Starburst substituted hexaazatriphenylene compounds: synthesis, photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.tetlet.2009.01.126

Starburst-substituted hexaazatriphenylene compounds have been designed and synthesized by introducing various peripheral aryl substituents to the central heterocyclic core. The effects of various substituent groups on the photophysical and electrochemical

Full text of DOI:10.1016/j.tetlet.2009.01.126

Tetrahedron Letters 50 (2009) 1649–1652
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Starburst substituted hexaazatriphenylene compounds:
synthesis, photophysical and electrochemical properties
a
b
b
Baoxiang Gao a, , Yueling Liu , Yanhou Geng , Yanxiang Cheng ,
*
b
b
Lixiang Wang b, , Xiabin Jing , Fosong Wang
*
a Key Laboratory of Medicinal Chemistry and Molecular Diagnosis, Ministry of Education, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, PR China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Starburst-substituted hexaazatriphenylene compounds have been designed and synthesized by introduc-
ing various peripheral aryl substituents to the central heterocyclic core. The effects of various substituent
groups on the photophysical and electrochemical properties of the substituted hexaazatriphenylene have
been investigated. Significant red-shifts of the absorption peak (from 413 nm to 530 nm) and emission
peak (from 432 nm to 700 nm) were observed when the electron-donating ability of the aryl substituents
was increased, corresponding to a decrease in the band gap from 2.90 eV to 2.05 eV. Introducing bulky
substituents with weak electron-donating ability enhances the fluorescence quantum yield from 23%
to 87%. In contrast, incorporating aryl substituents with strong electron-donating ability decreases the
fluorescence quantum yield. Also, due to the extended conjugation between the aryl substituents and
the hexaazatriphenylene core, the reduction potentials of the compounds were reduced and the LUMO
levels were thus increased.
Received 29 November 2008
Revised 20 January 2009
Accepted 24 January 2009
Available online 29 January 2009
Keywords:
Hexaazatriphenylene
Bandgap
Fluorescence
Electrochemical
Ó 2009 Elsevier Ltd. All rights reserved.
Recently, there has been a growing interest in organic semicon-
ductors due to their potential applications in electronic and
optoelectronic devices. The vast majority of synthesis and struc-
ture-property relationship studies in this field has been devoted
to organic semiconductors having p-type (electron donor, hole
transport) properties.1,2 N-type (electron acceptor, electron trans-
port) organic semiconductors are rather rare compared to p-type
semiconductors, and only a limited number of molecular building
blocks have been synthesized, such as fluorinated compounds,3,4
heteroaromatic ring compounds,5,6 fullerene derivatives,7 naph-
thalene and perylene diimide.8 Recently, pyrazine has been used
in the design and synthesis of N-type materials, owing to its elec-
tron-deficient nature.9,10 Our research group has studied the syn-
thesis and characterization of a series of pyrazine-containing
acene-type conjugated molecules, and some unique structure–
property relationships have been reported.11 Among the pyrazine
bandgap engineering and optical/electrochemical studies of hexa-
azatriphenylene-based compounds are necessary.
Herein, we report the synthesis of hexaazatriphenylene com-
pounds with peripheral aryl substituents. The corresponding opti-
cal properties (UV–vis and PL) and electrochemical properties
(redox potentials and HOMO/LUMO levels) of these materials were
also carefully investigated.
The synthetic approach of compounds 1a–f is outlined in
Scheme 1. The precursors 2b–f were synthesized by the palla-
dium-catalyzed Suzuki or Stille coupling reactions with good yield
(75–85%).14 Compounds 1a–f were obtained by tricondensation of
the hexaketocyclohexane with the aromatic diamines 2a–2f, in
acetic acid, with good yield (70–85%) and purity.15 Unfortunately,
the hexaketocyclohexane and precursors 2a–6a are air-sensitive,
therefore need to be freshly prepared. Otherwise, the yield of the
condensation products will drop significantly.
compounds, the hexaazatriphenylene that contains three
p
-defi-
To investigate the influence of aryl substituents on the optical
and electrochemical properties of hexaazatriphenylene, we syn-
thesized compound 1a as a reference compound by the reaction
of 4,5-dimethyl-o-phenylenediamine with hexaketocyclohexane
in acetic acid.
