Molecular Oligogermanes and Related Compounds: Structure, Optical and Semiconductor Properties

Article abstract of DOI:10.1002/asia.201700151

The optical (UV/Vis absorbance, fluorescence in the solid state and in solution) and semiconducting properties of a number of di- and trigermanes as well as related silicon- and tin-containing germanes, 1–6 ((p-Tol)₃GeGeMe₃ (1), Ph₃SnGe(SiMe₃)₃ (2), (C₆F₅)₃GeGePh₃ (3), (p-Tol)₃GeSiMe₂SiMe₃ (4), (p-Tol)₃GeGeMe₂Ge(p-Tol)₃ (5), (p-Tol)₃GeSiMe₂SiMe₂Ge(p-Tol)₃ (6)) were investigated. Molecular structures of 5 and 6 were studied by X-ray diffraction analysis. All compounds displayed luminescence properties. In addition, a band gap (of about 3.3 eV) was measured for compounds 1–6 showing that those molecules display semiconductor properties.

Full text of DOI:10.1002/asia.201700151

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Molecular Oligogermanes and Related Compounds: Structure,
Optical and Semiconductor Properties
Authors: Kirill V. Zaitsev, Viktor A. Tafeenko, Yuri F. Oprunenko,
Anastasia V. Kharcheva, Zhaisan Zhanabil, Yerlan Suleimen,
Kevin Lam, Vladimir B. Zaitsev, Anna V. Zaitseva, Galina S.
Zaitseva, and Sergey S. Karlov
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10.1002/asia.201700151
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Molecular Oligogermanes and Related Compounds: Structure,
Optical and Semiconductor Properties
Kirill V. Zaitsev,*[a] Viktor A. Tafeenko,[a] Yuri F. Oprunenko,[a] Anastasia V. Kharcheva,[a] Zhaisan
Zhanabil,[b] Yerlan Suleimen,[b] Kevin Lam,*[b] Vladimir B. Zaitsev,[c] Anna V. Zaitseva,[d] Galina S.
Zaitseva, [a] and Sergey S. Karlov[a]
Abstract: The optical (UV/vis absorbance, fluorescence in solid
state and in solution) and semiconducting properties of a number of
di- and trigermanes and related silicon and tin containing germanes,
elemental Si or Ge, and leads to a positive charge mobility within
the elemental chain itself. In fact, polygermanes/polysilanes
intrinsically display photoconductivity in which the electron
1-6 ((p-Tol)3GeGeMe3 (1), Ph3SnGe(SiMe3)3 (2), (C6F5)3GeGePh3 (3), transport is assured by a hole hopping process. Such as
(p-Tol)3GeSiMe2SiMe3 (4), (p-Tol)3GeGeMe2Ge(p-Tol)3 (5), (p-
Tol)3GeSiMe2SiMe2Ge(p-Tol)3 (6)) were investigated. Molecular
structures of 5 and 6 were studied by X-ray diffraction analysis. All
compounds displayed luminescence properties. In addition, a band
gap (of about 3.3 eV) was measured for the compounds 1-6 showing
that that those molecules display semiconductor properties.
phenomenon strongly depends on the nature and on the length
of the side-chain.[4] The construction of semiconducting devices
based on such polymer materials[5] (polysilanes, polygermanes,
polystannanes) relies on specific techniques, such as the
production of doped thin films.[6] Interestingly, the band gap in
elemental Ge is lower than the one in Si and hence one could
expect enhanced conductive properties in individual germanium
organic compounds. Therefore, the synthesis and the studies of
the shortest Group 14 element chain (especially containing Ge
atoms) which is necessary for the conductive properties to be
observed may be regarded as an essential task in the
development of new semiconducting devices based on
germanium. The conductivity mechanism in free oligogermanes
and related compounds is significantly different from the one
observed in other related conductive materials due to σ-bond
electrons delocalization.[7] In oligogermanes, it is well known that
the HOMO is distributed across the chain of Group 14
elements[2] and an interchain hopping mechanism has been
previously described.[5]
Introduction
Catenated oligosilanes[1] and oligogermanes[2] are attracting the
attention of the scientific community due to their unique physical
properties. These compounds possess strong UV/visible
absorption, luminescence, photo- and electric conductivity due
to presence of an effective σ-conjugation between the Group 14
elements (E = Si, Ge, Sn, Pb). A special attention should be paid
to germanium compounds since they exhibit semiconducting
properties due to their smaller band gap and higher electron and
hole mobility.
Investigations and studies of small molecules and their
Semiconducting properties in polymeric catenated Group
14 derivatives are usually observed after the molecule has
undergone a special treatment such as an oxidative doping by
SbF5 or AsF5.[3] In this case a partial oxidation results in the
formation of a cation-radical, similar to the “hole” usually found in
properties should help to establish
a “structure–property”
relationship. Recently, several works emerged in which single-
molecule conductance has been studied.[8] But in such studies a
very specific and complex method (scanning tunneling
microscope-based break-junction) was used to measure the
conductance.
In continuation of our research program on catenated
germanium compounds[9] we perform this detailed investigation.
Taking into account that oligogermanes as well as related silicon
and tin containing germanes include semiconducting (silicon and
germanium) atoms, we decided to investigate the optical and
semiconductor properties of several molecular organometallic
compounds (1-6) in their solid state. The optical and
electrochemical properties of oligogermanes were also
investigated in solution. Thus, this research contains subjects
from various fields of science including not only various
branches of chemistry (synthesis, studying the structure,
electrochemistry), but also spectroscopy (NMR, UV/vis), physics
and material science.
