Synthesis and Optical Properties of a Series of Green-Light-Emitting 2-(4-Phenylquinolin-2-yl)phenol-BF2 Complexes (Boroquinols)

Article abstract of DOI:10.1002/ejoc.201500461

A series of 2-(4-phenylquinolin-2-yl)phenol derivatives were efficiently synthesized and complexed with boron trifluoride to give fluorescent organoboron complexes. All the synthesized compounds were characterized by ¹H, ¹³C, ¹¹B, ¹⁹F NMR, FTIR, and HRMS analyses, and single-crystal structures of representative compounds were determined to examine the conformations of these compounds. The electronic properties of 2-(4-phenylquinolin-2-yl)phenol-BF₂ complexes were studied by absorption and fluorescence spectroscopy and the complexes were found to be highly emissive in the solid state. The electrochemical properties of boroquinols were measured by using cyclic voltammetry (CV) in acetonitrile with 0.1 M tetrabutyl ammonium hexafluorophosphate as a supporting electrolyte and the LUMO values of boroquinols were calculated to range from -2.64 to -3.46 eV, with HOMO values of -5.32 to -6.10 eV.

Full text of DOI:10.1002/ejoc.201500461

FULL PAPER
DOI: 10.1002/ejoc.201500461
Synthesis and Optical Properties of a Series of Green-Light-Emitting
2-(4-Phenylquinolin-2-yl)phenol–BF2 Complexes (Boroquinols)
Umamahesh Balijapalli[a] and Sathiyanarayanan Kulathu Iyer*[a]
Keywords: Dyes/Pigments / Boron / Fluorescence / Luminescence / Redox chemistry
A series of 2-(4-phenylquinolin-2-yl)phenol derivatives were
efficiently synthesized and complexed with boron trifluoride
to give fluorescent organoboron complexes. All the synthe-
sized compounds were characterized by 1H, 13C, 11B, 19F
NMR, FTIR, and HRMS analyses, and single-crystal struc-
tures of representative compounds were determined to ex-
amine the conformations of these compounds. The electronic
plexes were studied by absorption and fluorescence spec-
troscopy and the complexes were found to be highly emissive
in the solid state. The electrochemical properties of boroquin-
ols were measured by using cyclic voltammetry (CV) in
acetonitrile with 0.1
M tetrabutyl ammonium hexafluorophos-
phate as a supporting electrolyte and the LUMO values of
boroquinols were calculated to range from –2.64 to –3.46 eV,
properties of 2-(4-phenylquinolin-2-yl)phenol–BF2 com- with HOMO values of –5.32 to –6.10 eV.
Introduction
tronics.[6] Moreover, there is the possibility of modulating
the emission wavelength of such compounds by introducing
functional groups and/or extending the conjugation around
the fluorescent core.[7]
Luminescent organic and organometallic compounds
have attracted much interest because of their potential ap-
plications as biomolecular labels, molecular probes and
switches, solar cells, laser dyes, photovoltaics, and organic
Most BODIPY dyes, however, possess very small Stokes
light-emitting diode (OLEDs).[1] Among the various types shift values (400–600 cm–1) because of their molecular ri-
of luminescent compounds, boron complexes based on N- gidity with minimal differences between the ground- and
and O-donor ligands have received particular attention be- excited-state structures.[8] Such small Stokes shifts make it
cause they exhibit excellent photophysical properties such impossible for optical filters to cut off the excitation light,
as thermal, photo and chemical stability and high fluores- which makes it difficult to identify the fluorescence signal
cence quantum yield.[2] In particular, four-coordinate fluo- over the noise in a bioassay. It also results in the reabsorp-
rescent organoboron complexes (FOBCs) constitute par- tion of its own fluorescence, thereby quenching the fluores-
ticularly elegant fluorescent dyes and they serve as efficient cence intensity..[9] Furthermore, most BODIPY dyes pos-
emitters in OLEDs. Several classes of four-coordinate sess intermolecular interactions due to their high planarity,
boron chromophores including N,O-, N,N-, and N,C-che- which increases the concentration-quenching effect in the
late π-systems have been synthesized.[3] The four-coordinate solid state. Thus, these molecules exhibit almost no fluores-
boron molecules with four-, five- and six-membered ring cence in the solid state, which may explain why BODIPYs
fused skeletons have recently been produced as bright emit- are rarely applied as electroluminescence materials.[10] Al-
ting materials with blue, green, and yellow emission col- though some efforts have been made to induce larger Stokes
ors.[4] FOBCs such as azobenzene, diketone, pyridometh- shifts in BODIPY derivatives, the major problem has been
ene, pyrromethene, subphthalocyanine, subporphyrin, and the loss of energy through nonradiative decay of the excited
others have been reported.[5] Specifically, boron-dipyrro- fluorophores.[11] Recently N-benzylideneaniline, benzox-
methene (BODIPY) is well known to exhibit not only excel- azole, benzothiazole, pyridomethene, pyrazine, thiazole,
lent optical properties such as sharp and narrow absorption naphthyridine, pyrazolyl aniline, indocarbazole, phenanthro
and emission spectra, high photo and chemical stability, imidazole, coumarin, iminocoumarin and isatin–phenyl-
and good solubility, but it also plays a significant role in hydrazone–BF2 complexes have been studied extensively as
many fields including biomolecular probes and optoelec- an analogue of BODIPY dyes.[12]
Thus, in a continuation of this quest, we attempted to
[a] Chemistry Division, School of Advanced sciences, Vellore
Institute of Technology,
explore a new class of four-coordinated quinoline-based
boron complexes to substitute BODIPY derivatives as
multifunctional fluorophores for broader applications with
excellent emitting properties. In this paper, we outline the
synthesis and fluorescence properties of new quinoline–
boron complexes (boroquinols) that exhibit green fluores-
Vellore 632014, Tamilnadu, India
E-mail: sathiya_kuna@hotmail.com
https://www.sites.google.com/site/
sathiyanarayananresearchgroup/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500461.
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U. Balijapalli, S. K. Iyer
FULL PAPER
cence both in solution and in the solid state with large ide (Cu2O), which gave moderate to good yields (Scheme 1).
Stokes shift (3688–10231 cm–1). To our knowledge, there These compounds were very weakly fluorescent in solution
has only been one report describing related boroquinol because of an excited state intramolecular proton transfer
complexes with photophysical properties and no detailed (ESIPT) process. The reaction of the 2-(4-phenylquinolin-
electrochemical studies were reported.[13] The main advan- 2-yl)phenol (PQPs) with boron trifluoro diethyl etherate
tage of the current study was the introduction of the phenyl (BF3·OEt2) in the presence of triethylamine gave quinoline-
substituent on the quinoline, which should improve the so- containing BF2 complexes (Scheme 1). The reaction was
1
lid-state emission properties by reducing π–π interaction be- readily monitored by H NMR spectroscopy because the
tween chromophores, and could be a handle for future phenolic OH signal of the 2-(4-phenylquinolin-2-yl)phenol
derivatization.
at δ = 15.5 ppm disappeared progressively; this was further
affirmed by 11B and 19F NMR spectroscopy. In addition,
these compounds are strongly fluorescent in the solution
state because of an intramolecular charge transfer (ICT)
Results and Discussion
Substituted 2-(4-phenylquinolin-2-yl)phenol (PQPs) de- process. The quinoline structures and quinoline–boron
rivatives were obtained by heating the corresponding anil- complex structures were fully characterized and confirmed
ine, salicylaldehyde, and phenylacetylene to reflux in nitro- by 1H, 13C, 11B, 19F NMR, FTIR, HRMS, and X-ray crys-
methane solution in the presence of iodine and cuprous ox- tallography. Details of the experimental procedures and
Scheme 1. Synthesis of substituted 2-(4-phenylquinolin-2-yl)phenol derivatives 4a–l and their corresponding boron(III) complexes 5a–l.
