PH triggered dye chemosensor: Design, synthesis and optical switching properties

Article abstract of DOI:10.1080/15421406.2012.701851

A novel dye chemosensor based on indole as a donor unit and quinaldium as an acceptor unit have caused promising optical changes when used as a reversible, pH-induced, molecular switching in the presence of alkali/acid stimulus. The dye chemosensor also exhibited marked fluorescence quenching in the presence of alkali addition, which are based on intramolecular charge transfer (ICT) system. The level of energy potential and the electron flowing of this dye chemosensor such as HOMO and LUMO values were calculated by computational calculation approaches. Copyright Taylor and Francis Group, LLC.

Full text of DOI:10.1080/15421406.2012.701851

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pH Triggered Dye Chemosensor:
Design, Synthesis and Optical Switching
Properties
b
a
b
a
Eun-Mi Lee , Seon-Yeong Gwon , Young-A Son & Sung-Hoon Kim
c
a
BK21 FTIT Department of Advanced Organic Materials and Textile
System Engineering, Chungnam National University, Daejeon,
3
05-764, S. Korea
b
Department of Textile System Engineering, Kyungpook National
University, Daegu, 702-701, S. Korea
c
School of Chemistry Science & Technology, Zhanjiang Normal
University, Zhanjiang, 524048, P.R. China
Published online: 12 Sep 2012.
To cite this article: Eun-Mi Lee , Seon-Yeong Gwon , Young-A Son & Sung-Hoon Kim (2012) pH
Triggered Dye Chemosensor: Design, Synthesis and Optical Switching Properties, Molecular Crystals
and Liquid Crystals, 566:1, 106-111, DOI: 10.1080/15421406.2012.701851
To link to this article: http://dx.doi.org/10.1080/15421406.2012.701851
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Mol. Cryst. Liq. Cryst., Vol. 566: pp. 106–111, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.701851
pH Triggered Dye Chemosensor: Design, Synthesis
and Optical Switching Properties
1
2
1,∗
EUN-MI LEE, SEON-YEONG GWON, YOUNG-A SON,
AND SUNG-HOON KIM2
,3
1BK21 FTIT Department of Advanced Organic Materials and Textile System
Engineering, Chungnam National University, Daejeon, 305-764, S. Korea
2
Department of Textile System Engineering, Kyungpook National University,
Daegu, 702-701, S. Korea
3
School of Chemistry Science & Technology, Zhanjiang Normal University,
Zhanjiang 524048, P.R. China
A novel dye chemosensor based on indole as a donor unit and quinaldium as an acceptor
unit have caused promising optical changes when used as a reversible, pH-induced,
molecular switching in the presence of alkali/acid stimulus. The dye chemosensor also
exhibited marked fluorescence quenching in the presence of alkali addition, which are
based on intramolecular charge transfer (ICT) system.The level of energy potential and
the electron flowing of this dye chemosensor such as HOMO and LUMO values were
calculated by computational calculation approaches.
Keywords Dye chemosensor; Alkali addition; Anion recognition; Optical switching;
Homo; Lumo
1. Introduction
The promising sensing function of alkali anions using dye chemosensor compound has been
attracted great attention owing to its application importance in both chemical and environ-
mental application [1–6]. The pH triggered potential sensing ability of dye chemosensor
based on optical switching properties is also an interesting research topic. In addition,
the molecular framework design of dye chemosensor that shows sensitive reaction behav-
-
iors towards a specific target anion, name OH is a great challenge for dye chemists and
researchers [7].
This optical switching characteristics of dye chemosensor is importantly based on push
unit-π conjugation-pull unit of intramolecular charge transfer (ICT) system through dye
chromophore or fluorophore molecules and the resulting dye chemosensor can be utilized
∗
Correspondence to: Young-A Son, Department of Advanced Organic Materials and Textile
System Engineering, Chungnam National University, Daejeon 305-764, S. Korea +82 42 821 6620
+
82 42 823 3736. E-mail: yason@cnu.ac.kr
∗
Correspondence to: Sung-Hoon Kim, Department of Textile System Engineering, Kyung-
sp ho oo kk i mN @a t iko nn ua . la cU . kn ri versity, Daegu 702-701, S. Korea +82 53 950 5641 +83 53 950 6617. E-mail:
106

