Optimization and functionalization of red-shifted rhodamine dyes

Expanding the palette of fluorescent dyes is vital for pushing the frontier of biological imaging. Although rhodamine dyes remain the premier type of small-molecule fluorophore due to their bioavailability and brightness, variants excited with far-red or near-infrared light suffer from poor performance due to their propensity to adopt a lipophilic, nonfluorescent form. We report a general chemical modification for rhodamines that optimizes long-wavelength variants and enables facile functionalization with different chemical groups.