Widely Electronically Tunable 2,6-Disubstituted Dithieno[1,4]thiazines—Electron-Rich Fluorophores Up to Intense NIR Emission

Article abstract of DOI:10.1002/chem.202000859

2,6-Difunctionalized dithieno[1,4]thiazines were efficiently synthesized by (pseudo)five- or (pseudo)three-component one-pot processes based on lithiation-electrophilic trapping sequences. As supported by structure–property relationships, the thiophene anellation mode predominantly controls the photophysical and electrochemical properties and the electronic structures (as obtained by DFT calculations). From molecular geometries and redox potentials to fluorescence quantum yields in solution, the interaction of the dithieno[1,4]thiazine-core with the substituents causes striking differences within the series of regioisomers. Most interestingly, strong acceptors introduced in anti–anti dithieno[1,4]thiazines nearly induce a planarization of the ground-state geometry and a highly intense NIR fluorescence (Φ_F=0.52), whereas an equally substituted syn–syn dithieno[1,4]thiazine exhibits a much stronger folded molecular structure and fluoresces poorly (Φ_F=0.01). In essence, electrochemical and photophysical properties of dithieno[1,4]thiazines can be tuned widely and outscore the compared phenothiazine with cathodically shifted oxidation potentials and redshifted and more intense absorption bands.

Full text of DOI:10.1002/chem.202000859

A Journal of
Accepted Article
Title: Widely Electronically Tunable 2,6-Disubstituted Dithieno[1,4]thia-
zines – Electron-Rich Fluorophores up to Intense NIR Emission
Authors: Lars May and Thomas J. J. Müller
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Widely Electronically Tunable 2,6-Disubstituted Dithieno[1,4]thia-
zines – Electron-Rich Fluorophores up to Intense NIR Emission
Lars May,[a] and Thomas J. J. Müller*[a]
Dedication ((optional))
Abstract: 2,6-Difunctionalized dithieno[1,4]thiazines were efficiently
synthesized by (pseudo)five- or (pseudo)three-component one-pot
processes based on lithiation-electrophilic trapping sequences. As
supported by structure-property relationships, the thiophene
anellation mode predominantly controls the photophysical and
electrochemical properties and the electronic structures (as obtained
by DFT calculations). From molecular geometries and redox
potentials to fluorescence quantum yields in solution, the interaction
of the dithieno[1,4]thiazine-core with the substituents causes striking
differences within the series of regioisomers. Most interestingly,
strong acceptors introduced in anti-anti dithieno[1,4]thiazines nearly
induce a planarization of the ground-state geometry and a highly
phenothiazines, the oxidation potentials of dithieno[1,4]thiazines
2 are drastically shifted cathodically and the radical cations
formed by oxidation are more stable by several orders of
magnitudes, depending on the mode of thiazine-thiophene
anellation. Accordingly, the oxidizability and polarizability of the
regioisomeric dithieno[1,4]thiazines
addition to established dithieno[2,3-b:3',2'-e][1,4]thiazines (2-ss,
syn-syn isomer),[8]
in particular, dithieno[3,2-b:2',3'-
2 differ distinctively. In
e][1,4]thiazines (2-aa, anti-anti isomer) aroused our interest due
to even lower oxidation potentials and better stabilized radical
cations.[9] Similarly to phenothiazines, the electronic properties
of syn-syn dithieno[1,4]thiazines 2-ss can be widely fine-tuned
intense NIR fluorescence (F
=
0.52), whereas an equally
by
substitution.[8,10]
However,
functionalized
anti-anti
substituted syn-syn dithieno[1,4]thiazine exhibits a much stronger
folded molecular structure and fluoresces poorly (F = 0.01). In
essence, electrochemical and photophysical properties of
dithieno[1,4]thiazines can be tuned widely and outscore the
compared phenothiazine with cathodically shifted oxidation
potentials and redshifted and more intense absorption bands.
dithieno[1,4]thiazines 2-aa have not been investigated yet.
Introduction
Phenothiazines have become particularly attractive in numerous
applications in organic electronics due to their outstanding
electronic properties: fully reversible one-electron oxidations at
low potentials,[1] rather unusual among organic compounds on
the one hand, and furthermore, luminescence, rather unusual in
combination with redox activity on the other hand.
Advantageously, redox potentials and luminescence can both be
fine-tuned by substitution on the phenothiazine core.[2] Hence
the phenothiazine structure motif has been implemented
diversely in organic light emitting diodes (OLEDs),[3] redox
switchable luminophores,[4] and Grätzel-type sensitizers for
photovoltaics.[5] Moreover, bulk-heterojunction (BHJ) solar cells
have been constructed based on phenothiazines.[6] Thiophenes,
electron-rich heterocycles, are prominent scaffolds in molecular
electronics due to a high polarizability accompanied by favored
charge transport.[7] Consequently, electron-enriched, highly
polarizable phenothiazine analogues, namely dithieno[1,4]-
thiazines 2, were conceptualized by topological benzo-thieno
exchange (Scheme 1). Most remarkably, these small structural
changes of the phenothiazine mother system 1 strongly affect
the electronic structure. For instance, compared to
Scheme 1. Tuning of the widely used phenothiazine moiety 1 – gaining
access to improved electronic properties by construction of the
dithieno[1,4]thiazines 2 by topological benzo-thieno exchange and further 2,6-
difunctionalization of the dithieno[1,4]thiazine core.
