Near‐Infrared Fluorescence of Novel Pyridinium−Cyclic Enolate Betaine Dyes π‐Expanded by Condensed Cyclic Thiophene Spacers

Abstract

Herein, we report the synthesis of new near‐infrared (NIR) fluorescent dyes based on the pyridinium–cyclic enolate betaine (PCB) skeleton. The formation of the electron‐donor–π–electron‐acceptor (D–π–A) type dye with the electron‐donating N,N‐diphenylamino group and the electron‐accepting PCB skeleton through a series of π‐spacers allowed us to achieve NIR fluorescence. Particularly, a dye with a thienylisothianaohthene spacer showed NIR fluorescence with a significant intensity (λPL = 822 nm, ΦPL = 0.19 in DMSO). Detailed experimental and theoretical analyses suggested that the structural relaxation to a planar quinoidal form in the S1 state would be critical for intense NIR fluorescence.