Synthesis and photophysical properties of phenanthroimidazole-triarylborane dyads: intriguing ‘turn-on' sensing mediated by fluoride anions

Article abstract of DOI:10.1039/c6ra28559g

Phenanthroimidazole-based triarylborane compounds with an N-phenyl (1Ph, 2Ph) or N-biphenyl (1BP, 2BP) bridge were synthesized and characterized. All four compounds exhibit a dual emission pattern in their photoluminescence (PL) spectra, which can be separated into high- (λ_em = ca. 380 nm in THF) and low-energy (λ_em = ca. 480 nm) emissions. While the high-energy emission remains largely unchanged in different organic solvents, the low-energy emission exhibits clear signs of positive solvatochromism. The results of the photophysical analysis and theoretical calculations suggest that the high-energy emission corresponds to a π-π* transition band arising from the phenanthroimidazole, whereas the low-energy emission originates from an intramolecular charge transfer (ICT) transition between phenanthroimidazole and the triarylborane moiety. UV-vis titration experiments examining the association of 1Ph, 2Ph, 1BP, and 2BP with fluoride demonstrate that these compounds associate with a 1?:?1 binding stoichiometry in THF and binding constants (K_a) that are estimated to be around 1.0-3.0 × 10⁴ M^?1. These compounds show a ratiometrically increased fluorescence response in PL titration experiments upon binding of fluoride to the borane moiety, thereby giving rise to a ‘turn-on' chemosensor for detection of fluoride anions. The ‘turn-on' properties can be judged as a result of the reinforcement of π-π* transition on phenanthroimidazole and the restriction of ICT transition to triarylborane.

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Synthesis and photophysical properties of
phenanthroimidazole–triarylborane dyads:
intriguing ‘turn-on’ sensing mediated by fluoride
anions†
Cite this: RSC Adv., 2017, 7, 10345
Dong Kyun You,a Seon Hee Lee,b Ji Hye Lee,a Sang Woo Kwak,c Hyonseok Hwang,a
d
c
b
a
*
*
*
Junseong Lee, Yongseog Chung, Myung Hwan Park and Kang Mun Lee
Phenanthroimidazole-based triarylborane compounds with an N-phenyl (1Ph, 2Ph) or N-biphenyl (1BP,
2BP) bridge were synthesized and characterized. All four compounds exhibit a dual emission pattern in
their photoluminescence (PL) spectra, which can be separated into high- (lem ¼ ca. 380 nm in THF) and
low-energy (lem ¼ ca. 480 nm) emissions. While the high-energy emission remains largely unchanged in
different organic solvents, the low-energy emission exhibits clear signs of positive solvatochromism. The
results of the photophysical analysis and theoretical calculations suggest that the high-energy emission
corresponds to a p–p* transition band arising from the phenanthroimidazole, whereas the low-energy
emission originates from an intramolecular charge transfer (ICT) transition between phenanthroimidazole
and the triarylborane moiety. UV-vis titration experiments examining the association of 1Ph, 2Ph, 1BP,
and 2BP with fluoride demonstrate that these compounds associate with a 1 : 1 binding stoichiometry in
THF and binding constants (Ka) that are estimated to be around 1.0–3.0 ꢀ 104
M
ꢁ1. These compounds
Received 22nd December 2016
Accepted 31st January 2017
show a ratiometrically increased fluorescence response in PL titration experiments upon binding of
fluoride to the borane moiety, thereby giving rise to a ‘turn-on’ chemosensor for detection of fluoride
anions. The ‘turn-on’ properties can be judged as a result of the reinforcement of p–p* transition on
phenanthroimidazole and the restriction of ICT transition to triarylborane.
DOI: 10.1039/c6ra28559g
rsc.li/rsc-advances
diodes (OLEDs),1,4 dye-sensitized solar cells,5 and effective
mother-luminophores of uorescent chemosensors for detect-
ing cations.6 In recent years, conjugated phenanthroimidazole-
based compounds capable of detecting uoride anions have
been studied thoroughly as naked-eye detectable receptors
owing to their remarkable uorescence changes.7 For example,
Introduction
Phenanthroimidazole is a robust, heterocyclic compound,
which possesses two nitrogen atoms in the imidazole ring and
exhibits high quantum efficiency (F ¼ 0.18–0.93)1 and good
electron-transporting ability.2 The introduction of various
functional groups at the N1- and C2-positions of phenan-
throimidazole can lead to the emergence of intriguing proper-
ties.3 Accordingly, phenanthroimidazole-based derivatives have
received great attention as promising attractive molecular
scaffolds for optoelectronic materials of organic light-emitting
Mahapatra and co-workers demonstrated that
a carbon-
ohydrazone-bridged phenanthroimidazole dimer, driven by
hydrogen bonding interactions between –NH group and uo-
ride, can act as an efficient colorimetric and ratiometric uo-
rescence sensor that allows detection by naked eye (I in Chart
1).7a Liu and co-workers have also reported an ‘easy-to-prepare’
chemosensor, 2-(4-formyl-phenyl)phenanthroimidazole, suit-
able as a colorimetric and uorometric probe for uoride
sensing with good sensitivity and selectivity (II in Chart 1).7b
The binding of uoride to receptors can potentially result in
considerable changes in the electronic environment of the
phenanthroimidazole moiety. Indeed, Lee group has re-
ported triarylborane–phenanthroimidazole conjugates, where
a restricted intramolecular charge transfer (ICT) transition from
the phenanthroimidazole moiety to triarylborane group aer
the binding of uoride to the boron centre (III in Chart 1)
produced a change in the emission colour.7c In this regard, the
aDepartment of Chemistry, Institute for Molecular Science and Fusion Technology,
Kangwon National University, Chuncheon, Gangwon 24341, Republic of Korea.
