Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: New host materials for efficient sky-blue PhOLEDs

Article abstract of DOI:10.1039/c5tc01812a

Four high triplet organic semi-conductors based on the donor-spiro-acceptor design (D-spiro-A) have been synthesized. Their physicochemical and photophysical properties have been studied, compared and discussed in light of the nature of their respective donor/acceptor units. The four compounds have been used as host materials in efficient sky-blue (EQE > 10% at 10 mA cm^-2) phosphorescent organic light emitting diodes.

Full text of DOI:10.1039/c5tc01812a

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Berthelot, M. Romain, C. PORIEL, D. -. Tondelier, O. Jeannin and B. Geffroy, J. Mater. Chem. C, 2015,
DOI: 10.1039/C5TC01812A.
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Properties modulation of organic semi-conductors based on a
Donor-Spiro-Acceptor (D-Spiro-A) molecular design:
New host materials for efficient sky-blue PhOLEDs *
Maxime Romain,a Denis Tondelier,b Olivier Jeannin,a Bernard Geffroy,b,c*
Joëlle RaultꢀBerthelota* and Cyril Poriela*
a. UMR CNRS 6226 "Institut des Sciences Chimiques de Rennes"
Université de Rennes 1-Campus de Beaulieu-35042 Rennes cedex (France) Tel:(+33)2-2323-
5977
E-mail: cyril.poriel@univ-rennes1.fr/joelle.rault-berthelot@univ-rennes1.fr
b.UMR CNRS 7647, LPCIM, École Polytechnique, 91128 Palaiseau, France
c. LICSEN/NIMBE UMR 3685, CEA Saclay, 91191 Gif Sur Yvette, France
E-mail: bernard.geffroy@polytechnique.edu
Four high triplet organic semi-conductors based on the Donor-Spiro-Acceptor design
(D-Spiro-A) have been synthesized. Their physicochemical and photophysical properties have
been studied, compared and discussed at the light of the nature of their respective
Donor/Acceptor units. The four compounds have been used as host materials in efficient
sky blue (EQE>10 % at 10 mA/cm2
) Phosphorescent Organic Light Emitting Diodes.
In the last twenty years, many families of organic emissive materials have been developed for
flatꢀpanel displays and solid state lighting sources.1ꢀ3 The design of fluorescent organic semiꢀ
conductors for green and red emission has led to highly efficient organic light emitting diodes
(OLEDs).4 However, the blue colour despite fantastic recent progresses in the design of
fluorescent materials5ꢀ12 and in the device architectures13 remains less efficient and less stable
than the other colours.4 In addition, in the last fifteen years, it has been demonstrated that
highly efficient blue emission should not be obtained in OLED devices using pure organic
singlet exciton emissive layers (EML) but should require more sophisticated EML in which a
blue phosphor is doped in an organic host material to harvest both singlet and triplet excitons.
Such devices, the phosphorescent Organic LightꢀEmitting Diodes (PhOLEDs) have then
attracted fantastic interest.14ꢀ18 One of the weakest links of this technology remains the design
of highly efficient host materials for blue phosphors. Indeed, the prerequisites for an ideal
host for a blue dopant are (i) a high triplet energy (ET) to avoid reverse energy transfers from
the guest back to the host, (ii) a high glassꢀtransition temperature (Tg) and decomposition
temperature (Td) for stability, (iii) matching HOMOꢀLUMO levels for injection of the
charges (iv) good and balanced charge transporting properties to insure efficient
recombination in the dopant. For industrial production, long and sophisticated chemical
syntheses of the host should be also clearly proscribed. Bipolar molecules incorporating hole
* Electronic Supplementary Information (ESI) available: Materials and methods, experimental details;
synthesis and characterization of all compounds, absorption spectra in various solvents, DSC, copy of
NMR spectra, Green devices performance.
1

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and electron transporting units to adjust the energy levels of frontier orbitals appear to date the
most promising candidates for PhOLED applications.2, 16ꢀ22 Thus, when designing bipolar
hosts, one should carefully consider the selection of the donor/acceptor pair and their relative
positions within the dye. Indeed, the donor/acceptor combination often results in πꢀ
conjugation enlargement, which would accordingly reduce the singlet and triplet energies.
Therefore, to obtain bipolar hosts with high ET, the πꢀconjugation and the electronic coupling
between the donor and the acceptor units must be restricted. The choice of the linker and the
relative position of the donor and acceptor units must then be carefully performed.
To date, many successful design strategies have been developed to gather all the above
mentioned properties in a single host and particularly to disrupt the πꢀconjugation. Some
relevant examples are presented in Chart 1.
N
N
N
N
N
N
N
Taz-Cz
Ortho linkage
(steric hindrance)
PCb-PCz
Meta linkage
(electronic coupling)
N
N
Si
N
P
O
N
N
N
Sit-Cz
TCTP
Triphenylsilane linkage
(insulating Si atom)
Diphenylphosphine oxide linkage
(insulating P atom)
Chart 1.Different design strategies for πꢀconjugation disruption
First, it is known that electronic coupling through a meta linkage is inherently weaker than
through a para one5, 23ꢀ27 and this strategy has often been used to connect a hole transporting
and an electron transporting unit in a host material.21, 24, 28, 29 Thus, the meta linkages
efficiently disrupt the πꢀconjugation between a donor and an acceptor as for instance in 9ꢀ(3ꢀ
(diphenylcarbazolꢀ3ꢀyl)phenyl)ꢀαꢀcarboline (PCb-PCz, Chart 1), in which the Nꢀ
phenylcarbazole donor unit is separated from the pyridoindole acceptor unit by a metaꢀ
substituted phenyl unit (HOMO: ꢀ5.75 eV, LUMO: ꢀ2.31 eV, ET = 2.74 eV).21 A second
approach widely developed in the literature consists to introduce a steric congestion within the
dye to hinder the planarization between two connected πꢀsystems.30ꢀ33 This 'steric hindrance'
strategy is most of the time performed through the incorporation of a sterically hindered ortho
linkage efficiently leading to a πꢀconjugation restriction. For example, in Taz-Cz (Chart 1),
the carbazole and 1,2,4ꢀtriazole are both oriented with a dihedral angle larger than 45° with
the central benzene ring, efficiently disrupting the πꢀconjugation between the carbazole and
the triazole (HOMO: ꢀ5.7 eV, LUMO: ꢀ2.3 eV, ET of 3.09 eV) 31
.
2