Herein, we present efficient one-pot syntheses of six novel 2,6-
disubstituted syn-syn and anti-anti dithieno[1,4]thiazines 3 as
well as a comparative study on their ground and excited state
electronic properties and electronic structures. For comparison,
underlining the potential applicability of dithieno[1,4]thiazines as
phenothiazine substitutes in molecular electronics
a 3,7-
diacceptorsubstituted phenothiazine was prepared and studied.
Results and Discussion
Synthesis
The synthetic one-pot strategy is founded on the inherent -
acidity of thiophenes warranting direct dilithiation of
unfunctionalized dithieno[2,3-b:3',2'-e][1,4]thiazines.[10] Multicom-
ponent reactions, where several compounds are reacted in a
one-pot fashion, have become a valuable tool for accelerated
and more sustainable diversity oriented syntheses of complex
functional molecules.[11] Just recently we established a fast and
[a]
M. Sc. L. May, 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@uni-duesseldorf.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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efficient multicomponent access to diversely acceptor-
substituted thiophenes based on lithiation-electrophile trapping
sequences,[12] we now set out for one-pot pseudo five-com-
ponent syntheses of 2,6-diacceptor-substituted dithieno[1,4]-
thiazines (Scheme 2).
Scheme 3. One-pot LiForK synthesis of the 2,6-dimalononitrile-acceptor
substituted anti-anti (3a-aa) and syn-syn (3b-ss) dithieno[1,4]thiazines and the
corresponding phenothiazine
process.
6 via consecutive (pseudo)five-component
For
efficient
twofold
-arylation
of
N-phenyl
dithieno[1,4]thiazines we envisioned a (pseudo)three component
dilithiation-lithium-zinc exchange-Negishi coupling, which was
developed for syn-syn dithieno[1,4]thiazines like 2b-ss.[10] This
sequence was employed for synthesizing four additional 2,6-
diarylated dithieno[1,4]thiazines 3 in moderate to good yields (20
– 71%) of both anti-anti and the syn-syn isomers with diacceptor
(3c-aa and 3d-ss) and bisdonor (3e-aa and 3f-ss) substitution
pattern (Scheme 4).
Scheme 2. Retrosynthetic analysis of a (pseudo)five-component one-pot
synthesis of 2,6-diacceptor-substituted dithieno[2,3-b:3',2'-e][1,4]thiazines in
the sense of
sequence.
a lithiation-formylation-Knoevenagel-condensation (LiForK)
Hence, reaction conditions of the previously reported lithiation-
formylation-Knoevenagel-condensation (LiForK) sequence[12]
were applied for functionalizing dithieno[1,4]thiazines in a one-
pot fashion. For this comparative study, both anti-anti and syn-
syn dithieno[1,4]thiazine isomers of all targets were synthesized.
The regioisomeric dithieno[1,4]thiazines 2a-aa and 2b-ss as
starting materials were efficiently accessed employing previously
reported twofold intermolecular-intramolecular Buchwald-
Hartwig aminations with aniline.[8,9] Dilithiation of 2a-aa and 2b-
ss with n-BuLi/TMEDA in THF followed by trapping with DMF
and buffering with acetic acid selectively gave the isomeric
dialdehydes as intermediates. Upon addition of malononitrile (4)
the dithieno[1,4]thiazinyl dialdehydes are smoothly converted in
a
one-pot fashion into the 2,6-diacceptor-substituted
Scheme 4. One-pot synthesis of 2,6-diarylated anti-anti and syn-syn
dithieno[1,4]thiazines 3 via consecutive (pseudo)three-component dilithiation-
lithium-zinc exchange-Negishi coupling (exemplarily illustrated for 3c).
dithieno[1,4]thiazines by concluding Knoevenagel
3
a
condensation with malononitrile (4) at ambient temperature
(Scheme 3). In a similar fashion 3,7-dibromo-10-phenyl-10H-
phenothiazine (5),[13] however, by bromine-lithium exchange,
was reacted in the LiForK sequence to give the phenothiazine
derivative 6. This type of donor-acceptor conjugate can be often
found in efficient bulk-heterojunction (BHJ) solar cells.[14]
While diacceptor anti-anti dithieno[1,4]thiazine 3a-aa was
synthesized with an excellent yield of 83%, i.e. an average yield
of 95% per bond forming, the corresponding syn-syn
dithieno[1,4]thiazine 3b-ss was obtained in a moderate yield of
40%. Initiated by bromine-lithium exchange the LiForK synthesis
of a corresponding diacceptor phenothiazine 6 gives a good
yield of 74%. The required brominated phenothiazine 5 was
synthesized as indicated in the literature.[13]
Electronic Properties and Structures
With respect to potential applications of functionalized
dithieno[1,4]thiazines in organic electronics the electronic
properties and structures of the dithieno[1,4]thiazines 3 were
assessed experimentally and computationally. The electronic
ground states were examined by cyclic voltammetry experiments
and quantum chemical calculations on DFT level of theory. The
excited states were examined by absorption and emission
spectroscopy in dichloromethane solutions and by TDDFT
calculations. All ground and excited state (S1) geometries were
optimized using the Gaussian09 program package,[15] the
PBE1PBE functional[16] and the 6-31G** basis set[17] and were
confirmed as minima by analytical frequency analyses. The
excitation energies were calculated with TDDFT[18] methods
implemented in the Gaussian09 program package using the
PBE1PBE functional[16] and the 6-31G** or the 6-31+G** basis
set[17] as indicated. The polarizable continuum model (PCM) was
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always applied for the calculations with the same solvent used in
the corresponding experiment.[19]
a good linear correlation (r² = 0.9847 for E0/+1, r² = 0.9980 for
E1/+2) with the HOMO energy levels, underlining the continuous
impact of the substituents on the electron richness and thus on
the redox potentials.
