Synthesis of crown ethers containing a rubicene moiety

Article abstract of DOI:10.3390/50300620

A symmetrically disubstituted derivative of the highly fluorescing and photostable rubicene was incorporated in a macrocycle using high dilution conditions and a hydroxyrubicene was functionalized with a modified aminobenzo-15-crown-5.

Full text of DOI:10.3390/50300620

Molecules 2000, 5, 620-628
molecules
ISSN 1420-3049
http://www.mdpi.org
Synthesis of Crown Ethers Containing a Rubicene Moiety
Mario Smet and Wim Dehaen*
Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee Belgium
Tel.: 32-16-327439, Fax: 32-16-327990, E-mail: Wim.Dehaen@chem.kuleuven.ac.be
* Author to whom correspondence should be addressed.
Received: 10 February 2000 / Accepted: 14 March 2000 / Published: 22 March 2000
Abstract: A symmetrically disubstituted derivative of the highly fluorescing and photosta-
ble rubicene was incorporated in a macrocycle using high dilution conditions and a hy-
droxyrubicene was functionalized with a modified aminobenzo-15-crown-5.
Keywords: rubicene, crown ethers, macrocycle synthesis.
Introduction
Recently we have used the highly fluorescing and photostable rubicene as a core in dendrimer
chemistry [1]. A new approach towards disubstituted rubicenes was devised [2]. A metal catalyzed
ring closure was found to afford heterocyclic analogues of rubicene and asymmetrically disubstituted
derivatives [3]. In this paper we wish to report some more applications of rubicene in systems having
great potential as supramolecular building blocks, namely macrocycles and crown ethers. Macrocycles
containing a rubicene moiety could be interesting fluorescing units to be used in the synthesis of
[n]catenanes [4]. On the other hand, crown ethers bearing a rubicene unit could be used as selective
cation sensitive fluorescence indicators.
Results and Discussion
In previous work we have reported the preparation and alkylation of 5,12-dihydroxyrubicene [1-2].
Although alkylation of the latter compound with benzyl bromides proceeded quite smoothly, reaction
© 2000 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.

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Molecules 2000, 5
of this rubicene derivative with simple aliphatic bromides and tosylates turned out to fail. However, we
found that the use of more reactive bromoacetates or bromoacetamides afforded the alkylation pro-
ducts in very good yield. As we wished to use the 5,12-dihydroxyrubicene to prepare macrocycles, we
decided to prepare the tetraethylene glycol derivative 2 by reaction of the tetraethylene glycol monoto-
sylate 1 with bromoacetyl bromide (Scheme 1). The required monotosylate was prepared following the
literature conditions [5]. Compound 2 bears a bromoacetate moiety which could be substituted by
5,12-dihydroxyrubicene in refluxing acetone in the presence of K2CO3 and 18-crown-6. Under these
conditions the tosylate was not substituted. This fact was in accordance with our former observation of
the low reactivity of 5,12-dihydroxyrubicene towards simple aliphatic tosylates. Even in DMF at 80°C,
tetraethylene glycol ditosylate failed to react with 5,12-dihydroxyrubicene. However, the use of com-
pound 2, allowed a very convenient preparation of the ditosylate 3. Subsequent reaction of this ditosyl-
ate with 1,5-naphthalenedithiol (which was prepared following the literature procedure [6]) under high
dilution conditions and using Cs2CO3 as the base, afforded the desired macrocycle 4. This compound
was found to display a strong fluorescence (l max = 560 nm). Both the rubicene containing compounds 3
en 4 were found to have excellent solubility in CH2Cl2 and CHCl3. This behaviour is in sharp contrast
with unsubstituted rubicene or the 5,12-dihydroxy derivative which essentially behave as pigments.
O
bromoacetyl bromide
HO
O
O
O
OTs
O
O
O
O
OTs
Et3N
Br
1
2
O
O
O
O
O
O
O
O
O
OTs
O
O
O
5,12-dihydroxyrubicene
K2CO3
S
acetone
1,5-naphthalenedithiol
Cs2CO3
DMF
high dilution
S
O
3
O
O
O
O
O
O
O
O
O
OTs
4
O
O
Scheme 1.
In a second approach, we wanted to functionalize a rubicene with a crown ether derivative. There-

