Spin states of the iron(III) complexes of porphyrin, porphycene, hemiporphycene, and corrphycene bearing both 1-methylimidazole and azide as axial ligands were analyzed with infrared (IR) spectroscopy at 20 degreesC. The IR stretching band of coordinating azide split into two peaks around 2047 and 2017 cm(-1) reflecting an equilibrium between the high- (S = 5/2) and low- (S = 1/2) spin states. The high-spin fraction changed over a 0-90% range among the macrocycles, demonstrating that the tetrapyrrole array is essential to control the equilibrium.