Solvent-controlled metal ion binding selectivity and anion interaction of the acridinedione-based heteroditopic host

Article abstract of DOI:10.1021/jp107717z

Acridinedione-based heteroditopic hosts 1a-c, which contain a flexible oxyethylene moiety as a metal ion binding site and amino hydrogen as an anion receptor unit, were synthesized and characterized. Metal ion and anion binding studies were carried out in different solvents to understand the solvent-induced selectivity of the metal ion binding and anion interactions. In acetonitrile, Ca²⁺ alone shows a binding with the oxyethylene unit and -OCH ₃ group, which results in a fluorescence enhancement without any spectral shift due to the suppression of the photoinduced electron transfer (PET) process. In chloroform, in addition to Ca²⁺, Na⁺ also shows a fluorescence enhancement with a 14 nm red shift in the emission maximum. ¹H NMR titration studies confirm that the addition of Na⁺ does not involve binding at the oxyethylene moiety, while it shows a binding at the acridinedione carbonyl group and -OCH₃ group, which results in the red shift and fluorescence enhancement, respectively. In the anion interaction studies, F^- shows a neat deprotonation in polar aprotic solvents, whereas in chloroform, it shows a H-bond complex due to the lower anion desolvation energy. The present study clearly signifies the role of solvent in metal ion selectivity, which is an often unnoticed parameter in metal ion binding.

Full text of DOI:10.1021/jp107717z

84
J. Phys. Chem. B 2011, 115, 84–92
Solvent-Controlled Metal Ion Binding Selectivity and Anion Interaction of the
Acridinedione-Based Heteroditopic Host
P. Ashokkumar, V. T. Ramakrishnan, and P. Ramamurthy*
National Centre for Ultrafast Processes, UniVersity of Madras, Taramani Campus, Chennai- 600 113, India
ReceiVed: August 15, 2010; ReVised Manuscript ReceiVed: NoVember 21, 2010
Acridinedione-based heteroditopic hosts 1a-c, which contain a flexible oxyethylene moiety as a metal ion
binding site and amino hydrogen as an anion receptor unit, were synthesized and characterized. Metal ion
and anion binding studies were carried out in different solvents to understand the solvent-induced selectivity
of the metal ion binding and anion interactions. In acetonitrile, Ca2+ alone shows a binding with the oxyethylene
unit and -OCH3 group, which results in a fluorescence enhancement without any spectral shift due to the
suppression of the photoinduced electron transfer (PET) process. In chloroform, in addition to Ca2+, Na+
also shows a fluorescence enhancement with a 14 nm red shift in the emission maximum. 1H NMR titration
studies confirm that the addition of Na+ does not involve binding at the oxyethylene moiety, while it shows
a binding at the acridinedione carbonyl group and -OCH3 group, which results in the red shift and fluorescence
enhancement, respectively. In the anion interaction studies, F- shows a neat deprotonation in polar aprotic
solvents, whereas in chloroform, it shows a H-bond complex due to the lower anion desolvation energy. The
present study clearly signifies the role of solvent in metal ion selectivity, which is an often unnoticed parameter
in metal ion binding.
I. Introduction
in terms of similarities between metal ion size and the size of
the inner hole of the crown ether and also charge density of the
metal ion.13 Whereas, in pseudocyclic systems, combined effects
of the size of the pseudocrown ether cavity, number of oxygen
atoms, the charge density, and the coordination number of the
cations play a considerable role in the selectivity of metal ion
binding.8b Unlike cyclic crown ethers which are rigid, the acyclic
polyether changes its conformation from linear to pseudocyclic
structure upon variation in the solvent polarity or complex
formation with metal ions.7-9 Solvent-dependent conformational
changes in poly(oxoethylene) linkers were investigated by
theoretical as well as experimental methods.14 It is reported that
the carbon-carbon bond of the -OCH2CH2O- unit exists
predominantly in the gauche conformation in a high polar
medium but shifts to the trans conformation in a low polar
medium.15 Accordingly, it has been suggested that the poly-
(oxoethylene) chain forms a pseudo lipophilic hydrocarbon core
in low polar media, and in contrast, a pseudo hydrophilic
electron-rich cavity is formed in highly polar media. However,
until now, the effect of solvent on metal ion binding selectivity
of the acyclic polyether receptors has scarcely been investigated.
