Synthesis of pyrazole anchored three-coordinated organoboranes and their application in the detection of picric acid

Article abstract of DOI:10.1039/d1dt00586c

Three-coordinated organoboron fluorophores bearing 3,5-diphenyl pyrazoles have been synthesized. The pyrazole anchored boron fluorophores show selective fluorescence quenching response to trinitrophenol (or) picric acid (PA) and have the ability to discriminate picric acid over other analytes. We investigated nonlinear optical (NLO) properties of these three-coordinated organoboron compounds (in solutions) in the presence and absence of PA. In absence of PA, the two-photon-absorption coefficient (β) of organoboron fluorophores exhibits a variation from 2 × 10-12 cm W-1 to 4 × 10-12 cm W-1. The results also reveal that the NLO characteristics of organoboron fluorophores exhibit a discernible variation with PA addition which has correlations with quenching observed in fluorescence measurements.

Full text of DOI:10.1039/d1dt00586c

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Synthesis of pyrazole anchored three-coordinated
organoboranes and their application in the
detection of picric acid†
Cite this: Dalton Trans., 2021, 50,
6204
Shreenibasa Sa,‡a Vanga Mukundam,‡a Anupa Kumari,b Ritwick Das *b and
a
Krishnan Venkatasubbaiah
*
Three-coordinated organoboron fluorophores bearing 3,5-diphenyl pyrazoles have been synthesized.
The pyrazole anchored boron fluorophores show selective fluorescence quenching response to trinitro-
phenol (or) picric acid (PA) and have the ability to discriminate picric acid over other analytes. We investi-
gated nonlinear optical (NLO) properties of these three-coordinated organoboron compounds (in solu-
tions) in the presence and absence of PA. In absence of PA, the two-photon-absorption coefficient (β) of
organoboron fluorophores exhibits a variation from 2 × 10−12 cm W−1 to 4 × 10−12 cm W−1. The results
also reveal that the NLO characteristics of organoboron fluorophores exhibit a discernible variation with
PA addition which has correlations with quenching observed in fluorescence measurements.
Received 22nd February 2021,
Accepted 9th April 2021
DOI: 10.1039/d1dt00586c
rsc.li/dalton
usage in terrorism related activities. Among the nitro explo-
sives 2,4,6-trinitrophenol (picric acid, PA) shown superior
Introduction
Three-coordinated organoboranes are an important class of explosive capabilities as compared to its counterpart tri-
compounds which possess a vacant p-orbital through which nitrotoluene (TNT).12 Furthermore, picric acid can create
they can establish interaction with an organic π-system and severe health problems such as skin irritation, skin aller-
consequently could serve as a π-acceptor. Furthermore, the gies, nausea and damage to respiratory organs.13 Hence,
empty p-orbital of boron extends the π-conjugation in organic selective, reliable and convenient detection of picric acid is
systems, which leads to unique absorption and emission pro- in high demand. Even though various fluorescent sensors,
perties. These unusual properties which are facilitated by such as polymers, nanoparticles, mesoporous and metal
interactions with boron compounds have been exploited in organic frameworks have been reported for the detection of
many opto-electronic applications.1–4 For instance, Marder, nitro-aromatic explosives, most of these investigations were
Lambert, Fang, Mullen, Jäkle and Perry have carried out exten- directed towards the detection of TNT and discernibly
sive studies of such chromophores in the context of non-linear weaker importance was given to the more powerful explosive
optics.5–9 Shirota and others have demonstrated the use of tri- picric acid.14 Although ample reports exist for the detection
coordinated boranes for realizing electron transport as well as of picric acid, owing to similar electron affinities of poly
efficient emitters.10 Wang, Yamaguchi, Tamao and others have nitro-aromatics, the augmentation of an effective and
shown that tri-coordinated boron compounds could be reliable sensor for picric acid with high selectivity is still a
employed for realizing effective fluorescent and colorimetric challenging task. We propose a very efficient and selective
sensors for the detection of fluoride anion.11
sensor based on tri-coordinated boron. To the best of our
The recognition and sensing of nitro-explosives has knowledge this is the first example of the use of tri-co-
attracted substantial attention owing to ever-increasing ordinated borane based fluorophores for the selective detec-
tion of picric acid.
