Synthesis of new intramolecular charge transfer A-D-A tetrathiafulvalene- fused triads exhibiting large solvent sensitive emission behavior

Article abstract of DOI:10.1016/j.tetlet.2011.03.016

We have synthesized three new acceptor-donor-acceptor (A-D-A) triads incorporating the donor tetrathiafulvalene (TTF) fused with acceptors quinoxaline and dipyrido[3,2-a:2′,3′-c]phenazine (dppz) systems. Solution emission spectral studies of all these com

Full text of DOI:10.1016/j.tetlet.2011.03.016

Tetrahedron Letters 52 (2011) 2496–2500
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of new intramolecular charge transfer A–D–A
tetrathiafulvalene-fused triads exhibiting large solvent
sensitive emission behavior
⇑
Ramababu Bolligarla, Samar K. Das
School of Chemistry, University of Hyderabad, Hyderabad 500046, AP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesized three new acceptor–donor–acceptor (A–D–A) triads incorporating the donor tetra-
thiafulvalene (TTF) fused with acceptors quinoxaline and dipyrido[3,2-a:20,30-c]phenazine (dppz) sys-
tems. Solution emission spectral studies of all these compounds show large solvent sensitive behavior
with huge Stokes shifts. The large solvent dependence of the emission indicates that the excited state
is stabilized in more polar solvents due to the intramolecular charge transfer. We have also described
electrochemical studies of one of the title compounds (compound 1b) exhibiting two oxidation responses
at 1.02 and 1.31 V versus Ag/AgCl, that correspond to the oxidized species of TTF monocation and dica-
tion, respectively.
Received 29 December 2010
Revised 28 February 2011
Accepted 3 March 2011
Available online 8 March 2011
Keywords:
A–D–A TTF triads
Emission
Solvatocromic effects
Large Stokes shifts
Cyclic voltammetry
Ó 2011 Elsevier Ltd. All rights reserved.
Research interests on tetrathiafulvalene(TTF)-based compounds
have remained dynamic in the field of materials science, particu-
larly, in the context of molecular electronics and NLO materials,1–6
because this class of compounds can easily form donor–acceptor
(D–A) ensembles through facile oxidation of relevant sulfur atoms.
incorporating TTF and p-benzoquinone molecules, have been
synthesized and their opto-electronic properties were investigated
by Hudhomme and co-workers.20 Recently, Zhu et al. have studied
a Py–TTF–Py compound (A–D–A triad) which exhibits multi-color
solvatochromism.40 The photo-induced intramolecular charge
transfer studies in TTF–dppz molecule (D–A system) have been re-
ported by Liu and co-workers, in which they used electronic
absorption, fluorescence emission and electrochemical tech-
niques.41 There is considerable interest in the extended dppz-
based systems because its fascinating features include structural
Owing to their unique p-donor properties, TTF and its derivatives
are successfully used as versatile building blocks for the formation
of charge transfer salts giving rise to organic conductors and even
super conductors.7,8 Moreover, the molecular systems of D–A diads
as well as D–A–D/A–D–A triads, based on TTF derivatives, are of
current interest due to their potential applications in molecular
electronic devices9–11 and artificial photo-synthetic systems.12,13
The highly conjugated D–A systems are particularly important in
the development of nonlinear optical materials and in the
approach of producing high efficiency solar energy conversion.14,15
Even though the intramolecular charge-transfer TTF D–A diads
have been explored extensively,16–34 only few examples of
TTF-based D–A–D/A–D–A triads have been reported in the litera-
ture. To the best of our knowledge, D–A–D triads, such as
di-TTF–quionones,35 di-TTF–PI23,36,37 (pyromellitic diimide, PI)
and di-TTF–TCNAQ (tetracyanoantraquinodimethane, TCNAQ),16
A–D–A triads, such as TTF–diquionones,20,38,39 and Py–TTF–Py40
have been described in the literature. Fused A–D–A triad system,
planarity,
p-extended conjugation and metal-chelating diamine
functionality.42,43 However, a system of A–D–A triads, containing
TTF–dppz fused molecules, have not yet been reported in the liter-
ature. In this letter, we have described three newly synthesized A–
D–A triads (compounds 1a–c) that contain TTF as donor moiety as
shown in Schemes 1 and 2. Two of them are described as TTF-fused
quiunoxaline systems [Scheme 1(a)] and remaining one can be
described as TTF-fused dppz molecule [Scheme 1(b)]. The newly
synthesized A–D–A triads (1a–c) exhibit good emission in the
visible region with large solvatochromic effects and massive Stokes
shifts. We have also described redox properties of the compound
1b (Scheme 2) in DCM solution.
