Separation of electrical and optical energy gaps: Selectively adjusting the electrical and optical properties for a highly efficient blue emitter

Article abstract of DOI:10.1002/chem.201304544

The design concept of separation of optical and electrical bandgap for wide bandgap materials is further developed in DCzSiPI. The HOMO/LUMO levels can be tuned by incorporation of PI and DCz substituents. The tetraphenylsilane core avoids the intramolecular charge transfer from DCz to PI (DCz=dimer carbazole, PI=phenanthro[9,10-d]imidazole). The allowed transitions are found to be from HOMO-1 to LUMO providing DCzSiPI with sufficient bandgap. Copyright

Full text of DOI:10.1002/chem.201304544

DOI: 10.1002/chem.201304544
Communication
&
Organic Semiconductors
Separation of Electrical and Optical Energy Gaps: Selectively
Adjusting the Electrical and Optical Properties for a Highly
Efficient Blue Emitter
He Liu, Ping Chen, Dehua Hu, Xiangyang Tang, Yuyu Pan, Huanhuan Zhang,
Wenqiang Zhang, Xiao Han, Qing Bai, Ping Lu,* and Yuguang Ma[a]
the LUMO+n, therefore, the whole molecule maintains a wide
optical bandgap. The separation of optical and electrical
bandgap to achieve wide bandgap and good carrier injection
properties simultaneously provides us with a hopeful prospect
for the design of organic wide-bandgap materials.
Abstract: The design concept of separation of optical and
electrical bandgap for wide bandgap materials is further
developed in DCzSiPI. The HOMO/LUMO levels can be
tuned by incorporation of PI and DCz substituents. The
tetraphenylsilane core avoids the intramolecular charge
transfer from DCz to PI (DCz=dimer carbazole, PI=
phenanthro[9,10-d]imidazole). The allowed transitions are
found to be from HOMOꢀ1 to LUMO providing DCzSiPI
with sufficient bandgap.
To further develop and investigate the universality of this
new design strategy, we herein selectively modified the HOMO
and LUMO energy levels of a new series of tetraphenylsilane
compounds. We took phenanthro[9,10-d]imidazole (PI) as a n-
type model unit because it has good electron injection proper-
ty and exhibits blue light emission.[20–22] The carbazole (Cz) and
dimer carbazole (DCz), which are good electron-donating
units, were kept unchanged. Through this material design, we
developed two new bipolar materials CzSiPI and DCzSiPI to in-
vestigate the possibility of adjusting electrical and optical
bandgaps independently in these silane-cored compounds.
SiPI with only one PI substituent was also synthesized for com-
parison purposes.
Organic semiconductors have already attracted and will contin-
ue to attract great interest from researchers worldwide be-
cause they have achieved amazing accomplishments in many
fields, such as organic lighting-emitting diodes (OLEDs),[1–7] or-
ganic photovoltaics (OPVs),[8–11] organic field-effect transistors
(OFETs),[12–15] and organic light-emitting electrochemical cells
(OLECs).[16–18] Compared with their inorganic counterparts, the
intrinsic low carrier mobilities of organic semiconductors,
which result in a limited load-bearing capacity for current, is
still an obstacle to further applications. One representative
case would be the contradiction between the wide bandgap
and effective carrier injection and/or transport properties of
semiconductive materials that emit blue or violet light.
The structures of these three molecules are shown in
Figure 1. These materials were synthesized by using a Suzuki
coupling and Ullmann reaction (see Scheme S1 in the Support-
Recently, we have demonstrated a novel molecular design
strategy to optimize the optical bandgap and carrier injection
ability simultaneously in a series of silane-based wide-bandgap
materials by independently incorporating electron-donating
(carbazole) and electron-accepting (pyridine) moieties.[19] In
this series of wide-bandgap materials, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecu-
lar orbital (LUMO) levels are mainly located on corresponding
electron-donating and electron-accepting units, which benefits
their carrier injection. However, the allowed electronic transi-
tions are excited from the HOMO to higher energy levels of
Figure 1. Molecular structures of SiPI, CzSiPI, and DCzSiPI.
