Singlet Fission in a Flexible Bichromophore with Structural and Dynamic Control

Area of Science:

• Photochemistry
• Materials Science
• Physical Chemistry

Background:

• Singlet fission (SF) converts one high-energy singlet exciton into two lower-energy triplet excitons, promising enhanced solar energy harvesting.
• Understanding the complex cascade and factors limiting SF efficiency remains a significant challenge.

Purpose of the Study:

• To investigate the influence of interchromophore coupling and molecular conformation on singlet fission dynamics.
• To elucidate the role of solvent effects and aggregation in controlling SF pathways and efficiency.

Main Methods:

• Utilized a pentacene dimer with a flexible crown ether spacer to tune interchromophore coupling via solvent-induced aggregation and cation binding.
• Performed time-resolved spectroscopic studies across femtosecond to microsecond timescales.
• Systematically varied solvent polarity, viscosity, and excitation wavelength.

Main Results:

• High interchromophore coupling resulted in ultrafast SF (<2 ps), independent of solvent polarity, driven by intramolecular modes.
• Low interchromophore coupling led to slower SF (∼500 ps) dependent on viscosity, producing distinct triplet pair states.
• Spectroscopic distinction of triplet pair types allowed determination of their contribution to overall SF efficiency.

Conclusions:

• Molecular conformational restrictions and controlled dynamics are key to enhancing overall SF efficiency.
• The study provides a coherent picture of the SF cascade, differentiating pathways based on coupling strength and environmental factors.