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Figure 3. Schematic presentation of the secondary assembly of G
mediated by H-trans.
with a wall thickness of about 8 nm. Based on a computer
simulation study, the calculated wall thickness of nanotubes
was 7.9 nm, which nicely agrees with the value (ca. 8 nm)
determined from the TEM image (Figure 3). Interestingly, the
concentration-dependent TEM images revealed that, at
a relatively low concentration, a number of rod-like structures
were observed. Each rod-like structure was composed of
a number of spherical nanoparticles, the size of which was
very similar to that of the self-aggregate of G (Figure S17). At
higher concentrations of H-trans/G, these rod-like structures
turned to nanotubes. Combining these observations, we can
reasonably deduce that the H-trans-mediated secondary
assembly of G is responsible for the formation of nanotubes,
and that the interior and exterior surfaces of nanotubes are
composed of the highly stable H-trans/porphyrin-associated
units, whereas the hydrophobic alkyl chains of G interlace
with each other in the middle of the tubular walls, as
illustrated in Figure 3.
Figure 4. a) DLS, b) SEM (scale bar=1 mm), and c) TEM images of H/
G assembly after light (365 nm) irradiation for 20 minutes (scale
bar=200 nm; inset: scale bar=100 nm). ([H]=[G]=5 mm, pH 7.2).
probably by the small micelle aggregation mechanism or the
[13]
multicompartment micelle formation mechanism. In a con-
trol experiment, an equimolar mixture of H-cis and G gave
the DLS and TEM results similar to those shown in Figure 4.
Upon irradiation at 365 nm, the H-trans/G assembly exhib-
ited moderate fluorescence assignable to the emission of
porphyrin. This means that, in addition to the isomerization to
the cis-isomer, the excited trans-azobenzene transfers the
excitation energy at least in part to a nearby porphyrin,
leading to the porphyrin fluorescence.
It is also important to investigate the repetitiveness of the
photodriven morphology conversion. As shown in Figure 5,
the absorbance of H-trans at 357 nm, assigned to the
characteristic absorption of the p-p* transition of trans-
azobenzene, dramatically decreased under the light irradia-
tion at 365 nm, indicating photoisomerization of the azoben-
Significantly, after irradiation of the solution of H-trans/G
at 365 nm for 20 minutes, the original diameter of H-trans/G
(
thousands of nanometers, measured by DLS) was greatly
reduced to 180 nm (measured by DLS), as shown in Fig-
ure 4a. In addition, visible information of the photoinduced
morphological conversion was obtained from the SEM and
TEM images (Figure 4b and c). Therein, the long H-trans/G
nanotubes turned to a number of solid nanoparticles with an
average diameter of 180–200 nm. These nanoparticles were
appreciably larger and denser than those formed by the self-
aggregation of G. Job analysis gave the complexation
stoichiometry between the cis-isomer of H (H-cis) and G as
[10a,15]
zene moiety from trans to cis.
However, the decreased
intensity at 357 nm was recovered to its original level upon
subsequent irradiation at 450 nm because of the reverse
photoisomerization from cis to trans. In addition, the fluo-
rescence excitation spectrum of H-cis/G at 450 nm was very
weak, and H-cis/G barely fluoresced upon excitation at
450 nm. This indicates that the irradiation at 450 nm predom-
inantly excites the cis-azobenzene chromophore, although the
direct excitation of porphyrin cannot be rigorously ruled out.
Significantly, this cycle can be repeated tens of times. In
addition, TEM and DLS experiments confirmed that the
assembly morphology can be switched between nanotube and
nanoparticle by irradiating at different wavelengths (Figur-
es S18 and S19). These phenomena jointly indicate the good
reversibility and repetitiveness of the photocontrolled nano-
tube–nanoparticle morphological conversion.
1
1
:1. Moreover, the binding constant of H-cis with G (8.45
0 m ) was appreciably higher than that of H-trans probably
6
À1
because of the cooperative binding of two b-CD cavities in H-
cis with a G molecule. Based on this information, we deduce
that the morphological conversion is driven by the photo-
induced geometrical change of H from the trans-isomer (H-
trans) to cis-isomer (H-cis). That is, under the light irradi-
ation, the H-trans units in H-trans/G secondary assembly
convert to H-cis, and these H-cis units subsequently interact
with G through sandwich complexation. The resulting H-cis/
G sandwich complexes, possessing a larger hydrophilic head
than free G, first self-assemble to small micelles (computa-
tional structure in Figure 4d). Then, these micelles further
aggregate to form larger nanoparticles (Figure 4d), most
In summary, taking advantage of the strong binding
affinity between permethyl-b-CD and water-soluble tetraar-
ylporphyrins as well as the photoisomerization property of
azobenzene derivatives, an azobenzene-bridged bis(per-
methyl-b-CD) was successfully applied to the construction
of the secondary assembly of amphiphilic porphyrin deriva-
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Angew. Chem. Int. Ed. 2015, 54, 9376 –9380