A tunable ratiometric pH sensor based on phenanthro[9,10-d]imidazole covalently linked with vinylpyridine

Article abstract of DOI:10.1039/c5ra09536k

A rational design of the novel phenanthro[9,10-d]imidazole based pH sensing fluorophore PITP was developed, in which phenanthroimidazole acts as a donor and pyridine acts as an acceptor. Various spectral studies confirm that the protonation occurs at two different nitrogen centres, each of which produces an observable color change depending on the pH of the buffer solution. DFT calculations also support the experimental results. We show that PITP can be used as a ratiometric and colorimetric pH sensor in the acidic region.

Full text of DOI:10.1039/c5ra09536k

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A Tunable Ratiometric pH Sensor Based on Phenanthro[9,10-
d]imidazole covalently linked with vinylpyridine
Arockiam Jesin Beneto,a ViruthachalamThiagarajan,b and Ayyanar Siva*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A rational design of novel phenanthro[9,10-d]imidazole based pH In general, Phenanthro[9,10-d]imidazole based molecules act as a
10-12
sensing fluorophore PITP was developed, in which metal ion sensors for Hg2+ and Fe3+
to date only very few
phenanthroimidazole acts as a donor and pyridine acts as an reports are available for pH sensors that detect within a very
acceptor. Various spectral studies confirm that the protonation narrow pH region.13,14 In contrast PITP can act as a sensor molecule
occurs at two different nitrogen centres, each of which produces for the region between pH of 2.0 to 7.5. The presence of extended
an observable color change depending on the pH of the buffer π-conjugation in PITP results in large stoke shift, which in turn
solution. DFT calculations also support the experimental results. becomes an essential requirement to detect the wide range of pH
We have shown that PITP can be used as a ratiometric and of the analyte. Herein, we report the novel synthesis of
a
colorimetric pH sensor in the acidic region.
phenanthro[9,10-d]imidazole based fluorophore PITP containing
extended π-conjugation in the form of 3-hexyl-thiophene having
two protonation sites. We found that it act as an efficient
ratiometric and colorimetric pH sensor between a range of pH. The
sensing ability of PITP was carried out using UV-vis, fluorescence,
cyclic voltammetry studies and DFT calculations further support the
experimental results. The fluorophore PITP was synthesized from 2-
bromo-3-hexylthiophene as depicted in scheme 1. Compound 2 was
obtained initially by the Vilsmeier-Haack reaction of 2-bromo-3-
hexylthiophene and further it was treated with the 9,10-
phenoanthrodione in the presence of acetic acid and ammonium
acetate at 120° C to get compound PIT. The fluorophore PITP was
obtained by the Heck coupling reaction of vinyl pyridine with PIT
under the reaction conditions.
Design and synthesis of novel fluorophores are of great interest
because of their wide range of applications in clinical diagnostics
and environmental monitoring.1-4 Fluorophores can be used as
sensors for the detection of biomolecules, cations, anions,
biological and ecological pH.5,6pH sensors that works under the
acidic conditions usually have a single protonation site. Upon
protonation on such site there will be a physical or chemical
changes occurs either through photoinduced electron transfer (PET)
or intramolecular charge transfer (ICT) and in turn used to precisely
measure the pH of the medium. pH sensors that deals with single
protonation site can only detect a single pH and is widely known.7-9
To the best of our knowledge, pH sensor molecule having a multiple
protonation sites that can sense different pH of the solution is not
known till date.
We have designed
a molecule in such a way that it can
accommodate multiple functional groups having different pKa
values, which can alter the ICT depending on the pH of the medium
and hence it can sense a different pH under different reaction
conditions. For that purpose we chose Phenanthro[9,10-d]imidazole
in combination with the thiophene tethered pyridine as our target
molecule (PITP). Under the different pH condition either the
imidazolyl nitrogen or the pyridinyl nitrogen get protonated and
made significant red shift of more than 115 nm, which is very useful
to detect the pH of the analyte under visible region. It is important
to note that the PITP molecule can act both as ratiometric as well as
colorimetric pH sensor under the reaction conditions.
Structure 1
a.
School of chemistry, Madurai Kamaraj University, Madurai-21, Tamilnadu, India.
E-mail: drasiva@gmail.com; ptcsiva@yahoo.co.in; Tel: +91451-2458471.
School of Chemistry, Bharathidasan University, Tiruchirapalli, Tamilnadu, India.
b
Electronic Supplementary Information (ESI) available: Experimental procedures,
spectral data, copies of 1H &13C NMR and ESI-MS. See DOI: 10.1039/x0xx00000x
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Scheme 1 (a) POCl3, DMF, EDC, 80°C, (b) 9,10-phenanthroquinone,
NH4OAc, AcOH, 120°C, (4-vinylpyridine, Pd(OAc)2, DMF, 120°C,
TBAB, K2CO3.
