Fluorene analogues of xanthenes - low molecular weight near-infrared dyes.

Fluorene-based analogues of fluorescein, rhodol, and rhodamine exhibit absorption and fluorescence beyond 800-900 nm in water, 300-400 nm red-shifted compared to the original oxygen-bridged xanthene dyes, giving potential access to low molecular weight fluorescent markers for the second biological window (NIR-II, ca. 1000-1350 nm).