Dipolar Quinoidal Acene Analogues as Stable Isoelectronic Structures of Pentacene and Nonacene

Article abstract of DOI:10.1002/anie.201507573

Quinoidal thia-acene analogues, as the respective isoelectronic structures of pentacene and nonacene, were synthesized and an unusual 1,2-sulfur migration was observed during the Friedel-Crafts alkylation reaction. The analogues display a closed-shell qui

Full text of DOI:10.1002/anie.201507573

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International Edition: DOI: 10.1002/anie.201507573
German Edition: DOI: 10.1002/ange.201507573
Hydrocarbons
Dipolar Quinoidal Acene Analogues as Stable Isoelectronic Structures
of Pentacene and Nonacene
Xueliang Shi, Weixiang Kueh, Bin Zheng, Kuo-Wei Huang, and Chunyan Chi*
Abstract: Quinoidal thia-acene analogues, as the respective
isoelectronic structures of pentacene and nonacene, were
synthesized and an unusual 1,2-sulfur migration was observed
during the Friedel–Crafts alkylation reaction. The analogues
display a closed-shell quinoidal structure in the ground state
with a distinctive dipolar character. In contrast to their acene
isoelectronic structures, both compounds are stable because of
the existence of more aromatic sextet rings, a dipolar character,
and kinetic blocking. They exhibit unique packing in single
À
crystals resulting from balanced dipole–dipole and [C H···p]/
À
[C H···S] interactions.
F
unctionalized acenes/heteroacenes have been demon-
strated to be good active materials in organic electronics.[1]
However, the lack of efficient synthetic methods and the
unstable nature of longer acenes/heteroacenes limit their
potential applications. Typical decomposition pathways of
longer acenes/heteroacenes involve: 1) addition with singlet
oxygen to form an endoperoxide[2] and further oxidation to
the corresponding quinone;[3] 2) Diels–Alder reaction with
dienophiles (e.g. alkyne substituents) or [4+4] cycloaddition
of the acene backbone.[4] In addition, the solubility issue
should be also addressed to obtain characterizable and
processible materials. Chemists have developed various
strategies to stabilize and solubilize acenes/heteroacenes
including: 1) substitution with bulky aryl and silylethynyl
groups;[5] 2) substitution with electron-deficient carboximide,
fluorine and cyano groups;[6] 3) incorporation of imine-type
nitrogen atoms to the backbone;[7] and 4) annulation of
cyclopenta-moieties along the zig-zag edges.[8]
Figure 1. Structures of 5,12-dithiapentacene, pentacene, nonacene, and
their quinoidal thia-acene isoelectronic structures.
Our group has a longstanding interest in acenes/hetero-
acenes chemistry,[9] and recently we developed a new type of
quinoidally conjugated dithia-acene analogue (e.g., 5,12-
dithiapentacene; Figure 1) which displayed remarkable sta-
bility and distinctive electronic properties.[10] The switching
from a cis-1,3-butadiene conjugation to a quinoidal conjuga-
tion eliminates the possible cycloaddition reactions and at the
same time increases the benzenoid character by releasing one
additional aromatic sextet ring (highlighted in blue in
Figure 1), and thus improves the stability and dramatically
changes the ground-state electronic structure. We believe that
this concept can be further extended to longer acenes such as
highly reactive nonacene which was predicted to have an
open-shell singlet diradical ground state (Figure 1).[11] How-
ever, by counting two lone-pair electrons for each sulfur atom,
these quinoidal dithia-acenes possess two more p electrons
than the corresponding acene derivatives (e.g., 24pe for 5,12-
dithiapentacene while 22pe for pentacene; Figure 1). Hence,
we became interested in a new type of quinoidal acene
analogues such as A–C (Figure 1) by removing one sulfur
atom (thus 2pe) per p-quinodimethane (p-QDM) unit and
the obtained structures now can be regarded as the isoelec-
tronic structures of the respective acenes (Figure 1). They are
better described as “acene-like molecules” rather than
“acenes” because of existence of more than one aromatic
sextet rings. Besides the closed-shell quinoidal structure, one
open-shell diradical resonance form and one dipolar zwitter-
[*] X. Shi, W. Kueh, Prof. C. Chi
Department of Chemistry, National University of Singapore
3 Science Drive 3, 117543 (Singapore)
E-mail: chmcc@nus.edu.sg
Dr. B. Zheng, Prof. K.-W. Huang
KAUST Catalysis Center and Division of Physical Sciences &
Engineering, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900 (Saudi Arabia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507573.
