Copper(II) ion-sensing mechanism of oligo-phenylene vinylene derivatives: Syntheses and theoretical calculations

Article abstract of DOI:10.1016/j.tet.2012.02.066

Oligo-phenylene vinylene (oligo-PV) with two picolinamide side-groups and six methoxy end-groups was synthesized in order to be a fluorescent sensing molecule. Various metal ion solutions (1.5×10^-4 M) were added to the 1.5×10^-6 M acetonitrile solution of the fluorescent molecule. The fluorescent emission spectra showed that, at about the same concentration (1.5×10^-4 M), only Cu(II) ion can quench the fluorescent emission of the picolinamide-PV solution. Possible metal ion-sensing mechanisms could be either the binding at picolinamide side-groups or methoxy end-groups, or interchain-stacking driven by the metal ions. Hence, oligo-PVs with six methoxy end-groups but without substituted side-groups, and another oligo-PV with six methoxy end-groups and two ethoxy side-groups were synthesized for comparison. These molecules turned out to be inactive to any metal ion solutions. Moreover, quantum calculation was used to confirm the result. Binding energy and conformation were calculated and simulated. It could be concluded that nitrogen and oxygen atoms of the picolinamide group and one oxygen atom from the methoxy group are involved in the metal ion-binding process.

Full text of DOI:10.1016/j.tet.2012.02.066

Tetrahedron 68 (2012) 3329e3335
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Copper(II) ion-sensing mechanism of oligo-phenylene vinylene derivatives:
syntheses and theoretical calculations
,
Pratoomrat Tongkate a, Darinee Phromyothin b, Jitapa Sumranjit a
*
a National Nanotechnology Center (NANOTEC), Phahonyothin Rd., Klongluang, Pathumthani 12120, Thailand
b College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 August 2011
Received in revised form 30 January 2012
Accepted 27 February 2012
Available online 3 March 2012
Oligo-phenylene vinylene (oligo-PV) with two picolinamide side-groups and six methoxy end-groups was
synthesized in order to be a fluorescent sensing molecule. Various metal ion solutions (1.5ꢀ10ꢁ4 M) were
added to the 1.5ꢀ10ꢁ6 M acetonitrile solution of the fluorescent molecule. The fluorescent emission spectra
showed that, at about the same concentration (1.5ꢀ10ꢁ4 M), only Cu(II) ion can quench the fluorescent
emission of the picolinamide-PV solution. Possible metal ion-sensing mechanisms could be either the
binding at picolinamide side-groups or methoxy end-groups, or interchain-stacking driven by the metal
ions. Hence, oligo-PVs with six methoxy end-groups but without substituted side-groups, and another
oligo-PV with six methoxy end-groups and two ethoxy side-groups were synthesized for comparison.
These molecules turned out to be inactive to any metal ion solutions. Moreover, quantum calculation was
used to confirm the result. Binding energy and conformation were calculated and simulated. It could be
concluded that nitrogen and oxygen atoms of the picolinamide group and one oxygen atom from the
methoxy group are involved in the metal ion-binding process.
Keywords:
Copper sensor
Fluorescent sensor
Calculation
Gaussian
Phenylene vinylene
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
fluorescent sensing is the formation of excimer in the excited state,
which results in excimer fluorescent emission.10,11 In this case, the
The detection of metal ions has been developed through many
processes. Although there were reports on many practical methods
for detecting heavy metal ions,1e4 researchers are still looking for
more precise, sensitive and selective protocols. One of the popular
methods is a fluorescence technique due to its great sensitivity. The
general molecular composition of fluorescent sensing molecules
includes reporting fluorophores and metal ion-binding units. When
appropriate metal ions bind to the fluorescent molecules, the
fluorescent emission of the system is changed. The fluorescent
sensing processes can be categorized into turn-on and turn-off. The
term ‘turn-on’ is used when the fluorescent emission intensity of
the system is enhanced by the metal ions;5e8 and ‘turn-off’ when
the fluorescent emission intensity is quenched by the metal ions.9
In some cases, more than one type of fluorophore can be pre-
sented in the systems. This strategy employs fluorescence reso-
nance energy transfer (FRET) between fluorophores as a sensing
mechanism. A typical event is the disturbance of the FRET process
by certain analytes resulting in changes of the fluorescent emission.
The changes due to the FRET process could be either the emission
intensity or the shift of emission wavelength. Another example of
presence of metal ions resulted in a decrease of excimer fluorescent
emission and an increase of monomer fluorescent emission.
