Synthesis and Characterization of Oxazaborinin Phosphonate for Blue OLED Emitter Applications

Article abstract of DOI:10.1002/cphc.201801087

A blue-light emitting material based on a boron complex containing heteroaromatic phosphonate ligand is synthesized and characterized. The Phospho-Fries rearrangement is used in the synthesis route of the ligand as a convenient method of introducing phosp

Full text of DOI:10.1002/cphc.201801087

Accepted Article
Title: Synthesis and characterization of oxazaborinin phosphonate for
blue OLED emitter applications
Authors: Jonas Köhling, Volodymyr Kozel, Vladislav Jovanov, Romana
Pajkert, Sergey N. Tverdomed, Oleg Gridenco, Malte Fugel,
Simon Grabowsky, Gerd-Volker Röschenthaler, and Veit
Wagner
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10.1002/cphc.201801087
ChemPhysChem
ARTICLE
Synthesis and characterization of oxazaborinin phosphonate for
blue OLED emitter applications
Jonas Köhling,[a] Volodymyr Kozel,[b] Vladislav Jovanov, [a] Romana Pajkert,[b] Sergey N.
Tverdomed, [b] Oleg Gridenco,[c] Malte Fugel,[d] Simon Grabowsky,[d] Gerd-Volker Röschenthaler[b]
and Veit Wagner *[a]
Abstract: Blue-light emitting material based on a boron complex
containing heteroaromatic phosphonate ligand is synthesized and
characterized. The Phospho-Fries rearrangement is used in the
synthesis route of the ligand as a convenient method of introducing
phosphonate groups into phenols. Structural, thermal and
photophysical properties of the resulting oxazaborinin phosphonate
compound have been characterized. DFT geometry optimizations
are studied as well as the spatial position and symmetry of the
HOMO and LUMO. Good thermal stability up to 250 °C enables
vacuum deposition methods next to solution processing. Combining
the work function with the optical band gap from UV-Vis
measurement shows that the band alignment is possible with
standard contact materials. Photoluminescence reveals an emission
peak at 428 nm, which is suitable for the blue light-emitter.
emitters
generally
show
a
noticeably
inferior
electroluminescence (EL) performance with regard to
efficiencies, lifespan, colour quality, and charge-carrier
injection/transport.[4,10–12] According to the standards of the
full-colour display industry, blue emitting materials should
emit a saturated blue or deep-blue colour to meet the
demand of high quality displays. This indicates that the blue
emitters should possess a very wide band-gap, making it
difficult to design their molecular structure. Thus, the
development of efficient blue emitters with improved
electroluminescent (EL) performances is attractive and
challenging.
Among the EL materials, four-coordinated organoboron
complexes have attracted much attention due to their
intense luminescence, high carrier mobility, chemical and
thermal stability as well as facile synthetic accessibility.[6,13–
17]
Importantly the application of boron instead of transition
Introduction
metals (e.g. iridium, copper etc.) is environmentally and
economically preferable. The structures of the boron-
bridged molecules are π-conjugated and can easily be
tuned for desirable properties by modifying the chelating
ligand or substituent on boron.[6] Although, boron
Since the breakthrough made by C. W. Tang et al.[1] in
1987, organic light-emitting diodes (OLEDs) have been
seen as one of the most promising technologies for display
and solid state lighting applications. In the last two decades,
a number of materials have been developed and improved
by both academic and industrial research to fulfil the
requirements of these applications.[2–12] Intense effort has
been devoted to developing materials and device structures
for OLEDs in order to obtain higher efficiencies. Among the
primary-colour luminescent materials for OLEDs, the blue
complexes
with
N,N-chelating ligands (e.g. BODIPYs) are well investigated
and widely used as fluorescent dyes, their less investigated
N,O-analogues are becoming increasingly important
because of their stronger emission in solid state.[18–23]
Compared to widely used N,O-ligands, such as 8-
hydroxyquinolines, pyridylphenolates have
a
less
conjugated system which results in blue emission
(hypsochromic effect).[24] Indeed, only few blue emitters of
four-coordinate boron with pyridylphenolate ligands have
been reported in the literature[25,26] and additional
investigations are needed.