cient pyrazine nuclei at its core has been investigated as N-type
materials.12 However, the synthesis of hexaazatriphenylene com-
pounds that are substituted with various functional groups still re-
mains almost unexplored.13 Considering the vast number of
applications within the areas of electronics and optoelectronics,
The photophysical properties of the hexaazatriphenylenes 1a–f
are summarized in Table 1. The UV–vis spectrum of compound 1a
shows an absorption peak at 413 nm with a strong vibronic fine
structure (Fig. 1), which is attributed to the hexaazatriphenylene
* Corresponding authors. Tel.: +86 312 3382004; fax: +86 312 5079317 (B.G.),
tel.: +86 431 85262201; fax: +86 431 85685653 (L.W.)
core
atriphenylene perimeter, the longest absorption bands assigned
p–
p* transition. By increasing the conjugation at the hexaaz-
E-mail addresses: bxgao@hbu.cn (B. Gao), lixiang@ciac.jl.cn (L. Wang).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.126

1650
B. Gao et al. / Tetrahedron Letters 50 (2009) 1649–1652
CH3
N
H3C
O
NH2
CH3
H2N
H3C
N
O
O
AcOH
Reflux
O
O
+
8H2O
N
N
CH3
CH3
O
N
N
H3C
H3C
2a
3
1a
Ar
Ar
Ar
Ar
Br
Br
ArSnBu3
or ArB(OH)2
N
N
3/AcOH
N
N
Pd(PPh3)4
Reflux
H2N
NH2
H2N
NH2
Ar
N
N
Ar
4
2b-2f
Ar
Ar
1b-1f
C8H17
C8H17
S
S
N
C6H13
OCH3
Ar=
1f
1e
1d
1c
1b
Scheme 1. Synthetic sequence of hexaazatriphenylene compounds.
Table 1
to the electronic
the maximum absorption peak. For example, the k
compound 1a is 413 nm, while for compounds 1b (phenyl substi-
p
–
p* transition undergo a bathochromic shift of
Photophysical properties of hexaazatriphenylene compounds
abs
(nm) for
max
abs
max
fl
max
Compound
k
(nm)
e
(104 molꢀ1 dmꢀ3 cmꢀ1
)
k
(nm)
uf (%)
23a
74b
81b
87b
0.3c
0.8c
tuent), 1c (4-methoxyphenyl substituent) and 1d (fluorene substi-
1a
1b
1c
1d
1e
1f
413
435
460
479
526
530
3.06
3.88
2.11
3.49
2.05
0.97
432
466
543
552
689
700
abs
tuent) the k
(nm) is 435 nm, 460 nm and 479 nm, respectively.
max
The introduction of bulky aryl substituents on the hexaazatriphen-
ylene core produces line broadening in the absorption spectrum
and less pronounced vibronic fine structure (Fig. 1). The spectral-
broadening phenomena have been reported elsewhere for
a
a
Fluorescence quantum yield relation to 9,10-diphenylanthracene in ethanol
twisted perylene bisimide core.16
(95%).
b
c
Fluorescence quantum yield relation to Coumarin 6 in acetonitile (63%).
Fluorescence quantum yield relation to Nile red in 1,4-dioxane (68%).
1.2
1.2
1a
1b
0.9
0.9
1a
1c
1d
1b
1c
1e
1f
1d
1e
1f
0.6
0.6
0.3
0.0
0.3
0.0
300
400
500
600
700
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Figure 1. UV–vis absorption spectra of compound 1a–1f in dichloromethane.
Figure 2. Photoluminescence spectra of compound 1a–1f in dichloromethane.

