Synthesis of group 14 dipyridinometalloles with enhanced electron-deficient properties and solid-state phosphorescence

Article abstract of DOI:10.1021/om401019b

Silicon- and germanium-bridged bipyridyls were prepared, and their optical and electrochemical properties were investigated. It was found that they exhibited enhanced electron deficiency and phosphorescence properties in comparison to parent bipyridyl. The single-crystal structure of a dipyridinosilole and results of DFT calculations on models are also described.

Full text of DOI:10.1021/om401019b

Article
pubs.acs.org/Organometallics
Synthesis of Group 14 Dipyridinometalloles with Enhanced Electron-
Deficient Properties and Solid-State Phosphorescence
Joji Ohshita,*,† Kazuya Murakami,† Daiki Tanaka,† Yousuke Ooyama,† Tomonobu Mizumo,†
Norifumi Kobayashi,‡ Hideyuki Higashimura,‡ Takayuki Nakanishi,§ and Yasuchika Hasegawa§
†Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
‡Advanced Materials Research Laboratories, Sumitomo Chemical Co. Ltd., 6 Kitahara, Tsukuba 300-3294, Japan
§Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
S
* Supporting Information
ABSTRACT: Silicon- and germanium-bridged bipyridyls were pre-
pared, and their optical and electrochemical properties were
investigated. It was found that they exhibited enhanced electron
deficiency and phosphorescence properties in comparison to parent
bipyridyl. The single-crystal structure of a dipyridinosilole and results of
DFT calculations on models are also described.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Three dipyridinometalloles (DPyS1, DPyS2, and DPyG1) were
Silole (silacyclopentadiene) derivatives have received a great
deal of attention, because of their interesting electronic states
and functionalities.1 In the silole system, an in-phase interaction
between silicon σ* and butadiene π* orbitals (σ*−π*
interaction) lowers the LUMO energy level, giving rise to
high electron affinity and enhanced conjugation of the
compounds.2 In 1998, we reported the synthesis of
dithienosiloles (DTSs) as the first examples of siloles
condensed with heteroaromatic systems, which also possessed
low-lying LUMOs.3 Recently, we prepared also dithienoger-
moles (DTGs) as the germanium analogues of DTS.4
Currently, DTS and DTG are widely used as the building
units of functional π-conjugated polymers and oligomers, which
are applicable to active materials of organic devices, including
organic transistors5 and photovoltaic cells,4,6 and highly
emissive materials.7 However, to date much less attention has
been focused on group 14 metalloles condensed with other
heteroaromatic systems, except for some pyrrole- and indole-
condensed siloles.8 Bipyridyl is known as a phosphorescent and
highly electron deficient material, and therefore bipyridyls
condensed with a group 14 metallole system promise to be
exciting compounds. In this paper, we report the synthesis of
dipyridinosilole and dipyridinogermole (DPyS and DPyG) for
the first time, which exhibit enhanced electron affinity and
interesting solid-state phosphorescence.
prepared as shown in Scheme 1.9 The reaction of 3,3′-dilithio-
a
Scheme 1. Synthesis of Group 14 Dipyridinometalloles
a
Numbering is used for NMR assignments (see Experimental
Section)
4,4′-bipyridyl with dichlorodiphenylsilane gave 1,1-diphenyldi-
pyridinosilole (DPyS1) in 11% yield, together with many
unidentified products, including nonvolatile substances. Bis-
(methoxyphenyl)dipyridinosilole (DPyS2) was also prepared in
a similar fashion in 10% yield. In contrast to the synthesis of
DPyS, a similar reaction with dichlorodiphenylgermane
proceeded smoothly to provide 1,1-diphenyldipyridinogermole
(DPyG1) in 71% yield. Using difluorodiphenylsilane instead of
dichlorodiphenylsilane afforded DPyS1 in slightly higher yield
Received: October 18, 2013
© XXXX American Chemical Society
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(13%). These dipyridinometalloles are light brown solids, and
repetitive visual observations indicated that they melted without
decomposition. The thermal stability of DPyG1 was also
confirmed by repetitive DSC analysis (see Figure S-7 in the
Supporting Information). The crystal structure of compound
DPyS2 was determined by an X-ray diffraction study, and the
molecular structure and packing are depicted in Figure 1. The
Figure 2. UV spectra of dipyridinometalloles in chloroform.
