A novel antenna cyclodextrin: Synthesis and photosensitized reaction of an included guest

Article abstract of DOI:10.1021/ja963072s

A novel water-soluble photochemical microreactor has been prepared. A perbutylated cyclodextrin bearing seven naphthalenesulfonate antenna chromophores (Bu-CD-N₇) displays improved solubilization of hydrophobic guests in comparison with the unbutylated material (CD-N₇). The photophysical properties of Bu-CD-N₇ are similar to those of the CD-N₇. The naphthyl chromophores of Bu-CD-N₇ were used to sensitize the photolysis of 9-anthrylmethyl pivalate (AP). Inclusion of AP in the cyclodextrin leads to the selective formation of one of the two possible coupling products.

Full text of DOI:10.1021/ja963072s

4364
J. Am. Chem. Soc. 1997, 119, 4364-4368
A Novel Antenna Cyclodextrin: Synthesis and Photosensitized
Reaction of an Included Guest
Maria Nowakowska,† Nikolai Loukine, David M. Gravett,
Nicholas A. D. Burke, and James E. Guillet*
Contribution from the Department of Chemistry, UniVersity of Toronto,
Toronto, Ontario, Canada M5S 3H6
ReceiVed September 3, 1996X
Abstract: A novel water-soluble photochemical microreactor has been prepared. A perbutylated cyclodextrin bearing
seven naphthalenesulfonate antenna chromophores (Bu-CD-N7) displays improved solubilization of hydrophobic
guests in comparison with the unbutylated material (CD-N7). The photophysical properties of Bu-CD-N7 are similar
to those of the CD-N7. The naphthyl chromophores of Bu-CD-N7 were used to sensitize the photolysis of
9-anthrylmethyl pivalate (AP). Inclusion of AP in the cyclodextrin leads to the selective formation of one of the
two possible coupling products.
Introduction
guests.18 The antenna cyclodextrin bearing naphthalenesulfonate
groups on C6 of each of the seven sugar units possessed several
important features: (1) water solubility, (2) absorption in the
UV spectral region, (3) energy migration between the antenna
naphthyl groups, (4) binding of hydrophobic guests, and (5)
energy donation to molecules included in the cyclodextrin cavity.
Energy transfer is efficient because excimer formation between
antenna chromophores, an energy wasting process, is disfavored
by the mutual repulsion of the naphthalenesulfonate groups.
Subsequently, a very similar antenna system in which
â-cyclodextrin is labeled with seven naphthalenecarboxylate
groups was developed.18,19 This system was found to form quite
stable 1:1 complexes (K ) 1.2 × 105 M-1 in 5% ethanol) with
4-(dicyanomethylene)-2-methyl-6-(4-(bis(hydroxyethyl)amino)-
styryl)-4H-pyran), a derivative of DCM, a merocyanine laser
dye. The strong complexation was thought to be due to a good
match between the size of the guest and the dimensions of the
cavity. Highly efficient energy transfer (efficiency ) 1) from
the naphthoate groups to the guest was observed.
In order to use the antenna cyclodextrins as photochemical
microreactors, high association constants are required. Unfor-
tunately, the association constants for many cyclodextrin-guest
complexes are fairly low (typically K e 103 M-1).12,20 If high
concentrations of cyclodextrin are used to ensure that all guest
molecules are complexed, most of the light will be absorbed
by unoccupied cyclodextrins and the photochemical efficiency
will be low. On the other hand, if a high guest concentration
is used to ensure that all cyclodextrin molecules are occupied,
photochemical reactions of free guest molecules may dominate.
For this reason it is desirable to improve the binding of
hydrophobic guests by increasing the hydrophobicity of the
In this laboratory there has been an ongoing interest in the
antenna effect.1-3 This has led to the development of a number
of water-soluble antenna polymers which have been shown to
act as photocatalysts.4-9 With the desire to improve the
substrate selectivity of the photocatalysts, two antenna systems
which include a group capable of forming host-guest complexes
have been developed. In one of these systems, calix[4]arene
groups were incorporated into a water-soluble polymer.10
Naphthalenesulfonate groups acted as the antenna chromophores
as well as imparting water solubility.
