Efficient synthesis of π-extended phenazasilines for optical and electronic applications

Article abstract of DOI:10.1039/c4cc06636g

The rhodium-catalyzed synthesis of phenazasilines from readily achievable biarylhydrosilanes is presented. This highly efficient method offers opportunities for preparing π-extended phenazasilines with enhanced optoelectronic properties for device applications in organic electronics.

Full text of DOI:10.1039/c4cc06636g

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Efficient synthesis of p-extended phenazasilines
for optical and electronic applications†
Cite this:DOI: 10.1039/c4cc06636g
a
a
a
a
a
a
Huanhuan Li, Yang Wang, Kai Yuan, Ye Tao, Runfeng Chen,* Chao Zheng,
a
a
ab
Received 23rd August 2014,
Accepted 28th October 2014
Xinhui Zhou, Junfeng Li and Wei Huang*
DOI: 10.1039/c4cc06636g
www.rsc.org/chemcomm
The rhodium-catalyzed synthesis of phenazasilines from readily were generally required for high optical and/or electronic
14
achievable biarylhydrosilanes is presented. This highly efficient activities.
method offers opportunities for preparing p-extended phenazasilines Inspired by the recent studies on the metal-catalyzed double
with enhanced optoelectronic properties for device applications in activation of Si–H and C–H bonds with dehydrogenation to generate
organic electronics. a Si–C bond in the preparation of a five-membered Si-ring
1
5
(
silafluorenes), we propose a rhodium-catalyzed synthetic
Silicon-containing conjugated molecules are an emerging class method for the synthesis of the N-containing six-membered
of optoelectronic materials that have wide-spread applications Si-ring of phenazasilines (Scheme 1). Starting from readily
1
in organic electronics, owing to their singular optical and elec- achievable mono-halogen (Br or I) substituted precursors, this
tronic properties originating from the particular interactions new synthetic route to prepare aromatically substituted p-extended
(
s* - p* conjugation) between the Si atom and the p-electron phenazasilines is accomplished in two steps with 80–90% overall
2
3
system. Silicon-bridged diphenylamines (phenazasilines), bene- yield. As an added benefit, the highly rigid (carbazole-fused)
fitting from both Si and N atoms in a fused six-membered ring, phenazasilines that can be hardly prepared in traditional routes
show promising potential as hole transporting layers in organic were efficiently synthesized in high yields. The obtained p-extended
light-emitting diodes (OLEDs), DNA straightening and fixing phenazasilines show excellent solubility, ultra-small Stokes shift,
materials, electrochromic molecules, antioxidants, stabilizers,
lubricants etc. Impressively, their properties can be effectively phorescent OLEDs (PhOLEDs). The method represented here
tuned by changing substituents on Si or N atoms. However, the
synthesis of phenazasilines, either by the extended heating of
diphenylsilane with phenothiazine derivatives or by cyclization
reactions of 2,2 -dilithiodiarylamine intermediates with dichloro-
4
5
6
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8
high triplet energy, and excellent device performance in phos-
9
10
11
0
12,13
silanes,
suffers from the low yield, complicated precursor, and
a long synthetic route. These synthetic difficulties significantly
hinder the studies and applications of phenazasilines, especially
in the field of organic electronics, where p-extended phenazasi-
lines either via conjugation expansion from the diphenyl wing of
phenazasilines or via aromatic substitution on Si and N atoms
a
Key Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials (IAM), National Synergistic Innovation, Center for Advanced
Materials (SICAM), Nanjing University of Posts and Telecommunications,
9
Wenyuan Road, Nanjing, 210023, P. R. China. E-mail: iamrfchen@njupt.edu.cn
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
IAM), National Synergistic Innovation Center for Advanced Materials (SICAM),
b
(
Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816,
P. R. China. E-mail: wei-huang@njtech.edu.cn
†
Electronic supplementary information (ESI) available. CCDC 1020766 and
020767. For ESI and crystallographic data in CIF or other electronic format
Scheme 1 Schematic presentation of the synthesis of phenazasilines: (a)
classic route via C–M/Si–X coupling; (b) proposed rhodium-catalyzed
synthesis via Si–H/C–H coupling.
