Electron-Deficient Tetrabenzo-Fused Pyracylene and Conversions into Curved and Planar π-Systems Having Distinct Emission Behaviors

Article abstract of DOI:10.1002/anie.201503783

Polycyclic aromatic compounds containing fully unsaturated five-membered ring(s) have been intensively studied because of their unique properties, which include high electron affinity and reactivity. Reported herein is an efficient route for the synthesis

Full text of DOI:10.1002/anie.201503783

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201503783
German Edition: DOI: 10.1002/ange.201503783
Fused-Ring Systems
Electron-Deficient Tetrabenzo-Fused Pyracylene and Conversions into
Curved and Planar p-Systems Having Distinct Emission Behaviors**
Chaolumen, Michihisa Murata,* Yasunori Sugano, Atsushi Wakamiya, and Yasujiro Murata*
Abstract: Polycyclic aromatic compounds containing fully
unsaturated five-membered ring(s) have been intensively
studied because of their unique properties, which include
high electron affinity and reactivity. Reported herein is an
efficient route for the synthesis of tetrabenzo-fused pyracylene,
which comprises pyracylene and tetracene segments, using
als.[8] In this work, we focused on the tetrabenzo-fused
pyracylenes 1 (abbreviated as TBP), which includes both
pyracylene and tetracene moieties, to elucidate its electronic
properties and chemical reactivity (Scheme 1). Although the
synthesis of 1 was reported in 1956 by Theiling et al.,[9] this
À
intramolecular oxidative C H coupling. It was shown to
possess high electron affinity and was found to undergo
addition reactions with n-butyllithium or benzyne. These
reactions led to either a 1,4-addition compound or triptycene-
type adduct with a curved or planar p-system, respectively.
Although these compounds exhibited similar sky-blue emis-
sions in a dilute solution, the emission band of the 1,4-addition
compound was significantly red-shifted in the solid state and
exhibited intense yellow emission attributable to the excimer,
while the triptycene-type adduct retained the intense blue color
emission in the solid state.
Scheme 1. Structures of TBP (1) and its key segments, pyracylene and
tetracene.
method was recently reported to be low yielding.[10] Herein we
report an efficient route for the synthesis of TBPs by
P
olycyclic aromatic hydrocarbons (PAHs) containing fully
À
unsaturated five-membered rings, so called cyclopenta-fused
polycyclic aromatic hydrocarbons (CP-PAHs), have received
attention because of their unusual properties such as high
electron affinities[1] and chemical reactivities,[2] as well as for
their potential use in organic electronic devices.[3] CP-PAH
molecules such as rubicenes,[4] dithiarubicene (so called
emeraldicenes),[5] and cyclopenta[hi]aceanthrylenes[6] have
been reported as electron-accepting (n-type) and ambipolar
materials for either organic photovoltaics (OPVs) or organic
field-effect transistors (OFETs).
intramolecular oxidative C H coupling and its unique
reactivity, thus leading to the formation of intriguing p-
systems with distinct emission properties in the crystal form
with high quantum yields. These addition reactions can
provide ways to access highly fluorescent materials having
unique three-dimensional structures.
The syntheses of the TBPs 1a–c were conducted by Scholl
reaction of the 5,11-diaryltetracene derivatives 3a–c using
FeCl3 as an oxidant (Scheme 2). Monocyclization and double
cyclization proceeded regioselectively at ambient conditions
in a controlled manner, depending on the amount of FeCl3, to
Among the CP-PAHs, pyracylene has been known to
possess anti-aromatic character and high electron affinity,
thus resulting from the contribution of 12p-electron periphery
with a strongly localized interior double bond (1.35 Š).[7] In
contrast, tetracene and derivatives thereof have attracted
particular interest as excellent charge transporting materi-
[*] Chaolumen, Dr. M. Murata, Y. Sugano, Prof. Dr. A. Wakamiya,
Prof. Dr. Y. Murata
Institute for Chemical Research, Kyoto University
Uji, Kyoto 611-0011 (Japan)
E-mail: mmurata@scl.kyoto-u.ac.jp
yasujiro@scl.kyoto-u.ac.jp
[**] We are grateful to Prof. Jun Terao and Prof. Yasushi Tsuji for the
quantum yield measurements. Financial support was provided by
JSPS KAKENHI Grant Number 24350024. XRD measurements of
the compound 1b was carried out at SPring-8 (BL38B1) with the
approval of JASRI (2012B1319 and 2013A1489). This study was
carried out with the NMR spectrometer in the Joint Usage/Research
Center (JURC) at ICR, Kyoto University.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503783.
Scheme 2. Intramolecular oxidative cyclizations of the 5,11-diaryltetra-
cene derivatives 3a–c.
9308
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 9308 –9312

