2,5-Digermaselenophenes: Germanium Analogues of Selenophenes

Article abstract of DOI:10.1021/jacs.8b07588

A stable crystalline 2,5-digermaselenophene was synthesized. In contrast to hitherto reported selenophenes, this digermaselenophene exhibits a trans-pyramidalized structure, which is due to its electronic properties. The practical utility of this 2,5-digermaselenophene is reflected in its ability to activate dihydrogen and acetylene at room temperature in the absence of a transition-metal complex, and this behavior can be rationalized on the basis of its physicochemical properties, which are characterized by considerable electron-donating and -accepting abilities.

Full text of DOI:10.1021/jacs.8b07588

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2,5-Digermaselenophenes: Germanium Analogues of Selenophenes
Tomohiro Sugahara, Takahiro Sasamori, and Norihiro Tokitoh
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07588 • Publication Date (Web): 25 Aug 2018
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Journal of the American Chemical Society
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2,5-Digermaselenophenes: Germanium Analogues of Selenophenes
Tomohiro Sugahara,1 Takahiro Sasamori,*2 and Norihiro Tokitoh1,3
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1 Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
2 Graduate School of Natural Sciences, Nagoya City University, Yamanohata 1, Mizuho-cho, Mizuho-ku, Nagoya, Aichi 467-8501, Japan.
3 Integrated Research Consortium on Chemical Sciences, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
Supporting Information Placeholder
ic materials, but no example of a thiophene/selenophene
ABSTRACT:
A
stable
crystalline
2,5-
derivative with very small HOMO-LUMO gap enough
to activate small molecules. Moreover, the variety of
available organic functional groups and molecular de-
signs should be limited, since the physical properties of
thiophenes and selenophenes predominantly reflect the
p-electron-properties of the butadiene moiety, which
suggests that substitution with organic substituents
and/or replacement of the sulfur atom with selenium
should not induce a remarkable improvement. The re-
placement of the carbon atoms in the butadiene moiety
in thiophenes or selenophenes with heavier main-group
elements has not yet been investigated in detail, mostly
due to a lack of effective synthetic strategies or appro-
priate precursors.
digermaselenophene was synthesized. In contrast to
hitherto reported selenophenes, this digermaselenophene
exhibits a trans-pyramidalized structure, which is due to
its electronic properties. The practical utility of this 2,5-
digermaselenophene is reflected in its ability to activate
dihydrogen and acetylene at room temperature in the
absence of a transition-metal complex, and this behavior
can be rationalized on the basis of its physico-chemical
properties, which are characterized by considerable elec-
tron-donating and -accepting abilities.
Organic optoelectronic materials are considered
the next generation of electron-transporting materials.1
Suitable building blocks for such advanced optoelectron-
ic materials should be designed to exhibit the following
physical properties: (i) a low LUMO level, (ii) a high
HOMO level, and (iii) a corresponding small HOMO-
LUMO gap in order to promote the transfer of electrons.
These very requirements should also be advantageous
for functional main-group-element-containing molecules
that aim at the activation of small inert molecules.2 On
account of their potential electron-donating ability, thio-
phenes often serve as building blocks for organic optoe-
lectronic materials3 such as organic field-effect transis-
tors (OFETs), organic light-emitting diodes (OLEDs),
and organic photovoltaics (OPVs). Much research has
been focused on modifying thiophenes, e.g. via exten-
sion of their p-conjugation system and the introduction
of p-conjugated functional substituents, which are ex-
pected to diminish the HOMO-LUMO gap under con-
comitant improvement of the electron-transporting abil-
ity.4 Despite the small HOMO-LUMO gap of a thio-
phene, even such thiophene derivatives would not acti-
vate small inert molecules. Similarly, selenophenes, i.e.,
selenium analogues of thiophenes, are also excellent
building blocks for organic optoelectronic materials.
Specifically, they exhibit a smaller ionization energy
relative to thiophenes, i.e., an effective electron-donating
ability.5 While the HOMO-LUMO gaps of thiophenes
and selenophenes may be small enough as optoelectron-
Tbb
Tbb
Se
Tbb
Tbb
Ge Ge
Ge
Ge
(Me2N)3P=Se
+ (Me2N)3P
C6D6
r.t.
Ph
1 (quant.)
