A Viologen–Perylenediimide Conjugate as an Efficient Base Sensor with Solvatochromic Property

Article abstract of DOI:10.1002/cphc.201601053

A viologen–perylenediimide conjugate, denoted PDEV, is prepared for efficient base sensing. The conjugate shows solvatochromic behavior as well. The base sensitivity of viologen is purposefully coupled with the emission property of perylenediimide (PDI) to lower the detection limit. PDEV shows base-sensing ability at the ppb level, which is at least three orders of magnitude lower than those of previously reported sensors. The probe is sensitive toward solvent polarity and generates different shades of colors according to the polarity of the medium (solvent). The photophysical properties show a linear correlation with the solvent polarity, and this makes it an efficient solvatochromic agent. On the other hand, the generation of viologen radical cations by bases affects the aggregation and consequently the absorption and emission behavior of the PDI core. The effect of bases can also be visualized, because the probe generates different colors in the presence of bases, both under normal and under UV light. Organic amines can be detected even in the crystalline state, since the dark red color of the PDEV crystals changes to purple in a reversible fashion on exposure to amine vapors. An easy and practical paper-based tool created by using the probe can efficiently be used to detect solvent polarity and presence of bases optically.

Full text of DOI:10.1002/cphc.201601053

Accepted Article
Title: A Viologen-Perylenediimide Conjugate as an Efficient Base
Sensor with Solvochromic Property
Authors: Debapratim Das, Bapan Pramanik, Julfikar Hassan Mondal,
Nilotpal Singha, Sahnawaz Ahmed, and Jyotirmayee Mohanty
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A Viologen-Perylenediimide Conjugate as an Efficient Base
Sensor with Solvochromic Property
Bapan Pramanik,[a] Julfikar Hassan Mondal,[a] Nilotpal Singha,[a] Sahnawaz Ahmed,[a] Jyotirmayee
Mohanty*,[b] Debapratim Das*[a]
Dedication ((optional))
Abstract: A viologen-perylenediimide conjugate, termed as PDEV, is
prepared for efficient base sensing. The conjugate is showing
solvatochromic behavior as well. The base-sensitivity of viologen is
purposefully coupled with the emission property of perylenediimide
(PDI) in order to lower the detection limit. PDEV has shown base
sensing ability in the ppb level which is at least three orders of
magnitude lower than the previously reported sensors. The probe is
sensitive toward solvent polarity and generates different shades of
colors based on the polarity of the medium (solvent). The photo-
physical properties follow a linear correlation with the solvent polarity
and thus making it an efficient solvatochromic agent. On the other
hand, the generation of viologen cation radicals by bases affect the
aggregation and consequently the absorption and emission behavior
of the PDI core. The effect of bases can also be visualized as the
probe generates different colors in presence of bases both under
normal as well as under UV light. Organic amines can be detected
even in the crystalline state as dark red color of the PDEV crystals
change to purple in a reversible fashion upon exposure to amine
vapors. An easy and practical paper based tool is also created using
the probe which can efficiently be used to detect solvent polarity as
well as presence of bases optically.
molecularly imprinted polymers, fluorescent sensors etc. have
been developed and used successfully for sensing bases.[5-9]
However, requirement of sophisticated instrumental facilities
significantly limits the practical applicability of these methods. In
this regard, though many sensors have been developed that
allow detection of these chemicals by naked eye through color
change but many of them lack proper sensitivity.[10-12] Detectors
showing efficient visual sensing of solvent polarity as well as
bases are therefore of great importance.
In this regard, owing to the redox controlled and charge transfer
complexation induced color change, viologens (Scheme 1A) have
found its use as base sensors.[10,11] One important feature of
viologens is their electron deficient nature which makes them very
good electron acceptors.[13-15] Viologens undergo a fast and
reversible two step redox process and are capable of forming
three different species like, a dication, a radical cation, and the
neutral species (Scheme 1A).[16-18] The optical charge transfer
between N1+ and N0 of the radical cation species makes them
intensely colored.[16-18] Using this phenomenon, Ma et al. reported
a benzyl-substituted viologen based sensor for amines.[11]
Similarly, Zhu and co-workers recently enhanced the efficiency of
the viologen based sensors by delocalization of the radical
generated by bases.[10] Detectable color change was observed at
1 × 10-5 M sensor concentration in presence of 10 equivalents of
base and the sensitivity was in ppm level. Notably, all these
viologen based sensors rely on the absorbance behavior of
viologen and related species generated by the redox reactions
which decreases the overall sensitivity of the systems in solution
phase detection.