1
see DOI: 10.1039/c4cc06636g
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geometries of TPASi and PhCzSi are well in line with the ab initio
density functional theory (DFT) predictions (Scheme S3, ESI†). When
the free phenyl substituent on N atoms is forced to be connected to
the nearby phenyl of phenazasiline as in PhCzSi, the perfect planar
phenazasiline in TPASi cannot be reserved, because the newly
formed carbazole moiety is structurally in competition with phen-
azasiline; both the planar molecular geometries of phenazasiline
and carbazole are slightly distorted, resulting in a slightly bent
molecular geometry of PhCzSi with a bit torsion of the molecular
planes of both phenazasiline (B251) and carbazole (B101) (Table S3,
ESI†). This small single molecular structure change, however, leads
to significantly different molecular packing structures (Fig. S9, ESI†).
An obvious p–p stacking (3.320 Å) has resulted from the larger rigid
planar molecular structure of PhCzSi, which may have profound
18
effects on its optoelectronic properties.
Despite their high molecular rigidity as indicated by both single
crystal X-ray analysis and theoretical predictions, TPASi and PhCzSi
have good solubility in common solvents. Nevertheless, the more
rigid carbazole-fused PhCzSi exhibits much improved thermostability
than TPASi (Fig. S10, ESI†): the decomposition temperature (T ) of
d
PhCzSi (246 1C) is about 60 1C higher than that of TPASi (188 1C) as
revealed by the thermogravimetric analysis (TGA); the melting point
Fig. 1 (a) Synthetic route for the preparation of phenazasilines; (b) single
crystal structures of TPASi and PhCzSi.
(T
110 1C) according to the differential scanning calorimetry (DSC)
measurements; good amorphous stability of the PhCzSi film was
offers a new way for the efficient preparation of optically and observed by atomic force microscopy (AFM) before and after anneal-
m
) of PhCzSi (150 1C) is about 40 1C higher than that of TPASi
(
electronically active phenazasilines for optoelectronic devices.
The synthetic route for the construction of phenazasiline deri-
ing at 100 1C for 30 min (Fig. S11, ESI†).
The photophysical properties of the p-extended phenazasilines
vatives was outlined in Fig. 1a and Scheme S1 (ESI†). Compared to were investigated by UV-Vis absorption and photoluminescence
11,13
the classical route developed about 50 years ago,
where the key (PL) spectra in both dilute dichloromethane (DCM) and thin solid
intermediates are highly reactive organometallic reagents such as film (Fig. 2a and b) in comparison with that of triphenylamine
0
2
,2 -dilithio derivatives prepared from multi-halogen substituted (TPA) and 9-phenyl-carbazole (PhCz). Considering that TPASi is
precursors for cyclization with dichlorosilanes to generate phenaza- formed by connecting the nearby two phenyls in TPA with a silicon
silines, this route provides an alternative way. Firstly, from the bridge to extend the molecular planarity and the p-conjugation,
mono-brominated precursors of 2a and 2b that are facilely prepared the newly emerged absorption bands of TPASi in both dilute
16
by the Ullmann reaction, 3a and 3b were obtained in a high yield solution and aggregated solid film provide clear evidence of this
93% for 3a and 90% for 3b) after conventional halogen-metal p-extension. Similarly, PhCzSi can be assumed to be structurally
(
17
interconversion and Si–C formation. The second step is to treat derived from PhCz also via Si-bridging. The obvious red-shift of the
19
the stable diarylhydrosilanes of 3a and 3b with 0.5 mol% of the p–p* transition absorption band of carbazole moieties at around
rhodium complex (RhCl(PPh ) and 5 equiv. of 3,3-dimethyl-1- 330 and 340 nm in PhCz to around 340 and 355 nm in PhCzSi
butene (a H eliminator) at 135 1C in 1,4-dioxane for 24 h. The clearly supports the extended p-system after the Si-bridging.
double activation and subsequent dehydrogenation of Si–H and This red-shift in the absorption spectrum due to Si-bridging is in
3 3
)
20
2
C–H bonds (Scheme S2, ESI†) give a Si–C bond for the formation of accordance with the red-shifted PL band of PhCzSi (370 nm) com-
the desired phenazasiline derivatives (TPASi and PhCzSi) in a high pared to that of PhCz (349 and 365 nm). However, TPASi shows a
yield (see the ESI† for the detailed procedure and the characteriza- blue-shifted PL regardless of the p-extension of TPA via Si-bridging,
21
tion data, Fig. S1–S8). This two-step route results in the significantly which may be because of the localized emission from the newly
improved synthetic efficiency; the overall yields for TPASi and formed aromatic phenazasiline moiety. The absorption and emission
PhCzSi reach 87% and 83% respectively.