Tri-coordinated boron has been chosen as the fluorophore
aSchool of Chemical Sciences, National Institute of Science Education and Research
(NISER), HBNI, Bhubaneswar-752050, Odisha, India. E-mail: krishv@niser.ac.in
bSchool of Physical Sciences, National Institute of Science Education and Research
(NISER), HBNI, Bhubaneswar-752050, Odisha, India
unit because of its widely studied properties as discussed vide
supra. A pyrazole unit at the para position of the phenyl ring or
5-position of thiophene was placed to act as an H-bond accept-
ing site. We anticipate that the lone pair on the pyrazole nitro-
gen will help in forming H-bonding with picric acid through
N⋯H–O interaction and consequently it would help to detect
the picric acid selectively.
†Electronic supplementary information (ESI) available. CCDC 2063185 and
2063186. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d1dt00586c
‡These authors contributed equally.
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Results and discussion
Synthesis & characterisation
The starting material 1-(4-bromophenyl)-3,5-diphenyl-1H-pyr-
azole (1a) was prepared by following the literature reported
procedure15 and compound 1b was synthesized by Cu2O cata-
lyzed Ullmann-type coupling reaction of 3,5-diphenyl pyra-
zole and 2-bromothiophene followed by bromination using
N-bromosuccinimide (ESI†). Compound 1a or 1b was reacted
with n-butyllithium, then quenched with dimesitylboron flu-
oride, which yields the desired triaryl borane-pyrazole com-
pounds 2a and 2b in 73 and 55% respectively (Scheme 1). The
formation of compounds 2a and 2b were confirmed using 1H,
13C and 11B NMR spectroscopy. Compounds 2a and 2b were
further characterized using single crystal X-ray diffraction
crystallography (Fig. 1 & Table S2†). Compounds 2a and 2b
crystallizes in the monoclinic P21/n and P21/c space groups
respectively. The boron centre adopts a trigonal planar geo-
metry (SC–B–C = 359.9° for 2a & 360.0° for 2b) as shown in
Fig. 1. The boron–carbon bond distances (1.572(2)–1.583(2) Å
for 2a and 1.538(3)–1.579(3) Å for 2b) are comparable
to the literature reported analogues tri-coordinated boron
compounds.
Scheme 1 Synthetic route to the compounds 2a & 2b.
Photophysical properties
The absorption spectra of compound 2a shows a weak solvato-
chromism, however the emission spectra exhibit strong sol-
vatochromism (Fig. S1†). The emission maxima shifted from
λem = 370 nm (toluene) to λem = 403 nm (CH3CN), and the
Stokes shift increases from 2913 cm−1 to 5673 cm−1 with
increasing solvent polarity which indicates the presence of
different charge distribution in the excited state in polar sol-
vents compared to the ground state. The emission spectra of
Fig. 1 (left) Molecular structures of 2a and 2b. Hydrogen atoms are omitted for clarity. (right) Computed orbitals for compounds 2a and 2b.
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compound 2b did not show any solvatochromism, however, its pound 2a (E1/2 = −2.27 V). The quantum yield for compounds
absorption spectra showed weak solvatochromism. The 2a & 2b are 0.23 & 0.15 (in THF) respectively (Table S1†) which
absorption and emission maxima of compound 2b (358 nm & are comparable with other reported pyrazole based systems.14k
415 nm in CH3CN) exhibits a slight red shift (Fig. S1 & S2†) in To better understand the photophysical properties of com-
comparison with compound 2a (328 nm & 403 nm in CH3CN) pounds 2a and 2b, theoretical calculations were performed.
