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: tau congruent to 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, tau less than or equal to 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 (MLCT)-M-3 states, presumably (3)A(1) and B-3(2). The high-energy emission is due predominantly to W --> CO (MLCT)-M-3 state(s). Involvement of emissive (LF)-L-3 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; b(1) and a(2), respectively. (C) 2001 Elsevier Science B.V. All rights reserved.