Fully conjugated tri(perylene bisimides): An approach to the construction of n-type graphene nanoribbons

Article abstract of DOI:10.1021/ja807803j

We present an experimental study encompassing synthesis and characterization of fully conjugated tri(perylene bisimides) (triPBIs), having 19 six-membered carbon rings in the core and six imide groups at the edges. Two structural isomers of triPBIs resulting from the two probable coupling positions were successfully separated by HPLC. To assist the identification of the two structural isomers, quantum-chemical calculations of electronic structure, NMR, and optical spectra were carried out. Calculations predict stable helical and nonhelical configurations for both triPBIs isomers and allow the assignment of triPBIs 6 unequivocally to the most bathochromically shifted absorption spectrum. Increasing the number of PBI units in oligo-PBIs leads to an expansion of the ? system, in turn associated with a reduction of the transport and optical band gaps, and a remarkable increase in electron affinities, which make oligo-PBIs promising n-type functional components in optoelectronic devices.

Full text of DOI:10.1021/ja807803j

Published on Web 11/26/2008
Fully Conjugated Tri(perylene bisimides): An Approach to the
Construction of n-Type Graphene Nanoribbons
Hualei Qian,†,§ Fabrizia Negri,*,‡ Chunru Wang,† and Zhaohui Wang*,†
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China, Dipartimento di Chimica “G. Ciamician”, UnVersita` di
Bologna, Via F. Selmi 2, 40126 Bologna, Italy and INSTM, UdR Bologna, Italy, and Graduate
School of the Chinese Academy of Sciences, Beijing 100190, China
Received October 2, 2008; E-mail: fabrizia.negri@unibo.it; wangzhaohui@iccas.ac.cn
Abstract: We present an experimental study encompassing synthesis and characterization of fully
conjugated tri(perylene bisimides) (triPBIs), having 19 six-membered carbon rings in the core and six imide
groups at the edges. Two structural isomers of triPBIs resulting from the two probable coupling positions
were successfully separated by HPLC. To assist the identification of the two structural isomers, quantum-
chemical calculations of electronic structure, NMR, and optical spectra were carried out. Calculations predict
stable helical and nonhelical configurations for both triPBIs isomers and allow the assignment of triPBIs 6
unequivocally to the most bathochromically shifted absorption spectrum. Increasing the number of PBI
units in oligo-PBIs leads to an expansion of the π system, in turn associated with a reduction of the transport
and optical band gaps, and a remarkable increase in electron affinities, which make oligo-PBIs promising
n-type functional components in optoelectronic devices.
Introduction
Molecular systems based on perylene bisimides 1 (PBIs) have
been extensively investigated in recent years, primarily due to
their industrial applications as dyes and pigments.5 Moreover,
their versatile optical and electrochemical properties,6 the
possibility to tune these properties by grafting various substit-
uents at their bay positions,7 their high electron affinity,8 and
their chemical and thermal stabilities make PBIs attractive n-type
materials in electronics and optoelectronics.9 In these respects,
core-expanded PBIs such as coronene, terrylene, and quater-
rylene bisimides have been demonstrated to have important uses
for liquid-crystal materials, single-molecule spectroscopy, and
near-infrared-absorbing dyes.10 Since the imide groups have a
great influence on properties and applications of PBIs, it is
interesting to modify GNRs with arrays of electron-withdrawing
imide groups. Accordingly, we attempted to construct F-GNRs
by repeating the PBI unit in one dimension, that is, designing
Graphene nanoribbons (GNRs), for which the width of the
sheet is confined to be a finite size while the length is considered
infinitely long, have attracted considerable interest due to their
high potential for technological applications, mostly in GNRs-
based nanoelectronics.1 Driven by these, a great effort has been
devoted to developing methods for preparation of GNRs.
