Bis-Sulfone- and Bis-Sulfoxide-Spirobifluorenes: Polar Acceptor Hosts with Tunable Solubilities for Blue-Phosphorescent Light-Emitting Devices

Article abstract of DOI:10.1002/ejoc.201600247

Bis-sulfone- and bis-sulfoxide-spirobifluorenes are a promising class of high-triplet-energy electron-acceptor hosts for blue phosphorescent light-emitting devices. The molecular design and synthetic route are simple and facilitate tailoring of the solubilities of the host materials without lowering the high-energy triplet state. The syntheses and characterization (including single-crystal structures) of four electron-accepting hosts are reported; the trend in their reduction potentials is consistent with the electron-withdrawing nature of the sulfone or sulfoxide substituents. Emission maxima of 421-432 nm overlap with the MLCT absorption of the sky-blue emitter bis(4,6-difluorophenyl-pyridinato)(picolinato)iridium(III) (FIrpic), allowing effective energy transfer from the acceptor hosts to FIrpic. Theoretical calculations show that the introduction of sulfone groups leads to better electron acceptors compared to analogous phosphine oxide functionalized hosts and, at the same time, preserves the energy of the lowest-lying triplet above that of the FIrpic emitter. The new hosts have been tested in phosphorescent light-emitting electrochemical cells (LECs). Large effects of the various solubilizing moieties on the device performance are observed and discussed. Bis-sulfone- and bis-sulfoxide-spirobifluorenes are a promising class of high-triplet-energy electron-acceptor-hosts for blue phosphorescent light-emitting devices. Using a simple synthetic route amenable to tailoring host solubility, we have generated and evaluated a series of polar acceptor hosts. The effects of solubilizing moieties on host light emission capabilities are discussed.

Full text of DOI:10.1002/ejoc.201600247

DOI: 10.1002/ejoc.201600247
Full Paper
Host Materials in LECs
Bis-Sulfone- and Bis-Sulfoxide-Spirobifluorenes: Polar Acceptor
Hosts with Tunable Solubilities for Blue-Phosphorescent Light-
Emitting Devices
Cathrin D. Ertl,[a] Henk J. Bolink,[b] Catherine E. Housecroft*[a] Edwin C. Constable,[a]
Enrique Ortí,[b] José M. Junquera-Hernández,[b] Markus Neuburger,[a] Nail M. Shavaleev,[c]
Mohammad Khaja Nazeeruddin,[c] and David Vonlanthen*[‡]
Abstract: Bis-sulfone- and bis-sulfoxide-spirobifluorenes are a fluorophenyl-pyridinato)(picolinato)iridium(III) (FIrpic), allowing
promising class of high-triplet-energy electron-acceptor hosts effective energy transfer from the acceptor hosts to FIrpic. The-
for blue phosphorescent light-emitting devices. The molecular oretical calculations show that the introduction of sulfone
design and synthetic route are simple and facilitate tailoring of groups leads to better electron acceptors compared to analo-
the solubilities of the host materials without lowering the high- gous phosphine oxide functionalized hosts and, at the same
energy triplet state. The syntheses and characterization (includ- time, preserves the energy of the lowest-lying triplet above that
ing single-crystal structures) of four electron-accepting hosts of the FIrpic emitter. The new hosts have been tested in phos-
are reported; the trend in their reduction potentials is consist- phorescent light-emitting electrochemical cells (LECs). Large ef-
ent with the electron-withdrawing nature of the sulfone or fects of the various solubilizing moieties on the device perform-
sulfoxide substituents. Emission maxima of 421–432 nm overlap ance are observed and discussed.
with the MLCT absorption of the sky-blue emitter bis(4,6-di-
Introduction
Phosphine oxide (PO) functionalization of biphenyl-type struc-
tures has proved to be a good strategy for obtaining high-
triplet-energy electron-acceptor host materials with low-lying
LUMOs.[3] Highly efficient blue and white multi-stack OLEDs us-
ing various PO-acceptor hosts have been reported.[4] Moreover,
the polar PO group can be used in modifiers to lower the work
function of metal electrodes.[5] We recently reported on the ad-
vantages of a PO-acceptor host in phosphorescent light-emit-
ting electrochemical cell (LEC) devices.[6]
Organic light-emitting diodes (OLEDs) promise to have a great
potential for thin flat-panel displays and general lighting in the
forthcoming future.[1] In phosphorescent OLEDs, host materials
transfer energy from singlet and triplet states to the phosphor
resulting theoretically in 100 % internal quantum efficiency.[2]
Good electron-acceptor host materials for blue and white
electroluminescent devices possess high triplet energy, good
electron-transport properties, and improve the electron injec-
tion.[1a] However, the limited availability of electron-transport-
ing host materials, in particular those that are suitable for solu-
tion processing, is one of the bottlenecks to providing cheap,
economically competitive, white electroluminescent devices.
Kippelen and co-workers have shown that bis-sulfonyl-bi-
phenyl is a good acceptor host for blue OLEDs.[7] The sulfone
group lowers the LUMO energy more than the PO group.[7] Sas-
abe et al.[8] recently used a terphenyl-sulfone derivative to ob-
tain highly-efficient blue multi-layer OLEDs. However, the low
solubility of these hosts, coupled with the multi-layer device
architecture, requires the use of expensive vapor-deposition
techniques.
[a] Department of Chemistry, University of Basel,
Spitalstrasse 51, 4056 Basel, Switzerland
E-mail: catherine.housecroft@unibas.ch
http://www.chemie.unibas.ch/~housecroft/index.html
[b] Instituto de Ciencia Molecular, Universidad de Valencia,
46980 Paterna, Spain
The introduction of sulfone and sulfoxide groups in host
structures[9] is attractive for several reasons: i) the acceptor
groups exert a strong inductive electron-withdrawing effect,
which lowers both the HOMO and LUMO energies of the triplet
synthon and improves electron injection; ii) the heteroatom in-
terrupts conjugation to neighboring π-systems and preserves
the high-energy triplet of the host; iii) molecules with strongly
polarized SO bonds can be expected to function as electrode
modifiers analogously to those bearing PO groups;[5] iv) the
simplicity and broad scope of the sulfone/sulfoxide chemistry
is highly appealing.[7,10]
http://www.uv.es/bohenk/index.html
[c] Group for Molecular Engineering of Functional Materials, Institute of
Chemical Sciences and Engineering, École Polytechnique Fédérale de
Lausanne,
1951 Sion, Switzerland
[‡] Current address: Center for Polymers & Organic Solids,
University of California Santa Barbara,
Santa Barbara, CA 93106-5090, USA
E-mail: dvl@physics.ucsb.edu
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201600247
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Here we report the design, synthesis, and photophysical/
electrochemical properties of four electron-acceptor hosts:
SPSO1, SPSO2, SPSO3, and SPSX (Scheme 1). The experimental
study is complemented by theoretical calculations. We hypothe-
sized that sulfoxide functionalization (not previously reported in
host materials in LECs) should give rise to higher-energy triplet
materials.[11] The new hosts were tested in LECs, and the influ-
ence of various solubilizing moieties on the photophysical
properties and LEC performance was investigated. The spirobi-
fluorene moiety is generally preferred over smaller biphenyl
structures as it leads to more stable amorphous thin-film struc-
tures.[12] However, the structural rigidity of spirobifluorenes
tends to hamper solution processing.
Scheme 2. Synthetic routes to thioethers of interest.
Due to its simplicity, the SNAr reaction (Scheme 2, Method
a) was preferred over the transition-metal-catalyzed reaction to
give bis-thioethers. Thus, compounds 2 and 3 were obtained
using the SNAr method (Scheme 2). However, the Pd-catalyzed
cross-coupling methodology offers a viable alternative if the
thiol precursor is not readily accessible. A long reaction time
was required to obtain 2, probably due to the steric hindrance
of the mesitylene-CH3 groups in the ortho-positions of the thio-
phenol-based nucleophile.
