Incorporation of spiroxanthene units in blue-emitting oligophenylene frameworks: A new molecular design for OLED applications

Article abstract of DOI:10.1002/chem.201100790

We report herein the incorporation of xanthenyl units into two extended π-conjugated phenylene systems, namely indenofluorene and pentaphenylene. Thus, dispiroxanthene-indenofluorene (DSX-IF) and dispiroxanthene- ladderpentaphenylene (DSX-LPP) have been designed and synthesized through short and efficient synthetic approaches. These two molecules possess a 3π-2-spiro architecture (3π-systems/2-spiro bridges), in which two xanthenyl cores are spirolinked to a π-conjugated backbone either indenofluorene for DSX-IF or pentaphenylene for DSX-LPP. The structural, electrochemical, and photophysical properties of these blue/violet emitters have been studied in detail and compared to those of their 'all carbon' analogues with spirofluorenyl cores instead of spiroxanthenyl cores, namely dispirofluorene-indenofluorene (DSF-IF) and dispirofluorene-ladderpentaphenylene (DSF-LPP), previously reported in the literature. Finally, the application of DSX-IF and DSX-LPP as new light-emitting materials in nondoped organic light emitting diodes is reported. A detailed optical study of the different electroluminescence spectra is notably presented, with an emphasis 1) on the origin of the low-energy emission band observed in the case of DSX-LPP and 2) on the unexpected optical contribution of the well-known hole-transporting-layer NPB (N,N'-di(naphtyl)-N,N'-diphenyl(1, 1′-biphenyl)-4,4′-diamine). Energize your chemistry! New spiro-configured blue fluorophores containing an intracyclic oxygen atom, dispiroxanthene-indenofluorene (DSX-IF) and dispiroxanthene-ladder pentaphenylene (DSX-LPP), have been synthesized by different synthetic approaches (see graphic). Their properties have been investigated in detail and compared with those of their 'all-carbon' analogues.

Full text of DOI:10.1002/chem.201100790

FULL PAPER
DOI: 10.1002/chem.201100790
Incorporation of Spiroxanthene Units in Blue-Emitting Oligophenylene
Frameworks: A New Molecular Design for OLED Applications
Cyril Poriel,*[a] Nicolas Cocherel,[a] Joꢀlle Rault-Berthelot,[a] Laurence Vignau,[b] and
Olivier Jeannin[a]
Abstract: We report herein the incor-
poration of xanthenyl units into two
extended p-conjugated phenylene sys-
tems, namely indenofluorene and pen-
taphenylene. Thus, dispiroxanthene-in-
denofluorene (DSX-IF) and dispirox-
anthene-ladderpentaphenylene (DSX-
LPP) have been designed and synthe-
sized through short and efficient syn-
thetic approaches. These two molecules
possess a 3p-2-spiro architecture (3p-
systems/2-spiro bridges), in which two
xanthenyl cores are spirolinked to a p-
conjugated backbone either indeno-
fluorene for DSX-IF or pentapheny-
lene for DSX-LPP. The structural, elec-
trochemical, and photophysical proper-
ties of these blue/violet emitters have
been studied in detail and compared to
those of their ꢁall carbonꢂ analogues
with spirofluorenyl cores instead of spi-
roxanthenyl cores, namely dispirofluor-
ene-indenofluorene (DSF-IF) and di-
ACHTUNGTRENNsUNG pirofluorene-ladderpentaphenylene
(DSF-LPP), previously reported in the
literature. Finally, the application of
DSX-IF and DSX-LPP as new light-
emitting materials in nondoped organic
light emitting diodes is reported. A de-
tailed optical study of the different
electroluminescence spectra is notably
presented, with an emphasis 1) on the
origin of the low-energy emission band
observed in the case of DSX-LPP and
2) on the unexpected optical contribu-
tion of the well-known hole-transport-
Keywords: indenofluoroene · low-
energy emission bands · OLEDs ·
organic electronics · pentapheny-
lene · spiro compounds · xanthene
derivatives
ing-layer NPB (N,N’-di
ACHTUNGTREN(NGNU naphtyl)-N,N’-
diphenyl(1,1’-biphenyl)-4,4’-diamine).
Introduction
ing the thermal and morphological stabilities, increasing the
solubility etc..[31] Since the first example of blue electrolumi-
nescence based on a spiro derivative,[33] numerous oligo-
mers[12,17,18,31,32,34–41] incorporating spiro bridges have been
designed for blue OLED applications. Typically, the core of
most of spiro compounds is based on the known 9,9’-spirobi-
fluorene.[31] Recently, new synthetic approaches have been
developed to obtain spiro compounds incorporating heteroa-
toms to tune the optical, electrochemical, or charge-trans-
port properties of the materials.[18,42–46] For example, Salbeck
and co-workers have notably reported the first spiro-type
derivative constituted of four thiophene units, presenting
very appealing charge-transport properties.[47] However, in-
corporation of spiro centers in xanthene-based materials for
organic electronics is almost absent from the literature. One
of the simplest spiro-configured xanthene parents is spiro-
(fluorene-9,9’-xanthene) SFX, made of a fluorene unit spiro-
linked to a xanthene core (Scheme 1). The use of the SFX
backbone as an emissive layer in blue OLEDs either incor-
porated in polymers[48–50] or in oligomers[6,51,52] has only been
very recently investigated and is still very rare. For example,
Huang and co-workers have notably reported for OLED ap-
plications, new blue light emitters based on the SFX scaf-
fold.[6,51] Similarly, a SFX-based emitter incorporating oligo-
phenylene units has been recently investigated as an emis-
sive layer within an OLED. This device exhibits a sky-blue
emission with a maximum brightness of 1507 Cdmꢀ2 and a
current efficiency of 0.36 CdAꢀ1.[52] Despite these very
During the last two decades, organic electronics have at-
tracted considerable interest[1,2] and organic light emitting
diodes (OLEDs) appear to be the near future of display ap-
plications.[3,4] As the lifetime of blue OLEDs[5] is still shorter
than that of the green and red OLEDs, numerous research
groups have been involved in the design and synthesis of
novel small molecules,[6–30] with stable blue fluorescence
emission. One appealing strategy to obtain stable fluores-
cence in the solid state is to introduce within the oligomer
or the polymer backbone a spiro centre.[31,32] This approach
called the “spiro concept”, in which a common sp3-hybrid-
ized carbon atom links two orthogonal p-conjugated arms
has allowed outstanding breakthroughs in the field.[31]
Indeed, the spiro concept possesses several advantages, such
as suppressing excimer formation in the solid state, improv-
[a] Dr. C. Poriel, Dr. N. Cocherel, Dr. J. Rault-Berthelot, Dr. O. Jeannin
Universitꢀ de Rennes 1-UMR CNRS 6226
“Sciences Chimiques de Rennes”-MaCSE group. Bat 10C
Campus de Beaulieu - 35042 Rennes cedex (France)
E-mail: cyril.poriel@univ-rennes1.fr
[b] Dr. L. Vignau
Universitꢀ de Bordeaux - IMS/UMR CNRS 5218-site ENSCBP
16 Avenue Pey-Berland - 33607 Pessac cedex (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100790.
Chem. Eur. J. 2011, 17, 12631 – 12645
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12631

Scheme 1. Synthesis of SFX and plausible mechanism proposed.[75]
recent examples, xanthene-based oligomers have been very
weakly studied for organic electronic applications due to the
lack of efficient synthetic approaches and SFX is the only
example reported to date. To expand the scope of applica-
tions of spiro derivatives for electronics, it is hence of great
interest 1) to introduce xanthene units into various extended
p-conjugated phenylene systems, such as indenofluorene
and pentaphenylene, and 2) to develop efficient synthetic
methodologies towards these spiro-configured xanthene de-
rivatives.
observed in the case of DSX-LPP and 2) on the unexpected
optical contribution of the well-known hole-transporting-
layer NPB (N,N’-diACTHNUTRGNE(NUG naphtyl)-N,N’-diphenyl(1,1’-biphenyl)-
4,4’-diamine). This work constitutes, to the best of our
knowledge, the first incorporation of xanthene units within
indenofluorene and pentaphenylene frameworks and one of
the rare examples of nondoped SMOLEDs based on xan-
thene derivatives.
