Femtosecond Time-Resolved Optical and Raman Spectroscopy of Photoinduced Spin Crossover: Temporal Resolution of Low-to-High Spin Optical Switching

A combination of femtosecond electronic absorption and stimulated Raman spectroscopies has been employed to determine the kinetics associated with low-spin to high-spin conversion following charge-transfer excitation of a Fell spin-crossover system in solution. A time constant of T = 190 +/- 50 fs; for the formation of the 5T(2) ligand-field state was assigned based on the establishment of two isosbestic points in the ultraviolet in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes: additional dynamics observed in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time constant of ca. 10 ps. The results set an important precedent for extremely rapid, formally forbidden (Delta S = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.