The synthesis and photoelectric study of 6,13-bis(4-propylphenyl)pentacene, and its TiO2 nano-sized composite films

Article abstract of DOI:10.1016/j.jpcs.2009.12.080

The synthesized 6,13-bis(4-propylphenyl)pentacene(BPP) is much more soluble and stable than pentacene. The introduction of 4-propylphenyl group leads the solubility to 7.33 mg/mL in CHCl₃. In cyclic voltammetry (CV) test, it was found that its forbidden band gap is 1.88 eV. Moreover, a series of BPP-TiO₂ composites were constructed. Characterizations proved that an interaction between BPP and TiO₂ has probably occurred, leading to the generation of some novel properties. The most interesting result is that the forbidden band gaps of the composite materials are much smaller than that of BPP and TiO₂.

Full text of DOI:10.1016/j.jpcs.2009.12.080

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Journal of Physics and Chemistry of Solids 71 (2010) 296–302
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
The synthesis and photoelectric study of 6,13-bis(4-propylphenyl)pentacene,
and its TiO2 nano-sized composite films
Zhong Huang a, Yuansheng Jang c, Xiuying Yang b, Weiliang Cao a,b, Jingchang Zhang a,b,n
a Institute of Modern Catalysis, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
b Hainan Institute of Science and Technology, Haikou 571126, China
c School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesized 6,13-bis(4-propylphenyl)pentacene(BPP) is much more soluble and stable than
pentacene. The introduction of 4-propylphenyl group leads the solubility to 7.33 mg/mL in CHCl3. In
cyclic voltammetry (CV) test, it was found that its forbidden band gap is 1.88 eV. Moreover, a series of
BPP–TiO2 composites were constructed. Characterizations proved that an interaction between BPP and
TiO2 has probably occurred, leading to the generation of some novel properties. The most interesting
result is that the forbidden band gaps of the composite materials are much smaller than that of BPP and
TiO2.
Received 17 July 2009
Received in revised form
21 December 2009
Accepted 22 December 2009
Keywords:
A. Electronic materials
B. Chemical synthesis
D. Electrical properties
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
found that after the composite of BPP with TiO2, the composite
BPP–TiO2 materials have different optical and electrical perfor-
mance. The most interesting result is that the forbidden band gaps
of the composite materials are much smaller than those of both
BPP and TiO2, and these materials could overcome disadvantages
of the poor conductivity at reduction potential of BPP and the poor
conductivity at oxidation region of nano-sized TiO2.
Pentacene as one of the most important candidate for organic
semiconductor in electronic devices has been paid much attention
in the past decades [1–3], but its poor solubility and stability limit
its large-scale application. Actually, some efforts have been made
to overcome these demerits, leading to many new pentacene
derivatives [4–12]. However, most of these researches are only
focused on pentacenes themselves, and these pentacenes as p-
type semiconductors cannot overcome the poor conductivity in
reduction potential.
2. Experiment
2.1. Apparatus for measurements
Organic–inorganic composite has attracted continuous atten-
tion because of its remarkable properties [13]. One of these
composites is nano-scaled core-shell materials consisting of oxide
particles and a functional organic shell. The material may offer a
great potential for improving the novel electrical and optical
properties. It was expected that when pentacenes coat nano-
materials, the charge distribution of their atoms, the molecular
stacking, the lowest unoccupied molecular orbital (LUMO) level,
the highest occupied orbital (HOMO) level, and forbidden band
gap could be changed.
1H NMR spectra were recorded with Bruker, AV 600 NMR
spectrometer in CDCl3. Elemental analysis was conducted on a
Vario EL elemental analyzer. Infrared spectra were recorded by a
prostige-21IR spectrophotometer in KBr flake. Cyclic voltammetry
(CV) was performed on an IM6e Electrochemical Station. UV–vis
absorption spectra and experiment on stability were measured on
a
UV-2501PCS double spectrophotometer. The fluorescence
spectra (CHCl3 solution) were obtained with a Shimadzu RF-
5301PC spectrophotometer. Melting point was measured on X-4
binocular melting point measurement apparatus. X-ray photo-
electron spectroscopy (XPS) was characterized on ESCALAB 250.
In this paper, we would like to report a new synthesized 6,13-
bis(4-propylphenyl)pentacene (BPP) and its composites with
nano-sized TiO2 on the basis of our former works [9]. BPP has
much better solubility and stability than pentacene. It was indeed
2.2. Synthesis of BPP
n Corresponding author. Tel./fax: +86 01 64434904.
Fig. 1 shows the synthetic procedure. A solution of 4-propyl-1-
bromobenzene (5 ml, 32 mmol) in THF (7 mL) was added to the
E-mail address: zhangjc1@mail.buct.edu.cn (J. Zhang).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2009.12.080

