Highly sensitive and selective "turn-on" calcium cation sensing from a dendronic terthiophene tetraethylene glycol (TEG) molecule

Article abstract of DOI:10.1021/jp1068522

A molecule consisting of a branched or dendronic terthiophene with tetraethylene glycol moiety (3T5O) was synthesized for highly sensitive and selective calcium ion detection in solution. The sulfur atoms of the terthiophene dendron moiety contribute to t

Full text of DOI:10.1021/jp1068522

13084
J. Phys. Chem. B 2010, 114, 13084–13094
Highly Sensitive and Selective “Turn-on” Calcium Cation Sensing from a Dendronic
Terthiophene Tetraethylene Glycol (TEG) Molecule
Yushin Park, Dahlia C. Apodaca, Jonathan Pullen, and Rigoberto C. Advincula*
Department of Chemistry and Department of Chemical and Biomolecular Engineering, UniVersity of Houston,
Houston, Texas 77204-5003, United States
ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: September 7, 2010
A molecule consisting of a branched or dendronic terthiophene with tetraethylene glycol moiety (3T5O) was
synthesized for highly sensitive and selective calcium ion detection in solution. The sulfur atoms of the
terthiophene dendron moiety contribute to the metal-ligand complexation of calcium cations. The modification
of the terthiophene dendrons with a hydroxyl-terminated tetraethylene glycol (TEG) group produces a twisted
intramolecular charge transfer (TICT) event in which the terthiophene moiety is conformationally twisted,
affecting its chromophoric and fluorophoric activity. This twisted conformation resulted in an increase in
emission intensity at 320 nm, at the same time producing a blue shift in the absorption spectra. This process
enabled a “turn-on” system for calcium ion detection compared to most “turn-off” systems involving a
quenching route. The intensity of the emission is related to saturation of the terthiophene moiety at a
concentration of 3T5O:[Ca2+] 1:4, of ∼1 × 10-8 M, which indicates high sensitivity for calcium cations. No
response with other cations such as Na+, K+, and Mg2+ was observed, which also indicates high selectivity
for the system. Conformational analysis of the 3T5O and the twisted terthiophene dendron geometry was
done by semiempirical PM3 modeling. In an effort to distinguish specific interactions between monovalent
and divalent cations and the 3T5O, restricted Hartree-Fock level calculations using the STO-3G basis set
were also performed. The calculations correlated with the experimental results.
Introduction
rescence or “turn-off” is observed when the cation detection
causes structural changes from monomers to excimers.3,4a,20 In
addition, ionic bonds between cations and sensing molecules
can change the intramolecular structure of chromophores.
However, this “turn-off” system could lead to poorer sensitivity
for the reason that many conjugated polymers are prone to
fluorescence quenching brought about by aggregation due to
chromophore-chromophore interactions.45 Changes in the emis-
sion intensity would therefore be attributed to both sensing of
the ions and aggregation. Also, chemosensors based upon
photoinduced electron transfer (PET) often exhibit anomalous
emission properties displaying poor selectivity due to interfer-
ence of related cations.46 On the other hand, with this “turn-
on” sensing of calcium ions, sensitivity and selectivity are
greatly improved with the use of a terthiophene dendron
containing the TEG group. Apart from the expected twisting
on the conformation of the terthiophene units resulting in dual
fluorescence, the encapsulation of calcium ion by the hydroxyl
TEG moiety provides enhancement on both the absorption and
emission peaks brought about by a strong ionic bond and strong
ion dipole electrostatic forces of attraction between the calcium
cation and negative dipoles of the oxygen atoms of the TEG
moiety.
Synthesizing sensitive and selective chemosensors for specific
analyte detection is an interesting topic due to challenges in
molecular design and its applications in a number of research
fields.1 Moreover, the detection of Na+, K+, Mg2+, and Ca2+
cations using various chromophoric moieties under physiological
or buffered conditions is an active field in chemistry, biology,
and medicine.2-6 In addition, a sensing system that detects
calcium cations in the presence of sodium or potassium cations
is important since calcium cations trigger, regulate, and influence
a number of physiological processes.7,8 Most well-known ion-
detecting materials usually consist of ring-structured moieties
such as crown ethers,9-11 cyclodextrins,12,13 and calixarenes.14-16
In particular, the binding event between ether moieties in crown
ether derivatives and cations is essential for any sensing study.
Due to the size-dependent specific interaction with cations,
crown ether derivatives have been applied widely in biochem-
istry and environmental science. Recently, ethylene glycol
oligomeric derivatives have been reported for ion sensing. The
twisted ethylene glycol moieties act similarly to crown ethers
making complexes with cations at specific sizes.17,18 Calcium
ion specific detecting sensors have been synthesized by de Silva
et al. that utilized photoinduced electron transfer (PET).19
A
Nakamura and co-workers reported the detection of calcium
cations and alkali earth metal ions from a twisted intramolecular
charge transfer (TICT) behavior by fluorescence.21 The TICT
model was capable of explaining dual fluorescence along with
the large Stokes shift of the emission bands of p-(N,N-
dimethylamino)benzonitrile in a polar solvent.22 In another study
by Hu et al.,55 an examination was made to determine the effect
of temperature on the TICT process. It was reported that the
emission of a TICT luminogen is sensitive to temperature
variations. Results would show that by varying the temperature
typical design for these chemosensors involves a conformation
change that allows greater π-π stacking to occur, resulting in
the occurrence or extinction of chromophores which produce
different optical outputs. These output signals from the binding
event are achieved by an energy- or charge-transfer process that
occurs while the supramolecular stacking properties of the
chromophore are altered.2,7 Normally, self-quenching of fluo-
* To whom correspondence should be addressed. Phone: 713-743-1760.
