4H-Dithieno[2,3-b:3′,2′-e][1,4]thiazines - Synthesis and electronic properties of a novel class of electron rich redox systems

Article abstract of DOI:10.1039/c2cc32731g

Quantum chemical screening reveals that 4H-dithieno[2,3-b:3′, 2′-e][1,4]thiazines possess the highest HOMO among four constitutional isomers, even 0.27 eV higher in energy than the well established 10H-phenothiazine. N-Substituted 4H-dithieno[2,3-b:3′,2′-

Full text of DOI:10.1039/c2cc32731g

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4H-Dithieno[2,3-b:30,20-e][1,4]thiazines – synthesis and electronic
properties of a novel class of electron rich redox systemswz
Catherine Dostert,a Claudia Wanstrath,a Walter Frankb and Thomas J. J. Mu
¨
ller*a
Received 17th April 2012, Accepted 28th May 2012
DOI: 10.1039/c2cc32731g
Quantum chemical screening reveals that 4H-dithieno-
[2,3-b:30,20-e][1,4]thiazines possess the highest HOMO among
four constitutional isomers, even 0.27 eV higher in energy
than the well established 10H-phenothiazine. N-Substituted
4H-dithieno[2,3-b:30,20-e][1,4]thiazines are readily accessible
by twofold Pd-catalyzed amination. According to cyclic voltammetry
dithienothiazines are reversibly oxidized and can be considered as
new donors for functional p-systems.
Electroactive organic molecules and polymers are key constituents
of organic semiconductors1 and they have therefore become
increasingly important as charge carrier transporting materials in
technological applications ranging from organic light-emitting
diodes to organic photovoltaic devices and organic field-effect
transistors.2 Moreover, small redoxactive molecules can act as
molecular wires,3 thus enabling unimolecular electronics.4
Fig. 1 Structures of phenothiazine 1 and the four constitutional
isomeric dithieno[b,e][1,4]thiazines 2.
dithienothiazines 2 differentiating in the anellation of the
thiophene rings to the central thiazine ring (Fig. 1).
Here, we communicate the computational screening of
dithienothiazines 2 and the synthesis and electronic properties
of N-substituted 4H-dithieno[2,3-b:30,20-e][1,4]thiazine derivatives
of structure 2a.
Electron-rich organic p-systems qualify as hole conductors
(p-type semiconductors) per se as a consequence of their low
oxidation potential and low electron affinity. In particular,
polycyclic heterocycles can reversibly form stable radical
cations upon oxidation. For instance, 3,6-linked5 and 2,7-linked
oligocarbazoles6 as well as 3,7-linked oligophenothiazines7 have
displayed favorable hole conducting properties. Furthermore,
phenothiazine 1 as a tricyclic nitrogen–sulfur donor moiety has
found considerable application in various fields of materials
science.8 For enhancing the donor capacity of tricyclic thiazine
based heterocycles the formal replacement of the b,e-anellation of
benzene by that of thiophene appears to be a conceptual intriguing
goal, thereby suggesting four conceivable constitutional isomers of
A topological exchange of benzene against thiophene inevitably
causes an increase in electron density and polarizability of dithieno-
thiazines 2 in comparison to phenothiazine 1. As the energies of the
frontier molecular orbitals (FMO), i.e. the HOMO and the LUMO,
are mostly indicative of the electron richness or deficiency, we first
set out to computationally screen the FMO energies of the four
dithienothiazines 2 and of phenothiazine 1 at the DFT level of
theory.9 After geometry optimization (B3LYP10/6-311G*11) the
energy levels of the frontier orbitals were examined revealing
that isomer 2a, 4H-dithieno[2,3-b:30,20-e][1,4]thiazine, possesses
ꢀ4.99 eV, the highest HOMO energy (Fig. 2).
Surprisingly, N-substituted derivatives of dithienothiazine 2a
have practically remained unexplored, although some compounds
were prepared by Ullmann N-arylation and subsequent sulfura-
tion.12 Inspired by Jørgensen’s phenothiazine synthesis13 and other
recently published syntheses of 6-membered aza-heterocycles14
based upon a sequential inter/intramolecular Pd-catalyzed
N-arylation in a one-pot fashion, starting from 3,30-dibromo-
2,20-dithienylsulfide15 (3) and primary amines 4 a large number
of N-substituted dithienothiazines 5 are generally accessible in
good yields (Scheme 1).16
a Lehrstuhl fur Organische Chemie, Institut fur Organische Chemie
und Makromolekulare Chemie,
¨
¨
Heinrich-Heine-Universitat Dusseldorf, Universitatsstraße 1,
¨
¨
¨
D-40225, Dusseldorf, Germany.
