Development of New Pyridinium–Cyclic Enolate Betaine Dyes With the Planarized Intramolecular Charge Transfer Behavior

Herein, we report a series of donor–π–acceptor (D–π–A) type dyes based on a pyridinium–cyclic enolate betaine (PCB) skeleton that exhibits planarized intramolecular charge transfer (PLICT) behavior in the excited state. These dyes, composed of an electron‐accepting PCB skeleton, a π‐spacer, and an electron‐donating N , N ‐diphenylamino group, displayed solvent‐dependent fluorescence with minimal changes in absorption properties. Experimental and theoretical analyses revealed that structural relaxation from a twisted to a planar geometry on the excited state potential energy surface plays a crucial role in the red shift of fluorescence. This PLICT relaxation mechanism enabled independent control of fluorescence wavelength without altering absorption wavelength, facilitating the development of a series of dyes with distinct fluorescence colors excitable by a single‐wavelength light source.