The absorption and emission spectral studies of PITP were carried
out in a series of protic and aprotic solvents and we infer that the
spectral behaviours are very similar in all solvents studied. A strong
absorption and emission band is present around 400 and 500 nm
respectively, which occurs due to the charge transfer from the
phenanthro[9,10-d]imidazole donor to the pyridine acceptor within
the PITP fluorophore. Optical response of PITP at different pH was
studied in 0.2 M phosphate buffer (containing 60% methanol as a
co-solvent). As the pH of the solution is decreased from 7.51 to
4.12, the absorbance around 400 nm decreases with a simultaneous
appearance of a new peak at around 450 nm with an isobestic point
at 420 nm as shown in Figure 1a. As the pH of the solution
decreased further from 4, the newly formed absorption peak
around 450 nm is blue shifted (420 nm) along with an isobestic
point at 450 nm. The absorbance around 400 nm for PITP is due to
the neutral form and the new peaks around 450 nm and 420 nm
with decrease in pH are due to the successive protonation at
pyridine nitrogen (acceptor) and imidazole nitrogen (donor)
centers. It can be anticipated that protonation at the donor or
acceptor moiety will change the photophysical properties of
fluorophore because it affects the efficiency of intramolecular
charge transfer within PITP. The first protonation at pyridine
nitrogen increases the electron accepting character of the acceptor
group. This increase in charge transfer results in the red shift of the
absorption spectrum. The second protonation at imidazole
nitrogen decreases the electron donating character of the donor
group results in the blue shift15,16 of the absorption spectrum
(Figure 1b). Similar studies were carried out using PIT(PITP without
the pyridine moiety), upon decreasing the pH of the solution from
7.10 to 1.88 leads to red shift in the absorption spectrum from 355
nm to 361 nm, due to protonation at only the imidazole nitrogen
and this shift happened only at pH below 4.0 (Figure S-1). This
confirms the second protonation occurs in PITP below pH 4.0 is at
imidazole nitrogen (donor moiety).
Figure 1 UV-visible spectrum of PITP (a) changes in absorbance of
PITP (10 µM) in 0.2 M phosphate buffer (containing 60% methanol
as co-solvent) at pH 7.51 to 4.12 (b) changes in absorbance of PITP
(10 µM) in 0.2 M KCl-HCl buffer (containing 60% methanol as co-
solvent) at pH 3.64 to 1.88.
As similar to the absorption spectra, the emission spectra of PITP is
red shifted by 116 nm with a clear isoemissive point at 620 nm on
decreasing the pH from 7.51 to 4.12 (Figure 2). As the pH is
decreased, the intensity of the new emission peak around 646 nm
enhances with simultaneous decrease in the intensity of the 528 nm
peak on exciting at the isosbestic point as shown in Figure 2. The
emission around 500 nm for PITP is due to the neutral form. The
appearance of the new red shifted emission is caused by the
protonation at the pyridine moiety, which results in the increasing
the electron withdrawing ability of acceptor moiety.
Figure 2Fluorescence spectra of PITP(10 µM) in of 0.2 Mphosphate
2 | J. Name., 2012, 00, 1-3
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and KCl-HCl buffer solution (containing 60% methanol as a co- The electronic distributions in LUMO of mono-protonated form
solvent) at pH 7.51 to 4.12, excited at 450 nm.
located only on pyridine while in bis-protonated form it is located
on pyridine as well as imidazole ring; that results in blue shift of the
CT state (Figure 4). The energy difference between HOMO and
LUMO of PITP, PITP-H+ and PITP-2H+ forms is 3.23 eV, 2.77 eV and
3.06 eV respectively. These results are in concordant with our
experimental results.
The larger red shift makes this molecule suitable for ratiometric pH
sensing in acidic region. Upon decreasing the pH from 7.51 to 4.12
the emission intensity ratio (I528 / I646) changes from 8.16 to 0.33
and the pKa (5.47) value was calculated from emission ratio as a
function of pH (Figure S-4a).
In addition to UV-visible studies, cyclic voltammetric analysis of
PITP has also been studied at pH 2.0 and 4.2 (Figure S-2). The result
is as follows: Eox= 0.961 V, Ered= -0.066 V, as the ∆E is > 1000 mV, it
is irreversible redox couple. There is a shift in oxidation peak from
0.50 to 0.64 when we change the pH from 2 to 4.2 and the
corresponding ∆E values are 0.27 V and 0.59 V. We infer from these
values that there is a double protonation occurs (2e-transfer) at pH
2.0 while single protonation (1e- transfer) occurs at pH 4.2.18,19
In addition to absorption and emission spectral changes, the color
of the PITP solution is changed from colorless to orange-red upon
decreasing the pH from 7.5 to 4.0, then changes from orange-red to
pale yellow upon decreasing the pH from 3.5 to 2.0 and the color
changes are visible to naked eye (Figure 3a). Figure 3b shows the
emission color changes of PITP solution at three different pH values
(the photo was taken under illumination of a UV lamp). The color
and spectral changes are reversible, and reverse back the reaction
by increasing the pH using aqueous ammonia. Hence, PITP can be
used as a pH sensor within the three different pH’s such as
7.2/6.2/5.2; 4.2; and 3.2/2.2 region.