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Green-Light-Emitting Boroquinols
structural characterization data along with spectra are pre- Figure S6). The central B-atom possessed B–F1, B–F2, B–
sented in the Supporting Information.
N, B–O bond lengths of 1.378(2), 1.364(2), 1.607(2), and
Furthermore, to explore the solid-state fluorescence 1.410(2) Å, respectively, and bond angles around the B
properties of these compounds, X-ray crystallographic atom ranged from 106.8 to 111.35 Å [F2–B1–F1 110.03(2),
analysis was performed for compound 5h. A single crystal F2–B1–O1 108.45(2), F1–B1–O1 110.54(1), F2–B1–N1+
of ligand 5h that was suitable for X-ray diffraction studies 109.63(1), F1–B1–N1 106.83(1), O1–B1–N1 111.35(1)]. The
was grown by slow evaporation from ethanol and tetra- bond lengths and bond angles were similar to N,O-chelated
hydrofuran solutions at room temperature. ORTEP dia- BF2 complexes. As depicted in Figure 1, the phenyl moiety
grams of boroquinol 5h are given in Figure 1 (top and bot- at the C9-atom exhibited distorted geometry from the
tom). The complex crystallized as a triclinic system without planar quinoline complex. The molecule is essentially co-
solvation and the boron atom was coordinated in a tetrahe- planar, and all portions except for the phenyl moiety at the
dral geometry by nitrogen, oxygen, and two fluorine atoms C9-atom and two F-atoms were located in the same plane,
with one F–B–F center. In the crystal structure of 5h, the which suggests the formation of an extended π-conjugation
boron atom (B1) lies –0.065(2) Å in the least square plane system that is favorable for intramolecular charge transfer.
containing N1, O1, C1, C6, and C7 atoms, and the ring has The crystal structure of 5h thus showed a π-conjugated
almost planar structure (see the Supporting Information, framework exhibiting B–O interactions, which resulted in a
Figure 1. Top: ORTEP diagram for boroquinol 5h (CCDC-1049309); bottom: intermolecular π-π interactions among the complexes of
5h.
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FULL PAPER
rigid molecular structure. As shown in Figure 1 (bottom),
the molecules were arranged in a layer fashion and signifi-
cant intermolecular CH–π, and π–π interactions were found
to exist along the crystallographic axis direction with dif-
ferent intermolecular distance of CH–π···O (2.653 Å), CH–
π···F (2.629 Å), CH–π···B (3.155 Å), and CH–π···F2–B
(2.379 Å). The intermolecular π–π interactions linked the
neighboring molecules and hence resulted in the formation
of network π–π interactions. The π–π interactions are
known to cause fluorescence quenching in the solid state.[14]
Figure 2. UV/Vis absorption (left) and fluorescence spectra (right)
Surprisingly, despite the existence of π–π networks, the of 4a, 4b, 5a and 5b in chloroform measured at a concentration of
1 ϫ 10–5 moldm3 of substrate at 25 °C.
compounds exhibited strong fluorescence in the solid state.
The UV/Vis absorption and fluorescence spectra of 4a–l
were measured in chloroform and are shown in the Sup- attributed to πǞπ* transition and nǞπ* transition,
porting Information (Figure S14–S15). The UV/Vis absorp- respectively. Compound 5a exhibited maximum absorption
tion spectra of 4a showed two absorption peaks at 297 wavelength (λmax) at 375 nm, which is a 10 nm bathoch-
(weak) and 348 (strong) nm, which can be attributed to romic shifted compared with 5b (365 nm). In addition to
πǞπ* and hydrogen bonding between the phenolic OH the strong absorption band at 291 nm, weak absorption
and quinoline nitrogen atom respectively. The PQP ligands bands were present at 365, 415 nm along with the vi-
were weakly emissive but showed large Stokes shifts (Δss) brational peaks, and this could be attributed to π–π transi-
and high molar extinction coefficient values through an ex- tions present in 5b. Although 4a and 4b exhibited very weak
cited-state intramolecular proton transfer process; this is a fluorescence in chloroform solvent, 5a and 5b showed en-
characteristic feature of hydroxybenzoxazole family that hanced bluish green fluorescence at 476 and 490 nm, respec-
was responsible for the large Stokes shifts.[12b] Indeed, the tively. The steady-state fluorescence spectrum of 5a was ob-
tautomerization in the excited state led to the keto form served at 476 nm with mirror image relationship to the low-
that is responsible for the lower energy emission. The ab- est energy absorption band shown in Figure 3 and this is
sorption and emission data of PQP ligands are summarized common for all the boroquinols (see the Supporting Infor-
in the Supporting Information (Table S1). The photophysi- mation, Figure S2–S13). The emission spectra of boroquin-
cal properties of the newly synthesized boroquinols 5a–l ols 5a–l (see the Supporting Information, Figure S18) ex-
were recorded in chloroform solvent as shown in the Sup- hibited a major emission between 467 and 538 nm with
porting Information (Figure S16–S18) and the correspond- large Stokes shifts in the range 3688–10231 cm–1. The λmax
ing data are summarized in Table 1. The absorption and of 5c (531 nm) and 5d (538 nm) were redshifted by 55 and
fluorescence spectra of ligands 4a and 4b, and correspond- 62 nm, respectively, compared with that of 5a (476 nm).
ing complexes 5a and 5b in chloroform are shown in Fig- Unlike the low stokes shift values in BODIPYs, higher
ure 2.
stokes shift values were observed for these boroquinols in
The absorption spectra of complexes 5a–l exhibited a the range of 3688–10231 cm–1. Larger Stokes shift values
major absorption band between 250 and 415 nm. UV/Vis (Δss) observed for 5c (9543 cm–1) and 5d (10231 cm–1) were
absorption spectra of 5a showed three absorption peaks at higher than that of 5a (5658 cm–1) assigned to increased
288, 329, and 375 nm. The weak absorption peaks at 288, ICT character. This large Stokes shift may be attributed to
319 nm and the strong absorption peak at 375 nm could be the increased electron density as a result of the donating
Table 1. Optical properties of quinoline–boron complexes in chloroform.
[b]
5
Solution state[a]
Solid state
λem [nm]
Δss [cm–1]
Φfl
τ [ns][c] Kr [108 s–1] Knr [108 s–1]
λabs [nm] (ε 104 m–1 cm–1)
λem [nm]
5a
5b
5c
5d
5e
5f
5g
5h
5i
288(1.27), 329(1.32), 375(1.5)
289(1.5), 365(0.7), 415(0.38)
283(1.32), 348(1.57)
284(1.21), 347(1.43)
250(1.16), 297(1.62),351(1.72)
291(1.43), 376(1.33)
289(1.45), 334(1.05), 382(1.11)
288(1.56), 328(1.41), 378(1.37)
289(1.63), 338(1.13), 381(1.22)
352(2.42), 385(2.62)
291(1.18), 332(1.15), 376(1.44)
285(1.54), 375(1.64)
476
490
531
538
486
485
484
482
484
467
471
496
484, 597
470
535
556
502, 587
492
493
487
492
510
5658
3688
9543
10231
7913
5977
5517
5708
5586
4560
5290
6505
0.09
0.22
Ͻ 0.01
Ͻ 0.01
Ͻ 0.01
0.08
0.10
0.12
0.09
0.08
4.94
4.77
1.45
2.0
1.07
4.99
4.64
2.75
2.02
2.94
4.01
2.15
0.18
0.46
0.07
0.05
0.09
0.16
0.22
0.44
0.45
0.27
0.22
0.14
1.82
1.63
6.93
4.95
8.91
1.84
1.98
3.22
4.55
3.10
2.22
4.52
5j
5k
5l
486
481
0.09
0.03
[a] Measured in chloroform at a concentration of 1.0ϫ 10–5 moldm–3 at 25 °C. [b] The fluorescence quantum yields (ΦF) were estimated
with quinine sulfate (ΦF) 0.55 in 1 n H2SO4 solution as a standard, λex = 366 nm. [c] Fluorescence lifetime measured in chloroform by
exciting the sample with a 342 nm, 150 ps pulse using nano-LEDs.
