Baird aromaticity as a molecular design principle for enhancing excited-state proton affinity
- 1Department of Chemistry, SR.C, Islamic Azad University, Tehran, Iran; Corresponding authors.
- 2Department of Science, Payame Noor University (PNU), P.O. Box: 19395-3697, Tehran, Iran; Corresponding authors.
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.
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BibTeX
@article{mirjafary2026baird,
title = {{Baird aromaticity as a molecular design principle for enhancing excited-state proton affinity}},
author = {Zohreh Mirjafary and Hamid Saeidian},
journal = {Chemical Physics Impact},
year = {2026},
month = oct,
doi = {10.1016/j.chphi.2026.101199},
url = {https://doi.org/10.1016/j.chphi.2026.101199},
note = {CC BY},
}
RIS
TY - JOUR
TI - Baird aromaticity as a molecular design principle for enhancing excited-state proton affinity
AU - Zohreh Mirjafary
AU - Hamid Saeidian
PY - 2026
DA - 2026/10/30
JO - Chemical Physics Impact
DO - 10.1016/j.chphi.2026.101199
UR - https://doi.org/10.1016/j.chphi.2026.101199
AB - 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.
SN - 2667-0224
ER -