Bis[1]benzothieno[1,4]thiazines: Planarity, Enhanced Redox Activity and Luminescence by Thieno-Expansion of Phenothiazine

Article abstract of DOI:10.1002/chem.201805085

Twofold Buchwald–Hartwig aminations selectively furnish three regioisomers of bis[1]benzothieno[1,4]thiazines; X-ray structure analyses and DFT calculations were corroborated for correlation of their electronic properties. All regioisomers outscore the pa

Full text of DOI:10.1002/chem.201805085

A Journal of
Accepted Article
Title: Bis[1]benzothieno[1,4]thiazines – Planarity, Enhanced
Redox Activity and Luminescence by Thieno-Expansion of
Phenothiazine
Authors: Arno P. W. Schneeweis, Simone T. Hauer, Guido J. Reiss,
and Thomas J. J. Müller
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To be cited as: Chem. Eur. J. 10.1002/chem.201805085
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Bis[1]benzothieno[1,4]thiazines – Planarity, Enhanced Redox
Activity and Luminescence by Thieno-Expansion of
Phenothiazine
Arno P. W. Schneeweis,a Simone T. Hauer,a Guido J. Reiss,b and Thomas J. J. Müller*,a
Abstract: Twofold Buchwald-Hartwig aminations selectively furnish
all three regioisomers of bis[1]benzothieno[1,4]thiazines and X-ray
structure analyses and DFT calculations were corroborated for
correlation of their electronic properties. All regioisomers outscore
the parent compound phenothiazine with respect to a low lying
oxidation potential and reversible redox activity. The anti-anti
bis[1]benzothieno[3,2-b:2',3'-e][1,4]thiazines possess the lowest
oxidation potentials in this series and display pronounced green
luminescence in solution (F ≈ 20%) and in the solid state. Syn-anti
regioisomers are only weakly luminescent in solution, but show
aggregation induced emission enhancement and solid state
luminescence. Most interestingly, found by X-ray structure analyses
anti-anti derivatives reveal an amazingly coplanar structure of the
pentacyclic anellated 1,4-thiazine system, emphasizing a structural
similarity to heteroacenes. The calculated theoretical nucleus-
independent chemical shifts additionally suggest that these
8-electron core systems can be considered as the first
electronically unbiased anellated 1,4-thiazines with antiaromatic
character.
excellently with required electronic profiles.2 Interestingly,
anellated 1,4-thiazines, like phenothiazine, according to higher
semiquinone formation constants, form more stable reversible
redox systems than many other di- and triarylamines.3
Phenothiazine, a classic tricyclic heteroarene with a pronounced
pharmacological profile,4 can be readily modulated as a donor
moiety in functional -systems.5 The major challenge of
designing novel anellated 1,4-thiazine donors remains in
providing lower oxidation potentials than phenothiazine. Thieno
anellation of the 1,4-thiazine core furnishes dithieno[2,3-b:3',2'-
e][1,4]thiazines.6 Formal replacement of the benzo moiety by the
higher polarizable thiophene core consequently leads to a
significant cathodic shift, i.e. a decrease, of the oxidation
potential. However, on expense of the thieno anellation
luminescence characteristics typical for many phenothiazines
were significantly minimized. Yet, luminescence could be
restored after specific (hetero)aryl substitution at the -
positions.6 Therefore, we conceptualized restoring of
luminescence by anellation of an extended -system to 1,4-
thiazine. Based on the phenothiazine core structure, we
reasoned an expansion of the -system by formal insertion of
thiophene rings furnishing bis[1]benzothieno[1,4]thiazines
(BBTT), structural and electronic combinations of both
phenothiazine and dithieno[1,4]thiazine (Figure 1). These
pentacyclic -systems share topological similarities with the
highly topical class of heteroacenes,7 which became increasingly
important as semiconducting materials in organic field-effect
transistors.8 Herein, we present the first syntheses of three
regioisomeric bis[1]benzothieno[1,4]thiazines and investigations
on their ground and excited state electronic properties and
electronic structure.
Introduction
Electron-rich heterocycles adopt a prominent role in organic
electronics, and already find numerous applications in photonics
and photovoltaics.1 As organic donors with inherently reversible
oxidation potentials di- and tri(hetero)arylamines match
[a]
[b]
M. Sc. A. P. W. Schneeweis, S. T. Hauer, Prof. Dr. T. J. J. Müller
Institut für Organische Chemie und Makromolekulare Chemie
Heinrich-Heine-Universität Düsseldorf
Universitätsstraße 1, D-40225 Düsseldorf, Germany.
E-Mail: ThomasJJ.Mueller@hhu.de
Dr. G. J. Reiss
Institut für Anorganische Chemie und Strukturchemie
Heinrich-Heine-Universität Düsseldorf
Figure 1. Bis[1]benzothieno[1,4]thiazines
phenothiazines with N-substitution.
–
doubly thieno-expanded
Universitätsstraße 1, D-40225 Düsseldorf, Germany.