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Green-Light-Emitting Boroquinols
vents when compared with nonpolar toluene and CH2Cl2.
However, in the case of 5b, a redshifted absorption was ob-
served in CH2Cl2. The fluorescence maxima of 5a were sig-
nificantly affected by solvent polarity; in particular, a red-
shift in emission was observed in 5a upon moving from tol-
uene to dimethyl sulfoxide (DMSO), with the exception of
MeOH in which a 82 nm blueshift was observed. For com-
pound 5b, the fluorescence maxima were blueshifted upon
moving from toluene to DMSO. Notably, dissolving these
complexes in polar solvents such as MeOH, CH3CN, and
DMSO led to the progressive decomplexation of boron, as
shown by the recovery of the typical ESIPT emission of the
PQP ligand.
To gain information on the excited-state processes, the
fluorescence lifetime of boroquinols were measured by the
time correlated single photon counting method by exciting
the sample with a 342 nm, 150 ps pulse using nano-LEDs.
The decay profiles of boroquinols derivatives 5a–d in
chloroform are given in Figure 4. The decay profiles can be
fitted with a single exponential function. The lifetime data
are summarized in Table 1. The fluorescence lifetime of 5a
Figure 3. Excitation (dotted), luminescence (dotted) in chloroform
and solid state luminescence (solid line) spectra of 5b (black), 5h
(blue) and 5j (pink). Inset. Solution and solid-state fluorescence
images taken at 365 nm.
nature of methoxy and ethoxy groups present on the phenol and 5b were measured to be 4.94 and 4.77 ns, respectively.
ring of the boroquinol in the excited state. By increasing Generally, the fluorescence lifetimes of phenolic-substituted
the electron density on the quinoline side by substituting compounds are shorter than that of the unsubstituted com-
methyl or methoxy groups in the para-position (boroquinol pound 5a. However, the shorter lifetime values were mea-
5j–k), λmax of 5j (467 nm) and 5k (471 nm) were slightly sured for 5h, 5i, and 5j (2.75, 2.02, and 2.94 ns, respectively)
blueshifted by 9 and 5 nm compared with that of 5a when compared with 5a, even though they possess different
(476 nm). Switching from electron-donating methyl/meth- functional groups such as halogens (F, Br) or methoxy sub-
oxy groups to F-, Cl-, Br- (boroquinol 5g–i) shifted the stituents, respectively, present on the boroquinol.
emission bathochromically (484, 482, and 484 nm) by 8, 6
and 8 nm respectively to that of 5a. Due to the larger Stokes
shift that originated from internal charge transfer from the
phenol to the quinolinium at the excited state, the overlap
between the absorption and emission bands became mini-
mal (Figure 2), which is a desirable characteristic for the
dyes to be used in many fluorescence-based applications.[15]
The solid-state emission spectra of boroquinols 5a–l exhib-
ited major emission maxima between 481 and 556 nm ex-
cept for 5a (484, 597 nm) and 5e (502, 587 nm), which had
two emission maxima, represented in Figure 3. In general,
the solid-state emission maxima of the complexes were red-
shifted when compared with their emission maxima in solu-
tion except for 5b, for which a 20 nm blueshift was ob-
served. The broad shape of the emission band for boroquin-
ols 5a–l, the nonmirror symmetry with the absorption
Figure 4. The lifetime decay profiles of boroquinols derivatives 5a–
d in chloroform.
bands and a large Stokes shift (3688–10231 cm–1) are
suggestive of an intraligand charge transfer (ILCT) emissive
state.
These dyes exhibited quantum yields in the range of
The electrochemical properties of boroquinols were mea-
0.22–0.01 with good chemical and photochemical stability sured by using cyclic voltammetry (CV) in acetonitrile sol-
in apolar solvents. Compounds 5c–e exhibited very low vent with 0.1 m tetrabutylammonium hexafluorophosphate
quantum yields, which was attributed the destabilization of as a supporting electrolyte; a representative CV is illustrated
the boroquinol through increased electron-density as a re- in Figure 5. The first oxidation and reduction potential of
sult of the substituents present on the phenolic ring of the all the compounds are summarized in Table 2. The HOMO
complex. The absorption and fluorescence properties of 5a and LUMO levels were readily estimated from the onset
and 5b in various solvents were investigated are shown in oxidation (Eox) and onset reduction potentials (Ered) by
the Supporting Information (Figure S20–S21). Generally, using the equations EHOMO = –(Eox + 4.8) eV and ELUMO
the absorbance maxima of 5a were blueshifted in polar sol- = –(Ered + 4.8) eV and the data outlined in Table 2. All the
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U. Balijapalli, S. K. Iyer
Table 2. Electrochemical data of boroquinols 5a–l.
FULL PAPER
5
E1/2
E1/2
EHOMO
ELUMO
Eg
red
[V][oa]x
[V][a]
[eV][b]
[eV][b]
[eV][c]
5a
5b
5c
5d
5e
5f
5g
5h
5i
n.d./1.19
n.d./1.44
n.d./0.95
n.d./0.94
n.d./1.23
n.d./1.48
n.d./1.28
n.d./1.29
n.d./1.35
n.d./1.52
n.d./1.68
n.d./1.14
–1.20
–1.70
–1.20
–1.20
–1.54
–1.13
–1.11
–1.14
–1.10
–0.88
–0.86
–0.88
–5.99
–6.24
–5.75
–5.74
–6.03
–6.28
–6.08
–6.09
–6.15
–6.32
–6.48
–5.94
–3.60
–3.10
–3.60
–3.60
–3.26
–3.67
–3.69
–3.66
–3.70
–3.92
–3.94
–3.92
2.39
3.14
2.15
2.14
2.77
2.61
2.39
2.43
2.45
2.40
2.54
2.02
5j
5k
5l
[a] The redox potential of compounds obtained from cyclic voltam-
metry using glassy carbon as working electrode with reference to
the Fc/Fc+ couple; 0.1 m tetrabutylmmonium hexafluorophosphate
was used as supporting electrolyte; n.d.: not determined.
[b] EHOMO/LUMO HOMO and LUMO energy levels calculated from
the redox potentials using the equation EHOMO = Eox + 4.8 and
ELUMO = Ered + 4.8. [c] Eg = electrochemical HOMO–LUMO en-
ergy gap.
Figure 5. Cyclic voltammograms of compound 5a–c, 5e, 5h, and 5j
measured at a scan rate of 100 mVs–1 in acetonitrile solvent using
glassy carbon as working electrode and 0.1 m TBAHFP as support-
ing electrolyte.
Conclusions
compounds show irreversible anodic redox process in the
potential ranges of 0.94 to 1.68 V vs. ferrocenium/ferrocene
redox couple. The oxidation process of boroquinols could
be attributed to the irreversible oxidation of phenolic group
of quinolines. In contrast, the reversible cathodic redox pro-
cess spread over a larger potential range of –0.86 to –1.70 V
and this is characteristic of the substituents present on the
quinoline ring. Given that the electron-donating/with-
drawing groups were substituted on the quinoline moiety,
this significantly enhanced the electrochemical reduction
behavior of the species. Complexes 5b showed more nega-
tively shifted reduction waves compared with complex 5a,
which is indicative of the enhancement of π-conjugation
when replacing the phenyl group by a naphthyl group.
