Cruciform p-n diblock conjugated oligomers for electroluminescent applications

Article abstract of DOI:10.1039/b517067b

The optoelectronic properties of the cruciform p-n diblock oligomers 1-3 can be independently tuned by the oligofluorene and oxadiazole branches to increase the charge mobility. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b517067b

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Cruciform p–n diblock conjugated oligomers for electroluminescent
applicationsw
Hong-Yu Wang, Jia-Chun Feng, Gui-An Wen, Hong-Ji Jiang, Jun-Hua Wan,
Rui Zhu, Chuan-Ming Wang, Wei Wei and Wei Huang*
Received (in Montpellier, France) 1st December 2005, Accepted 3rd March 2006
First published as an Advance Article on the web 20th March 2006
DOI: 10.1039/b517067b
The optoelectronic properties of the cruciform p–n diblock
oligomers 1–3 can be independently tuned by the oligofluorene
and oxadiazole branches to increase the charge mobility.
the di(1,3,4-oxadiazole) phenylene branch, which is p-defi-
cient, acts as an electron-transporting channel. Variation of
the length of oligofluorenes affords the possibility of tuning
p-Conjugated organic molecules have attracted much inter-
est because of their applications in organic light-emittig diodes
(OLEDs), organic field-effect transistors (OFETs), photovol-
taics, and nonlinear optics, etc.1,2 Most of these applications
would benefit from a full understanding of charge transport
properties, which depend on the chemical and electronic
structures of the p-conjugated systems. So far, the most
intensely studied molecules are linear conjugated p-systems,
in which the molecule is composed of a p-dopable block (e.g.
an oligothiophene unit) and an n-dopable block (e.g. an
aromatic oxadiazole moiety).3 However, the n-dopable block
inserted into the p-type polymer mainchain will partially act as
a hole-blocking unit due to its high electron deficiency. Vice
versa, the hole-transporting unit will lower the electron mobi-
lity. Therefore, it is difficult to further improve the charge
injecting and transporting properties.4
Compared to linear conjugated molecules, the energy levels
of the HOMO and LUMO of most cruciform p-systems are
spatially separated and the independent properties of each
branch mostly maintained.5,6 These unique properties are
crucial for molecular electronic applications, e.g., adding
donor and/or acceptor substituents to cross-shaped chromo-
phores at suitable positions can lead to independent electronic
shifts in the LUMO and HOMO.6 However, alternative modes
of p-electron delocalization, in particular cross conjugation,
are much less frequently encountered, and the nature of cross-
conjugated p-delocalization is thus far less understood.7 In
2003, first functionalized cruciform p-system was developed by
Nuckolls et al. to test the electronic properties of the film.8
More recently, Bunz and co-workers reported a series of
cruciform p-systems dealing with hybrid phenylene-ethyny-
lene/phenylene-vinylene oligomers.5,6,9
Herein we report a new series of functionalized cruciform
p–n diblock oligomers (see Scheme 1) for electroluminescent
applications. In the cruciforms, the oligofluorene branch,
which is p-excessive, acts as a hole-transporting channel, while
Institute of Advanced Materials (IAM) Fudan University, 220
Handan Road, Shanghai, 200433, P. R. China. E-mail:
iamdirector@fudan.edu.cn; Fax: þ86 (21) 6565 5123; Tel: þ86 (21)
5566 4188
w Electronic supplementary information (ESI) available: Detailed
synthetic procedures. See DOI: 10.1039/b517067b
Scheme 1 Synthetic routes of the oligomers.
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Table 2 Electrochemical properties of the oligomers
EHOMO ELUMO Energy
c/eV
Oligomer Ered/onseta/V Eox/onsetb/V c/eV
gap/eV
1
2
3
ꢁ2.06
ꢁ2.03
ꢁ2.08
1.09
0.90
0.84
5.79
5.60
5.54
2.64
2.67
2.62
3.15
2.93
2.92
a
Reduction potentials measured by cyclic voltammetry in THF
(1 ꢂ 10ꢁ3 mol Lꢁ1). Oxidation potentials measured by cyclic vol-
b
tammetry in CH2Cl2 (1 ꢂ 10ꢁ3 mol Lꢁ1). Estimated from the onset
c
oxidation and reduction potential (vs. Ag/Ag1) plus 4.7.
diazoles. The decomposition temperatures of 5% weight loss
in nitrogen were determined to be above 400 1C according to
TGA experiments.
Fig. 1 Absorption and emission spectra of the oligomers in dilute
cyclohexane solution.
Detailed data for the cyclic voltammograms are listed in
Table 2. Most of the oligomers possess two anodic and
cathodic peaks, which can be attributed to two different blocks
of the oligomers. When scanning cathodically, the oligomers
show reversible reduction peaks around ꢁ2.0 V, which may be
due to the same oxadiazole segment in these oligomers.
