COMMUNICATION
Preliminary anion-titration experiments were independ-
ently carried out with 1 and 2, by both absorption and emis-
sion spectroscopy in acetone solution.[13] Receptors 1 and 2
revealed their ability to bind different anions, as could be
quantified by the determination of their stability constants
(Table 1).[14] All titrations fitted well a 1:1 stoichiometry
model, with most of the anions forming stronger complexes
with 1.
Table 1. Anion-binding constants (logKass)
[a] of 1 and 2.
À
À
PhCO2
AcOÀ
HSO4
BrÀ
1
2
6.1
4.1
5.3
3.9
5.2
3.8
4.7
5
[a] Kass [MÀ1] determined by UV/Vis spectroscopy with an error <10%,
at 258C. [1]=5ꢃ10À5 m, [2]=10À4 m in acetone. Association constants, for
1, have been further confirmed by emission spectroscopy at 258C, lexc
420 nm and [1]=5ꢃ10À5 m.
=
Figure 1. 1H NMR spectra of A) 1; B) 1/benzoate 1:1; C) 1/2/benzoate
1:1:1; D) 2; E) 2/benzoate 1:1 in deuterated acetone.
Since the initial condition of our dynamic system demand-
ed an unbalanced distribution of the complexed anion be-
tween both macrocycles, we focused our study on the ben-
zoate anion, which showed the larger binding constant ratio
between 1 and 2.
Although the low solubility of compounds 1 and 2 in deu-
terated acetone thwarted any possibility of quantifying bind-
1
ing constants by H NMR analysis, a qualitative study could
be indeed carried out (Figure 1).[15] It is worth highlighting
how the solubility of both receptors is remarkably improved
when one equivalent of benzoate is added to the suspension
in the NMR tube. This can be interpreted as an initial evi-
dence of the host–guest interaction. Furthermore, a down-
field shift is detected for the signals corresponding to the
protons more directly involved in the complexation of the
anion, in both macrocycles 1 and 2, as commonly results
from the hydrogen bonding interaction between the recep-
tor and the complexed anion (Figure 1).
Figure 2. Emission spectra (lexc =420 nm) of (A) 1 (5ꢃ10À5 m) in acetone
(c), 1/benzoate 1:1 (c) and B) 1/benzoate 1:1 (c), 1/2/benzoate
1:1:1 (c).
benzoate is added to a solution of 1 (5ꢃ10À5 m in acetone)
88% of the anion is bound by the macrocyclic receptor.
However, when 1, 2 and TBA-benzoate are dissolved to-
gether, at the same concentration, the ratio of anion com-
plexed by receptor 1 drops to 85%.[17]
The most important condition to be fulfilled by our
system in order to achieve an efficient intermolecular anion
transfer is that the affinities towards anionic guests can be
reversed. In our case, this will be done by a controlled elec-
trochemical potential. A square wave voltammetry titration
(Figure 3) of compound 2 with TBA-benzoate showed a
double wave behaviour with a cathodic shift in the oxidation
potential (DEp =0.218 V). This double wave observation is
the usual response to strong anion binding with slow kinetic
decomplexation on the voltammetric timescale. According
to the square Scheme applied to the equilibria of redox
Interestingly, an upfield shift of the protons, which are
complexing the anion in 1, could be detected after the addi-
tion of one molar equivalent of 2 to a 1:1 mixture of recep-
tor 1 and benzoate. Simultaneously, a downfield shift of the
1
thiourea protons is observed in the H NMR spectra of the
ternary mixture 1/benzoate/2. The structural information,
which can be drawn from this experiment, indicates that
competitive anion complexation equilibria are established
between both macrocycles.
Control experiments carried out by emission spectroscopy
also confirmed this result (Figure 2). An important enhance-
ment of the MLCT emission intensity was initially observed
when benzoate was added to a solution of 1. Subsequently,
the luminescence intensity of the anion complex was partial-
ly quenched after the addition of 2. In agreement with what
has been discussed for the 1H NMR experiments, the de-
crease in the luminescence can be related to the removal of
part of the complexed anion from 1.[16]
switchable host–guest systems,
a binding enhancement
factor of 4860 could be estimated for the complexation of
benzoate by 2 in its oxidized state.[18] Thus, the resulting
binding constant for the oxidized receptor would be approx-
imately logKass =7.8, which is significantly larger than the
binding constant determined for 1, logKass =6.1 (Table 1).
This promising result may mean that, when 1 and 2 are dis-
The anion distribution can be determined from the values
of the binding constants. Initially, when one equivalent of
Chem. Eur. J. 2009, 15, 7534 – 7538
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7535