the hydrogen-bonding ability and electron-donating ability
of the guest molecule. In contrast, the color of most
colorimetric sensing systems corresponds to a single param-
eter, which is usually the receptor-guest association constant.
Thus, a CT sensor can differentiate analytes that have similar
association constants based on the differences in their
electron-donating or -withdrawing abilities.5 A CT sensor
can also differentiate analytes with similar electron donating
or accepting abilities based on the differences in their
affinities for the sensor platform.
CT receptor 1 was designed to form hydrogen bond
complexes with phenols and diols (Scheme 1). The two
Figure 1. Crystal structure of receptor 1·1,7-naphthalenediol
cocrystal (1:2 complex).
Scheme 1. Synthesis and Binding Geometry of Receptor 1
in π-π stacking distance. The exclusive formation of the
more symmetrical anti-isomer in the solid state did not
preclude the formation of the convergent syn-isomer in
solution.7 In solution, the pyridine arms are free to rotate
and thus can adopt either the syn- or anti-conformers.8 The
crystal structure also revealed an additional degree of
preorganization provided by the C12 methyl group on the
pyridine arms. The C12 methyl group restricts the conforma-
tion about the C6-C7 bond to avoid steric interactions with
the N1 imide nitrogen. This fixes the pyridine N2 nitrogens
to point inward toward the central diimide surface.
Next, the ability of receptor 1 to bind and form colored
CT complexes with electron-rich guests in solution (4:1,
CHCl3/CH3CN) was assessed (Figure 2). This mixed solvent
system yielded the best balance between maximizing solubil-
ity of the diols without disrupting the host-guest hydrogen
bonding interactions. The solutions varied widely in color
and intensity and were easily visually differentiated, ranging
from yellow (e, f, g) to orange (b, k m), to purple (h). A
clear correlation between the ability of a guest to form
hydrogen bonds to receptor 1 and the formation of a colored
CT complex was observed. For example, diols b, c, e, f,
and g form strong colored CT complexes. In comparison,
guests that could not form hydrogen-bonding interactions
such as p-dimethoxybenzene d and 1,7-dimethoxynaphtha-
lene j showed little or no color, even though they have very
similar electron-donating abilities to the diols.
pyridine arms are positioned to form hydrogen bonds to a
diol guest and position it over the electron-poor 1,2,4,5-
benzenediimide surface.6 Unlike the yellow 1,4,5,8-naph-
thalenediimide CT sensors, receptor 1 is colorless in solution,
and thus the formation of colored CT complexes upon
addition of diol guests was easily visualized. In addition,
receptor 1 was readily soluble in organic solvents as an
unaggregated structure and thus the association constants of
its host-guest complexes could be easily measured via
UV-vis titration studies and correlated to the color and
intensity of the CT band.
There was also considerable variation in the color of the
CT complexes even with diols with very similar structures.
For example, the positional isomers hydroquinone b, catechol
e, and resorcinol f formed orange, pale yellow, and bright
yellow complexes, respectively. The differences in color were
even more dramatic for the isomeric dihydroxynaphthalenes
h, k, and l. Perhaps the most remarkable was that guests
Receptor 1 was easily prepared by condensation of 2 equiv
of aminopyridine 4 with 1,2,4,5-benzene dianhydride. Py-
ridine 4 was prepared in two steps from chloromethylpyridine
2. The methyl and methoxy groups on the pyridine 4 were
necessary to enhance the solubility of receptor 1 in organic
solvents.
First the structure and recognition abilities of receptor 1
were studied in the solid state. The receptor 1·1,7-naphtha-
lenediol crystal structure is shown in Figure 1. Both pyridine
nitrogens of 1 form hydrogen-bonding interactions with a
phenolic hydrogen of a 1,7-naphthalenediol. This positions
the naphthalenediol over the electron-poor diimide surface
(6) (a) Kishikawa, K.; Iwashima, C.; Kohmoto, S.; Yamaguchi, K.;
Yamamoto, M. J. Chem. Soc., Perkin Trans. 1 2000, 2217. (b) Fallon, G.;
Langford, S. J.; Lee, M. A. P. Chem. Lett. 2001, 578. (c) Kato, S.;
Matsumoto, T.; Ideta, K.; Shimasaki, T.; Goto, K.; Shinmyozu, T. J. Org.
Chem. 2006, 71, 4723. (d) Iijima, T.; Vignon, S. A.; Tseng, H. R.; Jarrosson,
T.; Sanders, J. K.; Marchioni, F.; Venturi, M.; Apostoli, E.; Balzani, V.;
Stoddart, J. F. Chem. Eur. J. 2004, 10, 6375.
(5) (a) Foster, R. Organic Charge Transfer Complexes; Academic Press:
New York, 1969. (b) Rathore, R.; Lindeman, S. V.; Kochi, J. K. J. Am.
Chem. Soc. 1997, 119, 9393.
(7) This has been observed elsewhere: Barooah, N.; Sarma, R. J.;
Batsanovq, A. S.; Baruah, J. B. J. Mol. Struct. 2006, 791, 122.
(8) Kacprzak, K.; Gawronski, J. Chem. Commun. 2003, 1532.
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