The second approach, of which this paper is an extension,
employs a â-cyclodextrin as the host group. In a previous paper,
the synthesis and photophysical properties of a derivatized
â-cyclodextrin displaying the antenna effect were presented.11
Cyclodextrin was chosen because of its ability to (1) form
inclusion complexes with a variety of guest molecules12 and
(2) influence the outcome of photochemical reactions.13 Several
other photoactive cyclodextrins14-17 were known but none that
utilized the antenna effect to enhance photoreactions of included
† Permanent address: Faculty of Chemistry, Jagiellonian University,
Krakow, Poland.
X Abstract published in AdVance ACS Abstracts, April 15, 1997.
(1) Holden, D. A.; Rendall, W. A.; Guillet, J. E. Ann. N.Y. Acad. Sci.
1981, 366, 11.
(2) Guillet, J. E.; Rendall, W. A. Macromolecules 1986, 19, 224.
(3) Guillet, J. E.; Wang, J.; Gu, L. Macromolecules 1986, 19, 2793.
(4) Nowakowska, M.; White, B.; Guillet, J. E. Macromolecules 1989,
22, 3903.
(5) Nowakowska, M.; White, B.; Guillet, J. E. Macromolecules 1988,
21, 3430.
(6) Nowakowska, M.; White, B.; Guillet, J. E. Macromolecules 1989,
22, 2317.
(7) Nowakowska, M.; Sustar, E.; Guillet, J. E. J. Am. Chem. Soc. 1991,
113, 253.
(8) Nowakowska, M.; Foyle, V. P.; Guillet, J. E. J. Am. Chem. Soc. 1993,
115, 5975.
(9) Guillet, J. E.; Burke, N. A. D.; Nowakowska, M.; Reese, H.; Gravett,
D. M. Macromol. Symp. 1995, 98, 53.
(10) Gravett, D. M.; Guillet, J. E. Macromolecules 1996, 29, 617.
(11) Gravett, D. M.; Guillet, J. E. J. Am. Chem. Soc. 1993, 115, 5970.
(12) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Akademiai
Kiado: Budapest, 1971.
(16) Neckers, D. C.; Paczkowski, J. J. Am. Chem. Soc. 1986, 108, 291.
Neckers, D. C.; Paczkowski, J. Tetrahedron 1986, 42, 4671.
(17) Aquino, A. M.; Abelt, C. J.; Berger, K. L.; Darragh, C. M.; Kelley,
S. E.; Cossette, M. V. J. Am. Chem. Soc. 1990, 112, 5819.
(18) During review of this manuscript, a paper (Wang, P. F.; Jullien, L.;
Valeur, B.; Filhol, J.-S.; Canceill, J.; Lehn, J.-M. New J. Chem. 1996, 20,
895) appeared describing the isomerization of a nitrone sensitized by an
antenna cyclodextrin. We thank a referee for bringing this publication to
our attention.
(13) Ramamurthy, V.; Eaton, D. F. Acc. Chem. Res. 1988, 21, 300.
Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood:
Toronto, 1991; p 295.
(14) Tabushi, I.; Fujita, K.; Yuan, L. C. Tetrahedron Lett. 1977, 2503.
(15) Gonzalez, M. C.; McIntosh, A. R.; Bolton, J. R.; Weedon, A. C. J.
Chem. Soc., Chem. Commun. 1984, 1138.
(19) Jullien, L.; Canceill, J.; Valeur, B.; Bardez, E.; Lehn, J.-M. Angew.
Chem., Int. Ed. Engl. 1994, 33, 2438. Jullien, L.; Canceill, J.; Valeur, B.;
Bardez, E.; Lefevre, J.-P.; Lehn, J.-M.; Marchi-Artzner, V.; Pansu, R. J.
Am. Chem. Soc. 1996, 118, 5432.
(20) Blyshak, L. A.; Dodson, K. Y.; Patonay, G.; Warner, I. M.; May,
W. E. Anal. Chem. 1989, 61, 955.
S0002-7863(96)03072-7 CCC: $14.00 © 1997 American Chemical Society

A NoVel Antenna Cyclodextrin
J. Am. Chem. Soc., Vol. 119, No. 19, 1997 4365
cyclodextrin. This paper describes the synthesis and properties
of a butylated cyclodextrin which bears seven naphthalene-
sulfonate groups as antenna chromophores. This system
displays improved binding of a hydrophobic guest and sensitizes
the selective photochemical reaction of the included guest.
