Functional Platinum(II) Complexes with Four-Photon Absorption Activity, Lysosome Specificity, and Precise Cancer Therapy.

Multiphoton materials are in special demand in the field of photodynamic therapy and multiphoton fluorescence imaging. However, rational design methodology for these brands of materials is still nascent. This is despite transition-metal complexes favoring optimized nonlinear-optical (NLO) activity and heavy-atom-effected phosphorescent emission. Here, three four-photon absorption (4PA) platinum(II) complexes (Pt1-Pt3) are achieved by the incorporation of varied functionalized C^N^C ligands with high yields. Pt1-Pt3 exhibit triplet metal-to-ligand charge-transfer transitions at ∼460 nm, which are verified multiple times by transient absorption spectra, time-dependent density functional theory calculations, and low-temperature emission spectra. Further, Pt1-Pt3 undergo 4PA. Notably, one of the complexes, Pt2, has maximum 4PA cross-sectional values of up to 15.2 × 10-82 cm8 s3 photon-3 under excitation of a 1600 nm femtosecond laser (near-IR II window). The 4PA cross sections vary when Pt2 is binding to lecithin and when it displays its lysosome-specific targeting behavior. On the basis of the excellent 4PA property of Pt2, we believe that those 4PA platinum(II) complexes have great potential applications in cancer theranostics.