All dithieno[1,4]thiazines exhibit reversible oxidations in
dichloromethane solutions. By different functionalization the
oxidation potentials of mono- and dioxidations of the anti-anti
dithieno[1,4]thiazines (3a-aa, 3c-aa and 3e-aa) (Figure 1, Table
1) can be tuned over a broad range (E0/+1(3a-aa,3e-aa) = 670
mV, E+1/+2(3a-aa,3e-aa) = 610 mV). On one hand, the redox
potentials can be shifted anodically up to 540 mV (230 mV for
E+1/+2) and on the other hand they can be shifted cathodically by
130 mV (380 mV for E+1/+2) against the unfunctionalized system
2a (E0/+1(2a-aa) = 374 mV, E+1/+2(2a-aa) = 1292 mV[9]) by
introducing strong acceptors or donors, respectively. This also
holds true for syn-syn isomers (3b, 3d and 3f), but the measured
range of redox potentials is narrower (E0/+1(3b-ss,3f-ss) = 600
mV).
Figure 2. Correlation of the oxidation potentials (0.1 M [Bu4N][PF6], v = 100
mV/s,
Pt-working,
Ag/AgCl-reference
and
Pt-counter
electrode,
[Cp*Fe]/[Cp*Fe]+ as an internal standard; Cp*Fe = decamethylferrocene) of
the monooxidation E0/+1 (red) and the dioxidation E+1/+2 (blue) with the HOMO
energies EHOMO of the compounds 3 and 6 (PBE1PBE/6-31G** PCM CH2Cl2).
However, there is one exception: anti-anti dithieno[1,4]thiazines
have been shown to be more electron-rich than the
regioisomeric syn-syn dithieno[1,4]thiazines.[9] But whereas the
oxidation potentials of both anti-anti diaryl dithieno[1,4]thiazines
(3c and 3e) are shifted about 40 mV cathodically against the
corresponding syn-syn isomers (3d-ss and 3f-ss) as expected,
surprisingly, a reverse behavior can be observed comparing 3a-
aa and 3b-ss (E0/+1(3b-ss – 3a-aa) = -22 mV) bearing strong
acceptors. Nevertheless, the hierarchy of the oxidation
potentials, especially of 3a-aa and 3b-ss, can be correctly
reproduced by DFT-calculated oxidation potentials (see
Supporting Information, chapter 6). Further studies concerning
the rotational barriers of dithieno[1,4]thiazine-substituent bonds
G‡rot (Table 2) imply that the strength of the
dithieno[1,4]thiazine-substituent interaction is larger in the anti-
anti isomers compared to the syn-syn isomers. All DFT-
computed rotational barriers of the anti-anti isomers are higher
than the barriers of the syn-syn isomers, so that a stronger -
bonding character can be plausibly assumed.
Figure 1. Selected cyclic voltammograms of the 2,6-disubstituted
dithieno[1,4]thiazines 3c-aa - 3f-ss (CH2Cl2, T = 298 K, 0.1 M [Bu4N][PF6], v =
100 mV/s, Pt-working, Ag/AgCl-reference and Pt-counter electrode,
[Cp*Fe]/[Cp*Fc]+ as an internal standard; Cp*Fe = decamethylferrocene, E0/+1
= -95 mV vs. ferrocene with E0/+1(Fc/Fc+) = 450 mV).[20]
Table 1. Electrochemical properties and HOMO-energy levels EHOMO of
compounds 3 and 6.
[a]
Compound
E0/+1 [mV][a]
E+1/+2 [mV][b]
KSEM
EHOMO [eV] [c]
3a-aa
3b-ss
6
3c-aa
3d-ss
3e-aa
3f-ss
915
893
1118
453
491
247
291
1522
1552
-
1202
1247
916
1.95·1010
1.46·1011
-
-5.594
-5.622
-5.923
-5.133
-5.232
-4.829
-4.914
4.95·1012
6.51·1012
2.18·1011
1.01·1012
1000
[a] Recorded in CH2Cl2, T = 298 K, 0.1 M [Bu4N][PF6], v = 100 mV/s, Pt-
working, Ag/AgCl-reference and Pt-counter electrode, [Cp*Fe]/[Cp*Fe]+ as an
internal standard (Cp*Fe = decamethylferrocene, E0/+1 = -95 mV vs. ferrocene
Table 2. Differences of the calculated rotational barriers of the
dithieno[1,4]thiazine-substituent bonds between the anti-anti and syn-syn
isomers G‡rot (PBE1PBE/6-31G** PCM CH2Cl2).
퐸
− 퐸
0/+1
+1/+2
with E0/+1(Fc/Fc+) = 450 mV).[20] [b] 퐾푆퐸푀 = 10
.
[c] PBE1PBE/6-
59 푚푉
31G**, PCM CH2Cl2.