To afford the final acceptor hosts, different oxidation condi-
tions were tested (Table 1). Selective and stepwise oxidation of
symmetric bis-thioethers 1–3 gave bis-sulfoxide SPSX and bis-
sulfones SPSO1, SPSO2, and SPSO3 (Scheme 3). All hosts were
obtained by metal-catalyst-free oxidation reactions using H2O2
or mCPBA.
Table 1. Conditions for oxidation of the bis-thioethers.[a]
Substrate Oxidant (equiv.) Solvent/conditions[b]
Host (yield [%])
SPSX (70)
Scheme 1. Chemical structures of the electron-acceptor host compounds.
1
1
1
1
2
3
H2O2 (2.0)
mCPBA (2.0)
H2O2 (exc.)
H2O2 (exc.)
H2O2 (exc.)
H2O2 (exc.)
A, r.t. (30 h)
B, 0 °C, then r.t. (overnight) SPSX (47)
A, r.t. (21 h)
A, reflux (3.5 h)
C, reflux (18 h)
A, r.t. (24 h),
SPSO1 (77)
SPSO1 (89)
SPSO2 (63)
SPSO3 (55)
Results and Discussion
then 40 °C (0.5 h)
Synthesis and NMR Spectroscopic Characterization
[a] Entries 1, 4–6 are described in the main paper; entries 2 and 3 are de-
scribed in the Supporting Information. [b] Solvent systems: A = AcOH/CHCl3,
B = CH2Cl2, C = AcOH/EtOAc.
All hosts were synthesized from commercially available 2,7-di-
bromo-9,9′-spirobifluorene. Reactions with the appropriate thi-
ols yielded bis-thioether derivatives 1–3 (Scheme 2) which were
Selective oxidation[14] of 1 with an equimolar amount of
subsequently oxidized to the final acceptor hosts. Thiolations[13] H2O2 at room temperature gave bis-sulfoxide SPSX. The pres-
in Scheme 2 were achieved through either aromatic nucleo- ence of two stereogenic sulfur centers leads to the formation
philic (SNAr) substitution (Method a) or Pd-cross-coupling of the enantiomeric R,R- and S,S-pair and the R,S-meso form.
(Method b, Supporting Information) to give 1 in good yield. The H and 13C NMR spectroscopic data show that SPSX exists
1
Alternatively, 1 was synthesized starting from 2,7-bis(phenyl- as a 1:1 mixture of diastereoisomers (R,R/S,S and meso), consist-
thio)-9H-fluoren-9-one (4, Supporting Information). We note ent with there being no inversion at sulfur on the NMR spectro-
that 4 can also be used to design mixed n- and p-type high- scopic timescale at 295 K. Oxidation of bis-thioether 1 with
energy triplet cores.[10] Compound 4 was treated with biphenyl- mCPBA[15] was not as selective giving the asymmetric sulfone
2-ylmagnesium bromide to give 1 in 44 % yield over 3 steps side-product 5 (see Supporting Information). Treatment of 1
(Method c, Supporting Information).
with excess H2O2 gave exclusively bis-sulfone SPSO1 in high
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Scheme 3. Selective oxidation of the various thioethers. Reaction conditions
are listed in Table 1.
yield. Notably, high temperatures allowed for a decrease in re-
action time and led to higher yields of 1.
Oxidation of 2 afforded bis-mesitylsulfone SPSO2 in good
yield. Similarly, oxidation of 3 gave SPSO3 in satisfactory yield.
Whereas conversions of 1 and 3 required the use of chlorinated
solvents during the oxidation, oxidation of 2 to SPSO2 pro-
ceeded smoothly in ethyl acetate. This observation points to
the improved substrate solubility attributable to the mesityl
1
group. All new compounds were fully characterized by H and
13C NMR and IR spectroscopy, mass spectrometry and elemen-
tal analyses.
Crystal Structures of SPSO1·2CH2Cl2, SPSO2, and SPSO3
Figure 1. X-ray structures of (a) SPSO1 in SPSO1·2CH2Cl2, (b) SPSO2, and
(c) SPSO3. Solvent molecules are omitted for clarity. Ellipsoids plotted at the
40 % probability level. Selected bond parameters: SPSO1: S1–O1 1.4419(13),
S1–O2 1.4429(12), S2–O3 1.4347(17), S2–O4 1.4417(18), C16–S1 1.7626(15),
C26–S1 1.7632(16), C23–S2 1.7679(17), C32–S2 1.7563(17) Å; C26–S1–C16
105.72(7), C26–S1–O1 108.05(8), C16–S1–O1 107.73(7), C26–S1–O2 107.78(7),
C16–S1–O2 107.25(7), O1–S1–O2 119.52(8), C23–S2–C32 105.08(8), C23–S2–
O3 107.33(10), C32–S2–O3 108.37(9), C23–S2–O4 107.73(9), C32–S2–O4
107.37(10), O3–S2–O4 120.02(12)°; SPSO2: S1–O1 1.4389(10), S1–O2
1.4373(10), C12–S1 1.7753(12), C14–S1 1.7859(13) Å; C14–S1–C12 106.93(6),
C14–S1–O1 107.60(6), C12–S1–O1 107.70(6), C14–S1–O2 109.28(6), C12–S1–
O2 107.09(6), O1–S1–O2 117.75(6)° (symmetry code i: –x, y, 1/2 – z). SPSO3:
S1–O1 1.4406(11), S1–O2 1.4427(12), C10–S1 1.7808(15), C14–S1 1.7705(17) Å;
C14–S1–C10 103.69(7), C14–S1–O1 109.70(7), C10–S1–O1 108.16(7), C14–S1–
O2 107.74(7), C10–S1–O2 108.13(7), O1–S1–O2 118.43(7)° (symmetry code i:
1 – x, y, 1/2 – z).
Single-crystals of SPSO1·2CH2Cl2 were grown from CH2Cl2 over-
laid with MeOH. X-ray quality crystals of SPSO2 were obtained
from recrystallization of the bulk material from cyclohexane and
toluene, and those of SPSO3 were grown from a CHCl3 solution
of the compound overlaid with n-hexane. The structures of the
three compounds were determined by single-crystal X-ray dif-
fraction. SPSO1·2CH2Cl2 crystallizes in the space group P21/c
with one molecule and two disordered CH2Cl2 molecules in the
asymmetric unit. Each solvent molecule has been modeled over
two positions with fractional occupancies of 0.70/0.30 and 0.84/
0.16 respectively. Figure 1 (a) shows the structure of the SPSO1
molecule with selected bond parameters given in the Figure
caption. Compounds SPSO2 and SPSO3 both crystallized in the
C2/c space group with half of each molecule in the asymmetric
unit; the second half, in each case, was generated by a twofold
axis (Figure 1, b and c).
The X-ray diffraction data confirm that the compounds pos-
sess the expected structures, with bond parameters that are
similar to one another and which are consistent with other or-
ganic sulfones.[16] Dominant packing interactions in
SPSO1·2CH2Cl2 involve short S–O···HC and Cl···HC contacts. In
addition to exhibiting short SO···HC contacts of 2.51 Å, mol-
ecules of SPSO2 pack so that centrosymmetric pairs of fluorene
domains interact through weak face-to-face π-interactions (Fig-
ure 2). Although the interplane separation is 3.1 Å, the inter-
centroid distance between arene rings is too large (4.9 Å) for
Figure 2. Packing of molecules of SPSO2 showing short S–O···HC and π-con-
tacts (see text) in black hashed lines.
