TD-DFT study on the fluorescent chemosensor for Hg2+, 2-(Benzo-d-thiazol-2-yl)quinoline

Article abstract of DOI:10.1016/j.molstruc.2011.02.002

2-(Benzo-d-thiazol-2-yl)quinoline (BTQ) was synthesized and tested for its use as a fluorescent chemosensor for Hg²⁺. The metal-ion recognition of BTQ was investigated by adding several metal ions (M²⁺) to a solution of BTQ in acetonitrile. The shape and intensity of the fluorescence spectra did not change upon addition of several metal ions, except for Hg ²⁺ and Cu²⁺. The time-dependent density functional calculations were carried out on BTQ and BTQ-M²⁺ complexes (M = Zn, Cd, and Hg) to clarify their spectroscopic properties. The results of the calculation showed that the energy levels of the excited singlet and triplet states are similar in Zn²⁺ and Cd²⁺ complexes, whereas the Hg²⁺ complex has lower transition energy. The character of their excited states was also different between zinc and cadmium complexes with the mercury complex.

Full text of DOI:10.1016/j.molstruc.2011.02.002

Journal of Molecular Structure 991 (2011) 73–78
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
TD-DFT study on the fluorescent chemosensor for Hg2+,
2-(Benzo-d-thiazol-2-yl)quinoline
⇑
⇑
Ryo Miyamoto , Jun Kawakami , Rie Ohtake, Masahiko Nagaki, Shunji Ito
Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
2-(Benzo-d-thiazol-2-yl)quinoline (BTQ) was synthesized and tested for its use as a fluorescent chemo-
sensor for Hg2+. The metal-ion recognition of BTQ was investigated by adding several metal ions (M2+
)
Received 13 November 2010
Received in revised form 4 February 2011
Accepted 6 February 2011
Available online 1 March 2011
to a solution of BTQ in acetonitrile. The shape and intensity of the fluorescence spectra did not change
upon addition of several metal ions, except for Hg2+ and Cu2+. The time-dependent density functional cal-
culations were carried out on BTQ and BTQ–M2+ complexes (M = Zn, Cd, and Hg) to clarify their spectro-
scopic properties. The results of the calculation showed that the energy levels of the excited singlet and
triplet states are similar in Zn2+ and Cd2+ complexes, whereas the Hg2+ complex has lower transition
energy. The character of their excited states was also different between zinc and cadmium complexes
with the mercury complex.
Keywords:
Hg-sensitive fluoroionophore
TD-DFT
Luminescence mechanisms
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
complexes of the BTQ (M = Zn, Cd, and Hg) to clarify their molecu-
lar and electronic structures and their spectroscopic properties.
The development of selective and sensitive chemosensors for
quantitative analysis of metal ions has become extremely impor-
tant for environmental and biological applications [1–5]. In partic-
ular, the detection of Hg2+ has become one of the most important
topics owing to the highly toxic nature of many mercury com-
pounds [6–8]. In a recent review [9], some fluoro- and chromo-
genic chemodosimeters for heavy metal ions, especially the
mercury(II) ion, were also summarized.
Therefore, we attempted to synthesize a new fluorescence
chemosensor for the mercury ion. Recently, 2-(Benzo-d-thiazol-
2-yl)quinolines (BTQs) were synthesized as fluorescent chemosen-
sors for Hg2+ [10]. The structural formula of BTQ is shown in Fig. 1;
the geometric (rotational) isomers are discussed below. The BTQs
showed more distinctive Hg2+-selective fluoroionophoric proper-
ties over the other tested metal ions. Therefore, the spectroscopic
properties and the mechanisms of their luminescence become
the subject of much interest.