Herein, we report the solvent-controlled metal ion selectivity
of an ADD-linked ditopic receptor, which contains acyclic
polyether as a metal ion binding site and amino hydrogen as an
anion receptor.
Anion recognition by neutral receptors through H-bonding
interactions stays at the basis of many important subdisciplines
of supramolecular chemistry.16 Both H-bonding and deproto-
nation processes are involved in anion interaction, which
depends on the basicity of the anion and acidity of the proton,17
which in turn is determined by the solvent employed in the
investigation. If the solvent polarity is high enough, it will lead
to deprotonation by highly basic anions due to the higher anion
desolvation energy.17d To better understand the role of solvent
on the anion recognition ability of amino hydrogen, we have
carried out anion binding studies of a heteroditopic host in
Molecular recognition is the first step in the design of
molecular sensors for detection of ions or neutral species in
organic or aqueous media.1 In the presence of target species,
the molecular sensor is designed to exhibit a physical response
which must be easily measured. Among various detection
techniques, fluorescence makes the best choice since it offers
high sensitivity, fast response, easy online detection, and local
observation by fluorescence imaging with subnanometer spatial
resolution.2 Furthermore, remote sensing is also possible by
means of optical fibers with a molecular sensor immobilized at
the tip.3 High selectivity is a hallmark of biological receptors
and has always been a most important aim of synthetic
supramolecular chemistry.4 Since Pedersen’s initial observa-
tions,5 on the affinity of macrocyclic polyethers to alkali and
alkaline earth metal ions, several new types of ligands of similar
structure have been synthesized to increase the stability of the
cation-crown complex or to improve the cation selectivity of
the ligand. Even though a large number of cyclic crown-ethers
and related macrocyclic-based chemosensors were reported, the
corresponding acyclic polyether-based sensors are relatively
rare.6-11 Among the early reports on acyclic polyether-based
receptors, reported by Nakamura et al. and Ajayaghosh et al.,
the signaling of the binding event was achieved by excimer
formation,7 exciton interaction,8 the twisted intramolecular
charge transfer (TICT)9 process, and the electron10 or energy
transfer11 process between two terminal chromophore units.
Recently, we have reported a photoinduced electron transfer
(PET) based acyclic polyether linked acridinedione (ADD)
derivative as a specific Ca2+ sensor.12
Cyclic crown ethers display a wide range of binding
specificities, and the association properties of crown ethers with
alkali and alkaline earth metal ions have been mainly described
* Corresponding author. Fax: 091-44-24546709. Tel.: 091-44-24540962.
E-mail: prm60@hotmail.com.
10.1021/jp107717z  2011 American Chemical Society
Published on Web 12/15/2010

Acridinedione-Based Heteroditopic Host
J. Phys. Chem. B, Vol. 115, No. 1, 2011 85
SCHEME 1: Synthetic Routes to Compounds 1a-c
different solvents. Simultaneous binding of both metal ions and
anions can be achieved with the use of a heteroditopic host,
which contains two different receptors for two kinds of ions.18
Since the present system consists of two distinct binding sites
for metal ions and anions, we have also carried out the effect
of competing anion on the binding ability of metal ions and
vice versa.
prepared and mixed to standard volumes and proportions so
that the total concentration remained constant. ∆F values were
calculated by measuring the fluorescence intensity of bichro-
mophores in the absence (F0) and presence (F) of the corre-
sponding concentration of metal ions or anions. Subsequently,
∆F values were plotted for the corresponding metal ion against
mole fraction (xi ) [bichromophore]/([bichromophore] + [metal
ion])).
General Procedure of Synthesis of 1a-c. The synthesis of
ditopic hosts 1a-c is outlined in Scheme 1.12a The acridinedione
3 was obtained by refluxing tetraketone with ammonium acetate
in acetic acid. Reaction of acridinedione with the corresponding
ditosylate in the presence of K2CO3 yielded bisacridinediones
1a-c, which were characterized by spectral analyses.
