Excited State Aromaticity Unveiled by Electron Localization Function Topology.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Chemical Topology

Background:

• Aromaticity is a fundamental concept in chemistry, crucial for understanding molecular stability and reactivity.
• Assessing aromaticity in excited states presents unique challenges compared to ground states.
• Existing methods for evaluating aromaticity may not fully capture the nuances of excited electronic configurations.

Purpose of the Study:

• To extend the application of π-electron localization function (ELFπ) bifurcation topology for assessing excited state aromaticity (ESA).
• To introduce and utilize two topological descriptors: ring-closure bifurcation value (RCBV) and span of bifurcation values (DBV).
• To provide a computationally efficient framework for probing aromaticity in excited states.

Main Methods:

• Application of ELFπ bifurcation topology to analyze aromaticity.
• Utilizing ring-closure bifurcation value (RCBV) to mark the onset of annular π-delocalization.
• Employing the span of bifurcation values (DBV) to measure the uniformity of π-delocalization.
• Testing the methodology on benzene, cyclobutadiene, cyclooctatetraene, and naphthalene in their S0, S1, and T1 states.

Main Results:

• The ELFπ bifurcation analysis successfully reproduced expected aromatic and antiaromatic trends for the studied molecules.
• The RCBV and DBV descriptors provided insights into the delocalization patterns in excited states.
• The method highlighted ambiguous regimes requiring further investigation, complementing existing magnetic and electronic indices.

Conclusions:

• ELFπ bifurcation analysis offers a chemically meaningful and computationally efficient method for evaluating excited state aromaticity.
• This topological perspective complements conventional energetic and magnetic descriptors.
• The framework provides valuable insights into the electronic behavior of molecules in excited states.