E-mail: radvincula@uh.edu.
10.1021/jp1068522  2010 American Chemical Society
Published on Web 09/28/2010

“Turn-on” System for Calcium Ion Detection
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13085
Figure 1. (a) Chemical structure of the dendron 3T5O with (b) a ball-stick model highlighting the planar region of the chromophore sensor.
from -75 to 60 °C, a continuous increase in the emission
intensity accompanied by a gradual blue shift in the emission
maximum was observed. Since then, the TICT model has been
used to explain other dual fluorescence properties with similar
chemicals.23,24 This model was also used to elucidate the
“off-on” fluorescence output mechanism of 1-pyrenyl aromatic
amides.21 However, in order to utilize this model efficiently,
the complex formed between sensing materials and cations needs
to be designed carefully and parametrized.22a
in solution which would then require more Ca2+ ions before
saturation can be reached.53 Each emitting molecule may assume
a different twisted molecular conformation depending on the
following factors: (1) solvent effects; (2) substitution on the
side chain of the terthiophene dendron which could affect the
regioregularity of the polymer backbone; (3) diameter of the
cation; (4) number of ligand binding sites; and (5) the relative
cation to ligand cavity size ratio. It is highly probable that each
factor may counteract each other. The extent of which may be
translated to different absorption and emission characteristics.55
Along with this sensing via TICT state, the solvating power
of the solvent to stabilize the twisted molecular conformation
can intensify both absorption and emission with incremental
changes on the concentration of the calcium ions. The correct-
ness of this molecular design is validated by our experimental
results and the semiempirical calculations as described in this
paper.
There are limited compounds that possess dual fluorescence
which can be used as molecular probes to sense changes in the
microscopic microenvironment. A signaling unit which produces
a sufficiently strong and specific spectroscopic change is an
essential criterion for designing chemosensors in solution.25 The
molecule shown in Figure 1 was designed and targeted to
assemble incremental changes in absorbance and fluorescence
activity in the presence of calcium cations. Normally, when the
primary chromophores are within a highly π-conjugated system,
the other end group is modified with a different conjugated
system that develops π-π stacking. For example, it has been
reported that poly(p-xylylene) moieties enable metal-ligand
cross-linking with metal ions.26 In our case, the terthiophene
dendron was modified with the TEG moiety to facilitate ion
capture. Recently, thiophene dendrons were synthesized in our
group due to their strong absorption properties in the 250-400
nm wavelength region. Another interesting feature of these
dendrons is the light-harvesting and energy-transfer properties
when synthesized as an antenna macromolecule.27 The ter-
thiophene dendron moiety exhibits (R,ꢀ-linkage) which breaks
the conjugation and allows a blue-shifted absorption compared
to a linear terthiophene (R,R-linkage). However, this dendron
sensor was also designed to allow the hydroxyl-terminated TEG
group to play an important role for cation trapping together with
the thiophene dendron moieties. The manifested sensitivity of
the hydroxyl-terminated TEG group is then expected to give
rise to an emission peak appearing at lower energy. On the other
hand, by adding a phenyl ring between the TEG and thiophene
dendrons, the sulfur atoms in the thiophene dendrons can
properly contribute to the interaction event with the cations,
which is then detected through twisting of the terthiophene
conformation. Both these phenomena give rise to dual fluores-
cence via a TICT. Furthermore, high sensitivity and selectivity
was observed for this sensing system compared to what can be
achieved if the TICT system were attempted on a one-
dimensional or linear π-conjugated oligothiophene system alone.
The expected sensitivity via this TICT mechanism is not greatly
affected by the saturation of Ca2+ ions that bind with the
terthiophene dendron due to the possibility of dimer formation
Experimental Section
General Methods. All chemicals were purchased from
Aldrich Chemical Co. and were used directly without further
purification. Tetrahydrofuran (THF) was freshly distilled over
sodium and benzophenone before use. All solvents were
aspirated with nitrogen gas before use.
Synthesis. The chemosensor molecule was prepared by
etherification of a terthiophene dendron (G-1) possessing phenol
groups at their focal point with brominated ethylene glycol
(3T5O) (Scheme 1). The 2,3-dibromothiophene has proven to
be a very useful AB2 monomer for the formation of poly-
thiophene dendrimers.27 The tetraethylene glycol was chosen
as a binding unit to interact with calcium cations. The hydroxyl
group at the end of TEG is present to increase the possibility
for ionic bonding or amphiphilicity. The absence of a phenyl
ring prevents the formation of a twisted structure of the
chromophore moiety as will be discussed later with the
calculations. Terthiophene dendrons (R,ꢀ linkage) were used
as the signaling unit to break the π-conjugation in a linear
terthiophene (all R,R linkage). This combination produces a
“turn-on” sensing system.
Thiophene Dendron (r,ꢀ Linkage) Synthesis. The synthetic
protocol for terthiophene dendron (tributyl(5,5′′-dihexyl-
[2,2′;3′,2′′]terthiophene-5′-yl)stannane) is included in Scheme
1. Our group reported the procedure for terthiophene dendrons
previously.27b
Synthesis of 1-((4-Bromophenoxy)methyl) (1). 1.2 g of
sodium hydride was added slowly to a solution of 4-bromophe-
nol (5 g) in tetrahydrofuran with stirring at 0 °C. After 30 min,

13086 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Park et al.
