Synthesis and characterization of S,N-heterotetracenes

Article abstract of DOI:10.3762/BJOC.16.214

The synthesis and optoelectronic properties of novel S,N-heterotetracenes consisting of fused heterocyclic thiophene and pyrrole rings are presented. Tetracyclic and benzannulated derivatives with a varying number and sequence of sulfur and nitrogen heteroatoms were synthesized in multistep synthetic routes. A Buchwald-Hartwig amination of brominated precursors, thermolysis of azide precursors, and a Cadogan reaction of nitro-substituted precursors were successfully applied to eventually build-up pyrrole rings to stable and soluble fused systems. The various obtained heteroatom sequences 'SSNS' (SN4), 'SNNS' (SN4''), and 'NSSN' (SN4') allowed for evaluation of structure-property relationships relative to the sulfur analogue tetrathienoacene ('SSSS'). In line with the results for the whole series of S,N-heteroacenes, we find that replacement of sulfur by nitrogen atoms in the tetra- and hexacyclic systems leads to a red-shift in absorption, a decrease in oxidation potential and energy gap. On the other hand, the replacement of a thiophene ring by benzene leads to the opposite effects.

Full text of DOI:10.3762/BJOC.16.214

Synthesis and characterization of S,N-heterotetracenes
Astrid Vogt1, Florian Henne1,2, Christoph Wetzel1,3, Elena Mena-Osteritz1
and Peter Bäuerle*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2636–2644.
1Institute of Organic Chemistry II and Advanced Materials, University
of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany, 2Carl Zeiss
SMT GmbH, Rudolf-Eber-Straße 2, 73447 Oberkochen, Germany and
3Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany
https://doi.org/10.3762/bjoc.16.214
Received: 21 July 2020
Accepted: 15 October 2020
Published: 26 October 2020
Email:
Peter Bäuerle* - peter.baeuerle@uni-ulm.de
Associate Editor: I. Baxendale
* Corresponding author
© 2020 Vogt et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
Buchwald–Hartwig amination; Cadogan reaction; optoelectronic
properties; S,N-heteroacene; structure–property relationship
Abstract
The synthesis and optoelectronic properties of novel S,N-heterotetracenes consisting of fused heterocyclic thiophene and pyrrole
rings are presented. Tetracyclic and benzannulated derivatives with a varying number and sequence of sulfur and nitrogen
heteroatoms were synthesized in multistep synthetic routes. A Buchwald–Hartwig amination of brominated precursors, thermolysis
of azide precursors, and a Cadogan reaction of nitro-substituted precursors were successfully applied to eventually build-up pyrrole
rings to stable and soluble fused systems. The various obtained heteroatom sequences ‘SSNS’ (SN4), ‘SNNS’ (SN4’’), and ‘NSSN’
(SN4’) allowed for evaluation of structure–property relationships relative to the sulfur analogue tetrathienoacene (‘SSSS’). In line
with the results for the whole series of S,N-heteroacenes, we find that replacement of sulfur by nitrogen atoms in the tetra- and
hexacyclic systems leads to a red-shift in absorption, a decrease in oxidation potential and energy gap. On the other hand, the
replacement of a thiophene ring by benzene leads to the opposite effects.
Introduction
Thienoacenes and related S,N-heteroacenes have been de- acenes, whereby the incorporation of the electron-rich pyrrole
veloped to important π-conjugated systems mainly for applica- rings not only modifies the electronic properties compared to
tion as p-type semiconductor in organic electronic devices with thienoacenes, but also allows for the attachment of side chains
excellent results [1-3]. The series of thienoacenes consisting of at the pyrrole nitrogen as a useful synthetic tool to enhance
only fused thiophene rings has been synthesized up to an solubility [7]. The smaller members of the series of S,N-hetero-
octamer, whereby the longer members in the series quickly lose acenes, dimeric thieno[3,2-b]pyrrole [8] and trimeric
solubility [4-6]. The implementation of pyrrole rings into the dithieno[3,2-b:2’,3’-d]pyrrole (DTP) [9], are long known but
fused ladder-type structure leads to the class of S,N-hetero- still frequently used as building block for organic electronic ma-
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terials [10]. By application of Pd-catalyzed Buchwald–Hartwig by nitrogen can lead to different isomeric SN4 structures with
amination/cyclization reactions of brominated thiophene-based possible sequences ‘NSSN’, which we denominate as SN4', and
precursors and amines [11,12], the series of S,N-heteroacenes ‘SNNS’ as SN4'' (Figure 1). In this respect, the only known
has been systematically extended from pentamer SN5 [13,14] structures were prepared by Gryko et al., who synthesized
up to a stable and still soluble S,N-heterotridecacene SN13 heterohexacenes with N-phenylated SN4'' cores [35]. The same
[7,15-17]. Correlation of, e.g., absorption or redox potentials vs group furthermore prepared heterotetracenes comprising four
conjugated chain length led to interesting structure–property pyrrole rings (‘NNNN’) as electron-rich, ladder-type fluoro-
relationships showing the influence of the number and se- phores by a two-step synthesis [36]. The possible asymmetric
quence of pyrrole rings. The highly planar π-conjugated SN4 isomer ‘SNSN’ and heterotetracenes comprising three
systems revealed interesting structural features such as nearly pyrrole rings are unknown so far.
