Figure 4. Proposed structures for complex formation of 1 with
(a) 1° amine, (b) 2° amine, and (c) 3° amine.
was synthesized and tested for the color change upon the
addition of amines. The results are given in the Supporting
Information, Table S1 and Figures S4 and S5. Unlike the
case of 1, addition of 1° and 2° amines to the chloroform
solution of 2 changed the color from yellow (380 nm) to
pink (500 nm), respectively (Supporting Information, Table
S1 and Figures S3 and S4, respectively), but lacked selectiv-
ity. On the other hand, addition of 3° amines to 2 gave no
color change. These results show that the crown unit in 1
plays an important role in discriminating among the type of
amines.
Figure 3. FAB-Mass spectrum of 1 + n-heptylamine in chloro-
form.
inducing a photoinduced charge transfer (PCT).15 In addition,
the selectivity of 1 toward the type of amines is closely
related to comparable H-bonding interactions between the
crown ring of 1 and the protons of the ammonium ion as
well as to a critical role of the adjacent R-CD which can
interact with the lipophilic alkyl chain of the amine by a
hydrophobic interaction (vide infra).
To elucidate the role of the R-CD in the amine selectivity,
3 with a p-chlorophenyl unit instead of the R-CD was
prepared. Addition of 1° and 2° amines to the chloroform
solution of 3 changed the color from yellow (380 nm) to
pale blue (∼560 nm) and to pink (∼540 nm), respectively
(Supporting Information, Table S1 and Figures S5-S7,
respectively). As with 1, no color change was noted with
the addition of 3° amines. However, both binding sensitivity
and selectivity of 3 toward the tested amines were found to
be much lower than those of 1. So, one can assert that the
less-selective interaction and low sensitivity are related to
the hydrophobic interaction between R-CD and the alkyl
chain of the ammonium ion. One isosbestic point appeared
in the spectrum of 3 with n-octylamine in CHCl3, indicating
the formation of a 1:1 complex as well (Supporting Informa-
tion, Figure S7).
Acidity, pKa, is also important to determine the selectivity
of the acidic ligands toward the target guest molecules. The
pKa values of 1-3 in a solution of H2O/1,4-dioxane (9:1
v/v) were determined to be 5.75, 5.99, and 7.30, respectively.
Hence, it is noteworthy that 1 can be easily deprotonated to
give rise to a phenoxide anion which develops photoinduced
charge transfer through the p-nitrodiazophenol to provide its
bathochromic shift.
The NMR spectrum of the complex 1‚n-heptylamine
(Supporting Information, Figure S1) shows that the protons
of the crown loop and C1-H of the R-CD are seen to high-
field shift. This observation is consistent with an idea that
there are H-bonds between H atoms of the ammonium and
the crown ring of 1 as well as hydrophobic interactions with
the target amine, respectively, as mentioned earlier.
The lack of color change of 1 toward 3° amines is
presumably due to a steric hindrance between the alkyl chain
of the 3° amine and the R-CD.
The log Ka’s of 1 for 1° and 2° amines are 4.19-4.85 and
2.02-2.32, respectively, as listed in Supporting Information,
Table S1. The 1° and 2° amine selectivity of 1 was calculated
to be 150-500. These results also indicate that the number
of H-bonds formed between the crown ring of 1 and the
protons of the ammonium ion is of critical importance to
discriminate between the amines.
It is noteworthy that upon addition of n-octylamine, the
intensity of the 380 nm absorption band of 1 decreased
with concomitant increase of a new band centered at 580
nm with only one isosbestic point at 521 nm, leading to solid
evidence of a 1:1 complex formation (Supporting Informa-
tion, Figure S2). For the complexation ratio, we took FAB
mass spectroscopy of the 1-n-heptylamine complex. As
shown in Figure 3, it is evident that the 1-n-heptylamine
complex has a 1:1 complexation ratio (1886.12 m/e, Figure
3).
According to UV-Vis, NMR, and mass spectra of the
complex, we propose the complex structures of 1-amine in
Figure 4. Previously, Kaneda et al. reported on the H-bonding
interactions between the crown ring of a macrocyclic
chromophore and amines through an XRD study.7,11 A 1°
amine forms three-pointed perching H-bonds between the
oxygen atoms of 18-crown-6 and the H atoms of the
ammonium ion, and a 2° amine participates only through
two-pointed H-bonding interactions.
To gain insight into the role of the crown unit on the amine
selectivity, 2, in which no crown ether unit was introduced,
(15) Cobben, P. L. H. M.; Egberink, R. J. M.; Bomer, J. G.; Bergveld,
P.; Verboom, W.; Reinhoudt, D. N. J. Am. Chem. Soc. 1992, 114, 10573-
10581.
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