DFT Study of Optical Properties for Potential Photosensitizers in Photo-Dynamic Therapy

Understanding the optical properties of metal-coordinated complexes is essential for their use as photosensitizers and red to near-infrared emitters, with potential applications in photocatalysis, isomerization, photovoltaics, and photodynamic therapy. Despite progress in this field, there is still a need for new photosensitizers that have fewer stereoisomers, improved absorption in the near-infrared (NIR) range, and are cost-effective and made from earth-abundant materials. We use density functional theory (DFT) and time-dependent DFT (TD-DFT) with spin-orbit correction (SOC) to analyze how the structure of coordinated complexes with rare metals like Ir(III), Ru(II), and Os(III), as well as earth-abundant metals like Cu(I) and W(0), affects their optical properties. We validate our computations by comparing the calculated optical spectra with experimental data. We found that for bis-terpyridine complexes with oligothienyl, SOC significantly affects the absorption spectra of Os(II) samples, enhancing the mixing of singlet and triplet contributions to the lowest energy transitions and increasing their optical activity. In contrast, SOC has a negligible effect on similar complexes with Ru(II) and Ir(III). For pyridyl-imidazole ligands coordinated to Cu(I), SOC also notably mixes singlet and triplet contributions in the lowest optical transitions, while its effect is minimal for W(0) complexes. For all complexes increase in the conjugation length of ligands slightly redshifts the lowest energy transitions to 550-600 nm. Replacing carbonyl ligands with pyridine or bipyridine in W(0) complexes significantly redshifts the spectra from around 500 nm to 700-1000 nm, while highly enhancing the optical activity of the lowest-energy transitions.