Conjugated push-pull salts derived from linear benzobisthiazole: Preparation and optical properties

Article abstract of DOI:10.2478/s11696-012-0251-2

A series of novel monomethylated salts derived from linear benzobisthiazole was prepared. The push-pull attributes of these new compounds are represented by a quaternised azolium cycle as the acceptor part at one end of the structure and the dialkylamino- or diarylamino-substituted benzene ring as the donor part at the opposite end. Both moieties are connected by a conjugated linker consisting of one or two double bonds. Such dipolar structures are promising candidates for non-linear optical materials. The quantum-chemical indices describing linear and non-linear optical properties were obtained from semi-empirical calculations. The relationships between the chemical structure and non-linear optical properties of the cations studied were obtained. Effective conjugation was confirmed by measuring the optical properties in the UV-VIS region.

Full text of DOI:10.2478/s11696-012-0251-2

Chemical Papers 67 (1) 110–116 (2013)
DOI: 10.2478/s11696-012-0251-2
ORIGINAL PAPER
Conjugated push–pull salts derived from linear benzobisthiazole:
preparation and optical properties
Alexandra Čibová, Peter Magdolen*, Andrea Fu¨lo¨pová, Pavol Zahradník
Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia
Received 26 April 2012; Revised 13 July 2012; Accepted 18 July 2012
Dedicated to Professor Štefan Toma on the occasion of his 75th birthday
A series of novel monomethylated salts derived from linear benzobisthiazole was prepared. The
push–pull attributes of these new compounds are represented by a quaternised azolium cycle as
the acceptor part at one end of the structure and the dialkylamino- or diarylamino-substituted
benzene ring as the donor part at the opposite end. Both moieties are connected by a conjugated
linker consisting of one or two double bonds. Such dipolar structures are promising candidates
for non-linear optical materials. The quantum-chemical indices describing linear and non-linear
optical properties were obtained from semi-empirical calculations. The relationships between the
chemical structure and non-linear optical properties of the cations studied were obtained. Effective
conjugation was confirmed by measuring the optical properties in the UV-VIS region.
c
ꢀ 2012 Institute of Chemistry, Slovak Academy of Sciences
Keywords: heterocycles, thiazolium salts, conjugation, non-linear optics, condensation
Introduction
are known and in commercial use, mainly as cya-
nine dyes (Thioflavine T, Thiazole Orange, Basic
Blue 41) (Mojzych & Henary, 2008). Some D-π-
A benzothiazolium salts show antibacterial and an-
thelmintic effects (dithiazanine). The NLO properties
of benzothiazolium salts, especially the unique two-
photon absorption parameters, were demonstrated
(Hrobáriková et al., 2010). In an effort to enhance
NLO response, the benzothiazole part of the molecule
was replaced with a more robust linear benzobisthia-
zole backbone which can occur in two isomers with
axial or central symmetry. This structure renders
it possible to prepare one- or two-armed condensa-
tion products. A further possibility for enhancing the
acceptor capacity of the heterocyclic unit is alky-
lation of one or both of the heterocyclic nitrogen
atoms. In this article, we focus on the synthesis and
study of monomethylated salts derived from linear
centrosymmetric benzobisthiazole and its one-armed
condensation products with donor-substituted aro-
matic aldehydes. The electron donor part of the de-
Organic heterocyclic compounds with a push–pull
structure are noted for their wide range of applica-
tions as chromophores in non-linear optics (NLO),
in the second harmonic generation (SHG) (Zhang
et al., 2001), molecular probes (Fox, 1992), fluores-
cent markers (Lakowicz, 1994), organic light-emitting
diodes (OLED) (Balaganesan et al., 2003), or pho-
tovoltaic cells (Loudet & Burgess, 2007). A typical
push–pull organic chromophore consists of a polar
A-π-D structure with a planar π-system end-capped
with a strong electron donor (D) and a strong elec-
tron acceptor (A). These systems use the follow-
ing conventional substituents: dialkylamino or diary-
lamino groups as donors and nitro, cyano, or car-
bonyl groups as acceptors. The special set of D-
π-A molecules comprises heteroaromatic cations as
the acceptor part; the heterocycles in most use are
pyridinium, tetrazolium, acridinium, and benzothia-
zolium fragments. Conjugated benzothiazolium salts
*Corresponding author, e-mail: magdolen@fns.uniba.sk

A. Čibová et al./Chemical Papers 67 (1) 110–116 (2013)
111
signed dipolar structures is represented by a dimethy-
laminophenyl fragment by default. Here, a varia-
tion in substituents renders it possible to modify
other physicochemical properties of the target com-
pounds, e.g. solubility in different solvents. In or-
der to enlarge the second order hyperpolarisabil-
ity we proposed exchanging the dialkylamino group
for the diphenylamino group and, in addition, for
N-carbazolyl residue with a stable conformation of
the benzene rings. The planar aromatic structures
are known for their poor solubility in common sol-
vents. Designed compounds with additional hydroxyl
or methoxy groups as well as the N-methylpiperazinyl
group with non-planar conformation are characteris-
tic of improved solubilities in solvents like methanol
and dimethyl sulphoxide (DMSO). The connection be-
tween the donor and acceptor parts ensured by one or
two double bonds originated in the condensation re-
actions.