SCHEME 1: Synthesis of Calcium Cation Detecting Sensor
1.2 g of benzyl bromide was added dropwise under nitrogen
atmosphere. Then, the reaction was allowed to warm to room
temperature overnight. The mixture was stirred with water to
kill excess of sodium hydride. After drying the mixture under
vacuum, it was extracted with methylene chloride. The product
was purified with silica gel column chromatography. 4.8 g of
125.14, 124.41, 123.33, 116.28, 116.19, 31.98, 31.91, 30.56,
30.52, 29.15, 23.05, 22.98, 14.49.
Synthesis of 5-(4-(2-(2-(2-(2-(4-(4,5-bis(5-hexylthiophen-
2-yl)thiophen-2-yl)phenoxy) ethoxy)ethoxy)ethoxy)ethox-
y)phenyl)-2,3-bis(5-hexylthiophen-2-yl)thiophene (3T5O).
0.2 g of compound 3, 57 mg of 2-(2-(2-(2-bromoethoxy)ethoxy)-
ethoxy)ethanol, and 54 mg of potassium carbonate were
dissolved in ethanol and refluxed overnight. The mixture was
dried under vacuum and simply extracted with methylene
chloride. The compound was purified with silica gel column
chromatography. 0.2 g of pure product was obtained (yield:
1
pure product was obtained (yield: 63%). HNMR (CDCl3): δ
7.33-7.42 (m, 7H), 6.85 (d, 2H), 5.04 (s, 2H).
Synthesis of 1-((4-Bromophenoxy)methyl)-4-(5,5′′-dihexyl-
[2,2′;3′,2′′]terthiophene-5′-yl)benzene (2). 1.3 g of tribu-
tyl(5,5′′-dihexyl-[2,2′;3′,2′′]terthiophene-5′-yl)stannane, 0.5 g of
1-((4-bromophenoxy)methyl), 50 mg of tetrakis(triphenylphos-
phine)palladium, and a catalytic amount of CuI was dissolved
in DMF under an inert atmosphere and refluxed overnight. The
mixture was cooled down to room temperature and DMF was
removed under vacuum. After simple extraction, the product
was purified with silica gel column chromatography. 0.5 g of
1
44%). HNMR (CDCl3): δ 7.5 (d, 4H), 7.20 (s, 2H), 6.94 (d,
2H), 6.89 (d, 2H), 6.84-6.86 (d, 4H), 6.67 (d, 4H), 4.17 (d,
4H), 3.88 (d, 4H), 3.68-3.74 (m, 6H), 3.62 (d, 2H), 2.74-2.81
(td, 4H), 1.60-1.66 (m, 4H), 1.25-1.40 (m, 12H), 0.88 (t, 6H).
13C NMR (in CDCl3): δ 158.92, 147.66, 142.18, 135.37, 132.90,
132.55, 132.42, 128.97, 128.81, 127.58, 127.22, 126.51, 125.14,
124.53, 124.41, 115.40, 72.91, 71.22, 71.05, 70.98, 70.71, 70.09,
67.89, 62.16, 31.98, 31.91, 30.56, 30.52, 29.15, 23.05, 22.98,
14.49. Anal. Calcd for C38H52O5S3: C, 66.63; H, 7.65. Found:
C, 66.43; H, 7.77. Exact mass calculated for [C38H52O5S3]+
requires m/z 684.30; found, 684.46.
1
pure product was obtained (yield: 50%). HNMR (CDCl3): δ
7.52 (d, 2H), 7.36-7.41(m, 5H), 7.20 (s, 1H), 6.95 (d, 1H),
6.90 (d, 1H), 6.85 (d, 2H), 6.67 (d, 2H), 5.10 (s, 2H), 2.74-2.81
(td, 4H), 1.60-1.66 (m, 4H), 1.25-1.40 (m, 12H), 0.88 (t, 6H).
Synthesis of 4-(4,5-Bis(5-hexylthiophen-2-yl)thiophen-2-
yl)phenol (3). 0.8 g of compound 2 was dissolved in a cosolvent
of methanol and methylene chloride followed by a 10% activated
Pd/C suspension. Then, hydrogen gas was bubbled into the
solution at room temperature for 2 days. The mixture was filtered
to remove Pd/C and the compound was purified with silica gel
column chromatography. 0.4 g of pure product was obtained
Results and Discussion
Absorption and Fluorescence Spectroscopy of 3T5O. The
absorption spectrum of 3T5O shows a strong peak around 300
nm, representing the terthiophene dendron (R,ꢀ linkage) moiety.
A smaller band around 350 nm, with half the intensity of the
primary band at 300 nm is due to the conjugation of the phenyl
and terthiophene dendron in solution (Figure 2). By contrast, a
linear terthiophene (R,R linkage) has absorbance at 345 nm and
emission at 435 nm (345 nm excitation).28 Linear terthiophenes
1
(yield: 50%). HNMR (CDCl3): δ 7.46-7.49 (d, 2H), 7.20 (s,
1H), 6.94 (d, 1H), 6.89 (d, 1H), 6.84-6.86 (d, 2H), 6.67 (d,
2H), 2.74-2.81 (td, 4H), 1.60-1.66 (m, 4H), 1.25-1.40 (m,
12H), 0.88 (t, 6H). 13C NMR (in CDCl3): δ 155.75, 151.88,
151.39, 147.70, 146.10, 142.13, 138.41, 135.78, 127.61, 127.51,

“Turn-on” System for Calcium Ion Detection
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13087
Figure 2. Absorbance and emission spectra of 3T5O: (a) UV spectrum (b ) 1 cm) and (b) fluorescence which was excited at 300 nm and 350 nm
in hexane, CHCl3, THF, methanol, and acetonitrile (emission and excitation slit widths ) 10 nm).