Ph
Ph
Ph
2
Se
Tbb
Ph
Tbb
Ge Ge
Se (excess)
C6D6, 60 ºC
Ph
3
Se
Se
Se Se
Se
Ge
Se (excess)
C6D6, 40 ºC
+
Tbb
Tbb Tbb
Tbb
Ge
Ge
Ge
SiMe3
Ph
Ph
Ph
Ph
Me3Si
Tbb =
Me3Si
4
5
75% from 1
80% from 2
21% from 1
20% from 2
(estimated by 1H NMR spectroscopy)
SiMe3
Scheme 1. Synthesis of 2,5-digermaselenophene 1, and
subsequent treatment with elemental selenium to give 4
and 5.
Recent developments in the area of main-group-
element chemistry have allowed the preparation of sta-
ble p-bonded species of heavier-group-14 elements,6
which demonstrates that the incorporation of heavier
main-group elements into p-systems significantly in-
creases the electron affinity of the latter due to their ex-
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Page 2 of 12
An X-ray crystallographic analysis (Figure 1)13
tremely low-lying p*-orbitals. Thus,
a
2,5-
1
2
3
4
5
6
7
8
revealed that 2,5-digermaselenophene 1 exhibits a non-
planar structure with a trans-pyramidalized geometry of
the five-membered [SeGe2C2] ring with a trans-
pyramidalized angle (a) of 7.8˚, which stands in sharp
contrast to the completely planar structure of previously
reported selenophenes.14 However, the bent structure of
1 should not be attributed to the sterically demanding
Tbb substituents on the Ge atoms, but rather to the in-
trinsic nature of the 2,5-digermaselenophene. The results
of theoretical calculations on the parent 2,5-
digermaselenophene SeGe2C2H4 suggested that its pla-
nar structure is a transition state; the energy minimum is
the trans-pyramidalized structure, which is 7.2 kcal/mol
lower in SCF-energy relative to the planar geometry.
This result can be rationalized in terms of the weakness
of the Ge-containing p-bond, i.e., given the small over-
lap of the 4p orbitals, a 2,5-digermaselenophene would
adopt a trans-pyramidalized geometry to form a C1=C2
p-bond rather than keeping its Ge1=C1 and Ge2=C2 p-
bonds. In other words, the contribution of resonance
structures B or C (Scheme 2) should become dominant.
dimetallaselenophene, which contains heavier-group-14
elements instead of carbon atoms at the 2- and 5-
positions, should represent fascinating physical and
chemical properties due to its unique electronic struc-
ture.7 Herein, we report the synthesis of digermaseleno-
phene 1 and its ability to activate small molecules,
which renders 1 also a promising prospective building
block for advanced optoelectronic materials.
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1,2-Digermacyclobutadiene 2 can serve as a
suitable precursor for heterocycles, given that insertion
reactions may occur at the reactive Ge=Ge p-bond.8,9
When 2, which was obtained from the reaction between
the stable digermyne TbbGeºGeTbb (Tbb = 2,6-
[CH(SiMe3)2]2-4-t-butyl-phenyl) with diphenylacety-
lene,10 was treated with an equimolar amount of
(Me2N)3P=Se in C6D6 at room temperature, the color of
the reaction mixture changed from dark red to purple.
Removal of the solvent and (Me2N)3P under reduced
pressure afforded 2,5-digermaselenophene 1 as purple
crystals (Scheme 1).11 While 1 is extremely sensitive
toward air and moisture, which results in complicated
product mixtures, 1 is thermally stable in benzene upon
heating to 80 ˚C under an atmosphere of inert gas. The
formation of 1 should most likely be interpreted in terms
of the formation of selenadigermirane 3 as an intermedi-
ate, which could easily undergo transformation into 1.12
Although the reaction of 2 with (Me2N)3P=Se was care-
fully monitored by 1H NMR spectroscopy, only the sig-
nals of 1 and 2 were observed under light shielding con-
ditions, due to the facile conversion of 2 into 1. In a pre-
vious study, the selenization of 2 with elemental seleni-
um resulted in the formation of heterocycles 4 and 5.9c
Since further selenization of 1 with elemental selenium
afforded 4 and 5 exclusively, 1 could be conceived as an
intermediate in the reaction of 2 with elemental selenium
that affords 4 and 5.
