Synthesis, molecular structure and optical spectral studies of syn-dithia benzothiazolophane and anti-bis-lactone benzothiazolophane

Article abstract of DOI:10.1016/j.molstruc.2004.04.014

Synthesis of syn-dithia-benzothiazolophane 1 and anti-bis-lactone benzothiazolophane 2 has been carried out by well established protocols as outlined in Schemes 1 and 2, respectively. The ¹H NMR spectra of heterophanes 1 and 2 were found to be temperature independent in the range of -55 to +120°C which indicates conformationally mobile nature of these molecules. In comparison to plain CHCl₃, the UV-visible spectra of 1 and 2 measured in CHCl₃ containing trifluoroacetic acid (TFAA) exhibited bathochromic shifts by 25 and 19 nm, respectively. The red shifts are attributed to the increased charge transfer interaction arising from the donor phenyl ring to the acceptor benzothiazolium ring. The emission spectra of thia-bridged 1, both in the neutral as well as the protonated forms are accompanied by large Stokes shifts (Δν=127-156nm) with the emissions most likely originating from the locally excited states. The bis-lactone 2 in neutral CHCl₃ solvent also exhibited a single locally excited emission, but with a relatively lower Stokes shift (Δν=69nm). Interestingly, unlike 1, bis-lactone 2 in the protonated form displayed both local as well as what appears to be an intramolecular charge transfer emission. The emission behavior of 1 and 2 has been tentatively rationalized on the basis of HOMO-LUMO interaction. Molecular modelling of 1 and 2 generated five reasonable conformations within 5kcal/mol depending upon the orientation of the benzothiazole rings and the heteroatoms in the connecting bridges. The transannular distances in 1 and 2 were found to be ca. 2.8 and 3.6A?, respectively. While the rings in 2 are nearly planar, however, in the case of 1 the stacks are slightly curved signifying molecular strain in the letter system. Based on dynamic ¹H NMR spectral analysis and molecular modelling, we propose that these molecules exist as a rapidly equilibrating mixtures of conformers.

Full text of DOI:10.1016/j.molstruc.2004.04.014

Journal of Molecular Structure 697 (2004) 221–230
www.elsevier.com/locate/molstruc
Synthesis, molecular structure and optical spectral studies of syn-dithia
benzothiazolophane and anti-bis-lactone benzothiazolophane
a,
Sabir H. Mashraqui , Sukeerthi Kumar , Elise Tran Huu Dau
a
b
*
ˆ
ˆ
aDepartment of Chemistry, University of Mumbai, Vidyanagari, Santacruz (E), Mumbai 400098, India
bInstitut de Chimie des Substances Naturelles, C.N.R.S., 91190 Gif-sur -Yvette, France
Received 27 February 2004; revised 16 April 2004; accepted 22 April 2004
Abstract
Synthesis of syn-dithia-benzothiazolophane 1 and anti-bis-lactone benzothiazolophane 2 has been carried out by well established protocols
as outlined in Schemes 1 and 2, respectively. The 1H NMR spectra of heterophanes 1 and 2 were found to be temperature independent in the
range of 255 to þ120 8C which indicates conformationally mobile nature of these molecules. In comparison to plain CHCl3, the UV–visible
spectra of 1 and 2 measured in CHCl3 containing trifluoroacetic acid (TFAA) exhibited bathochromic shifts by 25 and 19 nm, respectively.
The red shifts are attributed to the increased charge transfer interaction arising from the donor phenyl ring to the acceptor benzothiazolium
ring. The emission spectra of thia-bridged 1, both in the neutral as well as the protonated forms are accompanied by large Stokes shifts
(Dn ¼ 127–156 nm) with the emissions most likely originating from the locally excited states. The bis-lactone 2 in neutral CHCl3 solvent
also exhibited a single locally excited emission, but with a relatively lower Stokes shift (Dn ¼ 69 nm). Interestingly, unlike 1, bis-lactone 2 in
the protonated form displayed both local as well as what appears to be an intramolecular charge transfer emission. The emission behavior of 1
and 2 has been tentatively rationalized on the basis of HOMO–LUMO interaction. Molecular modelling of 1 and 2 generated five reasonable
conformations within 5 kcal/mol depending upon the orientation of the benzothiazole rings and the heteroatoms in the connecting bridges.
˚
The transannular distances in 1 and 2 were found to be ca. 2.8 and 3.6 A, respectively. While the rings in 2 are nearly planar, however, in the
1
case of 1 the stacks are slightly curved signifying molecular strain in the letter system. Based on dynamic H NMR spectral analysis and
molecular modelling, we propose that these molecules exist as a rapidly equilibrating mixtures of conformers.
q 2004 Published by Elsevier B.V.
Keywords: Benzothiazolophanes; Synthesis; Dynamic NMR; UV–visible; Fluorescence spectra; Local/charge transfer emissions
1. Introduction
described in Ref. [9]. 2-Aryl benzothiazoles, which find
applications as whitening agents [10], photoconducting
materials [11] and constituents of biologically active
molecules [12] have been investigated for their solvato-
chromic and emission properties [13]. Indeed, Tashiro et al.
[14] have recently described the synthesis and spectral
properties of metacyclophanes containing benzothiazole as
a peripheral substituent located outside the phane ring. In
continuation of our interest in the synthesis and confor-
mational properties of heterophanes [15], we herein
describe for the first time the incorporation of arylben-
zothiazole fluorophore as a constituent of the phane cavity
in the form of heterophanes 1 and 2. The synthesis of
heterophanes 1 and 2 were of interest to us to study their
absorption and emission characteristics with respect to
differently oriented p-stacked benzothiazole chromophores.