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Results and Discussion
derivatives was pursued accordingly.6 All regioisomers of BBTT
can be convergently synthesized by an intermolecular-
intramolecular9 Buchwald-Hartwig amination10 using the
respective dibrominated starting materials11 and various para-
substituted anilines. This concise synthetic strategy allows a
facile variation of the N-substituent (Scheme 1), thus enabling
fine-tuning of the electronic properties of the -system.
According to optimization studies the right choice of the
employed ligand for Pd catalyst is crucial for obtaining satisfying
yields of the desired BBTT. As a consequence of inherent steric
hindrance syn-syn BBTTs are generally formed in lower yields
compared to the other two regioisomers (see also Figure 2).
Synthesis
The
three
regioisomers
bis[1]benzothieno[2,3-b:3',2'-
e][1,4]thiazine, bis[1]benzothieno[2,3-b:2',3'-e][1,4]thiazine, and
bis[1]benzothieno[3,2-b:2',3'-e][1,4]thiazine differ by the mode of
anellation of the benzo[b]thiophene unit at the central 1,4-
thiazine core (Figure 1). According to the orientation of the
thiophene rings with respect to the 1,4-thiazine sulfur these
regioisomers are abbreviated as syn-syn, syn-anti, and anti-anti
BBTT for simplification. Motivated by quantum chemical
calculations on 4H-dithieno[2,3-b:3',2'-e][1,4]thiazine and our
syntheses of its derivatives the synthetic approach to BBTT
Scheme 1. Selective synthesis of the three BBTT regioisomers by twofold Buchwald-Hartwig-amination (yields are given after isolation by flash chromatography
and recrystallization; ligands used for the syntheses: route
bis(diphenylphosphano)ferrocene (DPPF).
A
and
B
1,1'-bis(dicyclohexylphosphano)ferrocene (DCPF) and route
C
1,1'-
Structure
substituent and the benzo[b]thiophene wings the syn-syn BBTTs
3a and 3d adopt extra conformations in the solid state. The
same reasoning can be applied for the syn-anti BBTT 5d with a
more relaxed extra conformation due to less steric hindrance.
Finally, the crystal structure of the anti-anti BBTT 7d adopts an
intra conformation, as a consequence of the absence of steric
biases. The intra conformation induces a larger folding angle 
and, therefore, increasing the interaction of 1,4-thiazine sulfur
and nitrogen with the fused -systems. This conformationally
induced interaction considerably influences the electronic
properties (vide infra, Figure 5).
The structures of the three regioisomers 3, 5, and 7 were
corroborated by X-ray structure analyses of several differently
substituted examples (Figure 2).12 Benzothieno anellated 1,4-
thiazines are characterized by their significant folding along the
S-N axis with folding angles

of these butterfly-like
conformations, as well as torsion () and orientation () angles
of the N-aryl substituents (Table 1).13
Table 1. Selected folding, torsion and orientation angles determined by X-ray
structure analyses.
Surprisingly, X-ray crystal structures of anti-anti BBTTs (7a, 7c,
7e) reveal almost completely coplanar conformations of the
pentacyclic core systems. For the crystals of 7c a remarkably
high folding angle  of 179.6° is found. Although thermal
ellipsoids of sulfur and nitrogen atoms indicate a certain
deviation of these atoms from the benzo[b]thiophene plane,
intermolecular interactions of the molecules in the crystals were
-
system in the X-ray structures of 7a (3.59 Å), 7c (3.60 Å) and 7e
(3.5 Å), i.e. larger distances than the van der Waals distance
between two sp2-hybridized carbon atoms (3.4 Å)16 (for an
illustration see supporting information). For the p-cyano-
substituted derivative 7e the molecular distance is the shortest
indicating more intermolecular interactions in the crystal.
folding
angle  [°]
136.9
131.8
125.8
136.5
166.0
177.6
179.6
159.2
155.6
175.0
162.6
150.7
torsion
angle  [°]
87.2
81.3
88.9
64.8
34.3
1.5
35.8
3.3
20.0
59.8
-
S-N-Cphenyl
angle  [°]
111.5
115.0
120.6
128.5
136.5
177.9
167.0
168.5
167.1
167.9
-
Compound
3a[a]
3d
5d
5d
7a
7c
7d[a]
7e
N-phenylphenothiazine[a]14
[a] Two molecules are found in the asymmetric unit of the single crystal.
The conformation of anellated 1,4-thiazines in the solid state can
be classified as intra (quasi-equatorial) or extra (quasi-axial)
(see Figure 2).6b,14-15 Due to steric hindrance between the N-
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Figure 3. (A) Crystals and crystal structures of the planar anti-anti BBTTs 7a,
7c and 7e. (B) Placement and numbering of ghost atoms used for NICS
calculations (see Table 2).
Figure 2. Crystal structures of compounds 3a, 3d, 5d, and 7d and illustration
of the angles shown in Table 1.
