Synthesis of large polycyclic aromatic hydrocarbons from bis(biaryl)acetylenes: Large planar PAHs with low π-sextets

Article abstract of DOI:10.1021/ol201854g

A new synthesis of large PAHs with low Clar sextets was developed. This synthesis involves initial bis(biaryl)acetylene 1, which undergoes initial ICl-aromatization and a subsequent Mizoroki-Heck coupling reaction to give dibenzochrysene derivative 3 that can be transformed into planar PAHs 4 using DDQ-oxidation.

Full text of DOI:10.1021/ol201854g

ORGANIC
LETTERS
2011
Vol. 13, No. 17
4644–4647
Synthesis of Large Polycyclic Aromatic
Hydrocarbons from Bis(biaryl)acetylenes:
Large Planar PAHs with Low π-Sextets
Tse-An Chen and Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua University, No. 101, Section 2,
Kuang-Fu Road, Hsinchu, Taiwan, 30013, R. O. C.
rsliu@mx.nthu.edu.tw
Received July 11, 2011
ABSTRACT
A new synthesis of large PAHs with low Clar sextets was developed. This synthesis involves initial bis(biaryl)acetylene 1, which undergoes initial
ICl-aromatization and a subsequent MizorokiꢀHeck coupling reaction to give dibenzochrysene derivative 3 that can be transformed into planar
PAHs 4 using DDQ-oxidation.
Scholl oxidation of the 1,2-diphenylbenzene units is a
powerful tool to access large polycyclic aromatic hydro-
carbons (PAHs).1 Due to the fully populated Clar π-
sextets,2 the reported large PAHs such as IꢀIII (Scheme 1)
typically have irreducible highest occupied molecular or-
bital and lowest unoccupied molecular orbital (HOMOꢀ
LUMO) energy gaps.3 To seek large PAHs with low Clar
π-sextets,4 we reported the use of bis(biaryl)diynes IV for
short syntheses of PAHs VI and their heavier congeners,5
as depicted in Scheme 1. Nevertheless, these PAHs have
nonplanar geometries because of the steric interactions
around the two cove regions. We envisage that planar
PAHs will be beneficial for organic optoelectronic devices
due to their strong intermolecular πꢀπ interactions.6
Previously, we reported the synthesis of dibenzochrysenes7,8
based on the 2-fold aromatizations of bis(biaryl)acetylenes.8
Here, we report further development of this method to
achieve the synthesis of planar PAHs 4aꢀ4e with low Clar
π-sextets in Figure 1; PAHs 4b and 4c are structurally
related to the parent skeleton 4a with two additional ben-
zenes, whereas PAHs 4d and 4e have four extra benzenes.
An examination of their photophysical properties with
increasing benzene arrays is the focus of this investigation.
As depicted in Scheme 2, treatment of bis(biaryl)ac-
etylene 1a with ICl (1.1 equiv) in cold dichloromethane
(DCE, ꢀ78 °C, 3 h) afforded iodo derivative 2a in 82%
yield. A subsequent MizorokiꢀHeck coupling reaction of
species 2a provided a large dibenzochrysene derivative 3a
€
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r
10.1021/ol201854g
Published on Web 08/09/2011
2011 American Chemical Society

