Oligonaphthofurans: Fan-shaped and three-dimensional π-compounds

Article abstract of DOI:10.1021/ja502209w

Using a bottom-up method, we prepared a series of oligonaphthofurans composed of alternating naphthalene rings and furan rings. The largest compound (compound 25) contained 8 naphthalene units and 7 furan units. DFT calculations revealed that these compounds were fan-shaped molecules and each naphthalene ring was oriented in an alternate mountain-valley fold conformation because of steric repulsion by the hydrogens at the peri-positions. We investigated the optical properties that derived from their fan-shaped and mountain-valley sequences. As the number of aromatic rings of the oligonaphthofurans increased, the peaks of the longest wavelength absorptions in the UV-vis spectra (HOMO-LUMO energy gap) of these compounds steadily red-shifted, although the shapes of spectra were not sustained because of the decreasing molar absorption coefficients (εs) of their λ_max. We compared our results with those reported for other types of oligoaromatic compounds such as acenes 1, ethene-bridged p-phenylenes 2, rylenes 3, oligofurans 4, and oligonaphthalenes 5. The slopes of the plots between the transition energies (HOMO-LUMO energy gap) of the oligoaromatic compounds and the reciprocal of the number of aromatic rings indicated that the efficiency of π conjugation of the oligonaphthofurans was comparable with that of linear and rigid acenes and rylenes. The higher-order compounds 22 and 25 aggregated even under high dilution conditions (～10^-6 M).

Full text of DOI:10.1021/ja502209w

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Article
Oligonaphthofurans: Fan-Shaped and Three-Dimensional #-Compounds
Kentaro Nakanishi, Daisuke Fukatsu, Kazuto Takaishi, Taiki Tsuji, Keita
Uenaka, Kouji Kuramochi, Takeo Kawabata, and Kazunori Tsubaki
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja502209w • Publication Date (Web): 17 Apr 2014
Downloaded from http://pubs.acs.org on April 23, 2014
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Oligonaphthofurans: FanꢀShaped and ThreeꢀDimensional πꢀCompounds
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a
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a
a
Kentaro Nakanishi, Daisuke Fukatsu, Kazuto Takaishi, Taiki Tsuji, Keita Uenaka,
a
b
*a
Kouji Kuramochi, Takeo Kawabata, Kazunori Tsubaki
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a
Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Shimogamo,
Sakyoꢀku, Kyoto 606ꢀ8522, Japan
b
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611ꢀ0011, Japan
c
Faculty of Science and Technology, Seikei University, Musashino, Tokyo 180ꢀ8633, Japan
d
RIKEN, Wako, Saitama 351ꢀ0198, Japan
ABSTRACT:
Using a bottomꢀup method, we prepared a series of oligonaphthofurans composed of
alternating naphthalene rings and furan rings. The largest compound (compound 25) contained 8
naphthalene units and 7 furan units. DFT calculations revealed that these compounds were fanꢀshaped
molecules and each naphthalene ring was oriented in an alternate mountainꢀvalley fold conformation
because of steric repulsion by the hydrogens at the periꢀpositions. We investigated the optical
properties that derived from their fanꢀshaped and mountainꢀvalley sequences. As the number of
aromatic rings of the oligonaphthofurans increased, the peaks of the longest wavelength absorptions in
the UVꢀvis spectra (HOMOꢀLUMO energy gap) of these compounds steadily red shifted although the
shapes of spectra were not sustained because of the decreasing molar absorption coefficients (εs) of
their λmax. We compared our results with those reported for other types of oligoaromatic compounds
such as acenes 1, etheneꢀbridged paraꢀphenylenes 2, rylenes 3, oligofurans 4, and oligonaphthalenes 5.
The slopes of the plots between the transition energies (HOMOꢀLUMO energy gap) of the
oligoaromatic compounds and the reciprocal of the number of aromatic rings indicated that the
efficiency of π conjugation of the oligonaphthofurans was comparable with that of linear and rigid
acenes and rylenes. The higherꢀorder compounds 22 and 25 aggregated even under high dilution
−
6
conditions (~10 M).
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INTRODUCTION
Oligoaromatic compounds, which are composed of connecting or conjugated aromatic rings,
have attracted much attention because of the beauty of their structures and their useful functions such
1
as in innovative electronic and nearꢀinfrared materials.
