The Effect of the Diphosphine Basicity on the Excited-State Properties of trans-(N2)2W(R2PCH2CH2PR2)2:  Identification of Near-Degenerate, Luminescent 3MLCT and 3LF Terms

Absorption spectra (77 and 298 K), luminescence spectra (5−80 K), and luminescence lifetimes (5−80 K) for the title complexes have been correlated to increasing diphosphine basicity (R = 4-CF3−Ph < 4-H−Ph < 4-CH3O−Ph < Et). As a consequence, spectral peaks have been assigned to 1,3MLCT (B1u, W → phosphorus) and 1,3LF (B2g) terms. As the ligand basicity increases, the 3MLCT bands observed in absorption blue-shift nearly 8000 cm-1 and the vibrationally structured 3LF bands observed in emission red-shift approximately 1300 cm-1. 3LF terms lie lowest in energy in the 4-H−Ph, 4-CH3O−Ph, and Et compounds, and temperature-dependent lifetime data suggest emission from each be assigned to the equilibrated, spin−orbit split levels of the 3LF term. The 3LF and 3MLCT excited-state terms lie close in energy in the 4-CF3−Ph compound, resulting in an emission band shape that is temperature-dependent. At 77 K, the emission band is broad and structureless and is assigned to arise primarily from the 3MLCT term. As the temperature is lowered toward 5 K, the 3MLCT emission diminishes in intensity accompanied by the development of a vibrational structure that is characteristic of emission from the 3LF term. These excited-state terms satisfy the requirements (different orbital origins, near-degeneracy) for separation by a Franck−Condon energy barrier, resulting in simultaneous emission from both terms between 5 and 77 K.