Breaking Kasha's Rule to Enable Higher Reactivity in Photoredox Catalysis

Area of Science:

• Photochemistry
• Photophysics
• Organic Chemistry

Background:

• Kasha's rule states photochemical reactions occur from the lowest excited state.
• Higher-energy excited states rapidly relax to the lowest state.
• This principle governs most known photochemical transformations.

Purpose of the Study:

• To challenge the universality of Kasha's rule in photochemical reactions.
• To demonstrate reactivity from higher-energy excited states.
• To establish a framework for anti-Kasha reactivity.

Main Methods:

• Time-resolved laser spectroscopy was employed.
• Investigated the 4,4″-dicyano-p-terphenyl radical anion.
• Analyzed photoinduced electron transfer dynamics.

Main Results:

• Demonstrated direct photoinduced electron transfer from a higher-energy excited state.
• Achieved subnanosecond electron transfer via preassociation and driving-force optimization.
• Observed reactivity not accessible from the lowest excited state.

Conclusions:

• Established a general framework for anti-Kasha photochemical reactivity.
• Bypassed the limitations imposed by Kasha's rule.
• Opened new avenues for photoredox catalysis and solar energy conversion.