Optical properties of disilane-bridged donor - Acceptor architectures: Strong effect of substituents on fluorescence and nonlinear optical properties

Article abstract of DOI:10.1021/ja511177e

A series of disilane-bridged donor-acceptor architectures 1-9 containing strong electron-donating and -withdrawing substituents were designed and synthesized in acceptable yields. The substituents substantially affected the fluorescence and nonlinear optical properties of the compounds. In the solid state, the compounds showed purple-blue fluorescence (λ_em = 360-420 nm) with high quantum yields (up to 0.81). Compound 3, which had p-N,N-dimethylamino and o-cyano substituents, exhibited optical second harmonic generation (activity 2.9 times that of urea, calculated molecular hyperpolarizability β = 1.6 × 10^-30 esu) in the powder state. Density functional theory calculations for the ground and excited states indicated that both the locally excited state and the intramolecular charge transfer excited state make important contributions to the luminescence behavior.

Full text of DOI:10.1021/ja511177e

Communication
pubs.acs.org/JACS
Optical Properties of Disilane-Bridged Donor−Acceptor
Architectures: Strong Effect of Substituents on Fluorescence and
Nonlinear Optical Properties
Masaki Shimada,† Yoshinori Yamanoi,*,† Tomonori Matsushita,‡ Takashi Kondo,‡ Eiji Nishibori,§
Akari Hatakeyama,∥ Kunihisa Sugimoto,⊥ and Hiroshi Nishihara*,†
†Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan
§Division of Physics, Faculty of Pure and Applied Sciences, Tsukuba Research Center for Interdisciplinary Materials Science (TIMS),
and Center for Integrated Research in Fundamental Science and Engineering (CiRfSE), University of Tsukuba, 1-1-1 Tennodai,
Tsukuba, Ibaraki 305-8571, Japan
∥Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan
⊥Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
S
* Supporting Information
because of their chemical versatility, which allows easy
modification of the optical parameters through the introduction
of donor−acceptor (D−A) substituents.
In the course of our study on the synthetic use of hydrosilanes,
we described a variety of examples of transition-metal-catalyzed
arylation of hydrosilanes with aryl iodides.6 A convenient
synthesis of disilane-bridged D−A compounds 1−9 is reported
in Scheme 1. The introduction of suitable substituents
ABSTRACT: A series of disilane-bridged donor−acceptor
architectures 1−9 containing strong electron-donating and
-withdrawing substituents were designed and synthesized
in acceptable yields. The substituents substantially affected
the fluorescence and nonlinear optical properties of the
compounds. In the solid state, the compounds showed
purple−blue fluorescence (λem = 360−420 nm) with high
quantum yields (up to 0.81). Compound 3, which had p-
N,N-dimethylamino and o-cyano substituents, exhibited
optical second harmonic generation (activity 2.9 times that
of urea, calculated molecular hyperpolarizability β = 1.6 ×
10−30 esu) in the powder state. Density functional theory
calculations for the ground and excited states indicated
that both the locally excited state and the intramolecular
charge transfer excited state make important contributions
to the luminescence behavior.
Scheme 1. Molecular Structures of the Unsymmetrical 1,2-
Diaryldisilane Derivatives Containing Electron-Donating and
-Withdrawing Groups Used in This Study
n recent years, push−pull chromophores have been
Iextensively studied because of their promise as light-emitting
and nonlinear optical (NLO) materials.1 A typical organic push−
pull chromophore consists of an electron donor and electron
acceptor connected by a π-conjugated spacer.2 To show good
photophysical properties, the compounds must absorb in the
visible region. The development of bright organic emitters and
nonlinear optical devices with transparency remains a synthetic
challenge. Cunjugation of Si−Si σ orbital and aryl π orbital is less
well studied, although materials with σ−π conjugation display
photoluminescence (PL) or electron-transporting properties.
