Highly efficient near-infrared organic light-emitting diode based on a butterfly-shaped donor-acceptor chromophore with strong solid-state fluorescence and a large proportion of radiative excitons

Article abstract of DOI:10.1002/anie.201308486

The development of near-infrared (NIR) organic light-emitting diodes (OLEDs) is of growing interest. Donor-acceptor (D-A) chromophores have served as an important class of NIR materials for NIR OLED applications. However, the external quantum efficiencies (EQEs) of NIR OLEDs based on conventional D-A chromophores are typically below 1 %. Reported herein is a butterfly-shaped D-A compound, PTZ-BZP. A PTZ-BZP film displayed strong NIR fluorescence with an emission peak at 700nm, and the corresponding quantum efficiency reached 16 %. Remarkably, the EQE of the NIR OLED based on PTZ-BZP was 1.54 %, and a low efficiency roll-off was observed, as well as a high radiative exciton ratio of 48 %, which breaks through the limit of 25 % in conventional fluorescent OLEDs. Experimental and theoretical investigations were carried out to understand the excited-state properties of PTZ-BZP. Copyright

Full text of DOI:10.1002/anie.201308486

Angewandte
Chemie
DOI: 10.1002/anie.201308486
Optoelectronic Materials
Highly Efficient Near-Infrared Organic Light-Emitting Diode Based on
a Butterfly-Shaped Donor–Acceptor Chromophore with Strong Solid-
State Fluorescence and a Large Proportion of Radiative Excitons**
Liang Yao, Shitong Zhang, Rong Wang, Weijun Li, Fangzhong Shen, Bing Yang,* and
Yuguang Ma*
Abstract: The development of near-infrared (NIR) organic
light-emitting diodes (OLEDs) is of growing interest. Donor–
acceptor (D–A) chromophores have served as an important
class of NIR materials for NIR OLED applications. However,
the external quantum efficiencies (EQEs) of NIR OLEDs
based on conventional D–A chromophores are typically below
1%. Reported herein is a butterfly-shaped D–A compound,
PTZ-BZP. A PTZ-BZP film displayed strong NIR fluores-
cence with an emission peak at 700 nm, and the corresponding
quantum efficiency reached 16%. Remarkably, the EQE of the
NIR OLED based on PTZ-BZP was 1.54%, and a low
efficiency roll-off was observed, as well as a high radiative
exciton ratio of 48%, which breaks through the limit of 25% in
conventional fluorescent OLEDs. Experimental and theoret-
ical investigations were carried out to understand the excited-
state properties of PTZ-BZP.
values of NIR POLEDs usually appeared at very low current
densities, and there were serious efficiency roll-offs at high
current density and high brightness, probably owing to the
long lifetime of triplet excitons in such NIR OLEDs.[7]
The development of metal-free organic materials is of
significance, considering their fluorescent nature (avoid
quenching of long-lifetime excitons) and cost advantage
(without expensive metals). The research is currently focused
on p-conjugated compounds with a donor–acceptor (D–A)
structure, because a strong D–A interaction could lead to
a much narrower band gap, thus resulting in an electronic
transition in the deep-red or NIR spectral range.[8] However,
these D–A compounds exhibit unexpectedly low efficiency in
both photoluminescence and electroluminescence (EL) devi-
ces. The main reason for the low efficiency could be ascribed
to an intrinsic limitation according to the energy-gap law,
which predicts that the quantum efficiency of chromophores
decreases as the energy gap is reduced, owing to vibrational
overlap of the ground and excited states.[9] Additionally, the
HOMO/LUMO overlap in D–A chromophores is usually
limited, which always leads to a lower radiative-transition
rate, and finally results in virtually no emission.[10] In a solid
