Octafunctionalized biphenylenes: Molecular precursors for isomeric graphene nanostructures

Article abstract of DOI:10.1002/anie.201309324

A straightforward method for the octafunctionalization of biphenylene based on the [2+2]-cycloaddition of an aryne intermediate has been developed. This enabled a North-South extension of biphenylene towards isomeric graphene nanoribbons composed of four-, six-, and eight-membered rings. This procedure furthermore allowed an East-West expansion to [n]phenylenes with different lengths. For the fabrication of isomeric nanongraphenes, octaarylbiphenylenes decorated with phenyl, pyrenyl, and thieno substituents were prepared. The subsequent oxidative cyclodehydrogenation provided an expanded helicene as a model compound. Carbon allotropes: A straightforward method for the octafunctionalization of biphenylene based on the [2+2]-cycloaddition of an aryne intermediate has been developed. This enabled the synthesis of isomeric graphene nanoribbons, [n]phenylenes, and nanographenes with even-membered rings embedded in a hexagonal lattice. Copyright

Full text of DOI:10.1002/anie.201309324

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DOI: 10.1002/anie.201309324
Graphene Very Important Paper
Octafunctionalized Biphenylenes: Molecular Precursors for Isomeric
Graphene Nanostructures**
Florian Schlꢀtter, Tomohiko Nishiuchi, Volker Enkelmann, and Klaus Mꢀllen*
Abstract: A straightforward method for the octafunctionali-
zation of biphenylene based on the [2 + 2]-cycloaddition of an
aryne intermediate has been developed. This enabled
a “North–South” extension of biphenylene towards isomeric
graphene nanoribbons composed of four-, six-, and eight-
membered rings. This procedure furthermore allowed an
“East–West” expansion to [n]phenylenes with different lengths.
For the fabrication of isomeric nanongraphenes, octaarylbi-
phenylenes decorated with phenyl, pyrenyl, and thieno sub-
stituents were prepared. The subsequent oxidative cyclodehy-
drogenation provided an expanded helicene as a model com-
pound.
Figure 1. Overview of the connection between graphene and isomeric
graphenes 1 and 2 with the smallest subunit, biphenylene (I). The
arrows indicate the direction of expansion: along the 1, 4, 5,
8 positions (North–South, red), and the 2, 3, 6, 7 positions (East–
West, blue).
T
he unique binding properties of carbon can lead to different
allotropes with exceptional structural and electronic proper-
ties.[1] In particular, the all-benzene family with different
dimensionality, including fullerenes (sp2, 0D),[2] carbon nano-
tubes (sp2, 1D),[3] graphene (sp2, 2D),[4] and the elusive cubic
graphite (sp2, 3D),[5] have sparked considerable attention. The
vanishing band gap of graphene, however, hampers its
utilization in organic electronics.[6] The lateral confinement
towards graphene nanoribbons (GNRs),[7] and the introduc-
tion of different ring sizes,[1g,8] open the band gap, thus making
these graphene materials application relevant. Biphenylene
(I) can be viewed as an appealing subunit of novel carbon
allotropes, such as isomeric graphenes 1 and 2 (Figure 1). The
aromatic and antiaromatic character of biphenylene, and the
resulting eight- and four-membered rings are thereby pre-
dicted to induce a band gap upon polymerization.[9]
red).[9] To further expand biphenylene architectures, we
present a novel functionalization strategy. This provides
access to unique North–South extended isomeric GNRs and
a one-step East–West expansion towards [n]phenylenes.
Moreover, the procedure enables the preparation of isomeric
nanographenes as model substances (2, Figure 1). The
influence of the even-membered rings on the band gap of
the resulting materials will be thereby emphasized.
