The Design and Bioimaging Applications of NIR Fluorescent Organic Dyes with High Brightness

Due to the merits of decreased photon attenuation, autofluorescence, and scattering, the near-infrared (NIR, 700–1700 nm) region is an important window in the field of biomedicine, such as in vivo fluorescence imaging, in which both the optical detection depth and the resolution/contrast have been significantly improved. In particular, for second NIR (NIR-II, 1000–1700 nm) dyes, biological tissues have almost no background interference. Typical fluorophores have excellent spectral performance and rich functional modification sites and can be optimized into NIR dyes for bioimaging. However, as the absorption/emission wavelength of fluorophores redshift to NIR, it is challenging to keep fluorophores with satisfying brightness. Therefore, for the purpose of increasing the absorption/emission wavelength of dye while promoting its brightness, it is necessary to study the structure–property relationship of the dyes. This review introduces the influences of fluorophores’ structure on their photophysical properties, summarizes the strategies for maintaining high fluorescence brightness along with redshifted absorption/emission wavelengths, and the latest advances of highly fluorescent brightness dyes. Finally, the opportunities and challenges in this emerging field are also provided. The authors aim to provide insightful design guidelines and clear overview of highly bright NIR fluorescent dyes, which might trigger new ideas and applications.