1.
FINAL PUBLISHABLE SUMMARY REPORT
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The EU funded research project CCMOBIUS was focused on the design of novel Möbius
aromatic compounds from expanded porphyrins with applications as non-linear optical
materials and molecular switches using computational chemistry methods. Although the
concept of Möbius aromaticity was proposed on 1964, the synthesis of the first Möbius
molecule was a challenge during forty years since the implementation of a smooth
conjugated π network and a twisted conformation in a single molecule is not easy. Only
recently, expanded porphyrins were shown to have definite advantages in the formation of
Möbius aromatic molecules such as (i) overall conformational flexibility, (ii) multiple
oxidation states which can be easily interconverted by two-electron redox reactions, (iii)
ability to invert, or “flip out”, the pyrrolic subunits and (iv) possibility of “locking in” Möbius
conformations through metallation by the formation of both N-metal and C-metal bonds.
Besides the Möbius topology, these large macrocycles adopt a variety of intriguing
structures, such as planar or chiral figure-eight conformations, which can be interconverted
under certain conditions (temperature, solvent, pH, etc).
The vast structural diversity and the extended π-conjugation pathway exhibited by
expanded porphyrins has led to diverse applications such as ion sensors, near-infrared dyes,
two-photon absorption materials, and nonlinear optical materials. Interestingly, the
photophysical properties are strongly dependent on the conformation and
aromaticity/antiaromaticity of the π-electron system. Therefore, the control of the
molecular topology is crucial for most applications. In order to find the optimum conditions
for Möbius aromatic expanded porphyrin, we have carried out a quantum chemical study
focusing on diverse aspects such as conformational analysis, dynamical switch between
Hückel and Möbius structures, aromaticity and metal-ligand interactions. Furthermore,
different conformational control methods have been explored in our research such as
solvent, protonation and meso-substituents. In this period, the conformations and
properties of penta-, hexa- and heptaphyrins has been investigated using mainly Density
Functional Theory (DFT) methods. As the role of the aromaticity, strain and metallation in
the stability of Möbius aromatic compounds is unclear in advance, we have evaluated them
independently using different combination of macrocycles (small vs large porphyrin ring),
substituents at the meso-positions and metals (group 10 vs. group 11 metals). First, we were
focused on expanded porphyrins without metals with different macrocycle size and
oxidation state, and then the metallation behaviour was investigated.
The conformational stability depends on various factors, such as the ring strain of the
macrocycle, intra/intermolecular hydrogen bonding, steric effects of meso-substituents and
aromaticity. We have proposed a set of descriptors to quantify independently the
contribution of the different factors. Aromaticity has been quantified using energetic,
magnetic, structural and reactivity criteria, whereas the steric effects of the substituents
were analysed in terms of the non-covalent interaction index. For the first time, aromatic
stabilization energies, magnetic susceptibility exaltation and relative hardness were
computed for expanded porphyrins. From the correlation analysis, it was proven that
magnetic descriptors together with the relative hardness are the best indices to quantify
both Möbius and Hückel aromaticity. The torsional descriptors together with the aromaticity
indices are shown to be very useful for identifying porphyrinoids with an optimum balance
between the ring strain imposed by the twist and the energy stabilization due to aromaticity.
First of all, the viability of Möbius topologies in [26] and [28]hexaphyrins(1.1.1.1.1.1) was
investigated since they are among the most studied macrocycles in the area of expanded
porphyrins. A large amount of experimental information on their conformations is currently
available, being certainly appropriate to test the novel computational approach. We found
that the conformation of the hexaphyrin macrocycle is strongly dependent on the oxidation
state. [26]hexaphyrin strongly prefers a Hückel conformation, planar and highly aromatic,
whereas Hückel and Möbius conformers coexist in dynamic equilibrium for the
[28]hexaphyrin. The conformational equilibrium of [28]hexaphyrin is quite sensitive to the
solvent and, importantly, the twist induces a dramatic change in the magnetic properties
and reactivity of the macrocycle. These features make [28]hexaphyrin suitable for the
development of molecular switches with practical applications.
Secondly, the developed computational approach was applied to determine the
conformational preferences and the topological interconversion pathways in neutral and
protonated [32]heptaphyrins(1.1.1.1.1.1.1). In the neutral state, the antiaromatic figureeight conformation is preferred over the aromatic Möbius topology due to its more effective
hydrogen bonding, although a Hückel-Möbius aromaticity switch is feasible with a low
activation barrier. The conformational equilibrium is solvent-dependent, so polar solvents
further stabilize the Möbius strip structure. In addition, conformational control of neutral
[32]heptaphyrins can be effectively achieved by meso-substituents and protonation. It is
found that the two-photon absorption cross-section value (σTPA) increases significantly in the
triprotonated heptaphyrins due to the formation of aromatic Möbius species.
A similar approach was then applied to the family of pentaphyrins. Pentapyrrolic
macrocycles are drug leads for photodynamic therapy. However, attempts to synthetize fully
meso-substituted pentaphyrins resulted unexpectedly in the isolation of N-fused systems in
two stable oxidation states. Interestingly, we found that removing only one substituent
provides stable aromatic non-fused [22]pentaphyrin with a high σTPA. On the contrary, the Nfusion reaction is thermodynamically favoured in [24]pentaphyrins due to the release of
strain, so they are expected to be quite unstable. A weakly aromatic N-fused
[24]pentaphyrins with Möbius topology is provided by the trifluoromethyl substituent,
although the large dihedral angles preclude effective delocalization in pentapyrrolic
macrocycles. In short, Möbius topologies turns out to be indeed accessible for [4n] 
expanded porphyrins, although fully aromatic Möbius structures are only achievable by
protonated [32]heptaphyrin and [28]hexaphyrin, among the investigated compounds. A
close relationship between the molecular topology, the number of  electrons, aromaticity
and photophysical properties is found.
Regarding to the metal effect, we found that the group 10 metals (Ni2+, Pt2+, Pd2+) locked the
Möbius conformation in the [28]hexaphyrin, whereas the group 11 metals (Cu3+, Ag3+, Au3+)
provide antiaromatic Hückel-type complexes. On the other hand, the metal complexes of
[26]hexaphyrin provides aromatic Hückel structures with novel metal-carbon bonds. The
computational results reveal novel coordination abilities and a variety of metal complexes
depending upon the oxidation state of the ligand and the metal ion. The computed metal
affinities might be important for the development of practical sensors for the quantification
of metal ions in environmental samples.
Our computational results are in excellent agreement with the experimental data available
for penta-, hexa- and heptaphyrins and provide a fully description and comprehension of the
Möbius aromaticity. It is shown that computational chemistry is a powerful tool in the
design of Möbius aromatic expanded porphyrins and promising molecular switches and our
theoretical results can be used as a guide for experimental groups in academic and industrial
environments. In addition, the knowledge derived from this project could assist the design
of novel porphyrin nanodevices for photovoltaic, photonic and biomedical applications.
Figure 1. Möbius topologies for several expanded porphyrins.