Combining meso -substitutions and core-modifications in the 28R → 30R hexaphyrin-based switch can synergistically improve the contrast between the nonlinear optical responses of the ON and OFF states by 10-fold compared to the unsubstituted switch.

Expanded porphyrins offer a unique platform for probing how (anti)aromaticity influences molecular properties. These macrocycles are remarkably flexible, capable of adopting diverse π ‐conjugation topologies, including Möbius strip‐like and twisted Hückel structures, which are difficult to realize with regular porphyrins. Their rich redox chemistry facilitates the formation of congeneric macrocycles with (4 n + 2) and (4 n ) π ‐electrons, making them ideal systems for testing the practical limits of Hückel, Möbius, and Baird aromaticity rules. Spectroscopic properties are commonly employed to experimentally probe the ground‐ and excited‐state aromaticity of expanded porphyrins. Nevertheless, quantifying aromaticity remains challenging from both experimental and theoretical standpoints due to the intricate interplay between local and macrocyclic ring currents, which often leads to discrepancies between descriptors based on different criteria. This review summarizes our efforts to unravel the aromaticity fingerprint on the photophysical and nonlinear optical properties of expanded porphyrins. A multidimensional framework to quantify Hückel and Möbius aromaticity is first introduced, integrating global and local descriptors derived from the energetic, reactivity, magnetic, electronic, and structural criteria. The complex structure–property relationships between aromaticity and spectroscopic features across redox‐ and topology‐controlled expanded porphyrins are examined. Lastly, innovative approaches to uncover the driving forces governing the spectroscopic properties of diverse hexaphyrin databases are introduced.

Spectroscopic properties are commonly used in the experimental evaluation of ground‐ and excited‐state aromaticity in expanded porphyrins. Herein, we investigate if the defining photophysical properties still hold for a diverse set of hexaphyrins with varying redox states, topologies, peripheral substitutions, and core‐modifications. By combining TD‐DFT calculations with several aromaticity descriptors and chemical compound space maps, the intricate interplay between structural planarity, aromaticity, and absorption spectra is elucidated. Our results emphasize that the general assumption that antiaromatic porphyrinoids exhibit significantly attenuated absorption bands as compared to aromatic counterparts does not hold even for the unsubstituted hexaphyrin macrocycles. To connect the spectroscopic properties to the hexaphyrins’ aromaticity behaviour, we analyzed chemical compound space maps defined by the various aromaticity indices. The intensity of the Q‐band is not well described by the macrocyclic aromaticity. Instead, the degeneracy of the frontier molecular orbitals, the HOMO–LUMO gap, and the |ΔHOMO–ΔLUMO|² values appear to be better indicators to identify hexaphyrins with enhanced light‐absorbing abilities in the near‐infrared region. Regions with highly planar hexaphyrin structures, both aromatic and antiaromatic, are characterized by an intense B‐band. Hence, we advise using a combination of global and local aromaticity descriptors rooted in different criteria to assess the aromaticity of expanded porphyrins instead of solely relying on the absorption spectra.

Molecular switches, in which a stimulus induces a large and reversible change in molecular properties, are of significant interest in the domain of photonics. Due to their commutable redox states with distinct nonlinear optical (NLO) properties, hexaphyrins have emerged as a novel platform for multistate switches in nanoelectronics. In this study, we employ an inverse design algorithm to find functionalized 26R→28R redox switches with maximal ${\beta }_{HRS}$ contrast. We focus on the role of core modifications, since a synergistic effect with meso-substitutions was recently found for the 30R-based switch. In contrast to these findings, the inverse design optima and subsequent database analysis of 26R-based switches confirm that core modifications are generally not favored when high NLO contrasts are targeted. Moreover, while push–pull combinations enhance the NLO contrast for both redox switches, they prefer a different arrangement in terms of electron-donating and electron-withdrawing functional groups. Finally, we aim at designing a three-state 26R→28R→ 30R switch with a similar NLO response for both ON states. Even though our best-performing three-state switch follows the design rules of the 30R-based component, our chemical compound space plots show that well-performing three-state switches can be found in regions shared by high-responsive 26R and 30R structures.

The HBr-assisted electrophilic aromatic bromination of benzene, anisole and nitrobenzene was investigated using static DFT calculations in gas phase and implicit apolar (CCl4) and polar (acetonitrile) solvent models at the ωB97X-D/cc-pVTZ level of theory. The reaction profiles corresponding to either a direct substitution reaction or an addition–elimination process were constructed and insight into the preferred regioselectivity was provided using a combination of conceptual DFT reactivity indices, aromaticity indices, Wiberg bond indices and the non-covalent interaction index. Our results show that under the considered reaction conditions the bromination reaction preferentially occurs through an addition–elimination mechanism and without formation of a stable charged Wheland intermediate. The ortho/para directing effect of the electron-donating methoxy-group in anisole was ascribed to a synergy between strong electron delocalisation and attractive interactions. In contrast, the preferred meta-addition on nitrobenzene could not be traced back to any of these effects, nor to the intrinsic reactivity property of the reactant. In this case, an electrostatic clash between the ipso-carbon of the ring and the nitrogen atom resulting from the later nature of the rate-determining step, with respect to anisole, appeared to play a crucial role.