Typical light-emitting materials are conjugated polymers, for
examples, poly(phenylene ethynylenes) (PPEs), poly(para-phenyl-
ene vinylenes) (PPVs) and poly(thiophene). PPEs have been ex-
tensively used in many applications, such as sensing molecules. The
structures have been modified by attaching water-soluble unit, so
they can be utilized as biosensors. There have been reports on
preparing PPE derivatives as sensor arrays for detecting biological
substances, such as cancerous cells and proteins.12,13 PPE was also
designed as a cruciform shape in order to meet varieties of appli-
cations. Cross-shaped PPEs have been reported to be fabricated as
a metal-sensing system14 and metal sensor arrays.15e17
Phenylene vinylene (PV) is another common conjugated organic
molecule well known as a light-emitting unit. The synthesis of PV is
quite straightforward so there have been many reported methods
for modifying the structures as desired.18 PVs have been used in
many applications, such as organic light-emitting diode
(OLED),19e21 organic field-effect transistor (OFET)22 and fluorescent
imaging.23 In this article, oligo-para-phenylene vinylene derivatives
with six methoxy end-groups were synthesized for use as fluo-
rescent sensing molecules. Picolinamide units were laterally at-
tached to the PV core unit of the first derivative in order to be
metal-binding sites. Oligo-PV with two ethoxy side-groups and
* Corresponding author. Tel.: þ66 2564 7100x6613; fax: þ66 2564 6981; e-mail
address: jitapa@nanotec.or.th (J. Sumranjit).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.02.066

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P. Tongkate et al. / Tetrahedron 68 (2012) 3329e3335
another oligo-PV without functional group were synthesized for
comparison. It was hypothesized that these two molecules cannot
interact with metal ions since they did not contain metal-binding
site.
Quantum chemical calculations were performed using Gauss-
ian0324 program in order to gain insight into the structural elec-
tronic properties of the sensing molecule.25 The 3D free and metal-
bound molecules were analysed and reported.
2. Results and discussion
2.1. Synthesis and experiments
A fluorescent metal ion-sensing molecule was designed by using
oligo-phenylene vinylene as a fluorescent-reporting unit. Six
methoxy groups were attached as end groups to help improve the
solubility. Two picolinamide units were incorporated to both sides
of the PV unit as metal-binding sites (Fig. 1). A typical reaction for
generating the main phenylene vinylene unit was Heck cross-
coupling reaction with palladium diacetate as a catalyst. The
picolinamide-functionalized phenylene vinylene (I) was soluble in
common organic solvents, such as acetonitrile and methanol. Ab-
sorption and fluorescent emission spectra of the molecule in ace-
tonitrile were shown in Fig. 2. The molecule emitted light in the
Fig. 3. Responses of picolinamide-PV (I) towards metal ions.
quench the fluorescent emission intensity of the picolinamide-PV.
It could be roughly explained that either the excited state in-
teraction between the metal ions and the picolinamide units or
other sites on the main phenylene vinylene chain caused the
fluorescent quenching.
A straightforward strategy to determine if the side-chain pico-
linamide units involved in the metal-binding process was to syn-
thesize another oligo-phenylene vinylene (II) for comparison. This
molecule had the same core unit and methoxy end-groups as in the
picolinamide-PV but contained no side-unit (Fig. 4). Compound II
also emitted light in blue region (lem(max)¼447 nm) with a quan-
blue region, lem(max)¼440 nm. The quantum efficiency (
f) was
measured to be 0.18 in methanol with quinine bisulfate in 0.1 M
sulfuric acid as a reference.
tum efficiency (f) of 0.6 in acetonitrile. The absorption and emis-
sion spectra of the molecule in acetonitrile were shown in Fig. 5.
Fig. 1. Picolinamide-functionalized phenylene vinylene (I).
Fig. 4. Oligo-phenylene vinylene II.
Fig. 2. Absorption and emission spectra of picolinamide-PV (I) in acetonitrile.
The metal ion-sensing experiments were performed in aceto-
nitrile to ensure the solubility of the picolinamide-PV and metal
ions. 1.5ꢀ10ꢁ6 M solution of the picolinamide-PV was mixed with
various metal ion solutions (1.5ꢀ10ꢁ4 M), one at a time, at a volume
ratio of 1:1. In this experiment 1 mL of the picolinamide-PV solu-
tion and 1 mL of the metal ion solution were mixed together, so the
final concentration of the picolinamide-PV was 7.5ꢀ10ꢁ7 M and the
concentration of the metal ion was 7.5ꢀ10ꢁ5 M. The fluorescent
emission intensities (Fig. 3) with fluorescent emission spectra (in
setdFig. 3) showed that only Ni(II) and Cu(II) ions can obviously
Fig. 5. Absorption and emission spectra of oligo-PV II in acetonitrile.
