Twisted Thienylene–Phenylene Structures: Through-Space Orbital Coupling in Toroidal and Catenated Topologies

Article abstract of DOI:10.1002/ejoc.201901637

Sterically crowded and twisted thienylene-phenylenes were synthesized and characterized in comparison to corresponding polyphenylene nanostructures. Spectroscopic, diffraction, and theoretical studies gave evidence of through-space delocalization of π-electrons of peripheral (hetero)aromatic rings in toroidal and catenated topologies.

Full text of DOI:10.1002/ejoc.201901637

DOI: 10.1002/ejoc.201901637
Full Paper
Extended π-Systems | Very Important Paper |
Twisted Thienylene–Phenylene Structures: Through-Space
Orbital Coupling in Toroidal and Catenated Topologies
Tanja D. Leitner,[a] Yannick Gmeinder,[a] Fynn Röhricht,[b] Rainer Herges,[b]
Elena Mena-Osteritz,[a] and Peter Bäuerle*[a]
Abstract: Sterically crowded and twisted thienylene-phenyl- fraction, and theoretical studies gave evidence of through-
enes were synthesized and characterized in comparison to cor- space delocalization of π-electrons of peripheral (hetero)aro-
responding polyphenylene nanostructures. Spectroscopic, dif- matic rings in toroidal and catenated topologies.
Introduction
Herein, we now present synthesis and characterization of
Since many years, the synthesis of polyphenylene nanostruc- deca(2-thienyl)biphenyl 1 and comparison to decaphenylbi-
tures has attracted great attention due to the improved accessi- phenyl 4, which was already synthesized by Becker et al. in
bility of highly complicated distinct structures.[1] Popular repre- 1965 and can be seen as an evident expansion of the HAB
sentatives of this class of compounds are hexaphenylbenzenes system.[24] Nevertheless, it took quite a long time until the
(HPB), that are strong candidates for application as photochemi- structure of 4 was fully elucidated by Pascal et al.[25] Ethynyl
cal and chemical switches,[2] redox materials,[3,4] liquid crystal- intermediates 2 and 5[26,27] and parent structures hexa(2-thi-
line materials[5], molecular receptors[6] as well as in organic enyl)benzene 3[11] and hexaphenylbenzene 6[1] are included in
light-emitting diodes[7,8] or molecular machines.[9,10] Related the discussion for comparison (Figure 1). Supported by single-
hexathienylbenzenes (HTB) are also known in literature.[5,11] Al- crystal X-ray structure analysis and theoretical calculations, we
though conjugated oligomers based on thiophenes show ex- found evidence of pz-orbital overlapping opening the possibil-
ceptional optical and electrochemical properties,[12] compared ity of toroidal through-space conjugation of π-electrons in
to HPBs HTBs were much less investigated. HPB is known to these compounds. Particularly intriguing are the catenated and
show a propeller-shaped geometry due to six peripheral aro- twisted topologies of this phenomenon for biphenyls 1 and 4.
matic rings.[13] In the radical cationic state, the affected π-elec- Toroidal delocalization and stabilization have only been de-
trons undergo toroidal delocalization of charge and energy, scribed for charged HABs so far.[16]
which was proven experimentally and theoretically.[13–18] In this
respect, stable radical cations of the investigated HPBs showed
characteristic near-infrared charge-resonance absorption.[19,20]
Furthermore, it was found that the involved ipso-carbon atoms
showed a distance of 2.9 Å from each other[21] which is closer
than the combined calculated van der Waals radii of 3.4–
3.5 Å.[22] Therefore, this finding indicates an overlap of the re-
spective π-orbitals of the ipso-C atoms that can lead to toroidal
delocalization[17] as Akita et al. also showed for thiophene-con-
taining hexaarylbenzenes (HAB).[23]
[a] Institute of Organic Chemistry II and Advanced Materials, University of Ulm,
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: peter.baeuerle@uni-ulm.de
https://www.uni-ulm.de/nawi/nawi-oc2/
[b] Otto Diels-Institute of Organic Chemistry,
Christian-Albrechts University Kiel,
Otto-Hahn-Platz 4, 24098 Kiel, Germany
E-mail: rherges@oc.uni-kiel.de
Figure 1. Sterically crowded phenylene-thienylene structures 1–3 and their
phenylene counterparts 4–6 under investigation.
http://scholle.oc.uni-kiel.de/herges/pages_de
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901637.
Results and Discussion
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This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
Synthetic Procedures
Preparation of deca(2-thienyl)biphenyl
1 is illustrated in
Scheme 1. 1,4-Di(thien-2-yl)buta-1,3-diyne 7[28] and tetra(2-thi-
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Scheme 1. Synthesis of thienylene-phenylenes 1 and 2.
