A comparative study of the electronic spectra, fluorescence quantum yields, cyclic voltammograms and theoretical calculations of phenanthrene-type benzodifurans

Article abstract of DOI:10.1016/j.tet.2016.05.019

Three isomeric difuran analogs of phenanthrene, including benzo[1,2-b;6,5-b′]difuran (3O), benzo[1,2-b;4,3-b′]difuran (4O) and benzo[1,2-b;3,4-b′]difuran (5O) have prepared and their properties investigated systematically. Compared with benzo[1,2-b;4,5-b′

Full text of DOI:10.1016/j.tet.2016.05.019

Accepted Manuscript
A comparative study of the electronic spectra, fluorescence quantum yields, cyclic
voltammograms and theoretical calculations of phenanthrene-type benzodifurans
Naoto Hayashi, Yoko Saito, Xiaoxi Zhou, Junro Yoshino, Hiroyuki Higuchi, Toshiki
Mutai
PII:
S0040-4020(16)30406-9
10.1016/j.tet.2016.05.019
TET 27746
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 9 March 2016
Revised Date: 6 May 2016
Accepted Date: 9 May 2016
Please cite this article as: Hayashi N, Saito Y, Zhou X, Yoshino J, Higuchi H, Mutai T, A comparative
study of the electronic spectra, fluorescence quantum yields, cyclic voltammograms and theoretical
calculations of phenanthrene-type benzodifurans, Tetrahedron (2016), doi: 10.1016/j.tet.2016.05.019.
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A comparative study of the electronic
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of phenanthrene-type benzodifurans
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a,
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a
a
a
b,
Naoto Hayashi ∗ , Yoko Saito , Xiaoxi Zhou , Junro Yoshino , Hiroyuki Higuchi and Toshiki Mutai ∗
a
b
Graduate School of Science and Engineering, University of Toyama, Gofuku, Toyama 930-8555, Japan, Institute of Industrial Science, The University of
Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan

1
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Tetrahedron
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A comparative study of the electronic spectra, fluorescence quantum yields, cyclic
voltammograms and theoretical calculations of phenanthrene-type benzodifurans
a,
a
a
a
a
b,
Naoto Hayashi ∗ , Yoko Saito , Xiaoxi Zhou , Junro Yoshino , Hiroyuki Higuchi and Toshiki Mutai ∗
a
Graduate School of Science and Engineering, University of Toyama, Gofuku, Toyama 930-8555, Japan
Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Three isomeric difuran analogues of phenanthrene, including benzo[1,2-b;6,5-b′]difuran (3O),
benzo[1,2-b;4,3-b′]difuran (4O) and benzo[1,2-b;3,4-b’]difuran (5O) have prepared and their
properties investigated systematically. Compared with benzo[1,2-b;4,5-b′]difuran (1O) and
benzo[1,2-b;5,4-b′]difuran (2O), there were pronounced differences in the fluorescence quantum
yields of 3O–5O. Consideration of the absorption and fluorescence spectra, cyclic
voltammograms and B3LYP/6-31G(d,p) calculations revealed that 4O had the highest electron-
donating/accepting characteristics of all of the compounds prepared in this study. The lower
electron-donating/accepting properties of 5O were attributed to the shorter chain length of it π-
conjugated system. The unexpectedly high electron-donating and low electron-accepting
properties of 3O were attributed to changes in the radical cationic and anionic states,
respectively. The energy levels of the highest occupied and lowest unoccupied molecular
orbitals of the thiophene and selenophene analogues of 3O–5O have also been calculated, and
their relative energies explained in a similar manner.
Available online
Keywords:
Benzodifurans
Oxidation potentials
Fluorescence quantum yields
HOMO/LUMO energy levels
2
009 Elsevier Ltd. All rights reserved.
—
——
∗
∗
Corresponding author. Tel.: +81-76-445-6613; fax: +81-76-445-6549; e-mail: nhayashi@sci.u-toyama.ac.jp (Naoto Hayashi)
Corresponding author. Tel.: +81 3 5452 6098 (ext. 57936); fax: +81 3 5452 6353; e-mail: mutai@iis.u-tokyo.ac.jp (Toshiki Mutai)