film, the fluorescence efficiency is further reduced because of
the nonradiative process induced by dipole–dipole interac-
tions of these polar chromophores.[11] Thus, for highly efficient
NIR OLEDs, the D–A chromophores should possess a rea-
sonable radiative-transition rate, and the fluorescence effi-
ciency should be maintained as well as possible in the solid
film. More importantly, further EQE improvement of fluo-
rescent NIR OLEDs (FOLEDs) based on D–A chromo-
phores requires an innovative molecular approach. An
increase in the radiative exciton ratio and the use of triplet
excitons in FOLEDs may be of great potential, as demon-
strated with some D–A compounds reported very recently.[12]
Herein, we report a butterfly-shaped NIR D–A com-
pound, PTZ-BZP (Scheme 1), in which phenothiazine serves
as the electron donor and benzothiadiazole as the electron
acceptor. The DFT-optimized ground-state geometry
(B3LYP/6-31G(d,p) method) reveals that the phenothiazine
moiety possesses a nonplanar “butterfly-like” structure with
a C-S-N-C dihedral angle (qN) of 1428.[13] Furthermore, the
phenothiazine and benzothiadiazole groups are twist-linked
with a distorted angle (qD–A) of 1458, which is a relatively
planar arrangement for D–A compounds. Generally, the
electronic transition of the charge-transfer (CT) excited state
is partially forbidden owing to the spatial separation of the
donor and acceptor moieties. In our case, in PTZ-BZP, the
O
rganic light-emitting diodes (OLEDs) that emit visible
and white light have found great success in flat-panel displays
and solid-state lighting over the past two decades.[1] Extension
of the spectrum of OLEDs from visible to deep-red and near-
infrared (NIR) has become a new target in the field of
OLEDs because of the urgent demand in chemosensing,
night-vision devices, and information-secured displays.[2]
There have been several types of organic or metal-complex
materials utilized for NIR OLEDs, including lanthanide-
metal complexes,[3] conjugated polymers,[4] phosphorescent
metal complexes,[5] and organic p-conjugated compounds
with donor–acceptor (D–A) structures.[6] The external quan-
tum efficiencies (EQEs) of these NIR OLEDs were typically
below 1%. Recently, a significant breakthrough in EQE has
been made in NIR phosphorescent OLEDs (POLEDs): An
EQE of up to 9.2% was reported for a platinum(II) porphyrin
with extended p-conjugation.[5d] However, these high EQE
[*] L. Yao, S. Zhang, R. Wang, Dr. W. Li, Dr. F. Shen, Prof. B. Yang,
Prof. Y. Ma
State Key Laboratory of Supramolecular Structure and Materials
Jilin University
2699 Qianjin Avenue, Changchun, 130012 (P. R. China)
E-mail: yangbing@jlu.edu.cn
ygma@jlu.edu.cn
[**] We are grateful for financial support from the National Science
Foundation of China (grant number 91233113, 51273078) and the
National Basic Research Program of China (973 Program, grant
numbers 2013CB834705 and 2013CB834805).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308486.
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Scheme 1. Molecular structure of PTZ-BZP and DFT-optimized geome-
try of a simplified structure with a methyl substituent.
Figure 1. a) Linear correlation of the orientation polarization (Df) of
solvent media with the Stokes shift (uaꢀuf; a: absorbed light; f:
fluorescence) for PTZ-BZP (see Table S1 for the solvents and corre-
sponding data). In low-polarity and high-polarity solvents, the S1 state
is the HLCT state and the CT state, respectively. b) PL spectra of PTZ-
BZP (10 mm) in THF/water mixtures with different water fractions.
near-planar arrangement of the phenothiazine and benzo-
thiadiazole groups (qD–A) may cause considerable orbital
overlap between these moieties and further enhance the
radiative-transition rate and fluorescence efficiency.