Phenothiazine 6, the heterocyclic topological analogue of 3a-aa
and 3b-ss, is about 200 mV anodically shifted against 3a-aa and
3b-ss, which is in line with the unfunctionalized systems[9] and
the calculated HOMO energy levels (Figure 2). The HOMO
energy levels scale with the electron richness and the ionization
potential of a compound. Therefore, the oxidation potentials give
Compared isomers
G‡rot.[kcal/mol]
3a-aa – 3b-ss
3c-aa – 3d-ss
3e-aa – 3f-ss
1.061
0.553
0.201
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Consequently, the offbeat oxidation potential of 3a-aa together
with the larger substituent effects on the oxidation potentials of
the anti-anti isomers can be rationalized (Table 1). This is also in
line with the higher tendency towards delocalization of the anti-
anti dithieno[1,4]thiazine mother system 2a-aa.[9] Additionally,
the radical cations showed unexpected relative stabilities. The
stabilities of radical cations can be compared by their
semiquinone formation constants KSEM (equilibrium constants of
the comproportionation of dications and neutral ground
states).[21] As indicated by KSEM, the radical cations formed upon
aa) = 436 nm to max,abs(3a-aa) = 641 nm, equaling an energy
difference of 0.91 eV. An equally wide range of 0.96 eV is
exhibited by the syn-syn isomers. Nevertheless, the max,abs of
syn-syn isomers are always hypsochromically shifted compared
to the corresponding anti-anti isomers. And in addition, the
longest wavelength absorption bands of all anti-anti isomers are
significantly more intense compared to the syn-syn isomers. For
example, the extinction coefficient  at the longest wavelength
absorption maximum of 3a-aa ((max,abs) = 39400 L∙mol-1∙cm-1)
exceeds that of 3b-ss ((max,abs) = 12350 L∙mol-1∙cm-1) by more
than three-times. This accounts for a more efficiently conjugated
push-pull system in 3a-aa than in 3b-ss, which again is in line
oxidation of the anti-anti isomers
– in contrast to the
unsubstituted systems[9] – are always one order of magnitude
lower than those of the corresponding syn-syn isomers (Table 1). with the stronger dithieno[1,4]thiazine-substituent interaction in
This can well be a consequence of stronger substituent effects
in the anti-anti isomers resulting in a higher destabilization of the
radical cations by the acceptors in 3a-aa and 3c-ss and a higher
stabilization of the dication in 3e-aa (E+1/+2(3f-ss – 3e-aa) = 84
mV) relative to the syn-syn isomers, respectively. Moreover, the
KSEM values of all substituted dithieno[1,4]thiazines 3 are several
magnitudes lower than the KSEM values of the unsubstituted
mother systems 2.[8,9] However, in comparison to other organic
redox systems[21] KSEM ≈ 1010 – 1012 are still remarkably high.
Similar to the electrochemical properties, the photophysical
properties of the dithieno[1,4]thiazine isomers 3 (Table 3, Figure
3) can be tuned over a broad spectrum of the UV/Vis by
functionalization as well. The longest wavelength absorption
maxima max,abs of the anti-anti isomers range from max,abs(3e-
the anti-anti isomers (Table 2).
Figure 3. UV/Vis absorption spectra of (a) dithieno[1,4]thiazines 3a-aa and
3b-ss and phenothiazine 6 and (b) dithieno[1,4]thiazines 3c-aa – 3f-ss (c(3,6)
= 10-5 MCH2Cl2, T = 298 K).
Table 3. Photophysical properties of compounds 3 and 6.
[e]
Compound

max,abs.[nm][a]
(max,abs)[L∙mol-1∙cm-1]
∫()d[1013 L∙mol-1]

max,em [nm][b]
719
E0-0 [eV][c]

̃
F
3a-aa
641
472
342
623
349
517
418
366
331
277
496
300
459
309
436
287
420
295
39400
3300
7.12
1.79
0.52[f]
41200
12350
35700
20300
8000
15150
37900
13960
14800
33740
6700
3b-ss
6
4.56
5.17
750
639
1.76
2.09
0.01[f]
0.57[g]
3c-aa
3d-ss
3e-aa
3f-ss
4.82
3.57
3.69
2.84
611
620
559
576
2.17
2.23
2.39
2.48
0.29[g]
0.03[g]
0.02[h]
<0.01[h]
38900
8200
42800
4300
31800
[a] Recorded in CH2Cl2 at T = 298 K, c(3,6) = 10-5 M. [b] Recorded in CH2Cl2 at T = 298 K, c(3,6) = 10-6 M. [c] E0-0 was determined from the crossing point of
absorption and emission spectra. [d] ̃ = 1/max,abs - 1/max,em. [e] F in CH2Cl2 at T = 298 K was determined relative to a fluorescence standard. [f] Nile Blue A
perchlorate in methanol as a standard (exc = 626 nm, F = 0.21[22]). [g] DCM in methanol as a standard (exc = 492 nm, F = 0.43[23]). [h] Coumarin 153 in ethanol
as a standard (exc = 422 nm, F = 0.38[24]).