UUA Ultragen (20 kHz, 300 W) GENTECH ultra-
sonic submersible generator. For reactions performed
under microwave irradiation, the InitiatorTM BIO-
TAGE (max. power 300 W) reactor was used. Melt-
ing points were measured on an Electrothermal IA-
9200 Kofler apparatus and were not corrected. 1H
(300 MHz) and 13C (75 MHz) NMR spectra were
recorded on a Varian Gemini 2000 spectrometer. IR
spectra were recorded on a Thermo Scientific Nicolet
iS10 spectrometer (Smart iTR diamond ATR). HRMS
were recorded on a Shimadzu LC-IT-TOF MS instru-
ment using both ESI positive and ESI negative modes.
Electronic absorption spectra were obtained on a Jen-
way 6705 UV-VIS spectrophotometer. Chromatogra-
phy was performed using Swambe Chemicals silica gel
60 A flash (230–400 mesh) or neutral aluminium ox-
ide.
Preparation of 2,6-dimethylbenzo[1,2-d:4,5-d ꢀ]
bisthiazole (I)
Computational details
Triethylamine (3.58 mL, 25.7 mmol) was added
dropwise to a stirred suspension of 2,5-diaminobenz-
ene-1,4-dithiol dihydrochloride (3.0 g, 12.2 mmol) in
dry 1,4-dioxane (100 mL) in a three-necked flask
(equipped with thermometer, condenser, and drop-
ping funnel) in an argon atmosphere below 35◦C.
Acetic anhydride (2.88 mL, 30.5 mol) was then added
dropwise and the reaction mixture was heated to re-
flux for 3 h. After cooling to ambient temperature, the
mixture was carefully neutralised with a 20 % aqueous
NaOH solution. The precipitate was collected by fil-
tration, then dried and purified by column chromatog-
raphy on silica gel using hexane/ethyl acetate (ϕr =
1 : 1) as the eluent to give pure product (53 %) as
a white solid; m.p. 229–230◦C (Mike et al. (2010) re-
The restricted Hartree–Fock method was chosen
for the quantum-chemical calculations. The optimised
geometries for each compound in vacuo were calcu-
lated in the Turbomole V6.2 program (Turbomole,
2010), using the DFT method (Treutler & Ahlrichs,
1995) and the B3LYP exchange-correlation functional
(Becke, 1993; Lee et al., 1988). On all atoms, the
TZVP basis set (Schäfer et al., 1994) was employed.
DFT total energy as well as orbital energies were ob-
tained in this way.
The DFT geometry was used as input for calcula-
tion of all the required linear and non-linear quanti-
ties. The final characteristics (polarisability α, first
and second hyperpolarisabilities β and γ were ob-
tained using the semi-empirical PM3 method (Stew-
art, 1989a, 1989b, 1991). A finite-field method for cal-
culation of the static first and second hyperpolarisabil-
ity developed by Kurtz et al. (1990) was applied. This
procedure, as well as the semi-empirical method used,
is implemented in the AMPAC molecular modelling
package (Semichem, 2004).
1
ported 232–233◦C). H NMR (300 MHz, CDCl3), δ:
8.36 (s, 2H, HAr), 2.88 (s, 6H, 2 × CH3); 1H NMR
(300 MHz, DMSO-d6), δ: 8.58 (s, 2H, HAr), 2.83 (s,
6H, 2 × CH3).
