Tridentate cyclometalated platinum(II) complexes with strong absorption of visible light and long-lived triplet excited states as photosensitizers for triplet–triplet annihilation upconversion

C^N*N cyclometalated platinum(II)L complexes with functionalized acetylide ligands were prepared (L = phenyl, pyrenyl or naphthalimide acetylides). The absorption of the pyrene and naphthalimide derived Pt(II) complexes in the visible region of the spectrum was enhanced relative to the phenylethenyl substituted model complex. Long-lived deep-red emissions were observed for the pyrene (83.7 μs) and naphthalimide (135.7 μs) derived Pt(II) complexes and compared to the phenylethenyl substituted model complex (9.2 μs). Room temperature and 77 K phosphorescence, time-resolved transient absorption and theoretical calculations indicated intraligand triplet excited states for the complexes. The photophysical properties of the complexes were fully rationalized by density functional theory calculations. The complexes were used as triplet photosensitizers for triplet–triplet annihilation based upconversion. Upconversion quantum yields up to 19.5% were observed. These results are useful for design of visible light-harvesting transition metal complexes that shows long-lived triplet excited states.

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► Pt(II) acetylide complexes that show strong absorption of visible light were prepared.
► A room temperature long-lived triplet excited state (136 μs) was observed.
► A general strategy of direct metalation of the organic chromophore is proposed.
► The Pt(II) complexes were used as efficient triplet sensitizers for triplet–triplet annihilation upconversion.