Introduction
Solvents and bases are the two extremely important constituents
of modern day chemical industry which are used or produced in
huge quantities. However, the inherent cytotoxicity and volatility
of these chemicals leave an adverse impact on environment and
effective ways to sense these toxic chemicals are essential.[1,2]
Compounds which show dependence of color on solvent polarity
are generally employed as solvatochromic sensors.[3] Pyridinium
N-phenolate betaine dyes are considered as the standard to
estimate the polarity of a solvent.[4] Though this dye shows the
most pronounced solvatochromism, the synthesis of this molecule
is somewhat expensive and tedious.[4] On the other hand, various
methods such as single molecule and array sensors, enzymes,
We envisioned that the sensitivity of viologen towards bases can
be amplified by conjugating viologen units with a suitable
fluorophore which will also act as an alternative for common
solvatochromic probes. In this regard, perylenediimides (PDI) are
a well-studied class of fluorophore with excellent electronic,
optical, and redox properties.[19,20] PDI have recently emerged as
a very efficient moiety to prepare fluorescent sensors for a
variety of species especially because of their outstanding
photochemical stability and high quantum yields.[21-23] In addition,
the PDI chromophores have added advantage in the design of
sensors because of their inclination towards self-assembly
through  stacking, electrostatic and solvation interactions.[21-
[a]
[b]
B. Pramanik, Dr. J. H. Mondal, N. Singha, S. Ahmed, Dr. D. Das
Department of Chemistry, Indian Institute of Technology Guwahati
North Guwahati, Assam 781039, India
E-mail: ddas@iitg.ernet.in
Dr. J. Mohanty
Radiation & Photochemistry Division, Bhabha Atomic Research
Center, Trombay, Mumbai 400085, India
23]
The presence of planar -conjugated core leads to the
formation of both J- and H- type aggregates depending upon the
functionalization at the imide positions as well as the medium
(solvent).[24-27] The aggregation of the PDIs is associated with
Supporting information for this article is given via a link at the end of
the document.
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drastic influence on their photophysical properties.[21-27] The
aggregation of PDIs and consequently the changes in their
photophysical properties are highly influenced by the medium and
thus with appropriate conjugation, PDIs can be solubilized in
solvents with a wide range of polarity and can be used as a
solvatochromic agent.[28,29]
the intensity order changes from 0-0> 0-1 to 0-1 > 0-0 and this
order reversal is characteristic of aggregation of the PDI core. In
this process, the A0-0/A0-1 ratio decreased from 1.54 (0.1 × 10-6 M)
to 0.85 (2.6 × 10-5 M or above). Thus, this particular concentration
(2.6 × 10-5 M) can be considered as the minimum aggregation
concentration (MAC, Table 1). The emission spectrum of PDEV
in water is the mirror image of the absorption spectrum with a
Stokes shift of 12 nm. The fluorescence spectra showed three
emission peaks at 549, 588, and 639 nm (Figure 2A).32 Notably,
the concentration dependent emission pattern also showed a
drastic change at 2.6 × 10-5 M. The initial linear enhancement of
the intensities with increase in PDEV concentration suddenly took
a negative slope above 2.6 × 10-5 M. Above a concentration of 1×
10-4 M, no measurable emission intensity was observed. The
decrease in the emission intensity and complete loss of emission
can presumably be explained in terms of self-quenching by
stacked arrangement of molecules. The inflection point at 2.6 ×
10-5 M is very similar to the MAC obtained from absorption spectra
and can be considered as the concentration above which all the
molecules remain in aggregated form.
As a proof of concept, we have selected PDI as the emissive
component of our molecule and a viologen-PDI conjugate has
been prepared (PDEV, Scheme 1). This bola-amphiphilic
viologen-PDI conjugate showed solvatochromic property by
generating distinguishable colors in different solvents. As
expected, the presence of two viologen units at the two terminals
of PDI make this molecule an effective sensor for bases.
Incorporation of the fluorophore significantly enhanced the base
sensing efficiency (sensitivity ~1-77 ppb for various bases)
compared to that of the previously reported viologen based base
sensors in solution phase (sensitivity ~130-1880 ppb for various
bases)[10] as well as in solid crystalline phase.