[a]
Dr. Kirill V. Zaitsev, Dr. Viktor A. Tafeenko, Dr. Yuri F. Oprunenko,
Anastasia V. Kharcheva, Dr. Galina S. Zaitseva, Prof. Sergey
S .Karlov
Department of Chemistry
Moscow State University
Leninskye Gory, 1, 3, Moscow 119991, Russia
E-mail: zaitsev@org.chem.msu.ru
Zhaisan Zhanabil, Dr. Yerlan Suleimen, Dr. Kevin Lam
Department of Chemistry
School of Science and Technology, Nazarbayev University
Astana, Kazakhstan, 010000
E-mail: kevin.lam@nu.edu.kz
Dr. Vladimir B. Zaitsev
Department of Physics
Moscow State University
Leninskye Gory, 1, Moscow 119991, Russia
Dr. Anna V. Zaitseva
[b]
[c]
[d]
Institute of Physical Chemistry
Results and Discussion
Russian Academy of Sciences
Leninskii pr. 31, Moscow, Russia
Several types of compounds were investigated in this work.
Germanes (p-Tol)3Ge-GeMe3 (1), Ph3Sn-Ge(SiMe3)3 (2)[9e] and
Supporting information for this article is given via a link at the end of
the document.
1
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(C6F5)3Ge-GePh3 (3)[9d] were prepared using known procedures.
Compounds 4-6 were prepared by reacting (p-Tol)3GeLi, formed
in situ, with a silicon or germanium halide (Scheme 1).
average value of torsion angle С-Ge(1)-Ge(2)-С is 175.76(19)o).
The value of Ge(1)-Ge(2)-Ge(1i) (120.37(4)o) angle is close to
120о,[10] which indicates an efficient delocalization of σ-electron
density between the Ge atoms. In general the structural
parameters of 5 are very close to those of (p-Tol)3Ge-GeMe3,
1.[9e] At the same time, 5 displays the shortest Ge-Ge bond
length among the known trigermanes investigated by X-ray
analysis.
Table 1. The main structural parameters of trigermanes investigated by X-ray
analysis.
Angle
Ge-Ge-
Geav, deg
Angle
C-Gecentral
Cav, deg
d(Ge-
Trigermane
-
Reference
Ge)av, Å
Scheme 1. Synthesis of compounds 4-6.
[11]
[12]
[13]
[14]
[Ph3Ge]2GePh2
[Ph3Ge]2GeMe2
2.440(2)
2.429(1)
2.423(4)
2.620(3)
121.3(1)
120.3(1)
108.7(4)
109.2(2)
Compounds 4-6 were isolated in good yields as white
crystalline air and moisture stable powders, soluble in common
organic solvents (toluene, ether, chloroform, THF).
The structures of compounds 5 and 6 in solid state were
studied by X-ray analysis (Figures 1 and 2; Table S1, Supporting
Information); in solution, the structures of those novel derivatives
were investigated by multinuclear NMR (see Supporting
Information, Figures S1-S11), UV/vis and emission spectroscopy
as well as by electrochemistry. For comparison purposes, the
luminescent properties of compounds 1-3 were also studied.
[ClPh2Ge]2GePh2
[Me(t-Bu)2Ge]2Ge(t-
Bu)2
113.52(12)
118.56(17)
111.82(13)
110.31(15)
[14]
[15]
[16]
[16]
[9d]
[17]
[Br(t-Bu)2Ge]2Ge(t-Bu)2
[I(t-Bu)2Ge]2Ge(t-Bu)2
[(p-Tol)3Ge]2GePh2
[(p-Tol)3Ge]2Ge(p-Tol)2
[(p-Tol)3Ge]2Ge(C6F5)2
[(Me3Si)3Ge]2GeMe2
[(p-Tol)3Ge]2GeMe2 (5)
2.609(2)
2.641(1)
2.4328(5)
2.4404(5)
2.459(5)
2.4616(8)
2.4219(6)
113.61(15)
115.38(10)
114.80(2)
117.54(1)
124.10(3)
125.00(4)
120.37(4)
109.25(12)
109.36(12)
106.2(1)
106.45(9)
108.0(2)
105.35(6)
106.5(4)
this work
Figure 1. Molecular structure of 5. Displacement ellipsoids are shown at 50%
probability level. Selected bond lengths (Å) and bond angles (deg): Ge-Ge
2.4219(6), Ge(1)-Cav 1.949(4), Ge(2)-C(22) 1.941(5); С-Ge(1)-Cav 108.39(19),
С-Ge(1)-Ge(2)av 110.46(12), С(22)-Ge(2)-C(22i) 106.5(4), С-Ge(2)-Ge(1)av
107.32(17), Ge(1)-Ge(2)-Ge(1i) 120.37(4).
Figure 2. Molecular structure of 6. Displacement ellipsoids are shown at 50%
probability level. Selected bond lengths (Å) and bond angles (deg): Ge(1)-Si(1)
2.3972(10), Si(1)-Si(1i) 2.3486(18), Ge(1)-Cav 1.952(3), Si(1)-Cav 1.871(3); С-
Ge(1)-Cav 107.29(13), С-Ge(1)-Si(1)av 111.56(9), С(22)-Si(1)-C(23) 108.6(2),
С-Si(1)-Cav109.77(14), C(22)-Si(1)-Ge(1) 108.51(14), Si(1i)-Si(1)-Ge(1)
110.31(5).
To the best of our knowledge, only 10 structures of linear
trigermanes have been reported so far (Table 1). The steric
hindrance and the electronic properties of the substituents
(electron donors or acceptors) are the main factors that impact
the structural parameters of oligogermanes.
