Self-assembly of amphiphilic aryl-squaramides in water driven by dipolar π-π interactions

Article abstract of DOI:10.1039/c9ob02085c

Squaramides are versatile compounds with a great capacity to interact via non-covalent interactions and therefore of interest for the development of supramolecular systems and functional materials. In the present work, a new series of aryl-squaramide amphiphiles (1-5) were prepared to form supramolecular polymers in water. Interestingly, only compounds 1 and 2 that contain electron-deficient aryl groups are capable of forming hydrogels (～10^-2 M) upon treatment with a base (NaOH or PBS). The aggregation behaviour of 1 and 2 was studied by static light scattering, UV-Vis, ¹H NMR, FT-IR, and atomic force microscopy, and it was found that these compounds aggregate forming well-defined 1D nanofibers below the critical gelation concentration (<10^-3 M). Moreover, the combination of these experiments with 1D and 2D NMR studies and theoretical calculations revealed that 1 and 2 self-assemble via an unprecedented interaction motif showing dipolar π-π interactions between the squaramide rings and the 4-nitrophenyl or 3,5-bis(trifluoromethyl)phenyl rings of 1 and 2, respectively. Such kinds of assemblies are stabilized by the compensation of the dipole moments of the stacked molecules. This interaction mode contrasts with those typically driving squaramide-based assemblies based on either hydrogen bonds or antiparallel stacking. We believe that this interaction motif is of interest for the design and development of new squaramide nanomaterials with free hydrogen bonding groups, which might be useful in drug delivery applications.

Full text of DOI:10.1039/c9ob02085c

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ARTICLE
Self-assembly in water of amphiphilic aryl-squaramides driven by
dipolar  interactions
Sergi Bujosa,a Eduardo Castellanos,a Antonio Frontera,a Carmen Rotger,a Antonio Costa*a and
Received 00th January 20xx,
Accepted 00th January 20xx
Bartolome Soberats*a
DOI: 10.1039/x0xx00000x
Squaramides are versatile compounds with a great capacity to interact via non-covalent interactions and therefore of
interest for the development of supramolecular systems and functional materials. In the present work, a new series of aryl-
squaramides amphiphiles (1-5) were prepared to form supramolecular polymers in water. Interestingly, only compounds 1
and 2, that contain electron-deficient aryl groups are capable of forming hydrogels (~10–2 M) upon treatment with base
(NaOH or PBS). The aggregation behaviour of 1 and 2 was studied by static light scattering, UV-Vis, 1H NMR, FT-IR, and atomic
force microscopy, and it was found that these compounds aggregate forming well-defined 1D nanofibers below the critical
gelation concentration (<10–3 M). Moreover, the combination of those experiments with 1D and 2D NMR studies and
theoretical calculations revealed that 1 and 2 self-assemble via an unprecedented interaction motif consisting of dipolar
 interactions between the squaramide rings and the 4-nitrophenyl or 3,5-bis(trifluromethyl)phenyl rings of 1 and 2,
respectively. Such kind of assemblies are stabilized by the compensation of the dipole moments of the stacked molecules.
This interaction mode contrasts with those typically driving squaramide-based assemblies based on hydrogen bonds or
either by antiparallel stacking. We believe that this interaction motif is of interest for the design and development of new
squaramide nanomaterials with free hydrogen bonding groups, which might be useful in drug delivery applications.