As our group is strongly involved for many years in the
design of spiro-configured materials for blue OLED applica-
tions,[53,54] we report herein, promising new blue spiroxan-
thene emitters based on an indenofluorene framework,
namely dispiroxanthene-indenofluorene (DSX-IF), and
based on a pentaphenylene framework, namely dispiroxan-
thene-ladderpentaphenylene (DSX-LPP). These molecules
Results and Discussion
Synthesis: Due to the lack of efficient and convenient syn-
thetic approaches, xanthene derivatives and especially spi-
roxanthene derivatives have been very rarely studied for or-
ganic electronics applications. As the synthesis of new mate-
rials is strongly sought worldwide for organic electronics ap-
plications it is crucial to develop efficient routes towards
novel fluorophores, such as spiroxanthene derivatives, to
fully explore their potential. For example, Huang and co-
workers have reported an elegant but unexpected one-pot
synthesis of SFX (Scheme 1) from a mixture of 9-fluore-
none, phenol, and MeSO3H, involving a thermodynamically
controlled cyclization reaction as the key step.[75] Thus, they
have hence postulated that the thermodynamic-controlled
product of the reaction is SFX and that the kinetic-con-
trolled product of the reaction is a para/para-substituted di-
phenol derivative (Scheme 1).
possess
a
3p-2-spiro architecture (3p-systems/2-spiro
bridges),[54] in which two xanthenyl cores are spirolinked to
a p-conjugated backbone either indenofluorene for DSX-IF
or pentaphenylene for DSX-LPP. As indenofluorene[3,53,55–65]
or pentaphenylene[54,55,66–74] derivatives are highly appealing
cores for organic electronics, it is of great interest to investi-
gate the effect of the incorporation of xanthenyl units in
such frameworks. In this work, as a development to our pre-
liminary note,[56] we first report our investigations towards
the synthesis of DSX-IF and DSX-LPP through different, ef-
ficient, and short synthetic approaches. The structural, elec-
trochemical, thermal, and photophysical properties of these
blue/violet emitters will be discussed, together with their
first application as an emissive layer in small molecule
OLED (SMOLED). Finally, a detailed optical study of the
different electroluminescence spectra is presented, with an
emphasis 1) on the origin of the low-energy emission band
In the light of these results, we decided to follow the
same strategy to introduce the xanthenyl units within the
pentaphenylene core (DSX-LPP) and within the indeno-
fluorenyl core (DSX-IF). In this context, the corresponding
diketones, pentaphenylenedione 1[53] and indenofluorenone
3
[54] were the key intermediates (Schemes 2 and 3).
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Blue-Emitting Oligophenylene Frameworks
FULL PAPER
ate yield of the reaction (35%).
Owing to the importance of in-
denofluorene[3,53,55–65] or penta-
phenylene[54,55,66–74] derivatives
in organic electronics, this ap-
proach offers a simple, easy to
perform, and highly adaptable
access to various p-conjugated
building blocks bridged by spi-
roxanthenyl units.
Despite the fact that this ap-
proach offered a rapid access to
DSX-IF, the yield of the reac-
tion was, however, too low for
future optoelectronic applica-
tions and we thus decided to re-
visit the synthetic approach.
Scheme 2. Synthesis of DSX-LPP and tetraphenol derivative 2.
The idea was to first introduce
the xanthenyl units prior to the
formation of the indenofluorenyl core. This new approach is
based on a coupling reaction between the known 2,2“-diodi-
nated terphenyl (2,2”-DITP)[53] and a ketone derivative with
the xanthenyl core already in place, namely xanthone
(Scheme 3, route 2). Thus, the lithium–iodine exchange of
2,2“-DITP with n-butyllithium followed by the addition of
commercially available xanthone afforded the dixanthenol
4, which was immediately involved in an intramolecular cyc-
lization reaction giving in fine DSX-IF with a 50% yield
over the two steps. Although route 2 has an extra step com-
pared with route 1, none of the steps requires the use of
column chromatography for both the purification of dixan-
thenol 4 and DSX-IF and the overall yield is higher than the
one-step approach.
1
It should be stressed that the H NMR spectra of DSX-
LPP and DSX-IF are very similar to those of their ꢁall
carbonꢂ analogues with spirofluorenyl cores instead of spi-
roxanthenyl cores, namely dispirofluorene-indenofluorene
Scheme 3. Synthesis of DSX-IF: Routes 1 and 2.
(DSF-IF)[53]
and
dispirofluorene-ladderpentaphenylene
Thus, DSX-LPP was synthesized from a mixture of dike-
tone 1,[54] MeSO3H, and phenol (54% yield; Scheme 2). By
decreasing the reaction time, the tetraphenol derivative 2,
with 4-para-hydroxyphenyl units, was isolated and further
cyclized into DSX-LPP under the same experimental condi-
tions. Tetraphenol 2 appears hence as the kinetic-controlled
product and DSX-LPP as the thermodynamic-controlled
product. These results are in accordance with the mecha-
nism postulated for the synthesis of SFX (Scheme 1).
Similarly DSX-IF was also obtained in a one-pot ap-
proach with 35% yield, from a mixture of diketone 3,[53]
MeSO3H, and phenol (Scheme 3, route 1). In contrast to the
synthetic approach of DSX-LPP presented above
(Scheme 2), we did not manage to isolate the indenofluor-
ene-tetraphenol derivative, analogue of 2, maybe due to the
very low solubility of the indenofluorene derivatives.
Indeed, diketone 3 and DSX-IF appear to be poorly soluble
in common organic solvents. This very low solubility of the
starting diketone 3 is also surely responsible for the moder-
(DSF-LPP)[54] previously reported. We note, however, in the
1H NMR spectra of DSX-LPP and DSX-IF, two sets of
shielded hydrogen atoms, d=6.85/6.55 ppm for DSX-LPP
and d=6.79/6.45 ppm for DSX-IF (see spectra in the Sup-
porting Information). These signals are not present in the
1H NMR spectra of DSF-LPP and DSF-IF and correspond
to the hydrogen atoms of the xanthenyl units in the a- and
ꢀ
b-position of the C O linkage. This shielding effect is due to
the electro-donating behavior of the intracyclic oxygen
atom. In 13C NMR spectra, the oxygen atom also leads to a
strong shielding of the spiro carbon chemical shift, recorded
around d=54 ppm instead of the ordinary chemical shift of
d=66 ppm, observed for most of the spiro compounds.[54]
Physicochemical properties of spiroxanthene derivatives
Structural properties: The poor solubility of DSX-IF does
not allow us to obtain a suitable single crystal. However,
single crystals of DSX-LPP were obtained by liquid–liquid
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C. Poriel et al.
diffusion of MeOH into a solution of CDCl3 and analyzed
by X-ray diffraction to elucidate its molecular structure and
packing characteristics. DSX-LPP crystallizes with one mol-
xanthenyl moieties of two neighboring molecules was esti-
mated to be 3.8 ꢄ, greater than the sum of the van der
Waals radii of two carbon atoms. Oppositely, the closest
C–C distance between two terminal phenyl rings of the pen-
taphenylene moieties of two neighboring molecules was esti-
mated to be 3.29 ꢄ. This short C–C distance is also observed
between the same carbon atoms in the case of its “all
carbon” analogue DSF-LPP with spirofluorene units instead
of spiroxanthene,[54] suggesting that the arrangement of the
pentaphenylene cores is not strongly influenced by the spi-
rolinked p-systems (fluorene or xanthene). These intermo-
lecular C–C distances are smaller than the sum of the Van
Der Waals radii of two sp2 carbon atoms and leads us to
conclude that intermolecular interactions may occur, which
can lead to parasite bands in fluorescence spectroscopy (see
the optical section below). We also note a rather short C–C
distance between methylene carbon atoms of the alkyls
chains (ca 3.56 ꢄ) and between the xanthene moieties and
the pentaphenylene moieties (ca 3.59 ꢄ; Figure 1, bottom).
¯
ecule of CDCl3, in the triclinic system, space group P1, with
the DSX-LPP unit on an inversion center and a CDCl3 mol-
ecule in general position.
DSX-LPP presents a linear antarafacial geometry[76] and
its pentaphenylene core (19.4 ꢄ long) does not adopt a
strict coplanar configuration as two distorsions are observed
on both sides (Figure 1). The dihedral angles between the
Electrochemical properties: The redox properties of DSX-IF
and DSX-LPP were investigated by cyclic voltammetry
(CV) and compared to their ꢁall carbonꢂ congeners DSF-IF
and DSF-LPP (Figure 2).[53,54] DSX-LPP presents in CV two
Figure 1. Molecular structure (top) and crystal packing diagram along the
c stacking axis (bottom) of DSX-LPP obtained from single-crystal X-ray
diffraction data. The hydrogen atoms have been omitted for clarity.
plane of the central phenyl ring 3 and those of side rings 1
and 5 are of approximately 78 (see phenyl rings labeling in
Figure 1, top). These distortions are comparable to those ob-
served for a similar derivative with fluorenyl units instead of
xanthenyl (8.48).[54] The xanthenyl cores do not add, hence
any significant structural deformations of the central penta-
phenylene compared to the fluorene units. The angles be-
tween the plane of the central phenyl ring 3 of the penta-
phenylene and those of the phenyl rings of the xanthenyl
units are recorded around 81.3 and 80.38 (see the Supporting
Information). Indeed, the phenyl rings of the xanthenyl
cores of DSX-LPP are not planar as a dihedral angle of
18.28 is observed between the planes of each phenyl ring.
