Intramolecular excimer emission as a blue light source in fluorescent organic light emitting diodes: A promising molecular design

Article abstract of DOI:10.1039/c2jm16774c

Intramolecular excimer emission arising from organic molecules as a blue light source in fluorescent Small Molecule Organic Light Emitting Diodes (SMOLEDs) is almost absent from the literature. In this work, three aryl-substituted DiSpiroFluorene-IndenoFl

Full text of DOI:10.1039/c2jm16774c

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PAPER
Intramolecular excimer emission as a blue light source in fluorescent organic
light emitting diodes: a promising molecular design†
*
Damien Thirion, Maxime Romain, Joelle Rault-Berthelot and Cyril Poriel
*
€
Received 22nd December 2011, Accepted 5th February 2012
DOI: 10.1039/c2jm16774c
Intramolecular excimer emission arising from organic molecules as a blue light source in fluorescent
Small Molecule Organic Light Emitting Diodes (SMOLEDs) is almost absent from the literature. In
this work, three aryl-substituted DiSpiroFluorene–IndenoFluorenes (DSF–IFs 1–3) possessing
different fluorescent properties due to their different main emitters have been investigated through
a structure–property relationship study. Due to its particular geometry, the rigid DSF–IF platform 2
allows an ‘aryl/fluorene/aryl’ dimer to be preformed in the ground state leading, in the excited state, to
a deep blue fluorescent emission through strong p–p intramolecular interactions between the two ‘aryl/
fluorene/aryl’ arms. 2 has been successfully used as an emitting layer in a SMOLED with
electroluminescence arising from electrogenerated intramolecular excimers and the properties of these
excimer-based OLEDs have been compared to those of two model compounds (1 and 3). The simple
and non-optimized double-layer device displays a deep blue colour (CIE coordinates: 0.19; 0.18)
exhibiting a luminance of 510 Cd mꢀ2 with a luminous efficiency of ca. 0.1 Cd Aꢀ1. This work is, to the
best of our knowledge, the first rational and comparative study describing an intramolecular excimer
based-SMOLED.
emission could be used as a light source in an OLED. Indeed, the
Introduction
strategy to generate blue light (and light in general) in an OLED
is most of the time always the same and consists of designing
a fluorophore with emission properties directly arising from its
p-conjugated backbone modulated (or not) by the electronic
effects of different substituents (for example, donor–acceptor
p-conjugated dyes).8–12 In this work, we wish to report a unique
and promising strategy to generate light in an OLED, that is
using intramolecular excimer emission. Recently, our group has
designed an original family of aryl-substituted DiSpiroFluorene–
IndenoFluorene (DSF–IF) derivatives, which possess two ‘‘aryl–
fluorene–aryl’’ moieties in a rigid face-to-face arrangement. This
particular face-to-face geometry predominantly leads for these
‘3p–2 spiro’‡ compounds13 to conformationally controllable
intramolecular excimer fluorescence emission.14–16 However,
these systems have been mainly studied in solution and never
incorporated in an optoelectronic device. In the present work, we
wish to report preliminary results on intramolecular excimer
emission as a deep blue light source in an OLED. Three struc-
turally related DSF–IFs 1–3 (Scheme 1) possessing different
emission properties have been investigated through a structure–
property relationship study. Their optical properties have first
been investigated in detail before successfully incorporating them
as an emitting layer (EML) in blue SMOLEDs. The perfor-
mances and electro-optic properties have been discussed. If the
Intermolecular p–p interactions play a key role in the field of
Materials Science and especially in Organic Electronics. Indeed,
in an Organic Field Effect Transistor (OFET), intermolecular
p–p interactions between molecules in the thin solid film must be
very strong to ensure high mobility of charge carriers.1–4
Conversely, in an Organic Light Emitting Diode (OLED),
intermolecular p–p interactions should be usually suppressed to
avoid any parasite and uncontrolled emission colour due to
intermolecular excimer formation (dimer in the excited state).
However, in white OLEDs the use of a single fluorescent emitter
which can form an excimer through intermolecular p–p inter-
actions has been designed in order to cover the whole visible
range from 400 to 700 nm.5–7 Thus, due to their importance in
OLED technology, intermolecular p–p interactions have been
hence extensively studied for the last twenty years, but intra-
molecular p–p interactions have not been deeply investigated.
Indeed, if one can control the formation of intramolecular p–p
interactions within a fluorescent dye and in the meantime avoid
intermolecular p–p interactions, then intramolecular excimer
Universiteꢀ de Rennes 1—UMR CNRS 6226 ‘‘Sciences Chimiques de
Rennes’’, Campus de Beaulieu, 35042 Rennes Cedex, France. E-mail:
Cyril.poriel@univ-rennes1.fr; Joelle.rault-berthelot@univ-rennes1.fr
† Electronic supplementary information (ESI) available: Some
1
SMOLED characterizations for 1–3, copy of H and 13C NMR spectra
and CV of 3, CIE coordinates of the different OLEDs. See DOI:
10.1039/c2jm16774c
‡ ‘3p-2 spiro’ is a geometric concept meaning three p-conjugated systems
linked by two spiro bridges. See ref. 13.