The UV/Vis absorption and photoluminescence (PL)
spectra of PTZ-BZP were recorded in various solvents with
different polarities (see Figures S4 and S5, respectively, and
the detailed data summarized in Table S1 in the Supporting
Information). The absorption spectra seldom changed in
terms of both their shape and position as the solvent polarity
increased, thus implying a rather small dipolar change at the
ground state in different solvents. The absorption bands at
approximately 447 nm were assigned to the intramolecular
CT transition. The fluorescence spectra broadened and
revealed a remarkable redshift as the polarity of the solvent
increased: from 590 nm in nonpolar hexane to 790 nm in polar
dichloromethane. The large solvatochromic shift indicated
a strong CT state. The PL quantum yields of PTZ-BZP in
different solvents exhibited a clear decreasing trend with
increasing solvent polarity: from 0.47 in hexane to 0 in
acetonitrile. To better understand the solvent-polarity effect,
we applied the Lippert–Mataga relation.[14] Figure 1a shows
a plot of the Stokes shift versus the orientation polarizability
(Df). Notably, the plot shows two sets of linearity indicative of
two different excited states: one for high-polarity solvents and
the other for low-polarity solvents. The dipole moment, me,
was calculated to be 31.1 and 13.2 D for high- and low-
polarity solvents, respectively. In high-polarity solvents, the
me value is larger than that of 4-(N,N-dimethylamino)benzo-
nitrile (DMABN), which is a typical CT molecule with
a me value of 23 D. Furthermore, with increasing solvent
polarity, the fluorescence quantum yield rapidly decreased
from 0.31 in isopropyl ether to 0.02 in dichloromethane. We
concluded that a CT state is dominant in high-polarity
solvents. In low-polarity solvents, the dipole moment of
13.2 D indicated that the S1 state still possessed CT character.
Nevertheless, this value was significantly smaller than that in
high-polarity solvents. Meanwhile, a relatively high quantum
yield (between 0.47 in hexane and 0.32 in butyl ether) could
be maintained with increased solvent polarity. These two
factors demonstrated that a certain degree of locally excited
(LE) character had been introduced. Thus, the S1 state in low-
polarity solvents contained CT and LE components simulta-
neously. Since single-exponential fluorescence decay was
observed for PTZ-BZP in low-polarity solvents, we supposed
that a new and unique excited state existed rather than
a simple mixture of the LE and CT states. As it combines
a large dipole moment and a high radiative-transition rate, we
named it the “hybridized local and charge-transfer” (HLCT)
state.[15] Very recently, we reported a series of highly luminous
D–A compounds based on the “HLCT” state.[15]
In the absorption spectra of the thin film, the compound
showed an absorption band at 479 nm, which is significantly
red-shifted (by 30 nm) from that in solution, probably as
a result of the domination of a more planar D–A conformer in
the aggregated state. In the PL spectrum of the film, PTZ-
BZP exhibited a broad emission ranging from 600 to 1000 nm
with a peak at 700 nm, thus covering the NIR region well. The
Stokes shift of 6591 cmꢀ1 lies between that of butyl ether
(6549 cmꢀ1) and that of isopropyl ether (7132 cmꢀ1). The
results indicate that the polarity of the PTZ-BZP film was
similar to that of a low-polarity solvent, and that the S1 state of
the PTZ-BZP film was also a HLCT state. A remarkable
quantum efficiency of 16% was observed for the neat film. To
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investigate the influence of aggregation effects on the
fluorescence behavior, we measured the PL spectra of PTZ-
BZP in THF/water mixtures with different water fractions (fw;
Figure 1b), which enabled direct tuning of the solvent polar-
ity and the extent of solute aggregation. The solutions with
fw ꢁ 60 vol% were nearly non-emissive, and the PL curve was
practically a flat line parallel to the x axis, mainly as a result of
the CTeffect induced by the strong polarity of the THF/water
mixtures. Fluorescence appeared from fw ꢂ 70 vol% and was
enhanced by further increases in fw. Evidently, PTZ-BZP is
aggregation-induced-emission (AIE) active.[16] The main
reason for the AIE phenomenon could be that the HLCT
state in the aggregated state is associated with a more efficient