Se
Se
Se
R
R
R
R
R
R
Ge
Ge
Ge
Ge
Ge
Ge
A
B
C
singlet open shell
Ge
Ge
Se
Se
Ge
Ge
B
C
weak π-interaction
donor-acceptor interaction
supported by the lone pair of Se between Se(0) and germylenes
Scheme 2. Conceivable resonance structures for a 2,5-
digermaselenophene.
It should be noted that the results of NBO cal-
culations15 revealed a double-bond character for the
C1=C2 bond, and p-type bonding interaction between
the Ge atoms, thus supporting a 3-center-4-electron p-
bonding16 through the stabilization of the electron-
deficient Ge atoms by coordination of lone pairs of the
Se atoms in resonance structure B. Conversely, the re-
sults of NPA calculations revealed a negative charge at
the Se atom and a positive charge at the Ge atoms, sup-
porting electron donation from the Ge atoms to the cen-
tral Se atom, indicative of non-negligible contributions
from the donor-acceptor model C (NPA charges for 1:
qSe = –0.35; qGe = +0.90). In their entirety, these results
suggest that such a 2,5-digermaselenophene should ex-
hibit a trans-pyramidalized structure at the energetic
minimum due to the predominant contribution of B to-
gether with minor contributions from the donor-acceptor
interactions of C. The results of the theoretical calcula-
tions on the real model (1) and the parent models of se-
C(Tbb)
Ge1
(b)
Se
C2
(a)
C1
Ge2
C(Tbb)
pyramidalized angle (α)a = 7.8º
C1C2
(c)
2.3153(4)
2.3118(4)
Ge2
Ge1
Se
Se
Ge1 Ge2
C(Tbb)
1.922(3)
C(Tbb)
1.921(3)
C1 C2
1.375(4)
Figure 1. Molecular structure of 1 (ORTEP drawing with
thermal ellipsoids set to 50% probability). The minor parts
of the disordered moieties, hydrogen atoms, and the sol-
vents are omitted for clarity; (a) entire molecule, (b) side
view of the [SeGe2C2] ring, and (c) top view of the
[SeGe2C2] ring; selected bond lengths are shown in Å. aThe
pyramidalized angle (a) is defined as the angle between the
Se–Ge1–Ge2 and Se–C1–C2 planes.
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Journal of the American Chemical Society
lenophene derivatives (1’, 6, 7) are summarized in Table Due to the trans-pyramidalized structure of 1,
1
2
3
4
5
6
7
8
1. The C1–C2 bond length of 1 is relatively short
[1.375(4) Å], suggesting a partial π-bond character,
while the Ge–C bond lengths in the five-membered ring
are elongated relative to those of previously reported
Ge=C bonds (ca. 1.83 Å).17 In addition, a comparison of
the structural parameters of the simple models
[SeE2C2H4] (E = C (7), Si (6), Ge(1’)) clearly suggests
an increasing contribution of resonance structure A as
the E atom descends the periodic table.
the energy levels of the slipped p-type HOMO (–5.04
eV) and LUMO (–2.69 eV) are raised and lowered, re-
spectively.6 In addition, the small HOMO-LUMO gap of
1 (2.35 eV)7 is responsible for its purple color, which
stands in contrast to conventional selenophenes, which
are typically colorless/pale yellow. The UV/vis spectrum
of 1 in hexane reveals a strong absorption at lmax = 536
nm (e 7,800), which was assigned to the HOMO-LUMO
transition based on TDDFT calculations (533.6 nm; f =
0.2164).19 Such an absorption at longer wavelengths
suggests that 2,5-digermaselenophenes could serve as
promising building blocks for optoelectronic materials.
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Table 1. Results of the theoretical calculations.
1a
1b
1’c
6c
7c
Se
R
R
(a)
(b)
E1
E2
E=Ge
R=Tbb
R’=Ph
E=Ge
E=Ge
E=Si
R=H
R’=H
2.258
2.258
E=C
R=H
R’=H
1.856
1.856
R=Tbb R=H
R’=Ph R’=H
C1 C2
R'
R'
Se–E1/Å
Se–E2/Å
E1–C1/Å
E2–C2/Å
C1–C2/Å
2.3118(4) 2.325
2.3153(4) 2.325
2.345
2.344
1.922(3)
1.918
1.915
1.809
1.376
1.921(3)
σ∗
σ
H
H
H
H
1.375(4)
91.21(2)
7.8
481.8e
176.8e
---
1.390
92.5
1.368
90.9
8.5
1.385
89.4
8.5
1.420
87.6
0.0
Ge
Se
Ge
Se
E1–Se–E2/º
a/º d
dSe/ppm
HOMO Ge
Ge
LUMO
7.0
520.6f
188.3f
–11.2f
–8.0f
---
---
---
d
C1/2/ppm
---
---
---
Figure 2. (a) HOMO of 1 (–5.04 eV) and schematic illus-
tration of the interaction with the s* orbital of H2. (b)
LUMO of 1 (–2.69 eV) and schematic illustration of the
interaction with the
311G(3df)<Se,Ge,Si>/6-311G(d)<C,H> level of theory.