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Angewandte
Chemie
afford 2a–c and 1a–c as deep-purple and deep-blue solids,
respectively. Whereas a similar reaction was recently reported
for pentacene derivatives by Chi et al.,[11] the construction of
a pyracylene moiety by the double cyclization is unprece-
dented. Although Scholl reactions are considered to proceed
via either radical cation or arenium cation intermediates,[12]
we confirmed that no cyclization of 3c proceeded under acidic
conditions with MeSO3H, and is indicative of the radical
cation mechanism. Actually, the DFT calculations (see the
(the corresponding bond length is 1.35 Š).[13] In contrast,
when we compare the structures of indeno moieties, bond c is
elongated in 1b [1.438(2) Š] after the second cyclization
relative to bond c in 2a [1.421(2) Š]. In addition there is
a slight elongation of bonds d and e in 1b [1.485(2) Š and
1.486(2) Š, respectively], compared to those bond lengths in
2a [1.474(2) Š and 1.478(2) Š, respectively]. It is notable that
bond c of 1b is clearly longer than the corresponding bond in
the parent pyracylene (1.366 Š),[13] and is likely a result of
avoiding contributing to the anti-aromatic character. These
data indicate that the major contribution to the resonance
structure is form A, which is among the possible resonance
structures, including form B, in which the 20p-antia-romatic
periphery can be drawn (as shown with a bold line in
Figure 1d).
+
+
Supporting Information) for the radical cations 3aC and 2aC
indicated that spin densities are relatively localized at C6 and
+
+
C12 for 3aC and C12 for 2aC . In addition, 1a was calculated
to be the most unstable in energy among the three possible
isomers, thus suggesting that the cyclizations likely proceeded
under kinetic control.
X-ray diffraction analyses were performed for 3a, 2a, and
1b (Figure 1a–c). The results show that the cyclizations of the
two phenyl rings to form five-membered rings caused
significant changes in the bond lengths. In particular, the
bonds denoted as a were shortened [1.451(3) Š for 3a,
1.417(2) Š for 2a, 1.381(3) Š for 1b], while the bonds b were
elongated [1.442(2) Š for 3a, 1.450(2) Š for 2a, 1.474(2) Š
for 1b]. The enhanced double-bond character of bond a in 1b
appeared to be similar to the characteristics of pyracylene
This consideration is supported by the results of DFT
calculations.[14] The structural optimization for 1a–3a calcu-
lated at the B3LYP/6-31G* level of theory reproduce the
crystal structures well (see the Supporting Information). The
NICS(0) (nucleus independent chemical shifts)[15] values for
the optimized structures are summarized in Figure 2.
Figure 2. NICS(0) values for 1a–3a, pyracylene, and dibenzo-fused
pyracylene 4 calculated at the HF/6-311+G**//B3LYP/6-31G* level of
theory.
Although the changes in the NICS(0) values after monocyc-
lization are small, both the negative values for six-membered
rings and the positive value for the five-membered ring
become more positive in 1a, with reference to 2a, after the
second cyclization. The NICS(0) calculations for the model
compound 4 indicated that introduction of two fused rings on
the naphthalene moiety in pyracylene significantly increases
the positive values for five-membered ring compared to those
for pyracylene (from + 12.9 for pyracylene to + 21.0 for 4),
and is indicative of the contribution of 20p-anti-aromatic
periphery. The introduction of two fused rings on the two five-
membered rings of 4 has a strong impact and weakens the
anti-aromatic contribution, while enhancing the aromatic
character in the tetracene moiety. Hence, we expected TBP to
possess electronic properties analogous to both pyracylene
and tetracene.
Figure 1. The ORTEP drawings (50% probability for thermal ellipsoids)
of 3a (a), 2a (b), 1b (c), and selected resonance structures of 1a (d).
Selected bond lengths [Š] for 3a: bond a 1.451(3); b 1.442(2). Selected
bond lengths [Š] for for 2a: bond a 1.417(2); b 1.450(2); c 1.421(2);
d 1.474(2); e 1.478(2). Selected bond lengths [Š] for 1b: bond
a 1.381(3); b 1.474(2); c 1.438(2); d 1.485(2); e 1.486(2).
To clarify the electronic properties, we conducted cyclic
voltammetry (CV) for 1b–3b. In the reduction process, 3b
and 2b exhibited one reversible reduction wave (E1/2
=
Angew. Chem. Int. Ed. 2015, 54, 9308 –9312
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9309