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this to be more than a very weak contact. Short S–O···HC con- tured absorption observed above 270 nm for all four com-
tacts are also a dominant feature of the molecular packing in pounds is attributed to π → π* transitions centered on the spi-
SPSO3. The n-pentyl chain is in a partially folded conformation robifluorene core. The bis-sulfonyl-functionalized hosts exhibit
and is accommodated in a cleft between two fluorene domains very similar PL spectra with a maximum emission, λem, in the
of adjacent molecules (Figure 3). Short CH···π contacts appear range of 421–432 nm. This emission overlaps well with the
to operate between the two methylene units and the arene metal-to-ligand charge transfer (MLCT) absorption of the sky-
ring (Figure 3).
blue emitter bis(4,6-difluorophenylpyridinato)(picolinato)irid-
ium(III) (FIrpic),[18] enabling efficient energy transfer from the
bis-sulfone acceptor hosts to FIrpic.[7] The PL emission maxima
of the bis-sulfoxide SPSX is shifted by 50 nm into the deep-
blue/near-UV region relative to the bis-sulfones.[11]
Figure 3. Packing of molecules of SPSO3 showing sandwiching of the folded
n-pentyl chains between pairs of fluorene domains of adjacent molecules.
Short CH···π-contacts are shown by black hashed lines.
Electrochemical and Photophysical Studies
Figure 5. UV and PL spectra of SPSO1–3 and SPSX in CH3CN. Solution concen-
trations: 1.0 × 10–5 mol dm–3
.
Reversible reduction processes were observed for compounds
SPSO1–3 at similar potentials (E1/2red, Table 2 and Figure 4). This
is in agreement with the Hammett constants reported for SO2-
aryl- (σ = 0.68) and SO2-alkyl substituents (σ = 0.72–0.77) indi-
cating
a similar electron-withdrawing strength for both
Theoretical Calculations
red
groups.[17] In contrast, E1/2 for SPSX is shifted to more nega-
tive potential (Figure 4 and Table 2).
To gain further insight into the electrochemical and photophys-
ical properties, the molecular and electronic structures of the
sulfoxide and sulfone hosts were investigated by performing
density functional theory (DFT) calculations at the B3LYP/6-
31G** level in the presence of solvent (CH3CN). Non-substituted
9,9′-spirobifluorene (SP) and the 2,7-bis(diphenylphosphoryl)-
9,9′-spirobifluorene phosphine oxide (SPPO13) were also calcu-
lated at the same theoretical level as reference systems for com-
parison purposes.
Table 2. Photophysical and electrochemical data.
red
Host
E1/2 [V][a]
λabs [nm][b]
λem [nm, eV][b]
SPSO1
SPSO2
SPSO3
SPSX
–2.00
–2.07
–2.12
–2.24
284, 330
285, 327
279, 322
273, 326
432, 2.87
421, 2.95
422, 2.94
380, 3.26
[a] First reduction wave, as measured in CH3CN with [TBA]+[PF6]– against Fc/
Fc+ as the internal reference. [b] Measured in CH3CN at 298 K (1 × 10–5
).
M
The molecular geometries of the substituted systems were
fully relaxed with the two substituent groups pointing to differ-
ent sides of the fluorene plane to which they are attached and
converged to C2-symmetry conformations (see Figure S1 in the
Supporting Information). Calculations correctly reproduce the
structural features obtained from X-ray single-crystal analysis
for SPSO1–3. The fluorene moieties are almost orthogonal form-
ing dihedral angles of about 88.5° and the sulfone groups ex-
hibit near-tetrahedral structures. For instance, SPSO1 is com-
puted to have average C–S–C, C–S–O, and O–S–O bond angles
of 105.85, 107.58, and 119.87°, respectively, in very good accord
with the experimental X-ray average values (105.40, 107.70, and
119.77°, respectively). Calculations predict a linearly extended
conformation for the n-pentyl chains attached to the sulfur at-
oms in the SPSO3 molecule. The folded conformation observed
Figure 4. Cyclic voltammograms of SPSO1–3 and SPSX in CH3CN with
[TBA]+[PF6]– against Fc/Fc+ (as an internal reference).
The absorption and the photoluminescence (PL) spectra of experimentally (Figure 1, c) is stabilized by interactions between
the sulfoxide and sulfone hosts are shown in Figure 5. The struc- adjacent molecules.
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Figure 6 compares the electron density contours calculated
for the highest-occupied (HOMO) and lowest-unoccupied mo-
lecular orbital (LUMO) of SPSO1, as a representative example,
with those computed for non-substituted spirobifluorene. The
topology of the frontier MOs of all the other hosts is similar to
that depicted for SPSO1 (see Figure S2, Supporting Informa-
tion). Table 3 depicts the energies calculated for the HOMO, the
LUMO, and the HOMO–LUMO gap for all the hosts and SP.
Table 3 also includes the electron affinities computed as the
energy difference between the neutral host and its radical an-
ion at their respective minimum-energy optimized geometries.
Figure 7. Unpaired-electron spin density contours (0.002 a.u.) calculated for
the anion of SPSO1.
HOMO–LUMO gap therefore decreases by almost 1 eV in going
from SP (4.85 eV) to SPSO1 (3.94 eV). Compared to SPPO13
(bearing phosphine PO groups), the sulfone SO2 groups in
SPSO1 lower the energy of the LUMO to a higher degree (–2.09
vs. –1.73 eV) and leads to a higher electron affinity (2.39 vs.
2.02 eV). This is in accordance with results previously found
for biphenyl-based hosts,[7] and confirms the higher electron-
withdrawing character of the sulfone group compared with the
phosphine group.
The LUMO of SPSO2 (–1.95 eV) and SPSO3 (–1.97 eV) are
calculated to be slightly higher in energy than the LUMO of
SPSO1 (–2.09 eV). This destabilization is due to the fact that this
orbital is delocalized over the terminal phenyl groups for SPSO1
(Figure 6), whereas it remains more confined over the fluorene
moiety for SPSO2 and especially for SPSO3 (Figure S2, Support-
ing Information). This observation suggests that SPSO1 presents
a more effective electronic conjugation through the sulfone
groups than do SPSO2 and SPSO3. The sulfoxide groups in SPSX
exert a weaker electron-withdrawing effect than the sulfone
groups and, as a consequence, induce a smaller stabilization of
the LUMO that is calculated to be –1.62 eV. The computed elec-
tron affinity decreases along the series SPSO1 (2.39 eV) > SPSO2
(2.29 eV) > SPSO3 (2.28 eV) > SPSX (2.09 eV) in good agreement
with the more negative reduction potentials measured along
this series (–2.00, –2.07, –2.12, and –2.24 V, respectively,
Table 2). The values obtained for the electron affinities suggest
that the sulfone and sulfoxide hosts are better electron accept-
ors than the phosphine oxide SPPO13 host for which an elec-
tron affinity of 2.02 eV is computed. Owing to the almost con-
Figure 6. Schematic representation showing the isovalue contours ( 0.03 a.u.)
and energies calculated for the frontier molecular orbitals of SP and SPSO1.
Hydrogen atoms are omitted.
Table 3. B3LYP/6-31G** values computed for the energy of the HOMO (EHOMO
)
and the LUMO (ELUMO), the HOMO–LUMO energy gap (ΔEH–L), and the elec-
tron affinity (EA). All values are given in eV units.
Host
EHOMO
ELUMO
ΔEHOMO–LUMO
EA
SP
–5.82
–6.04
–6.02
–6.04
–5.95
–5.98
–0.97
–2.09
–1.95
–1.97
–1.62
–1.73
4.85
3.95
4.07
4.07
4.33
4.25
1.20
2.39
2.29
2.28
2.09
2.02
SPSO1
SPSO2
SPSO3
SPSX
SPPO13
The SP molecule presents a D2d symmetry with two equiva- stant energy of the HOMO (Table 3), the HOMO–LUMO gap
lent fluorene moieties over which molecular orbitals are equally increases along the series SPSO1 (3.95 eV) SPSO2
<
distributed. Functionalization lowers the molecular symmetry (4.07 eV) < SPSO3 (4.07 eV) < SPSX (4.33 eV) pointing to a
and breaks the equivalence of the fluorene moieties. As ob- blueshift of the absorption and emission wavelengths along the
served in Figure 6 for SPSO1, the LUMO in the host is fully series.
localized on the fluorene to which the sulfone groups are at-
tached, whereas the HOMO mainly resides on the non-function-
alized fluorene. This suggests that, upon reduction, electron in-
jection takes place on the functionalized fluorene fragment.