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Optical properties
The absorption spectrum of 1, in solution in THF (Fig. 1A), is
quite well defined with one shoulder at 301 nm and four maxima
at l ¼ 314, 331, 337, 345 nm.14 As previously reported, the three
first main bands have been ascribed to the ‘‘aryl–fluorene–aryl’’
moieties as these bands are also found in other spirolinked oli-
goaryl derivatives.22–24 The band at 345 nm has been assigned to
the p–p* electronic transition of the (1,2-b)-indenofluorenyl core
in perfect accordance with that of its non-substituted (without
aryl rings) congener (1,2-b)-DSF–IF (lmax ¼ 345 nm).25,26 It
should be stressed that this band is red-shifted by 11 nm
Scheme 1 Molecules investigated in this work.
use of intermolecular excimer emission as a light source in
a SMOLED is known for organic5–7 and organometallic
compounds,17–20 the use of intramolecular excimer emission
arising from pure organic molecules has been reported, to the
best of our knowledge, only once.16 In the light of the marked
difference observed between the excimer-based SMOLEDs (with
2 as EML) and the ‘‘classical’’ SMOLEDs (with 1 or 3 as EML),
we are convinced that this strategy may open new avenues in the
design of efficient blue fluorophores for optoelectronic
applications.
compared to that of (1,2-b)-indenofluorene ((1,2-b)–IF, lmax
¼
334 nm), due to the electronic effects of the two spirolinked
diarylfluorene units.25–27 The absorption spectrum of 2, in solu-
tion in THF (Fig. 1B), is less defined compared to that of 1 with
nevertheless one shoulder at 314 nm and two maxima at 328 and
340 nm. As mentioned above, the bands at 314 and 328 nm have
been ascribed to the ‘‘aryl–fluorene–aryl’’ moieties. The transi-
tion recorded at 340 nm has been assigned to the (2,1-a)-inden-
ofluorenyl core in accordance with that of its non-substituted
congener (2,1-a)-DSF–IF (lmax ¼ 339 nm).25 The slight blue-shift
Results and discussion
The molecular design adopted is the following: the (2,1-a)-DSF–
IF platform found in 2 has been chosen in order to impose a rigid
arrangement of the two cofacial ‘‘aryl/fluorene/aryl’’ arms and to
allow these arms to interact with one another in the ground state.
This will then favour the facile and easily controllable formation
of excimers in the excited state. In addition, the 3D geometry of 2
should suppress the non-desired intermolecular p–p interactions
in thin film leading to a stable blue colour only arising from
intramolecular p–p interactions. Two other molecules (1 and 3),
with a very similar molecular structure but with ‘classical’ fluo-
rescence properties arising from their p-conjugated backbone
(without any excimer emission), will be also investigated, for
comparison purpose, in order to evaluate the efficiency of using
intramolecular excimer emission in an OLED. The molecule 1,
a regioisomer of 2, presents a (1,2-b)-DSF–IF core and two
‘‘3,4,5-trimethoxyphenyl/fluorene/3,4,5-trimethoxyphenyl’’ arms
but the latter do not interact with one another (as it is the case in
2). The molecule 3 is also constituted of a (1,2-b)-DSF–IF core,
but this time the indenofluorenyl unit is substituted by two
3,4,5-trimethoxyphenyl moieties.
observed between the maxima of 2 (lmax ¼ 340 nm) and 1 (lmax
¼
345 nm) has been assigned to a better delocalization of p-elec-
trons in 1 compared to 2 due to the different indenofluorenyl
cores (1,2-b vs. 2,1-a).14,25 In addition, a red shift of 18 nm is
detected between the maximum of 2 and that of (2,1-a)-indeno-
fluorene ((2,1-a)-IF, lmax ¼ 322 nm), due to the electronic effects
of the two spirolinked diarylfluorene units. The electronic influ-
ence on the indenofluorenyl core of a cofacial diaryl-fluorene
p-dimer found in 2, leading to a 18 nm red shift, is different from
that of two non-directly interacting diaryl-fluorene units as
found in 1 (red shift: 11 nm, vide supra). Such a difference was
also observed in the (2,1-a)-IF/(2,1-a)-DSF–IF (red shift: 17 nm)
and (1,2-b)-IF/(1,2-b)-DSF–IF (red shift: 11 nm) series. Thus,
isomers 1 and 2 present absorption properties governed by both
their indenofluorenyl and their ‘‘aryl–fluorene–aryl’’ moieties.
DSF–IF 3 displays a different behaviour due to its different
substitution pattern compared to 1 and 2. Indeed, 3 possesses
a (1,2-b)-indenofluorenyl core substituted by two 3,4,5-trime-
thoxyphenyl moieties. Thus, the optical properties of 3 should be
almost fully governed by the ‘‘aryl/indenofluorene/aryl’’ moiety.
Indeed, 3 presents an absorption spectrum with a maximum
recorded at 362 nm, red shifted by 17 nm compared to 1 due to
the extension of the conjugation length of the 2,6-diaryl-(1,2-b)-
indenofluorenyl core. This wavelength (362 nm) has been
assigned to a p–p* transition of the ‘‘aryl/indenofluorene/aryl’’
moiety as it fits well with the maximum reported for a structur-
ally related compound, that is 2,6-diphenylindenofluorene
(lmax ¼ 350 nm in DMF)28 with nevertheless a 12 nm red shift
caused by the electron donating behaviour of the six methoxy
groups borne by 3.24,29,30
Synthesis
The synthesis of 1 and 2 has been previously reported.21 3 has
been synthesized through a Pd-catalyzed Suzuki–Miyaura cross-
coupling reaction between the related dibromo-indenofluorene
substituted compound (1,2-b)-DSF(t-Bu)4–IF(Br)2 ref. 12 and
3,4,5-trimethoxyphenylboronic acid using Pd2(dba)3/P(t-Bu)3 as
the catalytic system, potassium carbonate as the base in
a mixture of toluene and water at 100 ꢁC (yield: 55%, Scheme 2).