enyl)cyclopentadienone 8[29] were synthesized following modi- (Scheme 2).[11] Phenylene derivative decaphenylbiphenyl 4 was
fied literature procedures. Diels-Alder cycloaddition of butadi- synthesized in 84 % yield through twofold Diels-Alder cyclo-
yne 7 and 2.5 equivalents of cyclopentadienone 8 in the micro- addition reaction of 1,4-diphenylbuta-1,3-diyne 10 and tetracy-
wave at 200 °C gave desired deca(2-thienyl)biphenyl 1 in 7 % clone 11 at 315 °C in the melt and therefore is the main product
yield accompanied with ethynylene derivative 2 as intermediate of the reaction.[24] Ethynylene derivative 5 was synthesized in
of the reaction and product of a single Diels-Alder cycloaddition 79 % yield in a single Diels-Alder cycloaddition of diyne 10
(74 % yield). Both products were isolated by column and high- and tetracyclone 11 at 200 °C in 1,2-dichlorobenzene
pressure liquid chromatography (HPLC). Because cyclopentadi- (Scheme 3).[26,27] Hexaphenylbenzene 6 was synthesized from
enone 8 decomposes above 200 °C, no higher temperature tolane according to a modified literature procedure.[30]
could be applied for the conversion. After reducing the excess
of cyclopentadienone 8 from 2.5 to 1.5 equivalents, ethynylene
derivative 2 was isolated in 90 % yield. This intentionally synthe-
sized intermediate was subsequently converted into deca(2-thi-
enyl)biphenyl 1 in 14 % yield by performing a separate second
Diels-Alder cycloaddition with cyclopentadienone 8.
Synthesis of basic hexa(2-thienyl)benzene 3 was achieved in
42 % yield by RhCl3-catalyzed cyclotrimerization of di-
(2-thienyl)ethynylene 9 in a modified literature procedure
Scheme 2. Synthesis of basic hexa(2-thienyl)benzene 3.
Scheme 3. Synthesis of phenylene derivatives 4 and 5.
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Structural Characterization of Derivatives 1–6 by NMR and
High-Resolution Mass Spectra
Interestingly, all six derivatives 1–6 showed up-field shifted sig-
nals in 1H-NMR spectra at room temperature (Supporting infor-
mation, SI, Figure S1–S7). This behaviour was most pronounced
for deca(2-thienyl)biphenyl 1, for which one set of signals be-
longing to protons in 3-position of thiophenes (integral of four)
appeared at δ = 6.25 ppm and therefore showed the largest
up-field shift in the series (Figure S1). At 230 K this signal was
furthermore shifted to δ = 6.0 ppm (Figure S2). This shielding
effect is induced by aromatic ring current[31] arising from a pre-
ferred conformation, in which the protons in 3-position of the
inner four thiophenes are lying above and beneath the centre
of the two nearly perpendicularly arranged phenylene rings
(Figure 2). The structures of 1–6 were further confirmed by
high-resolution mass spectra (HRMS, Figure S8–S13).
Figure 3. Molecular structure of hexa(2-thienyl)benzene 3: front view (a) and
side view (c) of the conformer with all thiophenes pointing into the same
direction. Front view (b) of the conformer with alternating up and down
orientation of the thiophenes. (d) Unit cell with four molecules. Ellipsoids
were depicted in all cases.
lowest deviation in the values of short contacts between the
ipso-Cs with values between 2.869(5)-2.918(5) Å. This might be
due to the highest symmetry of this molecule in the series of
1–3. Moreover, intramolecular short contacts occur between
C-C, C-S, and S–S atoms at distances between 3.148–3.552 Å.
Due to the propeller-like conformation of the thiophenes, the
pz-orbitals of their ipso-C atoms are almost aligned pointing to
each other and forming a toroid. The short distances between
the ipso-C atoms (< 3 Å) are of relevance because coupling
between the pz-orbitals should be present at these distances
and electron delocalization in this toroid seems to occur (vide
infra).
The packing mode of the molecules revealed quasi-herring-
bone intermolecular interactions between the thiophene π-sys-
tem and α-H atoms of thiophenes in adjacent molecules (cyan-
dotted lines in Figure 4a, c). In the perpendicular direction, the
molecules organize in stacked columns showing short contacts
between S- and H-atoms. Some S-π interactions at distances of
3.517 and 3.580 Å can be found as well (labelled by orange and
Figure 2. Protons in 3-position of inner thiophenes (blue) of deca(2-thienyl)bi-
phenyl 1 deshielded by the aromatic ring current of the adjacent phenylene
unit.
Structural Characterization of Thiophene-Based
Derivatives 1–3 by X-ray Structure Analysis and Evidence
for Toroidal Non-Covalent pz-Orbital Overlapping
The molecular structures of the novel thienylene-phenylene de-
rivatives 1–3 were investigated by single-crystal X-ray structure
analysis and compared to structures of the phenyl derivatives
4,[25] 5,[27] and 6[32] which are literature known. The interesting
question was if the exchange of the propeller-like arranged pe-
ripheral phenyl substituents in 4–6 by thiophenes in 1–3 would
result in different geometric constraints.
Single crystals of hexa(2-thienyl)benzene 3 were obtained by
slow diffusion of n-hexane into a solution of 3 in dichloro-
methane. The molecules crystallize in the Pna21 space group
with four molecules in the unit cell (Table S1, Figure 3d). Three
of the thiophene rings in the molecule showed orientational
disorder causing two conformers: in the first one at 52 % proba-
bility all thiophenes point into the same direction (Figure 3a, c),
whereas in the second one at 48 % probability, we observe an
alternating up and down orientation of the thiophenes related
to the plane of the benzene ring (Figure 3b).