9,9′-(4,4′(2,2′(Ethylenedioxy-bis(ethyleneoxy)))-bis(3-meth-
oxyphenyl))bis(3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-
1,8(2H,5H) Acridinedione) (1a). Yield, 62%; mp, 143-145 °C;
FTIR (KBr) (cm-1) 3194 (NH), 1639 (CO), 1485 (-CdC-);
1H NMR (500 MHz; CDCl3:DMSO-d6, TMS) δ (ppm) ) 8.77
(2H, s, NH), 6.90 (2H, d, J ) 1.8 Hz, ArH), 6.70-6.73 (4H,
m, ArH), 4.93 (2H, s, C9-H), 4.04 (4H, t, OCH2), 3.76-3.78
(10H, m, OCH2 and OCH3), 3.66 (4H, s, OCH2), 2.39 and 2.10
(8H, 2 d, J ) 16.8 Hz, C2 & C7 -CH2), 2.24 and 2.15 (8H,
2 d, J ) 16.0 Hz, C4 & C5 -CH2), 1.07 and 0.95 (24H, 2s,
gem-dimethyl); 13C NMR (125 MHz; CDCl3, DMSO-d6, TMS)
δ (ppm) ) 195.6 (CO), 149.1 (CdC), 148.7 (C), 145.9 (C),
140.6 (C), 119.7 (CH), 113.3 (CH), 112.5 (CH), 112.4 (CdC),
70.4 (CH2), 69.4 (CH2), 68.3 (CH2), 55.6 (CH3), 50.7 (CH2),
40.1 (CH2), 32.6 (C), 32.3 (CH), 29.5 and 26.8 (CH3); MS (ESI)
m/z ) 906.19 [M + 1]+. Elemental analysis (%) calcd for
C54H68N2O10 (905.12): C, 71.66; H, 7.57; N, 3.09. Found: C,
71.72; H, 7.54; N, 3.06.
II. Experimental Methods
All starting materials and reagents were purchased from
commercial suppliers and used without further purification. The
solvents used for the spectral studies were of HPLC grade.
Absorption spectra were recorded on an Agilent 8453 diode
array spectrophotometer. Emission spectra were recorded on a
HORIBA JOBIN YVON Fluoromax 4P spectrophotometer. The
fluorescence quantum yield was determined by exciting the
sample at 366 nm with the use of quinine sulfate as the standard
(φf ) 0.546 in 0.1 N H2SO4). Fluorescence decays were recorded
by using an IBH time-correlated single-photon counting tech-
nique as reported elsewhere.19 NMR spectra were recorded on
JEOL 500 MHz and Bruker Avance III 500 MHz instruments
in deuterated solvents as indicated. Chemical shifts are reported
in parts per million, and coupling constants (JX-X′) are reported
in hertz. ESI-MS were performed on an ECA LCQ Thermo
system with ion-trap detection in positive and negative mode.
Elemental analyses (C, H, and N) were taken on a Euro EA
Elemental analyzer.
General Procedure for Ion Binding Studies. Bichromophores
1a-c were dissolved in the respective solvents. Metal perchlo-
rate stock solutions (9.11 × 10-3 M) were prepared in
acetonitrile (due to the solubility reason), and the titrations were
carried out by adding small volumes (1-5 µL) of the metal ion
to the bichromophore (3.5 mL) in a quartz cuvette. After the
addition of metal ion to the cuvette, using a microliter syringe,
the solution was shaken well and kept for 1 min before recording
any measurements. To clarify the effect of added acetonitrile
to the chloroform solution of bichromophore 1a-c, we have
also purposely carried out a blank experiment by adding 1-5
µL of acetonitrile to the chloroform solution of dyes. The
addition of acetonitrile did not show any change in all the
