Facile synthesis and self-assembly of diazafluorenone-based p-n (donor-acceptor) organic semiconductors

Article abstract of DOI:10.1016/j.tet.2012.07.066

A mild and effective method to prepare 2,7-dibromo-4,5-diazafluoren-9-one (3) has been described involving tandem oxidation and rearrangement reactions. Diazafluorenone-based donor-acceptor (p-n) molecules via stille coupling reactions exhibit solvent-dep

Full text of DOI:10.1016/j.tet.2012.07.066

Tetrahedron 68 (2012) 8216e8221
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Facile synthesis and self-assembly of diazafluorenone-based pen
(donoreacceptor) organic semiconductors
,
a
Wei-Jie Li a, Hai-Mei Wu a, Yi-Bao Li b, Chao-Peng Hu a, Ming-Dong Yi a, Ling-Hai Xie a , Lin Chen ,
*
Jian-Feng Zhao a, Xiang-Hua Zhao a, Nai-En Shi a, Yan Qian a, Chen Wang c, Wei Wei a, Wei Huang a
,
*
a Jiangsu-Singapore Joint Research Center for Organic/Bio Electronics & Information Displays and Institute of Advanced Materials (IAM), Nanjing University of Posts and
Telecommunications, 9 Wenyuan Road, Nanjing 210046, PR China
b Key Laboratory of Organo-pharmaceutical Chemistry, Gannan Normal University, Ganzhou 341000, PR China
c National Center for Nanoscience and Technology, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 May 2012
Received in revised form 11 July 2012
Accepted 17 July 2012
Available online 22 July 2012
A mild and effective method to prepare 2,7-dibromo-4,5-diazafluoren-9-one (3) has been described
involving tandem oxidation and rearrangement reactions. Diazafluorenone-based donoreacceptor (pen)
molecules via stille coupling reactions exhibit solvent-dependent fluorescence and excellent self-
assembly behaviors at the solideliquid interface according to the characterization of scanning tunnel-
ing microscopy (STM).
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
2,7-Dibromo-4,5-diazafluoren-9-one
Oxidation and rearrangement mechanism
Self-assembly
pen (donoreacceptor)
Organic semiconductors
1. Introduction
perylene diimide, diketopyrrolopyrrole, and 1,10-phenanthroline
acting as electron-acceptor units in order to construct novel DeA
Donoreacceptor (DeA) molecules acting as organic semi-
conductors have been intensively investigated in the filed of or-
ganic and plastic electronics, such as organic light emitting diodes
(OLED),1 organic field effect transistors (OFET),2 organic photovol-
taics (OPV)3 owing to their unique charge transfer and energy
transfer behaviors. However, the devices are affected not only by
the properties of individual molecules but also the spatially in-
termolecular arrangement in the assembly.4 For example, Cho
et al.4b reported that the P3HT nanocrystals could adopt two dif-
ferent orientations-parallel and perpendicular to the insulator
substrate, the results in the field-effect mobilities differ by more
than a factor of four. In this context, a prerequisite for the de-
velopment of high-performance devices is to adjust and control the
spatial orientation and packing of molecules assemblies on the
surface.5 Therefore, it is important to investigate self-assembly
behaviors of the well-designed DeA structures.
organic molecules. As potential optoelectronic materials, diaza-
fluorene derivatives (DAFs) have been investigated in many re-
search fields, such as electron-transporting materials, organic light-
emitters, solar sensitizers, and antibodies.7 Moreover, diaza-
fluorene derivatives can smoothly form various complexes by co-
ordination to many metals. However, due to the lack of convenient
and effective synthetic routes to obtain the key intermediates,8
such as halogen-substituted DAFs, their electron-deficient proper-
ties have been rarely investigated to a large extent.9
In this work, a facile and mild method to prepare the in-
termediate-2,7-dibromo-4,5-diazafluoren-9-one has been well
developed via tandem oxidation and rearrangement reactions.
Based on that, a series of DeA molecules exhibiting solvent-
dependent fluorescence have been obtained. Furthermore, their
self-assemblies have been investigated on a highly oriented pyro-
lytic graphite (HOPG) surface.