In addition, conformational behavior of 1 and 2 was of
Short bridged cyclophanes have been extensively used as
molecular probes to study their conformational behavior
and the effects of ring proximity on physical, spectral and
chemical properties [1]. Numerous reports by Haenel,
Misumi, Staab and others [2] concerning the ground and
excited state properties of fluorescent active cyclophanes
have clearly established the dependency of photophysical
properties upon ring distortion, orientation and intrachro-
mophoric distance between p-stacked fluorophores.
Although, cyclophanes of polycyclic aromatics, viz
naphthalene, anthracene, pyrene, fluorene abound [3–8],
only a few examples of cyclophanes composed of hetero-
aromatic fluorophores, namely carbazolophanes have been
*
Corresponding author. Tel.: þ91-2226526119; fax: þ91-2226528547.
E-mail address: sh_mashraqui@yahoo.com (S.H. Mashraqui).
0022-2860/$ - see front matter q 2004 Published by Elsevier B.V.
doi:10.1016/j.molstruc.2004.04.014

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S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
interest, since potential exists in these heterophanes to adopt
a variety of conformations subject to conformational
rigidity and depending up on the orientations of benzothia-
zole nuclei and the bridging heteroatoms.
The reaction was diluted with water and refrigerated
overnight. The precipitated solid was filtered, washed with
cold water and finally dried in air to give essentially pure 6,
m.p. 350 8C decomp. in 86% yield (7.05 g); IR (KBr): 3420,
3010, 2928, 1700, 1605, 1565, 1480, 1415, 1330, 1300,
1245, 1122, 1005, 940 and 835 cm21 1H NMR (300 MHz,
CDCl3): d 4.78 (s, 2H, PhCH2 Br), 7.40–8.80 (m, 7H, Ar–
H), 13.2 (bs, 1H, –COOH); IR; Found: C, 51.49; H, 2.66; N,
4.23; S, 9.32; Br, 23.17%. Calcd for C15H10NSO2Br: C,
51.72; H, 2.87; N, 4.02; S, 9.19; Br, 22.99%.
2. Experimental
The melting points (uncorrected) were determined on a
Gallenkamp melting-point apparatus. IR spectral data were
recorded on a Shimadzu FTIR -4200 Spectrophotometer as
a KBr disk. Fluorescence spectra were recorded on a
Shimadzu spectrofluorometer RF-5301 PC. 1H NMR
spectra were recorded on Varian EM-360-L, 60 and
300 MHz spectrometers with tetramethylsilane as the
internal standard. Mass spectra were recorded on GCMS-
QP 5050A Shimadzu spectrophotometer.
2.4. Preparation of bromo- ester 7
Bromo-acid 6 (7.0 g, 20.12 mmol) was dissolved in dry
methanol (100 ml) and SOCl2 (15 ml) was added drop-wise
at 0–5 8C during 15 min. The reaction was allowed to come
to room temperature and then heated to reflux for 24 h. The
solvent was removed under reduced pressure and the
residual oil dissolved in chloroform, washed successively
with a saturated aqueous NaHCO3 and cold water. The
organic extract was dried over anhyd. Na2SO4 and solvent
removed to give a crude solid. Purification was effected by
SiO2 column chromatography (CHCl3 as eluent) to furnish 7
as white solid in 48% yield (3.5 g), m.p. 150–155 8C; IR
(KBr disk): 3015, 2914, 1710, 1619, 1595, 1460, 1435,
1380, 1310, 1225, 1162, 1000, 980 and 815 cm21; 1H NMR
(300 MHz, CDCl3): d 4.62 (s, 2H, PhCH2 Br), 3.98 (s, 3H, –
COOCH3), 7.40–8.72 (m, 7H, Ar–H); Found: C, 52.87, H,
3.47; N, 4.03; S, 8.93; Br, 22.33%. Calcd for C16H12NSO2-
Br: C, 53.04; H, 3.31; N, 3.87; S, 8.84; Br, 22.09%.
2.1. Preparation of p-methoxymethylbenzothioamide 4
p-methoxymethylbenzonitrile [16] (15 g, 102 mmol) was
dissolved in a solution consisting of dry pyridine (30 ml)
and triethylamine (3 ml) to which dry H2S was bubbled
through for 12 h. The reaction mixture was diluted with cold
water, precipitated solid filtered and washed once with 10%
HCl followed by cold-water washing. The air-dried solid
was crystallized from ethanol to give 4 as yellow crystal,
m.p. 113–116 8C in 76% yield (14 gm). IR (KBr): 3315,
3112, 3008, 1610, 1445, 1332, 1245, 1107, 964, 902,
827 cm21 NMR (CDCl3, 60 MHz): d 3.4 (3H, s, Ar
CH2OCH3), 4.40 (2H, s, Ar CH2OCH3), 7.25 (2H, d,
J ¼ 7 Hz, Ar–H), 7.85 (2H, d, J ¼ 7 Hz, Ar–H), 11.20
(2H,bs, –NH2); Found: C, 59.43; H, 5.89; N, 7.68; S, 17.85.
Calcd for C9H11NOS: C, 59.66; H, 6.07; N, 7.73; S, 17.68.
