D-π-A triarylboron compounds with tunable push-pull character achieved by modification of both the donor and acceptor moieties

DOI: 10.1002/chem.201405621

Source and publish data:

Chemistry - A European Journal p. 177 - 190 (2015)

Update date:2022-08-29

Topics:

Authors:

Zhang, Zuolun

Edkins, Robert M.

Nitsch, J?rn

Fucke, Katharina

Eichhorn, Antonius

Steffen, Andreas

Marder, Todd B.

Wang, Yue

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Article abstract of DOI:10.1002/chem.201405621

The push-pull character of a series of donor-bithienyl-acceptor compounds has been tuned by adopting triphenylamine or 1,1,7,7-tetramethyljulolidine as a donor and B(2,6-Me2-4-RC₆H₂)₂ (R=Me, C₆F₅ or 3

Full text of DOI:10.1002/chem.201405621

DOI: 10.1002/chem.201405621

Full Paper

Fluorescence |Very Important Paper|

D–p–A Triarylboron Compounds with Tunable Push–Pull Character

Achieved by Modification of Both the Donor and Acceptor

Moieties

Zuolun Zhang,^[a]Robert M. Edkins,^[a]Jçrn Nitsch,^[a]Katharina Fucke,^{[a, b]}Antonius Eichhorn,^[a]

Andreas Steffen,^[a]Yue Wang,^[c]and Todd B. Marder*^[a]

Abstract: The push–pull character of a series of donor–bi-

thienyl–acceptor compounds has been tuned by adopting

triphenylamine or 1,1,7,7-tetramethyljulolidine as a donor

and B(2,6-Me₂-4-RC₆H₂)₂(R=Me, C₆F₅or 3,5-(CF₃)₂C₆H₃) or

B[2,4,6-(CF₃)₃C₆H₂]₂as an acceptor. Ir-catalyzed CꢀH boryl-

ation was utilized in the derivatization of the boryl acceptors

and the tetramethyljulolidine donor. The donor and acceptor

strengths were evaluated by electrochemical and photo-

physical measurements. In solution, the compound with the

strongest acceptor, B[2,4,6-(CF₃)₃C₆H₂]₂((FMes)₂B), has strong-

ly quenched emission, while all other compounds show effi-

cient green to red (F_F=0.80–1.00) or near-IR (NIR; F_F=

0.27–0.48) emission, depending on solvent. Notably, this

study presents the first examples of efficient NIR emission

from three-coordinate boron compounds. Efficient solid-

state red emission was observed for some derivatives, and

interesting aggregation-induced emission of the (FMes)₂B-

containing compound was studied. Moreover, each com-

pound showed a strong and clearly visible response to fluo-

ride addition, with either a large emission-color change or

turn-on fluorescence.

Introduction

erties of this type of boron compounds: 1) Higher HOMO and

lower LUMO energies arising from stronger donors and accept-

ors, respectively, can enhance the carrier-injecting properties of

the materials in OLEDs,^[12]which may improve device per-

formance; 2) smaller HOMO–LUMO gaps may lead to red/near-

infrared (NIR) emissive materials, which are rare for three-coor-

dinate boron-based systems, particularly in the solid state;^[3i,j,13]

3) red-shifted emission, caused by a stronger acceptor, may in-

crease the difference in emission color before and after fluo-

ride or cyanide coordination to the boron center and, thus, fa-

cilitate naked-eye detection of these important analytes.^[4c]

Therefore, we were motivated to develop organoboron com-

pounds with strong push–pull character by strengthening the

acceptor and/or donor groups. Up until now, many different

donors have been employed in this type of compound;^[8–11]

however, the boryl acceptor is still quite limited, with the

group bis(mesityl)boryl ((Mes)₂B) typically used, in which the

sterically bulky mesityl substituents decrease the reactivity of

the boron center towards nucleophiles, providing air stability.

Although a few attempts to tune the boryl acceptors have

been reported,^[14]most of the modified boryl groups do not

provide sufficient air stability for the boron compounds, due to

a lack of steric protection of the boron center.^[4b,14a–d]

Three-coordinate organoboron compounds are an important

class of materials for optical and optoelectronic applica-

tions.^[1–7]In particular, donor–(p-spacer)–acceptor (D–p–A) or-

ganoboron systems with an aromatic amine donor group and

a boryl acceptor group have attracted tremendous research in-

terest.^[8–11]Such push–pull systems usually show strong intra-

molecular charge-transfer (ICT) emission, which make them

promising materials for applications in nonlinear optics

(NLO),^[8a,9]organic light-emitting diodes (OLEDs),^[10]and anion

sensing.^[11]Modification of each of the donor, p-spacer, or ac-

ceptor groups has been shown to be effective for tuning the

properties of the materials.^[8–11]Considering the “push–pull”

structure and the ICT character of the excited states of these

D–p–A compounds, the incorporation of strong donor and/or

acceptor groups is important for improving some of the prop-

[a] Dr. Z. Zhang, Dr. R. M. Edkins, J. Nitsch, Dr. K. Fucke, A. Eichhorn,

Dr. A. Steffen, Prof. Dr. T. B. Marder

Institut fꢀr Anorganische Chemie, Julius-Maximilians-Universitꢁt Wꢀrzburg

Am Hubland, 97074 Wꢀrzburg (Germany)

E-mail: todd.marder@uni-wuerzburg.de

[b] Dr. K. Fucke

School of Medicine, Pharmacy and Health, Durham University

University Boulevard, Stockton-on-Tees, TS17 6BH (UK)

Recently, we introduced the strongly electron-accepting bis-

(fluoromesityl)boryl ((FMes)₂B) group into D–p–A systems.^[15]In

the present study, we modified the (Mes)₂B group through

substitution of the methyl substituents para to the boron

atom by electron-withdrawing perfluorophenyl and 3,5-bis(tri-

fluoromethyl)phenyl substituents, to produce the acceptor

[c] Prof. Dr. Y. Wang

State Key Laboratory of Supramolecular Structure and Materials

College of Chemistry, Jilin University, Changchun 130012 (P.R. China)

Supporting information for this article is available on the WWW under

http://dx.doi.org/10.1002/chem.201405621.

Chem. Eur. J. 2014, 20, 1 – 15

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groups (2,6-Me₂-4-C₆F₅-C₆H₂)₂B ((Pfp)₂B) and (2,6-Me₂-4-(3,5-

(CF₃)₂-C₆H₃)-C₆H₂)₂B ((Tfp)₂B), respectively (Scheme 1). To allow

comparison of the acceptor strength of the known (Mes)₂B

and (FMes)₂B groups with the newly obtained (Pfp)₂B and

(Tfp)₂B groups, as well as to systematically study their influence

on material properties, compounds 1–4 with the same triphe-

nylamine (Tpa) donor group and each of these acceptors have

been synthesized. Furthermore, to elucidate the influence of

enhanced push–pull character, 5 and 6 have been prepared,

which are analogues of 2 and 3 with a stronger 1,1,7,7-tetra-

thienyl–Li reagents were prepared by lithiation of the products

of the Suzuki–Miyaura cross-coupling reactions of TpaꢀBr with

[(2,2’-bithiophene)-5-yl]boronic acid or TmjulꢀBpin with 5-

iodo-2,2’-bithiophene. TmjulꢀBpin was obtained in high yield

(88%) by regioselective iridium-catalyzed CꢀH borylation^[17]of

TmjulꢀH on a multigram scale in hexane with a simple workup

of filtration followed by washing with cold solvents (the crystal

structure is shown in the Supporting Information, Figure S1).

The CꢀH borylation of TmjulꢀH is sterically controlled and,

consequently, highly selective, leading to exclusive borylation

at the 9-position, that is, para to the nitrogen atom. This pro-

vides a facile and rapid route to the complementary nucleo-

philic coupling partner to 9-bromojulolidine, a compound that

is widely used in the construction of D–p–A chromophores.^[18]

Furthermore, we note that simple adaption of the method

might also be expected to expedite the synthesis of related

Bpin-substituted planarized triphenylamines^[19]and 2,6-substi-

tuted anilines^[20]of pharmaceutical relevance, both of which

otherwise require a two-step procedure of bromination and

Pd-catalyzed Miyaura borylation. Key precursors BrꢀPfp and

BrꢀTfp, required for modifying the boryl moiety, were prepared

by Ir-catalyzed CꢀH borylation of 2,6-dimethylbromobenzene

at the 4-position in 95% yield on a multigram scale, and subse-

quent Suzuki–Miyaura cross coupling with pentafluoroiodo-

benzene or 3,5-bis(trifluoromethyl)iodobenzene in 79–80%

yield; notably, the borylation reaction was regiospecific and

was, therefore, pivotal to ensuring high yields of the isolated

products. The Grignard reagents prepared from these two pre-

cursors were reacted with BF₃·OEt₂to produce the R₂BF precur-

sors for the synthesis of 2, 3, 5 and 6. These compounds were

Scheme 1. Molecular structures of 1–6.

methyljulolidine (Tmjul) donor

group in conjunction with the

(Pfp)₂B and (Tfp)₂B acceptor

groups. In all of these com-

pounds, 5,5’-substituted 2,2’-bi-

thienyl was chosen as the p-

spacer, because its electron-rich

character and often close-to-co-

planar structure were expected

to facilitate long-wavelength

emission.^[16]