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Using an insulating heteroatom is also an efficient strategy to obtain high ET materials with
short πꢀconjugated pathway. Thus, silicium (silane),2, 18, 34, 35 and phosphoryl group (P=O),2, 18,
20, 29, 36, 37
may act as an effective breaking point of the πꢀconjugation between the main core
of the molecule and the outer groups linked to the heteroatom (phenyl for example in the case
of the efficient electron transporting diphenylphosphine oxide unit). Thus, the electronic
structure of the bipolar molecules is almost identical to those of the corresponding central
core, which is a crucial point to avoid the decrease of ET.20 For example, in the 9’ꢀ
triphenylsilanylꢀ9’Hꢀ[9,3’,6’,9”]terꢀcarbazole (Sit-Cz), the direct connexion of the silane to a
carbazole unit through the nitrogen atom renders this carbazole electroꢀdeficient (HOMO: ꢀ
5.54 eV, LUMO :ꢀ2.3 eV, ET : 3.0 eV).38 Similarly, in the starꢀshaped molecule 4,4’,4”ꢀtri(Nꢀ
carbazolyl)triphenylphosphine oxide (TCTP), the electronꢀacceptor phosphine oxide unit is
used both as the core and as the electron donor, the carbazole moiety, acting as the branch.39
HOMO/LUMO levels respectively lie at ꢀ5.25 eV/ꢀ1.67 eV, and the disrupted conjugation via
phosphineꢀoxide linkage preserves a high ET of 3.03 eV.
More recently, another promising and simple molecular design based on a πꢀconjugation breaꢀ
king induced by an insulating spiro bridge (called D-Spiro-A design) has been introduced in
literature. This design which allows to separate the HOMO and LUMO levels has a great
potential in Thermally Activated Delayed Fluorescence (TADF)40, 41 but remains rarely used
to date in the field of host materials for blue phosphorescent dopants.35, 42ꢀ44 Our attention for
this design was to avoid the introduction of pendant hole or electron transporting units,
strongly simplifying the molecular structure and hence the chemical synthesis. In the present
work, we wish to report a structureꢀproperties relationship study of new donor/acceptor
molecules based on this DꢀSpiroꢀA design. These semiꢀconductors present different electronic
properties and have been used as host in sky blue PhOLEDs leading in some case to high
efficiencies (EQE>10% at 10 mA.cmꢀ2) The large differences observed in term of PhOLED
efficiencies highlight the importance of the donor/acceptor combination used in the molecular
design of the host. Such structureꢀproperties relationship study may provide interesting
insights for the development of future materials for optoelectronics. Thus, in this work, as a
development to our preliminary note on the dye SPA-TXO2 (Scheme 1),45 we have
investigated through a DꢀSpiroꢀA design, two hole transporting moieties, namely Nꢀ
phenylacridine (PA)43 and thioxanthene (TX) and two electron transporting moieties, namely
thioxanthene dioxide (TXO2) and diazafluorene (DAF) (Scheme 1). These four molecular
fragments are briefly described below.
In PA, two phenyl rings of a triphenylamine unit are connected by a methylene unit forming a
hexagon. Structurally, the presence of this hexagon renders the Nꢀphenyl acridine unit more
similar to a triphenylamine than to a Nꢀphenylcarbazole unit.46 The strong electron rich nature
of PA unit has been previously exploited in TADF,40, 41, 47 in nonꢀdoped blue OLEDs,48 and in
host materials for blue PhOLEDs.35, 42, 43, 49, 50
TX is the structural analogue of xanthene, possessing an intracyclic sulphur atom instead of
an oxygen atom. Recently, Yam et al have reported various spiroꢀconfigured dyes based on
the connection of a TX (and TXO2) unit with a 2,7ꢀbis(diphenylamine)fluorene leading to
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very promising donor materials in the construction of high performance organic photovoltaic
devices.51
TXO2 is the oxidized analogue of TX. The presence of the sulfone in TXO2 leads to better
electron injection and transport abilities due to the depress of the LUMO level.12 Indeed,
sulfones have been efficiently incorporated (i) in highly efficient blue emitting materials,52ꢀ54
(ii) in host materials for PhOLEDs,55ꢀ57 and (iii) in electronꢀtransporting materials.58 However,
the use of TXO2 fragment remains scarce and our group, in a preliminary note, has recently
demonstrated its potential in the design of host for blue PhOLEDs.45 Thus, by a simple
oxidation step, it is hence possible to switch from hole transporting properties in TX to
electron transporting properties in TXO2, highlighting the versatility of these systems.
Electron acceptor DAF may be regarded as a bipyridine unit possessing then a similar
molecular arrangement than that of the wellꢀknown bridged biphenyl, namely fluorene. The
strong electron affinity of this unit59, 60 could decrease the LUMO energy level, thereby
improving the electron injection and transport properties of the materials. Up to now, DAF
fragments have been investigated in many research fields, such as electronꢀtransporting
materials,61 organic emitters,59, 62 and sensors.63 DAF fragment is also an interesting platform
to coordinate various metals such as cadmium,64 zinc,64 rhenium,65 silver66 and iridium.67, 68
However, to the best of our knowledge, DAF unit remains rarely incorporated in host
materials for PhOLED.60, 61, 69
Results and discussion
Synthesis
It is crucial for the future of organic optoelectronics to establish short and very efficient
synthetic approaches of organic materials. Herein, all the dyes have been synthesised through
a one pot very efficient route using common and easily synthesizable intermediates. Thus, the
two halogenoꢀaryls 3 and 4, judiciously coupled with the two ketones 1 and 2 provide STX-
TXO2 (3 + 1), STX-DAF (3+2), SPA-TXO2 (4+1) and SPA-DAF (4+2) (Scheme 1).
Scheme 1. Retrosynthetic analysis of the four dyes investigated in this work: STX-TXO2, STX-DAF,
SPA-TXO2 and SPA-DAF
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Regarding the TXO2 derivative (Scheme 2, top), a lithiumꢀbromine exchange was first
performed on either 2ꢀbromoꢀN,Nꢀdiphenylaniline 4 or (2ꢀiodophenyl)(phenyl)sulfane 3
followed by the trapping of the lithiated intermediate by ketone 9Hꢀthioxanthenꢀ9ꢀoneꢀ10,10ꢀ
dioxide 1 (obtained by oxidation of commercially available 9Hꢀthioxanthenꢀ9ꢀone with
hydrogen peroxide, see SI). Electrophilic intramolecular cyclization reaction of the resulting
dioxothioxanthenols (not isolated) in acidic media (HCl in acetic acid), afforded SPA-TXO2
or STX-TXO2 in high yield (78 and 88 % respectively over the three steps). This one pot
synthetic strategy is very simple, easy to perform, highly efficient for a gram scale synthesis
and is versatile to many different donor and acceptors fragments.
Scheme 2 : Synthesis of STX-TXO2, SPA-TXO2 , STX-DAF and SPA-DAF
a\ (2ꢀiodophenyl)(phenyl)sulfane 3, nBuLi, THF from ꢀ80 °C to 20 °C then HCl, AcOH reflux 88%,
b\ 2ꢀbromoꢀN,Nꢀdiphenylamine 4, nBuLi, THF from ꢀ80 °C to 20 °C then HCl, AcOH reflux 78%. c\
(2ꢀiodophenyl)(phenyl)sulfane 3, nBuLi, THF from ꢀ80 °C to 20 °C then MsOH, 1,2ꢀdichlorobenzene
reflux 39%, d\ 2ꢀbromoꢀN,Nꢀdiphenylaniline, nBuLi, THF from ꢀ80 °C to 20 °C then MsOH, 1,2ꢀ
dichlorobenzene, reflux 47% .
An identical strategy has been developed to synthesise STX-DAF and SPA-DAF (Scheme 2,
bottom) involving first the synthesis of the known diazafluorenone 2 (obtained by oxidation
and ring contraction of phenanthroline in water in presence of KMnO4 and KOH, see SI).70
Reaction of 2 with the corresponding lithiated intermediates derived from 3 or 4 was then
performed leading to the corresponding diazafluorenol derivatives. However the intramoꢀ
lecular cyclization reaction in similar conditions than those exposed above (HCl/CH3CO2H,
Scheme 2) did not occur, highlighting the very different reactivity of diazafluorenols vs
dioxothioxanthenols. Xꢀray structure of the triphenylamine diazafluorenol shows short
distances (ca 2 Å, see Figure in SI) between hydrogen atom of the hydroxide unit of one
molecule and a nitrogen atom of the diazafluorene unit of a second molecule. These short
distances sign the presence of intermolecular interactions that may explain a stabilization of
the diazafluorenol and hence its weak reactivity. In addition, the nitrogen atoms of the
diazafluorene unit might be protonated during the cyclisation step decreasing again its
reactivity. Using more drastic conditions and 3 equivalents of methane sulfonic acid at high
temperature (180 °C in 1,2ꢀdichlorobenzene), target compounds STX-DAF (yield : 39%) and
SPA-DAF (yield : 47 %) were finally isolated. It should be noted that in the course of this
work, the synthesis of SPA-DAF as an intermediate building block has been reported by
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Adachi and coworkers using Eaton’s reagent to cyclise the diazafluorenol derivative.40
However, the properties of SPA-DAF were not reported in this work.
1H NMR studies
Figure 1. Portion 1H of NMR Spectra of (a) STX-TXO2 , (b) SPA-TXO2, (c) STX-DAF
and (d) SPA-DAF in CD2Cl2.
1H NMR spectroscopy is an interesting tool to evaluate the strength of electron withꢀ
drawing/donating moieties on the environment. Thus, the effect of the incorporation of heteꢀ
roatoms in the spirobifluorene like molecules described herein can be interestingly visualized
through NMR spectroscopy. The complete assignments of all signals have been performed by
2D NMR spectroscopy experiments (HMBC, HMQC, 1H/1H COSY, see SI).
For STX-TXO2 and SPA-TXO2, one can note that the hydrogen atoms (Haꢀd) of the TXO2
fragment (Figures 1a and 1b, red lines) are centred around 7.35 ppm except Ha, which appear
to be deshielded due to the proximity of the sulfone unit. Considering the chemical shift of
benzene (7.35 ppm in CD2Cl2)71 one can conclude that the TXO2 is a weak electron
withdrawing unit. On the opposite, the signals Hfꢀh of the TX unit and Hlꢀo of the PA core
appear to be shielded highlighting their electronꢀdonating nature. We can note that the
incorporation of the nitrogen atom within the PA unit, leads to a stronger shielding than that
observed for the sulphur in TX, highlighting the stronger electronꢀdonating nature of the
former. This will be confirmed by electrochemistry (see below). It should be mentioned that
the resonances of the pendant phenyl ring of the PA (Hiꢀk) are found at low field (δ 7.58/7.8
ppm) meaning that the free doublet of the nitrogen atom is conjugated with the acridine and
not with the phenyl unit.
Similar effects have been detected for STX-DAF and SPA-DAF with in addition two very
low field resonances recorded at 8 and 8.65 ppm (respectively assigned to Hc in γ position
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and to Ha in β position of the nitrogen atom of the DAF). This highlights the strong electronꢀ
withdrawing nature of DAF fragment.
Structural properties
The four compounds have been crystallized by vapour diffusion of pentane in CDCl3 solution
in order to confirm their molecular structures by Xꢀray crystallography and to study the
structural characteristics (See XꢀRay in SI). STX-TXO2 and SPA-TXO2 both crystallized in a
triclinic system. STX-TXO2 asymmetric unit contains two different molecules and one
molecule of CDCl3 whereas that of SPA-TXO2 contains only one molecule without solvent.
STX-DAF crystallizes in an orthorhombic system with only one molecule per unit and SPA-
DAF crystallizes with two molecules of solvents in a monoclinic system.
The molecular radius of each molecule (distance from the spiro carbon atom to the farthest
carbon atom, green arrow in figure 2) has been evaluated at 7.14/7.10 Å for SPA-TXO2 and
SPA-DAF resp. and at 4.36/4.34 Å for STX-TXO2 and STX-DAF resp. For the four
molecules, this radius is imposed by the donor groups (TX or DAF) which are the longest
units.
Figure 2. Molecular structure from XꢀRay crystallography of STX-TXO2 (top left), STX-DAF (top
right), SPA-TXO2 (bottom left), and SPA-DAF (bottom right) (ellipsoïd probability at 50% level),
hydrogen atoms have been omitted for clarity.
In all molecules, the donor and the acceptor fragments are almost orthogonal with a twist
angle (value in orange in figure 2) varying from 88° for both STX-DAF and SPA-TXO2, 89°
for STX-TXO2 to 90° for STX-DAF. This orthogonality between the donor and the acceptor
moiety is at the origin of the absence of significant πꢀconjugation between them. Additionally,
in the PA derivatives, the angle between the mean plane of the acridine units (indicated in
purple in figure 2) and that of the attached phenyl is of 85/88° in SPA-TXO2 and SPA-DAF
resp. This large angle indicates a second πꢀconjugation breaking at the nitrogen atom between
the acridine unit and the phenyl. As discussed below, these πꢀconjugation breakings are
essential to keep the high ET (see below).
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For each aromatic unit, a torsional angle has been defined as the dihedral angle between the
two external benzene rings of each unit (values in blue for the donor units and in red for the
acceptor units in figure 2).
For the donor units, the TX torsion angles are impressively different depending of the
acceptor core. Thus, a small angle of 6.4° is recorded for STX-TXO2 and a very large angle
of 31° is recorded for STX-DAF. Similarly, the torsion angles measured in the PA units are 5
times larger for SPA-DAF (16°) than for SPA-TXO2 (3.3°). Thus, the donor fragments in
SPA-DAF and STX-DAF are strongly more distorted than in their counterparts SPA-TXO2
and STX-TXO2. The presence of the DAF unit within the dyes leads hence to a very distorted
donor with large dihedral angles. This feature may be assigned to the packing forces induced
by the presence of the DAF. In addition and regardless of the acceptor, the PA units are
always less distorted than the TX units surely due to the presence in PA of a pendant phenyl
ring which provides a certain degree of rigidity.
For the acceptor units, the TXO2 torsion angle is of 7° for STX-TXO2 and of 12° for SPA-
TXO2 whereas this torsion angle is very weak in the case of the DAF unit: 2.6° for STX-
DAF, and 4.0° for SPA-DAF. Thus, the very rigid DAF unit only allows very weak
deformations due the presence of the CꢀC bond in α position of the nitrogen atoms. Such
conformational locking is very similar to that observed for fluorene unit. Oppositely, the
presence of the sulphur atom linking the two phenyl groups in the TXO2 unit allows some
deviations from planarity.
Thermal Properties
Before any possible OLED applications and in order to confirm the interest of the present Dꢀ
spiroꢀA design, the four compounds have been studied by thermogravimetric analysis (TGA)
(Figure 3) and differential scanning calorimetry (DSC) (see SI).
261 296
313
|
|
1.0
0.8
0.6
0.4
0.2
0.0
|
0.95
|
258
STX-TXO2
SPA-TXO2
STX-DAF
SPA-DAF
150
300
450
Temperature (°C)
Figure 3. TGA curves of STX-TXO2 (violet line), STX-DAF (green line), SPA-TXO2 (blue line) and
SPA-DAF (yellow green line)
The decomposition temperature Td, is defined as the temperature corresponding to 5% of
mass loss.72 Herein, these temperatures are recorded at 261/258 °C for STX-TXO2 / STX-
DAF and at 313/296 °C for SPA-TXO2 / SPA-DAF respectively (Figure 3). As a complete
mass loss then occurs, we believe that this mass loss is attributed to a sublimation process as
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previously observed for other πꢀconjugated systems.24 Thus, the presence of the PA unit
within the dye leads to a material with a higher Td (by ca 40/50°) than that obtained with the
TX unit. DSC studies reveal no phase transition between the room temperature and the
decomposition of the molecules (see SI). The rigid molecular structure of the two spiroꢀ
annulated fragments is at the origin of the interesting thermal properties of the four molecules.
Electrochemical Properties
Electrochemical properties have been investigated by cyclic voltammetries (CV) in CH2Cl2 in
oxidation and reduction (Figure 4 and Table 1, potentials given vs SCE). In oxidation, the two
TX derivatives present an irreversible oxidation with maximum at 1.55 V for STX-TXO2
(violet line) and at 1.48 V for STX-DAF (green line). From their respective onset oxidation
potentials (1.42/1.34 V), their HOMO energy levels were determined at ꢀ5.79/ꢀ5.74 eV for
STX-TXO2 / STX-DAF.
In the PA series, the two derivatives are oxidized at even lower anodic potentials and present
one reversible oxidation wave with maximum at 1.14 V for SPA-TXO2 and two very close
oxidation waves with maxima at 1.11/1.20 V for SPA-DAF. For SPA-DAF, recording the CV
up to the first oxidation wave only shows an irreversible oxidation wave indicating a high
reactivity of the SPA-DAF.+ species at the timescale of the CV (see Figure in SI). However,
when reaching the second oxidation wave, an adsorptionꢀlike reduction peak is observed
showing that the species formed during the two oxidation processes are strongly adsorbed at
the electrode surface. From their onset potentials measured at 1.03/0.95 V for SPA-TXO2 /
SPA-DAF resp, the HOMO levels were calculated at ꢀ 5.42 eV for SPA-TXO2 and ꢀ5.35 eV
for SPA-DAF.
In the light of electronic distribution of the HOMO (DFT calculations performed at the
Gaussian 09 B3LYP/6ꢀ311+G(d,p) level of theory, Figure 6), the first oxidations are assigned
to the oxidation of the electron rich fragment, namely TX unit in STX-TXO2 / STX-DAF and
to the PA core in SPA-TXO2 / SPA-DAF.
10
STX-TXO2
SPA-TXO2
10
0
0
0
ꢀ10
ꢀ20
ꢀ100
STX-DAF
SPA-DAF
ꢀ3
ꢀ2
E(V) vs SCE
ꢀ10
ꢀ3
ꢀ2
ꢀ1
0
1
2
ꢀ3
ꢀ2
ꢀ1
0
1
2
E(V) vs SCE
E(V) vs SCE
Figure 4. Cyclic voltammetry (100 mV/s) in CH2Cl2/[Bu4N][PF6] 0.2 M of STX-TXO2 (violet line)
and SPA-TXO2 (blue line) (left) and of STX-DAF (green line), and SPA-DAF (yellow green line)
(right).
In reduction (Figure 4), the TXO2 derivatives are reduced at potential values closed to that of
the electrolytic medium reduction and their onset potentials are detected at ca ꢀ2.4 V, the
LUMO levels lying hence around ꢀ2.0 eV. The DAF derivatives are reduced at less negative
potentials than those of TXO2 derivatives and an irreversible reduction wave is observed (see
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inset in figure 4, right) with a maximum recorded at ꢀ2.42/ꢀ2.5 V for STX-DAF / SPA-DAF
resp. Thus, the LUMO levels of STX-DAF and SPA-DAF have been evaluated at ꢀ2.34 /ꢀ
2.31 eV, being ca 0.3 eV lower than the LUMO levels of the TXO2 derivatives (ꢀ2.0 eV). This
feature clearly indicates the stronger electron accepting capability of DAF compare to that of
TXO2 and its potential to depress the LUMO level. Thus, SPA-DAF possessing the strongest
donor/acceptor combination, displays an electrochemical gap ꢁEelec of 3.04 eV being strongly
more contracted than that of STX-TXO2 (3.79 eV), which possesses the weakest donor/
acceptor combination (Figure 5). STX-DAF and SPA-TXO2 possess therefore intermediate
values of 3.36 and 3.43 eV respectively. For comparison purpose, structurally related 9,9'ꢀ
spirobifluorene (SBF)72, 73 possessing two spiroꢀconnected fluorene units possesses a wide
ꢁEelec of 4.05 eV (HOMO: ꢀ5.94 eV, LUMO: ꢀ1.89 eV),33 widened by more than 1 eV
compared to that of SPA-DAF. Thus, compared to SBF, we can note that the TX unit leads to
dyes with higher HOMO level (ꢀ5.94 eV for SBF vs ꢀ5.70/ꢀ5.79 eV for STX-DAF/STX-
TXO2) and that the TXO2 unit leads to dyes with slightly lower LUMO level (ꢀ1.89 eV for
SBF vs ꢀ2.0 eV for both STX-TXO2 and SPA-TXO2). The effect of these fragments on the
molecular orbitals energy levels are hence relatively weak compared to those of fluorene.
Oppositely, the PA/DAF units lead to a strong increase/decrease of the HOMO/LUMO energy
levels compared to SBF. Thus, the PA/DAF combination seems to be the more attractive
leading to the smallest gap, 3.04 eV, in the series whereas the TX/TXO2 combination leads to
the widest gap in the series, 3.79 eV, highlighting not only the efficiency of the present design
to tune the electronic properties of spiroꢀconnected compounds but also the importance of the
chosen Donor/Acceptor combination.
Compared to previously reported structurally related PA compounds bridged by a
spiroanthracenone (HOMO/LUMO: ꢀ5.50/ꢀ1.90 eV) or by two phenyls units (HOMO/LUMO:
ꢀ5.42/ꢀ1.74 eV),42 SPA-TXO2 and SPA-DAF (HOMO: ꢀ5.42 and ꢀ5.35 eV respectively) have
(i) a very similar HOMO energy level, due to the presence of the PA unit, but (ii) a lower
LUMO energy level (LUMO: ꢀ2 eV and ꢀ2.31 eV) due to the presence of either the TXO2 or
the DAF unit instead of the anthracenone core. Thus, the present design allows selectively
tuning the LUMO energy level without changing that of the HOMO.
Figure 5. HOMO/LUMO energy levels and ꢁEel (eV) obtained from electrochemical investigations
10

