Tri-N-annulated hexarylene: An approach to well-defined graphene nanoribbons with large dipoles

Article abstract of DOI:10.1021/ja100276x

We report a highly efficient synthetic methodology toward tri-N-annulated hexarylenes from easily available N-annulated perylenes, which Is favored by the oxidative ring fusion driven by DDQ/Sc(OTf)₃. To assist the characterization of the new compounds, quantum-chemical calculations of structural and spectroscopic properties have been carried out for three oligomers of N-annulated rylenes, It Is shown that tri-N-annulated hexarylene dyes display remarkably large dipole moments likely associated with the formation of H aggregates, as suggested by the marked concentration dependence of the measured UV-vis spectra. It is suggested that the combination of π-π stacking Interactions and dipole-dipole interactions may favor the formation of highly ordered supramolecular structures, resulting In enhanced charge carrier mobilities.

Full text of DOI:10.1021/ja100276x

Published on Web 03/10/2010
Tri-N-annulated Hexarylene: An Approach to Well-Defined
Graphene Nanoribbons with Large Dipoles
Yan Li,†,‡ Jing Gao,†,‡ Simone Di Motta,§ Fabrizia Negri,*,§ and Zhaohui Wang*,†
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, People’s Republic of China, Graduate School of the Chinese Academy
of Sciences, Beijing 100190, People’s Republic of China, Dipartimento di Chimica “G.
Ciamician”, UniVersita` di Bologna, Via F. Selmi 2, 40126 Bologna, Italy, and
INSTM, UdR Bologna, Italy
Received October 14, 2009; E-mail: wangzhaohui@iccas.ac.cn; fabrizia.negri@unibo.it
Abstract: We report a highly efficient synthetic methodology toward tri-N-annulated hexarylenes from easily
available N-annulated perylenes, which is favored by the oxidative ring fusion driven by DDQ/Sc(OTf)3. To
assist the characterization of the new compounds, quantum-chemical calculations of structural and
spectroscopic properties have been carried out for three oligomers of N-annulated rylenes. It is shown that
tri-N-annulated hexarylene dyes display remarkably large dipole moments likely associated with the formation
of H aggregates, as suggested by the marked concentration dependence of the measured UV-vis spectra.
It is suggested that the combination of π-π stacking interactions and dipole-dipole interactions may favor
the formation of highly ordered supramolecular structures, resulting in enhanced charge carrier mobilities.
p-type GNRs into n-type transistors.5 On the other hand, Mu¨llen
also reported a new synthetic approach to linear F-GNRs with
Introduction
Graphene nanoribbons (GNRs), in which the lateral quantum
confinement opens an electronic gap that is a function of the
ribbon width, have opened the way to a breakthrough in carbon-
based nanoelectronics.1 Driven by these, a great effort has been
devoted to developing methods for the preparation of graphene
nanoribbons. Lithographic patterning of graphene sheets has led
to the fabrication of GNRs down to widths of ∼20 nm thus
far,2 but there have been difficulties in obtaining smooth edges
and reaching true nanometer-scale ribbon width. Quite recently,
Dai and Tour have reported respectively simple and inexpensive
methods to prepare GNRs with defined shape by the longitudinal
unzipping of multiwalled carbon nanotubes (MWCNTs).3
Dresselhaus demonstrated an efficient shaping of defective rough
graphitic nanoribbon edges into atomically smooth zigzag or
armchair edges via Joule heating inside a TEM-STM system.4
As for functionalized graphene nanoribbons (F-GNRs), it is
promising that edge doping by nitrogen species can transform
branched alkyl chains exhibiting good solubility and length up
to 12 nm, which was never achieved before.6 As GNRs consist
of infinite repeating polycyclic aromatic units in one dimension,
which can be easily decorated, the bottom-up organic synthetic
methodologies can be regarded as the most promising way to
functionalized graphene nanoribbons (F-GNRs).