However, multiple redox waves, i.e., single two-electron pro-
cess, is observed in acetonitrile for few compounds. This
was attributed to a negative shift of the redox potential for
the first reduction because of the preferential solvation of
more positively charged species by acetonitrile. In addition,
the single two-electron process was split into two one-elec-
tron processes because of the large increase in ΔEp for the
second reduction relative to that of the first reduction and
to the small difference in the redox potentials. A greater
reduction potential of –1.70 V was observed for 5b. On the
We have developed quinoline-based boron complexes by
the reaction of 2-(4-phenylquinolin-2-yl)phenol with boron
trifluoride and investigated its optical properties. Signifi-
cant features such as strong green fluorescence in the solid-
state and moderate fluorescence in solution, unusually large
Stokes shifts of 3688–10231 cm–1, and lower LUMO and
higher HOMO levels were observed. The fluorescence
maximum and fluorescence quantum yields were in the
range 467–538 nm and 0.01–0.22, respectively. Due to the
existence of CH–π···O, CH–π···F, and CH–π···B networks,
the compounds exhibited strong fluorescence in the solid
state. The LUMO values of 5a–l were calculated to be in
the range –3.10 to –3.94 eV and HOMO values were –5.74
to –6.48 eV. These values are similar to that of electron-
transporting material Alq3 (Epc = –2.3 V, LUMO –3.0 eV,
and HOMO –5.7 eV). These properties qualify these novel
boroquinol complexes as not only multifunctional fluoro-
phores but also as an alternative material that can be em-
ployed in sensing applications.
Experimental Section
basis of the reduction potentials, the lowest unoccupied mo- General Information: All organic chemicals and solvents were pur-
chased from Sigma–Aldrich, Merck, or Himedia. Purity of all the
chemicals was greater than 99%. 1H and 13C NMR spectra were
taken with a Bruker 300 MHz spectrometer using CDCl3 or [D6]-
DMSO as the solvent with TMS as internal standard. Melting
points were measured with a Guna capillary-based melting-point
apparatus and were not corrected. HRMS values were obtained
with a Joel GC Mate II GC-Mass Spectrometer. FTIR spectra of
the synthesized organic compounds were recorded with a Jasco-
4100 spectrometer. UV/Vis spectra were taken with a Hitachi U-
2910 spectrophotometer. Fluorescence spectra in solution and solid
were measured with a Hitachi F-7000 fluorescence spectrometer.
lecular orbital (LUMO) energy levels were calculated to be
in the range of –3.10 to –3.94 eV. Moreover, the reduction
potentials of 5a–l shifted positively in the range of 0.80 and
1.64 V compared with that of tris(8-hydroxyquinolinato)-
aluminium (Alq3) (Epc = –2.3 V), which is one of the most
widely used electron-transporting materials. This confirmed
the high electron-accepting ability of these boron complex-
es.[4a] The HOMO values were calculated to be in the range
–5.74 to –6.48 eV, which are similar to that of Alq3
(–5.7 eV).[16]
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Green-Light-Emitting Boroquinols
The fluorescence quantum yields (ΦF) were measured with quinine
sulfate (ΦF 0.55 in 1 n H2SO4 solution) as standard, λex = 366 nm.
Analytical thin-layer chromatography (TLC) was performed on
precoated plates (Merck, silica gel 60 F254). Silica gel (60–
120 mesh) was used for column chromatography. Single-crystal X-
ray diffraction data were recorded with a Bruker Kappa APEXII.
The structures were solved by direct methods.
6.87–6.83 (t, J = 8.0 Hz, 1 H), 6.98–6.96 (d, J = 8.0 Hz, 1 H), 7.50–
7.46 (t, J = 8.0 Hz, 1 H), 7.56–7.55 (m, 6 H), 7.75–7.71 (t, J =
8.0 Hz, 1 H), 7.88–7.86 (d, J = 8.0 Hz, 1 H), 7.95 (s, 1 H), 8.07–
8.05 (d, J = 8.0 Hz, 1 H), 15.94 (s, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 15.0, 64.4, 114.6, 117.8, 118.6, 118.8, 125.3, 125.8,
126.7, 127.7, 128.7, 128.7, 129.4, 130.4, 137.9, 145.0, 148.8, 150.1,
151.7, 157.6 ppm. HRMS: m/z calcd. for C23H19NO2 [M+]
341.1416; found 341.1416.
General Procedure for the Preparation of 4a–l: A mixture of substi-
tuted salicylaldehyde 1 (10 mmol) and substituted aromatic amine
2 (10 mmol) was stirred in nitromethane at room temperature for
10 min. After addition of 10 mol-% anhydrous iodine and 20 mol-
% of Cu2O, phenyl acetylene 3 (10 mmol) was added and the reac-
tion mixture was heated to reflux for 24 h. Upon completion of the
reaction (monitored by TLC analysis), the reaction mixture was
cooled and diluted with saturated NaHCO3 (80 mL). The mixture
was extracted with ethyl acetate (3ϫ 20 mL) and the combined
organic extract was washed with brine and dried with (Na2SO4).
The solvent was distilled off and the resulting product was purified
by column chromatography over silica gel (60–120 mesh; n-hexane/
ethyl acetate, 9:1) to give the product 4a–m.
4-Bromo-2-(4-phenylquinolin-2-yl)phenol (4e): Yellow solid; m.p.
184–186 °C. IR (KBr): ν = 3267, 3091, 3041, 3011, 2987, 2335,
˜
1737, 1579, 1489, 1467, 1392, 1304, 1255, 1207, 1182, 1085, 975,
1
883, 812, 705, 680, 628, 597 cm–1. H NMR (400 MHz, CDCl3): δ
= 6.99–6.97 (d, J = 8.0 Hz, 1 H), 7.43–7.40 (dd, J = 16.0, 4.0 Hz,
1 H), 7.60–7.49 (m, 6 H), 7.77–7.73 (t, J = 8.0 Hz, 1 H), 7.90–7.88
(d, J = 8.0 Hz, 2 H), 8.04 (s, 1 H), 8.09–8.07 (d, J = 8.0 Hz, 1 H),
15.34 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 110.4, 117.3,
120.5, 125.5, 125.9, 127.0, 127.9, 128.8, 128.9, 129.4, 129.4, 130.5,
134.5, 137.6, 145.1, 150.6, 156.0, 160.2 ppm. HRMS: m/z calcd. for
C21H14BrNO [M+] 375.0259; found 375.0264.
2-(7-Chloro-4-phenylquinolin-2-yl)phenol (4f): Yellow solid; m.p.
2-(4-Phenylquinolin-2-yl)phenol (4a): Yellow solid; m.p. 153–155 °C.