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Table 1. Selected theoretical calculations, electrochemical and thermal data of the four dyes
Theoretical calculations
Thermal
properties
Td (°C)
Electrochemical properties a
Eonsetox (V) /
HOMO (eV)
HOMO/LUMO
Eonsetred (V) /
LUMO (eV)
ꢁEtheo
(eV)
Eox (V)
Ered (V)
ꢁEel (eV)
-6.03/-1.59
-5.57/-1.44
-5.87/-1.87
-5.49/-1.72
4.44
261
313
258
296
STX-TXO2
SPA-TXO2
STX-DAF 1.48/1.60
SPA-DAF 1.11/1.20
1.55
1.14
1.39 / -5.79
1.02 / -5.42
1.30 / -5.70
0.95 / -5.35
_
_
-2.40 / -2.00
-2.40 / -2.00
-2.06 / -2.34
-2.09 / -2.31
3.79
3.43
3.36
3.04
4.13
4.00
3.77
-2.41
~-2.5
a/ all potentials are given vs SCE
Theoretical calculations
Geometry optimization of the four dyes in the singlet and triplet states was performed using
Density Functional Theory (DFT) at the Gaussian 09 B3LYP/6ꢀ311+G(d,p) level of theory.
All the results are reported in Table 1 and in Figure 6. The electronic distribution and the
energy levels of HOMOs and LUMOs (and the corresponding energy gaps ꢁEtheo) have been
determined on optimized geometries. The same tendency is observed between the
HOMO/LUMO values determined by CVs and those obtained by theoretical calculations. The
HOMO levels (calculated at ꢀ5.49 and ꢀ5.57 eV) of SPA-DAF and SPA-TXO2 are
exclusively centred on the PA units (Figure 6) in accordance with an electron transfer centred
on this unit as suggested above in the electrochemical part. One can note that there is no
electron density on the pendant Nꢀphenyl meaning that the electronic properties of PA unit are
independent of this pendant phenyl. HOMO of STX-TXO2 and STX-DAF are mainly
localized on the TX unit, leading to deeper HOMO energy levels (ꢀ6.03 and ꢀ5.87 eV) than
those of PA derivatives.
11

