Supramolecular chirality in solvent-promoted aggregation of amphiphilic porphyrin derivatives: Kinetic studies and comparison between solution behavior and solid-state morphology by AFM topography

Article abstract of DOI:10.1002/chem.200901964

The solvent-promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L-proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macro- cycles give rise to aggregates featuring supramolecular chirality with high el- lipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cat- ionic (nitrogen-quaternized L-proline, 3H₂) or anionic (carboxylate residue, 6H₂) group. Formation of anionic 6H₂ aggregates shows a diffusion-limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H₂ aggregates are formed by a cooperative, fractal-type decay, and appear as long-fibrous, looser structures. In the templated aggregation of 3H₂ over preformed 6H₂ aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter in- terchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid-state aggregate structures, corroborating the sergeant- soldier effect observed in the templat- ed aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereose- lective sensors and devices. 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/chem.200901964

DOI: 10.1002/chem.200901964
Supramolecular Chirality in Solvent-Promoted Aggregation of Amphiphilic
Porphyrin Derivatives: Kinetic Studies and Comparison between Solution
Behavior and Solid-State Morphology by AFM Topography
[
a]
[a]
[b]
[b]
Donato Monti,* Massimo De Rossi, Alessandro Sorrenti, Giuseppe Laguzzi,*
[
a]
[a]
[a]
[b]
Emanuela Gatto, Manuela Stefanelli, Mariano Venanzi, Loredana Luvidi,
[
b]
[a]
Giovanna Mancini, and Roberto Paolesse
Abstract: The solvent-promoted aggre-
gation behavior of some amphiphilic
porphyrin derivatives bearing chiral
functionality in the form of a charged
l-proline group has been investigated
by UV/Vis, resonance light scattering,
fluorescence and circular dichroism
spectroscopy. The investigated macro-
cycles give rise to aggregates featuring
supramolecular chirality with high el-
lipticity. Kinetic studies reveal peculiar
differences in the fashion of aggrega-
tion, depending on the intimate nature
of the chiral functionality, namely, cat-
ionic (nitrogen-quaternized l-proline,
6H ) group. Formation of anionic 6H
globular structures with reduced sizes,
as a likely consequence of shorter in-
terchromophore distances, due to fa-
vorable Coulombic interactions. The
results obtained show an interesting
parallelism between the solution be-
havior and the solid-state aggregate
structures, corroborating the sergeant–
soldier effect observed in the templat-
ed aggregation. The results presented
give important insights for understand-
ing the complex mechanisms involved
in these issues, which are of key impor-
tance for the development of chiral
supramolecular materials and stereose-
lective sensors and devices.
2
2
aggregates shows a diffusion-limited ki-
netic behavior. AFM topography stud-
ies show formation of tighter globular
structures. On the other hand, the cor-
responding 3H aggregates are formed
2
by a cooperative, fractal-type decay,
and appear as long-fibrous, looser
structures. In the templated aggrega-
tion of 3H over preformed 6H aggre-
2
2
gates, AFM images show formation of
Keywords: aggregation · chirality ·
porphyrinoids
·
self-assembly
·
supramolecular chemistry
3
H ) or anionic (carboxylate residue,
2
Introduction
within multimolecular systems through noncovalent interac-
tions. It has profound implications in the field of nanotech-
[2]
Supramolecular chirality, one of the most fascinating aspects
nology and in the construction of sensor devices with ste-
[1]
[3]
of supramolecular chemistry, allows the transfer of asym-
metry information (i.e., sensing, induction, amplification)
reoselective properties. In particular, the evolution of
chiral porphyrin systems at the supramolecular level mainly
relies, among others, on the aggregation of achiral porphyrin
platforms on chiral templates such as DNA, RNA, and
other natural or synthetic polymers upon electrostatic inter-
[
a] Dr. D. Monti, Dr. M. De Rossi, Dr. E. Gatto, Dr. M. Stefanelli,
Prof. M. Venanzi, Prof. R. Paolesse
Dipartimento di Scienze e Tecnologie Chimiche
Universitꢀ degli Studi di Roma, Tor Vergata
Via della Ricerca Scientifica 1, 00134 Rome (Italy)
Fax : (+39)0672594328
[
4]
[5]
action, by transfer of chirality from micellar systems, by
[6]
[7]
layer-by-layer assembly, by anion-controlled assembly, on
chiral ligand coordination or by directional stirring.
[8]
[9]
E-mail: monti@stc.uniroma2.it
Some recent examples of obtaining chiral porphyrin films
upon symmetry breaking at air–liquid interfaces, by using
[
b] Dr. A. Sorrenti, Dr. G. Laguzzi, Dr. L. Luvidi, Dr. G. Mancini
Istituto di Metodologie Chimiche, IMC-CNR
Area della Ricerca di Roma, Monterotondo Stazione, and
Dipartimento di Chimica
Langmuir–Sheafer (LS) deposition protocols, have been re-
[10]
ported,
with the chirality of the films occurring in a
random fashion.
Universitꢀ degli Studi di Roma La Sapienza
P. le A. Moro 5. 00185 Rome (Italy)
[11]
A different approach, adopted by us and others, relies
on functionalization of the periphery of the tetrapyrrolic
macrocycle by a charged, chiral group that induces the for-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901964.
