Carbanion intermediates in the photodecarboxylation of benzannelated acetic acids in aqueous solution

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

“…This suggestion, put forward in 1972 on the basis of perturbation molecular orbital theory, predicted that the Hückel (anti)aromaticity observed for ground-state systems would become reversed in the lowest triplet state. In other words, annulenes with [4n]π-electrons would exhibit aromatic character, while [4n + 2]π-electron systems should show antiaromatic character in direct contrast to what is seen in the corresponding ground states 5 . Baird's pioneering prediction [5][6][7] has been supported by an enormous body of theoretical work, including studies based on aromatic indices such as nucleus-independent chemical shifts (NICS), the aromatic stabilization energy (ASE) and the harmonic oscillator model of aromaticity (HOMA) [7][8][9][10][11][12][13][14][15][16][17][18][19][20] . There have also been numerous experimental observations in relation to excited-state aromaticity, particularly by Wan and colleagues, for example, by studying stable planar [4n]π-annulenes in their lowest triplet (T 1 ) and singlet excited (S 1 ) states 6,[21][22][23][24] . Furthermore, Ottosson and colleagues observed the aromatic chameleon spectroscopic features of fulvene based on sensitization, in accord with their previous theoretical model 7,25,26 .…”

Photoinduced electron transfer of carbanions and carbocations

“…Examples of this type of photoinduced electron transfer reaction are not common because it requires either (i) that the carbanion is photogenerated adiabatically and hence can undergo electron ejection or (ii) that the carbanion photogenerated is sufficiently stable to be long-lived enough to absorb a second photon and hence result in electron ejection. Wan and coworkers [22,23] have reported two methods for the photogeneration of dibenzannelated carbanions: photodecarboxylation [22] and photochemical C-H bond heterolysis [23] (excited state carbon acids). Photolysis [22] of dibenzannelated acetic acids [3][4][5][6][7][8] in aqueous solution (pH > pKa) results in decarboxylation to give the corresponding protonate~ products with quantum yields in the range 0.06 to 1.0.…”

“…Wan and coworkers [22,23] have reported two methods for the photogeneration of dibenzannelated carbanions: photodecarboxylation [22] and photochemical C-H bond heterolysis [23] (excited state carbon acids). Photolysis [22] of dibenzannelated acetic acids [3][4][5][6][7][8] in aqueous solution (pH > pKa) results in decarboxylation to give the corresponding protonate~ products with quantum yields in the range 0.06 to 1.0. Photolysis in D20 solution gave the corresponding ~-deuterated products, which is indicative of a mechanism via initial formation of a carbanion intermediate.…”

“…The efficiency of PNB radical anion generation correlated with the efficiency of photodecarboxylation, i.e., the greater the quantum yield of photodecarboxylation, the stronger the ESR signal of PNB radical anion observed. Scheme 3 has been proposed [22] to account for these observations. The electron transfer step is believed to take place from the photogenerated carbanion to PNB.…”

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

“…[3] With increasing attention to Bairds pioneering work, the concept of aromaticity reversal in the lowest triplet state of annulenes has been an interesting research topic as illustrated by a significant body of theoretical and experimental research because of its substantial potential applications in organic synthetic photochemistry. [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.…”