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Intramolecular excimer emission as a blue light source in
uorescent organic light emitting diodes: a promising
molecular design
Damien Thirion, Maxime Romain, Joëlle Rault-Berthelot, Cyril Poriel
To cite this version:
Damien Thirion, Maxime Romain, Joëlle Rault-Berthelot, Cyril Poriel. Intramolecular excimer
emission as a blue light source in uorescent organic light emitting diodes: a promising molecu-
lar design. Journal of Materials Chemistry, Royal Society of Chemistry, 2012, 22, pp.7149-7157.
�10.1039/c2jm16774c�. �hal-00687064�
Intramolecular excimer emission as a blue light source in fluorescent organic
light emitting diodes: a promising molecular design†
Damien Thirion, Maxime Romain, Jo
€
elle 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 m
2
with a luminous efficiency of ca. 0.1 Cd A
1
. This work is, to the
best of our knowledge, the first rational and comparative study describing an intramolecular excimer
based-SMOLED.
Introduction
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
emission could be used as a light source in an OLED. Indeed, the
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’‡ compounds
13
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
Universit
e 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
SMOLED characterizations for 1–3, copy of
1
H and
13
C 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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use of intermolecular excimer emission as a light source in
a SMOLED is known for organic
5–7
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.
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.
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 Pd
2
(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).
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 (l
max
¼ 345 nm).
25,26
It
should be stressed that this band is red-shifted by 11 nm
compared to that of (1,2-b)-indenofluorene ((1,2-b)–IF, l
max
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 (l
max
¼ 339 nm).
25
The slight blue-shift
observed between the maxima of 2 (l
max
¼ 340 nm) and 1 (l
max
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, l
max
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
(l
max
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
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 1 Molecules investigated in this work.
Scheme 2 Synthesis of 3.
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shifted compared to those of 2,2
0
,7,7
0
-tetraphenyl-9,9
0
-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
between ‘‘aryl–fluorene–aryl’’ moieties in the excited state.x
14
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, l
max
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
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 (l
max
¼ 391/412 nm) are remarkably blue-shifted
compared to that of 2 (l
max
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.
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
Fig. 1 Absorption spectra of 1 (A), 2 (B) and 3 (C) in solution (blue line, THF, C ¼ 10
6
M) and in thin-solid films (red line), depositing solvent: 1,2-
dichlorobenzene, C ¼ 15 mg mL
1
(1 and 2); THF; C ¼ 10 mg mL
1
(3).
Fig. 2 Emission spectra of (A): 1, (B): 2 and (C): 3, l
exc
¼ 340 nm (1 and 2), l
exc
¼ 336 nm (3) in solution (THF 10
6
, blue line) and in thin-solid films (red
line); depositing solvent: 1,2-dichlorobenzene, C ¼ 15 mg mL
1
(1 and 2); THF; C ¼ 10 mg mL
1
(3).
x It should be noted that the fluorescence spectra of 2 are independent of
the concentration (10
7
to 10
3
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
compounds with the ‘aryl–fluorene–aryl’ frameworks.
24
3
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
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-
trimethoxyphenylethenyl-9,9
0
-diethylfluorene)), a structurally
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
Cis
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
HOMO
a
/eV LUMO
b
/eV DE
opt
c
/eV Ref.
1 5.49 2.04 3.45 14
2 5.33 1.94 3.39 14
3 5.43 2.26 3.17 This work
a
Calculated from the onset oxidation potential E
onset
ox
(HOMO
(eV) ¼[E
onset
ox
(vs. SCE) + 4.4], based on an SCE energy level of 4.4
eV relative to the vacuum).
b
Calculated from the HOMO energy level
and the optical band gap.
c
Optical band gap DE
opt
¼ hc/l(DE
opt
(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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