Photoluminescence mechanisms of BF2-formazanate dye sensitizers: a theoretical study

Abstract

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 ³O₂ to generate ¹O₂ (Type II), the toxicity of which leads to cancer cell death. In this work, the photoluminescence mechanisms of a BF₂-formazanate dye sensitizer (BF₂-FORM) and its iodinated derivative (BF₂-FORM-D) were studied using complementary theoretical approaches; the photoluminescence pathways in the S₁ and T₁ 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 S₁ → S₀ transition for the perpendicular structure, whereas two pathways were anticipated for the T₁ → S₀ transition, namely, [T₁ → S₀]₁ occurring immediately after the S₁/T₁ intersystem crossing (ISC) and [T₁ → S₀]₂ occurring after the S₁/T₁ ISC and T₁ equilibrium structure relaxation, with the T₁ → S₀ energy gap being comparable to the energy required for ³O₂ → ¹O₂. The PESs also showed that because of the heavy-atom effect, BF₂-FORM-D possessed a significantly smaller S₁/T₁ energy gap than BF₂-FORM. The TST results revealed that at room temperature, BF₂-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 S₁/T₁ ISC and T₁ → S₀ 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.