Synthesis and optical properties of salicylaldimine-based diboron complexes

Article abstract of DOI:10.1002/ejic.201301019

New salicylaldimine-based diboron complexes 1-3 have been synthesized and characterized by multinuclear NMR spectroscopy, mass spectrometry, and single-crystal X-ray analysis (1 and 3). The photophysical and electrochemical properties of these compounds h

Full text of DOI:10.1002/ejic.201301019

FULL PAPER
DOI:10.1002/ejic.201301019
Synthesis and Optical Properties of Salicylaldimine-Based
Diboron Complexes
Kunchala Dhanunjayarao,[a] Vanga Mukundam,[a]
Mamidala Ramesh,[a] and Krishnan Venkatasubbaiah*[a]
Keywords: Boron / Boron dyes / Schiff bases / Luminescence
New salicylaldimine-based diboron complexes 1–3 have
been synthesized and characterized by multinuclear NMR
spectroscopy, mass spectrometry, and single-crystal X-ray
analysis (1 and 3). The photophysical and electrochemical
properties of these compounds have been investigated ex-
perimentally and with theoretical calculations; their proper-
ties suggest that these compounds could have potential ap-
plications for the manufacture of optoelectronic devices.
ported a Schiff base route for the synthesis of boron-based
stackable pseudo-triphenylenes, which showed interesting
increased fluorescence upon aggregation in solution.[49]
Introduction
In recent years, tri-[1–11] and tetracoordinate[12–14] boron
compounds have received immense attention owing to their
use as new materials in different fields including organic
field-effect transistors, sensor materials, organic light-emit-
ting diodes (OLEDs), and photovoltaics. The overlap of the
empty p orbital of the tricoordinate boron atom with the
organic π system leads to interesting optoelectronic proper-
ties and also enables the detection of anions such as fluor-
ide and cyanide. Recently, efforts have been devoted to the
design and synthesis of tetracoordinate boron compounds
with N,O-, N,N-, and, N,C-chromophores.[15–31] Among the
tetracoordinate boron compounds, boron dipyrromethene
dyes (Figure 1, A)[32–35] have been studied to a greater ex-
tent owing to their potential application in artificial light
harvesters, fluorescent sensors, laser dyes, sensitizers for so-
lar cells, and molecular photonic wires. Boron quinolate
compounds are analogous to aluminum quinolato com-
pounds (Figure 1, B);[26,36–42] the photophysical tuning of
R2BQ compounds (Q = substituted quinolate) was recently
studied by Wang and co-workers[40,41] and Jaekle and co-
workers.[43–46] Schiff base boron compounds (Figure 1, C)
are yet another type of four-coordinate boron complex that
has gained interest owing to their greater stability than
tricoordinate boron compounds.[15,18,24,47–50] For example,
Ziessel, Ulrich, and co-workers reported the synthesis and
optical properties of Boranil complexes.[47] Ziessel and
Ulrich are also credited for expanding the scope of the Bor-
anil complexes in a model labeling experiment with bovine
serum albumin.[48] More recently, Lee and co-workers re-
Figure 1. Schematic representation of the skeleton of (A) boron
dipyrromethene, (B) boron quinolate, and (C) Schiff base boron
compounds.
With advances in the design of different boron fluoro-
phores, researchers are continually trying to synthesize con-
jugated tetracoordinate diboron or multiboron systems to
improve fluorescence and electron-transport proper-
ties.[17,19,51,52] For instance, Yamaguchi and co-workers re-
ported the synthesis of boron-bridged dipyridylvinylenes
and dithiazolylvinylenes.[51] More recently, Gomes and co-
workers revealed the synthesis of an iminopyrrolyl diboron
complex and its usefulness in nondoped OLEDs.[52] In view
of the important potential applications of boron com-
pounds, in particular the use of diboron compounds in
transistors, OLEDs, and lasers, the search for alternative
fluorescent dyes prompted us to synthesize new salicylald-
imine diboron complexes. We expect that a 1,1Ј-biphenyl
backbone will help to increase the conjugation and, thus,
result in better luminescent materials. The preparation and
structural, photophysical, and electrochemical properties of
these compounds have been investigated.