This dihedral angle is larger than that observed for SFX.[48]
As indicated in the crystal packing diagram of DSX-LPP
(Figure 1, bottom), the closest C–C distance, between two
Figure 2. Cyclic voltammetry of DSX-LPP (top, inset: the 0/1.4 V por-
tion) and of DSX-IF (bottom) (2ꢅ10ꢀ3 m in CH2Cl2 (Bu4NPF6, 0.2m);
working electrode: Pt disk diameter=1 mm; sweep rate=100 mVsꢀ1).
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Blue-Emitting Oligophenylene Frameworks
FULL PAPER
irreversible oxidation waves with maxima recorded at E1 =
1.92 and E2 =2.25 V versus SCE (Figure 2, top). This behav-
ior appears to be very different to that observed for its con-
gener DSF-LPP,[54] both in terms of intensity of the oxida-
tion waves and in terms of potential values. Indeed, for simi-
lar concentrations, the first oxidation wave of DSX-LPP
(1.92 V; Table 1) has an intensity greater than 150 mA,
HOMO and LUMO1 energy levels (obtained from electro-
chemical data and by using an SCE energy level of 4.4 V rel-
ative to vacuum)[77] were: HOMO: ꢀ5.45 eV and LUMO:
ꢀ2.2 eV for DSX-LPP and HOMO: ꢀ5.79 eV and LUMO:
ꢀ2.15 eV for DSX-IF. Thus, DSX-LPP possesses a HOMO
level lying 0.34 eV higher than that of DSX-IF due to the
extension of the p-conjugation of the central core. Opposite-
ly, the LUMO levels of DSX-IF
and DSX-LPP are very similar
and do not seem to be influ-
Table 1. Electrochemical data for DSF-IF, DSX-IF, DSF-LPP, and DSX-LPP (vs. SCE).
enced by the extension of the
p-conjugation of the central
core.
ox
onset
red
onset
E1
E2
E
E
HOMO[a]
LUMO[b]
LUMO[c]
DEel[d]
DEopt[e]
DSX-IF
1.5
1.86
1.95
1.92
1.66
1.39
ꢀ2.25
ꢀ2.23
ꢀ2.2
ꢀ5.79
ꢀ5.76
ꢀ5.45
ꢀ5.36
ox
ꢀ2.15
ꢀ2.17
ꢀ2.2
ꢀ2.23
ꢀ2.25
ꢀ2.36
ꢀ2.29
3.64
3.59
3.25
3.26
3.53
3.51
3.09
3.07
DSF-IF[78]
DSX-LPP
DSF-LPP[54]
1.47
1.36
Compared to their ꢁall
carbonꢂ analogues DSF-IF and
DSF-LPP,[53,54] DSX-IF and
DSX-LPP present very similar
gaps and energy levels clearly
signing that the main properties
of these systems are driven by
their corresponding central
cores (indenofluorene and pen-
taphenylene). However, the
[f]
–
1.05[g]
0.96
1.09
ꢀ2.3
ꢀ2.1
[a] Calculated from the onset oxidation potential E . [b] Calculated from the onset reduction potential
onset
red
E
. [c] Calculated from the HOMO energy level (obtained from redox data) and the edge of optical gap.
onset
[d] DEel = jHOMO–LUMOj from redox data. [e] Calculated from the edge of the absorption spectrum in so-
lution by using DEopt =hc/l (DEopt (eV)=1237.5/l (nm)). [f] A low-intensity wave is recorded around 1.1 V
with no clear maximum. [g] From the low-intensity wave at 1.1 V.
whereas the first oxidation wave of DSF-LPP (1.09 V) has
an intensity lower than 2 mA. This observation leads us to
focus DSX-LPP CV studies on the 0.2/1.4 V potential range
(see inset in Figure 2, top). In this portion, one can observe
a “quasi-reversible” wave, with an intensity of 1.5 mA in ac-
cordance with the first oxidation wave of DSF-LPP.[54] As
the onset potential of this wave is recorded around 1.05 V,
almost identical to that of DSF-LPP (0.96 V),[54] it is then
reasonable to contend that the first oxidation of DSX-LPP
occurs on the pentaphenylene central core.
Contrary to DSX-LPP discussed above, DSX-IF oxidation
occurs along two processes with maxima recorded at E1 =
1.5 and E2 =1.86 V (Figure 2, bottom), almost identical to
those reported for its ꢁall carbonꢂ congener DSF-IF (1.47
and 1.95 V).[53] Hence, the first oxidation of DSX-IF, as al-
ready observed for DSF-IF, involves the indenofluorenyl
core in a one-electron process, whereas the second oxidation
occurs on the two spirolinked xanthenyl units. The first oxi-
dation peak of DSX-IF (E1 =1.5 V) is, however, shifted
compared with that of the indenofluorene molecule (E1 =
1.31 V),[53] resulting from the electron-withdrawing effect of
the spirolinked xanthenes.
HOMO levels of DSX-IF and DSX-LPP present a slight sta-
bilization compared to those of DSF-IF and DSF-LPP
(Table 1). This stabilization is due to the presence of the
xanthenyl units, which hence allow a fine-tuning of the
HOMO/LUMO energy levels of these systems.
Optical properties: UV/Vis absorption spectrum of DSX-
LPP in solution presents a well-defined vibronic structure
with lmax =391 nm (Figure 4, top). Similarly, DSX-IF also
possesses a well-defined spectrum (lmax =344 nm, Figure 3,
top) but hypsochromically shifted by 47 nm (DEopt =3.09 eV
for DSX-LPP vs. 3.53 eV for DSX-IF; vide supra), clearly
signing the extension of the conjugation of the central p-
system.
The fluorescence maxima of DSX-LPP recorded at 395
and 419 nm (Figure 4, bottom) appear also to be redshifted
compared to those of DSX-IF, that is, 347 and 364 nm
(Figure 3, bottom). These two first transitions have been as-
signed to the 0–0 and 0–1 singlet transitions as already ob-
served for similar compounds.[79] The fluorescence quantum
yield in solution is very high (91% for DSX-LPP vs. 63%
for DSX-IF; Table 2), indicating that nonradiative decay
pathways are nearly suppressed in such structures. These
two molecules are hence very efficient violet (DSX-IF) and
blue (DSX-LPP) emitters. The Stokes shift[80] of both DSX-
LPP and DSX-IF is very small (4 and 3 nm, respectively)
and consistent with highly rigid structures with only very
DSX-IF and DSX-LPP reductions were studied in the
cathodic range and were both observed close to the di-
chloromethane electrolyte discharge (see the Supporting In-
formation). The shift of the discharge, however, allowed the
determination of the onset reduction potentials of DSX-IF
(ꢀ2.25 V) and DSX-LPP at (ꢀ2.2 V).
1
The gaps obtained from electrochemical data of DSX-IF,
DEel =3.64 eV, and DSX-LPP, DEel =3.25 eV, are wide and
are consistent with the measured optical gaps DEopt with,
however, a small deviation (0.15 eV or less). Thus, DSX-
LPP presents an electrochemical gap contracted by approxi-
mately 0.4 eV compared with that of DSX-IF. The estimated
The LUMO energy levels of DSX-LPP and DSX-IF have been ob-
tained not from a clear potential peak but from the intersection of the
tangent of the current line with the potential axis and hence these
values may contain a non-negligible experimental error (see CV in the
Supporting Information). Thus, the LUMO energy levels obtained from
red
onset
the onset reduction potential E
and those obtained from optical
data, appear to be slightly different (Table 1).
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C. Poriel et al.
Figure 4. Normalized absorption (top) and fluorescence (bottom, lexc
=
320 nm) of DSX-LPP in solution in cyclohexane (8ꢅ10ꢀ7 m, c) and in
the thin film: spin-coated: 20 mgmLꢀ1 from toluene (c); vacuum-de-
posited (c).
Figure 3. Normalized absorption (top) and fluorescence (bottom, lexc
=
275 nm) of DSX-IF in solution in cyclohexane (8ꢅ10ꢀ7 m, c) and in
spin-coated thin film (10 mgmLꢀ1 in pyridine, a).
This might be ascribed to self-absorption phenomena due to
the overlap of the 0–0 transition emission band and the ab-
sorption band that leads to a relative decrease of the intensi-
ty of the 0–0 transition.[79,83] This partial self-absorption of
the first emission band is due to the very small Stokes shift
observed for DSX-LPP.
