Synthesis, Photophysical Characterization, and Bioimaging with Benzoquinolizine‐Based Fluorescent Dyes Prepared through an Aryne‐Induced Domino Reaction

There is a pressing need to develop fluorescent dyes with light emission peaks in the wavelength range of 650–950 nm for optical live‐cell imaging applications. The advancement of fluorescent dyes with a large Stokes shift is imperative for their biological applications. A novel domino reaction is developed with success for the synthesis of diverse 9aH‐benzoquinolizines in 65–95% yields through reaction of α‐silylaryl triflates, pyridine derivatives, and activated alkynes at room temperature. This single‐flask reaction involves completion of a 1,2‐elimination, a 1,2‐addition, and an intermolecular (4 + 2) cycloaddition in situ. Among 15 new products, diethyl 9aH‐tribenzo[c,f,h]quinolizine‐1,2‐dicarboxylate with extension of π‐conjugation is identified as the top‐performing dye. It shows near‐infrared fluorescent emission at 674 nm with the Stokes shift of 197 nm, ε_max = 5.62 × 10⁴M^–1cm^–1, Φ_fl = 15.2%, and τ_av = 10.1 ns. It also exhibits excellent photostability with only 4.97% decomposed under irradiation with an Hg lamp for 60 min. With a remarkable 96 ± 2% viability, minimal toxicity toward CT‐26 cancer cells, and coupled with promising results from confocal microscopy, this 9aH‐tribenzoquinolizine emerges as an ideal fluorescent dye for imaging live cells.