E-mail: kangmunlee@kangwon.ac.kr
bDepartment of Chemistry Education, Chungbuk National University, Cheongju,
Chungbuk 28644, Republic of Korea. E-mail: mhpark98@chungbuk.ac.kr
cDepartment of Chemistry, Chungbuk National University, Cheongju, Chungbuk
28644, Republic of Korea. E-mail: yschung@chungbuk.ac.kr
dDepartment of Chemistry, Chonnam National University, Gwangju 61186, Republic of
Korea
† Electronic supplementary information (ESI) available: X-ray crystallographic
data of 1Ph and 1BP in CIF format, NMR spectra, and computational details.
CCDC 1521689 and 1521688. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6ra28559g
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(48%), respectively. Biphenylene-bridged compounds 1BP and
2BP were synthesized using the Suzuki–Miyaura coupling
between 1a or 2a and (4-(dimesitylboryl)phenyl)boronic acid
(3a) in relatively low yields (28% and 9%, respectively). All the
compounds were characterized by multinuclear NMR spec-
troscopy (Fig. S20–S26†) and elemental analysis (EA). The 1H
and 13C NMR spectra of all compounds were in agreement with
the predicted structures, and the chemical shis of the reso-
nances associated with the proton and carbon atoms in these
molecules were in expected range. Nevertheless, 11B NMR
signals for 1Ph, 2Ph, 1BP, and 2BP could not be observed
despite prolonged acquisition times (up to 12 hours). The solid-
state structures of 1Ph and 1BP were conrmed by single crystal
X-ray diffraction (Fig. 1, Tables S1 and S2†). Single crystals of
sufficient quality for structural analysis were obtained by slow
evaporation of mixed saturated solution of THF/EA at ambient
temperature. The central boron atom observed in both 1Ph and
1BP was found to adopt a perfectly trigonal planar geometry, as
Chart 1 Examples of phenanthroimidazole-based chemosensors for
detecting fluoride.
phenanthroimidazole group can play an important role as an
electron or charge donor which mediates ICT transition. These
ndings indicate that a well-dened molecular design of phe-
nanthroimidazole derivatives equipped with appropriate
charge-acceptors or uoride-receptor units can produce drastic
changes in the colour of uorescence.
Bearing in mind that the potential application of ratiometric
and ‘naked-eye’ detectable uorescence response is worthy of
an analytical point of view in chemosensors, we designed and
synthesised in this work four phenanthroimidazole–
triarylborane-based dyad systems (1Ph, 2Ph, 1BP, 2BP), linked
through the nitrogen atom of the phenanthroimidazole unit,
which exhibit ratiometric ‘turn-on’ uorescence upon uoride
binding. The details of synthesis and characterization,
including molecular structures in the solid state and the pho-
tophysical changes mediated by uoride sensing, are provided
and supplemented with the results of theoretical calculations.
evidenced by the sum of the three C–B–C angles (S(C–C–C)
¼
360ꢂ, 1Ph: 120.4ꢂ, 118.2ꢂ, and 121.4ꢂ and 1BP: 120.4ꢂ, 117.6ꢂ,
and 121.9ꢂ, respectively). Importantly, the X-ray analysis
revealed that the phenanthroimidazole plane in both structures
is signicantly distorted with respect to the N-phenyl group
connected to the dimesityl borane, resulting in dihedral angles
of 69.7ꢂ (1Ph) and 79.8ꢂ (1BP), respectively. Although those
structural features in solid state of both 1Ph and 1BP would
seem that delocalization of electrons between the bridged N-
phenyl group and the phenanthroimidazole moiety might be
unfavourable, both moieties could be electronically connected
by the free rotation of bridged N-phenyl group in solution at
least, which is certainly supported by reduction of p–p* tran-
sition bands for the phenanthroimidazole part upon the
Results and discussion
Synthesis and characterization
The synthetic procedures for the preparation of N-triarylborane
substituted phenanthro[9,10-d]imidazole compounds 1Ph, 2Ph,
1BP, and 2BP are shown in Scheme 1. Lithiation reaction of
bromophenyl-substituted phenanthroimidazole (1a and 2a)
with dimesitylboron uoride produced 1Ph (12%) and 2Ph
Fig. 1 X-ray crystal structures of 1Ph (top) and 1BP (down) depicted as
Scheme 1 Synthetic route of 1Ph–2BP. Reagents and condition: (i) n- 40% thermal ellipsoids. Hydrogen atoms and tetrahydrofuran (for 1BP)
BuLi, Mes2BF, THF, ꢁ78 ꢂC. (ii) Pd(PPh3)4, K2CO3, THF/H2O, 90 ꢂC.
are omitted for clarity.
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addition of uoride anions in UV-vis titration experiments (see
the uoride sensing properties below).
UV-vis absorption and emission properties
In order to investigate the photophysical properties of all phe-
nanthroimidazole–triarylborane dyads, 1Ph, 2Ph, 1BP and 2BP,
UV-vis absorption and photoluminescence (PL) measurements
were performed at room temperature (Fig. 1 and Table 1). All
compounds exhibited a major absorption band centred at ca.
