Highly luminescent π-conjugated dithienometalloles: Photophysical properties and their application in organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm33526c

We have disclosed the photophysical and electroluminescent properties of a series of hetero-annulated π-conjugated dithieno[3,2-b:2′,3′-d] metallole derivatives incorporating Ge, Si, P, and S atoms as a bridging center. The influence of the hetero-annulat

Full text of DOI:10.1039/c2jm33526c

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PAPER
Highly luminescent p-conjugated dithienometalloles: photophysical properties
and their application in organic light-emitting diodes†
a
Ryosuke Kondo, Takuma Yasuda,
ab
a
a
a
*
*
Yu Seok Yang, Jun Yun Kim and Chihaya Adachi
Received 1st June 2012, Accepted 5th July 2012
DOI: 10.1039/c2jm33526c
We have disclosed the photophysical and electroluminescent properties of a series of hetero-annulated
p-conjugated dithieno[3,2-b:20,30-d]metallole derivatives incorporating Ge, Si, P, and S atoms as a
bridging center. The influence of the hetero-annulated structures on their photophysical properties has
been investigated systematically by UV/vis and photoluminescence (PL) spectroscopy, transient PL
measurements, and DFT calculations. All these compounds show bright fluorescence with high
quantum yields both in solutions and in doped thin films with a host matrix. Furthermore, the OLEDs
employing the dithienometallole emitters exhibit high external quantum efficiencies of ꢀ6% at a
practical brightness.
Recently, various types of metalloles containing boron,7
phosphorus,8 and sulfur9 bridging elements have also emerged as
a new building module of organic semiconductors. The incor-
poration of the hetero-bridged architecture into the p-conju-
gated framework is a fascinating approach to tailor the electronic
properties and hence to develop photo- and electro-functional
materials.7–9 Despite these recent progresses, metalloles posses-
sing a high degree of p-conjugation have been very weakly
studied as an emitter in OLEDs so far.6b–d,8a,b
Introduction
Luminescent organic and organometallic materials have gained a
tremendous amount of attention during the last two decades,
with respect to practical applications in organic electronics such
as organic light-emitting diodes (OLEDs).1–3 The control of their
photophysical and electronic properties is one of the key issues in
the optimization of these functional materials for the applica-
tions. In this regard, new synthetic approaches have been
explored to produce luminescent p-conjugated materials incor-
porating main-group elements. Especially, metalloles,4 which are
hetero-annulated p-conjugated molecules, are attracting
renewed interest because of their unusual electronic structure and
resulting unique optoelectronic properties. Siloles5 and their
fused derivatives6 (e.g., silafluorene) are the most important
silicon-containing p-conjugated materials exhibiting high elec-
tron-transport and photoluminescent properties.5,6 An effective
s*–p* hyperconjugation between the s* orbital of the hetero-
atom and the p* orbital of the parent framework lowers the
lowest unoccupied molecular orbital (LUMO) energy level,5,6
making it possible to utilize these compounds as n-type electron-
transport materials, which is hardly realized in carbon-based
analogues.
In this paper, we report a detailed study on the structural and
photophysical properties of a family of luminescent p-conju-
gated dithieno[3,2-b:20,30-d]metalloles incorporating germanium,
aDepartment of Applied Chemistry, Graduate School of Engineering,
Center for Organic Photonics and Electronics Research (OPERA),
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
E-mail: yasuda@cstf.kyushu-u.ac.jp; adachi@cstf.kyushu-u.ac.jp; Fax:
+81 92-802-6921
bPRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku,
Tokyo 102-0076, Japan
† Electronic supplementary information (ESI) available: Synthetic details
and characterization data. CCDC reference numbers 875779, 875780 and
875781. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2jm33526c
Fig. 1 Chemical structures of luminescent p-conjugated dithieno[3,2-
b:20,30-d]metalloles incorporating group 14–16 elements.