Numerous efforts have been made to focusing on aza-aro-
matics,6 such as pyridine, tetrazine, oxadiazole, benzothiadiazole,
2. Results and discussion
Two synthetic routes have been demonstrated to synthesize 2,7-
dibromo-4,5-diazafluoren-9-one (3) as shown in Scheme 1 (route I
and route II). In the past, compound 3 was synthesized by direct
* Corresponding authors. Tel.: þ86 25 8586 6008; fax: þ86 25 8586 6999; e-mail
addresses: iamlhxie@njupt.edu.cn (L.-H. Xie), iamwhuang@njupt.edu.cn, iamdir-
ector@fudan.edu.cn (W. Huang).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.066

W.-J. Li et al. / Tetrahedron 68 (2012) 8216e8221
8217
bromination of 4,5-diazafluoren-9-one in the conditions of ex-
tremely excessive bromine and oleum with a low yield of 32%
according to the literature (route I).8 In this study, an effective and
smooth method to synthesize 3 is well developed and depicted as
route II. The target compound was prepared starting from 1,10-
phenanthroline, which suffered bromination to give 3,8-dibromo-
1,10-phenanthroline in 70% yield according to the literature.10 3,8-
Dibromo-1,10-phenanthroline was converted into the target com-
pound 3 in the mixture of H2SO4/HNO3/KBr, followed by solid so-
dium hydroxide with a moderate yield of 50%. The reaction is likely
to involve tandem oxidation rearrangement processes similarly to
our previous works,9 which has been unambiguously confirmed by
the separated intermediate, 3,8-dibromo-1,10-phenanthroline-5,6-
dione (2).
room temperature, were unambiguously characterized by X-ray
crystallographic analysis as shown in Fig. 1. Compound 3, crystal-
lizes in a triclinic form with space group Pꢀ1. The molecule is
planar as a whole and the dihedral angle between two pyridine
planes is 2.41ꢁ. Excellent co-planarity of conjugated moiety enables
the compound to be a high delocalized-electron system. In this
crystal, the molecules form two sets of parallel layers that are
linked by C3eH3/O1 intermolecular hydrogen bonds with the
ꢀ
length of 2.468 A along the c axis. Besides, Br/Br intermolecular
bonds play a significant role in the packing of the molecules. The
ꢀ
distance between Br1 and Br2 is 3.489 A, which doubles the value
of van der Waals radius of a bromine atom.12
O
a
70%
Br
Br
N
N
N
N
N
N
1
Route I:
Route II:
O
O
N
O
b
Br
Br
N
c
Fig. 1. ORTEP drawing (30% probability) showing (a) the X-ray structure of monomer 3
and (b) packing program of the monomer 3.
N
Br
Br
50%
N
3
2
Scheme 1. Two synthetic routes to compound 3. Conditions: (a) Br2/pyridine/S2Cl2/1-
chlorobutane, reflux, 24 h; (b) H2SO4/HNO3/KBr, reflux, 3 h; (c) solid sodium
hydroxide.
Fig. 2 shows UVevis spectra of compounds 5e7 in dichloro-
methane (DCM) solvent (10ꢀ5 mol/L) and spin-coated film. There
are two absorption bands in all three compounds. The high-
energy absorption bands appear in the range of 350e460 nm,
which are attributed to
charge-transfer (ICT) transition. The low-energy absorption band
in the range of 460e580 nm has small molar extinction co-
efficients, which is assigned to the symmetry-forbidden nep*
transition of the carbonyl groups in 4,5-diazafluoren-9-one units.
In contrast to compounds 5 and 7, the absorption maximum
pep* transition and the intramolecular
Subsequently, a series of DeA compounds were smoothly pre-
pared as shown in Scheme 2. The target compounds 5 and 6, were
obtained via stille reactions between 3 and the tris(stannyl) de-
rivatives in the presence of a catalytic amount of Pd(PPh3)2Cl2 with
the yields of 40% and 33%, respectively. Mono-substituted com-
pound 7 was synthesized from compound 4 in the similar condition
with a yield of 55%. In view of the toxicity of tin (II) reagents, we
initially made attempt to construct these molecules by Suzuki
cross-coupling reaction. Unfortunately, no target compounds were
obtained probably due to instability of 2-thienylboronic acid de-
rivatives,11 along with the low reactivity of 4,5-diazafluorenone
moiety.
(
lmax) of the absorption band of compound 6 is red-shifted ap-
proximately 40 nm, presumably because of the improved
p
-
conjugation by the addition of thiophene rings. The spectra
broaden and extend to 576, 598, and 622 nm in the solid state
with a peak emerging of 382, 395, and 420 nm for compounds 5,
6, and 7, respectively, indicating strong
pep interactions in their
crystalline forms.12 From the absorption edges in the solid
state, the optical energy gaps are estimated to be 2.15, 1.99, and
2.07 eV for 5, 6, and 7, respectively, which are also in close
agreement with the data obtained from electrochemical
measurements.