NICS(0)
NICS(1)
–12.5i
–10.5i
–12.6i
–10.4i
–13.4i
–10.9i
---
aObserved by XRD analysis. bOptimized at the B3PW91/6-
311G(3df)<Se,Ge,Si>/6-311G(d)<C,H> level of theory cOpti-
s orbital of H2. B3PW91/6-
d
mized at MP2/6-311G(3d) level of theory. Defined as the angle
e
between the Se–E1–E2 and Se–C1–C2 planes. Measured at r.t. in
C6D6.
fCalculated
at
the
GIAO-B3PW91/6-
i
Se
311G(3df)<Se,Ge,Si>/6-311G(d)<C,H> level of theory. Calculat-
ed at GIAO-MP2/6-311G(3d) level of theory.
Tbb
Tbb
Ge
Ge
(1 atm)
hexane, r.t.
Ph
47%a (quant.)b
Ph
6
Based on the calculated NICS values (NICS(0)
= –11.2; NICS(1) = –8.0), digermaselenophene 1 should,
despite its trans-pyramidalized structure, exhibit consid-
erable levels of aromaticity similar to parent seleno-
phene 7. In addition, NICSzz(r)18 scan calculations de-
livered an NICSzz(1.3) value of –24.1 for 1’, which is
slightly smaller but still comparable to the NICSzz(1.0)
value for a selenophene (–29.2). Accordingly, it can be
concluded that 2,5-digermaselenophenes should exhibit
aromaticity similar to that of selenophenes and 2,5-
disilaselenophenes. Due to the aromatic p-conjugation
system in digermaselenophene 1, a small ionization en-
ergy and a high electron affinity were calculated, sug-
gesting excellent electron-donating and -accepting abili-
SiMe3
Se
H
Se
SiMe3
(3 equiv.)
Tbb
Tbb
Ge
Ge
Ge
Ge
Tbb
Tbb
hexane, r.t.
Ph
Ph
Ph
Ph
7
1
46%a (quant.)b
H
H
Se
Ge
Ge
H2 (1 atm)
hexane, r.t.
Tbb
Tbb
Ph
Ph
8
39%a (quant.)b
Scheme 3. Reactions of 2,5-digermaselenophene 1 with
small molecules. aIsolated yield. bYield estimated based
on the 1H NMR spectrum of the crude mixture.
ties. The low-field shifted 77Se NMR resonance (dSe
=
481.8 in C6D6) for digermaselenophene 1, which could
be reproduced by GIAO calculations (dSe = 520.6), sug-
gests that 1 retains its trans-pyramidalized structure in
solution.19
Considering the frontier orbitals of 1, and espe-
cially the small HOMO-LUMO gap, 1 could be highly
susceptible toward pericyclic or addition reactions. The
HOMO and LUMO of 1 (Figure 2) exhibit a predomi-
nant contribution of the p-type Ge–Se–Ge 3-center-4-
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Page 4 of 12
electron p-bond.16 As expected, digermaselenophene 1
readily reacts with alkynes and H2 under ambient condi-
tions to form the corresponding adducts in moderate
yield (Scheme 3). Notably, 1 is able to activate and fix
molecular hydrogen and acetylene in the absence of any
transition metal.2a,20
4. For reviews, see: (a) Cinar, M. E.; Ozturk, T., Chem. Rev. 2015,
115, 3036-3140. (b) Anthony, J. E., Chem. Rev. 2006, 5028-
5048. (c) Murphy, A. R.; Fréchet, M. J., Chem. Rev. 2007, 107,
1066-1096. (d) Mishra, A.; Ma, C.; Bäuerle, P., Chem. Rev.