860
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Chem. Eur. J. 2010, 16, 860 – 870

FULL PAPER
mation of supramolecular aggregates expressing elements of
chirality. The “reading-out” process, that is, the critical step
in the transfer of the stored molecular information, is initiat-
ed by the presence of particular functional groups (i.e., chi-
rophores), exerted via noncovalent interactions such as hy-
drogen bonding, electrostatic forces, p–p and other disper-
sion forces or the hydrophobic effect. In particular, the pres-
monomeric form, as shown by the sharpness of the Soret
bands at 415 nm. The exact superposition of the spectra in-
dicates that peripheral substitution of the macrocycles does
not affect the energy of the electronic transitions (Figure 1a,
trace A). The self-aggregation process can be steered by
changing the bulk properties of the solvent. In ethanol/
water mixtures, for example, on going from ethanol to more
water-rich solvents, aggregation of the macrocycles takes
place to give porphyrin aggregates characterized by red-
shifted and broadened absorption features (Figure 1a). In
ence of an l-prolininium moiety (porphyrin 3H , see
2
Scheme 1) or its negative counterpart (porphyrin 6H , see
2
Scheme 2) steer the self-aggregation process toward forma-
tion of large porphyrin aggregates featuring supramolecular
particular, the aggregates of cationic 3H show a Soret ab-
2
[11b]
chirality.
The choice of an l-proline moiety as chirophore
sorption maximum at 445 nm, indicating the formation of J-
type, offset structures, whereas the anionic counterpart 6H2
gives rise to supramolecular structures in which only a mar-
ginal shift of about 1–2 nm is evident.
was suggested by its intrinsic properties, such as conforma-
[12]
tional rigidity and robustness toward racemization. Inter-
estingly, if the self-aggregation of 3H is carried out in the
2
presence of preformed chiral aggregates of 6H , a remark-
This, evidently, should be the consequence of different
modes of inter-macrocycle interaction. The aromatic nature
of the macrocycles governs the predominant p–p and elec-
trostatic interactions. These forces are additionally finely
modulated by specific interactions exerted by the appended
groups, which influence the mutual geometrical orientation
of the platforms and thus the final morphology of the aggre-
2
able amplification of the CD intensity is observed. This indi-
cates a strong template effect exerted by the presence of the
anionic assemblies, initiated by electrostatic forces. Reverse
[14]
experiments, that is, aggregation of 6H triggered by aggre-
2
gates of 3H , showed no amplification, that is, the phenom-
2
enon is under the control of the so-called sergeant–soldier
[13]
effect. This can be tentatively ascribed to the different in-
timate structures of the supramolecular aggregates. To shed
light on this important finding we carried out detailed spec-
troscopic and kinetic studies on the aggregation of the title
gates. In the case of anionic derivative 6H , for example, the
2
geometry of the assemblies would be triggered by the H-
bonding interaction between the carboxylate and inner NH
[15]
groups. This should result in a tighter, scarcely offset, ge-
porphyrins, and in particular on anionic 6H , which has not
ometry. On the other hand, the guiding interaction for 3H
2
2
[16]
been thoroughly investigated. Moreover, topographic atomic
force microscopy (AFM) studies on porphyrin aggregates
deposited onto highly oriented pyrolitic graphite (HOPG)
gave further insights into the morphology of the aggregates,
showed remarkable differences on their mesoscopic struc-
tures, and gave key hints for
should be of cation–p nature, between the quaternary am-
monium ion and the electron-rich tetrapyrrolic platform.
This would result in a looser, more offset, J-type structure,
with red-shifted Soret UV/Vis spectroscopic features, as ex-
[16c]
perimentally observed.
full comprehension of the phe-
nomena observed in solution.
Results and Discussion
Porphyrin derivatives were ob-
tained by following straightfor-
ward
synthetic
procedures
making efficient use of peptide
chemistry, as outlined in
Schemes 1 and 2. The EDCl/
HOBT-mediated coupling of 5-
(
4-aminophenyl)-10,15,20-tri-
phenylporphyrin (1H ) or 5-(4-
2
carboxyphenyl)-10,15,20-triphe-
nylporphyrin (4H2) with the
properly protected l-proline de-
rivative gave title porphyrin de-
rivatives 3H2 and 6H , respec-
2
tively, in satisfactory overall
yields. In the concentration
range of 1–50 mm, the macrocy-
Scheme 1. Preparation of 3H
2
. i) EDCl, HOBT, THF/CH
OH, RT, overnight.
2 2
Cl , 08C, 1 h. ii) N-methyl-l-proline, 48 h, 258C.
cles exist in ethanol solution in iii) CH
3
I, DMF, RT, 72 h. iv) NaCl/CH
3
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861

D. Monti et al.
2 2 2 2 2 3 2 4
Scheme 2. Preparation of 6H . i) EDCl, HOBT, CH Cl , 08C, 1 h, then RT 12 h. ii) TFA/CH Cl (1/1 v/v), 1 h. iii) Aqueous NaHCO , then Na SO .
The solvent composition at which the aggregation starts
i.e., critical aggregation solvent composition, CASC) de-
(
pends on the intimate nature of the appended charged l-
proline functionality. For example, the CASC for 3H is an-
2
ticipated at lower water content with respect to its negative-
ly charged counterpart 6H . Aggregation of the cationic por-
2
phyrin derivative is virtually complete in fact at about 75%
water, whereas at the same solvent composition aggregation
of the anionic derivative has reached only approximately
6
0% (see Figure S1 in the Supporting Information). This re-
flects the different hydrophilicity of the appended ionic
charges.
Aggregation occurs with concomitant quenching of fluo-
rescence emission and rise of resonance light scattering in-
tensity, corroborating formation of aggregates species. The
corresponding spectra are reported in Figure S2 in the Sup-
[11b,c]
porting Information. As shown in preliminary reports,
both of the architectures show strong supramolecular chiral-
ity, as shown by their spectroscopic CD features, indicating
the appearance of coupled, negative bisignated bands in sol-
utions with high water content (i.e., 75%). The presence of
negative bands in both of the aggregates likely indicates that
mutual orientation of the porphyrin chromophores occurs in
anticlockwise fashion, as a consequence of the reading-out
process of the stereogenic molecular information stored in
the l-proline group. Under the same conditions, the intensi-
ty of the CD bands, related to the extrinsic chirality of the
porphyrin assemblies, is higher for 6H derivatives than for
2
those of 3H , and the crossover points are shifted in accord-
2
ance with the UV/Vis spectral features (Figure 1b).