140–142 °C. IR (KBr): ν = 3270, 3096, 3057, 2939, 2871, 2331,
˜
IR (KBr): ν = 3263, 3088, 3057, 3030, 2917, 2876, 2331, 1737, 1726, 1573, 1544, 1487, 1460, 1415, 1371, 1296, 1278, 1199, 1156,
˜
1585, 1467, 1433, 1398, 1274, 1213, 1120, 893, 752, 698, 675, 663,
1153, 1076, 981, 877, 821, 737, 626 cm–1. 1H NMR (400 MHz,
646 cm–1. 1H NMR (400 MHz, CDCl3): δ = 6.95–6.91 (t, J = CDCl3): δ = 6.97–6.93 (q, J = 4.0 Hz, 1 H), 7.13–7.11 (d, J =
8.0 Hz, 1 H), 7.10–7.08 (t, J = 8.0 Hz, 1 H), 7.37–7.33 (t, J = 8.0 Hz, 1 H), 7.67–7.28 (m, 8 H), 7.84–7.82 (d, J = 8.0 Hz, 1 H),
8.0 Hz, 1 H), 7.50–7.46 (t, J = 8.0 Hz, 1 H), 7.58–7.51 (m, 5 H),
7.96–7.91 (m, 2 H), 8.10–8.04 (m, 2 H), 14.82 (s, 1 H) ppm. 13C
7.75–7.71 (t, J = 8.0 Hz, 1 H), 7.88–7.86 (d, J = 8.0 Hz, 1 H), 7.95– NMR (100 MHz, CDCl3): δ = 117.7, 118.3, 118.9, 121.2, 123.0,
7.94 (d, J = 4.0 Hz, 2 H), 7.50–7.46 (t, J = 8.0 Hz, 1 H), 8.09–8.07 123.7, 126.9, 127.5, 127.7, 127.8, 128.0, 128.8, 129.0, 129.9, 131.2,
(d, J = 8.0 Hz, 1 H), 15.29 (s, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 117.5, 118.6, 118.7, 118.9, 121.2, 125.3, 125.8, 126.7,
126.9, 126.9, 127.9, 128.7, 128.7, 129.4, 129.4, 130.3, 132.0, 137.9,
145.2, 150.2, 157.4, 161.1 ppm. HRMS: m/z calcd. for C21H15NO
[M+] 297.1154; found 297.1150.
132.4, 132.5, 136.3, 137.4, 140.7, 145.8, 146.9, 149.8, 150.1, 157.1,
158.5, 161.1 ppm. HRMS: m/z calcd. for C21H14ClNO [M+]
331.0764; found 337.0767.
2-(6-Chloro-4-phenylquinolin-2-yl)phenol (4g): Yellow solid; m.p.
156–158 °C. IR (KBr): ν = 3263, 3091, 3059, 3026, 2920, 2848,
˜
2-(4-Phenylbenzo[h]quinolin-2-yl)phenol (4b): Yellow solid; m.p. 2397, 2343, 1764, 1743, 1587, 1485, 1417, 1352, 1298, 1274, 1124,
180–182 °C. IR (KBr): ν = 3256, 3092, 3057, 3013, 2976, 2337,
1072, 869, 819, 738, 700, 684, 617 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 6.98–6.94 (t, J = 8.0 Hz, 1 H), 7.13–7.11 (d, J = 8.0 Hz,
1 H), 7.42–7.38 (t, J = 8.0 Hz, 1 H), 7.64–7.55 (m, 5 H), 7.71–7.69
˜
2328, 2297, 1944, 1815, 1705, 1583, 1492, 1365, 1296, 1280, 1215,
1
1195, 1068, 876, 829, 704, 628 cm–1. H NMR (400 MHz, CDCl3):
δ = 6.96–6.92 (t, J = 8.0 Hz, 1 H), 7.15–7.13 (d, J = 8.0 Hz, 1 H), (d, J = 8.0 Hz, 1 H), 7.87–7.86 (d, J = 8.0 Hz, 1 H), 7.95–7.94 (d,
7.39–7.35 (t, J = 8.0 Hz, 1 H), 7.55–7.53 (m, 5 H), 7.78–7.68 (m, 4 J = 4.0 Hz, 1 H), 7.99 (s, 1 H), 8.05–8.03 (d, J = 8.0 Hz, 1 H),
H), 7.89–7.87 (d, J = 8.0 Hz, 1 H), 7.97–7.95 (d, J = 8.0 Hz, 1 H), 14.90 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 118.4, 118.7,
8.05 (s, 1 H), 8.98–8.96 (d, J = 8.0 Hz, 1 H), 15.54 (s, 1 H) ppm.
118.9, 119.2, 122.4, 124.7, 126.0, 127.0, 128.9, 129.0, 129.3, 129.5,
13C NMR (100 MHz, CDCl3): δ = 118.3, 118.5, 118.9, 119.2, 122.7, 131.2, 132.3, 132.6, 133.4, 137.2, 143.7, 149.4, 157.7, 161.0,
123.2, 124.1, 126.9, 127.7, 128.2, 128.7, 128.7, 128.7, 129.6, 129.7,
131.9, 133.8, 138.2, 143.7, 150.3, 155.8, 160.6 ppm. HRMS: m/z
calcd. for C25H17NO [M+] 347.1310; found 347.1312.
162.9 ppm. HRMS: m/z calcd. for C21H14ClNO [M+] 331.0764;
found 337.0763.
2-(6-Fluoro-4-phenylquinolin-2-yl)phenol (4h): Yellow solid; m.p.
2-Methoxy-6-(4-phenylquinolin-2-yl)phenol (4c): Yellow solid; m.p.
167–169 °C. IR (KBr): ν = 3270, 3091, 3059, 2991, 2971, 2351,
˜
164–166 °C. IR (KBr): ν = 3250, 3087, 3053, 2985, 2945, 2825,
2336, 1741, 1591, 1550, 1431, 1363, 1240, 1049, 902, 758, 705,
2330, 1726, 1624, 1587, 1550, 1508, 1494, 1425, 1382, 1355, 1251,
˜
1195, 1109, 925, 873, 746, 702, 655 cm–1. 1H NMR (400 MHz,
601 cm–1. 1H NMR (400 MHz, CDCl3): δ = 3.97 (s, 3 H), 6.90– CDCl3): δ = 6.95–6.91 (t, J = 8.0 Hz, 1 H), 7.09–7.07 (d, J = 8.0 Hz,
6.86 (t, J = 8.0 Hz, 1 H), 6.99–6.97 (d, J = 8.0 Hz, 1 H), 7.52–7.48 1 H), 7.37–7.33 (t, J = 8.0 Hz, 1 H), 7.56–7.48 (m, 7 H), 7.93–7.91
(t, J = 8.0 Hz, 1 H), 7.59–7.56 (m, 6 H), 7.77–7.73 (t, J = 8.0 Hz, (d, J = 8.0 Hz, 1 H), 7.96 (s, 1 H), 8.07 (s, 1 H), 14.88 (s, 1 H) ppm.
1 H), 7.89–7.87 (d, J = 8.0 Hz, 1 H), 7.96 (s, 1 H), 8.09–8.07 (d, J
= 8.0 Hz, 1 H), 15.88 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 56.1, 113.3, 117.8, 118.6, 118.8, 125.3, 125.8, 126.7, 127.8,
13C NMR (100 MHz, CDCl3): δ = 109.4, 109.7, 118.2, 118.6, 118.8,
118.8, 120.2, 120.4, 126.1, 126.2, 126.9, 128.9, 129.0, 129.2, 130.3,
130.4, 132.1, 137.4, 142.3, 149.6, 149.7, 156.9, 156.9, 159.4, 160.8,
128.7, 128.7, 129.4, 130.4, 137.9, 145.0, 149.5, 150.2, 151.6, 161.9 ppm. HRMS: m/z calcd. for C21H14FNO [M+] 315.1059;
157.60 ppm. HRMS: m/z calcd. for C22H17NO2 [M+] 327.1259; found 315.1061.
found 327.1260.
2-(6-Bromo-4-phenylquinolin-2-yl)phenol (4i): Yellow solid; m.p.
2-Ethoxy-6-(4-phenylquinolin-2-yl)phenol (4d): Yellow solid; m.p.
