Synthesis and optical properties of novel unsymmetrically substituted benzothiadiazole-based luminophores

Article abstract of DOI:10.1016/j.mencom.2021.01.009

New unsymmetrically substituted benzothiadiazoles were synthesized from 4,7-dibromo-2,1,3-benzothiadiazole via the sequence of Pd-catalyzed Suzuki and Buchwald–Hartwig cross-coupling reactions with 4-methoxyphenylboronic acid and heterocyclic amines, respectively. Based on initially performed photophysical study as well as DFT calculation, these compounds, in particular with dibenzoazepine core, can be selected as promising scaffolds for further fine-tuning of their properties to be used in optoelectronics including OLED technologies.

Full text of DOI:10.1016/j.mencom.2021.01.009

Mendeleev
Communications
Mendeleev Commun., 2021, 31, 33–35
Synthesis and optical properties of novel unsymmetrically
substituted benzothiadiazole-based luminophores
Pavel S. Gribanov,a Dmitry A. Lypenko,b Artem V. Dmitriev,b Sergey I. Pozin,b
Maxim A. Topchiy,c Andrey F. Asachenko,c Dmitry A. Loginova,d and Sergey N. Osipov*a,e
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. E-mail: osipov@ineos.ac.ru
b A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
199071 Moscow, Russian Federation
c A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Moscow,
Russian Federation
d G. V. Plekhanov Russian University of Economics, 117997 Moscow, Russian Federation
e Moscow Region State University, 141014 Mytishchi, Moscow Region, Russian Federation
DOI: 10.1016/j.mencom.2021.01.009
OMe
New unsymmetrically substituted benzothiadiazoles were
synthesized from 4,7-dibromo-2,1,3-benzothiadiazole via the
sequence of Pd-catalyzed Suzuki and Buchwald–Hartwig
cross-coupling reactions with 4-methoxyphenylboronic acid
and heterocyclic amines, respectively. Based on initially
performed photophysical study as well as DFT calculation,
these compounds, in particular with dibenzoazepine core,
can be selected as promising scaffolds for further fine-tuning
of their properties to be used in optoelectronics including
OLED technologies.
R
R
N
N
N
,
S
R'
S
S
R
R
X = O, S
R' = H, CF3
N
,
N
N
,
X
N
N
=
Keywords: benzothiadiazoles, luminophores, cross-coupling, synthesis, photophysical properties.
R
R
Duringthepastdecade,4,7-disubstituted2,1,3-benzothiadiazole
N
Br
(BTD) derivatives with extended p-conjugation owing to their
N
S
N
N
N
outstanding electron affinity,1 carrier transport abilities,
Br
S
thermal and chemical stabilities have been extensively studied
N
i
ii
R2N–H
as privileged scaffolds for the construction of different opto-
electronic devices, e.g., organic light emitting diodes (OLEDs),2
field effect transistors (OFETs),3 solar cells (OSCs),4
bioprobes,5 and chemsensors.6 Despite prominent advances
highlighting the great potential of these compounds, particularly
attention is focused on direct substitution at the 4,7-positions
with amino groups to afford unsymmetrical BTDs of donor–
acceptor type. The most promising examples of such molecules
were designed for dye-sensitized solar cells with an apparent
increase in the power conversion efficiency7 and for thermally
activated delayed fluorescence (TADF) emitters with high
external quantum efficiency.8 Taking into account the fact that
electronic and steric nature of substituents at positions 4 or/and
7 can dramatically affect performance and photophysical
properties of the target molecules, herein we report on the
synthesis of a series of novel unequally substituted BTD
derivatives with phenoxazine, phenothiazine, dithienopyrrole
and dibenzazepine as electron donor units as well as our initial
investigation on their photophysical characteristics to estimate
a potential application of these compounds in materials science.
A synthetic route from commercially available 4,7-dibromo-
2,1,3-benzothiadiazole 1 (Scheme 1) to the amino BTDs 3a–f
included a sequence of two Pd-catalyzed cross-coupling
reactions. The first step, Suzuki mono-coupling of dibromide 1
with 4-metoxyphenylboronic acid, was accomplished by slightly
S
N
Br
OMe
OMe
2, 51%
1
3a–f, 50–85%
S
S
O
N
S
a R2N =
b R2N =
c R2N =
d R2N =
N
e R2N =
f R2N =
N
N
S
N
N
CF3
Scheme 1 Reagents and conditions: i, 4-MeOC6H4B(OH)2 (1 equiv.),
NaHCO3 (3 equiv.), 1,4-dioxane/water (3:1), Pd(PPh3)2Cl2 (1 mol%),
reflux, 24 h; ii, R2N–H (1.05 equiv.), Pd(OAc)2 (5 mol%), RuPhos
(10 mol%), ButONa (1.5 equiv.), 1,4-dioxane, 100°C, 24 h.
modified procedure previously described for selective formation
of monobromide 29 under catalysis with significantly reduced
© 2021 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 33 –