Results and Discussion

Synthesis

The procedures used to synthe-

size compounds 1–6 are sum-

marized in Scheme 2. 1–6 were

obtained by reactions between

the appropriate Tpa/Tmjul–bi-

thienyl–Li and R₂BF precursors in

Scheme 2. Syntheses of compounds 1–6. a) B₂pin₂, [{Ir(m-OMe)(cod)}₂] (cod=1,5-cyclooctadiene), 4,4’-di-tert-butyl-

2,2’-bipyridine (dtbpy), hexane, 808C; b) C₆F₅I or 3,5-(CF₃)₂C₆H₃I, Ag₂CO₃, [Pd(PPh₃)₄], THF, reflux; c) Mg, C₂H₄Br₂,

THF, RT to 608C; d) BF₃·OEt₂, 08C to 608C; e) [(2,2’-bithiophene)-5-yl]boronic acid, [Pd(PPh₃)₄], Na₂CO₃, THF, H₂O,

reflux; f) nBuLi, THF, ꢀ788C; g) (Mes)₂BF/(Tfp)₂BF, ꢀ788C to RT or (Pfp)₂BF, 08C to RT; h) (FMes)₂BF, toluene, ꢀ258C

an adaption of the typical

method for synthesizing three-

coordinate organoboron com-

pounds.^[4]The Tpa/Tmjul–bi- to RT; i) 5-iodo-2,2’-bithiophene, Ag₂CO₃, [Pd(PPh₃)₄], THF, reflux; j) (Pfp)₂BF, 08C to RT or (Tfp)₂BF, ꢀ788C to RT.

Chem. Eur. J. 2014, 20, 1 – 15

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characterized by multinuclear NMR spectroscopy, mass spec-

trometry, and elemental analysis. The room temperature ¹⁹F{¹H}

NMR spectrum of 4 shows three broad peaks at ꢀ50.4, ꢀ52.1,

and ꢀ56.5 ppm (1:1:2), for the four CF₃groups of the FMes

moieties that are in positions ortho to the boron atom, and

one singlet at ꢀ63.4 ppm, for the two CF₃groups at the para-

positions (see the Supporting Information, Figure S4), which in-

dicates restricted rotation of the boron-bonded aryl rings. A

similar phenomenon has been observed in our studies of Arꢀ

B(FMes)₂compounds.^[15]At 223 K, the signals for the six CF₃

groups are fully separated, resulting in four quartets (two pairs,

J=11 and 13 Hz, respectively) and two singlets for the ortho-

and para-CF₃groups, respectively (Figure S4). This is different

from the behavior of TpaꢀB(FMes)₂,^[15]which only displays two

quartets and one singlet at 223 K for the ortho- and para-CF₃

groups, respectively. The loss of the C₂symmetry along the Bꢀ

C(thienyl) bond in compound 4, accompanied by restricted ro-

tation, is responsible for the complete inequality of the six CF₃

groups at low temperature, and the observed splitting of the

signals into quartets for the ortho-CF₃groups is due to

through-space ¹⁹F–¹⁹F coupling.^[15]

Crystal structures

To confirm the molecular structures and to investigate the in-

fluence of different donors and acceptors on the molecular

conformations, single crystals of these compounds were

grown and their structures were determined by X-ray diffrac-

tion. Single crystals of 2 were obtained by cooling the seeded

melt and those of 4 were grown by crystallization from hexane

solution at ꢀ308C. The crystal structures are shown in

Figure 1, and selected bond lengths and dihedral angles are

given in Table 1. The boron atoms in both structures have

a nearly perfect trigonal planar configuration, with the sum of

the three C-B-C angles close to 3608. In 4, the B1ꢀC1

(1.605(4) ꢁ) and B1ꢀC9 (1.601(4) ꢁ) bonds in the boryl group

are significantly longer than the B1ꢀC17 bond (1.518(4) ꢁ),

while the corresponding differences between the two types of

BꢀC bonds in 2 are smaller (ca. 0.03–0.05 ꢁ). It has been re-

ported previously that replacement of (Mes)₂B, for example, by

more electron-withdrawing boryl groups, such as (C₆F₅)₂B^[14a,b]

and (FMes)₂B,^[15]leads to an increase in such bond-length dif-

ferences. Therefore, the large bond-length difference in 4 re-

flects the stronger acceptor strength of (FMes)₂B than (Pfp)₂B.

In 2, the dihedral angles between the BC₃plane and the 2,4,6-

trisubstituted phenyl rings (P1 and P2) are similar to each

other (59.9(3) and 57.8(2)8) and to the analogous dihedral

angles (ca. 57–598) in the compound 4-Me₂NꢀC₆H₄ꢀCH=CH-

2’,5’-thienylꢀB(Mes)₂,^[9o]whereas the related dihedral angles in

4 show a large difference (67.5(2) and 43.9(2)8). The dihedral

angles between the two thienyl rings (T1 and T2) are 19.0(2)

and 12.7(2)8 in 2 and 4, respectively, allowing good conjuga-

tion. The dithienyl p system is also relatively planar with re-

spect to the adjacent BC₃plane and the diphenylamine-substi-

tuted phenyl ring (P3), exhibiting dihedral angles of 15.8(2) in

2 and 19.1(2)8 in 4 between T1 and the BC₃plane and 25.5(2)

in 2 and 18.6(1)8 in 4 between T2 and P3. In compound 2, the

Figure 1. Molecular structures of 2 (top) and 4 (bottom) from single-crystal

X-ray diffraction. Hydrogen atoms are omitted for clarity. The phenyl rings

related to the discussion are labeled P1–P5, and the thienyl rings are labeled

T1 and T2. Element (color): carbon (gray), nitrogen (blue), sulfur (yellow),

boron (orange), and fluorine (green).

Table 1. Selected bond lengths [ꢁ] and dihedral angles [8] for 2 and 4, as

obtained by single-crystal X-ray diffraction.

B1ꢀC1

1.584(9)

1.565(9)

1.532(9)

59.9(3)

57.8(2)

15.8(2)

19.0(2)

25.5(2)

51.6(3)

61.8(2)

1.605(4)

1.601(4)

1.518(4)

67.5(2)

43.9(2)

19.1(2)

12.7(2)

18.6(1)

–

B1ꢀC15/B1ꢀC9

B1ꢀC29/B1ꢀC17

aP1ꢀBC3 plane

aP2ꢀBC3 plane

aT1ꢀBC3 plane

aT1ꢀT2

aT2ꢀP3

aP1ꢀP4

aP2ꢀP5

–

two C₆F₅groups (P4 and P5) and the adjacent phenyl rings (P1

and P2, respectively) exhibit dihedral angles of 51.6(3) and

61.8(2)8 and are, thus, expected to be relatively poorly conju-

gated in the solid state.

Absorption spectra

In toluene, all compounds gave rise to a broad and structure-

less lowest-energy absorption band with large molecular ex-

ꢀ1

tinction coefficients ranging from 32000 to 42000m^ꢀ1cm

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(Figure 2). Considering the push–pull structures of these com-

pounds, their absorption bands are assigned as ICT transitions.

The absorption maxima (l_abs) of 1–3 are at 433, 452, and

457 nm, respectively (Table 2). The very close values of l_absfor

2 and 3 indicates that (Pfp)₂B and (Tfp)₂B have similar acceptor

significantly red-shifted to 507 nm (a further 2200 cm^ꢀ1), indi-

cating that (FMes)₂B is a much stronger acceptor than (Mes)₂B,

(Pfp)₂B, or (Tfp)₂B. By enhancing the donor strength, the l_absof

5 (484 nm) and 6 (487 nm) are red-shifted by approximately

30 nm (1400 cm^ꢀ1) compared to those of the corresponding

Tpa-substituted analogues, 2 and 3, but still not to the same

extent as 4. The l_absof these compounds are slightly depen-

dent on the solvent polarities: from toluene to CH₃CN, a nega-

tive shift of 5–23 nm was observed.

strengths.

A spectral red shift of approximately 20 nm

(1000 cm^ꢀ1) from 1 to 2 and 3 shows the effect of increased

acceptor strength; however, the absorption maximum of 4 is

DFT and TD-DFT calculations

To understand further the electronic structures of these com-

pounds, and to examine the orbitals involved in the electronic

transitions, we carried out DFT (B3LYP/6-31G(d)) and time-de-

pendent DFT (TD-DFT; CAM-B3LYP/6-31G(d)) calculations for 1,

2, 4 and 5. Considering the quite similar profiles and peak

wavelengths of the absorption spectra of 2 and 3, and 5 and

6, the electronic structures of 3 and 6 are expected to be simi-

lar to those of 2 and 5, respectively. TD-DFT calculations show

that the S₁S₀transitions of these compounds have large os-

cillator strengths, greater than or equal to 1.466, and the exci-

tation wavelengths for the S₁S₀transitions are 392, 405, 424,

and 410 nm for 1, 2, 4, and 5, respectively (Table 3). These cal-

culated values overestimate the experimental l_absvalues by

0.30–0.46 eV, which is not untypical for such systems.^[9j,15]Im-

Figure 2. UV/Vis absorption spectra of 1–6 in toluene.

Table 2. Photophysical data for 1–6 in solution and in the solid state at room temperature.