Symmetrical TTF-based compounds (1a–c) are generally
synthesized by the phosphite-mediated (triethylphosphite) self
coupling reaction of corresponding 1,3-dithia-2-one derivatives
(2a–c) at 130–140 °C in good yields (52–70%) as described in
Scheme 1.44 The 1,3-dithia-2-one derivatives (2a–c) have been
⇑
Corresponding author. Tel.: +91 40 2301 1007; fax: +91 40 2301 2460.
E-mail address: samar439@gmail.com (S.K. Das).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.016

R. Bolligarla, S. K. Das / Tetrahedron Letters 52 (2011) 2496–2500
2497
prepared from corresponding 1,3-dithia-2-thione derivatives
(3a–c) by using Hg(OAc)2 in CHCl3/CH3COOH (3:1) in good yields
(90–98%) as presented in Scheme 1.45 5,6-Diamino-benzo[1,3]-
dithiole-2-thione (5) is the common precursor for the syntheses
of compounds 3a–c, which was prepared according to literature
procedure in two steps starting from o-phenylenediamine.41,46
a
X
X
N
S
S
X
X
N
S
(i)
(ii)
S
1a-b
O
N
S
1a, 53%
1b, 70%
N
X = Cl (3a) , 65%
= Ph (3b) , 83%
X = Cl (2a) , 95%
= Ph (2b) , 90%
b
N
N
N
N
N
N
S
S
S
S
(i)
(ii)
S
1c
52%
O
N
N
(3c) , 80%
(2c) , 98%
Scheme 1. (a) Synthetic route for compounds 1a and b; (b) compound 1c from their corresponding 1,3-dithia-2-thone derivatives. Reagents and conditions: (i) CHCl3/
CH3COOH (3:1), Hg(OAc)2, rt, 2 h; (ii) P(OEt)3, 130–140 °C, 2 h.
Cl
Cl
N
N
N
N
Cl
Cl
S
S
S
S
(1a)
N
N
N
N
S
S
S
S
(1b)
N
N
N
N
N
N
N
N
S
S
S
S
(1c)
Scheme 2. Structural representation of newly synthesized A–D–A TTF triads.
Table 1
Syntheses of 1,3-dithia-2-thione derivatives
Entry Precursor
Reagent
Conditions
Product
Yield
(%)
H
N
H2N
S
O
S
S
S
S
1
(5) Oxalic acid
MeOH, reflux, 24 h
66
(4)
S
H2N
O
N
H
2
3
4
5
SOCl2
Benzil
DMF (cat), reflux, 15 h
I2 (10 mol %), CH3CN, rt,
30 min
3a
3b
65
83
4
5
1,10-Phenanthroline-
5,6-dione
MeOH, reflux, 24 h
3c
96

2498
R. Bolligarla, S. K. Das / Tetrahedron Letters 52 (2011) 2496–2500
Table 2
The 1,3-dithia-2-thione derivatives 3a–c were prepared by con-
densation reaction of 5,6-diamino-benzo[1,3]dithiole-2-thione
with corresponding diones following the procedures as described
in Table 1. Compound 3a has been prepared by the chlorination
of 2-thioxo-5,8-dihydro-1,3-dithia-5,8-diaza-cyclopenta[b]naph-
thalene-6,7-dione (4) (di-oxime) with SOCl2 by using DMF as cata-
lyst (see the Supplementary data) by using modified literature
procedure.47 The di-oxime (4) was prepared by condensation reac-
tion between the 5,6-diamino-benzo[1,3]dithiole-2-thione and
oxalic acid in MeOH at reflux condition (see the Supplementary
data). Compound 3b was prepared according to modified literature
procedure,48 which was obtained by a condensation reaction be-
tween the 5,6-diamino-benzo[1,3]dithiole-2-thione and benzil by
using iodine as catalyst in acetonitrile solution at room tempera-
ture.49 But compound 3c was isolated by the simple condensation
of 5,6-diamino-benzo[1,3]dithiole-2-thione with 1,10-phenanthro-
line-5,6-dione50–52 in MeOH solution at reflux condition without
using any catalyst (see the Supplementary data).
