Multigram synthesis of bis[(trimethylsilyl)ethynyl]benzenes suitable for post-polymerization modification

Article abstract of DOI:10.1039/c4nj00011k

Novel substituted bis[(trimethylsilyl)ethynyl]benzenes have been prepared as versatile building blocks for organic functional materials. The resulting effects of replacing sulfur by selenium and tellurium on photophysical and electrochemical properties ha

Full text of DOI:10.1039/c4nj00011k

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LETTER
Multigram synthesis of
bis[(trimethylsilyl)ethynyl]benzenes suitable for
post-polymerization modification†
Cite this: New J. Chem., 2014,
38, 2229
Received (in Montpellier, France)
2nd January 2014,
Accepted 3rd March 2014
Florian Glocklhofer,a Daniel Lumpi,*a Berthold Stogerb and Johannes Frohlicha
¨
¨
¨
DOI: 10.1039/c4nj00011k
www.rsc.org/njc
Novel substituted bis[(trimethylsilyl)ethynyl]benzenes have been pre- properties.12 In contrast, characteristics of polytellurophenes are
pared as versatile building blocks for organic functional materials. The severely affected by Te–Te interactions, which are expected to
resulting effects of replacing sulfur by selenium and tellurium on result in strong interchain electronic coupling.13
photophysical and electrochemical properties have been examined.
We report on a straightforward, comparatively cheap and scalable
Polymerization via microwave-assisted Cu(I)-catalyzed azide–alkyne synthesis of novel sulfur, selenium and tellurium based analogs
cycloaddition (CuAAC) and subsequent post-polymerization modifica- (6a–c) of 1,4-bis[(trimethylsilyl)ethynyl]2,5-bis(hexyloxy)benzene.
tion by oxidation reveals the utility of the developed building blocks.
The introduction of these elements does not only impact the
material properties but also allows for a specific modification of
1,4-Bis[(trimethylsilyl)ethynyl]-2,5-bis(hexyloxy)benzene (X = O) and the photophysical and electrochemical characteristics by simple
the desilylated species 1,4-bis(ethynyl)-2,5-bis(hexyloxy)benzene oxidation, which is demonstrated using the example of post-
are of current interest in the synthesis of a growing number of polymerization modification.
functional organic materials such as fluorescent polymers,1
The discussion of the results is split into two subchapters: (1) the
conjugated sensor materials,2,3 conducting metallopolymers4 large-scale synthesis of novel building-blocks for functional organic
and special materials like shape-persistent macrocycles.5 In polymers and (2) an exemplary application of the developed mono-
terms of further applications, the two alkyne groups enable the mers in materials suitable for post-polymerization modification.
functionalization by a variety of established reactions such as
The synthetic pathway towards monomers 6a–c is illustrated in
alkyne metathesis,6,7 Glaser coupling,8 Sonogashira coupling,9 Scheme 1. A multigram synthesis of 2,5-dibromobenzoquinone 3
azide–alkyne cycloaddition,10 etc.
from 1,4-dimethoxybenzene 1 was previously reported by Lopez-
´
Despite the widespread utility of hexyloxy-substituted Alvarado et al.14 Following this procedure, the synthesis of 3 could
benzenes,1–5 the synthesis of the sulfur, selenium and tellurium be achieved on the 30 g scale (71%).
containing analogs 6a–c has not been reported to date. However,
Nucleophilic addition15 of TMS-acetylene applying n-BuLi
replacing the oxygen atom by these elements may significantly affect resulted in an isomeric mixture of 4a and 4b (3 : 1 (cis/trans), not
the properties of the material, as reported for heterocyclic polymers assigned) in a good yield of 85% (20 g). Since both isomers are
polyfuran, -thiophene, -selenophene and -tellurophene. Whereas converted to 5 at similar rates in the subsequent reductive
polyfuran has attracted less attention, polythiophenes are widely aromatization, separation of the two isomers is not required.
utilized due to their conducting properties.11 Replacing sulfur by Nevertheless, the removal of impurities by filtration through a pad of
selenium in polythiophenes results in reduced bandgaps and silica and trituration in boiling n-hexane proved to be advantageous
lower LUMO energy levels, leading to ambipolar charge transport to avoid the cleavage of the TMS-group in the subsequent reduction
step15 to form 5, which was accomplished in good yields
of 94% (12 g) using SnCl2Á2H2O. The hexylthio-, hexylseleno- and
a Institute of Applied Synthetic Chemistry, Vienna University of Technology,
hexyltelluro-groups were introduced by a lithium–halogen exchange
and subsequent addition of either elemental sulfur or selenium and
hexyl iodide to obtain 6a (2.5 g, 63%) and 6b (1.7 g, 73%) or dihexyl
ditelluride16 to obtain 6c (1.5 g, 53%).
