Aromaticity and photophysical properties of various topology-controlled expanded porphyrins

Abstract

Recently, expanded porphyrins have come to the forefront in the research field of aromaticity, and been recognized as the most appropriate molecular system to study both Hückel and Möbius aromaticity because their molecular topologies can be easily changed and controlled by various methods. Along with this advantage, many efforts have been devoted to the exploration of the aromaticity-molecular topology relationship based on electronic structures in expanded porphyrins so that further insight into the aromaticity--a very attractive field for chemists--can be provided. In this tutorial review, we describe the recent developments of various topology-controlled expanded porphyrins and their photophysical properties, in conjunction with the topological transformation between Hückel and Möbius aromaticity by various conformational control methods, such as synthetic methods, temperature control, and protonation.

Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule

“…In particular, as compared to the ground-state absorption spectrum, the ESA spectrum of the T 1 state in R26H is relatively broadened and its intensity is weaker (216,000 → 72,500 cm −1 M −1 at the absorption maxima, B-like band). In fact, this ESA spectrum is quite similar to the ground-state spectra of typical antiaromatic porphyrinoids 32 . This suggests that the T 1 state of R26H presumably becomes antiaromatic.…”

“…To address this challenge we have analysed two previously reported congeneric expanded porphyrins with welldefined [4n + 2] and [4n]π-electron ground-state peripheries, and show that these systems undergo 'reversal of (anti)aromaticity' in their lowest triplet states compared to the closed-shell singlet state, as inferred from their distinct spectroscopic signatures 29 Relative to other conjugated species, expanded porphyrins have an advantage in that they can release or capture two pyrrolic protons following two-electron reduction or oxidation of the macrocycle. These chemical features have made expanded porphyrins test beds for the evaluation of molecular aromaticity and exploration of novel aromatic concepts [30][31][32] . On the basis of considerable earlier work, it is now recognized that the photophysical properties of expanded porphyrins, including their absorption and fluorescence spectra, excited-state lifetimes and nonlinear optical properties, can be used as reliable experimental indices for aromaticity, at least within a comparable set of [4n]/[4n + 2]π-electron heteroannulenes 32, 33 .…”

Solvent‐Dependent Aromatic versus Antiaromatic Conformational Switching in meso‐(Heptakis)pentafluorophenyl [32]Heptaphyrin

“…Interestingly, in moderately polar solvent, ethyl ether, we find out that these two conformational isomers coexist with a dynamic equilibrium, as revealed by excitationwavelength-dependent TA, temperature-dependent absorption and Keywords: aromaticity · expanded porphyrins · Mçbius confomation · porphyrinoids · ultrafast spectroscopy efforts towards the experimental verifications and the theoretical explanation. [15] In 2003, Herges and his co-workers reported a [16]annulene molecule as the first example of stable Mçbius aromatic molecule. [16] Even though these results were confronted with skeptical debates about aromatic character of that molecule, [17] this work boosted scientific interests towards Mçbius aromatic molecules.…”

5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1): A Stable Hückel Antiaromatic Hexaphyrin Stabilized by Intramolecular Hydrogen Bonding and Protonation-Induced Conformational Twist To Gain Möbius Aromaticity

“…11 The UV/vis/NIR absorption spectrum of 7 showed three absorption bands at 345, 504, and 616 nm and a very weak absorption tail that reaches 1600 nm, which is characteristic of antiaromatic porphyrinoids because of the dark state ( Figure 2). 12 The structure of 7 was determined by X-ray 9-syn (yellow-green line), and 9-anti (blue line) in CH 2 Cl 2 . The absorption bands of 7 (red dashed line) and 9-anti (blue dashed line) in the NIR region were magnified 40-fold.…”