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Figure 6. Calculated frontier molecular orbitals by DFT (B3LYP/6ꢀ311+G(d,p) and the 4th first
calculated electronic transitions by TDꢀDFT after geometry optimization with DFT B3LYP/6ꢀ
311+G(d,p), shown with a isovalue of 0.04 [e bohrꢀ3]1/2
The calculated LUMO levels of STX-DAF and SPA-DAF are lower in energy (ꢀ1.87 and ꢀ
1.72 eV resp.) than those of STX-TXO2 and SPA-TXO2 (ꢀ1.59 and ꢀ1.44 eV) clearly
showing the stronger electron acceptor nature of the DAF unit compared to that of the TXO2
unit. For the two DAF derivatives, the LUMO level is localized on the DAF fragment (in
accordance with a first electrochemical reduction centred on this fragment, see above) and is
separated by more than 0.6 eV from the LUMO+1 level. However, for the TXO2 derivatives,
the situation is slightly different. Indeed, the LUMO of SPA-TXO2 is exclusively spread out
on the TXO2 core whereas that of STX-TXO2 seems to be not only spread out on the TXO2
core but also on the central TX unit. For both TXO2 derivatives, the gap between the LUMO
and the LUMO+1 is less than 0.1 eV (0.05/0.1 eV for STX-TXO2 / SPA-TXO2) indicating
some possible mixing of the LUMO and LUMO+1 orbitals. In conclusion the electronic
separation between the donor (HOMO) and the acceptor unit (LUMO), key point in host
12

