Two novel push-pull series of benzothiazole-based dyes: Synthesis and characterization

Article abstract of DOI:10.1002/hlca.201200080

New dichromophoric cyanine dyes based on benzothiazol-2-amines as push-pull systems were synthesized in two series of disperse and cationic forms (see 4 in Scheme1 and 5 in Scheme2, resp.). Their thermal stabilities, UV/VIS, fluorescence, and solvatochrom

Full text of DOI:10.1002/hlca.201200080

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Helvetica Chimica Acta – Vol. 95 (2012)
Two Novel PushꢀPull Series of Benzothiazole-Based Dyes: Synthesis and
Characterization
by Farahnaz Nourmohammadian*a)b), and Mahnaz Davoudzadeh Gholamib)
a) Center of Excellence for Color Science and Technology, Tehran, Iran
b) Department of Organic Colorants, Institute for Color Science and Technology, P.O. Box 654-16765,
Tehran, Iran (phone: þ 98-21-22956126; fax: þ 98-21-22947537; e-mail: nour@icrc.ac.ir)
New dichromophoric cyanine dyes based on benzothiazol-2-amines as pushꢀpull systems were
synthesized in two series of disperse and cationic forms (see 4 in Scheme 1 and 5 in Scheme 2, resp.).
Their thermal stabilities, UV/VIS, fluorescence, and solvatochromic behavior, which are important
parameters in pushꢀpull systems, were also studied.
Introduction. – In continuation of our recent investigations on novel hemicyanine
dyes [1] and considering the various applications of benzothiazol-2-amine-derived azo
dyes, we decided to synthesize a series of dichromophoric cyanine dyes based on
benzothiazol-2-amines as pushꢀpull systems in disperse and cationic variations and
study their thermal, UV/VIS, fluorescence, and solvatochromic behavior, important
parameters in pushꢀpull systems.
Results and Discussion. – The reaction of glyoxal (¼ethanedial; 1a) or tereph-
thaladehyde (¼ benzene-1,4-dicarboxaldehyde; 1b) with 2 mol-equiv. of aniline
(¼ benzenamine) under acidic conditions led within 1 h at 58 to the corresponding
Schiff bases 2a and 2b in high yields (95 and 98%, resp.; Scheme 1). Coupling of the
Schiff bases 2a and 2b with 1 mol-equiv. of diazotized 3a or 3b followed by 1 mol-equiv.
of diazotized 3c gave the unsymmetrically substituted diazo dyes 4a – 4d in good yields
(64 – 81%). The structures of compounds 2a and 2b and 4a – 4d were deduced from
1
their mass, IR, and H- and 13C-NMR spectra and CHN analysis.
N-Methylation of the benzothiazole moieties of 4a – 4d with dimethyl sulfate gave
5a – 5d in good yields (62 – 80%), which represent a second cationic category of these
novel dyes and are members of the important class of colorants with special
1
applications [2 – 7] (Scheme 2). MS, IR, H- and 13C-NMR data, and CHN analysis
confirmed the structures of 5a – 5d.
Compounds 4a – 4d are purple to red dyes (l
496 – 505 nm) with 2.0 – 2.7 · 104 l
max
mꢀ1 cmꢀ1 molar absorption coefficients. Their corresponding cationic N-methyl forms
5a – 5d show 67 – 85 nm bathochromic shifts (indigo to deep blue colors, lmax 572 –
584 nm) with a significant increase of the molar absorption coefficients (2.9 – 3.7 ·
104 l mꢀ1 cmꢀ1; see Table 1).
Thermal Stabilities. To investigate the applicability of the synthesized dyes as
pushꢀpull systems in advanced technologies, such as nonlinear optical (NLO) devices,
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