The crystal structure of compound 5 is highly symmetric
and displays a C2v symmetry. Geometry at each Ge atom may
be described as a slightly distorted tetrahedron. The substituents
at the neighboring Ge atoms are in an anti-conformation (the
Molecular structure of compound 6 is centrosymmetric
(space group P-1, Z = 1). The structural parameters are very
close to those found for (p-Tol)3Ge-SiMe3.[9e] In 6, the
substituents along the Ge-Si bond in (p-Tol)3Ge-SiMe2 fragment
are in a skewed conformation[10b] (torsion angle C-Ge-Si-C is
2
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86.58(14)°). At the same time the geometry along Me2Si-SiMe2
fragment may be described as an ideal anti-conformation
(torsion angle Ge-Si-Si-Ge is 180.00(5)o). Such a conformation
allows for an effective σ-conjugation in the chain of four atoms of
Group 14 elements.
As it can be seen from Figure 3, the corresponding growth
steps may be easily measured. Plotting of surface cross-
sections allows estimating the height of these growth steps.
Figure 4, shows an analysis of compound 1 surface profile for
one of the investigated sites.
Only three compounds with a related structure have been
reported so far (Table 2).
Table 2. Structural parameters of compounds with Ge-Si-Si-Ge chain.
Compound
d(Ge-
d(Si-Si),
Angle
Ge-Si-Si,
deg
Torsion
Ge-Si-
Si-Ge,
deg
reference
Si)av, Å
Å
[18]
[K(Me3Si)2Ge-
SiMe2-]2*2[18-
crown-6]
2.429(2)
2.378(3)
116.05(10)
180
[19]
[17]
[(Me3Si)3Ge-
SiMe2-]2
2.407(2)
2.410(2)
2.397(1)
2.379(2)
2.349(3)
2.349(2)
116.23(8)
114.81(5)
110.31(5)
180
180
180
[(Me3Ge)3Ge-
SiMe2-]2
[(p-
this work
Figure 4. AFM image of the surface of compound 1 single crystal and three
surface profile cross-sections.
Tol)3GeSiMe2-]2
(6)
The analysis of cross-sections obtained with several
samples of the same compound, provides an average value of
1.46 ± 0.01 nm for the step height growth. Such a value
When comparing the structural parameters of 6 with the
related compounds showed in Table 2 it is evident that the steric
volume of the substituents plays a key role in the geometry of
such molecules. Introduction of bulky groups, even at the ends
of the elemental chain leads to a noticeable increase in bond
lengths.
The chemical shifts in NMR spectra of related p-tolyl
substituted Ge and Si derivatives 4-6 are very similar. Thus in 1H
NMR there are two doublets in aromatic field (at δ 7.32-7.18 and
7.15-7.03 ppm with 7.8 Hz spin coupling constant) and singlet of
methyl group (at approximately δ 2.35 ppm). Very similar signals
of CH groups are observed in 13C NMR (at approximately δ
135.4, 128.9 ppm) typical for para-substituted tolyl group but the
ipso-carbons (at δ 137.9 and 134.6-135.2 ppm) are more
sensitive to the nature of substituent.
correlates with the size of elemental cell (a:b:c
=
10.6888(2):19.3569(4):12.2900(3) Å) of 1 according to X-ray.[9e]
In the case of 2 the step height growth is 0.97 ± 0.03 nm.
The UV/vis absorbance spectra of compounds 4-6 in
solution are given in Figure 5.
The topology of the single crystal surface of compounds 1,
2 and 5 was investigated by AFM. Figures 3 and 4 show the
AFM data for compound 1 (for other compounds see Supporting
Information, Figures S12-S17).
Figure 5. UV/vis absorption spectra (normalized) for 4-6 in CH2Cl2.
The data concerning UV/vis absorption of 4-6 and related
compounds are given in Table 3.
Figure 3. AFM image of different regions of compound 1, single crystal
surface.
3
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Table 3. UV/visible absorption for 4-6 and related compounds.
1,0
0,5
0,0
40
30
20
10
0
λmax, nm
Compound
(ε×10-4, M-
1cm-1)
reference
[20]
[21]
[22]
[22]
Me3Si-SiMe2-SiMe3
217 (0.8)
218 (0.8)
243 (1.95)
254 (3.22)
241 (2.5)
258 (4.7)
256 (4.60)
282 (4.5)
285
Me3Ge-GeMe2-GeMe3
PhMe2Si-SiMe2-SiMe2Ph
Ph3Si-SiPh2-SiPh3
200
400
600
h,nm
(p-Tol)3Ge-SiMe2-SiMe3 (4)
(p-Tol)3Ge-SiMe2-SiMe2-Ge(p-Tol)3 (6)
Ph3Ge-GeEt2-GeEt2-GePh3
Ph3Ge-GePh2-GePh2-GePh3
(p-Tol)3Ge-GePh2-GePh2-Ge(p-Tol)3
this work
this work
[23]
[11]
[16]
2
3
4
5
6
[23]
[23]
[23]
[23]
h
Ph3Ge-GeMe2-GePh3
245 (3.02)
244 (4.42)
250 (4.42)
250 (4.56)
251 (4.0)
Ph3Ge-SiMe2-GePh3
Figure 6. The plot of Kubelka–Munk function derivative for compound 3 used
for the optical band gap Eg calculation. Insert: spectral dependence of the
absorption coefficient of compound 3 in solid state.
Ph3Ge-GePh2-GePh3
Ph3Ge-Ge(Me)Ph-GePh3
(p-Tol)3Ge-GeMe2-Ge(p-Tol)3 (5)
(p-Tol)3Ge-GePh2-Ge(p-Tol)3
(p-Tol)3Ge-Ge(p-Tol)2-Ge(p-Tol)3
(p-Tol)3Ge-Ge(C6F5)2-Ge(p-Tol)3
this work
[16]
251 (3.17)
253 (2.55)
258 (1.4)
[16]
[9d]
From this spectral dependence it is seen that a threshold
wavelength exists below which the absorbance increases
dramatically. Since the solid state samples were powders or
single crystals the absorption spectra were recalculated from
measured diffuse reflection.