315 nm, which can be assigned to the p–p* transition of the
phenanthroimidazole unit. The spectra also showed a shoulder
absorption band in the region above 360 nm, which represents
the p(Mes)–pp(B) charge transfer (CT) transition in the borane
moiety (see theoretical calculations for details). The PL spectra
of all four compounds in THF exhibited an apparent dual
Fig. 2 UV-vis absorption spectra of 1Ph, 2Ph, 1BP, and 2BP in THF (2.5
ꢀ 10ꢁ5 M).
experiments in THF as the solvent (Fig. 3 and Table 1). The
addition of increasing quantities of uoride to a solution of
1Ph, 2Ph, 1BP, or 2BP in THF revealed gradual quenching of the
major absorption band around 315 nm, arising from phenan-
throimidazole centred p–p* transitions in all compounds, and
a decrease in the intensity of the absorption region at 360 nm,
which can be assigned to p(Mes)–pp(B) CT band. These spectral
emission band, which could be divided into high- (lem
¼
386 nm, Table 1) and low-energy (ca. 480 nm) emission regions.
In particular, the emission in the low-energy region showed
clearly solvent-dependent features. In contrast, the absorption
band (Fig. S1†) and the high-energy emission region (Fig. 2) of
all the compounds remained practically unaltered in solvents
(cyclohexane, THF, and DMSO) with different polarities. With
increasing solvent polarity, the low-energy emission band was
red-shied dramatically by over 100 nm for all compounds
except for 2Ph (Dl ¼ 103 nm for 1Ph, 133 nm for 1BP, and
126 nm for 2BP, Table 1). The emission of 2Ph exhibited less
signicant solvatochromic changes from 409 nm in cyclohexane
to 483 nm in THF (Table 1). These solvatochromic changes
exhibited by all four compounds suggest that the excited states
associated with the low-energy emission are principally polar in
character. This polar character, in turn, indicates that these are
emissions arising from a charge transfer (CT) state between
phenanthroimidazole and the triarylborane moiety. The broad
and unstructured shape of the low-energy emission band
further supports the CT transition state, which differs from the
high-energy emission band originating from the phenan-
throimidazole centred p–p* transition. The decay lifetimes of
both the high and low-energy emissions for 1Ph–2BP were
measured as 2.8–6.5 ns which can be assignable to uorescence
(Fig. S2–S5† and Table 1).
Fluoride sensing properties
Fig. 3 Photoluminescence emission spectra of (a) 1Ph (lex ¼ 326 nm),
(b) 2Ph (lex ¼ 325 nm), (c) 1BP (lex ¼ 296 nm), and (d) 2BP (lex ¼ 292
nm) in various organic solvents at 298 K.
The uoride anion binding properties of 1Ph, 2Ph, 1BP, and
2BP were investigated using both UV-vis and PL titration
Table 1 UV-vis absorption and fluorescence properties of 1Ph, 2Ph, 1BP, and 2BPa
lemb/nm
sobsa/ns
labsa/nm (log 3)
lex/nm
Cyclohexane
THF
DMSO
Fa,c
ꢃ385 nm
ꢃ480 nm
Ka/Mꢁ1
1Ph
2Ph
1BP
2BP
310 (4.57), 360 (4.00)
311 (4.58), 359 (3.99)
319 (4.98), 360 (4.29)
318 (4.53), 360 (3.77)
326
325
296
292
370, 414
385, 409
390, 402
384, 403
386, 479
386, 483
385, 485
386, 476
387, 517
387
385, 535
391, 529
0.125
0.042
0.033
0.015
4.35
6.51
3.45
2.77
4.19
6.28
5.44
5.05
1.4 ꢀ 104
1.1 ꢀ 104
1.0 ꢀ 104
3.0 ꢀ 104
c ¼ 2.5 ꢀ 10ꢁ5 M in THF. c ¼ 2.5 ꢀ 10ꢁ5 M. Quinine sulfate (F ¼ 0.55) was used as a standard.
a
b
c
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changes suggest that the binding of uoride anion to the boron emission band at 386 nm was enhanced tremendously.
results in the disruption of the CT transition centred at the Importantly, 69-fold enhancement was determined for 1Ph, 13-
triarylborane moiety and also reduction in the absorption fold for 2Ph, 40-fold for 1BP, and 67-fold for 2BP, compared to
arising from phenanthroimidazole centred p–p* transitions. the emission maxima observed prior to the addition of uoride.
The linear decrease in the absorption bands of all compounds In contrast, the low-energy emission was found to decrease only
from 310 to 320 nm is indicative of 1 : 1 binding between the slightly in the presence of uoride, producing a change in the
triarylborane and uoride anion. The binding constants (Ka) for colour of uorescence from blue-green to purple. These
1Ph, 2Ph, 1BP, and 2BP were estimated as 1.0–3.0 ꢀ 104 Mꢁ1 dramatic changes in emission patterns indicate unambiguously
(Fig. 4, inset and Table 1) respectively. These values are one that the uoride binding interrupts the ICT transition between
order of magnitude lower than that of Mes3B (3.3(ꢄ0.5) ꢀ 105 the triarylborane and phenanthroimidazole moiety, while
Mꢁ1).8a This result originates from the fact that the boron atom simultaneously intensifying the p–p* transition arising from
of amine (NR2)-appended triarylborane compounds would have the phenanthroimidazole moiety.
anionic character due to the resonance structures induced by
These turn-on features were supported further by the emis-
intramolecular charge transfer from an N (donor) to a B sion patterns of 1Ph and 1BP uoride adducts. These uoride
(acceptor) atom.8b Interestingly, the PL titration experiments of adducts were synthesised simply by the addition of 1.5 equiv. of
all compounds in THF revealed signicant changes in their PL Bu4NF to 1Ph or 1BP in deuterated THF and their structures
spectra following the addition of uoride (Fig. 4 for 1Ph and were characterized by 1H NMR spectroscopy (Fig. S9 and S10†).
