Naphthalimide-Modified Tridentate Platinum(II) Complexes: Synthesis, Characterization, and Application in Singlet Oxygen Generation

Singlet oxygen (1O2), representing an important reactive oxygen species, has promising applications in biomedical, material, and environmental sciences. Photosensitized production of 1O2 using organic dyes is highly desirable and the exploration of highly efficient photosensitizers has received considerable attention. Herein, two tridentate Pt(II) complexes, i.e., cationic 1(PF6) and neutral 2, modified with the ethynylnaphthalimide chromophore, were designed and prepared for the application in 1O2 generation. Spectroscopic studies and computational results suggest that 1(PF6) and 2 display the lowest-energy absorption bands centered at 435–465 nm with the molar extinction coefficients of 0.6–3.2 × 104 M−1 cm−1, originating from the singlet ligand-to-ligand charge transfer (1LLCT) and a mixture of 1LLCT and singlet ligand-centered (LC) transitions, respectively. Moreover, they show similar phosphorescence at 620–640 nm assigned to the Pt-perturbed triplet LC emission of the ethynylnaphthalimide moiety. Thanks to the relatively long phosphorescence lifetimes, these complexes exhibit O2-dependent phosphorescence intensities with good reversibility and stability. They are able to behave as efficient triplet photosensitizers to promote the 1O2 generation with high quantum yields (84–89%). This work indicates that the combination of an organic chromophore with Pt(II) complexes provides an effective method to obtain photosensitizers for 1O2 generation.