The bond lengths in the central benzene ring vary between
1.402–1.412 Å and the bond lengths between the benzenoid
carbons and the ipso-Cs of the thiophenes range from 1.474–
1.493 Å. Nevertheless, the torsion angles between the central
benzene and the thiophene rings showed values between
51.4(7) and 75.8(4) degrees allowing for some interaction be-
tween them (vide infra). Hexa(2-thienyl)benzene 3 showed the
Figure 4. Packing mode of hexa(2-thienyl)benzene 3 perpendicular (a), parallel
(b), and (c) in the [5 0 3] plane. Contacts between atoms at distances below
van der Waals radii are labelled by dotted lines (cyan, orange, and green lines
corresponding to H-C, H-S, and S-π contacts, respectively). Ellipsoids were
depicted in all cases.
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green-dotted lines, respectively, in Figure 4b). In phenylated
counterpart HPB 6, the peripheral phenyl rings are tilted by 75°
or 79° in average,[32] a value, which is substantially higher than
that in thienyl-substituted derivative 3 (ca. 65°, vide supra).
Single crystals of (2-thienyl)ethynylbenzene 2 were obtained
by slow evaporation from a dichloromethane (DCM) solution.
Ethynylene 2 crystallizes in the triclinic space group P1¯ with
two molecules in the unit cell (Table S2, Figure 5c). The thio-
phene ring located in the para-position to the ethynyl substitu-
ent showed orientational disorder causing two conformers. The
prevailing conformer with 67 % probability is depicted in Fig-
ure 5. The bond lengths in the central benzene ring vary be-
tween 1.404–1.411 Å and those between the benzenoid Cs and
the ipso-Cs of the thiophene rings range from 1.476(4)–1.484(4)
Å. Related to the central benzene unit, the torsion angles of the
thiophene rings showed values between 67.0(5)° and 70.0(4)°.
The separated thiophene attached at the ethynyl-bridge is dis-
torted by 46.4° from the benzene plane. Intramolecular short
contacts of 2.831(4)-2.924(3) Å have been found between the
ipso-Cs indicating pz-orbital overlapping as in the case of HTB
3. As well several intramolecular C-C and C-S interactions have
been found at distances between 3.21–3.39 Å.
Figure 6. Packing mode of (2-thienyl)ethynyl-benzene 2 perpendicular (a) and
parallel (b) to plane [16 17 8]. Contacts between atoms at distances below
van der Waals radii are labelled by dotted lines (cyan lines for H-C contacts,
pink lines for C-C contacts, denoting π–π stacking). Ellipsoids were depicted
in both cases.
dral angles between 56°-70°, is quite similar to the thiophenes
in 2 (67°-70°).
Single crystals of deca(2-thienyl)biphenyl 1 were obtained by
slow diffusion of n-hexane into a solution of 1 in DCM. Crystals
belong to the triclinic space group P1 with two molecules in
¯
the unit cell. Eight out of ten thiophene rings in the molecule
showed orientational disorder causing two conformers with al-
most equal probability (51 % and 49 %) (Figure 7, for clarity
only one conformer is shown). The thienyl substituents in each
half of the molecule have torsion angles between 55°-85° and
arrange as expected propeller-like. The torsion angle between
the planes of the two benzene rings amounts to 80° (Figure 7a).
The inter-ring bond length of 1.50(5) Å in the biphenyl unit of
1 is comparable to 1.501(4) Å[25] in polyphenyl 4 and to 1.49(5)
Å in non-constrained 1,1′-biphenyl.[33] The interring bonds be-
tween the phenyl and the thiophene rings vary between 1.44(3)
–1.52(1) Å and are larger than the 1.44 Å observed in non-
constrained 2,5-diphenylthiophene[34] due to the closely
packed and distorted thiophene rings in 1.
Figure 5. Molecular structure of (2-thienyl)ethynylbenzene 2: front view (a),
side view (b), and unit cell (c). Ellipsoids were depicted in all cases.
Figure 7. Molecular structure of deca(2-thienyl)biphenyl 1 with ellipsoids:
(a) front view. Short contacts between the ipso-C atoms are labelled with
magenta and purple lines in (b) and their catenated topology is sketched in
(c). The remaining intramolecular short contacts between the ipso-C and
S- or ꢀ-C-atoms are labelled with cyan rings in (d). Their twisted toroidal
topology over all ten thiophene rings is sketched in (e).
In the bulk, the molecules pack in dimers with an antiparallel
orientation such that π–π stacking between the thienyl-ethynyl
substituents have been found (pink-dotted lines in Figure 6a).
Moreover, several H-C intermolecular interactions force the di-
mers to order in a plane (cyan-dotted lines in Figure 6a). In the
perpendicular direction, the molecules interact through short
H-C contacts with the dimers in the planes above and below
(cyan-dotted lines in Figure 6b). In the case of the crystal struc-
ture of 2, both, intra- and intermolecular interactions are rele-
vant.
Like in case of derivatives 2 and 3, intramolecular short con-
tacts at distances far below the sum of van der Waals radii have
been found between ipso-Cs at 2.82(1)-3.04(1) Å (Figure 7b) and
C-C and C-S short contacts between 2.897–3.545 Å (Figure 7d).
The image of the interacting pz-orbitals of the ipso-C atoms
Corresponding phenylene derivative 5 as well showed two in 1 resembles two rings in a catenated topology (Figure 7c).
independent conformers in single-crystal X-ray structure analy- Remarkably, as well π–π stacking between the parallelly
sis.[27] In one conformer the central and ethynylene-linked rings stacked four inner thiophene rings (numbered 1–1′ and 5–5′ in
are coplanar which contrasts with thienyl derivative 2 (46°). The Figure 8a) at distances as short as 3.16(2) can be observed giv-
arrangement of the phenyl substituents in 5, which adopt dihe- ing rise to a second pathway for the intramolecular interactions
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Figure 8. Unit cell of decathienylbiphenyl 1 (a) and packing mode perpendicular to plane [010] (b). Four of the thiophene rings are labelled in a) (see text).