Planar 1,4-thiazine derivatives can be considered to be
inherently antiaromatic as a consequence of the 8-electron
system and they are not known in the literature. Only by
electronic biases, such as strongly electron withdrawing
substituents, 1,4-thiazines can adopt planar conformations, e.g.
for p-nitrophenyl substituted diquino[1,4]thiazines.17 However,
the discussed anellated 1,4-thiazines, such as anti-anti BBTTs
7a and 7c, are electron-rich systems bearing electron neutral
substituents at the nitrogen center. The observed exceptional
planarity of compounds 7a, 7c, and 7e raises the question about
their degree of antiaromaticity. Therefore, the antiaromaticity of
the 1,4-thiazine core was assessed by well-established
theoretical nucleus-independent chemical shifts (NICS)
calculations.18
For structures 7a, 7d, and 7e the NICS(0) and NICS(1) (1 Å
above (+1) and under (-1) the plane) method was tested using
the GIAO protocol in the Gaussian 09 package19 with
RB3LYP20/6-311+G**.21 The starting geometries were extracted
from the respective crystal structures and minimum structures
were examined by frequency analyses. NICS calculations were
performed for all six rings (Table 2). The positive NICS(0) and
NICS(1) values for the 1,4-thiazine rings strongly suggest a
significant paratropic ring current. To exclude that the calculated
positive NICS values of the 1,4-thiazine are primarily caused by
the neighboring aromatic thiophene ring currents a system
based on two benzo[b]thiophenes with the same coordinates as
the benzo[b]thiophene wings of the BBTT and the same
placement of ghost atoms (benzo[b]thiophene dimer test
system) was used for NICS calculations (see supporting
information). According to the NICS(1) values the calculated
paratropic ring current of in the center of BBTTs is mainly
caused by the ring current of the 1,4-thiazine. Therefore, the
central cores of all three molecules 7a, 7d, and 7e are plausibly
assumed to be antiaromatic.
Table 2. Calculated NICS values of 7a, 7c, and 7e (RB3LYP/6-311+G**).
benzo[b]thiophene
Ring No.
dimer test system
NICS(0/1)
Ring current
Aromaticity
7a NICS(0/+1/-1)
7c NICS(0/+1/-1)
7e NICS(0/+1/-1)
1
1*
2
2*
3
-8.42/-10.37/-10.32
-8.47/-10.47/-10.27
-9.50/-6.66/-6.76
-9.46/-6.93/-6.91
-8.08
-8.42/-10.35/-10.45
-8.57/-10.32/-10.26
-9.21/-6.67/-7.05
-9.48/-6.49/-6.56
-7.61
-8.37/-10.38/-10.26
-8.29/-10.32/-10.37
-8.44/-6.88/-6.37
-8.39/-6.88/-6.11
-7.50
-8.48/-10.81
-8.39/-10.85
-11.34/-8.13
-11.20/-9.15
-
diatropic
diatropic
diatropic
diatropic
diatropic
paratropic
aromatic
aromatic
aromatic
aromatic
aromatic
4
+8.32/+7.17/+7.18
+7.56/+6.50/+6.85/
+6.35/+6.13/+5.57
+4.53/+2.02
antiaromatic
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Electronic Properties and Electronic Structure
should additionally hamper the first oxidation. In the subsequent
second oxidation process to furnish the dication, no major
structural changes are expected. The anti-anti regioisomer 7b
The electronic properties of the three BBTT regioisomers 3, 5,
and 7 were examined in solution by cyclic voltammetry (Table 3), experiences a drastically reduced steric hindrance by the N-
UV/Vis and fluorescence spectroscopy. All BBTTs show two fully
chemical reversible oxidation processes (Figure 4A). Moreover,
the first and second oxidations of syn-anti and anti-anti BBTTs
are Nernstian reversible. In case of the syn-syn BBTT the first
oxidation is quasi-reversible whereas the second oxidation is
Nernstian reversible (for a detailed illustration see supporting
information). In comparison to N-phenylphenothiazine, all N-
phenyl BBTT regioisomers show lower potentials for the first and
second oxidations.6a However, in comparison to the respective
dithieno[2,3-b:3',2'-e][1,4]thiazines the first oxidation potentials
are shifted anodically. These findings are in agreement with the
potential differences between dithieno[3,2-b:2',3'-d]pyrroles and
bis[1]benzothieno[3,2-b:2',3'-d]pyrroles.22 In the series of the
three regioisomers anti-anti BBTTs are easiest to oxidize (for
HOMO energy levels see supporting information). Interestingly,
the gap of the first potential is largest between the syn-syn and
the syn-anti BBTT, also as a consequence of largest structural
differences between these two regioisomers. The comparison of
the second oxidation potentials reveals a reversed order for the
regioisomers. The radical cations of the syn-syn BBTTs are
generally easier to oxidize than the radical cations of the syn-
anti and anti-anti regioisomers.
phenyl substituent, favoring the molecule's intra conformation.