Scheme 1. A) Redox induced chromic process of viologens; B) Synthetic routes
to DEV, PDEV and PDTMA, a) ACN, 81 °C, 12 h; b) imidazole, DMAP, 1,2-
dichlorobenzene, 80 °C, 12 h; c) Et-Br, ACN, reflux,12 h; d) DMF, 130 °C, 5 h;
e) MeI, DCM, 8 h; f) Et-Br, 80 °C, 24 h.
Results and Discussion
The absorption spectra of PDEV in aqueous medium showed
several absorption bands generated from the viologen (253 nm)
and PDI chromophores (Fig. S1). As PDI core is mainly
responsible for the aggregation and the emission, for clarity,
absorption region (400-800 nm) corresponding to the PDI core is
shown in Figure 1A. In this region, two prominent vibronic bands
at 534, 494 and a shoulder at 464 nm attributed to the 0-0, 0-1
and 0-2 vibrational transitions were observed.30,31 The
Figure 1. Absorption spectra of PDEV in water with varying concentrations. A)
up to 2.6 × 10-5 M and B) above 2.6 × 10-5 M showing the reversal of order of 0-
0, 0-1 transitions above this concentration. Inset of A) absorbance at 534 nm vs
log [PDEV] plot showing the inflection point.
PDEV has a fairly high solubility in water, dimethylformamide
(DMF), dimethylsulfoxide (DMSO), methanol (MeOH), ethanol
(EtOH), acetonitrile (ACN), and acetone. Concentration
dependent study in these solvents showed similar behavior to that
in case of water (Figures S2-S8). Though a sudden change in the
concentration dependent absorption spectra results in
a
continuous increase in the absorption for all three transitions with
a change in the slope at 2.6 × 10-5 M. Above this concentration,
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Table 1. Photophysical Parameters of PDEV in Different Solvents
Solvent
ET(30)
(kcal mol-1)
ETN bs
 abs
m
(nm)
em



MAC
( × 10-6 M)
(nm)
(cm-1)
(cm-1)
(cm-1)
Water
MeOH
EtOH
ACN
63.1
55.4
51.9
45.6
45.1
43.2
42.2
1
534
530
528
526
524
523
522
18727
18868
18939
19011
19084
19120
19157
549, 588
536, 574
537, 575
533, 572
539, 577
542, 581
523, 562
18215
18656
18622
18762
18553
18450
19120
512
212
317
249
531
670
37
0.06
0.04
0.08
0.06
0.06
0.08
0.03
26
32
12
12
20
0.762
0.654
0.46
DMSO
DMF
0.444
0.386
0.355
8.2
5.2
Aceton
e
absorption in all these solvents was observed above a
particular concentration, no peak reversal was observed in any
of these organic solvents. As in case of water, the fluorescence
intensities increased with increase in concentration and after a
certain concentration, a sharp decrease was observed. The
inflection points in both absorption and emission spectra were
obtained at very similar concentrations for a particular solvent.
The UV-Vis and fluorescence spectra of PDEV in different
solvents at low concentration (1 × 10-6 M), under which
aggregation does not occur (below MAC), are shown in Figure
3. With increasing solvent polarity, a clear bathochromic shift
was observed in the peaks generated from 0-0, 0-1 and 0-2
vibrational transitions (Figure 3A). The absorption maxima of
PDEV displays good linear correlation with the ET(30) values
(parameter indicative of solvent polarity and hydrogen bond
donating ability) of the solvents and found to shift to lower
energies as the ET(30) value increases. This can presumably
be attributed to the presence of dicationic viologen units at both
ends of PDEV which stabilizes the excited states with
increasing solvent polarity.[22] The emission of PDEV in these
solvents except DMF and DMSO (hydrogen bond accepting
solvents) also show similar changes in the emission band
positions. A red shift was observed for all the peaks when
moved from the nonpolar to polar solvents (Figure 3B and
inset).[34,35] However, the quantum yield (Φ) values of PDEV in
these solvents using rhodamine 6G as the standard could not
reveal any correlation with the solvent polarity (Table 1).
Figure 2. Emission spectra of PDEV in water with varying concentration A)
up to 2.6 × 10-5 M and B) above 2.6 × 10-5 M showing the decrease in
emission intensities after this concentration. Inset of A) Intensity at 549 nm
vs log [PDEV] plot showing the inflection point.
