Spectroscopic, morphological, and mechanistic investigation of the solvent-promoted aggregation of porphyrins modified in meso-positions by glucosylated steroids

Article abstract of DOI:10.1002/chem.201101163

Solvent-driven aggregation of a series of porphyrin derivatives was studied by UV/Vis and circular dichroism spectroscopy. The porphyrins are characterised by the presence in the meso positions of steroidal moieties further conjugated with glucosyl groups

Full text of DOI:10.1002/chem.201101163

FULL PAPER
DOI: 10.1002/chem.201101163
Spectroscopic, Morphological, and Mechanistic Investigation of the Solvent-
Promoted Aggregation of Porphyrins Modified in meso-Positions by
Glucosylated Steroids
Karel Zelenka,[a] Tomꢀsˇ Trnka,[a] Iva Tisˇlerovꢀ,[a] Donato Monti,*[c] Stefano Cinti,[c]
Mario Luigi Naitana,[c] Luca Schiaffino,[c] Mariano Venanzi,[c] Giuseppe Laguzzi,[d]
[b]
Loredana Luvidi,[d] Giovanna Mancini,[d] Zdena Novꢀkovꢀ,[b] Ondrˇej Simꢀk,
ˇ
Zdeneˇk Wimmer,[e] and Pavel Drasˇar*[b]
Abstract: Solvent-driven aggregation
of a series of porphyrin derivatives was
studied by UV/Vis and circular dichro-
ism spectroscopy. The porphyrins are
characterised by the presence in the
meso positions of steroidal moieties
further conjugated with glucosyl
groups. The presence of these groups
makes the investigated macrocycles
amphiphilic and soluble in aqueous sol-
vent, namely, dimethyl acetamide/
water. Aggregation of the macrocycles
is triggered by a change in bulk solvent
composition leading to formation of
large architectures that express supra-
molecular chirality, steered by the pres-
ence of the stereogenic centres on the
periphery of the macrocycles. The ag-
gregation behaviour and chiroptical
features of the aggregates are strongly
dependent on the number of moieties
decorating the periphery of the por-
phyrin framework. In particular, exper-
imental evidence indicates that the
structure of the steroid linker dictates
the overall chirality of the supramolec-
ular architectures. Moreover, the por-
phyrin concentration strongly affects
the aggregation mechanism and the
CD intensities of the spectra. Notably,
AFM investigations reveal strong dif-
ferences in aggregate morphology that
are dependent on the nature of the ap-
pended functional groups, and closely
in line with the changes in aggregation
mechanism.
The
suprastructures
formed at lower concentration show a
network of long fibrous structures
spanning over tens of micrometres,
whereas the aggregates formed at
higher concentration have smaller rod-
shaped structures that can be recog-
nised as the result of coalescence of
smaller globular structures. The fully
steroid substituted derivative forms
globular structures over the whole con-
centration range explored. Finally, a ra-
tionale for the aggregation phenomena
was given by semiempirical calculations
at the PM6 level.
Keywords: aggregation · chirality ·
porphyrinoids · steroids · supra-
molecular chemistry
Introduction
ˇ
[a] Dr. K. Zelenka, Prof. Dr. T. Trnka, Dr. I. Tislerovꢁ
Charles University, Department of Organic Chemistry
Albertov 8, 12840 Prague 2 (Czech Republic)
The supramolecular chirality of porphyrin derivatives is an
important aspect of these macrocycles, in which chiral (or
even achiral) building blocks assemble and organise them-
selves into chiral architectures through a combination of
noncovalent interactions.[1] This is usually accomplished by a
number of well-established procedures: 1) templated aggre-
gation on chiral natural and non-natural polymer matrices,[2]
2) chiral-ligand coordination,[3] 3) spontaneous symmetry
breaking induced by stirring,[4] 4) electrostatic interactions,[5]
5) by Langmuir–Blodgett and Langmuir–Schꢀfer tech-
niques,[6] and, as recently reported, 6) by interaction with
chiral surfactants.[7]
ˇ
ˇ
[b] Z. Novꢁkovꢁ, Dr. O. Simꢁk, Prof. Dr. P. Drasar
Department of Natural Compounds Chemistry
Institute of Chemical Technology Prague
Technickꢁ 5, 16628 Prague 6 (Czech Republic)
Fax : (+420)220-443-422
[c] Dr. D. Monti, Dr. S. Cinti, Dr. M. L. Naitana, Dr. L. Schiaffino,
Prof. Dr. M. Venanzi
University of Rome Tor Vergata
Department of Chemical Technologies
Via della Ricerca Scientifica 1, 00133, Rome (Italy)
Fax : (+30)0672594328
E-mail: monti@stc.uniroma2.it
[d] Dr. G. Laguzzi, Dr. L. Luvidi, Dr. G. Mancini
IMR-CNR, c/o Department of Chemistry, University of Rome
La Sapienza, P.le A. Moro 5, 00185, Rome (Italy)
Our contribution to this field concerns functionalisation
of the periphery of a porphyrin macrocycle by a chiral
moiety, namely, a configurationally stable amino acid resi-
due,[8] a robust C-glycosidic group, or a steroid moiety.[9]
These groups, besides imparting an amphiphilic character to
the molecular frameworks, steer solvent-promoted self-ag-
gregation toward formation of supramolecular structures
[e] Prof. Dr. Z. Wimmer
Institute of Experimental Botany AS CR vvi, Isotope Laboratory
ˇ
Vꢂdenskꢁ 1083, 142 20 Prague 4 (Czech Republic)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101163.
