Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule

Fluorescence exclusively occurs from the lowest excited
state of
a given multiplicity according to Kasha’s rule. However, this
rule is not obeyed by a handful of anti-Kasha fluorophores whose underlying
mechanism is still understood merely on a phenomenological basis.
This lack of understanding prevents the rational design and property-tuning
of anti-Kasha fluorophores. Here, we propose a model explaining the
photophysical properties of an archetypal anti-Kasha fluorophore,
azulene, based on its ground- and excited-state (anti)aromaticity.
We derived our model from a detailed analysis of the electronic structure
of the ground singlet, first excited triplet, and quintet states and
of the first and second excited singlet states using the perturbational
molecular orbital theory and quantum-chemical aromaticity indices.
Our model reveals that the anti-Kasha properties of azulene and its
derivatives result from (i) the contrasting (anti)aromaticity of its
first and second singlet excited states (S1 and S2, respectively) and (ii) an easily accessible antiaromaticity relief
pathway of the S1 state. This explanation of the fundamental
cause of anti-Kasha behavior may pave the way for new classes of anti-Kasha
fluorophores and materials with long-lived, high-energy excited states.