Computational Investigation on the Photophysical Properties of Halogenated Tetraphenyl BODIPY

The electronic structure, transition probabilities, and corresponding quantum yields of fluorescence in a family of dihalogen-tetraphenyl-aza-BODIPY were calculated at the Time-Dependent Density Functional and post-Hartree-Fock levels of theory. Excellent agreement between theoretical and experimental spectral-luminescent data was achieved with the HSE06 functional and the 6-311G∗ basis set. Because the fluorescence can be quenched through nonradiative intersystem spin crossing transitions from the lowest photoactive singlet state to triplet excited states, spin-orbit coupling matrix elements were calculated and applied along with Marcus-Levich-Jortner theory, leading to satisfactory agreement for the lifetimes in comparison with available experimental data. The anomalous dependence of the fluorescence efficiency on the atomic number of the halogen congeners was elucidated and shown to be due to an inversion between the fluorescent and the nearest triplet states in the iodinated compounds. The high rate of fluorescence quenching by intersystem crossings and the probability of collisions in a solvent between oxygen molecules and the molecules studied show that these molecules can provide efficient triplet sensitization. The most preferable sites for such interactions were predicted using electrostatic potential mapping at the extreme positive and negative charge points.