Owing to the antiaromatic character and rehybridization
of the bonds forming the strained four-membered ring,
a direct halogenation of biphenylene at its 1, 4, 5, and
8 positions is impeded.[11] Therefore it was critical to attach
the (masked) halogen functionalities prior to the formation of
the biphenylene core. Among others, trimethylsilyl (TMS)
groups can be introduced at alkoxydibromobenzenes (3a/b)
by a deprotonation–silylation sequence using lithium di-iso-
propylamide (LDA) and chlorotrimethylsilane (Scheme 1).[12]
Subsequent lithiation of 4a/b gave rise to an aryne inter-
mediate, which underwent a [2+2]-cyclodimerization to
afford 1,4,5,8-tetra(trimethylsilyl)-2,3,6,7-tetraalkoxybiphe-
nylenes (5a/b). Owing to the steric hindrance of the TMS
groups, the formation of the corresponding triphenylenes was
not detected. Interestingly, by repeating the sequence with
1,2,4,5-tetrabromobenzene (3c) a similar silylation towards
4c was observed. The subsequent [2+2]-cyclodimerization of
the aryne intermediate allowed for the synthesis of 2,3,6,7-
tetrabromo-1,4,5,8-tetra(trimethylsilyl)biphenylene (5c).[13]
Moreover, using two equivalents of n-butyllithium led to
the instantaneous formation of [n]phenylenes 6, with n = 3, 4,
5, and 6. Unlike the multistep approach of Vollhardt,[10a–e,g] the
described method enabled the synthesis of [n]phenylenes in
one step, and thus allowed the anticipated East–West
expansion. DFT calculations[14] confirmed the crucial influ-
The pioneering work of Vollhardt allowed for the
functionalization of biphenylenes at the 2, 3, 6, and 7 positions
along the so-called “East–West” direction (Figure 1, blue).
This enabled further extension towards linear [n]phenylenes,
whereas 1,2-funtionalization allowed for the synthesis of
angular systems.[10] However, the controlled functionalization
of the 1, 4, 5, and 8 positions remained elusive; hence, apart
from a dimer, the synthesis of “North–South” connected
biphenylene ribbons has not been reported to date (Figure 1,
[*] Dr. F. Schlꢀtter, Dr. T. Nishiuchi, Dr. V. Enkelmann,
Prof. Dr. K. Mꢀllen
Max-Planck-Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: muellen@mpip-mainz.mpg.de
[**] This work is supported by the Transregio SFB TR 49 (Frankfurt,
Mainz, Kaiserslautern). We are grateful to Dr. Manfred Wagner for
the help with the NMR measurements. We thank Dr. Brenton
Hammer for carefully reading the manuscript.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309324.
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Having functionalized biphenylenes 8a–c in hand, the
polymerization along the North–South direction was targeted
(Scheme 2). By screening different transition-metal-mediated
polymerizations for 8a, the Cu-mediated Ullmann coupling
was found to be the most suitable.[17] Procedures such as the
Yamamoto reaction resulted in a ring opening dimerization,
owing to the reactivity of the central cyclobutadiene towards
Ni0 complexes (see the Supporting Information).[18] The
Ullmann reaction conditions were optimized with activated
copper in combination with 1,2-dichlorobenzene for seven
days at 2108C. Size-exclusion chromatography (SEC) of the
resulting polymer revealed a Mn = 2100 gmolÀ1 (PDI = 1.40),
which corresponds to seven biphenylene units (see the
Supporting Information). MALDI-TOF MS confirmed the
presence of defect-free dimers, trimers, and pentamers.
Moreover, a tetramer with one non-fused eight-membered
ring was observed (see the Supporting Information).
The obtained isomeric GNRs were separated into four
polydisperse fractions 9a–d (Scheme 2). This provided poly-
mer 9d with Mn = 6460 gmolÀ1 (PDI = 1.21), which is com-
posed of at least 22 connected biphenylenes. As MALDI-
TOF MS has limitations for the detection of high-molecular-
weight species with a high polydispersity, and SEC provides
only approximate relative values, the number of defects were
Scheme 1. Synthetic route towards polyphenylenes 6, 1,4,5,8-tetraiodo-
2,3,6,7-tetraalkoxybiphenylenes (8a/b), and 2,3,6,7-tetrabromo-1,4,5,8-
tetraiodobiphenylene (8c). LDA=lithium di-iso-propylamide, TMS=tri-
methylsilyl.