experiments, which shows the absence of any influence by
9,9′-(4,4′(2,2′(Oxybis(3-oxapentamethleneoxy)))-bis(3-meth-
oxyphenyl))bis(3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-
1,8(2H,5H) Acridinedione) (1b). Yield, 58%; mp 129-131 °C;
FTIR (KBr) (cm-1) 3194 (NH), 1640 (CO), 1486 (-CdC-);
1H NMR (500 MHz; CDCl3, TMS) δ (ppm) ) 8.74 (2H, s,
NH), 6.91 (2H, d, J ) 1.8 Hz, ArH), 6.64-6.70 (4H, m, ArH),
4.98 (2H, s, C9-H), 4.02 (4H, t, OCH2), 3.70-3.76 (10H, m,
OCH2 and OCH3), 3.59-3.64 (4H, m, OCH2), 3.57 (4H, s,
OCH2), 2.30 and 2.16 (8H, 2 d, J ) 16.0 Hz, C2 & C7 -CH2),
2.27 and 2.11 (8H, 2 d, J ) 16.5 Hz, C4 & C5 -CH2), 1.01 and
0.91 (24H, 2s, gem-dimethyl); 13C NMR (500 MHz; CDCl3,
TMS) δ (ppm) ) 195.9 (CO), 149.4 (CdC), 148.7 (C), 146.3
(C), 140.7 (C), 119.8 (CH), 113.2 (CH), 113.0 (CH), 112.4
(CdC), 70.6 (CH2), 70.2 (CH2), 68.3 (CH2), 55.8 (CH3), 50.8
(CH2), 40.6 (CH2), 32.5 (C), 32.4 (CH), 29.5 and 27.0 (CH3);
MS (ESI) m/z ) 950.23 [M + 1]+. Elemental analysis (%) calcd
for C56H72N2O11 (949.18): C, 70.86; H, 7.65; N, 2.95. Found:
C, 70.94; H, 7.67; N, 2.92.
1
the added solvent. For H NMR titration studies, 1-7 µL of
metal ions in CD3CN was added to a 4.0 mM solution of
bichromophores in CDCl3. Anions were used as their tetrabu-
tylammonium salts. Stock solutions of anions (0.07 M for
experiments in low polar solvents and 0.24 M for polar solvents)
were prepared in acetonitrile. Complex stoichiometry has been
determined from the continuous variation technique (Job plot),20
based on the difference in fluorescence intensity ∆F (∆F ) F0
- F) of bichromophores observed in the presence of metal ions
or anions. Equimolar solutions of 1a and metal ions were

86 J. Phys. Chem. B, Vol. 115, No. 1, 2011
Ashokkumar et al.
9,9′-(4,4′(2,2′(Ethylenedioxybis(3-oxapentamethyleneoxy)))-
bis(3-methoxyphenyl))bis(3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahy-
dro-1,8(2H,5H) Acridinedione) (1c). Yield, 57%; mp 117-119
°C; FTIR (KBr) (cm-1) 3193 (NH), 1640 (CO), 1486 (-CdC-);
1H NMR (500 MHz; CDCl3, TMS) δ (ppm) ) 8.77 (2H, s,
NH), 6.98 (2H, d, J ) 1.8 Hz, ArH), 6.72-6.78 (4H, m, ArH),
5.04 (2H, s, C9-H), 4.04 (4H, t, OCH2), 3.78-3.81 (10H, m,
OCH2 and OCH3), 3.64-3.67 (8H, m, OCH2), 3.63 (4H, s,
OCH2), 2.34 and 2.21 (8H, 2 d, J ) 16.5 and 16.0 Hz C2 & C7
-CH2), 2.23 and 2.16 (8H, 2 d, J ) 16.5 Hz, C4 & C5 -CH2),
1.06 and 0.96 (24H, 2s, gem-dimethyl); 13C NMR (500 MHz;
CDCl3, TMS) δ (ppm) ) 195.6 (CO), 149.0 (CdC), 148.7 (C),
145.9 (C), 140.6 (C), 119.7 (CH), 113.2 (CH), 112.6 (CH), 112.4
(CdC), 70.4 (CH2), 70.2 (CH2), 69.3 (CH2), 68.3 (CH2), 55.6
(CH3), 50.8 (CH2), 40.5 (CH2), 32.6 (C), 32.3 (CH), 29.5 and
26.9 (CH3); MS (ESI) m/z ) 994.27 [M + 1]+. Elemental
analysis (%) calcd for C58H76N2O12 (993.23): C, 70.14; H, 7.71;
N, 2.82. Found: C, 70.17; H, 7.68; N, 2.80.
Figure 1. Fluorescence spectra of 1a (11 µM) upon addition of Ca2+
in acetonitrile; λex ) 363 nm.
