1,5,9-triazacoronenes: A family of polycyclic heteroarenes synthesized by a threefold pictet-spengler reaction

Article abstract of DOI:10.1002/anie.201002369

We are family: Triazacoronene derivatives have been synthesized in four steps from veratrole by using a threefold Pictet-Spengler reaction as the key step (see picture). They have good photophysical and electronical properties, thermal stability, and solu

Full text of DOI:10.1002/anie.201002369

Angewandte
Chemie
DOI: 10.1002/anie.201002369
Heteroarenes
1,5,9-Triazacoronenes: A Family of Polycyclic Heteroarenes
Synthesized by a Threefold Pictet–Spengler Reaction**
Junfa Wei,* Bo Han, Qiang Guo, Xianying Shi, Wenliang Wang, and Ni Wei
The design and synthesis of polycyclic aromatic hydrocarbons
(PAHs) have been attracting considerable interest owing to
their potential applications in organic optoelectronics, such as
field-effect transistors, photovoltaic cells, and light-emitting
diodes.[1,2] One of the most commonly used methodologies for
this purpose is the incorporation of heteroatoms into the
aromatic framework of PAHs to improve their physicochem-
ical properties and enhance the carrier transport of the system
for applications as organic electronics.[3] For example, the
replacement of the CH groups with electronegative nitrogen
atoms is able to facilitate the electron injection and transport
properties of the materials.[3a–f] Coronene (1) is a representa-
tive example of graphitic fragments with a zigzag periphery
and just the right size for such processing techniques
(Scheme 1).[4–6] However, only a few reports describe routes
tions (Scheme 2). In addition to the preparative accessibility,
these TAC derivatives have several advantages over their
analogues, including: 1) the three electron-deficient pyridine
Scheme 2. The synthetic route to TACs: a) FeCl3, CH2Cl2, 97%;
b) fuming HNO3, CH2Cl2, Et2O, AcOH, reflux, 6 h, 28%; c) H2, Raney
Ni, EtOH, RT, 97%; d) RCHO, DMF, TfOH, 40–1008C, 32–66%.
DMF=N,N-dimethylformamide.
Scheme 1. Coronene and its polyaza derivatives.
rings that should enhance the electron acceptor character-
istics of the system; 2) three side-arms and six peripheral
methoxy groups that should permit further functionalization
to create new hetero-PAHs, starburst molecules, and den-
drimers;[9] and 3) three sets of 8-hydroxyquinoline and
catechol moieties that should allow for further elaboration
for constructing coordination supermolecules. Herein, we
describe their synthesis, structure, and some of their essential
properties.
to its nitrogenated analogues,[7] such as 1,2-diazacoronene 2[7a]
and 1,2,7,8-tetraazacoronene 3,[7b] which were obtained by the
Diels–Alder reaction of perylene with diethyl azodicarbox-
ylate and maleic anhydride at 3508C. Obviously, the harsh
preparative conditions and difficulties in selective modifica-
tion have limited the further investigation of these azacoro-
nenes.
We have designed and synthesized a new family of PAHs
containing a 1,5,9-triazacoronene (TAC) core 4, which can be
regarded as a triple-fused isoquinoline or quinoline, from
veratrole using a four-step sequence of trimerization, nitra-
tion, reduction, and a tandem threefold Pictet–Spengler
reaction[8] and oxidative aromatization under mild condi-
Hexamethoxytriphenylene
6
was readily available
through the tricondensation of veratrole.[10] Nitration of 6
with fuming nitric acid in a mixed solvent of acetic acid gave 7
as yellow needles in 28% yield. The next challenge was the
reduction of 7 to the corresponding triamine 8. Although
many protocols have been well-established for reducing
nitroarenes to arylamines, the catalytic hydrogenation over
Raney nickel in ethanol was found to be the most favorable,
giving triamine 8 in near-quantitative yield. The analytical
and spectral data of intermediates 6–8 were perfectly self-
consistent.