Consequently, less structural reorganization upon oxidation is
required for a planarization. Therefore, the electron transfer to
the electrode surface is almost non-inhibited, a favorable feature
for all relevant electron transfer processes in electronic materials.
Table 3. First and second oxidation potentials of the three regioisomers 3, 5,
and 7 (referenced to internal standard decamethylferrocene23 E0 = 552 mV
(vs. ferrocene24 E0(Fc/Fc+) = 0 V)), and semiquinone formation constant KSEM
0/+1
(E
+1/+2
– E
)
0
0
0.059 V
(KSEM = 10
).
Compound / R
E00/+1 [V]
0.17
0.15
0.19
0.21
0.00
0.00
0.03
0.05
0.20
-0.04
-0.04
-0.02
0.01
0.09
0.27
-0.06
E0+1/+2 [V]
0.67
0.67
0.67
0.68
0.71
0.71
0.72
0.72
0.76
0.73
0.73
0.74
0.75
0.78
1.07
0.81
KSEM
3a / tBu
3b / Me
3c / H
3.0 · 108
6.5 · 108
1.4 · 108
9.2 · 107
1.1 · 1012
1.1 · 1012
5.0 · 1011
2.3 · 1011
3.1 · 109
1.1 · 1013
1.1 · 1013
7.6 · 1012
3.5 · 1012
5.0 · 1011
3.6 · 1013
5.6 · 1014
3d / F
5a / tBu
5b / Me
5c / H
Figure 4. (A) Cyclic voltammograms of compounds 3c, 5c and 7c. (B)
Deconvolution diagram and cyclic voltammograms of 3b and 7b (CVs
recorded in CH2Cl2, rt, electrolyte 0.1 M [n-Bu4N][PF6];  = 100 mV/s; Pt
working and counter electrode, Ag/AgCl reference electrode, referenced to
decamethylferrocene23 E0 = -552 mV (vs. ferrocene24 E0(Fc/Fc+) = 0 V)).
5d / F
5e / CN
7a / tBu
7b / Me
7c / H
7d / F
7e / CN
Different anellation and substitution patterns of the BBTT
derivatives strongly affect the electronic ground state, and
likewise also the electronic excitations, and, thus, a significant
influence on the excited state properties can be expected. This
effect is nicely reflected by the UV/Vis spectra of the three
regioisomers (Figure 5). As a consequence of the more planar
structure the interactions between the heteroatoms of the 1,4-
thiazine and -systems is increased and, thus, the longest
wavelength absorption bands of the anti-anti derivatives 7 are
redshifted. Additionally, the conjugation pathway between the
nitrogen electron pair and the benzo[b]thiophene wing is best for
the anti-orientation.
N-phenylphenothiazine6a
N-phenyl-dithieno[1,4]thiazine6a
From the differences of first and second oxidation potentials the
semiquinone formation constants KSEM can be calculated.25
A
higher semiquinone formation constant indicates a higher
stability of the radical cation against disproportionation into the
reduced form and the dication. Here, the KSEM values indicate
that the anti-anti regioisomers 7 form most stable radical cations.
The comparison of the cyclic voltammograms of syn-syn and
anti-anti BBTTs 3b and 7b evaluated by semi-differential
deconvolution26 (Figure 4B) allows for a qualitative discussion of
electron transfer processes at the anode. The higher peak
+1/+2
separation for E00/+1 of syn-syn BBTT 3b in comparison to E0
indicates that the first oxidation process to give the radical cation
is inhibited. In analogy to phenothiazine this inhibition
corresponds to the barrier of structural changes from the
butterfly conformation of the reduced species to the planarized
radical cation.27 Taking into account steric hindrance in neutral
syn-syn BBTT the structural changes required for planarization
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Most impressively, the emission behavior of anti-anti BBTT
derivatives 7 is clearly superior to the other two regioisomers
(Figure 6B), with exception the p-cyano derivative 7e, which is
essentially non-emissive in solution and in the solid state. The
other derivatives 7 generally display green emission upon
excitation with UV light and notably higher quantum yields in the
range of 20% are measured. Additionally, the Stokes shifts are
drastically lower and range from 5300 to 5400 cm-1. Anellated
1,4-thiazines, such as phenothiazine without any additional
functionalization of the anellated -system, normally reach only
lackluster
fluorescence
quantum
yields,
while
the
dithieno[1,4]thiazine is essentially non-luminescent. The anti-anti
BBTTs 7 easily surpass similar substituted phenothiazines in
terms of fluorescence quantum yields.30
In the emission data (max,em and fluorescence quantum yields
F) no major influence of the corresponding para-substituents is
found. TD-DFT calculations were performed for exploring the
electronic nature of the vibrationally relaxed first excited state of
anti-anti BBTT 7c (Figure 7). The Franck-Condon excitation at
423 nm (max,exp = 424 nm) can be considered as a HOMO →
LUMO (96%, f = 0.1507) transition, which is associated with a
characteristic transfer of electron coefficient density from the
1,4-thiazine ring (HOMO) to the benzo[b]thiophene wings
(LUMO). This is generally considered as a charge transfer
transition. The vibrationally relaxed S1 geometry is essentially
planar in the pentacyclic bis[1]benzothieno[1,4]thiazine moiety.