A 1.5ꢀ10ꢁ6 M solution of oligo-PV II was also combined with the
same group of metal ion solutions (1.5ꢀ10ꢁ4 M) used in the ex-
periment of picolinamide-PV to test the metal-sensing property, at
the same volume ratio of 1:1. In each sample, solution of oligo-PV II
(1 mL) was combined with metal ion solution (1 mL) and the
fluorescent emission spectrum was measured. It was shown in

P. Tongkate et al. / Tetrahedron 68 (2012) 3329e3335
3331
Fig. 6 that the fluorescent emission of oligo-PV II cannot be obvi-
ously quenched by any metal ions used in this experiment. The
emission intensity scarcely dropped from the initial point, which
could possibly rise from a weak interaction between the metal ion
and the electron cloud on the main phenylene vinylene chain, or
the lone pair electrons of the methoxy groups.
The metal-sensing experiments were done at the same protocol
as the other two PV molecules. The solution of ethoxy-PV
(1.5ꢀ10ꢁ6 M) was prepared in acetonitrile and mixed with each
metal ion solution (1.5ꢀ10ꢁ4 M) at 1:1 ratio.
From the results in Fig. 9, the fluorescent quenching by metal
ions can be divided into two groups. The first group described the
metal ions those did not quench or scarcely quenched the emission
intensity of the ethoxy-PV, which included Cd(II), Cu(I), Co(II), Mn
(II) and Zn(II). The other group was the metals those can somewhat
quench the emission intensity of the ethoxy-PV, which were Cu(II),
Ni(II) and Pb(II). However, when compared these results with the
picolinamide-PV experiments, it can be concluded that this ethoxy-
PV did not interact very well with all the metal ions used in the
experiments.
Fig. 6. Responses of II towards metal ions.
Moreover, another oligo-PV containing two ethoxy side-groups
and six methoxy end-groups (III) was also synthesized as a com-
parison (Fig. 7). This was to prove that the oxygen atoms of the
ethoxy side-groups did not involve in the metal-binding phenom-
enon, and picolinamide units really played an important role in the
Cu(II)-binding process. This molecule emitted light in blue region
Fig. 9. Responses of III towards metal ions.
The results confirmed the hypothesis that the two oxygen atoms
of the ethoxy side-groups were not the dominant sites for the
metal-binding process.
(lem(max)¼446 nm) with a quantum efficiency (f) of 0.7 in aceto-
nitrile. The absorption and emission spectra of the molecule in
acetonitrile were shown in Fig. 8.
From the experiments on the three phenylene vinylene de-
rivatives, it can be interpreted that the phenylene vinylene main
chain cannot show a practical metal-sensing property without the
picolinamide side-groups. In order to confirm the crucial role of the
picolinamide side-groups, theoretical calculations have been in-
troduced in the next section.
2.2. Theoretical calculation
2.2.1. Free molecule. The HOMO and LUMO orbitals of the
picolinamide-PV sensing molecule are provided in Fig. 10 and Table
1. The obtained HOMO and LUMO levels were mainly localized on
the HOMO/LUMO transition of the fluorescent molecule with an
oscillator strength of 1.7146, and HOMO-2/LUMO, HOMO-
1/LUMO with an oscillator strength of 0.1932; whereas the
LUMOþ4 spread out over the picolinamide moiety. In addition, the
sensing molecule was divided into two parts, the fluorophore
(phenylene vinylene skeleton) and the receptor (picolinamide
moiety), in the calculations for a better understanding of the fluo-
rescent mechanisms. The HOMO and LUMO orbitals of the fluo-
rophore are illustrated as localized electrons on the skeleton as
shown in Fig. 11, and the calculated molecular orbitals are shown in
Table 2. The HOMO and LUMO orbitals of the receptor moiety are
provided in Fig. 12 and Table 3. The majority of electrons were lo-
cated within the picolinamide moiety but some electrons of the
HOMO orbital were confined between the linker and the picoli-
namide unit.
Fig. 7. Ethoxy-phenylene vinylene (III).