[a]
J. Köhling, Dr. V. Jovanov, Prof. Dr. V. Wagner
Department of Physics and Earth Sciences
Jacobs University gGmbH
Campus Ring 1, 28759 Bremen, Germany
E-mail: v.wagner@jacobs-university.de
Dr. V. Kozel, Dr. R. Pajkert, Dr. S. N. Tverdomed,
Prof. Dr. G-V. Röschenthaler
Department of Life Sciences and Chemistry
Jacobs University gGmbH
Campus Ring 1, 28759 Bremen, Germany
O. Gridenco
Semiconductor Optics, Institute of Solid State Physics
University of Bremen
28359 Bremen, Germany
M. Fugel, Prof. Dr. S. Grabowsky
Institute of Inorganic Chemistry and Crystallography
University of Bremen
An introduction of phosphonate functional groups has
recently shown improved electron injection/transporting
capability and excellent adhesion of emitters on cathode
material.[27] The high electronegativity of oxygen makes the
P=O group highly polar and electron withdrawing, which,
with its tetrahedral geometrical structure, is advantageous
for its application as host and EL material in OLEDs. The
electron withdrawing properties of the phosphonate group
can make the connected core electron deficient, improving
its electron transport properties.[28] HOMO and LUMO are
also shifted due to the electron withdrawing character of the
P=O bond.[29] The tetrahedral geometry is also beneficial to
the amorphous morphology of the organic materials
because it is difficult to stack molecules containing P=O
[b]
[c]
[d]
28359 Bremen, Germany
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/cphc.201801087
ChemPhysChem
ARTICLE
regularly. Thus, the introduction of this group into
pyridylphenol ligands of boron complexes could make it a
favourable technique for property tuning in order to obtain
superior blue emitters for OLEDs. Therefore, the aim of our
project is the introduction of a phosponate group into a
pyridylphenol ligand for the synthesis of boron complexes
which are promising as efficient blue emitting materials.
The Phospho-Fries-rearrangement is a convenient method
of introduction of phosphonate groups into phenols[30] which
previously has not been applied for heteroaromatic
compounds. Hence, in this work we present the first
example of application of Phospho-Fries-rearrangement for
the synthesis of 2-hydroxyphenylpyridine phosphonate
ligands for the N,O-difluoroboronic complex, a potential EL
material. Furthermore, we have fully characterized the
resulting compound including computational calculations of
the molecule and experimental measurements to determine
structural, thermal and photophysical properties. Finally,
possible device architectures are also discussed.
2.0
45b
[a] Determined by 31P{1H} NMR
[b] The presence of a phosphate group at δP = 0.5 ppm was detected
Moreover, it was found that the order of addition of reagents
had an important influence on the reaction outcome. When
a solution of compound
dropwise into pre-generated LDA (1.5 equiv.) in THF at -78
°C, full conversion of into was achieved. On the other
hand, the ‘reverse’ order of addition of reagents (LDA was
added to the solution of phosphate in THF at -78 °C) led
only to a partial rearrangement of and decomposition of
even when large excess of LDA (2 equiv.) was applied.
Despite of possible α-lithiation of the pyridine ring, product 3 was
obtained with high regioselectivity, and the products of
functionalization in the pyridine ring were not registered even in
trace amount. The reaction was performed at -78 °C in dry THF.
