Synthesis and properties of novel fluoroionophores for Ag+-optode sensing based on oligophenylenevinylenes

Article abstract of DOI:10.1016/s0040-4039(00)01679-8

Novel hexylsulfanyl substituted oligophenylenevinylenes have been designed and synthesized as fluoroionophores which selectively detect Ag⁺ ions in membrane optode sensors. In addition, the 4-phenyl-ring oligomer-based optode sensor exhibits higher sensitivity than that of the 3-phenyl-ring counterpart. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/s0040-4039(00)01679-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9293–9297
Synthesis and properties of novel fluoroionophores for
Ag+-optode sensing based on oligophenylenevinylenes
Man Shing Wong,* Wing Hong Chan, Wai Yin Chan, Jun Li and Xiao Dan
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
Received 31 May 2000; revised 13 September 2000; accepted 27 September 2000
Abstract
Novel hexylsulfanyl substituted oligophenylenevinylenes have been designed and synthesized as
fluoroionophores which selectively detect Ag+ ions in membrane optode sensors. In addition, the
4-phenyl-ring oligomer-based optode sensor exhibits higher sensitivity than that of the 3-phenyl-ring
counterpart. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: oligomers; optical properties; fluorescence.
It is of great interest and challenge to design and synthesize fluoroionophores for sensing
environmentally and/or biologically important cations.1,2 In the course of our studies on the
structure–property relationships of end-substituted oligophenylenevinylenes (OPV), we have found
that planar OPVs, in particular the higher homologues, are highly fluorescent,3 which indicated
that they may be potentially useful fluorophores for sensing applications. With an incorporation
of a specific ionophore onto the OPV framework, the spectral property/properties of OPV may be
modified upon binding of the ionophore with a target ion which give(s) rise to a sensing mechanism.
With this concept in mind, we have recently developed a novel I2-optode sensor with sensitivity
down to 5×10−8 M based on a polyalkenoxy-substituted 3-phenyl-ring OPV.4 In a continuation
of this, we report here our design, syntheses and optical properties of novel hexylsulfanyl-substituted
OPVs 1 and 2 as well as their application in membrane optodes for Ag+ ion sensing.
* Corresponding author. Tel: 852-2339-7069; fax: 852-2339-7348; e-mail: mswong@hkbu.edu.hk
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01679-8

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The syntheses of the hexylsulfanyl substituted 3-phenyl-ring oligomer 1 and the 4-phenyl-ring
oligomer 2 are shown in Scheme 1.5 The three hexylsulfanyl substituents were incorporated by
nucleophilic aromatic substitution of 1-bromo-3,4,5-trifluorobenzene 3. Lithium–halogen
exchange of 5 followed by the reaction with N-formylmorphine and hydrolysis afforded the
corresponding tri-hexylsulfanyl substituted benzaldehyde 6. Double Wadsworth–Emmons reac-
tions of benzaldehyde 6 with bis-phosphonate 7 afforded the 3-phenyl-ring oligomer 1. How-
ever, with an excess of bis-phosphonate 7, both styrylbenzyl–phosphonate 8 and oligomer 1
could be isolated.6 For the synthesis of the 4-phenyl-ring oligomer 2, the key intermediate,
mono-phosphonate 9 was prepared by LiAlH4 reduction of 6, followed by chlorination with
SOCl2 and then a Michaelis–Arbuzov reaction. The Wadsworth–Emmons reaction of 9 and
terephthalaldehyde monodiethylacetal 10 followed by an acidic work up afforded the corre-
sponding styrylaldehyde 11. Oligomer 2 was finally obtained by the Wadsworth–Emmons
reaction of 8 and 11.
Scheme 1. Synthesis of hexylsulfanyl substituted oligophenylenevinylenes 1 and 2, respectively. Conditions: (i) K2CO3,
DMSO, 140°C, 50%; (ii) n-BuLi, THF, −78°C; (iii) N-formylmorphine; (iv) HCl, 80–95%; (v) NaH, DME, rt,
50–65%; (vi) LiAlH4,THF, 0°C, 83%; (vii) SOCl2, pyridine, CH2Cl2, 0°C, 60%; (viii) P(OEt)3, 150°C, 69%

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In view of the electronic absorption spectra of 1 and 2, both oligomers show strong and
intense low-lying absorption bands with apparent vibronic structures suggesting the coplanar
conformation (Fig. 1) The absorption maximum of the higher homologous oligomer 2 (umax
=
398 nm) is 11 nm red-shifted in CHCl3 with a concomitant increase in extinction coefficient
(mmax=6.1×104 M−1 cm−1), compared to those of 1 (umax=377 nm, mmax=4.4×104 M−1 cm−1). In
addition, the fluorescence maxima of oligomer 2 are at longer wavelengths (uem=420 and 444
nm for 1 and 441 and 469 nm for 2). With an incorporation of the hexylsulfanyl substituents,
the fluorescence quantum yield of the three-phenyl-ring oligomer, 1 (b=0.15) decreases substan-
tially as compared to that of the unsubstituted counterparts;7 however, there is a fourfold
enhancement in the fluorescence quantum yield with an addition of a phenylenevinylene unit (2:
b=0.65).
Figure 1. Normalized absorption and emission spectra of 1 and 2 in chloroform
To investigate the sensing properties of these OPVs towards various heavy metal ions, optode
membrane sensors containing 3 wt% of oligomer 1 and 2 were fabricated.8 It has been found
that these OPV-based optode sensors show no significant change in fluorescence intensity upon
contact with Zn2+, Cd2+, Cu2+, Pb2+, and Co2+ ions in the buffer solution KNO3/HNO3,
respectively. However, there is a dramatic quenching in fluorescence intensity upon contact with
Ag+ ions, suggesting that these OPV-based optode sensors are sensitive and selective to Ag+ ions.
The fluorescence spectra of these optode sensors in response to various concentrations of Ag+
ions are shown in Figs. 2 and 3, respectively. It is evident that the 2-based optode sensor exhibits
higher sensitivity than that of its lower homologue.
In summary, novel hexylsulfanyl substituted oligophenylenevinylenes have been designed and
synthesized as fluoroionophores for membrane optode sensing. It has been shown that these
OPV-based optode sensors selectively respond to Ag+ ions. In addition, the 4-phenyl-ring
oligomer-based optode sensor exhibits higher sensitivity than that of the 3-phenyl-ring counter-

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Figure 2. Fluorescence spectra of the 1-based optode sensor in various concentrations of Ag+ ions in 2 M KNO3 +
2 M HNO3 buffer solution with excitation at 383 nm
Figure 3. Fluorescence spectra of the 2-based optode sensor in various concentrations of Ag+ ions in 2 M KNO3 +
2 M HNO3 buffer solution with excitation at 398 nm
part. These OPV-based Ag-optode sensors show potential for practical applications. Further
optimization of these hexylsulfanyl-substituted OPV-based optode sensors for Ag+ ion sensing is
still under investigation.

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Acknowledgements
This work was supported by Faculty Research Grants (FRG/99-00/II-11) from Hong Kong
Baptist University, and Competitive Earmarked Research Grant (HKBU 2054/98P).
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