Contacts between H-C atoms at distances below van der Waals radii are labelled by cyan-dotted lines in b). Ellipsoids were depicted in both cases.
involving all ten thiophene rings in a twisted toroidal topology Theoretical Calculations on Structures 1, 3, 4 and 6.
(Figure 7e).
Geometry Optimizations, Energies, and Evidence for
Toroidal Through-Space Conjugation
In the bulk, the molecules pack in perfectly aligned rows
perpendicular to the [0 1 0] plane. Surprisingly, only one H-C
intermolecular interaction can be found between molecules in
these rows (cyan-dotted lines in Figure 8b) opposite to the nu-
merous intramolecular short contacts (vide supra). The lack of
significant intermolecular interactions in the crystal structure
analysis of 1 indicates that the intramolecular short contacts
are structurally inherent of biphenyl 1 lacking relevant influence
from packing effects. The intramolecular interactions found for
biphenyl 1 apparently contribute to their molecular stabiliza-
tion energy.
The structural characterization of thiophene-based deriva-
tives 1–3 by analysis of intramolecular non-bonding interac-
tions of adjacent ipso-carbons, which were substantially shorter
than the sum of the van der Waals radii, showed evidence for
continuous overlap of the respective tilted pz-orbitals of the
ipso-Cs in a toroidal topology. This phenomenon is as well re-
flected in the corresponding occupied MOs (Figure 9) and was
already discussed by Rathore et al. from X-ray structure analysis
of HPB 6.[20] Comparison of the torsion angle and bond lengths
of the central biphenyl unit in derivatives 1 and 4 revealed
an orthogonal arrangement and thereby a larger distortion for
decaphenylbiphenyl 4 (90°) than for deca(2-thienyl)biphenyl 1
(77°) whereas the bond lengths are practically not affected.
Quantum chemical calculations were performed on sterically
crowded thienylene-based structures 1 and 3 and compared to
their polyphenylene counterparts 4 and 6. Because of disorder
in the crystal structures, the orientations of the thiophene rings
in structures 1–3 are not unambiguous. Geometry optimiza-
tions at the M062X-D3/def2TZVP level of density functional
theory (DFT) revealed that the two conformers of hexa(2-thi-
enyl)benzene 3, in which either all sulfur atoms point towards
the same direction or they point up and down in an alternating
sequence are almost isoenergetic (ΔE = 0.11 kcal mol–1, Fig-
ure 10).
Figure 10. Symmetrical conformers of hexa(2-thienyl)benzene 3. All sulfur
atoms pointing in one direction (left) or the sulfur atoms point up and down
in alternating sequence (right).
The two symmetrical conformers are indeed obtained in
crystal structure analysis, although other less symmetrical con-
formers are conceivable. According to our calculations (geome-
try optimizations of all possible structures at the M062X-D3/
def2T-ZVP level of DFT), corresponding energy differences in
deca(2-thienyl)biphenyl 1 and decaphenylbiphenyl 4 are some-
what larger but still in the range of solid-state packing effects.
For investigation of electron delocalization, our ACID (An-
isotropy of the Induced Current Density) method was em-
ployed.[35,36] The ACID scalar field is interpreted as the density
of delocalized electrons and can be plotted as isosurface (yel-
low surface in Figure 11). For the sake of simplicity and to save
computer time, the conformations with the highest symmetry
of derivatives 1, 3, 4, and 6 were analyzed. As expected, the
ACID plots of derivatives 3 and 6 exhibited strong π-electron
delocalization in each phenyl or thienylene ring and a diatropic
ring current in the central benzene ring. Furthermore, a notice-
ably, although subtle, through-space conjugation between the
ipso-Cs of the outer rings (Figure 11) was found, which theoreti-
cally confirms the toroidal delocalization proposed in the litera-
ture.[13–18]
Figure 9. Schematic representation of the three occupied MOs that result
from in-plane overlap of the pz-orbitals of the ipso C atoms.
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ments allowing for the determination of through-bond carbon–
carbon connectivities.[39]
Tenfold-substituted biphenyls 1 and 4 exhibited similar
through-bond, but topologically even more intriguing toroidal
delocalization as shown in Figure 12. ACID plots revealed that
through-space conjugation between the ipso-Cs of the deca-
phenyl and deca(2-thienyl)benzene units exhibit the topology of
catenated wedding rings.
Figure 11. ACID plots of hexa(2-thienyl)benzene 3 (left) and hexaphenyl-
benzene 6 (right) at isosurface values of IV=0.008. The magnetic field is or-
thogonal to the central benzene ring. The current density is plotted as green
arrows with red arrowheads on top of the ACID isosurface. The length of the
arrows is proportional to the current. The wave function was calculated at
the B3LYP/6-311G**-level of density theory and the current density tensor
field was computed with the option IOp (10/93=1) implemented in the Gaus-
sian program. The ipso through-space conjugation is clearly visible (see en-
largement). However, there is no diatropic current following the ipso conjuga-
tion pathway.
