Wavelength-dependent photosubstitution and excited-state dynamics of [Cr(CO)4(2,2′-bipyridine)]: A quantum yield and picosecond absorption study

Article abstract of DOI:10.1021/ja00053a029

Quantum yields of the photosubstitution of an axial-CO ligand in [Cr(CO)₄bpy] by PPh₃ were measured as a function of irradiation wavelength, temperature, and solvent. The dependence of quantum yields and of apparent activation energies on the excitation wavelength within the envelope of the metal to ligand charge transfer (MLCT) absorption band points to distinct chemical reactivities for the ligand field (LF) and MLCT excited states. Importantly, it also suggests a photodissociation from higher vibrational levels of directly excited spin-singlet, ¹MLCT, electronic excited state(s). This interpretation is corroborated by picosecond transient absorption spectra which show a presence of a short-lived (tens of picoseconds) transient, identified with a ¹MLCT excited state, and a long-lived (≥100 ns) transient assigned as a spin-triplet ³MLCT state. Spectroelectrochemical results indicate a resemblance between the ¹MLCT state and the one-electron reduced species, [Cr(CO)₄bpy]^-. Comparison of the picosecond spectra measured under 532 (MLCT) and 355 nm (LF) excitation shows that the electronic relaxation of LF states bypasses the ¹MLCT excited state(s), ¹LF(reactive) → ³LF(reactive) → ³MLCT(slowly) → ¹ground state, whereas the relaxation of ¹MLCT state(s) follows the ¹MLCT(reactive) → ³MLCT(slowly) → ¹ground state pathway.