These oligoaromatic compounds are
classified by their fragmental aromatic rings (thus benzenoids, nonꢀbenzenoids including hydrocarbons,
and hetero aromatic including compounds) and the direction of their π system extension (thus
twoꢀdimensional or threeꢀdimensional compounds). Representative aromatic compounds are shown
in Figure 1.
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Figure 1. Related oligoaromatic compounds.
R
R
1
: acenes (heptacene)
R
R
2
; ethene-bridged para-phenylene oligomers
OR
RO
O
N
O
O
R
N R
O
OR
RO
3
; rylenes (hexarylene)
O
O
O
O
O
O
O
O
O
4
; oligofurans (up to 9mer)
R R
R
R
R
R
R
R
R
R
R
R
n
R
R
R
R
5
; olgonaphthalnenes (up to all-(S)-32mer)
As for twoꢀdimensional benzenoid polycyclic aromatic hydrocarbons (BPAHs), linearly fused
2
benzene oligomers (acenes 1; up to 9 fused benzene rings = nonacene) and etheneꢀbridged
3
paraꢀphenylene oligomers 2 (up to 13 fused benzene rings) were reported by Anthony and Liu,
respectively. Müllen reported periꢀfused naphthalene oligomers (rylenes 3; up to a 6mer), and
4
introduced a hexarylene derivative with an absorption maximum around 950 nm. Recently, Peña et
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al. introduced a cloverꢀshaped cataꢀcondensed PAH (cloverphene with up to 16 fused benzene rings).
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Furthermore, as examples of polycyclic hydrocarbons with 5ꢀmembered rings (cyclopentadienes),
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ladderꢀtype oligonaphthalenes (up to 5 fused naphthalene rings and 4 cyclopentadienes) , and
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indenofluorenes, which have a singlet diradical character, have also been reported. Moreover, the
unique properties of the 3D framework of aromatic compounds such as fullerenes and their segmental
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corannulene and sumanene, carbon nanotubes and their segmental cycloparaphenylenes and related
11aꢀb
12
carbon nanocages,
have also been demonstrated.
With respect to typical examples of
13
14
oligoaromatic compounds with hetero aromatic rings, oligothiophenes and oligofurans, and
threeꢀdimensional helicene types containing thiophenes and furans have also been synthesized and their
functions have been reported.
With this background, our research initially focused on chiral helical oligonaphthalenes 5
where each naphthalene ring is connected at the 1,4ꢀpositions. We prepared compounds up to
1
5, 16
allꢀ(S)ꢀ32mers via a bottomꢀup procedure.
These oligonaphthalenes 5 exhibit peculiar functions
(
i.e., hierarchical energy transfer through the axes and CD properties because of their rigid rodꢀshaped
1
7
structures).
However, the nearly orthogonal arrangement causes the extensive πꢀconjugation
between the adjacent naphthalene rings to be ineffective.
More recently, we systematically investigated the characteristics of spreading the πꢀsystem and
controlling the dihedral angles by incorporating an axis in the ring structure. We have already
reported an oligonaphthodioxepine (fused 7ꢀmembered ring) in which the dihedral angle of the adjacent
naphthalene unit is controlled at ca 48° and its fluorescence properties in the solid state were
18
investigated.
Herein, we report that the incorporation of an axis into a 5ꢀmembered furan ring results in
19
fanꢀshaped oligonaphthofurans in which the dihedral angles are narrowed (ca 20°). Revealing the
chemical and physical properties of these 3D πꢀsystems and determining the number of furans and
20
naphthalenes that can be incorporated are of interest.
Figure 2 shows that these compounds have
unique fanꢀshaped structures composed of alternating furan and naphthalene rings. Additionally, the
3
D πꢀsystems are derived from repulsion between the hydrogens at the periꢀpositions.
To the best of our knowledge, previous studies have focused on the dihedral angle, Osuka et al.
have evolved a mesoꢀmesoꢀlinked porphyrin array in which each porphyrin is connected at the meso
2
1
position with a 90° dihedral angle into a porphyrin tape in which each porphyrin ring is fused in a
2
2
planar fashion.
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Figure 2. From orthogonal oligonaphthalenes to fanꢀshaped oligonaphthofurans
OR
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φ = ca 90°
φ = ca 20°
O
OR
O
OR
OR
O
OR
orthogonal type
fused 5-membered ring
RESULTS AND DISCUSSION
Synthesis of the naphthofuran 3mer.