2.5. Preparation of thia-diester 8
To a solution of bromo-ester 7 (2.65 g, 7.32 mmol) in
100 ml dry CH2Cl2 was added a catalytic amount of CTAB
(50 mg), followed by portion-wise addition of anhyd. Na2S
(1.5 g, 19.23 mmol) during 8 h with vigorous stirring. The
reaction was stirred further at room temperature for 48 h and
then filtered through a pad of celite. The filtrate was
concentrated to give a semi-solid residue which was purified
by SiO2 column chromatography (CHCl3 as eluent) to
furnish 8 as a pale-yellow crystalline solid in 89% yield
(1.95 gm), m.p. 180–185 8C.; IR (KBr): 3025, 2910, 1712,
1600, 1485, 1424, 1415, 1295, 1240, 1110, 960, 845 and
2.2. Preparation of methoxy-acid 5
To a solution of 4-amino-3-mercapto-benzoic acid 3 [17]
(5.50 g, 32.5 mmol) and 4-methoxymethyl benzothioamide
4 (4.525 g, 25 mmol) in dry N-methylpyrrolidone (50 ml)
was added conc. HCl (5 ml) and the reaction was heated on
water bath for 8 h. After allowing the reaction to cool to
room temperature, it was poured over crushed ice. The
precipitated solid was filtered, air-dried and crystallized
from alcohol to give 5, m.p. 260–265 8C in 80% yield (6 g);
IR (KBr): 3414, 3000, 2988, 1720, 1600, 1482, 1414, 1300,
1245, 1200, 1005, 966 and 844 cm21; 1H NMR (300 MHz,
CDCl3): d 4.53 (s, 2H, PhCH2OCH3), 3.46 (s, 3H,
PhCH2OCH3), 7.46–8.64 (m, 7H, Ar–H), 11.5 (bs, 1H, –
COOH; Found: C, 64.39; H, 4.18; N, 4.87; S, 10.65%. Calcd
for C16H13NO3S: C, 64. 21; H, 4.35; N, 4.68; S, 10.70%.
780 cm21 1H NMR (300 MHz, CDCl3): d 3.7 (s, 4H,
;
PhCH2S–), 4.0 (s, 6H, –COOCH3), 7.40–8.6 (m, 14H, Ar–
H); Found: C, 64.68; H, 4.29; N, 4.31; S, 16.35%. Calcd for
C32H24N2O4S3: C, 64.43; H, 4.03; N, 4.69; S, 16.11%.
2.6. Preparation of diol 9
The solid diester 8 (1.4 g, 2.35 mmol) was added portion-
wise to a slurry of LiAlH4 (500 mg, 15 mmol) in dry THF
(100 ml) under N2 atmosphere during 1 h. The reaction
mixture was stirred an additional 1 h and decomposed at
0 8C by careful addition of 3 ml of 15% NaOH and 10 ml
2.3. Preparation of bromo-acid 6
Methoxy-acid 5 (7 g, 23.4 mmol) was dissolved in 39%
aqueous HBr (100 ml) and heated on water bath for 10 h.

S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
223
H2O. The reaction mixture was stirred for 15 min, filtered
through celite and the filtrate concentrated under reduced
pressure to give an oily residue. Purification by SiO2 column
chromatography using CHCl3 as eluent provided 9 in 45%
yield (0.570 g), m.p. 185–190 8C; IR (KBr): 3005, 2950,
1610, 1495 and 980 cm21; H1 NMR (300 MHz, CDCl3): d
4.72 (s, 4H, PhCH2OH), 3.62 (s, 4H, PhCH2S–), 7.20–8.1
(m, 14H, Ar–H); Found: C, 66.35; H, 4.63; N, 5.32; S,
17.54%. Calcd for C30H24N2O2S3 requires: C, 66.66; H,
4.44; N, 5.19; S, 17.77%.
containing 1 ml of conc. HCl. The reaction was heated on
water bath for 70 h, then diluted with cold water and left at
room temperature overnight. The precipitated solid was
filtered, washed with cold water and air-dried. The crude
product was subjected to SiO2 column chromatography
(CHCl3 as eluent) to give yellow crystals of 12, m.p. 155–
158 8C in 58% yield (1.8 g). IR (KBr): 3100, 2912, 1713,
1508, 1456, 1405, 1310, 1216, 1100, 986 and 807 cm21; 1H
NMR (300 MHz, CDCl3 þ DMSO-d6): d 3.8 (s, 2H,
PhCH2COOH–), 3.45 (s, 3H, PhCH2OCH3), 4.55 (s, 2H,
PhCH2OCH3), 7.2–8.2 (m, 7H, Ar–H), 11.78 (bs, 1H, –
COOH); Found: C, 64.95; H, 4.51; N, 4.12; S, 10.46%.
Calcd for C17H15NO3S: C, 65.18; H, 4.79; N, 4.41; S,
10.22%.
2.7. Preparation of thia-dibromide 10
Diol 9 (900 mg, 1.66 mmol) was stirred with 33% HBr in
acetic acid (40 ml) for 4 h at room temperature. The reaction
was diluted with cold water and extracted with chloroform.
The organic extract was washed well with cold saturated
NaHCO3 solution and then with cold water. The organic
extract after drying over anyd. Na2SO4 was concentrated
under reduced pressure to afford a yellow solid, which was
further purified by SiO2 column chromatography (CHCl3–
petroleum ether 1:1 as eluent) to give 10 in 77% yield
(850 mg), m.p. 195–200 8C; IR (KBr): 3000, 2940, 1610,
1480, 1455, 1315, 974 and 822 cm21; 1H NMR (300 MHz,
CDCl3): d 4.65 (s, 4H, PhCH2 Br), 3.70 (s, 4H, PhCH2S–),
7.20–8.20 (m, 14H, Ar–H); Found: C, 54.38; H, 3.19; N,
3.98; S, 14.16; Br, 24.36%. Calcd for C30H22N2S3Br2: C,
54.05; H, 3.30; N, 4.20; S; 14.41; Br, 24.02%.
