Internal conversion dominates the excited state dynamics and fluorescence efficiency of mono-substituted TPE-BODIPY dyes

In this study, we employ Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) to systematically investigate the excited-state dynamics of a series of six mono-tetraphenylethylene (TPE)–substituted BODIPY dyes (BDP-TPE-1 to -6). By modulating the TPE moiety with electron-donating (methoxy) and -accepting (dicyanovinyl) groups, we unravel the structural factors controlling the competition between radiative and nonradiative decay. Our calculations, validated against experimental quantum yields for structurally analogous derivatives (mean absolute error < 0.5%), reveal that the nonradiative decay channel via torsional motion to a conical intersection is kinetically suppressed across the series due to high activation barriers (ΔE‡ = 18.1–81.4 kcal/mol). Consequently, the fluorescence quantum yield (Φf) is almost exclusively governed by the competition between radiative decay (kr) and internal conversion (IC) from the S1 minimum. BDP-TPE-2, featuring a single dicyanovinyl acceptor, exhibits the highest predicted quantum yield (Φf ≈ 22%), not due to a superior radiative rate, but because of a uniquely suppressed IC rate (2.67 × 108 s−1) arising from exceptionally weak electronic coupling (V = 0.0038 eV) and minimal low-frequency reorganization energy (λl). Conversely, the poorest emitters, BDP-TPE-5 and BDP-TPE-6, are shown to suffer from exceptionally fast IC ( > 1010 s−1) driven by large electronic coupling and, crucially, large low-frequency reorganization energies. These results demonstrate that enhancing fluorescence efficiency in this scaffold depends critically on the strategic suppression of internal conversion by minimizing both electronic and vibrational coupling between the ground and excited states. The predicted quantum yields (Φf = 0.16-22%) represent intrinsic molecular properties validated against weak-AIE experimental benchmarks, and may be enhanced in solid-state applications if strong aggregation-induced emission occurs.