1
analyzed by H NMR spectroscopy. Two sets of signals were
ence of the TMS groups for the [2+2]-cycloaddition by a p-
backbonding from the aryne to the d orbitals of the adjacent
silicon atoms, which increased the electron density of the
aryne intermediate (see the Supporting Information). The
deprotective iodination of 5a/b provided 1,2-diketones 7a/b.
This could be explained by the loss of the antiaromatic
character of the four-membered ring as a driving force.[11c]
Reductive alkylation[15] of 7a/b led to fully soluble 1,4,5,8-
tetraiodo-2,3,6,7-tetraalkoxybiphenylenes (8a/b)[16] with an
overall yield of around 10–15% over four steps. Iodination of
5c was conducted in a similar procedure, which afforded
poorly soluble 2,3,6,7-tetrabromo-1,4,5,8-tetraiodobipheny-
lene (8c) in 65% yield.
observed, with an intensity ratio of two (aromatic, Hb + Hc) to
33 (aliphatic, Ha) (Figure 2a). The obtained aromatic signals
corresponded to approximately 16 protons related to eight
non-fused eight-membered rings (iodine endgroup). How-
ever, taking (partial) protodehalogenation into account (four
proton endgroups), six non-fused eight-membered rings were
revealed. In other words, at least 13–15 eight-membered rings
were expected for isomeric GNR 9d.
Raman spectroscopy of 9a–d revealed G- and 2G-bands
at around 1601 and 3205 cmÀ1, which were similar to reported
values for polyphenylene-based GNRs.[7h,i,19] The measure-
ments were, however, accompanied by fluorescence (see the
Supporting Information).
Scheme 2. Synthetic route for the North–South and East–West expansion attempts towards 9a–d, and 10, as well as towards model compounds
11a–c. oDCB=1,2-dichlorobenzene.
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the materials. Emissions at around 420 nm were detected for
isomeric GNRs 9a–d, in contrast to their monomer 8a, which
supported the increased p-conjugation upon North–South
expansion. A moderate level of reorganization in the excited
state was revealed by comparably low Stokes shifts of around
1700 cmÀ1. DFT geometry optimizations (B3LYP/6-31G*) on
model oligomers were carried out,[14] which supported an out-
of-plane distortion between the individual biphenylenes
along the ribbon chains (see the Supporting Information).
This was attributed to the steric repulsion between the
methoxy groups of neighboring biphenylene subunits, which
significantly decreased the p-conjugation of the isomeric
GNRs, and thus increased their band gap in comparison to
fully planar all-benzene GNRs (see the Supporting Informa-
tion).[7f,h–j] The differences between the measured and calcu-
lated band gaps can be ascribed to the non-fused eight-
membered rings, which were expected to further weaken the
p-conjugation (see above). These results, however, verified
the anticipated band-gap opening upon a selective introduc-
tion of eight- and four-membered rings to GNRs.[1f,g]
Although the intrinsic properties of isomeric GNRs 9a–d
were mainly determined by the interplay between eight-, six-
and four-membered rings, their lateral expansion was
expected to govern their electronic characteristics. Thus,
biphenylene 8c was anticipated to initially polymerize along
the 1,4,5,and 8 positions (North–South direction) towards 10
(Scheme 2), which could subsequently lead to the formation
of two types of isomeric GNRs (types I and II; East–West
direction) upon activation of the bromines. Even though
multiple procedures were screened for the polymerization of
8c, they were unsuccessful, most likely due to the sterically
demanding bromines (see the Supporting Information).