166–168 °C. IR (KBr): ν = 3280, 3092, 3057, 3028, 2669, 2397,
˜
157–159 °C. IR (KBr): ν = 3258, 3099, 3053, 2981, 2904, 2872,
2337, 1717, 1593, 1548, 1462, 1440, 1357, 1238, 1049, 1031, 902,
2330, 1957, 1851, 1737, 1610, 1583, 1543, 1500, 1485, 1369, 1350,
˜
1294, 1244, 1120, 1039, 871, 823, 746, 700, 669, 630 cm–1. 1H NMR
875, 858, 769, 702, 574 cm–1. 1H NMR (400 MHz, CDCl3): δ = (400 MHz, CDCl3): δ = 6.99–6.95 (t, J = 8.0 Hz, 1 H), 7.13–7.11
1.57–1.53 (t, J = 8.0 Hz, 3 H), 4.20–4.14 (q, J = 8.0 Hz, 2 H, 3 H), (d, J = 8.0 Hz, 1 H), 7.42–7.38 (t, J = 8.0 Hz, 1 H), 7.65–7.55 (m,
Eur. J. Org. Chem. 2015, 5089–5098
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5095

U. Balijapalli, S. K. Iyer
FULL PAPER
5 H), 7.85–7.83 (dd, J = 16.0, 4.0 Hz, 1 H), 7.98–7.96 (d, J =
4.0 Hz, 2 H), 8.00 (s, 1 H), 8.04–8.03 (d, J = 4.0 Hz, 1 H), 14.91
(400 MHz, CDCl3): δ = 7.08–7.04 (t, J = 8.0 Hz, 1 H), 7.25–7.23
(d, J = 8.0 Hz, 1 H), 7.67–7.53 (m, 7 H), 7.99–7.91 (m, 3 H), 8.11
(s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 117.3, 118.4, (s, 1 H), 9.07–9.05 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR (100 MHz,
118.7, 118.9, 120.7, 122.8, 126.5, 127.0, 128.0, 128.9, 129.0, 129.3,
129.6, 132.3, 132.5, 133.7, 137.2, 144.0, 149.3, 157.8, 161.0, 161.1,
163.0 ppm. HRMS: m/z calcd. for C21H14BrNO [M+] 375.0259;
found 375.0259.
CDCl3): δ = 117.5, 120.3, 120.4, 125.0, 126.6, 126.7, 126.8, 127.8,
129.0, 129.3, 129.9, 132.6, 135.5, 136.4, 151.9, 155.3, 156.3 ppm.
11B NMR (128 MHz, CDCl3): δ = 1.57 (s, BF2) ppm. 19F NMR
(376 MHz, CDCl3): δ = –(132.50–132.44) (2 F) ppm. HRMS: m/z
calcd. for C21H14BF2NO [M+] 345.1137; found 345.1131.
2-(6-Methoxy-4-phenylquinolin-2-yl)phenol (4j): Yellow solid; m.p.
144–146 °C. IR (KBr): ν = 3266, 3088, 3043, 2999, 2943, 2331,
˜
8,8-Difluoro-15-phenyl-8H-benzo[h]benzo[5,6][1,3,2]oxazaborinino-
[3,4-a]quinolin-7-ium-8-uide (5b): Yellow solid; m.p. Ͼ300 °C. IR
1778, 1620, 1585, 1543, 1506, 1494, 1386, 1267, 1215, 1120, 1029,
1
906, 887, 754, 698, 624, 590 cm–1. H NMR (400 MHz, CDCl3): δ
(KBr): ν = 3053, 3015, 2926, 2854, 2313, 1737, 1620, 1581, 1512,
˜
= 3.80 (s, 3 H), 6.94–6.90 (t, J = 8.0 Hz, 1 H), 7.09–7.07 (d, J =
8.0 Hz, 1 H), 7.17–7.16 (d, J = 4.0 Hz, 1 H), 7.35–7.31 (t, J =
8.0 Hz, 1 H), 7.42–7.38 (dd, J = 16.0, 4.0 Hz, 1 H), 7.57–7.53 (m,
5 H), 7.91 (s, 1 H), 7.93–7.92 (d, J = 4.0 Hz, 1 H), 8.01–7.99 (d, J
= 8.0 Hz, 1 H), 15.13 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 55.5, 104.2, 117.9, 118.5, 118.7, 119.1, 122.4, 126.3, 126.6,
128.7, 128.8, 129.2, 129.4, 131.5, 138.2, 141.1, 148.8, 155.1, 158.1,
160.7 ppm. HRMS: m/z calcd. for C22H17NO2 [M+] 327.1259;
found 327.1259.
1
1384, 1294, 1112, 1080, 958, 902, 869, 852, 756, 705, 675 cm–1. H
NMR (400 MHz, CDCl3): δ = 6.80–6.76 (t, J = 8.0 Hz, 1 H), 6.92–
6.90 (d, J = 8.0 Hz, 1 H), 7.20–7.16 (t, J = 8.0 Hz, 1 H), 7.60–7.39
(m, 9 H), 7.77–7.75 (d, J = 8.0 Hz,1 H), 7.84–7.82 (d, J = 8.0 Hz,1
H), 7.92 (s, 1 H), 8.77–8.75 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 108.7, 118.8, 120.2, 122.5, 123.5, 126.5,
127.2, 128.1, 129.4, 129.7, 133.3, 136.8, 145.0, 154.3, 170.9 ppm.
11B NMR (128 MHz, CDCL3): δ = –1.16 (s, BF2) ppm. 19F NMR
(376 MHz, CDCl3): δ = –(148.45–148.38) (2 F) ppm. HRMS: m/z
calcd. for C25H16BF2NO [M+] 395.1293; found 395.1399.
2-(6-Methyl-4-phenylquinolin-2-yl)phenol (4k): Yellow solid; m.p.
168–170 °C. IR (KBr): ν = 3250, 3089, 3047, 2926, 2858, 2330,
˜
6,6-Difluoro-8-methoxy-13-phenyl-6H-benzo[5,6][1,3,2]oxaza-
borinino[3,4-a]quinolin-5-ium-6-uide (5c): Yellow solid; m.p. 247–
1741, 1583, 1546, 1490, 1355, 1246, 1124, 1076, 962, 871, 752, 744,
705, 594 cm–1. 1H NMR (400 MHz, CDCl3): δ = 2.46 (s, 3 H),
6.94–6.90 (t, J = 8.0 Hz, 1 H), 7.09–7.07 (d, J = 8.0 Hz, 1 H), 7.36–
7.32 (t, J = 4.0 Hz, 1 H), 7.57–7.53 (m, 6 H), 7.61 (s, 1 H), 7.91 (s,
1 H), 7.94–7.92 (d, J = 4.0 Hz, 1 H), 7.99–7.97 (d, J = 8.0 Hz, 1
H), 15.30 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.8,
117.6, 118.5, 118.6, 119.1, 121.0, 124.6, 125.3, 126.8, 127.7, 128.6,
128.7, 129.4, 130.0, 131.8, 132.4, 136.8, 138.1, 143.7, 149.5, 156.5,
161.0, 161.7 ppm. HRMS: m/z calcd. for C22H17NO [M+] 311.1310;
found 311.1316.
249 °C. IR (KBr): ν = 3053, 3005, 2903, 2835, 2346, 1743, 1604,
˜
1554, 1519, 1460, 1386, 1363, 1234, 1080, 1062, 1039, 935, 767,
727, 704 cm–1. 1H NMR (400 MHz, CDCl3): δ = 3.99 (s, 3 H),
6.99–6.95 (t, J = 8.0 Hz, 1 H), 7.10–7.08 (d, J = 8.0 Hz, 1 H), 7.54–
7.52 (d, J = 8.0 Hz, 1 H), 7.66–7.56 (m, 6 H), 7.97–7.89 (m, 2 H),
8.08 (s, 1 H), 9.09–9.07 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 56.2, 115.8, 118.0, 119.9, 125.0, 125.1,
126.1, 126.6, 126.8, 127.8, 128.9, 129.0, 129.3, 129.4, 129.8, 132.5,
136.4, 140.8, 150.2, 152.0, 155.2 ppm. 11B NMR (128 MHz,
CDCl3): δ = 1.68 (s, BF2) ppm. 19F NMR (376 MHz, CDCl3): δ
= –(132.06–131.99) (2 F) ppm. HRMS: m/z calcd. for
C22H16BF2NO2 [M+] 375.1242; found 375.1249.