radiative electron transition than the CT state in solutions in
THF. Therefore, the HLCT state provides a novel approach
for the design of highly luminous D–A chromophores. On the
other hand, the formation of intermolecular aggregates tends
to restrict the rotation described by qN and qD–A, and avoids
the nonradiative decay channel, which enhances fluorescence
efficiency.[16]
To verify the high potential of PTZ-BZP as a light-
emission material for use in optoelectronic devices, we
fabricated an undoped NIR OLED with the configuration:
ITO/PEDOT:PSS (40 nm)/NPB (80 nm)/TCTA (5 nm)/PTZ-
BZP (30 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al (120 nm; ITO =
indium tin oxide, PEDOT= poly(3,4-ethylenedioxythio-
phene), PSS = poly(styrene sulfonate), NPB = N,N’-bis(naph-
thalen-1-yl)-N,N’-bis(phenyl)benzidine, TCTA = tris(4-car-
bazoyl-9-ylphenyl)amine, TPBi = 1,3,5-tris(1-phenyl-1H-ben-
zimidazol-2-yl)benzene. As shown in Figure 2, the EL spec-
trum of the PTZ-BZP device was almost identical to the PL
spectrum of the evaporated film. Although our instrument
(PR-650 Spectroscan spectrometer) could not detect the EL
luminance signal over 780 nm, it can be inferred that the EL
spectrum was extended to nearly 1000 nm according to the PL
spectrum of the PTZ-BZP film. Additionally, the EL
spectrum showed little change under different driving vol-
tages, thus indicating that the PTZ-BZP device possesses
excellent spectral stability. A maximum EQE of 1.54% and
brightness of 780 cdmꢀ2 were observed for the nondoped NIR
OLED based on PTZ-BZP. At a high current density of
300 mAcmꢀ2, the EQE of the PTZ-BZP device still remained
as high as 1.17%, which is indicative of a relatively low
efficiency roll-off. To the best of our knowledge, the
performance of the PTZ-BZP device places it among the
best undoped NIR FOLEDs.[6,8,11]
The theoretical value of the radiative exciton ratio was
calculated by the following equation:
hext ¼ g hr hPL hout
ð1Þ
in which hr is the radiative exciton ratio, hext is the external
quantum efficiency, hout is the light out-coupling efficiency
(ca. 20%), hPL is the intrinsic photoluminescence efficiency
(ca. 16%), and g is the recombination efficiency of injected
holes and electrons, which is ideally 100% only if holes and
electrons are fully balanced and completely recombined to
form excitons. Thus, the hr value of the PTZ-BZP device was
calculated to be 48%, which breaks through the limit of the
radiative exciton ratio of 25% for conventional FOLEDs.
Since no delayed fluorescence was observed from transient
PL, and the luminance of EL displayed a linear increase with
increasing current density, the high radiative exciton ratio
does not seem to be in accordance with some mainstream
views, such as thermally activated delayed fluorescence
(TADF)[12] or triplet–triplet annihilation (TTA).[17]
To explain the high radiative exciton ratio in the PTZ-
BZP device, we calculated and analyzed the energy landscape
and the natural transition orbitals (NTOs) of the singlet and
triplet states on the basis of TDDFT results with M06-2X/6-
31G(d,p) (Figure 3). For the S1 state, the hole and particle
NTOs showed an excellent balance between spatial separa-
tion and orbital overlap. The well-separated orbitals led to CT
character with a large dipole moment. On the other hand,
certain orbital overlaps induced LE character and ensured
a reasonable radiative-transition rate. The calculated results
demonstrate the coexistence of CT and LE components, in
good agreement with our definition of the HLCT state. The
low-lying triplet state T1 was a LE state, and its hole and
particle were almost completely overlapped. The energy gap
between the S1 state and the T1 state reached 0.79 eV, and
correspondingly, reverse intersystem crossing (RISC) from
the T1 state to the S1 state by TADF is not a facile process. The
T2 and T1 states are degenerate states. The high-lying triplet
excited state T3 (referred to herein as a “hot” excited state)
was found to be a HLCT state whose configuration is quite
similar to that of the S1 state. As shown in Figure 3, the energy
levels of the S1 state (2.91 eV) and the T3 state (2.92 eV) are
almost identical. Such small singlet–triplet splitting could
offer a potential RISC T3!S1 process, which we refer to as
a “hot-exciton” process.[18] On the other hand, the energy gap
between the T3 state and the T2 or T1 state is 0.76 eV.