To construct a series of oligonaphthofurans using the bottomꢀup method, we selected
naphthofuran 2mer 6 as a suitable synthon. Compound 6 was synthesized by the dehydration of
2
3
binaphthol under acidic conditions (CH SO H, toluene reflux).
Scheme 1 shows the synthetic route
3
3
2
4
for the naphthofuran 3mer 10 from 6. Using a modification of a reported procedure, the initial
naphthofuran 2mer 6 was orthoꢀlithiated by nꢀBuLi, and allowed to successively react with B(OMe)3
and 30% aqueous H O to afford compound 7 in 92% yield over three steps. Heterocoupling between
2
2
compound 7 and an excessive amount of 2ꢀnaphthol (9 eq.) occurred in the presence of CuCl (20 eq.)
2
as an oxidant together with nꢀPr NH to give the desired (2+1)ꢀ3mer 8 (37% yield based on compound
2
7
) and the homocoupling adduct (2+2)ꢀ4mer 9 (60% yield based on 7) with a large amount of
25
binaphthol.
After the isolation and purification of (2+1)ꢀ3mer 8 by column chromatography, the
furan ring was constructed by a dehydration of the two hydroxy groups of 8 in the presence of
CH SO H (2 eq.) under toluene reflux conditions to give naphthofuran 3mer 10 in 76% yield.
3
3
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Scheme 1. Synthesis of naphthofuran 3mer 10.
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OH
(
a)
(b)
(c)
OH
OH
O
O
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O
OH
OH
O
(
d)
OH
OH
O
O
O
8
10
9
Conditions; (a) MeSO H, 94%; (b) 1) nꢀBuLi, 2) B(OMe) , 3) H O , 92% for 3 steps; (c)
3
3
2
2
2
ꢀnaphthol (9 eq.), CuCl , nꢀPr NH, 37%; (d) CH SO H, 76%
2
2
3
3
Synthesis of the naphthofuran 4mer.
Compound 7 was homocoupled using CuCl and nꢀPr NH to give the desired (2+2)ꢀ4mer 9 in
2
2
26
9
4% yield (Scheme 2).
The attempted dehydration of (2+2)ꢀ4mer 9 in the presence of CH SO H
3 3
gave a residue, which was inferred to be the naphthofuran 4mer. However, because of its very low
solubility in all solvents it was impossible to determine its structure and use it as an intermediate for
further higherꢀorder oligonaphthofurans. To enhance its solubility, we planned to introduce alkyl
chains onto the ortho position of (2+2)ꢀ4mer 9. Thus, two nꢀBu groups were tentatively introduced to
the central hydroxy groups of 9. These groups served both as a protecting group and a modulator of
solubility to give 11 in 83% yield. Compound 11 was orthoꢀmonoꢀlithiated by nꢀBuLi (1.2 eq.) and
trapped by DMF to convert it to the formyl derivative 12 and to recover 11 in respective yields of 56%
and 18%. A Wittig reaction was performed using compound 11 and nꢀoctyltriphenylphosphonium
bromide and then the double bond was hydrogenated by H under Pd/C conditions to introduce the
2
nꢀnonyl side chain as a new solubility modulator, affording compound 13 in 89% yield over two steps.
Compound 13 was treated with CF SO H (2 eq.) in oꢀxylene at 130 °C for 24 h to remove the two butyl
3
3
groups. The successive formation of the furan ring via the dehydration of the hydroxy groups
afforded the corresponding naphthofuran 4mer 14 in 78% yield. The important intermediate 15,
which possessed a scaffolding hydroxy group for further higherꢀorder oligonaphthofurans was
constructed through orthoꢀlithiation, the introduction of dimethoxyboran and oxidation using 30%
aqueous H O to give the product in 92% yield over three steps from compound 14. The same
2
2
sequence was applied to 13 to afford compound 16, which was another important intermediate for
further higherꢀorder oligonaphthofurans, and it was obtained in 92% overall yield.
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Scheme 2. Synthesis of naphthofuran 4mers 14 and 15, and (2+2)ꢀ4mer 16.