0.8
5
1.0
6
0.6
7
0.4
0.8
0.2
0.6
0.0
300
400
500
600
700
Wavelength (nm)
0.4
0.2
0.0
5
Scheme 2. Synthesis of 5e7. Conditions: (a) 2-tributylstannyl-5-hexylthiophene,
Pd(PPh3)2Cl2, DMF, 85 ꢁC, 48 h; (b) 5-tributylstannyl-50-hexyl-2,20-bithiophene,
Pd(PPh3)2Cl2, DMF, 110 ꢁC, 48 h; (c) 5-tributylstannyl-50-hexyl-2,20-bithiophene,
Pd(PPh3)2Cl2, DMF, 80 ꢁC, 24 h.
6
7
350 400 450 500 550 600 650 700 750
Wavelength (nm)
All the compounds were confirmed by 1H NMR, 13C NMR, X-ray
crystallography, and/or element analysis. Single crystals of mono-
mer 3, grown by slow evaporation of the chloroform solution at
Fig. 2. UVevis spectra of compounds 5e7 in solid film and solvent (M¼10ꢀ5 mol/L)
(inset).

8218
W.-J. Li et al. / Tetrahedron 68 (2012) 8216e8221
The absorption and emission spectra of compound 6 were in-
5
6
7
vestigated in different polar solvents, including toluene,
dichloromethane (DCM), tetrahydrofuran (THF), and N,N-dime-
thylformamide (DMF) (Fig. 3). The high-energy bands in the range
of 350e460 nm are somewhat dependent on solvent polarity, in-
dicating that there are very weak electronic coupling between
donor (thiophene) and acceptor (4,5-diazafluoren-9-one) moieties
in the ground state. However, it exhibits solvent-dependent fluo-
rescence, suggesting the occurrence of complicated excited state
and energy transfer process. There are two emission bands peaking
at 481 nm and 595 nm under the excitation of 414 nm in non-polar
solvent, such as toluene (Fig. 3), in which the intensity of blue
emission is equivalent to that of yellow emission. The blue and
50
100
150
200
250
O
Temperature ( C)
Fig. 4. DSC curves for compounds 5e7.
yellow emission are attributed to ICT transition and nep* state via
limited energy transfer process. Furthermore, the long wavelength
emissions vanish in polar solvents, such as DCM, THF, and DMF,
revealing that the strong quenching effect of these polar solvents
tetrahydrofuran (THF) and dichloromethane (DCM), respectively
(Fig. 5). A series of redox potentials are provided in Table 1. All the
occurs on the lowest excited state of nep* state feature of 4,5-
compounds present one quasi-reversible reduction peaks with
red
diazafluoren-9-one. In the emission spectra of compounds 5 and
7, the similarly incomplete energy transfer process and solvent-
dependent effect are also found in the above-mentioned solvents
(Supplementary data). While in the solid state, the emission spectra
of these three compounds are quenched due to aggregation.
E
onset
of ꢀ1.31 V for compound 5, ꢀ1.23 V for compound 6, and
ꢀ1.26 V for compound 7, respectively. In the oxidation region, the
oxidation waves for three compounds were observed at 1.08, 0.87,
and 1.02 V, respectively. The electrochemical gaps for compounds 5,
6, and 7 are 2.39, 2.10, and 2.28 V, respectively. The ability to adjust
the oxidation and reduction properties of these compounds leads to
the variation of electrochemical gaps from 2.39 V for compound 5
to 2.10 V for compound 6. DFT calculated highest occupied molec-
ular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energies are illustrated in Table 1. The HOMO is distributed
to both 4,5-diazafluoren-9-one core and thiophene units for these
compounds. While the LUMO is only localized on the 4,5-
diazafluoren-9-one core. The calculated ground-state singlet gaps
for compounds 5, 6, and 7 are found to be 3.01, 2.60, 2.79 eV, re-
spectively. The DFTcalculations confirmed that the HOMO energy is
increased by the addition of thiophene units owing to the in-
0.8
a)
Toluene
0.6
DCM
THF
DMF
0.4
0.2
0.0
300
400
500
600
700
creasing length of
p-conjugation. The electrochemical measure-
Wavelength (nm)
ments are also consistent with the results of the DFT calculations
that the oxidation and reduction potentials of these compounds
could be adjusted by varying the number of the thiophene units.
b)
600
400
200
0
Toluene
DCM
THF
DMF
6
0.16
5
7
0.08
0.00
-0.08
-0.16
400 450 500 550 600 650 700 750
Wavelength (nm)
Fig. 3. The UVevis (a) and PL spectra (b) of compound
(1ꢂ10ꢀ5 mol/L).
