Aromaticity, known for its special energetic stability in cyclic [4n + 2]π-conjugated electronic structures, is crucial in understanding molecular stability, reactivity, and properties. Colin Baird’s pioneering work on aromaticity reversal has garnered significant attention. This concept suggests that the aromatic character in the ground state reverses to antiaromatic in the excited triplet state and vice versa. However, experimental evaluation of this phenomenon remains challenging.
Recent breakthroughs using time-resolved optical spectroscopies offer new ways to experimentally assess excited-state aromaticity. These spectroscopies use ultrafast laser pulses to monitor changes in molecular systems’ excited states. Expanded porphyrins, with their distinct aromatic and antiaromatic properties, have proven ideal for such studies. These spectroscopies capture changes in electronic structures and conformations, providing clear evidence for aromaticity reversal in excited states. This advancement paves the way for new experimental indices, enhancing our understanding of aromaticity in stability, dynamics, and reactivity, with potential practical applications. -Scientific Journal cover design by scapiens