spectra of TPASi, PhCz, and PhCzSi in solution are quite close,
More importantly, besides the advantages of the high yield and leading to very small Stokes shifts (B15 nm) due to their rigid
22
easy operation, this route overcomes the difficulties in the prepara- molecular geometries before and after photo-excitation. Small
tion of p-extended phenazasilines (PhCzSi) with rigid molecular variations of their UV and PL spectra in solvents with different
structures that can be hardly synthesized by other routes. The polarities (Fig. S12, ESI†) further confirm the rigidity of these
19
single-crystal structure analysis (Fig. 1b) reveals that both of the molecules. The increased rigidity results in much increased photo-
compounds show the rigid molecular feature with low angles (o111) luminescence quantum yield (PLQY) in PhCzSi (37%) compared to
between the planes of phenyl groups (numbered as 1, 2, and 3 in that in TPASi (8%). In contrast to the similar absorption spectra in
Scheme S3, ESI†) around N atoms (Table S3, ESI†). Such molecular dilute solution and solid film, their PL spectra were significantly
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Fig. 2 UV-Vis absorption (solid symbol) and PL (hollow symbol) spectra (a) in DCM (1.3 ꢁ 10ꢀ mol L ) and (b) in thin film. PL was excited at the
6
ꢀ1
g
absorption peak at around 300 nm; (c) DFT calculated (in black) and experimental (in red) HOMO, LUMO, and bandgap (E ) energy levels (in eV) as well as
the isosurfaces of the HOMO, the LUMO, and electrostatic potential (ESP).
different (Fig. 2b). Additional new broad emission bands of TPASi used excellent hole transport units of TPA and PhCz, the p-extended
429 nm) and PhCzSi (B489 nm) appear in solid films, indicating the phenazasilines (TPASi and PhCzSi) also exhibit high hole and
possible formation of excimers owing to the enhanced molecular electron transport rates with close relaxation and reorganization
(
23
interactions of the p-extended phenazasiline in aggregated states.
energy values, suggesting the good electronic properties of these
The electrochemical properties of the phenazasilines were p-extended phenazasilines. The molecular packing structure
investigated by cyclic voltammetric (CV) measurements. From the (Fig. S9d, ESI†) of PhCzSi revealed by its single crystal structure
onset voltage of the irreversible oxidation wave of TPASi and PhCzSi analysis also supports the good charge transport mobility of the
at 0.67 and 0.85 V, their HOMO energy levels were determined to be rigid conjugated compound, which preserves intermolecular p–p
at ꢀ5.43 and ꢀ5.61 eV, respectively, according to the reference stacking as conducting channels for charge carriers.
24
energy level of ferrocene (Fig. S13, ESI†). Combined with the
1
The lowest triplet states (T ) of the p-extended phenazasilines were
optical bandgap results, the frontier molecular orbital energy levers measured from the 0-0 transition bands of their low-temperature
were identified (Fig. 2c). The introduction of the Si-bridge generally (77 K) time-resolved phosphorescence spectra (Fig. S14, ESI†). To our
leads to the lower LUMO (decreased B0.10 eV) due to particular delight, both the experimental measurements and theoretical predic-
s* - p* interaction between the silyl group and the p-electron tions indicate that the increase of the p-extension via Si-bridging does
20
1 1
system, when comparing TPA with TPASi and PhCz with PhCzSi. not lead to the lower T energies (ET1), because T is dominated by
25
The p-extended system usually has a low bandgap, which is in either the TPA or the PhCz unit in the p-extended phenazasilines,
accord with the observation that the most rigid phenazasiline of according to the spin density distributions (Fig. S15, ESI†). The higher
2
0
PhCzSi has the lowest optical bandgap. The DFT calculations well
1
T than that of iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C ]
predict the same changing trends of the HOMO, the LUMO, and picolinate (FIrpic), which is a famous blue phosphorescent emitter,
the bandgap upon the structural modifications in phenazasilines. suggests the potential of these phenazasilines as host materials for
26
Notably, the calculated iso-surface of the frontier orbitals distri- FIrpic-based blue PhOLEDs.
butes on the whole molecular plane; the delocalized distribution is In light of the high optical and electronic properties of the
of the most obvious in the most rigid phenazasiline of PhCzSi, p-extended phenazasilines (Table S5, ESI†), especially the high-
indicating the effective extending of the p-conjugation in these lying T energy levels, we fabricated blue PhOLED devices using
1
phenazasilines.