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silicon, phosphorus, and sulfur atoms as the bridging center
(Fig. 1), together with their application as an emitting material in
OLEDs. These building modules, conjoining two thiophene rings
and a central heteroatom via annulations, allow modulation of
the electronic functionality of the materials by virtue of their s*–
p* interactions and high planarity of the syn-configured bithio-
phene unit. We envision that the versatile molecular design of
luminescent p-conjugated dithienometalloles will offer great
promise for their application in OLEDs because the device
performance can be further enhanced by an appropriate design
of the hetero-bridged architectures.
Results and discussion
Synthesis and characterization
The syntheses of Ge–BT, Si–BT, PO–BT, and SO2–BT were
achieved by Suzuki–Miyaura cross-coupling of the correspon-
ding dibromo-precursors with two equivalents of phenylboronic
acid in the presence of Pd(PPh3)4 as a catalyst (see ESI†). The
synthetic protocol used to obtain Si–BT was previously estab-
lished by Ohshita, Kunai, and coworkers.6h–j The synthesis of
PO–BT was reported by Baumgartner and coworkers.8d–f Since
the selective oxidation of thiophene rings can increase effectively
its electron affinity,10 we also chose to target the dithienothio-
phene-S,S-dioxide derivative SO2–BT. After conventional puri-
Fig. 3 Frontier orbital distributions for dithieno[3,2-b:20,30-d]metalloles
derived from DFT calculations (B3LYP/6-31G(d)/LANL2DZ).
(DFT) calculations for the dithienometalloles (Fig. 3). The
highest occupied molecular orbital (HOMO) of these molecules
is expanded over the whole p-conjugated molecular framework
without obvious participation of the heteroatom orbitals. In
contrast, it is found that the bridging heteroatom center
substantially affects the lowest unoccupied molecular orbital
(LUMO) of the dithienometallole systems; s*–p* interactions
can operate to stabilize the LUMO energy levels (vide infra). For
these materials, the HOMO energy levels have been determined
experimentally by means of photoelectron spectroscopy, and
subsequently their LUMO energy levels can be deduced from the
HOMO energy levels and the optical energy gaps (Eg) (Table 1).
SO2–BT possesses the most electron-deficient p-system,
providing a low-lying LUMO energy level (ꢂ3.58 eV). Thus,
modulation of the electronic character or energy levels can be
attained by varying the hetero-bridged scaffold for this class of
materials.
fications
including
column
chromatography
and
recrystallization, the obtained materials were further purified by
temperature gradient sublimation under vacuum, and were
characterized by 1H and 13C NMR spectroscopy, mass spec-
trometry, and elemental analysis (see ESI†).
The solid-state structures of Ge–BT, Si–BT, and SO2–BT have
been determined by single-crystal X-ray analysis (Fig. 2). The
rigid tricyclic dithienometallole units are completely planar,
independent of the bridging heteroatom. The dihedral angles
between the central tricyclic unit and the peripheral phenyl rings
range from 4 to 14ꢁ, suggesting that an effective p-conjugation is
preserved over the entire molecular structure. In all these mole-
cules, each heteroatom center (Ge, Si, S) adopts a distorted
tetrahedral geometry with endocyclic angles of 89.6ꢁ for C–Ge–C
(in Ge–BT), 92.06ꢁ for C–Si–C (in Si–BT), and 92.24ꢁ for C–S–C
(in SO2–BT), which are much smaller than the standard value
(109.5ꢁ) of an sp3-hybridized silicon or carbon atom.
Photophysical properties
To better understand the electronic effects of the hetero-
bridged structures, we have performed density functional theory
UV/vis absorption and photoluminescence (PL) spectra of Ge–
BT, Si–BT, PO–BT, and SO2–BT have been measured in
CH2Cl2 solution, and the photophysical data are summarized in
Table 1. The absorption spectra of these compounds are very
similar, giving the absorption peaks (labs) in the range of 400–
420 nm (see ESI†), which are assigned to p–p* transition of the
p-conjugated backbone. In the PL spectra (Fig. 4a), PO–BT and
SO2–BT exhibit structureless emissions with the emission peak
(lem) at 529 and 509 nm, respectively, which takes place at a
lower energy with a larger Stokes shift, compared to those of Ge–
BT (lem ¼ 483 nm) and Si–BT (lem ¼ 493 nm). An intriguing
photophysical feature is that the annulated dithienometalloles
present much higher PL quantum yields (FPL) than that of the
non-bridged parent compound, 5,50-diphenyl-2,20-bithiophene
(BT).11 Incorporation of the bridging heteroatom results in a
remarkably increased FPL and excited-state lifetime, as well as in
Fig. 2 ORTEP drawings of the X-ray structures (50% probability
ellipsoids) of Ge–BT, Si–BT, and SO2–BT: (a) top view and (b) side view.