In this paper, the metal-ion recognition of BTQ was investigated
by adding Ca2+, Ba2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+ to an ace-
tonitrile solution. Consequently, it was shown that BTQ had dis-
tinctive Hg2+-selective fluoroionophoric properties over other
tested metal ions. Furthermore, the emission maxima of the
BTQ–Hg2+ complexes shifted to longer wavelengths with respect
to those of the other metal complexes and the BTQ itself. In the
next step, the ab initio calculations were performed on the M2+
2. Methods
2.1. Synthesis and measurements
BTQ was obtained by the reaction of 2-aminobenzenethiol with
quinoline-2-carbaldehyde. The MS spectra of BTQ showed corre-
sponding molecular ion peaks, and the NMR spectra were suitable
[10]. Stock solutions of BTQ were prepared by dissolving a weighed
amount of BTQ in acetonitrile. The titration of BTQ ([BTQ] = 10 lM,
M = mol dmꢀ3) against some metal ion solutions was performed in
a spectrophotometric cell of 1 cm path length. The UV–visible
spectra (between 50,000 and 16,700 cmꢀ1 (200 and 600 nm,
respectively)) of the resulting solutions were recorded at room
temperature with a Hitachi U-2001 spectrophotometer after the
addition of each of the metal salts (Cd(ClO4)2, Ni(ClO4)2, Zn(ClO4)2,
Co(ClO4)2, Ca(ClO4)2, Ba(ClO4)2, Cu(ClO4)2, and Hg(ClO4)2). The fluo-
rescence spectra were measured between 25,000 and 16,700 cmꢀ1
(400 and 600 nm, respectively) with a Hitachi F-4500 fluorometer
using an excitation wavelength of 28,600 cmꢀ1 (350 nm). The titra-
tions were performed with metal ions (1–100
and BTQ (10 M) as the titrate. The metal-ion sources were identi-
cal to those used to perform the UV–visible spectroscopic studies.
lM) as the titrant
l
2.2. Calculation procedure
⇑
Corresponding authors.
E-mail addresses: rmiya@cc.hirosaki-u.ac.jp (R. Miyamoto), jun@cc.hirosaki-
All the ab initio calculations were performed by the Gaussian 03
u.ac.jp (J. Kawakami).
program [11]. The molecular geometries were optimized by B3LYP
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.02.002

74
R. Miyamoto et al. / Journal of Molecular Structure 991 (2011) 73–78
0.50
(a)
(b)
N
0.40
N
N
S
0.30
0.20
0.10
0.00
S
N
50
45
40
35
30
25
20
Fig. 1. Structural formula of BTQ. (a) NS-conformer, (b) NN-conformer.
Wavenumber, σ / 103 cm-1
Fig. 2. UV–vis absorption spectra of BTQ with Hg2+ in acetonitrile solution at room
temperature. [BTQ] = 10
M, [Hg2+] = 0.1–1 equiv.
[12–14] with a 6-31G basis set for the CHNO elements, 6-31G(d)
for S [15], and Stuttgart/Dresden basis set/effective-core-potential
(ECP) for Zn, Cd, and Hg (using SDD keyword) [16,17]. The consis-
tency in the calculation through the group 12 elements was pre-
served by using the same basis set/ECP. The excitation energies
and molecular properties were calculated by a time-dependent
DFT (TD-DFT) method [18–22] with a B3LYP functional and 6-
31+G(d) basis set [15] for non-metal elements and SDD basis set/
ECP for metals.
According to the results of the titrating luminescence measure-
ment, the stoichiometry of 1:1 for the BTQ–M2+ complexes were
assumed (vide infra, [10]). BTQ works as a bidentate chelating li-
gand, and the divalent metal ions M2+ tested in this work usually
accept four donor atoms. Therefore, some water molecules were
used to fill the remaining coordinating site of the M2+ in the
BTQ–M2+ complex; the BTQ–M2+–2aq structure was assumed
(aq = water molecule, H2O). It must be pointed out that unrealistic
calculation results were obtained for the BTQ–M2+ complexes
without these water molecules, in which the triplet states were
energetically lower than the lowest singlet state that is normally
called the ground state, S0. This impropriety was corrected by plac-
ing these molecules in a polar solvent or by coordinating two water
molecules to the M2+ ion to construct the BTQ–M2+–2aq structure.