¨
E-mail: ThomasJJ.Mueller@uni-duesseldorf.de;
Fax: +49 0211 8114324; Tel: +49 0211 8112298
¨
b Lehrstuhl fur Material und Strukturforschung, Institut fur
Anorganische Chemie und Strukturchemie,
¨
Heinrich-Heine-Universitat Dusseldorf, Universitatsstraße 1,
¨
¨
D-40225, Dusseldorf, Germany
¨
¨
w Dedicated to Prof. Dr Herbert Mayr on the occasion of his 65th
birthday.
z Electronic supplementary information (ESI) available: Experimental
procedures, characterization, and selected spectra of compounds 5,
computational and structure analytical data. CCDC 873353 (5m). For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc32731g
In addition to unambiguous spectroscopic characterization
(by 1H and 13C NMR, mass spectrometry) the structure of
N-substituted dithienothiazines 5 was additionally corroborated
by an X-ray structure analysis of compound 5m (Fig. 3).17
c
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Table 1 Selected electronic properties of dithienothiazines 5
Absorption
Compound lmax [nm]
a
0/+1 b
E0
+1/+2 b
c
[V] E0
[V] KSEM
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
254, 318
242, 314
244, 318
247, 318
248, 319
248, 315
248, 316
249, 315
249, 318
249, 318
250, 328
253, 315
256, 325
0.31
0.37
0.36
0.37
0.39
0.42
0.45
0.46
0.43
0.44
0.47
0.49
0.54
1.03d
(0.02 ꢂ 1014
)
1.24
1.24
1.25
1.26
1.25
1.36
1.28
1.26
1.25
1.28
1.29
1.29
1.37d
1.22
1.25
5.57 ꢂ 1014
8.23 ꢂ 1014
8.23 ꢂ 1014
5.57 ꢂ 1014
1.17 ꢂ 1014
26.5 ꢂ 1014
7.91 ꢂ 1014
1.17 ꢂ 1014
0.54 ꢂ 1014
0.54 ꢂ 1014
0.36 ꢂ 1014
0.05 ꢂ 1014
(0.36 ꢂ 1014
26.5 ꢂ 1014
5.57 ꢂ 1014
Fig. 2 Computed energies [eV] of the frontier molecular orbitals of
phenothiazine 1 and the three dithienothiazines 2a–d.
249, 299, 391 0.57
248, 327
245, 328
)
0.31
0.38
a
Recorded in CH2Cl2, T = 293 K, maxima with highest extinction
b
coefficients are indicated in bold face. Recorded in CH2Cl2, T = 293 K,
0.1 M electrolyte [nBu4N][PF6], Pt working, Ag/AgCl–NaCl(sat.)
0/+1
reference calibrated vs. E0
= ꢀ95 mV for [FeCp*2]/[FeCp*2]+
,
(+1/+2)
(0/+1)
c
ꢀE0
and Pt counter electrodes. KSEM = 10[(E
)/0.059] (potential
0
d
differences are dimensionless). Recorded in anodic peak potential of
the irreversible second oxidation.
The electronic properties of the dithienothiazines 5a–n were
studied experimentally by absorption and emission spectro-
scopy as well as by cyclic voltammetry (Table 1). The absorp-
tion characteristics of the dithienothiazines 5 are very similar
to those of the corresponding phenothiazines19 and in most
cases a strong absorption band appears at around 250 nm and
the second band with a lower extinction coefficient is found at
around 320 nm (with the exception of the strongly electron
withdrawing substituted derivatives 5k, 5m, and 5n).
Scheme 1 Synthesis of 4H-dithieno[2,3-b:30,20-e][1,4]thiazines 5 by
inter/intramolecular Buchwald–Hartwig coupling. aCs2CO3 (3 equiv.)
b
was used as a base. Dielectric heating at 160 1C for 2 h.
Interestingly and in contrast to the analogous N-aryl-substituted
phenothiazines,20 the dithienothiazines 5 do not display any
detectable emission. This peculiar behavior can be simply
rationalized by the higher content of sulfur and the resulting
increased spin–orbit coupling.
Electrochemical data for the dithienothiazines 5 were
obtained by cyclic voltammetry. Nearly all dithienothiazines,
with the exception of 5a and 5n, show two clearly separated
reversible oxidations with Nernstian behavior (see ESIz).
In comparison to the analogous phenothiazines the first oxida-
tion potentials are considerably shifted cathodically. For instance
the first oxidation potential of N-phenyl-phenothiazine21 appears at
E00/+1 =717 mV, whereas the first half-wave potential of N-phenyl
dithienothiazine 5e is found at E00/+1 = 393 mV. In addition, the
large differences between first and second oxidations indicate
substantial stabilities of the electrochemically generated radical
cations 5+ꢁ, which are additionally quantified by large semiquinone
Fig. 3 ORTEP plot of the molecular structure of compound 5m
(rotational ellipsoids at the 50% probability level).
