Quantum Chemical and Trajectory Surface Hopping Molecular Dynamics Study of Iodine-based BODIPY Photosensitizer

A computational study of I-BODIPY (2-ethyl-4,4-difluoro-6,7-diiodo-1,3-dimethyl-4-bora-3a,4a-diaza-s-indacene) was conducted to investigate its photophysical properties as a potential triplet photosensitizer for singlet oxygen generation. Multireference CASPT2 and CASSCF methods were used to calculate vertical excitation energies and spin-orbit couplings (SOCs) in a model monoiodinated BODIPY molecule to assess the applicability of the single-reference ADC(2) method. Time-dependent density functional theory (TD-DFT) with the Tamm-Dancoff approximation (TDA) was tested against ADC(2) using different exchange-correlation functionals, employing a two-component pseudopotential basis set for iodine. SOC magnitudes between excited states were discussed using the Slater-Condon rules. The geometry dependence of SOCs for the lowest states was also examined. TD-DFT/B3LYP and TD-DFT(TDA)/BHLYP were selected for subsequent absorption spectra and trajectory surface hopping (TSH) molecular dynamics (MD) simulations. Two bright states were identified in I-BODIPY's visible spectrum, showing a red shift due to iodine substitution. Excited-state MD simulations, including nonadiabatic effects and SOCs, were performed to investigate relaxation after photoexcitation to the S1 state. TSH MD simulations revealed that intersystem crossings occur on a similar timescale to internal conversions. After triplet population growth, a "saturation" phase was reached with a triplet-to-singlet ratio of about 4:1. The calculated triplet quantum yield of 0.85 agrees qualitatively with the experimental singlet oxygen generation yield of 0.99.