2-(5,7-Dimethyl-4-phenylquinolin-2-yl)phenol (4l): Yellow solid; m.p.
149–151 °C. IR (KBr): ν = 3290, 3142, 3057, 2978, 2861, 2328,
˜
1735, 1597, 1494, 1411, 1381, 1363, 1300, 1222, 1153, 765, 756,
1
868, 659, 619, 588 cm–1. H NMR (400 MHz, CDCl3): δ = 2.00 (s,
3 H), 2.54 (s, 3 H), 6.94–6.90 (t, J = 8.0 Hz, 1 H), 7.12–7.10 (d, J
= 8.0 Hz, 1 H), 7.14 (s, 1 H), 7.40–7.28 (m, 3 H), 7.52–7.49 (m, 3
H), 7.79–7.77 (d, J = 8.0 Hz, 2 H), 7.92–7.90 (d, J = 8.0 Hz, 1 H),
15.44 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.4, 24.1,
118.5, 118.5, 118.7, 118.7, 118.9, 122.8, 125.9, 126.8, 128.0, 128.0,
128.6, 131.8, 132.3, 135.3, 140.1, 142.4, 146.6, 150.4, 155.9,
161.2 ppm. HRMS: m/z calcd. for C23H19NO [M+] 325.1467; found
325.1469.
8-Ethoxy-6,6-difluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxazaborinino-
[3,4-a]quinolin-5-ium-6-uide (5d): Yellow solid; m.p. 240–242 °C. IR
(KBr): ν = 3047, 2972, 2926, 2328, 1737, 1595, 1442, 1388, 1363,
˜
1234, 1095, 1074, 1053, 962, 918, 873, 769, 723, 698 cm–1. 1H NMR
(400 MHz, CDCl3): δ = 1.57–1.53 (t, J = 8.0 Hz, 1 H), 4.26–4.20
(q, J = 8.0 Hz, 2 H), 6.98–6.94 (t, J = 8.0 Hz, 1 H), 7.12–7.10 (d,
J = 8.0 Hz, 1 H), 7.55–7.53 (d, J = 8.0 Hz, 1 H), 7.66–7.57 (m, 6
H), 7.98–7.90 (m, 2 H), 8.08 (s, 1 H), 9.09–9.07 (d, J = 8.0 Hz, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 14.8, 65.0, 114.9, 117.9,
118.0, 118.1, 118.3, 119.9, 125.0, 125.9, 126.6, 126.8, 127.1, 127.8,
128.8, 129.0, 129.3, 129.4, 129.8, 132.5, 136.4, 152.1, 155.1 ppm.
11B NMR (128 MHz, CDCl3): δ = 1.67 (s, BF2) ppm. 19F NMR
(376 MHz, CDCl3): δ = –(132.25–132.32) (2 F) ppm. HRMS: m/z
calcd. for C23H18BF2NO2 [M+] 389.1399; found 389.1399.
General Procedure for the Preparation of 5a–m: To a stirred solution
of 2-(4-phenylquinolin-2-yl)phenol (1 mmol) in anhydrous di-
chloromethane (10 mL) at room temperature was added boron tri-
fluoride–diethyl ether complex (2.5 mmol). A color change oc-
curred or a precipitate appeared, revealing the formation of an in-
termediate adduct. Triethylamine (1.0 mmol) was then added to the
solution, which was stirred at room temperature for 12–18 h. The
course of the reaction was monitored by TLC analysis and, upon
completion of the reaction, water was added to the solution. The
solution was extracted with CH2Cl2 and the organic layer was
washed with water and dried with MgSO4. After concentration of
solvent, the residue was purified by column chromatography with
silica gel (CH2Cl2/n-hexane) to afford 5a (52 mg, 97%) as a yellow
solid.
10-Bromo-6,6-difluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxaza-
borinino[3,4-a]quinolin-5-ium-6-uide (5e): Yellow solid; m.p. 292–
294 °C. IR (KBr): ν = 3093, 3057, 2991, 2366, 1734, 1622, 1593,
˜
1467, 1392, 1354, 1249, 1205, 1101, 1001, 933, 871, 812, 759, 704,
680, 626, 597 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.04–7.02 (t,
J = 8.0 Hz, 1 H), 7.48–7.45 (dd, J = 16.0, 4.0 Hz, 1 H), 7.67–7.55
(m, 5 H), 7.83–7.79 (t, J = 8.0 Hz, 1 H), 7.92 (s, 1 H), 7.95–7.93
(d, J = 8.0 Hz, 1 H), 8.07–8.06 (dd, J = 4.0 Hz, 1 H), 8.16–8.14 (d,
J = 8.0 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 117.4,
120.5, 126.0, 127.1, 127.7, 128.8, 129.0, 129.4, 130.7, 134.7,
6,6-Difluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxazaborinino[3,4-a]-
quinolin-5-ium-6-uide (5a): Yellow solid; m.p. 252–254 °C. IR
(KBr): ν = 3055, 3026, 2922, 2852, 2333, 1717, 1604, 1550, 1267, 155.9 ppm. 11B NMR (128 MHz, CDCL3): δ = 1.51 (s, BF2) ppm.
˜
1101, 1073, 1002, 871, 837, 750, 742, 702, 663 cm–1
.
1H NMR 19F NMR (376 MHz, CDCl3): δ = –(132.38–132.44) (2 F) ppm.
5096
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 5089–5098

Green-Light-Emitting Boroquinols
HRMS: m/z calcd. for C21H13BBrF2NO [M+] 423.0242; found
423.0257.
(d, J = 8.0 Hz, 1 H), 8.04 (s, 1 H), 8.13–8.11 (d, J = 8.0 Hz, 1 H),
8.48–8.46 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 55.8, 104.8, 118.8, 119.4, 120.6, 126.3, 128.3, 128.9, 129.0,
129.0, 129.1, 129.2, 129.4, 129.9 ppm. 11B NMR (128 MHz,
CDCL3): δ = 1.56 (s, BF2) ppm. 19F NMR (376 MHz, CDCl3): δ
= –(132.72) (2 F) ppm. HRMS: m/z calcd. for C22H16BF2NO2 [M+]
375.1242; found 375.1246.
3-Chloro-6,6-difluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxaza-
borinino[3,4-a]quinolin-5-ium-6-uide (5f): Yellow solid; m.p. 240–
242 °C. IR (KBr): ν = 3136, 3061, 2976, 2339, 1789, 1607, 1598,
˜
1541, 1512, 1263, 1083, 1066, 931, 852, 833, 753, 740, 698, 686,
685 cm–1 1H NMR (400 MHz, CDCl3): δ = 7.07–7.03 (t, J =
.
8.0 Hz, 1 H), 7.24–7.22 (d, J = 8.0 Hz, 1 H), 7.70–7.53 (m, 8 H),
7.96–7.94 (d, J = 8.0 Hz, 1 H), 8.13 (s, 1 H), 9.10–9.01 (m, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 110.6, 110.8, 115.8,
118.4, 120.3, 120.5, 122.0, 122.2, 126.7, 127.8, 127.9, 128.1, 129.1,
129.3, 129.5, 130.1, 135.6, 135.9, 137.6, 151.6, 154.6, 156.2 ppm.
11B NMR (128 MHz, CDCl3): δ = 1.54 (s, BF2) ppm. 19F NMR
(376 MHz, CDCl3): δ = –(132.22–132.16) (2 F) ppm. HRMS: m/z
calcd. for C21H13BClF2NO [M+] 379.0747; found 379.0759.
