Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

Abstract

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.

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

“…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 . Later it was shown through experimental studies involving solvatochromatic shifts that the S 1 states of the substituted fulvenes display similar effects in the corresponding T 1 states 7, 27 . In addition to their work with fulvenes, this same research team recently reported on the basis of computational calculations that various substituted cyclopentadienes and siloles would display aromatic chameleon features 28 .…”

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

“…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] . Note that Baird's rule can also be applied to the S 1 state [28][29][30] .…”

Electron delocalization and aromaticity in low-lying excited states of archetypal organic compounds

“…16 More recently, excited state aromaticity has been found in the lowest-lying singlet excited state of fulvene derivatives. 17 It is usually accepted that 4nπ-electron monocycles are aromatic not only in the T 1 (Baird's rule) but also in the S 1 state. Finally, let us mention the work by Soncini and Fowler that represents a generalized form of Baird's rule.…”