Graphitic molecules with partial "Zig/Zag" periphery

Article abstract of DOI:10.1021/ja048580d

A new synthetic protocol that affords novel hexa-peri-hexabenzocoronene (HBC) derivatives with partial "zig/zag" periphery and dramatically different electronic properties has been developed, which represents an important step toward understanding structu

Full text of DOI:10.1021/ja048580d

Published on Web 06/05/2004
Graphitic Molecules with Partial “Zig/Zag” Periphery
Zhaohui Wang,† Zˇeljko Tomovic´,† Marcel Kastler,† Ramona Pretsch,† Fabrizia Negri,‡
Volker Enkelmann,† and Klaus Mu¨llen*,†
Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany,
and Dipartimento di Chimica ‘G. Ciamician’, UniVersita’ di Bologna, Via F. Selmi, 2, 40126 Bologna, Italy
Received March 11, 2004; E-mail: muellen@mpip-mainz.mpg.de
Polycyclic aromatic hydrocarbons (PAH) represent one of the
most intensively investigated classes of compounds.1 Recently, we
have developed a straightforward route to a series of all-benzenoid
PAHs with a broad range of sizes and shapes.2 In addition to
providing real-world examples to test theoretical modeling of the
properties of graphite segments,3 these molecules have attracted
considerable interest as active components in solar cells, field effect
transistors, and other molecular electronic devices.4
Because of its hexagonal symmetry, hexa-peri-hexabenzocoro-
nene (HBC, 1, Chart 1) serves as an intriguing homologue of
benzene. Inclusion of varying edge structures should be revealing
since it is known that periphery and overall size of PAHs are critical
factors that determine their electronic structure and chemical
reactivity.5 Herein, we present our preliminary result, which
promises the establishment of a new family of graphitic molecules
Figure 1. UV-visible absorption and photoluminescence spectra of 1 and
2a recorded at room temperature in 1.0 × 10 -6 M 1,2,4-trichlorobenzene
with “reactive” double bonds. The integration of “zig/zag” edges
should influence dramatically not only its electronic properties but
also its two- and three-dimensional self-assembly behaviors.
Whereas cyclotrimerization of phenyl-substituted ethynes is
restricted to polyphenylene precursors with 6-fold symmetry,
Diels-Alder [4 + 2] cycloaddition of tetraphenylcyclopentadienone
with diarylacetylenes is a well-established method to construct
polyphenylene derivatives.6a Therefore, the key design element here
is to incorporate a “zig/zag” edge into the cyclopentadienones.
Recently, we found that pyrene-4,5-dione derivatives form unique
helical columnar arrangements in single crystals,7 and it appears
that their Knoevenagel condensation with benzil would give the
aimed building block.6b,c Diels-Alder reaction of 4a with com-
mercially available diphenylacetylene resulted in the corresponding
9,10,11,12-tetraphenyl-benzo[e]pyrene (5a) in relatively low yield
(35%), probably because of the thermal decomposition in the
cycloaddition process (240 °C) (see below). Cyclodehydrogenation
of the unsubstituted 5a proceeded smoothly with copper(II)
trifluoromethane sulfonate and aluminum(III) chloride to give the
desired planar PAH 2a with one additional “zig/zag” edge. The
vanishing solubility of 2a precludes solution 1H NMR and 13C NMR
characterization; however, MALDI-TOF MS (solid-state analyte
in TCNQ matrix preparations)8 reveals a single species with isotopic
distribution in accord with the calculated one for compound 2a,
proving the loss of exactly 10 hydrogen atoms during the formation
of five new carbon-carbon bonds.
solution. Black line: 1, red line: 2a.
Chart 1. Molecular Structure of HBC (1) and Target Compounds
Reducing the symmetry increases the intensity of 0 f 0 band of
the R (or Lb) transition (486 nm), which is otherwise very weak
(symmetry-forbidden) for the D6h symmetric unsubstituted HBC,
as shown in the profile of the emission spectra. Interestingly, there
is also a characteristic band at 438 nm that is not found in the
all-benzenoid analogue. ZINDO-CIS quantum chemical calcula-
tions were carried out to estimate excitation energies and oscillator
strengths at the B3LYP/3-21G equilibrium structure of this
molecule, and it was found that a net remarkable transition dipole
moment is associated with the 0 f 0 band of the S0 f La (or S0
f p) transition. It is clear that this unusually strong band originates
from the strong electronic perturbation induced by the two additional
π-centers.
A further step toward “superbenzene” that carries reactive sites
in the periphery concerns improving the solubility by introducing
