Breaking the Kasha Rule for More Efficient Photochemistry.

Area of Science:

• Photochemistry
• Chemical Kinetics
• Spectroscopy

Background:

• Kasha's rule describes excited-state relaxation to the lowest energy state before reaction.
• Classical photochemistry assumes reactions initiate from the lowest excited singlet state.
• Deviations from Kasha's rule are increasingly recognized in various excited-state processes.

Purpose of the Study:

• To systematically review and analyze phenomena violating Kasha's rule in photochemical reactions.
• To elucidate the kinetic factors governing these anti-Kasha effects.
• To explore the potential applications of anti-Kasha phenomena in advanced technologies.

Main Methods:

• Systematic literature review and analysis of photochemical reaction mechanisms.
• Kinetic analysis of ultrafast excited-state processes.
• Comparison of reaction pathways involving direct high-energy state reactions versus relaxation to the lowest state.

Main Results:

• Identified several classes of excited-state reactions that bypass Kasha's rule.
• Demonstrated that kinetic factors, specifically ultrafast reaction rates, enable anti-Kasha effects.
• Showcased that these reactions compete with vibrational relaxation and other upper-state processes.

Conclusions:

• Anti-Kasha effects are crucial for understanding excited-state transformations.
• These phenomena enable selective control over photochemical reactions.
• Applications include enhanced artificial photosynthesis, photovoltaics, optical sensing, and imaging.