Development of Responsive Molecules through Proper Molecular Design and Organic Synthesis: Unique Functions Based on the Use of a Metastable Structure

In the field of physical organic chemistry, the important thing is how to design and synthesize beautiful molecules with a unique structure or construct molecules with desired functions. During molecular design, the author determines himself to focus on the following points: (1) highly distorted molecules and (2) molecules that can undergo redox interconversion. This is because he believes that imparting strain to a molecule not only gives it a structure with unusual geometrical parameters but also allows it to exhibit functions that normal molecules do not have. However, the more strain a molecule has, the more difficult it is to synthesize the molecule. To solve this issue, the author adopts the key strategy of designing a redox-active molecule, so that the final reaction yielding the desired product could be conducted by the redox reaction. Based on such a design concept, the author's group has developed highly strained redox-active molecules with unique geometries and functions. Herein, the author focuses on overcrowded ethylenes (OCEs), which exhibit nonplanar geometries due to the bulky substituents on the alkene moiety. For example, well-designed OCEs can adopt multiple conformations/configurations, such as folded and twisted forms, and their frontier orbital levels can change significantly based on easily interconvertible molecular structures, allowing for dynamic changes in physical properties. In this paper, the author describes his molecular design concept and successful examples of stimuli-responsive systems with metastable structures and their properties.