A Description of Vibrational Modes in Hexaphyrins: Understanding the Aromaticity Reversal in the Lowest Triplet State

Abstract

Aromaticity reversal in the lowest triplet state, or Baird's rule, has been postulated for the past few decades. Despite numerous theoretical works on aromaticity reversal, experimental study is still at a rudimentary stage. Herein, we investigate the aromaticity reversal in the lowest excited triplet state using a comparable set of [26]- and [28]hexaphyrins by femtosecond time-resolved infrared (IR) spectroscopy. Compared to the relatively simple IR spectra of [26]bis(rhodium) hexaphyrin (R26H), those of [28]bis(rhodium) hexaphyrin (R28H) show complex IR spectra the region for the stretching modes of conjugated rings. Whereas time-resolved IR spectra of R26H in the excited triplet state are dominated by excited state IR absorption peaks, while those of R28H largely show ground state IR bleaching peaks, reflecting the aromaticity reversal in the lowest triplet state. These contrasting IR spectral features serve as new experimental aromaticity indices for Baird's rule.

Molecular Photoswitching Aided by Excited‐State Aromaticity

“…The foregoing discussion makes it clear that, from a theoretical viewpoint, there is ample support for the occurrence of aromaticity reversal (relative to S 0 ) in low‐lying excited states. From an experimental viewpoint, spectroscopic evidence for this phenomenon has recently appeared in UV‐Vis and IR studies of hexaphyrins by Kim and co‐workers . Furthermore, direct structural evidence for the onset of ESA has been reported in studies of cyclooctatetraene‐featuring molecules by electron diffractometry .…”

Excited‐State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time‐Resolved Electronic and Vibrational Spectroscopy

“…The FT‐IR spectra of 1 , 2 , 3 , and 4 showed intense absorption bands around 1500 cm −1 , which were assigned to the vibrational mode of the meso ‐pentafluorophenyl groups (Figure S15 in the Supporting Information). These strong, distinctive bands in this region were used as important reference peaks in the vibrational analysis because they exist in the same region in all the IR spectra, regardless of the aromaticity of the compounds.…”

“…The FT‐IR spectra of 1 and 3 exhibit simple and silent IR bands, except for the reference bands. These simple spectral features represent the planar and symmetric structures of 1 and 3 , which originate from their stable aromatic nature . The planar structures of 1 and 3 make the C=C stretching motions in the hexaphyrin macrocycle vibrate in the same symmetric plane, so that the net dipole moment change becomes negligible.…”

“…In this regard, we have reported spectroscopic evidence of Baird aromaticity in various comparable sets of expanded porphyrin systems by comparing the electronic structures represented by their absorption spectral features: Hückel aromatic bis‐rhodium hexaphyrins, Möbius aromatic hexaphyrins and larger twisted molecular systems in the lowest triplet (T 1 ) state, and phenylene‐bridged hexaphyrins in the lowest excited singlet (S Q ) state. In particular, the distinct Hückel aromatic hexaphyrins arising from the rigid conformations by metalation with rhodium and phenylene bridges allowed us to obtain clearly contrasting IR spectra for the aromatic and anti‐aromatic congeners.…”

“…In particular, the distinct Hückel aromatic hexaphyrins arising from the rigid conformations by metalation with rhodium and phenylene bridges allowed us to obtain clearly contrasting IR spectra for the aromatic and anti‐aromatic congeners. This contrast depending on the aromaticity enabled vibrational analysis to compare the IR spectral features between the ground and excited states . The coordinated metal also assisted effective intersystem crossing (ISC) by means of the heavy atom effect, letting us detect the aromaticity reversal in the triplet state.…”