2,3,6-Trimethylbenzo[1,2-d:4,5-d ꢀ]bisthiazol-
3-ium iodide (II)
Experimental
Iodomethane (2.50 g, 17.6 mmol) was added to
2,6-dimethylbenzo[1,2-d:4,5-dꢀ]bisthiazole (I ) (0.775 g,
3.52 mmol) dissolved in methanol (20 mL) in the
glass vessel from the microwave reactor and the re-
action mixture was exposed to microwave irradiation
at 115◦C for 50 min. After cooling to ambient tem-
perature, the precipitated product was filtered and
washed with diethyl ether. An additional portion was
obtained by concentrating the filtrate at reduced pres-
sure. The crude product contaminated by 5–10 mass %
of diiodide was purified by crystallisation from ethanol
to give 0.583 g (46 %) of white solid; m.p. 262–
264◦C (Lochon et al. (1967) reported 264◦C); 1H NMR
(300 MHz, DMSO-d6), δ: 9.07 (s, 1H, H-4), 8.96 (s, 1H,
Solvents were purified and dried using standard
methods. Commercially unavailable substituted ben-
zaldehydes were synthesised according to previously
published methods: 4-diphenylaminobenzaldehyde
(IIIb) from triphenylamine (Hrobáriková et al., 2010),
4-(4-methylpiperazin-1-yl)benzaldehyde (IIId) from
4-fluorobenzaldehyde (Magdolen et al., 2001), and 4-
(bis-(2-hydroxyethyl)aminobenzaldehyde (IIIe) from
the corresponding 4-bis-(2-acetoxyethyl) derivative
(Le Bouder et al., 1998).
The reactions where the substrate was treated un-
der ultrasound conditions were performed using a

112
A. Čibová et al./Chemical Papers 67 (1) 110–116 (2013)
Table 1. Characterisation data of newly prepared benzobisthiazolium salts
wi(calc.)/%
wi(found)/%
Yield
M.p.
Compound
Formula
Mr
C
H
N
S
%
70
76
52
78
49
47
89
◦C
IVa
IVb
IVc
IVd
IVe
IVf
V
C20H20IN3S2
C30H24IN3S2
C30H22IN3S2
C23H25IN4S2
C22H24IN3O2S2
C24H28IN3O2S2
C22H22IN3S2
493.43
617.57
615.55
548.51
553.48
581.53
519.46
48.68
48.34
58.35
58.79
58.54
58.51
50.36
50.40
47.74
47.54
49.57
49.29
50.87
50.85
4.09
4.09
3.92
3.92
3.60
3.76
4.59
4.67
4.37
4.24
4.85
4.89
4.27
4.20
8.52
8.79
6.80
6.59
6.83
7.06
10.21
9.95
7.59
7.35
7.23
7.03
8.09
8.09
13.00
13.10
10.38
10.34
10.42
10.63
11.69
11.94
11.59
10.95
11.03
10.83
12.35
12.71
279–281
228–230
194–196
238–241
264–267
215–218
277–280
1
H-8), 4.22 (s, 3H, CH3N+), 3.20 (s, 3H, CH3 at C6),
2.92 (s, 3H, CH3 at C2).
158◦C); H NMR (300 MHz, CDCl3), δ: 10.12 (s, 1H,
CHO), 8.15 (d, J = 7.5 Hz, 2H, H-1ꢀ, H-8ꢀ), 8.13 (d,
J = 9.0 Hz, 2H, H-2, H-6), 7.80 (d, J = 9.0 Hz, 2H,
H-3, H-5), 7.51 (d, J = 8.1 Hz, 2H, H-4ꢀ, H-5ꢀ), 7.44
(td, J = 7.0 Hz, J = 1.2 Hz, 2H, H-3ꢀ, H-6ꢀ), 7.33 (td,
2H, H-2ꢀ, H-7ꢀ).
