DFT based predictive model for BODIPY triplet quantum yields

Triplet photosensitizers have been developed for use in a variety of applications such as photodynamic therapy, photocatalysis, and organic LEDs. All of these require a high triplet excited-state quantum yield, which is often measured as the singlet oxygen quantum yield (ФΔ). A common method to increase ФΔ is through the heavy-atom-effect achieved by the addition of heavy-atoms to the chromophore – such as the heavy halogens bromine and iodine. Previously, our group developed an empirical model using DFT to predict the ФΔ of heavy-atom containing chromophores. The model correlates a natural atomic orbital composition calculation of the heavy-atoms to the ФΔ of a chromophore. Herein, we modify the original model to apply specifically to halogenated boron dipyrromethene (BODIPY) dyes. In addition to developing a method to predict how changes in the structure of BODIPY dyes affects the ФΔ, this model provides insight into why different structural changes have differing impacts to the ФΔ. The BODIPY core has several unique substitution positions that can be halogenated; however, the model indicates that the 2- and 6-positions on BODIPY (IUPAC numbering) have the greatest impact on the ФΔ with yields changing from 0.00 to 0.90 when replacing the two protons with iodides. Although the original model made using xanthene type dyes provided reasonably good agreement with BODIPY dyes, the new reparametrized model allows for more accurate prediction of the ФΔ of BODIPY type chromophores and provides insight in the importance of substituent location to guide future chromophore design.