Exploiting Excited-State Aromaticity To Design Highly Stable Singlet Fission Materials

Area of Science:

• Organic electronics
• Photochemistry
• Materials science

Background:

• Singlet fission (SF) allows one high-energy singlet exciton to generate two lower-energy triplet excitons, a process crucial for enhancing solar cell efficiency.
• Achieving SF typically requires molecules with high diradical character, which compromises their stability under ambient conditions.
• Existing SF materials often degrade rapidly when exposed to oxygen and light, limiting their practical applications.

Purpose of the Study:

• To design and synthesize novel organic molecules capable of efficient singlet fission with enhanced ambient stability.
• To explore the use of Baird's rule of excited state aromaticity as a strategy to tune the singlet-triplet energy gap.
• To establish a new class of singlet fission materials based on indolonaphthyridine thiophene (INDT) derivatives.

Main Methods:

• Utilized Baird's rule by incorporating [4n] 5-membered heterocycles to promote triplet state aromaticity.
• Designed and synthesized a series of indolonaphthyridine thiophene (INDT) derivatives.
• Evaluated the photophysical properties and ambient stability of the synthesized materials through experimental testing and theoretical analysis.
• Compared the stability of the new materials with established SF chromophores like TIPS-pentacene.

Main Results:

• Successfully designed novel singlet fission candidates by stabilizing the triplet excited state through aromaticity.
• The synthesized INDT derivatives demonstrated highly tunable excited state energies.
• The new materials exhibited excellent ambient stability, retaining up to 85% of their activity after weeks of exposure to oxygen and light.
• In contrast, TIPS-pentacene films degraded completely within 4 days under similar conditions.
• Theoretical analysis identified thousands of potential SF candidates and confirmed the correlation between triplet aromaticity and the singlet-triplet energy gap.

Conclusions:

• Exploiting excited state aromaticity via Baird's rule offers a viable strategy to design stable singlet fission materials.
• The developed INDT-based scaffolds represent a promising new class of organic materials for efficient and stable singlet fission applications.
• This approach significantly enhances the stability of singlet fission materials, overcoming a major limitation for their use in devices like solar cells.