Scheme 1
Experimental Details
General. 1H and 13C NMR spectra were measured with a Varian
Gemini 200 MHz spectrometer. IR spectra were recorded on a Nicolet
8210E FT-IR. UV-vis absorption spectra were measured with a
Hewlett-Packard 8451A diode-array spectrophotometer. Steady state
fluorescence spectra (uncorrected) were measured at room temperature
with an SLM 4800S spectrofluorometer. Elemental analyses were
performed by Guelph Chemical Laboratories (Guelph, Canada). GC-
MS analyses were performed with a Hewlett-Packard 5890 GC and a
VG Analytical 70-250S mass spectrometer. Electrospray ionization
mass spectroscopy was performed at the Biological Mass Spectroscopy
Laboratory at the University of Waterloo (Waterloo, Canada).
Materials. The preparation of 6A,6B,6C,6D,6E,6F,6G-hepta(6-oxy-2-
naphthalenesulfonate, sodium salt)-â-cyclodextrin (CD-N7) has been
described.11 9-Anthrylmethyl pivalate (AP)21 and sodium 6-methoxy-
2-naphthalenesulfonate (MNSS)11 were prepared as described earlier.
2,2-Di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH; Aldrich) and meth-
ylene blue (MB; Aldrich) were used as received. Methanol (Caledon;
Spectro Grade) and benzene (Caledon; Spectro Grade) were used
without further purification. Deionized water from a Millipore Milli
Q water purification system was used to prepare aqueous solutions.
2A,2B,2C,2D,2E,2F,2G,3A,3B,3C,3D,3E,3F,3G-Tetradecabutyl-
6A,6B,6C,6D,6E,6F,6G-hepta(6-oxy-2-naphthalenesulfonate, sodium salt)-
â-cyclodextrin (Bu-CD-N7). Butylation was performed by a method
similar to that used by Brimacombe et al.22 A solution of CD-N7 (3.0
g, 1.1 mmol) in dry dimethylsulfoxide (100 mL) was transferred into
a 200 mL flask containing NaH (1.32 g, 55.1 mmol) under a nitrogen
atmosphere. The resulting suspension was stirred under nitrogen for
20 min and then cooled in an ice bath before 1-bromobutane (11.5
mL, 107.0 mmol) was added dropwise over 5 min. After 40 min, the
nitrogen inlet and cooling bath were removed, and the yellowish, nearly
solid suspension was left at room temperature for 40 h. Methanol (12
mL) was added carefully to the suspension with stirring. When
effervescence had ceased, the precipitate was filtered. The solvent was
removed under vacuum, and the resulting solid was reprecipitated twice
from water into methanol and once from water into ethanol. Vacuum
drying at 90 °C provided a white powder (1.8 g, 47%).
The NMR peaks (both 13C and 1H) were broad and overlapped. 13C
NMR (D2O, δ): 158.0, 138.5, 135.7, 131.3, 128.2, 126.0, 123.6, 118.6,
107.6, 102.4, 80.8, 73.4, 71.7, 68.6, 32.8, 20.2, 14.6. 1H NMR (D2O,
δ): 7.9, 7.2, 6.5, 4.8, 3.5, 1.4, 1.0. IR (cm-1): 3444, 2958, 2934,
2873, 1631, 1460, 1265, 1216, 1188, 1129, 1101, 1040. Anal. Calcd
for C168H217O56S7Na7: C, 57.36; H, 6.22; S, 6.38 (calcd for C168H217O56S7-
Na7 + 10 H2O: C 54.57; H, 6.46; S, 6.07). Found: C, 54.45; H, 6.2;
S, 5.76.
Electrospray mass spectra were measured for both the starting
material (CD-N7) and the product (Bu-CD-N7). Samples were dissolved
in 1% CH3CN or 1% NH4OH prior to introduction to the mass
spectrometer. For CD-N7 (1% NH4OH): 2665, 2644, 2622, 2600, 2578,
2476, 2454, 2416, 2398, 2394, 2376, 2354. For Bu-CD-N7 (1% NH4-
OH): 3385, 3363, 3307, 3251, 3195, 3139, 3083.