2
Tetrahedron
1
. Introduction
electronic materials, such as organic field-effect transistors
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8–10 11,12
(OFETs)
and photovoltaics (PVs).
A comparison of these
Fused heteroaromatic compounds have attracted considerable
materials revealed that their physical and electronic properties
varied considerably in some cases and only slightly in others,
depending on the differences in the electronic characteristics of
the different BDT moieties. Moreover, Müllen and the co-
workers reported differences in the OFET behaviors of isomeric
copolymers consisting of 1S–5S moieties, which they explained
in terms of their electronic characteristic, as well as the shape of
interest from researchers working in the field of organic
electronic materials chemistry. In contrast to fused aromatic
hydrocarbons, fused heteroaromatic compounds can exist as
structural isomers with respect to the direction of the fused
heteroaromatic rings. Although these isomers are alike in terms
of their molecular structure, their intrinsic properties, assembly
structures and intermolecular electronic interactions can vary
considerably. The appropriate use of these isomers and isomeric
moieties can therefore provide interesting opportunities to tune
the molecular and/or aggregation structures derived from these
fused heteroaromatic compounds to generate unique materials
with specific properties.
1
2
13
the polymeric molecules.
Several fused-furan compounds have also been studied in
terms of the differences in the properties of their structural
14
isomers. In contrast to fused-thiophene compounds, fused-furan
compounds likely form weaker intermolecular electronic
interactions because they cannot form S···S interactions. For this
reason, fused-furan compounds tend to have poorer OFET
properties (i.e., small mobility). However, the lack of sulfur
atoms in fused-furan systems can enhance their fluorescence
quantum yields compared with their fused-thiophene counterparts
because they do not experience a heavy atom effect. Based on the
differences in the intrinsic properties of fused-furan and fused-
thiophene compounds, it should be possible to use these systems
to develop new materials with specifically tailored properties.
Unfortunately, however, there have been very few studies aimed
at developing a thorough understanding of the differences
between these compounds, which currently remain unclear.
Fused-thiophene compounds are among the most frequently
and extensively studied of all the fused heteroaromatic
compounds reported in the literature, because of their highly
electron-donating character, chemical stability and ability to form
specific intermolecular interactions such as S···S bonds.
Benzodithiophenes (BDTs) are one of the simplest fused-
thiophene systems capable of existing as structural isomers with
respect to the direction of their fused thiophene rings.
Furthermore, the BDT derivatives 1S–5S (Chart 1) have been
investigated much more extensively than any other directional
isomers because of their stability and availability. BDTs 1S and
3
4–7
2
S are isomers of each other with respect to the direction of their
Benzodifurans (BDFs) are the furan analogue of BDTs. In a
similar manner to BDTs, the BDF derivatives 1O–5O have also
been studied extensively.
two thiophene moieties. Furthermore, the structures of 1S and 2S
are isoelectronic with anthracene, which is why they are
sometimes referred to as anthracene-type BDTs. Although BDTs
15–19
Given that the behaviors of these
derivatives are largely dependent on the intrinsic properties of
their BDF moieties, it is important to have a thorough
understanding of the characteristics of the BDF moieties
themselves. With this in mind, we previously conducted a series
of spectral, electrochemical and theoretical studies on the
3
S, 4S and 5S can exist as different isomeric structures in the
same way as 1S and 2S, they are isoelectronic with phenanthrene,
and are therefore referred to as phenanthrene-type BDTs. BDTs
1
S–5S have been used to develop a wide variety of organic
20
anthracene-type BDFs 1O and 2O. It is noteworthy that the
electron-donating abilities of 1O and 2O were found to be very
2
1
different to those of the corresponding BDTs 1S and 2S.
Furthermore, the fluorescence quantum yields of 1O and 2O
were virtually identical, whereas those of 1S and 2S were very
different. The differences in these results could be attributed to
the unique properties of BDFs compared with BDTs. Notably,
however, there have been no systematic studies on the properties
of the phenanthrene-type BDFs 3O–5O, despite the fact that a
large number of compounds have been synthesized consisting of
22–26
3
O–5O moieties and their properties examined.
For this
reason, discussions pertaining to the observed properties of these
material in the context of their individual BDF moieties have
been limited in most cases. To address this issue, we have
synthesized 3O–5O and systematically characterized these
compounds spectroscopically, electrochemically and theoretically
by comparing them to the isomeric compounds 1O and 2O, as
well as their thiophene (1S–5S) and selenophene (1Se–5Se)
analogues.
2
. Results
2
.1. Synthesis
Phenanthrene-type BDFs can be synthesized in a variety of
27–33
different ways,
and the BDFs prepared in the current study
(3O–5O) were synthesized in a similar manner to 1O and 2O, as
depicted in Scheme 1.
For the synthesis of 3O, 3,6-diiodocatechol (6) was initially
diesterified to give 7, which was subjected to a Sonogashira
Chart 1.
34,35
coupling
with trimethylsilylacetylene (TMSA) to afford 8.

3
ACCEPTED MANUSCRIPT
Scheme 1.
Compound
8
was subsequently subjected to sequential
values of 4O (3.36) and 5O (3.26) were similar, whereas the
value for 3O (2.84) was much lower. No significant changes
were observed in the spectra of these compounds when they were
run in ethanol (Figure S1), which indicated that these behaviors
could be attributed to the intrinsic properties of 3O–5O rather
than a solvent effect.
cyclization and desilylation reactions using tetrabutylammonium
fluoride and 4 Å molecular sieves to yield the target compound.
Isomers 4O and 5O were obtained in a similar manner using 10
36
(prepared from 9) and 13, respectively. It is noteworthy that the
yield for the final step in the synthesis of 5O was comparable to
those of 3O and 4O.
2
.3. Fluorescence spectra
2
.2. Electronic absorption spectra
The fluorescence spectra of 3O–5O and phenanthrene are also
Kellogg and co-workers previously reported the electronic
shown in Figure 1. The spectra of 3O–5O all contained three
principal peaks (or shoulder), although the intensity ratios of
these peaks varied for the different compounds. Notably, the
Stokes shifts of 3O–5O were much smaller (10, 1 and 0 nm,
respectively) than that of phenanthrene (52 nm). No discernible
differences were observed in the shapes of these spectra when
they were run in ethanol (Figure S1). In contrast to the absorption
spectra, large differences were observed in the peak wavelengths
of 3O–5O compared with that of phenanthrene, because of the
31
absorption spectra of 4O, 4S and 5S in cyclohexane. These
results revealed that the longest-wavelength π–π* absorption
peaks of 4O and 4S were observed at 294 and 317 nm,
respectively (Table 1). Notably, the longest-wavelength π–π*
absorption peak of 5S had a slightly higher bathochromic shift
than that of 4S at 320 nm. The molar absorption coefficients (ε)
of the longest-wavelength absorption peaks of 4O, 4S and 5S
were similar. In the present study, we not only measured the
electronic absorption spectra of 3O and 5O but also measured
those of 4O and phenanthrene for a fair comparison.
different Stokes shifts. The fluorescence quantum yields (Φ ) of
F
3O–5O were 0.03, 0.74 and 0.19, respectively.
As shown in Figure 1, there were considerable differences in
the shapes of the longest-wavelength absorption bands in the
electronic adsorption spectra of 3O–5O. The longest-wavelength
absorption peak of 4O appeared at λpeak = 294 nm, which was in
agreement with the value reported by Kellogg and the co-workers,
while that of 5O was observed at λpeak = 297 nm. The
bathochromic shifts of 4S and 5S were also very similar. In
contrast, the hypsochromic shift of 3O was observed at its
longest-wavelength absorption peak (λpeak = 284 nm). The logε
2.4. Cyclic voltammograms
The cyclic voltammograms of 3O–5O were obtained in
acetonitrile. All three cyclic voltammograms contained a similar
irreversible one-electron oxidative wave, as shown in Figure 2.
onset
The onset values for the oxidative waves (Ea ) in compounds
3
O and 4O were identical, whilst there was a pronounced
positive shift in the onset value of 5O. It is noteworthy that the
onset
onset
onset
a
gaps between E
(3O) and Ea (5O) (∆E
= 0.10 V),
a
onset
onset
onset
onset
Ea (4O) and Ea (5O) (0.07 V) and Ea (1O) and Ea (2O)