III. Results and Discussion
a. Photophysical Studies. Compounds 1a-c show the
absorption and emission maxima around 361 and 426 nm,
respectively, which are assigned to the intramolecular charge
transfer (ICT) from the ring nitrogen atom to the ring carbonyl
center within the ADD moiety. Lower fluorescence quantum
yield (φf ) 0.039 ( 5%) and lifetime (biexponential: 0.26 (
0.01 ns (22.3%) and 2.40 ( 0.03 ns (77.7%)) of 1a-c in
acetonitrile, compared to the constituent fluorophore without
the electron donor -OCH3 group (φf ) 0.21 ( 2% and τf )
2.01 ( 0.03 ns in acetonitrile), are attributed to the intramo-
lecular PET process through space from the electron-rich
-OCH3 group to the relatively electron-deficient excited state
of the ADD fluorophore.12,19 The bichromophores show rela-
tively lower quantum yield (φf ) 0.018 ( 5%) and lifetime
(biexponential: 0.14 ( 0.01 ns (68.5%) and 2.29 ( 0.03 ns
(31.5%)) in chloroform compared to acetonitrile due to the
change in the polarity of the medium. The earlier reported12a
bichromophores having a -CH3 group instead of H at the ring
nitrogen (ICT donor) showed relatively higher quantum yield
and lifetime due to the enhanced ICT from the electron-releasing
-CH3 group.
Figure 2. Metal ion selectivity plot of 1a (11 µM) upon addition of
different metal ions in acetonitrile; λex ) 363 nm.
b. Metal Ion Binding Studies. Among the metal ions
investigated in acetonitrile, the oxyethylene receptor shows
selective binding with Ca2+. Addition of Ca2+ results in the 3
nm red-shifted absorption maximum (Figure S1, Supporting
Information) and 6-fold fluorescence enhancement without any
spectral shift as shown in Figure 1. Addition of other metal
ions like Na+, K+, and Mg2+ did not show any considerable
change in both absorption and emission spectra, which is evident
from the metal ion selectivity plot (Figure 2). Binding of Ca2+
at the oxyethylene moiety and -OCH3 group suppresses the
PET process and results in the fluorescence enhancement
without any spectral shift as observed earlier for the similar
compound.12
In chloroform, both Na+ and Ca2+ show spectral changes,
which were analyzed thoroughly. Addition of Ca2+ to 1a in
chloroform did not show any considerable shift in the absorption
maximum (Figure S2, Supporting Information). The correspond-
ing fluorescence spectrum shows 8.5-fold enhancement in the
intensity without any spectral shift as shown in Figure 3. The
absence of any spectral shift indicates that the Ca2+ did not
involve any interaction with the carbonyl group of the ADD
moiety, which is an acceptor in the ICT electronic transition.
On the other hand, the addition of Na+ to chloroform solution
Figure 3. Fluorescence spectra of 1a (11 µM) upon addition of Ca2+
in chloroform; λex ) 363 nm.
of 1a shows a 9 nm red-shifted absorption maximum with an
isosbestic point at 371 nm as shown in Figure 4a. The existence
of a clear isosbestic point indicates the presence of 1:1
complexation with Na+. The corresponding emission spectrum
(Figure 4b), when excited at its isosbestic point, shows a 14
nm red-shifted emission maximum together with an enhance-
ment in the fluorescence intensity. Observation of a red shift in
both absorption and emission spectra indicates the involvement
of the ADD carbonyl group in Na+ binding. To determine the
stoichiometry of the complexes, a Job’s plot20 has been carried
out from the changes in the fluorescence intensity. For both the
metal ions, the Job’s plot (Figure S3, Supporting Information)
showed the maximum changes, when the mole fraction of 1a
was around 0.5, which is a characteristic of a host-guest binding

Acridinedione-Based Heteroditopic Host
J. Phys. Chem. B, Vol. 115, No. 1, 2011 87
Figure 6. Fluorescence decay profiles of 1a (11 µM) upon addition
of Na+ in chloroform; λex ) 371 nm and λem ) 440 nm. (a) Laser
profile.
shows a biexponential decay with a PET quenched lifetime of
0.14 ns (68.1%) and 2.25 ns (31.9%) in chloroform. Addition
of Na+ (Figure 6) to the chloroform solution of 1a shows a
gradual disappearance of a shorter-lifetime component with the
increase in the lifetime (2.25 to 3.09 ns) and amplitude of the
longer component. The disappearance of the shorter lifetime
component confirms the suppression of the PET process in the
Na+-bound complex of 1a. We observe a single exponential
decay with longer lifetime (3.09 ns) on complete complex
formation with Na+. Addition of Ca2+ (Figure S6, Supporting
Information) also shows a similar behavior with the longer
lifetime being 2.86 ns.