[*] Prof. Dr. J. Wei, B. Han, Q. Guo, Dr. X. Shi, Prof. W. Wang, N. Wei
School of Chemistry and Materials Science, Key Laboratory for
Macromolecular Science of Shaanxi Province
Shaanxi Normal University, Xi’an, 710062 (P.R. China)
Fax: (+86)29-8530-7774
The key step was the triannulation of triamine 8 by
inserting a methylidyne group between the nitrogen atom and
the adjacent benzene ring to build the isoquinoline motifs. A
Pictet–Spengler reaction of 8 with benzaldehyde was chosen
to furnish the triple ring-closure. After some investigation, we
found that the triannulation and subsequent oxidative aro-
E-mail: weijf@snnu.edu.cn
[**] This work was financially supported by the National Foundation of
Natural Science in China (Grant No. 21072123, 20873079, and
20906059).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002369.
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8209

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matization reactions were achieved directly, thus providing 4a
coplanar within 0.0066 ꢀ, whilst both of the fused benzene
and pyridine rings are slightly irregular. Interestingly, the
side-view shows that the core is a slightly bowl-shaped
structure wherein the fused rings bend out of the central
ring plane by 4.158, on average, to the same side to form a
curve conformation. A likely reason is that the undersized
in one-pot. The reaction was further improved to afford a
61% yield in N,N-dimethylformamide at 1008C in the
presence of triflic acid. Analytical and spectral data match
those expected from its C3-symmetrical geometry.
Encouraged by the success of such a one-pot triannulation
of 8 with benzaldehyde, we used a range of aldehydes, both
aromatic and aliphatic, to check the scope of our procedure,
and, more importantly, to generate more TAC derivatives as
candidate materials for the chemistry and materials commun-
ities (Table 1). The data revealed that both aromatic and
aliphatic aldehydes afforded the desired products in promis-
ing yields.
=
N C bonds (average length 1.342 ꢀ) makes the perimeter too
short to keep the core coplanar. In fact, the average bond
length of the rim (1.393 ꢀ) is shorter than that of the central
ring (1.418 ꢀ).
The six methoxy groups are oriented alternately above
and below the core plane with dihedral angles near 908 for
those adjacent to the nitrogen atoms and 60–708 for the rest.
Each ring plane of the phenyl side-groups is twisted 69.98
(phenyl appended on C1), 70.58 (on C6), and 60.68 (on C11),
respectively, from the pyridine moiety. Such orientations
make it difficult for the peripheral groups to conjugate with
the core in the solid state. In addition, the bowl-shaped core
renders the molecule chiral with a rough C3 symmetry in the
crystal.
Table 2 summarizes the photophysical properties and
thermal stabilities of five representative TACs, and Figure 2
shows the typical spectra of 4a and 4g. The strongest
absorption peak of the aromatic derivatives appears at
around 370 nm in the ultraviolet region of the spectrum,
Table 1: The triazacoronene derivatives synthesized by triannulation of
the triamine 8 with various aldehydes.
Entry
R
Yield [%][a] Entry
R
Yield [%][a]
4a
4b
4c
4d
C6H5
61
66
54
53
4e
4 f
4g
4h
4-CH3C6H4
CH3
CH3CH2CH2
CH3(CH2)4CH2
64
32
51
50
4-CH3OC6H4
4-BrC6H4
4-ClC6H4
[a] Conditions: triamine (0.55 mmol), aldehyde (3.3 mmol), N,N-dime-
thylformamide (2 mL), triflic acid (1%), 1008C (ArCHO) or 408C
(RCHO).
À
typical of a p p* transition, whilst the aliphatic analogues
All of the TAC derivatives are soluble in common
solvents, such as chloroform, tetrahydrofuran, dichlorome-
thane, ethyl acetate, and acetone, giving yellowish-green-
colored solutions with bright fluorescence. The good solubil-
ity (up to 25 mgmLÀ1) permitted them to be processible in
solution.
The characterization data of the products are consistent
with their structures; the structure of 4b was clearly
confirmed by X-ray crystallography (Figure 1).[11] Compound
4b has a three-blade propeller-like shaped structure possess-
ing the fundamental motif of coronene. The central benzene
ring is a regular hexagon in which the six carbon atoms are
Table 2: The photophysical properties, melting point, and temperature
of thermal decomposition of the triazacoronene derivatives.