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Molecules 2000, 5
fore, a rubicene with only one phenol function, such as 10, was required (Scheme 2). We decided to
introduce a tert.-butyl group to enhance the solubility and hence the ease of synthesis and manipula-
tion. The preparation of this compound 10 was achieved analogously to compounds previously pre-
pared in our group (Scheme 2) [7]. First, a solution of 4-methoxyphenylmagnesium bromide in THF
was slowly added to a suspension of 1,5-dichloroanthraquinone (5) in THF, affording the monoadduct
6. To this compound, an excess of 4-tert.-butylphenyllithium was added yielding the diol 7. Reduction
of the latter using NaH2PO2 and KI in refluxing acetic acid afforded the substituted 9,10-
diphenylanthracene 8. Ring closure of this compound was achieved by a palladium catalyzed reaction
as previously published [3]. Finally, the obtained 5-tert.-butyl-12-methoxyrubicene 9 could be depro-
tected by treatment with BBr3 yielding the desired phenol 10. The yields of these reactions are all good
to excellent.
OCH3
OCH3
Cl
Cl
Cl
O
HO
HO
4-t.-butylphenyl-
lithium
4-methoxyphenyl-
magnesium bromide
THF
OH
Cl
O
Cl
O
Cl
5
6
7
OCH3
R
Cl
Pd(OAc)2
Bu4NHSO4
NaH2PO2
KI
9: R = OCH3
10: R = OH
BBr3
CH3COOH
K2CO3
DMF
120°C
48h
Cl
8
Scheme 2.
In order to be able to conjugate a crown ether to this phenolic compound 10, we prepared the
bromoacetamide 12 by treatment of 4-aminobenzo-15-crown-5 (11) with bromoacetyl bromide
(Scheme 3). The obtained bromoamide 12 was found to be sufficiently reactive to be coupled with the
monophenol 10, affording the desired compound 13 in moderate yield. The latter was found to be
highy fluorescing and further investigations to evaluate the influence of cations on the fluorescence are
under way.

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Molecules 2000, 5
O
O
NH
O
O
O
O
O
10
O
K2CO3
O
RHN
18-crown-6
O
O
acetone
O
BrCH2COCl
Et3N
R = H 11
R = BrCH2CO 12
CH2Cl2
13
Scheme 3.
Experimental
General
THF was dried by distillation from sodium / benzophenone. All other reagents and solvents were
purchased from Acros Organics and used without further purification. Each new compound was fully
1
13
characterized by mass spectrometry (Perkin Elmer, EI 70 eV and ES) in addition to H-NMR and C-
NMR spectroscopy (Bruker AMX 400 MHz or Brucker WM 250 MHz). Chemical shifts are relative to
TMS as an internal reference.
Synthetic procedures and spectral data
Bromoacetate 2
Tetraethyleneglycol monotosylate (1) (22g; 63 mol) and Et3N (9.6 g; 95 mmol) were dissolved in
CH2Cl2 ( 130 mL) and placed under argon atmosphere. The mixture was cooled to -20°C. Through a
septum, bromoacetyl bromide (19 g; 95 mmol) was added and the resulting dark suspension was left at
room temperature for 1 h. The suspension was poured on crushed ice (ca. 150 g) and the mixture was
washed with a solution of HCl (3 x 50 mL; 2 M) and water (2 x 50 mL). The organic layer was dried
over MgSO4 and the solvent was evaporated in vacuum. The desired bromoacetate 2 was obtained as a
1
light brown oil (24 g; 81%) after column chromatography (SiO2; CH2Cl2-ethyl acetate 1:1): H NMR
(250 MHz, CDCl3) d 7.78 (d, J = 8 Hz, 2H, HCCSO2), 7.33 (d, J = 8 Hz, 2H, HCCCSO2), 4.31 (m, 2H,
CH2OTs), 4.14 (m, 2H, CH2OCOCH2Br), 3.89 (s, 2H, CH2Br), 3.72-3.56 (m, 12H), 2.43 (s, 3H, CH3);