6 in different solvents
-2
-1
Potential (v) vs Fc/Fc+
0
1
2
3
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were used to investigate the thermal properties
of these compounds. All of the present molecular materials dem-
onstrate good thermal stability. The decomposition temperatures
(lost 5% weight) of three compounds are 302, 335, and 321 ꢁC, re-
spectively (Supplementary data). Fig. 4 shows the DSC plots for
compounds 5e7. All the compounds exhibit crystalline-isotropic
transitions with melting temperature (Tm) ranging from 167 to
223 ꢁC. The melting temperature of compound 7 is 203 ꢁC, much
higher than the value of compound 5 (Tm¼167 ꢁC). Tm increases as
the number of thiophene rings increases from two to four, which is
confirmed by the highest melting temperature of compound 6 with
a value of 223 ꢁC.
Fig. 5. Cyclic voltammetry of compounds 5 (black), 6 (red), and 7 (blue) measured in
0.1 M solution of TBAPF6/THF (reduction) and in 0.1 M solution of TBAPF6/DCM
(oxidation).
Table 1
Physical properties of compounds 5e7
a
Compound Eg
Eox
Ered
(V)
D
(V)
E
HOMO LUMO HOMOe
(eV) (eV) LUMO (DFT), eV
(eV) (V)
5
6
7
2.15 1.08 ꢀ1.31 2.39 ꢀ5.84
1.99 0.87 ꢀ1.23 2.10 ꢀ5.63
2.07 1.02 ꢀ1.26 2.28 ꢀ5.78
ꢀ3.45 3.01
ꢀ3.53 2.60
ꢀ3.5
2.79
The reduction and oxidation behavior of the compounds was
investigated by cyclic voltammetry (CV) in anhydrous
a
From thin film.

W.-J. Li et al. / Tetrahedron 68 (2012) 8216e8221
8219
The self-assembled monolayers (SAM) of compounds 6 and 7
were observed by scanning tunneling microscopy (STM) at the
solid/liquid interface as shown in Fig. 6. The high-resolution STM
image in Fig. 6a reveals the structural details of compound 6, which
turns out to be a striple feature, forming regular molecular rows.
Owing to the large electronic density of aromatic rings, the aro-
matic part of the molecule appears in higher contrast than the alkyl
proposed and outlined by a black parallelogram in proportion
marked in Fig. 6d. There are two different arrangement manners for
the packing of compound 7: head-to-head or tail-to-tail. The reason
for the resultant structure might be found in the relatively bulky
characters in 4,5-diazafluoren-9-one moiety. The molecules prefer
to adopt the energetically most favorable arrangement to shape the
packing pattern. The moleculeesubstrate interactions may also
play an important role in the 2D self-assembly patterns of com-
pound 7. After further analysis and careful comparison of com-
pounds 6 and 7, the packing structures are found to be distinct. The
monolayer of compounds 6 is a large uniform and highly ordered
assembly structure. The alkyl chains are interdigitated with each
other between neighboring molecular rows. The van der Waals
forces play a significant role in the formation of the molecular
monolayers. While the STM image of compounds 7 reveals that the
moleculeesubstrate interactions are probably dominant in the
formation of 2D self-assembly patterns.
chains in the image. The bright slices are attributed to p-conjugated
molecular backbones, whereas the alkyl chains are darker.13 The
slices stack in parallel with each other. The average length of the
slices is measured to be 2.6ꢃ0.2 nm in the high-resolution STM
image, which matches well with the theoretical length of
jugated backbone.
p-con-
3. Conclusions
In summary, an effective and convenient protocol to synthesize
2,7-bromo-4,5-diazafluoren-9-one (3) has been proposed, in-
volving tandem oxidation and rearrangement processes. A series of
DeA molecules based on 4.5-diazafluoren-9-one units have been
prepared via stille coupling reactions, which exhibit solvent-
dependent fluorescence and low energy bandgaps. All three DeA
compounds show good thermal stability and volatility. The results
of their 2D self-assembly behaviors reveal that compound 6 tends
to construct a highly ordered and uniform 2D structure while
compound 7 is not very legible due to the competition of the van
der Waals forces and the moleculeesubstrate interactions.