However, the reduction potential of the oligomers show little
dependence on the increase of fluorene number in the oligo-
fluorene branch. This implies that the reduction properties of
the oligomers are dominated by the oxadiazole branch. On
sweeping anodically, unlike their reduction peaks, the oxida-
tion potentials of the oligomers are sensitive to the variation in
the fluorene number in the oligofluorene branch. The onset
potential for oxidation is reduced from 1.09 to 0.84 V, with
increasing fluorene number in the fluorene branch from oli-
gomer 1 to 3. It is demonstrated that the HOMO level of the
oligomers can be effectively adjusted by changing the fluorene
number in the oligofluorene branch.10 It is difficult to confirm
whether the two crossed p-systems are electronically coupled
or only topologically cross-conjugated. But it is acceptable to
say that attachement of the oxadiazole branch significantly
modulates the electronic properties of the oligofluorene.
In order to thoroughly understand the optical (i.e., band
gap, solvatochromicity) and the redox properties of the oligo-
mers, quantum chemical calculations were performed. The
geometries of all the molecules were optimized using the
Gaussian 03 program12 at the B3LYP/3-21G* level, and the
alkyl at the C9 position of the fluorene group was substituted
by an H atom due to its minor influence on the electrical and
optical properties. The HOMO is localized on the oligofluor-
ene branch of the cruciform, while the LUMO is localized on
the oxadiazole part (Fig. 2). HO and LU orbitals overlap only
in the central benzene ring. This result is also consistent with
redox behavior (related to the energy levels of the HOMO and
LUMO of the oligomers) and the emissive color of the
resultant oligomers.10
It is noticed that the oligomers exhibit two absorption peaks
in solution, corresponding to the oxadiazole branch and the
oligofluorene branch, respectively (Fig. 1a). The identical
absorption maximum of the oxadiazole segment of the oligo-
mers (ca. 280 nm) suggests that the electronic interactions
between the oxadiazole branch and the oligofluorene branch
are quite limited, which may be due to the twist of the two
branches.11 The longer wavelength peak in the region of
310–355 nm is attributed to the electron transition of p–p*
along the conjugated backbone of the oligofluorene. The other
predominant feature of the absorption spectra of the oligo-
mers is a steadily increasing molar absorption as the number
of fluorene monomer units is increased and the maximum
absorption peaks shift to the long wavelength region, appear-
ing at 310 nm for oligomer 1, 333 nm for oligomer 2 and 355
nm for oligomer 3. A strong solvatochromism was also
observed on the absorption spectra. In dilute chloroform, only
one absorption peak was observed and the maximum absorp-
tion of the oligomers shifted to longer wavelength. Compared
to the absorption, the oligomers only show an emission
maximum appearing at 415 nm for oligomer 1, and at 429
nm for oligomers 2 and 3, respectively. This indicates the
existence of efficient energy transfer from the oxadiazole
segment to the oligofluorene branch.6,11 The physical proper-
ties of the oligomers are presented in Table 1. It is noted that
all of the cruciforms are highly efficiently fluorescent (0.55 o
F o 0.75) in cyclohexane, while the emission quantum yields
are lower in chloroform. The thermal stability of the oliogmers
has substantially improved through the introduction of oxa-
Table 1 Physical properties of cruciform p–n diblock oligomers
Td/1C labs,sola/nm labs,filma/nm lPL,solc/nm lPL,filmc/nm Band gapb/eV FWHMd/nm (FWHM/eV) Ff,sol
e
Oligomer Mw
1
2
3
a
1480.3 418.2
2482.0 408.4
3483.6 404.1
310 (280)
333 (280)
355 (280)
316
342
356
415
429
429
429
441
446
3.26
3.07
3.05
70 (17.76)
57 (21.81)
67 (18.55)
0.55
0.75
0.70
Absorption maxima in solution (cyclohexane, 1 ꢂ 10ꢁ6 mol Lꢁ1) and in films on quartz substrates. Optical band gap derived from the onset of
b
UV-Vis absorption spectra of oligomer solutions. Emission maxima in solution (cyclohexane, 1 ꢂ 10ꢁ6 mol Lꢁ1) and in films on quartz
c
d
substrates; the excitation wavelength corresponds to the absorption maximum. Full width at half-maximum of the film PL spectra.
e
9,10-Diphenylanthrecene standard (FPL = 0.95 in cyclohexane).
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Fig. 3 Current–voltage and luminescence–voltage characteristics of
the ITO/PEDOT:PSS/oligomer 3/Ca/Al device. Inset is the EL spec-
trum of the OLED device from 3.
Fig. 2 HOMO and LUMO electron densities of oligomer 3 (alkyl
groups omitted). Top: HOMO of oligomer 3. Bottom: LUMO of
oligomer 3. The HOMO is almost completely localized on the oligo-
fluorenes branch of the cruciform, while the LUMO is almost fully
localized on the oxadiazole substructure.
and higher maximum brightness are due to better charge
injection and transport and efficient charge recombination,13
which may be due to the two independent channels in the
cruciform. The oligofluorene branch acts as a hole transport-
ing channel, while the di(1,3,4-oxadiazole)phenylene branch
acts as an electron-transport channel. The conventional fluor-
ene-based conjugated polymers containing oxadiazole pen-
dants such as polymer behave considerably more like
poly(fluorene-alt-phenyl). However, if the oxadiazole groups
and oligofluorenes laterally attach to the benzene ring, the
electronic properties of the resulting oligomer 3 are different
from both oligofluorenes and oxadiazoles. The independent
properties of both branches are mostly maintained.6 This
result suggests that the cross-shaped p–n diblock structure
can more efficiently increase charge mobility.