The charge mobility of the phenazasilines was assessed by the under the following device configuration (Fig. S16, ESI†): ITO/
relaxation and reorganization energies via DFT calculations to MoO (10 nm)/TAPC (25 nm)/PhCzSi:FIrpic (20 nm)/TmPyPB
provide a qualitative investigation of their charge-transport rates (35 nm)/LiF(1 nm)/Al(100 nm). In the device, MoO and LiF
at a single molecule level (Table S4, ESI†). Compared to the widely served as the hole- and electron-injecting materials; FIrpic
PhCzSi (ET1 = 2.9 eV) as the host material and FIrpic as the guest
x
x
16
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a project funded by the priority academic program development
of Jiangsu higher education institutions (PAPD, YX03001), the
Qing Lan project of Jiangsu province, the National Basic
Research Program of China (973 Program) (2012CB933301,
2
012CB723402, and 2014CB648300), and the Program for Post-
graduates Research Innovations in University of Jiangsu Province
CXZZ11_0412 and CXZZ12_0455).
(
Notes and references
1
(a) J. Wencel-Delord and F. Glorius, Nat. Chem., 2013, 5, 369;
b) Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and
(
A. J. Heeger, Nat. Mater., 2011, 11, 44; (c) C. Duan, W. Cai, F. Huang,
J. Zhang, M. Wang, T. Yang, C. Zhong, X. Gong and Y. Cao,
Macromolecules, 2010, 43, 5262; (d) W. W. Wong, J. F. Hooper and
A. B. Holmes, Aust. J. Chem., 2009, 62, 393.
(a) K. L. Chan, M. J. McKiernan, C. R. Towns and A. B. Holmes, J. Am.
Chem. Soc., 2005, 127, 7662; (b) R. Chen, C. Zheng, Q. Fan and
W. Huang, J. Comput. Chem., 2007, 28, 2091.
2
3
4
5
(a) H. Nakao, H. Hayashi and K. Okita, Anal. Sci., 2001, 17, 545;
(
b) H. Gilman and E. A. Zuech, J. Org. Chem., 1961, 26, 2013.
H. Hayashi, H. Nakao, S. Onozawa, A. Adachi, T. Hayashi and
K. Okita, Polym. J., 2003, 35, 704.
H. Nakao, H. Hayashi, T. Yoshino, S. Sugiyama, K. Otobe and
T. Ohtani, Nano Lett., 2002, 2, 475.
Fig. 3 (a) Current density (J)–luminance–voltage curves of the device of
6
7
8
H. Hayashi and H. Nakao, Synth. Met., 2009, 159, 859.
H. Hideki and N. Hidenobu, Jpn. J. Appl. Phys., 2013, 52, 5D.
H. Nakao, H. Hayashi and K. Okita, Polym. J., 2001, 33, 498.
ITO/MoO
b) EL spectra under varied voltages, insets: the efficiencies–luminance
curves.
x
/TAPC/PhCzSi:FIrpic/TmPyPB/LiF/Al, inset: the device structure;
(
9 H. Nakao, H. Hayashi, F. Iwata, H. Karasawa, K. Hirano, S. Sugiyama
and T. Ohtani, Langmuir, 2005, 21, 7945.
10 J. Braddock-Wilking, J. Y. Corey, L. M. French, E. Choi, V. J. Speedie,
M. F. Rutherford, S. Yao, H. Xu and N. P. Rath, Organometallics,
doped in PhCzSi with the doping level of 10% was used as the
emitting layer; TAPC [1,1-bis{(di-4-tolylamino)phenyl} cyclohexane]
2
006, 25, 3974.
1
1 H. Gilman and D. Wittenberg, J. Am. Chem. Soc., 1957, 79, 6339.
and TmPyPB [1,3,5-tri(m-pyrid-3-ylphenyl)benzene] act as the hole- 12 J. Y. Corey, K. A. Trankler, J. Braddock-Wilking and N. P. Rath,
Organometallics, 2010, 29, 5708.
3 D. Wasserman, R. E. Jones, S. A. Robinson and J. D. Garber, J. Org.
Chem., 1965, 30, 3248.