According to the energy-gap law, the internal-conversion (IC)
rate, kIC(T), from the T3 state to the T2 state may be lower than
the RISC rate, kRISC, from the T3 to the S1 state.[19] Besides, it
has been reported that the RISC rate can be greatly enhanced
in some heterocyclic systems with sulfur atoms owing to
improved spin–orbit coupling.[20] Therefore, when triplet
excitons relax to the lowest vibrational level of T1, a fraction
of them may be converted into singlet excitons through the
T3!S1 channel. Since the concentration of triplet excitons is
Figure 2. EQE–current-density characteristics of the device. The inset
graph is the EL spectrum.
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of the undoped PTZ-BZP device is 1.54%: among the highest
observed for undoped NIR FOLEDs. Notably, in the device,
a high radiative exciton ratio of 48% was observed, which
exceeds the limit of 25% in conventional FOLEDs, probably
as a result of efficient RISC through the hot-exciton process.
Therefore, PTZ-BZP could serve as an attractive light-
emitting material in NIR OLEDs, and our study should
provide new ideas for the design of efficient NIR-fluorescent
molecules by emphasizing the full use of both singlet and
triplet excitons.
Received: September 29, 2013
Revised: December 15, 2013
Published online: January 21, 2014
Keywords: chromophores · donor–acceptor systems ·
.
near-infrared fluorescence · optoelectronic materials ·
organic light-emitting diodes
[1] a) M. A. Baldo, D. F. OꢀBrien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151 – 154; b) Y.
Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R.
Forrest, Nature 2006, 440, 908 – 912; c) A. C. Grimsdale, K. L.
Chan, R. E. Martin, P. G. Jokisz, A. B. Holmes, Chem. Rev. 2009,
109, 897 – 1091; d) Q. Wang, D. Ma, Chem. Soc. Rev. 2010, 39,
2387 – 2398.
[2] a) G. Qian, Z. Y. Wang, Chem. Asian J. 2010, 5, 1006 – 1029;
b) N. Tessler, V. Medvedev, M. Kazes, S. Kan, U. Banin, Science
2002, 295, 1506 – 1508; c) B. Stender, S. F. Vçlker, C. Lambert, J.
Pflaum, Adv. Mater. 2013, 25, 2943 – 2947; d) W. Qin, D. Ding, J.
Liu, W. Zhang Yuan, Y. Hu, B. Liu, B. Z. Tang, Adv. Funct.
Mater. 2012, 22, 771 – 779.
[3] a) J. Kido, Chem. Rev. 2002, 102, 2357 – 2368; b) R. G. Sun, Y. Z.
Wang, Q. B. Zheng, H. J. Zhang, A. J. Epstein, J. Appl. Phys.
2000, 87, 7589 – 7591.
[4] a) T. T. Steckler, O. Fenwick, T. Lockwood, M. R. Andersson, F.
Cacialli, Macromol. Rapid Commun. 2013, 34, 990 – 996; b) B. C.
Thompson, L. G. Madrigal, M. R. Pinto, T.-S. Kang, K. S.
Schanze, J. R. Reynolds, J. Polym. Sci. Part A 2005, 43, 1417 –
1431; c) M. Sun, X. Jiang, W. Liu, T. Zhu, F. Huang, Y. Cao,
Synth. Met. 2012, 162, 1406 – 1410.
[5] a) C. Borek, K. Hanson, P. I. Djurovich, M. E. Thompson, K.
Aznavour, R. Bau, Y. R. Sun, S. R. Forrest, J. Brooks, L.
Michalski, J. Brown, Angew. Chem. 2007, 119, 1127 – 1130;
Angew. Chem. Int. Ed. 2007, 46, 1109 – 1112; b) H. Xiang, J.