In thin film, 1–3 display the same behaviour that is a broader
absorption spectrum and a slight red shift, usually assigned to the
different environment surrounding the molecules (solid vs.
liquid) and hence the different dielectric constants.22,31
The fluorescence spectrum of 1, in solution in THF (Fig. 2A),
which mainly arises from its ‘‘aryl/fluorene/aryl’’ moieties, pres-
ents two maxima at 381 and 393 nm.14 These maxima are red
Scheme 2 Synthesis of 3.
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Fig. 1 Absorption spectra of 1 (A), 2 (B) and 3 (C) in solution (blue line, THF, C ¼ 10ꢀ6 M) and in thin-solid films (red line), depositing solvent: 1,2-
dichlorobenzene, C ¼ 15 mg mLꢀ1 (1 and 2); THF; C ¼ 10 mg mLꢀ1 (3).
Fig. 2 Emission spectra of (A): 1, (B): 2 and (C): 3, lexc ¼ 340 nm (1 and 2), lexc ¼ 336 nm (3) in solution (THF 10ꢀ6, blue line) and in thin-solid films (red
line); depositing solvent: 1,2-dichlorobenzene, C ¼ 15 mg mLꢀ1 (1 and 2); THF; C ¼ 10 mg mLꢀ1 (3).
shifted compared to those of 2,20,7,70-tetraphenyl-9,90-spirobi-
fluorene (l ¼ 359, 378 nm), which can be considered as two
spirolinked ‘‘phenyl/fluorene/phenyl’’ units.22 This red shift,
ascribed to the electron-donating effect of the methoxy groups
(vide supra),24,29,30 allows a fine tuning of the emission colour of 1.
Compound 2 displays a drastically different fluorescent spectrum
constituted of a single, large, structureless and red-shifted band
(with respect to 1) with a maximum recorded at 457 nm (Fig. 2B).
This behaviour highlights the remarkable effect of the cofacial
fluorenes arrangement found in 2 and has been ascribed to the
emission of intramolecular excimers, due to the interactions
derivatives by ab initio quantum chemical methods.43 Compared
to 1, there is a red shift of the two main maxima (11 and 19 nm)
for 3 due to the extension of the conjugation length from diaryl-
fluorene in 1 to diaryl-indenofluorene units in 3. However and
despite this extension of the conjugation length, the emission
maxima of 3 (lmax ¼ 391/412 nm) are remarkably blue-shifted
compared to that of 2 (lmax ¼ 457 nm, vide infra). This feature
highlights the efficiency of the molecular design found in 2, which
allows reaching a deep blue emission with relatively short p-
conjugated molecular fragments (i.e. ‘‘aryl/fluorene/aryl’’),
whereas this deep blue emission wavelength is almost impossible
to reach with structurally related ‘aryl–fluorene–aryl’ deriva-
tives.24 For example, in the series of comparable spiro-‘aryl–
fluorene–aryl’ compounds, described by Salbeck and co-workers,
possessing two, four, six, or eight phenyl units connected to the
fluorene core, the fluorescence maxima in solution only gradually
increase in the order 359, 385, 395, and 402 nm.24 Thus, reaching
an emission wavelength of 460 nm, as it is the case for 2, seems to
be almost impossible (even with the introduction of appropriate
electron–donor substituents) as a limiting value for long chains
exists.24 Indeed, even bridged-polyphenylene derivatives such as
polyfluorene, polyindenofluorene or polypentaphenylene deriv-
atives, known to possess high wavelengths in the blue region,
only present an emission maximum at around 420, 430 nm and
445 nm respectively.44 The use of intramolecular excimers emis-
sion resulting from ‘aryl–fluorene–aryl’ fragments to obtain deep
blue fluorescent emission appears hence as a possible solution to
this issue.
14
between ‘‘aryl–fluorene–aryl’’ moieties in the excited state.x
Several groups have also reported similar behaviour for various
molecular systems with p–p interactions.32–40 An important
feature in the fluorescent spectrum of 2 is related to the large
Stokes shift i.e. 116 nm which leads to an emission in the deep
blue region. This feature is of key importance as no self-
absorption may hence occur in this blue emitter. The well-
resolved emission spectrum of 3 (Fig. 2C, lmax ¼ 391/412 nm)
compared to its absorption spectrum (Fig. 1C) suggests a more
rigid/planar structure of 3 in the excited state since the bonds
joining the indenofluorene unit and the aryl rings acquire some
double bond characters.16,41,42 Indeed, for species that comprise
torsional degrees of freedom in their p-conjugated core, varying
degrees of deviation from the classical mirror image behaviour
can be detected. Such differences between absorption and emis-
sion bandshapes may be related to the flexible character of the
molecules, as already studied for p-terphenyl and indenofluorene
In thin-solid film, 1 and 3 present a broader and red-shifted
spectrum compared to their solution ones due to the different
dielectric constants of the environments (liquid vs. solid) as
x It should be noted that the fluorescence spectra of 2 are independent of
the concentration (10ꢀ7 to 10ꢀ3 M).