tert-butyl groups to the parent graphitic molecule.10 Sufficient
solubility is also a prerequisite for the structural investigation and
characterization of physical properties.
We achieved hexa-tert-butyl-substituted “zig/zag” HBC (2b)
synthetically in a stepwise manner as outlined in Scheme 1. It should
be noted that the yield of Diels-Alder cycloaddition of 4b with
diarylacetylene is much higher (80%) than that of 4a, probably
because tert-butyl groups kinetically stablize the cyclopentadienone.
The key step for the synthesis of 2b is the oxidative cyclodehy-
A UV-visible absorption spectrum of 2a recorded in 1,2,4-
trichlorobenzene clearly shows three types (groups) of bands (R, p
9
(or Lb, La9) and â (Ba, Bb )) which are typical for aromatic
hydrocarbons (Figure 1).1b The absorption maximum of 2a (λmax
) 380 nm) shows a significant bathochromic shift with respect to
the corresponding band of 1 (λmax ) 359 nm) (0 f 0 band of the
S0 f â transition, assignments based upon calculations; see below).
† Max-Planck-Institute for Polymer Research.
‡ Universita’ di Bologna.
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10.1021/ja048580d CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Synthetic Route to “Zig/Zag” PAHsa
170 °C in good yield (70%). As the 2D representation of 6 suggests,
it can be cleanly cyclodehydrogenated to a graphitic molecule 3
with more extended “zig/zag” character, which can be identified,
for example, by its MALDI-TOF MS spectra and solid UV-visible
spectra recorded on mechanically “smeared” films on quartz. The
absorption maximum of 3 (λmax ) 535 nm) shows a significant
bathochromic shift (45 nm) with respect to its parent PAH without
“zig/zag” edge (see Supporting Information). Presumably, the
introduction of six tert-butyl groups is not enough to solublize this
large PAH,11a and we are currently focusing on the synthesis and
characterization of soluble derivatives by attaching long alkyl
chains.11b
In conclusion, a new synthetic protocol ensuring novel polycyclic
aromatics with partial “zig/zag” periphery and dramatically different
electronic properties has been developed. In addition to serving as
an interesting theoretical model for graphite, they also provide
reactive sites for further functionalization, e.g., preparing its
corresponding K-region oxide. Attaching long alkyl chains onto
the PAH core is supposed to give liquid crystalline properties.
a Conditions: (a) diphenylacetylene, Ph2O, 240 °C, 6 h, 35%. (b)
Cu(OSO2CF3)2, AlCl3, CS2, 25 °C, 20 h, 87%. (c) 1,3,5-Triethynylbenzene,
o-xylene, 170 °C, 48 h, 70%. (d) FeCl3, CH2Cl2, 25 °C, 10 h, 82%. (e)
Di-tert-butylbenzil, KOH, EtOH, 80 °C, 15 min, 78%. (f) Di-tert-
butyldiphenylacetylene, Ph2O, 240 °C, 6 h, 80%. (g) FeCl3, CH2Cl2, 25
°C, 20 min, 95%. (h) RuO2/NaIO4, DMF/H2O, 48h, 11%.
Acknowledgment. This work was supported by funds from the
Zentrum fu¨r Multifunktionelle Werkstoffe und Miniaturisierte
Funktionseinheiten (BMBF 03N 6500) and the EU project DISCEL
(G5RD-CT-2000-00321). M. Kastler gratefully acknowledges sup-
port from the Fonds der Chemischen Industrie (Doktorandensti-
pendium). We thank Dr. M. Wagner for NMR measurement and
T. Julius for the synthesis of 4a.
drogenation of 5b. Using anhydrous FeCl3 as oxidant and Lewis
acid, we obtained 2b in nearly quantitative yield after reductive
workup. H,H NOESY, H,H COSY, and H,X COSY NMR spectra
of 2b were obtained in THF-d8. The two protons in the “zig/zag”
edge were observed at δ ) 8.64, i.e., significantly shielded with
respect to the aromatic “armchair” protons, which resonate as
singlets between 9.1 and 9.5. The solubility of 2b allows further
derivatization to probe the chemical reactivity unique to the “zig/
zag” motif. The selective oxidation of the “zig/zag” edge to its
corresponding diketo structure was achieved by using ruthenium
dioxide/sodium periodate in aqueous N,N-dimethylformamide,
which provides a potentially versatile building block for giant
graphene molecules.
The UV-visible absorption spectrum of 2b is essentially
unchanged with reference to unsubstituted parent compound 2a,
in accord with the relatively weak donor effect of tert-butyl groups.
A small bathochromic shift by about 6 nm respective to 2a is
probably due to the expected nonplanarity caused by steric
congestion.
Supporting Information Available: Full experimental details,
MALDI-TOF mass spectra of compound 2a, 2b, and 3, and quantum
chemical calculations of 1 and 2a (CIF, PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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