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Molecules 2000, 5
MS (EI) m/z: 468 (M+).
Ditosylate 3
5,12-Dihydroxyrubicene (0.25 g; 0.70 mmol), bromide 2 (0.82 g; 1.7 mmol) and K2CO3 (0.25 g; 1.7
mmol) were suspended in acetone (20 mL) and the resulting mixture was refluxed overnight under ar-
gon atmosphere. After cooling to room temperature, the solvent was evaporated in vacuum and the de-
sired compound 3 was obtained as a dark red oil (0.65 g; 82%) after column chromatography (SiO2;
CH2Cl2-ethyl acetate 1:1): 1H NMR (250 MHz, CDCl3) d 8.01 (d, J = 8.5 Hz, 2H), 7.76 (d, J = 8.5 Hz,
2H), 7.73 (d, J = 8.5 Hz, 4H), 7.59 (d, J = 6 Hz, 2H), 7.39 (dd, J = 8.5 Hz, J = 6 Hz, 2H), 7.28 (d, J =
1.5 Hz, 2H), 6.81 (dd, J = 8.5 Hz, J = 1.5 Hz, 2H), 4.74 (s, 4H), 4.42 (t, J = 7 Hz, 4H), 4.10 (t, J = 7
Hz, 4H), 3.79-3.58 (m, 16H), 3.52 (s, 8H), 2.33 (s, 6H); 13C NMR (400 MHz, CDCl3) d 168.87, 157.08,
144.64, 140.82, 137.08, 133.57, 132.87, 132.60, 131.72, 129.67, 127.94, 127.78, 124.31, 124.15,
123.66, 119.76, 113.33, 108.47, 71.02, 70.54, 70.44, 70.35, 69.12, 68.82, 68.51, 65.48, 64.23, 30.28,
21.42; MS (ES) (m/z): 1136 (M+).
Cyclophane 4
Ditosylate 3 (0.55 g; 0.48 mmol) and 1,5-naphthalenedithiol (92 mg; 0.48 mmol) were dissolved in
DMF (50 mL). The resulting solution was added over a period of 24 h to a vigourously stirred suspen-
sion of Cs2CO3 (0.47 g; 1.4 mmol) in DMF (300 mL) at 70°C under argon atmosphere by means of an
infusion pump. After complete addition, the mixture was stirred for another 12 h at 70°C. After cooling
to room temperature, the solvent was evaporated in vacuum and the cyclophane 4 was obtained as a
1
dark red oil (0.13 g; 27%) after column chromatography (SiO2; CH2Cl2-ethyl acetate 1:1): H NMR
(400 MHz, CDCl3) d 8.32 (d, J = 8.6 Hz, 2H), 8.00 (d, J = 8.6 Hz, 2H), 7.98 (d, J = 7.6 Hz, 2H), 7.80
(d, J = 6.6 Hz, 2H), 7.58 (dd, J = 6.6 Hz, J = 8.5 Hz, 2H), 7.42 (d, J = 2.4 Hz, 2H), 7.25 (d, J = 7.6 Hz,
2H), 7.13 (t, J = 7.6 Hz, 2H), 6.88 (dd, J = 2.4 Hz, J = 8.5 Hz, 2H), 4.79 (s, 4H), 4.42-4.40 (m, 4H),
3.72-3.70 (m, 4H), 3.54-3.52 (m, 4H), 3.45-3.43 (m, 4H), 3.37-3.34 (m, 8H), 3.30-3.28 (m, 4H), 2.87-
2.83 (m, 4H); 13C NMR (400 MHz, CDCl3) d 169.11, 157.45, 141.07, 137.53, 133.85, 133.79, 133.05,
132.88, 132.20, 128.43, 127.35, 125.60, 124.70, 124.65, 123.98, 123.56, 120.24, 113.69, 108.79,
70.60, 70.42, 70.36, 70.06, 69.34, 68.80, 65.80, 64.45, 33.23; MS (ES) (m/z): 983 (M+).
Bromoacetamide 12
A solution of 4-aminobenzo-15-crown-5 (11) (0.50 g, 1.8 mmol) and Et3N (1.1 g, 11 mmol) in
CH2Cl2 (15 mL) was placed under argon and cooled to 0°C in an ice bath. Through a septum, bromo-
acetyl bromide (1.1 g, 5.3 mmol) was added and the resulting dark solution was left for 30 min at 0°C.
The reaction mixture was poured on crushed ice (ca. 20 g). The organic layer was separated and
washed with diluted (1N) HCl (2 x 20 mL) and water (20 mL). The organic layer was dried over

