Efficient Synthesis of All-Aryl Phenazasilines for Optoelectronic Applications

Article abstract of DOI:10.1071/CH15652

An efficient metal-free radical-catalyzed intramolecular silylation method has been developed for the preparation of all-aryl phenazasilines, which can be hardly synthesized by traditional synthetic methods. The easily prepared aromatic and rigid phenazasiline exhibits excellent solubility, high thermal stability, and good optoelectronic properties, which are highly attractive for optical and electronic applications in organic electronics. These advances, in the preparation of all-aryl phenazasilines, offer exciting opportunities for sophisticated molecular design and efficient synthesis of optoelectronic molecules based on the phenazasiline.

Full text of DOI:10.1071/CH15652

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH15652
Efficient Synthesis of All-Aryl Phenazasilines for
Optoelectronic Applications
A
B
A
A C
,
B C
,
Shen Xu, Huanhuan Li, Yuting Tang, Runfeng Chen, Xiaoji Xie,
A
B
A B C
, ,
Xinhui Zhou, Guichuan Xing, and Wei Huang
A
Key Laboratory for Organic Electronics and Information Displays and Institute of
Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications
(NUPT), Nanjing 210023, China.
B
Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials
(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),
Nanjing Tech University, Nanjing 211816, China.
C
Corresponding authors. Email: iamrfchen@njupt.edu.cn; iamxjxie@njtech.edu.cn;
wei-huang@njupt.edu.cn
An efficient metal-free radical-catalyzed intramolecular silylation method has been developed for the preparation of
all-aryl phenazasilines, which can be hardly synthesized by traditional synthetic methods. The easily prepared aromatic
and rigid phenazasiline exhibits excellent solubility, high thermal stability, and good optoelectronic properties, which are
highly attractive for optical and electronic applications in organic electronics. These advances, in the preparation of
all-aryl phenazasilines, offer exciting opportunities for sophisticated molecular design and efficient synthesis of
optoelectronic molecules based on the phenazasiline.
Manuscript received: 16 October 2015.
Manuscript accepted: 1 February 2016.
Published online: 18 February 2016.
Introduction
phenazasilines with high thermal stability, one potential strategy
is to replace alkyl groups (e.g. methyl group) with aromatic
phenyl substituents on both Si and N atoms in phenazasilines.[8]
However, previously reported methods cannot afford the all-
alkyl phenazasilines.
Phenazasilines, which have a Si-bridged diphenylamine
framework containing both Si and N atoms in a fused six-
membered ring, are promising candidates for hole-transporting
materials, electrochromic molecules, and antioxidants.[1–3]
Traditionally, phenazasilines can be synthesized through
extended heating of diphenylsilane with phenothiazines or
cyclization reactions of 2,20-dilithiodiarylamine intermediates
with dichlorosilanes.[4–5] Unfortunately, these methods com-
monly require special precursors, which are difficult to prepare,
and need long synthetic routes that typically generate low yields.
These difficulties, therefore, significantly hinder the further
studies and applications of phenazasilines.
To facilitate the application of phenazasilines, concise and
efficient synthetic methods are highly desired. Recently, a
highly efficient method was developed for the preparation of
p-extended phenazasilines by rhodium-catalyzed double
activation of Si–H and C–H bonds.[6] This method can give
high yields up to 93 %. More importantly, the as-synthesized
p-extended phenazasilines exhibited improved optoelectronic
properties as indicated by the high external quantum efficiency
(up to 11 %) when applied as host materials in blue phosphorescent
organic light-emitting diodes (OLEDs).
In this study, we established a concise synthetic method for
the preparation of a newly designed all-aryl phenazasiline,
5,10,10-triphenyl-5,10-dihydrophenazasiline (DPhPz). It should
be noted that all-aryl phenazasilines can be hardly obtained by
using other synthetic methods such as the facile rhodium-
catalytic method. We reasoned that the successful synthesis
of the all-aryl phenazasiline proceeds via a radical-mediated
catalysis mechanism. Inspiringly, the as-prepared DPhPz shows
high thermal stability, with a decomposition temperature (Td)
higher than 3108C and a melting point (Tm) above 2008C,
however, with photophysical and electrochemical properties
similar to those of its alkyl counterpart, 10,10-dimethyl-5-
phenyl-5,10-dihydrophenazasiline (DMePz). The newly deve-
loped metal-free radical-catalytic method could pave the way
for the facile and efficient preparation of all-aryl phenazasilines,
which are highly attractive owning to their unique photophysical
properties for optoelectronic applications.