2009, 109, 1141-1276.
1
2
3
4
5
6
7
8
5. (a) Ma, J.; Li, S.; Jiang, Y., Macromolecules 2002, 35, 1109-
1115. (b) Jeffries-EL, M.; Kobilka, B. M.; Hale, B. J., Macromo-
lecules 2014, 47, 7253-7271. (c) He, X.; Baumgartner, T., RSC
Adv. 2013, 3, 11334-11350. (d) Al-Hashimi, M.; Han, Y.; Smith,
J.; Bazzi, H. S.; Alqaradawi, S. Y. A.; Watkins, S. E.; Anthopou-
los, T. D.; Heeney, M., Chem. Sci. 2016, 7, 1093-1099.
6. (a) Weidenbruch, M., J. Organomet. Chem. 2002, 646, 39-52.
(b) Wang, Y.; Robinson, G. H., Chem. Commun. 2009, 5201-
5213. (c) Fischer, R. C.; Power, P. P., Chem. Rev. 2010, 110,
3877-3923. (d) Asay, M.; Sekiguchi, A., Bull. Chem. Soc. Jpn.
2012, 85, 1245-1261. (e) Sasamori, T.; Tokitoh, N., Bull. Chem.
Soc. Jpn. 2013, 86, 1005-1021.
7. The HOMO/LUMO levels of selenophene (–8.79/2.91), 2,5-
disilaselenophene (–6.79/–0.04), and 2,5-digermaselenophene (–
6.84/–0.61 eV) were calculated at the MP2/6-311G(d) level of
theory. Thus, calculated HOMO-LUMO gaps of parent mole-
cules of a selenophene and a 2,5-digermaselenophene are 11.7
eV and 6.23 eV.
8. Cui, C.; Olmstead, M. M.; Power, P. P., J. Am. Chem. Soc. 2004,
126, 5062-5063.
9. (a) Sugahara, T.; Sasamori, T.; Tokitoh, N., Angew. Chem., Int.
Ed. 2017, 9920-9923. (b) Sugahara, T.; Tokitoh, N.; Sasamori,
T., Inorganics 2017, 5, 79-86. (c) Sugahara, T.; Sasamori, T.;
Tokitoh, N., Chem. Lett. 2018, 47, 719-722.
10. Sugahara, T.; Guo, J.-D.; Sasamori, T.; Karatsu, Y.; Furukawa,
Y.; Ferao, A. E.; Nagase, S.; Tokitoh, N., Bull. Chem. Soc. Jpn.
2016, 89, 1375-1384.
11. Compound 1: purple crystals, mp 102 °C (dec), dSe = 481.8
(C6D6), UV−vis (n-hexane) λmax (nm, ε) = 530 (7,800). For de-
tailed data, see Supporting Information.
In conclusion, we have demonstrated the syn-
thesis of stable 2,5-digermaselenophene 1, and demon-
strated its ability to activate small molecules such as
dihydrogen and acetylenes. Given the electronic struc-
ture of 1 and its high reactivity toward the activation of
small molecules, 1 is also a promising prospective build-
ing block for advanced optoelectronic materials. Further
investigations into the electrochemical properties of such
digermaselenophenes are currently in progress in our
laboratories.
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ASSOCIATED CONTENT
Supporting Information. The Supporting Information is
available free of charge on the ACS Publications website.
Experimental section (PDF), spectral data (PDF) crystallo-
graphic data (CIF), and theoretically optimized coordinates
(XYZ) are included in the SI.
AUTHOR INFORMATION
Corresponding Author
sasamori@nsc.nagoya-cu.ac.jp
12. (a) Tsumuraya, T.; Sato, S.; Ando, W., Organometallics 1998, 7,
2015-2019. (b) Sasamori, T.; Miyamoto, H.; Sakai, H.; Furuka-
wa, Y.; Tokitoh, N., Organometallics 2012, 31, 3904-3910.
13. [1·hexane] (C62H108Ge2SeSi8, C6H14): M = 1388.51, l = 0.71069
Å, T = –170 ºC, triclinic, P–1 (no. 2), a = 13.3399(2) Å, b =
ORCID
Takahiro Sasamori: 0000-0001-5410-8488
Norihiro Tokitoh: 0000-0003-1083-7245
17.1478(3) Å, c
=
19.6332(3) Å,
a
=
95.455(1)˚,
b
=
=
ACKNOWLEDGMENT
108.200(1)˚, g = 105.365(2)˚, V = 4035.66(12) Å3, Z = 2, Dcalc
1.143 g cm–3, µ = 1.348 mm–1, 2qmax = 50.0˚, measd./unique refls.