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design, is clearly more efficient for the DAF derivatives than for the TXO2 derivatives. The
calculated energy gaps ꢁEtheo, follow the same trend than the electrochemical ones with the
lowest gap calculated for SPA-DAF (3.77 eV) and the highest recorded for STX-TXO2 (4.44
eV).
Absorption spectroscopy
The UVꢀVisible absorption spectra of the four DꢀspiroꢀA dyes, recorded in cyclohexane, are
presented in figure 7 and their main characteristics are summarized in table 2. The four
compounds present different absorption spectra with three or four absorption bands with low
absorption coefficients (between 5 ×103 to 20 ×103 L.molꢀ1.cmꢀ1) in the range 260ꢀ350 nm.
This is fully consistent with the presence of small and weakly conjugated units.
Absorption spectra of the two TXO2 derivatives (Figure 7, left) are similar in the high energy
range (maxima at 274/281 nm for STX-TXO2 and at 273/280 nm for SPA-TXO2) indicating
that these two absorption bands may be associated to the TXO2 fragment. A series of
previously reported TXO2 derivatives spiroꢀlinked to 2,7ꢀtriphenylamineꢀfluorene or to 2,7ꢀNꢀ
phenylcarbazole fluorene also present absorption bands of the TXO2 units close to 300 nm.51
2.4x104
STXꢀTXO2
SPAꢀTXO2
STXꢀDAF
SPAꢀDAF
7.5x103
5.0x103
319
|
297
|
306
|
323
|
1.6x104
319
|
|
|
|
309
307
8.0x103
0.0
2.5x103
0.0
275
300
325
350
375
400
275
300
325
350
375
400
Wavelength (nm)
Wavelength (nm)
Figure 7. Absorption spectra of the four dyes in cyclohexane.
In the lowest energy range, the two spectra are nevertheless different with one broad band
centred at 309 nm for STX-TXO2 and two broad bands centred at 297 and 323 nm for SPA-
TXO2. The band at 309 nm may therefore be associated to TX and the two bands 297 and 323
nm to PA. The absorption of TX has been previously reported and possesses a maximum
centred at 266 nm in ethanol.74 Thus, there is a redꢀshift of 43 nm between the absorption of
TX and that of the spiroꢀbridgedꢀTX recorded herein for STX-TXO2. A similar red shift of 32
nm has been reported for spiroꢀconnected TX (bridged with a 2,7ꢀtriphenylamineꢀfluorene,
λmax: 298 nm in CH2Cl2).51 This red shift is due to the different influence of a methylene
bridge and of a spiroaromatic bridge on the TX core. Interestingly, absorption spectra of
structurally related compounds built on the spiroconnection of PA units with anthracenone41
present the same profile than that presented herein for SPA-TXO2 with two absorption bands
with maxima at 300/320 nm.41
13

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The two DAF derivatives (Figure 7, right) present similar absorption spectra at lower energy,
the absorption being however broader for SPA-DAF than for STX-DAF. The two main
absorption bands centred at 306/307 and 319 nm may be assigned to the absorption of the
DAF core in accordance with literature data.64, 66, 69, 75 In addition, we have shown above that
the band at 309 nm in STX-TXO2 was assigned to the TX absorption. Thus, in STX-DAF,
there is a clear superimposition of the absorptions of TX and DAF cores leading to a large
band at 307 nm, being relatively more intense than that at 319 nm. Similarly, in the case of
SPA-DAF, there is an overlap between the PA and DAF fragments, the shoulder observed
above 327 nm being surely due to the absorption of the PA core. Finally, it should be stressed
that higher absorption coefficients are obtained for SPA-DAF than for STX-DAF (SPA-
DAF: ε319 nm=20.4×103 L.molꢀ1.cmꢀ1 and ε306 nm=16.4×103 L.molꢀ1.cmꢀ1, STX-
DAF: ε319 nm=12.1×103 L.molꢀ1.cmꢀ1 and ε307 nm=12.9×103 L.molꢀ1.cmꢀ1 ).
The calculated absorption spectra from TDꢀDFT (Figure 6) show that, except for STX-TXO2,
the HOMO/LUMO transitions are theoretically forbidden (oscillator strength: 0.000). This is a
crucial point, which finds its origin in the spatial separation of HOMO and LUMO levels
leading to through space forbidden transitions. The calculated energy of this HOMO/LUMO
transition varies from 327 nm for STX-TXO2 to 391 nm for SPA-DAF when going from
weak donor/acceptor units (TX and TXO2) to strong ones (PA and DAF). In STX-TXO2, as
electronic densities of the HOMO and the LUMO allow some orbital overlap, the oscillator
strength of the HOMO/LUMO transition is slightly higher (0.0553) and the transition is
weakly allowed. Thus, in the case of DAF derivatives SPA-DAF and STX-DAF, the
theoretical HOMO/LUMO transitions (391 and 362 nm resp.) are not detectable
experimentally in their corresponding absorption spectra. Similarly, no band at 350 nm is
observed for SPA-TXO2. Finally, the HOMO/LUMO theoretical transition calculated at 327
nm for STX-TXO2 would be overlapped with the other absorptions bands.
Regarding the other transitions, one can note that a high oscillator strength (f>0.17) transition
is observed for both DAF derivatives. Indeed for both STX-DAF and SPA-DAF, HOMOꢀ
1/LUMO transitions between DAF units are detected at 301 nm and 303 nm resp. These
oscillator strengths are in accordance with the high ε recorded for these molecules (see
above). For TXO2 derivatives, the situation is different. Indeed, for SPA-TXO2, all the
oscillator strengths are very low (f<0.033), the more intense transitions being observed at 345
nm between HOMO and LUMO+1 (f = 0.0324) and at 312 nm between HOMO and
LUMO+5 (f=0.0312). The transitions all involve PA units. In the case of STX-TXO2, all the
oscillator strengths are also very low except a HOMOꢀ1/LUMO transition (f= 0.1011) with
both orbitals involving TX and TXO2 molecular fragments.
Optical gaps ꢁEopt have been evaluated from the onset of the last absorption band, varying
from 3.54 eV to 3.79 eV (STX-TXO2 : 3.73 eV, SPA-TXO2 : 3.54 eV, STX-DAF : 3.79 eV
and SPA-DAF : 3.64 eV). Thus, one can note that TXO2 derivatives present a ꢁEopt in
accordance with the electrochemical gap ꢁEel, which is not the case for DAF derivatives
(Table 1). Indeed, STX-DAF and SPA-DAF possess a ꢁEopt strongly wider than their
corresponding ꢁEel (see Tables 1 and 2). Indeed and as exposed above, all molecules possess
a theoretical forbidden throughꢀspace HOMO/LUMO transition, calculated at higher energy
14