Support for the hypothesis on the different nature of self-
interactions was provided by CD spectroscopic investiga-
tions of the aggregation of the corresponding 3M and 6M
porphyrin derivatives (M=Cu and Zn ). In the case of the
anionic 6Cu, for example, no CD features are detectable
under aggregative conditions. This should be a consequence
ꢀ
6
Figure 1. a) UV/Vis spectra of title porphyrins (5ꢂ10 m) in different
EtOH/H O solutions: A) 6H and 3H in EtOH; B) 6H at 25% EtOH;
C) 3H at 25% EtOH. b) CD spectra of 6H (full line) and 3H (dotted
line) in EtOH/H O (25/75 v/v).
II
II
2
2
2
2
2
2
2
2
862
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Chem. Eur. J. 2010, 16, 860 – 870

Supramolecular Chirality in Porphyrin Aggregation
FULL PAPER
of the low propensity of square-planar copper porphyrins to
Table 1. Kinetic data for the aggregation of 6H
v).
2
in EtOH/H
2
O (25/75 v/
[17]
bind a further axial ligand.
Conversely, the 6Zn aggre-
[
a]
ꢀ1
Entry
A
H
U
G
R
N
U
G
2
] [m]
k [s
]
n
gates are CD-active, but with opposite positive Cotton
effect, due to coordination to the central metal ion. These
findings indicate the importance of the interaction of the
carboxylate group with the porphyrin core, which should be
ꢀ
ꢀ
ꢀ
ꢀ
6
5
5
5
1
2
3
4
5
6
7
8
4.5ꢂ10
1.0ꢂ10
2.0ꢂ10
3.0ꢂ10
0.0050
0.0205
0.0520
0.0680
1.7ꢂ10 (1.25ꢂ10
8.0ꢂ10 (7.3ꢂ10
0.23
0.21
0.23
0.19
0.44 (4.0)
4.7 (0.5)
1.2 (2.3)
1.1
also important in free-base 6H . On the other hand, in the
[b]
ꢀ6
ꢀ2
ꢀ4
2
5.0ꢂ10 , 0.10m NaBr
)
[
[
[
c]
d]
e]
ꢀ6
ꢀ4
ꢀ4
case of the cationic 3Cu and 3Zn, strong similarities to the
5.0ꢂ10 , 0.50m NaBr
)
ꢀ6
ꢀ3
ꢀ4
CD features of 3H were found, that is, in this case interac-
5.0ꢂ10 , 0.025m NaBr
2.3ꢂ10 (6.3ꢂ10
0.0004
)
2
ꢀ5
3.0ꢂ10
tion of the l-proline ammonium group with the porphyrin
core does not play a relevant role in chiral molecular recog-
nition. Further NMR or IR evidence for these kinds of in-
teractions could not be obtained because of the low concen-
tration of the solutions employed, owing to the sparing solu-
bility of the investigated free-base porphyrins.
[a] Entries 1–4: the kinetic parameters were obtained according to Equa-
tion (1). Entries 5–7: the fitting analysis was performed by using Equa-
tion (2). The values in parentheses refer to the second part of the bipha-
sic decay (see text). [b] Data refer to aggregation in 30% EtOH, 0.10m
2
NaBr. [c] 0.50m NaBr. [d] 0.025m NaBr. [e] Aggregation of 3H in EtOH/
[
11b]
2
H O (25/75 v/v).
Strong differences were found in the intimate nature of
the aggregation mechanisms of the title macrocycles. In the
case of the 3H we reported that aggregation follows a sig-
reveal that aggregation of 6H2 occurs with overall faster
2
moidal autocatalytic behavior that indicates formation of
rates, by about two orders of magnitude, with respect to cor-
[11b]
“fractal-type” architectures.
On the other hand, in the
responding cationic derivative 3H (e.g., compare entries 4
2
present study on the aggregation of 6H , we found that the
and 6, Table 1). The aggregation rate constant k was found
to increase with increasing initial porphyrin bulk concentra-
tion, whereas the fact that the “n factor” remains virtually
unchanged within experimental error suggests that no
change in aggregation mechanism occurs within the window
of concentration explored. The results are reported in
Table 1 (entries 1–4) and graphically in Figure 2.
More intriguing insights can be achieved by monitoring
aggregate growth by circular dichroism spectroscopy. Inter-
estingly, the appearance of the CD features occurs with a ki-
netic profile characterized by a well-defined induction time
2
self-interaction follows a so-called “stretched exponential”
decay, which indicate a different mode of aggregation with a
diffusion-limited mechanism. The experimental data points
can be nicely fit by a so-called “stretched exponential”
equation [Eq. (1)]
n
Ext ¼ Ext þ DExt½1ꢀexpfꢀðktÞ gꢁ
ð1Þ
0
where Ext is the extinction at time t, DExt=Ext ꢀExt
t
1
0
the difference between the initial extinction value (t=0)
and that at t=1, k the kinetic rate constant and n the ag-
gregation growth factor, related to the mean number of
binding sites available for the aggregate growth process.
(
Figure 3). This is in close analogy with the results formerly
reported by Pasternak et al. for porphyrin assembly on
[21a]
DNA surfaces
and for the induced aggregation of chloro-
[18]
This equation is valid in the case of monodisperse systems
[21b]
phyll a, and structurally related dyes,
in which a nuclea-
in which large clusters are formed by interaction between
initial smaller clusters (i.e., nucleation seeds) and mono-
mers. This is referred to as the diffusion-limited aggregation
tion process forming small species with weak stereochemical
relationship is followed by growth of larger species featuring
enhanced supramolecular chirality, due to extended three-
dimensional electronic conjugation.