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with a high degree of asymmetry. The strong dependence of
the resulting chiroptical features on bulk solvent properties
such as the water content, ionic strength, and so on, indi-
cates the occurrence of a subtle balance of forces during the
molecular recognition processes. Moreover, we recently re-
ported that simply changing one stereogenic centre in C-gly-
coside derivatives, from “C-d-gluco” to “C-d-galacto” con-
figuration, results in a change of sign of the CD spectra,[9b]
indicating an opposite mutual configuration of the chromo-
phores.
presence of two (trans-A2B2 type) or four glucosylated ste-
roid (A4 type) in the meso position (H2DGS-P and H2TGS-
P, respectively).
Hence, we decided to study the aggregation properties of
porphyrin derivatives, decorated with peripheral glucosylat-
ed steroid derivatives. This functionalisation offers the op-
portunity to exploit the combined effects of the glucosylated
steroid moieties in conjunction with the porphyrin aromatic
platform. In this particular field, little work has been done
so far. Studies on the photophysical properties of porphy-
rin–fullerene hybrids linked by a steroid moiety were repor-
ted.[11a] Moreover, porphyrin–steroid conjugates are studied
for comprehension of the processes involved in artificial an-
tenna systems.[11b] The catalytic properties of some oestro-
gen–porphyrin conjugates were reported,[12] and a steroid–
porphyrin conjugate was intended for saccharide sensing in
protic media.[13] An MnIII porphyrin–steroid conjugate was
used as a semi-synthetic antibody.[14] A review on dimeric
and oligomeric steroids highlights some supramolecular ap-
plications of the conjugation of steroids and porphyrins,[15]
and membrane-spanning porphyrins[16] have been employed
as mimics of cytochrome P450. Furthermore, some steroid-
capped porphyrins have been reported to show interesting
behaviour as molecular bowls.[17]
The aggregation properties of the title systems were stud-
ied in aqueous solvent mixtures by several spectroscopic
and kinetic means, to give insight into the mechanism of as-
sembly, and on the nature of the resulting supramolecular
architectures. This can be important in the field of molecular
materials, and for sensor applications, owing to the complex-
ity of the molecular frameworks, which offer a variety of in-
teraction sites for molecular recognition and discrimination
of complex matrices of analytes.[18] Moreover, as recently re-
ported for related structures, these sugar-conjugated por-
phyrins could be of importance in the photodynamic therapy
(PDT) of tumours. In this regard, the affinity of the photo-
sensitiser to the cell surface of neoplastic tissues is enhanced
by the presence of sugar moieties, and the further transport
into tissue is effectively modulated by the amphiphilicity of
the drug.[19]
Aggregation studies: The aggregation behaviour of porphy-
rin derivatives was studied in aqueous solvent mixtures such
as dimethylacetamide (DMAc)/water in various proportions.
Such solvent combinations have been found to give the best
performances in terms of solubility, stability of the porphy-
rin solutes, and reproducibility of the results, with respect to
other investigated hydro-organic media such as ethanol/
water, acetonitrile/water, and tetrahydrofuran/water (results
not reported).
In pure DMAc, H2TGS-P shows a sharp Soret Band at
418 nm, indicating that the macrocycle is present in solution
in monomeric form. Analogously, bis-pentafluorophenyl de-
rivative H2DGS-P features a sharp Soret band, hypsochrom-
ically shifted at 413 nm, due to the presence of strongly elec-
tron withdrawing fluorinated groups. In mixed solvent
media, an increasing fraction of water triggers aggregation
of the monomers with formation of supramolecular struc-
tures, as indicated by the typical spectroscopic UV/Vis
changes (hypochromicity, band broadening, and wavelength
shift; Figure 1). The solvent composition at which onset of
aggregation occurs, as well as its extent at equilibrium, de-
pends on the intimate nature of the groups decorating the
molecular frameworks. Detailed kinetic studies on the effect
of substrate concentration, and solvent properties, will be re-
ported and discussed below.
