Triplet Pair States in Singlet Fission

Area of Science:

• Photophysics and Photochemistry
• Materials Science
• Organic Electronics

Background:

• Singlet fission (SF) is a photophysical process where a molecular singlet excited state (S₁) generates two triplet excited states (2 × T₁).
• The field has seen significant growth, but confusion persists regarding triplet pair intermediates and SF rate definitions.
• Understanding SF is crucial for enhancing solar energy conversion efficiencies.

Purpose of the Study:

• To clarify the nature of triplet pair intermediate states in singlet fission.
• To provide a precise definition for singlet fission rates.
• To discuss challenges and future directions in singlet fission research.

Main Methods:

• Theoretical analysis of electronic coherence in triplet pair states.
• Experimental evidence distinguishing between coherent (¹(TT)) and incoherent (¹(T···T)) intermediates.
• Review of mechanisms (incoherent and vibronic coherent) for ¹(TT) formation.

Main Results:

• Distinguished between electronically coherent (¹(TT)) and incoherent (¹(T···T)) triplet pair states.
• Defined the singlet fission rate exclusively by the formation rate of the incoherent ¹(T···T) state.
• Presented experimental evidence supporting the existence and formation mechanisms of ¹(TT).

Conclusions:

• The precise definition of singlet fission rates is crucial for accurate interpretation of experimental and theoretical data.
• Further research is needed to address challenges beyond the dimer approximation and the role of delocalization.
• Overcoming these challenges is key to realizing the full potential of singlet fission in solar energy applications.