Furthermore, BTQ has conformational isomers with the C–C bond
connecting the quinoline moiety and the benzothiazole. The S-cis
conformer with respect to quinoline-N and benzothiazole-S is de-
noted by ‘NS-conformer’, and S-trans as ‘NN-conformer’ (see Fig. 1).
l
Changes in the fluorescence spectra were also observed upon
addition of Cd2+, Ni2+, Zn2+, Co2+, Ca2+, Ba2+, Cu2+, or Hg2+ to the
solution of BTQ in acetonitrile. The shape and intensity of the fluo-
rescence spectra after excitation at 28,600 cmꢀ1 did not change
upon addition of Cd2+, Ni2+, Zn2+, Co2+, Ca2+, and Ba2+. However,
the fluorescence emission of BTQ was quenched with the addition
of Hg2+ or Cu2+. The fluorescence spectra of BTQ in the presence of
several concentrations of Hg(ClO4)2 are shown in Fig. 3; here, a
weak red-shifted emission maxima was also observed. In other
words, the emission maxima of BTQ with Hg2+ was at
21,600 cmꢀ1, while the emission maxima of free BTQ was at
24,400 cmꢀ1
.
There may be some uncertainty concerning the complex forma-
tion between BTQ and the metal ions because the UV–visible
absorption spectra showed little change upon changing the metal
ions, except mercury or copper. However, according to the results
of the ab initio calculation, it could be possible that the complex
14
12
10
8
3. Results and discussion
3.1. Absorption and luminescence spectra
6
The changes in the UV–visible absorption spectra after adding
various metal ions to the solution of BTQ were observed. The
absorption spectra of BTQ in acetonitrile did not show changes in
shape and absorbance upon addition of the metal ions, such as
Cd2+, Ni2+, Zn2+, Co2+, Ca2+, or Ba2+; however, there were changes
in shape and absorbance with the addition of Cu2+ or Hg2+, and a
new absorption band appeared after 28,000 cmꢀ1. The absorption
spectra of BTQ in the presence of several concentrations of
Hg(ClO4)2 are shown in Fig. 2; here, an isosbestic point was ob-
served at 28,500 cmꢀ1. Using the absorbance at 38,000 and
32,500 cmꢀ1, titration curves were produced that show a sharp
endpoint at a 1:1 ligand-to-metal ion ratio for Hg2+ [10].
4
2
0
30
25
20
15
Wavenumber, σ / 103 cm-1
Fig. 3. Fluorescence spectra of BTQ excited at 28,600 cmꢀ1 (350 nm) with Hg2+ in
an acetonitrile solution at room temperature. [BTQ] = 10
M, [Hg2+] = 0.1–1 equiv.
l

R. Miyamoto et al. / Journal of Molecular Structure 991 (2011) 73–78
75
Table 1
Energy levels (E/cmꢀ1) and oscillator strengths (f) for BTQ–M(II)–2aq complexes.