In the case of H2TGS-P, bearing four steroid groups (A4
type), aggregation is complete with 20% of water, whereas
for the disubstituted trans-A2B2 congener H2DGS-P, having
two pentafluorophenyl groups, complete aggregation re-
quires a larger fraction of water (40%). This can certainly
be ascribed to the effect of the overall polarity of the com-
pounds, and on the specific interactions of the substituent
groups with the solvent molecules, which are more favour-
Results and Discussion
We used our previously reported procedures[20] to prepare
5,10,15,20-tetrakis
nosyloxy)-5b-24-norcholan-23-yl]porphyrin (H2TGS-P) and
5,15-bis(pentafluorophenyl)-10,20-bis[3a-(2,3,4,6-tetra-O-
ACHTUNGTRENNUNG[3a-(2,3,4,6-tetra-O-benzyl-b-d-glucopyra-
AHCTUNGTRENNUNG
benzyl-b-d-glucopyranosyloxy))-5b-24-norcholan-23-yl]por-
phyrin (H2DGS-P). The derivatives are characterised by the
AHCTUNGTREGaNNUN ble for the more polar pentafluorophenyl moieties.
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Solvent-Promoted Aggregation of Modified Porphyrins
FULL PAPER
These findings are further corroborated by resonance light
scattering spectroscopy (RLS) measurements, which show
strong features upon aggregation, indicating the formation
of large structures formed by a number of monomers in
electronic communication higher than 25.[21]
Circular dichroism spectroscopic studies showed that in
all cases the interaction results in formation of chiral supra-
molecular species, but with remarkably strong differences in
terms of band intensity and shape, depending on the kind
and degree of substitution of the ring periphery. This finding
strongly suggests different modes of molecular interaction
between the different macrocycles, as a consequence of the
structural diversity. The CD spectrum of H2TGS-P (ca.
1 mm) in pure DMAc features a weak dichroic negative band
at 419 nm (Figure 2A), which can be safely interpreted on
the basis of the induction effect of the bulky chiral substitu-
ent on the electronic transition dipoles (Soret B band),[22]
and on the effect of distortion of the porphyrin framework
from planarity. The “intramolecular” nature of this effect
could be proven by the fact that also in the presence of
added salts (i.e., 1.5m NaBr) the induced CD (ICD) intensi-
Figure 1. UV/Vis spectra of porphyrin derivatives in aqueous solvent mix-
tures. A) H2TGS-P, c: 100% DMAc, a: 90% DMAc, g: 80%
DMAc. B) H2DGS-P, c: 100–80% DMAc, g: 60% DMAc.
The differences in the self-assembly propensity are also
clearly reflected in the spectral patterns of the final aggre-
gated species (Figure 1). In the case of H2TGS-P at 2.7 mm
aggregation results in the formation of scarcely red-shifted
J-type bands in the UV/Vis spectra, whereas at higher con-
centration (ca. 3 mm) the aggregation process give rise to
mixed J- and H-type morphologies, in accordance with the
presence of an additional blue-shifted feature at 405 nm.
Similar behaviour was found in aggregation studies on simi-
lar tetra-meso-glucoporphyrin derivatives carried out in
aqueous acetonitrile.[9b] Conversely, in the case of the less
crowded disubstituted H2DGS-P aggregation gives species
characterised by excitonically coupled Soret B bands, with
the “spectroscopic barycentre” remaining fixed at 413 nm.
Concomitant fluorescence spectroscopic studies reveal
that in both cases aggregation occurs with quenching and
red shift of the fluorescence emission at the same solvent
composition as indicated by UV/Vis spectroscopy (results
not shown). Notably, the shift of emission maxima is higher
for aggregates of H2TGS-P (lmax =660 nm) than for conge-
ner H2DGS-P (lmax =645 nm). This again strictly parallels
the behaviour featured in the UV/Vis spectral patterns.
Figure 2. CD spectra of 2.5ꢄ10ꢀ6
B) DMAc/water (80:20).
m H2TGS-P in A) DMAc and
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ˇ
D. Monti, P. Drasar et al.
ty remains unchanged, ruling out, at this stage, the effect of
porphyrin aggregation, which should otherwise be influ-
enced by ionic strength. The same considerations hold for
the other derivatives studied (see below). When the studies
are carried out under aggregative conditions (i.e., >30%
water, see Experimental Section), only very weak CD fea-
tures are detected, which are scarcely distinguishable from
the background noise, and indicate mainly formation of
structures with aspecific, weakly chirally active morphology.
Interestingly, some more evident CD features become ap-
preciable at higher porphyrin concentration. In this case
both higher-energy and lower-energy, partially superposed,
bisignated negative bands are observed, indicating formation
of chiral species of either H-like or J-like character, as for-
merly shown by UV/Vis spectroscopy (Figure 2B).[23]
tive bisignated Cotton effect, as a consequence of strong
electronic coupling between the macrocycles.[3d,24,25] The
negative sign of the spectral patterns is suggestive of a
mutual anticlockwise arrangement of the tetrapyrrolic plat-
forms on forming the aggregate suprastructures.[26] Here,
too, good analogies between the results of different spectro-
scopic techniques are found, as the maximum CD effect is
reached at about 40% water content (Figure 3B), where ag-
gregation is virtually complete, as already indicated by UV/
Vis spectroscopy. On the other hand, aggregates formed in
nearly pure water show lower CD intensities, indicating for-
mation of less-ordered structures. This is due to the in-
creased aggregation rate, as already observed by our group
in the case of related chiral porphyrin derivatives.[8c]
The large differences in CD intensity observed for
H2TGS-P and H2DGS-P should be the consequence of
either the steric hindrance caused by the bulky moieties
being smaller in the trans-A2B2 frame, so that tighter inter-
action between the aromatic platforms is possible, or the dy-
namics of the sugar residues, which may hamper tighter in-
teraction. It is noteworthy that the CD spectra of these com-
pounds feature bands of opposite sign to that formerly ob-
served for simpler glycosylated porphyrin derivatives lacking
steroid spacers.[9b] This can interpreted on the basis of the
formation of chiral suprastructures, whose mutual spatial
configuration is governed by the molecular information in-
troduced by the rigid steroid structures.
