Excimer evolution hampers symmetry-broken charge-separated states.

Area of Science:

• Materials Science
• Photochemistry
• Physical Chemistry

Background:

• Artificial photosynthesis mimics require long-lived charge-separated states for high performance.
• Energy trap states and excimer formation impede exciton and charge transport, reducing power conversion efficiency in photovoltaic devices.

Purpose of the Study:

• To investigate the impact of excimer formation on symmetry-broken charge-separated states in bichromophoric systems.
• To understand the role of molecular geometry and dielectric environment in charge-separation dynamics.

Main Methods:

• Synthesis and characterization of core-annulated perylenediimide dimers (SC-SPDI2 and SC-NPDI2).
• Fragment-based excited state analysis and temperature-dependent photoluminescence measurements.
• Femtosecond transient absorption (fsTA) spectroscopy to study charge-separation dynamics.

Main Results:

• Bichromophoric systems exhibit a near-orthogonal ground state and a π-stacked foldamer structure in the excited state.
• Solvation-assisted ultrafast symmetry-breaking charge-separation (SB-CS) occurs in high dielectric environments.
• Undesired relaxed excimer formation (Ex) due to configuration mixing quenches the SB-CS state, especially in low dielectric environments.

Conclusions:

• Detrimental excimer formation significantly reduces the persistence of the symmetry-broken charge-separated state.
• Conformational rigidity is a crucial design principle for developing efficient molecular photovoltaics.