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Interestingly, the solutions of PDEV generate different colors in
these solvents (Figure 4) indicating its solvatochromic behavior.
The aqueous solution of PDEV was reddish orange in color
while the color changes to pale straw yellow to dark straw
yellow in ACN and DMSO respectively. Under UV-light (365
nm), the same solutions fluoresce differently as can be
expected from the emission studies. As we move from water to
less polar acetone, the color of the emitted light changes from
dark orange (water) to fluorescent yellow (ACN) to fluorescent
greenish yellow (acetone).
nm), these solutions emit differently, and notably, the visual
detection limit further dropped to 5 × 10-7 M of PDEV which is
the lowest ever reported to the best of our knowledge.
Figure 4. Photographic images of PDEV (1 × 10-6 M) in different solvents, A)
under normal light, and B) under UV light (= 365 nm).
Figure 3. A) Normalized absorption and B) normalized emission spectra of
PDEV in different solvents. [PDEV] = 1 × 10-6 M. Inset of A)
vs ET(30) of
 abs
solvents and Inset of B)
vs ET(30) of solvents plots.
em
To test the base sensitivity of PDEV, six different amines
(diisopropylethylamine (DIPEA), N, N-dimethyl-4-
Figure 5. Photographic images of PDEV in presence of different bases (2
equivalents), A) under normal light [PDEV] = 5× 10-6 M and B) under UV light
(= 365 nm), [PDEV] = 5 × 10-7 M. Photographs were taken two minutes
after the addition of aqueous (NaOH) or ACN (amines) solutions to an
aqueous solution of PDEV and mixing.
aminopyridine (DMAP), pyridine, piperidine, triethylamine
(TEA) and triisopropylamine (TPA)), and NaOH were used. As
seen in Figure 5, solutions of PDEV changed color dramatically
upon addition of amine/base. PDEV can act as a fast detector
of amines to the naked eye. This prominent changes in color
can be detected even at a concentration of 5 × 10-6 M and that
too in presence of 1-2 equivalent bases which makes PDEV a
far superior and efficient visual base sensor than other
viologen based sensors.[10,11] Under influence of UV light (365
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In order to gain an insight into the possible reason behind such
changes in color, we did examine the absorption and emission
behaviors of PDEV in presence of various bases. When titrated
with NaOH, the absorption profile of PDEV showed a decrease
in intensity up to an equimolar ratio of NaOH (Figure 6). Above
this concentration, the absorption corresponding to the 0-1
transition becomes higher than that of the 0-0 transition,
signifying the aggregation. With further increase in NaOH
concentration (> 2 equivalents), the absorption bands blue
shifted significantly along with broadening of the peaks. A
similar absorption profile was observed at high pH as well
(Figure S9). Along with these changes in the UV-visible region,
new bands started appearing in the NIR region at higher mole
ratio (3-4 equivalents, Figure 6B). The new bands in NIR region
and deep blue coloration suggest the radical cation
formation.[16,17] Above this concentration, the material
precipitated. The microscopic image of the precipitate showed
extremely ordered flower like arrangements (Supporting
Information S10). It is worth mentioning that the aqueous
solution of PDEV at a concentration below MAC showed no
prominent morphology while above MAC, a highly ordered
arrangement of brick like structures were found.
Figure 7. EPR spectra of PDEV, DEV and PDTMA in presence of NaOH or
sodium hyposulfite. Inset: EPR spectra of PDEV in presence of various
amines. For all measurements, the following parameters were used,
temperature: 295 K, power: 0.995 mw.
As expected, the fluorescence intensity decreased with
increase in NaOH concentration without any significant change
in the emission pattern (Figure 6C). The intensity vs
concentration plot shows an inflection point when one
equivalent NaOH was added. At higher equivalence of NaOH
(> 2), the emission property vanishes completely. The blue shift
of the absorption bands and complete loss of emission
suggests H-type aggregation of the PDI core.[35] Excitation
spectra of the 1:2 PDEV-NaOH solution collected at 575 nm,
produced a broad, structure-less band centered at 535 nm
suggesting the formation of H-type aggregates (Figure S11).