The CD spectrum of H2DGS-P in pure DMAc (Figure 3)
features an ICD band due to electronic interaction of the
steroid moieties with the tetrapyrrolic platform. Significant-
ly, in this case solvent-driven aggregation occurs with the
formation of strongly CD active species, with intense nega-
Kinetic studies: Kinetic studies on the aggregation of por-
phyrin derivatives H2TGS-P and H2DGS-P were performed
to shed light on the effect of the molecular structure on the
aggregation mechanism. The studies on H2DGS-P were per-
formed in DMAc/water (87:13), as precisely this solvent
composition allowed for optimal reaction rates for conven-
ient monitoring by conventional spectroscopic means.[27] All
reactions showed a clean isosbestic point at 426 nm indicat-
ing formation of aggregates with structurally similar mor-
phology. A close analysis of the kinetic behaviour showed
remarkable dependence of the self-assembly process on the
reaction conditions. The aggregation properties, in terms of
constant rates and mechanism, are strongly influenced by
the starting porphyrin concentration. Aggregation at low
concentration (i.e., 0.8 mm) showed a typical sigmoidal decay
indicative of a peculiar autocatalytic behaviour and likely
formation of fractal-type species (Figure 4A). The decay
could be conveniently fitted with the following “unconven-
tional” equation, developed by Pasternack and co-workers
[Eq. (1)][28]
ðExttꢀExt1Þ=ðExt0ꢀExt1Þ ¼
ð1Þ
nþ1 1=ðmꢀ1Þ
½1 þ ðmꢀ1Þꢁ½k0t þ ðn þ 1Þꢀ1ðkctÞ
ꢁ
where Ext0, Ext1 and Extt are the extinctions at initial, final
and time t, respectively,[29,30] k0 and kc the “uncatalysed” and
“catalysed” reaction rates, m the nucleation factor (i.e., the
minimum number of aggregated porphyrin monomers acting
Figure 3. CD spectra of H2DGS-P (1.6 mm) in different media. A) DMAc.
B) DMAc/H2O: i) 80% DMAc, ii) 40% DMAc, iii) 0% DMAc.
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Solvent-Promoted Aggregation of Modified Porphyrins
FULL PAPER
toward a mechanism involving formation of larger nuclea-
tion seeds (mꢂ8) that grow in a following step character-
ised by a reduced degree of cooperativity (nꢂ2–3).
Remarkably, by merely further increasing threefold the
initial porphyrin concentration (i.e., from 0.8 to 2.4 mm) a ki-
netic change occurs, toward stretched exponential decay be-
haviour, indicating the onset of a diffusion-limited aggrega-
tion (DLA) mechanism (Figure 4B). The decay can be
nicely fitted by Equation (2), which has been commonly em-
ployed, for example, in the aggregation of dyes onto charged
polymers[31] and in related studies[9a]
n
Extt ¼ Ext0 þ ðExt1ꢀExt0Þ½1ꢀexpðꢀktÞ ꢁ
ð2Þ
where k is the apparent first-order aggregation rate con-
stant, and n the stretching factor, which modulates the diffu-
sion growth of the aggregates, and for this kind of mecha-
nism is required to be n<1. This kinetic behaviour implies
the initial rapid formation of small porphyrin clusters (nu-
cleation seeds) subsequently growing by slow inclusion of
diffusing porphyrin monomers. Evidently, this latter pathway
is biased at high monomer concentration, at which the nu-
cleation phase is no longer the rate-determining step. This is
an unprecedented result, with respect to structurally related
porphyrin frameworks studied by us, for which constancy of
the self-assembly process upon changing the porphyrin con-
centration within a larger window was commonly observed.
A change of aggregation mechanism, however, has been
reported in the literature for highly charged, water-soluble
porphyrin platforms in which biasing of the mode of self-in-
teraction is promoted by a large change in the ionic strength
of the medium,[32] or by a severe variation of the porphyrin
concentration.[33] This is accompanied by a change from frac-
tal to nanorod structures of the J aggregates upon increasing
the porphyrin concentration. A change in kinetic profile was
also observed on changing the mixing protocol of the reac-
tants in porphyrin aggregation promoted by HCl.[34] In the
present case, the change of aggregation mechanism must be
due only to the effect of the macrocycle concentration, as
the medium composition, preparation of the solutions, and
their mixing procedures were kept constant for all of the
performed kinetic runs, as reported in detail in the Experi-
mental Section.
Concomitant CD studies gave further insights. The kinetic
behaviour closely follows that observed by UV/Vis spectros-
copy for the aggregation of 0.8 mm H2DGS-P (Figure 5). The
same mathematical treatment [Eq. (1)] gave results that
were in good agreement with those obtained by UV/Vis
spectroscopy (Table 1).
The final ellipticity of the CD bands strongly depends on
porphyrin concentration. The maximum q values increase by
one order of magnitude, from 3.5ꢄ105 to 3.1ꢄ
106 8cm2 dmolꢀ1, on going from 0.8 to 1.6 mm, respectively
(Figure 6). On further increasing the concentration a strong
decrease in CD intensity, by about one order of magnitude,
occurs (6ꢄ105 8cm2 dmolꢀ1 at 2.4 mm). This effect should be
the consequence of the observed change in mechanism,
Figure 4. A) Kinetic plot for the aggregation of H2DGS-P (0.8ꢄ10ꢀ6 m) in
*
DMAc/H2O (87:13) at 298 K, l=413 nm. : experimental points, c:
theoretical curve fit according to Equation (1). Inset: The corresponding
UV/Vis spectral variations. B) Kinetic plot for the reaction carried out at
2.4ꢄ10ꢀ6 m. The fit was accomplished according to Equation (2).
as a “nucleation seeds”, formed during the first delay
period) and n the fractal growth factor. A value of n>1 in-
dicates the occurrence of cooperative catalysed aggregate
growth. In this equation, contrary to classical kinetic treat-
ment, the “catalysed” rate constant is regarded as time-de-
pendent.
A regression fit according to the proposed model (see Ex-
perimental Section for details) gave the parameters k0 =
2.9ꢄ10ꢀ4 sꢀ1, kc =9.1ꢄ10ꢀ4 sꢀ1, with mean m value of 5, and
nꢂ7. The experiment carried out at intermediate concentra-
tion (i.e., 1.6 mm) revealed a progressive profile change, with
an evident reduction of the initial “lag phase”, that is, induc-
tion period.
A least-squares regression fit of the experimental data by
means of Equation (1), gave similar values for k0, and an in-
creased catalysed rate constant kc within the experimental
error. Moreover, the increased m value and decreased value
of the stretching factor n likely indicate gradual biasing
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D. Monti, P. Drasar et al.
Figure 6. CD spectra of H2DGS-P in DMA/water (87:13, 298 K) at differ-
ent porphyrin concentrations: i) 0.8, ii) 1.6, iii) 2.4 mm.
Table 2. Kinetic parameters for aggregation of H2TGS-P.[a]
A
k [sꢀ1
]
n
0.69
1.33
1.99
2.66
3.32
0.32ꢃ0.05
1.08ꢃ0.04
1.47ꢃ0.02
2.00ꢃ0.03
6.12ꢃ0.1
0.81ꢃ0.01
0.57ꢃ0.05
0.81ꢃ0.02
0.68ꢃ0.07
0.40ꢃ0.02
[a] Runs carried out in DMAc/H2O (91:9) at 298 K. Kinetics monitored
at l=413 nm. Values were calculated by using Equation (2).
Figure 5. A) CD spectral variation with time of H2DGS-P in DMA/water
(87:13) at 0.8 mm and 298 K. B) Corresponding kinetic plot at l=428 nm.
several orders of magnitude faster than that observed for
disubstituted H2DGS-P, owing to the increased hydrophobic
character brought about by replacement of pentafluoro-
phenyl moieties by steroid groups. This is also reflected in
the aggregation profiles, which show diffusion-limited be-
haviour (DLA) throughout the explored concentration
range (i.e., 0.7–3.3 mm). Evidently, the increased hydropho-
bicity of the molecular frameworks fosters rapid aggregation
of the macrocycles into a large number of small nucleation
seeds, biasing the rate-limiting step to that of their further
growth by diffusion-limited inclusion of residual monomers.
The rate constant regularly increases with increasing initial
porphyrin concentration up to 2.7 mm (Table 2). Above this
value an abrupt increase of the rate is observed (Figure 7),
probably associated with formation of morphologically dif-
ferent species (i.e., H versus J aggregates), as already indi-
cated by UV/Vis and CD techniques.
Table 1. Kinetic parameters for the aggregation of H2DGS-P.[a]
A
104 k0 [sꢀ1
]
104 kc [sꢀ1
]
m
n
0.80
1.60
2.9ꢃ0.3
3.5ꢃ0.4
3.3ꢃ0.6
4.5ꢃ0.5
9.07ꢃ0.03
15.70ꢃ0.05
13.12ꢃ0.06
15.1ꢃ0.2
5.1ꢃ0.4
7.6ꢃ0.3
8.1ꢃ0.1
9.7ꢃ0.6
6.6ꢃ0.1
2.5ꢃ0.2
2.8ꢃ0.1
1.60[b]
1.60[c]
2.40[d]
3.1ꢃ0.3
38.0ꢃ0.1
0.40ꢃ0.05
[a] Kinetic runs in DMAc/H2O (87:13) at 298 K monitored by UV/Vis
spectroscopy at l=413 nm according to Equation (1). [b] Monitored by
CD spectroscopy at l=428 nm. [c] Monitored by fluorescence spectros-
copy at l=413 nm. [d] Monitored by UV/Vis spectroscopy at l=413 nm.