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for TXO2 derivatives (327 and 350 nm) than for DAF derivatives (362 and 391 nm). ꢁEopt
should hence be determined with this absorption band. Thus, in the case of TXO2 derivatives,
this hypothetical transition would be overlapped in the tail of the large absorption band at 323
nm (SPA-TXO2) and 309 nm (STX-TXO2) whereas in the case of DAF derivatives this band
would be found at lower energy (362 and 391 nm). Since ꢁEopt does not correspond for DAF
derivatives to the energy difference between HOMO and LUMO, this feature explains the
difference observed between ꢁEopt and ꢁEel. As DAF derivatives present an almost identical
ꢁEopt (3.79 and 3.64 eV), the transition involved is surely a HOMOꢀ1/LUMO transition
implying for both compounds, only the DAF units (Figure 6).
Emission spectroscopy
TX derivatives present structureless emission spectra with maxima recorded at 338 nm for
STX-TXO2 and at 348 nm for STX-DAF (Figure 8, left). Literature reports the emission of
4,5ꢀdiazaspirobifluorene66 at 350 nm (in THF) in accordance with that of STX-DAF and
highlighting the weak influence of the spiroconnected unit (fluorene in diazaspirobifluorene
and thioxanthene in STX-DAF) on the DAF core. Emission of the PA derivatives also appear
structureless and are redꢀshifted compared to those of TX derivatives (λmax=348/350 nm for
SPA-TXO2 /λmax=388 nm for SPA-DAF). Thus, one can observed a red shift of the
emission maxima (and hence a gap contraction) as the donor and acceptor strength increases.
It is important to mention that the emission of all compounds fits well with the energy of the
HOMO/LUMO transition calculated by TDꢀDFT (STX-TXO2: 327 nm, STX-DAF: 350 nm,
SPA-TXO2: 362 nm, SPA-DAF: 391 nm, see above). This clearly confirms the above
mentioned assignment of a through space electronic transfer depending of the electrochemical
HOMO/LUMO gap. The emission of the four dyes is hence due to a photoinduced
intramolecular charge transfer (ICT) between the donor and the acceptor in the excited state
(see below solvatochromic experiments). The more the donorꢀacceptor strength, the more
contracted gap and the more redꢀshifted emission. Another signature of a through space
electronic transfer is the very low quantum yield (QY) translating the very weak probability
of this transition. In the present case, QY are indeed very low varying from 0.1 % for the two
DAF derivatives to 0.8/4.1% for SPA-TXO2/STX-TXO2 resp. in perfect accordance with our
above mentioned conclusions. In addition, it should be noted that another emission of low
intensity is detected at high energy (ca 330/350 nm) for SPA-DAF, which can be tentatively
assigned to an emission induced by the fragments (PA or DAF) alone corresponding to a
locally excited state or to an emission from a higher excited state. Indeed, for comparison
purpose, the emission of triphenylamine is reported at 358 nm (in THF)76 and that of Nꢀ
phenylꢀcarbazole at 347 nm (in THF)76 in accordance with the tiny band detected for SPA-
DAF.
15

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338 348
388
|
|
|
350
|
362 371 394
STX-TXO2
420
|
1.0
0.5
0.0
|
|
|
STX-TXO2
1,0
0,5
0,0
SPA-TXO2
STX-DAF0
SPA-DAF0
SPA-TXO2
STX-DAF0
SPA-DAF0
350
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 8. Emission spectra of the four dyes in cyclohexane (left) (concentration ~10ꢀ3 M) and in the
solid state (thin film, deposited on a sapphire plate from 50 µL of a 10 g/L solution) (right).
Regarding the solid state fluorescent properties (Figure 8ꢀright), the emission spectra appear
broader and red shifted (from 21 to 46 nm) compared to solution ones. This red shift is
assigned not only to the different dielectric constants between the two environments (liquid vs
solid) but also to the existence of intermolecular interactions in the solid state which may
appear surprising due to the 3D geometry induced by the spiro carbon.
Table 2. Photophysical properties of the four dyes
c
λ
ABS (nm)
Φ (%)
ET
λEM
λEM
ΔS-T
Δν
ꢁEOPT (eV)a
(cm-1/ eV)a
(eV)
(nm)a (nm)b
d (eV)
[103.ε (L.mol-1.cm-1)]
309 (5.1); 281 (6.6);
274* (6.4)
[λexc (nm)]a
STX-TXO2
3.73
338
362
2780 / 0.34
0.8 [308]
3.06
0.61
323(5.1); 297 (5.9);
280 (7.2); 273* (7.6)
319 (12.1); 307* (12.9)
330(sh); 319(20.4);
306 (16.4); 271 (11.8)
SPA-TXO2
STX-DAF
SPA-DAF
3.54
3.79
3.64
350
348
388
371
394
420
2390 / 0.30
2610 / 0.32
5570 / 0.69
4.1 [323]
0.1 [319]
0.1 [319]
3.08
3.03
2.98
0.46
0.53
0.22
*\ λmax, a\ in cyclohexane, b\ in thin film, c\ in frozen methylcyclohexane/2-methylpentane (1/1) (77 K), d\ ΔS-
T = 1239.84/λEM(cyclohexane)-ET
Solvatochromic experiments allow going deeper in the understanding of the photophysical
properties of the dyes through the determination of the polarity of the excited states (Table 3,
Figure 9). First, as the absorption spectra do not differ much as a function of the polarity, the
characteristics of the ground and FranckꢀCondon excited states are very similar (see SI).
Oppositely, the fluorescence spectra show an intense solvatochromic effect. Indeed, the
emission maxima of all fluorophores are redꢀshifted with the increase polarity of the solvent
(from cyclohexane to acetonitrile, see Figure 9). This shift is of 58, 93, 110 and 111 nm for
STX-TXO2, STX-DAF, SPA-TXO2 and SPA-DAF respectively. The large bathochromic
shift of the fluorescence emission is the consequence of the stabilization of intramolecular
charge transfer (ICT) excited state relative to the ground state, leading to an energy gap
contraction. This is caused by dipoleꢀdipole interactions between the dyes and polar solvent
molecules and hence of a photoinduced ICT between the donor and the acceptor. ꢁµ of 13.4
D (STX-TXO2), 32.6 D (SPA-TXO2), 14.3 D (STX-DAF) and 29.2 D (SPA-DAF) have
been evaluated using Lippert Mataga formalism (dipole moment at the ground state obtained
through DFT calculations were: 4.8, 7.5, 3.1 and 6.2 D for STX-TXO2, SPA-TXO2, STX-
DAF and SPA-DAF (see details for calculations in SI). We note that PA derivatives (SPA-
TXO2, ꢁꢀ: 32.6 D and SPA-DAF, ꢁꢀ: 29.2 D) lead to a twice larger ꢁµ than that calculated
16