The hydroxyphenyl phosphonate 3 was obtained in 71% yield
and fully characterized by NMR. It is known that
2-(pyridin-2-yl)phenol reacts with boron trifluoride etherate to
give a corresponding difluoroboron complex.[24] On other hand,
phosphonic units are known to be reactive towards fluorinating
agents forming fluorophosphates.[33] Indeed, we have found that
compound 3 reacts with BF3 · Et2O upon formation of the
2 in anhydrous THF was added
2
3
2
2
3
Results and Discussion
Synthesis route
difluoroborate complex
4
without affecting the diethyl
The classical Phospho-Fries-rearrangement involves the
conversion of an aryl phosphate to an ortho-
hydroxyarylphosphonate.[27–29] The transposition of the
phosphoryl moiety is initiated by formation of an aryl anion
adjacent to the phosphate substituent.[31] LDA being a
strong and non-nucleophilic base is used to generate an
aromatic anion to avoid substitution at the phosphonate
group.[32] We have revealed that 2-hydroxyphenylpyridine
phosphonate function. The reaction was carried out in dry
benzene in presence of triethylamine, and product 4 was
obtained in 53% yield after column chromatography purification.
1
The complex was fully characterized by H NMR (Fig 1.). The
proton resonances were assigned by H,H-COSY, H,C-HMQC
and H,C-HMBC spectra (ESI Fig. S1 – Fig. S6) and are in
agreement with the DFT calculations. Characteristic signals
were found for 19F at δF = -147.57, and 11B at δB = -0.24 ppm.
(
1
) reacts with diethyl chlorophosphate in presence of
triethylamine as a base to form diethyl 2-(pyridine-2-
yl)phenyl phosphate ( ) in 50% yield (Scheme 1). The
2
reaction was carried out in dry dichloromethane in presence
of catalytic amount of DMAP, the product was purified by
column chromatography and its structure has been
established by NMR. The obtained phosphate
2 was then
treated with LDA to form the typical product of a Phospho-
Fries-rearrangement (Scheme 1). The rearrangement was
performed in THF at -78 °C using various amounts of LDA.
Scheme
1. Synthetic route to diethyl (6,6-difluoro-6H-6λ4,7λ4-
When 1.1 equiv. of a base reacted with phosphate
conversion of achieved 70% and a mixture of ortho-
hydroxy phosphonate and starting material was
2, the
benzo[e]pyrido[1,2-c][1,3,2]oxazaborinin-4-yl)phosphonate.
2
3
2
obtained in a 7 : 3 ratio, respectively (Tab. 1). By increasing
the amount of a base up to 1.5 equiv., the ortho-lithiation
and 1,3-O→C migration proceeded smoothly giving rise to
the desired phosphonate
Treatment of with 2 equiv. of LDA led also to the
rearranged product , however, partial decomposition of
occurred, plausibly due to the C-P cleavage of the
phosphonate Tab. 1).
3 as a sole product (Tab. 1).
2
3
3
3
(
Table
1 Optimization of the LDA amount used for Phospho-Fries
rearrangement in THF from -78 °C to rt.
Amount of LDA
(equiv.)
Conversion of 2 into 3
(%)a
1.1
1.5
70
100
2
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10.1002/cphc.201801087
ChemPhysChem
ARTICLE
Figure 1. 1H NMR spectra of compound 4 measured in CDCl3.
Figure 2. A part of a crystal-packing pattern of compound 4 that shows no
stacking between adjacent interlayered crystals reviewed along the y-axis (a)
and along the x-axis (b).
Further, the crystal structure of a single crystal of 4 was
determined by XRD. It crystallizes in the space group P2(1)/c in
Theoretical calculations
a
monoclinic crystal system with the lattice parameters
a = 11.8968(18) Å, b = 8.7134(13) Å, c = 15.997(3) Å,
β = 104.962(6)°, V = 1602.05(40) Å and Z = 4 (Full information
in ESI Tab. S1). Selected bond lengths of the oxazaborinin part
are compared with an overall good agreement to the DFT
calculation in Tab. 2.
Table 2 Selected bond lengths of compound 4 calculated by DFT (B3LYP/6-
31G*) and measured with XRD.
Bond
DFT [Å]
1.3723
1.3723
1.4557
1.6240
1.8166
XRD [Å]
B – F1
B – F2
B – O1
B – N1
P – C1
1.3836 (22)
1.3778 (19)
1.4399 (22)
1.5999 (22)
1.7915 (15)
Figure 3. A part of a crystal-packing pattern of compound 4 that shows no
stacking between adjacent interlayered crystals reviewed along the y-axis (a)
and along the x-axis (b).
