1,4-bis(diarylamino)-2,5-bis(4-cyanophenylethenyl)benzenes: Fluorophores exhibiting efficient red and near-infrared emissions in solid state

Article abstract of DOI:10.1002/anie.201108943

Molecular fluorophores of type A-π-D-π-A (D=donor, A=acceptor) demonstrate solid-state emission in the red to near-infrared region with high efficiency. The emission color can be tuned through the substituents on the diarylamino and cyanophenyl moieties. The electroluminescence performance of the designed fluorophore confirms its potential as an emitter for use in organic light-emitting devices. Copyright

Full text of DOI:10.1002/anie.201108943

Angewandte
Chemie
DOI: 10.1002/anie.201108943
Materials Science
1,4-Bis(diarylamino)-2,5-bis(4-cyanophenylethenyl)benzenes:
Fluorophores Exhibiting Efficient Red and Near-Infrared Emissions in
Solid State**
Masaki Shimizu,* Rina Kaki, Youhei Takeda, Tamejiro Hiyama, Naomi Nagai,
Hideo Yamagishi, and Hiroyuki Furutani
The molecular design and characterization of organic materi-
als exhibiting highly efficient fluorescence in the solid state[1]
are some of the urgent research topics supporting advances in
optoelectronic devices, such as organic light-emitting diodes
(OLEDs),[2] organic solid-state lasers,[3] and fluorescent
molecular sensors.[4] Red fluorophores usually consist of
planar molecules having extended p conjugation, such as
porphyrins and rylenes, or p-conjugated molecules which are
substituted with an electron donor (D) and acceptor (A) at
the termini of the conjugated system, that is, D–p–A type
compounds.[5] Such structural characteristics generally cause
strong intermolecular p–p interactions in the densely aggre-
gated state and inevitably consumes the exciton energy,
thereby rendering the red fluorophores weakly emissive or
non-emissive in the solid state. Red is one of the three
primary colors (red, green, and blue) that are essential for the
realization of full-color displays and white lighting. In
addition, there has been growing interest in p-conjugated
compounds that exhibit efficient luminescence in the near-
infrared (NIR) region because of their potential applications
in bioimaging, night vision devices, and optical communica-
tions.[6]
are scarcely available owing to much more severe concen-
tration quenching than red fluorophores.[8] Therefore, the
development of fluorophores that exhibit efficient solid-state
luminescence in the red to NIR region is a challenging and
essential first step towards the realization and advancement
of solid-state light-emitting devices. We report that the 1,4-
bis(diarylamino)-2,5-bis(4-cyanophenylethenyl)benzenes 1–3
serve as a new class of fluorophores exhibiting highly efficient
solid-state photoluminescence (PL) and whose emission
maxima range from orange to the NIR region (Figure 1).
Moreover, the electroluminescence (EL) performance of 3b
is presented to demonstrate the high potential of the designed
fluorophores as emitters applicable to OLEDs.
Whereas various chromophores exhibiting efficient NIR
fluorescence in solution, such as cyanine dyes, porphyrins,
squaraines, benzoheteroles, xanthenes, and borondipyrrome-
thanes (BODIPYs) are frequently developed,[7] organic
fluorophores exhibiting efficient solid-state NIR emission
Figure 1. Structures of 1,4-bis(diarylamino)-2,5-bis(4-cyanophenyl-
ethenyl)benzenes 1–3.
[*] Prof. Dr. M. Shimizu, R. Kaki, Dr. Y. Takeda, Prof. Dr. T. Hiyama[+]
Department of Material Chemistry, Graduate School of Engineering
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: m.shimizu@hs2.ecs.kyoto-u.ac.jp
The D–p–A system is an attractive electronic structure for
organic light-emitting materials because the photophysical
and redox properties can be fine-tuned by the judicious
combination of a donor and an acceptor.[9] However, push-
pull type molecules frequently encounter severe emission
quenching in the solid state owing to dipole-induced inter-
Homepage: http://www.npc05.kuic.kyoto-u.ac.jp/npc05/
Home.html
N. Nagai, Dr. H. Yamagishi
Research & Development Group OLED AOMORI Co., Ltd.