General procedure for preparation
of benzobisthiazol-3-ium salts IVa–IVf and V
The aldehyde III (0.525 mmol, 1.05 eq) was
added to a suspension of 2,3,6-trimethylbenzo[1,2-
Preparation of 4-(bis-(2-methoxyethyl)amino)
d:4,5-dꢀ]bisthiazol-3-ium iodide (II) (0.181 g, 0.5 mmol) benzaldehyde (IIIf)
in methanol (20 mL) in the glass vessel from the mi-
crowave reactor and the mixture was exposed to irra-
diation at 100◦C for 20 min. The course of the reaction
was monitored by TLC. After cooling, the precipitated
product was filtered, washed with a small portion of
cold methanol, diethylether, and finally dried. In the
case of IVc, the reaction time was prolonged to 5 h,
the temperature increased to 115◦C, and two drops
of pyridine were added to the reaction mixture. The
yields and spectral characteristics of the salts obtained
are summarised in Tables 1 and 2.
Caesium carbonate (1.45 g, 4.5 mmol) was added
solution of 4-fluorobenzaldehyde (1.22 g,
to
a
10.0 mmol) and bis(2-methoxyethyl)amine (1.33 g,
10.0 mmol) in DMSO (10 mL) in the glass vessel from
the microwave reactor and the mixture was exposed to
irradiation at 160◦C for 1 h. The cooled reaction mix-
ture was poured into distilled water (200 mL) and ex-
tracted with diethyl ether (3 × 50 mL). The combined
ether layers were washed with water and brine, dried
with Na2SO4, and concentrated at reduced pressure to
give the crude product (oil) which was purified by col-
umn chromatography on silica gel using hexane/ethyl
acetate (ϕr = 2 : 1) as the eluent to give colourless oil
Preparation of 4-(carbazol-9-yl)benzaldehyde
(IIIc)
1
(0.12 g, 5 %); H NMR (300 MHz, CDCl3), δ: 9.73 (s,
Potassium carbonate (2.90 g, 21.0 mmol) was
added to the solution of carbazole (3.34 g, 20.0 mmol)
and 4-fluorobenzaldehyde (4.93 g, 40 mmol) in DMSO
(30 mL). The reaction mixture was exposed to ultra-
sonic irradiation generated by an ultrasonic horn in an
open flask for 30 min. The temperature of the reaction
mixture attained 140◦C at the end of sonication. After
cooling to 60◦C, the reaction mixture was poured into
distilled water (400 mL), the precipitate was filtered,
washed with water, and dried. The crude product was
purified by crystallisation from cyclohexane (starting
carbazole is insoluble) to give pure IIIc (3.18 g, 59 %);
m.p. 153–155◦C (Zhang et al. (2001) reported 156–
1H, CHO), 7.71 (d, J = 9.0 Hz, 2H, H-2, H-6), 6.74
(d, J = 9.0 Hz, 2H, H-3, H-5), 3.65 (t, J = 5.3 Hz, 4H,
2 × CH2O), 3.58 (t, J = 5.3 Hz, 4H, 2 × CH2N), 3.35
(s, 6H, 2 × CH3O).
Results and discussion
The condensation reaction between 2,5-diamino-
benzene-1,4-dithiol and acetic anhydride was cho-
sen for synthesis of the linear benzobisthiazole skele-
ton. The older method (Finzi & Grandolini, 1959),