To unravel the binding mode of metal ions, 1H NMR titration
studies were carried out in CDCl3. Addition of Na+ (Figure 7)
to the bichromophore 1a did not show any significant change
in the oxyethylene proton peaks, which shows the absence of
binding at the oxyethylene moiety. In contrast, the -OCH3
proton peak i shifted (∆δ ) 0.60) to higher magnetic field due
to the decreased deshielding effect of the metal-coordinated
oxygen atom of the -OCH3 group. The aromatic proton peak
g shifted (∆δ ) 0.16) toward a lower magnetic field due to the
decreased electron density at the oxygen atom of the OCH3
group. Aromatic proton h nearest to the Na+ binding site shifted
(∆δ ) 0.26) to higher magnetic field due to the shielding effect
of the counteranion, perchlorate. The acridinedione proton peaks
k and l nearest to the ADD carbonyl group shifted (∆δ ) 0.14
and 0.35) to lower magnetic field due to the deshielding effect
of the electron-deficient carbonyl carbon. Binding of Na+ at
the carbonyl oxygen decreases the electron density on the
carbonyl group, which creates electron deficiency at the carbonyl
carbon. The above changes clearly confirm that the Na+ involves
binding at the -OCH3 and ADD carbonyl group as shown in
Scheme 2.
Figure 4. (a) Absorption and (b) fluorescence spectra of 1a (11 µM)
upon addition of Na+ in chloroform; λex ) 371 nm.
Figure 5. Metal ion selectivity plot of 1a (11 µM) upon addition of
different metal ions in chloroform; λex ) 371 nm.
in a 1:1 stoichiometry. Quantitative binding constant values
could not be obtained accurately due to the possibility of the
ion-pair formation in less polar organic solvents. Addition of
K+ and Mg2+ did not show any considerable change in both
absorption and emission spectra, which is clear from the plot
(Figure 5) of fluorescence intensity against the ratio of the metal
ion and 1a. Metal ion binding studies of other derivatives 1b
and 1c also showed behavior similar to that observed in 1a.
Similar metal ion binding behavior is observed in other less
polar solvents, namely, toluene (Figure S4, Supporting Informa-
tion) and dichloromethane (Figure S5, Supporting Information),
which suggest that the polarity of the solvent plays a major
role in the metal ion interaction of acyclic polyether-based
receptors.
Binding of Na+ at the carbonyl group increases the ICT
character of the electronic transition, which results in the red
shift in both absorption and emission maxima. In addition,
binding at the -OCH3 group suppresses the PET process and
results in the fluorescence enhancement.
Addition of Ca2+ (Figure 8) to bichromophore 1a21 shows a
lower magnetic field shift of the oxyethylene proton peaks c
(∆δ ) 0.10) and d (∆δ ) 0.03) which indicates the binding of
Ca2+ at the middle oxygen atoms of the oxyethylene receptor.
The observed higher magnetic field shift of -OCH3 proton peak
i (∆δ ) 0.09) indicates the binding at the OCH3 group. Aromatic
protons show a similar shift as observed in the case of Na+;
The complex formation between metal ions and 1a has also been
investigated by time-resolved fluorescence studies. Bichromophore 1a

88 J. Phys. Chem. B, Vol. 115, No. 1, 2011
Ashokkumar et al.
1
Figure 7. Partial H NMR spectra of 1a in CDCl3 upon addition of (i) 0, (ii) 0.2, (iii) 0.4, (iv) 0.6, (v) 0.8, and (vi) 1.0 equiv of Na+.