Fig. S6–S8† for 2Ph, 1BP, and 2BP, respectively). As shown in The PL spectra of the prepared uoride adducts exhibited only
Fig. 2, all compounds exhibited dual emission bands: high- (lem the high-energy emission region (lem ¼ ca. 385 nm) (Fig. S11†).
¼ ca. 386 nm) and low-energy (lem ¼ ca. 480 nm) regions, giving In fact, the shape and intensity of the resulting spectra were
rise to a weak blue-green emission colour (Fig. 4 and S6–S8,† quite similar to those obtained at the end of the titration
inset). Upon addition of incremental amounts of uoride, the experiments. In particular, the high-energy emission of the
uoride adducts was found to change very little as a function of
solvent polarity (adduct of 1Ph: lem ¼ 374, 389 nm in cyclo-
hexane, lem ¼ 389 nm in THF, lem ¼ 388 nm in DMSO; adduct
of 1BP: lem ¼ 368, 385 nm in cyclohexane, lem ¼ 387 nm in THF,
lem ¼ 387 nm in DMSO, Fig. S11†). Consequently, these results
clearly demonstrate that the observed ‘turn-on’ uorescence
phenomena are closely associated with the concentration of p–
p* transitions of the phenanthroimidazole moiety and the
restricted ICT state between the borane and phenan-
throimidazole as a result of uoride binding.
Theoretical calculations and molecular orbital analyses
To gain further insight into the origin of the electronic transi-
tions and the ‘turn-on’ emission behaviour of 1Ph–2BP, time-
dependent density functional theory (TD-DFT) calculations on
the ground state (S0) and the rst excited state (S1) optimized
structures of four compounds were performed using the B3LYP
functional and 6-31G(d) basis sets (Fig. 5, S12–S19,† and Table
2). A conductor-like polarizable continuum model (CPCM) was
used in order to account for the solvent effects (THF).9,10 All
calculations were performed using the Gaussian 09 Package.11
The wavelengths calculated for 1Ph–2BP in the ground state (S0)
exhibited a major low-energy absorption (fcalc > 0.1, Table 2)
around 360 nm, which is mainly involved in the transition from
HOMOꢁ2 to LUMO. These results are indicative of a p(Mes)–
pp(B) ICT transition, centred at the triarylborane moiety, since
the HOMOꢁ2 and LUMO are localised predominantly on the
dimesityl borane (>94%, Tables S4, S8, S12, and S16†) or on
both the dimesityl borane (ꢃ62%) and the N-bridged phenyl
group (ꢃ35%), respectively. On the other hand, the second
Fig. 4 Spectral changes in (a) UV-vis absorption and (b) PL intensity of
major low-energy absorption (fcalc > 0.25) determined for 1Ph–
a solution of 1Ph (lex ¼ 326 nm) in THF (4.00 ꢀ 10ꢁ5 M) following the
2BP involves the HOMO / LUMO+1 transition which can be
addition of TBAF (0–5.83 ꢀ 10ꢁ5 M). The inset shows the changes in
the absorbance at 311 nm as a function of [Fꢁ]. The line corresponds to
assigned to the p–p* transition on the phenanthroimidazole
the binding isotherm calculated with Ka ¼ 1.4 ꢀ 104 Mꢁ1
.
moiety only as the molecular occupation of this unit for both
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Fig. 5 Frontier molecular orbitals for 1Ph–2BP at their first excited singlet state (S1) with their relative energies determined using TD-DFT
calculations. The transition energies (in nm) were calculated using the TD-B3LYP method with 6-31G(d) basis sets.
the HOMO and LUMO+1 is over 98% (Tables S4, S8, S12, and These results also indicate that the emission band around
S16†). These results of computational analysis are consistent 385 nm arises mostly from the p–p* transition of the phe-
with the absorption features observed experimentally and nanthroimidazole moiety. Put together, the results ob-
suggest that signicant absorptions take place independently of tained through computational analyses demonstrate that
the ICT from phenanthroimidazole to triarylborane and phe- the ratiometric ‘turn-on’ uorescent behaviour of the
nanthroimidazole centred p–p* transition.
phenanthroimidazole-based N-triarylborane system upon
TD-DFT calculations performed on the optimized S1 struc- uoride binding originates from the reinforcement of the
tures of 1Ph–2BP were used to predict the emission energies phenanthroimidazole moiety centred p–p* transition as
and transitions for these molecules. Although the calculated a result of the restricted ICT transition between the phenan-
oscillator strengths (fcalc) were not intense, the lowest-energy throimidazole and triarylborane unit.
transition in all compounds could be characterized as the
HOMO to LUMO transition (Table 2 and Fig. 5). Each HOMO is
located fully on the phenanthroimidazole moiety (99.8%,
Tables S6, S10, S14, and S18†), whereas the LUMO levels
involve mainly the triarylborane unit (>98%, bridged phenyl
Experimental
General considerations
All operations were performed under an inert nitrogen atmo-
sphere using standard Schlenk and glove box techniques.
