Selective and random syntheses of [n]cycloparaphenylenes (n = 8-13) and size dependence of their electronic properties

Article abstract of DOI:10.1021/ja2020668

[n]Cycloparaphenylenes (n = 8-13, CPPs) were synthesized, and their physical properties were systematically investigated. [8] and [12]CPPs were selectively prepared from the reaction of 4,4′-bis(trimethylstannyl) biphenyl and 4,4′-bis(trimethylstannyl)terphenyl, respectively, with Pt(cod)Cl₂ (cod = 1,5-cyclooctadiene) through square-shaped tetranuclear platinum intermediates. A mixture of [8]-[13]CPPs was prepared in good combined yields by mixing biphenyl and terphenyl precursors with platinum sources. Products were easily separated and purified by using gel permeation chromatography. In ¹H NMR spectra, the proton of the CPPs shifts to a lower field as n increased due to an anisotropic effect from the nearby PP moieties. Although the UV-vis spectra were rather insensitive to the size of the CPPs, the fluorescence spectra changed significantly in relation to their size. A larger Stokes shift was observed for the smaller CPPs. Redox properties of the CPPs were measured for the first time by using cyclic voltammetry, and the smaller CPPs had lower oxidation potentials. The results are consistent with the HOMO energies of CPPs, of which the smaller CPPs had higher energies.

Full text of DOI:10.1021/ja2020668

ARTICLE
pubs.acs.org/JACS
Selective and Random Syntheses of [n]Cycloparaphenylenes
(
n = 8ꢀ13) and Size Dependence of Their Electronic Properties
†
†
‡,§
‡,§
,†,§
Takahiro Iwamoto, Yoshiki Watanabe, Youichi Sakamoto, Toshiyasu Suzuki, and Shigeru Yamago*
†
Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan
‡
Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
§
CREST, Japan Science and Technology Agency
S Supporting Information
b
ABSTRACT:
[
[
n]Cycloparaphenylenes (n = 8ꢀ13, CPPs) were synthesized, and their physical properties were systematically investigated. [8] and
0 00
12]CPPs were selectively prepared from the reaction of 4,4 -bis(trimethylstannyl)biphenyl and 4,4 -bis(trimethylstannyl)terphenyl,
respectively, with Pt(cod)Cl (cod = 1,5-cyclooctadiene) through square-shaped tetranuclear platinum intermediates. A mixture of
2
[
8]ꢀ[13]CPPs was prepared in good combined yields by mixing biphenyl and terphenyl precursors with platinum sources.
1
Products were easily separated and purified by using gel permeation chromatography. In H NMR spectra, the proton of the CPPs
shifts to a lower field as n increased due to an anisotropic effect from the nearby PP moieties. Although the UVꢀvis spectra were
rather insensitive to the size of the CPPs, the fluorescence spectra changed significantly in relation to their size. A larger Stokes shift
was observed for the smaller CPPs. Redox properties of the CPPs were measured for the first time by using cyclic voltammetry, and
the smaller CPPs had lower oxidation potentials. The results are consistent with the HOMO energies of CPPs, of which the smaller
CPPs had higher energies.
’
INTRODUCTION
Cycloparaphenylenes (CPPs) are hoop-shaped π-conjugated
molecules, in which paraphenylene (PP) units are linked in a cyclic
manner (Figure 1). Because of their unique structures, CPPs
have attracted the attention of theoretical, physical, and synthetic
1
ꢀ6
chemists for more than a half-century.
In addition, they have
recently gained much interest in material science and technology
for photo-, electro-, and photoelectronic applications because
they are the simplest structural unit of armchair carbon nano-
tubes (CNTs). Despite the potential impact of CPPs in funda-
mental and applied chemistry, science, and technology, their low
availability has hampered the unveiling of their properties.
Because properties of CNTs are strongly affected by the struc-
Figure 1. Structure of CPP.