Helvetica Chimica Acta – Vol. 95 (2012)
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Scheme 1. Synthesis of the Dichromophoric Benzothiazole-Based Dyes 4a – 4d
organic light-emitting diodes (OLEDs), and liquid crystals, their thermal stability was
determined, which is an important requirement for their use in the mentioned
technologies [8]. The thermal stabilities were evaluated by measuring the decom-
position temperatures of 4a – 4d and 5a – 5d by means of thermal gravimetric analysis
(TGA) (Fig. 1). Comparing the decomposition temperatures of 4a – 4d revealed that
4d had the highest thermal stability. Thus, presence of an EtO group and a central 1,4-
phenylene linker (n ¼ 1) in the structure of 4d efficiently enhanced its thermal stability.
The thermal stabilities of the cationic dyes 5a – 5d were higher, and among them, 5c,

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Scheme 2. N-Methylation of the Benzothiazole Moities of 4a – 4d to Produce the Cationic Dyes 5a – 5d
Table 1. Summarized Spectroscopic Data for the PushꢀPull Systems in EtOH
n
R
lmax [nm]
e [l mꢀ1 cmꢀ1
]
Color
4a
4b
4c
4d
5a
5b
5c
5d
0
0
1
1
0
0
1
1
Me
Et
Me
Et
Me
Et
Me
Et
505
499
496
500
572
581
581
584
2.4 · 104
2.7 · 104
2.0 · 104
2.6 · 104
3.7 · 104
3.3 · 104
3.5 · 104
2.9 · 104
purple
red
red
purple
indigo
deep blue
deep blue
indigo
with an MeO group and a central 1,4-phenylene linker (n ¼ 1) present in its structure,
displayed the highest thermal stability.
Solvatochromic Effects. To study the solvatochromic effects, the maximum
absorption wavelength in the UV/VIS spectra of 4a – 4d and 5a – 5d were measured
in different solvents (Table 2). Thus, these dichromophore dyes revealed a negative
solvatochromic effect (blue shisft) in aprotic solvents and a positive solvatochromic
effect (red shift) in protic solvents. The blue shift is caused by the decrease of the
dichromophoreꢂs dipole moments an excitation and more stability of the ground-state
than of the excited state in the more polar aprotic solvents; consequently, the energy
difference between the ground and excited states will increase [9]. This means that the
compounds are highly polar because they contain pushꢀpull systems and linear
properties. Although polar and protic solvents stabilize the excited state more than the
ground state, the bathochromic shift or positive solvatochromic effect in these solvents
is evident.

Helvetica Chimica Acta – Vol. 95 (2012)
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Fig. 1. Thermal gravimetric analysis (TGA) of the chromophores 4a – 4d and 5a – 5d
Table 2. lmax Values [nm] of the UV/VIS Spectra of Dyes 4 and 5 in Different Solvents
4a
4b
4c
4d
5a
5b
5c
5d
THF
492
482
479
505
481
506
495
482
482
499
482
505
494
465
477
496
484
508
497
473
479
500
491
510
588
565
572
572
570
582
583
580
573
581
562
589
575
585
581
581
579
590
582
580
581
584
581
598
CHCl3
Me2CO
EtOH
MeCN
DMSO
Fluorescence Emission. All dyes 4a – 4d showed fluorescence emission in a broad
range at 2 · 10ꢀ5 mol lꢀ1 concentration in N,N-dimethylformamide (DMF) at 293 K
(Fig. 2). The dyes were excited at 325 – 350 nm, and the fluorescence peaks emerged at
400 – 500 nm, 4a showing the highest and 4d the lowest emission. Similarly, the
fluorescence emission spectra of the cationic dyes 5a – 5d were measured with an
excitation at 288 – 345 nm (Fig. 3).
Two important characteristics for fluorescent compounds, the Stokes shift (˜nA ꢀ˜nF)
and the oscillator strength (f) were studied for the selected dichromophoric dyes. The
Stokes shift is a parameter which indicates the difference in the properties and structure
of the fluorophores between the ground-state S0 and the first excited state S1. The
Stokes shifts (cmꢀ1) for 4a – 4d and 5a – 5d were calculated by Eqn. 1, which gave values
between 5529 and 8680 cmꢀ1 (Table 3).
(˜nA ꢀ˜nF) ¼ (1/lA ꢀ 1/lF) · 107
(1)
The oscillator strength (f) shows the effective number of electrons whose transition
from the ground to the excited state gives the absorption area in the electron spectrum.
The oscillator strengths for 4a – 4d and 5a – 5d were calculated with Eqn. 2 where D˜n1/2