B. Gao et al. / Tetrahedron Letters 50 (2009) 1649–1652
1651
Table 2
Oxidation and reduction E1/2 potentialsa for hexaazatriphenylene compounds
Compd
Oxidation E1/2 (V)a
Reduction E1/2 (V)a
2nd
LUMO/HOMOe (eV)
Band gap
1st
1st
Eeg (eV)c
Eog (eV)d
1a
1b
1c
1d
1e
1f
—
—
—
—
ꢀ0.96
ꢀ0.81
ꢀ0.82
ꢀ0.81
ꢀ0.79
ꢀ0.77
ꢀ1.25
ꢀ1.21
ꢀ1.28
ꢀ1.18
ꢀ1.04
ꢀ1.06
ꢀ3.45/ꢀ6.35f
ꢀ3.62/ꢀ6.29f
ꢀ3.57/ꢀ6.03f
ꢀ3.60/ꢀ5.99f
ꢀ3.66/ꢀ5.66f
ꢀ3.63/ꢀ5.28
—
—
—
—
2.35
1.61
2.90
2.67
2.48
2.39
2.00
2.05
1.65b
0.96
a
Potentials versus Ag/AgCl.
b
c
d
e
f
Irreversible.
ox
re
onset
Electrochemical band gap Eeg ¼ E
ꢀ E
.
onset
Optical band gap Eog ¼ 1240=k
.
abs
onset
re
onset
ox
onset
HOMO and LUMO energy were calculated with reference to ferrocene (4.8 eV) LUMO = ꢀ(4.80 + E
); HOMO = ꢀ(4.80+E
).
HOMO = LUMO ꢀ Eog.
Compared to compounds 1a–1d, dithiophene and triphenyl-
compound 1f exhibits a reversible multi-electron oxidation wave
at 0.96 V.
amine-substituted hexaazatriphenylene have quite different spec-
tral behavior (Fig. 1). The absorption spectra of compounds 1e and
1f show a new absorption band, with peaks located at 526 nm and
530 nm, respectively. The new absorption band is tentatively
attributed to the charge-transfer absorption involving the elec-
tronic transition from the donor moieties (dithiophene or triphen-
ylamine) to the electron-deficient hexaazatriphenylene core.17
Furthermore, introduction of aryl substituents leads to a reduction
in the HOMO–LUMO gap (from 2.90 eV to 2.05 eV, derived from the
absorption spectrum).
The fluorescence emission spectra of compounds 1a–1f in
dichloromethane are presented in Figure 2. With an increase in
the conjugated strength and electron-donor strength of aryl sub-
stituents, their emission wavelengths undergo red-shift. Compared
to compound 1a, which contains only peripheral methyl substitu-
ents, when the conjugation is extended by the addition of aryl sub-
stituents the emission wavelength is red-shifted from 432 nm
(compound 1a) to 700 nm (compound 1f). The introduction of
bulky conjugated substituents has a beneficial effect on the fluo-
rescence quantum efficiencies, and fluorescence quantum yields
increased from 23% (compound 1a) to 87% (compound 1d). How-
ever, compared with bulky conjugated substituents, aryl substitu-
ents with strong electron-donating ability have a different effect on
the fluorescence quantum yield. When the electron-donating abil-
ity of these aryl substituents increased, for example, with respect
to compounds 1e and 1f, the fluorescence quantum yields drop
dramatically (Table 1). This fluorescence quenching might be due
to the efficient intramolecular electron transfer between the elec-
tron donor (dithiophene and triphenylamine) and the electron
acceptor (hexaazatriphenylene core).
The lowest unoccupied molecular orbital (LUMO) was esti-
mated from the onset reduction potentials by comparison to ferro-
cene.18 The highest occupied molecular orbital (HOMO) of these
hexaazatriphenylene compounds was estimated from the onset
oxidation potentials (compound 1f) or from the equation
(LUMO = HOMO ꢀ Eog, for compounds 1a–e). When the electron-
donating ability of these aryl substituents increased, HOMO levels
of compounds 1b–f were enhanced from ꢀ6.29 eV (compound 1b)
to ꢀ5.28 eV (compound 1f). This indicates that the LUMOs lie on
the hexaazatriphenylene core and the HOMOs lie on the aryl
substituents.
In summary, starburst-substituted hexaazatriphenylene com-
pounds have been synthesized in good yields of 70–85%. The
photophysical properties of multi-aryl-substituted hexaazatri-
phenylene are strongly related to the electron-donating ability of
these aryl substituents. Fluorescence enhancement and fluores-
cence quenching can be controlled by adjusting the electron-
donating ability of the aryl substituents. The electrochemical
properties of these compounds were investigated by cyclic voltam-
metry at room temperature. By incorporating aryl substituents to
the peripheries of the hexaazatriphenylene core, the reduction
potentials were improved due to the extended conjugation
between the aryl substituents and the hexaazatriphenylene core.