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discussed above. However, such behaviour can also translate
some p–p intermolecular interactions which can further disturb
the purity of the colour. We also noted a marked difference
between solution and thin-film fluorescence spectra of 3 in the
relative intensity of the emission bands, with the second fluo-
rescence band at 422 nm being more intense compared to the first
at 402 nm (Fig. 2C). This might be ascribed to self-absorption
phenomena due to the overlap of the 0–0 transition emission
band and the absorption band which lead to a relative decrease
of the intensity of the 0–0 transition.45 However, the shift
observed for 3 is without any comparison with that described in
the case of the previously described 2,6-diphenylindenofluorene
for which the thin-film emission is detected at ca. 500–530 nm
clearly signing a stacking of the molecules in the solid state due to
strong intermolecular p–p interactions.28 Thus, the absence of
strong p–p interactions in the case of 3 is hence due to the
presence of the two spiro(t-Bu-fluorene) units on each side of the
indenofluorene core and highlights the efficiency of the present
molecular design to avoid any intermolecular p–p interaction.12
Interestingly, the fluorescence spectrum of 2 in solution is
almost identical to that in the solid state (Fig. 2B) with only
a slight red shift of around 6 nm. Thus, the intramolecular ‘‘aryl–
fluorene–aryl’’ dimer formed in the excited state seems to lead to
a stable deep blue fluorescence emission even in the solid state.
This feature shows that no intermolecular p–p-interactions
occur in the thin-film between the molecules of 2, due to the steric
protection induced by the geometry of the molecule. The
molecular design of 2 allows hence gathering in a single molecule,
a wide HOMO/LUMO gap (ca. 3.4 eV, Table 1) and its associ-
ated properties (high stability toward oxidation for example) and
a deep blue emission in the solid state. Finally, the quantum yield
of 1–3 has been evaluated. 1 possesses a high quantum yield, ca.
75% relative to quinine sulfate, as already observed for similar
closely, in a favourable conformation, and require only a very
slight spatial reorganization to form excimers. This close vicinity
between the two ‘‘aryl–fluorene–aryl’’ chromophores probably
leads to a decrease of any non-radiative decay processes and
hence keeps the quantum yield at a reasonable value (ca. 35%),
hence promising for further OLED applications.
To conclude on the optical properties of these molecular
systems, one can say that their fluorescences are drastically
different and are mainly driven: in 1, by the ‘‘aryl/fluorene/aryl’’
moieties, in 2 by the excited dimer constituted of two face-to-face
‘‘aryl/fluorene/aryl’’ moieties and in 3 by the ‘‘aryl/indeno-
fluorene/aryl’’ moiety. In addition, we noted for 2 very appealing
properties for a fluorophore in which the fluorescence arises from
excimer emission. Indeed, its quantum yield is in an acceptable
range and its thin-film fluorescence spectrum is almost
unchanged compared to its solution spectrum.
An important feature in OLED technology and particularly
for blue light is related to the stability of the colour as a function
of the temperature.48 Indeed, it is known that an OLED device
can reach a temperature of ca. 86 ꢁC and the stability of the
emitted colour upon heating in air is hence of key importance.49
In order to determine the stability of the colour, spin-coated
films of DSF–IFs 1–3 on quartz substrates were exposed to
thermal stress conditions under ambient atmosphere (Fig. 3).
Gradual heating of a spin-coated film of 1, in air, from room
temperature to 200 ꢁC (1 h for each stage and finally one day at
200 ꢁC), leads to a red-shift of around 6 nm and a broader
spectrum. However, even after 24 h at 200 ꢁC in the presence of
air, no sign of low-energy emission band usually called Green
Emission Band (GEB) was observed as it is usually the case for
fluorene derivatives.48,50 For example, Lahti and co-workers have
recently investigated the behaviour of fluorescent spectra as
a
function of the temperature for E,E-(2,7-bis(3,4,5-
compounds with the ‘aryl–fluorene–aryl’ frameworks.24
3
trimethoxyphenylethenyl-9,90-diethylfluorene)),
a
structurally
possesses a higher quantum yield, ca. 90% than that of 1. This
result is in accordance with the fact that the indenofluorene
molecule possesses a higher quantum yield than that of the flu-
orene or spirobifluorene molecule.26 2 presents again a drastically
different behaviour compared to its analogues 1 and 3 as its
quantum yield is of only 35%, fully consistent with the fact that
the fluorescence of 2 arises from intramolecular excimers.32,46,47
However, despite being smaller than those of 1 and 3, the
quantum yield of 2 remains relatively high for an excimer-based
emission. Thus, in the case of 2, due to the rigidity of the back-
bone, the ‘‘aryl–fluorene–aryl’’ chromophores are located
related compound to 1. The authors have notably reported the
existence of an important GEB (l ¼ 540 nm) after heating a thin
film at 200 C in the presence of air.51 This GEB emission has
ꢁ
been assigned to the presence of fluorenone moieties at the flu-
orene bridge due to the oxidation process. However, in the case
of 1 possessing spiro aryl bridges (and not alkyl bridges), no
oxidation degradation processes leading to a GEB is observed.
