Incorporation of perfluorohexyl-functionalised thiophenes into oligofluorene-truxenes: Synthesis and physical properties

Article abstract of DOI:10.3762/bjoc.9.141

Oligofluorene-functionalised truxenes containing perfluorohexylthiophene units at the terminal positions on the arms were synthesised, and their optical and electrochemical properties were investigated to determine the effect that the perfluorohexylthiophene unit has on the HOMO and LUMO properties of the oligomers. By synthesising a molecule with longer oligofluorene arms the effects of the perfluorohexylthiophene unit on larger oligomers was explored. The effect of steric hindrance from the perfluorohexyl chain was also evaluated by altering the position of the chain on the thiophene moiety.

Full text of DOI:10.3762/bjoc.9.141

Incorporation of perfluorohexyl-functionalised
thiophenes into oligofluorene-truxenes:
synthesis and physical properties
Neil Thomson, Alexander L. Kanibolotsky, Joseph Cameron, Tell Tuttle,
Neil J. Findlay and Peter J. Skabara*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1243–1251.
WestCHEM, Department of Pure and Applied Chemistry, University of
Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, U.K.
doi:10.3762/bjoc.9.141
Received: 27 February 2013
Accepted: 12 June 2013
Published: 27 June 2013
Email:
Peter J. Skabara* - peter.skabara@strath.ac.uk
*
Corresponding author
Associate Editor: D. O'Hagan
Keywords:
© 2013 Thomson et al; licensee Beilstein-Institut.
License and terms: see end of document.
oligomers; perfluorinated-materials; star-shaped molecules; synthesis;
truxene
Abstract
Oligofluorene-functionalised truxenes containing perfluorohexylthiophene units at the terminal positions on the arms were synthe-
sised, and their optical and electrochemical properties were investigated to determine the effect that the perfluorohexylthiophene
unit has on the HOMO and LUMO properties of the oligomers. By synthesising a molecule with longer oligofluorene arms the
effects of the perfluorohexylthiophene unit on larger oligomers was explored. The effect of steric hindrance from the perfluoro-
hexyl chain was also evaluated by altering the position of the chain on the thiophene moiety.
Introduction
Star-shaped oligomers share a common central unit with Truxene has been used as a starting compound, or a core unit,
multiple arms branching from the core [1]. The morphology and for larger star-shaped polyarenes such as fullerene fragments [4]
electronic properties of these oligomers often vary from the and liquid-crystalline compounds [5]. Positions C5, C10 and
linear versions of the arms alone.
C15 can be functionalised with a range of substituents,
commonly saturated alkyl chains, which can enhance the solu-
1
0,15-Dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene, also known bility and processability as well as reduce intermolecular π–π
as truxene (Figure 1), is a heptacyclic polyarene with C3 stacking [6], but there are also examples of unsaturated alkyl [7]
symmetry that can be envisaged as three fluorenes that are and phenyl [8] substituents at these positions. Cross-coupling
superimposed through a common central benzene ring [2,3]. reactions at the C2, C7 and C12 positions is the usual method of
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Figure 1: Labelled truxene and compounds T1 and T4.
attaching “arms” to the core to form the star-shaped oligomers, observe changes in the optical and electrochemical properties
such as T1 and T4 (Figure 1), extending the conjugation length by introducing perfluorohexylthiophene units at the terminal
and altering the photophysical properties of the resulting positions of the oligofluorene arms. Planarity of the backbone is
oligomer. There are also less common examples of functionali- an important concept in conjugated organic oligomers and poly-
sation at the C3, C8 and C13 positions [9].
mers, as any twist in the dihedral angle between aromatic repeat
units can lessen the p-orbital overlap, breaking the effective
Alkyl substituents attached to hetero- and carbocyles, which conjugation length [15]. This planarity can be perturbed by the
make up the conjugated backbone of oligomers and polymers, steric effects of substituents attached to the hetero- or carbo-
can also affect the properties of the material [10]. Alkyl chains cycles in the conjugated backbone. In order to observe if the
or other functional groups can donate or withdraw electron position of the perfluorohexyl chains would have an effect, two
density from the polymer chain, for example by inductive molecules that are structural isomers were synthesised and
effects, altering the HOMO–LUMO levels and subsequent band studied to compare the optical and electrochemical properties.
gap in the process. One type of substituent that can be used is a Furthermore, a significantly larger molecule was synthesised to
perfluoroalkyl chain, where the high percentage of fluorine observe how a molecule with longer conjugated arms would
present gives rise to a large electron-withdrawing effect [11]. also be affected by the perfluorohexylthiophene units.
