Effects of different directions and intensities of external electric fields on the dissociation and excitation characteristics of meta‑bromine-nitrobenzene and 5‑bromine-2-fluoro-nitrobenzene molecules

This study investigates the dissociation and excitation properties of meta-bromonitrobenzene (mBNB) and 5-bromo-2-fluoronitrobenzene (5Br2FNB) under different orientation (CBr or CN bond axis) and intensity (0–0.035 a.u.) external electric fields (EEFs) using density functional theory at B3LYP/6-31+G(d,p) and ωB97XD/def2-TZVP levels. Results show that EEFs along the CBr direction induce CBr bond elongation, electron redistribution from NO₂ to Br, increased dipole moment, and weakened molecular stability. CBr bond dissociation occurs at EEF strengths >0.030 a.u., accompanied by a transition of the S₀ → S₁ excitation from n → π* localized excitation to Rydberg excitation with π → p_x feature. In contrast, EEFs along the CN direction shorten CBr bond, without promoting dissociation. The S₀ → S₁ excitation under CN EEFs shifts from n → π* localized excitation to π → π* charge transfer. Fluorine substitution in 5Br2FNB exhibits limited influence on dissociation or excitation compared to mBNB. These findings demonstrate that directionally applied EEFs effectively modulate molecular stability and dissociation pathways, with CBr-oriented EEFs facilitating environmentally relevant Br elimination.