This is illustrated in Figure 7 for SPSO1, for which the unpaired
electron in the anion is fully localized on the substituted frag-
ment.
Time-dependent DFT (TD-DFT) calculations were performed
on the geometry of the electronic ground state (S0) to obtain
information about the nature of the singlet excited states (Sn)
involved in the absorption spectra. Calculations assign the low-
intensity band above 300 nm (Figure 5) to the S0 → S1 elec-
tronic transition calculated around 3.4–3.7 eV for SPSO1–3 and
SPSX (Table S1, Supporting Information). This transition has a
Functionalization with electron-withdrawing sulfone groups charge transfer (CT) nature since it implies an electron promo-
enables a drastic stabilization of the LUMO that lowers in en- tion from the HOMO, located on the non-functionalized
ergy from –0.97 eV in SP to –2.09 eV in SPSO1. The stabilization fluorene, to the LUMO, spreading over the functionalized
of the HOMO is significantly smaller (0.22 eV) because the sulf- fluorene (Figure 6). The CT character of the transition explains
one groups do not participate in this orbital (Figure 6). The the low intensity of the absorption band. The more intense
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band observed between 250 and 300 nm involves excitations duction of sulfone groups significantly reduces the energy of
to several excited singlets calculated in the 4.0–4.5 eV range the LUMO and leads to better electron acceptors compared to
with high oscillator strengths (f > 0.1, Table S1). These transi- SPPO13, as discussed above, it mainly preserves the energy of
tions mainly imply π → π* excitations within the spirofluorene the T1 triplet. The adiabatic T1 energies estimated for all hosts
core.
are indeed higher than the value of 2.73 eV calculated for FIrpic
(experimental value of 2.65 eV). This suggests that sulfone
SPSO1–3 and sulfoxide SPSX systems can be used as effective
hosts with the FIrpic phosphor as blue dopant.
To obtain an estimation of the emission energies, the geom-
etry of the first singlet excited state was fully relaxed using the
TD-DFT method. According to the calculations, emission takes
place from the charge transfer S1 state resulting from the
HOMO → LUMO excitation for all the hosts. The emission ener-
gies follow the trend expected from the HOMO–LUMO gaps,
with the value calculated for SPSO1 (2.77 eV, 448 nm) slightly
redshifted relative to SPSO2 (2.84 eV, 437 nm) and SPSO3
(2.83 eV, 438 nm) in excellent agreement with the emission
maxima measured experimentally (432, 421, and 422 nm, re-
spectively, Table 2). For SPSX (2.96 eV, 419 nm), calculations
reproduce the shift to bluer wavelengths observed experimen-
tally (Figure 5), although the predicted shift with respect to
SPSO1 (29 nm) underestimates the experimental value (52 nm).
The nature of the lowest-energy triplet excited state (T1) was
first investigated by a TD-DFT study at the optimized geometry
of S0. For all the hosts, TD-DFT calculations predict that T1
mainly results from the HOMO–1 → LUMO excitation, which
mostly involves the functionalized fluorene moiety (see Figure 6
for SPSO1), and that the triplet state associated to the charge
transfer HOMO → LUMO promotion appears higher in energy
(Table S2, Supporting Information). The vertical excitation ener-
gies calculated at the TD-DFT level from S0 to T1 [E(S0 → T1)]
are similar for all the hosts and are given in Table 4. In a second
step, the geometry of the lowest-energy triplet was fully opti-
mized using the spin-unrestricted UB3LYP approach. After full-
geometry relaxation, the unpaired-electron spin density com-
puted for T1 shows the same distribution for all the hosts, in-
cluding phosphine oxide compound SPPO13, and is mainly
confined to the functionalized fluorene moiety as depicted in
Figure 8 for SPSO1. This confirms the electronic nature pre-
dicted for T1 by TD-DFT calculations. The adiabatic energy of T1
[computed as the difference in the DFT energies of S0 and T1
at their respective optimized geometries, ΔE(T1–S0)] is nearly
identical for all the hosts, being between 2.77 eV for SPSO1 and
2.83 eV for SPSO3. The energy calculated for T1 in SPPO13
(2.81 eV) is intermediate between these two values and is in
very good accord with the experimental value of 2.73 eV ob-
tained from low-temperature photoluminescence measure-
ments.[4a] Calculations therefore show that, although the intro-
Figure 8. Unpaired-electron spin density contours (0.002 a.u.) calculated for
the fully-relaxed T1 triplet state of SPSO1.
Electroluminescent Devices
The main advantages of LECs over OLEDs are their simple struc-
tures (consisting of a single active layer processed from solu-
tion) and their insensitivity to the work function of the elec-
trodes employed.[19] However, attempts to fabricate blue phos-
phorescent LECs (electroluminescence emission below 480 nm)
based on ionic transition-metal complexes remain extremely
challenging due to self-quenching of the active material and
unbalanced carrier transport in the emissive layer. Due to these
factors, a maximum luminance of 94 cd m–2 (maximum efficacy
of 4.3 cd A–1) was reported for blue iridium phosphors.[20] Fur-
thermore, light-emitting devices that use 100 % rare-earth-
based materials are not economically viable. The host–guest
approach is an alternative that promises cheaper, more stable,
and brighter LEC devices.[6,21]
We have tested the solution-processible SPSO1–3 hosts as
electron transporters in blue host–guest LEC devices using FIr-
pic as the blue dopant and a previously synthesized ionic hole-
transporting material NMS25 (see Scheme S1, Supporting Infor-
mation, for the molecular structure).[6] Solubility tests for the
sulfone-host series are given in Table S3 in the Supporting In-
formation. The performance data of the blue host–guest LECs
are illustrated in Figure 9 and summarized in Table 5.
In Figure 9, we observe a drop in the driving voltage of 1.0 V
in LEC 1 (3.6 V) when using host SPSO1 compared to LEC 3
(4.6 V) with host SPSO3 (Table 5). The lowered operating volt-
age may be attributed to the strong polarization effect[5a] of
the aryl-sulfone group in SPSO1 forming a dipole layer at the
Al cathode,[5b] lowering the electron injection barrier. It can be
speculated that the highly flexible alkyl-chains in SPSO3 effect-
ively hinder the interaction of the sulfone-group with the Al-
cathode. A low operating voltage of 3.8 V was observed with
host SPSO2 (LEC 2), bearing rigid mesityl groups. From the X-
ray structures (Figure 1), it is obvious that the steric vicinity of
the sulfone groups in SPSO2 is very similar to that found in
Table 4. B3LYP/6-31G** values computed for the vertical TD-DFT excitation
energy from the ground state to the lowest triplet excited state [E(S0 → T1)]
and for the adiabatic energy difference between S0 and T1 [ΔE(T1–S0)] All
values are in eV.
Host
E(S0 → T1)
ΔE(T1–S0)
SP
3.05
2.86
2.88
2.92
2.90
2.90
2.83
2.98
2.77
2.78
2.83
2.80
2.81
2.73
SPSO1
SPSO2
SPSO3
SPSX
SPPO13
FIrpic
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vice was obtained with SPSX as the host and this may be re-
lated to a decomposition reaction at the Al-cathode. However,
it is clear that all members of the sulfone-acceptor host series
give working blue-phosphorescent LECs.
Figure 10. Luminance vs. time of LEC devices with SPSO3.
Conclusions
Figure 9. (top and bottom) Luminance and average voltage vs. time of LEC
devices with the acceptor hosts.
Four electron-acceptor hosts SPSO1, SPSO2, SPSO3, and SPSX
containing sulfone- or sulfoxide-functionalized spirobifluorenes
have been prepared and characterized. The single-crystal struc-
tures of SPSO1, SPSO2 and SPSO3 have been determined.
