Synthesis and conformational switching of partially and differentially bridged resorcin[4]arenes bearing fluorescent dye labels

Article abstract of DOI:10.1002/hlca.200390172

We report the synthesis of modified Cram-type cavitands bearing one or two fluorescent labels for single-molecule spectroscopic studies of vase - kite conformational switching (Scheme 3). Syntheses were performed by stepwise bridging of the four couples of neighboring H-bonded OH groups of resorcin[4]arene bowls (Schemes 2 and 3). The new substitution patterns enable the construction of a large variety of future functional architectures. ¹H-NMR Investigations showed that the new partially and differentially bridged cavitands feature temperature- and pH-triggered vase - kite conformational isomerism similar to symmetrical cavitands with four identical quinoxaline bridges (Table). It was discovered that vase - kite switching of cavitands is strongly solvent-dependent.

Full text of DOI:10.1002/hlca.200390172

Helvetica Chimica Acta Vol. 86 (2003)
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Synthesis and Conformational Switching of Partially and Differentially
Bridged Resorcin[4]arenes Bearing Fluorescent Dye Labels
Preliminary Communication
by Vladimir A. Azova), FranÁois Diederich*a), Yoriko Lillb), and Bert Hecht*b)
a) Laboratorium f¸r Organische Chemie, ETH-Hˆnggerberg, CH-8093 Z¸rich
(e-mail: diederich@org.chem.ethz.ch)
b) Nano-optics group, Institut f¸r Physik, Uni Basel, Klingelbergstrasse 82, CH-4056 Basel
(e-mail: bert.hecht@unibas.ch)
We report the synthesis of modified Cram-type cavitands bearing one or two fluorescent labels for single-
molecule spectroscopic studies of vaseÀkite conformational switching (Scheme 3). Syntheses were performed
by stepwise bridging of the four couples of neighboring H-bonded OH groups of resorcin[4]arene bowls
(Schemes 2 and 3). The new substitution patterns enable the construction of a large variety of future functional
architectures. 1H-NMR Investigations showed that the new partially and differentially bridged cavitands feature
temperature- and pH-triggered vaseÀkite conformational isomerism similar to symmetrical cavitands with four
identical quinoxaline bridges (Table). It was discovered that vaseÀkite switching of cavitands is strongly solvent-
dependent.
Cavitands, which consist of a resorcin[4]arene bowl bridged by four quinoxaline (1)
or pyrazine moieties, were introduced by Cram and co-workers [1] as a family of
macrocycles capable of adopting two preferred conformations with profound geometry
and property differences (Scheme 1). The vase conformation, with a deep cavity for
guest inclusion, is preferred at elevated temperatures (> 318 K), whereas the kite
conformation, with a large flat surface, dominates at low temperatures (<213 K).
Conformational switching can also be reversibly induced at ambient temperature by
pH changes, with the kite conformation being preferred at low pH [2]. The vaseÀkite
1
isomerization is conveniently monitored by H-NMR spectroscopy [1] as well as by
optical absorption and emission spectroscopy [2]. The molecular-recognition proper-
ties of the cavitands in the vase form, as well as of supramolecular capsules formed by
self-assembly of two cavitands with H-bonding rims, have been elegantly exploited by
Rebek and co-workers [3].
We are interested in the mechanistic understanding of the temperature- and pH-
triggered vaseÀkite conformational isomerism [2] as well as in the potential utilization
of resorcin[4]arene cavitands as miniaturized mechanical grippers for molecular
construction at the single-molecule level. Within this context, we recently reported the
STM-imaging at molecular resolution of highly ordered self-assembled monolayers
formed by alkylthiol-legged quinoxaline-bridged resorcin[4]arene cavitands on gold
[4]. Investigations of the switching dynamics by polarization-resolved single-molecule
microscopy [5] now required the formation of differentially bridged resorcin[4]arenes
bearing suitable fluorescent dyes. We chose ×borondipyrrylmethane× (BODIPY) dyes

