Time-resolved emission spectra and TD-DFT excited-state calculations of [W(CO)4(1,10-phenanthroline)] and [W(CO)4(3,4,7,8-tetramethyl-1,10-phenanthroline)]

The multiple emission from [W(CO)4(phen)] and [W(CO)4(tmp)] (phen=1,10-phenanthroline, tmp=3,4,7,8-tetramethyl-1,10-phenanthroline) was studied by nanosecond time-resolved emission spectroscopy, measured from a 2-MeTHF glass at 80 K. Two emission bands were observed for each complex. The low-energy emission decays with double-exponential kinetics: τ≅150 and 487 ns for [W(CO)4(phen)]; 612 and 2684 ns for [W(CO)4(tmp)]. The high-energy emission decays much faster, τ≤40 ns. The energies and characters of the singlet and triplet excited states were calculated for both complexes, using time-dependent density functional theory and were used to interpret the emission spectra. The low-energy emission band was attributed to the radiative decay of two unequilibrated W→phen/tmp 3MLCT states, presumably 3A1 and 3B2. The high-energy emission is due predominantly to W→CO 3MLCT state(s). Involvement of emissive 3LF excited states, assumed previously, has been excluded. The emission behavior is qualitatively similar for both [W(CO)4(phen)] and [W(CO)4(tmp)], despite the different characters of their LUMOs; b1 and a2, respectively.