are also strongly aggregated.29 A solvatochromic effect on the
absorption spectra was observed due to the amphiphilic property
of 3T5O. As a result, the absorption and fluorescence shifts
varied with the structural conformation of 3T5O, which
consequently gives an approximation on the relative solvating
power of the solvent. Amphiphilic polythiophenes such as 3T5O
exhibit a much lower hydrodynamic volume indicating a more
compact conformation in THF than in chloroform47 hence a
relatively higher λmax as shown in Figure 2a. Molar absorbance
was calculated at ε ) 10 000 L mol-1 cm-1 at 300 nm and ε )
5416.7 L mol-1 cm-1 at 350 nm in THF solution. On the other
hand, it is presumed that the blue shift observed in hexane may
be due to the strongly polar character of the hydroxyl-terminated
TEG group of the emitting state of 3T5O. The intermolecular
interactions of the strongly polar character of the side chain
eliminates the internal energy barrier even in the presence of a
nonrelaxing, rigid property in hexane, resulting in an increase
in the population of the charge-transfer state.48 While this was
the assumption for the observed red shift in hexane, it is believed
that the twisted conformation of the terthiophene units coupled
with the electrostatic interaction of the calcium ion with the
TEG group may have effectively counteracted interaction of
the solvent molecules with the dendron units. This could provide
a favorable complexation enthalpy to cause the desired com-
plexation between the dendron and the metal ion. This eventually
leads to a shift in the emission λmax toward the blue region.
However, only a single emission band at 460 nm was seen when
the solution was excited at both 300 and 350 nm. The single
emission indicates a larger conjugation between the terthiophene
dendron and the phenyl ring, i.e., energy transfer from the
terthiophene dendron moiety to the phenyl group resulting in a
single emission. It should be noted that the presence of a single
emission a priori is important in the context of how, by
disrupting the planar conformation of this molecule, a second
higher energy emission is eventually observed. Hence, it may
be generalized that molecular twisting of the amphiphilic 3T5O
molecule is accompanied with solvent effects. The 3T5O
molecule may have assumed different twisting angles, stabilized
by the solvating effect of the solvent, yielding distinct absorption
and emission characteristics.
solution. There were no significant differences in the NaCl, KCl,
and MgCl2 solution spectra. Mg2+ ions showed some weak
change at 1.6 × 10-7 M (Supporting Information, Figures S2
and S3). However, the predominant absorption band around 300
nm showed an increase and a blue shift when CaCl2 solution
was added. A concentration of 8 × 10-8 M of calcium cations
was added and stirred with the 4.8 × 10-8 M solution of 3T5O
(Figure 3) or 2:1 ratio. Further addition of calcium cations
increased the concentration of solution up to 12 × 10-7 M or
24 times the concentration of 3T5O and the UV-vis absorption
band increased linearly with increasing concentration. Thus, the
primary absorption band of 3T5O at 300 nm was increased and
blue-shifted as the concentration of calcium cations increased
(Figure 3a). However, the small band at 350 nm decreased and
the small shoulder around 320 nm increased. This can be
attributed to the phenyl ring-terthiophene dendron moieties’
increasing interaction with the calcium cation.27c The phenom-
enon indicates a disruption of the conjugated system in the
chromophore. However, other cations, such as potassium,
magnesium, and sodium did not exhibit this effect with the
3T5O in solution (Figure 3b), e.g., when potassium and
magnesium cations were added at over 8 × 10-7 M, which is
20 times of the concentration of 3T5O. Thus, no change was
produced even at higher concentrations.
The fluorescence spectra showed a much more dramatic
change than the UV-vis spectra. A large new band appeared
when the CaCl2 solution (Figure 3c) was added to the 3T5O
solution, whereas the response to the other cations NaCl, KCl,
and MgCl2 was minimal (Figure 3d). The term 3T5O3T is now
used to describe the calcium ion complex with the terthiophene
dendron, 3T5O. This new emission at 320 nm is in contrast to
most fluorescence sensing systems which results in quenching
or a turn-off. Thus, this system produces a “turn-on” mechanism
specifically with calcium cations. The emission band at around
450 nm can be attributed to the conjugated system of ter-
thiophene and phenyl ring as described earlier. However, the
large band that appeared at 320 nm and blue-shifted relative to
450 nm after calcium cations were added can be explained with
the same hypothesis presented with the UV band increase and
blue-shifting; the planar π-conjugated system of the terthiophene
dendron chromophore is twisted and disrupted. Raman spec-
troscopic studies would be useful in verifying the functional
group responsible for the large band at 320 nm. Nevertheless,
a comparison with previous studies conducted by Van Loon
et al.,49 Kapitan et al.,50 and Feng et al.,51 was done to confirm
Absorption and Fluorescence Spectroscopy of 3T5O with
Ca2+. For the cation detection experiment, 3T5O was dissolved
in methanol with tetrabutylammonium chloride (TBACl) as a
standard. Solutions of NaCl, KCl, CaCl2, and MgCl2 with
TBACl in methanol were used for titration with the 3T5O

13088 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Park et al.
