCl3) δ 60.0; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C46H51BNS 660.3835; Found 660.3804. 5-Bis(2,4,6-trimethylphenyl)boryl-2-[4-(N,Ndiphenylamino)phenyl]thiophene (7). This compound was synthesized by adopting a procedure previously reported by Shirota.3a To a solution of S10 (301 mg, 0.755 mmol) in anhydrous THF (2.0 mL) was added dropwise a hexane solution of n-BuLi (1.63 M, 0.460 mL, 0.750 mmol) at -78 °C over 3 min. After the mixture was stirred for 3 h, a solution of dimesitylboryl fluoride (204 mg, 0.759 mmol) in anhydrous THF (1.0 mL) was added at -78 °C and gradually warmed up to room temperature. The mixture was stirred for an additional 19 h and was subsequently quenched with water (20 mL). The organic layer was separated, and the aqueous layer was extracted with ethyl acetate (20 mL × 2). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was subjected to silica gel column chromatography (hexane/toluene = 3/1; Rf = 0.43), and further purified by HPLC (hexane/toluene = 10/1) to afford 7 (296 mg, 0.514 mmol) in 68% yield as a yellow solid: mp 148.7-149.5 °C; 1H NMR (400 MHz, acetone-d6) δ 7.67 (dt, J = 4.4 Hz, 2.4 Hz, 2H), 7.60 (d, J = 2.0 Hz, 1H), 7.39 (d, J = 2.2 Hz, 1H), 7.36–7.31 (m, 4H), 7.13–7.08 (m, 6H), 7.02 (dt, J = 4.2 Hz, 2.4 Hz, 2H), 6.85 (s, 4H), 2.28 (s, 6H), 2.14 (s, 12H); 13C{1H} NMR (100 MHz, CDCl3) δ 157.3, 148.6, 148.3, 147.4, 142.1, 141.4, 141.0, 138.5, 129.5, 128.3, 128.0, 127.1, 124.9, 124.6, 123.5, 123.3, 23.6, 21.4; 11B{1H} NMR (128 MHz, CDCl3) δ 64.9; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C40H39BNS 576.2891; Found 576.2889.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthetic schemes for compounds S8, S9, 4, and 7, X-ray crystallographic analysis of compound 6a, photophysical properties of all compounds, Lippert-Mataga diagrams and related data for all compounds, plots of kr vs Δf for compounds 3a-d, photostability assessment of all compounds, detailed theoretical calculations for all compounds, and NMR spectra of all compounds X-ray data for compound 6a

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Shigehiro Yamaguchi: 0000-0003-0072-8969 Masato Hirai: 0000-0002-3498-308X

Author Contributions §These authors contributed equally.

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI grants 15H02163 and 18H03909. ITbM is supported by the World Premier International Research Center (WPI) Initiative, Japan.

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