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
.
Angewandte
Communications
À2.11 V vs. Fc/Fc+ for 3b and À1.64 V for 2b). The formation
of one indeno moiety causes a positive shift of the potential of
2b, with reference to 3b, by DE1/2 = 0.47 V. The compound 1b
exhibited two reversible waves (E1/2 = À1.30 V and À1.76 V)
with the first reduction potential shifted to a more positive
value by DE1/2 = 0.34 V compared to that of 2b, and is
indicative of high electron affinity of the TBP skeleton. In the
oxidation process, 3b and 2b exhibited one reversible
oxidation wave (E1/2 =+ 0.43 V for 1b and + 0.55 V for 2b),
but the difference (DE1/2 = 0.12 V) is apparently smaller than
those observed in the reduction process. The shape of the
oxidation wave of 1b (Epa =+ 0.49 V) was broad but repro-
ducible for several cycles, in a range of + 1.0 to À2.0 V, and
indicative of the effect of aggregation.
Scheme 3. Regioselective addition reactions of 1a. THF=tetrahydro-
The absorption spectra were also measured and the data,
together with that of CV, are summarized in Table 1.
furan.
Table 1: Optical and electrochemical data.
Compd. Oxidation
potential[a]
Reduction
potential[a]
E1/2 [V]
LUMO[b]
(eV)
Absorption[c]
E1/2 [V]
lmax [nm] loge
3b
2b
1b
0.43
0.55
(0.49)
À2.11
À1.64
À1.30
À2.99
À3.46
À3.80
495
579
613
4.04
3.93
4.28
[a] In CH2Cl2 with (nBu)4N+PF6 (0.1m) as a supporting electrolyte at
a scan rate of 100 mVsÀ1 using ferrocene/ferrocenium as an internal
standard. Peak oxidation potential (Epa) in parenthesis. [b] LUMO energy
level was calculated from the half wave potential of the first reduction
wave by using the following equations: LUMO=À(E1/2 +5.1) (eV).[16]
[c] In toluene.
À
Figure 3. Plot of the LUMO of 1a (a; B3LYP/6-31G*) and the HOMO
of the anion intermediate 5a (b; B3LYP/6-31+G*).
an intriguing triptycene-type structure (Scheme 3), thus
indicating that TBP retains the reactivity characteristics of
tetracene.[18]
Reflecting the low-lying LUMO, 1b showed an intense
absorption band (lmax = 613 nm, loge = 4.28) which is notably
red-shifted by 118 nm compared to that of 3b (lmax = 495 nm,
In a toluene solution, 6a and 7 showed an absorption band
in a similar region with the maximum wavelength (labs) at
388 nm and 376 nm, respectively, which were hypsochromi-
cally shifted with reference to that of 1a (labs = 611 nm), thus
reflecting the partial scission of the p-system of 1a (Figure 4).
The compounds 6a and 7 (8.6 ” 10À6 m and 7.1 ” 10À6 m in
toluene, respectively) exhibited intense sky-blue fluorescence
with the maximum wavelength (lem) at 407 nm (FF = 0.99, t =
3.2 ns) and 383 nm (FF = 0.98, t = 2.3 ns), respectively. The
rigidly fixed conformations of 6a and 7 likely contribute to
the small Stokes shifts (19 nm and 7 nm, respectively) as well
as the high quantum yields. However, 6a and 7 showed
distinct emission behaviors in a concentrated solution and in
the solid state, while the absorption profiles were almost
unchanged. The compound 6a exhibited a new red-shifted
emission band (lem = 509 nm, FF = 0.51, t = 7.1 ns) in a higher
concentration (7.9 ” 10À3 m in toluene; Figure 4a).[19] The
longer fluorescence lifetime for 6a at a high concentration
indicates that the red-shifted emission is attributable to the
excimer.[20] Interestingly, the crystals of 6a showed intense
yellow emission at lem = 547 nm while keeping the high
quantum yield (FF = 0.88). In contrast, the red-shift of the
emission band for 7 was not remarkable and showed intense
sky-blue emission in a concentrated solution (lem = 404 nm,
loge = 4.04), and by 34 nm compared to that of 2b (lmax
=
579 nm, loge = 3.93). Considering the electron affinity and
the absorption profile, TBP can be used as a potential
building block for n-type materials in OPVs.[1c,4d]
Unique addition reactions[17] or substitution reactions[5a]
of CP-PAHs with alkyllithium reagents have been reported,
so we also studied the reactivity of TBP. When n-butyllithium
was added to a suspension of 1a in THF at À788C, the dark-
blue powder of 1a smoothly dissolved to afford a deep-purple
solution. Quenching this solution with n-butyliodide or acidic
water provided the 1,4-addition products 6a,b (Scheme 3).
The compounds 6a,b have a curved p-surface as confirmed by
the single-crystal X-ray diffraction analyses (see below). The
LUMO of 1a delocalizes over the entire molecular surface
except for the interior double bond (Figure 3a). We antici-
pated n-butyllithium to kinetically attack one of the carbon
atoms in the five-membered rings, atoms for which orbital
coefficients are relatively large, to give the cyclopentadienyl-
type anion 5a. The HOMO of anion 5a is localized on the
five-membered ring with the highest coefficient at C12
(Figure 3b). Thus, the quenching reactions selectively pro-
ceeded at C12 to give 6a,b under kinetic control. In addition,
benzyne was found to undergo addition to 1a to afford 7 with
FF = 0.50, t = 2.8 ns) as well as in the solid state (lem
441 nm, FF = 0.73; Figure 4b).
=
9310
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 9308 –9312