ing Information). The chemical structures were fully character-
ized by using NMR spectroscopy, mass spectrometry, and ele-
mental analysis, and correspond well with their expected struc-
tures. All materials showed good thermal stability. There was
no obvious glass transition temperature (Tg) of SiPI during the
whole heating process, whereas the Tg of CzSiPI and DCzSiPI
reached 166 and 1988C, respectively (Figure S1a in the Sup-
porting Information), which is rarely seen for organic mole-
cules. The decomposition temperatures (Td) of three com-
[a] Dr. H. Liu, Dr. P. Chen, Dr. D. Hu, X. Tang, Dr. Y. Pan, H. Zhang, W. Zhang,
X. Han, Q. Bai, Prof. P. Lu, Prof. Y. Ma
State Key Laboratory of Supramolecular Structure and Materials
Jilin University, 2699 Qianjin Avenue
Changchun, 130012 (P.R. China)
E-mail: lup@jlu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304544.
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Communication
pounds were 474, 506, and 5508C, respectively (Figure S1b in
the Supporting Information). The excellent thermal stability of
these materials would be beneficial in applications in OLEDs.
CzSiPI and DCzSiPI showed almost the same absorption
spectra as SiPI both in solution and film (Figure 2 and Fig-
&
*
Figure 3. The cyclic voltammetry curves for SiPI ( ), CzSiPI ( ), and DCzSiPI
ꢀ1
~
(
) at a scan rate of 100 mVs
.
Eop is dominated by one of the carrier injection units is consis-
tent with our design strategy.
Cyclic voltammetry (CV) was applied to determine the LUMO
and HOMO energy levels of these materials (Figure 3). If our
design strategy is successful, the reduction process should
occur on the electron-deficient PI unit and the oxidation pro-
cess should be controlled by the electron-rich Cz unit. The ob-
tained experimental results showed that SiPI, CzSiPI, and
DCzSiPI indeed underwent the same reduction process, which
originated from their n-type PI unit consistent with our molec-
ular design strategy. However, the oxidative onset potential of
CzSiPI was the same as SiPI, which suggests that the oxidation
process also occurred on the PI
&
*
Figure 2. The absorption and PL spectra of SiPI ( ), CzSiPI ( ), and DCzSiPI
~
(
) in dilute CH2Cl2.
ure S2 in the Supporting Information). They all exhibited major
absorption bands around l=330 and 365 nm, which arise
from the PI unit. The absorption peaks at l=292 and 342 nm
are characteristic of the Cz group. Thus, the optical bandgaps
(Eop) calculated from the absorption edge were 3.19, 3.20, and
3.19 eV for SiPI, CzSiPI, and DCzSiPI, respectively (Table 1). The
unit rather than on the Cz unit.
Table 1. Photophysical data for SiPI, CzSiPI, and DCzSiPI.
We found that the HOMO level
of PI calculated from the oxida-
[d]
[e]
Absmax [nm]
film
PLmax [nm]
solution[a]
PL efficiency [%]
Eop
Eel
tive onset was ꢀ5.55 eV, where-
as the HOMO level of Cz was
ꢀ5.61 eV.[19] Thus PI was more
easily oxidized than Cz, which
led to the oxidation occurring in-
itially on the PI unit for CzSiPI.
For DCzSiPI, the HOMO level of
DCz was ꢀ5.47 eV,[19] which is
lower than that of PI. Therefore,
solution[a]
Film
solution[b]
film[c]
[eV]
[eV]
SiPI
CzSiPI
DCzSiPI
330, 365
315, 373
422
422
422
433
433
433
94
97
99
77
65
67
3.19
3.20
3.19
3.16
3.20
3.09
292,329, 342, 365
293,331, 343, 365
297, 345, 373
299, 347, 273
[a] Recorded in dilute dichloromethane at 10ꢀ4 m. [b] Compared with quinine sulfate (PL efficiency 54.6% in
0.1m H2SO4). [c] Measured by using an integrated sphere. [d] Calculated from the edge of film absorption spec-
tra. [e] Calculated from the electrochemical data.
photoluminescent (PL) spectra (Figure 2) present the same in-
formation. All spectra exhibit blue emission with a maximum
peak at l=422 nm in solution and l=433 nm in a film, which
suggested that the theoretical transitions and radiations of
SiPI, CzSiPI, and DCzSiPI are dominated by the PI chromophore.
