New fluorogenic sensors for Hg2+ ions: Through-bond energy transfer from pentaquinone to rhodamine

Article abstract of DOI:10.1021/ic201990q

New pentaquinone derivatives 5 and 8 having rhodamine moieties have been designed and synthesized that undergo through-bond energy transfer (TBET) in the presence of Hg²⁺ ions among the various cations (Cu²⁺, Pb²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Ni ²⁺, Cd²⁺, Co²⁺, Ag⁺, Ba ²⁺, Mg²⁺, K⁺, Na⁺, and Li ⁺) tested in mixed aqueous media.

Full text of DOI:10.1021/ic201990q

Article
pubs.acs.org/IC
New Fluorogenic Sensors for Hg2+ Ions: Through-Bond Energy
Transfer from Pentaquinone to Rhodamine
Vandana Bhalla,*,† Roopa,† Manoj Kumar,*,† Parduman Raj Sharma,‡ and Tandeep Kaur‡
†Department of Chemistry, UGC Sponsored-Centre for Advanced Studies-I, Guru Nanak Dev University, Amritsar 143005, Punjab,
India
‡Department of Cancer Pharmacology, Indian Institute of Integrative Medicine, Canal Road, Jammu 180001, India
S
* Supporting Information
ABSTRACT: New pentaquinone derivatives 5 and 8 having rhodamine moieties have been designed and synthesized that
undergo through-bond energy transfer (TBET) in the presence of Hg2+ ions among the various cations (Cu2+, Pb2+, Fe2+, Fe3+,
Zn2+, Ni2+, Cd2+, Co2+, Ag+, Ba2+, Mg2+, K+, Na+, and Li+) tested in mixed aqueous media.
orbit coupling effect.10 Fluorescence quenching not only is
disadvantageous for a high signal output upon recognition but
INTRODUCTION
The developments of fluorogenic sensors for heavy and soft
■
also hampers temporal separation of spectrally similar
transition-metal ions, based on ion-induced changes in
complexes with time-resolved fluorometry.11 On the other
fluorescence, are particularly attractive because of their
simplicity, high sensitivity, and instantaneous response.1
hand, the sensors that undergo fluorescence enhancement in
the presence of metal ions are preferred because these allow a
Among soft transition-metal ions, mercury has received
lower detection limit and high-speed spatial resolution via
considerable attention because it is highly toxic. The wide-
microscopic imaging.12,13 Recently, a number sensors based on
spread contamination of mercury is due to a variety of natural
and anthropogenic sources,2 including oceanic and volcanic
rhodamine have been reported that show a selective “turn-on”
response in the presence of Hg2+ ions.14 However, the
rhodamine-based sensors have the limitation of having a very
small Stokes shift (around 25 nm), which may lead to self-
quenching and fluorescence detection errors because of
excitation back-scattering effects.15 Thus, it is important to
have sensors with improved properties; however, it is difficult
to design such types of organic dyes with desirable photo-
physical properties. Recently, the possibility of having organic
dyes with more than one fluorophore linked through a
nonconjugated linker with an energy donor−acceptor combi-
nation has been explored wherein energy from one
fluorophore, called the donor, is transferred to another
fluorophore, called the acceptor, without emission of a
photon.16 However, fluorescence resonance energy transfer
based systems require that the donor emission overlap with the
acceptor absorption, which makes their utility limited. On the
other hand, through-bond energy transfer (TBET) is
emission,3,4 solid waste incineration, combustion of fossil fuels,5
and gold mining.6 The Environmental Protection Agency
(EPA) standard for the maximum allowable level of inorganic
mercury(II) in drinking water is 2 ppb.5 Thus, exposure to
mercury, even at very low concentration, leads to digestive,
kidney, and especially neurological diseases.7 Further both
elemental and ionic mercury can be converted into
methylmercury by bacteria in the environment that enters the
food chain and accumulates in the higher organisms,8 which
can cause serious damage to the central nervous and endocrine
systems.9 Thus, imaging of Hg2+ ions in living cells is crucial.