the electrochemical analysis. The excited states of the oligo-
mers must show charge separation, which explains the sensi-
tivity of their emission wavelength towards the polarity of the
solvent. An increasingly polar solvent stabilizes the excited
state and leads to a bathochromically shifted emission.6
To evalutate our cross-shaped materials, a device of oligo-
mer 3 has been fabricated with the configuration ITO/ PED-
OT:PSS/oligomer 3/Ca/Al and its electroluminescence (EL)
behavior was compared with a device of fluorene-based con-
jugated polymers containing oxadiazole pendants (ITO/PED-
OT:PSS/Polymer/Ca/Al) (The chemical formula of the
polymer is depicted in Scheme 2).13 The EL data of the OLED
devices are listed in Fig. 3 and Table 3. The maximum
luminance of the oligomer 3 device is 2622 cd mꢁ2 at 16.5 V
and its luminance efficiency is 1.6 cd Aꢁ1, which is much
higher than that of the polymer. The lower turn-on voltage
In summary, these results together show that the optoelec-
tronic properties of the cruciform oligomers 1–3 can be
independently tuned by the oligofluorene and oxadiazole
branches. The cross-shaped p–n diblock structure can more
efficiently increase the charge mobility. These monomers can
be used in the synthesis of diverse sets of cross-conjugated
fluorene–oxadiazole derivatives with fine-tuned optical/elec-
tronic properties. Further studies of these materials are con-
tinuing to discern to what extent cross-conjugation exists as an
electronic coupling of the oxadiazole backbone to the oligo-
fluorene arm beyond the topological aspect.
Experimental
Materials
Chemicals were reagent grades and purchased from Aldrich,
Acros, and Lancaster Chemical Co. Toluene and tetrahydro-
furan (THF) were distilled to keep them anhydrous before use.
Table 3 EL data of the OLED devices
Lmax/cd mꢁ2
a
Von /V
Sample
lmax,EL/nm
Vonb/V
3
Polymer
456
452
5.5
7.5
6.5
7.5
b
2622
126
a
Von is the turn-on voltage for the current. Von is the turn-on
voltage for light.
Scheme 2 Chemical formula of the polymer
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Instruments
were synthsized by palladium-catalyzed Suzuki cross-coupling
reactions.
The NMR spectra were collected on a Varian Mercury Plus
400 spectrometer with tetramethylsilane as the internal stan-
dard. Molecular masses were determined by laser desorption/
ionization time-of-flight mass spectrometry (MALDI-TOF/
MASS). UV-Vis spectra were recorded on a Shimadzu 3150
PC spectrophotometer. The concentration of the oligomer
solution was adjusted to B0.001 mmol Lꢁ1. Fluorescence
measurement was carried out on a Shimadzu RF-5301 PC
spectrofluorophotometer with a xenon lamp as a light source.
Thermogravimetric analysis (TGA) was performed on a Shi-
madzu thermogravimetry and differential thermal analysis
DTG-60H at a heating rate of 10 1C minꢁ1 under N2.
Elemental microanalyses were carried out on a Vario EL III
CHNOS Elementar analyzer. Cyclic voltammetry (CV) was
performed at a scanning rate of 200 mV sꢁ1 on an AUTO-
LAB.PGSTAT30 potentiostat/galvanostat system (Eco-
chemie, Netherlands), which was equipped with a three-elec-
trode cell. Pt wires were used as the counter electrode and the
working electrode, and Ag/Ag1 was used as a reference
electrode. 0.1 mol Lꢁ1 tetrabutylammonium hexafluoro-
phosphate (n-Bu4NPF6) was used as a supporting electrolyte.
The reduction and oxidation behavior of the oligomers were
measured in solutions of THF and CH2Cl2 (1 ꢂ 10ꢁ3 mol
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China under Grants 60325412,
90406021, and 50428303, the Shanghai Commission of Science
and Technology under Grants 03DZ11016 and 04XD14002,
and the Shanghai Commission of Education under Grant
2003SG03.
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2
)
Funk = Fstd(Iunk/Aunk)(Astd/Istd)(Zunk/Zstd
(1)
General syntheses
The general synthetic routes of the desired oligomers are
outlined in Scheme 1. Oligofluorene derivatives were synthe-
sized according to the literature.15–17 Firstly, 2,5-dibromo-p-
xylene (1) was oxidized with potassium permanganate under
alkali conditions to afford 2,5-dibromoterephthalic acid. This
is the key step for the synthesis of the oxadiazole monomer
(20% yield). Then, the acid group was further esterified to give
2,5-dibromoterephthalic acid diethyl ester (2).18 After the ester
groups had been converted to benzoyl hydrazide (3), they were
reacted with aryl chloride to form compound 4. Finally,
oxadiazole monomer (5) was produced by the dehydrative
cyclization of hydrazides in POCl3. The resulting oligomers
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670 | New J. Chem., 2006, 30, 667–670 This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006