Solubility of Anthrylmethyl Pivalate in Aqueous Solutions.
Aqueous solutions were saturated with AP by making four injections
(2.5-12.5 µL each) of an AP solution (0.0513 M in ethanol) into 5.0
mL of water or aqueous cyclodextrin solution. The solutions were
vigorously shaken after each addition of AP and then left to stand for
3 days in the dark. The solutions were passed through a syringe filter
(Whatman GF/F glass microfiber filter) to remove any fine particles
of undissolved AP. The aqueous AP solutions were analyzed by UV-
vis and fluorescence spectroscopies.
lamps (RPR-3000). Quantitative and qualitative analyses of photolyzed
solutions were conducted with a Hewlett-Packard 5890 gas chromato-
graph equipped with an FID and a J&W Scientific DB-1 capillary
column (30 m × 0.25 mm (i.d.) × 0.25 µm (film thickness)). The
photoproducts were identified by GC-MS and/or NMR analysis. Prior
to NMR analysis the photoproducts were separated from the reactant
by preparative TLC (silica, heptane/benzene (1:1)). The photoproducts
consisted of 9-methylanthracene (mass spectrum (EI): 192 (M+). 1H
NMR (CDCl3, δ): 3.11 (s, 3H, CH3), 7.2-8.4 (m, ArH)), 9-neopen-
tylanthracene (mass spectrum (EI): 248 (M+). 1H NMR (CDCl3, δ):
0.98 (s, 9H, t-Bu), 3.68 (s, 2H, CH2), 7.2-8.4 (m, ArH)), and 9-t-
butyl-10-methylanthracene (mass spectrum (EI): 248 (M+). 1H NMR
(CDCl3, δ): 1.54 (s, 9H, t-Bu), 3.38 (s, 3H, CH3), 7.2-8.4 (m, ArH)).
Results and Discussion
Characterization of Bu-CD-N7. Bu-CD-N7 was prepared
by the butylation of CD-N7 (Scheme 1). The FT-IR spectrum
contains absorptions for CH3 stretching and bending vibrations
(2958, 2871, and 1463 cm-1) and CH2 stretching vibrations
(2933 cm-1) which reveals the presence of butyl groups in Bu-
CD-N7. Similarly, 13C NMR signals at 14.6, 20.2, and 32.8
ppm and 1H NMR signals at 1.0 and 1.4 ppm can be attributed
to the butyl groups. The NMR signals were broad, and it was
not possible to accurately quantify the degree of butylation of
the cyclodextrin. We believe that this is due to the presence of
a mixture of butylated isomers/homologues (Vide infra) and slow
tumbling of the large molecule in D2O. The elemental analysis
revealed lower than expected carbon and sulfur levels; however,
it is common for both cyclodextrins and sulfonate salts to include
water in their crystalline forms. It appears that the cyclodextrin
contains about 10 waters of crystallization.
Electrospray ionization mass spectroscopy (ES-MS) is a
“gentle” mass spectral technique applicable to high molecular
weight molecules.23 It has been used to characterize cyclodex-
trin compounds24-26 and inclusion complexes,23,27-33 although
there is some doubt that it is inclusion complexes that are
(23) Przybylski, M.; Glocker, M. O. Angew. Chem., Int. Ed. Engl. 1996,
35, 807.
(24) Huang, E. C.; Henion, J. D. Rapid Commun. Mass Spectrom. 1990,
4, 467.
(25) Tinke, A. P.; van der Hoeven, R. A. M.; Niessen, M. W. A.; van
der Greef, J.; Vincken, J.-P.; Schols, H. A. J. Chromatogr. 1993, 647, 279.
(26) Bates, P. S.; Parker, D.; Patti, A. F. J. Chem. Soc., Perkin Trans. 2
1994, 657.
(27) Bates, P. S.; Parker, D.; Green, B. N. J. Chem. Soc., Chem. Commun.
1993, 693.
(28) Camillieri, P.; Haskins, N. J.; New, A. P.; Saunders, M. R. Rapid
Commun. Mass Spectrom. 1993, 7, 949.
(29) Bates, P. S.; Kataky, R.; Parker, D. Analyst 1994, 119, 181.