originally employed for synthesis of isomeric 2,6-
dimethylbenzo[1,2-d:5,4-dꢀ]bisthiazole, was improved

A. Čibová et al./Chemical Papers 67 (1) 110–116 (2013)
Table 2. Spectral data of newly prepared compounds
113
Compound
Spectral dataa
IVa
IR, ν˜/cm−1: 3065, 2913, 1600, 1568, 1435, 1381, 1263, 1160
1H NMR (DMSO-d6), δ: 2.90 (s, 3H, CH3), 3.13 (s, 6H, (CH3)2N), 4.24 (s, 3H, CH3N+), 6.87 (d, 2H, J = 9.1 Hz,
—
—
HAr), 7.66 (d, 1H, J = 15.2 Hz, CH ), 7.94 (d, 2H, J = 9.1 Hz, H ), 8.11 (d, 1H, J = 15.2 Hz, CH ), 8.84 (s,
—
—
Ar
2H, HAr
)
13C NMR (DMSO-d6), δ: 20.1 (CH3), 35.6 (CH3), 48.5 (CH3), 106.1 (CH), 109.0 (CH), 111.9 (2 × CH), 116.3 (CH),
121.4 (C), 125.1 (C), 133.0 (2 × CH), 136.5 (C), 139.3 (C), 150.5 (CH), 151.6 (C), 153.5 (C), 170.6 (C), 171.5 (C)
IVb
IVc
IVd
IVe
IVf
V
IR, ν˜/cm−1: 3032, 1568, 1505, 1483, 1428, 1326, 1260, 1166
1H NMR (DMSO-d6), δ: 2.91 (s, 3H, CH3), 4.30 (s, 3H, CH3N+), 6.92 (d, 2H, J = 8.9 Hz, HAr), 7.20–7.28 (m, 6H,
—
HAr), 7.45 (t, 4H, J = 7.6 Hz, HAr), 7.80 (d, 1H, J = 15.5 Hz, CH ), 7.93 (d, 2H, J = 8.9 Hz, HAr), 8.17 (d, 1H,
—
—
J = 15.5 Hz, CH ), 8.91 (s, 1H, HAr), 8.95 (s, 1H, HAr
)
—
13C NMR (DMSO-d6), δ: 20.2 (CH3), 36.1 (CH3), 109.8 (CH), 110.0 (CH), 116.5 (CH), 118.8 (2 × CH), 125.5
(2 × CH), 125.6 (C), 126.0 (C), 126.1 (4 × CH), 130.0 (4 × CH), 131.9 (2 × CH), 136.9 (C), 139.2 (C), 145.3
(2 × C), 149.0 (C), 151.5 (CH), 152.0 (C), 171.2 (C), 172.0 (C)
IR, ν˜/cm−1: 3046, 1515, 1444, 1398, 1353, 1333, 1265, 1165
1H NMR (DMSO-d6), δ: 2.94 (s, 3H, CH3), 4.45 (s, 3H, CH3N+), 7.29 (d, 2H, J = 8.6 Hz, HAr), 7.33–7.38 (m, 2H,
—
HAr), 7.47–7.52 (m, 2H, HAr), 7.55 (d, 2H, J = 7.7 Hz, HAr), 8.21 (d, 1H, J = 15.7 Hz, CH ), 8.29 (d, 2H, J =
—
—
—
7.7 Hz, HAr), 8.40 (d, 2H, J = 8.6 Hz, HAr), 8.43 (d, 1H, J = 15.7 Hz, CH ), 9.02 (s, 1H, HAr), 9.08 (s, 1H, HAr
)
13C NMR (DMSO-d6), δ: 20.3 (CH3), 36.7 (CH3), 109.8 (2 × CH), 110.4 (CH), 114.6 (CH), 116.7 (CH), 120.6
(4 × CH), 123.1 (2 × CH), 126.1 (C), 126.4 (2 × CH), 126.7 (2 × CH), 132.7 (2 × CH), 132.7 (C), 137.3 (C), 139.3
(C), 139.4 (2 × C), 140.2 (C), 147.8 (C), 152.3 (CH), 171.8 (C), 172.3 (C)
IR, ν˜/cm−1: 3061, 1575, 1518, 1436, 1233, 1184, 1140
1H NMR (DMSO-d6), δ: 2.25 (s, 3H, CH3N), 2.46 (s, 4H, 2 × CH2), 2.90 (s, 3H, CH3), 3.48 (s, 4H, 2 × CH2), 4.27
(s, 3H, CH3N+), 7.10 (d, 2H, J = 9.1 Hz, HAr), 7.74 (d, 1H, J = 15.4 Hz, CH ), 7.95 (d, 2H, J = 9.1 Hz, HAr),
—
—
—
8.12 (d, 1H, J = 15.4 Hz, CH ), 8.88 (s, 1H, HAr), 8.89 (s, 1H, HAr
)
—
13C NMR (DMSO-d6), δ: 20.2 (CH3), 35.9 (CH3), 45.4 (CH3), 46.0 (2 × CH2), 54.0 (2 × CH2), 107.7 (CH), 109.3
(CH), 113.6 (2 × CH), 116.4 (CH), 123.0 (C), 125.3 (C), 132.6 (2 × CH), 136.7 (C), 139.2 (C), 149.3 (CH), 151.7
(C), 153.6 (C), 170.8 (C), 171.8 (C)
IR, ν˜/cm−1: 1567, 1517, 1489, 1436, 1395, 1336, 1257, 1165