pH Triggered Dye Chemosensor
107
Figure 1. Synthetic route for dye chemosensor (DCS).
as useful sensing probes for checking, determining and monitoring subjecting target anion
detection[1,2,8–13].
Of various types of push-π-pull charge transfer dye, indole moiety is a well-known,
representative, which undergoes intramolecular charge transfer. This is the reason to design
and choose the indole moiety as an electron-donation unit in the dye chemosensor molecular
structure. As the designed dye chemosensor has NH segment within the molecular structure,
-
it can act as H-bond donors for OH anions [7,14,15]. Thus, this indole moiety can be utilized
as electron-donatingand H-bond unit.
In this context, this workreportsthe synthesis of a novel dye chemosensor for pH
triggered optical switching effects based on a push-π-pull, ICT system. Hence, both the
-
promising potential of dye chemosensor (DCS, Fig. 1) to recognize OH anions and the
changes in its absorption/emission properties imparted by pH were discussed [16–20].
2. Experimental
All reagent and chemicals were purchased from Aldrich Chemical Co. and TCI. Solvents
were purified by standard procedures and used under moisture free atmosphere. The other
materials were commercial products and were used without further purification. Elemental
analyses were recorded on a Carlo Elba Model 1106 analyzer. UV-visible absorption
spectra were measured on an Agilent 8453 spectrophotometer whilst fluorescence spectra
were measured on a Shimadzu RF-5301 PC fluorescence spectrophotometer. Mass spectra
were recorded on a Shimadzu QP-1000 spectrometer using electron energy of 70 eV and
1
the direct probe EI method. H NMR spectra were recorded using a Varian Inova 400 MHz
FT-NMR spectrometer with TMS as internal standard. Melting point was determined using
anTA Q600 thermal analyzer.
Quinaldiumsalt1 (0.5g, 1.754 mmol) and indole-3-carboxaldehyde 2 moieties (0.26g,
1.175 mmol) were dissolved in 20 ml of 1-propanol. Piperidine was added dropwise and
the ensuing mixture was refluxed for 7h. The desired solid was obtained with decreasing
to room temperature and then filtration under reduced pressure and washing with hexane
were carried out. The aimed dye chemosensor product (namely Dye ChemoSensor: DCS)
was obtained in 78.8% yield (Fig. 1).
1H NMR(400 MHz) DMSO-d6: δ (ppm) 4.46 (s, 3H), 7.29 (m, 2H), 7.54 (m, 2H), 7.82
(
8
1
t, J = 7.48, 1H), 8.06 (t, J = 7.28, 1H), 8.20(m, 1H), 8.23(d, J = 9.08, 1H), 8.39 (s, 1H),
.43 (d, J = 8.92, 1H), 8.59 (s, J = 9.24, 1H), 8.63 (d, J = 15.44, 1H), 8.81 (d, J = 9.16,
H), 12.31 (s, 1H). EA: anal. calcd. C20H17IN2: C; 58.27, H; 4.16, N; 6.79. foundC; 58.25,
o
H; 4.08, N; 6.64%. Ms = 285.2.mp: 275∼276 C

108
3. Results and Discussion
Indole moiety has been attracted as a useful donor component to design D-π-A system
of dye chemosensor, which is capable of forming the host-guest complex by hydrogen-
anion binding at NH segment [a]. Herein, the selected acceptor component, 1,2-dimethyl-
qunaldium salt 1 was condensed with donor component, indole-3-carboxaldehyde 2 to
produce the target dye chemosensor (DCS).With based on a typical D-π-A structural
framework of dye chemosensor molecule, the intramolecular electron migration charac-
teristics can be developed through DCS structure. The characteristics of pH triggered
opto-switching/-chemosensing effects of DCS influenced by alkali/acid changes were ob-
5
served in acetonitrile (1×10– M), where very promising opto-switching/-chemosensing
behaviors were determined. These obtained results are caused by intramolucular charge
transfer (ICT) system, which are structurally framed with push-π conjugation-pull struc-
tural design. Reversible UV-Vis. absorption and fluorescence emission changes of DCS
with alkali/acid stimulus were represented in Figs 2 and 3.
The chemosensing potential of DCS imparted by pH stimulation was monitored by the
−5
addition ofalkali and acid to a 1×10 M solution of DCS in acetonitrile. The prominent
reversible absorption and fluorescence emission changes are shown in Figs 2 and 3, from
which it is clear that upon the addition of tetrabutylammonium hydroxide to a solution of
DCS in acetonitrile, the peak λmax at 461 nm decreased and a new peak at 526 nm appeared
(Fig. 2); the addition of trifluoroacetic acid then resulted in the absorption peak at 461nm
to re-appear. The fluorescence emission spectral behaviors (Fig. 3) also showed similar
observation. The emission spectrum peak at 548 nm was dramatically quenched with the
addition of tetrabutylammonium hydroxide to the DCS solution and the next step of addition
using trifluoroacetic acid to a emission quenched colorless solution of DCS resulted in a
return to the initial emission state. Therefore, very promising reversible chemosensing
behaviors with UV-Vis absorption and fluorescence emission were accomplished by the
alternate addition of tetrabutylammonium hydroxide and trifluoroacetic acid. There spectral
changing behaviors of DCS could be caused by the deprotonation of NH segment at the
Figure 2. Effect of pH triggered deprotonation/protonation on UV–Vis absorptionswitching of 1 ×
−
5
6
10
M solution of DCS in acetonitrile. [Bu
4
NOH] and [CF
3
COOH] are 1 × 10– M.