which is attributed due to electron donating nature of thio-
As shown in Fig. 1, the HOMO of compounds 2a and 2b are
phene (Fig. 2). Cyclic voltammetry studies showed a reversible dominated by the orbitals from the pyrazole, one of the phenyl
reduction wave for both 2a and 2b (Fig. S3†). Compound 2b ring with small dihedral angle attached to the pyrazole (the di-
(E1/2 = −2.42 V) showed more negative potential than com- hedral angles between the pyrazole and the phenyl rings are
46.0 & 5.0 for 2a; 50.7 & 4.3° for 2b) and the spacer (‘phenyl’ in
case of 2a and ‘thiophene’ in case of 2b); whereas the LUMO
gets in maximum contribution from the boron and the spacer
(phenyl or thiophene) indicates that the pyrazole unit is the
donor and the boron moiety acts as an acceptor. With this
well-defined triarylborane anchored pyrazole in hand, we
examined the application of compound 2a as a probe for picric
acid detection in THF. As shown in Fig. 3, gradual addition of
picric acid to a solution of compound 2a in THF causes sub-
stantial quenching of the emission. Addition of 14 equiv.
(140 μM) solution of picric acid quenches about 89% and 44%
of emission intensity for the compounds 2a and 2b respect-
ively (Fig. 3 and Fig. S4†). It is worth noting that 90% fluo-
rescence quenching was observed for 2a at 14 equiv. of PA and
that for the 2b it was observed at 70 equiv. of PA in THF. As
picric acid is freely soluble in water, determination of PA in
aqueous medium is necessary. Both the compounds 2a and 2b
are not soluble in water; in order to use them in aqueous
environment, mixed solvent system was adopted (THF : H2O;
70 : 30) for further sensing studies. Aliquots of PA in water was
added to compounds 2a and 2b in THF/H2O (70 : 30) as
described vide supra. About ∼20% emission quenching was
realized for 2a & 2b, upon addition of 0.5 equiv. of picric acid
in H2O. The emission further quenched to ∼90% upon further
addition of PA (92% quenching was observed for 2a at 7 equiv.
of PA and 90% quenching was observed for 2b at 4.4 equiv. of
PA) to compounds 2a and 2b respectively (see Fig. 4). We
further studied the fluorescence quenching of compounds 2a
and 2b using Stern–Volmer plot. At lower concentration of
Fig. 2 Normalized UV-Vis absorption and fluorescence spectra of
compounds 2a (top) and 2b (bottom). (Concentration = 10−5 M).
Fig. 3 (a) Fluorescence quenching of compound 2a with the addition of different concentrations of PA (0, 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0,
6.0, 7.0, 8.0, 9.0, 10.0, 12.0, 14.0, 16.0, 18.0, 23.0, and 27.0 equiv. of PA) in THF (10−5 M; excited at 333 nm). (b) Color change under a UV lamp
before (left) and after (right) the addition of PA.
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Fig. 5 (top) Fluorescence quenching efficiencies of compounds 2a and
2b in THF : H2O (70 : 30) (10−5 M; excited at 333 nm) toward of different
nitroaromatics and other analytes (6 equiv.) like picric acid (PA), 1,3-dini-
trobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4-DNB), 2,4-dinitrotoluene
(2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 4-nitrotoluene (4-NT), nitro-
benzene (NB), DDQ, nitromethane (NM), benzoic acid (BA), phenol and
trifluoroacetic acid (TFA). (bottom) Proposed quenching mechanism and
optimized geometries of 2a-PA & 2b-PA (H-bond interaction for 2a is
H⋯N 1.982 Å; O–H⋯N 136.3° and for 2b is H⋯N 2.177 Å; O–H⋯N
124.3°).
Fig. 4 (top) Fluorescence quenching of compound 2a (10−5 M, excited
at 333 nm) in a mixed solvent of THF : H2O (70 : 30) with different con-
centrations of PA (0, 0.25, 0.5, 0.75, 1.0, 1.25, 1.50, 1.75, 2.0, 2.5, 3.0, 3.5,
4.0, 4.5, 5.0, 6.0, 7.0, 10.0, 12.0, and 14.0 equiv. of PA). (bottom)
Fluorescence quenching of compound 2b (10−5 M, excited at 366 nm) in
a mixed solvent of THF : H2O (70 : 30) with different concentrations of
PA (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0,
3.3, 3.6, 4.0,4.4, 5.0, 5.5, 6.0, 7.0 and 8.0 equiv. of PA).