Excitation from the vibrationally excited ground state S0* to the
relaxed first excited state S1 translates into the process of
fluorescence. The involved HOMO → LUMO transition (99%,
f = 0.1754, calc = 585 nm, max,exp = 549 nm) reveals almost no
distribution of electron coefficient density on the N-phenyl
substituent. Therefore, the electronic effect of p-phenyl
substitution only has a minimal impact on the fluorescence,
which nicely rationalizes the experimental observations.
However, functionalization at the bis[1]benzothieno[1,4]thiazine
core system still needs exploration.
Figure 5. UV/Vis spectra of compounds 3c, 5c and 7c (recorded in CH2Cl2).
In contrast to N-aryl substituted dithieno[1,4]thiazine derivatives
BBTTs indeed reveal a distinct emission behavior.6a All syn-syn
BBTTs 3 do not show measurable emission in solution upon UV
excitation. However, the syn-anti BBTTs 5 are weakly orange
fluorescent with remarkably large Stokes shifts up to 8000 cm-1
(Figure 6A), a typical feature of phenothiazines.28 Here, the
electron deficient cyano derivative 5e, which is essentially non-
fluorescent in solution, is an exception.
Figure 6. (A) Normalized absorption and emission spectra of syn-anti
derivatives 5. (B) Normalized absorption and emission spectra of anti-anti
derivatives and determined quantum yields F (recorded in CH2Cl2, absolute
F: 7a, 7c, 7d; relative F (7b) determined with coumarin 153 as a standard in
Figure 7. Jablonski diagram of compound 7c (E(S0) = 0 eV; B3LYP/6-
311++G** IEFPCM CH2Cl2, isosurface value at 0.03 a.u.).
MeOH (F = 0.45)29
.
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Just like in solution, the crystals of the para-substituted syn-syn
derivatives 3 are essentially non-emissive. The maxima of
emissive crystals of syn-anti derivatives 5 are generally blue-
shifted and the fluorescence quantum yield in the crystal state is
apparently higher than in solution.
addition, the larger geometry change also causes a lower solid
state fluorescence quantum yield of 2%.
The emissive properties of the crystals of the anti-anti BBTTs
depend on the crystal type. The yellow crystals of the p-fluoro-
substituted BBTT 7d display yellow fluorescence with only a
minor loss of quantum yield (F = 16%) in comparison to
emission in solution (F = 22%). The corresponding crystal
structure is similar to the calculated structure in solution.
However, the orange crystals of the antiaromatic derivatives 7a
(Figure 9) and 7c show bathochromically shifted emission with
diminished quantum yields (F (7a) = 10%; F (7c) = 5%). The
Stokes shift in the crystals (2700 cm-1) is significantly lower
compared to the Stokes shift in solution (5300 cm-1) caused by
the structural similarity between the planar ground state
geometry in the solid state (see Figure 3) and the planar
vibrationally relaxed excited state geometry (see Figure 7).
Furthermore, the influence of the planar structure on the optical
properties of the crystals was additionally corroborated by TD-
DFT calculations (RB3LYP/6-311++G**). The orange color of
the crystals of compound 7a (Figure 9) is nicely reproduced
(calc = 460 nm, HOMO → LUMO, 98%, f = 0.1094) when
employing the X-ray data as a starting geometry.
Moreover, crystallization of the p-fluoro-substituted BBTT 5d led
to the formation of two polymorphs,31 which were analyzed by X-
ray diffraction and emission spectroscopy. The bright yellow
single crystals of 5d exhibit green fluorescence similar to the
yellow crystals of p-methyl-substituted BBTT 5b (F = 7%) with
increased quantum yields of 6%, while the colorless single
crystals of 5d show red-shifted orange fluorescence with lower
quantum yields of only 2% (Figure 8). The molecules in the
asymmetric unit of the two crystal structures display major
conformational differences. The BBTTs in colorless crystals are
significantly more folded ( = 136.5°) than molecules in bright
yellow crystals ( = 166.0°). Additionally, there are differences in
the orientation of the N-phenyl substituent represented by the 
angle (Table 1) allowing different interactions between the
nitrogen lone pair and the -system of the N-substituent. The
influence of the different ground state geometries on the
excitation properties was explored by TD-DFT calculations
(B3LYP/6-311++G**)
using
the
corresponding
crystal
Fluorescence intensities of the crystals of the syn-anti
geometries of 5d. For the colorless crystals
a
longest
derivatives 5 are higher than in solutions of the same
wavelength absorption band at 364 nm was calculated (HOMO
→ LUMO (95%, f = 0.0031). According to the optical appearance
of the bright yellow crystals a bathochromically shifted longest
wavelength absorption band at 410 nm (HOMO → LUMO (98%),
f = 0.0019) was determined (see Supp Inf for plots of the
associated frontier orbitals).
