An AIE-active hemicyanine fluorogen with stimuli-responsive red/blue emission: extending the pH sensing range by “switch + knob” effect

In this work, a red-emissive zwitterionic hemicyanine dye, named TPE–Cy, containing tetraphenylethene (TPE) and N-alkylated indolium is designed and synthesized. TPE–Cy inherits the aggregation-induced emission (AIE) feature of TPE and displays a large Stokes shift (>185 nm), overcoming the limitations of the concentration-quenching effect and small Stokes shift (from a few to 20 nm) encountered by conventional cyanine dyes. By taking advantage of the photophysical AIE property and chemical reactivity towards OH−/H+, TPE–Cy is able to sense pH in a broad range (the broadest to date) by showing different emission colors and intensities: strong to moderate red emission at pH 5–7, weak to no emission at pH 7–10, and no emission to strong blue emission at pH 10–14. The acid/base-switched red/blue emission transition is reversible and can be repeated for many cycles. By means of NMR and HRMS analyses, we have drawn a mechanistic picture at molecular level to illustrate how this dye works as a pH-sensitive fluorescent probe.