2-1-1, Hieitsuji, Otsu-shi, Shiga 520-0104 (Japan)
Dr. H. Furutani
molecular interactions. For example,
a neat film of
RD Planning & Administration Department, Kaneka Corporation
3-2-4, Naka-No-Shima, Kita-Ku, Osaka-Shi, Osaka 530-8288 (Japan)
4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-
4H-pyran (DCM), which was a standard red dopant for
OLEDs, reportedly exhibited PL with an emission maximum
at l = 665 nm and a quantum yield of only 0.05.[10] One
possible solution to the intrinsic issue of quenching in D–p–A
systems is the construction of a linear p-conjugated molecular
framework consisting of a D–p–A–p–D electronic structure,
wherein the dipoles originating from the left and right halves
of the symmetrical molecule cancel out because of antipar-
[+] Current address: Research & Development Initiative, Chuo Uni-
versity, 1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
[**] This work was supported by Grants-in-Aid for Creative Scientific
Research (No. 16GS0209), and Scientific Research (No. 22350081)
from the Ministry of Education, Culture, Sports, Science and
Technology (Japan), and the Adaptable and Seamless Technology
Transfer Program (No. AS2211079D) from the Japan Science and
Technology Agency.
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Communications
allelism. Successful examples of red-emitting materials of the
type D–p–A–p–D include diarylamino-terminated 1,5-
dicyano-2,6-bis(styryl)naphthalenes,[11]
bis(1-cyano-2-aryl-
ethenyl)benzene,[12] diarylfumaronitriles,[13] dithienylbenzo-
[1,2,5]thiadiazole,[14] and dimesitylboryl-substituted quater-
thiophene.[15] In contrast, the design and development of red-
emissive organic solids consisting of the A–p–D–p–A frame-
work, which is an electronic counterpart of the D–p–A–p–D
skeleton, remains unexplored.[16,17] We recently demonstrated
that
2,5-bis(2,2-diacetylethenyl)-1,4-dipiperidinobenzene,
which could be classified as an A–p–D–p–A fluorophore,
exhibited red emission in the solid state with a high quantum
yield.[18] However, there was little room for structural
modification of the 2,2-diacetylethenyl moiety and piperidyl
group to fine-tune the emission color and extend it to the
deep-red to NIR region. Then, we designed 1–3 to develop
new chromophores exhibiting efficient solid-state emission in
the red to NIR region on the basis of the molecular design of
molecules having a A–p–D–p–A framework.[19]
Figure 2. Molecular and crystal structures of 1a: a) View from a long
axis of the molecule, b) side view, c) packing diagram, and d) packing
diagram with mean planes of each central benzene ring. Hydrogen
atoms are omitted for clarity.
Compounds 1–3 were readily prepared by the double
Horner–Wadsworth–Emmons reaction of the corresponding
4-cyano-substituted benzaldehydes with 2,5-bis(diethylphos-
phonatomethyl)-1,4-dibromobenzene and subsequent palla-
dium-catalyzed N-arylation with secondary amines.[20] The
decomposition temperatures (Td) of 1–3, temperatures at
which the compounds lose 5% of their weight, ranged from
3168C to 3778C.[20] These data clearly indicate that all the
compounds are thermally stable. Such good thermal stability
is a desirable characteristic for the purification of materials by
vacuum sublimation, fabrication of organic devices by
vacuum deposition, and extended device lifetimes.
Single crystals of 1a suitable for X-ray analysis were
obtained by recrystallization from CH2Cl2/n-hexane. Com-
pound 1a crystallizes in the monoclinic space group C2
without inversion symmetry with respect to the centroid of
the central benzene ring.[20,21] The bis(styryl)benzene frame-
work is slightly distorted as the dihedral angles between two
Table 1: Photophysical properties of 1–3.
Cmpd labs [nm][a,b] (e)[c]
lem [nm][d] (Ff)[e]
Solution[a]
Powder
PS film[f]
1a
1b
2a
2b
3a
3b
456 (7200)
471 (5000)
473 (4500)
471 (7600)
473 (3400)
498 (4000)
565 (0.61) 576 (0.56) 563 (0.75)
596 (0.55) 605 (0.58) 573 (0.70)
609 (0.57) 619 (0.20) 591 (0.70)
643 (0.38) 634 (0.33) 620 (0.58)
622 (0.53) 667 (0.36) 611 (0.72)
657 (0.38) 702 (0.33) 643 (0.57)
[a] 1ꢀ10À5 m in toluene. [b] Absorption maximum at the longest wave-
length. [c] Molar absorption coefficient [mÀ1 cmÀ1]. [d] Emission maxi-
mum upon photoexcitation at l=400 nm. [e] Absolute quantum yield
determined with a calibrated integrating sphere system. [f] A polystyrene
film doped with 1–3.
=
4-NCC6H4CH CH- moieties and the central benzene ring are
8.58 and 21.38 (Figure 2a). The phenyl group of one amino
substituent orients itself above the central benzene plane and
the other orients itself below the plane (Figure 2b). There is
no p–p stacking between the bis(styryl)benzene skeletons of
adjacent molecules, presumably owing to the symmetric
electronic structure and the twisted conformation of the
bulky amino moieties (Figures 2c and d). This packing mode
is beneficial for preventing excimer formation and Dexter-
type energy transfer leading to the loss of the excited energy.