Diazafluorenone-based donoreacceptor molecules are potential
organic semiconductors for the application of solar cells, memory,
and photodetectors.
Fig. 6. (a) STM images on HOPG of 6. 17.6 nmꢂ17.6 nm, I¼223.
3 pA, and
V¼ꢀ854.6 mV; (b) Tentative molecular models and unit cell vectors for the 2D packing
of 6; (c) STM images on HOPG of 7. 18.6 nmꢂ18.6 nm, I¼221. 4 pA, and V¼ꢀ857.6 mV;
(d) Tentative molecular models and unit cell vectors for the 2D packing of 7.
4. Experimental section
4.1. General information
On the basis of molecular arrangement, a unit cell is determined
and outlined by a black parallelogram in Fig. 6a. The lattice con-
stants are a¼2.0ꢃ0.2 nm, b¼8.4ꢃ0.2 nm, and
a
¼58ꢃ2ꢁ. According
All the solvents and reagents were purchased from commercial
suppliers and used without further purification, unless noted oth-
erwise. All products were purchased by flash column chromatog-
raphy, which was carried out with Kanto Silica Gel 60 N
to the above observations, a plausible model is proposed in Fig. 6b,
giving a visual representation of adlayer structure. In this model,
the
a
p-conjugated moieties of compound 6 stack with an angle of
¼58ꢃ2ꢁ to the molecular row. The distance between neighboring
(40e63
mm). Spectrochemical-grade solvents were used for optical
molecules within one molecular row is 2.0ꢃ0.2 nm, indicating that
four thiophene rings and diazafluorenone moieties are all adsorbed
in parallel with the graphite plane. The alkyl chains (eC6H13) are
interdigitated with each other between neighboring molecular
rows.
measurements.
1,10-Phenanthroline, thiophene, 2-
bromothiophene, dichlorobis(triphenylphosphine) palladium(II),
and n-butyllithium were obtained from Aldrich Chemical Co, which
were used without further purification. Sulfuric acids, nitric acid,
bromine, sulfur monochloride, N,N-dimethylformamide, potassium
carbonate, magnesium sulfate, chloroform, and toluene were pur-
chased from Sinopharm Chemical Reagent Co., Ltd without further
purification. Tetrahydrofuran and toluene were dried over sodium
benzophenone ketyl anion radical and distilled under a dry nitro-
gen atmosphere immediately prior to use. 3,8-Dibromo-1,10-
phenanthroline,10 2-bromo-4,5-diazafluoren-9-one (4),9 2-
tributylstannyl-5-hexylthiophene, and 5-tributylstannyl-50-hexyl-
2,20-bithiophene,15 were prepared according to the corresponding
literature. 1H NMR and 13C NMR were recorded on a Bruker
400 MHz spectrometer in CDCl3-d or DMSO-d6 with tetrame-
thylsilane (TMS) as the interval standard. Mass spectra were
recorded on a Shimadzu GCeMS 2010 PLUS. Element analyses were
carried out on an Elementar Analysensysteme GmbH Vario EL III
Instrument. Absorption spectra were measured with a Shimadzu
UV-3150 spectrometer at 25 ꢁC, and emission spectra were recor-
ded on a Shimadzu RF-530XPC luminescence spectrometer upon
excitation at the absorption maxima. Differential scanning
In order to study the substituent effect, 2D self-assembly be-
haviors of compounds 7 were also taken into account. From the
high-resolution STM image (Fig. 6c), large vague arrangements can
be readily discerned among several ordered domains in this large
area due to the solvent effect.14 Besides, several regular rows con-
sist of bright sticks. Each bright stick represents a compound 7
molecule. All the sticks parallels to each other in the same row
along the lamella axis. The intermolecular distance between the
neighboring molecules along the lamella axis is w1.0 nm. The
length of the stick measured from Fig. 6c is about 1.4ꢃ0.1 nm,
which is identical to the theoretical length of p-conjugated moiety
in this molecule. Considering the length of alkyl chain (eC6H13),
they are probably interdigitated at the molecular side. A unit cell is
marked by a black parallelogram in the high-resolution STM image
(Fig. 6c), which includes four molecules from compound 7. The
parameters are measured to be a¼1.0ꢃ0.1 nm, b¼4.1ꢃ0.1 nm, and
a
¼84ꢃ2ꢁ. Based on the above observations, a plausible model is

8220
W.-J. Li et al. / Tetrahedron 68 (2012) 8216e8221
calorimetry (DSC) analyses were performed on a Shimadzu DSC-
60A Instrument at a heating rate of 10 ꢁC/min. Thermogravi-
metric analyses (TGA) were conducted on a Shimadzu DTG-60H
thermogravimetric analyzer under a heating rate of 10 ꢁC/min.