= 71755/14184 (Rint = 0.0557), param = 769, GOF = 1.145, R1 =
0.0401/0.0560 [I>2s(I)/all data], wR2
Dedicated to Emeritus Professor Kohei Tamao on the occa-
sion of his 77th birthday.
=
0.1041/0.1108
This work was supported by JSPS KAKENHI grants
JP15H03777 and JP 15K13640 from MEXT (Japan), a
Grant-in-Aid for Research at Nagoya City University, as
well as JURC, IRCCS, and KURCA at Kyoto University. T.
Sugahara gratefully acknowledges support from a grant-in-
aid for JSPS fellows (JP16J05501). We would like to thank
Mr. Toshiaki Noda and Ms. Hideko Natsume (Nagoya Uni-
versity) for the expert manufacturing of custom-tailored
glassware.
[I>2s(I)/all data], largest diff. peak and hole 1.989 and –1.137
e.Å–3 (CCDC-1849658).
14. For examples, see: (a) Masuda, T.; Inagaki, Y.; Momma, H.;
Kwon, E.; Setaka, W., New. J. Chem. 2016, 40, 8593-8599. (b)
Patra, A.; Wijsboom, Y. H.; Leitus, G.; Bendikov, M., Org. Lett.
2009, 11, 1487-1490. (c) Hua, G.; Henry, J. B.; Li, Y.; Mount, A.
R.; Slawin, A. M. Z.; Woollins, J. D., Org. Biomol. Chem. 2010,
8, 1655-1660. (d) Beswick, M. A.; Harmer, C. N.; Raithby, P.
R.; Steiner, A.; Tombul, M.; Wright, D. S., J. Organomet. Chem.
1999, 573, 267-271.
15. Glendening, E. D.; Landis, C. R.; Weinhold, F. J. Comp. Chem.
2013, 34, 1429-1437.
16. Sugamata, K.; Hashizume, D.; Suzuki, Y.; Sasamori, T.; Ishii, S.,
Chem.-Eur. J. 2018, 24, 6922-6926.
17. (a) Meyer, H.; Baum, G.; Massa, W.; Berndt, A., Angew. Chem.,
Int. Ed. Engl. 1987, 26, 798-799. (b) Couret, C.; Escudie, J.;
Satge, J.; Lazraq, M., J. Am. Chem. Soc. 1987, 109, 4411-4412.
(c) Tokitoh, N.; Kishikawa, K.; Okazaki, R., J. Chem. Soc.
Chem. Commun. 1995, 1425-1426.
18. Stanger, A., J. Org. Chem. 2006, 71, 883-893.
19. TDDFT calculations were carried out at the B3PW91/6-
311G(3df)<Se,Ge,Si>, 6-311G(d)<C,H> level of theory.
20. Spikes, G. H.; Fettinger, J. C.; Power, P. P., J. Am. Chem. Soc.
2005, 127, 12232-12233.
REFERENCES
1. (a) Ostroverkhova, O., Chem. Rev. 2016, 116, 13279-13412. (b)
Günes, S.; Neugebauer, H.; Sariciftci, N. S., Chem. Rev. 2007,
107, 1324-1338. (c) Ameri, T.; Khoram, P.; Min, J.; Brabec, C.
J., Adv. Mater. 2013, 25, 4245-4266. (d) Boudreault, P. T.;
Najari, A.; Leclerc, M., Chem. Mater. 2011, 23, 456-469.
2. (a) Chu, T.; Nikonov, G. I., Chem. Rev. 2018, 118, 3608-3680.
(b) Power, P. P., Nature 2010, 463, 171-177.
3. For examples, see: (a) Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang,
S.; Hou, J., Chem. Rev. 2016, 116, 7397-7457. (b) Sirringhaus,
H., Adv. Mater. 2014, 26, 1319-1335. (c) He, Y.; Hong, W.; Li,
Y., J. Mater. Chem. C. 2014, 2, 8651-8661. (d) Gather, M. C.;
Köhnen, A.; Meerholz, K., Adv. Mater. 2011, 23, 233-248.
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