Keeping in mind the ill effects of mercury in day-to-day life,
there is a need to develop an approach for simple and rapid
tracking of mercury ions in biological, toxicological, and
environmental monitoring. Fluorescence spectroscopy lends a
helping hand for the sensing and imaging of trace amounts of
mercury because of its high sensitivity and simplicity. Mercury-
selective fluorescent sensors have been reported in the past
where in most cases the presence of mercury causes
fluorescence quenching of the fluorophores via the spin−
Received: September 10, 2011
Published: February 2, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Compounds 3, 5, 6, 8, and 9
theoretically not subjected to the requirement of spectral
overlap between the donor emission and acceptor absorption
and is expected to have large Stokes and emission shifts.17
These spectral benefits are very important for the use of
fluorescent dyes in chemistry, biology, medicine, and materials
science. In TBET systems, the donor and acceptor are joined
by a conjugated spacer, which prevents them from becoming
flat and conjugated. These types of systems absorb at a
wavelength characteristic of the donor and then emit via a
receptor. Recently, Burgess et al.17a,b have developed excellent
TBET systems based on rhodamine and fluorescein for use in
biotechnology, but there is only one report of such systems for
fluorogenic sensing of metal ions.18 Thus, there is considerable
scope for the development of fluorogenic sensors for different
types of analytes that involve TBET.
RESULTS AND DISCUSSION
■
Our research program involves the design, synthesis, and
evaluation of novel artificial receptors selective for soft metal
ions and anions of clinical and environmental interest.19 In a
preliminary communication from our laboratory, we reported a
naphthalimide−rhodamine fluorescent dyad that undergoes
TBET in the presence of Hg2+ ions; however, the energy
transfer was not 100% because of leakage of some fluorescence
from the naphthalimide donor.18 Now, in a continuation of this
work, we have designed and synthesized a rhodamine−
pentaquinone dyad and a rhodamine−pentaquinone−rhod-
amine triad, both of which undergo TBET in the presence of
Hg2+ ions with nearly 100% efficiency. In addition, one of the
two sensors can be used for imaging Hg2+ ions in living cells.
Pentaquinone derivatives have found immense utility in the
design and synthesis of solution-processable functionalized
pentacene derivatives.20 The role of pentaquinone as a
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Figure 1. (A) UV−vis spectra of 5 (10 μM) in the presence of Hg2+ ions (0−200 equiv) in THF/H2O (9.5:0.5, v/v) buffered with HEPES, pH = 7.
Inset: Change in the color of the receptor before and after the addition of Hg2+ ions. (B) UV−vis spectra of 8 (10 μM) in the presence of Hg2+ ions
(0−200 equiv) in THF/H2O (9.5:0.5, v/v) buffered with HEPES, pH = 7. Inset: Change in the color of the receptor before and after the addition of
Hg2+ ions.
Figure 2. (A) Fluorescence response of receptor 5 (10 μM) upon the addition of Hg2+ ions (0−200 equiv) in THF/H2O (9.5:0.5, v/v) buffered
with HEPES, pH = 7; λex = 360 nm. Inset: (a) Change in the fluorescence intensity of receptor 5 as a function of the Hg2+ ion concentration. (b)
Fluorescence before and after the addition of Hg2+ ions. (B) Fluorescence response of receptor 8 (10 μM) upon the addition of Hg2+ ions (0−200
equiv) in THF/H2O (9.5:0.5, v/v) buffered with HEPES, pH = 7; λex = 360 nm. Inset: (a) Change in the fluorescence intensity of receptor 8 as a
function of the Hg2+ ion concentration. (b) Fluorescence before and after the addition of Hg2+ ions.
chemosensor for different metal ions has not yet been explored,
except for one report from our group.21 To the best of our
knowledge, this is the first report where a pentaquinone
scaffold has been appended with rhodamine moieties for
selective sensing of Hg2+ ions that involve energy transfer
through a conjugated spacer.
ppm and one quartet at 3.29−3.37/3.26−3.33 ppm corre-
sponding to N-ethyl protons and three doublets at 6.69, 7.08,
7.17/6.61, 6.83, and 6.94 ppm and six multiplets at 6.28−6.36,
7.50−7.53, 7.68−7.70, 7.71−7.82, 8.02−8.16, 8.88−8.92/6.28−
6.31, 7.15−7.17, 7.49−7.51, 7.68−7.71, 8.00−8.02, and 8.10−
8.13 ppm corresponding to aromatic protons. In addition,
compound 8 showed three singlets at 7.96, 8.86, and 8.93 ppm
corresponding to aromatic protons. In the MS spectra, the
parent ion peaks for compounds 5 and 8 were observed at
824.3 [(M + 1)+] and 1338.4 [(M)+] ppm, respectively. These
spectroscopic data corroborate with structures 5 and 8.
