Tetrahedron Letters
Competitive intra- and inter-molecular proton transfer in hydroxy-
naphthyl benzothiazole: selective ratiometric sensing of acetate
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Shyamaprosad Goswami , Avijit Kumar Das, Krishnendu Aich, Abhishek Manna
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The receptor, naphthalene based benzothiazole (NHBT) shows acetate selectivity in its absorbance and
emission behavior among the various interfering anions. Substantially red shifted absorption band and
fluorescence emission of NHBT in pure acetonitrile media were developed which drastically got enhanced
upon addition of tetrabutylammonium acetate. Excited-state intermolecular proton transfer in the sen-
sor–anion hydrogen-bonding complex was suggested to be the signaling mechanism. NHBT, a good
hydrogen-bonding donor, the ESIPT process is inhibited by the formation of a strong acetate-NHBT inter-
molecular hydrogen bond complex, and the inhibition mechanism consequently results in ratiometric
response in absorption and emission spectroscopy, respectively.
Received 22 February 2013
Revised 24 May 2013
Accepted 28 May 2013
Available online 7 June 2013
Keywords:
Naphthalene
Benzothiazole
ESIPT
Ó 2013 Elsevier Ltd. All rights reserved.
Chromogenic
Fluorogenic
The development of optical molecular or polymeric systems
capable of sensing various biologically and/or environmentally
important anions has generated significant interest in recent
years.1,2 Anions are mainly recognized through hydrogen-bonding
interactions3, electrostatic interactions,4 coordination through
metal ions5 etc. Among various non-covalent interactions, hydro-
gen-bonding interactions are particularly useful and effective in
this regard. Different signaling mechanisms, viz. photo-induced
electron transfer (PET),6 intramolecular charge transfer (ICT),7
metal-to-ligand charge transfer (MLCT),8 excited state intramolec-
ular proton transfer (ESIPT),9 fluorescence resonance energy trans-
fer (FRET),10 etc. have been used to design anion sensors. Among
these fluorescence phenomena, the excited state intramolecular
proton transfer (ESIPT) has attained a special status due to vast
applications as laser dyes,11 UV-photostabilizer,12 scintillators,13
membrane,14 and protein probes15 and as a potential component
for photoswitches and organic LEDs.16 In the ESIPT molecule, the
intramolecular hydrogen bond formed (IHB) usually takes place
in the molecules bearing an H-bond donor group usually a pheno-
lic/amino moiety associated with basic site (O, N) in the ground
state through H-bond interactions.17 This covalently attached
proton in the electronically excited state migrates to a neighboring
hydrogen-bonded atom which is less than 2 Å away. To achieve
high binding affinity and good selectivity, hydrogen bonding
moieties are arranged in space in a rigid and convergent manner.
In addition, receptors bearing multiple hydrogen bonding moieties
have been shown to be useful to promote cooperative binding,
which would result in enhanced binding affinity.18 Moreover, as
a subset of the anion sensors, the chromogenic anion sensors have
shown unique merit, because they can reveal the host–anion bind-
ing information through a change of color.19
The acetate ion plays an important role in biochemistry, envi-
ronment, and pharmaceutical science.20,2f,21 Selective sensing of
acetate is particularly topical22 due to its significance in numerous
metabolic processes for example, a significant number of metal en-
zymes having cations in their active sites influence various hydro-
lysis processes. The rate of acetate production and sedimentation
has frequently been used to indicate the organic decomposition
in marine sediments. Carboxylate anions play an important role
in nylon industry. They are also used in the manufacture of paper,
plastics, dyes, and paints. There are only a few reports on ratiomet-
ric fluorescence where the changes in fluorescence emission
wavelength occur on interaction with acetate ions. Ratiometric
fluorescence measurements can increase the selectivity and sensi-
tivity of the detection of the analyte because the ratio of fluores-
cence intensities at two wavelengths is independent of the
concentration of the sensor.
In the design of anion receptors based on proton transfer mech-
anisms, the proton transferring ability of the ligand controls the
sensitivity and selectivity. In the case of fluorescent molecules with
ESPIT ability, on addition of different anions, the intermolecular
proton transfer from ligand to anion may restrict the intramolecu-
lar proton transfer mechanism within the molecule. These two
competitive phenomena, if signaled by variation in absorption or
emission wavelengths and/or their intensities, would provide
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0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.