Getreidemarkt 9/163, A-1060 Vienna, Austria. E-mail: daniel.lumpi@tuwien.ac.at
b Institute of Chemical Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9/164, A-1060 Vienna, Austria
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of all synthesized compounds as well as crystal structures and crystal-
lographic information files (CIFs) for 6a, 6b and 6d. CCDC 955488–955490. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4nj00011k
To ensure scalability, all steps toward 5 were carried out
utilizing purification techniques such as extraction, crystal-
lization, trituration and flash filtration through silica. 6a–c
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Fig. 1 Crystal structure of 6a viewed down [010]. C, S and Si atoms are
represented by gray, yellow and orange ellipsoids drawn at 50% probability
levels, respectively. H atoms were omitted for clarity.
The crystal structures of 6a18 and 6b18 are isostructural and
possess C2/c symmetry. One crystallographically unique molecule is
located on a center of inversion. The hexyl chains extend in the
plane of the benzene ring. Indeed, with the exception of the TMS
groups the molecules are virtually flat: the largest distance of a non-
H atom with the exception of the TMS-methyl groups to the least
squares plane defined by the atoms of the p-conjugated core is
0.0958(13) Å (6a) and 0.121(3) Å (6b). The terminal atoms of the hexyl
chain feature enlarged atomic displacement parameters, indicating
static or dynamic disorder as expected for long alkyl chains. The
molecules are arranged in a three dimensional network controlled
solely by van der Waals interactions. As expected, the unit cell
volume of 6b is slightly larger than 6a (3164.8(8) vs. 3085.8(3) Å3)
due to the additional space required for the Se atom.18
Scheme 1 Synthetic route towards monomers 6a–c.
were purified by column chromatography; however, exemplarily
6a was alternatively purified by crystallization, resulting in only
slightly lower yields (56% instead of 63%).
An alternative modification approach to the heteroatom exchange
(S/Se/Te) represents the possibility to modify molecular properties by
selective oxidation of these elements. This enables us to directly
convert the electron donating +M-substituent (e.g. alkylthio) to an
electron accepting ÀM-substituent (e.g. sulfone), thus significantly
influencing electrochemical and photophysical characteristics.
In order to evaluate convenient oxidation conditions the conver-
sion of 6a to sulfone 6d was chosen as a model reaction (Scheme 2).
Dimethyldioxirane (DMDO), a mild and versatile oxidation reagent,
proved to be particularly advantageous with regard to purification,
selectivity as well as efficiency (98%). With regard to the applicability
of this oxidation reagent, our group reported on practical and
efficient large-scale preparation of DMDO.17
6d18 (crystals were grown by solvent evaporation from
ethanol) crystallizes in space group P21/c. Although, like in 6a
and 6b, the molecules are located on centers of inversion, 6d is
structurally unrelated to the former. Most notably, the hexyl
chains in 6d propagate nearly perpendicular to the plane of the
benzene ring. They are located in the pockets of the structure
and the three terminal aliphatic C atoms feature disorder.
Electrochemical and photophysical properties of 6a–d were
probed by cyclic voltammetry (CV) as well as photometry (UV-vis
absorption). CV revealed similar HOMO energy levels for compounds
6a (À5.35 eV) and 6b (À5.31 eV), whereas tellurium compound 6c
showed a significantly higher HOMO energy level of À5.08 eV.
This fact is also indicated by decomposition of 6c in DCM
solution in contrast to 6a and 6b. DFT calculations performed
did not reproduce the observed higher HOMO level for 6c;
however, a similar phenomenon for tellurium based compounds
has been described in the literature.19
Single crystals of 6a and 6b were obtained by solvent
evaporation from n-hexane;18 6c did not give suitable crystals.
The crystal structure of 6a is shown in Fig. 1.
UV-vis absorption spectra of 6a–c are depicted in Fig. 2 (right).
The high wavelength absorption bands of 6a–c are broad weak
peaks, which are characteristic for n - p* transitions. The
bands below 320 nm are attributed to p - p* transitions, given
their spectral shape (vibronic structures) and intensity. The optical
bandgaps were determined from the absorption onsets (Table 1).
Scheme 2 Oxidation protocol towards 6d.
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Fig. 2 CV charts of irreversible oxidation (left) and UV-vis absorption
spectra (right) of 6a–d.
Table 1 Photophysical and electrochemical data for 6a–d
b
lon-set [nm]/optical
bandgap [eV]
HOMO levelc [eV]
a
Eox [V]
6a
6b
6c
6d
0.55
0.51
0.28
0.51
406/3.05
408/3.04
425/2.92
347/3.57
À5.35
À5.31
À5.08
À5.31
Onset potential vs. Fc/Fc+. Absorption onset wavelength. From CV
a
b
c
on the premise that the Fc/Fc+ energy level is À4.80 eV.
Similar values for the optical bandgap have been observed
for 6a and 6b, indicating only a minor influence of the S–Se
exchange on photophysical properties. In contrast, for compound
6c a slightly reduced bandgap was observed.