[a] School of Chemical Sciences, National Institute of Science
Education and Research (NISER), Institute of Physics Campus,
Bhubaneswar 751005, Orissa, India
Results and Discussion
E-mail: krishv@niser.ac.in
http://www.niser.ac.in/~krishv/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301019.
The bis-salicylaldimine Schiff bases H2L1–H2L3
(Scheme 1) were synthesized by acid-catalyzed condensa-
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tion reactions between the dialdehyde[53] and commercially
available anilines in toluene or dichloromethane at reflux
temperature. These starting materials were fully charac-
1
terized by H and 13C NMR spectroscopy and LC–MS. As
shown in Figure 2, a distinctive H-bonded downfield phen-
olic proton (δ ≈ 13–14 ppm) was observed for all bis-salicyl-
aldimine Schiff bases. All three Schiff bases were also char-
acterized by single-crystal X-ray analysis. The molecular
structure of H2L3 is shown in Figure 2. The metric param-
eters along with the molecular structures of H2L1 and
H2L2 are available as Supporting Information. Notably, all
three Schiff bases are involved in intramolecular hydrogen
bonding to the respective imine nitrogen atoms to form six-
membered rings (Figures 2, S1, and S2). The observed hy-
drogen-bonding metric parameters (H···N 1.86–1.89 Å;
N···O 2.59–2.62 Å; O–H···N 147.7–148.6°) show similar
trends to those previously reported for these interacting
units.[54] The Schiff bases were deprotonated by using so-
dium hydride in anhydrous tetrahydrofuran (THF) at room
Figure 2. Molecular structure of H2L3 (top). 1H NMR spectrum
of H2L3 in C6D6 (bottom).
temperature. The sodium salt of the bis-salicylaldimine re- by the substitution. The 19F NMR signals for 2 and 3 at δ
acted with excess BF3·Et2O in tetrahydrofuran for 24 h to = –137.2 and –137.9 ppm are marginally upfield shifted in
afford the diboron salicylaldimine complexes in moderate comparison to that of 1 (δ = –135.4 ppm). The absence of
yields.
quartets in the 19F NMR spectra reveals that there may
be fast relaxation of the quadrupolar boron nuclei at room
temperature.[55] Moreover, the proposed structures of the
diboron compounds were confirmed by [M + Na]+ ion
peaks in LC–MS and single-crystal X-ray analysis of 1 and
3.
The molecular structures of 1 and 3 are shown in Fig-
ure 3, and selected bond lengths and angles are listed in the
caption. The B–N and B–O bond lengths in 1 and 3 are
similar to those of the other reported Schiff base–BF2 com-
plexes.[21,47] In both structures, the boron atom deviates
from the nine-atom plane defined by the six biphenyl car-
bon atoms and the imino carbon, nitrogen, and oxygen
atoms (Figures 4 and S6); the deviation is more pronounced
in 1 (0.39 Å for B1 and 0.37 Å for B2) than in 3 (0.30 Å for
B1 and 0.18 Å for B2). The dihedral angles between the two
planes (planes A and B, Figure S6) are 73.45 and 123.21°
in 1 and 3, respectively (Figures 4 and S6). Steric hindrance
in 3 plays a major role in this drastic difference, as is also
seen from the separation distance between the boron atoms;
the B–B distance in 1 is 5.74 Å, whereas in 3 it increases to
7.16 Å. Both boron atoms in 1 and 3 have slightly distorted
tetrahedral geometries with angles ranging from 106.1(3)
to 111.0(3)° for 1 and 107.30(11) to 110.96(12)° for 3. The
interplanar angles between planes A or B (six biphenyl car-
bon atoms and imino carbon, nitrogen, and oxygen atom)
Scheme 1. Synthesis of 1–4.
