Thermal and Light-Activated Spin Crossover in Iron(III) qnal Complexes

Three iron(III) complexes, [Fe(qnal)2]Y {qnal = 1‐[(8‐quinolinylimino)methyl]‐2‐naphthalenolate; Y = NO3 1, BPh4·CH2Cl2 2, NCS 3} have been prepared to explore anion effects in spin crossover systems. Structural studies on 2 reveal supramolecular chains formed by π–π and C–H···π interactions between the [Fe(qnal)2]+ cations. Magnetic susceptibility measurements show the onset of spin crossover in 1 above 200 K, while complete spin crossover is observed in 2 and 3 with T1/2 = 285 and 340 K, respectively. Unusually, 2 also undergoes light‐induced excited spin state trapping (LIESST) at 980 nm with ca. 65 % efficiency and T(LIESST) = 25 K and reverse‐LIESST at 660 nm with full reversibility over many cycles. The results highlight the important influence that the anion has on thermal and light‐activated spin crossover properties