Entry lmax (Abs) lmax (Em)
[nm][a] [nm][a]
lmax (Ex)
Stokes m.p.
Td
[8C]
[nm][a]
shifts
[8C]
4a
4b
4c
4g
4h
278, 369 504
379
405
406
362
370
135
132
136
120
121
>300 331
280–282 345
>300 374
149–151 316
99–100 346
277, 372
278, 372
272, 351
272, 351
504
508
471
472
[a] 2.00ꢀ10À5 m in CHCl3. Abs=absorbance, Em=emission, Ex=exci-
tation.
have an absorption maxima at
351 nm with a blue-shift of circa
20 nm relative to the aromatic
analogues. This shift means that
the side phenyl groups p-conju-
gate with the core to some
extent. Compared with their all-
carbon parent, 1,2,5,6,9,10-hex-
amethoxycoronene (HMC, 300
and 325 nm),[6a] they are signifi-
cantly red-shifted and have
higher values for their molar
extinction
coefficient
(4.1–
10.4 ꢁ 105 mÀ1 cmÀ1). The remark-
able differences between the
TACs and HMC in their absorp-
tion peaks and intensities reflect
the profound electronic effect of
Figure 1. Molecular structure of 4b: top view (left), side view (top right), and overall view (bottom
right). Thermal ellipsoids set at the 30% probability level.
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Figure 2. The electronic (left), emission, and excitation spectra (both right) of 4a (green) and 4g (blue) in CHCl3 (2.00ꢀ10À5 m).
replacing the nitrogen atoms on the p-electron density of the
system. The TACs show a strong blue-green emission under
UV light and display large Stokes shifts ranging from 120 to
135 nm in their excitation spectra.
Cyclic voltammetry of the TACs (Table 3, Figure 3)
revealed a reversible one-peak reduction with formal poten-
tials (E8’) of À1.58 to À1.91 V and irreversible oxidation
waves of between 1.37 and 2.50 V vs Ag/AgCl, respectively.
The LUMO and HOMO energy levels were estimated at
around À3.0 eVand À6.0 eV, based on the reduction and first
oxidation onsets. Both energy levels are near to those of the
commonly used hole-blocking materials, such as 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline (BCP) and 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBI),[1k,12] and the
HOMO levels are lower than those of HMC (À5.5 eV).[6a]
These results suggest that TACs constitute intrinsic n-type
semiconducting materials with good hole-blocking and elec-
tron transport properties[13] and agree with their p-deficient
characterization owing to the electron-poor pyridine rings.
The electrochemical energy gaps (Egel) were between 2.61–
2.91 eV, near the optical gaps (Egopt, 2.62–2.76 eV).
Table 3: The electrochemical properties and energy levels of five
representative compounds.
Prod. Eg (opt) Ere
Ere
Eox1 Eox1
EHOMO
ELUMO
[eV][a]
[V][b]
(onset) [V][b] (onset)
[eV][c]
[eV][d]
All TACs produced strong electrogenerated chemilumi-
nescense (ECL) emission for radical ion annihilation by
stepping from the first oxidation wave (Eox1 = +50 mV) to the
reduction wave (Ere = À50 mV; Table 3). Figure 4 shows the
ECL for 4b by stepping from 1.48 to À1.63 V, which can be
seen with the naked eye in a dark room. As shown, highly
stable and repeatable ECL emissions were observed under
continuous stepping for 100 cycles. The operational stability is
an important criteria when evaluating the performance of
electroluminescent materials in terms of their practical
[V]
[V]
4a
4b
4c
4g
4h
2.67
2.64
2.62
2.75
2.76
À1.59 À1.30
À1.58 À1.45
À1.91 À1.74
À1.68 À1.61
À1.77 À1.62
À1.35
1.47 1.36
1.43 1.34
1.55 1.26
1.48 1.28
1.37 1.29
À6.07
À6.05
À6.06
À5.99
À6.00
À5.5
À3.41
À3.26
À3.45
À3.10
À3.09
HMC[e] 2.45
[a] Calculated from the lmax values. [b] 0.1m [nBu4N][ClO4] at a GC
working electrode vs Ag/AgCl in CH2Cl2. Scan rate 0.1 VsÀ1. [c] EHOMO
À4.71ÀEox1 (onset). [d] ELUMO =À4.71ÀEre (onset). [e] See Ref. [6a].
