Towards better red emitters for bioimaging: Innovations in rhodamine and cyanine chemistry

Rhodamines and cyanines are widely used fluorescent dyes in biological and medical imaging because of their high brightness and well-established chemistry. Modern fluorescence imaging technologies, such as super-resolution and time-lapse imaging, have raised higher requirements for the photophysical, photochemical properties, and biocompatibility of rhodamine- and cyanine-based fluorescent dyes. In this review, we discuss strategies for designing rhodamine and cyanine cores to tune their properties. Particularly, we focus on the approaches to improve the brightness, photostability, fluorogenicity (signal-to-background ratio), and biocompatibility. These tailored molecules are privileged reagents for advanced microscopy and cell biology, as well as inspirations for next-generation molecular designs.