Looking at the data obtained, it is evident that the
substitution of a silicon atom by a germanium results in a red
shift of the absorption band of the molecule. This could be
explained by an effective conjugation between the identical
atoms in the chain. A more significant shift is observed in the
presence of aromatic substituents at Ge or Si centers.[9d, 23-24]
Increasing the number of atoms in the chain also resulted in a
bathochromic shift. Furthermore, in the case of compound 6 the
absorption band is clearly visible and may be due to an ideal
conjugated structure similar to the one found in the crystal. At
the same time, the nature of the substituting groups located at
the end and in the centre of the linear molecule clearly showed
to impact on the UV/vis absorption. On the one hand, electron
donating groups such as p-tolyl (when compared with phenyl)
present at the ends of the chain resulted in a bathochromic shift.
On the other hand, electron withdrawing groups such as
perfluorophenyl (when compared with methyl or phenyl) lead to
an even more significant red shift. Substitution on the central Ge
atom (Me, Ph or p-Tol) has only a small effect on the absorption
properties of the molecule. In conclusion, in solution, the
substitution (group at E atom, nature of E) in the chains of Group
14 elements significantly effects on UV/vis absorption and may
result in better conjugation between these atoms and in a
bathochromic shift.
The optical band gap energy Eg of the solid samples was
determined from the so-called intrinsic absorption edge. Several
methods exist to accomplish this. The first method allowed us to
make an approximate estimate of Eg directly from the plot of the
absorption coefficient α. At high λ, the energy of a quantum is
small and no absorption occurs. This corresponds to the right
part of the curve shown in the insert at Figure 6. Once λ attains
the critical value of λedge, the absorption abruptly rises. This
means that a sharp kink of the α(λ) dependence occurs at λ =
λedge (see Figure 6 insert). The energy of a light quantum is
related to the wavelength as E = hc/ = hν.
The main drawback of this approach lies in the difficulty to
estimate correctly the absorption coefficient from scattering
spectra. The α value of semiconductor materials varies within a
wide range from 10–2 to 105 cm–1. For this reason, when the
absorption coefficient is measured, the thickness of a specimen
is selected in such a manner that the absorbance D = αd (where
d is the thickness of a specimen) is nearly equal to 1. In this
case, it is possible to use, with an admissible error, the
expression
T (1 – R)2 = exp(–D),
which allow calculating the absorption coefficient from the
measured R (Fresnel reflection coefficient), T (transmittance),
and d as
When moving from solution to the solid state, the
compounds studied showed new optical properties. The
absorption bands not only start at longer wavelengths but also
display the characteristic shape of semiconductor absorption
bands. The Kubelka–Munk plot for compound 3 is shown on
Figure 6.
The situation becomes more complex for the detection of
scattered radiation. Although no strict multiple scattering theory
exists, the theory of the diffuse reflection and transmission of
optically opaque specimens, i.e., the so-called two-component
theory developed by Kubelka and Munk, is rather widely applied.
For scattering specimens, this theory has the same importance
4
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as the Bouguer–Beer law in the absorption spectroscopy of
transparent specimens. In Kubelka-Munk theory, it is assumed
that reflected radiation is isotropic, i.e., direction independent,
and radiating light is monochromatic. As a result of the solution
of the Kubelka–Munk equation system, it turns out that the
diffuse reflectance R∞ of a specimen depends only on the ratio
of the absorption coefficient α and the scattering coefficient S
instead of either the scattering coefficient or the absorption
coefficient, i.e.,
α/S = (1– R∞)2/2R∞ = F(R∞)
The function F(R∞) is called the Kubelka–Munk function. In
diffuse reflection spectroscopy, as well as in absorption and
emission spectroscopy, the dependence of the response of an
instrument on the wavelength must be eliminated. This is
provided by measuring the diffuse reflection spectrum of a
specimen itself logR(λ) and the spectrum of scattering from an
infinitely reflecting surface Rref(λ), for instance, from a surface
coated with a thin barium sulfate, magnesium carbonate, or
magnesium oxide layer; here, the ratio is calculated in the
logarithmic form log[R(λ)/Rref(λ)]. In the absence of reflection
from the bottom part of a specimen (for example, when a
specimen has a sufficient thickness for the light to be completely
absorbed), it is equal to logR∞. For the practical estimation of the
band gap energy in the case of direct interband transitions, the
Figure 7. Emission spectra of compounds 2–6 in solution in CH2Cl2.
Figure 8. Emission spectra of compounds 2, 3 and 5 in solid state.
experimental data are expressed in the form of the dependence
(αhν)2 = A2(hν –Eg),
which must be linear (Figure 6). As can be seen from Figure 6,
the value of Eg is determined by extrapolating the linear
dependence to the intersection with the abscissa axis.
The experimental dependence of α with hν for indirect
interband transitions are plotted in the form of curves
=
f(hν).
We analyzed the results using a variety of methods for
determining the width of the forbidden zone. All our results
proved that compounds 1–6 possess semiconductor properties.
The values of band gap for those materials were in the range 3.3
 0.1 eV. According to DFT calculations for semiconductor
polymers [Ph2Ge]n the bandgap width is 2.13 eV.[25] Furthermore,
partial substitution of phenyl groups by protons, i.e. [Ph(H)Ge]n
and [H2Ge]n, results in 2.72 and 3.03 eV bandgap width. This
reveals the critical role of aryl groups (σ-π conjugation) in the
bandgap width value. The comparison with related silicon
derivatives, [Phm(H)2–mSi]n, gives 3.61 (m = 2), 3.72 (m = 1) and
4.53 (m = 0) eV,[26] or 3.5 eV for [MePhSi]n[7] that reflects the role
of the nature of the catenated atom in the chain.[9e] Therefore we
are confident that molecular oligogermanes, due to their physical
properties are promising semiconducting precursors.
