Selective transportation of metal ions by photo-induced crown ether compounds

Article abstract of DOI:10.1016/S0009-2614(99)00727-7

A novel photoresponsive system that we call a photo-induced crown ether (PIC) is described, in which excimers are used to fix the conformation of a molecule for relatively long periods. 1,n-bis(3-(1-pyrenyl)propylcarboxy)-oxaalkanes (DP3n: n=3-6), linear oligooxyethylenes with pyrenyl groups at both ends, were prepared and investigated to determine whether they might work as PICs. DP35 molecules were found to capture and transport calcium ions more effectively when they were irradiated. This result is consistent with the finding that five-unit crown ether transported the greatest amount of calcium ions.

Full text of DOI:10.1016/S0009-2614(99)00727-7

20 August 1999
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Chemical Physics Letters 309 1999 402–406
www.elsevier.nlrlocatercplett
Selective transportation of metal ions by photo-induced crown
ether compounds
)
Hideyuki Itagaki , Wakako Masuda, Yukiko Hirayanagi
Department of Chemistry, Faculty of Education, Shizuoka UniÕersity, 836 Ohya, Shizuoka 422-8529, Japan
Received 23 April 1999; in final form 17 May 1999
Abstract
A novel photoresponsive system that we call a photo-induced crown ether PIC is described, in which excimers are used
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.
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.
.
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to fix the conformation of a molecule for relatively long periods. 1,n-bis 3- 1-pyrenyl propylcarboxy -oxaalkanes DP3n:
.
ns3–6 , linear oligooxyethylenes with pyrenyl groups at both ends, were prepared and investigated to determine whether
they might work as PICs. DP35 molecules were found to capture and transport calcium ions more effectively when they
were irradiated. This result is consistent with the finding that five-unit crown ether transported the greatest amount of
calcium ions. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction
cordingly, we refer to them as photo-induced crown
Ž
.
ether compounds PICs .
Most photoresponsive systems that have been de-
scribed so far employ photochemical reactions, such
as photochromism 1 . Here we describe a novel
reversible photoresponsive system that employs a
photophysical process. We introduced pyrenyl groups
onto both ends of linear oligooxyethylenes Fig. 1 .
When irradiated with UV light, the pyrenyl groups
link together to form an excimer with a lifetime in
the sub-microsecond range. This structure is analo-
gous to that of crown ethers, which are cyclic ethy-
lene glycols capable of transporting ionic compounds
into an organic phase. Here we show that one of
these compounds indeed has the properties of a
crown ether: when irradiated, it can capture calcium
ions and transport them through a CCl4 phase. Ac-
Several studies have investigated oligooxyeth-
ylenes having aromatic compounds at both ends.
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These studies can be classified into three types: 1
studies on the expansion or cyclization of an oligomer
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x Ž .
or polymer in fluid solution 2,3 ; 2 attempts to
develop a fluorescent reagent to monitor metal ions
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.
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x
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captured by them 4–7 ; and 3 attempts to develop
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a compound to capture metal ions by irradiation 8 .
In the first two types of study, excimers formed
between aromatic end groups were used as probes to
get the information on the physical properties or the
amounts of metal ions present. The third type of
study used photodimerization, which is a photochem-
ical reaction, to permanently link the aromatic end
groups. Accordingly, our work is the first attempt to
use excimers to fix the conformation of a molecule
for relatively long periods. Since the concentration of
metal ions in a PIC solution can be changed by
)
Corresponding author. Fax: q81-54-237-3354; e-mail:
itagaki@ed.shizuoka.ac.jp
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 99 00727-7