1H NMR (DMSO-d6), δ: 2.90 (s, 3H, CH3), 3.62 (bs, 8H, 2 × NCH2CH2O), 4.23 (s, 3H, CH3N+), 4.87 (bs, 2H,
—
OH), 6.91 (d, 2H, J = 8.9 Hz, HAr), 7.61 (d, 1H, J = 15.1 Hz, CH ), 7.89 (d, 2H, J = 8.9 Hz, HAr), 8.07 (d, 1H,
—
—
J = 15.1 Hz, CH ), 8.83 (s, 2H, H
)
—
Ar
13C NMR (DMSO-d6), δ: 20.2 (CH3), 35.7 (CH3), 53.1 (2 × CH2), 58.2 (2 × CH2), 105.9 (CH), 109.1 (CH), 112.1
(2 × CH), 116.3 (CH), 121.4 (C), 125.1 (C), 133.1 (2 × CH), 136.6 (C), 139.3 (C), 150.4 (C), 151.6 (CH), 152.7
(C), 170.6 (C), 171.4 (C)
IR, ν˜/cm−1: 2873, 1568, 1515, 1397, 1338, 1261, 1168, 1100
1H NMR (DMSO-d6), δ: 2.90 (s, 3H, CH3), 3.28 (s, 6H, 2 × OCH3), 3.55 (t, 4H, J = 5.7 Hz, 2 × NCH2), 3.71 (t,
4H, J = 5.7 Hz, 2 × OCH2), 4.24 (s, 3H, CH3N+), 6.92 (d, 2H, J = 9.2 Hz, HAr), 7.64 (d, 1H, J = 15.2 Hz, CH ),
—
—
—
7.90 (d, 2H, J = 9.2 Hz, HAr), 8.09 (d, 1H, J = 15.2 Hz, CH ), 8.84 (s, 1H, HAr), 8.85 (s, 1H, HAr
)
—
13C NMR (DMSO-d6), δ: 20.7 (CH3), 36.2 (CH3), 50.5 (2 × CH3), 58.8 (2 × CH2), 70.1 (2 × CH2), 106.9 (CH),
109.6 (CH), 112.7 (2 × CH), 116.8 (CH), 122.2 (C), 125.7 (C), 133.5 (C), 137.1 (2 × CH), 139.9 (C), 150.9 (C),
152.2 (CH), 152.9 (C), 171.2 (C), 172.1 (C)
IR, ν˜/cm−1: 2908, 1545, 1478, 1372, 1350, 1149, 1067, 996
1H NMR (DMSO-d6), δ: 2.90 (s, 3H, CH3), 3.05 (s, 6H, (CH3)2N), 4.17 (s, 3H, CH3N+), 6.80 (d, 2H, J = 9.0 Hz,
—
—
—
—
HAr), 7.21 (dd, 1H, J = 11.0 Hz, J = 14.4 Hz, CH ), 7.33 (d, 1H, J = 15.0 Hz, CH ), 7.50 (d, 1H, J = 15.0 Hz,
—
—
—
CH ), 7.56 (d, 2H, J = 9.0 Hz, H ), 8.03 (dd, 1H, J = 11.0 Hz, J = 14.4 Hz, CH ), 8.86 (s, 1H, HAr), 8.89 (s,
—
Ar
1H, HAr
)
13C NMR (DMSO-d6), δ: 20.7 (CH3), 36.1 (CH3), 49.0 (2 × CH3), 109.8 (CH), 112.5 (CH), 112.6 (2 × CH), 116.9
(CH), 122.7 (CH), 123.3 (C), 126.0 (C), 131.2 (2 × CH), 137.4 (C), 139.8 (C), 149.2 (C), 152.2 (CH), 152.3 (CH),
152.8 (C), 171.3 (C), 171.4 (C)
a) Aromatic (aryl) protones are indicated by subscript Ar.
when stable dihydrochloride of the starting diamino-
dithiol was used instead of disodium diaminodithiol-
ate that needed to be prepared afresh. The other
old method claimed as leading to linear benzobis-
tizole, starting from 6-amino-2-methylbenzothiazole,
in fact provides angular 2,7-dimethylbenzo[1,2-d:4,3-
dꢀ]bisthiazole (Kiprianov et al., 1956). Recently, an-
other condensation reaction of 2,5-diaminobenzene-
1,4-dithiol leading to linear benzothiazoles appeared,
using orthoesters and indium triflate as a catalyst
(Mike et al., 2010). In our case, acetic anhydride is
sufficiently effective and the reaction could be per-
formed simply with a high yield. Two methyl groups
in the prepared benzobisthiazole derivative are suf-
ficiently acidic to undergo a Knoevenagel-type reac-
tion with aromatic aldehydes. Carrying out this re-
action solely with one methyl group is problematic.