The emission colour of 1 appears then to be highly stable upon
heating in the presence of air. In the case of 3, gradual heating of
a spin-coated film, in air, from room temperature to 160 ꢁC does
not lead to any modification of the shape of the fluorescent
spectra, highlighting the high colour stability. This feature has
been assigned to the presence of two t-butyl-fluorene units on
each side of the indenofluorenyl core leading to a steric protec-
tion and then avoiding any p–p intermolecular interaction.12
The most striking feature here is again related to the
compound 2. Thus, from room temperature to 160 ꢁC, the
fluorescence spectrum of 2 remains perfectly stable with a main
emission centred at 463 nm (no additional GEB and no broad-
ening of the spectrum compared to the solution spectrum). This
feature is remarkable since the fluorescence emission of 2 results
from excimer emission. Interestingly, the heating at 200 ꢁC is
accompanied by a slight modification of the fluorescence spec-
trum which now presents a maximum at 453 nm, surprisingly in
accordance with that of its solution fluorescence spectrum.
Table 1 Electronic properties of compounds 1, 2 and 3
HOMOa/eV
LUMOb/eV
DEoptc/eV
Ref.
1
2
3
ꢀ5.49
ꢀ5.33
ꢀ5.43
ꢀ2.04
ꢀ1.94
ꢀ2.26
3.45
3.39
3.17
14
14
This work
a
ox
Calculated from the onset oxidation potential Eonset (HOMO
ox
(eV) ¼ ꢀ[Eonset (vs. SCE) + 4.4], based on an SCE energy level of 4.4
b
eV relative to the vacuum). Calculated from the HOMO energy level
and the optical band gap. c Optical band gap DEopt ¼ hc/l(DEopt (eV) ¼
1237.5/l in nm) has been estimated from the liquid UV-vis spectra in
THF (l is the edge of the absorption spectrum).
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Fig. 3 Normalized fluorescence spectra of 1 (A), 2 (B) and 3 (C) in the solid state at various temperatures (1 h for each stage and one day at 200 ꢁC),
lexc ¼ 300 nm (1 and 2), lexc ¼ 317 nm (3); depositing solvent: 1,2-dichlorobenzene, C ¼ 15 mg mLꢀ1 (1 and 2); THF; C ¼ 10 mg mLꢀ1 (3).
Similar changes of the solid-state fluorescence spectrum with
temperature are described in the literature for butadiene deriv-
atives,3,52,53 but in these examples, the monomer-like or excimer-
like fluorescence comes from specific ‘‘intermolecular packing’’
which may be modified with the temperature. In a ‘‘herringbone’’
arrangement, minimal electronic coupling between molecules
leads to a monomer-like emission, whereas in a ‘‘brickstone’’
arrangement, molecular packing leads to excimer-like fluores-
cence. In the case of 2, as previously observed for spiroter-
fluorene face-to-face fluorophores,16 heating of the films leads to
a slight reorganization of the face-to-face ‘‘aryl–fluorene–aryl’’
moieties in the solid with less p–p interactions and hence a blue
shift of the emission band.
evaluated from electrochemical data (see figures in the ESI†) and
optical data (Table 1).
We first decided to focus on single-layer devices to study the
intrinsic properties of 1–3 when used as EML. Single-layer
devices, ITO/PEDOT/EML/Ca (devices A), have been fabricated
and characterized. In these devices, ITO/PEDOT is referred to as
the anode, in which PEDOT (poly(3,4-ethylene dioxythiophene))
doped with PSS (poly(styrene sulfonate)) is the hole injecting
layer and calcium is the cathode. These devices are considered as
single-layer ones as PEDOT is not taken into account in the
number of layers. The choice of the low work-function calcium
cathode (ꢀ2.9 eV) was evident in the light of the very high
LUMO levels of all the compounds investigated (Table 1). The
luminance of the device A using 1 as EML (50 nm) reaches
around 52 Cd mꢀ2 with luminous and energetic efficiencies of
3.1 ꢂ 10ꢀ2 Cd Aꢀ1 and 1.1 ꢂ 10ꢀ2 Lm Wꢀ1 respectively (see
figures in the ESI†). The turn-on voltage of the device, defined as
the voltage for a luminance of 1 Cd mꢀ2, is recorded around
7.2 V. For 3, the performance of the device A appears to be very
similar to that discussed above for 1. Thus, the luminance of the
device A using 3 as EML (40 nm) reaches around 50 Cd mꢀ2 (see
figure in the ESI†) with luminous and energetic efficiencies of
1.6 ꢂ 10ꢀ2 Cd Aꢀ1 and 4 ꢂ 10ꢀ3 Lm Wꢀ1 respectively (turn on
voltage: 10.8 V). The device A with 2 as EML (45 nm) was finally
investigated and a maximum luminance of 100 Cd mꢀ2 with
luminous and energetic efficiencies of 3.7 ꢂ 10ꢀ2 Cd Aꢀ1 and
1.8 ꢂ 10ꢀ2 Lm Wꢀ1 were obtained. This device not only exhibits
the best performances among the investigated single-layer
devices but also presents the lowest turn on voltage (5 V).
