Design, synthesis, photophysical and electrochemical properties of 2-(4,5-diphenyl-1-p-aryl-1H-imidazol-2-yl)phenol-based boron complexes

Article abstract of DOI:10.1039/c5dt00867k

New hybrid organic-inorganic boron compounds using an imidazole core have been readily synthesized by a two-step procedure from commercially available simple starting materials. All boron compounds were fully characterized by multinuclear NMR spectroscopy

Full text of DOI:10.1039/c5dt00867k

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Design, synthesis, photophysical and
electrochemical properties of 2-(4,5-diphenyl-
1-p-aryl-1H-imidazol-2-yl)phenol-based boron
complexes†
Cite this: DOI: 10.1039/c5dt00867k
Vanga Mukundam,a Kunchala Dhanunjayarao,a Ching-Nan Chuang,b
Dun-Yen Kang,c Man-kit Leung,b,d Kuo-Huang Hsiehb,c and
Krishnan Venkatasubbaiah*a
New hybrid organic–inorganic boron compounds using an imidazole core have been readily synthesized
by a two-step procedure from commercially available simple starting materials. All boron compounds
were fully characterized by multinuclear NMR spectroscopy, LC-MS, thermogravimetric analysis, cyclic
voltammetry and single crystal X-ray diffraction analysis (for 2a & 2c). The photoluminescence measure-
ments of 2a–2c revealed distinct emission peak maxima located at 378, 379 and 387 nm, respectively.
Electroluminescent devices fabricated using these boron compounds (2a–2c) suggest that the boron
compounds are capable of transporting electrons. A maximum brightness of 6450 cd m−2 at 12.5 V was
realized when compound 2a was used as an electron-transporting material.
Received 3rd March 2015,
Accepted 16th April 2015
DOI: 10.1039/c5dt00867k
www.rsc.org/dalton
main group elements into organic frameworks with extended
conjugation has attracted much attention owing to their
Introduction
Over the past decade, there has been growing interest in the unusual optical and electronic properties.
development of organic electronics for applications in organic
Among various types of main group element-containing
field effect transistors, nonlinear optic materials and organic π-electron systems, tri-9–18 and tetra-19–21coordinated boron
light emitting diodes (OLEDs).1–6 Although organic materials compounds have received particular interest because of their
have noteworthy advantages over inorganic materials, such as applications in nonlinear optics, fluorescent sensors, organic
cost effectiveness, a wide array of processing options and the light emitting diodes and biomolecular probes. Much effort
ability to tune the photophysical properties using different syn- has been devoted to the development of different types of
thetic approaches, they lack long-term stability and perform- tetra-coordinated boron compounds owing to their high stabi-
ance. Intensive work has been carried out in both academia lity towards air and moisture over tri-coordinated boron com-
and industry with a view to preparing conjugated frameworks pounds. In particular, boron dipyrromethene22–26 (BODIPYs)
of enhanced stability and performance. Incorporation of dyes synthesized by the condensation of pyrrole, carbaldehyde
hetero atoms into the backbone of π-conjugated systems has and boron reagents have been used extensively as organic
turned out to be one of the widely used methods to tune the light-emitting diodes, fluorescent imaging agents and sensors.
electronic structures thus producing desirable photophysical Although BODIPYs exhibit strong fluorescence and high molar
and electronic properties.7,8 In this regard, incorporation of extinction coefficients, they suffer from small Stokes’ shift26
and weak solid-state emissions. Efforts have been made to syn-
thesise modified BODIPY fluorophores with pronounced
aSchool of Chemical Sciences, National Institute of Science Education and Research
Stokes’ shifts27–29 and better electroluminescent properties;
(NISER), Institute of Physics Campus, Bhubaneswar 751005, Orissa, India.
however, a distinct disadvantage of this approach is that the
assemblies made using this method are structurally complex
and require multi-step organic synthesis.