Notably, in case of PDTMA, under similar condition, only ~ 25%
Figure 6. A) Absorption spectra of PDEV in water with increase in
concentration of NaOH upto 2 eq. of NaOH; Inset: with higher amount of
NaOH showing the hypsocromic shift and broadening of the peaks; B) same
in the higher wavelength region showing the appearance of the viologen
radical cation peaks; C) emission spectra of PDEV in water with increase in
concentration of NaOH; Inset: Intensity vs molar ratio of NaOH/PDEV plot.
[PDEV] = 1 × 10-6 M.
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decrease (Figure S12) in emission intensity with no change in
the solution color detected. The absorption spectra of PDTMA
in presence of NaOH also did not show the appearance of any
new peak in the NIR region (Figure S13). Additionally,
treatment of DEV with NaOH produces a strong blue color and
the peaks corresponding to viologen radical cations in the NIR
region (Figure S14). These results conclusively point the effect
of the presence of viologen units in PDEV. Under strong basic
medium, the formation of viologen radical cations leads to
aggregation and consequently affect the emission of the PDI
core.
Table 2. Absorption Maxima of PDEV in Presence of Various Bases, Base
Sensitivity at abs = 0.01 and em = 10% of a 1.0 × 10-6 M Aqueous Solution
of PDEV Measured at Absorbance and Emission Maxima (588 nm).
Figure 8. A-B) Photographs of PDEV crystals A) before and B) after
exposure to amine vapor; C-D) photographs of test papers soaked in PDEV
solution and then exposed to different C) solvents and D) bases. C-D)
images were taken under UV light (365 nm).
Base Sensitivity (ppb)
Base
abs (nm)

abs = 0.01
em = 10%
Base sensitivity of PDEV in solution phase was quantified
using both absorbance and fluorescence. Only the absorbance
and fluorescence changes of the PDI core is considered as it
is not directly affected by the bases. The changes in absorption
and emission intensities against the equivalence of bases in all
cases follow a linear trend up to 2 equivalents of bases (Figure
6 and S15-20). These linear plots can be considered as a
calibration curve in order to quantitatively estimate the amount
of bases present. However, concentrations of bases required
to change the absorbance by 0.01 (abs = 0.01) and emission
intensity by 10% (em = 10%) in the initial linear part of these
calibration plots are considered to calculate the sensitivity
(Table 1). Remarkably, the sensitivities were found in ppb level
or less. The sensitivity observed in the present case was in
the range of 1-77 ppb whereas the previously reported best
viologen-containing sensor showed sensitivity in the order of
130-1880 ppb.[10] Overall, the sensitivity is observed to be three
orders of magnitude lower than the previously reported
viologen based sensors.[10,11] To the best of our knowledge,
these results suggest that PDEV is the most sensitive base
sensor both qualitatively and quantitatively.
NaOH
DIPEA
DMAP
Piperidine
Pyridine
TEA
535
533
522
524
523
533
521
1.01
1.89
19.95
29.10
17.78
36.76
77.41
40.84
12.27
14.68
3.69
17.45
41.01
18.91
TPA
In case of the organic bases, due to solubility issue, acetonitrile
solutions of the amines were used for the titration. In all cases,
the absorption corresponding to all three transitions decreased
with increase in the base concentration but no reversal of the
peak intensities was detected in any case (Figures S15-20).
Also in the emission spectra, a continuous decrease in the
emission intensity was noted with increase in the base
concentration. Owing to the weak nature of the organic bases,
no noticeable absorption band in the NIR region was detected.
In addition to the solution based detection, for a practical
colorimetric detection, it would be preferred if the compound
shows color change in solid state upon exposure to amines.
The EPR spectra of PDEV, PDTMA and DEV showed no
measurable signal (Figure 7). When treated with NaOH,
intense signals were observed only in case of PDEV and DEV.
As a standard, the EPR spectra of PDEV in presence of a
strong reducing agent (sodium hyposulfite) was monitored for
comparison purpose. Though the signal position remained
same, the intensity was found to be much higher than that of
the other samples at similar PDEV concentration. These
observations along with the appearance of absorption bands in
the NIR region in case of NaOH treated PDEV samples clearly
indicate the formation of viologen radical cations.[17] However,
presence of various amines resulted in extremely low signals
which indicate a minor population of the radical cation in these
cases (Inset of Figure 7).
-
For that purpose, the tetrafluoroborate (BF4 ) salt of PDEV was
prepared and crystallized from acetonitrile. Upon exposure to
amine vapors, the dark red crystals immediately changed to
purple (Figure 8 A, B). When kept under ambient condition, the
color of the crystals slowly returned to its original dark red color.
