Stable supramolecular complex of porphyrin macroring with pyridyl and fullerenyl ligands

Article abstract of DOI:10.1016/j.tetlet.2008.07.038

Binding constants and thermodynamic parameters for 1:1 complexation of a porphyrin macroring, self-assembled from three trisporphyrinatozinc through imidazole-Zn coordination on the terminal porphyrins, with several multidentate pyridyl and fullerenyl ligands were examined. In benzonitrile, the ligand having one fullerenyl and two coordinative pyridyl moieties surprisingly afforded the highest affinity. The thermodynamic data of the complexation indicated that an unusual and fairly large positive entropic change, which may be attributed to extensive desolvation of the solutes especially from the large cavity of porphyrin macroring and fullerene surface, significantly contributed to the enhancement of the binding constant.

Full text of DOI:10.1016/j.tetlet.2008.07.038

Tetrahedron Letters 49 (2008) 5484–5487
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stable supramolecular complex of porphyrin macroring
with pyridyl and fullerenyl ligands
b,
a,
Zafer Uyar a, Akiharu Satake a, , Yoshiaki Kobuke , Shun Hirota
*
*
*
a Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
b Institute of Advanced Energy, Kyoto University, Gokasho, Uji 611-0011, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Binding constants and thermodynamic parameters for 1:1 complexation of a porphyrin macroring, self-
assembled from three trisporphyrinatozinc through imidazole–Zn coordination on the terminal porphy-
rins, with several multidentate pyridyl and fullerenyl ligands were examined. In benzonitrile, the ligand
having one fullerenyl and two coordinative pyridyl moieties surprisingly afforded the highest affinity.
The thermodynamic data of the complexation indicated that an unusual and fairly large positive entropic
change, which may be attributed to extensive desolvation of the solutes especially from the large cavity
of porphyrin macroring and fullerene surface, significantly contributed to the enhancement of the bind-
ing constant.
Received 7 June 2008
Revised 26 June 2008
Accepted 4 July 2008
Available online 9 July 2008
Keywords:
Host–guest
Self-assembly
Porphyrins
Ó 2008 Elsevier Ltd. All rights reserved.
Macrocycles
Fullerenes
Supramolecular chemistry
Entropy
Coordination
Natural and synthetic macrocyclic molecules such as cyclodex-
trin,1,2 calixarene,3 cucurbituril,4,5 cyclic paraphenyleneacetylene,6
cyclotriveratrylene,7 and cyclic multiporphyrin8–10 have attracted
much attention in supramolecular science, since they are able to
incorporate various guest molecules inside their rings with high
association constants. We have reported several supramolecular
macrorings self-assembled from bis(imidazolylporphyrinatozinc)s
through imidazole coordination.11,12 The robust macroring skele-
tons provided stable nanometer-size pores even on metal sur-
faces13,14 and in a bilayer lipid membrane.15
A very strong complexation behavior of macroring 1 was ob-
served with 6 having tripyridyl and fullerenyl moieties (Fig. 1).16
The complex showed an unusual geometry, where fullerene over-
whelmed the pyridyl coordination and was entrapped in the mac-
roring. Here, we synthesized various multipyridyl and fullerenyl
ligands 2–9, in which the number of pyridine(s) and fullerene(s)
was varied systematically (Fig. 2). The thermodynamic parameters
evaluated for ligands 2, 3, 5, and 6 elucidated that the binding of
the porphyrin macroring with the multipyridyl and fullerenyl
ligands was stronger than that estimated from the conventional
enthalpy–entropy compensation plot. The combination of two
coordinating pyridyl moieties and one fullerenyl moiety afforded
the highest affinity with macroring 1.
Porphyrin macroring 117 and fullerene derivatives 7 and 818
were synthesized according to the reported procedures. Ligands
2, 3, 5, and 9 were synthesized from 1,1,1-tri(4-iodophenyl)-3-eth-
ylheptane by palladium-catalyzed coupling with 4-ethynylpyri-
dine and fullerene attachment (Scheme S1 in Supplementary
data). The ligands were characterized by NMR and MALDI-TOF
mass spectra, and their purities were checked chromato-
graphically.
The binding affinities of macroring 1 with the ligands were
examined by UV–vis and fluorescence titrations in benzonitrile,
which has been frequently used as an aprotic polar solvent in
photo-induced electron transfer studies related to artificial photo-
synthesis.19,20 Since benzonitrile coordinates to the porphyrinato-
zinc moiety,16 it should act as a competitive ligand for ligands 2–9.
UV–vis spectral change of macroring 1 upon the addition of
ligand 5 is shown in Figure 3a. The intensities of the Soret bands
at both 413 and 445 nm decreased gradually, and the change be-
came subtle when more than 1 equiv of 5 was added. The absorp-
tion change, j Aj, at 413 nm is plotted in the inset of Figure 3a. The
D
change corresponded very well with an assumption of 1:1 com-
plexation between macroring 1 and 5. This result was consistent
with the previous results obtained for 4 and 6.16,17 From the curve
fitting analysis,21 the binding constant was determined to be
8.0 ꢀ 107 Mꢁ1. Along with the UV–vis spectral change, decrease
in the fluorescence intensity due to energy and/or electron transfer
from the excited porphyrin to the fullerene moiety was also
* Corresponding authors. Tel.: +81 743 72 6113; fax: +81 743 72 6119 (A.S.).
E-mail address: satake@ms.naist.jp (A. Satake).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.038