It is worth noting that there was no low-energy absorption or
emission band, which illustrates that additional transitions,
such as intramolecular charge-transfer from Cz to PI, did not
exist. The p-type Cz unit did not affect the absorption and
emission spectra either in CzSiPI or DCzSiPI. This phenomenon
is similar to that observed in the donor–acceptor substituted
silanes system(CzSiPy and DCzSiPy) that we published previ-
ously, in which the n-type pyridine (Py) unit had no effect on
the absorption and emission spectra.[19] This phenomenon that
the oxidation of DCzSiPI happened on the DCz unit as expect-
ed and the HOMO level was calculated to be ꢀ5.46 eV. The
electrical band gaps (Eel) were accordingly found to be 3.16,
3.20, and 3.09 eV for SiPI, CzSiPI, and DCzSiPI, respectively
(Table 1). DCzSiPI presented the biggest difference (0.1 eV) be-
tween Eop and Eel; SiPI and CzSiPI showed differences of 0.03
and 0 eV, respectively. Independently adjusting the energy
levels without changing the optical properties was realized in
DCzSiPI.
To further understand the spectral and CV behavior, density
functional theory (DFT) calculations were carried out at the
B3LYP/6-31G level and the distribution of LUMO and HOMO
energy levels are shown in Figure 4a. The LUMO levels of all
these three materials were located on PI, which corresponds
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ration for HOMO and LUMO configuration is obtained, which
leads to forbidden transitions between the donor and acceptor
(from HOMO to LUMO), for example, in DCzSiPI, then local
transitions cause the compound to maintain a high optical
bandgap and provide it with sufficient bandgap.
Because all these compounds have the same LUMO level
but different hole affinities, the hole-only devices are formed
with following structures: ITO/MoO3 (10 nm)/compounds
(60 nm)/MoO3 (10 nm)/Al (100 nm) (ITO=indium tin oxide) to
investigate the hole-injection property. MoO3 (LUMO=
ꢀ2.3 eV) was applied to prevent electron injection from Al
(ꢀ4.3 eV). As shown in Figure S3 in the Supporting Informa-
tion, the current densities of both CzSiPI and DCzSiPI were
much higher than that of SiPI at the same electric field intensi-
ty. Introduction of a Cz unit could enhance the hole-transport-
ing ability due to its electron-rich nature. DCzSiPI shows higher
current density than CzSiPI because the hole-affinity of DCz
was higher than that of Cz. The hole-transporting ability was
obviously enhanced by the introduction of a donor unit.
To see how this improvement would affect the electrolumi-
nescent properties, light-emitting devices A–C were fabricated
with the following structures: ITO/PEDOT:PSS/NPB (40 nm)/
TCTA (10 nm)/EML (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al
(100 nm); PEDOT=ploy(3,4-ethylenedioxythiophene), PSS=
poly(styrenesulfonate), NPB=N,N’-di(1-naphthyl)-N,N’-diphenyl-
benzidine, TCTA=4,4’,4’’-tri(N-carbazolyl)triphenylamine, EML=
emitting layer, TPBi=1,3,5-tri(1-phenyl-1H-benzimidazol-2-yl)-
benzene. EMLs for devices A–C were SiPI, CzSiPI, and DCzSiPI,
respectively. NPB and TPBi were used as hole- and electron-
transporting layers. TCTA was used as a electron-blocking layer.
All the devices exhibited blue emission with maximum peak
around l=440 nm (Figure 5). The electroluminescent (EL)
Figure 4. a) The relevant molecular orbital density maps. b) A possible transi-
tion sketch map of DCzSiPI.
with the electrochemistry results. The HOMO levels of SiPI and
CzSiPI were distributed on PI, whereas the HOMOꢀ1 energy
level of CzSiPI is distributed on the Cz unit, which means that
Cz has lower oxidative activity than PI. This phenomenon was
also consistent with the phenomenon observed by using CV.
Unlike the other two molecules, the HOMO of DCzSiPI was
spread over the DCz group and exhibited higher oxidative ac-
tivity than that of PI, whereas the HOMOꢀ1 energy level of
DCzSiPI was located on PI and the allowed transitions are from
the HOMOꢀ1 to the LUMO (Figure 4b). The result indicated
that the separation of optical and electrical bandgap was fully
achieved in DCzSiPI.
~
&
*
Figure 5. The EQE curves of devices A ( ), B ( ), and C ( ). Inset: the EL
~
&
*
spectrum of devices A ( ), B ( ), and C ( ).
