Synthesis of polyaromatic hydrocarbons from bis(biaryl)diynes: Large PAHs with low Clar sextets

Article abstract of DOI:10.1002/chem.201101057

A new synthesis of large polyaromatic hydrocarbons (PAHs) with low Clar sextets is reported (see figure). The synthetic protocol involves starting bis(biaryl)diynes, which undergo initial PtCl₂-catalyzed aromatizations, and subsequent 2,3-dichloro-5,6-dicyano benzoquinone (DDQ) oxidation.

Full text of DOI:10.1002/chem.201101057

COMMUNICATION
DOI: 10.1002/chem.201101057
Synthesis of Polyaromatic Hydrocarbons from Bis
ACHTUNGTNER(NUNG biaryl)diynes: Large PAHs
with Low Clar Sextets
Tse-An Chen and Rai-Shung Liu*[a]
Large polyaromatic hydrocarbons (PAHs) have found var-
energy gaps (Eg >2.1 eV),[2,3] in addition to difficulties of
functionalization. Large PAHs with fewer aromatic p sextets
would not only facilitate electron delocalization to create
small energy gaps,[5] but also generate an open-shell ground
state with singlet or triplet biradicals.[6] Few practical meth-
ods have been available[5,6] for the preparation of large
PAHs with small Clar p sextets. Here, we report a new syn-
thetic approach toward such PAHs using readily prepared
ious applications in organic opto-electronic devices.[1] The
reported large PAHs were generally prepared from the
Scholl oxidation of 1,2-diphenylbenzene units; several repre-
sentatives[2,3] I–V are depicted in Scheme 1. Due to the fully
populated Clar p sextets,[4] the reported large PAHs typical-
ly have irreducible highest occupied molecular orbital and
lowest unoccupied molecular orbital (HOMO–LUMO)
bisACTHUNRTGNEUNG(biaryl)diynes (see the Supporting Information).
Scheme 2 shows the two-step synthesis of novel benzo[a]-
dinaphtho[2,1,8-cde:1’,2’,3’,4’-ghi]perylene 3a, of which the
Clar formula is represented by two resonance structures
comprising four aromatic sextets. The treatment of the start-
Scheme 2. A short synthesis of PAH 3a.
ing bisACHTUNTRGNE(NUG biaryl)diyne 1a with PtCl2 (10 mol%) in hot toluene
(908C, 24 h, 10ꢀ2 m) enabled a two-fold aromatization[7] to
form 5,5’-bichrysene 2a in 88% yield. Among various oxi-
dants, we found that the intramolecular oxidative cyclization
is performed best with 2,3-dichloro-5,6-dicyano benzoqui-
none (DDQ)/CH3SO3H[8] in CH2Cl2 (08C, 30 min, 10ꢀ2 m),
giving orange solid 3a in 95% yield. We also obtained
single crystals of 3a for X-ray diffraction;[9] the ORTEP
drawing in Figure 1 reveals the S2-symmetry, resulting from
the steric interaction between the two cove regions[10] at the
A’- and C-rings (or A- and C’-rings). The central portion of
the molecular geometry is highly distorted from planarity
Scheme 1. Large PAHs prepared from Scholl oxidations.
[a] Dipl.-Chem. T.-A. Chen, Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing-Hua University
No. 101, Section 2, Kuang-Fu Road, Hsinchu
30013 Taiwan
Fax : (+886)3-5711082
E-mail: rsliu@mx.nthu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101057.
Chem. Eur. J. 2011, 17, 8023 – 8027
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Figure 2. Temperature-dependent proton NMR spectra of compound 3a
in C2D2Cl4 (600 MHz).