3
utilize the sp -hybridized carbon in cyclohexa-2,5-diene-1,4-diol
or cyclohexane-1,4-diol derivatives, respectively, to induce the
curvature required for the cyclic structure of CPPs, and the diol
units are aromatized in the final step. Bertozzi has also reported
that the fluorescence of CPPs is strongly affected by their size,
and CPPs with smaller diameter exhibit larger Stokes shifts than
7
ꢀ9
ture, the effects of the size of CPPs on the properties would be
of great interest.
Although CPPs have a simple structure, their synthesis has
12
1
0,11
been a significant challenge.
and Itami
After extensive efforts, Bertozzi
1
3ꢀ15
have reported innovative synthetic methods to
Received: March 15, 2011
Published: May 04, 2011
prepare [n]CPPs (n = 9, 12, 14, 15, 16, and 18). Both groups
r 2011 American Chemical Society
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|J. Am. Chem. Soc. 2011, 133, 8354–8361

Journal of the American Chemical Society
ARTICLE
Scheme 1. Synthetic Route of [2(l þ m)]CPP from Tetranuclear Platinum Complex
1
2
those of the larger CPPs among [n]CPPs (n = 9, 12, and 18).
properties. Full experimental details together with theoretical
calculations to estimate the properties of CPPs are described.
The redox properties of CPPs are intriguing, but there are no
reports of these properties so far.
We have been interested in the synthesis of CPPs by using a
different strategy. We envisioned that quadrangular tetra(oligo-
paraphenylene)platinum complex 3 can serve as precursors for
’
RESULTS AND DISCUSSION
Structure, Strain Energy, and HOMO/LUMO Energies. Struc-
[
2(l þ m)]CPP 4 (Scheme 1). Because the bond angles of the
ture and electronic properties of [n]CPPs (n = 4ꢀ20) were es-
timated by using density functional theory calculations at the
B3LYP/6-31G* level of theory. Although conformations and strain
substituents cis to each other in platinum complexes are ∼90°, 3
should form from appropriate linear oligoparaphenylene pre-
cursors, such as 1 and 2, without inducing significant strain. Once
1
7
energies of several CPPs have recently been reported by Itami
18
3
forms, multiple reductive eliminations of platinum from 3 would
give 4. When precursors with the same PP units are used (l = m),
4l]CPP should selectively form from the square-shaped plati-
m),
and Bachrach, their orbital energies have not been reported. Struc-
tures and strain energies of [17] and [19]CPPs were obtained for
the first time. Structures and orbital energies of oligoparapheny-
lenes (OPPs, H[C H ] H) were also calculated as references.
[
num complex. When the precursors have different PP units (l¼
6
6
4 n
even CPPs should form from the rectangular platinum complex.
Preliminary result revealed that [8]CPP, which is the smallest
CPP so far synthesized, was selectively synthesized in excellent
All CPPs optimized in this study have conformations and
HartreeꢀFock energies virtually identical to those previously
17,18
reported (see the Supporting Information).
The most stable
1
6
overall yield. We report here details of the synthesis of CPPs by
this route; [8] and [12]CPPs were selectively synthesized, and a
mixture of [8]ꢀ[13]CPPs was obtained in good combined yields.
conformation of even [n]CPPs is an alternating zigzag orienta-
tion of PP units with nearly D(n/2)h point group symmetry, and
that of odd CPPs is a helical arrangement of the PP units with C1
point group symmetry. Diameters, strain energies, and HOMO/
LUMO energies of the CPPs are summarized in Table 1. We
defined the diameter of the CPPs to be the diameter of a circle
passed by the ipso-carbons of each PP unit. The diameter of the
odd CPPs reported by Itami was smaller than our value by about
0.01ꢀ0.02 nm, whereas that of even CPPs is the same.