In semiconducting organometallic compounds such as 1–6
the band gap is determined by the HOMO/LUMO gap. Decrease
of the energy gap by varying the nature of the substituents
(introduction of only electron withdrawing substituents,[9d] or only
electron donating groups, substitution by Sn atoms[9e] or
increasing the number of catenated atoms in the chain) should
result in the appearance of bulk semiconductor properties.
The luminescent properties of compounds 2–6 were
studied (Table 4) in solution (Figure 7) and in solid state (Figures
8–9, Figure S18, Supporting Information).
Figure 9. Emission spectra of compound 4 in solid state at different
excitation.
Table 4. Fluorescence emission data for compounds 1-6.
Compound
Solid State
λem (nm)[b]
Solution[a]
λem (nm)[b]
286(270)
Φf (%)[c]
3.27
357, 373,
393 (300)
398 (350)
373 (270)
400 (290),
427 (370)
Me3Ge-Ge(p-Tol)3 (1)[d]
Ph3Sn-Ge(SiMe3)3 (2)
Ph3Ge-Ge(C6F5)3 (3)
335 (285)
377 (265)
331, 350
(275)
0.93
12.50
(p-Tol)3Ge-SiMe2-SiMe3 (4)
10.64
1.85
1.98
(p-Tol)3Ge-GeMe2-Ge(p-
Tol)3 (5)
438 (380)
338 (285)
(p-Tol)3Ge-SiMe2-SiMe3-
Ge(p-Tol)3 (6)
416 (300),
445 (320)
363, 375
(260)
[a] Spectra were recorded in CH2Cl2. [b] Excitation wavelength (λex, nm)
shown in parentheses. [c] Quantum yield. [d] The data from the reference
[9e]
.
5
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2, 3, 4 and 6 were carried out under standard conditions in
dichloromethane containing [NBu4][PF6] (0.10 M) as supporting
electrolyte. All data are referred to standard for organometallic
compounds reference Fc/Fc+. None of the compounds showed
any cathodic reduction. Catenated aryl substituted Group 14
The fluorescence of compounds 1–6 in solution shows that
the electronic properties of the substituents on the Ge atom
significantly affect the Stokes shift and the quantum yield.
Introduction of electron withdrawing groups resulted in a red shift
with high quantum yield. In the case of the silyl substituted
derivatives, 4 and 6, two emission bands were observed,
indicating several excitation fragments.
On contrary, in solid state, silyl substituted derivatives 4
and 6 showed only one fluorescence band. The emission
wavelengths were red shifted due to the strong interaction
between molecules inside the molecular crystal in comparison
with the results previously obtained in solution.
28]
compounds are known to have irreversible oxidations[21,
(equal to n-1 waves, where n is an E atoms quantity in the
chain)[16] since the E-E bonds are rapidly consequentially
29]
cleaved when oxidized.[9e,
This is differed from alkyl
substituted derivatives where only one oxidation wave is
observed.
Compound 2 (Figure 10) showed a series of a very close
chemically-irreversible anodic oxidations at, respectively, Epa
=
1.06, 1.20, 1.41 and 1.62 V vs. Fc/Fc+ (approximately 1.55, 1.69,
1.90 and 2.11 V vs. Ag/AgCl).[30] In addition, 2 displays an
important degree of electrochemical irreversibility. The
measured Epa-Epa/2 value for the first oxidation of 2 is diagnostic
of a slow charge transfer process with coefficient (β) of 0.30.[31]
No re-reduction wave was observed during the back scan due to
rapid following up chemical reaction happening after the initial
electron transfers (EC type).[32] Whilst the first anodic event
could presumably be attributed to the oxidation of the Sn-Ge
bond (compare with 1.52 V vs. Ag/AgCl for (p-Tol)3GeSnMe3)[9]
followed by the rapid cleavage of the formed radical-cation,[9e]
the subsequent oxidations encompass the oxidations of the
decomposition products of the Sn-Ge radical cation as well as
the Ge-Si bond oxidation. This anodic behavior differs
significantly from what has been previously reported for the
other branched Group 14 derivatives, where only one oxidation
is observed.[30c,d]
The studies presented in earlier research[8] has shown that
the “conductance” of a molecule in solution could be measured
by using a complex scanning tunneling microscope technique
based on the break-junction (STM-BJ). Nevertheless, measures
obtained using this method are not representative of the physical
properties of the bulk material. Most of the conductivity studies[6]
carried on polysilanes (polygermanes) used time-resolved
methods (such as FP-TRMC or TOF). Those studies have
measured the intramolecular charge carrier mobility and
conductivity without predicting the carrier transport properties for
long distances in the bulk material, and thus having only limited
significance.
In this work, the conductivity study was carried out by
using DC and AC conductivity techniques, as well as
photoconductivity methods. Compounds 1–6 were analyzed as
pellets made from the polycrystalline powder for 1-4 and single
crystals for 1, 5 and 6. Furthermore, the doping procedure by
iodination was used for compound 6. All of the studied
compounds have shown resistance typical of dielectric materials
and no photoconductivity was observed. When compared to
published data[6d, 8a, b], our results tend to indicate that the charge
carriers movements are hampered within the oligogermane
molecules in the bulk material.
It is well known that the mobility of the charge carriers
(usually holes in related compounds) strongly depends on the
interfaces between molecules and between domains. In other
words the conductivity is determined by the structure of the
material and decreases when increasing the number of domain
borders.[27] There could be several reasons that might explain
the low bulk conductivity and photoconductivity of the materials
studied. The first possible reason is high energy barriers (either
between molecules inside the material or at the contact point
with the metallic electrodes) which may be preventing the
charge carriers from moving. Also, a high number of the
recombination centers can be present within the material itself.
