Spin crossover and optical properties

The d-d electron transitions for a low-spin iron(II) system cover the spin forbidden Ax = lAx - TX and A2 = XAX - 3T2 transitions with low intensity, as well as the spin allowed (orbital forbidden) A3 = XAX - XTX and A4 = XAX 1T2 transitions which are well resolved (Fig. 9.20). Typically, Ax 10000cm-1, J3 18000-19 500cm-1 and A4 26000cm-1 so that the LS system is coloured (purple). In contrast, the HS system exhibits only the single spin-allowed d-d transition As = 5T2 - SE with a typical value of d5 12000 cm-1, which implies that the HS system is colourless (no low-lying metal-to-ligand charge-transfer transitions are assumed). 

Upon thermal treatment the high-spin mole fraction increases in accordance with the formula 

LS to HS transition at Tc Tc increases HS to LS relaxation rate increases Tc decreases when the host lattice is bigger rc decreases slightly 

When the colourless crystal in the LIESST-generated HS state is subsequently irradiated by a monochromatic red light of v= 12200 cm-1, the excited state 5E of the HS complex is generated. Its decay proceeds through the SE 37 1 - 1 A pathway and results in the reappearance of the purple LS system. This is referred to as the reverse LIESST effect. The electron spectrum of the reverse-LIESST-generated LS state is almost identical to that of the starting LS system at the beginning of the experiment [7]. 

Spin crossover systems can be utilised for data recording if the following requirements are fulfilled [8]  

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