[
10], [11], and [13]CPPs were synthesized for the first time. All
CPPs were isolated in pure form and fully characterized, and their
photophysical and electrochemical properties were systemati-
cally studied. Redox properties of CPPs were obtained for the
first time. The studies showed that not only the cyclic structure
but also the size of the CPPs strongly affects their physical
a
Table 1. Calculated Diameters and Strain and HOMO/LUMO Energies of [n]CPPs
b
c
n
diameter (nm)
strain energy (kJ/mol) [kcal/mol]
HOMO energy (eV)
LUMO energy (eV)
4
5.70
7.05
602.7 [144.1]
490.5 [117.2]
406.8 [97.23]
356.5 [85.20]
307.1 [73.40]
279.6 [66.82]
246.6 [58.93]
229.4 [54.83]
205.21 [49.05]
194.33 [46.45]
176.14 [42.10]
168.33 [40.23]
153.82 [36.76]
148.27 [35.44]
136.63 [32.65]
132.76 [31.73]
122.96 [29.39]
ꢀ4.498
ꢀ4.665
ꢀ4.915
ꢀ4.968
ꢀ5.105
ꢀ5.125
ꢀ5.198
ꢀ5.209
ꢀ5.263
ꢀ5.259
ꢀ5.289
ꢀ5.289
ꢀ5.311
ꢀ5.309
ꢀ5.326
ꢀ5.324
ꢀ5.332
ꢀ1.990
ꢀ1.955
ꢀ1.780
ꢀ1.802
ꢀ1.699
ꢀ1.720
ꢀ1.666
ꢀ1.677
ꢀ1.632
ꢀ1.651
ꢀ1.638
ꢀ1.638
ꢀ1.622
ꢀ1.630
ꢀ1.617
ꢀ1.622
ꢀ1.621
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8.40
9.77
11.13
12.49
13.87
16.61
16.61
17.99
19.36
20.74
22.11
23.49
24.87
26.23
27.62
1
1
1
1
1
1
1
1
1
1
2
a
b
Data obtained from the DFT calculation at the B3LYP/6-31G* level of theory. Dtermined from the diameter of circle passed by the ipso-carbons of
each PP unit. A group equivalent reaction [(n þ 1) terphenyl f [n]CPP þ n biphenyl ] used for Itami and Bachrach was employed.
c
17
18
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ARTICLE
Strain energies of the CPPs at 0 K are within 0.4 kcal/mol of
HOMOꢀ1 and HOMOꢀ2 orbitals of [n]CPPs are comple-
tely or nearly degenerated in all cases, and the energies increase as
the increase of the size of CPP. LUMOþ1 and LUMOþ2 orbitals
are also completely or nearly degenerated except for [4]CPP, and
the energies decrease with the increase of the size of CPP. In
contrast to HOMO/LUMO energies, the size dependencies of
these orbital energies are identical to those observed for OPPs
18
those obtained by Bachrach. The strain energies of the CPPs
increase with a decrease in the number of PP units, and there is a
small but distinct oddꢀeven effect, as pointed out by Bachrach.
The strain energies of the odd CPPs are slightly higher than those
expected from even CPPs. This effect is due to the existence of
the helical conformation in the odd CPPs.
(
see the Supporting Information).
Selective Synthesis of [8] and [12]CPPs. Tetranuclear
HOMO and LUMO energies of the CPPs increase and decrease,
respectively, as the size of CPP decreases (Figure 2). The results
are in sharp contrast to those of OPPs, of which the HOMO and
LUMO energies increase and decrease, respectively, with an
increase in the number of PP units due to an increase in the effective
conjugation. The conflicting results for CPPs are due to the size
dependence of the aromaticity of the PP units. The aromaticity of
the PP units of CPPs tends to decrease, and the PP units tend to
take a quinodimethane form rather than a benzenoid form when
platinum complex 6c, which is a precursor for [8]CPP, was prepared
0
in a stepwise manner starting from 4,4 -bis(trimethylstannyl)bi-
23
phenyl (5a). Thus, 5a was heated with 2 equiv of Pt(cod)Cl2
cod = 1,5-cyclooctadiene) in a refluxing THF/toluene to give
(
bis-platinum biphenyl 5b in 90% yield (Scheme 2). Next, 5b was
treated with 1 equiv of 5a in 1,2-dichloroethane at 75 °C for 72 h
to give 6c in 34% yield. The same compound was also prepared in
one step in 51% yield by mixing a 1:1 mixture of 5a and
4,19ꢀ22
the number of PP units decreases.
