Sub-picosecond time resolved infrared spectroscopy of high-spin state formation in Fe(II) spin crossover complexes

The photoinduced low-spin (S = 0) to high-spin (S = 2) transition of the iron(ii) spin-crossover systems [Fe(btpa)](PF₆)₂ and [Fe(b(bdpa))](PF₆)₂ in solution have been studied for the first time by means of ultrafast transient infrared spectroscopy at room temperature. Negative and positive infrared difference bands between 1000 and 1065 cm^-1 that appear within the instrumental system response time of 350 fs after excitation at 387 nm display the formation of the vibrationally unrelaxed and hot high-spin ⁵T₂ state. Vibrational relaxation is observed and characterized by the time constants 9.4 ± 0.7 ps for [Fe(btpa)](PF₆)₂/acetone and 12.7 ± 0.7 ps for both [Fe(btpa)](PF₆)₂/acetonitrile and [Fe(b(bdpa)](PF₆)₂/acetonitrile. Vibrational analysis has been performed via DFT calculations of the low-spin and high-spin state normal modes of both compounds as well as their respective infrared absorption cross sections. The simulated infrared difference spectra are dominated by an increase of the absorption cross section upon high-spin state formation in accordance with the experimental infrared spectra.