Excited-state singlet-triplet inversion in hexagonal aromatic and heteroaromatic compounds

Area of Science:

• Computational Chemistry
• Photophysics
• Organic Electronics

Background:

• Singlet-triplet (S1-T1) energy inversion violates Hund's rule and is rare in stable organic molecules.
• S1-T1 inversion significantly affects the photophysical and photochemical properties of organic chromophores.

Purpose of the Study:

• To investigate the potential for S1-T1 inversion in hexagonal polycyclic aromatic and heteroaromatic compounds using computational methods.
• To identify molecular designs that promote S1-T1 inversion.

Main Methods:

• Wave-function based *ab initio* computational methods were utilized.
• Analysis focused on molecules with two-fold degenerate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).

Main Results:

• While larger polycyclic aromatics showed decreasing positive S1-T1 gaps, they remained positive.
• Substitution of C-C pairs with B-N groups in the interior of hexagonal structures, while preserving outer rim conjugation, led to significantly negative S1-T1 gaps.

Conclusions:

• A new class of boron carbon nitrides exhibiting inverted singlet-triplet energy gaps has been identified.
• These findings offer novel molecular designs for controlling photophysical properties in organic materials.