Lithographic patterning of graphene sheets has led to the
fabrication of GNRs down to widths of ∼20 nm thus far,2 but
there are difficulties in obtaining smooth edges and reaching
true nanometer-scale ribbon width. Quite recently, Dai devel-
oped a chemical method for making 10 nm wide GNRs just
one atom thick and demonstrated their promising semiconduct-
ing properties for electronic applications.3 Mu¨llen also reported
a new synthetic protocol yielding linear GNRs with length up
to 12 nm which was never achieved before.4 As GNRs consist
of infinite repeating polycyclic aromatic units in one dimension,
which can be easily decorated, bottom-up organic synthetic
methodologies can be regarded as the most promising routes to
functionalized graphene nanoribbons (F-GNRs).
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† Institute of Chemistry, Chinese Academy of Sciences.
§ Graduate School of the Chinese Academy of Sciences.
‡ Unversita` di Bologna and INSTM.
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J. J. Mater. Chem. 2001, 11, 1789–1799. (c) Wu¨rthner, F.; Sautter,
A.; Schilling, J. J. Org. Chem. 2002, 67, 3037–3044. (d) Serin, J. M.;
Brousmiche, D. W.; Frechet, J. M. J. J. Am. Chem. Soc. 2002, 124,
11848–11849. (e) Chen, Z.; Debije, M. G.; Debaerdemaeker, T.;
Osswald, P.; Wu¨rthner, F. ChemPhysChem 2004, 5, 137–140. (f) Chao,
C.; Leung, M.; Su, Y. O.; Chiu, K.; Lin, T.; Shieh, S.; Lin, S. J. Org.
Chem. 2005, 70, 4323–4331. (g) Osswald, P.; Leusser, D.; Stalke,
D.; Wu¨rthner, F. Angew. Chem., Int. Ed. 2005, 44, 250–253. (h) Qian,
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10.1021/ja807803j CCC: $40.75
2008 American Chemical Society

Fully Conjugated Tri(perylene bisimides)
A R T I C L E S
Figure 1. PBIs 1, poly(PBIs) 2 and 3, and diPBIs 4.
poly(PBIs) 2 or 3 (poly(PBIs) having several probable isomeric
structures, two of which are shown in Figure 1).
evolution of structures and properties from PBIs to diPBIs to
triPBIs, which are useful for predicting specific information on
photonic and electronic properties of the corresponding higher
oligomeric or polymeric analogues.
As shown in other studies,11 π-conjugated oligomers with
precisely defined length and constitution are regarded as ideal
model compounds for elucidating properties of high-molecular-
weight polymers. In the course of our research on oligomeric
or polymeric PBIs, we have prepared triply linked di(perylene
bisimides) 4 (diPBIs) by copper-mediated coupling of tetra-
chloro-PBIs,12 the aromatic cores of which are enlarged more
than twice along the bay positions. Herein we present an
experimental study encompassing synthesis and characterization
of fully conjugated tri(perylene bisimides) 5 and 6 (triPBIs),
which have 19 six-membered carbon rings in the core and 6
imide groups at the edges. The structures of triPBIs 5 and 6 are
Computational Details
Atomic structures of 1, 4, 5, and 6 were optimized with density
functional theory (DFT) calculations using the B3LYP hybrid
functional13 with the basis set limited to 3-21G owing to the large
dimension of the chromophores. Molecular orbital shapes and
energies discussed in the text are those calculated at the optimized
geometries. Orbital pictures were prepared with Molekel 4.3 visual
software.14
Chemical shifts were calculated with the gauge-including atomic
orbitals (GIAO) theory15 and were plotted with Gaussview16 using
as reference the B3LYP/3-21G results for TMS. Electronic excita-
tion energies and oscillation strengths were computed for the 60
lowest singlet excited electronic states of 4, 5, and 6 with time-
dependent (TD) DFT calculations. In plotting computed electronic
spectra, a Lorentzian line width of 0.1 eV was superimposed to
each computed intensity to facilitate comparison with experimental
spectra. All quantum-chemical calculations were performed with
the Gaussian03 package.17
1
determined by H NMR and MALDI-TOF spectroscopy. Due
to differences in the molecular structures and the degree of π
electron delocalization, two isomers of triPBIs 5 and 6 exhibit
different optical and electrochemical properties, providing an
ideal model for a systematic study of structure-property
relationships of GNRs. To assist the identification of the two
structural isomers of triPBIs, quantum-chemical calculations of
electronic structure, NMR spectra, and optical spectra are carried
out. Furthermore, computations are employed to investigate the
Synthesis and Characterization
As revealed by the synthesis of diPBIs 4,12 only two
chlorine substituents in one bay region of PBIs can participate
in copper-mediated coupling reaction, probably because of
the low reactivity of chlorides. The reactivity of the precursor
as well as the reaction temperatures are presumed to be key
factors to expand the PBI unit along the two bay regions.
