them to present a conformation of lipophilic ethylene outside and hydrophilic oxygen
atoms inside in organic solution or hydrophilic oxygen atoms outside and lipophilic
ethylene inside in aqueous solution (Fig. 2). In aqueous solution 18-crown-6 has to
undergo remarkable conformational readjustment to arrange oxygen atoms inward
which is in the fashion most complementary to guests. This process consumes extra
energy and lowers the binding strength of guests, which is ubiquitous for all aliphatic
crown ethers and aromatic crown ethers in aqueous solution.
Water-soluble aromatic crown ethers could be essentially realized as the ethylene
glycol chains extended by negatively charged aromatic groups. While the hosts are
complexation with π-electron-poor guests, the main noncovalent interactions
involved in the association are electrostatic attraction, π-stacking, hydrogen bond,
and dipole-dipole interaction. The hydrogen bond and dipole-dipole interaction
mainly occur between ethylene glycol chains and cationic guests, as well as 18crown-6. Thus, the ethylene glycol chains on water-soluble crown ethers should
show similar behaviors like 18-crown-6 in aqueous solution. The association constant of NH 4
+
& 18-crown-6 is only 17 M
À1 in aqueous solution; thus it is reasonable
to declare that the hydrogen bond and dipole-dipole interaction contributed far
smaller than a value of 17 M
À1 . The other factors containing electrostatic attraction
and π-stacking play the dominant role in the binding process.
A large collection of cationic guests have been reported by several groups. To
generate strong electrostatic attraction and π-stacking with water-soluble crown
ethers, the complementary guest should be π-electron-poor aromatic cations. Some
of the most common cationic guests, containing pyridiniums, bipyridinium,
phenanthroliium, 2,7-dimethyldiazapyrenium, pyromellitic diimide, naphthalene
diimide, NAD
+
, and stilbazolium dye, are listed in Fig. 3.
1.3
Sulfonated Aromatic Crown Ethers
Back in 1997, Shigeo et al. discovered that, by reacting sulfuric acid with benzocrown ethers in acetonitrile, monosulfonated benzo-12-crown-4, benzo-15-crown-5,
benzo-18-crown-6 were obtained in good yields (Fig. 4) [19]. These sulfonated
O
O
O
O
O
O
O
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
O
O
H
H
H
H
H
H
H
H
O
O
O
H
H
H
H
H
H
H
H
O
O
O
O
O
O
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
K +
K +
(a)
( b)
(c)
conformational
readjustment
Fig. 2 Conformation of 18-crown-6 in (a) aqueous solution, (b) organic solution, and (c) bound
state with K
+
6
L. Chen and Y. Liu