Cyclic voltammetric (CV) studies were conducted at room tem-
perature on the CHI660E system in a typical three-electrode cell
with a platinum sheet working electrode, a platinum wire counter
electrode, and a silver/silver nitrate (Ag/Agþ) reference electrode.
All electrochemical experiments were carried out under a nitrogen
atmosphere at room temperature in an electrolyte solution of 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in CH2Cl2 at
a sweeping rate of 0.1 V/s. According to the redox onset potentials
of the CV measurements, the highest occupied molecular orbital
(HOMO)/lowest unoccupied energy levels (LUMO) of the materials
are estimated based on the reference energy level of ferrocene
(4.8 eV below the vacuum): HOMO/LUMO¼ꢀ(Eonsetꢀ0.04 V)ꢀ
4.8 eV, where the value 0.04 V is for ferrocene versus Ag/Agþ. X-ray
crystallographic data for 3 were collected on a P4 Bruker diffrac-
tometer equipped with a Bruker SMART 1K CCD area detector and
mixture was poured to 300 mL water and neutralized with solid
sodium hydrate quickly. The residue was extracted with chloroform
(3ꢂ100 mL) and dried over anhydrous MgSO4. Yellow organic layer
was concentrated by rotary evaporation and washed with hexane
to give 2,7-dibromo-4,5-diazafluoren-9-one (200 mg, 0.59 mmol)
as a yellow solid in 50% yield. Mp: 257e259 ꢁC; Rf 0.54 (silica gel,
EtOAc/petroleum ether, 1:3); GCeMS (EI-m/z): 340 [Mþ]; 1H NMR
(400 MHz, CDCl3-d):
d
8.88 (d, J¼2.1 Hz, 1H), 8.11 (d, J¼2.1 Hz, 1H);
13C NMR (100 MHz, CDCl3-d):
d
186.67, 160.61, 156.28, 134.44,
130.23, 122.58; Anal. Calcd for C11H4N2Br2O: C, 38.86; H, 1.19; N,
8.24; found: C, 39.01; H, 1.27; N, 8.13.
4.1.3. 2,7-Bis(5-hexylthiophen-2-yl)-4,5-diazafluoren-9-one (5). In
a
three-necked, oven-dried 250 mL round-bottom flask, 2,7-
dibromo-4,5-diazafluoren-9-one (340 mg, 1 mmol) and 2-
tributylstannyl-5-hexylthiophene (2.5 mmol, 2.5 equiv) was dis-
solved in 120 mL of anhydrous DMF. Bis(triphenylphosphine)pal-
ladium dichloride (40 mg, 0.06 mmol) was added quickly. After
degassed for 30 min, the mixture was stirred at 80 ꢁC for 24 h under
nitrogen. The solvent was evaporated under vacuum. The residue
was dissolved in a mixture of dichloromethane (200 mL) and THF
(80 mL), and the resulting solution was washed with statured KF
solution (100 mL), water and brine, respectively, and dried over
anhydrous MgSO4. The crude product was purified by column
chromatography using dichloromethane as eluent to give a yellow
solid (206 mg, 40%). Mp: 166e169 ꢁC; Rf 0.52 (silica gel, EtOAc/
a rotating anode utilizing graphite-monochromated Mo K
a
radia-
ꢀ
tion (
l¼0.71073 A). Data processing was carried out by using the
program SAINT, while the program SADABS was utilized for the
scaling of diffraction data, the application of a decay correction and
an empirical absorption correction based on redundant reflections.
The structures were solved by using the direct-method procedure
in the Bruker SHELXL program library and refined by full-matrix
least-squares methods on F2. All non-hydrogen atoms were
treated with riding model at calculated positions, with isotropic
thermal parameters based on the carbon atom to which they are
bonded. The self-assembled monolayers (SAMs) of the compounds
petroleum ether, 1:3); 1H NMR (400 MHz, CDCl3-d):
d 8.96 (d,
J¼2.2 Hz, 1H), 8.09 (d, J¼2.1 Hz, 1H), 7.30 (d, J¼3.6 Hz, 1H), 6.83 (d,
J¼3.6 Hz, 1H), 2.86 (t, J¼7.6 Hz, 2H), 1.72 (dt, J¼7.6 Hz, 2H), 1.42 (m,
6H), 0.90 (t, J¼7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3-d):
d 189.60,
were prepared by placing a drop (ca. 3
mL) of solution (the con-
160.87, 151.55, 148.59, 136.55, 131.93, 129.90, 127.36, 125.72, 125.07,
31.55, 30.33, 28.75, 22.58, 14.09; Anal. Calcd for C31H34N2OS2: C,
72.33; H, 6.66; N, 5.44; found: C, 72.01; H, 6.70; N, 5.33; MALDI-
TOF-MS (m/z) calcd for C31H34N2OS2 [Mþ] 514.21, found 513.77.