2.2.2. Cu(II)-bound molecule. The sensing molecule was expected
Fig. 8. Absorption and emission spectra of ethoxy-phenylene vinylene (III) in
acetonitrile.
to bind divalent or trivalent metal ions where the picolinamide

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P. Tongkate et al. / Tetrahedron 68 (2012) 3329e3335
Fig. 10. Molecular orbitals (MOs) of the picolinamide-PV (I).
Table 1
Table 3
Calculated excitation energy, oscillator strengths and molecular orbitals (MOs) in-
Calculated excitation energy, oscillator strengths and molecular orbitals (MOs) in-
volved in the excitation of the picolinamide moiety, using B3LYP/6-311G(d)
volved in the excitation of the picolinamide-PV (I), using B3LYP/6-311G(d)
Energy (eV)
labs (nm)
Oscillator strength
MOs
Energy (eV)
4.9533
labs (nm)
Oscillator strength
0.0831
MOs
2.7699
3.4729
447.62
357.00
1.7146
0.1932
HOMO/LUMO (99%)
HOMO-2/LUMO (27%)
HOMO-1/LUMO (68%)
250.30
HOMO-3/LUMO (19%)
HOMO-1/LUMO (12%)
HOMO/LUMO (51%)
units were proposed as recognition sites that bind to the ions.
When the binding occurred, the HOMO and LUMO levels of the
picolinamide-PV changed and a fluorescent process was altered.
Theoretical calculation with fully optimized structure of Cu(II)-
bound picolinamide-PV suggested that the nitrogen atom of the
picolinamide unit and one oxygen atom of a methoxy group in-
volved in the metal-binding process. The geometry-optimized
structure, selected bond distances and selected atomic charges
were shown in Fig. 13. The computed binding energy (BE) of bound
state was ꢁ6.65 eV as defined by
Fig. 11. Molecular orbitals (MOs) of molecule II.
BEðeVÞ ¼ Eðbound stateÞ ꢁ ½Eðunbound stateÞ þ Eðmetal ionÞꢂ
where E(bound state) is the energy of Cu(II)-bound picolinamide-
PV; and E(unbound state) and E(metal ion) are the energies of
picolinamide-PV and Cu(II) ion, respectively.
Table 2
Calculated excitation energy, oscillator strengths and molecular orbitals (MOs) in-
volved in the excitation of molecule II, using B3LYP/6-311G(d)
The perturbation of electrons in the HOMO and LUMO of the
Cu(II)-bound picolinamide-PV was shown in Fig.14. In the unbound
state, all its electron pairs presented as a closed shell, whereas in
Energy (eV)
2.984
labs (nm)
Oscillator strength
2.1342
MOs
415.49
HOMO/LUMO (85%)
the bound state, the complex became an open shell in which
a and
Fig. 12. Molecular orbitals (MOs) of the picolinamide moiety.

P. Tongkate et al. / Tetrahedron 68 (2012) 3329e3335
3333
Fig. 13. Geometry-optimized structure, selected bond distances and selected atomic charges of Cu(II)-bound molecule.
Fig. 14. Molecular orbitals (MOs) of the Cu(II) bound sensing molecule.
b
electrons were investigated. The energies of the HOMO and LUMO
quenched by Cu(II) ion in solution. The crucial role of picolinamide
groups was confirmed by using oligo-PV II and oligo-PV III as
comparisons. Moreover, theoretical calculations suggested that the
nitrogen atom at the picolinamide moiety and one oxygen atom of
a methoxy group were involved in the metal-binding process. In
the Cu(II)-bound state, the fluorescent signal of the compound was
predicted to be quenched by a charge-transfer process from the
metal-binding site to the phenylene vinylene main chain [HOMO-
of the free molecule, and the metal-bound molecule were calcu-
lated and compared. The excited state of the Cu(II)-bound sensing
molecule composed of the transitions from HOMO(
HOMO( )/LUMO( ) and HOMO-2( )/LUMO( ) with an oscilla-
tion strength of 0.3757 (Table 4). The transitions of HOMO( )/
LUMO( ) and HOMO( )/LUMO( ) were considered to be charge-
transfer processes within the fluorophore skeleton, whereas
HOMO-2( )/LUMO( ) was a charge-transfer process from the
a)/LUMO(a),
b
b
b
b
a
a
b
b
b
b
2(b)/LUMO(b)].
metal-binding site to the phenylene vinylene skeleton. This process
suggested that the fluorescent efficiency of the molecule will be
reduced.