Accordingly, we used tetrabromo-PBI instead of chloride as
the precursor and conducted the coupling reaction at 110 °C
under the system of CuI, L-proline, and K2CO3 (Scheme 1).
According to the MALDI-TOF spectroscopy, diPBI, triPBIs,
and tetraPBIs were observed in the products of the coupling
reaction. After purification on silica column chromatography,
triPBIs were obtained as dark-green solids in 16%, which
indicates that simultaneous coupling in both bay regions of
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Qian et al.
Scheme 1. Synthesis of triPBIs 5 and 6
1
PBI is achieved, and therefore opens the door to higher
oligomeric and polymeric PBIs. Notably, for the central PBI
unit, the participant C-H bonds can be in different or the
same naphthalene rings, the two possibilities resulting in two
isomeric structures of triPBIs 5 and 6. Using toluene as the
eluent, triPBIs were successfully separated by HPLC to two
fractions with peaks at 17 and 23 min, namely the first and
second fraction (Figure 2), MALDI-TOF spectroscopy of
which displayed the parent-ion peaks at m/z ) 2118.1 and
2118.2, respectively (calcd for triPBI (C144H114N6O12), 2118.8
[M]-). The structure of tri(perylene bisimides) was verified
by the well-resolved 1H and 13C NMR spectroscopy obtained
As can be seen from the H spectra of the first and second
fractions recorded in CDCl3 at room temperature (Figure 4),18
the NMR signals that mark the difference between the two
structural isomers are those due to Ha and Hb. The chemical
shift of these protons is considerably different for the first
fraction and is rather similar for the second fraction.
Helical and Nonhelical Structures of F-GNRs. To investigate
the structural changes of imide group functionalized GNRs
occurring upon increasing the number of PBI units, and to assist
the assignment of the NMR spectra of the two triPBI isomers,
quantum chemical calculations were carried out on models for
PBI 1, diPBI 4, and triPBIs 5 and 6, featuring methyl
substituents instead of isopropyl units on the phenyl rings. The
computed structures of 1 and 4 (Figure S5 and S6, Supporting
Information) indicate that the addition of one PBI unit leads to
an out-of-plane twisting of the aromatic core,19 driven by the
steric congestion between oxygen and the neighboring hydrogen
atom.
1
from the second fraction. The H spectrum of the second
fraction recorded in o-C6D4Cl2 at 383 K (Figure 3) shows
the characteristic signals at 2.81 (6H) and 3.11 ppm (6H)
for 12 protons of isopropyl groups, and two sets of singlet
peaks at 10.44 (2H) and 10.57 ppm (2H), respectively, for
the aromatic protons neighboring oxygen atoms.
A similar trend, namely a considerable twisting of each PBI
unit with respect to the others, is computed also for triPBIs
whose equilibrium structures are distorted out of plane for both
isomeric forms 5 (Figure 5) and 6 (Figure 6). More precisely,
two low-energy conformers were optimized for both 5 and 6,
representing two different out-of plane deformations of the three
PBI moieties. In both isomers, each PBI unit is twisted with
respect to the adjacent one, similar to the observation with 4.
However, for three PBI units there are two pairs of adjacent
PBI units, and one obtains two different out-of-plane confor-
mational isomers. In one isomer, the two pairs of adjacent PBI
units bend in the same direction so as to form a helical structure
(with two enantiomeric forms), while in the second isomer the
two pairs of adjacent PBI units bend in opposite directions,
resulting in a nonhelical structure (a meso-configuration).
