Synthesis and unexpected reactivity of Si-H functionalized dithieno[3,2-b:2′,3′-d]phospholes

Article abstract of DOI:10.1021/ol0529288

Si-H functionalized, blue light-emitting dithieno[3,2-b:2′,3′- d]phospholes are accessible by reaction of an appropriate bithiophene precursor with a dichlorophosphane. Subsequent functionalization of the central phosphorus center allows for a fine-tuning

Full text of DOI:10.1021/ol0529288

ORGANIC
LETTERS
2006
Vol. 8, No. 3
503-506
Synthesis and Unexpected Reactivity of
Si H Functionalized
Dithieno[3,2-b:2 ,3 -d]phospholes
−
′
′
Thomas Baumgartner* and Wolfram Wilk
Institute of Inorganic Chemistry, RWTH-Aachen UniVersity, Landoltweg 1,
52074 Aachen, Germany
thomas.baumgartner@ac.rwth-aachen.de
Received December 2, 2005
ABSTRACT
Si
with a dichlorophosphane. Subsequent functionalization of the central phosphorus center allows for a fine-tuning of the optoelectronic properties
of the material. Pt-catalyzed reaction of the Si H functionalities with alkynes affords the hydrosilation products including a polymer by reaction
with 1,7-octadiyne. By contrast, the absence of any substrate leads to the exclusive formation of a polymer via dehydrogenative homocoupling.
−H functionalized, blue light-emitting dithieno[3,2-b:2′,3′-d]phospholes are accessible by reaction of an appropriate bithiophene precursor
−
The incorporation of phosphorus centers into π-conjugated
materials for molecular electronics and optoelectronics is a
young, emerging field of research.1 Phosphole-based materi-
als, in particular, are very intriguing candidates for respective
applications owing to their unique electronic structure (e.g.,
low degree of lone-pair delocalization; low lying LUMO).2
The versatile reactivity of the central phosphorus atom allows
for an efficient fine-tuning of the electronic properties of
the materials by very simple chemical modifications.3,4 These
can include oxidation or complexation reactions via Lewis
acids or transition metal complexes. In this context, we have
recently established the novel dithieno[3,2-b:2′,3′-d]phos-
phole system with very advantageous and readily tunable
photophysical properties that can be attributed to the nature
of the bridging phosphorus atom.4
To successfully utilize novel π-conjugated materials in
optoelectronic applications, it appears necessary, however,
(3) (a) Hay, C.; Hissler, M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet,
L.; Nyula´szi, L.; Re´au, R. Chem. Eur. J. 2001, 7, 4222. (b) Fave, C.; Cho,
T.-Y.; Hissler, M.; Chen, C.-W.; Luh, T.-Y.; Wu, C.-C.; Re´au, R. J. Am.
Chem. Soc. 2003, 125, 9254. (c) Fave, C.; Hissler, M.; Ka´rpa´ti, T.; Rault-
Berthelot, J.; Deborde, V.; Toupet, L.; Nyula´szi, L.; Re´au, R. J. Am. Chem.
Soc. 2004, 126, 6058.
(4) (a) Baumgartner, T.; Bergmans, W.; Ka´rpa´ti, T.; Neumann, T.; Nieger,
M.; Nyula´szi, L. Chem. Eur. J. 2005, 11, 4687. (b) Baumgartner, T.;
Neumann T.; Wirges, B. Angew. Chem., Int. Ed. 2004, 43, 6197. (c) Dienes,
Y.; Eggenstein, M.; Neumann, T.; Englert, U.; Baumgartner, T. Dalton
Trans. In press.
* Address correspondence to this author. Fax: +49-241-809-2644.
Phone: +49-241-809-9684.
(1) (a) Mathey, F. Angew. Chem., Int. Ed. 2003, 42, 1578. (b) Jin, Z.;
Lucht, B. L. J. Organomet. Chem. 2002, 653, 167. (c) Wright, V.; Gates,
D. P. Angew. Chem., Int. Ed. 2002, 41, 2389. (d) Smith, R. C.; Protasiewicz,
J. D. J. Am. Chem. Soc. 2004, 126, 2268. (e) Hissler, M.; Lescop, C.; Re´au,
R. J. Organomet. Chem. 2005, 690, 2482. (f) Gates, D. P. Top. Curr. Chem.
2005, 250, 107. (g) Hissler. M.; Dyer, P. W.; Re´au, R. Top. Curr. Chem.
2005, 250, 127.
(2) Mathey, F. Chem. ReV. 1988, 88, 429.
10.1021/ol0529288 CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/04/2006

to be able to form very thin films of the respective materials.5
One of the major strategies to approach this issue is the spin
coating of a solution of the material onto a substrate, which
is suitable for oligomeric/polymeric systems as they tend to
form films easily upon removal of the solvent.5a,6
Here we report on the synthesis of Si-H functionalized
dithieno[3,2-b:2′,3′-d]phospholes and the fine-tuning of their
optoelectronic properties by simple chemical modifications
performed at the phosphorus center. Their hydrosilation
reactivity toward alkynes and the serendipitous dehydroge-
native homocoupling to access polymeric dithienophosphole
materials potentially suitable for optoelectronic applications
are also presented.
The Si-H functionalized dithienophosphole 2 is accessible
in good yield (76%) via a similar protocol as reported for
other silylated dithienophospholes.4 Reaction of 5,5′-bis-
(dimethylsilyl)-3,3′-dibromo-2,2′-dithiophene7 1 with n-BuLi
in the presence of TMEDA in Et2O at -78 °C affords the
product 2 after addition of phenyl(dichloro)phosphane (Scheme
1). The 31P NMR spectrum of 2 shows a resonance at δ
As already indicated, the electronic nature of the phos-
phorus center has a significant impact on the optoelectronic
properties of the dithienophosphole moiety. Chemical modi-
fications performed at the trivalent phosphorus center in
general lead to materials with significantly red-shifted
absorption and emission wavelength maxima that are coher-
ent with a lowered energy of the π*-LUMO level.4a This is
also observed with the chemically modified dithienophos-
pholes 3-5 that are accessible in almost quantitative yields
by reaction of 2 with borane (added as H3B‚SMe2), hydrogen
peroxide, or sulfur, respectively (Scheme 1). Their downfield
shifted 31P NMR resonances at δ 12.5 (3), 17.1 (4), and 23.6
ppm (5) correlate well with those observed for related
compounds4a and correspond to the increased electron
acceptor character of the phosphorus center. The same applies
to the optoelectronic properties of the three species showing
red-shifted absorption and emission maxima, compared with
the values for 2 (see Table 1). The presence of the Si-H
Table 1. Photoluminescence Data of the Dithienophospholes
2-8 (c ) 1 × 10-4 M in CH2Cl2)
Scheme 1. Synthesis and Functionalization of Si-H
8
compd
λex [nm]a
λem [nm]b
φPL
Substituted Dithienophospholes
2
3
4
5
6
7
8
366
376
383
383
357 (sh), 399
378
353 (sh), 393
420
447
457
457
459
460
459
0.810
0.634
0.581
0.615
0.548
0.562
0.566
a Maximum wavelength of absorption. b Maximum wavelength of emis-
sion.
1
resonance in the H NMR spectra of the three compounds
3-5 indicates that this functionality is not affected by the
reaction conditions applied and that it can be utilized in
subsequent experiments.
To probe the hydrosilation reactivity of the Si-H func-
tionalized dithienophospholes, we first focused upon suitable
model compounds to optimize the reaction conditions. The
primary substrate for these reactions was 1,2-diphenylacety-
lene (tolane). Furthermore, all hydrosilation experiments were
performed with the oxidized dithienophosphole 4 exclusively,
to prevent potential poisoning of the employed catalyst
(PtDVDS, “Karstedt catalyst”);9 as shown recently by our
group, the phosphorus center in dithienophospholes is an
excellent ligand for corresponding platinum complexes.4c
The reaction of 4 with 2 equiv of tolane works best in
dichloromethane at room temperature with a few drops of
Karstedt catalyst solution9 added (Scheme 2). These condi-
tions afford a quantitative conversion to the desired product
6 overnight, which could be isolated in good yield (75%) as
a light yellow, sticky oil. The successful generation of 6 is
supported by the disappearance of the Si-H resonance in
-24.1 ppm that is slightly downfield shifted compared to
other related silyl-functionalized dithienophospholes (δ 31P
-25.0 to -28.2 ppm).4a The 1H NMR spectrum exhibits the
characteristic Si-H resonance at δ 4.54 ppm with the
expected septet splitting due to the coupling to six equivalent
methyl protons. Similar to all known dithienophospholes,4
compound 2 also exhibits very pronounced optoelectronic
properties with a maximum wavelength of absorption at λex
) 366 nm, a maximum wavelength of emission at λem
)
420 nm (blue), and an exceptionally high photoluminescence
quantum yield efficiency of φPL ) 0.810.8
(5) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002,
14, 99. (b) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34,
359.
(6) Middleman, S.; Hochberg, A. K. Process Engineering Analysis in
Semiconductor DeVice Fabrication; McGraw-Hill:New York, 1993; p 313.
(7) Baumgartner, T. Appl. Organomet. Chem. 2005, 19, 859.
(8) Relative to quinine sulfate (0.1 M H2SO4 solution) (10%; see:
Demas, N. J.; Crosby, G. A. J. Chem. Phys. 1971, 75, 991.
(9) Solution in xylenes, ca. 2% metal.
504
Org. Lett., Vol. 8, No. 3, 2006