State
Free basec
NS
Zn(II)
NS
Cd(II)
NS
Hg(II)
NS
NN
NN
NN
NN
DfEa
S0
S1
(f1)
S2
(f2)
S3
(f3)
S4
(f4)
S5
(f5)
T1
–
–
–
–
–
ꢀ1435
–
–
ꢀ1176
–
–
ꢀ1244
b
2301.55
29572.3
(0.6183)
30136.1
(0.0096)
30220.0
(0.0006)
30988.6
(0.0260)
34626.2
(0.0008)
19434.7
23964.3
26148.496
26216.2
29601.4
ꢀ9001.51
23315.9
(0.0329)
24249.9
(0.0591)
25129.8
(0.5066)
31366.1
(0.0001)
32004.1
(0.0172)
16878.8
19572.7
19829.1
25590.4
29052.1
ꢀ7442.70
23780.45
(0.0263)
24704.0
(0.0991)
24894.3
(0.0030)
25425.8
(0.4667)
26129.1
(0.0034)
17272.4
19931.6
20105.8
24631.4
25984.0
ꢀ6178.53
18160.4
(0.0000)
19376.7
(0.0000)
20519.6
(0.0000)
23519.9
(0.0271)
24473.3
(0.1147)
17258.7
17761.1
19190.4
19803.3
19899.3
28960.1
(0.2900)
29925.6
(0.3475)
31008.8
(0.0377)
32247.7
(0.0007)
35376.3
(0.2442)
19778.3
23520.7
25885.6
28435.9
29602.2
25266.9
(0.3249)
25909.8
(0.0248)
26409.8
(0.0477)
28181.8
(0.0274)
30382.9
(0.0044)
18430.6
20271.9
23909.5
25623.4
27597.9
21012.4
(0.0224)
23012.6
(0.0074)
23643.3
(0.0151)
26465.47
(0.2528)
27417.2
(0.0189)
18205.5
20211.4
21587.4
21933.4
23560.3
14413.9
(0.0044)
17069.9
(0.0277)
17653.9
(0.0217)
22890.0
(0.0001)
25043.5
(0.1274)
13630.8
14467.2
16745.7
19175.0
20865.6
T2
T3
T4
T5
a
b
c
Complex formation energy in kJ molꢀ1 unit. See text for details.
Energy difference between NS-conformer and NN-conformer in cmꢀ1 unit.
CS symmetry.
Fig. 4. Optimized structures of BTQ–M2+–2aq complexes.

76
R. Miyamoto et al. / Journal of Molecular Structure 991 (2011) 73–78
formation makes little change in the spectra upon changing the
metal ions other than mercury (vide infra). Regardless of whether
or not several metal ions make a coordination complex with BTQ,
it is clear that Hg2+ and BTQ form a complex in the 1:1 manner
[10]; this result indicates that BTQ showed distinctive Hg2+-selec-
tive fluoroionophoric properties over other tested metal ions.
nar BTQ are broken in the NS-conformers. This means that the
lone-pair or the orbital lobe of the sulfur is directed toward a tilted
angle with the in-plane and/or perpendicular to the five-mem-
bered ring. Because the C–S–C angle is approximately 90°, the sul-
fur has sp3 hybridization rather than sp2.
The transition energy levels of BTQ in the NS-conformation and
those of the BTQ–M2+ complexes with two water molecules are
shown in Fig. 5; note that only the lowest five levels of the singlet
and triplet states are depicted. The solid bar at each singlet energy
level indicates the oscillator strength of an individual transition
from the ground state. Detailed values are shown in Table 1. The
observed electronic absorption/emission spectra can be inter-
preted using the results of the ab initio calculation.
3.2. Ab initio calculation
The calculated transition energies and oscillator strengths are
summarized in Table 1. A series of the optimized structures of
the BTQ–M2+–2aq complexes (M = Zn, Cd, Hg) are depicted in
Fig. 4, in which these complexes have tetrahedral coordination
structures for all metal ions.
The lowest singlet-excited state of BTQ, S1 at 28,960 cmꢀ1, is
characterized as a pp⁄ transition, which is constructed mainly from
the HOMO(68)–LUMO(69) transition; note that the word ‘‘molecu-
lar orbital’’ means Kohn–Sham orbital in this context. The lowest
For the BTQ molecule two conformers are possible (see Fig. 1),
and each conformer gives rise to a different type of the coordina-
tion complex. The energy of S0 in the table shows the energy differ-
(D ) between the NS-conformer and the NN-
E/cmꢀ1
n
p⁄ transition of MO(65)–LUMO(69) appears at 32,248 cmꢀ1 as
ences
S4, for which the MO(65) has a lone pair of nitrogen at the quino-
line ring and has some contribution from the lone pairs of another
nitrogen and sulfur (Fig. 6).
conformer of each compound in the ground state. It is interesting
that the NS-conformer has lower energy than the NN-conformer
in BTQ (D
E = 2302 cmꢀ1 = 27.53 kJ/mol), while the NN-conformer
The results show that the lowest singlet-excited state, S1, for all
complexes under study is lowered by forming a metal complex.
