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

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-diphenyl amino group and the electron-accepting PCB skeleton through a series of π−spacers allowed us to achieve NIR fluorescence. Particularly, a dye with a thienylisothianaphthene spacer showed NIR fluorescence with a significant intensity (λ_PL=822 nm, Φ_PL=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.