An important feature, in the case of DSX-LPP, is never-
theless related to the difference observed in the fluorescence
spectra, depending of the deposition process (spin-coated vs.
vacuum-evaporated; Figure 4, bottom). Indeed, a vacuum-
evaporated thin-film of DSX-LPP reveals the presence of a
small broad band at low energy (around 525 nm), whereas a
spin-coated thin-film does not. Such a difference between
the two fluorescence spectra, depending of the deposition
process, may be assigned to p–p interchromophore interac-
tions of the pentaphenylene unit. Indeed, the packing ar-
rangement of DSX-LPP (Figure 1, bottom) has revealed
some short distances that can be responsible for such optical
little deformations in the excited state. The thin-film absorp-
tion spectrum of DSX-IF (Figure 3, top) is broader than its
solution spectrum with only a slight bathochromic shift
(2 nm) assigned to the difference between the dielectric con-
stant around the molecules.[81,82] Similarly, a comparison of
solution and thin-film fluorescence spectra of DSX-IF also
reveals a broader spectrum with a 25 nm redshift between
the two first emission peaks (Figure 3, bottom).
The thin-film UV/Vis and fluorescence spectra of DSX-
LPP present a fine vibronic structure and are almost identi-
cal to their solution spectra, being redshifted only by ap-
proximately 5 and 7 nm, respectively (Figure 4), due to the
different dielectric constants.[81,82] We also noted a marked
difference between solution and thin-film fluorescence spec-
tra of DSX-LPP in the relative intensity of the emission
bands, the second fluorescence band at 427 nm being much
more intense than the first at 403 nm (Figure 4, bottom).
Table 2. Optical data and decomposition temperatures Td.
[a]
[d]
[e]
labs liq. [nm]
lem[a] liq. [nm]
labs solid [nm]
lem solid [nm]
Fsoln.
Td
[%]
[8C]
DSX-LPP
DSX-IF
338 (2.70), 371 (3.27), 391 (3.46)
298 (2.03), 309 (2.13), 328 (2.08), 335 (2.08), 344 (2.40)
395, 419, 445, 475
347, 355, 364
338, 375, 396[b]
315, 346[c]
403, 427, 453, 480[b]
372, 388, 411 (sh)[c]
91
63
397
380
[a] In cyclohexane (8.10ꢀ7 m) with log e between brackets. [b] Spin-coated thin film (20 mgmLꢀ1 in toluene) [c] Spin-coated thin film (10 mgmLꢀ1 in pyri-
dine). [d] The relative quantum yield was measured with reference to quinine sulfate in 1n H2SO4 (F=0.546). The values are estimated ꢁ10%.[61]
[e] Decomposition temperatures Td corresponding to 5% loss have been determined by thermogravimetric analysis (under N2 atmosphere, rate:
58Cminꢀ1).
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Blue-Emitting Oligophenylene Frameworks
FULL PAPER
phenomena. This will be discussed in detail below in the
OLED section. Such phenomenon was not observed in the
case of DSX-IF.
Before any OLED application, the stability of the DSX-
IF and DSX-LPP fluorescence spectra as a function of the
temperature must be carefully studied.
OLED technology.[55,85] Indeed, the appearance of the well-
known low-energy emission band (also called g-band) has
been the subject of debate about its origin.[85] If this low-
energy emission band was initially explained by the excimer
emission (the formation of dimerized units in the excited
state that emit at lower energies) due to the p–p interchro-
mophore interactions of the main p-system,[86] it seems to be
agreed nowadays that the low-energy emission band is usu-
ally linked to the presence of keto-defects appearing in
phenylene derivatives.[87,88]
Despite being still debated, one possible manner to deter-
mine whether this low-energy emission band arises from
keto-defects or excimer emission due to p–p interchromo-
phore interactions of the main p-system is to study the be-
havior of a DSX-LPP thin-film in the strict absence of
oxygen. Indeed, several groups have proposed degradation
mechanisms involving atmospheric oxygen as an explanation
for the formation of these keto-defects.[88,89] Thus, a spin-
coating thin-film of DSX-LPP has been annealed at 2008C
for 24 h under an argon atmosphere and no low-energy
band was detected in the corresponding fluorescence spec-
trum (Figure 6, c). Then, the film was exposed to ambient
Gradual heating of the DSX-IF films in air from room
temperature to 2008C (1 h for each stage) does not lead to
any modification of the UV/Vis absorption spectrum neither
in shape, wavelength, nor in intensity (see the Supporting
Information). For DSX-LPP, the behavior is almost identical
with nevertheless a decrease in intensity (see the Supporting
Information). However, no shift to higher wavelengths, usu-
ally linked to strong p–p interchromophore interactions due
to aggregation in the solid state, is observed for both com-
pounds.[84]
Gradual heating of a spin-coated thin-film of DSX-IF in
air from room temperature to 2008C (1 h for each stage)
does not lead to clear photoluminescent spectral changes
(Figure 5, top) and DSX-IF presents hence a high color sta-
bility in the solid state.[56] DSX-LPP displays a drastically
different behavior as a broad band with two maxima at 493
and 523 nm appears and gradually increases upon heating in
ambient atmosphere (Figure 5, bottom). The origin of such
contributions at low energy has been extensively studied for
the last decade as it remains one important problem in blue
Figure 6. Normalized (425 nm) thin-film fluorescence spectra of DSX-
LPP (spin-coated: 20 mgmLꢀ1 in toluene, lexc =320 nm) at room temper-
ature (c), after annealing under argon (c, 24 h at 2008C) and after
annealing in air (c, 24 h at 2008C).
atmosphere and heating for a further 24 h. The appearance
of a broad low-energy emission band between 480 and
650 nm with two maxima recorded at approximately 493
and 523 nm was clearly evidenced (Figure 6, c), which in-
dicated that atmospheric oxygen has a strong influence on
the appearance of this band. These experiments, in the light
of other similar reports,[55,66,69,70,88] support the fact that this
low-energy emission band mainly arises from keto-defects
formation due to the oxidation of dialkyl bridgeheads and
not from aggregation leading to excimer emission of the
pentaphenylene units.2 However, as exposed above, the fluo-
rescence spectrum of a DSX-LPP thin-film at room temper-
Figure 5. Fluorescence spectra after annealing of top) DSX-IF (spin-
coated from pyridine at 10 mgmLꢀ1, lexc =275 nm, normalized at 373 nm)
and bottom) DSX-LPP (vacuum-deposited, lexc =320 nm, normalized at
425 nm) in air at different temperatures (c: after preparation, c: 1 h
at 1008C, c: 1 h at 1308C, c: 1 h at 1608C, c: 1 h at 2008C, c:
24 h at 2008C).
2
It should be noted that when a degraded film of DSX-LPP was redis-
solved after thermal annealing, its fluorescence spectrum is identical to
that of pristine compound in solution. Similar features have been previ-
ously observed for related oligomers.[88]
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12637

C. Poriel et al.
ature is slightly different depending on the deposition pro-
cess (absence of the low-energy emission band when spin-
coated and presence of this band when vacuum-evaporated,
Figure 4, bottom), which may be due to some close p–p
interchromophore contacts of the pentaphenylene core.
Hence, we believe that the large low-energy emission band
arises from two different phenomena: p–p interchromo-
phore interactions of the pentaphenylene unit and keto-de-
fects due to the oxidation of the dialkyl bridgeheads.
The thermal properties of DSX-IF and DSX-LPP were fi-
nally evaluated by means of thermogravimetric analysis
(Table 2). Decomposition temperature (Td, corresponding to
5% loss) shows that both compounds exhibit very good
thermal stabilities with high Td (3978C for DSX-IF and
3808C for DSX-LPP), which is highly important to improve
the lifetime of OLEDs. The xanthene derivatives DSX-LPP
and DSX-IF present therefore a higher Td compared with
those of their ꢁall carbonꢂ analogues, DSF-LPP and DSF-IF
(365 and 3558C, respectively).[53,54] Thus, the incorporation
of xanthene units within indenofluorene and pentapheny-
lene units leads to molecules with enhanced thermal stabili-
ty.
been assigned to the wide gap of DSX-IF leading to a high
injection barrier for both electrons and holes. The turn-on
voltage of the device, defined as the voltage for a luminance
of 1 Cdmꢀ2 is hence high, at around 12.7 V, translating the
difficulty in injecting charges in DSX-IF.
The luminance of the single-layer device with DSX-LPP
(50 nm) as EML reaches 220 Cdmꢀ2 (Table 3) signing hence
a better injection of holes due to the higher HOMO level of
DSX-LPP (ꢀ5.45 eV for DSX-LPP vs. ꢀ5.79 eV for DSX-
IF). As the LUMO level of DSX-LPP is also lower than
that of DSX-IF (ꢀ2.36 eV vs. ꢀ2.23 eV) the injection of
electrons from the Ca cathode is also slightly improved. The
turn-on voltage, 5.8 V, is therefore strongly decreased com-
pared with that of DSX-IF device A and the efficiencies are
impressively magnified by one order of magnitude (Table 3).