The copper-mediated condensation of perylene-3,4:9,10-
tetracarboxylic acid bis-imides (PBIs) along the bay region (short
molecular axis) was used to construct graphene-type nanoribbons
that are functionalized by arrays of imide groups (Figure 1, 1).7
However, due to the two possible coupling positions, there are
structural isomers of oligo-PBIs, which hinder further attempts
toward the effective synthesis of higher homologues and
structurally perfect poly(bay-PBIs).8
While the intriguing structural and electro-optical properties
of poly(peri-naphthalene) (PPN) have led to the pursuit of its
rational synthesis, to date, hexarylene9 and the higher homo-
logues remain elusive targets for chemists due to the lack of
suitable building blocks and effective synthetic methods for the
long-range extension of π conjugation as well as their impro-
† Beijing National Laboratory for Molecular Sciences.
‡ Graduate School of the Chinese Academy of Sciences.
§ Universita` di Bologna and INSTM.
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10.1021/ja100276x  2010 American Chemical Society

Tri-N-annulated Hexarylene
A R T I C L E S
Figure 1. Poly(bay-PBIs) 1 and poly(peri-N-annulated-perylenes) 2.
the lines described in previous work15 and as detailed in the
Supporting Information. To model the possible aggregates, we
considered a dimer and a trimer maximizing the dipole-dipole
interaction, with molecules at a distance of 3.5 Å. It is interesting
to note that the orientation determined by the attractive dipole-dipole
interaction implies a parallel orientation of the transition dipole
moments, which is typical for H aggregates. All quantum-chemical
calculations were performed with the Gaussian03 package.16
cessability caused by their poor solubility.10 Recently we
reported the one-pot synthesis of bis-N-annulated quaterrylene
(BNQ, 16) based on the practical synthesis of a new building
block, N-annulated perylene (NP).11 Poly(peri-N-annulated-
perylene) (PPNP) can be regarded as perfect graphene ribbons
incorporating nitrogen atoms into the armchair edge structures,
providing ideal model compounds for chemically modified
graphene ribbons with large dipoles. In this contribution, we
present tri-N-annulated hexarylenes containing three space-
demanding, branched alkyl chains and a variety of end-cap
pending groups to guarantee their good processability, which
represent a new approach toward the rational synthesis of
graphene nanoribbons with well-defined edges. Tri-N-annulated
hexarylenes have been characterized both experimentally and
computationally. Electric and electronic properties have been
determined, and evidence for the formation of H aggregates has
been provided by comparing measured and computed absorption
spectra.
Synthesis and Characterization
The synthesis is shown in Scheme 1. Regioselective NBS
bromination of NP with dovetailed alkyl substituents provided
the key building blocks, denoted monobrominated NP (3) and
dibrominated alkylated NP (12), for the construction of N-
modified GNRs. Suzuki-Miyaura coupling of 3 and arylboronic
acid afford mono-end-capped NP, which can be further selec-
tively brominated to 9-11. Compound 12 was converted to the
diboronate 13 via bromine-lithium exchange followed by
reaction of the lithium intermediate with 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane. Subsequently, Suzuki-Miyaura
coupling of 9-11 with 13 affords the key precursor 14 for the
final ring fusion. When 14 was treated with 5 equiv of 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and scandium
trifluoromethanesulfonate (Sc(OTf)3) in toluene at 110 °C for
24 h, the expansion of the π system was successfully achieved
along the long molecular axis to afford tri-N-annulated
hexarylenes (TNH) 15 in 61-73% yield. For comparison, bis-
N-annulated quaterrylene was prepared according to the known
procedure.11a Although the use of DDQ/Sc(OTf)3 has been
proven to be powerful in the extension of electron-rich
π-conjugated systems to prepare triply linked oligoporphyrins,17
there are only a few examples of the more accessible polycyclic
aromatics.18 It is remarkable that, under this simple condition,
the expansion of the π system was successfully and highly
effectively achieved along the long molecular axis, which opens
a new avenue toward graphene ribbon-type molecules.
