Broadband Visible Light Harvesting N^N Pt(II) Bisacetylide Complex with Bodipy and Naphthalene Diimide Ligands: Förster Resonance Energy Transfer and Intersystem Crossing

A N^N Pt(II) complex, Pt-1, with two heteroleptic ligands was prepared, which is a rarely reported molecular structure. The two different acetylide ligands, i.e., boron-dipyyromethane (BDP) and naphthalenediimide (NDI) chromophores, show strong absorption in the visible region. The photophysical properties of the complex were investigated by using steady-state and femtosecond/nanosecond time-resolved optical spectroscopies, as well as electrochemical characterization. Upon selective photoexcitation of the coordinated BDP acetylide ligand at 503 nm, the Förster-resonance energy transfer (FRET, k_FRET = 1.2 × 10¹¹ s^-1) process from the BDP to NDI ligand was observed, which leads to the population of the singlet excited state of the latter. After that, intersystem crossing (ISC) process occurs (k_ISC = 3.3 × 10⁹ s^-1), which generates the triplet excited state of the NDI ligand (τ = 28.1 μs). The overall excited-state dynamics are fairly similar in both nonpolar toluene and polar benzonitrile, indicating that photoinduced charge separation dynamics between BDP and NDI is insignificant. This is presumably due to the strong interaction between the NDI ligand and central Pt atom which can give rise to strong spin-orbit coupling. This hypothesis can be further supported by the excited-state dynamics obtained after photoexcitation at the S₂ state of the NDI ligand. The ultrafast ISC from the S₂ state of the NDI ligand to a higher triplet state, which corresponds to the breakdown of the Kasha-Vavilov's rule, was observed, suggesting that the NDI core strongly interacts with the heavy Pt atom. Finally, we identified that this broadband visible light-excitable Pt(II) complex can be used as a triplet photosensitizer for two-color excitable triplet-triplet annihilation upconversion, and the upconversion quantum yield was determined as 4.1%.