Z. Uyar et al. / Tetrahedron Letters 49 (2008) 5484–5487
5485
N
R
R
N
R
R
R
N
R
N
R
Zn
Zn
Zn
Zn
Zn
Zn
N
N
N
N
R
R
R
R
Zn
N
R
N
R
N
N
R
R
N
N
R
N
N
R
N
Self-assembly
N
Zn
R
Zn
R
R
3
R
R
N
N
R
R
L
Zn
Zn
Zn
Zn
N
N
N
N
R
N
N
N
R
R
N
Zn
R
Zn
R
R
N
R
R
N
N
N
Zn
Zn
- L
R
N
R
N
N
N
R
R
R
R
R1 = -(CH2)3OCH2CH=CH2
R2 = -(CH2)2CO2Me
N
N
N
N
Zn
Zn
Zn
Zn
R
R
R
R
1/L complex
1
Figure 1. Schematic image of the equilibrium between the porphyrin macroring 1 and a guest ligand L.
N
N
R
N
CN
N
N
R
N
N
R
R
N
4
3
5
2
N
N
N
N
R1=CH2(CH2)16CH3
R=-CH2CH(C2H5)(CH2)3CH3
R
N
N
O
Ph
Ph Ph
O
N
O
N
N
N
N
R
N
6
7
9
N
8
R
N
Figure 2. Guest ligands L for the porphyrin macroring 1.
observed (Fig. 3b). In the inset of Figure 3b, the integrated fluores-
cence intensity is plotted as a function of the concentration of the
added ligand. The change became subtle upon the addition of
1 equiv of 5, and the fluorescence was quenched by 82% at the
addition of 1 equiv of 5. The binding constant (8.0 ꢀ 107 Mꢁ1) esti-
mated from the fluorescence titration was identical with that ob-
tained from the UV–vis titration.
In a similar manner, binding constants of 1 with other ligands
were estimated by UV–vis titration along with fluorescence titra-
tion for fullerene-containing ligands. In the case of monofullerene
7, the fluorescence was quenched by only 9% when 300 equiv of 7
were added, and the binding constant was too small to determine
(the data are listed in Table 1).
the binding constant of porphyrinatozinc and pyridine (ca.
103 Mꢁ1).22,23
To investigate the factors affecting the complexation, thermo-
dynamic parameters for the formation of complexes between mac-
roring 1 and the ligands (2, 3, 5, and 6) were estimated from
variable temperature UV–vis titrations (Table 2). The enthalpy
and entropy terms were determined by the van’t Hoff plot as
shown in Figure S19 and in
D
R
H0
1
T
D
S0
ꢁ ln K ¼
ꢁ
R
ð1Þ
where K, R, and T are binding constant, gas constant, and tempera-
ture, respectively.
The binding constant of 1 with 2 (2.2 ꢀ 106 Mꢁ1) is three orders
of magnitude larger than that of tetraarylporphyrinatozinc with
pyridine in non-coordinative solvents.22,23 This enhancement is
due to cooperative coordinations between the two pyridyl moieties
and the two zinc porphyrin sites in 1. Introduction of further pyr-
idyl moieties in the trispyridyl ligand 3 and the tetrapyridyl ligand
4 increased their binding constants only by approximately 5–10
times compared to that of ligand 2, which suggests that the bind-
ing ability is also affected by a factor other than the coordination.
Bidentate interaction was also effective in the bisfullerene ligand 8
to give a moderate binding constant, 3 ꢀ 104 Mꢁ1, in contrast to the
weak binding of the monofullerene ligand 7. Although introduction
of a pyridyl group to ligand 8 improved the affinity by approxi-
mately 4 times in 9, the effect was smaller than that expected from
Complexation of 1 with bis- and trispyridyl ligands 2 and 3 in
benzonitrile exhibited large negative
D
H0 values. A positive and
significantly large
D
S0 value is unusual for such a strong coordina-
tion event and is best explained by hypothesizing that a certain
amount of benzonitrile molecules solvated to the host was released
upon complexation. Introduction of additional pyridyl moiety in 3
increased the
D D
H0 value by 1.5 times but decreased the S0 value
to almost half of that of 2. The behavior indicates that the three
pyridyl coordination is advantageous for stronger overall binding
in terms of
D
H0, but must be compensated to some extent by the
loss of flexibility due to the formation of a rigid three-dimensional
complex. This is normal enthalpy–entropy compensation behavior
observed in various host–guest systems.1,2 When the measure-
ment was undertaken in chloroform, the
D
H0 value of 3 became