R2
O
O
(
a)
1
1
(c)
OR
OR
OBu
OBu
0
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0
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O
O
9: R1 = H
1: R1 = Bu
12: R2 = CHO
(
b)
(d)
13: R2 = n-C
H
9 19
1
C H
9 19
(e)
O
O
(
14: R3 = H
O
f)
1
5: R3 = OH
13
R3
C H
9
19
(
g)
O
OBu
OBu
O
OH
1
6
Conditions; (a) CuCl , nꢀPr NH, 94%; (b) nꢀBuBr, K CO , 83%; (c) nꢀBuLi, DMF, 56%; (d) 1)
2
2
2
3
Ph P=CHC H , 2) H , Pd/C, 89% for 2 steps; (e) CF SO H, 78%; (f) 1) nꢀBuLi, 2) B(OMe) , 3)
3
7
15
2
3
3
3
H O , 92% for 3 steps; (g) 1) nꢀBuLi, 2) B(OMe) , 3) H O , 92% for 3 steps
2
2
3
2
2
In contrast, starting from a binaphthol possessing two nꢀoctyl chains on the 6, 6′ positions was
not fruitful. Because of steric repulsion between the alkyl side chains of the adjacent naphthalenes, no
furan ring formation upon the second conversion from 17 to 18 occurred (Scheme 3). Thus,
compound 17 was subjected to the same conditions as the transformation from 13 to 14. Because the
methoxy groups on the top and bottom naphthalenes of 17 were partially removed under these reaction
conditions, the residue was successively treated with MeI and K CO . However, the desired
2
3
compound 18 was not obtained at all, and methylated 19 was obtained in 50% yield instead (for the
investigated synthetic route, see the Supporting Information).
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Scheme 3. Attempted furan ring formation from compound 17.
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C H
8 17
C H
8
17
OMe
OMe
O
O
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C H
8 17
C H
(a)
8
17
OR
OR
O
C H
8
17
C H
8
17
O
O
OMe
OMe
C H
8
17
C H
8 17
1
1
7: (R = H)
1
8
9: (R = Me)
Conditions; (a) (1) CF SO H, (2) MeI/K CO , 50% for 19, not obtained for 18.
3
3
2
3
Synthesis of the naphthofuran 6mer.
Upon repeating the procedure, (4+2)ꢀ6mer 20 was derived from the heterocoupling of 7 (9 eq.)
and naphthofuran 4mer 15 (1 eq.) in 29% yield. For this reaction, the homocoupling of each
component occurred instead of the desired heterocoupling. In contrast, for the coupling between
(
2+2)ꢀ4mer 16 and 2mer 7 under the same reaction conditions, the yield of the desired (2+2+2)ꢀ6mer
2
1, which was obtained as a diastereomeric mixture with respect to the two axes bonds, improved
significantly to 59%. The naphthofuran 6mer 22 from both 20 and 21 was obtained in moderate
yields. Thus, treating 20 with pꢀTsOH (20 eq.) in refluxing oꢀxylene for 24 h gave the desired
naphthofuran 6mer 22 in 24% yield. The treatment of (2+2+2)ꢀ6mer 21 with CF SO H (5 eq.) in
3
3
oꢀxylene at 160 °C (sealed tube conditions) for 3 days led to the removal of the butyl side chains and
two furan rings formed to afford naphthofuran 6mer 22 in 27% yield.
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Scheme 4. Synthesis of naphthofuran 6mer 22.
O
C H
(
a)
O
9
19
1
5 + 7
O
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OH
OH
(b)
C H
9 19
O
O
O
2
0
O
C H
9
19
O
O
O
(
d)
OBu
OBu
(
c)
2
2
O
1
6 + 7
OH
OH
O
2
1
Conditions; (a) CuCl , nꢀPr NH, 29%; (b) pꢀTsOH, 24%; (c) CuCl , nꢀPr NH, 59%; (d) CF SO H,
2
2
2
2
3
3
2
7%
Synthesis of naphthofuran 8mer.
We then tried to synthesize naphthofuran 8mer 25 using the following two approaches
Scheme 5). The first approach was through (4+4)ꢀ8mer 23, and the other was through
(
(2+2+2+2)ꢀ8mer 24. Precursors 23 and 24 were derived from the corresponding monomers 15 and 16
using optimized oxidative coupling conditions (CuCl and nꢀPr NH) in 68% and 84% yields,
2
2
respectively. The construction of a central furan ring from (4+4)ꢀ8mer 23 seemed to be the most
direct and reasonable way to achieve our goal; however, this strategy came to nothing. Even though
(
4+4)ꢀ8mer 23 was treated with CF SO H (10 eq.) in oꢀxylene at 160 °C for 30 days (sealed tube), no
3 3
desired product was obtained. A DFT calculation at the B3LYP/6ꢀ31G(d) level indicated that the
(4+4)ꢀ8mer is about 3.3 kcal/mol more stable than the (7+1)ꢀ8mer and, therefore, the drastic increase in
2
7
strain energy during the final step may prevent cyclization (see Supporting Information).