Figure 12. ACID plots of deca(2-thienyl)biphenyl 1 (top left) and decaphenyl-
biphenyl 4 (top right), at isosurface values of IV: 0.0015. The topology of the
through-space conjugation of the ipso-C atoms is isomorphic to a pair of
catenated wedding rings (bottom).
Optical and Electrochemical Properties
The optoelectronic properties of biphenylene derivatives 1 and
4 and of corresponding ethynylene derivatives 2 and 5 were
investigated and compared to the parent structures hexa-
(2-thienyl)benzene 3 and hexaphenylbenzene 6. Absorption and
emission maxima, optical energy gaps, oxidation potentials, and
the calculated frontier orbital energies are summarized in
Table 1. The absorption maxima in DCM solutions of intermedi-
ates 2 and 5 are red-shifted compared to target derivatives 1
and 4 due to a higher delocalization in the elongated π-system
(Figure 13). Derivatives containing thiophene units (1, 2, and 3)
showed red-shifted absorption compared to the corresponding
phenylene derivatives (4, 5 and 6) and consequently lower opti-
cal gap energies. The unstructured absorption bands at lower
energy of biphenyls 1 and 4 showed bathochromic shifts and
smaller energy gaps compared with parent compounds 3 and
6 which can be rationalized by a partial electronic delocaliza-
tion over the entire molecule in 1 and 4 with, fairly speaking,
Notwithstanding the clearly visible toroidal through-space
ipso-conjugation, there is no ring current induced along the
ipso-Cs in the outer periphery (Figure 11). A simple qualitative
orbital consideration, however, predicts that the ipso pz-orbitals
of each phenyl substituent should form the basis of a six-elec-
tron, in-plane aromatic π-system (Figure 9). Hence, a diatropic
ring current should be induced in a magnetic field. Orbital-sep-
arated ACID plots based on e1 and b1, which include the ipso
pz-orbitals, confirm this picture (Figure S19; for an analogous
treatment of hexaphenylbenzene 6, see Figure S18). The di-
atropic ring currents in the σ-system of the benzene ring and
the ipso through-bond conjugation induce six local paratropic
currents (red circles in Figure S18 and S19). However, if the com-
plete set of MOs is included in ACID plots these currents vanish.
Obviously, there are paratropic ring currents induced by other
MOs that cancel the diatropic ipso C-atom current.
Orbital contributions to ring currents can be qualitatively an-
alyzed based on their symmetry. If the product of the occupied
and unoccupied pair of MOs involved in the transition trans-
forms as the same representation as the rotation around a
Cartesian axis, the contribution is paratropic, if it transforms
as the translation along one of the axes the contribution is
diatropic.[37,38] The contributions from HOMO-2 → LUMO and
HOMO-3 → LUMO excitations are paratropic and therefore can-
cel the diatropic contribution of the b2u → LUMO excitation.
This might be the reason why the effects due to homoconjuga-
tion do not give rise to measurable effects in NMR. Therefore,
we calculated 13C-13C coupling constants using the GIAO
method. The resulting 1.06 Hz were well below the detection
limit of about 4–5 Hz in 2D 13C-13C INADEQUATE NMR experi-
Table 1. Optical and electrochemical data of thienylene derivatives 1–3 and
phenylene counterparts 4–6.
opt
ox1
ox2
λabs
Eg
[eV]
λem
Ep
Ep
EHOMO
[eV]
ELUMO
[eV][c]
[nm][a]
[nm][b]
[V]
[V]
1
2
3
240, 294, 337(s) 3.45
240, 307, 347 3.32
240, 289, 320(s) 3.68
424
425
420
0.91
0.99
1.04
1.07
–
–
–5.95
–6.03
–5.94
–2.50
–2.71
–2.26
4
5
6
254, 277(s)
259, 304, 319
254, 282(s)
4.04
3.72
4.26
352
344
333
1.26
1.19
1.36
–
–
–
–6.31
–6.23
–6.14
–2.27
–2.51
–1.88
[a] Measured in DCM, maximum in italics, (s) denotes shoulder. [b] Measured
opt
in DCM. [c] Calculated with Eg
.
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very low transition probability. The gain in vibronic structure in
the absorption spectra of 2 and 5 accounts for a better planari-
zation of the π-system compared to the higher torsion of the
aromatic units in all other derivatives.
Figure 13. UV/Vis absorption spectra of (a) deca(2-thienyl)biphenyl 1 (black
line), ethynylene derivative 2 (black dashed line), and hexathienylbenzene 3
(black dotted line); (b) decaphenylbiphenyl 4 (blue line), ethynylene derivative
5 (blue dashed line), and hexaphenylbenzene 6 (blue dotted line) measured
in dichloromethane at room temperature.
In order to gain a better understanding of the experimentally
observed optical transitions, theoretical calculations were per-
formed on hexa(2-thienyl)benzene 3 (C6-symmetry) and hexa-
phenylbenzene 6 (D6-symmetry). After optimization at the
M062X-D3/def2TZVP level of DFT, time-dependent DFT calcula-
tions at the same level of theory including 30 singlet excitations
each were performed (Figure 14). The lowest energy excitations
for 3 (λmaxcalc=317 nm) and in 6 (λmaxcalc=286 nm) are in good
agreement with the experimental values (λmax=320 nm and
282 nm, respectively) and can be characterized as HOMO →
LUMO transitions. Both molecules exhibit degenerate HOMOs
and LUMOs, which give rise to three transitions, one of which
is allowed (B in 3 and E2 in 6). Fluorescence spectra of all com-
pounds were analyzed in DCM solution (Table 1, Figure S20).