For practical applications of organic materials in opto-
electronic devices, molecules with high thermal stability are
generally required to form uniform and amorphous nanofilms
under vacuum thermal evaporation conditions.[7] To obtain
Results and Discussion
As depicted in Scheme 1a, the synthesis of DPhPz starts
from diphenylamine and 2-bromoiodobenzene in three steps.
Journal compilation Ó CSIRO 2016
www.publish.csiro.au/journals/ajc

B
S. Xu et al.
The detailed synthetic procedures and characterizations of the
related compounds were described in the Supplementary
Material and shown in Figs S1–S4. Notably, this method
employed a cheap and commercially available tert-butyl
hydroperoxide (TBHP) for Si–C bond formation and a small
amount of tetrabutylammonium iodide (TBAI) as an initiator.
The overall yield of DPhPz is 27 %. For the key cyclization
reaction in the silylation, the use of aromatic solvents of
benzene, chlorobenzene, and toluene produces relatively higher
yields of 54 %, 47 % and 41 %, respectively. Single-crystal
structure analysis (Scheme 1b and Table S1 in the Supplementary
Material) demonstrates that DPhPz has a rigid molecular struc-
ture with highly planar and p-conjugated diphenylphenazasiline
unit to serve as the main functional moiety for optoelectronic
properties.
rhodium-catalyzed approach. In fact, the rhodium-catalyzed
approach can generate DMePz in high yield (Scheme 2a).[6]
However, the attempts to extend this synthetic method to the
preparation ofDPhPzfailed, probablybecause ofthe large spatial
hindrance of diphenyl substituents towards the bulky rhodium
catalyst of RhCl(PPh3)3, which significantly blocks the catalytic
cycling reaction.[9–10] Meanwhile, inspired by the intramolecular
radical silylation of 2-diphenylsilylbiaryls via base-promoted
homolytic aromatic substitution for preparing a five-membered
Si-ring of 9-silafluorene,[11] we propose that the synthesis of DPhPz
follows a radical-catalyzed silylative cross-dehydrogenative
coupling mechanism (Scheme 2b). In the initiation step, an
electron transfers from the iodide of TBAI to TBHP, subse-
quently generating a hydroxyl anion along with a tert-butoxyl
radical. The resulting tert-butoxyl radical then abstracts the H
atom from silane A to give the corresponding Si-centred radical
B and cyclohexadienyl radical C. The acidic proton in C gets
deprotonated by the hydroxyl anion to give the radical anion D.
Finally, liberation of the electron from D produces target
product of DPhPz and the liberated electron then serves as the
catalyst for the next reaction cycle.
We then characterized the thermal stability of as-prepared
DPhPz. As expected, DPhPz, with a high molecular weight, has
a significantly improved thermal stability with Td up to 3138C
and Tm of 2188C (Fig. S6, Supplementary Material) when
compared with its alkyl counterpart, DMePz (Td ¼ 1918C and
Tm ¼ 1118C). These results confirm the success in designing
thermal stable phenazasilines via all-aryl substitution strategy.
On a separate note, similarly to DMePz, the all-aryl phenazasiline,
DPhPz, has good solubility in common solvents.
To elucidate the mechanism of the newly developed synthetic
method for the synthesis of all-aryl phenazasilines, we compared
our newly developed reaction with the previously reported
(a)
(b)
Br
I
n-BuLi, THF
NH
N
Cul, K2 CO3
Ph2HSiCl
Ϫ78ЊC to r.t.
Xylene, 120ЊC
Br
TBHP, TBAI
HSi
N
N
Si
PhH, 90ЊC, 24 h
In the following set of experiments, we studied the opto-
electronic properties of the stable all-aryl phenazasiline. Structur-
ally, DPhPz is formed by connecting the nearby two phenyls in
triphenylamine (TPA) with a silicon bridge, thus exhibiting
an extended molecular planarity and p-conjugation. Such a
p-system extension is evidenced by the emerging red-shifted
absorption bands in both dilute DCM and thin solid film
(Fig. 1a, b) when compared with that of TPA. The molar
absorptivity of DPhPz can be up to 104 Mꢀ1 cmꢀ1 (Fig. S9,
DPhPz
N
Si
Scheme 1. (a) Synthetic route and (b) single-crystal structure of DPhPz
(CCDC 1431510).