The binding behavior of compounds 5 and 8 toward
different cations (Cu2+, Hg2+, Fe2+, Fe3+, Co2+, Pb2+, Zn2+, Ni2+,
Cd2+, Ag+, Ba2+, Mg2+, K+, Na+, and Li+) as their perchlorate
salts was investigated by UV−vis and fluorescence spectrosco-
py. The UV−vis spectrum of compound 5/8 exhibits
absorption bands at 290/275 and 320/322 nm in THF/H2O
(9.5:0.5, v/v) due to the pentaquinone moiety (Figure 1A/B).
However, upon the addition of Hg2+ ions (0−200 equiv), the
intensities of these absorption bands increase and a new band
appears at 538/554 nm for receptor 5/8, respectively (Figure
1A/B). These changes are accompanied by a gradual change of
the color from colorless to pink, visible to the naked eye (inset,
The Suzuki−Miyaura cross-coupling of boronic ester 222
with 2-bromo-6,13-pentacenequinone (1)23 catalyzed by Pd-
(Cl)2(PPh3)2 furnished compound 3 in 32% yield (Scheme 1).
1
The H NMR spectrum of compound 3 showed two doublets
at 6.84 and 7.94 ppm, three multiplets at 7.59−7.73, 8.12−8.16,
and 8.93−8.98 ppm, one singlet at 8.24 ppm corresponding to
aromatic protons, and one broad signal corresponding to amino
protons at 3.89 ppm (Supporting Information, Figure S1). The
mass spectrometry (MS) spectrum of compound 3 showed a
parent ion peak at m/z 400.1 [(M + 1)+; Supporting
Information, Figure S2]. The reaction of pentaquinone amines
3 and 721 with rhodamine acid chloride 424 in tetrahydrofuran
(THF) furnished compounds 5 and 8 in 34% and 50% yields,
respectively (Scheme 1). The structures of compounds 5 and 8
were confirmed from their spectroscopic and analytical data
(Supporting Information, Figures S3−S8). The 1H NMR
spectrum of compound 5/8 showed one triplet at 1.16/1.11
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Scheme 2. Hg2+-Induced TBET-OFF−ON
Figure 3. (A) Fluorescence response of receptor 5 (10 μM) upon the addition of various cations (200 equiv) in THF/H2O (9.5:0.5, v/v) buffered
with HEPES, pH = 7; λex = 360 nm. (B) Fluorescence response of receptor 8 (10 μM) upon the addition of various cations (200 equiv) in THF/
H2O (9.5:0.5, v/v) buffered with HEPES, pH = 7; λex = 360 nm. Bars represent the emission intensity ratio (I − I0/I0). I0 is the fluorescence intensity
of each free host, and I is the fluorescence intensity after the addition of metal ions. The blue bars represent the addition of various metal ions, while
the red bars represent the change in the emission that occurs upon the subsequent addition of Hg2+ (200 equiv) to the above solution.
Figure 1A/B). The formation of a new band at 538/554 nm is
attributed to the interaction of Hg2+ ions with the receptor 5/8,
leading to the opening of the spirolactam ring. Thus, in the
presence of mercury ions, receptors 5 and 8 show the
absorption characteristics of both donor and acceptor
components. We also carried out UV−vis studies of model
compounds 6 and 9 (pentaquinone donor) and rhodamine
acceptor 1125 (Supporting Information, Figures S9−S11 and
S12−S16) with Hg2+ ions independently. These compounds
exhibit similar results under experimental conditions parallel
with those for receptors 5 and 8 in which two moieties are
covalently linked to each other through a conjugated spacer,
thus suggesting the absence of any electronic interaction
between pentaquinone and rhodamine moieties in receptors 5
and 8 in the ground state in the presence of Hg2+ ions. In other
words, receptors 5 and 8 behave like a cassette and not as a
planar conjugated dye. However, no significant variation in the
absorption spectra was observed in the presence of other metal
ions (Cu2+, Fe2+, Fe3+, Co2+, Pb2+, Zn2+, Ni2+, Cd2+, Ag+, Ba2+,
Mg2+, K+, Na+, and Li+; Supporting Information, Figures S17
and S18).