Anhydrous grade solvents (Aldrich) were dried by passing them
through an activated alumina column and stored over activated
rings and dimesityl borane). These results suggest strongly that
the experimentally observed low-energy emission, which is
turned off upon uoride binding, originates from the ICT
between the phenanthroimidazole donor and triarylborane
acceptor. The other major transition observed is that from
HOMO to LUMO+1, which is associated with the high-energy
emission since the LUMO+1 level is, similarly to HOMO,
predominantly located on the phenanthroimidazole moiety.
˚
molecular sieves (5 A). Spectrophotometric grade tetrahydro-
furan (THF), cyclohexane, dimethyl sulfoxide (DMSO) (Aldrich)
was used as received. Commercial reagents were used without
any further purication aer purchasing from Aldrich
(1-bromo-4-iodobenzene, 9,10-phenanthrenequinone, 3-bro-
moaniline, 4-bromoaniline, benzaldehyde, ammonium acetate,
trimethyl borate, tetrakis(triphenylphosphine)palladium(0),
Table 2 The major low-energy electronic transitions determined for
1Ph, 2Ph, 1BP, and 2BP for the first excited singlet state (S1) using TD-
DFT calculationsa
potassium carbonate, n-butyllithium (n-BuLi, 1.6
M in
n-hexane), acetic acid) and TCI chemicals (dimesitylboron
uoride). Deuterated solvents from Cambridge Isotope Labo-
ratories were used aer drying over activated molecular sieves
lcalc/nm
fcalc
Assignment
˚
(5 A). NMR spectra were recorded on a Bruker Avance
1Ph
2Ph
1BP
2BP
520.27
361.20
522.86
360.37
508.14
359.61
509.59
359.76
0.0003
0.7249
0.0003
0.6975
0.0024
0.7675
0.0008
0.7166
HOMO / LUMO (99.4%)
HOMO / LUMO+1 (88.7%)
HOMO / LUMO (99.7%)
HOMO / LUMO+1 (88.0%)
HOMO / LUMO (98.9%)
HOMO / LUMO+1 (89.8%)
HOMO / LUMO (99.3%)
HOMO / LUMO+1 (90.0%)
400 spectrometer (400.13 MHz for H and 100.62 MHz for 13C)
1
at ambient temperature. Chemical shis are given in ppm and
are referenced against external Me4Si (1H and 13C). Elemental
analyses were performed on an EA3000 (Eurovector) in the
Central Laboratory of Kangwon National University. UV-vis
absorption and PL spectra were recorded on a Jasco V-530
a
and
respectively.
a Spex Fluorog-3 Luminescence spectrophotometer,
Singlet energies calculated for the vertical transition in the optimized
S1 geometries.
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1a (1.0 mmol, 0.47 g) in THF (10 mL) and the mixture was
stirred for 1 h at ambient temperature. Aer cooling to ꢁ78 ꢂC,
a solution of dimesitylboron uoride (1.0 mmol) in THF (5 mL)
was added to the mixture. The reaction mixture was then slowly
heated to room temperature and stirred overnight. Aer
quenching with water (20 mL), the organic layer was extracted
with DCM. The organic layer was dried over anhydrous MgSO4
and ltered. Purication by column chromatography (ethyl
Synthesis of 1-(4-bromophenyl)-2-phenyl-1H-phenanthro
[9,10-d]imidazole (1a). To
a
solution of 9,10-phenan-
threnequinone (5.0 mmol) in acetic acid (20 mL) was slowly
added a mixture of benzaldehyde (5.0 mmol), ammonium
acetate (20.0 mmol) and 4-bromoaniline (25.0 mmol) in acetic
acid (30 mL) at ambient temperature. The reaction mixture was
ꢂ
heated to 120 C and stirred overnight. Aer cooling to room
temperature and pouring distilled water (50 mL), a precipitation
was obtained by lteration. Washing with water and ethanol
gives a pale blue solid (1a, 1.92 g); yield ¼ 86%; 1H NMR
acetate : n-hexane ¼ 1 : 10, v/v) afforded the pale brown solid
3
(72 mg); yield ¼ 12%; 1H NMR (CDCl3): d ¼ 8.90 (d, JHH
¼
3
3
9.2 Hz, phenan-CH, 1H), 8.78 (d, JHH ¼ 9.6 Hz, phenan-CH,
(CDCl3): d ¼ 8.88 (d, JHH ¼ 8.0 Hz, phenan-CH, 1H), 8.78 (d,
1H), 8.71 (d, 3JHH ¼ 9.20 Hz, phenan-CH, 1H), 7.74 (m, 3H), 7.65
3JHH ¼ 8.4 Hz, phenan-CH, 1H), 8.71 (d, 3JHH ¼ 8.0 Hz, phenan-
CH, 1H), 7.73 (m, 3H), 7.66 (t, 3JHH ¼ 8.4 Hz, 1H), 7.54 (m, 3H),
(m, 3H), 7.52 (t, 3JHH ¼ 9.8 Hz, 3H), 7.34 (m, 4H), 7.22 (t, 3JHH
¼
3
8.4 Hz, 1H), 6.90 (s, 4H), 2.35 (s, Mes-p-CH3, 6H), 2.13 (s, Mes-o-
CH3, 12H) ppm; 13C NMR (CDCl3): d ¼ 150.64 (s, imidazole-
NCN), 148.33 (s), 141.47 (s), 141.39 (s), 140.85 (s), 139.44 (s),
137.49 (s), 137.27 (s), 130.37 (s), 129.50 (s), 129.45 (s), 129.33 (s),
128.93 (s), 128.86 (s), 128.79 (s), 128.53 (s), 128.33 (s), 128.28 (s),
128.10 (s), 128.00 (s), 127.33 (s), 127.23 (s), 125.99 (s), 125.65 (s),
124.91 (s), 124.17 (s), 123.13 (s), 122.96 (s), 122.80 (s), 120.95 (s),
23.50 (s, Mes-o-CH3), 21.32 (s, Mes-p-CH3) ppm; anal. calcd for
7.35 (m, 6H), 7.22 (d, J ¼ 9.6 Hz, 1H), 7.66 (t, JHH ¼ 8.4 Hz,
1H) ppm; 13C NMR (CDCl3): d ¼ 151.07 (s, imidazole-NCN),
137.88 (s), 137.63 (s), 133.46 (s), 130.83 (s), 130.33 (s), 129.62 (s),
129.44 (s), 129.16 (s), 128.48 (s), 128.42 (s), 127.97 (s), 127.48 (s),
127.22 (s), 126.52 (s), 125.88 (s), 125.16 (s), 124.34 (s), 123.86 (s),
123.23 (s), 122.92 (s), 120.78 (s) ppm; anal. calcd for C27H17BrN2:
C, 72.17; H, 3.81; N, 6.23. Found: C, 71.88; H, 3.49; N, 6.00.