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Helvetica Chimica Acta – Vol. 95 (2012)
Fig. 2. Fluorescence emission spectra of 4a – 4d in DMF
Fig. 3. Fluorescence emission spectra of 5a – 5d in DMF

Helvetica Chimica Acta – Vol. 95 (2012)
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Table 3. Absorption and Fluorescence Characteristics of the Dichromophoric Dyes 4 and 5 in DMF at
293 K (c ¼ 2 · 10ꢀ5 mol lꢀ1
)
lex [nm]
lem [nm]
emax [l mꢀ1 cmꢀ1
]
˜nA ꢀ˜nF [cmꢀ1
]
Intensity [a.u.]
ƒ
4a
4b
4c
4d
5a
5b
5c
5d
325
335
340
350
345
315
288
330
439
433
431
434
435
421
384
435
2.4 · 104
2.7 · 104
2.0 · 104
2.6 · 104
3.7 · 104
3.3 · 104
3.5 · 104
2.9 · 104
7990
6756
6209
5529
5997
7993
8680
7314
628
0.400
0.299
0.198
0.158
0.399
0.405
0.362
0.369
531.6
302.7
182.6
840
999
741.4
272.9
is the half-band width of the absorption band [cmꢀ1] at emax [10]. The f values 0.158 –
0.400 were thus obtained for 4a – 4d and 0.362 – 0.405 for 5a – 5d (Table 3).
f ¼ 4.32 · 10ꢀ9 · D˜n1/2 · emax
(2)
Conclusion. – A series of novel dichromophoric cyanine dyes based on benzothia-
zol-2-amines as pushꢀpull systems in two series of disperse and cationic forms, 4 and 5,
respectively, were synthesized and their thermal stabilities studied up to 200 – 2208. The
disperse dyes 4 in EtOH revealed absorptions at 496 – 505 nm in red and purple with
2.0 – 2.71 · 104 l mꢀ1 cmꢀ1 molar extinction coefficients. The corresponding cationic dyes
5 showed 67 – 85 nm bathochromic shifts (indigo to deep blue colors, l max 572 – 584 nm)
with a significant increase in the molar absorption coefficients (2.9 – 3.7 · 104 l mꢀ1
cmꢀ1). These dyes were excited at 288 – 350 nm whereupon they showed fluorescence
emission in the broad range of 400 – 500 nm. Moreover, these dyes revealed negative
solvatochromic effects in aprotic solvents and positive solvatochromic effects in protic
solvents.
Experimental Part
General. Chemicals were purchased from Merck and used without further purification. M.p.: Bꢀchi
B-545 melting-point apparatus; uncorrected. UV/VIS Spectra: Multispec-1501-Shimadzu UV/VIS
spectrophotometer. IR Spectra: Perkin-Elmer-Spectrum-One-BX FT-IR spectrometer; in KBr; ˜n in cmꢀ1
.
Fluorescence spectra: Perkin-Elmer-LS-55 spectrometer. 1H- and 13C-NMR Spectra: Bruker-500-Avance
Fourier-transform (FT) NMR instrument, at 500 and 125.7 MHz, resp., in dimethyl sulfoxide
((D6)DMSO) and CDCl3; d in ppm rel. to SiMe4, J in Hz. MS: Finnigan-Mat-8430 mass spectrometer;
ionization potential 20 eV, electrospray ionization (ESI), pos. made; in m/z (rel. %). Elemental analyses
for C, H, and N: Heraeus-CHN-O-Rapid analyzer.
1. Compounds 2a and 2b: General Procedure. Aniline (0.02 mol) was added dropwise to a stirred
soln. of glyoxal (1a; 0.01 mol) or terephthalaldehyde (1b; 0.01 mol) in 30% aq. AcOH soln. (20 ml) at 58.
The mixture was stirred for 1 h and filtered. Compounds 2a and 2b were crystallized from H2O to get the
pure forms.
N,N’-Diphenylethane-1,2-diimine (¼ N,N’-Ethane-1,2-diylidenebis[benzenamine]; 2a). Yield 95%.
1
Cream-colored powder. M.p. 103 – 1058. IR (KBr): 2935 (CH¼), 1608 (C¼N), 1529 (C¼C). H-NMR
(CDCl3): 6.65 (S, 2 CH¼N); 6.73 (d, J ¼ 7.8, 4 CH); 6.84 (t, J ¼ 7.8, 2 CH); 7.2 (dd, J ¼ 7.8, 4 CH).
13C-NMR (CDCl3): 158.9; 152.1; 129.8; 125.8; 121.4 (14 C). Anal. calc. for C14H12N2 (208.26): C 80.74, H
5.81, N 13.45; found: C 80.80, H 5.72, N 13.44.