[b]

Compound

Medium

l_abs[nm]^[a]

l_em[nm]

Stokes shift [cm^ꢀ1

]

F_F

t_F[ns]

k_r[10⁷s^ꢀ1

]

k_nr[10⁷s^ꢀ1

]

toluene

THF

CH₃CN

solid

433

428

–

504

546

599

548

3300

4800

6700

–

0.95

0.96

0.92

0.30

1.87

2.46

3.20

–

50.8

39.0

28.8

–

2.67

1.63

2.50

–

toluene

THF

CH₃CN

solid

452

449

439

–

530

583

638

545

3300

5100

7100

–

0.98

1.00

0.96

0.46

2.19

2.91

3.59

–

44.7

34.3

26.7

–

0.91

0.00

1.11

–

toluene

THF

CH₃CN

solid

457

451

442

–

535

583

636

563

3200

5000

6900

–

0.96

0.93

0.85

0.27

2.23

2.89

3.51

–

43.0

32.1

24.2

–

1.79

2.42

4.27

–

[c]

toluene

THF

CH₃CN

solid

507

495

484

–

644

4200

–

[c]

–

[c]

646

–

0.31

–

toluene

THF

CH₃CN

solid

484

478

–

582

664

745

635

3900

5600

7500

–

0.93

0.87

0.48

0.23

2.69

3.53

2.58

–

34.6

24.6

18.6

–

2.60

3.68

20.2

–

toluene

THF

CH₃CN

solid

487

485

474

–

588

666

744

618

3500

5600

7700

–

0.95

0.80

0.27

0.05

2.76

3.42

1.56

–

34.4

23.4

17.3

–

1.81

5.85

46.8

–

[a] Lowest-energy absorption maximum; [b] absolute fluorescence quantum yields measured using an integrating sphere; [c] not determined due to very

weak emission.

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Table 3. TD-DFT-calculated photophysical data for 1, 2, 4 and 5 at the

CAM-B3LYP/6-31G(d) level.

Transition [f]

E [eV]^[a]

l [nm]^[a]

Dominant components^[b]

S₁S₀(1.618)

3.16 (2.86)

392 (433)

LUMO HOMO (70%)

LUMO HOMOꢀ1 (20%)

S₁S₀(1.676)

3.06 (2.74)

2.93 (2.50)

3.02 (2.56)

405 (452)

424 (507)

410 (484)

LUMO HOMO (66%)

LUMO HOMOꢀ1 (22%)

S₁S₀(1.470)

LUMO HOMO (63%)

LUMO HOMOꢀ1 (23%)

S₁S₀(1.466)

LUMO HOMO (77%)

LUMO HOMOꢀ1 (14%)

[a] Values in parentheses are experimental longest-wavelength absorption

maxima in toluene; [b] components with greater than 10% contribution

shown. Percentage contribution approximated by 2ꢂ(c_i)²ꢂ100%, where

c_iis the coefficient for the particular orbital rotation.

Figure 4. Cyclic voltammograms of 1–6. Oxidation and reduction processes

were measured in CH₂Cl₂and THF, respectively.

portantly, the results reproduce the variation in observed

values of l_abs. According to the calculations, for all three com-

Table 4. Cyclic voltammetric data^[a]and experimental HOMO and LUMO

energies.

pounds, the S₁S₀transitions are predominantly LUMO

HOMO with a small contribution from LUMO HOMOꢀ1. As

can be seen from Figure 3, the HOMO and HOMOꢀ1 are dis-

tributed on the donor and the bithienyl spacer for all com-

pounds, whereas the LUMO is mainly distributed on the ac-

ceptor and the bithienyl spacer. Therefore, these transitions

possess ICT character, which is consistent with our assignment

of the experimental lowest-energy absorption bands as being

due to ICT.

[V]^[b]

[V]^[c]

HOMO [eV]^[d]

LUMO [eV]^[d]

E_g[eV]^[e]

1=2

red

+0.39

+0.41

+0.43

+0.04

ꢀ2.23

ꢀ2.04

ꢀ1.61

ꢀ2.10

ꢀ2.11

ꢀ5.19

ꢀ5.21

ꢀ5.23

ꢀ4.84

ꢀ2.57

ꢀ2.76

ꢀ3.19

ꢀ2.70

ꢀ2.69

2.62 (2.53)

2.45 (2.42)

2.45 (2.39)

2.04 (2.14)

2.14 (2.24)

2.15 (2.21)

[a] Potentials are given vs. ferrocene/ferrocenium (Fc/Fc⁺); [b] measured

in CH₂Cl₂; [c] measured in THF; [d] estimated assuming that the HOMO of

Fc lies 4.8 eV below the vacuum level;^[21][e] values in parentheses are op-

tical band gaps calculated from the low-energy edge of the absorption

spectra in toluene.

1=2

red

much easier than that of 2 or 3, exhibiting an E positively

shifted by approximately 0.4 V. These results confirm an in-

creased acceptor strength in the order of (Mes)₂B<(Pfp)₂Bꢁ

(Tfp)₂B !(FMes)₂B. The oxidation potentials of 1–4 are very

similar, exhibiting little increase with enhanced acceptor

strength and, thus, the donor and acceptor groups are relative-

ly electronically decoupled in the ground state. The stronger

1=2

Tmjul donor groups of 5 and 6 make their oxidation (E

+0.04 V) much easier than that of either 2 or 3 (E

Figure 3. DFT (B3LYP/6-31G(d))-calculated frontier orbitals for 1, 2, 4, and 5.

ꢀ3

Surface isovalue: ꢂ0.02 [e a₀

] .

+0.41 V) bearing triarylamine donors. The increased donor

strength also has a slight influence on E¹_re⁼_d², making the reduc-

tion of 5 and 6 slightly more difficult than that of either 2 or 3

by approximately 0.06 eV. From the electrochemical data, the

HOMO and LUMO energy levels and energy gaps were calcu-

lated (Table 4). Compared to 1, the LUMOs of 2 and 3 are stabi-

lized by 0.19 eV, whereas their HOMO levels are stabilized by

only 0.02 eV, resulting in reduced HOMO–LUMO gaps. Moving

from 2 and 3 to 5 and 6, the HOMOs are strongly elevated

whereas LUMO levels are only slightly raised, further leading to

reduced energy gaps. The extremely strong acceptor in 4 re-

sults in a significantly stabilized LUMO level, which serves as

a key factor leading to its small energy gap, the smallest

amongst the compounds presented here. Therefore, the

Electrochemistry

Electrochemical measurements were carried out to evaluate

further the acceptor strength and to confirm experimentally

the influence of different donors and acceptors on the HOMO

and LUMO levels and energy gaps. Cyclic voltammetry mea-

surements (Figure 4 and Table 4) show that both the reduction

and oxidation processes of these compounds are reversible, in-

dicating their bipolar character. Compared to 1 (E¹_re⁼_d²=ꢀ2.23 V),

compounds 2 (E_r¹_e⁼_d²=ꢀ2.04 V) and 3 (E_r¹_e⁼_d²=ꢀ2.04 V) are more

easily reduced with reduction potentials positively shifted by

approximately 0.2 V. The reduction of 4 (E¹_re⁼_d²=ꢀ1.61 V) is

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energy gaps obtained from electrochemical measurements

show the same trend as the optical band gaps.

markable and uncommon for push–pull systems, since emis-

sion from ICT states is usually quenched in more polar solvents.

Only a few D–p–A-type organoboron compounds showing sol-

vent-independent F_Fvalues close to unity have been report-

ed.^[8b]The emission spectra of 3 are similar to those of 2 in

each solvent, consistent with their similar absorption spectra,

as discussed above. Compound 3 also has high F_F(0.85–0.96)