-extended amphiphiles in order to obtain aggregates with
different morphologies i.e. fibers, toroids, tubes, etc. that in
turn exhibit stimuli responsive optoelectronic and photonic
Introduction
Supramolecular polymers are attracting a great deal of
attention due to its potential in material science and chemical
biology.1-13 Thus, many efforts have been addressed to develop
new molecular design strategies to prepare monomers that
self-assemble forming specific nanostructures and in turn,
exhibit the desired functions. This task is specially challenging
when water is used as solvent and the hydrophobic effects are
dominating the assembly processes.14-19 While the simplest
surfactants, based on aliphatic chains functionalized with ionic
heads, self-assemble forming micelles or vesicles, the utilization
of alternative molecular structures are required to develop
more complex functional structures in water.14-19 For these
purposes it is necessary to design molecules with specific
functional groups that can direct the self-assembly processes
via additional weak and reversible non-covalent interactions
such as hydrogen bonds20-24 (H-bonds) or  interactions.25-33
Amphiphilic -extended aromatic scaffolds have been
widely exploited to create complex supramolecular
architectures in water via  interactions.25-33 For example,
Lee and coworkers extensively studied the molecular design of
properties.29,30 However, despite  interactions are strong in
aqueous environments, they are not directional and the
resulting assemblies often suffer of translational and rotational
freedom which prevents higher level of structural control. Other
kind of interactions such as charge transfer complexes (donor-
acceptor) or dipole-dipole -interactions can provide
additional directional control over the assemblies.34-40 For
example, the group of Ghosh studied the self-assembly by the
alternate stacking of electron poor and electron rich aromatic
scaffolds as strategy to build multicomponent assemblies.24-36
On the other hand, Würthner and co-workers focused on the
self-assembly studies of merocyanine dyes through dipole-
dipole interactions in low polarity environments.37-40 Such
specific and directional  interactions are proved to offer
additional control on the aggregates and therefore could be
utilized in water systems for the further development of novel
functional aqua materials.14-19
In the present work, we unveil two aryl-squaramide systems
that aggregate in water via hydrophobic effects and dipolar 
interactions to form fibrous supramolecular aggregates and
hydrogels. This interaction motif was discovered thought the
exploration of aryl-squaramides 1-5 (Fig. 1a) as new hydrogel
materials. However, it was found that only 1 and 2, bearing
electron-deficient aryl groups, form fibrillar supramolecular
aggregates (at lower concentrations) and hydrogels (at higher
concentrations) under the appropriate conditions (pH=7-8).
a. Universitat de les Illes Balears, Crta. de Valldemossa Km 7.5, 07122 Palma de
Mallorca, Spain.
† Electronic Supplementary Information (ESI) available: [Experimental methods,
synthetic procedures, NMR experiments, AFM, UV-Vis experiments, FT-IR
experiments and theoretical studies ]. See DOI: 10.1039/x0xx00000x
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moiety provide to the molecule a highly ionizable group, an
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aromatic system and a H-bond donor group from which it can
be built several intermolecular interations while the carboxylate
gives only an moderate ionisable group. We reckoned that the
structural modification would permit to study the aggregation
features in more detail. For building the new series of
compounds, we selected five different aryl groups to
incorporate at the squaramidic end of the molecule in order to
tune the material properties. Thus, the compounds bear 4-
nitrophenyl (1), 3,5-(bis(trifluoromethane))phenyl (2), 4-
methoxyphenyl (3), 3,5-dimethoxyphenyl (4) and phenyl (5)
rings. Squaramides 1-5 were prepared easily in a two steps
process involving the formation of the corresponding aryl-
squaramide esters and followed by the condensation with 4-
(aminomethyl)benzoic acid. For synthetic details see the ESI†,
Scheme S1, and Figures S2-S11.
Hydrogels based on compounds 1-5 were prepared
according to our previously described procedure,51 consisting
on heating (80-90 ⁰C) the insoluble squaramide compounds (1.0
wt%) in a basic water solution (NaOH or phosphate buffer (PBS),
pH 7-8), followed by slow cooling of the samples (see
Experimental section). This process warrants the total
deprotonation of the benzoic acid (pKa= 4.2) group and
facilitates the solubilization of compounds 1-5 at high
temperatures (Fig. 2a). We observed that after the slow cooling
Fig. 1. Molecular structure of aryl squaramides 1-5. b) Schematic representation of the
self-assembly in water of compound 1 and 2 via dipolar  interactions. Electron
withdrawing groups (EWG); Base (B:); dipole moment () is represented as pink arrows.
c) Common interaction modes of squaramides via quadruple H-bonds (left) and
antiparallel stacking (right).
Interestingly, NMR, UV-Vis, and FT-IR, as well as theoretical
calculations, permitted to conclude that 1 and 2 self-assemble
via dipolar  interactions by the cancellation of the dipole
moments () between vicinal molecules as shown in Fig. 1b.
This interaction motif contrasts with the common interaction
modes of squaramides consisting of H-bonds41-46 or antiparallel
stacking (Fig. 1c)47-49 and opens new possibilities for the design
and development of squaramide assemblies50 with free NH
positions as H-bonding acceptors.