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Scheme 1. Temperature- or pH-Triggered Conformational vaseÀkite Equilibrium inBridged Resorcin[4]arene
1
for their favorable electronic absorption and emission properties, and their low
sensitivity to pH (important in proton-induced switching experiments) and environ-
mental polarity [6]. Here, we report the synthesis of the first dye-labeled resorcin-
[4]arenes and demonstrate that these differentially bridged cavitands as well as their
partially bridged precursors
undergo efficient pH- or temperature-triggered
vaseÀkite conformational switching. Besides, solvent-dependency of the vaseÀkite
equilibrium was discovered and studied for the first time.
Starting resorcin[4]arene 2 was obtained by condensation [7] of resorcinol with
heptanal, recrystalliation several times from MeOH, and drying over P2O5 (Scheme 2).
Selective bridging of three of the four couples of neighboring, H-bonded OH groups
was achieved as described in [8] in a reaction of tetrakis[benzene-diol] 2 with 3 equiv. of
2,3-dichloroquinoxaline, providing triply-bridged cavitand 3 in up to 35% yield. For the
synthesis of doubly-bridged resorcin[4]arenes, 2 equiv. of 2,3-dichloroquinoxaline were
used. The latter conversion yielded a mixture of several products, from which the
desired bowls 4 and 5, with the novel double bridging patterns, could be isolated by
column chromatography (SiO2; CH2Cl2/AcOEt 95 :5 ! 85 :15) together with 3 (up to
20%) and fully bridged 1 (3 5%). Anhydrous conditions and purity of the starting
tetrakis[benzene-diol] 2 proved to be critical for the partial bridging reactions;
otherwise excessive tar formation was observed.
Dichloropyrazinedicarboximide 6 (Scheme 3) was selected for the introduction of
the dyes into cavitands 7 9. In 6 as well as in the corresponding cavitands 7 9 (cf.
Scheme 4), the plane of the phenyl ring is nearly orthogonal to the plane of the
BODIPY dye, and their p-systems are electronically decoupled; this ensures a very
small influence of changes in the residual molecule (e.g., from protonation) on the
position of the absorption/emission bands of the dye. In the synthesis of 6, anhydride 10
was prepared in two steps from 2,3-dichloroquinoxaline [1c]. The BODIPY fragment
11 was obtained in several steps starting from 2,4-dimethyl-3-ethyl-1H-pyrrole and 4-
nitrobenzaldehyde (Scheme 3). Subsequent reaction of anhydride 10 with 11 under
activation with oxalyl chloride provided imide 6.
Finally, the BODIPY-substituted cavitands 7 9 were prepared from diol 3 and
tetraols 4 and 5, respectively, as shown in Scheme 4. All three cavitands are brightly red-
colored, featuring sharp optical absorption/emission bands in CHCl3 with maxima at
530/540 nm, respectively (Fig.). The substitution patterns of cavitands 8 and 9 are
entirely novel and open multiple opportunities for future functionalization. For

Helvetica Chimica Acta Vol. 86 (2003)
Scheme 2. Synthesis of Resorcin[4]arenes 3
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5
a) Conc. HCl, EtOH, 908, 14 h; 51%. b) 2,3-Dichloroquinoxaline (2 equiv.), K2CO3, Me2SO, 208 (8 h), then 508
(18 h); 35% (3). c) 2,3-Dichloroquinoxaline (3 equiv.), K2CO3, Me2SO, 208 (18 h), then 508 (6 h); 16% (3),
3.5% (4), 20% (5).
Scheme 3. Synthesis of Dichlorodiazaphthalimide 6
a) KMnO4, H2O, 95 978, 2 h; 49%. b) (COCl)2, Py (cat.), THF, 508, 20min; 49%. c) TFA, CH2Cl2, 208, 2 h. d)
DDQ, toluene, 1 h. e) Et3N, 208 (10min), then BF 3 ¥ Et2O, 208 (30min), then 50 8 (1 h); 31% for c e. f) H2
(1 atm), Pd/C (10%), CHCl3/EtOH (1:1), 208, 12 h; 67%. g) THF, 1 h, then (COCl)2, Py, 508, 12 h; 72%. Py ˆ
pyridine, TFA ˆ CF3COOH, DDQ ˆ 2,3-dichloro-5,6-dicyano-p-benzoquinone.
example, receptor sites can be attached instead of the dye moieties for specific guest
binding.
Before starting polarization-resolved single-molecule fluorescence-microscopic
investigations, it was important to demonstrate that the substantial structural
modification of cavitands 7 9 as compared to parent 1 was not interfering with
reversible temperature- or pH-induced vaseÀkite switching. This conformational
isomerism is conveniently monitored by measuring the 1H-NMR chemical shift of the
methine H-atom in the skeleton of the octol bowl. In 1, this resonance appears at d ꢀ
5.5 ppm in the vase and at d ꢀ 3.7 ppm in the kite conformation (Table). Acid-triggered