To improve the performances of the single-layer devices
A, various double and triple-layer OLEDs were fabricated
and characterized (devices B–D).
As DSX-IF possesses a relatively low HOMO level
(ꢀ5.79 eV), it seemed appropriate to insert an intermediate
layer of a hole transporter, namely NPB, to adjust its
HOMO level with the HOMO level of the anode (ꢀ5.1 eV).
The turn-on voltage of the ITO/PEDOT/NPB (30 nm)/
DSX-IF (40 nm)/Ca (device B) appears at approximately
10 V with a luminance impressively magnified by nearly two
orders of magnitude, reaching approximately 2800 Cdmꢀ2
with luminous and energetic efficiencies of 0.6 CdAꢀ1 and
0.16 LmWꢀ1, respectively (Figure 7, top).[56] Device B pres-
ents performances impressively magnified by two orders of
magnitude compared with those of device A (Table 3). Simi-
larly, device B with DSX-LPP as the EML also leads to a
strong enhancement of the performances (Figure 7, bottom).
Indeed, the maximum luminance is recorded at approxi-
mately 1400 Cdmꢀ2 with a luminous efficiency of approxi-
mately 0.3 CdAꢀ1. The performances of the DSX-LPP-
based device B are lower than those of the DSX-IF-based
device B exposed above. This might be explained by the fact
that NPB presents a HOMO level lying at ꢀ5.4 eV, almost
identical to that of DSX-LPP (ꢀ5.45 eV), but different to
the low-lying HOMO level of DSX-IF (ꢀ5.79 eV). Thus,
due to its HOMO level intermediate between that of
PEDOT/PSS (ꢀ5.1 eV) and that of DSX-IF (ꢀ5.79 eV),
NPB is hence more efficient in the case of DSX-IF than in
OLEDs with DSX-LPP and DSX-IF as emitting layers: To
investigate the potential applications of DSX-LPP and
DSX-IF as blue-emitting layers, several single-layer and
multilayer OLEDs were fabricated and characterized. The
energy-level diagrams and devices configuration of the dif-
ferent OLEDs are presented in the Supporting Information.
OLED performances: As there are only a few examples of
nondoped OLEDs based on spiroACTHNUGRTNEUNGxanthene oligomers in the
literature,[51,52] we first decided to focus on single-layer devi-
ces to study the intrinsic properties of DSX-LPP and DSX-
IF when used as the emitting layer (EML). Single-layer de-
vices, ITO/PEDOT/DSX-IF or DSX-LPP/Ca (devices A)
have been hence fabricated and their current density–volt-
age–luminance characteristics and luminous/energetic effi-
ciencies are presented in the Supporting Information. The
luminance of the single-layer device (device A) with DSX-
IF (40 nm) as the EML reaches around 31 Cdmꢀ2 with lumi-
nous and energetic efficiencies of 8.10ꢀ3 CdAꢀ1 and
2.10ꢀ3 LmWꢀ1, respectively. These poor performances have
Table 3. Selected data of different devices investigated.
Device Turn-on Maximum Luminous
Energetic
efficiency
Chromatic
coordinates
(x, y)
EL l [nm]
Thickness
of the organic
layers [nm]
voltage
luminance efficiency
2
[b]
[b]
[V][a]
[Cdmꢀ
]
[CdAꢀ1
]
[LmWꢀ1
]
A
B
C
D
A
B
C
D
5.8
8
5.9
10
12.7
9.9
7.3
8
220
1360
398
1600
31
2770
190
3740
8ꢅ10ꢀ2/4.5ꢅ10ꢀ2
0.35/0.32
4ꢅ10ꢀ2/1.5ꢅ10ꢀ2 0.35, 0.38
403, 425, 452, 525, 558, 590
406, 425 (sh), 452, 525, 560
404, 425, 452, 560
50
35/50
40/10
0.1/7 10ꢀ2
0.30, 0.31
0.30, 0.28
0.22, 0.22
0.18 , 0.08
0.19 , 0.08
–
DSX-LPP
DSX-IF
3.6ꢅ10ꢀ2/3.5ꢅ10ꢀ2 1.4ꢅ10ꢀ2/10ꢀ2
0.33/0.22
9ꢅ10ꢀ2/4ꢅ10ꢀ2
2ꢅ10ꢀ3/10ꢀ3
0.16/0.08
404, 425, 452, 535, 560, 584 (sh) 35/40/10
8ꢅ10ꢀ3/5ꢅ10ꢀ3
0.6/0.4
413, 590, 650
414, 438, 590, 650
–
40
30/40
40/10
35/40/10
3.6ꢅ10ꢀ2/3.5ꢅ10ꢀ2 1.5ꢅ10ꢀ2/10ꢀ2
1/0.75
0.3/0.19
0.19, 0.14
413, 438, 590, 650
[a] For 1 Cdmꢀ2. [b] Left value: maximum value obtained, right value: efficiency at the maximum of luminance.
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Blue-Emitting Oligophenylene Frameworks
FULL PAPER
Figure 7. Current density–voltage–luminance characteristics (left) and luminous/energetic efficiencies (right) of devices B. Top) ITO/PEDOT/NPB
(30 nm)/DSX-IF (40 nm)/Ca. Bottom) ITO/PEDOT/NPB (35 nm)/DSX-LPP (50 nm)/Ca.
the case of DSX-LPP. However, the performances of DSX-
LPP-based device B are nevertheless better than those of
device A because NPB avoids the quenching of the holes
near the anode.
Finally, to prevent holes from crossing the device and
reaching the cathode without recombination, a hole block-
ing layer (HBL) of bathocuproine (BCP, 2,9-dimethyl-4,7-di-
sive decrease of the turn-on voltage from 12.7 V in the
single-layer device A to 7 V in device C (see the Supporting
Information). The luminance and resulting efficiencies are
also enhanced by approximately one order of magnitude
(see the Supporting Information) leading us to conclude
that the BCP layer effectively avoids holes leakage and
allows a better charge recombination in the DSX-IF layer.
For DSX-LPP, device C presents a turn-on voltage almost
unchanged relative to the single-layer device A (5.9 V) and
the luminance is doubled (~400 Cdmꢀ2). However, the effi-
ciencies of device C do not vary much (see the Supporting
Information).
In conclusion, the insertion of a HTL, namely NPB, leads
to an impressive increase of the performances of DSX-LPP
and DSX-IF-based devices. Thus, NPB appears to compen-
sate unbalanced charge transport, both by enhancing holes
transport and by preventing electrons from reaching the
emissive layer/anode interface. Oppositely, the insertion of a
HBL, namely BCP, has a less impressive effect on the per-
formances of DSX-IF-based OLEDs and does not improve
so much that of DSX-LPP-based OLEDs. In the case of
DSX-LPP, this effect can be caused by a stronger un-
phenyl-1,10-phenanthroline) with
a
HOMO level of
ꢀ6.4 eV[18,90,91] has been added (devices D).
The ITO/PEDOT/NPB/DSX-IF/BCP/Ca device D shows
enhanced properties with a high luminance of approximately
3800 Cdmꢀ2 at 12.5 V and efficiencies of 1 CdAꢀ1 and
0.3 LmWꢀ1 (Figure 8, top). The performances are hence
almost doubled thanks to the insertion of BCP. As the
device structure remains very simple and not-optimized, the
NPB/DSX-IF/BCP system appears to be efficient compared
to other simple blue OLEDs recently reported in the litera-
ture.[6,26,52]
Alas, the addition of BCP as a HBL in a DSX-LPP-based
device (ITO/PEDOT/NPB/DSX-LPP/BCP/Ca: device D)
does not lead to any impressive enhancement of the per-
formances, which remain almost unchanged with a maxi-
mum luminance at 1600 Cdmꢀ2 and a luminous efficiency of
0.33 CdAꢀ1 (Figure 8, bottom).
To find out about the influence of HBL, BCP was inserted
between the emissive layer and the cathode in NPB-free de-
vices (device C). The addition of BCP in the case of DSX-IF
appears to be efficient. Indeed, device C presents an impres-
AHCTUNGTREGbNNNU alanced charge transport within the device than for DSX-
IF. As BCP appears almost unnecessary for DSX-LPP-based
devices, the holes do not need to be blocked at the DSX-
LPP/Ca interface. This means that all the holes in the DSX-
LPP layer recombine with electrons within the emissive
layer. In the case of DSX-IF, there is a need for holes block-
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12639

C. Poriel et al.
Figure 8. Current density–voltage–luminance characteristics (left) and luminous/energetic efficiencies (right) of devices D. Top) ITO/PEDOT/NPB
(35 nm)/DSX-IF (40 nm)/BCP (10 nm)/Ca. Bottom) ITO/PEDOT/NPB (40 nm)/DSX-LPP (45 nm)/BCP (10 nm)/Ca.
