| Abstract
| - 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 twoelectron oxidation/reduction processes. The excited singlet and triplet states of [26]hexaphyrin in solutionexhibit lifetimes of 125 ps and 1.8 μs, respectively, as revealed by various time-resolved spectroscopicmeasurements. 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 electronformulation 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 valuesof 2600 and 810 GM of [28]hexaphyrin and perfluorinated [28]hexaphyrin, respectively, match well withtheir relatively short excited-state lifetimes. Overall, the enhanced excited-state lifetimes for varioushexaphyrins go in line with the increased TPA cross-section values and the ring planarity.
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