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The above results provide real evidence that the choice of the
protecting group, the nature of the dipeptide and the solvent
mixture can lead to distinctive supramolecular architectures. One
can conclude that the Ile-Ile dipeptide is able to induce more
distinctive architectures, namely spheres, flakes and spikes, while
Ala-Ile can lead only to spherical assemblies.
The self-assembling behavior of another chromophore,
namely boron-dipyrromethene, was also investigated in this
work. Therefore we synthesized hybrids Boc-Ala-Ile-BDP and
Boc-Ile-Ile-BDP and tested their behavior in the same solvent
systems as the corresponding porphyrin hybrids. Interestingly,
no self-assembling ability was observed in HFIP–EtOH and
HFIP–MeOH mixtures (Fig. S25, ESI†). This can be attributed
to the partial solubility of BODIPY molecules in ethanol and
methanol. Since BDP-dipeptide hybrids are quite soluble in
EtOH and MeOH, there is not an actual ‘‘bad’’ solvent to induce
self-assembly in these two systems. On the other hand, when
water was used as a ‘‘bad’’ solvent, well defined spherical
nanostructures with a uniform size were formed (Fig. 5).
The synthesized hybrids were studied with UV-vis absorption
spectroscopy in solution and in the solid state. The broadening
and red shifts of all bands of the chromophore (and especially the
Soret band) observed in all porphyrin hybrids in relation to the
respective bands of the compounds in solution (DCM) indicate
the formation of J-aggregates (side-by-side) of the porphyrin
moiety in the assemblies (Fig. S26–S29 and Table S1, ESI†).
Equivalent observations were made during the fluorescence
studies, where the observed red shift supports the formation of
J-aggregates (Fig. S30, S31 and Table S2, ESI†).
In conclusion, this work describes the synthesis and char-
acterization of porphyrin and BODIPY hybrids coupled with
aliphatic peptides that are able to self-assemble in mixed
solvent systems. More specifically, two dipeptides namely
Ile-Ile and Ile-Ala bearing a series of protecting groups (Fmoc-,
Boc-, -Z, -OCH3) were connected with porphyrin and BODIPY
chromophores. To the best of our knowledge, this is the first
example where Ile-Ile and Ala-Ile dipeptides are covalently
attached to chromophores in order to transfer their self-
assembling properties. The resulting conjugates were able to
form spherical and spiky nanostructures depending on the
protecting group, the solvent mixture and the nature of the
dipeptide and the chromophore. Moreover, the Fmoc-Ile-Ile-TPP
hybrid was able to form hydrogels in HFIP–H2O 2 :8 solvent
mixture at 1 mM final concentration. The BODIPY hybrids were
Fig. 3 Self-assembly behavior of (a) Fmoc-Ile-Ile-TPP in HFIP–MeOH
2 : 8 and (b) Fmoc-Ile-Ile-TPP in HFIP–EtOH 2 : 8, (c) TPP-Ile-Ile-OMe in
HFIP–MeOH 2 : 8 and (d) TPP-Ile-Ile-OMe in HFIP–H2O 2 : 8.
The influence of another aromatic protecting group, Fmoc-,
was investigated as well. The Fmoc-Ala-Ile-TPP hybrid (Fig. 2
and Fig. S22, ESI†) was able to form spherical architectures in
HFIP–MeOH and HFIP–EtOH 2 : 8 solvent conditions. In the
above cases of Ala-Ile dipeptide hybrids, no flakes were obtained
and water was not an appropriate solvent for the formation of self-
assembled nanostructures.
A completely different supramolecular architecture was
observed in the case of the Fmoc-Ile-Ile-TPP hybrid (Fig. 3a, b
and Fig. S23, ESI†). It was able to form spiky nano-assemblies,
which is not common in these kinds of systems, after dissolving
in HFIP and using MeOH or EtOH as a bad solvent. Surprisingly, by
using water as a ‘‘bad’’ solvent, hydrogels were formed (Fig. 4a, b
and Fig. S24, ESI†). Hydrogels consist of an interconnected fibrillar
network with high water content (Fig. 4b) and can be used in many
biomedical applications. This was an unexpected result since, as far
as we know, it is the first chromophore molecule that forms a
hydrogel in the absence of any other gelator molecules. Moreover,
we examined the effect of protecting the carboxyl terminus of the
dipeptide, as far as it may influence the self-assembly process. More
specifically, the TPP-Ile-Ile-OMe hybrid was studied in HFIP–MeOH
2 : 8 and HFIP–EtOH 2 : 8 and formed flakes by applying MeOH or
spheres when EtOH was used as a ‘‘bad’’ solvent (Fig. 3c and d).
Fig. 4 (a) Photographs of the hydrogel formed from the Fmoc-Ile-Ile-
TPP hybrid in HFIP–H2O 2 : 8 solvent mixture at 1 mM final concentration.
(b) FESEM image of the formed hydrogel.
Fig. 5 Self-assembly behavior of (a) Boc-Ala-Ile-BDP in HFIP–H2O 2 : 8,
and (b) Boc-Ile-Ile-BDP in HFIP–H2O 2 : 8.
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 14103--14106 | 14105