Furthermore, the spectral integrals ∫()d(Table 3)highlight
that the overall absorption of the anti-anti isomers is significantly
intensified. In contrast the analogous phenothiazine 6 exhibits a
way more hypsochromically shifted absorption maximum of
max,abs = 517 nm and almost a halved  of the longest
wavelength absorption maximum in comparison to 3a-aa. The
favorable impact of the benzo-thieno exchange leading from
phenothiazines to dithieno[1,4]thiazines[8,9] is thereby clearly
emphasized. DFT calculations on the UV/Vis absorption spectra
(Table 4) correctly reproduce the experimental data and indicate
that characteristic longest wavelength absorption maxima mostly
originate from HOMO-LUMO transitions. All dithieno[1,4]-
thiazines 3 fluoresce in dichloromethane solutions (Table 3,
Figure 4), but the Stokes shifts ̃ differ quite noticeably. The
Stokes shifts are about 2000 cm-1 for the dithieno[1,4]thiazines
3a-aa and 3b-ss bearing strongly electron withdrawing groups,
whereas they are in a range of 5000–6000 cm-1 for the donor
substituted dithieno[1,4]thiazines 3e-aa and 3f-ss.
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Table 4. TDDFT calculations on the UV/Vis-absorption maxima of the compounds 3 and 6 (PBE1PBE/6-31G**, PCM CH2Cl2).
Compound

max,exp [nm] ( [L∙mol-1∙cm-1])[a]
max,calcd [nm]
Oscillator strength
Most dominant contributions
3a-aa[b]
641 (39400)
472 (3300)
625
477
356
342
636
0.9395
0.0504
0.3642
0.1856
0.4468
0.5038
0.6349
0.0706
0.7812
0.8492
0.9124
0.3337
1.2624
0.5106
HOMO  LUMO (99%)
HOMO  LUMO+1 (97%)
HOMO-2  LUMO (97%)
HOMO-1  LUMO (97%)
HOMO  LUMO (99%)
HOMO-2  LUMO (96%)
HOMO  LUMO (98%)
342 (41200)
3b-ss
6
632 (12350)
349 (35700)
517 (20300)
418 (8000)
350
535
421
363
502
299
482
311
436
HOMO  LUMO+1 (96%)
HOMO-1 LUMO (95%)
HOMO  LUMO (97%)
HOMO  LUMO+4 (82%)
HOMO  LUMO (97%)
HOMO-2  LUMO (85%)
HOMO  LUMO (96%)
HOMO-2  LUMO (49%)
HOMO-1  LUMO+1 (46%)
HOMO  LUMO (96%)
HOMO-1  LUMO+1 (73%)
366 (15150)
496 (14800)
3c-aa
3d-ss
300 (33740)
459 (6700)
309 (38900)
436 (8200)
3e-aa[b]
287 (42800)
284
0.9076
3f-ss[b]
420 (4300)
433
297
0.1836
1.2881
295 (31800)
[a] Recorded in CH2Cl2, c(3,4) = 10-5 M, T = 293 K. [b] PBE1PBE/6-31+G**, PCM CH2Cl2 was used instead.
DFT calculations on the ground and excited states of the
dithieno[1,4]thiazines 3 and phenothiazine 6 reveal that the
Stokes shifts can be explained with geometry changes
accompanied by a planarization after photoexcitation (Figures 5
and 6), in analogy to phenothiazines.[25] The ground state
geometries of all functionalized dithieno[1,4]thiazines 3 are
folded along the S,N-axis as a consequence of an unfavored
Hückel antiaromaticity of a planarized thiazine and, thus, they
possess a phenothiazine-like[26] butterfly structure. Interestingly,
the Stokes shifts are at least 1000 cm-1 larger for the syn-syn
isomers than for the anti-anti isomers.
Figure 6. DFT-computed Jablonski diagrams and Kohn-Sham FMOs
corresponding to the S0–S1*-transition (longest wavelength absorption) and
the S1–S0*-transition (fluorescence) of 3a-aa and 3b-ss (PBE1PBE/6-31G**,
PCM CH2Cl2, isosurface value at 0.04 a.u.).
Accordingly, the S,N-folding angles  are larger in anti-anti than
in corresponding syn-syn isomers, respectively. In addition, 
increases concomitantly with the electron withdrawing character
of the substituents implying a tunable antiaromatic character of
the thiazine rings. The more electron-rich the thiazine ring
becomes the more folded it will be. This rationalizes the
hierarchy of the Stokes shifts of compounds 3a-aa – 3d-ss and
6 (Figure 7). For example, the remarkably almost planar ground
state geometry of 3a-aa ( = 174°) is in accordance with the
implemented strongly electron withdrawing acceptor substituents.
The weaker substituent interactions in the corresponding syn-
syn isomer 3b-ss ( = 160°) explain the larger Stokes shift of
3b-ss due to a higher thiazine located electron density.
Otherwise, we did not observe a significant effect of donor
substituents on (3e-aa,3f-ss) = 145°) compared to the 2,6-
unsubstituted dithieno[1,4]thiazines ((2a-aa,2b-ss) = 144° [8]),
which indicates that the stabilization of the thiazine by folding
predominates and delocalization is less favored.
Figure 4. Normalized UV/vis absorption (solid lines) and emission spectra
(dashed lines) of (a) dithieno[1,4]thia-zines 3a-aa and 3b-ss and
phenothiazine 6 and (b) dithieno[1,4]thiazines 3c – 3f (recorded in CH2Cl2 at T
= 298 K, c(3,6) = 10-6 M, exc(3,6) = max,abs(3,6)).
Figure 5. Optimized ground-state (S0) and excited state (S1) geometries, S,N-
folding angles  and stokes shifts ̃ of 3a-aa and 3b-ss (PBE1PBE/6-31G**,
PCM CH2Cl2).