Figure 3. Absorbance and emission spectra of 3T5O3T with cations: (a) UV-vis spectrum in methanol with increasing Ca2+ concentration, i.e.,
from 8 × 10-8 M to 12 × 10-7 M) (b ) 1 cm), (b) with increasing Na+ concentration; (c) fluorescence which was excited at 300 nm with
increasing Ca2+ concentration and (d) with increasing Na+ concentration (emission and excitation slit widths ) 10 nm).
that the peak at 320 nm was not due to methanol. With reference
to the Raman peak assignments, they have reported for neat
methanol, aqueous salt solutions (CaCl2, MgCl2, and NaCl) and
powdered polyethylene glycol (PEG), respectively. Results
would show that the distinct band at 320 nm, visibly seen as
giving incremental changes in absorption and fluorescence
intensities, pertains to that of the twisted terthiophene dendron
chromophore. There is also a possibility of contribution from
the hydroxyl TEG group of the 3T5O terthiophene dendron.
As reported by Tummler et al.,56 solvent molecules of methanol
are not too tightly bound, and hence will not strongly interfere
with electrostatic interaction between the metal ion and the
coordinating sites of the ligand. It is therefore presumed that
methanol posed no potential interference in the determination
of the absorption and fluorescence peaks for the “turn-on”
sensing of calcium ions by 3T5O terthiophene dendron.
The addition of Na+ ions upon solution of 3T5O resulted to
increasing but broad, undefined absorption and emission peaks
which may be attributed to the relative weak affinity of the
monovalent Na+ with ethylene glycols.52 Still higher binding
constants have been reported in the case of doubly charged
all the thiophenes and phenyl group to lie flat, constructing a
more extended π-conjugated system of a particular minimum
energy. However, in the presence of calcium cations interacting
with the S on the thiophenes and the effect of the TEG unit
complexing with the calcium ions in a crown ether effect, the
molecule is conformed to a lower energy conformation that then
disrupts the conjugation of the chromophore. This in turn causes
the blue shift and a higher energy emission at 320 nm since the
energy transfer pathway is removed from the fluorophore and
replaced by a metal-ligand charge transfer (MLCT).23,24 The
results observed in the absorption and emission spectrum in
essence are due to the twisted structure that can be more aptly
explained by a TICT. It is also consistent with higher concentra-
tions of Ca2+ relative to 3T5O giving a linear plot, since more
ions are made available. This will be shown by the semiem-
pirical PM3 and Hartree-Fock calculations at the latter part of
this paper. First there is a need to complete all the experimental
results:
Sensitivity Studies. Normally, the saturation of the peaks
prevents quantitative experiments for cation detection. Specif-
ically a 1:1 matching system can only detect a 1 equiv
concentration of the cationic species; i.e., no more band changes
can be observed with continued addition of the analyte.
However, for the 3T5O3T system, it was shown that an
incremental increase of absorbance with no saturation after
addition of up to 24 equiv of the calcium cations can still be
observed. This system can also detect smaller portions of the
cations at 1 × 10-8 M. This means that this system has a high
linear dynamic range compared to other solution cation sensors.
Thus, the I/I0 versus [Ca2+]/[3T5O] can be plotted as shown in
Figure 3. A calculated slope of 1.7 indicates that the amount of
calcium cations against the 3T5O amplifies the emission peak
cations such as Ca2+ 53
.
A twisted structure of the terthiophene-phenyl conjugated
chromophore should explain the blue shift and intensity
increases in the absorption and emission spectra using the TICT
theory. Twisted thiophene molecules within the terthiophene
dendron moiety form a complex with the calcium cation. This
paired interaction with the calcium cation should represent a
minimum energy conformation condition with a synergistic
effect on enhancing fluorescence attributed to a higher energy
phenyl-terthiophene excited state. Initially, the presence of a
phenyl group next to the terthiophene dendrons, should conform

“Turn-on” System for Calcium Ion Detection
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13089
Figure 4. (a) Absorption spectra (b ) 1 cm) and (b) emission spectra with 3T5O solution and up to 30% molar ratio of calcium cations for
sensitivity (excitation and emission slit widths ) 10 nm).
Figure 5. Change of the absorbance and emission when calcium cation is added to the pool of sodium, potassium and magnesium cations with
compound of 3T5O: (a) UV-vis absorption change (b ) 1 cm) and (b) emission of the solution of cations and 3T5O (excitation and emission slit
widths ) 10 nm).
to be about twice the expected 1:1 correspondence in most
solution sensing systems.9-12 Furthermore, a solution of 3T5O
can detect calcium cations as small as 10% molar ratio (Figure
4). Since the 3T5O3T solution has more than 24 times the molar
ratio of calcium cations, the UV-vis spectrum changed at a
minimum of 30% molar ratio and the fluorescence changed at
a minimum of 10% molar ratio. When calcium was added to
the solution with 30% molar ratio, the small absorbance shoulder
around 350 nm was reduced, and a separate shoulder appeared
around 330 nm, while the primary band at 300 nm blue-shifted.
The response in the fluorescence was more sensitive compared
to the UV-vis spectra and as a result, even with 5% molar
ratio of calcium cation, a band around 360 nm appeared, which
was not as predominant a signal compared to the common
emission band at 460 nm. However, as little as 10% of calcium
cation addition produced the small peak around 320 nm. This
region would eventually produce a broad emission upon addition
of larger amounts of calcium cation indicating its relevance in
calcium ion detection. Thus, this higher energy emission is a
consequence of reduced conjugation by the twisting of the
terthiophene rings in complexation with the calcium ions alone
even at lower ion concentrations. Moreover, a larger dynamic
range can be utilized. Semiempirical PM3 modeling studies
confirm this favorable twisted conformation in the presence of
the complexed calcium ion with the presence of possible cross-
linking around the Ca2+ ions at higher 3T5O concentration.