complete bond length equalization and S–S and S–π dipolar
interactions in the solid state. Due to tunable optoelectronic In continuation of our work on heteroacenes, we now report in
properties, a variety of functionalized derivatives, mainly detail the syntheses and characterization of fused S,N-heterote-
trimeric DTP (SN3) [18], pentameric SN5 [19-23], and hexa- tracenes, SN4 (‘SSNS’), benzannulated SN4' (‘NSSN’), and
meric SN6 [24-28], have been successfully implemented as SN4'' (‘SNNS’) by application of various cyclization methods
active semiconducting components in organic solar cells [18- to build up pyrrole rings in the tetraheterocyclic systems.
21,24-26], perovskite solar cells [22,27], organic field-effect Among them, a Buchwald–Hartwig amination of brominated
transistors [28], or sensors [23]. The interesting structure–prop- precursors, thermolysis of azide precursors, and a Cadogan
erty relationships of the S,N-heteroacene series inspired as well cyclization of nitro-substituted precursors were applied to
theoretical work. In this respect, De Simone et al. computed prepare various unknown derivatives. These represent novel
electronic spectra of the neutral, charged radical cationic, and core molecules, which by further derivatisation, for example
dicationic species [29], whereas Sarkar et al. undertook quan- with terminal acceptor groups, would lead to interesting
tum chemical investigations on the photovoltaic performance π-conjugated materials for application in organic electronics.
revealing an odd-even relation of the charge separation [30].
Furthermore, they calculated the single molecule transport be- Results and Discussion
havior of various S,N-heteroacenes with different conjugation Synthesis of N-alkylated S,N-heterotetracene 9 by Pd-cata-
length between nanocontacts [31].
lyzed amination. Asymmetric S,N-heterotetracene Hex-SN4 9
comprises the sequence of heteroatoms ‘SSNS’ in the tetra-
The obvious missing link in the series of S,N-heteroacenes is cyclic conjugated π-system [37]. In a multistep synthesis, 2,6-
represented by heterotetrameric SN4, which is very rarely de- dibromothienothiophene 2 [39], which was obtained from
scribed in the literature. SN4 is derived from well-known parent thieno[3,2-b]thiophene (1) [38], was triisopropylsilyl (TIPS)-
tetrathienoacene (TTA, ‘SSSS’, Figure 1) by replacing one “S” protected by lithium-halogen exchange with n-BuLi and triiso-
by “N” or in other words one fused thiophene unit by a pyrrole propylsilyl chloride to give thienothiophene 3 in 69% yield. A
ring. For TTA various synthetic methods have been developed halogen dance reaction of 3, induced by lithium diisopropyl-
and it has been broadly implemented as building block into ma- amide (LDA) at −78 °C, gave the corresponding β-brominated
terials for organic electronic devices [32,33]. We have disclosed thienothiophene 4 in 91% yield [6]. Compound 4 was selec-
the first SN4 derivative with the heteroatom sequence ‘SSNS’ tively brominated in α-position by using NBS in DMF at room
in the context of complementing the series, but did not yet temperature to give dibrominated thienothiophene 5 in 92%
describe synthetic details [7]. Xue et al. published a yield which was coupled in a Negishi-type cross-coupling reac-
donor–acceptor photosensitizer for dye-sensitized solar cells, in tion with zincated 2,3-dibromothiophene 6 and Pd(dppf)Cl2 as
which the N-phenylated SN4 unit serves as bridge between catalyst to give the corresponding dibromide 7 in a yield of
donor and acceptor moieties [34]. Further replacement of sulfur 70%. Organozinc species 6 was obtained from 2,3-dibromothio-
Figure 1: Heteroacenes: tetrathienoacene (TTA), S,N-heteroacenes SN4, SN4', and SN4''.
