Design, synthesis, magnetic properties of a π-radical ligand with photo-excited high-spin state and its Fe(II) complex. The first stage of a new strategy for LIESST materials.

A π-radical ligand (9-[4-(6-oxo-1,5-dimethylverdazyl)phenyl]-10-[5-(2,2'-bipyridyl)ethynyl]anthracene, L1) with a photo-excited high-spin quartet state (S = 3/2) and its iron(II) complex [Fe(L1){H(2)B(Pz)(2)}(2)] (1) {H(2)B(Pz)(2)(-) = dihydrobis(1-pyrazolyl)borate} were synthesized as a candidate for a new strategy for spin-crossover compounds exhibiting light-induced excited spin state trapping (LIESST), which is via the photo-excited high-spin state of the π-conjugated aromatic system. Control compounds, ligand L2 and [Fe(L2){H(2)B(Pz)(2)}(2)] (2), in which the verdazyl radical moiety in L1 was removed, were also synthesized. The photo-excited quartet state of the π-radical ligand L1 was confirmed by the time-resolved ESR technique. Temperature dependence of the magnetic behaviors of 1 and 2 were investigated from 5 K to 350 K, showing spin-crossover transition at T(c) = 222 K and at T(c) = 162 K for complexes 1 and 2, respectively. The transition enthalpies and entropies were determined to be ΔH = 8.09 kJ mol(-1) and ΔS = 36.4 J K(-1) mol(-1) for 1 and to be ΔH = 22.39 kJ mol(-1) and ΔS = 138 J K(-1) mol(-1) for 2. LIESST phenomena were also observed below ca. 50 K for both complexes. The effects of the attachment of radical moiety are discussed based on the results.