Synthesis of 1-(3-bromophenyl)-2-phenyl-1H-phenanthro
[9,10-d]imidazole (2a). This compound was prepared in
a manner analogous to the synthesis of 1a using 3-bromoaniline
(25.0 mmol). Pale gray solid (0.72 g); yield ¼ 32%; 1H NMR
C
45H39BN2: C, 87.37; H, 6.35; N, 4.53, found: C, 87.42; H, 6.79; N,
4.25.
Synthesis of 1-(3-(dimesitylboryl)phenyl)-2-phenyl-1H-phe-
3
nanthro[9,10-d]imidazole (2Ph). This compound was prepared
in a manner analogous to the synthesis of 1Ph using 2a
(CDCl3): d ¼ 8.87 (d, JHH ¼ 9.6 Hz, phenan-CH, 1H), 8.78 (d,
3JHH ¼ 8.4 Hz, phenan-CH, 1H), 8.71 (d, 3JHH ¼ 8.0 Hz, phenan-
CH, 1H), 7.76 (m, 2H), 7.67 (m, 2H), 7.55 (m, 3H), 7.47 (m, 2H),
7.33 (m, 4H), 7.19 (d, 3JHH ¼ 9.6 Hz, 1H) ppm; 13C NMR (CDCl3):
d ¼ 151.04 (s, imidazole-NCN), 140.11 (s), 137.63 (s), 133.19 (s),
132.31 (s), 131.37 (s), 130.27 (s), 129.59 (s), 129.46 (s), 129.24 (s),
128.52 (s), 128.43 (s), 128.09 (s), 128.04 (s), 127.50 (s), 127.22 (s),
126.59 (s), 125.90 (s), 125.19 (s), 124.37 (s), 123.49 (s), 123.25 (s),
122.92 (s), 122.89 (s), 120.83 (s) ppm; anal. calcd for C27H17BrN2:
C, 72.17; H, 3.81; N, 6.23. Found: C, 71.72; H, 3.44; N, 5.88.
Synthesis of 4-(dimesitylboryl)phenyl)boronic acid (3a). A
hexane solution of n-BuLi (1.6 M, 4.08 mL, 6.52 mmol) was
added at ꢁ78 ꢂC to a solution of (4-bromophenyl)dimesityl
borane (5.42 mmol) in THF (40 mL) and the mixture was stirred
for 1 h at ambient temperature. Aer cooling to ꢁ78 ꢂC, a solu-
tion of trimethyl borate (2.2 mL, 19.5 mmol) in THF (15 mL) was
added to the mixture. The reaction mixture was then slowly
heated at room temperature and stirred overnight. It was
quenched by addition of saturated NH4Cl (55 mL) and extracted
with dichloromethane (50 mL). The organic layer was dried over
anhydrous MgSO4 and passed through silica pad (5 cm). The
solvent was removed by rotary evaporation and the residue was
washed with n-hexane (100 mL) to give white solid (1.30 g, 65%).
1
(1.5 mmol, 0.71 g). Pale brown solid (0.45 g); yield ¼ 48%; H
NMR (CDCl3): d ¼ 8.86 (d, 3JHH ¼ 8.0 Hz, phenan-CH, 1H), 8.76
3
3
(d, JHH ¼ 8.4 Hz, phenan-CH, 1H), 8.69 (d, JHH ¼ 8.4 Hz,
phenan-CH, 1H), 7.72 (m, 2H), 7.64 (t, 3JHH ¼ 8.6 Hz, 1H), 7.57
(m, 5H), 7.51 (t, 3JHH ¼ 8.4 Hz, 1H), 7.31 (m, 3H), 7.21 (m, 2H),
6.78 (s, 4H), 2.26 (s, Mes-p-CH3, 6H), 1.99 (s, Mes-o-CH3,
12H) ppm; 13C NMR (CDCl3): d ¼ 151.21 (s, imidazole-NCN),
149.26 (s), 141.28 (s), 140.83 (s), 139.53 (s), 138.92 (s), 137.38 (s),
136.83 (s), 135.53 (s), 132.12 (s), 130.62 (s), 130.05 (s), 129.86 (s),
129.36 (s), 128.93 (s), 128.53 (s), 128.30 (s), 128.29 (s), 128.25 (s),
127.38 (s), 127.35 (s), 126.06 (s), 125.67 (s), 124.97 (s), 124.25 (s),
123.21 (s), 123.19 (s), 122.90 (s), 121.04 (s), 23.51 (s, Mes-o-CH3),
21.35 (s, Mes-p-CH3) ppm; anal. calcd for C45H39BN2: C, 87.37;
H, 6.35; N, 4.53, found: C, 87.50; H, 6.53; N, 4.52.