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Molecules 2000, 5
MgSO4 and evaporated in vacuum. Bromoacetamide 12 was obtained after column chromatography
(SiO2, EtOAc-CH3OH 1:1) as an amorphous solid (0.49g, 67%), nmax (KBr)/cm-1 3443 (NH), 1670
(C=O), 1631 (C=O) ca. 1100 (broad) (C-O); dH (400 MHz ; CDCl3) 3.73-3.79 (8 H, br m,
Ph(O(CH2)2O(CH2)2)2O), 3.86-3.92 (4 H, br m, Ph(OCH2CH2O(CH2)2)2O), 4.07 (2 H, s, -CH2Br), 4.09
(4 H, br m, Ph(OCH2CH2O(CH2)2)2O), 6.76 (1 H, d, J 8.5, benzo 6-H), 7.04 (1 H, dd, J 1.5 and 8.5,
benzo 5-H), 7.30 (1 H, d, J 1.5, benzo 3-H), 8.56-8.63 (1 H, br s, NH) ; dC (100 MHz ; CDCl3) 163.8,
148.5, 145.6, 131.6, 114.1, 113.0, 106.9, 70.39, 70.38, 69.94, 69.93, 69.19, 68.99, 68.98, 68.42, 29.78;
m/z 405 (M+, 20%), 403 (M+, 20), 273 (49), 271 (49), 151 (100).
1,5-Dichloro-10-hydroxy-10’-(4-methoxyphenyl)(10H)anthracene–9-one (6)
1,5-Dichloroanthraquinone (5) (7.0 g, 25 mmol) was suspended in dry THF (200 mL) and placed
under argon. A solution of 4-methoxyphenylmagnesium bromide was prepared from 4-bromoanisol
(5.7 g, 28 mmol) and magnesium turnings (0.73 g, 30 mmol) in dry THF (25 mL) and added dropwise
(during 30 min) to the anthraquinone suspension. After the addition was complete, the resulting clear
solution was stirred for 12 h at room temperature. Water was added (100 mL) and after vigorous
shaking, the organic layer was separated. The water layer was extracted with CH2Cl2 (2 x 80 mL). The
combined organic layers were dried over MgSO4 and evaporated in vacuum. To the pasty residue,
toluene (150 mL) was added and the resulting suspension was refluxed for 15 min. The hot solution
was filtered and the precipitate purified by column chromatography (SiO2) with CH2Cl2/petroleum
ether (4 :1) as the eluent. The title compound was obtained as a white solid (4.62 g, 48%), mp 199°C ;
nmax (KBr)/cm-1 3445 (OH), 3078 (sp2 C-H), 2927 (sp3 C-H), 1649 (C=O), 1580 (C=C); dH (400 MHz ;
DMSO d6) 3.67 (3 H, s, OCH3), 6.79 (2 H, d, J 9, phenyl 3,5-H), 6.96 ( 1H, s, OH), 7.21 (2 H, d, J 9,
phenyl 2,6-H), 7.47 (1 H, dd, J 7.7 and 1.3, 2-H anthrone), 7.56 (1 H, t, J 7.7, 7-H anthrone), 7.56 (1
H, t, J 7.7, 3-H anthrone), 7.63 (1 H, dd, J 7.7 and 1.3, 4-H anthrone), 7.68 (1 H, dd, J 7.7 and 1.3, 6-H
anthrone), 8.16 (1 H, dd, J 7.7 and 1.3, 8-H anthrone) ; dC (100 MHz ; DMSO d6) 181.5, 157.6, 153.4,
142.2, 136.7, 136.2, 134.2, 133.4, 133.3, 131.9, 130.6, 129.4, 128.6, 126.7, 125.6, 124.0, 113.4, 71.6,
54.9; m/z 386 (M+, 55), 387 (M+, 21), 384 (M+, 82), 351 (23), 349 (63), 333 (31), 332 (21), 331 (82),
279 (66), 277 (100).
1,5-Dichloro-9,10-dihydroanthracene-9-(4-(1,1-dimethylethyl)phenyl)-10-(4-methoxy-phenyl)-9,10-
diol (7)
1-Bromo-4-(1,1-dimethylethyl)benzene (1.1 g, 5.2 mmol) was dissolved in dry THF (10 mL). The
solution was placed under argon and cooled to 0°C. BuLi (2.1 mL 2.5 M solution in hexanes, 5.2
mmol) was added through a septum and the mixture was allowed to warm to room temperature. The
anthrone 6 (0.50 g, 1.3 mmol) was added and the resulting suspension was stirred overnight. The
reaction mixture was quenched with water (20 mL) and extracted with CH2Cl2 (3 x 15 mL). The