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for TX derivatives (STX-TXO2, ꢁꢀ: 13.4 D and STX-DAF, ꢁꢀ: 14.3 D). This reflects the
stronger electronꢀdonating ability of PA compared to TX and hence the stronger ICT induced.
348
440
STX-DAF
µ(S1) : 14.3 D
STX-TXO2
1.0
0.8
0.6
0.4
0.2
0.0
396
338
1.0
0.8
0.6
0.4
0.2
0.0
µ(S1) : 18.2 D
ꢁ
µ : 17.4 D
ꢁµ : 13.4 D
350
400
450
500
550
600
350
400
450
550
600
Wavelength (5n0m0 )
Wavelength (nm)
SPA-DAF
µ(S1) : 35.4 D
387
497
1.0
0.8
0.6
0.4
0.2
0.0
SPA-TXO2
350
459
1.0
ꢁ
µ : 29.2 D
µ(S1) : 40.1 D
0.8
0.6
0.4
0.2
0.0
ꢁ
µ : 32.6 D
350
400
450
550
600
Wavelength (n5m00)
350
550
600
400 Wave4l5e0ngth (n5m00)
Figure 9. Normalized PL spectra of the four dyes in cyclohexane (black line); toluene (blue line),
chloroform (green line), ethylacetate (orange line) and acetonitrile (red line).
Table 3. Photophysical properties of the four dyes as a function of the polarity of the solvents.
λabs (nm)/λem (nm)/Φ (%)
Δμ /μ(S1)/μ(S0)a
[λexc (nm)]
Chloroform
312/354/0.7
[312]
(D)
Cyclohexane
309/338/0.8
[309]
324/350/4.1
[323]
319/348/0.1
[319]
319/388/0.1
[319]
Toluene
311/348/0.7
[309]
Ethyl Acetate
310/353/0.9
[309]
Acetonitrile
310/396/1.1
[310]
300/460/1.4
[323]
320/441/0.1
[319]
321/499/0.5
[320]
STX-TXO2
SPA-TXO2
STX-DAF
SPA-DAF
13.4/18.2/4.8
32.6/ 40.1/7.5
14.3/ 17.4/3.1
29.2/ 35.4/6.2
326/382/2.3
[325]
328/421/1.2
[328]
326/410/1.7
[326]
322/391/0.1
[321]
322/398/ 0.1
[322]
320/395/0.1
[319]
321/429/0.1
[321]
323/440/0.2
[332]
320/465/0.1
[319]
a. μ(S0) has been obtained through DFT calculations, Δμ has been obtained using Lippert-Mataga
formalism, Δμ = μ(S1)-μ(S0)
Phosphorescence of the four dyes was studied at 77 K in frozen methylcyclohexane/2ꢀmethylꢀ
pentane (1/1) (figure 10). TXO2 derivatives present a first phosphorescent transition at
405/402 nm corresponding to an ET of 3.06/3.08 eV for STX-TXO2 and SPA-TXO2
17

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respectively. For DAF derivatives, a first phosphorescence contribution is observed at 409 nm
(ET: 3.03 eV) for STX-DAF and at 416 nm (ET: 2.98 eV) for SPA-DAF. In the light of these
results, the spin density of the triplet state of the four molecules is probably located on their
corresponding acceptor unit. This is a key point to be considered for the future design of
structurally related host materials with a DꢀSpiroꢀA architecture. The four compounds present
hence a very high ET (3.03±0.05 eV), enabling their use as host for blue dopant.
STXꢀTXO2
SPAꢀTXO2
(
λ
exc = 300 nm)
(
λexc = 300 nm)
1.0
0.5
0.0
STXꢀDAF (
SPAꢀDAF (
λ
exc = 300 nm)
λexc = 300 nm)
|
|
350
400
450
500
550
600
Wavelength (nm)
Figure 10. Emission spectra of the four dyes recorded at 77 K in frozen methylcyclohexane/2ꢀ
methylpentane (1/1)
Phosphorescent OLEDs.
Skyꢀblue (FIrpic) PhOLEDs were fabricated and characterised using the four host materials.
The device configuration was ITO/CuPc (10 nm)/NPB (40 nm)/TCTA (10 nm)/EML: FIrpic
(20 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm). ITO is used as the anode, CuPc (copper
phtalocyanine) is the hole injecting layer, NPB (N,N’ꢀdi(1ꢀnaphthyl)ꢀN,N’ꢀdiphenylꢀ[1,10ꢀ
biphenyl]ꢀ4,4’ꢀdiamine) is the hole transporting layer, TCTA (4,4’,4’’ꢀTris(carbazolꢀ9ꢀyl)ꢀ
triphenylamine) is the electron/exciton blocking layer, TPBI (1,3,5ꢀTris(1ꢀphenylꢀ1Hꢀ
benzimidazolꢀ2ꢀyl)benzene) is both the electron transporting layer and the hole blocking layer
and a thin film of lithium fluoride covered with aluminium is the cathode. The device
architecture and the relative energy levels of the successive layers can be found in our
previous works.32, 33
The four dyes were used as host for the skyꢀblue dopant FIrpic (ET: 2.62 eV).*77 Different
FIrpic concentrations were tested and the best performances were obtained with FIrpic 20 %
doped EML. Those performances are reported in figure 11 and the main device characteristics
summarized in table 4. The more efficient blue PhOLEDs were based on SPA-DAF, STX-
DAF and SPA-TXO2, whereas devices using STX-TXO2 as host were clearly less efficient.
* The four dyes have been also used as host for green Ir(ppy)3 and the devices performances are
gathered in the SI.
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PE STX-TXO2 : FIrpic 20%
STX-TXO2: FIrpic 20%
PE SPA-TXO2 : FIrpic 20%
20
18
16
14
12
10
8
40
30
20
10
0
1.25
1.00
0.75
0.50
0.25
0.00
-0.25
PE STX-DAF : FIrpic 20%
PE SPA-DAF : FIrpic 20%
CE STX-TXO2 : FIrpic 20%
SPA-TXO2: FIrpic 20%
STX-DAF: FIrpic 20%
SPA-DAF: FIrpic 20%
CE SPA-TXO2 : FIrpic 20%
CE STX-DAF : FIrpic 20%
CE SPA-DAF : FIrpic 20%
6
4
2
0
0
20
40
60
400
500
600
700
800
Current Density (mA/cm2)
Wavelength (nm)
Figure 11. Current (empty symbol) and power efficiencies (filled symbol) versus current density of the
blue devices using STX-TXO2, SPA-TXO2, STX-DAF and SPA-DAF doped with FIrpic 20 % in
mass) as emitting layer (left). Corresponding EL spectra recorded at 10 mA/cm2 (right).
In the TXO2 family, PhOLEDs using SPA-TXO2 as host reach a high EQE of 10.6 % (at 10
mA/cm2), whereas those using STX-TXO2 as host are impressively less efficient displaying a
very low EQE of 1.9 %. The corresponding luminous (CE) and power (PE) efficiencies are
recorded at 27.7 cd/A and at 14.7 lm/W for SPA-TXO2 and at 4 cd/A and at 1.2 lm/W for
STX-TXO2. The devices incorporating DAFꢀderivatives do not display such difference as
both display interesting performance at 10 mA/cm2 with EQE of 8 % (CE= 22 cd/A and PE=
10.4 lm/W) and 10.2 % (CE= 24.3 cd/A and PE= 14 lm/W) for STX-DAF and SPA-DAF
respectively.
Table 4. Selected EL data of blue devices
Von (V)
L = 1a
CE (cd/A)
J = 1 b J = 10 b J = 1 b J = 10 b
PE (lm/W)
EQE (%)
J = 10 b
CIE (x;y)
J=10
Lmax (cd/m²)
(J b)
EML
STX-TXO2 + 20%
6.7
3.1
3.4
4.0
1.2
1.2
1.9
0.21; 0.41
0.17; 0.42
1300 (70)
9200 (90)
SPA-TXO2 + 20%
17.1
27.7
12.3
14.7
10.6
STX-DAF + 20%
SPA-DAF + 20%
3.6
2.9
22.5
21.7
22.0
24.3
13.7
16.4
10.4
14.0
8.0
0.17; 0.42
0.19; 0.43
4300 (60)
6100 (70)
10.2
a\ en cd/m², b\ en mA/cm²
From these blue PhOLED performances, several conclusions can be drawn. First, it is clear
that the incorporation of PA units within the structure is strongly correlated to the high
performance of the PhOLED. Thus, PhOLED performances based on the PA units are always
higher than those based on the TX unit due to the higher HOMO energy levels measured for
the former. In addition, STX-TXO2 with the deepest HOMO and the highest LUMO levels in
the series appears to be a very bad host material highlighting how an unsuitable combination
can dramatically decrease the efficiency of a device. The potential of the DAF core is more
difficult to evaluate. Indeed, the association of the DAF and TXO2 fragments with the PA unit
19

