Organometallic Platinum(II) Photosensitisers that Demonstrate Ligand‐Modulated Triplet‐Triplet Annihilation Energy Upconversion Efficiencies

A series of 2‐phenylquinoxaline ligands have been synthesised that introduce either CF(3) or OCF(3) electron‐withdrawing groups at different positions of the phenyl ring. These ligands were investigated as cyclometalating reagents for platinum(II) to give neutral complexes of the form [Pt(C^N)(acac)] (in which C^N=cyclometalating ligand; acac=acetyl acetonate). X‐ray crystallographic studies on three examples showed that the complexes adopt an approximate square planar geometry. All examples revealed strong Pt−Pt linear contacts of 3.2041(6), 3.2199(3) and 3.2586(2) Å. The highly coloured complexes display efficient visible absorption at 400–500 nm (ϵ ≈5000 M(−1) cm(−1)) and orange red photoluminescent characteristics (λ (em)=603–620 nm; Φ (em) ≤37 %), which were subtly tuned by the ligand. Triplet emitting character was confirmed by microsecond luminescence lifetimes and the photogeneration of singlet oxygen with quantum efficiencies up to 57 %. Each complex was investigated as a photosensitiser for triplet–triplet annihilation energy upconversion using 9,10‐diphenylanthracene as the annihilator species: a range of good upconversion efficiencies (Φ (UC) 5.9–14.1 %) were observed and shown to be strongly influenced by the ligand structure in each case.