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6). Since the comparatively high heavy-atom effect of sulfur can
be assumed to cause an enhanced intersystem crossing, a
larger kISC of the syn-syn isomers, along with smaller kR values,
which, in turn, can plausibly rationalize the observed weaker
fluorescence of all syn-syn isomers compared to the
corresponding anti-anti isomers.
Table 5. Fluorescence lifetime , radiative rate constant kR, TDDFT-computed
DFT
rate constant kR
and resulting non-radiative rate constant kNR of 3a-aa
and 3b-ss.
kR [s-1][a]
kRDFT [s-1][b,c]
Compound
[ns]
kNR [s-1][d]
3a-aa
3b-ss
2.48
0.21
2.10∙108
4.70∙107
1.26·108
5.49·107
1.49·108
4.66·109
[a] kR = F/[b] kRDFT= 2/3 fem ∙ em
̃
².[27] [c] PBE1PBE/6-31G** PCM CH2Cl2. [d]
 kNR = (1-F)/.
Table 6. LUMO coefficients localized on the thiazine- and the two thiophene-
sulfur atoms SLUMO[28] (PBE1PBE/6-31G** PCM CH2Cl2).
Compound
SLUMOThiazine [%]
SLUMOThiophene [%]
Figure 7. Dependence of the experimental Stokes shifts ̃ of the DFT-
computed S,N-folding angle  of the diacceptor phenothiazine 6 and the
diacceptor dithieno[1,4]thiazines 3a-aa 3d-ss (PBE1PBE/6-31G** PCM
CH2Cl2). For illustration, we added an exponential fit (r² = 0.9881).
3a-aa
3b-ss
6
3c-aa
3d-ss
3e-aa
3f-ss
0.05
0.65
0.31
0.32
0.81
1.62
2.11
11.20
10.11
-
8.58
8.47
14.97
12.16
–
Regarding the fluorescence quantum yields F in
dichloromethane solutions there are also striking differences
between the isomers (Table 3). Anti-anti diacceptor-
dithieno[1,4]thiazines 3a-aa (F = 0.52) and 3c (F = 0.29)
fluoresce intensely, whereas their syn-syn regioisomers 3b-ss
(F = 0.01) and 3d-ss (F = 0.03) fluoresce only weakly. The
same trend holds true for the donor substituted
dithieno[1,4]thiazines 3e-aa (F = 0.02) and 3f-ss, (F < 0.01)
with their much weaker luminescence. In terms of F anti-anti
dithieno[1,4]thiazine 3b-ss represents a direct analogue to
phenothiazine 6 (F = 0.57). In general, F depends on the ratio
of the radiative (kR) and the sum over all radiative and non-
radiative rate constants (kNR) of the excited-state decay
(Equation 1). The larger kR and the smaller kNR become, the
more F will increase.
Increased intersystem crossing rates of syn-syn isomers are
also supported by the phosphorescence of 3f-ss in degassed
toluene solutions at 77 K (Figure 8) and the absence of any
detectable phosphorescence of 3e-aa under the same
conditions.
푘
푅
∑
훷퐹 =
with
푘푁푅 ≈ 푘퐼퐶 + 푘퐼푆퐶
(Equation 1)
∑
+ 푘
푁푅
푘
푅
For the anti-anti dithieno[1,4]thiazines 3a-aa, 3c-aa and 3e-aa
the extinction coefficients  as well as the TDDFT-computed
oscillator strengths f of the S0–S1 transitions are larger than for
the corresponding syn-syn isomers, respectively (Table 4),
suggesting that kR is larger as well. Furthermore, fluorescence
lifetime measurements and TDDFT calculations on kR of 3a-aa
and 3b-ss support this assumption and reveal that there is also
a large difference in kNR (Table 5): kNR(3b-ss) is about 30-
times higher than kNR(3a-aa). As the rate constants of the
internal conversion (kIC) can be assumed to be approximately
similar for both dithieno[1,4]thiazine regioisomers due to their
similarly rigid structures and S0–S1 energy gaps (for E0-0, see
Table 3), the major non-radiative decay could arise from
different intersystem crossing rates (kISC). The LUMO coefficient
densities located on the sulfur atoms, representing their
electronic participation in the S1-state, are significantly smaller in
each anti-anti isomer on the thiazine sulfur atoms and relatively
similar on the thiophene sulfur atoms of pairs of isomers (Table
Figure 8. Phosphorescence (red,p), phosphorescence lifetime p,
fluorescence (blue,F) of 3f-ss (recorded in toluene, exc = 420nm, c(3f-ss) =
10-6 M, T = 77 K, degassed with N2) and TDDFT-calculated phosphorescence
pDFT (PBE1PBE/6-31G** PCM toluene).
The acceptor substituted dithieno[1,4]thiazines 3a-aa – 3d-ss
show positive emission solvatochromism as their emission
maxima are shifted more bathochromically with increasing
solvent polarity (Figure 9). Hence, the dipole moments increase
upon photo excitation and the transitions corresponding to the
longest wavelength absorption maxima possess charge transfer
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(CT) character. Via Lippert-Mataga plots (see Supporting
Information chapter 5.1) the ground to excited state dipole
moment changes (S0→S1) were obtained (Table 7).
dipole moment of syn-syn 3b-ss is around 1 D larger than of
anti-anti 3a, whereas the difference 3c-aa and 3d-ss is even
around 2 D (Table 7). This pronounced larger CT-character of
the syn-syn isomers is visualized by the difference plots of the
Kohn-Sham FMOs suggesting a more clarified separation of the
thiazine an acceptor -systems (Figure 11).
