Light-Induced Excited Spin State Trapping and Charge Transfer in Trigonal Bipyramidal Cyanide-Bridged Complexes

Three members of the family of trigonal bipyramidal (TBP) complexes of general formula [M(tmphen)₂]₃[M′(CN)₆]₂ (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) or [M₃M′₂], which are known to exhibit thermally induced spin crossover and charge transfer, have been investigated for optical and photomagnetic properties. The light-induced excited spin-state trapping (LIESST) effect found in classical spin crossover compounds, such as [Fe(phen)₂(NCS)₂], was explored for the [Fe₃Fe₂] and [Fe₃Co₂] compounds. Similarly, inspired by the light-induced charge-transfer properties of K_0.2Co_1.4[Fe(CN)₆]·6.9H₂O and related Prussian blue materials, the possibility of photo-induced magnetic changes was investigated for the [Co₃Fe₂] TBP complex. Optical reflectivity and magnetic susceptibility measurements were used to evaluate the photoactivity of these compounds. A comparison of these data before and after light irradiation demonstrates that (i) the spin crossover of the Fe^II centers in the [Fe₃Fe₂] and [Fe₃Co₂] analogues and the (ii) charge transfer events in the [Co₃Fe₂] complex occur with temperature and irradiation. In addition, photomagnetic behavior is exhibited by all three compounds. The photo-conversion efficiency has been estimated at 20% of photo-induced high spin Fe^II centers in [Fe₃Co₂], 30% of paramagnetic Co^II−Fe^III pairs in [Co₃Fe₂], and less than 2% of photo-induced high spin Fe^II centers in [Fe₃Fe₂].