Three main information may be extracted from this dataset: (i)
the three single-layer devices A display modest performances
translating the difficulty in injecting charges in the three
compounds. This feature was assigned to the wide HOMO/
LUMO gap of these materials coupled to their very high LUMO
energy level (around ꢀ2.0/ꢀ2.2 eV, Table 1) and low HOMO
energy level (around ꢀ5.3/ꢀ5.5 eV, Table 1), leading to a very
high injection barrier for both electrons and holes. Even if the
performances are modest, single-layer devices allow a perfect
comparison of the intrinsic properties of each molecule and
a clear definition of their potential for OLED applications.{ (ii)
The molecules 1 and 3 which possess a similar HOMO/LUMO
These studies showed the high colour stability upon heating of
these three DSF–IFs derivatives. As the fluorescent emission of 2
arises from intramolecular excimer, the high stability of the deep
blue colour appears even remarkable and to the best of our
knowledge without any precedent.
Small Molecule Organic Light-Emitting Diodes (SMOLEDs)
using 1, 2 and 3 as the emitting layer
Several single-layer and double-layer OLEDs using 1, 2 and 3 as
the emitting layer were fabricated and characterized (Fig. 4). The
HOMO/LUMO levels of the three compounds have been then
Fig. 4 Current density–voltage–luminance characteristics (left) and
luminous/energetic efficiencies (right) of device B. Top: ITO/PEDOT/
NPB (40 nm)/1 (50 nm)/Ca device and bottom: ITO/PEDOT/NPB
(40 nm)/2 (45 nm)/Ca device.
{ Very efficient single-layer OLEDs emitting a blue colour, key point for
the future of OLED technology, requires to use very specific molecules
with accurate molecular design such as donor–acceptor p-conjugated
fluorescent dyes (see ref. 8–12).
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gap present nearly identical performances. (iii) The best results
were obtained with 2 as EML. Indeed, despite 2 possessing
a similar HOMO/LUMO gap compared to that of 1, the emis-
sion of 2 results from an ‘‘aryl–fluorene–aryl’’ dimer and then the
electrogenerated exciton is delocalized over this dimer, leading to
a red-shifted emission band (vide infra) and hence to enhanced
performances.
contribution at low energy has been extensively studied for the
last decade as it remains one important problem in blue OLED
technology.48,50,54,55 It is widely agreed nowadays that the GEBs
appearing in poly or oligo(dialkyl)fluorene arise from keto-
defects in the form of fluorenone moieties.50 Similarly, GEBs
have also been observed in structurally related poly-p-phenylenes
such as poly(alkylindenofluorene) or ladder-type poly or oli-
go(alkylpentaphenylene).48,55–59 However, in the case of 1, the
bridges are protected by a spirofluorene unit, not sensitive to
oxidation degradation processes55,60 and hence the oxidation of
the bridges leading to keto-defects appears to be impossible. In
addition, we have also shown above that even annealing in the
presence of oxygen, the fluorescent emission spectra do not
possess any GEB (Fig. 3A). Thus, it is difficult to assign the GEB
observed in the EL spectra of 1 to intermolecular p–p interac-
tions leading to excimer emissions. We believe that this GEB may
arise from interactions with the electrodes55,57 and/or exciplex
emissions at the organic/organic interface,61,62 as previously
described. Indeed, Ca-EML and/or PEDOT/PSS-EML interac-
tions at the interface between the EML and the electrode have
been previously suggested for similar extended p-conjugated
backbones.55,57 The chromatic coordinates of the devices A and B
with 1 as EML are (0.24; 0.24)/(0.25; 0.27) respectively and
correspond to a light blue colour (see the CIE diagram in
the ESI†).
In order to increase the performances, a thin layer of NPB (N,N0-
di(1-naphthyl)-N,N0-diphenyl-[1,10-biphenyl]-4,40-diamine)
was
deposited between the anode and the EML. The turn-on
voltage of the ITO/PEDOT/NPB (40 nm)/1 (50 nm)/Ca (device
B) appears at ca. 6.8 V with a luminance magnified by around
5 times compared to device A, reaching ca. 230 Cd mꢀ2 with
efficiencies also magnified, 6.2 ꢂ 10ꢀ2 Cd Aꢀ1 and 2 ꢂ 10ꢀ3 Lm
Wꢀ1 (Fig. 4, top). Similarly, the ITO/PEDOT/NPB (40 nm)/2
(45 nm)/Ca device B with 2 as EML also leads to a strong
enhancement of the performances (Fig. 4, bottom). Indeed, the
maximum luminance reaches around 510 Cd mꢀ2 with a lumi-
nous efficiency of ca. 0.1 Cd Aꢀ1. The turn-on voltage of the
double-layer device B (5.8 V) is slightly higher than that of
the single-layer device A due to the increase thickness of the
organic layers. It is obvious that the performances of this
excimer-based SMOLED are modest but: (i) the devices were
not optimized, (ii) such intramolecular excimer based devices
were without any precedent, (iii) the performances are magnified
compared to a classical ‘‘aryl–fluorene–aryl’’ emitter such as 1,
(iv) with a more accurate molecular design of the molecules
(hole/electron transporting moieties) and/or of the devices
(‘‘band engineering’’ with hole/electron transporting/blocking
layers, modification of the electrodes) the performances of the
SMOLEDs will be surely and easily enhanced.