Figure 11. HOMO → LUMO difference plot of 3a and 3b (red = decrease, blue
= increase, PBE1PBE/6-31G**).
The larger (S0→S1) of phenothiazine 6 in comparison to 3a-aa
and 3b-ss points out that the electronic interaction with the
acceptor substituents is weaker in phenothiazines than in
dithieno[1,4]thiazines. Therefore, the benzo-thieno exchange
emphasizes the favorable impact of increased delocalization on
the electronic properties, in particular, on charge transmission
within the chromophores.
Figure 9. Solvatochromism study on
a selected dithieno[1,4]thiazine –
Normalized absorption (solid lines) and emission spectra (dashed lines) of 3a-
aa recorded in diethyl ether, toluene, dichloromethane, ethyl acetate, THF,
isopropanol, acetone and acetonitrile (c(3a-aa) = 10-6 M, T = 298 K, exc(3a-aa)
= 641 nm).
Table 7. Ground to excited state dipole moment change (S0→S1) and FMO
centroid distance dFMO (PBE1PBE/6-31G**) of compounds 3 and 6.
Compound
3a-aa
(S0→S1)[D]
2.77
dFMO [Å][27]
0.50
Conclusion
3b-ss
6
3c-aa
3d-ss
3.78
5.22
6.07
8.50
0.85
1.24
1.44
2.53
A complementary series of six novel acceptor- and donor-
functionalized dithieno[1,4]thiazine regioisomers was efficiently
accessed by employing straightforward one-pot processes. In
addition, the acceptor substituted phenothiazine 6 was also
synthesized in a one-pot fashion for electronic comparison.
The acceptor interaction in anti-anti isomers surpasses that in
syn-syn isomers accompanied by a stronger intramolecular
charge transfer, as supported by absorption and emission
spectroscopy and calculations. For instance, the DFT calculated
ground state geometry of the anti-anti dithieno[1,4]thiazine 3a-aa
is almost planar, but its syn-syn regioisomer 3b-ss is folded
noticeably due to a less efficient depopulation of the 8-electron
containing thiazine ring. It is noteworthy that in phenothiazine 6
substituent interactions are smaller than in the corresponding
dithieno[1,4]thiazines 3a-aa and 3b-ss according to
solvatochromicity studies. As a consequence of the fact that
dithieno[1,4]thiazines form much stronger push-pull systems
than phenothiazines. Furthermore, the absorption of the
relatively small chromophores 3a-aa and 3b-ss (MW = 440
g/mol) almost reaches the near infrared region. In comparison,
more extended chromophores (-system with MW = 660 g/mol)
applied in efficient bulk-heterojunction solar cells based on
Since the CT-absorption bands are dominated by HOMO–LUMO
transitions (Table 4), the different CT-character of the isomers is
illustrated by correlation of the centroid distances of these
frontier molecular orbitals dFMO with (S0→S1) (Figure 10, r² =
0.9768).
dithieno pyrrole as
a donor exhibit more hypsochromic
absorption bands.[14] Therefore, especially the very intensively
absorbing 3a-aa is a promising candidate for photovoltaics in the
miniaturization of dye-architectures.[6,29] Dithieno[1,4]thiazines
can be considered as phenothiazine substitutes in donor-
acceptor conjugates for improving the properties while
maintaining structural changes and molecular sizes small.
Figure 10. Ground to excited state dipole moment change (S0→S1) vs.
FMO centroid distance dFMO of the compounds 3 and 6.
The change of dipole moment is larger for the syn-syn isomers
3b-ss and 3d-ss than for the corresponding anti-anti isomers
3a-aa and 3c-aa. Thus, the electronic dithieno[1,4]thiazine-
substituent transmission depending on the thiophene anellation
mode becomes apparent again. For instance, the change of
Finally, acceptor-functionalized anti-anti dithieno[1,4]-thiazines
fluoresce very intensively in contrast to the syn-syn regioisomers,
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10.1002/chem.202000859
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solution was stirred at 70 °C for 1 h. The volatiles were removed by
evaporation and the crude product was purified by flash column
chromatography using gradient elution (n-hexane/ethyl acetate 4:1 with
1% triethyl amine → n-hexane/ethyl acetate 1:1 with 1% triethyl amine)
and suspension in acetone giving compound 3c-aa (249 mg, 71%) as a
as a consequence of faster intersystem crossing in syn-syn
isomers inter alia. Hence, in particular acceptor-functionalized
anti-anti dithieno[1,4]thiazines can be also considered as intense
red-light emitters (F = 0.52) or even as NIR-emitters for
biomedical imaging[30] or OLED-devices.[3] Further studies on
dithieno[1,4]thiazine based donor-acceptor conjugates and their
applications are currently underway.