Scheme 1. All Benzenoid PAHs and Low Clar Sextets PAHs
Scheme 2. Synthetic Route toward Compound 4a (R = t-Bu)
81% yield. To our delight, the second fusion was subsequently
achieved with a DDQ-oxidation in a sealed tube at 70 °C (1,2-
dichloroethane, 8 h), giving product 4c in 90% yield.
Scheme 3 depicts the synthesis of large PAH 4e using
dibenzochrysene as the starting building block. Treatment
of species 3a withNBSinCHCl3/DMF produced dibromide
derivative 1e in 88% yield; compound 1e was subse-
quently transformed into species 2e (51% yield) via Suzuki
Table 1. A Short Synthesis of PAHs 4bꢀ4d (R = t-Bu)
Figure 1. Structure of Targeted PAHs (R = t-Bu).
in 71% yield. Species 3a possesses two nonplanar fjord regions
that are subjected to a DDQ-induced oxidative coupling9
(DDQ = 2,3-dichloro-5,6-dicyanobenzoquinone), giving tar-
geted molecule 4a in 91% yield. 1H NMR of PAHs 3a and 4a
in CDCl3 show only nine and seven separate aromatic signals
over the range ꢀ213ꢀ333 K that exhibit no dynamic behavior.
For species 3a, we envisage that an alteration at the fjord
chiralty is impeded by a strong steric hindrance.
Table 1 highlights the applicability of this new synthesis
to additional bis(biaryl)acetylenes 1bꢀ1d, which delivered
extended dibenzochrysene species 3bꢀ3d efficiently through
a sequential ICl-cyclization and MizorokiꢀHeck coupling.
For the resulting 3b and 3d, the corresponding DDQ-oxida-
tion in CH2Cl2 (0 °C, 6 h) gave the desired PAHs 4b and 4d
in 79% and 85% yields respectively. Nevertheless, the same
DDQ-oxidation of PAH 3c under the same conditions effected
the coupling of only one fjord region, giving compound 4c0 in
a ICl (1.1 equiv), CH2Cl2, ꢀ78 °C. b [Pd(PPh3)2Cl2] (5 mol %),
Na2CO3 (4.0 equiv), N,N-dimethylacetamide (0.01 M), 110 °C. c DDQ
(2.1 equiv), CH3SO3H/CH2Cl2 (1/10, v/v), 0 °C. d DDQ (1.1 equiv),
CH3SO3H/1,2-dichloroethane (1/10, v/v), 70 °C. e Yields are given after
purification by column chromatography on silica gel.
(9) DDQ-oxidation: (a) Zhai, L.; Shukla, R.; Rathore, R. Org. Lett.
2009, 11, 3474. (b) Zhai, L.; Shriya, R.; Wadumethrige, S. H.; Rathore,
R. J. Org. Chem. 2010, 75, 4748.
Org. Lett., Vol. 13, No. 17, 2011
4645