SCHEME 2: Binding Mode of Na+ with 1a in
Chloroform
H-bonding interaction of AcO- with an amino hydrogen. The
corresponding fluorescence spectrum shows a 17 nm red-shifted
emission peak with fluorescence enhancement as shown in
-
Figure 9. Addition of H2PO4 also shows a similar behavior
with a 6 nm red-shifted absorption maximum and a 12 nm red-
shifted emission maximum with fluorescence enhancement
(Figure S8, Supporting Information). A similar change has been
observed in polar aprotic solvents like acetronitrile, dimethyl
formamide, and dimethyl sulfoxide for AcO- and H2PO4-, but
the extent of red shift was found to be higher due to the
increased polarity of the medium.22
Addition of F- to an acetonitrile solution of 1a shows a
decrease in the absorbance at 365 nm with the simultaneous
however, the peaks are considerably broadened due to the
appearance of a new peak at 457 nm (Figure 10a). The
turbidity of the solution, and the extent of shift was found to
corresponding emission spectrum shows a decrease in the
be relatively low. The absence of any shift of acridinedione
1
emission intensity at a 427 nm peak with formation of a new
proton peaks k and l in the H NMR indicates the absence of
Ca2+ binding at the ADD carbonyl group. The absence of a red
shift in both absorption and emission spectral studies is in line
with the observed NMR spectral changes, which reveals that
the Ca2+ binds at the middle oxygen atoms of the oxyethylene
moiety and the -OCH3 group as shown in Scheme 3. Binding
of Ca2+ at the oxyethylene moiety and the -OCH3 group results
in the suppression of the PET process and fluorescence
enhancement without any spectral shift.12
emission peak at 505 nm (Figure 10b). As the smallest and
highly electronegative anion, F- deprotonates the amino hy-
drogen, which results in the formation of a new peak in both
absorption and emission spectra in polar aprotic solvents.22 In
contrast, the addition of F- to 1a in low polar aprotic solvents
like chloroform, dichloromethane, and toluene did not show any
new peak in both absorption and emission spectra. Addition of
F- to a chloroform solution of 1a shows a 17 nm red-shifted
absorption maximum as shown in Figure 11a. The corresponding
emission spectra show a 32 nm red-shifted emission maximum
together with fluorescence enhancement (Figure 11b). Binding
c. Anion Binding Studies. Addition of AcO- to a chloroform
solution of 1a shows an 8 nm red-shifted absorption maximum
(Figure S7, Supporting Information), which indicates the

Acridinedione-Based Heteroditopic Host
J. Phys. Chem. B, Vol. 115, No. 1, 2011 89
1
Figure 8. Partial H NMR spectra of 1a in CDCl3 upon addition of (i) 0, (ii) 0.2, (iii) 0.5, and (iv) 1.0 equiv of Ca2+
.
SCHEME 3: Binding Mode of Ca2+ with 1a in
Chloroform
ratio of 1:2 (dye:anion) were proved from a Job’s plot (Figure
S9, Supporting Information) analysis, which showed maximum
fluorescence intensity changes when the mole fraction of 1a
was around 0.33. The fluorescence spectral changes of 1a with
the addition of F- in toluene and DCM are presented in Figures
S10 and S11 (Supporting Information), respectively. The
observed difference in the spectral response of F- in chloroform
compared to acetonitrile may be due to the absence of
deprotonation of an amino hydrogen in chloroform. To confirm
this, we have carried out similar experiments in chloroform with
the addition of OH-, which is a well-known deprotonating agent.
Addition of OH- to a chloroform solution of 1a shows a
decrease in the absorbance at 361 nm with the formation of a
new absorption peak at 453 nm (Figure S12a, Supporting
Information). In emission spectral studies, also a new peak is
formed at 500 nm (Figure S12b, Supporting Information). This
clearly proves that the deprotonated form of dye absorbs and
emits at 453 and 500 nm, respectively, in chloroform. So, the
observed red shift in absorption and emission spectra of 1a with
F- is due to the H-bonding complexation with the amino
Figure 9. Fluorescence spectra of 1a (12 µM) upon addition of AcO-
in chloroform; λex ) 362 nm.
hydrogen. The decreased anion desolvation energy in chloroform
leads to the formation of a stable H-bond complex instead of
deprotonation by F-. H-bonding interaction of anions with
amino hydrogen increases the electron density of the ICT donor,
which in turn leads to the red shift in both absorption and
emission maxima together with an increase in the intensity. The
extent of red shift depends on the charge density of anions,
which accounts for the observed red shift for F-, AcO-, and
H2PO4-. So, by changing the polarity of the medium, one can
tune the anion-receptor H-bonding complexation and deproto-
nation of the receptor. Addition of other anions like Cl-, Br-,
-
I-, HSO4-, and ClO4 did not show any such changes in both
absorption and emission spectra.