1
All boron compounds 1–3 were characterized by H and and the N-phenyl rings in 1 are 44.18 and 48.88°, and in 3
13C NMR spectroscopy; the disappearance of the downfield these angles are 77.86 and 85.19° (Figure S6).
H-bonded phenolic proton (δ ≈ 13–14 ppm) of H2L1, H2L2,
All three compounds showed limited solubility in non-
and H2L3 gives the first indication for the formation of 1– polar solvents; hence, the photophysical studies were per-
3. The 11B NMR spectra of the diboron compounds each formed in tetrahydrofuran. The photophysical properties of
show a narrow signal at δ ≈ 0–1 ppm, which is in agreement these compounds were unaffected by solvent change (see
with the existence of a four-coordinate boron atom. Figure S3); this indicates that the interactions of the fluoro-
Furthermore, compounds 1–3 were also characterized by phores with solvent molecules in the excited state is less
19F NMR spectroscopy. The chemical shifts of the fluorine significant. The absorption and fluorescence spectra of di-
nuclei in the 19F NMR spectra were not strongly affected boron compounds 1–3 in tetrahydrofuran are shown in Fig-
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Figure 3. Molecular structures of 1 and 3. Solvent THF and
CH3CN molecules and hydrogen atoms are omitted for clarity. Se-
lected bond lengths [Å] and angles [°] for 1: B1–F1 1.377(5), B1–
F2 1.388(5), B2–F3 1.367(5), B2–F4 1.386(4), B1–O1 1.450(5), B2–
O2 1.465(5), B1–N1 1.591(5), B2–N2 1.577(5), F1–B1–F2 110.9(3),
F1–B1–O1 108.7(3), F2–B1–O1 111.0(3), F1–B1–N1 110.9(3), F2–
B1–N1 106.1(3), O1–B1–N1 109.4(3), F3–B2–F4 110.7(3), F3–B2–
O2 108.9(3), F4–B2–O2 109.8(3), F3–B2–N2 108.1(3), F4–B2–N2
108.5(3). Selected bond lengths and angles for 3: B1–F1 1.3640(17),
B1–F2 1.3751(18), B2–F3 1.3670(19), B2–F4 1.3723(19), B1–O1
1.4485(17), B2–O2 1.4238(17), B1–N1 1.5905(18), B2–N2
1.5812(17), F1–B1–F2 110.01(12), F1–B1–O1 109.57(11), F2–B1–
O1 110.96(12), F1–B1–N1 109.54(11), F2–B1–N1 107.30(11), O1–
B1–N1 109.42(10), F3–B2–F4 109.43(12), F3–B2–O2 109.13(12),
F4–B2–O2 110.47(13), F3–B2–N2 109.35(11), F4–B2–N2
108.24(11).
Figure 4. Illustration of the boron deviation from the nine-atom
plane (defined by the six biphenyl carbon atoms and the imino
carbon, nitrogen, and oxygen atoms) and the dihedral angle be-
tween the planes for 1 (top) and 3 (bottom).
ure 5. As shown in Figure 5 and Table 1, all three com-
pounds exhibit absorption at ca. 370–380 nm and emission
at ca. 460–490 nm. Both the absorption and the emission
of 1 are redshifted in comparison to those of 2 and 3. Com-
pounds 1–3 showed moderate quantum yields (12, 28, and
16% respectively); however, a remarkably higher quantum
yield was observed for 2 compared to those of 1 and 3. To
gain more insight into the effect of the biphenyl moiety in
our design, we also synthesized the salicylaldimine mono-
boron complex 4 (Scheme 1 and Supporting Information)
and studied its photophysical properties. The mononuclear
compound 4 showed substantial blueshifts both in absorp-
tion (20 nm) and in emission (18 nm) when compared to
those of the diboron compound 3 (Figures S4 and S5); this
might be because of the decreased conjugation length, and
such a trend is similar to that reported by Gomes and co-
workers in their study of iminopyrrolyl boron complexes.[52]
Density functional theory (DFT)[56] computations were
performed to model the photophysical properties of the di-
boron compounds. The geometries of 1–3 were optimized
by DFT [B3LYP, 6-31G(d)] calculations, and the excitation
energies were computed by using time-dependent DT (TD-
DFT) calculations. The computed results are consistent
with the experimental results, although the absolute exci-
tation data deviate significantly (Table 2). Figure 6 shows
Figure 5. Normalized absorption (top) and fluorescence emission
plots of the highest occupied molecular orbitals (HOMOs) (bottom) spectra of 1–3.