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Helvetica Chimica Acta Vol. 86 (2003)
Scheme 4. Synthesis of the Dye-Labeled Cavitands 7
9
a) 3 (1 equiv.), 6 (1 equiv.), K2CO3 (1 equiv.), Me2SO, 208, 24 h; 73%. b) 4 (1 equiv.), 6 (2.2 equiv.), K2CO3 (2.2
equiv.), Me2SO, 208, 4 h, then K2CO3 (2.2 equiv.), 30min; 54%. c) 5 (1 equiv.), 6 (2 equiv.), K2CO3 (2 equiv.),
Me2SO, 208, 24 h; 66%.
vase ! kite isomerization is readily reversed upon addition of base (K2CO3 plus
catalytic Et3N). The following results were obtained.
i) vaseÀkite Switching is strongly solvent-dependent and was observed only in the
apolar, nonaromatic solvents CDCl3/CS2 1:1, CD2Cl2, and C2D2Cl4. In (D8)toluene or
1
(D6)benzene, addition of TFA led to minor changes in the H-NMR spectrum that
cannot be ascribed to vaseÀkite equilibration; no temperature variations in (D8)tol-
uene (except for line broadening) were detected. In the polar solvents ((D6)acetone
and (D8)THF), no spectral changes upon TFA addition or temperature variation were
observed. This supports the explanation advaned by Cram and co-workers that

Helvetica Chimica Acta Vol. 86 (2003)
2153
Figure. Electronic absorption and emission spectra of cavitand 7. Intensity not to scale. Cavitands 8 and 9 feature
similar spectra.
Table. 1H-NMR Investigation of the Temperature- or pH-Triggered vaseÀkite Isomerization, with Monitoring of
the Methine Resonance of the Octol Bowl
Cavitand
Solvent
dH [ppm]/T [K]a)
dH [ppm]/T [K]b)c)
dH [ppm] at [TFA] ˆ
0.5mb)
1
3
4
5
7
8
9
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CDCl3/CS2 (1 : 1)
CDCl3/CS2 (1 : 1)
5.51/293
3.65/203
3.65(3 H), 4.40/193
3.76
5.61, 5.51 (2 H), 4.29/308
5.39 (2 H), 4.36 (2 H)/293
5.52 (2 H), 4.22 (2 H)/293
5.52 (3 H), 5.40/308
5.56 (2 H), 5.37 (2 H)/333
5.56 (2 H), 5.35 (2 H)/333
4.37, 3.96 (2 H), 3.85
4.43 (2 H), 3.74 (2 H)
4.35 (2 H), 3.79 (2 H)
3.80
3.89 (2 H), 3.83 (2 H)
3.97 (2 H), 3.91 (2 H)
3.65/193
3.71/193
3.71/193
a) Corresponds to vase conformation. b) Corresponds to kite conformation. c) Multiplet is not resolved.
solventÀcavitand interactions represent a key factor in controlling the switching
process [1a,b].
ii) All BODIPY-dye-substituted cavitands undergo reversible vaseÀkite switching
both upon temperature change and upon TFA addition. The differences in temperature
at which the vase conformer is present exclusively, were, however, noticeable: in the
1H-NMR spectrum of cavitand 7 with a single pyrazinedicarboximide wall component,
all signals of the vase conformation became resolved at 308 K (CD2Cl2), as compared to
293 K for 1. In the spectra of cavitands 8 and 9 with two pyrazinedicarboximide wall
components, signals of the vase conformer became resolved only at 323 K (CDCl3).