(30) Camillieri, P.; Haskins, N. J.; Howlett, D. R. FEBS Lett. 1994, 341,
256.
(31) Haskins, N. J.; Saunders, M. R.; Camillieri, P. Rapid Commun. Mass
Spectrom. 1994, 8, 423.
(32) Ramanathan, R.; Prokai, L. J. Am. Soc. Mass Spectrom. 1995, 6,
866.
Photolysis. AP solutions were irradiated in quartz tubes with a
Rayonet RPR-100 photochemical reactor equipped with 16 300 nm
(21) Holden, D. A.; Guillet, J. E. Macromolecules 1980, 13, 289.
(22) Brimacombe, J. S.; Jones, B. D.; Stacey, M.; Willard, J. J.
Carbohydr. Res. 1966, 2, 167.
(33) Kelly, P. M.; Kataky, R.; Parker, D.; Patti, A. F. J. Chem. Soc.,
Perkin Trans. 2 1995, 1955.

4366 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Nowakowska et al.
two naphthalenesulfonates in the sodium salt form and five in
the acid form and that at 2578 g/mol is for the cyclodextrin
bearing seven naphthalenesulfonic acid groups. It should be
emphasized that the naphthalenesulfonic acid is formed by the
vaporization and ionization processes during the ES-MS experi-
ment with the cyclodextrin/1% NH4OH solution and does not
reflect the concentration of this species in aqueous solution. In
aqueous solution, the naphthalenesulfonate groups of CD-N7
exist as the sodium salt.
The portion of the mass spectrum due to cyclodextrin with
six naphthalenesulfonates indicates that several species are
present. It appears that C6 of the unsubstituted sugar unit may
exist as a CH3 (peaks at 2354, 2376, and 2398 g/mol), CH2OH
(2372, 2394, and 2416 g/mol), or CH2Br (2454 and 2476 g/mol)
group. Both the CH3 and CH2Br groups are remnants of earlier
steps in the synthesis of CD-N7.11 In addition, the small peak
at 2151 g/mol is consistent with â-cyclodextrin bearing five
naphthalenesulfonates (acid form) with the two unlabeled C6
positions in the form of a CH3 and CH2OH. The peak at 2943
g/mol is consistent with a γ-cyclodextrin bearing eight naph-
thalenesulfonic acids, where the γ-cyclodextrin occurred as a
minor impurity in the â-cyclodextrin starting material. The
results for CD-N7 illustrate the great utility of ES-MS for the
characterization of cyclodextrin compounds.
Figure 1. Electrospray mass spectrum of CD-N7 in 1% NH4OH.
The spectrum of Bu-CD-N7 measured for a sample dissolved
in 1% CH3CN was rather complex, since the peaks due to
cyclodextrins of differing degreees of butylation were further
split by the conversion of some naphthalenesulfonates from the
salt to the acid form during vaporization and ionization. In 1%
NH4OH, the conversion of the sodium sulfonates to sulfonic
acid groups is more complete, and the spectrum (Figure 2) is
considerably simplified. The peaks at 3363 and 3385 g/mol
are from the fully butylated â-cyclodextrin bearing seven
naphthyl groups, Bu-CD-N7 (Scheme 1). The peaks at 3307,
3251, 3195, and 3139, and 3083 g/mol are for this cyclodextrin
substituted with 13, 12, 11, 10, and 9 butyl groups, respectively.
From the intensity of these peaks it can be estimated that, on
average, 12.6 of the 14 possible OH groups have been butylated.
Thus, butylation has been extensive but not complete, which is
not surprising since alkylation of cyclodextrins are often
incomplete.26 The mass spectrum also contains several smaller
peaks at about 3155, 3177, 3215, and 3234 g/mol, which are
consistent with Bu-CD-N6-CH2OBu (Scheme II) bearing 14 and
15 butyl groups. Thus, with the aid of ES-MS it is possible to
conclude that the antenna cyclodextrin denoted as Bu-CD-N7
is composed of â-cyclodextrin bearing seven naphthalene-
sulfonates (as the sodium salt) on the primary rim and, on
average, 12.6 butyl groups on the secondary rim.
Figure 2. Electrospray mass spectrum of Bu-CD-N7 in 1% NH4OH.