2.10. Preparation of bromo-acid 13
Methoxy-acid 12 (2.05 g, 6.4 mmol) was dissolved in
50 ml of 37% aqueous HBr and heated on water bath for
10 h. The reaction mixture was diluted with cold water, the
precipitated solid filtered, washed with cold water and
finally dried in open air to afford essentially pure 13 in 80%
(1.84 g), m.p. 215–218 8C; IR (KBr): 3406, 2912, 1712,
1586, 1500, 1405, 1376, 1310, 1212, 1000, 845 and
805 cm21
;
1H NMR (300 MHz, CDCl3 þ DMSO-d6): d
3.77 (s, 2H, PhCH2COOH), 4.78 (s, 3H, PhCH2 Br), 7.4–8.2
(m, 7H, Ar–H), 12.8 (bs, 1H, –COOH); Found: C, 52.89;
H, 3.25; N, 4.13; S, 9.02; Br, 22.35%. Calcd for C16H12-
NO2SBr: C, 53.04; H, 3.31; N, 3.87; S, 8.84; Br, 22.09%.
2.8. Preparation of syn-dithia-phenylbenzothiazolophane 1
A solution of dibromide 10 (500 mg, 0.751mmol) in dry
THF (100 ml) was added drop-wise to a stirred solution of
dry THF-benzene (1:1, 200 ml) containing anhyd. Na2S
(500 mg, 6.4 mmol) and CTAB (20 mg) during 10 h under
N2 atmosphere. The reaction mixture was further stirred at
room temperature for 72 h and filtered through a pad of
celite. The filtrate was concentrated to provide a yellow
solid which was purified by SiO2 column chromatography
(CHCl3–petroleum ether, 3:1 as eluent) to give 1 as a pale-
yellow solid in 37% yield (150 mg), m.p. 270–275 8C; IR
(KBr): 3012, 2914, 1610, 1485, 1465, 1410, 1305, 1208,
2.11. Preparation of anti-bis-lactone benzothiazolophane 2
A solution of bromo-acid 13 (1.452 g, 4 mmol) in dry
THF was added drop-wise to a stirred THF (250 ml)
containing anhyd. K2CO3 (1.38 g, 10 mmol) and CTAB
(50 mg) during 10 h with vigorous stirring at 50–55 8C
under N2 atmosphere. The reaction was additionally stirred
at this temperature for 24 h and then filtered through celite.
After usual work up, a semi-solid residue obtained was
subjected to SiO2 column chromatography (CHCl3 –
CH3OH, 95:5 as eluent) to afford 2 as a white crystalline
solid, m.p. .300 8C in 13% yield (0.150 g). IR (KBr disk):
3000, 2980, 1723, 1485, 1454, 1412, 1360, 1310, 1242,
1
1106, 964, 842, 802 and 750 cm21; H NMR (300 MHz,
CDCl3): d 3.75 (s, 4H, PhCH2S), 3.82 (s, 4H, BzCH2S–),
6.8 (d, 1H, J ¼ 1:5 Hz, H7 of Bz), 6.95 (d, 2H, J ¼ 7:5 Hz,
ArH), 7.30 (d, 1H, J ¼ 7 Hz, H4 of Bz), 7.45 (d, 2H,
J ¼ 7:5 Hz, ArH), 7.72 (dd, 1H, J ¼ 7:0 and 1.5 Hz, H5 of
Bz); Found: C, 66.75; H, 4.35; N, 5.05; S, 23.62%. Calcd for
C30H22N2S4 requires: C, 66.91; H, 4.09; N, 5.20; S, 23.79%;
MS: m=z 538 (Mþ), 268, 238 (base peak), 121, 77.
1122, 1105, 986 and 810 cm21 1H NMR (300 MHz,
;
CDCl3): d 5.2 (s, 4H, 4H, PhCH2OCO–), 3.70 (S, 4H, –
OCO–CH2–Ph), 7.05 (d, 2H, J ¼ 7:5 Hz, ArH), 7.15 (d,
1H, J ¼ 1:5 Hz, H7 of Bz), 7.42 (d, 1H, J ¼ 7 Hz, H4 of
Bz), 7.51 (d, 2H, J ¼ 7:5 Hz, ArH), 7.92 (dd, 1H, J ¼ 7:0
and 1.5 Hz, H5 of Bz); Found: C, 68.62; H, 3.75; N, 5.01; S,
11.48%. Calcd for C32H22N2O4S2: C, 68.33; H, 3.91; N,
4.98; S, 11.39%; MS: m=z 562 (Mþ), 474, 259, 237 (base
peak), 121, 77, 44.
2.9. Preparation of methoxy-acid 12
4-Amino-3-mercapto-benzeneacetic acid 11 [18] (1.83 g,
10 mmol) and 4-methoxymethyl benzothioamide 4 (1.81 g,
10 mmol) were dissolved in N-methylpyrrolidone (25 ml)

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3. Results and discussion
(11) [18] with benzothioamide 4 in NMP under acid
catalysis. The reaction gave the desired methoxy-acid 12 in
58% yield. The compound 12 was demethylated with 49%
aq. HBr at 100 8C for 6 h to produce bromo-acid 13 in 80%
yield. The final step in the sequence involved lactonisation
of 13 which was accomplished under the Regen’s protocol
[19] in dry THF solvent in the presence of anhydrous K2CO3
and a catalytic amount of CTAB as the phase transfer
catalyst. The purification of the crude product on SiO2
column chromatography afforded the required bis-lactone 2
as a colorless solid in 13% yield.