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Helvetica Chimica Acta Vol. 86 (2003)
iii) Interestingly and unexpectedly, even the partially bridged resorcin[4]arenes 3, 4,
and 5 undergo switching. For diol 3, low-temperature experiments revealed that the
switching was not complete, i.e., both kite and vase conformers were present at 193 K
(CD2Cl2). On the other hand, complete switching from vase to kite occurred upon
addition of TFA. For tetrols 4 and 5, low-temperature scans could not be performed,
since they started to precipitate from a solution at temperatures below 273 K; addition
of TFA at room temperature, on the other hand, led to complete transition from vase to
kite. These experiments clearly underline the power of the pH-switching protocol
recently introduced by us [2].
In summary, the BODIPY-dye-labeled cavitands 7 9 were prepared and shown in
solution to undergo reversible vaseÀkite switching triggered either by temperature or
pH changes. Samples immobilized at the solidÀliquid interface support are now being
investigated by polarization-resolved single-molecule microscopy in real time. The
optical spectra in solution, as well as preliminary fluorescence-microscopy studies on 6
and 7 in spin-coated films in poly(methyl methacrylate) (PMMA) or neat on glass
microscope cover slips, clearly confirmed the stability of the BODIPY dye labels and
their compatibility with methods of single-molecule microscopy. Monitoring the
switching at the level of single molecules should allow assessment of important
parameters such as switching time and orientational distribution under various
conditions, and these investigations are currently being pursued.
Support by the Swiss National Science Foundation, via the NFP ×Supramolecular Functional Materials×, the
NCCR ×Nanoscience×, and a SNF Fˆrderprofessur (B. H. ) is gratefully acknowledged.
Experimental Part
7-[4-(2,8-Diethyl-5,5-difluoro-1,3,7,9-tetramethyldipyrrolo[1,2-c :2,1-f][1,3,2]diazaborinin-4-ium-5-uid-10-
yl)phenyl]-60-endo,64-endo,68-endo,72-endo-tetrahexyl-2,12,16,27,31,42,46,57-octaoxa-4,7,10,18,25,33,40,48,55-
nonaazaheptadecacyclo[56.15.1.159,73.03,11.05,9.013,71.015,69.017,26.019,24.028,67.030,65.032,41.034,39.043,63 .045,61.047,56.049,54]penta-
heptaconta-1(73),3,5(9),10,13,15(69),17,19(24),20,22,25,28,30(65),32,34(39),35,37,40,43,45(61),47,49(54),50,52,-
55,58(74),59(75),62,66,70-triacontaene-6,8-dione (7). Bright-red solid. Rf (SiO2; CH2Cl2/AcOEt, 98 :2) 0.55.
UV/VIS (CHCl3): lmax 530( e 71000 mÀ1 cmÀ1). Fluorescence (CHCl3): lmax 542. IR (CCl4): 3075, 2962, 2930,
2859, 1744, 1543, 1481, 1415, 1400, 1363, 1335, 1265, 1192, 1161, 1118, 1087, 981, 898. 1H-NMR (CDCl3,
300 MHz): 0.90 0.96 (m, 12 H); 1.0 6 (t, J ˆ 7.5, 6 H); 1.28 1.53 (m, 32 H); 1.45 (s, 6 H); 2.21 2.32 (br. m, 8 H);
2.39 (q, J ˆ 7.5, 4 H); 2.59 (s, 6 H); 5.48 (t, J ˆ 7.8, 1 H); 5.54 5.63 (m, 3 H); 7.21 7.31 (m, 8 H); 7.41 7.52 (m,
6 H); 7.79 7.85 (m, 4 H); 7.96 8.00 (m, 2 H); 8.16 (s, 2 H); 8.17 (s, 2 H). 13C-NMR (CDCl3, 75 MHz): 12.30;
12.94; 14.39; 15.01; 17.49; 22.97; 28.20; 29.66; 32.15; 32.70; 34.63; 118.56; 118.93; 123.47; 123.81; 126.74; 127.57;
127.90; 128.99; 129.30; 130.74; 131.40; 133.29; 135.42; 135.58; 135.83; 136.18; 136.91; 138.09; 138.51; 139.56;
139.64; 139.72; 141.05; 152.09; 152.24; 152.31; 152.41; 152.58; 152.63; 152.86; 154.46; 158.56; 161.15. 19F-NMR
(CDCl3, 282.5 MHz): À145.12 (q, J ˆ 35). HR-MALDI-MS (2,5-dihydroxybenzoic acid (DHB)): 1706.7853
‡
‡
‡
‡
([M À F] , C105H102BFN11O10 ; calc. 1706.78882), 1726.7983 (MH , C105H103BF2N11O10 ; calc. 1726.79505).
7,36-Bis[4-(2,8-diethyl-5,5-difluoro-1,3,7,9-tetramethyldipyrrolo[1,2-c:2,1-f][1,3,2]diazaborinin-4-ium-5-
uid-10-yl)phenyl]-59-endo,63-endo,67-endo,71-endo-tetrahexyl-2,12,16,27,31,41,45,56-octaoxa-4,7,10,18,25,33,-
36,39,47,54-decaazaheptadecacyclo[55.15.1.158,72.03,11.05,9.013,70.015,68.017,26.019,24.028,66.030,64.032,40.034,38.042.62.044,60.-
046,55.048,53]tetraheptaconta-1(72),3,5(9),10,13,15(68),17,19(24),20,22,25,28,30(64),32,34(38),39,42,44(60),-
46,48(53),49,51,54,57(73),58(74),61,65,69-octacosaene-6,8,35,37-tetrone (8). Bright-red solid. Rf (SiO2; CH2Cl2/
AcOEt, 97:3) 0.4. UV/VIS (CHCl3): lmax 529 (e 122000 mÀ1 cmÀ1). Fluorescence (CHCl3): lmax 542. IR (CCl4):
3080, 2963, 2930, 2859, 1746, 1544, 1481, 1412, 1363, 1322, 1264, 1192, 1160, 1088, 981, 900. 1H-NMR ((D8)THF,
300 MHz): 0.93 0.99 (m, 24 H); 1.44 (s, 12 H); 1.35 1.55 (m, 32 H); 2.27 2.49 (m, 16 H); 2.51 (s, 12 H); 5.71 (t,
J ˆ 8.1, 2 H); 5.80( t, J ˆ 8.1, 2 H); 7.32 7.36 (m, 4 H); 7.50( s, 4 H); 7.51 7.59 (m, 8 H); 7.98 8.02 (m, 4 H); 8.26
(s, 4 H). 13C-NMR ((D8)THF, 125 MHz): 12.06; 12.63; 14.42; 14.96; 17.64; 23.59; 25.87; 28.94; 28.99; 30.18;