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phene by lithiation with n-BuLi and reaction with zink dichlo- The synthesis started from the above mentioned brominated
ride. Annulation to TIPS-protected SN4-Hex 8 was achieved in thieno[3,2-b]thiophene 3, which was coupled with zincated 2,3-
87% yield by a tandem Buchwald–Hartwig amination of dibro- dibromothiophene 6 in a Negishi-type cross-coupling reaction
mide 7 with n-hexylamine in the presence of sodium tert- with Pd(dppf)Cl2 as catalyst to give thiophene-substituted
butoxide as base and Pd(dba)2/dppf as the catalytic system. The thienothiophene 10 in 72% yield. In the next step, an azide
TIPS group was removed from Hex-SN4 8 upon treatment with group was introduced to 10 by lithiation with n-BuLi followed
tetra-n-butylammonium fluoride (TBAF) and the parent system by the reaction with tosyl azide giving azide 11 in an excellent
Hex-SN4 9 was obtained in 80% yield (Scheme 1).
yield of 92%. Thermally induced ring-closure under release of
nitrogen and formation of a nitrene intermediate was achieved
Synthesis of S,N-heterotetracene H-SN4 13 by thermolysis by refluxing azide 11 in chlorobenzene for 30 min to yield
of azide precursors. Smaller parent heterotriacene dithienopyr- TIPS-protected H-SN4 12 in 46% yield. Finally, the TIPS group
role (H-DTP) was first synthesized by Zanirato et al. by thermo- of S,N-heterotetracene 12 was smoothly removed using TBAF
lysis of 3-azido-2,2’-bithiophene as the key step [9]. We there- in THF solution to yield the basic structure H-SN4 13 in almost
fore tried to build up tetracyclic H-SN4 13 via the azide route. quantitative yield (Scheme 2).
Scheme 1: Synthesis of fused S,N-heterotetracene SN4 9 starting from thieno[3,2-b]thiophene (1).
Scheme 2: Synthesis of parent H-SN4 13 via the azide route.
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Synthesis of S,N-heterotetracene H-SN4 13 and related de- in 16. 2-(2-Nitrophenyl)thieno[3,2-b]thiophene (18) was
rivatives by Cadogan reaction of nitro-substituted precur- similarly prepared by Pd-catalyzed Stille-type reaction of
sors. Another possibility for the preparation of fused pyrrole monostannylated thienothiophene 14 and o-iodonitrobenzene
rings is established by the Cadogan reaction [40]. Well-known (17) in 86% yield. Successive Cadogan cyclization of 18
examples are the reductive cyclization of 2-nitro-1,1’-biphenyls with triethyl phosphite under reflux gave tetracyclic
with triethyl phosphite or phosphanes as reducing agent to 9H-thieno[2’,3’:4,5]thieno[3,2-b]indole (19) in an increased
provide carbazoles covering a wide substrate and functional yield of 58% (Scheme 4).
group spectrum [41]. Moreover, penta- and heptafused hetero-
acenes were prepared by the Cadogan reaction by annulation of Comparison of the reactions indicated that o-nitrophenyl sub-
nitrophenyl or nitrobenzothienyl precursors [42-45]. In this stituents as in 18 give higher cyclization yields than the corre-
respect, we recently reported a Cadogan cyclization of 3-nitro- sponding 3-nitrothienyls as in 16. We rationalize this finding by
2,2’-bithiophene with triethyl phosphite under microwave irra- a higher reactivity of the intermediate thienyl nitrene, which is
diation and surprisingly obtained targeted heterotriacene H-DTP formed during the cyclization reaction of 16 compared to
(vide supra) with only 11% yield [45]. This result prompted us phenyl nitrene [42]. Besides H-SN4 13, which is formed by
to have a closer look on the applicability of the Cadogan reac- insertion of the nitrene N-atom into the C3–H σ-bond of the
tion/cyclization in order to provide S,N-heterotetracenes and thienothiophene in 16 and simultaneous migration of the
related derivatives from nitro-substituted thiophene-based pre- H-atom to the nitrogen, mostly undefined black oligomeric or
cursors. First of all, the reductive cyclization of nitrothienyl- polymeric products were obtained.
substituted thienothiophene 16 to H-SN4 13 was investigated.
For this purpose, 2-stannylthienothiophene 14, which was ob- We further tested a twofold nitrophenyl-substituted thienothio-
tained from thienothiophene 1, was converted in a Pd-catalyzed phene in a Cadogan cyclization. Thus, distannylated thienothio-
Stille-type cross-coupling with 2-bromo-3-nitrothiophene (15) phene 20 was obtained in 68% yield from thienothiophene 1
to precursor 2-(3-nitrothien-2-yl)thieno[3,2-b]thiophene (16) in with two equivalents each of n-BuLi at −78 °C and trimethyl-
64% yield. Cadogan cyclization of nitro derivative 16 with stannyl chloride in THF. The stannyl reacted with o-iodo-
triethyl phosphite at reflux gave 4H-thieno[3,2-b]thieno[2’, nitrobenzene (17) and Pd[PPh3]4 as catalyst in DMF to give
3’:4,5]thieno[2,3-d]pyrrole (H-SN4, 13) in a similar low yield 2,5-bis(2-nitrophenyl)thieno[3,2-b]thiophene (21) in 86% yield.
(16%) as before H-DTP (vide supra) (Scheme 3). The change of Subsequent Cadogan cyclization with triethyl phosphite under
the reducing agent to triphenyl phosphite did not lead to a reac- reflux gave hexacyclic S,N-heteroacene 22 in 38% yield which
tion at all.
reflects a benzannulated SN4' system with the heteroatom se-
quence ‘NSSN’ (Scheme 5). The use of triphenyl phosphite as
We turned then to related thienothiophene 18, which bears an reducing agent gave a decreased yield of 24%. The N-alkylated
o-nitrophenyl substituent instead of a 3-nitrothienyl residue derivative of hexacyclic SN4' 22 was prepared by Wong et al.,
Scheme 3: Synthesis of tetracyclic H-SN4 13 via the Cadogan route.
Scheme 4: Synthesis of tetracyclic indole derivative 19 via the Cadogan route.
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Scheme 5: Synthesis of hexacyclic heteroacene SN4' 22 via the Cadogan route.
who applied the Cadogan reaction to cyclize a dibrominated Final removal of the TIPS-protecting group by TBAF gave the
precursor similar to 21. Without isolation, the SN4’-intermedi- novel SN4''-system 33 in 98% yield (Scheme 6).
ate was directly alkylated with 2-ethylhexyl bromide and the
corresponding dialkylated SN4' system was subsequently ob- All prepared S,N-heterotetracenes and -hexacene were clearly
tained by removal of the bromines by lithium–halogen identified, fully characterized, and their structures confirmed by
exchange and acidic work-up [28].
1H and 13C NMR spectroscopy and high-resolution mass spec-
trometry (HRMS) via matrix-assisted laser desorption/ioniza-
Synthesis of S,N-heterotetracenes SN4''. In contrast to SN4’, tion (MALDI) (Supporting Information File 1, Figures S1–S5).
S,N-heterotetracene system SN4'' comprising the heteroatom se- In the 1H NMR spectra, the S,N-heterotetracenes typically
quence ‘SNNS’ is built-up by annulation of two ‘outer’ thio- showed two doublets with 3J-coupling constants of 5.2–5.3 Hz
phene and two ‘inner’ pyrrole rings resulting in the symmetric for the α- and β-protons of the terminal thiophene units in the
sequence of heteroatoms in the tetracyclic conjugated π-system. fused skeleton. Thereby the α-protons typically occur slightly
The multistep synthesis of Pr-SN4'' 33 started from 2-thienyl- downfield-shifted relative to the ß-protons [20]. If one
carbaldehyde (23), which was put to reaction with methyl compares the shifts of these protons of the basic systems TTA
2-azidoactetate [46] and sodium methanolate in a Knoevenagel (Hα 7.52, Hß 7.41 ppm) with those of SN4 13 (Hα 7.39, Hß
condensation to give azide 24 in 81% yield. A solution of the 7.33 ppm) and SN4'' 33 (Hα 7.06, Hß 7.02 ppm), both protons
azide 24 was added to boiling toluene and cyclization to gradually shift up-field the more electron-rich pyrrole rings are
thienopyrrole 25 occurred via a nitrene intermediate in nearly present in the system. In contrast, for benzofused heteroacene
quantitative yield [47]. Single crystals of intermediate thieno- 19 these protons are shifted down-field (Hα 7.67, Hß 7.49 ppm)
pyrrole 25 were obtained by slow evaporation of a chloroform compared to SN4 13. MALDI–HRMS of all synthesized S,N-
solution and an X-ray structure analysis was performed. Struc- heteroacenes showed only a single peak for the molecular ions
tural details and packing motifs of the molecule are described in accounting for their purity.
