Anti-Kasha emission in DCM-IFC: computational evaluation of the type III separated wavefunction hypothesis

Area of Science:

• Photochemistry and Photophysics
• Computational Chemistry
• Materials Science

Background:

• Kasha's rule governs photochemistry, stating emission originates from the lowest excited state of a given multiplicity.
• Exceptions, like anti-Kasha (AK) emission, occur with large energy gaps (S1-S2) or spatially separated wavefunctions.
• A DCM-IFC fluorophore with a spirolactane switch exhibited dual emission, interpreted as S1 and S2 state emission.

Purpose of the Study:

• To theoretically investigate the excited state pathways and energy landscape of the DCM-IFC fluorophore.
• To elucidate the mechanisms governing the observed dual emission and challenge the anti-Kasha interpretation.
• To determine the electronic character and relative brightness of the S1 and S2 excited states.

Main Methods:

• Theoretical investigation using implicit solvation models.
• Calculation of excited state energies and oscillator strengths via time-dependent density functional theory (TD-DFT) and high-level wavefunction theories.
• Analysis of potential energy surfaces and internal conversion pathways.

Main Results:

• The S1 state exhibits locally excited character and is brighter than the S2 state, which has charge-transfer character.
• Calculations reveal overlapping spectral absorption profiles for S1 and S2 states.
• Barrierless internal conversion pathways from the S2 surface to the S2/S1 intersection region were identified, facilitating rapid S2 population decay.

Conclusions:

• The rapid S2 population decay and unlikely thermal equilibration make anti-Kasha emission improbable.
• The observed anomalous emission in the DCM-IFC fluorophore likely requires an alternative mechanistic interpretation.
• This study provides critical theoretical insights into the photophysical behavior of complex fluorophores.