Whereas hexa(hetero)arylbenzenes 3 and 6 were only very
weakly fluorescent, thiophene-based derivatives 1 and 2
showed stronger fluorescence and unstructured, strongly
bathochromically shifted emission spectra compared to the cor-
responding polyphenylene derivatives 4 and 5.
Redox potentials of biphenylene derivatives 1 and 4 and of
corresponding ethynylene intermediates 2 and 5 were deter-
mined by cyclic voltammetry and compared to parent hexathi-
enylbenzene 3 and hexaphenylbenzene 6 (Figure S21). All deriv-
atives showed irreversible oxidation waves, which indicate the
formation of reactive radical cations undergoing subsequent
follow-up reactions due to free terminal positions. In the nega-
tive potential regime, no reductions were visible in the electro-
chemical window. The first oxidation potential (0.91–1.04 V) of
the three thiophene derivatives 1–3 expectedly were at lower
values than those of the polyphenylene derivatives 4–6 (1.19–
1.36 V). Only for deca(2-thienyl)biphenyl 1 two separated (irre-
versible) oxidation waves at 0.91 V and 1.07 V were detected
indicating that each half unit of the molecule is consecutively
oxidized by one electron.
Figure 14. Theoretically calculated UV/Vis absorption spectra of 3 (top, C6
symm.) and 6 (bottom, D6 symm.). The insets represent the degenerated
frontier orbitals that are involved in the corresponding transitions.
Conclusions
The synthesis of sterically crowded and congested thienylene-
phenylene structures 1–3 has been presented and their struc-
tural characterization compared to the polyphenylene counter-
parts 4–6. Single crystal X-ray structure analysis revealed pro-
peller-like arrangement of the substituents around the central
basic benzene (2, 3, 5, 6) or biphenyl units (1, 4), respectively.
Typically, steric congestion and distortion effects of the thio-
phene-based derivatives 1–3 are smaller than in 4–6 due to the
smaller geometric extension of thiophene compared to benz-
ene rings. Evidence for through-space pz-orbital interaction has
been found by X-ray structure analyses on all derivatives.
Through-space π-conjugation in a toroidal topology involving
all ipso-carbons has been found theoretically for benzenoid de-
rivatives 3 and 6 in their neutral state. For more complex bi-
phenyls 1 and 4 through-space π-conjugation in a catenated
toroidal topology was rationalized from advanced theoretical
calculations (ACID) and suggested by the X-ray structure analy-
sis. The characterization of the optoelectronic properties re-
The analysis of the optical and redox properties of the six vealed the expected influence of the more electron-rich thio-
derivatives 1–6 clearly revealed that the replacement of phenyl- phene units in 1–3 compared to 4–6. Finally, we expect that
enes in 4–6 by thienylene substituents in 1–3 leads to batho- the subtle and small conjugation effects for the neutral mole-
chromic shifts in the optical spectra and lower oxidation poten- cules under investigation here should be much larger in
tials in electrochemistry due to the higher electron-donating charged species. In this respect, model compounds are under-
effect of thiophenes.
way in our laboratories.
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Full Paper
5H), 6.67–6.61 (m, 11H), 6.51 (dd, J = 3.5, 1.2 Hz, 2H), 6.47 (dd, J =
3.5, 1.2 Hz, 4H), 6.24 (dd, J = 3.6, 1.2 Hz, 4H); 13C NMR (101 MHz,
CD2Cl2): δ = 142.1, 141.5, 140.9, 140.0, 137.8, 137.6, 136.7, 131.1,
129.8, 129.7, 126.8, 126.54, 126.3, 126.2, 126.1, 125.6; HRMS (MALDI-
FT-ICR) m/z: calcd. for C52H30S10, 973.95491; found 973.95476 [M]+
(δm/m = 0.2 ppm), 1224.09250 [M + DCTB]+; CCDC 1884726.
