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 mus, 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 pi 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 >10(2) 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.