4
Tetrahedron
ACCEPTED MANUSCRIPT
Figure 2. Cyclic voltammograms of 3O (a), 4O (b) and 5O (c) in
-1
3
CH CN at a scan rate of 100 mV s .
EHOMO(4O), whereas EHOMO(3S) and EHOMO(3Se) were lower than
EHOMO(4S) and EHOMO(4Se), respectively.
A comparison of the energy levels of the lowest unoccupied
molecular orbitals (LUMO), ELUMO, of these compounds revealed
that ELUMO(4X) was much lower than ELUMO(5X), irrespective of
the identity of X (i.e., X = O, S or Se). In contrast, ELUMO(3O)
was much higher than ELUMO(4O) and ELUMO(5O). Although
ELUMO(3S) was slightly higher than ELUMO(4S) and much higher
than ELUMO(3Se), it was lower than ELUMO(4Se).
Figure 1. Electronic absorption spectra (thick lines) and fluorescence
spectra (thin lines) of 3O (a), 4O (b), 5O (c) and phenanthrene (d) in
cyclohexane.
We subsequently compared the HOMO and LUMO energy
levels of the isoelectronic compounds, 3O–3Se, 4O–4Se and
5
O–5Se, which only varied in terms of the nature of their
chalcogen atoms. The results revealed that the fused-furan and
fused-selenophene compounds had the highest and the lowest
LUMO energy levels, respectively. The energy gaps between the
fused-furan and fused-thiophene compounds (0.16–0.56 eV)
were larger than those between the fused-thiophene and fused-
selenophene compounds (0.04–0.14 eV). A similar trend was
observed for a comparison of compounds 1O–1Se, where the
LUMO energy levels decreased in the order of ELUMO(1O) >
ELUMO(1S) > ELUMO(1Se). However, the order of the LUMO
energy levels for compounds 2O–2Se was slightly different as
ELUMO(2O) > ELUMO(2Se) > ELUMO(2S), with the furan-fused
compound still acting as the strongest electron-accepter and the
thiophene-fused and selenophene-fused compounds swapping
places. Notably, the energy gaps were much larger between the
fused-furan and fused-thiophene compounds (1O/1S, 0.28 eV;
2O/2S, 0.16 eV) than they were between the fused-thiophene and
fused-selenophene compounds (1S/1Se, 0.05 eV; 2S/2Se, –0.04
eV).
(
0.08 V) were very similar. On the other hand, no reductive
waves were observed under our experimental conditions.
2
.5. Theoretical calculations
Hybrid DFT calculations were performed at the B3LYP/6-
1G(d,p) level to develop a deeper understanding of the different
3
characteristics of compounds 3O–5O. Compounds 3S–5S and
Se–5Se were also studied in the same way for comparison. The
3
results of the calculation are shown in Figure 3 and Table 2.
We initially compared the energy levels of the highest
occupied molecular orbital (HOMO), EHOMO, of the isomeric
compounds 3O–5O, 3S–5S and 3Se–5Se. The results revealed
that EHOMO(4O) was higher than EHOMO(5O). Furthermore,
EHOMO(4S) and EHOMO(4Se) were higher than EHOMO(5S) and
EHOMO(5Se), respectively. However, EHOMO(3O) was similar to

5
In contrast to the LUMO energy levels of
A
3O
C
–
C
3S
E
e,
P
4
T
O
E
–4
D
Se MA
w
N
ere
U
o
S
bs
C
er
R
ve
I
d
P
i
T
n the HOMO/LUMO energy gaps of these
and 5O–5Se, the order of the HOMO energy levels was much
more complicated. For example, EHOMO(3O) was higher than
EHOMO(3S) and EHOMO(3Se), and EHOMO(4O) was determined to
be higher than EHOMO(4S) and EHOMO(4Se). However, EHOMO(5Se)
was found to be the highest value for compounds 5O–5Se.
Furthermore, the energy gaps between the HOMO levels of the
different compounds were smaller (0.01–0.09 eV) than the
corresponding gaps between the LUMO energy levels. A
comparison of the HOMO energy levels for compounds 3O–3Se,
compounds, it was predicted that the S –S transitions of 4O and
0
1
5O would give an identical λpeak value. However, the oscillator
strength (f) of 4O was much larger than that of 5O. Compared
with 4O and 5O, there was a slight hypsochromic shift in the S0–
S1 transition of 3O of 4 nm. It is noteworthy that f value of this
transition was very small, whereas the f value for the S –S
0
2
transition of 3O was large and around 11 nm shorter than those
of the longest-wavelength absorptions of 4O and 5O.
Consequently, the longest-wavelength absorption of 3O was
found to be hypsochromically shifted to a much greater extent
than expected.
4
O–4Se and 5O–5Se revealed that they were contrary to those of
compounds 1O–1Se and 2O–2Se, in which the HOMO energy
levels of the fused-furan and fused-selenophene compounds were
the lowest and highest, respectively.
3
. Discussion
3
.1. Synthesis
TD-DFT calculations were performed to analyze the
electronic absorption spectra of the different compounds prepared
in the current study. The S –S absorptions (longest-wavelength
It was envisaged that the cyclization and desilylation of 15 to
0
1
give 5O would afford a low yield of the desired product because
5 could potentially proceed down three different cyclization
absorptions) and S –S absorptions of compounds 3O–5O were
0
2
1
assigned as shown in Table 3. Although considerable differences
Table 1. Collection of spectroscopic properties and oxidation potentials for 3O–5O, 4S and 5S.
a
a
3
O
4O
5O
4S
5S
λ
max (log ε)
218(4.29),
216(3.99),
261(3.98),
269(4.14),
225(4.54),
244(3.95),
250(4.05),
258(4.11),
270(3.15),
219 (4.47), 234 sh
(4.11), 248 sh (4.09),
252 (4.16), 257 (4.10), 255 (4,58), 275
268 (3,95), 277 (4.11), (4.03), 286 (3.90),
288 (4,29), 300 (4.20), 296 (3.23), 307
205 (4.08), 223
(4.13), 248 sh (4.48),
2
2
2
22(4.30),
59(3.93),
67(3.80),284(2 281(4.00),
b
/
nm
.
84)
294(3.36)
2
2
2
80(3.18),
86(3.29),
97(3.26)
317 (3.53)
(3,23), 315 (2,93),
320 (3.20)
f
f
λ
F, max
304, 315,
295, 306, 312,
317, 330(sh)
297, 308, 321
–
–
3
27(sh)
b,c
/
nm
d
f
f
Φ
F
0.03
0.74
0.19
–
–
onset
e
f
f
E
a
/ V
+0.98
+1.01
+1.08
–
–
a
b
c
d
Ref. 31. In cyclohexane. Excitation wavelengths of 268, 280 and 258 nm for 3O, 4O and 5O, respectively. In EtOH.
Electrode; Pt (working), Pt (counter) and Ag/AgCl (reference). Supporting electrolyte n-Bu NPF . Solvent; CH CN. Scan rate; 100
e
4
6
3
f
mV/s. Versus ferrocene/ferrocenium. c ~ 1 mmol/L. Not reported.
Table 2. HOMO and LUMO energy levels (in eV) for 1O–5O, 1S–5S and 1Se–5Se calculated using DFT at the B3LYP/6-
3
1G(d,p) level.
a
a
1
X
2X
3X
4X
5X
[
HOMO]
X = O
X = S
X = Se
–5.54
–5.48
–5.42
–5.70
–5.50
–5.44
–5.63
–5.76
–5.74
–5.63
–5.67
–5.64
–5.76
–5.78
–5.71
[
LUMO]
X = O
X = S
X = Se
–0.82
–1.10
–1.15
–0.70
–0.90
–0.86
–0.41
–0.97
–1.11
–0.71
–1.02
–1.09
–0.45
–0.81
–0.90
a
Ref. 20.