1
violet powder, Mp 274-275 °C. Rf (n-hexane/ethyl acetate 3:1) = 0.36. H
NMR (600 MHz, DMSO-d6, 393 K):  6.23 (s, 2H), 7.54 –7.57 (m, 1H),
7.57 – 7.60 (m, 4H), 7.62 – 7.66 (m, 2H), 7.66 – 7.70 (m, 6H). 13C NMR
(150 MHz, DMSO-d6, 393 K):  108.3 (Cquat), 108.6 (Cquat), 117.8 (Cquat),
122.9 (CH), 124.0 (CH), 126.7 (CH), 128.9 (CH), 130.1 (CH), 130.4
(Cquat), 132.1 (CH), 136.7 (Cquat), 142.2 (Cquat), 142.7 (Cquat). MS(MALDI-
Experimental Section
TOF) m/z: 489.115 ([M]+). IR:  [cm-1] 3057 (w), 2990 (w), 2886 (w), 2218
̃
(w), 1559 (m), 1493 (m), 1435 (s), 1408 (s), 1362 (w), 1296 (w), 1283 (w),
1271 (w), 1227 (w), 1177 (m), 1165 (m),1111 (w), 1045 (w), 1016 (w),
984 (w), 964 (w), 945 (w), 918 (w), 880 (w), 835 (w), 818 (s), 802 (m),
772 (w), 743 (w), 719 (w), 691 (m), 651 (w). Anal. calcd. for C28H15N3S3
(489.6): C 68.69, H 3.09, N 8.58, S 19.64; Found: C 68.67, H 3.00, N
8.40, S 19.93.
Experimental details, full characterizations, 1H and 13C NMR spectra of
compounds 3 and 6, additional cyclic voltammograms, absorption and
emission spectra, as well as all DFT computed XYZ-coordinates,
energies and absorption spectra are compiled in the Supporting
Information.
Typical procedure for the preparation of 2,6-diacceptor-substituted
dithieno[1,4]thiazine 3a-aa via Lithiation-Formylation-Knoevenagel
sequence (LiForK): In a flame-dried Schlenk vessel with magnetic stir
bar under nitrogen atmosphere 8-phenyl-8H-dithieno[3,2-b:2',3'-
e][1,4]thiazine (2a-aa) (146 mg, 0.51 mmol) and tetramethylethylene-
diamine (0.19 mL, 1.28 mmol) were dissolved in dry THF (5.10 mL) and
cooled down to -78 °C (isopropanol/dry ice). Then, n-butyllithium (0.80
mL, 1.28 mmol, 1.6 Min hexane) was added dropwise slowly via syringe
to the vigorously stirred solution. Stirring was continued at -78 °C for 2 h.
Then dry DMF (118 L, 1.53 mmol) was added, stirring was continued at
-78 °C for another 90 min and then at ambient temperature for 30 min. To
the reaction mixture acetic acid (0.15 mL, 2.55 mmol) was added. After
stirring at ambient temperature for 15 min, malononitrile (4) (101 mg,
2.62 mmol) was added and the stirring was continued at ambient
temperature for 20 min. The volatiles were removed by evaporation and
the crude product was purified by flash column chromatography using
Acknowledgements
The authors cordially thank the Fonds der Chemischen Industrie
for financial support (scholarship of LM) and M. Sc. Kristoffer
Thom (Institute of Physical Chemistry, HHU Düsseldorf) for
measuring the fluorescence lifetimes.
Keywords: Fluorescence • Multicomponent reactions • NIR
fluorescence • Redox systems • Structure-property relationships
[1]
For comprehensive reviews on the chemistry of phenothiazines, see
e.g. a) R. A. Aitken, K. M. Aitken, 1,4-Thiazines and their Benzo
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suspension in ethanol giving compound 3a-aa (185 mg, 83%) as a dark
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2:1) = 0.23. 1H NMR (600 MHz, DMSO-d6, 372 K):  7.51 (s, 2H), 7.69-
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DMSO-d6, 372 K):  69.2 (Cquat), 110.8 (Cquat), 114.5 (Cquat), 115.2 (Cquat),
124.8 (Cquat), 127.7 (CH), 131.6 (CH), 131.7 (CH), 132.0 (CH), 139.8
(Cquat), 150.1 (CH), 152.2 (Cquat). MS(MALDI-TOF) m/z: 438.980 ([M]+).
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[cm-1] = 2212 (w), 1561 (s), 1555 (s), 1501 (w), 1489 (w), 1423 (w),
1368 (s), 1325 (s), 1314 (s), 1287 (s), 1273 (s), 1254 (s), 1209 (s), 1169
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dithieno[1,4]thiazine 3c-aa via dilithiation-lithium-zinc exchange-
Negishi coupling: In a flame-dried Schlenk vessel with magnetic stir bar
under nitrogen atmosphere 8H-dithieno[3,2-b:2',3'-e][1,4]thiazine (2a-aa)
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hexane) was added dropwise slowly via syringe to the vigorously stirred
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the reaction mixture had slowly warmed up to ambient temperature,
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5 mol%) and 4-
bromobenzonitrile (7) (328 mg, 1.80 mmol) were added and the reaction
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10.1002/chem.202000859
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COMMUNICATION
Four isomeric
Lars May, and Thomas J. J. Müller*
dithieno[1,4]thiazines,
thieno congeners of
pheno-thiazines, are
synthesized by inter-intra-
molecular Buchwald-
Hartwig coupling. Their
electronic properties open
new avenues as donor
systems for molecular
electronics
Page No. – Page No.
Widely Electronically Tunable 2,6-
Disubstituted Dithieno[1,4]thia-zines
– Electron-Rich Fluorophores up to
Intense NIR Emission
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