(a)
(b)
TBAI
Ph
Ph
Me
Me
Si
Si
N
N
t-BuOOH
N
SiHMe2
Ph
Ph
eϪ
Rh
HSi
N
t-BuO•
DMePz
DPhPz
OHϪ
A
SiMe2
Rh
N
Ph
Ph
N
Si
t-BuOH
Rh
D
N
Si
Me2
H
ϪH2
Ph
Ph
SiMe2
N
Si
N
H
Rh
H2O
H
Ph
Ph
N
Si
H
B
C
Rhodium-catalyzed Si–H/C–H coupling
Metal-free radical silylation
Scheme 2. Proposed mechanisms for (a) rhodium-catalyzed synthesis of DMePz and (b) radical-catalyzed synthesis of DPhPz.

All-Aryl Phenazasilines
C
Supplementary Material). Due to the high rigid molecular struc-
ture, the absorption and emission peaks of DPhPz in solution are
quite close with very small Stokes shifts (,15 nm); these features
are very similar to those of DMePz.[12] The enhanced UV
absorption around 310–340 nm of DMePz in solid film may be
due to the smaller spatial hindrance of methyl substitutions when
compared with that of the bulky phenyl groups in DPhPz, leading
tostronger intermolecular interactions inDMePzfilm. Contrary to
the sharp and slightly shifted photoluminescence (PL) spectra in
solution, broad PL spectra, with additional emission bands, were
observed in the thin solid film state. The red-shifted broad PL
spectra in the film state are probably due to the strong molecular
interactions of the rigid phenazasilines in aggregated states with
apparent excimer emission.[13–15] Particularly, in the all-aryl
phenazasiline film formed by DPhPz, the intermolecular interac-
tions are even stronger than those in TPA and DMePz films,
leading to a more suppressed single molecular emission at
,360 nm. Taken together, the UV-visible absorption and PL
spectra of DMePz and DPhPz are quite similar, suggesting
the limited influence of peripheral aromatic substituents on the
electronic structure of the rigid phenazasilines (Table 1).
wave of DMePz (0.67 V) and DPhPz (0.76 V) (Fig. S7, Sup-
plementary Material), the HOMO energy levels were deter-
mined to be at –5.43 and –5.55 eV, correspondingly, on the basis
of the reference energy level of ferrocene under identical CV
conditions. Taking into account the optical band gaps (optEg)
from the onset absorption spectra in the film state, the LUMO
energy levels can be determined. As listed in Table 1, the
HOMO and LUMO of DPhPz are very close to those of DMePz,
with a small variation of smaller than 0.1 eV, indicating the
limited effects of peripheral substituents of Si on the electro-
chemical properties of phenazasilines.
Density functional theory (DFT) calculations were further
performed to understand the optoelectronic properties of the
obtained all-aryl phenazasiline theoretically.[17,18] Compared
with DMePz, DPhPz exhibits slightly lower predicted HOMO
and LUMO energy levels, which is well in line with the
experimental results (Fig. 1c). The DFT calculations also reveal
that the peripheral phenyl substituents of Si have a very small
contribution to the HOMO and LUMO isosurfaces, offering
theoretical insights on the intrinsic mechanism for their limited
influence on absorption and PL spectra as well as frontier orbital
energy levels. In addition, the DFT-predicted triplet energies of
DPhPz and DMePz are also very similar, with high values of
.3.1 eV (Table 1). Such a high triplet energy of DPhPz with
Subsequently, the electrochemical properties of the optically
active all-aryl phenazasiline were investigated by cyclic
voltammetry (CV).[16] From the onset voltages of the oxidation
(a)
1.0
(c)
1.0
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
TPA
Ϫ0.43
4.59
DMePz
Ϫ0.53
DPhPz
Ϫ1.83
Ϫ1.91
4.56
(b)
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
3.60
3.64
Ϫ5.02
Ϫ5.09
Ϫ5.43
Ϫ5.55
300
350
400
450
500
550
DMePz
DPhPz
Wavelength [nm]
Fig. 1. UV-visible absorption (closed markers) and photoluminescence (opened markers, excited at 300 nm) spectra of TPA,
DMePz, and DPhPz measured in (a) DCM (1.3 ꢁ 10ꢀ6 mol Lꢀ1) and (b) in thin film. (c) DFT-calculated (in black) and
experimentally determined (in red) HOMO, LUMO, and band gap (Eg) energy levels (in eV) as well as the frontier orbital
isosurfaces of DMePz and DPhPz.