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Table 1. Experimental photophysical data of 1–3.
feature. Owing to this π conjugation, the LUMO level of 1
is lowered and this helps to decrease the band gap and,
thus, results in a redshift. Pronounced twists of the N-
phenyl rings were observed in 2 and 3 [the rings in 3 are
twisted by 77.86 and 85.19°, whereas the rings in 1 are
twisted by 48.88 and 44.18° (from X-ray data)] and may be
responsible for restricted electronic communication in 2 and
3.
λmax [nm]
ε [L/molcm]
λem
[nm] (Φ)[c]
[a]
[a,b]
1
2
3
380
373
375
13500
8700
15548
490 (0.12)
464 (0.28)
460 (0.16)
[a] Concentrations were 4.35ϫ10–5 m in THF. [b] Excited at the
absorption maximum. [c] Quantum yield.
The electron-accepting abilities of the diboron com-
pounds were studied by cyclic voltammetry in dimethyl-
formamide (DMF). All compounds 1–3 exhibit two sepa-
rate reduction waves. The redox potentials for 1 at E1/2(1)
= –1.63 V and E1/2(2) = –1.83 V are slightly less negative
than those of 2 and 3 [E1/2(1) = –1.83 V, E1/2(2) = –2.03 V
for 2; E1/2(1) = –1.81 V, E1/2(2) = –2.0 V for 3 (Figure 7)],
maybe because of the presence of electron-donating groups
in 2 and 3.[57] The first reduction potentials of diboron com-
pounds 1–3 are comparable with those of the diboron flu-
and lowest unoccupied molecular orbitals (LUMOs) of 1–3;
according to the theoretical calculations, the lowest energy
excitation corresponds to a π–π* transition for all three
compounds. The HOMOs are mainly contributed to by the
π orbitals of the biphenyl and oxygen moieties, whereas the
LUMOs are composed of π* orbitals of the biphenyl and
imine moieties (Tables S4 and S5). There is a clear differ-
ence in the electron distribution of 1 and those of 2 and 3.
For 2 and 3, the electron distribution in the LUMO is con-
fined to the biphenyl and oxygen moieties, whereas the elec-
tron distribution in the LUMO of 1 shows a delocalized
Table 2. Calculated electronic transitions for 1–3 from TD-DFT
(B3LYP) calculations.
Transition MO contribution
Energy gap Oscillator
[eV] ([nm]) strength (f)
1
2
S0ǞS1
S0ǞS2
S0ǞS3
HOMOǞLUMO
HOMOǞLUMO+1
HOMO–2ǞLUMO+1 3.54 (350) 0.0142
HOMO–1ǞLUMO
HOMOǞLUMO
HOMOǞLUMO+1
HOMO–6ǞLUMO+1 3.71 (333) 0.0080
HOMO–5ǞLUMO
3.01 (411) 0.2030
3.09 (400) 0.0001
S0ǞS1
S0ǞS2
S0ǞS3
3.16 (391) 0.1325
3.19 (387) 0.0000
HOMO–4ǞLUMO+1
HOMO–1ǞLUMO
HOMOǞLUMO
HOMOǞLUMO+1
HOMO–3ǞLUMO+1 3.67 (337) 0.0103
HOMO–2ǞLUMO
3
S0ǞS1
S0ǞS2
S0ǞS3
3.16 (391) 0.1324
3.19 (387) 0.0000
Figure 7. Cyclic voltammogram of 3 with 0.1 m Bu4N(PF6) in
DMF as the supporting electrolyte (scan rate 100 mV/s). Refer-
enced relative to Fc/Fc+ couple.