pH Triggered Dye Chemosensor
109
Figure 3. Effect of pH triggered deprotonation/protonation on fluorescence emissionswitching of
−
5
6
1
× 10 M solution of DCS in acetonitrile. [Bu
4
NOH] and [CF
3
COOH] are 1 × 10– M.
indole moiety of DCS, which directly influenced on chromogenic and fluorogenic system
of DCS molecule. The distribution of electron density of DCS was rearranged by forming
-
N at indole moiety (see Fig. 1), which greatly imparted intramolecular charge transfer
of DCS. Owing to its well-explained, push and pull conjugation structure that includes
(
NH) hydrogen atom, it isexpected that DCS could function as a receptor molecule for the
-
recognition of OH anions via H-bonding interaction.
pH stimuli-color switching image of DCS is shown in Fig. 4, which was measured by
-
+
the addition of OH and H with increasing molar equivalence. As shown in Fig. 4, DCS
chromophore showed reversible color switching images according to the pH deprotonation
and protonation stimuli-effects.
For modeling calculation of the electron density distribution of DCS (Fig. 5), the
3
quantum chemical DMol determination was conducted. All theoretical calculations were
Figure 4. UV-Vis absorption image photographs of DCS in pH deprotonation (a) and protonation
(
b) reversible effects.

110
Figure 5. Electron distribution of HOMO and LUMO energy levels of DCS.
3
proceeded by DMol program of Materials Studio 4.3 package [12,13], which has the
quantum mechanical code using density functional theory. Perdew-Burke-Ernzerhof (PBE)
function of generalized gradient approximation (GGA) level [14] with double numeric
polarization basis set was used to calculate the energy level of the frontier molecular
orbital. In this context, Fig. 5 displays HOMO and LUMO energy values and electron
density distribution of DCS.
-
Figure 5 shows the electron distribution states in the frontier MOs. After OH addition
reaction to DCS molecule, the electron density distribution was dramatically changed.
The presence of electron clouds was shown in the indole donor part in HOMO state,
but the electrons totally moved to the quinaldium acceptor part in LUMO state. In addition,
-
the resulting energy gapbetween HOMO and LUMO with OH addition was decreased
from ꢀE = 2.523eV to ꢀE = 1,610eV: that of the reacted DCS is smaller than that of
DCS. This means that the alkali reacted DCS is expected to exhibit the bathochromic
absorption compared with that for DCS. This finding can be supported by Fig. 2. The
-
OH addition resulted in a decrease of the shorter absorption band and an increase of new
peak in the longer wavelength region. Thus, this observation is well agreed with a strong
-
electron flowing character of DCS caused by OH nucleophilic addition reaction to the dye
molecules, namely intramolecular charge transfer system (ICT).
4. Conclusions
In this work, we have synthesized the novelpush-π conjugation-pull ICT dye chemosensor
having quinaldium salt as an electron-withdrawing unit and indole as an electron-donating
unit. This dye chemosensor displayed marked changes in UV-Vis absorption and fluo-
rescence emission intensity upon the pH stimulation. The dye exhibitedsizeable color
changes when used as a reversible, pH-induced, molecular switching observation. And
HOMO/LUMO energy potentials and electron density distribution were modeled and sim-
ulatedwith computational calculation.

pH Triggered Dye Chemosensor
111
Acknowledgement
This work was supported by the Basic Science Research Program through the National
Research Foundation (NRF) grant funded by the Korea government (MEST) (No. 2011-
0001084). This research was supported by a grant from the Fundamental R&D Program for
Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic
of Korea.
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