However, perfluoroalkyl halides are not suitable alkylating
reagents for SN2 type reactions, which makes the attachment of Results and Discussion
these types of chains to aromatic cyclic compounds difficult The synthesis of the oligofluorene-functionalised truxenes is
without the use of a spacer group, such as an ethylene chain, outlined in Schemes 1–4. 3-Perfluorohexylthiophene (3) was
which can lessen the inductive effect [12]. Due to the wide- synthesised from 3-bromothiophene (1) by initial halogen
spread industrial use of polythiophenes, such as P3HT, analo- exchange, using the method described by Buchwald et al. [16],
gous poly(perfluoroalkyl)thiophenes have received some to afford 3-iodothiophene (2), which then underwent a copper-
interest due to their ability to affect certain properties of the mediated coupling reaction with perfluorohexyl iodide
polymer, such as the electrochemistry and stability, as well as (Scheme 1). Lithiation of 3 and treatment with perfluorohexyl
the improved chemical resistance and thermal stability that iodide gave the product 2-iodo-3-perfluorohexylthiophene (4).
replacing alkyl hydrogen atoms with fluorine atoms affords The regioselectivity of this reaction was poor, and the product
[
13].
was obtained as a mixture with its isomer, 2-iodo-4-perfluoro-
hexylthiophene, which was separated by column chromatog-
Our group has previously published work on oligofluorene- raphy. Compound 4 then underwent Suzuki–Miyaura coupling
functionalised truxenes [14], and it was in our interest to with compound 5, which was synthesised according to our
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Scheme 1: Synthesis of the thiophene-fluorene arm for the 3-isomer.
previously published method [17], and the product obtained, 6, The synthetic route to 2-iodo-4-perfluorohexylthiophene (13)
was deprotected by bromination to give compound 7 and then (Scheme 2) began with treatment of 3-bromothiophene (1) with
reacted with bis(pinacolato)diboron to afford boronic ester 8.
lithium bis(trimethylsilyl)amide and chlorotrimethylsilane to
Scheme 2: Synthesis of the thiophene-fluorene arm for the 4-isomer.
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Beilstein J. Org. Chem. 2013, 9, 1243–1251.
afford 2-trimethylsilyl-3-bromothiophene (9). However, with perfluorohexanes from a 5% water/ethanol solution. Lithi-
although the conditions were unchanged, the halogen exchange ation and treatment with perfluorohexyliodide afforded 12,
reaction used to afford 10 was poorer yielding than that which which was deprotected with tetrabutylammonium fluoride to
produced 2. This was due to incomplete conversion from the afford 13. The boronic ester 16 was then synthesised by the
bromide to the iodide. The inability to separate 9 and 10 meant same steps applied to obtain compound 8 (see Scheme 1).
that this mixture was not further purified, and the mixture was
taken forward to the next stage of the synthesis. The subse- T1-3FTh and T1-4FTh were then synthesised by Suzuki
quent copper-mediated coupling reaction to afford 11 was also coupling of tribromohexahexyltruxene 17, which was synthe-
poor-yielding and a mixture of 9, 10 and 11 was obtained. Com- sised according to our previously published method [14], and
pound 11 was obtained pure by a fluorinous extraction process the relevant boronic ester, 8 or 16 (Scheme 3).
Scheme 3: Coupling of arms to the truxene core.
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T4-4FTh (Scheme 4) was synthesised by coupling compound π–π* transitions. The maximum absorption band was observed
1
6 with T3Br. The latter compound was obtained by our previ- at 349 nm for T1-3FTh, 362 nm for T1-4FTh, and 380 nm for
ously published method [17].