Scheme 3. Synthetic Route toward Compounds 4e (R = t-Bu)
Figure 2. ORTEP drawing of 3c (left) and bond lengths (right).
is planar, as reflected by the dihedral angle (32.9°) between
rings A and D in the cove region, less than that (51.6°) of
molecule 3c. Species 4c consists of an S2 axis with both the
A0 and D rings lying below the D0 and A rings. Examination
of the bond lengths indicates that the inner rings C, F, C0, and
F0 are unlikely to have Clar sextet character, as evidenced
˚
by the two large C(14)ꢀC(15) (1.464 A) and C(3)ꢀC(4)
2
˚
(1.460 A) lengths that approximate a standard C(sp )ꢀ
C(sp2) bond.11 Molecule 4c accordingly appears to have six
Clar sextets with rings Dand Eas two benzene sextets and the
A/B rings representing a naphthalene group.
coupling, followed by removal of a silyl group, which sub-
sequently underwent Pt-catalyzed aromatization affording
the cyclized product 3e in 90% yield. For species 3e, a final
DDQ-oxidation enabled a closure of the two fjord regions,
giving desired compound 4e in 78% yield.
The characterization of 3aꢀ3e relies on appropriate ana-
lytic methods including elemental analysis, matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF)
1
high resolution mass spectrometry (HRMS), and H and
13C NMR spectra. We obtained single crystals of 3c for
X-ray diffraction.10 Crystallographic data of its dibenzo-
chrysene precursor 3c were also acquired to compare their
molecular planarity. The unit cells of 3c and 4c contain two
independent molecules, Z = 2. Figure 2 shows the ORTEP
drawing for molecule 3c that reveals a great distortion from
planarity in the fjoid geometry, as revealed by a large
dihedral angle 77.8° between the AꢀE0 and EꢀA0 rings. In
contrast, the dihedral angle between the A/D and A0/D0
benzenes, in the so-called cove areas, is relatively small
(51.6°). Species 3c comprises an S2 axis with both A and E
rings lying below the E0 and A0 rings, thus generating a
double helicene, defined by rings E0ꢀC0ꢀB0ꢀBꢀA and
A0ꢀB0ꢀBꢀCꢀE respectively. Notably, the six carbonꢀ
carbon distances around the outer rings A, D, and E fall
Figure 3. ORTEP drawing of 3d (left) and bond lengths (right).
Table 2 summarizes the photophysical properties of
3aꢀ3e and 4aꢀ4e. Compounds 3aꢀ3e have the same
UV/vis absorption patterns including three maxima in
the β-bands (300ꢀ360 nm)12 and two maxima in the p-
bands (365ꢀ460 nm) (Figure 4). Here, we observed bath-
ochromic shifts for the p-bands increasing with the array
size 3af 3b/3cf 3d/3e, each with two-benzene alteration.
For example, the last maxima of the p-bands appear at
375 nm for 3a, 383/403 nm for 3b/3c, and 409/428 nm for 3d/
3e. In the photoluminescence (PL) spectra (Figure s1,
Supporting Information (SI)), the emission maxima of
3aꢀ3c are near each other (439ꢀ441 nm) but are red-shifted
to 453 and 463 nm, respectively, for large arrays 3d and 3e.
Of particular interest are the photophysical properties
for planar PAHs 4aꢀ4e derived from 3aꢀ3e. As shown
˚
within a narrow range, 1.368ꢀ1.416 A, indicating benzene
character. In contrast, we observed large distances (1.60ꢀ1.68
˚
A) for the inner C(1)ꢀC(14), C(2)ꢀC(3), and C(1)ꢀC(15)
2
2
11
˚
bonds, near a standard C(sp )ꢀC(sp ) bond (1.467 A).
Accordingly, 3c is appropriately described by six Clar sextets,
with the benzene character located in the six outer rings.
Figure 3 presents an ORTEP drawing of 4c that con-
firms our hypothetical structure. The molecular geometry
(12) Discussions of the R-, β-, and p-bands of PAHs: (a) Clar, E. J. Am.
Chem. Soc. 1952, 62, 6235. (b) Kastler, M.; Schmidt, J.; Pisula, W.;
Sebastiani, D.; Mullen, K. J. Am. Chem. Soc. 2006, 128, 9526. (c) Karcher,
(10) Crystallographic data for compounds 3c and 4c are provided in
the Supporting Information.
€
W. Dibenzanthracenes and Environmental Carcinogenesis; Cambridge
(11) Kveseth, K.; Seip, R.; Kohl, D. A. Acta Chem. Scand. 1980, A34, 31.
University Press: Cambridge, NY, 1992.
4646
Org. Lett., Vol. 13, No. 17, 2011