The special tetrahedral configuration of silane can effectively
prevent the donor–acceptor interactions. No charge transfer
from donor to acceptor is observed in these bipolar wide-
bandgap materials, which is commonly observed in a series of
pyridine- and carbazole-substituted silanes we published previ-
ously (e.g., CzSiPy).[19] Their theoretical transitions and radia-
tions are all dominated by one donor or acceptor chromo-
phore, that is, the corresponding carbazole–silane chromo-
phore in CzSiPy and PI–silane in CzSiPI. When full spatial sepa-
spectra for the three compounds were very stable under vari-
ous driving voltages (Figure S5 in the Supporting Information).
As expected, DCzSiPI exhibited the maximum external quan-
tum efficiency (EQE) with a value of 3.5%. To evaluate the in-
fluence of charge-balance factor, the use of excitons was calcu-
lated according to Equation (1):
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hext
5.15, N 3.97; found: C 85.56, H 4.97, N 3.92; MS (55 eV): m/z calcd
for C51H36N2Si: 704.93; found: 705.50 (100) [M+H]+.
h ¼
ð1Þ
gFPhout
2-(4’-((4-(9H-Carbazol-9-yl)phenyl)diphenylsilyl)-(1,1’-biphen-
yl)-4-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (CzSiPI)
In this equation, h represents the use of excitons for radia-
tive decay, hext is the external quantum efficiency, g refers to
the ratio of electrons to holes (be regarded as 1), Fp is the PL
quantum efficiency in solid state, and hout represents the out-
coupling efficiency (accounts for 20%). The maximum h calcu-
lated for SiPI, CzSiPI and DCzSiPI were 16.2, 19.2, and 26.1%,
respectively (Table S2 in the Supporting Information). CzSiPI
and DCzSiPI showed lower PL efficiency than SiPI, however,
they achieved higher use of excitons than SiPI, which indicates
the importance of the charge-balance factor. More balanced
carrier injection was achieved and more excitons were used in
DCzSiPI and its use of excitons was improved by 61% com-
pared with SiPI. The selectively modified electrical properties
of CzSiPI and DCzSiPI have greatly enhanced the EL properties
without changing the optical properties.
9-(4-((4-Bromophenyl)diphenylsilyl)phenyl)-9H-carbazole was react-
ed with1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
phenyl)-1H-phenanthro[9,10-d]imidazole in a Suzuki coupling reac-
tion. The process was the same as SiPI. The crude product was pu-
rified by using column chromatography with petroleum ether/di-
chloromethane (v/v 1:1) as the eluent to give a white solid (0.43 g,
1
yield: 33%). H NMR (500 MHz, CDCl3, 258C, TMS): d=8.95–8.94 (s,
1H, Ar-H), 8.82–8.80 (d, J=8.7 Hz, 1H, Ar-H), 8.32–8.31 (s, 1H, Ar-
H), 8.22–8.21 (d, J=7.9 Hz, 2H, Ar-H), 8.16–8.13 (d, J=7.9 Hz, 1H,
Ar-H), 7.92–7.90 (d, J=8.2 Hz, 2H, Ar-H), 7.81–7.42 (m, 37H, Ar-H),
7.37–7.31 (m, 4H, Ar-H), 7.22–7.20 ppm (d, J=7.6 Hz, 1H, Ar-H);
13C NMR (125 MHz, CDCl3, 258C, TMS): d=141.5, 140.6, 139.1,
137.9, 137.0, 136.4, 133.8, 133.5, 133.2, 130.3, 129.9, 129.8, 129.2
128.33, 128.26, 128.1, 127.4, 126.9, 126.6, 126.3, 126.2, 126.0, 125.0,
124.2, 123.5, 123.1, 120.9, 120.3, 120.1 ppm; elemental analysis
calcd (%) for C63H43N3Si: C 89.96, H 4.98, N 4.83; found: C 90.11, H
4.88, N 5.07; MS (89 eV): m/z calcd for C63H43N3Si: 870.12; found:
871.10 (100) [M+H]+.
In conclusion, following the idea to further develop the
design concept of separation of the optical and electrical
bandgaps for wide-bandgap materials, we synthesized a new
series of bipolar materials by using silane to couple electron-
donor Cz and electron-acceptor PI units. The HOMO/LUMO
levels of the materials can be tuned by the incorporation of PI
and Cz substituents. The tetraphenylsilane core prevents the
intramolecular charge transfer from donor to acceptor. In
DCzSiPI, the allowed transitions are found to be from the
HOMOꢀ1 to the LUMO. The separation of the optical and elec-
trical bandgaps is fully achieved in DCzSiPI. DCzSiPI possesses
much higher exciton use than SiPI due to the more balanced
carrier flux. Our design concept could be very useful for con-
structing excellent organic semiconductors.