Therefore, the smaller
Pt(cod)Cl in 1,2-dichloroethane at 70 °C for 59 h. The yield
2
CPPs have stronger polyene character than do the larger analogues.
The HOMO/LUMO energies of CPPs also exhibit oddꢀeven
effects. The HOMOs and LUMOs of the odd CPPs are higher
and lower, respectively, than those expected from the even CPPs.
The most probable explanation is the conformational differences
of the odd and even CPPs as described above. The calculations
suggest that the smaller CPPs have narrower HOMOꢀLUMO
gaps than the larger CPPs and thus would be better lead com-
pounds for electronic applications.
increased to 84% when the reaction was carried out in refluxing
THF for 36 h. Complex 6c is less soluble and precipitates more
easily from THF than from 1,2-dichloroethane. Thus, precipitation
may shift the equilibrium for the formation of 6c and also reduce
the possibility of 6c to transform into unwanted side products, such
as linear oligomeric products, under thermodynamic conditions
(see below).
0
The cod ligand in 6c was substituted with 1,1 -bis(diphenyl-
phosphino)ferrocene (dppf, 4.0 equiv) to give 6d in 91% yield.
Reductive elimination from 6d forming [8]CPP proceeded in
24
4
9% yield by the addition of bromine (7.0 equiv) and heating at
9
5 °C for 17 h in toluene. [8]CPP was prepared in three steps
1
starting from 5a in 37% overall yield. The H NMR (7.48 ppm in
CDCl ) and the C NMR (127.6 and 137.8 ppm) and the
13
3
MALDI TOF MS (m/z = 608.2504) spectra are consistent with
the structure of [8]CPP.
25
26
Addition of iodine (4.8%) or triphenylphosphine (2.4%)
instead of bromide, or simple reductive elimination without additives
<2%), did not improve the efficiency of the reductive elimination.
(
The ligand exchange reaction of 6c withdiphenylphosphinoethane,
diphenylphosphinopropane, and xantphos proceeded smoothly
to give the corresponding platinum phosphine complexes. How-
ever, reductive elimination of platinum from these complexees to
[
8]CPP did not proceed efficiently (<2% yield).
We next examined the synthesis of [12]CPP starting from
0
0
Figure 2. HOMO and LUMO energies of CPPs (blue) and oligopar-
4,4 -bis(trimethylstannyl)terphenyl 7 (Scheme 3). The reaction
aphenylenes (red).
of 7 with 1 equiv of Pt(cod)Cl proceeded in refluxing THF, and
2
a
Scheme 2. Selective Synthesis of [8]CPP
a
Conditions and reagents: (a) 5a, Pt(cod)Cl (2.0 equiv), THF/toluene, reflux, 10 h, 90%; (b) 5b, 5a (1.0 equiv), 1,2-dichloroethane, 75 °C, 72 h, 34%; (c) 5a,
2
Pt(cod)Cl (1.0 equiv), THF, reflux, 36 h, 84%; (d) dppf (4.0 equiv), CH Cl , room temperature, 20 h, 91%; (e) Br (7.0 equiv), toluene, 95 °C, 17 h, 49%.
2
2
2
2
8
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Journal of the American Chemical Society
ARTICLE
a
Scheme 3. Selective Synthesis of [12]CPP
a
Conditions and reagents: (a) 7, Pt(cod)Cl
7.0 equiv), toluene, 95 °C, 17 h, 58%.
2
(1.0 equiv), THF, reflux, 36 h, 20%; (b) dppf (4.3 equiv), CH
2
Cl
2
, room temperature, 36 h, 96%; (c) Br
2
(
a
Scheme 4. Random Synthesis of [8]ꢀ[13]CPPs
observed at 7.48, 7.52, 7.56, 7.58, and 7.61 ppm were determined
to be [8], [9], [10], [11], and [12]CPP, respectively, by using
MALDI TOF MS analysis. The overall yields of each compound
from 5b were 2.4%, 3.2%, 5.3%, 4.7%, and 2.3%, respectively.