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Scheme 5. Synthesis of naphthofuran 8mer 25.
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
O
O
O
C H
9
19
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
OH
OH
(
a)
15
O
23
O
O
(b)
O
O
O
C H
9
19
C H
9
19
O
O
C H
9
19
O
C H
9
19
O
O
OBu
OBu
(
d)
25
O
OH
OH
(
c)
16
O
OBu
OBu
C H
9
19
O
2
4
Conditions; (a) CuCl , nꢀPr NH, 68%; (b) CF SO H, not obtained; (c) CuCl , nꢀPr NH, 84%; (d)
2
2
3
3
2
2
CF SO H, 16%.
3
3
In contrast, (2+2+2+2)ꢀ8mer 24 was treated under the same conditions and the reaction was
monitored by TLC. Many reactions (deregulated removal of the four butyl chains, three random
closings of the furan rings, and the disorderly epimerization of the three connecting axes) were initially
observed, but they gradually converged into a single yellow fluorescent spot. The desired
naphthofuran 8mer 25 was finally isolated in 16% yield. The structure of naphthofuran 8mer 25 was
confirmed via changes in its NMR spectra (although the signals of 8mer 25 were extremely broad
because of selfꢀaggregation, the signals from the butyl side chains and phenolic hydroxy protons
clearly disappeared) and a good correlation was obtained between the theoretical and experimental
values for the isotopic distribution in the highꢀresolution mass spectrum of 25.
With respect to the conformation of the naphthofuran 8mer, DFT calculations at the
B3LYP/6ꢀ31G(d,p) level after a zeroꢀpoint energy correlation revealed that the alternating
mountainꢀvalley fold conformation was about 11 kcal/mol more stable than the homochiral
screwꢀshaped conformation (Figure 3). The reported Xꢀray structure of the thiophene analog of
2
8
naphthofuran 3mer 10 also showed an alternating mountainꢀvalley folds conformation.
These
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
results can be explained by considering that the warped naphthalene ring along its long axis is slightly
more stable than the twisted naphthalene ring and/or the conformation cancels the effect of the dipole
moments for each distorted naphthalene ring, which is preferred. The energy barrier of inversion for
naphthofuran 2mer 6 is estimated to be about 1.7 kcal/mol by DFT calculations at the
B3LYP/6ꢀ31G(d,p) level. This value is smaller than that of the rotation barrier of ethane (2.88
kcal/mol). Therefore, the conformation of the oligonaphthofurans is not fixed in the alternating
mountainꢀvalley shape but the respective naphthalene rings can easily flipꢀflop within the
oligonaphthofuran molecules.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. Two optimized conformational structures of the naphthofuran 8mer.
(
a)
H
H
H
H
H
H
H
O
O
H
H
O
O
O
(b)
O
H
H
O
H
H
H
H
H
H
H
H
H
H
H
O
H
O
O
H
H
O
O
O
H
H
O
H
H
Optimized structures obtained by DFT calculations at the B3LYP/6ꢀ31G(d,p) level for the (a)
alternating mountainꢀvalley folds conformation and (b) the homochiral screwꢀshaped conformation.
The nonyl side chain was replaced by hydrogen for the DFT calculations.
Optical Properties of Oligonaphthofurans.
With the series of oligonaphthofurans (2mer 6, 3mer 10, 4mer 14, 6mer 22, and 8mer 25) in
hand, we then studied their optical properties. Figure 4 shows the UVꢀvis spectra of the
oligonaphthofurans.
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Figure 4. UVꢀvis spectra of the oligonaphthofurans (6, 10, 14, 22, and 25).
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
3
mer
mer 10
mer 14
6
1
.0
4
0
.8
.6
6mer 22
8mer 25
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0.4
0.2
0
3
50
400
450
500
550
wavelength (nm)
−5
Conditions: CH Cl , 20 °C, concentration = 1.0 × 10 M, light path length = 1 cm.