Cheng, X. Ma, X. Zhou, J. J. Chruma, Chem. Soc. Rev. 2013, 42,
6128 – 6185; c) O. Fenwick, J. K. Sprafke, J. Binas, D. V. Kon-
dratuk, F. Di Stasio, H. L. Anderson, F. Cacialli, Nano Lett. 2011,
11, 2451 – 2456; d) K. R. Graham, Y. Yang, J. R. Sommer, A. H.
Shelton, K. S. Schanze, J. Xue, J. R. Reynolds, Chem. Mater.
2011, 23, 5305 – 5312.
[6] a) G. Qian, Z. Zhong, M. Luo, D. Yu, Z. Zhang, Z. Y. Wang, D.
Ma, Adv. Mater. 2009, 21, 111 – 116; b) G. Qian, B. Dai, M. Luo,
D. Yu, J. Zhan, Z. Zhang, D. Ma, Z. Y. Wang, Chem. Mater. 2008,
20, 6208 – 6216; c) G. Qian, Z. Zhong, M. Luo, D. Yu, Z. Zhang,
D. Ma, Z. Y. Wang, J. Phys. Chem. C 2009, 113, 1589 – 1595; d) Y.
Yang, R. T. Farley, T. T. Steckler, S. H. Eom, J. R. Reynolds, K. S.
Schanze, J. Xue, J. Appl. Phys. 2009, 106, 044509.
[7] a) N. C. Giebink, S. R. Forrest, Phys. Rev. B 2008, 77, 235215;
b) W. Staroske, M. Pfeiffer, K. Leo, M. Hoffmann, Phys. Rev.
Lett. 2007, 98, 197402.
Figure 3. a) Natural transition orbitals for S1, T1, and T3. b) The energy
landscape for singlet and triplet excited states. c) Model for exciton
relaxation in the EL process. RISC: reverse intersystem crossing; IC(T):
internal conversion between the triplet states; fl: fluorescence; ph:
phosphorescence.
much higher than that of singlet excitons, the dynamic
equilibrium will promote the conversion from triplet excitons
into singlet excitons in the electroluminescence process. We
deduce that the efficient RISC provided by the hot-exciton
process is responsible for the radiative exciton ratio of over
25% in the PTZ-BZP device.
In summary, we have investigated the optoelectronic
properties of an NIR D–A compound, PTZ-BZP. Photo-
physical and DFT analysis demonstrated that the S1 state of
PTZ-BZP in low-polarity solvent and as a thin film shows
HLCT character. It benefits from the large dipole moment of
the CT state and a certain degree of orbital overlap of the LE
state, thus making the compound a potentially highly
luminous molecule. In the aggregated state, PTZ-BZP
displays strong NIR emission and AIE behavior. The EQE
[8] a) S. Ellinger, K. R. Graham, P. Shi, R. T. Farley, T. T. Steckler,
R. N. Brookins, P. Taranekar, J. Mei, L. A. Padilha, T. R. Ensley,
H. Hu, S. Webster, D. J. Hagan, E. W. V. Stryland, K. S. Schanze,
2122
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 2119 –2123

Angewandte
Chemie
J. R. Reynolds, Chem. Mater. 2011, 23, 3805 – 3817; b) X. Du, J.
Qi, Z. Zhang, D. Ma, Z. Y. Wang, Chem. Mater. 2012, 24, 2178 –
2185.
W. Li, F. Shen, D. Liu, B. Yang, Y. Ma, J. Mater. Chem. 2012, 22,
4401 – 4408; c) W. Li, Y. Pan, R. Xiao, Q. Peng, S. Zhang, D. Ma,
F. Li, F. Shen, Y. Wang, B. Yang, Y. Ma, Adv. Funct. Mater. 2013,
DOI: 10.1002/adfm.201301750; d) S. Zhang, W. Li, L. Yao, Y.
Pan, F. Shen, R. Xiao, B. Yang, Y. Ma, Chem. Commun. 2013, 49,
11302 – 11304.
[9] a) J. V. Caspar, E. M. Kober, B. P. Sullivan, T. J. Meyer, J. Am.