Supporting Information File 1. Alkylation to N-propyl-substi-
tuted thienopyrrole 26, saponification to carbonic acid 27, and Optical and redox properties of S,N-heteroacenes 9, 13, 19,
subsequent Cu-mediated decarboxylation in quinoline resulted 22, and 33 in comparison to tetrathienoacene (TTA). Optical
in thienopyrrole 28 in more than 80% yield over three steps. spectroscopy of the various SN4-heteroacenes was performed in
Lithiation of 28 with n-BuLi and reaction with TIPS chloride THF solutions. UV–vis absorption spectra of TTA, SN4 9, and
selectively occurred at the thiophene α-position to give TIPS- SN4'' 33 are shown in Figure 2, left, those of heteroacenes 13,
protected thienopyrrole 29 in 92% yield which was further bro- 19, and 22 in Figure S6 (Supporting Information File 1) and
minated with N-bromosuccinimide (NBS) to dibrominated data is listed in Table 1. In general, the heteroacenes exhibited
thienopyrrole 30 in 99% yield. Pd-catalyzed Neghishi-type cou- one vibronically well-resolved strong absorption band in the
pling of 30 with 3-bromo-2-thienylzinc chloride (6) resulted in UV-regime of 250–360 nm which we address to the π–π* tran-
thienyl-substituted thienopyrrole 31 in moderate 37% yield. sition corresponding to the HOMO–LUMO energy gap. The
Cyclization to form the second pyrrole ring was achieved by absorption maximum is gradually red-shifted from 318 nm for
selective Buchwald–Hartwig amination of dibromide 31 with TTA and with increasing number of pyrrole rings to 330 nm for
n-propylamine to give TIPS-protected SN4'' 32 in 61% yield. SN4'' 33 including molar extinction coefficients in the range
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Scheme 6: Synthesis of heterotetracene SN4'' 33 via the azide and Buchwald–Hartwig amination route.
Figure 2: UV–vis absorption spectra of TTA, Hex-SN4 9, Pr-SN4'' 33 and fluorescence spectrum of 33 in THF at rt (left) and corresponding cyclic
voltammograms in dichloromethane/tetrabutylammonium hexafluorophosphate (0.1 M), 100 mV/s (right).
from 30 500 M−1 cm−1 for TTA to 33 700 M−1 cm−1 for SN4'' Supporting Information File 1). The absorption maximum
derivative 33. In comparison to H-SN4 13, the influence of the (378 nm) and the shape of the band are comparable to those of
hexyl side chain in Hex-SN4 9 on the optical properties is as ex- SN6 (379 nm) [17]. The pronounced vibronic fine splitting of
pected marginal and the absorption maximum is only slightly the main absorption bands is typical for the planar and stiff-
red-shifted. The effect of benzannulation in 19 compared to fused backbones of the heteroacenes [20]. The optical gap,
H-SN4 13 as well results only in small differences, which, how- which is determined from the absorption onset, gradually
ever, become larger in comparison to hexacyclic heteroacene 22 decreases with increasing number of pyrrole rings in the hetero-
showing the largest bathochromic shift in the series (Figure S6, acenes from 3.62 eV for TTA to 3.50 eV for 33 (Figure 2, left).
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Beilstein J. Org. Chem. 2020, 16, 2636–2644.
Table 1: Thermal, optical, and electrochemical data of S,N-heterotetracenes 9, 13, and 33 in comparison to tetrathienoacene (TTA) and benzofused
S,N-heteroacenes 19 and 22.
Heteroacene
TTA
Mp
[°C]
λabs
[nm]a
ε
Egopt
[eV]b
λem
[nm]a
ϕem
[%]c
Stokes shift
[cm−1]
EpOx1
[V]
HOMO LUMO
[eV]e
[M−1 cm−1]
[eV]d
222.0
332, 318,
305
30 500
3.62
356
<<1
965f
0.81
−5.76
−2.14
H-SN4
(13)
186.3
66.3
333, 320,
305
29 300
30 700
33 700
3.57
3.55
3.50
n.d.g
364
355
n.d.g
<<1
<<1
n.d.g
1433f
861
0.37
0.39
0.07
−5.37
−5.39
−5.10
−1.80
−1.84
−1.60
Hex-SN4
(9)
336, 322,
310
Pr-SN4’’
(33)
141.0
344, 330,
317
H-SN3B
(19)
225–226
452
339, 325,
313
29 500
52 500
3.55
3.20
n.d.g
388
n.d.g
1.9
n.d.g
682
0.56
0.33
−5.56
−5.27
−2.01
−2.07
H-BSN4’B
(22)
378, 358,
336
aAbsorption and emission measured in THF. bCalculated by Eg = 1240/λonset. cQuantum yields referred to anthracene in ethanol (ϕem = 0.27).
dCalculated from the onset value of the oxidation wave; Fc/Fc+ was set to −5.1 eV vs vacuum. eCalculated with EHOMO and Egopt sol. fValue calcu-
lated through Gaussian deconvolution of the unstructured emission spectrum. gn.d. = not detectable.