The sequence used for the synthesis of syn-dithia [3.3]
phenyl benzothiazolophane 1 is depicted in Scheme 1. To
start with, the known 4-amino-3-mercapto-benzoic acid 3
[17] was condensed with 4-methoxymethyl benzothioamide
4 in N-methyl pyrrolidone (NMP) solvent containing ca. 2
equivalent of conc. HCl. The reaction provided benzothia-
zole derivative 5 in 80% yield. The demethylation of 5 was
achieved with 49% aq. HBr under reflux to afford bromo-
acid 6 as a colorless solid (86% yield). Esterification of 6
with SOCl2 in dry methanol gave quantitatively the methyl
ester 7. Next, to form the mono-thia bridged 8, bromo-ester
7 was reacted with Na2S in methylene chloride solvent
containing a catalytic amount of cetyl trimethylammonium
bromide (CTAB) as the phase transfer catalyst. The diester 8
thus obtained was reduced to diol 9 with LiAlH4 in dry THF.
Treatment of 9 with 33% HBr–AcOH afforded dibromide
10 as colorless solid in good yield. The final step of the
sequence, namely the formation of the second thia-bridge
was effected under high dilution conditions by reacting 10
with Na2S in 1:1 THF-benzene solvent containing a
catalytic amount of CTAB. The work-up of the reaction,
followed by purification of the crude by SiO2 column
chromatography afforded heterophane 1 in reasonable yield
of 37%.
3.1. Structures and conformational analysis of 1 and 2
by 1H NMR spectroscopy
The structures of heterophanes 1 and 2 were character-
ized by 1H NMR spectral data (CDCl3, 500 MHz) which are
compiled in Table 1. The dithia syn-1 (Mþ at m=z 538) in its
1H NMR spectrum showed singlets at 3.75 and 3.82 d for
the bridged –CH2– groups attached to the phenyl and
benzothiazole moieties, respectively. The benzothiazole
protons H4, H5 and H7 were observed at 7.30
(d, J ¼ 1:5 Hz), 7.72 (dd, J ¼ 7 and 1.5 Hz) and 6.85 d
(d, J ¼ 1:5 Hz), respectively. As a consequence of shielding
ring anisotropic effect, the H7 proton of the benzothiazole
moiety is shifted higher field by 0.9 d relative to the position
of the corresponding proton in the reference compound, 2-
phenylbenzothiazole (PBT) [20]. The phenyl protons appear
as a doublet each (J ¼ 7:5 Hz) at 7.45 and 6.95 d,
The synthesis of lactone bridged [4.4] anti-phenyl
benzothiazolophane 2 (Scheme 2) starts with the conden-
sation of known 4-amino-3-mercaptobenzene acetic acid
Scheme 1. Synthesis of syn-dithia-1 Reagents and conditions: (i) dry NMP, 70–908 C, 72 h. (ii) 33% aq. HBr, D, 8 h. (iii) SOCl2, CH3OH, reflux. 1 h (iv) Na2S,
CH2Cl2, CTAB, 24 h. (v) LiAlH4, dry THF, 0 8C, 8 h. (vi) 33%HBr-AcOH, D, 8 h. (vii) Na2S, dry THF-benzene, CTAB, R.T., 72 h.

S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
225
Scheme 2. Synthesis of anti-bislactone-2. Reagents and conditions. (i) dry NMP, 70–908C, 72 h. (ii) 33% aq. HBr, D, 8 h. (iii) K2CO3, dry THF, N2,
60–658C, 48 h.
respectively. For the case of bis-lactone 2, the molecule
(Mþ at m=z 562) exhibited two sharp singlets at 3.70 and
5.26 d which are assigned to the bridged –CH2CO2– and –
CH2O– methylene groups, respectively. The benzothiazole
ring protons H4, H5 and H7 appear at 7.92 (d, J ¼ 7:6 Hz),
7.42 (dd, J ¼ 7:6 and 1 Hz) and 7.15 d (d, J ¼ 1 Hz),
respectively. The shielding being experienced by the H7
proton of the benzothiazole ring in 2 (0.60 d with respect to
PBT) is found to be lower than that observed in dithia syn-1
(0.90 d). In comparison to 1, lower shielding of H7 proton in
bis-lactone 2 is not unexpected on account of longer bridges
(i.e. increased transannular distance) between the proximate
aromatic rings. In keeping with this reduced degree of
shielding, the phenyl protons in 2 appear slightly less upfield
at 7.51 and 7.05 d compared to the corresponding protons in
1 (7.45 and 6.95 d).
The sharp singlets observed for the bridged methylene
protons for both 1 and 2 clearly imply free bridge inversions
in these molecules on the NMR time scale at ambient
condition. The dynamic NMR measurements carried out
from room temperature down to 255 8C indicated no signal
broadening in the spectra of 1 and 2, with the singlets
associated with the bridged –CH2– retaining their singlet
character. Thus, on the basis of unhindered bridge
inversions occurring at least up to 255 8C in 1 and 2, we
can estimate the energy barrier for this process to be
,40 kJ/mol [21]. Further, to check whether or not the rings
are undergoing flipping, we measured NMR up to þ120 8C.
