Bis-dibenzo[a.i]fluorenylidene, does it exist as stable 1,2-diradical?

Area of Science:

• Theoretical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• Bis-[dibenzo[a.i]fluorenylidene] exhibits paramagnetic properties, suggesting an unusual electronic state.
• Understanding the electronic structure of polycyclic aromatic hydrocarbons is essential for materials science.

Purpose of the Study:

• To theoretically investigate the geometries, energies, and electronic properties of bis-[dibenzo[a.i]fluorenylidene].
• To elucidate the reasons behind the observed paramagnetic properties and low singlet-triplet gap.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP functional.
• High-level basis set (UB3LYP/6-311+G(2d,p) // UB3LYP/6-31+G(d,p)) employed for accurate electronic structure determination.

Main Results:

• The singlet state was found to be 3.4 kcal mol(-1) lower in energy than the triplet state at room temperature.
• The calculated singlet-triplet gap is significantly smaller compared to similar alkenes like ethylene and tetra-tert-butylethylene.
• The results align with experimental findings regarding the paramagnetic nature of the compound.

Conclusions:

• The low singlet-triplet gap is attributed to steric destabilization of the singlet state and delocalization stabilization of the triplet state.
• The electronic structure of the triplet state, with facial hydrogen atom interactions, plays a key role.
• These findings provide a theoretical basis for the experimental observations of bis-[dibenzo[a.i]fluorenylidene]'s electronic properties.