Acknowledgements
This work is supported by the National Natural Science Founda-
tion of China (20804012 and 20574067) and the Science Research
Project of Department of Education of Hebei Province (2007413).
The redox properties of these compounds were studied by cyclic
voltammetry (CV), and the results are shown in Table 2. The CV
scan for compound 1a with anodic scanning from 0 to 2 V shows
no peaks. On cathodic scanning, the CV scan of compound 1a
exhibits two reversible one-electron reduction waves. This indi-
cates that the hexaazatriphenylene core has electron-accepting
properties. By introducing aryl substituents to the hexaazatriphen-
ylene core, the reduction potentials of compound 1b–d are com-
pared to those of compound 1a, in which a difference of ꢀ0.15 V
is observed. The increase in reduction potentials could attribute
to the conjugation between the aryl substituents and the hexaaz-
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tended conjugation moieties.
p-ex-
Reduction potentials of compounds 1e and 1f were not further
enhanced by strong electron-donor substituents on the hexaazatri-
phenylene core (Table 2). On anodic scanning, compound 1e exhib-
its an irreversible one-electron oxidation wave at 1.65 V and
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B. Gao et al. / Tetrahedron Letters 50 (2009) 1649–1652
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chromatography on silica gel, eluting with a 2:1 petroleum ether/ethyl
acetate mixture. Yellow solid (yield: 86%); 1H NMR (CDCl3): d 7.04 (m, 6H),
6.85 (s, 2H), 6.71 (d, J = 7.3 Hz, 2H), 3.43 (br, 4H), 2.81 (t, J = 7.6 Hz, 4H), 1.73 (t,
J = 7.4 Hz, 4H), 1.42–1.33 (m, 12H), 0.91(t, J = 6.5 Hz, 6H).
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G.; Geng, Y. H.; Cheng, Y. X.; Ma, D. G.; Xie, Z. Y.; Wang, L. X.; Wang, F. S. Mater.
Chem. Phys. 2006, 99, 247.
15. General procedure for the cyclization of hexaazatriphenylene: A mixture of
hexaketocyclohexane(1 mmol) and the corresponding diamine (4 mmol) in
acetic acid (60 mL) was heated at 100 °C for 24 h under nitrogen atmosphere.
After the reaction mixture was cooled to room temperature, it was poured into
water (100 mL) and extracted with dichloromethane (30 mL ꢁ 3). The organic
layer was washed with saturated aqueous sodium hydrogen carbonate
solution (100 mL ꢁ 2) and brine (100 mL ꢁ 2), dried over anhydrous Na2SO4
and evaporated in vacuo to dryness. The residue was absorbed on silica gel and
purified by column chromatography eluting with
dichloromethane mixture.
a 2:1 petroleum ether/
Compound 1a. Yellow solid (yield: 85%), 1H NMR (DMSO-d6): d 8.13 (s, 6H),
3.34 (s, 18H). Elemental Anal. Calcd for C30H24N6: C, 76.90; H, 5.16; N, 17.94.
Found: C, 76.81; H, 5.01; N, 18.33. MALDI-TOF: calcd: 468.2, found: 469.0.
Compound 1b. Yellow solid (yield: 82%), 1H NMR (CDCl3): d 8.81 (s, 6H), 7.63 (s,
30H). 13C NMR (CDCl3): d 145.96, 143.89, 142.93, 139.85, 131.50, 129.98,
128.25, 127.75 Elemental Anal. Calcd for C60H36N6: C, 85.69; H, 4.31; N, 9.99.
Found: C, 85.43; H, 4.41; N, 10.15. MALDI-TOF: calcd: 840.3, found: 840.5.