6
Tetrahedron
ACCEPTED MANUSCRIPT
Figure 3. Diagram of the HOMO and LUMO energy levels of 1O–5O, 1S–5S and 1Se–5Se, which were calculated using DFT at the B3LYP/6-31G(d,p)
level.
pathways (i.e., path a, b and c). Whilst pathways a and c would
provide access to the desired product 5O pathway b would afford
the undesired product 16 and/or its derivatives (Figure 4).
However, the actual yield for this step was higher than expected,
and comparable with those of the reactions to give 3O and 4O.
The success of this reaction was attributed to the first
saponification occurring at the less sterically crowded acetoxy
group (at the 1-position), followed by the formation of the furan-
ring via pathway a before the second saponification reaction. The
validity of this mechanism was supported by the fact that none of
the undesired product 16 was detected in the reaction mixture.
absorption spectra of compounds 3O–5O appeared in a shorter-
wavelength region (284–297 nm) compared with those of
compounds 1O and 2O (302–307 nm). Similar hypsochromic
shifts were also observed in the spectra of the analogous
hydrocarbon compounds, phenanthrene and anthracene. The ε
values varied considerably for the longest-wavelength absorption
peaks of 3O–5O, whereas those of 1O and 2O were virtually
identical. No discernible differences were observed in the peak
positions (∆λpeak < 10 nm) based on a comparison of the λpeak
value of phenanthrene with those of 3O–5O. However, this result
was contrary to the large ∆λpeak values observed for 1O and 2O
compared with anthracene. Furthermore, there was a considerable
difference in the λpeak values of 4O and 4S (∆λpeak = 22 nm), as
well as those of 5O and 5S (∆λpeak = 23 nm).
3
.2. Electronic absorption spectra
The longest-wavelength absorption peaks in the electronic
Table 3. Assignments, wavelengths and oscillator strengths (f) of the S –S and S –S transitions of 3O, 4O and 5O, which
0
1
0
2
were obtained by TD-DFT calculations at the B3LYP/6-31G(d) level.
transitions
3O
4O
5O
0
.52 (HOMO – 1 → LUMO) + 0.11
–
0.49 (HOMO – 1 → LUMO) + 0.51
–0.17 (HOMO – 1 → LUMO + 2) +
0.63 (HOMO → LUMO)
(HOMO – 1 → LUMO + 1) + 0.26
(HOMO → LUMO) + 0.37 (HOMO
0 1
S –S
(HOMO → LUMO + 1)
→
LUMO + 1)
wavelength
nm), f
2
60, 0.002
264, 0.317
264, 0.017
(
–
0.22 (HOMO – 1 → LUMO) + 0.19
0
.59 (HOMO – 1 → LUMO) + 0.33
(HOMO → LUMO + 1) + 0.18 (HOMO
LUMO + 2)
0
.26 (HOMO – 1 → LUMO + 1) + 0.61
(HOMO – 1 → LUMO + 1) + 0.56
(HOMO → LUMO) + 0.17 (HOMO
→
0 2
S –S
(HOMO → LUMO)
→
LUMO +1)
wavelength
nm), f
2
53, 0.196
258, 0.039
248, 0.116
(