addition of incremental amounts of Hg2+ ions (0−200 equiv) to
the solution of receptors 5 and 8 in THF/H2O leads to the
appearance of emission bands at 572 and 582 nm, respectively,
due to the rhodamine (acceptor) moiety (Figure 2A/B). The
emission intensity of receptor 5/8 increased linearly as a
function of the Hg2+ ion concentration (inset, Figure 2A/B).
We propose that emission enhancement at 572 and 582 nm is
attributed to the opening of the spirolactam ring of rhodamine
to the amide form, thus indicating the TBET process in
receptors 5 and 8, i.e., via the conjugated linker from donor to
acceptor (Scheme 2). The characteristic emission of the
pentaquinone moiety at ∼520 nm was not observed
(Supporting Information, Figures S19 and S20), suggesting
nearly 100% energy-transfer efficiency26 within experimental
error with large pseudo-Stokes shifts of up to 210 nm. We also
carried out fluorescence titrations of compound 5/8 with Hg2+
ions at different pH values. It was observed that the compounds
5 and 8 operate well in the pH = 4.0−7.0 range (Supporting
Information, Figures S21 and S22). Under sets of conditions
similar to those for compounds 5 and 8, we also carried out
fluorescence studies of an equimolar mixture of pentaquinone
donor 6/9 and rhodamine acceptor 10 and found that no
visible quenching of 6/9 and no enhancement in the
fluorescence emission of the rhodamine acceptor was observed
when the mixture was excited at the pentaquinone absorption
band, i.e., at 360 nm, which clearly indicates that there is no
The solution of receptor 5/8 in THF/H2O (9.5:0.5, v/v) is
nonfluorescent when excited at 360 nm (Figure 2A/B). The
quenched fluorescence emission is probably due to photo-
induced electron transfer (PET) from the nitrogen atom of the
spirolactam ring to the pentaquinone moiety. Interestingly, the
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Figure 4. Fluorescence and bright-field images of PC3 cell lines: (a) bright-field image of cells treated with probe 8 (1.0 μM) only for 20 min at 37
°C; (b and c) fluorescence images of part a in green and red channels, respectively. (d) Overlay image of parts a−c; (e) bright-field image of cells
upon treatment with probe 8 (1.0 μM) and then with Hg(ClO4)2 (30.0 μM) for 20 min; (f and g) fluorescence images of part e in green and red
channels, respectively; (h) overlay image of parts e−g. (A) λex = 488 nm. (B) λex = 405 nm.
intermolecular energy transfer between the pentaquinone
donor and rhodamine acceptor in the mixture (Supporting
Information, Figures S23 and S24). Thus, the advantage of the
TBET system for energy transfer is obvious. Further, for
practical applications, it is very important that the fluorescence
intensity of the acceptor in the cassette is greater than that of
the acceptor without a donor when it is excited at the donor
absorption wavelength. The fluorescence enhancement factors
for receptors 5 and 8 are 56-fold and 160-fold, respectively,
compared to 10 when excited at 360 nm (Supporting
Information, Figures S25 and S26). Under the same conditions
as those used above for Hg2+, we also tested the fluorescence
response of receptors 5 and 8 (Figure 3A/B) to other metal
ions (Pb2+, Cu2+, Fe2+, Fe3+, Ni2+, Zn2+, Cd2+, Co2+, Mg2+, Ba2+,
Ag+, K+, Na+, and Li+), and a negligible change in fluorescence
occurred in the presence of these metal ions (Supporting
Information, Figures S27 and S28). The competitive experi-
ments were conducted in the presence of 200 equiv of Hg2+
mixed with 200 equiv (Pb2+, Cu2+, Fe2+, Fe3+, Ni2+, Zn2+, Cd2+,
Co2+, Mg2+, Ba2+, Ag+, K+, Na+, and Li+) of various metal ions,
respectively (Figure 3A/B), and no significant variation in the
fluorescence intensity change was found by comparison with or
without the other metal ions. Further, to study the influence of
addition of potassium iodide to the solutions of 5−Hg2+ and
8−Hg2+ complexes resulted in quenching of the fluorescence
intensity. The quenching of fluorescence is due to the strong
affinity of iodide ions for the Hg2+ ions, which resulted in
decomplexation of the receptor−Hg2+ complex; i.e., Hg2+ ions
were not available for binding with the receptor. Upon further
addition of Hg2+ ions, the fluorescence was revived again along
with the appearance of a pink color, which proved the reversible
behavior of Hg2+ for receptors 5 and 8 (Supporting
Information, Figures S33 and S34). To elucidate the binding
mode of the receptor 5/8 with Hg2+ ions, the 1H NMR
spectrum of its complex with mercury perchlorate was also
recorded. The protons corresponding to NCH2CH3 and
NCH2CH3 undergo downshifts of 0.14/0.19 and 0.24/0.30
ppm, respectively, and the aromatic protons Hc,d, He, Hf, and
Hg corresponding to the rhodamine moiety of receptor 5/8
also undergo downfield shifts of 0.34/0.44, 0.22/0.31, 0.07/
0.26, and 0.03/0.24 ppm, respectively, in the presence of 1.0/
2.0 equiv of Hg2+ ions. (Supporting Information, Figures S35
and S36), which indicates transformation of the nonfluorescent
spirocyclic form of the rhodamine moiety in receptor 5/8 to
the fluorescent ring-opened amide form (Scheme 2).
The potential biological application of receptor 8 was
evaluated for the in vitro detection of Hg2+ ions in prostate
cancer (PC3) cell lines (Figure 4A,B). The prostate cancer
(PC3) cell lines were incubated with receptor 8 [1.0 μM in
THF/H2O (9.5:0.5, v/v) buffered with HEPES, pH = 7.0] in a
RPMI-1640 medium for 20 min at 37 °C and washed with a
phosphate-buffered saline (PBS) buffer (pH = 7.4) to remove
excess receptor 8. Microscopic images showed no fluorescence
in both the green and red channels respectively as shown in
Figure 4A,B (b-c). The cells were then treated with mercury
perchlorate (30.0 μM) in the RPMI-1640 medium, incubated
again for 20 min at 37 °C, and washed with a PBS buffer. After
treatment with Hg2+ ions, the cells showed significant red
fluorescence emission [Figure 4A,B (g)]. These results suggest
that 8 is an effective intracellular Hg2+ imaging agent with the
appearance of red emission attributed to the working of the
TBET phenomenon within the cells.
different counteranions such as Cl− and NO3 , we examined
−
the sensing behavior of receptor 5/8 with HgCl2 and
Hg(NO3)2. It was observed that there was no change in the
sensing performance of the receptor 5/8 when these counter-
anions were used except in the case of receptor 8, where there
−
was a small decrease in the sensing performance when NO3
was used as the counterion (Supporting Information, Figures
S29 and S30). The fluorescence quantum yields (Φfs) of
compounds 5 and 8 in the free state were found to be 0.03 and
0.01, respectively, and the Hg2+-bound states were found to be
0.15 and 0.21, respectively. Fitting the changes in the
fluorescence spectra of compounds 5 and 8 with Hg2+ ions
using the nonlinear regression analysis program SPECFIT27
gave a good fit and demonstrated that 1:1 and 1:2
stiochiometry (host/guest) were the most stable species in
the solution with the binding constant (log β) = 5.00 and 7.92,
respectively. The method of continuous variation (Job’s plot;
Supporting Information, Figures S31 and S32) was also used to
prove the 1:1 and 1:2 stiochiometry, respectively.28 The
detection limits of compounds 5 and 8 as fluorescent sensors
for analysis of the Hg2+ ions were found to be 5 × 10−6 and 7 ×
10−7 M, respectively, which were sufficiently low for detection
of the submillimolar concentrations of Hg2+ ions, as found in
many chemical systems. To test if the proposed complex could
be reversed, we also carried out a reversibility experiment. The
CONCLUSION
■
In conclusion, new rhodamine−pentaquinone dyad 5 and
rhodamine−pentaquinone−rhodamine triad 8 have been
synthesized that show TBET in the presence of Hg2+ ions in
mixed aqueous solution. Complexation of the Hg2+ ion opens
the spirolactam ring of rhodamine moieties to give specific
color change as well as fluorescence enhancement at 572 and
582 nm, respectively. In addition, in vitro properties of
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compound 8 showed good selectivity toward Hg2+ ions with
“on” fluorescence response.