XRD studies of similar structures show that the basic
oxazaborinin molecule class tends to form π-π stacking in the
solid
state,[34]
which
can
lead
to
fluorescence
DFT calculations at a B3LYP/6-31g* level were carried out to get
insights into electronic and structural properties at a molecular
level. The relaxed structure and the spatial resolution of the last
two highest occupied (HOMO-1 and HOMO) and first two lowest
unoccupied (LUMO and LUMO+1) molecular orbitals are shown
in Fig 3. Overall the molecule shows a flat structure, with a small
twist at the heteroatoms oxygen and boron in the backbone. It is
expressed by the dihedral angle of θCOBN = -45.2°. The induced
stress twists the pyridine ring with respect to the phenyl ring by
quenching.[13,14,19,20,35] An approach of introducing bulky side
substituents is used to overcome this problem. Indeed, the X-ray
crystal analysis of compound 4 reveals that the alignment of
diethylphosphonate groups between layers reduces the
formation of the π-π stacking between phenyl or pyridine rings
(Fig. 2) compared to a basic oxazaborinin molecule class.[34]
θ
CCCC = -13.6°. The energy levels of the molecular orbitals are
calculated to be -6.891 eV, -6.295 eV, -2.266 eV and -1.702 eV,
respectively. The LUMO molecular orbitals are located mainly in
the 2-phenyl-pyridine ring of the backbone. The LUMO is
predominately located in the pyridine part and also distributes
slightly over the oxazaborinine. The LUMO+1 is located only
over the 2-phenyl-pyridine. Both, LUMO and LUMO+1 do not
spatially distribute towards the phosphonate group. The
occupied molecular orbitals are also mainly located at the 2-
phenyl-pyridine ring of the backbone. However, the HOMO and
HOMO-1 are predominantly located at the phenyl ring and are
less pronounced at the pyridine part. They differ from the spatial
point of view from each other that the HOMO is more distinct at
the oxygen of the oxazaborinine than at the phosphonate group,
whereas the opposite holds true for the HOMO-1. However,
spatial positon and symmetry of molecular orbitals provide
sufficient overlap which promotes transitions.
3
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10.1002/cphc.201801087
ChemPhysChem
ARTICLE
the absorption spectra. All measurements show absorption band
maxima at λ = 332 nm, λ = 299 nm, λ = 260 nm and λ = 216 nm,
which are expected to be π-π* transitions within the aromatic
backbone of the molecule and n-π* transitions of free electron
pairs. The optical band gap is estimated to be EG = 366 nm
(3.39 eV) as the onset of the absorption. The absorption spectra
for the evaporated film is multiplied by 6 to be directly
comparable with other results.
Basic oxazaborinin molecules exhibit promising fluorescence
quantum yields for OLED-emitters.[20,34] The PL spectra of the
bladed thin film of compound 4 is given in Fig 5. and shows two
emission maxima at λ = 428 nm and λ = 500 nm. The shoulder
features indicate underlying emissions which give only a small
contribution and cause the broadening of the spectra. The onset
of the PL is estimated to be at λ = 401 nm (3.09 eV) which is
red-shifted by 35 nm with respect to the absorption of the
material. This is caused by thermalization and different
geometries between the ground state and the excited state, the
so called Stokes shift.
Figure 4. TGA (blue) and DSC (red) measurement of compound 4 under
nitrogen.
Thermal properties
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) are used to investigate the thermal properties
of compound 4. The weight loss below T = 100 °C is assigned to
desorbing water. The material is stable until the decomposition
temperature of TD = 251 °C. Until this temperature the DSC
curve is in the endothermic region and shows a melting
temperature in the region between T = 144 – 147 °C. At
temperatures higher than T = 251 °C, the curves indicate that
the material is first evaporating and then thermally decomposing.
