Open-shell π-electronic molecules exhibit spin and magnetic properties, facile redox properties, and unique photophysical properties derived from their unpaired electron(s) and the presence of singly-occupied molecular orbital(s). Especially, stable open-shell π-electron molecules are continuously attracting attention in interdisciplinary research areas as promising components of magnetic, electric, and related materials. Among them, radical cation species generated by oxidation of π-electron molecules with superior electron donating ability are one of the promising components of the materials because their physical properties and functionality of molecular entities and assemblies are able to be controlled by substituents and counter anions. Here, we have introduced detailed molecular structures and intriguing physical properties of highly stable radical cations based on phenothiazine, dihydrophenazine, and phenoxiazine π-electronic systems, such as controls of magnetic, near-IR absorption, and liquescent properties, that have been elucidated as results of our continuous research.