Theoretical investigation of NˆCˆN-coordinated Pt(II) and Pd(II) complexes for long-lived two-photon photodynamic therapy

•Pd(NˆCˆN)Cl is fully characterized and first proposed as two-photon Ps.•Pd(NˆCˆN)Cl can replace Pt(NˆCˆN)Cl as promising Ps due to longer T1 lifetimes.•Pd(NˆCˆN)Cl is found to be more effective and economical than Pt(NˆCˆN)Cl for PDT.

Transition metal complexes capable of near-infrared light triggered-cytotoxicity are actively being developed as potential photosensitizers (PS) for photodynamic therapy (PDT) of cancers. In the present work, the structures and photophysical properties of complexes Pt(NˆCˆN)Cl and Pd(NˆCˆN)Cl (NˆCˆN = 2,6-dipyrido-4-methyl-benzenechloride) are investigated with a theoretical approach. The features of low-lying singlet and triplet excited states are discussed in detail. Their potential therapeutic use as two-photon PS in PDT is proposed on the basis of their strong absorbance in near-infrared region (NIR), vertical triplet energies resulting higher than 0.98 eV, and the spin-orbit matrix elements lager than 0.24 cm−1. Moreover, an evaluation of triplet excited states lifetime is presented. The longer lifetime of the triplet excited states would lead to higher 1O2 yields for Pd(NˆCˆN)Cl compared to Pt(NˆCˆN)Cl. It is expected that the low-cost Pd(NˆCˆN)Cl will replace the reported Pt(NˆCˆN)Cl as a promising two-photon PS for improving clinical PDT efficacy.

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