22
As for phenothiazines18 also the N-substituted dithienothia-
zine 5m displays a butterfly conformation and the folding
angle y of the two thienyl planes was determined to be 132.61.
This is in good agreement with the computed folding angle
y = 140.41 (by geometry optimization at the DFT level
of theory9–11). Both the X-ray structure analysis and the
geometry optimized computation reproduce this structural
finding. Therefore, a crystal packing effect can be excluded
and an electronic effect can be plausibly assumed.
formation constants KSEM for the comproportionation of
5 and 52+ to give two 5+ꢁ. In comparison to N-phenyl-
phenothiazine (KSEM = 0.36 ꢂ 1014) the corresponding
N-phenyl-dithienothiazine 5e (KSEM = 5.57 ꢂ 1014) is by
more than one order of magnitude more stable.
Interestingly, the oxidation potentials in the series of
4-N-arylsubstituted dithienothiazines 5a–5f and 5i–5n vary
by the electronic nature of the remote para-substituent. This
strong dependence of the reversible first oxidation potential on
c
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0/+1
Fig. 4 Linear correlation plot of the first oxidation potentials E0
[mV] of N-aryl-dithienothiazines 5 against the Hammett parameters sp
0/+1
(E0
= 401 + 179.5sp [mV]; r2 = 0.9685).
the electronic substituent effect is shown by a reasonably good
linear correlation (r2 = 0.9685) with the Hammett substitution
23
parametersꢁ sp (Fig. 4). Therefore, the stability of the radical
cations 5+ obtained by oxidation is sustained by remote sub-
stituents operating through both inductive and resonance effects.
In conclusion, we have disclosed a straightforward and efficient
access to the practically unknown dithieno[b,e][1,4]thiazines by
applying sequential twofold Pd-catalyzed C–N coupling. The
electronic properties were investigated by absorption spectroscopy
and cyclic voltammetry. Interestingly, the oxidation potential is
considerably lower in comparison to the analogous phenothiazine
compound; however, in contrast to phenothiazines dithienothia-
zines are not fluorescent. Further studies directed towards the
functionalization of dithienothiazines as novel donor entities for
functional p-systems are currently underway.
13 T. Dahl, C. W. Tornøe, B. Bang-Andersen, P. Nielsen and
M. Jørgensen, Angew. Chem., Int. Ed., 2008, 47, 1726.
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16 Synthesis of 5b. In a screw-cap Schlenk vessel 3 mL of dry toluene
were placed under a nitrogen atmosphere. Then, 178 mg (0.5 mmol)
of 3,30-dibromo-2,20-dithienylsulfide (3),14 71 mg (0.58 mmol) of
p-anisidine, 22 mg (7.5 mol%) of bis(dibenzylideneacetone)palladium,
42 mg (15 mol%) of 1,10-bis(diphenylphosphino)ferrocene, and
144 mg (1.50 mmol) of sodium tert-butoxide were added to the
solvent. The mixture was stirred in a preheated oil bath at 100 1C for
30 h. After workup and purification by chromatography on silica
gel (hexane with 2.5% triethylamine; Rf = 0.09) analytically
pure 4-(4-methoxyphenyl)-4H-dithieno[2,3-b:30,20-e][1,4]thiazine (5b)
(149 mg, 94%) was obtained as yellow crystals. 1H NMR (500 MHz,
The support of this work from the Fonds der Chemischen
Industrie is gratefully acknowledged.
Notes and references
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3
acetone-d6): d = 3.87 (s, 3H), 6.07 (d, J = 5.5 Hz, 2H), 7.12–7.08
(m, 2H), 7.18 (d, 3J = 5.5 Hz, 2H), 7.35–7.30 (m, 2H). 13C NMR
(125 MHz, acetone-d6): d = 55.9 (CH3), 102.0 (Cquat), 116.3 (CH),
120.4 (CH), 124.6 (CH), 131.1 (CH), 137.3 (Cquat), 145.5 (Cquat), 160.2
(Cquat). EI + MS (70 eV, m/z (%)): 319 ([C15H11ON34S32S2]+, 14),
318 (19), 317 ([C15H11ON32S3]+, 100), 285 (12), 284 (31), 210
([C8H4N32S3]+, 35), 134 ([C7H4N32S]+, 12). UV/Vis (CH2Cl2): lmax
(e) 242 nm (19 000), 317 (4400). Anal. calcd for C15H11ONS3 (317.5):
C 56.75, H 3.49, N 4.41. Found: C 56.75, H 3.62, N 4.27%.
17 CCDC 873353 (5m)z.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.