values in all three solvents used; however, the emission is

somewhat more easily quenched by polar CH₃CN compared to

1 and 2. Compound 4, with the strongest acceptor, displays

a huge spectral red shift in toluene (l_em=644 nm) of 109–

140 nm (3200–4300 cm^ꢀ1) compared to 1–3. However, the

emission of 4 in toluene is too weak to obtain a reliable value

of F_F, and in more polar solvents, 4 is almost nonemissive. 5

and 6 have close values for l_em, similar to the case of 2 and 3,

but their emission spectra are red-shifted and more sensitive

toward solvents in relation to those of 2 and 3, resulting from

their stronger donors. These compounds show efficient orange

(l_emꢁ585 nm) and red (l_emꢁ664 nm) emission in toluene and

THF, respectively, with F_F=0.80–0.95. It is notable that these

compounds display NIR (l_em>700 nm) emission in CH₃CN solu-

tion; this emission is somewhat quenched relative to that in

less polar solvents (Table 2), but the F_Fvalues are still good to

high; 0.48 for 5 and 0.27 for 6. As far as we are aware, these

two compounds represent the first examples of three-coordi-

nate boron compounds showing efficient NIR emission. We

note, however, that a (Mes)₂B-containing iridium complex with

two emission bands, one in the visible and one in the NIR

region, was reported previously,^[22]but this compound had

a very low emission quantum yield of 5.7ꢂ10^ꢀ4. Moreover, re-

cently, a boron–dipyrromethene derivative with (Mes)₂B-con-

taining substituents was reported to show NIR emission; how-

ever, F_Fwas not mentioned.^[23]In addition, in our previous

work, we observed that TpaꢀB(FMes)₂displays NIR emission in

THF; however, the F_Fcould not be determined because the

emission was weak.^[15]As compounds exhibiting NIR emission

normally have relatively small F_Fvalues, the quantum yield of

5 is, to our knowledge, among the highest reported.^[24]In addi-

tion, our new NIR-emitting compounds also possess sizeable

Stokes shifts of over 7000 cm^ꢀ1(ca. 270 nm) in CH₃CN. To un-

derstand the factors influencing the F_Fvalues of the present

systems, the F_Fand fluorescence lifetimes (t_F) of 1–3, 5, and 6

were measured in all three solvents and the radiative (k_r) and

nonradiative (k_nr) rate constants were calculated from these

data. Due to its very weak emission, related data could not be

obtained for 4. The k_rvalues of these compounds show the

following trends: 1) Independent of solvent, k_rbecomes small-

er in the order of 1>2ꢁ3>5ꢁ6, due to enhanced push–pull

character; and 2) for each compound, k_rdecreases with in-

creased solvent polarity. The k_nrvalues of 1–3 are very small

and do not show obvious changes between the three solvents,

which results in high and solvent-independent F_Fvalues for

these compounds. In contrast, both 5 and 6 possess obviously

increased k_nrvalues upon changing from toluene to CH₃CN sol-

utions, with a more significant increase for 6. The combined

decrease in k_rand increase in k_nrwith increasing solvent polari-

ty leads to the relatively low F_Fvalues of 5 and 6 in polar sol-

Photoluminescence

In solution, the emission wavelengths and observed emission

colors of these compounds strongly depend on the solvent

(Figure 5 and Table 2). With increased solvent polarity, the

emission spectra of all of the compounds broaden and exhibit

a significant red shift. This is attributed to a highly polarized

ICT excited state, which is typical for D–p–A compounds. From

toluene to CH₃CN, the emission maxima (l_em) of 1 shifts from

504 to 599 nm (3100 cm^ꢀ1), and the color of the emission

varies from green to orange. With its increased acceptor

strength, 2 shows red-shifted emission in each solvent com-

pared to 1, exhibiting emission ranging from yellow-green to

red with l_emshifting from 530 to 638 nm. It is notable that

compounds 1 and 2 show very high F_Fvalues (0.92–1.00),

even for the orange/red emission in polar solvents, which is re-

Figure 5. Emission spectra of 1–6 in toluene (red), THF (blue), and CH₃CN

(green).

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vents, whereas the lower value of F_Ffor 6 in CH₃CN compared

to 5 is attributed to the larger increase of k_nr. As for compound

4, with the strongest push–pull character, as deduced from its

significantly red-shifted absorption and emission spectra, the

very weak emission is possibly related to a fairly small k_rvalue,

caused by a twisted ICT (TICT) excited state, which could re-

strict the radiative deactivation of the S₁state. Our previous

study of TpaꢀB(FMes)₂showed that this compound has

a highly twisted geometry in the S₁excited state with a large

deformation of the B(FMes)₂group, which forms even in the

gas phase.^[15]To investigate whether the same process occurs

for compound 4 in the present study, the excited-state (S₁)

structure of this compound was calculated using TD-DFT opti-

mization (CAM-B3LYP/6-31G(d)). In the relaxation process from

the S₁Franck–Condon state following absorption, one FMes

group rotates towards a more planar conformation (50.7 to

24.88), while the second FMes group rotates somewhat and

the bithienyl unit rotates to a significantly larger dihedral

angle with respect to the BC₃plane (63.8 to 74.98 and 19.3 to

76.78, respectively; see the Supporting Information, Figure S5);

therefore, the S₁state of 4 is described as a TICT state and is

analogous to that observed for TpaꢀB(FMes)₂. In the TICT state,

there is electronic decoupling between the molecular orbitals

involved in the emission (S₁!S₀) process (see the Supporting

Information, Figure S6), leading to a small oscillator strength of

0.035, consistent with the observed weak emission from this

Figure 6. Emission spectra of 1 (black), 2 (red), and 5 (blue) before (solid

line) and after (dashed line) excess F^ꢀ(>5 equiv) is added to the THF solu-

tion. The inset shows the emission-color changes of 1 (left), 2 (middle) and

5 (right) following F^ꢀbinding in THF under 365 nm UV-light irradiation.

compounds 3 and 6 are expected to have responses similar to

2 and 5, respectively. Compound 4 is nonemissive in THF;

however, bright blue emission is observed after fluoride bind-

ing (Figure 7). Such a “turn-on” response behavior^[5c,25]is quite

different from the case of the other compounds. The elimina-

tion of the nonemissive ICT state of 4 upon fluoride binding

and the generation of an emissive state similar to those for

fluoride-bound 1, 2 and 5 reasonably explains the “turn-on” re-

sponse behavior. Following fluoride binding, 4 could not be re-

covered by washing with water; instead, excess BF₃·OEt₂was

used to extract the more strongly bound fluoride, which fur-

ther demonstrates the enhanced Lewis acidity of 4 compared

to 2 (see the Supporting Information, Figure S8). We suggest

that the binding of fluoride used herein to confirm our inter-

pretation of the photophysical studies could possibly be ap-

plied to the development of efficient fluoride probes, but we

note that this is already a very mature area.^{[3b–d,f,4a–c,5a–e,6,11,14g]}

state in solution. This is in contrast to the allowed S₁S₀tran-

sition at the more planar ground-state geometry, which has

a large oscillator strength of 1.470.

Because binding of anions to the Lewis-acidic boron atom in

the acceptor group occupies the previously vacant p_zorbital

thus destroying its accepting ability, this can be used to con-

firm the ICT nature of the optical transitions. The different

push–pull strengths of these compounds led to different

changes in emission color upon treatment with fluoride in THF.

After an excess of fluoride was added to the solution, the emis-

sion of 1, 2 and 5 changed from yellow-green to blue, yellow-

orange to blue, and red to blue, respectively (Figure 6). A

¹⁹F NMR study of 2 in CD₂Cl₂confirmed that a 1:1 complex be-

tween the compound and fluoride was formed and that com-

plete recovery of 2 can be achieved by washing with water

(see the Supporting Information, Figure S7). These results clear-

ly confirmed that the original emission of these compounds

comes from an ICT state. The gradually larger color difference

upon fluoride binding from 1 to 2 to 5 can be easily under-

stood; initially, the enhanced push–pull character from 1 to 2

to 5 leads to red-shifted ICT emission; after fluoride is bound

to the boron atom of the acceptor, the original ICT state

cannot be formed; therefore, bright blue emission arising from

the electronic transition within the donor-spacer moiety is ob-

served. The emission-color change from red to blue for 5 is

particularly remarkable. Such a large color difference has not

been observed in sensing studies of three-coordinate-boron

systems to the best of our knowledge. The distinct color

change and the bright emission of both free and bound states

facilitates naked-eyed recognition.^[4c]Considering the similar

photophysical properties of 2 and 3 and of 5 and 6 in THF,

Figure 7. Emission spectra of 4 before (black line) and after (red line) excess

^ꢀ(>5 equiv) is added to the THF solution. The inset shows the “turn-on” of

fluorescence after F^ꢀbinding under 365 nm UV irradiation.

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As solid powders, 1–3 show yellow emission, while 4–6

show red emission. The emission maxima are located at 548,

545, 563, 646, 635 and 618 nm for 1–6, respectively (Figure 8).

The solid-state F_Fvalues of 1–3, 5 and 6 are 0.30, 0.46, 0.27,

Figure 9. Emission spectra of 4 in toluene at different temperatures. Inset:

Normalized spectra showing the blue-shift upon cooling.

emission in the solid state, are especially interesting, given the

lack of efficient red emitters based on organoboron

compounds.

Figure 8. Emission spectra of solid (powder) samples of 1–6.

Conclusions

0.23 and 0.05, respectively, which, although still moderate, are

much lower than those of their solutions (Table 2). Such aggre-

gation-induced quenching of fluorophores is commonly ob-

served in the solid state.^[26]Interestingly, compound 4 shows

the opposite behavior, exhibiting a much higher F_Fvalue

(0.31) in the solid state than in solution: such behavior is

known as aggregation-induced emission (AIE),^[26a,27]but has

rarely been observed for three-coordinate boron com-

pounds.^[28]Several studies indicate that the AIE phenomenon

of compounds possessing donor and acceptor subunits may

be caused by restricted formation of a TICT state,^[29]which is

likely the reason for the AIE of 4. As discussed in regard to the

solution photophysics, the nonemissive nature of 4 in solution

should be related to a highly twisted ICT excited-state geome-

try with a low S₁!S₀oscillator strength. However, as disclosed

in the crystal structure of 4, the NC₃ꢀPhꢀdithienylꢀBC₃moiety

is relatively planar with small interplanar dihedral angles in the

ground state, close to the optimized S₀structure. Once the

compound is excited in the solid state, the rigid environment

restricts the formation of the TICT state; therefore, more effi-

cient emission might be expected from this geometry. The

low-temperature (77 K) emission spectrum of 4 in frozen tolu-

ene (l_em=597 nm) shows a significant blue shift of 47 nm

(1200 cm^ꢀ1) compared to the solution spectrum in toluene

(Figure 9), which is similar to the behavior of TpaꢀB(FMes)₂

and consistent with the lack of formation of a TICT state in

a rigid environment.^[15]In addition, the emission in frozen tolu-

ene solution shows significantly increased intensity, similar to

the case in the rigid solid and consistent with our hypothesis

of an efficient emission from a more planar excited-state con-

formation. We note that l_emof 4 in the solid state is red-shifted

compared to that in frozen toluene by 49 nm (1300 cm^ꢀ1),

which may be related to aggregation in the solid state. Com-

pounds 4 (F_F=0.23) and 5 (F_F=0.31), with their efficient red

A series of donor–bithienyl–acceptor compounds containing

Ar₂B acceptor and arylamine donor groups was synthesized.