The energy levels of the first three singlet states and their oscillator
strengths match well in Zn2+ and Cd2+ complexes; the first four
triplet states match as well. It is very interesting that the S1 level
of the Zn2+ complex is similar to that of the Cd2+ complex but is
not similar to the Hg2+ complex. Moreover, the Sn levels with large
is more stable compared to the NS-conformer in all the BTQ–
M2+–2aq complexes. According to the hard-soft-acid-base (HSAB)
theory, the heavier metal ion, Hg2+, is preferred over the soft donor
atom, S, and vice versa. However, in our results, the Hg2+ complex
has a more stable conformation with the NN-conformer, which is
also true for the Zn2+ and Cd2+ complexes. Furthermore, it is
remarkable that the optimized structure of the BTQ–M2+ com-
plexes shows that the NS-conformation makes a distortion or twist
between the quinoline–thiazole bond (Fig. 4). The dipole moment
of BTQ is 1.39 Debye in the NS-conformer, which is smaller than
the 2.31 Debye for the NN-conformer. In addition, the NN-con-
former has steric hindrance in sulfur and hydrogen, such that the
BTQ in the NS-conformer is slightly more stable. These properties
are very similar to 2,20-bipyridine in that the S-trans form is stable
because of the steric hindrance between the hydrogens at the 3,30-
position. While in the metal complexes of the BTQ, the NN-con-
oscillator strengths have nearly the same energy through Zn2+
,
Cd2+, and Hg2+ complexes (ca. 25,000 cmꢀ1), which are pp⁄ transi-
tions (vide infra). These results coincide with the observed elec-
tronic absorption spectra in the visible region; a new absorption
band can be measured by adding metal salts to BTQ, and a similar
change was observed for all the Zn2+, Cd2+, and Hg2+ salts (Fig. 2).
As well as the energy levels, the properties of the excited states
of the metal complexes are also interesting. The excited states of
S1–S3 of the Zn2+ complex have pp⁄ character; these states are cre-
ated by the various combination of the transitions from HOMO(87),
formers are stable because p-electron conjugation-systems in pla-
40000
35000
30000
25000
20000
15000
10000
5000
S
T
S
T
S
T
S
T
BTQ, NS
Zn, NN
Cd, NN
Hg, NN
0
Fig. 5. Transition energy diagram of BTQ–M2+–2aq complexes. M2+ = metal free, Zn2+, Cd2+, Hg2+. The lowest five levels are shown for both singlet (S) and triplet (T) states of
the complexes; and the bold part of the singlet level indicates the oscillator strength.

R. Miyamoto et al. / Journal of Molecular Structure 991 (2011) 73–78
77
LUMO(69)
MO(89)
S1
S4
ππ*
ππ*
HOMO(68)
LUMO(88)
S4
nπ*
MO(65)
HOMO(87)
S1
LMCT
Fig. 6. Selected MOs of BTQ as NS-conformer.
MO(86), and MO(85) to LUMO(88) (Fig. 7). However, in the Hg2+
complex, the lowest three excited states have ligand-to-metal
charge-transfer (LMCT) character; those states consist of the tran-
sition of HOMO(87)–LUMO(88), MO(86)–LUMO(88), and MO(85)–
LUMO(88), respectively, for which the LUMO(88) has mainly a cen-
tral metallic character. The S4 of the Hg2+ complex is constructed
by the transitions from HOMO(87), MO(86), and MO(85) to next-
LUMO(89), which are pp⁄ transitions (Fig. 8).