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1,1’-(1,4-Phenylene)bis[N-phenylmethanimine] (¼ N,N’-(1,4-Phenylenedimethylidene)bis[benzen-
amine]; 2b). Yield 97%. Cream-colored powder. M.p. 155 – 1578. IR (KBr): 2922 (CH¼), 1614
1
(C¼N), 1541(C¼C). H-NMR (CDCl3): 7.29 (d, J ¼ 7.2, 4 CH); 7.30 (t, J ¼ 7.2, 2 CH); 7.45 (dd, J ¼ 7.2,
4 CH); 8.05 (s, 4 CH); 8.54 (s, 2 CH). 13C-NMR (CDCl3): 159.9; 152.2; 130.2; 129.7; 129.6; 126.8; 121.4
(20 C). Anal. calc. for C20H16N2 (284.35): C 84.48, H 5.67, N 9.85; found: C 84.56, H 5.59, N 9.91.
2. Preparation of Compounds 4a – 4d: General Procedure. A soln. of benzothiazol-2-amine derivative
3a or 3b (10 mmol) in 50% H2SO4 soln. (12.5 ml) was cooled in an ice-water bath, and an aq. NaNO2 soln.
(10 mmol in 2.5 ml H2O) was added within 15 min at 08. The resulting yellow-to-orange soln. was stirred
at 08 for 1 h. Then, a suspension of 2a or 2b (10 mmol) in 1% HCl soln. (20 ml) was added dropwise to
the diazonium salt at 0 – 58. The cooling bath was removed, and the mixture was stirred at r.t. for 1 h.
The second diazonium salt was papered separately by dissolution of 5-nitrobenzothiazol-2-amine
(3c; 10 mmol) in 50% H2SO4 soln. (12.5 ml). The mixture was cooled in an ice-water bath, and an aq.
NaNO2 soln. (10 mmol in 2.5 ml H2O) was added within 15 min at 08. The resulting yellow-to-orange
soln. was stirred at 08 for 1 h.
Then, the mixture of the first soln. (obtained from 3a or 3b) was added to this second diazonium salt
(from 3c) at 0 – 58 and stirred at r.t. for 1 h and then neutralized by adding 10% NH3 soln. The
precipitated dye was isolated by filtration, washed with excess H2O, and crystallized in EtOH to afford
pure 4.
4-[(1E)-2-(5-Methoxybenzothiazol-2-yl)diazenyl]-N-{(1E,2E)-2-{{4-[(1E)-2-(5-nitrobenzothiazol-
2-yl)diazenyl]phenyl}imino}ethylidene}benzenamine (4a). Yield 81%. Purple powder. M.p. 160 – 1628.
IR (KBr): 2929 (CH¼), 1633 (C¼N), 1598 (C¼C), 1278 (CꢀN). 1H-NMR (CDCl3): 3.81 (s, MeO); 6.72
(s, 2 CH¼N); 6.91 – 8.00 (m, 14 arom. H). ESI-MS: 604 (3, Mþ), 310 (9), 295 (12), 180 (61), 165 (75), 106
(12), 92 (100). Anal. calc. for C29H19N9O3S2 (605.65): C 57.51, H 3.16, N 20.81; found: C 57.57, H 3.11, N
20.86.
4-[(1E)-2-(5-Ethoxybenzothiazol-2-yl)diazenyl]-N-{(1E,2E)-2-{{4-(1(E)-2-(5-nitrobenzothiazol-2-
yl)diazenyl]phenyl}imino}ethylidene}benzenamine (4b): Yield 78%. Red powder. M.p. 145 – 1478. IR