Notably, for both 5 and 6, the difference in absorption spectra
(and molecular orbital energies) between the helical and
nonhelical structures is almost negligible. From the above
discussion it can be inferred that F-GNRs (PBI)n will be
characterized by having as many as n - 1 possible out-of-plane
(18) Notably, the dramatic difference in appearance between the NMR
spectra in Figures 3 and 4 is probably due to the solvent effect and
aggregations that are not uncommon for large PAHs.
(19) (a) Osswald, P.; Wu¨rthner, F. J. Am. Chem. Soc. 2007, 129, 14319–
14326. (b) Wu¨rthner, F. Pure Appl. Chem. 2006, 78, 2341–2349.
Figure 2. HPLC (Buckyprep column, toluene eluent at 12 mL/min)
chromatograms: (a) triPBIs before separation, (b) the first fraction after
separation, and (c) the second fraction after separation.
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Fully Conjugated Tri(perylene bisimides)
A R T I C L E S
Figure 3. 1H spectrum of the second fraction recorded in o-C6D4Cl2 at 383 K.
1
Figure 4. Selected part of the H spectra of the first (top) and second (bottom) fractions recorded in CDCl3 at room temperature.
structures of similar energy, of which one is helical (all the PBI
pairs twisted in the same direction) and the remainder are
nonhelical.
The NMR spectra were computed for the helical and
nonhelical structures of 5 and 6. The portions of the computed
NMR spectra pertinent to Ha and Hb are shown in Figure 7.
The spectra computed for the two isomers (helical and nonhe-
lical) of 5 show similar chemical shift differences for Ha and
Hb. Such computed chemical shift difference is larger for 5 than
for 6, especially for the nonhelical structures, in agreement with
the experimental spectrum of the first fraction, compared to that
measured for the second fraction. For 6, in addition, the NMR
spectra computed for the two forms differ in that the chemical
shift of the two Hb hydrogens is not equivalent for the nonhelical
isomer. This computed result, in better agreement with the
observed spectrum of the second fraction, showing a single but
broader band for Hb, suggests a predominant nonhelicial
structure for the second fraction. Accordingly, the first and
second fractions can be assigned to the structures of 5 and 6,
respectively, an assignment which is further supported by
comparison of the computed and experimental absorption
spectra.
Figure 5. Helical (a) and nonhelical (b) structures of triPBI 5.
Absorption Spectra. Compared to diPBI 4 (Figure S7,
Supporting Information), triPBIs 5 and 6 (Figure 8) display
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A R T I C L E S
Qian et al.
Figure 6. Helical (a) and nonhelical (b) structures of triPBI 6.
Figure 8. (Top) TDDFT-calculated absorption spectra of triPBI 5 (red)
and triPBI 6 (blue). (Bottom) Absorption spectra of triPBI 5 (red) and triPBI
6 (blue) in CHCl3.
1
Figure 7. Portions of the B3LYP/3-21G computed H spectra (chemical
shift in ppm) of the helical (a) and nonhelical (b) configurations of 5 (1)
and 6 (2).
broad and red-shifted spectra with major bands at 378, 426,
467, 508, 610, 681, and 750 nm and 372, 437, 508, 591, 700,
and 805 nm, respectively. Examining the fully conjugated oligo-
PBI series (PBIs to diPBIs to triPBIs), it is apparent that the
energy of the lowest allowed electronic transition, namely the
optical HOMO-LUMO gap, decreases and the number of
absorption bands increases upon the addition of PBI units. The
UV/vis spectrum of 6 is bathochromically shifted by about 55
nm with respect to that of 5, suggesting a slightly more efficient
conjugation in this as compared to triPBI 5.
Figure 9. B3LYP/3-21G computed energies and shapes of the frontier
orbitals of PBI, diPBI, and triPBIs at their equilibrium structures. The orbital
shapes of the nonhelical structures of 5 and 6 are very similar to those of
the helical species.