Mignani,3 Hadziioannou and van Hutten,4 and Hiratsuka5 have
independently described the synthesis of unsymmetrical
disilanes; however, no systematic study of the relationship
between the chemical structures and physical properties of these
compounds has been published. Systematically designed
disilanes with electron-donating and electron-accepting sub-
stituents are suitable models for studying push−pull chemistry
significantly altered the photophysical properties and enhanced
the PL quantum yield. Importantly, unsymmetrical disilane 3,
which absorbed solely in the UV region, displayed nonlinear
optical properties. Computational studies were also performed to
support and provide further insight into the experimental
findings.
The compounds contained a donor group on one disilane
silicon atom and an acceptor group on the other. The synthesis of
compounds 1−9 involved Pd-catalyzed cross-coupling reactions
of hydrosilanes as a key step, which were achieved using our
previously reported method (see the Supporting Information
(SI)). Si−Si bond cleavage did not occur under the mild Pd-
Received: October 30, 2014
© XXXX American Chemical Society
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Table 1. Photophysical Data for Disilane-Bridged Donor−Acceptor Compounds 1−9
a
in CH2Cl2
in the solid state
em
ε
λex
λ
SHG efficiency
max
b
c
d
e
c
d
f
abs
em
compd
λ
max
(nm)
g
(104 M−1cm−1
)
λ
(nm)
max
Φ
τ (ns)
(nm)
(nm)
Φ
τ (ns)
h
(vs urea)
h
i
1
2
3
4
5
6
7
8
9
273, 304
276, 306
275, 305
2.37, 0.78
2.61, 0.51
2.30, 0.62
503
488
501
0.13
0.06
0.10
29.1
37.7
26.3
331
331
362
420
360
411
0.05
0.14
0.13
13.9 (85), 49.4 (15)
−
g
g
i
3.7
3.9
−
2.9
g
g
j
j
j
j
j
j
j
i
276, 306, 347
3.11, 1.14, 0.30
2.79, 0.84
2.47, 1.17
2.62, 1.45
2.12, 1.26
2.53, 1.66
−
−
−
−
−
−
−
−
g
k
i
272, 300
348, 511
0.02
0.21
0.18
0.21
0.34
3.3
3.9
3.4
3.6
2.8
335
421
0.13
13.2
−
l
l
l
l
l
236, 271
238, 270
462
−
−
−
−
−
i
471
292
297
302
401
396
395
0.53
0.68
0.81
1.7
2.1
2.0
−
g
i
240, 268
464
−
g
i
244, 268
465
−
a
b
c
Measured in anhydrous degassed dichloromethane. Molar extinction coefficient. Absolute quantum yields determined using an integrating sphere
d
e
system. Fluorescence lifetimes detected at the maximum fluorescence wavelengths. Excitation wavelength in the solid state determined by
f
g
h
excitation spectra. Measured by the powder method using a Q-switched Nd3+:YAG laser (1064 nm). Shoulder peak. The number in parentheses is
i
j
k
l
the amplitude contribution (%). Not active. Fluorescence was below the detection limit. Detected at 511 nm. Colorless oil.
catalyzed reaction conditions.7 The analytically pure compounds
were isolated in up to 41% yield and were fully characterized by
1H and 13C NMR spectroscopy and high-resolution mass
spectrometry or elemental analysis.
The optical properties of 1−9 were investigated by UV−vis
and fluorescence spectroscopy (Table 1 and Figures S1−S9 in
the SI). These compounds showed absorption bands at high
energies (λmax ≈ 275 nm for 1−5 and 240 nm for 6−9) and weak
absorption bands at low energies (λmax ≈ 305 nm for 1−5 and
270 nm for 6−9) in dichloromethane. The difference between
the absorption wavelengths of N,N-dimethylaminophenyl (1−5)
and methoxyphenyl or phenoxyphenyl (6−9) derivatives
resulted from the donor properties.
Figure 1. Photographs of (top) the appearance of 1−5 and 7−9 and
(bottom) their photoluminescence in the solid state upon illumination
with a 365 nm (1−5) or 254 nm (7−9) UV lamp.