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Fig. 1. Synthesis of 6, 13-bis(propylphenyl)pentacene.
nitrogen-purged mixture of dried Mg powder (0.77 g, 32.1 mmol)
Table 1
Solubility of BPP and pentacene (20 1C).
in THF (5 mL) at room temperature to synthesize the Grignard
reagent. The reaction system was stirred for 30 min. The
suspension of 6,13-pentacenequinone (1.0 g, 3.2 mmol) in THF
(20 mL) was added slowly on ice bath, and the system was then
heated to 60 1C for 1 h. After cooling to room temperature, a
mixture of 20% HCl (30 mL) saturated with SnCl2 was added very
carefully. The reaction was heated at 60 1C for 2 h, followed by
cooling to room temperature. The mixture was filtered, and the
purple cake as crude product was washed with water (5 mL Â 3),
ethanol (5 mL Â 3) and diethyl ether (5 mL Â 3), respectively. After
drying in vacuum, the crude product was purified by column
chromatography using petroleum ether/CHCl3 (20:1) as eluent
and silica gel as sorbent. The solvents were removed in vacuo and
the purple-red powder product yielded 44.3%. During the
isolation process, light was avoided.
Compounds
Solubility (mg mLÀ1
)
CHCl3
THF
DMF
Toluene
Pentacene
BPP
0.21
7.33
0.11
2.25
0.10
0.52
0.18
5.00
3. Results and discussion
3.1. Configuration characterizations of BPP
The characteristic signals of 1H NMR spectra of as-obtained
BPP are displayed as follows (in ppm unit): 1.14 (t, 6H), 1.90 (m,
4H), 2.86 (t, 4H), 7.22(m, 4H), 7.52(m, 8H), 7.73(d, 4H), 8.34(s,
4H). FT-IR (cmÀ1): 3042, 2957, 2925, 2857, 1460, 876, 825, 802,
743. The result of elemental analysis is that the contents are
93.40% (C) and 6.54% (H), very approximate to the theoretical
values of 93.34% (C) and 6.66% (H). M.P.: 185.7–187.7 1C
(decomposition).
2.3. Preparation of BPP–TiO2 nano-composite
BPP (0.063 mmol) in CHCl3 (2 mL), and nano-sized TiO2 (about
8 nm, 1–10 equiv.) were added into a sealed flask. The system was
processed by ultrasound irradiation for 3 h, and CHCl3 was then
instantly removed by vacuum to give BPP–TiO2 nano-composite.
Noticeably, light must be avoided in the process.
3.2. Solubility of BPP
Table 1 shows the solubilities of BPP and pentacene [9] in some
common organic solvents. The solubility of BPP was largely
enhanced after the introduction of 4-propylphenyl group, which
is caused by the fact that 4-propylphenyl group largely enhances
steric hindrance and reduces the strong force between pentacene
backbones. However, BPP still has the same soluble trend with
that of pentacene.
2.4. Preparation of BPP and composite films
After ultrasound process, the suspension was evenly dropped
to ITO and quartz glass instantly. After CHCl3 was evaporated, two
films were prepared. The film on ITO was used to get cyclic
voltammetry, and the other was used for UV–vis spectra, and
fluorescence spectra. BPP film was prepared by the same way
except ultrasound process.
3.3. UV–vis and fluorescence spectra of BPP
Fig. 2 shows three absorption peaks at 518, 557, and 602 nm of
BPP in CHCl3, and the absorption becomes stronger in order. The
molar absorptivities at the three wavelengths are 512, 1128, and
1568 MÀ1 cmÀ1, which show its good absorption. Its maximum
absorption wavelength (lmax) is 602 nm, which only has a red shift
of 2 nm compared with 6,13-diphenylpentacene (DPP) [9]. The
reason is that the electron-donating ability of propyl group is so
weak that its effect becomes very limited. However, BPP has a large
2.5. Electrochemical measurements by cyclic voltammetry
Cyclic voltammetry (CV) was performed on an IM6e Electro-
chemical Station (ZAHNER ELEKTRIK), with a scanning rate of
150 mV/s. The CV studies of pentacenes were carried out in
acetonitrile with 0.1 M LiClO4, using Pt wires and 0.01 M Ag/
AgNO3 in acetonitrile as electrodes. The values are expressed in
potentials versus Fc/Fc+. All solutions were insufflated with N2 for
30 min.
red shift of 25 nm compared with pentacene (
can be explained by the increased delocalization of
the introduction of 4-propylphenyl group. According to the onset
absorption edge in visible light, the optical forbidden band gap (Eg1
of BPP is calculated to be 2.00 eV, smaller than 2.02 eV of DPP [9].
Fluorescence spectrum (Fig. 2) shows the emitting peak of BPP is at
621 nm, with a Stokes shift of 19 nm.
l
max=577 nm), which
p
electrons after
)

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3.4. Research on stability of BPP
the electron density of pentacene backbone through the p–p
conjugation.
Fig. 3 records the IR spectra of BPP exposure to the ambient
light in air. The result shows that as the exposure time passed, no
obvious change of BPP was observed. Therefore, BPP is very stable
to ambient light and air.
3.5. XRD of BPP–TiO2 nano-composites
Fig. 5 shows XRD spectra of BPP, BPP–TiO2 composites and
nano-sized TiO2. BPP has good crystal form, with main
In order to get information about BPP’s stability, our group
a
adopted
a very detailed measurement. Four decomposition
diffraction peaks at 5–301. Sharp diffraction peaks appear at
18.441, 21.041, and 26.321. After coating on nano-sized TiO2, the
crystal form of BPP slowly disappears as the mol ratio becomes
smaller, such as its characteristic peaks at 18.441 and 21.041.
Instead of these peaks, a broad peak at 15–251 appears, which
derives from the destroying of BPP molecular stacking after
coating nano-TiO2. At the mol ratio of 1:5, BPP characteristics fully
disappear. The crystal form of TiO2 is not obviously changed after
processes (a, b, c, d) of BPP in CHCl3 are shown in Fig. 4. Under
condition a, the half life of BPP is 13 min, and under condition b,
the half life is 34 min. However, under conditions c and d, the
color of solution could be retained for weeks. At last, three results
can be concluded from the processes as follows: (1) The new
pentacene is sensitive to light in solution. (2) It is more sensitive
to light than to oxygen, and its decomposition is mainly caused by
light. (3) When combined with light, oxygen can accelerate its
decomposition, but the effect of oxygen is very limited without
light. BPP is much more stable than pentacene (about 3 min of
half life at the same condition). The factor may be that (1) BPP has
larger steric hindrance in 6,13 positions, and molecules of BPP are
more difficult to be attacked by O2. (2) The substituent decreases
being coated by BPP, but 2y of all peaks increases by one degree,
leading to a smaller interplanar spacing of TiO2. The transfer of
electrons from BPP to TiO2 may give an explanation. After
acceptance of
while O with a good electric affinity can attract
p
electrons, the electric density of Ti increases,
electrons from
p
Fig. 2. (a)UV–vis spectra and (b) fluorescence spectra (
lex=532 nm) of BPP in
Fig. 4. Different oxidative processes of the BPP–CHCl3 solution under different
CHCl3.
conditions.
Fig. 3. IR spectra of BPP (solid states) exposure to ambient and strong light.

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Fig. 5. XRD spectra of BPP, BPP–TiO2, and TiO2.
Fig. 6. XPS spectra of: (a) Ti2p; (b) C1s and (c) O1s.

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BPP through Ti. As a result, the Ti–O bond is strengthened, the
bond length and interplanar spacing are shortened. XPS can
further define the interaction.
Therefore,
p electrons of BPP are inclined to transfer to TiO2, and a
p-d interaction may happen.
3.6. XPS of BPP–TiO2 nano-composites
3.7. IR spectra
Fig.
6
gives comparative XPS of Ti, C, and
O
without
Fig. 7 gives IR spectra of BPP, BPP–TiO2 nano–composites, and
TiO2. Three peaks at 700–800 cmÀ1, belonging to the out-of-plane
bending of C–H in C=C and the characteristic peaks of aromatic
rings, disappear gradually, and peaks at the region fully disappear
at a mol ratio of 1:5. The disappearance indicates the complete
composition of BPP molecules and nano-sized TiO2, and that the
coating of BPP at nano-sized TiO2 hinders these vibrations.
Additionally, peak at 1277 cmÀ1 belonging to the in-plane
bending vibration of C–H bond of pentacene framework sharply
strengthens, while peak at 876 cmÀ1 belonging to out-of-plane
bending of C–H suddenly weakens. These obvious changes
confirm that the interaction existed between BPP and nano-
sized TiO2.
adjustment. The binding energy of Ti decreases from 463.40 to
460.10 eV, and that of O decreases from 534.85 to 531.35 eV. The
result shows that the two elements accept electrons after
composition to give decreased valence states. Inversely, the
binding energy of C of BPP increases from 286.95 to 287.20 eV,
which shows C lost electrons to result in increased valence state.
Another important information is that shoulder peak at 533.49 eV
for O XPS appears after composition, which indicates the splitting
valence for O, and not all of oxygen atoms have the same
electrons. It is well known that titanium (IV) with 5 empty orbits
is a strong Lewis acid, and easy to accept electrons, while BPP is
rich in delocalized
p
p electrons, and easy to donor electrons.
Fig. 7. IR spectra of BPP, BPP–TiO2 and TiO2.
Fig. 8. UV–vis absorption spectra of BPP, BPP–TiO2 and TiO2.