dried over anhydrous Na2SO4. The organic layer was evaporated under
reduced pressure, and the crude product was purified by column
chromatography (EtOAc/hexane, 1:4, v/v) to give 70 mg (34%) of
1
compound 5 as a yellow solid. Mp: >250 °C. H NMR (300 MHz,
EXPERIMENTAL SECTION
■
CDCl3, ppm): δ 1.16 (t, 12H, J = 7.05, CH3), 3.29−3.37 (q, 8H,
NCH2), 6.28−6.36 (m, 4H, ArH), 6.69 (d, 2H, J = 9.3, ArH), 7.08 (d,
2H, J = 7.5, ArH), 7.17 (d, 1H, J = 6.6, ArH), 7.50−7.53 (m, 4H,
ArH), 7.68−7.70 (m, 2H, ArH), 7.71−7.82 (m, 1H, ArH), 8.02−8.16
(m, 5H, ArH), 8.88−8.92 (m, 4H, ArH). 13C NMR (75.45 MHz,
CDCl3, cm−1): δ 12.56, 44.29, 67.50, 97.88, 108.25, 123.40, 123.99,
127.06, 127.47, 128.18, 128.74, 128.82, 129.32, 129.55, 129.73, 129.99,
130.28, 130.48, 130.57, 130.72, 133.01, 134.02, 135.08, 135.13, 135.35,
148.87, 153.07, 167.93, 182.67. TOF-ES+-MS: m/z 824.3 [(M + 1)+].
Anal. Calcd for C56H45N3O4: C, 81.63; H, 5.50; N, 5.10. Found: C,
81.78; H, 5.35; N, 5.25.
2-(N-Phenylacetamide)-6,13-pentacenequinone (6). To a
solution of 3 (0.03 g, 0.075 mmol) in acetic acid (5.0 mL) was
added an excess of acetic anhydride in ice-cold conditions, and the
resulting reaction mixture was stirred at room temperature for 2 h. The
reaction mixture was then poured into ice-cold water for precipitation.
The precipitate thus formed was filtered, washed with water, and dried
to give 32 mg (91%) of compound 6 as a yellow solid. Mp: >250 °C.
1H NMR (300 MHz, CDCl3, ppm): δ 2.10 (s, 3H, CH3), 7.73−7.87
General Information. All reagents were purchased from Aldrich
and were used without further purification. THF (AR grade) was used
to perform analytical studies. UV−vis spectra were recorded on a
Shimadzu UV-2450 spectrophotometer with a quartz cuvette (path
length, 1 cm). The fluorescence spectra were recorded with a Varian
Cary Eclipse spectrofluorimeter. 1H and 13C NMR spectra were
recorded on a JEOL-FT NMR-AL 300 MHz using CDCl3 as the
solvent and tetramethylsilane (SiMe4) as internal standards. Data are
reported as follows: chemical shifts in ppm (δ), multiplicity (s =
singlet, d = doublet, q = quartet, br = broad singlet, m = multiplet, dd
= doublet of doublet), coupling constants (Hz), integration, and
interpretation. Silica gel 60 (60−120 mesh) was used for column
chromatography. The fluorescence quantum yield29 was determined
using optically matching solutions of rhodamine B (Φfr = 0.65 in
ethanol) as standards at an excitation wavelength of 540 nm, and the
quantum yield is calculated using the equation
−A L
s s
−A L
2
2
r
r
Φ
= Φ × 1 − 10
fr
/1 − 10
× N /N × D /D
s r s r
fs
Φfs and Φfr are the radiative quantum yields of the sample and
reference, respectively, As and Ar are the absorbances of the sample
and reference, respectively, Ds and Dr are the respective areas of
emission for the sample and reference, respectively, Ls and Lr are the
lengths of the absorption cells of the sample and reference,
respectively, and Ns and Nr are the refractive indices of the sample
and reference solutions (pure solvents were assumed), respectively.
