Synthesis and properties of a blue bipolar indenofluorene emitter based on a D-π-A design

Article abstract of DOI:10.1021/ol201751p

Through a rational design, a novel Donor-Acceptor π-conjugated (D-π-A) blue fluorescent indenofluorene dye, DA-DSF-IF, has been synthesized for application in single-layer Small Molecule Organic Light Emitting Diodes (SMOLEDs). This new blue emitter posse

Full text of DOI:10.1021/ol201751p

ORGANIC
LETTERS
2011
Vol. 13, No. 16
4418–4421
Synthesis and Properties of a Blue Bipolar
Indenofluorene Emitter Based on a D-π-A
Design
†
Damien Thirion, Joelle Rault-Berthelot, Laurence Vignau, and Cyril Poriel*
†
‡
,†
€
ꢀ
Universite de Rennes 1 - UMR CNRS 6226 “Sciences Chimiques de Rennes”-MaCSE group,
Bat 10C, Campus de Beaulieu - 35042 Rennes cedex, France, and Universite de Bordeaux -
ꢀ
IMS/UMR CNRS 5218 - site ENSCBP, 16 Avenue Pey-Berland -33607 Pessac cedex, France
cyril.poriel@univ-rennes1.fr
Received June 29, 2011
ABSTRACT
Through a rational design, a novel DonorꢀAcceptor π-conjugated (D-π-A) blue fluorescent indenofluorene dye, DA-DSF-IF, has been synthesized
for application in single-layer Small Molecule Organic Light Emitting Diodes (SMOLEDs). This new blue emitter possesses bipolar properties as
well as good morphological and emission color stabilities and has been successfully used in a blue emitting single-layer SMOLED, with
performances impressively magnified compared to a nonbipolar indenofluorene emitter.
For the past two decades of research in Organic Electro-
nics, significant efforts have been devoted to the design and
the synthesis of original π-conjugated molecules with spe-
cific properties.1ꢀ5 In this context designing efficient blue
emitting fluorophores for Organic Light Emitting Diodes
(OLEDs) is probably one of the most challenging tasks.
Indeed, OLED technology is considered the most promis-
ing technology for the future generation of flat panel
display3,6ꢀ8 but still suffers from the shorter lifetime of blue
emitting materials compared to the green and redmaterials.
Despite the significant improvements in Small Molecule
OLED (SMOLED) performances, the quest for a molecule
with stable and efficient blue fluorescent emission continues
to hold the attention of a number of research groups.9 Of
particular interest for the future of this technology are the
simple single-layer SMOLEDs, which do not require,
theoretically, the use of additional hole/electron transport-
ing layers, strongly simplifying the device structure.10
An appealing strategy to obtain efficient blue single-layer
SMOLEDs involves the use of DonorꢀAcceptor π-
†
ꢀ
Universite de Rennes 1.
‡
ꢀ
Universite de Bordeaux.
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r
10.1021/ol201751p
Published on Web 07/26/2011
2011 American Chemical Society