Chem. Soc. 1982, 104, 630 – 632; b) S. D. Cummings, R. Eisen-
berg, J. Am. Chem. Soc. 1996, 118, 1949 – 1960.
[10] M. Van der Auweraer, Z. R. Grabowski, W. Rettig, J. Phys.
Chem. 1991, 95, 2083 – 2092.
[11] M. Shimizu, R. Kaki, Y. Takeda, T. Hiyama, N. Nagai, H.
Yamagishi, H. Furutani, Angew. Chem. 2012, 124, 4171 – 4175;
Angew. Chem. Int. Ed. 2012, 51, 4095 – 4099.
[12] a) H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi,
Nature 2012, 492, 234 – 238; b) G. Mꢁhes, H. Nomura, Q. Zhang,
T. Nakagawa, C. Adachi, Angew. Chem. 2012, 124, 11473 –
11477; Angew. Chem. Int. Ed. 2012, 51, 11311 – 11315; c) Q.
Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki, C.
Adachi, J. Am. Chem. Soc. 2012, 134, 14706 – 14709; d) J. Li, T.
Nakagawa, Q. Zhang, H. Nomura, H. Miyazaki, C. Adachi, Adv.
Mater. 2013, 25, 3319 – 3323.
[13] a) L. Yao, S. Sun, S. Xue, S. Zhang, X. Wu, H. Zhang, Y. Pan, C.
Gu, F. Li, Y. Ma, J. Phys. Chem. C 2013, 117, 14189 – 14196; b) G.
Kim, H. R. Yeom, S. Cho, J. H. Seo, J. Y. Kim, C. Yang,
Macromolecules 2012, 45, 1847 – 1857.
[16] a) X. Y. Shen, W. Z. Yuan, Y. Liu, Q. Zhao, P. Lu, Y. Ma, I. D.
Williams, A. Qin, J. Z. Sun, B. Z. Tang, J. Phys. Chem. C 2012,
116, 10541 – 10547; b) C. Y. K. Chan, Z. Zhao, J. W. Y. Lam, J.
Liu, S. Chen, P. Lu, F. Mahtab, X. Chen, H. H. Y. Sung, H. S.
Kwok, Y. Ma, I. D. Williams, K. S. Wong, B. Z. Tang, Adv. Funct.
Mater. 2012, 22, 378 – 389; c) Y. P. Li, F. Li, H. Y. Zhang, Z. Q.
Xie, W. J. Xie, H. Xu, B. Li, F. Shen, L. Ye, M. Hanif, D. Ma, Y.
Ma, Chem. Commun. 2007, 231 – 233.
[17] a) C.-J. Chiang, A. Kimyonok, M. K. Etherington, G. C. Grif-
fiths, V. Jankus, F. Turksoy, A. P. Monkman, Adv. Funct. Mater.
2013, 23, 739 – 746; b) D. Y. Kondakov, T. D. Pawlik, T. K.
Hatwar, J. P. Spindler, J. Appl. Phys. 2009, 106, 124510.
[18] a) J. Guo, H. Ohkita, H. Benten, S. Ito, J. Am. Chem. Soc. 2009,
131, 16869 – 16880; b) J. Lee, K. Vandewal, S. R. Yost, M. E.
Bahlke, L. Goris, M. A. Baldo, J. V. Manca, T. V. Voorhis, J. Am.
Chem. Soc. 2010, 132, 11878 – 11880.
[19] N. J. Turro, Modern Molecular Photochemistry, University
Science Books, Sausalito, 1991, pp. 148 – 149.
[20] a) W. Barford, Phys. Rev. B 2004, 70, 205204; b) G. Cui, W.-h.
Fang, J. Chem. Phys. 2013, 138, 044315.
[14] a) V. E. Z. Lippert, Electrochemistry 1957, 61, 962 – 975; b) N.
Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 1956, 29,
465 – 470.
[15] a) W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu, B.
Yang, Y. Ma, Adv. Funct. Mater. 2012, 22, 2797 – 2803; b) S. Tang,
Angew. Chem. Int. Ed. 2014, 53, 2119 –2123
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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