Computational Details
The chosen models for N-annulated rylenes did not include the
naphthyl substituents, since these contribute negligibly to the final
structural and electronic properties of the extended conjugated
chromophores. Atomic structures were optimized with density
functional theory (DFT) calculations using the B3LYP hybrid
functional12 with the basis set limited to 3-21G owing to the large
dimension of the chromophores. The small basis set results were
validated with additional calculations carried out with the 6-31G*
basis set. Molecular orbital shapes and energies discussed in the
text are those calculated at the optimized structures. Orbital pictures
were prepared with Molekel 4.3 visual software.13 Electronic
excitation energies and oscillatior strengths were computed with
time-dependent (TD) DFT calculations. In plotting computed
electronic spectra, a Lorentzian line width of 0.1 eV was super-
imposed on each computed intensity to facilitate the comparison
with experimental spectra. The Franck-Condon (FC) vibronic
structure14 associated with electronic transitions was evaluated along
Electric and Electronic Properties of N-annulated
Rylenes
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K.; Luettke, W.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1990, 29,
525–527.
The B3LYP/3-21G optimized geometries of NP, BNQ, and
TNH are collected in Figure S1. It is seen that the extended
(11) (a) Li, Y.; Wang, Z. Org. Lett. 2009, 11, 1385–1387. (b) Jiang, W.;
Qian, H.; Li, Y.; Wang, Z. J. Org. Chem. 2008, 73, 7369–7372.
(12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
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F. J. Phys. Chem. A 2000, 104, 9121–9129. (b) Di Donato, E.; Vanzo,
D.; Semeraro, M.; Credi, A.; Negri, F. J. Phys. Chem. A 2009, 113,
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(13) Molekel Version 4.3, http://www.cscs.ch/molkel/: Portmann, S.; Lu¨thi,
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V. J. Chem. Phys. 2007, 126, 084509. (c) Santoro, F.; Lami, A.;
Improta, R.; Barone, V. J. Chem. Phys. 2007, 126, 184102. (d)
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3945–3947.
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A R T I C L E S
Li et al.
Scheme 1. Synthesis of N-annulated Rylenesa
a Conditions: (a) 4-(methoxycarbonyl)benzeneboronic acid, Pd(PPh3)4, K2CO3, THF, 66 °C, 12 h, 87%; (b) 3,5-bis(methoxycarbonyl)benzeneboronic
acid, Pd(PPh3)4, K2CO3, THF, 66 °C, 12 h, 83%; (c, e) (i) KOH, THF, 66 °C, 24 h, 98%, (ii) 2-decyl-1-tetradecanol, EDAC, DMAP, CH2Cl2, 25 °C, 12 h,
86% for 6, 80% for 8; (d) 1-naphthaleneboronic acid, Pd(PPh3)4, K2CO3, THF, 66 °C, 12 h, 90%; (f-h) NBS, CH2Cl2, 25 °C, 30 min, 96% for 9 and 11,
92% for 10; (i) n-BuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF, -78 °C, 12 h, 63%; (j) Pd(PPh3)4, K2CO3, THF, 66 °C, 18 h, 76% for
14a, 72% for 14b and 14c; (k) Sc(OTf)3, DDQ, toluene, 110 °C, 24 h, 73% for 15a, 65% for 15b, 61% for 15c; (l) Sc(OTf)3, DDQ, toluene, 50 °C, 24 h,
23%.
conjugation results in a large planar carbon skeleton. The
computed bond length changes (bond numbering defined in
Figure S2) upon excitation are collected in Figures S3 and S4.
The computed dipole moments for the three systems, oriented
along the short molecular axis as shown in Figure S1, are 2.69,
5.08, and 7.41 D. The remarkably large dipole moment of TNH
may result in an increased ability to form aggregates. The
frontier molecular orbitals in Figure 2 indicate a marked
reduction of the electronic gap, consistent with the remarkable
red shift of the absorption spectrum of TNH in comparison to
NP and BNQ (see below). We note that the energy of the
HOMO level increases remarkably, which may result in
exceptional electron-donating properties of TNH.