14 W. Lu, J. Kuwabara, T. Iijima, H. Higashimura, H. Hayashi and
and electron-transporting materials, respectively. The preliminary
1
device results of the blue PhOLED showed the highest current
ꢀ1
ꢀ1
efficiency (CE) of 24.2 cd A , power efficiency (PE) of 20.3 lm W
,
T. Kanbara, Macromolecules, 2012, 45, 4128.
and external quantum efficiency (EQE) of 11.2% with a stable
electroluminescence (EL) spectrum under the various voltages
during the operation of the device (Fig. 3). The first attempt to
use phenazasilines as host materials for PhOLEDs is exciting to
afford a high EQE of the blue PhOLED device (over 10%) that is
1
5 (a) T. Ureshino, T. Yoshida, Y. Kuninobu and K. Takai, J. Am. Chem.
Soc., 2010, 132, 14324; (b) Y. Kuninobu, K. Yamauchi, N. Tamura,
T. Seiki and K. Takai, Angew. Chem., Int. Ed., 2013, 52, 1520.
1
6 (a) S. Chun To and F. Yee Kwong, Chem. Commun., 2011, 47, 5079;
(
b) S. Zhang, R. Chen, J. Yin, F. Liu, H. Jiang, N. Shi, Z. An, C. Ma,
B. Liu and W. Huang, Org. Lett., 2010, 12, 3438; (c) M. Yan, Y. Tao,
R. Chen, C. Zheng, Z. An and W. Huang, RSC Adv., 2012, 2, 7860.
7 R. Chen, Q. Fan, C. Zheng and W. Huang, Org. Lett., 2006, 8, 203.
8 S. Yagai, S. Okamura, Y. Nakano, M. Yamauchi, K. Kishikawa,
T. Karatsu, A. Kitamura, A. Ueno, D. Kuzuhara, H. Yamada,
T. Seki and H. Ito, Nat. Commun., 2014, 5, 4013.
9 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and
D. Ma, Angew. Chem., Int. Ed., 2008, 48, 8104.
0 (a) K. Tamao, M. Uchida, T. Izumizawa, K. Furukawa and S. Yamaguchi,
J. Am. Chem. Soc., 1996, 118, 11974; (b) S. Yamaguchi, C. Xu and
K. Tamao, J. Am. Chem. Soc., 2003, 125, 13662.
27
still achievable in a relatively small number of host materials,
1
1
suggesting bright future of the p-extended phenazasilines for
optical and electronic applications.
In summary, we have succeeded in extending the rhodium-
catalyzed Si–H/C–H coupling to nitrogen-containing substrates,
offering a new route for the highly efficient synthesis of phenaza-
silines, especially of the p-extended phenazasilines. The obtained
phenazasilines exhibit excellent solubility and unique optical and
electronic properties. For the first time, the preliminary blue
1
2
2
1 Z. An, C. Zheng, R. Chen, J. Yin, J. Xiao, H. Shi, Y. Tao, Y. Qian and
W. Huang, Chem. – Eur. J., 2012, 18, 15655.
PhOLED device based on phenazasilines as a host material was 22 J. He, H. Liu, Y. Dai, X. Ou, J. Wang, S. Tao, X. Zhang, P. Wang and
ꢀ
1
D. Ma, J. Phys. Chem. C, 2009, 113, 6761.
3 F. Wang, X. Li, W. Lai, Y. Chen, W. Huang and F. Wudl, Org. Lett.,
fabricated; the high device performance with high CE (24.2 cd A ),
2
ꢀ1
PE (20.3 lm W ) and EQE (11.2%) highlights the great potential of
the optoelectronically active phenazasilines in organic electronics.
This study was supported in part by National Natural
Science Foundation of China (21274065, 21304049, 21001065,
2014, 16, 2942.
24 C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale and G. C. Bazan, Adv.
Mater., 2011, 23, 2367.
5 Y. Matano, H. Ohkubo, Y. Honsho, A. Saito, S. Seki and H. Imahori,
2
Org. Lett., 2013, 15, 932.
6
1136003, and 51173081), The Ministry of Education of China 26 Y. Tao, J. Xiao, C. Zheng, Z. Zhang, M. Yan, R. Chen, X. Zhou, H. Li,
Z. An, Z. Wang, H. Xu and W. Huang, Angew. Chem., Int. Ed., 2013,
2, 10491.
(
(
No. IRT1148), Natural Science Foundation of Jiangsu Province
BK2011751, BM2012010), Natural Science Foundation of the
5
2
7 (a) K. S. Yook and J. Y. Lee, Adv. Mater., 2014, 26, 4218; (b) Y. Tao,
Jiangsu Higher Education Institutions of China (12KJB150017),
C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943.
Chem. Commun.
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