Porphyrinoids, a unique platform for exploring excited-state aromaticity

Recently, Baird (anti)aromaticity has been referred to as a description of excited-state (anti)aromaticity. With the term of Baird's rule, recent studies have intensively verified that the Hückel aromatic [4n + 2]π (or antiaromatic [4n]π) molecules in the ground state are reversed to give Baird aromatic [4n]π (or Baird antiaromatic [4n + 2]π) molecules in the excited states. Since the Hückel (anti)aromaticity has great influence on the molecular properties and reaction mechanisms, the Baird (anti)aromaticity has been expected to act as a dominant factor in governing excited-state properties and processes, which has attracted intensive scientific investigations for the verification of the concept of reversed aromaticity in the excited states. In this scientific endeavor, porphyrinoids have recently played leading roles in the demonstration of the aromaticity reversal in the excited states and its conceptual development. The distinct structural and electronic nature of porphyhrinoids depending on their (anti)aromaticity allow the direct observation of excited-state aromaticity reversal, Baird's rule. The explicit experimental demonstration with porphyrinoids has contributed greatly to its conceptual development and application in novel functional organic materials. Based on the significant role of porphyrinoids in the field of excited-state aromaticity, this review provides an overview of the experimental verification of the reversal concept of excited-state aromaticity by porphyrinoids and the recent progress on its conceptual application in novel functional molecules.

Benzo-Tetrathiafulvalene- (BTTF-) Annulated Expanded Porphyrins: Potential Next-Generation Multielectron Reservoirs

Two sterically crowded benzo-tetrathiafulvalene (BTTF)-annulated expanded porphyrins (BTTF7-F and BTTF8) are synthesized. Detailed photophysical investigations reveal their intrinsic intramolecular charge transfer (CT) character, originated from partial electron transfer from electron-rich TTF units to the relatively electron-deficient macrocyclic core. This finding stands in contrast to what was observed in the previously reported Figure-of-eight conformer of BTTF-annulated [28]hexaphyrin (BTTF6), in which a typical π-π* electronic transition from HOMO to LUMO was observed. However, core expansion in BTTF7-F and BTTF8 makes the oligopyrrole macrocyclic cores relatively more electron-deficient, facilitating the effective intramolecular CT process. Comparative electrochemical investigations reveal that the current generated at the oxidative region is directly proportional to the number of TTF units attached to the macrocyclic core. This work demonstrates the control of the intramolecular CT process through incremental addition of TTF units to the macrocyclic core. Facile multielectron electrochemical oxidations of these expanded porphyrins suggest that they behave like potential multielectron reservoirs.

Embedding heteroatoms: an effective approach to create porphyrin-based functional materials

Incorporation of planarized heteroatom(s) onto the porphyrin periphery is an effective approach to create porphyrin-based functional materials. In the last three decades, such an "embedding heteroatom" strategy has been actively explored in order to realize attractive electronic, optical, and electrochemical properties. This review aims to cover a variety of synthetic methodologies that have been developed for the construction of heteroatom-embedded porphyrins. Moreover, we also summarize their structure-property relationships as well as possible applications in various research fields including artificial photosynthesis, molecular engineering, organic electronics, and bioimaging.

Retrieving aromaticity of dithiadiazuliporphyrin by oxidation: illustration by experimental and theoretical investigation

The photophysical properties of neutral, monocation radical, and dication of dithiadiazuliporphyrin have been examined with a particular focus on their aromaticity.

Current advances in fused tetrathiafulvalene donor–acceptor systems

Development and applications of tetrathiafulvalene-fused electron donor–acceptor systems will be covered within this tutorial review.

Intramolecular charge transfer character in tetrathiafulvalene-annulated porphyrinoids: effects of core modification and protonation

The effect of the core modification and protonation on the intramolecular charge transfer phenomenon was studied in the core-modified families of TTF-annulated porphyrinoids.

Effective tuning of the electronic and photophysical properties of tetrathiafulvalene pyrroles via aromatic heterocycle annulation

The electronic and optical properties of tetrathiafulvalene-pyrrole may be systematically tuned through aromatic heterocycle-annulation.