HOMO–1ǞLUMO+1
Figure 6. Computed orbitals for 1–3.
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orophores reported by Zhang et al.[17] but notably less nega-
tive than that of AlQ3.[58] The HOMO and LUMO energy
levels were also calculated from the onset absorption and
onset reduction potentials (Table 3). The HOMO–LUMO
gaps from these measurements are in agreement with the
results obtained from the TD-DFT computations.
Single-crystal X-ray diffraction data were collected with a Bruker
KAPPA APEX-II four angle rotation system with Mo-Kα radiation
(0.71073 Å). The crystallographic data for 1, 3, H2L1, H2L2 and
H2L3 and details of the X-ray diffraction experiments and crystal
structure refinements are given in the Supporting Information.
SADABS[59] absorption corrections were applied. Structures were
solved by direct methods and completed by subsequent difference
Fourier syntheses and refined by full-matrix least-squares pro-
cedures on reflection intensities (F2). All non-hydrogen atoms were
refined with anisotropic displacement coefficients. The H atoms
were placed at calculated positions and were refined as riding
atoms. All software and source scattering factors are contained in
the SHELXTL (v. 5.10) program package.[60]
Table 3. Frontier orbital energies [eV] derived from UV/Vis onset
absorption and electrochemical data.
HOMO–LUMO gap[a]
LUMO[b]
HOMO[c]
1
2
3
2.91
2.98
2.97
–3.28
–3.11
–3.18
–6.19
–6.09
–6.15
CCDC-932528 (for 1), -932529 (for 3), -942656 (for H2L1), -942657
(for H2L2), and -942655 (for H2L3) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. DFT calculations were
performed with the Gaussian03 program.[52] Excitation data were
determined by TD-DFT (B3LYP) calculations.
[a] Estimated from the absorption onset of the longest-wavelength
UV band. [b] Calculated from Epc of the first reduction wave refer-
enced to Fc/Fc+. [c] Calculated from the HOMO–LUMO gap and
the LUMO.
[1,1Ј-Biphenyl]-2,2Ј-diol-3,3Ј-bis(phenylimino)methyl (H2L1): Anil-
ine (0.88 mL, 9.70 mmol) and 2,2Ј-dihydroxybiphenyl-3,3Ј-dicarb-
aldehyde (1.12 g, 4.62 mmol) were dissolved in anhydrous toluene
(20 mL). To this solution a catalytic amount of p-toluenesulfonic
acid was added. The reaction mixture was heated to reflux for 15 h
under Dean–Stark reaction conditions. The reaction mixture was
cooled to room temperature, the solvent was removed under vac-
uum, and the solid was purified by crystallization in chloroform
Conclusions
We have designed and synthesized new salicylaldimine-
based diboron fluorophores by a simple synthetic pro-
cedure. All three boron compounds showed high thermal
stability and interesting photophysical and electrochemical
properties. We expect that the new diboron complexes re-
ported here will have potential applications in optoelec-
tronic devices. Further studies on the modification of conju-
gated diboron system are in progress.
1
and ethanol, yield 1.50 g, 83%, m.p. 169 °C. H NMR (400 MHz,
C6D6): δ = 6.85–6.9 (m, 6 H, ArH), 6.95–6.99 (m, 4 H, ArH), 7.02–
7.07 (m, 4 H, ArH), 7.73 (dd, J = 1.6, 8 Hz, 2 H, ArH), 8.07 (s, 2 H,
CH=N), 13.97 (s, 2 H, ArOH) ppm. 13C NMR (100 MHz, C6D6): δ
= 118.54, 119.83, 121.56, 126.74, 126.92, 129.43, 132.26, 135.95,
148.97, 159.96 (ArC), 163.34 (CH=N) ppm. HRMS (ESI): calcd.
for C26H20N2O2 [M + H]+ 393.1598; found 393.1595. C26H20N2O2
(392.46): calcd. C 79.57, H 5.14, N 7.14; found C 79.31, H 5.22, N
7.01.
