Similar fluorescence enhancements were observed when the cor-
responding alkali hydroxides (LiOH, NaOH, KOH, RbOH, and
CsOH) were added, indicating that not Cl- but alkali metal ions are
triggers of bathochromic shift and fluorescence enhancement. It is
noted that only fluorescence intensity gave good linearity against
alkali salt concentration without precipitation. Monovalent metal
ion Cu+, multivalent metal ions Ca2+, Cu2+, Fe2+, and Fe3+ led to
precipitation of the anionic amphiphiles. Among alkali metal ions,
Li+ induced the most intense fluorescence as well as the largest
bathochromic shift in St-4C1. The differences in the fluorescence
intensities were very small for K+, Rb+, and Cs+. The effectiveness
of the alkali metal ions for induction of intense fluorescence is
in the following order: Li+ > Na+ > K+ > Rb+ > Cs+. This
order is in good agreement with the lyotropic series (Hofmeister’s
series: Li+ > Na+ > K+ > Rb+ > Cs+) which indicates the
order of ability of salting-out effect. It is generally accepted that
neutralization of charge and dehydration of solutes/dispersoids
by added electrolytes are responsible for the salting-out effect.
Although precipitations were not observed in the present systems,
the strongest dehydration and relaxation of electrostatic repulsion
by Li+ must have happened simultaneously with increasing alkali
salt concentration at pH 10. It is likely that this process displaces
the equilibrium in Fig. 4 to the right, and that the electron
density distribution in the sensitive dye molecule will be changed
in this process, as will the geometry of the amphiphile matrix,
and both effects would be expected to induce spectroscopic and
fluorescence changes in the dye. A novel type of amphiphile 15
may be potentially applicable to a OFF-ON type fluorescence
sensor for monovalent alkali metal ions, especially for Li+. Such
salting-out-like effects that promotes hydrophobic interaction
between adjacent amphiphiles by addition of alkali metal cations
is consistent with the fact that aggregate morphologies of 15
in water changed from small particles to helical ribbon upon
addition of LiCl and NaCl, respectively, at pH 10 (see Fig. S5
in ESI-3†). Original small particles and fragmentary aggregates
(a) were converted to the mixture of sea urchin-like assemblies
of divergently assembled nanotubes and untwisted tapes (b),
and helical aggregates (c) in the presence of Li+ and Na+,
respectively. Also, DSC thermograms showed increased Tc and
DH values upon addition of LiCl and NaCl, respectively (see
Fig. S6 in ESI-3†). These results are consistent with the increased
molecular packing of 15 due to the cooperation of dehydration
and relaxation of electrostatic repulsion by addition of alkali
metal ions. Amphiphiles 13 and 14 with two dodecanoyl-b-alanyl
groups and two decanoyl-b-alanyl groups, respectively, originally
induced much more enhanced fluorescence than 15 at pH 10
even without addition of alkali metal ions. In view of these
results, it is concluded that the Lys-derived anionic double-chain
amphiphile 15 with two octanoyl-b-alanyl groups and a b-Ala-
headgroup enabled the formation of a loosely packed self-assembly
at pH 10, and changed to densely packed aggregates upon addition
of alkali metal ions, accompanied by remarkable fluorescence
enhancement, especially by addition of Li+. The increased hy-
drophilicity by having additional amide groups must have been
compensated by the intramolecular hydrogen bond formation
responsible for the denser molecular packing as schematically
shown in Fig. 4. To the best of our knowledge, there has
been no example of double-chain peptide amphiphiles behaving
like this.
Conclusions
We have demonstrated that it is possible to induce intense
fluorescence in cationic dyes using appropriately designed double-
chain anionic amphiphiles that can form extremely hydrophobic
sites in water. In this study, it could be concluded that the more
hydrophobic the inside of the amphiphile assembly is, the closer
the molecular packing of amphiphile attained. This results in
rigidification of the dye molecules upon binding to the inside of
the aggregates (even through simple hydrophobic interactions).
Such rigidification will result in increased planarity (hence higher
conjugation, therefore red absorption shift), as well as minimizing
the role of vibrationally coupled nonradiative deactivation of the
excited state (hence higher fluorescence intensity). It is noted
that the introduction of b-Ala residues into two long-chain alkyl
group moieties was most effective for the amphiphiles derived
from L-glutamic acid with relatively shorter side-chain methylenes.
The related L-Lys-derived amphiphiles with a longer side-chain
were found to be capable of inducing intense fluorescence even
without such additional amide groups in the double-chain alkyl
groups. As a whole, it seems difficult for the intense fluorescence-
inducing amphiphiles to recognize detailed dye structure. The
general trend that increased fluorescence intensity led to decreased
dye specificity was observed. The cationic dyes in which intense
fluorescence was induced were hemicyanines and thiacarbocya-
nines with planar and relatively compact molecular structures.
An azo dye that is essentially non-fluorescent did not show
fluorescence induction at all, although it was incorporated in
the highly hydrophobic supramolecular cavities. These results
indicate that whether the dye is essentially fluorogenic or not
is important in induction of intense fluorescence as well as the
whole molecular planarity and compactness of the dye molecule.
The amphiphile with the shortest octanoyl-b-alanyl double-chain
alkyl groups, longer side-chain, and shorter methylene spacer
was found to be most sensitive to Li+, probably depending on
lyotropic series. Therefore, an aqueous amphiphile–dye complex
system that originally does not fluoresce so intensely was found to
be potentially applicable to an OFF-ON type fluorescence sensor
for Li+.
In conclusion, newly designed amphiphiles such as 1 were
found to form highly specific sites hydrophobic enough to induce
intense fluorescence emission in solvatochromic stilbazolium dyes
with compact push-pull substituents and other related cationic
hemicyanine, and thiacarbocyanine dyes except for the corre-
sponding azo dye. The largest fluorescence enhancement of St-4C1
through amphiphile 15 in the presence of Li+ is of current intense
interest to us and we hope it could be extended to a selective
recovery system of Li+ by e.g., immobilization of self-assemblies of
amphiphile 15 or coprecipitation of its analogues with Li+. Further
investigations including improved molecular design are now
ongoing.
Experimental
Materials and methods
All the amphiphiles except for 4 were newly synthesized and
identified by Fourier transform infrared spectroscopy (FTIR)
2334 | Org. Biomol. Chem., 2009, 7, 2327–2337
This journal is
The Royal Society of Chemistry 2009
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