Photophysical properties of A–D–A TTF compounds 1a–c (Amax and Emax = Absorption
and emission maxima, respectively,
quantum yield)
D
mst (cmꢁ1) = Stokes shift, UF = fluorescence
Compound
Solvent
Amax (cmꢁ1
)
Emax (cmꢁ1
)
D
mst (cmꢁ1
)
UF
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1c
1c
1c
Toluene
CHCl3
DMF
21,097
21,097
21,186
21,505
21,505
21,505
21,739
21,834
21,884
21,929
18,867
18,867
19,047
16,474
15,060
13,869
18,796
17,574
16,233
15,797
15,503
15,060
14,662
15,772
14,814
13,315
4623
6037
7317
2709
3931
5272
5942
6331
6824
7267
3095
4053
5732
0.03
0.05
0.026
0.04
0.063
0.102
0.117
0.082
0.126
0.097
0.024
0.019
0.012
CCl4
Toluene
CHCl3
DCM
Acetone
DMF
DMSO
Toluene
CHCl3
DMF
All the synthesized materials have been characterized by NMR
spectroscopy including elemental and LC–MS analyses (see the
Supplementary data). Molecular structures of the newly synthe-
sized TTFs (1a–c) have been determined by 1H and 13C NMR, LC–
MS spectral studies including their elemental analyses and the
knowledge of the precursors from which they have been synthe-
sized. Compound 1a could not be characterized by 13C NMR studies
because of its poor solubility in common deuterated solvents.
The electronic absorption spectrum of the compound 1b shows
a strong absorption at 465 nm in the visible region in CHCl3 as
shown in Figure 1(a). In solutions of other solvents the visible band
positions vary with slight change over the region 457–465 nm
(Table 2). It shows that the absorption band positions do not
change much with polarity.
Compound 1b exhibits strong emission in solution at room tem-
perature. In contrast to its electronic absorption spectral behavior
in various solvents, the emission spectra of compound 1b are
strongly dependent on polarity of the solvents (Table 2). The band
maxima shift to red region with increasing the polarity of the sol-
vent as shown in Figure 1. As shown in Figure 1(a), the excitation
spectrum of compound 1b is identical to its absorption spectrum
in chloroform and indeed same is true for all the solvents of the
series. The emission light of the compound 1b varies from green
to red with increasing the polarity of the solvent (CCl4 to DMSO).