Table 1. Physical, optical, and electrochemical properties of DMePz and DPhPz
calEg, calculated bandgap; calET, calculated lowest triplet energy level
Comp. Td/Tm [8C]
l
abs [nm]
optEg [eV]
l
em [nm]
CV [eV]
DFT [eV]
HOMO LUMO calEg calET
DCM
Film
DCM
Film
HOMO LUMO
DMePz
DPhPz
191/111
313/218
290, 312, 335 290, 325, 339
291, 316, 341 296, 329, 341
3.60
3.64
346
356
363, 429
360, 438
ꢀ5.43
ꢀ5.55
ꢀ1.83
ꢀ1.91
ꢀ5.02
ꢀ5.09
ꢀ0.43
ꢀ0.53
4.59 3.15
4.56 3.13

D
S. Xu et al.
significantly improved thermal stability indicates the great
potential of the all-aryl phenazasiline as high-performance host
materials for blue phosphorescent OLEDs.[19–21]
Supplementary Material
Experimental details and H NMR and 13C NMR spectra of
DPhPz are available on the Journal’s website.
1
Conclusion
Acknowledgements
In summary, an efficient transition-metal-free intramolecular
radical silylation method has been developed, for the first time, to
prepare all-aryl phenazasilines with excellent optoelectronic
properties. The novel synthetic process is important for overcoming
the difficulties of rhodium-catalyzed Si–H/C–H coupling in the
synthesis of spatially hindered phenazasilines. Compared with
alkyl phenazasiline of DMePz, the obtained all-aryl phenazasiline,
DPhPz, shows excellent solubility and significantly improved
thermal stability with almost identical optical and electronic
properties, highlighting the bright future of optoelectronically
active all-aryl phenazasilines in organic electronics.
This study was supported in part by the National Natural Science Foundation
of China (21274065, 21304049, 21001065, and 61136003), Qing Lan
project of Jiangsu province, and Science Fund for Distinguished Young
Scholars of Jiangsu Province of China (BK20150041).
References
[1] H. Hayashi, H. Nakao, S. Onozawa, A. Adachi, T. Hayashi, K. Okita,
Polym. J. 2003, 35, 704. doi:10.1295/POLYMJ.35.704
[2] H. Hayashi, H. Nakao, Synth. Met. 2009, 159, 859. doi:10.1016/
J.SYNTHMET.2009.01.039
[3] H. Hayashi, H. Nakao, T. Miyabayashi, M. Murase, Jpn. J. Appl. Phys.
2013, 52, 05DA13. doi:10.7567/JJAP.52.05DA13
[4] H. Gilman, D. Wittenberg, J. Am. Chem. Soc. 1957, 79, 6339.
doi:10.1021/JA01580A060
Experimental
General Procedure for the Preparation of 5,10,10-Triphenyl-
5,10-dihydrophenazasiline (DPhPz)
[5] (a) J. Y. Corey, K. A. Trankler, J. Braddock-Wilking, N. P. Rath,
Organometallics 2010, 29, 5708. doi:10.1021/OM100544F
(b) D. Wasserman, R. E. Jones, S. A. Robinson, J. D. Garber, J. Org.
Chem. 1965, 30, 3248. doi:10.1021/JO01020A534
2-Bromo-N,N-diphenylaniline
A mixture of diphenylamine (10 g, 59 mmol), 2-bromoiodo-
benzene (11.43 mL, 89 mmol), copper(I) iodide (7.88 g,
41 mmol), and anhydrous potassium carbonate (16.34 g,
118 mmol) in xylene (40 mL) were heated at 1208C for 2 days,
followed by cooling to room temperature. The resulting reaction
mixture was then poured into deionized water to quench the
reaction and extracted with DCM for three times. The organic
phase was collected, dried, and concentrated under vacuum. The
resulting residue was further purified by flash column chroma-
tography on silica gel to afford 2-bromo-N,N-diphenylaniline as
a white solid.