Hydrogen atoms are omitted for clarity (see ESI†).
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Table 1 Photophysical properties of dithieno[3,2-b:20,30-d]metallole derivatives and the corresponding non-bridged analogue BT
CH2Cl2 solution (10ꢂ5 M)
Doped thin film in TBADN
HOMO (eV)a LUMO (eV)b Eg (eV)b labs (nm)c lem (nm) FPL (%)d s (ns)e kr (108 sꢂ1
)
knr (108 sꢂ1
)
lem (nm) FPL (%)d s (ns)e
Ge–BT ꢂ5.60
Si–BT
ꢂ3.00
ꢂ3.04
ꢂ2.96
2.60
2.55
2.48
401
408
419
414
370
483
493
529
509
455
50
81
75
90
13
1.3
2.8
6.0
5.7
0.3
3.9
2.9
1.3
1.6
4.3
3.9
0.7
0.4
0.2
29
466
479
531
519
440
70
86
90
94
18
2.9
3.6
8.9
5.7
0.6
ꢂ5.59
PO–BT ꢂ5.44
SO2–BT ꢂ5.82
ꢂ3.58
2.24
f
f
f
BT
a
Determined by photoelectron spectroscopy. b LUMO ¼ HOMO + Eg, the values of Eg were estimated from the absorption onset of thin films. c Only
the lowest energy absorption maxima are given. d Absolute PL quantum yield measured using an integrating sphere. e PL lifetime. Not determined.
f
red-shifted emission (Table 1). The hetero-bridged structure
would prevent structural relaxation in the excited state by
comparison with the non-bridged BT, and therefore suppress
non-radiative decay. The trend of FPL is SO2–BT > Si–BT >
PO–BT > Ge–BT in CH2Cl2 solution.
are in the same order of magnitude (108 sꢂ1), the non-radiative
rates are observed to decrease considerably in the case of Si–BT,
PO–BT, and SO2–BT, by introducing the rigid hetero-bridged
structure. It is noteworthy that the suppression of non-radiative
decay processes leads to progressive enhancement of the lumi-
nescent properties for the dithienometalloles. As for Ge–BT, a
relatively large knr value should mainly be ascribed to the inter-
system crossing into the triplets manifold, as a consequence of
the heavy-atom effect of germanium.
The transient PL signals for the dithienometalloles (Fig. 4b)
exhibit monoexponential decay with the lifetime in the range of
1–6 ns, indicative of their fluorescent origin. One can calculate
the radiative (kr) and non-radiative (knr) decay rates from the
data of s (¼ 1/(kr + knr)) and FPL (¼ kr/(kr + knr)), as listed in
Table 1. While the radiative decay rates of all these compounds
Since the present hetero-bridged p-conjugated compounds
were being developed as emitting materials for OLEDs, it is of
importance to investigate their PL properties not only in
solution but also in doped thin films with a host matrix.
We have employed 2-tert-butyl-9,10-di(2-naphthyl)anthracene
(TBADN)12 as a host material for Ge–BT, Si–BT, PO–BT, and
SO2–BT guest emitters on account of its ambipolar ability,13
wide energy gap, and spectral overlap between the emission of
TBADN and the absorption of the guest emitters. With the
doping concentration of 3 wt%, the emission from the TBADN
host centered around 440 nm disappears under photoexcitation
€
(see ESI†), confirming the occurrence of efficient Forster-type
energy transfer from the host to the guest emitters. The FPL
values of the thin films tend to depress at higher doping
concentrations (>6%) owing to concentration quenching.14 The
3 wt% dithienometallole:TBADN codeposited films give FPL of
70–94% and s of 2–9 ns (Table 1). Of great interest, the FPL
values are higher than those recorded in solution, which should
arise from the restriction of intramolecular fragmental rotation
in the solid state.
