Ultrafast Photochemistry of the [Cr(NCS)6]3- Complex in Dimethyl Sulfoxide and Dimethylformamide upon Excitation into Ligand-Field Electronic State

The ultrafast photochemistry of the [Cr(NCS)(6)](3-) complex upon excitation to the T-4(2) ligand-field (LF) state was studied in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) in a wide temporal range (100 fs to 9 ms) by a combination of femtosecond and nanosecond transient absorption spectroscopy techniques and supported by quantum-chemical DFT/TD-DFT calculations. The initially excited T-4(2) state undergoes intersystem crossing to the vibrationally hot E-2 state with time constants of 1.1 +/- 0.2 and 1.8 +/- 0.1 ps in DMSO and DMF, respectively. Vibrational relaxation occurs in the same time scale and takes 1-5 ps. A major part of the [Cr(NCS)(6)](3-) complex in the E-2 state undergoes intersystem crossing to the ground state with time constants of 65 +/- 5 and 85 +/- 5 ns in DMSO and DMF, respectively. A minor part of electronically excited [Cr(NCS)(6)](3-) undergoes irreversible photochemical decomposition. In DMSO, the photolysis of the [Cr(NCS)(6)](3-) complex results in single or double isothiocyanate ion release followed by the coordination of the solvent molecules with a time constant of 1 +/- 0.2 ms.