Intense Fluorescence of Metal-to-Ligand Charge Transfer in [Pt(0)(binap)₂] [binap = 2,2′-Bis(diphenylphosphino)-1,1′ -binaphthyl]

[Pt(0)(binap)₂] (binap = 2,2′ -bis(diphenylphosphino)-1,1′-binaphthyl) is found to exhibit a luminescence from metal-to-ligand charge transfer state (MLCT) with a quantum yield of 0.12 and a lifetime of 1.2 μs in toluene at an ambient temperature. Prompt fluorescence with a quantum yield of 1.6 × 10^-4 is observed by means of a picosecond time-correlated single photon counting technique. The spectrum of the steady-state luminescence is almost identical to that of the prompt fluorescence, indicating that the intense luminescence is mainly delayed fluorescence from thermally activated ¹MLCT. The analysis of the temperature-dependent emission indicates that the energy difference between the ¹MLCT and ³MLCT is 1.15 × 10³ cm^-1. The lifetime of the prompt fluorescence is determined to be 3.2 ps from the decay of stimulated emission overlapped on subpicosecond transient absorption spectra. The lifetime of the ¹MLCT is much longer than expected from the large spin-orbit coupling constant of 5d (Pt) electrons (4000 cm^-1). Theoretical analysis based on density functional theory reveals that structural distortion in the MLCT states causes large energy splitting between HOMO and HOMO - 1, which prevents a very fast ISC induced by strong spin-orbit interactions between these orbitals. The relatively slow ISC is therefore induced by weak spin-orbit interactions (ca. 50 cm^-1) between ligand-centered molecular orbitals. Theoretical calculations indicate that the phosphorescence observed at lower temperatures is due to intensity borrowing from 4 ¹B₂ → GS transition. However, the large energy difference between HOMO and HOMO - 2 reduces the extent of mixing between the lowest ³MLCT and 4¹B₂ due to spin-orbit interaction, thereby decreasing the radiative rate of the phosphorescence.