Experimental Section
Materials and Methods: 1,2-Dichlorobenzene (Merck) was dried
with CaCl2 and distilled prior to use. Dichloromethane, petroleum
ether, and n-hexane were purchased from VWR and distilled prior to
use. Hexaphenylbenzene 6,[40] 1,4-Di(thien-2-yl)buta-1,3-diyne 7,[28]
2,3,4,5-tetra(thien-2-yl)cyclopenta-2,4-dien-1-one 8,[29] di(thien-2-
yl)acetylene 9,[41] 1,4-diphenylbuta-1,3-diyne 10,[28] and tetra-
cyclone 11[42] were prepared according to literature. Thin-layer
chromatography was performed on aluminum plates, precoated
with silica gel, Merck Si60 F254. Preparative column chromatogra-
phy was carried out on glass columns packed with silica gel, Merck
Silica 60, particle size 40–63 μm. HPLC was performed on a Shim-
adzu CBM-20A equipped with a SPD-20A UV/VIS detector and a
LC-8A solvent system with a Macherey-Nagel column (Nucleosil
100-5 NO2). Melting points were measured using a Büchi Melting
Point M-565. Thermogravimetric analyses (TGA) were performed on
a TGA/SDTA 851e from Mettler Toledo. NMR spectra were recorded
on a Bruker Avance 400 (1H NMR: 400 MHz, 13C NMR: 101 MHz) at
293 K. Chemical shift values (δ) are given in parts per million (ppm)
and were calibrated on residual non-deuterated solvent peaks (1H
NMR: δH = 5.32 for CD2Cl2; 13C NMR: δC = 54.00 for CD2Cl2) as an
internal standard. The splitting patterns are labeled as follows: d
(doublet), dd (doublet of doublet) and m (multiplet). High-resolu-
tion MALDI mass spectra (HRMS) were recorded on a MS Bruker
Reflex 2 (Bruker Daltonik GmbH, Bremen, Germany), using trans-
2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile
(DCTB) as a matrix. Cyclic voltammetry experiments were carried
out using a computer-controlled Autolab PGSTAT30 potentiostat in
a three-electrode single-compartment cell with a platinum working
electrode, a platinum wire counter electrode, and an Ag/AgCl refer-
ence electrode. All potentials were internally referenced to the fer-
rocene/ferricenium couple. UV/Vis absorption spectra were re-
corded on a Perkin Elmer Lambda 19 spectrometer. Fluorescence
spectra were recorded on a Perkin Elmer LS 55 Luminescence Spec-
trometer.
2,2′,2′′,2′′′,2′′′′-[6-(Thien-2-ylethynyl)benzene-1,2,3,4,5-pentayl]pen-
tathiophene (2). A mixture of 1,4-di(thien-2-yl)buta-1,3-diyne 7
(100 mg, 0.47 mmol) and 2,3,4,5-tetra(thien-2-yl)cyclopenta-2,4-
dien-1-one 8 (286 mg, 0.70 mmol) in 2.5 mL of dry 1,2-dichloro-
benzene was stirred for 18 h under reflux under Ar atmosphere.
After cooling to room temperature, the solvent was removed under
reduced pressure and the crude product was subjected to flash
column chromatography (SiO2, petroleum ether/dichloromethane,
3:2). The desired product 2 (250 mg, 0.42 mmol, 90 %) was obtained
as a white solid after recrystallization from dichloromethane/n-hex-
ane and HPLC (n-hexane/dichloromethane, 3:2). Additionally, 2,2′,
2′′,2′′′,2′′′′,2′′′′′,2′′′′′′,2′′′′′′′,2′′′′′′′′,2′′′′′′′′′-([1,1′-biphenyl]-2,2′,3,3′,
4,4′,5,5′,6,6′-decayl)decathiophene 1 (11.0 mg, 0.01 mmol, 2 %
yield) was obtained as side product. Melts under decomposition, Td
323 °C (TGA) 1H-NMR (400 MHz, CD2Cl2): δ = 7.34 (dd, J = 5.1, 1.2 Hz,
2H), 7.23 (dd, J = 5.2, 1.2 Hz, 1H), 7.12 (dd, J = 5.1, 1.2 Hz, 2H), 7.09
(dd, J = 5.1, 1.2 Hz, 1H), 7.03 (dd, J = 3.5, 1.2 Hz, 2H), 6.97 (dd, J =
5.1, 3.5 Hz, 2H), 6.89 (dd, J = 5.2, 3.7 Hz, 1H), 6.79 (dd, J = 3.7, 1.2 Hz,
1H), 6.71 (dd, J = 5.1, 3.5 Hz, 2H), 6.68–6.66 (m, 3H), 6.62 (dd, J =
3.5, 1.2 Hz, 1H); 13C NMR (101 MHz, CD2Cl2): δ = 140.8, 140.5, 140.4,
138.4, 137.4, 137.4, 132.7, 130.2, 130.1, 129.8, 128.7, 127.6, 126.9,
126.9, 126.8, 126.8, 126.6, 126.3, 126.2, 123.3, 92.4, 92.3; HRMS
(MALDI-FT-ICR) m/z: calcd. for C32H18S6, 593.97273; found 593.97099
[M]+ (δm/m = 0.3 ppm), 844.11555 [M + DCTB]+; CCDC 1884740.
Hexa(thien-2-yl)benzene (3). All steps were carried out under Ar
atmosphere. Di(thien-2-yl)acetylene 9 (0.09 g, 0.47 mmol) was dis-
solved in 4 mL of 2-propanol. Rhodium(III)chloride (7.1 mg,
0.03 mmol) and triethylamine (0.5 mL) was added and the solution
was stirred for 3 days at 85 °C. The solution was poured into 20 mL
of water, extracted with dichloromethane and dried with Na2SO4.
The product was purified by column chromatography (SiO2, 1. pe-
troleum ether and 2. petroleum ether/ethyl acetate, 20:1) and iso-
lated as colourless solid (152 mg, 0.27 mmol, 41 % yield). No melt-
ing up to 450 °C. 1H-NMR (400 MHz, CDCl3): δ = 7.08 (dd, J = 5.1,
1.2 Hz, 1H, H-5), 6.68 (dd, J = 5.0, 3.5 Hz, 1H, H-4), 6.58 (dd, J = 3.5,
1.2 Hz, 1H, H-3); 13C NMR (101 MHz, CD2Cl2): δ = 141.1, 137.7, 129.1,
126.6, 126.2; HRMS (MALDI-FT-ICR) m/z: calcd. for C30H18S6,
569.97273; found 569.97241 [M]+ (δm/m = 0.6 ppm); CCDC
1884759.