The absorption and fluorescence data of compounds 1–3
in toluene are summarized in Table 1. The spectra of 1a, 2a,
and 3a are shown in Figure 3.[20] Each compound shows
a strong absorption band at l = 334–353 nm, which is assign-
able to a p–p* transition, and a weaker band at l = 456–
498 nm, which can be ascribed to an intramolecular charge-
transfer (CT) transition. Fluorescence maxima appeared in
the region ranging from yellow to red with large Stokes shifts.
There was very little overlap in the absorption and fluores-
cence spectra, thus suggesting that the designed structures of
1–3 were advantageous for suppressing energy transfer
through the Fçrster mechanism, which results in lumines-
Figure 3. Absorption and fluorescence spectra of 1a, 2a, and 3a in
toluene at 1ꢀ10À5 m.
cence quenching. The absorption and fluorescence spectra of
1–3 in CH2Cl2, which are shown in the Supporting Informa-
tion, also support the intramolecular CT transition from
dipole-cancelled ground states.
Compounds 1–3 exhibited fluorescence in the orange to
NIR region with good to high efficiency, both in powder form
and dispersed in a thin film of polystyrene (PS). The
fluorescence data are summarized in Table 1 and the spectra
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the electron-withdrawing substituents such as fluorine or
trifluoromethyl group in the phenyl groups of the styryl
moieties greatly lowered the LUMO levels and slightly
lowered the HOMO levels. These results suggest that the
transition between the S0 and S1 states has an intramolecular
CT character.
To gain insight into the intramolecular CT character of 1–
3, we carried out density functional theory (DFT) calculations
(B3LYP/6-31G(d) level) of 1a using the structure determined
by X-ray analysis. The HOMO is primarily localized over the
diphenylamino moieties and the central benzene rings,
whereas the LUMO is localized over the bis(styryl)benzene
framework.[20] These results are consistent with the substitu-
ent effects on the energy levels of the HOMO and LUMO.
The time-dependent DFT calculations indicate that the
absorption band at l = 452 nm could be the transition from
the HOMO to the LUMO; in other words, the large Stokes
shifts leading to the suppression of Fçrster-type energy
transfer are ascribed to the intramolecular CT from the
diarylamino moieties to the cyano-substituted styryl moieties.
To verify the high potential of 1,4-bis(diarylamino)-2,5-
bis(4-cyanophenylethenyl)benzenes as light-emitting materi-
als for use in optoelectronic devices, we fabricated the OLED
devices I–III using 3b as a nondoped host emitter (device I)
and a dopant (devices II and III) by vacuum deposition. The
EL performances of the devices are summarized in Table 2,
and the emission spectra are shown in Figure 5.[20] The
nondoped device I exhibited EL with the emission maximum
at l = 666 nm, which was much shorter than that of the PL of
3b as a powder (Figure 5). Vacuum-deposited thin films of 3b
exhibited PL with an emission maximum at l = 667 nm and
Figure 4. Fluorescence spectra of 1–3 as a powder.
of 1–3 as a powder are displayed in Figure 4. The emission
maxima in powder generally red-shifted compared with those
in toluene except for 2b. The red-shifted emission is
presumably attributed to the formation of a physical dimer
in which transition dipoles of the monomer are arranged
either in-line or in an oblique manner,[1e] whereas the reason
for the blue-shifted emission of powder 2b is unclear at
present. Red fluorophores in the pure solid state intrinsically
tend to cause aggregation, thus resulting in severe concen-
tration quenching, and thus, it is a formidable challenge to
obtain quantum yields exceeding 0.3 with regard to red
emission from neat organic solids.[1a,5] Therefore, it is note-
worthy that 2b and 3a in powder form exhibited intense red
emissions at l = 634 nm and 667 nm with excellent quantum
yields of 0.33 and 0.36, respectively. Moreover, 3b was found
to exhibit a highly efficient NIR emission at l = 702 nm with
a remarkable quantum yield of 0.33.[22] The highly efficient
NIR emission was almost retained (F = 0.30) even when
luminescence was measured using a suspension of powder 3b
in water.[20] This behavior is a very attractive feature for
applications in biological imaging because bioimaging is
generally performed in aqueous media where organic fluo-
rescent probes are prone to aggregation which results in
emission quenching. In addition, the large Stokes shift of 3b is
advantageous for avoiding imaging interference by excitation
and scattered light.