(KBr): 2979 (CH¼), 1614 (C¼N), 1509 (C¼C), 1256 (CꢀN). 1H-NMR (CDCl3): 1.49 (t, J ¼ 6.8, Me); 4.14
(q, J ¼ 6.8, CH2O); 6.81 (s, 2 CH¼N); 6.96 – 8.22 (m, 14 arom. H). ESI-MS: 618 (4, Mþ), 310 (5), 309
(10), 194 (68), 165 (71), 120 (15), 92 (100). Anal. calc. for C30H21N9O3S2 (619.68): C 58.15, H 3.42, N
20.34; found: C 58.14, H 3.42, N 20.39.
4-[(1E)-2-(5-Methoxybenzothiazol-2-yl)diazenyl]-N-{(1E)-{4-{(1E)-{{4-[(1E)-2-(5-nitrobenzothia-
zol-2-yl)diazenyl]phenyl}imino}methyl}phenyl}methylene}benzenamine (4c): Yield 70%. Red powder.
M.p. 197 – 1998. IR (KBr): 2928 (CH¼), 1634 (C¼N), 1525 (C¼C), 1294 (CꢀN). 1H-NMR (CDCl3): 3.87
(s, MeO); 7.21 (s, 4 CH); 7.48 – 8.23 (m, 14 arom. H); 8.49 (s, 2 CH). ESI-MS: 680 (3, Mþ), 348 (11), 333
(9), 348 (12), 241 (74), 180 (66), 106 (18), 92 (100). Anal. calc. for C35H23N9O3S2 (681.75): C 61.66, H
3.40, N 18.49; found: C 61.64, H 3.33, N 18.52.
4-[(1E)-2-(5-Ethoxybenzothiazol-2-yl)diazenyl]-N-{(1E)-4-{(1E)-{{4-[(1E)-2-(5-nitrobenzothia-
zol-2-yl)diazenyl]phenyl}imino}methyl}phenyl}methylenebenzenamine (4d): Yield 64%. Red powder.
M.p. 226 – 2288. IR (KBr): 2970 (CH¼), 1614 (C¼N), 1522 (C¼C), 1282 (CꢀN). 1H-NMR (CDCl3): 1.52
(t, J ¼ 6.8, Me); 4.17 (q, J ¼ 6.82, CH2O); 7.23 (s, 4 CH); 7.34 – 8.28 (m, 14 arom. H); 8.50 (s, 2 CH). ESI-
MS: 694 (2, Mþ), 348 (11), 347 (7), 241 (69), 194 (64), 120 (16), 92 (100). Anal. calc. for C36H25N9O3S2
(695.77): C 62.14, H 3.62, N 18.12; found: C 62.14, H 3.64, N 18.17.
3. Compounds 5a – 5d: General Procedure. To a magnetically stirred suspension of 4 (10 mmol) and
NaHCO3 (1.7 g, 20 mmol) in H2O (4 ml), dimethyl sulfate (4.4 ml, 0.036 mol) was added dropwise at 308.
The mixture was stirred at 608 for 2 h (TLC (CHCl3/MeOH 8 :1) monitoring). Then, the pH was adjusted
to 4 by addition of 1m HCl. The precipitated dye was filtered and crystallized in H2O to afford pure 5.
5-Methoxy-3-methyl-2-{(1E)-2-{4-{{(1E,2E)-2-{{4-[(1E)-2-(3-methyl-5-nitrobenzothiazol-3-ium-2-
yl)diazenyl]phenyl}imino}ethylidene}amino}phenyl}diazenyl}benzothiazol-3-ium Bis(methyl Sulfate)
(5a): Yield 80%. Indigo powder. M.p. 267 – 2698. IR (KBr): 2941 (CH¼), 1645 (C¼N), 1588 (C¼C),
1297 (CꢀN). 1H-NMR ((D6)DMSO): 3.85 (s, MeO); 4.12 (s, 2 MeN); 6.83 (d, J ¼ 8.1, 2 CH); 7.00 – 8.70
(m, 14 arom. H). ESI-MS: 634 (1, Mþ), 620 (8), 325 (12), 310 (8), 165 (18), 133 (100). Anal. calc. for
C33H31N9O11S4 (857.91): C 46.20, H 3.64, N 14.69; found: C 46.18, H 3.59, N 14.77.

