Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule

Abstract

The reversal of (anti)aromaticity in a molecule's triplet excited state compared with its closed-shell singlet ground state is known as Baird's rule and has attracted the interest of synthetic, physical organic chemists and theorists because of the potential to modulate the fundamental properties of highly conjugated molecules. Here we show that two closely related bis-rhodium hexaphyrins (R26H and R28H) containing [26] and [28] π-electron peripheries, respectively, exhibit properties consistent with Baird's rule. In the ground state, R26H exhibits a sharp Soret-like band and distinct Q-like bands characteristic of an aromatic porphyrinoid, whereas R28H exhibits a broad absorption spectrum without Q-like bands, which is typical of an antiaromatic porphyrinoid. In contrast, the T-T absorption of R26H is broad, weak and featureless, whereas that of R28H displays an intense and sharp Soret-like band. These spectral signatures, in combination with quantum chemical calculations, are in line with qualitative expectations based on Baird's rule.

Global Aromaticity in Macrocyclic Cyclopenta‐Fused Tetraphenanthrenylene Tetraradicaloid and Its Charged Species

“…[1] A(nearly) planar monocyclic system follows the Hückel rule [2] in the singlet ground state and the Baird rule [3] in the lowest triplet excited state.O nt he other hand, a p-conjugated monocycle with aMçbius twist topology obeys the Mçbius aromaticity rule, [4] which is inverse to the Hückel rule.T he aromaticity in polycyclic p-conjugated systems is more complicated. [1] A(nearly) planar monocyclic system follows the Hückel rule [2] in the singlet ground state and the Baird rule [3] in the lowest triplet excited state.O nt he other hand, a p-conjugated monocycle with aMçbius twist topology obeys the Mçbius aromaticity rule, [4] which is inverse to the Hückel rule.T he aromaticity in polycyclic p-conjugated systems is more complicated.…”

Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium

“…Schleyer and co-workers supported Baird's rule by theoretical investigation of aromaticity in 4nπ annulenes in their singlet and triplet states 16 . Recently, Kim and Osuka characterized the excited-state (anti)aromaticity in expanded porphyrins [17][18][19][20][21][22] . Ottosson and co-workers reported a series of "aromatic chameleons" that are prone to be aromatic in both the T 1 and S 1 states [23][24][25][26][27] .…”

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

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][21][22][23] At the early stage in the development of this interesting concept, only theoretical works based on various aromatic indices, such as aromatic stabilization energy, nucleus-independent chemical shifts, and harmonic oscillator model of aromaticity, were reported because the experimental aromaticity indices for the excited state have not yet been established. [27,28] In previous work, we observed the changes in the electronic transitions of [26]-and [28]hexaphyrins in the excited states, hexaphyrins having aromatic character ( [26]hexaphyrins in the ground state and [28]hexaphyrins in the triplet state) exhibit sharp and intense absorption spectra, while antiaromatic hexaphyrins ( [26]hexaphyrins in the triplet state and [28]hexaphyrins in the ground state) show broad and weak absorption spectra. [24][25][26] Recently, we have demonstrated the aromaticity reversal in the excited singlet and triplet states of a comparable set of [26]-and [28]hexaphyrins by investigating their absorption spectral differences between the ground and excited states along with quantum mechanical calculations, which provides the spectroscopic evidence of Bairds rule.…”

“…In the 2000s, with an analysis of fulvene having chameleon spectroscopic features reported by Ottosson et al [5,[17][18][19][20][21] quantitative experimental research for the aromaticity reversal has been conducted. [27][28][29] Considering the relationship between the electronic structures and molecular structures, we could suggest that aromatic hexaphyrins exhibit different molecular structures from antiaromatic hexaphyrins in the process of aromaticity reversal in the excited triplet state. [27,28] In previous work, we observed the changes in the electronic transitions of [26]-and [28]hexaphyrins in the excited states, hexaphyrins having aromatic character ( [26]hexaphyrins in the ground state and [28]hexaphyrins in the triplet state) exhibit sharp and intense absorption spectra, while antiaromatic hexaphyrins ( [26]hexaphyrins in the triplet state and [28]hexaphyrins in the ground state) show broad and weak absorption spectra.…”