compounds. Consequently, aggregates of the chromophores
should also express intensive fluorescence.32 Indeed, the
fluorescence of syn-anti BBTT 5d is significantly enhanced by
the formation of aggregates, i.e. aggregation induced enhanced
emission (AIEE).33 An AIEE titration experiment visualizes this
effect (Figure 10A). For a given concentration of syn-anti BBTT
5d various ratios of binary THF/water mixtures indicate the
formation of fluorescent aggregates and thus unambiguously
identifying AIEE behavior (Figure 10B). Further assessments of
this unusual luminescence effect will be studied in more detail in
ongoing investigations.
Figure 8. (A) Normalized emission spectra of the two crystal types of 5d. (B)
The two crystal types of 5d (above: daylight; below: UV-light exc= 365 nm;
absolute F of the crystals) and their associated crystal structures.
The apparent and measurable difference in fluorescence of the
presented polymorphs is obviously based on geometrical
changes between ground state and the assumed planar
geometry of the common vibrationally relaxed excited state S1.
The higher geometrical deviation between ground and planar
Figure 9. Normalized excitation (solid line) and emission (dashed line) spectra
of the orange crystals of 7a (F = 10%).
excited state translates into
a larger Stokes shift and
pronounces the red shift from green to orange fluorescence. In
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1
7, H and 13C NMR spectra of compounds 3, 5, and 7, UV/Vis
and emission spectra of BBTTs 3, 5, and 7, AIEE experiments,
cyclic voltammetry of BBTTs 3, 5, and 7, DFT computed XYZ-
coordinates, energies, and absorption spectra of structures of
selected BBTTs 3, 5, and 7, NICS calculations of anti-anti
BBTTs 7a, 7c, and 7e, X-ray structure data tables.
Experimental Section
Synthesis of N-(4-tert-butylphenyl)bis[1]benzothieno[2,3-b:3',2'-
e][1,4]thiazine (3a). A typical procedure. In a dry screw-cap Schlenk
tube with a magnetic stir bar bis(3-bromobenzo[b]thiophene-2-yl)sulfane
(1) (0.23 g, 0.50 mmol), bis(dibenzylideneacetone)palladium (22 mg,
0.038 mmol) and 1,1′-bis(dicyclohexylphosphano)ferrocene (43 mg,
0.074 mmol), and sodium tert-butoxide (0.14 g, 1.5 mmol) were placed
under argon. 4-tert-Butyl-aniline (2a) (80 L (0.50 mmol) and dry toluene
(3 mL) were then added by syringe. The reaction mixture was stirred at
100 °C for 43 h. After cooling to room temperature a saturated aqueous
sodium sulfite solution was added to the reaction mixture. The aqueous
phase was extracted with dichloromethane (3 x 20 mL) and the combined
organic phases were dried (anhydrous sodium sulfate). After evaporation
of the solvents under reduced pressure the residue was absorbed on
Celite®. The crude product was purified by flash chromatography on silica
gel (cyclohexane) to give compound 3a as a colorless solid (82 mg, 36%),
Mp 175 °C. 1H NMR (600 MHz, THF-d8):  1.23 (s, 9 H), 6.80 – 6.83 (m,
2 H), 7.13 – 7.16 (m, 2 H), 7.35 – 7.38 (m, 2 H), 7.39 – 7.43 (m, 2 H),
7.82 – 7.85 (m, 4 H). 13C NMR (151 MHz, THF-d8):  31.9 (CH3), 34.8
(Cquat), 117.1 (CH), 122.3 (CH), 123.9 (CH), 125.9 (CH), 125.9 (CH),
126.7 (CH), 132.4 (Cquat), 136.4 (Cquat), 139.8 (Cquat), 141.0 (Cquat), 144.6
Figure 10. AIEE experiments: (A) Emission spectra of 5d in THF with different
water fractions (c(5d) = 6 · 10-5 mol/L). (B) Plots of I/I0 of 5d vs. water fractions
in THF/water mixtures (I0 = emission intensity in pure organic solvent, I =
emission intensity in water mixture).
(Cquat), 145.8 (Cquat). IR: ̃
[cm-1] 3057 (w), 3034 (w), 2959 (w), 2899 (w),
2864 (w), 1611 (w), 1512 (m), 1472 (w), 1460 (w), 1427 (w), 1391 (w),
1350 (w), 1317 (w), 1286 (w), 1255 (w), 1246 (m), 1202 (w), 1173 (w),
1159 (w), 1130 (w), 1117 (w), 1090 (w), 1045 (w), 1018 (w), 1011 (w),
988 (m), 906 (w), 851 (w), 824 (m), 810 (w), 791 (w), 760 (s), 732 (s),
716 (w), 694 (w), 642 (w), 633 (w). HRMS (ESI) calcd. for C26H21NS3:
443.0831; Found: 443.0831. Anal. calcd. for C26H21NS3 (443.6): C 70.39,
H 4.77, N 3.16, S 21.68. Found: C 70.20, H 4.88, N 3.11, S 21.45.