2
2
As the number of naphthalene rings increased from 2mer 6 to 4mer 14, consistent red and
hyperchromic shifts were observed. Thus, the spectrum maintained its shape, and shifted to a longer
wavelength with a deeper absorption intensity. However, for 6mer 22 and 8mer 25, although
consistent red shifts were observed, the spectral shape deviated from the original form. These
observations indicated that the HOMOꢀLUMO energy gaps were constantly decreasing. On the other
hand, the molar absorption coefficients (ε) that represent the probabilities of the corresponding
HOMOꢀLUMO transitions were gradually suppressed as the number of aromatic rings in the
oligonaphthofurans increased.
Figure 5 shows an estimation of the HOMO and LUMO by DFT at the B3LYP/6ꢀ31G(d,p)
level for the naphthofurans 4mer, 6mer and 8mer. These calculations provided the following
information. (1) Both the HOMO and LUMO were spread throughout the entire molecule. (2) The
orbital coefficients of the HOMO and LUMO increased from the edge to the center of the molecule,
which is likely a major factor for the convex curve. (3) With respect to one naphthalene unit, larger
coefficients were observed for the inner benzene ring. Therefore, the πꢀconjugates on the molecule
mainly came from the furan rings and the inner benzene rings. (4) Furthermore, the conjugates of the
double bonds of the inner benzene rings and the furan rings appeared to be similar to polyacetylenes
linked in a sꢀcis fashion (relevant s-cis bonds shown in red in Figure 5). These HOMO and LUMO
properties, especially for (2) are reasons for the observed constant red shifts in absorption and the
decrease in ε of the fanꢀshaped oligonaphthofurans.
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 5. Optimized structures of the naphthofuran 4mer, 6mer and 8mer with their HOMOs and
LUMOs.
HOMO
LUMO
O
O
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
HOMO
LUMO
O
O
O
O
O
HOMO
LUMO
O
O
O
O
O
O
O
DFT calculations at the B3LYP/6ꢀ31G(d,p) level. The nonyl side chain was replaced by hydrogen for
the DFT calculations.
It is important to compare the properties of these compounds to discuss other types of
oligoaromatic compounds. Figure 6 shows the maximum absorption in the long wavelength region
(
λmax) as a function of the number of aromatic rings in oligoaromatic compounds for (a)
1
5f, 29
2
1d
14
oligonaphthofurans, (b) oligonaphthalenes 5,
(c) acenes 1, (d) rylenes 3, (e) oligofurans 4, and
3
(f) etheneꢀbridged paraꢀphenylenes 2.
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Figure 6. Correlation between λmax and the number of the aromatic rings in oligoaromatic
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
compounds.
(
a) oligonaphthofurans
00
(b) oligonaphthalenes
350
5
4
50
300
250
200
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
400
350
R
R
n
5
10
15
20
10 20 30 40 50 60
number of the aromatic rings
number of the aromatic rings
(c) acenes
000
(d) rylenes
000
1
1
R
R
8
00
800
n
R
R
O
N
O
O
N
O
600
600
400
R
R
n
4
00
2
4
6
8
5
10
15
number of the aromatic rings
number of the aromatic rings
(e) oligofurans
(f) ethene-bridged para-phenylenes
O
4
4
50
00
n
4
40
430
20
n
4
350
410
400
2
4
6
8
6
8
10 12
number of the aromatic rings
number of the aromatic rings
The plotted data are taken from (b) ref. 15f, 29 (c) ref. 2, (d) ref. 1d, (e) ref. 14, and (f) ref. 3,
respectively. For the precise chemical structures of these oligoaromatic compounds see the
Supporting Information.
For oligonaphthalenes 5 (Figure 6b), a negligible red shift in λmax was observed because of the
orthogonal arrangement of the neighboring naphthalene rings. In contrast, the planar acenes 1 and the
rylenes 3 have sharp linear relationships because of the effective spread of the π resonance system over
the entire molecule (Figure 6c and d). The correlation curves for the oligonaphthofurans, oligofuran 4,
and etheneꢀbridged paraꢀphenylene 2 are convex in shape (Figure 6a, e, and d). These data indicate
that the extension of the π resonance system depends on the shape of the aromatic systems, the
direction of extension of the aromatic rings, and the conjugation of the π fragments.
Furthermore, Figure 7 shows plots of the transition energies (eV) of the oligoaromatic
30
compounds and the reciprocal of the number of aromatic rings (n).