However, no changes either in the chemical shift position or
multiplicities of aromatic protons were discernible. This
result indicates one of the two possibilities; either the rings
are undergoing free ring flippings giving rise to a time
averaged conformation (i.e. fast interconversion between
various conformers) or the molecules 1 and 2 exist as a
single predominant, non-convertible conformer up to a
temperature of 120 8C.
Since, benzothiazole ring contains a basic nitrogen, it
was of interest to examine the 1H NMR spectra of 1 and 2 in
the presence of TFAA which is acidic enough to protonate
the benzothiazole ring (pKa ¼ 1.2) [22]. In CDCl3 contain-
ing TFAA, the molecules 1 and 2 were found to undergo bis-
protonation as evidenced by the appearance of only a single
set of resonance corresponding to half of the molecules
(Table 1). In the case of 1 2 2Hþ, all the signals although
slightly broad are reasonably well resolved and their
assignments are based on the chemical shift positions and
spin multiplicities. As expected, placing positive charge on
the benzothiazole nuclei resulted in the downfield shifts of
all the protons, the magnitude of shielding being dependent
upon the positions of protons on the molecules. In 1 2 2Hþ
system, the benzylic protons at C6 and C10 are located
slightly downfield at d 4.03 and 3.94 compared to neutral 1
(Table 1) and the downfield shifts observed for benzothia-
zole ring protons range from d 0.13 to 0.57, the highest
Table 1
1H NMR data of 1 and 2 in CDCl3 and CDCl3 þTFAA
Proton type
1
1 þ 2Hþ
2
2 þ 2Hþ
C6(–CH2)
C10(–CH2)
H4
3.82 (s)
4.03 (s)
3.70 (s)
4.03 (s)
3.75 (s)
3.94 (s)
5.20 (s)
5.30 (s)
7.74a (bs)
7.3 (d, J ¼ 7 Hz)
7.72 (d, J ¼ 1:5 Hz)
6.8 (dd, J ¼ 7 and 1.5 Hz)
7.4 (d, J ¼ 7:5 Hz)
6.95 (d, J ¼ 7:5 Hz)
7.66 (d, J ¼ 7 Hz)
7.85 (d, J ¼ 1:5 Hz)
7.37
7.42 (d, J ¼ 7 Hz)
7.92 (dd, J ¼ 7 and 1.5 Hz)
7.15 (d, J ¼ 1:5 Hz)
7.51 (d, J ¼ 7:5 Hz)
7.05 (d, J ¼ 7:5 Hz)
H5
8.08 (dd, J ¼ 7 and 1.5 Hz)
7.44 (d, J ¼ 1:5 Hz)
7.74a (bs)
H7
H8,12
H9,11
7.73 (d, J ¼ 7:5 Hz)
7.37 (d, J ¼ 7:5 Hz)
7.27 (d, J ¼ 7:5 Hz)
a
Protons H4, H8, H12 showed identical chemical shifts.

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S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
downfield shift of d 0.57 being noted for the C7 proton. The
protons on the benzo-ring in 1 þ 2Hþ showed downfield
shift in the range of d 0.28–0.42 compared to the neutral 1.
Almost similar trend is noticeable for the bis-lactone 2.
However, protons located on C4 and C8, C12 could not be
separately assigned due to accidentally identical chemical
shift. The assignments for remaining protons are shown in
Table 1. Interestingly, the downfield shifts observed for the
benzothiazole protons in 2 2 2Hþ are markedly lower
(d 0.23–0.32) compared to 1 2 2Hþ (d 0.28–0.42). This
observation clearly suggests that the impact of positive
charge on the chemical shifts in 2 2 2Hþ is relatively less
than is the case for 1 þ 2Hþ. To account for the difference
in the shielding of benzothiazole protons, we propose that
since in the case of 1 þ 2Hþ, the proximate benzothizolium
rings are ecliptically oriented, they will mutually reinforce
the deshielding (downfield shifts) of the protons associated
with these ring systems. On the other hand, in 2 2 2Hþ the
positively charged benzothiazolium ring is eclipsed with
the neutral benzo-ring in the opposite stack and thus the
magnitude of downfield shift of benzothiazolium ring
protons should be relatively less. Alternatively, this result
can also be interpreted by invoking the presence of through-
space charge transfer from the neutral (p-rich) benzo-ring to
the positively charged (p-deficient) benzothiazolium ring in
2 2 2Hþ. This could slightly offset the effects of positive
charge, thereby accounting for the lower downfield shifts in
this system. Such intramolecular charge transfer phenomena
are reminiscent of a variety of donor–acceptor cyclophanes
reported in Ref. [1].
Fig. 1. UV–visible spectra of 1 and 2: (a) 1 in CHCl3 (I) and 2% TFAA in
CHCl3 (II) and (b) 2 in CHCl3 (I) and 2% TFAA in CHCl3 (II).
