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 σ-bonded to Re, the π 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 . ν(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 ν(CO) bands.