Another strategy employed for the development of boron
fluorophores30–51 with improved properties relies on principles
of ligand design. Several examples of boron fluorophores with
different architectures have been reported to intrinsically
exhibit better photophysical properties, not only in solution
E-mail: krishv@niser.ac.in; Fax: (+) 091-674-2302436
bInstitute of Polymer Science and Engineering, National Taiwan University, Taipei
106, Taiwan
cDepartment of Chemical Engineering, National Taiwan University, Taipei 106,
Taiwan
dDepartment of Chemistry, National Taiwan University, Taipei 106, Taiwan
†Electronic supplementary information (ESI) available. CCDC 1044387 and
1044388. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5dt00867k
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but also in the solid state. For example, Massue et al.52,53 tion mass spectrometry. The 11B NMR spectra of complexes
reported a series of boron complexes based on 2-(2′-hydoxy- 2a–2c show resonance at around 1 ppm, which is characteristic
phenyl)benzoxazole. Ahn and co-workers51 reported the dra- of tetracoordinated boron and the 19F chemical shifts appear
matic substituent effect of boron complexes based on at around −139 ppm for the fluorine atoms attached to
2-(benzothiazol-2-yl)phenols. More recently, Wagner and co- boron.66 Compounds 2a and 2c were further characterized by
workers54 reported aryl(hydro)boranes as versatile boron-con- X-ray crystallography (Table S5†). Complexes 2a and 2c crystal-
ˉ
taining π-electron materials. In pursuit of a new ligand system, lizes in the triclinic P1 space group and monoclinic C2/c space
we noticed that imidazoles are highly attractive synthons, group, respectively.
especially tetraphenylimidazole based excited-state intra-
The molecular structures of 2a and 2c along with selected
molecular proton-transfer (ESIPT) molecules that showed good bond lengths and bond angles are depicted in Fig. 1 and 2,
electroluminescence, high thermal and morphological respectively. The boron atoms in both 2a and 2c adopt a
stability.55–60 Different synthetic approaches have been slightly distorted tetrahedral geometry. The B–O and B–N
employed to modify the optical and charge transport pro- bond lengths in 2a and 2c are comparable to those of the
perties of arylimidazoles.60–62 In an effort to tune the optical other literature reported boron complexes.62,67 In both struc-
properties of imidazoles, Ziessel and co-workers62 introduced tures, the six-membered ring formed by the boron chelation
boron into the frame work of imidazoles through N,O-chela- takes a twisted conformation. The boron and oxygen atoms
tion. Herein, we report the synthesis and characterization of deviate from the imidazole plane by 0.06 Å and 0.66 Å in the
tetraarylimidazoles based boron complexes from simple start- case of 2a and 0.15 Å and 0.83 Å in the case of 2c. The dihedral
ing materials using simple synthetic methodology and also angle between the imidazole and the phenolate is 19.46° (for
present their photophysical and electroluminescent properties. 2a) and 22.13° (for 2c), which indicates that the π-system is
We chose 2-(4,5-diphenyl-1-p-aryl-1H-imidazol-2-yl)phenol as distorted from the coplanar framework. The phenyl rings at
the building unit due to its ease of synthesis from simple start- the 1, 4 and 5-positions are severely twisted, the twist observed
ing materials and its versatility to construct four coordinated
boron difluoride complexes.63,64
Results and discussion
Synthesis & characterization
The tetraarylimidazoles (1a–1c) were readily synthesized
according to a literature procedure55,65 using commercially
available reagents. The corresponding boron complexes 2a–2c
were prepared by the reaction of 1a–1c with boron trifluoride
etherate in the presence of diisoprolylethylamine, as shown in
Scheme 1. All three boron complexes were fully characterized
1
using H, 13C, 11B and 19F NMR spectroscopy and high-resolu-
Fig. 1 Molecular structure of 2a. (a) ORTEP view of 2a with thermal
ellipsoids at the 30% probability level. (b) Side view of 2a (capped stick
model). Hydrogen atoms are omitted for clarity. Selected bond distances
(Å) and bond angles (°) are as follows: B1–F1 1.383(2), B1–F2 1.357(2),
B1–O1 1.455(3), B1–N1 1.582(2), F1–B1–F2 111.76(16), F1–B1–O1
110.55(16), F2–B1–O1 108.89(17), F1–B1–N1 107.49(15), F2–B1–N1
Scheme 1 Synthetic route to compounds 2a–2c.
