association constants. Crystal structures show that the positively charged
bipyridinium units are located nearby the negatively charged sulfonate groups of
H4 and stacked with naphthalene planes (Fig. 11). The flexible glycol chains do not
play a major role in the association progress, only few weak C–HÁ Á ÁO (d
H Á Á Á O > 2.5 Å) hydrogen bonds are found in a typical complexation crystal structure
like G9 & H4. For the case of G17 & H4 the association constant is really small due
to the single charge and lack of π-conjugation size. Therefore, the binding process
between H4 and pyridinium guests should be governed by electrostatic attraction
and π-stacking. For the complexation of H5, the K a values are obviously different
from that of H4. Both bipyridinium and monopyridinium guests form more stable of
two orders of magnitude complex with H5 than that of H4, which is accompanied by
even more negative enthalpy change. The dominant enthalpy changes are mainly
contributed by the π-stacking and electrostatic interactions, and the favorable (or
slightly unfavorable) entropy changes generally originate from the positive contribution of desolvation effect accompanied by π-stacking and electrostatic contact as
well as the slight positive entropy change for H4 complexes. The entropy change
slightly decreases upon increasing the N-substituted aryl chain length on guests, in
Table 4 Association constants (K a , M
À1
), enthalpy change (-ΔH
, kJ/mol), and entropy change
(TΔS
, kJ/mol) for intermolecular complexation of H4 and H5
Hosts
Guests
K a
-ΔH
TΔS
Ref.
H4
G8
3.2 Â 10
5
30.13
1.33
[24]
G9
1.8 Â 10
5
27.20
2.86
G10
1.9 Â 10
5
27.27
2.83
G11
7.1 Â 10
5
30.99
2.54
[25]
G12
2.5 Â 10
6
29.80
6.84
G13
1.1 Â 10
8
47.84
À1.96
G14
2.2 Â 10
8
40.06
1.87
G15
8.1 Â 10
4
20.59
7.40
[26]
G16
2.3 Â 10
6
36.31
0.01
G17
4.2 Â 10
2
14.71
0.38
[24]
H5
G8
4.0 Â 10
7
38.93
4.47
[24]
G9
5.2 Â 10
7
41.54
2.50
G10
4.7 Â 10
7
43.92
À0.17
G15
5.8 Â 10
5
24.85
8.05
[26]
G16
9.8 Â 10
5
23.17
11.01
G17
1.1 Â 10
5
29.23
À0.39
[24]
G18
2.2 Â 10
3
67.90
À48.87
G19
8.1 Â 10
5
39.37
5.65
[27]
G20
1.6 Â 10
6
40.54
5.08
G21
2.8 Â 10
6
35.47
1.34
G22
1.8 Â 10
6
41.42
5.89
G23
4.9 Â 10
6
33.38
4.82
G24
4.3 Â 10
6
41.39
À3.54
G25
7.9 Â 10
6
–
–
[28]
12
L. Chen and Y. Liu
bipyridinium units are located nearby the negatively charged sulfonate groups of
H4 and stacked with naphthalene planes (Fig. 11). The flexible glycol chains do not
play a major role in the association progress, only few weak C–HÁ Á ÁO (d
H Á Á Á O > 2.5 Å) hydrogen bonds are found in a typical complexation crystal structure
like G9 & H4. For the case of G17 & H4 the association constant is really small due
to the single charge and lack of π-conjugation size. Therefore, the binding process
between H4 and pyridinium guests should be governed by electrostatic attraction
and π-stacking. For the complexation of H5, the K a values are obviously different
from that of H4. Both bipyridinium and monopyridinium guests form more stable of
two orders of magnitude complex with H5 than that of H4, which is accompanied by
even more negative enthalpy change. The dominant enthalpy changes are mainly
contributed by the π-stacking and electrostatic interactions, and the favorable (or
slightly unfavorable) entropy changes generally originate from the positive contribution of desolvation effect accompanied by π-stacking and electrostatic contact as
well as the slight positive entropy change for H4 complexes. The entropy change
slightly decreases upon increasing the N-substituted aryl chain length on guests, in
Table 4 Association constants (K a , M
À1
), enthalpy change (-ΔH
, kJ/mol), and entropy change
(TΔS
, kJ/mol) for intermolecular complexation of H4 and H5
Hosts
Guests
K a
-ΔH
TΔS
Ref.
H4
G8
3.2 Â 10
5
30.13
1.33
[24]
G9
1.8 Â 10
5
27.20
2.86
G10
1.9 Â 10
5
27.27
2.83
G11
7.1 Â 10
5
30.99
2.54
[25]
G12
2.5 Â 10
6
29.80
6.84
G13
1.1 Â 10
8
47.84
À1.96
G14
2.2 Â 10
8
40.06
1.87
G15
8.1 Â 10
4
20.59
7.40
[26]
G16
2.3 Â 10
6
36.31
0.01
G17
4.2 Â 10
2
14.71
0.38
[24]
H5
G8
4.0 Â 10
7
38.93
4.47
[24]
G9
5.2 Â 10
7
41.54
2.50
G10
4.7 Â 10
7
43.92
À0.17
G15
5.8 Â 10
5
24.85
8.05
[26]
G16
9.8 Â 10
5
23.17
11.01
G17
1.1 Â 10
5
29.23
À0.39
[24]
G18
2.2 Â 10
3
67.90
À48.87
G19
8.1 Â 10
5
39.37
5.65
[27]
G20
1.6 Â 10
6
40.54
5.08
G21
2.8 Â 10
6
35.47
1.34
G22
1.8 Â 10
6
41.42
5.89
G23
4.9 Â 10
6
33.38
4.82
G24
4.3 Â 10
6
41.39
À3.54
G25
7.9 Â 10
6
–
–
[28]
12
L. Chen and Y. Liu