Understanding contrasting S ₂ → S ₁ internal conversion rates in boron-dipyrromethene derivatives <i>via</i> multi-configuration time-dependent hartree method

Internal conversion (IC) dynamics from higher-lying to lower electronic states following photoexcitation are often described as ultrafast, occurring in much less than 1 ps. However, IC processes can exhibit a range of time scales, depending critically on the energetic landscape of excited-state manifolds and the strength of vibronic couplings that drive nonadiabatic transitions. These dynamics play a fundamental role in most photochemical and photophysical applications. A previous time-resolved fluorescence study revealed that two structurally analogous boron-dipyrromethene (BODIPY) molecules, PM650 and PM597, exhibit markedly different IC rates, despite both undergoing ultrafast IC in <100 fs from the S₃/S₂ to the S₁ state. Nuclear wave packets persisting in the S₁ state after the IC were also observed by time-resolved fluorescence. To elucidate the origin of these divergent IC rates and the nature of vibronic interactions among excited states, we performed theoretical simulations using the multiconfiguration time-dependent Hartree (MCTDH) method. Our results reveal nonadiabatic decay pathways mediated by vibronically coupled S₁, S₂, and S₃ potential energy surfaces, with multiple conical intersections (CIs) enabling the IC processes. The IC rates obtained from the MCTDH simulations are in good agreement with the experimental observations, including the contrasting rates for PM650 and PM597. Importantly, the proximity of CIs to the Franck-Condon region was found to significantly influence IC efficiency. As more vibrational modes were incorporated into the model, a consistent acceleration of the IC dynamics was observed, underscoring the role of multimode effects in nonadiabatic transitions through CIs. Additionally, coherent vibrational spectra of the S₁ state, generated from nuclear densities computed <i>via</i> MCTDH following excitation to higher states, were found to match experimental results closely, further supporting the conclusions of this study. Overall, these findings advance our understanding of the intricate excited-state dynamics and highlight the critical role of vibronic coupling and CIs in ultrafast IC.