110.71(15), O1–B1–N1 107.35(14).
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Fig. 3 Normalized UV-Vis absorption (top) and fluorescence (bottom)
spectra of compounds 2a–2c. Insets: fluorescence photograph under
UV light (wavelength of light is 365 nm).
Fig. 2 Molecular structure of 2c. (a) ORTEP view of 2c with thermal
ellipsoids at the 50% probability level. (b) Side view of 2c (capped stick
model). Co-crystallized solvent molecule and hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and bond angles (°) are described in the following section. Upon excitation of a
as follows. B1–F1 1.3751(13), B1–F2 1.3894(14), B1–O1 1.4585(14), B1–
N1 1.5851(15), F1–B1–F2 111.58(9), F1–B1–O1 108.39(9), F2–B1–O1
111.09(9), F1–B1–N1 110.16(9), F2–B1–N1 108.13(9), O1–B1–N1
107.40(8).
dichloromethane solution of boron complexes 2a–2c, a broad
emission band with λmax = 378 (2a), 379 (2b) & 387 (2c) nm
(Fig. 3) is observed. All three complexes showed moderate
emission quantum yields (quantum yields in CH2Cl2: 0.44
(2a); 0.45 (2b); 0.50 (2c)) with larger Stokes’ shifts in compari-
son to typical BODIPYs systems. There is a considerable red
for 2a is 70.34, 45.06 and 58.83°, and for 2c is 64.17, 46.50 and
53.03°, respectively (Fig. S2†). This ring twist leads to a propel-
ler type conformation for the chromophore, which helps in
preventing the π–π stacking interactions. The π–π stacking
interactions are blamed for the quenching of the fluorescence
in the solid state.
shift for complex 2c, both in the absorption and fluorescence
spectra, which can be attributed to the electron-withdrawing
nature of the –CF3 substituent. We tested the solvent effect of
all three boron complexes (2a–2c) using different solvents
(Table 1), ranging from toluene to dimethylformamide, and
found that our boron complexes (2a–2c) are not sensitive
towards solvent polarity, which is quite a contrasting result
compared to the majority of BODIPY dyes.
Photophysical properties
As shown in Fig. 3 & Table 1, all three boron complexes (2a–2c)
exhibited similar UV-Visible absorption and emission spectra.
The absorption spectra of 2a–2c in dichloromethane (Fig. 3)
consist of two bands. The first band is centered at around
Thermal and electrochemical properties
284 nm while the second band appeared at 325 nm (ε = 20 500 The thermal properties of complexes 2a–2c were studied using
M−1 cm−1) for 2a, 325 nm (ε = 21 200 M−1 cm−1) for 2b and thermogravimetric analysis (TGA) and differential scanning
328 nm (ε = 19 100 M−1 cm−1) for 2c. The origin of this absorp- calorimetry (DSC) under a nitrogen atmosphere at a heating
tion was studied using theoretical calculations and will be rate of 20 °C min−1. All three boron complexes exhibit good
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Table 1 Computed and experimental photophysical data of compounds 2a–2c
Experimental data
Theoretical calculations
a
λmax
εmax
Oscillator
strength
Compound
Solvent
(nm)
(M−1 cm−1 × 103)
λema,b (nm)((Φ)c)
MO contributions
λexc (nm)
2a
CH3CN
THF
CH2Cl2
DMF
321
326
325
323
328
17.9
17.6
20.5
18.7
19.4
379 (0.44)
382 (0.40)
379 (0.44)
382 (0.45)
385 (0.46)
HOMO–LUMO
331
329
362
0.274
Toluene
2b
2c
CH3CN
THF
CH2Cl2
DMF
321
326
325
323
328
16.6
16.6
21.2
17.7
17.9
379 (0.44)
382 (0.58)
379 (0.45)
381 (0.36)
384 (0.40)
HOMO–LUMO
HOMO–LUMO
0.313
0.065
Toluene
CH3CN
THF
CH2Cl2
DMF
323
327
328
325
331
16.5
15.1
19.1
18.0
16.8
386 (0.48)
390 (0.49)
387 (0.50)
388 (0.39)
392 (0.51)
Toluene
a Concentrations were 4.00 × 10−5 M. b Excited at the absorption maximum. c Quantum yields were measured according to literature reported69,70
methods using quinine sulphate as the reference.
thermal stability (decomposition starts above 300 °C for 2a & Molecular orbital calculations
2b-see ESI†) with high melting points (>270 °C).
