Electronic structure and excited states of rhenium(I) amido and phosphido carbonyl-bipyridine complexes studied by picosecond time-resolved IR spectroscopy and DFT calculations

UV-vis absorption and picosecond time-resolved IR (TRIR) spectra of amido and phosphido complexes fac-[Re-(ER2)(CO)(3)(bpy)] (ER2 = NHPh, NTol(2), PPh2, bpy = 2,2'-bipyridine, Tol = 4-methylphenyl) were investigated in conjunction with DFT and TD-DFT calculations in order to understand their ground-state electronic structure, low-lying electronic transitions and excited-state character and dynamics. The HOMO is localized at the amido/phosphido ligand. Amide and phosphide ligands are sigma-bonded to Re, the pi interaction being negligible. Absorption spectra show a weak band at low energies (1.7-2.1 eV) that arises from essentially pure ER2 -> bpy ligand-to-ligand charge transfer (LLCT). The lowest excited state is the corresponding triplet, (LLCT)-L-3. Low triplet energies and large distortions diminish the excited-state lifetimes to 85 and 270 ps for NHPh and NTol(2), respectively, and to ca. 30 ps for PPh2. nu(CO) vibrations undergo only very small (<= 10 cm(-1)) shifts upon excitation, attesting to its LLCT character, which hardly affects the electron-density distribution on the Re(CO)(3) moiety. Relaxation of the (LLCT)-L-3 state occurs with complex dynamics ranging from units to tens of picoseconds. The "pure" LLCT excitation, which does not mix with the Re -> bpy MLCT character, is a unique feature of the amido/phoshido complexes, whose lowest excited state can be viewed as containing a highly unusual aminyl/phosphinyl radical-cationic ligand. For comparison, the amino and phosphino complexes fac-[Re(NHPh2)(CO)(3)(bpy)](+) and fac-[Re(PPh3)(CO)(3)(bpy)](+) are shown to have the usual Re -> bpy (MLCT)-M-3 lowest excited states, characterized by upshifted nu(CO) bands.