The products and their ratio depended on the reaction con-
ditions for the formation of the platinum complex. When 5b and
7
[
3
were heated for 20 h under otherwise identical conditions, [9],
10], [11], [12], and [13]CPPs were formed in 5.4%, 9.6%, 7.7%,
.8%, and 0.8% yields, respectively. [8]CPP was not observed. A
mixture of CPPs was also obtained by mixing 5a, 7, and Pt-
cod)Cl in a ratio of 1:1:2 at 70 °C in 1,2-dichloroethane for
(
2
2
4 h. After the ligand exchange and bromine-induced reductive
elimination reactions, [8]ꢀ[12]CPPs were obtained in 0.7%, 3.0%,
7
.3%, 9.8%, and 3.7% yields, respectively.
[10]CPP should form from expected rectangular platinum
a
Conditions and reagents: (a) 5b, 7 (1.0 equiv), 1,2-dichloroethane,
5
7
0 °C, 32 h; (b) 5b, 7 (1.0 equiv), 1,2-dichloroethane, 50 °C, 20 h; (c) 5a,
complex 10, and [8], [9], [11], and [12]CPPs should form from
tetranuclear platinum complexes 6, 9, 11, and 8, respectively
(Scheme 5). For [13]CPP to form, three terphenyl and two
biphenyl units or one terphenyl and five biphenyl units must couple,
indicating the formation of a penta- or hexanuclear platinum
intermediate. It has already been reported that the transmetala-
tion between arylstannanes and platinum is reversible and that
the aryl-group on the diarylplatinum complexes undergoes scram-
(1.0 equiv), Pt(cod)Cl (2.0 equiv), THF, reflux, 24 h.; (d) dppf
2
(
2.0 equiv), CH Cl , room temperature, 13ꢀ15 h; Br (2.0 equiv),
2
2
2
toluene, 90ꢀ95 °C, 12ꢀ13 h. See main text for the yields of CPPs.
the desired platinum complex 8c formed in 20% yield. The low
yield may be due to the low solubility of oligoaryl platinum
intermediates in THF. After ligand exchange from cod to dppf,
the bromine-induced reductive elimination from the resulting
platinum complex 8d afforded [12]CPP in 58% yield. The yield
for the reductive elimination step from 8d to give [12]CPP was
higher than that from 6d to give [8]CPP due to the lower ring
27
bling. Therefore, the formation of unexpected platinum com-
plexes must be due to an equilibrium during the transmetalation
and/or the ligand exchange reactions. Although the detailed
mechanism is not fully understood, this synthetic route is attractive
because it affords various CPPs, including odd CPPs, which can
be easily separated by using GPC depending on their molecular size.
1
13
strain in [12]CPP. H NMR (7.61 ppm in CDCl ), C NMR
3
þ
1
13
(127.3 and 138.5 ppm), and MALDI TOF MS spectra (M =
NMR Spectroscopy of CPPs. The H and C NMR chemical
12
9
12.3754) were consistent with the data reported by Bertozzi
shifts of the CPPs, including the CPPs prepared by Bertozzi and
1
3
1
and Itami.
Itami, are summarized in Table 2. A single peak in the H NMR
13
Random Synthesis of [n]CPPs (n = 8ꢀ13). Encouraged by
these results, we attempted to synthesize [10]CPP starting from
bis(platinum)biphenyl 5b and bis(stannyl)terphenyl 7 (1.0 equiv)
by heating at 50 °C in 1,2-dichloroethane for 32 h (Scheme 4).