7
3
.3. Fluorescence quantum yields
oxidation potentials, they did show quite similar qualitative
ACCEPTED MANUSCRIPT
behaviors. However, it is not possible to discuss the reduction
potentials in the same context because they cannot be measured
directly by cyclic voltammetry. It was envisaged that the half-
wave reduction potentials of 3O–5O would be in the range of –
.1 to –4.4 V (vs Fc/Fc ), which would be outside of the standard
potential window. Furthermore, it was not possible to estimate
these values based on the oxidation potentials and optical gaps
measured in their electronic spectra, because the S –S transition
energies of 3O–5O could not be related to their HOMO/LUMO
energy gaps in a simple manner, as shown in Table 3. For this
reason, it was only possible to estimate the electron-accepting
characteristics of 3O–5O using theoretical calculations.
There were significant differences in the ΦF values of the
compounds belonging to series 3O–5O. For example,
compounds 3O and 4O were found to be weakly and intensely
fluorescent, respectively, whereas the fluorescence of 5O was
intermediate. These results contrasted with the ΦF values
observed for 1O and 2O, which were almost identical (0.43 and
+
4
0
.41, respectively). In most cases, there are discernible
0
1
differences in the Φ values of the different directional isomers of
fused heteroaromatic rings.
5
exception to this general rule. Comparing 3O–5O, the compound
bearing larger Φ value has the larger oscillator strength (f) in the
S –S transition. This should indicate that the lager Φ value is
F
20,21,37,38
In this case, compounds 3O–
O exhibited typical behaviors, whereas 1O and 2O acted as an
F
3
.5. The HOMO/LUMO energy levels
0
1
F
due to significant overlap of the involved orbitals in the S –S1
transition. Phenanthrene-type fused aromatic compounds
0
It is generally accepted that the canonical structure of an
isomeric compound with the longest series of alternating single
and double (or triple) bonds should have the most efficiently
extended π-conjugated system and therefore exhibit greater
electron-donating/electron-accepting characteristics (i.e., higher
generally have larger Φ values than their anthracene-type
F
39
isomers. However, there were no obvious trends between the
Φ values of compounds 1O–5O. For example, the Φ value of
F
F
4
5
O was larger than those of 1O and 2O, while those of 3O and
O were smaller. A comparison of the ΦF value of the
E
E
and lower ELUMO levels, respectively). Indeed, different
values have been explained in this manner for several
HOMO
onset
40
a
isoelectronic hydrocarbon phenanthrene (Φ = 0.13), with those
F
isomeric compounds, including 1O and 2O, isomeric
of 3O–5O, revealed that the value of 3O was smaller, whereas
those of 4O and 5O were larger. In contrast, compounds 1O and
21,42
thienobisbenzothiophenes
and
isomeric
38
40
tetrathienoanthracenes. Based on this guideline, the π-systems
for the compounds in the current study would be extended in the
order of 5O < 3O < 4O, because the longest series of alternating
single and double bonds would be consistent with the structures
depicted in Figure 5. However, this expected order was
inconsistent with the experimental and theoretical results. In fact,
the observed oxidation potentials and calculated HOMO energy
levels were of the order of EHOMO(5O) < EHOMO(3O) =
EHOMO(4O), whereas the LUMO energy levels were of the order
of ELUMO(3O) > ELUMO(5O) > ELUMO(4O).
2
O both had larger Φ values than anthracene (Φ = 0.27).
F
F
3
.4. Evaluation of the experimental and theoretical results for
compounds 3O–5O
onset
As shown in Table 1, the Ea
values of 3O, 4O and 5O were
determined to be +0.98, +1.01 and +1.08 V, respectively, while
+
their half-wave oxidation potentials (E1/2 vs Fc/Fc ) were
estimated to be +0.83, +0.83 and +0.96 V, respectively, using the
41
equation E1/2 = – (EHOMO + 4.8). Although these values cannot
be compared qualitatively because of the irreversibility of the
This inconsistency can be explained by using canonical
structures as follows. Although the theoretical description based
on the molecular orbital theory is generally better than the
explanation by means of the canonical structures, the latter one
would be accepted more easily to the experimental researcher.
The order of the HOMO and LUMO energy levels of a given
series of isomeric molecules can be determined at a fundamental
level based on the canonical structures with the longest series of
alternating single and double bonds. Furthermore, we
hypothesized that the energy levels could be raised or lowered to
some extent by the stabilizing/destabilizing effects on the radical
cationic or radical anionic states. According to the Koopmans’
43
theorem, when the HOMO energy level of a given molecule is
high or the molecule itself has a strong electron-donating ability,
the radical cation state should be highly stabilized (or vice versa).
Alternatively, when the LUMO energy level of a given molecule
is low, or the molecule itself has a strong electron-accepting
ability, the radical anion state should be highly stabilized (or vice
versa).
With regard to the HOMO energy levels of 3O–5O, the
EHOMO(3O) value was much higher than expected, most likely
because of the stabilizing effect of the radical cation state of 3O,
Figure 4. Plausible pathways for the cyclization of 15.
·
+
·+
3
O . Consideration of the resonance forms of 3O revealed that
canonical structure I (Figure 6) would make the greatest the
contribution to the overall structure, with the positive charge
residing on the α position of the furan ring and the unpaired
electron on the β position. This form would be favored because
the central benzene ring can described as a complete Kekulé
structure, with the unpaired electron being stabilized by a benzyl-
type resonance, and the positive charge being stabilized through a
resonance effect with the adjacent oxygen atom (see canonical
Figure 5. The longest series of alternating single and double bonds in
3
O–5O (black lines).