5486
Z. Uyar et al. / Tetrahedron Letters 49 (2008) 5484–5487
Table 2
Thermodynamic parameters for complex formation of macroring 1 with ligands 2, 3,
5, and 6 in benzonitrile and chloroforma
L
Number of moieties
D
H0 (kJ molꢁ1
)
D
S0 (J molꢁ1 Kꢁ1
)
R
2c, p = 4
Py
C60
2
3
5
2
3
2
3
3
0
0
1
1
0
ꢁ24.1
ꢁ36.3
ꢁ32.8
ꢁ32.3
ꢁ75.7
40.7
19.8
41.1
43.8
0.999
0.999
0.997
0.998
0.999
6
3b
ꢁ66.5
a
Standard deviation <5%.
In chloroform.
b
c
R2: value for the linear fit of the van’t Hoff plot.
point interaction of 5 and 6 probably due to additional
action between the fullerene moiety and the porphyrins. Although
the
H0 values increase, their S0 values are almost the same as
p–p inter-
D
D
that of 2. This seems to be contradictory to the enthalpy–entropy
compensation observed in the case of 2 and 3. The behavior can
be explained if more benzonitrile molecules interact with the ful-
lerene ligands, 5 and 6, compared to ligand 2 prior to the ligand-
to-macroring coordination. Therefore, the configurational entropy
penalty for the three-point interaction of 1 with 5 or 6 must be
compensated by the larger positive entropy change gained by the
desolvation of fullerenyl moieties upon complexation. Actually, li-
gands containing two pyridyl and one fullerenyl moieties, 5 and 6,
interacted more strongly than trispyridine 3 (Table 1), although
interaction of monofullerene with porphyrinatozinc is usually
significantly weaker than that of the pyridine ligand.22,23 These
results show the importance of the desolvation effect of
fullerene24,25 along with the fullerene–porphyrin interaction upon
the complex formations. The results also support the previous
suggestion from fluorescence quenching experiment of macroring
1 by ligand 6, where two pyridyl and one fullerenyl moieties were
accommodated, excluding coordination of one of the pyridyl
moieties.16
Figure 3. (a) UV–vis and (b) fluorescence (ex. 558 nm) titrations for macroring 1
with ligand 5 in benzonitrile. [1] = 3.05 ꢀ 10ꢁ7 M, [5] = 0–2.5 equiv, 25 °C. Inset in
(a) absorption change at 413 nm (filled circle) and the theoretical curve for
Ka = 8.0 ꢀ 107 Mꢁ1 (plain line), inset in (b) the change of the signal integration
(570–750 nm) (filled circle) and the theoretical curve for Ka = 8.0 ꢀ 107 Mꢁ1 (plain
line).
To gain further insight into the complexation of macroring 1
with the ligands, the present data are compared with the enthal-
py–entropy compensation plots for complexation of zinc porphyrin
derivatives26 (Fig. 4) and -cyclodextrins1 (not shown)
a-, b-, and c
with various guest molecules. The data obtained in the complexa-
tion of 1 exhibited similar slopes to those of other zinc porphyrin
enthalpy–entropy compensation plots. However, the present data
series locate at much higher positions over these enthalpy–entropy
compensation plots.1,9,26 The results suggest that macroring 1 is
more advantageous than other hosts for binding ligands due to
Table 1
Binding constants (Ka) and fluorescence quenching ratios of macroring
complexation with ligands 2–9 in benzonitrile
1 for
a
L
Number of moieties
Ka (Mꢁ1
)
Fluorescence
quenchinga,b
Reference
Py
C60
2
3
4
5
6
7
8
9
2
3
4
2
3
0
0
1
0
0
0
1
1
1
2
2
2.2 ꢀ 106
2.5 ꢀ 107
1.2 ꢀ 107
8.0 ꢀ 107
9.0 ꢀ 107
—
—
—
—
This work
This work
16
This work
This work
This work
This work
This work
82% (1 equiv)
85% (1 equiv)
9% (300 equiv)
77% (300 equiv)
77% (40 equiv)
3.0 ꢀ 104
1.1 ꢀ 105
a
At 25 °C.
b
F/F0 (%), where F0 is the initial fluorescence intensity of 1 in the absence of a
ligand, and F is the fluorescence intensity on the addition of a ligand ([L]/[1] equiv).
more negative and the
D
S0 value was reversed to a large negative
value. The strong solvent effect supports that benzonitrile is coor-
dinated to the macroring.
The thermodynamic parameters for complex formation of 1
with 5 or 6 showed an interesting tendency. The large incremental
D
Figure 4. The thermodynamic data (T = 298 K) obtained for porphyrin macroring 1
and ligands (2, 3, 5, and 6) complexation (diamonds), and tetraarylporphyrinatozinc
with amine ligands26 (open circles).
H0 values (ꢁ8.7 and ꢁ8.2 kJ molꢁ1) from that of 2 suggest three-