3.2. Optical spectral properties of 1 and 2
The absorption and emission properties of 1 and 2 were
of interest in view of the facts that (a) these heterophanes
possess a donor acceptor, 2-arylbenzothiazole structural
motif; and (b) the transannular distance and relative
orientation of the chromophores are different in these
molecules. The UV–visible spectra of syn-1 and anti-2
recorded in chloroform solvent (Fig. 1) showed lmax at
317 nm (1max 2.8 £ 1024) and 305 nm (1max 3.1 £ 1024),
respectively, while for the reference compound, PBT the
lmax appears at 299 nm (1max 2.1 £ 1024) [23]. The
observed bathochromic shifts for 1 and 2 in comparison to
the model PBT may be attributed at least in part to
transannular p–p interaction [24]. On the other hand,
marked difference in the lmax values between 1 and 2 could
be a consequence of varying degree of through-bond vs.
through-space charge transfer interactions, which in turn
could differently affect the excitation energies in these
molecules. Addition of 2% TFAA in CHCl3 solution
induces a bathochromic shifts in the absorption maxima
by 25 nm (lmax at 342 nm) and 19 nm (lmax at 324 nm) for 1
and 2, respectively. The model system, PBT in CHCl3-
TFAA solvent exhibits a red shift of 21 nm (lmax at 320 nm)
[23]. These red shifts most likely arise from increased
charge transfer interaction due to enhanced acceptor
property of the N-protonated benzothiazole moieties
compared to the parent, neutral systems.
The emission spectra of 1 and 2 are shown in Fig. 2.
When excited at its absorption wavelength at 317 nm, 1 in
CHCl3 showed a structureless emission at 440 nm register-
ing a Stokes shift of 127 nm. For the anti-bislactone 2, when
excited at 305 nm, a single structureless emission occurs at
369 nm with a Stokes shift of just 69 nm. For the model,
PBT, the emission occurs at 375 nm (lex at 315 nm) with a
Stokes shift of 60 nm. Despite the fact that the emission
bands in heterophanes 1 and 2 are broad and structureless,
these emissions are unlikely to be due excimer/exciplex
formation on the ground that (a) even in the reference PBT,
we observe a similar broad, structureless emission; and (b)
the emissions are not quenched in the presence of oxygen;
and (c) no literature precedent seems to exist for 2-
arylbenzothiazole systems exhibiting excimer type emis-
sions. We, therefore propose that the emissions in 1 and 2
are most likely originating from a locally excited state
(S1 ! S0). A considerably large Stokes shift of 127 nm for
1 in comparison to both anti-2 and the model PBT suggests
that the emission in 1 takes place from a highly relaxed,

S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
227
Stokes shifts. Furthermore, nearly similar values of the
Stokes shifts for 2 and the model PBT (69 and 60 nm)
indicate similar excited state structures in these molecules
[25].
In CHCl3–TFAA, a single emission band believed to be
from a locally excited state for 1 appeared at 498 nm upon
excitation at 342 nm. The red shift for the emission band in
1 in the presence of TFAA compared to the neutral solution
is in accord with the red shift also observed in its absorption
spectrum (Fig. 1). In contrast to 1, bislactone-2 exhibited a
dual emission, a strong higher energy emission (lem
405 nm) appears to be due to a locally excited state
(S1 ! So), whereas a broad low intensity emission band
located at longer l (503 nm) is proposed to occur from an
intramolecular charge transfer excited state (CT ! So). It is
noteworthy that in 2-arylbenzothiazole systems, both
the HOMO and LUMO are reported to be located on the
aryl ring with the benzothiazole moiety serving as a
substituent [23b]. Though, such may be the case in the
neutral 2-aryl benzothiazole systems, in the presence of
TFAA, we presume that the LUMO shifts position from the
aryl ring to the protonated, electron deficient benzothiazo-
lium ring. However, the HOMO is expected to retain its
position on the aryl ring, as illustrated in Chart 1.
Accordingly, for bislactone-2, due to the eclipsed orien-
tation of donor and acceptor rings, intramolecular charge
transfer from the HOMO of one chromophore to the LUMO
of the another chromophore across space appears eminently
feasible. Thus, in addition to the local emission, the
presence of a new longer wavelength emission in 2 can be
reasonably accounted for due to the charge transfer emission
(CT ! S0). In contrast, in 1 the donor and the acceptor
components in the opposite stacks are oriented obliquely
from each other thereby precluding effective HOMO–
LUMO interaction through space. Thus, on this premise the
presence of only local emission with the complete exclusion
of charge transfer emission in 1 is understandable. It is
noteworthy that the emission band 1 in the presence of
TFAA was of much reduced intensity compared to that
observed for 2 (Fig. 2). Relatively, higher molecular
Fig. 2. Emission spectra of 1 and 2: (a) 1 in CHCl3 (I) and 2% TFAA in
CHCl3 (II) and (b) 2 in CHCl3 (I) and 2% TFAA in CHCl3 (II).
equilibrated S1 excited state, i.e. Frank–Condon state [26].
This may be rationalized in terms of relatively rigid nature
of 1 that might allow rapid reorganization of the solvent
molecules to stabilize the excited state structure. For the
case of relatively less strained 2 (and the model PBT)
entailing greater conformational flexibility, solvent reor-
ganization accompanying the excited state could be a
relatively less efficient process, resulting in less pronounced
Chart 1. Proposed HOMO–LUMO assignments.

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S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
Table 2
Stable conformations and energies of 1 and 2 by MM2 calculations
Dithia-1
Bislactone-2
Conformers
Conformers
DE in kJ/mol
Interring distance
˚
CH2–CH2 (A)
DE in kJ/mol
Interring distance
˚
CH2–CH2 (A)
Syn–syn (a)
Anti–anti
Syn–anti
28.7
28.7
28.7
29.0
29.8
2.81
2.81
2.81
2.81
2.81
Syn–syn (a)
Anti–anti
Syn–anti
296.7
296.7
297.8
301.4
301.4
3.59
3.59
3.61
3.59
3.60
Anti–syn
Anti–syn
Syn–syn (b)
Syn–syn (b)
Fig. 3. Selected computer generated conformations: syn–syn 1(A), syn–syn 2 (B), and a higher energy conformation of 2 (C).

