Abstract
Amphiphilic porphyrins are of great interest in the field of supramolecular chemistry because they can be fabricated into highly ordered architectures that are stabilized by π-π stacking of porphine rings as well as by non-covalent interactions between their hydrophilic substituents. Protoporphyrin IX (PPIX) has two flexible propionic acid tails and is one of the most common amphiphilic porphyrins. However, unlike other PPIX analogues, PPIX does not form stable extended nanostructures, and the reason for this is still not understood. Here, we employ ion mobility mass spectrometry in combination with infrared multiple photon dissociation spectroscopy to investigate early aggregates of PPIX. The ion mobility results show that growth occurs via single-stranded face-to-face stacking of PPIX. From the infrared spectroscopy on well-defined aggregates, it can be concluded that pairing of the carboxylic acid groups of the tails is a stabilizing element and that such a pairing occurs across a third residue from residue n to residue n+2. The tetramer appears to be especially stable, because all of its propionic acid tails are optimally paired and no free tails to promote further growth are present, which possibly prevents PPIX from forming larger, well-ordered assemblies.
Highlights
Their flat heterocyclic structure with its delocalized π-electron system gives porphyrins unique electronic properties and makes them valuable templates for building functionalized supramolecular assemblies
Amphiphilic porphyrins are of great interest in the field of supramolecular chemistry because they can be fabricated into highly ordered architectures that are stabilized by π−π stacking of porphine rings as well as by non-covalent interactions between their hydrophilic substituents
From the infrared spectroscopy on well-defined aggregates, it can be concluded that pairing of the carboxylic acid groups of the tails is a stabilizing element and that such a pairing occurs across a third residue from residue n to residue n+2
Summary
Their flat heterocyclic structure with its delocalized π-electron system gives porphyrins unique electronic properties and makes them valuable templates for building functionalized supramolecular assemblies. Examples include porphyrin−fullerene complexes,[1−5] one-dimensional fibrillar or tubular arrays of porphyrin derivatives,[6−13] discotic liquid crystals,[14−18] metal− organic frameworks,[19,20] and several others.[21−23] In such supramolecular assemblies, the well-defined redox, photochemical, and spectroscopic properties of porphyrins can be tuned, and various intermolecular interactions can drive porphyrins into specific supramolecular structures. These properties make porphyrin assemblies valuable in diverse applications in the fields of, for example, photovoltaics and photocatalysis.[24−28]. In PPIX assemblies containing amide-derived tails, stabilization occurs via cooperative hydrogen bonding between -C O and -N−H groups,[6,10] and in those containing aminederived tails, amine−ammonium interactions are suggested to stabilize their supramolecular structures (Figure 1c).[7]
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