OLED device performances
The high PL efficiency of the p-conjugated dithienometalloles in
doped films suggests that they are promising candidates for the
fabrication of efficient OLEDs. To this end, multilayer OLEDs
have been fabricated with the following device configuration
(Fig. 5): ITO/4,40-bis[N-(1-naphthyl)-N-phenyl]biphenyl diamine
(a-NPD, 40 nm)/3 wt% dithienometallole:TBADN (30 nm)/4,7-
diphenyl-1,10-phenanthroline (BPhen, 50 nm)/LiF (0.8 nm)/Al
(80 nm), where a-NPD serves as the hole-transporting layer,
BPhen functions as the hole-blocking and electron-transporting
layer, and LiF is the electron-injection material. As schematically
shown in Fig. 5, the LUMO and HOMO levels of TBADN
(ꢂ2.8 eV and ꢂ5.8 eV, respectively) as a host are above and
below those of the dithienometalloles, respectively, implying that
Fig. 4 (a) PL spectra and (b) PL decay profiles for Ge–BT, Si–BT, PO–
BT, SO2–BT, and BT in CH2Cl2 solution after excitation at 405 nm at
293 K.
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Fig. 5 Energy-level diagram of OLED devices (top) and chemical
structures of the materials used for the devices (bottom).
the guest emitters will possibly trap electrons and holes. More-
over, the BPhen layer possessing a deep HOMO level effectively
prevents hole leakage and allows a better confinement of charge
carriers and excitons within the emitting layer.
Fig. 6 displays the current density–voltage (J–V) and lumi-
nance–voltage (L–V) characteristics and the external quantum
efficiencies (hext) of the OLEDs featuring Ge–BT, Si–BT, PO–
BT, and SO2–BT as emitters. The electroluminescence (EL)
properties are summarized in Table 2, which are dependent on
the optoelectronic features of the hetero-bridged scaffold. The
EL spectra testify the emission exclusively from the dithieno-
metallole emitters (Fig. 6a); no emission from the TBADN host
is observed. All of the devices exhibit good OLED perfor-
mances, as they possess a low turn-on voltage (VON) of 2.8–
2.9 V and a high maximum luminance (Lmax) exceeding 26 000
cd mꢂ2 (Fig. 6b). Among the devices, the SO2–BT-based one
shows the highest external EL quantum efficiency (hext) of 6.1%
(Fig. 6c), which is greater than those of the devices based on
Ge–BT (hext ¼ 3.4%), Si–BT (5.2%), and PO–BT (4.0%), as well
as conventional fluorescent OLEDs. Notably, even when the
luminance reaches 10 000 cd mꢂ2, the hext of the SO2–BT-based
device still remains nearly 6%, indicative of a balanced injection
and the transport of holes and electrons in the device, at high
current densities.
Fig. 6 Performance of multilayer OLEDs incorporating dithieno[3,2-
b:20,30-d]metalloles as emitters: (a) current density–voltage (inset: EL
spectra operated at ca. 10 V), (b) luminance–voltage (inset: photographs
showing EL emission), and (c) external EL quantum efficiency–current
density characteristics of the devices.
As for the SO2–BT-based OLED, the theoretical value of the
external EL quantum efficiency, hext, can be estimated to be 4.7%
15
using the following equation: hext ¼ g ꢃ hst ꢃ FPL ꢃ hout
.