The complexation between anions and 1a has also been
investigated by time-resolved fluorescence studies. Addition of

90 J. Phys. Chem. B, Vol. 115, No. 1, 2011
Ashokkumar et al.
Figure 11. (a) Absorption and (b) fluorescence spectra of 1a (11 µM)
upon addition of F- in chloroform; λex ) 368 nm.
Figure 10. (a) Absorption and (b) fluorescence spectra of 1a (12 µM)
upon addition of F- in acetonitrile; λex ) 395 nm.
F- to a chloroform solution of 1a shows a triexponential decay
as shown in Figure 12. Both the lifetime components of free
dye decrease with the formation of a longer-lifetime (6.59 ns)
component. We observed a single exponential decay with longer
lifetime component on complete complex formation with F-.
Addition of AcO- and H2PO4 also shows a similar behavior,
-
the longer lifetime being 4.69 and 4.18 ns, respectively. The
observed enhancement in the lifetime also follows the order of
the basicity of the anion.
d. Simultaneous Binding Studies of Metal Ions and
Anions. To understand the binding ability of an ion in the
presence of its competing ion, we have carried out two sets of
simultaneous binding studies of 1a with metal ions and anions.
First, we have titrated 1a with anion in the presence of metal
ion. Figure 13 shows the absorption and emission spectra of
the 1a·Na+ complex with the addition of F-. As can be seen,
the addition of 1 equiv of F- brings back the red-shifted
absorption maximum of 1a to 361 nm. This suggests the
sequestering of the metal ion from the binding site and the
existence of free dye 1a. However, when sequestering of
the metal ion is complete, further addition of F- binds at the
amino group and results in a red shift of the absorption
maximum as observed in the anion binding studies, which is
further confirmed by emission spectral studies. Up to 1 equiv
of F-, the emission intensity at 440 nm decreases and reaches
the intensity of the free 1a together with a shift from 440 to
426 nm; after the complete sequestering of Na+, further addition
leads to a 32 nm red-shifted emission peak with the increase in
the intensity. These results confirm the sequestering of Na+ by
Figure 12. Fluorescence decay profiles of 1a (11 µM) upon addition
of F- in chloroform; λex ) 370 nm and λem ) 460 nm. (a) Laser profile.
the added F- (Figure S13, Supporting Information). In the
second experiment, Na+ is titrated with the 1a·F- complex,
which also shows a similar sequestering process (Figure S14,
Supporting Information). Other metal Ca2+ ions and anions such
as AcO- and H2PO4- also show the sequestering process. Even
though 1a contains distinct binding sites for both metal ions
and anions, positive cooperative binding of both types of
analytes, as observed exploiting the allosteric effect,23 through
electrostatic interactions between the ion pairs24 or to the host
as an associated ion pair,25 is not possible in this case due to
the absence of close proximity of two binding sites.26 The weak
binding of the host-guest interaction and the strong ion-pairing
equilibria between two types of ions lead to the sequestering
of ions. In most organic solvents, the metal ions and anions do

Acridinedione-Based Heteroditopic Host
J. Phys. Chem. B, Vol. 115, No. 1, 2011 91
finding clearly illustrates that the effect of solvent on ion
recognition should also have been included in the design of
selective metal ion and/or anion receptors.
Acknowledgment. We thank the Department of Science and
Technology (DST), Government of India, for financial support
through SERC scheme project no. DST/SR/S1/PC-31/2005.
Financial support by DST-IRHPA is also gratefully acknowledged.
Supporting Information Available: NMR spectra of 1a;
absorption and emission spectra of 1a with various metal ions
and anions; Job’s plot for 1a toward metal ions and anions.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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