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

Area of Science:

• Photophysics
• Quantum Chemistry
• Organic Chemistry

Background:

• Kasha's rule states fluorescence originates from the lowest excited state of a given multiplicity.
• Anti-Kasha fluorophores violate this rule, but their mechanisms remain poorly understood.
• This knowledge gap hinders the rational design and tuning of anti-Kasha fluorophores.

Purpose of the Study:

• To propose a theoretical model explaining the anti-Kasha photophysical properties of azulene.
• To elucidate the fundamental mechanisms underlying anti-Kasha behavior in fluorophores.

Main Methods:

• Analysis of electronic structure for ground and excited states (singlet, triplet, quintet) of azulene.
• Application of perturbational molecular orbital theory.
• Calculation of quantum-chemical aromaticity indices.

Main Results:

• Azulene's anti-Kasha properties stem from contrasting aromaticity in its first (S1) and second (S2) excited singlet states.
• An accessible antiaromaticity relief pathway exists for the S1 state.
• The model successfully explains the photophysical behavior of azulene.

Conclusions:

• The proposed model provides a fundamental explanation for anti-Kasha behavior based on excited-state (anti)aromaticity.
• This understanding may facilitate the development of novel anti-Kasha fluorophores and materials.
• Potential for creating materials with long-lived, high-energy excited states.