Synthesis of 1-(40-(dimesitylboryl)-[1,10-biphenyl]-4-yl)-2-
phenyl-1H-phenanthro[9,10-d]imidazole (1BP). A mixture of 1a
(1.36 mmol, 0.64 g), Pd(PPh3)4 (0.068 mmol) and THF (20 mL)
was stirred for 10 min. The above boronic acid 3a (1.50 mmol,
0.56 g) in 10 mL of THF and K2CO3 (4.50 mmol) in 10 mL of H2O
were subsequently added. The reaction mixture was heated to
ꢂ
80 C and stirred for 24 h. Aer cooling to room temperature,
1H NMR (CDCl3): d ¼ 8.19 (d, 3JHH ¼ 8.0 Hz, 2H), 7.64 (d, 3JHH
¼
the water layer was separated and extracted with DCM (30 mL).
The combined organic layers were dried over MgSO4 and the
solvents were evaporated under reduced pressure. Purication
of the crude product by column chromatography (ethyl
acetate : n-hexane ¼ 1 : 10, v/v) afforded white solid (0.29 g) as
8.0 Hz, 2H), 6.86 (s, 4H), 2.34 (s, Mes-p-CH3, 6H), 2.03 (s, Mes-o-
CH3, 12H) ppm; 13C NMR (CDCl3): d ¼ 150.76 (s, Ar-C-B(OH)2),
141.90 (s), 141.00 (d), 139.06 (d), 135.54 (s), 135.19 (d), 133.16 (d),
128.40 (s), 23.57 (s, Mes-o-CH3), 21.40 (s, Mes-p-CH3) ppm; anal.
calcd for C24H28B2O2: C, 77.89; H, 7.63. Found: C, 77.42; H, 7.12.
Synthesis of 1-(4-(dimesitylboryl)phenyl)-2-phenyl-1H-phe-
nanthro[9,10-d]imidazole (1Ph). A hexane solution of n-BuLi
(1.6 M, 0.69 mL, 1.1 mmol) was added at ꢁ78 ꢂC to a solution of
1
3
a product; yield ¼ 28%; H NMR (CDCl3): d ¼ 8.91 (d, JHH
¼
3
8.0 Hz, phenan-CH, 1H), 8.79 (d, JHH ¼ 8.4 Hz, phenan-CH,
3
3
1H), 8.72 (d, JHH ¼ 8.4 Hz, phenan-CH, 1H), 7.89 (d, JHH
¼
3
3
8.4 Hz, 2H), 7.76 (t, JHH ¼ 8.0 Hz, 3H), 7.68 (t, JHH ¼ 7.2 Hz,
3
3
3H), 7.64 (m, 2H), 7.58 (d, JHH ¼ 8.4 Hz, 2H), 7.52 (t, JHH
¼
10350 | RSC Adv., 2017, 7, 10345–10352
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8.1 Hz, 1H), 7.31 (m, 5H), 6.89 (s, 4H), 2.36 (s, Mes-p-CH3, 6H),
2.09 (s, Mes-o-CH3, 12H) ppm; 13C NMR (CDCl3): d ¼ 151.17 (s,
imidazole-NCN), 145.73 (s), 142.48 (s), 141.94 (s), 141.78 (s),
140.96 (s), 138.39 (s), 138.32 (s), 137.65 (s), 137.27 (s), 131.28 (s),
130.66 (s), 129.61 (s), 129.54 (s), 129.46 (s), 129.40 (s), 128.99 (s),
128.77 (s), 128.41 (s), 128.25 (s), 127.41 (s), 127.37 (s), 126.74 (s),
126.47 (s), 125.76 (s), 125.04 (s), 124.26 (s), 123.24 (s), 123.18 (s),
122.90 (s), 121.02 (s), 23.67 (s, Mes-o-CH3), 21.40 (s, Mes-p-
CH3) ppm; anal. calcd for C51H43BN2: C, 88.17; H, 6.24; N, 4.03,
found: C, 88.35; H, 6.51; N, 4.45.