Fluorescence maxima of PS films doped with 1–3
appeared at longer wavelengths than those for the toluene
solutions, and at shorter wavelengths than those of the
powder (Table 1).[20] Fluorescence quantum yields of the PS
films ranged from 0.57 to 0.75, which were higher than both
those of the toluene solution and powder in all cases. These
results indicate that 1–3 can also be used as dopant emitters.
The energy gap between the HOMOs and LUMOs of 1–3
were calculated from the absorption edge, and the energy
levels of the HOMOs were determined by cyclic voltammetry.
The LUMO energies were estimated from the HOMO–
LUMO gaps and HOMO energies because the reduction
waves were structureless. The data and cyclic voltammograms
are shown in the Supporting Information. All the compounds
showed two reversible oxidation waves,[20] thus suggesting
that the radical cations of 1–3 were stable. The substitution of
the tBu groups onto the phenyl rings of the amino moieties
significantly raised the HOMO energies, while having little
effect on the LUMO energies. In contrast, the introduction of
Table 2: EL performance of devices I–III.
Device[a] Von
[V][b] [nm][c] [cdAÀ1 [d]
lmax
hc
hp
hext
[%][f]
CIE (x, y)
]
[lmWÀ1 [e]
]
I
II
III
3.2
3.7
3.6
666
662
657
0.71
0.91
1.44
0.87
1.05
1.38
1.36
1.66
2.09
0.63, 0.36
0.64, 0.36
0.61, 0.37
[a] For details of device structures, see the Supporting Information.
[b] Turn-on voltage at a brightness of 1 cdmÀ2. [c] Emission maxima.
[d] hc =Maximum current efficiency. [e] hp =Maximum power efficiency.
[f] Maximum external quantum efficiency.
Figure 5. EL spectra of devices I–III.
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high quantum yield of 0.28 upon irradiation at l = 400 nm.[20]
Hence, the blue-shift of the EL compared with the powder PL
is ascribed to the morphological difference between the thin
film prepared by vacuum deposition and the powder obtained
by evaporation of its solution.[23] The PL spectrum and
quantum yield of a vacuum-deposited film were constant even
when the film was heated at 608C for 24 hours.[20] The
emission maxima of devices II and III appeared at l = 662 and
666 nm, respectively, and were very similar to those of
device I and the toluene solutions, thus indicating that the
EL emission of all the devices originates from the singlet
excited state of 3b. Small emission bands were observed at
around l = 450–550 nm in device I, and l = 500–570 nm in
devices II and III. These bands were attributed to emissions
from Alq3 or rubrene, thus suggesting that the energy transfer
from those chromophores was incomplete.[24] Additional
optimization of the device configuration is in progress. The
external quantum efficiency of the nondoped device I was
high (1.36%)[25] and the efficiency increased to 2.09% with
the doped device III. These preliminary results imply that
1,4-bis(diarylamino)-2,5-bis(4-cyanophenylethenyl)benzenes
serve as attractive candidates for light-emitting materials
which can be used in nondoped and doped OLEDs, although
further studies are necessary for practical applications.
In summary, we have developed 1,4-bis(diarylamino)-2,5-
bis(4-cyanophenylethenyl)benzenes as novel fluorophores
exhibiting efficient solid-state emission in the region ranging
from orange to NIR. The quantum yields in the solid state are
good to high, and the emission color can be tuned by changing
the substituents on the diarylamino and cyanophenyl moi-
eties. In particular, one of the designed fluorophores exhib-
ited solid-state NIR emission with an excellent quantum yield
of 0.33. The long-awaited development of NIR-emissive
probes that do not cause aggregation-induced emission
quenching in aqueous biological systems is very important
in the field of biomolecular imaging, and these results should
provide new ideas for the molecular design of efficient NIR-
emissive probes. Furthermore, the good EL performance of
nondoped and doped devices are presented. These results
demonstrate that the molecular design of 1,4-bis(diaryla-
mino)-2,5-bis(4-cyanophenylethenyl)benzenes based on an
A–p–D–p–A framework is definitely valid for the invention
of organic fluorophores exhibiting highly efficient solid-state
emission in the red to NIR region.
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Published online: March 15, 2012
Keywords: arenes · chromophores · fluorescence ·
.
luminescence · materials science
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[20] The Supporting Information includes the preparation and
thermal properties of 1–3, the absorption and fluorescence
spectra of 1b, 2b, and 3b in toluene and CH2Cl2, the optical and
fluorescence images of 1–3 as a powder, the fluorescence spectra
of a suspension of 3b in water, the fluorescence spectra of 1–3
Angew. Chem. Int. Ed. 2012, 51, 4095 –4099
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4099