0.70, 1.08ꢀ1.17 V); the first oxidation is assigned to a
simultaneous oxidation of two half arrays. We estimated
the HOMO energy levels from the UV/vis spectra. As
shown in Table 2, the electron delocalization of non-
planar 3aꢀ3e appears to be less efficient because varia-
tions of their energy gaps (Eg) are small, within a small
range (2.84ꢀ2.71 eV). In contrast, a significant decrease
in the Eg values is shown for planar PAHs 4aꢀ4e, within
a reasonable range (2.51ꢀ2.27 eV). Notably, species 4c
bearing two cove regions has Eg = 2.42 eV which is even
smaller than that of planar 4b (Eg = 2.48 eV) that lacks a
cove region. For species 4d and 4e bearing four cove
regions, their energy gaps are distinct, 2.42 vs 2.27 eV.
These observations reveal that large planar PAHs are
not the decisive factor to obtain small Eg among various
geometric isomers.
Figure 4. UV/vis absorption spectra of compounds 3aꢀ3e.
in Figure 5, we observed significant bathochromic shifts
for the absorption maxima of 4aꢀ4e in both β-bands and
the para-band relative to those of 3aꢀ3e. For example, the
last maxima of their p-bands are shifted to larger wave-
lengths up to 86ꢀ103 nm. Large bathochromic shifts
(50ꢀ74 nm) were also observed for the PL spectra of species
4aꢀ4e (Figure s2, SI) relative to those of 3aꢀ3e. Figures 5
and s2 also reveal gradual bathochromic shifts in the last
absorption maxima (478ꢀ519 nm), as well as the emission
maxima (491ꢀ537 nm), with increasing sizes 4af 4b/4cf
4d/4e (see Table 2). This information reveals that the
increasing array size on such planar skeletons significantly
affects both UV and PL wavelengths.
Table 2. Optical and Electrochemical Data
PL HOMO LUMO Eg
PAHs
Absmax (nm)a
(nm)b (eV)c (eV)d (eV)e Φb,f
3a 313, 327, 342, 362, 375
3b 316, 326, 345, 369, 383
3c 329, 337, 386, 403
441 ꢀ5.54 ꢀ2.70 2.84 0.12
442 ꢀ5.51 ꢀ2.67 2.84 0.33
439 ꢀ5.47 ꢀ2.63 2.84 0.20
453 ꢀ5.44 ꢀ2.67 2.77 0.18
463 ꢀ5.40 ꢀ2.69 2.71 0.40
491 ꢀ5.13 ꢀ2.62 2.51 0.61
498 ꢀ5.07 ꢀ2.59 2.48 0.49
506 ꢀ5.12 ꢀ2.70 2.42 0.17
3d 326, 340, 356, 387, 409
3e 332, 345, 407, 428
4a 316, 330, 424, 448, 478
4b 332, 348, 424, 449, 482
4c 332, 348, 432, 460, 489
4d 354, 370, 387, 458, 477, 489 509 ꢀ5.20 ꢀ2.78 2.42 0.38
4e 321, 341, 357, 454, 484, 519 537 ꢀ5.28 ꢀ3.01 2.27 0.21
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 Coumarin
1 as the standard.
In summary, we have developed a new synthesis of large
PAHs with low Clar sextets. This synthesis involves initial
bis(biaryl)acetylene 1, which undergoes initial ICl-aroma-
tization and a subsequent MizorokiꢀHeck coupling reac-
tions to give dibenzochrysene derivatives 3 that can be
transformed into planar PAH 4 using DDQ-oxidation.
X-ray diffraction studies reveal highly nonplanar skeleta
for dibenzochrysene species 3c, but a planar geometry for
PAHs4c. Thephotophysicalproperties of 4aꢀ4eincluding
UV/vis, photoluminscence, and cyclic voltammetry indi-
cate that these properties are greatly affected by the
increasing array size, whereas those of nonplanar 3aꢀ3e
appear to be less sensitive to such size variations.
Figure 5. UV/vis absorption spectra of compounds 4aꢀ4e.
According to cyclic voltammetry (Figure s3, SI), species
3aꢀ3e show one reversible oxidation (0.75ꢀ0.93 V vs
Agþ/Ag) due to a simultaneous oxidation of two nonpla-
nar half sizes whereas planar PAHs 4aꢀ4e reveal two
reversible oxidations. For small PAHs 4aꢀ4c, the two
oxidation potentials (0.52ꢀ0.54, 0.68ꢀ0.71 V), given from
the successive oxidation of the two half arrays, are close to
each other but recognizable. Large array sizes 4dꢀ4e
have two well separated oxidation potentials (0.66ꢀ
Acknowledgment. We thank National Science Council,
Taiwan, for financial support of this work.
Supporting Information Available. Spectral data, NMR
spectra, MALDI-Mass, PL, CV measurements of new
compounds, and X-ray crystallographic data of com-
pounds 3c and 4c are provided in Supporting Informa-
tion. This materials is available free of charge via the
Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 17, 2011
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