Spin-crossover dendrimers: Generation number-dependent cooperativity for thermal spin transition

Poly(benzyl ether) dendrons having a focal triazole unit (Gntrz: trz = triazole; n = generation number = 0-2) were found to react with (MeSO ₃)₂Fe to form dendritic coordination polymers ([Fe(Gntrz)₃]-(MeSO₃)₂·2H₂O) that undergo the thermal spin transition. When the generation number of the dendritic unit was larger (n = 0 → 1 → 2), the average degree of polymerization (D_p = 20 → 10 → 3) and spin-crossover temperature (T_c = 335 → 315 → 300 K) of the resulting polymer were lower. However, the abruptness of the spin transition was not monotonically dependent on the generation number; (G1trz)Fe exhibited an abrupt spin transition with a temperature width of only 10 K, while the smallest and largest members of the (Gntrz)Fe family both displayed a rather broad spin-transition temperature width (30 (n = 0) and 50 K (n = 2)). X-ray diffraction and calorimetric analyses indicated the presence of a discotic columnar core-shell assembly with a crystal lattice best occupied by a C _3v symmetric array of medium-sized (G1trz)-Fe.