This book focuses on the construction of redox systems composed of doubly fused para-quinodimethane (p-QD) units and on the elucidation of the unique redox behavior. p-QD is a class of non-aromatic π-conjugated compounds that is well-known to be interconvertible scaffolds in many response systems due to its contribution of resonance structure with a planar aromatic sextet ring. This book describes studies on a strategy for precisely controlling the frontier orbital levels of p-QD-based redox systems and a nature of cationic states of π-electron systems with doubly fused p-QD units. The author revealed that multi-redox unimolecular systems with cross-linking of the two sterically strained p-QD units exhibit unique hysteretic character or have a domino effect upon electron transfer, which cannot be achieved in a monomeric p-QD-based system. Thus, these studies are milestones in creating functional redox systems unprecedented dynamic redox (dyrex) behaviors. A study on cationic states of π-electron systems with doubly fused p-QD units and their intrinsic redox properties will lead to the development of unprecedented dyrex systems, the frontier orbital levels of which can be precisely controlled by input of external stimuli such as electric potential and heat. This book benefits chemists with a deeper understanding of the redox-active molecules.