conjugated (D-π-A) fluorescent dyes with both electron-
donating and -accepting groups linked by a π-conjugated
bridge.10ꢀ15 Using this strategy, Bryce and co-workers have
for example recently reported a very efficient blue single-
layer SMOLED.12 Alas, designing and synthesizing D-π-A
blue fluorescent molecules which can be used in single-layer
SMOLEDs is far from an easy task. Indeed, in designing a
D-π-A blue fluorescent dye, one should notably avoid a
strong charge transfer between the donor and the acceptor
which causes the emission to shift to higher wavelengths
while maintaining a high quantum yield. The nature of the
donor, acceptor, and π-conjugated bridge is hence of key
importance. In this context, the π-conjugated system (1,2-b)-
indenofluorene, a very promising building block for blue
OLED applications3,5,16ꢀ22 and for Organic Electronics in
general,23ꢀ26 has not been used to date as a π-spacer in a
D-π-A dye. Indeed, up to now, only symmetric indeno-
fluorene derivatives have been reported for Organic Elec-
tronic applications, due to the synthetic complexity.
units spirolinked to the central bridges of the indenofluorenyl
core have also been introduced to give the molecule excellent
morphological and thermal stabilities.4 Finally, the 2,7-posi-
tions of the fluorenyl units have been protected with tert-
butyl groups to avoid intermolecular πꢀπ interactions in the
solid state and parasite anodic coupling processes.
Scheme 1. Synthesis of DA-DSF-IF
Herein, we report the design and the synthesis of a
bipolar D-π-A blue emitter called DA-DSF-IF (Donor-
Acceptor DiSpiroFluorene-IndenoFluorene) using an in-
denofluorenyl core as the π-conjugated bridge. Its electro-
chemical, optical, and morphological properties are
disclosed as well as its first application as an emissive layer
within a nondoped single-layer blue SMOLED. This work
is, to the best of our knowledge, the first example of a D-π-A
dye using the promising indenofluorenyl core.
The molecular design adopted in DA-DSF-IF is the follow-
ing: a diarylamino and a phenylbenzimidazolyl group, two
well-known hole and electron accepting units, have been
integrated at each extremity of an indenofluorenyl moiety,
known to possess a high quantum yield in the near-UV
region.21 Because of its moderate electron affinity,13 the ben-
zimidazolyl group has been chosen to avoid a strong charge
transfer character in the molecule that will notably result in a
red shift of the emission wavelength. In addition, two fluorene
The synthesis of DA-DSF-IF (Scheme 1) starts with the
synthesis of the key DiSpiroFluorene-IndenoFluorene scaf-
fold (DSF-IF(t-Bu)4 1), constituted of an indenofluorenyl
core spirolinked to two 2,7-tert-butyl fluorenyl moieties.
DSF-IF(t-Bu)4 1 has been prepared from 2,7-di-tert-butyl-
fluoren-9-one and 2,200-diiodoterphenyl27 and will be used in
this study as a relevant model to DA-DSF-IF. With 1 in
hand, the next step was to introduce the bromine atoms on
the indenofluorenyl core, prior to the selective Pd-catalyzed
reaction. Since the 2,7-positions of the fluorenyl moieties
have been protected, electrophilic bromination of 1 cleanly
leads to the dibromination of the indenofluorenyl core with
90% yield. The key desymmetrization of 2 was then carried
out through a careful Pd-catalyzed SuzukiꢀMiyaura cross-
coupling reaction between 2 and 4-formylbenzeneboronic
acid. After intensive scouting, the best conditions found to
synthesize 3 (30% yield) include Pd2(dba)3/P(t-Bu)3 as the
catalytic system and sodium carbonate as the base in a
mixture of toluene and water (6/1) at 100 °C. It should be
noted that decreasing the temperature of the reaction or
switching to other Pd catalysts (Pd(dppf)Cl2, Pd(PPh3)4, or
Pd(OAc)2) only leads to lower yields, highlighting the very
poor reactivity of the Bromine atoms. The benzimidazolyl
group was then constructed through the condensation of 3
with N-phenyl-o-phenylenediamine in methoxyethanol lead-
ing to 4 in 70% yield. The Hartwig Pd-catalyzed CꢀN
coupling reaction was finally adopted to introduce the
dimethoxydiphenylamine unit providing the target molecule
DA-DSF-IF with an overall yield of 10%. This synthetic
strategy is straightforward and versatile and appears
adaptable to synthesize other D-π-A indenofluorenyl dyes.
We investigated the bipolar character of DA-DSF-IF
using cyclic voltammetry (CV) and differential pulse
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Org. Lett., Vol. 13, No. 16, 2011
4419