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Molecules 2000, 5
combined organic layers were dried over MgSO4 and the solvent was evaporated in vacuum. The title
compound was obtained after column chromatography (SiO2, CH2Cl2) as a white solid (0.57 g, 85%),
dH (250 MHz ; CDCl3) 1.23 (9 H, s, C(CH3)3), 3.72 (3 H, s, OCH3), 4.48 (1 H, s, OH), 4.53 (1 H, s,
OH), 6.73 (2 H, d, J 8, CHCOCH3), 7.15-7.35 (10 H, m), 7.88-7.93 (2 H, 2 x dd, J 7.5 and 1.3, 4-H and
8-H anthracenediol) ; dC (100 MHz ; CDCl3) 158.4, 149.8, 143.8, 142.9, 142.6, 139.2, 134.2, 134.1,
132.5, 130.7, 130.6, 129.1, 127.8, 127.7, 127.2, 125.6, 125.3, 125.1, 124.9, 113.3, 74.3, 74.1, 55.1,
34.3, 31.3.
5-(1,1-Dimethylethyl)-12-methoxyrubicene (9)
The diol 7 (0.57 g, 1.1 mmol), KI (0.91 g, 5.5 mmol) and NaH2PO2 (0.48 g, 5.5 mmol) were
suspended in acetic acid (6 mL) and the stirred mixture was heated to reflux for 1 h. The resulting
yellowish suspension was allowed to cool to room temperatue and the solids were collected by
filtration, washed with water (2 x 5 mL) and methanol ( 2 x 5 mL) and dried in vacuum. The desired
anthracene 8 (0.39 g, 73%) thus obtained was used without further purification, m/z 487 (24%), 486
(72), 485 (37), 484 (100), 393 (33), 358 (27), 357 (26), 57 (44). This anthracene 8 ( 0.39 g, 0.80
mmol), K2CO3 (0.98 g, 7.0 mmol), Bu4NHSO4 (0.62 g, 1.6 mmol) and Pd(OAc)2 (38 mg, 0.16 mmol)
were suspended in DMF (15 mL) and the stirred mixture was heated at 120°C under argon for 24 h.
After cooling to room temperature, water (20 mL) was added and the precipitate was collected by
filtration. Rubicene 9 was obtained after Soxhlet extraction (toluene) as dark red crystals (0.30 g,
91%), mp > 300°C ; nmax (KBr)/cm-1 3067 (sp3 C-H), 2957 (sp2 C-H), 1614 (C=C), 1458, 1097 (C-O),
1025 (C-O); dH (400 Mhz ; CDCl3) 1.48 (9 H, s, C(CH3)3), 3.98 (3 H, s, OCH3), 6.98 (1H, dd, J 8.3 and
2.2, 13-H), 7.49 (1 H, dd, J 8.0 and 2.0, 6-H), 7.53 (1 H, d, J 2.2, 11-H), 7.73-7.77 (2 H, 2 x t, J 8.5
and 6.7, 2-H and 9-H), 7.98 (1 H, d, J 6.7, 3-H or 10-H), 8.02 (1 H, d, J 2.0, 4-H), 8.03 (1 H, d, J 6.7,
3-H or 10-H) 8.20 (1 H, d, J 8.3, 14-H), 8.22 (1 H, d, J 8.0, 7-H), 8.52 (1 H, d, J 8.5, 1-H or 8-H), 8.58
(1 H, d, J 8.5, 1-H or 8-H); m/z 413 (35%), 412 (100), 398 (26), 397 (76).
5-(1,1-Dimethylethyl)-12-hydroxyrubicene (10)
Rubicene 9 (0.15 g, 0.36 mmol) was dissolved in CH2Cl2 (15 mL) and the resulting solution was
cooled to 0°C. Through a septum, BBr3 was added (0.55 mL 1M solution in CH2Cl2, 0.55 mmol). The
mixture was stirred overnight at room temperature and then poured on crushed ice (ca. 10 g). The
organic layer was separated and evaporated. Methanol (5 mL) was added to the residue and the
precipitate was collected by filtration, yielding the desired compound 10 as a dark red amorphous solid
(0.12 g, 84%), mp > 300°C, dH (400 MHz ; DMSO d6) 1.44 (9 H, s, C(CH3)3), 6.91 (1 H, dd, J 8.2 and
2.3, 13-H), 7.51 (1 H, dd, J 8.0 and 2.0, 6-H), 7.54 (1 H, d, J 2.3, 11-H), 7.85 (1 H, t, J 8.8 and 6.6, 2-
H or 9-H), 7.86 (1 H, t, J 8.8 and 6.6, 2-H or 9-H), 8.18 (1 H, d, J 6.6, 3-H or 10-H), 8.20 (1 H, d, J
2.0, 4-H), 8.30 (1 H, d, J 8.2, 14-H), 8.31 (1 H, d, J 6.6, 3-H or 10-H), 8.37 (1 H, d, J 8.0, 7-H), 8.68 (1

