Scope and Limitations of Baird's Theory on Triplet State Aromaticity: Application to the Tuning of Singlet–Triplet Energy Gaps in Fulvenes

Abstract

Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (DeltaE(ST)) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol(-1). This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S(0)) to the lowest pipi* triplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of DeltaE(ST) by triplet sensitization. The findings can also be generalized to fulvenes other than the pentafulvenes, even though the effect is attenuated as the size of the fulvene increases. Our studies thus reveal that triplet-state aromaticity can greatly influence the properties of conjugated compounds in the T1 state.

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

“…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 6,21-24 . 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 . 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 .…”

Triplet‐State Aromaticity of 4nπ‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function

“…[6] Owing to the reversal of polarity in the S 0 and T 1 states, the singlet-triplet energy gaps of fulvenes can be varied widely. [7] We also deduced that the profile of the potentialenergy surface that describes T 1 -state Z/E photoisomerization of annulenyl-substituted olefins is closely connected to the number of p electrons of the substituent. [8] The possibility to use Baird's theory of triplet-state aromaticity to tailor the excited-state properties of molecules suggests that the theory could also be applied as a qualitative back-of-an-envelope tool for the design of molecules for functionalized materials.…”

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] .…”