In the case of 3, the device A presents an EL spectrum with
a maximum recorded at ca. 452 nm and a long wavelength tail
until 800 nm, whereas the device B presents a maximum red
shifted by 38 nm (lmax ¼ 490 nm) and an even larger long
wavelength tail with a shoulder at 700 nm, Fig. 5C. This large
band (with its shoulder at 700 nm) found in device B can be
tentatively assigned to exciplex emission due to the interactions
at the organic/organic interface.61,62 This is confirmed by the fact
that both devices possess EL spectra strongly red-shifted
compared to the fluorescent spectrum of 3 (Fig. 5C, black line).
Finally for 2, the EL spectrum of the device A exhibits a nice
structureless band with a maximum at 447 nm (Fig. 5B, red line),
whereas the device B displays a maximum slightly red-shifted at
462 nm (Fig. 5B, blue line). These two EL spectra, opposite to
those of 1 and 3, are in accordance with the thermally evaporated
PL spectrum of 2 (Fig. 5B, black line) clearly signifying that the
fluorescent emission of the device arises, without any doubt,
from electrogenerated intramolecular excimer. A slight blue shift
is nevertheless noted between the EL spectrum of device A
(lmax ¼ 447 nm) and the PL spectrum of 2 (lmax ¼ 461 nm). Such
The last and crucial feature is finally related to the emission
colour of these devices. The electro-optic properties of the
devices A and B are presented below (Fig. 5). For 1, two
important features were noted. First, the Electroluminescent
(EL) spectra seem to be dependent on the nature of the device.
Indeed, the device A presents an EL spectrum with a blue
emission recorded at 432 nm (Fig. 5A, red line) red shifted by 18
nm compared to its thermally evaporated thin film, l ¼ 414 nm
(Fig. 5A, black line). The device B also presents a red-shift of its
EL spectrum, nevertheless more pronounced with a maximum
recorded at 448 nm (Fig. 5A, blue line). The second important
feature is related to the shape of the EL spectra which appears to
be not only larger than the PL spectrum but also with an addi-
tional GEB recorded at ca. 540 nm. The origin of such
Fig. 5 Electroluminescent spectra using 1 (A), 2 (B) and 3 (C), as EML: device A (red), device B (blue) and thermally evaporated (ꢃ260 ꢁC for 1 and 2)
and spin-coated (3) thin-film PL spectra (black).
7154 | J. Mater. Chem., 2012, 22, 7149–7157
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€
a feature has been also recently reported by Mullen and co-
relationship study may open new avenues in the design of effi-
cient blue OLEDs.
workers for dendrimer based blue OLEDs.63 This point needs to
be nevertheless elucidated in further studies. In addition, no sign
of intermolecular excimer leading to undesired GEB was detec-
ted, clearly signifying that the molecules of 2 in the thin-film are
isolated enough to avoid any p–p intermolecular interaction.
This site isolation is a key feature to ensure an efficient fluores-
cence and highlights the potential of the present molecular design
for blue OLED applications. In addition, no band arising from
keto-defects or from exciplex formation was detected as it was
the case for 1 and 3. The chromatic coordinates of the devices A
and B with 2 as EML are almost identical (0.19; 0.19) and (0.19;
0.18) respectively, and correspond to a deep blue colour (see the
CIE diagram in the ESI†). In summary, the intramolecular
excimer based-OLEDs not only display a stable deep blue colour
with no trace of GEB due to ‘‘site isolation’’ of the molecules in
the film but also present magnified performances compared to
those of other devices with structurally related compounds (1 and
3). Generating intramolecular excimers emission through the
highly rigid (2,1-a)-DSF–IF molecular platform appears hence as
an interesting new strategy to design electroluminescent devices.
Indeed, this strategy allows obtaining a deep blue emission (PL
and EL) centred at 460 nm with a short p-conjugated ‘aryl–flu-
orene–aryl’ fragment, whereas this emission wavelength is almost
impossible to obtain with structurally related oligo ‘aryl–fluo-
rene–aryl’ derivatives even with a large number of aryl
substituents.24
Experimental section
Synthesis
Commercially available reagents and solvents were used without
further purification other than those detailed below. THF was
distilled from sodium/benzophenone prior to use. Toluene was
distilled from sodium prior to use. Light petroleum refers to the
ꢁ
fraction with bp 40–60 C. Reactions were stirred magnetically,
unless otherwise indicated. Analytical thin layer chromatog-
raphy was carried out using aluminium backed plates coated
with Merck Kieselgel 60 GF254 and visualized under UV light
(at 254 and 360 nm). Chromatography was carried out using
silica 60A CC 40–63 mm (SDS). H and 13C NMR spectra were
1
recorded using Bruker 300 MHz instruments (1H frequency,
corresponding 13C frequency: 75 MHz); chemical shifts were
recorded in ppm and J values in Hz. In the 13C NMR spectra,
signals corresponding to CH, CH2 or Me groups, assigned from
DEPT, are noted; all others are C. The residual signals for the
NMR solvent CD2Cl2 are 5.32 ppm for the proton and
53.80 ppm for the carbon. The following abbreviations have
been used for the NMR assignment: s for singlet, d for doublet,
t for triplet and m for multiplet. High resolution mass spectra
ꢀ
were recorded at the Centre Regional de Mesures Physiques de
l’Ouest (Rennes). The synthesis and the full characterization of
1, 2 and DSF(t-Bu)4-IF(Br)2 may be found in our previous
works.12,21
Conclusion
DSF–IF 2 has been successfully used as an emitting layer in
a SMOLED leading to a stable deep blue emission arising from
electrogenerated intramolecular excimers with better perfor-
mances compared to those of structurally related compounds.