The acceptor strength was systematically tuned by varying

Ar₂B, namely by using Ar=mesityl ((Mes)₂B), 2,6-dimethyl-4-

pentafluoropenylbenzene ((Pfp)₂B), 2,6-dimethyl-4-[3,5-bis(tri-

fluoromethyl)phenyl]benzene ((Tfp)₂B) and 2,4,6-tris(trifluoro-

methyl)benzene ((FMes)₂B). Tuning the donor strength by ex-

change of a triphenylamine donor for a stronger 1,1,7,7-tetra-

methyljulolidine donor gave further fine control over the opto-

electronic properties. The key to the high-yield syntheses of

these compounds was the use of iridium-catalyzed CꢀH boryl-

ation, which was successfully employed in the process of deri-

vatizing both the boryl acceptors and the tetramethyljulolidine

donor.

The acceptor groups were evaluated using a complementary

combination of crystallographic, photophysical, electrochemi-

cal and theoretical studies, which established that the order of

increasing strength is (Mes)₂B<(Pfp)₂Bꢁ(Tfp)₂B !(FMes)₂B. In

CH₃CN, near-infrared emission with quantum yields of 0.27–

0.48 was obtained for derivatives containing the moderately

strong (Pfp)₂B and (Tfp)₂B acceptors and tetramethyljulolidine

donors, which is the first time such efficient low-energy emis-

sion has been observed from a three-coordinate organoboron

system.

A compound containing the strongest acceptor,

(FMes)₂B, and a triphenylamine donor suffered from strongly

quenched emission in solution, whereas the analogous deriva-

tives containing (Pfp)₂B and (Tfp)₂B, had near-unity quantum

yields, even in polar CH₃CN solution, making them particularly

attractive for practical applications. Although very high quan-

tum yields (>0.80) were obtained for (Pfp)₂B- and (Tfp)₂B-con-

taining compounds with tetramethyljulolidine donors in tolu-

ene and THF, reduced values of k_rand increased values of k_nr

led to quantum yields in CH₃CN solution being decreased by

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can be obtained free of charge from The Cambridge Crystallo-

graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

a factor of approximately two to three. Thus, our systematic

studies suggest that employing donors and acceptors of

medium strength is essential for achieving efficient emission in

polar solvents, insight that should facilitate the future design

of bright, organoboron-based NIR emitters. Efficient solid-state

red emission was observed for some derivatives and interest-

ing aggregation-induced emission of 4 containing the (FMes)₂B

group was studied. Moreover, each compound showed

a strong and clearly visible response to the addition of fluoride,

with either a large emission-color change, for example, from

red to blue, or turn-on fluorescence.

General photophysical measurements

All solution-state measurements were made in standard quartz

cuvettes (1 cmꢂ1 cm cross-section). UV/Vis absorption spectra

were recorded using an Agilent 8453 diode array UV/Vis spectro-

photometer. The emission spectra were recorded using an Edin-

burgh Instruments FLSP920 spectrometer equipped with a double

monochromator for both excitation and emission, operating in

right-angle geometry mode, and all spectra were fully corrected

for the spectral response of the instrument. All solutions used in

photophysical measurements had a concentration lower than

10^ꢀ5m.

Experimental Section

General information

Fluorescence quantum yield measurements

(FMes)₂BF (bis(fluoromesityl)boron fluoride),^[30](2,2’-bithiophen)-5-

ylboronic

acid,^[31]

5-iodo-2,2’-bithiophene^[32]

and

[{Ir(m-

The fluorescence quantum yields of solutions and powders were

measured using a calibrated integrating sphere (150 mm inner di-

ameter) from Edinburgh Instruments combined with the FLSP920

spectrometer described above. For solution-state measurements,

the longest-wavelength absorption maximum of the compound in

the respective solvent was chosen as the excitation wavelength,

while for solid-state (powder) measurements, the longest-wave-

length absorption maximum in toluene was selected.

OMe)(cod)}₂]^[33]were prepared according to literature procedures.

All other starting materials were purchased from commercial sour-

ces and were used without further purification. The organic sol-

vents for synthetic reactions and for photophysical and electro-

chemical measurements were HPLC grade, further treated to

remove trace water using an Innovative Technology Inc. Pure-Solv

Solvent Purification System and deoxygenated using the freeze–

pump–thaw method. All synthetic reactions were performed in an

Innovative Technology Inc. glovebox or under an argon atmos-

phere using standard Schlenk techniques. H, ¹³C{¹H} and ¹¹B NMR

Fluorescence lifetime measurements

spectra were measured on either a Bruker Avance 500 (¹H,

500 MHz; ¹³C, 125 MHz; ¹¹B, 160 MHz) or Bruker Avance 300 (¹H,

300 MHz; ¹³C, 75 MHz; ¹¹B, 96 MHz) NMR spectrometer. ¹⁹F{¹H} and

¹⁹F NMR spectra were measured on either a Bruker Avance 200 (¹⁹F,

188 MHz) or Bruker Avance 500 (¹⁹F, 470 MHz) NMR spectrometer.

The abbreviation dm stands for doublet of multiplets. Mass spectra

were recorded on Agilent 7890 A/5975C Inert GC/MSD or Varian

320MS-GC/MS systems operating in EI mode. Elemental analyses

were performed on a Leco CHNS-932 Elemental Analyzer.

Fluorescence lifetimes were recorded using the time-correlated

single-photon counting (TCSPC) method using an Edinburgh In-

struments FLS980 spectrometer equipped with a high speed pho-

tomultiplier tube positioned after a single emission monochroma-

tor. Measurements were made in right-angle geometry mode, and

the emission was collected through a polarizer set to the magic

angle. Solutions were excited with a 418 nm pulsed diode laser at

repetition rates of 10 or 20 MHz, as appropriate. The full-width-at-

half-maximum (FWHM) of the pulses from both diode lasers was

approximately 75 ps with an instrument response function (IRF) of

approximately 230 ps FWHM. The IRFs were measured from the

scatter of a LUDOX aqueous suspension at the excitation wave-

length. Decays were recorded to at least 4000 counts in the peak

channel with a record length of at least 1000 channels. The band

pass of the monochromator was adjusted to give a signal count

rate of <60 kHz. Iterative reconvolution of the IRF with one or two

decay functions and non-linear least-squares analysis were used to

analyze the data. The quality of all decay fits was judged to be sat-

isfactory, based on the calculated values of the reduced c²and

Durbin-Watson parameters and visual inspection of the weighted

residuals.

Single-crystal X-ray diffraction

Crystals suitable for single-crystal X-ray diffraction were selected,

coated in perfluoropolyether oil, and mounted on MiTeGen sample

holders. Diffraction data of TmjulꢀBpin and 2 were collected on

a Bruker D8 Quest three-circle diffractometer utilizing mirror-mono-

chromated Mo_Karadiation (l=0.71073 ꢁ) from an ImS microfocus

sealed X-ray tube (Incoatec, Germany) operated at 50 kV and 1 mA,

and equipped with a Photon area detector. Diffraction data of 4

were collected on a Nonius Kappa three circle diffractometer utiliz-

ing graphite monochromated Mo_Karadiation (l=0.71073 ꢁ) from

a rotating anode tube run at 50 kV and 30 mA, equipped with an

APEXII area detector. Both instruments were operated with an

open-flow N₂Cryoflex II (Bruker) device and measurements were

performed at 100 K. For data reduction, the Bruker Apex2 software

suite (Bruker AXS) was used. Subsequently, utilizing Olex2,^[34]the

structures were solved using the Olex2.solve charge-flipping algo-

rithm, and were subsequently refined with Olex2.refine using

Gauss-Newton minimization. All non-hydrogen atom positions

were located from the Fourier maps and refined anisotropically.

Hydrogen atom positions were calculated using a riding model in

geometric positions and refined isotropically.

Electrochemical measurements

All cyclic voltammetry experiments were conducted in an argon-

filled glovebox using a Gamry Instruments Reference 600 potentio-

stat. A standard three-electrode cell configuration was employed

using a platinum disk working electrode, a platinum wire counter

electrode, and a silver wire reference electrode separated by

a Vycor frit. Compensation for resistive losses (IR drop) was em-

ployed for all measurements. A 0.1m solution of [NBu₄][PF₆] was

used as the supporting electrolyte. A scan rate of 250 mVs^ꢀ1was

adopted.

CCDC 1025373 (TmjulꢀBpin), 1025372 (2) and 1025371 (4) contain

the supplementary crystallographic data for this paper. These data

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mixture was heated at reflux for 24 h, cooled to RT and filtered.