Selectivity Studies. Calcium detection was also performed
with a mixture of cations present. No change was observed in
the spectrum when each of the 2.4 × 10-7 M sodium, potassium,
and magnesium ions was added directly to the 3T5O solution.
However, the addition of calcium cations to the solution
containing 3T5O and the other three cations brought a dramatic
change in UV-vis and fluorescence spectrum (Figure 5) similar
to that observed with calcium ions alone. The same amount of
calcium cations analogous with the other cations made the
system respond strongly even at this concentration. This means
that even with the presence of the other ions, both monovalent
Na+ and K+ and the divalent Mg2+, the molecule is strongly
selective and specific for Ca2+. Again, this selectivity will be
confirmed by the Hartree-Fock level calculations described
below.
Molecular Modeling Studies. The proposed fluorescent
TICT mechanism for the “turn-on” sensing system for calcium
ion by this terthiophene dendron with hydroxyl-terminated TEG
group offers very important applications in analytical chemistry,
medical analysis, and environmental monitoring.46 TICT mech-
anism as proposed by Grabowski et al.23 is anchored on
observing dual fluorescence due to a twisted charge transfer
state in which planarity is lost. To this end, it is only appropriate
to illustrate the changes on the conformation of this terthiophene
dendron upon sensing of Ca ion through modeling/computational
studies using the Spartan 08 software. Computations were

13090 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Park et al.
Figure 6. Optimized geometry of the (a) 3T5O molecule and the (b) terthiophene dendron with the space-filling model and ball and spoke model
and electron (density) cloud map. The space filling model is given to ascertain the molecular size of the optimized geometry relative to each other.
TABLE 1: Bond Angles Observed in the Twisted
Conformation of the Thiophene Units
performed using the semiempirical model, PM3, to obtain the
equilibrium geometry at the ground state for each Ca to 3T5O
atoms
bond angle, deg
ratio. Additionally, bond angles, bond distances, stabilization
energies for complexes and others as obtained from these
calculations would be useful in supporting the experimental
results generated from the ionoselective behavior of the ter-
thiophene dendron toward calcium ions. The computational
method was chosen to avoid extensive computing time and
resources. Geometry optimizations using other models for large
systems are expected to be difficult due to the increased number
of basis functions as well as the increase in the degrees of
freedom. Moreover, the total charge for the complex was
assumed neutral since calcium atom was used to illustrate the
sensing response of the 3T5O molecule toward calcium.
The structures shown in Figure 6 are obtained for (a) 3T5O
(emphasis was made to show the conformation of the thiophene
units and the phenyl ring) and (b) a terthiophene dendron after
geometry optimization using the semiempirical PM3 model
(Spartan 08, wave function). It is clearly seen that the thiophene
units comprising the terthiophene dendron assume a slightly
twisted yet planar conformation. Sulfur atoms are denoted as
S2 and S3 and are about 2.4 Å apart from each other. The bond
angles formed by close association of carbon and sulfur atoms
comprising the twisted thiophene units are given in Table 1.
This observation corroborates earlier studies which concluded
that, in the gas phase, the 2,2′-bithiophene is twisted around
the 2,2′-bond and occurs in at least two conformations.31,32
Previous studies have also indicated that the polythiophene
configuration is affected and modified by the environment.33
Direct evidence for chain flexibility in solutions obtained from
C10, C12, S2
C11, C12, C10
C12, C10, S1
C18, C9, C7
C17, C18, C9
C9, C18, S3
122.40
127.06
120.59
121.76
125.94
123.89
small-angle neutron scattering of undoped poly(butylthiophene)
has also been previously investigated.34,35 This slightly twisted
conformation of the thiophene units is retained similarly in the
3T5O molecule even with a phenyl group and ethylene glycol
moieties attached to the polymer backbone. The presence of
the additional phenyl chromophoric group would explain the
observed blue shift as well as the increase in the intensities both
in the absorption and emission maxima. Moreover, there has
been indirect evidence which suggests that soluble poly-
thiophenes modify their absorption spectra (color) from the solid
to the dissolved state, and also upon melting.36
The geometry optimization performed on the 1:1 Ca2+/3T5O
complex would show that there is also a significant interaction
between the calcium atom and the TEG moiety of the 3T5O.
A very strong ionic bond coupled with electrostatic interactions
aptly describes the interaction of the hydroxyl-terminated TEG
group with calcium. The electronic configuration of alkaline
earth ions such as Ca2+ promotes stronger ion-dipole electro-
static forces, enabling the formation of a critical ionic bond via
donation of the lone s electron to an oxygen atom.54 This

“Turn-on” System for Calcium Ion Detection
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13091
Figure 7. Predicted geometry of 1:1 Ca/3T5O complex with the space-filling model and ball and spoke model.
TABLE 2: Calculated Distances between Calcium Atom and
interaction is manifested through a change in the geometry
of the 3T5O molecule in the presence of the calcium atom.