The SN4-heteroacenes fluoresce, if at all, only very weakly and substantially higher than this for SN6 (Epox1 = 0.06 V) [7,37]
emission maxima lay in between 356 nm for TTA and 388 nm clearly showing the effect of the two-fold benzannulation.
for 22 with the most extended conjugated π-system (Table 1).
Fluorescence quantum yields were far below 1% except for From the optoelectronic data a schematic energy level diagram
hexacene 22, which showed a very moderate value of 1.9%. of the frontier orbitals and the optical transitions in the series
This result is in line with the series of S,N-heteroacenes where can be derived (Figure 3). The HOMO energy levels were de-
noticeable fluorescence started with heptacene SN7 termined from the onset of the oxidation wave and are continu-
(ϕem = 19%) reaching 93% for the longest tridecamer SN13 ously destabilized in the series TTA, SN4 13 and 9, and SN4''
[17,37]. The Stokes shifts, which were determined from the 0,0- 33. Due to the absence of reduction waves in the cyclic voltam-
transitions, lay in between 682 cm−1 for hexacene 22 and mograms, the LUMO energy levels were calculated from the
1433 cm−1 for SN4'-derivative 9. These relatively small values optical energy gaps Egopt and the HOMO energy levels. As well
indicate small geometric changes and structural deformation be- the LUMOs are destabilized with increasing number of pyrrole
tween the ground and excited states which is typical for such rings.
planar and conformationally restricted systems [48].
Conclusion
In order to get information about the redox properties and ener- In summary, we have presented the synthesis and characteriza-
getics of the frontier orbitals, the S,N-heteroacenes were studied tion of novel S,N-heterotetracenes and -hexacene with varying
by cyclic voltammetry and all potentials were referenced the number and sequence of the sulfur and nitrogen heteroatoms
against the ferrocene/ferricenium couple (Table 1). In the series which complement the general series of S,N-heteroacenes.
TTA, SN4 9, and SN4'' 33 irreversible oxidation waves were Various cyclization methods to eventually build up pyrrole
obtained due to follow-up reactions of the formed radical rings in the tetraheterocyclic systems have been tested and used
cations and peak potentials gradually decrease from 0.81 V for to prepare SN4, SN4'', and benzannulated SN4' derivatives.
TTA to 0.07 V for SN4'' 33 (Figure 2, right). As expected, the Among them, the Buchwald–Hartwig amination of brominated
oxidation potential is substantially influenced by the increasing precursors, the thermolysis of azide precursors, and a Cadogan
number of electron-rich pyrrole rings and drops continuously. cyclization of nitro-substituted precursors were successfully
As expected, if an electron-rich fused thiophene ring in H-SN4 applied. The various heteroatom sequences ‘SSNS’, ‘SNNS’,
13 is replaced by a phenyl ring in 19, the oxidation potential is and benzannulated ‘NSSN’ allowed for evaluation of
increased by 190 mV. The value for hexacyclic 22 structure–property relationships relative to sulfur analogue
(Epox1 = 0.33 V) is in the same regime than for H-SN4 13, but tetrathienoacene (‘SSSS’). In line with the results for the whole
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Beilstein J. Org. Chem. 2020, 16, 2636–2644.
Figure 3: Energy diagram of the frontier molecular orbitals of heterotetracenes TTA, 9, 13, 19, 22, and 33, and heterohexacene 22.
series of S,N-heteroacenes, we found that replacement of sulfur
by nitrogen atoms in the tetracyclic systems leads to a red-shift
in absorption, a decrease in oxidation potential and energy gap.
On the other hand, the replacement of a thiophene ring by
benzene leads to the opposite effects. The basic SN4 and SN4''
frameworks represent novel core molecules for further functio-
nalization, for example with terminal acceptor groups, leading
to highly absorbing dye molecules for application in organic
solar cells.
ORCID® iDs
Peter Bäuerle - https://orcid.org/0000-0003-2017-4414
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