Synthesis and utilization of perylene-based n-type small molecules in light-emitting electrochemical cells

Article abstract of DOI:10.1039/b814913e

We report the synthesis of a soluble perylene-based small molecule for use as an n-type emissive material for organic optoelectronic device applications, and demonstrate the material in a light-emitting electrochemical cell configuration. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b814913e

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Synthesis and utilization of perylene-based n-type small molecules in
light-emitting electrochemical cellsw
a
a
a
b
Zachary B. Hill, Deanna B. Rodovsky, Janelle M. Leger and Glenn P. Bartholomew*
Received (in Cambridge, UK) 27th August 2008, Accepted 30th October 2008
First published as an Advance Article on the web 12th November 2008
DOI: 10.1039/b814913e
We report the synthesis of a soluble perylene-based small
molecule for use as an n-type emissive material for organic
optoelectronic device applications, and demonstrate the material
in a light-emitting electrochemical cell configuration.
lead to fluorescent quenching in the solid state, the bay region
1
6,17
is tetra-substituted with 4-tert-butylphenoxy groups.
These substitutions offered the desired molecule 1,6,7,12-tetra-
0
(4-tert-butylphenoxy)-N,N -bis(2,6-diisopropylphenyl)perylene-
1
8
3
,4,9,10-bis(dicarboximide).
The synthesis to obtain our target PDI molecule is outlined
Light-emitting electrochemical cell (LEC) device operation in-
volves in situ redistribution of ions under an applied bias with
subsequent electrochemical doping of the p-conjugated material
in Scheme 1. Bromination of 3,4,9,10 perylene tetracarboxylic
dianhydride afforded a product that could not be purified or
analyzed by NMR due to its poor solubility in organic
1
at the electrodes. A variety of primarily p-type, or partially
Common
2–6
oxidized, materials have been utilized for LECs.
¨
solvents. Wurthner and co-workers reported that the tetra-
bromo product was difficult to obtain in high yield because a
reported materials include poly(p-phenylene vinylene) (PPV)-
based emissive polymers, among others. While these materials
have been shown to be successful in LECs, there is growing
evidence that the use of materials that are relatively easy to
oxidize but are difficult or impossible to reversibly reduce could
introduce asymmetry in the location of the emissive, or junction,
1
9
mixture of di-, tri-, and tetrabromo product formation.
Alternatively, Qui et al. reported elevated temperatures and
extended time led to a majority of the tetrabromo product
2
0
being formed. Utilizing the approach of Qui et al., we took
the product forward after filtration and washing. This product
was subsequently condensed with 2,6-diisopropylaniline in
propionic acid to afford the diimide (2). Based on the integra-
tion ratios of the crude NMR sample of 2, the tetrabrominated
species was the major product (490% yield) in the first step.
The condensation product 2 was obtained in moderately low
yield (33%) after column chromatography and recrystalliza-
tion. Further reaction with base and t-butylphenol led to 3 in
23% yield. Since compounds 2 and 3 exhibited significant
solubility in methylene chloride, purification using silica was
possible. However, both 2 and 3 had di- and tri-substituted
impurities making purification a challenge, leading to lower
overall yields.
7–10
region.
The use of more n-type materials that are more easily
and reversibly reduced could be advantageous in balancing
doping effects in this device architecture.
In addition to improving the electrochemical effects in
these materials, n-type materials, such as perylene-based
compounds, could offer additional advantages in organic
optoelectronic devices through enhanced stability to ambient
conditions that cause device degradation due to oxidation of
the emissive material. In particular, perylenetetracarboxylic
diimide (PDI)-based materials are increasingly attractive
n-type materials due to efficient emission characteristics, high
molar extinction coefficients, as well as high thermal, chemical,
1
1–13
and photostability.
‘‘Color tuning’’ for light-emitting
The target molecule, 3, showed solubility in methylene
chloride, chloroform, and a 50 : 50 mixture of chloroform
applications can also be achieved by rational alterations of
PDI at the terminal imide as well as the bay region (1,6,7,12
1
substitution). Potential applications include photovoltaic
2
1
1
devices, dye lasers, and molecular switches. In this commu-
nication, we report the synthesis of perylene-based
a
compound modified for device applications (processability,
solid-state fluorescence) and demonstrate its utility as the
emissive material in an LEC.
Substitution at the terminal and bay region of PDI modifies
both the solubility and processability of the resultant material
and significantly influences the photophysical properties of the
1
4–17
product.
terminal imide, primarily to enhance solubility.
A 2,6 diisopropyl phenyl group is appended at the
1
4,15
To more
directly disrupt p–p interactions that influence solubility and
a
University of Washington, Department of Chemistry, Box 351700,
Seattle, Washington, 98195-1700, USA
Sirigen, Inc., 11494 Sorrento Valley Road, 92121 San Diego, USA.
E-mail: glenn.bartholomew@sirigen.com
b
Scheme 1 Synthesis of perylene-3,4,9,10-tetracarboxylic dianhydride.
(a) Br
anilinepropionic acid, 140 1C, 17 hours, 33% yield; (c) 4-t-butylphenol,
CaH , NMP, 20 hours, 23% yield.
2 2 4
, H SO , 110 1C, 168 hours, 450% yield; (b) 2,6-diisopropyl-
w Electronic supplementary information (ESI) available: Experimental
details and CV data. See DOI: 10.1039/b814913e
2
6
594 | Chem. Commun., 2008, 6594–6596
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and acetonitrile. Solutions of 3 were made in both chloroform
and chloroform–acetronitrile (50 : 50) with no notable differ-
ences in the solution absorption spectra. The absorption
spectra of thin films of 3 were slightly blue-shifted relative to
solution spectra. Solution absorption spectra were collected in
LECs were fabricated in a single-layer device architecture by
spin-casting a mixture of 3, PEO, and LiTf in chloroform to a
thickness of 200 to 500 nm onto patterned ITO glass substrates.
Solution composition was 3, PEO, and LiTf in a 10 : 2 : 2 weight
ratio at approximately 1% total weight in chloroform. The films
were annealed on a hot plate under an inert atmosphere at
801 C for 1 h. Gold top electrodes were deposited by thermal
evaporation at 10 torr following overnight drying under
vacuum. All device testing took place in a dry nitrogen glove-
box using a Keithley 2400 Sourcemeter. Electroluminescence
measurements were taken using a calibrated Ocean Optics
USB2000 fiber optic spectrometer. After applying an initial
bias of +8 to +10 V, the current reached equilibrium and
emission was observed. Red emission could be observed by the
eye in a lighted lab. The electroluminescence spectrum obtained
(Fig. 1), exhibited a lmax of emission at 622 nm. The electro-
luminescence and photoluminescence spectra are similar
with the exception of a more defined shoulder on the electro-
luminescence spectra at approximately 660 nm.
ꢁ4
ꢁ6
a 10 to 10 M concentration range, and a molar absorp-
ꢁ1
ꢁ1
tivity of 44 000 M cm was measured in chloroform.
Absorbance spectra of thin films of 3 showed a maximum at
ꢁ
7
5
substituted perylene compounds.
68 nm, consistent with the range of previously reported
1,14,16,17,21
1
Photolumines-
cence spectra showed an emission maximum at 613 nm giving
a Stokes shift of 45 nm (Fig. 1). The optical band gap of 2.0 eV
was calculated from the onset of absorption at 618 nm. This
agrees with the observed photoluminescence and the band gap
measured using cyclic voltammetry.
To determine the band gap of 3, electrochemistry experi-
ments were performed (see ESIw). As previously reported
1
6,21–23
in the literature for perylene-based compounds,
we
observe a quasi-reversible, one-electron oxidation wave and
two quasi-reversible, one-electron reduction waves. Our data is
consistent with the mechanism for the two-electron reduction
The use of perylene-based, n-type small molecules as the
emissive material in LECs has been demonstrated. The
bay- and imide-substituted PDI, 1,6,7,12-tetra(4-tert-butyl-
1
1
previously proposed. The half wave oxidation potential (E1/2
)
+
0
of the perylene compound versus Ag/Ag was +0.9625 V.
Using E[vacuum] = EAg/Ag+ + 4.66 V, a HOMO of 5.6 eV is
calculated. The half wave reduction potential (E1/2) of the
phenoxy)-N,N -bis( 2,6-diisopropylphenyl )perylene-3,4,9,10-
bis(dicarboximide) (3) was synthesized in useable yields. The
material demonstrates good solubility and high photolumines-
cence in the solid state. LEC devices fabricated with 3 have
shown red light emission indicating that devices utilizing PDIs
are possible. We expect that improvements in device perfor-
mance will be likely with continued optimization of film and
device parameters. Studies to determine the effects of using an
n-type material in an LEC with regards to doping symmetry
and environmental stability are ongoing.
+
perylene compound versus Ag/Ag was ꢁ1.0 V. Using
E[vacuum] = EAg/Ag+ + 4.66 V, a LUMO of 3.66 eV is
calculated. This correlates to a band gap of 1.96 eV, which is
in good agreement with the optically calculated band gap. The
LUMO level is much lower than the typical PPV derivatives
often employed in LECs, indicating that in relative terms, 3 is
an n-type material. The lower lying LUMO level may lead
to enhanced stability of the perylene film in comparison to
PPV-based films. This is supported by observed, quasi-reversible
reduction waves. In contrast to PPV-based materials where only
the oxidation wave is observable, both oxidation and reduction
waves can be measured for 3 (see ESIw for CV data).
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Fig. 1 Target PDI molecule, 3, absorption, photoluminescence, and
electroluminescence spectra.
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