4-phenoxy groups, which effectively suppress the intermolecular
π−π stacking in the solid state.
To examine the photoemission dynamics, the fluorescence
lifetimes (τ) of the compounds were measured. They depended
on the donor and acceptor moieties (Table 1): the lifetimes
achieved using the various terminal donor moieties were ranked
in the order phenoxy ≤ methoxy < dimethylamino, while for the
acceptor moieties, the ester group led to shorter lifetimes than
the nitrile group. The τ values in the solid state were shorter than
those in dichloromethane, except in the case of 5. The
fluorescence rate constant (kf) and the nonradiative rate constant
(knr) (Tables S7 and S8) observed for 1−3 and 5 were
substantially smaller than those for 6−9 because of the stability of
the radical cation (Me2N+•) in the charge-separated state.11
To investigate the effect of the two terminal aromatic rings on
the photophysical properties, the molecular orbitals (MOs) and
transition states of 5, 7, and 9 were calculated using density
functional theory (DFT) and time-dependent DFT (TD-DFT).
Their representative frontier MOs and their energy levels are
shown in Figures 2 and S15. By comparison with the transitions
of the three compounds, the lowest-energy absorption band for 7
(270 nm) and 9 (268 nm) was assigned to the HOMO →
LUMO transition. The fluorescence of 7 and 9 is assignable to
All of the PL spectra had a broad peak at around 480 nm in
dichloromethane, except for that of 4. The absorption spectra did
not depend strongly on the solvent used, whereas the emission
maxima changed significantly depending upon the solvent. The
UV−vis absorption and PL spectra of representative disilane 3 in
cyclohexane, dichloromethane, and acetonitrile are shown in
Figure S10 and Table S5. Compound 3 showed dual fluorescence
spectra with peaks around 350 nm and above 400 nm. The
fluorescence peaks of 3 at around 350 nm were not shifted
depending on the solvent polarity, whereas those above 400 nm
showed a large red shift as the solvent polarity was increased. The
emissions located at 350 nm and 400−600 nm were assigned to
the locally excited (LE) emission and the intramolecular charge
transfer (ICT) excited one, respectively.8 The slope of the
Lippert−Mataga plot was used to estimate the difference
between the excited-state and ground-state dipole moments
(μe − μg) of 3 as 20.3 D (Table S6 and Figure S11).9,10
Fluorescence quantum yields (Φ) are shown in Table 1. The
quantum yield depended on the terminal donor group and was
independent of the acceptor group. The emission quantum yields
of 1−5 (Φ = 0.02−0.13) were lower than those of 6−9 (Φ =
0.18−0.34). Subsequent investigation of the fluorescence
properties of these compounds in the solid state revealed blue
fluorescence (Figure 1) over a broad, unstructured emission
profile with maxima at 360−420 nm. The emission maxima in the
solid state were blue-shifted relative to those in the dichloro-
methane solutions as a result of the rigid medium. Attaching a 4-
phenoxy donor group dramatically improved Φ. Compound 9 in
the solid state exhibited a maximum emission band at 395 nm
with a high Φ value of 0.81. This may be attributed to the twisted
Figure 2. Selected frontier MOs for 5 calculated using DFT at the
B3LYP/6-31G(d,p) level.
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ICT emission via a charge-separated state, as shown by Hiratsuka
et al.5 (Tables S10 and S11). The calculated excited states of 5
indicated the HOMO → LUMO transition (the lowest excited
state; 300 nm) and a combination of the HOMO → LUMO+1
(minor) and HOMO → LUMO+2 (major) transitions (second
lowest excited state; 272 nm) (Table S9). Because the HOMO
→ LUMO+2 transition can be assigned to a π(D−A) →
π*(D*−A) excitation on the donor aryl group, compound 5 has
fluorescence of the D*−A state to the D−A state assignable to
LE emission in addition to ICT emission (Figures 2 and S14).