This is to the best of our knowledge the first rational and
comparative study describing the use of intramolecular excimers
arising from organic molecules as blue light source in
a SMOLED. Thus, due to the geometry of the rigid (2,1-a)-DSF–
IF platform, the ‘aryl/fluorene/aryl’ dimer is preformed in the
ground state favouring the facile and easily controllable forma-
tion of excimers in the excited state and leading to a deep blue
fluorescent emission (l ¼ 460 nm) through strong p–p intra-
molecular interactions. This emission wavelength is almost
impossible to obtain with the direct connection of simple aryl
units to a fluorene core24 and highlights the interest of the present
strategy. The simple and non-optimized double-layer device
using 2 as EML displays a deep blue colour (CIE coordinates:
0.19; 0.18) exhibiting a luminance of 510 Cd mꢀ2 with a luminous
efficiency of ca. 0.1 Cd Aꢀ1. More sophisticated SMOLED
designs, such as the use of carrier blocking layers, will easily
allow the device performances to be significantly optimized. In
addition, more experiments are nevertheless necessary in order to
perfectly understand the physical and electronic properties of
these intramolecular excimer based devices and to take full
advantage of the favourable properties of this new molecular
design. Regarding the importance of fluorene derivatives in
organic electronics and the number of molecular platforms
(pyrene, naphthalene, etc.)36 which have been used to induce
intramolecular excimer emission, the present structure/property
Spectroscopic studies
UV-visible spectra were recorded using a UV-Visible spectro-
photometer Shimadzu UV-1605. The optical band gap was
calculated from the absorption edge of the UV-vis absorption
spectrum using the formula DEopt (eV) ¼ hc/l, l being the
absorption edge (in metres). With h ¼ 6.6 ꢂ 10ꢀ34 J s (1 eV ¼
1.6 ꢂ 10ꢀ19 J) and c ¼ 3.0 ꢂ 108 m sꢀ1, this equation may be
simplified as: DEopt (eV) ¼ 1237.5/l (in nm). Photoluminescence
spectra were recorded with a PTI spectrofluorimeter (PTI-814
PDS, MD 5020, LPS 220B) using a xenon lamp. Quantum yields
in solution (fsol) were calculated relative to quinine sulfate
(fsol ¼ 0.546 in H2SO4 1 N) using standard procedures. fsol was
determined according to the following eqn (1):
ꢀ
ꢁ
2
ðTs ꢂ ArÞ ns
fsol ¼ fref ꢂ 100 ꢂ
(1)
ðTr ꢂ AsÞ nr
where subscripts s and r refer respectively to the sample and
reference. The integrated area of the emission peak in arbitrary
units is given as T, n is the refracting index of the solvent (ns ¼
1.4072 for THF) and A is the absorbance (A < 0.1). IR spectra
were recorded on a Bruker Vertex 70 using a diamond crystal
MIRacle ATR (Pike).
Electrochemical studies
All electrochemical experiments were performed under an argon
atmosphere, using a Pt disk electrode (diameter 1 mm), the
counter electrode was a vitreous carbon rod and the reference
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J. Mater. Chem., 2012, 22, 7149–7157 | 7155

View Online
electrode was a silver wire in a 0.1 M 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 the internal standard. The three electrode cell
was connected to a PAR Model 273 potentiostat/galvanostat
(PAR, EG&G, USA) monitored with the ECHEM Software.
Activated Al2O3 was added to the electrolytic solution to remove
excess moisture. All potentials are referred to the SCE electrode
that was calibrated at ꢀ0.405 V vs. Fc/Fc+ system.
IR (ATR, cmꢀ1) n ¼ 3066, 3031, 2953, 2902, 2867, 2832, 1585,
1511, 1494, 1459, 1427, 1402, 1361, 1342, 1243, 1173, 1126.
Acknowledgements
ꢀ
DT thanks the Region Bretagne for a studentship. MR thanks
ꢀ
the Region Bretagne and l’Agence de l’Environnement et de la
^
Maıtrise de l’Energie (ADEME) for a studentship. The OLED
fabrication/characterization has been performed at the ENSCBP
(IMS/UMR CNRS 5218-Bordeaux) and we wish to highly thank
Sokha Khiev and Dr Laurence Vignau for their help and fruitful
discussions.
OLED fabrication and testing
OLEDs were fabricated using the following procedure. Indium-
tin oxide (ITO) substrates on glass from Merck underwent
a solvent ultrasonic cleaning using acetone, ethanol and iso-
propanol followed by a 15 min UV-ozone treatment. A layer of
poly(3,4-ethylene dioxythiophene) doped with poly(styrene
sulfonate) (PEDOT/PSS from Aldrich) was then deposited onto
ITO by spin-coating at 6000 rpm, from a 3 wt% water dispersion
to form a 40 nm thick layer. PEDOT/PSS was subsequently
annealed at 120 ꢁC under vacuum for 40 minutes. This layer
improves hole injection from the ITO to the HOMO level of the
organic material and increases the performances and the lifetime
of the device. Then a layer of 1, 2 or 3 or a NPB (N,N0-di(1-
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