The differences found by following aggregate growth by
UV/Vis spectroscopy can be due to the fact that this tech-
nique is able to monitor only the overall process, which sug-
gests that the final mesoscopic aggregation step (i.e., coales-
cence of the assemblies) should occur with conformational
evolution and only marginal electronic and, consequently,
spectral changes.
(
DLA) mechanism. Notable examples are reported for ag-
gregation of charged cyanine dyes on charged polymeric
[19]
templates, such as polyvinylsulfonate and in our previous
[11d]
work.
An alternative mechanism, implying fast initial formation
of small clusters, followed by their slow collapse into larger
structures, is known as diffusion-limited cluster–cluster ag-
gregation (DLCCA). The corresponding kinetic profiles, al-
though defined by a stretched exponential decay curve, are
characterized by an “initial burst”, followed by slower final
evolution. This was recently reported for the case of a spe-
cific ion-pairing effect on cationic–anionic porphyrin inter-
action and on salt-promoted aggregation of glycoside-func-
Further, we carried out some experiments on aggregation
of 6H in the presence of added salts, such as 0.10m NaBr in
2
[22]
EtOH/H O (30/70).
The results obtained indicate some
2
remarkable differences in terms of mechanism of aggrega-
tion and supramolecular chirality of the final species, com-
pared to that obtained in the absence of added salts. In this
case a kinetic investigation by UV/Vis spectroscopy showed
an aggregation process with biphasic decay. A typical exam-
ple is shown in Figure 4.
[11e,20]
tionalized porphyrin derivatives.
In both cases the n factor is required to be <1, but in the
case of the DLCCA mechanism it should be strictly ꢂ0.25.
In the present case, we found a mean value of n=0.21 (ꢃ
0.02), which indicates likely occurrence of the DLCCA
The kinetic analysis was carried out by using multipara-
meter Equation (2), recently employed in related cases of
mechanism. The results, summarized in Table 1, clearly
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863

D. Monti et al.
ꢀ
6
Figure 3. a) CD spectral changes for the aggregation of 6H
EtOH/H O (25/75 v/v). b) Corresponding plot of [q] versus time, at l=
55 nm.
2
5.0ꢂ10 m in
2
Figure 2. a) Kinetic profile for the UV/Vis spectral variation of the aggre-
gation of 6H in H O/EtOH (25/75 v/v) at 298 K. b) Corresponding k
4
2
2
versus porphyrin concentration plot.
DLA mechanism and subsequent growth of the aggregates
by a cooperative process, respectively) in the proper tempo-
ral window, in which the relative extinction versus time plots
show clear isosbestic points (see Figure S3 in the Supporting
Information). The first stage promotes aggregates with spec-
troscopic features characterized only by broadening and hy-
pochromic effect of the Soret band. Conversely, the second
step gives rise to severely broadened and red-shifted spec-
tra, with a J-type structure, indicating marked evolution at
the supramolecular level.
[
23]
porphyrin aggregation by Purrello and co-workers,
The
factor b used in this equation corresponds to the term n in
Equation (1)
b1
b2
Ext ¼ Ext1 þ DExt expðꢀk tÞ þ DExt expðꢀk tÞ
ð2Þ
t
1
i
2
i
where k and Ext are the usual terms, and b is the stretch-
ing factor that modulates the aggregate growth. A stretching
factor b<1 generally indicates the occurrence of a diffu-
sion-limited phenomenon, whereas a value of b>1 indicates
cooperative growth. A value of b=1 indicates a reaction-
limited aggregation (RLA) mechanism corresponding to a
first-order process. At micromolar porphyrin concentration
and 0.10m NaBr, the non-linear regression fit according to
Equation (2), gives a first diffusion-limited step (DLA) with
Circular dichroism spectroscopy gave further insights into
this interesting phenomenon. The CD spectra show some
differences, in terms of curve shapes and intensity, with re-
spect to those of the structures formed in the reaction pro-
moted in the absence of salt, in that the negative feature at
higher wavelengths virtually disappeared, and the positive
band remained practically the only one visible in the spectra
ꢀ
2
ꢀ1
k1 =1.7ꢂ10
s
and b =0.44 (ꢃ0.03), followed by a coop-
1
ꢀ
4
ꢀ1
[24]
erative decay with k =1.2ꢂ10
s
and b =4.03 (ꢃ0.05).
of the final aggregate architectures. The CD intensity, al-
2
1
The input values for the non-linear regression fit could be
independently obtained by examining the early and late ag-
gregation stages (i.e., formation of aggregate nuclei by the
though reduced twofold, is not cancelled out, as is instead
observed in the case of the aggregation of 3H , in which
2
quite strong annihilation of the CD features occurs upon ad-
864
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Chem. Eur. J. 2010, 16, 860 – 870

Supramolecular Chirality in Porphyrin Aggregation
FULL PAPER
ions. However, an overall asymmetry of the aggregates is
still present, as witnessed by the still-strong dichroic band in
the CD spectra. A qualitative kinetic evaluation indicates a
close analogy of the CD spectral evolution with the reaction
carried out without added ions. Also in this case, CD spec-
troscopy reveals that the first rapid aggregation step gives
rise to the formation of non-chiral species (aspecific aggre-
gates, formed in the first, “lag” period), which only in the
second step start to transform themselves into chiral archi-
tectures. Noteworthily, this behavior nicely follows the sig-
moidal decay shown by UV/Vis spectral changes of the
Soret band, in support of this hypothesis (see Figure S4 in
the Supporting Information). Fluorescence and RLS spec-
troscopy studies further corroborate the observed aggrega-
tion behavior. The RLS intensities are weaker than those of
the species obtained in the salt-free aggregation process.
