Excited State Aromaticity Unveiled by Electron Localization Function Topology

The application of π-electron localization function (ELF_π) bifurcation topology is extended here to assess excited state aromaticity (ESA). The analysis employs two topological descriptors: the ring-closure bifurcation value (RCBV), which marks the onset of annular π-delocalization, and the span of bifurcation values (DBV), which measures its uniformity. Thresholds proposed for ground-state systems are applied here, while values below the aromatic limit (RCBV < 0.70) are interpreted with caution, since delocalization-based indices cannot unambiguously separate nonaromatic from antiaromatic cases. The methodology is applied to benzene, cyclobutadiene, cyclooctatetraene, and naphthalene across their S₀, S₁, and T₁ states. As benchmarks, the nonaromatic 1,4-dihydrobenzene in its ground state is also included, and the problematic case of borazine is discussed based on literature values. ELF_π descriptors reproduce the expected aromatic and antiaromatic trends and complement magnetic and electronic indices reported in the literature, while highlighting ambiguous regimes where additional analysis is required. Overall, ELF_π bifurcation analysis provides a chemically meaningful and computationally efficient framework for probing aromaticity in excited states, offering a topological perspective that complements conventional energetic and magnetic descriptors.