Trends in the electrochemical behaviour of these compounds
are consistent with the electron-withdrawing properties of the
sulfone or sulfoxide groups. The emission maxima of all four
hosts lie in the range 421–432 nm, which overlaps with the
MLCT absorption of the sky-blue emitter FIrpic, and thus leads
to energy transfer from the acceptor hosts to FIrpic. Theoretical
calculations show that, although the introduction of sulfone
groups significantly reduces the energy of the LUMO and leads
to better electron acceptors compared to analogous phosphine
oxide functionalized hosts, it does not affect the energy of the
lowest lying triplet (ca. 2.80 eV) which is maintained above the
triplet of FIrpic. The acceptor hosts have been tested in LEC
devices. LECs containing the sulfone hosts SPSO1 and SPSO2
(aryl sulfones) perform better than those with SPSO3 (long
chain alkyl sulfone substituents); no working LEC was obtained
with the sulfoxide-functionalized host material.
Table 5. Performance data for LEC devices.[a]
LEC Host mixture
ton
Lummax
t1/2
Efficacy[e] V[f]
[b]
[c]
[d]
[min] [cd m–2
]
[min]
[cd A–1
]
[V]
1
2
3
4
5
SPSO1:NMS25 (1:1)
0.83 21
< 0.1 49
0.18 56
0.29 123
19.3
4.6[g]
1.2
1.6
23.0
0.10
0.23
0.25
0.55
0.43
3.6
3.8
4.6
5.2
4.0
SPSO2:NMS25 (1:1)
SPSO3:NMS25 (1:1)
SPSO3:NMS25 (2:1)
SPSO3:TCTA (2:1) + IL 3.4
92
[a] FIrpic was 10 % in all LEC devices. [b] The turn-on time (ton) is the time
to reach the maximum luminance. [c] Maximum luminance. [d] The lifetime
(t1/2) is the time to reach half of maximum luminance. [e] Maximum efficacy.
[f] Operating average voltage. [g] Lower lifetime mainly due to higher lumi-
nance, not an indication of lower stability.
SPSO1. The lifetimes of LECs 1 and 2 with hosts SPSO1 and
SPSO2, respectively, are significantly longer than that of LEC 3
(Table 5).
However, the performance of the LECs with di-n-alkyl sulfone
host, SPSO3, was considerably improved when the ratio
SPSO3:NMS25 was increased in LEC 4 (Figure 10 and Table 5).
This results in a doubling of the brightness and efficacy
(123 cd m–2 and 0.55 cd A–1, respectively), and can be attrib-
uted to an improved balance of the hole and electron charge
carriers within the emissive layer.[1a,6,21] Moreover, the lifetime
of the SPSO3 LEC was increased 14 times in LEC 5 when using
the lipophilic ionic liquid (IL) [THA]+[BF4]– ([THA]+ = tetra-n-hex-
ylammonium) and the non-ionic hole transporting material
TCTA (see Figure 10, and Scheme S1 in Supporting Information
for the chemical structure of TCTA). This improvement in life-
time can be attributed to the better compatibility of the IL and
the acceptor host SPSO3 resulting in increased stability of the
emissive thin film. At the same time, the turn-on time in LEC 5
is increased. This trend of increase in turn-on time leading to
an increase in lifetime is often observed in LECs and is due to
the role of ionic motion in these devices. In LEC 5, using lipo-
philic ions, the mobility is reduced explaining the observed re-
This study has revealed a promising class of acceptor hosts
for use in LECs. The synthetic routes used to prepare SPSO1,
SPSO2 and SPSO3 can be readily adapted using the pool of
commercially available aryl- and alkyl-thiols, functionalized with
ethers, alcohols, esters, and carboxylic acids to produce a wide
range of polar acceptor-hosts with tailored solubility properties
required for solution-based deposition processes.
Experimental Section
General: All starting materials were commercially available, of rea-
gent grade, and used without further purification. The solvents
were reagent grade or distilled, except for the p-xylene which was
dried by refluxing over NaH. Column chromatography was per-
formed using Fluka silica gel 60 (40–63 μm), Silicycle SilicaFlash P60
(40–63 μm). 1H and 13C{1H} NMR spectra were recorded with a
Bruker DRX400 (400 MHz) or a Bruker DRX500 (500 MHz) spectrom-
eter at 295 K. The chemical shifts were referenced with respect to
residual solvent peaks with δ(TMS) = 0. IR spectra were recorded
sults in lifetime and turn-on time. Notably, no working LEC de- with a Shimadzu FTIR-8400S spectrophotometer using neat samples
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1
and a Golden Gate attachment for solid state samples. EI mass spec-
tra were recorded using a Finnigan MAT 95Q spectrometer. MALDI- the two diastereomers are marked with and without an asterisk: 1H
powder, yield 0.10 g, 0.18 mmol, 70 %. H and 13C NMR signals of
TOF mass spectrometry was done using a Voyager-DE PRO spec-
trometer. Measurement of UV/Vis spectra was carried out with an
Agilent Technologies UV/Visible 8453 spectrophotometer and a Shi-
madzu RF-5301PC spectrofluorometer was used to measure the
photoluminescence spectra. Cyclic voltammetry was recorded using
a CH Instruments 900B potentiostat with glassy carbon working and
platinum auxiliary electrodes; a silver wire was used as a pseudo-
reference electrode and ferrocene as internal reference.
NMR (500 MHz, CDCl3): δ = 7.88–7.85 (m, 4 H, H4,5,4*,5*,4′,5′,4′*,5′
*), 7.54–7.51 (m, 2 H, H3,6,3*,6*), 7.48–7.44 [m, 4 H, S(O)Ph-H2,2*],
7.44–7.34 [m, 8 H, H3′,6′,3′*,6′*,S(O)Ph-H3,4,3*,4*], 7.15–7.14 (m, 2
H, H1,8,1*,8*), 7.12–7.05 (m, 2 H, H2′,7′,2′*,7′*), 6.61–6.57 (m, 2 H,
H1′,8′,1′*,8′*) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 150.88 (C2,7/
C2*,7*), 150.87 (C2,7/C2*,7*), 146.45 (C8′a,9′a,8′a*,9′a*), 146.19
(C8a,9a,8a*,9a*), 145.52 [S(O)Ph-C1/C1*], 145.51 [S(O)Ph-C1/C1*],
143.31 (C4a,4b/C4a*,4b*), 143.30 (C4a,4b/C4a*,4b*), 142.14 (C4′a,4′
b/ C4′a*,4′b*), 142.13 (C4′a,4′b/C4′a*,4′b*), 131.192 [S(O)Ph-C4/C4*],
131.186 [S(O)Ph-C4/C4*], 129.41 [S(O)Ph-C3], 128.61 (C3′,6′/C3′*,6′
*), 128.59 (C3′,6′/C3′*,6′*), 128.56 (C3′,6′/C3′*,6′*), 128.21 (C2′,7′/C2′
*,7′*), 128.18 (C2′,7′/C2′*,7′*), 128.15 (C2′,7′/ C2′*,7′*), 124.98
[S(O)Ph-C2/C2*], 124.97 [S(O)Ph-C2/C2*], 124.87 (C3,6,3*,6*), 123.94
(C1′,8′/C1′*,8′*), 123.91 (C1′,8′/C1′*,8′*), 123.86 (C1′,8′/C1′*,8′*),
121.56 (C4,5/C4*,5*), 121.54 (C4,5/C4*,5*), 121.27 (C1,8/C1*,8*),
121.23 (C1,8/C1*,8*), 120.64 (C4′,5′,4′*,5′*), 66.13 (C9,9*) ppm. IR
The syntheses of compounds 4 and 5, and alternative syntheses
of SPSO1 and SPSX are given in the Supporting Information. See
Supporting Information for the atom labelling for NMR assign-
ments.