Thus, compound 5 has two fluorescent states and displays dual
fluorescence, whereas 7 and 9 have only ICT emission.12
Finally, we investigated the second harmonic generation
(SHG) efficiency of compounds 1−9 with the powder method
(Figure S16).13 This method, which uses a Q-switched
Nd3+:YAG laser (1064 nm), is one of the most convenient
methods for screening powdered materials for NLO activity. The
SHG efficiency was determined by comparison with a reference
compound,14 and the results are summarized in Table 1. Only
compound 3 exhibited significant powder SHG efficiency, which
was 2.9 times the intensity of urea. The molecular hyper-
polarizability (β) which evaluates the second-order NLO
efficiency, can be predicted from two-level models according to
the literature,1h,15,16 and that of 3 was estimated to be 1.6 × 10−30
esu, 5.0 times higher than that of urea (0.32 × 10−30 esu).17 This
result is roughly consistent with the experimental data.
symmetric crystal structure, which has D−A dipole moments
since all of the molecules are oriented in the same direction.
Meanwhile, the crystal structure of 4 shows that the two aryl
groups are parallel and staggered in a steplike conformation. The
torsion angles of the C6−Si1−Si2−C4 moiety and the Si−Si
bond/p-nitrophenyl group were ca. 178° and ca. 90°, respectively
(Table S4). The alternate directional packing ensures that the
D−A dipoles avoid parallel alignments, minimizing intermo-
lecular repulsion. Thus, these differences in the crystal packing of
3 and 4 reflect the difference in SHG activity.
In conclusion, we synthesized a series of structurally well-
defined unsymmetrical disilanes 1−9 via Pd-catalyzed arylation
of hydrosilanes with aryl iodides as a key step. The light-emitting
efficiencies were improved considerably, with quantum yields of
up to 0.34 in dichloromethane and 0.81 in the solid state. DFT
and TD-DFT calculations supported the experimental data and
provided a deeper understanding of the tuning of the
photophysical properties of asymmetric disilanes.
Moreover, disilane 3 powder showed a second-order nonlinear
optical response 2.9 times that of urea. The β value was calculated
to be 1.6 × 10−30 esu, which is 5.0 times that of urea. The results
reveal that the substituents of the donor and acceptor moieties in
NLO chromophores dramatically affect their NLO properties in
a properly designed system. Furthermore, these asymmetric
disilanes are interesting building blocks for constructing linear
and nonlinear optoelectronic materials.
The magnitude of the SHG signal obtained by the powder test
is mainly governed by crystallographic factors. Single crystals of 3
(SHG-active) and 4 (SHG-inactive) for X-ray diffraction analysis
were obtained by recrystallization from chloroform and
acetonitrile, respectively.18 The molecular and packing structures
are shown in Figure 3. Compound 3 crystallizes in the non-
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, crystallographic data for 3 and 4 (CIF),
1
spectroscopic data, copies of H and 13C NMR spectra for the
reported compounds, and additional spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*yamanoi@chem.s.u-tokyo.ac.jp
*nisihara@chem.s.u-tokyo.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Ms. Kimiyo Saeki and Dr. Aiko Kamitsubo at the
Elemental Analysis Center of The University of Tokyo for
performing elemental analysis. The work was financially
supported by Grants-in-Aid for Scientific Research (C) from
the Ministry of Education, Culture, Sports, Science, and
Technology, Japan, and a Kurata Grant awarded by the Kurata
Memorial Hitachi Science and Technology Foundation. The
synchrotron radiation experiments were performed at SPring-8
with the approval of the JASRI (Proposal 2014B0078).
Figure 3. Crystal structures of 3 and 4. (a) Molecular structure of 3. (b)
Packing structure of 3. (c) Molecular structure of 4. (d) Packing
structure of 4. Thermal ellipsoids are drawn at the 50% probability level,
and H atoms have been omitted for clarity.
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D
DOI: 10.1021/ja511177e
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