The filtrate was evaporated to dryness and the residue was purified

by column chromatography (Al₂O₃, 8:1 hexane/CH₂Cl₂) and subse-

quent crystallization from hexane to provide the title compound as

Theoretical studies

All calculations (DFT and TD-DFT) were carried out with the pro-

gram package Gaussian 09 (Revision B.01)^[35]and were performed

on a parallel cluster system. Gaussview 5.0 was used to visualize

the results, to measure calculated distances and bond lengths, and

to plot orbital surfaces. The ground-state geometries were opti-

mized without symmetry constraints using the B3LYP function-

al^[36–38]in combination with the 6-31G(d) basis set.^[39]The molecular

structures of 2 and 4 as determined by X-ray crystallography were

used as the input for optimizing their ground-state geometries;

the input structure of 1 was obtained by replacing the C₆F₅groups

of 2 with methyl groups; and the input structure of 5 was ob-

tained by exchanging the Tpa group of 2 for a Tmjul group. The

optimized geometries were confirmed to be local minima by per-

forming frequency calculations and obtaining only positive (real)

frequencies. Based on these optimized structures, the lowest-

energy gas-phase vertical transitions were calculated (singlets, six

states) by TD-DFT using the Coulomb-attenuated functional CAM-

B3LYP^[40]in combination with the 6-31G(d) basis set as this pairing

has been shown to be effective for ICT systems.^[41]The S₁state of 4

was optimized using TD-DFT optimization at the CAM-B3LYP/6-

31G(d) level of theory with six singlet states. No symmetry con-

straint was used in any of the calculations.

a yellow solid (1.80 g, 54%). H NMR (500 MHz, [D₈]THF): d=7.26 (s,

2H), 7.24 (dd, J=5, 1 Hz, 1H), 7.15(dd, J=4, 1 Hz, 1H), 7.09 (d, J=

4 Hz, 1H), 7.03 (d, J=4 Hz, 1H), 6.98 (dd, J=5, 4 Hz, 1H), 3.20–3.18

(m, 4H), 1.77–1.74 (m, 4H), 1.30 ppm (s, 12H); ¹³C{¹H} NMR

(126 MHz, [D₈]THF): d=146.2, 141.1, 138.7, 134.3, 131.1, 128.3,

124.9, 124.1, 123.2, 122.2, 122.1, 121.2, 47.2, 37.6, 32.8, 31.3 ppm;

MS (EI⁺) m/z: 393 [M]⁺; elemental analysis calcd (%) for C₂₄H₂₇NS₂:

C 73.23, H 6.91, N 3.56, S 16.29; found C 73.05, H 6.85, N 3.56, S

16.05.

4-Bromo-3,5-dimethylphenyl boronic acid pinacol ester (4-Br-

3,5-Me₂PhꢀBpin). 2,6-Dimethylbromobenzene (18.5 g, 100 mmol),

B₂pin₂(33.0 g, 130 mmol), [{Ir(m-OMe)(cod)}₂] (333 mg, 0.50 mmol),

dtbpy (403 mg, 1.50 mmol) and hexane (116 mL) were added to

a flask. The reaction mixture was heated at 808C for 65 h, cooled

to RT, and then filtered through a short silica-gel pad. The obtained

solution was evaporated to give an oil, which was purified by

column chromatography (silica gel, hexane). The obtained oily

product solidified under vacuum to give the title compound as

a white solid (29.3 g, 95%). H NMR (500 MHz, CD₂Cl₂): d=7.49 (s,

2H), 2.43 (s, 6H), 1.34 ppm (s, 12H); ¹³C{¹H} NMR (126 MHz, CD₂Cl₂):

d=138.1, 134.7, 131.6, 127.9 (br), 84.3, 25.1, 23.9 ppm; ¹¹B NMR

(160 MHz, CD₂Cl₂): d=29.9 ppm (s, 1B); MS (EI⁺) m/z: 310 [M]⁺; el-

emental analysis calcd (%) for C₁₄H₂₀BBrO₂: C 54.06, H 6.48; found

C 54.45, H 6.55.

Synthesis

5-(4-Bisphenylaminophenyl)-2,2’-bithiophene (TpaꢀThꢀTh):^[42]4-

Bromo-N,N-diphenylaniline (926 mg, 2.86 mmol), [(2,2’-bithio-

phene)-5-yl]boronic acid (500 mg, 2.38 mmol), Na₂CO₃(1.26 g,

11.9 mmol), [Pd(PPh₃)₄] (110 mg, 0.10 mmol), THF (35 mL), and de-

gassed H₂O (6 mL) were added to a flask. The mixture was heated

at reflux for 24 h and then cooled to RT. Water and diethyl ether

were added to the mixture for extraction. The organic phase was

evaporated to dryness, and the residue was purified by column

chromatography (silica gel, 8:1 hexane/CH₂Cl₂). Subsequent crystal-

lization from hexane provided the title compound as a yellow solid

4-Perfluorophenyl-2,6-dimethylbromobenzene (PfpꢀBr). 4-Br-3,5-

Me₂PhꢀBpin (8.00 g, 25.8 mmol), C₆F₅I (12.9 g, 43.9 mmol), Ag₂CO₃

(17.8 g, 64.5 mmol), [Pd(PPh₃)₄] (1.48 g, 1.28 mmol) and THF

(120 mL) were added to a flask. The mixture was heated at reflux

for 25 h, cooled to RT, and then filtered. The filtrate was evaporat-

ed to dryness and the residue was purified by column chromatog-

raphy (silica gel, hexane) to provide the title compound as a white

solid (7.25 g, 80%). ¹H NMR (500 MHz, CDCl₃): d=7.15 (s, 2H),

2.49 ppm (s, 6H); ¹³C{¹H} NMR (126 MHz, CDCl₃): d=144.2 (dm,

(695 mg, 71%). H NMR (500 MHz, [D₆]DMSO): d=7.58 (d, J=8 Hz,

¹J_CF=248 Hz), 140.6 (dm, ¹J_CF=254 Hz), 139.2, 138.0 (dm, J_CF

2H), 7.52 (d, J=4 Hz, 1H), 7.38–7.29 (m, 7H), 7.11–7.06 (m, 7H),

6.99 ppm (d, J=8 Hz, 2H); ¹³C{¹H} NMR (125 MHz, [D₈]THF): d=

148.5, 148.4, 143.8, 138.3, 136.7, 130.1, 129.1, 128.6, 127.1, 125.3

(2C, overlap), 125.1, 124.5, 124.2, 124.0, 123.8 ppm; MS (EI⁺) m/z:

409 [M]⁺; elemental analysis calcd (%) for C₂₆H₁₉NS₂: C 76.25, H

4.68, N 3.42, S 15.66; found C 76.91, H 4.81, N 3.46, S 15.90.

251 Hz), 129.7, 129.1, 124.9, 115.4 (td, ²J_CF=17 Hz, ³J_CF=4 Hz),

24.0 ppm; ¹⁹F{¹H} NMR (188 MHz, CDCl₃): d=ꢀ142.9 (dd, J=23,

8 Hz, 2F), ꢀ155.3 (t, J=21 Hz, 1F), ꢀ162.0 to ꢀ162.2 ppm (m, 2F);

MS (EI⁺) m/z: 350 [M]⁺; elemental analysis calcd (%) for C₁₄H₈BrF₅:

C 47.89, H 2.30; found C 47.98, H 2.46.

4-(3,5-Bis(trifluoromethyl)phenyl)-2,6-dimethylbromobenzene

(TfpꢀBr). 4-Br-3,5-Me₂PhꢀBpin (3.10 g, 10.0 mmol), 3,5-bis(trifluoro-

methyl)iodobenzene (5.78 g, 17.0 mmol), Ag₂CO₃(6.90 g,

25.0 mmol), [Pd(PPh₃)₄] (577 mg, 0.50 mmol) and THF (45 mL) were

added to a flask. The mixture was heated at reflux for 21 h, cooled

to RT and then filtered. The filtrate was evaporated to dryness and

the residue was purified by column chromatography (silica gel,

hexane) and subsequent crystallization from methanol to provide

the title compound as a white solid (3.11 g, 79%). ¹H NMR

(500 MHz, CD₂Cl₂): d=8.05 (m, 2H), 7.91 (m, 1H), 7.35 (m, 2H),

2.51 ppm (m, 6H); ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): d=142.9, 139.9,

1,1,7,7-Tetramethyljulolidin-9-yl boronic acid pinacol ester

(TmjulꢀBpin). 1,1,7,7-Tetramethyljulolidine (6.00 g, 26.2 mmol),

bis(pinacolato)diboron

(B₂pin₂,

8.65 g,

34.1 mmol),

[{Ir(m-

OMe)(cod)}₂] (174 mg, 0.26 mmol), 4,4’-di-tert-butyl-2,2’-bipyridine

(dtbpy; 138 mg, 0.51 mmol) and hexane (36 mL) were added to

a flask. The reaction mixture was heated at 808C for 48 h, then

cooled to RT and then further cooled in ice-water to crystallize the

product. The mixture was filtered, and the solid product was

washed with cold hexane (80 mL) and dried in vacuum to provide

the title compound as an off-white solid (8.22 g, 88%). ¹H NMR

(300 MHz, C₆D₆): d=8.05 (s, 2H), 2.86 (t, J=6 Hz, 4H), 1.50 (t, J=

6 Hz, 4H), 1.20 (s, 12H), 1.19 ppm (s, 12H); ¹³C{¹H} NMR (125 MHz,

CD₂Cl₂): d=143.5, 130.8, 129.5, 114.3 (br), 83.3, 47.1, 37.1, 32.4,

31.2, 25.1 ppm;¹¹B NMR (96 MHz, C₆D₆): d=31.3 ppm (s, 1 B); MS

(EI⁺) m/z: 355 [M]⁺; elemental analysis calcd (%) for C₂₂H₃₄BNO₂: C

74.37, H 9.64, N 3.94; found C 74.23, H 9.58, N 3.96.

137.0, 132.5 (q, J_CF=33 Hz), 128.9, 127.5 (m), 127.2, 124.0 (q, J_CF

273 Hz), 121.5 (septet, J_CF=4 Hz), 24.2 ppm; ¹⁹F{¹H} NMR (188 MHz,

C₆D₆): d=ꢀ61.4 ppm (s, 6F); MS (EI⁺) m/z: 396 [M]⁺; elemental

analysis calcd (%) for C₁₆H₁₁BrF₆: C 48.39, H 2.79; found C 48.83, H

2.79.