Procedure for Metal-Ion Sensing. Solutions of compounds 5, 6,
8, 9, and 11 and metal perchlorates were prepared in THF/H2O
(9.5:0.5, v/v) buffered with HEPES, pH = 7.0. In titration
experiments, each time a 3 mL solution of 5/8 (10 μM) was filled
in a quartz cuvette (path length, 1 cm) and metal ions were added into
the quartz cuvette by using a micropippet. For fluorescence
measurements, excitation was provided at 360 nm, and emission was
collected from 350 to 650 nm.
(m, 6H, ArH), 8.08−8.11 (m, 1H, ArH), 8.31−8.35 (m, 2H, ArH),
8.39 (s, 1H, ArH), 8.59 (s, 1H, ArH), 8.90−8.95 (m, 4H, ArH); Anal.
Calcd for C30H19NO3: C, 81.62; H, 4.34; N, 3.17. Found C, 81.49; H,
4.45; N, 3.39. IR (KBr, cm−1): νmax 1674 (CO).
2,3-Bis(di--N-phenylrhodamine B)-6,13-pentacenequinone
(8). The acid chloride 424 (0.11 g, 0.22 mmol) was added to the
stirred solution of diamine 7 (0.05 g, 0.10 mmol) in dry THF and
triethylamine. The reaction mixture was stirred overnight at room
temperature. The reaction mixture was treated with water, extracted
with dichloromethane, and dried over anhydrous Na2SO4. The organic
layer was evaporated under reduced pressure, and the crude product
was purified by column chromatography (EtOAc/hexane, 1:1, v/v) to
give 138 mg (50%) of compound 8 as a yellow solid. Mp: >250 °C. 1H
NMR (300 MHz, CDCl3, ppm): δ 1.11 (t, 24H, J = 6.9, CH3), 3.26−
3.33 (q, 16H, NCH2), 6.28−6.31 (m, 8H, ArH), 6.61 (d, 4H, J = 9.3,
ArH), 6.83 (d, 4H, J = 8.4, ArH), 6.94 (d, 4H, J = 8.4, ArH), 7.15−
7.17 (m, 2H, ArH), 7.49−7.51 (m, 4H, ArH), 7.68−7.71 (m, 2H,
ArH), 7.96 (s, 2H, ArH), 8.0−8.02 (m, 2H, ArH), 8.10−8.13 (m, 2H,
ArH), 8.86 (s, 2H, ArH), 8.93 (s, 2H, ArH). 13C NMR (75.45 MHz,
CDCl3, cm−1): δ 13.01, 44.72, 68.07, 98.38, 107.09, 108.62, 123.74,
124.37, 126.45, 128.49, 128.97, 129.81, 130.01, 130.32, 130.43, 130.98,
131.06, 131.23, 132.04, 133.25, 134.69, 135.58, 136.99, 137.82, 142.37,
149.18, 153.55, 153.80, 167.97, 183.16. MALDI-TOF: m/z 1338.36
[(M)+]. Anal. Calcd for C90H78N6O6: C, 80.69; H, 5.87; N, 6.27.
Found: C, 80.43; H, 5.65; N, 6.56.
Procedure for Fluorescence Imaging. The prostate cancer
(PC3) cell lines were incubated with receptor 8 [1.0 μM in THF/H2O
(9.5:0.5, v/v) buffered with HEPES, pH = 7.0] in a RPMI-1640
medium for 20 min at 37 °C and washed with a PBS buffer (pH = 7.4)
to remove excess receptor 8. The cells were then treated with mercury
perchlorate (30.0 μM) in the RPMI-1640 medium, incubated again for
20 min at 37 °C, and washed with a PBS buffer. The cells were imaged
by a confocal fluorescence microscope with excitation wavelengths of
488 and 405 nm.
Compounds 2,22 7,21 and 1125 were synthesized according to the
literature procedure.
2,3-Bis(di-N-phenylacetamide)-6,13-pentacenequinone (9).
To a solution of 7 (0.03 g, 0.06 mmol) in acetic acid (5.0 mL) was
added an excess of acetic anhydride in ice-cold conditions, and the
resulting reaction mixture was stirred at room temperature for 2 h. The
reaction mixture was then poured into ice-cold water for precipitation.
The precipitate thus formed was filtered, washed with water, and dried
to give 0.02 g (57%) of compound 9 as a yellow solid. Mp: >250 °C;
1H NMR (300 MHz, CDCl3, ppm): δ 2.20 (s, 6H, CH3), 7.20 (d, 4H,
J = 6.03, ArH), 7.44 (d, 4H, J = 6.27, ArH), 7.73 (br, 2H, ArH), 8.13
(br, 4H, ArH), 8.97 (s, 4H, ArH); TOF-ES+-MS: 575.3 [(M + 1)+].
Anal. Calcd for C38H26N2O4: C, 79.43; H, 4.56; N, 4.88. Found: C,
79.1; H, 4.24; N, 4.95. IR (KBr, cm−1): νmax 1666 (CO).
2-(4-Aminophenyl)-6,13-pentacenequinone (3). To a solution
of 1 (0.6 g, 1.55 mmol) and 2 (0.41 g, 1.87 mmol) in dry dioxane were
added K2CO3 (0.42 g, 3.1 mmol), H2O (10 mL), and [Pd-
(Cl)2(PPh3)2] (0.271 g, 0.25 mmol) under N2. The mixture was
degassed and purged with N2 for 15 min. The mixture was refluxed
overnight. The dioxane was then removed under vacuum, and the
residue so obtained was treated with water, extracted with dichloro-
methane, and dried over anhydrous Na2SO4. The organic layer was
evaporated, and the compound was purified by column chromatog-
raphy using CHCl3/MeOH (9.5:0.5, v/v) as an eluent to give 0.20 g
1
(32%) of compound 3 as a red solid. Mp: >260 °C. H NMR (300
MHz, CDCl3, ppm): δ 3.89 (br, 2H, NH2), 6.84 (d, 2H, J = 8.4, ArH),
7.59−7.73 (m, 4H, ArH), 7.94 (d, 1H, J = 7.8, ArH), 8.12−8.16 (m,
3H, ArH), 8.24 (s, 1H, ArH), 8.93−8.98 (m, 4H, ArH). TOF-ES+-MS:
m/z 400.12 [(M + 1)+]. Anal. Calcd for C28H17NO2: C, 84.19; H,
4.29; N, 3.51. Found: C, 83.95; H, 4.32; N, 3.33.
ASSOCIATED CONTENT
■
S
* Supporting Information
2-(N-Phenylrhodamine B)-6,13-pentacenequinone (5). The
acid chloride 424 (0.15 g, 0.3 mmol) was added to the stirred solution
of 3 (0.1 g, 0.25 mmol) in dry THF and triethylamine. The reaction
mixture was stirred overnight at room temperature. The reaction
mixture was treated with water, extracted with dichloromethane, and
Characterization data including H and 13C NMR, IR, MS,
1
UV−vis, fluorescence spectra and energy transfer efficiency
calculation. This material is available free of charge via the
Internet at http://pubs.acs.org.
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dx.doi.org/10.1021/ic201990q | Inorg. Chem. 2012, 51, 2150−2156

Inorganic Chemistry
Article
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AUTHOR INFORMATION
Corresponding Author
*E-mail: vanmanan@yahoo.co.in (V.B.), mksharmaa@yahoo.
co.in (M.K.). Tel.: 91182258802 ext. 3302.
■
ACKNOWLEDGMENTS
■
We are thankful to CSIR (New Delhi, India) for financial
support [Reference No. CSIR Scheme 01 (2167)07/EMR-II].
We are also thankful to Central Drug Research Institute
(CDRI), Lucknow, India, for ES+-MS spectra and to Guru
Nanak Dev University for providing research facilities. Roopa is
thankful to CSIR (New Delhi, India) for a senior research
fellowship.
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