The characterization of 6d by CV and absorption measure-
ments revealed a minor influence on the HOMO energy levels
(À5.31 eV), but significant changes in the absorption spectra with a
blue-shift of the absorption onset of B60 nm/B0.5 eV compared to
Scheme 3 Microwave-assisted CuAAC polymerization towards 8 and
post-polymerization modification (oxidation) yielding 9.
The effects of post-polymerization modification were
6a (Fig. 2). The marginal shift of the HOMO level indicates a minor evaluated by absorption and fluorescence measurements for
conjugation between the heteroatom groups and the p-system of the polymers 8 and 9.
benzene, which was also observed for the selenium derivative 6b.
Despite the drastic structural and electronic modification,
The blue-shift is explained by the elimination of the lone pair converting the electron donating into an electron withdrawing
electrons and, thus, the lack of n - p* transitions. The p - p* substituent, qualitatively similar absorption characteristics but
transition of 6d (320–350 nm) is red-shifted by B30 nm compared to reduced absorbance values were observed (Fig. 3, left). In contrast,
6a, indicating a reduced p–p* bandgap.
the fluorescence characteristics alter significantly shifting the
To outline the effect of selective oxidation on material properties emission maximum from 438 nm (8) to 516 nm (9). This severe
post-polymerization modification20 was chosen as a proof of concept red-shift of B80 nm changes the emission color from blue to
(Scheme 3). Polymer 8 was obtained by microwave-assisted yellow (Fig. 3, right) suggesting a variety of potential applications
Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) polymeriza-
tion of 6a and 7. The optimized procedure allows for in situ
deprotection of 6a and significantly reduces reaction times
compared to conventional CuAAC polymerization.10
The oxidation of 8 was performed in analogy to the model
reaction towards 6d. Indeed, post-polymerization modification
was accomplished quantitatively yielding polymer 9 (conver-
1
sion monitored by H NMR spectroscopy). Since acetone is the
only by-product of the oxidation using DMDO, 9 was simply
purified by solvent evaporation.
GPC measurements of both 8 and 9 indicate no degradation
of the polymers during oxidation. While the number average
molecular weight slightly decreased from 3.6 kDa to 3.5 kDa,
the weight average molecular weight increased from 7.2 kDa to
Fig. 3 UV-vis absorption and photoluminescence spectra of 8 (lex
=
7.5 kDa. These variations are within the limitations of the
chosen relative method for molecular weight determination.
288 nm) and 9 (lex = 287 nm) (left), solutions of 8 (blue) and 9 (yellow)
in DCM at lex = 366 nm (right).
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such as specific color tuning in the field of OLEDs, new strategies Acknowledgements
for device processing, etc. Further experimental and photophysical
We gratefully thank H. Wutzel for performing GPC measure-
investigations are currently performed to analyze this effect and to
prove the general applicability of this approach. Determination of
quantum yields using an Ulbricht sphere afforded similar values of
13% and 15% for polymers 8 and 9, respectively.
¨
ments and M. Taublander for contributing to synthetic experi-
ments. B. Holzer, E. Horkel and C. Hametner are acknowledged
for performing NMR experiments.
In conclusion, we have established a straightforward, cheap
and reliable synthesis of versatile building blocks 6a–c on a
(multi)gram-scale. The application of the developed building
blocks potentially yields functional organic materials with entirely
new properties compared to widely applied 1,4-bis[(trimethylsilyl)-
ethynyl]-2,5-bis(hexyloxy)benzene (X = O). Thus, microwave-
assisted CuAAC polymerization of 6a and post-polymerization
modification were demonstrated. The observed alteration in
photophysical properties by this simple oxidation procedure
utilizing DMDO gives rise to several possible applications
ranging from color tuning to post-processing modification of
polymer layers.
Notes and references
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Experimental section
General procedure for the synthesis of 6a–c
5 (1.0 eq.) was dissolved in degassed dry Et2O (0.1 M) under an
argon atmosphere in a pressure resistant glass vial. The solution
was cooled to À78 1C and t-BuLi (1.7 M in pentane, 4.0 eq.) was
slowly added. The resulting mixture was stirred for 45 min at
À78 1C before allowing the solution to warm above 0 1C. Sulfur,
selenium or dihexyltelluride (2.1 eq.) was then added and the
yellow suspension was stirred for 3 h at room temperature/60 1C.
If elemental reagents were used, hexyl iodide (2.2 eq.) was addi-
tionally added and the suspension was stirred at 60 1C overnight.
The solution was poured into water and extracted three times with
Et2O. The combined organic layers were dried over anhydrous
Na2SO4 and the solvent was removed in vacuo. The residue was
purified by column chromatography (PE) or by crystallization from
EtOH followed by trituration in boiling EtOH.
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Microwave-assisted CuAAC polymerization
´
˜
´
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into separate vials. CuI (10 mol%), KF (3.0 eq.) and H2O (8.0 eq.)
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were added to 6a and the vial was sealed and flushed with 15 V. B. Van, K. Miki and T. M. Swager, Org. Lett., 2010, 12,
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¨
¨
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18 See ESI† for details.
Post-polymerization modification
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