Mendeleev Commun., 2021, 31, 33–35
Table 1 Photophysical properties of compounds 3a–f.
Table 2 General analysis of the absorption bands by TD DFT calculations
at the B3LYP/DZP level.
Compound
lambasx /nm
lPmLax /nm
jPL (%)
Experimental Calculated Calculated
Com-
pound
Main MOs responsible
for excitationa
3a
3b
3c
3d
3e
3f
383
384
385
437
455
467
480
540
530
542
587
591
0.2
absorption
absorption oscillator
0.1
maxima/nm maxima/nm strengths
0.5–1
2–3
3a
3b
3c
3d
383
384
385
437
413
665
408
571
413
549
399
479
528
403
446
334
450
0.179106 HOMO–1 ® LUMO (99%)
0.000579 HOMO ® LUMO (99%)
0.185355 HOMO–1 ® LUMO (99%)
0.004237 HOMO ® LUMO (99%)
0.191621 HOMO–1 ® LUMO (99%)
0.007352 HOMO ® LUMO (99%)
0.070450 HOMO–2 ® LUMO (99%)
0.141245 HOMO–1 ® LUMO (99%)
0.000678 HOMO ® LUMO (100%)
0.003554 HOMO ® LUMO+1 (98%)
0.15404 HOMO ® LUMO (98%)
0.004909 HOMO–1 ® LUMO (99%)
0.16546 HOMO ® LUMO (98%)
26–33
32–40
amount of Pd-complex. For the next Buchwald–Hartwig
coupling of compound 2 with selected cyclic amines, the
combination of 5 mol% Pd(OAc)2, 10 mol% RuPhos (2-dicyclo-
hexylphosphino-2',6'-diisopropoxybiphenyl) and 1.5 equiv. ButONa
has proved to be the most effective catalytic system among
tested.
All compounds 3a–f were isolated by flash chromatography
on silica gel and additionally purified by sublimation
(200–220°C/0.1 Torr). Their structures were proved by 1H NMR,
13C NMR and HRMS (see Online Supplementary Materials).
Photoluminescent properties of compounds 3a–f are
summarized in Table 1. The optical spectra were recorded for
nondeaerated 0.2–20 μm solutions in toluene at room temperature
(see Online Supplementary Materials, Figures S1–S6). The
fluorescence quantum yield was estimated by Parker–Rees
method. Rhodamine B in ethanol solution (Q = 65–70%) was
chosen as a standard.10
All compounds exhibit intense absorption bands at
383–467 nm and fluorescence emission in the green-yellow
region (480–591 nm). Figure 1 shows the absorption and
emission spectra for compound 3f. Surprisingly, the phenoxazine,
phenothiazine and dithienopyrrole derivatives 3a–d showed very
low quantum yields (0.1–3%). In contrast, the benzoazepine
compounds 3e,f have rather high fluorescence efficiency, with
the best quantum yield being 40% for 3f.
To make spectral assignments, we calculated the electronic
absorption spectra and the frontier molecular orbitals using
the TD-DFT method at the B3LYP/DZP level. Earlier we
showed that B3LYP is the best exchange correlation functional
for estimation of the differences in energy between the
frontier orbitals of organic dyes.11 The calculated absorption
bands for all studied compounds are only 10–40 nm differ
relative to experimental values (Table 2). In the case of the
dibenzoazepine (3e) and dihydrodibenzoazepine (3f)
derivatives, the most intense absorption band corresponds to
the pꢀ® p* transition, which is formed by HOMO ® LUMO
orbitals. Both these frontier orbitals are mainly located at the
benzothiadiazole moiety (see Online Supplementary
Materials, Figure S7). In contrast, in the case of phenoxazine
(3a) and phenothiazine (3b,c) compounds, HOMO is located
at the phenoxazine and phenothiazine fragments, while
3e
3f
455
467
aValues in parentheses give the percentage contribution of the corresponding
orbitals to the total transition.
LUMO remains at the benzothiadiazole. However, the band
corresponding to this charge transfer transition has very low
intensity because of the absence of conjugation between
cyclic moieties in these compounds (for example, the dihedral
angle phenoxazine/benzothiadiazole in molecule of 3a is
86.4°). Therefore, for compounds 3a,b the most intense
absorption band is provided by HOMO−1 ® LUMO transition,
where the HOMO−1 orbital is mainly located at
benzothiadiazole and 4-methoxyphenyl substituent. The
introduction of the CF3 group into the phenothiazine moiety
leads only to a slight bathochromic shift of the most intense
band and the increase of the HOMO–LUMO gap by 0.08 eV.
Dithienopyrrole derivative 3d exhibits closely related
photophysical behavior involving HOMO−1 and HOMO−2 to
the main transitions instead of HOMO.
Noteworthy, according to DFT calculations, the heterocyclic
moieties in all studied compounds are not coplanar that prevents
its conjugation. It can be explained by the sterical hindrance
between these moieties due to its bulkiness and close proximity.
We propose that the addition of acetylene or phenyl spacer
between them should prominently improve photophysical
properties of the final compounds.
In conclusion, an efficient scheme for the preparation of a
series of new unsymmetrically substituted benzothiadiazoles has
been elaborated via sequence of two Pd-catalyzed cross coupling
Suzuki and Buchwald–Hartwig reactions. Based on our results,
these compounds, in particular with dibenzoazepine core, can be
selected as promising scaffolds for further fine-tuning of their
properties to be used in optoelectronics including OLED
technologies.
0.0ꢁ
This work was supported by the Russian Foundation for
Basic Research (grant no. 19-29-08038). NMR studies and
spectral characterization were performed with the financial
support from the Ministry of Science and Higher Education of
the Russian Federation using the equipment of the Center for
Molecular Composition Studies of INEOS RAS. Part of this
work was carried out by M. A. Topchiy and A. F. Asachenko
within the framework of the State Program of A. V. Topchiev
Institute of Petrochemical Synthesis of the Russian Academy
of Sciences.
100
ꢄ0
0.0ꢀ
ꢁ0
ꢀ0
0.02
20
0
ꢅ00
0
ꢀ00
ꢆ00
lꢈnm
ꢁ00
ꢇ00
ꢄ00
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2021.01.009.
Figure 1 The absorption and emission spectra of compound 3f in toluene
(C = 0.2×10–5 m, lex = 460 nm, 20°C).
– 34 –

Mendeleev Commun., 2021, 31, 33–35
6 J. Wu, G. Lai, Z. Li, Y. Lu, T. Leng, Y. Shen and C. Wang, Dyes Pigm.,
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