[6] H. Li, Y. Wang, K. Yuan, Y. Tao, R. Chen, C. Zheng, X. Zhou, J. Li,
W. Huang, Chem. Commun. 2014, 50, 15760. doi:10.1039/
C4CC06636G
[7] K. S. Yook, J. Y. Lee, Adv. Mater. 2014, 26, 4218. doi:10.1002/
ADMA.201306266
[8] K. Y. Sandhya, C. K. S. Pillai, M. Sato, N. Tsutsumi, J. Polym. Sci.,
Part A: Polym. Chem. 2003, 41, 1527. doi:10.1002/POLA.10695
[9] T. Ureshino, T. Yoshida, Y. Kuninobu, K. Takai, J. Am. Chem. Soc.
2010, 132, 14324. doi:10.1021/JA107698P
[10] S. Furukawa, J. Kobayashi, T. Kawashima, J. Am. Chem. Soc. 2009,
131, 14192. doi:10.1021/JA906566R
[11] D. Leifert, A. Studer, Org. Lett. 2015, 17, 386. doi:10.1021/
OL503574K
2-(Diphenylsilyl)-N,N-diphenylaniline
[12] J. J. Michels, M. J. O’Connell, P. N. Taylor, J. S. Wilson, F. Cacialli,
H. L. Anderson, Chem. – Eur. J. 2003, 9, 6167. doi:10.1002/CHEM.
200305245
[13] Z. An, C. Zheng, Y. Tao, R. Chen, H. Shi, T. Chen, Z. Wang, H. Li,
R. Deng, X. Liu, W. Huang, Nat. Mater. 2015, 14, 685. doi:10.1038/
NMAT4259
[14] Z. An, C. Zheng, R. Chen, J. Yin, J. Xiao, H. Shi, Y. Tao, Y. Qian,
W. Huang, Chem. – Eur. J. 2012, 18, 15655. doi:10.1002/CHEM.
201202337
[15] Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361.
doi:10.1039/C1CS15113D
To a freshly distilled THF solution of 2-bromo-N,N-
diphenylaniline (1 g, 3.1 mmol) at –788C, a hexane solution of
n-butyllithium (1.86 mL, 4.65 mmol) was added dropwise.[6]
The resulting mixture was stirred at –788C for 1.5 h, followed by
the addition of chlorodiphenylsilane (3.35 mL, 15.5 mmol). The
reaction was subsequently kept at –788C for 0.5 h and at room
temperature for 12 h, quenched with water, and extracted with
DCM. The organic layer was collected and dried, and the solvent
was removed under vacuum. The resulting crude product was
purified by column chromatography on silica gel to give a white
solid.
[16] Y. Tao, J. Xiao, C. Zheng, Z. Zhang, M. Yan, R. Chen, X. Zhou, H. Li,
Z. An, Z. Wang, H. Xu, W. Huang, Angew. Chem., Int. Ed. 2013, 52,
10491. doi:10.1002/ANIE.201304540
[17] T. Chen, L. Zheng, J. Yuan, Z. An, R. Chen, Y. Tao, H. Li, X. Xie,
W. Huang, Sci. Rep. 2015, 5, 10923. doi:10.1038/SREP10923
[18] F. May, M. Al-Helwi, B. Baumeier, W. Kowalsky, E. Fuchs,
C. Lennartz, D. Andrienko, J. Am. Chem. Soc. 2012, 134, 13818.
doi:10.1021/JA305310R
[19] J. Yin, S. Zhang, R. Chen, Q. Ling, W. Huang, Phys. Chem. Chem.
Phys. 2010, 12, 15448. doi:10.1039/C0CP00132E
[20] Y. Tao, X. Guo, L. Hao, R. Chen, H. Li, Y. Chen, X. Zhang, W. Lai,
W. Huang, Adv.Mater. 2015, 27, 6939. doi:10.1002/ADMA.201503108
[21] Y. Tao, C. Yang, J. Qin, Chem. Soc. Rev. 2011, 40, 2943. doi:10.1039/
C0CS00160K
5,10,10-Triphenyl-5,10-dihydrophenazasiline (DPhPz)
A mixture of 2-(diphenylsilyl)-N,N-diphenylaniline (0.2 g,
0.47 mmol), tert-butyl hydroperoxide (0.28 mL, 1.55 mmol),
and tetrabutylammonium iodide (1.7 mg, 0.0047 mmol) dis-
solved in benzene (2 mL) were stirred at 908C for 24 h.[11] After
cooling to room temperature, the reaction mixture was extracted
with DCM for three times. The organic layer was collected and
dried. After removing the solvent, the crude product was
purified by column chromatography on silica gel to afford
DPhPz as a white solid.