Results and discussion
Molecular design and hydrogels formation
The molecular design of the new squaramides 1-5 was
inspired by our previously described materials Sq(NO2) and
Sq(CF3) consisting of an arylsquaramide with an appended
benzylsquaramate group (Fig. S1).51 These compounds form
thixotropic hydrogels and were found to be useful for the
controlled-release of organic molecules in water. Initially, we
thought that those compounds would associate by H-bonds or
either by antiparallel stacking as it is well-known in
squaramides.41-50 For example, we previously reported a
tripodal receptor bearing three squaramide groups that forms
1D nanofibres in CHCl3 via H-bonding interactions.41 Similar
supramolecular polymers were obtained in water using
squaramide amphiphiles by H-bonds reinforced by an aromatic
gain.44,45 However, we could not find enough evidence about
the interaction mode for Sq(NO2) and Sq(CF3) hydrogels due to
the strong interactions involved in the system.51 In the present
work, we designed the next generation of aryl-squaramide
hydrogels bearing a benzoic acid group (Fig. 1a). We expected
to decrease the aggregation tendency by replacing the benzyl-
squaramate group for a benzoate group since the squaramate
Fig. 2. a) Scheme of the deprotonation of the benzoic acid group of 1 and 2 upon
treatment with NaOH or PBS. b) Inversion test of the vials with compounds 1 (top) and 2
(bottom) at different concentrations (% wt) after treatment with PBS (pH=7-8). AFM
images of spin coated solutions (1.0  10-4 M, PBS) of c) 1 and d) 2 on mica substrates
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of the samples, compounds 1 and 2 form gels (Fig. 2b), while
compounds 3-5 precipitate in powder form. It is noteworthy
that compounds 1 and 2 are the only ones of the series that bear
electron-withdrawing groups in the aryl rings. Additional pH
dependent UV-Vis experiments indicate that compounds 1 and
2 do not suffer the deprotonation of the first squaramidic NHs
until pHs higher than 9 (Fig. S19).52,53 Fig. 2b shows the inversion
tests of the vials containing samples of 1 (top) and 2 (bottom)
at different concentrations after the basic treatment in PBS at
high temperatures. In the case of 1 the approximate critical
gelation concentration (CGCs) is 0.2 wt%, while it is required 0.6
wt% of compound 2 to form self-consistent hydrogels. The sol-
gel transition temperatures for both 1 (0.6 wt%, PBS) and 2 (1
wt%, PBS) hydrogels were estimated to be around 65 °C,
according to the variable temperature NMR experiments (Figs.
S16 and S17). Rheology experiments of the hydrogels prepared
in NaOH were also measured and are shown in Fig. S18. These
results suggest that compounds 1 and 2 form specific
assemblies suitable for the formation of firm hydrogels.
To observe in detail the morphology of the aggregates, PBS
solutions of 1 and 2 below the CGC (ca. 110-4 M) were spin-
coated on mica substrates and were analysed by AFM (Figs. 2c,
S12, and S13). Fig. 2c shows the AFM image of the spin-coated
solution of 1 (110-4 M) that evidences the formation of rigid
fibers of 18 nm of width and 0.4 nm of thickness (Fig. S12).
Compound 2 (110-4 M) also aggregates forming fibrillar
structures, but in this case the fibers are longer and less rigid
tending to form helical structures (Figs. 2d and S13). These
results confirm the aggregation of compounds 1 and 2 into
fibrillar nanostructures which enables at higher concentration
the formation of networks and subsequently hydrogels (Fig.