Without the calcium atom, the ethylene glycol units of the
3T5O, though the repeating -CH2-CH2-O- group is bent
but arranged in a linear fashion. However, in the presence of
the calcium atom, these glycol units curled up, with the calcium
atom apparently trapped beneath this conformation. The con-
formational flexibility of the side chain-substituted poly-
thiophenes has also been observed in several studies.37-42
Consequently, this phenomenon causes a shift in the absorption
maxima in the visible region. Changes in the optical properties
are commonly associated with the chemical and the geometrical
structure of a polymer.43 As shown in Figure 7, simultaneous
with the encircling of calcium by the hydroxyl terminated TEG
group is the enhanced by the twisting of the terthiophene
dendron resulting to lost of the planar conformation which have
been observed in Figure 6a. The two thiophene units have
arranged themselves perpendicular to the plane of the thiophene
unit that is sandwiched by these two thiophene units. This
observation confirms that the terthiophene dendron has assumed
a different twisting angle upon the addition of calcium ions
resulting to variations on experimentally obtained absorption
and emission spectra. The calcium ion induced self-assembly
of the TEG group may be thought of as a major cocontributor
in the peak observed at 320 nm shown in the emission spectrum.
This band at 320 nm further affirms the strongly polar character
and relative mobility of the TEG hydroxyl segment of the
terthiophene dendron, enhancing the signaling response in the
presence of calcium ions. Moreover, it has been clearly
demonstrated in Figure 7 that the enhancement in the absorption
and emission peak is due to the less stretched conformation of
the terthiophene dendron resulting in a lower hydrodynamic
volume similar to coillike polymers brought about by the coiling
of the hydrophilic TEG side chain, dictating the “turn on” mode
for the sensing of calcium ions. Differences in the energy are
normally found and in the case of sensing of calcium by the
3T5O molecule, the average calculated energy (heat of forma-
tion) of an optimized 1:1 Ca2+/3T5O complex is -1457.46 kJ/
mol.
the Oxygen Atoms of 3T5O, 1:4 Ca to 3T5O Molar Ratio
labels
distance, Å
O3, Ca1
O17, Ca1
O2, Ca1
O15, Ca1
O19, Ca1
O20, Ca1
O18, Ca1
2.668
2.760
2.491
2.498
2.590
2.496
2.532
TEG group of the dendron, dendron-dendron dimerization, or
solvent-dendron interactions. Cross-linking arises due to the
interplay of electrostatic interactions, hydrogen bonding, and
van der Waals interactions among different components present
in solution. It is ascertained that the calcium atom is somewhat
encapsulated during the cross-linking process by the TEG groups
and likewise the terthiophene units by a favorable π-π stacking.
Table 2 gives the calculated distances between the calcium atom
and the oxygen atoms with 4 units of 3T5O molecule.
The calculated heat of formation for the 1:4 Ca/3T5O
complex is -3779.88 kJ/mol and that for the 4 units of 3T5O
only (without calcium) is around -2463.89 kJ/mol which
accounts for the 3T5O/3T5O self-interactions. Using the
equation below, the stabilization energy for the complex is
determined to be -1494.23 kJ/mol.
∆E ) ECa/3T5O complex - [ECa/3T5O + ECa]
(1)
In the determination of the energy differences for this
particular system, the procedure employed by Cece et al.44 in
treating their calculated data can fully explain the interaction
of ethane and hexafluoroethane with carbon dioxide. Table 3
summarizes the calculated energies for the different Ca/3T5O
complex systems. The first column of values is the energy (heat
of formation) of the optimized geometry of the complex formed
by the interaction of calcium with varying numbers of 3T5O
units. On the other hand, the second column contains the sum
of the energies of the individual components which make up
the complex: ∑E(heat of formation) ) ∑E(3T5O units) +
E(calcium atom). The third column gives the single-point energy
of the 3T5O units calculated at different ratios while the fourth
column shows the energy of the calcium atom extracted from
the optimized geometries for each Ca:3T5O molar ratio. The
Calculations were also performed for the 1:2 and 1:4 Ca/
3T5O systems using the semiempirical model (PM3). Results
suggest that in the presence of additional 3T5O units (or at
smaller concentration of the ions), a cagelike structure is formed
due to possible cross-linking or aggregation arising from
chromophore-chromophore interactions such as the hydroxyl

13092 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Park et al.
TABLE 4: Interaction Energies for Ca/3T5O Systems
TABLE 3: Energies (kJ/mol) of Ca/3T5O Interactionsa
(kJ/mol)
single-point calcns of
heat of
molar ratio, formatn of
Ca to 3T5O complex components
heat of formn of
3T5O
(varying ratios)
∆E of
Ca from
complex
energy of
interaction of
3T5O with
one another
interaction energy
between the 3T5O
molecule and the Ca
atom in the system
∑E of
no. of
total energy of
3T5O interaction between
units
Ca and 3T5O
1:1
1:2
1:4
-1457.4624 -401.462
-2470.524
-3779.88
-579.70
-1202.695
-2463.889
178.238
221.533
323.327
-981.162
-2140.562
1
2
4
-1056.0004
-1489.362
-1639.318
0.0
-43.295
-145.089
-1056.0004
-1446.067
-1494.229
a Heat of formation of an isolated Ca atom ) 178.238 kJ/mol;
heat of formation of an isolated 3T5O unit ) -579.70 kJ/mol.
TABLE 5: Calculated Stabilization Energies for 1:1
Cation/3T5O Complex System (kJ/mol)a
tabulated results imply that the presence of varying molar
amounts of 3T5O may have induced changes upon the calcium
atom. The change in the energy (heat of formation) of calcium
extracted from the complex was found to have deviated from
the single-point calculation performed for calcium atom alone.
As the amount of 3T5O unit was increased, the change in energy
for calcium from the complex has been noted to increase.