Therein, the charge balance factor (g), the production efficiency
of spin-allowed excitons (hst), the quantum yield of the SO2–
BT:TBADN film (FPL), and the light out-coupling efficiency
(hout) could be assumed as approximately 1, 0.25, 0.94, and 0.2,16
respectively. As a result, the experimentally obtained hext of 6.1%
exceeds the theoretical limit for the present system. A plausible
origin accounting for a relatively high EL efficiency is out-
coupling enhancement by orientation of the luminescent mole-
cules within the emitting layer. Recently, it has been revealed that
the emission dipoles of linearly p-extended fluorescent17a–c or
phosphorescent17d molecules are not isotropic but aligned hori-
Table 2 Summary of OLED performancesa
Emitter lEL (nm) VON (V) Lmax (cd mꢂ2
) hext,max (%) CIE (x,y)
Ge–BT
Si–BT
PO–BT 520
SO2–BT 524
490
502
2.8
2.9
2.8
2.8
29 700
26 100
43 800
46 300
3.4
5.2
4.0
6.1
0.17,0.39
0.18,0.42
0.29,0.57
0.28,0.55
a
Device configuration: ITO/a-NPD (40 nm)/3 wt% emitter:TBADN (30
nm)/BPhen (50 nm)/LiF (0.8 nm)/Al (80 nm). Abbreviations: lEL: EL
maximum; VON
: turn-on voltage at 1 ; Lmax: maximum
cd mꢂ2
luminance; hext,max: maximum external EL quantum efficiency; CIE
(x,y): Commission Internationale de l’Eclairage color coordinates
zontally on the surface of the OLEDs, giving rise to an increase
17
measured at 10 mA cmꢂ2
.
of hout
.
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compared with those of their non-bridged counterparts. The
multilayer OLED devices employing the luminescent dithieno-
metallole guest emitters, doped into a host matrix, have been
fabricated and evaluated. We have achieved a high external EL
efficiency of approximately 6% for the fluorescent OLED
incorporating SO2–BT at a practical brightness. The polarized
photoluminescence measurements indicated the possibility of the
enhancement of the light out-coupling efficiency by the hori-
zontal orientation of the emitting molecules in the devices.
Fig. 7 Polarized emission spectra from the edge of a 3 wt% SO2–
BT:TBADNfilm witha thickness of100nmona quartzglass substrate (left)
and optical setup used for the anisotropic PL measurements (right).16,18 The
electric field in TE mode is polarized parallel to the substrate and that in the
TM mode is polarized in the plane perpendicular to it.
Experimental
General procedure
Commercially available reagents and solvents were used without
further purification unless otherwise noted. 3,30-Dibromo-2,20-
bithiophene and dithieno[3,2-b:20,30-d]thiophene were purchased
from Aldrich. If necessary, the compounds were purified by
recycling preparative HPLC (LC-9130 NEXT, Japan Analytical
Industry) on GPC columns. 1H and 13C{1H} NMR spectra were
recorded on a Bruker Avance III 500 spectrometer. Chemical
shifts of 1H and 13C NMR signals were quoted to tetrame-
thylsilane (d ¼ 0.00) and CDCl3 (d ¼ 77.0) as internal standards.
Mass spectra were collected on a Waters 3100 mass spectrometer
with an atmospheric solid analysis probe. Elemental analyses
were carried out with a Yanaco MT-5 CHN corder. UV/Vis
absorption and photoluminescence (PL) spectra were measured
with a Shimadzu UV-2550 spectrometer and a JASCO FP-6500-
ST spectrophotometer, respectively. PL quantum yields were
measured using an integration sphere system coupled with a
photonic multichannel analyzer (Hamamatsu Photonics C9920-
02, PMA-11). The experimental HOMO energy levels of thin
films were determined using a Riken-Keiki AC-2 photoelectron
spectrometer (see ESI†); the LUMO energy levels were estimated
by subtracting the optical energy gap from the measured HOMO
energy levels. Transient PL decay curves were measured with a
Hamamatsu Photonics Quantaurus-Tau spectrometer at room
temperature. The density-functional theory (DFT) calculations
were performed on the Gaussian 03 program package, using the
B3LYP functional with the 6-31G(d)/LANL2DZ basis sets. The
LANL2DZ effective core potential was applied for Ge atom and
6-31G(d) for other atoms.