very well resolved fluorescence spectrum and a small Stokes
shift (25 nm), consistent with a highly rigid molecular
structure. (The Stokes shift is defined in this work as λem
ꢀ
λabs (in nm).) In addition, we note that the fluorescence
spectrum of DA-DSF-IF is better resolved than its absorp-
tion spectrum, suggesting that in the excited state the bond
joining the indenofluorene and the aryl ring of the benzimi-
dazolyl unit acquires some double bond character, hence a
more rigid/planar structure.30,31
Figure 1. Cyclic voltammetry (and DPV inset) of DA-DSF-IF using
Pt anode; sweep rate: 100 mV sꢀ1. Left: reduction in THF-[NBu4]-
[PF6] 0.2 M. Right: oxidation in CH2Cl2-[NBu4][PF6] 0.2 M.
3
voltammetry (DPV). DA-DSF-IF presents four successive
oxidation processes (in CH2Cl2, Figure 1, right), the first
two being monoelectronic (Eox1 =0.575VandEox2 =1.2V),
and the other bielectronic (Eox3 = 1.61 V andEox4 = 1.89 V),
clearly pointed out by DPVs. In addition, DA-DSF-IF
presents (in THF, Figure 1, left) two reversible monoelec-
tronic reduction waves at ꢀ2.2 V (Ered1) andꢀ2.37 V (Ered2).
Compared to the model compound DSF-IF(t-Bu)4 1, which
exhibits three reversible oxidations (1.33, 1.61, and 1.79 V, in
CH2Cl2)27 and one quasi reversible reduction (ꢀ2.6 V, in
THF; see Supporting Information (SI)), DA-DSF-IF exhi-
Figure 2. Absorption and emission spectra (λexc = 404 nm) of
DA-DSF-IF: in cyclohexane (A) and as vacuum evaporated thin
film (B).
DA-DSF-IF also presents a very high quantum yield of ca.
85%, in the pure blue region, slightly increased compared to
that of DSF-IF(t-Bu)4 1 (70%) and ensuring hence its
potential as an efficient D-π-A blue emitter. From a careful
inspection of the dependence of the absorption behavior of
DA-DSF-IF on solvent polarity, we note that the absorption
maximum is almost insensitive to the dielectric constant of
the environment (see SI). This indicates that the electronic
and structural characteristics of the ground and Franckꢀ
Condon excited states do not differ much with a change in
solvent polarity.32 Oppositely, the fluorescence spectra re-
veals a clear solvatochromic effect. Indeed, DA-DSF-IF is
highly sensitive to the solvent polarity with fluorescence
maxima ranging from 435 nm in cyclohexane to 537 nm in
dichloromethane (Figure 3). This dependence of the emis-
sion wavelength on the solvent polarity is indicative of
dipoleꢀdipole interactions between DA-DSF-IF and polar
solvent molecules and hence of a photoinduced intramole-
cular charge transfer.33
A vacuum evaporated (at T ∼250 °C) thin-film absorp-
tion spectrum of DA-DSF-IF appears to be slightly broader
compared to its solution spectrum but nevertheless identical
in shape and wavelength (Figure 2B). We can hence con-
clude that DA-DSF-IF does not aggregate in the solid state
due to steric protection induced by the 2,7-di-tert-butyl-
fluorene units. The fluorescence spectrum of DA-DSF-IF in
thin-solid film presents a maximum in the blue region (476
nm), located between that in THF (504 nm) and that in
1
bits one additionnal oxidation (Eox = 0.575 V) and one
additional reduction (Ered1 = ꢀ2.2 V). These two reversible
electronic processes have been respectively assigned to the
oxidation of the diphenylamine unit28 and to the reduction
of the phenylbenzimidazolyl unit.13 The existence of stable
radical cationic and radical anionic species for DA-DSF-IF
clearly indicates that this molecule has a bipolar character
and hence a great potential for efficient electron/hole trans-
port in OLEDs. From electrochemical data,29 the HOMO/
LUMO levels of DA-DSF-IF have been respectively esti-
mated to be around ꢀ4.90/ꢀ2.3 eV leading to an electro-
chemical HOMO/LUMO gap ΔEelec of 2.6 eV. (HOMO and
LUMO levels were determined either in CH2Cl2 or in THF
with consistent results (see figures in SI).)
The UVꢀvis absorption spectrum of DA-DSF-IF, in
cyclohexane, presents a maximum recorded at 410 nm,
significantly red-shifted compared to that of its constituting
building block DSF-IF(t-Bu)4 1 (λmax = 345 nm),27 clearly
indicating that the π-conjugation was effectively extended
once the D and A groups were connected into the D-π-A
chromophore (Figure 2A). Thus, in cyclohexane, DA-DSF-
IF presents an optical HOMO/LUMO gap ΔEopt of 2.83 eV,
strongly contracted compared to that of DSF-IF(t-Bu)4 1
(ΔEopt = 3.49 eV). The π-conjugation extension in DA-
DSF-IF is also clearly observed in fluorescent spectroscopy
(Figure 2A) as DA-DSF-IF presents a maximum at 435 nm
(in cyclohexane) red-shifted by 87 nm compared to that of
DSF-IF(t-Bu)4 1(λmax =348nm).27 DA-DSF-IF possesses a
^
ꢀ
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4420
Org. Lett., Vol. 13, No. 16, 2011