T4-4FTh. The maximum wavelength of emission was observed
at 412 nm for T1-3FTh, 402 nm for T1-4FTh, and 421 nm for
The absorption and emission spectra of the oligomers in solu- T4-4FTh. In comparison to the corresponding spectra of T1 and
tion (dichloromethane), shown in Figure 2, all exhibit strong T4 (Table 1), the introduction of perfluorohexylthiophene units
Scheme 4: Synthesis of T4-4FTh.
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Table 1: Optical data for TX-xFTh and TX materials.
compound
λmax (nm) absorption
onset
(nm)
ε
HOMO–LUMO gap
(eV)
λmax (nm) emission
ΦPL
(CH2Cl2)
(CH2Cl2)
(mM−1 cm−1)
(CH2Cl2)
T1-3FTh
T1-4FTh
T4-4FTh
T1a
349
362
380
343
374
387
397
409
—
306
323
385
—
3.20
3.12
3.03
3.29
3.05
412
—
0.86
0.93
—
402, 418
421, 440sh
375sh, 396, 416sh
411, 436, 460sh
T4a
—
—
—
aValues obtained from [14].
emission spectrum of T4-4FTh also reveals the vibronic struc-
ture, although it is less pronounced, with a shoulder at 440 nm.
The Stokes shifts for T1-4FTh and T4-4FTh are 40 and 41 nm
respectively; however, the shift for T1-3FTh is larger, at around
6
3 nm. This is an indication that the excited state of T1-3FTh
undergoes a greater conformational change before emission
than that of T1-4FTh, which is further evidence of the greater
planarity in the T1-4FTh ground state compared to that of its
isomer. The optical HOMO–LUMO gaps were estimated from
the onset of absorption and were recorded as 3.20, 3.12 and
3
.03 eV for T1-3FTh, T1-4FTh and T4-4FTh, respectively.
The HOMO–LUMO gap for T1-3FTh and T1-4FTh is slightly
narrower than that for T1; however, there is not much differ-
ence between the HOMO–LUMO gap of T4-4FTh and that of
T4.
Figure 2: Normalised absorbance (solid) and emission (dashed) of
materials in solution (dichloromethane).
to the terminal positions on the arms leads to a red shift for both The results for the cyclic voltammetry of the oligomers are
absorption and emission. The red shift within the 4FTh-substi- given in Table 2 (see Supporting Information File 1, Figures
tuted series (T1-4FTh and T4-4FTh) increases with increasing S1–S6 for the voltammograms). T1-3FTh showed one irre-
chain length. The emission spectrum for T1-4FTh also shows a versible oxidation peak at +1.09 V, and a reduction peak was
pronounced vibronic feature at 418 nm, which is an indication found at −2.22 V. From the onset of the oxidation and reduc-
of the planarity of the molecular ground state in comparison to tion peaks the HOMO level was estimated to be −5.72 eV and
that of T1-3FTh, where no vibronic splitting is observed. The the LUMO −2.74 eV with reference to ferrocene, which was
Table 2: Electrochemical data for TX-XFTh and TX.
compound
Eox (V) peaks
EPa/EPc or EPaa
HOMO
Ered (V) peaks
EP
LUMO
HOMO–LUMO gap
(eV)
T1-3FTh
T1-4FTh
+1.09
−5.72
−5.55
−2.22
−2.84
−2.74
−2.39
2.98
3.16
+0.86/0.79
+1.03/0.96
+
+
1.26b
1.44b
T4-4FTh
+0.91/+0.82c
−5.53
−2.59
−2.35
3.18
+1.11c
T1d
T4d
—
−5.6
−5.5
—
—
−2.2
−2.3
3.4
3.2
—
aNo cathodic peak is reported where the wave is irreversible. bPeak exists as a shoulder. cTwo unresolved single-electron peaks. dValues obtained
from [14].