Nevertheless, it is safe to assume that the conductivity could be
improved by tweaking the oligogermanes. For instance
introducing longer chain of σ-conjugated Group 14 atoms or
introducing novel substituents (like thiomethyl groups[8a, b]) on the
Ge should improve the intermolecular electron transport and
interaction with the electrodes and therefore this should result in
observing an increase of the conductivity for these derivatives in
the bulk material. Another way of solving this issue could be
mixing the oligogermanes with an electron transporting materials
such as fullerenes.[5]
Figure 10. Cyclic voltammogram of 1mM 2 in dichloromethane - [NBu4][PF6]
(0.1 M) solution. Sweeping rate 200 mV/s at room temperature.
The anodic electrochemistry of compounds 3 (Figure S19,
Supporting Information) showed only one chemically-irreversible
oxidation at Epa = 1.62 V vs. Fc/Fc+ (approximately 2.11 V vs.
Ag/AgCl). The presence of electron withdrawing groups on the
aromatic rings pushed the oxidation of the Ge-Ge bond to a very
positive value and partially merges it with the solvent’s wall;
therefore no charge transfer coefficient could accurately be
measured. The introduction of an electron withdrawing group
within the core of 3 increases the oxidation potential by
stabilizing the HOMO. Such a phenomenon is typically observed
To measure the level of HOMO (which correlates with
oxidation potential) electrochemical investigations for compound
6
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10.1002/asia.201700151
Chemistry - An Asian Journal
FULL PAPER
for oligogermanes containing a small number of element-
element bonds.[9d]
the molecule the first oxidation that occurs is the oxidation of the
bond with this atom (Sn>Ge>Si; compare 1.52 V vs. Ag/AgCl for
(p-Tol)3Ge-SnMe3[9e] and 1.55 V for 2).
Compound
4 (Figure 11) displayed two successive
chemically-irreversible oxidations at Epa = 1.01, 1.30 V vs. Fc/Fc+
(approximately 1.50, 1.79 V vs. Ag/AgCl). Again, the first
oxidation displayed a slow electron transfer (β = 0.34). It is safe
to assume that the first anodic event is due to the oxidation of
the Ge-Si bond. The second oxidation is either due to the
presence of a decomposition product resulting from the rapid
cleavage of the Ge-Si radical-cation or to the oxidation of the Si-
Conclusions
We have disclosed new improved structure-semiconductor
properties relationships for several oligogermanes as well as for
several related silicon and tin compounds containing germanium
atom. Although, the overall conductivity of the bulk material
remains an issue, we are confident that this problem could be
solved by doping the material, increasing the number of atoms in
the catenated chain or by introducing functional groups (suitable
for better interaction with the electrodes) on the germanium
center.
Si bond (compare with 1.79
V
vs. Ag/AgCl for (p-
Tol)3GeSiMe3).[9] It is evident that elongation of the catenated
atoms in the chain decreases the oxidation potentials
(destabilizes the HOMO energy) and the introduction of aryl
groups on the E atoms stabilize the radical-cation.
We are confident that further studies on individual
molecules similar to the ones reported in this work with known
bulk (NMR, XRD, AFM) structure, optical (UV/vis, luminescence)
and electrochemical properties (CV) will open new possibilities
and will lead to new practical applications of catenated Group 14
compounds in the future.
Experimental Section
General methods and remarks. All operations with germanium
derivatives were conducted in a dry argon atmosphere using standard
Schlenk techniques. 1H NMR (400.130 MHz), 13C NMR (100.613 MHz),
and 29Si (79.495 MHz) NMR spectra were recorded on Bruker 400 or
Agilent 400 spectrometers (at 295 K). Chemical shifts in the spectra are
given in ppm relative to internal Me4Si. Elemental analyses were carried
out using HeraeusVarioElementar instrument. UV/visible spectra were
recorded using two ray spectrophotometer Evolution 300 «Thermo
Scientific» with cuvette of 0.10 cm long. Fluorescence (room
Figure 11. Cyclic voltammogram of 1 mM 4 in dichloromethane - [NBu4][PF6]
(0.1 M) solution. Sweeping rate 200 mV/s at room temperature.
temperature)
spectra
were
recorded
with
Hitachi
F-7000
spectrofluorimeter. Diffuse light scattering and adsorption was
investigated with the use of LS-55 Perkin Elmer spectrophotometer. The
surface structure of the obtained crystal was investigated using an atomic
force microscope (AFM) "Multimode V" (Production Veeco) in tapping
mode. DC conductivity and photoconductivity of the samples was
measured with Keithley 6487 picoammeter. AC conductivity and
photoconductivity was measured by NR 4192 impedance analyzer. Silver
epoxy paste was used to produce electric contacts to the samples. For
the photoconductivity measurements the samples were illuminated by 1
kW high pressure xenon lamp (with white spectrum) through MDR-12
monochromator. Mass spectra (EI-MS, 70 eV) were recorded on a
quadropoule mass spectrometer FINNIGAN MAT INCOS 50 with direct
insertion; all assignments were made with reference to the most
abundant isotopes.
Compound
6
(Figure S20, Supporting Information)
displayed series of successive ill-defined chemically-
a
irreversible oxidations. Three oxidations appear at Epa = 0.99,
1.15 and 1.30 V vs. Fc/Fc+ (approximately 1.48, 1.64 and 1.79 V
vs. Ag/AgCl). Presumably, the first oxidation wave is due to the
oxidation of the Ge-Si bond (compare with 1.01 V for 4). The
oxidation potential is shifted to less positive value due to
increased E-E chain length. Apparently, the second event is also
due to Ge-Si bond oxidation in intermediate radical cation. The
third process is typical for Si-Si bond (compare with 1.30 V for 4).