2-(4’-((4-(9H-[3,9’-Bicarbazol]-9-yl)phenyl)diphenylsilyl)-(1,1’-
biphenyl)-4-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole
(DCzSiPI)
9-(4-((4-Bromophenyl)diphenylsilyl)phenyl)-9H-3,9’-bicarbazole was
reacted with 1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole in a Suzuki coupling
reaction. The process was the same as SiPI. The crude product was
purified by using column chromatography with petroleum ether/
dichloromethane (v/v 1:1) as the eluent to give a white solid
1
(0.43 g, yield: 58%). H NMR (500 MHz, CDCl3, 258C, TMS): d=8.83–
8.81 (d, J=8.2 Hz, 1H, Ar-H), 8.76–8.74 (d, J=8.4 Hz, 1H, Ar-H),
8.76–8.74 (d, J=8.5 Hz, 1H, Ar-H), 7.84–7.83 (d, J=8.2 Hz, 2H, Ar-
H), 7.87–7.86 (d, J=8.2 Hz, 2H, Ar-H), 7.81–7.59 (m, 21H, Ar-H),
7.56–7.44 (m, 11H, Ar-H), 7.34–7.30 (m, 3H, Ar-H), 7.24–7.22 ppm
(d, J=8.2 Hz, 1H, Ar-H); elemental analysis calcd (%) for C75H50N4Si:
C 87.01, H 4.87, N 5.41; found: C 86.29, H 5.09, N 5.26; MS (89 eV):
m/z calcd for C75H50N4Si: 1034.38; found: 1036.50 (100) [M+H]+.
Experimental Section
1-Phenyl-2-(4’-(triphenylsilyl)-(1,1’-biphenyl)-4-yl)-1H-
phenanthro[9,10-d]imidazole (SiPI)
In a 100 mL round flask, ethanol (2.5 mL) and K2CO3 (2m, 2.5 mL)
were added to
a solution of (4-bromophenyl)triphenylsilane
Acknowledgements
(713 mg, 1.65 mmol) and 1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (821 mg,
1.65 mmol) in toluene (5 mL). [Pd(PPh3)4] (38 mg, 0.033 mmol) was
added and the resultant solution was degassed and stirred at 908C
for 2 d under nitrogen. Water was added to quench the reaction,
the mixture was then extracted with dichloromethane and the or-
ganic solution was washed with water and dried over anhydrous
sodium sulfate. The crude product was purified by using column
chromatography with ethyl petroleum ether/dichloromethane (v/v
This work was financially supported by the National Basic Re-
search Program of China (973 Program, 2013CB834701,
2013CB834800), the National Science Foundation of China
(grant nos. 21174050, 21374038), and the Graduate Innovation
Fund of Jilin University (grant no. 20121044).
Keywords: bipolar · carbazole · donor–acceptor systems ·
silanes · bandgap
1
1:1) as the eluent to give a white solid (0.72 g, yield: 62%). H NMR
(500 MHz, [D6]DMSO, 258C, TMS): d=8.96–8.94 (d, J=8.4 Hz, 1H,
Ar-H), 8.91–8.89 (d, J=8.2 Hz, 1H, Ar-H), 8.73–8.72 (d, J=7.6 Hz,
1H, Ar-H), 7.81–7.70 (m, 13H, Ar-H), 7.59–7.46 (m, 18H, Ar-H), 7.37–
7.34 (t, J=8.0 Hz, J=7.6 Hz, 1H, Ar-H), 7.10–7.08 ppm (d, J=8.2 Hz,
1H, Ar-H); 13C NMR (125 MHz, CDCl3, 258C, TMS): d=141.2, 136.9,
136.4, 134.1, 133.6, 130.3, 129.9, 129.8, 129.7, 129.3, 129.2, 128.3,
128.2, 127.9, 127.3, 126.4, 126.3, 125.7, 125.0, 124.2, 123.1, 122.9,
120.9 ppm; elemental analysis calcd (%) for C51H36N2Si: C 86.89, H
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Received: November 20, 2013
Revised: December 11, 2013
Published online on January 29, 2014
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