The results confirm that the material possesses good thermal
properties for deposition under high vacuum utilizing organic
molecular beam deposition (OMBD).
Figure 6 He Iα UPS spectra after sputter-cleaning OMBD compound 4
(20 nm) on top of gold (Vbias = 10 V).
Figure 5. UV-Vis absorption spectra of compound 4 evaporated (20 nm),
bladed (200 nm) and in solution (ACN) as well as normalized PL emission
spectra of compound 4 bladed (200 nm).
Photophysical properties
In order to determine suitability of the material as blue emitter
UV-VIS and photoluminescence measurements were conducted.
For this purpose, we prepared a solution in ACN and deposited
thin films on glass substrates by evaporation and solution
processing.
To determine the energy levels of molecular orbitals of
compound
4 and find potential contact materials UPS
measurements were conducted.
Fig. 6 shows the He Iα UPS spectra of compound 4 using gold
as a reference. The ionization energy for this molecule is equal
to the energy of the HOMO level, which is measured to be
Fig. 5 shows the results of UV-Vis and PL measurements of
compound 4. UV-Vis spectra are measured in solution (ACN),
for bladed and evaporated thin films. Solution and solid state
absorptions are very similar except for a small broadening that
occurs in the solid state. This indicates that intermolecular
interactions are minor and give only a negligible contribution to
E
HOMO = -5.288 eV, while the Fermi energy is EF = -4.153 eV. By
adding the optical band gap energy to the energy of the HOMO
level allows us to estimate the energy for the LUMO as ELUMO = -
2.038 eV, relative to the vacuum level. With the Fermi energy
4
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10.1002/cphc.201801087
ChemPhysChem
ARTICLE
General information
located closer to the HOMO level, the material can be
characterized as p-type. Based on the determined absolute
energy levels of the molecular orbitals, materials like ITO and
Liq/Al are suitable contact materials as shown in Fig. 6. Further
alignment between the contact materials and compound 4 can
be achieved by additional hole transport layers (HTL) and
electron transport layers (ETL), which are not shown in Fig. 7.
All chemicals and solvents were purchased from Sigma-Aldrich and used
without further purification.
Sample preparation and characterization
NMR and XRD. 1H NMR (400 MHz), 11B NMR (128 MHz), 13C NMR
(100 MHz), 19F NMR (376 MHz) and 31P NMR (162 MHz) were recorded
on JEOL ECX 400 MHz spectrometer. 1H, 11B, 13C, 19F and 31P NMR
chemical shifts (δ) are reported in ppm from tetramethylsilane, BF3·OEt2,
CFCl3, and H3PO4, using the residual solvent resonance as an internal
reference. Coupling constants (J) are given in Hz. XRD of a single crystal
was measured with a Bruker D8 Venture diffractometer fitted with a IμS
microfocus radiation source using MoKα radiation and a Photon-100
CMOS detector. The data collection was performed at a temperature of
100 K.
TGA and DSC. TGA and DSC measurements were carried out with a
Thermal Analysis Q 600 under nitrogen with a heating ramp of 5 °C/min
to determine the melting point and thermal properties of the material.
Sample preparation. Quartz substrates were cleaned before deposition
by 2 min ultrasonication in acetone (tech. grade), followed by 10 min
ultrasonication in a mixture of acetone and isopropanol (1:1, VLSI grade)
and finally rinsed with deionized water (18.3 MΩ) for 1 min. The
substrates were dried first with nitrogen and then on a hotplate at 120 °C
before 10 min UV/ozone treatment. For UPS measurements a 200 nm
thick gold layer was sputtered on top of the substrate (Quorum QT150T).
Thin films with a thickness of 200 nm were deposited by doctor blading
(Erichsen COATMASTER 510) 40 µL of a 20 mg/mL solution of the
compound in chlorobenzene. The blade was operated at a speed of
50 mm/s with 2 mm height. Afterwards the samples were dried at 75 °C
Figure 7 Energy level diagram for compound 4 with suitable contact materials.
Possible charge transport layers are not displayed.
for 3 min. The crystallization occurred over night in
a glovebox
Conclusions
environment (MBraun, nitrogen, O2 < 2 ppm, H2O < 2 ppm). Thin films
with a thickness of 20 nm were deposited by organic molecular beam
deposition OMBD in UHV at 170 °C with a rate less than 1 nm/min and
monitored with a quartz scale (intellimetrics, IL 150) utilizing the density
obtained from XRD.
In this study, we present the design, synthesis and
characterization of the luminescent organic semiconductor
diethyl
(6,6-difluoro-6H-6l4,7l4-benzo[e]pyrido[1,2-
c][1,3,2]oxaza-
UPS. UV photoelectron spectroscopy was carried out in UHV (10-8 mbar)
using a Specs PHOIBOS 100 hemispherical energy analyzer and He Iα
borinin-4-l)phosphonate. The molecular structure was confirmed
with NMR and XRD. Thermal stability was examined by TGA.
Selected structural properties were compared to DFT
calculations. Also, the DFT calculations were used to visualize
the spatial distribution of the last two occupied and first two
unoccupied molecular orbitals. HOMO and LUMO are mainly
located in the conjugated aromatic backbone, which includes the
oxazaborinin structure. The compound shows good thermal
stability and thin layers can be deposited under high vacuum by
radiation (E = 21.2 eV).
A bias of V = 10 V was applied while
measurement. Before UPS measurement the surfaces were cleaned by
argon sputtering.
UV-VIS and PL. UV-VIS measurements were carried out with a Perkin
Elmer Lambda 12. For PL measurements, a microscope setup with a 36
x objective, numerical aperture 0.5 into which a 325 nm cw (He-Cd)
laser was coupled, was used to collect the PL and recorded using a
single-grating,
spectrometer.
thermos-electric
cooled
AvaSpec-ULS2048LTEC
thermal
evaporation.
UV-Vis
and
photoluminescence
measurements were carried out for thin films as well as
dissolved material to determine photophysical properties such
as the wide optical band gap (366 nm, 3.39 eV). Also the
material exhibits two emission maxima in the blue range of
visible light around 428 nm and 500 nm in the blue region of
visible light. The exact energy levels (EHOMO = -5.288 eV, EF = -
4.153 eV and as ELUMO = -2.038 eV) are determined by UV
photoelectron spectroscopy and suitable contact materials for
device fabrication are given. The reported data show that this
class of molecule has a strong potential for applications in OLED
devices.
DFT calculations
First, the geometry of a single molecule in vacuum was optimized by
density functional theory (DFT) with B3LYP[36–39] (Becke three parameter
Lee-Yang-Parr) exchange-correlation functional and the Gaussian 09
code.[40] 6-31G*[41] was used as a basis set for all atoms (H, B, C, N, O, F
and P). Afterward, the frequency calculations were carried out, and it was
ensured that no imaginary vibrations were calculated. Avogadro
software[42] was used to visualize the computational structure and the
molecular orbitals for the following states HOMO-1, HOMO, LUMO, and
LUMO+1. Finally, the NMR shielding tensors and magnetic
susceptibilities were calculated using the Gauge-Independent Atomic
Orbital method (GIAO) approximation. All NMR shielding tensors were
referenced to the tetramethylsilane (TMS) calculated spectra utilizing the
beforehand mentioned parameters. For the NMR calculations solvent
Experimental Section
5
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10.1002/cphc.201801087
ChemPhysChem
ARTICLE
effects were taken into account with the Polarizable Continuum Model
(PCM).
M.p. 144-147 °C. 1H NMR (400 MHz, CDCl3) δ 8.70 (d, J = 5.5 Hz, 1H),
8.23 (ddd, J =1.4 Hz, 7.3 Hz, 8.7 Hz, 1H), 8.13 (d, J =8.7 Hz, 1H), 8.09
(ddd, J = 1.8 Hz, 7.3 Hz, 14.2 Hz, 1H), 8.01 (d, J = 7.8 Hz, 1H), 7.63 (ddd,
J = 0.9 Hz, 5.7 Hz, 7.3 Hz, 1H), 7.12 (ddd, J = 33.7 Hz, 7.8 Hz, 14.2 Hz,
1H), 4.23 (m, 4H), 1.36 (td, J = 3.4 Hz, 7.0 Hz, 6H), 13C NMR (101 MHz,
Synthesis
CDCl3)
δ 16.42 (d, J = 6.7 Hz), 62.84 (d, J = 5.8 Hz), 116.86 (d,
J = 10.5 Hz), 119.42 (s), 120.33 (d, J = 14.4 Hz), 120.96 (s),123.61 (s),
129.97 (d, J = 1.9 Hz), 140.60 (d, J = 6.7 Hz), 141.28 (s), 142.77 (s),
149.61 (s), 157.56 (d, J = 2.9 Hz), 11B NMR (128 MHz, CDCl3) δ -0.23
(s)., 19F NMR (376 MHz, CDCl3) δ -147.56 (d, J = 23.1 Hz), 31P NMR
(162 MHz, CDCl3) δ 15.48 (s)
Synthesis of 2-(pyridin-2-yl)-phenol (1). 2-(Pyridin-2-yl)-phenol was
synthesized according to the reported literature.[43]
A solution of 2-phenylpyridine (4.37 g, 28 mmol), PdCl2 (0.5 g, 2.8 mmol),
and 35% aqueous H2O2 (13.7 g, 1.5 mmol) in 4-methyl-2-pentanone
(10 mL) was stirred for 12 h at 100 °C. Then, the reaction mixture was
filtered through a celite pad and washed with EtOAc. The solution was
treated with a 0.5 M aqueous Na2S2O3 solution, and then the organic
layer was dried over MgSO4 and concentrated in vacuo. The crude
product was purified by flash chromatography on silica gel (petroleum
ether / DCM, 3 : 7) and the product 2 was obtained with a yield of 78%
(3.76 g). The analytical data are in a good agreement with those from the
publication.
Acknowledgements
We thank Ali S. Mougharbel and Prof. Dr. Ulrich Kortz,
Department of Life Sciences and Chemistry, Jacobs University
Bremen, for TGA/DSC measurements.
Keywords: Fluorescence • Materials science • N,O-
Synthesis of diethyl 2-(pyridine-2-yl)phenyl phosphate (2). To a
solution of DMAP (488 mg, 4.4 mmol) TEA (1.07 g, 4.4 mmol) and 2-
(pyridin-2-yl)-phenol (1) (680 mg, 4.0 mmol) in 20 mL dry DCM diethyl
chlorophosphate (690 mg, 4.4 mmol) was added drop-wise. Then, the
solution was stirred for 24 h at room temperature. Afterwards, the
reaction mixture was treated twice with saturated NaHCO3 (2 x 30 mL)
and brine (1 x 20 mL) before drying in vacuo. The crude product was
purified through column chromatography on silica gel (eluent: EtOAc).
The final product was obtained as colorless oil with a yield of 50%
(1.23 g).
difluoroboronic compound • Phospho-Fries rearrangement
[1]
[2]
C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913–915.
A. R. Bin Mohd Yusoff, A. J. Huckaba, M. K. Nazeeruddin, Top. Curr.
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A. de Bettencourt-Dias, Dalt. Trans. 2007, 2229.
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22, 5410.
[5]
[6]
[7]
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1H NMR (400 MHz, CDCl3) δ 8.70 (ddd, J = 4.6, 1.4, 1.4 Hz, 1H), 7.80 –
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J = 8.1, 7.4, 1.8 Hz, 1H), 4.08 – 3.91 (m, 1H), 1.19 (tt, J = 7.1, 1.0 Hz,
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136.07, 132.10 (d, J = 6.7 Hz), 131.48, 130.03 (d, J = 1.2 Hz), 125.45 (d,
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