centration is ca. 10ꢀ4 M) on a freshly cleaved atomically flat surface
of highly ordered pyrolytic graphite (HOPG, ZYB quality, Veeco
Metrology). The solvent for preparing all solutions was octylben-
zene. The samples were investigated at the liquidesolid interface. A
nanoscope IIIa scanning probe microscope (Veeco Metrology) was
employed to carry out the STM experiments using a standard
constant-current mode under ambient conditions. STM tips were
mechanically cut Pt/Ir wire (80/20). All the STM images shown
herein are presented without further processing, except flattening.
Experiments were repeated in several sessions using different tips
to check for reproducibility and to avoid artifacts. The tunneling
conditions used are given in the corresponding figure captions. The
molecular models were built with a HyperChem software package.
4.1.4. 2,7-Bis(50-hexyl-[2,20-bithiophen]-5-yl)-4,5-diazafluoren-9-
one (6). A mixture of compound 3 (340 mg, 1 mmol, 1 equiv), 5-
tributylstannyl-50-hexyl-2,20-bithiophene (1350 mg, 2.5 mmol,
2.5 equiv) and bis(triphenylphosphine)palladium dichloride
(70 mg, 0.1 mmol, 0.1 equiv) was reacted in a similar fashion to that
of preparing compound 5, except the reaction temperature was
increased to 110 ꢁC, and the reaction time was extended to 48 h. A
brown power was obtained with a yield of 33%. Mp: 222e225 ꢁC; Rf
0.51 (silica gel, EtOAc/petroleum ether, 1:3); 1H NMR (400 MHz,
CDCl3-d):
d
9.00 (d, J¼2.2 Hz, 1H), 8.12 (d, J¼2.1 Hz, 1H), 7.38 (d,
4.1.1. 3,8-Dibromo-1,10-phenanthroline-5,6-dione (2). To a mixture
of HNO3 (1.4 g/mL, 1.35 mL, 19 mmol) and H2SO4 (1.84 g/mL,
2.70 mL, 50 mmol) were added 3,8-dibromo-l,10-phenanthroline
(400 mg, 1.18 mmol) and KBr (170 mg, 1.42 mmol), the mixture
was stirred at 90 ꢁC for 3 h. After cooling to room temperature, the
mixture was poured to 300 mL water and neutralized with diluted
sodium hydroxide solution. The solution was extracted with chlo-
roform (3ꢂ100 mL) and dried over anhydrous MgSO4. After re-
moving organic solvent, a yellow power was obtained in 70% yield.
Mp: 308e312 ꢁC; Rf 0.46 (silica gel, EtOAc/petroleum ether, 1:3);
J¼3.8 Hz, 1H), 7.13 (d, J¼3.8 Hz, 1H), 7.07 (d, J¼3.5 Hz, 1H), 6.72 (d,
J¼3.5 Hz, 1H), 2.81 (t, J¼7.7 Hz, 2H), 1.67 (dt, J¼7.7 Hz, 2H),
1.40e1.30 (m, 6H), 0.90 (t, J¼6.9 Hz, 3H); 13C NMR (100 MHz, CDCl3-
d):
d 189.25, 160.94, 151.48, 146.65, 139.92, 136.88, 133.87, 131.42,
129.94, 127.24, 125.99, 125.05, 124.16, 31.56, 31.54, 30.22, 28.76,
22.58, 14.08; Anal. Calcd for C39H38N2OS4: C, 68.99; H, 5.64; N, 4.13;
found: C, 69.12; H, 5.83; N, 4.02; MALDI-TOF-MS (m/z) calcd for
C39H38N2OS4 [Mþ] 678.19, found 678.12.