ing, which means that either hole mobility is higher or elec-
tron mobility is lower.
similar low-energy emission broad band was also observed
in solid-state fluorescence studies after annealing in air
(Figure 5, bottom). However, the relationship between the
low-energy band observed in fluorescence and that observed
in electroluminescence is clearly not evident as they may
arise from different causes. First, because their maxima are
different, and second because fluorescence and EL spectra
have been obtained under very different conditions, that is,
under an ambient atmosphere for the fluorescence spectra
or under an inert atmosphere for the EL spectrum. At this
early stage and in the light of literature, we can tentatively
assign the green emission band observed in the EL spectrum
of device A to intermolecular p–p stacking of the pentaphe-
nylene units. Indeed, Mꢆllen and co-workers have recently
studied a very similar pentaphenylene derivative and have
also reported the presence of a low-energy emission band in
the EL spectrum, assigned to intermolecular p–p stacking.[71]
However, the low-energy emission band observed in DSX-
LPP-based devices seems to have multiple origins. Indeed, it
is known in OLED technology that interactions between the
electrodes (cathode and anode) and the EML may occur,
and then the EL spectrum of the devices B and C, in which
the cathode/anode is not directly in contact with the EML
should be highly informative. The EL spectrum of device B
(Figure 9, top, c) with the anode covered with NPB pre-
Optical characterization: A detailed optical study of the dif-
ferent electroluminescence (EL) spectra is presented in this
last part, with an emphasis 1) on the origin of the low-
energy emission band observed in the case of DSX-LPP and
2) on the unexpected optical contribution of the well-known
hole-transporting-layer NPB.
The normalized EL spectra of DSX-LPP- and DSX-IF-
based devices are presented in Figure 9. All EL spectra of
DSX-LPP-based devices are made of two main contribu-
tions: a blue one with two main peaks at around 403 and
425 nm due to the fluorescence of the pentaphenylene core
and a green one with different maxima. The origin of these
green low-energy emission bands observed in all the EL
spectra of DSX-LPP-based devices will be discussed first.
Device A (Figure 9, top, c) presents two main contribu-
tions: a sharp well-resolved contribution, in the blue region,
with two maxima recorded at 403 and 425 nm in perfect ac-
cordance with the fluorescence spectrum of DSX-LPP
(Figure 5, bottom) and a very broad contribution in the
green region with three maxima recorded at 525, 558, and
590 nm. These two contributions in the EL spectrum of the
single-layer device A leads to a pseudo-white color with
chromatic coordinates of 0.35; 0.38 (see the CIE diagram in
the Supporting Information). It is important to stress that a
AHCTUNGTRENNUNG
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Blue-Emitting Oligophenylene Frameworks
FULL PAPER
devices A–C. Indeed, the EL spectrum of device D still pos-
sesses a green contribution but lower than that of device-
s A–C, assigned hence to intermolecular p–p stacking of
DSX-LPP. Thus, the low-energy emission bands observed in
all the DSX-LPP-based devices result from multiple origins,
which renders the precise interpretation very complicated
and notably their respective ratio. However, devices A–C
present a color close to white, therefore making them highly
interesting in lighting applications (see the CIE diagram in
the Supporting Information).
The second important feature is related to the blue contri-
butions in the EL spectra of the DSX-LPP- and DSX-IF-
based devices.
In the case of DSX-LPP, devices A and C present EL
spectra with two sharp bands at 403 and 425 nm (Figure 9,
top), in accordance with its fluorescence spectra (Figure 5,
bottom). In device B (with the NPB layer), these two bands
are still observed but in a drastically different intensity
ratio; the band at 403 nm being much more intense than
that at 425 nm (Figure 9, top, c). This difference seems to
indicate a non-negligible contribution of the NPB layer in
the EL spectra of DSX-LPP-based devices and leads us to
go deeper in the knowledge of such systems. Indeed, despite
being commonly used in OLED technology, reports describ-
ing the contribution of NPB in the EL spectra are very
rare.[92] We hence first fabricated a single-layer device using
NPB as the EML. The device ITO/NPB/Ca exhibits an ex-
tremely low luminance (<10 Cdmꢀ2) and its EL spectrum
presents a maximum at 415 nm (Figure 10, top). This value
does not perfectly fit with that of the ITO/NPB/DSX-LPP/
Ca device B (Figure 9, top, c). To find out about the pos-
sible energy transfer between NPB and DSX-LPP, the fluo-
rescence spectrum of a DSX-LPP/NPB blend (1:1 in weight)
has been recorded and has shown the presence of a maxi-
mum at 405 nm (Figure 10, bottom, c), in perfect accord-
ance with the maximum observed 1) in the EL spectrum of
the DSX-LPP-based device B (Figure 9, top, c) and 2) in
the fluorescence spectrum of the DSX-LPP thin-film
(Figure 10, bottom, c). Hence, the electron/hole recombi-
nation within the device appears to take place predominant-
ly in the DSX-LPP layer but also slightly in the NPB layer.
This recombination in two layers is probably due to the
close HOMO/LUMO levels of both compounds (ꢀ5.45 eV/
ꢀ2.36 eV for DSX-LPP and ꢀ5.4 eV/ꢀ2.3 eV for NPB). It
is, however, obvious in the light of the interesting perform-
ances of the single-layer DSX-LPP-based device A (see
above) and the very poor performances of the single layer
NPB-based device, that the recombination predominantly
takes place in the DSX-LPP layer. However, we believe that
an energy transfer occurs between NPB and DSX-LPP lead-
ing hence to EL spectra containing a little contribution of
the NPB layer.
Figure 9. EL spectra of DSX-LPP-based devices (top c: device A=
ITO-PEDOT/DSX-LPP/Ca, c: device B=ITO-PEDOT/NPB/DSX-
LPP/Ca, c: device C=ITO-PEDOT/DSX-LPP/BCP/Ca, c: de-
vice D=ITO-PEDOT/NPB/DSX-LPP/BCP/Ca) and DSX-IF-based devi-
ces (bottom c: device A=ITO-PEDOT/DSX-IF/Ca, c: device B=
ITO-PEDOT/NPB/DSX-IF/Ca, c: device D=ITO-PEDOT/NPB/
DSX-IF/BCP/Ca).
er, strongly decreased in intensity. Similarly, the EL spec-
trum of device C (Figure 9, top, c) with the cathode cov-
ered with BCP, also presents a broad band in the green
region but completely nonresolved, centered around
560 nm, and strongly reduced in intensity compared with
that of device A. This is the proof that the low-energy emis-
sion band in the case of DSX-LPP-based devices arises from
different causes. The first is clearly the Ca/EML interactions
occurring in devices A and B, which are suppressed in devi-
ce C. Indeed, such interactions are often seen despite the
fact that the exact nature of the phenomenon has not been
fully unraveled.[70] Similarly, the second cause is surely the
interactions occurring between PEDOT/PSS and the EML.
Indeed, such phenomena have been also previously suggest-
ed for similar extended p-conjugated backbones.[70] The EL
spectrum of device D (Figure 9, top, c), in which the elec-
trodes are both covered by NPB and BCP clearly confirms
that interactions occur between electrodes and the EML in
In the case of DSX-IF, the contribution of the NPB layer
appears to be more difficult to rationalize. Indeed, the nor-
malized EL spectrum of the ITO/NPB/DSX-IF/Ca device B
reveals two main peaks in the blue region, at 413 and
438 nm (Table 3), almost superimposable to the EL spec-
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12641

C. Poriel et al.
3.25 eV for DSX-LPP. Compared with their ꢁall carbonꢂ ana-
logues, DSF-LPP and DSF-IF, xanthene derivatives DSX-
LPP and DSX-IF showed enhanced thermal stability while
maintaining similar optical (high quantum yield, blue fluo-
rescence, and very small Stokes shift) and electrochemical
properties. The xanthene-based materials hence appear
promising for organic electronics applications as they could
replace the ꢁall carbonꢂ materials. The SMOLED application
studies have been carried out with DSX-LPP and DSX-IF
and have evidenced the potential of these new materials as
emissive layers. For example, the nonoptimized three-layer
DSX-IF-based device reaches a brightness of approximately
3800 Cdmꢀ2 with
1 CdAꢀ1, which is promising for
a
maximum luminous efficiency of
blue fluorescent
a
SMOLED (chromatic coordinates: 0.19; 0.14) and that can
be further improved by adjusting the nature and the thick-
nesses of the different layers. Similarly, the nonoptimized
double-layer DSX-LPP-based device reaches an interesting
brightness of approximately 1400 Cdmꢀ2 with a maximum
luminous efficiency of 0.35 CdAꢀ1. A detailed optical study
of the different EL spectra has been also carried out and
has notably allowed us to conclude that the low-energy
emission band observed in the EL spectra of DSX-LPP has
multiple origins (interactions with anode and cathode and
intermolecular p–p stacking) that cannot be directly linked
to the fluorescence spectra. This work, which appears to be
one of the rare examples of nondoped blue SMOLEDs
based on xanthene derivatives, may pave the way to the de-
velopment of this promising but very weakly studied class of
materials.
