A new type of cation responsive chromoionophore with spectral sensitivity in the near-infrared spectral range

Article abstract of DOI:10.1246/cl.2001.552

A long wavelength absorbing cation responsive chromoionophore 1 based on the strongly π-electron accepting dicyanovinylindane subunit and an electron donating N-phenylazacrown ether binding site was synthesized. The absorption spectra of this compound in

Full text of DOI:10.1246/cl.2001.552

552
Chemistry Letters 2001
A New Type of Cation Responsive Chromoionophore with
Spectral Sensitivity in the Near-Infrared Spectral Range
Daniel Citterio,†,†† Shin-ichi Sasaki,† and Koji Suzuki*†,††
†Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522
††Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012
(Received March 2, 2001; CL-010176)
A long wavelength absorbing cation responsive chromo-
ionophore 1 based on the strongly π-electron accepting
dicyanovinylindane subunit and an electron donating N-phenyl-
azacrown ether binding site was synthesized. The absorption
spectra of this compound in CH3CN show strong changes in the
near-infrared (NIR) spectral range upon binding of cations in
the crown-ether moiety.
The near-infrared spectral range (λ > 750 nm) is favored as
the spectral working range for optical chemical sensors because
of the absence of naturally occurring absorbing species and the
availability of low-cost compact size solid-state devices for
optical measurement systems. Operating in the deep-red region
of the spectrum also eliminates the photodegradation of biologi-
cal compounds often found in the case of UV irradiation. So
far, several NIR absorbing and emitting ion selective chromo-
ligands have been presented in the literature, based on the long
wavelength absorbing squaraine chromophore, characterized by
narrow and extremely intensive absorption bands.1–6 Despite
these useful characteristics, many of the squaraine-based sen-
sing molecules suffer from the drawback of small ion-induced
spectral effects, low solubility and dye aggregation.
Merocyanine dyes incorporating the dicyanovinylindane
moiety are an example of a typical donor–acceptor chromophore
with relatively long wavelength absorption bands of moderate
intensity.7 Because of their spectral characteristics and high
chemical and photochemical stability, they have been used as
pH-sensitive indicators for NIR optical chemical sensors.8
In this letter, we report on the synthesis and the spectral
characteristics of a new dicyanovinylindane incorporating mero-
cyanine-type chromoionophore 1, designed according to the
basic principles of donor–acceptor chromophores. Compound 1
is composed from an aromatic nitrogen π-electron donor as part
of an azacrown ether and the strongly electron-withdrawing
dicyanovinylindane acceptor,9 connected through a nitrogen
substituted methine bridge. This combination results in a cation-
responsive chromoionophore with spectral sensitivity reaching
into the NIR wavelength range.
In order to investigate the usefulness and the spectral char-
acteristics of dicyanovinylindane-connected chromogenic
crown ethers, a commercially available monoaza-15-crown-5-
ether 2 was used as the starting material. However, any other
monoazacrown ether could be used instead, making the present-
ed synthetic pathway a general route to obtain a variety of chro-
moionophores. Chromoionophore 1 was synthesized according
to Scheme 1. Crown ether 2 was condensed with 1,3,5-trihy-
droxybenzene (phloroglucinol) with azeotropic water removal,
making use of the presence of a small amount of the keto tau-
tomer of phloroglucinol.10 In the next step, the resulting N-
phenylazacrown compound 3 was converted into its nitroso
derivative 4, and then condensed with 1,3-bisdicyanovinyl-
indane 6, prepared by the reaction of malononitrile with 1,3-
indandione, to yield the chromoionophore 111 as a green crys-
talline solid. During the last reaction step, the phenolic hydroxy
groups of 4, originating from phloroglucinol, are converted
into acetoxy groups due to the presence of acetic anhydride.
This transformation reduces the pH cross-sensitivity of the
chromoionophore and increases the degree of spectral changes
during cation binding. The response mechanism is based on a
photoinduced intramolecular charge transfer (ICT)12 from the
crown nitrogen center to the dicyanovinylindane acceptor. Due
to its interaction with the positively charged cation bound in the
crown cavity, the π-electron donating strength of the anilinic
nitrogen is weakened, resulting in a hypsochromic spectral
shift. The disturbance of the π-electron distribution by ion
binding is rather small, especially for monovalent alkali metal
cations with low charge density. Therefore, the presence of π-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
553
electron donor groups in the chromoionophore that are not
involved in cation binding such as the phenolic hydroxy group,
is expected to decrease the cation induced signal shifts. With
the acetoxy group being a weaker π-electron donor than the
hydroxy group, this undesired side-effect is reduced.
ion concentration dependent spectral changes between 700 nm
and 800 nm have been observed. Due to the promising results,
further examples of dicyanovinylindane chromoligands will be
prepared, using modified azacrown ethers bearing bulky side-
walls14 to obtain NIR chromoionophores with high ion selectiv-
ity. Such compounds will be useful for optical ion determina-
tion in solvent extraction experiments. Furthermore, lipophilic
derivatives obtained by replacing the acetoxy group, can be
incorporated into ion selective ion-exchanger optode mem-
branes made from plasticized PVC.
D.C. gratefully acknowledges a postdoctoral fellowship
granted by the Japan Society for Promotion of Science (JSPS)
as well as financial support by the Japan International Science
and Technology Exchange Center (JISTEC).
References and Notes
1
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9
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The longest wavelength absorption band was found in chloro-
form solution at 762 nm. In order to study the effect of the
presence of metal cations, measurements were performed in
acetonitrile solution. In this case, the absorption maximum was
found at 741 nm wavelength. As shown in Figure 1a, the
expected hypsochromic spectral shift together with a strong
decrease in absorbance was observed, when increasing the sodi-
um ion concentration in the sample. In Figure 1b, the normal-
ized response curves for sodium cations extracted from the
absorbance values at 740 nm are presented. A signal decrease
is found in the concentration range of 10–4 M to 10–1 M. The
association constant, Ka, was calculated as 30 M–1. For lithium
cations, Ka was found to be 200 M–1. Even though the ionic
diameter of Na+ matches the size of the crown ether, this differ-
ence in binding strength reflects the higher charge density of
the lithium ion. The rather low complex stability might be
explained by the very strong π-electron accepting properties of
the two dicyanovinylindane groups, reducing the electron den-
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1
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8.60–8.62 (m, 2H); ESI-MS ([M+Na]+) found 687.01,
calcd for C35H32N6NaO8 687.22.
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In the present letter, a model compound for a new type of
NIR sensitive chromoionophore has been presented. Strongly