Increasing Radical Character of Large [n]cyclacenes Unveiled by Wave Function Theory

Area of Science:

• Quantum chemistry
• Materials science
• Organic electronics

Background:

• Cyclic polyacenes ([n]cyclacenes) are graphene fragments with potential applications in organic electronics.
• Understanding their electronic structure, particularly radicality and energy gaps, is crucial for predicting their properties.
• Previous studies have often relied on simplified models that may not capture complex electronic behaviors.

Purpose of the Study:

• To investigate the electronic structure, specifically radicality and singlet-triplet energy gaps, of [n]cyclacenes.
• To analyze how these properties evolve with increasing system size (n).
• To provide a systematic method for active space selection in computational studies of large polycyclic aromatic hydrocarbons.

Main Methods:

• Utilizing the complete active space self-consistent field (CASSCF) method for accurate electronic structure calculations.
• Employing second-order n-electron valence perturbation theory (NEVPT2) for refining energy calculations.
• Developing a systematic approach for active space selection to ensure balanced wave function description.

Main Results:

• The ground state of [n]cyclacenes was identified as an open-shell singlet.
• A progressive increase in multireference character was observed with increasing system size (n).
• The singlet-triplet energy gap was found to decrease with n, approaching a finite positive value.

Conclusions:

• [n]Cyclacenes exhibit a polyradical character for larger system sizes.
• The electronic properties of [n]cyclacenes are strongly dependent on system size.
• Accurate computational methods are essential for describing the complex electronic wave functions of extended polycyclic systems.