Kinetic decay fitted by Equation (2).
which leads to the formation of supramolecular structures
with different intimate morphologies. Nevertheless, the CD
signs of the spectra remained unchanged throughout the
changes in concentration, and this indicates constancy of the
mutual porphyrin configuration.[26]
Analogous kinetic experiments were carried out on
H2TGS-P. The results are summarised in Table 2. The runs
were performed in DMAc/water (91/9). Despite the lower
polarity of the medium, the overall self-assembly process is
AFM studies: Atomic force microscopy studies of the aggre-
gates of H2DGS-P and H2TGS-P were carried out on freshly
cleaved graphite sheets (HOPG). The samples were pre-
pared by drop casting (see Experimental Section) from ag-
gregate solutions formed in DMAc/water of the same com-
position as used for the kinetic investigations. Reproducible
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Solvent-Promoted Aggregation of Modified Porphyrins
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Figure 7. Plot of kinetic constants versus porphyrin concentration for the
aggregation of H2TGS-P in DMAc/H2O (91:9). The point at higher con-
centration was omitted from the regression fit.
results, in terms of different batches and different regions
scanned, were obtained. The same holds for topographies
on glass surfaces, acquired by optical spectroscopy (see Ex-
perimental Section and ESI for details), which showed for-
mation of very similar structures, ruling out the effect of the
surface on the morphology of the aggregates.[34]
Figure 8. AFM topographies (HOPG) of aggregates of H2DGS-P formed
at different porphyrin concentrations (DMAc/H2O, 87:13): A) 0.8, B) 1.6,
C) 2.4 mm (inset: an enlargement of the central area). D) AFM image of
H2TGS-P aggregates (2.4 mm, DMAc/H2O, 91:9).
In particular, in the case of the aggregates of H2DGS-P
formed at lower concentration (0.8 mm), the images show the
presence of fibril structures, up to tens of micrometers in
length and 300–500 nm wide (see Supporting Information,
Figure S1), bundled in larger fibres (Figure 8, top left). At
intermediate concentration (i.e., 1.6 mm, Figure 8, top right),
the structures appear as wider (350–550 nm, see Supporting
Information, Figure S2), mostly isolated rods, with a mean
length of about 10 mm. Interestingly, the structures formed
at higher concentration (2.4 mm, at which a DLA mechanism
is manifested) show a different morphology (Figure 8, lower
left). In this case the porphyrin structures appear as aggre-
gate rods (10 mm in length, 150–200 nm wide; see Support-
ing Information, Figure S3), which on closer inspection ap-
peared to be the result of coalescence of smaller globular
entities into longer fibril-like structures. A related issue was
recently exploited by Liu et al, who demonstrated evolution
of porphyrin globular nanostructures into nanofibers.[35] The
different morphologies of the aggregates are the conse-
quence of the progressive change of mechanism, driven by
changing concentration.
vent mixtures, were shown to be dependent on the mono-
mer structures and on the conditions of formation.[36]
Besides, we serendipitously found that aggregates of por-
phyrin H2DGS-P precipitated from solution (0.8 mm in 87:13
DMAc/H2O) upon standing overnight after their formation,
to form a coagulated gel of brownish porphyrin material
flocculated from the solution. These porphyrin macroaggre-
gates are very stable, and remain unchanged in solution
after months of storage. This is in close analogy to the re-
sults of Balaban and co-workers describing the formation of
an artificial antenna system of photosynthetic bacteria (syn-
thetic chlorosomes) in hydrocarbon solvents.[37] The forma-
tion of such structures is due to the synergic presence of
both cholesterol and sugar groups, which are known to be
the key functionality for the achievement of self-assembled
fibril structures and gels.[38] SEM and conventional electron
microscopy studies on this colloidal air-dried material lay-
ered on glass cover slides showed formation of fibres up to
a millimetre in length (Supporting Information, Figure S6).
In the case of tetrasubstituted H2TGS-P, which has the
least CD active self-assembled structures, only aspecific ran-
domly aggregated nanoparticle-like species are formed
(Figure 8, lower right) with diameters of about 300–350 nm
(estimated for isolated structures; see Supporting Informa-
tion, Figure S4). The same result was found for structures
formed at different concentrations. These results are consis-
tent with some recent works, in which the structures of am-
phiphilic porphyrin nanoparticles, formed in good/poor sol-
Computational studies: Semiempirical calculations were car-
ried out with the aim of gaining information on the struc-
tures of the H2DGS-P porphyrin aggregates, and on the
driving force steering the specificity of the self-recognition
process. We chose this compound as a model owing to the
lower complexity of the structure and its more pronounced
chiroptical properties. As, in a first instance, the nature of
the sugar has no effect on the sign of the CD spectra of the
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D. Monti, P. Drasar et al.