8
Tetrahedron
ACCEPTED MAse
N
len
U
op
S
h
C
en
R
e
I
c
P
ompounds were comparable. The lower LUMO
T
energy levels of the fused-thiophene and fused-selenophene
compounds compared with the fused-furan compounds could be
attributed to the stabilizing effects of the sulfur and selenium
atoms in the radical anion states, as mentioned above.
In contrast to the LUMO energy levels, there were no obvious
trends in the HOMO energy levels of XO–XSe, where X = 1–5.
It was previously reported that the HOMO energy levels of 1O–
1
Se and 2O–2Se increased in the order of EHOMO(1O) <
EHOMO(1S) < EHOMO(1Se) and EHOMO(2O) < EHOMO(2S) <
11
EHOMO(2Se), respectively. Similar orders to these have also been
observed following a comparison of the electron-donating
characteristics of fused-furan compounds with their thiophene
and selenophene analogues. For example, the vertical ionization
energies measured by photoelectron spectroscopy for benzofuran,
benzothiophene and benzoselenophene have been reported to be
51
Figure 6. Principal canonical structures contributing to
8
.37, 8.13 and 8.03 eV, respectively. However, the HOMO
·+
·–
stabilization/destabilization of 3O (a) and 3O (b).
energy levels of 3O–3Se and 4O–4Se showed different orders,
which increased in the order of EHOMO(3S) < EHOMO(3Se) <
structure II). In the neutral state, the two oxygen atoms of 3O
would be slightly negative (i.e., δ–) because of their high
electronegativity, which would lead to electrostatic repulsion
between these two atoms. The extent of this repulsive interaction
EHOMO(3O) and EHOMO(4S)
< EHOMO(4Se) < EHOMO(4O),
respectively. Fused-furan compounds are generally the most
electron-donating of these systems, whereas fused-thiophene
compounds are generally the least. In contrast, the order of the
HOMO energy levels of 5O–5Se was determined to be
EHOMO(5S) < EHOMO(5O) < EHOMO(5Se), where the most electron-
donating compound was no longer a fused-furan compound but a
fused-selenophene compound instead, although the fused-furan
compound was still more electron-donating than the thiophene
compound. Since the gaps between the HOMO energy levels
were generally small (0.01–0.09 eV), various factors should be
intrinsically intertwined, resulting in the complicated ordering of
the HOMO energy levels of these compounds.
·
+
would be smaller in 3O compared with 3O because the δ–
characteristics of the oxygen atoms would be diminished by
resonance between canonical structures I and II, as shown in
·
+
Figure 6. These effects would enhance the stability of 3O ,
thereby raising the HOMO energy level of 3O. This effect was
not observed in 4O because the oxygen atoms in this compound
were not adjacent to each other.
A comparison of the LUMO energy levels of compounds 3O–
O (i.e., ELUMO(3O) > ELUMO(5O) > ELUMO(4O)) revealed that
5
ELUMO(3O) was higher than expected. This effect was attributed
4
. Conclusions
·
–
to the destabilizing effect of the radical anion state of 3O, 3O .
·
–
Consideration of the different resonance forms of 3O suggested
that canonical structure III would make the greatest contribution
to the overall structure, with the negative charge located on the α
position of the furan ring and the unpaired electron located at the
β position. The close proximity of the oxygen atom next to the
negatively charged carbon atom would most likely destabilize the
system because of its strong π-donating character. However, the
inductive effect of the oxygen atoms in canonical form III would
also have a stabilizing effect on this structure. It is noteworthy
that similar stabilization effects have been observed in
methoxymethyl anions, where the conjugated acid, dimethyl
ether, was found to be more acidic than the carbon analogue,
Compared with 1O and 2O, there were considerable
differences in the electronic absorption and fluorescent spectra of
compounds 3O–5O. Furthermore, there were significant
differences in the Φ values of 3O–5O, whereas those of 1O and
F
2
O almost identical. The electron-donating characteristics of 3O–
5O increased in the order of 5O < 3O = 4O based on the results
of CV measurements and DFT calculations. In contrast, their
electron-accepting characteristics, which were estimated by DFT
calculations, increased in the order of 5O < 3O < 4O. The
differences in these orders have been successfully explained in
terms of the differences in the length of the alternating single and
double bonds in the canonical structures of these compounds.
The stabilizing effects achieved through a reduction in the
44
·–
propane. The oxygen atoms in 3O would have greater δ–
character than those in 3O because of the inductive effect (as
depicted in III), which would lead a greater degree of
·
+
Coulombic repulsion in 3O , as well as the destabilizing effects
·
–
achieved by increasing the Coulombic repulsion in 3O have
also been discussed in detail. The orders of the HOMO and
LUMO energy levels of 3S–5S and 3Se–5Se have been be
explained in a similar manner, although enhanced Coulombic
·
–
electrostatic repulsion. Consequently, 3O would be destabilized,
which would lead to the observed increase in the LUMO energy
level of 3O. A similar process was also applied to develop a
deeper understanding of the order of the HOMO and LUMO
energy levels in 3S–5S and 3Se–5Se (See Supplementary data).
·
+
repulsion effects would result in the destabilization of 3S and
·
+
3
Se , whereas decreased Coulombic repulsion would result in
·– ·–
the stabilization of 3S and 3Se . These results will provide
important information for the design of electronic materials
consisting of 3O–5O moieties. Given that several 3S-, 4S- and
As mentioned above the LUMO energy levels of 1O–1Se
decreased in the order of ELUMO(1O) > ELUMO(1S) > ELUMO(1Se).
Similar results were also observed following a comparison of the
LUMO energy levels of 3O–3Se, 4O–4Se and 5O–5Se, where
the LUMO energy levels decreased in the order of XO > XS >
XSe, where X = 3, 4 and 5. In contrast, there were considerable
differences in the order of the LUMO energy levels of 2O–2Se,
where ELUMO(2O) > ELUMO(2Se) > ELUMO(2S). In all cases, the
fused-furan compounds showed much lower electron-accepting
characteristics than any of the other isomers, while electron-
accepting characteristics of the fused-thiophene and fused-
5
S-based polymers have shown enhanced properties compared
with their 1S–2S analogues, it is envisaged that compounds
consisting of 3O–5O moieties will provide better materials than
their 1O–2O analogues.
5
. Experimental details
5.1. Synthesis