Table 1 shows the applicability of this new synthesis to
bisACTHNUGRTENUNG(biaryl)diynes 1b–1d, which delivered novel PAHs 3b–
3d of medium sizes, and of which the hypothetic Clar repre-
sentations are based on an assumption that the central ben-
zene rings are nonplanar due to the steric congestion of the
cove regions. Both steps gave desired compounds 2b–2d
and 3b–3d with yields exceeding 71%. Targeted molecules
Figure 1. a) ORTEP drawing of 3a measured at 100 K, b) bond lengths.
with
a large dihedral angle
(36.68) between the A and C’
rings. The crystallographic data
support our proposed Clar for-
mula 3a and 3a-1 that the A-
ring represents a benzene unit
and the B–C rings resembles a
naphthalene moiety. The long
ꢀ
CACHTUNGTRENNUNG(18’) CACHTUNGTRENNUNG(19’) length (1.439 ꢁ)
supports resonance structure
Scheme 3. Interconversion of the enantiomeric pair 3a.
ꢀ
3a, whereas the short C(25)
C(26) bond (1.354 ꢁ) implies
structure 3a-1.
In C2D2Cl4, we observed seven proton signals for species
3a in the aromatic regions (d=7.9–9.4 ppm) at 253 K.
Among them, four doublets nearby the cove and bay re-
gions became broad as temperatures were raised to 293 K
with the rest of the signals remaining unchanged. As the
temperatures were raised to 353 K, all proton NMR signals
became well resolved again without alteration of the pattern
(Figure 2). This dynamic behavior suggests the racemization
of the enantiomeric pair of 3a via intermediate 3a’, which
has C2-symmetry (see Scheme 3). Such a motion is achieva-
ble by rotation of the sigma bond around the A-ring.
3b–3d are soluble in common organic solvents including
chloroform, dichloromethane, and tetrahydrofuran; purifica-
tions were conducted by column chromatograph. The char-
acterization of compounds 3b–3d relies on elemental analy-
sis, matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass, high resolution mass spectrometry
(HRMS), 1H and 13C NMR spectra; herein, we also ob-
served similar dynamic proton NMR spectroscopy behavior
caused by the racemization of the enantiomeric pair 3b–3d.
To our delight, this new synthesis is applicable to a large
PAH 4 containing only six Clar-sextets; the modified proto-
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Synthesis of Polyaromatic Hydrocarbons from BisACHTUNTRGNEU(GN biaryl)diynes
COMMUNICATION
Table 1. A short synthesis of PAHs 3b–3d.
dynamical behavior over the
range 223 to 353 K. We envis-
age that only one diastereo-
meric product is preferably pro-
duced because of its particular
stability.[12] The dynamic behav-
ior is impeded by a simultane-
ous alternation of both cove
and fjord chiralty.[12] Further ox-
idative coupling of compound
species 3e at the fjord region is
Substrate
Reaction 1[a,c]
Reaction 2[b,c]
accomplished
by
DDQ/
CF3SO3H in dichloromethane
(08C, 30 min) to give a large
rhombus-shaped PAH 4 (84%)
as an air-stable purple solid, of
which the structure is identified
by 1H NMR spectroscopy and
its nuclear overhauser effect
(NOE).
1b
2b (28 h, 88%)
3b (30 min, 93%)
Table 2 presents the photo-
physical properties of com-
pounds 3a–3e and 4. Notably,
these six PAHs have the same
UV/Vis absorption patterns
(Figure 3a),
including
two
major maxima between 300–
400 nm, and three maxima in
the region 400–550 nm; this ob-
servation indicates that their
electronic structures belong to
the same family. For the family
3a–3c, we observed bathochro-
mic shifts with increasing ben-
zene arrays in their UV/Vis and
photoluminescence (PL) spec-
tra (Figure 3b). This phenom-
enon truly reflects an effective
delocalization since there is a
two-benzene alteration for the
3a!3b!3c sequence. As we
expected, the large PAH 4 has
the largest wavelengths in both
absorption (574 nm) and emis-
sion spectra (593 nm). The
1c
2c (28 h, 71%)
3c (30 min, 89%)
1d
2d (24 h, 75%)
3d (30 min, 90%)
[a] PtCl2 (10 mol%), toluene (10ꢀ2 m). [b] DDQ (2.1 equiv), CH3SO3H/CH2Cl2 (1/10, v/v). [c] Yields are given
after purification by column chromatography on silica gel.