=
Figure 3. Cyclic voltammograms of 4a (blue line) and 4h (red line) in
CH2Cl2 (2.00ꢀ10À3 m) containing 0.1m nBu4NClO4. Scan rate:
Figure 4. Stepping ECL (annihilation) of 4b (2.00ꢀ10À3 m) in CH2Cl2.
Pulse width, 0.8 s; stepping direction is from the first oxidation peak
to the reduction peak.
0.1 VsÀ1
.
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applications.[14] A possible radical-ion annihilation can be
and the easily designable and modifiable groups on the
periphery allow the TACs a new n-type center to create
various star-shaped molecular networks, such as large hetero-
arenes, dendrimers, and supermolecules, or to tune their
properties by installing desired subunits on the arms or
extending the p-system. Synthetic efforts towards compounds
of this type are currently underway.
proposed, in which TAC0 is oxidized to the radical cation
TAC·+ on the cathode and reduced to the radical anion TAC·À
on the anode; the emission occurs in the annihilation of TAC·À
and TAC·+, wherein the singlet excited state of TAC* was
formed.
Density functional theory (DFT) computations at the
B3LYP/6-31G* level in a suite of Gaussian 03 programs
showed that the optimized structure of the cores was in
agreement with the X-ray crystal data (for the optimized
structure, the electrostatic potentials, and the frontier molec-
ular orbital energies, see the Supporting Information). The
HOMOs in 4b and 4g were partially delocalized to the
peripheral groups, whilst the LUMOs were mainly localized
on the core (Figure 5). The calculated HOMO and LUMO
Experimental Section
7: Fuming nitric acid (3.1 mL, 1.52 gcmÀ3) was added dropwise to a
stirred suspension of 2,3,6,7,10,11-hexamethoxytriphenylene (3.00 g)
in a mixture of AcOH, Et2O, and CH2Cl2 (15.0 mL each). The mixture
was stirred at reflux for 6 h and then the volatile solvents were
removed under reduced pressure. The solution was poured into water
and the resulting solid was filtrated, washed with water, and dried in
vacuum. Column chromatography on silica gel (eluted with CHCl3)
and recrystallization from CHCl3/EtOH gave 7 as yellow needles
(1.11 g, 28% yield); m.p. 267–2688C. 1H NMR (300 MHz, CDCl3):
d = 7.59 (s, 3H), 3.99 (s, 9H), 4.06 ppm (s, 9H); 13C NMR (75.45 MHz,
CDCl3): d = 55.81, 62.09, 107.34, 114.05, 122.89, 141.64, 143.35,
152.03 ppm; elemental analysis calcd. (%) for C24H21N3O12: C 53.04,
H 3.89, N 7.73; found: C 53.34, H 3.82, N 7.45.
8: A solution of 7 (0.50 g, 0.92 mmol) in EtOH (6.0 mL) was
stirred with freshly prepared Raney nickel[16] (1.07 g, 18.4 mmol)
under a hydrogen atmosphere at room temperature and atmospheric
pressure. After the absorption of hydrogen ceased, the catalyst was
removed by filtration. The filtrate was evaporated to dryness in a
vacuum and the residue was recrystallized from EtOH to give 8 as a
grayish-white solid (0.40 g, 97% yield); m.p. 170–1718C. 1H NMR
(300 MHz, CDCl3): d = 3.95 (s, 9H), 3.97 (s, 9H), 4.58 (s, 6H),
8.10 ppm (s, 3H); 13C NMR (75.45 MHz, CDCl3): d = 55.26, 59.73,
98.17, 113.68, 127.71, 134.39, 137.66, 150.04 ppm; elemental analysis
calcd. (%) for C24H27N3O6: C 63.56, H 6.00, N 9.27; found: C 63.53,
H 6.11, N 9.17.