When the benzobisthiazole skeleton is quaternised at

114
A. Čibová et al./Chemical Papers 67 (1) 110–116 (2013)
Fig. 1. Synthesis of 2-substituted 3,6-dimethylbenzo[1,2-d:4,5-dꢀ]bisthiazol-3-ium iodides (IV and V). Reaction conditions: i) Ac2O;
ii) MeI, microwave irradiation.
one nitrogen atom, the acidic character of the adjacent
methyl is increased, and the subsequent condensation
results in only one product. The requisite methylation
was performed with various reagents (dimethyl sul-
phate, trimethyloxonium tetrafluoroborate) and the
Fig. 2. Preparation of 4-substituted benzaldehydes IIIc and
IIIf via nucleophilic substitution. Reaction conditions:
best results were achieved using iodomethane un-
der microwave conditions. The reaction was appro-
priately selective and efficient in 5–7 molar excess
of iodomethane. The second nitrogen atom in some
portion of the starting material was also methylated
but the yield of a double salt did not exceed 10 %
and was easily removed by crystallisation. Methyla-
tion under thermal conditions without irradiation re-
quired a substantially longer reaction time and the
yield was poorer. Microwave conditions were also used
in the final step. Irradiation at 100◦C for 20 min led
to completion of the reaction in the case of most ben-
zaldehydes with dialkylamino or diarylamino groups.
Both the temperature and the reaction time needed
to be increased when the aldehyde with the car-
bazole fragment was used. The relatively harsh con-
ditions needed for reaction of this substituted ben-
zaldehyde are due to the reluctance of elimination
in the second step of the condensation because of
the weaker push–pull character of the resultant prod-
uct in comparison with other target compounds. All
the final compounds were prepared as solely E iso-
mers and they did not show any E/Z isomerisation
on standing for several months. The complete syn-
thesis is shown in Fig. 1 and the data characteris-
ing the new products are summarised in Tables 1 and
2.
i) carbazole or bis(2-methoxyethyl)amine, potassium
carbonate or caesium carbonate, DMSO, 140–160
◦C.
of carbazolyl and bis(2-methoxyethyl)amino deriva-
tives.
As described by Magdolen et al. (2001), the reac-
tion under ultrasound conditions showed low conver-
sion of carbazole, so both the reaction time and the rel-
ative amount of 4-fluorobenzaldehyde were increased
until the yield of 4-(carbazol-9-yl)benzaldehyde (IIIc)
was sufficient. These improvements did not affect the
substitution with bis(2-methoxyethyl)amine, which
emerged as being notably unreactive. Only a low yield
was achieved under quite harsh conditions – irradi-
ation in the microwave reactor at 160◦C for 1 h.
These conditions caused a remarkable decomposition
of the starting material to the complex mixture; al-
though we isolated the crude product containing the
desired compound and 4-fluorobenzaldehyde in about
the same amount, after chromatographic purification
only a low-percentage yield was achieved.
The experimental UV-VIS absorption spectra of
the compounds studied measured in CH3OH are pre-
sented in Fig. 3. Table 3 summarises the UV-VIS ex-
perimental data as well as the results of quantum-
chemical calculations.
The aromatic aldehydes that were not commer-
cially available were synthesised mostly via nucle-
ophilic substitution (Fig. 2) from 4-fluorobenzaldehy-
de according to the published procedure (Magdolen et
al., 2001). Some modifications were made in the cases
The quantum-chemical calculations verified the
planar geometry of the central unit including conju-
gated linkers in each of the compounds under study.