Unfortunately, the obtained crystals could not be analyzed by
X-ray crystallography owing to poor diffraction.
For another practical application, a paper based sensor was
developed for in situ on-site detection of both solvents as well
as bases. A strip of filter paper was immersed in methanol (for
base sensing) or aqueous (for solvents) solutions of PDEV and
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air dried. As can be seen in Figure 8, papers from the aqueous
solution did not fluoresce under UV light (365 nm) while the
same from methanol solution emits strongly. Upon exposure to
various solvents, the test papers from aqueous medium
showed different colors (Figure 8C). Spraying dilute solutions
of different bases on the test papers (methanol) quench the
emission (Figure 8D) completely. These results show that
PDEV can be used to prepare efficient and easy paper based
sensors for both solvents and bases.
dissolved in acetonitrile at high concentration and appropriate amounts
from these stocks were added to the aqueous solutions of PDEV. In
order to avoid any effect from added acetonitrile, only 0.1% v/v of the
base solutions were added. An appropriate control experiment carried
out by adding exactly the same amount of acetonitrile in aqueous
solution of PDEV resulted in no observable change in the absorption
as well as emission spectra.
Electron paramagnetic resonance (EPR): For EPR measurements,
prior to the addition of the bases, all the samples were degassed by
purging argon gas for 10 mins. After the addition of bases, Argon was
flushed thoroughly and the sample tubes were sealed properly to
prevent any areal oxidation. All measurements were carried out with 1
× 10-6 M samples (PDEV, PDTMA, DEV) with 2 equivalents of
bases/reducing agent on a JES-FA200 instrument from JEOL.
Conclusions
In summary, the conjugation of PDI unit with viologen allowed
us to prepare an efficient base sensor as well as a
solvatochromic probe. PDEV shows linear dependence of
photophysical properties on solvent polarity and generates
distinguishable colors in different solvents. In presence of
bases, viologen radical cations are produced which affect the
photophysical behavior of the PDI core and thus allowing
optical sensing of bases. The bases can be detected by PDEV
in ppb level in solutions as well as in crystalline state. For a
practical application, a paper strip based technique is
developed which allows to detect bases and solvents through
color changes. These findings will certainly open up the
possibility to create new base and solvent sensors with higher
efficacy.
Quantum Yield Measurements: The fluorescence quantum yields of
PDEV in different solvents were determined by using Rhoamine-6G as
a standard fluorophore following equation (1),
u = (AsFunu2)/ (AuFsns2)s
(1)
where, Φs is the quantum yield of the reference (Rhodamine, 0.95) in
ethanol, Φu is the quantum yield of PDEV, As and Au are the
absorbance of Rhodamine and PDEV at the excitation wavelength, Fs
and Fu are the area of integrated fluorescence intensity of the
Rhodamine and PDEV sample when excited at the same excitation
wavelength. The refractive indices of the solvents for the PDEV and
Rhodamine are denoted by nu and ns respectively. To minimize the
reabsorption of the fluorescence light passing through the samples their
absorption maximum was kept 0.1.
Experimental Section
General: All the chemicals and reagents used were obtained from
Sigma-Aldrich (USA) and used without further purification. All solvents
were procured from Merck, India and Spectrochem, India. Amberlite IR
400 anion exchange resin was obtained from Merck, India. To prepare
samples, Milli-Q water with a conductivity of less than 2 μS·cm−1 was
used. 1H NMR and 13C NMR were recorded either on a Bruker Ascend
600 MHz (Bruker, Coventry, UK) or on an Oxford AS400 (Varian)
spectrometer and referenced to deuterated solvents. ESI-MS was
measured using a Q-tof-Micro Quadrupole mass spectrophotometer
(Micromass). UV visible data was recorded using a PerkinElmer
Lambda 750 spectrometer. Fluorescence measurements were
Calculation: The initial linear parts of the absorption (at absorption
maximum) or emission intensity (at 588 nm) vs concentration of added
bases are considered. The concentration of base required ([base
conc.]) to change the absorbance by 0.01 (ΔAbs = 0.01) or initial
emission intensity by 10% (Δem = 0.01) used in equation 2 to calculate
the base sensitivity.[10]
obtained using
a Carry Eclipse spectrophotometer (Agilent). All
emission spectra were recorded by exciting the sample at 485 nm.