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3.8. UV–vis absorption and fluorescence spectra of BPP–TiO2 nano-
composites film
easily. Additionally, the ratio of the gross charge at the reduction
potentials and the gross charge at the oxidation potentials (Qred
/
Qox) became larger, and the characteristics for n semiconductors
were more obvious as the mol ratio of BPP and nano-TiO2 was
smaller. It can be concluded that these new composites have the
mixed property of n and p semiconductors.
Fig. 8 gives UV–vis spectra of BPP, BPP–TiO2 nano-composites,
and nano-sized TiO2. The ability of absorbing UV–vis light was
enhanced after composition, especially at the region under
450 nm. Peaks at 610, 562, and 520 nm of BPP had shifts of 2–
5 nm. The phenomenon may be caused by
p-d coordination
4. Conclusions
interaction between BPP and nano-sized TiO2. Fluorescence
spectra (Fig. 9) show that on the influence of nano-sized TiO2,
the main emitting peak at 607 nm changed little, but new
emitting peaks at 634 and 666 nm appeared. The conclusion can
also confirm the interaction between BPP and nano-sized TiO2
(Fig. 10).
(1) The new pentacene in CHCl3 has a maximum absorption
peak at 602 nm, and a maximum emitting peak at 621 nm. (2)
XRD shows that BPP has a good crystal form. Owing to the good
stacking, its forbidden band gap is 1.88 eV. (3) BPP is much more
Table 2
3.9. CV of BPP–TiO2 nano-composites film
Electrochemical data for BPP, BPP–TiO2, and TiO2.
a
onset
ox
a onset
E
BPP/TiO2 (in mol
ratio)
E
bEHOMO
bELUMO
cEg2
Qred
Qox
/
red
Table 2 exhibits CV data of BPP, BPP–TiO2 nano–composites,
(V)
(eV)
(V)
(eV)
(eV)
and nano-sized TiO2. From the onset oxidation potential (Eoonxset
)
and the onset reduction potential (Eorendset), the electric forbidden
band gap (Eg2) of BPP is 1.88 eV. Eg2 for composites were
calculated to be 1.14–1.35 eV, much smaller than those of BPP
and TiO2. The change of Eorendset further confirms the interaction
BPP
0.40
0.42
0.47
0.35
0.32
0.47
2.14
À5.11
À5.13
À5.18
À5.06
À5.03
À5.18
À6.85
À1.44
À0.72
À0.75
À0.88
À1.02
À0.88
À0.94
À3.27
À3.99
À3.96
À3.83
À3.69
À3.83
À3.77
1.88
1.14
1.22
1.23
1.34
1.35
3.08
–
1:1
0.152
0.227
0.478
0.585
0.686
–
1:2.5
1:5
1:7.5
1:10
TiO2
between BPP and nano-sized TiO2. However, no obvious change
onset
ox
for E
is found. The reason may be that the difference between
HOMO levels of BPP and nano-TiO2 is too big to match (Fig. 11),
but their LUMO levels are so approximate that the match happens
a
vs. 0.01 M Ag+/Ag in CH3CN.
onset
ox
onset
red
E
and E
b
onset
red
EHOMO and ELUMO calculated by EHOMO= À(Eonset +4.71) eV; ELUMO= À(E
ox
+4.71) eV.
c
Eg2 calculated by Eg2=Eoonxset ÀEroendset
.
Fig. 9. Fluorescence spectra of BPP (excited by 490 nm), BPP–TiO2 (in mol ratio of
Fig. 11. Energy levels of BPP, BPP–TiO2 (1:5 in mol ratio), and TiO2.
1:5, excited by 490 nm) and TiO2 (excited by 360 nm).
Fig. 10. CV of: (a) BPP and (b) BPP–TiO2 (in mol ratio of 1:5).

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soluble and stable than pentacene, and particularly stable in solid
state. The property provides the basis for its easy use in preparing
composites and electronic devices. (4) BPP–TiO2 nano-composites
are new materials with smaller forbidden band gaps than those of
BPP and nano-sized TiO2, which may be derived from the possible
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