Compound 1c. Yellow solid (yield: 78%), 1H NMR (CDCl3): d 9.07 (s, 6H), 7.30 (d,
J = 8.7 Hz, 12H), 6.90 (d, J = 8.8 Hz, 12H). 13C NMR (CDCl3): d 159.31, 145.50,
143.61, 142.83, 132.43, 131.19, 131.03, 113.80, 55.27 Elemental Anal. Calcd for
C66H48N6O6: C, 77.63; H, 4.74; N, 8.23. Found: C, 77.38; H, 4.68; N, 8.41.
MALDI-TOF: calcd: 1020.4, found: 1020.9.
12. (a) Lemaur, V.; da Silva Filho, D. A.; Coropceanu, V.; Lehmann, M.; Geerts, Y.;
Piris, J.; Debije, M. G.; van de Craats, A. M.; Senthilkumar, K.; Siebbeles, L. D. A.;
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Warman, J. M.; Bredas, J.-L.; Cornil, G. J. Am. Chem. Soc. 2004, 126, 3271; (b)
Kaafarani, B. R.; Kondo, T.; Yu, J.; Zhang, Q.; Dattilo, D.; Risko, C.; Jones, S. C.;
Barlow, F.; Domercq, B.; Amy, F.; Kahn, A.; Bre´das, J.-L.; Kippelen, B.; Marder, S.
R. J. Am. Chem. Soc. 2005, 127, 16358; (c) Lehmann, M.; Kestemont, G.; Aspe, R.
G.; Buess-Herman, C.; Koch, M. H. J.; Debije, M. G.; Piris, J.; de Haas, M. P.;
Warman, J. M.; Watson, M. D.; Lemaur, V.; Cornil, J.; Geerts, Y. H.; Gearba, R. I.;
Ivanov, D. A. Chem. Eur. J. 2005, 11, 3349.
13. (a) Ishi-i, T.; Hirayama, T.; Murakami, K.-i.; Tashiro, H.; Thiemann, T.; Kubo, K.;
Mori, A.; Yamasaki, S.; Akao, T.; Tsuboyama, A.; Mukaide, T.; Ueno, K.; Mataka,
S. Langmuir 2005, 21, 1261; (b) Chang, T.-H.; Wu, R.-R.; Chiang, M. Y.; Liao, S.-
C.; Ong, C. W.; Hsu, H.-F.; Lin, S.-Y. Org. Lett. 2005, 7, 4075; (c) Ishi-i, T.;
Murakami, K.; Imai, Y.; Mataka, S. Org. Lett. 2005, 7, 3175.
Compound 1d. Orange solid (yield: 74%), 1H NMR (CDCl3): d 8.93 (s, 6H), 7.69,
7.64 (m, 12H), 7.50 (s, 6H), 7.43 (d, J = 7.8 Hz, 6H), 7.35 7.34 (m, 18H), 1.96 (t,
J = 7.7 Hz, 24H), 1.25, 1.07 (m, 48H), 0.87 (t, J = 6.7 Hz, 18H). 13C NMR (CDCl3): d
150.97, 146.43, 143.95, 142.99, 140.97, 140.60, 138.82, 131.77, 128.99, 127.34,
126.82, 124.45, 122.81, 120.00, 119.51, 55.22, 40.59, 31.89, 30.09, 29.45, 29.31,
23.96, 22.66, 14.10. Elemental Anal. Calcd for C198H252N6: C, 87.55; H, 9.35; N,
3.09. Found: C, 87.29; H, 9.03; N, 3.29. MALDI-TOF: calcd: 2174.0, found:
2175.1.
14. General procedure for synthesis of precursor 2b, 2c, 2d and 2f: To a mixture of
4,5-dibromo-o-phenylenediamine (2 mmol, 532 mg) and corresponding
boronic acid (5 mmol) and Pd(PPh3)4 (0.05 mmol, 58 mg) in toluene (50 mL),
2 M K2CO3 solution (15 mL) was added under nitrogen atmosphere. The
resulting mixture was heated at 85 °C for 24 h. The reaction mixture was
poured into water and extracted with dichloromethane (20 mL ꢁ 3). The
organic layer was washed with brine (50 mL), dried over Na2SO4 and
evaporated in vacuo to dryness. The crude product was purified by column
chromatography on silica gel, eluting with a 2:1 petroleum ether/ethyl acetate
mixture.
