Figure 3.

Single crystal UV–vis spectra of the spin crossover compound [Fe(ptz)₆](BF₄)₂ (ptz = 1-propyltetrazole). At 300 K, the crystal is white and in the HS state. The spectrum shows a weak ⁵T₂→⁵E band at ca. 12000 cm⁻¹. At ca. 135 K, a spin transition takes place to the deeply colored LS state. At 80 K the crystal is red–purple, and the spectrum shows two spin-allowed d–d bands, ¹A₁→¹T₁ and ¹A₁→¹T₂ above 20000 cm⁻¹. The LS state can be switched back to the long-lived metastable HS state by irradiation with green light. The optical spectrum recorded at 10 K shows the typical HS band again. This observation, known as the LIESST effect, is described in more detail below. (Reproduced with permission from [36]. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA).