Fluorescent aggregates of hetero-oligophenylene derivative as no quenching probe for detection of picric acid at femtogram level

Article abstract of DOI:10.1039/c3tc32516d

Fluorescent aggregates of hetero-oligophenylene derivative 3 serve as a no quenching probe for the detection of picric acid (PA) in aqueous media. Photo-induced intermolecular excited-state proton transfer from PA to pyridyl nitrogen results in the formation of protonated species, which exhibit emission at a different wavelength; hence, there is no quenching detection of PA in aqueous media.

Full text of DOI:10.1039/c3tc32516d

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Materials Chemistry C
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PAPER
Fluorescent aggregates of hetero-oligophenylene
derivative as “no quenching” probe for detection of
picric acid at femtogram level†
Cite this: J. Mater. Chem. C, 2014, 2,
3936
*
*
Sharanjeet Kaur, Vandana Bhalla, Varun Vij and Manoj Kumar
Received 19th December 2013
Accepted 19th March 2014
Fluorescent aggregates of hetero-oligophenylene derivative 3 serve as a “no quenching” probe for the
detection of picric acid (PA) in aqueous media. Photo-induced intermolecular excited-state proton
transfer from PA to pyridyl nitrogen results in the formation of protonated species, which exhibit
emission at a different wavelength; hence, there is “no quenching” detection of PA in aqueous media.
DOI: 10.1039/c3tc32516d
www.rsc.org/MaterialsC
interest.26 Keeping this in view, we designed and synthesized
hetero-oligophenylene derivative 3 having pyridine groups for
Introduction
the sensitive detection of PA. Pyridine exhibits Lewis basicity
and is a good proton acceptor. On the other hand, electron-
decient PA has an acidic character. We envisaged that forma-
tion of protonated species due to intermolecular transfer of
protons from PA to the pyridine group of derivative 3 could
inuence the photophysical properties of the system. Interest-
ingly, derivative 3 forms uorescent aggregates in aqueous
media and exhibits reversible photo-induced intermolecular
excited-state proton transfer from PA to pyridine nitrogen of
aggregates of derivative 3 and subsequent formation of
protonated species exhibited emission at different wavelengths;
hence, a “no quenching” response towards PA was observed in
aqueous media. The uorescent aggregates of derivative 3 serve
as a better probe for detection of PA in comparison with the
previously reported chemosensors for PA.27 To the best of our
knowledge, this is the rst report where uorescent aggregates
of hetero-oligophenylene derivative 3 serve as “no quenching”
chemosensors for PA based on a photo-induced, intermolecular
excited-state proton transfer mechanism with a detection limit
at the femtogram level.
Among various nitroaromatics, picric acid (PA) is a health
hazard as it can cause skin/eye irritation and can affect the
organs involved in the respiratory system.1,2 PA is a strong
organic acid,3 and its vapors are hazardous and cause headache,
weakness, anemia and liver injury.4 Furthermore, with the its
electron-decient character, the degradation of PA is more
difficult in the biosystem, which is probably responsible for
many chronic diseases such as sycosis and cancer.5 PA is also
used in dye manufactures, pharmaceuticals, and chemical
laboratories.6,7 Like many polynitrated aromatic compounds, it
is a powerful explosive and its explosive nature is equivalent to
105% of trinitrotoluene (TNT).8,9 Further, because of its high
solubility in water, it can easily contaminate soil and ground-
water when exposed. As a result, there have been considerable
efforts for development of cost-efficient, selective,10,11 sensitive,
fast and portable detection methods for PA in aqueous
media.12–15 Thus, various uorescent materials have been
developed that serve as turn-off sensors for PA.16–22 However,
uorescent sensors showing no quenching response are
attractive due to less interference from uctuation of back-
ground uorescence,23 but uorescent materials showing no
quenching response towards nitroaromatics are still in their
infancy.24,25 Motivated by better sensitivity and reliability of “no
quenching” sensors, we were interested in the development of
uorescent assemblies showing a “no quenching” response
towards PA.
Results and discussion
The target compound 3 was synthesized via Diels–Alder reaction
of 3-(2-(pyridine-3-yl)ethynyl)pyridine 1a (ref. 28) with 2,5-
diphenyl-3,4-di(pyridin-2-yl)cyclopenta-2,4-dienone 2a (ref. 29)
in diphenylether at 240 ꢀC in 70% yield (Scheme 1). The 1H NMR
spectrum of compound 3 showed ve doublets at 8.16, 8.12,
7.24, 7.17 and 7.02 ppm, one singlet at 8.08 ppm, one multiplet
at 6.83–6.91 ppm and one triplet at 6.8 ppm corresponding to
aromatic protons. A parent ion peak for (M + H+) was observed
at m/z 539.2302 in ESI mass spectrum (pS15 to S17 in ESI†).
These spectroscopic data corroborate the structure 3 for this
compound. To get insight into the role played by 3-pyridyl/2-
pyridyl groups in the “no quenching” response, we also
Recently, stimuli-responsive smart materials with tunable
photophysical properties have attracted considerable research
Department of Chemistry, UGC-Centre for Advanced Studies-1, Guru Nanak Dev
University, Amritsar, Punjab, India. E-mail: vanmanan@yahoo.co.in; mksharmaa@
yahoo.co.in
† Electronic supplementary information (ESI) available: Synthesis of compound 3
and 5, characterization data, UV-vis and uorescence studies. See DOI:
10.1039/c3tc32516d
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Scheme 1 Synthesis of compounds 3–5.
Fig. 1 (A) UV-vis absorption spectra of 3 (50 mM) in EtOH and H2O–
EtOH (6 : 4) buffered with HEPES, pH ¼ 7.0. (B) (i) Scanning electron
microscopy (SEM) image of aggregates of compound 3 (10ꢁ4 M) in
H2O–EtOH (6 : 4) buffered with HEPES, pH ¼ 7.0. (ii) SEM image of
derivative 3 (10ꢁ4 M) in presence of 7 equiv. of PA in H2O–EtOH (6 : 4)
buffered with HEPES pH ¼ 7. (C) Fluorescence emission spectra of 3
(50 mM) with the addition of picric acid (125 mM) in H2O–EtOH (6 : 4)
buffered with HEPES, pH ¼ 7.0; lex ¼ 273 nm. (D) Fluorescence
emission spectra of derivative 3 (50 mM) up to the addition of 350 mM of
PA in H2O–EtOH (6 : 4) buffered with HEPES pH ¼ 7; lex ¼ 273 nm.
Inset shows the emission band at 339 nm completely quenched with
the addition of 200 mM of PA.
synthesized hetero-oligophenylene derivatives 4 and 5 (ref. 28)
as model compounds incorporating 2-pyridyl and 3-pyridyl
groups, respectively. The derivative 4 was synthesized by Diels–
Alder reaction of 1,2-diphenylethyne 1b (ref. 30) with 2,5-
diphenyl-3,4-di(pyridin-2-yl)cyclopenta-2,4-dienone 2a (ref. 29)
in diphenylether at 240 ꢀC in 65% yield (Scheme 1). The 1H NMR
spectrum of compound 4 showed two doublets at 8.13 and 6.98
ppm, one triplet at 7.19 ppm and one multiplet at 6.73–6.86
ppm corresponding to aromatic protons. A parent ion peak for
(M + H+) was observed at m/z 537.2332 in ESI mass spectrum
Derivative 3 in H2O–EtOH (6 : 4) exhibits uorescence
(pS18 to S20 in ESI†). The derivative 5 was prepared by the emission at 339 nm (FF ¼ 0.35)35 when excited at 273 nm
reported method.28
(Fig. 1C). The strong emission of aggregates of derivative 3
The UV-vis spectrum of compound 3 in EtOH exhibits an prompted us to explore their potential application as chemo-
absorption band at 213 nm with two shoulders at 245 nm and sensor for the detection of nitro derivatives such as trinitrotol-
271 nm (Fig. 1A). On addition of water (60% volume fractions) uene (TNT), trinitrobenzene (TNB), picric acid (PA), etc. Thus,
to the EtOH solution of derivative 3, the absorption band at we carried out the uorescence titrations of aggregates of
213 nm is slightly red shied to 220 nm and a well-dened compound 3 in H2O–EtOH (6 : 4) toward various nitro deriva-
absorption band appeared at 271 nm. The red shiing of the tives such as picric acid (PA), 2,4,6-trinitrololuene (TNT), 2,4-
absorption band indicates aggregation of derivative 3 to dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB), 1,4-benzoqui-
produce J-type assemblies.31,32 Since it is known that deag- none (BQ), nitromethane (NM), and 2-nitrotoluene (NT). Upon
gregation takes place on increasing the temperature,33 we addition of PA up to 2.5 eq. to the solution of derivative 3 in the
carried out temperature-dependent UV-vis studies of derivative H2O–EtOH (6 : 4) mixture, the intensity of emission band at 339
3 in H2O–EtOH (6 : 4).ꢀIt was observed that, with an increase in nm decreased gradually and a new band appeared at 446 nm
temperature up to 70 C, the absorption band at 220 nm blue (Fig. 1C). On addition of PA up to 4 eq., the intensity of emission
shied to 213 nm (Fig. S1 in ESI†). This revival of an absorption band at 339 nm quenched completely (inset, Fig. 1D) and the
band at 213 nm clearly indicates the deaggregation of J-type intensity of emission band at 446 nm increased (Fig. 1D). On
assemblies. To further prove the formation of J-assemblies, we further addition of PA up to 7 eq., the intensity of emission
1
carried out concentration-dependent H NMR studies of deriv- band at 446 nm increased along with a red shi of 14 nm to give
ative 3 in CDCI3 (Fig. S2 in ESI†). These studies show the upeld nal emission at 460 nm (Fig. 1D). The formation of a new band
shiing of signals corresponding to aromatic protons. Such an at 460 nm suggests the formation of protonated species due to
upeld shi is attributed to the intermolecular shielding from excited-state intermolecular proton transfer from PA to pyridyl
the neighboring aromatic molecules, suggesting their tendency nitrogen. The factor responsible for this transfer is the match-
to undergo self-association to form aggregates.34 The scanning ing of the acidity of the phenolic group of PA with the basicity of
electron microscopy (SEM) image of compound 3 in the solvent the pyridine group in the excited state.
mixture of H2O–EtOH (6 : 4) shows the presence of aggregates
On the other hand the absorption studies of derivative 3
[Fig. 1B(i)]. The solution of aggregates is visibly transparent and aggregates in the presence of PA do not suggest formation of
stable at room temperature for several weeks.
protonated species in the ground state (Fig. S3 in ESI†). The
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absorption studies of derivative 3 aggregates in the presence of band at 339 nm is due to energy transfer and very little charge
7 eq. of PA show the appearance of a band at 356 nm corre- transfer interaction between aggregates of derivative 3 and PA.
sponding to PA (Fig. S4 in ESI†) and an increase in absorption To conrm the excited-state intermolecular proton transfer, we
intensity of the band at 218 nm, whereas slight change in the carried out uorescence studies of aggregates of derivative 3
intensity of absorption band at 273 nm is observed (Fig. S3 in with triuoroacetic acid (TFA), which is strong non-aromatic
ESI†). However, the uorescence spectrum of the same solution acid, under the same set of conditions as was used for PA. The
shows the formation of a new band at 460 nm (Fig. S5 in ESI†). emission spectra show the appearance of an emission band at
Thus, the absorption studies rule out the charge transfer 439 nm upon addition of 1.5 mL of TFA (10ꢁ1 M), which
interactions between aggregates of derivative 3 and PA in the conrms the protonation of pyridyl nitrogen of derivative 3
ground state. On the basis of uorescence studies of aggregates aggregates in the excited state (Fig. S11 in ESI†). Interestingly,
of 3 in the presence of PA, we propose that the sensing mech- no signicant quenching of the emission band at 339 nm was
anism involves two steps (Fig. 2). In the rst step, interaction observed. Thus the nature of interaction between aggregates of
between the aggregates of 3 and PA takes place, which is derivative 3 and PA is considerably different from that for other
responsible for quenching of the emission band at 339 nm acidic molecules. Furthermore, the emission signal corre-
(Fig. 1C and inset Fig. 1D). In the next step, we believe that upon sponding to protonated species is more red shied in the
light absorption, the basicity of pyridyl nitrogen is enhanced,36 presence of PA (Dl ¼ 121 nm) (Fig. 1C) as compared with that in
which facilitates proton transfer from PA to pyridyl nitrogen in the presence of TFA (Dl ¼ 100 nm) (Fig. S11 in ESI†). We believe
the excited state. This excited-state intermolecular proton that in comparison with electron-decient and acidic PA, TFA
transfer from PA to pyridine groups leads to the formation of has only an acidic character, and thus a different sensing
protonated species, which is responsible for appearance of the response is observed. We recorded the absorption spectra of
new band at 460 nm (Fig. 1D). Further, we determined the pKa aggregates of derivative 3 in H2O–EtOH (6 : 4) in the presence of
of picric acid in H2O–EtOH (6 : 4) solution buffered with HEPES TFA (Fig. S12 in ESI†). No new band corresponding to the
at pH ¼ 7 using the UV-vis spectroscopic method,37,38 and it protonated species was observed, which also conrms that
comes out to be 6.45 (Fig. S6 and PS6 in ESI†). This result shows protonation of aggregates is an excited-state phenomenon
that the presence of a signicant portion of picric acid in its (Fig. S12 in ESI†). Further, we also carried out uorescence
conjugate acid form supports the proposed mechanism. To titration of derivative 3 aer protecting the hydroxyl group of PA
further support our theory of the mechanism, we carried out with the methoxy group (Fig. S13 in ESI†) and, interestingly, no
absorption and emission studies of derivative 3 in acetonitrile new band appeared. However, there is only quenching of the
with PA (the pKa of PA in acetonitrile is 11),39 and similar emission band at 339 nm. These studies clearly indicate that the
absorption and emission results were obtained that suggest the formation of an emission band at 460 nm is due to excited-state
formation of protonated species due to excited-state intermo- intermolecular proton transfer from PA to pyridyl nitrogen. To
lecular proton transfer from PA to pyridyl nitrogen (Fig. S8 in conrm that the protonation of aggregates of derivative 3 is not
ESI†). Further, we recorded the 1H NMR spectrum of derivative a ground-state phenomenon, we studied the effect of triethyl-
3 in a solvent mixture of D2O–CD3OD (6 : 4) in the presence of amine on emission spectra of aggregates of derivative 3 in the
PA, which shows an average downeld shi of 0.05 ppm in presence of PA. The addition of 8 equiv. of triethylamine
signals corresponding to aromatic protons (Fig. S9 in ESI†). (10ꢁ2 M) results in gradual weakening of the band at 460 nm;
This very low downeld shi in the signal corresponding to however, the emission band at 339 nm was not revived (Fig. S14
aromatic protons indicates the very little charge transfer inter- in ESI†). In contrast, no signicant change was observed in the
action between derivative 3 and PA. However, the considerable absorption spectra of aggregates of derivative 3 in the presence
spectral overlap between the emission spectrum of aggregates of PA with the addition of triethylamine (Fig. S15 in ESI†). These
of derivative 3 and the absorption spectrum of PA indicates the results further conrm that the protonation of aggregates of
possibility of energy transfer between aggregates and molecules derivative 3 is an excited-state phenomenon and the protonated
of PA (Fig. S10 in ESI†). Thus the quenching of the emission species cannot be observed in the ground state. Further the SEM
Fig. 2 Mechanism of the recognition behavior of aggregates of 3 towards picric acid in H2O–EtOH (6 : 4).
3938 | J. Mater. Chem. C, 2014, 2, 3936–3941
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image of derivative 3 in H2O–EtOH (6 : 4) in the presence of 7
equiv. of PA shows layered akes, thus indicating modulation of
a self-assembled structure of derivative 3 on addition of PA
[Fig. 1B(ii)].
Under the same set of conditions as was used for the
detection of PA, we also carried out uorescence studies of
aggregates of derivative 3 in presence of NT (40 equiv.), DNT
(62 equiv.), TNT (65 equiv.), BQ (120 equiv.) and NM
(135 equiv.), DNB (82 equiv.), but no new band corresponding to
the protonated species was observed (Fig. S16 in ESI†). Further,
we also carried out uorescence studies of aggregates of deriv-
ative 3 with phenolic derivatives (catechol, Br-phenol, I-phenol,
2,4-dinitrophenol (DN), 4-nitrophenol (NP) and phenol) but no
new band appeared at 460 nm upon titrations with phenolic
derivatives; however, emission bands at 302 and 325 nm are the
characteristic emission bands of phenol and catechol itself
(Fig. S17 in ESI†). Thus, derivative 3 is selective towards PA and
Fig. 4 (A) Fluorescence emission spectra of derivative 4 (50 mM) with
the addition of picric acid (500 mM) in H2O–EtOH (3 : 7) buffered with
HEPES pH ¼ 7; lexc ¼ 273 nm. (B) Fluorescence emission spectra of
derivative 5 (5 mM) with the addition of picric acid in H2O–EtOH (6 : 4)
buffered with HEPES pH ¼ 7; lexc ¼ 290 nm.
To study the role of 3-pyridyl/2-pyridyl groups in a “no
the “no quenching” response is observed only where PA exists quenching” response, we recorded the uorescence spectra of
among the various nitro and phenol derivatives tested (Fig. S18 derivatives 4 and 5 in the presence of PA. Both derivatives have
in ESI†). The uorescence lifetime studies of derivative 3 rotors and form aggregates in aqueous media. Interestingly, a
aggregates monitored at 339 nm show shorter decay time solution of derivative 4 having 2-pyridyl groups in H2O–EtOH
(0.606 ns); however, at 460 nm in the presence of PA (6 equiv.), a (3 : 7) mixture exhibits a “no quenching” response towards PA
long-lived component (0.840 ns) is observed (Fig. S19 in ESI†). (Fig. 4A), whereas under the same set of conditions derivative 5
This result suggests the formation of a new uorescent species having 3-pyridyl groups exhibits quenching of emission in the
due to interaction between derivative 3 and PA. The detection presence of PA (Fig. 4B). No new band was observed in the case
limit in this case was found to be 26 nM (Fig. S20 in ESI†). We of derivative 5 having 3-pyridyl groups with the addition of PA,
also carried out pH studies and found that emission spectra of a which suggests that the site of protonation is only in the
derivative 3 solution in buffer–EtOH (6 : 4) mixture using a 2-pyridyl groups. We also carried out uorescence studies of
universal buffer is independent of pH in the range of 6 to 14 derivative 5 in the presence of TFA; however, no signicant
(Fig. 3A). Further, the effect of PA on derivative 3 at different pH change in the emission spectra was observed (Fig. S22 in ESI†).
values was also studied. At pH ¼ 2 (acidic), the uorescence The SEM image of derivative 5 shows the presence of spherical
spectra initially shows the emission band at 439 nm, and with aggregates (Fig. S23 in ESI†). It is expected that 3-pyridyl
the addition of 16 equiv. (0.8 mM) PA, the emission band at 439 nitrogens will be present at the periphery and may be hydrogen
nm shied to a longer wavelength and nally reached emission bonded to the solvent molecules. We believe that the 3-pyridyl
at 460 nm, which corresponds to the protonated species group has relatively weaker basicity that is not enough to enable
(Fig. 3B). However, at pH ¼ 12 (basic), the effect of PA on the excited-state proton transfer. These results suggest the
derivative 3 is same as that at neutral pH ¼ 7. At acidic pH ¼ 2, importance of 2-pyridyl groups for “no quenching” detection of
more PA equiv. are required for protonation because of already- PA in aqueous media.
protonated pyridyl groups, which slows down the transfer of
protons from PA to pyridyl nitrogen (Fig. S21 in ESI†).
Further, PA can contaminate the human body, clothing and
other materials in the surroundings during the manufacture of
rocket fuel and reworks. Detection of these traces is a major
concern in the eld of analytical and forensic sciences. In this
context, we prepared TLC strips by dip-coating a solution of
aggregates of 3 onto TLC strips, followed by drying them in a
vacuum in order to check for residual contamination in contact
mode. We prepared several samples of solution-coated TLC
strips and studied the response of their uorescence towards
picric acid in contact mode and solution phase. PA crystals were
placed over coated TLC strips for 5 s to test the contact mode
response of derivative 3 nanoakes towards PA. Upon illumi-
nation with a UV lamp, blue spots were observed in the contact
area [Fig. 5(i)(a) and (b)]. We also checked the effect of various
concentrations of PA, DNT and TNT solutions on the uores-
cent TLC strips by applying small spots of different concentra-
tions of analytes to the TLC strips [Fig. 5(ii)(a)–(f)]. The
minimum amount of PA detectable by the naked eye was as low
as 10 mL of 1 ꢂ 10ꢁ12 M solution, thereby registering a detection
Fig. 3 (A) Fluorescence emission spectra of derivative 3 (50 mM) at
different pH in universal buffer–EtOH (6 : 4); lex ¼ 273 nm. (B) Fluo-
rescence emission spectra of derivative 3 (50 mM) at pH ¼ 2 with the
addition of 16 equiv. (0.8 mM) of PA in H2O–EtOH (6 : 4); lexc
273 nm.
¼
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As and Ar are the absorbance of the sample and the reference,
respectively; Ds and Dr are the areas of emission for the sample
and reference; Ls and Lr are the lengths of the absorption cells;
and Ns and Nr are the refractive indices of the sample and
reference solutions (pure solvents were assumed).
2
1 ꢁ 10ꢁArLr Ns
Nr
Ds
Dr
ffs ¼ ffr ꢂ
ꢂ
ꢂ
(1)
2
1 ꢁ 10ꢁAsLs
Experimental details of nding the detection limit
Fig. 5 Photographs of derivative-3-coated TLC strips under different
experimental conditions. (i) (a) PA crystals on top of derivative-3-
coated TLC strips; (b) removal of PA crystal after 5 s. (ii) Photographs of
fluorescence quenching of derivative-3-coated TLC strips by PA, DNT
and TNT in contact mode (10 mL of analyte with a contact area of 0.05
cm2) when viewed under 365 nm UV illumination.
To determine the detection limit, uorescence titration of
compound 3 with picric acid was carried out by adding aliquots
of a picric acid solution of micromolar concentration and
plotting the uorescence intensity as a function of picric acid.
From this graph, the concentration at which there was a sharp
change in the uorescence intensity multiplied by the concen-
tration of receptor gave the detection limit.
limit at the 2.29 femtogram level, whereas these TLC strips were
not found to be sensitive towards DNT and TNT [Fig. 5(ii)(a)–(f)].
However, no visible quenching was observed by applying blank
solvent (ethanol) on uorescent paper strips [Fig. 5(ii)(a)].
Preparation of TLC strips
TLC strips (5 cm ꢂ 2 cm) were prepared by coating with deriv-
ative 3 (50 ꢂ 10ꢁ6 M), followed by removal of solvent in a
vacuum at room temperature. The ensemble-coated lter
papers were then cut into 8 pieces (1 cm ꢂ 1 cm) to get the test
strips and used for the detection of explosives.
Conclusion
We designed and synthesized hetero-oligophenylene derivative
3, which forms uorescent aggregates in aqueous media; and
these aggregates work as an efficient and selective “no
quenching” uorescent sensor for the detection of picric acid in
solution and solid state. In addition, we prepared uorescent
TLC strips carrying derivative 3 aggregates that can detect the
PA up to femtogram level.
Contact mode visual detection of PA
Aqueous samples were prepared by dissolving PA in EtOH. The
explosive solutions were spotted onto the TLC strips at the desired
concentration using a micropipette. A solvent blank was spotted
near the spot of each explosive. In order to ensure consistent
analysis, all depositions were prepared from a 10 mL volume,
thereby producing a spot ꢃ0.5 cm in diameter. Aer solvent
evaporation, the lter paper was illuminated with 365 nm UV
light. The blue-coloured spots (under UV light) were identied by
an independent observer, and each set of experiments was
repeated three times for consistency. The detection limits were
calculated from the lowest concentration of the explosive that
enabled an independent observer to detect the quenching visually.
General experimental methods
All the uorescence spectra were recorded on SHIMADZU 5301
PC spectrouorometer. UV spectra were recorded on Shimadzu
UV-2450PC spectrophotometer with a quartz cuvette (path
length: 1 cm). The cell holder was thermostated at 25 ꢀC.
Elemental analysis was done using a Flash EA 1112 CHNS/O
analyzer of Thermo Electron Corporation. 1H and 13C NMR
spectra were recorded on a JEOL-FT NMR-AL 300 MHz spec-
trophotometer and Bruker (Avance II) FT-NMR 400 MHz spec-
trophotometer using CDCl3 and DMSO-d6 as solvent and
tetramethylsilane (Si(CH3)4) for internal standards. Data are
reported as follows: chemical shis in parts per million (d),
multiplicity (s ¼ singlet, br ¼ broad signal, d ¼ doublet, m ¼
multiplet), coupling constants (Hz), integration, and interpre-
tation. All spectrophotometric titration curves were tted with
SPECFIT 32 soware. All spectral characterizations were carried
out in HPLC-grade solvents at 20 ꢀC within a 10 mm quartz cell.
Experimental details
The compounds 1a,28 2a,29 1b,30 and 5 (ref. 28) were synthesized
according to literature procedure.
Synthesis of compound 3
A solution of 3-(2-(pyridin-3-yl)ethynyl)pyridine 1a (ref. 28) (1.4
mmol) and 2,5-diphenyl-3,4-di(pyridin-2-yl) cyclopenta-2,4-
dienone 2a (ref. 29) (1.4 mmol) in diphenyl oxide (2 mL) was
reuxed under an atmosphere of nitrogen. Aer 18 h, the
reaction mixture was cooled to room temperature followed by
the slow addition of hexane (5 mL) to the reaction mixture. The
Fluorescence quantum yield
9,10-Diphenylanthracene (ff ¼ 0.95) in ethanol has been used hexane was decanted off and the rest of the crude product was
as standard in the measurement of uorescence quantum yield puried by column chromatography using CHCl3–hexane (8 : 2)
by using eqn (1), in which ffs is the radiative quantum yield of as eluent to give a beige solid (70%); 1H NMR (400 MHz, CDCl3):
the sample; ffr is the radiative quantum yield of the reference; d ¼ 8.16 [d, J ¼ 4 Hz, 2H, ArH], 8.12 [d, J ¼ 4 Hz, 2H, ArH], 8.08
3940 | J. Mater. Chem. C, 2014, 2, 3936–3941
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[s, 2H, ArH], 7.24 [d, J ¼ 4 Hz, 2H, ArH], 7.17 [d, J ¼ 4 Hz, 2H, 13 D. Li, J. Liu, R. T. K. Kwok, Z. Liang, B. Z. Tang and J. Yu,
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158.25, 151.55, 151.27, 147.88, 146.98, 140.87, 138.68, 138.31,
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122.26, 120.66; ESI-MS: calculated: 538.2157; found: 539.2302 Chem., 2012, 22, 11574.
(M + 1)+; elemental analysis: calcd for C38H26N4: C 84.73; H 4.87; 16 S. S. Nagarkar, B. Joarder, A. K. Chaudhari, S. Mukherjee and
N 10.40; found: C 84.41%; H 4.72%, N 10.15%.
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Synthesis of compound 4
A solution of 1,2-diphenylethyne 1b (ref. 30) (1.4 mmol) and 2,5-
diphenyl-3,4-di(pyridin-2-yl) cyclopenta-2,4-dienone 2a (ref. 29)
(1.4 mmol) in diphenyl oxide (2 mL) was reuxed under an
atmosphere of nitrogen. Aer 18 h, the reaction mixture was
cooled to room temperature followed by the slow addition of
hexane (5 mL) to the reaction mixture. The hexane was decanted
off and the rest of the crude product was puried by column
chromatography using CHCl3–hexane (8 : 2) as eluent to give a
beige solid (65%); 1H NMR (400 MHz, CDCl3): d ¼ 8.13 [d, J ¼ 4
Hz, 2H, ArH], 7.19 [t, J ¼ 4 Hz, 2H, ArH], 6.98 [d, J ¼ 4 Hz, 2H,
ArH], 6.73–6.86 [m, 22H, ArH]; 13C NMR (100 MHz, CDCl3):
120.29, 125.37, 125.49, 126.63, 126.71, 131.14, 131.27, 134.44,
139.71, 140.12, 141.21, 147.66; ESI-MS: calculated: 536.2252;
found: 537.2332 (M + 1)+; elemental analysis: calcd for
C
40H28N2: C 89.52; H 5.26; N 5.22; found: C 89.31%; H 5.01%, N
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5.11%.
26 M. Irie, T. Fukaminato, T. Sasaki, N. Tamai and T. Kawai,
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We are thankful to DST (ref. no. SR/S1/OC-63/2010 and SR/S1/
OC-69/2012) and CSIR (ref. no. 02(0083)/12/EMR-II) for nancial
support. SK is thankful to the “UGC-BSR” Scheme for providing
a fellowship.
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