Helvetica Chimica Acta Vol. 86 (2003)
2155
30.25; 30.63; 32.84; 32.89; 33.14; 35.09; 119.78; 124.84; 127.25; 129.50; 129.93; 130.17; 131.46; 132.96; 133.71;
136.84; 136.99; 137.97; 138.07; 139.72; 140.70; 142.73; 153.21; 153.29; 153.96; 155.08; 159.04; 156.32. 19F-NMR
(CDCl3, 282.5 MHz): À143.99 (q, J ˆ 35). HR-MALDI-MS (trans-2-{3-[4-(tert-butyl)phenyl]-2-methylprop-2-
‡
‡
enylidene}malononitrile (DCTB)): 2103.9791 ([M À F] , C126H124B2F3N14O12 ; calc. 2103.96614), 2145.9598
‡
‡
([M ‡ Na] , C126H124B2F4N14O12Na ; calc. 2145.95431).
7,21-Bis[4-(2,8-diethyl-5,5-difluoro-1,3,7,9-tetramethyldipyrrolo[1,2-c:2,1-f][1,3,2]diazaborinin-4-ium-5-
uid-10-yl)phenyl]-59-endo,63-endo,67-endo,71-endo-tetrahexyl-2,12,16,26,30,41,45,56-octaoxa-4,7,10,18,21,24,-
32,39,47,54-decaazaheptadecacyclo[55.15.1.158,72.03,11.05,9.013,70.015,68.017,25.019,23.027,66.029,64.031,40.033,38.042.62.044,60.-
046,55.048,53]tetraheptaconta-1(72),3,5(9),10,13,15(68),17,19(23),24,27,29(64),31,33(38),34,36,39,42,44(60),-
46,48(53),49,51,54,57(73),58(74),61,65,69-octacosaene-6,8,20,22-tetrone (9). Bright-red solid. Rf (SiO2; CH2Cl2/
EtOAc, 97:3) 0.44. UV/VIS (CHCl3): lmax 530( e 135000 mÀ1 cmÀ1). Fluorescence (CHCl3): lmax 543. IR (CCl4):
3075, 2963, 2930, 2860, 1744, 1543, 1481, 1414, 1364, 1322, 1264, 1192, 1162, 1116, 1085, 980, 899. 1H NMR
((D8)THF, 300 MHz): 0.93 0.99 (m, 24 H); 1.26 (s, 12 H); 1.36 1.54 (m, 32 H); 2.29 2.46 (m, 16 H); 2.50( s,
12 H); 5.68 (t, J ˆ 8.1, 2 H); 5.79 (t, J ˆ 8.1, 2 H); 7.28 7.65 (m, 16 H); 7.89 8.08 (m, 5 H); 8.20 8.24 ( m, 3 H).
13C-NMR ((D8)THF, 75 MHz): 12.17; 12.52; 14.34; 14.96; 17.51; 23.47; 28.80; 28.85; 30.05; 30.12; 32.77; 32.92;
34.96; 35.06; 119.46; 119.54; 119.81; 124.36; 124.79; 125.43; 127.86; 128.66; 129.08; 129.43; 129.93; 130.14; 131.19;
132.67; 133.43; 136.34; 136.67; 136.75; 137.30; 137.97; 138.38; 139.74; 140.16; 142.62; 142.94; 152.61; 153.05;
153.38; 153.53; 154.41; 157.81; 158.22; 161.62; 161.89. 19F-NMR (CDCl3, 282.5 MHz): À144.19 (q, J ˆ 35). HR-
‡
‡
‡
MALDI-MS (DHB): 2103.9735 ([M À F] , C126H124B2F3N14O12 ; calc.: 2103.96614), 2145.9580 ([M ‡ Na] ,
‡
C
126H124B2F4N14O12Na ; calc. 2145.95431).
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Received February 27, 2003