Experimental Section
Reagents were used as received unless otherwise noted. THF and
toluene were distilled from Na/benzophenone prior to use. Chlorin-
ated solvents were distilled from CaH2. 2,2Ј-Dihydroxybiphenyl-
3,3Ј-dicarbaldehyde was prepared according to a literature pro-
[1,1Ј-Biphenyl]-2,2Ј-diol-3,3Ј-bis({[2,6-bis(methyl)phenyl]imino}-
methyl) (H2L2): 2,6-Dimethylaniline (0.63 mL, 5.16 mmol), 2,2Ј-di-
hydroxybiphenyl-3,3Ј-dicarbaldehyde (0.50 g, 2.06 mmol), and for-
mic acid (0.1 mL) were dissolved in anhydrous dichloromethane
(15 mL). The reaction mixture was heated to reflux for 15 h and
then cooled to room temperature. The solvent was removed under
vacuum, and the solid was purified by crystallization in chloroform
1
cedure.[53] All H (400 MHz), 13C (100 MHz), 11B (128 MHz), and
19F (376 MHz) NMR spectra were recorded with a Bruker ARX
400 spectrometer. All 1H and 13C NMR spectra were referenced
internally to solvent signals. 11B NMR spectra were referenced ex-
ternally to BF3·Et2O in CDCl3 (δ = 0 ppm), and 19F NMR spectra
were referenced to α,α,α-trifluorotoluene (0.05% in CDCl3; δ =
–63.73 ppm). All NMR spectra were recorded at ambient tempera-
ture. Elemental analyses of C, H, and N were performed with a
Perkin–Elmer 240C elemental analyzer. ESI mass spectra were re-
corded with a Bruker microTOF-QII mass spectrometer. The ab-
sorbance spectra were recorded with a Perkin–Elmer Lambda 750
UV/Visible spectrometer. The fluorescence spectra were recorded
with a Perkin–Elmer LS-55 Fluorescence Spectrometer and cor-
rected for the instrumental response. Quinine sulfate was used as
the standard for the determination of the quantum yields.
1
and ethanol, yield 0.65 g, 70%, m.p. 175 °C. H NMR (400 MHz,
C6D6): δ = 1.95 (s, 12 H, CH3), 6.85 (t, J = 8 Hz, 2 H, ArH), 6.90–
6.93 (m, 8 H, ArH), 7.71 (s, 2 H, CH=N), 7.73 (d, J = 8 Hz, 2 H,
ArH), 13.72 (s, 2 H, ArOH) ppm. 13C NMR (100 MHz, C6D6): δ =
18.45 (CH3), 118.54, 119.42, 125.14, 126.73, 128.58, 128.63, 132.16,
136.19, 148.74, 159.91 (ArC), 167.50 (CH=N) ppm. HRMS (ESI):
calcd. for C30H28N2O2 [M + H]+ 449.2224; found 449.2234.
C30H28N2O2 (448.56): C 80.33, H 6.29, N 6.25; found C 80.12, H
6.01, N 6.08.
Electrochemical measurements were performed with a conventional
three-electrode cell and an electrochemical workstation (CH Instru-
ment 1100A). The three-electrode system consisted of a glassy car-
bon working electrode, a Pt wire as the secondary electrode, and a
Ag wire as the reference electrode. The voltammograms were re-
[1,1Ј-Biphenyl]-2,2Ј-diol-3,3Ј-bis({[2,6-bis(methylethyl)phenyl]-
imino}methyl) (H2L3): By a similar procedure as that for H2L2,
the reaction of 2,6-diisopropyl aniline (1.94 mL, 10.32 mmol) and
biphenol dialdehyde (1.00 g, 4.13 mmol) gave a yellow solid, which
was purified by recrystallization, yield 2.00 g, 86%, m.p. 192 °C.