2b).51,54-57
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Fig, 3. a) Scattered intensity (s-1) as function of the concentration (log[M]) of 1 (top) and
2 (bottom) in PBS. Inset is shown the CAC. b) UV-Vis spectra of 1 in PBS at different
concentrations
for 2 and shows two bands at 325 and 280 nm (Fig. S21). It is
noteworthy that the bands corresponding to the aryl groups of
1 and 2 are batochromically shifted 100 and 60 nm compared
Aggregation studies
to
the
N-(4-nitrophenyl)acetamide
and
N-(3,5-
The aggregation behaviour in solution of compounds 1 and
2 was then analysed by Static Light Scattering (SLS), 1H NMR, FT-
IR, and UV-Vis experiments. First, we analysed the scattered
intensities (SLS experiments) of 1 and 2 in PBS (pH=7-8) as a
function of the concentration (Fig. 3a).‡ We found that the
intensities of the scattered light increas after reaching certain
concentration, as result of the aggregation of the systems. From
the experimental data, we could calculate the critical
aggregation concentrations (CACs) for 1 and 2 to be 3.310-4 M
and 3.710-4 M, respectively (Fig. 3a). The formation of
aggregates was further supported by running variable
concentration NMR experiments. The 1H NMR spectra of 1 and
2 exhibit broad signals at concentrations of 1-2 10-2 M, while
the signals become sharper at lower concentrations around
110-4 M (Figs. S14 and S15). This sharpening of the NMR signals
by decreasing concentration was attributed to the system de-
aggregation.5
bis(trifluoromethyl)phenyl)acetamide bands (Figs. S20, S21,
respectively, which suggest the extension of the aryl  system
towards the squaramide ring.44,58,59 Variable concentration
experiments of compound 1 revealed a marked hypochromic
(overall decrease of the molar absorption coefficient) effect
upon increasing concentration (Figs. 3b and S22) between
2.010-4 M and 2.510-3 M which is the same concentration
range of the CAC calculated from the SLS studies. This effect is
accompanied by slight blue-shift of the band at 385 nm.
Compound 2 also exhibits analogous hypochromic effect in the
concentration range between 110-5 and 510-3 M as shown in
Fig. S23. The hypochromic effect is attributed to an aggregation
process and indicates a relatively broad distribution of
translational and rotational displacements between molecules
in the aggregates suggesting that the molecules are not highly
constrained within the assembly.60,61 The experimental data
points of the concentration dependent UV-Vis experiments
were fitted to the nucleation-elongation aggregation model.62
While compound 2 could be fitted to the cooperative model
(Fig. S23), the experimental points of compound 1 did not fit
(Fig. S22) to this model, probably due to the
gelification/precipitation of the sample during the elongation
process.§
To get more details about the aggregation process,
concentration dependent UV-Vis studies of 1 and 2 were carried
out in PBS. The UV-Vis spectrum of 1 in PBS shows two main
broad bands at 385 and 280 nm (Fig. 3b) which correspond to
electronic transitions of the 4-nitrophenyl and squaramide
parts, respectively (Fig. S20). Similar spectral profile is observed
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The SLS and UV-Vis experiments point out that compounds
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1 and 2 self-assemble at relatively high concentrations and the
resulting aggregates are not very strong and undergo molecular
displacements. These features are not consistent with an
assembly based on H-bonds (Fig. 1c).41-45 Indeed, Kieltyka and
co-workers previously reported on H-bonded squaramide
amphiphiles forming well-defined fibers in water.44-45 They
described that, upon aggregation, two sharp UV-Vis bands at
255 and 328 nm appear while the monomer exhibits a single
band around 300 nm. The appearance of these two bands is
indicative of the formation of H-Bonds between squaramides.44-
45 However, these bands do not appear in the UV-Vis spectra of
1 and 2 even at high concentrations where the molecules are
highly aggregated (Figs S20-S23). To discard the formation of H-
bonds, we prepared 1 and 2 xerogels (PBS) and analysed them
by IR spectroscopy (Figs S24,S25). The FT-IR spectra of hydrogels
1 and 2 show two sharp bands at 1796 cm-1 and 1693 cm-1, and
1799 cm-1 and 1692 cm-1 respectively. These peaks correspond
to the squaramide breathing and the CO stretching.44,45,63 Above
3000 cm-1 appear the bands corresponding to phosphate groups
and water at 3086, 3128, 3385 and 3448 cm-1 (Fig. S26).
However, no bands are observed between 3128 and 3385 cm-1,
where usually the stretching of H-bonded squaramidic NHs
appears. Figs S24,S25).44 This observation supports that the the
squaramidic compounds are not H-bonded and the NH
stretching signals are probably below the broad band between
3384 and 3500 cm-1.
To learn more about the parts of the molecules involved in
the self-assembly process, H NMR experiments of 1(Na) and
Fig. 4. a) Molecular structure of compound 2(Na) and the labels of the distinct NMR
protons. b) Stacked 1H NMR spectra of 2(Na) in H2O:DMSO-d6 mixtures (1.010-3
M). c) Relevant part of the NOESY spectrum of 2 in D2O/NaOD (6.710-3 M).
1
2(Na)§§ (see Experimental part) in DMSO-d6/H2O mixtures were
carried out at 1.010-3 M (Figs. 4a-b and S27). It was assumed
that both compounds are highly aggregated in water at these
concentrations, while they are monomerically dissolved in
DMSO-d6.51 Interestingly, we could observe that the aromatic
1H NMR signals of 1 and 2 in H2O/DMSO-d6 mixtures show
significant shifts upon increasing the ratio of H2O (Fig. 4b and
S27). The NMR signals of frotons Ha, Hc and Hd (Fig. 4a) of
compound 2 undergo first a deshielding and then a shielding
upon increasing the ratio of water. On the other hand, the Hb
proton of the 3,5-(bis(trifluoromethane))phenyl ring of 2
appears at 8.7 ppm in pure DMSO and is shifted up to 7.7 ppm
in a 5:95 DMSO-d6/H2O mixture (Fig. 4b). These shifts of the
aromatic signals were attributed to a conformational change of
the molecules and the concomitant shielding resulting from the
aggregation via  interactions in the water mixtures. It is
noteworthy that these experiments show no signals of the
squaramidic NHs in the presence of any ratio of water. This
supports our assumption that 1 and 2 are not forming H-bonds
and therefore the squaramidic NH groups that are not shielded
into the aggregates can exchange protons with the aqueous
medium.
are observed. Similar results were obtained by using PBS (Figs.
S35-S38). These NOE signals indicate the proximity in the space
of these protons, which can only be the result of intermolecular
proximities in the aggregated structure. This is supported by the
fact that no NOE signals are observed for 2(Na) between those
protons in DMSO-d6 in the monomeric state (Fig. S41 and S42).
Analogous results were found for compound 1 (Figs S28, S29,
S31-S34, S39, S40), where NOE signals are observed between
the protons of the nitrobenzene ring and the CH2 in aqueous
solutions.
Theoretical calculations
Experimental data results were supported by theoretical
calculations. The self-assembled structures of squaramides 1
and 2 were theoretically investigated by using the Gaussian-09
software and the B3LYP-D/6-31+G* level of theory.64 Initially,
we optimized the monomeric species and obtained the
molecular electrostatic potential MEP surfaces and then,
dimeric structures were geometry optimized in order to figure
out their aggregation model. Figs. 5a-b and S43 show the MEP
surfaces and the dipole moment of the molecules alone.
Remarkably, compounds 1 and 2 possess the largest dipole
moments (10.1 and 8.5 D, respectively) of the 1-5 series (Table
S1). Furthermore, while for compounds 3-5 the dipole moment
is oriented perpendicular to the aryl-squaramide structure, in
the case of 1 and 2 the direction of the vector is tilted along the
More precise information about the molecular
environments in the 1 and 2 aggregates was obtained by 2D
NMR NOESY experiments in D2O and in DMSO-d6 (Figs 4c and
S28-S42). Fig. 4c shows the relevant part of the NOESY spectrum
of 2 in D2O/NaOD, where NOE contacts between the aryl
protons Ha and Hb with the methylene protons (HCH2) (Fig. 4a)
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molecular axis. These features are determinant for the self- Preparation of gels and supramolecular polymers
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assembly behaviour (vide infra).
The appropriate amount of 1 or 2 is suspended in few ml of mQ water, and the pH
For the modelling of the dimeric structures (as model of
supramolecular assembly) of compounds 1 and 2, it was
assumed that the benzoate groups are located at the periphery
of the aggregates while the less polar arylsquaramide parts are
located in the interior (Fig. 1b), according to the typical
assembly of amphiphilic molecules in water.14-19 Accordingly,
simplified structures of 1 and 2 without the benzoate groups
were utilized for the theoretical studies, in order to shorten the
calculation time and avoid undesired interactions between
squaramides and carboxylate groups. The dimeric structures
were then geometry optimized, and the results are shown in
Figs. 5c and S44. As shown for compound 2 (Fig. 5c), the dimer
is formed by  interactions between the squaramide moiety
and the 3,5-(bis(trifluoromethyl))phenyl rings while the dipole
moments of the vicinal molecules are oriented in opposite
directions and, consequently, the total dipole moment of the
assembly is zero. Thus, the infinite stack of molecules via 
dipolar interactions leads to the formation of 1D assemblies
where the CO and NH groups of the squaramides point to the
side of the aggregates. Analogous results were obtained for
compound 1 (Fig. S44). We have also included two water
molecules in the optimization (not shown in Fig. 5c) to model
the experimental water environment. This interaction pattern
agrees with the NOESY experiments that indicate that the
methylene groups and the aryl groups of vicinal squaramidic
compounds are proximal in the assembled structure. We
believe that this interaction pattern combined with the
hydrophobic effects drive the assembly of compounds 1 and 2
into 1D aggregates, that bundle forming fibers, as observed by
AFM (Fig 2c,d).
was set to 7.5-8 with PBS (or NaOH 0.1 M). Then the appropriate amount of
additional mQ water is added until the desired concentration is reached. The
mixture is heated to 80ºC for 1 hour and then solution is allowed to cool at room
temperature. The samples prepared with NaOH requires 3-5 days to form gels
(high concentrations) or reach the equilibrium (lower concentrations) however the
samples prepared with PBS reach the equilibrium in 1-2 days after preparation.
Preparation of 1(Na) and 2(Na) salts
Compounds 1 and 2 were suspended in mQ water and a stochiometric amount of
NaOH was added. The mixture was heated until everything is dissolved. Then the
samples were cooled at r.t. and filtrated if appear a precipitate. Finally, solvent was
removed by cold evaporation to give 1(Na) and 2(Na) as solids.
Theoretical calculations
The geometries of all compounds reported herein were fully optimized using the
Gaussian-09 program64 at the B3LYP65-D/6-31+G* level of theory. This level
includes Grimme’s dispersion correction that is adequate to study π-stacking
assemblies.66 The MEP surfaces and dipole moments have been computed at the
same level of theory. The MEP surfaces have been represented using the 0.001 a.u.
isosurface since it is the best estimate of the van der Waals surface. The minimum
nature of the complexes has been confirmed by doing frequency calculations.
Conclusions
In this work we unveil a new interaction motif based on dipolar
 interactions in squaramides 1 and 2 aggregates, which is
distinct to the previously described hydrogen bonding and
antiparallel stacking. Certainly, this is enabled by strong
hydrophobic effects but also by the strong dipole moments in
the arylsquaramide part with electron poor aryl rings. It might
be highlighted that, while dipolar  interactions are well
known in organic solvents, they are rarer in aqueous
assemblies. We believe this discovery opens new strategies for
the design of squaramide aggregates and which still have free
squaramidic NHs groups suitable for the recognition of other
species. Accordingly, and as we showed in our previous works,
these squaramidic compounds are very attractive for the
development of new aqua materials for drug delivery
applications.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank G. Martorell and J. Cifre of the SCT of the UIB for the
technical support. We thank the MINECO/AEI (projects
CTQ2017-85821-R and CTQ2017-90852-REDC, FEDER
funds) for financial support. B. S. is thankful to the MINCIU and
AEI for the Ayuda Ramón y Cajal (RYC-2017-21789) and the
Govern de les Illes Balears (GOIB) for the AAEE 116/2017. S. B.
and E. C. are thankful to SOIB and the Comunitat Autonoma de
les Illes Balears for the financial support through the program
“Joves Qualificats 2017”.
Fig. 5. Open MEP surfaces of compounds 1 (a) and 2 (b). Red and blue coloured
isosurfaces represent negative and positive values of MEP. c) B3LYP-D/6-31+G*
optimised dimer of compound 2. The dipole vectors are represented using pink arrows.
Experimental part
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24 C. Rest, M. J. Mayoral, K. Fucke, J. Schellheimer, V.
Notes and references
Stepanenko and G. Fernández, Angew. Chem. IVnietw. EArdti.c,le2O0n1lin4e,
DOI: 10.1039/C9OB02085C
‡
PBS (pH=7-8) is used as media in all the aggregation
experiments to avoid variation on the pH and keep the
benzoic acid group deprotonated.
The samples for the concentration dependent experiments
were allowed to rest for 2 days. After this period no changes
in the UV-Vis spectra were observed with time indicating that
the thermodynamic equilibrium was reached.
53, 700-705.
25 F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and A. P. H. J.
Schenning, Chem. Rev., 2005, 105, 1491-1546.
26 Z. J. Chen, A. Lohr, C. R. Saha-Moller and F. Würthner, Chem.
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