General procedure for the synthesis of TACs (4a–h): A mixture
of 8 (0.25 g, 0.55 mmol) and aldehyde (3.3 mmol) in DMF (2 mL)
containing 1% triflic acid was stirred at 1008C (408C for aliphatic
aldehydes) until 8 could not be detected by TLC. Then the reaction
mixture was diluted with water (10 mL), the solution was made basic
with 15% aqueous NaOH solution, and extracted with CHCl3
(20 mL). The organic phase was washed with brine (10 mL), dried
with Na2SO4, and evaporated to dryness. The residue was purified by
column chromatography on silica gel using hexane/ethyl acetate (1:8,
v/v) as eluent to afford the product.
Figure 5. The orbital diagrams of 4b (top) and 4g (bottom): HOMO
(left) and LUMO (right).
1
4b: (0.29 g, 66% yield), yellow solid, m.p. 275–2778C; H NMR
levels were systematically over-estimated in the DFT calcu-
(300 MHz, CDCl3): d = 7.16 (d, J = 6.9 Hz, 6H); 7.95 (d, J = 6.9 Hz,
6H); 4.57 (s, 9H); 3.99 (s, 9H); 3.78 ppm (s, 9H); 13C NMR
(75.45 MHz, CDCl3): d = 159.8, 159.0, 150.2, 147.6, 139.8, 136.3,
131.5, 115.9, 112.8, 112.6, 63.1, 62.0, 55.5 ppm; elemental analysis
calcd. (%) for C48H39N3O9: C 71.90, H 4.90, N 5.24; found: C 71.77, H
4.95, N 5.29.
lations. The HOMO–LUMO energy gaps Egs (2.82–3.02 eV)
el
were uniformly higher than Egopts and Eg s by approximately
0.2 eV. The discrepancies between the experimental and
calculated values was presumably owing to the fact that the
calculations were performed in a vacuum.[15]
Thermogravimetric analysis revealed that the TACs were
stable to above 3008C, thus proving them to be thermally
robust materials suitable for application in organic electron-
ics.
In conclusion, a new family of triazacoronene derivatives
has been designed and the first eight members have been
synthesized from veratrole using a threefold Pictet–Spengler
reaction as the key step. Their unique structure, admirable
photophysical and electronical properties, good solubility, and
high thermal stability should make this class of hetero-PAHs
promising candidates for emissive and electron-transport
materials. Moreover, the electron-deficient nature of the core
4g: (0.17 g, 51% yield), yellow solid, mp 146–1478C. 1H NMR
(300 MHz, CDCl3): d = 4.54 (s, 9H), 4.47 (s, 9H), 4.12 (d, 6H), 2.26 (t,
6H), 1.27 ppm (t, 9H); 13C NMR (75.45 MHz, CDCl3): d = 162.8,
150.7, 146.9, 139.5, 123.3, 116.3, 112.4, 62.7, 62.3, 42.8, 29.7, 23.8,
14.7 ppm; elemental analysis calcd. (%) for C36H39N3O6: C 70.92, H
6.45, N 6.89; found: C 70.75, H 6.48, N 6.96.
Received: April 21, 2010
Published online: September 23, 2010
Keywords: electrochemistry · heterocycles · optoelectronics ·
.
polycycles · triazacoronenes
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[11] CCDC 788011 (4b) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif. Crystal data for 4b: single
crystals suitable for X-ray diffraction were obtained by slowly
evaporating a solution of 4b in tetrahydrofuran over one week.
ꢀ
C48H39N3O9·2.5(C4H8O), M = 982.08, Triclinic, space group P1,
a = 11.993(4), b = 13.599(4) ꢀ, c = 18.240(6) ꢀ, a = 111.631(4)8,
b = 107.684(4)8, g = 90.120(4)8, V= 2612.8(14) ꢀ3, T= 296(2) K,
Z = 2, m(MoKa) = 0.71073 ꢀ, colorless square-plates, crystal
dimensions 0.40 ꢁ 0.25 ꢁ 0.19 mm, crystal density 1.248 MgmÀ3
.
Full matrix least-squares based on F2, gave R1 = 0.0779 and
wR2 = 0.2410 for 9092 (I ꢀ 2s(I)), GOF = 1.033 for 686 param-
eters.
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