The resulting mixture was treated with dppf (2.0 equiv to 5b) without
and two peaks in the C NMR spectra were observed in all cases,
indicating that the PP units freely rotate at room temperature on
1
the NMR time scale. In the H NMR spectrum, the chemical shift
depends on the size of the CPPs and shifts downfield with an
increase in the number of PP units, reaching a maximum at
∼7.7 ppm (Figure 3). The chemical shift of the aromatic hy-
drogen in [n]paracyclophanes shows an opposite dependence on
the size. The proton appears at lower magnetic field in smaller
isolating the intermediate, followed by treatment with bromine
1
(4.0 equiv). In the H NMR spectrum of the crude mixture, five
singlet signals in the range of 7.48ꢀ7.61 ppm were exhibited signals,
indicating that there were five products. The products were easily
separated by using size exclusion chromatography, and the products
28ꢀ31
cyclophanes
due to a decrease in aromaticity and the de-
shielding effect caused by an increase in the strain. Therefore, the
8
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Journal of the American Chemical Society
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Scheme 5. Possible Synthetic Precursors of [9], [10], and [11]CPPs
1
13
Table 2. Summary of H NMR, C NMR, and UVꢀVis Spectral Data of [n]CPPs
1
a
13
a
ꢀ1
ꢀ1
n
H NMR (ppm)
C NMR (ppm)
UV (nm)/ε (M cm
)
5
8
7.48
127.60, 137.81
340/1.0 ꢁ 10
b
b
5
9
0
1
2
3
7.52 (7.53)
7.56
127.51, 138.02 (127.4, 137.9)
127.51, 138.29
341/1.2 ꢁ 10
5
1
1
1
1
1
1
1
1
341/1.3 ꢁ 10
5
7.58
127.40, 138.49
340/1.3 ꢁ 10
b
c
b
c
5
7.61 (7.62) [7.61]
127.33, 138.49 (127,3, 138.5) [127.33, 138.52]
339/1.4 ꢁ 10
5
7.64
7.65
7.67
7.68
7.71
127.49, 138.77
127.3, 138.8
127.3, 138.8
127.3, 138.9
127.3, 139.0
338/1.6 ꢁ 10
d
e
4
5
6
8
ꢀ
d
d
b
e
ꢀ
e
ꢀ
∼340
a
b
c
d
e
Measured in CDCl . Data taken from ref 12. Data taken from ref 13. Data taken form ref 14. Not reported.
3
observed size dependence of the CPPs cannot be explained by
the strain. It is possible that this trend is attributed to the dif-
ference in the anisotropic shielding effect of the adjacent PP units.
1
3
The C NMR chemical shifts also depend on the size of the
CPPs. The peak for the ipso carbon atoms shifted downfield with
an increase of PP units and reaches a maximum of ∼139.0 ppm,
whereas the ortho- and meta-carbon atoms shift upfield, plateau-
ing at ∼127.3 ppm.
UV and Fluorescence Spectra of the CPPs. The UVꢀvis
absorption maxima (λ ) of the CPPs are summarized in
max
Table 2. All of the CPPs absorb UVꢀvis light at around λ
≈
max
40 nm regardless of their size (Figure 4), and the absorption
3
coefficient (ε) slightly increased with an increase in the size of the
1
Figure 3. H NMR chemical shift of [n]CPPs in CDCl3.
CPP. The insensitivity of λ to the CPP size is consistent with
max
12
results reported by Bertozzi, but the results contradict the size
dependence of the HOMO/LUMO gap discussed above.
To clarify this discrepancy, we carried out the time-dependent
DFT calculation at the B3LYP/6-31þG*//B3LYP/6-31G* level
of theory. The calculation reveals that the HOMOꢀLUMO
transition is forbidden with no or very small oscillator strength in
all cases, and that the strong absorption can be assigned to the sum
of HOMOꢀ2 to LUMO, HOMOꢀ1 to LUMO, HOMO to
LUMOþ1, and HOMO to LUMOþ2 transitions (see the
Supporting Information). The results are consistent with the
insensitivity of λmax to the HOMO/LUMO gap of CPPs. The
small CPPs have small shoulder peaks at λ ≈ 380ꢀ450 nm,
and the absorptions may be related to the HOMOꢀLUMO
transition.
