The Role of Electronic Triplet States and High‐Lying Singlet States in the Deactivation Mechanism of the Parent BODIPY: An ADC(2) and CASPT2 Study

Abstract The potential tunability of the spectroscopic properties of the boron‐dipyrromethene (BODIPY) parent dye by suitable functionalization makes it attractive for a number of applications. The intrinsic remarkable fluorescence of the parent compound, which strongly competes with intersystem crossing to the triplet states, can be reverted by structural tuning of the BODIPY core, leading to BODIPY derivatives suitable for photodynamic therapy purposes. With the perspective of rationally designing BODIPY derivatives with enhanced intersystem crossing, the goal of this work is two‐fold: a) To investigate the main deactivation channels of the parent BODIPY following irradiation, paying particular attention to the accessibility of the triplet‐state potential energy surfaces, as well as the non‐radiative pathways involving the second brightest, most stable singlet electronic state S 2 , and b) to evaluate the performance of the computationally efficient second‐order algebraic‐diagrammatic construction scheme for the polarization propagator (ADC(2)) against the complete active space second‐order perturbation theory (CASPT2) method. Three singlet/triplet crossings were found, all of them with small spin−orbit couplings, with the S 1 /T 2 crossing being the most plausible for the observed intersystem crossing yield. Methodologically, it is found that the ADC(2) method qualitatively reproduces the landscape of the potential energy profiles for the photophysical processes investigated; however, it systematically underestimates the energies of the stationary points and crossings of the same and different multiplicity, with the largest discrepancies found at S 1 /S 0 crossing points. Our CASPT2 results provide a comprehensive picture of the landscape of the excited‐state potential energy surfaces of the parent BODIPY that might serve as a basis for the rational design of photosensitizers with a particular photophysical profile.