4.1.5. 2-(50-Hexyl-[2,20-bithiophen]-5-yl)-4,5-diazafluoren-9-one
(7). A mixture of compound 4 (261 mg, 1 mmol, 1 equiv), 5-tribu-
tylstannyl-50-hexyl-2,20-bithiophene (650 mg, 1.2 mmol, 1.2 equiv)
and bis(triphenylphosphine)palladium dichloride (0.1 mmol,
0.1 equiv, 70 mg) was reacted in a similar fashion to that of pre-
paring compound 5, except the reaction temperature was increased
to 85 ꢁC, and the reaction time was extended to 48 h. A red power
was obtained in 55% yield. Mp: 201e204 ꢁC; Rf 0.49; 1H NMR
GCeMS (EI-m/z): 366 [Mþ]; 1H NMR (400 MHz, CDCl3-d):
d 8.61 (d,
J¼2.1 Hz, 1H), 9.14 (d, J¼2.1 Hz, 1H); 13C NMR (100 MHz, CDCl3-d):
d
177.13, 157.60, 150.47, 139.53, 128.44, 123.58; Anal. Calcd for
C12H4Br2N2O2: C, 39.17, H, 1.10, N, 7.61; found: C, 39.21; H, 1.29; N,
7.52.
4.1.2. 2,7-Dibromo-4,5-diazafluoren-9-one (3). To a blend solution
of HNO3 (1.4 g/mL, 1.35 mL, 19 mmol) and H2SO4 (1.84 g/mL,
2.70 mL, 50 mmol) was added 3,8-dibromo-l,10-phenanthroline
(400 mg, 1.18 mmol) and KBr (170 mg, 1.42 mmol), the mixture
was stirred at 90 ꢁC for 3 h. After cooling to room temperature, the
(400 MHz, CDCl3-d):
d
9.01 (d, J¼2.1 Hz, 1H), 8.80 (d, J¼4.7 Hz, 1H),
8.13 (d, J¼2.2 Hz, 1H), 8.00 (d, J¼7.6 Hz, 1H), 7.41e7.30 (m, 2H), 7.13
(d, J¼3.8 Hz, 1H), 7.07 (d, J¼3.6 Hz, 1H), 6.72 (d, J¼3.5 Hz, 1H), 2.81
(t, J¼7.6 Hz, 2H), 1.69 (dt, J¼7.6 Hz, 2H), 1.53e1.19 (m, 6H), 0.90 (t,

W.-J. Li et al. / Tetrahedron 68 (2012) 8216e8221
8221
J¼6.7 Hz, 3H); 13C NMR (100 MHz, CDCl3-d):
d
189.35, 163.25,
References and notes
161.07, 155.29, 151.28, 146.58, 139.86, 136.76, 133.83, 131.53, 129.60,
127.14, 126.00, 125.04, 124.30, 124.01, 31.55, 30.20, 28.77, 22.59,
14.10; Anal. Calcd for C25H22N2OS2: C, 69.73; H, 5.15; N, 6.51; found:
C, 69.93; H, 5.25; N, 6.17; MALDI-TOF-MS (m/z) calcd for
C25H22N2OS2 [Mþ] 430.12, found 430.42.
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C
11H4Br2N2O, M¼339.98, triclinic, space group Pꢀ1, Z¼2,
a¼5.918(4) A, b¼8.445(6) A, c¼68.687(7) A,
a
ꢀ
¼68.687(7) (7)ꢁ,
ꢀ ꢀ ꢀ
3
b
¼80.926(7)ꢁ,
g
¼83.203(9)ꢁ, V¼527.7(6) A , F(000)¼324,
Dc¼2.140 Mg/m3,
m
(Mo K
a
)¼7.656 mmꢀ1. Reflection collected/
2383, Goodness of fit on F2(s)/1.062, Final R indices (I>2
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R1¼0.0459, wR2¼0.1270,
R
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Acknowledgements
This work was financially supported by the National Key Basic
Research and Development Program of China (Grant No.
2009CB930601 and 2010CB327701), National Natural Science
Foundation of China (Grant Nos. 21144004, 60876010, 61136003,
20804020, 60976019, and 20974046, 21003076), Natural Science
Foundation of Jiangsu Province (Grant No. SBK201122680,
BK2008053, 11KJB510017, BK2009025, JHB09-36), The Program for
New Century Excellent Talents in University (NCET-11-0992),
NJUPT (NY210030, NY211022) and Jiangxi Province Education De-
partment Youth Science Fund (GJJ12578).
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1H NMR, 13C NMR, MALDI-TOF MS spectral data for all corre-
sponding products are provided. Supplementary data related to
this article can be found online at http://dx.doi.org/10.1016/
j.tet.2012.07.066.