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ionic resonance form are also supposed to contribute to the
ground-state structure because of the recovery of one more
aromatic sextet ring in the latter two forms (Figure 1). In this
context, we started to investigate the chemistry, structure, and
physical properties of the derivatives of A (isoelectronic
pentacene) and B (isoelectronic nonacene), 1 and 2, respec-
tively, and compare them with the corresponding acenes such
as 4 and reported nonacene derivatives.[2b] Bulky mesityl
substituents are attached at the methylene site of the p-QDM
moiety to stabilze the possible diradical structure. However,
instead of the target compound 2, an isomer 3 was obtained
through an unusual 1,2-sulfur migration process, and it also
serves as a good isoelectronic structure of nonacene.
cationic intermediate during the ring cyclization reaction in
the presence of BF3·Et2O (Scheme 1).[13] However, the main
driving force for this particular migration and the reason for
the exclusive generation of 3, rather than the proposed 2, are
not clear at this stage. For comparison, the 6,13-dimesityl
pentacene 4 was synthesized according to a published proce-
dure with minor modification (see the Supporting Informa-
tion).[3a]
The compounds 1 and 3 are extremely stable in air, and is
in contrast to their corresponding reactive pentacene (such as
4) and nonacene derivatives.[2b] The compound 1 displays
distinctly different absorption spectrum from that of 4 in
dichloromethane, with an intense absorption band at l =
360 nm and a broad band at l = 650–900 nm (Figure 2a).
Time-dependent density functional theory (TD DFT) calcu-
lations (B3LYP/6-31G*; see the Supporting Information)
indicated that the longest-wavelength absorption band orig-
inates from HOMO!LUMO transition (lmax = 677.7 nm,
oscillator strength f = 0.1940; see Table S1 and Figure S2 in
the Supporting Information). In contrast, the pentacene
derivative 4 shows a well-resolved p-band with a maximum
at l = 601 nm. The compound 3 exhibits a similar band
structure to that of 1, but both bands are red-shifted (by 44 nm
for the longest absorption band), consistent with an extension
of p-electron delocalization, and in agreement with the TD
DFT calculations (lmax = 728.6 nm, f = 0.9865; see Table S2
and Figure S2). The corresponding nonacene derivatives
show a weak p-band with an absorption maximum shifted
beyond l = 1000 nm.[2b] Such a dramatic difference between
the acenes and our acene-like molecules can be explained by
the existence of more aromatic sextet rings in our new
quinoidal systems, which increase the energy gap. The optical
energy gap (Egopt) was determined as 1.34, 1.28, 2.02, and
1.20 eV for 1, 3, 4, and Anthonyꢀs nonacene derivatitve,
respectively, from the onset of the lowest-energy absorption.
It was hypothesized that the long-wavelength broad absorp-
tion band is attributed to the intramolecular charge transfer
The syntheses of 1–3 were based on an intramolecular
Friedel–Crafts-alkylation/dehydrogenation
strategy
(Scheme 1). Suzuki coupling between the dibromo diketone
5 (see the Supporting Information) and one equivalent of the
phenylboronic acid gave the key intermediate 6, and subse-
quent nucleophilic substitution with thiophenol in the pres-
ence of CuI and K2CO3 afforded the asymmetric diketone 7.
Reduction of 7 gave the diol 8 and subsequent BF3·Et2O-
mediated Friedel–Crafts alkylation generated the dihydro
compound 9. Finally the target product 1 was obtained by
oxidative dehydrogenation with p-chloranil in refluxing
toluene. Following a similar protocol, reaction between 6
and benzene-1,4-dithiol failed to give the desired tetraketone
À
10. Alternatively, palladium-catalyzed C S coupling afforded
10 in 65% yield. After a similar reduction/Friedel–Crafts
alkylation/dehydrogenation sequence from 10, surprisingly,
the meta-dithia isomer 3, instead of the para-dithia compound
2, was obtained in 50% yield over three steps, as confirmed by
X-ray crystallographic analysis. The structure of 10 was also
identified by X-ray crystallographic analysis.[12] The rear-
rangement reaction likely does not happen during either the
reduction (with LiAlH4) or oxidative dehydrogenation (with
p-chloranil) steps. Therefore, the formation of the meta-dithia
compound is likely due to 1,2-sulfur migration via a spirocyclic
Scheme 1. Reagents and conditions: a) phenylboronic acid, [Pd(PPh3)4], Na2CO3, toluene/water (5:1), 1008C, overnight; b) thiophenol, CuI,
.