For example, in the solution of nonpolar solvent, such as CCl4, this
shows green emission and emission maximum is centered at
532 nm [Fig. 1(b)]. In the solution of moderate polar solvent, such
as toluene, it shows yellowish emission as shown in Figure 1(b),
and the relevant emission maximum centers at 570 nm. On the
other hand, in the solutions of relatively more polar solvents, such
as DMF and DMSO, this shows red emission and the emission max-
ima are centered at 665 and 682 nm respectively [Fig. 1(b)]. Thus
the solvent dependence of the emission shows that the excited
state is stabilized in more polar solvents, which is due to an intra-
molecular charge transfer.41 Besides, the fluorescence quantum
yield of the compound 1b increases from 4.0% to 12.6% with
increasing the polarity of the solvent (CCl4 to DMF) at room tem-
perature. Notably, the quantum efficiency of the compound 1b in
relatively more polar solvents is ꢀ0.1. This suggests that the com-
pound 1b is good fluorescent as compared to the other TTF systems
of D–A diads as well as D–A–D and A–D–A triads, reported
earlier.41 In the similar fashion, the observed Stokes shifts of
compound 1b increases from 2709 to 7267 cmꢁ1 with increasing
the polarity of the solvent (CCl4 to DMSO). Compound 1a also
shows similar absorption band at 465 nm in the visible region in
CHCl3 (Fig. S1, see the Supplementary data) and it does not change
much with the solvent polarity (Table 2).
The emission spectrum of the compound 1a is also strongly
dependent on the polarity of the solvents (Fig. S1, see the Supple-
mentary data) as shown by compound 1b; however, in the case of
compound 1a, emission maxima are further shifted to lower
energy in red region as shown in Table 2.
The absorption spectrum of the compound 1c in the CHCl3 solu-
tion is shown in Figure S2 in the section of Supplementary data. In
this case, the absorption band shifts to low energy region com-
pared to compounds 1a and b. It absorbs at 530 nm in CHCl3 solu-
tion. The emission spectrum of the compound 1c is also strongly
dependent on the polarity of the solvents (Fig. S3, see the Supple-
mentary data) as observed in the case of compounds 1a and b;
interestingly, in this case, the emission maxima are further shifted
to still low energy region as shown in Table 2.
Figure 1. (a) Emission spectra (––, kex = 460 nm) of the TTF triad (1b) in different
solvents at room temperature, (a) CCl4, (b) toluene, (c) CHCl3, (d) DCM, (e) acetone,
(f) DMF, (g) DMSO, and absorption (——) and excitation spectra (. . ., kem = 610 nm)
of 1b in CHCl3 (excitation and emission spectra are normalized to 1); (b) color
response of compound 1b with different solvents under UV lamp.

R. Bolligarla, S. K. Das / Tetrahedron Letters 52 (2011) 2496–2500
2499
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Figure 2. Cyclic voltammograms of 1b in DCM to the monocation (dashed line) and
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and E1=2
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We are thankful to Department of Science and Technology
(DST), Government of India for a fund (project number: SR/S1/
MBD-05/2007). RB thanks CSIR, New Delhi, India for his fellowship.
We are grateful to Professor S. Pal and his group for providing us
electrochemical data.
Supplementary data
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44. Typical procedure for the preparation of 6,7,60,70-tetraphenyl-[2,20]bi[1,3-dithia-
Supplementary data (absorption and emission spectra of
compounds 1a and 1c, analytical data, 1H and 13C NMR, LC–MS
and elemental analyses) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2011.03.016.
5,8-diaza-cyclopenta[b]napthalenylidene] (1b): A solution of compound 2b
(125 mg, 0.336 mmol) in triethylphosphite (3 mL) was refluxed at 130–
140 °C for 2 h under N2 atomsphere. After cooling to room temperature,
MeOH (20 mL) was added and the resulting orange precipitate was filtered off.
Yield: 70.0%; orange solid; IR (KBr, cmꢁ1): 3040, 1658, 1585, 1493, 1450, 1431,

2500
R. Bolligarla, S. K. Das / Tetrahedron Letters 52 (2011) 2496–2500
1340, 1253, 1188, 1093, 1055, 1022, 864, 825, 767, 694, 549; 1H NMR
46. Brusso, J. L.; Clements, O. P.; Haddon, R. C.; Itkis, M. E.; Leitch, A. A.; Oakley, R.
T.; Reed, R. W.; Richardson, J. F. J. Am. Chem. Soc. 2004, 126, 8256–8265.
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6345–6348.