Unless otherwise mentioned, all studies were performed at room
temperature. Samples were prepared by casting a drop of the solution
on a silicon wafer and dried under ambient conditions. Standard 10 mm
path quartz cuvettes were used for all spectroscopic measurements.
Sensitivity (ppb) = MW [base conc.] 1000000 (2)
Synthesis: All the compounds were synthesized following the routes
shown in Scheme 1B.
Sample Preparation: The stock solutions of PDEV, PDTMA, and DEV
were prepared in a 10 mL of volumetric flasks in different solvents.
These stock solutions were diluted to desired concentration as required
for the experiments. For amine sensitivity, the organic bases were
Compound 1: To a stirring solution of 4, 4-bipyridyl (1 g, 6.4 mmol) in
acetonitrile (10 mL), 2-bromoethylamine hydrobromide (262 mg, 1.28
mmol) was added and was refluxed. After 12 h, the reaction mixture
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was cooled to room temperature and volatiles were removed under
reduced pressure. The crude product was then washed several times
with ether to get the white solid. Yield: 300 mg (83%). 1H NMR (600
MHz, D2O) δ = 9.09 (d, J = 6.6 Hz, 2H), 8.80 (d, J = 4.8 Hz, 2H), 8.53
(d, J = 6.6 Hz, 2H), 7.96 (d, J = 4.8 Hz, 2H), 5.07 (t, J = 6.6 Hz, 2H),
3.79 (t, J = 3.6 Hz, 2H) ppm; Anal calcd. for C12H14N3Br: C, 51.44; H,
5.04; N, 15.00. Found: C, 51.42; H, 5.09; N, 15.07; HRMS (ESI) m/z
calcd. for C12H14N3: 200.1184; found 200.1174 [M-Br-]+.
40 °C overnight yielded the title compound as a dark purple solid. Yield:
610 mg (90 %). 1H NMR (400 MHz, CDCl3) δ = 8.676 (s, 4H,), 8.62 (s,
4H) 4.37(s, 4H), 2.70 (s, 4H), 2.37(s, 12H) ppm; Anal. calcd. for
C32H28N4O4: C, 72.17; H, 5.30; N, 10.52. Found: C, 72.14; H, 5.34; N,
10.55. HRMS (ESI) m/z calcd. for C32H29N4O4: 533.2183; found
533.2183 [M+H]+.
PDTMA: Compound 3 (50 mg, 0.094 mmol), iodomethane (58 µL, 0.94
mmol) were taken in DCM and the mixture was stirred for 8 h. The
reaction mixture was centrifuged and the precipitate was washed with
DCM, finally with THF. The resulting red precipitates were dried under
vacuum. Yield: 65 mg (84 %). 1H NMR (600 MHz, DMSOd6) δ = 9.02
(d, J = 7.8, 4H), 8.64 (d, J = 7.8 Hz, 4H), 4.51(t, J = 7.2 Hz, 4H), 3.67
(t, J = 7.2 Hz, 4H), 3.24 (s, 12H) ppm; 13C NMR (125 MHz, DMSOd6)
δ = 162.69, 133.93, 130.92, 128.36, 125.26, 124.21, 122.36, 61.89,
52.56, 33.76 ppm; Anal calcd. for C34H34N4O4Br2: C, 56.52; H, 4.74; N,
7.75. Found: C, 56.55; H, 4.71; N, 7.73. HRMS (ESI) m/z calcd. for
Compound 2: Perylene-3, 4, 9, 10-tetracarboxylic anhydride (PTCDA,
100 mg, 0.255 mmol), compound 1 (184 mg, 0.501 mmol), and
imidazole (68 mg, 1 mmol) were taken in 1, 2-dichlorobenzene (8 mL),
4-N, N-dimethylaminopyridine (56 mg, 0.5 mmol), and was heated at
80 °C with stirring for 12 h. The reaction mixture was then cooled to
room temperature and the precipitate was collected by centrifugation.