toluene (454 nm). The thin film of DA-DSF-IF was then
gradually heated from rt to 200 °C (see SI). As no low energy
emission bands beyond 500 nm were detected, the emission
color of DA-DSF-IF appears to be stable and hence very
promising for blue OLED applications.
turn on voltage of the device (for 1 Cd mꢀ2) is low (5.2 V),
and the luminance reaches 320 Cd m with a luminous
3ꢀ2
3
efficiency of 0.13 Cd Aꢀ1 (see SI). The EL spectrum of the
3
device reveals a main peak in the blue region (λmax = 470
nm, Figure 4, right) in accordance with the thin-solid film
fluorescence spectrum (Figure 2B) and shows no obvious
sign of troublesome parasite emission. The single-layer
nonoptimized SMOLED using DA-DSF-IF as an emissive
layer presents hence a blue EL with a luminous efficiency
magnified by ∼25 compared to the double layer device using
DSF-IF(t-Bu)4 1as an emissive layer. This effect is due to the
incorporation of electron and hole transporting units in DA-
DSF-IF and highlights the efficiency of the D-π-A strategy
to design indenofluorene based materials for single-layer
blue SMOLEDs. The blue EL of this single-layer SMOLED
is one of the highest reported among those of indenofluor-
ene based single-layer OLEDs.16,17,22,34
Figure 3. Emission spectra (λexc = 404 nm) of DA-DSF-IF in
different solvents.
The morphology of DA-DSF-IF thin films has been
studied by atomic force microscopy in acoustic mode
(AC-AFM) to evaluate the effect of temperature on the
thin-film structure (see SI), a key parameter before any
OLED application. The surface has been hence exposed
to air under thermal stress conditions from rt to 130 °C.
The nonheated film surface presents a regular and smooth
morphology. The surface roughness (Ra) of the film
appears to be very low, around 0.2/0.3 nm, and is kept
almost unchanged until 130 °C (see SI), highlighting the
very good quality of the film surface and the high stability
of DA-DSF-IF upon heating. DA-DSF-IF presents hence,
due to its spiro linkages,4 a very good morphological
stability in harsh conditions.
Figure 4. ITO/PEDOT/DA-DSF-IF (40 nm)/LiF/Al device.
Left: IꢀVꢀL characteristics. Right: Normalized EL spectrum.
In summary, we have designed and synthesized, via an
expedient route, a new efficient indenofluorenyl blue
fluorophore DA-DSF-IF based on a D-π-A molecular
design. This rational design allows DA-DSF-IF to pre-
sent (i) a high quantum yield in the blue region, (ii)
bipolar properties, and (iii) good morphological and
emission color stabilities. The nonoptimized single-layer
SMOLED using DA-DSF-IF as an emissive layer emits a
blue color with performances impressively magnified com-
pared to a classical indenofluorene emitter such as DSF-
IF(t-Bu)4 1. This workrepresents the first example of a blue
fluorescent D-π-A dye using the promising indenofluor-
enyl core as π-conjugated bridge. Regarding the impor-
tance of indenofluorene derivatives for the future of
Organic Electronics, this work may pave the way to the
development of this class of molecules.
Several OLEDs have been finally investigated using two
different emissive layers, namely DA-DSF-IF or its constitut-
ing building block DSF-IF(t-Bu)4 1. Indeed, to study the
effect of the incorporation of electron and hole transporting
units in DA-DSF-IF, we first decided to investigate for
comparison several devices using the nonsubstituted indeno-
fluorene emitter DSF-IF(t-Bu)4 1as an emissive layer. Thus, a
nondoped device with a simple single-layer structure, ITO/
PEDOT/DSF-IF(t-Bu)4 1/Ca, has been fabricated and char-
acterized. The performance of this single-layer device using 1
as an emissive layer was nevertheless too low to be recorded
(Luminance < 2 Cd mꢀ2) due to the large electron/hole
3
injection barrier. A double layer device containing a hole-
transporting layer (N,N0-di-(1-naphtyl)-N,N0-diphenyl-[1,10-
biphenyl]-4,40-diamine-NPB) has then been investigated. The
ITO/PEDOT/NPB/1/Ca device presents an electrolumines-
cence (EL) spectrum in the blue region (λmax = 415/436 nm)
but possesses poor performance, with a maximum luminance
Acknowledgment. D.T. thanks the Region Bretagne for
a studentship. The authors thanks Dr. Bergamini (Rennes)
for AFM images and the CRMPO (Rennes).
Supporting Information Available. Synthetic proce-
dures, complete characterizations, and devices fabrica-
tion. This material is available free of charge via the
Internet at http://pubs.acs.org.
of ca. 25 Cd mꢀ2, a luminous efficiency of 0.005 Cd Aꢀ1
,
3
3
and a high turn on voltage (ca. 9 V); see SI.
A single-layer device, ITO/PEDOT/DA-DSF-IF/LiF/
A1 has been then fabricated using DA-DSF-IF as an
emissive layer. Current densityꢀvoltageꢀluminance char-
acteristics of the device are presented in Figure 4, left. The
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€
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Org. Lett., Vol. 13, No. 16, 2011
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