Synthesis of 1-(40-(dimesitylboryl)-[1,10-biphenyl]-3-yl)-2-
phenyl-1H-phenanthro[9,10-d]imidazole (2BP). This compound
was prepared in a manner analogous to the synthesis of 1BP
using 2a (1.1 mmol, 0.52 g) and 3a (1.2 mmol, 0.45 g). Puri-
cation of the crude product by column chromatography (tol-
uene : n-hexane ¼ 10 : 1, v/v) afforded white solid (0.078 g); yield
¼ 9%; 1H NMR (CDCl3): d ¼ 8.92 (d, 3JHH ¼ 8.0 Hz, phenan-CH,
Theoretical calculations
The geometries of the ground state (S0) and rst excited state
(S1) structures of 1Ph–2BP were optimized using the density
functional theory (DFT) method with the B3LYP functional13
and the 6-31G(d)14 basis sets. The structures of the rst excited
state (S1) of 2 were optimized using the conguration interac-
tion single (CIS) with the 3-21G basis sets. Time-dependent
density functional theory (TD-DFT)15 was used with 6-31G(d)
to obtain the electronic transition energies which include some
accounts of electron correlation. To include the solvation effect
of THF, the conductor-like polarizable continuum model
(CPCM) was used in the calculations.9,10 All calculations were
carried out using the GAUSSIAN 09 program.16 The GaussSum
3.0 was used to calculate the percent contribution of a fragment
to each molecular orbital in the molecule.17
3
3
Conclusions
¼
1H), 8.80 (d, JHH ¼ 8.4 Hz, phenan-CH, 1H), 8.73 (d, JHH
8.4 Hz, phenan-CH, 1H), 7.91 (d, 3JHH ¼ 8.0 Hz, 1H), 7.81 (t, 3JHH
¼ 1.8 Hz, 1H), 7.77 (t, 3JHH ¼ 1.8 Hz, 1H), 7.67 (m, 4H), 7.60 (d,
3JHH ¼ 8.4 Hz, 2H), 7.53 (m, 4H), 7.33 (m, 5H), 6.85 (s, 4H), 2.33
(s, Mes-p-CH3, 6H), 2.04 (s, Mes-o-CH3, 12H) ppm; 13C NMR
(CDCl3): d ¼ 151.13 (s, imidazole-NCN), 145.62 (s), 142.97 (s),
142.48 (s), 141.70 (s), 140.90 (s), 139.34 (s), 138.92 (s), 137.66 (s),
137.23 (s), 130.66 (s), 130.55 (s), 129.64 (s), 129.43 (s), 129.02 (s),
128.56 (s), 128.40 (s), 128.35 (s), 128.21 (s), 128.11 (s), 127.79 (s),
127.42 (s), 127.38 (s), 126.74 (s), 126.46 (s), 125.76 (s), 125.03 (s),
124.29 (s), 123.24 (s), 123.16 (s), 122.92 (s), 121.03 (s), 23.64 (s,
Mes-o-CH3), 21.36 (s, Mes-p-CH3) ppm; anal. calcd for
We have synthesized and characterized
a
series of
phenanthroimidazole-based N-phenyl (1Ph and 2Ph) and
biphenyl (1BP and 2BP) bridged arylborane compounds using
lithiation and Suzuki–Miyaura coupling reactions, respectively.
The PL spectra of these compounds exhibit a clear dual emis-
sion pattern, which can be separated into high- (lem ¼ ca.
380 nm in THF) and low-energy (lem ¼ ca. 480 nm) emission
regions. Photophysical data and the results of theoretical
calculations indicate that the high-energy emission originates
solely from the p–p* transition of phenanthroimidazole unit
and the low-energy emission can be assigned to the ICT tran-
sition between phenanthroimidazole and triarylborane moiety.
The prepared phenanthroimidazole-N-arylborane dyads show
ratiometrically increased uorescence response upon binding
of uoride to the borane moiety, resulting in a ‘turn-on’ che-
mosensor for the detection of uoride anions. These ‘turn-on’
properties arise from the reinforcement of p–p* transition of
phenanthroimidazole moiety as a result of restricted ICT tran-
sition upon uoride binding to the boron centre.
C
51H43BN2: C, 88.17; H, 6.24; N, 4.03, found: C, 87.72; H, 6.44;
N, 4.13.
X-ray crystallography
Single crystals of 1Ph and 1BP were coated with Paratone oil and
mounted onto a glass capillary. The crystallographic measure-
ment was performed on a Bruker SMART Apex II CCD area
detector diffractometer with a graphite-monochromated Mo-Ka
˚
radiation (l ¼ 0.71073 A). The structure was solved by direct
methods and all non-hydrogen atoms were subjected to aniso-
tropic renement by full-matrix least-squares on F2 by using the
SHELXTL/PC package, resulting in X-ray crystallographic data
of 1Ph and 1BP in CIF format (CCDC 1521689 and 1521688†).
Hydrogen atoms were placed at their geometrically calculated
positions and rened riding on the corresponding carbon
atoms with isotropic thermal parameters. The detailed crystal-
lographic data and selected bond lengths and angles are given
in Tables S1–S2,† respectively.
Acknowledgements
This work was supported by the Nano Material Technology
Development Program (NRF-2016M3A7B4909246 for K. M. Lee)
and Basic Science Research Program (NRF-2016R1C1B1008452
for M. H. Park) through the National Research Foundation of
Korea (NRF) funded by the Ministry of Science, ICT and Future
Planning. K. M. Lee is thankful for the Basic Science Research
Program (NRF-2015R1D1A1A01057396) through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Education. This study was further supported by the 2016
Research Grant from Kangwon National University (No.
520160510).
UV-vis absorption and photoluminescence (PL)
measurements
UV-vis absorption and PL measurements were performed with
a 1 cm quartz cuvette at ambient temperature. Solution PL
experiments were performed in degassed solution for all
compounds. The solution quantum efficiencies were
measured with reference to that of quinine sulfate (0.5 M
H2SO4, F ¼ 0.55).12
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