Helvetica Chimica Acta – Vol. 95 (2012)
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5-Ethoxy-3-methyl-2-{(1E)-2-{4-{{(1E,2E)-2-{{4-[(1E)-2-(3-methyl-5-nitrobenzothiazol-3-ium-2-
yl)diazenyl]phenyl}imino}ethylidene}amino}phenyl}diazenyl}benzothiazol-3-ium Bis(methyl Sulfate)
(5b): Yield 71%. Blue powder. M.p. 195 – 1978. IR (KBr): 2971 (CH¼), 1613 (C¼N) 1558 (C¼C) 1287
1
(CꢀN). H-NMR ((D6)DMSO): 1.53 (t, J ¼ 6.6, Me); 4.10 (q, J ¼ 6.6, CH2O); 4.15 (s, 2 MeN); 6.95 (d,
J ¼ 8.3, 2 CH); 7.11 – 8.43 (m, 14 arom. H). ESI-MS: 648 (3, Mþ), 635 (7), 325 (6), 324 (13), 165 (19), 133
(100). Anal. calc. for C34H33N9O11S4 (871.94): C 46.83, H 3.81, N 14.46; found: C 46.87, H 3.83, N 14.42.
5-Methoxy-3-methyl-2-{(1E)-2-{4-{{(1E)-{4-{(1E)-{{4-[(1E)-2-(3-methyl-5-nitrobenzothiazol-3-
ium-2-yl)diazenyl]phenyl}imino}methyl}phenyl}methylene}amino}pheny})diazenyl}benzothiazol-3-ium
Bis(methyl Sulfate) (5c): Yield 65%. Blue powder. M.p. 208 – 2108. IR (KBr): 2935 (CH¼), 1622 (C¼N),
1514 (C¼C), 1278 (CꢀN). 1H-NMR ((D6)DMSO): 3.93 (s, MeO); 4.16 (s, 2 MeN); 7.16 (s, 4 CH); 7.44 –
8.28 (m, 14 arom. H); 8.52 (s, 2 CH). ESI-MS: 710 (3, Mþ), 696 (5), 363 (11), 348 (17), 241 (38), 133
(100). Anal. calc. for C39H35N9O11S4 (934.01): C 50.15, H 3.78, N 13.50; found: C 50.12, H 3.79, N 13.47.
5-Ethoxy-3-methyl-2-{(1E)-2-{4-{{(1E)-{4-{(1E)-{{4-[(1E)-2-(3-methyl-5-nitrobenzothiazol-3-ium-
2-yl)diazenyl]phenyl}imino}methyl}phenyl}methylene}amino}phenyl}diazenyl}benzothiazol-3-ium Bis-
(methyl Sulfate) (5d): Yield 62%. Indigo powder. M.p. 237 – 2398. IR (KBr): 2956 (CH¼), 15612
(C¼N), 1528 (C¼C), 1294 (CꢀN). 1H-NMR ((D6)DMSO): 1.49 (t, J ¼ 6.7, Me), 4.12 (q, J ¼ 6.7, CH2O),
4.08 (s, 2 MeN); 7.21 (s, 4 CH); 7.42 – 8.46 (m, 14 arom. H); 8.59 (s, 2 CH). ESI-MS: 724 (2, Mþ), 710 (6),
343 (9), 342 (11), 241 (25), 133 (100). Anal. calc. for C40H37N9O11S4 (948.04): C 50.68, H 3.93, N 13.30;
found: C 50.71, H 3.87, N 13.35.
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Received February 17, 2012