The collected red solid was dissolved in water (50 mL) and washed with
dichloromethane (DCM) to remove water insoluble residues. The
volume of the aqueous layer was lowered by removing water on a rotory
evaporator and to it was added a portion of saturated aqueous
ammonium hexafluorophosphate (NH4PF6) solution (10 mL). The
resulting red precipitate was collected by filtration and dried under
vacuum. Yield: 166 mg (62%). 1H NMR (400 MHz, DMSOd6): δ = 9.37
(d, J = 5.6 Hz, 4H), 8.87 (d, J = 4.4 Hz, 4H), 8.72 (d, J = 8 Hz, 4H), 8.63
(d, J = 6 Hz, 4H), 8.35 (d, J = 8 Hz, 4H), 8.04 (d, J = 4 Hz, 4H), 5.0 (s,
4H), 4.70 (s, 4H) ppm; Anal calcd. for C48H32N6O4P2F12: C, 55.08; H,
C34H34N4O42: 281.1284; found 281.1280 [M-2I-]2+
.
DEV: DEV was prepared following a previously reported protocol.[36] In
brief, excess (10 equivalent) ethyl bromide was mixed with 4, 4′-
dipyridyl in a glass tube and the mouth of the tube was sealed. The tube
was heated to 80 °C for 24 h. After being cooled to room temperature,
the seal was broken and the material was concentrated on a rotory
evaporator, and the residue was crystallized three times from
methanol-diethyl ether to get a yellow solid (Yield: 35%). 1H NMR (400
MHz, D2O) δ = 9.15 (d, J = 7.2 Hz, 4H), 8.56 (d, J = 7.2 Hz, 4H), 4.77
(m, 4H), 1.73 (t, J = 8.0 Hz, 6H) ppm; 13C NMR (100 MHz, D2O) δ =
150.17, 146.12, 129.21, 57.92, 15.91 ppm; Anal calcd. for C14H18N2Br2:
C, 44.95; H, 4.85; N, 7.49. Found: C, 44.98; H, 4.86; N, 7.43; HRMS
(ESI): m/z calcd. for C14H18N2Br: 293.0648, found: 293.0651 [M-Br-]+.
3.08; N, 8.03. Found: C, 55.10; H, 3.04; N, 8.06; HRMS (ESI) m/z calcd.
- 2+
for C48H32N6O4: 378.1237; found 378.1227 [M-2PF6 ]
.
PDEV: A mixture of 2 (100 mg, 0.09 mmol) and ethyl bromide (104 mg,
0.9 mmol) in acetonitrile was refluxed for 12 h. After cooling to room
temperature, the resulting precipitates were collected by centrifugation
and washed with acetonitrile several times before drying it under
reduced pressure. The material was then taken in water and subjected
to ion exchange by passing the solution through a column of bromide
functionalized amberlite IR 400 anion exchange resin. The fractions
containing the compound was freeze dried and crystalized from
methanol-diethyl ether solvent mixture to get the desired compound.
Yield: 62 mg (63 %). 1HNMR (600 MHz, DMSOd6): δ = 9.58 (d, J = 5.4
Hz, 4H), 9.44 (d, J = 5.4 Hz, 4H), 8.92 (d, J = 7.2 Hz, 4H), 8.85 (q, J =
5.4, 6.0 Hz, 8H), 8.47 (d, J = 7.2 Hz, 4H), 5.10 (s, 4H), 4.74 (t, J = 7.8
Hz, 8H), 1.61 (t, J = 7.2 Hz, 6H) ppm; 13C NMR (100 MHz, DMSOd6) δ
= 163.17, 162.91, 148.79, 148.13, 146.68, 145.68, 134.18, 131.08,
126.54, 126.18, 125.61, 124.28, 122.63, 56.53, 40.95, 16.33 ppm; Anal
calcd. for C52H42N6O4Br4: C, 55.05; H, 3.73; N, 7.41. Found: C, 55.04;
H, 3.74; N, 7.43; MS (MALDI-TOF, DHB matrix) m/z calcd. for
C52H42N6O4: 814.3246; found 814.3510 [M-4Br].
Acknowledgements
We want to thank BRNS, India (BRNS/2013/RP/37C/60), CSIR,
India (01(2757)/13/EMR-II), and SERB, DST, India
(SR/FT/CS-154/2011) for financial support. DD wants to thank
AvH foundation for instrument grant and Dr. A. Dasgupta for
helpful discussion.
Keywords: Sensors • Viologen • Solvatochromism • Base •
Fluorescence
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Entry for the Table of Contents
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Base sensing
Author(s), Corresponding Author(s)*
A Viologen-PDI conjugate with
efficient base sensing ability and
solvatochromic behavior
Page No. – Page No.
Title
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