2,7-Bis(phenylthio)-9,9′-spirobifluorene (1). Method a: A dried
flask was charged with 2,7-dibromo-9,9′-spirobifluorene (371 mg,
0.782 mmol, 1.00 equiv.), K2CO3 (554 mg, 4.01 mmol, 5.12 equiv.),
and thiophenol (0.82 mL, 884 mg, 8.02 mmol, 10.3 equiv.) under an
argon atmosphere. Dry DMF (4 mL) was added and the suspension
was stirred for 22 h at 140 °C. CH2Cl2 was added to the mixture and
the precipitate was filtered off. The filtrate was washed with H2O,
the aqueous layer was extracted with CH2Cl2 and the combined
organic layers were washed three times with H2O and dried with
Na2SO4. The solvent was removed under reduced pressure. Cyclo-
hexane was added to the residue and the suspension was heated
until everything had dissolved. The precipitate was filtered, washed
with cyclohexane, dissolved with CHCl3, and the solvent was re-
moved under reduced pressure. The residue was recrystallized from
cyclohexane and dried under vacuum, the filtrate was concentrated
under reduced pressure and the residue was again recrystallized
from cyclohexane and dried under vacuum. From both recrystalliza-
tions, the product 1 was obtained as colourless crystals, yield
320 mg, 0.60 mmol, 77 %. 1H NMR (500 MHz, CD2Cl2): δ = 7.84
(solid): ν = 3056 (w), 2925 (w), 1474 (m), 1442 (m), 1398 (m),
˜
1082 (s), 1070 (m), 1040 (s), 1020 (m), 1002 (m), 997 (m), 818 (m),
761 (s), 744 (s), 734 (s), 726 (s), 686 (s), 636 (s) cm–1. UV/Vis (MeCN,
1.0 × 10–5 mol dm–3): λmax (ε) = 230 (sh, 77000), 273 (42000),
326 nm (21000 dm3 cm–1 mol–1). Fluorescence (MeCN,
1.0 × 10–5 mol dm–3, λex = 300 nm): λem = 380 nm. MS (EI, 70 eV):
m/z (%) = 564.1 [M]+ (100, calcd. 564.1). C37H24O2S2 (564.72): calcd.
C 78.70, H 4.28; found C 78.52, H 4.58.
2,7-Bis(phenylsulfonyl)-9,9′-spirobifluorene (SPSO1): Oxidation
at elevated temperature: 0.89
M H2O2 (13.4 mmol, 7.13 equiv.) solu-
tion (15 mL) in AcOH (100 %) were added to a solution of 2,7-
bis(phenylthio)-9,9′-spirobifluorene (1, 1.00 g, 1.88 mmol,
1.00 equiv.) in CHCl3 (15 mL) at 0 °C. The solution was stirred for
1 h at room temperature, heated to reflux for 2 h, then stirred at
room temperature overnight, heated to reflux again for 1.5 h, and
cooled to room temperature. The solution was poured onto H2O
and extracted with CH2Cl2. The combined organic layers were
washed with H2O until no peroxide was present any more, dried
with Na2SO4, and the solvent was removed under reduced pressure.
The residue was purified by column chromatography (SiO2; cyclo-
hexane/EtOAc, 1:1), recrystallized from 1,4-dioxane, and dried under
vacuum to yield the SPSO1 as colourless crystals, yield 1.0 g,
3
[pseudo-dt, J (H,H) = 7.8, 4,5J (H,H) = 0.9 Hz, 2 H, H4′,5′], 7.77 [dd,
3J (H,H) = 8.1, 5J (H,H) = 0.6 Hz, 2 H, H4,5], 7.39 [pseudo-td, 3J (H,H) =
4
3
4
7.6, J (H,H) = 1.1 Hz, 2 H, H3′,6′], 7.29 [dd, J (H,H) = 8.1, J (H,H) =
1.7 Hz, 2 H, H3,6], 7.23–7.14 (m, 12 H, H2′,7′, SPh-H2,4), 6.77–6.75
(m, 4 H, H1,8,1′,8′) ppm. 13C{1H} NMR (126 MHz, CD2Cl2): δ = 150.54
(C2,7), 148.25 (C8′a,9′a), 142.33 (C4′a,4′b), 140.94 (C4a,4b), 136.53
(SPh-C1), 135.56 (C8a,9a), 131.54 (C3,6), 130.70 (SPh-C2/C3), 129.62
(SPh-C2/C3), 128.59 (C3′,6′), 128.44 (C2′,7′), 127.42 (C1,8/SPh-C4),
127.36 (C1,8/SPh-C4), 124.27 (C1′,8′), 121.46 (C4,5), 120.79 (C4′,5′),
1
1.7 mmol, 89 %. H NMR (500 MHz, CDCl3): δ = 7.95–7.88 (m, 6 H,
3
H3,4,5,6,4′,5′), 7.78–7.76 (m, 4 H, SO2Ph-H2), 7.51 [tt, J (H,H) = 7.5,
4J (H,H) = 1.2 Hz, 2 H, SO2Ph-H4], 7.45–7.40 (m, 8 H, H1,8,3′,6′,
66.21 (C9) ppm. IR (solid): ν = 3062 (w), 1573 (w), 1472 (m), 1446
˜
SO2Ph-H3), 7.08 [pseudo-td, 3J (H,H) = 7.5, 4J (H,H) = 1.1 Hz, 2 H, H2′
(m), 1439 (s), 1399 (m), 1252 (w), 1179 (w), 1154 (w), 1070 (m), 1024
(m), 1000 (w), 959 (w), 920 (w), 860 (m), 809 (s), 763 (s), 751 (s), 744
3
4
5
,7′], 6.54 [pseudo-dt, J (H,H) = 7.6, J (H,H) = 0.9, J (H,H) = 0.6 Hz,
2 H, H1′,8′] ppm. 13C{1H} NMR (126 MHz, CDCl3): δ = 151.40 (C2,7),
145.44 (C8′a,9′a), 144.40 (C4a,4b), 142.30 (C8a,9a/C4′a,4′b), 142.19
(C8a,9a/C4′a,4′b), 141.40 (SO2Ph-C1), 133.36 (SO2Ph-C4), 129.39
(SO2Ph-C3), 128.88 (C3′,6′), 128.36 (C3,6/C2′,7′), 128.34 (C3,6/C2′,7′),
127.68 (SO2Ph-C2), 123.87 (C1′,8′), 123.77 (C1,8), 121.84 (C4,5),
(s), 733 (s), 723 (s), 704 (s), 692 (s), 684 (s), 679 (m), 673 (s) cm–1
.
UV/Vis (MeCN, 1.0 × 10–5 mol dm–3): λmax (ε) = 220 (sh, 82000), 270
(sh, 37000), 334 nm (29000 dm3 cm–1 mol–1). Fluorescence (MeCN,
1.0 × 10–5 mol dm–3, λex = 320 nm): λem = 374 nm. MS (EI, 70 eV):
m/z (%) = 532.1 [M]+ (100, calcd. 532.1).
120.85 (C4′,5′), 66.18 (C9) ppm. IR (solid): ν = 3069 (w), 1446 (m),
˜
See the Supporting Information for Methods b and c.
1402 (w), 1314 (m), 1306 (s), 1177 (w), 1145 (s), 1089 (s), 1063 (w),
1004 (m), 935 (w), 821 (m), 769 (s), 750 (s), 721 (s), 691 (s),
682 (s) cm–1. UV/Vis (MeCN, 1.0 × 10–5 mol dm–3): λmax (ε) = 227 (sh,
60400), 260 (sh, 33400), 286 (42600), 299 (41300), 323 nm
2,7-Bis(phenylsulfinyl)-9,9′-spirobifluorene (SPSX): Oxidation
with H2O2: 0.89
M H2O2 (0.508 mmol, 2.0 equiv.) solution (0.57 mL)
in AcOH (100 %) were added to a solution of 2,7-bis(phenylthio)-
9,9′-spirobifluorene (1, 135 mg, 0.254 mmol, 1.00 equiv.) in a 1:1
mixture of CHCl3 and AcOH (3 mL) at 0 °C. The solution was stirred
for 30 h at room temperature, poured onto H2O and extracted with
CH2Cl2. The combined organic layers were washed with H2O until
no peroxide was present any more, dried with Na2SO4, and the
(13400 dm3 cm–1 mol–1). Fluorescence (MeCN, 1.0 × 10–5 mol dm–3
,
λex = 297 nm): λem = 432 nm. MS (EI, 70 eV): m/z (%) = 596.1 [M]+
(100, calcd. 596.1). C37H24O4S2 (596.71): calcd. C 74.48, H 4.05; found
C 74.13, H 4.05.