idinophosphole oxide (DPyPO, λmax 276 nm) reported by
Durben and Baumgartner (Chart 1).9 The absorption maxima
Chart 1. 4,4′-Bipyridyl and Phenyldipyridinophosphole
Oxide
of DPyS1 and DPyS2 were at lower energies in comparison to
that of DPyG1. The effects of bridging units pronouncing the
conjugation of bipyridyl derivatives thus seem to be enhanced
in the order diphenylgermane < diarylsilane < phenyl-
phosphorus oxide. Table 2 represents the results of quantum
chemical calculations at the B3LYP/6-31G(d,p) level of theory
on nonsubstituted model compounds (DPyS3 and DPyG3 in
Figure 3). For comparison, we also examined BPy molecules in
either a fully optimized or planarly fixed structure, and the
results are also summarized in Table 2. All the computed
molecules possess an n-type HOMO and a π-type LUMO, and
the TD-DFT calculations indicated that the absorption maxima
are assignable to π-type transitions from the HOMO-1 or
HOMO-2 to the LUMO. Planar BPy exhibited a π−π* band
gap smaller than that of the optimized species with a twisted
structure, while introduction of a silicon or germanium bridge
resulted in an even smaller band gap. Destabilization of the
highest occupied π orbital (HOMO-1 or HOMO-2) by
introduction of a bridge is likely due to inductive electron
donation from the silicon and germanium atoms. For the
LUMO, an in-phase σ*−π* interaction is clearly seen to
stabilize it, as observed for DTS, DTG, and other silole
derivatives (Figure 2 for DPyS3).2−4 DPyG3 has a π−π* band
gap slightly larger than that of DPyS3, in accordance with the
optical data (Table 2). Cyclic voltammograms of the
dipyridinometalloles were examined in acetonitrile containing
TBAP (tetrabutylammonium perchlorate) as a supporting
electrolyte. As shown in Figure 4, DPyG1 revealed pseudo-
reversible reduction properties with a suppressed oxidation
counterpart by approximately 70% in the peak height. For
DPyS1 and DPyS2, rather weak reduction peaks appeared. The
reduction peaks of the metalloles were at potentials higher than
that of BPy (Epc = −2.09 V vs Fc/Fc+), in accordance with the
DFT calculations (Table 1), although the differences were only
0.03−0.15 V. They were at potentials lower than that of
DPyPO by approximately 0.1−0.2 V.9
Figure 1. Molecular (a), packing (b), and chain (c) structures of
DPyS2 with thermal ellipsoids at the 50% probability level.
tricyclic core of DPyS2 is highly planar (vide infra), and the
bond distances and angles of the silole ring system closely
resemble those reported for 1,1-dimethyldibenzosilole
(DBS),10 but DPyS2 shows an endocyclic C−Si−C angle
(90.69(6)°) slightly smaller than those of DBS (91.05(14) and
91.21(14)°), likely due to the effects of sterically large
methoxyphenyl substituents and/or the packing situation.
Other silole endocyclic bond angles are 109.55(9) and
109.77(9)° for Si−CC and 115.07(11) and 114.83(12)°,
respectively. The sum of the endocyclic angles is 539.91°, and
the silole CC−CC dihedral angle is only 0.45°, indicative
of high planarity of the silole ring. The crystal packing exhibits
an intermolecular face to face interaction between the pyridine
rings with the shortest contact of 3.608 Å. In addition, an
intermolecular pyridine CH- - -N interaction of 2.595 Å is also
observed, thus forming DPyS chains, as shown in Figure 1c.
