Nucleophilic Control of BODIPY Decay Pathways: A Quantum Mechanical Study.

This study employs density functional theory to investigate nucleophilic substitution effects on photophysical properties of 3,5-substituted meso-(4-bromophenyl) BODIPY derivatives. BODIPY compounds with chloride, methoxy, cyano, fluoro, ethoxy, hydroxy, and azido substituents are examined, focusing on radiative and nonradiative decay mechanisms. Calculations reveal modest variations in radiative decay rates (k_r: 1.08 × 10⁸ to 3.05 × 10⁸ s^-1) across derivatives, while nonradiative rates show significantly greater variation. Internal conversion (IC) rates span 2.76 × 10⁷ to 3.25 × 10⁸ s^-1, whereas decay via the minimum energy conical intersection pathway is negligible due to high activation barriers (>0.44 eV). Electron-withdrawing groups enhance radiative decay, while electron-donating groups promote nonradiative pathways. Huang-Rhys factors (S_M), internal reorganization energies (λ_l), and electronic coupling (V) critically determine IC rates, with λ_l and S_M being most influential. Electronic coupling varies modestly (0.23-0.30 eV), with CN₂-BrPh-BODIPY exhibiting the lowest coupling (0.23 eV) and nonradiative rate (2.76 × 10⁷ s^-1). These findings confirm IC as the dominant nonradiative channel and provide insights for optimizing BODIPY-based fluorescent probes for biological imaging and chemical sensing applications.