aggregates, which is indicative of their helicity, the calcula-
tions were performed on the model compound H2DGS-P,
pruned of the sugar pendant groups. This protocol gave us
the fundamental advantage of a reasonable calculation
speed. All calculations were carried out with the PM6
method[39] as implemented in the program MOPAC 2009.[40]
A thorough conformational search around the four rotat-
ACHTUNGTRENNUNGable bonds of the steroid pendants of H2DGS-P revealed
that the most stable structure corresponds to the expected
all-anti conformer (Supporting Information, Figure S6). Sev-
eral starting geometries for optimization of an l-handed
dimer of porphyrin H2DGS-P (i.e., the anticlockwise L
isomer of M helicity) were built by placing two monomers
in their minimum-energy conformations close to each other
in proper relative orientations. Interestingly, most of the
starting geometries evolved into the same compact structure,
which proved to be the most stable among all optimized ge-
ometries. The steroid pendants on different molecules are in
close contact, and this suggests that van der Waals interac-
tions between their surfaces play a major role in aggregate
formation. Furthermore, some deviations from planarity,
with a mean value of about 0.21 ꢅ, are observed in the por-
phyrin rings (Supporting Information, Figure S8).
When the same study was performed on d-handed dimers
(i.e. the clockwise D isomer, of P helicity) again most start-
ing geometries converged to a unique structure. In this case,
minor deviations from planarity are observed in the porphy-
rin ring, and the steroid pendants of different monomers
mutually interact (Supporting Information, Figure S9). The
contact surface between these pendants is clearly smaller
than in the related L structure. Since all other structural fea-
tures are almost identical in the two cases, it can be reason-
Figure 9. Representation of a tetramer of porphyrin H2DGS-P in which
the l-handedness is the result of favourable van der Waals interactions
between the steroid pendant moieties.
should interact in a similar way with both of the differently
handed growing suprastructures.
Conclusion
The aggregation properties of some elaborate porphyrin ar-
chitectures, decorated with steroid linked to glucosidic moi-
eties, have been studied in water/DMAc by different spec-
troscopic techniques, which revealed that aggregation leads
to formation of species featuring supramolecular chirality.
The results obtained indicate that it is possible to modulate
the chirality of the overall supramolecular structures by
changing the nature of the ancillary group present on the
periphery of the porphyrin macrocycles, their concentration
and the composition of the media. These experimental and
molecular parameters also play a fundamental role in deter-
mining the overall morphology and the size of the aggre-
gates. Moreover the experimental findings suggest that the
intimate mutual arrangement of porphyrins, as well as their
electronic coupling, is dictated by the chiral information
stored on the linking steroid groups. In the case of the dis-
ubstituted H2DGS-P derivative, semiempirical calculations
gave a rationale for the experimental results. The complexi-
ty of the molecular structure of the investigated systems ren-
ders them amenable to further investigation in terms of mo-
lecular recognition properties, either in the solid state for
the detection of complex volatile matrices,[41] or in solu-
tion.[42] The results presented should be of importance for
the construction of sensor devices featuring stereoselective
features,[43] or for obtaining solid-state materials with chiral
properties,[44] toward asymmetric catalysis.[45,15d] Studies de-
voted to exploiting the electrochemical behaviour of these
interesting systems are underway.
ACHTUNGTRENNUNGably argued that such a looser interaction between the pend-
ants should cause a lower stability of the D dimer, which
indeed has been calculated to be about 37 kJmolꢀ1. To
obtain a pictorial structure that well illustrates how in our
opinion the above finding can explain the experimentally
observed handedness, we built a porphyrin tetramer by
ACHTUNGTRENNUNGplacing two L dimers one above the other (Supporting In-
formation, Figure S10).
The steroid pendants were positioned in relative orienta-
tions that allow alternate interactions between the mono-
mers, and the resulting geometry was subsequently opti-
mized at the PM6 level by using the MOZYME function of
MOPAC 2009 (Figure 9). This optimized geometry is a rea-
sonable “short model” for porphyrin aggregates of H2DGS-
P that shows, and corroborates, how the favourable van der
Waals interactions between the steroid pendants, responsible
for the highest stability of the l-handed dimer, and subse-
quently of its tetramer, should also govern the handedness
of higher aggregates. We are aware that these “gas-phase”
calculations overlook the role of the solvent, which always
plays an important role in molecular recognition. However,
the discriminating effect should be that of a “differential sol-
vation” between the oppositely handed structures. The re-
sulting overall effect in our opinion would be negligible, as
the solvents considered in our studies are not chiral, and
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Chem. Eur. J. 2011, 17, 13743 – 13753

Solvent-Promoted Aggregation of Modified Porphyrins
FULL PAPER
RT, thoroughly rinsed with Milli-Q (Millipore) distilled water, then dried
with an ultrapure nitrogen stream.
Experimental Section
Circular dichroism spectroscopy: CD spectra were recorded by using a
JASCO J-600 spectrometer equipped with a thermostated cell holder at
298 K, and purged with ultrapure nitrogen gas. The samples were pre-
pared by following the procedure described for the UV/Vis and kinetic
studies.
General: Spectroscopic measurements were performed by using a Varian
Cary 1E UV/Vis spectrophotometer at RT. Fluorescence and resonance
light scattering studies were carried out by using FluoroMax-2 spectro-
fluorophotometers; concentrations used for UV/Vis and fluorescence
measurements were (0.8–5.0)ꢄ10ꢀ6 molLꢀ1, unless otherwise indicated.
Resonance light scattering: RLS experiments were performed by using a
FluoroMax-2 spectrofluorophotometer. Spectra were acquired at (298ꢃ
0.5) K in a “synchronous scan” mode, in which the emission and excita-
tion monochromators were preset to identical wavelengths. Solutions
were prepared by following the protocol used in the UV/Vis aggregation
studies.
Aggregation studies: All spectroscopic studies were carried out at 298 K,
unless otherwise indicated. Solutions suitable for aggregation studies
were prepared as follows: Appropriate aliquots of a stock solution in
DMAc (15–150 mL) were added to a suitable amount of solvent in a glass
vial, then the solution was made up to 4.0 mL with water to give a solu-
tion with the desired water/organic solvent ratio and a range of porphyrin
concentrations (0.8–5.0 mmolLꢀ1), unless otherwise indicated. A sample
Fluorescence spectroscopy experiments: Fluorescence spectra were re-
corded at (298ꢃ0.5) K by using a JASCO 780 or a FluoroMax-2 spectro-
fluorophotometer. Solutions were prepared by following the protocol
used in the UV/Vis aggregation studies. Excitation wavelengths corre-
spond to that of the most intense Q-band maxima.
(ꢂ3 mL) was transferred to a quartz cuvette and relative UV/Vis spectra
were acquired. The corresponding absorbance versus solvent composition
(i.e., % water) plots indicate the critical aggregation solvent composition
at which aggregation occurs. Spectra were further acquired at different
times to get indications of the proper solvent mixture for kinetic experi-
ments.
Kinetic studies: Kinetic experiments were performed at (298ꢃ0.5) K by
using a Varian Cary 1E spectrophotometer equipped with a thermostat-
ing apparatus, by measuring the UV/Vis spectroscopic changes (Soret B
band) of porphyrin derivatives over time. Porphyrin aqueous solutions
suited for kinetic studies were prepared as follows. Appropriate aliquots
of a stock solution of porphyrin in DMAc (15–150 mL) were added to a
suitable amount of DMAc, then water was added to give an 87%
DMAc/water solution with a final porphyrin concentration in the range
of 0.8–2.4 mm. A suitable portion (ꢂ2.5 mL) was rapidly transferred to a
quartz or optical-glass cuvette, and relative UV/Vis spectra acquired over
time. In the case of faster reactions (higher porphyrin concentrations),
the solutions were prepared directly in the measurement cuvette. The
stock solutions were always pretreated by filtration through nylon syringe
filters (Albet, 0.45 mm diameter) and used within a week of preparation.
Values of k were obtained by analysing the absorbance (extinction)
versus time data points by the above equations (see text). The kinetic pa-
rameters were obtained by nonlinear least-squares regression fit (Kalei-
dagraph program, Synergy Software, 2003) over hundreds of experimen-
tal data points. The quality of the fits is generally very good, with R2 ꢄ
0.9994. For faster reactions, the “zero-time” input values were calculated
by measuring the B-band absorbance under non-aggregative conditions
(i.e., solutions with higher DMAc contents closer to that employed for
kinetic experiments). The nonlinear regressions were run accordingly to
give the other kinetic parameters with the smallest calculated errors.
Final regressions were then made with “free” zero-time parameters, and
the calculated values for Ext1 and Ext0 were found to be always in excel-
lent agreement with the experimental values. Data reported are the aver-
age values of at least two different runs with uncertainties within 5%.
Acknowledgements
This work was supported by NATO (grant CBP.EAP.CLG.982972) and
by the Ministry of Education, Youth and Sports of the Czech Republic
(project nos. MSM6046137305, 2B06024 SUPRAFYT). We also thank
MiUR-PRIN, Italy (project no. 20088NTBKR; 2008) for partial financial
support. The helpful discussion of Profs. Gianfranco Ercolani and Rob-
erto Paolesse (University of Rome, Tor Vergata) is gratefully acknowl-
edged. We also wish to thank Dr. A. DꢆEpifanio (University of Rome,
Tor Vergata) for SEM studies.
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not find direct data on the hydrogen-bond accepting capability
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that it should be similar to that of the closely structurally related
N,N-dimethylformamide (DMF), which is b=0.69. We made this as-
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Chem. Eur. J. 2011, 17, 13743 – 13753

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Received: April 15, 2011
Published online: November 3, 2011
Chem. Eur. J. 2011, 17, 13743 – 13753
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13753