Diradical and Ionic Characters of Open-Shell Singlet Molecular Systems.

Area of Science:

• Quantum Chemistry
• Materials Science

Background:

• Open-shell singlet systems exhibit complex electronic structures with both diradical and ionic characteristics.
• Understanding and controlling these electronic natures are crucial for developing advanced materials with tunable properties.

Purpose of the Study:

• To investigate the diradical and ionic natures of open-shell singlet systems using novel definitions and computational models.
• To explore how external stimuli, such as electric fields and structural modifications, influence these electronic characters.

Main Methods:

• Employed the valence configuration interaction (VCI) model with two electrons in two active orbitals.
• Introduced new definitions for diradical and ionic characters and their densities.
• Examined symmetric and asymmetric diradical models, diradicaloid diphenalenyl, graphene nanoflakes, and π-stacked phenalenyl-derivative dimers.

Main Results:

• An external static electric field was found to decrease diradical character and induce ionic character in the ground state of diradicaloids.
• Increasing intermonomer distance in B/N substituted phenalenyl dimers shifted the electronic state from open-shell singlet to closed-shell ionic.
• The first excited state transformed from ionic to diradical-dominant with increasing electric field amplitude.

Conclusions:

• External static electric fields and asymmetric substitution in π-dimers can effectively tune diradical/ionic characters.
• These findings suggest a pathway to control nonlinear optical responses by manipulating the electronic states of diradicaloid systems.