Unraveling Excited-Singlet-State Aromaticity via Vibrational Analysis

The concept of excited-state aromaticity is receiving much attention in that completely reversed aromaticity in the excited state (so-called aromaticity reversal) provides crucial insight into photostability, photoreactivity, and its application to the photosynthetic mechanism and photoactive materials. Despite this significance, experimental elucidation of excited-state aromaticity is still unsolved, particularly for the excited singlet state. Here, as an unconventional approach, time-resolved IR (TRIR) spectroscopy on aromatic and anti-aromatic hexaphyrin congeners shed light on excited-singlet-state aromaticity. The contrasting spectral features between the Fourier transform IR and TRIR spectra reveal the aromaticity-driven structural changes, corroborating aromaticity reversal in the excited singlet states. Our paradigm for excited-state aromaticity, the correlation of IR spectral features with aromaticity reversal, provides another fundamental key to understanding the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited singlet state of π-conjugated molecular systems. Whereas ground-state aromaticity governs molecular properties, excited-state aromaticity can not only rationalize photostability and photoreactivity but also offer fruitful insight into designing photoactive materials. However, evaluation of excited-state aromaticity is still challenging and elusive, particularly for excited singlet states, because estimating aromaticity with conventional magnetic and energetic indices is still inaccessible to transient excited states. As an unconventional approach, time-resolved IR spectroscopy, which offers information on structural changes in photoreactions, can facilitate a study of excited-state aromaticity. As reported here, IR spectral changes allow us to monitor the change in vibrational modes and structural distortions in the excited singlet state, which is correlated with aromaticity changes. This report opens another avenue for unraveling excited-state aromaticity and its application to photosynthetic protocols and photoactive materials. Kim and colleagues employed time-resolved IR (TRIR) spectroscopy to scrutinize excited-singlet-state aromaticity from the viewpoint of molecular geometry. The interconvertible spectral features were observed in the IR spectra between the ground and excited singlet states; the contrasting spectral features of simple and complicated Fourier transform IR spectra between aromatic and anti-aromatic hexaphyrins were reversed in their TRIR spectra. Qualitative analyses revealed that the interconvertible spectral features arise from aromaticity-driven structural changes, demonstrating aromaticity reversal in the excited singlet states.