Conclusion
In
summary,
all
three
(BBTT)
regioisomers
were selectively
of
bis[1]benzothieno[1,4]thiazine
synthesized by twofold Buchwald-Hartwig amination and their
structures were extensively characterized by X-ray structure
analyses. Although, all three regioisomers share many
similarities with respect to their electronic properties they also
display distinct peculiarities as a consequence of their mode of
anellation. While syn-syn BBTTs are essentially non-
luminescent, the emission behavior is restored for syn-anti and
anti-anti BBTTs in solution and often also in the solid state.
Besides distinct solid state emission also polymorphous
emission was observed for the p-fluoro-substituted syn-anti
BBTT derivative. X-ray structure analyses and DFT calculations
additionally support the influence of structural features on the
excitation properties of the molecule in the solid state. It is
noteworthy to mention that some of the solid state emissive
derivatives display AIEE, a particularly favorable feature for
application in fluorescence sensor systems.34
Finally, three planar antiaromatic anti-anti BBTTs were identified
by X-ray structure analyses and the antiaromatic character of
these systems was assessed by NICS(0) and NICS(1)
calculations. Further efforts to enhance the fundamentally
interesting inherent antiaromaticity, also with respect to small
singlet-triplet separations, as well as enhancing the optical and
materials properties of novel multifunctional -systems are
currently underway.
Synthesis of N-(4-tert-butylphenyl)bis[1]benzothieno[2,3-b:2',3'-
e][1,4]thiazine (5a). A typical procedure. In a dry screw-cap Schlenk
tube with a magnetic stir bar 2-bromo-3-((3-bromo-benzo[b]thiophen-2-
yl)thio)benzo[b]thiophene
bis(dibenzylideneacetone)palladium
(4)
(0.23 g,
(22 mg,
0.50 mmol)),
0.038 mmol) and
1,1′-bis(dicyclohexylphosphano)ferrocene (43 mg, 0.074 mmol), and
sodium tert-butoxide (0.14 g, 1.5 mmol) were placed under argon. 4-tert-
Butyl-aniline (2a) (80 L, 0.50 mmol) and dry toluene (3 mL) were then
added by syringe. The reaction mixture was stirred at 110 °C for 47 h.
After cooling to room temperature a saturated aqueous sodium sulfite
solution was added to the reaction mixture. The aqueous phase was
extracted with dichloromethane (3 x 20 mL) and the combined organic
phases were dried (anhydrous sodium sulfate). After evaporation of the
solvents under reduced pressure the residue was absorbed on Celite®.
The crude product was purified by flash chromatography on silica gel
(cyclohexane) to give compound 5a after recrystallization from acetone
as yellow crystals (104 mg, 47%), Mp 238 °C. 1H NMR (600 MHz, THF-
d8):  1.32 (s, 9 H), 6.93 – 6.97 (m, 1 H), 7.07 – 7.12 (m, 1 H), 7.18 –
7.22 (m, 1 H), 7.24 – 7.29 (m, 1 H), 7.34 – 7.38 (m, 1 H), 7.39 – 7.42 (m,
1 H), 7.42 – 7.48 (m, 4 H), 7.67 – 7.71 (m, 1 H), 7.72 – 7.75 (m, 1 H). 13
C
NMR (151 MHz, THF-d8):  31.8 (CH3), 35.3 (Cquat), 109.4 (Cquat), 120.5
(CH), 121.3 (Cquat), 122.0 (CH), 123.3 (CH), 123.5 (CH), 125.1 (CH),
125.2 (CH), 125.2 (CH), 125.4 (CH), 126.0 (CH), 127.4 (CH), 134.1
(Cquat), 135.6 (Cquat), 136.0 (Cquat), 136.5 (Cquat), 140.0 (Cquat), 144.5
(Cquat), 145.7 (Cquat), 150.0 (Cquat). IR: ̃
[cm-1] 2965 (w), 2901 (w), 2868
(w), 1566 (s), 1557 (w), 1537 (w), 1508 (m), 1456 (w), 1431 (m), 1348
(m), 1256 (w), 1231 (w), 1204 (w), 1159 (m), 1115 (m), 1067 (m), 1051
(w), 1014 (w), 959 (w), 930 (w), 843 (m), 822 (w), 791 (w), 750 (s), 723
(s), 691 (w), 625 (w), 612 (w). MS (EI) (70 eV, m/z (%)): 443
([C26H21NS3]+, 100), 428 ([C25H18NS3]+, 6), 386 ([C22H12NS3]+, 4), 310
Supporting information for this article is available on the WWW
under
http://dx.doi.org/10.1002/chem.2014xxxxx.