Compound 1e. Black solid (yield: 70%), 1H NMR (CDCl3): d 8.79 (s, 6H), 7.05,
7.04 (m, 18H), 6.71 (d, J = 6.4 Hz, 6H), 2.84 (t, J = 7.5 Hz, 12H), 1.751.65 (m,
12H), 1.43, 1.34 (m, 36H), 0.93 (t, J = 6.3 Hz, 18H). 13C NMR (CDCl3): d 146.17,
143.95, 142.89, 140.45, 138.58, 138.20, 134.32, 131.45, 129.73, 124.98, 124.05,
123.50, 31.61, 31.23, 30.26, 28.79, 22.61, 14.10. Elemental Anal. Calcd for
C108H108N6S12: C, 69.19; H, 5.81; N, 4.48; S, 20.52. Found: C, 69.32; H, 5.73; N,
4.63. MALDI-TOF: calcd: 1872.5, found: 1874.1.
Compound 2b. White solid (yield: 83%); 1H NMR (CDCl3): d 7.22–7.17 (m, 6H),
7.13 (d, J = 7.6 Hz, 4H), 6.84 (s, 2H), 3.52 (br, 4H).Compound 2c. White solid
(yield: 80%); 1H NMR (CDCl3) d 7.44 (d, J = 9.0 Hz, 4H), 6.96 (s, 2H), 6.84 (d,
J = 9.0 Hz, 4H), 3.43 (br, 4H).
Compound 1f. Red solid (yield: 72%), 1H NMR (CDCl3): d 8.74 (s, 6H), 7.327.22
(m, 36) 7.18 (d, J = 6.9 Hz, 24H), 7.09 (t, J = 7.8 Hz, 24H). 13C NMR (CDCl3): d
147.50, 145.65, 143.66, 142.94, 133.56, 130.85, 130.60, 129.40, 124.89, 123.33,
122.32 Elemental Anal. Calcd for C132H90N12: C, 85.97; H, 4.92; N, 9.11. Found:
C, 85.72; H, 5.13; N, 9.32. MALDI-TOF: calcd: 1844.2, found: 1845.0.
16. Wurthner, F.; Thalacker, C.; Diel, S.; Tschierske, C. Chem. Eur. J. 2001, 66, 6109.
17. (a) Albert, I. D. L.; Marks, T. J.; Ratner, M. A. J. Am. Chem. Soc. 1997, 119, 16358;
(b) Justin Thomas, K. R.; Lin, Jiann T.; Tao, Yu-Tai; Chuen, Chang-Hao Adv.
Mater. 2002, 14, 822.
Compound 2d. White solid (yield: 78%); 1H NMR (CDCl3) d 7.63 (d, J = 8.5 Hz,
2H), 7.52 (d, J = 8.2 Hz, 2H), 7.31–7.29 (m, 2H), 7.23–7.21 (m, 4H), 7.03–7.01
(m, 4H), 6.66 (s, 2H), 4.76 (br, 4H), 1.83–1.67 (m, 8H), 1.18–0.97 (m, 48H), 0.78
(t, J = 6.8 Hz, 12H).
Compound 2f. Yellow solid (yield: 75%); 1H NMR (CDCl3) d 7.39 (d, J = 7.3 Hz,
4H), 7.35–7.25 (m, 6H), 7.14–7.09 (m, 12H), 7.07 (d, J = 7.4 Hz, 4H), 7.00 (d,
J = 7.2 Hz, 4H), 6.97 (s, 2H), 3.43 (br, 4H).
Compound 2e. To a mixture of 4,5-dibromo-o-phenylenediamine (2 mmol,
532 mg), corresponding tributyl-stannyl (6 mmol,) and Pd(PPh3)4 (0.05 mmol,
58 mg), toluene (60 mL) was added under nitrogen atmosphere. The resulting
18. (a) Agrawal, A. K.; Jenekhe, S. A. Chem. Mater. 1996, 8, 579589; (b) Yang, C. J.;
Jenekhe, S. A. Macromolecules 1995, 28, 1180; (c) Alam, M. M.; Jenekhe, S. A. J.
Phys. Chem. B 2002, 106, 11172.