S2
LMCT
MO(86)
MO(85)
When the Hg2+ ion was added to the solution of BTQ, the lumi-
nescence spectrum was very different from those that added other
S3
LMCT
LUMO(88)
Fig. 8. Selected MOs of BTQ–Hg2+–2aq as NN-conformer.
cations. The transition energy diagram depicted in Fig. 5 shows
that the energy levels of the Zn2+ and Cd2+ complexes were similar
to each other; however, that of Hg2+ was different from the others.
The former two have triplet states lying apart from the S1 state,
such that a nonradiative decay process may be prohibited in these
cases. Whereas in the latter case, the lowest singlet state ap-
proaches the lower triplet states. Then, its luminescence is de-
creased in intensity and is red-shifted in wavelength. Another
possible interpretation of the experimental results would be that
BTQ does not make a coordination complex with Zn2+ or Cd2+ ions,
while only Hg2+ makes a BTQ–Hg2+–2aq complex, which leads to
the change in the spectra. However, it cannot be explained from
our calculations that Hg2+ takes precedence to form a complex
with BTQ over the other metal ions because the complex formation
energies of these ions estimated by the following equation were
comparable with each other (see Table 1):
S1~S3
ππ*
HOMO(87)
MO(86)
Df E ¼ Ecomplex ꢀ ðEM2þ þ EBTQ þ 2 ꢁ EwaterÞ;
where Es are the total electronic energies of each compound.
Finally, it may be necessary to comment on a failure of TD-DFT
in the excited state calculations. It is known that TD-DFT some-
times produces spurious charge-transfer excited states for large
molecules or long-range charge-transfer systems [23,24], although
DFT is one of the most powerful and successful quantum chemical
tools for investigating molecular and electronic structures. The sys-
MO(85)
Fig. 7. Selected MOs of BTQ–Zn2+–2aq as NN-conformer.

78
R. Miyamoto et al. / Journal of Molecular Structure 991 (2011) 73–78
tems under study, BTQ–M2+–2aq, are not as large as the systems in
the literature discussed above, and the moieties of the electron do-
nor and accepter have some orbital overlap, which does not indi-
cates ‘‘long-range’’. However, the low-lying LMCT states for BTQ–
M2+–2aq were investigated by configuration interaction singles
(CIS and CIS(D)) [25–27] to confirm the TD-DFT results. The results
of the HF calculation show that excited energy levels and the char-
acter of MOs around the HOMO and LUMO were coincident within
all three complexes (M = Zn, Cd, Hg), while in the results by DFT,
the Hg2+ complex had different properties from the other two com-
plexes (Figs. 5, 7, and 8). It seems that this HF result does not di-
rectly indicate a failure of the DFT calculation because DFT
usually provides more reliable electronic structure in metal com-
plexes than does HF. In fact, the orbital energies of LUMO(88)
and next-LUMO(89) of the Hg2+ complex given by HF were very
close (almost 1/10 order) with respect to the Zn2+ complex case,
and the lowest LMCT state of the Hg2+ complex was only about
1000 cmꢀ1 above the ligand-centered S1 state (by CIS(D) calcula-
tion). Therefore, it would be difficult for the HF level calculation
to provide an accurate estimation for the electronic structures of
the compound including heavy metal elements; further progress
is required in this region.
mercury complex, and the S1 of the former is a pp⁄ transition,
while that of the latter is LMCT.
The present work was partially supported by a Grant-in-Aid for
Scientific Research (C) (No. 17550070) from JSPS.
References
[1] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition,
Kluwer Academic Publishers, London, 1997.
[2] A.W. Czarnik, Fluorescent Chemosensors for Ion and Molecule Recognition,
American Chemical Society, Washington, DC, 1993.
[3] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.T.M. Huxley, C.P. McCoy, J.T.
Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515.
[4] L. Prodi, F. Bolletta, M. Montalti, N. Zaccheroni, Coord. Chem. Rev. 205 (2000)
59.
[5] C.W. Rogers, M.O. Wolf, Coord. Chem. Rev. 233–234 (2002) 341.
[6] D.W. Boening, Chemosphere 40 (2000) 1335.
[7] D. Mohan, V.K. Gupta, S.K. Srivastava, S. Chander, Colloids Surface A 177 (2001)
169.
[8] M.F. Yardim, T. Budinova, E. Ekinci, N. Petrov, M. Razvigorova, V. Minkova,
Chemosphere 52 (2003) 835.
[9] D.T. Quang, J.S. Kim, Chem. Rev. 110 (2010) 6280.
[10] J. Kawakami, R. Ohtake, R. Miyamoto, M. Nagaki, S. Ito, Trans. MRS-J 33 (2008)
9.
[11] Gaussian 03, Revision D.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E.
Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin,
J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M.
Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J.
Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas,
D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G.
Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,
C. Gonzalez, J.A. Pople, Gaussian, Inc., Wallingford, CT, 2004.
[12] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[14] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200.
[15] W.J. Hehre, L. Radom, P.v.R. Schleyer, J.A. Pople, Ab Initio Molecular Orbital
Theory, Wiley-Interscience, New York, 1989.
[16] M. Dolg, U. Wedig, H. Stoll, H. Preuss, J. Chem. Phys. 86 (1987) 866.
[17] D. Andrae, U. Häußermann, M. Dolg, H. Stoll, H. Preuß, Theor. Chim. Acta 77
(1990) 123.
[18] E. Runge, E.K.U. Gross, Phys. Rev. Lett. 52 (1984) 997.
[19] E.K.U. Gross, W. Kohn, Phys. Rev. Lett. 55 (1985) 2850.
[20] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218.
[21] R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 454.
[22] M.E. Casida, C. Jamorski, K.C. Casida, D.R. Salahub, J. Chem. Phys. 108 (1998)
4439.
[23] A. Dreuw, M. Head-Gordon, J. Am. Chem. Soc. 126 (2004) 4007.
[24] R.J. Magyar, S. Tretiak, J. Chem. Theory Comput. 3 (2007) 976.
[25] J.B. Foresman, M. Head-Gordon, J.A. Pople, M.J. Frisch, J. Phys. Chem. 96 (1992)
135.
4. Conclusion
The metal-ion recognition of BTQ was investigated by adding
several metal ions (M2+) to a solution of BTQ in acetonitrile. The
shape and intensity of the fluorescence spectra did not change
upon addition of several metal ions, including Cd2+, Ni2+, Zn2+
,
Co2+, Ca2+, and Ba2+. However, the fluorescence emission of BTQ
was quenched with the addition of Hg2+ or Cu2+, and a red shift
of the emission maxima was observed when Hg2+ was added to
the solution. While the emission maxima of free BTQ was at
24,400 cmꢀ1, the emission maxima of BTQ with Hg2+ was at
21,600 cmꢀ1
.
The ab initio calculations using the time-dependent density
functional method with a 6-31+G(d) basis set and ECP for metals
were carried out on the zinc, cadmium, and mercury complexes
of BTQ. Then, the source of the spectroscopic properties shown
above was investigated. The results of the calculation showed the
following: (1) all the metal complexes calculated are more stable
for the NN-conformer rather than NS-conformer, while BTQ itself
has lower energy for the NS-conformer; (2) the energy levels of
the excited singlet and triplet states are similar in the Zn2+ and
Cd2+ complexes, while the Hg2+ complex has lower transition ener-
gies than others; and (3) the character of the excited states were
also different between the zinc and cadmium complexes with the
[26] J. Baker, J. Comput. Chem. 7 (1986) 385.
[27] J. Baker, J. Comput. Chem. 8 (1987) 563.