Multichromophoric Cyclodextrins
J. Am. Chem. Soc., Vol. 118, No. 23, 1996 5441
excitation of the reactants included in the cavity by means of
energy transfer from the appended chromophores. Investigations
are in progress along this line. We have recently studied the
case of unimolecular photoreactions exemplified with the
photoisomerization of a nitrone within CD-NA’s cavity.30
1
ΦT )
(9)
1 + (R/R0)6
For instance, the transfer efficiency is expected to be 0.9999 at
a donor-acceptor distance of 5 Å and 0.994 at 10 Å; these
values are very close to 1, which agrees with the experimental
value.
Experimental Section
General Procedures. Anhydrous solvents (SDS), kept on molecular
sieves (3-4 Å), were used as obtained. All catalytic hydrogenations
were performed at 1 bar of pressure. Column chromatography (CC):
silica gel 60 (0.040-0.063 mm) Merck. Analytical and preparative
thin layer chromatography (TLC): silica gel plates Merck; detection
by UV (254 nm), I2, 5% H2SO4 or [MoO4(NH4)2 (2.5 g), (NH4)2Ce-
(NO3)6 (1.2 g), H2SO4 (100 mL, 3.6 M)]. Melting point: Kofler hot-
stage. 1H-NMR spectra: AM-200-SY-Bruker (4.7 T); Aspect 3000
calculator; chemical shifts in parts per million related to protonated
solvent as internal reference (1H: CHCl3 in CDCl3, 7.26 ppm; CHD2-
SOCD3 in CD3SOCD3, 2.49 ppm. 13C: 13CDCl3 in CDCl3, 76.9 ppm,
13CD3SOCD3 in CD3SOCD3, 39.6 ppm); coupling constants J in hertz.
Mass spectrometry: FAB-MS (positive mode) was performed by the
Service de Spectrome´trie de masse du CNRS, Vernaison. Microanaly-
ses were performed by the Service de Microanalyses de l’Universite´
Pierre et Marie Curie, Paris. The per-2,3-diacetyl,per-6-iodo-â-
cyclodextrin was synthesized according to the literature.13
With regard to the rate constant for transfer, the expression
for the Fo¨rster mechanism is
6
R0
1
τD
kdT-d
)
(10)
( )
R
where τD is the lifetime of the donor (in the absence of acceptor).
For distances of 5-10 Å, the rates are 1.8 × 1012 to 2.8 × 1010
s-1 for τD ) 5.95 ns (this value was measured with Me-NA).
If we assume that some excimers are not broken upon binding
of DCM-OH, the decomposition of the fluorescence spectrum
allowed us to calculate the overlap integral which leads to R0
) 27.5 ( 1.5 Å. The larger integral overlap explains that this
value of R0 is greater than in the case where transfer would
occur only from the monomer species. Consequently, if transfer
can occur from excimer species, it is even more efficient and
faster.
Competition between Homotransfer and Heterotransfer.
The question arises as to whether a naphthoate group that has
just been excited by absorption of a photon transfers directly
its excitation energy to an included DCM-OH molecule or
whether energy hopping among the naphthoate groups has time
to occur before transfer to DCM-OH. At the present stage of
this study, it is not possible to answer this question; information
on this point is in principle contained in the fluorescence decay
of the donor groups (naphthoate) but, as shown above, it was
not possible to observe such a decay because it was much too
fast with respect to the time resolution of our apparatus.
Nevertheless, it is worth noting that the emission spectrum of
the naphthoate groups overlaps more strongly the absorption
spectrum of DCM-OH than the absorption spectrum of these
groups, so that energy transfer to DCM-OH is expected to be
faster than homotransfer; this supposes of course that the average
distance between a naphthoate group and a DCM-OH molecule
is not significantly larger than the distance between naphthoate
groups. Further studies using faster time-resolved spectroscopic
techniques are required for a better understanding of the primary
events of the excited-state processes.
6-(Benzyloxy)-2-bromonaphthalene (2). A solution of 6-bro-
monaphth-2-ol (1) (10.0 g, 45 mmol) and benzyl bromide (12 mL, 49
mmol) in dry DMF (80 mL) was added dropwise into a suspension of
NaH 60% (1.5 g, 45 mmol) in dry DMF (20 mL). The mixture was
stirred at 70 °C for 22 h. After cooling, the solution was poured onto
crushed ice. The resulting precipitate was filtered and thoroughly
washed with water, then pentane. After drying, 2 was obtained as white
1
crystals (10.6 g, 75%): mp 110 °C; H-NMR (CDCl3) 7.93 (s, 1H),
7.67 (d, J ) 9 Hz, 1H), 7.60 (d, J ) 9 Hz, 1H), 7.49 (d, J ) 9 Hz,
1H), 7.47-7.34 (m, 5H), 7.24 (d, J ) 9 Hz, 1H), 7.20 (s, 1H), 5.21 (s,
2H). Anal. Calcd for (C17H13BrO) (313.2): C, 65.19; H, 4.18.
Found: C, 65.17; H, 4.19.
6-(Benzyloxy)-2-cyanonaphthalene (3). A suspension of CuCN
(5.43 g, 61 mmol) and 2 (9.50 g, 30 mmol) in dry DMF (100 mL) was
stirred at 150 °C for 24 h. The mixture was extracted with ethyl acetate,
the organic phase was extensively washed with NaCl brine and was
then dried on Na2SO4. After evaporation of the solvent, the crude
residue was purified by column chromatography (elution: CH2Cl2) to
1
provide 3 as a white powder (4.80 g, 61%): mp 138 °C; H-NMR
(CDCl3) 8.14 (s, 1H), 7.81 (d, J ) 8 Hz, 1H), 7.77 (d, J ) 7 Hz, 1H),
7.57 (d, J ) 7 Hz, 1H), 7.51-7.39 (m, 5H), 7.33 (d, J ) 7 Hz, 1H),
7.23 (s, 1H), 5.21 (s, 2H). Anal. Calcd for (C18H13NO) (259.3): C,
83.37; H, 5.05. Found: C, 83.25; H, 5.09.
6-(Benzyloxy)-2-naphthoic Acid (4). A mixture of 3 (3.90 g, 15
mmol) and NaOH 10 M (8.5 mL, 85 mmol) in ethylene glycol
monomethyl ether (60 mL) was refluxed for 8 h. After cooling, the
mixture was extracted with ethyl acetate. The aqueous phase was
neutralized with HCl (0.1 M). The precipitate was filtered and
extensively washed with water to give 4 as a white powder (2.80 g,
67%): mp 236 °C; 1H-NMR (CDCl3/DMSO-d6) 8.54 (s, 1H), 8.01 (d,
J ) 8.5 Hz, 1H), 7.94 (s, 1H), 7.85 (d, J ) 8.5 Hz, 1H), 7.55 (d, J )
8.5 Hz, 1H), 7.48-7.29 (m, 6H), 7.34 (d, J ) 7 Hz, 1H), 5.27 (s, 2H).
Anal. Calcd for (C18H14O3) (278.3): C, 77.68; H, 5.07. Found: C,
77.55; H, 5.03.
Methyl 6-(Benzyloxy)-2-naphthoate (5). DCC (1.96 g, 9.5 mmol)
was added portionwise at 0 °C into a solution of 4 (2.66 g, 9.5 mmol),
DMAP (1.20 g, 9.5 mmol) in CH2Cl2 (30 mL), and MeOH (2 mL, 38
mmol). After the solution was stirred at room temperature for 5 h, the
suspension was filtered off. The filtrate was washed with HCl (1.2
M), then saturated NaHCO3 aqueous solution, and was dried on Na2SO4.
After evaporation of the solvent, 5 was obtained as a white powder
(2.25 g, 81%): mp 150 °C; 1H-NMR (CDCl3) 8.53 (s, 1H), 8.12 (d, J
) 8.5 Hz, 1H), 7.86 (d, J ) 8.5 Hz, 1H), 7.74 (d, J ) 8.5 Hz, 1H),
7.57-7.25 (m, 9H), 5.20 (s, 2H), 3.97 (s, 3H). Anal. Calcd for
(C19H16O3‚0.25H2O) (296.8): C, 76.90; H, 5.56. Found: C, 76.91; H,
5.46.
Conclusion
The transfer of excitation energy from the naphthoyl chro-
mophores of CD-NA toward a merocyanine, DCM-OH, in-
cluded in the cavity takes place with an efficiency close to 1.
The stability of the 1:1 complex is higher than expected from
previously reported data on complexes with â-CD owing to the
contribution of the naphthoate residues. Fluorescence polariza-
tion and 13C-NMR experiments provide evidence for tight
complexes. In spite of the short distances between like or unlike
chromophores in such complexes, it is likely that the Coulombic
interaction has a much larger contribution in the mechanism of
energy transfer than the short-range interactions.
Complexes of CD-NA with DCM-OH can be considered as
supramolecular photochemical devices capable of efficient light
conversion via the antenna effect and thus mimick important
features of photosynthetic units. Another important aspect of
multichromophoric cyclodextrins is their use as photochemical
microreactors performing antenna-induced photoreactions within
the cavity. Specific effects are to be expected from selective
(30) Wang, P. W.; Jullien, L.; Valeur, B.; Filhol, J.-S.; Canceill, J.; Lehn,
J.-M. New J. Chem., in press.