To obtain a better understanding of the electronic structures
of complexes 2a–2c, we carried out density functional theory
The electrochemical properties of complexes 2a–2c were
measured in DMF (and CH3CN) solution using Bu4NPF6 as the
(DFT) calculations by the Gaussian 03 programmes using the
electrolyte and the data are summarized in Table S1 (ESI†). As
B3LYP/6-31G basis set. The plots and the calculated HOMO
and LUMO energy levels for complexes 2a–2c are provided in
shown in Fig. 4, all three complexes exhibit one electron
reduction; the reduction potential for 2c is less negative than
Fig. 5, Table 1 and Tables S2–S4 (ESI†), respectively. As shown
2a and 2b, attributed to the electron withdrawing nature of the
in Fig. 5, the HOMO levels for all three complexes are π-orbi-
–CF3 moiety present in 2c. The reduction potentials of our
tals that involve contributions from the central imidazole, 2,4-
phenyl and the oxygen atom, whereas the LUMO levels for 2a
boron complexes (2a–2c) are comparable with those of the
anilido–benzoxazole boron complexes reported by Lee and co-
and 2b are π* orbitals consisting of atomic orbitals from
workers,50 however notably more highly negative than those of
central imidazole, 1, 2 and 5-phenyl moieties. An important
BODIPYs.68
observation is that the LUMO for 2c is more effectively stabil-
ized than the HOMO so that the gap gets smaller for 2c (4.000 eV)
compared to 2a (4.245 eV) and 2b (4.274 eV). The electronic
excitations for complexes 2a–2c were calculated using
TD-B3LYP/6-31-G. The calculated excitation values are compar-
able with the experimental results, especially the calculation
nicely reproduced the red shift for the complex 2c.
Electroluminescent properties
To assess the electron transfer properties of the complexes
(2a–2c) in polymer LEDs, we fabricated solution-processed
electrophosphorescent devices using a green phosphor, Ir-
(ppy)3, as the dopant. Single layer devices with the configur-
ation of ITO/PEDOT:PSS/PVK:Ir(ppy)3 (8 wt%–16 wt%): 2(40 wt%)/
Mg/Ag were fabricated (Fig. 6), where the ITO glass was used
as a transparent anode; the conducting polymer poly(3,4-ethyle-
nedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was used
as the hole-injection layer; poly(N-vinylcarbazole)(PVK) was
Fig. 4 Cyclic voltammograms of compounds 2a–2c (vs. ferrocene/
used as the host material; the electron-transporting material 2
was mixed into the host materials to facilitate electron trans-
ferrocenium) with 0.1 M Bu4NPF6 in DMF as the supporting electrolyte
(scan rate 100 mV s−1).
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Fig. 7 EL characters of the devices using 2a–2c. (a) Brightness–voltage
and (b) current efficiency–current density.
Table 2 EL performances of the device 2a–2c
Turn-on voltage at
1 cd m−2 (V)
Max. brightness
Efficiency (cd A−1)/
voltage (V)
Device
(cd m−2
)
2a
2b
2c
12.5
23.5
15
4760
1600
200
10.6
5
0.8
Fig. 5 Computed orbitals for compounds 2a–2c.
group (–CH3) of 2b not only exhibits poor efficiency but also
reduces the brightness, which is attributed to steric-hindrance
limiting electron injection.
To balance the hole and electron injection (compound 2a),
the amount of guest molecule (Ir(ppy)3) was increased from
8 wt% to 16 wt%. The B–V curves of these two samples are
summarized in Fig. S3 and Table S6.† The device with 16 wt%
of guest molecule (Ir(ppy)3 exhibits maximum brightness of
6450 cd m−2 at 12.5 V, with maximum current efficiency
(ηc,max) of 11.8 cd A−1. The improved device performance may
be attributed to the balanced recombination of holes and
electrons.
Fig. 6 Schematic illustration of EL device structure.
port in the emitting layer; Ir(ppy)3 with a concentration of 8
and 16 wt% was used as the emitter. Fig. 7(a) shows the bright-
ness–voltage (B–V) characteristics of the devices. According to
the resulting characteristic curves, the turn-on voltages of 2a,
2b, and 2c were 12.5 V, 23.5 V and 15 V, respectively (corres-
ponding to 1 cd m−2). The maximum brightnesses obtained
for 2a–2c were 4760, 1600, and 200 cd m−2, respectively; and
highest current efficiencies of 10.6, 5, and 0.8 cd A−1 respecti-
vely (Fig. 7(b) and Table 2). The device characteristics suggest
that the electron withdrawing groups (2c) are likely to prevent
the current inject into the system and the electron donating
Experimental section
All reagents and starting materials were purchased from
Sigma-Aldrich, Alfa Aesar and Spectrochem chemical compa-
nies and used as received unless otherwise noted. Chlorinated
solvents and acetonitrile were distilled from CaH2. THF and
toluene were distilled from Na/benzophenone prior to use.
Tetraarylimidazoles (1a–1c) were prepared according to litera-
ture procedures.55,65 All 400 MHz H, 100 MHz 13C, 128 MHz
1
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11B, and 376 MHz 19F NMR spectra were recorded using a Bruker dard. Thermogravimetric analyses (TGA) were recorded using a
ARX 400 spectrometer operating at 400 MHz. All 1H and 13C PerkinElmer Pyris 6 TGA model in a nitrogen atmosphere at a
NMR spectra were referenced internally to solvent signals. 11B heating rate of 20 °C min−1. Differential scanning calorimetric
NMR spectra were referenced externally to BF3·Et2O in CDCl3 (DSC) analyses were recorded using a PerkinElmer Pyris 6 DSC
(δ = 0), 19F NMR spectra, to α,α,α-trifluorotoluene (0.05% in model in a nitrogen atmosphere at a heating rate of 10 °C
CDCl3; δ = −63.73). All NMR spectra were recorded at ambient min−1. Electroluminescence properties were measured using a
temperature. ESI mass spectra were recorded using a Bruker Minolta CS-100A instrument. The I–V and L–V characteristics
micrOTOF-QII mass spectrometer. The absorbance spectra of the devices were measured by integrating a Keithley 2400
were recorded using a PerkinElmer Lambda 750 UV–visible source-meter as the voltage and current source and a Minolta
spectrometer. The fluorescence spectra were recorded using a CS100A instrument as the Luminance detector. All of the
PerkinElmer LS-55 fluorescence spectrometer. The fluo- measurements and device fabrications were performed at
rescence spectra were corrected for the instrumental response. room temperature in a dust-controlled environment.
The quantum yield was calculated by measuring the integrated
Device fabrication
area under the emission curves and by using the following
equation: Φsample = Φstandard × (Isample/Istandard) × (ODstandard
/
The indium-tin oxide (ITO) glass plates were cleaned by soni-
ODsample) × (ηsample2/ηstandard2) where ‘Φ’ is the quantum yield, cation and rinsed in a series of solvents: deionized water,
‘I’ the integrated emission intensity, ‘OD’ the optical density at Triton-100 water solution, deionized water, acetone, and then
the excitation wavelength, and ‘η’ the refractive index of the methanol. The solution composed of poly(ethylenedioxythio-
solvent. The subscripts “standard” and “sample” refer to the phene):poly(styrene sulfonic acid) (PEDOT:PSS) with Triton-
fluorophore of reference and unknown respectively. In this 100 (10 : 1 vol%) was spin-coated (5000 rpm, 60 s) on top of the
case, the unknown is 2a–2c and the reference is quinine sul- ITO substrates to form ca. 30 nm layer. The samples were then
phate (quantum yield of quinine sulphate in 1 N H2SO4 is baked for 30 min at 130 °C under vacuum. Thin films of the
0.55). Optically matched solutions with very similar optical emitting layer (EML) were made by spin-casting the com-
densities of the “sample” and “standard” at a given absorbing pounds dissolved in chloroform (10 mg mL−1). These solutions
wavelength were used for quantum yield calculations.
were filtered using a membrane filter with the pore size of
Elemental analyses were carried out using a Thermo quest 0.45 µm. The thin film samples were then thermally annealed
CE instrument model EA/110 CHNS-O elemental analyzer. in a vacuum at 80 °C for 5 min. After cooling down to the
Single-crystal X-ray diffraction data were collected on a Bruker room temperature, the substrate was transferred into a vacuum
APEX-II diffractometer equipped with an Oxford Instruments thermal evaporator. A 2 nm thick layer of Mg was deposited at
low-temperature attachment. The data were collected at 296 K a pressure below 2 × 10−6 torr through a mask. Another layer
(2a) and 100 K (2c) using Mo-Kα radiation (0.71073 Å). Crystal- of 100 nm thick Ag was deposited as a protecting layer for Mg.
lographic data for 2a and 2c, and details of X-ray diffraction The deposition rates for Mg and Ag cathodes were 1 and
experiments and crystal structure refinements are given in 4 Å s−1, respectively, to form the active layer with an area of
Table S5.† SADABS absorption corrections were applied in 0.126 cm2. The thickness for each layer was measured by an
both cases.71 The structures were solved and refined with the Alpha step instrument.
SHELX suite of programs.72 All non-hydrogen atoms were
refined with anisotropic displacement coefficients. The H
General procedure for the synthesis of imidazole boron
difluoride complexes
atoms were placed at calculated positions and were refined as
riding atoms. Crystallographic data for the structures of 2a and To a solution of imidazole (1 equiv.) in anhydrous 1,2-dichloro-
2c have been deposited with the Cambridge Crystallographic ethane under nitrogen, BF3·Et2O (6 equiv.) was added. After
Data Centre as supplementary publication no. CCDC 1044387 5 minutes, diisopropylethylamine (DIEA) (6 equiv.) was added
and 1044388.
and the resulting mixture was stirred at 40 °C for 1 h. The
DFT calculations were performed with the Gaussian03 crude solution was filtered through a column of basic Al2O3,
program.73 The structures were optimized using 6-31G(d) eluting with CH2Cl2. The resulting solid was recrystallized
(B3LYP) as the basis set. The input files were generated using using CH2Cl2/hexane or CH3CN to obtain the pure product.
X-ray data. Excitation data were determined using TD-DFT
Synthesis of boron difluoride complex 2a
(B3LYP) calculations.
Cyclic voltammetry measurements were performed with a The quantities involved are as follows: compound 1a (2.00 g,
conventional three electrode cell using an electrochemical 5.15 mmol), BF3·Et2O (3.8 mL, 30.90 mmol) and diisopropyl-
workstation (CH Instrument, Model: 1100A) The three-elec- ethylamine (5.34 mL, 30.90 mmol). Yield: 1.80 g (80%). mp:
1
trode system consisted of a glassy carbon working electrode, a 301 °C. H NMR (400 MHz, CDCl3): δ = 6.44 (d, 1H, J = 8 Hz,
Pt wire as the secondary electrode, and a Ag wire as the refer- ArH), 6.56 (t, 1H, J = 8 Hz, ArH), 6.93 (d, 2H, J = 8 Hz, ArH),
ence electrode. The voltammograms were recorded with ca. 7.12–7.22 (m, 4H, ArH), 7.27–7.34 (m, 6H, ArH), 7.47–7.57 (m,
1.0 × 10−3 M solution in DMF containing Bu4NPF6 (0.1 M) as 5H, ArH) ppm. 13C NMR (100 MHz, CDCl3): δ = 109.85, 119.15,
the supporting electrolyte. The scans were referenced after the 120.55, 124.87, 126.74, 128.24, 128.37, 128.44, 128.56, 129.01,
addition of a small amount of ferrocene as the internal stan- 129.15, 130.47, 130.56, 130.75, 131.23, 132.42, 132.50, 133.13,
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135.23, 156.81 ppm. 19F NMR (376 MHz, CDCl3):
δ
=
boron compounds showed good quantum yields in solution
−138.22 ppm. 11B NMR (128 MHz, CDCl3): δ = 1.17 (s) ppm. and one electron electrochemical irreversible reduction. Fur-
HR-MS (ESI): calcd for C27H19B1F2N2O1 ([M]+): 436.1558, thermore, they also showed high thermal stability. Single layer
found: 436.1519. Elemental analysis calcd (%) for organic light emitting devices were fabricated using these
C27H19B1F2N2O1: C 74.33, H 4.39, N 6.42; found: C 74.20, H boron compounds as the electron-transporting materials. With
4.28, N 6.29. IR (KBr): ν(cm−1) = 3056 (s), 1570 (m), 1503 (s), a doping concentration of 16 wt% (Ir(ppy)3), compound 2a
1311 (m), 1266 (m), 1165 (m), 1150 (m), 1086 (m), 1053 (s), 921 showed a maximum brightness as high as 6450 cd m−2 at
(m), 905 (s), 864 (m), 757 (m), 718 (m), 705 (m), 540 (m).