9
5
.1.1. General
All commercially available chemicals were used without
further purification except THF, which was distilled over
sulfate, and evaporated to dryness. The crude product was
ACCEPTED MANUSCRIPT
purified by column chromatography (SiO , 1:4 ethyl acetate-
hexane) to yield the target compound 8 as a white powder (83.2
2
1
13
1
benzophenone-ketyl before use. H and C NMR spectra were
recorded on a JEOL JNM-ECP600 (600 MHz for H and 150
MHz for C) with tetramethylsilane as internal reference. Mass
spectra were conducted on a JEOL MStation JMS-700 (EI).
mg, 93%). Mp 112–114 ºC. H NMR (CDCl
ar), 2.31 (s, 6H, COCH ), 0.23 (s, 18H, SiCH
CDCl ): δ = 167.33, 144.46, 129.82, 119.40, 102.37, 98.44,
): δ = 7.30 (s, 2H,
3
13
1
). C NMR
3
3
13
(
3
+
+
20.39, –0.28. MS: m/z = 386 (M ). HRMS (m/z): 386.1363 (M ,
calcd. 386.1370 for C H O Si ). Compounds 12 and 15 were
20
26
4
2
5
.1.2. Synthesis of 2,3-diiodohydroquinone (10)
synthesized from 11 and 14, respectively, in similar manners.
52
To a solution of 2,3-diiodo-1,4-benzoquinone (0.15 g, 0.42
1
,4-Diacetoxy-2,3-bis(trimethylsilylethynyl)benzene (12). A
white powder. Yield 53%. Mp 94–96 ºC. H NMR (CDCl ): δ =
.03 (s, 2H, ar), 2.32 (s, 6H, COCH ), 0.25 (s, 18H, SiCH ). C
NMR (CDCl ): δ = 168.39, 149.56, 122.76, 121.31, 104.83,
mmol) in chloroform (20 mL) was added an aqueous solution (15
mL) of sodium dithionite (Na S O ) (0.15 g, 0.83 mmol)
1
3
2
2
4
1
3
7
dropwisely at ambient temperature in air. After stirring for 3
hours in dark, the reaction mixture was acidified by using 3
mol/L hydrochloric acid. The organic layer was separated, and
the aqueous layer was extracted with chloroform. The combined
chloroform solution was washed by burin, dried over anhydrous
magnesium sulfate, then concentrated. The target compound 10
3
3
3
+
9
7.27, 20.77, –0.17. MS: m/z = 386 (M ). HRMS (m/z): 386.1363
+
(M , calcd. 386.1370 for C H O Si ).
20 26 4 2
1
,3-Diacetoxy-2,4-bis(trimethylsilylethynyl)benzene (15). A
1
yellow powder. Yield 48%. Mp 73–75 ºC. H NMR (CDCl ): δ =
.43 (s, J = 8.4 Hz, 1H, ar), 6.97 (s, J = 8.4 Hz, 1H, ar), 2.33 (s,
3H, COCH ), 2.31 (s, 3H, COCH ), 0.23 (s, 9H, SiCH ), 0.23 (s,
9H, SiCH3). C NMR (CDCl ): δ = 168.03, 167.33, 154.03,
3
52.25, 132.66, 119.82, 115.82, 113.10, 105.15, 100.12, 98.30,
93.96, 20.77, 20.39, –0.21, –0.24. MS: m/z = 386 (M ). HRMS
3
was obtained as a white powder (0.12 g, 81%). Mp 203–204 ºC
7
1
(dec). H NMR (acetone-d ): δ = 6.91 (s, 2H, ar), 2.86 (s, 2H,
6
3
3
3
1
3
13
OH). C NMR (acetone-d ): δ = 157.79, 115.88, 99.56. MS: m/z
=
C H O I ).
6
+
+
362 (M ). HRMS (m/z): 361.8301 (M , calcd. 361.8301 for
1
+
6
4
2 2
+
(
m/z): 386.1379 (M , calcd. 386.1370 for C H O Si ).
5
.1.3. Synthesis of 2,3-diacetoxy-1,4-diiodobenzene
7)
To a mixture of 3,6-diiodocatechol 8 (1.10 g, 3.01 mmol),
acetic anhydride (9.0 mL, 95 mmol) and triethylamine (13 mL,
20 26 4 2
(
5
.1.5. Synthesis of benzo[1,2-b:6,5-b′]difuran (3O)
A mixture of tetra-n-butylammonium fluoride (1 mol/L in
THF, 4.2 mL, 4.2 mmol) and 4 Å molecular sieves (1.0 g) in
THF (30 mL) was degassed by purging with argon and stirred at
room temperature for 1 hour. Then a solution of 8 (0.54 g, 1.4
mmol) in THF (20 mL) was added and the whole was stirred at
53
9
0
5 mmol) was added 4-dimethylaminopyridine (DMAP) (59 mg,
.48 mmol) at ambient temperature in air. After stirring for 2
hours, iced water was added. The organic substrate was diluted
with diethyl ether, washed with aqueous sodium bicarbonate
6
0 ºC for 12 hours. The suspension was cooled to room
(saturated), 3 mol/L hydrochloric acid and burin, and dried over
temperature, then water was added. The organic substrate was
extracted with n-pentane. The combined organic phase was
washed with 3 mol/L hydrochloric acid and brine, dried over
anhydrous magnesium sulfate, and evaporated to dryness. The
crude product was purified by column chromatography (SiO , n-
pentane) to yield the target compound 3O as a colorless oil (77
anhydrous magnesium sulfate. Evaporation of the solvents gave a
light brown solid, which was recrystallized from ethanol to yield
the target compound 7 as colorless plates (1.21 g, 89 %). Mp
1
2
03–205 ºC. H NMR (CDCl ): δ = 7.44 (s, 2H, ar), 2.35 (s, 6H,
3
13
2
COCH3). C NMR (CDCl ): δ = 167.03, 144.42, 137.60, 91.87,
2
4
synthesized from 10 and 2,4-diiodoresorcinol 13, respectively,
in similar manners.
3
+
+
0.77. MS: m/z = 446 (M ). HRMS (m/z): 445.8504 (M , calcd.
45.8512 for C H O I ). Compounds 11 and 14 were
1
mg, 35%). H NMR (600 MHz, CDCl ): δ = 7.67 (d, J = 1.8 Hz,
H, ar), 7.46 (s, 2H, ar), 6.90 (d, J = 1.8 Hz, 2H, ar). C NMR
3
13
10
8
4 2
2
54
(
150 MHz, CDCl ): δ = 144.05, 140.05, 125.44, 116.11, 107.55.
3
+ +
MS: m/z = 158 (M ). HRMS (m/z): 158.0366 (M , calcd.
54.0368 for C H O ). Compounds 4O and 5O were synthesized
1
1
,4-diacetoxy-2,3-diiodobenzene (11). Colorless needles.
10
6
2
1
from 12 and 15, respectively, in similar manners.
Yield 83%. Mp 182–184 ºC. H NMR (CDCl ): δ = 7.16 (s, 2H,
ar), 2.37 (s, 6H, COCH3). C NMR (CDCl ): δ = 168.34, 149.38,
1
4
3
13
1
3
Benzo[1,2-b:4,3-b′]difuran (4O). Yield 41%. H NMR
CDCl ): δ = 7.73 (d, J = 1.8 Hz, 2H, ar), 7.46 (s, 2H, ar), 6.97
d, J = 1.8 Hz, 2H, ar). C NMR (CDCl ): δ = 151.62, 145.21,
19.87, 107.77, 105.54. MS: m/z = 158 (M ). HRMS (m/z):
+
22.92, 106.69, 21.45. MS: m/z = 446 (M ). HRMS (m/z):
45.8505 (M , calcd. 445.8512 for C H O I ).
(
(
+
3
13
10
8
4 2
3
+
1
1
1
,3-Diacetoxy-2,4-diiodobenzene1 (14). Slightly brown
+
58.0364 (M , calcd. 154.0368 for C H O ).
powder. Yield 86%. Mp 77–79 ºC. H NMR (CDCl ): δ = 7.81
10
6
2
3
(d, J = 8.4 Hz, 1H, ar), 6.78 (d, J = 8.4 Hz, 1H, ar), 2.42 (s, 3H,
1
Benzo[1,2-b:3,4-b′]difuran (5O). Yield 52%. H NMR
CDCl ): δ = 7.67 (d, J = 1.8 Hz, 1H, ar), 7.66 (d, J = 1.2 Hz, 1H,
ar), 7.49–7.44 (m, 2H, ar), 7.05 (s, 1H, ar), 6.87 (d, J = 1.8 Hz,
H, ar). C NMR (CDCl ): δ = 154.21, 147.71, 144.61, 143.78,
21.90, 116.65, 113.11, 107.41, 107.23, 102.98. MS: m/z = 158
13
COCH ), 2.37 (s, 3H, COCH ). C NMR (CDCl ): δ = 168.06,
3
3
3
(
3
1
2
4
67.04, 152.91, 152.80, 139.05, 121.96, 89.23, 86.81, 21.38,
+
+
1.18. MS: m/z = 446 (M ). HRMS (m/z): 445.8515 (M , calcd.
13
1
1
(
3
45.8512 for C H O I ).
10
8
4 2
+
+
M ). HRMS (m/z): 158.0372 (M , calcd. 154.0368 for C H O ).
5
.1.4. Synthesis of 2,3-diacetoxy-1,4-
10 6 2
bis(trimethylsilylethynyl)benzene (8)
5
.2. Electron absorption/emission spectra
Electronic absorption spectra were acquired in cyclohexane
A mixture of 7 (0.10 g, 0.23 mmol) and CuI (4.1 mg, 0.022
mmol) in diisopropylamine (10 mL) was degassed by purging
with argon. After dichlorobis(triphenylphosphine)palladium (18
mg, 0.026 mmol) and trimethylsilylacetylene (0.10 mL, 0.98
mmol) were added, the reaction mixture was stirred at ambient
temperature for 3 hours. After addition of water, organic layer
was separated, and aqueous layer was extracted with ethyl
acetate. Combined organic phase was washed with 3 mol/L
hydrochloric acid and brine, dried over anhydrous magnesium
and ethanol on a Shimadzu UV-2400PC spectrometer (Shimadzu,
Kyoto, Japan). Fluorescent spectra were recorded in cyclohexane
and ethanol on a Shimadzu RF-5300PC spectrometer using a 450
W xenon short arc and sample detection with a geometry of 90°.
Fluorescence quantum yields were measured in ethanol using an
absolute PL quantum yield measurement system consisting of a