picric acid, the quenching constant was calculated using an interaction between compound 2a or 2b and picric acid. In
Stern–Volmer plot and was found to be 5.2 × 104 M−1 and 0.5 × general, pyrazoles are prone to form hydrogen bonding with
104 M−1 for compounds 2a and 2b in THF (Fig. S5 and S6†) themselves through N–H⋯N intermolecular hydrogen bonding
respectively which confirms the high sensitivity of both the and with other molecules through intramolecular hydrogen
compounds towards picric acid in THF, whereas quenching bonding (N⋯H–X; X may be O or N). We propose that N⋯H–O
constant is 5.4 × 104 M−1 for 2a in THF : H2O (70 : 30) and 6.1 × interaction between compound 2a or 2b and picric acid
104 M−1 for 2b in THF : H2O (70 : 30) (Fig. S7 and S8†). Time- resulted in selective fluorescence quenching (Fig. 5).
resolved fluorescence measurements were carried out to Compounds 2a & 2b showed a much better or comparable
realize the origin of the quenching process. As shown in the detection limits compared to other fluorescent probes reported
ESI,† the fluorescence life time of 2a and 2b is invariant in the in the literature (Tables S3 & S4†).
presence of picric acid, which suggests that the mechanism of
To get deeper insight into the quenching mechanism, we
1
quenching is static (Fig. S9 and S10†). In order to judge the carried out H NMR studies of compound 2a in DMSO-d6. In
selectivity of compounds 2a & 2b, fluorescence response of the the presence of picric acid, a slight shift of all the protons
probes with different nitroaromatics (Fig. 5) and other analytes corresponding to compound 2a was observed, which confirm
such as 1,3-dinitrobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4- that the proposed mechanism involves the electrostatic inter-
DNB), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6- action (N⋯H–O) between compound 2a and picric acid
DNT), 4-nitrotoluene (4-NT), nitrobenzene (NB), DDQ, nitro- (Fig. S11†). Our efforts to get co-crystals of compounds 2a and
methane (NM), benzoic acid (BA), phenol and trifluoroacetic (or) 2b with picric acid were not fruitful. To gain more insights
acid (TFA) were studied. Among the different nitroaromatics about the interactions involved between 2a (or) 2b with PA, we
and analytes tested, picric acid results in 90% quenching of performed DFT calculations. Both, 2a-PA and 2b-PA were opti-
emission of the probes (2a & 2b), however the emission (2a & mized using 6-31(d,p) basis set, and the optimized molecular
2b) quenching by other analytes was negligible. The selective structures are presented in Fig. 5. Both the optimized 2a-PA
emission response towards picric acid indicates that there is and 2b-PA showed weak H-bond interaction between PA and
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compounds 2a & 2b, thus helps to stabilize the complex
The measured normalised z-dependent transmission is rep-
formed between the PA and the compounds 2a & 2b. The resented by black dots. It is to be noted that the laser intensity
HOMO–LUMO energy difference of 2a (3.97 eV) and 2b (3.78 (I) is relatively small and uniform in the regions which are far
eV) are considerably reduced upon forming a complex with from the focal point. This leads to a negligible nonlinear
picric acid (2.77 eV for 2a-PA & 2.55 eV for 2b-PA). The HOMO optical absorption (NLA = βI) in this region and consequently,
of 2a-PA and 2b-PA is localized on 2a or 2b, whereas the LUMO the actual transmission is normalised with respect to this
is localized on picric acid, which suggest that photoinduced value of transmission (far from focal point). Near the focal
electron transfer occurs from 2a (or) 2b to picric acid point, the nonlinear optical effects are more pronounced,
(Table S6†).
which results in significant alternation in transmission.
