Energy Upconversion Using Platinum(II)-BPI Photosensitizers

Five heteroleptic Pt(II) complexes structurally defined by a diethyl-substituted bis(2-pyridylimino)isoindoline (BPI^Et) core and an ancillary alkynyl coligand are reported. Structural variation across Pt(BPI^Et)(1-5) was achieved through different alkyne coligands: phenylacetylene (1), 4-ethynylanisole (2), 3-ethynylthiophene (3), 1-ethynyl-4-fluorobenzene (4), 1-ethynyl-4-dimethylaniline (5). Complexes were fully characterized using a range of spectroscopic and analytical techniques: ¹H and ¹³C NMR, IR, UV-vis, luminescence, and transient absorption spectroscopies, HRMS, and cyclic voltammetry. Two X-ray structures revealed a subtle deviation from idealized square planar geometry where the Pt atom lies within the plane defined by the three nitrogen donors of BPI^Et. The redox behavior of the complexes showed one irreversible oxidation between +0.61 and +0.82 V (attributed to Pt^2+/3+ couple) and two well-defined ligand-based reductions with fully or quasi-reversible character between -1.70 and -2.09 V. Photophysical studies and supporting DFT calculations describe the phosphorescent nature of the complexes (λ_em = 625-644 nm in toluene) with strong metal-to-ligand charge transfer (MLCT) character and notable singlet oxygen photogeneration (up to 73%). Triplet-triplet annihilation energy upconversion (TTA-UC) investigations in toluene indicated that this class of triplet emitting complex are viable photosensitizers with an impressive maximum efficiency of Φ_UC = 29.6% for Pt(BPI^Et)(4).