5-(1,1,7,7-Tetramethyljulolidin-9-yl)-2,2’-bithiophene (TmjulꢀThꢀ

Th). TmjulꢀBpin (3.00 g, 8.45 mmol), 5-iodo-2,2’-bithiophene

(2.95 g, 10.1 mmol), Ag₂CO₃(5.80 g, 21.0 mmol), [Pd(PPh₃)₄]

(484 mg, 0.42 mmol) and THF (45 mL) were added to a flask. The

TpaꢀThꢀThꢀB(Mes)₂(1). nBuLi (2.5m in hexane, 0.23 mL,

0.58 mmol) was added to a THF (5 mL) solution of TpaꢀThꢀTh

(216 mg, 0.53 mmol) at ꢀ788C, and then the mixture was stirred at

this temperature for 1 h. After a THF (2 mL) solution of (Mes)₂BF

Chem. Eur. J. 2014, 20, 1 – 15

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(170 mg, 0.63 mmol) was added, the mixture was slowly warmed

to RT and stirred for 2 h. The solvent was removed under vacuum,

and the residue was purified by column chromatography (silica

gel, 4:1 hexane/CH₂Cl₂) and subsequent precipitation from diethyl

ether by addition of methanol to provide 1 as a yellow solid

(303 mg, 87%).¹H NMR (500 MHz, [D₈]THF): d=7.51 (d, J=9 Hz,

2H), 7.39 (d, J=4 Hz, 1H), 7.33–7.32 (m, 2H), 7.27–7.23 (m, 5H),

7.09–7.07 (m, 4H), 7.03–7.00 (m, 4H), 6.81 (s, 4H), 2.27 (s, 6H),

2.13 ppm (s, 12H); ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): d=150.4, 149.1

(br), 148.2, 147.8, 145.1, 142.2, 141.5 (br), 141.2, 139.0, 135.8, 129.8,

128.6, 127.9, 126.8, 126.5, 125.9, 125.1, 123.8, 123.6, 123.5, 23.6,

21.4 ppm; ¹¹B NMR (160 MHz, CD₂Cl₂): d=64 ppm (sbr); MS (EI⁺)

m/z: 658 [M]⁺; elemental analysis calcd (%) for C₄₄H₄₀BNS₂: C 80.35,

H 6.13, N 2.13, S 9.75; found C 80.65, H 6.15, N 1.94, S 9.72.

(0.24 mL, 1.91 mmol) was added dropwise to the prepared

Grignard reagent at 08C (ice bath). The mixture was heated at

608C for 1 h, and then cooled to RT. Hexane (10 mL) was added to

the solution, the mixture was filtered and the filtrate was evaporat-

ed under vacuum to give a viscous oil. As-prepared (Tfp)₂BF was

dissolved in THF (5.5 mL) without further purification. nBuLi (1.6m

in hexane, 0.18 mL, 0.29 mmol) was added to a THF (2 mL) solution

of TpaꢀThꢀTh (120 mg, 0.29 mmol) at ꢀ788C, and the mixture was

stirred at this temperature for 1 h. To this lithium reagent, a portion

of the prepared (Tfp)₂BF solution (2 mL) was added. The mixture

was warmed to RT and stirred overnight. The solvent was removed

under vacuum, and the residue was purified by column chroma-

tography (silica gel, 3:1 hexane/CH₂Cl₂) and subsequent precipita-

tion from CH₂Cl₂by addition of methanol to provide compound 3

as a yellow solid (198 mg, 65% based on TpaꢀThꢀTh). ¹H NMR

(500 MHz, CD₂Cl₂): d=8.14 (s, 4H), 7.89 (s, 2H), 7.49–7.46 (m, 3H),

7.41 (d, J=4 Hz, 1H), 7.35 (s, 4H), 7.31(d, J=4 Hz, 1H), 7.30–7.27

(m, 4H), 7.20 (d, J=4 Hz, 1H), 7.12–7.10 (m, 4H), 7.08–7.03 (m, 4H),

2.34 ppm (s, 12H); ¹³C{¹H} NMR (126 MHz, [D₈]THF): d=152.7,

149.0, 148.4, 148.2 (br), 146.3, 145.4 (br), 144.5, 143.8, 142.8 (br),

139.4, 135.7, 132.7 (q, ²J_CF=33 Hz), 130.2, 128.4, 128.2 (m), 127.7,

127.2, 127.1, 126.6, 125.6, 124.3, 124.2 (q, ¹J_CF=274 Hz), 124.2,

124.1, 121.7 (m), 24.0 ppm; ¹⁹F{¹H} NMR (188 MHz, CD₂Cl₂): d=

ꢀ63.1 ppm (s, 12F); ¹¹B NMR (160 MHz, CD₂Cl₂): ca. 66 ppm (weak

and broad); MS (EI⁺) m/z: 1054 [M]⁺; elemental analysis calcd (%)

for C₅₈H₄₀BF₁₂NS₂: C 66.10, H 3.83, N 1.33, S 6.09; found C 66.42, H

3.75, N 1.57, S 6.04.

TpaꢀThꢀThꢀB(Pfp)₂(2). THF (1 mL) and C₂H₄Br₂(4 drops) were

added sequentially to magnesium turnings (1.03 g, 42.9 mmol) at

RT, and the resulting mixture was stirred for 5 min. A solution of

PfpꢀBr (1.50 g, 4.29 mmol) in THF (4.3 mL) was prepared. A portion

(0.4 mL) of the PfpꢀBr solution was added at RT to the magnesium

turnings, and then the mixture was heated to 508C before the re-

maining PfpꢀBr solution was added dropwise. After addition, the

reaction mixture was heated at 608C for 20 min, before it was

cooled to RT and the solution was transferred to another flask by

syringe to leave the excess Mg as a solid residue. BF₃·OEt₂

(0.21 mL, 1.67 mmol) was added dropwise to the prepared

Grignard reagent at 08C (ice bath). The reaction mixture was

heated at 608C for 15 min and then cooled to RT. After the addi-

tion of hexane (10 mL), the mixture was filtered, and the filtrate

was evaporated under vacuum to give a viscous oil. As-prepared

(Pfp)₂BF was dissolved in THF (3 mL) and used without further pu-

rification. nBuLi (1.6m in hexane, 0.45 mL, 0.72 mmol) was added

to a THF (5 mL) solution of TpaꢀThꢀTh (295 mg, 0.72 mmol) at

ꢀ788C, and the mixture was stirred at this temperature for 1 h.

The prepared lithium reagent was transferred to the prepared

(Pfp)₂BF solution (08C) by cannula. The obtained mixture was

warmed to RT and stirred overnight. The solvent was removed

under vacuum, and the residue was purified by column chroma-

tography (silica gel, 4:1 hexane/CH₂Cl₂) and subsequent precipita-

tion from CH₂Cl₂by addition of methanol to provide compound 2

as a yellow solid (300 mg, 43% based on TpaꢀThꢀTh). ¹H NMR

(500 MHz, CD₂Cl₂): d=7.51 (d, J=4 Hz, 1H), 7.48 (d, J=9 Hz, 2H),

7.41 (d, J=4 Hz, 1H), 7.33 (d, J=4 Hz, 1H), 7.30–7.27 (m, 4H), 7.20

(d, J=4 Hz, 1H), 7.14 (s, 4H), 7.13–7.11 (m, 4H), 7.08–7.04 (m, 4H),

2.31 ppm (s, 12H); ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): d=152.1, 148.3,

147.7, 147.4 (br), 145.8, 145.7 (m, CF), 145.2 (br), 143.7 (m, CF),

143.7, 141.7 (br), 141.7 (m, CF), 139.6 (m, CF), 138.3 (dm, CF), 135.3,

129.8, 129.2, 127.7, 127.2, 127.1, 126.8, 126.2, 125.2, 123.8, 123.6,

123.5, 116.5 (m), 23.9 ppm; ¹⁹F{¹H} NMR (188 MHz, CD₂Cl₂): d=

ꢀ143.5 (dd, J=23, 8 Hz, 4F), ꢀ156.9 (t, J=21 Hz, 2F), ꢀ163.2 to

ꢀ163.5 ppm (m, 4F); ¹¹B NMR (160 MHz, CD₂Cl₂): d=65 ppm (sbr);

MS (EI⁺) m/z: 962 [M]⁺; elemental analysis calcd (%) for

C₅₄H₃₄BF₁₀NS₂: C 67.44, H 3.56, N 1.46, S 6.76; found C 67.71, H

3.20, N 1.56, S 6.67.