4. Experimental
4.1. General
Table 4
Calculated excitation energy, oscillator strengths and molecular orbitals (MOs) in-
volved in the excitation of the Cu(II)-bound PV, using B3LYP/6-311G(d)
All chemicals were purchased and used as received. Solvents for
syntheses were anhydrous reagent grade. The synthesis of amino-
functionalized phenylene vinylene (amino-PV) was previously
reported elsewhere.26 UVevis spectra and fluorescent emission
spectra were recorded by PerkineElmer Lambda 650 and Per-
kineElmer LS 55 spectrometers, respectively. 1H NMR spectra were
obtained using Bruker 500 MHz spectrometer.
Energy (eV)
1.0537
labs (nm)
Oscillator strength
0.3757
MOs
1176.65
HOMO(
b
)/LUMO(
b
) (91%)
) (5%)
) (4%)
HOMO(
a
)/LUMO(
a
HOMO-2(
b)/LUMO(
b
3. Conclusion
4.2. Synthesis of picolinamide-PV (I) (Scheme 1)
Picolinamide-PV (I) was synthesized as a metal ion-capturing
molecule. The molecule contained phenylene vinylene core as
a fluorescent-reporting unit and methoxy groups as solubilizing
groups. Picolinamide moieties were attached in order to interact
with metal ions. The molecule was tested to be specifically
In a dry flask, picolinic acid (0.96 mmol, 0.118 g) and triphe-
nylphosphine (1.44 mmol, 0.378 g) were dissolved in dichloro-
methane (2 mL) at 0 ꢃC. A solution of carbon tetrabromide
(1.44 mmol, 0.378 g) in dichloromethane was slowly added to the

3334
P. Tongkate et al. / Tetrahedron 68 (2012) 3329e3335
reaction mixture. The mixture was allowed to warm up to room
temperature and stirred for 30 min. Next, triethylamine
(2.88 mmol, 0.4 mL) and amino-PV (0.1 mmol, 0.058 g) were added
subsequently. The mixture was stirred at room temperature for
30 min, washed with solutions of 10% HCl, CaHCO3 and water and
dried over anhydrous MgSO4. After evaporation, the residue was
purified by column chromatography (silica, hexanes/ethyl acetate,
1:1) to give yellow powder of picolinamide-PV (0.0298 g, 34%).
Scheme 3. Synthetic route of ethoxy-PV III.
1,4-Diethoxy-2,5-diiodobenzene
(B):
1,4-Diethoxybenzene
(0.29 mmol, 1.75 g) was added into a flask followed by water
(0.29 mL), glacial acetic acid (8.75 mL) and sulfuric acid (96%)
(0.12 mL). The mixture was stirred, then potassium iodate
(0.88 mmol, 0.187 g) and iodine (resublimed) (2.1 mmol, 0.533 g)
were added. The reaction was stirred and heated to reflux for 18 h.
Next, the reaction was allowed to cool and ice water (5 mL) was
added. The precipitate was collected by filtration and dried. The
crude product was redissolved in chloroform and washed with
a solution of Na2O3S2. Afterwards, the organic layer was separated
and dried to give product as brown oil (0.716 g, 98%).
Scheme 1. Synthetic route of picolinamide-PV (I).
1H NMR (500 MHz CDCl3):
d 3.86 (s, 18H), 3.95 (m, 4H), 4.26 (m,
4H), 6.76 (s, 4H), 7.04 (d, 2H, J¼16.5 Hz), 7.13 (s, 2H), 7.36 (m, 4H),
7.81 (t, 2H, J¼7.5 Hz), 8.15 (d, 2H, J¼7.5 Hz), 8.35 (d, 2H, J¼3.6 Hz),
8.54 (m, 2H); LRMS (NaFormate): [MþH]þ, found 791.33. UVevis
(methanol, lmax): 388 nm. Photoluminescence (methanol,
lex¼380 nm): ¼440 nm (max), 466 nm (shoulder).
l
Compound III: Into a dry flask under a nitrogen atmosphere,
3,4,5-trimethoxystyrene (2.00 mmol, 0.39 g), 1,4-diethoxy-2,5-
diiodobenzene (1.00 mmol, 0.39 g), tri(o-tolyl)phosphine
(0.48 mmol, 0.15 g) and palladium diacetate (0.035 mmol, 0.0078 g)
were added. Anhydrous dimethylformamide (12.5 mL) was added
as a solvent. Triethylamine (5.02 mmol, 0.7 mL) was added when
the temperature has reached 80 ꢃC. The reaction mixture was then
stirred at 100 ꢃC for 2 days. The reaction was allowed to cool, and
then poured into water. The precipitate was collected by filtration
and further purified by column chromatography to yield a yellow
powder (0.069 g, 7%).
