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A polymer compound has been studied by optical response and electron spin resonance (EPR) using and as dopants and paramagnetic probes. The transition between low-spin (LS) and high-spin (HS) states of ions occurs with a broad hysteresis loop around room temperature and is accompanied by a colour change from bright pink in the LS state to chalky white in the HS state. With increasing doping level the hysteresis loop narrows and shifts to lower temperatures. In the HS state of the EPR spectra of and are severely broadened by spin-spin interactions with paramagnetic HS modulated by a rapid spin-lattice relaxation of the latter ions. The EPR data give evidence of the presence of domains of the LS and HS ions in the transition region.

The EPR of Fe 3 § ions has been used for the first time to evidence a low-spin (S = 0) to high-spin (S = 2) transition of Fe 2 § ions in an octahedral ferrous complex [Fe(trz)(Htrz)2](BF4). The temperature dependence of the intensity of the Fe 3 § EPR lineat g = 4.3 reveals a spin transition which occurs for the Fe 2 § ions, with hysteresis. The transition temperatures are To~ = 374 K in the warming mode and Tc~ = 345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.