1H NMR (400 MHz, C6D6): δ = 1.04 (d, J = 8 Hz, 24 H, CH3),
2.93–3.03 (m, 4 H, CH), 6.82 (t, J = 8 Hz, 2 H, ArH), 6.96 (d, J =
8 Hz, 2 H, ArH), 7.09–7.10 (m, 6 H, ArH), 7.74 (d, J = 8 Hz, 2 H,
ArH), 7.99 (s, 2 H, CH=N), 13.75 (s, 2 H, ArOH) ppm. 13C NMR
corded with ca. 1.0ϫ10–3
m solutions in DMF containing
Bu4N(PF6) (0.1 m) as the supporting electrolyte. The scans were
referenced after the addition of a small amount of ferrocene as the
internal standard.
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(100 MHz, C6D6): δ = 23.60 (CH3), 28.53 (CH), 118.75, 119.36,
123.56, 125.87, 126.60, 132.21, 136.46, 139.08, 147.00, 159.88
(ArC), 167.58 (CH=N) ppm. HRMS (ESI): calcd. for C38H44N2O2
[M + H]+ 561.3476; found 561.3441. C38H44N2O2 (560.78): calcd.
C 81.39, H 7.91, N 5.00; found C 81.23, H 7.99, N 4.92.
C38H42B2F4N2O2 [M
C38H42B2F4N2O2 (656.37): calcd. C 69.54, H 6.45, N 4.27; found
C 69.83, H 6.31, N 4.11.
+
Na]+ 679.3273; found 679.3211.
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis of 4, additional experimental and analytical details.
Complex 1: Under nitrogen, H2L1 (0.40 g, 1.02 mmol) was added
to a suspension of NaH (0.056 g, 2.34 mmol) in anhydrous tetra-
hydrofuran (15 mL) at 0 °C with stirring. The reaction mixture was
warmed to room temperature and stirred at that temperature for
2 h. The resulting solution was added to a solution of BF3·Et2O
(2.51 mL, 20.40 mmol) in tetrahydrofuran, and the mixture was
stirred for 24 h. The reaction mixture was filtered through Celite,
and the resulting filtrate was concentrated to yield a pale yellow
solid, which was purified by recrystallization in acetonitrile, yield
260 mg, 52%, m.p. 297–303 °C (dec). 1H NMR (400 MHz, CDCl3):
δ = 7.17 (t, J = 4 Hz, 2 H, ArH), 7.47–7.58 (m, 12 H, ArH), 8.05
(d, J = 8 Hz, 2 H, ArH), 8.49 (s, 2 H, CH=N) ppm. 13C NMR
(100 MHz, CDCl3): δ = 116.61, 120.40, 123.78, 126.15, 129.49,
129.84, 132.43, 142.01, 142.56, 157.65 (ArC), 163.84 (CH=N) ppm.
19F NMR (376 MHz, CDCl3): δ = –135.44 (br) ppm. 11B NMR
(128 MHz, CDCl3): δ = 1.01 (s) ppm. Quantum yield (Φ) = 0.12.
Acknowledgments
K. V. thanks the Department of Science and Technology (DST),
New Delhi for a Ramanujan fellowship (SR/S2/RJN-49/2011) and
the National Institute of Science Education and Research (NISER)
for financial support. K. D., V. M. and M. R. thank the Council of
Scientific and Industrial Research (CSIR), New Delhi for research
fellowships. The authors also thank the reviewers for their helpful
suggestions.
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IR (KBr): ν = 3030 (w), 2944 (m), 2872 (s), 1622 (s), 1589 (m),
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Published Online: December 10, 2013
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