For each oligoaromatic
compound, a clear linear correlation exists between λmax and 1/n. The slope of each linear
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
approximation reflects the pitch of the decrease in the HOMOꢀLUMO energy gaps for the aromatic
rings. Thus, the slope shows the efficiency of π conjugation for each oligoaromatic compound. The
linear approximation equations for the oligoaromatic compounds are as follows: (a) oligonaphthofurans,
E = 2.41 + 5.66/n (R = 0.986), (b) oligonaphthalenes, E = 3.98 + 1.11/n (R = 0.983), (c) acenes, E =
0
.52 + 7.02/n (R = 0.998), (d) rylenes, E = 1.02 + 5.82/n (R = 0.983), (e) oligofurans, E = 2.42 + 3.95/n
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(R = 0.999), (f) etheneꢀbridged paraꢀphenylenes, E = 2.63 + 2.28/n (R = 0.997). It is logical that rigid
and planar acenes (c) and rylenes (d) have large slopes (7.02 and 5.82, respectively), whereas the
orthogonal oligonaphthalene has a small slope (1.11). The slope of the oligonaphthofuran (5.66) is
comparable to those of acenes and rylenes, and is clearly larger than that of the oligofuran (3.95) and
the etheneꢀbridged paraꢀphenylene (2.28). Despite their fanꢀshape (not linear), their 3D (not planar),
and dynamic (not rigid) πꢀsystem, oligonaphthofurans show an effective spreading of the π resonance
system.
Figure 7. Linear correlations between λmax and 1/n for the oligoaromatic compounds.
(b) oligonaphthalenes
4
3
.0
.0
(a) oligonaphthofurans
(
e) oligofurans
(
f) ethene-bridged p-phenylenes
2.0
(
d) rylenes
(
c) acenes
0.2
1
.0
0
0.1
/n (n = number of aromatic rings)
0.3
1
Figure 8 and Table 1 summarize the standardized fluorescence spectra of the
oligonaphthofurans and their characteristic parameters, respectively. Each oligonaphthofuran shows a
high quantum yield with a small Stokes shift.
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Figure 8. Fluorescence spectra of the oligonaphthofurans (6, 10, 14, 22, and 25).
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
mer 1
mer 5
3
4
mer 9
6
mer 14
mer 17
8
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
00
450
500
550
600
650
wavelength (nm)
−7
Conditions; CH Cl , 20 °C, concentration = 5.0 × 10 M, light path length = 1 cm. λ = 340 nm for 6,
2
2
ex
370 nm for 10, 380 nm for 14, 22 and 25.
Table 1. Optical properties of the oligonaphthofurans.
a
naphthofuran
λabs, max
nm)
λem
(nm) shift
nm)
Stokes Φ (%)
(
(
6
355.5
389
364
397
429
464
486
8.5
8.5
12
47
74
69
76
50
10
14
417
22
453
11
25
478.5
7.5
a; Fluorescence quantum yields were determined using a solution of quinine sulfate in 1 N H SO as a
2
4
reference standard (Φ = 0.546)
Aggregation behavior of the oligonaphthofurans
For higherꢀorder oligonaphthofurans, especially 8mer 25, aggregation behavior was observed.
Figure 9a–c shows the variableꢀconcentration UVꢀvis spectra of 2mer 6, 6mer 22 and 8mer 25 in CHCl3.
−5
−6
When the concentration of 2mer 6 was changed from 2 × 10 to 1 × 10 M its molar absorption
coefficient (ε) remained the same (Figure 9a). In contrast, the corresponding extinction coefficients of
6
mer 22 and 8mer 25 obviously decreased as the concentration increased (Figure 9b and c).
Furthermore, in the variableꢀtemperature UVꢀvis spectra of 8mer 25 (Figure 10), as the temperature
was lowered from 60 °C to 20 °C, the UVꢀvis spectrum of 8mer 25 gradually red shifted and several
isosbestic points were present (expanded in Figure 10b–d). This spectral behavior is attributed to the
formation of aggregates at higher concentrations or at lower temperatures.
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 9. Variableꢀconcentration UVꢀvis spectra of 2mer 6, 6mer 22 and 8mer 25.
(
a) 2mer 6
(b) 6mer 22
2
1
5
.0 x 10-5
.0 x 10-5
.0 x 10-6
3
2
1
6
4
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2.5 x 10-6
.0 x 10-6
1
3
00
400
500
300
400
500
wavelength (nm)
wavelength (nm)
(
c) 8mer 25
6
4
2
3
00
400
500
wavelength (nm)
Conditions: CHCl , 20 °C, light path length = 1 cm.
3
Figure 10. Variableꢀtemperature UVꢀvis spectra of 8mer 25.