To confirm the assignment of structural isomers 5 and 6 and
to assist the interpretation of the experimental absorption spectra,
the absorption spectra were calculated for helical and nonhelical
structures and are compared with the observed absorption spectra
in Figure 8. The spectrum computed for the diPBI analogue of
5 and 6 is presented for comparison in the Supporting Informa-
tion (Figure S7). It is seen that the TDDFT-calculated bands
agree very well with the most prominent observed features in
the experimental spectra, which further supports our assignment
of 5 and 6 to the first and second fractions, respectively. The
smaller number of bands in the computed spectra is due to the
limited number of computed excitation energies and to the
neglect of vibronic structures associated with the electronic
transitions. Interestingly, it can be seen that the spectra simulated
for the helical and nonhelical forms of each triPBI compound
are very similar, and do not allow for discrimination between
them. More importantly, the lowest energy excitation is
predicted, invariably, for compound 6, which may suggest a
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Fully Conjugated Tri(perylene bisimides)
A R T I C L E S
Figure 10. Reductive cyclic voltammograms of 5 (a) and 6 (b) in CH2Cl2. Scan rate, 0.1 V/s (electrolyte, 0.1 M TBAPF6).
Table 1. Redox Potentials (in V vs Fc/Fc+)a
energy transitions of 5 and 6 is small (0.11 eV), and the H-L
gap is even smaller.
E1r
E2r
E3r
E4r
E5r
E6r
In the arrays of fully conjugated PBIs, the imide substituents
have a negligible influence on the HOMO and LUMO orbitals
because of the nodes at the imide nitrogens. As expected, the
HOMOs of diPBI20 and triPBIs21 are linear combinations of
the monomer’s HOMO, and the lowest (two or three) unoc-
cupied orbitals correspond to different linear combinations of
the LUMO of PBI 1. Calculations show that the energy of the
LUMO lowers considerably when moving from PBI to diPBI
to triPBIs. The computed trend suggests a concomitant increase
of the electron affinity of these compounds. The increasing
number of low-energy, unoccupied orbitals with similar parent-
age suggests an increasing ability to accept electrons, which is
confirmed by the electrochemical properties discussed below.
Electrochemical Properties. The electrochemistry of fullerene
derivatives exhibits six one-electron reduction waves, which is,
to the best of our knowledge, the largest number for n-type
organic materials.22 According to the MO calculations, for
diPBIs there are two unoccupied orbitals with the same
parentage, the energies of which are very close. Thus, four
electrons can be accommodated by diPBI, in agreement with
the observed four reduction waves in electrochemistry.12
Compared with diPBI, triPBIs have the lowest LUMOs and three
unoccupied orbitals with the same parentage, implying their
stronger electron affinity and the ability to accept up to six
electrons. Cyclic voltammograms of triPBIs 5 and 6 display
well-defined, single-electron reduction waves, two reversible,
two quasireversible, and two irreversible (six in all, Figure 10).
The half-wave reduction potentials vs Fc/Fc+ of PBI 1, diPBI
4, triPBI 5, and 6 are collected in Table 1. With the expansion
of the π system by PBI units, the first reduction potentials of
PBI arrays undergo a positive shift from -0.96 V for PBI 1 to
-0.46 V for diPBI 4 to -0.33 V for triPBI 5, and -0.30 V for
triPBI 6, indicating the increase in the electron-accepting ability.
As revealed by the first reduction potentials, triPBI 6 is more
PBI 1
-0.96
-0.46
-0.33
-0.30
-1.22
-0.73
-0.60
-0.57
diPBI 4
triPBI 5
triPBI 6
-1.49
-1.12
-1.14
-1.66
-1.26
-1.30
-1.80
-1.80
-2.00
-1.99
a Half-wave potentials. The data have been recalculated to the
reference of Fc/Fc+. The oxidation potential of Fc/Fc+ was measured as
0.40 V against Ag/AgCl.
Figure 11. Computed LUMO energies (eV) versus the first reduction
potentials (E1r in V versus Fc/Fc+) of PBI 1, diPBI 4, and triPBIs 5 and 6.
slightly more efficient conjugation in this as compared to the
parent triPBI 5.