All CPPs fluoresced, and the Stokes shift became larger with a
decrease in the size of the CPP. The results are consistent with
those reported by Bertozzi. The larger Stokes shift of the smaller
CPPs can be explained by the release of strain energy in the
excited state. Because smaller CPPs are more strained, they would
undergo a larger structural change in the excited state than would
the larger CPPs. All of the fluorescence spectra can be divided
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Journal of the American Chemical Society
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Figure 4. (a) UVꢀvis (ꢀ) and fluorescence (ꢀ ꢀ ꢀ) spectra and
(b) fluorescence emission of [8], [9], [10], [11], [12], and [13]
iCn Pt eP ns s. i Ft yl u. orescent spectrum is normalized to its UVꢀvis absorption
into two or three components. Further analysis of the UVꢀvis
and fluorescence spectra is needed.
Redox Properties of the CPPs. Oxidation potentials of the
CPPs were measured by using cyclic voltammetry (CV) in
1
,1,2,2-tetrachloroethane solutions containing 0.1 mol/L of
Bu NPF at room temperature. Voltammograms are shown in
4
6
Figure 5, and the oxidation potentials are summarized in Table 3.
Reduction wave(s) could not be observed within this solvent’s
potential window. All of the CPPs showed reversible oxidation
waves, indicating that the oxidized intermediates are stable
under the conditions used. The oxidation potential was strongly
affected by the size of the CPPs, and [8]CPP has the lowest
half-wave oxidation potential of 0.59 V (vs the ferrocene/
ferrocenium couple). The oxidation potential increases as the
size of the CPP increases, and the oxidation of [12] and
[
13]CPPs was at the edge of the solvent’s potential window.
However, a clear reversible oxidation wave was observed with
a half-wave oxidation potential of 0.83 V for [12]CPP and
0
.85 V for [13]CPP.
The oxidation potential increases monotonically with an increase
in the number of PP units in the CPPs and reaches a plateau
around 0.85 V. This trend is in good agreement with the trend in
the HOMO energy of the CPPs. Although the HOMO energy
shows oddꢀeven effect, no such clear effect was observed in the
oxidation potential (Figure 6). We believe that the results can
be explained by considering the conformation of the CPPs. All
4 6 2 2 4
Figure 5. Cyclic voltammograms of CPPs in Bu NPF /C H Cl at
room temperature.
averaged HOMO energy of the dynamic structures of the CPPs
at room temperature. It is also possible that solvent plays some
role because the calculation was carried out for molecules in a
vacuum.
1
13
of the PP units in CPP freely rotate on the basis of H and
C
NMR spectroscopy and are chemically equivalent at room
temperature. However, the HOMO energies were calculated
on the basis of the static structures with unequivalent PP units
at 0 K. Because the HOMO energy is highly sensitive to the
structure, the oxidation potential is likely to reflect the time-
’
SUMMARY
A new synthetic strategy for preparing CPPs through multi-
nuclear arylplatinum complexes was successfully used for the
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Journal of the American Chemical Society
ARTICLE
Table 3. Half-Wave Oxidation Potential of [n]CPPs Ob-
tained by Using Cyclic Voltammetry
material is available free of charge via the Internet at http://pubs.
acs.org.
a
þ
n
E (vs Fc/Fc )
’
AUTHOR INFORMATION
8
0.59
0.70
0.74
0.79
0.83
0.85
Corresponding Author
yamago@scl.kyoto-u.ac.jp
9
0
1
2
3
1
1
1
1
’ ACKNOWLEDGMENT
This work was partly supported from the Core Research for
Evolution Science and Technology (CREST), Japan Science and
Technology Agency. We thank Dr. Yasuyuki Nakamura of our
group for valuable discussions and suggestions for the time-
dependent DFT calculation and Prof. Norihiro Tokitoh and his
group members of our institute for the measurement of fluores-
cence spectra. Computation time was provided by the Super
Computer Laboratory, Institute for Chemical Research, Kyoto
University.
a
4 6
V versus ferrocene/ferrocenium couple; Bu NPF (0.1 M) in 1,1,2,2-
tetrachloroethane; scan rate = 20 mV/s; scan range = 0ꢀ1.25 V.
’
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