Absorption maxima of the present dipyridinometalloles were
observed at approximately 270 nm (Table 1 and Figure 2), red-
shifted from that of 4,4′-bipyridyl (BPy; λmax 237, 270
(shoulder) nm) but slightly blue-shifted from phenyldipyr-
Table 1. Optical and Electrochemical Properties of
Dipyridinometalloles
a
ab
,
c
compd
UV abs λmax/nm
PL λem/nm
Epc/V
DPyS1
DPyS2
DPyG1
273
273
269
420
408
400
−1.98
−2.06
−1.94
a
b
In chloroform at room temperature. Emission maximum excited at
the absorption maximum. Absolute PL quantum yields were
c
determined to be less than 2%, by using an integral sphere. Peak
potential from Fc/Fc+ (2 mM in acetonitrile with 0.1 M TBAP at 50
mV/s) .
These dipyridinometalloles were nearly nonemissive in
solution at room temperature with quantum efficiencies lower
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Table 2. Frontier Orbital Energies and Characteristics of BPy, DPyS3, and DPyG3, Derived from Calculations at B3LYP/6-
31G(d,p)
a
compd
HOMO-2/eV
HOMO-1/eV
HOMO/eV
LUMO/eV
π−π*/eV
S0 → T1/eV
interplane angle/deg
b
BPy
−7.28 (π)
−7.16 (π)
−7.16 (n)
−7.16 (n)
b
−7.12 (n)
−7.15 (n)
−7.14 (π)
−7.16 (π)
c
−7.08 (n)
−7.14 (n)
−7.04 (n)
−7.03 (n)
−1.66 (π*)
−1.88 (π*)
−1.95 (π*)
−1.92 (π*)
5.61
5.28
5.18
5.24
3.46
36.3
c
d
BPy (planar)
nd
0
0
0
b
DPyS3
3.16
3.20
b
DPyG3
a
LUMO − (HOMO(π)). Fully optimized. The dihedral angle of CC−CC was fixed to 0° and other structural parameters were optimized.
d
Not determined.
Figure 3. HOMO-1 (left) and LUMO (right) profiles of DPyS3,
derived from DFT calculations at the B3LYP/6-31G(d,p) level of
theory.
Figure 6. Decay plots of DPyG1 in the solid state at 77 K.
determined in an integration sphere to be 7% for fluorescence
and 22% for phosphorescence at 77 K. Phosphorescence
radiative and nonradiative decay rate constants were 3 × 104
and 1 × 105 s−1, respectively. DPyS1 and DPyS2 exhibited
similar low-temperature phosphorescence, in contrast to BPy,
which showed phosphorescence only in solution. The S0 → T1
transition energies obtained by the DFT calculations decrease
in the order BPy > DPyG3 > DPyS3 (Table 2). The quantum
yields of DPyS1 as solids were found to be less than 1% for
both the fluorescence and phosphorescence, while those of
DPyS2 could not be determined because of the low intensities.
The phosphorescence lifetimes for DPyS1 and DPyS2 were 1.4
and 1.1 μs, respectively. Some dibenzosiloles were recently
reported to be phosphorescent at low temperature in glassy
solution with the emission quantum efficiencies ranging from 4
to 11%. However, no solid-state phosphorescence was
studied.11
Figure 4. CV profiles of BPy and DPyG1 in acetonitrile.
