Baird aromaticity as a molecular design principle for enhancing excited-state proton affinity
Year: 2026
Type: article
Abstract: The concept of Baird aromaticity provides a valuable perspective on how electronic excitation affects the stability and reactivity of conjugated systems. Using spin-unrestricted density functional theory calculations, this study investigated the link between excited-state aromaticity and protonation thermodynamics in cyclopenta-2,4-dien-1-imine derivatives 1–8 . Geometry optimizations, proton affinity (PA), gas-phase basicity (GB), aromaticity descriptors, and natural bond orbital analyses were carried out for both the ground (S 0 ) and first triplet excited (T 1 ) states. Benchmarking different basis sets revealed that UB3LYP/6–311+G(d,p) provides results consistent with those obtained using the larger basis sets while offering a favorable balance between computational cost and accuracy. Enhanced Baird aromaticity in the T 1 state was consistently observed with protonation, indicated by more negative NICS values, higher HOMA and MCI indices, and lower SA values. The augmented enhanced aromatic character was further corroborated by LOL-π and ELF-π analyses, which indicated enhanced π-electron delocalization following excitation and protonation. Additionally, electronic excitation resulted in an increase in electron density at the protonation center (nitrogen atom), significantly boosting PA and the gas-phase basicity. All investigated derivatives exhibited positive ΔPA values, defined as ΔPA= PA(T 1 ) − PA(S 0 ), while excellent linear correlations between PA(T 1 ) and the NBO charge (R 2 = 0.742) and between ΔPA and Δq NBO (R 2 = 0.872) demonstrated that local charge redistribution governs excited-state basicity. Electron-donating substituents produced the strongest bases, whereas electron-withdrawing groups exhibited the largest excitation-induced increases in proton affinity. These findings suggest that excited-state aromaticity may provide a useful framework for designing highly basic transient species.
Source: Chemical Physics Impact
Authors: Zohreh Mirjafary, Hamid Saeidian
Institutions: Islamic Azad University, Tehran, Payame Noor University
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,520
OpenAlex ID: W7223172176