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planar guest molecules. The assemblies are robust even after
the complete evaporation of water. Dry-state AFM images
showed large spherical particles of uniform dimensions
(height) of approximately 2.0 nm (see Figure S31).[23]
0.07), which most likely arises from anthracene excimer
aggregates (Figure 5b).[26] The excitation spectrum of 2a
resembles the absorption band of 1a and 2a (Figure 5a) and
indicates that the emission results from the excitation of
orthogonal and thus decoupled anthracene moieties. The CIE
chromaticity diagram (CIE = International Commission on
Illumination) of 1a and 2a was used to quantify the total
emission color ((0.16, 0.04) and (0.25, 0.34), respectively;
Figure 5c). The color of 2a approaches pure white (0.31,
0.34), unlike that of most anthracene derivatives or their
assemblies.
Simple modification of the hydrophilic groups on the exo-
cyclic surface of amphiphile 1 gave improved micelle
behavior. Amphiphile 1b, obtained in one step from the
precursor to 1a, has pendant zwitterionic sulfobetaine hydro-
philic groups and gave rise to spherical assemblies 2b in H2O.
AFM analysis revealed that the average diameter of 2b is
3.9 nm, although the modeled aromatic shell of 2b is
comparable to that of 2a (see Figure S40). Notably, 2b
showed a lower CMC value (0.03 mm), which indicates that
the micellar structure of 2b is more stable than that of 2a.[23]
The large hydrophilic sulfobetaine groups cover the exo-cyclic
surface of 1b, as revealed in detail by the X-ray crystal
structure (see Figure S41). The pale-blue fluorescence of 2b is
different from that of 2a and indicates that the large
sulfobetaine groups alter the intermolecular anthracene–
anthracene interactions of 2b (Figure 5b,c).
Like micellar assemblies, the capsule 2a is sensitive to
concentration. Concentration-dependent NMR spectroscopic
studies (at 0.25–4.0 mm as based on 1a) demonstrated that the
CMC value of 2a is ꢀ 1.0 mm and thus approximately 10 times
smaller than that of sodium dodecyl sulfate (SDS) micelles.
At a lower concentration (< 0.25 mm), the aromatic signals of
2a in the 1H NMR spectrum shifted back downfield, and the
DOSY spectrum showed a broadened band around D = 7.6 ꢀ
10À10 m2 sÀ1. However, in stark contrast to typical micelles, 2a
is insensitive to both temperature and the pH value. The
proton signals of 2a (2.0 mm) in the 1H NMR spectrum
remained almost unchanged up to 708C and in the pH range
1–13 (see Figures S27 and S28).[23,25] We believe the higher
stability of 2a stems from intermolecular aromatic–aromatic
interactions of the anthracene frameworks.
Micelle 2a is effectively a dense molecular cluster of
anthracene fluorophores and exhibited unusual emission
behavior. The UV/Vis spectra of 1a and 2a were quite
similar and exhibited absorption bands at 320–420 nm, which
were assigned to the p–p* transitions of the anthracene
moieties (Figure 5a).[21] There were significant differences,
The hydrophobic cavity of capsule 2, defined by the
fluorescent anthracene shell, successfully encapsulated Nile
red (3) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethyla-
minostyryl)-4H-pyran (4), well-known hydrophobic fluores-
cent dyes, to produce photoactive host–guest complexes
(Figure 6a). A slight excess of water-insoluble 3 was added
to an aqueous solution of 2a (2.0 mm as based on 1a), and the
resulting suspension was stirred at room temperature for 1 h.
A clear blue solution of 2a containing 3 (denoted 2aꢁ3) was
obtained after the removal of the remaining free 3 by
filtration (Figure 6b).[27] The UV/Vis spectrum of the solution
showed a new absorption band at lmax = 610 nm due to
encapsulated 3. Dye 4 was enclathrated under similar
conditions to give a red solution of 2aꢁ4 with a UV/Vis
absorption band at lmax = 508 nm (Figure 6b). The host–guest
complex 2aꢁ3 exhibited blue emission upon irradiation of the
host anthracene absorption band at 370 nm. Two weak, broad
emission bands were present in the emission spectrum at l =
420–570 and 640–760 nm (Figure 6c) and were assigned to the
anthracene moieties of 2a and enclathrated 3, respectively.
These bands indicate that FRET efficiency from host 2a to
guest 3 is moderate (66% as calculated from the fluorescence-
quenching data). The aqueous solution of 2aꢁ4 displayed
efficient FRET, and only red emission was observed (Fig-
ure 6c). The absorption band (lmax = 508 nm) of 4 overlaps
greatly with the emission band of 2a; accordingly, strong red
emission (lmax = 642 nm, F = 0.23) from guest 4 was observed
upon irradiation of the anthracene bands of 2aꢁ4 at 370 nm.
The apparent efficiency of the energy transfer was estimated
to be 97% from the fluorescence-quenching profile of the
anthracene moieties. Interestingly, the red emission of 2aꢁ4
was enhanced by a factor of 1.3 when the host–guest complex
Figure 5. Spectroscopic properties of capsules 2a and 2b as compared
with 1a and 1b. a) Normalized absorption spectra and b) normalized
fluorescence spectra (lex =370 nm) of 1a (red dotted line) and 1b
(green dotted line) in MeOH and 2a (red line) and 2b (green line) in
H2O at room temperature. c) CIE coordinate diagram of the fluores-
cence color of 1a and 1b in MeOH and 2a and 2b in H2O.
however, in the emission behavior of 1a and 2a. A typical
blue anthracene-like emission (lmax = 415 nm) was observed
for a solution of 1a in methanol upon irradiation at 370 nm. In
contrast, the irradiation of an aqueous solution of 2a at
370 nm resulted in a pale-green emission due to a broad
fluorescent band from 400 to 700 nm (lmax = 505 nm, FF =
2310
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 2308 –2312