It is to note that the CA normalized transmission in
Fig. 6(a) and (b) exhibits a familiar valley–peak behaviour
which indicates a self-focusing behaviour in both the com-
pounds 2a and 2b. Alternately, the variation of normalized
transmittance results in a positive value of n2, thereby depict-
ing a strong electrostrictive effect. According to this, both the
compounds exhibit a tendency to have a higher density in
regions where the laser intensity is high and consequently,
such compounds are expected to offer resistance to thermal
lensing effect. In order to estimate the values of n2, the experi-
mental measurements have been fitted with the mathematical
relation given by,18
Optical nonlinearities
Griesbeck et al. showed that three-coordinated boron com-
pounds possess small dipole moment in ground state and
large dipole moment in first excited singlet state. In addition,
they have tendency to show intense intramolecular charge-
transfer transitions when attached with a suitable electron
enrich chromophore which can greatly enhance the probability
of nonlinear absorption.5c In order to investigate the nonlinear
optical behaviour and subsequently, determine the nonlinear
absorption coefficient (β) and nonlinear refractive index (n2),
we used the single beam Z-scan technique.16 The details about
the experimental setup could be found elsewhere.17 In order to
measure n2 and β, we irradiated the solution (concentration:
0.04 mM in dichloromethane) to an intense laser beam from
Yb-doped ultrashort pulse (Δτ = 370 fs) fiber laser (Model:
Cazadero, M/S Calmar Inc., USA) at wavelength of λ = 515 nm
and at 1 kHz repetition rate. The solution was contained in a
1.0 mm thick optical cell which was translated across the focal
point of the focused Gaussian laser beam along the beam
propagation direction. In closed-aperture (CA) Z-scan, the
optical power of transmited beam from the sample is recorded
through a finite-size circular aperture which is placed in the
far field. For an open aperture (OA) Z-scan, the entire laser
beam transmitted through the sample is measured. Fig. 6
shows the normalized transmittance of CA and OA in the
Z-scan experiment for compounds 2a and 2b.
4Δϕ0x
2ðx2 þ 3ÞΔΨ0
Tðz; Δϕ0Þ ¼ 1 ꢀ
ꢀ
ð1Þ
ðx2 þ 9Þðx2 þ 1Þ ðx2 þ 9Þðx2 þ 1Þ
where T represents the normalised transmittance, x = z/z0, z0
being Rayleigh range. Δϕ0 is the on-axis phase-shift. The value
of n2 can be estimated by the relation Δϕ0 = kn2I0Leff, k = 2π/λ
is the wave vector and λ is the pump wavelength, ΔΨ0 = β I0Leff
/
2 is the phase change due to nonlinear absorption. It is worth-
while to point out that NLA has been included in eqn (1) so as
to account for any kind of asymmetric behaviour. Fig. 6(c and
d) shows the normalized transmission for the OA Z-scan
measurement. There is an apparent transmission drop at the
focus (z = 0) for the compounds 2a and 2b. Transmission drop
near the focus is essentially a consequence of two-photon
absorption (TPA) or multi-photon absorption (MPA) which is
characterized by a positive value of β. In absence of other non-
linear optical effects or parasitic effects, the normalized trans-
mission is symmetric with respect to the focus (z = 0), where it
has a minimum transmission.
The absorption coefficient (β) can be estimated from the
OA normalized transmission as16
βI0Leff
Tðz; S ¼ 1Þ ¼ 1 ꢀ
ð2Þ
3
2
2
2 ð1 þ x Þ
where x = z/z0, I0 is the on-axis peak irradiance at focus (z = 0),
Leff is the effective sample length. The results are summarized
in Table 1 for compounds 2a and 2b along with the two-
photon absorption cross-section (TPAC) in GM units (1 GM =
10−50 cm4 s per photon).
Recently, there have been a concerted effort to identify and
ascertain a plausible mechanism behind the high nonlinearity
in PA coordinated crystals. However, such investigations have
rarely been carried out in the context of PA in solutions where
the flexibility in realizing π-conjugations are significantly
Fig. 6 Normalized Z-scan (a and b) CA transmittance. (c and d) OA
transmittance of 2a and 2b respectively in CH2Cl2.
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Table 1 Optical nonlinear coefficients of 2a and 2b at 1 kHz repetition
rates
n2
β
TPAC
(GM)
Compound
(×10−16 cm2 W−1
)
(×10−12 cm W−1
)
2a (w/o picric acid)
2b (w/o picric acid)
Picric acid
2a (with 7% picric acid)
2b (with 14% picric acid) 1.9
2b (with 40% picric acid)
2.3
1.14
—
2.0
4.0
6.59
10.9
4.8
7
13
23
38
17
66
1.73
—
25.1
higher. It has been pointed out by Karthikeyan et al. that NO2
and OH groups in PA are discernibly easy to polarise.