The oxidation waves are too convoluted to allow measuring any
charge transfer coefficient accurately. No significant
improvement of the resulting data was observed when the cyclic
voltammetry experiments were conducted in dichloromethane
containing the weakly coordinating salt [NBu4][B(C6F5)4] (0.05 M)
as supporting electrolyte.[33]
Solvents were dried by usual procedures. Diethyl ether were stored
under solid KOH and then distilled under sodium/benzophenone. n-
Hexane were refluxed and distilled over sodium. Dichloromethane was
distilled over CaH2. C6D6 was distilled over sodium under argon. CDCl3
was distilled over CaH2 under argon.
The measured electrochemical data are in good
agreement with what has been previously reported in the
literature for similar compounds. Increasing the length of the
catenated Group 14 atoms chain decreases the first oxidation
potential (1.79 V vs. Ag/AgCl for (p-Tol)3GeSiMe3,[9e] 1.50 V for 4
and 1.48 V for 6); in addition, branching of the structure resulted
in a smaller increase of the oxidation potential (1.06 V for 2 and
0.99 V for 6). Apparently, if the heavier E atom is present within
nBuLi (“Aldrich”), Me3Si-Me2SiCl (“Aldrich”), ClMe2Si-Me2SiCl
(“Aldrich”) were the commercial reagents and were used as received.
Compounds (p-Tol)3GeH,[34] Me4Ge,[9e] (p-Tol)3Ge-GeMe3 (1),[9e] Ph3Sn-
Ge(SiMe3)3 (2),[9e] (C6F5)3Ge-GePh3 (3)[9d] were synthesized according to
literature procedures. Acetyl chloride was distilled over PCl5 under flow of
argon.
7
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10.1002/asia.201700151
Chemistry - An Asian Journal
FULL PAPER
Dimethyldichlorogermane, Me2GeCl2. The improved procedure was
used.[35] At 0°C under strong stirring acetyl chloride (51.00 mL, 56.50 g,
720.00 mmol) was added dropwise to the mixture of Me4Ge (26.00 g,
200.00 mmol) and anhydrous AlCl3 (112.00 g, 840.00 mmol). Reaction
mixture was slowly warmed to room temperature and stirred for 5 h, then
heated at 100°C for 1 h. Volatile materials were distilled twice using
effective condenser, giving colorless liquid, b.p. 120-123 °C,
b.p.124 °C.[36] Yield: 18.40 g (53%). 1H NMR (CDCl3, 400.130 MHz): δ
1.20 (s, 6H, CH3). 13C{1H} NMR (CDCl3, 100.613 MHz): δ 10.77 (CH3).
under reduced pressure. Residue was recrystallized from the mixture of
n-hexane/dichloromethane. Compound 6 was isolated as a white powder,
m.p. 233-234 oC. Yield: 0.32 g, 68%. 1H NMR (CDCl3, 400.130 MHz): δ
7.32 (d, 12H, 3JH-H = 7.8 Hz, p-C6H4CH3), 7.15 (d, 12H, 3JH-H = 7.8 Hz, p-
C6H4), 2.37 (s, 18H, p-C6H4CH3), 0.23 (s, 12H, SiMe2). 13C{1H} NMR
(CDCl3, 100.613 MHz): δ 137.89, 135.22 (2 ipso-C6H4CH3), 135.36,
128.93 (p- and m-C6H4CH3) (aromatic carbons), 21.40 (p-C6H4CH3), -
3.03 (SiMe2). 29Si NMR (CDCl3, 79.495 MHz): δ -34.66 (SiMe2). MS
(EI, %): 809 ([M+], 1), 687 ([M – 2Me - Tol]+, 1), 462 ([M – GeTol3]+, 41),
346 ([Tol3Ge]+, 100), 405 ([Tol3GeSiMe2]+, 2), 255 ([Tol2Ge]+, 10). UV
(CH2Cl2), λmax, nm (ε, M-1 cm-1): 258 (4.7×104). Elemental analysis calc.
for C46H54Ge2Si2 (808.312): C 68.35, H 6.73. Found: C 68.23, H 6.68.
Dimethyl(tris(p-tolyl)germyl)trimethylsilylsilane,
(p-Tol)3Ge-SiMe2-
SiMe3 (4). a) Synthesis of [tris(p-tolyl)germyl]lithium. At room temperature
solution of nBuLi in n-hexane (2.5 M, 0.46 mL, 1.15 mmol) was added
dropwise to the solution of (p-Tol)3GeH (0.40 g, 1.15 mmol) in ether (20
mL). Reaction mixture was stirred for 6 h. The solution of lithium
compound was used further without purification.
Electrochemistry. Electrochemical measurements were carried out
using an Autolab 302N potentiostat interfaced through Nova 2.0 software
to a personal computer. Electrochemical measurements were performed
in a glovebox under oxygen levels of less than 5 ppm using solvent that
had been purified by passing through an alumina-based purification
system. Diamond-polished glassy carbon electrodes of 3 mm diameter
were employed for cyclic voltammetry (CV) scans. CV data were
evaluated using standard diagnostic criteria for diffusion control and for
chemical and electrochemical reversibility. The experimental reference
electrode was a silver wire coated with anodically deposited silver
chloride and separated from the working solution by a fine glass frit. The
electrochemical potentials in this paper are referenced to
ferrocene/ferrocenium couple, as recommended elsewhere.[37] The
ferrocene potential was obtained by its addition to the analyte solution.[38]
At an appropriate time in the experiment [NBu4][B(C6F5)4] was prepared
as previously described.[39]
b) Synthesis of 3. Solution of Me3Si-Me2SiCl (0.24 mL, 1.15 mmol) in
ether (20 mL) was added dropwise to the ethereal solution of (p-
Tol)3GeLi obtained as described above. Reaction mixture was stirred
overnight. Then water (20 mL) was added, water phase was extracted
with ether (3х20 mL), combined organic phases were dried over
anhydrous Na2SO4. All volatile materials were removed under reduced
pressure. Residue was recrystallized from n-hexane. Compound 4 was
isolated as a white microcrystal material, m.p. 80-81 oC. Yield: 0.32 g,
3
58%. 1H NMR (CDCl3, 400.130 MHz): δ 7.32 (d, 6H, JH-H = 7.8 Hz, p-
C6H4), 7.14 (d, 6H, 3JH-H = 7.8Hz, p-C6H4), 2.35 (s, 9H, p-С6Н4CH3), 0.33
2
2
(s, 6H, JH-29Si = 3.1 Hz, SiMe2), -0.01 (s, 9H, JH-29Si = 3.1 Hz, SiMe3).