6,6-Difluoro-2-methyl-13-phenyl-6H-benzo[5,6][1,3,2]oxaza-
borinino[3,4-a]quinolin-5-ium-6-uide (5k): Yellow solid; m.p. 250–
252 °C. IR (KBr): ν = 3059, 3026, 2987, 2816, 2361, 1749, 1604,
˜
1552, 1494, 1408, 1386, 1296, 1265, 1132, 1103, 1078, 1008, 937,
752, 704, 613 cm–1. 1H NMR (400 MHz, CDCl3): δ = 2.52 (s, 3 H),
7.06–7.02 (t, J = 8.0 Hz, 1 H), 7.24–7.22 (d, J = 8.0 Hz, 1 H), 7.53–
7.51 (d, J = 8.0 Hz, 1 H), 7.67–7.55 (m, 5 H), 7.70 (s, 1 H), 7.76–
7.74 (d, J = 8.0 Hz, 1 H),7.96–7.94 (d, J = 8.0 Hz, 1 H), 8.06 (s, 1
H), 8.95–8.93 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 21.5, 117.5, 120.2, 120.3, 124.7, 125.6, 126.6, 126.7,
129.0, 129.2, 129.7, 134.6, 135.1, 136.6, 138.3, 139.2, 150.9, 154.6,
156.1 ppm. 11B NMR (128 MHz, CDCl3): δ = 1.54 (s, BF2) ppm.
19F NMR (376 MHz, CDCl3): δ = –(132.74–132.68) (2 F) ppm.
HRMS: m/z calcd. for C22H16BF2NO [M+] 359.1293; found
359.1298.
2-Chloro-6,6-difluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxa-
zaborinino[3,4-a]quinolin-5-ium-6-uide (5g): Yellow solid; m.p. 283–
285 °C. IR (KBr): ν = 3140, 3082, 3026, 2976, 2339, 1730, 1602,
˜
1546, 1448, 1384, 1263, 1155, 1099, 1078, 1064, 923, 844, 752,
700 cm–1 1H NMR (400 MHz, CDCl3): δ = 7.09–7.05 (t, J =
.
8.0 Hz, 1 H), 7.25–7.23 (d, J = 8.0 Hz, 1 H), 7.68–7.55 (m, 5 H),
7.87–7.84 (dd, J = 16.0, 4.0 Hz, 1 H), 7.93–7.92 (d, J = 4.0 Hz, 1
H), 7.97–7.94 (dd, J = 16.0, 4.0 Hz, 1 H), 8.13 (s, 1 H), 9.02–9.00
(d, J = 8.0 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 118.5,
120.4, 120.5, 125.5, 126.7, 129.1, 129.3, 130.2, 133.1, 134.2, 135.8,
154.7, 156.2 ppm. 11B NMR (128 MHz, CDCL3): δ = 1.50 (s,
BF2) ppm. 19F NMR (376 MHz, CDCl3): δ = –(132.10–132.03) (2
F) ppm. HRMS: m/z calcd. for C21H13BClF2NO [M+] 379.0747;
found 379.0753.
6,6-Difluoro-1,3-dimethyl-13-phenyl-6H-benzo[5,6][1,3,2]oxaza-
borinino[3,4-a]quinolin-5-ium-6-uide (5l): Yellow solid; m.p. 276–
278 °C. IR (KBr): ν = 3099, 3053, 3013, 2987, 2346, 1737, 1624,
˜
1604, 1587, 1514, 1446, 1354, 1246, 1101, 1049, 1008, 976, 862,
1
765, 713, 650, 603 cm–1. H NMR (400 MHz, CDCl3): δ = 2.07 (s,
3 H), 2.66 (s, 3 H), 7.14–7.10 (t, J = 8.0 Hz, 1 H), 7.36–7.34 (d, J
= 8.0 Hz, 1 H), 7.63–7.42 (m, 7 H), 7.96–7.86 (m, 2 H), 8.00 (s, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.9, 23.8, 119.3, 119.4,
120.8, 121.9, 123.4, 127.9, 127.9, 128.8, 128.8, 129.2, 129.8, 135.4,
136.0, 137.3, 139.6, 146.9, 157.0, 159.0 ppm. 11B NMR (128 MHz,
CDCl3): δ = 0.19–0.15 (d, J = 5.1 Hz, BF2) ppm. 19F NMR
(376 MHz, CDCl3): δ = –(150.17.74–150.19) (2 F) ppm. HRMS:
m/z calcd. for C23H18BF2NO [M+] 373.1450; found 373.1455.
2,6,6-Trifluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxazaborinino[3,4-
a]quinolin-5-ium-6-uide (5h): Yellow solid; m.p. 217–219 °C. IR
(KBr): ν = 3138, 3084, 3028, 2971, 2364, 1913, 1604, 1587, 1548,
˜
1508, 1492, 1388, 1134, 1097, 1078, 1064, 875, 775, 754, 700 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.07–7.03 (t, J = 8.0 Hz, 1 H),
7.24–7.22 (d, J = 8.0 Hz, 1 H), 7.68–7.41 (m, 7 H), 7.92–7.90 (d, J
= 8.0 Hz, 1 H), 7.96–7.94 (d, J = 4.0 Hz, 1 H), 8.09 (s, 1 H), 9.06–
9.04 (d, J = 8.0 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
117.7, 120.4, 120.5, 120.6, 124.4, 126.8, 127.9, 128.0, 128.2, 128.3,
128.7, 128.9, 129.2, 130.1, 135.9, 135.9, 139.4, 155.1, 156.2 ppm.
11B NMR (128 MHz, CDCL3): δ = 1.47 (s, BF2) ppm. 19F NMR
(376 MHz, CDCl3): δ = –(131.99–131.92) (2 F), –(132.54–132.48)
(d, J = 22.5 Hz, 2 F) ppm. HRMS: m/z calcd. for C21H13BF3NO
[M+] 363.1042; found 363.1053.
Acknowledgments
U. B. thanks the Council of Scientific and Industrial Research
(CSIR), New Delhi for providing SRF. The DST-FIST NMR facil-
ity at VIT University is greatly acknowledged. The authors thank
Dr. P. G. Aravindan, Associate Professor, SAS, VIT University, In-
dia, for solving the X-ray data. The authors thank Dr. S. Easwara-
moorthi, Scientist, CLRI, Chennai, India, for electrochemical stud-
ies.
2-Bromo-6,6-difluoro-13-phenyl-6H-benzo[5,6][1,3,2]oxaza-
borinino[3,4-a]quinolin-5-ium-6-uide (5i): Yellow solid; m.p. 256–
258 °C. IR (KBr): ν = 3140, 3078, 3026, 1948, 1602, 1546, 1448,
˜
1382, 1153, 1107, 1076, 999, 875, 750, 663, 605, 575 cm–1. 1H NMR
(400 MHz, CDCl3): δ = 7.07–7.03 (t, J = 8.0 Hz, 1 H), 7.24–7.22
(d, J = 8.0 Hz, 1 H), 7.68–7.54 (m, 6 H), 8.00–7.93 (m, 2 H), 8.09–
8.08 (d, J = 4.0 Hz, 1 H), 8.12 (s, 1 H), 8.94–8.92 (d, J = 8.0 Hz,
1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 115.8, 118.5, 120.3,
120.5, 122.3, 126.7, 126.8, 127.8, 128.8, 129.1, 129.3, 130.2, 135.7,
135.8, 154.2, 156.4 ppm. 11B NMR (128 MHz, CDCL3): δ = 1.49
(s, BF2) ppm. 19F NMR (376 MHz, CDCl3): δ = –(132.13–132.07)
(2 F) ppm. HRMS: m/z calcd. for C21H13BBrF2NO [M+] 423.0242;
found 423.0249.
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Received: April 10, 2015
Published Online: July 15, 2015
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Eur. J. Org. Chem. 2015, 5089–5098