Excited State Aromaticity Unveiled by Electron Localization Function Topology.

The application of π-electron localization function (ELFπ) 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. ELFπ 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, ELFπ 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.