TpaꢀThꢀThꢀB(FMes)₂(4). nBuLi (2.5m in hexane, 0.16 mL,

0.40 mmol) was added to a THF (4 mL) solution of TpaꢀThꢀTh

(150 mg, 0.37 mmol) at ꢀ788C, and then the mixture was slowly

warmed to ꢀ258C during 1 h. After the solvent was removed

under vacuum, toluene (2 mL) was added to the residue at approx-

imately ꢀ258C. A toluene (5 mL) solution of (FMes)₂BF (220 mg,

0.37 mmol) was added to the prepared lithium reagent, the reac-

tion mixture was warmed to RT and stirred overnight. The solvent

was removed under vacuum, and the residue was purified by

column chromatography (silica gel, 4:1 hexane/CH₂Cl₂) and subse-

quent crystallization from hexane at ꢀ308C to provide compound

4 as a red solid (40 mg, 11%). H NMR (500 MHz, [D₈]THF): d=8.44

(s, 4H), 7.52 (d, J=9 Hz, 2H), 7.48 (d, J=4 Hz, 2H), 7.44 (d, J=4 Hz,

1H), 7.34 (d, J=4 Hz, 1H), 7.32 (d, J=4 Hz, 1H), 7.27–7.24 (m, 4H),

7.09–7.07 (m, 4H), 7.04–7.02 ppm (m, 4H); ¹³C{¹H} NMR (126 MHz,

[D₈]THF): d=154.8, 149.2, 148.3, 147.7, 146.4, 134.6, 133.9, 133.6,

130.2, 129.1, 128.1, 127.3, 126.5, 125.7, 124.9, 124.5, 124.3, 123.9,

122.7 ppm; ¹⁹F{¹H} NMR (188 MHz, CD₂Cl₂, RT): d=ꢀ50.4 (sbr, 3F),

ꢀ52.1 (sbr, 3F), ꢀ56.5 (sbr, 6F), ꢀ63.4 ppm (s, 6F); ¹⁹F{¹H} NMR

(188 MHz, CD₂Cl₂, 223 K): d=ꢀ50.0 (q, J=13 Hz, 3F), ꢀ51.8 (q, J=

11 Hz, 3F), ꢀ56.0 (q, J=11 Hz, 3F), ꢀ56.4 (q, J=13 Hz, 3F), ꢀ62.8

(s, 3F), ꢀ62.9 ppm (s, 3F); ¹¹B NMR (160 MHz, [D₈]THF): not ob-

served; MS (EI⁺) m/z: 981 [M]⁺; elemental analysis calcd (%) for

C₄₄H₂₂BF₁₈NS₂: C 53.84, H 2.26, N 1.43, S 6.53; found C 54.34, H

2.44, N 1.66, S 6.33.

TmjulꢀThꢀThꢀB(Pfp)₂(5). nBuLi (1.6m in hexane, 0.63 mL,

1.01 mmol) was added to a THF (2.3 mL) solution of TmjulꢀThꢀTh

(375 mg, 0.95 mmol) at ꢀ788C, and the mixture was stirred at this

temperature for 1 h. The prepared lithium reagent was transferred

to a THF solution of (Pfp)₂BF at 08C, which was prepared using the

same method and in the same amount as that used in the synthe-

sis of 2. The mixture was warmed to RT and stirred overnight. The

solvent was removed under vacuum, and the residue was purified

by column chromatography (silica gel, 3:1 hexane/CH₂Cl₂) and sub-

sequent precipitation from CH₂Cl₂by addition of methanol to pro-

vide compound 5 as a red solid (430 mg, 48% based on Tmjulꢀ

TpaꢀThꢀThꢀB(Tfp)₂(3). THF (1 mL) and C₂H₄Br₂(4 drops) were

added sequentially to magnesium turnings (909 mg, 37.9 mmol) at

RT, and the resulting mixture was stirred for 5 min. A solution of

TfpꢀBr (1.50 g, 3.79 mmol) in THF (5.70 mL) was prepared. A por-

tion (0.5 mL) of the TfpꢀBr solution was added at RT to the magne-

sium turnings, and then the mixture was heated to 508C before

the remaining solution was added dropwise. After addition, the re-

action mixture was heated at 608C for 30 min, before it was

cooled to RT and the solution was transferred to another flask by

syringe to leave the excess Mg as a solid residue. BF₃·OEt₂

Chem. Eur. J. 2014, 20, 1 – 15

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2008 , 120 , 84 6 –8 50; Angew. Chem. Int. Ed. 2008 , 47 , 83 4 –8 38; l) N. Mat-

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ThꢀTh). H NMR (500 MHz, [D₈]THF): d=7.52 (d, J=4 Hz, 1H), 7.46

(d, J=4 Hz, 1H), 7.38 (d, J=4 Hz, 1H), 7.31 (s, 2H), 7.19 (s, 4H),

7.15(d, J=4 Hz, 1H), 3.22 (t, J=6 Hz, 4H), 2.32 (s, 12H), 1.76 (t, J=

6 Hz, 4H), 1.30 ppm (s, 12H); ¹³C{¹H} NMR (126 MHz, [D₈]THF): d=

154.0, 149.2, 147.0 (br), 145.8 (br), 145.6 (dm, CF), 144.5, 142.3–

142.1 (m, 2 C), 141.7, 140.2 (br, CF), 138.8 (dm, CF), 133.4, 131.3,

129.7, 127.8, 127.7, 126.0, 122.3, 121.9, 121.7, 117.2 (m), 47.4, 37.6,

33.0, 31.3, 23.9 ppm; ¹⁹F{¹H} NMR (188 MHz, CD₂Cl₂): d=ꢀ143.5

(dd, J=23, 8 Hz, 4F), ꢀ157.0 (t, J=21 Hz, 2F), ꢀ163.2 to

ꢀ163.5 ppm (m, 4F); ¹¹B NMR (160 MHz, [D₈]THF): ca. 65 ppm

(weak and broad); MS (EI⁺) m/z: 946 [M]⁺; elemental analysis calcd

(%) for C₅₂H₄₂BF₁₀NS₂: C 66.03, H 4.48, N 1.48, S 6.78; found C

66.39, H 4.25, N 1.49, S 6.54.

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TmjulꢀThꢀThꢀB(Tfp)₂(6). nBuLi (1.6m in hexane, 0.40 mL,

0.64 mmol) was added to a THF (1.5 mL) solution of TmjulꢀThꢀTh

(250 mg, 0.64 mmol) at ꢀ788C, and the mixture was stirred at this

temperature for 1 h. A solution of (Tfp)₂BF in THF (3.5 mL), pre-

pared in the same batch as that used in the synthesis of 3, was

added to the to the lithium reagent. The mixture was warmed to

RT and stirred overnight. The solvent was removed under vacuum,

and the residue was purified by column chromatography (silica

gel, 3:1 hexane/CH₂Cl₂) and subsequent precipitation from CH₂Cl₂

by addition of methanol to provide compound 6 as a red solid

(330 mg, 50% based on TmjulꢀThꢀTh). H NMR (500 MHz, CD₂Cl₂):

d=8.15 (s, 4H), 7.90 (s, 2H), 7.47 (d, J=4 Hz, 1H), 7.38 (d, J=4 Hz,

1H), 7.36 (s, 4H), 7.29 (d, J=4 Hz, 1H), 7.27 (s, 2H), 7.08 (sbr, 1H),

3.21 (sbr, 4H), 2.35 (s, 12H), 1.76 (t, J=6 Hz, 4H), 1.30 ppm (s,

12H); ¹³C{¹H} NMR (126 MHz, [D₈]THF): d=153.7, 149.1, 147.4 (br),

145.4 (br), 144.6, 144.0, 142.8 (br), 141.7, 139.3, 133.4, 132.7 (q,

²J_CF=33 Hz), 131.3, 128.2 (m), 127.6, 127.1, 125.9, 124.7 (q, J_CF

273 Hz), 122.3, 121.9, 121.7, 121.6 (m), 47.4, 37.6, 33.0, 31.3,

24.0 ppm; ¹⁹F{¹H} NMR (188 MHz, CD₂Cl₂): d=ꢀ63.2 ppm (s, 12F);

¹¹B NMR (160 MHz, [D₈]THF): ca. 65 ppm (weak and broad); MS (EI⁺)

m/z: 1038 [M]⁺; elemental analysis calcd (%) for C₅₆H₄₈BF₁₂NS₂: C

64.80, H 4.66, N 1.35, S 6.18; found C 65.12, H 4.84, N 1.55, S 6.09.

Acknowledgements

We are grateful for generous financial support by the Bavarian

State Ministry of Science, Research, and the Arts for the Collab-

orative Research Network “Solar Technologies go Hybrid”. Z.Z.

and R.M.E. thank the Alexander von Humboldt Foundation for

Postdoctoral Research Fellowships. The authors thank Dr. Ste-

fan Wagner for mass spectrometry measurements, Dr. Rꢃdiger

Bertermann for NMR measurements, Dr. Ivo Krummenacher for

help with electrochemical measurements, and Prof. Dr. Chris-

toph Lambert for access to the FLS980 spectrofluorimeter.

Keywords: boron · charge transfer · fluorescence · near

infrared · photophysics

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Received: October 13, 2014

Published online on && &&, 0000

Chem. Eur. J. 2014, 20, 1 – 15

www.chemeurj.org

ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ÝÝ These are not the final page numbers!

Full Paper

FULL PAPER

Tune in, turn on: D–p–A organoboron

compounds with tunable push–pull

character have been prepared by vary-

ing the donor and acceptor groups. Effi-

cient green to near-IR emission has

been achieved from these compounds

in solution. Aggregation-induced emis-

sion was observed for one compound.

Each compound showed a strong and

clearly visible response to fluoride addi-

tion, with either a large emission-color

change or turn-on fluorescence.

Fluorescence

Z. Zhang, R. M. Edkins, J. Nitsch, K. Fucke,

A. Eichhorn, A. Steffen, Y. Wang,

T. B. Marder*

&& – &&

D–p–A Triarylboron Compounds with

Tunable Push–Pull Character Achieved

by Modification of Both the Donor

and Acceptor Moieties

Chem. Eur. J. 2014, 20, 1 – 15

www.chemeurj.org

ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

These are not the final page numbers! ÞÞ

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