This should be accounted for by the formation of smaller ag-
gregates, in analogy to that reported in related studies by
Pasternack et al. on the electrostatically driven aggregation
[25]
of cyanine dyes onto polyvinylsulfonate matrices.
Peculiar results were obtained on further varying the
ionic strength of the solution. At low salt concentration, that
is, 0.025m NaBr, the self-interaction still follows a biphasic
decay, but the first step apparently follows a reaction limited
ꢀ
3
ꢀ1
decay, with k =2.3ꢂ10
s
and b =1.2 (ꢃ0.3), and it is
1
1
followed by a slower, cooperative decay, with k =6.3ꢂ
2
ꢀ
4
ꢀ1
1
0
s
and b =2.26 (ꢃ0.03). Further changes occur at
1
higher ionic strength. At 0.50m NaBr, for example, the ag-
gregation takes place in an initial highly cooperative step
ꢀ
4
ꢀ1
with k =8.0ꢂ10
s
and b =4.7 (ꢃ0.1), which is followed
1
1
ꢀ
4
ꢀ1
by a diffusion-limited decay with k =7.3ꢂ10
s
and b =
1
2
0
.50 (ꢃ0.04). The corresponding plots are reported in Fig-
ure S5 in the Supporting Information, and the results sum-
marized in Table 1, entries 5–7. The change of cooperativity
in the subsequent step should be attributed to the shielding
effect of the ionic solute on the charged groups, which
strongly influences the interaction potential between the in-
[20b]
teracting species.
However, a thorough investigation of
this interesting topic, underway in our laboratories, is
beyond the scope of the present work. The detailed results
will be presented elsewhere in the near future.
Templated aggregation: We found a remarkable template
effect in the aggregation of 3H in the presence of pre-
2
[11a]
formed 6H aggregates.
Some detailed studies on this in-
2
teresting topic, in terms of the effect of the solution bulk
properties on kinetic and solid-state morphology (AFM),
were carried out in order to shed light on this phenomenon.
An example is reported in Figure 5, which shows the UV/
Vis and CD spectral variations.
Figure 4. a) Kinetic profile (UV/Vis, l=415 nm) for aggregation of 5.0ꢂ
ꢀ
6
1
2 2
0 m 6H in EtOH/H O (30/70 v/v) with 0.10m NaBr at 298 K. The full
circles are the experimental points and the continuous line represents the
theoretical curve fit according to Equation (2). b) Corresponding CD
spectral variations. c) CD intensity profile at l=411 nm.
The kinetic decays follow a biexponential progress, in
which the first stage expresses the aggregate growth, where-
as the second step reasonably follows the collapse of the
formed templated structures onto the quartz walls of the
[11b]
[11a]
dition of NaBr.
This should indicate that the specificity
cuvette, as formerly established.
of the stereochemical induction is looser, due to the shield-
ing effect on the charged chiral groups exerted by the added
Equation (3), proposed by Mallamace et al. for related
[18a,20c]
cases,
was employed.
Chem. Eur. J. 2010, 16, 860 – 870
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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865

D. Monti et al.
effect on the rate, which is about one order of magnitude
lower (see Table 2, entries 5 and 4).
The electrostatic nature of the interaction has been con-
firmed by the results obtained by performing aggregation
studies in the presence of added salts such as NaBr. In such
a case, the presence of salt strongly reduces the CD ampli-
tudes of the corresponding spectra, as a consequence of
shielding of the polar moieties bearing the stereochemical
information, which biases the effect of aspecific p–p interac-
tions and promotes formation of aggregated species with a
[11e]
lower degree of supramolecular chirality.
Competitive ex-
periments, that is, the effect of adding an anionic, non-chiral
porphyrin derivative, yielded some interesting insights into
the observed phenomena. For example, on adding an equi-
molar amount of achiral 4H (see Scheme 1) to a solution of
2
3
H aggregates (in EtOH/H O 25/75 v/v), the CD amplitude
2 2
rapidly vanishes with time and goes to zero in a few minutes
Figure 6). Clearly, the electrostatic effect exerted between
(
the carboxylate and prolininium moieties disrupts the inti-
mate stereochemical relationship of the aggregates and re-
sults in achiral structures, likely by formation of intercala-
Figure 5. a) UV/Visible spectral changes for templated aggregation of
ꢀ
6
ꢀ5
3
2 2 2
H (7.5ꢂ10 m) onto aggregates of 6H (1.5ꢂ10 m; H O/EtOH 75/25
v/v). b) Corresponding CD plot; lowermost trace: t=0 min; uppermost
trace: t=250 min.
Ext ¼ Ext þ Ext expðꢀk tÞ þ Ext expðꢀk tÞ
ð3Þ
t
0
1
1
2
2
ꢀ
6
Figure 6. CD spectral evolution of the deaggregation of 3H
2
(7.5ꢂ10 m)
O/EtOH (75/25 v/v). Solid line: 3H
solution at t=0 (the corresponding intensity has been scaled down 50-
fold for clarity); dotted lines: CD profiles at t =0.5 min, t =3 min, t
0 min.
ꢀ
5
on addition of 4H
2
(1.5ꢂ10 m) in H
2
2
The input values of the first-order rate constant for the
aggregation step can be easily obtained by taking into con-
sideration the first part of the spectral variations with time.
The results are summarized in Table 2. It could be conclud-
ed, on inspection of the table, that the presence of the
anionic template accelerates the aggregation rate of the cat-
ionic derivative, and that the effect is more pronounced on
increasing the concentration of the aggregating species.
Aging of the template solution has an evident detrimental
1
2
3
=
2
tion species. Evidently, the presence of the intercalated achi-
ral platforms hampers a successful reading-out process of
the stereochemical information, and final structures lacking
the supramolecular chiral features result.
Remarkably, if the same competition experiment is car-
ried out with templated “heteroaggregates”, only a marginal
effect can be observed. This should imply an increased ro-
bustness of these species toward the detrimental action of
the negative achiral counterpart.
Moreover, the same reduced amplification effect is ob-
served in the case of simultaneous aggregation of the chiral
derivatives. This finding can be explained in terms of several
concomitant factors. Co-aggregation, again driven by elec-
trostatic interaction, would be detrimental to the reading-
Table 2. Kinetic data for the templated aggregation of 3H
gregates in EtOH/H O (25/75 v/v) at T=298 K.
2
2
onto 6H
2
ag-
[
a]
ꢀ
1
Entry
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[6H
2
] [m]
A
H
U
G
R
N
U
G
2
] [m]
k [s ]
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
6
6
5
5
5
ꢀ5
ꢀ4
1
2
3
4
5
5.0ꢂ10
7.5ꢂ10
1.0ꢂ10
1.0ꢂ10
1.0ꢂ10
1.5ꢂ10
1.5ꢂ10
1.5ꢂ10
3.0ꢂ10
3.0ꢂ10
1.5ꢂ10
1.2ꢂ10
4.7ꢂ10
5.8ꢂ10
8.5ꢂ10
ꢀ5
ꢀ5
ꢀ5
ꢀ5
ꢀ3
ꢀ3
ꢀ3
ꢀ5[a]
[
2
a] Templating solution of 6H aggregates was aged for 30 days.
866
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 860 – 870

Supramolecular Chirality in Porphyrin Aggregation
FULL PAPER
out process of the stereochemical information, by a mis-
matching effect of stereogenic centres with different struc-
tures. Furthermore, despite the fact that the aggregation
rate of 6H is higher than that of its 3H counterpart (see
2
2
Table 1), templation would occur at the first stage of 6H2
evolution, in which, according to the reported CD results, a
very small amount of aggregates expresses supramolecular
chirality. The complexity of the phenomena involved result
in more complex CD spectral features, and in irregular ki-
netic profiles. Furthermore, a sergeant–soldier effect is
found in the observed phenomenon. Indeed, on carrying out
the reverse experiment, that is, templated heteroaggregation
of 6H onto 3H aggregates, no increase in CD amplitudes
2
2
was observed, and the CD intensities were a virtual super-
position of those of the independently aggregated species.
The origin of this phenomenon would be ascribed, in the
first instance, to the different lipophilicities of the charged
[26]
groups. According to the Hofmeister series, the carboxyl-
ate moiety, a relatively hydrophilic group, will be more ex-
posed at the surface of the aggregates, toward the bulk
water. This would result in a more favorable interaction
with the incoming positively charged ammonium group.
Conversely, the ammonium moiety, a more lipophilic ion,
will be embedded in a less exposed region of the 3H aggre-
2
gates, and thus reading out by the incoming anionic porphyr-
ins is less efficient. Moreover, polarisable species, such as
tetralkylammonium ions and the like, are known as “ionic
chaotropes”, which show higher tendency to interact with
large polarisable surfaces and are surrounded by a layer of
low-density water (LDW). Less polarisable, harder ions such
as halides and carboxylates (ionic kosmotropes), prefer to
be surrounded by high-density water (HDW) and show less
propensity to bind to large polarisable surfaces, such as pro-
Figure 7. AFM images of porphyrin aggregates layered onto HOPG from
EtOH/H O (25/75 v/v) solutions. a) 6H aggregates; b) 3H aggregates.
2 2 2
[27]
teins and the like, or in our case, to porphyrin aggregates.
Synergism of these effects would explain the order observed
in the template effect.
Topographic AFM studies: Atomic force microscopy topo-
graphic studies on the morphology of the aggregates were
carried out to shed light on their intimate structure, and to
correlate the morphology with the aggregation behavior and
the magnitude of the final supramolecular chirality. Samples
were prepared by drop casting a solution of porphyrin ag-
gregates (see Experimental Section) on highly oriented py-
rolitic graphite (HOPG). Use of different substrate (i.e.,
freshly cleaved mica) did not have any influence on the
mesoscopic structures of the aggregates; for example, the
These globular structures are condensed together into larger
mesoscopic arrangements, several micrometers wide.
Remarkably, the aggregates of 3H showed a very differ-
2
ent appearance, constituted by long ramified mesostructures,
with diameters of the isolated thinner entities of about
0.10 mm and lengths of several micrometers. These are
woven into larger bundles several hundreds of nanometres
wide. The evident superposition of some of the branches in-
dicates that these structures collapsed from solution and did
not grow directly on the surface.
[28]
structures observed for 3H aggregates deposited on freshly
cleaved mica are very similar to those obtained on HOPG
Formation of tighter structures by the negatively charged
compounds should be a consequence of the greater rigidity
of the appended framework, which is constituted by a small-
er number of rotors (i.e., freely rotating single s bonds) and
more extensively conjugated double bonds. This should
result in a more effective reading-out process of the stereo-
chemical information stored on the porphyrin framework
during the aggregation process. Conversely, the cationic con-
gener, which has a chiral moiety with a larger number of
rotors, forms looser structures. This is also reflected in the
2
(
see Figure S6 in the Supporting Information). The results
are reported in Figure 7.
The AFM topographic images clearly show major differ-
ences between the morphologies of the chiral aggregates. In
particular, the structures formed by the 6H aggregates have
2
a globular-like appearance, with a mean diameter of the iso-
lated structures of about 0.45 to 0.50 mm, as shown in the
profiles depicted in Figure S7 in the Supporting Information.
Chem. Eur. J. 2010, 16, 860 – 870
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
867

D. Monti et al.
different intensities of the observed CD features, which are
higher in the case of the anionic porphyrin derivative.
For the heteroaggregated species, the AFM images again
show formation of globular structures with mean diameters
of about 200 nm (Figure 8 a). The globular appearance of
the structures can undoubtedly be ascribed to the templating
effect exerted by the anionic aggregates, which drive the
final form of the templated structures. Moreover, the de-
creased mean size of the structures (i.e., 0.20–0.25 mm; see
Figure S8 in the Supporting Information) could be ascribed
to a shrinking effect exerted by favorable electrostatic inter-
actions. This should also explain the observed chiral amplifi-
cation, due to enhanced electronic coupling among the
more closely interacting chromophores.
spheroids of 0.35–0.45 mm, that is, similar to that observed
for untemplated 6H aggregates. This increased size could
2
be accounted for by the reduced propensity of the carboxy-
lated derivative to interact with polarisable surfaces, which
leads to a larger interchromophore distance, and hence to
the reduced chiral-amplification effect observed by CD
[11a]
spectroscopy.
Conclusions
We have reported on spectroscopic studies on the solvent-
promoted aggregation of chirally functionalized amphiphilic
porphyrin derivatives. The aggregation mechanism is strong-
ly dependent on the intimate nature of the appended func-
tionality and on bulk properties of the solvent such as polar-
ity and ionic strength. Noteworthily, aggregation occurs with
transfer of the stereogenic information stored on the periph-
ery of the macrocycles (i.e., charged l-proline derivatives)
to the whole final supramolecular architectures. Close paral-
lelism between solution behavior and solid-state morpholo-
gy of the aggregates was revealed by AFM studies, which
showed remarkable differences in the structures of the ar-
chitectures at the mesoscopic level. Preliminary studies on
the Langmuir–Blodgett deposition of films indicate that this
transfer of chirality also occurs in the solid state, and it is
dependent on the stereogenic centre present on the porphy-
rin molecular framework. This is of importance in the field
Interestingly, the structures obtained in the reverse-tem-
plated experiment, that is, aggregation of derivative 6H2
onto aggregates of cationic 3H , show the same coalescence
2
to globular structures, but with a mean size of the single
[29]
[30]
of sensors and chiral molecular materials, and the re-
sults will be reported in the near future.
Experimental Section
General information on the instrumentation, synthesis of intermediates,
protocols for solution preparation, and kinetic calculations, are reported
in the Supporting Information.
Porphyrin preparation: Chiral charged porphyrin derivatives 3H
2
and
were prepared by the synthetic strategies described in Schemes 1 and
, respectively. Syntheses of porphyrin precursors 1H and 4H were car-
was
prepared as previously described in the literature, while cationic ana-
logue 3H was prepared by following an improved version of a procedure
previously reported in the Supporting Information of reference [11b].
6
H
2
2
2
2
[31]
ried out as reported in the literature. Anionic chiral porphyrin 6H
2
[
32]
2
5
-(4-Aminophenyl)-10,15,20-triphenylporphyrin (1H
2
) was synthesized by
[
31a]
literature methods.
Analytical and spectroscopic data were in full
agreement with published values. Detailed procedures are reported in
the Supporting Information.
N-[5-(4-Aminophenyl)-10,15,20-triphenylporphyrinyl]-l-prolin
yl)amide (2H ): 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydro-
chloride (EDCl, 606 mg, 3.16 mmol) and 1-hydroxy-1H-benzotriazole
HOBT, 427 mg, 3.16 mmol) were added to a stirred solution of N-
methyl-l-proline monohydrate (58 mg, 0.45 mmol) in dry THF/CH Cl
1/1 v/v; 40 mL) at 08C. After complete dissolution of all reagents, N-
methylmorpholine (85 mL, 0.77 mmol) was added and the reaction mix-
ture stirred for 1.5 h at 08C. Porphyrin 1H (283 mg, 0.45 mmol) was then
ACHTUNGTRENUNNG( N-meth-
2
(
2
2
(
2
added and the reaction mixture was stirred for an additional 48 h at
room temperature, with monitoring of the reaction progress by TLC on
Figure 8. AFM images of templated porphyrin aggregates layered onto
HOPG from EtOH/H
of 3H onto 6H aggregates; b) templated aggregation of 6H
aggregates.
2
O (25/75 v/v) solutions. a) templated aggregation
silica gel with CHCl
ther progress of the reaction, the solvent was removed under reduced
pressure and the residue dissolved in CHCl (100 mL) and evaporated to
3
/MeOH (1%) as eluent. When TLC showed no fur-
2
2
2
onto 3H
2
3
868
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 860 – 870

Supramolecular Chirality in Porphyrin Aggregation
FULL PAPER
give a red solid that was applied to a short neutral alumina column
Acknowledgements
(
2 2
Brockmann grade III) and eluted with CH Cl /petroleum ether (7/3 v/v).
The unconsumed aminophenyl porphyrin derivative was separated. The
second red fraction corresponding to the title l-proline–porphyrin conju-
The Authors wish to thank Dr. Veronica Cantonetti for the early involve-
ment in the synthesis of some of the porphyrin derivatives. We also grate-
fully acknowledge MiUR-PRIN (Project no. 2007C8RW53) and NATO
(Grant no. CBP.EAP.CLG.982972) for financial support.
gate was isolated and then crystallized from CH
bright purple crystals (86 mg, 0.116 mmol; 26% yield). UV/Vis (CHCl
max (loge)=419 (5.1), 514 (3.8), 546 (3.6), 586 (3.5), 645 nm (3.2);
2
Cl
2
/hexane to afford
3
):
l
1
3
H NMR (300 MHz, CDCl ): d=9.78 (s, 1H, NHC(O)), 8.89 (m, 8H, b-
pyrr), 8.24 (m, 8H, ArH), 8.05 (d, 2H, J=8.3 Hz, ArH), 7.78 (m, 9H,
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(
2
+
N
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(CH
3
) and 1H, proline d-H), 2.46 (m, 1H, proline b-H), 2.20 (m, 1H,
proline b-H), 1.99 (m, 2H, proline g-H), ꢀ2.74 ppm (s, 2H, pyrrole NH);
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+
FAB-MS (NBA): m/z: 741 [MꢀH] .
[
[
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4
04–420, and references therein.
prolininium amide chloride (3H
dissolved in dry DMF (10 mL). To this solution, stirred under an inert at-
mosphere, CH I (600 mg 4 mmol) was added. The reaction mixture was
stirred at room temperature for 72 h. Then the solvent was removed and
the residue dissolved in CHCl . Purification of the crude product was per-
formed on a neutral alumina column (Brockmann grade III) by first elut-
ing with CH Cl , then CHCl and finally CHCl /CH OH (95/5 v/v), to
2 2
): Porphyrin 2H (80 mg, 0.11 mmol) was
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2
2
3
3
3
give a red microcrystalline solid corresponding to the methylated prod-
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14 (3.7), 546 (3.6), 586 (3.5), 645 nm (3.1); H NMR (300 MHz, CDCl ):
d=12.41 (s, 1H, NHC(O)), 8.87 (m, 8H, b-pyrr), 8.25 (m, 10H, ArH),
.78 (m, 9H, ArH), 6.05 (m, 1H, proline a-H), 3.90 (m, 1H, proline d-
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1
5
3
7
+
3 2
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H), 2.63 (m, 2H, proline g-H), 2.47 (m, 1H, proline b-H), ꢀ2.77 ppm (s,
+
2
H, pyrrole NH). FAB-MS (NBA), m/z: 756 [MꢀCl] .
[
5
-(4-Carboxyphenyl)-10,15,20-triphenylporphyrin (4H
2
) was synthesized
[
31b]
by following methods reported in the literature.
Analytical and spec-
[
[
troscopic data were in full agreement with published values. Detailed
procedures are reported in the Supporting Information.
5
2
1–54; b) S. Muniappan, S. Lipstman, I. Goldberg, Chem. Commun.
008, 1777–1779, and references therein.
5
-[4-{Carboxyphenyl-(N-l-proline tert-butyl ester)}]-10,15,20-triphenyl-
porphyrin (5H ): EDCl (0.041 g, 0.2111 mmol) and HOBT (0.029 g,
.211 mmol) were added to a stirred solution of 4H (0.14 g, 0.211 mmol)
in anhydrous CH Cl (25 mL) at 08C (ice bath). The resulting solution
was stirred at 08C for 1 h, and then l-proline tert-butyl ester (0.11 g,
.633 mmol) was added. The reaction mixture was gently brought to
2
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room temperature and left under stirring overnight. The solvent was then
removed under reduced pressure and the reddish residue dissolved in
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organic phases were dried (Na
duced pressure to give pure 5H
line solid. Further chromatographic purification (SiO
CHCl /CH OH 9/1 v/v as eluant) gave 5H (105 mg, 0.13 mmol; 61%
yield) as bright purple crystals. UV/Vis (CHCl ): lmax (loge)=419 (5.1),
14 (3.8), 546 (3.6), 586 (3.7), 645 nm (3.2); FABMS (NBA): m/z: 811
2
SO
(105 mg, TLC run) as a purple crystal-
chromatography,
4
) and the solvent evaporated under re-
2
1
2
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3
2
3
5
[
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15; h) Y. Kubo, Y. Ishii, T. Yoshizawa, S. Tokita, Chem. Commun.
+
M+H] .
-[4-{Carboxyphenyl(sodium-N-l-prolinatoyl)}]-10,15,20-triphenylpor-
phyrin (6H ): Compound 5H (100 mg, 0.123 mmol) was dissolved in
CH Cl /trifluoroacetic acid (1/1 v/v; 50 mL). The reaction mixture was
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004, 1394–1395; i) D. Monti, L. La Monica, A. Scipioni, G. Manci-
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2
2
2
2
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(
50 mL) and washed with a saturated solution of NaHCO
organic layer was dried over anhydrous Na SO , evaporated under re-
duced pressure, and the red solid crystallized (CHCl /hexane) to give 6H
): lmax (loge)=419 (5.1), 514
3.8), 546 (3.8), 586 (3.7), 642 nm (3.5); FABMS (NBA): m/z: 756
3
(50 mL). The
2
4
3
2
(
75 mg) as purple crystals. UV/Vis (CHCl
3
(
+
1
[
M+H] ; H NMR (CDCl
J=8.2 Hz, 2H, 3’-C COR), 8.15 (d, J=8.2 Hz, 2H, 2’-C
.5 (m, 1H, proline a-H), 4.8–3.7 (m, 2H, proline d-H), 2.5–1.9 ppm (m,
H, proline g-H+b-H), ꢀ2.79 ppm (brs, 2H, pyrrole NH).
3
): d=9.1–8.5 (brs, 8H, pyrrole b-H), 8.25 (d,
6
H
4
6 4
H
COR), 4.7–
4
4
Chem. Eur. J. 2010, 16, 860 – 870
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
869

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nature of the self-assembly process. Note, however, that the overall
rate is lower than those observed in the absence of added salt, un-
doubtedly a consequence of the chosen ethanol-richer composition
of the medium, in which detailed kinetic studies can be conveniently
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2
[
11b]
tion of the CD features is observed.
This indicates that the ste-
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[
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[
Received: July 15, 2009
Published online: November 20, 2009
870
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 860 – 870