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H. Itagaki et al.rChemical Physics Letters 309 1999 402–406
403
counting machine and were analyzed by the decon-
w x
volution method after O’Connor and Phillips 9 ,
w
x
using the Durbin–Watson factor 10 to assess the
validity of the trial fitting function. The concentra-
tion of calcium ion was determined with a Hitachi
Z-8270 polarized Zeeman atomic absorption spectro-
photometer. NMR signals of DP3n in CD3CN were
measured on a JEOL GX 400 FT-NMR spectrome-
ter.
3. Results
Information on the conformation of DP3n in fluid
solution can be obtained by measuring its photosta-
tionary and transient fluorescence. Fig. 2 shows the
fluorescence spectra of DP3n in aerated acetonitrile
at 258C. The peaks near 378 and 480 nm were
assigned to the pyrenyl monomer singlet and ex-
Fig. 1. A schematic drawing of a photo-induced crown ether
compound together with the chemical structure of DP3n. The
probability that a compound will be in the closed form, which is
similar to a crown ether, is usually very small in the absence of
light. However, if one of the pyrenyl groups of DP3n is irradiated,
it links to the other pyrenyl group to form an excimer that
stabilizes this conformation.
w
x
cimer emission, respectively 11 . In fact, the excita-
tion spectra for the fluorescence between 380 and
550 nm of all samples were almost identical. They
were also identical with their absorption spectra. No
concentration dependence was observed for the fluo-
rescence spectra of the DP3ns below 10 mM, mean-
ing that all the excimers shown in Fig. 2 are formed
intramolecularly. The fluorescence spectra of DP3n
in THF, water, chloroform and dichloromethane all
irradiation, such solutions might have applications as
photo-switches.
2. Experimental
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.
The structure of the 1,n-bis 3- 1-pyrenyl pro-
.
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.
pylcarboxy -oxaalkanes DP3n studied in the pre-
sent work is shown in Fig. 1. To prepare the DP3ns,
1-pyrenebutanoic acid was first converted to the
acid-chloride by using thionyl chloride in benzene.
Each acid-chloride was then reacted in benzene in-
cluding a small amount of pyridine under N2 flux
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with triethylene glycol OE3; ns3 , tetraethylene
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glycol OE4; ns4 , pentaethylene glycol OE5;
.
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ns5 and hexaethylene glycol OE6; ns6 to pro-
duce DP33, DP34, DP35 and DP36, respectively.
The products were separated with a Sephadex L-20
chromatographic column using THF as the eluent.
Fluorescence spectra and fluorescence excitation
spectra were measured on a Hitachi F-4500 spectro-
fluorometer. Fluorescence decay curves were ob-
tained by using a Horiba NAES550 single-photon
Fig. 2. Fluorescence spectra of DP3n in aerated acetonitrile. All
the spectra are normalized to the peak. The excitation wavelength
is 342 nm.

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H. Itagaki et al.rChemical Physics Letters 309 1999 402–406
showed strong excimer fluorescence and weak
monomeric fluorescence, meaning that DP3n readily
takes a closed form similar to a crown ether.
The ratio of excimer fluorescence intensity, IE , to
monomer fluorescence intensity, IM , was inversely
related to the number of oxyethylene units: the order
of IErIM was DP33)DP34)DP35)DP36.
Transient measurements of the pyrenyl monomer
and excimer fluorescence give the time constant for
excimer formation from the excited monomers, tem.
The values of tem for DP33, DP34, DP35 and DP36
in degassed THF were 5.2, 5.6, 6.8 and 7.9 ns,
respectively. All the lifetimes of excimer fluores-
cence, te, were 50"2 ns. The decay component of
50 ns was not detectable in the monomer fluores-
cence. Thus the excimer dissociation of DP3n can be
neglected at room temperature. In this case, IErIM is
proportional to the rate of excimer formation, i.e., to
the rate of the cyclization process. The order of
IErIM among the DP3ns was identical to the order
of the tem values, and indicates that the rate of the
cyclization process is dependent on the average dis-
tance between the two pyrenyl end groups.
Ž .
The fluorescence results show that:
1 DP3n
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.
Ž
normally i.e., in the dark exists in the open form I
. Ž .
in Fig. 1 ; 2 it requires a few nanoseconds to form
. Ž .
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a closed form II in Fig. 1 ; 3 tem becomes longer
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.
with longer chains; and 4 roughly 40% 1re of
DP3n would remain in the closed conformation even
50 ns after the irradiation.
Next, we examined whether the addition of cal-
cium ions influences the excimer formation of DP3n.
We measured the dependence of DP3n fluorescence
on calcium concentration using 3 mM DP3n and
w
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. x
Ca SCN
ranging from 10y5 to 10y2 M. Since
2
DP3n is more stable in acetonitrile than in water, the
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Fig. 3. A quartz cell for determining whether irradiated PICs of different sizes are capable of transporting calcium ions. Only part A is
irradiated.