Interestingly, the data shown in Table 3 validates that the sensing
of calcium ions by the terthiophene dendron having a hydroxyl-
terminated TEG chain is accompanied by changes in the
enthalphy of ligand complexation arising from a number of
factors which include intermolecular 3T5O-3T5O interactions
(as also shown in Table 4), π-π stacking of the phenyl ring
attached to the TEG side chain of the terthiophene dendron and
the steric deformation brought about by the coiling of calcium
by the TEG group. Also, results of the calculations carried out
for each molar ratio of Ca to 3T5O suggests that a single 3T5O
dendron unit provides better stabilization for the calcium ion
(∆E ) 178.238 kJ/mol) during the complexation reaction,
resulting in an efficient sensing of calcium ions. More so, the
relative polarizability of the dendron has been established, thus
realizing the desired conformational change on the dendron as
induced by the calcium ion, which is a pivotal requirement in
the occurrence of TICT phenomenon.
energy of the stabilizn energy,
cation energy of the complex
cation
∆E
Ca2+
Mg2+
K+
-3670.4347
-3197.4895
-3593.6728
-3160.395
-669.4335
-196.5119
-593.0093
-159.7846
-0.4483
-0.4247
-0.0711
-0.0626
Na+
a Energy of single 3T5O molecule ) -3000.552 826 kJ/mol.
the restricted Hartree-Fock level theory using the STO-3G
basis set were performed. The semiempirical model (PM3)
is not parametrized for calculations involving sodium and
potassium ions. The charge of the cation is taken as the total
charge for each complex/system. Single-point calculations
were also performed at the STO-3G level on varying ratios
of 3T5O units and on separate cations to determine if any
of the observed energy differences were due to 3T5O/3T5O
interactions. Calculations were carried out with the assump-
tion that the solvent-induced changes for the formation of
complexes are negligible; i.e., concentrations of the solutions
used in generating the experimental data were not very diluted
to make the system/complex highly solvated to cause
significant changes. The values shown are average of three
calculations per complex/system.
The first column in Table 4 gives the difference between the
energy of the optimized geometry of the complex formed by
the interaction of 3T5O with calcium and the summation of
the energies of the individual components which make up the
complex. The difference between the values obtained from
single-point energy calculations of varying 3T5O units (energy
of the cluster of 3T5O molecules in the complex) and the sum
of the energies of individual 3T50 units (-579.70 kJ/mol)
represents the energy of interaction of the 3T5O units with one
another is given in the second column. The last column indicates
the interaction energy between the 3T5O molecule and the Ca
atom in the system. This value is the difference between column
1 and column 2 of Table 4.
Thus, the data given in Table 4 provides for the calculated
change in the energy for systems with varying molar ratios
of Ca/3T5O. The change in the energy accounts for the
changes in the geometry of the 3T5O as it accommodates/
interacts with the calcium atom. Consequently, this change
in geometry would explain the observed sensing of calcium
by the 3T5O molecule and with increasing ratio; it gives
unfavorable interaction primarily due to cross-linking or
“aggregating” around the ion by the TEG group. Also, the
calculated change in the energy upon interaction of a 3T5O
unit with calcium ion (∆E ) 1056.004 kJ/mol) is more
significant compared with the change in the energy for the
3T5O/3T5O interactions, particularly in the case of 1:1 molar
ratio, which is marked by the absence of dendron-dendron
interaction (∆E3T5O/3T5O ) 0 kJ/mol).
Results of these calculations as summarized in Table 5
show that, between monovalent and divalent ions, the divalent
ions such as calcium and magnesium provide a more stable
complex with the 3T5O unit over the monovalent species
such as potassium and sodium cations. Moreover, it is the
calcium dication which gives a relatively more stable complex
with 3T5O with net stabilization energy equal to -0.4483
kJ/mol compared with magnesium, which may be used to
explain for the observed preferential sensing of calcium ions
by the terthiophene dendron in the presence of the other three
cations used for the selectivity experiments. The stabilization
energy calculated further validates the ability of the TEG
group to optimally bind with the Ca2+ ions. Optimal binding
may then be correlated with maximized interactions within
the cavity/cage that is formed from the encircling of the
hydroxyl-terminated TEG group in the presence of a Ca2+
ion. The size of the cage/cavity thus created from the curling
of the TEG group is commensurate to that of the size of the
Ca2+ ion and, accordingly, may be regarded as the most
favorable conformation for the desired TICT “turn on”
sensing of calcium ion by the 3T5O dendron. Similarly,
cyclic receptors such crown ethers demonstrate the same
behavior toward cation binding, termed as hole-size relation-
ship.54 Moreover, calculated data presented in Table 5 further
support the experimental data obtained from sensing of Na+
ions by terthiophene dendron 3T5O. It has been previously
discussed that the weak affinity of the monovalent cations
such as Na+ to ethylene glycol resulted to broadened,
undefined increments on the emission intensity upon addition
of increasing concentrations of Na+ ions. This could be due
In an effort to distinguish specific interactions between
monovalent and divalent cations and 3T5O, calculations at

“Turn-on” System for Calcium Ion Detection
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13093
Figure 8. Predicted geometry for the 1:4 Ca/3T5O mole ratio as illustrated using the ball and stick (left) and space-filling models (right).
to the weak electrostatic forces existing between the TEG
group and the Na+ ion. Such weak affinity can be ascribed
to the inert np6 electronic configuration of Na+.54
In summary, the findings in this work have yet to realize
practical sensor applications, But the molecular/macromolecur
design properties have been laid out for any future synthesis.
As discussed, the possibility of using a “turn-on” system has
many advantages over “turn-off” systems.
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