Chiang Mai Journal of Science

     The effects of the position of fluorophenyl substituents and the number of fluorine atoms on the structural, electronic, and photophysical properties of cyclometalated Pt(II) complexes (PtM1, PtM2, PtM3, PtP1, PtP2, and PtP3) were investigated computationally using DFT and TD-DFT. Specifically, the frontier molecular orbitals, charge-transfer characteristics, absorption and emission spectra, phosphorescence behavior, and oxygen-quenching mechanisms of the complexes were examined. All complexes exhibit characteristic MLCT and LLCT transitions. The meta-substituted derivatives (PtM1-PtM3) display slightly improved electron transfer characteristics relative to the para-substituted analogs (PtP1-PtP3). The meta-substituted complexes exhibit redshifts in their emissions, indicating a reduced HOMO-LUMO energy gap. Furthermore, all complexes display high triplet exciton generation fractions, indicating efficient intersystem crossing (ISC). The radiative decay rates (k_r) are notably higher for the meta-substituted complexes, 1.53 x 10⁵ s^-1 (PtM1), 1.68 x 10⁵ s^-1 (PtM2), and 1.72 x 10⁵ s^-1 (PtM3), compared to the para-substituted complexes, 1.22 x 10⁵ s^-1 (PtP1), 1.23 x 10⁵ s^-1 (PtP2), and 1.23 x 10⁵ s^-1 (PtP3). Additionally, increasing the number of fluorine atoms further enhances the k_r values, suggesting an improved phosphorescence efficiency. Hence, fluorophenyl-substituted cyclometalated Pt(II) complexes represent promising candidates for photodynamic therapy (PDT). DFT-based theoretical investigations offer meaningful interpretations of the observed photophysical behavior and provide design principles for developing new, efficient photosensitizers for PDT applications.