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H. Itagaki et al.rChemical Physics Letters 309 1999 402–406
405
measurements were made in acetonitrile. Addition of
Ca2q did not change the shapes of the spectra of any
of the DP3ns. Thus, even if there is some interaction
between DP3n and calcium ions, it did not have any
effect on the intramolecular excimer formation pro-
cess.
Moreover, we examined the ability of the DP3ns
1
to form a complex with Ca2q by H-NMR measure-
ments using CD3CN as a solvent. In the case of
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.
oligooxyethylenes OEn; ns3–6 , the addition of
Ca2q increased the chemical shifts of methylene
protons by at most 0.23 ppm, but the strength of the
interaction was not significantly different among the
different OEns. However, in the case of DP3n, the
chemical shifts of DP35 changed most by the addi-
tion of Ca2q, while those of the other DP3ns changed
by smaller amounts. The signal assigned to the hy-
drogen atoms of the oxyethylene part was shifted as
Fig. 4. Transport of calcium ions by DP3n, where n is the number
of oxyethylene units.
6.4=1018 photonsrs for 10 min, and then an addi-
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follows: unit, ppm 0.015 DP33 , 0.11 DP34 ,
tional 10 min was allowed to pass to provide time
0.33 DP35 and 0.19 DP36 in mid-chain –O–
CH–C– , 0.019 DP33 , 0.12 DP34 , 0.18 DP35
and 0.079 DP36 in end-chain –CO–O–CH–C–
O– , and 0.017 DP33 , 0.10 DP34 , 0.32 DP35
for the diffusion of Ca2q-bearing PIC molecules to
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.
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2q
.
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.
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.
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Ž .
B . When part A was not irradiated, Ca was not
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observed in B , regardless of which DP3n was
dissolved in C . However, when part A was irradi-
ated, Ca2q was detected in the water cell B Fig.
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.
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.
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.
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.
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Ž .
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.
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and 0.11 DP36 in the end-chain –CO–O–C–CH–
.
.
4 .
O– . Thus, the NMR results show that DP35 has the
strongest interaction with Ca2q among the DP3ns
studied in this work.
The values shown in Fig. 4 are averaged over
Ž .
several measurements. Since C was not stirred, we
estimate that the errors in these averages are 20–30%.
In any case, the PIC with 5 oxyethylene units trans-
ported the greatest amount of Ca2q: the Ca2q con-
There is no effective direct way to measure the
ability of DP3n to capture metal ions when it is
Ž
irradiated. Therefore, we employed a quartz cell 1
.
Ž
.
Ž .
cm=1 cm=6 cm high with a partition Fig. 3 to
determine the amount of Ca2q transported by a PIC
in the presence and absence of UV light, after a
method for measuring transportation yield of metal
centration in B reached 0.1 mM after irradiation for
10 min.
w
x
ions 12 . A solution of 1 mM DP3n in CCl4 was
Ž .
4. Discussion
placed in part C . An aqueous solution of 10 mM
Ž
.
Ca NO3 and 10 mM DP3n, and pure water were
The data in Fig. 4 clearly show that some DP3ns
can work as photo-induced crown ether compounds
and transport Ca2q only when the DP3ns are irradi-
2
Ž . Ž .
Ž .
Ž .
simultaneously layered on top of C in A and B ,
respectively. The transportation yield of Ca2q in B
ated with light, because Ca2q was detected in B
Ž .
was measured by atomic absorption spectroscopy. In
the case of Ca2q, the limit of detection is 5=10y9
M.
Calcium ions are normally insoluble in CCl4.
Thus, when we did not dissolve any DP3n in A
only when DP3ns were irradiated. The Ca2q is
Ž .
assumed to be transported to B by the following
Ž .
mechanism. When irradiated, PICs in A capture
Ca2q. Some of those near C would diffuse into C .
Ž .
Ž .
Ž .
2q
2q
Ž .
Ž .
Ž .
and C , no Ca was observed in B even after 1 h
Although C is not irradiated, the Ca ion cannot
either with and without UV irradiation. To measure
be released from the PIC because it is not soluble in
transportation yields of Ca2q by DP3n, part A was
CCl4. Some of the Ca2q-bearing PIC molecules
Ž .
Ž .
would diffuse into B , where they could then release
either kept in the dark for 10 min, or irradiated at

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H. Itagaki et al.rChemical Physics Letters 309 1999 402–406
the Ca2q ion. Thus, some of the Ca2q would end up
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Acknowledgements
in B .
The greater transport of Ca2q by DP35, which
This work was supported by a Grant-in-aid for
Ž . Ž
.
Scientific Research B 09450348 from the Min-
istry of Education, Science, Sports and Culture of
Japan, and by the Iwatani Naoji Foundation.
has five units of oxyethylene, is consistent with its
size: the diameter of the hole in 5-unit-crown ether
2q
Ž
.
w x
15C5 is 170–220 pm 13 while that of Ca
200 pm.
is
We assume that DP35 molecules do not start with
capturing Ca2q after forming an excimer, but rather
they have some interaction with Ca2q before being
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Ž
.
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