Accordingly, if the linear-shaped light-emitting SO2–BT
molecules have a tendency towards horizontal orientation within
the amorphous TBADN matrix, the out-coupling efficiency of
the device will be enhanced. To verify this notion, we have
measured polarized PL spectra from the edge of the SO2–
BT:TBADN codeposited film (Fig. 7). The intensity of the
transverse electric (TE) emission from the edge of the doped thin
film is found to be much larger than that of the transverse
magnetic (TM) emission. This result unambiguously indicates
that the transition dipoles for emission lie preferentially parallel
even in the isotropic TBADN host matrix on the substrate,
leading to an enhancement of hout of the OLEDs. The S]O
groups in SO2–BT are orthogonal to the dithienothiophene
plane (Fig. 2), but are sterically less hindered compared to the
phenyl groups of the other dithienometallole emitters. It is
anticipated that the preferential horizontal orientation of the
emitting molecules in the doped films should be affected by steric
effects of the orthogonal substituents.
Another possible mechanism causing such high hout is triplet–
triplet annihilation (TTA), in which interactions between two
triplet states lead to the production of one singlet excited state
and one ground state.19 To promote deeper insight, the transient
EL responses have been examined for the SO2–BT-based device
(see ESI†). While the delayed EL component originating from
the TTA process can be observed at high current densities (ca.
10–100 mA cmꢂ2), the contribution of the TTA in the EL effi-
ciency is found to be negligible at low current densities less than
10 mA cmꢂ2. Hence, the higher hext than the theoretical limit
should predominantly be ascribed to the enhancement of out-
coupling efficiency on account of horizontal orientation of the
emitters in the devices.
X-ray crystallography
Single crystals of Ge–BT, Si–BT, and SO2–BT were obtained by
slow diffusion of methanol vapour into their CHCl3 solutions at
room temperature. X-ray crystal analyses were made on a
Rigaku VariMax with a Saturn 724+ system with graphite
monochromated MoKa radiation. The structures were solved by
direct methods (SIR2008) and refined by full-matrix least-square
methods based on F2 (SHELXL-97). CCDC-875779 (Ge–BT),
CCDC-875780 (Si–BT), and CCDC-875781 (SO2–BT) contain
the supplementary crystallographic data for this paper.
Conclusions
In summary, we have synthesized and characterized a series of
hetero-annulated dithieno[3,2-b:20,30-d]metalloles (Ge–BT, Si–
BT, PO–BT, and SO2–BT) as new light-emitting materials. The
versatile molecular design strategy provides access to a wide
variety of luminescent hetero-annulated p-conjugated materials.
The incorporation of heteroatoms into main p-systems allows
their electronic structure and photophysical properties to be
modulated effectively. The photoluminescent properties of the
dithienometalloles are remarkable in terms of quantum yields,
OLED device fabrication and measurements
In a general procedure, indium-tin oxide (ITO)-coated glass
substrates were cleansed with detergent, deionized water,
acetone, and isopropanol, followed by UV-ozone treatment.
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Organic layers were successively deposited onto the ITO
substrate by vacuum thermal evaporation at deposition rates of
0.2–0.4 nm sꢂ1. The layer thickness and the deposition rate were
monitored in situ during the deposition by an oscillating quartz
thickness monitor. LiF and Al cathodes were finally deposited
through a shadow mask on the organic stacks. The current
density–voltage–luminance characteristics and EL spectra of the
devices were collected using a Keithley 2400 source measurement
unit and a Hamamatsu Photonics PMA-12 photonic multi-
channel analyzer in an ambient condition. For edge photo-
luminescence measurements of thin films, the sample was excited
by an N2 laser (337 nm), and the light emission was detected
through a linear polarizer and an optical fiber scope connected to
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Acknowledgements
This work was partially supported by a Grant-in-Aid for
Precursory Research for Embryonic Science and Technology
(PRESTO) (no. 10111) from JST, Grant-in-Aid for Young
Scientists (B) (no. 23750218) from the Japan Society for the
Promotion of Science (JSPS), Grant-in-Aid for the Funding
Program for World-Leading Innovative R&D on Science and
Technology (FIRST), and Grant-in-Aid for the Global COE
Program ‘‘Science for Future Molecular Systems’’ from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan. T.Y. is grateful for the financial support from
the Inamori Foundation, the Hattori-Hokokai Foundation, and
the Sumitomo Foundation. We would like to thank Dr Masat-
sugu Taneda and Dr Katsuyuki Shizu for helpful discussions,
and the Rigaku Co. for technical support in X-ray crystallo-
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