Theoretical studies as a tool for understanding the aromatic character of porphyrinoid compounds

Abstract

The scientific interest in porphyrinoid based materials is steady growing, since porphyrinoids are not only of biological relevance, but they also show interesting spectroscopic properties that link them to many possible applications such as near infrared dyes, photovoltaic dyes, field-effect transistors, nonlinear optical materials and nanoelectronic devices. 1-8 Biomedical applications of porphyrinoids are of particular importance specially for photomedical applications in cancer treatment, such as photodynamic therapy, multimodal imaging, drug delivery and biosensing. 9-12 Porphyrinoids show also an ability to form complexes with metals with unusual oxidation states and are therefore relevant for catalysis. 13-18 The classic porphyrin molecule can formally be regarded as four pyrrole rings connected to each other by methin bridges. 19 Depending on the localization of the two inner pyrrolic hydrogen atoms the molecule is labeled cis-or trans-porphyrin. However, at room temperature the inner hydrogens generally move around inside the porphyrin ring. 20,21 The more general term porphyrinoids is used for a class of molecules that share the classical porphyrin structure for the macroring but differ for example by bearing various substituents or heteroatoms. Classic porphyrins, chlorins and bacteriochlorins are aromatic molecules satisfying Hückel's (4n þ 2) p-electron count rule for aromaticity. 22,23 There is no doubt that aromaticity is an important concept in chemistry albeit it is still not fully understood and thus continuously under debate. 24-27 Theoretical calculations have shown that the aromatic pathways of

Computational Studies of Aromatic and Photophysical Properties of Expanded Porphyrins

“…The calculations show that the ring-current splits for all the studied molecules at the pyrrolic rings. Thus, the current pathways of the expanded porphyrins are very similar to those obtained for other porphyrinoids 28. There is no evidence for strong local currents in the pyrrolic rings suggesting that all π-electrons participate in the aromatic pathways as also obtained for other porphyrinoids.…”

“…12,13 The aromatic character of porphyrinoids has therefore been investigated computationally in a number of works. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Computational and experimental studies have revealed novel properties of antiaromatic porphyrinoids such as that closed-shell strongly antiaromatic porphyrinoids can be paramagnetic. 29,30 Thus, one can expect that the magnetic properties of such molecules change when being exposed to a strong magnetic field.…”

Performance of Localized Coupled Cluster Methods in a Moderately Strong Correlation Regime: Hückel–Möbius Interconversions in Expanded Porphyrins

“…They may therefore constitute an obvious solution for the larger expanded porphyrins. 24,25 That being said, the systems under consideration are known to be strongly delocalized: 26,27 thence, one may intuitively expect that localized orbital-based correlation approaches, such as the above-mentioned DLPNO-type ones, would prove to be inadequate.…”

Aromatic character of Oh‐C24N24. A cavernous nitride fullerene bearing N4‐macrocycle motifs

“…Such structure exhibits six N 4 ‐macrocyclic faces resembling porphyrin‐like species, and eight 1,3,5‐triazine units at each vertex of the octahedral cage, resulting in a 48‐ π electrons system, where boron clusters and fullerenes exhibit interesting aromatic properties . In last years, the aromatic, nonaromatic, and antiaromatic properties of planar N 4 ‐macrocycles have been well discussed by Fliegl and Sundholm, among other authors, highlighting the capabilities of sustaining extended ring currents under an applied magnetic field. These are informative in the formation of host‐guest structures standing in the characterization and quantitative analysis provided by regular NMR experiments .…”