Absorption Spectra
TNHs are dark green solids and are soluble in common
organic solvents. Room-temperature absorption spectra of 17,
16, and 15a-c in THF are shown in Figure 3. In sharp contrast,
the spectrum of 15 displays a drastic bathochromic shift (403
nm, 173 nm) compared with those of NP (17) and BNQ (16),
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Tri-N-annulated Hexarylene
A R T I C L E S
Figure 2. B3LYP/3-21G frontier molecular orbitals of (a) NP, (b) BNQ, and (c) TNH.
Figure 3. UV/vis absorption spectra of 17 (cyan), 16 (magenta), 15a (black), 15b (red), and 15c (blue) in THF (1 × 10-6 M). The apparent reduction of
ε for 15a-c is due to the fact that only a fraction of the monomer is present, while the remaining TNH is present in the form of aggregate.
respectively, which reflects how the ladder-type conjugated
structure leads to a great degree of planarity and correspondingly
large delocalization of electronic wave functions with an
absorption maximum at 846 nm. Interestingly, the introduction
of sterically bulky end groups has a strong influence on the
absorption coefficient as a reflection of the strong aggregation
of 15 (see below).
The observed peak separation is nicely reproduced by the
simulated spectra, indicating the activity of a number of
vibrations with frequencies ranging between 1238 and 1336
cm-1. In addition, band shapes are modulated by the activity
of a low-frequency mode at ca. 350 cm-1, which describes an
in-plane expansion of the whole molecule along its long
molecular axis. For BNQ the most active vibration is computed
at ca. 1250 cm-1, but band shapes are modulated by the
remarkably active vibration at 166 cm-1 in the acoustic phonon
region, corresponding to the molecular expansion in the long
molecular axis whose activity has been documented in other
PAH systems.19
The simulated vibronic spectra of NP and BNQ are compared
with the experimental counterparts in Figure 4, from which it
is clear that computed activities agree very nicely with the
observed vibronic structure. On the basis of calculations we can
formulate an assignment of the most prominent FC structure in
the two spectra, which results in association with the normal
coordinates shown in Figures S5 and S6. The peaks observed
at 443, 418, 396, and 375 nm in the experimental absorption
(19) Di Donato, E.; Tommasini, M.; Fustella, G.; Brambilla, L.; Castiglioni,
C.; Zerbi, G.; Simpson, C. D.; Mu¨llen, K.; Negri, F. Chem. Phys.
2004, 301, 81–93.
spectrum of NP correspond to a separation of ca. 1350 cm-1
.
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A R T I C L E S
Li et al.
Figure 4. Comparison between observed (dashed) and computed (solid) spectra (including computed vibronic structure) for NP in black and BNQ in red.
The fine structure shown under the broad computed spectra represents the FC activity contributions to each broad band. The origins of the computed spectra
were translated to the origins of the respective experimental spectra.
Figure 5. Dimer (top) and trimer (bottom) of TNH as examples of H aggregates. Cyan arrows represent the directions of dipole moments. Red arrows
represent the directions of the S0 f S1 transition dipole moments of each molecule.
The observed spectrum of TNH (Figure 3) is similar to those
of NP and BNQ, although it shows a strong dependence on
substituents. In addition, the relative intensity distribution among
the observed bands is markedly concentration dependent (see
Figure 6).
Calculations predict a vibronic structure similar to those of
NP and BNQ (see Figures S7 and S8) and a remarkable increase
in transition dipole moment from NP to TNH (see Tables S1
and S2) as a result of the expanded conjugation along the long
molecular axis, which corresponds to the direction of the
transition dipole moment (see Figure 5). Thus, the apparent
reduction of the observed ε values for 15a-c (Figure 3) must
be due to the fact that only a fraction of the TNH monomer is
present, while the remaining amount is in the form of aggregate.
Indeed, the large computed ground state dipole moment (cyan
arrows in Figure 5) induces a remarkable dipole-dipole
interaction which justifies the formation of aggregates. Because
the transition dipole moment is directed along the long molecular
axis, while the ground-state dipole is directed along the short
molecular axis, the aggregates are expected to be H type
(aggregates showing hypsochromically shifted bands). To model
the spectroscopic features associated with aggregates, we
considered a dimer and a trimer maximizing the dipole-dipole
interaction, oriented as shown in Figure 5 with molecules at a
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Tri-N-annulated Hexarylene
A R T I C L E S
similar features for NP, BNQ, and TNH with a remarkable
activity for frequencies around 1250-1350 cm-1 and an
increased modulation by low-frequency acoustic modes as the
extension of the conjugated framework increases. Because the
transition dipole moment associated with the allowed electronic
transition is polarized along the long molecular axis, the
extension of conjugation is reflected not only in a bathochromic
shift of the first transition but also in a strong enhancement of
its intensity. At the same time, the large dipole moment (directed
along the short molecular axis) favors aggregation of the larger
dye (TNH), which is reflected by a decrease of its observed
intensity in absorption in combination with a marked concentra-
tion dependence of the spectrum, displaying increased intensity
on the blue side of the first maximum, consistent with the
computed results indicating the formation of H aggregates.
These dyes display remarkably large dipole moments that may
favor the formation of highly ordered supramolecular structures,
resulting in enhanced charge carrier mobilities. Extension of
this synthetic strategy to higher oligomeric graphene nanorib-
bons as well as further investigation of the structure/properties
relationship to tune their electronic, electrostatic, and optic
properties are currently underway.
Figure 6. Comparison between observed (bottom) and simulated (top)
absorption spectra of TNH. The computed spectra were obtained by
superimposing, in different proportions, the spectra of the monomeric and
trimeric species with their associated FC structures.
distance of 3.5 Å. Accordingly, the TDDFT computed vertical
excitations show a blue shift of the intense excitonic transition
(see Table S1 and Figure S9). Assuming, for simplicity, similar
vibronic structures for the spectra of the monomer and the
aggregate, the computed results are compatible with the intensity
dependence of the second, third, and higher bands in the
observed absorption spectrum as a function of concentration
(see Figure 6). We note in passing that the apparently different
aggregation behaviors displayed by 15a-c are not related to
the electronic properties of the substituents but rather to their
steric hindrance. Indeed, quantum-chemical calculations carried
out for 15b,c show similar dipole moments (see Figure S10).
However, the large dimensions and the directions of the
substituents in 15c (with respect to the rylene plane) are likely
to force the molecules even farther apart, thereby reducing the
efficiency of aggregation.
Acknowledgment. For financial support of this research, we
thank the National Natural Science Foundation of China (Grant
Nos. 50873106 and 20721061), 973 Program, NSFC-DFG joint pro-
ject TRR61, Chinese Academy of Sciences, and an Italian MIUR
grant (PRIN 2008 JKBBK4).
Supporting Information Available: Text, figures, and tables
giving experimental details, MS, 1H NMR, and 13C NMR
spectra, computational and modeling details, the optimized
structures of 15-17, comparison between computed and
experimental absorption spectra of monomeric TNH, active
vibrations in electronic spectra, the computed hypsochromic shift
for the electronic transition of the aggregates of TNH compared
to the monomer, quantum-chemical results for 15b,c, MO
energies, absolute energies, and optical and transport gaps for
isolated molecules and for small aggregates of TNH, and the
largest predicted FC activity of NP, BNQ, and TNH. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Conclusions
We have reported a highly efficient synthetic methodology
toward tri-N-annulated hexarylenes from easily available N-
annulated perylenes, which is favored by the oxidative ring
fusion driven by DDQ/Sc(OTf)3.
The measured absorption spectra combined with quantum-
chemical calculations and modeling of vibronic activities show
JA100276X
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