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

Abstract

[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.