Experimental details and full characterization of BBTTs 3, 5, and
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10.1002/chem.201805085
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FULL PAPER
([C23H21NS3]+, 86), 57 ([C4H9]+, 57). Anal. calcd. for C26H21NS3 (443.6): C
70.39, H 4.77, N 3.16, S 21.68. Found: C 70.55, H 4.68, N 3.14, S 21.74.
787, 118 124; c) J. Wang, K. Liu, L. Ma, X. Zhan, Chem.
Rev. 2016, 116, 14675 14725; d) E. Mondal, W.-Y. Hung,
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Mater. Chem. C 2017, 5, 10715 10720.
Synthesis of N-(4-tert-butylphenyl)bis[1]benzothieno[3,2-b:2',3'-
e][1,4]thiazine (7a). A typical procedure. In a dry screw-cap Schlenk
tube with a magnetic stir bar 2-bromo-3-((3-bromo-benzo[b]thiophen-2-
yl)thio)benzo[b]thiophene
bis(dibenzylideneacetone)palladium
(6)
(0.23 g,
(22 mg,
0.50 mmol)),
0.038 mmol),
1,1′-bis(diphenylphosphano)ferrocene (42 mg, 0.076 mmol), and sodium
tert-butoxide (0.14 g, 1.5 mmol) were placed under argon. 4-tert-Butyl-
aniline (2a) (80 L, 0.50 mmol) and dry toluene (3 mL) were then added
by syringe. The reaction mixture was stirred at 100 °C for 44 h. After
cooling to room temperature a saturated aqueous sodium sulfite solution
was added to the reaction mixture. The aqueous phase was extracted
with dichloromethane (3 x 20 mL) and the combined organic phases
were dried (anhydrous sodium sulfate). After evaporation of the solvents
under reduced pressure the residue was absorbed on Celite®. The crude
product was purified by flash chromatography on silica gel (n-
hexane/triethylamine 100:3) to give compound 7a after recrystallization
from acetone as orange crystals (127 mg, 57%), Mp 279 °C. 1H NMR
(600 MHz, THF-d8):  1.40 (s, 9 H), 7.15 – 7.20 (m, 2 H), 7.29 – 7.32 (m,
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2 H), 7.33 – 7.37 (m, 2 H), 7.56 – 7.60 (m, 4 H), 7.62 – 7.66 (m, 2 H). 13
C
NMR (151 MHz, THF-d8):  31.8 (CH3), 35.8 (Cquat), 99.8 (Cquat), 120.1
(CH), 122.8 (CH), 124.0 (CH), 126.0 (CH), 128.3 (CH), 129.1 (CH), 134.6
(Cquat), 137.3 (Cquat), 141.2 (Cquat), 142.8 (Cquat), 153.9 (Cquat). IR: ̃
[cm-1]
3065 (w), 2959 (w), 2901 (w), 2864 (w), 1595 (w), 1568 (m), 1508 (s),
1454 (m), 1437 (s), 1364 (w), 1288 (m), 1275 (s), 1244 (m), 1184 (m),
1159 (w), 1128 (w), 947 (w), 928 (w), 845 (m), 791 (w), 739 (s), 719 (s),
640 (w), 602 (m). MS (EI) (70 eV, m/z (%)): 443 ([C26H21NS3]+, 2), 310
([C16H8NS3]+, 3), 298 (15), 297 (20), 296 ([C26H8S3]+, 100), 251 (8), 148
([C9H8S]+, 20). Anal. calcd. for C26H21NS3 (443.6): C 70.39, H 4.77, N
3.16, S 21.68. Found: C 70.32, H 4.79, N 3.02, S 21.97.
[7]
[8]
Acknowledgments
The authors gratefully acknowledge the support by the Fonds
der Chemischen Industrie, and Tobias Wilcke for the
photographs, and the Hamamatsu Photonics Deutschland
GmbH for measuring the absolute fluorescence quantum yields.
Keywords: Absorption • Aggregation induced emission
enhancement • Antiaromaticity • Buchwald-Hartwig coupling •
Cyclic voltammetry • DFT calculations • Fluorescence •
Heterocycles • Polymorphism • 1,4-Thiazines
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This article is protected by copyright. All rights reserved.

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FULL PAPER
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Entry for the Table of Contents
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Bis[1]benzothieno[1,4]thiazines
(BBTT) are a novel class of anellated
thiazines with remarkable features:
anti-anti BBTTs are planar according
to X-ray structure analysis and NICS
calculations support an antiaromatic
character; all BBTT show high radical
cation stabilities and lower oxidation
potentials than phenothiazines;
depending on the anellation mode
solid state luminescence and even
aggregation induced emission
Arno P. W. Schneeweis,a Simone T.
Hauer,a Guido J. Reiss,b and Thomas J.
J. Müller*,a
Page No. – Page No.
Bis[1]benzothieno[1,4]thiazines –
Planarity, Enhanced Redox Activity
and Luminescence by Thieno-
Expansion of Phenothiazine
enhancement is found.
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