Figure 10. Top) EL spectra of the ITO/NPB/Ca device. Bottom) Fluores-
cence spectra (lexc =370 nm) of the spin-coated thin film of DSX-LPP
(c), NPB (c), and a blend (1:1 in weight) of DSX-LPP/NPB (c).
trum of the ITO/DSX-IF/Ca device A (Figure 9, bottom,
c) but also almost superimposable to the EL spectrum of
the ITO/NPB/Ca device (Figure 10, top). Thus, single-layer
devices with DSX-IF or NPB as EML present surprisingly
almost identical EL spectra.3 It is, however, obvious that the
good efficiencies obtained from DSX-IF-based devices
cannot be attributed to the luminescence of NPB, even if an
energy transfer may occur between NPB and DSX-IF, as de-
picted above in the case of DSX-LPP. The chromatic coordi-
nates in the CIE 1964 chromaticity diagram calculated from
the EL spectra of DSX-IF-based devices correspond to a
blue color and are suitable for display applications (see the
CIE diagram in the Supporting Information and Table 3).
Experimental Section
Synthesis: Commercially available reagents and solvents were used with-
out further purification other than those detailed below. THF was dis-
tilled from sodium/benzophenone. Dichloromethane was distilled from
P2O5 drying agent Sicapent (Merck). 1,2-Dichlorobenzene was distilled
from CaCl2 drying agent. Light petroleum refers to the fraction with b.p.
40–608C. Reactions were stirred magnetically, unless otherwise indicated.
Analytical TLC was carried out by using aluminum-backed plates coated
with Merck Kieselgel 60 GF254 and visualized under UV light (at 254
and/or 360 nm). Chromatography was carried out by using silica 60A CC
40–63 mm (SDS). 1H and 13C NMR spectra were recorded by using
Bruker 300 MHz instruments (1H frequency, corresponding 13C frequency
is 75 MHz); chemical shifts were recorded in ppm and J values in Hz.
The residual signals for the NMR solvents are: CDCl3, d=7.26 for the
proton and 77.00 ppm for the carbon. The following abbreviations have
been used for the NMR spectroscopic assignments: s for singlet, d for
doublet, t for triplet, q for quintet, and m for multiplet. HRMS were re-
corded either at the Centre Rꢀgional de Mesures Physiques de l’Ouest
(Rennes) or with a Microflex LT (Bruker). Diketones 1 and 3 and 2,2“-
DITP were prepared in a multistep synthesis as previously reported.[53,54]
Full characterization of DSX-IF can be found in our preliminary note.[56]
Names of chemicals have been generated with the naming service of
ACD-I lab, which determines the chemical name according to systematic
application of the nomenclature rules agreed upon by the International
Union of Pure and Applied Chemistry and International Union of Bio-
chemistry and Molecular Biology.
Conclusion
In summary, we have designed and synthesized, through dif-
ferent and efficient synthetic approaches, very rigid spiro-
configured blue fluorophores, DSX-LPP and DSX-IF, con-
taining an intracyclic oxygen atom. Since the central p-con-
jugation is extended going from DSX-IF to DSX-LPP, we
observed a gap contraction from 3.64 eV for DSX-IF to
3
It should be noted that DSX-IF-based OLEDs also present a small con-
tribution in the red region (590/650 nm) attributed to interactions with
the calcium cathode. Indeed, switching to LiF/Al instead of Ca leads to
the complete disappearance of this band (see the Supporting Informa-
tion).
Dispiro(xanthene-9,6’-indeno
[1,2ꢀb]fluorene-12’,9“-xanthene) (DSX-IF):
In schlenk tube under an argon atmosphere, 2,2”-DITP (0.30 g,
a
12642
www.chemeurj.org
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12631 – 12645

Blue-Emitting Oligophenylene Frameworks
FULL PAPER
0.63 mmol) was dissolved in dry THF (7 mL) and the solution was de-
gassed. The mixture was cooled to ꢀ788C and stirred at this temperature
for 10 min. A solution of nBuLi (1.6m in hexane, 1.6 mL, 2.56 mmol) was
added dropwise over 3.5 min. The resulting cloudy yellow solution was
stirred for a further 3.5 min and xanthone (0.27 g, 1.37 mmol), dissolved
in dry and degassed THF (20 mL), was added dropwise over 4 min by a
cannula. The reaction was allowed to stir overnight (from ꢀ708C to
room temperature) and the resulting mixture was poured into a saturated
solution of ammonium chloride (10 mL) and THF (20 mL) was added
before extraction with dichloromethane. The combined extracts were
dried (MgSO4) and the solvent was removed in vacuo. The resulting
crude mixture was taken up with dichloromethane/light petroleum (8:2)
and filtered to afford the dixanthenol intermediate as a pure product.
This dixanthenol was then suspended in acetic acid (30 mL) with concen-
trated hydrochloric acid (0.5 mL). The resulting mixture was allowed to
stir for 90 min at 1008C and water (30 mL) was added. After cooling to
room temperature, the colorless precipitate was filtered to afford the title
compound DSX-IF as a colorless solid (0.18 g, 50%). Elemental analysis
calcd (%) for C44H26O2: C 90.08, H 4.47; found: C 89.48, H 4.55. The
spectroscopic analyses and purity of DSX-IF were in perfect accordance
with our previous work.[56]
30.0 (CH2), 29.7 (CH2), 29.2 (CH2), 23.7 (CH2), 22.6 (CH2), 14.0 ppm
(Me); HRMS (ESI): m/z: calcd for C90H102O4: 1246.77781 [M]+; found:
1246.7774.
X-ray analysis: Crystal data of DSX-LPP are presented below. The crys-
tal was picked up with a cryoloop and then frozen at 100 K under a
stream of dry N2 on a APEX II Brucker AXS diffractometer for X-ray
data collection (MoKa radiation, l=0.71073 ꢄ). The structure was solved
by direct methods (SIR97),[93] and then refined by full-matrix least-
square methods based on F2 (SHELXL-97)[94] as implemented in the
WinGX software package.[95] An empirical absorption correction was ap-
plied. Hydrogen atoms were introduced at calculated positions (riding
model) included in structure factor calculation but not refined. CCDC-
816681 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Data for DSX-LPP: C92H100Cl6O2; M=1450.42; pale-yellow prism; crys-
¯
tal size=0.4ꢅ0.4ꢅ0.3 mm; triclinic; P1; a=12.1508(6), b=12.7242(6),
c=13.5848(7) ꢄ; a=95.064(2), b=109.720(2), g=98.741(2)8; V=
1932.09(17) ꢄ3; Z=1; 1calcd =1.247 Mgmꢀ3; MoKa radiation l=0.71073 ꢄ;
m=0.272 mmꢀ1; T=100 K; 24060 data (8746 unique, Rint =0.0297, 1.61 <
2
2
q <27.508); wR={S[w
G
ACHTUNGTRENNUNG
0.0446 for F values of reflections with Fo2 >2s
G
2
9’,9’,18’,18’-Tetraoctyl-9’,18’-dihydrodispiro(xanthene-9,6’-benzo-
A
ACHTUNGTRENNUNG[1,2-b]indenoACHTNUGTRENNUNG
tions); S=1.066 for 451 parameters. Residual electron density extremes
were 0.563 and ꢀ0.417 eꢄꢀ3
.
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Spectroscopic studies: Pyridine was purchased from Acros. Toluene (sem-
iconductor grade) was purchased from Alfa Aesar. DSX-IF (10 mgmLꢀ1
in pyridine, 90 mL) and DSX-LPP (20 mgmLꢀ1 in toluene, 90 mL) were
deposited on a quartz substrate by using a ꢁhome madeꢂ spin coater and
UV/Vis and photoluminescence spectra were immediately recorded. UV/
Vis spectra were recorded by using a UV/Vis spectrophotometer SHI-
MADZU UV-1605. The optical gap was calculated from the absorption
edge of the UV/Vis absorption spectrum by using the formula DEopt
(eV)=hc/l, in which l is the absorption edge (in meters), h=6.6262ꢅ
10ꢀ34 Js (1 eV=1.602ꢅ 10ꢀ19 J), and c=2.99ꢅ108 msꢀ1 (DEopt (eV)=
1237.5/l (nm). Photoluminescence spectra were recorded with a PTI
spectrofluorimeter (PTI-814 PDS, MD 5020, LPS 220B) by using a xenon
lamp either in solution (cyclohexane) or in thin-film. Quantum yields in
solution (lsol) were calculated relative to quinine sulfate (lsol =0.546 in
H2SO4 1n) by using standard procedures.[2] lsol was determined according
to Equation (1):
(414 mg, 4.0 mmol), and methane sulfonic acid (114 mL, 170 mg,
1.8 mmol) in 1,2-dichlorobenzene (20 mL) was heated at 1608C under an
argon atmosphere for 24 h. The reaction was quenched with water
(30 mL) and the mixture extracted with dichloromethane (3ꢅ10 mL).