Consequently, PA interact with nearby species through rapid
change delocalization.19 It is worthwhile to note that the pres-
ence of three electron withdrawing NO2 groups in PA makes it
a good π-acceptor for neutral carrier donor molecule. However,
NO2 group present at the para-position exhibits maximum
charge delocalization which gives rise to the possibility of a
longer conjugation length when interacts with electron-donat-
ing species. Thus, detection of PA through enhancing the non-
linear optical interactions could be plausible option. In order
to investigate this aspect, we measured the nonlinear optical
coefficients for pure PA solution which resulted in β = 6.59 ×
10−12 cm W−1. This value is higher than the compounds 2a
and 2b which could be attributed to the presence of phenolic
(OH) group. This favours the formation of hydrogen-bonding
interactions, which leads to increased molecular hyperpolariz-
ability.20 In our work, we observed the fluorescence quenching
on addition of PA to the pyrazole anchored boron fluorophore
which encouraged us to have a further investigation of how
does it affect the nonlinearity of these compounds. Since, PA
is explosive in nature, we have added limited concentrations
(7.0 equiv. and 14.0 equiv. to the compounds 2a and 2b
respectively). This concentration quenches the fluorescence of
each solution by approximately 40%. The normalized CA and
OA transmittance of compounds 2a and 2b after adding PA is
shown below.
Fig. 7 Normalized Z-scan (a and b) CA transmittance. (c and d) OA
transmittance after adding 7.0 equiv. and 14.0 equiv. PA in compounds
2a and 2b respectively. Solvent: CH2Cl2.
the probability of the formation of 2b-PA is appreciably small,
however, at high PA concentration the equilibrium shits to
form 2b-PA via intermolecular hydrogen bond, which, in turn
results in a gradual increase in the effective conjugation
length. For a comparison, we have listed the values of n2 and β
for some similar organic compounds in Table S8.† The com-
parison reveals that 2a and 2b possesses appreciable third-
order nonlinear response which scales up higher with the
addition of picric acid. Table S8† also presents a few measure-
ments which were carried out using continuous-wave (CW)
laser which yields exceptionally high values of NLO coefficients
due to large thermo-optic manifestations. Therefore, NLO
measurements and material stability of compounds 2a and 2b
suggest that they are potential candidates for optical switching
and limiting applications. To probe the effect of triarylborane
linkage with triaryl pyrazole, we synthesized triaryl pyrazole15
and studied its photophysical properties. Although, the
quenching phenomenon was similar to that of 2a, the
observed absolute quantum yield was lower than that of 2a,
which is the advantage of linking triaryl pyrazole with
triarylborane.
It is evident from the Fig. 7(a) and (b) above, that the
addition of PA does not change the behaviour of the CA trans-
mittance and hence, it does change the sign of n2 for 2a as
well as 2b. From Table 1, it could be observed that there is a
marginal change in the value of n2 with the addition of 7.0
equiv. of PA in 2a and 2b which indicates that the optical
phase changes are insignificant. On the other hand, we To
Experimental section
Synthesis of compound 1a
a
solution of 4-bromo phenylhydrazine (5.48 g,
observed a significant change in the value of β in case of 2a. 29.30 mmol) and 1,3-diphenylpropane-1,3-dione (5.00 g,
For 7.0 equiv. PA addition to 2a, we observed more than five- 22.30 mmol) in methanol (80 mL) was added AcOH (80 mL).
fold increase in the value of β. This change can be attributed The reaction mixture was heated to reflux for 24 h. After
to the significant alteration in π-conjugation of the compound cooling to RT, the mixture was poured into water and extracted
due to the association of OH from PA to the lone pair of elec- with CH2Cl2. The combined extracts were washed with satu-
trons at nitrogen in pyrazole. In case of 2b, however, there is rated sodium carbonate solution followed by brine and dried
small change in the value of β after addition of 14.0 equiv. of over anhydrous Na2SO4. Solvent was removed under reduced
PA. Further, β increases significantly with increase in PA con- pressure and the residue was purified by silica gel column
centration. In fact, we observed that β = 25.1 × 10−12 cm W−1 chromatography using ethyl acetate and n-hexane as eluent.