(
a)
(b)
2
3
4
5
0 ºC
0 ºC
0 ºC
0 ºC
397 nm
0
.8
0
0
0
.8
.7
.6
0
0
0
.6
.4
.2
60 ºC
409 nm
3
00
400
wavelength (nm)
500
380
(d)
390
400
410 420
wavelength (nm)
(
0
c)
.6
0.6
446 nm
.5
0
477 nm
0.5
461 nm
4
33 nm
0
.4
0
.4
420
430
440
450
460
470
480
wavelength (nm)
wavelength (nm)
−5
Conditions: CHCl , concentration = 1.0 × 10 M, light path length = 1 cm. Figure 10b–d shows an
3
enlargement of 10a to clarify the isosbestic points.
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Figure 11a and b shows the variableꢀtemperature fluorescence spectra of 6mer 22 and 8mer 25.
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
With an increase in temperature the intensity of the emission of 6mer 22 decreased whereas that of
mer 25 increased. With an increase in temperature the thermal relaxation process took precedence
8
over the fluorescence process for 6mer 22, while dissociation from the aggregate of 8mer 25 enhanced
the fluorescence from the monomer or the lowerꢀorder aggregate.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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Figure 11. Variableꢀtemperature fluorescence spectra of 6mer 22 and 8mer 25.
(
a) 6mer 22
(b) 8mer 25
20 ºC
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0 ºC
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50 ºC
0 ºC
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00
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wavelength (nm)
wavelength (nm)
−
7
Conditions: CHCl , 5.0 × 10 M, light path length = 1.0 cm.
3
CONCLUSION
We synthesized fanꢀshaped oligonaphthofurans (up to an 8mer) by repeating a bottomꢀup
method. As the number of units increased, the construction of the furan ring by dehydration became
more difficult. The formation of a central furan during the final stage was especially troublesome
because of an abrupt increase in the distortion strain. To avoid this problem, we developed a
continuous furan ring formation route via a (2+2+2+2)ꢀ8mer to achieve the desired fanꢀshaped
naphthofuran 8mer in 16% yield at 160 °C for 1 month.
An evaluation of the optical properties of these oligonaphthofurans revealed that as the
number of aromatic rings increased, (1) the λmax of the UVꢀvis spectrum steadily shifted to longer
wavelengths, (2) the shapes of the UVꢀvis spectra became deformed compared with the original form.
Although the 3D πꢀresonance system was unevenly spread over the fanꢀshaped oligonaphthofurans,
even as the number of aromatic rings increased, the HOMOꢀLUMO coefficients were distributed in a
gradual manner over the central part of the fanꢀshaped molecules. The oligonaphthofurans showed an
effective spreading of the π resonance system as well as rigid acenes and rylenes. Furthermore,
calculations showed the potency of the sꢀcis fixed polyacetylene property on the oligonaphthofuran
skeletons. We believe that these properties provide extremely useful basic knowledge for the
development of new features in 3D πꢀsystems. We are currently working on synthesizing macrocyclic
oligonaphthofurans.
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ASSOCIATED CONTENT
1
13
Supporting Information. Experimental procedures and H and C NMR spectra for all new
compounds, the coordinates of oligonaphthofurans used in the DFT calculations, and the precise
chemical structures of the oligoaromatic compounds in Figure 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
tsubaki@kpu.ac.jp
Author Contributions
The manuscript was written through contributions of all authors.
Funding Sources
This work was supported in part by the Collaborative Research Program of Institute for Chemical
Research, Kyoto University (#2012ꢀ60 and #2013ꢀ48), the Iketani Science and Technology Foundation
(#0221002ꢀA), and the Sumitomo Foundation (#100010).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors are grateful to Prof. Yasujiro Murata, Prof. Atsushi Wakamiya, and Prof. Michihisa Murata
(ICR, Kyoto University) for their useful suggestions and discussions. We also want to thank Ms.
Akiko Fujihashi (ICR, Kyoto University) for the mass spectral analyses and Ms. Kyoko Ohmine (ICR,
Kyoto University) for the NMR measurements.
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(26) The oxidative coupling was quite sensitive toward some amines; iꢀpropylamine (10% yield),
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3
07.0 nm (for the allꢀ(S)ꢀnaphthalene 8mer), 308.5 nm (for the allꢀ(S)ꢀnaphthalene 16mer) and 309.5
nm (for the allꢀ(S)ꢀnaphthalene 32mer), respectively.
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