col is shown in Scheme 4. The PtCl2-catalyzed aromatization
of bis(biaryl)diyne (1e) was unsuccessful with protracted
emission maxima of compounds 3a–3e and 4 are highly de-
pendent on their concentrations (Figures S1–S6 in the Sup-
porting Information). The emission maxima of species 3a
appeared at 512 nm at 10ꢀ6–10ꢀ7 m, gradually becoming red-
shifted to 514–519 nm at 10ꢀ5–10ꢀ4 m, and further to 548 nm
at 10ꢀ3 m due to increasing molecular aggregation. According
to cyclic voltammetry (CV; Figure S8 in the Supporting In-
formation), compounds 3a and 3b, 3d and 3e, and 4 show
two reversible oxidations (0–1.25 V vs. Fc+/Fc) and one irre-
versible reduction (ꢀ1 to ꢀ1.75 V). In addition to these
common CV features, species 3c exhibits an additional oxi-
dation curve at 0.25 V. This information indicates a superior
ACHTUNGTRENNUNG
heating (908C, 72 h), but the use of ICl[11] (2.2 equiv) in cold
CH2Cl2 (ꢀ788C, 3 h) effected the aromatization smoothly to
give the desired compound 2e–2i in 83% yield. The treat-
ment of species 2e–2i with n-butyl lithium (nBuLi) in cold
tetrahydrofuran (ꢀ788C) provided the desired 2e in 96%
yield. The oxidative cyclization of species 2e with DDQ/
CH3SO3H in cold CH2Cl2 (08C, 30 min) produced the de-
sired product 3e in 89% yield. Although compound 3e pos-
sess both cove and fjord chiralty,[10] we observed only one
set of well-defined proton NMR signals, which showed no
Chem. Eur. J. 2011, 17, 8023 – 8027
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www.chemeurj.org
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R.-S. Liu and T.-A. Chen
Scheme 4. Synthesis of PAH 4: a) ICl (2.2 equiv), CH2Cl2, ꢀ788C 83%;
b) nBuLi (2.2 equiv), THF, ꢀ788C; H2O, 91%; c) DDQ (2.1 equiv),
CH3SO3H/CH2Cl2 (1/10, v/v), argon, 08C, 89%; d) DDQ (2.1 equiv),
CF3SO3H/CH2Cl2 (1/10, v/v), argon, 08C, 84%.
Figure 3. a) UV/Vis absorption spectra of compound 3a (black), 3b
(red), 3c (blue), 3d (dark cyan), 3e (orange), and 4 (purple) in CH2Cl2
(10ꢀ5 m), b) photoluminescence spectra in CH2Cl2 (10ꢀ6 m).
cation-delocalization for compound 3c. We speculate that
the initially generated cation at one naphthalene affects the
same oxidation on the other naphthalene to give two sepa-
rate oxidation curves.[7b] We estimated the HOMO energy
levels from their oxidation potentials, and derived their
LUMO energy levels from the energy gaps provided by the
UV/Vis spectra. For compounds 3a–3c we observed a grad-
ual decrease in the energy gap (2.41–2.28 eV), reflecting an
extensive conjugation. Notably, the large PAH 4 has a small
gap (2.07 eV), which is smaller than the gaps reported for
the large PAHs I–V.
planar cove regions. Photophysical evidence indicates an ef-
ficient electron delocalization within the frameworks of
these PAHs. This convenient approach is applicable to a
large PAH 4 that has small energy gap (2.07 eV) even
though the molecule is nonplanar.
In summary, we have devel-
oped
a
new synthesis of
medium-sized PAHs with low
Clar sextets. This two-step syn-
thesis involves starting bis-
Table 2. Optical and electrochemical data.
