Comparative Photophysics of [26]- and [28]Hexaphyrins(1.1.1.1.1.1): Large Two-Photon Absorption Cross Section of Aromatic [26]Hexaphyrins(1.1.1.1.1.1)

We have explored the electronic natures of representative expanded porphyrins, [26]- and [28]hexaphyrins, to investigate the interplay between the aromaticity and antiaromaticity that is brought by two
electron oxidation/reduction processes. The excited singlet and triplet states of [26]hexaphyrin in solution
exhibit lifetimes of 125 ps and 1.8 μs, respectively, as revealed by various time-resolved spectroscopic
measurements. On the other hand, [28]hexaphyrin shows faster singlet and triplet lifetimes than those of
[26]hexaphyrin, which is largely in accordance with the perturbation of aromaticity due to the π electron
formulation of [4n] in [28]hexaphyrins. The two-photon absorption cross-section values at 1200 nm for
[26]hexaphyrins show ca. 9890 GM which is >102 larger than those of porphyrins. The reduced TPA values
of 2600 and 810 GM of [28]hexaphyrin and perfluorinated [28]hexaphyrin, respectively, match well with
their relatively short excited-state lifetimes. Overall, the enhanced excited-state lifetimes for various
hexaphyrins go in line with the increased TPA cross-section values and the ring planarity.