Excited-State Hund's Rule Violations in Bridged [10]- and [14]Annulene Perimeters

Area of Science:

• Photophysics and Molecular Design
• Organic Chemistry
• Computational Chemistry

Background:

• Hund's rule violations in molecules are key for novel fluorescent emitters.
• Few molecular classes exhibit this property, limiting development.
• Understanding these violations is crucial for designing advanced materials.

Purpose of the Study:

• To identify novel molecular classes exhibiting Hund's rule violations.
• To investigate the mechanism of Hund's rule violations in bridged annulenes.
• To establish design principles for new fluorescent emitters.

Main Methods:

• High-throughput screening of the FORMED database.
• Computational analysis of bridged [10]- and [14]annulene perimeters.
• Comparison with known polycyclic systems exhibiting Hund's rule violations.

Main Results:

• A new class of bridged [10]- and [14]annulenes with Hund's rule violations was identified.
• The mechanism involves contributions from two molecular orbital configurations.
• Hund's rule violation assignment depends on the symmetry of excited states.
• These compounds structurally link nonalternant and alternant classes.

Conclusions:

• Bridged annulenes offer a new platform for exploring Hund's rule violations.
• The findings provide design principles for novel fluorescent materials.
• This work expands the scope of molecules with potential for inverted excited state ordering.