The extracts were dried over MgSO4. The solvent was removed in vacuo
and the residue was purified by column chromatography on silica gel
eluting with dichloromethane/light petroleum (1:4) to give the title com-
pound (143 mg, 54%) as a colorless solid. M.p.=2878C (MeCN); Rf =0.4
1
(dichloromethane/light petroleum (1:9), H NMR (300 MHz; CDCl3) d=
7.54 (d, J=3 Hz, 4H; ArH), 7.49–7.47 (m, 2H; ArH), 7.42 (s, 2H; ArH),
7.38–7.34 (dd, J=8.4 Hz, J=1.5 Hz, 4H; ArH), 7.31–7.20 (m, 10H;
ArH), 6.87–6.82 (td, J=7.4 Hz, J=1.5 Hz, 4H; ArH), 6.57–6.54 ppm (dd,
J=7.4 Hz, J=1.5 Hz, 4H; ArH), 1.98 (t, J=7.2 Hz, 8H; CH2), 1.25–0.98
(m, 40H; CH2), 0.79 (t, J=7.2 Hz, 12H; Me), 0.70–0.54 ppm (m, 8H;
CH2); 13C NMR (75 MHz; CD2Cl2) d= 156.1 (C), 154.0 (C), 151.6 (C),
151.2 (C), 150.9 (C), 141.7 (C), 140.8 (C), 139.6 (C), 139.6 (C), 128.5 (C),
128.1 (CH), 126.9 (CH), 126.6 (CH), 125.7 (CH), 123.4 (CH), 122.8
(CH), 119.7 (CH), 117.0 (CH), 116.8 (CH), 116.7 (CH), 114.3 (CH), 54.9
(C), 53.7 (C), 40.6 (CH2), 31.7 (CH2), 30.0 (CH2), 29.2 (CH2), 29.2 (CH2),
23.8 (CH2), 22.6 (CH2), 14.0 ppm (Me); IR (KBr): n˜ =2926, 2853, 1600,
ꢀ
ꢁ
2
ðTs ꢂ ArÞ ns
ð1Þ
ꢀsol ¼ ꢀref ꢂ 100 ꢂ
nr
ðTr ꢂ AsÞ
1571, 1477, 1441, 1415, 1285, 1244 cmꢀ1 (Ph O); HRMS (ESI): m/z:
in which subscripts s and r refer to the sample and reference, respectively.
The integrated area of the emission peak in arbitrary units is given as T,
n is the refracting index of the solvent (ns =1.42662 for cyclohexane),
ꢀ
calcd for C90H98O2 +Na: 1233.74645 [M+Na]+; found: 1233.7429; ele-
mental analysis calcd (%) for C90H98O2: C 89.21, H 8.15; found: C 89.12,
H 8.14.
and
A
is the absorbance (ꢃ0.1). IR spectra were recorded on
a
BIORAD IRFTS175C.
4,4’,4’’,4’’’-(9,9,18,18-Tetraoctyl-6,9,15,18-tetrahydrobenzo
[1,2-b]indeno[2,1h]fluorene-6,6,15,15-tetrayl)tetraphenol (2): A mixture
of 9,9,18,18-tetraoctyl-9,18-dihydrobenzo[5,6]-s-indaceno[1,2-b]indeno-
[2,1h]fluorene-6,15-dione (1)[54] (70 mg, 0.08 mmol), phenol (75 mg,
ACHTUNGTREN[NUGN 5,6]-s-indaceno-
Thermal analysis: Thermogravimetric analyses (TGA) were carried out
A
ACHTUNGTRENNUNG
with a Rigaku Thermoflex instrument under a nitrogen atmosphere be-
A
ACHTUNGTRENNUNG
tween room temperature up to 10008C with a heating rate of 58Cminꢀ1
.
ACHTUNGTRENNUNG
Melting points were determined by using an electrothermal melting-point
apparatus.
0.8 mmol), and methane sulfonic acid (52 mL, 76 mg, 0.8 mmol) was
heated at 1608C under an argon atmosphere for 12 h. The reaction was
quenched with water (30 mL) and the mixture extracted with dichloro-
methane (5ꢅ10 mL). The extracts were dried over MgSO4. The solvent
was removed in vacuo and the residue was purified by column chroma-
tography on silica gel eluting with dichloromethane/AcOEt (95:5) to give
Electrochemical studies: All electrochemical experiments were per-
formed under an argon atmosphere by using a Pt disk electrode (diame-
ter 1 mm), the counter electrode was a vitreous carbon rod and the refer-
ence electrode was a silver wire in a 0.1m AgNO3 solution in CH3CN.
Ferrocene was added to the electrolyte solution at the end of a series of
experiments. The ferrocene/ferrocenium (Fc/Fc+) couple served as an in-
ternal standard. The three-electrode cell was connected to a PAR Model
273 potentiostat/galvanostat (PAR, EGandG, USA) monitored with the
ECHEM software. Dichloromethane with less than 100 ppm of water
(ref. SDS 02910E21) were used without purification. Activated Al2O3
was added in the electrolytic solution to remove excess moisture. For a
further comparison of the electrochemical and optical properties, all po-
tentials are referred to the SCE electrode that was calibrated at
ꢀ0.405 V versus the Fc/Fc+ system. By following the work of Jenekhe,[77]
the title compound (25 mg, 26%) as
a
colorless solid; 1H NMR
(300 MHz; CDCl3): d=7.99 (d, J=7.5 Hz, 4H; ArH), 7.51–7.46 (m, 6H;
ArH), 7.35 (d, J=8.4 Hz, 2H; ArH), 7.28–7.11 (m, 12H; ArH), 6.95 (s,
2H; ArH), 6.90 (d, J=7.5 Hz, 4H; ArH), 1.91 (t, J=7.2 Hz, 8H; CH2),
1.55 (s, 4H; OH), 1.35–1.02 (m, 40H; CH2), 0.79 (t, J=7.2 Hz, 12H;
Me), 0.67–0.47 ppm (m, 8H; CH2); 13C NMR (75 MHz; CDCl3): d=
151.0 (C), 150.8 (C), 149.6 (C), 149.1 (C), 147.8 (C), 141.9 (C), 141.9 (C),
141.0 (C), 140.9 (C), 140.8 (C), 127.9 (CH), 127.7 (CH), 126.6 (CH),
126.4 (CH), 124.4 (CH), 122.6 (CH), 120.1 (CH), 119.6 (CH), 115.2
(CH), 115.0 (CH), 114.4 (CH), 65.6 (C), 54.7 (C), 40.6 (CH2), 31.7 (CH2),
Chem. Eur. J. 2011, 17, 12631 – 12645
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12643

C. Poriel et al.
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red
onset
lated from: LUMO (eV)=ꢀ[E
(vs SCE) +4.4] and the HOMO level
ox
onset
from: HOMO (eV)=ꢀ[E
(vs SCE) +4.4], based on an SCE energy-
level of 4.4 eV relative to the vacuum. The electrochemical gap was cal-
culated from DEel = jHOMO–LUMOj (in eV).
EL fabrication and testing: OLEDs were fabricated by using the follow-
ing procedure. Indium–tin oxide (ITO) substrates on glass from Merck
underwent a solvent ultrasonic cleansing by using acetone and isopropa-
nol followed by a 15 min UV–ozone treatment. A layer of poly(3,4-ethyl-
ene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT/PSS
from HC Starck) was then deposited onto ITO by spin-coating at
5000 rpm, from a 3 wt.% water dispersion to form a 40 nm-thick layer.
PEDOT/PSS was subsequently annealed at 808C under vacuum for
30 min. This layer improves hole injection from the ITO to the HOMO
level of the organic material and increases the performances and the life-
time of the device. Then, NPB, DSX-IF, DSX-LPP, and finally BCP
layers were thermally evaporated under vacuum (ca. 10ꢀ6 mbar) at depo-
sition rates of 5 ꢄminꢀ1. The layer thickness was monitored in-situ
during the evaporation by a piezoelectric quartz. Calcium cathodes were
finally evaporated through a shadow mask. The OLEDs were then stored
and characterized under an inert atmosphere in a nitrogen glove box
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N.C. thanks the MENRT for a studentship. We wish to highly thank Dr.
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Received: March 15, 2011
Revised: June 16, 2011
Published online: September 27, 2011
Chem. Eur. J. 2011, 17, 12631 – 12645
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12645