1
0
Tetrahedron
1/2
JASCO FP-6600 spectrometer (JACSO
A
,
C
To
C
ky
E
o,PTJa
E
pa
D
n) MAfe
N
rro
U
ce
S
ne
C
/fe
R
rr
I
icinium redox couple (E = +0.46 V), which was
P
T
equipped with an integrating ILF-533 sphere.
measured under the same conditions.
5
.3. Electrochemical analyses
Cyclic voltammetry (CV) experiments were performed in 1
5.4. Theoretical studies
Our theoretical investigations of compounds 3O–5O, 1S–5S
and 1Se–5Se were performed on the optimized structures using
density functional theory calculations at the B3LYP/6-31G(d,p)
level with Spartan ’04 for Windows (version 1.0.3, Wavefunction
Inc., City, Country) on Microsoft Windows XP. Time-dependent
DFT (TD-DFT) calculations were performed at the B3LYP/6-
mM of substrate on an ALS-600a electrochemical analyzer
Bioanalytical Systems, Inc., West Lafayette, IN, USA). All of
the oxidation CV measurements were conducted in anhydrous
acetonitrile containing 0.1 tetrabutylammonium
hexafluorophosphate (nBu NPF ) as a supporting electrolyte,
(
M
4
6
55
which was purged with argon prior to the experiment. A platinum
electrode was used as the working electrode with a platinum wire
as the counter electrode. All of the potentials were recorded
against an Ag/AgCl electrode, which was used as a reference
electrode. The values were then calibrated against the standard
31G(d) level using the Gaussian 98 package. The symmetries of
the molecular structures were assumed to be C2v (for 3O, 4O, 3S,
4S, 3Se and 4Se) and Cs (for 5O, 5S and 5Se) for all of the
calculations, because the molecules bearing lower symmetries
were energetically less favorable.

1
1
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.
ACCEPTED MANUSCRIPT
Acknowledgments
A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, P. Salvador, J. J.
Dannenberg, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B.
Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon,
E. S. Replogle, and J. A. Pople, Gaussian, Inc., Pittsburgh PA,
2001.
This work was supported by JSPS KAKENHI Grant Number
2
4550151.
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