Figure
5 A schematic representation of single and double
Figure 3 Change in color of PITP at pH 7.5, 4.2 and 2.0 respectively
(a) under visible light and (b) under UV light (365 nm).
protonation of PITP at pyridine and imidazole nitrogen centres
respectively.
To establish the optical response of sensor PITP observed at
different pH, we carried out density functional theory (DFT)
calculations using DFT-B3LYP-6-31G level under Gaussian 03
package.17DFT study shows that the charge transfer state
originating from the highest occupied molecular orbital (HOMO) to
the lowest unoccupied molecular orbital (LUMO) excitation. The
HOMO is localized on the phenanthro[9,10-d]imidazole(donor), and
the LUMO is localized on the thiophene and the vinyl pyridine
moiety(acceptor).
To confirm the protonation site, we recorded the 1H NMR spectra of
PITP with and without passing HCl vapour (Figure 6b). After which
we observe that the protons adjacent to pyridine nitrogen (i.e. Ha
and Hb) are shifted from δ 8.20–8.30 to 8.60–8.70 ppm and δ 8.04–
8.10 to 8.40–8.50 ppm respectively. Also the imidazole –NH proton
at 13.9 ppm is disappeared.This is due to the possible bis-
protonation of the pyridine as well as the phenanthro[9,10-
d]imidazole N atoms, which in turn leads to a fast exchange of
protons between the imidazole nitrogens (Figure 6a). The He-
hprotons are also shifted downfield to obtain a well-resolved 1H
NMR spectrum of PITP.
Figure 4 Molecular orbital plots showing HOMO - LUMO energy
gaps of PITP, mono and bis-protonated forms of PITP.
Figure 6(a) Schematic representation of tautomerisation of
protonated imidazole, (b) H NMR of PITP with and without passing
1
HCl vapour.
Upon mono- and bis- protonation, the electronic distributions in
HOMO is similar to PITP (only on phenanthro[9,10-d]-imidazole).
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In order to show that the PITP can act as a pH sensor, we did the 13. W. Lin, L. Long, L. Yuan, Z. Cao and J. Feng, Analytica. Chimica.
following experiment. First the PITP flurophore was dissolved in
methanol and few drops of it was placed over the Whatman paper
and dried in air for few minutes. It was then dipped in different
acidic buffer solution (Figure 7). As illustrated in the Figure 7, the
change in colour is in accordance with the spectral results
suggesting that, the designed fluorophore could act as a pH sensors
with in the neutral to acidic region.
Acta., 2009, 634, 262-266.
14 Y. Yan, R. Zhang, X. Yu, and H. Xu,Chem.Plus.Chem., 2014, 79,
1676-1680.
15. A.J.Zucchero, J. Tolosa, L.M. Tolbert and U.H.F. Bunz, Chem. Eur.
J., 2009, 15, 13075-13081.
16 V. Schmitt, S.Moschel and H. Detert, Eur. J. Org. Chem., 2013,
25, 5655-5669.
17 M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci andG. A.
Petersson, Gaussian 09, revision C.01; Gaussian, Inc.:
Wallingford, CT, 2010.
18 P.L. Runnels, D.J. Joseph, M.J. Logman and M.R. Wightman,Anal.
Chem.,1999, 71, 2782-2789.
Figure 7Change in color of PITP absorbed on paper upon adding
buffer solutions (containing 60% methanol) at pH- 7.5, 4.2, 2.0 (left
to right).
19 R. Gulaboski, I. Bogeski, V. Mircˇeski , S. Saul , B. Pasieka , H. H.
Haeri , M. Stefova , J. P. Stanoeva , Sasˇa Mitrev , M. Hoth and
R. Kappl, ScientificC Reports , 2013, 3-1865, DOI: 10.1038,srep
01865.
In summary we have successfully synthesized
a
novel
phenanthro[9,10-d]imidazole based fluorophore having an
extended π-conjugation. We observed multiple protonation at two
different nitrogen centres of PITP by conducting UV-vis,
1
fluorescence, CV and H NMR studies. DFT studies of PITP supports
our experimental results. In conclusion we demonstrated the utility
of PITP as a ratiometric and calorimentric pH sensor in acidic region.
We acknowledge the financial support of the Department of
Science and Technology, New Delhi, India (Grant No.
SR/F/1584/2012-13) and Council of Scientific and Industrial
Research, New Delhi, India (Grant No. 01(2540)/11/EMR-II),
University Grants Commission, New Delhi, India (Grant No. UGC
No.41-215/2012 (SR), VT acknowledge the UGC-FRP(Grant no. F.4-
5(24-FRP)/2013(BSR)), New Delhi, India for its financial support.
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