Ó 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 12, 1589–1594 (2008) 1589
Structural Effect of Amphiphilic Crown Ether Azoprobes on Alkali
Metal Ion Recognition and Aggregation Behavior in Water
Fuyuki Sato, Miki Tsukano, Koki Sakamoto, Wakako Umemoto,
ꢀ
Takeshi Hashimoto, and Takashi Hayashita
Department of Chemistry, Faculty of Science and Technology, Sophia University,
7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554
Received May 26, 2008; E-mail: ta-hayas@sophia.ac.jp
Amphiphilic azoprobes having a crown ether moiety (15C5–Azo–Cn; n ¼ 4, 6, 8, and 12 and 18C6–Azo–Cn;
n ¼ 4, 6, and 8) were synthesized. We examined the effect of alkyl spacer length of the amphiphilic azoprobes on
1
the spectral response to alkali metal ions in water by UV–vis spectral and H NMR measurements. 15C5–Azo–Cn ex-
hibited self-assembly in response to Kþ selectively and the most selective response to Kþ was observed for an azoprobe
possessing a hexyl spacer unit (15C5–Azo–C6). 15C5–Azo–C12 formed nano-size aggregates. 18C6–Azo–Cn (n ¼ 6
and 8) showed Csþ selectivity and its molecular aggregation behavior was similar to that of 15C5–Azo–Cn.
Alkali metal ion recognition in water is an emerging topic of
interest in analytical and biological chemistry. Although vari-
ous efficient chemosensors that function as unimolecular sen-
sors have been developed, most of them require precise design
of the ion recognition sites to achieve sufficient selectivity and
sensitivity for alkali metal ions in water. Meanwhile, new ap-
proaches for the versatile design of recognition sites, which are
based on supramolecular chemistry, have been attracting much
attention. Recently, we have endeavored to combine supramo-
lecular chemistry and self-assembly to fabricate supramolecu-
lar sensors that exceed the sensitivity and selectivity of uni-
molecular sensors and mimic the functions of biomolecules.
Supramolecular sensors for alkali metal ions utilize various
types of metal-binding induced aggregates for signal transduc-
tion, including fluorescent bis-15-crown-5 derivatives,1 gold
nanoparticles and quantum dots,2 oligonucleotides,3 and cyclo-
dextrin complexes.4 It is well known that amphiphiles are wa-
ter-soluble and their critical micelle concentration (cmc) and
aggregate structures are dependent on their structure and alkyl
chain length, i.e., hydrophilicity–lipophilicity balance (HLB).5
Therefore, supramolecular sensors possessing an amphiphilic
skeleton are considered to be useful for the versatile design
of chemosensors that function in water, and recognition ability
can be tuned easily by controlling the strength of aggregation.
Here, we report cationic azo-amphiphiles, 15C5–Azo–Cn
and 18C6–Azo–Cn, possessing 15-crown-5 and 18-crown-6
moieties, respectively, as the alkali metal ion recognition site;
a phenylazo group for signal transduction; and a quaternary
ammonium unit with different lengths of alkyl spacers to in-
duce the amphiphilic nature. We have recently reported the
Kþ recognition selectivity of 15C5–Azo–Cn (n ¼ 2, 4, and
6) in water by forming H-aggregate dimers triggered by Kþ
binding based upon exciton interaction of the phenylazo
groups in the azoprobes.6 Among these azoprobes, 15C5–
Azo–C6 exhibited the best performance. In this study, alkyl
spacer lengths were varied from butyl to dodecyl to estimate
the most suitable structure to recognize target ion species in
water. As a result, 15C5–Azo–C6 exhibited the best perform-
ance, and azoprobes having long alkyl spacers showed de-
creased Kþ binding ability due to strong aggregation. In partic-
ular, 15C5–Azo–C12 formed nano size aggregates to exhibit
the lowest Kþ selectivity. In addition, the effect of crown ether
ring size on metal ion selectivity was examined. While 18-
crown-6 is known to bind with Kþ, Xia et al. have reported
an 18-crown-6 fluoroionophore that selectively binds Csþ by
forming a 2:2 sandwich complex.7 In this study, we have syn-
thesized amphiphilic azoprobes possessing 18-crown-6 as the
ion recognition moiety (18C6–Azo–Cn; n ¼ 4, 6, and 8), and
compared the ion recognition characteristics with those of
15C5–Azo–Cn to clarify the ring size effect. These derivatives
showed Csþ selectivity and similar aggregation behavior was
obtained in comparison to 15C5–Azo–Cn. 18C6–Azo–C6
and –C8 showed higher Csþ/Naþ selectivity than 18C6–
Azo–C4.
Experimental
General. Aqueous sample solutions were prepared with a
Millipore milli-Q water system (electric conductivity: 18.2
Mꢀ cmꢁ1). To retain the trans configuration of the azoprobes,
all sample preparation steps were accomplished in the dark. All
organic solvents and reagents were commercially available and
used without further purification.
Apparatus. UV–vis absorption spectra were recorded on a V-
560 (JASCO Co.) and a U-3000 (Hitachi High-Technologies Co.)
spectrophotometer equipped with a Peltier thermocontroller with a
0.01-cm quartz cell at 298 K. 1H NMR spectra were measured
with a Lambda500 (JEOL Ltd.; 500 MHz) at 300 K. Nonlinear
least-squares fitting was done with KaleidaGraph (Synergy Soft-
ware).
Synthesis of 15C5–Azo–Cn and 18C6–Azo–Cn. Amphiphil-
ic crown ether azoprobes (15C5–Azo–Cn (n ¼ 4, 6, 8, and 12) and
18C6–Azo–Cn (n ¼ 4, 6, and 8)) were synthesized, as shown in
Scheme 1. The preparation of 15C5–Azo–Cn started from 40-ami-