Intersystem crossing in the [Fe(ptz)6](BF4)2 spin crossover system (ptz=1-propyltetrazole)

[Fe(ptz)6](BF4)2 (ptz=1-propyltetrazole) is an Fe(II) spin crossover system, which shows a light-induced low-spin (1A1)→high-spin (5T2) conversion below ∼50 K by irradiating into the spin allowed 1A1→1T1 d–d absorption band. This phenomenon, known as light-induced excited spin state trapping (LIESST), is reversible, and a subsequent irradiation into the 5T2→5E band results in a light-induced 5T2→1A1 conversion (reverse LIESST). Single crystal absorption spectra of the title compound in the region of d–d transitions are reported. In addition to the well-established spin allowed 1A1→1T1 and 1A1→1T2 transitions of the low-spin species and the 5T2→5E transition of the high-spin species two weak bands in the NIR are assigned to the spin forbidden 1A1→3T1 and 1A1→3T2 transitions. Direct irradiation into the 1A1→3T1 absorption band at 20 K results in a quantitative 1A1→5T2 conversion, proving that this low lying triplet state plays an important role in the mechanism of LIESST. A full kinetic scheme for LIESST and reverse-LIESST with the 3T1 state as intermediate state is developed, and the quantum efficiencies for the various intersystem crossing steps involved are given: they are of the order of unity for the first step from the initially excited 1T1 and 5E states to the intermediate 3T1 state, respectively. The branching ratio from the 3T1 state to the 1A1 and the 5T2 states is 1:4.