than 2% (emission maxima are in Table 1). However, they
exhibited aggregation-induced emission and showed fluores-
cence in the solid state at room temperature (lifetime for
DPyS1 2.27 ns). Figure 5 depicts temperature-dependent
It may be likely that the π−π interaction in the solid state
affects the optical properties of dipyridinometalloles. To
understand this, we carried out DFT calculations of DPyS2
on a single molecule and a π−π dimeric form based on the
crystal structure derived from the X-ray diffraction study (vide
supra), indicating intermolecular interaction between the
dipyridinosilole units in the LUMO of dimeric form (Figure
S-12, Supporting Information). The LUMO is destabilized in
the dimer in comparison to the monomer by 0.11 eV. On the
other hand, the HOMO is localized on the methoxyphenyl
group and is not electronically influenced by the formation of a
dimeric form, thus leading to a larger HOMO−LUMO energy
gap for the dimer than the monomer by 0.08 eV, in accordance
with the fluorescence band of DPyS2 in the solid state being
slightly blue shifted from that in chloroform (Figures S-10 and
S-11, Supporting Information). However, we could not obtain
information about how the π−π interaction affects the solid-
state phosphorescence of DPyS2.
Figure 5. Temperature-dependent emission spectra of DPyG1 in the
solid state.
emissive properties of DPyG1 as an example. Interestingly,
below −80 °C, new low-energy bands most likely due to
phosphorescence appeared and were intensified by lowering the
temperature. The phosphorescence of DPyG1 in the solid state
at 77 K is composed of multiexponential decays, and the
lifetime in longer components is estimated to be approximately
6 μs (Figure 6). The absolute quantum efficiencies were
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Using Ph2SiF2 instead of Ph2SiCl2 resulted in a slightly higher yield
of DPyS1 (13%). DPyS2 was synthesized in a fashion similar to that
above, by using dichlorobis(4-methoxyphenyl)silane instead of
dichlorodiphenylsilane, as light brown solids in 10% yield after
purification by silica gel chromatography, followed by recycling
preparative GPC with toluene as eluent: mp 162.1−163.2 °C; EI-
CONCLUSIONS
■
In summary, we prepared group 14 dipyridinometalloles and
investigated their electrochemical and optical properties. These
compounds exhibit pronounced electron affinity in comparison
to the parent bipyridyl. In addition, we observed enhanced
solid-state phosphorescence of the present dipyridinometal-
loles. It is likely that introduction of the silicon or germanium
bridge fixing the bipyridyl ring minimizes the molecular
vibration, thus suppressing the nonradiative decay. Two aryl
groups on the bridge may also enhance the solid-state
phosphorescence by sterically preventing the aggregation-
induced quenching. However, as presented in Figure 1,
DPyS2 exhibits intermolecular π−π stacking in the solid state,
which may influence the optical properties in the solid state.
Detailed studies on the optical properties of these dipyr-
idinometalloles and structural optimization to enhance the
phosphorescence properties are underway.
1
MS m/z 396 [M+]; H NMR (CDCl3) δ 3.81 (s, 6H, Me), 6.93 (d,
4H, J = 8.2 Hz, Ph), 7.55 (d, 4H, J = 8.2 Hz, Ph), 7.79 (d, 2H, J = 5.2
Hz, pyridine ring H), 8.78 (d, 2H, J = 5.2 Hz, pyridine ring H), 8.99 (s,
2H, pyridine ring H); 13C NMR (CDCl3) δ 55.11 (Me), 114.30 (m-
Ph), 116.94 (C3), 120.69(i-Ph), 131.42 (C5), 137.06 (o-Ph), 151.98
(C2), 153.75 (C4), 154.67 (C6), 161.85 (p-Ph); ESI exact MS calcd
for C24H21O2N2Si [M + H+] 397.13668, found 397.13745.