The optical gaps determined from absorption spectra are
expected to reflect the trend in the HOMO-LUMO (transport)
gaps since calculations show that the lowest allowed transition
is dominated by the HfL single excitation. The shapes of the
frontier orbitals of PBI and diPBI, and of the helical structures
of triPBIs 5 and 6, along with a schematic representation of
their energy levels, are depicted in Figure 9, while energies and
energy gaps are collected in the Supporting Information (Tables
S1 and S2), together with the computed TDDFT excitation
energies for the lowest allowed electronic transition.
(20) Note that compared with the LUMO energy of PBI, for diPBI the
LUMO energy is below, and that of the LUMO+1 is slightly above,
as expected when making a linear combination of two orbitals.
(21) Notably, the LUMO+1 orbitals of triPBIs correspond to the combina-
tion of two LUMOs localized on external PBI units. The external units
interact little, and hence the energy (-3.68 eV) of the corresponding
orbital is very close to that of unperturbed PBI (-3.56 eV).
(22) (a) Xie, Q.; Pe`rez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992,
114, 3978–3980. (b) Bendikov, M.; Wudl, Fred, Chem. ReV. 2004,
104, 4891–4945.
The H-L energy gap follows the trend of the lowest energy
transition upon the addition of PBI units, which is in agreement
with the observed red-shifted absorption spectra. It should be
noted, however, that the energy difference between the lowest
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Qian et al.
easily reduced than 5, which is consistent with their calculated
LUMO energies.
spectra for the two isomers of triPBIs agree very well with the
observed spectra and allow the assignment of 5 and 6 to the
first and second HPLC fraction, the computed NMR spectra
suggesting also a predominance of the nonhelical isomer for
compound 6. With 19 benzene rings and 6 imide groups in the
structures, triPBIs 5 and 6 display broad and red-shifted
absorption spectra and six reduction waves in electrochemistry.
Increasing the number of PBI units in oligo-PBIs (from PBIs
to diPBIs to triPBIs) leads to an expansion of the π system, in
turn associated with a reduction of the transport and optical band
gaps (reflected in the UV spectra), the lowering of LUMO
energies, mild reduction of the ionization potentials, and a
remarkable increase in electron affinities, suggesting their
potential applications as F-GNRs in organic electronics.
Accurate computational predictions of redox potentials require
comparison of energies for both the starting molecule and its
reduced forms. However, Koopmans’s theorem23 enables us to
correlate the first redox potential of xPBI (x ) mono, di, tri)
systems with their LUMO energies. The result of such correla-
tion is shown in Figure 11, where a plot of the computed LUMO
energies versus the first reduction potentials shows good
linearity. As shown in the plot, the change in LUMO energy
and reduction potential from PBI to diPBI is quite big, while
that observed from diPBI to triPBI is comparably smaller. The
correlation suggests that, by adding new PBI units, the LUMO
energy will lower further and the reduction potential will
undergo a further positive shift, although both changes are
expected to be small.
Acknowledgment. For financial support of this research, we
thank the National Natural Science Foundation of China (Grant
50873106, 20421101), 973 Program (Grant 2006CB932101,
2006CB806200), and Chinese Academy of Sciences.
Conclusions
In summary, we report the copper-mediated trimerization of
PBIs along bay regions to construct graphene nanoribbons that
are functionalized by arrays of imide groups. Due to the two
possible coupling positions, there are two structural isomers of
triPBIs, 5 and 6, which were successfully separated by HPLC.
According to calculations, each of the two isomers has two
possible conformers, corresponding to helical and nonhelical
arrangements of the PBI units. The computed 1H and absorption
Supporting Information Available: Complete ref 17, experi-
mental section, MS, 1H and 13C NMR spectra, DPV of 5 and 6,
optimized structures of 1 and 4, comparison between computed
and experimental absorption spectra of 4, and tables with MO
energies, absolute energies, and optical and transport gaps. This
information is available free of charge via the Internet at http://
pubs.acs.org.
(23) Koopmans, T. Physica 1934, 1, 104–113.
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