Owing to the fact that the laterally extended isomeric
GNRs were so far not accessible, octaarylbiphenylenes 11a–c
were designed as model compounds for type II GNRs
(Scheme 2). Comparing this type of isomeric nanographenes
with well-studied polycyclic aromatic hydrocarbons (PAHs),
such as hexa-peri-hexabenzocoronene (HBC), provides
insights into the influence of selectively introduced non-
hexagonal rings on the resulting properties. Precursors 11a–c
were easily accessed from 8c by a one-step method utilizing
the Suzuki–Miyaura reaction for sterically demanding eight-
fold cross-coupling (Scheme 2).[21] The procedure provided
octaphenyl- (11a), octcapyrenyl- (11b) and octathienobiphe-
nylene (11c) in 30–70% yield. Single crystal X-ray analysis of
11a and 11c revealed two sets of different external aryl
moieties, which are located at the 1, 4, 5, and 8 (equatorial),
and the 2, 3, 6, and 7 (axial) positions.[22] Expectedly, they are
significantly twisted for both molecules (54.1–74.78 and 42.9–
68.88 for 11a and 11b, respectively; hexaphenylbenzene =
ca. 82.58),[23] whereas the biphenylene core remained almost
planar (Scheme 3; see also the Supporting Information). With
octaarylbiphenylenes 11a–c in hand, the subsequent intra-
molecular cyclodehydrogenation was targeted. Thus, 11a was
treated with dichlorodicyano-p-benzoquinone (DDQ) and
methanesulfonic acid (Scheme 3).[24] Aside from the expected
loss of 12 protons, MALDI-TOF MS confirmed the removal
of two additional protons (m/z = 1218.72), which was identi-
fied as the [M+Na]+ peak of 12 (see the Supporting
Figure 2. a) 1H NMR spectrum of isomeric GNR fraction 9d in CD2Cl2
at 258C together with proton assignment. b) UV/Vis absorption (solid
line) and emission (marked with squares) spectra of 8a (black), 9a
(red), 9b (blue), 9c (green) and 9d (violet). All spectra obtained in
CH2Cl2 (1ꢁ10À6 m).
Isomeric GNRs 9a–d and 1,4,5,8-tetraiodo-2,3,6,7-tetra-
methoxybiphenylene (8a) were characterized by UV/Vis
absorption and emission spectroscopy (Figure 2b and
Table 1). By increasing the molecular weight towards ribbons
9a–d small hypsochromic shifts for the longest wavelength
transitions were revealed in comparison to monomer 8a,
reflecting large optical band gaps (Egopt) of around 2.8 eV for
Table 1: Selected photophysical data for 8a, 9a–d, 11a–c and 12.
[a]
[a,b]
opt[c]
Compound labs,max [nm]
lPL,max
[nm]
Stokes Eg
Shift
[eV]
[cmÀ1
]
8a
265(s), 297(s), 308, 361(s),
–
–
2.96
380(s), 399
9a
9b
9c
9d
277, 351(s), 373(s), 392
276, 353(s), 373(s), 391
269, 353(s), 372(s), 390
278, 353(s), 376(s), 394
309, 378, 394
276, 320(s), 333, 349, 393
320, 388,
299, 395(s), 452
418
417
419
423
433
538
538
552
490
1590
1595
1775
1740
2290
6860
7190
4010
1100
2.87
2.84
2.83
2.81
2.90
2.84
2.84
2.74
2.66
11a
11b
11c
12
HBC[d]
362, 392, 465
[a] All spectra obtained in CH2Cl2 (1ꢁ10À6 m). [b] Excited at the
absorption maxima. [c] Egopt =hc/l0.1max. [d] Values were taken from
Ref. [20]. (s)=shoulder.
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Received: October 25, 2013
Published online: January 13, 2014
Keywords: biphenylene · graphene · organic electronics ·
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polycylic compounds
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Scheme 3. X-ray crystal structure of 11a with atom numbering and the
synthetic route of its oxidative cyclodehydrogenation towards expanded
helicene 12 (H atoms omitted for clarity).
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