Scheme 2
Inclusion of AP. UV-vis and fluorescence spectroscopies
revealed that 9-anthrylmethyl pivalate (AP) is sparingly soluble
(9 × 10-7 M) in water containing 1% ethanol, but that the
solubility increases considerably in an aqueous solution of Bu-
CD-N7. As displayed in Figure 3, the emission spectrum of
AP solubilized in aqueous Bu-CD-N7 is about 10 times more
intense than that from water. In addition, the AP spectrum is
broadened and shifted toward longer wavelengths in Bu-CD-
N7. The enhanced solubility and the changes in the fluorescence
spectrum are presumably due to inclusion of AP into the
hydrophobic Bu-CD-N7. The association constant (K )
[AP‚‚‚Bu-CD-N7]/([AP][Bu-CD-N7])) for AP in Bu-CD-N7 in
1% ethanol/water was determined to be 7.8 × 104 (log K )
4.9). This is at least 35 times higher than that observed for AP
in CD-N7 or â-cyclodextrin or for anthracene in â-cyclodextrin
(Table 1).20
detected.34 ES-MS was used to study CD-N7 (Figure 1) and
Bu-CD-N7 (Figure 2). The spectrum of CD-N7 consists of a
set of peaks, centered at 2622 g/mol due to â-cyclodextrin
bearing seven naphthalenesulfonates (CD-N7), and a smaller set,
centered at 2394 g/mol arising from â-cyclodextrin bearing six
naphthalenesulfonates (see for example CD-N6-CH2OH, Scheme
2). Within each set, many of the peaks arise from cyclodextrins
in which some of the sodium sulfonates have been converted
to sulfonic acids during the electrospray experiment. For
example, the peak at 2622 g/mol is for a â-cyclodextrin with
(34) Cunniff, J. B.; Vouros, P. J. Am. Soc. Mass Spectrom. 1995, 6, 437.

A NoVel Antenna Cyclodextrin
J. Am. Chem. Soc., Vol. 119, No. 19, 1997 4367
Figure 3. Steady state fluorescence spectra of saturated solutions of
AP dissolved in (s) aqueous Bu-CD-N7 (2.8 × 10-4 M) and (- - -)
water (λex ) 363 nm).
Figure 4. Steady state fluorescence spectra of aqueous solutions of
(s) Bu-CD-N7 (1 × 10-5 M) and (‚‚‚) CD-N7 (1 × 10-5 M) (λex
)
280 nm).
Table 1. Association Constants (K) for Complexes of
Cyclodextrin with Anthryl Compounds
cyclodextrin
compound
K (M-1
)
Bu-CD-N7
CD-N7
â-cyclodextrin
â-cyclodextrin
APa
AP
AP
anthracene
78000
2200
80
2060b
a AP ) 9-anthrylmethyl pivalate. b Reference 20.
ES-MS has proven to be a useful technique for the detection
of cyclodextrin-guest complexes,27-33 as well as other host-
guest complexes.35-39 When an aqueous solution containing
AP (2 × 10-5 M) and Bu-CD-N7 (2.8 × 10-4 M) was analyzed
by ES-MS, no evidence for a host-guest complex could be
detected. This may indicate that the complex is not stable
enough to survive introduction to the gas phase. While
anthracene (molecular dimensions 9.2 × 5.0 Å) appears to fit
into the cavity of â-cyclodextrin (diameter 6.0-6.4 Å), the
addition of the methyl pivalate ester group to the 9-position of
anthracene will certainly rule out complete inclusion of AP in
the cyclodextrin cavity. Thus, AP may only be partially
included in the cavity or it may be dissolved in the naphthyl or
butyl groups of Bu-CD-N7, and these complexes may dissociate
upon introduction to the gas phase. A complex containing a
more appropriately shaped guest may exhibit an even higher
association constant and be detectable by ES-MS. However,
the AP/Bu-CD-N7 complex displays strong enough binding for
efficient solubilization, energy transfer, and a selective photo-
reaction (Vide infra).
Figure 5. Steady state fluorescence spectra of (s) Bu-CD-N7 (2.8 ×
10-4 M) and saturated solutions of AP in (‚‚‚) Bu-CD-N7 (2.8 × 10-4
M) ([AP] ≈ 1.8 × 10-5 M) and (- - -) CD-N7 (3.5 × 10-4 M) ([AP] ≈
1 × 10-6 M) (λex ) 314 nm, front-face cell).
Photophysical Properties. The steady state fluorescence
spectrum of Bu-CD-N7 (Figure 4) is dominated by monomer
emission (λmax ≈ 355 nm) from the naphthyl chromophores,
and the fraction of excimer emission is nearly identical to that
from CD-N7 (Figure 4). In the presence of AP, the naphthyl
emission (λex ) 314 nm) of Bu-CD-N7 is efficiently quenched
and intense AP emission is observed (Figure 5). Since
essentially all of the light is absorbed by the naphthyl chro-
mophores, the strong AP fluorescence demonstrates that energy
transfer is occurring. The emission spectrum of CD-N7 contain-
ing AP (Figure 5) displays a negligible amount of energy transfer
which confirms that the host-guest complex is not formed in
high concentrations for the unbutylated antenna cyclodextrin.
Figure 6. Steady state fluorescence spectrum of 60% methanol solution
containing MNSS (2 × 10-3 M) and AP (2.05 × 10-5 M) (λex ) 314
nm, front-face cell).
The emission spectrum (Figure 6, λex ) 314 nm) of a 60%
methanol solution containing AP and sodium 6-methoxy-2-
naphthalenesulfonate (MNSS), a model chromophore for the
naphthalenesulfonate groups in Bu-CD-N7, displays nearly
indiscernible AP emission on the tail of the strong naphthyl
emission of MNSS. The AP (2.05 × 10-5 M) and MNSS (2
× 10-3 M) concentrations were selected to match those in the
AP/Bu-CD-N7 system. The inefficient energy transfer for AP/
MNSS demonstrates that the energy transfer observed for the
AP/Bu-CD-N7 system was not simply the result of energy
transfer between two independent species in solution (i.e.,
collisional quenching).
(35) Lippmann, T.; Wilde, H.; Pink, M.; Scha¨fer, A.; Hesse, M.; Mann,
G. Angew. Chem., Int. Ed. Engl. 1993, 32, 1195.
(36) Ganem, B.; Li, Y.-T.; Henion, J. D. J. Am. Chem. Soc. 1991, 113,
6294.
Photosensitized Reaction of AP Included in the Bu-CD-
N7 Cavity. Upon direct excitation in deoxygenated methanol
or benzene, AP produced a mixture of three primary products
as determined by GC-MS. One of the products was identified
as 9-methylanthracene (MA) with M+ ) 192 g/mol. The two
(37) Ganem, B.; Li, Y.-T.; Henion, J. D. J. Am. Chem. Soc. 1991, 113,
7818.
(38) Smith, R. D.; Light-Wahl, K. J. Biol. Mass Spectrom. 1993, 22,
493.
(39) Li, Y.-T.; Hsieh, Y. L.; Henion, J. D.; Ocain, T. D.; Schiehser, G.
A.; Ganem, B. J. Am. Chem. Soc. 1994, 116, 7487.

4368 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Nowakowska et al.
Table 2. Ratio of Products Formed during Irradiation of
Scheme 4
9-Anthrylmethyl Pivalate (AP) in Isotropic Media and in Aqueous
a,b
Solutions of Bu-CD-N7
product ratio
system
AP/benzene
AP/MeOH
AP/MNSS/MeOH
AP/MNSS/DPPH/MeOH
AP/Bu-CD-N7/water
AP/Bu-CD-N7/water
105 [AP]0 (M)
MA
BMA
NA
1
1
1
1
1.4
5.2
1
1
1
1
1
1
1.9
1.5
1.4
0.3
0
2.1
2.7
2.1
0.4
3.4
2.6
0.3
a λirr ) 300 nm; irradiation time ) 30 min. b AP ) 9-anthrylmethyl
pivalate; MA ) 9-methylanthracene; BMA ) 9-tert-butyl-10-methyl-
anthracene; NA ) 9-neopentylanthracene; MNSS ) 6-methoxy-2-
naphthalenesulfonate, sodium salt; DPPH ) 2,2-di(4-tert-octylphenyl)-
1-picrylhydrazyl.
thermodynamic and steric factors. It has been shown that the
reaction between radical traps (e.g., nitroxides) and carbon-
centered resonance-stabilized radicals is reversible even at
ambient temperature.40 Since the 9-anthrylmethyl radical is
among the most stable of all resonance-stabilized radicals,41-43
•
the AnCH2 /DPPH adduct may be fairly labile (Scheme 4).
Scheme 3
Steric interactions between the anthryl group and the three
phenyl rings of DPPH may further destabilize the adduct.
When Bu-CD-N7 (1 × 10-3 M) is used to solubilize and
sensitize the photolysis of AP ([AP]total ) 1 × 10-5 M), only
two major photoproducts are formed: MA and NA. We
propose that the NA is formed by the “cage” recombination of
the tert-butyl radical with the anthrylmethyl radical while it is
still in the cyclodextrin cavity. MA is produced by hydrogen
abstraction by the anthrylmethyl radical in the event that the
tert-butyl radical escapes from the immediate vicinity of the
cyclodextrin complex. The absence of BMA from the reaction
products indicates that the 10-position of AP is protected upon
inclusion in Bu-CD-N7. This is in agreement with the earlier
observation that cyclodextrins influence the regiochemistry of
the coupling of arylmethyl radicals with other radicals (i.e., the
photo-Fries reaction).13
Conclusions
A â-cyclodextrin bearing seven naphthalenesulfonate antenna
chromophores was made more hydrophobic by butylation of
the 2- and 3-OH groups on each sugar unit. Electrospray
ionization mass spectroscopy proved to be a powerful technique
for the characterization of these cyclodextrin compounds and
showed that the butylation was successful. The butylated
cyclodextrin Bu-CD-N7 solubilized AP much more effectively
than either the unbutylated material, CD-N7, or â-cyclodextrin.
The naphthyl groups of Bu-CD-N7 displayed principally mono-
mer emission and efficiently transferred energy to solubilized
AP. The naphthyl antenna chromophores of Bu-CD-N7 were
shown to sensitize a selective photoreaction of AP included in
the cyclodextrin cavity. Inclusion in Bu-CD-N7 modifies the
photoreactivity of AP, suppressing reaction of the tert-butyl
radical with the 10-position of the anthryl ring. The preparation
of an antenna cyclodextrin which better solubilizes hydrophobic
molecules is an important development in the search for an
effective photochemical microreactor.
other products (both with M+ ) 248 g/mol) were tentatively
identified as 9-neopentylanthracene (NA) and 9-tert-butyl-10-
methylanthracene (BMA). A H NMR spectrum of a mixture
of the three photoproducts was consistent with the assigned
structures.
When AP was irradiated in methanol, a fourth photoproduct
was formed in trace amounts (ca. 2%). This product was
identified as 9-anthrylmethyl methyl ether (AnCH2OCH3) on
the basis of GC-MS analysis (222 (M+), 191 (M+ - OCH3).
Presumably, heterolytic photolysis producing AnCH2+ is a minor
photochemical pathway for AP in methanol.
1
The product distribution (Table 2) is not particularly depend-
ent on the polarity of the solvent. The triplet state quencher,
methylene blue, does not effect the rate of the photoreaction,
indicating that reaction occurs from the singlet state of AP.
Photolysis sensitized by the model chromophore (MNSS) leads
to the same product mixture. The photoreaction of AP can be
described by the reaction scheme given in Scheme 3.
In the presence of the free radical scavenger DPPH, the main
product of photolysis was MA (Table 2). Following photolytic
cleavage, t-Bu radicals which escape the solvent cage may be
rapidly scavenged by DPPH rather than recombining with the
anthrylmethyl radical. Scavenging of the anthrylmethyl radical
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada for financial
support.
JA963072S
(40) Howard, J. A.; Tait, J. C. J. Org. Chem. 1978, 43, 4279.
(41) Modi, P. J.; Guillet, J. E.; Scaiano, J. C.; Wintgens, V. J. Photochem.
Photobiol. A 1994, 81, 87.
(42) Stein, S. E.; Golden, D. M. J. Org. Chem. 1977, 42, 839.
(43) Herndon, W. C. J. Org. Chem. 1981, 46, 2119.
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(AnCH2 ) by DPPH may be much less efficient for both