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Molecules 2000, 5
H, d, J 8.8, 1-H or 8-H), 8.71 (1 H, d, J 8.8, 1-H or 8-H), 9.75-9.88 (1 H, br s, OH) ; dC (100 MHz ;
DMSO d6) 157.8, 149.8, 140.9, 138.5, 137.6, 137.5, 136.4, 133.4, 132.3, 131.9, 130.8, 130.3, 129.4,
129.1, 125.2, 124.85, 124.78, 124.61, 124.60, 123.8, 123.0, 121.1, 102.9, 119.1, 115.0, 109.8, 34.8,
31.2 ; m/z 398 (28%), 383 (29), 73 (54), 44(100), 43 (34).
Crown ether derivative 13
Hydroxyrubicene 10 (0.11 g, 0.28 mmol), bromoacetamide 12 (0.13 g, 0.33 mmol) and K2CO3 (92
mg, 0.66 mmol) were suspended in acetone (4 mL) and the mixture was placed under argon. The
stirred suspension was maintained at reflux temperature for 72 h. After cooling to room temperature,
the reaction mixture was poured in water (5 mL) and extracted with CH2Cl2 (2 x 5 mL). The organic
layers were combined and dried over MgSO4. After column chromatography (SiO2) with
EtOAc/CH3OH 1:1 as the eluent, the desired crown ether 13 was obtained as an amorphous solid (79
mg, 39%) (Found: C, 76.65, H, 5.95. C46H43NO7 requires C: 76.54; H: 6.00%); dH (400 MHz ; CDCl3)
1.48 (9 H, s, C(CH3)3), 3.72-3.74 (8 H, br m, Ph(O(CH2)2O(CH2)2)2O), 3.89 (4 H, br m,
Ph(OCH2CH2O(CH2)2)2O), 4.12-4.16 (4 H, br m, Ph(OCH2CH2O(CH2)2)2O), 4.71 (2 H, s, OCH2CON),
6.81 (1 H, d, J 8.4, 6-H benzo), 6.97 (1 H, d, J 8.2, 13-H rubicene), 7.08 (1 H, d, J 8.4, 5-H benzo),
7.42 (1 H, s, 3-H benzo), 7.47 (1 H, d, J 8.1, 6-H rubicene), 7.50 (1 H, s, 11-H rubicene), 7.66 (2 H, br
m, 2-H and 9-H rubicene), 7.88 (1 H, d, J 6.6, 3-H or 10-H rubicene), 7.92 (1 H, d, J 6.6, 3-H or 10-H
rubicene), 7.96 (1 H, s, 4-H rubicene), 8.07 (1 H, d, J 8.2, 14-H rubicene), 8.13 (1 H, d, J 8.1, 7-H
rubicene), 8.35 (1 H, d, J 8.3, 1-H or 8-H rubicene), 8.47 ( 1H, d, J 8.3, 1-H or 8-H rubicene); m/z 721
(M+ ; 5%), 664 (48), 663 (100).
Acknowledgement: We thank the University of Leuven, the FWO-Vlaanderen and the Ministerie voor
Wetenschapsbeleid for financial support. M.S. thanks the FWO Vlaanderen for a predoctoral fellow-
ship.
References and Notes
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lecular meccano, 29 The synthesis and spectroscopic properties of a [2]catenane incorporating an

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Samples Availability: Available from the authors.
© 2000 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.