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Angewandte
Chemie
Figure 5. The face-to-face dimeric structures in the crystal of 6a (a)
and 7 (b).
TBP possesses structural features similar to both tetracene
and pyracylene. We also disclosed that TBP undergoes unique
addition reactions with n-butyllithium or benzyne to give
either a 1,4-addition compound or triptycene-type adduct
with a curved or planar p-surface, respectively. These com-
pounds showed distinct emissions in the concentrated solu-
tion and solid state with high quantum yields. X-ray diffrac-
tion analyses demonstrated that the 1,4-addition compound
forms dimers with effective p-overlap, which can cause
a significant red-shift in the fluorescence by the stabilization
of excimer. Further studies on larger CP-PAHs containing
pyracylene segment(s) and their modifications are currently
underway in our group.
Figure 4. UV-vis absorption spectra (solid line) and normalized fluo-
rescence spectra (dashed line) of 6a (a) and 7 (b) in toluene and in
the solid state. The pictures of the toluene solution and crystals of 6a
(c) and 7 (d) with irradiation at 365 nm (the concentrations of the
solutions: 8.6”10À6 m and 7.1”10À6 m for 6a and 7, respectively).
The packing structures of 6a and 7 in the solid state
provided a better understanding of the difference in the
emission behaviors. The compound 6a has a curved p-system
and forms a dimeric structure in a face-to-face fashion
between the concave surfaces (Figure 5a). The distance
between the mean planes of the overlapping naphthalene
moieties (3.56 Š) is within a range that is typical of a p-
stacked structure (3.4–3.7 Š). In contrast, the nearly-planar
p-surface of 7 is arranged in a significantly slipped fashion
(Figure 5b). Although the p-stacked distance of 7 measured
between the mean plane of the naphthalene moiety and the
closest carbon atom in the benzene moiety (3.45 Š) is shorter
than that of 6a, contacts between the p-systems in the dimer
of 7 are small because of the slipped-stacking arrangement.
From these data, we propose that the curved p-skeleton of 6a
restricts the geometry for the p–p interactions in such a way
that the central naphthalene moieties can overlap effectively.
This effect leads to the stabilization of the excimer of 6a
which shows markedly red-shifted emission in the solid state
as compared to those of 7.
Keywords: fluorescence · fused-ring systems · hydrocarbons ·
p interactions · X-ray diffraction
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9308–9312
Angew. Chem. 2015, 127, 9440–9444
[1] a) F. G. Brunetti, X. Gong, M. Tong, A. J. Heeger, F. Wudl,
Angew. Chem. Int. Ed. 2010, 49, 532 – 536; Angew. Chem. 2010,
122, 542 – 546; b) D. T. Chase, A. G. Fix, B. D. Rose, C. D.
Weber, S. Nobusue, C. E. Stockwell, L. N. Zakharov, M. C.
Lonergan, M. M. Haley, Angew. Chem. Int. Ed. 2011, 50, 11103 –
11106; Angew. Chem. 2011, 123, 11299 – 11302; c) J. D. Wood,
J. L. Jellison, A. D. Finke, L. Wang, K. N. Plunkett, J. Am. Chem.
Soc. 2012, 134, 15783 – 15789; d) H. Xia, D. Liu, X. Xu, Q. Miao,
Chem. Commun. 2013, 49, 4301 – 4303.
[2] K. N. Plunkett, Synlett 2013, 898 – 902.
[3] a) J. Nishida, S. Tsukaguchi, Y. Yamashita, Chem. Eur. J. 2012,
18, 8964 – 8970; b) D. T. Chase, A. G. Fix, S. J. Kang, B. D. Rose,
C. D. Weber, Y. Zhong, L. N. Zakharov, M. C. Lonergan, C.
Nuckolls, M. M. Haley, J. Am. Chem. Soc. 2012, 134, 10349 –
10352.
[4] a) M. Smet, J. V. Dijk, W. Dehaen, Synlett 1999, 495 – 497;
b) H. A. Wegner, L. T. Scott, A. de Meijere, J. Org. Chem. 2003,
68, 883 – 887; c) H.-H. Hseuh, M.-Y. Hsu, T.-L. Wu, R.-S. Liu, J.
In summary, we demonstrated an efficient route for the
synthesis of electron-deficient tetrabenzo-fused pyracylene
(TBP) and its derivatives by using intramolecular oxidative
À
C H coupling. The X-ray diffraction analysis showed that
Angew. Chem. Int. Ed. 2015, 54, 9308 –9312
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9311