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Beilstein J. Org. Chem. 2013, 9, 1243–1251.
used as an internal standard and has a HOMO of −4.8 eV. In In order to rationalise the difference in optical and structural
comparison to the HOMO and LUMO values for T1 [14] (−5.6 properties between the two isomers, DFT calculations were
and −2.2 eV) there is a decrease in the HOMO and a large performed on the smaller truxenes. The T1-3FTh and T1-4FTh
decrease in the LUMO, with a reduction in the HOMO–LUMO structures were optimised in the gas phase by using the BP86-D
gap. For T1-4FTh, quasi-reversible oxidation peaks were found [18-20] functional with the def2-TZVP [21] basis set imple-
at +0.86 and +1.03 V, while irreversible peaks were found as mented in TURBOMOLE 6.3.1 [22]. The optimisations were
+
1.26 and +1.44 V. A reduction peak was observed at −2.84 V. carried out by using the RI-J approximation [23]. The hexyl
From the onset of the oxidation and reduction peaks the HOMO chains on the fluorene units were shortened to methyl groups in
level was estimated to be −5.55 eV, and the LUMO −2.39 eV. the model in order to decrease the computational cost of the
Comparing these values to that of T1, the HOMO is essentially simulation. The main structural difference observed is the
the same with the LUMO decreased, resulting in a narrower degree of twisting between the fluorene and thiophene in the
electrochemical band gap. A similar effect is observed for two structures. The average dihedral angle in T1-3FTh is 40.9°,
T4-4FTh in comparison with T4 [14], where the HOMO is whilst there is a much smaller twist of 6.4° in T1-4FTh. The
−
5.53 eV compared to −5.5 eV for T4 and the LUMO is lower frontier orbitals of the isomers were also compared and are
(
−2.35 eV for T4-4FTh and −2.3 eV for T4). The electrochem- shown below (Figure 3 and Figure 4). The HOMO−1 and
ical HOMO–LUMO gaps for T1-3FTh, T1-4FTh and T4-4FTh HOMO for both molecules (ΔE[T1-3FTh] = 0.0054 eV and
were close to those derived from the absorption spectra, (within ΔE[T1-4FTh] = 0.0030 eV) are essentially degenerate and
ca. 0.2 eV), which is in line with the differences observed for show a splitting due to a slight break in symmetry. This is also
T1 and T4.
the case for the LUMO and LUMO+1 of each isomer
Figure 3: HOMO−1 (bottom, left), HOMO (bottom, right), LUMO (top, left) and LUMO+1 (top, right) of T1-3FTh.
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Beilstein J. Org. Chem. 2013, 9, 1243–1251.
Figure 4: HOMO−1 (bottom, left), HOMO (bottom, right), LUMO (top, left) and LUMO+1 (top, right) of T1-4FTh.
(
ΔE[T1-3FTh] = 0.013 eV and ΔE[T1-4FTh] = 0.0047 eV). well as the optical HOMO–LUMO gaps remain essentially the
The HOMO−1 and HOMO of the compounds show there to be same for the substituted and the parent systems. The electro-
a larger contribution of the thiophene unit to the conjugation of chemical properties of T1-3FTh are more affected by the pres-
the molecule in T1-4FTh compared to T1-3FTh. This reaf- ence of the perfluoroalkyl thiophene substituent than those of
firms that the thiophene unit of T1-3FTh is too twisted to T1-4FTh.
contribute significantly to the conjugation of the molecule.
Supporting Information
Conclusion
In conclusion we have presented three new star-shaped
Supporting Information File 1
oligomers, namely T1-3FTh, T1-4FTh, and T4-4FTh. For the
smaller molecules, the presence of the powerfully electron-
withdrawing perfluorohexyl chain within the molecules leads to
a more stabilised LUMO, in comparison to the hydrogen-termi-
nated molecule T1. The optical and electrochemical properties
Experimental procedures and cyclic voltammetry details.
http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-141-S1.pdf]
[
of the molecules with shorter oligofluorene arms are altered to a Acknowledgements
much greater extent than those of the molecules containing We would like to thank the EPSRC and Cambridge Display
longer oligofluorene arms, where HOMO and LUMO levels as Technology for funding N.T.
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Beilstein J. Org. Chem. 2013, 9, 1243–1251.
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