Photoluminescence mechanisms of BF-formazanate dye sensitizers: a theoretical study

Photodynamic therapy (PDT) is an alternative, minimally invasive treatment for human diseases such as cancer. PDT uses a photosensitizer to transfer photon energy directly to cellular O to generate O (Type II), the toxicity of which leads to cancer cell death. In this work, the photoluminescence mechanisms of a BF-formazanate dye sensitizer (BF-FORM) and its iodinated derivative (BF-FORM-D) were studied using complementary theoretical approaches; the photoluminescence pathways in the S and T states were studied using density functional theory (DFT) and time-dependent (TD)-DFT methods, the kinetic and thermodynamic properties of the pathways using the transition state theory (TST), and the time evolution and dynamics of key processes using non-adiabatic microcanonical molecular dynamics simulations with surface-hopping dynamics (NVE-MDSH). Evaluation of the potential energy surfaces (PESs) in terms of the rotations of the phenyl rings suggested a pathway for the S → S transition for the perpendicular structure, whereas two pathways were anticipated for the T → S transition, namely, [T → S] occurring immediately after the S/T intersystem crossing (ISC) and [T → S] occurring after the S/T ISC and T equilibrium structure relaxation, with the T → S energy gap being comparable to the energy required for O → O. The PESs also showed that because of the heavy-atom effect, BF-FORM-D possessed a significantly smaller S/T energy gap than BF-FORM. The TST results revealed that at room temperature, BF-FORM-D was thermodynamically more favorable than the parent molecule. Analysis of the NVE-MDSH results suggested that the librational motions of the phenyl rings play an important role in the internal conversion (IC) and ISC, and the S/T ISC and T → S transitions could be enhanced by varying the irradiation wavelength and controlling the temperature. These findings can be used as guidelines to improve and/or design photosensitizers for PDT.