4.3. Synthesis of oligo-PV II (Scheme 2)
Into a dry flask under a nitrogen atmosphere, 1,2,3-trimethoxy-
5-vinylbenzene (1 mmol, 0.19 g), 1,4-diiodobenzene (0.5 mmol,
0.16 g), tri(o-tolyl)phosphine (0.24 mmol, 0.073 g) and palladium
diacetate (0.018 mmol, 0.0039 g) were charged. Anhydrous dime-
thylformamide (6 mL) was added as a solvent. Triethylamine
(2.51 mmol, 0.35 mL) was added when the temperature has
reached 80 ꢃC. The reaction mixture was then stirred at 100 ꢃC for 3
days. The reaction was allowed to cool, and then evaporated to give
crude product. After purification by column chromatography (silica,
hexanes/ethyl acetate, 1:1), the product was obtained as brown
solid (0.15 g, 66%).
1H NMR (CDCl3):
d 1.42 (m, 6H), 3.87 (s, 6H), 3.92 (s, 12H), 4.17
(m, 4H), 6.85 (s, 4H), 6.97 (d, 2H, J¼16.0 Hz), 7.02 (s, 2H), 7.38 (d, 2H,
J¼16.0 Hz). UVevis (acetonitrile, lmax): 390 nm. Photo-
luminescence (acetonitrile, lex¼365 nm): ¼446 nm (max), 472 nm
l
(shoulder).
4.5. Metal ion-sensing experiments
Picolinamide-PV solution (1.5ꢀ10ꢁ6 M) was prepared in aceto-
nitrile. Metal ions, such as Zn(II), Mn(II), Cu(II), Ni(II), Co(II), Cu(II),
Cd(II) and Pb(II) were also prepared in acetonitrile at a concentra-
tion of 1ꢀ10ꢁ4 M. The picolinamide-PV solution was mixed with
each metal ion solution at the volume ratio of 1:1. Fluorescence
emission spectra of the mixtures were measured to observe the
changes. The oligo-PVs II and III were prepared and mixed at the
same manner.
Scheme 2. Synthetic route of oligo-PV II.
1H NMR (500 MHz CDCl3):
d 3.86 (s, 6H), 3.91 (s, 12H), 6.73 (s,
4H), 7.01e7.03 (dd, 4H), 7.49 (s, 4H); LRMS (NaFormate): [MþH]þ,
found 463.21. UVevis (acetonitrile, lmax): 368 nm. Photo-
luminescence (acetonitrile, lex¼365 nm): ¼448 nm (max), 478 nm
l
(shoulder).
4.6. Quantum chemical calculations
4.4. Synthesis of ethoxy-PV (III) (Scheme 3)
Quantum chemical calculations were performed to gain insight
into the electronic properties of picolinamide-PV. The ground state
was fully optimized using the B3LYP/6-31G(d) by Gaussian03 pro-
gram. The calculations of the excitation energies were calculated at
the TD-B3LYP/6-31G(d). All calculations were performed with PCM
calculation using methanol as the solvent. Moreover, the metal-
bound molecules were then investigated at different electronic
states using B3LYP/6-31G(d) with full geometry optimizations
without any constraints. The absorption spectra were obtained
1,4-Diethoxybenzene (A): In a dry flask, a solution of hydroqui-
none (6.00 mmol, 0.661 g) and bromoethane (14.4 mmol, 1.57 g) in
60 mL of DMF was stirred and heated to 90 ꢃC. A powder of K2CO3
(14.4 mmol, 1.99 g) was slowly added into the reaction mixture.
After that, the solution was stirred at 90 ꢃC overnight. The resulting
mixture was allowed to cool and then cold water was added. The
resulting brown precipitate was collected by filtration. The product
was collected and dried to give light brown crystals (0.318 g, 32%).

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The authors would like to thank the Sensor Cross-Cutting Plat-
form of the National Science and Technology Development Agency
(NSTDA), Thailand for the financial support (Project number P-10-
10066). The computational resources at the Laboratory for Com-
putational
& Applied Chemistry (LCAC), Kasetsart University,
Thailand and the large scale simulation research laboratory of
NECTEC, Thailand are also acknowledged.
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