12.5 V. We expect that our boron compounds have the poten-
tial to serve as electron-transporting material for further OLED
applications. Further studies to improve the brightness and
efficiency by modification of boron compounds and optimi-
zation of the electroluminescence device are in progress in our
laboratories.
Synthesis of boron difluoride complex 2b
The quantities involved are as follows: compound 1b (1.50 g,
3.73 mmol), BF3·Et2O (2.76 mL, 22.38 mmol) and diisopropyl-
ethylamine (3.87 mL, 22.38 mmol). Yield: 1.31 g (78%). mp:
1
274 °C. H NMR (400 MHz, CDCl3): δ = 2.44 (s, 1H, CH3), 6.49
(d, 1H, J = 8 Hz, ArH), 6.58 (t, 1H, J = 8 Hz, ArH), 6.93 (d, 2H,
J = 8 Hz, ArH), 7.12–7.24 (m, 6H, ArH), 7.28–7.33 (m, 6H, ArH),
7.50–7.53 (m, 2H, ArH). 13C NMR (100 MHz, CDCl3): δ = 21.40,
109.83, 118.99, 120.35, 124.79, 126.72, 128.09, 128.39, 128.82,
128.94, 130.44, 130.93, 131.11, 132.18, 132.43, 132.46, 132.91,
140.93, 141.38, 156.64 ppm. 19F NMR (376 MHz, CDCl3): δ =
−138.24 ppm. 11B NMR (128 MHz, CDCl3): δ = 1.16 (s) ppm.
HR-MS (ESI): calcd for C28H21B1F2N2O1 ([M]+): 450.1714,
found: 450.1687. Elemental analysis calcd (%) for
C28H21B1F2N2O1: C 74.69, H 4.70, N 6.22; found: C 74.55,
H 4.55, N 6.04. IR (KBr): ν(cm−1) = 3040 (m), 1610 (m),
1570 (m), 1497 (s), 1311 (m), 1267 (m), 1172 (m), 1051 (s),
904 (s), 855(m), 699 (m), 540 (m).
Acknowledgements
K.V. thanks the Department of Science and Technology (DST),
New Delhi for financial support (SR/S2/RJN-49/2011 and SR/
S1/IC-58/2012). V.M., K.D. thank CSIR, New Delhi for research
fellowship. C.N.C., K.H.H., and M.K.L. thank the financial
support by National Science Council (NSC 103-2221-E-002-108)
and Ministry of Economic Affairs (MOEA 103-EC-17-A-08-S1-
205). D.Y.K. thanks the financial support by National Science
Council (NSC 102-2218-E-002-015-MY2). We are grateful to
Prof. V. Chandrasekhar for TGA and DSC analysis and helpful
discussions.
Synthesis of boron difluoride complex 2c
The quantities involved are as follows: compound 1c (1.60 g,
3.51 mmol), BF3·Et2O (2.60 mL, 21.06 mmol) and diisopropyl-
ethylamine (3.64 mL, 21.06 mmol). Yield: 1.21 g (69%). mp:
Notes and references
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