S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
229
1
or not could not be settled since the H NMR spectra were
strain in 1 could contribute to internal quenching in this
molecule [27].
found to be temperature independent up to þ120 8C. The
1
presence of a single set of well resolved signals in the H
3.3. Molecular modelling of 1 and 2
NMR spectra of both 1 and 2, and rather low energy
differences between various reasonable conformations
(Table 2) lead us to speculate that these molecules probably
exist as time-averaged, freely interconverting confor-
mations. Indeed, literature reports several examples of
[3.3] [33] and [4.4] type cyclophanes [34] exhibiting free
conformational rotations. Nevertheless, due to constricted
Since, the thiazole nucleus is unsymmetric owing to 1,3-
disposition of two different heteroatoms, possibility exists
for the presence of many conformations of 1 and 2
depending upon the relative orientation of two aryl-
benzothiazole stacks. Since, crystals of 1 and 2 were
unsuitable for single X-ray crystallographic structural
analysis, we carried out molecular modelling to probe the
conformational aspects of these systems. Two thousand
conformations of 1 and 2 were generated by random search
Monto Carlo method [28] and optimized by PRCG
Conjugated Gradient molecular mechanics minimization
method [29] using the Macromodel (version 5.5) program
[30] with the MM2 force field [31] and GB/SA chloroform
solvation [32]. From these conformational searches, all
possible conformations within 12 kcal/mol from the global
minima were analyzed. In both cases, we are able to get five
stable conformations within 5 kcal/mol from the global
minimum. The difference between these five conformers is
the relative orientation of the heteroatoms in the bridges and
in the thiazole rings. The first designation (syn or anti) refers
to the orientation of the heteroatoms (S or lactone CyO) in
the bridges. The second designation corresponds to the
orientation of the heteroatoms in the thiazole rings. The third
designation (a or b) for syn–syn indicates that the sulfur
atoms in the thiazole ring have the same (or opposite)
orientation with respect to those of sulfur atoms in 1 or the
lactone carbonyl in 2 in the bridges. The corresponding
steric energies DE are presented in Table 2 and for
illustration, only syn–syn conformers for 1 and 2 are
shown in Fig. 3.
˚
cavity (ca. 2.8 A), the energy barriers to conformational
inversions in 1 can be assumed to be higher compared to 2
which possesses a relatively larger cavity dimension (ca.
˚
3.6 A).
4. Conclusion
In the present work, we have reported for the first time
incorporation of photoemittive benzothiazole heteronuclei
into the cyclophane framework in the form of structures 1
and 2. The dynamic 1H NMR spectra of 1 and 2 were found
to be temperature independent from 255 to þ120 8C thus
suggesting the presence of free conformational rotation in
these systems. The importance of the relative orientation of
the benzothiazole chromophores in 1 and 2 was borne out by
optical spectra, in particular the fluorescence spectra data.
The short bridged dithia-1, having the syn orientation of
aryl-benzothiazole components showed both in the neutral
and acidic conditions only a single emission arising from the
locally excited state. However, bislactone-2 having anti
oriented aryl-benzothiazole chromophores displayed under
acidic condition a dual emissions; a higher energy band is
assigned to a locally excited emission, whereas a broad,
lower energy emission is believed to be originating from an
intramolecular charge transfer interaction. This is a
consequence of eclipsed orientation of donor phenyl ring
and the acceptor benzothiazolium ring in the opposite
stacks. The HOMO–LUMO interaction has been proposed
to explain the disparity in the emission behavior of
heterophanes 1 and 2. Along expected lines, the energy
minimization indicted the short bridged 1 to be relatively
more strained compared to the longer bridged 2. On the
basis of dynamic 1H NMR analysis coupled with the
molecular modelling, we speculate that both 1 and 2 exist as
time averaged conformations. Work is in progress to
synthesize [2.2] syn- and [2.2] anti-phenyl benzothiazolo-
phanes to more closely examine the effects of chromophoric
orientations on conformational and photophysical
properties.
Low energy differences between the five most stable
conformations clearly suggest no particular bias for any of
the syn/anti forms of 1 and 2. The comparison of geometries
of five conformers of 2 reveals that aryl-benzothiazole
components are oriented nearly parallel with interring
˚
separation of ca. 3.6 A. The transannular gap appears to
be sufficiently large in 2 to allow for unhindered ring
rotations through the phane cavity. Further, we also obtain a
higher energy conformation in 2 (E 301 kcal/mol, Fig. 3C)
in which rings in one stack are nearly perpendicular with
respect to those in the opposite stack. This particular
conformation resembles the transition state during the ring
inversion process, a feature, which lends supports to the
possibility of ring rotation in this molecule. For the case of
1, the frame in various conformations is slightly curved with
˚
interring separation of ca. 2.8 A. Relatively, closely spaced
chromophores in 1 support higher magnitude of shielding
and transannular interaction as discussed earlier.
Acknowledgements
On the basis of dynamic NMR analysis, we have
concluded the presence of free bridge inversions in
heterophanes 1 and 2. However, whether rings are rotating
We thank the CSIR, New Delhi (India) for generous
research grant and Professor R.M. Kellogg, University of

230
S.H. Mashraqui et al. / Journal of Molecular Structure 697 (2004) 221–230
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