13C{1H} NMR (CDCl3, 100.613 MHz): δ 137.78 (ipso-C6H4), 135.34,
128.87 (p- and m-C6H4) (aromatic carbons), 21.41 (p-C6H4CH3), -1.46
(SiMe2), -4.56 (SiMe3). One ipso aromatic carbon atom was not found
due to overlapping with an another signal. 29Si NMR (CDCl3, 79.495
MHz): δ -15.36 (SiMe3), -38.94 (SiMe2). MS (EI, %): 477 ([M+], 17), 462
([M – Me]+, 4), 404 ([M – SiMe3]+, 11), 346 ([Tol3Ge]+, 100), 313 ([M –
SiMe3 – Tol]+, 16), 255 ([Tol2Ge]+, 41), 165 ([TolGe + H]+, 17), 131
([Si2Me5]+, 12). UV (CH2Cl2), λmax, nm (ε, M-1 cm-1): 241 (2.5×104).
Elemental analysis Calc. for C26H36GeSi2 (477.345): C 65.42, H 7.60.
Found: C 65.24, H 7.49.
X-ray crystallography. Experimental intensities were measured on a
STADIVARI Pilatus (for 5 and 6) diffractometer using -scan mode.
Absorption correction based on measurements of equivalent reflections
was applied. The structures were solved by direct methods and refined
by full matrix least-squares based on F2 with anisotropic thermal
parameters for all non-hydrogen atoms. All aromatic hydrogen atoms
were placed in calculated positions. All H atoms were refined using a
riding model. Details of X-ray studies are given in Table S1 (Supporting
Information).
2,2-Dimethyl-1,1,1,3,3,3-hexakis(p-tolyl)trigermane,
(p-Tol)3Ge-
GeMe2-Ge(p-Tol)3 (5). Solution of Me2GeCl2 (0.20 mL, 1.69 mmol) in
ether (20 mL) was added dropwise to the ethereal solution of (p-
Tol)3GeLi obtained as described above from (p-Tol)3GeH (1.17 g, 3.37
mmol) and nBuLi in n-hexane (2.5 M, 1.35 ml, 3.37 mmol). Reaction
mixture was stirred overnight. Then water (20 mL) was added, water
phase was extracted with ether (3х20 mL), combined organic phases
were dried over anhydrous Na2SO4. All volatile materials were removed
under reduced pressure. Residue was recrystallized from the mixture of
n-hexane/dichloromethane. Compound 5 was isolated as a white powder,
m.p. 205-206oC. Yield: 1.12 g, 84%. 1H NMR (CDCl3, 400.130 MHz): δ
7.18 (d, 12H, 3JH-H = 7.8 Hz, p-C6H4CH3), 7.03 (d, 12H, 3JH-H = 7.8 Hz, p-
C6H4CH3), 2.35 (s, 18H, p-C6H4CH3), 0.59 (s, 6H, GeMe2). 13C{1H} NMR
(CDCl3, 100.613 MHz): δ 137.91, 134.65 (2 ipso-C6H4CH3-p), 135.45,
128.87 (p- and m-C6H4CH3) (aromatic carbons), 21.35 (p-C6H4CH3), -
2.14 (GeMe2). MS (EI, %): 795 ([M+], 3), 780 ([M – Me]+, 2), 449 ([M –
GeTol3]+, 9), 346 ([Tol3Ge]+, 100), 255 ([Tol2Ge]+, 21), 165 ([TolGe + H]+,
14). UV (CH2Cl2), λmax, nm (ε, M-1 cm-1): 251 (4.0×104). Elemental
analysis calc. for C44H48Ge3 (794.6819): C 66.50, H 6.09. Found: C 66.40,
H 6.03.
Crystallographic Data Centre as supplementary publications under the
CCDC numbers 1510022-1510023. This information may be obtained
free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was financially supported by the Russian President
Grant for Young Russian Scientists (MK-1790.2014.3 - KVZ), in
part by M.V. Lomonosov Moscow State University Program of
Development (KVZ), Nazarbayev University (ORAU grant for
Medicinal Electrochemistry - KL) and the Ministry of Education
and Science of Kazakhstan (KL).
Keywords: oligogermanes • silagermanes • luminescence •
organic semiconductor • X-ray analysis
1,1,2,2-Tetramethyl-1,2-bis[tris(p-tolyl)germyl]disilane,
(p-Tol)3Ge-
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10.1002/asia.201700151
Chemistry - An Asian Journal
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Semiconducting
molecular
Kirill V. Zaitsev,* Viktor A. Tafeenko, Yuri
F. Oprunenko, Anastasia V. Kharcheva,
Zhaisan Zhanabil, Yerlan Suleimen,
Kevin Lam,* Vladimir B. Zaitsev, Anna V.
Zaitseva, Galina S. Zaitseva and Sergey
S. Karlov
oligogermanes: A series of molecular
catenated Group 14 derivatives was
obtained and their structure, optical
(UV/vis absorbance, luminescence)
and electrochemical properties were
studied. A band gap (about 3.3 eV)
was measured for such compounds
what indicates that these molecules
have semiconductor properties.
Page No. – Page No.
Molecular oligogermanes and related
compounds: structure, optical and
semiconductor properties
10
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