2,7-Bis(mesitylthio)-9,9′-spirobifluorene (2): A dried flask was
solvent was removed under reduced pressure. The residue was puri- charged with 2,7-dibromo-9,9′-spirobifluorene (1), (400 mg,
fied by column chromatography (SiO2; cyclohexane/EtOAc,
9:1 → 1:1) and dried under vacuum to yield SPSX as a colourless
0.844 mmol, 1.00 equiv.), K2CO3 (587 mg, 4.25 mmol, 5.03 equiv.),
and 2,4,6-trimethylthiophenol (0.38 mL, 2.52 mmol, 2.99 equiv.) un-
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der an argon atmosphere. Dry DMF (4 mL) was added and the
Fluorescence (MeCN, 1.0 × 10–5 mol dm–3, λex = 285 nm): λem
=
suspension was stirred for 18 h at 140 °C. More 2,4,6-trimethylthio- 421 nm. MS (EI, 70 eV): m/z = 680.2 [M]+ (100, calcd. 680.2).
phenol (0.25 mL, 1.66 mmol, 1.97 equiv.), more K2CO3 (485 mg,
2,7-Bis(pentylsulfonyl)-9,9′-spirobifluorene (SPSO3): 2,7-Di-
bromo-9,9′-spirobi-[fluorene] (4.34 g, 9.14 mmol), K2CO3 (7.58 g,
54.9 mmol, 6.0 equiv.) and 1-pentanethiol (3.9 mL, 31.1 mmol,
3.4 equiv.) was added to dry DMF (4 mL) and the suspension was
stirred for 16 h at 140 °C. More 1-pentanethiol (9.14 mmol,
1.0 equiv.) was added and the mixture was stirred for another 6 h
at 140 °C. The mixture was poured into H2O and extracted with
CH2Cl2. The combined organic layers were washed several times
with H2O and dried with Na2SO4. The solvent was removed under
reduced pressure and the crude bis-thioether 3 (4.11 g, 86 %) was
used without further purification for the next step. Bis-thioether 3
(2.5 g, assuming 4.8 mmol) was dissolved in a mixture of AcOH
(25 mL), ethyl acetate (40 mL) and H2O2 (30 %, 4.8 mL, 10.0 equiv.).
After stirring at room temperature for 2 d, CHCl3 (20 mL) was added.
The clear solution was stirred again at room temperature for 1 d
and then at 40 °C for 30 min. The solvents were evaporated under
reduced pressure and the residue was purified by column chroma-
tography (SiO2; ethyl acetate/cyclohexane, 2:5 → 4:5). The white
product was further purified by recrystallization from dry EtOH. The
fine white needles were filtered off and washed with pentane/di-
3.51 mmol, 4.16 equiv.), and more dry DMF (2 mL) were added and
the mixture was stirred for 4 d at 140 °C. The mixture was poured
into H2O (100 mL) and extracted with CH2Cl2 (4 × 50 mL). The com-
bined organic layers were washed with H2O (3 × 50 mL) and dried
with Na2SO4. The solvent was removed under reduced pressure and
the residue was diluted with CH2Cl2 (100 mL) and washed twice
with H2O. The organic layers were again dried with Na2SO4 and the
solvent was removed under reduced pressure. The residue was
again diluted, washed, and dried and the residue was then purified
by column chromatography (SiO2; cyclohexane/CH2Cl2, 1:0 → 9:1)
1
and dried under vacuum, yield 255 mg, 0.414 mmol, 49 %. H NMR
(500 MHz, CDCl3): δ = 7.82 [pseudo-dt, 3J (H,H) = 7.5, 4,5J (H,H) =
0.9 Hz, 2 H, H4′,5′], 7.43 [dd, 3J (H,H) = 7.7, 5J (H,H) = 1.1 Hz, 2 H,
3
4
H4,5], 7.38 [pseudo-td, J (H,H) = 7.5, J (H,H) = 1.1 Hz, 2 H, H3′,6′],
7.14 [pseudo-td, 3J (H,H) = 7.5, J (H,H) = 1.1 Hz, 2 H, H2′,7′], 6.93 (s,
4
4 H, SMes-H3,5), 6.75 [pseudo-dt, J (H,H) = 7.6, 4,5J (H,H) = 0.8 Hz,
3
2 H, H1′,8′], 6.57–6.54 (m, 4 H, H1,3,6,8), 2.28 (s, 6 H, SMes-C4-CH3),
2.26 (s, 12 H, SMes-C2,6-CH3) ppm. 13C{1H} NMR (126 MHz, CDCl3):
δ = 149.37 (C2,7), 148.59 (C8′a,9′a), 143.51 (SMes-C2,6), 141.81 (C4′
a,4′b), 139.21 (SMes-C4), 138.49 (C4a,4b), 137.66 (C8a,9a), 129.38
(SMes-C3,5), 127.91 (C2′,7′/C3′,6′), 127.87 (C2′,7′/C3′,6′), 127.35
(SMes-C1), 124.47 (C3,6), 124.25 (C1′,8′), 122.27 (C1,8), 120.12 (C4,5/
C4′,5′), 120.08 (C4,5/C4′,5′), 65.70 (C9), 21.83 (SMes-C2,6-CH3), 21.23
1
ethyl ether (4:1), yield 1.54 g, 55 %. H NMR (500 MHz, CDCl3): δ =
8.09 (d, J = 8.0 Hz, 2 H, H4,5), 7.98 (dd, J = 8.1, 1.7 Hz, 2 H, H3,6),
7.88 (pseudo-dt, J = 7.6, 0.9 Hz, 2 H, H4′,5′), 7.42 (pseudo-td, J = 7.6,
1.1 Hz, 2 H, H2′,7′), 7.30 (d, J = 1.6 Hz, 2 H, H1,8), 7.11 (pseudo-td, J =
7.5, 1.1 Hz, 2 H, H3′,6′), 6.62 (pseudo-dt, J = 7.7, 0.9 Hz, 2 H, H1′,8′),
2.99–2.87 (m, 4 H, HSO2CH2), 1.61–1.50 (m, 4 H, HSO2CH2CH2), 1.28–
1.11 (m, 8 H, HCH2), 0.78 (t, J = 7.1 Hz, 6 H, HCH3) ppm. 13C{1H} NMR
(126 MHz, CDCl3): δ = 151.4 (C2,7), 145.5 (C4′a,4′b), 144.8 (C4a,4b), 142.2
(C8′a,9′a), 140.1 (C8a,9a), 129.0 (C2′,7′), 128.5 (C3,6), 128.4 (C3′,6′), 124.4
(C1,8), 123.7 (C1′,8′), 121.8 (C4,5), 120.9 (C4′,5′), 66.1 (C9), 56.4 (CSO2CH2),
30.3 (CCH2), 22.3 (CSO2CH2CH2), 22.1 (CCH2), 13.7 (CCH3) ppm. UV/Vis
(MeCN, 1.0 × 10–5 mol dm–3): λmax (ε) = 197 (47650), 211 (51000),
223 (sh, 42400), 259 (sh, 27500), 277 (36700), 290 (29800), 304
(9660), 318 nm (6900 dm3 cm–1 mol–1). MS (MALDI-TOF, without
matrix): m/z (%) = 585.5 [M]+ (calcd. 584.2). C35H36O4S2 (584.79):
calcd. C 71.88, H 6.20; found C 71.65, H 6.37. Fluorescence (MeCN,
1.0 × 10–5 mol dm–3): λem = 422 nm.
