Exploration of the π-Electronic Structure of Singlet, Triplet, and Quintet States of Fulvenes and Fulvalenes Using the Electron Localization Function

Abstract

The singlet ground states and lowest triplet states of penta- and heptafulvene, their benzannulated derivatives, as well as the lowest quintet states of pentaheptafulvalenes, either the parent compound or compounds in which the two rings are intercepted by either an alkynyl or a phenyl segment, were investigated at the (U)OLYP/6-311G(d,p) density functional theory level. The influence of (anti)aromaticity was analyzed by the structure-based aromaticity index HOMA, the harmonic oscillator model of aromaticity. The extent of (anti)aromatic character was also evaluated in terms of the π-electron (de)localization as measured by the π component of the electron localization function (ELF(π)). The natural atomic orbital (NAO) occupancies were calculated in order to evaluate the degree of π-electron shift caused by the opposing electron-counting rules for aromaticity in the electronic ground state (S(0); Hückel's rule) and the first ππ* excited triplet state (T(1); Baird's rule). Pentaheptafulvalene (5) shows a shift of 0.5 π electrons from the 5-ring to the 7-ring when going from the S(0) state to the lowest quintet state (Qu(1)). The pentaheptafulvalene 5 and [5.6.7]quinarene 7 were also investigated in their 90° twisted conformations. From our study it is apparent that excitation localization in fulvalenes, but not in fulvenes, to a substantial degree is determined by aromaticity localization to triplet biradical 4n π-electron cycles. Isolated benzene rings in these compounds tend to remain as closed-shell 6π-electron cycles.

Characterization of Local Aromaticity in Polycyclic Conjugated Hydrocarbons Based on Anisotropy of π‐Electron Density

“…It is very interesting to note that five‐membered rings in these systems display different behaviours to the loss or gain of aromaticity as compared to their corresponding monocyclic molecules (cyclopentadienyl cation and anion and pentafulvene, see references 36 and 37 to look at their plots of Hessian eigenvalues of ρ π ). The five‐membered ring in 11 gains minor aromatic character compared to cyclopentadienyl cation while five‐membered ring in 12 loses minor aromatic character compared to cyclopentadienyl anion, and the aromaticity of five‐membered ring in 13 is approximately the same as that of pentafulvene, which is in line with previous findings by Ottosson …”

“…In the case of the six‐membered ring of 16 , the lost aromatic character is smaller than the corresponding rings in 14 and 15 . Also, the aromaticity of seven‐membered ring in 16 is very similar to that of heptafulvene and, therefore, the seven‐membered ring in 16 has the antiaromatic character …”

Impact of Ground‐ and Excited‐State Aromaticity on Cyclopentadiene and Silole Excitation Energies and Excited‐State Polarities

“…We have previously shown, both computationally and experimentally, that pentafulvenes act as “aromatic chameleons” that change their electronic structures to adapt to the different rules for aromaticity in the S 0 , T 1 , and S 1 states, respectively 12–15. This chameleon feature can be understood by use of dipolar aromatic resonance structures (Figure 1a).…”

Visualizing and Quantifying Interactions in the Excited State

“…Whereas these functions have become very insightful for probing the structure of ground states, similar analysis of bonding in the excited state is a field relatively unexplored. Exceptions are the lowest triplet or quintet states of single organic molecules and coordination compounds (i.e. ground‐state computations enforcing triplet‐ or quintet‐spin state), and the analysis of benzene and naphthalene excimers by means of the non‐covalent interaction index (NCI) or alternative real‐space partitioning (e.g., quantum theory of atoms in molecules QTAIM or the parity function), which are somewhat more cumbersome to interpret .…”