Scheme 2. Synthesis of Extended Dithienophosphole Materials
the H NMR spectrum and a slightly highfield-shifted 31P
NMR resonance at δ 14.7 ppm (cf. δ 17.1 ppm for 4).
Compound 6 also exhibts very pronounced optoelectronic
features with λex ) 399 nm, λem ) 459 nm, and φPL ) 0.548.8
As observed earlier, the modification of the substitution
pattern of the silyl group only alters the absorption properties
of the dithienophosphole moiety whereas its emission
remains almost unaffected.4a The same holds true for the
diphenylethylene-substituted species 6 (∆λex ) 15 nm, ∆λem
) 2 nm). The presence of a second, very strong transition
at λex ) 357 nm appearing as a shoulder in the excitation
spectrum could arise from a charge transfer from the
diphenylethylene (dpe) moiety to the dithienophosphole (dtp)
moiety (πdpe f π*dtp) indicating a potential extension of the
π-conjugated system through the silicon centers; a corre-
sponding transition is not commonly observed in the excita-
tion spectra of dithienophospholes.4 This finding, on the other
hand, shows that the preservation of the intriguing blue light-
emitting properties of the monomers would be possible in
oligomeric/polymeric systems accessed via a similar strategy.
An extended, polymeric material can be generated by using
a suitable R,ω-bis(alkyne) as substrate. Reaction of the
oxidized dithienophosphole 4 with 1 equiv of 1,7-octadiyne
in dichloromethane and a catalytic amount of Karstedt
catalyst at room temperature afforded a polymeric material
after 3 days (Scheme 2). The almost complete disappearance
successful hydrosilation step. The polymeric nature of the
material was further confirmed by an end group analysis via
1H NMR showing an integral ratio of ca. 13:1 for Ph vs SiH.
The optoelectronic properties of the polymer 7 are compa-
1
rable to those of the monomeric species 4 and 6 with λex
)
378 nm, λem ) 460 nm, and φPL ) 0.562,8 supporting the
preservation of the photophysical features of the dithieno-
phosphole monomers within the polymer 7.
To further explore the reactivity of our system toward
alkenes, we also performed hydrosilation experiments with
trans-stilbene and cyclohexene, respectively, to access the
corresponding model compounds. However, the reaction with
trans-stilbene was found to be very slow. Even after
prolonged reaction times (10 d) and increased temperatures
only a minor portion (e15%) of the dithienophosphole 4
was consumed. This could be attributed to the unfavorable
trans geometry of the stilbene substrate employed. The
reaction with cyclohexane present in the mixture, on the other
hand, showed complete consumption of the starting material
4 after 3 d at room temperature, evident in the disappearance
of the Si-H resonance in the 1H NMR spectrum. However,
1
to our surprise, the H NMR spectrum of the product did
not show the expected resonances of a cyclohexyl moiety
suggesting a different reaction pathway being valid under
these conditions. The generation of a polymeric material 8
(δ 31P 14.6 ppm) instead of the targeted model compound
was supported by the reduced solubility of the obtained
product (insoluble in ether, pentane, heptane) and was
confirmed by GPC analysis (molecular weight up to 10 000;
n e 20). The presence of two sets of 29Si satellites (J(H,Si)
1
of the Si-H resonance at δ Η 4.54 ppm as well as the
presence of two major olefinic H resonances at δ Η 6.19
and 5.68 ppm with a vicinal trans coupling of J(H,H) )
18.8 Hz support a cis-1,2-addition of silicon and hydrogen
to the alkyne units to be favored.10 Analysis by gel
permeation chromatography (GPC) of the dark orange, glassy
oil obtained after precipitation from CH2Cl2 into heptane
indicated the generation of a polymeric material with
molecular weights of up to 8000 (n e 15) supporting the
1
1
3
1
) 17.2, 25.4 Hz), detectable at the H NMR resonance for
the methyl groups attached to silicon, indicated that a
dehydrogenative homocoupling occurred exclusively.
This finding was further supported by only one resonance
at δ -4.7 ppm present in the 29Si NMR. The fact that the
alkene was indeed not involved in this reaction was
confirmed by an experiment where a few drops of Karstedt
(10) Multinuclear NMR data also indicate the presence of the 2,1-addition
product in ca. 14% abundance.
Org. Lett., Vol. 8, No. 3, 2006
505

