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 4T2 ligand-field (LF) state was studied in dimethylsulfoxide (DMSO) and N,N-dimethylformamide (DMF) in a wide temporal range (100 fs - 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 4T2 state undergoes intersystem crossing to the vibrationally hot 2E 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 timescale and takes 1-5 ps. A major part of [Cr(NCS)6]3- complex in the 2E state undergoes intersystem crossing to the ground state with time constants 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 [Cr(NCS)6]3- complex results in single or double thiocyanate ion release followed by the coordination of the solvent molecules with time constant of 1±0.2 ms.