Aromaticity in the Spectroscopic Spotlight of Hexaphyrins

• Published in: Chemistry : a European journal 2024-09, Vol.30 (52), p.e202401933-n/a
• Main Authors: Desmedt, Eline, Casademont‐Reig, Irene, Monreal‐Corona, Roger, De Vleeschouwer, Freija, Alonso, Mercedes
• Format: Article
• Language: English
• Subjects: Absorption; Absorption spectra; Aromaticity; Banded structure; chemical compound space; Chemical compounds; Excitation spectra; expanded porphyrins; Luminous intensity; meso-substitutions and core-modifications; Molecular orbitals; Near infrared radiation; Porphyrins; Topology; UV/vis spectroscopy
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202401933

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. Through TD‐DFT calculations, aromaticity descriptors, and chemical compound space maps, we connected the spectroscopic fingerprints of functionalized hexaphyrins to the macrocycle's aromaticity. While the intensity of the Q‐band in the NIR region is well described by (ΔHOMO–ΔLUMO)2 and not by their aromaticity descriptors, highly planar and hexaphyrin structures showing large aromaticity and antiaromaticity exhibit an intense B‐band.