Z. Uyar et al. / Tetrahedron Letters 49 (2008) 5484–5487
5487
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from the robust and large macrocyclic structure. The pore size of 1
(diameter, D, >2 nm) is much larger than that of c-cyclodextrin (D
0.96 nm),2 which is one of the largest well-known macrocycles.
Therefore, 1 can include more solvent molecules and the entropy
gained by the release of solvent molecules upon complexation
must be significant. The plot obtained in CHCl3 is also on the line
in Figure 4, suggesting that the mobility of the chloroform mole-
cules inside the pore might be restricted prior to complexation.
In conclusion, the robust porphyrin macroring 1 exhibited
strong binding of several polypyridyl and fullerenyl ligands. In ben-
zonitrile, combination of two coordinative pyridyl and one fullere-
nyl moieties afforded higher affinities with macroring 1 than the
trispyridyl ligands. In this system, the positive entropic term con-
tributed significantly along with negative enthalpies. The positive
entropy deviating from conventional enthalpy–entropy compensa-
tion plots may come from desolvation upon complexation.
17. Kuramochi, Y.; Satake, A.; Kobuke, Y. J. Am. Chem. Soc. 2004, 126, 8668.
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Acknowledgments
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20. Tkachenko, N. V.; Lemmetyinen, H.; Sonoda, J.; Ohkubo, K.; Sato, T.; Imahori,
H.; Fukuzumi, S. J. Phys. Chem. A 2003, 107, 8834.
This work was supported by KAKENHI (20550098) (A.S.) from
JSPS and Kyoto-Advanced Nanotechnology Network.
21. Connors, K. A. In Binding Constants; Wiley Interscience: Canada, 1987.
22. Tabata, M.; Nishimoto, J.. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic: New York, 2000; Vol. 9, pp 221–419.
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Yanase, M.; Matsuoka, M.; Tatsumi, Y.; Suzuki, M.; Iwamoto, H.; Haino, T.;
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.07.038.
25. Tashiro, K.; Hirabayashi, Y.; Aida, T.; Saigo, K.; Fujiwara, K.; Komatsu, K.;
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