[b,f]
PAHs
Absmax [nm][a]
PL
HOMO
[eV][c]
LUMO
[eV][d]
Eg
FPL
[nm][b]
[eV][e]
ACHTUNGTRENNUNG(biaryl)diynes, which undergo
3a
3b
3c
3d
3e
4
332, 348, 436, 464, 496
354, 370, 440, 468, 502
356, 374, 454, 489, 522
338, 350, 434, 465, 496
348, 360, 448, 476, 508
358, 376, 494, 532, 574
512
515
539
512
530
593
ꢀ5.05
ꢀ5.04
ꢀ4.94
ꢀ5.16
ꢀ5.10
ꢀ4.93
ꢀ2.64
ꢀ2.66
ꢀ2.66
ꢀ2.80
ꢀ2.77
ꢀ2.86
2.41
2.38
2.28
2.36
2.33
2.07
0.34
0.25
0.47
0.31
0.55
0.34
initial PtCl2-catalyzed aromati-
zations and subsequent DDQ
oxidation. The resulting PAH
products have aromatic charac-
ters on the outer benzenes and
polyene properties on the cen-
[a] 10ꢀ5 m in CH2Cl2. [b] 10ꢀ6 m in CH2Cl2. [c] HOMO was calculated from the oxidation potential of CV.
[d] LUMO was calculated by the sum of HOMO and Eg. [e] Calculated from the UV/Vis absorption. [f] Cou-
tral framework due to the non- marin 1 as the standard.
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Synthesis of Polyaromatic Hydrocarbons from BisACHTUNTRGNEU(GN biaryl)diynes
COMMUNICATION
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Experimental Section
Synthesis of compound 1a: Nitrogen was bubbled through a mixing solu-
tion of toluene (80 mL), EtOH (20 mL), and water (20 mL) for 30 min.
1,4-Bis(2-bromo-4-tert-butylphenyl)buta-1,3-diyne (2.34 g, 4.96 mmol),
[Pd
ACHTUNGTRENNUNG
butylnaphthalen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
9.95 mmol) were added to this solution. The mixture was heated at 808C
for 24 h. The solution was extracted three times with ethyl acetate. After
removal of the solvent in vacuo, the crude material was purified by
column chromatography (dichloromethane/hexane=1:10) to afford com-
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Synthesis of compound 2a: A reaction tube containing PtCl2 (20 mg,
0.07 mmol) was dried in vacuo for 1 h, and vacuum was filled with nitro-
gen with a nitrogen balloon. To this round bottom flask was added com-
pound 1a (500 mg, 0.74 mmol) and toluene (74 mL), and the mixture was
stirred at 258C for 5 min before it was heated at 908C for 24 h. After re-
moval of the solvent in vacuo, the crude material was purified by column
chromatography (dichloromethane/hexane=1:5) to afford compound 2a
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Synthesis of compound 3a: Methanesulfonic acid (1 mL) was added, by
using a syringe, to a mixture of bichrysene 2a (65 mg, 0.096 mmol) and
DDQ (46 mg, 0.2 mmol) in dry CH2Cl2 (10 mL) at 08C under argon at-
mosphere. After 30 min, the mixture was quenched with saturated
NaHCO3 solution (10 mL), and then extracted with CH2Cl2 twice. The
combined extracts were washed with brine and dried over MgSO4. After
removal of the solvent in vacuo, the crude material was purified by
column chromatography (dichloromethane/hexane=1:5) to afford com-
pound 3a (61 mg, 95%) as an orange solid.
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Acknowledgements
This work was supported by funds from National Science Council,
Taiwan.
[12] For compound 3e, we envisage that the two cove regions have the
same stereochemistry as that of 3a; hence, the chirality of fjord
region is dependent on the cove stereochemistry because the A-ring
should have the three hydrogen atoms coplanar with the benzene
ring. Therefore an alteration of the cove chirality is always accompa-
nied by the change of fjord chirality.
Keywords: aromaticity · hydrocarbons · oxidation · synthetic
methods · UV/Vis spectroscopy
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Received: April 7, 2011
Published online: June 7, 2011
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8027