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leads to high performance devices with very similar efficiencies (Table 4, EQE of 10.6 % for
SPA-TXO2 and of 10.2 % for SPA-DAF at 10 mA/cm2). However, the association of the
DAF and TX fragments in STX-DAF provides a PhOLED with an EQE four times higher
than that of STX-TXO2 (8% vs 1.9% resp.). The strong decrease of the LUMO level of STX-
DAF compare to that of STX-TXO2 leading to more efficient electron injection, may be
involved in this impressive different efficiency. However, this effect is not found for SPA-
DAF vs SPA-TXO2 and more experiments would be therefore necessary to fully unravel this
peculiar behaviour. We believe nevertheless that the potential of the TX unit is clearly not as
high as that of the PA unit.
Except for STX-TXO2 for which the performance is very low both in term of EQE (1.9 %)
and in term of Von (6.7 V), the DꢀSpiroꢀA design seems to be promising for hosting blue
phosphors. Indeed, the present device performances are higher than those reported for non
bipolar structurally related hosts such as the parent molecule SBF (EQE: 6.5 % at 10
mA/cm2).33 As the device architecture of these previous works with SBF is the same as that
presented herein (except the host material), the different performance can only be attributed to
the nature of the host and more particularly to the adjustments of HOMO/LUMO energy
levels. Similarly, 4ꢀsubstituted SBF hosts, recently developed by our group such as 4ꢀphenylꢀ
SBF (EQE: 6 %),33 4ꢀ5ꢀpyrimidinylꢀSBF (5 %)78 or 4ꢀpyridinylꢀSBF32 (3.9 to 5.1%) all leads
to lower blue PhOLED performance. Moreover, the present devices using SPA-DAF, SPA-
TXO2 and STX-DAF as host emit light at lower Von (2.9 to 3.1 V for L=1 cd/m²) than the
above mentioned 4ꢀphenylꢀSBF,33 4ꢀ5ꢀpyrimidinylꢀSBF78 and 4ꢀpyridinylꢀSBF32 for which
Von are higher than 4 V. SPA-DAF and SPA-TXO2 based blue PhOLEDs even surpass the
performance of those based on the known and efficient host N,Nꢀdicarbazolylꢀ3,5ꢀbenzene
(mCP, ET: 2.9 eV) previously reported in literature with exactly the same device architecture
(EQE= 8.6 % at 10 mA/cm2, Von = 3.2 V).45 The better performance of the present hosts is
due to their bipolar character, which allow a good adjustment of their HOMO/LUMO energy
levels.
Interestingly, comparing the results obtained at 100 cd.mꢀ2 to those recorded at higher
luminance (1000 cd.mꢀ2), see SI, shows an increase of EQE for the three devices with SPA-
TXO2/STX-DAF and SPA-DAF as host from 5.8/8.1 and 8.7 % to 10.8/8.3 and 10.2 % resp.
These results show the stability and the efficiency of the devices even at high luminance.
All these features clearly highlight the efficiency of the DꢀSpiroꢀA design to host the sky blue
phosphor FIrpic. Except the device using STX-TXO2 as host, the EL spectra of the three
other devices are identical (Figure 11 right), exclusively showing the emission of the blue
dopant at 473 and 500 nm close to the photoluminescence of pure FIrpic film (475/500 nm)
with no parasite emission.77 The absence of other high energy emissions demonstrates an
efficient energy transfer from the host to the guest. On the opposite, the EL spectrum of the
device using STX-TXO2 as host, presents in addition to the emission band of the dopant
(FIrpic) another less intense emission centred at 400 nm in the range of the nonꢀdoped device
emission (figure 11). This satellite emission and the lower efficiency of the device signs a less
efficient energy transfer from STX-TXO2 to FIrpic.
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Conclusion
The present work reports the synthesis, the physicochemical and photophysical properties and
the application in sky blue PhOLED of four high triplet organic semiꢀconductors based on
the DꢀSpiroꢀA design. This promising chemical design is based on a πꢀconjugation disruption
induced by an insulating spiro bridge between a hole transporting unit, that is phenylacridine
(PA) or thioxanthene (TX) moiety and an electron transporting unit that is dioxothioxanthene
(TXO2) or diazafluorene (DAF) moiety. This molecular design leads to (i) a spatial separation
of HOMO and LUMO retaining a high ET, (ii) HOMO/LUMO levels of the constituting
building blocks adapted to efficient charge injections and (iii) good physical properties, key
feature for device stability and performance. These host materials can be easily synthesized
through a very short, efficient and highly adaptable synthetic strategy. In addition, we have
shown that the properties of the dyes can be easily modulated depending of the strength of the
donor/acceptor combination used, allowing to adjust the HOMO/LUMO levels without
disturbing the ET. These molecules have been incorporated as host in sky blue PhOLEDs with
EQE at 10 mA/cm2 ranging from 2% to more than 10%. The best performance was obtained
with SPA-DAF and SPA-TXO2, highlighting the importance of the chosen donorꢀacceptor
combination on the device performance. We believe that the DꢀSpiroꢀA design is very
promising to elaborate efficient host materials for blue PhOLED applications.
ACKNOWLEDGMENT
We wish to thank the CDIFX (Rennes) for XꢀRay diffraction data, the C.R.M.P.O for mass
analysis, GENCI for allocation of computing time under project c2015085032 (Dr. F.
Barrière, Rennes), the Institut des Sciences Analytiques (Villeurbanne) for TGA, the Service
de MicroanalyseꢀCNRS (Gif sur Yvette) for CHN analyses. MR, JRB and CP warmly thank
the region Bretagne and the ADEME for a studentship (MR), Dr B. Laffite (ADEME), the
CNRS and the ANR (Projects HOMEꢀOLED n°11ꢀBS07ꢀ020ꢀ01 and MEN IN BLUE n° 14ꢀ
CE05ꢀ0024ꢀ01) for financial support.
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