1
when 40.0 equiv. PA is added to 2b. At low PA concentration, Yield: 6.28 g, (75%). Mp: 280 °C. H NMR (400 MHz, CDCl3): δ
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= 7.99 (d, J = 7.5 Hz, 2H), 7.51–7.47 (m, 4H), 7.43–7.28 (m, 8H), CH2Cl2 were added to the mixture and the organic phase was
6.86 (s, 1H). 13C NMR (100 MHz, CDCl3): δ = 152.27, 144.36, collected and the aqueous phase extracted with (2 × 50 mL) of
139.14, 132.83, 131.98, 130.30, 128.76, 128.72, 128.66, 128.55, CH2Cl2. The combined organic phases were dried over Na2SO4,
128.18, 126.54, 125.83, 120.92, 105.70. HR-MS (ESI): calcd for and solvent was removed under reduced pressure. The
C21H15Br1N2 ([M + H]+): 375.0491, found: 375.0464.
obtained residue was purified by silica gel column chromato-
graphy using ethyl acetate and n-hexane as eluent. Yield:
3.96 g, (90%). Mp: 138 °C. 1H NMR (400 MHz, CDCl3): δ = 7.92
Synthesis of compound 2a
Compound 1a (4.41 g, 11.75 mmol) was dissolved in anhy- (d, J = 7.1 Hz, 2H), 7.48–7.36 (m, 8H), 6.82 (d, J = 4.0 Hz, 1H),
drous THF (150 mL) under nitrogen atmosphere and the 6.78 (s, 1H), 6.44 (d, J = 4.0 Hz, 1H).13C NMR (100 MHz,
resulting solution was cooled to −78 °C with stirring. Then, CDCl3): δ = 152.77, 145.54, 142.64, 132.44, 129.77, 129.19,
n-butyllithium (8.1 mL, 12.93 mmol, 1.6 M solution in hexane) 129.07, 128.85, 128.79, 128.57, 128.43, 126.05, 121.29, 109.77,
was slowly added to the stirred solution over 20 min. After 1 h, 105.77. HR-MS (ESI): calcd for
a solution of dimesitylfluoroborane (3.47 g, 12.93 mmol) in H]+):381.0056, found: 381.0052.
35 mL of dry THF was added over 10 min. The reaction
C
19H13N2SBr ([M
+
mixture was allowed to warm to room temperature and stirred
for 24 h. After 24 h, water was added to the reaction mixture
Synthesis of compound 2b
Compound 2b was prepared by following a procedure similar
to that used for compound 2a. The quantities involved are as
follows: Compound 1b (3.00 g, 7.87 mmol), n-butyllithium
(5.41 mL, 8.66 mmol, 1.6 M solution in hexane), dimesityl-
fluoroborane (2.32 g, 8.66 mmol). Yield: 2.38 g, (55%). Mp:
192 °C. 1H NMR (400 MHz, CDCl3): δ = 1H NMR (400 MHz,
CDCl3) δ = 7.91 (d, J = 7.2, 8.0 Hz, 2H), 7.46–7.34 (m, 8H), 7.18
(d, J = 4 Hz, 1H), 6.82 (s, 4H), 6.78 (s, 1H), 6.75 (d, J = 3.9 Hz,
1H), 2.31 (s, 6H), 2.14 (s, 12H). 13C NMR (100 MHz, CDCl3): δ =
153.87, 152.87, 145.33, 140.94, 139.71, 138.71, 132.45, 130.16,
129.31, 129.19, 128.80, 128.70, 128.54, 128.30, 126.12, 121.80,
106.53, 23.58, 21.33. HR-MS (ESI): calcd for C37H35N2SB ([M +
H]+): 551.2693, found: 551.2701.