catalyst solution were added to a dichloromethane solution
of 4 without any substrate leading to a similar polymeric
material after 4 d (molecular weight up to 10 000; n e 20).
These results are intriguing insofar, since they represent the
first case of a successful quantitative dehydrocoupling of a
tertiary silane. Although there are some examples known in
the literature, where tertiary silanes give a homocoupled
product, they generally only occur as a side reaction to give
a maximum of about 10% conversion. Furthermore, these
reactions require very drastic conditions (e.g., T g 100 °C)
to give notable amounts of dehydrocoupled products.11 In
the present case, a complete conversion is achieved at room
temperatureswith a catalyst that is known to be highly active
for the hydrosilation of alkenes, even to access polymeric
systems.12 It is interesting to note that the observed dehy-
drocoupling process also seems to be a competitive reaction
for the hydrosilation of alkynes, at least to some extend, as
the 29Si NMR of the polymer 7 also shows a corresponding
resonance for a dehydrocoupled segment at δ -4.7 ppm of
about 4% abundance. The emission properties of the dehy-
drocoupled polymer 8 are very similar to those of the
monomer (λem ) 459 nm), whereas the maximum wave-
length of absorption experiences a red shift of about 10 nm
now appearing at λex ) 393 nm, comparable to the value
observed for compound 6. The relation of the polymer 8 to
the diphenylethylene-functionalized dithienophosphole 6 in
terms of optoelectronic properties is also evident in a second,
strong transition of the excitation spectrum at λex ) 353 nm,
suggesting an interaction of the disilanylene linker with the
dithienophosphole moiety (σSiSi f π*dtp)13 in the polymer 8
(see Figure 1).
Figure 1. Fluorescence spectra (excitation, left; emission, right)
of the dithienophospholes 4 and 6-8 (c ) 1 × 10-4 M in
CH2Cl2).
properties of the respective monomers. The absence of any
substrate (or the presence of alkenes) affords a polymeric
material via an unexpected, and for tertiary silanes unprec-
edented, Pt-catalyzed dehydrogenative homocoupling, with
a quantitative conversion of the employed dithienophosphole.
Further investigations will include a detailed study of this
unusual dehydrocoupling process for future polymerization
reactions to optimize the molecular weights of the materials.
The fine-tuning of the optoelectronic properties of the
polymers by regenerating the trivalent phosphorus center is
also currently under detailed investigation.
In conclusion, we have synthesized the first Si-H func-
tionalized dithieno[3,2-b:2′,3′-d]phospholes that can be fine-
tuned conveniently by simple chemical modifications per-
formed at the phosphorus center. Utilizing the Si-H func-
tionality to access polymeric systems via a Pt-catalyzed
hydrosilation protocol provides materials in the presence of
suitable alkynes that possess the intriguing optoelectronic
Acknowledgment. Financial support from the Fonds der
Chemischen Industrie, the BMBF, the Deutsche Forschungs-
gemeinschaft (DFG), and Prof. Dr. J. Okuda is gratefully
acknowledged. We thank K. Beckerle for his assistance with
the polymer analysis.
(11) Corey, J. Y. AdV. Organomet. Chem. 2004, 51, 1 and references
therein.
(12) Pawluc, P.; Marciniec, B.; Kownacki, I.; Maciejewski, H. Appl.
Organomet. Chem. 2005, 19, 49.
(13) Ohshita, J.; Nodono, M.; Takata, A.; Kai, H.; Adachi, A.; Sakamaki,
K.; Okita, K.; Kunai, A. Macromol. Chem. Phys. 2000, 201, 851.
Supporting Information Available: Experimental pro-
cedures and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0529288
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Org. Lett., Vol. 8, No. 3, 2006