(400 MHz, CDCl3): d 8.02 (s, 4H), 7.49 (d, 8H), 7.32–7.37 (m, 12H) ppm; 13C
NMR (100 MHz, CDCl3): d 153.23, 140.93, 140.07, 138.70, 129.79, 128.98,
128.30, 120.23 ppm; LC–MS (positive mode): m/z = 713 (M++H)+; elemental
Anal. calcd for C42H24N4S4/C, 70.76; H, 3.39; N, 7.86. Found: C, 70.61; H, 3.45;
N, 7.70. UV–vis: kmax = 465 nm (in chloroform, ꢀ7 ꢂ 10ꢁ6 M concentration);
fluorescence: kem = 616 nm (in chloroform, ꢀ7 ꢂ 10ꢁ6 M concentration).
45. Typical procedure for the preparation of 6,7-diphenyl-1,3-dithia-5,8-diaza-
cyclopenta[b]napthalen-2-one (2b): To a solution of compound 3b (210 mg,
0.541 mmol) in chloroform and acetic acid (3:1, v/v, 40 mL), Hg(OAc)2 (552 mg,
1.73 mmol) was added and it was stirred for 2 h at room temperature under N2
atmosphere. The resulting suspension was filtered using celite and washed
with chloroform. The filtrate was extracted with NaHCO3 solution (3 ꢂ 33 mL)
and water (50 mL). To remove any traces of water anhydrous Na2SO4 was
added. The solvent was removed in vacuum, which produces pale yellow
colored compound 2b as product. Yield: 90.0%; pale yellow solid; IR (KBr,
cmꢁ1): 3049, 1734 (C@O), 1651, 1446, 1388, 1336, 1257, 1184, 1097, 1022,
968, 868, 769, 696, 596, 543; 1H NMR (400 MHz, CDCl3): d 8.25 (s, 2H), 8.57 (s,
2H), 7.52 (d, 4H), 7.33–7.41 (m, 6H) ppm; 13C NMR (100 MHz, CDCl3): d 188.90
(C@O), 154.14, 139.72, 138.48, 135.74, 129.84, 129.27, 128.37, 122.64 ppm;
LC–MS (positive mode): m/z = 373 (M++H)+; elemental Anal. calcd for
49. Typical procedure for the preparation of 6,7-diphenyl-1,3-dithia-5,8-
diaza-cyclopenta[b]napthalene-2-thione (3b):
A
mixture containing 5,6-
diaminobenzene-1,3-dithiole-2-thione (5) (200 mg, 0.934 mmol), benzil
(196 mg, 0.932 mmol) and I2 (10 mol %) in CH3CN (10.0 mL) was stirred for
30 min. The resulting yellow precipitate was separated by filtration, washed
with little CH3CN, and dried in vacuo. Yield: 83.0%; yellow solid; IR (KBr,
cmꢁ1): 3067, 1531, 1496, 1433, 1392, 1340, 1195, 1059 (C@S), 1020, 966, 877,
769, 696, 545; 1H NMR (400 MHz, CDCl3): d 8.21 (s, 2H), 7.53 (d, 4H), 7.34–7.43
(m, 6H) ppm; 13C NMR (CDCl3): d 211.96 (C@S), 154.47, 143.46, 139.64, 138.40,
129.83, 129.38, 128.40, 120.83 ppm; LC–MS (positive mode): m/z = 389
(M++H)+; elemental Anal. calcd for C21H12N2S3/C, 64.92; H, 3.11; N, 7.21.
Found: C, 65.23; H, 3.40; N, 6.96.
50. Paw, W.; Eisenberg, R. Inorg. Chem. 1997, 36, 2287–2293.
51. Smith, G. F.; Cagle, F. W., Jr. J. Org. Chem. 1947, 12, 781–784.
52. Guo, W.; Obare, S. O. Tetrahedron Lett. 2008, 49, 4933–4936.
C21H12N2OS2/C, 67.72; H, 3.25; N, 7.52. Found: C, 67.46; H, 3.59; N, 7.68.