and extracted with ethyl acetate. The combined extracts were
washed with brine and dried over anhydrous Na2SO4. Solvent
was removed under reduced pressure and the obtained residue
was purified by silica gel column chromatography using ethyl
acetate and n-hexane as eluent. Yield: 4.67 g, (73%). Mp:
192 °C. 1H NMR (400 MHz, CDCl3): δ = 8.02 (d, J = 7.5 Hz, 2H),
7.59 (d, J = 8.1 Hz, 2H), 7.50 (t, J = 7.5 Hz, 2H), 7.45–7.35 (m,
8H), 6.91 (s, 5H), 2.38 (s, 6H), 2.11 (s, 12H). 13C NMR
(100 MHz, CDCl3): δ = 152.43, 144.86, 144.70, 142.79, 141.61,
140.87, 138.91, 137.13, 133.00, 130.66, 128.89, 128.75, 128.53,
128.49, 128.36, 128.21, 125.96, 124.64, 105.75, 23.55, 21.31.
HR-MS (ESI): calcd for C39H37B1N2 ([M + H]+): 545.3129, found:
545.3138.
Synthesis of compound Th-Pz-diPh
To
a
mixture of 3,5-diphenyl-1H-pyrazole, (6.00 g,
Conclusions
27.24 mmol), 2-bromo thiophene (3.91 mL, 40.86 mmol),
Cu2O (389 mg, 2.724 mmol), Cs2CO3 (9.76 g, 29.96 mmol) was
added degassed dry DMF under nitrogen atmosphere. The
reaction mixture was heated at reflux for 48 h. After cooling to
room temperature, 200 mL of H2O and 200 mL of CH2Cl2 were
added, and the phases were separated. The aqueous phase was
extracted with (3 × 7 5 mL) of CH2Cl2 and the combined
organic phases were dried over Na2SO4, and the solvent was
removed under reduced pressure. The obtained residue was
purified by silica gel column chromatography using ethyl
acetate and n-hexane as eluent. Yield: 3.95 g, (48%). Mp:
In summary, 3,5-diphenylpyrazole anchored tri-coordinated
boron compounds have been synthesized. The fluorescence
studies revealed that compounds 2a and 2b are highly selec-
tive and sensitive for the detection of picric acid. The pres-
ence of 3,5-diphenyl pyrazole moiety endowed our design to
have the potential to discriminate picric acid over other ana-
lytes. Subsequently, we measured the NLO properties (namely
n2 and β) for the compounds 2a and 2b which revealed a
direct dependence on addition of concentration of PA to the
solution. The addition of PA essentially provides a plausible
route to enhance the effective conjugation length of the com-
pounds thereby, larger values of nonlinear optical coeffi-
cients. In future, it would be worthwhile to investigate and
corroborate the precise impact of quenching of linear absorp-
tion due to PA addition on the nonlinear absorption.
The results presented here hold a great capability for develop-
ment of new selective chemosensors based on tri-coordinated
boron system.
1
132 °C. H NMR (400 MHz, CDCl3): δ = 7.93 (d, J = 8 Hz, 2H),
7.47–7.35 (m, 8H), 7.14 (d, J = 8 Hz, 1H), 6.87–6.84 (m, 1H),
6.81 (s, 1H), 6.75 (d, J = 4 Hz, 1H). 13C NMR (100 MHz, CDCl3):
δ = 152.45, 145.75, 142.22, 132.70, 130.10, 128.98, 128.88,
128.78, 128.60, 128.37, 126.04, 125.65, 123.05, 121.75, 105.39.
HR-MS (ESI): calcd for C19H14N2S ([M + H]+): 303.0950, found:
303.0945.
Synthesis of compound 1b
Compound Th-Pz-diPh (3.50 g, 11.57 mmol) was dissolved in
80 mL and N-bromosuccinimide (2.26 g, 12.70 mmol) was
added. Then the reaction mixture was stirred for 24 h in dark
Conflicts of interest
condition. To this mixture, 100 mL of H2O and 70 mL of There are no conflicts to declare.
6210 | Dalton Trans., 2021, 50, 6204–6212
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C. Wang, Y. Sato, R. M. Edkins, Z. Zhang, M. Taki,
C. Lambert, S. Yamaguchi and T. B. Marder, Chem. – Eur.
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Acknowledgements
The authors thank Science & Engineering Research Board
(SERB) (EMR/2017/000620), New Delhi and Department of
Atomic Energy (DAE) for financial support.
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