Synthetic Methods
2,2′,2′′,2′′′,2′′′′,2′′′′′,2′′′′′′,2′′′′′′′,2′′′′′′′′,2′′′′′′′′′-([1,1′-Biphenyl]-2,2′,3,3′,
4,4′,5,5′,6,6′-deca-yl)decathiophene (1). Synthesis route 1: A mix-
ture of 1,4-di(thien-2-yl)buta-1,3-diyne 7 (50.0 mg, 0.23 mmol) and
2,3,4,5-tetra(thien-2-yl)cyclopenta-2,4-dien-1-one 8 (238 mg,
0.58 mmol) in 2.5 mL of dry 1,2-dichlorobenzene was stirred for 4.5
h at 200 °C in the microwave under Ar atmosphere. After cooling
to room temperature, the solvent was removed under reduced pres-
sure and the crude product was subjected to flash column chroma-
tography (SiO2, petroleum ether/dichloromethane, 5:3). Further im-
purities had to be removed by HPLC (n-hexane/dichloromethane,
5:3). The desired biphenyl 1 (16 mg, 0.02 mmol, 7 %) was obtained
as a white solid. 2,2′,2′′,2′′′,2′′′′-[6-(Thien-2-ylethynyl)benzene-
1,2,3,4,5-pentayl]pentathiophene 2 (103 mg, 0.17 mmol, 74 %) was
obtained as main product.
3′,3′′,4′,4′′,5′,5′′,6′,6′′-Octaphenyl-1,1′:2′,1′′:2′′,1′′′-quaterphenyl (4).
In a Schlenk-tube a mixture of 1,4-(diphenyl)buta-1,3-diyne 10
(70.0 mg, 0.35 mmol) and tetracyclone 11 (399 mg, 1.04 mmol) was
stirred in the melt for 3 h at 315 °C under Ar atmosphere. After
cooling to room temperature, the crude product was subjected to
flash column chromatography (SiO2, petroleum ether/dichloro-
Synthesis Route 2: A mixture of 2,2′,2′′,2′′′,2′′′′-[6-(thien-2-yl- methane, 3:2). After recrystallization from dichloromethane/n-hex-
ethynyl)benzene-1,2,3,4,5-penta-yl]pentathiophene 2 (40.0 mg,
0.07 mmol) and 2,3,4,5-tetra(thien-2-yl)cyclopenta-2,4-dien-1-one 8
(68.7 mg, 0.17 mmol) in 8 mL of dry 1,2-dichlorobenzene was stirred
for 4.5 h at 200 °C in the microwave under Ar atmosphere. After
cooling to room temperature, the solvent was removed under re-
duced pressure and the crude product was subjected to flash col-
umn chromatography (SiO2, petroleum ether/dichloromethane, 5:3).
Further impurities had to be removed by HPLC (n-hexane/dichloro-
methane, 5:3). The desired biphenyl 1 (10 mg, 0.01 mmol, 14 %
yield) was obtained as a white solid. Mp 335–339 °C; 1H-NMR
(400 MHz, CD2Cl2): δ = 7.17 (dd, J = 5.1, 1.2 Hz, 4H), 7.07–7.04 (m,
ane the desired quaterphenyl 4 (265 mg, 0.29 mmol, 84 % yield)
was obtained as a white solid (Lit. 60 %).[24] Additionally, 3′,4′,5′-
triphenyl-6′-(phenylethynyl)-1,1′:2′,1′′-terphenyl 5 (11.0 mg,
0.02 mmol, 6 % yield) was obtained as side product. Mp 329–332 °C
(Lit. 322–324 °C);[24] 1H-NMR (400 MHz, CD2Cl2): δ = 7.00–6.94 (m,
4H), 6.94–6.88 (m, 9H), 6.83–6.76 (m, 18H), 6.75–6.69 (m, 13H), 6.69–
6.65 (m, 8H); 13C NMR (101 MHz, CD2Cl2): δ = 142.2, 141.7, 141.4,
141.1, 140.8, 140.3, 137.8, 132.8, 132.2, 131.9, 126.8, 126.7, 126.1,
125.6, 125.5; HRMS (MALDI-FT-ICR) m/z: calcd. for C72H50, 914.39070;
found 914.38938 [M]+ (δm/m = 1.4 ppm), 824.34257 [M-2(C6H5) +
2H++ K + Na]+, 937.37828 [M + Na]+, 953.35234 [M + K]+.
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Acknowledgments
T. L. gratefully acknowledges financial support by the State of
Baden-Württemberg and the University of Ulm. Special ac-
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Received: November 6, 2019
Eur. J. Org. Chem. 2019, 1–11
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Full Paper
Extended π-Systems
T. D. Leitner, Y. Gmeinder,
F. Röhricht, R. Herges,
E. Mena-Osteritz, P. Bäuerle* ....... 1–11
Twisted
Thienylene–Phenylene
Structures: Through-Space Orbital
Coupling in Toroidal and Catenated
Topologies
Twisted thienylene-phenylenes give ero)aromatic rings in a catenated toroi-
evidence of through-space delocaliza- dal topology.
tion of π-electrons of peripheral (het-
DOI: 10.1002/ejoc.201901637
Eur. J. Org. Chem. 2019, 1–11
www.eurjoc.org
11
© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim