Role of the hydrophobic effect in the transfer of chirality from molecules to complex systems: from chiral surfactants to porphyrin/surfactant aggregates

Article abstract of DOI:10.1021/ja805669v

The interaction between the achiral sulfonated porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H₂TPPS ₄^4-, and two chiral cationic surfactants has been studied by optical absorption, fluorescence, and circular dichroism (CD) spectroscopies. At surfactant concentrations above the critical micellar concentration (cmc) the porphyrin is included in the micellar aggregates, but it is CD silent. Below the cmc at a definite porphyrin/surfactant stoichiometry the formation of heteroaggregates with transfer of chirality to the porphyrin chromophore occurs. The preferred surfactant/porphyrin stoichiometry is 3:1, which suggests a structure driven by electrostatic and hydrophobic interactions between porphyrin and surfactant and dipolar and ionic interactions with the water solution. At surfactant concentrations above the cmc, depending on the protocol of preparation of the samples, the formation of the two kinds of aggregates can be observed, reversible for the simple surfactant micelles incorporating the porphyrin, but irreversible for the heteroaggregates.

Full text of DOI:10.1021/ja805669v

Published on Web 10/17/2008
Role of the Hydrophobic Effect in the Transfer of Chirality
from Molecules to Complex Systems: From Chiral Surfactants
to Porphyrin/Surfactant Aggregates
Zoubir El-Hachemi,§ Giovanna Mancini,*,†,‡ Josep M. Ribo´,*,§ and
Alessandro Sorrenti†
Dipartimento di Chimica, and CNR, Istituto di Metodologie Chimiche, Sezione Meccanismi di
Reazione and Dipartimento di Chimica, UniVersita` degli Studi di Roma “La Sapienza”,
P.le A Moro 5, 00185 Roma, Italy, and Departament de Química Orga`nica, UniVersitat de
Barcelona, c/Mart`ı Franque`s 1, 08028 Barcelona, Spain
Received July 21, 2008; E-mail: giovanna.mancini@uniroma1.it; jmribo@ub.edu
Abstract: The interaction between the achiral sulfonated porphyrin 5,10,15,20-tetrakis(4-sulfonato-
phenyl)porphyrin, H2TPPS44-, and two chiral cationic surfactants has been studied by optical absorption,
fluorescence, and circular dichroism (CD) spectroscopies. At surfactant concentrations above the critical
micellar concentration (cmc) the porphyrin is included in the micellar aggregates, but it is CD silent. Below
the cmc at a definite porphyrin/surfactant stoichiometry the formation of heteroaggregates with transfer of
chirality to the porphyrin chromophore occurs. The preferred surfactant/porphyrin stoichiometry is 3:1, which
suggests a structure driven by electrostatic and hydrophobic interactions between porphyrin and surfactant
and dipolar and ionic interactions with the water solution. At surfactant concentrations above the cmc,
depending on the protocol of preparation of the samples, the formation of the two kinds of aggregates can
be observed, reversible for the simple surfactant micelles incorporating the porphyrin, but irreversible for
the heteroaggregates.
Introduction
detect chirality by the formation of heteroassociates with chiral
compounds5,6 or by the formation, in spontaneous symmetry-
Chirality is observed in nature at different levels of complex-
ity, from molecules to the macroscopic level of biological
systems, such as snails, flowers, and twining vines.1 Most chiral
species are present in nature only in one of the enantiomeric
forms; at molecular level, for example, racemic mixtures of
biomolecules and biopolymers are excluded to form part of the
basic processes that define life. Fundamental discussions on this
suggest that the biological homochirality could be a consequence
of the atom “homochirality” arising from the violation of the
parity of the electroweak force.2 Independently from this basic
issue, there are the pure chemical topics of the transfer of
chirality and of the chiral amplification in the building of
biological complex systems from simple molecules. The study
of transfer of chirality in the self-assembly of hierarchical
supramolecular associates can give basic knowledge on this.3
The transfer and amplification of chirality has been studied
on chiral porphyrins.4 Achiral porphyrins have been used to
breaking processes, of chiral homoassociates whose chiral sign
is determined by the influence of weak chiral polarizations.3d,i
In fact, porphyrins are some of the most powerful and versatile
(3) (a) Laursen, R. A.; Wen, D.; Knight, C. A. J. Am. Chem. Soc. 1994,
116, 12057–12058. (b) Orme, C. A.; Noy, A.; Wierzbicki, A.;
McBride, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; De Yoreo,
J. J. Nature 2001, 411, 775–779. (c) De Feyter, S.; Gesquie`re, A.;
Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew.
Chem. 2001, 113, 3317–3320. (d) Ribo´, J. M.; Crusats, J.; Sague´s, F.;
Claret, J.; Rubires, R. Science 2001, 292, 2063–2066. (e) Zepik, H.;
Shavit, E.; Tang, M.; Jensen, T. R.; Kjaer, K.; Bolbach, G.;
Leiserowitz, L.; Weissbuch, I.; Lahav, M. Science 2002, 295, 1266–
1269. (f) Geva, M.; Frolow, F.; Eisenstein, M.; Addadi, L. J. Am.
Chem. Soc. 2003, 125, 696–704. (g) Levy-Lior, A.; Weiner, S.; Addadi,
L. HelV. Chim. Acta 2003, 86, 4007–4017. (h) Purrello, R. Nat. Mater.
2003, 2, 216–217. (i) Crusats, J.; Claret, J.; Diez-Perez, I.; El-Hachemi,
Z.; Garcı´a-Ortega, H.; Rubires, R.; Sague´s, F.; Ribo´, J. M. J. Chem.
Soc., Chem. Commun. 2003, 158, 8–1589. (j) Zhang, L.; Yuan, J.;
Liu, M. J. Phys. Chem. B 2003, 107, 12768–12773. (k) Mateos-
Timoneda, M. A.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Soc.
ReV 2004, 33, 363–372. (l) Zhang, Y.; Chen, P.; Liu, M. Chem. Eur.
J. 2008, 14, 1793–1803.
† Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapi-
enza”.
‡ Istituto di Metodologie Chimiche, Sezione Meccanismi di Reazione,
Universita` degli Studi di Roma “La Sapienza”.
(4) (a) Monti, D.; Venanzi, M.; Mancini, G.; Di Natale, C.; Paolesse, R.
Chem. Commun. 2005, 2471–2743. (b) Monti, D.; Venanzi, M.;
Stefaneli, M.; Sorrenti, A.; Mancini, G.; Di Natale, C.; Paolesse, R.
J. Am. Chem. Soc. 2007, 129, 6688–6689. (c) Monti, D.; Cantonetti,
V.; Venanzi, M.; Bombelli, C.; Ceccacci, F.; Mancini, G. Chem.
§ Universitat de Barcelona.
(1) Janoschek, R., Ed. ChiralitysFrom Weak Bosons to the R-Helix;
Springer Verlag: Berlin, 1991.
(2) (a) Zanasi, R.; Lazzaretti, P.; Ligabue, A.; Soncini, A. Phys. ReV. E
1999, 59, 3382–3385. (b) Lazzaretti, P.; Zanasi, R.; Faglioni, F. Phys.
ReV. E 1999, 60, 871–874. (c) Berger, R.; Quack, M. ChemPhysChem
2000, 1, 57–60. (d) Johnson, L. N. Eur. ReV. 2005, 13, 77–95. (e)
Fitz, D.; Reiner, H.; Plankensteiner, K.; Rode, B. M. Curr. Chem.
Biol. 2007, 1, 41–52.
ˇ
ˇ
Commun. 2004, 972–973. (d) Steˇpa´nek, P.; Dukh, M.; Saman, D.;
Moravcova´, J.; Kniezˇo, L.; Monti, D.; Venanzi, M.; Mancini, G.;
Drasˇar, P. Org. Biomol. Chem. 2007, 5, 960–970. (e) Fuhrhop, J. H.;
Demoulin, C.; Boettcher, C.; Koning, J.; Siggel, U. J. Am. Chem. Soc.
1992, 114, 4159–4165.
9
15176 J. AM. CHEM. SOC. 2008, 130, 15176–15184
10.1021/ja805669v CCC: $40.75
2008 American Chemical Society

Transfer of Chirality from Molecules to Systems
A R T I C L E S
Chart 1
insights into the mechanisms of transfer of the chiral information
from the molecular level to the nanoscale level of polymolecular
aggregates.
Materials and Methods
The tetrasodium salt of 5,10,15,20-tetrakis(4-sulfonatophenyl)por-
4-
phyrin H2TPPS4
(Na4H2TPPS4) was prepared as previously
reported.9
(S)- and (R)-N,N-Dimethyl-N-(1-phenylethyl)hexadecan-1-am-
monium bromide (1a and 1b) were prepared according to a reported
procedure.10 1H NMR, δ (CDCl3): 0.833 (t, CH3, 3H, J ) 6.5 Hz),
1.249 (m, CH2 chain, 26H), 1.783 (m, CH2, 2H), 1.830 (d, CH3,
3H, J ) 7.1 Hz), 3.160 (s, CH3, 3H), 3.200 (s, CH3, 3 H), 3.470 (t,
CH2, 2H, J ) 7.1 Hz), 5.389 (q, CH, 1H J ) 7.1 Hz), 7.38-7.47
(m, ar, 3H), 7.620 (m, ar, 2H).
(1S,2S)-N-(2-Hydroxy-1-methyl-2-phenylethyl)-N,N-dimethyl-
hexadecan-1-ammonium bromide, 2, was prepared according to a
reported procedure.10 1H NMR, δ (CDOD3): 0.894 (t, CH3, 3H, J
) 6.7 Hz), 1.090 (d, CH3, 3H, J ) 6.7 Hz), 1.289 (m, CH2 chain,
26H), 1.800 (m, CH2, 1H), 1.943 (m, CH2, 1H), 3.220 (s, CH3,
3H), 3.292 (s, CH3, 3H), 3.525 (m, N+CH2, 1H), 3.732 (m, N+CH2,
1H), 3.911 (m, CH, 1H), 4.910 (d, CH, 1H, J ) 9.9 Hz), 7.33-7.48
(m, ar, 5H).
(R)-N,N,N-Trimethyl-1-phenylethanammonium bromide, 3, was
prepared as follows. An amount of 228 mg of CH3I (1.6 mmol)
was added to a solution of 130 mg (0.88 mmol) of (R)-N,N-
dimethyl-1-phenylethylamine in 1 mL of methanol; the reaction
mixture was kept under stirring at room temperature for 2 days.
The iodide salt was precipitated from methanol by Et2O, filtered,
and washed with Et2O. The bromide salt was obtained by dissolving
the iodide salt in a methanol solution saturated with NaBr.
Purification by chromatography on silica gel (CHCl3/MeOH ) 80/
20) of the residue obtained after filtration of the solution and
removal of the solvent under vacuum yielded the pure product, as
probes to gain information on the structure and on the chirality
of the systems where they are embedded.4-7
Here we report on the formation of different kinds of ag-
gregates formed by the tetrasodium salt of the achiral water-
soluble 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H2
TPPS44-, with chiral cationic surfactants: the enantiomers of
N,N-dimethyl-N-(1-phenylethyl)hexadecan-1-ammonium bro-
mide, 1a and 1b, and (1S,2S)-N-(2-hydroxy-1-methyl-2-phe-
nylethyl)-N,N-dimethylhexadecan-1-ammonium bromide, 2 (Chart
1). The methodology used was based on the porphyrin chro-
mophore as target of absorption (UV-vis and circular dichro-
ism) and emission spectroscopy.
It is known that the interaction of porphyrins with surfactants
can give different systems depending upon the [surfactant]/
[porphyrin] ratio and the nature of the two species.8 At high
[surfactant]/[porphyrin] ratios the inclusion of monomeric
porphyrin in surfactant micellar aggregates is generally observed,
whereas at low ratios the formation of so-called premicellar
surfactant-porphyrin aggregates is observed.8c-f However, to
the best of our knowledge, studies on the interaction between
achiral anionic porphyrins and chiral cationic surfactants have
not been reported. The investigation of these systems might give
1
a light yellow solid. H NMR, δ (CD3OD): 1.823 (d, CH3, 3H, J
) 7.0 Hz), 3.140 (s, CH3, 9H), 4.890 (q, CH, 1H J ) 7.0 Hz),
7.50-7.53 (m, ar, 3H), 7.63-7.66 (m, ar, 2H).
Water of Millipore Q quality (18.2 MΩ cm) was used for the
preparation of all the aqueous solutions.
Instrumentation. Conductivity measurements were carried out
at 298 K on a Hanna conductimeter HI-9932, equipped with a
thermostatting apparatus.
Circular dichroism (CD) spectra were recorded on Jasco spec-
tropolarimeters J-715 and J-810.
UV-vis spectra were recorded on a Carey-300 UV-vis spec-
trophotometer. The spectral changes induced by the increase of
temperaturewererecordedbyVarian-Carey(500SCAN)UV-vis-NIR
spectrophotometer equipped with a Peltier temperature controller.
Fluorescence spectra were recorded on a PTI spectrofluorimeter
in L geometry equipped with a Xe lamp (LPS 220B) and a
photomultiplier detection system PTI 814.
(5) (a) Bellacchio, E.; Lauceri, R.; Guerrieri, S.; Scolaro, L. M.; Romeo,
A.; Purrello, R. J. Am. Chem. Soc. 1998, 120, 12353–12354. (b)
Purrello, R.; Raudino, A.; Scolaro, L. M.; Loisi, A.; Bellacchio, E.;
Lauceri, R. J. Phys. Chem. B 2000, 104, 10900–10908. (c) Pasternack,
R. F.; Bustamante, C.; Collings, P. J.; Giannetto, A.; Gibbs, E. J. J. Am.
Chem. Soc. 1993, 115, 5393–5399. (d) Lauceri, R.; Purrello, R.; Shetty,
S. J.; Graca, M.; Vicente, H. J. Am. Chem. Soc. 2001, 123, 5835–
5836.
Resonance light scattering measurements5c,13 were carried out
on a Horiba Jobin Yvon Fluoromax-4 spectrofluorimeter, with a
right angle geometry, by operating in the synchronous scanning
mode in which the excitation and emission monochromators are
coupled to scan simultaneously.
Determination of Critical Micellar Concentration. Critical
micellar concentration (cmc) of surfactant 2 was measured by
conductivity experiments according to a described procedure.11
Known volumes of 25 mM aqueous solution of surfactant were
(6) Hembury, G. A.; Borovkov, V.; Inoue, Y. Chem. ReV. 2008, 108, 1–
73.
(7) Pescitelli, G.; Gabriel, S.; Wang, Y.; Fleischhauer, J.; Woody, R. W.;
Berova, N. J. Am. Chem. Soc. 2003, 125, 7613–7628.
(8) (a) Andrade, S. M.; Teixeira, R.; Costa, S. M. B.; Sobral, A. J. F. N.
Biophys. Chem. 2008, 133, 1–10. (b) Andrade, S. M.; Costa, S. M. B.
Chem. Eur. J. 2006, 12, 1046–1057. (c) Maiti, N. C.; Mazumdar, S.;
Periasamy, N. J. Phys. Chem. B 1998, 102, 1528–1538. (d) Maiti,
N. C.; Mazumdar, S.; Periasamy, N. J. Porphyrins Phthalocyanines
1998, 2, 369–376. (e) Gandini, S. C. M.; Yushmanov, V. E.;
Borissevitch, I. E.; Tabak, M. Langmuir 1999, 15, 6233–6243. (f)
Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem.
1991, 95, 4074–4086.
(9) Rubires, R.; Crusats, J.; El-Hachemi, Z.; Jaramillo, T.; Valls, E.;
Farrera, J.-A.; Ribo, J. M. New J. Chem. 1999, 189–198.
(10) Moss, R. A.; Sunshine, W. L. J. Org. Chem. 1974, 39, 1083–1089.
(11) Frinii, M.; Michelis, B.; Zana, R. J. Phys. Chem. 1992, 96, 6095.
(12) Andreani, R.; Bombelli, C.; Borocci, S.; Lah, J.; Mancini, G.;
Mencarelli, P.; Vesnaver, G.; Villani, C. Tetrahedron: Asymmetry
2004, 15, 987–994.
9
J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008 15177

A R T I C L E S
El-Hachemi et al.
4-
Figure 1. Soret region of the absorption spectrum of 1 µM H2TPPS4 in the presence of surfactants 1a (a) and 2 (b), at [surfactant]/[porphyrin] ratio of
10 000:1 (solid line), compared with the spectrum of porphyrin in water (dashed line).
4-
Figure 2. Soret region of the absorption spectrum of 1 µM H2TPPS4 in the presence of surfactants 1a (a) and 2 (b), at [surfactant]/[porphyrin] ratio of
4:1 (solid line), compared with the spectrum of porphyrin in water (dashed line).
added to a known volume of Milli-Q water. The cmc’s of 112 and
solution was studied by means of UV-vis spectroscopy,
2 are 2.40 × 10-4 and 2.15 × 10-4 M, respectively.
fluorescence, and CD spectroscopies.
Sample Preparation. Samples for UV-vis, fluorescence, and
CD experiments were prepared, when not differently specified, as
follow: (a) samples at [surfactant] < 1 mM were prepared by adding
the necessary volume of a 0.1-1 mM porphyrin stock solution to
the proper amount of water. To the resulting dilute porphyrin
solution a proper amount of a stock aqueous surfactant solution
was then added to obtain the required porphyrin/surfactant ratio;
(b) samples at [surfactant] > 1 mM, i.e., above the cmc, were
prepared by adding the proper amount of porphyrin stock solution
to the surfactant solution. The concentration of the porphyrin stock
solutions was checked by UV-vis spectroscopy (ε ) 480.000
mol-1 L cm-1 at 413 nm).
UV-Vis and Fluorescence Experiments. Spectra were regis-
tered within 5-15 min of the sample preparation, in quartz cuvettes
(path length 1 cm). Experiments were performed at room temper-
ature, when not specified.
The stoichiometry of the porphyrin-surfactant heteroaggregate
was determined by the method of continuous variations (Job’s
method plot analysis).14 The total concentration of the two
components (porphyrin and surfactant) was kept constant at 10 µM.
The absorption spectra were recorded immediately after the
preparation of the samples.
4-
The UV-vis experiments were carried out at [H2TPPS4
]
) 1 µM at [surfactant]/[porphyrin] ratio ranging between 1:1
and 10 000:1 (i.e., at surfactant concentration ranging between
1 µM, well below, and 10 mM, well above, cmc), the cmc of
1 and 2 being 2.40 × 10-4 and 2.15 × 10-4 M, respectively.
For the sake of clarity we report here only the UV-vis spectra
relative to some selected [surfactant]/[porphyrin] ratios (Figures
1 and 2); other UV-vis spectra are reported as Supporting
Information.
The absorption spectrum of H2TPPS44- (1 µM) in pure water
exhibits a sharp Soret band at 413 nm (dashed line in Figures
1 and 2) and four distinguishable Q-bands at 516, 552, 579,
and 634 nm (Supporting Information). The absorption spectrum
of H2TPPS44- (at [H2TPPS44-] ) 1 µM) is not affected by the
4-
presence of the chiral ammonium salt 3 (at [3]/[H2TPPS4
]
ratios ranging between 1 and 22). Notice that this chiral
ammonium salt is the homologue of surfactant 1b with a methyl
group instead of the hexadecyl hydrophobic tail. This result
indicates that whatever is the interaction between these op-
positely charged species, it does not perturb the porphyrin
electronic transitions. On the other hand, both Soret and Q-bands
of the absorbance spectrum of H2TPPS44- are strongly affected
by the presence of the cationic surfactants 1a, 1b, and 2
indicating a significant interaction with the charged porphyrin.
For example, at [1a]/[H2TPPS44-] ) 10 000:1 for 1 µM
H2TPPS44- the Soret band shows a red shift to 419 nm (Figure
Results and Discussion
The interaction between nonchiral sulfonated porphyrin
H2TPPS44-, and chiral cationic surfactants 1 and 2, in water
(13) PasternacK, R. F.; Collings, P. J. Science 1995, 269, 935.
(14) Job, P. Ann. Chim.(Paris) 1928, 9, 113–203.
9
15178 J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008

Transfer of Chirality from Molecules to Systems
A R T I C L E S
Figure 3. Fluorescence emission spectra of 1 µM H2TPPS44- in water (panels a and b, dashed line) and in the presence of (a) surfactant 1a at [surfactant]/
[porphyrin] ratio of 4:1, λex 408 nm (dotted line), at surfactant/porphyrin ratio of 10 000:1, λex ) 413 nm (solid line); (b) surfactant 2 at [surfactant]/
[porphyrin] ratio of 4:1, λex 408 nm (dotted line), at [surfactant]/[porphyrin] ratio of 10 000:1, λex 407 nm (dash-dotted line), and λex ) 420 nm (solid line).
1a), which indicates the incorporation of the porphyrin in the
surfactant cationic micelles. The presence of monomeric por-
phyrin included in micellar aggregates (micellized monomeric
porphyrin) has been reported previously.8 On the other hand,
at lower [surfactant]/[porphyrin] ratios the Soret band shows a
broad shape (Supporting Information), that cannot be assigned
to the presence of either free porphyrin in water or micellized
porphyrin, because the maximum absorptions, also those of
Q(0,0) band, appear to slightly different wavelengths with
respect to those of the free porphyrin and of the porphyrin
incorporated in the surfactant micelles (Supporting Information).
In the case of surfactant 2 the band relative to this species is
intense bands evolve with time or with the increase of surfactant/
porphyrin ratio to a higher intensity of the red-shifted band.
This pattern of the Soret band arising from aggregation is
significantly different from that observed in the typical π-stack-
ing of the free base porphyrins (H-aggregation with a slightly
hypsochromic shift) or in the side-to-side aggregation (J-
aggregation with a bathochromic shift) characteristic of homo-
associated diprotonated porphyrins. Therefore, the observed
splitting indicates an uncommon geometrical arrangement of
the porphyrins.
The porphyrin-surfactant heteroaggregate spectrum also was
observed at higher porphyrin concentrations. For example, in
the range of 0.1 µM e [H2TPPS44-] e 22.5 µM, at [2] ) 1
mM (i.e., above the surfactant cmc), the absorption spectrum
shows the split Soret band of the heteroaggregate (not shown).
Moreover, the plot of the absorbance at the two Soret maxima
as a function of porphyrin concentration is linear, thus indicating
that the porphyrin-surfactant heteroaggregate is the only species
in solution under these conditions. Note that the obtainment of
heteroaggregates at concentration above the cmc implies that
the formation of the heteroaggregate decreases the surfactant
concentration below the cmc, thus suggesting either a higher
stability of the heteroaggregates compared to that of the micelles
or a kinetic barrier in the transformation of heteroaggregates
into micelles. In contrast, previous results concerning analogous
systems show that the premicellar aggregates disappear by
increasing the surfactant concentration.8c
4-
also clearly observable in the absorption spectrum of H2TPPS4
at [surfactant]/[porphyrin] ratio of 10 000:1 as a shoulder at
lower wavelengths of the absorbance of micellized porphyrin
(Figure 1b), whereas the single sharp band of micellized
porphyrin is observed in the case of surfactant 2 only at higher
[surfactant]/[porphyrin] ratio (Supporting Information). Finally,
the lack of an isosbestic point in the UV-vis experiments in
4-
the absorption spectra of H2TPPS4 at various surfactant/
porphyrin ratios demonstrates the formation of different kinds
of aggregates in solution, depending on the experimental
conditions. Therefore, a detailed study on the porphyrin-surfactant
interactions at surfactant concentrations below the cmc was
performed.
At [surfactant]/[H2TPPS44-] ) 4:1 and below the cmc (Figure
2) a strong decrease of the Soret band absorption together with
a splitting with maxima at 408 and 418 nm is observed for both
surfactants together with an ∼16 nm red shift of the two lowest
energy Q-bands (absorbance values of Q-bands are given as
Supporting Information). At [surfactant]/[ H2TPPS44-] e 3:1
for both the surfactants broadening and strong intensity decrease
of the Soret band is observed (Supporting Information). Such
spectral changes observed at surfactant concentration below cmc
must be attributed to the formation of “premicellar” porphyrin-
surfactant aggregates8c,d that we call here as porphyrin-surfactant
heteroaggregates. The formation of a porphyrin-surfactant
heteroaggregate was confirmed by fluorescence and resonance
light scattering experiments (see below).
Fluorescence spectroscopy experiments confirm the presence
of different species in solution depending on the [surfactant]/
4-
[H2TPPS44-] ratio. The emission spectra of the [H2TPPS4
]
) 1 µM at some selected [surfactant]/[H2TPPS44-] ratios (for
both the surfactants) are shown in Figure 3. The spectrum of
H2TPPS4 in water exhibits the characteristic two bands of
4-
the free base porphyrin at 644 and 704 nm (dashed line in Figure
3), but the presence of surfactant induces changes in the emission
spectrum of the porphyrin showing the formation of new species.
At [surfactant]/[H2TPPS44-] ) 1:1 the emission spectrum (not
shown) is very similar to that of porphyrin in water; however,
at [surfactant]/[porphyrin] ) 4:1, for both surfactants (1 and
2), the emission spectrum shows a strong decrease in quantum
yield and a significant red shift of the emission maxima (657
and 721 nm) with respect to the spectrum of the porphyrin in
The fresh prepared diluted solutions of heteroaggregates show
a symmetric splitting ((290 cm-1) of the Soret band in two
absorption bands of the same intensity. However, these equally
9
J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008 15179

A R T I C L E S
El-Hachemi et al.
of time (decrease of the intensity and a broadening of the split
Soretband)thatsuggestanorganizationoftheporphyrin-surfactant
heteroaggregates into larger aggregates (Figure 5, parts a and
b). On the other hand, similar spectral changes are not observed
when the porphyrin–surfactant heteroaggregates are formed at
[H2TPPS44-] ) 10 µM, at the same [surfactant]/[porphyrin]
ratios (Figure 5, parts c and d).
However, porphyrin-surfactant large heteroaggregates (de-
tected by the UV-vis) featuring the characteristic broad
spectrum shown in Figure 5, parts a and b, were also obtained
at [H2TPPS44-] ) 10 µM (at [2]/[ H2TPPS44-] ) 6:1 by adding
successive proper amounts of concentrated surfactant and
porphyrin solutions to a previously equilibrated solution of 1
4-
µM H2TPPS4 and 6 µM 2, i.e., containing large hetero-
aggregates. The experiment was carried out by adding to the
preformed heteroaggregate, first the surfactant and then the
porphyrin concentrated solution. After each addition the sample
was allowed to equilibrate for 1-2 h following the evolution
into larger aggregates by the decrease of intensity in the
absorption spectra. The UV spectra, performed at different times
after each addition, are reported as Supporting Information.
Noteworthy, the addition of the surfactant to a solution
containing porphyrin-surfactant heteroaggregates does not
induce any spectral change, i.e., it does not perturb the
heteroaggregate structure, whereas the addition of the concen-
trated porphyrin solution induces the formation of new small
heteroaggregates (featuring a more intense absorption) that
slowly organize into larger aggregates. Finally, we observed
that the growth to larger heteroaggregates is faster in the
presence of preformed large heteroaggregates. The hydrophobic
effect plays an important role in the formation of the hetero-
aggregates and in the kinetics of the aggregation, as shown in
the following experiment. In a fresh prepared solution at
[H2TPPS44-] ) 2.5 µM and [2]/[porphyrin] ) 4:1 the increase
of temperature irreversibly promotes aggregation (Figure 6), thus
demonstrating that the driving force of the heteroaggregation
process is the hydrophobic effect (direct relationship with
temperature) and that the formation of the heteroaggregates is
irreversible.
A Job’s plot analysis allowed us to determinate the stoichi-
ometry of the porphyrin-surfactant heteroaggregates. Figure 7
shows the plot obtained for 1a by plotting absorbance at 416
nm versus the porphyrin molar fraction (10 µM, total concentra-
tion). The minimum was obtained at porphyrin molar fraction
0.25 corresponding to an unexpected surfactant/porphyrin 3:1
stoichiometry. An analogous result was obtained for surfactant
2.
This stoichiometry implies that the porphyrin building block
in the aggregate features a free sulfonato group able to interact
with water and that hydrophobic interactions between the
hexadecyl tails of neighboring surfactant molecules can construct
an organized hydrophobic network (Scheme 1). A parallel array
of two one-dimensional (1D) chains (Scheme 1) explains the
3:1 stoichiometry as the consequence of the stabilization exerted
by the formation of polar surfaces that interact with water, and
that would not occur at a 4:1 stoichiometry. However, the
asymmetric shape of the Job’s plot suggests the formation of
aggregates with stoichiometry between 3:1 and 4:1. On the basis
of the changes observed, as a function of time and/or temper-
ature, in the UV-vis spectra of porphyrin-surfactant hetero-
aggregates at surfactant/porphyrin ratios >3:1, the hypothesis
is that the heteroaggregate would grow once formed the 3:1
bilayer by incorporating 4:1 layers between the external layers
Figure 4. RLS profile of 1 µM H2TPPS44-, in the presence of surfactant
1a (dotted line) and surfactant 2 (dashed line) at [surfactant]/[porphyrin]
ratio of 4:1, compared with the RLS profile of porphyrin in water (solid
line). Spectra are not corrected for photomultiplier response.
water. This, in agreement with the results obtained in the
absorption experiments, demonstrates the formation of por-
phyrin-surfactant heteroaggregates at ∼4:1 [surfactant]/[por-
phyrin] ratio. Further, the lack of a dependence of the shape
and the intensity of the emission spectra upon the excitation
wavelength (and of the excitation spectra upon the selected
emission wavelength) indicates the porphyrin-surfactant hetero-
aggregate as the highly predominant species in solution at this
surfactant/porphyrin ratio.
At [surfactant]/[porphyrin] ) 10 000:1 and surfactant con-
centration (10 mM), well above the cmc, a strong increase of
the intensity of the emission spectrum was observed with the
more intense maximum slightly blue-shifted with respect to the
maximum of the emission spectrum of the sample 4:1 (650 and
717 nm, i.e., red-shifted with respect to the spectrum of free
porphyrin in water). This result suggests the incorporation of
monomeric porphyrin into the micelles. Note that, in the case
of surfactant 2 (Figure 3b), the dependence of the intensity and
of the wavelengths of the fluorescence maxima upon the selected
excitation wavelength (at [surfactant]/[porphyrin] ) 10 000:1)
indicates the presence of both micellized porphyrin and
porphyrin-surfactant heteroaggregate, in agreement with the
results obtained in the UV-vis experiments. Actually, the
simultaneous presence of heteroaggregate and micellized por-
phyrin in the solutions depends not only on the concentrations
and their ratio but also on the hierarchical order of preparation
of the two component solutions, which is discussed later.
Resonance light scattering (RLS)5c,13 experiments gave further
insight on the nature of the different species in solution. The
evidence of the heteroaggregate in the RLS spectrum is the
emergence of an intense scattering signal in the region of Soret
absorption for both surfactants (Figure 4), thus indicating the
presence of extended porphyrin arrays within the porphyrin-
surfactant heteroaggregates, with long-range porphyrin inter-
actions. On the other hand the monomeric micellized porphyrin
(as well as the free monomeric porphyrin) shows no enhanced
scattering in the Soret region; rather a minimum appears at the
absorption maximum due to the self-absorption of the incident
and scattered radiation.13
It is of note that the samples containing the porphyrin-
4-
surfactant heteroaggregate as main species at low H2TPPS4
concentration (1 µM) show strong spectral changes as a function
9
15180 J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008

Transfer of Chirality from Molecules to Systems
A R T I C L E S
Figure 5. UV-vis spectrum of the surfactant-porphyrin heteroaggregates at [surfactant]/[porphyrin] ratio of 6:1, at t ) 0 (solid line) and t ) 24 h (dashed
line), (a) at [H2TPPS44-] ) 1 µM in the presence of surfactant 1a; (b) at [H2TPPS44-] ) 1 µM in the presence of surfactant 2; (c) at [H2TPPS44-] ) 10
µM in the presence of surfactant 1a; (d) at [H2TPPS44-] ) 10 µM in the presence of surfactant 2.
Figure 7. Job plot relative to the porphyrin-surfactant 1a heteroaggregate,
obtained by plotting absorbance at 416 nm vs porphyrin molar fraction.
the kinetic reluctance of the heteroaggregates to transform into
micelles incorporating the porphyrin as put in evidence by the
experiments discussed below. This irreversibility suggests a
hierarchical aggregation process which is confirmed by the
4-
Figure 6. UV-vis spectrum of 2.5 µM H2TPPS4 in the presence of
surfactant 2 at [surfactant]/[porphyrin] ratio of 4:1 taken at different
temperatures, starting from 15 °C and increasing up to 60 °C.
of 3:1 stoichiometry. This would lead to an overall stoichiometry
between 3:1 and 4:1.
Notice that an orthogonal arrangement of the 2D networks
reported in Scheme 1, with the hexadecyl chains crossing the
cavities occurring in the 2D network between four porphyrin
sites, would give a 3D structure. Such a structure would explain
dependence of the aggregation on the order of mixing. In fact,
we could observe that in the case of surfactant 1a at surfactant
concentration above the cmc the type of the obtained aggregates
also depends on the protocol of the sample preparation (Figure
8a). Actually, the dissolution of the surfactant powder in a dilute
porphyrin solution yields micellized monomeric porphyrin,
9
J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008 15181

A R T I C L E S
El-Hachemi et al.
Scheme 1
decreases as a function of time, analogously to what is observed
in the absorption spectra (decrease and broadening of the band).
This change must be attributed to the increase of the size of the
aggregate leading to an apparently lower extinction coefficient.
Interestingly a clear isodichroic point is observed for the CD
spectra registered at different times (Supporting Information),
thus suggesting that the structural motif responsible for the CD
band is the same in the small and in the large heteroaggregates.
With respect to the structure of the porphyrin chromophore
leading to the CD response, note that the 3:1 stoichiometry must
lead to the breaking of the degeneracy of the x and y electronic
transitions (Scheme 1). Actually, in the porphyrin-surfactant
heteroaggregate the anisotropic interactions on the macrocycle
periphery (Coulombic interactions in one directions and water/
Coulombic on the other) would differentiate the two degenerate
Soret Bx and By transitions of the symmetric porphyrin, as seen
in the UV-vis spectrum.6,7 The presence of chiral surfactants
on the periphery of the achiral porphyrin simply transforms it
in a chiral, optically active, chromophore by stabilizing the
phenyl group conformational angles in one of the two enantio-
morphous forms. As a consequence, the Bx and By transitions
are CD active. The rigidity of the heteroaggregate structure
surely plays a decisive role in the transfer of chirality (dia-
stereoselection). Note that the chiral transfer is not due the chiral
induction exerted in homoassociates, as is the case of the well-
known J-aggregates of this type of porphyrin, but rather to the
formation of heteroassociates with defined structural trends.
whereas the addition of a concentrated porphyrin solution to a
micellar solution gives the formation of the heteroaggregate.
Sonication of both samples does not produce any spectral
change, and samples remain unchanged for days, thus showing
that there is no interconversion between the two kinds of
aggregates. However the interconversion of heteroaggregates
into porphyrin including micelles was obtained by precipitation
at low temperature of the surfactant from heteroaggregate
followed by redissolution. In the case of surfactant 2 both
protocols of sample preparation yield the formation of the
porphyrin-surfactant heteroaggregate as the most abundant
species (Figure 8b).
Transfer of Chirality. Our principal objective being to detect
the transfer of chirality from the chiral surfactant to the achiral
porphyrin, the investigation described previously was necessary
in order to detect unambiguously the species occurring in
solution.
Concluding Remarks
We have investigated the interaction of two chiral cationic
surfactants with an achiral anionic porphyrin. Depending on the
experimental conditions these may result either in the inclusion
of the monomeric porphyrin in the micellar aggregates of the
surfactant or in the formation of porphyrin-surfactant hetero-
aggregates whose absorption spectrum is characterized by a split
of the Soret band centered at 413 nm. The fact that above the
cmc different protocols of mixing the porphyrin and surfactant
1 yield either micelles including monomeric porphyrin or
porphyrin-surfactant heteroaggregates which do not inter-
convert indicates the complexity of the dynamics of the overall
system. In fact, though micelles are equilibrium systems in fast
exchange with the monomeric surfactant, the inclusion of
H2TPPS44- solutions in the presence of the chiral ammonium
salt 3 (at [3]/[H2TPPS44-] ratio ranging between 1 and 22) are
CD silent in the region of the porphyrin absorption bands. This
was an expected result as consequence of the absence of
electronic perturbations between surfactant and porphyrin.
The CD experiments on the porphyrin/surfactant samples
were carried out at [H2TPPS44-] ) 10 µM (i.e., a concentration
that ensures proper absorbance values) on two type of samples:
monomeric micellized porphyrin at [surfactant]/[porphyrin] )
250:1 prepared in conditions were the heteroaggregate is absent;
samples of surfactant-porphyrin heteroaggregate ([surfactant]/
[porphyrin] at 4.1 and 6:1 ratios).
4-
H2TPPS4 in the micelles is almost irreversible, due to the
high association constant of the porphyrin with the micellar
aggregates. Further, though the formation of the 1:3 por-
phyrin-surfactant structural motif is probably reversible, its
hierarchical aggregation into more complex and organized
heteroaggregates driven by hydrophobic interactions is also
irreversible. Similar effects, only understandable assuming a
hierarchical association, have been described for the homo-
associationofH2TPPS44-andsimilarwater-solubleporphyrins.3d,h,i
Unexpectedly, the micellized porphyrin is CD silent for both
surfactants. This result excludes porphyrin as a probe of chirality
of these chiral micellar aggregates. Actually, the porphyrin in
the chiral micelle features an electronic perturbation (as seen
in the UV-vis spectrum), but the structural arrangement of the
porphyrin in the micelle is probably a racemic mixture of
conformational chiral porphyrin structures, because a loose
interaction with the chiral aggregate does not allow a dia-
stereoselection. In contrast, the heteroaggregate shows intense
bisignate bands at the absorption wavelengths of the porphyrin
(Figure 9). As expected the sign of the CD bands is opposite in
the CD spectra of the heteroaggregates formed by the surfactant
enantiomers (Figure 10).
The CD spectra of the large porphyrin-surfactant hetero-
aggregates obtained at [H2TPPS44-] ) 10 µM by the protocol
described above also show a bisignate band, similar to the band
obtained for the smaller heteroaggregates. The intensity of CD
spectrum of the solution obtained immediately after the last
addition of the concentrated surfactant and porphyrin solutions
This is in contrast with the multiple equilibrium picture invoked
previously in the interaction of H2TPPS4
4-
other cationic
surfactants (e.g., CTAB).8c,d However, in the case of surfactant
2, the formation of the porphyrin-surfactant heteroaggregates
occurs more easily also in the conditions in which 1 yields
micellar aggregates including monomeric porphyrin, thus dem-
onstrating a fundamental role of the molecular structure of the
surfactant in the building and in the organization of the
heteroaggregates, as demonstrated also by the pattern of CD
spectra.
We found that hydrophobic interactions have a fundamental
role in the formation of the heteroaggregates; further, for both
9
15182 J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008

Transfer of Chirality from Molecules to Systems
A R T I C L E S
4-
Figure 8. UV-vis spectrum of 1 µM H2TPPS4 in the presence of surfactant 1a (a) and 2 (b) at [surfactant]/[porphyrin] ratio of 2500:1 prepared (i) by
dissolving the surfactant powder in a dilute porphyrin solution (solid line) and (ii) by adding a concentrated porphyrin solution to a micellar solution (dashed
line).
4-
Figure 9. CD spectrum (solid line) and UV-vis spectrum (dashed line) of 10 µM H2TPPS4 at [surfactant]/[porphyrin] ratio of 6:1 in the presence of
surfactants 1a (a) and 2 (b).
acterized by a decrease of intensity, by a broadening of the split
Soret band as a function of time, and finally, by an increase of
the stoichiometry from 3:1 toward 4:1. Because of its organiza-
tion, the heteroaggregate is therefore an excellent model for
investigating the transfer of chirality from molecules to complex
systems. Actually, the porphyrin embedded in the heteroaggre-
gate, differently from the porphyrin included in micellar
aggregates, features a bisignate CD band, thus demonstrating
that due to tight interactions in the highly organized system
governed by Coulombic and hydrophobic interactions, both
chiral surfactants are able to transfer their stereochemical
information to the whole aggregate by selecting one of the
enantiomeric forms of the porphyrin where phenyl groups are
tilted with respect to orthogonality. We do not know the extent
of enantioselection (i.e., the extent of enantiomeric excess);
however, the important point is the finding of transfer of chirality
in systems controlled by a combination of Coulombic and
hydrophobic interactions; in fact, neither Coulombic or hydro-
phobic interactions per se were shown to be sufficient to observe
it.
A further point of interest is given by the fact that the
formation of the highly organized porphyrin-surfactant hetero-
aggregates occurs at very low concentration of components. This
highlights the possibility that very low concentrations of
amphiphiles in the primordial soup might have been sufficient
Figure 10. CD spectrum of 12.5 µM H2TPPS44- at [surfactant]/[porphyrin]
ratio of 4:1 in the presence of surfactants 1a (solid line) and 1b (dashed
line).
surfactants the (surfactant/porphyrin) stoichiometry in the hetero-
aggregates is 3:1, thus leaving a sulfonate group free to interact
with water and confirming the lyotropic-like liquid crystal
behavior of ionic amphiphilic porphyrins.9 We also observed a
progressive hierarchical association of the heteroaggregates
(formed at [porphyrin] ) 1 µM) into larger aggregates char-
9
J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008 15183

A R T I C L E S
El-Hachemi et al.
to settle the conditions of organization and recognition necessary
for the development of life and might have had a role on the
history of chiral homogeneity of biomolecules.
Supporting Information Available: Absorption spectra (Soret
region) of 1 µM H2TPPS4 at various [surfactant]/[porphy-
4-
rin] ratios for both the surfactants, wavelengths of the
Q-bands for the same absorption spectra, and UV-vis and
CD spectra of the large porphyrin-surfactant heteroaggre-
gates (at [H2TPPS44-] ) 10 µM) performed at different times.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We express thanks for the financial support
from Dipartimeto di Progettazione Molecolare and from the Spanish
Government project AYA2006-1648-C02-01. A.S. thanks COST
Action D27 for an STMS travel bourse. We thank Professor Roberto
Purrello for the communication of unpublished results on the
hierarchical dependence on the mixing of the components in
noncovalent synthesis of aggregates of amphiphilic porphyrins.
JA805669V
9
15184 J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008