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-(ER(2))(CO)(3)(bpy)] (ER(2 =) NHPh, NTol(2), PPh(2), 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 ER(2) -> bpy ligand-to-ligand charge transfer (LLCT). The lowest excited state is the corresponding triplet, (3)LLCT. 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 PPh(2). 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 (3)LLCT 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(NHPh(2))(CO)(3)(bpy)](+) and fac-[Re(PPh(3))(CO)(3)(bpy)](+) are shown to have the usual Re -> bpy (3)MLCT lowest excited states, characterized by upshifted nu(CO) bands.