(SMes-C4-CH ) ppm. IR (solid): ν = 2916 (m), 2847 (w), 1597 (m),
˜
3
1566 (w), 1443 (s), 1404 (m), 1373 (w), 1281 (w), 1250 (w), 1173 (w),
1157 (w), 1057 (m), 1034 (w), 957 (w), 849 (m), 810 (s), 764 (m),
733 (s), 687 (m), 640 (m) cm–1. UV/Vis (MeCN, 1.0 × 10–5 mol dm–3):
λmax (ε)
=
225 (sh, 71000), 317 (37000), 340 nm
(34000 dm3 cm–1 mol–1). Fluorescence (MeCN, 1.1 × 10–5 mol dm–3
,
λ
ex = 320 nm): λem = 359 nm. MS (EI, 70 eV): m/z = 616.2 [M]+ (100,
calcd. 616.2), 308.1 [M]2+ (8.6).
2,7-Bis(mesitylsulfonyl)-9,9′-spirobifluorene
(SPSO2):
2,7-
Bis(mesitylthio)-9,9′-spirobifluorene (2, 158 mg, 0.256 mmol,
1.00 equiv.) was dissolved in EtOAc (20 mL) and AcOH (100 %, 5 mL).
H2O2 (30 %, 0.250 mL, 352 mg, 3.10 mmol, 12.1 equiv.) was added
and the solution was heated to reflux for 18 h. The solution was
poured onto H2O and extracted with EtOAc. The combined organic
layers were washed with H2O until no peroxide was present any
more, dried with Na2SO4, and the solvent was removed under re-
duced pressure. The residue was recrystallised from a mixture of
cyclohexane and toluene and dried under vacuum. Purification by
column chromatography (SiO2; cyclohexane/EtOAc, 3:1) yielded
SPSO2 as a colourless powder, yield 110 mg, 0.26 mmol, 63 %. 1H
NMR (500 MHz, CDCl3): δ = 7.88 [d, 3J (H,H) = 7.6 Hz, 2 H, H4′,5′],
7.85 [d, 3J (H,H) = 8.1 Hz, 2 H, H4,5], 7.59 [dd, 3J (H,H) = 8.1, 4J (H,H) =
1.7 Hz, 2 H, H3,6], 7.43–7.40 (m, 4 H, H1,8,3′,6′), 7.10 [pseudo-td,
3J (H,H) = 7.6, 4J (H,H) = 1.1 Hz, 2 H, H2′,7′], 6.86 (s, 4 H, SO2Mes-
H3), 6.58 [d, 3J (H,H) = 7.6 Hz, 2 H, H1′,8′], 2.40 (s, 12 H, SO2Mes-C2-
CH3), 2.26 (s, 6 H, SO2Mes-C4-CH3) ppm. 13C{1H} NMR (126 MHz,
CDCl3): δ = 150.57 (C2,7), 145.47 (C8′a,9′a), 143.92 (C4a,4b,8a,9a),
143.55 (SO2Mes-C4), 142.22 (C4′a,4′b), 139.92 (SO2Mes-C2), 134.05
(SO2Mes-C1), 132.33 (SO2Mes-C3), 128.79 (C3′,6′), 128.21 (C2′,7′),
126.93 (C3,6), 123.60 (C1′,8′), 123.01 (C1,8), 121.52 (C4,5), 120.90 (C4′
,5′), 66.18 (C9), 22.71 (SO2Mes-C2-CH3), 21.13 (SO2Mes-C4-
Crystallography: Data were collected on
a Bruker-Nonius
KappaAPEX diffractometer with data reduction, solution, and refine-
ment by using the programs APEX2.[22] Mercury v. 3.1[23] was used
to analyze and display the X-ray structures.
SPSO1·2CH2Cl2: C39H28Cl4O4S2, M = 766.59, colorless block, mono-
clinic, space group P21/c, a = 11.9690(6), b = 12.9602(6), c =
23.1478(11) Å, ꢀ = 93.801(3)°, U = 3582.8(3) Å3, Z = 4, Dc
=
1.421 Mg m–3, μ(Mo-Kα) = 0.488 mm–1, T = 123 K. Total 89315 reflec-
tions, 13057 unique, Rint = 0.036. Refinement of 9162 reflections
(498 parameters) with I >2σ (I) converged at final R1 = 0.0572 (R1
all data = 0.0744), wR2 = 0.0597 (wR2 all data = 0.0968), gof =
1.0852. CCDC 974892.
SPSO2: C43H36O4S2, M = 680.89, colorless block, monoclinic, space
group C2/c, a = 19.2422(4), b = 15.6495(4), c = 13.1743(3) Å, ꢀ =
123.096(2)°, U = 3323.54(15) Å3, Z = 4, Dc = 1.361 Mg m–3, μ(Mo-
Kα) = 0.206 mm–1, T = 173 K. Total 21904 reflections, 6040 unique,
Rint = 0.036. Refinement of 4110 reflections (222 parameters) with
I >2σ (I) converged at final R1 = 0.0458 (R1 all data = 0.0610), wR2 =
0.0484 (wR2 all data = 0.0775), gof = 1.0852. CCDC 974893.
CH3) ppm. IR (solid): ν = 2932 (w), 1605 (w), 1558 (w), 1443 (m),
˜
1396 (w), 1381 (w), 1304 (s), 1180 (w), 1165 (w), 1142 (s), 1126 (m),
1072 (m), 1034 (w), 1003 (w), 964 (w), 933 (w), 879 (w), 864 (w), 849
(w), 818 (w), 764 (s), 733 (m), 717 (m), 694 (s), 663 (s), 640 (s), 617 SPSO3: C35H36O4S2, M = 584.80, colorless plate, monoclinic, space
(w) cm–1. UV/Vis (MeCN, 1.0 × 10–5 mol dm–3): λmax (ε) = 230 (sh, group C2/c, a = 19.5503(8), b = 10.7566(4), c = 15.8860(7) Å, ꢀ =
66000), 285 (44000), 297 (40000), 327 nm (14000 dm3 cm–1 mol–1). 118.486(2)°, U = 2936.3(2) Å3, Z = 4, Dc = 1.323 Mg m–3, μ(Cu-Kα) =
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0.488 mm–1, T = 123 K. Total 11690 reflections, 2698 unique, Rint
0.033. Refinement of 2380 reflections (186 parameters) with I >2σ
(I) converged at final R1 = 0.0376 (R1 all data = 0.0404), wR2 =
0.0400 (wR2 all data = 0.0487), gof = 1.0884. CCDC 993287.
=
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LEC Device Preparation and Characterization: The solvents were
supplied by Aldrich. The thickness of films was determined with an
Ambios XP-1 profilometer. Indium tin oxide ITO glass plates were
patterned by conventional photolithography. The substrates were
cleaned by sonication in water-soap, water, and 2-propanol baths.
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light 42–220) for 20 min. The blue phosphorescent devices were
made as follows. First, a 70–80 nm layer of PEDOT/PSS (CLEVIOS P
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NMS25 (see Supporting Information for structure) or TCTA (Lumtec,
see Supporting Information for structutre) and 10 % FIrpic (Lumtec)
as the dopant. The total concentration of dissolved hosts and do-
pants was 20 mg/mL. In the LEC 5, a molar ratio of the host-FirPic
blend and the ionic liquid [THA]+[BF4]– (Sigma Aldrich) was 8:1. The
device life-time was measured by applying pulsed current (average:
100 A m–2) with a duty cycle of 50 % and monitoring the average
voltage and luminance by a True Colour Sensor MAZeT (MTCSiCT
Sensor) with a Botest OLT OLED Lifetime-Test System.
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The authors thank the Swiss National Science Foundation, the
University of Basel, the Spanish Ministry of Economy and Com-
petitiveness (MINECO) (MAT2014-55200, CTQ2012-31914,
CTQ2015-71154, and Unidad de Excelencia María de Maeztu
MDM-2015-0552), the Generalitat Valenciana (Prometeo/2012/
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Keywords: Donor–acceptor systems · Photophysics ·
Phosphorescence · OLEDS · LECs · Sulfones
Analytical X-ray Systems, I. Bruker, APEX2, version 2, User Manual, M86-
E01078, Madison, WI, 2006.
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Received: March 3, 2016
Published Online: March 29, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim