Fingerprint of Aromaticity and Molecular Topology on the Photophysical Properties of Octaphyrins

Expanded porphyrins are currently recognized as the ideal test bed to explore the correlation between molecular properties and (anti)aromaticity since they can adopt different pi-conjugation topologies and change easily the number of pi-electrons along the conjugation pathway. From recent studies, aromaticity indeed emerges as the guiding concept to rationalize the spectroscopic features and the twophoton absorption cross sections in expanded porphyrins. However, from the theoretical point of view, the complex structure - property relationship between aromaticity, pi-conjugation topology, and photophysical properties is not fully understood yet. To unravel such structure - property relationships, we focused on octaphyrin(1.1.1.1.1.1.1.1) macrocycles since they are flexible enough to provide twisted-Huckel, Mobius, and Huckel untwisted topologies with distinct aromaticity character. In this work, the (anti)aromaticity of the different states was first quantified using different aromaticity criteria. Second, the fingerprints of aromaticity on UV/vis spectra were elucidated. Importantly, we found that the absorption spectra of certain antiaromatic Huckel structures are characterized by more intense absorption bands than its aromatic homologues, contrary to the general statement that antiaromatic expanded porphyrins exhibit significantly attenuated absorption bands compared to aromatic ones. Finally, the role of aromaticity and pi-conjugation topology on linear and nonlinear optical properties was scrutinized and our results pinpoint the importance of molecular topology and symmetry on the first and second hyperpolarizabilities. Overall, we demonstrate that expanded porphyrins upon topology interconversions can act as efficient nonlinear optical switches.