Compound DPyG1 was synthesized in a fashion similar to that
above, by using dichlorodiphenylgermane instead of dichlorodiphe-
nylsilane, as off-white solids in 71% yield after purification by silica gel
chromatography: mp 173.9−174.9 °C; EI-MS m/z 382 [M+]; 1H
NMR (CDCl3) δ 7.38−7.47 (m, 6H, m- and p-Ph), 7.57 (dt, 4H, J =
6.4, 1.6 Hz, o-Ph), 7.85 (dd, 2H, J = 5.2, 0.8 Hz, pyridine ring H), 8.80
(d, 2H, J = 5.2 Hz, pyridine ring H), 9.00 (d, 2H, J = 0.8 Hz, pyridine
ring H); 13C NMR (CDCl3) δ 117.60 (C3), 128.87 (m-Ph), 130.31(p-
Ph), 132.50 (C5 or i-Ph), 132.79 (C5 or i-Ph), 134.50 (o-Ph), 151.58
(C2), 152.49 (C4), 154.34 (C6); ESI exact MS calcd for C22H17N2Ge
[M + H+] 383.05980, found 383.06036.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out in dry
argon. THF that was used as the reaction solvent was distilled from
CaH2 and stored over activated molecular sieves until use. The starting
compound 2,2′-dibromo-4,4′-bipyridyl was prepared as reported in the
literature.9 NMR spectra were recorded on Varian 400-MR and
System-500 spectrometers. ESI mass spectra were measured on a
Thermo Fisher Scientific LTQ Orbitrap XL spectrometer at N-BARD
Hiroshima University, while EI mass spectra were recorded on a
Shimadzu QP-2020A spectrometer. Room-temperature UV−vis
absorption and PL spectra were measured on Hitachi U-3210 and
HORIBA FluoroMax-4 spectrophotometers, respectively. The emis-
sion quantum yields excited at 350 nm were estimated using a JASCO
F-6300-H spectrometer attached to a JASCO ILF-533 integrating
sphere unit (φ = 100 mm). The wavelength dependences of the
detector response and the beam intensity of the Xe light source for
each spectrum were calibrated using a standard light source.
Measurements at low temperature (77−300 K) were performed with
a nitrogen bath cryostat (Oxford Instruments, Optistat DN) and a
temperature controller (Oxford Instruments, ITC 502S), or with a
CoolSpeK UV temperature controller (Unisok Co. Ltd., USP-203-A).
The quantum yield at 77 K was estimated using the temperature
dependence of emission and excitation spectra, following the equation
Φ(phosphorescence at 77 K) = Φ(all at 77 K) − Φ(fluorescence at
300 K), assuming that the temperature dependence of the fluorescence
intensity is negligible.
ASSOCIATED CONTENT
* Supporting Information
■
S
A CIF file, text, figures, and tables giving crystallographic data,
details of the theoretical calculations, NMR spectra of the
presently prepared dipyridinometalloles, a DSC profile of
DPyG1, CVs and temperature dependent emission spectra of
DPyS1 and DPyS2, and room-temperature emission spectra of
DPyS1, DPyS2, and DPyG1 in chloroform. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.O.: jo@hiroshima-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “New Polymeric Materials Based
on Element-Blocks (No. 2401)” of The Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
Synthesis of Dipyridinometalloles. To a solution of 0.314 g
(1.00 mmol) of 2,2′-dibromo-4,4′-bipyridyl in 45 mL of THF was
added dropwise 1.22 mL (2.00 equiv) of a 1.64 M n-butyllithium
solution in hexane at −85 °C, and the mixture was stirred at this
temperature for 1 h. To this was added 0.21 mL (1.0 equiv) of
dichlorodiphenylsilane, and the resulting mixture was immediately
heated to reflux and stirred for 15 min. The mixture was cooled to
room temperature and was poured into water. The organic layer was
separated, and the aqueous layer was extracted with chloroform. The
organic layer and the extracts were combined and dried over
anhydrous magnesium sulfate. After evaporation, the residue was
chromatographed on silica gel with hexane/ethyl acetate 1/1 as eluent,
followed by recrystallization of the crude solids from hexane/ethyl
acetate to afford DPyS1 in 11% yield (0.038 g, 0.011 mmol) as an off-
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dx.doi.org/10.1021/om401019b | Organometallics XXXX, XXX, XXX−XXX