Enabling Ultrafast Intramolecular Singlet Fission in Perylene Diimide Tetramer with Saddle-Shaped Linker

Area of Science:

• Photovoltaics
• Organic electronics
• Photochemistry

Background:

• Intramolecular singlet fission (SF) is crucial for enhancing photovoltaic performance.
• Perylene diimide (PDI) oligomers are promising for SF-based devices, but efficient SF remains a challenge.
• Controlling interchromophore geometry is key to facilitating SF.

Purpose of the Study:

• To develop a strategy for efficient intramolecular SF in covalent PDI tetramers.
• To investigate the role of a rigid core structure in predefining inter-PDI geometry and electronic coupling.
• To compare SF efficiency in systems with rigid versus flexible linking.

Main Methods:

• Synthesis of covalent PDI tetramers with a cyclooctatetrathiophene (COTh) core.
• Utilizing fused linking for rigid inter-PDI geometry in COTh-FPDI.
• Employing ultrafast spectroscopy to study charge separation and SF dynamics.
• Comparing COTh-FPDI with a flexible-linked analogue (COTh-αPDI).

Main Results:

• Achieved inter-PDI SF with a lifetime (τ_SF) of ~150 ps and a triplet yield of ~150% in COTh-FPDI.
• Observed ultrafast charge separation (τ_CS = ~100 fs) in the flexible-linked COTh-αPDI.
• Demonstrated that the rigid COTh core dictates inter-PDI geometry and electronic coupling, crucial for SF.
• Minimized excited-state structural relaxation in COTh-FPDI enhances SF efficiency.

Conclusions:

• A rigid COTh core effectively predefines interchromophore geometry, enabling efficient intramolecular SF in PDI tetramers.
• This strategy provides a new pathway for designing multichromophore SF systems for photovoltaics.
• The findings highlight the importance of structural control in optimizing SF processes.