K2CO3, DMF, 1008C, overnight; c) LiAlH4, anhydrous THF, for 8: 08C to RT, overnight; for 11: 08C to RT. À508C overnight; d) BF3 Et2O, anhydrous
CH2Cl2, 08C to RT, 3 h; e) p-chloranil, toluene, reflux, 5days; f) benzene-1,4-dithiol, [Pd2(dba)3], dppf, iPr2NEt, DMF, 1008C, overnight.
dba=dibenzylideneacetone, DMF=N,N-dimethylformamide, dppf=1,1’-bis(diphenylphosphino)ferrocene, Mes=mesityl, THF=tetrahydrofuran.
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Figure 3. Calculated (B3LYP/6-31G*) frontier molecular orbital profiles
and energy levels of 1 (a), 3 (b), and 4 (c), and the dipoles of 1 and 3
(d).
cations of 1, 3, and 4 by oxidative titration with SbCl5 in
CH2Cl2, and characteristic absorption bands at l = 419, 638,
974 nm for 1+·, l = 453, 835, 1071, 1224 nm for 3+·, and l = 430,
835, 943, 1195 nm for 4+·, were observed (see Figure S4).
Single crystals of 1, 3, and 4, suitable for X-ray crystallo-
graphy analysis, were successfully grown and analyzed. The
ORTEP drawings and three-dimensional (3D) packing struc-
tures are shown in Figure 4.[14] The solid-state packing of 1 is
highly symmetric (tetragonal; space group I41/a), even though
the molecule itself is asymmetric. The backbone of 1 is
essentially planar, with the two mesityl groups oriented
almost perpendicularly to the backbone. This orientation
disrupts the regular p stacking or herringbone arrangements
observed in common acenes/heteroacenes derivatives. Alter-
natively, molecules of 1 are packed into a squarelike tetra-
meric structure through intermolecular dipole–dipole inter-
actions and [C-H···p] interactions (2.692/2.895 Š) between
the methyl groups of one molecule to the p backbone of
another molecule. Such tetrameric structures are further
packed into a highly symmetric 3D structure through dipole–
dipole interactions. No close [S···S] interaction was observed.
In comparison, 3 crystalizes in a triclinic lattice system, with
space group P1. One toluene molecule per molecule is
incorporated into the crystal lattice. The backbone of 3 is
slightly distorted, with the four mesityl rings oriented almost
perpendicularly to the backbone. Interestingly, 3 packs in
Figure 2. a) UV-vis-NIR absorption spectra of 1, 3, and 4 recorded in
CH2Cl2. b) Cyclic voltammograms of 1, 3, and 4 in CH2Cl2 with 0.1m
Bu4NPF6 as supporting electrolyte, Ag/AgCl as reference electrode, Pt
wire as counter electrode, and scan rate at 50 mVsÀ1. Fc=ferrocene.
The electrode potential was externally calibrated by Fc+/Fc couple.
(ICT) and thus the absorption spectra of 1 and 3 have been
studied in different solvents (see Figure S3). However, it is
found that the positions of the absorption peak for both
compounds are almost independent of solvent polarity, thus
indicating that there are only weak ICTs in this system. No
emission can be observed from either 1 or 3. The HOMO and
LUMO profiles of 1 and 3 indeed show some disjointed
characters compared to that of 4, thus resulting in a significant
dipole moment of 3.1436 D and 2.8714 D, respectively
(Figure 3). The broadened and red-shifted absorption bands
in 1 and 3 are thus better ascribed to a weak intramolecular
donor–acceptor interaction in
skeleton.
a quinoidally conjugated
The compounds 1 and 3 display excellent amphoteric
redox behavior with one or two reversible reduction waves
(half-wave potential E1/2red = À1.79 for 1 and À1.84, À1.70 V
for 3, vs Fc+/Fc) and two reversible oxidation waves (half-
wave potential E1/2ox = À0.08, 0.78 V for 1 and 0.00, 0.51 V for
3; Figure 2b). For comparison, 4 exhibits one reversible
oxidation wave (E1/2ox = 0.22 V) and one reversible reduction
wave (E1/2red = À1.92 V). The HOMO and LUMO energy
levels were estimated to be À4.79 (1), À4.72 (3), À4.94 eV (4),
and À3.10 (1), À3.20 (3), À2.96 eV (4), respectively. The
electrochemical energy gap (EgEC) was thus determined to be
1.69, 1.52, and 1.98 eV for 1, 3, and 4, respectively. The high-
lying HOMO energy levels allow us to access the radical
À
a slipped face-to-face manner mainly through [C H···S]
interactions (2.895/2.994 Š) between one methyl group in
one molecule with sulfur atom in the neighboring molecule
À
and [C H···p] interactions (2.744/2.780 Š) between methyl
groups of one molecule and the p backbone of another
molecule. These interactions suppress the normally observed
anti-parallel dipole–dipole interaction in a dipolar molecule
and the dipole moments of all molecules in this case point to
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Chemie
spectra in different solvents and DFT calculations.
For comparison, the BLA of 4 exhibits a typical
diene conjugation motif.
In summary, two quinoidal thia-acene analogues,
1 and 3, as isoelectronic structures of pentacene and
nonacene,respectively, were successfully prepared
and an unusual 1,2-sulfur migration was discovered
during the synthesis. The ground-state geometry and
electronic properties of 1 and 3 were systematically
investigated both experimentally and theoretically.
The compounds 1 and 3 show excellent solubility and
stability and distinctively different absorption spec-
tra compared to their acene counterparts, and can be
explained by their unique dipolar quinoidal struc-
ture. No typical p stacking or herringbone arrange-
ment is observed in the crystalline form of both
compounds because of the bulky mesityl substitu-
ents. However, a balance between the dipole–dipole
À
À
interaction and intermolecular [C H···p]/[C H···S]
interaction result in unique packing structures in the
solid state. All experimental data and theoretical
calculations point to the same conclusion, that is,
both 1 and 3 have a typically closed-shell quinoidal
structure in their ground state with a small diradical
or bipolar/zwitterionic character. This work again
demonstrates the efficiency of the quinodimethane
approach to acenes and will shed light on the
synthesis of stable higher-order acene and hetero-
acene analogues in the future.
Acknowledgements
Figure 4. X-ray crystallographic structures and 3D packing of 1 (a), 3 (b) and
4 (c), and bond lengths of the p-conjugated core frameworks. Hydrogen atoms
are omitted for clarity. The arrows roughly denote the dipole moments.
C.C. acknowledges financial support from the MOE
Tier 1 grant (R-143-000-573-112), Tier 2 grant
(MOE2014-T2-1-080), and Tier
3
programme
(MOE2014-T3-1-004). K.-W.H. acknowledges finan-
the same direction. The compound 4 has a slightly distorted
backbone with the mesityl groups nearly perpendicular to the
backbone. Because of the bulky substituent, no close p–p
interaction was observed. The molecules are packed into
a compact structure with tetragonal symmetry, space group
cial support from KAUST. We thank Dr. Bruno Donnadieu
and Dr. Tan Geok Kheng for the crystallographic analysis.
Keywords: aromaticity · density functional calculations ·
polycyclic aromatic hydrocarbons · structure determination ·
X-ray diffraction
À
I41/a, mainly through intermolecular [C H···p] interactions.
As mentioned above, dipolar quinoidal polycyclic hydro-
carbons could exhibit unique singlet diradical and zwitter-
ionic character and diminished bond-length alternation
(BLA).[15] Hence, bond length analysis was performed to
analyze the ground-state geometry of 1 and 3, in comparison
to that of 4 (Figure 4). As observed, the central ring C of
1 displays noticeable BLA, thus indicating a significant
quinoidal conjugation. The two terminal rings, A and E,
possess relatively constant bond lengths, thus implying their
large aromatic character. Similarly, rings C and G of 3 also
show large BLA, while the central ring E and the two
terminal rings, A and I, possess relatively constant bond
lengths. These data confirm that both 1 and 3 have a quinoidal
structure in their ground state and have little contribution
from the diradical and dipolar zwitterionic resonance forms,
which is in good agreement with the UV-vis-NIR absorption
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14412–14416
Angew. Chem. 2015, 127, 14620–14624
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Received: August 13, 2015
Published online: October 8, 2015
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