Direct Exciton Harvesting from a Bound Triplet Pair

Area of Science:

• Photochemistry
• Materials Science
• Organic Electronics

Background:

• Singlet fission (SF) is a photophysical process where one high-energy photon generates two lower-energy triplet excitons.
• Traditional SF models involve a singlet exciton decaying into a bound triplet-pair biexciton, which then dissociates into free triplet excitons.
• Ambiguities exist regarding the exact nature of double triplet states and interconversion pathways in SF chromophores.

Purpose of the Study:

• To investigate the direct harvesting of individual triplet excitons from bound triplet-pair biexciton states in SF.
• To clarify the mechanism of singlet fission and the role of the triplet-pair intermediate.
• To quantify triplet-pair harvesting yields and explore their potential for chemical and electrical applications.

Main Methods:

• Utilized unique spectral and kinetic signatures characteristic of geminate triplet-triplet annihilation in intramolecular SF compounds.
• Quantified triplet-pair harvesting yields through spectroscopic analysis.
• Assessed the internal quantum efficiency of triplet exciton transfer from the triplet pair.

Main Results:

• Demonstrated direct extraction of individual triplet excitons from a bound triplet pair, challenging the necessity of a dissociation step.
• Achieved internal quantum efficiency for triplet exciton transfer from the triplet pair exceeding 50%.
• Harvesting efficiency was primarily limited by compound solubility, not the net multiplicity of the triplet-pair state.

Conclusions:

• The study provides direct evidence for harvesting triplet excitons from bound triplet pairs without requiring explicit dissociation.
• This clarifies the complex mechanisms of singlet fission and highlights the utility of triplet pairs for energy conversion.
• The findings suggest that triplet pairs can be directly utilized for chemical or electrical work, independent of their net spin multiplicity.