An intensive absorption band in the region of 450–
600 nm caused by the π–π* transition due to intra-

A. Čibová et al./Chemical Papers 67 (1) 110–116 (2013)
115
Fig. 3. UV-VIS absorption spectra of studied compounds (1 – IVa, 2 – IVb, 3 – IVc, 4 – IVd, 5 – IVe, 6 – IVf, 7 – V) measured
in methanol.
Table 3. Position and intensities of the long-wave band in the UV-VIS absorption spectra measured in methanol, calculated values
of energies of frontier orbitals (DFT), linear polarisabilities α, first and second order hyperpolarisabilities β and γ (PM3)
of studied benzobisthiazol-3-ium cations, respectively
Compound
Parameter
IVa
IVb
IVc
IVd
IVe
IVf
V
λmax/nm
540
531
52600
–7.93
450
73600
–7.86
516
39700
–7.92
539
541
588
74900
–7.85
ε/(dm3 mol−1 cm−1
HOMO/eV
LUMO/eV
)
45600
–8.22
–5.59
4.2029
6.4044
9.3064
61500
–8.22
–5.59
4.5031
0.9006
2.1682
84300
–8.19
–5.58
4.7366
5.5372
2.4350
–5.53
–5.93
–5.52
–5.55
α · 1032/C
5.7707
9.2064
22.9159
5.4705
22.182h0
84.7920
4.7033
3.2022
11.0743
5.7373
11.4746
22.8158
β · 1038/C
γ · 1044/C
molecular charge transfer is characteristic of the pre-
pared push–pull salts. All of the salts with dialkyl
or diarylamino substituents, in which free rotation
around the N—CAr single bond is possible, absorb
at negligibly different λmax near 535 nm. A distinct
bathochromic shift of 50 nm is observed when the
conjugation is extended with one more double bond
in compound V; this is in accordance with the lower
value of the calculated HOMO–LUMO energy gap
(∆EIVa = 2.63 eV; ∆EV = 2.31 eV). Compound IVc
exhibits a different UV-VIS spectrum. The most in-
tensive transition is shifted to 450 nm, probably ow-
ing to the twisting of the carbazole residue as was
also confirmed by the quantum-chemical calculations
(in the DFT optimised structure the carbazole moi-
ety is turned through about 42◦ from the remaining
planar part of the molecule). The values of linear po-
larisability α corroborate the more extensive conjuga-
tion in compound V compared with IVa and the do-
nating preference of NPh2 and carbazole substituents
compared with N(CH3)2. It is known that third-order
polarisability increases significantly when the energy
associated with the transition from the ground to
the first excited state (HOMO–LUMO gap) decreases
(Armstrong et al., 1962). This could explain the un-
expectedly high value of γ for compound IVc with the
carbazole substituent (∆EIVc = 1.92 eV).
The number of double bonds in the conjugated
bridge enhances the values of β and γ. Out of the
donor substituents studied, the diphenylamino- and
carbazole groups increased the hyperpolarisabilities
β and γ. These findings will determine the direction
of subsequent research aimed at predicting structures
with possible applications in non-linear optics.
Conclusions
Dipolar conjugated salts based on the linear
benzobisthiazol-3-ium building block were synthe-
sised in a three-step sequence starting from 2,5-
diaminobenzene-1,4-dithiol. Electron-donating frag-
ments represented by the dialkylamino, diarylamino,
or N-heterocyclic substituted benzene rings were in-
troduced into the molecule via a Knoevenagel-type
condensation with substituted benzaldehydes. Mi-
crowave irradiation was used to accelerate the reac-

116
A. Čibová et al./Chemical Papers 67 (1) 110–116 (2013)
tions in both the condensation and quaternisation
steps. Intensive absorption in the visible region due to
intra-molecular charge transfer is characteristic of all
the prepared final salts. The farthest wavelength of the
absorption maximum for the long-wave band was ob-
served for compound V with two double bonds in the
conjugated bridge in accordance with the difference
between HOMO–LUMO orbitals. The values of the
first and second hyperpolarisabilities were calculated
for the synthesised salts. Remarkably high values were
achieved for compounds with diphenylamino and N-
carbazolyl substituted benzene fragments. These cal-
culations predict the newly prepared structures as be-
ing promising NLO materials.
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Acknowledgements. This work was supported by the Slovak
Research and Development Agency (grants Nos. APVV 0424-
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Comenius University grant (no. UK/151/2011).
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